GAS TURBINE POWER PLANT FIRED BY A WATER-BEARING FUEL

Abstract: This publication discloses a method and an apparatus for the utilization of the heat value of a water-bearing fuel in a gas turbine power plant. The apparatus comprises a high-­ pressure combustion unit (7), a gas turbine (15), a generator (16), and heat recovery units, the latter connected the system close to the gas turbine (15) outlet. According to the invention the water-bearing fuel is dried at a high pressure by the heat energy of the turbine exhaust gases, and the steam generated in the dryer stage is fed to the high-pressure section of the combustion process, at a point between a compressor (9) and the turbine (15), that is, for instance, into the combustion or gasification unit (7). The system in accordance with the invention provides for the utilization of the heat value of fuels having a high moisture content.

Description: The present invention relates to a gas turbine power plant fired by a waterbearing fuel in accordance with the preamble of claim 1. The invention also concerns a method for utilizing the heat value (calorific value) of a water-bearing fuel. Presently, a predried solid fuel, e.g., peat, wood chips, or coal, is burned at a low pressure using, for instance, a stoker fired boiler, pulverized fuel fired boiler, or fluidized bed boiler. The art also conventionally utilizes fuel oil and natural gas for generation of heat and electric energy in a gas turbine process. The aforementioned process can also be complemented by a process referred to as the Rankine cycle where steam is generated in the exhaust gas boiler of the turbine and fed into a separate steam turbine. In a more advanced modification of the method, steam is fed as injection steam into the gas turbine itself whereby both the mass flow through the turbine and the heat value of the gas are increased and the output power at the gas turbine shaft as well as the process efficiency are improved. Test plants utilizing coal for firing the gas turbine also exist. A disadvantage of the conventional technique is that particularly the utilization of peat for firing conventional boilers in small power plants has been uneconomical due to the high cost of the preceding de-watering process of the peat. Furthermore, the conventional construction of a gas turbine is not applicable to peat-firing. A gas turbine power plant utilizing solid water-bearing fuel, e.g. peat, is disclosed in the document US-A-2 677 237. The aim of the present invention is to overcome the disadvantages of prior art technology and provide a totally new kind of gas turbine power plant fired by a water-bearing fuel as well as a method for utilizing the heat value of said fuel. The invention is based on the idea of de-watering the fuel in a high-pressure dryer heated by the waste heat of the gas turbine and feeding the generated steam into the high-pressure section of the process. More specifically, the gas turbine power plant in accordance with the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the method in accordance with the invention is characterized by what is stated in the characterizing part of claim 4. The invention provides outstanding benefits. The gas turbine power plant in accordance with the invention permits the utilization of the heat value of a fuel without resorting to a complicated pretreatment. Particular benefits are achieved in the combustion of peat. Here, the moisture content of the peat does not lower the efficiency of the process; rather, this moisture is beneficially utilized. In optimal cases, a mere mechanical compaction of peat is sufficient obviating peat preprocessing and de-watering at the peatland site. In spite of the high initial cost of the high-pressure dryer construction, a cost-effective solution is provided due to the low market price of the fuel, in comparison with fuels presently used. The market price of peat utilizable in the process is essentially lower than that of dried peat. The auxiliary equipment in the system in accordance with the invention does not impose extra costs in respect to conventional technology. A particularly advantageous situation is attained if the gas turbine power plant can be located in the vicinity of a peatland, resulting in minimized fuel transportation costs. The invention is next examined in detail with help of the following exemplifying embodiment illustrated by the attached drawing. The figure shows schematically a gas turbine power plant utilizing a water-bearing fuel in accordance with the invention. Typical design values for the process depicted in the figure are given in Table 1, supplemented by an energy balance sheet in Table 2. The fuel is fired in a combustor, or combustion unit 7, which comprises a combustion chamber brought to a positive pressure with help of a compressor 9. The compressor 9 delivers the required combustion air which is fed into the combustion unit 7 via a compressed air piping 10. Via an auxiliary steam pipe 6, the combustion unit 7 is provided with a proportion of the steam separated in a steam separator 5 from the peat fuel flow, while some of the steam is recirculated to a dryer 2. In addition, steam is fed into the combustion unit 7 via a pipe 21 from a separate steam generator 20. One purpose of feeding the combustion unit with steam is to provide means for controlling the outlet gas temperature in that the steam makes up for the excess inlet air normally required. Because of the steam supply, the compressor power losses are decreased, resulting in an increased net output power from the process. A portion of the fuel ashes is tapped immediately from the combustion unit 7 via channels 22, while the rest is carried along the flue gas flow to a flue gas piping 8, and further to a hot scrubber 13, where it is removed from the process via a tap joint 14. The flue gas line may be provided with a separate scrubber 11 that removes the largest particles from the flue gases and recycles them via a return pipe 12 to the combustion unit 7. Next to the flue gas scrubber 13, the flue gases are further fed via the flue gas piping 8 to a gas turbine 15 where the gases expand and generate inertial energy. The inertial energy is used for rotating the compressor 9 and an electric power producing generator 16, which are both attached to the turbine shaft. The turbine used in the exemplifying embodiment is a gas turbine, type GT 35 C, by Asea Stal Ab. Next to the turbine 15, the exhaust gases are fed into a separate exhaust heat recovery unit 17, in which a water-88307359 circulating circuit generates hot steam into a piping 18, along which the energy contained by the steam is fed into the dryer 2 to be used for the de-watering of the fuel. The piping 18 has a water-circulating pump 19 attached to it. From the exhaust heat recovery unit 17, the exhaust gases are fed into a steam generator 20, in which the heat energy still remaining in the exhaust gases is utilized for generating steam into the steam piping 21. As required, the order of the steam generator 20 and the heat recovery unit 17 may be interchanged. These two units may conceivably be replaced by a combination unit. Further, instead of generating steam, the heat recovery unit 17 can be used either for generating hot water or for superheating the steam used in fuel de-watering. The fuel feed stock is carried via a high-pressure joint to the dryer 2. The calculations of the 88307359 88307359 exemplifying embodiment are based on the use of milled peat having a 70 % moisture content. The process in accordance with the invention can also be fed with peat coming directly from a mechanical dewatering process and having a moisture content in excess of 80 %. If the drying (de-watering) process is carried out at the pressure of the combustion process, the water contained by the peat can be recovered as process medium by feeding it in the form of saturated steam into the combustion chamber. In principle, the fuel used may be any water-bearing solid or liquid fuel. The milled peat is dried in the high-pressure dryer 2 to a moisture content of 20 %. In the exemplifying embodiment, the dryer 2 is a fluidized bed steam dryer, but in regard to the process, it can be any high-pressure type of dryer. Part of the generated steam is recirculated by a fluidizing draft fan 3 in order to fluidize the material in the bed. Energy for drying is fed via the piping 18 from the exhaust gas heat recovery unit 17 of the gas turbine 15. The combustion unit 7 may also be replaced by total or partial gasification of fuel and burning of the generated gas. In these embodiments, gas scrubbing is usually provided prior to combustion, thus making the scrubbing temperature of gases independent of the inlet temperature of the gas turbine. Furthermore, in this case, the maximum temperature of the gasification unit is not dependent on the inlet temperature of the gas turbine 15 in the same way as the temperature of the combustion unit 7. Gasification can be implemented either as air, oxygen, or pyrolytic gasification. In oxygen gasification, the compressor 9 is replaced by an oxygen generator or storage unit, or by an oxygen generator and compressor. The process is compliant with all gasification reactor types and firing methods. Gas scrubbing can be done either at the combustion or the gasification temperature, or at any lower temperature. Steam flows from pipes 6 and 21 can be fed as injection steam into the combustion chamber or the gasification unit, or to any part of the high-pressure gas line - either before or after the combustion or the gasification unit. The steam flows may also be conducted to a point between the stages of the gas turbine 15. If there is a need to cool the firing or gasification gas, the energy released in the cooling process can be utilized to generate, for example, steam, which is then conducted principally in the manner as the steam flow from the steam pipe 21. Separation of steam and peat in the steam separator 5 is not necessary; instead, the fuel and generated steam can also be fed intermixed into the combustion or gasification unit. In the exemplifying embodiment, the electrical efficiency of the process is about 45 %. By increasing the inlet temperature of the gas turbine 15 from the value of 850 °C employed in the embodiment to a higher level, appreciable improvement in the efficiency may further be achieved. This is particularly the case in conjunction with gasification technology, where the temperatures of the reactor 7 or the gas scrubber 13 do not set limits to the inlet temperature of the gas turbine 15 in the same manner as with embodiments based on combustion. In the illustrated embodiment, the referred inlet temperature of 850 °C results from the maximum allowable combustion temperature in the fluidized bed. It will be appreciated that with a decreasing level of moisture content in the fuel, the amount of steam generated in the steam generator 20 is increased and the energy recovered in the heat recovery unit 17 is decreased. with an increasing moisture content, the opposite is true. In respect to the applicability of the invention, the feed of supplemental steam via the piping 21 is not absolutely essential. Process design values and mass flow balance sheet (see Fig.) Point no. in Fig. Mass flow m (kg/s) Temp. t (°C) Pressure p (bar) Material 1 10.1 15 13.5 Peat, m.c. 70 % 4 3.8 150 13.5 Peat, m.c. 70 % 6 6.3 212 13.5 Steam 22 0.06 150 13.0 Ash 23 87.9 850 12.5 Flue gas 24 87.8 850 12.0 Flue gas 26 87.8 425 1 Flue gas 27 87.8 256 1 Flue gas 29 9.3 237 22 Steam 30 9.3 227 22 Water 25 74.9 0 1 Air 10 74.9 349 13.0 Air 14 0.09 150 13.0 Ash 31 3.1 5 12.5 Water 21 3.1 190 12.5 Steam 28 87.8 173 1 Flue gases (m.c. = moisture content) Process energy balance sheet excluding internal electric power consumption (see Fig.) Input energy in fuel 45.9 MJ/s Turbine inertial energy 47.8 MJ/s Compressor input energy 27.1 MJ/s Net output energy (available for electric power generation) 20.7 MJ/s Electrical efficiency 45.0 %

Claims: A gas turbine power plant fired by a water-bearing fuel, e.g. peat, comprising a high-pressure combustion or gasification unit (7), a high-pressure dryer (2), from which the fuel can be fed into the combustion or gasification unit (7), fuel feed means (1,2,3) for feeding the fuel into the combustion or gasification unit (7), a gas turbine (15) for utilizing the inertial energy of flue gases, at least one flue gas pipe (8), attached to the combustion or gasification unit (7), through which the combustion products of the fuel can be fed into the gas turbine (15), gas scrubber elements (11,13), attached to the flue gas pipe or pipes (8), with help of which the flue gases can be scrubbed, a compressor (9), driven by the gas turbine (15), by which a positive pressure is provided in the combustion or gasification unit (7), a generator (16), driven by the gas turbine (15), by which electric energy is suitably produced, and exhaust gas heat recovery elements (17,20), attached to the outlet of the gas turbine (15), suitable for the recovery of heat energy from the gases exhausted from the turbine, characterized by first heat transfer means (18) interconnecting the dryer (2) and the turbine exhaust gas heat recovery elements (17) whereby heat energy recovered from the exhaust gases can be transferred to the dryer (2) in order to dry the water-bearing fuel under pressure, and second heat transfer means (6,4) through which steam generated in the drying process is transferred from the dryer (2) to a point between the compressor (9) and the turbine (15), e.g. into the combustion or gasification unit (7). A gas turbine power plant in accordance with claim 1, wherein the high-pressure dryer (2) is a fluidized bed steam dryer. A gas turbine power plant in accordance with claim 1 or claim 2, including steam feed pipes (21) attached to the exhaust gas heat recovery elements (17,20) and arranged to feed injection steam into the high-pressure section of the process at a point between the compressor (9) and the turbine (15), e.g. into the combustion or gasification unit (7). A method for recovering the heat energy of a water-bearing fuel, e.g. peat, in a gas turbine power plant, in which method fuel is fed into a high-pressure combustion or gasification unit (7) in order to combust or gasify the fuel, the generated flue gases are led via gas scrubber elements (11,13) to a gas turbine (15) in order to utilize the inertial and heat energy of the flue gases, the generated inertial energy is used to drive a compressor (9), which provides positive pressure in the combustion or gasification unit (7), and to drive a generator (16) for the production of electric power, and the heat energy of flue gases after passing through the gas turbine (15) is recovered by means of heat recovery elements (17,20) in order to generate or superheat steam, or to heat water, characterized in that the fuel is dried under high pressure by means of the heat energy available from the flue gases after passing through the gas turbine (15), and the steam generated in the drying process is fed into the high-pressure section of the process, at a point between the compressor (9) and the turbine (15), e.g. into the combustion or the gasification unit (7). A method in accordance with claim 4, in which method steam generated by the heat recovery elements (17,20) is fed as injection steam into the high-pressure section of the process, at a point between the compressor (9) and the turbine (15) to, e.g. the combustion or the gasification unit (7). A method in accordance with claim 4 or claim 5, wherein the fuel is dried in a fluidized bed dryer.

Inventors: RAIKO, MARKKU
Applicants: IMATRAN VOIMA OY
Application year: 1988
Main classification: F02C3
IPC codes: F02C6, F01K21, F02C3