1-s2.0-S1876610214011370-main

Energy Procedia 54 ( 2014 ) 166 – 176

Available online at www.sciencedirect.com

ScienceDirect

1876-6102 © 2014 Sudipta De. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of Organizing Committee of ICAER 2013doi: 10.1016/j.egypro.2014.07.260

4th International Conference on Advances in Energy Research 2013, ICAER 2013

Biomass Integrated Combined Power Plant with Post Combustion CO2 Capture – Performance Study by ASPEN Plus®

Kuntal Janaa, Sudipta Dea,* aDepartment of Mechanical Engineering, Jadavpur University, Kolkata – 700032, India

Abstract

Energy is one of the basic needs modern civilizations. Fossil fuels presently are the major primary energy source. However, it contributes maximum to the climate change problem also. With suitable technology greenhouse gas emission may be reduced. Employing CO2 capture process for energy efficient system using CO2 neutral fuels is one possible solution in this context. In this paper a biomass based combined power plant with CO2 capture has been proposed. Thermodynamic modelling for the detail process of this plant has been simulated by using ASPEN Plus®. Results show that post- combustion CO2 capture process has potential in CO2 negative energy system. Artificial thermal efficiency is also calculated and its variation with carbon capture efficiency is studied. The degree of CO2 capture has to be decided on the basis of overall performance of the plant specifically beyond certain value as CO2 capture efficiency is quit flexible for a net-CO2 negative emission plant. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Organizing Committee of ICAER 2013.

Keywords: biomass; gasification; CO2 capture; ASPEN Plus®; sensityvity analysis

1. Introduction Per capita energy consumption grossly estimates the economic prosperity and living standard of a country [1].

Energy demand (more specifically electricity) is ever increasing with improving living standard and population.

* Corresponding author. Tel.: 919831245561; fax: +913324146890. E-mail address: [email protected], [email protected]

© 2014 Sudipta De. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of Organizing Committee of ICAER 2013

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Kuntal Jana and Sudipta De / Energy Procedia 54 ( 2014 ) 166 – 176 167

Most of the electricity supply of the world is presently from fossil fuels, mostly coal [2]. CO2 emission from these fossil fuel based plants, is the major source of climate change, presently greatest challenge of human survival on earth [3]. For large scale power generation, carbon capture and sequestration (CCS) is the only possible options for using fossil fuels without affecting the climate change [4]. Different options of CO2 capture are available at different technological levels of maturity [5, 6]. Three most commonly known processes are post-combustion CO2 capture [7], pre-combustion CO2 capture [8], or oxy-fuel combustion [9]. Basic penalty for these processes are energy consumption during CO2 capture and storage and this energy penalty reduces the overall efficiency of the plants with CO2 capture [10]. Alternative ways of meeting increasing energy demand with no/low CO2 emission are either increasing energy efficiency during conversion or use of it or switching over to CO2-neutral fuels, say biomass [11]. Efficient way of using biomass is through gasification [12]. Integrated process of biomass gasification with efficient combine power plant is thus a future sustainable option for CO2-neutral power generation. Integrating utility heat with power, i.e. cogeneration even increases the overall thermodynamic efficiency of the plant. However CO2 capture process developed for fossil fuel based system may also be integrated with biomass based energy conversion. Energy penalty for the CO2 capture process is compensated by additional capture of CO2 making the plant to be a net CO2 negative plant. Thus biomass integrated gasification combined power with CO2 capture process is a very prospective future sustainable option to meet the CO2 mitigation target as a future action to flight the climate change problem.

In this paper a biomass integrated gasification combined cycle (BIGCC) with post-combustion CO2 capture has been proposed. Performance of this plant (for amine-based post-combustion CO2 capture) with different output parameters (say power output, utility heat, etc.) has been studied. Thermodynamic model for this BIGCC with CO2 capture is developed using ASPEN Plus®. Net reboiler heat duty and artificial thermal efficiency are also studied. To study the performance of this configuration sugarcane bagasse was assumed as fuel. It is a representative biomass with assumed mass flow rate of 1000 kg/hr.

2. System description

The schematic of the biomass integrated gasification combined power plant with post-combustion CO2 capture is shown in Fig. 1(a). The ASPEN Plus® model of BIGCC is given in Fig. 1(b). The schematic consists of four main islands- gasification, power generation, process steam generation and CO2 capture. Power was generated in topping GT-cycle and bottoming ST-cycle. Sugarcane bagasse was input to the system as sources of energy which were utilized in processes as described below. The detailed thermo-chemical properties of the sugarcane bagasse used for analysis are given in Table 1.

Table 1. Feedstock properties and their ultimate and proximate analyses (mass percent)

Sugarcane bagasse

Higher calorific value(MJ/Kg) 17.33

Lower calorific value(MJ/Kg) 16.24 FC 14.95 VCM 73.78

Ash 11.27

C 44.80 H 5.35 O 39.55 N 0.38 S 0.01 Cl 0.12 Residue 9.79

168 Kuntal Jana and Sudipta De / Energy Procedia 54 ( 2014 ) 166 – 176

2.1. Gasification

Based on present available technology, gasification section is modelled for sugarcane bagasse with atmospheric air in down draft gasifier [13-15]. For the modelling purpose gasification products were modelled according to following reactions. First of all, moisture content of the sugarcane bagasse was partially reduced by drying process. Then it was fed to the gasification chamber and reacted with air according to following reactions [16, 17]. 2C + O2 ↔ 2CO ∆H298= -111 KJ/ mole Partial combustion of char 2H2 +O2 ↔ 2H2O ∆H298= -242 KJ/mole Hydrogen combustion C+ H2O↔ H2+CO ∆H298=131 KJ/mole Water gas reaction CO2+C ↔ 2CO ∆H298=172 KJ/mole Boudouard reaction CO+H2O ↔ CO2+H2 ∆H298= -41 KJ/mole CO shift reaction C+2H2↔ CH4 ∆H298= -75 KJ/mole Methanation reaction CH4 + H2O ↔ CO + 3H2 ∆H298=206 KJ/mole Steam-meathane reforming 2N2+3H2 ↔ 2NH3 (not reported) NH3 formation H2+S↔ H2S (not reported) H2S formation

Biomass gasification is considered in steady state and equilibrium condition [18]. In this study 2% carbon loss in ash [19] was also considered. For gasification process, it was assumed that all sulphur of the fuel produced H2S [20, 21] and tars were assumed to be converted to combustible syngas [20]. Mass flow rate of air was determined by equivalence ratio and it was 25% of the stociometric air for the gasification. The gasified products were then cleaned for dust particles (ash, chars and other removable impurities) and cooled in superheater- reheater of the steam power cycle and subsequently in the syngas cooler. The produced syngas was utilized for power generation in combined cycle.

2.2. Power generation

Depending upon feedstock properties and gasification parameters syngas was produced. Heat value of the syngas was then utilized for power generation in a combined cycle. For the gas turbine of the combined cycle, cleaned and cooled syngas was compressed by syngas compressor and then fed to the combustion chamber. 25% excess air was also fed to the combustion chamber through the air compressor. Hot and pressurized combustion products then expanded in a gas turbine to generate net GT-power.

After exiting from the gas turbine at atmospheric pressure, flue gas still had enough heat content to generate power in bottoming steam turbine cycle. Dual-pressure reheat cycle was used in steam turbine cycle. Heat of flue gas was then utilized in superheater-reheater and subsequently in evaporator and economizer of the steam power cycle.

2.3. Process steam generation

The process water was fed to the waste heat recovery (WHR) unit after the economizer to utilize the residual heat of the flue gas. However flue gas temperature was always maintained above acid condensation temperature in the simulation to avoid corrosion of equipment. Hot water utilizing waste heat in the WHR was then fed to the syngas cooler. Wet process-steam was generated utilizing heat available there. This process-steam may be used in heating purpose in process industry.


AIR COMPRESSOR

GASIFIER

GAS

COOLER

SYNGAS

CLEANER

COMBUSTOR

SUPERHEATER-

REHEATER

SYNGAS COMPRESSOR

GAS

TURBINE ECONOMIZER-

EVAPORATOR

HEATER

PUMP

CONDENSER

Biomass

Air Ash Syngas

Air

Water

STEAM

TURBINE

Steam

DRIER

Gasification GT-Cycle

ST-Cycle

CO2

CAPTURE

CO2

Vent

gas

Fig. 1 (a). Schematic of biomass integrated gasification combined cycle power plant with post-combustion CO2 capture

Fig. 1 (b). ASPEN Plus® model of biomass integrated gasification combined cycle power plant

RSTOIC

DRY-FLSH

RYIELD

RGIBBS

BIOMASS

WET-BIODRY-BIO

WATER

DECOMP

GIBS-OUT

Q-DECOMP

GASI-AIR

SEPARATE

SOLID

SYN-GAS

COLD-SYN

GAS-SEP

H2S

AMONIA

GT-SYN

SYN-COMP

GT-COMB

COMP-SYN

AIR-COMP

GT-AIR

COMP-AIR

COMB-GAS GAS-TURB

HOT-FLUECOMP-WRK

SYN-COMW

GT-WORKW

SPRH

FLUE-OUT

RH-IN

RH-OUT

SPH-INSPH-OUT

HP-ST

ECO-EVAFLU-EXIT

LP-ST

PUMP

FEED-WTR

ECO-IN

LP-OUT

HP-WRKW

LP-WRKW

PUMP-WRK

COND

Q-COND

Q

FLU-EXHS

SYN-OUT

DRIER

HOT-BIO

PR-WTRIN

PRWTROUT

C-SEPS3

C-ASH

B20TO-ATMP

S48

B18

PRO-HT

S5


2.4. Post-combustion CO2 capture

As Amine based chemical solvents is a matured technology and commercially available, was used to capture CO2 from flue gas. The schematic of CO2 capture unit is shown in Fig. 2(a). The ASPEN Plus® model of amine based post-combustion model is given in Fig. 2(b). Cooled flue gas (at 400C) is introduced at the bottom and amine solution is introduced to top of the absorber column. Flue gas gets scrubbed in counter flow direct contact between flue gas and amine. CO2 free flue gas leaves from the top of the absorber while CO2 rich amine solution leaves at the bottom of it. Subsequent heating in stripper column release CO2 gas from the solution and it leaves from the top of the column while CO2-stripped amine solution is recycled back from the bottom of the column. Heating requirement for this process is divided in preheater (relatively smaller amount) and in stripper column (majority of heating). The electrolyte reactions [22] were used in absorber and stripper columns are given in Table.2 with the equilibrium constants. 3. Simulation by using ASPEN Plus®

The schematic of the BIGCC with and without CO2 capture was simulated by ASPEN Plus® [23]. For each configuration ASPEN Plus® model was created and simulated for sugarcane bagasse as input.

CONDENSER

CO2 Product

ABSORBER

FLUE GAS COOLER

AMINE TREATMENT PLANT

RICH-LEAN AMINE HEAT EXCHANGER

STRIPPER

Flue gas

Flue gas (40oC)

PUMP

Make-up amine

Amine (40oC)

Rich-amine solution

Lean-amine solution

Vent-gas

Reboiler

CO2 Product

Fig. 2 (a). Schematic of amine based CO2 capture process

AB

LEANMEA

FLUEGAS

TREATGAS

RICHMEA

PUMP

POUT

HOUT

STRIP

CO2OUT

MEAOUT

Q-REB

Q

COOLER

COL-FLU

HX

COL-MEA

H2O-IN H2O-OUT

HEATER

STRIPIN

Q-MEAQ

Fig. 2 (b). ASPEN Plus® model for amine based post-combustion CO2 capture


3.1. Specification and operating parameters

To estimate the performance of the biomass integrated combined cycle with CO2 capture, the Aspen Plus® flow sheet model as described in previous section was used for sugarcane bagasse as a representative of CO2 neutral fuel. The system analysis was carried out for the developed model as described in section 2. A BIGCC scheme with post-combustion CO2 capture was simulated to estimate the performance. An equal amount (1000 kg/hr) of sugarcane bagasse was considered as input to the system with identical operating parameters as shown in Table 3. The operating parameters for amine based CO2 capture system was given in Table 4. Table 2. Equilibrium constant for reactions of CO2 with aqueous MEA solution

Reactions A B C D

2H2O ↔ H3O+ + OH- 132.89888 -13445.9 -22.477301 0

CO2 + 2H2O ↔ H3O+ + HCO3- 231.465439 -12092.1 -36.781601 0

HCO3- + H2O ↔H3O+ + CO3

2 216.050446 -12431.7 -35.481899 0

MEAH++H2O ↔ MEA+ H3O+ -3.038325 -7008.3569 0 -0.003135

MEACOO-+H2O ↔ MEA+ HCO3- -0.52135 -2545.53 0 0

ln(Keq) = A + B/T + C ln(T) + DT , T in Kelvin 3.2. Physical property method To determine all physical properties of the conventional components for gasification and power generation in gas turbine, Peng-Robinson equation of state with Boston Mathias alpha function (PR-BM) was used. As alpha is a temperature dependent variable, it gives good result for correlation of the pure component vapour pressure at high temperature. HCOALGEN and DCOALIGHT model is used for enthalpy and density estimation of non-conventional components (say, sugarcane bagasse and ash). For non-conventional components ultimate and proximate analyses are used. Aqueous monoethyl amine (MEA) was chosen as solvent for CO2 capture process. Electrolyte Non Random Two Liquid (ELECNRTL) property method has good accuracy to estimate thermo-physical properties of carbon capture process. As STEAM-TA is directly related to the steam table data, it gives good result for steam turbine power generation and process-steam generation. Table 3. Operating parameters for the simulation of BIGCC

Configurations Parameters Value Biomass feed

Temperature Pressure Mass flow rate

250C 1atm 1000kg/hr

Reaction in gasification

Pressure Equivalence ratio

1atm 25% of stoichiometric air

Air compression, Syngas compression

Pressure ratio Isentropic efficiency

14 0.9

Combustion air

Mass flow rate 25% excess of stoichiometric air

Gas cleaning Separation efficiency of solids particles 85%

Gas turbine Pressure ratio Discharge pressure Isentropic efficiency

14 1atm 0.9


Superheater-Reheater HP stage temperature HP stage pressure LP stage temperature LP stage pressure

5380C 12.4MPa 5000C 3.2MPa

Feed water for ST cycle Temperature Pressure

250C 1atm

HP and LP Steam turbine

Isentropic efficiency LP-ST discharge pressure

0.92 0.07MPa

Ambient air Temperature Pressure N2 mass fraction O2 mass fraction

250C 1atm 0.77 0.23

Table 4. Operating parameters for the simulation of CO2 capture

Configurations Parameters Value

Lean amine solution

Temperature Pressure Amine concentration

400C 1.7 bar 30% by mass

Lean loading

CO2/amine (mole basis)

32%

Flue gas cooling

Outlet temperature

400C

Absorber column

Calculation type No. of stages Condenser pressure

Equilibrium 10 1 atm

Pump

Pressure increases

20 psia

Rich-lean heat exchanger Hot inlet- cold outlet temperature difference 150C

Stripper column

Calculation type No. of stages Condenser type

Equilibrium 20 Partial vapor

4. Results and discussion

In this work results are shown as simulated in ASPEN Plus®. Relative gain in CO2 capture and corresponding penalty in reboiler heating are combined in a non-dimensional number to estimate the net effect of these two counter-balancing effects as defined below. Moreover utility heat obtained from the plant is being utilized for CO2 capture process as part of total heat input required for that process. Performance of the plant is shown considering both the utility outputs i.e., power and utility heat as also defined in equation 1 and 2.

Fig.3 shows the power outputs from gas turbine, steam turbine and in total power from the plant. The plant fuelled with 1000kg/hr of sugarcane bagasse can generate around 2 MW net power. However out of 2 MW of power ~1.2 MW is generated in gas turbine and ~0.8 is generated in steam turbine.


Fig. 3. Power output of BIGCC with post-combustion CO2 capture

Carbon capture efficiency depends on reboiler heat duty. Part of which is supplied by the process-steam. Net

reboiler heat duty is defined as the additional heat requirement for this CO2 capture process. In Fig.4, this net reboiler heat duty is plotted against carbon capture efficiency (i.e. fraction of total CO2 captured). It is observed that net-reboiler heat duty increases sharply after 50% of CO2 capture from flue gas. So it is better to capture CO2 below 50%. Hence optimum operation as a combined effect of better carbon capture efficiency with relatively lower net reboiler heat duty may not be obtained beyond this value.

Fig. 4. Variation of net-reboiler heat duty with carbon capture efficiency

Utility heat as well as reboiler heat duty and in effect net reboiler heat duty for these three plants are shown in

Fig.5. Figure shows that reboiler heat duty for CO2 capture is ~0.7 MW but 0.2 MW heat is by-product of the plant itself. So the amount of net-reboiler heat duty supplied externally is 0.5 MW.

0

0.5

1

1.5

2

2.5

NET GT-POWER (MW)

LPST-POWER (MW)

HPST-POWER (MW)

TOTAL POWER (MW)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6

Net

rebo

iler h

eat d

uty

(MW

)

Carbon capture efficiency


Fig. 5. Heat consumption and production rate of BIGCC with post-combustion CO2 capture

For CO2 capture process, overall performance depends on both carbon capture efficiency and reboiler heat duty. Carbon capture efficiency is defined as fraction of CO2 captured of the total amount in flue gas. Obviously, this indicates the degree of desired effect obtained in the process. On the other hand, reboiler heat duty is the necessary input for this process of CO2 capture. Thus overall performance of the CO2 capture process may be assessed by a combine parameter involving both carbon capture efficiency and net reboiler heat duty. For non-dimensionalization, net reboiler heat duty is considered as a fraction of total heat input to the plant through fuel. A non-dimensional parameter ‘capture performance’ (CP) involving these two input and output quantities are defined as follows,

(1)

(2)

As operation of this plant promotes net negative CO2 emission, optimum operation of this plant is recommended for a carbon capture efficiency of ~0.45 beyond which more capture of CO2 may cause significant penalty through energy input leading to drastic reduction of the overall performance of CO2 capture process i.e., CP as shown in Fig.6.

Fig. 6. Variation of capture performance with carbon capture efficiency

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

REBOILER HEAT DUTY (MW)

UTILITY HEAT (MW) NET REBOILER HEAT DUTY (MW)

00.5

11.5

22.5

33.5

44.5

5

0 0.1 0.2 0.3 0.4 0.5 0.6

Capt

ure

perf

orm

ance

(CP)



Artificial thermal efficiency is an important performance parameter suitable for cogeneration and it is defined as -

(3)

Where, is the artificial efficiency of the plant, is the power output of the plant, Ta is the atmospheric temperature in kelvin, Ti is the temperature of the utility heat Qi, Qfuel is the fuel energy input.

Fig. 7. Variation of artificial thermal efficiency with carbon capture efficiency

In Fig.7, variation of artificial thermal efficiency is plotted against carbon capture efficiency. It is noted that artificial thermal efficiency decreases rapidly when carbon capture efficiency is beyond 50%. This is because net reboiler heat duty increases rapidly when carbon capture efficiency is more than 50%. 5. Conclusion

A BIGCC plant with amine based post combustion carbon capture is modelled in ASPEN Plus®. Analysis with the developed model for a typical biomass i.e., sugarcane bagasse with a constant mass flow rate (1000 kg/hr) has been reported. Results show that reboiler heat duty beyond 50% of CO2 capture increases sharply. As a result, artificial thermal efficiency decreases accordingly. For plants with CO2 capture, utility heat obtained from this plant is fully utilized for CO2 capture process. As this is a net CO2 negative emission plant, operational condition may be flexibly decided within a range of carbon capture efficiency (0-50%), depending on real need.

Acknowledgements

Mr. Kuntal Jana acknowledges the fellowship by Council of Scientific and Industrial Research (CSIR-India) for his research towards PhD. References [1] Amie Gaye, Access to Energy and Human Development - Fighting climate change: Human solidarity in a divided world. Human

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