GB2109100A - Rotary kiln coal gasifier with tar injection to suppress dust entrainment in gas - Google Patents

Abstract

A method and apparatus is disclosed for reducing entrainment of fines of feed stock material in tar- laden gases produced by a rotary kiln in the gasification of coal or other carbonaceous material. The fines and tar are both separated from the gas and from each other. The separated fines are recycled by admixing and churning such fines with newly provided solid carbonaceous feed material and recycled tar in a screw conveyor (8) prior to introduction of such mixture into the kiln (6). The mixing and churning action in the conveyor (8) causes the tar to coat particles of the newly provided feed material with tar and thereafter cause the fines to adhere to the tar-coated particles to form a composite agglomerate thereof. Such agglomerates when discharged into the kiln (6) by the conveyor (8), will travel a greater distance into the kiln (6) and away from the gas outlet (17) than the distance travelled by discrete fines. Fines adhering to the tar-coated particles as part of a composite agglomerate are, therefore, less likely to be carried out of the kiln by gas flow prior to gasification of such fines than are discrete fines. <IMAGE>

Description

SPECIFICATION Rotary kiln coal gasifier with tar injection to suppress dust entrainment in gas BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an apparatus for agglomerating particulates and fines of carbonaceous material for use as feed in a rotary kiln for the gasification of coal such as described in U.S.

Patent No. 3,990,865 to Cybriwsky and Petersen dated November 9, 1 976. Particularly, this invention is directed toward an improved apparatus for utilizing tar produced from the gasification of coal as a binder which is blended with newly provided coal particles and recycled fines in a screw conveyor prior to being fed into a coal gasification kiln.

Description of the Prior Art It is known to those familiar with the technology, and as disclosed in the aforesaid copending application of P. G. Garside and U.S. Patent No. 3,990,865 to introduce carbonaceous material, such as coal, to a rotary kiln gasifier by feeding the carbonaceous material into the kiln through a screw conveyor located on one end of the kiln and allowing the material to drop from the screw conveyor onto a bed of material within the kiln. However, this practice produces certain problems which adversely affect the efficiency of the gasification process. The end of the kiln where the material is fed into the kiln is the same end of the\ kiln where tar-laden gas produced in the process exits the kiln. The tar-laden gas flows above the bed of material in the kiln and against its direction of travel.Additionally, the direction of flow of the tar-laden gas is generally perpendicular to the direction of travel of coal free-falling from the screw conveyor to the bed of material. Additionally, since such rotary kilns operate at elevated internal-pressures, the ends of the kiln are tapered, or necked-down, in order to keep the diameter of kiln seals to a minimum. The existence of the neck-down at the end of the kiln where carbonaceous material is introduced affects the flow of tar-laden gas exiting the kiln at this end. Namely, the velocity of flow of the tar-laden gas is increased and the flow becomes increasingly turbulent at the point in the kiln where coal is being dropped upon the material bed. Of the coal particles being admitted to the kiln, a portion (i.e., the particulates) are sufficiently small as to be entrainable in the gas flow.Entrainable particulates may be subclassified as including fines, which are separable from gases by conventional cyclone separators, and dust, which cannot be separated from gases by conventional cyclone separators. The flow of the tar-laden gas and, most particularly, the turbulent nature of the flow results in small particulates of coal being entrained in the gas flow and exiting the kiln along with the tar-laden gas.

The entrainment of coal particulates within the flow of the tar-laden gas necessitates equipment to remove the particulates from the gas after leaving the kiln and recycle them to the gasifier-the greater the amount of entrained particulates in the gas then the greater the required capacity and cost of such equipment. Furthermore, a substantial amount of particulates are again picked up by the gas stream and carried out of the kiln. Indeed, the recycled particulates which, due to their size, are particularly susceptible to reentrainment, and particulates introduced with the newly provided coal are entrained resulting in an ever increasing load of particulates in the exiting gas.While conventional equipment such as cyclone separators are satisfactory for separating most coal particles from the gas, they are ineffective for removing the smallest of coal particles, or dust, from the gas necessitating additional and more expensive dust removal equipment. This problem is particularly troublesome since the gas will be destined for use as fuel for combustion turbines in a combined cycle power generating plant. Such combustion turbines are intolerant of even the smallest particulate material in the gaseous fuel as it causes turbine blade damage due to erosion and corrosion.

The use of liquid hydrocarbons to suppress dust and particulates is reported in Steam-lts Generation and Use, Babcox a Wilcox Co., 38th Ed, p. 8-6 (1975), where spraying oil on coal in amounts of 0.1 % to 0.8% oil to coal by weight is shown effective for causing dust to adhere to coal particles.

A process utiiizing tars produced in coal coking and gasification on a horizontal traveling grate is disclosed in U.S. Patent No. t,978,945 to A.J.A. Hereng, dated October 30, 1934. The Hereng process separates dust-laden heavy tar from educted gases and drops the dust-laden tar upon the top surface of a bed of coal on the horizontal traveling grate prior to the grate carrying the coal into a distillation chamber of the gasification process. The purpose of the foregoing is to minimize the amount of particulates falling through the grate, and thereby maximize the conversion of particulates both in the bed and in the heavy tar to fuel gas, as well as redistilling the heavy tar.

U.S. Patent No. 4,239,500 to Ratzeburg, dated December 16, 1980, discloses a process whereby small grain-sized coal particles are separated from larger particles prior to the larger particles being fed into the coal gasification unit. The small particles are agglomerated with dust laden tar derived from the gasification process. The agglomeration occurs in a pelletizer and pellets are subsequently fed into the gasification unit along with the large coal particles.

In U.S. Patent No. 3,560,369 to C. A. Rowland et al, dated February 2, 1971, a process is disclosed which utilizes tar derived in an oil shale retorting process as a binder for agglomerating raw shale fines prior to the agglomerates being fed into the retorting system. In the Rowland et al disclosure, raw shale fines are separated from larger shale particles. The fines are mixed with subsequently derived tar in an agglomerating drum. The agglomerates are introduced with the large shale particles prior to being fed into the retorting system by placing the agglomerates in a layer above and beneath a layer of large shale particles.

U.S. Patent No. 3,692,505 to E. H. Reichl, dated September 19, 1972, suggests the use of tars derived from a fixed-bed gasifier to assist in the pelletization of sized-coal fines in a rotary pelletizer prior to the pellets being introduced into the gasifier.

SUMMARY OF THE INVENTION The object of the present invention is to reduce the entrainment of coal particulates by agglomerating the particulates with larger coal particles in a screw conveyor using tar produced in the gasification process as an agglomerating binder.

It is a further object of the present invention to eliminate the entrainment of coal dust by agglomerating the dust with larger coal particles or fines in a screw conveyor using tar produced in the gasification process as an agglomerating binder.

The present invention will be described as applied as a further improvement to a coal gasification system in which carbonaceous material, such as coal, is gasified in a rotary kiln by processing a bed of coal through at least three treatment zones in the kiln. Coal is fed through the material inlet end of the kiln and enters a preheating zone. In this zone, the bed of coal is dried by the counterflow of hot gases moving above the bed. The coal next enters a devolatilization zone where tar-laden gas is educted from the coal and flows above the bed of coal toward the material inlet end where the tar-laden gas exits and entrains fines and dust from the coal being fed to the kiln. The coal next moves into a gasification zone where tar-free gas is educted from the coal. The tar-free gas is exited through the material outlet end of the kiln.The tar-laden gas is processed to separate entrained fines from the tar-laden gas. The fines may be separated from the tar-laden gas by a conventional cyclone separator. The fines are returned to the kiln through the material inlet end of the kiln. The tar-laden gas is processed to separate the tar from the tar-laden gas and thereby produce therefrom a tar-free fuel gas. The tar may be cooled, condensed and removed from the gas through conventional processes.

The problems that arise with a system such as described in the aforesaid U.S. Patent No.

3,990,865 are solved by an apparatus according to the present invention in which unsized and untreated coal, including an undetermined amount of fines, is fed into a gasification kiln through an inlet port of a screw conveyor. The fines removed by the cyclone are returned to the kiln through a second inlet port in the aforesaid screw conveyor.

The condensed tar is heated to maintain a temperature of 140OF or other suitable temperature to maintain a pumpable liquid and pumped to the screw conveyor discharging the unsized coal and fines into the kiln. The tar is injected into the screw conveyor through a port and the tar flow is adjusted to provide a ratio of tar to coal by weight of at least 0.1%. The mixture of tar, coal particles and fines is admixed and churned to coat the coal particles with tar which then acts as a binder to which the fines adhere to form a composite agglomerate of coal particles, tar and fines.

An agglomerate formed as described, discharged into the kiln by the conveyor will free-fall onto the tumbling bed of material with a minimum entrainment of fines and will travel a greater distance into the kiln and away from the gas outlet than the distance traveled by discrete fines.

Fines adhering to the tar-coated particles as part of a composite agglomerate are, therefore, less likely to be carried out of the kiln by the discharging gases prior to gasification than are discrete fines.

BRIEF DESCRIPTION OF THE DRAWINGS A manner, in which the foregoing and other objects of the subject invention have been achieved, will become apparent as the following description is read in light of the attached drawings in which: Figure 1 is a general material and gas flow and apparatus diagram of a rotary kiln coal gasification process equipped with a single screw conveyor; Figure 2 is a view in cross-section of the material inlet end of the rotary kiln shown in Fig. 1 taken along line ll-ll in Fig. 1; Figure 3 is a general material and gas flow and apparatus diagram of the rotary kiln coal gasification process equipped with two screw conveyors; Figure 4 is a view in cross-section of the material inlet end of the rotary kiln shown in Fig. 3 taken along line lV-lV in Fig. 3; and, Figure 5 is a view in cross-section of a screw conveyor taken along line V-V in Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the accompanying drawings, in Fig. 1, a gasifier 5 is shown as including an inclined refractory lined rotary kiln shell 6. Coal or other solid carbonaceous material is fed from a delivery conduit 7 into a screw conveyor 8 located at the uphill end 9 of the kiln shell 6.

Conventional means well known in the art may be provided to rotate the kiln shell 6. Since means for rotating a kiln shell forms no part of this invention, and is well known in the art, it is not shown in the drawings. The screw conveyor 8 is driven by commercially available drive units 31. The screw conveyor 8 contains a coal inlet port 10, a coal fines inlet port 11, a tar inlet port 12, and a material discharge port 1 3. As depicted in Fig. 5, carbonaceous material within screw conveyor 8 assumes an angular repose. Tar inlet port 1 2 is positioned within screw conveyor 8 above the carbonaceous material. The screw conveyor 8 extends through the uphill end 9 of the kiln 6 having a neck-down portion as shown generally at 32.

The kiln shell 6 is inclined downwardly from the material inlet end 9 to a solid ash discharge end 1 5. This inclination, together with the rotary action of the kiln shell 6, causes the coal to form the bed 1 4 within the kiln shell 6 that slowly moves down the incline as it is gasified.

The solid material inlet end 9 of the kiln 6 is provided with a stationary hood 1 6 having a gas outlet conduit 17. A vent 47 is provided connecting screw conveyor 8 with conduit 17. The ash discharge end 1 5 of the kiln 6 is provided with a stationary hood 1 8 having a gas outlet conduit 19 and an ash outlet passage 20. Suitable means such as a burner 21 is provided in the outlet hood 1 8 to preheat the kiln 6 and the coal to operating temperature during start-up.

The diagrammatic showing contains ports 22 for delivery of air and steam to the rotating kiln shell 6 and coal bed 1 4. The method and apparatus for delivering air and steam to the ports 22 form no part of this invention and are not shown in the drawings.

As will appear from the description to follow, gas is discharged simultaneously from the material inlet end 9 of the kiln 6 and from the ash outlet end 1 5 of kiln 6. A flow of off-gas from the material inlet end 9 of kiln 6 flows through conduit 1 7 and discharges into a particle gas cyclone separator 23, with off-gas free of coal fines then flowing from separator 23 through a conduit 24 to a cleaning facility 25 for condensing and separating tar from the off-gas. Off-gas free of tar flows from the cleaning facility 25 through a conduit 26 to a steam boiler burner or combined cycle power plant (not shown). Tar from the cleaning facility 25 flows through a conduit 27 to a tar reservoir 28 which may incorporate means such as steam coils (not shown) for heating the tar.Fines removed from gases by the separator 23 pass through a conduit 29 to the screw conveyor 8 for return to the gasifier 6. A flow of off-gas from the ash outlet end 1 5 of kiln- 6 flows through a conduit 1 9 for further processing and use as a fuel in a steam boiler burner or combined cycle power plant (not shown).

Tars from the reservoir 28 pass through a conduit 42 to a pump 43 where the tars are pumped through a conduit 44 and flow regulating valve 45 to the screw conveyor 8 for return to the gasifier 5. As an alternative to flow regulating valve 45, the flow of tar may be regulated by a variable speed positive displacement pump.

The screw conveyor 8 extends axially into kiln 6 through the stationary hood 16. As depicted in Fig. 2, the rotary action of the kiln 6 results in the coal bed 1 4 forming an angular repose in the direction of the rotation of the kiln 6. The screw conveyor 8 is positioned away from the center of the kiln 6 and toward the coal bed 1 4 such that the distance from the material discharge port 13 to the coal bed 14 is minimized.

Fig. 3 shows a variation of the embodiment as shown in Fig. 1. Coal or other solid carbonaceous material is fed from a delivery conduit 7 into a primary screw conveyor 36 through a coal inlet port 37. Coal fines from the separator 23 pass through a conduit 29a to a secondary screw conveyor 35 through a coal fines inlet port 34.

The screw conveyors 35, 36 are equipped with tar inlet ports 38, 39. Tar inlet ports 38, 39 are positioned within screw conveyors 35, 36 above the carbonaceous material within screw conveyors 35, 36. The conveyors 35, 36 extend axially into kiln 6 through the stationary hood 1 6. As depicted in Fig. 4, the conveyors 35, 36 are positioned away from the center of the kiln 6 so as to conform with the angular repose of the coal bed 1 4 such that material discharge ports 40, 41 of said conveyors 35, 36 are maintained at a minimum distance above the coal bed 14 in kiln 6. A vent 46 is provided connected screw conveyor 35 with conduit 17.

In the operation of the apparatus of Fig. 1, unsized coal containing fines is fed from a delivery conduit 7 into a material inlet end 9 of a kiln shell 6, through a screw conveyor 8. The coal is discharged from the screw conveyor 8 at a material discharge port 1 3 where the coal is allowed to fall freely onto a bed 14 of coal within the kiln 6. When the kiln 6 is partially filled and a bed 14 of coal has essentially covered the length of the kiln 6, air and steam injection is begun through ports 22.

The process for qualifying solid coal particles in the bed 1 4 in rotatirig kiln 6 produces simultaneously and continuously two distinctly different fuel gas streams from the opposite ends of the single kiln 6. A first of the gas streams which contains substantially all the tars educted from the coal during the gasification process is generated as the coal is heated from 400OF to 1,600OF. This tar-laden gas flows above the coal bed 14 and is discharged at the material inlet end 9 of the kiln 6 through a conduit 1 7. A second gas stream is generated in an atmosphere of steam and after the coal has been heated above approximately 1,600OF and converted to char.This second gas stream is discharged from the ash outlet end 1 5 of kiln 6 through a conduit 19, and is substantially tar free.

The tar-laden gas flows through conduit 1 7 to a particle-gas separator 23 where coal fines are removed from the tar-laden gas. The coal fines so removed flow through a conduit 29 and are fed into the screw conveyor 8 through a port 11 following the delivery of the unsized coal to the conveyor 8. The rotary action of the screw conveyor 8 causes the coal fines to be admixed with the unsized coal. The addition of hot coal particles with the untreated coal may cause drying and evaporation of water contained in the untreated coal. The evaporated water is permitted to pass to conduit 1 7 through vent 47.

The tar-laden gas flows from the separator 23 through dffi conduit 24 to a cleaning facility 25 for separation of the tar from the gas by cooling, condensing ansd removing the tar from the gas through commercially available apparatus. The separated tar flows from the cleaning facility 25 through a conduit 27 to a reservoir 28.

Tar contained in the reservoir 28 is heated to maintain a temperature of approximately 140 to 200OF and passed through a conduit 42 to a pump 43. Said heated tar is pumped through a conduit 44 to the screw conveyor 8 and discharged into said conveyor 8 through a port 1 2 at a point following the addition to said conveyor 8 of coal fines. The positioning of port 1 2 above the carbonaceous material in screw conveyor 8 prevents port 1 2 from clogging with coal particles. The introduction of tar to hot coal particles within screw conveyor 8 may cause evaporation of lighter tar fractiions. The evaporated light tar fractions are permitted to pass to conduit 1 7 through vent 47.

The rotary action of the screw conveyor 8 causes the mixture of tar, coal fines and unsized coal to be admixed and churned. During the said admixing, the coal particles are coated with tar which then acts as a binder to which the fines and dust adhere to form a composite agglomerate of coal particle, tar, fines and dust. The agglomerates so formed travel through the screw conveyor 8 and are discharged into the kiln 6 at the material discharge port 1 3 of the conveyor 8 and allowed to fall freely onto the coal bed 14. The discharge of the heated tar through the port 1 2 into the conveyor 8 is monitored and adjusted through regulating valve 45 to maintain a ratio of tar to coal in the conveyor 8 of at least 0.1 % by weight.

In the operation of the apparatus of Fig. 3, according to this invention, unsized and untreated coal containing fines and dust is fed to screw conveyor 36 through port 37. Recycled tar is admitted to screw conveyor 36 through port 38. The rotary action of the screw conveyor causes the mixture of tar and unsized coal to be admixed and churned. During the said admixing , the coal particles are coated with tar which then acts as a binder to which fines and dust adhere to form a composite agglomerate of coal particle, tar, fines and dust. Coal fines separated from the tar-laden gas in the cyclone separator 23 are passed through conduit 29a and introduced to screw conveyor 35 through port 34. Recycled tar is admitted to screw conveyor 35 through port 39.When the recycled tar is introduced to the hot recycled fines, evaporation of lighter tar fractions may occur, said evaporated tar fractions are permitted to pass to conduit 1 7 through vent 46. Remaining tar fractions are admixed and churned with the hot fines by reason of the rotary action of screw conveyor 35. During the admixing, the remaining tar acts as a binder for agglomerating the recycled fines in conveyor 35.

The inclination and rotary action of the kiln 6 causes the agglomerates to be admixed and churned within the coal bed 14 and move down the incline of the kiln 6 for gasification along with uncoated particles of coal.

From the foregoing, it can be seen that the present invention has achieved the objective of minimizing the amount of coal fines entrained in the tar-laden gas and maximizing the processing of coal fines in the gasifier without the need for presizing coal and agglomerating fines extrinsic to the gasifier.

Other advantages of the present invention will be apparent from the hereinafter described example with regard to controlled fines entrainment of particular carbonaceous material.

EXAMPLE Material: Unsized particles and fines of, 60% Coal-lllinois #6 40% Char Produce: Material fed through a screw conveyor as disclosed. First without and then with admixed recycled, reheated tar.

Screen size Percent By Weight Percent By Weight Openings Passing-No Tar Passing-10% Tar (micrnnsjt) Added Added 75p 1% 150y 3% 300y 6% 600y 12% 1.5% 1,200y 25% 15% 2,400,u 40% 28% The foregoing example illustrates that fines in the size range most easily entrained in turbulent discharge gases at the material inlet end 9 of the kiln 6 and fines most difficult to separate from gas through conventional processes (i.e., 0-600 ) almost totally adhere to larger coal particles, to achieve the object of this invention.

Modifications and equivalents of the disclosed concepts such as readily occur to those skilled in the art are intended to be included in the scope of this invention. Thus, the scope of the invention is intended to be limited only by the scope of the claims such as or may hereafter be, appended hereto.

Claims (9)

1. An apparatus for gasifying particles and fines of carbonaceous material and continuously blending therewith recycled tar to form agglomerates thereof to be used as fuel to the apparatus, said apparatus comprising: (a) a rotary kiln inclined downwardly from a material feed inlet end thereof to an ash discharge end thereof; (b) a screw conveyor projecting into the material feed inlet end of said kiln for feeding a continuous supply of carbonaceous material consisting of solid material particles and fines into the inlet end of the rotary kiln with rotation of said kiln transporting said carbonaceous material therethrough for gasification therein; (c) a stationary hood adjacent to the feed inlet end of said kiln with a gas outlet for discharging tar-laden gas from said kiln counterflow to said carbonaceous material and out the inlet end of the kiln;; (d) a tar separating device connected to said gas discharge port for condensing and separating the tar from the tar-laden gas discharged from the kiln; (e) said screw conveyor having a tar inlet port and (f) a conduit connecting said tar separating device to said tar inlet port of said screw conveyor for admitting tar into said carbonacrous material in said screw conveyor with the conveying movement of said screw conveyor mixing said tar with said carbonaceous material to provide composite agglomerates of tar-coated particles and fines which are fed into said rotary kiln.

2. An apparatus for gasifying agglomerates of carbonaceous particles and fines according to claim 1 comprising a vent in said screw conveyor for permitting gases to escape from said screw conveyor.

3. An apparatus for gasifying agglomerates of carbonaceous particles and fines according to claim 2 wherein said vent is connected to said gas outlet.

4. An apparatus for gasifying agglomerates of carbonaceous particles and fines according to claims 1, 2, or 3 comprising a heater for heating said condensed tar; a pump for transferring said heated tar from said tar separating device to said screw conveyor; and a regulating device for controlling the ratio of tar to particulates and fines admixed within said feeder.

5. An apparatus for gasifying agglomerates of carbonaceous particles and fines according to claim 4 wherein said regulating device comprises a flow regulating valve connected between said pump and said feeder.

6. A method of gasifying carbonaceous material continuously blending particulates of carbonaceous material with tar to form agglomerates to be used as feed for a gasifier in the production of gas from such carbonaceous materil, the method comprising the steps of; (a) providing and feeding a continuous supply of carbonaceous material consisting of solid material particles and fines into an inlet of a gasifier having at least preheating and devolitization zones, for transport through said gasifier and gasification therein; (b) discharging tar-laden gas from said gasifier counterflow to said carbonaceous material and out the inlet of the gasifier; (c) separating the tar from the tar-laden gas discharged from the gasifier;; (d) admixing and churning said tar with newly provided particles of carbonaceous material and fines to coat said newly provided particles with said tar and adhere said fines to the tar-coated particles and thereby provided a composite agglomerate of particles, tar and fines; and (e) feeding said agglomerates into said gasifier whereby said agglomerates carry said fines against the flow of the discharging gas and into the devolitization zones.

7. A method as claimed in claim 6 comprising the step of regulating the amount of heated tar admixed with newly provided particulates of carbonaceous material to at least 0.1% by weight of the particles and fines.

8. Apparatus for gasifying particles and fines of carbonaceous material substantially as hereinbefore described with reference to and as shown in Figs. 1, 2 and 5 or Figs. 3 and 4 of the accompanying drawings.

9. A method of gasifying carbonaceous material substantially as hereinbefore described with reference to and as shown in the accompanying drawings.