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DOWNDRAFT GASIFICATION OF BIOMASS
EXPERIMENTAL INVESTIGATION AND ASPEN PLUS SIMULATION
By Antonio Oliveira Dr. John Brammer (Supervisor)
1
THESIS OBJECTIVES
Measure temperature and gas profiles in axial and longitudinal direcFons in a conFnuous fixed bed reactor fed with charcoal;
Modify a commercially available throated biomass gasifier to measure axial and longitudinal temperature in the reducFon zone;
Develop a gas sampling line according to the orientaFon of European tar protocol;
Apply restricted equilibrium (temperature approach) correcFons to Aspen Plus gasifier models to improve results accuracy.
2
Study the biomass gasificaFon process and its behaviour under changes of operaFonal parameter and feedstock, with focus on the reducFon zone. As well as developing a Aspen Plus model based on thermodynamic equilibrium able to predict producer gas concentraFon.
INTRODUCTION
Biomass Energy
environmental polluFon
energy security
depleFon of fossil
climate change
3
BIOMASS
“plant material, vegetaFon, or agricultural waste used as a fuel or energy source”
4
BIOMASS UTILIZATION
Raw Material Process Intermediate Product Final Product
Vegetal Oil
Sugar & Starch
Lignocellulosics
Wet Biomass
Hydrolysis - Fermentation -
Destilation
Pyrolysis - Hydrogenation
Fisher - Tropsh
Gasification
Biogas
Pellets
Producer Gas
HydrocarbonsBio-oil
EthanolETBE
Biodiesel
Pelletization
Anaerobic Digestion
Transesterification
Transport BiofuelsChemicals
Electricity
Heating
Combustion
5
GASIFICATION
BIOMASS GASIFICATION
“thermochemical process in which parFal oxidaFon of organic maYer at high temperatures results in a mixture of products, but mainly consisFng of a gaseous fuel that can be uFlized for energy applicaFons”
6
TYPES OF GASIFICATION
AIR GASIFICATION
Oxygen gasificaFon
HydrogasificaFon
PyrolyFc gasificaFon
Near-‐ and super-‐criFcal water
7
GASIFICATION THERMODYNAMICS
8
DRYING
wet biomass
biomass
PYROLYSIS
pyrolysis gas
charcoal
COMBUSTION
C+O2→ CO2 4H+O2→ 2H2O
CnHm+(n/2+m/4)O2→ nCO2 + m/2H2O
REDUCTION
C+CO2↔2CO
C+H2O↔CO+H2
CnHm+nH2O↔nCO+(m/2+n)H2
CnHm+nCO2↔2nCO+m/2H2
H2O
Tat CH4
PRO
DU
CER
GA
S
CO2 H2O
CO H2
HEAT
H2O dry biomass
TYPES OF GASIFIER
According to the reactor design, there are 4 different types of gasificaFon.
FIXED BED
Fluidized bed
Entrained flow
Twin-‐bed 9
FIXED BED GASIFIERS
DowndraE gasifier Co-‐current flow design; thus, both the biomass and the air and producer gas follow a downward movement
10
FIXED BED GASIFIERS
Two-‐stage Gasifier EssenFally a downdra` gasifier. However, the pyrolysis and char reducFon zones have been separated into two reactors by an intermediate high temperature oxidaFon zone.
11
SIMULATION OF GASIFICATION PROCESSES
“EssenFally, all models are wrong, but some models are useful”
(Box & Draper 1987)
Determining opFmal operaFng condiFons
CreaFng the most appropriate reactor design
Studying a wider range of condiFons that cannot be obtained experimentally
Understanding experimental results and analysing improper performance of a gasifier
Choosing an appropriate feedstock and evaluaFng its yield
Scaling-‐up a reactor
12
SIMULATION OF GASIFICATION PROCESSES
GASIFICATION MODELS
CFD Thermo.
equilibrium
kinecFcs based
ASPEN PLUS
neural network
13
ASPEN PLUS
14
PREVIOUS WORK
KINECTIC AND CFD MODELS
THERMODYNAMIC EQUILIBRIUM
MODELS
EXPERIMENTAL
15
GASIFICATION EXPERIMENTS
Experimental study on 75 kWth downdraE (biomass) gasifier system (Sharma 2009)
• Fed with woodchips • Longitudinal temperature • Longitudinal pressure • Outlet gas composiFon
16
GASIFICATION EXPERIMENTS
Experimental invesZgaZon of a downdraE biomass gasifier (Zainal et al. 2002)
• Fed with wood furniture chunks
• Several equivalent raFo • Longitudinal temperature • Outlet gas composiFon
17
GASIFICATION EXPERIMENTS
GasificaZon of charcoal wood chips: Isolated parZcle and fixed bed (Tagutchou 2008)
• Emulates a 2-‐stage gasifier • Fed with charcoal from
woodchips • Several equivalent raFo • Longitudinal temperature
and pressure and gas profile
18
THERMODYNAMIC EQUILIBRIUM MODELS
Thermochemical equilibrium modelling of a gasifying process (Melgar et al. 2007)
19
Uses the approach equilibrium constant together with thermodynamic equilibrium of the global reacFon. The temperature of reacFon is the adiabaFc flame temperature. The system was solved in EES.
THERMODYNAMIC EQUILIBRIUM MODELS
Performance analysis of a biomass gasifier (Mathieu & Dubuisson 2002)
Modelled wood gasificaFon in a fluidized bed using Aspen Plus/minimizaFon of the Gibbs free energy.
20
THIS WORK
Char gasificaZon in a conZnuous fixed bed reactor -‐ CFiBR GasificaZon in a 25kW Throated fixed bed biomass gasifier
Modelling work – Aspen Plus
21
CHAR GASIFICATION IN A CONTINUOUS FIXED BED REACTOR -‐ CFIBR
22
EXPERIMENTAL APPARATUS
23
!M
!M
!V
mass!flowmeter/controller
volume!flowmeter/controller
thermocouple!/pressure!sensor!andgas!sampling!probe
!V
!M
C3H8
Air
H2O
!2
!12
!!11
!!10
!9
!8
!7
!6
!5
!4
!3
!1
!i
a
b
c
d
e
f
g
200mm
1600!m
m
100!mm
Flare
The CFiBR was designed and manufactured by CIRAD. It is essenFally of a refractory stainless steel tube of, surrounded by refractory insulaFon. At the top of the reactor, there is a conveyor belt (a) that enables the feeding of charcoal to the top of the reactor. A system of two pneumaFc valves (b) ensures that no air can enter the reactor when the char is introduced. The combusFon (c) chamber provides the reacFve atmosphere.
REACTIVE ATMOSPHERE
CombusZon chamber Steam generator
• The steam generator is designed to provide up to 6 kg/h of steam at a temperature of up to 1050 °C. It consists of a furnace and a heat exchanger equipped with a control system.
24
900#mm
500#mm
refractory#concrete#burner#cover
refractory#concrete#disk
burner
ceramic#insulator
200#mmReactor#centre
CONTINUOUS FIXED BED OPERATION
Charcoal feeding systems Ash and residues removal system
25
12#cm
!
11#cm
10#cm
Closed Open
PRODUCTION AND CHARACTERIZATION OF THE BIOMASS USED
Charcoal from woodchips Granulometric analysis and parZcles size distribuZon
26
20#mm 20#mm
(A) (B)
180 2 4 6 8 10 12 14 16
100
0
20
40
60
80
dp)(mm)
mass)(%)
differen5al
cumula5ve
INSTRUMENTATION, MEASUREMENTS AND CALCULATIONS
Temperature • Fixed
– CombusFon chamber (T1); – Outlet of the steam generator
(T2); – 10 cm above the charcoal bed
(T3); – Below the ash removal (T11); – Outlet of the cyclone (T12).
• Movable – These thermocouples (T4 to T10)
Pressure
Two pressure sensors (0-‐500 mbar) are placed before and a`er the char bed, in order to measure pressure drop across the bed. The pressure can also be measured everywhere in the bed via the thermocouple probes.
27
INSTRUMENTATION, MEASUREMENTS AND CALCULATIONS
Gas composiZon
28 GC
Reactor*wall
Reactor*interior
Filter*and*dryer
Gas
Temperature*readings
Flow*control/*measurement*(4)
Filter*(2)
Condenser*(3)
Sampling*probe*(1)
MASS AND ENERGY BALANCES
Mass Energy
29
The inlet reagents are charcoal and the reacFve atmosphere gases are composed of O2, N2, CO2, H2O. The outlet products are the producer gas (H2, CO, CH4, H2O, CO2 and N2) in addiFon to solid residues removed from the boYom.
There is no mechanical work being produced by the system and kineFc and potenFal energy are negligible
CHAPTER 5 – CHAR GASIFICATION ON A CONTINUOUS FIXED BED REACTOR - CFIBR
104
5.3.5 Mass and energy balances
The energy balance is calculated according to the principle of conservation of
energy (1st Law of thermodynamics) as expressed in Eq. 5.10.
0 = �̇� + �̇� + �̇� ℎ +
𝑢2 + 𝑔𝑧
− �̇� ℎ +𝑢2 + 𝑔𝑧
5.10
where Qlost represents the heat lost by the system, Wcv is the variation of the
mechanical work and m, h, u2/2 and gz are respectively the mass flow, enthalpy,
kinetic and potential energy in and out of the control volume (cv).
As there is no mechanical work being produced by the system and kinetic and
potential energy are negligible, Eq. 5.10 can be reduced to
0 = �̇� + �̇� ℎ − �̇� ℎ 5.11
The mass balance is given by the difference between inlet reagents and outlet
products (producer gas and residues). It can be mathematically expressed by Eq. 5.12.
0 = �̇� − �̇� 5.12
The inlet reagents are charcoal and the reactive atmosphere gases are
composed of O2, N2, CO2, H2O. The outlet products are the producer gas (H2, CO, CH4,
H2O, CO2 and N2) in addition to solid residues removed from the bottom.
Therefore,
ℎ �̇� = �̇� ℎ + �̇� ℎ 5.13
and,
ℎ �̇� = �̇� ℎ + �̇� ℎ 5.14
CHAPTER 5 – CHAR GASIFICATION ON A CONTINUOUS FIXED BED REACTOR - CFIBR
105
where h and m are respectively the specific enthalpy and mass flow of each
gaseous compound going in (i) or out (j) of the reactor. The specific enthalpy can be
calculated as:
ℎ , (𝑇) = ℎ , (𝑇) + 𝐶 ( , )(𝑇)𝑑𝑇 5.15
where ℎ , is the standard enthalpy of formation of the component i,j, Cp(j) is
the specific heat and T is the medium temperature.
Combining Eq. 5.11 and Eq. 5.14, the final equation for calculating the energy
balance is:
�̇� ℎ + �̇� ℎ −�̇� ℎ − �̇� ℎ − �̇� = 0 5.16
The heat loss is calculated according to Eq. 5.17
�̇� = ℎ 𝐴𝑑𝑇 5.17
where hc is the convective heat transfer coefficient of the process, A is heat
transfer area of the surface and dT is the temperature difference between the surface
and the ambient.
5.4 Operational parameters
Three experiments were performed. Experiments A and B were performed with
the same conditions. They were intended to provide two set of gas measurements, one
in the wall and another in the centre of the reactor, thus they would allow the study of
the gas variation in the radial direction. Experiment C was performed with a different
atmosphere.
Airflow was constant for all experiments while propane and water vapour flow
were changed.
OPERATIONAL PARAMETERS CHAPTER 5 – CHAR GASIFICATION ON A CONTINUOUS FIXED BED REACTOR - CFIBR
108
Table 5.5: Operating conditions of the CFiBR gasification experiments
Experiment A and B Experiment C
Reactants (inlet conditions) Char feeding rate (mC) 2.1 (mol/min) 25 (g/min) 2.1 (mol/min) 25 (g/min) Qair
10 8.031 (mol/min) 235.50 (g/min) 8.103 (mol/min) 237.61 (g/min) QN2 6.494 (mol/min) 181.93 (g/min) 6.553 (mol/min) 183.57 (g/min) QO2
11 1.674 (mol/min) 53.57 (g/min) 1.689 (mol/min) 54.05 (g/min) QC3H8 0.286 (mol/min) 12.59 (g/min) 0.303 (mol/min) 13.35 (g/min) QH2O (added water vapour) 0.67 (mol/min) 12.20 (g/min) 1.02 (mol/min) 18.41 (g/min) Products (attack gases) QO2 235.50 (mol/min) 7.883 (g/min) 0.18 (mol/min) 5.61 (g/min) QCO2 181.93 (mol/min) 37.699 (g/min) 0.91 (mol/min) 39.98 (g/min) QH2O 53.57 (mol/min) 33.238 (g/min) 2.30 (mol/min) 40.73 (g/min) QN2 12.59 (mol/min) 181.928 (g/min) 6.55 (mol/min) 183.57 Total Flux of attack gases 235.50 (mol/min) 260.748 (g/min) 9.94 (mol/min) 269.89 (g/min) Properties (attack gases) Superficial Velocity 0.55 (m/s) 0.57 (m/s) Products Temperature 1060 °C 1080 °C Total Pressure 1.01 atm 1.01 atm
10 Q stands for gas flow. 11 Oxygen is provided in excess of 2.70% in Experiment A/B and 1.78% in Experiment C
30
DATA COLLLECTION AND PROCESSING
Temperature and pressure every 10 seconds
Gases are sampled and the condensates are stored
Charcoal bed is cooled in an inert atmosphere to avoid further chemical reacFon
31
ESTABLISHMENT OF STEADY STATE
From start to steady state Bed level
32
!!!0 1 2 3 4 5 6 7 8 9 10 11
1100
0
100
200
300
400
500
600
700
800
900
1000
Time!(h)
!Tem
perature!(°C)
T2
T3
T5:T10
T11
T12
T4
Hea<ng Steady!state
Cooling!down
12
Bed!level!Thermal!stabilisa<onstabilisa<on !!!0 10 20 30 40 50
1010
750
800
850
900
950
Time!(min)
!Tem
perature!(°C)
T3
T4
T6
T8
T10
60
EXPERIMENTAL RESULTS
Over 100 hours of gasificaFon
Every experiment could only last a maximum of 13h
The results are analysed to provide: mass and energy balances, profiles of temperature, pressure, mole concentraFon and conversion, both in transient and steady states.
33
Temperature
34 !!!0 1 2 3 4 5 6 7 8 9 10 11
1100
0
100
200
300
400
500
600
700
800
900
1000
Time!(h)
!Tem
perature!(°C)
T2
T3
T5:T10
T11
T12
T4
Hea<ng Steady!state
Cooling!down
12
Bed!level!Thermal!stabilisa<onstabilisa<on
REACHING STEADY STATE
STEADY STATE: VARIATION OF PROPERTIES ACROSS THE REACTOR
Temperature Region 1: Above bed level a decrease of temperature is observed due to convecFve heat loss to the wall only.
Region 2: Between T4 and T6, reacFve atmosphere reaches the charcoal bed and the temperature drops rapidly due to the endothermic reacFons, heaFng up and drying of the charcoal.
Region 3: Under T6, the temperature decrease is less pronounced and the longitudinal gradient reduces. The radial gradient becomes stable.
35
STEADY STATE: VARIATION OF PROPERTIES ACROSS THE REACTOR
Gas composiZon profiles
36
200 5 10 15
Inlet
Outlet
T9
T8
T7
T6
T5
T4
T3
333
Concentra9on3(%)
Prob
e3loca9o
ns
CO
H2
O2 CO2 H20
CH43x310
430 5 10 15 20 25 30 35
Inlet
Outlet
T9
T8
T7
T6
T5
T4
T3
333
Species3mass3Flow3(g/min)
Prob
e3locaEo
ns
O2 H20
H2
CO2
CH43x310
CO
Longitudinal profiles of concentraFon and species mass flow in Experiment A.
STEADY STATE: VARIATION OF PROPERTIES ACROSS THE REACTOR
CHAPTER 5 – CHAR GASIFICATION ON A CONTINUOUS FIXED BED REACTOR - CFIBR
122
Table 5.6: Comparison of concentration on the radial and longitudinal profile of Experiment A/B.
H2 (%) CO (%) CH4 (%)
Centre Wall Diff Centre Wall Diff Centre
Wall Diff
T4 9.47 X
11.05 X
0.93 X
T5 10.91 11.04 -0.13 11.25 11.63 -0.38 0.95 0.97 -0.02 T6 12.97 13.40 -0.43 11.02 12.10 -1.08 0.98 1.00 -0.02 T7 13.30 12.37 0.92 11.65 11.22 0.42 1.94 1.92 0.02 T8 13.23 12.95 0.27 11.77 11.03 0.73 1.95 1.92 0.03 T9 13.38 13.20 0.18 12.10 11.37 0.72 1.98 1.93 0.05 Outlet 13.94 X
11.34 X
1.00 X
Figure 5.18: Comparison of concentration on the radial and longitudinal profile of Experiment A/B. Lines represent samples in the centre of the reactor and larger
symbols represent samples extracted by the wall.
5.6.2.3 Charcoal conversion and bed bulk density
For experiment A/B, It can be seen in Figure 5.19 that in the first 10 cm of the
bed, coal has a conversion rate of about 85%. This conversion rate then slowly evolves
37
Gas composiFon profiles: Comparison of concentraFon on the radial and longitudinal profile of Experiment A/B.
MASS AND ENERGY BALANCES
Experiment A/B Experiment C
38
7.4$g/min
276.6$g/min
Gasifica'onReac'ons
6.3631$kWTin$=$1060$°C
28$g/min
260.6$g/min
6.252.30$kWTout$=$770$°C
$0.533$kW
Char Gas Enthalpy$flux Heat$lost
9.9#g/min
283.3#g/min
Gasifica'onReac'ons
7.8556#kWTin#=#1080#°C
28#g/min
269.1#g/min
7.1887#kWTout#=#760#°C
#0.533#kW
Char Gas Enthalpy#flux Heat#lost
Mass balance error of 1.6% and an energy balance error of 6%.
Mass balance error of 1.3% and an energy balance error of 1.9%.
MAIN ACHIEVEMENTS
commissioning of the CFiBR
Temperature profiles
Irrelevant variaFon of PG concentraFon in the radial direcFon Existence of 3 disFnct regions of temperature and gas concentraFon
39
!M
!M
!V
mass!flowmeter/controller
volume!flowmeter/controller
thermocouple!/pressure!sensor!andgas!sampling!probe
!V
!M
C3H8
Air
H2O
!2
!12
!!11
!!10
!9
!8
!7
!6
!5
!4
!3
!1
!i
a
b
c
d
e
f
g
200mm
1600!m
m
100!mm
Flare
GASIFICATION IN A 25KW THROATED FIXED BED BIOMASS GASIFIER
40
EXPERIMENTAL APPARATUS
41
This is the GEK Gasifier Experimenters Kit
Reactor
Cyclone
PyroCoil
Auger
Drying2Bucket
Filter
Flare Hopper
GEK
Reactor
• Imbert type reactor • 60-‐75 kWth (20-‐25kWe) • 20-‐25 kg/h of lignocellulosic
biomass
42
Hopper&Mount&Flange
Air&Inlet
Gas&Exit
Gas&Cowling
Insula8on&Tube
Nozzles5@&0.6&ID&caps
17.8
7.6
10.2
10.2
45.7
19.0
Rotary&Crank/Drive
37.55
15.2
28
Rotary&Support&Grate
INSTRUMENTATION AND MEASUREMENTS
Temperature
• Three k-‐type thermocouples • 16 temperature points • Covers the reducFon zone • Error is less than 1%
43
Thermocouples Move.ver/cally.
INSTRUMENTATION AND MEASUREMENTS
Pressure
Fixed pressure measuring points are located at the boYom of the reactor and a`er the filter.
44
Reactor
Cyclone
PyroCoil
Auger
Drying2Bucket
Filter
Flare Hopper
INSTRUMENTATION AND MEASUREMENTS
Gas composiZon
45
GC
Massflowmeter
Vacuum0Pump
Condenser0(2)
Sampling0tube
0(3)
Flow0control/0measurement0(4)
Vent
Probe0and0Filtre0(1)
Gas is sampled and analysed every 30min following the European Tar Protocol.
INSTRUMENTATION AND MEASUREMENTS
Data collecZon Quantity Items 1 Atmel ATmega 1280 processor 16 K-type thermocouple inputs 6 Differential or gauge pressure/vacuum inputs 8 PWM FET outputs 4 Auxiliary analogue inputs 1 Frequency counter input 3 R/C hobby servo outputs 1 Display and four button keypad 1 USB serial host interface 1 SD-card slot 1 CANbus interface 1 Auxiliary RS-232 interface
!
46
PRODUCTION AND CHARACTERIZATION OF THE BIOMASS USED
47
EXPERIMENTAL PROCEDURES AND PARAMETERS
Commissioning
• Cold and hot trials were performed
• Check for leakages • Physical limits • OperaFonal parameters • Findings:
– Load with charcoal – Maximum temperature
supported by TC and reactor – Control pressure drop – Setup of the grid
48
EXPERIMENTAL PROCEDURES AND PARAMETERS
OperaZonal parameters
• Three types of pellets comprising mixed wood, Miscanthus and wheat straw
• 11 experiments • Air inlet varies
49
GASIFICATION RUNS
Run Feedstock Airflow (kg/h)
1 100% mixed wood pellets 8.10
2a 75% mixed wood pellets and 25% Miscanthus pellets
10.7
2b 12.8
3a 50% mixed wood pellets and 50% Miscanthus pellets
10.7
3b 12.8
4a 25% mixed wood pellets and 75% Miscanthus pellets
10.7
4b 12.8
5a 100% wheat straw pellets
10.7
5b 12.8
6a 50% mixed wood pellets and 50% wheat straw pellets
10.7
6b 12.8
!
50
RUN 1: 100% MIXED WOOD PELLETS
Mass balance
Mass balance Run 1
Air flow (kg/h) 8.1
Pellets flow (kg/h) 4.6
Flow of unreacted material (kg/h) 0.14
Gas outlet flow (kg/h) 12.2
Tar (g/Nm3) 1.5
ER – equivalence raFo 0.33
Closure 97.4%
Temperature profile
51
RUN 2: 75% MIXED WOOD AND 25% MISCANTHUS
Mass balance
Mass balance Run 2a Run 2b
Air flow (kg/h) 10.7 12.8
Pellets flow (kg/h) 7.4 7.66
Flow of unreacted material (kg/h) 0.22 0.38
Gas outlet flow (kg/h) 17.2 19.2
Tar (g/Nm3) 1.30 1.10
ER – equivalence raFo 0.27 0.31
Closure 96.5% 95.7%
Producer gas concentraZon
52
Species Run 2a (vol %) Run 2b (vol %)
CO 22.3 21
CO2 8.4 7.7
CH4 1.7 1.8
H2 24.4 19
H2O 8.3 11.4
N2 34.9 39.1
Higher the ER, lower the HHV
MAIN ACHIEVEMENTS
commissioning of the GEK
gas sampling line
Temperature profiles and beYer understanding of the behaviour in the reducFon zone
53 Reactor
Cyclone
PyroCoil
Auger
Drying2Bucket
Filter
Flare Hopper
SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
54
The model developed to simulate the CFiBR is based on Gibbs free energy minimizaFon (RGIBBS block in ASPEN). Restricted equilibrium parameters were used to calibrate the results against experimental.
55
PRINCIPLES OF RGIBBS AND GASIFICATION MODELLING
Calculate phase equilibrium and chemical equilibrium;
Restricted chemical equilibrium – specify temperature approach (or duty and temperature) of enFre system;
Restricted chemical equilibrium – specify temperature approach or molar extent for specified reacFon stoichiometry
Non-‐stoichiometric methods do not require reacFons to be specified, while stoichiometric methods require the specificaFon of the reacFons.
56
PRINCIPLES OF RGIBBS AND GASIFICATION MODELLING
Non-‐stoichiometric equilibrium method (min. of the Gibbs)
Applies minimizaFon of the Gibbs free energy to model the equilibrium of a reacFng system
NO reacFons needed
Restricted equilibrium *Temperature approach *Heat duty
Stoichiometric method (reacZons enabled)
Based on equilibrium constant method. Mimics kineFc-‐controlled behaviour.
Needs chemical reacFons
Restricted equilibrium *ReacFons Tapp *Heat duty
57
ASPEN PLUS GASIFICATION MODEL
58
ASPEN PLUS GASIFICATION MODEL
59
Yield reactor – converts the non-‐convenFonal stream BIOMASS into convenFonal components (C, H, O, N and ash)
ASPEN PLUS GASIFICATION MODEL
60
Separator – extracts a porFon of the carbon on the feedstock to represent un-‐
reacted charcoal removed from the boYom of the reactor
ASPEN PLUS GASIFICATION MODEL
61
Gibbs free energy reactor – calculates the equilibrium composiFon of the
combusFon and gasificaFon products
ASPEN PLUS GASIFICATION MODEL
Non-‐stoichiometric equilibrium method (minimizaFon of the Gibbs free energy);
Non-‐stoichiometric restricted equilibrium method with system temperature approach;
Stoichiometric restricted chemical equilibrium method with reacFon-‐specific temperature approach.
Three soluZons methods are used to simulate the gasifier, each involving only a change to the block GASIFIER
62
SIMULATION INITIAL PROPERTIES
63
Experiment A and B Experiment C
Reactants flow (g/min) Char feeding rate 25 25 Air 235.50 237.61 Propane 12.59 13.35 Added water vapour 12.20 18.41 Unreacted carbon removed via UC 7.4 8.8 Block temperature (°C) PROP-AIR 25 25 STEAM 1000 1000 BIOMASS 25 25 GAS-ATM 1060 1080 GASIFIER 870 870 Total Pressure (atm) 1.01 1.01
!
NON-‐STOICHIOMETRIC EQUILIBRIUM METHOD
WITHOUT temperature approach: Gasifier temperature is the equilibrium temperature.
ASPEN Experiment Difference O2 6.75E-18 0.00% 0.00 N2 58.67% 60.67% 2.00 H2O 7.53% 6.35% -1.18 H2 11.76% 13.52% 1.76 CO 14.28% 10.99% -3.29 CH4 3.66E-06 0.10% 0.10 CO2 7.76% 8.37% 0.60 Total Mole 100.00% 100.00%
!
64
NON-‐STOICHIOMETRIC EQUILIBRIUM METHOD
WITH temperature approach: Gasifier temperature is the equilibrium temperature.
65
500#510 #400 #300 #200 #100 0 100 200 300 400
0.2
0
0.04
0.08
0.12
0.16
Tapp.(K)
Concentra9on
CO
H2
CO2
H2O
CH4
#170.K
ASPEN Experiment Difference O2 3.00E-22 0.00% 0.00 N2 58.74% 60.67% 1.93 H2O 6.04% 6.35% 0.31 H2 13.19% 13.52% 0.33 CO 12.63% 10.99% -1.64 CH4 3.99E-04 0.10% 0.06 CO2 9.36% 8.37% -0.99 Total Mole 100.00% 100.0%
!
NON-‐STOICHIOMETRIC EQUILIBRIUM METHOD
WITH temperature approach: Gasifier temperature is the equilibrium temperature.
66
REACTIONS ENABLED -‐ STOICHIOMETRIC METHOD
System of equaZons ReacZons
67
The use of the stoichiometric method requires the specificaFon of the reacFons, such that the number of products is equal to the sum of the number of reacFons and elements.
(H2, CO, CO2, CH4, H2O, O2, N2 and C ) = 8 products. 3 elements (C, H, O) + 5 reacFons
CHAPTER 7 – SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
176
reactions and elements. Furthermore, the equations must be linearly independent
(Schefflan 2011).
Based on that, the following reactions (Eq. 7.17 to Eq.7.21) are used to
calculate the products (H2, CO, CO2, CH4, H2O, O2, N2 and C) that are formed by the
elements C, H, O. This results in 9 products, 3 elements and 5 reactions.
𝑪 + 𝟐𝑯𝟐 → 𝑪𝑯𝟒 7.17
𝑪𝑯𝟒 +𝑯𝟐𝑶 → 𝑪𝑶 + 𝟑𝑯𝟐 7.18
𝑪𝑶 +𝑯𝟐𝑶 → 𝑪𝑶𝟐 +𝑯𝟐 7.19
𝑪 + 𝑶𝟐 → 𝑪𝑶𝟐 7.20
𝑵𝟐 + 𝟐𝑶𝟐 → 𝟐𝑵𝑶𝟐 7.21
Sensitive analysis was applied to every equation, except Eq. 5.7 that has no
influence on the results, as N2 is considered inert. This equation was used only to
satisfy solution process restriction. A variation of ±500 degrees was applied to each
reaction in turn, while the remaining reactions were kept with no temperature
approach.
7.2.3.1 Sensitivity analysis – Reaction 7.17
Figure 7.4 and Figure 7.5 show the results for the sensitivity analysis of
temperature approach in Eq. 7.17. The result shows that this reaction only gives a
variation in the product mole fractions under -350 degrees of temperature approach.
SENSITIVITY ANALYSIS
ReacZon
68
CHAPTER 7 – SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
176
reactions and elements. Furthermore, the equations must be linearly independent
(Schefflan 2011).
Based on that, the following reactions (Eq. 7.17 to Eq.7.21) are used to
calculate the products (H2, CO, CO2, CH4, H2O, O2, N2 and C) that are formed by the
elements C, H, O. This results in 9 products, 3 elements and 5 reactions.
𝑪 + 𝟐𝑯𝟐 → 𝑪𝑯𝟒 7.17
𝑪𝑯𝟒 +𝑯𝟐𝑶 → 𝑪𝑶 + 𝟑𝑯𝟐 7.18
𝑪𝑶 +𝑯𝟐𝑶 → 𝑪𝑶𝟐 +𝑯𝟐 7.19
𝑪 + 𝑶𝟐 → 𝑪𝑶𝟐 7.20
𝑵𝟐 + 𝟐𝑶𝟐 → 𝟐𝑵𝑶𝟐 7.21
Sensitive analysis was applied to every equation, except Eq. 5.7 that has no
influence on the results, as N2 is considered inert. This equation was used only to
satisfy solution process restriction. A variation of ±500 degrees was applied to each
reaction in turn, while the remaining reactions were kept with no temperature
approach.
7.2.3.1 Sensitivity analysis – Reaction 7.17
Figure 7.4 and Figure 7.5 show the results for the sensitivity analysis of
temperature approach in Eq. 7.17. The result shows that this reaction only gives a
variation in the product mole fractions under -350 degrees of temperature approach.
0"500 "450 "400 "350 "300 "250 "200 "150 "100 "50
0.2
0
0.04
0.08
0.12
0.16
Tapp..".EQ2.(K)
Mol.Frac:on
."260.K
H2
COCO2
CH4
H2O
CH4.*10
SENSITIVITY ANALYSIS
ReacZon
69
CHAPTER 7 – SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
176
reactions and elements. Furthermore, the equations must be linearly independent
(Schefflan 2011).
Based on that, the following reactions (Eq. 7.17 to Eq.7.21) are used to
calculate the products (H2, CO, CO2, CH4, H2O, O2, N2 and C) that are formed by the
elements C, H, O. This results in 9 products, 3 elements and 5 reactions.
𝑪 + 𝟐𝑯𝟐 → 𝑪𝑯𝟒 7.17
𝑪𝑯𝟒 +𝑯𝟐𝑶 → 𝑪𝑶 + 𝟑𝑯𝟐 7.18
𝑪𝑶 +𝑯𝟐𝑶 → 𝑪𝑶𝟐 +𝑯𝟐 7.19
𝑪 + 𝑶𝟐 → 𝑪𝑶𝟐 7.20
𝑵𝟐 + 𝟐𝑶𝟐 → 𝟐𝑵𝑶𝟐 7.21
Sensitive analysis was applied to every equation, except Eq. 5.7 that has no
influence on the results, as N2 is considered inert. This equation was used only to
satisfy solution process restriction. A variation of ±500 degrees was applied to each
reaction in turn, while the remaining reactions were kept with no temperature
approach.
7.2.3.1 Sensitivity analysis – Reaction 7.17
Figure 7.4 and Figure 7.5 show the results for the sensitivity analysis of
temperature approach in Eq. 7.17. The result shows that this reaction only gives a
variation in the product mole fractions under -350 degrees of temperature approach.
500#500 #400 #300 #200 #100 0 100 200 300 400
0.2
0
0.04
0.08
0.12
0.16
Tapp..#.EQ3.(K)
Mol.Frac:on
H2
CO
CO2 H2O
.#190.K
OPTIMIZED METHOD
Tapp = -‐260
Tapp = -‐170
70
ASPEN Experiment Difference O2 0.00% 0.00% 0.00 N2 59.30% 60.67% 1.37 H2O 5.98% 6.35% 0.37 H2 12.45% 13.52% 1.07 CO 11.74% 10.99% -0.75 CH4 0.53% 0.10% -0.43 CO2 10.00% 8.37% -1.63 Total Mole 100.00% 100.00%
!
CHAPTER 7 – SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
176
reactions and elements. Furthermore, the equations must be linearly independent
(Schefflan 2011).
Based on that, the following reactions (Eq. 7.17 to Eq.7.21) are used to
calculate the products (H2, CO, CO2, CH4, H2O, O2, N2 and C) that are formed by the
elements C, H, O. This results in 9 products, 3 elements and 5 reactions.
𝑪 + 𝟐𝑯𝟐 → 𝑪𝑯𝟒 7.17
𝑪𝑯𝟒 +𝑯𝟐𝑶 → 𝑪𝑶 + 𝟑𝑯𝟐 7.18
𝑪𝑶 +𝑯𝟐𝑶 → 𝑪𝑶𝟐 +𝑯𝟐 7.19
𝑪 + 𝑶𝟐 → 𝑪𝑶𝟐 7.20
𝑵𝟐 + 𝟐𝑶𝟐 → 𝟐𝑵𝑶𝟐 7.21
Sensitive analysis was applied to every equation, except Eq. 5.7 that has no
influence on the results, as N2 is considered inert. This equation was used only to
satisfy solution process restriction. A variation of ±500 degrees was applied to each
reaction in turn, while the remaining reactions were kept with no temperature
approach.
7.2.3.1 Sensitivity analysis – Reaction 7.17
Figure 7.4 and Figure 7.5 show the results for the sensitivity analysis of
temperature approach in Eq. 7.17. The result shows that this reaction only gives a
variation in the product mole fractions under -350 degrees of temperature approach.
CHAPTER 7 – SIMULATION OF CHAR GASIFICATION PROCESS IN A CONTINUOUS FIXED BED REACTOR USING ASPEN PLUS
176
reactions and elements. Furthermore, the equations must be linearly independent
(Schefflan 2011).
Based on that, the following reactions (Eq. 7.17 to Eq.7.21) are used to
calculate the products (H2, CO, CO2, CH4, H2O, O2, N2 and C) that are formed by the
elements C, H, O. This results in 9 products, 3 elements and 5 reactions.
𝑪 + 𝟐𝑯𝟐 → 𝑪𝑯𝟒 7.17
𝑪𝑯𝟒 +𝑯𝟐𝑶 → 𝑪𝑶 + 𝟑𝑯𝟐 7.18
𝑪𝑶 +𝑯𝟐𝑶 → 𝑪𝑶𝟐 +𝑯𝟐 7.19
𝑪 + 𝑶𝟐 → 𝑪𝑶𝟐 7.20
𝑵𝟐 + 𝟐𝑶𝟐 → 𝟐𝑵𝑶𝟐 7.21
Sensitive analysis was applied to every equation, except Eq. 5.7 that has no
influence on the results, as N2 is considered inert. This equation was used only to
satisfy solution process restriction. A variation of ±500 degrees was applied to each
reaction in turn, while the remaining reactions were kept with no temperature
approach.
7.2.3.1 Sensitivity analysis – Reaction 7.17
Figure 7.4 and Figure 7.5 show the results for the sensitivity analysis of
temperature approach in Eq. 7.17. The result shows that this reaction only gives a
variation in the product mole fractions under -350 degrees of temperature approach.
VALIDATION
Data of Van de Steene (2010)
71
Experiment A - B Experiment C Van de Steene (2010)
Reactants flow (g/min) Char feeding rate 25 25 25 Air 235.50 237.61 231.01 Propane 12.59 13.35 11.78 Added water vapour 12.20 18.41 35 Unreacted carbon removed via UC
7.4 8.8 3.1
Block temperature (°C) PROP-AIR 25 25 25 STEAM 1000 1000 1000 BIOMASS 25 25 25 GAS-ATM 1060 1080 1020 GASIFIER 870 870 850 Total Pressure (atm) 1.01 1.01 1.01
!
MAIN ACHIEVEMENTS
Non-‐stoichiometric
Non-‐stoichiometric with Tapp
Stoichiometric with Tapp
72
GENERAL CONCLUSION
73
The scope of this was to invesFgate the reducFon zone of a downdra` gasifier, to provide the necessary data for development and validaFon of 2D CFD codes to simulate the behaviour of the gasificaFon zone of a downdra` gasifier, and to develop an Aspen Plus model for char gasificaFon.
74
SUGGESTIONS FOR FURTHER WORK
2D/3D CFD modelling of charcoal gasificaFon. This could be validated with the data presented in the chapter 5;
2D/3D CFD modelling of biomass gasificaFon. This could be validated with the data presented in the chapter 6;
Aspen modelling using reacFon kineFcs to model fixed bed gasificaFon;
Development of technique to perform longitudinal and radial gas measurements in a GEK;
75
76
Recommended