CA1130740A - Converting coal with gaseous and liquid fuels - Google Patents

Abstract

ABSTRACT: A coal conversion process wherein the coal is car-bonized in a series of fluidized beds to produce gas, an oil/tar fraction containing suspended solids which can be hydrogenated to produce a commercially useful product, and highly reactive char which is gasified with steam in a gasifier, in which substantial advantages are obtained, both in capital and operating costs, by separating the solids and the heavy oil component from the oil/tar fraction and recycling them to the gasifier.

Description

~C37~0 CONVERTING COAL WITH GASEOUS AND LI~UID FUELS
_ This invention relates to coal conversion processes, particularly to improvements in the production of gaseous and liquid fuels from coal.
In one known coal conversion process, coal is carbo-nized in a series of fluidized beds to produce char, com-bustible gas and an oil/tar fraction which can be hydro-genated to give valuable synthetic liquid fuels. A de-tailed description of the process is set forth in U.S.
Patent No. 3,375,175 issued March 26, 1968 to Eddinger, l0 et al.
Char from the Eddinger, et al carbonization tech-nique is highly reactive/ being especially useful as the carbonaceous component in the steam/carbon reaction for making synthesis ga~. A coal conversion system for manu-15 facturing high BTU synthetic liquid and gaseous fuels,utilizing the methods and materials of the aforecited Eddinger, et al patent is disclosed in U.S. Patent No.
3,~66,633 issued June 29, 1976 to Friedman and U.S.
Patent No~ 3,966,634 issued June 29, 1976 to Sacks. In 20 the procedure of these patents, char obtained via Ed-dinger, et al is gasified in a fluidized bed reactor, heated by a recirculating stream of solids which may be recycle char or an inert particulate material and the resultant water gas then methanated to give high BTU gas.
25 Gas from the carbonization stage is used for process heat, or mixed with the product gas. Oils and tars from the carbonizer are upgraded to liquid fuel products by hydrogenation using hydrogen from the synthesis gas.
Hydrogenating the coal tar converts it into a more suit-30 able form, such as for example, a feedstock for an oilrefinery. As is well known, the modern refinery is de-signed primarily to operate with petroleum or crude oil;
it cannot be operated using unprocessed coal tar.
In carrying out the aforedescribed coal conversion 35 process, there are incurred certain losses of hydrocar-113~74~

bonaceous material during upgrading the oil/tar fractioninto usable liquid fuels. These losses arise mainly in two ways. One such loss occurs from treatment of the oil/tar fraction to remove therefrom the finely divided S carbonaceo~s material which was entrained in the tar forming vapors evolved during fluidized carbonization of the coal. The separated solids still retain considerable quantities of oil and tar which canno~ be readily re-moved. This difficulty can be mitigated by first hydro-l0 genating the oil/tar fraction and then separating thesolids from the hydrotreated material, which by virtue of its low viscosity does not adhere to the solids to the extent as does the raw tar. Consequently, there is at-tained a cleaner and more effective separation of hydro-15 carbonaceous material from the solids. Nevertheless, anappreciable amount of the material remains associated with the solids fraction.
The other principal loss of hydrocarbonaceous ma-terial arises as a result of the presence, in the oil/tar 20 fraction, of a heavy oil component which is resistant to hydrogenation. This substance can be isolated directly from the raw oil/tar mixture such as by fractional con-densation or as a re$idue from fractionation of the hy-drotreated material. In an~ event, the heavy oil com-25 ponent is not readily upgraded to usable fuel value andrepresents an undesirable by-product which is a minus economic factor in the commercial realization of the coal conversion process aforesaid.
It is ~nown to reprocess heavy oil residues by pas-3~ sing them through a fluidized coal carbonization zone inaccordance with a procedure described in U.S. Patent No.
3,565,766 issued February 23, 1971 to Eddinger, et al for upgrading petroleum residue~ However, the recycling of such hydrocarbonaceous by-products through a fluidized coal carbonizer tends to promote agglomeration of the coal particles and may in severe cases, bring about total collapse of the fluidized bed. Agqlomeration can be 79~0 lessened by limiting the quantity of the recycl stream but then the technique becomes impractical since a large surplus of by-product would build up.
In accordance with the present invention, in the 5 gasification of coal in a series of fluidized beds to produce gas, an oil/tar fraction having entrained solids and which is hydrogenated to give useful liquid fuels, a highly reactive char which is gasified in a fluidized bed with steam, and by-products of heavy oil and separated lO solids having hydrocarbonaceous material associated therewith, an improvement in plant efficlency and opera-tion is realized by recycling the by-products to the gaslf ler.
The drawing represents a flow-sheet showing treat-15 ment, in accordance with the invention, of the hydrocar-bonaceous by-products disclosed in aforecited U.S. Patent No. 3,375,175 to Eddinger, et al. According to the patent, coal is introduced into a fluidized bed where it is treated with an inert gas stream at a relatively low 20 temperature, to reduce agglomeration. The coal is next transferred to a second bed, which is heated with recycle char and gas from the third sta~e to temperatures de-pending on the type of coal but typically about 427C to 482C. It then goes to a third stage, heated by recycle 25 char and gas from the fourth sta~e from about 510C to about 566C, and finally to a fourth stage, in which heat is obtained by partial combustion of the char and the temperature is of the order of 816C to 982C. The over-head volatiles (from stage 2) enter a product recovery 30 section where they are separated as gas, an oil/tar frac-tion containing entrained solids and an aqueous li~uor.
Char from the third heating stage is used in the process of the present invention as the carbonaceous feed for a fluidized gasifier. Heat for the gasifier is obtained by 35 recycling some of the char to a heater such as a slagging combustor, where a portion of the recycle char is burned, and the balance is recycled to the gasifier to provide ~30740 heat. When the process of U.S. Patent ~,375,175 afore-said is combined with the herein gasification system, char heating stage 4 is not used and heat for the coal pyrolysis is provided by hot recycle material from the gasifier. An alternate method utilizes an inert mass of pebbles as the heat transfer medium. The manner of heat-ing heating the gasifier is not germane to the instant invention, which resides in the oil~tar recovery and treatment section of the operation.
In carrying out the invention, the solids by-product is obtained first. It can be isolated from the oil/tar fraction either before or after hydrogenation. The solids are separated from the hydrocarbonaceous phase using know~ techni~ues such as centrifugation or filtra-tion, preferably the latter. Where the separation is effected prior to hyd~ogenation, more of the hydrocarbon-aceous material is retained by the solids since the crude oil/tar fraction is much more viscous than the hydro-genated product.
The isolated solids are comprised of finely divided particles, for example, char, coal and ash having a coating of the hydrocarbonaceous material. This causes the particles to cling together thereby forming a non-flowing contiguous mass. Such hydrocarbonaceous coated solids can be recycled to the gasifier while the fil-trate can be hydrogenated to give a liquid product. In the gasifier, the organic and carbonaceous moieties as-sociated with the recycle solids undergo gasification thereby increasing the output of hydrogen and carbon monoxide.
As previously noted, not all of the oil/tar fraction is susceptible to hydrogenation, there being a heavy oil component which is resistant to such treatment; this is the second type of by-product from the herein coal con-version process. This heavy oil can be isolated from thecrude oil/tar or as the residue remaining after fractio-nating the hydrotreated material to produce refined 1~307~0 liquid fuels. Because of its highly viscous nature and the presence of solids, the crude oil/tar is not usually distilled to separate the heavy oil component.
The heavy end consists almost entirely of unsatur-ated materials. It contains about 80% of the metal con-tent of the total crude oil/tar fraction of 19% of the sulfur, 27% of the nitrogen and 10% of the oxygen. The removal of these mate~rials produces a very marked change in the operation of the hydrogenator, a~d changes the product mix somewhat, increasing the yield of gas and de-creasing the yield of oil.
Turning to the drawing, char 4, obtained in ac-cordance with the carbonization process of U.S. Patent 3,375,175, is fed into fluidized gasifier 5. The char is fluidized by steam 8 e~tering the bottom of gasifier 5.
The steam reacts with the char to form water gas product 12 which exits from the top of gasifier 5. The gas can be sent to storage or used in further processing such as hydrocarbon synthesis. Gasifier 5 is provided with in.
ternal cyclones to free the water gas from dust particles and return these to the gasification zone, A portion of char is removed from the bottom of gasifier 5 by way of line 14 and conveyed to slagging combustor 17 where the char is heated above gasification temperatures and re-2S turned to the upper part of gasifier 5 through line 20.The hot char provides the heat to maintain the endo-thermic gasification reaction. The char is continuously recycled back and forth between gasifier 5 and combustor 17. Air or oxygen containing gas 22 and char, preferably char fines from gasifier 5 cyclones are supplied to com-bustor 17 to produce the hot combustion gases for heating the recycle char stream. Molten ash residue is withdrawn from combustor 17 through line 25, quenched and sent to a disposal area. Flu~ gas exits from top of combustor 17, via line pipe 28, after passage through cyclones to re-move fly ash.
The oil/tar fraction 30 pro~uced in the carboniza-~13~7~0 tion process of U.S. Patent 3,375,175, is freed of sus-pended solids in accordance with the invention herein, and the separated solids with adhering hydrocarbonaceous matter are then fed into gasifier 5. In the gasifier, the hydrocarbonaceous matter and any entrained char re-act with steam to give synthesis gas while suspended ash is removed from the synthesis gas product by gasifier 5 cyclones. Eventually such suspended ash will pass through combustor 17 and be rejected as molten ashO In removing the suspended solids from oil/tar fraction 30, two methods can be employed. In the first method or scheme I, oil/tar fraction 30 is conveyed to separation zone 32A where it is subjected to fractional condensation or filtration to remove solids. The separated solids containing adhering film of hydrocarbonaceous matter is introduced by way of line 36 into gasifier 5. The solids free filtrate 39 from the separation zone 32~ is then hydrogenated at hydrotreating station 40 to give a syn-thetic crude product oil 45. In the second method or scheme II, oil/tar fraction 30 is hydrogenated at hydro-treating station 40a, which is identical to 40, and the resulting hydrotreated product 50 is then conveyed to separation zone 32B, which is identical to 32A, where the suspended solids are removed and sent to gasifier 5 via line 52. In operating separation zone 32B, the hydro-treated material may be distilled first to give product oil 45 and then the residue freed of suspended solids or the solids may be separated prior to distillation.
In a 9,000 ton per day coal converstion operation with Illinois No. 6 seam coal, using the prior art pro-cess described in the Friedman and Sacks patents afore-said and carried out with and without the improvement of the present invention, the following results were ob-tained:
1. The yield of hydrotreated oil decreases about 7%, by 725 barrels per day from 10,000; the gas yield from the plant increases by about 8~. In effect, the 113~7~0 carbon and hydrogen in the heavy oil become gas instead of remaining as oil.

2. Catalyst life is increased by a factor of 5, since 80% of the metals (the effective catalyst poisons) are removed.

3. Since the synthesis gas volume will be larger because of the gasified oil, there will be more sensible heat available in the gasifier gas for heating of the pyrolysis stages. This can either decrease the amount of char recirculation or decrease the amount of externally burned fuel required for heat up of the pyrolysis sec-tion.

4. Since more gasification will occur, more combus-tion will occur, and the net effect is an increase i~ the amount of by-product ,steam and power.

5. The changes in the hydrogenation section allows the hydrotreating reactors to be smaller by 53.5~ and the rest of the plant by 7%; the hydrogen capacity necessary is decreased by 28.5%, while the gasifier capacity in-creases by 3.2%.

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Claims (5)

Claims:

1. A coal conversion process wherein the coal is car-bonized in a series of fluidized beds to produce gas, an oil/tar fraction having entrained solids and which is hydrogenated to give useful liquid fuels, a highly re-active char which is gasified in a fluidized bed with steam and by-products of heavy oil and separated solids having hydrocarbonaceous material associated therewith, characterized by recycling said by-products to the gasi-fier.

2. The process of claim 1 characterized in that the solids are recovered from the hydrogenated material.

3. The process of claim 1 characterized in that the solids are recovered from the oil/tar fraction.

4. The process of claim 1 characterized in that the heavy oil is isolated from the oil/tar fraction.

5. The process of claim 1 characterized in that the heavy oil is recovered as the residue from distilling the hydrogenated oil/tar fraction.