US3884649A

US3884649A - Coal pretreater and ash agglomerating coal gasifier

Info

Publication number: US3884649A
Application number: US410476A
Authority: US
Inventors: Charles W Matthews
Original assignee: Institute of Gas Technology
Current assignee: GTI Energy
Priority date: 1973-10-29
Filing date: 1973-10-29
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20

Abstract

Caking coal is converted to fuel gas by treatment in specific pretreatment and gasification zones. Pulverized coal is first pretreated by a mild oxidation in a fluid bed, with heat removal, at a temperature of about 700*-800 *F. to destroy the caking properties of the coal. The resulting gases and the pretreated coal are then passed to a fluid bed gasification zone maintained at the same pressure as the pretreatment zone. The fluid bed is positioned in the lower portion of the gasification zone and converts the coal to a gaseous mixture consisting essentially of CO, H2, CO2, and CH4. However, small amounts of oils and tars are also evolved. The oils and tars in the gaseous mixture, are then thermally cracked in the upper portion of the gasification zone to produce a fuel gas relatively free of oils and tars. A specific feature of the pretreatment zone enables removal of high density inorganic materials.

Description

United States Patent Matthews COAL PRETREATER AND ASH AGGLOMERATING COAL GASIFIER [75] Inventor: Charles W. Matthews, Darien, lll. Attorney, Agent, FirmMlinare, Allegretti, Newitt & Witcoff [73] Assignee: Institute of Gas Technology,

Chicago 57 ABSTRACT [22] Filed: Oct. 29, 1973 Cakmg coal is converted to fuel gas by treatment in PP BIO-1410376 specific pretreatment and gasification zones. Pulver-' ized coal is first pretreated by a mild oxidation in a 52 US. Cl. 48/202; 48/203; 48/206; fluid with heat removah at a tempmwm of 201/9; 201/31 about 700-800 F. to destroy the caking properties of [51] Int. Cl. Cj 3/16 the coal- The resulting gases and the pretreated coal [58] Field of Search 48/202, 197 R, 206, 209, are Passed to a fluid bed gasification Zone main- 48/21O 203; 201/9, 31, 38 41 tained at the same pressure as the pretreatment zone. The fluid bed is positioned in the lower portion of the [56] References Cited gasification zone and converts the coal to a gaseous mixture consisting essentially of CO, H CO and UNITED STATES PATENTS CH However, small amounts of oils and tars are also 2,336,466 12/1943 Chatterton et al 48/206 X evolved The oils and tars in the gaseous mixture, are 2582-712 3 m 53 2? then thermally cracked in the upper portion of the 41 a; 48/202 X gasification zone to produce a fuel gas relatively free 307O515 12/1962 g; 201/9 X of oils and tars. A specific feature of the pretreatment zone enables removal of high density inorganic materials.

6 Claims, 2 Drawing Figures RAW 6 727 TREAT/N6 COAL FEED LOCK 1 /04 5? PfiE-TflfATAlf/VT F01? B/I'l/M/IV0U5 co/ms H 6 arm/W f :1 GENEQAWO/V i 56 AIR 5 7o soups Fit'Dl-R Za 5 A/R STEAM 20 ASH LOCK HOPPER 28 Primary ExaminerS. Leon Bashore Assistant Examiner--Peter F. Kratz (WATER-F/LLED) COAL PRETREATER AND ASH AGGLOMERATING COAL GASIFIER BACKGROUND OF THE INVENTION This invention relates to the production of fuel gas from coal. In particular, this process relates ,to coal gasification wherein the coal is processed through special pretreatment and gasification zones to prevent agglomeration of the coal during gasification and to produce a gas product that is relatively free of oils and tars.

One of the main sources of atmospheric pollutants today is from coal fired electric utility boilers. In these installations, a clean fossil fuel, such as natural gas, is not a practical substitute for coal in the generation of electricity because of its scarcity and cost. Furthermore, the available supply of clean fuel may combat pollution more effectively when used to fulfill residential and small commercial needs.

By way of example, the combustion products of coal contribute one-eighth of the total atmospheric pollutants emitted in the United States, including one-half of the sulfur oxides and one-quarter of both the nitrogen oxides and of the particulates. Sulfur emissions from coal combustion may be reduced by (a) using lowsulfur coal, (b) cleaning high-sulfur coal by physical methods, (c) removing sulfur oxides from coal combustion gases, ((1) removing sulfur during the combustion step, (e) producing de-ashed lowsulfur fuel by solvent processing of coal, and (f) gasifying coal and removing sulfur from the gas before combustion.

The last method, coal gasification with gas cleaning before combustion, appears to offer the greatest reduction in sulfur emission since most of the sulfur in gasified coal appears as hydrogen sulfide. The removal of this hydrogen sulfide presents no great problem, however, since several different commercial gas cleaning processes are available today which can reduce the hydrogen sulfide content of gas streams from coal gasification to less than ppm, and some processes can remove hydrogen sulfide to 1 ppm or less.

As mentioned, processes are known for the conversion of coal to a fuel gas. In Williams, US. Pat. No. 2,805,189, coal is pretreated in a fluid bed prior to gasification at a temperature below about 600F. The resultant off gases are not recovered and a special solids transfer line is provided to insure that these gases do not enter the gasification reactor. This results in a loss of some volatile hydrocarbons, lowers overall process yield, and presents an additional gas stream that must be purified before disposal.

In Howard, US. Pat. No. 2,582,712, particulate coal is pretreated at 900 l400F. in admixture with a large volume of residue from a gasification reaction zone. Specifically, about 30 units of hot residue from the gasification reactionare admixed with a single unit of fresh feed coal to rapidly heat the fresh feed to a noncaking temperature. This process requires the circulation of large amounts of residue thereby increasing the cost of the unit and subjecting the unit to considerable wear due to the abrasive nature of the residue.

SUMMARY OF THE INVENTION It is an object of this invention to provide a process for efficiently converting a caking carbonaceous'mate rial such as bituminous coal, by gasification, to a fuel gas relatively free of oils and tars containing hydrogen, carbon monoxide and methane.

It is another object of this invention to effectively pretreat a coal such as a caking coal, in a pretreatment zone to provide a noncaking feed for a gasification reaction zone wherein the pretreatment and gasification zones are in direct fluid communication and are maintained at the same pressure.

It is still another object of this invention to provide a method for removing gangue and other inorganic solid material from a coal feed prior to gasification.

The process of the present invention relates to the conversion of a finely divided carbonaceous material containing ash such as bituminous coal, to a fuel gas. In addition to a caking material such as bituminous coal, other solid carbonaceous material such as oil shale, tar sands, etc. may be processed according to the present invention. This fuel gas, also known in the art as producer gas or low BTU gas, has a heating value of between 300 BTU per cubic foot and prferably between 250 BTU per cubic feet, and is particularly suited for use in a tired boiler for the generation of electricity. In the first step of this process, a finely divided carbonaceous material, such as a coal pulverized to a size of about 0 X /1 inches to O X /zinches is pretreated by mild oxidation with limited amounts of air at a temperature of about 700 800 F. for a time sufficient to destroy the caking properties of the coal. Preferably the coal is pretreated at this temperature for about 10 to 30 minutes. This pretreatment is effected in a fluidized pretreatment zone and results in the oxidation of the surface of the particles to prevent agglomeration in the subsequent gasification step. Since heat is evolved during this mild oxidation step, it is necessary to withdraw heat from the pretreatment zone to maintain close control on the pretreatment temperature and to avoid inadvertant agglomeration or ignition of the coal particles in the fluidized pretreatment bed. This temperature control is preferably effected by indirect heat exchange with water to produce steam. This steam in turn is produced in sufficient quantities to supply all the steam required in the subsequent gasification step.

The pretreated carbonaceous material and the resultant gases are passed from the upper portion of the pretreatment chamber or zone directly to an adjacent gasification zone which is maintained at the same pressure as the pretreatment zone. The pressure of the gasification zone can be maintained at atmospheric or superatmospheric pressures depending on the quality of the gas desired. Higher pressures, in general, increase the BTU value of the product gas. The pretreated carbonaceous material is then maintained as a fluidized bed in the lower portion of the gasification zone by means coarse particulate material having a recoverable carbon value. This gas-solids mixture is separated in two stages to provide a coarse particulate fraction, and a fine particulate mixture. The coarse particle mixture is returned to the fluidized bed for recovery of the carbon contained therein and the fine particles in turn are passed to the bottom ash agglomerating section of the gasification zone. This agglomeration-section is maintained at a temperature slightly higher than that of the fluidized bed by means known to the art to cause these fine materials to agglomerate with the ash removed from the fluidized'gasification bed.

In addition, this invention removes shale, gangue, dirt, or rocks present in the coal prior to the gasification of the coal. If this material is passed to the gasification reaction, it can interfere with the removal of ash from: the bottom of the fluidized bed and dilute the fluidized bed. Therefore, according to the present invention, this material,.which as a density higher than that of the pulverized coal, is withdrawn from the lower portion of the fluidized pretreatment zone for disposal.

Other objects and embodiments will become apparent by reference to the following more detailed descrip tion of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view of a preferred apparatus used in the present invention and the various process flows embodied therein to convert coal to a fuel gas.

FIG. 2 is a top cross-sectional view of the pretreatment and gasification chambers taken along section .line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION and the off gases containing hydrocarbons and carbon oxides are essentially oxygen free. The temperature within pretreatment zone 8 is carefully controlled between 700800F., preferably between 750800F., by a cooling water stream 18 passing through heat exchange coil 16 positioned within fluidized bed 10. The heat generated in the fludized bed by the controlled combustion or oxidation of the coal converts the water to steam thereby efficiently removing heat from the pretreatment zone as steam stream 20. This steam stream 20 in turn is then passed to the gasification zone in a manner to be described in greater detail hereinafter to supply the steam required therein.

A specific feature of pretreatment zone 8 is the ability to remove slate, gangue, rocks, etc., present in the feed coal from the bottom of the pretreatment zone to prevent the passage of this material to the gasification zone. More particularly, the pulverized coal particles per se have a density appreciably less than the higher density gangue, rocks etc. As a consequence, as the air entering through line 12 passes through the openings in grate 1 1, a selective separation of the lighter coal particles from the more dense gangue particles occurs. The concentration of gangue particles occurs immediately above grate 11 from which they may be selectively withdrawn via line 14 for suitable disposal.

The top portion of fluid bed 10 is allowed flow over and pass through conduit 9 to gasification chamber or Referring to FIG 1, a pulverized caking bituminous --l, rnaintained as a fluidized bed 10 above grate 11 by an 'o x y gen containing gas such as air entering via line 12.

The amount of gas or air which enters pre-treatment zone 8 and the oxygen content of the gas are adjusted to oxidize about 10% of the carbon value of the coal passed to pretreatment zone 8. This oxidation serves to merely oxidize the surface of the coal to prevent the coal from agglomerating when it is passed to the higher temperature gasification zone 30. Typically, about 1 to about 1.5 SCF of oxygen minimum (as an air stream) are used per pound of coal passed to the pretreatment zone. As a consequence, the overall conditions within pretreatment zone 8 result in a deficiency in oxygen zone 30. It should be noted that gasification zone 30 is in direct fluid communication with fluid bed 10. As a consequence, all of the gases generated in fluid bed 10 are passed directly to gasification zone 30, and the pretreatment zone and gasification zone are maintained at the same pressure. The coal passed to gasification zone 30 is maintained as a fluid bed 32 positioned in the gasification section 33 of zone 30 by air and stream entering the bottom portion of the fluidized bed through

conduits

27 and 29 in a manner known to the art which will be described in further detail hereinafter.

A single fluid bed 32 is illustrated in FIG. 1. However, for production of higher heating value gas more than one fluidized bed may be used. In this embodiment, the pretreated coal would flow into an upper fluidized gasification bed which reacts with gases from a lower gasification bed. Unreacted char from the upper bed in turn would flow to the lower gasification bed. The coal in fluid bed 32 is converted by a known reaction with the air and steam to a mixture of hydrogen, carbon monoxide, methane, nitrogen and small amounts of oils and tars. The majority (i.e., -70%) of the steam required in the gasification reactor is withdrawn from the steam in line 20 after intermediate processing, not shown, as generated in pretreatment zone 10, via line 24 and is admixed with air entering via line 26. The resultant air steam mixture is passed to the lower portion of gasification zone 30 via line 27, into a concentric solids free space 35 and passes upwardly through the openings in grate 34 to maintain fluid bed 32 in constant suspension, motion and circulation. Either make up steam or excess steam when produced can be added or removed respectively via line 70. Of course, all of the steam required in the gasification zone can be added from an independent source via line 70.

Fluid bed 32 proper is maintained at a temperature below the agglomeration temperature of the ash in the coal and preferably is maintained at a temperature above l,500 and preferably about 1,700 1,900F. This exact temperature is a function'of. a specific coal processed in the gasification zone and is readily determined without undue experimentation by those trained in the art.

The bottom section of gasification zone 30, however, is maintained at a temperature above the agglomeration temperature of the ash and comprises an inverted conical shaped grate 34 sloped downwardly toward Venturi nozzle 36 in the manner illustrated in .lequier,

et al., US. Pat. No. 2,906,608, the teachings of which of an-air steam mixture entering via line 29 and is in the range in whichv the coal ash becomes sticky. At these high temperatures (i.e. 50 200F. higher than the bed proper) the ash particles stick together and agglomerates are formed which can be withdrawn when they reach a predetermined size via pipe 40 and passed to lock hopper 42. A portion of the steamproduced in pretreater 8 is removed from line 20 via line 22 and admixed with air entering via line 28 and the resultant air steam mixture passed via line 29 to a point below constriction 37 in venturi nozzle 36 to create the high temperatures and separation desired. As described in greater detail in Jequier, et al., this air steam stream in constriction 37 is of sufficient velocity (such a greater than 40 ft/sec and preferably about 60 to 100 ft/sec) to prevent fine coal particles from flowing through constriction 37. The ash particles, however, have a somewhat higher density than the coal particles, and although these ash particles do not initially have sufficient mass to fall downward through the gas flowing in venturi 36, at the higher temperatures present within the throat of this nozzle the individual ash particles agglomerate together until they form a particle having sufficient mass to pass down through constriction 37.

As indicated, the hot agglomerated ash particles are then passed to water filled lock hopper 42 wherein the ash is quenched by water entering lock hopper 42 through line 47. The ash then settles as a layer 43 near the bottom of the hopper and is periodically discharged by closing valve 41, thereby isolating the lock hopper from the gasifier, and, upon the opening of valve 45, the water ash mixture is discharged, as a slurry, through conduit 44.

The gases produced in fluid bed 32, in admixture with the gases from pretreatment zone 8, comprise a mixture of fuel gas (i.e. a mixture of hydrogen, carbon monoxide, carbon dioxide and methane), oils and tars and solid particulate matter. This gas-solids mixture is held in the upper cracking section 38 of the gasification zone, which is maintained at a temperature above 1,000F., preferably l,200-l ,500, for at least seconds to thermally crack the oils and tars that are present into gases and carbon. Cracking of these oils and tars increases the fuel value of the gas and avoids fouling problems in subsequent recovery sections. In any event, the gas solids mixture is passed to cyclone 46 and the larger particles are removed and passed via conduit 48 to fluid bed 32 for recovery of their carbon values. The resultant gas-solids mixture comprising fuel gas in admixture with very fine particulate matter is passed via line 50 to second cyclone 52. In cyclone 52 raw product gas is recovered via line 60 for further art. For example, product-gasstream 60 after suitable heat recovery and pressure reduction can be passed to a-Stretford'unit for removal-ofany sulfurcontained in the gas thereby producing a relatively pollution free fuel.

Fine solids are removed from cyclone 52 ,via line 54 and passed to storage bin 56 Periodically, these fine solids are withdrawn via line by solids-feeder 58 and admixed with the air-steam mixture'of line 29. The resultant mixture is then passed--to-venturi nozzle 36,

below constriction 37, wherein'athe carbon, present on these fine particles is gasificedandthewery fine dust agglomerates with the ashifor withdrawal from the system.' E

DESCRIPTION. OF THE PREFE RED EMBODIMENT 'Tabulated in Table 1 beloware' the product gases that can be produced according to'theprocess, of the present invention, as illustrated schematically in FIG. 1, when processing an Ireland Mine Coal..The product gases are the gases recovered after'heat exchange to convert sensible heat to steam; expansionacross a turboexpander to generate electricity, andsulfur removal by a Stretford process. a

- The operating conditions:

PROCESS CONDITIONS Pretreater Pressure, psia 300 1000 Temperature, F. 800 800 Residence Time, Min. 30 30 Gasifier Pressure, psia 300 1000 Temperature, F. 1900 1900 Residence Time, Min. 4060 40-60 PRocEss PERFORMANCE Gasifier Pressure, psia 300 1000 Product Gas Wet Dry Wet Dry Heating Value, Btu/SCF Composition, ol. 153 164 CO 17.8 19.4 12.5 13.7 C0,, 9.2 10.0 13.6 14.8 H 12.1 13.3 11.6 12.6 H O 8.5 8.5 CH 4.3 4.7 7.1 7.8 N, 48.1 52.6 46.7 51.1 Total 100.0 100.0 100.0 100.0

Thermal Efficiency, I

To all products 865 88.2 To gas only 73.2 73.9 To steam only 12.7 12.6

l. A process for the conversion of a finely divided carbonaceous material containing ash to a fuel gas hav-' ing a heating value of between 100 300 BTU per cubic foot which comprises:

i. pretreating a finely divided carbonaceous feed material by mild oxidation at a temperature of about 700800F for about 10 to minutes to destroy any caking properties of the carbonaceous feed material in a' fluidized pretreatment zone;

ii. said fluidized pretreatment zone being in direct fluid communication withan adjacent gasification zone; 1

iii. withdrawing heat from the pretreatment zone by indirect heat exchange with water to produce steam and to maintain said pretreatment temperature;

iv. passing the pretreated carbonaceous material and the resultant gases from the upper portion of the pretreatment zone to said gasification zone and passing at least a portion of the steam produced in said pretreatment zone to said gasification zone;

v. said gasification zone being maintained at the same pressure as the pretreatment zone;

vi. maintaining the pretreated carbonaceous material as a fluidized bed in-the lower portion of the gasification zone at conditions sufficient to convert said pretreated material to ash and a gaseous mixture of fuel gas and oils and tars;

vii. passing the gaseous mixture to the upper portion of said gasification zone above the fluidized bed and maintaining the gaseous mixture in the upper portion of the gasification zone at a temperature of about 1,200- l,500F for about 10 100 seconds to crack the oils and tars in the gaseous mixture;

and viii. withdrawing ash from the bottom of said gasification zone.

2. A process as in claim 1 wherein said ash is agglomerated in the bottom portion of the gasification zone at a temperature above the temperature of the fluidized bed.

3. A process as in claim 2 wherein said gaseous mixture includes fine and coarse particulate material and said mixture is separated to provide a fine particulate fraction and a coarse particulate fraction, said coarse particulate fraction being passed to said fluidized bed in the gasification zone and said fine particulate fraction passed to the bottom portion of the gasification zone.

4. A process as in claim 1 wherein said finely divided carbonaceous material includes higher density inorganic material, said higher density material being withdrawn from the lower portion of said fluidized pretreatment zone. r r

5. A process as in claim 1 wherein the fluidized bed in said gasification zone is maintained 'at a temperature of greater than l500F and below the agglomeration temperature of the ash.

6. A process as in claim 1 wherein said carbonaceous material is bituminous coal.

Claims

1. A PROCESS FOR THE CONVERSION OF A FINELY DIVIDED CARBONACEOUS MATERIAL CONTAINING ASH TO A FUEL GAS HAVING A HEATING VALUE OF BETWEEN 100 - 300 BTU PER CUBIC FOOT WHICH COMPRISES: I. PRETREATING A FINELY DIVIDED CARBONACEOUS FEED MATERIAL BY MILD OXIDATION AT A TEMPERATURE OF ABOUT 700:-800:F FOR ABOUT 10 TO 30 MINUTES TO DESTROY AND CAKING PROPERTIES OF THE CARBONACEOUS FEED MATERIAL IN FLUIDIZED PRETREATMENT ZONE; II. SAID FLUIDIZED PREPTREATMENT ZONE BEING IN DIRECT FLUID COMMUNICATION WITH AN ADJACENT GASIFICATION ZONE; III. WITHDRAWING HEAT FROM THE PRETREATMENT ZONE BY INDIRECT HEAT EXCHANGE WITH WATER TO PRODUCE STEAM AND TO MAINTAIN SAID PRETREATMENT TEMPERATURE; IV. PASSING THE PRETREATED CARBONACEOUS MATERIAL AND THE RESULTANT GASES FROM THE UPPER PORTION F THE PRETREATMENT ZONE TO SAID GASIFICATION ZONE AND PASSING AT LEAST A PORTION OF THE STEAM PRODUCED IN SAID PRETREATMENT ZONE TO SAID GASIFICIATION ZONE; V. SAID GASIFICATION ZONE BEING MAINTAINED AT THE SAME PRESSURE AS THE PRETREATMENT ZONE; VI. MAINTAINING THE PRETREATED CARBONACEOUS MATERIAL AS A FLUIDIZED BED IN THE LOWER PORTION OF THE GASIFICATION ZONE AT CONDITIONS SUFFICIENT TO CONVERT SAID PRETREATED MATE-

4. A process as in claim 1 wherein said finely divided carBonaceous material includes higher density inorganic material, said higher density material being withdrawn from the lower portion of said fluidized pretreatment zone.

5. A process as in claim 1 wherein the fluidized bed in said gasification zone is maintained at a temperature of greater than 1500*F and below the agglomeration temperature of the ash.