Heat and Fuel Recovery Cycles

Simple representations of three fuel cell based heat and fuel recovery cycles are shown in Figures 9-12, 9-13, and 9-16.

Regenerative Brayton Cycle: The regenerative Brayton cycle, Figure 9-13, shows a gas turbine compressor for the air flow to the cell. The flow then passes through a countercurrent, recuperative heat exchanger to recover heat from the combustion product gases leaving the gas turbine. The air and the fuel streams then pass into the cathode and anode compartments of the fuel cell(s). The air and fuel streams leaving the cell(s) enter the combustor where they mix and the residual fuel burns. The combustion products enter the turbine, expand, and generate additional power. The turbine exhaust gases pass through the recuperative exchanger to the stack.

The most significant variables characterizing the cycle are the fuel cell operating temperature range and the temperature and pressure at the gas turbine expander inlet. These variables are directly related to certain operating variables: the air/fuel ratio entering the fuel cell, the fraction of the fuel leaving the cell unburned, and the temperature difference between the combustion products and air at the high temperature end of the recuperative heat exchanger. The operating variables must be selected and controlled to allow effective operation of the fuel cell, combustor, and gas turbine. There may well be an optimal quantity of unburned fuel leaving the fuel cell, depending on the acceptable fuel cell operating temperature range and turbine inlet temperature.

Further insight can be gained from the idealized T - S diagram for the cycle, Figure 9-14. The compression of the air and fuel streams is represented here as a single adiabatic reversible (constant S) process in which the temperature of the gases rises above ambient. The heating of

Brayton Cycle Plants
Figure 9-13. Regenerative Brayton Cycle Fuel Cell Power System

the air and also the fuel streams first in the recuperative exchanger, then in the fuel cell and finally in the combustor is assumed to occur along a single line of constant pressure. The subsequent expansion of the combustion gases in the turbine is also represented as an adiabatic reversible (constant S) process in which the temperature of the gases drops to a value close to that of the gases entering the fuel cell. The pressure ratio (PR) of the turbine (and of the compressor) is therefore established by the turbine nozzle inlet temperature (NIT) and the fuel cell operating temperature. In general, the pressure ratio of a regenerative Brayton cycle is low compared with that of a combined Brayton-Rankine cycle. A low pressure ratio allows a low outlet temperature of the exhaust gases from the recuperative exchanger as heat is transferred to the air leaving the compressor (and possibly also the fuel) and consequently results in low heat rejection and a high cycle efficiency.

The practical aspects of the cycle involve the efficiencies of the gas compressors, the turbine expander, and the fuel cell; the pressure losses as the gases flow through the system; and the temperature differences and the difference in heat capacities of the streams flowing through the recuperative heat exchanger. Other aspects of the fuel cell operation must be considered in greater detail for the design and evaluation of the power system. These include the possible need for fuel reforming external to the cell and the recycle of combustion product streams to provide the steam required to carry out the reforming process, to avoid carbon deposition, and to provide H2 for effective cell operation.

Table 9-23. Performance Calculations for a Pressurized, High Temperature Fuel Cell (SOFC) with a Regenerative Brayton Bottoming Cycle; Approach Delta T=30F

COMPRESSOR EFF = 0.83 n = number of moles

TURB EXPANDER EFF = 0.89 Cp = specific heat

FUEL CELL EFF= 56.9 Hf = heat of formation at standard conditions

CYCLE EFF= 82.1 So = entropy at standard conditions

COMPRESSOR EFF = 0.83 n = number of moles

TURB EXPANDER EFF = 0.89 Cp = specific heat

FUEL CELL EFF= 56.9 Hf = heat of formation at standard conditions

CYCLE EFF= 82.1 So = entropy at standard conditions

STREAM #

1

2

3

4

5

6

7

Cycle

p, PRESSURE, atm

1

1.48

1.48

1.48

1.48

1

1

T, TEMPERATURE, K

298

337

1200

1311

1332

1216

352

CH4, n

1

1

1

0.07

0

0

0

CO, n

H2, n

CO2, n

0

0

0

0.93

1

1

1

H2O, n

0

0

0

1.86

2

2

2

O2, n

16.23

16.23

16.23

14.37

14.23

14.23

14.23

N2, n

64.92

64.92

64.92

64.92

64.92

64.92

64.92

SU M (n)

82.15

82.15

82.15

82.15

82.15

82.15

82.15

SUM(nCp)

629.72

629.72

629.72

628.97

628.92

628.92

628.92

SUM(nHf)

-17.9

-17.9

-17.9

-196.181

-209.6

-209.6

-209.6

SUM(nSo)

3813.11

3813.11

3813.11

3811.99

3811.91

3811.91

3811.91

GAMMA

1.350

1.351

Q, HEAT, kcal/molCH4

0.0

543.5 0.0

-0.2

0.0

543.5

1086.8

W, WORK, kcal/molCH4

-24.4

0.0 109.1

0.0

72.7

0.0

157.4

The performance of a solid electrolyte fuel cell (SOFC) system (Hirschenhofer et al., 1994) operating with a regenerative Brayton bottoming cycle for heat and fuel recovery has been calculated. Table 9-23 illustrates the results. The work from the fuel cell burning CH4 is assumed to be 60% the theoretical maximum; the corresponding fuel cell voltage is 0.63 volts. The efficiencies of the fuel and air compressors are 83%; and the expander of the turbine, 89%. It is assumed that the cell makes direct use of CH4 fuel, or that oxidation and reforming are coincident; operation of the cell thus provides both the heat and the H2O required for CH4 reforming. Pressure losses in the fuel cell, combustor, recuperative exchanger, and the ducts of the system are ignored.

The results of the performance calculations are summarized in Table 9-24. The efficiency of the overall power system, work output divided by the lower heating value (LHV) of the CH4 fuel, is increased from 57% for the fuel cell alone to 82% for the overall system with a 30 F difference in the recuperative exchanger and to 76% for an 80 F difference. This regenerative Brayton cycle heat rejection and heat-fuel recovery arrangement is perhaps the simplest approach to heat recovery. It makes minimal demands on fuel cell heat removal and gas turbine arrangements, has minimal number of system components, and makes the most of the inherent high efficiency of the fuel cell.

Table 9-24. Performance Computations for Various High Temperature Fuel Cell (SOFC)

Heat Recovery Arrangements

General Conditions Notes

SOFC, solid oxide fuel cell PR = pressure ratio of the gas turbine

Operating temperature, 1700-1900 F NIT = nozzle inlet temperature of the turbine expander

Fuel cell output: 60% of theoretical maximum from CH4 fuel

Gas turbine compressor, expander efficiences: 83, 89%

Steam turbine efficiency: 90%

Work Output, %

Overall

Heat Recovery

Fuel

Gas

Steam

System

Arrangement

Cell

Turbine

Turbine

Eff., %

Remarks

Regenerative Brayton Cycle

69.3

30.7

n/a

82.1

30 F Approach in Recuperative Exchanger Gas Turbine PR=1.48, NIT=1938 F

Regenerative Brayton Cycle

74.5

25.5

76.3

80 F Approach in Recuperative Exchanger Gas Turbine PR=1.35, NIT=1938 F

Combined Brayton-Rankine Cycl

75.3

10.3

14.3

75.6

Gas Turbine PR=12, NIT=2300 F Steam Turbine: 1600 psia, 1000 F, 1.5" Hg

Rankine Cycle

79.1

20.9

72.4

Steam Turbine: 1600 psia, 1000 F, 1.5" Hg

Combined Brayton-Rankine Cycle: The combined Brayton-Rankine cycle, Figure 9-14, again shows the gas turbine compressor for the air flow to the cell. This flow passes through a heat exchanger in direct contact with the cell; it removes the heat produced in cell operation and maintains cell operation at constant temperature. The air and fuel streams then pass into the cathode and anode compartments of the fuel cell. The separate streams leaving the cell enter the combustor and then the gas turbine. The turbine exhaust flows to the heat recovery steam generator and then to the stack. The steam produced drives the steam turbine. It is then condensed and pumped back to the steam generator.

Combined Cycle Fuel
Figure 9-14. Combined Brayton-Rankine Cycle Fuel Cell Power Generation System

The air/fuel ratio entering the fuel cell and the fraction of the CH4 fuel consumed in the cell are selected to achieve the desired fuel cell operating temperature range and gas turbine NIT and PR. These are selected to correspond with those of a conventional, large-scale, utility gas turbine.

Further insight can be gained from an idealized T- S diagram for the cycle, Figure 9-15, in which both the Brayton and the Rankine cycles are illustrated. Both the pressure and the temperature increase during fuel and air compression in this combined cycle will be significantly greater than in the regenerative Brayton cycle described above. The heating of the air and fuel, the operation of the fuel cell, and the burning of the residual fuel are assumed to occur at constant pressure. The expansion of the combustion product gases in the gas turbine again is represented as an adiabatic, reversible (constant S) process. Next, heat is removed from these gases at nearly constant pressure in the heat recovery steam generator; and they pass out through the stack.

Ideal Brayton Cycle
Figure 9-15. Combined Brayton-Rankine Cycle Thermodynamics

The Rankine cycle diagram placed adjacent the Brayton cycle in Figure 9-15 is indicated as a simple steam cycle with superheat, but no reheat and no multi-pressure steam generation. The thermodynamic advantage of the Rankine bottoming cycle is the lowered temperature of heat rejection, in the steam condenser, from the overall combined cycles.

The performance of a SOFC system with a Brayton-Rankine bottoming cycle for heat and fuel recovery has been calculated. Gas turbine compressor and expander efficiencies of 83% and 89% and a steam turbine efficiency of 90% have been assumed.

The significant operating conditions of the gas and steam turbines and the results of the computations are summarized in Table 9-24. The principal result is that the efficiency of the overall system, work output divided by the CH4 LHV, is increased from 57% for the fuel cell alone to 75% for the overall system. This combined Brayton-Rankine cycle heat-fuel recovery arrangement is significantly more complex and less efficient than the simple regenerative Brayton cycle approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger.

The key link between the Brayton and the Rankine cycles is the heat recovery steam generator whose operation is illustrated by the temperature-heat (T-Q) plot in Figure 9-16. The temperatures of the gases and of the water, T, are plotted as a function of the heat, Q, transferred from the combustion product gases to the water-steam between their entrance and any point in the steam generator. The area between the temperature curves for the two flowing streams is an indication of the irreversibility, or loss in available work, resulting from the transfer of heat over a finite temperature difference. Reducing this area, moving the gas and steam curves closer, requires increased heat transfer surface area in the steam generator. Steam reheat and multi-pressure level heat recovery boilers are frequently proposed to minimize the loss in available work.

13UU.U

E to

Q, Heat Transferred to Steam from Hot Gas, kcal

Figure 9-16 T-Q Plot for Heat Recovery Steam Generator (Brayton-Rankine)

Rankine Cycle: The fuel cell Rankine cycle arrangement in Figure 9-17 employs a heat recovery steam generator operating on the exhaust combustion product stream from the fuel cell and combustor at atmospheric pressure. This exhaust stream first provides the heat required to preheat and reform the CH4 fuel, providing CO and H2 at temperature to the fuel cell. Partially combusted fuel from the cell is recycled to provide the H2O required for reforming the fuel. Depleted air from the cell exhaust is recycled to the air feed stream to raise its temperature to the desired value at the cell inlet. The operating conditions and the T - S diagram for the Rankine cycle are identical to those illustrated for the combined Brayton-Rankine cycle in Figure 9-15 and Table 9-24.

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, summarized in Table 9-24, indicate that the efficiency of the overall system is increased from 57% for the fuel cell alone to 72% for the overall system. This Rankine cycle heat-fuel recovery arrangement is less complex but less efficient than the combined Brayton-Rankine cycle approach, and more complex and less efficient than the regenerative Brayton approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. And in applications where cogeneration and the supply of heat is desired, it provides a source of steam.

The T - Q plot for the heat transfer processes involved in this fuel cell Rankine cycle arrangement is shown in Figure 9-18. Because heat is removed from the exhaust gases to heat and reform the CH4 fuel feed, the temperature of the hot gas entering the heat recovery steam generator in this

Combined Cycle Plot Stage Steam
Figure 9-17 Fuel Cell Rankine Cycle Arrangement

1800 r

1600

en

1400

E

TO

CD

1200

00

0

1000

CD

800

TO

CD

600

E

a)

1—

400

1-

200

0

L

0 gas leaving combustor —CH4 fuel gas feed

[-1

boiiei w

aiei-siec

m

Figure 9-18 T-Q Plot of Heat Recovery from Hot Exhaust Gas particular Rankine cycle fuel cell arrangement is significantly lower than in the previous combined Brayton-Rankine cycle arrangement. Increased surface area is, therefore, required in the heat recovery steam generator for this fuel cell Rankine cycle arrangement.

These three approaches to reject heat and exhaust fuel recovery with power generation apply primarily to the higher temperature, solid oxide (1800 F) and molten carbonate (1200 F), fuel cell systems operating on CH4 fuel. The lower operating temperatures of the phosphoric acid (400 F) and polymer electrolyte (175 F) fuel cells severely limit the effectiveness of thermal cycle based power generation as a practical means of heat recovery.

All three of the heat recovery arrangements have calculated overall efficiencies greater that 70% as indicated in Table 9-24. None have been optimized in any sense -- in terms of efficiency, capital and operating costs, maintainability or availability. Each of the arrangements has its advantages and disadvantages. It appears, however, that the regenerative Brayton cycle has the advantage of greatest simplicity and highest potential overall efficiency over the combined Brayton-Rankine and Rankine cycle approaches.

The consideration of heat recovery and use in such fuel cell systems requires some consideration of heat generation and transfer within the cells of the system. Direct oxidation of CH4 at the anode of the cell, if possible, would implement the overall process:

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the

CH4; the efficiency of the fuel cell power generation process could, therefore, approach 100%. However, work is lost and a corresponding quantity of heat is produced by irreversibilities both in fuel cell operation --

• the electrical resistance of the electrolyte to ion flow and of the electrodes, current collectors, and leads to electron flow;

• the kinetics of the processes involving reactants, ions, and electrons at the anode and cathode of the cell;

• the transport, or diffusion, of reactants within the anode and cathode chambers to the electrode;

• and also in overall system operation -

• the preheating of the air and fuel streams;

• the pretreating, or reforming, of the CH4 fuel to provide more reactive H2 and to prevent the deposition of carbon (C).

The heat resulting from these irreversibilities must then be removed in order to maintain the fuel cells at a desired operating temperature. Irreversibilities and the resulting quantity of heat produced can be reduced, in general, by increasing the active area of the fuel cells, heat exchangers, and fuel reformer; but increased equipment costs result.

In general, reforming of the CH4 fuel with excess H2O outside the cell has been practiced both in molten carbonate and solid oxide fuel cell systems in order to produce H2, more reactive on a fuel cell anode, and to avoid the possible deposition of C. This reforming reaction

is associated with an increase in entropy and absorbs heat. Excess H2O produces additional H2 and reduces the CO content of the reformed gases, which may adversely affect anode reactions, by the shift reaction

This reaction is thermally neutral. The heat absorbed in the CH4 reforming reaction is released by the subsequent reaction of the H2 product at the anode of the fuel cell. If, therefore, the reforming process can be carried out in close proximity to and in thermal contact with the anode process, the thermal neutrality of the overall CH4 oxidation process can be approximated. And the heat removal and recovery process for the fuel cell system can deal merely with the heat produced by its operational irreversibilities.

Heat removal from fuel cells, and cell batteries, can be accomplished:

• directly through the flow of reactants to and products from them.

• indirectly through heat transfer surfaces in contact with the cell or included within a battery.

A specific fuel cell system is viewed here as having a fixed range of operating temperature between a maximum and minimum; heat must therefore be removed in such a manner to maintain the temperature within these limiting values. If heat is removed directly by reactant flows, then the quantity of flow must be adjusted so that inlet and outlet temperatures (as well as the intermediate temperatures) of the cell and of the flow streams are within the permissible range. Practically, the air stream is adjusted to achieve this result, since the purpose of the fuel cell is to consume the fuel in the production of electrical energy. Increasing the fuel flow to remove heat from the cell increases the quantity of unburned fuel in the exhaust from the cell. If heat is removed from the fuel cell indirectly through adjacent or embedded surface, then the flow and temperature of the coolant stream can be selected somewhat independent of the cell operating temperature. But the distribution of heat transfer surface in the cell (or battery) and the rate of heat transfer across that surface must be carefully adjusted and controlled to maintain the temperature throughout the cell (or battery) within the prescribed temperature range.

The regenerative Brayton cycle, as presented, depends primarily on its fuel cell component for conversion of the fuel and thus for its overall efficiency. The gas turbine merely provides the means for recovery of the waste heat and residual fuel in the combustion product stream. The gas turbine operates, therefore, at a temperature only slightly elevated above that of the cell by the combustion of the residual fuel. The pressure ratio selected for the turbine in this regenerative cycle is determined by the ratio of the temperature of the gases leaving the auxiliary combustor to the temperature of the reactant gases entering the fuel cell. In general, for either molten carbonate or solid oxide cells, this selected pressure ratio will be less than two. The proposed method of cell cooling is air flow, which will be increased significantly, by a factor of 4-8 above that required for oxidation of the fuel. The feasibility of this cycle will depend on the availability of air compressor and turbine expander units with:

• the pressure ratio and temperature capability compatible with the fuel cell operation.

• a capacity appropriate to market applications.

The effectiveness of the regenerative Brayton cycle performance will depend on the efficiency of the fuel cell, compressor, and turbine units; the pressure loss of gases flowing through the system; the approach temperatures reached in the recuperative exchanger; and, most importantly, the cost of the overall system.

The combined Brayton-Rankine cycle depends on both the fuel cell and the gas turbine components for conversion of the fuel and thus for its overall efficiency. The extent of conversion of the fuel occurring in the fuel cell increases as the cell operating temperature and the range of coolant temperature rise increase. For this reason, the cycle as presented is based on indirect heat removal from the cell, heating the air stream temperature from the compressor outlet to the cell operating temperature. This provision maximizes the cell contribution to the energy output of the combined cycle. The PR and NIT of the turbine are those selected to match those of the current utility scale equipment -- a PR of 12 and an NIT of 2300 F -- resulting in a combined cycle efficiency of perhaps 45-50%, not considering the electrical energy output of and the fuel input to the fuel cell. The fuel combustion occurring in the combustor and overall air/fuel ratio is then determined by the combination of the cell and the turbine inlet temperatures.

The fuel cell Rankine cycle arrangement has been selected so that all fuel preheating and reforming are carried out external to the cell and air preheating is accomplished by mixing with recycled depleted air. The air feed flow is adjusted so that no heat transfer is required in the cell or from the recycled air. Consequently, the internal fuel cell structure is greatly simplified, and the requirement for a heat exchanger in the recycle air stream is eliminated.

Summary

Advantages, Disadvantages of Various Fuel Cell, Power Cycles

Regenerative Brayton

Advantages:

• simple cycle arrangement, minimum number of components.

• relatively low compressor and turbine pressure ratio, simple machines.

• relatively low fuel cell operating pressure, avoiding the problems caused by anode/cathode pressure differential and high pressure housing and piping.

• relatively low turbine inlet temperatures, perhaps 1950 F for solid oxide and 1450 F for molten carbonate fuel cell systems. Turbine rotor blade cooling may not be required.

• relatively simple heat removal arrangements in fuel cells, accomplished by excess air flow. No internal heat transfer surface required for heat removal.

• fuel conversion in cells maximized, taking full advantage of fuel cell efficiency.

• adaptability to small scale power generation systems.

Disadvantages:

• tailoring of compressor and turbine equipment to fuel cell temperature and cycle operating pressure required. (It is not clear to what extent available engine supercharging and industrial compressor and turbine equipment can be adapted to this application.)

• large gas to gas heat exchanger for high temperature heat recuperation required.

• efficiency and work output of the cycle sensitive to cell, compressor, and turbine efficiencies; pressure losses; and temperature differentials.

Combined Brayton-Rankine

Advantages:

• integrated plant and equipment available for adaptation to fuel cell heat recovery.

• high efficiency system for heat recovery.

Disadvantages:

• complex, multi component, large scale system for heat recovery.

• adaptation of existing gas turbine required to provide for air take off and return of hot depleted air and partially burned fuel.

• high pressure operation of the bulky fuel cell system required.

• precise balancing of anode and cathode pressures required to prevent rupture of fuel cell electrolyte.

• indirect heat removal required from fuel cells with compressed air, initially at low temperature, to enable significant conversion of the fuel flow in the cells.

Rankine

Advantages:

• ambient pressure operation within the fuel cell.

• heat recovery in a boiler, avoiding the high temperature gas to gas exchanger of a regenerative Brayton cycle.

• no gas turbine required, only fans for air and exhaust product gas flow.

• steam available for cogeneration applications requiring heat. Disadvantages:

• inherently lower efficiency than regenerative Brayton and combined Brayton-Rankine cycles.

• requirement for cooling and feed water.

• greater complexity than regenerative Brayton cycle arrangement. 9.4 Fuel Cell Networks

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