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ANALYSIS OF MUNICIPAL SOLID WASTE
GASIFICATION PLANT INTEGRATED
WITH SOFC AND GAS TURBINE
DTU, Danmarks Tekniske Universitet
Department of Mechanical Engineering
Thermal Energy Systems
Supervisor: Prof. Masoud Rokni
Università degli Studi di Padova
Dipartimento di Ingegneria Meccanica
Corso di Laurea Magistrale in Ingegneria Energetica
Relatore: Prof. Alberto Mirandola
Filippo Bellomare
Master Student
Student Number: s104481
Matricola: 623162
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“ Dai diamanti non nasce niente,
dal letame nascono i fior ”
(Nothing grows from diamonds
whereas flowers can be born from manure)
Fabrizio De Andrè.
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Preface The present project has been developed at DTU (Technical University of Denmark), under
the supervision of Prof. Masoud Rokni, Department of Mechanical Engineering, in
collaboration with the University of Padova, under the supervision of Prof. Alberto
Mirandola, Department of Mechanical Engineering.
First of all I am grateful to my family for giving me the opportunity to study in Denmark
and to make the great experience to work with qualified people from all over the world;
thank you for encouraging and loving me so much every day.
I express my gratitude to my Danish Supervisors, Prof. Masoud Rokni, for helping and
following me with steadiness and dedication and for teaching me how useful is the
comparison and the dialogue in a scientific work.
I thank my Italian Supervisor, Prof. Alberto Mirandola, to having suggested me to make
this unforgettable experience abroad and to put me in touch with a so good Professor at
DTU.
Thanks a lot to Luca for having been so close despite the distance, to Ilenia for
remembering me every day that important people in life will never go away, to Dalila for
demonstrating me that a sister can be also an exclellent confidant and to Serena to show
me that a true friend is forever.
Thanks a lot to my Italian flatmates and to all my friends from Italy for having the patience
to bear with me at any time; in particular a thanksgiving to Satiri Danzanti for all the
moments in which we shared something so deep like music.
Thanks also to all good people I met in Denmark for all the best moments we shared
together during this wonderful experience of life.
Kgs. Lyngby, Denmark.
August 2011.
Filippo Bellomare
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Abstract Biomasses are an interesting source to produce energy with low pollutants emission and
reduced environmental impact; in particular to use Municipal Solid Waste as fuel can be a
solution for decreasing the quantities to storage in landfills. Costs are really negligible due
to the use of ―waste materials‖ to produce energy.
In this project a Municipal Solid Waste Gasification Plant Integrated with SOFC and Gas
Turbine has been studied; simulations have been run using DNA (Dynamic Network
Analysis) a component-based simulation tool for energy systems analysis developed at the
Thermal Energy Systems department (DTU, Danmarks Tekniske Universitet).
Different plant configurations have been studied; the best one includes a Regenerative Gas
Turbine. Obtained Thermodynamic Efficiency achieves 51,93% under optimized
conditions.
Simulations suggest that a critical parameter to control is the fuel moisture; different
configurations can be in fact realized to optimize the efficiency.
Exhausted gases coming from the plant have a high energetic content that, as studied
through simulations, can be used in an Absorption Cooling System with really interesting
results for the total efficiency; others internal hot flows inside the plant can be used for this
aim.
Thermoeconomic Analysis of the considered plant has been developed; a system of
equations has been obtained and solved through EES (Engineering Equation Solver).
Results suggest that SOFC is the most critical component not only for the irreversibility
due to the chemical reactions, but in particular for the high capital cost; future decrements
of the price of this component could make it more competitive on the energetic market.
However the use of a ―low-cost‖ fuel like the Municipal Solid Waste makes this plant
really interesting in terms of price of the produced electricity, around 0,09 €/kWh; a
comparison with the other renewable sources shows a very great vantage for this plant.
A comparison with a similar system fed by Natural Gas has also been studied; simulations
show competitive results under a thermodynamic point of view, but an interesting
advantage for the use of MSW Gasification under an economic point of view. The Pay-
Back Time is in fact minor like the price of the produced electricity. The plant is proved to
be theoretically feasible under both energetic and thermoeconomic point of view.
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Nomenclature
Acronyms
DNA Dynamic Network Analysis
EES Engineering Equation Solver
SOFC Solid Oxide Fuel Cell
GT Gas Turbine
HR Hybrid Recuperator
HHV High Heat Value
LHV Low Heat Value
MSW Municipal Solid Waste
NG Natural Gas
TIT Turbine Inlet Temperature
TEC Theory of the Exergetic Costs
O&M Operating and Maintenance
PEC Purchase Equipment Cost
DC Direct Cost
IC Indirect Cost
DCF Discounted Cash Flow
NPV Net Positive Value
PB Pay-Back Time
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DK Denmark
IT Italy
CPO Catalytic Partial Oxidation
ASR Adiabatic Steam Reformer
Symbols
U moisture content
r water heat of vaporization
efficiency
T temperature
p pressure
∆G Gibbs free energy variation
E Potential
F Faraday constant
n electron number
W mechanical work
R universal gas constant
Uf utilization factor
P power
q heat flow
h enthalpy
s entropy
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∆s difference of entropy
x quality
y mass fraction
molar volume
V volume
u internal energy
c unitary cost rate
cost rate
component cost rate
exergy flow
I investment cost
mass flow
A surface area
K global heat transfer coefficient
rc pressure ratio
int interest rate
ri rate of inflection
qi interest factor
f annuity factor
n equipment lifespan
M maintenance factor
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CP Construction Period
Hr operating hours
∆r cost difference factor
fk exergoeconomic factor
e relative exergy destruction
Superscripts
0 reference state
CI capital investment cost
O&M operating and maintenance cost
TOT total
Subscripts
th k-th element or component
i i-th element or component
0 dried basis or ideal part
w wet basis
el electric
max maximum
P product
F fuel
W mechanical power
i inlet
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o outlet
e exit
IN input
OUT output
L lost
D destroyed
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Table of contents
Preface
Abstract
Nomenclature
Table of contents
1. Biomass energy
1.1. Introduction to biomass
1.2. Municipal Solid Waste
1.3. Properties of Municipal Solid Waste
1.3.1. Composition
1.3.2. High Heat Value and Low Heat Value
1.3.3. Moisture content
1.3.4. Heat Capacity
1.4. Energy from Municipal Solid Waste
1.5. Municipal Solid Waste management and costs
1.6. Municipal Solid Waste production in Italy
2. General Overview
2.1. Block scheme of the plant
2.2. Gasification technology
2.2.1. Gasification reactions
2.2.2. Types of gasifiers
2.2.3. Gasifier parameters
2.2.4. Viking Two-Stages Gasifier
2.2.5. Upscale of the Two-Stages gasification process
2.2.6. Features of the Two-Stage gasification process
2.3. Fuel Cell technology
2.3.1. Introduction to Fuel Cell
2.3.2. Fuel Cell types
2.3.3. Nernst equation and Gibbs energy
2.3.4. Efficiency and Utilization factor
2.3.5. Losses in a Fuel Cell
2.3.6. Solid Oxide Fuel Cell (SOFC)
2.3.6.1. SOFC technologies
2.3.6.2. SOFC components
2.3.6.3. Pre-Reforming in SOFC
2.3.6.4. Fuels in SOFC
2.3.6.5. Application areas of SOFC plants
2.3.6.6. SOFC modeling through Lumped method
2.4. Gas Turbines
2.4.1. Introduction to Gas Turbines
2.4.2. Principle of operation
2.4.3. Components
2.4.4. Thermodynamic cycle
2.4.4.1. Idealized closed cycle
2.4.4.2. Idealized open cycle
2.4.4.3. Simple real cycle
2.4.5. Gas Turbine emissions
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2.4.5.1. CO
2.4.5.2. NOX
2.4.6. Gas Turbines for market
3. Municipal Solid Waste Gasification Plant Integrated with SOFC and Gas
Turbine
3.1. Plant layout
3.2. Size of the plant
3.3. Fuel features
3.4. Gasification plant
3.4.1. Gasifier
3.4.2. Dryer
3.4.3. Steam Blower
3.4.4. Splitter
3.4.5. Steam Generator
3.4.6. Air Pre-Heater
3.4.7. Mixer
3.4.8. Desulphuriser
3.4.9. Data Input Gasification plant
3.5. SOFC plant
3.5.1. SOFC stack
3.5.2. Anode Pre-Heater
3.5.3. Cathode Pre-Heater
3.5.4. Air compressor
3.5.5. Syngas blower
3.5.6. Burner
3.5.7. Data Input SOFC plant
3.6. Gas Turbine
3.6.1. Turbine
3.6.2. Data Input Gas Turbine
3.7. Simulation Results
3.7.1. Plant Efficiency
3.7.2. Electric power production
3.7.3. Auxiliary consumption
3.7.4. Net power production
3.7.5. Energy losses
3.7.6. Low Heat Value
3.8. Hybrid Regeneration
3.8.1. Hybrid Recuperator
3.8.2. Data Input Hybrid Recuperator
3.9. Effect of Hybrid Recuperator on Simulation Results
3.9.1. Plant Efficiency
3.9.2. Electric power production
3.9.3. Auxiliary consumption
3.9.4. Net power production
3.9.5. Energy losses
3.9.6. Lowe Heat Value
3.10. Sensitivity analysis
3.10.1. Environmental conditions
3.10.2. Fuel moisture content
3.10.3. Fuel Low Heat Value
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3.10.4. Fuel mass flow
3.10.5. Gasification temperature
3.10.6. SOFC operating temperature
3.10.7. Number of Stacks
3.10.8. Turbine Inlet Temperature
3.10.9. Pressure Ratio
3.11. Optimized configuration
3.12. Pressure Drops in Heat Exchangers
3.13. Effect of the removal of the Anode Pre-Heater
3.13.1. Plant Performance
3.13.2. Economic benefit
3.13.3. Pressure ratio increment
3.14. Integration of internal flows of heat
3.14.1. Recuperator features
3.14.2. Plant performance
3.14.3. Effect on the economic investment
3.14.4. Pressure ratio increment
4. Integration with Absorption Cooling Units
4.1. Introduction to Absorption Plants
4.2. Features of the used Absorption System
4.3. Integration of one Absorption Cooling Unit
4.3.1. Simulation results
4.3.2. Sensitivity Analysis
4.4. Integration of two Absorption Cooling Units
4.4.1. Simulation results
4.4.2. Sensitivity Analysis
5. Thermoeconomic and Investment Analysis
5.1. Thermoeconomic principles
5.2. Components equations
5.2.1. Dryer
5.2.2. Gasifier
5.2.3. Desulphuriser
5.2.4. Heat Exchangers
5.2.5. Blowers and Compressor
5.2.6. Mixer
5.2.7. Splitter
5.2.8. SOFC
5.2.9. Burner
5.2.10. Gas Turbine and Electric Generator
5.2.11. Other Auxiliary Equations
5.3. Total Capital Investment
5.4. Capital Investment and Operating & Maintenance Cost
5.5. Exergetic Analysis of the plant
5.6. Results of the Thermoeconomic Analysis
5.7. Results of the Investment Analysis
5.8. Price of the produced electricity
5.9. Future Scenario
6. Comparison with another system
6.1. SOFC plant fed by Natural Gas Integrated with Gas Turbine
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6.2. Thermodynamic comparison
6.3. Thermoeconomic and Exergetic comparison
6.4. Investment Analysis comparison
7. Conclusions
References
Figure references
APPENDIX I: DNA FILES
1. Municipal Solid Waste Gasification Plant Integrated with SOFC and
Gas Turbine: Optimized configuration with Hybrid Recuperator.
2. Absorption Cooling System.
3. SOFC plant fed by Natural Gas Integrated with Gas Turbine, CPO
unit.
4. SOFC plant fed by Natural Gas Integrated with Gas Turbine, ASR
unit.
APPENDIX II: EES FILES
1. Pressure Drops in Heat Exchangers.
2. Thermoeconomic Analysis
a. Municipal Solid Waste Gasification Plant Integrated with SOFC
and Gas Turbine: Optimized configuration with Hybrid
Recuperator
b. SOFC Plant fed by Natural Gas Integrated with Gas Turbine,
CPO unit.
c. SOFC Plant fed by Natural Gas Integrated with Gas Turbine, ASR
unit.
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1. Biomass energy
1.1 Introduction to biomass
The word ―Biomass‖ refers to vegetables and animals substances, not from fossil origin;
these can be used as fuel in a power plant for the production of electrical energy.
Biomasses derive from living or recently living biological organisms; so they can be
considered a particular kind of renewable energy source, because the carbon dioxide
placed in atmosphere by them use as pollution derives from the carbon amount absorbed
during them life.
In this way, the most important pollutions linked to biomass utilizations are relative to
transport, manufacture and transformation processes.
During them life biomasses absorb solar energy for growing; this is in fact important for
the life processes and for all the chemical reactions that take place in all the biological
organisms. Under this point of view they can be considered a particular kind of solar
energy storage.
Some kinds of biomass don‘t need particular treatments, as for example woodchips, that
can require only a drying process.
Other biomasses require particular treatments before use; for example to obtain fuels as
bio-ethanol or biogas, some chemical processes, as fermentation or anaerobic digestion,
are necessary.
Also municipal solid waste can be considered a valid biomass for a power plant.
Generally from biomass it is possible to obtain:
- Solid fuels, as for example woodchips or municipal solid waste;
- Liquid, as for example bio-ethanol or bio-diesel;
- Gaseous, as for example biogas.
Some benefits can be obtained; the principal is the reduction of pollutions and greenhouse
gases emissions. Another advantage is that in most cases these are wastes to landfill; by
them use it is possible to reduce the among of landfill and to destine these spaces to other
human activities.
It‘s also important to say that this kind of renewable energy suffers much less the
variability that characterizes other type of renewable energy, as for example solar or wind
energy.
Some problems are also connected to the use of biomass; in fact it‘s important to estimate
the indirect pollutions from transport, transformation and manufacture treatments.
Another critical point is the evaluation of spaces sheltered to human activities and food
production, for example in the case of driven culture. In the particular case of municipal
solid waste is also really important to control the accuracy of separation processes, to
avoid to burn dangerous substances that can could damage people‘s health; in fact is
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necessary a good control system of all kinds of pollutions, also to not affect adversely the
public opinion about plants that use this fuel. The public opinion is in fact a critical point
of the development and building of plants that use municipal solid waste as fuel.
Figure 1.1.1: Biomass cycle.
1.2 Municipal Solid Waste
A crucial point of all developed countries is the generation of excessive amounts of waste
per inhabitant; the waste management is in fact becoming a critical issue in a lot of parts of
the world. The amount of waste, both municipal and industrial, is in fact increasing day by
day.
This situation suggest the need for many local authorities of a plan for sustainable and
integrated strategies for handling and treating waste materials .
In a good management of waste it should be considered the following handling
( ref. [10] ) :
- Prevention of generation of waste;
- Reuse of waste materials for new product;
- Incineration with energy recovery at efficiencies comparable with alternative
technologies and sophisticated exhaust gas cleaning equipment;
- Reduction at minimum of landfilling disposal;
- Gasification processes.
A crucial point is that in the most efficient waste recycling systems there will always be
parts of the waste that cannot be recycled or reused in an economic way.
The use of this particular biomass for the production of electric energy presupposes a valid
and accurate separation of the different types of solid waste (plastic, paper, metal, organic
part, etc.).
All the different parts can be used in useful ways; for example plastic, paper and some
kinds of metallic waste can be differentiated and reused, organic part can be used to
produce compost or other products for agriculture and the undifferentiated waste, with a
little amount of other substances that can increase the heat value, can be used for the
production of electrical energy or of course also of heat in a CHP solution.
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1.3 Properties of Municipal Solid Waste
The principal waste properties as fuel are:
- chlorine, sulfur, nitrogen content;
- ash content;
- calorific value;
- moisture content;
- specific volume.
Chlorine, sulfur, nitrogen traces are capable of forming sulfur and nitrogen compounds
(SOx, NOx) and hydrochloric and sulfuric acid (HCl, H2SO4).
High ash content results in high cost of ash disposal and problems with fouling, corrosion
and erosion of boilers or gasifiers.
Calorific value and moisture content are linked; low and high heat values are a linear
function of moisture content. The higher is moisture content the lower are both heat
values. High specific volumes significantly affect transport and storage costs.
1.3.1 Composition
Approximately a sample of waste is a mixture of five different components of ( ref. [11] ) :
- 24,37 % kitchen garbage;
- 12,28 % papers;
- 12,95 % textiles;
- 13,9 % wood;
- 36,5 % plastic.
Figure 1.3.1.1: Municipal Solid Waste composition.
24,37%
12,28%
12,95%13,90%
36,50%
MSW composition
kitchen garbage papers textiles wood plastic
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With this mean composition, the five principle chemical elements are Carbon ( C ),
Hydrogen ( H ), Oxygen ( O ), Nitrogen ( N ), Sulfur ( S ); a percentage is also composed
of Ash ( between 1 % and 20 % ).
This word refers to all components that don‘t contribute to the syngas production under an
energetic point of view.
Tracks of Chlorine and Calcium could be also present, but their quantity is really
negligible (between 0,1 % and 2 % ).
The following table contains all the different compositions of the waste, in terms of
chemical elements, found in the references of this project; it refers to proximate analysis
on dry basis, in terms of mass fraction.
C H O S N Cl Ca ASH
waste 1 [11] 0,5181 0,0576 0,3022 0,0036 0,0026 0,0564
waste 2 [12] 0,7371 0,0857 0,1633 0,0074 0,0038 0,0027 0,0275
waste3.1 [13] 0,467 0,062 0,441 0,005 0,012 0,008
waste 3.2 [13] 0,44 0,057 0,472 0,007 0,014 0,011
waste 4 [14] 0,448 0,078 0,292 0 0,007 0,135
waste 5 [15] 0,3527 0,031 0,191 0,0042 0,019 0,1495
waste 6 [16] 0,5158 0,059 0,4244 0 0,0008 0,009
waste 7 [17] 0,4 0,069 0,354 0,001 0,006 0,17
waste 8.1 [18] 0,612 0,082 0,266 0,002 0,013 0,025 0,189
waste 8.2 [18] 0,691 0,074 0,196 0,003 0,019 0,017 0,177
waste 8.3 [18] 0,574 0,038 0,368 0,002 0 0,018 0,204
waste 9 [19] 0,417 0,06 0,363 0,0017 0,0075 0,01 0,034 0,125
waste 10.1 [20] 0,466 0,068 0,3451 0,0013 0,0128 0,0108 0,0267 0,102
waste 10.2 [20] 0,417 0,05 0,363 0,0017 0,0075 0,01 0,027 0,125
waste 11.a [21] 0,459 0,068 0,337 0 0,011 0,123
waste 11.b [21] 0,483 0,076 0,316 0,001 0,006 0,002 0,116
waste 11.c [21] 0,408 0,067 0,389 0,006 0,009 0,007 0,114
waste 11.d [21] 0,422 0,061 0,399 0,001 0,008 0,005 0,104
waste 12 [22] 0,476 0,06 0,329 0,003 0,012 0,12
Table 1.3.1.1: Some Municipal Solid Waste chemical compositions.
If we refer to these data, the mean composition is the following:
- C = 0,490
- H = 0,06
- O = 0,34
- S = 0,003
- N = 0,009
- ASH = 0,10
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Figure 1.3.1.2: Waste mean composition in terms of mass fraction.
1.3.2 High Heat Value and Low Heat Value
The High heat Value (HHV) is defined as the amount of heat that is available after the
completely combustion, in conditions of constant pressure, of the unit mass of fuel when
the combustion products achieve the initial temperature of the reagents.
It‘s possible to measure directly HHV by some sophisticated instrument; the most famous
is the calorimetric bomb, called also Mahler‘s bomb. In this one happens a completely
stoichiometric reaction between a defined amount of fuel and oxidant; the produced heat is
absorbed by a known mass of water. By the knowledge of the temperature increase it‘s
possible to obtain the exchanged amount of heat.
Exist also some experimental formulas to estimate HHV, based on the elementary
composition of the fuel; the most common is the ―Dulong formula‖, whose expression, in
Btu/lb is the following ( ref. [23] ):
HHV = 145 C + 6101. ( H – O/8 ) + 40 N + 10 S [Btu/lb] (1.1),
where C is the Carbon percentage, H is the Hydrogen percentage, O is the Oxygen
percentage, N is the Nitrogen percentage, S is the Sulfur percentage.
Another expression of Dulong formula, in terms of MJ/kg, is the following ( ref. [22] ) :
HHV = 0,3383 C + 1,443 ( H – O/8 ) + 0,0942 S [MJ/kg] (1.2).
The same authors that report the last one studied another correlation to estimate HHV,
whose expression is ( ref. [22] ) :
48,9%
6,0%
33,9%
0,3%0,9%
10,0%
Waste mean composition in terms of chemical elements
C H O S N ASH
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HHV = 0,3491 C + 1,1783 H + 0,1005 S – 0,1034 O – 0,0151 N – 0,0211 ASH
[MJ/kg] (1.3).
They refer that this correlation goes well for a lot of kind of fuel, also waste, and it‘s valid
under the following conditions for the composition ( ref. [22] ):
0% < C < 92,25%
0,43% < H < 25,15%
0% < O < 50%
0% < N < 5,60%
0% < S < 94,08%
0% < ASH < 71,4%
In the last expression is interesting that is included the contribute of all elements; the
authors refer that the absolute error is around 1,45% .
The term Lower Heating Value (LHV) refers to the difference between the HHV and the
latent heat of condensation during combustion; this is the value that is used to calculate the
efficiency of a energy plant.
In the combustion, the production of steam is linked to the elementary composition in
terms of Hydrogen and moisture; so, if we assume that the latent heat of condensation for
water is approximately 2500 kJ/kg, the LHV, on dry basis, is calculated by the following
formula:
LHV = HHV – ( H * 2500 ) – ( MOI * 2500 ) (1.4).
Some papers report the LHV of waste, calculated by experimental way; the following table
compares these values (LHV*) with those calculated by empiric formulas:
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DULONG CHANNIWALA
MOI LHV* [kJ/kg] HHV [kJ/kg] LHV [kJ/kg] HHV [kJ/kg] LHV [kJ/kg]
waste 1 [11] 0,07 20287 20422 20103 21662 21343
waste 2 [12] 0,0215 33000 34427 34159 34152 33884
waste3.1 [13] 0,032 16838 16603 19064 18829
waste 3.2 [13] 0,095 14663 14283 17222 16842
waste 4 [14] 0,095 19900 21144 20712 21516 21083
waste 5 [15] 0,095 17185 13000 12685 13689 13374
waste 6 [16] 0,0825 18308 17954 20550 20196
waste 7 [17] 0,04 18900 17113 16840 18076 17804
waste 8.1 [18] 0,037 20800 27757 27460 27878 27581
waste 8.2 [18] 0,017 23300 30548 30320 30444 30216
waste 8.3 [18] 0,197 12600 18283 17695 20300 19713
waste 9 [19] 0,111 18392 16234 15806 17616 17188
waste 10.1 [20] 0,037 20637 19365 19102 20491 20228
waste 10.2 [20] 0,111 18392 14791 14388 16438 16035
waste 11.a [21] 0,04 19262 18992 20275 20005
waste 11.b [21] 0,045 21616 21314 22305 22003
waste 11.c [21] 0,155 16511 15956 17922 17367
waste 11.d [21] 0,04 15891 15639 17573 17320
waste 12 [22] 0,095 19879 18855 18468 20044 19656
Table 1.3.2.1: Comparison between measured LHW and calculated value for different compositions.
Referred to the mean composition, the calculated values are the following:
LHVDULONG = 19252 [kJ/kg]
LHVCHANNIWALA = 20473 [kJ/kg]
In some papers moisture content is reported; in the other cases is considered a guessed
value. The effect of moisture over LHV will analyzed in the following paragraph.
It‘s possible to calculate the LHVw, on wet basis, through the following demonstrated
formula ( ref [27] ):
LHVw = LHV – MOI ( LHV + r ) (1.5),
where LHV is the Low Heat Value on dry basis, MOI is the content of moisture of
biomass and r is the water heat of vaporization.
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1.3.3 Moisture content
When is used in a power plant a biomass as woodchips or waste, moisture is an important
parameter. Processes to decrease this value are necessary; so in this kind of plants is
present a dryer, to obtain low value of humidity.
As shown in the last paragraph, moisture is linked with the LHV; an high value can
influence the properties of fuel.
As we can see in the following diagram, if moisture increase, of course LHV decrease;
using parameters given upon for the mean composition, LHV is printed in the following
diagram as function of moisture:
Figure 21: Pattern of LHV at varying of moisture content.
1.3.4 Heat Capacity
Heat capacity, or thermal capacity, is the measurable physical quantity that characterizes
the amount of heat required to change a body's temperature by a given amount.
It‘s really difficult to find in literature a formula or a model to calculate heat capacity of a
solid fuel; some studies have been conducted for example by Yingwei Fei and Saxena
( ref. [24] ).
But this model requires the knowledge of empiric coefficients, really difficult to find for
waste; in fact in the mentioned work are analyzed in particular six different kinds of fuel,
once known the empiric coefficients.
Derived quantities that specify heat capacity as an intensive property, independent from
the size of a sample, are the ―molar heat capacity‖, which is the heat capacity per mole of a
pure substance, and the ―specific heat capacity‖, often simply called specific heat, which is
the heat capacity per unit mass of a material.
17700
17800
17900
18000
18100
18200
18300
18400
18500
18600
0 0,1 0,2 0,3 0,4
LHV
[kJ
/ k
g ]
MOISTURE
LHV as function of fuel moisture
LHV
Lineare (LHV)
29
In this work is assumed with good approximation that specific heat capacity can be
estimated as the weighted average of the values of all the elementary components of fuel:
(1.6),
where xi are the mass fractions of every component and cp,i is the specific heat capacity of
every elementary component.
Reliable values for specific heat capacity of each one component can be the following, at a
temperature of 300K (ref. [25] ) :
ELEMENT cp [kJ/kg K] , T=300K
C 0,71
H 14,31
O 0,920
S 0,71
N 1,041
Table 1.3.4.1: Specific Heat Capacity of chemical elements present in the waste composition.
1.4 Energy from Municipal Solid Waste
Two solutions to produce energy from waste are:
- Incineration with energy recovery;
- Gasification for syngas production.
Destroying waste through incineration has been practiced for many years. In the 1800s
waste incinerators were use commonly in England; in 1896 the streets of Oldham were lit
by electricity produced from waste ( ref. [10] ). In 1993 in these European countries, the
percentage of incinerated municipal solid waste was the following ( ref. [10] ):
Table 1.4.1: Percentage of incinerated waste in different countries.
The principle of an incineration plant with energy recovery is to use the heat provided by
waste combustion to produce superheated steam that cross a normal Rankine cycle for the
production of electrical energy through a steam turbine connected to an electric generator;
30
normally in these plants there‘s only one steam overheating and one regenerative heat
exchanger, usually a deaerator.
The efficiency of this plants are around 22-26%, for a production of 20-30 MWe of
electric power; in the average, the mass flow of waste inlet is around 20-25 kg/s.
In a gasification process, waste is subject of chemical treatments through air or steam
utilization; the result is a synthesis gas, called ―Syngas‖, composed principally of
Hydrogen and Carbon Monoxide. Tracks of Hydrogen Sulfure could be present, but it can
be easily separated through a desulphuriser.
The gasification process is usually based on an atmospheric-pressure circulating fluidized
bed gasifier coupled to a tar-cracking vessel; the gas produced is then cooled and cleaned.
Syngas can be used as fuel in a power plant; energy systems that can be used are for
example:
- Gas Turbine;
- Steam Cycle;
- Combine Cycle;
- Internal and External Combustion Engine;
- Fuel Cell, in particular SOFC (Solid Oxide Fuel Cell).
The last one is really interesting to obtain good values of efficiency, overall in Hybrid
Cycles, where SOFC are integrated with other power plants, as for example a Gas Turbine
or a Steam Cycle; SOFC is a really flexible system, that permit to use a lot of different
fuels, but it could be very sensible to poisoning phenomena. Is this the reason why it could
be necessary to desulfurize the syngas.
Compared with modern waste incinerators with heat recovery, the gasification process, as
say Morris and Waldheim ( ref. [10] ), will permit an increase in electricity output up to
50%; also in an economical way gasification of solid waste can compete with incineration
technology. In fact waste incinerators have been required to install sophisticated exhaust
gas cleaning equipment; this can be large and expensive.
1.5 Municipal Solid Waste management and costs
Municipal Solid Waste cost includes the following factors ( ref. [26] ):
- Direct management;
- Sweeping and Cleaning;
- Municipality costs;
- Capital costs for transportation.
The mean values associated are ( ref. [26] ):
31
Costs [ € / inhabitant year]
Direct Management 86,47
Sweeping and Cleaning 19,53
Municipality 18,00
Transportation 6,64
TOTAL 130,64
Table 1.5.1: Municipal Solid Waste costs.
If we want to use waste to produce energy in a power plant, the only one cost linked with
this aim is correlated with the transportation; in fact all the others costs are in any case
necessary, even if waste is not used as fuel and it‘s possible to consider as only cost of fuel
the one for the transportation.
So, if we consider for example ( ref. [26] ) that the production of municipal waste in Italy
is around 550 kg/inhabitant year and in Denmark is around 801 kg/inhabitant year (so the
costs in terms of €/inhabitant year will be higher), the cost of waste as fuel, in terms of
€/kg is:
CWASTE = 0,012 €/kg .
If we consider for this kind of fuel a Low Heat Value of 19879 kJ/kg, as seen in the
paragraph 1.3.2, the cost per unit of energy is:
cWASTE = 0,0022 €/kWh .
1.6 Municipal Solid Waste production in Italy
As is shown in the last report of ISPRA (Istituto Superiore per la Protezione e la Ricerca
Ambientale), the waste production in all the italian provinces is the following ( ref. [26] );
data are reported in terms of kg/s instead of ton/years:
DISTRICT INHABITANTS SEPARATED COLLECTION
[kg/s]
UNDIFFERENTIATED WASTE [kg/s]
BULKY WASTE [kg/s]
TOTAL PRODUCTION
[kg/s]
SEPARATED COLLECTION
[%]
Torino 2.290.990 18,1 18,7 0,0 36,8 49,3
Vercelli 180.111 0,8 2,2 0,0 3,0 26,5
Novara 366.479 3,7 2,1 0,0 5,8 63,4
Cuneo 586.020 4,2 5,5 0,0 9,6 43,3
Asti 220.156 1,7 1,3 0,0 3,0 56
Alessandria 438.726 3,6 4,4 0,0 7,9 45,3
Biella 187.314 1,2 1,6 0,0 2,8 41,8
Verbania 162.775 1,6 1,1 0,0 2,7 58,4
Piemonte 4.432.571 34,8 36,8 0,0 71,6 48,5
32
Aosta 127.065 0,9 1,4 0,1 2,4 38,6
Valle d’Aosta 127.065 0,9 1,4 0,1 2,4 38,6
Varese 871.448 7,8 4,7 1,0 13,4 57,7
Como 584.762 4,1 4,2 0,6 8,9 46,6
Sondrio 182.084 1,1 1,4 0,1 2,6 43,4
Milano 3.930.345 29,4 32,2 2,6 64,2 45,8
Bergamo 1.075.592 8,3 6,1 1,0 15,4 53,9
Brescia 1.230.159 9,3 13,2 1,3 23,8 39,1
Pavia 539.238 2,6 6,8 0,3 9,6 26,6
Cremona 360.223 3,2 2,1 0,5 5,8 56,1
Mantova 409.775 3,3 3,6 0,3 7,2 46,1
Lecco 335.420 2,7 2,1 0,3 5,0 53,4
Lodi 223.630 1,7 1,4 0,2 3,2 51,5
Lombardia 9.742.676 73,5 77,6 8,1 159,2 46,2
Bolzano 498.857 4,0 3,2 0,2 7,4 53,8
Trento 519.800 5,1 3,1 0,4 8,6 59,4
Trentino A.A. 1.018.657 9,1 6,3 0,6 16,0 56,8
Verona 908.492 7,1 6,8 0,6 14,5 48,8
Vicenza 861.768 6,5 4,6 0,5 11,5 56,2
Belluno 214.026 1,4 1,5 0,1 3,1 46,4
Treviso 879.408 7,3 3,2 0,4 10,9 66,9
Venezia 853.787 7,4 9,8 0,5 17,7 41,6
Padova 920.903 8,3 6,1 0,3 14,7 56,7
Rovigo 247.164 2,6 1,6 0,2 4,3 59,7
Veneto 4.885.548,00 40,5 33,5 2,6 76,6 52,9
Udine 539.723 3,8 4,9 0,2 8,9 42,5
Gorizia 142.461 1,3 0,9 0,1 2,3 56,3
Trieste 236.393 0,8 3,0 0,0 3,7 20,3
Pordenone 312.359 2,4 1,9 0,2 4,5 54,2
Friuli V.G. 1.230.936 8,3 10,7 0,5 19,4 42,6
Imperia 220.712 0,9 3,8 0,0 4,7 19,6
Savona 286.646 1,6 4,8 0,0 6,4 24,8
Genova 884.635 3,3 12,2 0,3 15,9 20,8
La Spezia 223.071 1,0 3,2 0,1 4,3 23,4
Liguria 1.615.064 6,8 24,0 0,5 31,3 21,8
Piacenza 285.922 2,6 2,9 0,4 5,9 44,4
Parma 433.154 4,0 4,2 0,2 8,4 47,8
Reggio Emilia 519.458 6,2 6,1 0,0 12,4 50,4
Modena 688.286 6,2 7,5 0,3 14,0 44
Bologna 976.175 6,3 11,6 0,0 18,0 35,1
Ferrara 357.980 3,3 4,5 0,1 7,9 42,3
Ravenna 385.729 4,3 5,1 0,0 9,4 45,8
Forli’-Cesena 388.019 3,9 5,5 0,2 9,6 40,5
Rimini 303.256 3,2 4,9 0,1 8,2 38,9
Emilia R. 4.337.979 40,0 52,2 1,4 93,6 42,7
33
Nord 27.390.496 213,9 242,6 13,7 470,2 45,5
Massa Carrara 203.698 1,0 3,4 0,1 4,5 22,9
Lucca 390.200 3,7 5,7 0,1 9,6 38,9
Pistoia 290.596 1,9 4,2 0,0 6,1 30,6
Firenze 984.663 7,4 12,9 0,1 20,5 36,3
Livorno 340.691 2,5 5,2 0,1 7,9 32,3
Pisa 410.278 2,6 5,6 0,1 8,4 31,6
Arezzo 346.324 1,8 4,7 0,1 6,6 27,1
Siena 269.473 2,4 3,2 0,2 5,7 41,3
Grosseto 225.861 1,4 3,8 0,1 5,4 26,6
Prato 246.034 2,4 3,8 0,0 6,2 37,8
Toscana 3.707.818 27,1 52,5 1,0 80,7 33,6
Perugia 661.682 3,9 9,3 0,1 13,3 29,6
Terni 232.540 1,1 2,9 0,1 4,1 26,8
Umbria 894.222 5,0 12,2 0,2 17,4 28,9
Pesaro-Urbino 381.730 1,9 5,4 0,2 7,5 26
Ancona 476.016 2,3 5,8 0,1 8,2 28,2
Macerata 322.498 1,5 3,5 0,1 5,1 29,2
Ascoli Piceno 389.334 1,5 5,1 0,0 6,7 22,2
Marche 1.569.578 7,2 19,8 0,4 27,4 26,3
Viterbo 315.523 0,5 4,2 0,3 5,1 10,6
Rieti 159.018 0,1 2,2 0,1 2,5 5,5
Roma 4.110.035 11,1 69,2 1,1 81,4 13,7
Latina 545.217 1,5 8,8 0,1 10,4 14,5
Frosinone 496.917 0,3 6,4 0,0 6,7 5
Lazio 5.626.710 13,7 90,8 1,6 106,0 12,9
Centro 11.798.328 53,1 175,4 3,1 231,6 22,9
L’Aquila 309.131 0,6 4,4 0,0 5,0 12,2
Teramo 309.838 1,8 3,7 0,0 5,6 32,7
Pescara 319.209 1,1 4,1 0,0 5,2 20,4
Chieti 396.497 1,4 5,0 0,0 6,4 21,4
Abruzzo 1.334.675 4,9 17,3 0,0 22,2 21,9
Campobasso 231.900 0,2 2,9 0,0 3,2 6,9
Isernia 88.895 0,1 1,0 0,0 1,1 5,2
Molise 320.795 0,3 4,0 0,0 4,3 6,5
Caserta 904.197 1,5 11,7 0,0 13,3 11,5
Benevento 288.726 0,8 2,5 0,0 3,4 23,8
Napoli 3.074.375 7,4 42,8 0,0 50,2 14,8
Avellino 439.565 1,8 3,1 0,0 5,0 36,9
Salerno 1.106.099 4,8 9,7 0,0 14,5 33,3
Campania 5.812.962 16,4 69,8 0,1 86,4 19
Foggia 682.260 1,0 9,6 0,0 10,7 9,7
Bari 1.601.412 3,2 23,2 0,0 26,4 12
Taranto 580.481 0,7 9,5 0,0 10,2 6,8
34
Brindisi 402.891 0,7 6,9 0,0 7,6 9,3
Lecce 812.658 1,6 11,2 0,1 12,9 12,4
Puglia 4.079.702 7,2 60,4 0,2 67,7 10,6
Potenza 386.831 0,5 4,0 0,0 4,5 10,7
Matera 203.770 0,2 2,5 0,0 2,7 6,3
Basilicata 590.601 0,7 6,6 0,0 7,2 9,1
Cosenza 733.508 1,4 8,9 0,0 10,3 13,6
Catanzaro 367.990 0,9 4,9 0,0 5,8 15,7
Reggio Calabria 566.507 0,8 7,2 0,0 8,0 10,2
Crotone 173.370 0,4 2,4 0,0 2,8 13
Vibo Valentia 167.334 0,2 2,1 0,0 2,3 8,8
Calabria 2.008.709 3,7 25,5 0,0 29,2 12,7
Trapani 435.913 0,8 6,1 0,0 6,9 11,6
Palermo 1.244.680 1,5 20,8 0,2 22,4 6,5
Messina 654.601 0,5 9,9 0,0 10,4 4,7
Agrigento 455.083 0,7 6,0 0,0 6,7 10,7
Caltanissetta 272.289 0,2 3,8 0,0 4,0 5,5
Enna 173.515 0,1 2,2 0,0 2,3 4,8
Catania 1.084.977 1,2 18,5 0,0 19,7 6,3
Ragusa 313.901 0,3 4,5 0,0 4,9 7
Siracusa 402.840 0,3 6,4 0,0 6,7 4
Sicilia 5.037.799 5,7 78,1 0,2 84,0 6,7
Sassari 336.451 1,6 3,5 0,0 5,2 30,9
Nuoro 161.444 0,7 1,4 0,0 2,2 32,3
Cagliari 559.820 3,2 5,7 0,0 8,9 35,6
Oristano 167.295 0,9 1,3 0,0 2,2 40,6
Olbia-Tempio 154.319 1,2 3,1 0,0 4,3 28,5
Ogliastra 58.097 0,3 0,3 0,0 0,6 52,3
Medio Campidano 103.020 0,8 0,6 0,0 1,4 58,7
Carbonia-Iglesias 130.555 0,6 1,5 0,0 2,1 28,5
Sardegna 1.671.001 9,3 17,4 0,1 26,8 34,7
Sud 20.856.244 48,1 279,1 0,7 327,9 14,7
Italia 60.045.068 315,1 697,1 17,5 1029,7 30,6
Table 1.6.1: Waste production in Italy; data in terms of kg/s.
35
2. General overview
2.1 Block scheme of the plant
The studied plant can be represented through the following block scheme:
Figure 2.1.1: Block scheme of the plant.
The principal components of plant are:
- Gasification Plant;
- SOFC (Solid Oxide Fuel Cell) plant;
- Gas Turbine.
Through the Gasification plant, Municipal Solid Waste is converted in Syngas, a mixture
of H2, N2, CO, CO2, H2O, CH4 and Ar; the produced Syngas is previously cleaned to
remove tracks of H2S that could poison the SOFC.
The cleaned Syngas is sent to the SOFC plant to produce electric power; but the reaction is
not of course complete. So the used fuel and the flue gas from the anode and the cathode
side are used in a burner to complete the combustion; the produced heat feeds a bottoming
cycle, consists of a Gas Turbine. In this one is produced also electric power; heat is
36
released through the exhausted gases; it can be used for others applications (i.e. district
heating or district cooling).
In fact for the second principle of the thermodynamic not all the heat coming from the
combustion process can be converted in electric energy; a part of this is lost as heat in the
exhausted gases.
From the block scheme we can also see another loss source: Ashes and Tar from the
gasification plant.
Over the fuel, the other input of the plant are:
- Air to feed the gasifier;
- Air to feed SOFC plant.
To introduce these, auxiliary energy is necessary for a compressor; another use of auxiliary
energy is necessary for blowing the Syngas out from the Gasification Plant to the SOFC.
The efficiency of the plan can be expressed as the ratio between the net produced electric
power and the fuel power, where with ―net power‖ we mean the difference between the
whole produced power and the power used in the auxiliary components (compressors,
blowers, control systems, etc.).
It can be expressed as:
=
(2.1),
where PTOT is the produced electric power, PAUX is the electric power used for the auxiliary
systems and PFUEL is the power introduced in the plant through the fuel; the last one can be
referred to the Low Heat Value or the High Heat Value of the fuel.
2.2 Gasification technology
Generally, a ―gasifier‖ is a reactor with three incoming flows:
- Fuel;
- Oxidand (air or oxygen);
- Water.
The outgoing flows are the following:
- Synthesis gas;
- Ashes, the solid residual of the process.
The reaction that has place in the gasifier is the conversion, through partial oxidation at
elevated temperature, of a carbonaceous feedstock such as coal or biomass into gaseous
energy.
37
This process consists of three steps:
- ―Drying‖; by this operation, moisture inside fuel is reduced down to 10-15 %
before entering in the gasifier.
- ―Pyrolysis‖; chemical boundaries are broken to form volatile components at
temperature below 600 °C. Biomass consists of 75-85 percent volatile matter
therefore this step plays an important part in the global process. Char, tars and ashes
are also present.
- ―Gasification‖; solid char, pyrolysis tars and gases are oxidized. Temperatures are
up to 700-800 °C.
Figure 2.2.1: Gasifier scheme.
2.2.1 Gasification reactions
The most important reactions that take place are ( ref. [1] ) :
PARTIAL COMBUSTION: C + 1/2O2 ↔ CO + 110,6 kJ/mol
TOTAL COMBUSTION: C + O2 ↔ CO2 + 393,7 kJ/mol
GASIFICATION: C + H2O ↔ H2 + CO – 131,4 kJ/mol
WATER GAS SHIFT: CO + H2O ↔ CO2 + H2 + 41,2 kJ/mol
METHANATION: CO+3H2 ↔ CH4 + H2O + 206,4 kJ/mol
38
The partial combustion reaction produces only the 28% of the heat that we can obtain from
the total combustion reaction; the remaining 72% is available in the synthesis gas as heat
value.
Usually, if is introduced more oxygen, the second reaction can advance and the synthesis
gas temperature is higher , but the heat value is lower. The gasification reaction is
endothermic; for this reason is better if the temperature is high.
The water inlet in this process has two aims: moderate temperatures and make hydrogen;
H2 and CO are the most important components of synthesis gas.
Water gas shift reaction determines the ratio between H2 and CO in the synthesis gas;
methanation reaction is instead important at low temperatures.
Other elements in small amount are the following:
- ―Sulfur‖, that with less oxygen becomes Hydrogen Sulfide ( H2S ).
- ―Nitrogen‖, that could be in the synthesis gas in the molecular state or as Ammonia
( NH3 ) or Hydrocyanic Acid ( HCN ); it could be better remove these elements,
because them presence is relevant for the formation of NOx in the synthesis gas.
- ―Ash‖, that is melted at a temperature of 1100-1200 °C ; through the next cooling it
becomes ―slag‖, an inert product that can be easily removed.
2.2.2 Types of Gasifiers
Depending on the type of reactions that take place, it‘s possible distinguish three principal
types of gasifier:
1- ―Moving bed gasifier‖, where the fuel flows in counter flow with the hot produced
gases; the synthesis gas in at a temperature around 450-550 °C.
Figure 2.2.2.1: Moving bed gasifier.
2- ―Fluidized bed gasifier‖, where the fuel bed is fluidized from a steam and oxidant
flow; the operative temperature is around 800-100 °C, With these temperatures is
really simple removing sulfur , but is more difficult the carbon conversion.
39
Figure 2.2.2.2: Fluidized bed gasifier.
3- ―Entrained flow gasifier‖; this kind of gasifier is very similar with a combustor, but
they work with lack of oxygen. Operative temperatures are very high, around 1200-
1400 °C, to obtain very low residence times and to bring the ashes to the melting
point. They are more simple and cheaper than the other two. The critical point are
the highest temperatures; to achieve these values is necessary, for a good efficiency
of the process, a good amount of heat recovery.
Figure 2.2.2.3: Entrained flow gasifier.
2.2.3 Gasifier parameters
Parameters that most influence the gasification process are:
- ―Type of oxidant‖; the most common used oxidants are air or pure oxygen. Using
oxygen the quality of the produced gas is better; but in this case is necessary to
separate oxygen from air, with a significant spending of energy for example in an
ASU (Air Separation Unit). If we use air as oxidant, is produced a gas with about
40
half of the calorific value due to the diluting effect of the nitrogen; in this case
steam could be used to increase hydrogen content in the gas.
- ―Temperature‖; it is the most important parameter because it defines gasification
rates and reactor design. Ash disposal is also strongly influenced by it. Most
biomass gasifiers utilize dry ash removal systems; therefore ash melting
temperature (1100-1200 °C) must be avoided.
- ―Pressure‖; this parameter doesn‘t have any influence on system design and costs.
But the higher is the pressure the higher is the rate of the processes. Pressure has
only a modest effect on the chemical reactions; but pressurized gasifiers are more
expensive than environmental pressure one.
2.2.4 Viking two-stages Gasifier
Gasifier model used in this project is the two-stage biomass gasification process developed
at the Technical University of Denmark (DTU).
This gasifier was established in 2002 and it had during 2003 more than 2000 hours of
operation (ref. [9] ).
Figure 2.2.4.1: Viking two-stage gasifier at DTU.
As is shown in the following figure, Viking gasifier consists of:
- drying section with superheater steam;
- High temperature pyrolysis and partial oxidation sections;
- gasification part;
41
- exhaust super-heater and air pre-heater;
- cleaning section;
- engine.
Figure 2.2.4.2: Layout of a two-stage gasification plant.
Fuel enters in the drying chamber; pyrolysis process reaches a temperature of about 600
°C. In the gasification section, gaseous mixture is partially oxidized at 1100 °C; ashes are
separated from the produced syngas which comes out at 750-800 °C.
Drying, air preheating and pyrolysis are fed by flows of the thermal energy inside the
syngas. The exhaust superheat syngas is cooled down releasing its energy to warm up
exhaust gases coming out from the engine; then exhaust gases feed the drying and
pyrolysis chamber. To enable high energy efficiency, air for the oxidation is also preheated
by syngas. After particles are removed, cleaned gas feeds a Diesel engine for the
production of electric energy.
Viking gasifier offers some interesting features ( ref. [9] ):
- low tar content in produced syngas (<5 [mg/Nm3]);
- stable unmanned operation;
- high coldgas efficiency (>95 %);
- low environmental impact (clean condensate, high carbon conversion ratio);
- gasification at environmental pressure.
2.2.5 Upscale of the two-stage Gasification process
This model of gasifier combines the following advantages ( ref. [9] ) :
42
- Stage divided gasification;
- Drying with superheated steam;
- Pyrolysis with superheated steam;
- Partial oxidation;
- Gasification with steam.
The following figure shows the principle of drying with superheat steam:
Figure 2.2.5.1: Principle of drying with superheat steam.
Steam is generated using an external source; in a modified plant it could be also possible to
use steam extracted from the wet biomass in the drying chamber.
This solution can offer some advantages as:
- Environmental friendly drying;
- No fire hazards;
- No loss of product;
- Improved drying rate.
Pyrolysis process requires a certain amount of moisture (around 10%) inside the fuel;
therefore superheated steam makes gasification process applicable for fuel with high
humidity content (up to 60%). Depending on the type of used fuel, moisture can be
significantly variable; this makes the solution suitable for example for biomass with high
moisture content.
When dry fuels are used, other heat sources must be added to the pyrolysis reactor, for
example hot product gas from the gasifier, returned charcoal, additional water, steam and
air mixture, etc.
Higher process rates are achieved when a mixture of air and steam is used as gasification
agent; temperature can be also lowered. Hydrogen content is also increased and this makes
the produced syngas composition suitable for a SOFC plant.
Advantages from the steam gasification are (ref. [9] ) :
- Lower temperature in partial oxidation zone;
- Lower soot production;
- Lower emission after engine.
43
2.2.6 Features of the two-stage Gasification process
The most important features of this kind of plant are ( ref. [9] ) :
- Low tar content in produced gas;
- High cold gas efficiency efficiency
- The gas cleaning will be relatively simple and robust.
- Stable process
- Clean fluegas to be used for district heat in condensing mode without cleaning of
condensate.
- No fire hazards in dryer.
- Low maximum temperatures in gasifier.
- Low soot production and low emissions from combuster.
- Well suited for fuels with moist content of 40-60%.
In a size between 3-10 MW, the pyrolysis and the gasification reactor can be of a moving
bed type; in this design, the pyrolysis reactor is heated with two heatsources: direct heat by
hot steam and indirect heat produced by hot gases from the gasifier. The retention time of
the biomass in the reactor can be reduced from half an hour to a few minutes; in this way
the size of the pyrolysis reactor can reduced considerable.
The pyrolysis and gasification reactors can also be designed as fluid bed; with this
solution, this kind of gasifier can be scaled up to large applications. With the separation of
the pyrolysis and gasification parts, the volatile gases from the pyrolysis, containing tar,
are partially oxidized and the tar content decrease dramatically; in this way, the investment
and running costs of a gas cleaning system are reduced and the reliability can be increased.
In this process, pyrolysis and gasification chamber use fluid beds; fluidized with steam; the
gasifier can be integrated with a steam drying process, where the produced steam is used
for the gasification.
This system has been studied with the following systems:
- Gas engine;
- Gas turbine and Recuperated gas turbine;
- Integrated gasification combine cycle.
The most important parameters for the models are the following (ref. [9] ) :
- Biomass is dried with superheated steam to 10% moist;
- Gasification at atmospheric pressure;
- Steam is used as the ‗agent‘ for both the pyrolysis and the gasification processes;
- The air is preheated for the partial oxidation;
- Condensing and cooling of syn- and flue gas by means of district heating (45°C);
50% moist in the fuel.
44
2.3 Fuel Cell technology
2.3.1 Introduction to Fuel Cell
The word ―Fuel Cell‖ refers to an electrochemical system that converts the chemical
energy of a fuel (e.g. Hydrogen) directly in electrical energy, through an oxidation
reaction with an oxidant (e.g. Oxigen), in the presence of an electrolyte; it happens without
combustion and emissions.
Fuel is supplied to the anode, oxidant is supplied to the cathode.
Plants with Fuel Cell are generally composed from three sections:
- ―Pre-treatment of fuel‖; this section generally consists of a De-sulfurizer, for the
separation of Hydrogen Sulfide, and a Pre-reformer, that converts fuel in a gas with
more Hydrogen; this one could be an Adiabatic Steam Reforming (ASR), a
Catalytic Partial Oxidator (CPO) or an Auto-Thermal Reactor (ATR).
- ―Electrochemical section‖, that consists of cells and electrolyte, for the production
of electricity.
- ―Power conditioning system‖, for the DC/AC transformation of the produced
electrical current.
At the anode takes place the hydrogen ionization reaction, with the realizing of electrons
and H+ ions:
2H2 ⟶ 4H+ + 4e- .
At the cathode oxygen reacts with H+ ions coming by the electrolyte and with electrons
taken from the electrode; the reaction product is water, as shown in the following reaction:
O2 + 4H+
+ 4e- ⟶ 2H2O.
The two reactions account for the hydrogen oxidation:
2H2 + O2 ⟶ 2H2O .
The functioning of a fuel cell is shown in the following figure :
45
Figure 2.3.1.1: Scheme of a fuel cell: anode and cathode reactions.
Anode and cathode are electrically linked for the production of electricity through
electrons that go across the external circuit; theoretically is not allowed to electrons to pass
through electrolyte and to complete the circuit H+ ions must pass from anode to cathode.
Figure 2.3.1.2: Australia‘s first commercial fuel cell (200 kW); PAFC.
46
2.3.2 Fuel Cell types
We can distinguish fuel cell from the kind of electrolyte that is used; different fuel cell
types have different operating temperature and a different flexibility for the using fuel.
The most common type are shown in the following table (ref. [5] ) :
Table 2.3.2.1: Different types of fuel cell.
2.3.3 Nernst equation and Gibbs energy
Fuel cells convert the fuel chemical energy directly into electric power; if we suppose that
takes place the following generally reaction :
aA + bB ⇄cC + dD ,
the cell voltage can be written as:
E = E° +
(2.2),
where R is the idea gases constant, T the operation temperature, n the electrons number
that takes place to the reaction and F is the Faraday constant (96487 Coulomb/electrons
mole); E° is the reversible standard cell voltage.
For an ideal gas, we can also write:
E = E° +
(2.3),
47
where p are the partial pressure values of reagents and products and vi are the
stoichiometric coefficients; this is the general form of Nernst Equation .
The maximum electrical work that an electrochemical cell can perform is equal to the
opposite of the Gibbs energy variation as the reactants go to products:
Wel, max = - ∆G = nFE (2.4)
If we considerer that the standard Gibbs energy variation is :
∆G° = -nFE° (2.5),
we can calculate the Gibbs free energy variation with the following expression:
∆G = ∆G° +
(2.6).
Gibbs free energy is a function of temperature and pressure; for hydrogen oxidation it can
be written as ( ref. [5] ) :
∆G = ∆G°(T) +
(2.7),
where p are the values of partial pressures of all the components.
2.3.4 Efficiency and utilization factor
The maximum electrical efficiency of a fuel cell is usually defined as:
ηel, max =
(2.8),
where LHV is the Lower Heating Value of the used fuel.
The higher is the temperature the higher is the theoretic efficiency; pressure can increase
or decrease the efficiency depending by the number of moles of reactants and products.
The real electric efficiency is calculated as:
ηel =
(2.9).
Real efficiency is influenced by polarization, ohmic and activation losses; therefore fuel
cells efficiency is higher at low temperatures, when the Nernst potential is higher; the
following figure shows the variation of the reversible potential of an SOFC fed by methane
when the temperature increases ( ref. [6] ) :
48
Figure 2.3.4.1: Variation of SOFC equilibrium potential with the increasing temperature.
For electric efficiency reasons not all the fuel reacts inside the fuel cell. To guarantee the
presence of non-oxidized fuel in all anode surface, a fraction of fuel input does not take
part to the reaction. A utilization factor is therefore defined as:
Uf =
(2.10).
Common values for utilization factor are between 0.75 and 0.90.
In theory, we can write:
ηel,FC,max =
=
(2.11).
The maximum cell voltage, referred to LHV (for hydrogen combustion ∆H = -241,83
kJ/mol ) , is:
Vcell,max = -
= 1,25 V (2.12).
So, the real cell efficiency can be written as:
ηcell =
(2.13).
If we use the utilization factor, we can write:
ηel,FC = Uf ηcell = Uf
(2.14),
that is the fuel cell efficiency as a function of the utilization factor and the cell potential.
49
2.3.5 Losses in a Fuel Cell
The maximum potential difference between electrodes is reached when there‘s no current
flow in the external circuit; when the current flow go across this one, we can see a
detachment from equilibrium state, due to three principal kinds of loss:
- Ohmic; the principal reason of this loss is the resistance to the electrons flow in the
electrolyte and in the materials of the external circuit and of the electrodes. We can
reduce this losses reducing the distance between electrodes and achieving
electrodes and electrolyte conduttivity.
- Concentration; this loss is caused by mass transport phenomena that hinder the
chemical reaction at the electrodes.
- Activation; this condition is verified when the kinetic conditions in the reactions
are too low.
Globally, the real potential will be:
Vreal = Erev – ( ∆Vohm + ∆Vconc + ∆Vact ) (2.15).
The following diagram shows the behavior of a fuel cell (ref [7] ) :
Figure 2.3.5.1: Fuel cell voltage as a function of fuel cell current: losses effect.
50
2.3.6 Solid Oxide Fuel Cell (SOFC)
SOFC is a particular type of fuel cell; it‘s a complete solid-state device that uses an oxide
ion-conducting ceramic material as the electrolyte.
The chemical reaction at the cathode is:
O2 + 4e- 2O
2-.
The chemical reaction at the anode is:
2H2 + 2O2-
2H2O.
Until recently, SOFCs have all been based on an electrolyte of Zirconia (ZrO2) stabilized
with the addition of a small percentage of Yttria (Y2O3); the state-of-art zirconia based
SOFC has an operative temperature between 800 and 1100 °C.
Lowering SOFC working temperature should lead to cheaper and more reliable materials;
making electrolytes and electrodes that work well at lower temperature is a major focus of
current SOFC research.
SOFCs show an enhanced performance with increasing cell pressure; the improvement is
mainly due to the increase in the change of the free Gibbs energy of reactants and
products. Operating at higher pressure is advantageous when SOFC is integrated with a
gas turbine (ref. [5]).
Temperature has a strong influence in the conductivity of materials; for example ohmic
losses are decreased at high temperature and therefore SOFC efficiency is increased. On
the contrary one of the main advantages of operating at lower temperature is the possibility
of using cheaper construction materials and methods.
2.3.6.1 SOFC technologies
The two most important SOFC structures are the ―Tubular‖ technology and the ―Planar‖
technology; these are shown in the following figures:
Figure 2.3.6.1.1: Tubular and Planar SOFC technology.
51
With the tubular design high temperature gas tight seals are eliminated; thermal robustness
and SOFC life is increased.
Planar SOFC technology has a superior stack performance (lower ohmic losses) and a
much higher power density. Another advantage is that low-cost fabrication methods such
as screen printing and tape casting can be used. One of the major disadvantages is the need
for gas-tight sealing around the edge of the cell components. Whit the planar technology
three configurations are possible:
- Counter flow;
- Cross flow;
- Co-flow.
Figure 2.3.6.1.2: Counter, Cross and Co flow schemes.
The following table shows some typical characteristics of the three flow designs (ref. [8]):
Table 2.3.6.1.1: Features of the three different configurations
52
2.3.6.2 SOFC components
The most important parts of an SOFC plant are the following ones:
- ―Cathode‖; the cathode is the negative side towards which electrons go. It‘s
exposed to the oxidant (pure oxygen or air) and consists of a porous ceramic
material, to allow oxidant flow through it. Various materials can be used; of course
they have to be electrically conductive. Its function is to reduce oxygen molecules
in the air flow to oxygen ions.
- ―Anode‖; the anode is the positive side from which electrons come. It‘s exposed to
the fuel and consists of a very porous material, to allow fuel flow to the electrolyte;
usually it‘s made up of nickel mixed with ceramic materials, with a thick and
strong layer that provides mechanical support. Its purpose is to use oxygen ions
diffused through electrolyte for the fuel oxidation.
- ―Electrolyte‖; it‘s a membrane that separate air on the cathode from fuel on the
anode. This membrane has to be dense and with a gas-tight layer; the most
common utilized material is zirconium. The electrolyte must be electrically
insulating; in this way electrons are forced to go through the external circuit before
reaching the cathode and the ohmic losses decrease.
This SOFC component should be able to conduct oxygen ions from cathode to
anode; is this the reason why its material is typically measured in ionic
conductivity.
- ―Interconnect‖; this word refers to a metallic or ceramic layer between each
individual cell. Its function is to connect each cell in series, so that it‘s possible
increase the produced electricity. It must be stable reaching the cathode, because it
is exposed to both oxiding and reducing side at very high temperature; initially
were very successful ceramic materials at high temperature. But these materials are
really expensive; therefore metallic materials have been promising at lower
temperature.
2.3.6.3 Pre-reforming in SOFC
Usually is necessary a pre-reformer unit before a SOFC; this is a chemical dispositive that
increase the hydrogen in the fuel input to fuel cell.
The most common pre-reformer units are the following:
- ASR (Adiabatic Steam Reforming); in this one is necessary superheated steam.
Because takes place and endothermic reaction, it needs heat; the chemical reaction
that takes place is:
CnHm+ nH2O ↔nCO + (2n+m)/2 H2
53
- CPO (Catalytic Partial Oxidation); in this unit is not necessary steam, but air. So if
a CPO is present, it could be necessary a blower for the inlet air. An exothermic
reaction takes place; this one is the following:
CnHm + n/2 O2↔nCO + m/2 H2
- ATR (Auto-Thermal Reactor); this reactor is a combination between CPO and
ASR; in fact less steam than an ASR and no, or very low, heat is needed.
In SOFC, internal reforming occurs thanks to the high temperatures; this can be
accomplished by choosing suitable materials. Internal reforming is also desirable, because
reduces the cost for external reforming. Depending on the SOFC type, complete reforming
may lead to problems such as carbon formation on the anode and thermal gradient at
entrance.
Before the pre-reforming, we have a gas mixture of: H2, CO, CH4, H2O, CO2; the first
three are fuel, while H2O is present for internal reforming and as emission and CO2 is
emission.
2.3.6.4 Fuels in SOFC
The most common used fuel in an SOFC is Hydrogen; it can be obtained from methane
through steam reforming reaction, from CO through water gas shift reaction or from water
through electrolysis.
So it‘s very usually feed a SOFC through Methane. Operative temperature provides the
hydrogen needed at the anode by means of reforming and water-gas shift reaction. The fuel
reforming reaction produces hydrogen and carbon monoxide from methane according to
the following equation:
CH4 + H2O ↔ 3H2 + CO.
The water-gas shift reaction provides hydrogen and carbon dioxide from carbon monoxide
and water according to the following equation:
CO + H2O ↔ H2 + CO2.
It‘s possible feed SOFC through alternative fuels, for example:
- Ethanol, produced from agriculture; it could be used as alternative fuel instead of
oil.
- Methanol, more toxic and with less energy density than ethanol; it‘s also colorless
and flammable.
54
- De-Methyl Ether (DME), produced from natural gas or coal gasification through
two steps catalyst process.
- BioDME, a synthetic fuel manufactured from biomass.
- Ammonia, produced in many chemical plants.
2.3.6.5 Application areas of SOFC plants
The most important application fields of SOFC, in a commercial way, are the following:
- Micro CHP such in houses;
- Hotels, Shopping centers, Hospital, etc. ;
- Auxiliary Power Unit;
- Marine;
- Combined with traditional cycles;
- Maybe, in the future, for mobile application.
2.3.6.6 SOFC modeling through Lumped method
An SOFC can be represented, according to the lumped method, as a black box whose
inputs are the air and fuel mass flows ( and ) and the outputs are the produced
electric energy and heat ( EFC and QFC ).
This model is shown in the following figure ( ref. [8] ) :
According to this model, we can write:
Pi = EFC + QFC (2.16).
The utilization factor can be written as:
Uf =
=
(2.17).
55
The cell efficiency is the ratio between the produced electric Energy and the addition of
EFC and QFC :
ηcell =
(2.18).
So, the electrical energy can be written also as follows:
EFC = ηcell Pi = ηcell Uf Pfuel,in = ηel,FC Pfuel,in (2.19).
Values of ηel,FC are normally included between 40% and 65% .
In this way, ηcell can be written also as:
ηcell =
(2.20).
If we call αFC the ratio between EFC and QFC and we guess that it‘s constant
αFC =
~ constant (2.21),
it‘s possible writing ηcell as a function of αFC :
ηcell =
(2.22).
56
2.4 Gas Turbines
2.4.1 Introduction to Gas Turbines
The term ―Gas Turbine‖ refers to a power system for the production of electrical energy;
it‘s composed from three principal machines:
- compressor;
- combustion chamber;
- turbine.
These are the basic elements of this system; there are some particular cases where the hot
exhausted gases from turbine are used for other aims, for example to increase temperature
of air that comes out from compressor (regenerative cycle) or to produce steam in a
bottoming cycle (combined cycle) or for the district heating or cooling.
The latter are only some of the solutions used to increase the efficiency, as we will see.
The gas turbine was born for the aeronautical propulsion ( ref. [1] ), first for the military
sector, later in the civil applications; since the Second World War it has been object of
studies, development and big investments. It‘s for these reasons that today is one of the
most developed energy systems for industrial applications; is in fact in this field that is
widely used for the production of electrical or mechanical energy with good efficiency.
Figure 2.4.1.1: A modern jet engine used to power Boeing 777 aircraft (left)
and a Land-based Gas Turbine, 40 MW (right).
2.4.2 Principle of operation
The thermodynamic process of a gas turbine involves the pressurization of intake air by
the compressor. The compressed air and a suitable fuel are mixed and burned in a
combustion chamber. The resulting hot combustion gas expands in the turbine, which
drives the compressor and provides power by rotating the compressor turbine shaft. With a
recuperator, the hot exhaust gases can pre-heat the inlet air in the combustion chamber.
The compressor-turbine group is mounted on a single shaft, sometimes also along with the
57
electric generator; two bearings support the single shaft. The physical size of components
and rotational speed of gas turbine systems are strongly influenced by the specific turbine
and compressor design characteristics. For a specific design, as the power rating decreases,
the shaft speed increases; hence the high shaft speed of the small turbines [4].
Figure 2.4.2.1 : Simple scheme of a Gas Turbine.
2.4.3 Components
In detail, the principal components of a Gas Turbine are the following:
- Compressor; it‘s always dragged from the turbine through a mechanical link that
consists of one or more shafts. It‘s in most cases an axial compressor; only for
small power, 1-2 MWe ( ref. [1] ), are used radial compressors. In an axial
compressor the number of stages is between 10 and 20; the rotation speed is
included between 3000 and 30000 rpm. Of course the first value is referred to
powers higher that 50-60 MWe.
- Burner; the aim of this component is to increase the temperature of the
thermodynamic cycle through the heat that comes from the combustion of a fuel.
The maximum temperature of the cycle is limited by technological factors related to
the strength of materials at high temperatures; are not possible values of temperature more
than 1200-1400 °C. For this reason it works with high excess of air; in this way the
maximum temperature is limited, but the exhausted gases have an high oxygen content and
they can used as oxidant in a post-combustion chamber. But if the mix air/fuel is less than
the low flammability limit, a good combustion is not possible; so the design of the
combustion chamber is particular. We can distinguish a primary and a secondary zone; in
this way air is mixed with fuel in two steps.
- Turbine; this is the most particular and important element of this kind of plants,
for the temperatures that here take place. In fact the maximum temperature has a
big influence on the efficiency and the net produced power. There are two methods
58
used to increase the temperature inlet turbine: to use non metallic materials or to
cool turbine blades.
- Electric generator; this component is not integral part of a gas turbine because
these can be used also for the mechanical drive. If the rotation speed of the turbine
shaft is compatible with the frequency of the electrical grid, turbine and electric
generator are coupled through a mechanical system. When the gas turbine requests
higher velocities, it‘s necessary a ―power conditioning system‖ for the lowering of
this value; in this case it‘s necessary to not neglect the losses of this system.
- Recuperator; the utilization of this component could be necessary because the
pressure ratio can be low if we use a compressor with a little pressure ratio. The
inlet temperature to the burner is not so high and the outlet temperature from the
turbine is high; these two conditions are not good for the efficiency of the plant and
the regeneration can be an interesting solution.
- Exhaust heat recovery; this component can be used in this type of plants for CHP
applications, due to the high temperature value of exhausted gases.
Figure 2.4.3.1: Regenerative gas turbine block scheme.
2.4.4 Thermodynamic cycle
2.4.4.1 Idealized closed cycle
The thermodynamic idealized close cycle for this kind of plant is a simple Brayton-Joule
cycle; the cycle is shown in the following figure, both in a P-v diagram and in a T-s
diagram:
59
Figure 2.4.4.1.1: Thermodynamic idealized regenerative Brayton-Joule cycle.
The shown thermodynamic transformations are:
1 – 2 : isentropic compression;
2 – 3 : heat inlet;
3 – 4 : isentropic expansion;
4 – 1 : exhausted gases cooling.
This cycle can be considered ideal if the adiabatic transformations are also isentropic, if
the pressure is constant along an isobar, if all gases can be considered to have an idealized
behavior and if the mass flow doesn‘t change along the cycle (close cycle).
It can be demonstrated that the efficiency of this ideal cycle is calculated as ( ref. [1] ):
1 -
= 1 -
= 1 –
(2.23),
where r is the pressure ratio ( p2 / p1 ) and is a gas property, the ratio between the specific
heat capacity at constant volume and constant pressure.
It‘s really important to note that the efficiency doesn‘t depend on the maximum
temperature, but is the same as a Carnot cycle between temperatures T1 and T2.
But to produce energy it‘s necessary that T3 > T2; so if we fix the extreme temperatures of
the cycle, the efficiency that we can obtain is less than the one that we have in a Carnot
cycle between the same temperatures.
The specific work that we can obtain from this cycle is the difference between the work
produced in the turbine and the one consumed in the compressor; this definition is of
course valid only for a close cycle ( ref. [1] ):
60
w = wt – wc = qin = ( 1 -
)
T1 (
-
) (2.24).
So, on the contrary of efficiency, the specific work depends also on the maximum
temperature of cycle T3 .
2.4.4.2 Idealized open cycle
If we remove the hypothesis of closed cycle, we obtain a cycle more similar to the real
one, but where exhausted gases are rejected in the atmosphere.
The cycle in this case can be represented through the following scheme:
Figure 2.4.4.2.1: Representative pattern of an idealized open Brayton-Joule cycle.
The differences with the close cycle are the following:
- operative fluid is necessary air;
- the introduction of heat take place trough an internal combustion; the flue gases are
the operative fluid after the burner;
- heat is rejected from the cycle dispersing the exhausted gases in atmosphere.
In this way it‘s not necessary to use heat exchangers to transfer heat and to absorb heat
from the cycle; so dimensions of plant really decrease. But under another point of view it‘s
necessary to use good materials for the turbine, because is direct the contact with the
exhausted gas, and the only fluid that it‘s possible to use is air in environmental
conditions.
2.4.4.3 Simple real cycle
In previous cycles the only irreversibility were in the heat transfer processes; when we use
real machines, we have others sources of losses, that can be resumed in the following
( ref. [1] ):
61
- compression and expansion processes are not isentropic;
- transformation between points 2 and 3 and 4-1 is not at constant pressure;
- another pressure loss is caused by the filter of compressor;
- after the turbine there‘s another pressure drops;
- thermal losses in the hot parts of machines;
- chemical losses for an incomplete combustion;
- mass flow losses;
- mechanical losses:
- loss in the transformation from mechanical energy to electrical energy.
The real cycle, compared with the idealized one, is represented in the following diagram:
Figure 2.4.4.3.1: Comparison between idealized and real Brayton-Joule cycle.
The first loss is the most important; so if we consider the isentropic efficiencies of
compressor and turbine (is,c and is,t) the useful work that we can obtain is ( ref. [1] ) :
Wu = Wt – Wc = Wis,t is,t – Wis,c / is,c (2.25).
2.4.5 Gas Turbine emissions
The level of emissions in a Gas Turbine is function of the physic and chemical conditions
of the combustion process and of the geometric features of the components where this
process takes place; reactions are governed from chemical kinetic.
For some pollutions, as for example carbon monoxide, formation process is linked directly
to the combustion reaction; for others, as for example NOx ,processes of formation and
destruction are more complicated.
The most important pollutants in a combustion process, as for example in a Gas Turbine,
are Carbon Monoxide ( CO ) and NOx.
62
2.4.5.1 CO
CO formation is one of the principal consequence of hydrocarbons combustion; this
pollutant forms quickly in the first part of flame, where the concentration of radicals and
aldehydes is major. CO can be destroyed through reaction with oxygen or radicals OH;
it‘s in fact residual ―fuel‖ that can be still oxidized in CO2 to obtain other energy. Presence
of CO in the products of combustion is in fact symptom of incomplete combustion.
2.4.5.2 NOx
The main formed NOx in a combustion process is the nitrogen monoxide NO;
concentrations are higher in fact that nitrogen dioxide NO2 .
NO are after slowly oxidized in NO2; the principal formation processes are ( ref. [1] ):
- ―Thermal‖; this process takes place in presence of high temperatures. In these
conditions the nitrogen contained in the air is oxidized in NO.
- ―Prompt‖; NO forms directly in the flame zone from the reaction between nitrogen
contained in the air and the radicals contained in the fuel.
- ―Fuel‖; in this process, NO are generated from nitrogen contained in the fuel not
under a molecular form, but as cyan and amino, that, with high temperatures,
originate simpler species as for example NH3 and HCN. From those is after formed
NO.
In presence of fuels without nitrogen, the main process is the ―Thermal‖; ―Prompt‖ process
is important only when are realized solutions to decrease formation of NO through a
―Thermal‖ process. The ―Fuel‖ NO are a problem only with fuels as for example coal or
heavy fuels.
About ―Thermal‖ NO, the principal reactions that take place are ( ref. [1] ):
O + N2 NO + N
N + N2 NO + O
N + OH NO + H .
The following figure shows the concentration of NO [%] in the products of combustion as
function of the flame temperature [K]:
63
Figure 2.4.5.2.1: NO concentration [%] as function of flame temperature [K].
2.4.6 Gas Turbines for market
Gas turbine market for land-based applications includes a limited amount of builders and
models; in fact design features are not so flexible and it is sometime difficult to satisfy the
customer.
The industrial situation is really complex and in continued progress.
The most important parameter is the efficiency; about it we can notice that ( ref. [1] ):
- the increment of efficiency is linked with the produced power; for low powers this
plants show an efficiency around 30%. With larger powers it‘s also possible to
achieve 35-42% .
- the growth performance with the power output is linked to the best isentropic
efficiency of compressor and turbine;
- the higher values for efficiency are obtained from modified cycles (regenerative,
HAT, STIG, intercooling, etc. ).
In the following tables are shown some features of commercial Gas Turbines ( ref. [1] ):
64
Builder Model Year Power [kW]
Efficiency [%]
r Mass flow [kg/s]
T4 [°C]
RPM
Ansaldo V94.3A4 2004 279000 39.10 17.7 666.7 577.2 3000
Aviadvigatel GTU-25PER 2004 24850 37.80 27.7 80.2 471.1 5000
GE Energy LM6000PD 2003 46903 41.26 30.9 132.6 445.6 3627
Iskra
energetika
GTES-6 2001 6200 26.7 8.7 33.5 477.8 55
3000
GTES-12 2001 12000 33.32 15.8 47.1 470.0 65
3000
GTES-16 2001 16000 34.87 19.9 57.1 482.2 55
3000
OPRA OP16-2AG 2003 1860 26.21 6.7 8.7 558.9 15
1800
OP16-2AL 2002 1730 25.71 6.7 8.7 542.2 15
1800
OP16-2HD 2004 1910 26.91 6.7 8.7 553.9 15
1800
Siemens SGT-100 1997 5050 30.22 14.3 19.5 546.1 17384
SGT-100 1998 5250 30.47 14.8 20.9 530.0 17384
Table 2.4.5.1: Gas Turbines from companies catalogues.
65
3. Municipal Solid Waste Gasification
Plant integrated with SOFC and Gas
Turbine
3.1 Plant layout
The following figure shows the general layout of the studied plant:
Figure 3.1.1: General plant layout.
Basically plant is composed from three sections:
- Gasification plant;
- SOFC;
- Gas Turbine.
66
Waste enters in the Gasification plant and is obtained a synthesis gas called ―Syngas‖; this
fuel, after a cleaning process that removes H2S tracks, is used in an SOFC plant. The used
fuel and the flue gas that come respectively from the anode and cathode side, are sent in a
burner; the products of combustion expand in a Gas Turbine to obtain other electric power.
Auxiliary systems are:
- Steam blower;
- Syngas blower;
- Compressor.
The efficiency of the plant can be expressed as:
η =
(3.1).
3.2 Size of the plant
An important step of a power plant project is to establish the size of the plant; the used
criterion cannot be the same for all the different kinds of plants, due to the different
features.
In all cases a good beginning to decide the size of plant can be to value the critical
parameters for the specific situation.
In this plant is used Municipal Solid Waste as fuel; so it could be interesting to study the
production of waste in some realities, to realize a plant that allows to use the maximum
part of this.
But it‘s also really important for a power plant to have a good value of efficiency; in fact if
this parameter is too low, it can means that too much power is used for the auxiliary
systems in comparison with the power introduced with the fuel.
A good compromise between the amount of produced waste, the mass flow of fuel inlet in
the plant and the efficiency seems the best way to establish the size of this kind of plant;
this is in fact the line followed in this project.
Under this point of view, the critical parameters are:
- Municipal Solid Waste production;
- Fuel mass flow;
- Plant efficiency.
If we increase the value of the mass flow of fuel inlet to the plant, we can see from the
following diagrams that the size of course increase, but efficiency decrease, due to the
increment of electric power absorbed from the auxiliary systems:
67
Figure 3.2.1: Total plant size as function of fuel mass flow.
Figure 3.2.2: Plant efficiency as function of fuel mass flow.
Diagrams are obtained from simulations increasing the amount of fuel.
For example, if we see the table about waste production in Italy, there‘re some cities where
this value is around 2,4 kg/s ; from diagrams we can see that if we establish this one as
value of mass flow of fuel inlet the plant, we can obtain a good value of efficiency (around
52%) and a medium value for size (around 35 MW of total produced electric power).
In this project the size of the plant is established through the fuel mass flow; the used value
for simulations is 2,4 kg/s.
The bottoming cycle is a Gas turbine; in this one are used the product of combustion
coming from a burner where are used the flue gas and the used fuel from the SOFC plant.
It can be interesting to observe how is divided the production of electrical energy between
SOFC plant and Gas Turbine; in the following diagrams are plotted the sizes of these plant
for different values of the mass flow of fuel:
0
10000
20000
30000
40000
50000
60000
70000
0,92 1,18 1,46 1,73 2,01 2,29 2,57 2,86 3,15 3,45 3,76 4,07
PLA
NT
SIZE
[kW
]
FUEL MASS FLOW [kg/s]
Total plant size
PLANT SIZE
50
50,5
51
51,5
52
52,5
53
53,5
0,92 1,18 1,46 1,73 2,01 2,29 2,57 2,86 3,15 3,45 3,76 4,07
EFFI
CIE
NC
Y [
%]
FUEL MASS FLOW [kg/s]
Efficiency
EFFICIENCY
68
Figure 3.2.3: SOFC size as function of fuel mass flow.
Figure 3.2.4: Gas Turbine size as function of fuel mass flow.
We can see that if the value of fuel mass flow is around 2,4 kg/s , the SOFC plant has a
size of about 19 MW and the Gas Turbine of about 16 MW; around the 55% of the electric
power is produced in the SOFC, the remaining 45% in the Gas Turbine.
It‘s important of course to consider that SOFC is an expensive system; if increase the
produced electric power of course increase also the initial investment. It‘s necessary to
consider how increase the capital investment; if we consider that in this moment the
market price of SOFC is 3000 €/kW, it‘s possible to calculate the initial capital investment
as:
ISOFC = 3000 [€/kW] * Pel (3.2),
0
5000
10000
15000
20000
25000
30000
35000
0,92 1,18 1,46 1,73 2,01 2,29 2,57 2,86 3,15 3,45 3,76 4,07
SOFC
SIZ
E [k
W]
FUEL MASS FLOW [kg/s]
SOFC size
SOFC SIZE
0
5000
10000
15000
20000
25000
30000
35000
0,92 1,18 1,46 1,73 2,01 2,29 2,57 2,86 3,15 3,45 3,76 4,07
GA
S TU
RB
INE
SIZE
[kW
]
FUEL MASS FLOW [kg/s]
Gas Turbine size
GAS TURBINE SIZE
69
where Pel is the produced electric power.
The following diagram refers to the initial investment as function of SOFC size:
Figure 3.2.5: Initial capital investment as function of SOFC size.
It‘s important of course to value also the effective power that we can obtain from the plant
(net power); it‘s an important parameter that permit to observe the power necessary for
auxiliary systems. With increasing values of fuel mass flow, the obtained values of ―net
power‖ are reported in the following diagram:
Figure 3.2.6: Net power plant as function of fuel mass flow.
If we establish for the plant a fuel mass flow of 2,4 kg/s , we can obtain a net power of
about 23 MW; it means that around 12 MW are necessary to feed auxiliary systems.
This size for plant is reasonable also in terms of initial capital investment for the SOFC
system; all simulations will be develop taking this one as size for plant.
40
42
44
46
48
50
52
54
[M€
]
Initial capital investment for SOFC system as function of electric power
CAPITAL INVESTMENT
05000
10000150002000025000300003500040000
0,92 1,18 1,46 1,73 2,01 2,29 2,57 2,86 3,15 3,45 3,76 4,07
NET
PO
WER
[kW
]
FUEL MASS FLOW [kg/s]
Net power production
NET POWER
70
It can be also interesting to evaluate the expected efficiency of the plant; this value can be
calculated as:
= gasifier [ sofc + gt ( 1 - sofc )] (3.3),
where:
- gasifier =
=
=
=
(3.4),
- sofc =
(3.5),
- gt =
(3.6).
If we consider that for the studied gasifier gasifier is around 95% and we suppose that in
average sofc is around 45% and gt is around 30%, it‘s obtained an expected efficiency
of 58%. The obtained value with a fuel mass flow of 2,4 kg/s is not so far from this one.
It is also interesting to note the following thing; if we hypothesize that gasifier is 100%
(no ash production), deriving respect to gt we obtain:
= 1 - sofc (3.7).
With an SOFC efficiency of 50%, if gt increase of 1%, a 0,5% increment for η is
obtained.
3.3 Fuel features
The studied plant is fed by Municipal Solid Waste (MSW); as is common to know, the
generation of excessive amounts of waste per inhabitant is a crucial point of all developed
countries. The waste management is in fact becoming a critical issue in a lot of parts of the
world; the amount of waste, both municipal and industrial, is increasing day by day.
This situation suggest the need for many local authorities of a plan for a sustainable
politics to manage this situation; one of the ways is to use the undifferentiated waste to
produce energy.
It‘s an interesting solution, because is used ―unusable‖ things to produce useful energy for
human activities.
In this project the studied solution to produce energy from waste is to submit this to a
Gasification process; in this way is obtained a synthesis gas called ―Syngas‖ that can be
71
used in an energy system. The composition of fuel considered in this paper is the following
( ref. [22] ):
- C = 0,476
- H = 0,06
- O = 0,329
- S = 0,003
- N = 0,012
- ASH = 0,12
Figure 3.3.1: Waste composition referring to a dry basis.
Compared to the mean composition analysis, it‘s possible to see that the two compositions
are really close; is this the reason why has used this one.
If we estimate this parameter through Dulong formula, the obtained value is 18855 kJ/kg;
with the Channiwala formula it‘s 19656 kJ/kg.
With good approximation in this project the Low Heat Value (LHV) of fuel is assumed
(ref. [22] ):
LHV = 19879 kJ/kg .
It‘s really difficult in literature to find an accurate formula to calculate the Specific Heat
Capacity cp for a solid fuel; normally it is calculated through empiric measurements.
In this paper has been used the following formula to estimate cp for the considered
composition of waste:
(3.8)
47,6%
6,0%
32,9%
0,3%1,2%
12,0%
Fuel composition referring to a dry basis
C H O S N ASH
72
where xi are the mass fractions of every component and cp,i is the specific heat capacity of
every elementary component. As mentioned before, reliable values for specific heat
capacity of each one component can be the following, at a temperature of 300K
(ref. [25] ) :
ELEMENT cp [kJ/kg K] , T=300K
C 0,71
H 14,31
O 0,920
S 0,71
N 1,041
Table 3.3.1: Specific heat capacity values for the chemical elements present in the fuel.
In this work, the obtained and used value for cp is:
cp = 1,71 kJ / kg K .
Of course Municipal Solid Waste is a particular fuel, whose composition can change
during the different seasons and the different days in a year; it‘s necessary an accurate
control of fuel before using this one in a Gasifier, to be sure that its composition is not so
different.
A careful selection of the different kind of waste (plastic, paper, metallic, etc. ) can be a
good beginning to permit to the plant to work near the design conditions.
Another important parameter to consider is the moisture content; in fact an increment can
influence the LHV of the fuel and the good operation of the Gasifier. Consequence of this
is that efficiency and produced power of plant decrease.
Normally it‘s a non fixed value; a sensitivity analysis is conducted in this work under the
variation of this parameter.
It‘s really important to consider also the composition not only on a dryed basis, but also
considering the content of water; under this point of view, if we hypothesize for example a
moisture percentage of 9,5 % , the mean composition in terms of mass fractions is:
- C = 0,4308
- H = 0,0543
- O = 0,2977
- S = 0,002715
- N = 0,01086
- ASH = 0,1086
- H2O = 0,095
The following diagram shows this composition:
73
Figure 3.3.2: Waste composition referring to a wet basis.
3.4 Gasification plant
In the following section are explained all the singular parts that compose the Gasification
plant; the following scheme shows in detail the Gasification Plant with all the components:
Figure 3.4.1: Gasification Plant.
43,081%
5,430%
29,771%
0,272%
1,086%10,860%
9,5%
Fuel composition referring to a wet basis
C H O S N ASH H2O
74
The principle one is of course the Gasifier, where takes place the Gasification process to
convert the introduced fuel in a synthesis gas. Other organs are useful for this process, to
increase the efficiency; these ones are:
- Dryer;
- Steam blower;
- Splitter;
- Steam Pre-Heater;
- Air Pre-Heater;
- Mixer;
- Desulfurizer.
The scheme of the plant is true with the model of Gasifier, called ―Viking Gasifier‖,
studied at the DTU (Technical University of Denmark) and previously considered; all
components are considered below.
3.4.1 Gasifier
Gasification process is useful to pull out from the introduced fuel a synthesis gas, called
―Syngas‖, that it‘s possible to use in a power plant for the electric energy production.
Fuel is previously dried; as in the Viking Gasifier, Gasification process take place through
injection of a mixture of steam and air. The only one loss of this process is represented
from Ashes; the product is Syngas.
Inside gasifier both pyrolysis and gasification are performed. Syngas out of gasifier is
cooled down first to preheat the air and then to generate the steam for the drying process.
Figure 3.4.1.1: Gasifier scheme.
In this case the fuel is Municipal Solid Waste, whose composition has been previously
analyzed; the obtained Syngas has in the average the following composition:
75
- H = 0,305
- O = 0
- N = 0,2868
- CO = 0,3507
- CO2 = 0,02212
- H2O = 0,01777
- H2S = 0,0009116
- CH4 = 0,01334
- Ar = 0,0033584
The following diagram shows the Syngas composition in term of percentages:
Figure 3.4.1.2: Syngas mean composition in terms of mass fraction.
3.4.2 Dryer
In this part of plant the content of moisture is decreased for a good operation in the
Gasifier. The steam obtained from this process is recycled; in this way we can avoid to
produce steam with other fuel consumption. Part of the produced steam is injected in the
Gasifier after a mixing with air; the following figure shows a scheme of the dryer:
30,5%
28,68%
35,07%
2,212%1,777%
0,09116%1,334% 0,33584%
Syngas compositionH N CO CO2 H2O H2S CH4 Ar
76
Figure 3.4.2.1: Dryer scheme
After drying process, the dry waste is introduced in the Gasifier; The mean composition of
this one is the following:
- C = 0,4522
- H = 0,057
- O = 0,3126
- S = 0,00285
- N = 0,0114
- ASH = 0,114
- H2O = 0,05
The water content decrease from 0,095 to 0,05 ; efficiency of this component is around
52,6% .
The following figure shows the composition of the dry waste in terms of mass fractions:
Figure 3.4.2.2: Dry waste composition.
45,218%
5,7%
31,258%
0,285%
1,14%
11,399%5,0%
Dry waste composition
C H O S N ASH H2O
77
3.4.3 Steam blower
All components can present pressure drops; for this reason is necessary to introduce in the
steam side a blower, to increase the pressure level.
Figure 3.4.3.1: Steam blower
This components increase pressure level until the value useful for the gasification process;
as explained before, Gasifier works with a pressure really close to the environmental value.
So the consumption of electrical energy in the blower is not so high; it‘s necessary only to
overcome the pressure drops.
3.4.4 Splitter
The function of this component is to separate a part of the produced steam; this fraction is
after sent in the mixer where is mixed with air.
Figure 3.4.4.1: Splitter scheme.
Steam comes from the steam blower, where the pressure increase; in this component a
fraction is separated and sent in the mixer. The rest goes in the steam pre-heater, where
temperature is increased.
78
3.4.5 Steam generator
The function of this heat exchanger is to increase the temperature level of the steam flow
used for the drying process; for this aim is used the hot flow of syngas that comes from the
Gasifier. Pressure drops in this heat exchanger are really important; an estimation of these
ones will be examined below.
Figure 3.4.5.1: Steam generator scheme.
Before going in this heat exchanger, the hot syngas is also used to increase the temperature
of air to introduce for the gasification process.
3.4.6 Air pre-heater
This is another heat exchanger present in the gasification plant; the aim of this component
is to increase the temperature level of the air from the environmental to the gasification
conditions.
Figure 3.4.6.1: Air pre-heater scheme.
79
The hot stream used to increase the air temperature is also in this component the syngas
coming from the Gasifier; this hot flow is used before in the air pre-heater and after in the
steam heater, as shown before.
Also in this component pressure drops are really important for the efficiency of plant; an
estimation will be considered below.
3.4.7 Mixer
The aim of this component is to mix the flow of heated air with the pressurized steam; this
mixture is after sent in the Gasifier for the gasification process.
Figure 3.4.7.1: Mixer scheme.
For a good mixing process is of course necessary that both flows are at the same level of
pressure; this one is of course the gasification pressure level, that, as said before, is really
close to the environmental value.
3.4.8 Desulphuriser
In this component the Hydrogen Sulfide is separated from the produced syngas; this
process is really important when this fuel is used in a delicate system as SOFC. Poisoning
phenomena can in fact take place; the good efficiency of system can be compromised and
of course it‘s possible to damage the SOFC plant.
The following figure shows a simple scheme of desulphuriser:
80
Figure 3.4.8.1: Desulphuriser scheme.
In this component is present another loss in the plant; Hydrogen Sulfide is in fact separated
from fuel. The clean syngas is after this process ready to be used in the SOFC plant.
The desulphurization process can require a temperature between 150°C and 450 °C; the
higher is this value, the better materials it‘s necessary to use.
The composition of syngas after this component of course change; chemical reactions in
fact take place. The mean composition of clean syngas is the following:
- H = 0,3053
- O = 0
- N = 0,2870
- CO = 0,3510
- CO2 = 0,02214
- H2O = 0,01779
- H2S = 0
- CH4 = 0,01336
- Ar = 0,003367
The following figure show this composition in terms of mass fraction:
Figure 3.4.8.2: Clean syngas composition.
30,53%
28,70%
35,10%
2,214% 1,779% 1,336% 0,3367%
Clean Syngas compositionH N CO CO2 H2O CH4 Ar
81
3.4.9 Data Input Gasification plant
Data input used for the introduced waste are previously reported; the main data input for
all gasification plant components are as follows:
COMPONENT DATA INPUT VALUE
Dryer Inlet mass flow 31 2,4 [kg/s]
inlet temperature t31 25 [°C]
inlet pressure p31 1,01325 [bar]
outlet temperature t32 1,008 [°C]
outlet pressure p32 150 [bar]
heat losses q320 0 [kW]
pressure drops fuel side 0,05 [bar]
pressure drops steam side 0,005 [bar]
Gasifier outlet temperature t33 800 [°C]
inlet water temperature t45 150 [°C]
outlet pressure H2S p46 1,01325 [bar]
heat losses q321 0 [kW]
operating pressure 0,998 [bar]
operating temperature 800 [°C]
pressure losses 0,005 [bar]
water to fuel ratio 0
carbon conversion factor 1
non equilibrium methane 0,01
Air pre-heater inlet temperature t41 25 [°C]
heat losses q322 0 [kW]
pressure drops syngas side 0,005 [bar]
pressure drops air side 0,005 [bar]
Steam heater outlet temperature t40 200 [°C]
heat losses q323 0 [kW]
pressure drops syngas side 0,005 [bar]
pressure drops steam side 0,005 [bar]
Steam blower isentropic efficiency 80%
mechanical efficiency 98%
Desulphuriser heat losses q325 0 [kW]
pressure drops 0,0049 [bar]
Table 3.4.9.1: Data Input Gasification plant.
82
3.5 SOFC plant
In the SOFC plant is used the syngas produced in the gasifier and after cleaned in the
desulphuriser. In order to reach the operating temperature, two heat exchangers are
introduced:
- anode pre-heater, where the syngas temperature increase until the operating
temperature; the hot used stream is the used fuel that comes out from the anode
side of the fuel cell.
- cathode pre-heater, where the air temperature increase; the hot used stream is
represented from the flue gas that comes out from the cathode side of the fuel cell.
Both syngas and air are compressed to increase the pressure level until the operating value
in the stack; for this reason are used a syngas blower and a compressor.
Not all the fuel reacts inside the SOFC; a part of the LHV is still inside the used fuel.
Therefore used fuel and flue gas are sent to a burner to generate heat, as we will see.
In the following figure is shown the SOFC plant:
Figure 3.5.1: SOFC plant scheme.
83
All components that take part in the SOFC plant are:
- SOFC stack;
- Anode pre-heater;
- Cathode pre-heater;
- Syngas blower;
- Compressor;
- Burner.
3.5.1 SOFC stack
This is an electrochemical system that converts the chemical energy of syngas directly in
electrical energy, through an oxidation reaction with air, in the presence of an electrolyte;
it happens without combustion and emissions.
Fuel is supplied to the anode, oxidant is supplied to the cathode.
Important operating parameters are the operating temperature and pressure; to increase the
produced electrical energy, a stack is composed from more cells. More stacks are after
linked in series connection.
The following figure shows the SOFC stack scheme:
Figure 3.5.1.1: SOFC stack scheme.
It‘s really interesting to analyze the composition of used fuel and flue gas after the
chemical reaction in the SOFC stack; the mean used fuel composition is the following:
- H = 0,06604
- N = 0,2805
- O = 0
84
- CO = 0,06445
- CO2 = 0,3133
- H2O = 0,2758
- CH4 = 0,000003551
- Ar = 0
As we can see in comparison with the clean syngas composition, the percentage of Argon
becomes 0 and the percentage of methane decreases a lot; also the content of hydrogen is
lower. The amount of carbon dioxide and water increases, because these represent the
principal product of the chemical reaction
The presence of hydrogen, carbon monoxide and methane, also if negligible, means that
the reaction is not completed; it‘s so possible to use these in a burner, as we can see, to
produce other energy through a combustion process.
The following scheme shows the mean used fuel composition:
Figure 3.5.1.2: Used fuel mean composition.
If we hypothesized that the inlet air has a standard composition, mean values for different
components can be the following:
- O = 0,2075
- N = 0,7729
- CO2 = 0,0003
- H2O = 0,01010
- Ar = 0,0092
6,60338%
28,04738%
6,44440%
31,32707%
27,57742%
0,00036%
Used Fuel composition
H N CO CO2 H2O CH4
85
Figure 3.5.1.3: Standard air composition.
In the average the composition of flue gas doesn‘t change too much from the air
composition; generally values of mass fractions for the different components are really
close to the following:
- O = 0,1917
- N = 0,7883
- CO2 = 0,0003060
- H2O = 0,01030
- Ar = 0,009383
The following figure shows the flue gas composition:
20,750%
77,290%
0,030%1,010% 0,920%
Standard Air composition
O N CO2 H2O Ar
86
Figure 3.5.1.4: Flue Gas composition.
Percentage of oxygen decreases, because a part reacts with the syngas; if the chemical
reaction was complete, all the oxygen should be used. It means that the flue gas stream can
be used as oxidant in a combustion process.
3.5.2 Anode pre-heater
This heat exchanger increases the syngas temperature until the operating stack value; for
this aim is used the hot stream of used fuel coming out from the anode side.
Figure 3.5.2.1: Anode pre-heater scheme.
It‘s really important for a good efficiency of plant to estimate approximately pressure
drops in this heat exchanger; it‘s better that this value is including between 0,01 bar and
0,05 bar.
19,1702%
78,8309%
0,0306%1,0300% 0,9383%
Flue Gas composition
O N CO2 H2O Ar
87
3.5.3 Cathode pre-heater
The aim of this heat exchanger is to increase the air temperature; the hot used flow is the
flue gas coming out from the cathode side.
Figure 3.5.3.1: Cathode pre-heater scheme.
Also in this one it‘s really important to estimate pressure drops, as we will see below; in
fact this parameter can compromise a good efficiency of plant.
3.5.4 Air compressor
Compressor increases the pressure level of air until the operating value good for the
considered SOFC stack; generally it has to process a big mass flow but is not requested an
high value of outlet pressure. Pressure ratio is of course if fact an important parameter; an
increment of pressure involves of course an increment of temperature also. The inlet
values of pressure and temperature are the environmental conditions.
Figure 3.5.4.1: Air compressor scheme.
Others two important parameters for this component are the isentropic and mechanical
efficiency.
88
3.5.5 Syngas blower
Syngas blower increases the pressure level of this stream until the operating value in the
SOFC stack; this value is the same as in the air compressor, because both cathode and
anode side works at the same pressure level.
Figure 3.5.5.1: Syngas blower scheme.
Clean syngas comes from the gasification plant, where the pressure level is very close with
the environmental value; after increasing pressure, the clean syngas is sent in the SOFC
stack through the anode pre-heater.
Also in this case isentropic and mechanical efficiency are two important parameters.
3.5.6 Burner
As shown before, the chemical reaction in the SOFC stack is not complete; for this reason
the used fuel and the flue gas can be used in a Burner as fuel and oxidant. The produced
smoke can be sent in a bottoming cycle to produce other electric energy or heat; a scheme
of this component is shown in the following figure:
Figure 3.5.6.1: Burner scheme.
An important parameter to declare for this component is the pressure ratio; normally is put
equal to one. This is only of course an idealization.
It‘s really interesting to see what can be the mean composition of produced smoke after the
combustion; in terms of mass fraction, the percentage of all components is really close to
the following values:
89
- O = 0,1753
- N = 0,7576
- CO2 = 0,02565
- H2O = 0,03261
- Ar = 0,008794
Figure 3.5.6.2: Smoke composition.
It‘s really interesting to see that there‘s no presence of Hydrogen and Carbon Monoxide; it
means that combustion is complete. The addition of a bottoming cycle that uses the heat
produced through this process can be increase both efficiency and produced power in the
plant;
In this work is in fact considered the introduction of a Gas Turbine after the burner; in this
way the produced smoke expands and is produced additional electric energy.
Another interesting result is that there‘s no presence of NOx; in fact, as seen in paragraph
2.4.5, in this particular case the only one process that can form these pollutants is the
―Thermal‖. But maximum temperature in the cycle is really low ( around 730 °C ); this
condition is not good for the formation of NOx. Also through ―Fuel‖ and ―Prompt‖
processes is not possible formation of NOx.
3.5.7 Data Input SOFC plant
In the following table are shown the main data input used to built the SOFC plant model:
17,5308%
75,7635%
2,5651% 3,2612% 0,8794%
Smoke composition
O N CO2 H2O Ar
90
COMPONENT DATA INPUT VALUE
SOFC stack heat losses q309 0 [kW]
inlet temperature cathode side t4 650 [°C]
inlet temperature anode side t11 650 [°C]
utilization factor 0,8
operating temperature 780
pressure drops anode side 0,01 [bar]
pressure drops cathode side 0,04 [bar]
cells per stack 75
number of stacks 50500
Anode pre-heater heat losses q308 0 [kW]
pressure drops used fuel side 0,01 [bar]
pressure drops syngas side 0,01 [bar]
Cathode pre-heater heat losses q303 0
pressure drops flue gas side 0,04 [bar]
pressure drops air side 0,04 [bar]
Air compressor inlet pressure p1 1,01325 [bar]
inlet temperature t1 25 [°C]
outlet pressure p2 2,5 [bar]
isentropic efficiency 0,8
mechanical efficiency 0,98
Syngas blower isentropic efficiency 0,8
mechanical efficiency 0,98
Burner heat losses q310 0 [kW]
pressure drops 0,95
Table 3.5.7.1: Data Input SOFC plant.
3.6 Gas Turbine
As we saw earlier, it‘s possible to use the flue gas and the used fuel that come from the
cathode side and the anode side of the SOFC stack in a burner; the content of oxygen in
the flue gas is in fact enough for a combustion process. In the used fuel, tracks of
Hydrogen, Carbon Monoxide and Methane there are; it means that the oxidation process in
the SOFC stack is not completed and it‘s possible to use this stream as fuel in a
combustion process.
The produced heat can be used in a bottoming cycle; in this work is considered a Brayton-
Joule cycle:
91
Figure 3.6.1: Brayton-Joule cycle, ideal and real process.
The heat coming from the burner through the produced smoke represent the inlet heat qin
in the cycle; after the increment of temperature until the value in the point 3 of the figure,
the smoke expands in a Turbine to produce electric energy.
In this way the efficiency of plant increases until 6-8% more than without bottoming cycle.
From simulations with the previously data it‘s in fact possible to see that without Gas
Turbine efficiency is around 34%; with the introduction of this system, this value achieves
41%.
In fact with the introduction of the turbine more electric power is produced; but another
important reason is that there isn‘t fuel consumption in the bottom cycle, because to feed
this one is used the ―waste fuel‖ from the top cycle.
Another important observation is that the top cycle is pressurized and for this aim are
present a compressor and a fuel blower; in a Gas Turbine compressor is a very important
component, that increases the pressure and temperature level of air inlet in the cycle. If we
use in the bottom cycle the smoke produced from the combustion of used fuel and flue gas,
these two streams are pressurized; it means that it‘s not necessary to use another
compressor for the Gas Turbine and that there isn‘t more energy consumption for the
introduction of another auxiliary component.
In this way of course the pressure level is limited from the operating pressure in the SOFC
system and from the presence of the cathode pre-heater, where the temperature profiles can
be violated if the outlet temperature of air from compressor is high. It means that the
pressure level of the plant is low and that the turbine inlet temperature is not so high; so
the single Gas Turbine, without the top cycle, would not have a very high efficiency. The
interesting thing is in fact the integration of both cycles; this operation increases the total
efficiency of the plant and of course the amount of produced electric energy. It‘s also
possible to increase slightly the pressure level; in fact the added consumption of electric
energy in the compressor is offset by the introduction of the Turbine and by the higher
production of energy. Without Gas Turbine the level of pressure of the system is around
1,5 bar; with the introduction of the bottom cycle it‘s possible to increase this value until
2,5 bar.
To increase more the efficiency, hybrid regeneration can be a good solution.
92
3.6.1 Turbine
Turbine is the system where is sent smoke at high temperature; this stream expands and
the pressure level decreases. The enthalpy difference between inlet and outlet is converted
in mechanical energy through the shaft; this organ is linked to an electric generator, where
mechanical energy is transformed in electric energy.
Figure 3.6.1.1: Gas Turbine scheme.
As previously mentioned, inlet temperature is not high in comparison with a normal Gas
Turbine; this value is around 900°C. It means under another point of view that there aren‘t
problems for the used materials in the turbine.
Two important parameters to declare are the isentropic and mechanical efficiency; also
electric efficiency is important to declare for the generator.
Outlet smoke from turbine is hot; the temperature level is in fact good, as we will see, for a
regenerative cycle, for district heating and cooling or for both solutions.
3.6.2 Data Input Gas Turbine
In the following table are shown the most important data input for the system composed
from turbine and electric generator.
COMPONENT DATA INPUT VALUE
Turbine outlet pressure p18 1,01325 [bar]
isentropic efficiency 0,86
Electric Generator electric efficiency 0,98
Table 3.6.2: Data Input Gas Turbine.
93
3.7 Simulation results
Simulations, calculations and analysis have been carried out by DNA (Dynamic Network
Analysis), a component-based simulator for energy systems analysis developed at the DTU
Thermal Energy Systems department.
DNA contains models for most of energy system compounds (compressors, pumps, heat
exchanger, fuel cells, etc.).
Solution is provided solving a system of nonlinear equations with the Newton-Raphson
modified algorithm.
Viking plant DNA model developed at Thermal Energy Systems department is used to
design gasification plant. The sofceq_1dn2.for file is utilized as SOFC model; this model
in fact permits also to define the number of cell per stack and the number of stacks.
3.7.1 Plant efficiency
Plant efficiency, as seen, can be calculated as difference between power production and
auxiliary power consumption through the equation:
η =
=
=
=
(3.9).
The analysis of plant shows an efficiency of:
ηLHV = 40,7 %
referred to Low Heat Value of fuel, and
ηHHV = 37,7 %
referred to High Heat Value of fuel.
If we refer to the first value, we can see that the remaining 59,3% of input energy is lost;
only the 40,7% of the energy that comes from the combustion of the waste can be used as
output from plant.
94
Figure 3.7.1.1: Fractions of useful output energy and loss energy in the power plant
We will see later that the biggest part of this loss is due to off-gases; an integration of heat
stream inside plant can to reduce this loss and to increase plant efficiency.
3.7.2 Electric Power production
The obtained electric power production, with the previously data, is:
PTOT = 29,5 MW .
Of this amount, part is produced in the SOFC and another part in the Gas Turbine;
respectively:
PSOFC = 18,5 MW ,
PGT = 11 MW .
It means that the 62,7% of the total electric power is produced through SOFC, the 37,3% is
produced through Gas Turbine.
40,7%
59,3%
Output and loss energy
OUTPUT ENERGY ENERGY LOSSES
95
Figure 3.7.2.1: Total power production: SOFC and Gas Turbine fractions.
3.7.3 Auxiliary consumption
The total consumption of electric power in the plant is:
PAUX = 12,22 MW .
Components that contribute to power consumption in the plant are:
- Steam blower;
- Syngas blower;
- Compressor.
The single contribute of each component is:
PSTEAM BLOWER = 0,02 MW ,
PSYNGAS BLOWER = 1,6 MW ,
PCOMPRESSOR = 10,6 MW .
It means that the 86,7% of auxiliary energy is consumed by compressor; the syngas blower
uses the 13% of auxiliary energy and only 0,3% is used in the steam blower.
62,7%
37,3%
Total power production
SOFC GAS TURBINE
96
Figure 3.7.3.1: Auxiliary electric power consumption.
3.7.4 Net Power production
The net electric power production is calculated as difference between the total electric
power production and the auxiliary electric power consumption; in this case, from the
previous values, it‘s possible to see that it‘s obtained the following value for the net
electric power:
PNET = PTOT - PAUX =
= ( PSOFC + PGT ) – (PSTEAM BLOWER + PSYNGAS BLOWER + PCOMPRESSOR ) (3.10),
PNET = 29,5 – 12,2 = 17,3 MW
The 41,3% of the total produced power is used for the auxiliary components; the
remaining 58,7% can be used as net output from plant.
0,3%
13%
86,7%
Auxiliary consumption
STEAM BLOWER SYNGAS BLOWER COMPRESSOR
97
Figure 3.7.4.1: Total electric power production: auxiliary consumption and net power production fractions.
3.7.5 Energy Losses
As seen before, the 59,3% of input energy comes from the waste combustion is lost.
The mainly sources of energy loss in the plant are:
- loss through ashes in the gasifier, 1120 kJ (3%) ;
- loss through H2S separation in the desulphuriser, 1,645 kJ (0,004%) ;
- heat losses in the mechanical components, 244,2723 kJ (0,66 %) ;
- heat losses in the generators, 224,5 kJ (0,636%);
- heat losses through exhausted gases, 35211,679 kJ (95,7%) .
41,3%
58,7%
Auxiliary consumption and Net Power production
AUXILIARY CONSUMPTION NET POWER PRODUCTION
98
Figure 3.7.5.1: Energy losses in the plant.
As we can see from the analysis of the plant, the biggest loss is due to off gases outlet from
Gas Turbine; temperature of these is in fact really high. This loss can be reduced reusing
their energy content inside the plant to increase temperature level of a cold stream.
3.7.6 Low Heat Value
Low Heat Value quantifies the amount of energy contained in fuel; it‘s a good parameter
to see how much energy is available.
Along the plant fuel changes; in fact we have:
- waste, that enter in the Gasification plant though the Dryer;
- dry waste, coming from the Dryer and sent to the Gasifier;
- Syngas, produced through the Gasification process;
- clean Syngas, sent, after the desulphurization process, to the SOFC plant;
- used fuel, coming from the anode side in the SOFC.
It can be really interesting to see how changes the LHV to estimate how is utilized in the
plant in order to produce electric energy:
- LHVWASTE = 19879 kJ / kg K ;
- LHVDRY_WASTE = 20130 kJ /kg K ;
- LHVSYNGAS = 15920 kJ /kg K ;
- LHVSYNGAS_CLEAN = 15910 kJ /kg K ;
- LHVUSED_FUEL = 3313 kJ /kg K :
3% 0,004% 0,660%
0,636%
95,7%
Energy losses
ASH H2S MECHANICAL COMPONENTS GENERATORS OFF GASES
99
Figure 3.7.6.1: LHV trend along the plant.
Of course from the analysis of plant we can see that LHVSMOKE = 0 kJ /kg K ; it means that
all the energy contained in the used fuel is converted in heat through combustion in the
burner.
Components where LHV changes are:
- dryer, where LHV increases due to moisture decrement;
- gasifier, where LHV decreases in order to provide heat for the gasification process;
- desulphurises, where hydrogen sulfide is removed, but LHV is marginally lowered
due to the small content of H2S;
- SOFC, where part of syngas LHV is used to produce electric power;
- burner, where the remaining LHV content inside the used fuel is totally converted
in heat.
3.8 Hybrid Regeneration
As mentioned, an high loss in the plant is due to the high temperature of the off gases
outlet from the gas turbine; this heat can be used to increase plant efficiency through an
hybrid regenerative heat exchanger.
Off gases are in fact used to increase air temperature level coming from the compressor; in
fact, as mentioned, pressure ratio is not high and in this way it‘s possible to increase more
air temperature before entering in the cathode pre-heater. The principle consequences of
adding this component are:
WASTEDRY
WASTESYNGAS
SYNGAS CLEAN
USED FUEL SMOKE
LHV TREND 19879 20130 15920 15910 3313 0
0
5000
10000
15000
20000[
kJ /
kg
K ]
LHV trend
100
- larger available heat for the bottom cycle;
- higher temperature of flue gas entering in the burner;
- higher thermodynamic mean temperature;
- higher efficiency;
- better integration of heat flows inside plant.
- lower losses caused by the high temperature of smoke outlet from turbine.
The plant scheme with hybrid regeneration is shown below:
Figure 3.8.1: Plant scheme with hybrid recuperation.
3.8.1 Hybrid Recuperator
Hybrid recuperator is an heat exchanger used to increase temperature level of air coming
from compressor; as shown before, this solution has more interesting benefits.
The hot flow is represented from the off gases coming from the gas turbine; the cold
stream is the air coming from compressor and entering in the cathode pre-heater.
101
Figure 3.8.1.1: Hybrid recuperator scheme.
Important parameters to declare in the model of this component are pressure drops in both
cold and hot side; it‘s in fact better having a good value to not compromise plant
efficiency.
3.8.2 Data Input Hybrid Recuperator
The following table shows the most important data for this component:
COMPONENT DATA INPUT VALUE
Hybrid Recuperator pressure drops smoke side 0,04 [bar]
pressure drops air side 0,04 [bar]
heat losses q302 0 [kW]
outlet pressure p19 1,01325 [bar]
Table 3.8.2.1: Data Input Hybrid Recuperator.
3.9 Effect of Hybrid Recuperation on simulation
results
3.9.1 Plant efficiency
With the introduction of the hybrid recuperator to increase temperature of air outlet from
compressor though the hot smoke, the analysis of plant shows an efficiency of:
ηLHV = 51,9 %
referred to Low Heat Value of fuel, and
ηHHV = 48,1%
referred to High Heat Value of fuel.
102
If we refer to the first value, we can see that this value is 11,2% more than the case without
regeneration.
Also losses of course decrease; in fact the lost energy is only 48,1% . It means that 11,2%
more of the input energy contained in the fuel is used to produce useful energy in output.
Figure 3.9.1.1: Fractions of useful output energy and loss energy with regeneration.
It‘s verified that using heat contained in the off gases to heat a cold stream inside plant,
efficiency increase.
3.9.2 Electric Power production
The obtained electric power production is in this case:
PTOT = 34,3 MW .
Of this amount, part is produced in the SOFC and another part in the Gas Turbine;
respectively:
PSOFC = 18,5 MW ,
PGT = 15,8 MW .
We can see that the amount produced in the SOFC system is the same that the previous
case; instead the electric energy produced in the Gas Turbine increase at 8,8% more.
51,9%
48,1%
Output and loss energy
OUTPUT ENERGY ENERGY LOSSES
103
The reason is that using the heat contained in the off gases to heat air coming from
compressor, in the cathode pre-heater less heat must be used from the flue gas stream to
achieve the operating temperature in the SOFC; consequence of this is that temperature of
flue gas inlet in the burner is higher and of course also temperature inlet in the Gas
Turbine. The value of this temperature is around 380°C without regeneration and around
750°C with regeneration; also off gases outlet temperature from Gas Turbine increases
from around 280 °C to 590 °C. But the difference of temperature is of course higher in the
plant with regeneration; it means that more enthalpy is available to produce mechanical
energy (and after electric through generator) in the Turbine.
53,9% of total electric energy is produced in the SOFC, 46,1% in the Gas Turbine.
Figure 3.9.2.1: Total power production: SOFC and Gas Turbine fractions in the regenerative plant.
3.9.3 Auxiliary consumption
The total consumption of electric power in the plant is:
PAUX = 12,2 MW .
The single contribute of each component is:
PSTEAM BLOWER = 0,02 MW ,
PSYNGAS BLOWER = 1,6 MW ,
PCOMPRESSOR = 10,6 MW .
53,9%
46,1%
Total power productionSOFC GAS TURBINE
104
It‘s possible to see that auxiliary consumption doesn‘t change; also the different
percentages are the same as in the previous case.
3.9.4 Net Power production
If the total production increases and the auxiliary consumption doesn‘t change, also the net
power production increases:
PNET = PTOT - PAUX =
= ( PSOFC + PGT ) – (PSTEAM BLOWER + PSYNGAS BLOWER + PCOMPRESSOR ) (3.11),
PNET = 34,3 – 12,2 = 22,1 MW
The net production increases of 4,8 MW in comparison with the case without regeneration;
but is also interesting to observe that even if the auxiliary power consumption doesn‘t
change, the percentage of total production used for this aim decreases of 5,7% .
The 35,6% of the total produced power is used for the auxiliary components instead of
41,3% as in the non regenerative plant; the remaining 64,4% can be used as net output
from plant.
Figure 3.9.4.1: Total electric power production: auxiliary consumption and net power production fractions in the
regenerative plant.
35,6%
64,4%
Auxiliary consumption and Net Power production
AUXILIARY CONSUMPTION NET POWER PRODUCTION
105
3.9.5 Energy losses
In the regenerative plant only the 48,1% of input energy comes from the waste gasification
is lost.
The loss sources are the same of the other case; but it can be interesting to observe how
much decreases the percentage of heat lost through off gases.
The mainly sources of energy loss in the plant are:
- loss through ashes in the gasifier, 1120 kJ (2,8%) ;
- loss through H2S separation in the desulphuriser, 1,645 kJ (0,004%) ;
- heat losses in the mechanical components, 243,9 kJ (0,596 %) ;
- heat losses in the generators, 322,2 kJ (0,7%);
- heat losses through exhausted gases, 41107,31 kJ (95,9%) .
Figure 3.9.5.1: Energy losses in the regenerative plant.
Also in this case the biggest loss is due to off gases outlet from Gas Turbine; temperature
of these is in fact really high. The regenerative solution decreases the total losses, increases
the power production but doesn‘t change the percentage distribution of the different losses;
the loss of energy through off gases is the same really high. Enthalpy of them is really
high; they can be used for other aims, as for example cogeneration, district heating or
district cooling.
2,8% 0,004% 0,596%
0,7%
95,9%
Energy losses
ASH H2S MECHANICAL COMPONENTS GENERATORS OFF GASES
106
3.9.6 Low Heat Value
It‘s of course obvious that the trend of LHV along the plant is the same as previously;
values obtained for this parameter for the different kinds of fuel are the same:
- LHVWASTE = 19879 kJ / kg K ;
- LHVDRY_WASTE = 20130 kJ /kg K ;
- LHVSYNGAS = 15920 kJ /kg K ;
- LHVSYNGAS_CLEAN = 15910 kJ /kg K ;
- LHVUSED_FUEL = 3313 kJ /kg K .
A comparison can be obtained also from the following diagram:
Figure 3.9.6.1: LHV trend along regenerative plant.
3.10 Sensitivity Analysis
A sensitivity analysis to vary of certain parameters is conduced; all the others values are
kept constant. The most critical parameters for this plant are identified in the following:
- Environmental conditions of temperature and pressure;
- Moisture content in the fuel;
- Fuel Low Heat Value;
- Fuel mass flow;
- Gasification temperature;
- SOFC operating temperature;
- Number of Stacks;
- Turbine Inlet Temperature (TIT);
- Compressor Pressure Ratio.
WASTEDRY
WASTESYNGAS
SYNGAS CLEAN
USED FUEL SMOKE
LHV TREND 19879 20130 15920 15910 3313 0
0
5000
10000
15000
20000
[ kJ
/ k
g K
]
LHV trend
107
The most important parameters influenced by those values are:
- Power production;
- Efficiency.
In the following analysis are reported diagrams for both; are separately considered:
- SOFC power production,
- Gas Turbine total power production,
- Total power production,
- Auxiliary power consumption,
- Net power production,
in the diagrams about power, and:
- SOFC efficiency,
- Gas Turbine efficiency,
- Plant efficiency,
- Gasification efficiency,
in the diagram about efficiency.
Total power production is sum of SOFC power production and Gas Turbine total power
production:
PTOT = PSOFC + PGT (3.12).
Net power production is the difference between the total production and the power
necessary for the auxiliary organs:
PNET = PTOT - PAUX (3.13).
SOFC efficiency is calculated as ratio between produced electric power and fuel inlet
power:
ηSOFC =
(3.14).
Gas Turbine efficiency is the ratio between the net power produced and the fuel inlet
power; the net power for this part of plant is considered as difference between the
produced power in the Turbine and the consumed power in the Compressor:
ηGT =
(3.15);
108
fuel for Gas Turbine is of course the Used Fuel coming from the SOFC anode side.
Total plant efficiency is calculated as:
ηPLANT =
=
=
=
(3.16).
Gasification efficiency is the ratio between the energy content in the coming out syngas
and the energy content in the coming in fuel in the gasifier:
ηGASIFICATION =
(3.17).
3.10.1 Environmental conditions
Environmental conditions can influence the good function of plant; in fact some organs, as
for example the air compressor, are really sensible to variations of inlet temperature and
pressure.
In the following diagrams are represented variations of electric power and efficiency as
functions of different environmental values of temperature:
Figure 3.10.1.1: Electric power as function of environmental temperature.
10
15
20
25
30
35
40
-15 -10 -5 0 5 10 15 20 25 30 35 40 45 50
P [
MW
]
Electric power as function of environmental temperature[°C]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
109
Figure 3.10.1.2: Efficiency as function of environmental temperature.
We can see that the production of total electric power in the SOFC and in the Gas Turbine
doesn‘t change when increases temperature; total production is in fact constant. But the
power required from auxiliary organs increases, in particular in the compressor, because of
the higher energy to be transmitted to the inlet air; for this reason net power decreases and
of course also plant efficiency. SOFC efficiency is not influenced by environmental
temperature instead of Gas Turbine efficiency, due to an higher required energy in the
compressor.
Environmental pressure doesn‘t has the same important influence as temperature, but little
variation of both power and efficiency can be noticed, as shown in the following diagrams:
Figure 3.10.1.3: Electric power as function of environmental pressure.
20,00
30,00
40,00
50,00
60,00
70,00
80,00
90,00
-15 -10 -5 0 5 10 15 20 25 30 35 40 45 50
[%
]
Efficiency as function ofenvironmental temperature[°C]
SOFC
GAS TURBINE
PLANT
GASIFICATION
10
15
20
25
30
35
0,95 0,98 1,01 1,03 1,05 1,07 1,09 1,11
P [
MW
]
Electric power as function of environmental pressure [bar]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
110
Figure 3.10.1.4: Efficiency as function of environmental pressure.
Variations of environmental pressure may be a consequence of air humidity; in fact the
higher is the moisture content in environmental air, the higher is air density and, in
conditions of same temperature, if this last value increases, of course also environmental
pressure increases.
Electric power required from auxiliary components, in particular from compressor,
decreases when environmental pressure increases; the produced electric power in the Gas
Turbine decreases and consequently decreases also the total produced power. But the net
power remains constant because the produced power decreases as the auxiliary power; so
the difference between these remains more or less constant. Plant efficiency decreases a
bit; Gas Turbine efficiency is more influenced. Also in this case we can conclude that
SOFC system is not influenced from variations of environmental conditions.
3.10.2 Fuel moisture content
Fuel moisture is a really important parameter; variations of this value in fact influence
properties of fuel, in particular of Low Heat Value, as we can see from the following
formula ( ref [27] ):
LHVw = LHV – MOI ( LHV + r ) (3.18),
where LHVw is the Low Heat value on wet basis, LHV the one on dry basis, MOI the
moisture content in the fuel and r the vaporization heat of water (2500 kJ/kg).
This result is confirmed by simulations, as shown in the following diagram:
20,00
30,00
40,00
50,00
60,00
70,00
80,00
0,95 0,98 1,01 1,03 1,05 1,07 1,09 1,11
[
%]
Efficiency as function of environmental pressure [bar]
SOFC
GAS TURBINE
PLANT
GASIFICATION
111
Figure 3.10.2.1: LHV as function of Fuel Moisture.
Values of moisture content in MSW are between around 8% and 20% as it‘s possible to
see from different scientific papers; but the lower is this parameters the better it is. In fact
MSW composition could change day by day as a function of the different composition; it
can be a good thing to control this value before introduce fuel in the gasifier.
Power production and efficiency are in fact influenced by fuel moisture content, as shown
in the following figures:
Figure 3.10.2.2: Electric power as function of Fuel Moisture.
14
15
16
17
18
19
20
9 10 12 14 16 18 20 22 24 26 28 30
LHV
[M
J/kg
]
Low Heat Value as function of Fuel Moisture [%]
Low Heat Value
5
10
15
20
25
30
35
9 10 12 14 16 18 20 22 24 26 28 30
P [
MW
]
Electric power as function of Fuel Moisture [%]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
112
Figure 3.10.2.3: Efficiency as function of Fuel Moisture.
As we can see from the previous diagrams, both SOFC and Gas Turbine produced power
decrease; also auxiliary consumption decreases, in particular in the syngas blower and in
the compressor, due to a reduction of produced amount of syngas and consequently of
required air. But the decrement of auxiliary consumption is lower that the reduction of
total power production; this is the reason why net power decreases.
Variations of plant efficiency are negligible; in fact to a decrement of net power
corresponds a decrement of inlet fuel power and the ratio between these two values
remains practically constant.
SOFC efficiency is not affected of moisture variation, on the contrary of Gas Turbine,
whose efficiency decreases due to a decrement of power production higher than the
decrement of inlet fuel power.
The most peculiar result is the Gasification efficiency; this value in fact increases when
moisture increases; it means that Gasifier works better when the water content in the fuel is
higher. Syngas output energy in fact also decreases of course, but slower than fuel input
energy; in any case it‘s better to have a low value of moisture, around 10%, due to a
decrement of output net power production.
3.10.3 Fuel Low Heat Value
Municipal Solid waste composition may be subject to variations of moisture, as saw, and
of composition, due to different percentages of all components; LHV is in fact another
parameter that for this reason can change. Variations can influence the work of the plant,
as we can see in the following diagrams:
20,00
30,00
40,00
50,00
60,00
70,00
80,00
90,00
100,00
9 10 12 14 16 18 20 22 24 26 28 30
[
%]
Efficiency as function of Fuel Moisture [%]
SOFC
GAS TURBINE
PLANT
GASIFICATION
113
Figure 3.10.3.1: Electric power as function of fuel LHV.
Figure 3.10.3.2: Efficiency as function of fuel LHV.
All analyzed parameters increase when fuel LHV increases, except for Gas Turbine
efficiency; in fact the difference between the required power in the compressor and the
produced electric power in the turbine doesn‘t change, but the LHV of Used Fuel
increases. In this way it‘s higher the input energy to Gas Turbine and of course efficiency
decreases.
3.10.4 Fuel mass flow
The analysis results to variations of this parameter are shown in the following diagrams:
5
10
15
20
25
30
35
14000 15000 16000 17000 18000 19000 20000
P [
MW
]Electric power as function of fuel Low Heat Value [kJ / kg]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
20,00
30,00
40,00
50,00
60,00
70,00
80,00
14000 15000 16000 17000 18000 19000 20000
[
%]
Efficiency as function of fuel Low Heat Value [kJ / kg]
SOFC
GAS TURBINE
PLANT
GASIFICATION
114
Figure 3.10.4.1: Electric power production as function of fuel mass flow.
Figure 3.10.4.2: Efficiency as function of fuel mass flow.
Both power production and power consumption increase due to an increment of inlet fuel
mass flow; for this reason also net power production decreases and the output energy from
plant is lower than the input energy in the fuel. So plant efficiency and SOFC efficiency of
course decrease; only Gas Turbine efficiency increases, due to a bigger production of
energy in the turbine, consequence of major inlet mass flow. SOFC system instead is more
sensible to mass flow variations and with a fixed number of stacks, efficiency decreases if
fuel mass flow decreases, because chemical reactions inside don‘t take place in a good
way; to use more fuel it‘s necessary to increase the number of stacks, but, as previously
shown, initial investment increases. It‘s a good solution to find a compromise between
produced electric power, efficiency and initial investment.
0
20
40
60
80
100
120
0,5 2,50 5,00 7,5
P [
MW
]Electric power as function of
fuel mass flow [kg/s]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
20,00
30,00
40,00
50,00
60,00
70,00
80,00
0,5 2,50 5,00 7,5
[
%]
Efficiency as function of fuel mass flow [kg/s]
SOFC
GAS TURBINE
PLANT
GASIFICATION
115
3.10.5 Gasification temperature
Gasification temperature is an important parameter because it affects SOFC fuel
composition; consequences of increasing gasification temperature are:
- an higher air mass flow is required for oxidation (more nitrogen is diluted inside
syngas); hence the produced amount of syngas increases but SOFC fuel heat value
is lowered, as shown in figure 3.10.5.3;
- chemical equilibrium, modeled on free Gibbs energy minimization, leads to higher
CO and lower CO2, CH4 and H2 content.
Figure 3.10.5.3: Syngas LHV as function of Gasification temperature.
Produced electric power and efficiency are influenced by this parameter, as shown in the
following diagrams:
14
14,5
15
15,5
16
16,5
17
17,5
18
700 750 800 850 900
LHV
[M
J/kg
]
Syngas Low Heat Value as function of Gasification temperature [°C]
SYNGAS LHV
116
Figure 3.10.5.1: Electric power as function of Gasification temperature.
Figure 3.10.5.2: Efficiency as function of Gasification temperature.
SOFC produced power decreases due to a decrement of syngas LHV produced during the
Gasification process; higher air mass flow is required for oxidation and for this reason
auxiliary consumption in the air compressor increases. But the produced electric power in
the Gas Turbine increases consequently to an increment of elaborated mass flow; for this
reason total produced power remains more or less the same. All efficiency values decrease,
except for Gas Turbine, due to an higher production of electric power.
10
15
20
25
30
35
700 750 800 850 900
P [
MW
]Electric power as function of Gasification temperature [°C]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
20,00
30,00
40,00
50,00
60,00
70,00
80,00
90,00
700 750 800 850 900
[
%]
Efficiency as function of Gasification temperature [°C]
SOFC
GAS TURBINE
PLANT
GASIFICATION
117
3.10.6 SOFC operating temperature
SOFC operating temperature has been changed from 780 °C to 950 °C; anode and cathode
pre-heater outlet temperatures are adjusted in order to maintain a constant difference with
SOFC operating temperature ( around 130°C ).
Figure 3.10.6.1: Electric power as function of SOFC operating temperature.
Figure 3.10.6.2: Efficiency as function of SOFC operating temperature.
SOFC power production and SOFC efficiency decrease when operating temperature
increases due to a decrement of Fuel Cell Equilibrium Potential (Figure 2.3.4.1, ( ref. [6] ))
and a thermal expansion among materials ( ref. [43] ), but auxiliary consumption increases;
9
14
19
24
29
34
39
44
780 800 850 900 950
P [
MW
]
Electric power as function of SOFC operating temperature [°C]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
20,00
30,00
40,00
50,00
60,00
70,00
80,00
780 800 850 900 950
[
%]
Efficiency power as function of SOFC operating temperature [°C]
SOFC
GAS TURBINE
PLANT
GASIFICATION
118
consequence of this is that plant efficiency decreases. Gas Turbine power production and
efficiency increase due to an higher energy input through the used fuel coming from
SOFC.
3.10.7 Number of Stacks
An increment of number of stacks in the SOFC can be used to produce major electric
power with better values of efficiency, in particular, as previously saw, when it‘s necessary
to increase the introduced fuel mass flow and consequently the size of the plant; in the
following analysis fuel mass flow is kept constant (2,4 kg/s).
Figure 3.10.7.1: Electric power as function of number of stacks.
Figure 3.10.7.2: Efficiency as function of number of stacks.
10
15
20
25
30
35
25
50
0
30
50
0
35
50
0
40
50
0
45
50
0
50
50
0
55
50
0
60
50
0
65
50
0
70
50
0
P [
MW
]
Electric power as function ofNumber of stacks
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
20,00
30,00
40,00
50,00
60,00
70,00
80,00
25
50
0
30
50
0
35
50
0
40
50
0
45
50
0
50
50
0
55
50
0
60
50
0
65
50
0
70
50
0
[
%]
Efficiency as function of Number of stacks
SOFC
GAS TURBINE
PLANT
GASIFICATION
119
As we can see from diagrams, both SOFC and plant efficiency increase; also net power
production increases, due to a decrement of required auxiliary consumption.
We can see also that Gas Turbine works worse even if compressor requires minor power;
this is consequence of a minor air mass flow and of elaborated mass flow in the turbine.
We can see also that all values change until a determined number of stacks; after this value
variations are negligible. It means that it‘s necessary also to increase the fuel mass flow to
obtain a major power production. It‘s also important to consider that an increment of
number of stacks has also an economic consequence on the initial investment, as seen
before.
3.10.8 Turbine Inlet Temperature
According to the second principle of thermodynamics, the higher is the maximum
temperature in a thermodynamic system, the higher is the efficiency; so an increment of
this parameter leads to an increase of efficiency and produced power in the Gas Turbine.
According to the previous analysis, factors that can increase this value are:
- decrement of moisture content in the fuel;
- increment of fuel LHV;
- increment of fuel mass flow;
- decrement of Gasification temperature;
- decrement of SOFC operating temperature;
- increment of number of stacks.
Variations of power and efficiency are shown in the following diagrams:
Figure 3.10.8.1: Electric power as function of TIT.
10
15
20
25
30
35
40
45
700 750 800
P [
MW
]
Electric power as function of Turbine Inlet Temperature [°C]
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
120
Figure 3.10.8.2: Efficiency as function of TIT.
Obviously the only one influenced system from this parameter is Gas Turbine; increments
of efficiency and power production in this system lead to increments of course of power
production and efficiency for the whole plant.
But because Gas Turbine is the bottom cycle, it‘s necessary to find a compromise between
values of parameters that allow a good work to SOFC system and values that may increase
the TIT; for this combined cycle a range of reasonable values for this parameter is between
700°C and 750 °C. It is not of course a problem for materials used in the turbine; a saving
can be obtained under an economic point of view.
3.10.9 Pressure Ratio
Pressure ratio is another important parameter to consider; an increment of this value leads
to a major auxiliary consumption, but of course also to an increment of produced electric
power both in SOFC and in the Gas Turbine. In fact SOFC works better when is high the
partial pressure of elements and Gas Turbine can produce more energy; but there‘s a limit
beyond which no longer can increase pressure, because SOFC can work in a specific range
of values.
25,00
35,00
45,00
55,00
65,00
75,00
85,00
700 750 800
[
%]
Efficiency as function of Turbine Inlet Temperature [°C]
SOFC
GAS TURBINE
PLANT
GASIFICATION
121
Figure 3.10.9.1: Electric power as function of pressure ratio.
Figure 3.10.9.2: Efficiency as function of pressure ratio.
It could be better to increase pressure ratio value; but there are two reasons why pressure
ratio cannot be increased after a specific value; in fact SOFC system works in a specific
range of values.
In addition, in the considered plant there are different heat exchangers, in particular anode
and cathode pre-heater and hybrid recuperator; an increment of pressure level leads to also
an increment of temperature. It means that if pressure increases beyond a specific value,
temperature profiles in the heat exchangers can be violated. We will see that there‘s a
reason why this problem is more heard in the anode pre-heater; but also in the other ones
it‘s better to take care of this problem.
0
5
10
15
20
25
30
35
40
2,00 2,50 2,80
P [
MW
]
Electric power as function of pressure ratio
SOFC
GAS TURBINE
TOTAL
AUXILIARY
NET
0,00
10,00
20,00
30,00
40,00
50,00
60,00
70,00
80,00
90,00
2,00 2,50 2,80
[
%]
Efficiency as function of pressure ratio
SOFC
GAS TURBINE
PLANT
GASIFICATION
122
With a SOFC operating temperature of 780 °C and a difference of temperature between
output and input of 130 °C, reasonable values for pressure ratio are between 2,2 bar and
2,5 bar; after, if we want to increase this value, it‘s necessary also to increase SOFC
operating temperature and SOFC inlet temperatures to maintain the same difference
between in and out, in order to increase the level of temperature in the system.
3.11 Optimized configuration
After the previous analysis we can conclude that optimized system is obtained by means of
the following devices:
- Hybrid recuperation;
- Municipal Solid Waste moisture content 9,5 %;
- Municipal Solid Waste LHV 19879 kJ/kg;
- Fuel mass flow 2,4 kg/s;
- Gasification temperature 800 °C;
- SOFC operating temperature 780 °C;
- 50500 stacks;
- Compression pressure ratio 2,5.
Simulations were run in order to provide plant efficiency; environmental conditions are
fixed at 25 °C and 1,01325 bar. Optimized system main data input and output are listed in
the following tables; DNA code and detailed results are provided in APPENDIX.
COMPONENT DATA INPUT VALUE
Dryer Inlet mass flow 31 2,4 [kg/s]
inlet temperature t31 25 [°C]
inlet pressure p31 1,01325 [bar]
outlet temperature t32 1,008 [°C]
outlet pressure p32 150 [bar]
heat losses q320 0 [kW]
pressure drops fuel side 0,05 [bar]
pressure drops steam side 0,005 [bar]
Gasifier outlet temperature t33 800 [°C]
inlet water temperature t45 150 [°C]
outlet pressure H2S p46 1,01325 [bar]
heat losses q321 0 [kW]
operating pressure 0,998 [bar]
operating temperature 800 [°C]
pressure losses 0,005 [bar]
water to fuel ratio 0
carbon conversion factor 1
non equilibrium methane 0,01
Air pre-heater inlet temperature t41 25 [°C]
123
heat losses q322 0 [kW]
pressure drops syngas side 0,005 [bar]
pressure drops air side 0,005 [bar]
Steam heater outlet temperature t40 200 [°C]
heat losses q323 0 [kW]
pressure drops syngas side 0,005 [bar]
pressure drops steam side 0,005 [bar]
Steam blower isentropic efficiency 80%
mechanical efficiency 98%
Desulphuriser heat losses q325 0 [kW]
pressure drops 0,0049 [bar]
SOFC stack heat losses q309 0 [kW]
inlet temperature cathode
side t4
650 [°C]
inlet temperature anode side
t11
650 [°C]
utilization factor 0,8
operating temperature 780
pressure drops anode side 0,01 [bar]
pressure drops cathode side 0,04 [bar]
cells per stack 75
number of stacks 50500
Anode pre-heater heat losses q308 0 [kW]
pressure drops used fuel side 0,01 [bar]
pressure drops syngas side 0,01 [bar]
Cathode pre-
heater
heat losses q303 0
pressure drops flue gas side 0,04 [bar]
pressure drops air side 0,04 [bar]
Air compressor inlet pressure p1 1,01325 [bar]
inlet temperature t1 25 [°C]
outlet pressure p2 2,5 [bar]
isentropic efficiency 0,8
mechanical efficiency 0,98
Syngas blower isentropic efficiency 0,8
mechanical efficiency 0,98
Burner heat losses q310 0 [kW]
pressure drops 0,95
Turbine isentropic efficiency 0,86
Electric Generator electric efficiency 0,98
Hybrid
Recuperator
pressure drops smoke side 0,04 [bar]
pressure drops air side 0,04 [bar]
heat losses q302 0 [kW]
outlet pressure p19 1,01325 [bar]
Table 3.11.1: Optimized data input.
124
DATA OUTPUT VALUE
Plant Total power production 34,32 [MW]
Total power consumption 12,19 [MW]
Net power production 22,13 [MW]
Fuel consumption 42,62 [MW]
Efficiency 51,93 [%]
SOFC Power production 18,53 [MW]
Efficiency 43,5 [%]
Gas Turbine Total power production 15,79 [MW]
Total power consumption 10,58 [MW]
Net power production 5,5 [MW]
Heat power input 21,57 [MW]
Efficiency 25,5 [%]
Table 3.11.2: Optimized data output.
3.12 Pressure Drops in Heat Exchangers
In this power plant are present different heat exchangers; their position is studied to
optimize the heat exchange between different flows in the plant and to increase the
efficiency.
The heat exchangers present are the following:
- Air pre-heater, to heat the incoming air to the gasifier using the hot syngas flow;
- Steam generator, to heat the steam used to dry the biomass through the hot syngas;
- Cathode pre-heater, to increase the incoming air temperature in the SOFC system
using the heat output from the cathode side;
- Anode pre-heater, to increase the incoming fuel temperature in the SOFC system
using the hot stream from the anode side;
- Hybrid recuperator, where the hot exhausted gases output from the gas turbine are
used to increase the temperature level of the incoming air in the burner.
It‘s better to have a good value of pressure drops in the heat exchangers to increase the
efficiency of the plant; in all the heat exchangers it‘s better to have pressure losses between
0,005 bar and 0,04 bar depending on fuel conditions. With reasonable values for all
different heat exchangers is estimated approximately the size of the heat exchangers; it‘s
hypothesized to use ―shell and tube‖ heat exchangers.
For this calculation is at first necessary to know the efficiency and the global heat transfer
coefficient for both cold and hot side; in this way are used the following equations.
Heat capacity is calculated as
Cc = c cp,c (3.19),
for cold side and
125
Ch = Cc
(3.20),
for hot side. Specific heat capacity for cold stream is assumed, at mean temperature, as:
cp,c = 1135 [ J/ kg K ]
For the hot stream, this value is calculated as:
cp,h =
(3.21).
The heat exchanger efficiency is:
ε =
(3.22).
If we call C the ratio between Cmin and the other heat capacity, for counter flow the
Number of Transport Unit (NTU) is:
NTU =
(3.23).
The product between the global heat transfer coefficient and the heat exchange area is:
UA = Cc NTU (3.24).
The input data are the temperatures in inlet and outlet and the mass flow in both cold and
hot side; the obtained values are shown in the following table:
Th,in
[°C]
Th,out
[°C]
Tc,in
[°C]
Tc,out
[°C] h
[kg/s]
c
[kg/s]
ε
[%]
NTU UA
[W/K]
AIR
PRE_HEATER
800 539 25 780 4,49 2,35 97,5 4,962 13234
STEAM
GENERATOR
539 425 152 200 4,49 8,38 29,5 0,374 3557
CATHODE
PRE_HEATER
780 650 520 650 91,72 93,74 50 1 106395
ANODE
PRE_HEATER
780 775 644 650 6,51 4,48 4,4 0,046 233,8
HYBRID
RECUPERATOR
587 231 134 519 98,23 93,74 85 4,716 501719
Table 3.12.1: Obtained values for efficiency, NTU and heat transfer coefficient.
126
Pressure drops are calculated through the following formulas:
∆pc = 2 fc
+
(3.25),
for cold side and
∆ph = 2 fh
+
(3.26),
for hot side.
fc and fh are the ―friction factors‖; it‘s possible to read these values by diagrams as function
of Reynolds number and relative roughness (Moody diagram) or they could be calculated
by the following empiric formulas:
f = 0,316 Re-0,25
if Re < 20000 (3.27),
f = 0,184 Re-0,20
if Re > 20000 (3.28).
Reynolds numbers are calculates as:
Rec =
D Reh =
Dh (3.29).
In this work are supposed to be:
fc = 0,01 0,006 fh = 0,01 0,006
L is the length of the heat exchanger and D the piper diameter; Gc and Gh are :
Gc =
Gc =
(3.30).
Dh is the hydraulic diameter
Dh =
(3.31).
Aflow is calculated as:
Aflow = Afr – n
(3.32),
where Afr is the frontal area.
Density values in input and output are known; for the dynamic viscosity are used the
following values:
127
- c = 4,1 * 10-5
[ Pa s ]
- h = 4,57 * 10-5
[ Pa s ]
To solve this system of equations and to obtain the values of all the geometrical
parameters of heat exchangers are necessary other equations; at this point we can also
write that:
(3.33),
where hc and hh are the convective coefficients:
hc = cp,c Gc Stc hh = cp,h Gh Sth (3.34).
St is the Stanton number and it‘s calculated as:
Stc = 0,5 fc Prc-2/3
Sth = 0,5 fh Prh-2/3
(3.35),
where Prandtl number is assumed to be:
- Prc = 0,71, for cold side;
- Prh = 0,72, for hot side .
All these equations are used to compose an equation system; EES is the software used to
solve the latter,file is attached in APPENDIX. The obtained results are shown in the
following table; all geometrical variables are unknown, except for D and n. It‘s also
assumed that for commercial pipes is D/D0 = 1,2 .
∆ph
[bar]
∆pc
[bar]
Aflow
[m2]
Afr
[m2]
A
[m2]
L
[m]
Dh
[m]
D0
[m]
D
[m]
n
AIR
PRE_HEATER
0,00
5
0,00
5
2,19
8
2,33
4
101,
6
2,696 0,194
4
0,01
2
0,0
1
1200
STEAM
GENERATOR
0,00
5
0,00
5
9,46
9
10,3
7
243,
1
0,967
7
0,125
7
0,01
2
0,0
1
8000
CATHODE
PRE_HEATER
0,04 0,04 18,3
8
20,1
9
591,
6
1,178 0,122 0,01
2
0,0
1
1600
0
ANODE
PRE_HEATER
0,01 0,01 14,1
7
14,4
5
84,1
2
1,072 0,601
5
0,01
2
0,0
1
2500
HYBRID
RECUPERAT
OR
0,04 0,04 3,86
3
5,55
9
537,
4
1,141 0,027
3
0,01
2
0,0
1
1500
0
Table 3.12.2: Obtained values for geometric size of all heat exchangers.
128
As we can see in the Table 3.11.1, the two heat exchangers with the lower efficiency are:
- steam generator (29,5 %);
- anode pre-heater (4,4 %).
The reason why the efficiency in the steam generator is not so high is because to produce
steam at 200 °C to dry the waste in the dryer, the syngas temperature decreases only of
about 115 °C, from 539 °C to 424 °C; in fact the moisture content of fuel is lower than for
example woodchips (around 10% against 30% in lingo-cellulosic biomass). It means that
more syngas is produced in the Gasifier; in this way, in the hot side of the steam generator,
there‘s about twice mass flow as for example using woodchips. So, remembering that the
exchanged heat is
Q = ∆H (3.36),
that, under the hypothesis of ideal fluid can be written as
Q = cp ∆T (3.37).
To exchange the same amount of heat, if the mass flow is higher, a lower difference of
temperature is necessary; this is the reason why the syngas temperature outside from the
steam generator is so high. Of course if the moisture content increases, the production of
syngas decreases and also the temperature of syngas outlet from steam generator; an higher
difference of temperature is in fact necessary to have the same heat exchanged.
The following figure shows the temperature profiles in the steam generator:
Figure 3.12.1: Temperature profiles in the Steam Generator.
100
150
200
250
300
350
400
450
500
550
600
0 20 40 60 80 100
T [°
C]
Q [%]
Steam Generator temperature profiles
COLD SIDE
HOT SIDE
129
Temperature outlet from steam generator is also the value inlet in the Desulphuriser;
fortunately the obtained level of temperature is not a problem for this component
( ref. [28] ). Of course the more is the level of temperature, the better should be the used
materials.
Anode pre-heater efficiency is really low, only 4,4%; it means that the syngas blower
increases, with the pressure level, also the temperature level of syngas until a value very
close to that of entry in the anode side of SOFC. As we can see in the following paragraph,
this component can be also removed, without any consequence on the power production
and efficiency.
The following diagram shows the temperature profiles in the anode pre-heater:
Figure 3.12.2: Temperature profiles in the Anode pre-heater.
3.13 Effect of removal of the Anode Pre-Heater
In this paragraph effect of removal of the Anode pre-heater is studied; in fact, as
previously seen (figure 3.11.1), temperature level of the cold side increases of only 6 °C,
from 644 °C to 650 °C, and in the hot side decreases of only 5 °C, from 780 °C to 775 °C.
Syngas blower in fact has to increase the pressure level of fuel until the operative value in
the SOFC plant; this value is fixed to 2,5 bar. With the pressure level increases of course
also the temperature level; clean syngas arrives from the Gasification plant at a
temperature of about 425 °C, as seen in the paragraph 3.11, and a pressure of about 1 bar.
To increase this value until 2,5 bar, temperature increases until 644 °C; this temperature is
very close to the inlet temperature in the anode side of the SOFC plant. For this reason is
600620640660680700720740760780800
0 20 40 60 80 100
T [°
C]
Q [%]
Anode pre-heatertemperature profiles
COLD SIDE
HOT SIDE
130
not useful the presence of an heat exchanger that increases the temperature level; the
scheme of plant without anode pre-heater is shown in the following figure:
Figure 3.13.1: Scheme of the plant without anode pre-heater.
Syngas coming from syngas blower goes directly in the SOFC, without a pre-heating; level
of temperature is in fact really close to the fixed operating temperature in the SOFC.
As seen before, if moisture content increases, temperature outlet from steam generator
decreases and also the level of temperature after the syngas blower; in this case it‘s
necessary the anode pre-heater to anchieve the inlet temperature in the SOFC. For example
with a moisture content of 30%, temperature outlet from steam generator is around 280 °C;
with a pressure ratio of 2,5 , after syngas blower the level of temperature is around 450 °C;
in this case of course an Anode pre-heater is necessary to achieve 650 °C in the inlet of
SOFC system. The following diagrams show the production of syngas and the outlet
temperature coming from steam generator and syngas blower at variations of fuel
moisture:
131
Figure 3.13.2: Obtained Syngas mass flow as function of fuel moisture.
Figure 3.13.3: Outlet temperature from steam generator and from syngas blower as function of fuel moisture.
Removal of Anode pre-heater can be possible only if moisture content of fuel is around
9% - 10%; after this value the production of syngas and consequently temperature levels
after steam generator and syngas blower decrease too much and it‘s necessary to pre-heat
syngas before introduction in the SOFC.
Moisture content in the Municipal Solid Waste is usually very low; variations of this value
can take place, but normally in a limited range between 9% and 19% ( Table 1.3.2.1 ).
An analysis of removal effects is conduced, under the hypothesis that moisture content is
9,5%.
5,2
5,4
5,6
5,8
6
6,2
6,4
6,6
9 10 12 14 16 18 20 22 24 26 28
mSY
NG
AS
[kg/
s]
Produced Syngas mass flow as function of waste moisture [%]
PRODUCED SYNGAS MASS FLOW
0
200
400
600
800
9 10 12 14 16 18 20 22 24 26 28 30
T OU
T[°
C]
Outlet temperature from steam generator and from syngas blower as function of
waste moisture [%]
OUTLET TEMPERATURE FROM STEAM GENERATOR
OUTLET TEMPERATURE FROM SYNGAS BLOWER
132
3.13.1 Plant Performance
Used input values for simulation are the same as in the case with anode pre-heater ( Table
3.11.1 ); DNA code and detailed results are provided in APPENDIX. The following table
shows the main obtained results from simulation without anode pre-heater; these are
compared with the others obtained in the simulation at paragraph 3.11:
With Anode pre-
heater
Without Anode pre-
heater
Plant Total power
production
34,32 [MW] 34,27 [MW]
Total power
consumption
12,19 [MW] 12,13 [MW]
Net power production 22,13 [MW] 22,14 [MW]
Fuel consumption 42,62[MW] 42,62 [MW]
Efficiency 51,93 [%] 51,95 [MW]
SOFC Power production 18,53 [MW] 18,54 [MW]
Efficiency 43,5 [%] 43,5 [%]
GT Total power
production
15,79 [MW] 15,73 [MW]
Total power
consumption
10,58 [MW] 10,53 [MW]
Net power production 5,5 [MW] 5,52 [MW]
Heat power input 21,57 [MW] 21,57 [MW]
Efficiency 25,5 [%] 25,6 [MW]
Table 3.13.1.1: Comparison between plant with and without anode pre-heater.
As we can see from the obtained results, plant is not affected by the removal of the Anode
pre-heater; indeed power consumption decreases a bit and consequently efficiency
increases, but only of 0,02 %. The results is negligible; the following diagram shows the
comparison between the two situations for the analyzed parameters:
133
Figure 3.13.1.1: Comparison between output obtained data with and without Anode pre-heater.
3.13.2 Economic benefit
As shown in the previous paragraph, under a thermodynamic point of view the removal of
the Anode pre-heater doesn‘t involve any alteration; but under an economic point of view,
a lower initial capital investment for the plant is necessary, because we haven‘t to buy this
heat exchangers.
The capital investment of an heat exchanger is function of the exchange area; usually the
used formula to calculate this value is ( ref. [29] ):
IHEAT-EXCHANGER = 130 (
[$] (3.38),
where Aex is the value of exchange area.
With the result obtained from EES simulation in the paragraph 3.12 ( 84,12 m2 ), the
initial investment for this component is:
IAP = 26300 $ = 18330 € (3.39),
where, as conversion factor from $ to €, is used 0,697 (July 2011).
As we can see later, the influence of this part of investment is negligible in comparison
with the one for others components (e.g. SOFC and Gasifier); but it‘s always an economic
saving.
0 10 20 30 40 50 60
Total power production [MW]
Total power consumption [MW]
Net power production [MW]
Fuel consumption [MW]
Efficiency [%]
SOFC power production [MW]
SOFC efficiency [%]
GT Total power production [MW]
GT Total power consumption [MW]
GT Net power production [MW]
GT Heat power input [MW]
GT efficiency[%]
Comparison between output obtained data
With Anode pre-heater
Without Anode pre-heater
134
3.13.3 Pressure ratio increment
As seen in the paragraph 3.10.9, it can be a good solution to increase the pressure ratio to
have major power production and higher efficiency; these values in fact increase with
increments of pressure ratio, as shown in figures 3.10.9.1 and 3.10.9.2.
But for the used values in the simulation, a limit is established at 2,5; in fact it‘s not
possible to use an higher value, because temperature conditions can be violated. Under this
aspect the most delicate component is of course the Anode pre-heater, where inlet
temperature in the cold side is really close with the inlet temperature in the SOFC; with a
pressure ratio of 2,6 temperature conditions are already violated, because after syngas
blower it‘s achieved a temperature of 656 °C, higher than the inlet temperature in the
SOFC anode side.
But, with the removal of this heat exchanger, a further increase of pressure ratio is
possible; in this situation the maximum rateable value is 2,6 if we use as SOFC operating
temperature the same as the other case. It‘s not so higher than the used in the other plant;
but 0,1 bar more of pressure can be useful to increase a bit the plant performance, as we
can see in the following table:
With Anode pre-
heater ( rp = 2,5 )
Without Anode pre-
heater ( rp = 2,6 )
Plant Total power
production
34,32 [MW] 35,17 [MW]
Total power
consumption
12,19 [MW] 12,76 [MW]
Net power production 22,13 [MW] 22,41 [MW]
Fuel consumption 42,62[MW] 42,62 [MW]
Efficiency 51,93 [%] 52,56 [MW]
SOFC Power production 18,53 [MW] 18,58 [MW]
Efficiency 43,5 [%] 43,6 [%]
GT Total power
production
15,79 [MW] 16,59 [MW]
Total power
consumption
10,58 [MW] 11,08 [MW]
Net power production 5,5 [MW] 5,85 [MW]
Heat power input 21,57 [MW] 21,57 [MW]
Efficiency 25,5 [%] 27,2 [MW]
Table 3.13.3.1: Comparison output results with a plant without Anode pre-heater and a pressure ratio of 2,6.
As we can see from the following diagram, the most sensible system to increment of
pressure level is Gas Turbine; both efficiency and produced power increase. Generally the
whole plant reflects the benefit of an increment of rp, even if this increment is limited to
only 0,1 bar; total power production and efficiency increase.
135
Figure 3.13.3.1: : Comparison output results between a plant with Anode pre-heater and a pressure ratio of 2,5 and the
other one without Anode pre-heater and a pressure ratio of 2,6.
2,6 bar is the maximum value for a SOFC operating temperature of 780 °C and an inlet
temperature of 650 °C; as mentioned in paragraph 3.10.9, an increment of pressure ratio
over this value can be obtained only increasing the SOFC temperature level.
3.14 Integration of internal flows of heat
As seen before, due to the low content of moisture in the fuel, temperature outlet from
steam generator and inlet in the desulphuriser is around 410 °C; consequently the
temperature level after the syngas blower is high enough to be not necessary the anode pre-
heater.
But desulphurization process can take place in a range of temperature between 200 °C and
450 °C ( ref. [28] ) and the higher is the temperature, the better should be the used
materials; so another interesting solution could be to decrease temperature inlet in the
desulphuriser until 200 °C and to use the hot stream between steam generator and
desulphuriser to increase the level of temperature of a cold stream inside the plant; in this
way the level of temperature decreases, also after the syngas blower, and becomes
necessary to have the anode pre-heater.
The most interesting cold stream inside the plant, whose temperatures are compatible with
the hot ones, is identified between the compressor and the hybrid recuperator; the layout of
the modified plant is shown in the following figure:
0 10 20 30 40 50 60
Total power production [MW]
Total power consumption [MW]
Net power production [MW]
Fuel consumption [MW]
Efficiency [%]
SOFC power production [MW]
SOFC efficiency [%]
GT Total power production …
GT Total power consumption …
GT Net power production [MW]
GT Heat power input [MW]
GT efficiency[%]
Comparison between output obtained data
With Anode pre-heater (rp = 2,5)
Without Anode pre-heater ( rp = 2,6 )
136
Figure 3.14.1: Scheme of the plant with the introduction of an heat exchanger between the steam generator and the
desulphuriser.
3.14.1 Recuperator features
The following table shows the main thermodynamics features of this heat exchanger;
values are calculated through the same formulas used in the paragraph 3.12. Equations are
solved through EES software; file is reported in APPENDIX.
Th,in
[°C]
Th,out
[°C]
Tc,in
[°C]
Tc,out
[°C] h
[kg/s]
c
[kg/s]
ε
[%]
NTU UA
[W/K]
RECUPERATOR 424 200 134 150 4,49 93,87 77,24 1,533 163327
Table 3.14.1.1: Obtained values for efficiency, NTU and heat transfer coefficient for Recuperator.
The following figure shows the profile of temperature inside the heat exchanger:
137
Figure 3.14.1: Temperature profiles in the Recuperator.
As we can see, temperature in the hot side decreases more than how increases in the cold
side; the reason is the difference between the two mass flows. In fact in the hot side is
lower than the cold side.
In the following table are reported all the geometric parameters calculated for this heat
exchanger; the same equations of paragraph 3.12 are used. EES is the software used to
solve the system of equations; the file is attached in APPENDIX.
∆ph
[bar]
∆pc
[bar]
Aflow
[m2]
Afr
[m2]
A
[m2]
L
[m]
Dh
[m]
D0
[m]
D
[m]
n
RECUPERATOR 0,04 0,04 0,6405 2,336 538,5 1,143 0,0045 0,012 0,01 15000
Table 3.12.2: Obtained values for Recuperator geometric size.
3.14.2 Plant performance
Used input values for simulation are the same as in the case studied in paragraph 3.11 (
Table 3.11.1 ); DNA code and detailed results are provided in APPENDIX. In the
simulation is fixed as ―add condition‖ the temperature inlet in the desulphuriser ( 200 °C ).
The following table shows the main obtained results from simulation with heat recuperator
between steam generator and desulphuriser; these are compared with the others obtained in
the simulation at paragraph 3.11:
130
180
230
280
330
380
430
0 20 40 60 80 100
T [°
C]
Q [%]
Recuperatortemperature profiles
COLD SIDE
HOT SIDE
138
With Recuperator Without Recuperator
Plant Total power
production
33,71 [MW] 34,32 [MW]
Total power
consumption
11,73 [MW] 12,19 [MW]
Net power production 21,98 [MW] 22,13 [MW]
Fuel consumption 42,62[MW] 42,62[MW]
Efficiency 51,56 [%] 51,93 [%]
SOFC Power production 18,52 [MW] 18,53 [MW]
Efficiency 43,5 [%] 43,5 [%]
GT Total power
production
15,19 [MW] 15,79 [MW]
Total power
consumption
10,59 [MW] 10,58 [MW]
Net power production 4,6 [MW] 5,5 [MW]
Heat power input 21,57 [MW] 21,57 [MW]
Efficiency 21,3 [%] 25,5 [%]
Table 3.14.2.1: Comparison between plant with and without Heat Recuperator.
As we can see from the obtained results, plant is a bit affected by the introduction of
another heat exchanger; power consumption decreases a bit but also power production
decreases; consequently efficiency decreases, but only of 0,39 %. The result is negligible,
even if can allow to the plant more flexibility, in particular about pressure ratio, as we will
see; the following diagram shows the comparison between the two situations for the
analyzed parameters:
Figure 3.14.2.1: Comparison between output obtained data with and without Anode pre-heater.
0 10 20 30 40 50 60
Total power production [MW]
Total power consumption [MW]
Net power production [MW]
Fuel consumption [MW]
Efficiency [%]
SOFC power production [MW]
SOFC efficiency [%]
GT Total power production [MW]
GT Total power consumption [MW]
GT Net power production [MW]
GT Heat power input [MW]
GT efficiency[%]
Comparison between output obtained data
With Recuperator
Without Recuperator
139
3.14.3 Effect on the economic investment
As shown in the previous paragraph, under a thermodynamic point of view the
introduction of the Recuperator doesn‘t involve particular alterations; but under an
economic point of view, an higher initial capital investment for the plant is of course
necessary to buy this heat exchanger.
The capital investment of an heat exchanger is function of the exchange area, as seen
before; usually the used formula to calculate this value is ( ref. [29] ):
IHEAT-EXCHANGER = 130 (
[$] (3.40),
where Aex is the value of exchange area.
With the result obtained from EES simulation, reported in APPENDIX ( 538,5 m2 ), the
initial investment for this component is:
IAP = 111900 $ = 78000 € (3.41),
where, as conversion factor from $ to €, is used 0,697 (July 2011).
As we can see later, the influence of this part of investment is negligible in comparison
with the one for others components (e.g. SOFC and Gasifier); but it‘s always an economic
expenses more.
3.14.4 Pressure ratio increment
As seen in the paragraph 3.10.9, it can be a good solution to increase the pressure ratio to
have major power production and higher efficiency; these values in fact increase with
increments of pressure ratio, as shown in figures 3.10.9.1 and 3.10.9.2.
But for the used values in the simulation reported in the paragraph 3.11, a limit is
established at 2,5; in fact it‘s not possible to use an higher value, because temperature
conditions can be violated. Under this aspect the most delicate component is of course the
Anode pre-heater, as seen before. But, with the introduction of another heat exchanger
before the desulphuriser, a further increase of pressure ratio is possible; temperature level
in fact decreases also after syngas blower. In this situation the maximum rateable value is
3,4 if we use as SOFC operating temperature the same as the other case. The effect over
the efficiency and the produced power is really interesting, as we can see in the following
table:
140
With Recuperator
( rp = 3,4 )
Without Recuperator
( rp = 2,5 )
Plant Total power
production
39,82 [MW] 34,32 [MW]
Total power
consumption
16,11 [MW] 12,19 [MW]
Net power production 23,71 [MW] 22,13 [MW]
Fuel consumption 42,62[MW] 42,62[MW]
Efficiency 55,63 [%] 51,93 [%]
SOFC Power production 18,87 [MW] 18,53 [MW]
Efficiency 44,3 [%] 43,5 [%]
GT Total power
production
20,95 [MW] 15,79 [MW]
Total power
consumption
14,45 [MW] 10,58 [MW]
Net power production 6,5 [MW] 5,5 [MW]
Heat power input 21,57 [MW] 21,57 [MW]
Efficiency 30,2 [%] 25,5 [%]
Table 3.14.4.1: Comparison output results with a plant without Recuperator and a pressure ratio of 3,4.
As we can see from the following diagram, the most sensible system to increment of
pressure level is Gas Turbine; both efficiency and produced power increase. Generally the
whole plant reflects the benefit of an increment of rp; total power production and efficiency
increase. It‘s also interesting to see that in this case Gas Turbine produces more power that
SOFC, on the contrary as in the case without Recuperator.
Figure 3.14.4.1: : Comparison output results between a plant with Recuperator and a pressure ratio of 3,4
and the other one without Recuperator and a pressure ratio of 2,5.
3,4 bar is the maximum value for a SOFC operating temperature of 780 °C and an inlet
temperature of 650 °C; as mentioned in paragraph 3.10.9, an increment of pressure ratio
over this value can be obtained only increasing the SOFC temperature level.
0 10 20 30 40 50 60
Total power production [MW]
Net power production [MW]
Efficiency [%]
SOFC efficiency [%]
GT Total power consumption …
GT Heat power input [MW]
Comparison between output obtained data
With Recuperator (rp = 3,4)Without Recuperator ( rp = 2,5 )
141
4. Integration with Absorption Cooling
Units
4.1 Introduction to Absorption Plants
A Cooling Unit works according to a reverse cycle; in fact, the work absorbed from the
outside is used to transfer heat from a low temperature source to an high temperature
source.
According to the second principle of thermodynamic, is not natural the transfer of heat
from a cold source to an hot one; for this reason is necessary the administration of work
from the outside.
We have two possibilities to produce cooling:
- to use electric energy to feed a compressor; in this case the system is a
―Compression Cooling System‖.
- to use heat coming from combustion or from exhausted gases of another process; in
this case the system is called ―Absorption Cooling System‖.
To explain the operation of this system, we refer to the following T-s diagram ( ref. [30] ):
Figure 4.1.1: Reverse cycle.
142
The thermodynamic involved transformations are:
1 – 2 : the fluid is compressed from the low pressure level to the high one;
2 – 3 : the steam is desuperheated until the temperature level T3;
3 – 4 : the operative fluid is condensed;
4 – 5 : the fluid expands from high pressure level to low one, commonly through a rolling
valve;
5 – 1 : the fluid evaporates absorbing heat from an external source.
In particular, the Absorption Cooling System works using the features of binary mixtures;
the thermodynamic process is really similar with the previous one, but instead of a
compressor ( 1 – 2 ) it‘s present a system composed from an absorber and a desorber.
Between these two is placed a pump; the electric consumption of this one is usually
negligible in comparison with the compressor consumption of the other kind of system.
The complex of these components serves to increase the temperature and pressure level of
the steam coming from the evaporator; in this way the low pressure steam coming from the
evaporator is not compressed using mechanical (and consequently electric) energy, but is
sent to an absorber.
In this component the refrigerant is absorbed by the strong absorption solution (rich in
absorption medium); this process is exothermic due to both condensing of refrigerant and
mixing of refrigerant and solution. The heat is removed by an external cooling circuit.
After, the liquid weak solution (poor in absorption media) is pressurized by a pump in
order to increase the pressure level; the solution is preheated in a solution heat exchanger.
At this point the weak solution is heated, by an external high temperature heat source, in
another component called desorber (or generator) which increase the refrigerant
temperature; the strong solution is sent through the solution heat exchanger and heat is
recovered by the weak solution.
The high pressure of the strong solution is reduced to the low pressure by an expansion
valve before entering in the absorber.
The used solutions for an Absorption Plant are principally two:
- Lithium Bromide absorption plants;
- Ammonia-Water mixture absorption plants.
In the first one the refrigerant is distilled water, that is stable, non toxic, readily available
and with a good value of vaporization latent heat; of course levels of pressure must be
really low. The adsorbent is Lithium Bromide, a crystalline salt with a good affinity with
water and non toxic.
In the second one water is the adsorbent and ammonia is the refrigerant; this technique was
used more before discovering the Li-Br technology. The only one difference with the other
one is that are used two others components: a rectifier and an analyzer, to separate with a
good efficiency steam and ammonia vapors.
In this project will be used a Lithium Bromide Absorption Plant.
143
To evaluate the efficiency of an absorption cooling system we have to consider the inlet
heat flow used in the desorber, coming from an external source, the effectiveness in the
evaporator, to have cooling, and the electric power used in the pump; we call ―Coefficient
of Performance‖ (COP) the ratio between outlet effectiveness and inlet heat expense:
COP =
(4.1).
Values for this parameter are usually included between 0,5 and 0,7 ( ref. [30] ).
4.2 Features of the used Absorption System
The Absorption Cooling System used in this paper is a One-Stage LiBr plant, with only
one level of condensation; the layout is shown in the following scheme:
Figure 4.2.1: Scheme of the used Absorption Plant.
With different colors are marked the different flows in the system; it‘s possible to
distinguish the Water flow, that comes from the desorber until the absorber through the
condenser, the valve and the evaporator, the LiBr-Water rich mixture that comes from the
desorber until the absorber through the heat exchanger and the second valve, and at last the
LiBr-Water weak mixture that comes from the absorber until the desorber through the
pump and the heat exchanger.
144
Also the external flows are marked with different colors; we can see for example the
external heat flow coming in the desorber.
Main components of this plant are:
- ―Desorber‖, where is used an external heat source to obtain a LiBr-Water mixture
rich of absorbent ( LiBr ) and to increase temperature in the system; the rich LiBr-
Water mixture is sent in the heat exchanger, where heats the cold stream coming
from the pump, and water is sent to condenser.
- ―Condenser‖, where refrigerant is at first desuperheated and after condensed,
through an external cold flow, until the condition of saturated liquid.
- ―Valve 1‖, that permit the refrigerant expansion until the low pressure level in the
system.
- ―Evaporator‖, where refrigerant changes state and evaporates until the condition of
saturated vapor; during this process, an external flow heats the refrigerant stream.
The cooled water is used as effectiveness in the system to cool for examples houses
or others ambient through district cooling systems.
- ―Absorber‖, where refrigerant is mixed with the rich solution to obtain a weak
LiBr-Water mixture; an external cold stream absorbs heat from the mixture.
- ―Pump‖, that increases the pressure level in the system; the consumption of electric
energy is negligible in comparison with the compressor of a cooling system with
vapor compression.
- ―Heat exchanger‖, where the hot flow of rich mixture heats the cold flow of Water
and LiBr weak mixture coming from the pump; in this way an internal integration
of flows is realized.
- ―Valve 2‖, that decrease the pressure level of the adsorbent coming from the
desorber until the low value in the system.
The following table shows the principal fixed data in the system for simulations:
145
COMPONENT DATA INPUT VALUE
Desorber external flow outlet temperature
t20
135 [°C]
external flow outlet pressure p20 1,01325 [bar]
inlet mixture mass fraction y56 0,548
outlet adsorbent mass fraction
y57
0,592
heat losses q330 0 [kW]
Condenser external flow inlet temperature
t62
20 [°C]
external flow outlet temperature
t63
32 [°C]
external flow inlet pressure t62 16 [bar]
pressure losses 0 [bar]
Valve 1 outlet pressure p53 0,008 [bar]
Evaporator external flow inlet temperature
t64
11 [°C]
external flow outlet temperature
t65
4 [°C]
external flow inlet pressure t64 16 [bar]
outlet quality x54 1
pressure losses 0 [bar]
heat losses q331 0 [kW]
Absorber external flow inlet temperature
t60
3 [°C]
external flow outlet temperature
t61
30 [°C]
external flow inlet pressure t60 16 [bar]
inlet adsorbent mass fraction
y58
0,592
pressure losses 0 [bar]
heat losses q332 0 [kW]
Pump outlet pressure p66 0,05 [bar]
isentropic efficiency 0,9
Heat Exchanger heat losses q335 0 [kW]
pressure losses 0 [bar]
Valve 2 outlet pressure p58 0,008 [bar]
outlet adsorbent mass fraction
y58
0,592
Table 4.2.1: Main Absorption Plant Input Data for simulations.
As we can see, the LiBr-Water weak mixture has a 54,8% of LiBr in terms of mass
fraction; the rich solution has a 59,2% of LiBr.
The high pressure level is 0,05 bar and the low one is 0,008 bar; with this value is possible
to obtain a cooling stream in the evaporator at the fixed temperature of 4°C.
146
4.3 Integration of one Absorption Cooling Unit
In this paragraph is studied the integration of one Absorption Cooling Unit with the
Municipal Solid Waste Gasification Plant integrated with SOFC and Gas Turbine
previously analyzed in chapter 3.
As seen before in paragraph 3.9.5, also with hybrid recuperation the biggest percentage of
lost energy is due to exhausted gases; temperature is in fact high, around 230°C.
It can be interesting to use this hot stream to feed the desorber in an Absorption Plant; heat
loss is reduced and a cooling flow is produced.
In the following figure is shown the integration of the plant with this cooling system:
Figure 4.3.1: Integration of one Absorption Cooling System.
In the figure Absorption Plant is drawn as a black-box; of course the complete scheme for
this system is the one shown in figure 4.2.1.
The used layout for the Municipal Solid Waste Gasification Plant integrated with SOFC
and Gas Turbine is the one with the anode pre-heater; as explained before (paragraph
3.13), this component in this case can be removed due to the moisture content in the fuel
and consequently due to the temperature levels.
147
4.3.1 Simulation results
Simulations, calculations and analysis have been carried out by DNA (Dynamic Network
Analysis), a component-based simulator for energy systems analysis developed at the DTU
Thermal Energy Systems department.
DNA contains models for most of energy system compounds (compressors, pumps, heat
exchanger, fuel cells, etc.).
Solution is provided solving a system of nonlinear equations with the Newton-Raphson
modified algorithm.
The model for the Absorption Plant is previously built using models for all components
and after added to the model of the plant; particular components are used for the part of the
plant where mixture streams are present:
- ―solu_pump‖ for pump;
- ―solu_hex‖ for heat exchanger;
- ―solu_valcc‖ for valve between heat exchanger and absorber.
Used data for Municipal Solid Waste Gasification Plant integrated with SOFC and Gas
Turbine are the ones reported in table 3.11.1; for Absorption Plant the ones contained in
table 4.2.1.
Obtained results are the following:
OUTPUT DATA VALUE
Plant Total power production 34,32 [MW]
Total power consumption 12,19 [MW]
Net power production 22,13 [MW]
Fuel consumption 42,62 [MW]
Efficiency 51,93 [%]
SOFC Power production 18,53 [MW]
Efficiency 43,5 [%]
GT Total power production 15,79 [MW]
Total power consumption 10,58 [MW]
Net power production 5,5 [MW]
Heat power input 21,57 [MW]
Efficiency 25,5 [%]
Absorption
Plant
Pump consumption 0,1 [kW]
Inlet heat 9,8 [MW]
Outlet cool stream 6,1 [MW]
COP 62%
Table 4.3.1.1: Analysis data output.
148
As we can see, output data for plant are not modified by the introduction of Absorption
Plant; electric consumption in the pump is negligible.
With this solution 9,8 MW of thermal power are used and not lost and 6,1 MW of
available cooling are produced; losses in the plant through exhausted gases are reduced,
even if in comparison with the others loss sources the percentage is the higher.
Thermal energy losses in the Absorption Plant can be identified in the condenser and in the
absorber:
- ―Condenser‖ 6455,72 kJ (40,4 %);
- ―Absorber‖ 9525,9 kJ ( 59,6 %).
The biggest one is located in the absorber; the following figure shows these losses in terms
of percentages:
Figure 4.3.1.1: Thermal energy losses in the Absorption Cooling System.
40,4%
59,6%
Thermal energy losses in the
Absorption PlantCondenser Absorber
149
4.3.2 Sensitivity Analysis
A sensitivity analysis is conduced to study the effect of the variation of some parameters
on the work of the Absorption Plant; of course the most important parameter to evaluate is
the COP.
The following diagram shows the trend of COP with variations of Environmental
Temperature:
Figure 4.3.2.1: COP as function of Environmental Temperature.
It‘s possible to see that if external temperature increases, COP decreases between 62% and
47%; in fact temperature of smoke coming from the plant increases, but temperature out
from desorber has fixed and smoke mass flow:
QDESORBER = ( (4.2).
So, if is constant and if h19 increases, of course QDESORBER increases and COP
decreases because it‘s hypothesized that Cooling power in the evaporator remains
constant.
Another parameter that can influence the COP can be the return temperature in the
evaporator.
40,00
45,00
50,00
55,00
60,00
65,00
27 30 35 40 45 50
CO
P [
%]
COP as function of Environmental Temperature
COP
150
Figure 4.3.2.2: COP as function of return temperature from users in the evaporator.
As we can see, if return temperature increases COP decreases, but in a really little range of
values between 62,4% and 62,23%; the effect of this variation is negligible.
This decrement of COP is due to a variation of Cooling Power available in the evaporator,
as we can see in the following diagram:
Figure 4.3.2.3: Cooling available power as function of return temperature from users in the evaporator.
Cooling available power doesn‘t change so much; variations are included in a range
between 6,115 MW and 6,098 MW.
62,10
62,15
62,20
62,25
62,30
62,35
62,40
62,45
11 13 15 17 19 21 23 25
CO
P [
%]
COP as function of return temperature in the evaporator
COP
6,09
6,095
6,1
6,105
6,11
6,115
6,12
11 13 15 17 19 21 23 25
QC
OO
LIN
G [M
W]
Cooling Power as function of return temperature in the evaporator
Q_cool
151
4.4 Integration of two Absorption Cooling Units
The introduction of another Absorption Plant is studied; in fact, as seen in paragraph 3.14,
an hot stream can be used between the steam generator and the desulphuriser. In this way,
as explained before, the Anode Pre-Heater can be reintroduced due to the decrement of
temperature levels. Temperature before Desulphuriser is fixed at 250 °C as in the case of
introduction of Recuperator; features of the second introduced Absorption Plant are the
same of the first one, reported in the table 4.2.1.
It‘s interesting to observe that in this case, like in the one explained in paragraph 3.14, the
presence of Anode Pre-Heater is necessary, due to a decrement of the level of temperature
after the introduction of the second Absorption Plant.
Two Absorption Cooling Units are integrated with the plant, as shown in the following
figure:
Figure 4.4.1: Integration of two Absorption Cooling Systems.
152
4.4.1 Simulation results
Simulations, calculations and analysis have been carried out by DNA (Dynamic Network
Analysis), a component-based simulator for energy systems analysis developed at the DTU
Thermal Energy Systems department.
DNA contains models for most of energy system compounds (compressors, pumps, heat
exchanger, fuel cells, etc.).
Solution is provided solving a system of nonlinear equations with the Newton-Raphson
modified algorithm.
The model for the Absorption Plants is previously built using models for all components
and after added to the model of the plant; particular components are used, as seen before,
for the part of the plant where mixture streams are present:
- ―solu_pump‖ for pump;
- ―solu_hex‖ for heat exchanger;
- ―solu_valcc‖ for valve between heat exchanger and absorber.
Used data for Municipal Solid Waste Gasification Plant integrated with SOFC and Gas
Turbine are the ones reported in table 3.11.1; for Absorption Plants the ones contained in
table 4.2.1.
Obtained results are the following:
153
OUTPUT DATA VALUE
Plant Total power production 34,04 [MW]
Total power consumption 11,68 [MW]
Net power production 22,35 [MW]
Fuel consumption 42,62 [MW]
Efficiency 52,45 [%]
SOFC Power production 18,54 [MW]
Efficiency 43,5 [%]
GT Total power production 15,51 [MW]
Total power consumption 10,58 [MW]
Net power production 5,5 [MW]
Heat power input 21,57 [MW]
Efficiency 25,5 [%]
Absorption Plant
1
Pump consumption 0,1 [kW]
Inlet heat 9,6 [MW]
Outlet cool stream 6,1 [MW]
COP 63,5%
Absorption Plant
2
Pump consumption 0,016 [kW]
Inlet heat 1,5 [MW]
Outlet cool stream 0,96 [MW]
COP 64 %
Table 4.4.1.1: Analysis data output.
As we can see, output data for plant are not much changed by the introduction of another
Absorption Plant; plant efficiency is a bit higher, 52,45 % against 51,93 % in the case
without the second Absorption System. In fact total power production is the same but total
power consumption is lower due to a decrement of used electric power in the Syngas
Blower, before the Anode Pre-Heater; temperature level is lower and less energy is
necessary to increase the pressure level.
Electric consumption in the pumps is negligible, more in the second Absorption Plant than
in the first one.
With this solution 9,6 MW of thermal power are used and not lost through exhausted gases
and 6,1 MW of available cooling are produced; losses in the plant through exhausted gases
are reduced, even if in comparison with the others loss sources the percentage is the
higher.
Others 1,5 MW of available thermal power are used in the second Absorption Plant to
produce 0,96 MW of cooling; mass flow of Syngas is less than Smoke mass flow, 4,49
kg/s against 98,23 kg/s and of course thermal power is lower.
Thermal energy losses in the two Absorption Plants can be identified in the condenser and
in the absorber:
154
- ―Condenser 1‖ 6458,23 kJ (40,9 %);
- ―Absorber 1‖ 9299,9 kJ ( 59,1 %);
- ―Condenser 2‖ 1010 kJ (40,5 %);
- ―Absorber 2‖ 1485,95 (59,5 %).
The biggest one is located in the absorbers; the total losses in the condensers and in the
absorbers for the two plants are:
- ―Condensers‖ 7468,23 kJ (40,9 %);
- ―Absorbers‖ 10785,85 kJ (59,1 %).
The following figure shows these losses in terms of percentages:
Figure 4.4.1.1: Thermal energy losses in the two Absorption Cooling Systems.
4.4.2 Sensitivity Analysis
A sensitivity analysis is conduced to study the effect of the variation of some parameters
on the work of the two Absorption Plants; of course the most important parameter to
evaluate is the COP.
The following diagram shows the trend of COP with variations of Environmental
Temperature for the two Absorption Plants:
40,9%
59,1%
Thermal energy losses in the two
Absorption Plants
Condensers Absorbers
155
Figure 4.4.2.1: COP as function of Environmental Temperature.
It‘s possible to see that if external temperature increase, COP for the two plants decreases
between 62% and 47% for the first plant and between 62% and 56% for the second one;
the second plant is subjected to variations of external temperature less than the first one
due to a lower increment of temperature inlet in the hot side of the desorber.
Another parameter that can influence the COP can be the return temperature in the
evaporator.
Figure 4.4.2.2: COP as function of return temperature from users in the evaporator.
As we can see, if return temperature increases COP decreases, but in a really little range of
values between 62,4% and 62,23% for the first plant and between 63,7% and 63,5% for
the second one; the effect of this variation is negligible.
40,00
45,00
50,00
55,00
60,00
65,00
27 30 35 40 45 50
CO
P [
%]
COP as function of Environmental Temperature
COP 1
COP 2
61,00
61,50
62,00
62,50
63,00
63,50
64,00
11 13 15 17 19 21 23 25
CO
P [
%]
COP as function of return temperature in the evaporator
COP 1
COP 2
156
This decrement of COP is due to a variation of Cooling Power available in the evaporator,
as we can see in the following diagram:
Figure 4.4.2.3: Cooling available power as function of return temperature from users in the evaporator.
Cooling available power doesn‘t change so much; variations are included in a range
between 6,115 MW and 6,098 MW for the first system and in a range between 0,956 MW
and 0,953 for the second one.
0
1
2
3
4
5
6
7
11 13 15 17 19 21 23 25
QC
OO
LIN
G [M
W]
Cooling Power as function of return temperature in the evaporator
Q_cool 1
Q_cool 2
157
5. Thermoeconomic and Investment
Analysis
5.1 Thermoeconomic principles
Thermoeconomic analysis combines exergy analysis and economic principles to provide
informations not available through conventional energy analysis and economical
evaluations, but essential to the design and operation of a cost-effective system.
The following analysis is developed according to the theory of exergetic cost (TEC)
formulated by Lozano and Valero.
The cost balance expresses that the ―cost rate‖ associated with the product of system (ĊP
[€/h]) equals the total rate of expenditures made to generate the product, namely the fuel
cost rate (ĊF [€/h]) and the cost rates associated with capital investment (ŻCI [€/h]) and
operating and maintenance (ŻOM [€/h]). For the kth component, operating at steady state, it
is formulated as:
ĊP,K = ĊF,K + ŻK = ĊF,K + ŻCI,K + ŻOM,K (5.1).
The cost rates Ċ are expressed in €/h, resulting from the product of exergy flow Ė
(expressed in J/h) and the specific exergy cost c (expressed in €/J); for the kth component:
ĊK = cK ĖK = cK ( eK) (5.2).
The rates ŻCI and ŻOM are calculated by dividing the annual contribution of capital
investment and the annual operating maintenance (O&M) costs, respectively by the
number of hours of system operation per year.
Product and Fuel for each component of the system must be defined; a system of equations
is built with a cost-balance equation for each unit, unit cost equations for external flows
into the system for which costs are externally defined, and losses for which the unit cost is
set equal to zero.
It‘s possible to solve the system when auxiliary equations, according to the two following
principles, are added:
- if definition of ―fuel‖ of a component includes a stream that goes through another
component and is used in it, then the unit cost of stream flowing into and out of the
component is the same;
- if the product of a component is composed of two or more streams then the unit
cost of those streams is equal.
158
TEC method requires to know exergy for each node of the system; exergetic analysis of
the plant is carried out by DNA, fixing a thermodynamic state for environment (Text = 25
°C and pext = 1 bar).
It‘s possible to calculate the destroyed exergy ĖD,K in the kth component through an
exergetic balance as the following:
ĖD,K = ĖF,K – ĖP,K – ĖL,K (5.3),
where ĖL,K is the lost exergy in the kth component.
Exergetic efficiency can be calculated as:
ηEX =
=
= 1 -
(5.4).
Through this kind of analysis it‘s possible to calculate also the price to sell the produced
electric energy:
Pel =
(5.5).
For each kth component of the plant the following equations have been set:
- exergy balance;
- investment cost;
- costs balance;
- auxiliary equations.
Collecting all those equations, a solvable linear system is created.
5.2 Components Equations
The studied configuration for the plant is the one explained in paragraph 3.9; a scheme is
reported below. For all components thermoeconomic and exergetic balances are developed
in the following paragraphs.
159
Figure 5.2.1: Scheme of the studied configuration of the plant for the Thermoeconomic Analysis.
5.2.1 Dryer
The aim of this component is to obtain a dry fuel to introduce in the gasifier:
Figure 5.2.1.1: Dryer scheme.
Costs and Exergy balances are formulated by the following equations:
c37 Ė37 – c40 Ė40 + ŻDRYER = c32 Ė32 - c31 Ė31 (5.6),
160
Ė37 - Ė40 + Ė31 = Ė32 + ĖD,DRYER + ĖL,DRYER (5.7),
ĖL,DRYER = 0 (5.8).
Two auxiliary equations can be added:
c31 = cWASTE = 0,0022 €/kWh (5.9),
as seen in paragraph 1.5, and
c37 = c40 (5.10),
according to the principle before explained.
Dryer purchase cost is assumed ( ref. [32] ):
IDRYER = 130000 € (5.11).
5.2.2 Gasifier
Outputs from this components are Syngas and Ashes; inlets are represented by dry waste
and streams of air and steam:
Figure 5.2.2.1: Gasifier scheme.
Costs and Exergy balances are formulated by the following equations:
c32 Ė32 + c44 Ė44 + ŻGASIFIER = c33 Ė33 + c46 Ė46 (5.12),
Ė32 + Ė44 = Ė33 + Ė46 + ĖD,GASIFIER + ĖL,GASIFIER (5.13),
ĖL,GASIFIER = Ė46 (5.14).
161
One auxiliary equation can be added; in fact it‘s possible to use ashes for the following
aims:
- cement production;
- light weight aggregate;
- road paving.
Because ashes production is 10% of the inlet fuel, also ashes cost is hypothesized to be
10% of cWASTE :
cASH = 0,00022 €/kWh (5.15).
Atmospheric gasifier purchase cost [$] is given as a function of waste mass flow input
( ref. [33] ):
IGASIFIER = 2,9 106 (3,6 )
0,7 [$] (5.16).
5.2.3 Desulphuriser
Fuel is represented by syngas coming from the gasifier; products are the cleaned syngas
and the separated Hydrogen Sulfide.
Figure 5.2.3.1: Desulphuriser scheme.
Costs and Exergy balances are formulated by the following equations:
c35 Ė35 + ŻDESULPHURISER = c36 Ė36 + c47 Ė47 (5.17),
Ė35 = Ė36 + Ė47 + ĖD,DESULPHURISER + ĖL,DESULPHURISER (5.18),
ĖL,DESULPHURISER = Ė47 (5.19).
The following auxiliary equation is set equal to zero:
c47 = cH2S = 0 €/kWh (5.20).
162
Desulphuriser cost is assumed to be ( ref. [34] ):
IDESULPHURISER = 800000 € (5.21).
5.2.4 Heat Exchangers
Heat Exchangers present in the plant, according to the scheme explained in paragraph 3.8
with the introduction of hybrid recuperation, are the following:
- Air Pre-Heater;
- Steam Generator;
- Anode Pre-Heater;
- Cathode Pre-Heater;
- Hybrid Recuperator.
The scheme of a general heat exchanger is the following:
Figure 5.2.4: Heat Exchanger general scheme.
Costs and Exergy balances are formulated by the following equations:
co,h Ėo,h – ci,h Ėi,h + ŻHEAT-EXCHANGER = co,c Ėo,c + ci,c Ėi,c (5.22),
Ėo,h – Ėi,h = Ėo,c – Ėi,c + ĖD,HEAT-EXCHANGER + ĖL,HEAT-EXCHANGER (5.23).
Lost exergy is assumed to be zero when this component is linked to another one
ĖL,HEAT-EXCHANGER = 0 (5.24),
and equal to exergy in the outlet of the hot side when gases are expelled in the atmosphere,
like in the case of Hybrid Recuperator:
ĖL,HEAT-EXCHANGER = Ėo,h (5.25).
If inlet fluid in the cold side comes from atmosphere, as for example in the Air Pre-Heater
in the Gasification Plant, the unit cost is equal to zero; so, for the Air Pre-Heater:
163
c41 = 0 (5.26).
According to the TEC method, one auxiliary equation can be added:
co,h = ci,h (5.27).
Heat Exchanger purchase cost is assumed to be a function of exchange air ( ref. [35] ):
IHEAT-EXCHANGER = 130 (
)
0,78 [$] (5.28).
Values for AEX are calculated and reported in paragraph 3.12 .
5.2.5 Blowers and Compressors
The aim of this component is to increase the pressure level of a fluid using mechanical
energy as fuel; heat losses are negligible.
In the plant are present:
- Steam Blower;
- Syngas Blower;
- Air Compressor.
Figure 5.2.5.1: Compressor scheme.
Costs and Exergy balances are formulated by the following equations:
cw Ėw + ŻCOMPRESSOR = cout Ėout – cin Ėin (5.29),
Ėw = Ėout – Ėin + ĖD,COMPRESSOR + ĖL,COMPRESSOR (5.30).
Lost exergy is assumed to be zero:
ĖL,COMPRESSOR = 0 (5.31).
164
If inlet fluid comes from atmosphere, its unit cost is hypothesized to be equal to zero, as
for example in the case of the compressor:
c1 = 0 (5.32).
Blowers and Compressor purchase cost is calculated by the following formula ( ref. [33] ):
ICOMPRESSOR = 75
rc ln rc [$] (5.33),
where is the fluid mass flow, are the mechanical and isentropic efficiency
and rc is the compression ratio.
5.2.6 Mixer
Product is represented by the mixture of two inlet streams:
Figure 5.2.6.1: Mixer scheme.
Costs and Exergy balances are formulated by the following equations:
c42 Ė42 + c43 Ė43 + ŻMIXER = c44 Ė44 (5.34),
Ė42 + Ė43 = Ė44 + ĖD,MIXER + ĖL,MIXER (5.35).
Lost exergy is assumed to be zero:
ĖL,MIXER = 0 (5.36).
No auxiliary equations are necessary. Gasification mixer purchase cost is fixed equal to
zero:
IMIXER = 0 (5.37).
165
5.2.7 Splitter
Splitter products are the two streams of steam coming out from the component; fuel is the
entering fluid.
Figure 5.2.7.1: Splitter scheme.
Costs and Exergy balances are formulated by the following equations:
c38 Ė38 + ŻSPLITTER = c39 Ė39 + c43 Ė43 (5.38),
Ė38 = Ė39 + Ė43 + ĖD,SPLITTER + ĖL,SPLITTER (5.39).
Lost exergy is assumed to be zero:
ĖL,SPLITTER = 0 (5.40).
Since two products come out of the component, one auxiliary equation is added, according
to TEC theory:
c39 = c43 (5.41).
Splitter purchase cost is fixed equal to zero:
ISPLITTER = 0 (5.42).
5.2.8 SOFC
The aim of this component is to produce electric power and heat, converting the chemical
energy of a fuel.
166
Figure 5.2.8.1: SOFC scheme.
Costs and Exergy balances can be expressed by the following equations:
c12 Ė12 - c11 Ė11 + ŻSOFC = c5 Ė5 - c4 Ė4 + c201 Ė201 (5.43),
Ė12 - Ė11 = Ė5 - Ė4 + Ė201 + ĖD,SOFC + ĖL,SOFC (5.44).
Lost exergy is assumed to be zero:
ĖL,SOFC = 0 (5.45).
The exergy difference between the outgoing used fuel and the inlet clean syngas is
considered as fuel. Products are electric power and heat absorbed by the flue gas coming
out from the SOFC. One auxiliary equations can be added:
c11 = c12 (5.46).
A price of 3000 €/kW is considered for this component, according to the current market in
this field; the initial investment can be calculated as:
ISOFC = 3000 Pel,SOFC [€] (5.47).
Since SOFC market is not yet developed, it is reasonable to assume that price could be
reduced significantly in the next future; efforts are being made to have in 2020 a price of
300 €/kW.
167
5.2.9 Burner
This component uses chemical energy from a combustion reaction to produce heat.
Products is smoke coming out at high temperatures.
Figure 5.2.9.1: Burner scheme.
Costs and Exergy balances can be expressed by the following equations:
c16 Ė16 + c13 Ė13 + ŻBURNER = c17 Ė17 (5.48),
Ė16 + Ė13 = Ė17 + ĖD,BURNER + ĖL,BURNER (5.49).
Lost exergy is assumed to be zero:
ĖL,BURNER = 0 (5.50).
No auxiliary equations are necessary.
Purchase cost for this component is assumed ( ref. [32] ):
IBURNER = 192000 € (5.51).
5.2.10 Gas Turbine and Electric Generator
Turbine produces mechanical power by means of a reduction in the working fluid
enthalpy.
Figure 5.2.10.1: Gas Turbine and Electric Generator.
168
For the Gas Turbine costs and exergy balances can be expressed by the following
equations:
c17 Ė17 - c18 Ė18 + ŻTURBINE = c102 Ė102 (5.52),
Ė17 - Ė18 = Ė102 + ĖD,TURBINE + ĖL,TURBINE (5.53).
Lost exergy is assumed to be zero:
ĖL,TURBINE = 0 (5.54).
One auxiliary equation is necessary, because of unit cost between inlet and outlet are
equal:
c17 = c18 (5.55).
Purchase cost [$] for Gas Turbine is calculated through the following expression
( ref. [35] ):
ITURBINE = ( -98,328 ln WTURBINE +1318,5 ) WTURBINE (5.56).
where WTURBINE is the produced mechanical power.
Electric generator converts in electric power the mechanical energy produced by the Gas
Turbine; no heat losses are considered. Cost and Exergy balances can be expressed in the
following way:
c102 Ė102 + ŻGENERATOR = c202 Ė202 (5.57),
Ė102 = Ė202 + ĖD,GENERATOR + ĖL,GENERATOR (5.58).
Lost exergy is assumed to be zero:
ĖL,GENERATOR = 0 (5.59).
Generator purchase cost [$] is calculated as ( ref. [36] ):
IGENERATOR = 60 Pel0,5
(5.60),
where Pel is the produced electric power.
169
5.2.11 Other Auxiliary Equations
To solve the linear system of equations, other auxiliary equations are necessary.
For this aim, unit cost of absorbed power in the auxiliary systems are set equal to the
weighted average unit cost of electric power produced by the SOFC plant and the Gas
Turbine:
cAUXILIARY =
(5.61).
5.3 Total Capital Investment
Purchase equipment cost (PEC) is only a small part of the total capital investment of a
plant; cost estimates consist of two major elements ( ref. [37] ):
- Direct costs (DC) are the costs of all permanent equipment, materials, labor, and
other resources involved in the fabrication, erection, and installation of the
permanent facilities.
- Indirect costs (IC) are required for the orderly completion of the project. No other
outlays (startup costs, working capital) are accounted.
The following table shows the principle direct and indirect costs considered for
calculations
( ref. [37] ):
TOTAL CAPITAL INVESTMENT
A.DIRECT COSTS 1. Onsite costs
1.a Purchased Equipment Costs (PEC) 1.b Purchased Equipment Installation 45%
PEC 1.c Piping 35%
PEC 1.d Instrumentation and controls 1.e Electrical Equipment and Materials
20% PEC
11% PEC
2. Offsite costs 2.a Civil, Structural, Architectural Work 30%
PEC 2.b Service Facilities 50%
PEC
170
B. INDIRECT COSTS 1. Engineering and Supervision 2. Construction costs and
Construction Profit 3. Contingency
8% DC 15% DC
15%
of 1 and 2
Table 5.3.1: Estimation of the Total Capital Investment.
Total Capital Investment for the kth component is calculated through the following
formula:
IKTOT
= IK ( 1 + 1,91 ) [ 1 + 0,23 ( 1 + 0,15 ) ] (5.62).
The following table shows the Total Capital Investment for each one component of the
plant; a conversion factor of 0,697 ( June 2011 ) is considered to convert from USA Dollar
to Euro.
COMPONENT TOTAL INVESTMENT
COST [€]
COMPONENT COST
[%]
Dryer € 130.000 0,24472
Gasifier € 8.876.710 16,71033
Desulphuriser € 800.000 1,50599
Air Pre-Heater € 21.240 0,03998
Mixer € 0 0
Splitter € 0 0
Steam Generator € 41.944 0,07896
Steam Blower € 401 0,00076
Syngas Blower € 4.242 0,00799
Anode Pre-Heater € 18.331 0,03451
SOFC € 38.683.500 72,82135
Cathode Pre-
Heater
€ 83.934 0,15801
Burner € 192.000 0,36144
Turbine € 4.088.893 7,69731
Electric generator € 5.257 0,00990
Hybrid
Recuperator
€ 77.874 0,14660
Compressor € 96.768 0,18217
Total € 53.121.095 100
Table 5.3.2: Total Investment Costs for all components in monetary and percentage terms.
Total Investment Cost is around 53 M€; the following diagram shows the capital
investment in terms of percentages:
171
Figure 5.3.1: Total Percentage Capital Investment.
The most expensive component is the SOFC; after follow Gasifier and Gas Turbine. Other
Components have a very little influence on the investment.
5.4 Capital Investment and Operating & Maintenance
Cost
For the kth component, Capital Investment IK
TOT is amortized in n years, as shown by the
following formula:
= f IK
TOT (5.63),
where f is the ―Annuity factor‖, calculated as:
f =
(5.64).
qi is the ―Interest factor‖:
qi = ( 1 +
(5.65).
Used values for n, CP, int, ri and Hr are reported in the following table ( ref. [36] ):
72,82%
16,71%
7,70%
0,46%0,20%2,11%
Total Capital Investment
SOFC Gasifier Gas Turbine
Heat Exchangers Blowers and Compressor Other Components
172
Parameter Value
Equipment Life-Span ―n‖ 15 [years]
Construction Period ―CP‖ 1 [year]
Interest Rate ―int‖ 6 [%]
Rate of Inflation ―ri‖ 2 [%]
Operating Hours ―Hr‖ 7000 [h/year]
Table 5.4.1: Economic used parameters.
With 7000 Operating Hours per Year, an 80% of Utilization Factor is obtained, calculated
as ratio between this value and the amount of hours in a year (8760 h).
Capital Investment Cost is calculated as:
=
(5.66).
Maintenance & Operating Cost is considered by a ―Maintenance Factor‖ M, assumed
to be 1,1 ( ref. [36] ); the relation with the total Cost is:
= M
(5.67).
In the following table are reported the calculated values for for each component in
the plant:
173
COMPONENT CAPITAL
INVESTMENT
COST
[€/h]
OPERATING
AND
MAINTENANCE
COST
[€/h]
TOTAL COST
[€/h]
Dryer 4,59 0,51 5,10
Gasifier 313,33 34,81 348,14
Desulphuriser 28,24 3,14 31,38
Air Pre-Heater 0,75 0,08 0,83
Mixer 0,00 0,00 0,00
Splitter 0,00 0,00 0,00
Steam Generator 1,48 0,16 1,65
Steam Blower 0,01 0,00 0,02
Syngas Blower 0,15 0,02 0,17
Anode Pre-Heater 0,65 0,07 0,72
SOFC 1365,44 151,72 1517,15
Cathode Pre-
Heater
2,96 0,33 3,29
Burner 6,78 0,75 7,53
Turbine 144,33 16,04 160,36
Electric generator 0,19 0,02 0,21
Hybrid
Recuperator
2,75 0,31 3,05
Compressor 3,42 0,38 3,80
Total 1875,05 208,34 2083,39
Table 5.4.2: Capital Investment, Operating & Maintenance and Total Cost.
5.5 Exergetic Analysis of the plant
Equations provided in Paragraph 5.2 allow to conduce the exergetic analysis of the plant;
exergy in each node of the plant is calculated by DNA (Dynamic Network Analysis),
destructions and losses are provided by EES (Engineering Equation Solver). Full results
are given in APPENDIX.
Lost and Destroyed Exergy for each one component is calculated through a system of
equation; for each component can be evaluated three parameters, to evaluate the waste
exergy:
- εD ( Destroyed Exergetic Efficiency ),
- εL ( Lost Exergetic Efficiency ),
- εTOT = εD + εL ( Total Exergetic Efficiency ),
calculated through the following formulas:
174
εD =
(5.68),
εL =
(5.69),
εTOT = εD + εL =
(5.70).
COMPONENT εD
[%]
εL
[%]
εTOT
[%]
Dryer 0,2026 0 0,2026
Gasifier 8,569 0,4389 9,008
Desulphuriser 0,001325 0,361 0,362325
Air Pre-Heater 0,727 0 0,727
Mixer 0,1395 0 0,1395
Splitter 0 0 0
Steam Generator 0,4913 0 0,4913
Steam Blower 0,007344 0 0,007344
Syngas Blower 0,2953 0 0,2953
Anode Pre-Heater 0,01458 0 0,01458
SOFC 5,529 0 5,529
Cathode Pre-Heater 1,951 0 1,951
Burner 6,881 0 6,881
Turbine 2,002 0 2,002
Electric generator 0,7002 0 0,7002
Hybrid Recuperator 6,593 13,25 19,85
Compressor 3,633 0 3,633
Table 5.5.1: Destroyed, Lost and Total Exergetic Efficiency for each component.
175
Figure 5.5.1: Waste Exergy for each component.
Principle losses can be identified in the following components:
- Gasifier, because of the chemical reactions that take place;
- Burner, due to the oxidation of fuel that requires the conversion of chemical in
thermal energy;
- SOFC, for the chemical reactions that have place;
- Cathode Pre-Heater, because difference between flue gas and air temperature profiles
is high; flue gas and SOFC air mass flow is also high, so exergy destruction affects
more on global losses;
- Compressor and Turbine, due to the high mass flow of fluid;
- Hybrid Recuperator, the principle source of loss, due to the high mass flow and
temperature of the off gases.
Total Exergetic Efficiency for the total plant can be evaluated as:
0 5 10 15 20
Dryer
Gasifier
Desulphuriser
Air Pre-Heater
Mixer
Splitter
Steam Generator
Steam Blower
Syngas Blower
Anode Pre-Heater
SOFC
Cathode Pre-Heater
Burner
Turbine
Electric generator
Hybrid Recuperator
Compressor
Waste Exergy
ε_TOT [%]
ε_L [%]
ε_D [%]
176
= 1-
(5.71),
where and are the total Destroyed and Lost Exergy for the complete plant; is
the inlet exergy through the introduced waste.
For the analyzed plant is obtained an exergetic efficiency of 48,62%.
5.6 Results of the Thermoeconomic Analysis
The aim of the Thermoeconomic Analysis is to build a system of equations through the
costs balances for each component reported in paragraph 5.2; the unknown variables are
the Unitary Costs c [€/kWh] for each node.
Once we know the values of Exergy for each node, it‘s possible to calculate for the kth
node the Thermoeconomic Cost in term of cost per hour:
Ck = ck k [€/h] (5.72).
The following table reports the obtained results for each node for Exergy k, Unitary Cost
ck and for Thermoeconomic Cost Ck; results are obtained through the solution of a system
of equation, as told before. The used software is EES (Engineering Equation Solver).
NODE k
[kW]
ck
[€/kWh]
Ck
[€/h]
1 198,8 0 0
2 9107 0,1143 1041
3 27609 0,1472 4063
4 36385 0,1471 5353
5 44868 0,133 5969
10 42212 0,01498 632,4
11 42241 0,015 633,7
12 12678 0,015 190,2
13 12642 0,015 189,7
16 35193 0,133 4682
17 44669 0,1092 4879
18 27636 0,1092 3018
19 6100 0 0
31 46022 0,002161 101,2
32 45891 0,01087 99,16
33 42718 0,01087 464,2
34 41411 0,01087 450
35 40920 0,01168 444,6
36 40754 0,03205 476
37 4328 0,0324 138,7
38 4346 0,0324 140,8
177
39 4288 0,03205 138,9
40 4553 0 145,9
41 6,45 0,01535 0
42 979,3 0,0324 15,04
43 58,2 0,01739 1,886
44 973,3 0,002161 16,92
45 0 0 0
46 202 0,00022 0,04444
47 166,2 0 0
101 10580 0,09806 1037
102 16112 0,1254 2021
104 1593 0,09806 156,3
110 21,12 0,09806 2,071
201 18536 0,07257 1345
202 15789 0,128 2021
Table 5.6.1: Values of Exergy, Unitary Cost and Thermoeconomic Cost for each node.
After it‘s possible to define and to calculate for each component a Unitary Cost for the
―Fuel‖ cF [€/kWh] and a Unitary Cost for the ―Product‖ cP [€/kWh]; the following
table refers to the values of cF and cP for each component:
COMPONENT cF
[€/h]
cP
[€/h]
Dryer 0,002211 0,002161
Gasifier 0,002477 0,01082
Desulphuriser 0,01087 0,01159
Air Pre-Heater 0,01087 0,01546
Mixer 0,01631 0,01739
Splitter 0,0324 0,0324
Steam Generator 0,01087 0,02638
Steam Blower 0,09806 0,1179
Syngas Blower 0,09806 0,1073
Anode Pre-Heater 0,015 0,04339
SOFC 0,015 0,04536
Cathode Pre-Heater 0,133 0,147
Burner 0,1018 0,1092
Turbine 0,1092 0,1254
Electric generator 0,1254 0,128
Hybrid Recuperator 0,1402 0,1633
Compressor 0,09806 0,1169
Table 5.6.2: Fuel and Product Unitary Cost for each component.
Two principal thermoeconomic parameters are calculated to evaluate and to optimized the
performance of a component:
178
- Relative Cost Difference Factor ∆rK;
- Exergoeconomic Factor fK.
These two variables are calculated for the kth component through the following
formulas:
∆rK =
and fK =
(5.73),
where and are the unit costs of product and fuel for the kth component.
The Relative Cost Difference Factor ∆rK suggests which one component it‘s possible to
enhance to have a major efficiency; but this evaluation is not enough to have a complete
analysis.
The Exergoeconomic Factor fK in fact suggests in which one way it‘s better to set in; this
parameter expresses the ratio between the lost exergoeconomic cost calculated in the
paragraph 5.4 and the total lost exergoeconomic cost, sum of the economic investment and
the inefficiency for irreversibility. A low value of this parameter (close to zero) suggest to
decrease exergetic losses at the expense of a higher total cost of investment for the
component; on the other hand, a high value (close to 1) suggest to decrease purchase
equipment cost by enhancing the exergetic losses of the component.
When a complex energy system is analyzed, optimization of one component by means of
exergoeconomic factor might not lead to a more optimized system. Modifications may
reflect negatively on other components; major attention should be paid to components that
have high exergetic losses and investment costs.
Suggested values for fK for example for Heat Exchangers, Turbines and Compressors are
below reported ( ref. [37] ):
- f < 55% for Heat Exchangers;
- 35% < f < 75% for Compressors and Turbines.
The following table refers to the obtained values for each component:
179
COMPONENT ∆rK
[%]
fK
[%]
Dryer 2,262 10,42
Gasifier 336,7 97,13
Desulphuriser 6,624 6,624
Air Pre-Heater 42,25 18,59
Mixer 6,599 0
Splitter 0 0
Steam Generator 142,8 40,18
Steam Blower 20,2 5,691
Syngas Blower 9,442 1,26
Anode Pre-Heater 189,2 87,73
SOFC 202,4 97,55
Cathode Pre-Heater 10,51 2,681
Burner 7,255 2,282
Turbine 14,83 61,44
Electric generator 2,051 0,5169
Hybrid Recuperator 16,52 0,2377
Compressor 19,2 2,265
Table 5.6.3: Exergoeconomic parameters for each component in the plant.
In the following figure is shown a comparison between all the components:
Figure 5.6.1: Exergoeconimic parameters for each component in the plant.
0 50 100 150 200 250 300 350
Dryer
Desulphuriser
Mixer
Steam Generator
Syngas Blower
SOFC
Burner
Electric generator
Compressor
Exergoeconomic Parameters
f_K [%]
∆r_K [%]
180
We can see that major values of ∆rK are in the following components:
- Gasifier and SOFC, where are present irreversibility due to chemical reactions that
take place;
- Anode Pre-Heater and Steam Generator, because of the inefficiency in the thermal
exchange due to the little difference between the inlet and outlet temperatures of
fluids.
The major values of fK are in those components where both exergy losses and total
investment cost are high; these are in particular Gasifier and SOFC.
Gasifier investment cost is dependent from biomass input; decreasing waste mass flow
may allow to obtain a lower exergoeconomic factor and a more optimized system. The
moisture content in the fuel, and consequently the LHV, could be also another source of
inefficiency; exergoeconomic factor for gasifier is around 97,13%.
In SOFC system, enhancement could be obtained decreasing SOFC temperature; for this
component exergoeconomic factor is around 97,55%. Decreasing the operating
temperature, as seen before, purchase cost will be lower and exergetic losses reduced; in
this way exergoeconomic factor should be strongly reduced.
Also the Thermoeconomic Analysis confirms that, as studied in paragraph 3.13, the Anode
Pre-Heater is a strong source of loss in the plant, due to the inefficiency in the thermal
exchange; a good solution could be to remove this component or to add a Recuperator
between the Steam Generator and the Desulphuriser, to decrease the temperature level.
5.7 Results of the Investment Analysis
In this analysis, the following parameters are evaluated:
- ―Pay-Back Time‖ ( PB ), calculated as the ratio between the initial required
investment I0 and the net annual cash flow C; through this value it‘s possible to
know after how many years it‘s possible to recover the initial investment:
PB =
(5.74);
- ―Net Positive Value‖ ( NPV ), calculated as the sum of the discounted cash flows
DCF in n years less the initial investment I0 :
NPV = (
) – I0 (5.75).
Economic parameters reported in table 5.4.1 are used; a period f 15 years is considered.
181
The net annual cash flow is calculated as difference between profits and expenses; profits
are from the sale of electricity, considered expenses are for fuel required and for
Operating&Maintenance:
CF = IEL – ( IFUEL + IO&M ) (5.76).
The three terms are calculated as:
IEL = PEL PPLANT Hr (5.77),
IFUEL = cFUEL PPLANT Hr (5.78),
IO&M = 10% I0 (5.79).
where Pel is the sale electricity price [€/kWh], PPLANT is the produced electric power [kW],
Hr is the number of operating hours [h/year], cFUEL is the unitary cost for waste [€/kWh], I0
is the initial capital investment [€].
From the previous analysis a price of electricity of 0,098 € is obtained; it‘s calculated as:
Pel =
(5.80).
The following figure shows the trend of the discounted cash flow in a period of 15 years:
Figure 5.7.1: Discounted Cash Flow trend.
-60000000
-50000000
-40000000
-30000000
-20000000
-10000000
0
10000000
20000000
30000000
40000000
50000000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15DC
F [€
]
Discounted Cash Flow
Discounted Cash Flow
182
A Pay-Back Time of around 5 years and half is obtained:
PB = 5 years and 5 months .
After 30 years it‘s possible to obtain a NPV of around:
NPV = 66.429.334 €.
5.8 Price of the produced electricity
Through the solution of the Linear system it‘s possible to obtain the unit costs of the
produced electric power by the SOFC and the Gas Turbine; as seen before, price of
electricity is calculated as:
Pel =
(5.81).
With the considered variables, is obtained:
Pel = 0,098 €/kWh .
SOFC purchase cost strongly affects the price of electricity, as expected after Investment
Analysis; at present time, SOFC purchase cost is estimated to be around 3000 €/kW.
The average price of electricity in 2010 is around 0.064 €/kWh in Italy ( ref. [38] ) and
0.051 €/kWh in Denmark ( ref. [39] ); a comparison between the price of the produced
electricity in the studied plant and the situation in the two considered countries during
2010 is shown in the following figure:
Figure 5.8.1: Price of Electricity, comparison with Italian and Danish trend during year 2010.
0
0,05
0,1
0,15
0,2
0,25
[€/k
Wh
]
Price of Electricity
PLANT
ITALY
DENMARK
183
The trend shows that the price of the produced electricity is a bit higher than average in the
two countries during 2010, but really competitive in particular if compared with the price
of electricity produced with other plants that use also renewable energy sources
( ref. [40] ):
Kind of Plant Price of Electricity
[€/kWh]
Hydroelectric 0,116 – 0,206
Wind Turbine 0,136 – 0,127
Photovoltaic 0,410 – 0,501
Biomass direct combustion (15-20 MWe) 0,234
Biogas (0,5 MWe) 0,149
MSV Gasification Plant Integrated with SOFC and GT 0,098
Table 5.8.1: Price of Electricity for different kinds of plants that use renewable sources.
The following figure shows the previous data:
Figure 5.8.2: Comparison between Price of Electricity by different renewable sources.
The price for the studied plant is really competitive and lower, due to the negligible price
of fuel; this way in fact can be interesting to help the diffusion in the energetic market of
SOFC. In any case the investment cost of SOFC influences the price of the produced
electricity.
00,05
0,10,15
0,20,25
0,30,35
0,40,45
Price of Electricity [€/kWh]
Price of Electricity [€/kWh]
184
5.9 Future Scenario
Present SOFC purchase cost is around 3000 €/kW; it is reasonable to assume that price
could be reduced significantly in the next future. Competitive plants as the one studied in
this paper could help the diffusion and the development of SOFC and consequently the
lowering of the market price.
Assuming in 2020 a SOFC price of 300 €/kW ( ref. [41] ), the hypothesized trend is shown
in the following figure:
Figure 5.9.1: Future scenario for SOFC price.
As told before, SOFC price influences the price of the produced electricity; under the
previous hypothesis, the expected trend could be the following. Price of Electricity could
achieve 0,04 €/kWh in 2020 in really the investment cost for SOFC will be 300 €/kW.
Figure 5.9.2: Future scenario for produced electricity.
0
500
1000
1500
2000
2500
3000
3500
20
11
20
12
20
13
20
14
20
15
20
16
20
17
20
18
20
19
20
20
[€/k
W]
SOFC Price: Future Scenario
SOFC Price [€/kW]
0
0,02
0,04
0,06
0,08
0,1
[€/k
Wh
]
Price of Electricity: Future Scenario
Price of Eectricity
185
6. Comparison with another system
6.1 SOFC plant fed by Natural Gas integrated with Gas
Turbine
In this section a comparison with another system is conduced; it can be in fact interesting
to see the differences with a similar plant fed by a conventional fuel: Natural Gas.
Of course in this way it‘s not present the Gasification Plant, but is necessary a Pre-
Reformer; all the others components are the same of the plant studied previously.
Simulations are conducted using two different kinds of Pre-Reformer:
- Catalytic Partial Reformer (CPO), where is not necessary steam, but air; if a CPO
is present, it could be necessary a blower for the inlet air. An exothermic reaction
takes place; this one is the following:
CnHm + n/2 O2 ↔ n CO + m/2 H2
- Adiabatic Steam Reforming (ASR), that needs superheated steam and heat because
takes place and endothermic reaction; the chemical reaction that takes place is:
CnHm + n H2O ↔ n CO + (2n+m)/2 H2
The two schemes of the plant with the two different kinds of Pre-Reformer are shown
below:
186
Figure 6.1.1: Scheme of the plant with CPO unit.
Figure 6.1.2: Scheme of the plant with ASR unit.
187
All components, except for the Pre-Reformer, are the same of the plant studied before; two
heat exchangers are present to increase the fuel temperature inlet in the SOFC system,
using the hot stream of used fuel coming from the anode side in the SOFC:
- Fuel Pre-Heater;
- Reformer Pre-Heater.
Configuration with Hybrid Recuperation is considered; in fact, as seen before, this solution
permit to increase efficiency and plant performances.
Pressure Drops in the Heat Exchangers are estimated following the same way of paragraph
3.12; for calculations is used EES to solve the formulated system of equations.
Thermodynamics variables are obtained through DNA software; same values are obtained
and used for both configurations, with CPO and ASR. Obtained results are below reported:
Th,in
[°C]
Th,out
[°C]
Tc,in
[°C]
Tc,out
[°C] h
[kg/s]
c
[kg/s]
ε
[%]
NTU UA
[W/K]
FUEL
PRE_HEATER
654,52 591,11 25 200 3,68 0,85 28,2 0,3476 335,4
REFORMER
PRE_HEATER
722,81 645,52 200 400 3,68 0,84 38,25 0,5251 500,6
CATHODE
PRE_HEATER
780 650 526,46 650 29,21 31,60 51,27 1,026 36788
ANODE
PRE_HEATER
780 722,81 550 650 3,68 1,29 43,48 0,6649 973,6
HYBRID
RECUPERATOR
721,91 412,75 170,12 526,46 32,88 31,60 64,58 1,633 58575
Table 6.1.1: Obtained values for efficiency, NTU and heat transfer coefficient.
The following table refers to the obtained geometric variables; it‘s in fact important, as
explained before, to estimate the exchange air, to have an idea about the initial capital
costs and about size of plant:
∆ph
[bar]
∆pc
[bar]
Aflow
[m2]
Afr
[m2]
A
[m2]
L
[m]
Dh
[m]
D0
[m]
D
[m]
n
FUEL
PRE_HEATER
0,01 0,01 0,9651 1,022 9,173 0,5843 0,2049 0,012 0,01 500
REFORMER
PRE_HEATER
0,01 0,01 0,8378 0,9056 10,72 0,5691 0,1482 0,012 0,01 600
CATHODE
PRE_HEATER
0,05 0,05 5,475 6,063 169,2 1,036 0,1118 0,012 0,01 5200
ANODE
PRE_HEATER
0,01 0,01 2,644 2,734 24,08 0,9585 0,3509 0,012 0,01 800
HYBRID
RECUPERATOR
0,05 0,05 1,673 2,261 153,2 0,9381 0,0341 0,012 0,01 5200
Table 6.1.2: Obtained values for geometric size of all heat exchangers.
188
The plant is fed by Natural Gas with the following composition:
- Methane ( CH4 ) = 87 %
- Hydrogen Sulfide ( H2S ) = 0,375 %
- Ethane ( C2H6 ) = 8,1 %
- Carbon Dioxide ( CO2 ) = 2,925 %
- Propane ( C3H8 ) = 1 %
- Butane ( C4H10 ) = 0,6 %
Figure 6.1.3: Used Natural Gas composition.
A little percentage of H2S is present; to not poison the SOFC it‘s in fact necessary in the
plant a Desulphuriser.
In the following table are reported all input data used for simulations; comparisons with
the Municipal Solid Waste Gasification Plant Integrated with SOFC and Gas Turbine are
conduced to have the same output in terms of produced electric power.
COMPONENT DATA INPUT VALUE
Fuel pre-heater inlet fuel mass flow m6 0,85 [kg/s]
inlet temperature t6 25 [°C]
outlet temperature t7 200 [°C]
heat losses q304 0 [kW]
pressure drops used fuel side 0,01 [bar]
pressure drops fuel side 0,01 [bar]
Desulphuriser heat losses q305 0 [kW]
pressure drops 0,01 [bar]
Reformer pre-heater outlet temperature t9 400 [°C]
heat losses q306 0 [kW]
pressure drops used fuel side 0,01 [bar]
87%
0,375%8,1%
2,925% 1% 0,6%
Natural Gas Composition
CH4 H2S C2H6 CO2 C3H8 C4H10
189
pressure drops fuel side 0,01 [bar]
Compressor 1 inlet pressure p1 1,01325 [bar]
inlet temperature t1 25 [°C]
CPO / ASR outlet temperature t10 550 [°C] / 450 [°C]
heat losses q307 0 [kW]
pressure drops 0 [bar]
SOFC stack heat losses q309 0 [kW]
inlet temperature cathode side t4 650 [°C]
inlet temperature anode side t11 650 [°C]
utilization factor 0,8
operating temperature 780
pressure drops anode side 0,01 [bar]
pressure drops cathode side 0,05 [bar]
cells per stack 75
number of stacks 50500
Anode pre-heater heat losses q308 0 [kW]
pressure drops used fuel side 0,01 [bar]
pressure drops fuel side 0,01 [bar]
Cathode pre-heater heat losses q303 0
pressure drops flue gas side 0,05 [bar]
pressure drops air side 0,05 [bar]
Air compressor inlet pressure p1 1,01325 [bar]
inlet temperature t1 25 [°C]
outlet pressure p2 2,5 [bar]
isentropic efficiency 0,8
mechanical efficiency 0,98
Burner heat losses q310 0 [kW]
pressure drops 0,95
Turbine isentropic efficiency 0,86
Electric Generator electric efficiency 0,98
Hybrid Recuperator pressure drops smoke side 0,05 [bar]
pressure drops air side 0,05 [bar]
heat losses q302 0 [kW]
outlet pressure p19 1,01325 [bar]
Table 6.1.3: Input Data for simulations.
All used data are equal to the others used for the MSW Gasification plant except for the
fuel mass flow; a lower amount is in fact enough to obtain as output the same quantity of
electric power. Natural Gas has in fact an higher Low Heat Value: 51010 kJ/kg against
15920 kJ/kg for the obtained Syngas from the Gasification process.
Pressure Drops and geometric features for Heat Exchangers are evaluated through the
same process illustrated in paragraph 3.12.
190
6.2 Thermodynamic comparison
Simulations are carried out by DNA software; code and complete results are attached in
APPENDIX.
The following table reports the principal obtained results from simulations; the two
configurations are separately considered. In a column are reported all results from
simulations with CPO unit; in another one all results from simulations with ASR unit.
CPO ASR
Plant Total power production 29,56 [MW] 28,37 [MW]
Total power
consumption
4,96 [MW] 4,27 [MW]
Net power production 24,6 [MW] 24,09 [MW]
Fuel consumption 38,94 [MW] 36,65 [MW]
Efficiency 63,17 [%] 65,74 [%]
SOFC Power production 23,34 [MW] 22,96 [MW]
Efficiency 59,9 [%] 62,65 [%]
GT Total power production 6,22 [MW] 5,4 [MW]
Total power
consumption
4,9 [MW] 4,27 [MW]
Net power production 1,32 [MW] 1,13 [MW]
Heat power input 8,18 [MW] 9,18 [MW]
Efficiency 16,14 [%] 15,3 [%]
Table 6.2.1: Output Data from simulations.
A major efficiency is obtained using an ASR unit instead of a CPO Reformer; a less
quantity of energy is in fact necessary for the chemical process due to the introduction of
splitted used fuel coming from SOFC.
Obtained results for Municipal Solid Waste Gasification Plant Integrated with SOFC and
Gas Turbine are reported in the table 3.11.2; the following diagram shows a comparison
between the different obtained values for the Plant Efficiency:
191
Figure 6.2.1: Comparison between the obtained values for the Thermodynamic Efficiency.
Efficiency is higher in the plant fed by Natural Gas, due to a lower auxiliary consumption
and an higher fuel LHV; if it‘s used an ASR instead of a CPO unit, this parameter
increases a bit because there‘s not electric consumption to compress air in the Pre-
Reformer and it‘s used an internal flow.
However in the plant with Gasification of MSW a ―low cost‖ fuel it‘s used, that should
throw out in landfills, and despite the low LHV, a competitive value of Efficiency is
obtained.
The mean composition for smoke coming from the plant is the following:
COMPONENTS CPO ASR
O2 [%] 11,57 10,72
N2 [%] 74,07 73,78
CO2 [%] 4,428 4,829
H2O [%] 9,066 9,8
Ar [%] 0,866 0,871
Table 6.2.2: Smoke composition.
As it‘s possible to see, in both cases there‘s no presence of NOx and CO in the expelled
smoke; maximum temperature in fact achieves a non high level, 878,5 °C in the plant with
CPO and 885,9 °C in the configuration with ASR.
More or less in both two cases composition is similar; the only one major difference is that
if is used an ASR unit, less oxygen and less nitrogen are present in the smoke, because is
not introduced air for the Pre-Reforming process.
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
51,93%63,17% 65,74%
Thermodynamic Efficiency Comparison
192
The following diagram shows a comparison with the plant fed by MSW; as it‘s possible to
see, the most evident difference is in the content of Oxygen, Carbon Dioxide and Steam.
The first one is in fact major in the smoke coming from the MSW Gasification Plant and
the other two pollutions in the other plant.
Figure 6.2.2: Comparison between the different smoke compositions in terms of mass fraction.
6.3 Thermoeconomic and Exergetic comparison
To have a comparison with the studied plant, a Thermoeconomic Analysis is conduced;
used equations for components are the same as in paragraph 5.2.
The only one component not considered before is the Pre-Reformer; costs and Exergy
balances are formulated by the following equations:
c9 Ė9 + c21 Ė21 + ŻCPO = c10 Ė10 (6.1),
Ė9 + Ė21 = Ė10 + ĖD,CPO + ĖL,CPO (6.2),
ĖL,CPO = 0 (6.3),
for CPO unit, and:
c9 Ė9 + ŻASR = c10 Ė10 (6.4),
Ė9 = Ė10 + ĖD,ASR + ĖL,ASR (6.5),
SOFC Plant fed by Natural Gas Integrated with GT (CPO)
SOFC Plant fed by Natural Gas Integrated with GT (ASR)
MSW Gasification Plant Integrated with SOFC and GT
11,57
10,72
17,53
74,04
73,78
75,76
4,428
4,829
2,565
9,066
9,8
3,261
0,866
0,871
0,8794
Smoke composition comparison[%]
O2 N2 CO2 H2O Ar
193
ĖL,ASR = 0 (6.6),
for ASR unit.
Pre-Reformer cost, in both cases, is assumed to be ( ref. [35] ):
IPRE-REFORMER = 45124 $ (6.7).
About the Capital Investment and other costs, as for example Operation & Maintenance,
the equations of paragraphs 5.3 and 5.4 are used; the following table reports all costs for
each component:
COMPONEN
T
CPO ASR
TOTAL
INVESTMENT
COST
[€]
COMPONEN
T COST
[%]
TOTAL
INVESTMEN
T COST
[€]
COMPONEN
T COST
[%]
Fuel Pre-Heater € 3.254,76 0,01 € 3.254,76 0,01
Desulphuriser € 800.000,00 1,54 € 800.000,00 1,54
Reformer Pre-
Heater € 3.675,46 0,01 € 3.675,46 0,01
Mixer ---------------------
-
------------------- € 0,00 0,00
Splitter ---------------------
-
------------------- € 0,00 0,00
Recycler -----------------------
-
--------------------- 0 0,00
Compressor 1 € 133,52 0,00
---------------------
- ---------------------
Pre-Reformer € 31.451,43 0,06 € 31.451,43 0,06
Anode Pre-
Heater € 6.909,58 0,01 € 6.909,58 0,01
SOFC € 48.812.304,00 93,96 € 48.812.304,00 93,96
Cathode Pre-
Heater € 31.616,35 0,06 € 31.616,35 0,06
Burner € 192.000,00 0,37 € 192.000,00 0,37
Turbine € 2.025.208,22 3,90 € 2.025.208,22 3,90
Electric
generator € 3.299,28 0,01 € 3.299,28 0,01
Hybrid
Recuperator € 29.259,12 0,06 € 29.259,12 0,06
Compressor 2 € 11.466,75 0,02 € 11.466,75 0,02
Total € 51.950.578,47 100 € 51.950.441,44 100
Table 6.3.1: Total Investment Costs for all components in monetary and percentage terms.
194
Total investment cost is more or less the same in both cases; it‘s a bit lower in the plant
with ASR, because there‘s not the Compressor to introduce air the Pre-Reformer. As we
can see also from the following diagram, the biggest percentage is, in either case, due to
the purchase cost for SOFC; in terms of percentage, in both cases each component has the
same weight:
Figure 6.3.1: Total Percentage Capital Investment .
The following diagram shows a comparison between the three different plants in terms of
total investment; in the plant where is present the Gasification section, initial investment is
a bit higher, but the difference is not so evident, because the most expensive component is
in all cases the SOFC.
93,96%
0,15%0,02%
3,90%1,97%
Total Capital Investment
SOFC Heat Exchangers Compressors Gas Turbine Other Components
195
Figure 6.3.2: Comparison in terms of Total Capital Investment.
Used economic parameters to calculate the Capital Investment and the Operating &
Maintenance costs are the same as the table 5.4.1; calculated values for ,
and
are reported in the following table. With both CPO and ASR plant are obtained the
same results:
COMPONENT
CAPITAL
INVESTMENT
COST
[€/h]
OPERATING AND
MAINTENANCE
COST
[€/h]
TOTAL
COST
[€/h]
Fuel Pre-Heater 0,11 0,01 0,13
Desulphuriser 28,24 3,14 31,38
Reformer Pre-Heater 0,13 0,01 0,14
Mixer 0,00 0,00 0,00
Splitter 0,00 0,00 0,00
Recycler 0,00 0,00 0,00
Compressor 1 0,00 0,00 0,01
Pre-Reformer 1,11 0,12 1,23
Anode Pre-Heater 0,24 0,03 0,27
SOFC 1722,96 191,44 1914,40
Cathode Pre-Heater 1,12 0,12 1,24
Burner 6,78 0,75 7,53
Turbine 71,49 7,94 79,43
Electric generator 0,12 0,01 0,13
Hybrid Recuperator 1,03 0,11 1,15
Compressor 2 0,40 0,04 0,45
Total 1833,73 203,75 2037,48
Table 6.3.2: Capital Investment, Operating&Maintenance and Total Cost.
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
53.121.095 51.950.578 51.950.441
Total Capital Investment comparison
196
Through the Exergetic Analysis, the same parameters of before are evaluated:
- εD ( Destroyed Exergetic Efficiency ),
- εL ( Lost Exergetic Efficiency ),
- εTOT = εD + εL ( Total Exergetic Efficiency ).
EES is the used software for calculations; values for exergy for each node and
consequently for each component are obtained through DNA. The following table refers to
the obtained results:
CPO ASR
COMPONENT
εD
[%]
εL
[%]
εTOT
[%]
εD
[%]
εL
[%]
εTOT
[%]
Fuel Pre-Heater 0,3832 0 0,3832 0,3601 0
Desulphuriser 0,6834 0,3445 1,028 0,6834 0,3445 1,028
Reformer Pre-Heater 0,2637 0 0,2637 1,621 0 1,621
Mixer --------------- --------------- ------------- 0,1678 0 0,1678
Splitter --------------- --------------- ------------- 0 0 0
Recycler ---------------- ---------------- -------------- 0,000812 0 0,000812
Compressor 1 0,05007 0 0,05007 ------------ --------- ---------
Pre-Reformer 1,305 0 1,305 0,2051 0 0,2051
Anode Pre-Heater 0,05278 0 0,05278 0,1148 0 0,1148
SOFC 5,825 0 5,825 5,576 0 5,576
Cathode Pre-Heater 0,7555 0 0,7555 0,691 0 0,691
Burner 6,973 0 6,973 7,116 0 7,116
Turbine 0,5259 0 0,5259 0,4819 0 0,4819
Electric generator 0,3131 0 0,3131 0,289 0 0,289
Hybrid Recuperator 4,423 14,48 18,91 4,255 13,66 17,92
Compressor 2 3,916 0 3,916 3,625 0 3,625
Table 6.3.3: Destroyed, Lost and Total Exergetic Efficiency for each component.
As we can see, the highest losses are in the SOFC, in the Burner and in the Hybrid
Recuperator, more in the plant with CPO than in the other one with ASR unit; also in the
Pre-Reformer, if we use an ASR losses are lower.
Total Exergetic Efficiency for the total plant can be evaluated as:
= 1-
(6.8),
where and are the total Destroyed and Lost Exergy for the complete plant; is
the inlet exergy through the introduced waste.
197
In the two cases are obtained the following values:
- Plant with CPO Unit: 59,7 % ;
- Plant with ASR Unit: 64,05 % .
As we can see, the plant with ASR unit has a better exergetic efficiency, due to a better
work of the Pre-Reformer that uses an internal splitted flow instead of an external flow.
The following diagram shows the comparison between the three different values for the
Exergetic Efficiency:
Figure 6.3.3: Comparison between the three obtained values for Exergetic Efficiency.
A lower value is obtained in the plant with the Gasification section; more irreversibility are
in fact present in this case as seen before.
The following table reports the obtained results for each node for Exergy k, Unitary Cost
ck and for Thermoeconomic Cost Ck; results are obtained through the solution of a system
of equation, as told before. The used software is EES (Engineering Equation Solver).
CPO ASR
NODE k
[kW]
ck
[€/kWh]
Ck
[€/h] k
[kW]
ck
[€/kWh]
Ck
[€/h]
1 100,7 0 0 87,74 0 0
2 3417 0,1806 617 2977 0,154 458,5
3 9446 0,3248 3068 8254 0,2974 2455
4 12241 0,325 3978 10665 0,2986 3184
5 14315 0,2932 4197 12354 0,2723 3365
6 40562 0,026 1055 38176 0,026 992,6
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
48,62%59,70% 64,05%
Exergetic Efficiency comparison
198
7 40637 0,02611 1061 38247 0,0261 998,3
8 40500 0,02698 1092 38117 0,02701 1030
9 40737 0,02706 1102 39050 0,02659 1038
10 40250 0,0276 1111 38972 0,02667 1039
11 40495 0,02763 1119 39391 0,02671 1052
12 12718 0,02763 351,4 12611 0,02671 336,8
13 12451 0,02763 344 11376 0,02671 303,8
14 12107 0,02763 334,5 11062 0,02671 295,4
15 11876 0,02763 328,1 10854 0,02671 289,9
16 11213 0,2932 3288 9679 0,2723 2636
17 20261 0,1788 3623 17816 0,1646 2933
18 13697 0,1788 2449 12116 0,1646 1995
19 5874 0 0 5215 0 0
20 1,41 0 0 38341 0,02653 1017
21 42,23 0,1822 7,694 773,6 0,02692 20,83
22 139,8 0 0 131,5 0 0
23 0 0 0 0 0 0
24 ----------------- ----------------- ----------------- 0 0 0
25 ----------------- ----------------- ----------------- 11839 0,02671 0
26 ----------------- ----------------- ----------------- 771,9 0,02671 0
101 4905 0,1257 616,6 4273 0,1072 458,1
102 6351 0,1973 1253 5516 0,1845 1018
103 61,13 0,1257 7,684 1,97 0,1072 0,2112
201 23341 0,1055 2463 22962 0,1067 2449
202 6224 0,2014 1253 5406 0,1094 591,4
Table 6.3.4: Values of Exergy, Unitary Cost and Thermoeconomic Cost for each node
in the two considered configurations.
As made before it‘s possible to define and to calculate for each component a Unitary
Cost for the ―Fuel‖ cF [€/kWh] and a Unitary Cost for the ―Product‖ cP [€/kWh]:
CPO ASR
COMPONENT
cF
[€/h]
cP
[€/h]
cF
[€/h]
cP
[€/h]
Fuel Pre-Heater 0,02763 0,08626 0,02671 0,08024
Desulphuriser 0,02611 0,02688 0,0261 0,02692
Reformer Pre-Heater 0,02763 0,0276 0,02671 0,009139
Mixer --------------- --------------- 0,02654 0,02659
Splitter --------------- --------------- 0,02671 0,02671
Recycler ---------------- ---------------- 0,1072 0,1272
Compressor 1 0,1257 0,1885 -------------- ------------
Pre-Reformer 0,02722 0,0276 0,02659 0,02667
Anode Pre-Heater 0,02763 0,03113 0,02671 0,03014
SOFC 0,02763 0,02763 0,02671 0,09757
Cathode Pre-Heater 0,2932 0,3258 0,2723 0,3026
Burner 0,1566 0,1788 0,1425 0,1646
199
Turbine 0,1788 0,1973 0,1646 0,1845
Electric generator 0,1973 0,2014 0,1845 0,1094
Hybrid Recuperator 0,3131 0,4065 0,2891 0,3783
Compressor 2 0,1257 0,186 0,1072 0,1587
Table 6.3.5: Fuel and Product Unitary Cost for each component for both configurations.
The two principal thermoeconomic parameters are calculated also in this case to evaluate
and to optimized the performance of a component:
- Relative Cost Difference Factor ∆rK;
- Exergoeconomic Factor fK.
The following table refers to the obtained values for each component in both cases:
CPO ASR
COMPONENT
∆rK
[%]
fK
[%]
∆rK
[%]
fK
[%]
Fuel Pre-Heater 212,2 2,939 200,4 3,419
Desulphuriser 2,951 74,24 3,138 75,39
Reformer Pre-Heater 47,21 4,524 65,78 0,8542
Mixer --------------- --------------- 0,164 0
Splitter --------------- --------------- 10-14
0
Recycler ---------------- ---------------- 18,67 0
Compressor 1 49,95 0,3902 -------------- ------------
Pre-Reformer 1,428 7,864 0,3196 37,14
Anode Pre-Heater 12,7 31,34 12,86 18,74
SOFC 241,4 96,7 265,3 97,12
Cathode Pre-Heater 11,11 1,361 11,13 1,697
Burner 14,2 1,672 15,55 1,908
Turbine 10,35 67,55 12,08 72,4
Electric generator 2,051 0,516 40,71 0,6345
Hybrid Recuperator 29,82 0,04787 30,86 0,05813
Compressor 2 48 0,2249 48,04 0,3025
Table 6.3.6: Exergoeconomic parameters for each component in the two studied configurations.
Higher values of ∆rK are in the following components:
- SOFC, where are present irreversibility due to chemical reactions that take place;
- Fuel Pre-Heater and Reformer Pre-Heater, because of the inefficiency in the
thermal exchange due to the little difference between the inlet and outlet
temperatures of fluids.
200
The major values of fK are in those components where both exergy losses and total
investment cost are high; these are in particular SOFC, Desulphuriser and Pre-Reformer.
Also for the Gas Turbine the value for this parameter is high, due to initial capital
investment.
In the three considered plants higher values of Exergoeconomic Parameters are obtained in
particular in the SOFC, where Investment cost is really high.
Figure 6.1.3.2: Exergoeconimic parameters for each component in the plant.
0 50 100 150 200 250 300
Fuel Pre-Heater
Desulphuriser
Reformer Pre-Heater
Mixer
Splitter
Recycler
Compressor 1
Pre-Reformer
Anode Pre-Heater
SOFC
Cathode Pre-Heater
Burner
Turbine
Electric generator
Hybrid Recuperator
Compressor 2
fK_ASR
fK_CPO
∆rK_ASR
∆rK_CPO
201
6.4 Investment Analysis comparison
The same parameters that in paragraph 5.7 are evaluated:
- ―Pay-Back Time‖ ( PB );
- ―Net Positive Value‖ ( NPV ).
Economic parameters reported in table 5.4.1 are used; a period f 15 years is considered.
The net annual cash flow is calculated as difference between profits and expenses; profits
are from the sale of electricity, considered expenses are for fuel required and for Operating
& Maintenance:
CF = IEL – ( IFUEL + IO&M ) (6.9).
The three terms are calculated as:
IEL = PEL PPLANT Hr (6.10),
IFUEL = cFUEL PPLANT Hr (6.11),
IO&M = 10% I0 (6.12),
where Pel is the sale electricity price [€/kWh], PPLANT is the produced electric power [kW],
Hr is the number of operating hours [h/year], cFUEL is the unitary cost for Natural Gas
[€/kWh], I0 is the initial capital investment [€].
For cFUEL for Natural Gas, a comparison between the Italian and the Danish market is
considered; the used values are the following, from the European Commission Statistics (
ref. [42] ):
cNG,IT = 0,028 [€/kWh] (6.13),
cNG,DK = 0,024 [€/kWh] (6.14).
A mean value is used for simulations:
cNG = 0,026 [€/kWh] (6.15).
Through the solution of the Linear system it‘s possible to obtain the unit costs of the
produced electric power by the SOFC and the Gas Turbine; as seen before, price of
electricity is calculated as:
202
Pel =
(6.16).
For the price of produced electricity, the following mean values are obtained in the two
cases:
CPO ASR
PEL [€/kWh] 0,126 0,108
Table 6.4.1: Obtained values for produced electricity.
A comparison with the obtained price for electricity in the plant with MSW Gasification is
reported in the following diagram; a lower price it‘s obtained if is used waste as fuel.
Figure 6.4.1: Comparison in terms of price of electricity.
The following figure shows the trend of the discounted cash flow in the three plants in a
period of 15 years; higher entrances are obtained in the SOFC plant fed by Natural Gas
Integrated with GT, with a CPO unit as Pre-Reformer, due to the higher price of sale for
the electricity.
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
0,09
0,1260,108
Price of electricity comparison
203
Figure 6.4.2: Discounted Cash Flow trend for the three plants.
The following values are obtained in the two considered cases for PB and NPV:
CPO ASR
PB [years] 4 and 4 moths 5 and 8 months
NPV [€] € 247.074.537,00 € 40.935.544,98
Table 6.4.2: Obtained values for Pay-Back Time and Net Positive Value in the two studied configurations.
In the case with ASR unit both parameters are lower, due to a minor price of sale for
electricity and consequently less entrances; NPV and PB are lower in the plant with MWS
Gasification, as shown in the following diagrams, due to a lower price of sale for
electricity, but really competitive, due to a really negligible expense to buy fuel.
-€ 60.000.000,00
-€ 40.000.000,00
-€ 20.000.000,00
€ 0,00
€ 20.000.000,00
€ 40.000.000,00
€ 60.000.000,00
€ 80.000.000,00
1 3 5 7 9 11 13 15
DC
F [€
]
Discounted Cash Flow comparison
SOFC Plant fed by Natural Gas Integrated with GT (CPO)
SOFC Plant fed by Natural Gas Integrated with GT (ASR)
MSW Gasification Plant Integrated with SOFC and GT
204
Figure 6.4.3: Pay-Back Time comparison.
Figure 6.4.4: Net Positive Value comparison.
SOFC purchase cost strongly affects the price of electricity, as expected after Investment
Analysis; at present time, SOFC purchase cost is estimated to be around 3000 €/kW.
The average price of electricity in 2010 is around 0.064 €/kWh in Italy ( ref. [38] ) and
0.051 €/kWh in Denmark ( ref. [39] ); a comparison between the price of the produced
electricity in the studied plant and the situation in the two considered countries during
2010 is shown in the following figure:
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
5,54,4
5,8
Pay-Back Time comparison
Pay-Back Time comparison
MSW Gasification Plant Integrated with SOFC
and GT
SOFC Plant fed by Natural Gas Integrated
with GT (CPO)
SOFC Plant fed by Natural Gas Integrated
with GT (ASR)
66.429.334
247.074.537
40.935.544
Net Positive Value comparison
Net Positive Value comparison
205
Figure 6.4.5: Price of Electricity, comparison with Italian and Danish trend during year 2010 and the previously studied
plants.
Sale price of electricity in the plant with CPO is a bit higher; but it‘s really interesting to
see that in both cases, with ASR and with CPO, the obtained price is higher than the MSV
Gasification Plant Integrated with SOFC and Gas Turbine.
However the price is really competitive if compared with the price of electricity produced
with other plants that use also renewable energy sources ( ref. [40] ):
Kind of Plant Price of Electricity
[€/kWh]
Hydroelectric 0,116 – 0,206
Wind Turbine 0,136 – 0,127
Photovoltaic 0,410 – 0,501
Biomass direct combustion (15-20 MWe) 0,234
Biogas (0,5 MWe) 0,149
MSV Gasification Plant Integrated with SOFC and GT 0,098
CPO 0,126
ASR 0,108
Table 6.4.3: Price of Electricity for different kinds of plants that use renewable sources.
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
[€/k
Wh
]
Price of Electricity: comparison with Italian and Danish market
MSW Gasification Plant Integrated with SOFC and GT
SOFC Plant fed by Natural Gas Integrated with GT (CPO)
SOFC Plant fed by Natural Gas Integrated with GT (ASR)
ITALY
DENMARK
206
Figure 6.4.6: Comparison between Price of Electricity by different renewable sources.
As told before, SOFC price influences the price of the produced electricity; under the
previous hypothesis, the expected trend could be the following:
Figure 6.4.7: : Future scenario for produced electricity.
Price of Electricity could achieve 0,055 €/kWh in the plant with CPO unit and 0,038
€/kWh in the one with ASR in 2020, if really the investment cost for SOFC will be 300
€/kW; increment will be higher in the plant with ASR and in the other one with MSW
Gasification than in the one with CPO unit.
00,05
0,10,15
0,20,25
0,30,35
0,40,45
Price of Electricity [€/kWh]
Price of Electricity [€/kWh]
0,000
0,020
0,040
0,060
0,080
0,100
0,120
0,140
2011201220132014201520162017201820192020
[€/k
Wh
]
Price of Electricity as function of SOFC price
SOFC Plant fed by Natural Gas Integrated with GT (CPO)
SOFC Plant fed by Natural Gas Integrated with GT (ASR)
MSW Gasification Plant Integrated with SOFC and GT
207
7. Conclusions Gasification technology allows to use biomass energy with high efficiency; in particular
the Gasification of Municipal Solid Waste has the vantage to obtain energy by a ―low-
cost‖ fuel that should throw out in landfills or used for other aims, as for example
incinerators, with problems of pollutions.
Integration with a SOFC plant and a Gas Turbine permits to achieve good efficiency,
51,93 % for the optimized configuration; no dangerous pollutions are present in the off-
gases.
The best performance is obtained with a Regenerative Gas Turbine; with this solution
efficiency increases of 11 % due to the recovery of the energy content in the exhausted off-
gases.
Energetic content of smoke is in fact high and permit also, with good values for the
efficiency, the introduction of an Absorption Cooling System.
An important parameter to consider is the fuel moisture content; modified configurations
in fact can permit to enhance the system not only under a thermodynamic point of view,
but also under an exergetic and an economic point of view.
According to this value, integrations of internal flows are possible; another Absorption
Cooling Unit can also integrated.
Thermoeconomic Analysis provides a price of produced electric power of 0.098 €/kWh;
this result is really competitive in the actual market with obtained values from other
energetic systems that use renewable energy sources. A decrease of SOFC investment cost
permits to achieve lower values for the price of electricity; in 2020 expected plant
electricity price is reduced down to 0.04 €/kWh.
Those low values are obtained due to the really cheap costs for the fuel; this is a very
interesting vantage for this plant, that could help SOFC technology to spread with very
competitive costs.
Investment Analysis suggests an interesting value for the Pay-Back time, around 5 years,
competitive with a traditional Combine Cycle.
With the substitution of the Gasification section with a Pre-Reformer it‘s possible to use
also a conventional fuel as Natural Gas; higher efficiencies are achieved due to a better
fuel LHV.
208
But an higher price of produced electric power is also obtained, due to costs to buy the
fuel; the advantage to integrate the system with a MSW Gasifier is in fact to obtain a
synthesis gas to feed an energy system from a fuel with negligible costs.
MSW Gasification Plant Integrated with SOFC and GT is more competitive with an SOFC
Plant fed by Natural Gas Integrated with GT that uses an ASR unit as Pre-Reformer;
thermodynamic are better but economic variables are really comparable.
This comparison suggests that it could be possible also an integration of two similar
systems to use both fuels: MSW and Natural Gas; the latter in fact could be used for
starting and when MSW features are not so good as integrating fuel.
In this case both Gasification and Pre-Reforming sections should be present, with systems
of by-pass for the heat exchangers depending on the used fuel.
Actually the most suitable plant size is around 25 MWe, according to the actual SOFC
market and consequently to obtain a good plant efficiency with a fixed input fuel mass
flow; future scenario for SOFC price suggests that it will be possible to increase the plant
size with good values of Capital Investment and price of produced electricity.
However with this size it‘s yet possible to remove quantities of MSW comparable with the
mean production in some small cities; it can be concluded that in the immediate future the
analyzed system is going to be feasible both by an energetic and thermoeconomic point of
view.
209
210
211
References
[1] Giovanni Lozza, ―Turbine a gas e cicli combinati‖, Progetto Leonardo, second
edition.
[2] H.I. Onovwiona, V.I. Ugursal, ―Residential cogeneration systems: review of the
current
technology‖ .
[3] Pilavachi PA, ―Mini- and micro-gas turbines for combined heat and power‖.
[4] Energy Nexus Group, ―Technology characterization—micro-turbines‖. USA:
Environmental Protection Agency; 2002.
[5] James Larminie, Andrew Dicks ―Fuel Cell System Explained‖, John Wiley & Sons.
[6] Meng Ni, Dennis Y.C. Leung, Michael K.H. Leung, ―Modeling of methane fed
solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen
ion conducting electrolyte‖.
[7] E. Durán, J.M. Andújar, F. Segura, A.J. Barragán ―A high-flexibility DC load for
fuel cell and solar arrays power sources based on DC–DC converters‖.
[8] Masoud Rokni, ―Fuel cell in power plants‖, Department of Mechanical
Engineering, Damarks Tekniske Universitet.
[9] Jens Dall Bentzen, Reto M. Hummelshøj, Ulrik Henriksen, Benny Gøbel, Jesper
Ahrenfelt, Brian Elmegaard ―Upscale of the two-stage gasification process‖.
[10] M. Morris, L. Waldheim ―Energy recovery from solid waste fuels using advanced
gasification technology‖ .
[11] Yanwen Guan, Siyi Luo, Shiming Liu, Bo Xiao, Lei Cai ―Steam catalytic
gasification of municipal solid waste for producing tar-free fuel gas‖ .
[12] Ladislav Be´bar, Petr Stehlı´k, Leosˇ Havlen, Jaroslav Oral ―Analysis of using
gasification and incineration for thermal processing of wastes‖ .
[13] Ajay K. Dalai, Nishant Batta, I. Eswaramoorthi, Greg J. Schoenau ―Gasification
of refuse derived fuel in a fixed bed reactor for syngas production‖ .
[14] S. Galvagno, G. Casciaro, S. Casu, M. Martino, C. Mingazzini, A. Russo, S.
Portofino ―Steam gasification of tyre waste, poplar, and refuse-derived fuel: A
comparative analysis‖ .
212
[15] Gerardo Gordillo, Kalyan Annamalai ―Adiabatic fixed bed gasification of dairy
biomass with air and steam‖ .
[16] P. Baggio, M. Baratieri, L. Fiori, M. Grigiante, D. Avi, P. Tosi ―Experimental and
modeling analysis of a batch gasification/pyrolysis reactor‖ .
[17] W. K. Buah, A. M. Cunliffe, P. T. Williams ―Characterization of products from
the pyrolysis of municipal solid waste‖ .
[18] Francisco D. Hernandez-Atonal, Changkook Ryu, Vida N. Sharifi, Jim
Swithenbank ―Combustion of refuse-derived fuel in a fluidised bed‖ .
[19] Guilin Piao, Shigeru Aono, Shigekatsu Mori, Seiichi Deguchi, Yukihisa Fujima,
Motohiro Kondoh, Masataka Yamaguchi ―Combustion of refuse derived fuel in a
fluidized bed‖.
[20] Guilin Piao, Shigeru Aono, Motohiro Kondoh, Ryohei Yamazaki, Shigekatsu
Mori ―Combustion test of refuse derived fuel in a fluidized bed‖ .
[21] Valerio Cozzani, Luigi Petarca and Leonardo Tognotti ―Devolatilization and
pyrolysis of refuse derived fuels: characterization and kinetic modelling by a
thermogravimetric and calorimetric approach‖ .
[22] S.A. Channiwala, P.P. Parikh ―A unified correlation for estimating HHV of solid,
liquid and gaseous fuels‖ .
[23] S.N.M. Menikpura, B.F.A. Basnayake ―New applications of ‗Hess Law‘ and
comparisons with models for determining calorific values of municipal solid wastes in
the Sri Lankan context‖ .
[24] Yingwei Fei, S.K. Saxena ―An equation for the heat capacity of solids‖ .
[25] Incropera, Dewitt, Bergman, Lavine ―Introduction to Heat Transfer‖ .
[26] ISPRA, ―Rapporto rifiuti urbani‖, Edizione 2009 .
[27] D. Cocco, C. Palomba, P. Puddu ―Tecnologie delle energie rinnovabili‖.
[28] G. Lozza, A. Giuffrida, M. Romano ―Sviluppo di modellistica di impianti di
generazione elettrica integrati con processi di gassificazione di carbone che utilizzano
tecnologie di desolforazione a caldo del syngas prodotto‖.
[29] A. Arsalis, M.R. Spakovsky, F. Carlise ―Thermoeconomic modeling and
parametric study of hybrid solid oxide fuel cell-gas turbine-steam turbine power plants
ranging form 1.5 MWe to 10 MWe‖.
[30] Sandro Petrizzelli ―Appunti di Fisica Tecnica‖.
[31] Leonardo Pierobon ― Analysis of a Gasification Plant fed by woodchips integrated
with SOFC and Steam Cycle‖.
213
[32] www.matche.com/EquipCost
[33] S.B. Ferreira ―Thermoeconomic Analysis and optimization of biomass fuel Gas
Turbine‖.
[34] F. Scappin, M. Rokni ―Integrating a SOFC with a Steam Turbine‖.
[35] A. Arsalis, M.R. Spakovsky, F. Carlise ―Thermoeconomic modeling and
parametric study of hybrid solid oxide fuel cell-gas turbine-steam turbine power plants
ranging from 1.5 MWe to 10 MWe‖.
[36] Z.T. Lian, K.J. Chua, S.K. Chou ―A Thermoeconomic Analysis of biomass energy
for trigeneration‖.
[37] Bejan A., Tsatsaronis G., Moran M. ― Thermal Design & Optimization‖.
[38] (2011, July) GME - Gestore dei Mercati Energetici.
http://www.mercatoelettrico.org/It/Statistiche/ME/DatiSintesi.aspx
[39] (2011, July) Nord Pool ASA. http://nordpool.com/marketinfo/powersystemdata/
[40] APER, Studio sui costi di generazione di energia elettrica da fonti rinnovabili.
[41] J. Hirschenhofer, D. Stauffer, R. Engleman ―Fuel Cells Handbook‖ 5th Edition,
U.S. Department of Energy, Morgantown, PA, (2000).
[42] epp.eurostat.ec.europa.eu
[43] Hirschenhofer, J.H. ―I‘ts time to take Fuel Cell seriously, Part II‖ .
214
215
Figure References
Figure 1.1.1 : Jürgen O. Metzger, Marco Eissen ―Concepts on the contribution of
chemistry to a sustainable development. Renewable raw materials‖ .
Figure 2.2.1 : http://www.ankurscientific.com/whatisgasification.htm
Figure 2.2.2.1 : Giovanni Lozza, ―Turbine a gas e cicli combinati‖, Progetto Leonardo,
second edition.
Figure 2.2.2.2 : Giovanni Lozza, ―Turbine a gas e cicli combinati‖, Progetto Leonardo,
second edition.
Figure 2.2.2.3 : Giovanni Lozza, ―Turbine a gas e cicli combinati‖, Progetto Leonardo,
second edition.
Figure 2.2.4.1: Jens Dall Bentzen, Reto M. Hummelshøj, Ulrik Henriksen, Benny
Gøbel, Jesper Ahrenfelt, Brian Elmegaard ―Upscale of the two-stage gasification
process‖.
Figure 2.2.4.2: Jens Dall Bentzen, Reto M. Hummelshøj, Ulrik Henriksen, Benny
Gøbel, Jesper Ahrenfelt, Brian Elmegaard ―Upscale of the two-stage gasification
process‖.
Figure 2.2.5.1: Jens Dall Bentzen, Reto M. Hummelshøj, Ulrik Henriksen, Benny
Gøbel, Jesper Ahrenfelt, Brian Elmegaard ―Upscale of the two-stage gasification
process‖.
Figure 2.3.1.1 : http://www.usawaterandpower.com/hosted/yours//SANBCWCP/
Figure 2.3.1.2 : http://www.cheresources.com/ausseycellzz.shtml
Figure 2.3.4.1 : ―Modeling of methane fed solid oxide fuel cells: Comparison between
proton conducting electrolyte and oxygen ion conducting electrolyte‖, Authors Meng
Ni, Dennis Y.C. Leung, Michael K.H. Leun
Figure 2.3.5.1 : ―A high-flexibility DC load for fuel cell and solar arrays power sources
based on DC–DC converters‖ Authors E. Durán, J.M. Andújar, F. Segura, A.J.
Barragán
Figure 2.3.6.1.1 : Masoud Rokni, ―Fuel cell in power plants‖, Department of
Mechanical Engineering, Damarks Tekniske Universitet.
Figure 2.3.6.1.2 : Masoud Rokni, ―Fuel cell in power plants‖, Department of
Mechanical Engineering, Damarks Tekniske Universitet.
216
Figure 2.4.1.1 : ―Turbine, Gas‖ Lee L. Langston, University of Connecticut Storrs,
Connecticut, United States (left)
Figure 2.4.1.1 : http://www.manufacturer.com (right)
Figure 2.4.2.1 : http://images.yourdictionary.com/gas-turbine
Figure 2.4.3.1 : ―Residential cogeneration systems: review of the current technology‖,
Authors H.I. Onovwiona, V.I. Ugursal .
Figure 2.4.4.1.1 : http://koolkampus.com/engineering-notes-2/mechanical/gas-turbine-
combustion-turbine/
Figure 2.4.4.2.1 : ―Residential cogeneration systems: review of the current
technology‖, Authors H.I. Onovwiona, V.I. Ugursal .
Figure 2.4.4.3.1 : ―Directions of the development of thermal barrier coatings in energy
applications‖, D. StoÈver, C. Funke
Figure 2.4.5.2.1 : ―Caldaie a gas per la produzione di acqua calda‖, Michele De Carli,
University of Padova, Department of Technical Physics.
Figure 3.6.1 : ―Directions of the development of thermal barrier coatings in energy
applications‖, D. StoÈver, C. Funke
Figure 4.1.1: ―Appunti di Fisica Tecnica‖, Sandro Petrizzelli.
217
218
219
APPENDIX I : DNA FILES
1. Municipal Solid Waste Gasification Plant Integrated with SOFC and Gas
Turbine: Optimized configuration with Hybrid Recuperator ( Figure 3.8.1 ).
220
221
222
223
224
225
226
227
228
229
2. Absorption Cooling System (Figure 4.2.1) .
Model Refers to two generic Absorption Cooling Plants; results are relative to the
introduction in the studied plant of two Absorption Cooling Plants (Figure 4.4.1).
230
231
232
233
234
235
236
237
3. SOFC Plant fed by Natural Gas Integrated with Gas Turbine, CPO unit (Figure
6.1.1).
238
239
240
241
242
243
244
245
4. SOFC Plant fed by Natural Gas Integrated with Gas Turbine, ASR unit (Figure
6.1.2).
246
247
248
249
250
251
252
APPENDIX II: EES FILES
1. Pressure Drops in Heat Exchangers
In this section is reported a generic file for a generic Heat Exchanger; for each
one are used the values in paragraph 3.12
"(Shell and Tube Heat Exchanger) " T_h_in = 800+273,15 T_h_out = 539+273,15 T_c_in = 25+273,15 T_c_out = 780+273,15 m_h = 4,49 m_c = 2,35 c_p_c = 1135 " At mean temperature" C_c = m_c*c_p_c C_h = (C_c*(T_c_out-T_c_in))/(T_h_in-T_h_out) c_p_h = C_h/m_h C = C_c/C_h "In this case C_min=C_c" epsilon = (T_c_out-T_c_in)/(T_h_in-T_c_in) NTU = (ln(((1-(epsilon*C))/(1-epsilon))))/(1-C) (UA) = C_c*NTU f = 0,01 f_0= 0,01 DeltaP_h = 0,005 rho_h_in = 1/4,4379 rho_h_out = 1/3,3756 rho_h = (rho_h_in+rho_h_out)/2 mu_h = 0,0000457 DeltaP_c = 0,005 rho_c_in = 1/0,8523 rho_c_out = 1/3,0255 rho_c = (rho_c_in+rho_c_out)/2 mu_c = 0,000041 DeltaP_h = (2*f_0*(L/D_h)*((G_0^2)/rho_h))+((G_0^2)*((1/rho_h_out)-(1/rho_h_in))) G_0 = m_h/A_flow A_flow = A_fr-((n*3,14*D_0^2)/4) D_h = (4*A_flow)/(n*3,14*D_0) Re_h = (G_0*D_h)/mu_h Pr_h = 0,72 St_0 = 0,5*f_0*(Pr_h^(-2/3)) "Stanton number, defined as St=Nu/Re*Pr " h_h = c_p_h*G_0*St_0 DeltaP_c = (2*f*(L/D)*((G^2)/rho_c))+((G^2)*((1/rho_c_out)-(1/rho_c_in))) A = n*3,14*D*L G = m_c/A Re_c = (G*D)/mu_c Pr_c = 0,71 St = 0,5*f*(Pr_c^(-2/3)) "Stanton number, defined as St=Nu/Re*Pr " h_c = c_p_c*G*St D_0/D=1,2 D = 0,01 n=1200
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2. Thermoeconomic Analysis
a. Municipal Solid Waste Gasification Plant Integrated with SOFC and Gas
Turbine: Optimized configuration with Hybrid Recuperator ( Figure
3.8.1 ).
"THERMOECONOMIC ANALYSIS" "MUNICIPAL SOLID WASTE GASIFICATION PLANT INTEGRATED WITH SOFC AND GAS TURBINE" "(Regenerative Gas Turbine)" "1. GASIFICATION PLANT" "Dryer" Z_dryer=5,10 "[Euro/h]" cu[37]*e[37]-cu[40]*e[40]+Z_dryer=cu[32]*e[32]-cu[31]*e[31] cu[37]=cu[40] cu[31]=0,0022"[Euro/kWh]" e[37]-e[40]=e[32]-e[31]+Ed_dryer CP_dryer=cu[32] CF_dryer=((cu[37]-cu[40])*(e[37]-e[40])+cu[31]*e[31])/(e[31]+(e[37]-e[40])) R_dryer=100*(CP_dryer-CF_dryer)/CF_dryer F_dryer=100*Z_dryer/(Z_dryer+CF_dryer*(Ed_dryer+El_dryer)) El_dryer=0 epsilon_D_dryer=Ed_dryer/e[31] epsilon_L_dryer=El_dryer/e[31] epsilon_dryer=epsilon_D_dryer+epsilon_L_dryer "Gasifier" Z_gasifier=348,14 "[Euro/h]" cu[44]*e[44]+Z_gasifier=cu[33]*e[33]-cu[32]*e[32]+cu[46]*e[46] cu[46]=0,00022 e[44]=e[33]-e[32]+e[46]+Ed_gasifier CP_gasifier=(cu[33]*e[33]+cu[46]*e[46])/(e[33]+e[46]) CF_gasifier=(cu[44]*e[44]+cu[32]*e[32])/(e[44]+e[32]) R_gasifier=100*(CP_gasifier-CF_gasifier)/CF_gasifier F_gasifier=100*Z_gasifier/(Z_gasifier+CF_gasifier*(Ed_gasifier+El_gasifier)) El_gasifier=e[46] epsilon_D_gasifier=Ed_gasifier/e[31] epsilon_L_gasifier=El_gasifier/e[31] epsilon_gasifier=epsilon_D_gasifier+epsilon_L_gasifier "Air pre-heater" Z_airpreheater=0,83 "[Euro/h]" cu[33]*e[33]-cu[34]*e[34]+Z_airpreheater=cu[42]*e[42]-cu[41]*e[41] cu[41]=0 cu[33]=cu[34] e[33]-e[34]=e[42]-e[41]+Ed_airpreheater CP_airpreheater=(cu[42]*e[42]-cu[41]*e[41])/(e[42]-e[41]) CF_airpreheater=(cu[33]*e[33]-cu[34]*e[34])/(e[33]-e[34]) R_airpreheater=100*(CP_airpreheater-CF_airpreheater)/CF_airpreheater F_airpreheater=100*Z_airpreheater/(Z_airpreheater+CF_airpreheater*(Ed_airpreheater+El_airpreheater)) El_airpreheater=0 epsilon_D_airpreheater=Ed_airpreheater/e[31] epsilon_L_airpreheater=El_airpreheater/e[31] epsilon_airpreheater=epsilon_D_airpreheater+epsilon_L_airpreheater "Mixer" Z_mixer=0 "[Euro/h]" cu[42]*e[42]+cu[43]*e[43]+Z_mixer=cu[44]*e[44] CP_mixer=cu[44] CF_mixer=(cu[42]*e[42]+cu[43]*e[43])/(e[42]+e[43])
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R_mixer=100*(CP_mixer-CF_mixer)/CF_mixer F_mixer=100*Z_mixer/(Z_mixer+CF_mixer*(Ed_mixer+El_mixer)) e[42]+e[43]=e[44]+Ed_mixer El_mixer=0 epsilon_D_mixer=Ed_mixer/e[31] epsilon_L_mixer=El_mixer/e[31] epsilon_mixer=epsilon_D_mixer+epsilon_L_mixer "Steam blower" Z_steamblower=0,02 "[Euro/h]" cu[110]*e[110]+Z_steamblower=cu[38]*e[38]-cu[37]*e[37] e[110]=e[38]-e[37]+Ed_steamblower CP_steamblower=(cu[38]*e[38]-cu[37]*e[37])/(e[38]-e[37]) CF_steamblower=cu[110] R_steamblower=100*(CP_steamblower-CF_steamblower)/CF_steamblower F_steamblower=100*Z_steamblower/(Z_steamblower+CF_steamblower*(Ed_steamblower+El_steamblower)) El_steamblower=0 epsilon_D_steamblower=Ed_steamblower/e[31] epsilon_L_steamblower=El_steamblower/e[31] epsilon_steamblower=epsilon_D_steamblower+epsilon_L_steamblower "Splitter" Z_splitter=0 "[Euro/h]" cu[38]*e[38]+Z_splitter=cu[39]*e[39]+cu[43]*e[43] cu[39]=cu[43] CP_splitter=(cu[39]*e[39]+cu[43]*e[43]) /(e[39]+e[43]) CF_splitter=cu[38] R_splitter=100*(CP_splitter-CF_splitter)/CF_splitter F_splitter=100*Z_splitter/(Z_splitter+CF_splitter*(Ed_splitter+El_splitter)) e[38]=e[39]+e[43]+Ed_splitter El_splitter=0 epsilon_D_splitter=Ed_splitter/e[31] epsilon_L_splitter=El_splitter/e[31] epsilon_splitter=epsilon_D_splitter+epsilon_L_splitter "Steam generator" Z_steamgenerator=1,65 "[Euro/h]" cu[34]*e[34]-cu[35]*e[35]+Z_steamgenerator=cu[40]*e[40]-cu[39]*e[39] cu[35]=cu[34] e[34]-e[35]=e[40]-e[39]+Ed_steamgenerator CP_steamgenerator=(cu[40]*e[40]-cu[39]*e[39])/(e[40]-e[39]) CF_steamgenerator=(cu[34]*e[34]-cu[35]*e[35])/(e[34]-e[35]) R_steamgenerator=100*(CP_steamgenerator-CF_steamgenerator)/CF_steamgenerator F_steamgenerator=100*Z_steamgenerator/(Z_steamgenerator+CF_steamgenerator*(Ed_steamgenerator+El_steamgenerator)) El_steamgenerator=0 epsilon_D_steamgenerator=Ed_steamgenerator/e[31] epsilon_L_steamgenerator=El_steamgenerator/e[31] epsilon_steamgenerator=epsilon_D_steamgenerator+epsilon_L_steamgenerator "Desulphuriser" Z_desulphuriser=31,38 "[Euro/h]" cu[35]*e[35]+Z_desulphuriser=cu[36]*e[36]+cu[47]*e[47] cu[47]=0 e[35]=e[36]+e[47]+Ed_desulphuriser CP_desulphuriser=(cu[36]*e[36]+cu[47]*e[47])/(e[35]+e[47]) CF_desulphuriser=cu[35] R_desulphuriser=100*(CP_desulphuriser-CF_desulphuriser)/CF_desulphuriser F_desulphuriser=100*Z_desulphuriser/(Z_desulphuriser+CF_desulphuriser*(Ed_desulphuriser+El_desulphuriser)) El_desulphuriser=e[47] epsilon_D_desulphuriser=Ed_desulphuriser/e[31] epsilon_L_desulphuriser=El_desulphuriser/e[31] epsilon_desulphuriser=epsilon_D_desulphuriser+epsilon_L_desulphuriser "2. SOFC PLANT"
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"syngas blower" Z_syngasblower=0,17 "[Euro/h]" cu[104]*e[104]+Z_syngasblower=cu[10]*e[10]-cu[36]*e[36] e[104]=e[10]-e[36]+Ed_syngasblower CP_syngasblower=(cu[10]*e[10]-cu[36]*e[36])/(e[10]-e[36]) CF_syngasblower=cu[104] R_syngasblower=100*(CP_syngasblower-CF_syngasblower)/CF_syngasblower F_syngasblower=100*Z_syngasblower/(Z_syngasblower+CF_syngasblower*(Ed_syngasblower+El_syngasblower)) El_syngasblower=0 epsilon_D_syngasblower=Ed_syngasblower/e[31] epsilon_L_syngasblower=El_syngasblower/e[31] epsilon_syngasblower=epsilon_D_syngasblower+epsilon_L_syngasblower "Anode preheater" Z_anodepreheater=0,72 "[Euro/h]" cu[12]*e[12]-cu[13]*e[13]+Z_anodepreheater=cu[11]*e[11]-cu[10]*e[10] cu[12]=cu[13] e[12]-e[13]=e[11]-e[10]+Ed_anodepreheater CP_anodepreheater=(cu[11]*e[11]-cu[10]*e[10])/(e[11]-e[10]) CF_anodepreheater=(cu[12]*e[12]-cu[13]*e[13])/(e[12]-e[13]) R_anodepreheater=100*(CP_anodepreheater-CF_anodepreheater)/CF_anodepreheater F_anodepreheater=100*Z_anodepreheater/(Z_anodepreheater+CF_anodepreheater*(Ed_anodepreheater+El_anodepreheater)) El_anodepreheater=0 epsilon_D_anodepreheater=Ed_anodepreheater/e[31] epsilon_L_anodepreheater=El_anodepreheater/e[31] epsilon_anodepreheater=epsilon_D_anodepreheater+epsilon_L_anodepreheater "SOFC" Z_SOFC=1517,15"[Euro/h]" cu[11]*e[11]-cu[12]*e[12]+Z_SOFC=cu[201]*e[201]+(cu[5]*e[5]-cu[4]*e[4]) cu[11]=cu[12] "cu[5]=0" "cu[12]=0" (cu[5]*e[5]-cu[4]*e[4])/(e[5]-e[4])=cu[201] e[11]-e[12]=e[201]+e[5]-e[4]+Ed_SOFC CP_SOFC=((cu[5]-cu[4])*(e[5]-e[4])+cu[201]*e[201])/(e[201]+(e[5]-e[4])) CF_SOFC=(cu[12]*e[12]-cu[11]*e[11])/(e[12]-e[11]) R_SOFC=100*(CP_SOFC-CF_SOFC)/CF_SOFC F_SOFC=100*Z_SOFC/(Z_SOFC+CF_SOFC*(Ed_SOFC+El_SOFC)) El_SOFC=0 epsilon_D_SOFC=Ed_SOFC/e[31] epsilon_L_SOFC=El_SOFC/e[31] epsilon_SOFC=epsilon_D_SOFC+epsilon_L_SOFC "Cathode preheater" Z_cathodepreheater=3,29 "[Euro/h]" cu[5]*e[5]-cu[16]*e[16]+Z_cathodepreheater=cu[4]*e[4]-cu[3]*e[3] cu[5]=cu[16] e[5]-e[16]=e[4]-e[3]+Ed_cathodepreheater CP_cathodepreheater=(cu[4]*e[4]-cu[3]*e[3])/(e[4]-e[3]) CF_cathodepreheater=(cu[5]*e[5]-cu[16]*e[16])/(e[5]-e[16]) R_cathodepreheater=100*(CP_cathodepreheater-CF_cathodepreheater)/CF_cathodepreheater F_cathodepreheater=100*Z_cathodepreheater/(Z_cathodepreheater+CF_cathodepreheater*(Ed_cathodepreheater+El_cathodepreheater)) El_cathodepreheater=0 epsilon_D_cathodepreheater=Ed_cathodepreheater/e[31] epsilon_L_cathodepreheater=El_cathodepreheater/e[31] epsilon_cathodepreheater=epsilon_D_cathodepreheater+epsilon_L_cathodepreheater "compressor" Z_compressor=3,80 "[Euro/h]" cu[101]*e[101]+Z_compressor=cu[2]*e[2]-cu[1]*e[1] cu[1]=0
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e[101]=e[2]-e[1]+Ed_compressor CP_compressor=(cu[2]*e[2]-cu[1]*e[1])/(e[2]-e[1]) CF_compressor=cu[101] R_compressor=100*(CP_compressor-CF_compressor)/CF_compressor F_compressor=100*Z_compressor/(Z_compressor+CF_compressor*(Ed_compressor+El_compressor)) El_compressor=0 epsilon_D_compressor=Ed_compressor/e[31] epsilon_L_compressor=El_compressor/e[31] epsilon_compressor=epsilon_D_compressor+epsilon_L_compressor "Burner" Z_burner=7,53 "[Euro/h]" cu[16]*e[16]+cu[13]*e[13]+Z_burner=cu[17]*e[17] e[16]+e[13]=e[17]+Ed_burner CP_burner=cu[17] CF_burner=(cu[16]*e[16]+cu[13]*e[13])/(e[16]+e[13]) R_burner=100*(CP_burner-CF_burner)/CF_burner F_burner=100*Z_burner/(Z_burner+CF_burner*(Ed_burner+El_burner)) El_burner=0 epsilon_D_burner=Ed_burner/e[31] epsilon_L_burner=El_burner/e[31] epsilon_burner=epsilon_D_burner+epsilon_L_burner "3. GAS TURBINE AND HYBRID RECUPERATOR" "Gas Turbine" Z_gasturbine=160,36 "[Euro/h]" cu[17]*e[17]-cu[18]*e[18]+Z_gasturbine=cu[102]*e[102] cu[17]=cu[18] e[17]-e[18]=e[102]+Ed_gasturbine CP_gasturbine=cu[102] CF_gasturbine=(cu[17]*e[17]-cu[18]*e[18])/(e[17]-e[18]) R_gasturbine=100*(CP_gasturbine-CF_gasturbine)/CF_gasturbine F_gasturbine=100*Z_gasturbine/(Z_gasturbine+CF_gasturbine*(Ed_gasturbine+El_gasturbine)) El_gasturbine=0 epsilon_D_gasturbine=Ed_gasturbine/e[31] epsilon_L_gasturbine=El_gasturbine/e[31] epsilon_gasturbine=epsilon_D_gasturbine+epsilon_L_gasturbine "Electric Generator" Z_electricgenerator=0,21 "[Euro/h]" cu[102]*e[102]+Z_electricgenerator=cu[202]*e[202] e[102]=e[202]+Ed_electricgenerator CP_electricgenerator=cu[202] CF_electricgenerator=cu[102] R_electricgenerator=100*(CP_electricgenerator-CF_electricgenerator)/CF_electricgenerator F_electricgenerator=100*Z_electricgenerator/(Z_electricgenerator+CF_electricgenerator*(Ed_electricgenerator+El_electricgenerator)) El_electricgenerator=0 epsilon_D_electricgenerator=Ed_electricgenerator/e[31] epsilon_L_electricgenerator=El_electricgenerator/e[31] epsilon_electricgenerator=epsilon_D_electricgenerator+epsilon_L_electricgenerator "Hybrid recuperator" Z_hybridrec=3,05 "[Euro/h]" cu[18]*e[18]-cu[19]*e[19]+Z_hybridrec=cu[3]*e[3]-cu[2]*e[2] cu[19]=0 e[18]-e[19]=e[3]-e[2]+Ed_hybridrec CP_hybridrec=(cu[3]*e[3]-cu[2]*e[2])/(e[3]-e[2]) CF_hybridrec=(cu[18]*e[18]-cu[19]*e[19])/(e[18]-e[19]) R_hybridrec=100*(CP_hybridrec-CF_hybridrec)/CF_hybridrec F_hybridrec=100*Z_hybridrec/(Z_hybridrec+CF_hybridrec*(Ed_hybridrec+El_hybridrec)) El_hybridrec=e[19] epsilon_D_hybridrec=Ed_hybridrec/e[31] epsilon_L_hybridrec=El_hybridrec/e[31] epsilon_hybridrec=epsilon_D_hybridrec+epsilon_L_hybridrec
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"Auxiliaries condictions" cu[101]=(cu[201]*e[201]+cu[202]*e[202])/(e[201]+e[202]) cu[101]=cu[104] cu[104]=cu[110] "Exergy output" TOT=e[31]-(Ed_dryer+Ed_gasifier+Ed_airpreheater+Ed_mixer+Ed_steamblower+Ed_splitter+Ed_steamgenerator+Ed_desulphuriser+Ed_syngasblower+Ed_anodepreheater+Ed_SOFC+Ed_cathodepreheater+Ed_hybridrec+Ed_compressor+Ed_burner+Ed_gasturbine+Ed_electricgenerator+e[46]+e[47]+e[19]) "Exergetic efficiency" eta_ex=TOT/e[31] "Price of electricty" Pel=(e[201]*cu[201]+e[202]*cu[202])/(e[201]+e[202]) "Euro/kWh" " Exergy [kW] flow for each node of the system" e[31]=46021,67 e[40]=4552,67 e[32]=45890,60 e[37]=4328,38 e[320]=0 e[45]=0 e[44]=973,25 e[33]=42718,16 e[46]=202 e[321]=0 e[34]=41410,74 e[41]=6,45 e[42]=979,27 e[322]=0 e[35]=40919,88 e[39]=4287,92 e[43]=58,2 e[323]=0 e[38]=4346,12 e[324]=0 e[110]=21,12 e[36]=40754,33 e[47]=166,16 e[325]=0 e[10]=42211,87 e[312]=0 e[104]=1593,43 e[12]=12678,05 e[13]=12642,43 e[11]=42240,78 e[308]=0 e[4]=36385,27 e[5]=44867,65 e[201]=18535,98 e[309]=0 e[16]=35193,36 e[3]=27608,75 e[303]=0 e[17]=44668,99 e[310]=0 e[18]=27635,93
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e[102]=16111,65 e[202]=15789,42 e[311]=0 e[19]=6099,74 e[2]=9106,90 e[302]=0 e[1]=198,79 e[301]=0 e[101]=10579,87 "Cost per hours calculation" C[1]=cu[1]*e[1] C[2]=cu[2]*e[2] C[3]=cu[3]*e[3] C[4]=cu[4]*e[4] C[5]=cu[5]*e[5] C[10]=cu[10]*e[10] C[11]=cu[11]*e[11] C[12]=cu[12]*e[12] C[13]=cu[13]*e[13] C[16]=cu[16]*e[16] C[17]=cu[17]*e[17] C[18]=cu[18]*e[18] C[19]=cu[19]*e[19] C[31]=cu[31]*e[31] C[32]=cu[32]*e[32] C[33]=cu[33]*e[33] C[34]=cu[34]*e[34] C[35]=cu[35]*e[35] C[36]=cu[36]*e[36] C[37]=cu[37]*e[37] C[38]=cu[38]*e[38] C[39]=cu[39]*e[39] C[40]=cu[40]*e[40] C[41]=cu[41]*e[41] C[42]=cu[42]*e[42] C[43]=cu[43]*e[43] C[44]=cu[44]*e[44] C[46]=cu[46]*e[46] C[47]=cu[47]*e[47] C[101]=cu[101]*e[101] C[102]=cu[102]*e[102] C[104]=cu[104]*e[104] C[110]=cu[110]*e[110] C[201]=cu[201]*e[201] C[202]=cu[202]*e[202]
b. SOFC Plant fed by Natural Gas Integrated with Gas Turbine, CPO unit
(Figure 6.1.1).
"THERMOECONOMIC ANALYSIS" "SOFC AND GAS TURBINE PLANT FED BY NATURAL GAS" "(CPO unit as Pre-Reforming)" "1. SOFC PLANT" "Fuel preheater" Z_fuelpreheater=0,13 "[Euro/h]" cu[14]*e[14]-cu[15]*e[15]+Z_fuelpreheater=cu[7]*e[7]-cu[6]*e[6] cu[14]=cu[15] "cu[6]=0,028[Euro/kWh]" "Natural Gas Italian Price" "cu[6]=0,024[Euro/kWh]" "Natural Gas Danish Price" cu[6]=0,026"[Euro/kWh]" "Mean Price"
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e[14]-e[15]=e[7]-e[6]+Ed_fuelpreheater CP_fuelpreheater=(cu[7]*e[7]-cu[6]*e[6])/(e[7]-e[6]) CF_fuelpreheater=(cu[14]*e[14]-cu[15]*e[15])/(e[14]-e[15]) R_fuelpreheater=100*(CP_fuelpreheater-CF_fuelpreheater)/CF_fuelpreheater F_fuelpreheater=100*Z_fuelpreheater/(Z_fuelpreheater+CF_fuelpreheater*(Ed_fuelpreheater+El_fuelpreheater)) El_fuelpreheater=0 epsilon_D_fuelpreheater=Ed_fuelpreheater/e[6] epsilon_L_fuelpreheater=El_fuelpreheater/e[6] epsilon_fuelpreheater=epsilon_D_fuelpreheater+epsilon_L_fuelpreheater "Desulphuriser" Z_desulphuriser=31,38 "[Euro/h]" cu[7]*e[7]+Z_desulphuriser=cu[8]*e[8]+cu[22]*e[22] cu[22]=0 e[7]+e[22]=e[8]+Ed_desulphuriser CP_desulphuriser=(cu[8]*e[8]+cu[22]*e[22])/(e[8]+e[22]) CF_desulphuriser=cu[7] R_desulphuriser=100*(CP_desulphuriser-CF_desulphuriser)/CF_desulphuriser F_desulphuriser=100*Z_desulphuriser/(Z_desulphuriser+CF_desulphuriser*(Ed_desulphuriser+El_desulphuriser)) El_desulphuriser=e[22] epsilon_D_desulphuriser=Ed_desulphuriser/e[6] epsilon_L_desulphuriser=El_desulphuriser/e[6] epsilon_desulphuriser=epsilon_D_desulphuriser+epsilon_L_desulphuriser "Reformer preheater" Z_reformerpreheater=0,14 "[Euro/h]" cu[13]*e[13]-cu[14]*e[14]+Z_reformerpreheater=cu[9]*e[9]-cu[8]*e[8] cu[13]=cu[14] e[13]-e[14]=e[9]-e[8]+Ed_reformerpreheater CP_reformerpreheater=(cu[9]*e[9]-cu[8]*e[8])/(e[9]-e[8]) CF_reformerpreheater=(cu[13]*e[13]-cu[14]*e[14])/(e[13]-e[14]) R_reformerpreheater=100*(CP_reformerpreheater-CF_reformerpreheater)/CF_reformerpreheater F_reformerpreheater=100*Z_reformerpreheater/(Z_reformerpreheater+CF_reformerpreheater*(Ed_reformerpreheater+El_reformerpreheater)) El_reformerpreheater=0 epsilon_D_reformerpreheater=Ed_reformerpreheater/e[6] epsilon_L_reformerpreheater=El_reformerpreheater/e[6] epsilon_reformerpreheater=epsilon_D_reformerpreheater+epsilon_L_reformerpreheater "compressor 1" Z_compressor1=0,01 "[Euro/h]" cu[103]*e[103]+Z_compressor1=cu[21]*e[21]-cu[20]*e[20] cu[20]=0 e[103]=e[21]-e[20]+Ed_compressor1 CP_compressor1=(cu[21]*e[21]-cu[20]*e[20])/(e[21]-e[20]) CF_compressor1=cu[103] R_compressor1=100*(CP_compressor1-CF_compressor1)/CF_compressor1 F_compressor1=100*Z_compressor1/(Z_compressor1+CF_compressor1*(Ed_compressor1+El_compressor1)) El_compressor1=0 epsilon_D_compressor1=Ed_compressor1/e[6] epsilon_L_compressor1=El_compressor1/e[6] epsilon_compressor1=epsilon_D_compressor1+epsilon_L_compressor1 "CPO" Z_CPO=1,23 "[Euro/h]" cu[9]*e[9]+cu[21]*e[21]+Z_CPO=cu[10]*e[10] e[9]+e[21]=e[10]+Ed_CPO CP_CPO=cu[10] CF_CPO=(cu[9]*e[9]+cu[21]*e[21])/(e[9]+e[21]) R_CPO=100*(CP_CPO-CF_CPO)/CF_CPO F_CPO=100*Z_CPO/(Z_CPO+CF_CPO*(Ed_CPO+El_CPO)) El_CPO=0 epsilon_D_CPO=Ed_CPO/e[6] epsilon_L_CPO=El_CPO/e[6] epsilon_CPO=epsilon_D_CPO+epsilon_L_CPO
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"Anode preheater" Z_anodepreheater=0,27 "[Euro/h]" cu[12]*e[12]-cu[13]*e[13]+Z_anodepreheater=cu[11]*e[11]-cu[10]*e[10] cu[12]=cu[13] e[12]-e[13]=e[11]-e[10]+Ed_anodepreheater CP_anodepreheater=(cu[11]*e[11]-cu[10]*e[10])/(e[11]-e[10]) CF_anodepreheater=(cu[12]*e[12]-cu[13]*e[13])/(e[12]-e[13]) R_anodepreheater=100*(CP_anodepreheater-CF_anodepreheater)/CF_anodepreheater F_anodepreheater=100*Z_anodepreheater/(Z_anodepreheater+CF_anodepreheater*(Ed_anodepreheater+El_anodepreheater)) El_anodepreheater=0 epsilon_D_anodepreheater=Ed_anodepreheater/e[6] epsilon_L_anodepreheater=El_anodepreheater/e[6] epsilon_anodepreheater=epsilon_D_anodepreheater+epsilon_L_anodepreheater "SOFC" Z_SOFC=1914,40 "[Euro/h]" cu[11]*e[11]-cu[12]*e[12]+Z_SOFC=cu[201]*e[201]+(cu[5]*e[5]-cu[4]*e[4]) cu[11]=cu[12] (cu[5]*e[5]-cu[4]*e[4])/(e[5]-e[4])=cu[201] "cu[5]=0" "cu[12]=0" e[11]-e[12]=e[201]+e[5]-e[4]+Ed_SOFC CP_SOFC=((cu[5]-cu[4])*(e[5]-e[4])+cu[201]*e[201])/(e[201]+(e[5]-e[4])) CF_SOFC=(cu[12]*e[12]-cu[11]*e[11])/(e[12]-e[11]) R_SOFC=100*(CP_SOFC-CF_SOFC)/CF_SOFC F_SOFC=100*Z_SOFC/(Z_SOFC+CF_SOFC*(Ed_SOFC+El_SOFC)) El_SOFC=0 epsilon_D_SOFC=Ed_SOFC/e[6] epsilon_L_SOFC=El_SOFC/e[6] epsilon_SOFC=epsilon_D_SOFC+epsilon_L_SOFC "Cathode preheater" Z_cathodepreheater=1,24 "[Euro/h]" cu[5]*e[5]-cu[16]*e[16]+Z_cathodepreheater=cu[4]*e[4]-cu[3]*e[3] cu[5]=cu[16] e[5]-e[16]=e[4]-e[3]+Ed_cathodepreheater CP_cathodepreheater=(cu[4]*e[4]-cu[3]*e[3])/(e[4]-e[3]) CF_cathodepreheater=(cu[5]*e[5]-cu[16]*e[16])/(e[5]-e[16]) R_cathodepreheater=100*(CP_cathodepreheater-CF_cathodepreheater)/CF_cathodepreheater F_cathodepreheater=100*Z_cathodepreheater/(Z_cathodepreheater+CF_cathodepreheater*(Ed_cathodepreheater+El_cathodepreheater)) El_cathodepreheater=0 epsilon_D_cathodepreheater=Ed_cathodepreheater/e[6] epsilon_L_cathodepreheater=El_cathodepreheater/e[6] epsilon_cathodepreheater=epsilon_D_cathodepreheater+epsilon_L_cathodepreheater "compressor 2" Z_compressor2=0,45 "[Euro/h]" cu[101]*e[101]+Z_compressor2=cu[2]*e[2]-cu[1]*e[1] cu[1]=0 e[101]=e[2]-e[1]+Ed_compressor2 CP_compressor2=(cu[2]*e[2]-cu[1]*e[1])/(e[2]-e[1]) CF_compressor2=cu[101] R_compressor2=100*(CP_compressor2-CF_compressor2)/CF_compressor2 F_compressor2=100*Z_compressor2/(Z_compressor2+CF_compressor2*(Ed_compressor2+El_compressor2)) El_compressor2=0 epsilon_D_compressor2=Ed_compressor2/e[6] epsilon_L_compressor2=El_compressor2/e[6] epsilon_compressor2=epsilon_D_compressor2+epsilon_L_compressor2 "Burner" Z_burner=7,53 "[Euro/h]" cu[16]*e[16]+cu[15]*e[15]+Z_burner=cu[17]*e[17] e[16]+e[15]=e[17]+Ed_burner CP_burner=cu[17] CF_burner=(cu[16]*e[16]+cu[15]*e[15])/(e[16]+e[15])
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R_burner=100*(CP_burner-CF_burner)/CF_burner F_burner=100*Z_burner/(Z_burner+CF_burner*(Ed_burner+El_burner)) El_burner=0 epsilon_D_burner=Ed_burner/e[6] epsilon_L_burner=El_burner/e[6] epsilon_burner=epsilon_D_burner+epsilon_L_burner "2. GAS TURBINE AND HYBRID RECUPERATOR" "Gas Turbine" Z_gasturbine=79,43 "[Euro/h]" cu[17]*e[17]-cu[18]*e[18]+Z_gasturbine=cu[102]*e[102] cu[17]=cu[18] e[17]-e[18]=e[102]+Ed_gasturbine CP_gasturbine=cu[102] CF_gasturbine=(cu[17]*e[17]-cu[18]*e[18])/(e[17]-e[18]) R_gasturbine=100*(CP_gasturbine-CF_gasturbine)/CF_gasturbine F_gasturbine=100*Z_gasturbine/(Z_gasturbine+CF_gasturbine*(Ed_gasturbine+El_gasturbine)) El_gasturbine=0 epsilon_D_gasturbine=Ed_gasturbine/e[6] epsilon_L_gasturbine=El_gasturbine/e[6] epsilon_gasturbine=epsilon_D_gasturbine+epsilon_L_gasturbine "Electric Generator" Z_electricgenerator=0,13 "[Euro/h]" cu[102]*e[102]+Z_electricgenerator=cu[202]*e[202] e[102]=e[202]+Ed_electricgenerator CP_electricgenerator=cu[202] CF_electricgenerator=cu[102] R_electricgenerator=100*(CP_electricgenerator-CF_electricgenerator)/CF_electricgenerator F_electricgenerator=100*Z_electricgenerator/(Z_electricgenerator+CF_electricgenerator*(Ed_electricgenerator+El_electricgenerator)) El_electricgenerator=0 epsilon_D_electricgenerator=Ed_electricgenerator/e[6] epsilon_L_electricgenerator=El_electricgenerator/e[6] epsilon_electricgenerator=epsilon_D_electricgenerator+epsilon_L_electricgenerator "Hybrid recuperator" Z_hybridrec=1,15 "[Euro/h]" cu[18]*e[18]-cu[19]*e[19]+Z_hybridrec=cu[3]*e[3]-cu[2]*e[2] cu[19]=0 e[18]-e[19]=e[3]-e[2]+Ed_hybridrec CP_hybridrec=(cu[3]*e[3]-cu[2]*e[2])/(e[3]-e[2]) CF_hybridrec=(cu[18]*e[18]-cu[19]*e[19])/(e[18]-e[19]) R_hybridrec=100*(CP_hybridrec-CF_hybridrec)/CF_hybridrec F_hybridrec=100*Z_hybridrec/(Z_hybridrec+CF_hybridrec*(Ed_hybridrec+El_hybridrec)) El_hybridrec=e[19] epsilon_D_hybridrec=Ed_hybridrec/e[6] epsilon_L_hybridrec=El_hybridrec/e[6] epsilon_hybridrec=epsilon_D_hybridrec+epsilon_L_hybridrec "Auxiliaries condictions" cu[101]=(cu[201]*e[201]+cu[202]*e[202])/(e[201]+e[202]) cu[101]=cu[103] "Exergy output" TOT=e[6]-(Ed_fuelpreheater+Ed_desulphuriser+Ed_reformerpreheater+Ed_compressor1+Ed_CPO+Ed_anodepreheater+Ed_SOFC+Ed_cathodepreheater+Ed_hybridrec+Ed_compressor2+Ed_burner+Ed_gasturbine+Ed_electricgenerator+e[22]+e[19])
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"Exergetic efficiency" eta_ex=TOT/e[6] "Price of electricty" Pel=(e[201]*cu[201]+e[202]*cu[202])/(e[201]+e[202]) "Euro/kWh" " Exergy [kW] flow for each node of the system" e[14]=12107,21 e[15]=11876,33 e[6]=40561,77 e[7]=40637,22 e[304]=0 e[8]=40499,77 e[22]=139,75 e[305]=0 e[13]=12451,42 e[9]=40737,03 e[306]=0 e[20]=1,41 e[21]=42,23 e[312]=0 e[103]=61,13 e[23]=0 e[10]=40249,77 e[307]=0 e[12]=12718,44 e[11]=40495,38 e[308]=0 e[4]=12241,41 e[5]=14315,05 e[201]=23340,61 e[309]=0 e[16]=11213,07 e[3]=9445,86 e[303]=0 e[17]=20260,84 e[310]=0 e[18]=13696,95 e[102]=6350,56 e[202]=6223,55 e[311]=0 e[19]=5874,15 e[2]=3417,26 e[302]=0 e[1]=100,71 e[301]=0 e[101]=4904,85 "Cost per hours calculation" C[1]=cu[1]*e[1] C[2]=cu[2]*e[2] C[3]=cu[3]*e[3] C[4]=cu[4]*e[4] C[5]=cu[5]*e[5] C[6]=cu[6]*e[6] C[7]=cu[7]*e[7] C[8]=cu[8]*e[8] C[9]=cu[9]*e[9] C[10]=cu[10]*e[10] C[11]=cu[11]*e[11] C[12]=cu[12]*e[12] C[13]=cu[13]*e[13] C[14]=cu[14]*e[14]
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C[15]=cu[15]*e[15] C[16]=cu[16]*e[16] C[17]=cu[17]*e[17] C[18]=cu[18]*e[18] C[19]=cu[19]*e[19] C[20]=cu[20]*e[20] C[21]=cu[21]*e[21] C[22]=cu[22]*e[22] C[101]=cu[101]*e[101] C[102]=cu[102]*e[102] C[103]=cu[103]*e[103] C[201]=cu[201]*e[201] C[202]=cu[202]*e[202]
c. SOFC Plant fed by Natural Gas Integrated with Gas Turbine, ASR unit
(Figure 6.1.2).
"THERMOECONOMIC ANALYSIS" "SOFC AND GAS TURBINE PLANT FED BY NATURAL GAS" "(ASR unit as Pre-Reforming)" "1. SOFC PLANT" "Fuel preheater" Z_fuelpreheater=0,13 "[Euro/h]" cu[14]*e[14]-cu[15]*e[15]+Z_fuelpreheater=cu[7]*e[7]-cu[6]*e[6] cu[14]=cu[15] "cu[6]=0,028[Euro/kWh]" "Natural Gas Italian Price" "cu[6]=0,024[Euro/kWh]" "Natural Gas Danish Price" cu[6]=0,026"[Euro/kWh]" "Mean Price" e[14]-e[15]=e[7]-e[6]+Ed_fuelpreheater CP_fuelpreheater=(cu[7]*e[7]-cu[6]*e[6])/(e[7]-e[6]) CF_fuelpreheater=(cu[14]*e[14]-cu[15]*e[15])/(e[14]-e[15]) R_fuelpreheater=100*(CP_fuelpreheater-CF_fuelpreheater)/CF_fuelpreheater F_fuelpreheater=100*Z_fuelpreheater/(Z_fuelpreheater+CF_fuelpreheater*(Ed_fuelpreheater+El_fuelpreheater)) El_fuelpreheater=0 epsilon_D_fuelpreheater=Ed_fuelpreheater/e[6] epsilon_L_fuelpreheater=El_fuelpreheater/e[6] epsilon_fuelpreheater=epsilon_D_fuelpreheater+epsilon_L_fuelpreheater "Desulphuriser" Z_desulphuriser=31,38 "[Euro/h]" cu[7]*e[7]+Z_desulphuriser=cu[8]*e[8]+cu[22]*e[22] cu[22]=0 e[7]+e[22]=e[8]+Ed_desulphuriser CP_desulphuriser=(cu[8]*e[8]+cu[22]*e[22])/(e[8]+e[22]) CF_desulphuriser=cu[7] R_desulphuriser=100*(CP_desulphuriser-CF_desulphuriser)/CF_desulphuriser F_desulphuriser=100*Z_desulphuriser/(Z_desulphuriser+CF_desulphuriser*(Ed_desulphuriser+El_desulphuriser)) El_desulphuriser=e[22] epsilon_D_desulphuriser=Ed_desulphuriser/e[6] epsilon_L_desulphuriser=El_desulphuriser/e[6] epsilon_desulphuriser=epsilon_D_desulphuriser+epsilon_L_desulphuriser "Reformer preheater" Z_reformerpreheater=0,14 "[Euro/h]" cu[13]*e[13]-cu[14]*e[14]+Z_reformerpreheater=cu[9]*e[9]-cu[8]*e[8] cu[13]=cu[14] e[13]-e[14]=e[9]-e[8]+Ed_reformerpreheater CP_reformerpreheater=(cu[9]*e[9]-cu[8]*e[8])/(e[9]-e[8]) CF_reformerpreheater=(cu[13]*e[13]-cu[14]*e[14])/(e[13]-e[14])
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R_reformerpreheater=100*(CP_reformerpreheater-CF_reformerpreheater)/CF_reformerpreheater F_reformerpreheater=100*Z_reformerpreheater/(Z_reformerpreheater+CF_reformerpreheater*(Ed_reformerpreheater+El_reformerpreheater)) El_reformerpreheater=0 epsilon_D_reformerpreheater=Ed_reformerpreheater/e[6] epsilon_L_reformerpreheater=El_reformerpreheater/e[6] epsilon_reformerpreheater=epsilon_D_reformerpreheater+epsilon_L_reformerpreheater "Splitter" Z_splitter=0 "[Euro/h]" cu[12]*e[12]+Z_splitter=cu[25]*e[25]+cu[26]*e[26] cu[25]=cu[26] e[12]=e[25]+e[26]+Ed_splitter CP_splitter=(cu[25]*e[25]+cu[26]*e[26])/(e[25]+e[26]) CF_splitter=cu[12] R_splitter=100*(CP_splitter-CF_splitter)/CF_splitter F_splitter=100*Z_splitter/(Z_splitter+CF_splitter*(Ed_splitter+El_splitter)) El_splitter=0 epsilon_D_splitter=Ed_splitter/e[6] epsilon_L_splitter=El_splitter/e[6] epsilon_splitter=epsilon_D_splitter+epsilon_L_splitter "Recycler" Z_recycler=0 "[Euro/h]" cu[103]*e[103]+Z_recycler=cu[21]*e[21]-cu[26]*e[26] e[103]=e[21]-e[26]+Ed_recycler CP_recycler=(cu[21]*e[21]-cu[26]*e[26])/(e[21]-e[26]) CF_recycler=cu[103] R_recycler=100*(CP_recycler-CF_recycler)/CF_recycler F_recycler=100*Z_recycler/(Z_recycler+CF_recycler*(Ed_recycler+El_recycler)) El_recycler=0 epsilon_D_recycler=Ed_recycler/e[6] epsilon_L_recycler=El_recycler/e[6] epsilon_recycler=epsilon_D_recycler+epsilon_L_recycler "Mixer" Z_mixer=0 "[Euro/h]" cu[20]*e[20]+cu[21]*e[21]+Z_mixer=cu[9]*e[9] e[20]+e[21]=e[9]+Ed_mixer CP_mixer=cu[9] CF_mixer=(cu[21]*e[21]+cu[20]*e[20])/(e[21]+e[20]) R_mixer=100*(CP_mixer-CF_mixer)/CF_mixer F_mixer=100*Z_mixer/(Z_mixer+CF_mixer*(Ed_mixer+El_mixer)) El_mixer=0 epsilon_D_mixer=Ed_mixer/e[6] epsilon_L_mixer=El_mixer/e[6] epsilon_mixer=epsilon_D_mixer+epsilon_L_mixer "ASR" Z_ASR=1,23 "[Euro/h]" cu[9]*e[9]+Z_ASR=cu[10]*e[10] e[9]=e[10]+Ed_ASR CP_ASR=cu[10] CF_ASR=cu[9] R_ASR=100*(CP_ASR-CF_ASR)/CF_ASR F_ASR=100*Z_ASR/(Z_ASR+CF_ASR*(Ed_ASR+El_ASR)) El_ASR=0 epsilon_D_ASR=Ed_ASR/e[6] epsilon_L_ASR=El_ASR/e[6] epsilon_ASR=epsilon_D_ASR+epsilon_L_ASR "Anode preheater" Z_anodepreheater=0,27 "[Euro/h]" cu[25]*e[25]-cu[13]*e[13]+Z_anodepreheater=cu[11]*e[11]-cu[10]*e[10] cu[25]=cu[13] e[25]-e[13]=e[11]-e[10]+Ed_anodepreheater CP_anodepreheater=(cu[11]*e[11]-cu[10]*e[10])/(e[11]-e[10]) CF_anodepreheater=(cu[25]*e[25]-cu[13]*e[13])/(e[25]-e[13])
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R_anodepreheater=100*(CP_anodepreheater-CF_anodepreheater)/CF_anodepreheater F_anodepreheater=100*Z_anodepreheater/(Z_anodepreheater+CF_anodepreheater*(Ed_anodepreheater+El_anodepreheater)) El_anodepreheater=0 epsilon_D_anodepreheater=Ed_anodepreheater/e[6] epsilon_L_anodepreheater=El_anodepreheater/e[6] epsilon_anodepreheater=epsilon_D_anodepreheater+epsilon_L_anodepreheater "SOFC" Z_SOFC=1914,40 "[Euro/h]" cu[11]*e[11]-cu[12]*e[12]+Z_SOFC=cu[201]*e[201]+(cu[5]*e[5]-cu[4]*e[4]) cu[11]=cu[12] (cu[5]*e[5]-cu[4]*e[4])/(e[5]-e[4])=cu[201] "cu[5]=0" "cu[12]=0" e[11]-e[12]=e[201]+e[5]-e[4]+Ed_SOFC CP_SOFC=((cu[5]-cu[4])*(e[5]-e[4])+cu[201]*e[201])/(e[201]+(e[5]-e[4])) CF_SOFC=(cu[12]*e[12]-cu[11]*e[11])/(e[12]-e[11]) R_SOFC=100*(CP_SOFC-CF_SOFC)/CF_SOFC F_SOFC=100*Z_SOFC/(Z_SOFC+CF_SOFC*(Ed_SOFC+El_SOFC)) El_SOFC=0 epsilon_D_SOFC=Ed_SOFC/e[6] epsilon_L_SOFC=El_SOFC/e[6] epsilon_SOFC=epsilon_D_SOFC+epsilon_L_SOFC "Cathode preheater" Z_cathodepreheater=1,24 "[Euro/h]" cu[5]*e[5]-cu[16]*e[16]+Z_cathodepreheater=cu[4]*e[4]-cu[3]*e[3] cu[5]=cu[16] e[5]-e[16]=e[4]-e[3]+Ed_cathodepreheater CP_cathodepreheater=(cu[4]*e[4]-cu[3]*e[3])/(e[4]-e[3]) CF_cathodepreheater=(cu[5]*e[5]-cu[16]*e[16])/(e[5]-e[16]) R_cathodepreheater=100*(CP_cathodepreheater-CF_cathodepreheater)/CF_cathodepreheater F_cathodepreheater=100*Z_cathodepreheater/(Z_cathodepreheater+CF_cathodepreheater*(Ed_cathodepreheater+El_cathodepreheater)) El_cathodepreheater=0 epsilon_D_cathodepreheater=Ed_cathodepreheater/e[6] epsilon_L_cathodepreheater=El_cathodepreheater/e[6] epsilon_cathodepreheater=epsilon_D_cathodepreheater+epsilon_L_cathodepreheater "compressor 2" Z_compressor2=0,45 "[Euro/h]" cu[101]*e[101]+Z_compressor2=cu[2]*e[2]-cu[1]*e[1] cu[1]=0 e[101]=e[2]-e[1]+Ed_compressor2 CP_compressor2=(cu[2]*e[2]-cu[1]*e[1])/(e[2]-e[1]) CF_compressor2=cu[101] R_compressor2=100*(CP_compressor2-CF_compressor2)/CF_compressor2 F_compressor2=100*Z_compressor2/(Z_compressor2+CF_compressor2*(Ed_compressor2+El_compressor2)) El_compressor2=0 epsilon_D_compressor2=Ed_compressor2/e[6] epsilon_L_compressor2=El_compressor2/e[6] epsilon_compressor2=epsilon_D_compressor2+epsilon_L_compressor2 "Burner" Z_burner=7,53 "[Euro/h]" cu[16]*e[16]+cu[15]*e[15]+Z_burner=cu[17]*e[17] e[16]+e[15]=e[17]+Ed_burner CP_burner=cu[17] CF_burner=(cu[16]*e[16]+cu[15]*e[15])/(e[16]+e[15]) R_burner=100*(CP_burner-CF_burner)/CF_burner F_burner=100*Z_burner/(Z_burner+CF_burner*(Ed_burner+El_burner)) El_burner=0 epsilon_D_burner=Ed_burner/e[6] epsilon_L_burner=El_burner/e[6] epsilon_burner=epsilon_D_burner+epsilon_L_burner
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"2. GAS TURBINE AND HYBRID RECUPERATOR" "Gas Turbine" Z_gasturbine=79,43 "[Euro/h]" cu[17]*e[17]-cu[18]*e[18]+Z_gasturbine=cu[102]*e[102] cu[17]=cu[18] e[17]-e[18]=e[102]+Ed_gasturbine CP_gasturbine=cu[102] CF_gasturbine=(cu[17]*e[17]-cu[18]*e[18])/(e[17]-e[18]) R_gasturbine=100*(CP_gasturbine-CF_gasturbine)/CF_gasturbine F_gasturbine=100*Z_gasturbine/(Z_gasturbine+CF_gasturbine*(Ed_gasturbine+El_gasturbine)) El_gasturbine=0 epsilon_D_gasturbine=Ed_gasturbine/e[6] epsilon_L_gasturbine=El_gasturbine/e[6] epsilon_gasturbine=epsilon_D_gasturbine+epsilon_L_gasturbine "Electric Generator" Z_electricgenerator=0,13 "[Euro/h]" cu[103]*e[102]+Z_electricgenerator=cu[202]*e[202] e[102]=e[202]+Ed_electricgenerator CP_electricgenerator=cu[202] CF_electricgenerator=cu[102] R_electricgenerator=100*(CP_electricgenerator-CF_electricgenerator)/CF_electricgenerator F_electricgenerator=100*Z_electricgenerator/(Z_electricgenerator+CF_electricgenerator*(Ed_electricgenerator+El_electricgenerator)) El_electricgenerator=0 epsilon_D_electricgenerator=Ed_electricgenerator/e[6] epsilon_L_electricgenerator=El_electricgenerator/e[6] epsilon_electricgenerator=epsilon_D_electricgenerator+epsilon_L_electricgenerator "Hybrid recuperator" Z_hybridrec=1,15 "[Euro/h]" cu[18]*e[18]-cu[19]*e[19]+Z_hybridrec=cu[3]*e[3]-cu[2]*e[2] cu[19]=0 e[18]-e[19]=e[3]-e[2]+Ed_hybridrec CP_hybridrec=(cu[3]*e[3]-cu[2]*e[2])/(e[3]-e[2]) CF_hybridrec=(cu[18]*e[18]-cu[19]*e[19])/(e[18]-e[19]) R_hybridrec=100*(CP_hybridrec-CF_hybridrec)/CF_hybridrec F_hybridrec=100*Z_hybridrec/(Z_hybridrec+CF_hybridrec*(Ed_hybridrec+El_hybridrec)) El_hybridrec=e[19] epsilon_D_hybridrec=Ed_hybridrec/e[6] epsilon_L_hybridrec=El_hybridrec/e[6] epsilon_hybridrec=epsilon_D_hybridrec+epsilon_L_hybridrec "Auxiliaries condictions" cu[101]=(cu[201]*e[201]+cu[202]*e[202])/(e[201]+e[202]) cu[101]=cu[103] "Exergy output" TOT=e[6]-(Ed_fuelpreheater+Ed_desulphuriser+Ed_reformerpreheater+Ed_mixer+Ed_splitter+Ed_recycler+Ed_ASR+Ed_anodepreheater+Ed_SOFC+Ed_cathodepreheater+Ed_hybridrec+Ed_compressor2+Ed_burner+Ed_gasturbine+Ed_electricgenerator+e[22]+e[19]) "Exergetic efficiency" eta_ex=TOT/e[6] "Price of electricty" Pel=(e[201]*cu[201]+e[202]*cu[202])/(e[201]+e[202]) "Euro/kWh"
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" Exergy [kW] flow for each node of the system" e[14]=11062,22 e[15]=10853,72 e[6]=38175,78 e[7]=38246,80 e[304]=0 e[8]=38117,43 e[22]=131,53 e[305]=0 e[13]=11376,18 e[20]=38340,74 e[306]=0 e[26]=771,91 e[21]=773,57 e[311]=0 e[103]=1,97 e[9]=39050,27 e[24]=0 e[23]=0 e[10]=38971,97 e[307]=0 e[25]=11839,43 e[11]=39391,39 e[308]=0 e[4]=10665,01 e[5]=12354,13 e[201]=22962,25 e[309]=0 e[12]=12611,34 e[16]=9678,98 e[3]=8253,65 e[303]=0 e[17]=17815,96 e[310]=0 e[18]=12116,03 e[102]=5515,98 e[202]=5405,66 e[312]=0 e[19]=5215,23 e[2]=2977,20 e[302]=0 e[1]=87,74 e[301]=0 e[101]=4273,22 "Cost per hours calculation" C[1]=cu[1]*e[1] C[2]=cu[2]*e[2] C[3]=cu[3]*e[3] C[4]=cu[4]*e[4] C[5]=cu[5]*e[5] C[6]=cu[6]*e[6] C[7]=cu[7]*e[7] C[8]=cu[8]*e[8] C[9]=cu[9]*e[9] C[10]=cu[10]*e[10] C[11]=cu[11]*e[11] C[12]=cu[12]*e[12] C[13]=cu[13]*e[13] C[14]=cu[14]*e[14] C[15]=cu[15]*e[15] C[16]=cu[16]*e[16] C[17]=cu[17]*e[17] C[18]=cu[18]*e[18]
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C[19]=cu[19]*e[19] C[20]=cu[20]*e[20] C[21]=cu[21]*e[21] C[22]=cu[22]*e[22] C[101]=cu[101]*e[101] C[102]=cu[102]*e[102] C[103]=cu[103]*e[103] C[201]=cu[201]*e[201] C[202]=cu[202]*e[202]
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