Thermodynamic modeling and evaluation of high efficiency heat pipe integrated biomass GasifiereSolid Oxide Fuel CellseGas Turbine systems

By using heat pipes between the SOFC acting as a heat source and the allothermal gasifier acting as a heat sink, the excess heat pro-duced at SOFC can be transferred to the gasifier, where

Thermodynamic modeling and evaluation of high ef ficiency heat pipe

systems

S Santhanam1, C Schilt, B Turker, T Woudstra, P.V Aravind*

Delft University of Technology, Energy Technology Section, Leeghwaterstraat, 2628 CB, Delft, The Netherlands

a r t i c l e i n f o

Article history:

Received 25 November 2015

Received in revised form

25 April 2016

Accepted 28 April 2016

Keywords:

Solid Oxide Fuel Cell

Gas Turbine

Biomass gasification

Heat pipes

Exergy

a b s t r a c t

This study deals with the thermodynamic modeling of biomass GasifiereSOFC (Solid Oxide Fuel Cell) eGT (Gas Turbine) systems on a small scale (100 kWe) Evaluation of an existing biomass GasifiereSOFC eGT system shows highest exergy losses in the gasifier, gas turbine and as waste heat In order to reduce the exergy losses and increase the system's efficiency, improvements are suggested and the effects are analyzed Changing the gasifying agent for air to anode gas gave the largest increase in the electrical efficiency However, heat is required for an allothermal gasification to take place A new and simple strategy for heat pipe integration is proposed, with heat pipes placed in between stacks in series, rather than the widely considered approach of integrating the heat pipes within the SOFC stacks The devel-oped system based on a GasifiereSOFCeGT combination improved with heat pipes and anode gas recirculation, increases the electrical efficiency from approximately 55%e72%, mainly due to reduced exergy losses in the gasifier Analysis of the improved system shows that operating the system at possibly higher operating pressures, yield higher efficiencies within the range of the operating pressures studied Further the system was scaled up with an additional bottoming cycle achieved electrical effi-ciency of 73.61%

© 2016 Elsevier Ltd All rights reserved

1 Introduction

Solid Oxide Fuel Cells are highly efficient devices Their second

law efficiencies are usually above 90%, as they produce electricity

and heat at very high temperatures By utilizing this heat to

pro-duce mechanical work, subsequently, electricity is expected to

result in very high efficiencies in electricity production [1e8]

Efficient heat management, helps to achieve high efficiencies in

such systems especially with a direct or indirect internal reforming

in the fuel cell when carbonaceous fuels are used SOFCeGT (Solid

Oxide Fuel CelleGas Turbine) systems using natural gas as fuel are expected to attain thermal efficiencies of 60%e80%[9] Operating such systems loaded with biosyngas and produced in a biomass gasifier, are expected to result in highly efficient and sustainable electricity production Biosyngas can be produced by endothermic reactions, when gasification agents, such as steam and CO2 are used The heat produced in SOFCs being partly used for gasification,

is expected to help minimize exergy losses in such integrated sys-tems The use of heat pipes for exchanging heat between solid oxide fuel cells and gasifiers, has been studied in the past[10e12] The EU (European Union)-funded project ‘BioHPR’ was successfully completed, using heat pipes in gasifiers while producing biosyngas with a high LHV (Lower Heating Value) Mol fractions of 40% H2, 20% CO and 5% CH4in the biosyngas are reported[13] Another EU funded project, Biocellus, has gained significant progress in inte-grating heat pipes with the gasifiers System studies at Delft, Im-perial College and other places have shown that electrical

efficiencies above 60% from solid fuels are achievable for SOFC based systems[14,15] Recently it has been shown a better thermal

* Corresponding author.

E-mail addresses: Santhanam.Srikanth@dlr.de (S Santhanam), A.

PurushothamanVellayani@tudelft.nl (P.V Aravind).

1 Permanent address: German Aerospace Center (DLR), Institute of Engineering

Thermodynamics, 38-40 Pfaffenwaldring, 70569, Stuttgart, Germany Tel.: þ49

7116862755.

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0360-5442/© 2016 Elsevier Ltd All rights reserved.

Energy 109 (2016) 751e764

integration between the SOFC and biomass gasifier can further

assist in reducing the exergy losses in the gasifier and thereby

in-crease the efficiency[16] In this work a new and simple strategy for

heat pipe integration is proposed, with heat pipes placed in

be-tween stacks in series, rather than the widely considered approach

of integrating the heat pipes within the SOFC stacks Further the use

of SOFC anode off gas as gasification agent is an additional new

concept proposed in this work To the best of authors knowledge,

solutions presented in this work are new This paper presents a

detailed description of the second law of evaluation and

optimi-zation of very high efficiency, small power level, gasifiereSOFCeGT

systems, having novel integration concepts A scaled up version of

the proposed system model with a steam rankine bottoming cycle

is later presented in this paper to evaluate the efficiency at higher

capacities

2 Description of the employed base-case model and

subsystems

The model presented earlier by our team[17]is considered as

the basis for the present study In this model (Fig 1), biosyngas is

formed in the gasifier and it is cleaned using a set of high

tem-perature gas cleaning devices Clean biosyngas is fed to the SOFC

which operates at an average temperature of 950C, and part of the

anode and cathode gas is recycled to maintain the SOFC inlet

temperature at 900C Since not all the fuel is utilized in the fuel

cell (85%), anode gas is combusted with cathode gas before the

turbine Turbine exhaust is used to preheat the cathode airflow, the

gasification agent and for generating steam required to prevent

carbon deposition Detailed process system scheme is shown in the

Fig 9inAppendix B The model presented in this study which is a

modified system involving heat pipes integration and anode off gas

recirculation, is similar to the model presented in the earlier

pub-lication by the team of present authors[14]

2.1 Biomass gasification

Biomass gasification is a thermochemical process of converting

solid biomass fuel to high calorific gas product when the biomass

reacts with a suitable gasifying agent at high temperatures The

product of the gas composition is primarily a mixture of H2

(hydrogen), CO (carbon monoxide), CO2 (carbon dioxide), CH4

(methane), as well as other hydrocarbons such as ethane and so

on Some amounts of H2O (water) and N2 (nitrogen), are also

present depending on the gasification agent used Air, oxygen,

steam or carbon dioxide is majorly used as the gasification agents

Autothermal gasification take place when air or oxygen is used,

and where the heat required for endothermic gasification re-actions are supplied by the oxidation reaction occurring in the gasifier The advantage of autothermal gasification is that no external heat is required, but it produces a significant amount of nitrogen in the gas product Allothermal gasification occurs when steam or carbon dioxide is used as the gasifying agent In such process, an external heat source is required to support the endo-thermic gasification reactions, more so, a higher energy content of gas product can be obtained The main advantages and technical challenges of using different gasifying agents are summarized in

Table 1given below Gasifier design, operating parameters and bed catalysts majorly determine the gas composition and the formed contaminants The gasification process is modeled in the study, while assuming chemical equilibrium However, simplified assumptions are taken, based on literature, from the percentage of char/carbon and methane produced in the high temperature gasification process and the carbon and hydrogen required for the production of char/carbon and methane are bypassed in the gasifier model

2.2 Carbon deposition

As the syngas is heated or cooled down, solid carbon may get deposited When carbon deposition occurs, it can lead to blockage

in the pipes and apparatuses This deposition is dependent on the thermodynamic CeHeO equilibrium composition of the bio-syngas at different temperatures and pressures In general, a higher carbon content in the gas tends to increase carbon depo-sition, while hydrogen and oxygen helps to reduce it Addition of steam thus, helps to reduce carbon deposition A discussion on carbon deposition in gasifiereSOFC systems is shown in Refs

[33,34]

2.3 Heat pipes

Steam or carbon dioxide gasification is an allothermal process, where high LHV biosyngas can be obtained, but additional heat is required for the process Meanwhile, SOFCs produce heat as a result

of exothermal electrochemical oxidation and internal losses The heat produced by SOFC is usually removed by providing excess air

on the cathode side, which affects the system's performance By using heat pipes between the SOFC (acting as a heat source) and the allothermal gasifier (acting as a heat sink), the excess heat pro-duced at SOFC can be transferred to the gasifier, where heat is required for gasification Such an integration using heat pipes result

in the cooling of SOFC stack, leading to a reduction in the required cathode airflow Furthermore, integrating SOFC with gasifier using heat pipes, also reduces the exergy losses in the gasifier Therefore,

a higher system performance can be achieved, due to the two effects

2.3.1 Principle Heat pipes are simple and effective heat transfer equipment, without moving parts A heat pipe is a hollow tube with layers of wire screen along the inner wall-the so-called wick The wick is filled with the liquid, having properties similar to the evaporation and condensation temperature of the application Heat pipes utilize the vaporizing liquid in order to create high heatfluxes from any heat source, in our case, it is utilized from the SOFC to the gasifier, where the endothermic gasification reactions take place High temperature heat pipes are usually metallic pipes containing an alkali metal (Na, K, and so on) Heat is transferred into the heat pipe

at the evaporation zone This heat is released at the condensation zone, from the heat pipe to its environment For application in Fig 1 Process flow diagram of the base case, biomass GasifiereSOFCeGasTurbine,

S Santhanam et al / Energy 109 (2016) 751e764

gasifiereSOFC sodium systems is a suitable working fluid with a

boiling point at 883C[35]

2.4 Gas cleaning

Cleaning biosyngas either at ambient temperatures or at higher

temperatures to meet the requirements of SOFCs is a challenging

task Further research on the gas cleaning requirements of SOFCs

and gas cleaning technologies is required for developing a suitable

biosyngas cleaning systems for gasifiereSOFC systems A detailed

analysis of the gas cleaning options is beyond the scope of this

paper, but it is available (from the team of present authors) in

literature[36]

2.5 SOFCeGT systems

SOFCs can be connected to gas turbines with the SOFC anode off

gas directly combusted in the Gas Turbine combustor or the heat

from the SOFC off gas transferred to an externally heated, closed gas

turbine cycle The latter case will decrease the Turbine Inlet

Tem-peratures, which often leads to reduced system efficiencies The

first option is widely considered in the study of systems available in

literature[37e51]

3 System configuration & assumptions

3.1 Biomass gasification

The biomass input is selected from the Phyllis database

pro-vided by ECN (Energy research Centre of Netherlands)[52] The

selected biomass is casuarina, which has a composition of; C

49.3%, H 5.9%, O 44%, N 0.6%, S 0.02%, Cl 0.162%; by weight The

biomass composition is shown inTable 2, with a lower heating

value of 15.500 kJ/kg, and a water content of 15 wt% The biomass

is fed to the gasifier at a temperature of 25C and at an operating

pressure of gasifier The biomass feed rate to the gasifier is kept

constant at 0.011 kg/s The biomass input has an energy source of

170.50 kW, which has an exergetic value of 192.87 kW

The gasifier operates at a temperature of 800 C and the

chemical equilibrium is assumed at the outlet To obtain a more realistic situation, 2 mol% of the carbon is bypassed in the gasifier as

an unconverted carbon Additionally, 2 mol% of methane is bypassed in the gasifier, as biosyngas from air gasifiers, which is often reported as containing methane around this concentration In the base case situation, no heat is added to or released from the gasifier

3.2 Gas cleaning

After the gasifier, biosyngas is cleaned close to a gasification temperature of 800 C Thefirst step is a cyclone in which the large particles are removed (the bypassed carbon from the gasifier), and a barrier filter, for removing the small particles Tar cleaning is also done at high temperatures, around 800 C, the suggested catalysts are based on dolomite and nickel After the dust and tar cleaning, the syngas is allowed to cool down to a temperature of 600 C, for HCl and H2S removal When steam gasification or recirculation of anode gas is applied, significantly less steam is required to prevent carbon deposition The biosyngas

is cooled down in two steps In thefirst step, the heat exchanger decreases the temperature, by increasing the temperature of the cleaned biosyngas, while in the second step, the heat exchanger reduces the temperature to 600C by cooling with an external air flow In the model HCl and H2S, cleaning is modeled as a pressure drop Just before the SOFC, an alkali getter is modeled as a pres-sure drop, operating at a temperature of 900 C No change in composition of the main gas components due to gas cleaning devices is assumed Only pressure drop due to gas cleaning is assumed and heat loss to the environment is not considered Please refer to for a detailed discussion on the development of such, and similar gas cleaning concepts for gasifiereSOFC inte-gration in the reference

3.3 SOFCeGas Turbine The SOFC system is implemented with a recycling of anode and cathode gases, in which a fraction of output gas is recycled in order to increase the SOFC inlet temperatures to 900 C The SOFC is assumed to operate at a temperature of 950C with an outlet temperature of 1000 C Reasonable assumptions were made for various input parameters for the SOFC, based on the measurements of Ni/GDC anodes, as described in literature For the present calculations, cell resistance is taken as 5 105Ohm

m2at an average SOFC temperature of 950C The mean current density is taken as 2500 A/m2 The fuel cell is elaborated as a direct-internal-reforming fuel cell, which means that methane is reformed in the fuel cell Methane reforming is an endothermic process and thus, a fraction heat will be consumed within the fuel cell The fuel utilization isfixed at 85% of the inlet flow, and the pressure drop in the fuel cell is 0.05 bars on both sides The

Table 1

Summary of the main advantages and challenges of the different gasifying principles.

Gaisfying agent

Air 1 Partial combustion for heat supply of gasification.

2 Moderate char and tar content

1 Low heating value (3e6 MJ N m3)

2 Large amount of N 2 in biosyngas

[18,19]

Steam 1 High heating value (10e15 MJ N m3)

2 H 2 rich biosyngas (>50% by vol)

1 Require indirect or external heat supply for gasification

2 High tar content in biosyngas

3 Require catalytic tar reforming

[20e26]

Carbon dioxide 1 High heating value of the biosyngas

2 High H 2 and CO, low CO 2 in the biosyngas

1 Require indirect or external heat supply

2 Require catalytic tar reforming

[27e32]

Table 2

Biomass composition in mass fractions.

S Santhanam et al / Energy 109 (2016) 751e764

anode product gas consisting of the unutilized combustible gases

of H2, CO and CH4, is combusted with the remaining oxygen in

the cathode output gas, found in the combustor The pressure

drop in the combustor is assumed to be 0.02 bars The amount of

cathode airflowing through the SOFC unit, determines the mass

of flue gas entering the gas turbine Secondly, the amount of

cathode air flowing through the system is determined by the

energy balance of the SOFC unit The flue gas from the turbine

exit is fed to the heat recovery unit to preheat the cathode air,

steam and gasification air (for the base case system) The

exhaust stack inlet temperature and pressure are measured at

100 C and 1.013 bar Due to the high level of cleanliness

ex-pected for the syngas, such low stack temperatures are

consid-ered reasonable

3.4 Heat pipes and anode off gas recirculation for gasifier

For the modified system with heat pipe and anode gas

recir-culation for gasifier (gasification), the base case system is

altered accordingly Normally, the heat pipes are integrated

within SOFC stack, which makes the stack design complex and

difficult to fabricate [53e56] Here, we propose to place the

heat pipes in between the two stacks, rather than integrating the

heat pipes within the SOFC stack The proposed method is

intended to make the fabrication significantly simpler, as no new

stack design is required The SOFC is now separated into two

parts, each operating within a temperature range of

900Ce1000C Heat is removed from thefirst and the second

fuel cell by cooling hot product gases (1000 C) to 900 C,

before entering the second fuel cell The heat pipes usually

operate with sodium, however, for modeling purpose, steam is

used The exergy loss is not affected by the modeling

approach, as long as heat pipe temperatures are equal to the

existing temperatures in practice The heat pipe model only

transfers an amount of heat and is independent of the heat

pipefluid medium Apart from the heat extracted from the fuel

cell system, a small fraction of the total heat required for the

gasification is extracted from the flue gas after combustion, using

heat pipes A fraction of anode off gas is extracted, before

entering into the combustor and is fed to the gasifier as

gasifi-cation agent Detailed process system scheme is shown in theFig

10inAppendix C

3.5 Cycle tempo component models and exergy efficiency

The Cycle Tempo is a thermodynamic modeling software,

developed by the Department of Process and Energy at TU Delft

The program calculates mass and energy balances of all the

indi-vidual apparatus in the system model Based on this approach, a

system matrix is developed, which is later solved to obtain the

individual values

In the present work, the biomass input feed determines the total

power of the process system Cycle Tempo utilizes the Gibbs energy

minimization routine to calculate the outlet gas compositions of

the Gasifier, Combustor and internal reforming Fuel cell

compo-nents The massflow of air to the cathode side is determined by the

energy balances of the fuel cell based on the cooling required to

maintain the specific outlet temperatures of the exhaust streams

The outlet temperature of the fuel cell is specified as input and it is

assumed that both anode streams and cathode streams exit at same

temperature

The fuel cell component is based on the following set of

equa-tions Firstly, the gas inlet is taken to equilibrium state Two

different methods can be used for fuel cell calculations In thefirst

method, operating voltage and current density are provided as

input to determine the area-specific resistance and active area of the fuel cell whereas in the second method, the area-specific resistance and active are input to determine the operating voltage and current density Additionally, in both cases the power output is calculated by specifying the fuel utilization This is performed based

on the assumption that temperature, pressure and compositions are constant in cross section perpendicular to the fuelflow The total cell current is calculated based on the anode inletflow rate and gas composition

I¼ 4m;a in

Ma 2F

y0H2þ y0

COþ 4y0

CH 4



In the above equation4m;a inrepresents the massflow rate of anode gas at the inlet, y0

i denotes the mass fractions of the gas components at the inlet F represents the faradays constant and final Uf is the fuel utilization Mais the molar mass of the anode gas mixture The transfer of oxygen from the cathode side to the anode side is related to the cell current via the faraday relation Based on the specified outlet temperature of the gas, the mass flow rate of cathode is determined using the energy balance The local reversible or Nernst voltage, current density and gas com-positions are calculated based on the following set of equations The local Nernst equations across a cross section area x is given by

Vrev;x¼ V0

revþRT 2Fln

8

<

:

y1=2O

2 yh2

yH2O P1=2

9

=

The voltage losses across the cross sectional area of the elec-trodes along the x-direction are assumed to be negligible, this leads

to the following relation for the voltage losses and the local current density

ix¼DVx

In the above equations, V0

rev refers to the standard reversible

voltage for hydrogen oxidation reaction and R is the universal gas constant DVx indicates the voltage losses, ix the local current density in the cross section x and Reqis the equivalent area specific resistance Equations(1)e(4)are encircled by afinal equation over the entire cell as given below

I

Req

Z Uf 0

dl=ðVrev VÞ

(5)

I is the total cell current, A is the cell area and dlis the local reaction coordinate given by the ratio of local extent of electrochemical reaction to the maximum extent of electrochemical reaction The mass balance of the fuel cell is given by the following set of equations

4m;a inþ 4m;c in 4m;a out 4m;c out¼ 0 (6)

S Santhanam et al / Energy 109 (2016) 751e764

The massflow rate of air entering cathode is denoted by 4m ;c in

and the massflow rate of anode gas and cathode gas leaving the

fuel cell is given by4m;a outand4m;c out respectively The mass of

oxygen atoms transferred from cathode to anode is denoted by

4m ;ca which is obtained from faradays law The energy balance

over the fuel cell is used to calculated the inletflow of cathode air

when the outlet temperatures are specified from the following

relation

4m;a inha; inþ 4m;c inhc;in 4m;a outha;out 4m;c outhc;out¼ Pdc

(8)

Where ha;in, ha;outare the inlet and outlet enthalpy of the anode gas

respectively and hc;in, hc;outare the inlet and outlet enthalpy of the

cathode gas respectively The termPdc is the DC (Direct Current)

power produced by the fuel cell

In order to calculate the power produced by the gas turbine,

outlet enthalpy of the stream exiting the gas turbine is calculated

based on the equation given below The isentropic efficiency of the

gas turbine is taken as an input

ho¼ hihs



hi ho;s

(9)

Once the outlet enthalpy is calculated, the power can be

ob-tained by multiplying the outlet enthalpy with the massflowing

through the turbine Likewise a similar calculation routine is the

utilized to calculate the power consumed by the compressor The

outlet enthalpy of the compressor is calculated as follows

ho¼ hiþ



ho;s hi



hs

(10)

where ho is the outlet enthalpy exiting the turbine or the

compressor, hiis the inlet enthalpy of the gas entering the turbine

or compressorhsis the isentropic efficiency of taken as the input

and ho ;s is the outlet enthalpy of the gas leaving the turbine or

compressor when expanded isentropically

Cycle Tempo software also allows for the computation of the

exergyflows and exergy efficiencies In our work, the exergy

effi-ciency is defined as the ratio of the total exergy of products of the

process to the total exergy of the source supplied as input to the

process The exergy of products includes all the available process

mass and energyflows exiting the process and exergy of the source

are defined as all the process mass and energy flows supplied to the

process required to produce the necessary products The exergy of

products and exergy of sources varies from component to

compo-nent and from one process to another Based on this definition, two

important exergy efficiencies calculated for the process system in this work are electrical exergy efficiency and total system exergy

efficiency which are defined as

hex;el¼

P

Pel;outPPel;in

hex;tot¼

P

Pel;outþPExheat;outPPel;in

whereP

Pel;outis the total output electrical power andP

Pel;inis the total electrical power consumed by the balance of plant compo-nents such as the compressor and pumps Exheat;outis the exergy of the heat available as waste heat and Exfuel;inis the exergy of fuel supplied to the system

4 Results and discussion

4.1 Integration of heat pipes between Gasifier & SOFC With the modified system, which is the base case system modified with heat pipes as shown inFig 2, heat is removed from the SOFC by employing two SOFCs in series, instead of one and introducing heat pipes The thermodynamic voltage of a fuel cell is affected by the fuel utilization; the thermodynamic voltage de-creases as fuel utilization inde-creases and hence, lowers the perfor-mance This can be overcome by multistage oxidation, that is, by connecting SOFC stacks in series with respect to fuelflow, such that the total fuel utilization is distributed over the SOFC's connected in series This increases the thermodynamic voltages of the SOFC stacks and hence, a higher system performance The performance can be further increased by employing interstage cooling, using heat pipes, such that temperature of inlet gas entering second SOFC, is reduced and hence, further increase in Nernst voltage is possible The heat pipes are placed in the middle of the two fuel cells, with a fuel utilization of 42% for thefirst SOFC stack and a total system fuel utilization of 85%, respectively, for the incoming bio-syngas An equal distribution of fuel utilization values over two SOFC stacks in series is approximated, such that the required total fuel utilization being maintained, is expected to result in a higher percentage increase in SOFC performance[57] It is observed that the total amount of heat removed from the SOFC is 33 kW This heat

is added to the gasifier, operating at 800C As a result, the

equi-librium composition of the biosyngas formed in the gasifier changes

Fig 2 Modified base case with heat pipes and anode off gas recirculation Fig 3 Exergy losses in Base case System compared with modified system.

S Santhanam et al / Energy 109 (2016) 751e764

In both systems, the biomass input is kept constant at 0.011 kg/s,

representing an exergetic value of 192.87 kW When heat pipes are

added to the base case model (Fig 1), and the anode gas is

re-circulated as a gasification agent, the net system electrical

effi-ciency at operating pressure of 6 bars is increased from 54.45% to

69.8%.Fig 3gives an overview of the exergy losses per component,

comparing the base case system and modified base case system

with heat pipes and anode gasification agent The base case model

also produces a significant waste heat, when this heat is utilized;

the total system efficiency (electricity þ heat) amounts to 72% In

the modified system, the waste heat generated is less, hence, it is

not accounted for

A comparison between the compositions of biosyngas streams

produced using the base case system and the modified system

(with heat pipes and anode gasification), is presented inTable 3 In

the base case model, air is used as the gasification agent, the carbon

conversion in the gasifier is 95 mol%, and the methane formation is

2 mol% Steam is added (15 wt%) to avoid carbon deposition in the

downstream equipment Steam addition results in a significant

reduction in the LHV of biosyngas to 107.42 kJ/mol, which result in

lower SOFC performance With the application of heat pipes,

cathodeflow is reduced However, with the addition of heat pipes

and using anode off gas as the gasifying agent, the steam addition to

the syngas is reduced to 3 wt% This resulted in a syngas with LHV of

126 kJ/mol

Table 3clearly indicates that the concentration of the

combus-tible products, that is, H2, CO and CH4are higher when the heat

pipes and anode gasification agent is applied to the system The net

quantities of the combustible products are almost equal in both

cases, however, the nitrogen and argon content in the biosyngas is

negligible with anode gasification agent, resulting in a higher LHV

of the biosyngas

The total exergy loss in the base case system is 92.41 kW For the

modified model, the total exergy loss is reduced to 73 kW In both

cases, the largest amount of exergy is lost in the gasifier

Imple-mentation of heat pipes and anode gasifying agent reduced the

relative exergy loss by approximately 5%, in relation to the gasifier's

exergy loss in the base case system The exergy loss in the turbine is

decreased by the application of heat pipes, since less air needs to be

compressed for cooling the SOFC, whose function is now done by

the heat pipes A logical result is therefore, a decreased flue gas

flow, resulting in a low power output of the gas turbine and a

significantly lower exergy losses contribution from the gas turbine,

since the gas turbine has a lower efficiency By contrast, the output

from SOFC stacks is increased proportionally, due to a higher LHV of

the syngas When heat pipes and anode gasification is utilized, less waste heat is produced as compared to the base case The heat is instead utilized for the gasifiers by the heat pipes In the modified model, the remaining heat is not utilized after the heat recovery unit, unlike in the base case In the modified system, all the remaining heat exits through the exhaust stack, hence the exhaust stack loss increases marginally

When the exergy losses are normalized with the total losses in the systems, it shows that the distribution of the losses over the system components only changes marginally Still, the exergy los-ses in the gasifier determine the total losses

4.2 Anode off gas as gasifying agent

The largest exergy losses in the gasifier take place due to partial oxidation of the biomass in the base case model If exergy losses in the gasifier need to be reduced further, oxidation needs

Table 3

Comparison of biosyngas compositions in mole fractions between base case model

and model with heat pipes and anode off gas recirculation.

Biosyngas composition Base case

Steam addition 15%

Heat pipes and anode gas recirculation

Steam addition 3%

Mole fractions (e) Dry

(e)

Wet (e)

Dry (e)

Wet (e)

Avg mol mass [kg/kmol] 22.55 21.83 22.78 22.61

Fig 4 Variation of efficiency with ATBR at different Pressures.

Fig 5 Variation of LHV with ATBR at different pressures.

S Santhanam et al / Energy 109 (2016) 751e764

to be minimised Allothermal gasification processes with steam

or anode gas is possible, if additional heat is supplied to the

gasifier

The integration of the gasifier and the SOFC via heat pipes makes

steam or anode gas gasification more attractive Heat required

for the endothermic gasification reaction can be obtained from

the SOFC and added to the gasifier, providing the necessary heat,

instead of using an additional external source for the same The

cycle tempo scheme of the optimized model of the base case

sys-tem with heat pipes and anode off gas recirculation is shown in

Fig 4 The amount of additional heat needed when anode off gas

is used for gasification varies from 50 kWth at an ATBR (anode

off-gas to biomass ratio) of 0.1e43 kWthat an ATBR of 2.5 There are

3 options required to increase the amount of transferred heat (1)

The first option is to lower the pressure in the system, which

will result in a lower power output of the SOFC and higher

pro-duction of heat, (2) The second option is to change the position of

the heat pipes in the SOFC, by increasing the fuel utilization in

thefirst fuel cell This would mean that a higher proportion of

electrical work would be produced at the first fuel cell and

consequently, more heat is also generated The more heat is then available, since the calculated cathode air flow in the first fuel cell is higher (3) The remaining amount of heat can be extracted from theflue gas exiting the combustor As most of the electric power is produced in the SOFC, decreasing pressure causes a drop in the efficiency hence, option one is given up and a combination of the options 2 and 3 are employed for the calcula-tions The system with anode gas gasification and heat pipes is evaluated at pressures varying from 6 to 11 bars and at ATBR's from

1 to 2.5

In the plot, Fig 5, the electrical efficiency is plotted at varying ATBR, and operating pressure The operating pressure is limited to 11 bars This operating pressure limit is as a result of the air factor in the combustor At high operating pressures, the SOFC performance improves (more power, less heat) Since the amount of heat necessary for gasification is more or less equal at all pressures, less cathode air is required at high operating pressures, which results in a lower demand for cathode air If the required amount of cathode air becomes too low, the air factor

in the combustor decreases to a value below 1 When this occur, high stack losses are found, since unburned H2and CO leave the system

For a given operating pressure, the ATBR is limited between 1 and 2.5 In our model, the cathode air mass flow is computed based on the energy balance of the second SOFC, that is placed after the heat pipes At low ATBRs, more heat is extracted from the

Fig 6 Optimization operating pressure of Base case model with anode recycle and

heat pipes.

Table 4

Parameters used for auxiliary components of modified base case system with

bot-toming cycle

Isentropic efficiency of steam turbine 0.7

Isentropic efficiency of gas turbine 0.88

Isentropic efficiency of compressor in GT cycle 0.88

Isentropic efficiency of auxiliary compressors 0.75

Pressure drop in gas cleaning units 0.02 bar

Fig 7 Process flow scheme of the modified base case system with bottoming steam cycle.

Table 5 Composition of syngas.

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fuel cells by the heat pipe which lowers the second SOFC inlet

temperature and hence, the amount of cathode air required

re-duces Meanwhile, the concentrations of hydrogen, carbon

mon-oxide and methane are higher in the anode gas and also in the

anode product gas This result in higher concentrations of the

combustible products at the combustor inlet and therefore, a

higher airflow in the combustor is required, which leads to an air

factor less than 1

However, as the recycle ratios are increased, the air factor in

the combustor increases This is because, as the recycle ratio

increases, the amount anode off gas entering the combustor

re-duces Furthermore, due to a higher recycle ratio, the heat

sup-plied to gasifier from SOFC also reduces, since part of the heat

required by the gasifier is supplied by the recycle anode off gas

This implies, more heat is now entering the second SOFC and

hence, to cool the second SOFC, a higher air flow is required

Hence, the cathode air, which is determined by energy balance of

the second SOFC increases Therefore, the air factor in the

combustor increases As the recycle ratio increases, the

concen-trations of H2, CO and CH4with value higher than 2.5, decreases

in the syngas produced and the concentrations of CO2 and H2O

increase in the syngas produced, which lowers the LHV of syngas

as shown in Fig 5 The biosyngas have lower LHV and high

concentrations of CO2and H2O, which results in the fact that, the

SOFC output would start to decrease further, and due to the

higher air factor in the combustor, the TIT (Turbine Inlet

Tem-perature) reduces, reducing the performance of the gas turbine

Evaluation of the system at varying operating pressures and

ATBRs shows an optimal electric efficiency at highest ATBRs, and a

higher pressure within the pressure ranges considered and as

seen as achievable The increase in electric efficiency at higher

ATBR's can be explained by two processes Firstly, there is an

in-crease in the fuel cell performance, although, there is a dein-crease

in LHV This is can be explained by the slightly complex behavior

When the anode gas is being recycled to the gasifier, along with

CO2and H2O, some amount of the combustibles is also recycled

This implies that, instead of the combustibles being utilized in

the combustor as it would be in the base case, they are recirculated

back to SOFC, through the gasifier The energy conversion in SOFC

is more efficient compared to CombustoreGT unit Hence, even

though the LHV of syngas is lower with increasing ATBR, the

massflow rate of fuel flowing into the SOFC unit increases with an increasing ATBR, even though the biomass fed to gasifier is constant This results in a higher output from the SOFC unit Second,

an increase in the gas turbine power output is seen This is due to the fact that higher ATBRs, results in a higher mass flow of cathode air in the system, as explained in the previous paragraphs This increases the mass flow rate of flue gas entering the gas turbine, resulting in increased power output It is observed that the TIT decreases when ATBR increases, thereby, affecting the Gas Turbine efficiency However, the decrease in efficiency is overcompensated by the increased mass flow rates through the turbine This advantage is offset when ATBR is increased to 2.5 and beyond for some pressures, as shown inFig 4due to the decreasing TIT and lower LHV of syngas, produced by the gasifier at higher ATBRs

The implementation of anode off-gas recirculation and heat pipe shows a large increase in efficiency In the case of air gasification implementation of heat pipes, an increase in electric efficiency of 8% can be gained, because exergy losses in the gasifier are reduced The heat required for the gasification can be provided by the heat pipes and anode gas Gasification by anode gas is more beneficial than steam gasification, since a part of the heat required for the gasification would be supplied by the heat available in the anode off gas stream In the system in which anode gas is recycled and heat pipes are applied, the electrical efficiency is increased to 72% due to

a further decrease in exergy losses in the gasifier and gas turbine Operating the system with the highest achievable anode gas recycle and highest achievable operating pressure, delivers a higher elec-trical efficiency and can be further increased if a bottoming cycle is applied

4.3 Operating pressure

The base case biomass GasifiereSOFCeGas turbine system shows a clear optimum in the operating pressure, at a pressure ratio

of 6, the found optimum is logical according to the Nernst equation, since the performance of the SOFC is expected to improve at higher pressures However the performance of recuperated gas turbines tends to have higher efficiencies at lower pressures, since at high pressures, the air temperature out of the compressor is too high and

Table 6

System energy and exergy efficiencies.

Delivered

S Santhanam et al / Energy 109 (2016) 751e764

the recuperator cannot transfer much heat, resulting in an

opti-mum, seeFig 6

Although, an optimum level in the operating pressure is

reached, the variance in efficiency is not large, from 70% at 6 bars to

72% at 11 bars As the pressure is increased beyond 8 bars, to

11 bars, no significant change in efficiency was observed The

effi-ciency almost remained constant at 72.6% from 8 bars onwards to

11 bars The pressure is limited to 11 bars, since further increase in

pressure reduces the air factor to below 1, as explained in Section

4.2

In the base case system, 66 kWeis produced by the fuel cell, and

33 kWeby the gas turbine at 6 bars In the improved system and at

the same operating pressure, 93 kWeis produced by the fuel cell

and only 27 kWe, by the gas turbine This change makes fuel cell,

and thus, the Nernst equation, much more important for the total

system performance

5 Modified base case system with steam rankine bottoming

cycle

Based on the results obtained for the Base case system with heat

pipes and anode gasification, a higher capacity system with a

bot-toming cycle was modeled The system capacity isfixed at 34 MW

as steam power plants are in general used efficiently at larger

po-wer levels The present model considers a 800 kW steam popo-wer

plant The system components of the modified system are retained

with the addition of a steam bottoming cycle However, isentropic

efficiencies of components such as turbines and compressors are

increased as large systems have better efficiencies.Table 4provides

the input parameters used for the system calculations The gasifier,

gas-cleaning unit, heat pipe unit and SOFCeGT are similar to the

system defined earlier

A steam bottoming cycle is included after the heat recovery

unit as shown inFig 7 A detailed process system model is provided

inFig 11ofAppendix D After the heat recovery unit, theflue gas

passes through the economizer, evaporator and super-heater,

where steam is produced The flue gas is then released to

the environment through the stack, where the temperature of

theflue gas is 100C Water is fed to the system at standard

con-ditions It passes through the economizer, evaporator and

super-heater, where it is converted to steam, before being fed to the

turbine The pressure across the steam turbine was set at 8 bars

The operating pressure of the SOFCeGT system is reduced to 9 bars,

so that enough heat is available for the bottoming cycle For the

modified system with bottoming cycle, the Biomass

Gas-ifiereSOFCeGT system is operated at a pressure of 9 bars At

this pressure ratio, sufficient heat was available after the heat

re-covery unit, to run a steam bottoming cycle The input parameters

of the system's main and auxiliary components are presented in

Table 4

5.1 Results

The net power of the system is determined by the feed rate of

the biomass feedstock It is assumed that a biomass feed of 2.2 kg/s

is fed to the gasifier at an operating pressure of 9 bars and at a

temperature of 25C The given feed rate of the biomass to the

gasifier corresponds to 34,100 kW (34.1 MW), of input energy to the

system The gasification agent fed to the gasifier, is obtained from

the anode off gas of the SOFC unit This results in a producer syngas,

with a gas composition as shown inTable 5and at a temperature of

800C and pressure of 9 bars The massflow rate of the produced

syngas is equal to 7.87 kg/s

As explained in the earlier section, the high temperature gas cleaning is employed, for the system with bottoming cycle The gas cleaning is performed at a temperature of 600C As the temper-ature is reduced, steam is added to prevent carbon deposition The syngas composition (after the removal of tar and particulates, represented by star in thefigure), lies below the carbon deposition boundary at 800 C (1073 K) As the temperature decreases to

600C, the syngas, lies in the carbon deposition region Hence, by adding some amount of steam, the steam to carbon ratio increases, therefore, the syngas, with its new composition, lies in the carbon safe region The corresponding massflow of steam added to the syngas is 3% of the massflow rate of syngas, after tar cleaning The new composition of syngas lies just beneath the carbon boundary line at a temperature of 600C This can be defined as the minimum amount of steam which is to be added to the syngas, to prevent carbon deposition

The cathode air is compressed to 9 bars It passes through the heat recovery unit, where the air is preheated to 680C, before it

is mixed with recirculated cathode air The mass flow of air before cathode recirculation is found to be 13.776 kg/s The air enters the SOFC unit with the cathode inlet temperature of

900C The temperature of gases at the exit of thefirst SOFC, was

1011C The excess air used, helps to maintain the SOFC, within thermal constraints of SOFC's The outlet gases at the exit offirst SOFC pass through the heat pipe unit The anode and cathode streams are cooled to a temperature of 900 C and 927 C respectively, which correspond to the inlet temperatures of the second SOFC The total heat required for the gasifier to gasify the biomass supplied, was equal to 8871 kW of which 311 kW, was extracted from the flue gas, after the combustor, the rest

8560 kW, was supplied by heat extracted from the SOFC unit The outlet temperature at the anode and cathode of the SOFC unit was fixed at 1000C Part of the anode and cathode streams is

recirculated Later on, a part of the anode off gas was supplied

to the gasifier as a gasifying agent The remaining anode and cathode off gas is finally combusted in the combustor The turbine inlet temperature after the heat extraction was 1152C, passing through the heat pipe unit after the combustor The temperature of theflue gas at the turbine exit was found to be

689C Theflue gas after being passed through the heat recovery unit is fed to the bottoming cycle Using the remaining heat in theflue gas, steam is generated for the bottoming steam cycle The steam turbine inlet temperature was found to be 359.50C The power generated by the SOFC unit, gas turbine unit and bottoming cycle, along with the system efficiencies is presented in

Table 6 From an exergy analysis, the relative exergy losses of the com-ponents compared to a total exergy loss in the system were highest for the gasifier at 10.59%, followed by the exhaust stack losses at 5.02% Moreover, it is assumed that a part of the carbon in the biomass remains unconverted, which is considered as a loss

6 Conclusions

Biomass gasifier system with SOFCeGas Turbine, using air as gasification agent generally results in higher exergy losses In this present study, an improved system of biomass GasifiereSOFCeGas Turbine system, with heat pipes and anode gas recycling to gasifier,

as gasification agent is proposed Parametric analysis performed on the system, has delivered the following conclusions

1 Gasification of biomass with steam or anode gas is more beneficial than air gasification The formed biosyngas has a higher LHV, since the concentrations of H2, CO and CH4 are

S Santhanam et al / Energy 109 (2016) 751e764

higher A high LHV, positively influence the fuel cell

perfor-mance, resulting in higher system efficiency From this

perspective, gasification by anode gas is shown to be

advan-tageous over steam gasification, since anode gas having a high

temperature delivers part of the heat necessary for

gasification

2 Implementation of heat pipes between the fuel cell and the

gasifier is a good solution in order to enable an allothermal

gasification process In our proposed configuration, we intend to

place the heat pipes between the stacks and not integrate it

within the stack itself Integrating the heat pipes within the

stacks makes the design and manufacturing of the stacks

com-plex and difficult On the contrary, our proposed method is

easier and a novel approach is intended to make the

manufacturing easier

3 The system calculations performed indicate that high system,

electrical efficiencies around 72% are achievable in small scale

GasifiereSOFCeGas Turbine systems, when anode recycle and

heat pipes are applied

4 The modified base case system with heat pipe integration and

anode gas gasification, was scaled up to higher capacity, by

adding a steam rankine bottoming cycle, after the heat recovery

unit

5 For the scaled up model, the total energy input in the form of

biomass was 34 MW The biomass Gasifier eSOFCeGT system

was operated at a pressure of 9bars A net electrical power

output of 25.1 MW was produced with an electric efficiency of

73.6% and exergy efficiency of 65%

Abbreviations and nomenclatures

Abbreviations and name

ATBR Anode off gas To Biomass Ratio

ECN Energy Center Netherlands

LHV Lower Heating Value

SOFC Solid Oxide Fuel Cell

STBR Steam To Biomass Ratio

Chemical formula and name

HCl Hydrogen chloride

H S Hydrogen sulfide

CO2 Carbon dioxide

CO Carbon monoxide

H2 Hydrogen

CH4 Methane

N2 Nitrogen

O2 Oxygen

Appendix

A Heat pipe

Heat pipes are heat transferring devices which uses a vapou-rizing liquid to absorb heat from an heat source and deliver to a heat sink where the vapor condenses back to liquid form Depending on the range of operation temperature, different liquids can be used For high temperature heat pipes, liquid metals such as Sodium, Potassium or an alkali metal is used In this process the heat is transferred from the heat source to the evaporation zone of the heat pipe where the liquid metal evaporates to form vapor The vapor moves towards the condensation zone where it delivers the heat to the heat sink while the vapor condenses A wetted wick is used to transport the condensed liquid back to the evaporator zone The amount of heat transferred from the heat source to the heat sink is determined by the temperature difference between the heat source and sink and thermal resistances to the heat transfer

The heat transferred is given by

qhp¼TsofcP T9 gasifier

where qhpis the heat transferred by a heat pipe, Tsofcand Tgasifierare the temperatures of heat source (SOFC in our case) and heat sink (gasifier) respectively and Riis the thermal resistances for the heat transfer InFig 8, R1and R9 corresponds to the resistance due to convective heat transfer, R2 and R8 are resistance due to the heat conduction through the heat pipe wall, R3 and R7 are the re-sistances due to heat transfer through the wetted wick, R4and R6

are the heat transfer resistance due to liquidevapor interfaces and finally R5 is the heat transfer resistance due to the temperature drop along the length of the heat pipe

Fig 8 Function of high temperature heat pipe.

S Santhanam et al / Energy 109 (2016) 751e764