CA1086477A - Process for so.sub.2 reduction - Google Patents

Abstract

ABSTRACT
A continuous process is provided for reducing sulfur dioxide to sulfur with coal in a fluidized bed. The coal is admixed with a particulate, solid, diluent material and fluidized by stream of SO2-containing gas introduced into the reaction zone at a velocity at least 1 foot per second greater than the minimum velocity required to fluidize the bed. Sulfur is recovered from the gases exiting the bed.

Description

BACKGROUND OF THE INVENTION
This invention relates to a process for reducing sulfur dioxide. More particularly~ this invention relates to a process for reducing the sulfur dioxide content of gases by reaction with coal.
Appreciable amounts of sulfur dioxide are contained in many industrial gases vented in~o the atmosphere from plants involved in roasting, smelting and sintering sulfide ores, in stack gases from power plants burning sulfur-bearing coal, or in exit ~` 10 gases from other industrial operations involving the combustion of sulfur-bearing fuels, such as fuel oilO Removal of the sulfur dioxide from these off ~ases is important for two reasons, first, the sulfur dioxide is an environmental pollutant and secondly, the sulfur dioxide emitted into the atmosphere results in the loss of sulfur values, a natural resource. Sulfur dioxide which is res:ov~red from these off gases can be converted to sulfur.
There are, of course, many processes for reducing the sulfur dioxide content o gases, most of which involve the use of reclucing agents such as natural gas or other gaseous reductants ~uch as methane, carbon monoxide, liqui.d petroleum gases and the like. The supply of these gaseous reductants is limited and therefore their use becomes more and more expensive, when and if they are available.

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It is known that coal can be a reductant for sul~ur dioxide resulting in the production of elemental sulfur and rela-tively harmless carbon dioxide~ Coal is also in abundance and, therefore, relatively inexpensive. As such~ it is a very desirable reductant for sulur dioxide-containing gasesO
; 8ritish Patent 1,390,694 is directed to a process for the reduction of sulfur dioxide in gases with the use of coal. The sulfur dioxide gases are introduced into a moving bed of particu-late coal and reduced to elemental sulfur and/or hydrogen sulfideO

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A potential problem with the use of such a moving bed is the agglomeration of the coal particles at the temperatures required for the reduction of the sulfur dioxide. Coal occurs naturally in various forms, some of which are agglomerating and some of which are non-agglomerating. With the high temperatures required for the reduc~ion of sulfur dioxide with coal, potential problems ; are foreseen in the use of a movin~ bed with a~glomerating coals unless the coal is first pretreated by, e.g., calcination, or some other method to prevent agglomexation in the bed. It is apparent that agylomeration of the coal in the bed would cause operating difficulties and frequent shutdowns. Additionally, since this process involves the use of a large excess of coal in the reaction zone; the purity of the product sulfur can be adversely affected by tar carry-over in the exiting ~ases. Some coals contain sub-stantial amounts of tars, and when heated to reaction temperatures these tars can be driven off with the product gases. Separation of these impurities from the sulfur is difficult.
The reduction of sulfur dioxide in flue gases employing coal in a fluidized bed is reported by Sinah and Walker in "Air Pollution and its Control" AIChE Symposium Series, No. 126, Vol. 68, pages 160 to 167 (197~). The authors report that the coal employed in ~he fluidization process was anthracite ~hich was calcined at 650C. for 2 hours prior to its use. The step of calcination prior to use is, of course, undesirable in the sense that this adds a step to the process and consumes substantial amounts of energy~ rendering the process less economical.
` It is an object of this invention to provide an economi-cal process for the reduction of SO2 with coal. It is a further object of this invention to provide a process for reducing sulfur dioxide with coal in a fluidized bed. These and other objects will become apparent from the description which follows.

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:- 5UMMARY OF THE INVENTION
_ _ In accordance with this invention there is provided a continuous process for reducing the sulfur dioxide content of gases which comprises continuously introducing a sulfur dioxide-containing gas stream into a reaction zone containing a bed of particulate :.
material comprising coal and a solid diluent, the velocity of the gas stream being maintained at least about 1 foot per second greater than the minimum fluidization velocity of the bed of particulate material and insufficient to carry substantial amounts of particu-late material from the reaction zone, continuously introducingcoal into the bed while maintaining the temperature of the reac-tion zone between about 1100~o and 2000F., continuously removing product gases from the reaction zone and recovering sulfur from the product gasesO
When operating in accordance with the process of this invention, sulfur dioxide is effectively and efficiently reduced, bed agglomeration is prevented regardless of the type of coal employed in the process, the process may be run continuously by regulating the introduction of additional coal into ~he reaction zone, and high purity sulfur can be readily recovered.
DETAILED DESCRIPTION OF THE INVENTION
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The use of coal to reduce sulfur dioxide pollutant to elemental sulfur has several advantagesO The supply of coal is not as limited as are other SO2 reductants such as natural gas.
Therefore, the use of coal as a reductant not only is more eco-; nomical, but also conserves natural gas. It also represents an additional ecological use for high sulfur coal.
Reduction of sulfur dioxide by crushed coal in a fluidbed of inert material also offers several practical advantages over other types of reactors. The Eluid bed reactor is well suited for the reaction because of the excellent gas solid contact. Thus, ~' although the reaction is exothermic, ~he heat transfer is very good resulting in good temperature and reaction control. As a conse-quence, the reaction can be readily conducted in a fluid bed - reactor at conditions favorable to high sulfur yields.
The source of the sulfur dioxide-containing gas is not believed to be critical to the process of the present invention.
:., Stack yases which contain sulfur dioxide are generally treated to concentrate the sulfur dioxide before the reduction process is institutedO The use of gases containing a low concentration of sul~ur dioxide would not be as efficient as the use of ~ases con-taining higher concentrations and for this reason it is preferred to employ a sulfur dioxide-containing gas having a concentration above 5~ by volume, more preferably above 50%, and most preferably above 80~ sulfur dioxide in the process of this invention. The feed gases should be low in oxygen because it can compete with the ; sulfur dioxide for reaction with the coal. In some instances, however, small amounts of oxygen may be desirable to maintain the .;
desired heat balance in the reactor.
, The process of this invention may be carried out utiliz-ing various types of coal to reduce the sulfur dioxideO Both agglomerating and non-agglomerating coals may be used at the pre-ferred operating conditions without bed sticking problems. Among the coals that may be used are lignite, subbituminous, bituminous and anthracite. Calcination or other similar pretreatment steps to render the coal nonagglomerating are not required for the proc~ss of the present invention. Thus, the coal is merely dried if necessary, pulverized to the desired particle size and fed to the fluidized bed in the amounts necessary for efficient reduction of the sulfur , di~xideO
The reaction of sulfur dioxide with coal may be illustrated `~ by the equation:
ClH0.8OO 2(1ignite) ~ 1.1 SO2 ~ C 2 2 "

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e ~ L'7~7 Side reactions can also occur producing COS, CS2 and H2S as well as H2, CO and hydrocarbons. The formation of side products will depend on the reaction conditions. It is of course most . desirable to maximize the direct conversion of SO2 to sulfur.
The amount of SO2 conversion will be affected by the feed ratio of coal to sulfur dioxide. ~igh ratios increase the amount of sulfur dioxide conversion. However, high ratios also tend to produce increased amounts of sulfur containing by-products such as H2S and COS in the product gas. The optimum feed ratio 10 will depend on the type of coal used in the process. For example, for lignite, it has been found that the conversion of sulfur dioxide does not increase significantly above a 130% stoichiometric feed ratio (based on the above equation) of coal to sulfur dioxide. In some instances, sulfur cont~ining by-product formation is minimized by employing less than stoichiometric ratios. Consequently, a feed ratio between 80~ and 130% stoichiometric is preferred when lignite is employed as the reductant.
The fluid bed in the reaction zone can be con-trolled at a uniform reaction temperature by known means. Sulfur 20 dioxide is reduced by coal at any temperature between about 1100F.
and 2000F. However, reaction temperatures of 1300F. to 1800F.
are preferred to maximize SO2 reduction to sulfur. A temperature of at least 1300F. appears necessary for practical, economical conversion of sulfur dioxide. At temperatures above 1800F. there is an increased danger of fusion in the bed which could result in bed agglomeration.
Coals which are classified as "agglomerating coals" will, as the classification indicates, tend to form lumps when heated to the temperatures required for reaction with suflur dioxide.
30 Furthermore, regardless of the type of coal employed to reduce sulfur dioxide, it is believeù, that at the reaction temperatures, ; .

certain low melting eutectics, which are formed from the coal ash, build up as the reaction progresses and these tend to ca~se bed agglomeration or sticking. It has been found, according to this invention, that this tendency to agglomer-ate can be combatted by providing a particulate, solid diluent as a major portion of the fluidized bed. The use of such a diluent ma~erial has also been found to result in stable bed operation ; and temperature uniformity. Thus, whether employing an agglomer-ating or a non-agglomerating coal, the diluent solid material is an essential part of this invention. The particulate, solid diluent material must be one which will not adversely affect the desired operation of the bed, i.e., should be substantially unreactive with coal, sulfur dioxide and sulfur~ While inert materials are satisfactory for this purpose, the solid diluent may be composed in whole or in part of catalyst or other reaction aidc Sand is a preferred diluent because of its abundance, its cost and its abrasion resistance. Among other diluent materials useful in the process of this invention are alumina, magnesium oxide, alumino-silicates, quartz, silicon carbide, and the like.
The proportional make-up of the bed is not critical except to the extent that there must be sufficient solid diluent particles present to attain the desired result of preventing bed sticking and there must be sufficient coal particles to react with the 52 to the extent desired. Within these broad guidelines it has been found advantageous to operate with a bed of particulate solids containing between 0.1 and lOoO weight percent coal at any given timec The relative amount of coal present in the bed will also be affected by other factors, eOg., coal size, coal type, i chemical composition of the coal and reactivity of the coal. Thus, determination oi optimum be composition will vary accordin~ly.

. i .. ~: ' The particle size of both the coal reductant and the particulate material in the bed will depend on many fac~ors such as the diameter of the reaction vessel, the abili~y of the particulates to be fluidized and the efficiency of reaction which is required.
For example, obviously very large particle size ma~erial will not be efficiently fluidized and very small material will be carried ~- away from the reaction zone with the gas. It has been found that an average solid diluent particle size of between 150 and 1500 microns is appropriate for use in the process of this invention but this could vary depending on the above-mentioned factors.
The coal particles may be the same size or different size than the diluent. Since there usually is a relatively small amount of coal present, compared to the diluent, the coal particle size has little effect on the fluidization characteristics of the bed.
Of course, the coal size must be small enough to be fluidized and large enough to remain in the bed for a time sufficient to react with the sulfur dioxide. The depth of the bed will like-wise be dependent on factors such as size of the reaction vessel and the required contact time depending on the velocity of the incoming gas. Further, bed depth is a factor which can easily be selected based on the desired result.
It has been found, surprisingly, that mere ~luidization of the coal even when combined with large amounts of particulate diluent material is insufficient to provide an operating process for the reduction of sulfur dioxide. At certain velocities, even though the bed is in a fluidized state and the reaction is proceeding as desired, bed sticking occurs and after a short period of operation large lumps are formed which prevent fluidiza-tion and cause a process shutdown. While not wishing to be bound to any specific theory of operation it is believed that despite the presence of the particulate diluent material, and despite the fact that the particles in the bed are in continuous motion, the -7~

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reaction of sulfur dioxide with the coal particles causes low melting eutectics to form on the bed particles and when they come in con~act with other bed particles, agglomeration occurs when :: there is insufficient bed movement. It is thus a critical aspect of the present invention to provide sufficient bed movement by maintaining the velocity of the fluidizing gases at least about 1 foot per second, preferably 1.1 feet per second, greater than the minimum fluidization velocity necessary to fluidize the particular bed of particulate solids~
The minimum fluidization velocity (Vmf) of any specific bed of particulate material is determined by using the correlation of Wen and Yu (C~.P. Symposium Series No. 62, Vol. 62, pp.
100-111, 1966), (Re)mf = ~33.7)2 + 0.0408 (Ga~ 1/2 _ 33.7 where (Re)~f is the particle Reynolds number at onset of fluidiza-tion and Ga is the Galileo number, defined as, . ( p) ~f (s - Pf)9 ~ ~ ....
.'. (Y) The minimum fluidization velocity (Vmf) is thusly calculated by, '~ V = ( ~f ( ) mf where dp is the effective particle diameter in the bed and is defined as, dp ~ xi i=l --~ 30 wherein Pf - the density of the feed gas at reaction temperature and pressure . -8-.

; Ps = the density of solid particle in bed = the viscosity of the feed gas at reaction tempera-ture g = gravitational acceleration (32.2 feetjsec.2) Xi = weight fraction of particles in th~ fluid bed having diameter dio Xi and di are readily deter-mined by standard sieve analysis Operation of the process of this invention in accordance with the parameter of velocity of feed gases defined above results in the ability to conduct the process over long periods of time without the detrimental bed sticking occuring. While this is the minimum gas velocity for successful operation of the sulfur dioxide . ~ recluction process, it will be apparent that the velocity of the i incoming gas should not be so great as to result in loss of sub-`~ stantial amounts of particulate materials from the reaction zone by being carried away with the gas coming from the fluidized bed.
The gases leaving the reaction zone and the fluidized bed may be treated to recover the elemental sulfur, e.g., by con-densation. Furthermore, in some cases it may be desirable to thereafter react the effluent gases, as in a Claus reactor.
A particularly advantageous feature of operating in accordance with this invention lies in the ability to obtain rather high purity sulfur in a relatively easy manner. Although substantially all of the coal ash~ i.e., the ash remaining after ; the coal reacts with the sulfur dioxide, is carried out of the bed with the exit gases, the majority of this coal ash may be readily removed from the gases prior to sulfur condensation by known means, for example, in a cyclone separator. Furthermore, ~ residual coal ash which may deposit in the liquid sulfur as the .~ .
3Q product gases are cooled to allow the sulfur to condense may be easily removed from the sulfur by simple filtration, resulting ., . - g_ .
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in high purity sulfur product. Additionally, when operating in accordance with this invention, product sulfur is essentially uncontaminated with coal tars.
As indicated, the process of the present invention is run in a continuous manner by continuously feeding coal to the fluidized bed which may be accomplished in a known manner, for example, by means of a ~eed screw, at a rate consistent with the reaction rate of the coal in the bed. Preferably the coal is fed ~o a point near the bottom of the bed.
The following examples are given by way of illustration only O
In the following examples a six-inch diameter fluid bed reactor was used. The bed was primarily composed of sand and usually had a static depth of about 2 ft unless otherwise noted.
Based on the fluid and solid properties in the bed, the minimum fluidization velocity was calculated using the correlation of Wen and Yu. SO2 and N2 gases were measured by rotameters, preheated and continuously fed to the bottom of the bed at a velocity to sustain bed fluidity. The superficial gas velocity in the bed was based on the inlet gas feed rate at the operating conditions. Dried pulverized coal was continuously fed into the ~luid bed at prescribed rates by means of a feedscrew. Reaction temperatures were measured by thermocouples in the fluid bed.
The reaction temperature was controlled by the degree of feed gas preheating. Because of the large heat losses from the six-inch reactor~ it was sometimes necessary to include oxygen in the feed gas (0-5 vol.%) to maintain the reaction temperature (i.e., compensate heat losses by burning coal). Ash was removed from the exit gas by cyclones. A~ter condensing the sulfur (and water) the exit gases were sampled and analy~ed by gas chromatography.

7~7 Example 1 A gas composed of 78 vol.~ SO2 and 22 vol.~ N2 was preheated and fed to the bottom of a 2 foot deep fluid bed at a rate to produce a superficial gas velocity of 1.27 ft/sec. in the bed. The average particle size of the sand was 350 u. The minimum fluidization velocity of the sand was 0.093 ft/sec. The bed temperature was controlled at 1375F. The reactor was continuously operated for 80 hours at these conditions without bed sticking problems. During this time, the feed rate of dried lignite (170 microns) was varied~ When the lignite feed rate was ; 102~ of the stoichiometric amount needed to convert the feed SO2 (to sulfur, carbon dioxide~ and water), the SO2 conversion was 80%.
83~ of the SO2 converted formed sulfur. After sulfur and water condensation the reactor exit gas had the followin~ composition (vol~%) 22.2% N2, 0.6% CH4, 0.6~ CO, 50.9% C02, 1.9% COS, 7.1 H2~ 0~ CS2, 15-4% SO2 Example 2 An attempt was made to operate at conditions similar -to those in Example 1, except the average sand particle size was 550 microns. The minimum fluidization velocity of this sand was , 0~28 ft/sec. The superficial gas velocity in the bed was 1.27 ~t/sec. The bed was fluid initially, but bed sticking occurred ~fter only four hours of operation, causing loss of bed fluidity and necessitating shutdown and discharge of the bed.
Example 3 Lignite from North Dakota was dried, pulverized to an a~erage particle si2e of 170 microns, and used to reduce SO2 in the fluid bed reactor, The lignite/SO2 feed ratio was 96% stoichio-~etric. The gas fed to the reactor was composed of 80 vol.~ SO2, 19% N2 and 1~ 2 The fluid ~ed temperature was maintained at 1490F. The superficial gas velocity in the 2-foot deep bed was . .

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1.3 ft/sec., and the average sand particle si~e in the bed was 350 microns~ The minimum fluidization velocity of the sand was 0.11 ft/sec. These conditions were maintained for 26 hours, and bed sticking never occurred~ The exit gas analyzed (vol.%): 20.7%
N2, 0~9% CO~ 52.4~ CO2, 2.5% COS/ 6.9~ H2S, 0.7% CS2, 10-5% SO2-88% of the SO2 in the feed yas was converted~ and the conversion was 86% selective to sulfur.
Example 4 An agglomeratins subbituminous coal was fed to the fluid bed reactor to reduce SO2. The coal was dried (220F) and pulverized to an average size of 170 microns but received no other pretreatment. The coal/SO2 feed ratio was 98~ stoichio-metric. The feed gas contained 68~ SO2, 27% N2t 5% 2 and was fed at a rate to effect 1.77 ft/sec. superficial gas velocity in a 2-foot deep bed. The sand in the fluid bed had an average particle size of 350 microns and a minimum fluidization velocity of 0.092 ft/sec. The bed temperature was 1500~F. These conditions were maintained for 14 hours and bed fluidity was never lost. The reactor exit gas analyzed 25.9 vol.% N2, 2.8~ CO, 33% CO2, 3.0%
20 COS, 3.6~ H2S, 1.9~ CS2 and 26O5~ SO2. 60% of the SO2 fed was converted and the conversion was 74% selective to sulfur.
Example S
Dried pulverized (1~0 microns) anthracite coal was used to reduce SO2 in the fluid bed reactor. A 2-foot deep sand bed was usedO The sand had an average particle size~'of 350 u and a minimum fluidization velocity of 0~095 ft/sec. The operating super~icial gas velocity in the bed was 1.2 ft/sec. The feed gas contained 86 vol.~ SO2, 6% N2, and 8% 2 The coal/SO2 feed ratio was 165% stoichiometric, but the SO2 conversion was only 43~, which illustrates the lower reactivity of anthracite. 96~ of the SO2 which was converted formed sulfur. On a water and sulfur free ' '7~7 basis, the reactor exit gas contained 6.9 vol.% N~, 0.6~ CO,41.8% CO2, 1.8% COS, 0.1% H2S, 0.1 CS2, and 50.6% SO2.
Example 6 ~` Dried pulverized tl70 microns) lignite was fed to the fluid bed reactor at a rate equivalent to 61% of the stoichiometric amount needed to reduce the SO2 fed to the reactor (to sulfur, carbon dioxide and watex). The feed gas contained 71% SO2, 26% N2 and 3% 2 and was fed at a rate to effect a superficial gas velocity of 1.7 ft/sec in the 1500F fluid bed. 350 microns sand with a minimum fluidization velocity of 0.09 ft/sec was used in the bed. The reactor exit gas analyzed 27.5 vol.% N2, 0.7%
CO, 37.0% CO2, 1.0~ COS, 1.5% H2S, 1.1% CS2, and 33.7% SO2.
The SO2 conversion was 55%. 89% of the converted SO2 formed sulfur.
Example 7 The fluid bed reactor was operated at the same condi-tions as in previous Example 6, except the lignite/SO2 feed ratio was 94~ stoichiometric. At this higher lignite feed rate, the average SO2 conversion was 74~ and 84% of the converted SO2 formed sulfur. The reactor exit qas composition was 24.5 vol.% N2, 0~1% CH4, 0.8~ CO, 50.9% CO2, 105% COS/ 4.2% H2S, 1.3~ CS2, and 17.3~ SO . The sulfur condensed from the exit gas was filteredO
The filtered product analyzed 99.9~ sulfur and was considered high quality.
Example 8 Dried pulverized (170 microns3 lignite was fed to the ~luid bed reactor at essentially the same rate (94~ stoichiometric) as in Example 7. Other reaction conditions were also the same, except the fluid bed temperature was 1300F. At this lower tempera-ture the average SO2 conversion was 66~, and the conversion was 89%
selective toward sulfur formation. The reactor exit gas analyzed .

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2307 vol.~ N2, 0.5~ CH4, 0.3% CO, 45.1% CO2, 0.9~ COS, 1.1% H2S, 1.3% CS , and 20O7% SO .

Example 9 The fluid bed reactor was operated at conditions similar to those in Example 8, except a shorter gas contact time was effected by a higher gas velocity in the bed o~ 2.4 - ft/sec. A 2 foot deep bed of sand was used~ The average particle size of the sand was 550 microns, and the minimum fluid-ization velocity was 0.26 ft/sec. ~ried 170 microns lignite was continuously fed at a 95% stoichiometric feed ratio. Because of the shorter contact time, the average SO2 conversion was a lower 4B%. 80% of the converted SO2 formed sulfur. The reactor exit gas contained 20.4 vol.% N2, 0.6% CH4, 0.7% CO, 29.9% CO2, 0.7%
COS, 3.5% H2S, 1.5% CS2, and 38.6~ SO2.
Example 10 ` The mean gas contact time was increased by increasing the sand bed depth to 3.5 feet (static). The sand had an average ., .
,~ particle size of 350 microns and a minimum fluidization velocity of 0.09 ft/sec. The feed gas, composed of 70 vol.% SO2 and and 30 vol.~ N2, was fed at a rate to effect a gas velocity of 2.2 ft/sec in the bed. Dried lignite, with an average particle size of 170 microns, was continuously fed at 104% of the stoichio-metric amount needed to reduce the SO2 fed. The bed temperature was maintained at 1500F. Because of the longer gas contact time, 94% of the SO2 feed was converted, and 79% of the SO2 feed was reduced to sulfurO The reactor exit gas contained 33.5 vol.%
N2, 2.1% CO, 47.6% CO2, 4.6% COS, 6.1% H2S, 0.4% C5~ 4.7% SO2.

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Claims (9)

What is claimed is:

1. A continuous process for reducing the sulfur dioxide content of a gas stream which comprises introducing a sulfur dioxide-containing gas stream into a reaction zone containing a bed of particulate material comprising coal and a solid diluent, the velocity of the gas stream being maintained at least about 1 foot per second greater than the minimum fluidization velocity of the bed of particulate material and insufficient to carry substantial amounts of particulate material from the reaction zone, continuously introducing coal into the bed while maintaining the temperature of the reaction zone between about 1100°F. and 2000°F., continuously removing product gases from the reaction zone and recovering sulfur therefrom.

2. The process as defined in claim 1 wherein the coal is lignite.

3. The process as defined in claim 1 wherein the solid diluent material is sand.

4. The process of claim 1 wherein the temperature of the reaction zone is maintained between 1300 and 1800°F.

5. The process as defined in claim 1 wherein the coal is lignite and wherein the stoichiometric ratio of coal to sulfur dioxide is between 100 and 130 percent.

6. A process as defined in claim 1 wherein the gas stream contains at least 50 volume percent of sulfur dioxide.

7. A process as defined in claim 1 wherein the coal is subbituminous.

8. A process as defined in claim 1 wherein the coal is anthracite.

9. A process as defined in claim 1 wherein coal comprises 0.1 to 10.0 weight percent of the bed.