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Modeling of an integrated SOFC and biomass gasification system
Can Ozgur Colpan Feridun Hamdullahpur
Ibrahim Dincer Yeong Yoo
MODELING OF AN INTEGRATED SOFC AND
BIOMASS GASIFICATION SYSTEM
Outline
• Introduction to integrated SOFC and biomass gasification systems • Literature survey on modeling of SOFC and biomass gasification
systems
• Objective of this study • Modeling
• Transient heat transfer modeling of SOFC • System level modeling
• Results
• Conclusions
Introduction to Integrated SOFC and Biomass
Gasification Systems
Researches focus on finding the optimum performance and the most economic solution for given biomass fuels and desired
outputs.
GASIFICATION GAS CLEANUP SOFC
Biomass Product gas Cleaned gas Reactor type Gasification agent Feedstock Operating conditions Electricity Heat Temperature level Components Design type Temperature level Reforming type Flow configuration Catalyst materials Operating conditions
Literature Survey
• Omosun et al. (2004): They compared cold gas cleanup and hot gas cleanup systems to be used in biomass gasification/SOFC system. Hot gas cleanup should be preferred.
• Panopoulos et al. (2006): They investigated the integration of a SOFC with a novel allothermal biomass steam gasification process. the electrical efficiency of the system as 36% and exergetic efficiency as 32%.
• Cordiner et al. (2007): They studied the integration of a downdraft gasifier with a SOFC in which woody material is used as the fuel. Electrical efficiency of the system is calculated as 45.8%.
• Athanasiou et al. (2007): They studied integrated SOFC/steam
turbine/gasifier system. Electrical efficiency of the system is found to be 43.3%.
Objective
• To model integrated SOFC and biomass gasification systems for predicting the performance of the system.
• Transient heat transfer model of a SOFC
• Thermodynamic models for the rest of the components
• Effect of gasification agent on the performance of the system is studied. Performance assessment parameters are:
• Electrical efficiency
• Fuel utilization efficiency • Power-to-heat ratio
Transient Heat Transfer Model of SOFC
• A control volume around the repeat element of a planar, co-flow, DIR-SOFC is taken.
• Solid structure is modeled in 2-D, whereas gas channels are modeled in 1-D.
• Cell voltage, Reynolds number at the fuel channel inlet, excess air
coefficient are input parameters. Current density, temperature, and molar gas composition distributions, fuel utilization, power output and electrical efficiency of the cell are output parameters.
• Six gas species, CH4, H2, CO, CO2, H2O and N2 at the fuel channel inlet and two gas species O2 and N2 at the air channel inlet are considered. • Fully developed laminar flow conditions are assumed in channels.
• Convection in the rectangular ducts and surface-to-surface radiation effects,
conduction heat transfer at the section where the interconnects are in contact with PEN structure, ohmic, activation and concentration
polarizations are considered.
System Level Modeling-II
• Approach in the modeling:
• The syngas composition (from the thermodynamic modeling of the gasifier)
• The number of SOFC stacks that must be used in this system (using the output of the heat transfer model of the SOFC)
• The molar flow rate of dry biomass • The enthalpy flow rate of all states • Work input to the blowers and pump • Performance assessment parameters
• Electrical efficiency
• Fuel utilization efficiency • Power-to-heat ratio
• Exergetic efficiency
System Level Modeling-III
• Electrical efficiency
• Fuel utilization efficiency • Power-to-heat ratio • Exergetic efficiency
fg
O H C net el h m LHV n W z y x 1
fg
O H C process net h m LHV n H W FUE z y x 1 process net H W PHR ) ( , 15 ,C H O chH2O l ch process net e n x E x E W z y x Input Data
Environmental temperature 25 °C
Fuel
Type of biomass Wood
Ultimate analysis of biomass [%wt dry basis] 50% C, 6% H, 44% O
Moisture content in biomass [%wt] 40%
Gasifier
Moisture content in biomass entering gasifier [%wt] 20%
Temperature of syngas exiting gasifier 900 °C
Molar ratio of steam entering to gasifier to drybiomass
0.1
Molar composition of enriched oxygen 0.35 O2, 0.65 N2
Balance of Plant
Temperature of exhaust gas leaving the system 127 °C
Pressure ratio of blowers 1.18
Isentropic efficiency of blowers 0.53
Pressure ratio of pump 1.2
Isentropic efficiency of pump 0.8
Inverter efficiency 0.95
SOFC
Power requirement of SOFC 10 kW
Number of cells per stack 50
Temperature of syngas entering SOFC 850 °C
Temperature of air entering SOFC 850 °C
Pressure of the cell 1 atm
Cell voltage 0.65
Excess air coefficient 7
Active cell area 10x10 cm2
Number of repeat elements per single cell 18
Flow configuration Co-flow
Manufacturing type Electrolyte-supported
Thickness of air channel 0.1 cm
Thickness of fuel channel 0.1 cm
Thickness of interconnect 0.3 cm Thickness of anode 0.005 cm Thickness of electrolyte 0.015 cm Thickness of cathode 0.005 cm Emissivity of PEN 0.8 Emissivity of interconnect 0.1 Diffusivity of anode 0.91 cm2/s Diffusivity of cathode 0.22 cm2/s Porosity of anode 0.5 Porosity of cathode 0.5 Turtuosity of anode 4 Turtuosity of cathode 4 10
Results-I
4 CH x 2 H x xCO 2 CO x xHO 2 xN2 Case1: Air 0.14% 11.22% 8.16% 12.95% 22.68% 44.84% Case2: Enriched O2 0.28% 15.74% 11.40% 16.37% 28.80% 27.41% Case3: Steam 2.15% 43.37% 27.38% 8.98% 18.12% 0.00% Syngas compositionOutput of SOFC model
r Re ic,ave[A/cm2] U F Wsofc [W/cm2] nstack Case1: Air 0 10.0 0.240 0.85 0.156 13 Case2: Enriched O2 0 6.5 0.246 0.85 0.160 13 Case3: Steam 0 1.5 0.343 0.85 0.223 9 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0 1 2 3 4 5 6 7 8 9 10 M a x . ca rb o n a ct iv it y Case 1 Case 2 Case3
Results-II
Case-1 Case-2 Case-3
12
Results-III
Power demand for auxiliary components, net power and heat output
SOFC W [W] 1 blower W [W] 2 blower W [W] pump W [W] net W [W] process H [W] Case1: Air 10140 227.5 1332.1 0.2 8073.2 19741.3 Case2: Enriched O2 10384 204.0 1366.9 0.2 8293.7 17032.9 Case3: Steam 10031 - 1319.6 0.02 8210.2 1765.9
Performance assessment parameters
FUE PHR Case1: Air 18.5% 63.9% 0.409 30.9% Case2: Enriched O2 19.9% 60.9% 0.487 30.7% Case3: Steam 41.8% 50.8% 4.649 39.1% el
Mass flow rate of substances entering the system
biomass m [g/s] air m (B1) [g/s] air m (B2) [g/s] water m [g/s] steam m [g/s] Case1: Air 4.048 7.796 45.648 7.654 7.654 Case2: Enriched O2 3.867 6.989 46.841 6.604 6.604 Case3: Steam 1.826 - 45.219 0.7670 0.6847
• A model for a SOFC and biomass gasification system is developed(heat transfer model for SOFC, thermodynamic models for the rest of the
components).
• The effect of gasification agent on the performance of the system is discussed.
• This study shows that steam gasification yields the highest electrical efficiency, power-to-heat ratio and exergetic efficiency, but the lowest fuel utilization efficiency.
• As a future study, exergy analysis will be applied to the system to calculate the exergy destructions and losses; hence identify the locations and magnitudes of the irreversibilities within the system.
