Exergetic analysis of solid oxide fuel cell and biomass gasification integration with heat pipes

ARTICLE IN PRESS

Energy 33 (2008) 292–299 www.elsevier.com/locate/energy

Exergetic analysis of solid oxide fuel cell and biomass gasification integration with heat pipes L. Frydaa, K.D. Panopoulosa,b,, J. Karlc, E. Kakarasa,b a

Laboratory of Steam Boilers and Thermal Plants, School of Mechanical Engineering, Thermal Engineering Section, National Technical University of Athens, 9 Heroon Polytechniou Ave., Zografou 15780, Greece b Institute for Solid Fuels Technology and Applications, Centre for Research and Technology Hellas, 4th km N.R. Ptolemais-Kozani, P.O. Box 95, 50200 Ptolemais, Greece c Institute of Thermal Power Systems, Technical University of Munich, Boltzmannstrasse 15, 85747 Garching, Germany Received 12 December 2006

Abstract This paper presents an exergetic analysis of a combined heat and power (CHP) system, integrating a near-atmospheric solid oxide fuel cell (SOFC) with an allothermal biomass fluidised bed steam gasification process. The gasification heat requirement is supplied to the fluidised bed from the SOFC stack through high-temperature sodium heat pipes. The CHP system was modelled in AspenPlusTM software including sub-models for the gasification, SOFC, gas cleaning and heat pipes. For an average current density of 3000 A m2 the proposed system would consume 90 kg h1 biomass producing 170 kWe net power with a system exergetic efficiency of 36%, out of which 34% are electrical. r 2007 Elsevier Ltd. All rights reserved. Keywords: Exergy; SOFC; Biomass; Gasification; AspenPlusTM

1. Introduction Significant effort is being put into the combination of fuel cell to work with gas produced from biomass gasification [1–2]. Omosun et al. [3] performed modelling and energetic efficiency evaluation of biomass-fuelled solid oxide fuel cell (SOFC) systems based on air gasification. In the present paper, an integrated combined heat and power (CHP) system based on SOFC and steam biomass gasification is modelled and analysed in exergy terms. Exergetic analysis is commonly adopted [4–8] to evaluate the advantages of novel systems based on high-temperature fuel cells. The CHP system under study has a nominal output range of less than 1 MWe and integrates an atmospheric pressure SOFC with a novel allothermal biomass steam gasification process, called BioHPR [9]. Corresponding author. Laboratory of Steam Boilers and Thermal

Plants, School of Mechanical Engineering, Thermal Engineering Section, National Technical University of Athens, 9 Heroon Polytechniou Ave., Zografou 15780, Greece. Tel.: +30 210 7721213; fax: +30 210 7723663. E-mail address: [email protected] (K.D. Panopoulos). 0360-5442/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2007.07.006

Allothermal biomass gasification is performed around 800 1C and requires external heat supply. SOFCs require large amounts of air throughputs for cooling. The system under study combines these two features into a useful outcome by thermally coupling the fluidised bed (FB) gasifier with the SOFC stack via high-temperature sodium heat pipes. Therefore, in this particular system, cooling of the SOFC stack and providing heat for the gasification are accomplished in a single step. A steady-state model of the integrated system was built in AspenPlusTM process simulation software. Four major sub-sections were incorporated: gasification, heat pipes, SOFC and gas cleaning. Second law efficiencies were defined for the major process steps together with an effort to optimise the overall CHP exergetic efficiency. 2. System configuration The proposed system flowchart consists of an FB reactor, a product gas cleaning train, an SOFC stack and its power conditioning, an air blower/compressor, two

ARTICLE IN PRESS L. Fryda et al. / Energy 33 (2008) 292–299

gas-to-gas heat exchangers (HX1 and 2), a heat recovery steam generator (HRSG) (HX3) and a heat exchanger (HX4) to make use of the remaining flue gas energy (Fig. 1). The gasifier FB operates at 1073 K, 1.5 bar and the SOFC stack at 1173 K, 1.1 bar. This allows heat transfer at 100 K temperature difference from the SOFC to the gasifier through the sodium heat pipes. The hot product gas stream is cooled down to 673 K in HX1 by reheating the (halogen/sulfur)-free gas. The raw gas is cleaned from particulates with a barrier-type filter and halogen and sulfur compounds harmful to the SOFC are removed in high-temperature sorbent trap beds. The gas temperature drops to around 573 K due to thermal losses in the gascleaning train. The catalytic compact tar-cracking reactor is heated by the post-stack combustor, and then additional steam is added to the clean product gas to avoid carbon deposition on the SOFC anode. Atmospheric air is blown to the SOFC operating pressure, heated up to 883 K in HX2. Depleted fuel and air from the SOFC are combusted in the post-stack combustor. The flue gases heat up the air (HX2) and pass through a heat recovery steam generator (HX3) to produce steam for the gasifier and product gas

293

moistening. Steam is introduced into the gasifier at 573 K and 2 bar. Finally, useful heat in (HX4) is produced by the flue gas in the form of hot water or steam. The system is based on an SOFC of 100 m2 active surface resulting in electrical output range of 200 kWe [10,11]. Ambient air was considered at 293 K and 1.013 bar and the blower compressor isentropic efficiency was 0.7, while the electronic inverter efficiency was ninv ¼ 95%. Minimum DTs for HX1–HX4 were above 100 K, while pressure and thermal losses for each unit operation were assumed 2%. The calculated efficiency could possibly be increased by assuming lower minimum DTs and less thermal losses to the expense of increased heat transfer surfaces and, therefore, capital costs. 3. Exergy analysis 3.1. Methodology Exergy is the maximum work that can be produced when a heat or material stream is brought to equilibrium in relation to a reference environment, which is kept at

Product Gas 573 K 673 K

FB Biomass Gasifier 1073 K

HX1

Alternative Catalytic tar destruction

Particulate Filter

HCl, H2S removal

Biomass

Steam

Air

Char/Ash

Water

C Heat Pipes

HX2

Combustor & integrated tar cracker

HX3

Flue Gases Product Gas 1123 K

Heat

HX4

Hot Air 923 K

Steam 573K

Power A

SOFC 1173K

C

Inverter

Stack 400 K

Fig. 1. Flowchart of the combined SOFC/allothermal biomass gasification system.

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reference pressure and temperature (To ¼ 298.15 K, po ¼ 1.013 bar) consisting of reference components, and characterized by the absence of pressure and temperature gradients. This state of the environment is called ‘dead state’ and the reference atmospheric composition can be found in exergy textbooks such as [12]. The exergy of each material stream with a molar flow of N (mol s1) is expressed as the sum of two components, physical and chemical exergy, taking into account the deviation between actual environmental and reference conditions: E m ¼ Nðph þ ch Þ ðWÞ,

(1)

where molar physical exergy is ph ¼ ðh  ho Þ  T o ðs  so Þ ðJ mol1 Þ.

(2)

Potential, kinetic energy, etc. were neglected in this work. Mole flows, molar fractions and molar enthalpy and entropy of material streams were taken from the AspenPlusTM flow sheet results; each material stream was duplicated and brought to reference environmental conditions for the evaluation of the reference molar enthalpy and entropy (ho, s). The reference conditions used in these calculations were considered equal to the standard environmental conditions in AspenPlusTM (To ¼ 298.15 K, po ¼ 1.013 bar). The molar chemical exergy is obtained when the components of the energy carrier are first converted to reference compounds and then diffuse into the environment, which is in reference (dead) state. For a gaseous stream flow, the molar chemical exergy is given by X X ch;gas ¼ xi oi þ RT o xi ln xi ðJ mol1 Þ, (3) i

i

where xi is the mole fraction of each component i, which has a standard molar chemical exergy eoi (J mol1) taken from reference tables [12], and R is the ideal gas constant (8314 J mol1 K). The chemical exergy of biomass was calculated according to the method proposed by Szargut et al. [13], for the evaluation of the exergy of solid fuels. The solid fuel data that were used, which correspond to olive kernel residues, are given in Table 1.

Exergy of power equals power itself and exergy of a heat stream Q was evaluated with the help of the Carnot factor 1  To , (4) T where T is the temperature at which Q is available. The exergy balance of a steady state process is given by the following expression: X X Q X X Q X Em þ ET ¼ Em þ ET þ E W þ I, (5)

EQ T ¼ Q

IN

IN

OUT

OUT

OUT

where the irreversibility I represents the loss of quality of materials and energy due to dissipation. Exergetic efficiencies were defined and examined parametrically for gasification, heat transfer through heat pipes, SOFC operation, and are presented in Sections 3.2, 3.3 and 3.4, respectively. 3.2. Allothermal gasification exergy analysis Steam gasification processes of a solid fuel (with major composition consisting of C, H and O) can be generally described with the following chemical equation: heat

CHx Oy þ n1 H2 O ! n2 CO þ n3 CO2 þ n4 H2 þ n5 CH4 þ n6 H2 O þ n7 Cs .

The gasification control volume is presented in Fig. 2. The evaluation of the product gas composition was based on chemical equilibrium calculations performed in AspenPlusTM applying the Gibbs energy minimisation method for the possible products presented in Eq. (6). The main assumptions for the products that are not predicted by thermodynamics were that (a) un-reacted char (modelled as graphite C(s)) was 5% (w/w) of the biomass carbon input (similar work but for circulating FB used a value of 15% (w/w) [14]), and that (b) CH4 concentration was around 5 vol%, together with a tar load of 5 g m3 in dry basis n product gas (typical value from experimental results of catalytic in situ FB tar reduction [15]). The remaining gas constituents (H2, CO, CO2 and H2O) were assumed at thermodynamic equilibrium. Main parameters affecting the product gas composition were operating temperature, pressure and steam-to-biomass ratio (STBR): :

Steam þ Fuel moisture ðkg s1 Þ STBR ¼ . Dry biomass ðkg s1 Þ Table 1 Biomass fuel data

Heating values HHV (kJ kg1 dry) LHV (kJ kg1 wet)

(7)

Gasification temperature was chosen Tgas ¼ 1073 K; higher temperatures would inhibit effective heat transfer

Proximate analysis

Volatiles Fixed carbon Moisture

ð6Þ

Ultimate analysis (% w/w dry) 72.64 24.78 10.0 18900 15567

C H O N S Ash

51.19 6.06 39.32 0.76 0.09 2.58

Ebiomass Esteam

Egas Gasifier Tgas = 1073 K Q

ETgas

Echar

Fig. 2. Gasification control volume for exergy analysis.

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whereas lower temperatures would create undesirable higher tar yields. The pressure chosen was pgas ¼ 1.5 bar, enough to overcome subsequent pressure drop in gas cleaning, SOFC, FB combustor, heat exchangers and flue gas disposal. Based on the general cold gas efficiency definition of gasification processes [16] and taking into account the heat required (Qreq), the cold gas energetic efficiency was expressed as

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caused by water vapour dilution and (b) larger thermal exergy requirements. Therefore, biomass allothermal gasification processes should theoretically be realised with the minimum steam requirement to accomplish carbon conversion. Similar conclusions are found in the literature based on experiments [18]. The value of STBR ¼ 0.6 was chosen as a base case for the rest of the analysis. 3.3. Exergy analysis of heat transfer through heat pipes

:

Zcg ¼

LHV in product gas . LHV in feedstock þ Qreq

Heat pipes’ control volume is defined in Fig. 4, receiving heat from and at the same time cooling the SOFC operating at TSOFC ¼ 1173 K and delivering it at Tgas ¼ 1073 K. Their exergetic efficiency is

(8)

Exergetic efficiency takes into account the entropy increase due to conversion of a solid to a gaseous fuel [17]; based on the definition of the degree of perfection for a process by Szargut et al. [13], the exergetic efficiency for allothermal gasification is expressed as Zex;gas ¼

E gas þ E char E biomass þ

EQ Tgar

þ E steam

Zex;HP ¼

EQ T SOFC

,

(10)

which with Eq. (4) becomes .

(9) Zex;HP ¼

In the present study, produced char is not further utilised, so neither the physical nor the chemical exergy of char is included in the expression above (Echar ¼ 0). The cold gas efficiency vs. STBR is plotted in Fig. 3 and shows the value for the STBR that maximises carbon conversion (according to the gasification model used), while it decreases slowly thereafter. For increasing STBR additional heat exergy E Q T gas is mainly consumed to heat up the additional steam up to the reaction temperature rather than enhance combustible gas species production. The exergetic efficiency decrease is continuous with STBR due to the combination of (a) product gas exergy decrease 100

ðT gas  T o ÞT SOFC ðT SOFC  T o ÞT gas

90

(11)

and results Zex,HP ¼ 96.8%. Higher exergetic efficiency could be achieved with higher number of heat pipes and thus reduced TSOFC to the expense of additional costs for more heat pipe surface area. Note that in energetic terms, the heat transfer efficiency is 100%. 3.4. SOFC exergy analysis A zero-dimensional SOFC model was built in AspenPlusTM using available blocks and a FORTRAN routine

STBR = 0.6 % v/v: H2: 43.9 CO: 26.6 CH4: 4.2 CO2: 9.9 H2O: 15.0 STCR = 0.4

95

efficiency %

EQ T gas

STBR = 2 % v/v: H2: 31.2 CO: 7.7 CH4: 2.2 CO2: 12.4 H2O: 46.4 STCR = 2.1 ex.gas (exergy efficiency)

85

80 cg (cold gas efficiency) 75

70 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

STBR

Fig. 3. Cold gas efficiency and exergetic efficiency of allothermal gasification vs. STBR at Tgas ¼ 1073 K, and pgas ¼ 1.5 bar. Major species gas composition is noted for STBR ¼ 0.6 (base case) and STBR ¼ 2.

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molar Gibbs free energy change is expressed as DG o ¼ DH o  T SOFC DSo , calculated at TSOFC and standard pressure. Finally, pout are the partial pressures of the i participating components in reaction (14) at the SOFC exit. Using the above partial pressures and temperature data, a FORTRAN calculator was used for the estimation of the overpotentials due to Ohmic (VOHM), activation (VACT) and polarisation (VPO) losses. The SOFC stack’s power output is

Gasifier Tgas = 1073K Q ETgas

Heat Pipes

PSOFC ¼ V  I

Q

ETSOFC SOFC TSOFC = 1173K Fig. 4. Heat pipe control volume for exergy analysis.

for the electrochemical calculations based on literature models from Chan et al. [8,19], Campanari and Iora [20], Selimovic [21] and Costamagna et al. [10]. The two operational characteristics of the SOFC are the steam-tocarbon ratio (STCR) and the fuel utilisation factor (Uf). The STCR is defined as nH2 O STCR ¼ , (12) nCH4 þ nCO þ nCO2 where nH2 O , nCH4 and nCO are the mole flows of the product gas components. Supplementary steam is added to the product gas before it enters the SOFC anode, so as to ensure carbon-deposition-free operation. In the present work, a STCR value of 2 was chosen, which is suggested when pre-reformed methane is fed to SOFCs [10,11]. It should be noted that large amounts of steam require large available water quantities and, furthermore, the efficiency penalty deriving from exergy destruction associated with steam production has to be considered as well. The fuel utilisation factor of the stack is defined as nH2 ;REACT U f ¼ IN , (13) IN nH2 þ nIN CO þ 4nCH4

where the current is evaluated as I ¼ 2FnH2 ;REACT . The corresponding current density is J ¼ I/ASOFC (A m2) where ASOFC is the active cell surface area equal to 100 m2. By specifying the fuel utilisation factor, the anode flow throughput and composition, iterative calculations of the overall energy balance are performed over the SOFC stack control volume to result in the air throughput adjustment to fulfil the gasification heat demand and allow 5 kWth thermal loss at the desired SOFC temperature. The modelling results of the net SOFC electric power vs. current density for various fuel utilisation factors Uf, are shown in Fig. 5. In order to draw more power from the SOFC stack, it has to be operated in reduced Uf values and increased current densities. For current densities up to 3000 A m2, higher Uf values can produce higher electric outputs. The control volume for the exergetic analysis of the SOFC is shown in Fig. 6, where the incoming exergy streams correspond to the product gas (Egas) and air (Eair), and the outgoing exergy streams correspond to depleted gas (Edepleted gas) and air (Edepleted air), while PSOFC is the electric power produced and E Q SOFC exergy of heat which is transferred to the gasifier through heat pipes. The exergetic electrical efficiency of the SOFC is defined as the ratio of the electric power output PSOFC to input exergy of product gas and air entering the SOFC stack, 240

refers to the anode’s fuel species input and where nIN i nH2 ;REACT is the H2 (mol s1) reacting in the hydrogen electrochemical reaction (14)

The output voltage of the cell is V ¼ V OC  V OHM  V ACT  V PO

(15)

ðVÞ.

The Nernst open circuit cell voltage VOC was evaluated at an average operating temperature TSOFC, i.e. the average between the anode and cathode inlet flows and the outlet of the SOFC at 1173 K, o

V OC ¼ 

DG RT SOFC þ ln 2F 2F 23

out 1=2 pout H2 ðpO2 Þ pout H2 O

,

19

(16)

Uf = 0.7

0.75

220 0.8 Electric power output (kWe)

H2 þ 12O2 2H2 O:

(17)

ðWÞ,

200 180 0.85

0.65

160 140 120 100 80 60 40

1

where F ¼ 6.023  10  1.602  10 C mol is the Faraday constant, 2 is the number of e produced per H2 mole that reacts through reaction (14) of which the

0

1000

2000

3000

4000

5000

6000

7000

Current density (Am-2)

Fig. 5. SOFC electric power vs. current density for Uf ¼ 0.65–0.85.

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Egas and Eair Zex;el;SOFC ¼

PSOFC . E gas þ E air

(18)

In the proposed configuration, the SOFC provides direct heat for the gasification; therefore, another exergy efficiency ratio is defined, expressing the exergy of heat output EQ SOFC , at the SOFC operating temperature TSOFC, again with respect to material streams exergy input Zex;heat;SOFC ¼

EQ SOFC . E gas þ E air

(19)

Eq. (19) refers to the effectiveness in delivering heat exergy for driving the gasification. In accordance to Eq. (5), the total SOFC exergetic efficiency is the sum of Eqs. (18) and (19), and are shown in Fig. 7, vs. the current density for fuel utilisation factors

PSOFC

297

0.65–0.85. Increasing current densities correspond to increasing gasifier biomass throughput and thus product gas throughput in the SOFC. This leads to increasing voltage drops and penalties in electrical exergetic efficiency. In accordance with modelling results shown in Fig. 5, the electrical exergetic efficiencies of the stack are higher for the maximum Uf ¼ 0.85 at lower current densities; lower Uf could only be preferable at higher current densities. In the present SOFC stack configuration, reduced air flows are required to fulfill the stack cooling since part of the dissipated heat is drawn from the heat pipes. Nevertheless, special consideration has to be paid to avoid system operations under air starvation conditions. In order to assess this, the air utilisation factor Ua is defined for the air side similar to the fuel utilisation and is graphed in Fig. 8 vs. fuel utilisation factor Uf. Higher Uf causes the increase of current density and, therefore, the additional air requirement (i.e. Ua decrease). It is notable that options for reduced Uf and lower fuel throughputs result in unacceptable increased Ua values: in these cases, part of the stack would remain inactive in oxygen starvation conditions.

Q

ETSOFC

Egas

4. Integrated exergetic analysis

SOFC Eair

TSOFC=1173 K

Edepleted gas

4.1. Integrated system exergy analysis The exergetic efficiency of the CHP system for electricity production is

Edepleted air

Zex;el;CHP ¼

Fig. 6. Control volume of the SOFC for exergy analysis.

Zinv PSOFC  PCOMP þ E Q usefull E biomass

.

100 90 80

Exergetic efficiency (%)

70 60

0.85

50

0.75

ex el SOFC + ex heat SOFC

Uf = 0.65

40

0.85 30 0.75 20

ex el SOFC

Uf = 0.65 10 0 0

1000

2000

3000

4000

5000

6000

7000

8000

Current density (Am-2)

Fig. 7. SOFC electrical efficiency and useful heat exergy efficiency for the gasification vs. current density J (A m2) for Uf ¼ 0.65–0.85.

(20)

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5. Conclusions

1 0.9

50 kg/hr

The combination of allothermal biomass gasification based on the BioHPR reactor system with an SOFC into a small-scale CHP was analysed exergetically. Driving heat from the SOFC stack to the biomass gasifier with the help of high-temperature sodium heat pipes results in significantly reduced air necessary to cool the fuel cell stack. High fuel utilisation factors would then have to be applied to avoid air starvation conditions to the cell. The system has a high electrical efficiency but suffers from low heat energy efficiency because high-quality heat is consumed for the gasification, the pre-heating of the SOFC air and the production of the SOFC required steam, leaving the flue gases at low-quality temperature levels, sufficient only to produce low-quality useful heat.

0.8

Ai utilisation (Ua)

0.7 0.6 100 kg/hr

0.5 0.4 0.3

150 kg/hr

0.2 200 kg/hr

0.1 0 0.6

0.65

0.7

0.75

0.8

0.85

0.9

Fuel utilisation (Uf)

References

Fig. 8. Air utilisation factor Ua vs. fuel utilisation factor Uf at different biomass fuel throughputs (50–200 kg h1).

50

Exergy efficiency (%)

40

Uf = 0.85 0.8 0.75

30

0.7 0.65

20

10

0 0

1000

2000

3000

4000

5000

6000

7000

2

Current density (A/m )

Fig. 9. Total exergy efficiency of the CHP system vs. current density for Uf ¼ 0.65–0.85.

Fig. 9 shows the total exergy efficiency of the system vs. current density, which is maximised for a wide range of current densities when Uf ¼ 0.85. For an average current density of 3000 A m2 the system would consume 90 kg h1 biomass producing 170 kWe net power with a system exergetic efficiency of 36%, out of which 34% are electrical. The thermal exergy output of the system is low because part of the high-quality dissipated SOFC heat is used to drive the gasification process, and, after pre-heating the SOFC air and producing the SOFC required steam, the flue gases reach low-quality temperature levels, sufficient only to produce warm water at 350 K.

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