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Gas-turbine total energy systems
Energy
Vol. 11, No. 10, pp. Printed in Great Britain
1023-1025,
1986
0360-5442/86
$3.00
t0.00
Pergamon Journals Ltd
NOTE GAS-TURBINE YOUSEF Mechanical
and Chemical
S. H.
TOTAL ENERGY NAJJAR
Engineering
and AWAD
Departments,
(Received 29 August Abstract-A useful approach for energy savings (cogeneration). These technologies are discussed.
SYSTEMS
R. MANSOUR
Yarmouk
University,
Irbid, Jordan
1985)
involves
combined
heat and power
production
1. INTRODUCTION
Among improvements in energy-generating systems, a convenient approach is combined heat and power (CHP) production or cogeneration, with a gas-turbine integrated into the total energy system. Gas-turbines return high plant efficiency with minimum down times. They have proved to be reliable sources of power in a wide variety of industries and locations. They are useful for CHP systems in cement drying, biscuit manufacture, etc. This form of energy conservation may lead to substantial savings in energy costs and result in acceptable payback periods.
2. ANALYSIS
AND
DISCUSSION
Combustion-engine efficiencies are limited by the Carnot cycle and part of the energy is lost to the atmosphere through the exhaust gases. This effect reduces the thermal efficiency of power stations using fossil fuels to z 32%. If part of this energy is recovered by using a heat-recovery steam generator to produce process heat, the overall thermal efficiency may be increased to 75%, as is shown in Fig. 1.
22 “1, Slack
45% Condenser losses
Fig.
I. Typical fuel-utilization
effectiveness
with a gas-turbine.
There are 2 widely used methods in energy recovery. The first involves the transfer of heat to liquid water to produce steam, which turns engines. The second involves preheating the air to improve combustion efficiency. In both cases, the hot liquid or air may be used directly for space heating. A convenient method for improving the efficiency of energy conversion is heat recovery from the exhaust gases of the gas-turbine. The gas-turbine has small size and weight, high specific power, relatively clean exhaust gases for high air-fuel ratios, acceptable temperatures 1023
1024
YOUSEF S. H. NAJJAR and AWAD R. MANSOUR
(500-600 “C), and relatively high oxygen concentrations in the exhaust gases which allow burning of additional fuel if needed. The following examples demonstrate some applications of gas-turbines for energy utilization and may improve the economics of these power units. 3. COGENERATION
SYSTEMS
Figure 15 of Ref. [l] shows a system for energy recovery, which consists of 2 steam boilers. Steam is injected in the gas turbine combustion chamber to control NO, and is taken from the low-pressure boiler. The high-pressure steam expands in the steam turbine to generate additional power. Table 1 of Ref. [2] shows energy balances for combined gasturbine and steam-injection cycles. The addition of 1.82 kg/set of steam increases the compressor pressure ratio from 9.3 to 10.3, while it decreases the turbine-entry temperature from 982 to 913°C. This cogeneration system produces 3 MW and 8860 kg/set of saturated steam. When steam is not needed for industrial purposes, the steam-injection arrangement is used to raise the power to 4 MW. There are recent applications of Rankine cycles with gas-turbine exhausts to improve energy recovery by using water-ammonia solution as the working substance instead of water (Fig. 2 of Ref. [3]). Table 1 of Ref. [3] shows the important parameters at different cycle points. Comparison of Rankine and the suggested cycles shows that the thermal efficiency exceeds 50% for simple gas-turbines, when the exhaust gases are cooled to 77 instead of 149°C as in the Rankine cycle. Thus, the suggested system improves the thermal efficiency by 20%. Low-grade energy is used in distillation, which is more efficient than evaporation. With solutions, the cost decreases because of decrease in surface areas for the evaporator and condenser. The suggested system recovers the investment in 25% of the recovery period required for a Rankine cycle. 4. COMBINED
HEAT
AND
POWER
The waste heat in the gas-turbine exhaust may be used to cool the compressor air when it is associated with an absorption-refrigeration machine (Fig. 1 of Ref. [4]). This system increases thermal efficiency and power above those for a gas-turbine, mainly because of higher ratios of maximum to minimum temperatures. The performance becomes relatively less sensitive to changes in ambient temperature. In this cycle, with a pressure ratio of 6 and an ambient temperature of 15°C the thermal efficiency improves by 4% and the specific power by 16%. The waste heat in the gas-turbine exhaust can be used for central heating or airconditioning systems (Fig. 1 of Ref. [S]). Utilization of waste energy in a boiler may recover 60&85% of the associated energy to shaft power and available heat. In this arrangement, an additional quantity of fuel may be burnt in the waste-heat boiler since sufficient oxygen is available. 5. OTHER
MEANS
FOR
IMPROVING
THERMAL
EFFICIENCY
In gas-turbines with a regenerator, energy recovery may be boosted by water injection. The working substance consists of air, water and steam. The compressed air temperature is reduced to 60°C while the exhaust gases are at 80°C. The efficiency is improved to 53% at a turbine inlet temperature of 1100°C. Figure 2 and Table 3 of Ref. [6] show the basic cycle and performance of different arrangements, respectively. Energy may be recovered from the gas-turbine exhaust in a boiler to produce steam and run a steam turbine that drives a supercharger, which pressurizes the air before entering the main compressor. The exhaust steam is injected in the compressor and the combustor. An aero-derived engine could be used as a gas generator, whereas a separate free turbine is used for power generation (Fig. 9 of Ref. [7]). This arrangement should produce an increase of about 30% in efficiency and 100% in power.
Gas-turbine
total energy
systems
1025
The gas-turbine efficiency is improved by using H, as fuel and employing over expansion in the turbine (Figs 1 and 2 of Ref. [S]). The specific power remains nearly constant. The performance of the cycle may be improved by using a twin-spool arrangement, especially when the maximum cycle temperature is low. Inter-blade combustion of Hz may be used until the constant temperature cycle is approached. The isothermal combined cycle has been suggested as a means of improving the gasturbine efficiency. The principles are illustrated in Fig. I, the line diagram in Fig. 2 and the T -S diagram in Fig. 3 of Ref. [9]. If this cycle is combined with a steam cycle, a maximum temperature of 1600 K and a 100: 1 pressure ratio may be reached at an efficiency of 60”/0. Aeroengines have been used in industrial applications, especially for electric power generation, because of their reliability in continuous operation. With coal as fuel, an integrated system consisting of a coal gasifier is preferably used with the combined steam and gas-turbine power plant. By using one of the specified arrangements, the central powergeneration system may not be needed, without affecting the reliability of the power supply.
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO.
R. P. Allen and J. M. Kovacik. J. Engng Gas Turbines Power 106, 725 (19x4). H. Leibovitz and E. Tabb. J. Engng Gus Turbine Power 106, 731 (19X4). A. I. Kalina. J. Erring Gas Turbines Power 106, 737 (1984). 0. Yang, M. Tsujita and G. T. Sato. Bull. J.S.M.E. 13, 1I I (1970). 0. Yang and G. T. Sato. Bull. J.S.M.E. 14, 234 (1971). Y. Mori, H. Nakamura, T. Takahashi and K. Yamamoto. ‘Ii,k~, /n(. Ciu.5 Turbinr Conxr. Paper Tokyo-IGIC-38 (1983). G. Pilkington and D. R. Carlisle. Proc. Inst. Mrch. En,qrs 183, Pt 3 N, X5 (1969). K. Enomoto and G. T. Sato. Tokyo Int. Gus Turbine Congr. Paper no. X3-Tokyo-IGIC-39 (1983). M. A. El-Maxi, J. H. Magnusson. J. Engn~ Gas Turbines Power 106, 743 (1984). J. A. Roberts, Tokyo fnt. Gas Turbine Congr. Paper no. 83-Tokyo-IGIC-199 (1983).
no. X3-
Vol. 11, No. 10, pp. Printed in Great Britain
1023-1025,
1986
0360-5442/86
$3.00
t0.00
Pergamon Journals Ltd
NOTE GAS-TURBINE YOUSEF Mechanical
and Chemical
S. H.
TOTAL ENERGY NAJJAR
Engineering
and AWAD
Departments,
(Received 29 August Abstract-A useful approach for energy savings (cogeneration). These technologies are discussed.
SYSTEMS
R. MANSOUR
Yarmouk
University,
Irbid, Jordan
1985)
involves
combined
heat and power
production
1. INTRODUCTION
Among improvements in energy-generating systems, a convenient approach is combined heat and power (CHP) production or cogeneration, with a gas-turbine integrated into the total energy system. Gas-turbines return high plant efficiency with minimum down times. They have proved to be reliable sources of power in a wide variety of industries and locations. They are useful for CHP systems in cement drying, biscuit manufacture, etc. This form of energy conservation may lead to substantial savings in energy costs and result in acceptable payback periods.
2. ANALYSIS
AND
DISCUSSION
Combustion-engine efficiencies are limited by the Carnot cycle and part of the energy is lost to the atmosphere through the exhaust gases. This effect reduces the thermal efficiency of power stations using fossil fuels to z 32%. If part of this energy is recovered by using a heat-recovery steam generator to produce process heat, the overall thermal efficiency may be increased to 75%, as is shown in Fig. 1.
22 “1, Slack
45% Condenser losses
Fig.
I. Typical fuel-utilization
effectiveness
with a gas-turbine.
There are 2 widely used methods in energy recovery. The first involves the transfer of heat to liquid water to produce steam, which turns engines. The second involves preheating the air to improve combustion efficiency. In both cases, the hot liquid or air may be used directly for space heating. A convenient method for improving the efficiency of energy conversion is heat recovery from the exhaust gases of the gas-turbine. The gas-turbine has small size and weight, high specific power, relatively clean exhaust gases for high air-fuel ratios, acceptable temperatures 1023
1024
YOUSEF S. H. NAJJAR and AWAD R. MANSOUR
(500-600 “C), and relatively high oxygen concentrations in the exhaust gases which allow burning of additional fuel if needed. The following examples demonstrate some applications of gas-turbines for energy utilization and may improve the economics of these power units. 3. COGENERATION
SYSTEMS
Figure 15 of Ref. [l] shows a system for energy recovery, which consists of 2 steam boilers. Steam is injected in the gas turbine combustion chamber to control NO, and is taken from the low-pressure boiler. The high-pressure steam expands in the steam turbine to generate additional power. Table 1 of Ref. [2] shows energy balances for combined gasturbine and steam-injection cycles. The addition of 1.82 kg/set of steam increases the compressor pressure ratio from 9.3 to 10.3, while it decreases the turbine-entry temperature from 982 to 913°C. This cogeneration system produces 3 MW and 8860 kg/set of saturated steam. When steam is not needed for industrial purposes, the steam-injection arrangement is used to raise the power to 4 MW. There are recent applications of Rankine cycles with gas-turbine exhausts to improve energy recovery by using water-ammonia solution as the working substance instead of water (Fig. 2 of Ref. [3]). Table 1 of Ref. [3] shows the important parameters at different cycle points. Comparison of Rankine and the suggested cycles shows that the thermal efficiency exceeds 50% for simple gas-turbines, when the exhaust gases are cooled to 77 instead of 149°C as in the Rankine cycle. Thus, the suggested system improves the thermal efficiency by 20%. Low-grade energy is used in distillation, which is more efficient than evaporation. With solutions, the cost decreases because of decrease in surface areas for the evaporator and condenser. The suggested system recovers the investment in 25% of the recovery period required for a Rankine cycle. 4. COMBINED
HEAT
AND
POWER
The waste heat in the gas-turbine exhaust may be used to cool the compressor air when it is associated with an absorption-refrigeration machine (Fig. 1 of Ref. [4]). This system increases thermal efficiency and power above those for a gas-turbine, mainly because of higher ratios of maximum to minimum temperatures. The performance becomes relatively less sensitive to changes in ambient temperature. In this cycle, with a pressure ratio of 6 and an ambient temperature of 15°C the thermal efficiency improves by 4% and the specific power by 16%. The waste heat in the gas-turbine exhaust can be used for central heating or airconditioning systems (Fig. 1 of Ref. [S]). Utilization of waste energy in a boiler may recover 60&85% of the associated energy to shaft power and available heat. In this arrangement, an additional quantity of fuel may be burnt in the waste-heat boiler since sufficient oxygen is available. 5. OTHER
MEANS
FOR
IMPROVING
THERMAL
EFFICIENCY
In gas-turbines with a regenerator, energy recovery may be boosted by water injection. The working substance consists of air, water and steam. The compressed air temperature is reduced to 60°C while the exhaust gases are at 80°C. The efficiency is improved to 53% at a turbine inlet temperature of 1100°C. Figure 2 and Table 3 of Ref. [6] show the basic cycle and performance of different arrangements, respectively. Energy may be recovered from the gas-turbine exhaust in a boiler to produce steam and run a steam turbine that drives a supercharger, which pressurizes the air before entering the main compressor. The exhaust steam is injected in the compressor and the combustor. An aero-derived engine could be used as a gas generator, whereas a separate free turbine is used for power generation (Fig. 9 of Ref. [7]). This arrangement should produce an increase of about 30% in efficiency and 100% in power.
Gas-turbine
total energy
systems
1025
The gas-turbine efficiency is improved by using H, as fuel and employing over expansion in the turbine (Figs 1 and 2 of Ref. [S]). The specific power remains nearly constant. The performance of the cycle may be improved by using a twin-spool arrangement, especially when the maximum cycle temperature is low. Inter-blade combustion of Hz may be used until the constant temperature cycle is approached. The isothermal combined cycle has been suggested as a means of improving the gasturbine efficiency. The principles are illustrated in Fig. I, the line diagram in Fig. 2 and the T -S diagram in Fig. 3 of Ref. [9]. If this cycle is combined with a steam cycle, a maximum temperature of 1600 K and a 100: 1 pressure ratio may be reached at an efficiency of 60”/0. Aeroengines have been used in industrial applications, especially for electric power generation, because of their reliability in continuous operation. With coal as fuel, an integrated system consisting of a coal gasifier is preferably used with the combined steam and gas-turbine power plant. By using one of the specified arrangements, the central powergeneration system may not be needed, without affecting the reliability of the power supply.
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO.
R. P. Allen and J. M. Kovacik. J. Engng Gas Turbines Power 106, 725 (19x4). H. Leibovitz and E. Tabb. J. Engng Gus Turbine Power 106, 731 (19X4). A. I. Kalina. J. Erring Gas Turbines Power 106, 737 (1984). 0. Yang, M. Tsujita and G. T. Sato. Bull. J.S.M.E. 13, 1I I (1970). 0. Yang and G. T. Sato. Bull. J.S.M.E. 14, 234 (1971). Y. Mori, H. Nakamura, T. Takahashi and K. Yamamoto. ‘Ii,k~, /n(. Ciu.5 Turbinr Conxr. Paper Tokyo-IGIC-38 (1983). G. Pilkington and D. R. Carlisle. Proc. Inst. Mrch. En,qrs 183, Pt 3 N, X5 (1969). K. Enomoto and G. T. Sato. Tokyo Int. Gus Turbine Congr. Paper no. X3-Tokyo-IGIC-39 (1983). M. A. El-Maxi, J. H. Magnusson. J. Engn~ Gas Turbines Power 106, 743 (1984). J. A. Roberts, Tokyo fnt. Gas Turbine Congr. Paper no. 83-Tokyo-IGIC-199 (1983).
no. X3-