US 3847566 A
Fines entrained by the product gas from a fluidized bed reactor used for the gasification of coal or similar material are separated from the product gas stream and fed to a transfer line burner in which carbonaceous solids withdrawn from the fluidized bed are heated before being returned to the reaction vessel. The fine particles burn preferentially in the presence of air admitted to the burner to provide heat for the process. Fines carried overhead with the combustion gases from the burner are separated and recycled to the burner until substantially all of the carbon has been consumed. This results in more complete utilization of the carbon in the feed material and greater thermal efficiency for the process.
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United States Patent [191 Wilson Nov. 12, 1974 221 Filed:
 Assignce: Exxon Research and Engineering Company, Linden, NJ.
Apr. 12, 1973 21 App]. No: 350,668
Edward L. Wilson, Baytown, Tex.
 US. Cl 48/202, 48/197 R, 252/373  Int. Cl. Cl0j 3/90  Field Of Search 48/202, 204, 206, 210,
48/197 R, DIG. 4; 201/38; 252/373  References Cited UNITED STATES PATENTS 2,654,665 10/1953 Phinney..... 48/206 2,729,552 1/1956 Nelson et a1 48/197 2,884,303 4/1959 Metrailer 23/1 2,761,772 9/1956 Atwell 48/203 com. FEED Ludcmzm 48/206 Welly, .lr. ct al. 48/197 [5 7 ABSTRACT Fines entrained by the product gas from a fluidized bed reactor used for the gasification of coal or similar material are separated from the product gas stream and fed to a transfer line burncr in which carbonaceous solids withdrawn from the fluidized bed are heated before being returned to the reaction vessel. The fine particles burn preferentially in the presence ofair admitted to the burner to provide heat for the process. Fines carried overhead with the combustion gases from the burner are separated and recycled to the burner until substantially all of the carbon has been consumed. This results in more complete utilization of the carbon in the feed material and greater thermal efficiency for the process.
18 Claims, 1 Drawing Figure DPRODUCT GAS BURNER GAS STEAM PATENTEUNUY 12 1914 F|NES RETURN GAS COAL FEED STEAM STEAM STEAM STEAM F LUIDIZED BED GASIFICATION PROCESS WITH REDUCTION OF FINES ENTRAINMENT BY UTILIZING A SEPARATE TRANSFER LINE BURNER STAGE BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to the gasification of coal and other carbonaceous solids and is particularly concerned with fluidized bed gasification processes in which transfer line burners or similar devices are used to generate heat required to react the feed material with steam.
2. Description of the Prior Art One of the problems encountered with fluidized bed processes for the gasification of coal and similar carbonaceous solids is that of maintaining the required gasification temperatures. The reaction of steam with carbon is endothermic and hence such processes normally require the continuous input of large quantities of heat. It has been suggested that this heat can be generated by continuously withdrawing a stream of particulate solids from the fluidized bed gasification vessel, passing the solids through a transfer line burner with sufficient oxygen to burn a portion of the carbon and heat the particles to the desired bed temperature, and then recycling the heated, unburned solids to the gasification vessel. One of the characteristics of such a system is that it results in the production of fine particles which are entrained in the gas streams withdrawn from the reactor and burner. If these solids contain substantial quantities of carbon, they must be recovered and utilized to avoid excessive carbon losses and low thermal efficiencies. The conventional method for handling the fines is to remove them from the gas by means of cyclone separators and return them to the gasification vessel. Since the fine particles elutriate through the fluidized bed very rapidly, almost at the velocity of the gas, the holding time is quite small and hence little additional carbon is gasified. The quantity of fine material recirculating through the bed therefore builds up, becomes excessive, and creates operating difficulties. It has been suggested that these difficulties might be alleviated by burning the fines separately in a-slagging-type combusthe fluidized bed at the required reaction temperature.
The fluidized bed reaction zone will preferably comprise a two-stage reactor including an upper hydrogasification zone in which feed particles of coal or similar finely divided carbonaceous solids are devolatilized to produce liquid and gaseous products and char and a lower steam gasification zone in which steam introduced into the zone reacts with the char particles to tion chamber and then injecting the hot combustion products into a transfer line to heat the solidssream from the fluidized bed. This type of a process is efficient but very difficult to operate and obtain adequate service factors for the equipment.
SUMMARY OF THE INVENTION The present invention provides an improved fluidized bed process which at least in part eliminates the difficulties outlined above. The process of the invention involves the reaction of steam with carbonaceous solids in a fluidized bed reaction zone, the separation of solid fines from the product gas taken overhead from the reaction zone, the continuous withdrawal of solid carbonaceous particles from the reaction zone to a combustion zone, introduction of the fines separated from the productgas into the combustion zone vwhere they are burned preferentially in the presence of the solid particles from the reaction zone to generate heat for the process, and recycling of the hot solid particles from the combustion zone to the reaction zone to maintain form principally hydrogen and carbon monoxide. The product gas taken overhead from the gasifier will thus preferably comprise a methane-rich gas formed by reaction of the coal or similar material with hydrogen gas obtained from the reaction of steam with char. Fines are separated from this product gas and introduced into a combustion zone, preferably a transfer line burner. A stream of char particles is continuously withdrawn from the reaction zone and introduced into the burner. Oxygen supplied to the burner results in the preferential burning of the fines in the presence of the char particles and the generation of sufficient heat to increase the temperature of the solid particles to the desired level. The heated particles flow from the burner and re turn to the gasifier for further reaction with steam; while combustion gases containing entrained fines are withdrawn overhead from the burner or combustion zone.
The hot combustion products taken overhead from the burner are preferably passed through a series of cyclone separators or similar solids separation devices which are successively more efficient in removing finer particles from the gas. The fines taken out of the gas in the initial separation zones are recovered by means of diplegs or similar devices and entrained in a carrier gas, preferably flue gas, for return to the burner where they serve as additional fuel. The fines removed from the gas in the final separation zone will have relatively low carbon contents and are preferably passed through a heat exchanger and then withdrawn from the system as ash. This recycle of the fines to the combustion zone until the carbon content has been reduced to a suitably low level alleviates difficulties encountered when the fines are returned to the fluidized bed reaction zone, results in better utilization of the carbon in the feed to the reaction zone, improves the overall thermal efficiency of the gasification process, and alleviates pollution prob lems which may otherwise be encountered upon discharge of the flue gasfrom the combustion zone. These and other advantages make the process of the invention considerably more attractive than gasification processes employed in the past.
BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE in the drawing is a schematic flow sheet of a process for the production of a methanerich gas from coal which is carried out in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawing depicts a process for the production of a product gas of relatively high methane content from bituminous coal, sub-bituminous coal, lignite, solid petroleum residuum, or a similar carbonaceous solid. The solid feed material employed-in the process, preferably a bituminous or lower rank coal, is introduced into the system through line 10 from a preparation plant in which the coal or other material may be dried, ground and screened or from a storage facility which is not shown in the drawing. To facilitate handling of the solid feed material in a fluidized state, the coal or other carbonaceous solid is introduced into this system in the form of finely-divided particles which are generally less than about one-eighth inch in diameter.
The process of the invention is operated at elevated pressure preferably from about 50 to about 1,000 psig and hence the coal or other feed material introduced into the system through line is fed into vessel 11, from which it is discharged through star wheel feeder or similar device 12 in line 13 at the system operating pressure or at a slightly higher pressure. In lieu of or in addition to this type of an arrangement, parallel lock hoppers, pressurized hoppers, or aerated standpipes operating in series may be used to raise the input stream of coal or similar solids to the system operating level. Equipment for the handling of solids at elevated pressures which will be suitable for purposes of the invention has been described in the literature and will be familiar to those skilled in the art.
A feed gas stream is introduced into the system through line 14 to permit the entrainment of solid particles from line 13 and introduction of the solids into gasifier 15. High pressure steam or product gas may be employed as the feed gas stream. The use of recycled product gas increases the hydrogen concentration in the gasifier and improves methane yields and is therefore normally preferred. The feed gas stream is introduced into the system at a pressure between about 100 and about 1,500 psig, depending upon the pressure at which gasifier 15 is operated and the solid feed material employed. The feed stream is introduced into the gasifier through one or more shrouded nozzles 16 into which steam is injected through line 17 to keep the nozzle temperature below about 600 F. and thus minimize difficulties which may otherwise be encountered due to fouling of the nozzle with agglomerating coal solids. If an agglomerating coal feed material is employed, an injection nozzle designed to promote intimate and extremely rapid mixing of the injected coal with the hot fluidized solids present in the gasifier will normally be used. Nozzles especially designed for such service have been described in the literature.
The gasifier vessel 15 which is employed in the system shown in the drawing comprises a refractory-lined vessel containing a fluidized bed of char particles introduced into the lower part of the vessel through inlet line 18. Steam for maintaining the particles in a fluidized state and reacting with the char to produce a synthesis gas containing substantial quantities of hydrogen and carbon monoxide is introduced into line 18 by means of steam inlet line 19. Additional steam is introduced through line 20 and is distributed over the crosssection of the gasifier by grid or similar device 21. The total steam rate will normally range between about 0.5 and about 2.0 pounds of steam per pound of coal feed. The upflowing steam and suspended char particles form a fluidized bed which extends upwardly in the gasifier to a level above the point at which the coal particles are introduced through nozzle 16. The upper surface of this bed is indicated in the drawing by reference numeral 22.
The lower portion of the fluidized bed in gasifier 15 between grid 21 and the level at which coal particles are introduced into the nozzle, indicated generally by reference numeral 23, serves as a steam gasification reaction zone. Here the steam introduced through inlet line 19 reacts with carbon in the hot char to form synthesis gas in accordance with the reaction: H O +C H +CO. At the point of steam injection near the lower end of the gasifier, the hydrogen concentration in the gaseous phase of the fluidized bed is essentially zero. As the steam moves upwardly through the fluidized char particles, it reacts with the hot carbon to produce synthesis gas and the hydrogen concentration in the gaseous phase thus increases. The temperature in the steam gasification zone will generally range between about l,450 and about 1,750 F. Depending upon the particular feed material and particle sizes employed, the gas velocities in the fluidized bed will normally range between about 0.2 and about 2.0 feet per second.
The upper part of the fluidized bed in reactor vessel 15, indicated generally by reference numeral 24, serves as a hydrogasification zone where the feed coal is devolatilized and a portion of the volatile matter thus liberated reacts with hydrogen generated in the steam gasification zone below to produce methane as one of the principal products. The point at which the coal feed stream is introduced into the gasifier through nozzle 16 and hence the location of the steam gasification and hydrogasification zone depends primarily upon the properties of the particular coal or other carbonaceous solid which is employed as the feed material for the process. It is generally preferred to select the nozzle location so that the methane yield from the gasifier will be maximized and the tar yield minimized. In general, the amount of methane produced increases as the coal feed injection point is moved nearer the top of the vessel. The tar formed from the input coal, which hasa tendency to foul downstream processing equipment, normally increases in amount as the coal injection point is moved upwardly and decreases as the coal input point is moved nearer the bottom of the reaction vessel, other operating conditions being the same. The coal feed stream should therefore generally be introduced into the gasifier at a point where the hydrogen concentration in the gas phase is in excess of about 20 percent by volume, preferably between about 30 percent and about 50 percent by volume. To secure acceptable methane concentrations in the product gas stream, the upper surface 22 of the fluidized bed should nonnally be located at a level sufficiently above the nozzle 16 to provide at least about 4 seconds of residence time for the gas phase in contact with the fluidizedsolids in the hydrogasification zone. A residence time for the gas in contact with the solid phase above the coal injection point of between about 10 and about 20 seconds is normally preferred. It will be understood, of course, that the optimum hydrogen concentration at the coal injection point and the gas residence time above that point will vary with different types and grades of feed coal and will also change with variations in the gasifier temperature, pressure, steam rate, and other process conditions. Higher rank coals normally require somewhat more severe reaction conditions and longer residence times to obtain high methane yields than do coals of lower rank. Similarly, high reaction temperatures and steam rates generally tend to increase the hydrogen concentration in the gas phase and thus reduce the solids residence time needed to secure acceptable methane yields from a particular feed coal.
The heat required to sustain the overall endothermic reaction taking place within gasifier l5 and maintain the gasifier operating temperature within the range between l,450 and about 1,750 F. is provided by withdrawing a portion of the char solids from the lower part of the fluidized bed through line 25 and passing this material into the lower end of transfer line burner 26. Steam or flue gas is injected into line 25 through one or more injection lines 27 in the vicinity of bends in the line in order to control the solids flow rate and to promote smooth flow of the solids. The entrained solid particles moving upwardly through line 25 into the burner 26 will be in dense phase flow. Recycle flue gas or a similar gas stream is introduced into the solids stream through line 28 to aid in lifting the particles and provide part of the velocity increase necessary for the transition from dense phase to dilute phase flow. Air or other oxygen-containing gas is injected through line 29 to furnish the oxygen necessary for combustion within the burner and provide additional gas velocity. The oxygen content of the input gas stream introduced through line 29 may be varied so that the required.
amount of heat is generated without producing excessive velocities and burner temperatures. In lieu of injecting oxygen-containing gas through line 29 as indicated in the drawing, it will often be preferred to introduce the oxygen-containing gas at two or more vertically-spaced points along the length of the burner. A plurality of injection nozzles will normally be provided at each injection point in order to secure effective distribution of the gas and minimize the danger of localized overheating which may lead to fusion of the ash in the particles and burner wall fouling.
The combustion reaction in transfer line burner 26 tends to take place at a rate proportional to the external surface area of the char particles and hence fines present in the entrained solids stream are burned preferentially. The amount of oxygen introduced is normally limited to the quantity necessary to generate sufficient heat to raise the temperature of the solids stream to be within the range between about 1,500 and about 1,950F or about 50 to about 300 F., preferably about 200 F., above the solids inlet temperature of from l,450 to about l,750 F. The amount of oxygen thus required will depend in part upon the quantity of solids being circulated, the gas rate, and the amount of heat losses from the burner and can be readily calculated. Because of the rapidity with which the reaction occurs, the injected oxygen is normally consumed in about 0.01 to about 0.1 second. This rapid combustion may produce over-heating in the lower portion of the burner and hence good distribution of the oxygen is necessary.
The hot combustion products and entrained solids reaching the upper end of the burner pass into separation zone 30 where the coarse particles to be returned to the fluidized bed are removed from the gas stream by means of a cyclone separator or similar device and the gas containing entrained fines is taken overhead through line 31. The coarse particles separated from the gas pass downwardly in densephase flow through dipleg 32 and are fed into inlet line 18 at the bottom of the gasifier. Steam or other gas may again be introduced through lines 33 and 34 as necessary to aid in conveying the particles. The hot particles thus reintroduced provide the heat necessary to maintain the reactions occurring within the gasifier and compensate for heat losses which occur.
The product gas formed in gasifier 15 is taken overhead from the gasifier through line 35 and passed into separation zone 36, which will normally comprise a cyclone separator designed to remove from the gas entrained particles greater than about 5 microns in diameter. These particles, which will consist primarily of fines much smaller than the feed coal particles, are withdrawn from the separation zone through dipleg 37 and injected into the stream of char particles being transmitted from the fluidized bed to the transfer line burner through line 25. These fines, along with fines from other sources, serve as fuel for the combustion process taking place in the burner and thus tend to reduce degradation of the larger particles due to combustion. Introduction of the fines into the burner instead of recycling them back to the gasifier as is conventionally done also makes possible better control of the fluidized bed and the use of somewhat higher gas velocities within the bed than might otherwise be feasible. The product gas taken overhead from separator 36 is passed through line 38 to a second, high efficiency separation zone 39 where entrained solids not removed in zone 36 are taken out of the gas stream. The product gas from zone 39 is withdrawn through line 40 and will normally be passed through a heat exchanger or similar device to permit the recovery of heat energy and then treated in conventional downstream facilities to produce a final gas of the desired quality. The solids recovered in separation zone 39 are passed through dipleg 41 and may be combined with the solid stream in dipleg 37 or entrained in a'stream of flue gas or similar carrier gas introduced through line 42 and injected into the stream of char solids through line 28 as shown in the drawing. These fines provide further fuel for the combustion reaction in the burner. Their recovery and utilization in this manner permits an improvement in the carbon utilization for the process and results in greater overall efficiency.
The gases from the transfer line burner, after removal of the entrained solid particles in separation zone 30, are transmitted through line 31 to an initial fines separation zone 43 where the larger fines still containing significant quantities of carbon are removed from the gas stream. These fines are taken off through dipleg 44 and returned to the burner by entraining them in the gas introduced through line 42. This recycling of the fines recovered from the burner gas results in the consumption of additional carbon which would otherwise have to be disposed of and thus improves the process efficiency. If the carbon content of this gas stream becomes low, the stream can be passed directly to the final fines removal equipment to permit heat recovery from the fines and solids disposal. The overhead gas from separation zone 43 is passed through line 45 to a two-stage final treatment for the removal of fines. The larger particles still remaining in the gas are removed by cyclone separator 46 and passed downwardly through dipleg 47 to a heat recovery zone 48. The smaller, lighter particles not taken out in separator 46 are removed from the gas in separator 49 and passed through dipleg 50 to heat recovery zone 51. The burner gas from which substantially all of the fines of significant size have been removed is discharged through line 52 and may be processed for the recovery of heat and the removal of additional constituents necessary to comply with applicable pollution regulations as desired. The two heat recovery zones 48 and 51 will normally be heat exchangers in which water introduced through lines 53 and 54 is converted into steam and withdrawn through lines 55 and 56 for use as process.
steam or for preheating the gases employed in the pro-' cess. The ash recovered through line 57 may be processed as necessary and be used in the preparation of building blocks or other products.
It will be apparent from the foregoing that the invention provides an improved process which is more efficient, results in more effective carbon utilization, and has numerous other advantages over other processes employed in the past. The product gas stream can be scrubbed for the removal of particulates and sulfur compounds and passed to a methanization plant or other unit without extensive further treatment. As a re sult of these advantages, the process has many potential applications.
1. In a process for the preparation of a product gas by reacting steam with carbonaceous solids in a fluidized bed reaction zone wherein said product gas is taken off overhead from said reaction zone and wherein a stream of solid carbonaceous particles is continuously withdrawn from the said reaction zone and heated by contacting said particles with an oxygencontaining gas in a transfer line burner, the improvement which comprises separating solid fines from said product gas, introducingsaid fines into said transfer line burner for combusting said fines with said oxygencontaining gas while in the presence of said stream of carbonaceous particles to heat said carbonaceous particles, and thereafter returning the heated carbonaceous particles from said transfer line burner to said reaction zone.
2. A process as defined by claim 1 including the additional steps of withdrawing gaseous combustion products from said transfer line burner, separating solid fines from said combustion products, and introducing the separated fines from said combustion products into 8. A process as defined by claim 2 wherein said fines are separated from said combustion products in a plurality of stages.
9. A process as defined by claim 2 including the further steps of separating additional fines from said combustion products and cooling said additional fines in a heat exchanger.
10. A process as defined by claim 2 wherein at least part of said fines separated from said combustion products are entrained in a carrier gas before being introduced into said transfer line burner.
l l. A process as defined by claim 2 including the further steps of separating additional fines from said combustion products and withdrawing part of said additional fines as ash.
12. A process as defined by claim wherein said carrier gas is flue gas.
said transfer line burner for combusting with said oxygen-containing gas in the presence of said stream of solid carbonaceous particles.
3. A process as defined by claim 1 wherein said fines separated from said product gas are injected intosaid stream of carbonaceous particles withdrawn from said reaction zone and said stream of solid particles containing said fines is then introduced into said transfer line burner.
4. A process as defined by claim 1 wherein said carbonaceous particles are coal char particles.
5. A process as defined by claim 1 wherein said reaction'zone comprises a gasifier including an upper hydrogasification zone in which coal particles are carbonized and a lower steam gasification zone from which said stream ofcarbonaceous particles is withdrawn and to which said heated carbonaceous particles are returned.
6. A process as defined by claim 1 wherein said oxygen-containing gas is air.
7. A process as defined by claim 2 wherein said fines separated from said product gas and said fines separated from said combustion products are separately introduced into said transfer line burner.
13. A process for the manufacture of a methane-rich gas from bituminous" coal of sirriilar cafbonaceous solid which comprises:
a. introducing finely divided particles of said carbonaceous solid and steam into a fluidized bed reaction vessel maintained at a temperature in the range of from about l,450 to about l,750 F. and a pressure in the range of from about to about 1,000 psig, said vessel containing an upper hydrogasification zone and a lower steam gasification zone; V
b. withdrawing a methane-rich gas overhead from said fluidized bed reaction vessel and removing particulate solids less than about 5 microns in diameter from said methane-rich gas;
c. withdrawing char particles from said steam gasification zone of said fluidized bed reaction vessel;
d. passing said particulate solids and said char particles to a transfer line burner;
e. introducing sufficient oxygen-containing gas into said transfer line burner to burn a portion of said particulate solids and heat said char particles to a temperature in the range between about l,500 and about 1,950 E; and
f. recycling the heated char particles to said steam gasification zone in said fluidized bed reaction vessel.
14. A process as defined by claim 13 including the additional steps of:
g. withdrawing gaseous combustion products from said transfer line burner;
h. separating entrained'fines from said bustion products; and v i. recycling at least a part of the separated fines to said transfer line burner.
A process as defined by claim 13 wherein said finely-divided particles "and said' stearri are introduced into said fluidized bed reaction vessel in a ratio of from about 0.5 to about 2.0 pounds of steam per pound of carbonaceous solid.
16. A process as defined by claim 13 wherein said gaseous comfinely-divided 52656168" are ie s 'iiiafi amr "(frieeighth inch in diameter.
17. A process as defined by claim 14 wherein said entrained fines are separated from said gaseous combustion products in two stages, the coarser fines being removed in a first stage and recycled to said transfer line burner and the smaller, lighter fines being removed in a second stage and withdrawnas ash.
18. A process as defined by claim 17 including the additional step of recoveringheat from said smaller, lighter fines.
UNITED STATES PATENT OFFICE QETINCATE 0F CORRECTION Patent No. 5, 847, 566 Dated November 12, 197% In t bs) Edward L. Wilson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Claim 15, line Bl, cancel less and insert -greater-- Signed and Sealed this thirtieth D f March 1976 [SEAL] A ttes r:
RUTH C. MASON C. MARSHALL DANN Arresting Officer (mmnissimwr of Pare/11s and Trademarks Patent No. 5,847,566 Dated November 12, 1974 Cx) Edward L. Wilson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Claim 15, line Bl, cancel "less" and insert -greater- Signed and Sealed thisthirtieth I) 3y 0f March 1 9 76 [SEAL] A ttest:
RUTH C. MASON C. MARSHALL DANN Allesting Officer (ummissivner oj'Patenls and Trademarks
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