REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending utility application U.S. Serial No. 08/54,986

(Newby) , filed April 30, 1993, entitled SYSTEM AND METHOD FOR CLEANING HOT FUEL GAS.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for cleaning and burning a hot coal-derived fuel gas. More specifically, the present invention relates to a method for removing particulates, sulfur and alkali species from high temperature, high pressure coal-derived fuel gas without excessive cooling of the fuel gas and then burning the fuel gas in a combustor so as to generate a minimum amount of nitrogen oxides (NOx) . The high efficiency, low capital cost and short lead time of gas turbine based power plants make them particularly attractive to electric utilities as a means of producing electrical power. Unfortunately, traditionally, gas turbines have been limited to operation on expensive, sometimes scarce, fuels -- chiefly, distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power that can utilize coal as its primary fuel. In one approach, referred to as an integrated combined gasification power plant, compressed air from the gas turbine compressor, or compressed oxygen, is used to partially combust coal in a gasifier to produce a hot, low

to medium heating value fuel gas. This hot fuel gas is combusted and then expanded in the turbine section of the gas turbine. The fuel gas contains particulate matter, as well as sulfur and alkali species, all of which can be harmful to the turbine components. In addition, the fuel gas, when combusted, should be capable of satisfying environmental emission standards.

Therefore, it is important that the fuel gas be cleaned prior to expansion in the turbine. Both cold and hot gas cleanup systems have been proposed. In the tradi¬ tional "cold" fuel gas cleanup system, the cleaned fuel gas is discharged to the combustor at near ambient temperature. Several types of hot fuel gas cleanup systems have been proposed. One such system utilizes ceramic barrier filter technology to remove particulates and zinc-based sorbents to remove sulfur. Another type is disclosed in the afore¬ mentioned U.S. application Serial No. 08/54,986 (Newby) . Hot gas cleanup systems offer improved thermodynamic performance since the sensible heat in the hot fuel gas is not lost from the cycle as a result of the cooling neces¬ sary with a cold cleanup system.

Regardless of whether a cold or hot gas cleanup system is utilized, the fuel gas exiting the cleanup system must be further heated by combustion in order to realize maximum power output during expansion in the turbine. Such combustion is accomplished in a topping combustor. Unfor¬ tunately, however, the combustion of the fuel gas results in the formation of nitrides of oxygen ("NOx") , which are considered an atmospheric pollutant. Also, if a hot gas cleanup system is utilized, the high temperature of the hot fuel gas from the cleanup system, while being thermodynami- cally desirable, makes its difficult to cool the walls of the topping combustor in which the fuel gas is burned. Accordingly, it would be desirable to incorporate a combus- tor into the power plant along with the hot gas cleanup system that would allow the fuel gas to be burned prior to expansion in a turbine without creating excessive NOx or

overheating the walls of the combustor in which the fuel gas is burned.

SUMMARY OF THE INVENTION Accordingly, it is the general object of the current invention to provide a power plant having (i) a sulfur removal system that is capable of operating on high temperature coal-derived fuel gas containing sulfur and nitrogen species and (ii) a combustor capable of burning the reduced sulfur fuel gas without creating excessive NOx or overheating the walls of the combustor in which the fuel gas is burned.

Briefly, this object, as well as other objects of the current invention, is accomplished in a method of reducing the concentrations of sulfur and NOx resulting from the generation of power from a hot coal-derived fuel gas containing sulfur and nitrogen species in a power plant. The method comprises the steps of (i) bringing a hot coal-derived fuel gas having a temperature of at least approximately 870°C (1600°F) and containing sulfur and nitrogen species and combustibles into contact with a first sulfur sorbent so as to remove a first portion of the sulfur species from the hot coal-derived fuel gas by converting the first sorbent into a first sulfur compound, at least portions of the first sorbent and the first sulfur compound being entrained in the hot coal-derived fuel gas, (ii) removing at least a portion of the entrained first sorbent and the entrained first sulfur compound from the hot coal derived fuel gas, (iii) mixing the hot coal derived fuel gas into a first flow of compressed air, thereby producing a fuel/air mixture, the ratio of the combustibles in the hot coal-derived fuel gas to the first flow of compressed air in the fuel/air mixture being greater than that associated with stoichiometric combus¬ tion, (iv) combusting only a first portion of the combusti- bles in the hot coal derived fuel gas so as to heat the fuel/air mixture to a temperature in the range of approxi¬ mately 1430-1650°C, thereby producing a combustion gas containing a second portion of the combustibles and

molecular nitrogen converted from the nitrogen species in the hot coal-derived fuel gas, and (v) mixing the combus¬ tion gas into a second flow of compressed air, the ratio of the second portion of the combustibles to the second flow of compressed air being less than that associated with stoichiometric combustion, so as to combust the second portion of the combustibles and quench the combustion gas to a temperature below approximately 1565°C in less than 10 millisec, thereby preventing the molecular nitrogen from being converted to NOx.

The current invention also encompasses a power plant for generating power from a hot coal-derived fuel gas containing sulfur and nitrogen species without emitting high concentrations of sulfur and NOx, comprising (i) means for generating a hot sulfur-bearing gaseous fuel from coal, (ii) sulfur removing means for removing at least a portion of the sulfur from the hot gaseous fuel, thereby producing a reduced-sulfur-bearing hot gaseous fuel having a tempera¬ ture of at least approximately 870°C (1600°F) , (iii) a combustor for burning the reduced-sulfur-bearing hot gaseous fuel, thereby producing a further heated reduced- sulfur-bearing gaseous fuel, and (iv) a turbine for expand¬ ing the further heated reduced-sulfur-bearing gaseous fuel, thereby producing power. According to the preferred embodiment of the current invention, the combustor includes (i) first, second and third concentrically arranged liners, the first and second liners forming a first annular passage therebetween, the second and third liners forming a second annular passage therebetween, with at least one of the liners enclosing a first combustion zone, (ii) means for introducing the reduced-sulfur-bearing hot gaseous fuel from the sulfur removing means into the first combustion zone, and (iii) means for inducing swirl disposed in the first and second annular passages. In one embodiment, the power plant further comprises (i) a compressor for produc¬ ing compressed air, (ii) means for directing at least a portion of the compressed air to the combustor so that inlets of the first and second annular passages of the

combustor each receive some of the portion of the com¬ pressed air, and (iii) means for transferring heat from the hot sulfur-bearing gas to the compressed air, thereby pre¬ heating the compressed air prior to the compressed air being received by the annular passage inlets.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of an integrated coal gasification gas turbine power plant using the hot gas cleanup system and topping combustor of the current inven- tion.

Figure 2 is a schematic diagram of the hot gas cleanup system shown in Figure 1.

Figure 3 is a schematic diagram of an alternate embodiment of the integrated coal gasification gas turbine power plant shown in Figure 1.

Figure 4 is a longitudinal cross-section through the topping combustor shown in Figures 1 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, there is shown in Figure 1 a schematic diagram of an integrated coal gasific¬ ation gas turbine power plant. The plant comprises a compressor 1 that inducts ambient air 9 and produces compressed air 10. The compressed air 10, which in the preferred embodiment is at a temperature in the range of approximately 370-425°C (700-800°F) , is directed to a gasifier 2, via piping, wherein it is used to gasify coal 11. The gasifier 2 produces a fuel gas 12 that may have a temperature and pressure as high as 1650°C (3000°F) and 2760 kPa (400 psia) , respectively, and that is laden with particulates (chiefly coal slag and ash) , as well as sulfur species (chiefly hydrogen sulfide and COS) , and alkali species. The fuel gas 12 is passed through a cyclone separator 3 in which a portion of the particulate matter is removed. In the embodiment of the invention shown in Figure 1, the fuel gas 12 then flows through a heat exchanger 4 supplied with feedwater or steam 13 and in which the temperature of the fuel gas is reduced to approx¬ imately 925°C (1700°F) .

The partially cooled but still hot fuel gas 20 from the heat exchanger 4 is then processed in a gas cleanup system 5. After cleaning, the fuel gas 15 is combusted in a topping combustor 6, as discussed further below, and then expanded in a turbine 7, thereby producing power. In the embodiment shown in Figure 1, the expanded gas 18 from the turbine 7 flows through a heat recovery steam generator 8, supplied with heated feedwater or steam from the heat exchanger 4, and the expanded gas 19 is then exhausted to atmosphere.

The gas cleanup system 5 is shown in an overall fashion in Figure 2. The system has three major subsystems -- a primary sulfur removal and oxidizer subsystem 21, a polishing sulfur removal and regenerator system 22, and an alkali removal unit 23. A primary sulfur sorbent 24 is injected into the fuel gas 20 upstream of a primary filter 25, thereby removing a substantial portion of the sulfur from the fuel gas. The sorbent laden fuel gas 44 then flows through the primary filter 25, wherein a substantial portion 73 of the used and unused primary sorbent is removed and directed to an oxidizer 26. The filtered fuel gas 40 is then directed to the alkali removal unit 23, containing a fixed alkali sorbent bed, wherein a substan¬ tial portion of the alkali species is removed from the fuel gas.

From the alkali removal unit 23, the fuel gas 41 is directed to a polishing de-sulfurizer 33, in which a polishing sulfur sorbent 32 in maintained in a bed fluidized by the fuel gas 41. The polishing de-sulfurizer 33 removes a substantial portion of the sulfur remaining in the fuel gas and discharges the fuel gas 42 to a cyclone separator 27 for particulate removal. The fuel gas 43 then flows through a secondary filter 35 and the clean fuel gas 15 is discharged from the system. The solids 97 captured by the filter, which include used and unused sorbent, are then removed for reprocessing, which may include regenera¬ tion, as discussed further below.

As shown in Figure 2, used polishing sorbent 87 from the polishing de-sulfurizer 33 is directed to a regenerator 34 into which air 29 is drawn to fluidize a bed of the used sorbent, thereby producing regenerated polish- ing sorbent 92 that is recycled to the polishing de-sulfur¬ izer 33. The regeneration also produces sulfur dioxide 93, which is directed, via a cyclone separator 28, to the oxidizer 26. In the oxidizer 26, the used and unused primary sorbent 73 is maintained in a bed fluidized by the sulfur dioxide rich gas stream 93 from the regenerator 34 and by air 78. This results in the used primary sorbent being converted to a more stable compound for waste dis¬ posal 31. The gas 85 discharged from the oxidizer 26 is directed to a cyclone separator 81 and the cleaned gas 36 is then discharged to a stack (not shown) . The various components of the system and the reactions that occur in these components are discussed in more detail in the aforementioned U.S. application Serial No. 08/54,986 (Newby) , hereby incorporated by reference in its entirety. Figure 3 shows another embodiment of the inven¬ tion, in which only a minor portion 10' of the compressed air 10 from the compressor 1 is directed, via piping, to the gasifier 2. The major portion 10" of the compressed air 10, which in the preferred embodiment consists of approximately 80% of the compressed air 10, is directed, via piping, to a heat exchanger 304. The hot fuel gas 12 from the gasifier 2 also flows through the heat exchanger 304. In the heat exchanger 304, which may be of the shell and tube type, heat is transferred from the hot fuel gas 12 to the compressed air 10". In the preferred embodiment, the hot fuel gas 12 is cooled according to the current invention from approximately 1650°C (3000°F) to approxi¬ mately 925°C (1740°F) and the compressed air 10" is heated from approximately 400°C (750°F) to approximately 540°C (1000°F) .

From the heat exchanger 304, the heated compressed air 30 is directed, via piping, to the combustor 6, where it forms the combustion air for the burning of the clean

fuel gas 15. Such pre-heating of the compressed air 30 results in an increase in the temperature of the hot gas 17 expanded in the turbine 7 and, therefore, the power output from the turbine. Alternatively, if a supplemental fuel is burned in the combustor 6, the pre-heating of the com¬ pressed air 30 allows a reduction in the amount of supple¬ mental fuel burned, thereby increasing the thermodynamic efficiency of the power plant .

As previously discussed, unlike conventional cleanup systems, the cleanup system 5 does not require that the fuel gas be cooled to ambient temperature. Thus, the fuel gas 15 discharged from the cleanup system 5 will typically be approximately 870°C (1600°F) . Unfortunately, this temperature is still too low to obtain maximum perfor- mance from the turbine 7 since modern turbines are capable of operation at gas inlet temperatures in excess of 1370°C (2500°F) . Thus, to obtain optimum thermodynamic perfor¬ mance, it is necessary to combust the fuel gas 15 so as to raise its temperature into the range suitable for optimum performance in the turbine 7. As shown in Figure 1, a supplemental fuel -- such as oil or natural gas -- may be added to the topping combustor 6 to further raise the temperature of the hot gas expanded in the turbine 7.

Unfortunately, combusting the fuel gas 15 in a topping combustor 6 presents several problems. First, although the hot gaseous fuel 15 discharged from the cleanup system 5 is relatively free of sulfur and alkali species, a significant quantity of the nitrogen that was bound in the coal 11 appears in the gaseous fuel -- typi- cally as NH ₃ (ammonia) and HCN. Therefore, combustion of the fuel gas 15 will result in the formation of NOx, which is considered an atmospheric pollutant. Second, since the fuel gas 15 is a relatively low BTU gas, typically having a heating value of less than approximately 90-250 BTU/SCF, the volumetric flow rate of the fuel is large. Therefore, the sensible heat from this large volumetric flow of fuel gas will make it more difficult to maintain the temperature of the walls of the combustor 6 within acceptable limits.

This situation is further exacerbated by the use of the heat exchanger 304 to pre-heat the compressed air 30 supplied to combustor 6 as combustion air. While such pre¬ heating improves efficiency, it makes cooling of the combustor walls more difficult since the compressed air 30 is relied upon for cooling.

The inventors have found that, according to the current invention, combusting of the hot gaseous fuel 15 can be accomplished without the formation of NOx or the over-heating of the combustor wall by the use of a multiple annular swirl type burner 6, as described in detail below. Multiple annular swirl type combustors are also disclosed in U.S. patent no. 4,845,940 (Beer) and U.S. application Serial No. (Bachovchin, Dowdy and Beer) , filed May 25, 1994 (attorney docket no. 58,214) , entitled AN IMPROVED GAS TURBINE TOPPING COMBUSTOR, each of which is hereby incorporated by reference in its entirety, .

As shown in Figure 4, preferably, the combustor 6 comprises six substantially cylindrical liners 311-316 that are concentrically arranged around the axial center line of the combustor. As discussed further below, liner 313 encloses a first combustion zone 300 in which a rich mixture of fuel 15 and air 30 is formed, while liner 315 encloses a second combustion zone 301 in which a lean mixture of fuel and air is formed. The liners 311-316 are interconnected by struts 352 that give the combustor 6 a unitized structure.

The liners 311-316 form five axially extending annular passages 321-325 between themselves. Each of the annular passages 321-325 has an air inlet so that the compressed air 30 from the heat exchanger 304 enters the combustor 6 as five annular streams that flow through the annular passages 321-325. Swirlers 345 are disposed in the annular passages 321-325 and impart a swirl to the incoming compressed air 30.

The outlets of the annular passages 321-325 are staggered in the axial direction so that the outlets of the annular passages are displaced successively further down-

stream as the radii of the passages increases. Thus, the outlet of annular passage 322 is disposed downstream of the outlet of annular passage 321 that is encircled by annular passage 322, the outlet of annular passage 323 is disposed downstream of the outlet of annular passage 322 that is encircled by annular passage 323, etc.

Annular rings 330 are attached to the inside diameter of liners 312-315. Each of the rings 330 forms a radially inward extending projection that is shaped so as to create a converging/diverging passage through the liner to which it is attached. Consequently, the rings 330 serve to form toroidal vortices 350 and 360.

A centrally disposed fuel nozzle 320 is located at the upstream end of liner 311 and serves to inject the hot fuel gas 15 into the combustor 6. If necessary, fuel may also be introduction at other locations around the combus¬ tor 6 -- for example, by circular manifolds, around which a plurality of fuel discharge ports are distributed, that are located upstream of one or more of the annular passages 321-325.

A plurality of swirlers 345 are circumferentially distributed around the annular passages 321-325. The swirlers 345 may be formed from vanes attached to the liners at an angle to the axial direction and serve to impart a swirl velocity component to compressed air 30 that aids in the mixing of the fuel 15 and helps control the temperature in the combustion products so as to reduce the formation of NOx, as discussed further below.

It is known that NOx is generated in two ways -- (i) by the formation of NOx from atmospheric nitrogen in the combustion air, which occurs at high temperatures, and (ii) by the conversion of organically bound nitrogen compounds in the fuel, such as HN ₃ and HCN, to NOx. The rate of formation of NOx from atmospheric nitrogen is primarily dependent upon the combustion temperature and, therefore, is typically referred to as "thermal NOx." The rate of formation of NOx from fuel bound nitrogen is

dependent largely upon the local fuel-air ratio and, to a lesser extent, upon temperature.

The specific composition of the fuel gas 15 will vary with the composition of the coal 11 but will typically be comprised chiefly of CO, C0 ₂ , H ₂ 0, H ₂ , N ₂ , and CH ₄ , along with the aforementioned fuel bound nitrogen. According to the current invention, in the combustor 6, the conversion of the fuel bound nitrogen to NOx is minimized by combust¬ ing a rich mixture of fuel and air in the first combustion zone 300 so that only a portion of the combustibles in the fuel gas 15 are burned. The combustion gases produced by this partial combustion -- primarily C0 ₂ , N ₂ , CO, NO, N0 ₂ , H ₂ , and H ₂ 0 -- are heated into a temperature in the range of approximately 1430-1650°C (2600-3000°F) . Due to the oxygen deficient environment in which the combustion occurs, the fuel undergoes complex chemical reactions that result in the fuel bound nitrogen being converted to molecular nitrogen. This oxygen deficient environment is created by regulating the introduction of compressed air 30 into the first combustion zone 300 so that the ratio of combustibles in the fuel gas to the combustion air is greater than that associated with stoichiometric combustion (i.e., the fuel/air ratio is "rich") . The ratio of combustibles in the fuel gas to the combustion air that is associated with stoichiometric combustion depends on the specific composi¬ tion of the fuel gas but for many types of coal-derived fuel gas will typically be in the range of 0.14 to 1.40 kg/kg. When such fuel gas is burned, the fuel/air ratio in the first combustion zone 300 should be at least 1.4 kg-mol of combustibles to kg-mol of oxidant .

Downstream of the first combustion zone 300, locally high temperatures that would result in the conver¬ sion of molecular nitrogen to thermal NOx are avoided by rapidly mixing a relatively large portion of the compressed air 30 into the combustion gases. This mixing permits combustion of the remaining combustibles -- primarily CO and H ₂ -- and rapidly quenches the combustion gases, in no more than about 10 millisec, to a temperature sufficiently

low to result in little conversion of molecular nitrogen to NOx -- that is, to a temperature less than approximately 1565°C (2850°F) , and preferably to approximately 1480°C (2700°F) . The combustion gases are then further quenched to the final turbine inlet temperature, preferably approxi¬ mately 1325°C (2400°F) , in no more than an additional 20 millisec. Thus, by maintaining a rich fuel/air ratio and sufficient residence time and temperature in the first combustion zone 300, followed by rapid quenching, little of the fuel or molecular nitrogen will be converted to NOx in the lean second combustion zone 301. Although a tempera¬ ture rise within the second combustion zone 301 is unavoid¬ able, the temperature history is carefully controlled to ensure that the combustion of soot and hydrocarbons pro- ceeds to completion within the residence time in the combustor while maintaining sufficiently low temperatures in the second combustion zone 301.

In operation, the hot fuel gas 15 is injected into the cavity 326 formed by the innermost liner 311 and then into the first combustion zone 300, where it mixes with a portion of the swirling hot compressed air 30. The swirl¬ ing of the compressed air 30, along with the divergence caused by the annular rings 330, generates the toroidal vortex 350 associated with the first combustion zone 300. A second toroidal vortex 360 is similarly formed in the second combustion zone 301.

The toroidal vortices 350 and 360 extend longitu¬ dinally within their respective combustion zones 300 and 301 and have a recirculating flow pattern along the axis of the combustor 6. A stagnation pressure develops slightly downstream of each toroidal vortex. To achieve proper flame stabilization and combustion in the first combustion zone 300, the axial spacing between diverging passages formed by the annular rings 330 attached to liners 313 and 314 must be sufficiently large to ensure that the vortices 350 and 360 remain separate.

The first vortex 350 creates a fuel rich mixture within the first combustion zone 300 that consists of the

hot gas fuel 15 from the nozzle 320 and only a portion of the compressed air 30. As previously discussed, the ratio of combustibles in the gas fuel 15 to compressed air in the first combustion zone 300 is greater than that associated with stoichiometric combustion. This portion of the compressed air consists, by and large, of the compressed air that flows through only the two innermost annular passages 321 and 322. It should be noted that, according to the current invention, typically, two-thirds of the stoichiometric combustion air is introduced through the three innermost annular passages 321-323. Vigorous mixing of the fuel and air within the first combustion zone 300 is essential for the efficient conversion of fuel bound nitrogen to molecular nitrogen, and also to avoid excessive formation of soot.

In the second combustion zone 301, the second toroidal vortex 360 forms a lean mixture of the combusti¬ bles in the combustion gas from the first combustion zone 300 and a second portion of the compressed air 30. Conse- quently, the ratio of combustibles to compressed air in the second combustion zone 301 is less than that associated with stoichiometric combustion. The second portion of compressed air consists, by and large, of the compressed air that flows through passages 323 and 324. As a result of the mixing with the additional portion of the compressed air 30, the combustion gases from the first combustion zone 300 are rapidly cooled so as to quench the combustion products to a temperature below that which permits the generation of thermal NOx. However, the temperature is maintained high enough to complete the combustion of carbon monoxide, hydrocarbons and soot leaving first combustion zone 300. Subsequently, a third portion of the compressed air 30, which flows through passage 325, further quenches the combustion products to a temperature suitable for the hot gas 17 entering the turbine 7.

The swirling compressed air 30 introduced into the combustor by the annular passages 321-325 forms streams of air that provide high velocity, long lasting layers of

cooling air flowing over the liners 312-316. Such layers of cooling air provide more effective cooling of the walls of the liners 312-316 than that associated with the film cooling traditionally directed at the liner walls, for example by corrugations. Such high efficiency cooling allows the liners to better tolerate the high temperatures associated with the increased temperature of the compressed air 30, due to the heating in the heat exchanger 304, and the increased temperature of the hot fuel gas 15, due to the absence of the need for extensive cooling of the fuel gas in the gas cleanup system 5, as previously discussed.

In addition, the cooling of the walls of the liners provided by the swirling compressed air 30 is essentially recuperative -- that is, heat transferred from the liners is returned to the combustion zones 300 and 301 by the vortices 350 and 360. This reduction in heat transfer from the fuel rich first combustion zone 300 maintains a high temperature therein that increases the rate of the chemical reactions that convert the fuel bound nitrogen to N ₂ .

The current invention may be embodied in other specific forms without departing from the spirit or essen¬ tial attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the fore- going specification, as indicating the scope of the inven¬ tion.