A study of thermochemically recuperative power generation systems using natural gas

Fuel Vol. 76, No. 7, pp. 593-601, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/97 $17.00+0.00

PII: SOO16-2361(97)00059-S

ELCEVIf:R

A study of thermochemically recuperative power generation systems using natural gas Marcio L. de Souza-Santos UNICAMP-University of Campinas, Department of Energy-FEM-Faculty of Mechanical Engineering, Cidade Universidria Zeferino Vaz, Cx. Postal 6122, Campinas, SP 13083-970, Brazil. E-mail: [email protected] (Received 25 July 1995; revised 31 January 1997)

A study is made of power generation based on thermochemically recuperative (TCR) systems. The main feature of such systems is the recuperation of energy from the turbine exhaust through the reforming of a gas-steam stream. This type of cycle shows promising theoretical results for future systems which would use an inlet turbine temperature of 1700 K. The present study follows previous ones which indicated that TCR would give efficiencies superior to those of more conventional cycles (combined cycles, STIG). From that earlier work, the best two configurations were selected for more refined computations: TCR with reheating (TCRR) and TCR with reheating and intercooling (TCRRI). The TCRRI configuration leads to an efficiency almost 1% above the TCRR configuration. Routes for future studies are suggested. 0 1997 Elsevier Science Ltd. (Keywords:

power generation;

thermochemically

recuperative

need for highly efficient cycles, combined with environmental concerns, has pushed several countries towards increasing utilization of natural gas as fuel for power generation. Combined cycles have been the most favourable choice, due to their superiority over the more traditional systems such as STIG. At the same time, research on maximum temperatures allowed at the turbine inlet has progressed rapidly. According to information from colleagues at several institutions (see Acknowledgements) an acceptable goal of 1700 K is not far off. This would lead to higher temperatures of the turbine exhaust. To recover that energy, increasingly elaborate cycles have been proposed, among them the thermochemically recuperative or TCR system. A simple TCR process is illustrated in Figure I. The basis of such system is the possibility of using turbine exhaust enthalpy to promote gas reforming after proper mixing with steam. The basic reactions are:

The

systems; natural gas reforming)

surface through the following reactions:

CO+Hz +H20+C

(4)

2CO~CO*+C

(5)

CH4 +2H2+C

(6)

Studies on the various possibilities as well limitations of TCR-based cycles have been reported’. The work presented here is a continuation of an earlier study’ and includes a comparison between the two optimized reheating arrangements. The reheating is represented by block 5 in Figures 2 and 3. In addition, one of these concepts uses intercooling, which is represented by block 23 in Figure 3. BASIC COMPUTER CODE

CH, + HZ0 + CO + 3H2

(1)

To accomplish the studies, a special simulation program was developed. The basic characteristics of the program are:

CO+H20+CO;?+Hz

(2)

(1) Rigorous computation of physicochemical properties of

Other hydrocarbons usually present in natural gas (see Table I) also lead to hydrogen production through the following possible reaction: C,H,+mH,O+mCO+

(

m+;

>

H2

(3)

Proper control of the reactor and reformer conditions can avoid the possibility of carbon deposition on the catalyst

the gas or liquid mixture in each flow stream. The Redlich-Kwong-Soave equations automatically correct enthalpies, entropies and other properties to account for deviations from ideal behaviour. Up to 500 different gas and liquid chemical components can be accepted. (2) A routine based on the Levenberg-Marquardt algorithm and a finite difference to the Jacobian is used to allow the solution for complex systems. Convergence for processes and cycles with large amounts of equipment, streams and chemical components is possible.

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M. L. de Souza-Santos

compressor

turbine

1111

reformer

compressor

steam generator

pump Simple TCR system

Figure 1

Table 1

Pumps

Natural gas composition

Methane Ethane Propane Butanes“ Pentanes” HexanesB Nitrogen Carbon dioxide

Mel%

Wt%

92.47 3.57 0.79 0.35 0.07 0.07 2.01

85.12 6.16 1.99 1.17 0.29 0.35 3.23

0.67

1.69

“Assumed to he straight-chain in the calculations

The code was tested and, within deviations < OS%, reproduced the stream temperatures and pressures of a TCR concept demonstrated in other studies’.

SIMULATION STRATEGY The power unit is composed of equipment and streams. The variables are temperatures, pressures and compositions in each flow stream. Some of these values are fixed or known. The unknown values are provided by the mass and energy balances for control volumes around each piece of equipment. The program contains routines for the following equipment: combustor compressors ?? heat exchangers 0 mixers

?? ??

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natural gas reformers splitters turbines valves.Whenever possible, these routines have been built to allow improvements which could lead to computations of internal temperature, pressure and composition profiles. For instance, a one-dimensional approach is used to simulate the reformer. Each piece of equipment is regarded as a near-isenthalpic or near-isentropic unit. For the equipment listed above, the near-isenthalpic model is used for combustor, heat exchangers, mixers, splitters, valves and reformers. The near-isentropic model is used for compressors, pumps and turbines. For the equipment to which the near-isentropic balance is applied, an enthalpy balance is also used to provide the total power consumed or produced. Deviation from ideal behaviour is accounted for by a specific value of efficiency, which is imposed for each piece of equipment. The definitions of efficiencies vary somewhat in the literature. Although the program can be modified to accommodate any concept, the present study uses the informed values of efficiencies which, for each class of equipment, are as follows: for the combustor, the fraction of the power input (mass flow times the heating value of the injected fuel) that represents losses due to heat transfer to the environment; for compressors, pumps and turbines, a level of entropy creation, representing a loss in potential production (or utilization) of useful mechanical power;

Thermochemically

Figure 2

TCR

recuperative

generation

systems:

M. L. de Souza-Santos

system with reheating, without intercooling (TCRR)

for heat exchangers and reformers, the fraction of the enthalpy change of the hot (or the cold stream) which is assumed to be lost to the environment, the effect of which is to decrease the amount of heat transferred between the hot and cold streams; for mixers and splitters, a fraction of the incoming enthalpy (total mass flow of entering streams times their enthalpy) that represents losses due to heat transfer to the environment. The literature’ provided data which were used to determine the following values for these efficiencies: combustor, between 0.98 and 0.96; compressors, between 0.91 and 0.87; heat exchangers (reformers included), between 0.98 and 0.96; mixers and splitters, between 1.00 and 0.99; turbines, average 0.90 and 0.84. Together with these efficiencies, and depending on the equipment, other characteristics are provided, such as: splitting ratios of exit streams, which should be set in cases of splitters or compressors with several stages; ?? identifications of mechanically coupled equipment which should be known for turbines which drive compressors; ?? rates of externally applied heat or work for isenthalpic or near-isenthalpic equipment; the possibility of imposing ??

power

these rates is included to allow balances for equipment where, for instance, electrical heating, mechanical mixing and other external sources (or sinks) of energy are present. For each stream, known or guessed values of its total mass flow, temperature, pressure and composition should be given as data. With all the required data, the system of equations formed by the mass, enthalpy and entropy balances can be established. Once the solution is achieved, the complete set of temperatures, pressures and compositions of each stream is printed. Also, all basic parameters that describe the operational behaviour of each piece of equipment, as well as for the complete process plant, are provided. Gas refomzer

In the TCR cycle, the performance of the gas reformer is crucial. Therefore some effort has been made to allow more detailed information than that obtainable by zero-order balances. In this way, it is possible to verify whether favourable conditions for carbon deposition exist. Such deposits may impair the catalyst surface and should be avoided. At the present stage, the program takes into account the kinetics and equilibrium of reactions (1) to (3). Computations are also performed to compare the equilibrium

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0-J 1

1

Figure 3

air

(

TCR system with reheating and intercooling

(TCRRI)

relations of reactions (4) to (6) with the actual conditions of the exit gas. The equilibrium parameters for each of these have been taken from the literature3-5. Any Runge-Kutta variable-step method can be used to solve the differential system which describes the concentrations throughout the reformer. However, the variable order predictor corrector Gear method6 is used, since it accelerates the solution and is very convenient for problems involving several units or pieces of equipment.

APPLICATIONS OF THE CODE TO TCR CYCLES Studies comparing various configurations of STIG, combined cycles and TCR have been performed’,*. These led to the conclusion that TCR cycles are among the most promising for advanced application of gas turbines. Encouraged by those conclusions, two configurations of gas turbine systems using thermochemical reforming have been evaluated: (1) system with reheating and without intercooling (TCRR), as shown in Figure 2. (2) System with reheating and intercooling (TCRRI), as shown in Figure 3. Assumed conditions

In the present work, some of the basic requirements have

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been imposed. Some have been found in the literature’ or are based on the experience (IGT team) and information from colleagues (see acknowledgements to Fluor Daniels and Allison/GM). They are listed below: maximum GT (gas turbine) compression ratio, 33.91 for the no-intercooling system and 40.26 for the intercooled system; 0 GT inlet pressure loss, 1.1 kPa; ?? GT intercooler plus high-pressure compressor inlet pressure loss, 4.24% of the inlet; . GT combustor pressure loss, 3.07% of the inlet; . minimum temperature difference in the intercooler (between exit air and inlet cooling water), 11 K; . steam generator pinch and boiler feedwater subcooling temperatures, between 8 and 11 K; 0 maximum GT exhaust temperature, 950 K; . maximum GT firing temperature, 1700 K; . maximum GT reheat combustor exhaust temperature, 2255 K minus t,-&mt,where toxi&t is the temperature Of the oxidant stream entering the combustor; . GT generator efficiency (electrical), 98.6%; . GT combustor heat loss plus margin, 2.28% of the lower heating value of the fuel; . HIRSG (heat recovery and steam generator system) heat loss, 2% of the heat released; . fuel enters the system at 0.3 MPa (absolute) and 288 K; . turbine mechanical efficiency, 90%; 0

Thermochemically

recuperative

compressor mechanical efficiency, 91%; cooling air to turbines, 10% of the total air from the main compressor; exit streams from reformers at chemical equilibrium conditions; fuel combustion assumed complete (e.g. there is always excess oxidant); maximum compression ratio at the h.p. compressor, 15. The H20/CH4 ratio of the stream to be reformed was set at 6 (mass fraction). This is around the value found in the literature ’. Some tests performed with various values confirmed that this ratio should be in the neighbourhood of the optimum. Results for each system

Once the above had been assumed, the remaining variables were: total pressure ratio in the system; intermediate pressures between the h.p. and 1.~. branches of the system; splitting ratio of the gas turbine exit for the parallel heat recovery systems. A note on the first parameter above is necessary. It is well

Table 2

Stream

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

power

generation

systems:

M. L. de Souza-Santos

known that in classical power generation cycles the global efficiency generally increases with increasing total pressure ratio. On the other hand, for systems with heat recovery based on chemical reforming, this is not always true, due to the following considerations:

(1) The heating value of the reformed gas decreases with increasing pressure. The volume change in reaction (1) is the main reason: an increase in pressure forces the equilibrium to the left. This can be verified if the properties of streams 21 and 33 are compared (see Tables 3 and 6). The former is that obtained for the h.p. reformer and the latter that for the 1.~. reformer. In the TCRIR case, the heating value of the gas obtained at 900 K and 3.32 MPa is -7% lower than that obtained at the same temperature but at 0.63 MPa. The equilibrium of the other important reaction (shift reaction) is not subject to the influence of total pressure because it is equimolar. (2) As the reforming reaction is less complete at higher pressures, the heat recovery by the reformer is less efficient, leading to higher stack temperatures. To allow a first comparison between the two basic concepts, a pressure ratio of 34 was chosen. The results are presented in Tables 2-7.

TCRR stream conditions” Temperature (K)

Pressure (MPa)

Mass flow (kg s-‘)

288.0 416.6 416.6 832.6 832.6 1735.3 1390.9 1324.7 951.0 951.0 759.4 463.7 375.6 288.0 288.0 450.0 600.0 288.0 501.4 580.1

0.1013 0.3319 0.3319 3.4000 3.4000 3.2900 1.4600 0.6110 0.1200 0.1190 0.1166 0.1143 0.1020 0.1013 3.6500 3.5700 3.4986 0.3000 3.5000 3.4638 3.3253 0.1190 0.1166 0.1143 0.1020 0.1013 0.7000 0.6800 0.6600 0.3000 0.6600 0.6534 0.6237 3.4000 3.4000 3.3660 0.6300

1.000 1.OOo 0.000 0.900 0.100 1.068 1.167 1.202 1.203 0.950 0.950 0.950 0.950 1.440 1.440 1.440 1.440 0.024 0.024 0.168 0.168 0.253 0.253 0.253 0.253 0.030 0.030 0.030 0.030 0.005 0.005 0.035 0.035 0.099 0.001 0.001 1.167

900.0 951.0 742.1 490.2 443.3 288.0 288.0 400.0 600.0 288.0 351.5 553.6 900.0 832.6 832.6 832.6 1172.9

‘Basis is 1 kg s-’ of air in stream 1 (see Figure 2)

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597

Thermochemically

Table 3

recuperative power generation systems: M. L. de Souza-Santos

TCRR stream compositions’ Composition

Stream Hz0

H2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

(mass fraction)

N2

02

co

co2

0.000 0.000 -

0.000 0.000 -

0.767 0.767 -

0.233 0.233

0.000 0.000 -

0.000 -

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.027 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.045 0.000 0.000 0.000 0.000

0.000 0.000 0.185 0.170 0.199 0.199 0.199 0.199 0.199 0.199 1.ooo 1.ooo 1.ooo 1.oOO 0.000 0.000 0.857 0.739 0.199 0.199 0.199 0.199 1.ooo 1.ooo 1.000 1.oOO 0.000 0.000 0.857 0.659 0.000 0.000 0.000 0.170

0.767 0.767 0.646 0.656 0.637 0.637 0.637 0.637 0.637 0.637 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.637 0.637 0.637 0.637 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.767 0.767 0.767 0.656

0.233 0.233 0.107 0.117 0.098 0.098 0.098 0.098 0.098 0.098 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.098 0.098 0.098 0.098 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.233 0.233 0.233 0.117

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.013 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.039 0.000 0.000 0.000 0.000

0.000 0.000 0.062 0.057 0.066 0.066 0.066 0.066 0.066 0.066 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.134 0.066 0.066 0.066 0.066 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.215 0.000 0.000 0.000 0.057

0.000

“See Figure 2

Table 4 Component

TCRR enthalpy and entropy variations in each component (no.)

Enthalpy variation (W)

L.p. compressor (1) H.p. compressor (2) 1st combustor (3) H.p. turbine (4) 2nd combustor (5) L.p. turbine no.2 (6) Splitter (7) H.p. reformer (8) H.p. evaporator (9) H.p. economizer (10) H.p. pump (11) Natural gas h.p. compressor (12) Mixer (13) L.p. reformer (14) L.p. evaporator (15) L.p. economizer (16) L.p. pump (17) Natural gas 1.~. compressor (18) Mixer (19) Cooling air splitter (20) Cooling air mixer (21) L.p. turbine no.1 (22) Total

0.13080E 0.44349E -064008E -0.44349E -0.11962E -0.63519E 0.45093E -0.48833E -0.70861E -0.20155E 0.52248E 0.13085E -0.25588E -0.14149E -0.16092E -0.28879E 0.18364E 0.71544E -0.12282E 0.17435E -0.22737E -0.35488E -0.86983E

“Basis is 1 kg s-’ of air in stream 1 (see Figure 2)

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+ + + + + + + + + + + + + + + + + +

06 06 04 06 04 06 05 04 04 04 03 05 06 04 04 03 02 03 07 07 12 06 06

and cycle total’ Entropy variation (W K- ‘) 0.33413E 0.63901E 0.11899E 0.31090E 0.28518E 0.62022E 0.32085E -0.21798E -0.87516E 0.65269E 0.36282E 0.29838E 0.16934E -0.79768E -0.15314E 0.19699E 0.12752E 0.20170E 0.15478E 0.20901E 0.28963E 0.30761E 0.46489E

+ + + + + + + + + + + + + + + + + + +

02 02 04 02 03 02 01 03 03 02 01 01 01 02 03 02 02 00 01 10 02 02 03

CH4

0.000 0.000 -

0.000 0.000

0.000 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.OOo 1.OOo 0.143 0.087 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.ooo 1.ooo 0.143 0.042 0.000 0.000 0.000 0.000

Thermochemically

recuperative

Under the above restrictions and conditions, it is observed that the global performance of a TCRRI (reheating with intercooling) system surpasses that of a TCRR (reheating without intercooling) system. The global efficiencies, defined as the total output of useful work divided by the total input of work and enthalpy, are: TCRR: 57.93% TCRRI: 58.80%The net power outputs that could be used to drive the generators are: TCRR: 844.93 kW per unit of mass flow (kg s-‘) of inlet air in the LPC (low-pressure compressor); TCRRI: 871.91 kW per unit of mass flow (kg s-l) of inlet air in the LPC. Other conditions

To verify the influence of other chemical species usually present in natural gas on the performance of the TCRR and TCRRI configurations, the computations were repeated. A typical composition for natural gas was used (Table I). The calculations confirmed the basic trend of better efficiency of the TCRRI than the TCRR cycle. To verify the influence of total pressure on the overall

Table 5

Stream

TCRRI

power

generation

systems:

M. L. de Souza-Santos

efficiency, simulations for those systems were repeated for a pressure ratio of 20. A decrease in the overall efficiency was observed when compared with the same cycles operating with pressure ratio of 34. At the same time, trials showed that the global efficiency decreases at ratios above 34. Therefore it seems that an optimum exists between these values. In addition it has been verified that at a pressure ratio of 20, systems including intercooling (TCRRI) do not provide a significant advantage over systems without that feature (TCRR). This is basically due to the relative simplicity of the TCRR configuration. With less equipment, TCRR leads to fewer losses and certainly lower capital cost. Therefore more detailed studies should combine pressure ratio optimization and economic analysis. Added to the above points, it seems that further improvement of each system can be obtained if part of the stack gas is recycled to the reformer. It may be feasible to achieve a reduction in the amount of fuel and/or steam consumption, due to a possible residual shift of CO2 to CO. It should be remembered that, in the limit, methane can be reformed with COz: CH4 + CO2 G 2C0 + 2H2

(7)

stream conditions” Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

288.0 395.3 395.3 785.1 785.1 1755.0 1403.5 1305.5 937.24 937.2 744.1 418.3 396.5 288.0 288.0 400.0 600.0 288.0 501.4 580.1 900.0 937.2 710.8 448.6 390.8 288.0 288.0 416.8 600.0 288.0 350.5 553.6 900.0 785.1 785.1 785.1 1184.8 348.1 360.0

Pressure (MPa)

Mass flow (kg SC’)

0.1013 0.2800 0.2800 3.4000 3.4000 3.2900 1.4600 0.6110 0.1200 0.1190 0.1166 0.1143 0.1020 0.1013 3.6500 3.5055 3.4354 0.3000 3.5000 3.4110 3.2746 0.1190 0.1166 0.1143 0.1020 0.1013 0.7000 0.6800 0.6600 0.3000 0.6600 0.6534 0.6273 3.4000 3.4000 3.3660 0.6300 0.2275 3.5770

1.ooo 1.000 0.000 0.900 0.100 1.082 1.181 I .209 1.210 1.020 1.020 1.020 1.020 0.156 0.156 0.156 0.156 0.026 0.026 0.182 0.182 0.190 0.190 0.190 0.190 0.024 0.024 0.024 0.024 0.004 0.004 0.028 0.028 0.099 0.001 0.001 1.181 1.ooo 0.156

“Basis is 1 kg SC’ of air in stream 1 (see Figure 3)

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Thermochemically Table 6

recuperative power generation systems: M. L. de Souza-Santos

TCRRI stream compositions”

Stream

Composition

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

(mass fraction)

0.000 0.000 -

0.000 0.000 -

0.767 0.767 -

0.233 0.233 -

0.000 0.000 -

0.000 0.000 -

0.000 0.000 -

0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.198 0.182 0.205 0.204 0.204 0.204 0.204 0.204 1.ooo 1.ooo 1.ooo 1.OOo 0.000 0.000 0.857 0.739 0.204 0.204 0.204 0.204 1.000 1.oOo 1.000 1.oOO 0.000 0.000 0.857 0.665 0.000 0.000 0.000 0.182 0.000 1.oOO

0.767 0.767 0.638 0.649 0.634 0.634 0.634 0.634 0.634 0.634 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.634 0.634 0.634 0.634 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.767 0.767 0.767 0.649 0.767 0.000

0.233 0.233 0.098 0.109 0.093 0.094 0.094 0.094 0.094 0.094 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.094 0.094 0.094 0.094 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.233 0.233 0.233 0.109 0.233 0.000

0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.062 0.061 0.068 0.068 0.068 0.068 0.068 0.068 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.134 0.068 0.068 0.068 0.068 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.204 0.000 0.000 0.000 0.061 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000

iEz o:ooo 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.027 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.048 0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.013 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.039 0.000 0.000 0.000 0.000 0.000 0.000

“See Figure 3

Table 7 Component

TCRRI enthalpy and entropy variations in each component (no.)

Enthalpy variation (W)

L.p. compressor (1) H.p. compressor (2) 1st combustor (3) H.p. turbine (4) 2nd combustor (5) L.p. turbine no.2 (6) Splitter (7) H.p. reformer (8) H.p. evaporator (9) H.p. economizer (10) H.p. pump (11) Natural gas h.p. compressor (12) Mixer (13) L.p. reformer (14) L.p. evaporator (15) L.p. economizer (16) L.p. pump (17) Natural gas 1.~. compressor (18) Mixer (19) Cooling air splitter (20) Cooling air mixer (21) L.p. turbine no.1 (22) Intercooler (23) Total

0.10901E + 0.46064E + -0.69336E + -0.46064E + -0.96552E + -0.63155E + -0.40745E -0.52906E + -0.83589E + -0.53880E + 0.56602E + 0.14176E + O.OOOOOE+ -0.11506E + -0.12523E + -0.26609E + 0.1469lE + 0.57235E + -0.58208E 0.36369E O.OOOOOE+ -0.36470E + -0.9623lE + -0.89763E +

‘Basis is 1 kg s-’ of air in stream 1 (see Figure 3)

600

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06 06 04 06 03 06 09 04 04 03 03 05 00 04 04 03 02 03 10 06 00 06 03 06

and cycle total0 Entropy variation (W K- ‘) 0.28714E 0.73174E 0.12997E 0.32040E 0.23185E 0.62575E 0.32368E -0.24112E -0.96570E 0.405 13E 0.39306E 0.32324E 0.18345E -0.65245E -0.13070E 0.12560E 0.10202E 0.16136E 0.12382E 0.42329E 0.28963E 0.31312E 0.76325E 0.49572E

+ + + + + + + + + + + + + + + + + + + +

02 02 04 02 03 02 01 03 03 02 01 01 01 02 03 02 02 00 01 09 02 02 02 03

0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.OOo 1.oOO 0.143 0.087 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.ooo 1.ooo 0.143 0.045 0.000 0.000 0.000 0.000 0.000 0.000

Thermochemically

recuperative

CONCLUSIONS Previous studies on future advanced systems, assuming inlet turbine temperatures of 1700 K, have shown that thermochemically recuperative (TCR) systems are among the most attractive configurations, mainly for pressure ratios above 25. These promising results encouraged further comparisons involving various TCR configurations. A pressure ratio of 34 was chosen. Among the concepts tested, the one including intercooling and reheating showed a global efficiency near 59%, almost 1% higher than that obtained for the system with reheating but without intercooling. As a possible route for future studies of TCR cycles, it is suggested that systems allowing partial recycling of flue gas to the reformer should be included. Also, more refined optimization studies should consider the pressure ratio as a variable and include exergy as well as economic analyses. ACKNOWLEDGEMENTS The author is grateful to colleagues

at the Institute

of Gas

power

generation

systems:

M. L. de Souza-Santos

Technology (IGT) (especially John O’Sullivan), to A. Rao of Fluor Daniel Inc. and D. Mukavetz of Allison/GM, concerning basic restrictions and conditions for gas turbine generation systems.

REFERENCES Kesser, K. F., Hoffman, M. A. and Baughn, J., Journal of Engineering for Gas Turbines and Power, 1994, 116, 277. Souza-Santos, M. L., Development of computational procedures for process analysis and evaluation. Third and Final Report of the IR&D Project, Institute of Gas Technology, Chicago, 1992. Hottel, H. C. and Howard, J. B., New Energy Technology. MIT Press, Cambridge, MA, 1971. Parent, J. D., and Katz, S., Equilibrium Compositions and Enthalpy for the Reaction of Carbon, Oxygen and Steam.

IGT Research Bulletin No. 2, Institute of Gas Technology, Chicago, 1948. Souza-Santos, M.L., Fuel, 1989, 68, 1507. Hindmarsh, A. C., Gear, Ordinary Differential Equation System Solver, UCID-30001, Rev. 3. Lawrence Livermore Laboratory, Livermore, CA, 1974.

Fuel 1997 Volume

76 Number

7

601