Staged furnaces for firing coal pyrolysis gas and char

Abstract: A pyrolyzer unit and two-staged furnaces are provided in a compact furnace assembly for combusting coal to generate power. Coal is reacted in the pyrolyzer to produce a clean hot fuel gas and char. The hot fuel gas is combusted in the first-stage furnace, which contains a high-temperature heat exchanger utilized to heat a pressurized air stream for expansion in a gas turbine and a heat exchanger for superheating steam for expansion in a steam turbine to produce power. Char is combusted in the second-stage furnace and the resulting combustion products are mixed with primary combustion gas from the first-stage furnace to product a secondary combustion gas and preheat the pressurized air stream, and also to superheat steam. The heated air stream is expanded in a gas turbine to produce shaft power to drive an air compressor and an electric power generator. The hot secondary combustion flue gases and hot air exhaust from the gas turbine are used for preheating pressurized water to generate the steam for the steam turbine cycle.

Description: This invention pertains to staged combustion furnaces used for burning pyrolysis gas and char to produce hot combustion gases and generate power from coal in a process utilizing both air and steam turbines. It pertains particularly to two-stage furnaces in which a pyrolysis gas derived from coal is combusted in a first stage containing a high temperature compressed air heat exchanger and steam superheater. Char from the first stage and coal as required, are combusted in a second stage furnace in which the compressed air is preheated and steam is initially superheated. In a high-efficiency coal-fired combined power cycle using a high temperature (Brayton Cycle) air turbine together with a steam (Rankine Cycle) turbine for generating electric power on both turbine shafts, it is desirable to raise the temperature of the Brayton Cycle working fluid (air) to at least 982°C (18⊘⊘°F). This requires high-temperature combustion gases which, when derived directly from coal, will be at a temperature at which both vaporized and molten ash constituents are present. Such ash constituents in the hot gases could cause corrosion and plugging of heat exchanger tubes, which need to be made of a suitable high temperature material e.g. a ceramic such as silicon carbide for the air heat exchanger, and metallic alloy for a steam superheater. Because the cost of such heat exchanger tubing is high, a closely packed heat exchanger is desirable so as to minimize the amount of heat transfer surface required. Processes utilizing staged combustion of coal are known, as disclosed by U.S. Patent Nos. 3,84⊘,354 to Donath, U.S. 4,229,185 and U.S. 4,322,222 to Sass, U.S. 4,312,639 to Johnson, U.S. 4,469,488 to Calderon, and U.S. 4,9⊘⊘,429 to Richardson and GB-A-2 095 762. However, these processes and associated furnace equipment have various deficiencies, so that improved more compact furnace combustion equipment and a process for more efficient staged combustion of coal to generate power has been needed. An improved combustion furnace assembly including a pyrolyzer unit and two-staged combustion furnaces has now been developed, in which coal is first pyrolyzed to produce pyrolysis gas and char. The pyrolysis gas is used to fire a first stage furnace containing an air heater and high temperature steam superheater. The char is used to fuel a second stage furnace in which the combustion gas therein is quenched by the primary combustion gases from the first stage furnace. Therefore according to one aspect of the invention there is provided a staged furnace assembly adapted for combustion of coal to produce hot flue gases for heating air and generating steam, comprising: (a) a first-stage furnace having inlet and outlet connections and adapted for combusting a coal-derived pyrolysis gas to produce a primary combustion gas effluent, (b) a second stage furnace located adjacent to the first stage furnace and adapted for combusting char from the pyrolysis of the coal to produce a secondary combustion gas and for mixing the resulting secondary combustion gas with the primary combustion gas from the first-stage furnace, the second stage furnace having an upper conduit connection for discharging combustion gas and bottom conduit connections for coal feed and ash withdrawal, the second-stage furnace containing within its upper portion a heat exchanger for heating pressurised air and a heat exchanger for heating steam, and (c) a high temperature heat exchanger provided within the upper portion of the first-stage furnace for further heating the pressurised air passing via conduit means from the heat exchanger for pressurised air in the second-stage furnace, the high temperature heat exchanger containing tubes utilizing heat resistant ceramic surfaces. Also according to another aspect of the invention there is provided a process for combusting coal utilising two staged furnaces and generating power, the process comprising: (a) reacting coal feed with air in a pyrolyzer reactor, and producing a clean fuel gas and char; (b) combusting the clean fuel gas with additional air in a first-stage furnace and producing a primary combustion gas, while heating a pressurised air stream in a high temperature heat exchanger therein; (c) feeding the char to a second-stage furnace and burning the char to produce secondary combustion gas; (d) feeding the primary combustion gas to the second-stage furnace to quench and reduce the temperature of the resulting secondary combustion gas produced therein so as thereby to protect heat transfer surfaces, and to preheat the pressurised air stream and superheat steam therein; (e) expanding the heated pressurised air stream in a gas turbine to generate shaft power; and (f) expanding the superheated steam in a steam turbine to produce additional shaft power. In a preferred embodiment, the second stage combustion furnace is provided adjacent to or below the first-stage furnace, and ducts for conveying hot flue gases extend vertically and transversely from the second-stage furnace to heat recovery exchangers, all units being arranged in a compact assembly. For this two-stage furnace assembly, coal feed is first pyrolyzed with limited air in the pyrolyzer reactor unit to produce a fuel gas from the coal volatiles and form char. This fuel gas is combusted with additional air in the first-stage furnace, which contains high-temperature heat exchangers for final heating pressurized air as a Brayton Cycle fluid, and final superheating steam for a Rankine cycle. Char from the pyrolysis reactor is then used to fuel the second-stage furnace, in which a secondary combustion gas is produced and quenched by introducing the primary combustion gases from the first-stage furnace. The combined secondary combustion gas streams are utilized in the second stage furnace to preheat the pressurized air as the Brayton Cycle fluid, before it enters the high temperature heat exchanger located in the first stage furnace for further heating before expansion in a gas turbine to generate power. The secondary combustion gases in the second stage furnace are also utilized to superheat pressurized steam prior to its final superheating in the first-stage furnace and expansion in a steam turbine to generate power. In order to protect the air preheater tubing in the second stage furnace from being exposed to molten ash derived from the char combustion therein, the secondary combustion gases are quenched by the fuel gas primary combustion products from the first stage furnace. Without such gas quenching, it would be impractical to provide heat exchange surfaces in the second-stage furnace for preheating air and superheating steam for efficient use in the gas and steam turbines. This improved furnace assembly for burning coal and generating power according to this invention advantageously provides a compact and cost effective facility for coal combustion and power generation that has increased reliability and reduced installation costs. The invention will now be described with reference to the following drawings, in which: Fig. 1 shows an elevation view of a furnace assembly consisting of a two-stage furnace and auxiliary heat exchangers utilized for coal combustion and fluid heating in a power generation process; Fig. 2 shows a combined process for power generation from coal in which coal feed is first pyrolyzed and the resulting fuel gas and char are combusted separately in two staged furnaces, and heated pressurized air and superheated steam are each expanded to produce shaft power or electricity; and Fig. 3 is a partial flowsheet showing an alternative arrangement for a power generation process from coal, in which the compressed air is intercooled, heat is exchanged between the compressed air and turbine exhaust streams, and the pressurized preheated air is further heated in a fuel fired burner so as to improve the overall cycle efficiency of the process. As shown in the Fig. 1 elevation drawing, furnace assembly 1⊘ utilizes a pyrolyzer reactor unit (not shown) into which coal and a suitable sorbent material can be fed for reaction therein to produce a desulphurized fuel gas. Located downstream from the pyrolyzer unit is a cyclone separator (not shown) used for separating clean fuel gas from entrained pyrolyzer char material. The clean pyrolysis gas from the cyclone separator is piped in through an inlet 14 to a first-stage combustion furnace 16 in which the gas is combusted to produce a primary combustion gas. The first-stage furnace 16 contains a high temperature heat exchanger 18, which is made of a suitable high temperature ceramic material such as silicon carbide or equivalent material, for heating compressed air to be expanded in a gas turbine (not shown) to produce shaft power. In heat exchanger 18 the tubes 18a are closely spaced so as to provide a high surface area in a small volume of furnace. The first-stage furnace 16 also preferably contains a heat exchanger 19 for final superheating steam generated in other heat exchangers (not shown). This superheated steam is conveyed in a piping system (not shown) to a steam turbine (not shown) where it is expanded to generate additional shaft power. The first stage furnace 16 has refractory-lined walls 17 and is located above or adjacent to a larger second-stage combustion furnace 20, in which the char from the pyrolyzer unit optionally together with some additional coal, if necessary, are combusted in a combustor unit 21 and the resulting hot combustion gas is tempered by mixing with relatively cool combustion gas discharged from the first-stage furnace 16. In second stage furnace 2⊘, an air preheater unit 22 is provided in the furnace upper portion for preheating pressurized air, which air is further heated up to 760 to 982°C (14⊘⊘ to 18⊘⊘°F) temperature in the heat exchanger 18 located within the first stage furnace 16. Ash is withdrawn from the lower portion of second stage furnace at 24 as controlled by inclined baffle surfaces 24a. The second stage furnace 2⊘ and associated heat exchangers are suitably supported by structural means 26. From the upper end of the second-stage furnace 2⊘, vertical duct 27, horizontal duct 28 and vertical duct 29 convey hot flue gases past preheater 22 to a primary compressed air preheater 3⊘ for heating a Brayton Cycle compressed air stream. During operation, pressurized air is supplied to the primary air preheater 3⊘ from a compressor (not shown). Ash particulates can be removed at 31 from the primary air preheater 3⊘, as needed. Heated compressed air conveyed by piping 32 from the preheater 3⊘ is further heated in the air heater platens 22 contained within the ducts 27, 28 and 29 and conveyed through piping 33 to the high temperature ceramic heat exchanger 18, from which it is conveyed through piping 34 to a gas turbine (not shown) where it is expanded to generate shaft power. The hot exhaust gas from the gas turbine is used to heat water in feed water heaters and boilers (not shown) to generate steam. The flue gas ducts 27, 28 and 29, as well as the staged furnaces 16 and 2⊘, are lined with tubes 35 terminating in upper and lower headers 36. These tubes, together with screening tubes 38, comprise a primary superheater in which steam generated elsewhere in the power cycle is heated before entering the finishing superheater 19. The screening tubes 38 are bent upwardly in a single plane where they are welded together to form a gas-tight wall 39 separating the first-stage furnace 16 from the vertical duct 27. The second stage furnace 2⊘ has thermally-insulated walls 2⊘a. A thermally-insulated casing or enclosure 4⊘ is provided around the staged furnace assembly 1⊘ so as to minimize undesired heat losses to the atmosphere. This compact and unique furnace arrangement including a pyrolyzer unit together with first and second stage combustion furnaces is advantageously provided in a compact, easily installed assembly for producing power from the combustion of coal reliably and economically. Operation of the two-staged furnace assembly will now be described in greater detail based upon the Fig. 2 process flowsheet. As shown in Fig. 2, coal provided at 5⊘ along with hot air supplied at 51 and optionally a sulfur-capturing sorbent material such as calcium carbonate supplied at 53 are fed to a pyrolyzer reactor unit 52, where they are reacted together to produce a pyrolysis or fuel gas at 54 derived mainly from the coal volatile matter, and to produce partly combusted coal particles or char. The fuel gas at 54 is passed to a gas-solids separator 56 and the unreacted char is removed at 55. The resulting clean fuel gas at 57 is passed to a first stage furnace 58, where it is combusted to produce a primary combustion gas at 59 and to heat a compressed air stream therein to about 982°C (18⊘⊘°F) in high temperature heat exchanger 76 provided therein as further described hereinbelow. From the first-stage furnace 53, the primary combustion gas stream 59 is passed to a second-stage furnace 6⊘, where it is mixed with secondary combustion gases therein resulting from burning char supplied at 55 with additional air supplied at 81 in the second stage furnace 60. The char at 55 includes some coal ash and partially sulphided sorbent material, resulting from the coal pyrolysis reaction at 52. Additional coal that may be required to support combustion of the char in furnace 6⊘ is supplied at 5⊘a, and sorbent may be supplied at 53a. The primary combustion gas 59 serves to quench and cool the secondary combustion gas so as to limit its temperature and prevent damage to heat exchanger surfaces in the second-stage furnace 6⊘. The furnace 6⊘ preheats compressed air at 74 as the fluid for a Brayton cycle to about 760°C (14⊘⊘°F), before it enters the high-temperature heat exchanger 76 in first-stage furnace 58 for further heating as described below. The second stage furnace 6⊘ also superheats pressurized steam provided at 9⊘ in superheater 92, which steam is expanded in steam turbine 94 to produce shaft power. From the second stage furnace 6⊘, ash is withdrawn at 64 and the hot flue gases at 65 are passed successively through heat exchangers 66 and 68 to recover heat by heating recycled condensate water, and the flue gases are then discharged at 69. Atmospheric air at 7⊘ is compressed by compressor 72 and the pressurized air stream 73 is heated by the preheater surface 74 provided in the second stage furnace 6⊘. Heated air stream 75 is further heated in the high-temperature ceramic heat exchanger 76 76 to about 982°C (18⊘⊘°F), and is then expanded in gas turbine 78 to produce shaft power needed to drive air compressor 72 and generate electric power at generator 77. A portion 79a of the turbine expanded air stream 79 is supplied as combustion air to the first-stage furnace 58, another portion is supplied at 51 to pyrolyzer reactor unit 52, and a third portion is supplied at 81 to the second-stage furnace 6⊘. The remaining expanded air portion 8⊘ is cooled successively in heat exchangers 82, 84 and 86 by heating recycled condensate water streams 98a and 99 to form steam, after which the cooled air is discharged to the atmosphere at 87. Because the hot air stream 8⊘ passing through heat exchangers 82, 84 and 86 does not have ash or combustion products entrained therein, stream 87 does not require pollution abatement equipment or provision for corrosion control. The pressurized recycled steam at 9⊘ is further heated in heat exchanger 92 provided in the second-stage furnace 6⊘, is preferably further superheated in heat exchanger 93 in first-stage furnace 58, and the superheated steam is then expanded through steam turbine 94 to produce shaft power and generate electric power at generator 95. The resulting low pressure steam from turbine 94 is condensed at condenser 96, and the resulting condensate water is repressurized by condensate pump 97 and further by boiler feed pumps 67 and 83. A first portion 98 of the pressurized condensate water from pump 97 is heated successively in the economizer exchangers 68 and 66 against the hot flue gases 65, and provides a first portion 98a of the recycled steam at 9⊘. A second portion 99 of the condensate water is heated successively in the heat exchangers 86 and 84 against hot waste gas stream 87, to provide a second portion of the recycled steam at 9⊘. The portions 98a and 99 are combined at 1⊘⊘ so that the full flow of steam at 9⊘ is heated in exchanger 82 against hot waste gas stream 8⊘. The overall thermal efficiency for this coal-fired power generation cycle per Fig. 2 can be improved by utilizing two-stage air compression and also preheating the compressed air stream to the second-stage furnace by utilizing heat from the gas turbine hot exhaust. As shown by the Fig. 3 partial flowsheet, the thermal efficiency of air compressor 72 can be improved by utilizing two stages 72a and 72b of compression separated by an intercooler 11⊘, in which the air is cooled against a cooler stream such as steam condensate stream 98 or 99. Also, the compressed air stream 73 discharged from air compressor 72 can be advantageously heated against the gas turbine hot exhaust stream 79 in a direct or an indirect type heat exchanger 112. If desired, the heat exchanger 112 may be a known type indirect liquid-coupled heat exchanger, which advantageously reduces the compressed air stream 73 pressure drop. Also if desired, the heat exchanger 112 can be replaced by a heat pipe type air heater (not shown). The use of intercooling at 11⊘ for staged air compressor 72 in combination with the heat exchanger 112 not only results in improved cycle efficiency, but also advantageously partially decouples the air turbine cycle from the coal fired equipment, which simplifies start-up procedures for the process. In a further desired arrangement, the preheated compressed air stream 75 which is additionally heated at surface 76 in the first-stage furnace 58, can be further heated from about 982°C (18⊘⊘°F) up to 1093 to 1316°C (2⊘⊘⊘ to 24⊘⊘°F), by utilizing an auxiliary fuel fired burner 114 located upstream of the gas turbine 78 so as to provide hot air stream 115 to the gas turbine. The burner 114 can be fired with a portion of the clean fuel gas 57, or by an auxiliary fuel gas or oil stream provided at 116 along with combustion air provided at 118. Flue gas is removed from burner 114 at 119. This invention will be further described by the following Example. EXAMPLE A furnace assembly is provided which includes a coal pyrolyzer reactor unit, a first stage combustion furnace having a high-temperature air heat exchanger and high-temperature steam superheater located in its lower portion, and a second stage combustion furnace having an air preheater and steam superheater surfaces provided therein. The pyrolyzer unit and first-stage furnace are mounted above the second stage furnace. The first-stage furnace air heat exchanger is formed of tubing made of silicon carbide or equivalent material and adapted to withstand the high temperatures therein. Coal is fed to the pyrolyzer reactor unit where it is reacted with a sub-stoichiometric amount of air and calcium carbonate sorbent material to produce a desulphurized hot fuel gas and char. The fuel gas and char are separated, and the clean gas is combusted in a first-stage furnace to produce a primary combustion gas and to preheat a compressed air stream therein. Both the primary combustion gas and the char are fed to a second-stage furnace. The char is combusted together with additional coal, if necessary, as required to support combustion, and the combustion products therefrom are mixed with the primary combustion gas to generate a secondary combustion gas that is used to preheat the pressurized air stream and to generate and superheat steam. The primary combustion gas serves to limit temperature of the secondary combustion gas by admixture therewith. The hot pressurized air stream heated in the first-stage furnace is expanded in a gas turbine, and the steam superheated in the second and first stage furnaces is expanded in a steam turbine to produce shaft power. Important parameters for the staged furnace assembly and power producing process are as follows: Pyrolysis gas exit temp., °C (°F) 760 (1,4⊘⊘) First stage furnace gas exit temperature, °C (°F) 816 (1,5⊘⊘) Second stage furnace gas exit temp., °C (°F) 410 (77⊘) Air temperature to gas turbine, °C (°F) 982 (1,8⊘⊘) Air pressure to gas turbine, N/m 2 (psig) 1·03x10 6 (15⊘) Steam temperature to turbine, °C (°F) 621 (1,15⊘) Steam pressure to turbine, N/m 2 (psig) 9.25x10 6 (1,35⊘)

Claims: A staged furnace assembly adapted for combustion of coal to produce hot flue gases for heating air and generating steam, comprising: (a) a first-stage furnace (16, 58) having inlet and outlet connections and adapted for combusting a coal-derived pyrolysis gas to produce a primary combustion gas effluent, (b) a second-stage furnace (20, 60) located adjacent to the first-stage furnace (16, 58) and adapted for combusting char from the pyrolysis of the coal to produce a secondary combustion gas and for mixing the resulting secondary combustion gas with the primary combustion gas from the first-stage furnace (16, 58), the second stage furnace (20, 60) having an upper conduit connection (65) for discharging combustion gas and bottom conduit connections for coal feed (50a) and ash withdrawal (64), the second-stage furnace containing within its upper portion a heat exchanger (22, 74) for heating pressurised air and a heat exchanger (92) for heating steam, and (c) a high temperature heat exchanger (18, 76) provided within the upper portion of the first-stage furnace for further heating the pressurised air passing via conduit means from the heat exchanger (22, 74) for pressurised air in the second-stage furnace (20, 60), the high temperature heat exchanger containing tubes utilizing heat resistant ceramic surfaces. The staged furnace assembly of Claim 1 in which the secondstage furnace (20, 60) also has a connection (53a) in its lower portion for feeding a sorbent material into the furnace. The staged furnace assembly of Claim 1 or Claim 2, in which a pyrolyzer reactor unit (52) is provided adjacent to the second-stage furnace (20, 60) for providing the pyrolysis gas to the first-stage furnace. The staged furnace assembly of any preceding claim in which a heat exchanger (19, 93) for superheating steam is provided within the first-stage furnace. The staged furnace assembly of any preceding claim in which a char combustion unit (21) is attached to a lower portion of the second-stage furnace (20, 60). The staged furnace assembly of any preceding claim in which upright and horizontal ducts (27, 28, 29, 65, 69) are connected to the outlets of the second-stage furnace (20, 60) for conveying combustion gases to heat recovery exchangers. The staged furnace assembly of Claim 6 in which screening heat exchanger tubes (38) are provided for separating the first-stage furnace (16) from the upright duct (27). The staged furnace assembly of any preceding claim in which a thermally-insulated casing (40) encloses the first- and second-stage furnaces (16, 20). A process for combusting coal utilising two staged furnaces and generating power, the process comprising: (a) reacting coal feed with air in a pyrolyzer reactor (52), and producing a clean fuel gas and char; (b) combusting the clean fuel gas with additional air in a first-stage furnace (16, 58) and producing a primary combustion gas, while heating a pressurised air stream in a high temperature heat exchanger (18, 76) therein; (c) feeding the char to a second-stage furnace (20, 60), and burning the char to produce secondary combustion gas; (d) feeding the primary combustion gas to the second-stage furnace (20, 60) to quench and reduce the temperature of the resulting secondary combustion gas produced therein so as thereby to protect heat transfer surfaces, and to preheat the pressurised air stream and superheat steam therein; (e) expanding the heated pressurised air stream in a gas turbine (78) to generate shaft power; and (f) expanding the superheated steam in a steam turbine (94) to produce additional shaft power. The process of Claim 9 in which raw combustion gases derived from the pyrolyzer reactor are separated to produce the clean fuel gas combusted in the first-stage furnace (16, 58), and the char is fed to the second-stage furnace (20, 60). The process of Claim 9 or Claim 10 in which the pressurised. air stream preheated in the second-stage furnace (20, 60) is first heated against an exhaust gas stream from the gas turbine (78). The process of any one of claims 9 to 11 in which the pressurised heated air from the first-stage furnace (16, 58) is further heated by a fuel fired burner (114) located upstream of the gas turbine (78). The process of any one of claims 9 to 12 in which additional coal as needed to produce combustion of the char is fed to the second-stage furnace (20, 60). The process of any one of claims 9 to 13 in which a sorbent material is fed to the pyrolyzer reactor to produce a desulphurised fuel gas. The process of any one of claims 9 to 14 in which the steam heated in the second-stage furnace (20, 60) is additionally superheated in the first stage furnace (16, 58).

Inventors: DAMAN ERNEST LUDWIG, FITZGERALD FRANCIS DAVID, ZOSCHAK ROBERT A, DAMAN, ERNEST LUDWIG, FITZGERALD, FRANCIS DAVID, ZOSCHAK, ROBERT A.
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1993
Main classification: F23C6
IPC codes: F23C6, F22B31, C10J3, F01K23