Patent Publication Number: US-3971639-A

Title: Fluid bed coal gasification

Description:
The invention will be more completely understood by reference to the accompanying drawing. As shown in the drawing, non-agglomerating carbonaceous materials such as sub-bituminous coal, lignite, anthracite, char, petroleum coke, or other carbonaceous substance enters the process through line 10 and is subdivided to a particle size of preferably about one-quarter inch (0.64 cm) and finer in grinder 12. The maximum particle size may be one-half inch or 1 inch (1.27 or 2.54 cm) or even larger as long as the largest particles have no pronounced tendency to settle and separate from other particles in the gasifier&#39;s fluidized bed. Bituminous coal has the property of agglomeration at the conditions encountered in the gasifier and, therefore, is not suitable as a feed without prior treatment. Agglomeration is caused by temperture and hydrogen atmosphere in the gasifier and refers to the condition of softening of particle surfaces and the sticking of one particle to another. Serious operational problems might occur as a result of formation of massive agglomerates, such as, attachment of larger masses of agglomerates to vessel walls, interfering with desired flow patterns, and attachment to and pluggage of gas distribution grids. Agglomeration of bituminous coal can be prevented by pretreatment, a process in which the surface of the coal particles is oxidized under moderate conditions. Pretreatment to prevent agglomeration of coal particles is well known to those knowledgeable in the art of coal gasification. Following pretreatment, the treated bituminous coal particles, known as coal char, are suitable as feed for the process of this invention. 
     Crushed coal flows from grinder 12 through conduit 14, from which it is caught up by an elutriating gas stream entering through line 16. The entrained coal flows through conduit 18 to vessel 20 in which the larger particle sizes settle as a result of the decreased velocity of the gas. Preferably, most of the finer particles, of about 100 mesh particle size and finer, are elutriated from the coarser particles and continue in upward flow with the gas through overhead line 22. Coarse coal particles drop to the bottom of settler 20 for passage through bottom outlet line 24 and valve 26 to feed lockhopper 28. In this manner, fine particles are removed which, if contained in the gasifier feed, would be quickly elutriated from the gasifier fluid bed, requiring substantially increased gas-solids separating equipment in the costly high pressure-high temperature system. Separation of fine from coarse particles could also be performed by recourse to a massive system of vibrating sieves or screens but such apparatus is unwieldy and costly. 
     Coal enters the pressurized gasifier system by means of feed lockhopper 28 through manipulation of valves 26 and 30. When feed lockhopper 28 is filling, valve 26 is open and valve 30 is closed, and when feed lockhopper 28 is emptying, valve 26 is closed and valve 30 is opened, thereby preventing loss of gasifier pressure. For crushed coal to be continuously supplied to the gasifier, one or more additional feed lockhoppers, not shown are arranged in parallel with lockhopper 28. 
     Crushed coal, primarily of size ranging from about 100 mesh to about one-quarter inch (0.63 cm), flows from feed lockhopper 28 through line 32 to gasifier 34. Gasifier 34 contains a fluidized bed 38 of disperse coal particles which reacts with hot combustion gas and steam rising through grid 168 from combustor 156. The chemical reactions of coal gasification take place at conditions preferably ranging from 1400° to 2000°F. (760° to 1093°C) temperature and 10 to 500 psi (0.7 to 35 Kg/cm 2 ) pressure. Average particle residence time will vary markedly depending on its chemical constitution, its initial size, the actual temperature and the composition of reacting gas from the combustor, but approximately 30 minutes will be typical. 
     The choice of reaction conditions is briefly described as follows: At temperatures lower than the preferred minimum, reaction rates are too low and formation of methane is enhanced, which is not desired. At temperatures above the preferred maximum, ash contained in the particles softens, causing agglomeration problems. The minimum pressure chosen is necessary to force the flow of gas through downstream equipment without the need for intermediate compression. The maximum pressure is established on the basis of reliable operation of lockhopper valves and is about the greatest pressure at which lockhopper valves have operated satisfactorily on a commercial basis until this time. The indicated typical residence time is adequate to avoid serious complications which might otherwise result from short-term feed system malfunction, and represents a safety factor by providing ample capacity of carbonaceous substance under reducing conditions to safely separate the oxidizing combustor zone into which oxygen is injected, from the reducing raw gas system. 
     The gasifier fluidized bed 38 has an upper pseudo-liquid surface or interface 40. Some particles, in general smaller than average size, are entrained by rising gas into the space above interface 40. The larger vessel diameter zone 44 causes a reduced velocity of gas flow, permitting some of the entrained particles to drop back into the fluid bed 38. Gasifier effluent passes through first stage cyclone 46 which includes separation of additional solids from the gas. The separated solids are returned through leg 48 to the interior of fluid bed 38. One or more first stage cyclones 46 may be required in the top space of the gasifier 34. Gas effluent from first stage cyclones 46 passes out of the gasifier through line 50 to one or more second stage cyclones 52. Additional char fines are removed in cyclone 52 and these fines pass through dip-leg 54 to fines quench chamber 56. Only a small amount of the smallest sizes of fines are contained in the gas passing from the second stage cyclones 52. 
     In the gasifier 34 small amounts of tar vapors may be evolved from the coal feed as a result of the high temperatures. Condensation of tar vapors in the gas handling parts of the process could cause fouling, pluggages, and substantially interfere with downstream gas treating and downstream handling of condensed water streams. This is prevented by designing the volume of the gasifier 34 which is above the fluid bed interface 40 so that residence time of gases will be approximately 10 to 20 seconds. As a result of time and temperature in zone 44 any tars and other potential hydrocarbonaceous liquids are thermally cracked to gases and carbon, thereby avoiding a serious problem with which some gasification processes must deal. 
     Even though case is used in choice of feed to the gasifier, some agglomerating constituents may be included in the feed inadvertently or some ash agglomerates may form in the fluid bed as a result of local, short-term deviations from normal operating conditions. If formed, agglomerates are purged from the fluid bed as follows, taking advantage of the property of large particles to segregate at the bottom of the fluid bed. Grate 168 is shaped in the form of an inverted dish for structural strength and to collect any non-fluidized ash agglomerates formed in gasifier 34 and to aid in their concentration and discharge out of gasifier 34. Agglomerates flow through line 190 to classifier 192. A pressurized recycle stream of raw gas taken from line 92 is passed through line 194 to elutriate any fines from agglomerates, and to transport fines separated in the classifier 192 back to the gasifier through line 196. Large agglomerates, free of fines, pass through line 198 to lockhoppers 200 and 202 provided with valves 204, 206 and 208, for maintaining gasifier pressure when withdrawing solids. The hot agglomerates are quenched in lockhoppers 200 and 202 by immersion in water. The resulting agglomerate slurry is removed from the system through line 210 for further processing or disposition, while recycle water is added to lockhopper 200 through line 212. 
     Except for removal of agglomerates through line 190, the removal of ash from gasifier 34 is entirely overhead as finely divided solids entrained in the raw gas. There is no flow of solids or gases downwardly through grate 168. Feed coal particles remain in the gasifier until their carbonaceous content is mostly gasified. Swelling of feed particles due to heat and removal of carbon by gasification creates a fragile particle structure of high ash content which breaks up into fine, low-bulk-density particles as a result of inter-particle contacting in the fluid bed. The fine, high-ash-content particles are carried from the gasification zone by the flow of gases and are separated from the gases outside of the gasifier mainly in the second stage cyclone 52 but also in the venturi scrubber 68 and in the water wash tower 74. The heating value of these particles is recovered by injecting them as fuel into the combustor 156 and thereby supplying part of the heat needed for gasification. In the combustor 156 the ash contained in the particles is melted and withdrawn from the system as slag through lower throat 164. 
     All of the coarser particles are removed by passage of the raw gas through first stage cyclones 46 and second stage cyclones 52. The high temperature of the gas is reduced and the sensible heat content recovered by heat exchange of the gas with boiler feed water in steam generator 60. Boiler feed water enters the steam generator 60 through line 62 and is converted into process steam which exits through line 64. The steam may be used in the present process, in a different process, for electrical power generation, or for heating as desired. Cooled raw gas flowing in line 66 contains as major gaseous constituents carbon monoxide, hydrogen, carbon dioxide, and water vapor, and as minor constituents ammonia, hydrogen sulfide, methane, cyanides, carbonyl sulfide and possibly, traces of phenols and chlorides. 
     Furthermore, the raw gas also contains some very-fine particulates. To prepare the gas for further treatment it is desirable to cool and condense most of the water vapor and to remove essentially all remaining dust from the gas. The gas in line 66 passes through venturi scrubber 68 where it is scrubbed utilizing condensed reactant steam and recycle water entering through line 70. The mixture of gas, liquid, and particulates formed in venturi scrubber 68 passes through line 72 to water wash tower 74 which is equipped with baffle plates 76. In water wash tower 74 the gas is further scrubbed with water which enters through line 86. Raw gas, free of particulates, is removed from the system through line 92. Subsequent processing of the gas can be performed by a variety of well-known methods, depending on the desired ultimate use of the gas. The gas may be scrubbed for removal of carbon dioxide, hydrogen sulfide, and ammonia using well-known commercial processes. The cleaned gas may be used as medium-heat-content fuel gas, as a reducing gas, or may serve as feed for processing into a hydrogen-rich stream for use in chemical processing, petroleum refineries, steel mills, and coal liquefaction and gasification (for high Btu gas) processes. 
     Water from line 70 injected into the venturi scrubber 68 cools and condenses water vapor entering the scrubber through line 66, in addition to removing fine particulates from the gas. Water separates from the gas in the base of the water wash tower 74 and a reservoir of water 78 is maintained in the tower. This water contains fine particulates which have been scrubbed from the gas, and water soluble gas components such as ammonia, part of the hydrogen sulfide and carbon dioxide, cyanides, chlorides, and dissolved fixed gases. The water is tranferred from the bottom of the wash tower 74 by pump 80 through line 82. A portion of the flow in line 82 enters line 84 and is cooled by air cooler 85 before flowing through line 86 as wash liquor for water wash tower 74 and through line 88 to venturi scrubber 68. Recycle water is added through line 90. 
     The remainder of the aqueous stream in line 82 passes through air cooler 92 and line 94 into slurry thickener 96. As a substitute for or together with fresh makeup water, contaminated or solids-containing water from an external process, such as boiler or cooling tower blow-down water containing slaggable salts, or difficult to treat waste water such as water containing combustible pollutants such as phenols or cyanides, can be charged to thickener 96 through lin 97 as makeup water. For example, an aqueous slurry of waste from a coal liquefaction process, a mixture containing diatomaceous earth used as filter aid, ash, and high sulfur undissolved coal residue from a coal liquefaction process, can be passed through line 97 for use within the present process. The ash contained in any residues from an external process is conveniently slagged with the ash from the coal charged to the present process. In this manner, the present process can supply hydrogen-rich gas to and receive waste from an associated coal liquefaction process. 
     The purpose of the thickener is to produce a clarified, low-solids-content water for recycle within this process for scrubbing, cooling, and quenching of various streams and to produce a thickened slurry of relatively constant content of combustible material. Any other aqueous clarifying means can be utilized in place of thickener 96, such as a centrifuge or rotary filter. Clarified water flows over weir 98 of thickener 96 to trough 100, from which pump 102 discharges it through line 104 to supply the process recycle water system. Recycle water is used in the following locations: enters wash tower 132 through line 146, lockhopper 200 through 212, suction of pump 184 through line 188, combustor 156 through line 160, fines quench 56 through line 122, and venturi scrubber 68 through line 90. Thickened slurry concentrates in the lower portion of thickener 96 and flows through line 106 to pump 108 and is discharged into line 110 and into slurry tank 112 which is equipped with stirrer 114. Slurry tank 112 also receives makeup of slurry from fines quench 56 and coal fines slurry tank 136. These streams may also be routed through thickener 96, if desired. The slurry tank 112 contains the supply of feed slurry which, for best operation, can be adjusted for constant heating value and water content for combustor 156. 
     Particulates separated by second stage cyclones 52 flow through dip-leg 54 and by flapper valve 118 to fines quench tank 56. Fines entering the quench tank 56 are quenched by an aqueous spray entering from line 120. The water slurry of fines 126 collects in the bottom of the fines quench tank 56. This water slurry 126 is recycled by pump 128 into line 120 from which it sprays onto and forms a slurry with particulates, or the slurry 126 is transferred to either slurry tank 112 or to thickener 96. Makeup water to the fines quench tank system may enter as recycle water through line 122 or as non-clarified process water from water wash tower 74 through line 124. 
     Elutriated coal fines from coarse coal settler 20 flow in line 22 to cyclone separator 128 which removes most of the largest particle sizes. Gas carrying the smallest fines discharges from cyclone 128 through line 130 to wash tower 132. Water enters wash tower 132 through line 144 from coal fines slurry tank 136 and as clarified recycle water through line 146. The water scrubs the remaining fine particles from the entering gas which vents from wash tower 132 through line 148, essentially free of particles. Wash water containing fines scrubbed from the gas flows through line 135 into coal fines slurry tank 136. Also entering the coal fines slurry tank 136 are solids separated from the gas by cyclone 128 through dip-leg 134. These solids are also slurried in the tank 136. Water slurry from tank 136 is recycled by pump 138 through lines 140 and 144 for additional gas scrubbing. Pump 138 transfers excess slurry through lines 140 and 142 to the gasifier fines slurry tank 112 or to thickener 96. 
     Combustor fuel which is stored in slurry tank 112 as an aqueous slurry 116 is made up from the following sources: coal fines which are formed during grinding of coal feed from coal fines slurry tank 136, and fine high-ash particulates separated from the raw gas stream and transferred from fines quench 56 and water wash tower 74. In addition, solids from an external process may be introduced through line 97. Practically all of the ash content of raw coal feed plus associated carbonaceous matter is recovered as part of combustor fuel. These fines are not suitable for gasification because of their small size and high ash content. The heating value contained in the fines is usefully recovered in the combustor when burned with oxygen to create the heat needed for gasification of the coarser particles and the heat to generate the steam needed for the gasification reactions. So that the combustor will operate reliably with controlled heat release. slurry 116 carbonaceous solids content is controlled as is water content of the slurry by operation of thickener 96 and by operation of coal grinder 12 for production of greater or lesser amounts of coal fines. 
     The purpose of combustor 156 is to burn fuel to generate the necessary heat for coal gasification and to generate the steam needed for gasification. An additional purpose of combustor 156 is to cause all normally solid ash constituents of the combustor feed to be melted into slag and thereby to be readily separated from the system in a form which is oxidized, of low sulfur content, and stable for environmentally acceptable disposition as land fill or other purposes. An additional purpose of the combustor 156 is to cause the oxidation and destruction of water soluble pollutants such as phenols, cyanides, sulfur compounds, and ammonia contained in process water streams from this process and from external processes, thereby enormously reducing waste water treatment requirements of this and associated processes and providing the means that stringent environmental regulations may be readily met. 
     Combustor 156 is a high-temperature, exothermic, reaction zone which is maintained at temperatures greater than about 2200°F. (1204°C.) and, in any case, high enough that ash contained in the feed is melted into slag, which temperature may be most often between 2400°F. (1316°C.) and 2900°F. (1593°C.). Certainly, combustor temperature must exceed the prevailing temperature in gasifier bed 38 because the combustor supplies heat for the endothermic reaction occurring in gasifier bed 38. Combustor 156 pressure is virtually the same as the pressure of the fluidized bed gasifier 36. 
     An aqueous mixture 116 as a slurry or paste is pumped or injected from slurry tank 112 by pump 150 through lines 152 and 154 into pairs of opposing burners mounted in combustor 156. Slurry water is flashed into steam by radiation from the hot flames and refractory walls of the combustor. Oxygen enters combustor 156 through lines 158 and oxidizes the carbonaceous portion of the fuel in tenths of a second. The high temperature causes ash to melt into slag which collects on combustor walls and flows by gravity to the slag discharge throat 164. Part of the slag forms into tiny molten particles which are swept upward by the combustor gas flow. These entrained molten particles are solidified by injection of quench water sprayed through line 160 into the upper throat of the combustor, causing a moderate reduction in gas temperature. Solidification of entrained slag particles is essential to avoid coating and pluggage of grid 168 and the cool parts of the gasifier. Normally, heat evolved in combustor 156 and contained in combustor gases is adequate to sustain gasifier 34 temperature at the desired level. However, for improved temperature control in the gasifier, additional oxygen may be introduced through line 218 into the gasifier for oxidation within the gasifier fluid bed 38. 
     Flux can be added to combustor feed slurry in slurry tank 112, by means not shown, if required to raise or lower the slagging temperature of ash, salts, metals, diatomaceous earth, or other material being slagged in combustor 156 so that the combustor temperature can be easily maintained in the desired range. 
     Perforated grid 168 supports fluidized bed 38 in gasifier 34, distributes gas flow to the bed for satisfactory fluidization, and constitutes a physical boundary between the combustor zone beneath the reducing zone of the gasifier above. Gas flow is upward through grate 168, and essentially no downward solids flow occurs. The grid 168 is preferably shaped as an inverted dish to concentrate agglomerates that may form in fluidized bed 38 so that they may be readily removed laterally from the system through line 190. 
     Molten slag formed in combustion zone 156 collects on the vessel walls and runs by gravity through the lower combustor throat 168 and falls into slag quench drum 166. Slag quench drum 166 contains a water quench, which is introduced through line 214, into which the molten slag drops, is cooled, and is solidified. Heat given up by the hot slag causes part of the water quench to vaporize, thereby returning heat to the combustion zone in the form of steam. Cooled, solidified slag in slag quench drum 166 passes through crusher 169 to ensure that large particles of solidified slag will not interfere with the operation of or damage lockhopper valves 216 or pump 176. From crusher 169 cooled slag passes into line 170 and slag slurry lockhoppers 172 and 174. The operation of lockhoppers 172 and 174 serves to retain the elevated pressure in the combustor 156 while withdrawing solidified slag in a water slurry. The slag slurry is transferred by pump 176 to a slag thickener and filter system 178 from which dewatered slag is recovered for disposal through line 180. Clarified water is recycled through line 182, pump 184, and line 186 to slag slurry lockhoppers 172 and 174. Recycle water enters through line 188 to make up for moisture losses due to vaporization or to wetting of slag to disposal 180. 
     As a result of the cooling, quenching, and solidified slag, transferral system, most of the heat contained in the molten slag is returned to the combustion zone as steam. It will also be appreciated that any solids such as ash, salts, or diatomaceous earth introduced to the present process through line 97 from another process such as a coal solvent liquefaction process can be conveniently slagged and disposed of together with ash of coal feed to the present coal gasification process and simultaneously the heating value of any carbonaceous material associated with the ash will be recovered to aid in additional coal gasification. 
     It will be apparent from the above process description that the description covers the best mode of performing an integrated gasification process and that the invention has been described within the context of the broad battery limits of a fully integrated gasification process. It will further be apparent that within the overall battery limits of the integrated process individual features of the integrated process can be practiced independently of other features, if desired. For example, the improved fluid bed gasifier system as described and the condensate product gas scrubbing system for removing pollutants and burning these pollutants within the process can be practiced independently of each other. The system for elutriating feed coal fines, slurrying these fines and feeding the slurry to the combustor can be practiced independently of the product gas condensate scrubbing step. And all of these systems can be practiced without introducing into the process either contaminated water from another process or high sulfur coal residue from another process, while each of these latter two features can be practiced independently of the other and of the aforementioned systems. Therefore, each of these independent systems and features are claimed as independent inventions in separate patent applications filed on even date herewith.