PROCESS FOR POWER GENERATION FROM PRESSURIZED COMBUSTION OF PARTICULATE COMBUSTIBLE MATERIALS

Abstract: A process for the pressurized combustion of particulate solid waste materials in a cyclonic type burner (20) for producing a hot effluent gas used for producing power. In the process, a particulate combustible solid material such as wood chips is pressurized and fed together with sufficient combustion air pressurized to 3-20 atmospheres pressure into a cylindrical shaped cyclonic type burner containing a primary combustion chamber (22) and having a choke portion (25). In the chamber, the fuel is combusted at volumetric heat release rates exceeding about 400,000 Btu/hr ft<3>. The resulting hot effluent gas is effectively air quenched in the choke portion and passed with the additional air to a secondary combustion chamber (30). The resulting cooled gas at 1400-1800<o>F temperature is centrifugally cleaned to substantially remove any fine particulate solids, and is then expanded in a gas turbine (40) driving a compressor (42) for providing the pressurized combustion air needed in the burner. The gas turbine also drives a loading device such as an electric generator (50) to produce electric power.

Description: PROCESS * FOR POWER GENERATION FROM PRESSURIZED COMBUSTION OF PARTICULATE COMBUSTIBLE MATERIALS BACKGROUND OF INVENTION This invention pertains to a process for pressurized combustion of particulate solid combustible materials to generate power. It pertains particularly to a process for the pressurized combustion of particulate solid waste materials such as wood chips in a cyclonic type burner to produce hot effluent gas, which is cooled, cleaned and expanded through a gas turbine driving an air compressor and an electric generator. Some processes for burning particulate solid fuels to produce hot combustion gases for expanding through a gas turbine and generating electric power are known. For example, U.S. Patent 2,625,791 to Yellott describes a process and equipment developed by Bituminous Coal Research, Inc. for pressurized combustion of pulverized coal to pro¬ duce a combustion gas which is expanded in a gas turbine to drive an electric generator. U.S. 2,735,266 to Atherton discloses a basic process and apparatus for the pressurized combustion of wood wastes in dual vertical combustion cham¬ bers to generate hot effluent gas used for driving a gas turbine and electric generator. Also, U.S. 4,152,890 to Weiland discloses a process for pressurized burning of wood to produce an effluent gas which is expanded in separate gas turbines to drive a compressor and also to generate shaft power. However, none of these prior art processes have demonstrated sufficiently high burner heat release rates and high efficiency and reliability to be economically viable and useful. Thus, further improvements are needed in such combustion processes for particulate solid fuels for gene¬ rating useful electric power from the hot effluent gases derived from the pressurized combustion of particulate fuels. SUMMARY OF INVENTION The present invention provided a process for pressu¬ rized combustion of particulate solid combustible materials to produce a hot pressurized effluent gas used for producing power, such as by expanding the gas through a gas turbine to drive an air compressor and a shaft load, ususally an elec¬ tric generator. A particulate "solid combustible material, such as wood chips or coke, is fed by suitable means into an air stream pressurized to 3-atια. and is pneumatically conveyed through a conduit and fed into the inlet port of a cylindrical cyclonic type burner at superficial gas velocity exceeding about 75 ft/sec. Additional combustion air is supplied to the burner tangentially through multiple tuyeres spaced-apart along the burner length at a tangential velo¬ city exceeding about 100 ft/sec. In the burner the par¬ ticles are rapidly heated and combusted with the added secondary combustion air to provide a high volumetric heat 3 release rate exceeding about 400,000 Btu/hr ft and produce a hot effluent gas at an initial temperature below the slagging temperature of the fuel ash, usually about 2600- 2800° F. The hot gas from the burner primary combustion chamber passes through a choke section and is quenched and cooled in a secondary combustion chamber to a temperature suitable for expanding in a gas turbine, usually about 1400- 2000° F. Any remaining particulate solids in the effluent gas leaving the burner are mechanically separated from the gas in a cyclone separator, after which the clean gas is then expanded to a lower pressure through a gas turbine for driving a compressor to provide the pressurized combustion air required in the burner. The gas turbine provides net shaft power output for driving a load, which is usually an electric power generator. The pressurized-combustion process of the present invention is useful for various combustible solids materials, including but not limited to wood bark, wood chips, sawdust, trim, and shavings, petroleum coke, and mixtures thereof. It is an advantage of the process of this invention that the particulate combustible solids, such as wood chips, are pressurized and burned efficiently at high volumetric heat release rates to provide a clean hot effluent gas at controlled temperature suitable for expanding through a gas turbine for producing power. Because of the burner compact¬ ness and high heat release rates, the overall process and equipment is more efficient and cost effective and the process equipment requires less space for a particular power rating than for prior conventional processes. BRIEF DESCRIPTION OF DRAWINGS The invention will be further described by reference to the accompanying drawing, in which: Fig. 1 is a schematic diagram of the basic process for the pressurized combustion of particulate combustible solids to produce power. DESCRIPTION OF INVENTION As shown in the Fig. 1 flow diagram, a source 10 pro¬ vides wood chips having size not exceeding about 0.70 inch major dimension, and preferably smaller than about 0.130 inch, which chips are collected at 11 in the hopper 12 maintained at substantially atmospheric pressure. The chips 11 are fed from the hopper 12 by a variable speed screw conveyor 14 driven by motor 14a into a vertically oriented chute 15, and are then passed to a suitable feeder means 16 for delivering the wood particulate solids material into pressurized conveying conduit 18. Feeder 16 preferably consists of two rotary valves 16a and 16b connected in series and arranged for transferring the particulate solids material by gravity flow from chute 15 into the conduit 18 at a pressure of about 3-20 atm. absolute, and preferably at 4-15 atm. pressure. The pressurized transport air from conduit 17 flows in conduit 18 at 40-120 ft/sec superficial velocity and preferably at 60-100 ft/sec velocity and pneu¬ matically conveys the particulate solids material tangen- tially to the pressurized burner 20. The particulate solids fuel material is fed pnue- matically into burner 20 at near its inlet end through tangential inlet port 21 at superficial gas velocity exceeding about 80 ft/sec and preferably at 90-150 ft/sec into primary combustion chamber 22. Additiona'l combustion air is introduced tangentially into the primary combustion chamber 22 at superficial velocity exceeding about 100 ft/sec, and preferably 110-150 ft/sec, through multiple spaced-apart openings or tuyeres 24a, 24b, 24c, etc., located axially along the length of chamber 22. If pre¬ heating or drying the solids in conduit 18 is desired, such preheating can be provided in heat exchanger 19 using any convenient source of heat such as turbine exhaust gas flowing through a jacket surrounding an elongated heat exchanger. In the combustion chamber 22, the fuel solids are made to swirl around at high rotational velocity exceeding about 80 ft/sec and preferably at 100-150 f /sec and produce high centrifugal forces exceeding about 140 gravitational units 'g', while the particles are rapidly heated by the hot chamber walls and progressively devolatized and burned to produce a hot pressurized effluent gas at a temperature of about 2800° F. The particles are also advantageously retained in the primary chamber 22 for prolonged combustion therein, not only by the high centrifugal forces but also by the effect of choke opening 25, located at the exit end of the primary chamber 22. The choke opening 25 has a • " smaller cross-sectional opening area than the combustion zone 22 so as to prolong the particle solids combustion time therein and thereby provide for more complete combus¬ tion of the particulate fuel solids and produce very high volu etric heat release rates exceeding about 400,000 3 Btu/hr ft of primary chamber volume and preferably 6000,000-3,000,000 Btu/hr ft 3 . It has been found advantageous that the primary combustion chamber 22 should have a length/diameter aspect ratio for the chamber at least about 2.5:1 and usually need not exceed about 10:1 to provide for adequate combustion time for the solids. The combustion chamber inside diameter should be at least about 1.5 ft. for achieving a reasonable throughput rate for the combustible solids material and usually should not exceed about 3 ft. diameter to achieve adequate rotational velocity for the solid particles there¬ in. In the choke section 25 of chamber 22, the hot effluent gas at about 2800° F temperature is mixed with additional combustion air provided through conduit 28, to quench and cool the hot effluent gas to lower temperature such as 1600-1800° F suitable for extended use in a gas turbine. The secondary or quench air is introduced in the choke zone through dual openings-26 oriented in a tangential direction opposite to that for tuyeres 24 in the primary combustion chamber 22, thereby producing highly turbulent shear type mixing of the two streams in the choke zone leading to secondary combustion zone 30. The flow of supplementary air at conduit 28 is controlled relative to combustion air in conduits 23a, 23b,.23c, etc. to the tuyeres 24a, 24b, 24c, etc. by controller 32, which monitors the air flows at flow meters 31a, 31b, and operates control valve 29 in condiut 28. The resulting cooled effluent gas in the secondary com¬ bustion chamber 30, which may still contain a very small concentration of incombustible particulate solids, is passed through a cyclone type separator device 34 for substantially complete removal of such fine solids. The cyclone separator 34 preferably uses an axial flow type element 35 to provide for a more compact separator overall arrangement. From TITUTE SHEET separator 34, a clean hot effluent gas stream at 1600-1800° F temperature is removed at 36, while the particulate solids removed are withdrawn through valve 37 for suitable disposal. The cleaned effluent gas at 36 at 3-10 atm. pressure is then passed through conduit 38 to the inlet of gas tur¬ bine 40, which is connected to drive air compressor 42 for supplying pressurized air source at 44 for the combustion air at tuyers 24 and the quench air at 28. Also, a portion of the compressed air stream at 44 is cooled at 45 against stream 45a sufficient to avoid combustion of the particulate solids such as to about 200° F, usually by heat exchange with ambient air. The air at 47 is further compressed at 46, preferably by a positive displacement type compressor, to a differential pressure such as 2-10 psi and preferably 4-8 psi to provide the pressurized air 17 required in conduit 18 for pneumatically conveying the wood chips into the burner 20. Turbine 40 also rotatively drives a load device 50, which is usually an electric generator for generating electric power. From turbine 40, exhaust stream 41 at near atmospheric pressure and at 900-1000° F temperature can be passed to a heat recovery step at 52 and used as a heat source for generating steam, for heating another fluid used for heating purp.oses, or as a hot gas for preheating and/or drying the particulate feed material in- heat exchanger 19. The gas turbine unit 40 can be divided into two sepa¬ rate turbines each operating at different rotational " shaft speeds, with the first turbine 40a used for driving the compressor 42 at a high rotational speed, and the inter¬ mediate exhaust gas stream at 41a from the first turbine 40a being passed to second turbine 40b which is.gear- connected to an electric generator_ ^ 50 for driving the gene¬ rator at a lower rotational speed. Alternatively, a single shaft type turbine-compressor unit can be used in which both the compressor and electric generator are driven by a single turbine. During start-up of the process, an auxiliary burner (not shown) using a hydrocarbon fuel source such as propane is used to initially heat the refractory walls of primary combustion chamber 22 to a temperature sufficient to ignite the particulate solid fuel introduced at 21. Also, an auxiliary drive motor 54 is used to drive compressor 42 to provide the hot air source initially needed for combustion. Also, air further compressed by compressor 46 is used for initially pneumatically conveying the particulate fuel solids through conduit 18 into the burner 20. The solid fuel pressurized combustion and power gene¬ ration process of this invention will be further described with reference to the following example of operations, which should not be construed as limiting the scope of the invention. EXAMPLE Wood chips and shavings, such as produced from a wood processing mill source and having nominal size of about 1/8 inch, were transferred from an atmospheric pressure col¬ lection hopper through tandem rotary feeder valves into a pressurized transfer pipe operating at about 5 atm. pressure. The wood chips were pneumatically * conveyed at superficial gas velocity of about 80 ft/sec and fed tan- gentially into the inlet end of a horizontally oriented cylindrical cyclonic burner primary combustion chamber having dimensions as shown in Table I below. Pressurized combustion air was also supplied tangentially into the combustion chamber through 6 sets of dual tuyeres spaced- apart axially along the chamber length and at superficial gas velocity of about 100 ft/sec. Numerous observations of burner operation made through viewing ports indicated that the particulate solids were circulated in a swirling he¬ lical flow path in the combustion chamber at calculated tangential velocity of about 100 ft/sec until consumed. In the primary combustion chamber, the wood particles being circulated at the high rotational velocity developed high centrifugal forces of about 200 'g' , which provided for prolonged total combustion of the particles at high Reynolds number and produced high volumetric heat release rates of about 1,800,000 Btu/hr ft 3 . Thus, the solid fuel particles were rapidly devolatized and combusted to produce a hot effluent gas at 2700-2800° .F temperature, which passed through a restricted choke opening at the exit end of the combustion chamber. The resulting hot effluent gas at about 2700-2800° F " temperature was quenched by additional pressurized secondary air injected tangentially into the throat portion of the choke opening. The quench air was injected tangentially in a direction opposite to that of the swirling effluent gas from the primary combustion chamber, thus producing highly turbulent shear type mixing of the two gas streams so that the hot effluent gas was effectively cooled to about 1700° F and then passed into ' a secondary combustion chamber located immediately downstream from the choke. From the secondary combustion chamber, a portion of the cleaned effluent gas containing about 250 ppm (wt.) fine particulate solids was then passed through a centrifugal type gas-solids separator in which the fine particulate solids in the gas were substantially all centrifugally separated and removed. The resulting cooled and cleaned gas -at about 1600° F temperature is then expanded through a gas turbine driving a rotary air compressor to provide the pressurized transport and combustion air-, and also driving an electric generator to produce net electric power. Based on burner operating data and related experience, the projected continuous operating period for this process is in excess of 30,000 hours. Performance data obtained for the pressurized com¬ bustion step and typical performance for the power- producing process of this invention are provided in Table I below: TABLE I Solid Fuel Pressurized Combustion and Process Characteristics Test Unit Prototype Primary combustion chamber: Inside diameter, in. 20 27 Length/diameter, ratio 3 >3 Choke diameter, in. 6 6.5 Wood Chip feed rate, lb/hr 2020 6100 Transport and combustion air flow rate, lb/hr 26,300 85,500 Combustor pressure, psia 66 95 Combustor pressure, atm. abs. 4.5 6.5 Volumetric heat release rate, Btu/hr f 3 1,866,000 1, ,900,000 Quench air flow rate, lb/hr 9,000 85,000 Secondary combustion chamber effluent: Gas Temperature, °F 1780 1780 Solids concentration, ppm (wt.) 250 250 Solids concentration of separator effluent, ppm (wt.) ' 38 30 Gas turbine: Inlet temperature, °F ' 1700 Inlet pressure, psia 90 Exhaust temperature, °F 900 Exhaust pressure, psia 15 Gas flow rate, lb/hr . 176,600 Net power produced, kw 3000 From the above data, it is seen that the present process utilizes improved pressurized combustion of wood chips or other particulate solid combustible material to provide high volumetric heat release rates in the burner. The process also utilizes effective quenching and cooling of the hot effluent gas together with gas-solids separa¬ tion to provide a clean pressurized effluent gas suitable for extended use in a gas turbine to produce electrical power. Although the present invention has been described broadly and also in terms of a preferred embodiment, it will be understood that various modification and * variations can be made to the process which is defined solely by the append¬ ed claims.

Claims: WE CLAIM: 1. A process for the pressurized combustion of particulate solid combustible materials for producing power, comprising: (a) feeding a particulate solid combustible material from an ambient pressure source through metering means and transferring the combustible material into a conveying conduit at a pressure of about 3-20 atm. abs. ; (b) mixing said pressurized particulate solid material with a pressurized transport air stream and ' pneumatically conveying the particulate solids tangentially at superficial gas velocity exceeding about 80 ft/sec. into primary combustion chamber of a cylindrical cyclonic burner; (c) supplying pressurized combustion air into said primary chamber and combusting said particulate solids material in said chamber at high rotational velocity so as to provide a high volumetric heat 3 release rate exceeding about 400,000 Btu/hr ft , and producing a hot pressurized effluent gas containing minimal particulate solids; (d) quenching and cooling said hot effluent gas by mixing with it sufficient quench air in a secon¬ dary combustion chamber to produce a gas mixture at temperature not exceeding about 2000° F; (e) centrifugally separating any existing particulate solids from said gas mixture stream to produce a clean cooled pressurized product gas stream; and (f) expanding the cooled gas stream through a gas turbine driving an air compressor for providing said pressurized transport and combustion air, said gas turbine also driving a mechanical load to produce a net output of power. 2. The pressurized combustion and power producing process of Claim 1; wherein said particulate solid material is wood chips having particle size smaller than about 0.130 inch major dimension and available from an atmospheric pressure source. 3. The pressurized combustion and power producing process of Claim 1, wherein the primary combustion chamber pressure is 4-15 atmospheres absolute. 4. The pressurized combustion and * power producing process of Claim 1, wherein the particulate solid combustible material is fed into the combustion chamber at a tangential velocity of 100-120 ft/sec. 5. The pressurized combustion and power producing process of Claim 1, wherein the particulate solids in the primary combustion chamber are subjected to centrifugal forces exceeding about 140 gravitational units. 6. The pressurized combustion and power producing process of Claim 1, wherein the volumetric heat release rate in the burner is 600,000-3,000,000 Btu/hr ft 3 . 7. The pressurized combustion and power producing process of Claim 1, wherein the flow of quench air is con¬ trolled relative to the flow of combustion air. 8. The pressurized combustion and power producing process of Claim 1, wherein said gas quenching step is provided by quench air flowing tangentially in a direction opposite to the helical flow of said effluent gas through said choke zone. 9. The pressurized combustion and power producing process of Claim 1, wherein said gas-solids separation step occurs in a centrifugal type separator. 10. The pressurized combustion and power generating process of Claim 1, wherein the cleaned mixed gas is expanded in a first gas turbine for driving the air compressor at a first rotational shaft speed and is then further expanded in a second gas turbine for driving an electric generator at a second lower rotational shaft speed to .generate electrical power. 11. The pressurized combustion and power producing process of Claim 1, wherein the hot compressed air for pneu¬ matically conveying the particulate solids to the burner is cooled to below the ignition point of the particulate solids. 12. The pressurized combustion and power producing process of Claim 1, wherein said solid combustible material is transferred into said pressurized conveying conduit by two rotary feeder valves connected in series. 13 ' . The pressurized combustion and power producing process of Claim 1, wherein said solid combustible material is preheated while being pneumatically conveyed through the conduit to the combustion chamber, using heat from the gas turbine exhaust gas. 14. The pressurized combustion and power producing process of Claim 1, wherein a portion of said compressed combustion air is further compressed by 1-10 psi differential pressure to provide said transport air stream'. 15. The pressurized combustion and power producing process of Claim 1, wherein the flow of quench air to the secondary combustion chamber is controlled relative to the combustion air supplied to the primary com¬ bustion chamber. 16. A process for the pressurized combustion of particulate solids combustible materials for producing power, comprising: (a) feeding wood chips having particulate size smaller than about 0.130 inch major dimension from an ambient pressure source through a rotary metering means for transferring the combustible material into a conveying conduit at a pressure of about 4-5 atm. abs. ; (b) mixing said pressurized wood chips with a pressu¬ rized transport air stream and pneumatically conveying the wood chips tangentially into a primary combustion chamber of a cylindrical eyeIonic burner; (c) supplying pressurized combustion air into said primary combustion chamber and combusting said particulate wood chips in said primary combustion chamber at high rotational velocity with excess secondary combustion air to provide a high volume¬ tric heat release rate of 600,000-3,000,000 Btu/hr 3 ft and producing a hot pressurized effluent gas containing minimal particulate solids; (d) quenching and cooling said hot effluent gas by mixing with it sufficient quench air in a secon¬ dary combustion chamber to produce a gas mixture at temperature of about 1400-2000° F; (e) centrifugally separating any existing fine particulate solids from said gas mixture stream to produce a clean cooled pressurized product gas stream; and (f) expanding the cooled gas stream through a gas turbine arranged for driving an air compressor for providing said pressurized transport air and combustion air, said gas turbine also driving an electric generator to produce a net output of electric power.

Inventors: HOFFERT FRANKLIN D, MILLIGAN DAVID J, MORRISON JAMES A, HOFFERT, FRANKLIN, D., MILLIGAN, DAVID, J., MORRISON, JAMES, A.
Applicants: POWER GENERATING INC, POWER GENERATING, INC.
Application year: 1985
Main classification: F02C3
IPC codes: F02C3