Conceptual design of a fuel cell system for an electrothermal phosphorus process

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May 1995

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U

Delft

Delft University of Technology

.

Conceptual Design of a Fuel Cell

System tor

an Electrothermal

Phosphorus Process

Ir. J.C.J.M. Goossens

Faculty of Chemical Technology and Materials Science Post-graduate Education

-_-

T

w

AIO

Laboratory for Chemical Process T echnology

Process Systems and Contral Graup Process & Pracess Equipment Design

,

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Delft University of Technology

Department of Chemical Technology and Materials Science

Laboratory for Chemical Process Technology

Process Systems and Control Group

Delft University Clean Technology Institute (Interduct)

Conceptual Design

of a Fuel

Cel I

System

for an Electrothermal

Phosphorus Process

May 1995

Ir. J.C.J.M. Goossens

Julianalaan 136 2628 BL DELFT tel/fax.: +33 15784383/4452 Supervisors:

prof. ir. J. Grievink (CT&MS)

ir. J. Schinkel (Interduct, Project manager) dr.ir. P.J.T. Verheijen (CT&MS)

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Fuel Geil System tor

a

Phosphorus Process Summary

Summary

This report presents a conceptual design for a Molten Carbonate Fuel Cell (MCFC) system to produce electricity from carbon monoxide off-gas of the phosphorus production plant of Hoechst Holland N.V. at Vlissingen, The Netherlands. The study has been carried out as the final part of the two-year post-M.Sc. Technological Designers Course of the Delft University of Technology. The objective of this study was to design an energy-optimal fuel cell system for a fluctuating industrial low-caloric oft-gas.

To design and optimise this system, an existing first principle model of a fuel cell was expanded and translated into a model for Aspen PIUSTM. The model, which is written as a FORTRAN subroutine, is robust and can simulate different types of fuel cells. The total fuel cell system has been simulated with the flowsheet simulation software package Aspen PIUSTM. This results in a calculated overall electrical efficiency of 50% and 9 MW of electricity produced.

The choice for a Molten Carbonate Fuel Cell as fuel cell type is discussed, together with the design choices. The design choices of the energy-optimal system are presented together with a sensitivity analysis of key variables and an exergy analysis. The fluctuations of the feed (0 and 73 MW with an average of 18 MW) led to a special study to optimise the total system with respect to the electricity produced. Not the fuel cell, but the compressors are the Iimiting factor, which was solved by using two parallel compressors. This way, only 5% of the fuel is flared instead of 20% using single compressors.

The fuel cell system has been designed as an add-on unit to keep interference with the phosphorus production plant to a minimum. From a technical point of view the most important weakness of implementing this fuel cell system is the tight restrietion on control performance because of the very small pressure difterence allowable between the various parts of the fuel cell. Although it is a general problem with MCFC-systems, fluctuations of the feed amplify the control problem.

A preliminary economie evaluation shows that the energy-optimal fuel cell system requires an investment of Mf68 (about kf6/kW). It will have a Return on Investment of 10% if the price of electricity is doubled to fO.26/kWh. Seventy percent of this investment is attributed to rotating equipment. The system requires an investment of Mf34 if the system is optimised for the production costs of electricity by using single compressors. The overall efficiency then drops to circa 38%, and the price of electricity must still increase by 50%.

A fuel cell system with only small fluctuations (70-110% of the average)·in the feed requires an investment of Mf38 and has a Return on Investment of 10%, which increases with increasing feeds. This suggests that a fuel cell system is potentially interesting for applications in the process industries if the feed is relatively constant and the scale is larger than 10 MW.

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a

Phosphorus Process Table ot Contents

a

Phaspharus Process 6 Economie Evaluation 6.1 Capital costs . . 6.2 Operating costs 6.3 Revenues . . . 6.4 Profitability calculation 6.5 Profitability optimisation

6.6 A tuel cell system at constant teed

7 Conclusions and Recommendations . 7.1 Conclusions . . . 7.2 Recommendations 8 Literature . . . . . 8.1 General . . . . 8.2 Cost Estimation 8.3 Fuel Ce lis . . .

8.4 Carbon Dioxide removal 8.5 Simulation . . . . 8.6 Personal Communication Table at Gantents . 65 .65 .73 . 75 .75 76 79 81 81 82 85 85 86 87 90 91 92 Dankwoord . . . . . . . . . . . . . . . . . . 93

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Fuel Cell System for

a

Phosphorus Process

Appendix I: Flowsheet of the Process

Appendix 11: Mass and heat balances

Appendix 111: Equipment lists . .

Appendix IV: Model of a fuel cell IV.1 Introduction . . . . . . . IV.2 Assumptions. . . . IV.3 Degrees of freedom analysis IV.4 Equations .

IV.S Balances . IV.6 Equilibrium

Appendix V: Aspen+ User Guide Pages for a Fuel Cell

Appendix VI: FORTRAN code of the Fuel Cell model

VI.1 Introduction . . . . . . VI.2 Aspen Plus version 8.5 . VI.3 Aspen Plus version 9.13

Appendix VII: Aspen+ Input and Output File VI1.1 Description of the Aspen+ Input file . VI1.1 Description of the Aspen+ Output file

page -ix-Table of Contents . 1.1 11.1 111.1 IV.1 IV.1 IV.2 IV.2 IV.S IV.? IV.8 V.1 VI.1 VI.1 VI.3 VI.15 . VI1.1 . VI1.2 VI1.13

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Phosphorus Process Introduction

1 Introduction

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical energy. The electrical efficiency of a fuel cell can be as high as 60-70% based on the lower heating value of the fue!. Much research has been done on different types of fuel cells, because the maximum electrical efficiency can be higher than the maximum reached in conventional power stations. One university where research has been done is the Delft University of Technology. The Delft University Clean Technology Institute (Interduct), has proposed the integration of fuel cells in the process industry. It is a spinoff of the research project 'Technica I options for a more effective management for primary and secondary

resources' of this institute. The use of fuel ce lis implemented in the production of methanol (Dijkema [35J page 299) or zinc (Schinkel [60J page 416) has been reported earlier. Also

some undergraduate students have studied some possibilities (e.g. the integration of fuel cells with an ammonia plant). A case study with more aspects than the fuel cell system itself,

is the design of a fuel cell system that uses the off-gas of a phosphorus production plant for the production of electricity.

1.1 Phosphorus production

Hoechst Holland N.v. (Vlissingen, the Netherlands) produces phosphorus using an electrothermal process. Thereby, a large amount of off-gas (200.106 Nm3_{/year) is produced}

with up to 85% carbon monoxide. The main part of the gas is burnt to supply heat to the phosphorus and other plants at the Hoechst site, but still a large amount (30%) is flared. Alternative applications are prohibited because of its contaminations. Flaring also means squandering a relative large amount of energy. In the future, legislation might require the reduction of emissions, especially sulphur dioxide.

If the off-gas has to be cleaned anyway, alternative uses can now be considered. The most obvious solution is to use it as a fuel, because yet natural gas is used at the site for heating purposes. This option is not used at Hoechst, because of the fluctuations of and contaminations in the feed. The fluctuations can be eliminated by using a gasholder, but the corrosion properties of the flared gas prohibit this option. Recently a study has been completed to investigate the possibilities of sulphur removal (Badger [1]). The use of a gasholder for eliminating fluctuations does not solve the problem totally, because the total amount of energy of the flared gas (22 MW= 22.106 Nm3 natural gas equivalents) is larger than the energy of the used natural gas (10-15.106

Nm3

natural gas:: 10-15 MW) (Hoechst

[96 pcJ, Huizinga and Hoogenkamp [7]).

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-1-Fuel Geil System for

a

Phosphorus Process Introduction

A solution for the problem of flaring the oft-gas is the production of e.g. methanol. A group of undergraduate students now studies this option (Luteijn [99 pc]).

The use of tuel ce lis for the production ot electricity trom this oft-gas seems a logical option, because the phosphorus production plant itself needs much electricity, and it is possible to use low caloric waste streams in a fuel cell instead ot using high grade fuels, like methane. The consequences ot using fuel cell technology tor such purposes have been discussed mostly qualitatively so tar (e.g. Kartha [41]). However, a thorough understanding of these consequences requires more detailed design studies on particular cases to assess the relative me rits and demerits. This project is such a design study.

1.2 Hoechst Project

The project, which is commissioned and partly sponsored by the Dutch Society tor Energy and Environment (NOVEM), is a cooperation ot

• the Department of Chemical Technology and Materials Science, Laboratory tor Chemical Process Technology, Process Systems and Control Group(Delft University ot Technology),

• Delft University Clean Technology Institute (Interduct),

• the Department of Mechanical Engineering and Marine Technology, Laboratory tor Thermal Power Engineering (Delft University of Technology) and

• Hoechst Holland N.V., site Vlissingen.

The objective of this project is to develop a conceptual design of a tuel cell system integrated with the phosphorus plant ot Hoechst and to evaluate the technological, environmental and economic consequences of the system. It was decided after the tirst progress meeting to concentrate on an autonomous system that can deal with fluctuations in the teed (Schinkel

[10]).

The project exists ot two main parts: the conceptual design of a tuel cell system for an electrothermal phosphorus plant and the technological and economical comparison of the tuel cell system with other techniques.

The first part of the project (one man year) is carried out at as the final part of the two-year post M.Sc. Technological Designers course. This course for Chemical Engineers is in the area of process and equipment design for the chemical process industry at the Department ot Chemical Technology and Materials Science trom the Delft University ot Technology, The Netherlands. In The Netherlands, at the three Universities of Technology, a two-year post M.Sc. course for chemical engineers is grounded (TwAIO-ontwerpers opleiding). The main part of the second year of th is course is an individual design project.

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Fuel Cell System far

a

Phaspharus Pracess Introductian

The second part of the project, which is a comparison with a conventional alternative (the off-gas used in a gas turbine) will be carried out by Interduet.

1.3 This report

This report reflects the conceptual design. The system envelope and the main considerations of the design of the flowsheet are described in Chapter 2 (Basis of Design).

Chapter 3 (Fuel Cells) gives a short introduction to fuel cells, the choice of the type of fuel cell, the design considerations and a model of the used fuel cell. In Chapter 4 (Design of the Flowsheet) the flowsheet of the fuel cell system is presented, with the considerations made.

The chapter explains also the calculation methods of the different types of equipment. The optimisation procedure for optimal use of the off-gas, the optimisation results of process parameters and the analysis and options for decreasing of the efficiency losses are reflected in Chapter 5 (Optimisation and Analysis). Chapter 6 (Economie Evaluation) gives the method and the results for the investment costs calculation and an economie evaluation.

The report is completed with a tew appendices, including a flowsheet, mass and heat balances, equipment lists, a describtion of a theoretical based model for a tuel cell, the souree code for the model, the practical implementation in a commonly used flowsheet simulator and finally the flowsheet simulation input and report file.

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Fue! Geil System for a Phosphorus Process Basis of Design

2 Basis of Design

This chapter deals with the specifications and the boundary conditions within which the design had to be made. Firstly, the fuel as supplied by the Phosphorus plant is given,

secondly assumptions had to be made about the technical viability of a potential fuel cell system. Thirdly the outlets are specified.

2.1 Design objectives

A process which uses all or part of the flared oft-gas of the Phosphorus Production plant of the Hoechst Holland N.V. site Vlissingen has to be designed. The fluctuations in the feed have to be taken into account. Interactions with the Phosphorus production plant should be avoided, only utilities may be used. The cleaning of the feed of the fuel cell system is not taken into account, except for the hydrogen sulphide present in the oft-gas.

The system designed should use the oft-gas for electricity production using a fuel cell system. The design objectives are to design

• an energy optimal system,

• which is as simple and cheap as possible, • which can deal with the fluctuations in the feed, • which can operate for at least ten years.

2.2System

Figure 2.1 shows the inlets and outlets at norm al operation of the system. The main inlet streams of the system are:

• cleaned Phosphorus Furnace Gas which is normally flared,

• boiling feed water, • air, P.F.G. (Fuel) B.F. Water Air N₂ A.C. Electricity A.C. Electricity System Flue Gas

• nitrogen (purge gas for the fuel cell pressure box).

Figure 2.1: Inlets and out/ets ot the tue/ cell system

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-5-Fuel Cell System for

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Phosphorus Process Basis of Design

When the fuel cell system is stand-by, or in start-up or in shut-down situations, some other feeds will be used also including:

• natural gas (heating the fuel cell system),

• medium pressure steam (instead of boiling feed water),

• A.c.

electricity (pumps, compressors). The system has three outlets:

• A.C. electricity,

• oxygen poor / carbon dioxide enriched flue gas,

• nitrogen (purge gas of the fuel cell pressure box).

2.3 Feeds

Air (no costs) is delivered at 1 atm., 15°C. The composition of the air used in volume tractions is (Cycle Tempo [81])

Oxygen: 0.2075 Nitrogen: Water: Argon: Carbon dioxide: 0.7729 0.0101 0.0092 0.0003

The composition ot the air used tor calculations is the same as used in the tlowsheeting program Cycle Tempo. This program has been developed tor designing energy conversion systems such as power stations.

The tuel Phosphorus Furnace Gas (PFG) (72.4 ktonne/yr, no costs) is delivered with an overpressure ot at 500 mmHp, 25°C. The composition ot the PFG tuel in mole tractions given by Hoechst and used in this study is (Hoechst [96 pc] and Badger [1]):

raw gas clean gas

Carbon monoxide: 0.864 => 0.864 Nitrogen: 0.068 => 0.068 Hydrogen: 0.029 => 0.029 Water: 0.023 => 0.023 Carbon dioxide: 0.008 => 0.008 Methane: 0.006 => 0.008 Hydrogen Sulphide: 0.0008 => 0.0008 Hydrogen Cyanide: 0.0015 => 0.000 Hydrogen Phosphide+Phosphorus: 0.000036 => 0.000

The tuel is contaminated with many components (e.g. hydrogen sulphide, phosphorus, phosphorus oxide) which have to be removed betore the tuel can be used in the tuel Geil. In this report it is assumed that the tuel is contaminant-tree except tor hydrogen sulphide, i.e.

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Fuel Geil System far

a

Phaspharus Pracess Basis af Design

the fuel consists only of carbon monoxide, nitrogen, hydrogen, water, carbon dioxide and methane. The amount of hydrogen sulphide in the feed stream is used for calculation of the amount of zinc oxide.

Boiling Feed Water (BFW) (f1.40/m3) is expected to enter the system de-ionised de-aired,

at 25°C and 1 atm.

A special consideration in the design is the fluctuation of the fuel supply (see also Figure 2.2). This is caused by the policy of "Load Management" of Hoechst. The power of the three phosphorus furnaces (in total 180 MW) is shut oft in the order of 10 minutes, when the demand of electricity in The Netherlands is high. The electricity used in the furnaces is for resistance heating purposes, so after a while the production of Phosphorus Furnace Gas is also down. The fuel cell system has to deal with these fluctuations, because it is an add-on for the existing plant at Hoechst Vlissingen.

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Figure 2.2: Cumulative time probability density function of the fluctuations of the Phosphorus Furnace Gas feed.

The fuel cell system operates continuously, but will be shut oft and brought into a stand-by mode if not enough fuel is available, because no alternative feed is available at Hoechst. Therefore the on-stream time is assumed to be 7500 hours/year and not 8000 hours/year.

The minimum, average and maximum feed capacities are 0.46, 2.13 and 5.07 kg/s respectively, which produces 0.8, 8.7 and 25.4 MW electricity respectively.

2.4 Uti I ities

Nitrogen is expected to enter the system at at least 6.5 bara. The nitrogen is used as inert purge gas for the fuel cell pressure box. The needed purity is therefore only limited to circa 98%. The inert gas (containing nitrogen and carbon dioxide) produced at Hoechst is also expected to be within the specifications.

The Medium Pressure Steam used should have a pressure of at least 7 bara. This Medium Pressure Steam is used when the fuel cell system cannot produce enough steam.

Superheating of this steam will occur in the fuel cell system and is therefore allowed but not necessary.

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-7-Fue! Geil System for

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The Natural Gas used in case of stand-by situations for heating the fuel cell system, should be desulphurised up to at least 0.5 ppm hydrogen sulphide, because it is a contaminant for the used fuel cell.

2.5 Produets and emissions

The A.C. electricity produced (f0.13/kWh) is used for heating the phosphorus furnaces, and should have a voltage of at least 1000 V. During normal operation, the system generates more electricity than it consumes. Only in start-up situations A.C. electricity is netto used.

The nitrogen oft-gas can be blown oft into the air via the tlare. In principle no contaminations are present in this flow, because the flow is mostly used for pressure control objectives.

The flue gas (no costs) of the fuel cell system is almost contaminant free (sulphur components <0.05 ppmv, nitrogen oxides <1 ppmv), and can be flared without special measurements.

2.6 Catalysts

Inside the system some catalysts are used:

• zinc oxide for hydrogen sulphide removal (f8/kg: the waste product is a product which can be used in the zinc industry, and will therefore have no costs tor disposal), • water gas shift catalysts (iron or chromium oxide catalyst + nickel oxide catalyst, f30/1),

• catalytic burner catalyst (platina on alumina, f30/1).

• fuel cell (anode and cathode, made of nickel and nickel oxide, f500/kW)

2.7 Technology

The highest pressure and temperature in the system exists in the Molten Carbonate Fuel Cell stack (maximum temperature 700°C, maximum pressure 6.5 bara) and the recycle blower (maximum temperature 750°C, maximum pressure 6.5 bara). These two parts are in a pre-commercial phase; the remainder parts of the plant are proven technology. It is possible to buy a recycle blower for this kind of system, but it is very expensive. Molten Carbonate Fuel Cell stacks have been built up to 2 MW, the expected Iife time of the tuel cell stack is circa 5 years. Expected is that Molten Carbonate Fuel Cell stacks are in a commercial phase

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a

in 5 to 10 years. It is expected that licence tees have to be paid tor parts ot the tuel cell system, because the tuel cell system and its components are not developed by Hoechst.

2.8 Safety

A tew dangerous compounds exist in the tuel cell stack: carbon monoxide, hydrogen sulphide, and hydrogen. Table 2.1 shows some physical constants with respect to safety.

Table 2.1: Physical constants with respect ta safety (Chemiekaarten [13])

component MAG Molar Boiling self-ignition explosion minimum relative vapour weight point temperature limits ignition energy density (air=1)

mg/m3 _g_/_mo_l _o_e _o_e _vol_%_{in air} _mJ _[o_l methane, eH. 16.04 -162 537 5-16 0.28 0.6 carbon dioxide, e02 9000 44.01 -79 1.5 carbon monoxide,eO 55 28.01 -191 605 12-75 0.1 0.97 hydrogen, H2 2.016 -253 400 4-76 0.01 0.07 _.. water, H₂0 18.02 100 0.46 nitrogen, N2 28.01 -196 0.97 oxygen, O2 32.00 -183 1.1 hydrogen sulphide, H2S 15 34.08 -60.7 260 4-46 0.07 1.2

• subhmatlOn pomt "density steam at 100°l;

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-9-Fue! Cell System tar a Phaspharus Process Fue! cells

3 Fuel Cells

This chapter gives a general introduction to fuel cells in general in Section 3.1. The different types of fuel cells are described in Section 3.2. Section 3.3 gives the criteria for the choice of the fuel cell type, the final choice and its evaluation. In Section 3.4 the design considerations of the chosen type of fuel cell is given. The model of the tue I cell necessary for simulation of the fuel cell system is presented in Section 3.5.

3.1 Introduction

A part of the following introduction in fuel cells is based on a recently published review by

Kinashita [43]. Fuel ce lis are electrochemical devices that convert the chemical energy of

a fuel directly into electrical and thermal energy. In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment, and an oxidant, e.g. oxygen or air, is fed continuously to the cathode (positive electrode) compartment. Electrochemical reactions occurring at the electrodes produce a direct electrical current. Theoretically the fuel cell has the capability of producing electrical energy for so long as fuel and oxidant are fed to the electrodes. In reality, degradation or malfunction of components limits the operating life of fuel cells.

Besides electricity, heat is produced as a byproduct in fuel cells. This heat is available for generation of additional electricity or for other purposes.

The maximum electrical efficiency in conventional power stations is a result of the indirect production of electricity with steam as an intermediate. The maximum efficiency of conversion of heat into power is defined by the Carnot efficiency

Tl elec

=

with

Thigh - 00w

T_high

T_,ow[K]

=

lowest temperature, T_hi9h[K]

=

highest temperature,

ll_el_{ec [-]}

=

maximum efficiency to electricity.

(3.1)

This limit is not active for a fuel cell system, because here electricity is produced directly·from the fue!. In case of direct conversion of fuel into power, the maximum efficiency can be defined as the ratio of the change in the Gibbs energy and the internal energy

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Fuel Cell System tor

a

Phosphorus Process Fuel cells

l::..G

TJelec

=

l::..H (3.2)

with

.ilG [J/mole]

=

change in Gibbs energy of the conversion of fuel, .ilH [J/mole]

=

change in internal enthalpy of the conversion ot tuel.

Theretore the efficiency of fuel cell systems, which runs from 45-70% based on the lower heating value (LHV) of a fuel, is higher than most other energy conversion systems. In addition, high temperature fuel ce lis can produce high-grade heat which is available for cogeneration applications.

Because fuel ce lis operate at nearly constant efficiency, independent of size, sm all fuel ce lis operate nearly as efficiently as large ones. The system around a fuel cell (compressors, etc.) is not size independent and therefore the total system will not be independent of size. Fuel cells are quiet and operate virtually without noxious emissions, although -depending on the type of fuel cell- they are sensitive to certain contaminants. A disadvantage of fuel cells is that the electricity produced is Direct Current (DC) instead of Alternating Current (AC) as produced by conventional power stations. The conversion of DC to AC results in some efficiency losses, which means that tuel cells are particularly attractive for applications such as electrolyis, where DC is required. At the present stage of development the three primary impediments to the widespread use of fuel cells are their high initial costs, their short operationallifetime (especially with respect to the Molten Carbonate Fuel Cell), and the still unapproved application on an industrial scale.

3.2 Types of

fuel

cell

A variety of tuel cells have been developed tor terrestrial and space applications. Fuel cells are usually classitied according to the type of electrolyte used in the cell and can be ordered in the basis of ave rage operating temperatures, as follows:

• Polymer Electrolyte Fuel Cell (PEFC, 80°C), • Alkaline Fuel Cell (AFC, 80-260°C),

• Phosphoric Acid Fuel Cell (PAFC, 200°C),

• Molten Carbonate Fuel Cell (MCFC, 650°C) • Solid Oxide Fuel Cell (SOFC, 1000°C).

The (electro)chemical reactions tor the different types

Figure 3. 1: Schema tic representation of a

typical fuel cel! showing the reactant/product gases and conduction flow (Hirschenhofer [38])

ot fuel cells are given in Figure 3.1 and Table 3.1. The standard reaction enthalpies ot the overall reactions are given in Table 3.2.

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-12-Fuel Cell System far a Phospharus Pracess Fuel cells

Table 3.1: (Electro)chemical reactions for the different types of fuel cells

Fuel Cell Type Anode Cathode

PEFC H2-72H+ +2e- 1f2q+2H+ +2e-- 7 H 20

AFC _{H2+20H- -72H20}+2e- 1f2q+ _H20+2e- -720H-PAFC H2-72H++2e- 1f2q+2H+ +2e-- 7 H

20 H2+ CO; -7H20+ Cq+2e- 112q+ Cq+2e--7 CO; MCFC _CO+H20=Cq+H2 (/R-MCFC) (CH₄+ _{H20= CO+3H2)} H2+ 0"_{-7 H2O+2e}- 1120₂₊2e--70" SOFC CO+_H20=Cq+H2 CH₄+ _{H20= CO}+_3H2

Table 3.2: Standard reaction enthalpies of the overall reactions in a fuel cell

name ( overall) reaetion

Eleetrieity generation: H₂+ 1fz

q

- 7 H 20 Shift reaetion: CO + H₂0 - 7

cq

+ H 2 o func6.H₂₉₈

-242

-41 Reform reaetion: CH₄+ H20 - 7 CO + 3 H2 +206

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Fue! Geil System for

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Phosphorus Process Fue! cells

Table 3.3: Some characteristics and component impact of the avai/able types of tuel cell (Kinoshita [43J and Hirschenhoter [38])

Characlerislic PEFC AFC AFC PAFC MCFC SOFC

anode PI black or PtlC 80%PI-20%Pd Ni PVC Ni-10%Cr Ni-Zr02 cermeI

calhode PI black or PtlC 90%Au-10%PI Li-doped NiO PVC Li-doped NiO Sr-doped LaMn03

pressure (bara) 1-5 4 4 HO HO 1

lemperalure (OC) 80 80-90 260 200 650 1000

eleclrolyte (wl%) Nation 35-45%KOH 85%KOH 100%H3PO, 62%Li2C03-38%K,C03 yttria-stabilized Zr02

H₂ tue I tuel tuel tuel tuel tue I

CO poison (>10 ppm) poison poison poison (>0.5%) tuel tuel

CH, diluent diluent diluent diluent diluent (tuel tor IR- tuel MCFC)

CO2 & HP diluent diluent diluent diluent diluent diluent

S (as H2S/COS) unknown poison poison poison (>50 ppm) poison (>0.5 ppm) poison (>1.0 ppm)

The physicochemical and thermomechanical properties of materials used for the cell components, i.e., electrodes, electrolyte, bipolar separator, current collector, etc., determine the operating temperatures and the effective life of the cells. Especially the properties of the electrolyte are important. Solid polymer and aqueous electrolytes are limited to temperatures of circa 200°C or lower because of high water-vapour pressure and/or rapid degradation at higher temperatures, whereas the operating temperatures of high-temperature fuel cells are determined by the melting point (MCFC) or the ionic-conductivity requirements (SOFC) of the electrolyte.

3.3 Choiee of fuel eell

The choice of a particular type of fuel cell tor a particular application depends on four criteria: • state of development of the fuel cell,

• efficiency of the tuel cell, • allowable feed for the fuel cell, • costs.

State of development of fuel cell

Low-temperature fuel ce lis are much further in time developed, and some types are already (semi-)commercially available (e.g. the PAFC). A few demonstration plants (0.25-2 MW) for the MCFC type of fuel cell will be in operation soon. It is anticipated that semi-commercial operation will be reached within 10 years. The SOFC is about 5 years behind in development compared to the MCFC, but corrosion (which is a major problem with the

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-14-Fuel Geil System far

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Phaspharus Pracess Fuel cells

MCFC) is not an critical issue in our case. The internal refarming variant of the MCFC is a litlle less far developed than the normal MCFC. The main problem for th is type of tue I cell are the poisoning of the internal retorming catalyst by the carbonate and the uneven temperature distribution over the cel I.

Efficiency of fuel cell

Figure 3.2 shows the cell voltage for the different types of fuel cells. The high temperature tuel cell has a lower overall internal resistance. On the other hand, the reversible cell voltage will be lower. The net effect is a higher efficiency. Another advantage of the high temperature fuel cell is that the heat which is produced with temperatures of 600 to 1000°C,

can be used for electricity production via steam or for heating purposes.

Allowable feed for fuel cell

In most practical applications, low temperature fuel cells using aqueous electrolytes are restricted to hydrogen as the fuel. The presence of carbon monoxide and sulphur-containing

~ Q) Ol al

g

äi () 1.2 1.1 1.0 AFC

/

0

IMCFC

0.9 SOFC APEFC 0.8 0.7 !PAFC O. 300 _{500 700 900 1100 1300 1500} Temperature ("K)

Figure 3.2: Dependence of the initial operation voltage of typical fuel cells on temperature (Kinoshita [42]) AFC=Alkaline Fuel Cell,

PAFC=Phosphoric Acid Fuel Cell;

MCFC=Molten Carbonate Fuel Cell;

SOFC=Solid Oxide Fuel Cell;

PEFC=Proton Exchange Fuel Cell

gases in low temperature fuel cells is detrimental to cell performance because they poison the anode. In high temperature fuel cells, the list of usabie fuels is more extensive for two reasons: the inherently rapid electrode kinetics and the lower need for high electrocatalytic activity at high temperatures. In addition, there are options for hydrocarbon fuels which can be used either directly or indirectly.

Costs

Only estimations of the initial costs of fuel ce lis are available. The high temperature fuel cells have the advantage that no noble metals are necessary as a catalyst. This is one of the reasons why in the long term, these types of fuel cell are expected to be cheaper than low temperature fuel cells.

Choice of fuel cell type

Phosphorus Furnace Gas consists mainly (:=:85%) of carbon monoxide, which can be transformed into hydrogen via the water-gas-shift reaction. A small percentage (0.6%) of the feed is methane. This can be transformed into carbon monoxide and hydrogen via the reforming reaction. The high temperature fuel cells can use the carbon monoxide in the feed directly. The methane in the feed can be used as feed in an MCFC with internal reforming

(23)

Fue! Geil System far a Phospharus Pracess Fue! cells

(IR-MCFC) and in an SOFC. The fuel contains also some containments, which can act as poisoning agent. Thorough cleaning of the feed is necessary for most types of fuel cells. The type of fuel cell chosen for the present project is the MCFC-type. The reasons for th is choice are the expected high efficiency (the produced heat can also be used for indirect electricity generation); the expected time to a pre-commercial status of th is type of fuel cell (:==10 years); and the type of fuel used (hydrogen and carbon monoxide). No internal reforming is necessary because hardly any methane is present in the fuel.

A variety of electrolytes can be used in an MCFC, except the standard electrolyte (Figure 3.3) because of the high content of water in the anode and cathode. The molair water fractions in the anode inlet, anode outlet, cathode inlet and cathode outlet are respectively 0.41, 0.46, 0.17 and 0.19. The potassium in the matrix will react with water to form potassium hydroxide,

HZ + co + COz ::> . . • . . . .. HzO. COz .(Hz • CO) ::> collector • • • • • • • • • • anode matrix cathode 65% porous Ni (.2·10% Cr) " l i l liAJOZ matrix 45% lizC03 26.2% KZ C03 28.8% 70·80% porous NiO (.1·2% Li) collector • • • • • • • • • • 0,6 ·1.5 mm 0,5 ·1.2 mm 0,4·0,8 mm Oz • COZ ::> •••••••• + • • • • • • • • • • • • • • (02' C02) ::>

Figure 3.3: Schema tic representation of the different cell components (Boersma [30])

which has substantial vapour pressure. The electrolyte will'disappear'. A solution for this problem is the use of sodium instead of potassium carbonate in the matrix. Sodium and lithium hydroxide have a much lower vapour pressure and will not 'disappear' (Hemmes [95

pc}).

3.4 Design considerations tor the MCFC-stack

Manifolding

Two main types of manifolding for fuel cells exist: cross flow with external manifolding and cocurrent flow with internal manifolding. The danger of hot spots (places in the fuel cell where the temperature is too high, resulting in declining fuel cell performance) is smaller with cocurrent flow than with cross flow.

Fue'lI'1

Air in

Figure 3.4: Types of manifolding (Ieft: internal, right extemal) (Kinoshita [43])

The reason is that the highest temperature with cross flow operation will be inside the fuel cell, whereas the highest temperature for cocurrent flow will be at the outlet of the fuel Geil. Therefore cocurrent flow is used.

(24)

-16-Fuet Cell System for

a

Phosphorus Process Fuet cells Temperature

The heat produced in the fuel cel! wil! result in a temperature increase of the outlet gas streams. The amount of cooling gas wil! be smaller if the temperature increase over fuel cell wil! increase. The temperature range of the fuel cell is limited by efficiency considerations (the overall internal resistance increases with lower temperature); operating conditions (the eutectic mixture of molten carbonate must be above the eutectic temperature of circa 520°C) and by lifetime considerations (the corrosive properties of molten carbonate increase with higher temperatures). In most references, the average temperature is 650°C (e.g. Hirschenhofer [38/) with a temperature increase over the fuel cell of 100°C. In our case,

therefore, inlet and outlet temperatures are set at 600°C and 700°C, respectively.

Pressure

The pressure range of a fuel cell is limited by the ambient pressure, by its lifetime (concentrations of corrosive components increase with increasing pressure), by the electrical

overall system efficiency (increasing cell potential with increasing power but also increasing power demand for compression), and by the size of the apparatus (volumetric flows decreases with increasing pressure). The amount of added steam wil! increase, because the tendency of carbon deposition increases at higher pressures. Constraints are 1 atm (less than ambient pressure is dangerous) and circa 7 bara (corrosion reactions are too quick above this pressure). The 'best' pressure will have to be found by optimisation on the basis of, e.g. maximum amount of electricity produced. This is explained in Chapter 5 (Optimisation and Analysis).

Intemal cooling

In high temperature fuel cells the heat produced is normally removed by increasing the cathode stream. This results in lower efficiency. A few literature references to internal cooling of a Molten Carbonate Fuel Cell exist (e.g. Yamashita [67/), but to date na bench scale units appear have been built. The internal cooling option is rejected, because of lack of short time perspectives.

Heat loss

The fuel cell is built inside a pressure vessel, sa the loss of heat to the environment is limited, because of the small area of this pressure vessel. Although the pressure vessel is purged with nitrogen, the heat loss is neglected because the nitrogen flow is small.

Pressure drop

The maximum pressure drop over a fuel cell and the pressure difference over the matrix should be very small. The reason is that the molten carbonate which is present in the matrix of the fuel cell, must stay in this matrix and may not be blown into the anode, the cathode or the pressure vessel. The matrix has an open connection with the pressure vessel which surrounds the fuel Geil. Via th is pressure vessel the matrix at the inlet of the fuel cell has a

(25)

Fuel Geil System for

a

connection with the end ot a tuel cell: the pressure drop over the tue I cell and the carbonate scrubbers tollowing the tuel cell is expected not to exceed 0.03 bar (Machielse [100 pc]).

Current density

The most trequent used current density ot an MCFC is 1.5 kAlm2 (e.g. Kordesch [44]) tor a system with a conversion ot circa 70-80% conversion and 0.8 V as cell voltage. This value is also used tor this conceptual design study. Lower values tor the current density will increase efficiency at the expense ot increased costs.

3.5 Model of an

MCFC

3.5.1 Introduction

For the conceptual design it was necessary to use a tlowsheet simulation software package. However most commercial tlowsheet simulators do not include a model tor a tuel Geil. The only known exception is the tlowsheeting program Cycle Tempo developed by the Dutch Organisation tor Applied Research (TNO) and the Delft University ot Technology, which contains simple models tor a molten carbonate fuel cell and solid oxide tuel Geil. In the end,

however, it was decided to use the Aspen PIUS™ flowsheeter because of its widespread use in the process industry and because ot the advanced options (like electrolyte systems) available in it. As a consequence, a new fuel cell model applicable within Aspen Plus had to be developed.

The literature (e.g. White [65]) offers several models which describe the behaviour of a tue I cell system. These models are divided into three types:

• phenomenological/empirical (e.g. IHI-model), • extended theoretical (e.g. PSI),

• simplified theoretical (e.g. Cycle Tempo).

The choice of a particular model to simualte a fuel cell system is governed by the following considerations:

• applicability in a tlowsheet simulation program,

• consistencyat limits of operation (e.g. pressure, temperature, concentrations), • (possibility of) validation by measurements,

• short computation time,

• possibility to calculate the same tuel cell under different process conditions.

The extended PSI model (available at e.g. ECN) runs standalone and was not available to the author. Therefore this option had to be rejected. The first option (IHI-model) has been used and tested, but tinally rejected because of the problems of using the model outside the

(26)

a

limits of measurement. The implementation of the fuel cell model used in Cycle Tempo was only available in Cycle Tempo itself. The decision was therefore made to use the theoretical background of th is model (de Groot [91 pc]) and to remove the main simplifications (like ideal

gas, constant temperature and constant internal resistance approximation). The IHI-model and the Cycle Tempo model will be discussed in more detail in the two sections.

3.5.2 IHI-empirical correlation

Izaki [40J of the IHI company (Japan) has proposed an empirical correlation which predicts

the cell voltage

v=

V_s- 0.17·(1;-1;.J + 0.085.10_I09[:)+ 0.110.10_I09[

~:~

1

(3.3) 5 YC,02'S Y C,Co2,S with V [V] V_s[V]

ç

[mole/mole] Çs [mole/mole] Ps [bara] Yc,comp [mole/mole] Yc,comp,s[mole/mole]

= entorced potentialof the fuel cell,

= enforced potential at standard state (0.820 V), = conversion of hydrogen plus carbon monoxide,

= conversion of hydrogen plus carbon monoxide at standard state (0.7 mole/mole),

= pressure of the system at standard state (5*1.01325 bara), = ave rage male fraction of component camp in the cathode,

= ave rage male fraction of component camp in the cathode at standard state (Yc,02,s·Yc,C02,S=0.03 mole/mole).

The stack used (3,6 m2 active area) has internal manifold and a co-flow pattern. The current density used was 150 mAlcm2_{. The fuel inlet (H;CO:!H20=66/17/17%) and the oxidant inlet} (Air/C02=70/30%) were heated to 600°C, the average temperature of the inlet and outlet gas streams was 650°C. The base conditions were 5 ata as operating pressure, 70% fuel and 30% oxidant utilisation. In the tests the fuel utilisation was va ried between 50 and 80%, the oxidant utilisation between 50 and 80% and the operating pressure between 1.2 and 7 ata. The advantages of th is model were the validation by measurements and the availability as

FORTRAN blocks in Aspen Plus (Schinkel [104 pc]). It is a very fast algorithm, but quite rough.

The main limitations of th is correlation are:

• In the correlation the cathode recycle was already taken into account. Therefore this overall model is difficult to transtorm into a general model.

• The model is only valid tor a limited range of conditions. Extrapolation is possible, but the results are unreliable.

(27)

Fuel Geil System for a Phosphorus Proeess Fuel eells

• In the empirical correlation the simp Ie average is taken between the inlet and outlet concentration. If one or more of these concentrations approach zero, the model overestimates the fuel cell potential.

• The influence of the anode gas composition is not implemented. The measurements performed by Izaki [40J were all done with standard reformed fuel (66% H₂, 17% CO₂,

17% H20). The fuel delivered by Hoechst consists for ab out 90% of carbon monoxide,

which is a very different feedstock. A way to overcome this limitation is to implement a kind of Nernst relation for the different concentrations. This correction has asolid theoretical foundation, but it remains an idealization of the real behaviour.

According to Nernst the equilibrium potentialof the electrochemical system in a molten carbonate fuel cell (see also Table 3.1), with the overall reaction

is

E =

Eo

+ --'In R· T [ PH. 2(8)

1

+ --'In R· T

(

~

Po 'Pea ) .

2 . F P . P 2 . F 2(<) 2(<)

H2 0(8) e02(a)

with

E [V]

=

the equilibrium potential,

Eo [V]

=

the equilibrium potential at standard state (1 bara, 25°C),

F [C/mole]

=

the Faraday constant,

T [K]

=

the temperature of the system,

py(a) [bara]

=

the vapour pressure of component y in the anode,

py(C) [bara]

=

the vapour pressure of component yin the cathode.

(3.4)

(3.5)

The fuel used in th is design study a very different from the standard reformed fuel this relation is based on. By using the Nernst equation, a first attemp was made to calculate the influe of the different feed. The difference in potential is calculated from the ave rage concentrations using V - R-T 'In[

~

/

(

~

.~

)

. 1 /

p]

n·F

_Ya

,

H

₂,S/

(

Ya

,eo 2

,

s . Ya,H

2

o

,s)

1 /

Ps

(3.6) with

Vcor [V]

=

the corrected potential,

Yc.comp [mole/mole]

=

average mole fraction of component comp in the anode,

Yc,comp.s[mole/mole]

=

ave rage mole fraction of component comp in the anode at standard state.

As already stated, th is model is limited and only based on a small set of observations of one author on one setup.

(28)

-20-Fue! Cell System for

a

Phosphorus Process Fue! cells

3.5.3 Cycle Tempo model

The research version of the flowsheeter Cycle Tempo (developed at the Delft University of Technology and at Dutch Organisation of Applied Research (Cyc!e Tempo [81])) already offers an implemented model for a fuel Geil. This model uses an equilibrium potential which depends on concentrations, temperature, and pressure. The temperature and the equivalent electrical resistance are kept constant. The Cycle Tempo model is used to compare it with the newly developed model, because the Cycle Tempo model has been compared with measurements of ECN in Petten at 1 bara. As a result of this comparison both models have been improved (de Groot [91 pc]).

3.5.4 New model

Finally a new model has been developed, which can meet all the above mentioned considerations. The basic idea of the Cycle Tempo model is used: an equilibrium potential of the electricity generating reaction and an overall electrical resistance. The main differences are

• the change in the Gibbs energy at the actual temperature is used to calculate the equilibrium voltage,

• The internal resistance is not kept constant but is a function of temperature, pressure and concentrations.

A fuel cell can be described by a membrane reactor: one si de is the anode (=fuel side), the other the cathode (=oxidant side). The matrix through which the electrolyte is transported is regarded as a kind of membrane. Figure 3.5 gives a schematic representation.

Fuel Electricity

I-t--+-+-+-+-t-l Product Gas Oxidant

' - - - , - - - '

Figure 3.5: Schema tic representation of a fuel cell

1r=~--~~~~~~~~---'

O.91···j···;···""""""j-"""

0.801-.. ···-'···,··· .. ··"··· ... , ... 7"'-"""""

i

~::

::~~::

:

:I~:::

:

:::I:::

:~~

I

~

::~:

~'

i

~

::::::::r::::::::I.:.:~::

~ i j ~ ~ i á:. 0.4 .. ···_··· .. :-· .. ·· .... 1··· .. ·:-.. ··· ··· .. ·T· .. ···r-··· .. · 0.3 ... ; ...

,...+...

. ...

~...~

....

.

... .

02 ···· .... ····~··· .. ··· .. · .. · .. · ... ··· .. i·· .... ·· ... + ...

j ...

.

0.1 .... ·· .. · ..

· +

· ··· ..

···f···· .. ···· ..

+··· ....

··

..

+·

..

· ..

···+··· .. ··r···· .. · .. · .. · 00 5000 1()()()() 15000 20000 25000 30000 35000 Surlace [m2]

Figure 3.6: Typical profile of the equilibrium potential and the fixed potential

(29)

I

-Fue! Cell System far

a

Phospharus Process Fue! cells

Through the matrix only ions can be transported. The electrolyte (ion) transport across the matrix is caused by the driving force of the system: the difference in concentrations. The difference in concentrations can be expressed as an equilibrium potential. The difference of this equilibrium potential and a fixed potential (with which electricity is produced) is the driving force for the fuel cello Figure 3.6 shows an example for values of the equilibrium potential and the given potentialover the whole tuel cell.

To calculate the current in the tuel cell, the tue I cell has to be divided into slices (Figure 3.7). In every slice an anode and a cathode stream enter, an anode and a cathode stream leave, carbonate is transported

trom the cathode to the anode and electricity is produced. Electricity I

I

I i Anode in

---!

! ! Cathode in ...--.i

1

i' - - - -Anode out )

I---

Cathode out

i !

1

I

Figure 3.7: Schematic representation of a

slice of a fuel cell

3.5.4.1 Equations

In a slice ot the tuel cell, the system can be described by the tollowing differential equation, when cocurrent flow (ot the reactants) is assumed:

. dl E-V

1 = =

-dA

'int

(3.7)

with

i [Alm2]

=

the local current density, I [A] = the current,

A [m2]

=

the surface of the matrix,

E [V]

=

the equilibrium potential, a 50le function of concentrations, temperature and pressure,

V [V]

=

the tixed potential, r_int[Qm2

]

=

the total internal resistance.

The internal resistance is calculated with the formulas ot Mugikura [52J and Se/man [61J.

'int

with 0.4567'1O-7'e 109R.T .0'10 . log(10.174975 'P) . .+.-'.80' . .+.'.533 .cp-l.48 3 ( )-1.801 +1.533-1.48 qJ a 'P ~.a 'P CO.a CO2.a +7.504'1O-6'e 77R.T .8'10 . log(10.174975·P) .cp-0.43.cp-0.09 3 ( )-0.43-0.09

cp

c ~.c CO2.c 40.9'103 + 2.71'10-3'e fH page -22-(3.8)

(30)

a

Phosphorus Process

<Pcomp,x [kmoleIs] = the moleflow of comp in the anode (x=a) or cathode (x=c)

The equilibrium potential is: llG_r

E =

-n·F

with

ilGr [J/mole] = the change in Gibbs energy of reaction 3.4,

n [-] = the amount of electrons involved in the reaction 3.4 (=2).

Fuel cells

(3.9)

The Gibbs energy depends on the concentration of the different components, the temperature and pressure of the system. The concentration of the different components is not only changing by the reaction 3.4, but by the shift reaction

(3.10)

and -in case of internal reforming- the reform reaction

(3.11)

It is assumed that the shift and the reform reaction are in equilibrium at every place at the anode side of the fuel cell.

The Gibbs energy of the components depends on the actual temperature. The fuel cell is expected to operated adiabatic and in equilibrium .. Therefore the temperature profile can be calculated by fulfilling the heat balance (The electricity produced is a product stream for the heat balance.).

d(qJa'Ha) + d(qJc'Hc) + V- dl = 0 (3.12)

dA dA dA

After integration this equation is transformed into an algebraic equation: qJ a . Ha +

rIJ

c . Hc + V·

IA -

qJ a.O . Ha,o -

rIJ

c,O . Hc,o

=

0

with

= the moleflow in the anode (x=a) or cathode (x=c) = the inlet moleflow in the anode (x=a) or cathode (x=c)

the molar enthalpy of the flow in the anode (x=a) or cathode (x=c)

(3.13) <1>x [maleIs] <1>x,o [molels] Hx [J/mole] = Hxo [J/mole] IA [A]

= the inlet molar enthalpy of the flow in the anode (x=a) or cathode (x=c), = the actual current.

(31)

Fue/ Cell System far a Phospharus Proeess Fue/ eells

3.5.4.2 Implementation

In order to get a good 'feel' for the operation of the model, the model was set up with some simplifieations (e.g. the ideal gas law) and with very simp Ie numerical approximations. This was done in a spreadsheet (Quattro Pro 5.0). Subsequently, the model was implemented as a FORTRAN routine with all appropriate precautions with regard to numerical implementation of what is essentially a system of differential equations. The FORTRAN routine

was implemented as a USER block in Aspen Plus. This two-pronged approach gave a simple tooi to test basic ideas, as weil as a solid interface to a general purpose flowsheeter.

The used correlations for the overall electrical resistance according to Mugikura {52J and

Se/man [61J are based on experiments at one bara, 50 the influence of higher pressures is uncertain. The integration has been done using an adaptive stepsize fifth order Runge Kutta method. The equilibrium potential is calculated by fulfilling a small conversion step, and calculating the difference in the Gibbs energy before and after the step. The implementation of the heat balance and equilibria was done with two nested Reguia Falsi procedures. The

FORTRAN code and comments to the code are given in Appendix VI. The corresponding

Manual Pages for the Aspen Plus User Guide are given in Appendix V.

With the newly developed model, the limitations of the other mode Is have been bypassed and the requirements for a user model have been fulfilled: the model is applicable in a standard flowsheeter, is consistent at limits and validated, does have a relatively short calculation time (circa %-1f2 5 on a HP1735 series workstation with 147 as Specmark89) and is flexible towards the feed of the fuel Geil.

A spin-off of the developed fuel cell block is a model for a membrane. The major difference is the driving forces, instead of the change in Gibbs energy it is the difference in partial pressure. This model is developed in cooperation with Fakhri {90 peJ, a Ph.D Student at the Mechanical Engineering Department, Laboratory for Process Equipment.

(32)

a

Phosphorus Process Design of the Flowsheet

4 Design of the Flowsheet

In this chapter, a flowsheet for the production of electricity from the flared oft-gas at Hoechst Vlissingen is presented. Agiobal description of the flowsheet is given in Section 4.1. In the next section, the supposed equipment is described, including the results of the calculations for the flowsheet and for the equipment. The flowsheet simulator Aspen Plus ™ of Aspen Technology is used for solving the mass and heat balances (see also Appendix 11). The input file for the flowsheet simulator Aspen Plus, including an explanation of the final simulation and the matching report file are given in Appendix VII. Alternative options and considerations are given in the third section. Finally, the control aspects of the flowsheet are described.

4.1 Description of the process

The concept of the flowsheet is

quite simpie. It can be divided into three main parts: feed pre-treatment, feed conversion as the focal point of the whole process, and energy recovery from the flue gas. This concept, which is shown in Figure 4.1, is very much the same as for the main competitor of

Water

0

· ---

0 __ - 0 ____

-l

i

Fuel

C?---i

Combus;;on _

~

l@

_

I

ChO'i'8C' / , ,

~---~.~- .

~

-

1 0i

Fuel Cel '

Air~

pretreatmen'.: conversion erergy recovery

Figure 4.1: Basic components in the flowsheet

a fuel cell: a gas turbine. However, two essential difterences exist with a gasturbine with steam injection:

• The fuel cell produces electricity and heat; the combustion chamber of a gas turbine produces only heat,

• The inlet of the expander is circa up to 1200°C for a gas turbine, and 700°C for a fuel cell.

Figure 4.2 depicts a diagram of the chosen set-up with the control loops. The input feeds needs to be brought up to the system pressure in the feed pretreatment section. Subsequently it passes through the fuel cell where electricity production takes place and the carbon monoxide is converted into carbon dioxide. The produced carbon dioxide is recycled to the cathode part of the fuel cell, where it is mixed with fresh air. About half of the cathode outlet stream is released, the remainder is recycled over the cathode.

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Fuel Cell System for a Phosphorus Process WATER MP STEAM ~~ ______________________ ~~ AIR NATURAL GAS -.;---, ~ - --C 1 Fuel Compressor C2 Air COrilpressor C3 Recycle Blower C4 Expcnder W Shift Reactor R2 Fuel Cell R3 Cotalytic aurner H1 Heat Exchonger P 1 Water Pump V1 KO Vessel

Design of the Flowsheet

exc:longer Streo~ name Ter.1:Jeroture Joruori 1995 J.C.J.M. Goossens PS&C/C I&MS/ DUT Figure 4.2: Flowsheet of the process. AVG=average; PdC=pressure difference controller; FC=flow controller;

LC=level controller; NC=number of revolutions controller; PC=pressure controller; FrC=ratio controller; TC=temperature controller; V=voltage

The system has three main feeds: the fuel gas stream (Phosphorus Furnace Gas) F1, the air stream A 1 and the water stream W1. The fuel stream F1 enters at a very small overpressure (circa 500 mmH₂0). It needs to be mixed with water to avoid carbon formation

via the Boudouard reaction in the fuel cell R2. Compressors C1 and C2 and pump P1 are used to reach the optimal pressure of the fuel Geil. A part of reactor R1 removes traces of poisons for the fuel cell (sulphur components). The remainder part of the reactor R1 brings the feed to water-gas shift equilibrium. This exothermic water-gas shift reaction takes place mainly to increase the temperature to the requested inlet temperature of the fuel Geil. Knock-Out drum V1 is used to circulate the boiling feed water and to avoid water droplets in the super heating part H1 c of the heat exchanger H1.

The fuel cell R2 produces electricity. A part (stream AR6) of the cathode outlet stream ARS

of the fuel cell is released to the energy recovery section. The anode outlet stream FW3

contains the carbon dioxide to be used by the cathode. The remainder stream AR1 is

(34)

a

Phosphorus Process Design of the Flowsheet

combined with the anode outlet stream FW3 of the fuel Geil. In the catalytic burner R3 the flow is burned, leading to stream AR2. Before the catalytic part of the burner, a carbonate scrubber (not drawn in Figure 4.2) re moves traces of carbonate to avoid deactivation of the catalytic burner. High temperature blower C3 is used to compensate the pressure drop over the recycle. The recycle stream AR3 is mixed with the fresh air feed A2 to cathode inlet stream AR4 in order to reach the optimal pressure of the tuel Geil.

The cathode oft-gas stream AR6, is expanded over the expander C4, which is directly coupled with compressor C2, to produce electricity in a generator and to generate the power for the three compressors (C1, C2, C3). The outlet stream AR7 of the expander is used to heat the inlet water stream W2to the super heated steam stream W3 in heat exchanger H1. The cooled oft-gas stream ARa is flared.

4.2 Equipment calculations

The method of calculation and the values of the critical parameters and variables of the difterent types of equipment are given in this section. A few variables and parameters have been optimised. A more extensive treatment is given in Chapter 5 (Optimisation and Analysis).

4.2.1 Fuel Cell 5tack

The method of calculation of the fuel cell R2 is already described in Chapter 3 (Fuel Cells). Figure 4.3 shows the temperature, the (equilibrium) potential, the conversion, the internal resistance, the local and the average current density as function ot position in the fuel cell, expressed by the total cell-surface.

· .

2.

· .

· --

-

·---

-·

-t·.··--·

···

-

-·

·.·1-·-

-

--

---

-·

-

-·i

·.···

····

·

· ···

·1

····

. ... ~ ... 680

Averag~ Current Den$ity (kA'm2) ~ .

: . ~ ~. Ü

1 5 ... .:. ... , ... '-... '-... :... ... 660

.2-. : : : Temperature ~oC) : ~

Intemal Resisiance (OhmaTÎ2) : ~: "ê

: : : : : Ql 1. . ... ~.... . ....... L. .... .;.. ...

-j--...

.

....

.

-+ ..

...

.

..

.

..

.

640 ~ .

f--·

.

· .

.

+-

...

..

.

..

...

..

.

-j--.

..

.

...

+

...

.

...

.

..

620 : Conve~on Fu~ (.)

I

~ ~~--~----~----_r----_r----_r----_t600 1000 2000 3000 4000 5000 6000

Surfaee of fuel eell (m2)

Figure 4.3: Profiles of important variables in the tuel ce"

The used average current density of the stack is 1.5 kAlm2

, the inlet temperature is set at 600°C, the outlet temperature is set at 700°C, according to the values given in Chapter 3 (Fuel Cells). The average fuel gas flow, which follows out of the optimisation study (see Chapter 5(Optimisation and Analysis», is used to calculate the surface. The fuel cell is