Exergetic Assessment of a Hybrid Steam Biomass Gasificationand SOFC System for Hydrogen, Power, and Heat Production 4Abdussalam Abuadala and Ibrahim Dincer

Abstract

In this chapter, an integrated process of steam biomass gasification and a solid oxide fuel cell (SOFC) for

multi-generation purposes (hydrogen, power, and heat) is thermodynamically studied, and its performance is assessed

through exergy efficiency. The scheme combines SOFC at 1,000 K and 1.2 bar and a gasifier which is used to gasify saw

dust with a steam–biomass ratio of 0.8 kmol/kmol and a gasification temperature range of 1,023–1,423 K at an

atmospheric pressure. A parametric study is performed to assess exergetic efficiency and investigate the effect of various

parameters related to the different system components such as airflow rate and preheating temperature on the efficiency.

The results show that SOFC is a major source of the system destruction exergy. For the gasification temperature range

studied here, the system exergetic efficiency increases with hydrogen yield from about 22 to 32 % and the overall exergy

efficiency, which considers electricity production, decreases from 57.5 to 51 %, respectively.

Keywords

Thermodynamics � Gasification � Biomass � Hydrogen � Solid oxide fuel cell � Exergy � Efficiency

Introduction

Conventional energy conversion systems fed by fossil fuels are known by their negative impact on the environment through

greenhouse gas emissions and air pollution, and these impacts will increase as energy demand increases by this rhythm

worldwide. These impacts can be reduced or eliminated by using alternative resources that are environmentally friendly to

produce fuels and use alternative energy conversion technologies, such as solid oxide fuel cell (SOFC), to generate heat and

power that are required for different applications.

There is a growing, worldwide interest in the development of technologies allowing the exploitation of renewable energy

sources, for both environmental and economical reasons [1]. To avoid the limitations on the use and applicability of biomass

in producing energy, it must compete with other renewable energy sources as well as fossil fuels [2]. Energy systems based

on the use of hydrogen offer a great promise for the future [3].

Biomass gasification-derived hydrogen is a renewable and sustainable fuel, which can be used as an alternative fuel and to

fuel SOFC for various applications. It was reported that the highest yield of hydrogen from lignocellulosic biomass is from

steam gasification [4].

The interest for future energy studies and solutions lies in hybrid systems to increase efficiency, reduce cost, and mitigate

greenhouse gas emissions. Such hybrid systems effectively show interaction between each other, which enables one system

to utilize products from other systems. The hybrid systems can differ from each other by including different numbers of

components or by way of interaction between them, which enables the system to perform different duties. The most typical

A. Abuadala (*) � I. DincerFaculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT),

2000 Simcoe Street North, Oshawa, ON, Canada L1H 7K4

e-mail: Abdussalam.Abuadala@uoit.ca; Ibrahim.Dincer@uoit.ca

I. Dincer et al. (eds.), Progress in Exergy, Energy, and the Environment,DOI 10.1007/978-3-319-04681-5_4, # Springer International Publishing Switzerland 2014

33

hybrid configuration suggested in the literature is a recuperated gas turbine process with an SOFC as the core unit of the

system [5]. Barvarsad [5] reported that electrical efficiency predictions for the system that combines the two units in a range

of 58–65 %. Costamagna et al. [6] energetically investigated a small-size hybrid system, combining a gas turbine that

produces about 50 kWe and a tubular SOFC. Costamagna et al. [6] found that the thermal efficiency of the system was

always higher than 50 %. Balli et al. [7] studied the exergetic performance assessment of a combined heat and power (CHP)

system installed in the Turkish city of Eskisehir. The system did not include a gasifier or an SOFC. They found from

the performed exergy analysis, along with system essential components, that the highest exergy consumption between the

components occurs in the combustion chamber.

Many researchers (e.g., Barvarsad [5]; Calise et al. [8]; Akkaya et al. [9]) indicated that limited studies have been

conducted on the exergetic performance of hybrid SOFC/GT systems and the effects of design and operating parameters on

the exergetic system efficiencies and destructions.

Fryda et al. [10] investigated a combination of an air-blown fluidized bed biomass gasifier with a high-temperature SOFC

and/or a gas micro-turbine in a CHP system of less than 1 MWe, which could operate at two pressure levels, near

atmospheric and about 4 bar, respectively. They used Aspen Plus software to simulate the integrated system. They found

that the pressurized SOFC operation is greatly improved and with power from a gas micro-turbine achieves efficiencies of

�35% when the current density value was 400 mA m�2.

Akkaya et al. [9] analyzed exergy performance by an exergetic performance coefficient which would give maximum total

exergy output possible for a given entropy generation rate. They used lumped control volumes to thermodynamically study

the system components. The analysis was conducted on a combination of a methane-fed SOFC and gas turbine in a CHP

system. They found that for a given total exergy output the maximum exergetic performance coefficient is achieved at the

least entropy generation rate.

Baravsad [5] analyzed a methane-fed internal reforming SOFC–gas turbine power generation system, based on the first

and the second law of thermodynamics. They found that increasing the fuel flow rate does not have a satisfactory effect on

system performance. Also they found that cycle efficiency increased when fuel or air flow rates decreased.

In the present study, exergetic assessment of a hybrid system is performed, and an influence of various operating

parameters of the system components on the performance is investigated. The system was proposed, and its energy

efficiency was studied in a previous work [11]. In order to improve this system, it is essential to understand parametric

impacts on the exergetic efficiency and hence enhanced evaluation of the system. This applies in particular to those

parameters which are related to different components like SOFC preheated airflow rate, burner preheated airflow rate,

and SOFC preheated air temperature. A comprehensive EES code is developed for system simulation. It is also designed to

calculate destroyed exergy as a result of exchanging energy in the steam-reforming reactor, water gas shift reactor, SOFC

stack, burner, gas turbine, air compressors, and heat exchangers. An assessment of the system via exergy analysis confirms

an ability of the system to competitively stand against other systems. Accordingly, this study investigates an exergetic

assessment of the system. Also, a parametric investigation is conducted to study the effect of single components through the

impact of their related parameters on exergy efficiencies.

System Description

We know that combining/hybridizing systems results in a better efficiency and effectiveness. This study includes an exergy

assessment to show how efficiently and effectively the system produces hydrogen, heat, and power. The main components of

the system are gasifier, SOFC, compressors, turbine, and heat exchangers (Fig. 4.1i). The performed analysis contains the

application of mass conservation, energy conservation, and entropy balance for the system components. The analysis is

performed under some general assumptions: steady state with negligible kinetic and potential energies and the gases obey

the ideal gas relations.

Analysis

The mass, energy, entropy, and exergy analyses of the system components are performed where the outlet stream from a

component considers the input stream for the next neighbor component. Each stream may constitute a single element

or a sum of elements. Separation of hydrogen from the produced gas is typically based on the separation by filters.

34 A. Abuadala and I. Dincer

3

DC/AC Inverter

Char &TarSeparation

Unit

S O F C

BurnerGas

Turbine

Biomass

1

2

5

611

78

10

12

19

14

4

13

15

16

H2

Gasifier

Flue gases

H2O

FilterCO2

H2

Steam ReformingReaction

Filter1H2

20

22

17

33

34

Compressor

26

27

21Water Gas Reaction

5W6•

7W8•

18

Air24

25

36

24W 25•

9

H2 StorageCO2 Storage

0

Steam

Air

Gas

0W9•

Air

35

i

Fig. 4.1 (i) System layout and (ii) schematic diagram of system components: (a) Compressor 5-6. (b) Turbine 7-8. (c) Heat exchanger 17-18-9-10.(d) The steam-reforming reactor. (e) The water gas shift reactor. (f) The SOFC. (g) The burner

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 35

T6T=T5

Compressor

P6P=P5

65W�

m6�m5�Turbine

7-8

T7

P7

P8

T8

m7�

m8�

87W�

9 10

18

17

Air

T18=300K

T17

T9P9

P17

P18

T10

P10

18m�

m9� m10�

17m�

Gas

P15

T15

P16

T16

P17

T17

Steam Reforming Reactor15m�

16m�

17m�

P21

T21

P18

T18=300 K

P22

T22

Water Gas Shift Reactor

18m�

22m�

21m�

Cathode Channel

ELECTROLYTE

CATHODE

ANODE

m10� 11m�

13m�m14�

Wac� Wdc

�

Anode Channel

35

11

7Burner

26

7m�

11m�26m�

35m�

7T

11T

26T

35T

a b

c d

e

g

f

ii

Fig. 4.1 (continued)

36 A. Abuadala and I. Dincer

These devices are assumed to have a neutral effect regarding energy: i.e., they do not destroy a significant amount of exergy;

and hence, their effect on the destroyed exergy is neglected. The processes taking place in the different components are

assumed to be of steady state, and the change in potential and kinetic forms of energy is neglected. Also, the energy losses

from a body of component to the environment are considered negligible by assuming the process adiabatic.

Energy and Exergy Balance EquationsThe biomass feedstock is sawdust wood and is totally gasified to H2, CO, and CO2 (char represents 5 % [10] of the biomass

carbon content; C6H6 [12, 13] represents the tar; and methane represents the other hydrocarbons). Hydrogen is oxidized in

the SOFC to water (steam), and methane is gasified in reforming reaction to CO and H2. CO is completely oxidized to CO2 in

water gas shift reaction. The remaining products (tar and char) are further processed to combust in the burner. Only hydrogen

from gasification is used in the SOFC, and therefore all SOFC calculations are based on these conditions. However, the

secondary hydrogen derived from downstream processing is stored.

In this study, the exergy of the used biomass is calculated by the method of Szargut et al. [14] as follows:

Exbiomass ¼ βLHVbiomass ð4:1Þ

where the biomass lower heating value is given by Shieh et al. [15]:

LHVbiomass ¼ 0:0041868 1þ 0:15 O½ �ð Þ 7837:667 C½ � þ 33888:889 H½ � � O½ �=8ð Þ ð4:2Þ

Here, C, H, and O are, respectively, carbon, oxygen, and hydrogen elements in sawdust wood, and they are obtained from

the sawdust ultimate analysis. The ultimate and proximate analysis data of the used wood are given in Table 4.1. The quality

coefficient β is given in terms of oxygen–carbon and hydrogen–carbon ratios, and according to the following equation:

β ¼ 1:0414þ 0:0177 H=C½ � � 0:3328 O=C½ � 1þ 0:0537 H=C½ �f g1� 0:4021 O=C½ � ð4:3Þ

The exergy flow rate is primarily calculated from the following equation:

_Exi ¼ _miExi ð4:4Þ

where the subscript i represents fuel (reactant) or agent or product and Ex is exergy. One part of the exergy depends on matter

composition which is known as chemical exergy, Exch, and for a mixture is given by

Exch ¼Xi

XiExO, i þ RTO

Xi

Xi ln Xi ð4:5Þ

Table 4.1 Ultimate

and proximate analysis

of sawdust wood

Element Weight on dry basis (%)

C 48.01

H 6.04

O 45.43

N 0.15

S 0.05

Ash 0.32

HHV (MJ/kg) 18.4

Volatile matter 76.78

Fixed carbon 18.7

Ash 0.32

Source: Turn et al. [16]

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 37

where Xi is the mole fraction of component i and Exo is standard exergy. The other part of exergy depends on the matter

temperature and matter pressure. It is known as physical exergy, Exph, and is given by

Exph ¼ h� hOð Þ � TO s� sOð Þ ð4:6Þ

where h and s are enthalpy and entropy at T and P and h0 and s0 are enthalpy and entropy at standard operating state

(T0 ¼ 298.15 K and P0 ¼ 1 atm). The total exergy, Ex, is the sum of the two above exergy parts.

Enthalpy and entropy data are necessary to perform both energy and exergy analyses. Gases are assumed to obey the ideal

gas behavior, and their enthalpies and entropies, respectively, are

h ¼ hOf þ Δh and s ¼ sO þ Δs

The properties of enthalpy and entropy change as a function of temperature, and they obey the ideal gas laws. To find

enthalpy and entropy values, the constant pressure-specific heat in kJ/(kmol k), Cp, is utilized as a third-degree polynomial

equation [17]. The enthalpy of formation, hfO, and entropy at standard state, sO, are obtained from thermodynamic tables.

Entropy of the gases changes as a function of temperature and pressure.

The specific heat of tar in coal gasification was developed by Li et al. [18, 19] and modified by Hyman et al. [20].

The same equation is used for derived tar from biomass gasification in kJ/kgtar K:

Cptar ¼ 0:00422T ð4:7Þ

The enthalpy and entropy values of tar are calculated in the same way as described above. However, its enthalpy

of formation and standard entropy are calculated from the following equations [20]. The enthalpy of formation, htaro ,

in kJ/kmol is given by

hotar ¼ �30:980þ XCO2

hoCO2

þ XH2O hoH2O

ð4:8Þ

where Xi is the mole fraction and hiO is the standard enthalpy of formation for specie i. The term related to sulphur is omitted

since the used biomass has negligible sulphur content. The standard tar entropy, staro , in kJ/(kmol K) is given by

sotar ¼ a1 þ a2exp �a3H

Cþ N

� �� �þ a4

O

Cþ N

� �þ a5

N

Cþ N

� �þ a6

S

Cþ N

� �ð4:9Þ

where a1–a6 coefficients are a1 ¼ 37.1635, a2 ¼ �31.4767, a3 ¼ 0.564682, a4 ¼ 20.1145, a5 ¼ 54.3111, and a6 ¼ 44.6712.

C, H, N, O, and S are, respectively, carbon, hydrogen, nitrogen, oxygen, and sulphur weight fractions in the sawdust.

Energy analysis is done in terms of the enthalpy. Processes in the system components are steady-state steady-flow

processes. Therefore, energy conservation for the adiabatic process takes place in the system component and is found from

the first law of thermodynamics as follows: rate of energy at the inlet state(s) is equal to rate of energy at the exit state(s).

Mathematically, on molar basis, it can be expressed by the following equation:

Xi

_Nihi ¼Xe

_Nehe þ i_W ð4:10Þ

The continuity equation becomes

Xi

_NiMWi ¼Xe

_NeMWe ð4:11Þ

where i and e refer to inlet and exit state point(s) of the system component under the study, h is the specific enthalpy, _N is the

molar flow rate, and MW is the molecular weight. The term i_W is less than zero when the system component consumes

power, greater than zero when the system component produces power, and greater zero when the system component does not

produce or consume power.

38 A. Abuadala and I. Dincer

The exergy destruction in a system component is calculated from the following equation:

_Exdes

¼ T0_Sgen ð4:12Þ

For each system component, the entropy generation, _Sgen, term is calculated from the entropy balance. The entropy

balance for the process takes place in the system component that is written through the second law of thermodynamics as

follows: rate of entropy at the inlet state(s) plus rate of the entropy generation in the system component is equal to rate of

entropy at the exit state(s). Thus, it can be expressed in a molar basis by the following equation:

Xi

_Nisi þ _Sgen ¼Xe

_Nese ð4:13Þ

Compression ProcessesThe compressor 5–6 is used to increase the pressure needs in a filtration process and to increase the gas temperature to the

temperature that is preferred for the reformation reaction to occur as well as to protect the gasifier from the backflow that can

happen (Fig. 4.1ii.a). The species, namely, H2, CO, CO2, and CH4, are compressed here. The temperatures of the gases at the

compressor exit and inlet are related to the corresponding pressures and compressor isentropic efficiency. The compression

process is also needed to compress the air as required for electrochemical reaction that takes place in the SOFC. The amount

of air is that air necessary for the electrochemical reaction to take place in the SOFC which is related to fuel with a

hydrogen–air ratio of 2. The pressure and the temperature of air at the compressor upstream are given at atmospheric

conditions. The temperature after the preheating process is calculated from the energy balance that is conducted on the

former SOFC heat exchanger. The temperature and pressure of the other streams are known. Streams which exit the SOFC

have the same temperature and pressure of the SOFC; the fuel (H2) stream has properties after filtration process where

pressure exceeds by 5 % of the SOFC operating pressure. The preheated air temperature is found based on the required

temperature delivered at the SOFC and the energy balance of the SOFC.

The compression process needs to compress air to the burner. The amount of air that will compress is that amount used to

control the burner temperature on the one hand and on the other hand to make sure that there is a sufficient amount of air to

completely burn the residuals that are sent to the burner from the SOFC and the gasifier. The power that drives this

compressor is calculated through energy analysis. The temperature after the preheating process is assumed to be 430 K and a

pressure which equals an operating pressure of the SOFC. The preheated air temperature is found based on a sufficient

amount of air and reasonable temperature needed at the burner. The energy required for the preheating process is extracted

from by-product gases when they pass through the heat exchanger that is located after the separation process.

Gas TurbineThe flue gas leaves the burner and gets expanded in the turbine to extract its energy content for power output (Fig. 4.1ii.b).

The stream properties at the turbine inlet are the same as those of the burner exit. According to the analysis done for the

burner, the gas consists of steam, carbon dioxide, air, and nitrogen. The properties of the stream at the turbine exit (state 8)

are given at the surrounding conditions (P0 and T0). The species that undergo expansion are water, air, nitrogen, and carbon

dioxide. One can look to the expansion process that takes place in the turbine and describe it as an opposite process to the

compression process that happens in the compressor. The produced power, when flue gases expand in the turbine, is found by

applying energy analysis. All species behave like an ideal gas at both states, and therefore their enthalpies become a function

of temperature only, and they are given in terms of constant pressure-specific heat. The temperature of the flue gas at the

turbine exit is assumed such that it obeys the environmental restrains.

Heat ExchangersIn heat exchangers 17-18-9-10, the 17-18 shows the hot stream, while the second one 9-10 indicates the cold line (Fig. 4.1ii.c).

The presence of this heat exchanger aims to extract heat from the gasification product gases to preheat air that passes through the

heat exchanger and utilizes in the SOFC. Three species constitute the hot stream H2, CO, and CO2, while the cold stream is air.

The temperature of the hot stream at state 17 is obtained from the energy balance of the steam-reforming reactor, while the

temperature at state 18 is assumed equal to the ambient and the pressure is decreased by about 5 % at state 17. Therefore, the

parameters of the hot line are known. Also, the properties of air at the heat exchanger inlet are known from the compressor 0-9

analysis. Air properties at the heat exchanger outlet are known from the energy balance of the heat exchanger. Accordingly,

a number of cells in the SOFC stack are known from the SOFC analyses. The same principles are applied to heat exchanger

36-5-25-35.

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 39

In heat exchangers 20-21-3-4, the 20-21 shows the states on the hot stream, while the second one 3-4 indicates the

states on the cold stream. The presence of this heat exchanger aims to extract heat from the high-temperature steam, 20-21,

to produce steam and use it as the gasification agent in the gasification process. In the present study, the amount of steam _m4

delivered to the gasifier is known. Also, the amount of steam flow in the hot stream and its inlet temperature (temperature

at state 20) are known from the SOFC analysis. Only the temperature of the hot stream at state 21 is unknown and is

calculated from the performed energy balance. The energy balance of the heat exchanging process simply says that energy

removed from the hot streamline is absorbed by the steam flow in the cold line.

Steam-Reforming ReactorAs a potential method to increase hydrogen yield from the system, the gas produced from the gasification process is further

processed to the steam-reforming reactor (Fig. 4.1ii.d). The reaction in the reactor is governed by the following reaction

equation:

CH4 þ H2O ! 3H2 þ CO ð4:14Þ

According to this reaction, H2–CO ratio of three is used in the analyses. A part of the steam of the SOFC electrochemical

reaction by-product is used as a reaction medium. The amount of steam that is required for the steam-reforming reaction is

calculated based on the molar balance of the reaction equation. It is clear from the reaction equation that a ratio of the

number of methane moles to that of the used steam is one. The molar flow rate of methane is known from the gasification

process analyses, while the molar flow rate of both the needed steam by the reaction and that of the reaction products are

known from the molar balance equation of the reaction.

The steam-reforming reaction is endothermic. The reactants of the steam-reforming reactor are H2O, CH4, CO, and CO2,

and its product gases are H2, CO, and CO2. The molar rates of carbon monoxide, methane, and carbon dioxide in the steam-

reforming reactor are known from the gasification analysis, while the steam is used according to the steam-reforming

reaction equation. Thermodynamic properties at the steam-reforming reactor inlet states are known, and the mole flow rates

at the steam-reforming reactor exit are known. Only the temperature of the exiting stream is unknown, and this can be

calculated from the energy balance of the steam-reforming reactor.

Water Gas Shift ReactorProcessing the gases further to the water gas shift reactor also aims to increase a hydrogen yield of the system. In this

process, the carbon monoxide from the gasification process as well as that from the steam-reforming reactor will shift by

steam to hydrogen and carbon dioxide according to the following reaction:

COþ H2O ! H2 þ CO2 ð4:15Þ

Here, the properties at state point 21 are known from the performed analysis on the SOFC while properties of state 18 are

known from the performed analysis on the heat exchanger 17-18-9-10 (Fig. 4.1ii.e). From the thermodynamic point of view,

the water gas shift reactor will be treated in a manner similar to that of the steam-reforming reactor. However, in this case,

the reaction is exothermic and takes place at lower temperatures. The process is assumed to take place adiabatically, and the

reactants of the water gas shift reactor are H2O, CO, and CO2 and the products of the water gas shift reactor are H2 and CO2.

The molar flow rate of the carbon monoxide will be the sum of the one from the gasification process and that from the steam-

reforming reaction. The other species molar flows are known from the mole balance of the reaction equation. The hydrogen

in this case is called secondary hydrogen and is stored after the filtration process, while the hydrogen from the gasification

process is called primary hydrogen and is used to fuel the SOFC after it is purified from the contaminants.

Solid Oxide Fuel CellA fuel cell is a device that converts the energy released from a reaction of matter, in this case, hydrogen with oxygen, directly

into electricity without the intermediate step that is seen in conventional thermal cycles where the chemical energy converts

first into thermal and then into electricity. The most common classification of fuel cells is by the used electrolyte type,

operating temperatures, and the mechanism by which charge is conducted in the cells and the SOFC operates in a

temperature range of 650–1,000 �C [21]. Because inherent properties tolerate well with contaminants from the gasification

process and operate in a temperature range similar to that of biomass gasification, the SOFC is used in the system. The

depleted air at the SOFC temperature from the SOFC’s cathode chamber is fed directly to the burner.

40 A. Abuadala and I. Dincer

The SOFC is the device that converts chemical energy available in matter to electricity, and the process produces

electricity (and some heat) and water via the reaction that happens between oxygen from air and hydrogen from gasification

according to the following reaction:

H2 þ 1

2O2 ! H2O ð4:16Þ

The open-circuit voltage is calculated at an average temperature between the mixed anode and cathode inlet flow and the

outlet of the SOFC from Nernst’s equation as follows:

VSOFC ¼ �ΔG0

2F� RTSOFC

2Fln

PSOFCH2O

PSOFCH2

ffiffiffiffiffiffiffiffiffiffiffiffiffiPSOFCO2

q0B@

1CA ð4:17Þ

where ΔG0 is the standard Gibbs free energy change per mole, R is the universal gas constant (8.314 kJ/kmol K), and F is the

Faraday constant (96,485 C/g mol). PSOFCH2O

, PSOFCH2

, and PSOFCO2

are, respectively, the partial pressure of H2O and H2 at the

cathode and of O2 at the anode. The voltage is obtained by subtracting the overpotential voltages from the above voltage.

The overpotential losses are originated from three sources: concentration, Vcon; ohmic, Vohm; and activation, Vact:

V ¼ VSOFC � Vcon � Vohm � Vact ð4:18Þ

The overpotentials due to activation, Vact, is calculated from the Butler–Volmer equation with a reaction rate constant

of 0.5 [22]:

Vact ¼ 2RTSOFC

nH2Fsinh�1 i

2io

� �ð4:19Þ

This equation is applied for the electrodes, cathode and anode, where i is the current density and io is the apparent

exchange current density. The ohmic overpotential, Vohm, obeys ohm’s law and is given by

Vohm ¼ iRres ð4:20Þ

The resistance of all materials, Rres, and those used in SOFC components can be obtained from Costamagna et al. [23],

and the respective resistivity is a function of temperature and is calculated by Bessette II et al. [24] from the following

equation:

ρ ¼ a expb

TSOFC

� �ð4:21Þ

where a and b are constants depending on cell material, see Table 4.2.

The polarization or the concentration overpotential, Vpol, is a summation of polarization over potential from anode, Vpol,a,

and that from cathode, Vpol,c, and can be obtained from Costamagna et al. [23].

The electric power produced by the SOFC is

_WSOFC,dc ¼ VI ð4:22Þ

Table 4.2 Cell material

resistivity and its dependence

on temperature

Cell material (carrier type) Resistivity formula Ω-cm

Air electrode (electronic) 0.008114exp (600/TSOFC)

Electrolyte (ionic) 0.00294exp (10,350/TSOFC)

Fuel electrode (electronic) 0.00298exp (�1,392/TSOFC)

Interconnection (electronic) 0.1256exp (4,690/TSOFC)

Source: Bessette II et al. [24]

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 41

For H2 fuel, the current I is calculated by

I ¼ 2F _nH2

2 ð4:23Þ

where 2 is a number of electrons transferred per molecule of fuel and _n 2H2

is the H2 (mol/s) that reacts in the hydrogen

electrochemical reaction which was solely considered. _n 2H2

is the utilized part from the hydrogen that is supplied to

the SOFC.

The fuel cell model developed in this study is based on a planar geometry in which its dimensions and material-related

data are given according to the data listed in Table 4.3. The preheating air is fed into the cathode inlet (state 10), and excess

depleted air and nitrogen flow out from the cathode exit (state 11). On the anode side from the cell, hydrogen is fed into the

anode inlet (state 13) and steam and excess depleted hydrogen flow out from the anode exit (state 14). If the fuel cell utilizes

fuel by a factor of UF, the depleted hydrogen that flows out will be _nH2

2 (1 � UF). One mole from water contains a H2–O2

mole ratio of 2. Therefore, a molar flow rate of hydrogen, _NH2, 13, that is used from the gasification process is twice the molar

flow rate of oxygen that is used from the supplied air. It means that the consumed oxygen will change according to the

utilized hydrogen and both of them will depend on the assumed utilization factor. It is well known that air has a N2–O2 ratio

of 79-21 and the nitrogen is treated as an inert substance. Therefore, from the molar flow rate of the utilized oxygen, the total

amount of air that is needed to supply to the SOFC can be calculated.

The energy balance for the adiabatic SOFC is obtained by applying the first law of thermodynamics for the states shown

on the schematic diagram of the SOFC (Fig. 4.1ii.f), where the reactants of the SOFC are H2 that flows at state point 13 and

air that flows at state point 10. The products of the SOFC are H2, N2, and O2 that flow at state point 11 and H2O (g) that flows

at state point 14.

BurnerA burner is used to convert the chemical energy of the unutilized fuel in the SOFC stack to heat. In this process, more

chemical energy is converted to thermal energy. After the SOFC stack, the excess depleted fuel and air, and the separated

char and tar from the gasification product, are sent to the burner (Fig. 4.1ii.g). An extra amount of preheated air via the

stream 36 is fed to the burner to make sure that all materials are completely burnt. The products of the burning process

contain mainly steam, carbon dioxide, and nitrogen according to the following reactions:

char26Cþ char26O2 ! char26CO2 ð4:24Þ

tar26C6H6 þ 7:5tar26O2 ! 3tar26H2Oþ 6tar26CO2 ð4:25Þ

H2,11H2 þ H2,11

2O2 ! H2,11H2O ð4:26Þ

Table 4.3 SOFC geometries and material-related data

Parameter Value Reference

Utilization factor, Uf 0.95 Bessette II et al. [24]

DC/AC inverter efficiency 0.95 Bessette II et al. [24]

Temperature of SOFC, TSOFC 1,000 K Bessette II et al. [24]

Active surface area, ASOFC 100 cm2 Colpan et al. [25]

Effective gaseous diffusivity through the anode, Daeff 0.2 cm2 s�1 Colpan et al. [25]

Effective gaseous diffusivity through the cathode, Dceff 0.05 cm2 s�1 Colpan et al. [25]

Thickness of the anode, ta 0.05 cm Colpan et al. [25]

Thickness of the cathode, tc 0.005 cm Colpan et al. [25]

Thickness of the electrolyte, te 0.001 cm Colpan et al. [25]

Thickness of the interconnect, tint 0.3 cm Colpan et al. [25]

Pre-exponential factor, γa 5.5 � 104 A/cm2 Costamagnaet al. [13]

Pre-exponential factor, γc 7 � 104 A/cm2 Costamagna et al. [13]

Eact,a 100 � 103 J/mol Costamagna et al. [13]

Eact,c 120 � 103 J/mol Costamagna et al. [13]

42 A. Abuadala and I. Dincer

The excess depleted oxygen from the former combustion process is the oxygen that flows at state 11, O2,11, and is known

from the SOFC analysis. Therefore, the minimum oxygen that is needed to feed to the burner becomes

O2,min ¼ O2,consumed � O2,11 ð4:27Þ

where O2,consumed is the oxygen that the above reactions need. The oxygen supplied to the burner has to satisfy at least the

minimum amount of oxygen and results in a reasonable temperature in the burner. Therefore, the preheated burner air, air35,

that flows at state 35 on the system flow diagram is greater than 4.762 times O2, min.

The molar flow rates of char and tar are known from the gasification process, while the molar flow rates of unutilized

hydrogen, H2,11; unutilized oxygen, O2,11; and nitrogen, N2,11, are known from the SOFC analyses. The properties of states

11, 35 (air), and 26 (char and tar) and the molar flow rate at state 7 are known. The only unknown is the temperature at the

burner exit which can be determined from the energy balance equation for the adiabatic burner.

Exergy EfficienciesA study of the system exergetic efficiency (or second-law efficiency) shows how efficiently the system works to increase the

secondary hydrogen yield from gasification via downstream processes, from external steam reforming and external water

gas shift reactions, and to utilize the primary hydrogen in producing electricity and heat. Four exergy efficiencies are defined

for this system based on the exergy of the fed sawdust: the exergy efficiency for producing power from the SOFC, exergy

efficiency for producing power from the gas turbine, exergy efficiency that considers producing secondary hydrogen from

gasification downstream processes, and exergy efficiency that considers all power from the system:

ηEX,SOFC ¼_ExSOFC_Exbiomass

ð4:28Þ

The exergy efficiency that considers a production of electricity and accompanies an expansion process of gases in the gas

turbine is

ηEX, t ¼_Ext,net_Exbiomass

ð4:29Þ

The third exergy efficiency that considers the hydrogen derived from gasification downstream reactions is defined as

ηEX,H2¼

_ExH2

_Exbiomassð4:30Þ

The system exergetic efficiency for electricity production is calculated from the exergetic efficiency that considers

producing electricity from the gas turbine and the SOFC. _ExH2is the exergy flow rate of the secondary hydrogen, _Exbiomass is

the exergy flow rate with biomass, and the subscript t stands for turbine. The exergy that flows with species at different states

is calculated in a way similar to that discussed above. The exergy of power is equal to the power itself.

Results and Discussion

Exergy Destructions

The rates of exergy destruction are calculated for the system components at the gasification temperature. Figure 4.2 shows

the exergy destructions of the system components at a gasification temperature of 1,023 K. It is clear from the figure that a major

part of the exergy destruction occurs in the SOFC stack followed by the turbine and the burner. Also, it is found that the total

exergy destruction in the system components has minimum value when the gasification temperature is 1,175 K (Fig. 4.3).

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 43

Exergy Efficiencies

In the gasification temperature range considered, and for a given utilization factor and steam–biomass ratio, the overall exergy

efficiencies for electricity production, based on the exergy content of biomass, are shown in Fig. 4.4a. The efficiency decreases

from 57.5 to 51% in the studied gasification temperature range because of decrease in the exergy efficiency of turbine. From the

exergy destruction results, it is found that a major part of exergy destruction occurred in the SOFC. Also, its exergy destruction

increasedwith an increase in the gasification temperature. The results show that secondary hydrogenyield increases and hence its

exergy increases. Thus, the exergy efficiency with the hydrogen production increases from about 22 % to about 32 %.

0

20

40

60

80

100

120

140

160

180

200

HE

17-

18-9

-10

HE

36-

5-25

-35

HE

3-4

-20-

21

Bur

ner

Com

pres

sor

0-2

5

Com

pres

sor

5-6

Com

pres

sor

0-9

SO

FC

Ste

am R

efor

min

g

Wat

er G

as S

hift

Tur

bine

Des

troy

ed E

xerg

y [k

W]

HE: Heat ExchangerFig. 4.2 Exergy destruction

rates in the system at gasification

temperature of 1,023 K

387

389

391

393

395

397

399

401

403

1000 1100 1200 1300 1400 1500

Exe

rgy

Des

truc

tion

[kW

]

Gasification Temperature [K]

1175 [K]

Fig. 4.3 Exergy destruction

in system components versus

gasification temperature

44 A. Abuadala and I. Dincer

Effect of Burner Exit Temperature

The overall exergy efficiency of the system increases as the burner temperature increases (Fig. 4.4b). Increasing the burner

preheated air enhances the energy available in the burner and thus its exit temperature which is the same as the turbine inlet

temperature. Higher inlet turbine temperature means higher power or exergy, and thus it improves the turbine exergy efficiency

which leads to improvement of the overall electrical exergy efficiency. Higher preheated burner temperature means a reduction

in the gases’ energy content which in turn decreases the exergy flow with product gas streams and among them hydrogen.

21

23

25

27

29

31

33

50

51

52

53

54

55

56

57

58

1000 1100 1200 1300 1400 1500

Exergy efficiency with electricity productionExergy efficiency with hydrogen production

Exe

rgy

Effi

cien

cy [

%]

Gasification Temperature [K]

Exe

rgy

Effi

cien

cy [%

]

a

y = 0.035x2- 1.7522x + 72.448

R2 = 0.9997

21

23

25

27

29

31

33

50

51

52

53

54

55

56

57

58

10 12 14 16 18 20 22

Exergy efficiency for electricity generation

Exergy efficiency for hydrogen yield

Exe

rgy

Effi

cien

cy [%

]

Burner Preheated Air Flow [kg]/ Biomass Flow [kg]

Exe

rgy

Effi

cien

cy [%

]

c

21

23

25

27

29

31

33

50

51

52

53

54

55

56

57

58

600 700 800 900 1000

Exergy efficiency with electricity production

Exergy efficiency with hydrogen production

Exe

rgy

Effi

cien

cy [%

]

Burner Breheating Temperature [K]

Exe

rgy

Effi

cien

cy [%

]

b

Fig. 4.4 Variations of exergy

efficiencies with (a) gasificationtemperature, (b) burnertemperature, and (c) burnerpreheated airflow–biomass ratio

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 45

Burner Preheated Airflow

The air supplied to the burner mainly aims to provide the oxygen and the energy needed for the burning process. In the

studied temperature range, an increase of preheated air per biomass throughput leads to a decrease in the overall exergy

efficiency of the system. This decrease is in good fitting with second-degree polynomial (Fig. 4.4c). Also, more gasification

by-products are sent to the burner at a higher gasification temperature. This will increase the burner temperature which in

turn increases the turbine inlet temperature.

SOFC Preheated Air Temperature

The temperature is found to be based on the amount of air needed for the combustion of hydrogen to take place via the

electrochemical reaction for the specific SOFC. The system has an exergy efficiency of 55 % when the preheated air has the

highest temperature of 445.5 K. In the studied gasification temperature range, the exergetic efficiency for electricity

production reaches a value of 57.5 % and that is when the preheated air temperature is 444.5 K (Fig. 4.5a). In the same

gasification temperature range, the system has the potential to increase hydrogen yield from about 22 % to about 32 %.

50

51

52

53

54

55

56

57

58

21

22

23

24

25

26

27

28

29

30

31

32

442 443 444 445 446

Exergy efficiency for hydrogen production

Exergy efficiency for electricity production

Exe

rgy

Effi

cien

cy [%

]

a

b

SOFC Air Breheating Temperature [K]

Exe

rgy

Effi

cien

cy [%

]

1067

1067

1112

1112

1156

1156

1201

1201

1245

1245 1290

1290

1397

1397

1423

1423

1334

1334

1023

1023

21

23

25

27

29

31

33

50

51

52

53

54

55

56

57

58

2.4 2.42 2.44 2.46 2.48 2.5 2.52 2.54 2.56 2.58

Exergy efficiency with electricity production

Exergy efficiency with hydrogen production

Exe

rgy

Effi

cien

cy [%

]

SOFC Preheated Air Flow [kg]/ Biomass Flow [kg]

Exe

rgy

Effi

cien

cy [%

]

Fig. 4.5 Variations of exergy

efficiencies with (a) SOFCpreheated air temperature

and (b) mass ratios of SOFC

preheated airflow

and biomass flow

46 A. Abuadala and I. Dincer

The maximum preheated temperature reaches its maximum value when the exergy efficiency of hydrogen yield is 26 %

(Fig. 4.5i). After that a steep decrease in preheating temperature is observed. This is attributed to the fact that at a

higher gasification temperature, more gases are produced, which results in a higher hydrogen concentration from the side

reactions that take place in the steam-reforming and water gas shift reactors, which in turn increases the energy content of the

product gases. After this temperature, the product gas energy content is dominant compared to the energy content of the air

sent to the SOFC.

Effect of SOFC Preheated Air Flow

It is found that increasing airflow per biomass throughput results in a reduction in the system overall exergy efficiency

(Fig. 4.5b). More preheated airflows increase the energy supplied to the SOFC which results in an increase in the destroyed

exergy. Also, an increase in the airflow results in a decrease of its temperature; therefore, more heat content is available with

product gas flow, which results in more hydrogen yield and thus higher exergy efficiency that considers hydrogen. Figure 4.6

shows that the preheated air fed to the SOFC and that fed to the burner affect the destroyed exergy in the system components

and both show a similar trend. The exergy destruction has the lowest value when the SOFC preheated airflow–biomass ratio

is 2.495, while the exergy destruction has the lowest value when the burner preheated air–biomass ratio has a value of 14.50.

Conclusions

The present study performed through exergy analysis investigates and assesses the potential of a new hybrid system which

combines both steam biomass gasification and SOFC with external water gas shift and steam-reforming reactions for multi-

generation purposes, including power, heat, and hydrogen. This chapter further studies the exergy efficiency of the system

that considers the hydrogen yield and the electricity production. It is found that the system efficiency with secondary

hydrogen yield increases from about 22 to 32 %, and this is attributed to the increase of hydrogen yield from the side

reactions that take place in the steam-reforming and water gas shift reactors. Also, for the same gasification temperature

range, the system exergy efficiency that considers electricity production decreases from 57.5 to 51 %. The SOFC has a major

contribution in the system exergy destruction, and any reduction in its exergy destruction results in an improvement in

electrical efficiency. The effects of the preheated air in the system on exergy efficiency were also studied. It was found that

the system’s electrical exergy efficiency increases, and that efficiency with hydrogen production decreases, when both

preheated airflows per biomass throughput decrease.

Acknowledgements This work is supported by University of Ontario Institute of Technology (UOIT). The first author would like to acknowledge

a support of Libyan Ministry for Education via Libyan Embassy in Canada.

2.4

2.42

2.44

2.46

2.48

2.5

2.52

2.54

2.56

2.58

10

12

14

16

18

20

22

387 389 391 393 395 397 399 401 403

Burner preheated air flow perbiomass throughputSOFC preheated air flow perbiomass throughput

Bur

ner

Pre

heat

ed A

ir [k

g]/B

iom

ass

[kg]

Total Exergy Destruction [kW]

SO

FC

Pre

heat

ed A

ir [k

g]/B

iom

ass

[kg]

389.61023

1067

11121156

1201

1245

14231397

1334

1290

1290

1334

1067

1112

1201

13971245

1156

1423

1023

14.50

2.495

Fig. 4.6 Variation of destroyed

exergy with preheated air fed

to the system in the gasification

temperature range

4 Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System. . . 47

Nomenclature

C Carbon content in biomass (w %)

Daeff Effective gaseous diffusivity through the anode

(cm2/s)

Dceff Effective gaseous diffusivity through

the cathode (cm2/s)

E Ohmic symmetry factor

Ex Exergy (kJ/kg or kJ/kmol)

Exo Standard exergy (kJ/kmol)

F Faraday constant (96,485 coulombs/g mol)

H Hydrogen content in biomass (w %)

h Specific enthalpy (kJ/kg or kJ/kmol)

I Circuit current (A)

i Current density (mA/cm2)

io Apparent exchange current density (A/cm2)

LHV Lower heating value (kJ/kg)

_m Mass flow rate (kg/s)

N Nitrogen content in biomass (w %)_N Molar flow rate (kmol/s)

_nH2 Hydrogen fed to SOFC (kmol/s)

O Oxygen content in biomass (w %)

P Pressure (pa or atm)

R Universal gas constant (8.314 kJ kmol�1 K�1)

S Total entropy (kJ/K)

s Specific entropy (kJ/kg K or kJ/kmol K)

T Temperature (K)

t Thickness (cm)

UF Utilization factor (–)

V Circuit or overpotential volt (volts)_W Power (W or kW)

X Mole fraction (–)

Subscripts

a Anode

act Activation

biomass Biomass

c Cathode

ch Chemical

con Concentration

dc Power from DC

des Destroyed

e Exit

el Electrical

gen Generation

H2 Hydrogen

H2O Water

i Inlet

o Reference or ambient

O2 Oxygen

ohm Ohmic

ph Physical

pol Polarization

res Resistance

SOFC Solid oxide fuel cell

t Turbine

tar Tar

Superscript

Over dot Quantity per time

Over bar Quantity per kmol

SOFC Solid oxide fuel cell

Greek Letters

β Quality coefficient (�)

ΔG Standard Gibbs function of reaction (kJ/kg)

η Efficiency (�)

ρ Resistivity (Ω-cm)

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