Supplementary fired combined cycle power plants
BACKGROUND OF THE INVENTION 
     The basic concept of thermodynamically coupling a Rankine-cycle steam turbine and a Brayton-cycle gas turbine to achieve overall efficiency higher than would be obtainable for either cycle alone (working in the same range of fluid temperatures and pressures) is well known. The higher efficiency achieved by combining cycles reduces fuel requirements and costs, and conserves energy that otherwise would be rejected to the environment as heat. The quantity and type of undesirable effluents in the plant exhaust gases can also be limited if a combined cycle plant is restricted to burning natural gas or refined or specially treated petroleum oils. Unfortunately, at the present time, these clean burning fuels are relatively scarce as compared to carbonaceous solid fuels such as coal, and alternative means have been sought to make coal, regardless of its rank or quality, an acceptable fuel for combined cycle power plants. 
     An important development in the field of power generation technology has been the use of coal-fueled sulfur-sorbing fluidized bed combustors to provide the basic energy source while minimizing power plant emissions. Although burning coal, it has been found that environmentally safe, sulfur-sorbing fluidized bed combustors can be utilized in combined cycle power plants to provide a gas turbine motive fluid that is free from undue corrosion and fouling effects which otherwise would cause severe reliability problems. Such uses are disclosed in U.S. Pat. No. 4,116,005 to Willyoung, and in the allowed U.S. patent application of Willyoung filed concurrently herewith under application Ser. No. 063,469 and assigned to the assignee of the present invention. 
     With these coal burning combined cycle plants, however, limitations of the sulfur-sorbing fluidized bed combustor have constrained the gas turbine to operate at less than state-of-the-art inlet temperatures, resulting in the gas turbine making less than optimal contribution to overall plant efficiency. Present-day gas turbines are able to operate at firing temperatures much higher than can now be obtained with a fluidized bed heater. The temperature discrepancy (which ma be as much as 450° F. for current commercial gas turbines and 800° F. or more for gas turbine designs projected for the future) is caused by two operational limitations on the fluidized bed combustor. First, because coal contains significant levels of sulfur, the bed temperature must be limited to a range which will insure sufficient sulfur capture by reaction with the in-bed sorption material to satisfy air emission standards. The second constraint that acts to limit fluidized bed combustor temperatures arises from the limitations and capabilities of presently available metallic materials which are suitable for in-bed heat transfer surfaces or for meeting structural requirements. 
     It is one object of the present invention, therefore, to provide an improved combined cycle power plant having a coal-fueled fluidized bed combustor as its principal source of energy and wherein the gas turbine is operated at state-of-the-art levels of specific power and firing temperature so that the highest overall power plant efficiency is attained. 
     Another object of the invention is to significantly increase the overall efficiency of a combined cycle steam turbine and gas turbine power plant without using a large proportion of premium fuel such as natural gas or petroleum. 
     Yet another object of the invention is to provide a highly efficient combined cycle power plant including a reliable gas turbine not exposed to coal combustion products. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are attained by providing a clean hydrocarbon fuel (CHF) combustor to supplement the gas turbine portion of the system so that the temperature of the working fluid driving the gas turbine is increased. The gas turbine working fluid thus contains the CHF combustion products, but since CHF meets normal gas turbine fuel specifications, sulfidation, hot corrosion, and erosion rates are not excessive. The CHF, which may be oil or other clean hydrocarbon fuel, contributes less than 10 percent of the total fuel energy (based on present state-of-the-art gas turbine inlet temperatures), but introduced to generate topping heat, provides incremental efficiency well in excess of 50 percent. That is, the CHF is used at a rate which is higher than would be attainable were such fuel to comprise the total fuel supply. Overall plant efficiency is increased by approximately 10 percent relative to a fluidized bed combined cycle plant without supplemental firing. The invention is advantageously applied with either atmospheric pressure fluidized bed combustors or with pressurized fluidized bed combustors in which the combustion gas is cleaned, cooled somewhat, and used to drive a second gas turbine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as the invention, the invention will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 shows a combined cycle steam turbine and gas turbine power plant having an atmospheric pressure fluidized bed combustor wherein air, preheated by heat derived from the fluidized bed, has supplemental heat added by a CHF combustor to provide a higher temperature motive fluid for the gas turbine; and 
     FIG. 2 shows a combined cycle steam turbine and gas turbine power plant having a pressurized fluidized bed combustor and including a CHF combustor to add supplemental heat energy to the gas turbine cycle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a preferred embodiment of the combined cycle power plant as shown in FIG. 1, a gas turbine unit 10 is combined with a steam turbine 11, with heat energy for driving both turbines furnished in whole or in part by atmospheric pressure fluidized bed combustor 12. Gas turbine unit 10, comprising a compressor section 13 and a turbine section 14, is connected in driving relationship to load 15 which may be a dynamoelectric machine. The steam turbine 11, comprising a high-pressure section 17 and lower pressure sections generally designated 18, is connected in a driving relationship to a second load 19 which may also be a dynamoelectric machine. 
     The fluidized bed combustor 12 is of the type disclosed in U.S. Pat. No. 4,116,005 (which disclosure is incorporated herein by reference thereto) and includes a plurality of sulfur-sorbing particles 20 forming a fluidized bed 21 which is suspended above a grate 22 by a stream of gas which passes therethrough. The sulfur-sorbing particles 20 are preferably calcined limestone (calcium carbonate) or dolomite (calcium magnesium carbonate) typically 1/16 to 1/2 inch in maximum dimension. The particles 20 capture sulfur oxides during combustion of coal or other carbonaceous fuel and also promote heat transfer. The fluidized bed combustor 12 includes a first inlet 23 for receiving a gas stream containing air to fluidize the bed 21 and support combustion, a second inlet 24 for receiving coal or other carbonaceous fuel, and an outlet 25 for discharge of combustion gases and other exhaust products including particulate matter. Fresh sulfur-sorbing particles 20 are supplied to the fluidized bed combustor 12 through a third inlet 26, and spent-sorbing particles and ash solids are removed through exit port 28. The percentage of coal in the bed 21 is typically rather small--of the order of one percent or less by weight, and as supplied to the bed 21, the coal is preferably in the form of solid particles. 
     Exhaust and combustion gases from the fluidized bed combustor 12 pass through particulate separation equipment 29a and 29b which may be multiple stage cyclone-type devices, through one side of heat exchanger 30, through feedwater heat-exchanger 31 and a final clean-up device 33 to pass to stack 34 from which the cleansed gases pass to the atmosphere. Solid matter which contains a significant amount of unburned carbon is separated from the gas stream by separator 29a and is recycled to the fluidized bed. This material may, for example, be fed back in with fresh fuel, as indicated by line 42. Solid material separated in the secondary separation stage 29b, typically contains little unburned carbon and is sent to ash disposal bin 44. 
     Fuel burned in fluidized bed combustor 12 provides the principal source of heat energy for sustaining plant operation. A portion of the fluidized bed heat energy is transferred by steam generating heat exchangers 36, 37, and 38, located within the combustor 12, to furnish throttle and reheat steam for turbine 11 which is arranged for high efficiency steam extraction for feedwater heating in the conventional manner (indicated schematically by feedwater heater system 40). In the steam turbine cycle, condensate from condenser 39 is heated in feedwater heater system 40, passed through parallel heat exchangers 31 and 41 for additional heat value prior to passing to the inbed heat exchanger 36 wherein steam is generated for high-pressure turbine section 17. Steam exhausted from high-pressure turbine 17 is reheated in heat exchangers 37 and 38. 
     For the gas turbine cycle, air taken in and pressurized by compressor 13 is preheated in heat exchanger 30 by heat extracted from the combustion gas stream exhausted by the fluidized bed combustor 12. The preheated air then passes through in-bed heat exchanger 43 to gain additional heat energy, raising the temperature of the air to nearly that of the bed. The bed temperature is limited, however, by the availability of high-temperature alloys from which the in-bed heat exchanger 43 may be constructed. Presently available economic alloys do not provide the required corrosion resistance and mechanical strength for temperatures above about 1550° F. 
     A further limitation on bed temperature (and hence in the temperature of the air passing from heat exchanger 43) results from the need to maintain the most effective temperature for removal of sulfur compounds by the sorption particles. When introduced into the combustor 12 the particles are typically crushed limestone (calcium carbonate) or dolomite (calcium magnesium carbonate), the latter undergoing chemical transformations analogous to those described herein for limestone. After entry into the combustor 12, the limestone particles are calcined by combustor heat, giving off carbon dioxide to form chemically active calcium oxide. The calcium oxide is highly reactive with the oxides of sulfur produced during combustion of the sulfur-bearing fuel, but only over a limited temperature range; the upper limit of which is around 1650° for combustion at atmospheric pressure. 
     To provide gas turbine 14 with motive fluid at a temperature high enough to take advantage of its state-of-the art capabilities and thus increase cycle efficiency, a clean hydrocarbon fuel (CHF) combustor 45 adds supplemental heat energy to the gas turbine cycle. The CHF (e.g., natural gas or petroleum oil), comprising approximately ten percent of the total plant fuel or less (depending on the compressor discharge temperature, the temperature rises in heat exchangers 30 and 43, and the inlet temperature of turbine 14), is fed into the combustor 45 at inlet 46 and is burned with the preheated air stream from heat exchanger 43. There is thus provided a high-temperature working fluid for the gas turbine 14 which approaches state-of-the-art capability (1800°-2100° F.). Although the working fluid contains CHF combustion products, turbine sulfidation, hot corrosion and erosion will not be excessive since the CHF meets normal gas turbine fuel specifications. Relative to a gas turbine fueled entirely with CHF of the same grade, these effects are less with supplemental firing, due to a substantially lower weight ratio of air to CHF. 
     The high-temperature motive fluid discharged from the CHF combustor 45 flows to the gas turbine section 14 where it expands to drive compressor 13 and load 15. 
     A flame cooling sidestream of air from the compressor, as shown by dashed line 47 of FIG. 1, may be taken into the CHF combustor 45 for sustaining combustion with low-temperature air if necessary to prevent generation of thermally fixed NO x  compounds. In such case, the CHF combustion may be essentially stoichiometric and the combustion products mix with air from heat exchanger 43 to achieve the desired gas turbine firing temperature. 
     The CHF combustor 45 is constructed according to principles applied in the design of combustors for gas turbines, emphasizing those principles which result in minimum production of NO x  compounds. The combustor 45 is preferably of the two-stage combustion type having means for introduction of cooling air to maintain metal temperatures below values at which strength of materials becomes a problem. The combustor 45 is provided with means for premixing the fuel with a portion of the combustion air (consistent with safety considerations of flashback) and for operation of the first stage of combustion fuel-rich and the second stage fuel-lean. These features promote efficient operation with minimum production of NO x  compounds. 
     A different embodiment of a supplementary fired combined cycle power plant is shown in FIG. 2. A gas turbine unit 50 is combined with a steam turbine 51 with both turbine portions 50 and 51 deriving heat energy from pressurized fluidized bed combustor 52. The fluidized bed combustor 52 differs from the fluidized bed combustor 12 of FIG. 1 principally in that combustor 52 operates at an elevated internal pressure. Gas turbine unit 50 comprises an air compressor section 54, a combustion gas turbine 55, a gas expander turbine 56, and is connected to drive load 57 which may be an electric generator. The steam turbine 51, including high-pressure section 59 and lower pressure sections 60 and 61, is mechanically coupled to drive electric generator 63. 
     The fluidized bed combustor 52, of the type disclosed in the above mentioned U.S. patent application of Willyoung, Ser. No. 063,469, which disclosure is incorporated herein by reference thereto, includes fluidized bed 64 comprising sulfur-sorbing particles 65 of, preferably, crushed limestone or dolomite. The particles 65 capture sulfur oxides formed from the combustion of coal or other sulfur-bearing carbonaceous fuel and also promote heat transfer. Combustor 52 includes a first inlet 66 for receiving a stream of pressurized air from the compressor 54, a second inlet 67 for receiving fuel, and an outlet 68 for discharging combustion gas and entrained particulate matter. Fresh sulfur-sorbing particles 65 are supplied through a third inlet 69, and spent-sorbing particles and ash solids are removed through exit port 70. Coal, supplied to the combustor 52 as fuel, is preferably in the form of small particles, and is a constituent of the fluidized bed 64. 
     Hot combustion gas from the fluidized bed combustor 52 passes into first stage particulate separator 72a which removes entrained particles consisting of ash, attrited sorbent particles, and unburned char and returns them to the fluidized bed 52 through return line 73. The combustion gases then pass to further particle separation as in separator 72b, where additional particulates (mostly fine ash) are removed and sent to ash disposal bin 74. The particulate stripped combustion gas then flows from the separator sections 72a and 72b to heat exchanger 75 which cools the combustion gas significantly, e.g., from about 1600° F. to about 1100° F. or less (typically 800° F.). The cooled combustion gas from heat exchanger 75 expands in combustion gas turbine 55 providing driving energy thereto. A portion of the remaining heat energy contained in the gas passing from turbine 55 is transferred in heat exchanger 76 to feedwater for steam turbine 51. The realtively low temperature of the combustion gas as it passes through turbine 55 avoids corrosion and erosion problems and minimizes the need for more extensive gas clean-up. The combustion gas from heat exchanger 76 passes to the stack 77 from which it is exhausted to the atmosphere. 
     A second pressurized air stream from compressor 54 is taken through heat exchanger 75 wherein heat removed from the combustion gas is transferred to the air. The second air stream then passes through an in-bed heat exchanger 79 gaining additional heat energy and raising the air temperature to approach that of the fluidized bed 64. For the reasons detailed above in describing the atmospheric fluidized bed combustor, the second pressurized air stream from in-bed heat exchanger 79 is lower in temperature than is required for highly efficient operation of gas expander turbine 56. As a remedy for the deficiency, supplemental heat is added to the second air stream by CHF combustor 80 to increase the firing temperature of gas turbine 56 and promote higher efficiency operation. The high-temperature gas stream from combustor 80 expands through the turbine 56 providing driving energy therefor. Gas exhausted from turbine 56 passes through economizer heat exchanger 81 releasing a portion of its remaining heat energy to feedwater for the steam turbine 51, and then is vented to the atmosphere through stack 77. 
     Among the advantages provided by the present invention over prior art combined cycle fluidized bed power plants, the following are noteworthy: 
     1. There is a higher overall power plant efficiency, i.e., more power produced per unit of fuel consumed. 
     2. The gas turbine portion of the plant operates at conventional levels of specific power and firing temperature without exposing the turbine to coal combustion products. 
     3. There is an ability to follow power plant load demands rapidly over a moderate range by regulating the CHF combustor. For example, 10 percent of the combined cycle plant&#39;s fuel may be CHF supplying 17 percent of the power to a load, so that a rapid turn-down from 100 percent load to 83 percent can be made. 
     4. As the fluidized bed combustor art advances toward higher operating temperatures the fractional portion of energy supplied by CHF may be reduced further without extensive plant changes. 
     5. Ability to operate the plant at a substantial fraction of its maximum rating, using coal only, in periods when CHF may be unavailable or so costly that its use is undesirable. 
     While there has been shown and described what is considered a preferred embodiment of the invention, it is understood that various modifications may be made therein. For example, the use of a heat recovery steam generator (HRSG) coupled with a second, bottoming steam turbine may be used in recovering waste heat from the gas turbine exhaust streams rather than recovery of such heat in feedwater heat exchangers as shown in FIGS. 1 and 2. Also it will be understood that gas turbine elements 55 and 56 in FIG. 2 may be mounted on independent shafts, each driving a section of the total plant compressor or generator load. Furthermore, it will be appreciated by those skilled in the art that a supplementary fired gas turbine cycle as described herein may be used in the cogeneration of steam to meet a heat load as well as for generating power to meet an electrical load. It is intended to claim all such modifications which fall within the true spirit and scope of the present invention.