SLAG PROCESSING SYSTEM FOR DIRECT COAL-FIRED GAS TURBINES

Abstract: In a direct coal-fired gas turbine system of the type having primary and secondary combustion zones (10, 30) delivering hot combustion gases to an expander section (40) of a gas turbine, a separator structure (20), is arranged between said primary and secondary combustion zones for removing debris from said hot combustion gases, and in pressurized communication therewith, and said separator structure (20) comprises an impact separator (22, 130) for at least separating a portion of molten slag from said hot combustion gases before they enter said secondary combustion zone.

Description: This invention relates to techniques for removing waste from direct coal-fired gas turbine systems, and more particularly, to specific processing techniques for removing slag from such gas turbine systems during their operation. Recently, much effort has been directed to developing advanced direct coal-fired gas turbine systems for electric utility applications. One approach is based on a multi-stage, slagging combustor with specified emissions control. Instead of requiring a low-ash, highly beneficiated coal, the slagging combustor concept allows the use of a raw or moderately cleaned, utility grade coal of higher ash content, thus maintaining a lower fuel cost. The economics of such a design are considered more favorable than conventional pulverized coal steam plants. The success of direct coal-fired combustion systems relies heavily on the effectiveness of the slagging combustor itself. The slagging combustor is a modular unit with three principal stages. The first stage is the primary combustion zone or compartment. Coal and pre-heated compressed air, enter the primary zone coaxially through a plurality of injection nozzles which are equally spaced around the combustion chamber. This coaxial injection promotes intense air and coal mixing and rapid particle heat-up and devolatilization, which minimize carbon burn-out time. The incoming coal-air jets converge at the combustor center line and form one vertical jet which impacts the combustor dome. This forms a toroidal vortex which provides the mechanisms for flame stabilization and centrifugal separation of larger ash and slag particles. The separated ash and slag form a stable flowing layer on the combustor walls. The vertical geometry of this stage allows gravity to assist in the removal of the molten slag. Fuel-rich conditions in the primary combustion zone inhibit NO x formation from fuel-bound nitrogen. They also provide the necessary reducing conditions for removal and capture of sulfur. The sulfur sorbent, limestone, dolomite, hematite or magnetite is counter-flow injected into the downstream end of the primary zone. The first stage is designed for a coal particle residence time of about 100 ms (for 75 micron sized particles). The second stage of the direct coal-fired combustor is an efficient impact separator which is closely coupled to the first stage. The impact separator removes particulates carried over with the gas from the primary zone, whether they are sorbent or fine particles of ash. To meet government regulations regarding particulate standards, a slagging cyclone separator has been suggested to be combined with the impact separator. See Diehl, et al. "Development of an Advanced Coal-Fired Gas Turbine Combustor" AVCO Research Laboratory, Heat. Engine Contractors' Conference, June 14, 1988; Loftus, et al. "The Use of 3-D Numerical Modelling in the Design of a Gas Turbine Coal Combustor", AVCO Research Laboratory, presented at the Winter Annual Meeting of the American Society of Mechanical Engineers (December, 1988). This combination of slag and ash removal promotes cleaner inlet gases for the third stage, also referred to as the secondary combustion zone. However, significant heat loss occurs, since the gases are channeled over a relatively large expanse of cooler surfaces of an additional pressure vessel, containing the cyclone separator, located between the exit of the impact separator and the entrance of the secondary combustion zone. Although the additional pressure vessel is water-cooled and refractory-lined for receiving the high temperature gases leaving the impact separator, it causes a substantial additional heat loss from the cycle. Moreover, the combination of impact and cyclone separators necessitates the use of a larger pressure vessel environment, with attendant expense and maintenance. For slagging to occur, temperatures throughout the combustion chamber, and particularly at the walls, must not fall below about (2,600°F) 1427°C, depending upon the type of coal. These temperatures are achieved by employing a proper air to fuel ratio, by providing adequate design for refractory walls, and sufficient residence time for the reacting coal to air mixture. In the third stage of the slagging combustor, complete combustion is accomplished and the combustion gases are tempered to meet turbine inlet requirements. This stage preferably consists of a single module with a pressure shell as is found in the first two stages. To prevent the possibility of refractory particles in the exhaust system passing to the turbine, this section has no refractory lining. Combustion and dilution air are introduced through two rows of injection tubes with approximately sixteen tubes per row circumferentially spaced. While for the most part, the early designs of direct coal-fired gas turbine systems have shown much promise, there is a current need for a more effective way of removing slag particles and ash prior to forwarding gases into the secondary combustion zone. The US-PS4 768 446 discloses a combustion system wherein the combustible hot gas and molten slag issued from the rich zone combustor are separated by guiding it through a coolant screen of water stretching across a throat of a quench stage to cool down the combustible gas and molten slag below the slagging temperature of the coal so that the slag freezes and shatters into small pellets. The cooling down process removes energy from the system so that the effectiveness is reduced. Since gas turbine systems operate at high pressures, i.e. 10, 12 or 14 atmospheres, it is the principal object of the present invention to provide a deslagging procedure which minimizes system high pressure working fluid losses and which can move a high volume of slagging products out of the gas turbine systems within a relatively short time and with a minimum of heat and gas losses for maintaining high efficiency. With this object in view, the present invention resides in a direct coal-fired gas turbine system of the type having a primary combustion zone for burning coal in the presence of compressed air to produce hot combustion gases and debris, such as molten slag, and a secondary combustion zone for the lean combustion of said hot combustion gases, said secondary combustion zone coupled to a gas turbine for generating power, wherein a separator structure comprising a cyclone separator formed by a pressure vessel of circular cross-section having an inner wall with a refractory lining is arranged between said primary and secondary combustion zones in pressurized communication therewith for removing said debris from said hot combustion gases, characterized in that an impact separator consisting of a deflector having a water-cooled refractory surface is disposed within said pressure vessel adjacent the vessel wall so as to be impacted by the combustion gas including the molten slag from said primary combustion zone as it enters said pressure vessel and to direct said molten slag onto the inner wall of said pressure vessel while diverting the combustion gas in a circumferential direction along the vessel wall, for separating said slag in molten form from said hot combustion gas before it enters said secondary combustion zone. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a diagrammatic view of a direct coal-fired gas turbine system including an impact slag separator, quenching and processing mechanisms for solidifying molten slag and forming a slurry therefrom, and a settling tank and recycling system for conserving water; Figure 2 is a partial front diagrammatic view of a direct coal-fired gas turbine system including cyclone separation means with an integrally disposed impact separator; and Figure 3 is a partial side diagrammatic view of the turbine system of Figure 2. Referring now to Figure 1, a direct coal-fired gas turbine system 100 includes a primary combustion zone (rich) 10, impact slag separator region 20, final combustion zone (lean) 30, combustion turbine 40, quenching apparatus 50, slurry processing apparatus 60, and recirculation system 70. The primary combustion zone 10 includes injector 11 for primary combustion of air and coal. It also may include sorbent ports 12 for receiving sulfur sorbent, e.g., limestone, dolomite, hematite, magnetite, etc. The injector 11 preferably is one of four separate coaxial injection nozzles equally spaced around the periphery of the combustor. Coaxial injection promotes intense mixing of the reactants and rapid coal particle heat-up and devolatilization. The inflowing jets coalesce near the combustor center line and form a vertically directed jet, which impinges the stagnation region at the top of the combustor dome. The reaction flow is turned to form a radial wall jet, which in turn, forms a toroidal vortex in the corner of the dome. The toroidal vortex mixes the incoming coal and air flow with hot combustion products, thus providing for favorable ignition characteristics and promoting highly efficient, rapid and stable combustion. Jet impact and radial acceleration of the recirculating vortex flow in the upper dome is the prime mechanism for inertial separation of particles from the combustion gases onto the slagging walls. A small amount of wall burning of large char particles deposited on the walls with the ash material usually occurs. The combustion products leave the dome as a subsonic flow. The combustor second stage consists of an impact separator region 20, which is closely integrated to the primary zone 10. The fuel-rich gases exiting the primary combustion zone 10 are accelerated through a nozzle 13 and directed at a collector plate 22, or center body, which is perpendicular to the flow. Particles which are too large to follow the gas stream lines, impinge on the collector plate 22 and are separated from the gas stream. The slag 21 flows down the collector plate 22, while the gas exits above the collector plate 22. Leaving the impact separator 20, the molten slag 21 is water-quenched by a plurality of nozzles 51 of the quenching zone 50. This cools the slag until it is substantially solid pieces 54. These pieces 54 drop into a water bath 53. A moving chain conveyer 62 in the bottom of the water bath 53 moves the chunks of slag 54 to a series of roller crushers 61 in the slurring processing region 60. The crushed slag is slurred in water, preferably with impeller 63 and allowed to return to atmospheric pressure by passing out of the apparatus through a pressure let-down valve 64. In a further embodiment of the invention, the slurry exiting the pressure let-down valve 64 is fed into a recycling zone 70. The slurry is preferably pumped employing pump 75 into a settling tank 71. The settling tank 71 separates out the water 72 from the slag particles 73, to enable the removal of slag tailings and the recirculation of water back to the system. The slag tailings can be removed in a road or rail vehicle 74, or equivalent conveyer or transportation means. The recycled water can be pumped via pump 76 and 78 and heat exchanger 77 either back into the slurring step or back into nozzles 51 for quenching molten slag 21. At the top of the slag separator, the hot gases having been freed of their loading of slag particles, pass on into the final, or lean combustion zone 30. Here they are mixed with additional compressor discharge air 34, through bellmouth scoops 35, in a can-type combustor, re-ignited, and burned to completion, this time in a lean-burn mode. Discharge air from the combustion turbine compressor exit (not shown) passes into annulus 45, which is circumscribed by the combustion turbine cylinder 42, which carries said air to annular duct, 34. Compressor discharge air is removed through nozzles 31 and conveyed through pipes (not shown) back into the injectors 11 for primary zone operation. The combustion gases 33 exiting the combustion chamber 32 of the final combustion zone 30 then pass to the annular duct 41, circumscribed by the metal cylinder 46, which carries them to the expander inlet. Not shown in Figure is the second slagging combustor apparatus, which is a mirror image of the first one shown. Combustion gases 44 are received from the second slagging combustor apparatus. Approximately half the compressed air exiting from the combustion turbine compressor, 43, feeds the second slagging combustor (not shown). Referring to Figures 2 and 3, there are shown in front and side views, comprising cyclone separators which are disposed between the primary and secondary combustion compartments of a turbine system. Such a cyclone separator 120 includes an integral impact separator 130 for separating molten slag from the hot combustion gases of the primary combustion zone 110. Partially burned combustion gases exit primary zone 110 through a nozzle at high velocity. They preferably strike a water-cooled refractory-coated deflector 132. The deflector 132 separates much of the slag, due to the sudden change of the gas stream direction. The slag 115 flows down the deflector 132 to the refractory walls of the cyclone separator 120, and then to a slag disposal system at 117, such as that described with respect to Figure 1. After leaving the deflector 132, hot gases spiral downward in the cyclone separator, losing additional slag and particulate matter to the slag-coated walls due to centrifugal forces. After losing the particles, the gases return to the top of the cyclone separator and exit through a vortex-finder exit pipe 122. After leaving the cyclone separator 120, the gases turn and pass into the lean combustion zone 125, where additional air is added and combustion is completed. The hot combustion products from the lean zone 125 then pass to an expander section of the gas turbine, as previously described with respect to Figure 1.

Claims: A direct coal-fired gas turbine system of the type having a primary combustion zone (10) for burning coal in the presence of compressed air to produce hot combustion gases and debris, such as molten slag, and a secondary combustion zone (30) for the lean combustion of said hot combustion gases, said secondary combustion zone (30) coupled to a gas turbine for generating power, wherein a separator structure (20, 120) comprising a cyclone separator (120) formed by a pressure vessel of circuit cross-section having an inner wall with a refractory lining is arranged between said primary and secondary combustion zones in pressurized communication therewith for removing said debris from said hot combustion gases, characterized in that an impact separator (130) consisting of a deflector having a water-cooled refractory surface is disposed within said pressure vessel adjacent the vessel wall so as to be impacted by the combustion gas including the molten slag from said primary combustion zone as it enters said pressure vessel and to direct said molten slag onto the inner wall of said pressure vessel while diverting the combustion gas in a circumferential direction along the vessel wall, for separating said slag in molten form from said hot combustion gas before it enters said secondary combustion zone. A system according to claim 1, characterized in that a quenching structure (50) is associated with said separator structure (20) for solidifying said molten slag after removal thereof from the combustion gas by said impact separator (20) and also a processing structure (60) for forming a slurry from said solidified slag thereby facilitating removal of said solidified slag from said system. A system according to claim 2, characterized in that said quenching structure (20) comprises nozzles (51) for applying a water spray to said molten slag for aiding in the solidification of said molten slag, and a water bath (53) disposed below said impact separator (20) for receiving said solidified slag. A system according to claim 2 or 3, characterized in that said processing structure (60) comprises crusher means (61) for crushing said solidified slag into slag particles, and a conveyor (62) for transferring the solidified slag to said crushing means (61). A system according to claim 2, 3 or 4, wherein said system is maintained at an operating pressure greater than atmospheric pressure within said system, characterized by a pressure let-down valve (64) for releasing said slurry from said system into atmospheric pressure environment. A system according to claim 5, characterized in that a settling tank (71) is associated with slurry processing structure (60) so as to receive said slurry from said pressure let-down valve (64) and for substantially separating said water from said slag particles. A system according to claim 6, characterized by recirculation means (76, 78) for pumping a portion of said water form said settling tank (71) to said nozzles (51) or said slurring mean (63). A system according to any one of claims 1 to 7, characterized in that said pressure vessel comprises a vortex-finder exit pipe having an open end disposed in said pressure vessel below said impact separator, for receiving the hot gases exiting said vessel.

Inventors: PILLSBURY, PAUL WALTER
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1990
Main classification: F23J1
IPC codes: F23J1, F02C3, F23J15, F23C6