Claims
- 1. An improved process for reducing SO.sub.2 using a carbonaceous fuel gasifier, comprising:
- (a) gasifying a carbonaceous fuel in a gasifier in the presence of steam and oxygen to produce a raw gas rich in H.sub.2 and CO;
- (b) passing the raw gas from the gasifier at a temperature between 2000.degree. F. and 3500.degree. F. into an organic sulfur reducer along with recycle gas containing SO.sub.2, H.sub.2 S, COS, CS.sub.2 and mercaptans and maintaining sufficient residence time within the organic sulfur reducer to react the recycle gas with the raw gas, thereby reducing the COS, CS.sub.2 and mercaptans in the recycle gas to predominantly H.sub.2 S and S.sub.2 ;
- (c) passing the raw gas and recycle gas mixture from the organic sulfur reducer into a SO.sub.2 reducer and adding the SO.sub.2 to be reduced to the SO.sub.2 reducer;
- (d) maintaining the temperature and residence time within the SO.sub.2 reducer to reduce the SO.sub.2 to predominantly H.sub.2 S vapor along with unconverted SO.sub.2 and lesser amounts of COS, CS.sub.2 and mercaptans;
- (e) treating the gas from the SO.sub.2 reducer for selective condensation of sulfur vapor to produce a sulfur vapor-free gas at a minimum temperature of 340.degree. F.; and
- (f) cooling and condensing water vapor from said sulfur vapor-free gas and then treating the said cooled gas for additional removal of remaining sulfurous gases, with said sulfurous gases recycled to said organic sulfur reduction stage;
- (g) and recovering a sulfur free product gas rich in H.sub.2 and CO.
- 2. The process of claim 1, wherein the gasifier so used operates at a minimum temperature of 2000.degree. F. and pressures ranging from 1-50 atmospheres absolute.
- 3. The process of claim 1, wherein the organic sulfur reducer operates at a temperature of at least 1800.degree. F.
- 4. The process of claim 1, wherein the SO.sub.2 reducer operates at a temperature of about 2000.degree. F.
- 5. The process of claim 1, whereby no sulfurous gases are ordinarily added directly to the gasifier.
- 6. The process of claim 1, whereby a portion of the recycle gas added to the organic sulfur reducer is added directly to the gasifier, as long as the portion of recycle gas added to the gasifier is not performed in admixture with any of the other gasifier feeds and as long as the recycle gas added to the gasifier does not directly contact the flame zone of said gasifier.
- 7. The process of claim 1, whereby the SO.sub.2 reduced on an overall basis to elemental sulfur equates to as high as 150 pounds per million Btus of net calorific heat content of carbonaceous fuel added to said gasifier.
- 8. The process of claim 1, whereby the concentrated sulfurous gases produced in step (f) are first processed through a sulfur recovery process to thereby convert 70-95 percent of the sulfur contained in the concentrated sulfurous gases to elemental form, with the sulfurous gases unconverted to elemental form by the sulfur recovery process then returned to the said organic sulfur reduction stage of step (b) of claim 1.
- 9. The process of claim 8, whereby the sulfur recovery process is a Claus process.
- 10. The process of claim 8, whereby the sulfur recovery process is a Stretford process.
- 11. The process of claim 8, whereby the sulfur recovery process is a sulfur recovery process which does not remove organic sulfur compounds.
- 12. The process of claim 8, whereby the overall amount of SO.sub.2 added to the SO.sub.2 reducer equates to up to 50 pounds per million Btu of net calorific heat content of said carbonaceous fuel added to said gasifier.
- 13. The process of claim 1, wherein all or a portion of the cooled gas produced in accordance with step (f) is then sent to an external process, with the SO.sub.2 generated by said external process recovered by a regenerable SO.sub.2 removal process, with SO.sub.2 recovered by the regenerable SO.sub.2 removal process then sent to the SO.sub.2 reducer.
- 14. The process of claim 1, whereby the cooled gas produced in accordance with step (f) is first treated in a CO shift reactor employing sulfur-resistant CO shift catalyst before the sulfurous gases contained in the gas are concentrated by subsequent removal from the cooled gas.
- 15. The process of claim 14, whereby the sulfurous gases leaving the CO shift reactor are concentrated by means of removing CO.sub.2 from the gases leaving the CO shift reactor, with the CO.sub.2 -free gas which results then sent to the said organic sulfur reducer.
- 16. The process of claim 1, wherein said carbonaceous fuel is selected from the group comprised of coal, petroleum coke, char, heavy residuals, oil, or pumpable slurries of carbonaceous materials.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 806,541 filed June 14, 1977 (now abandoned), which was a continuation-in-part of application Ser. No. 660,058 filed Feb. 23, 1976 (now abandoned), which was a continuation-in-part of application Ser. No. 531,064 filed Dec. 9, 1974, and now abandoned. su
Increasing concern over pollution of the atmosphere by SO.sub.2 has led to the development of numerous SO.sub.2 removal processes as would be applied, for example, to the recovery of SO.sub.2 from the flue gas of fossil fuel power plants or to recovery of SO.sub.2 from the off-gas of ore smelters. The various SO.sub.2 removal processes are characterized as non-regenerable or regenerable processes. In non-regenerable processes the SO.sub.2 is converted irreversibly to a fixed salt which must be disposed or sold as a by-product. Examples of non-regenerable processes include limestone scrubbing, where the SO.sub.2 is converted to gypsum, or processes such as taught by Tatterson in U.S. Pat. No. 3,798,308 where ammonia, if available, is reacted with the SO.sub.2 to form ammonium sulfite. Regenerable processes, on the other hand, can recover a concentrated SO.sub.2 stream. An example of a regenerable SO.sub.2 removal process is the Wellman-Lord process. The SO.sub.2 so recovered is then converted to saleable sulfuric acid or else must be converted to elemental sulfur. Carbonaceous fuels including coal have been suggested as an energy source for the reduction of SO.sub.2 to sulfur in regenerable SO.sub.2 systems.
The concept of using coal as an agent for the removal or reduction of SO.sub.2 dates back to at least the year 1879. As concern increased over the atmospheric pollution caused by SO.sub.2, progress was made on improvement of the basic teachings involved with the use of carbon or coal as an SO.sub.2 reducing agent. The earliest extensive demonstration of using coal for the reduction of SO.sub.2 is believed to have occurred in the 1930's when the Consolidated Mining and Smelting Company of Canada, Ltd., of Trail, British Columbia, operated a so-called "sulfur producer" whereby SO.sub.2 recovered from a smelting operation was introduced to a coal-fed, fixed-bed gas producer. Within the gas producer the carbon in the coal directly reduced the SO.sub.2 to sulfur vapor which in turn was condensed to give a solid elemental sulfur product. A discussion of data obtained at the Trail, British Columbia, plant was made by Robert Lepsoe in Industrial and Engineering Chemistry, Vol. 32, No. 7, pages 910-918, July 1940. In more recent times, additional but conceptually similar teachings have been made on the use of coal or hot char as an SO.sub.2 reducing agent. Kertamus et al., in Hydrogen Processing, February 1974, pages 95-96, suggested that sulfur dioxide be used in lieu of oxygen as a gasification agent for coal to produce carbon monoxide by feeding concentrated SO.sub.2 along with hot char to produce carbon monoxide and sulfur, along with carbonyl sulfide. In the process of Kertamus et al., the sulfur vapors so produced would then be condensed and then separately burned to SO.sub.2, with the SO.sub.2 then separated from the relatively inert combustion products so that concentrated SO.sub.2 could then be recycled for the gasification of additional quantities of hot char.
The most recent disclosures on the use of coal for reduction of SO.sub.2 are believed to be those which relate to the development of the Foster Wheeler RESOX.TM. process. Here, an external source of SO.sub.2 would be reacted with coal at a moderate temperature of typically 1400.degree. F. in the presence of substantial surplus of steam in order to reduce the SO.sub.2 to sulfur vapor or to hydrogen sulfide. Sulfur vapor would be condensed from the effluent gas to yield a solid sulfur product. Hydrogen sulfide would be recovered from the gas whereupon it could be reduced to sulfur by reduction with additional quantities of SO.sub.2, such as in the well-known Claus process. A description of the Foster Wheeler RESOX.TM. process can be found in the May-August 1974 issue of Heat Engineering, which is a Foster Wheeler corporate publication. Further description of the process can be found in patents assigned to Foster Wheeler Energy Corporation, and include U.S. Pat. No. 4,082,519, British Pat. No. 1,390,694 and French Pat. No. 2,195,584. The novel teachings of these patents which distinguish them from the prior work done at Trail, British Columbia, relate to using sufficient steam to permit a relatively low gasification temperature of 1200.degree.-1400.degree. F.
Reducing agents for SO.sub.2 include carbon, carbon monoxide, hydrogen, hydrogen sulfide, and carbonyl sulfide. The reduction reactions and associated standard heats of reactions (kcal/gram mole of SO.sub.2) are as follows, with a negative sign indicating an exothermic reaction.
Carbon Reduction:
Carbon Monoxide Reduction:
Hydrogen Reduction:
Hydrogen Sulfide Reduction (Basic Claus Reaction):
Carbonyl Sulfide Reduction:
The above reactions along with additional reactions can occur simultaneously and thus represent a complex system, where performance depends on initial concentration of reactants, temperature, pressures, and rate of reaction. However, several points are important for an understanding of the present invention.
(1) The most rapid and effective reduction of SO.sub.2 generally occurs with hydrogen.
(2) The reduction of SO.sub.2 with CO or COS is, generally speaking, very slow and usually demands catalytic inducement when conducted at temperatures considerably lower than about 1600.degree. F.
(3) None of the reduction reactions can be effected to 100% completion without complete removal of reaction products.
(4) All of the reactions which produce CO or H.sub.2, both of which are useful as gasification products or as reducing agents for SO.sub.2, demand an input of heat. Equation 2, for example, requires a heat input of 16.81 kcal/g-mole of SO.sub.2. Additional CO or H.sub.2 generation reactions which can occur in a coal gasification system are as follows:
Prior known methods for the reduction of SO.sub.2 using carbonaceous fuels have a number of limitations which the invention disclosed herein overcomes.
First of all, the prior known methods are not well suited for the production of CO and/or H.sub.2, which are useful by-products of SO.sub.2 reducing agents, with H.sub.2 being an excellent reducing agent. This is because the heat associated in the generation of CO and H.sub.2, as well as the heat associated in raising the reactants to the specified temperature, is barely provided, if at all, by reacting in-situ all of the generated CO and H.sub.2 with SO.sub.2, exothermically, thereby leaving little or no CO or H.sub.2 as a by-product. For example, in the teachings of the previously mentioned patents assigned to Foster Wheeler Energy Corporation the numerous independent reactions which occur to form H.sub.2 S from SO.sub.2 may be summed together to represent either of the following two overall idealized net reactions:
Preheating reactants or generation of steam is very inefficient, with the latter being exceptionally inefficient since the latent heat of vaporization is irreversibly lost in the overall process in accordance with the second law of thermodynamics. Therefore, it is preferable to use a minimal amount of steam in the SO.sub.2 reduction process. The disclosed invention does use a minimal amount of steam by incorporating a high temperature oxygen blown gasifier in the overall process. With the use of oxygen there is no problem in generation of CO and/or H.sub.2 and in supplying necessary heat directly to the process and maintaining a high temperature since heat is rapidly generated by highly exothermic oxidation of carbon to CO (Eqn. 10) or to CO.sub.2 as shown below:
Using a minimal amount of steam, as is a feature of the disclosed invention, offers additional advantages over prior art teachings. For example, when sulfur vapors are generated by the reduction of SO.sub.2 it is necessary, of course, to condense these sulfur vapors as well as to condense the water vapor which accompanies the gas. It is preferable, however, to avoid simultaneously condensing sulfur and water since this leads to plugging of equipment and highly corrosive conditions. In the Claus sulfur recovery process, for example, the removal of water vapor by its condensation from the tailgas would be theoretically beneficial in extending the degree of overall sulfur necessary in the Claus process, which presently is thermodynamically limited at about 93-95%. A number of serious attempts have been made to eliminate water in the Claus process, but all have been unsuccessful. This problem is recited by Beavon in Chemical Engineering, Dec. 13, 1971, pages 70-73. In prior known gasification processes for reduction of SO.sub.2, the high usage of steam can result in an exit gas where some water can condense from the gas simultaneously with the sulfur even though there is a big difference in boiling points between sulfur and water. With the disclosed invention, the minimal steam usage results in a gas which ordinarily contains less than 30 vol. % water and no more than about 10 vol. % of diatomic sulfur vapor. Over the range of anticipated operating pressures of the disclosed process it is possible to condense nearly 100% of the sulfur vapor from the gas before any water vapor begins to condense, thereby minimizing corrosion and operating problems.
An additional advantage to the use of a minimal amount of steam would be a reduction in the quantity of aqueous effluents to be treated. Generally the water condensed from the gas cannot practically be evaporated again for recycle to the gasifier because of contamination of this condensate, especially with sulfur compounds. Therefore this condensate must be treated to acceptable limits for discharge to a waterway. This problem is particularly serious for prior art SO.sub.2 reduction methods which stressed the use of low temperatures for gasification of the coal, such as in Steiner U.S. Pat. No. 4,082,519, where 1200.degree.-1450.degree. F. is the preferred temperature. At such temperature tars, phenols, and other environmentally hazardous compounds are evolved from the volatile matter of the coal and are subsequently condensed along with water. Thus in the prior methods of SO.sub.2 reduction, due consideration must be given as to whether the usefulness of the prior methods in regard to abating air pollution is offset by contributions to increased water pollution. In the disclosed invention a minimal amount of aqueous effluent would be generated and, in addition, no tars, phenols, or other condensible hydrocarbons are generated due to the high gasification temperature employed. It is recognized by those skilled in the art that gasification temperatures of at least 1800.degree.-2000.degree. F. are necessary to pyrolyze any condensible hydrocarbons generated from coal. Steiner, on the other hand, in Column 2, lines 49-54 of U.S. Pat. No. 4,082,519, expressly teaches against the use of high temperature.
A final major distinction between the disclosed invention and all of the prior teachings is that in all of the prior methods the reduction of SO.sub.2 occurs directly in the gasifier, whereas the disclosed invention teaches the reduction of SO.sub.2 externally to the gasifier. This is done in the disclosed invention in order to overcome certain disadvantages which are not obvious from the prior teachings. First of all, when SO.sub.2 is intimately contacted with carbon within a gasifier most of the SO.sub.2 can be reduced to sulfur, but, unfortunately, in the presence of steam some sulfuric or sulfurous acid can form, particularly at low temperature. As an example, in the Westvaco carbon absorption process for removal of SO.sub.2 from flue gas, as reported in The Oil and Gas Journal for Sept. 11, 1978, on page 90, carbon is intentionally used to absorb SO.sub.2 as sulfuric acid which can then be regenerated. It is of course resonable to expect that not all of the sulfuric acid can be regenerated and, if so, a portion of it should be regenerated to form sulfur trioxide. An additional reason for not introducing SO.sub.2 directly to the gasifier of the disclosed invention is that free oxygen could otherwise oxidize some of the SO.sub.2 to SO.sub.3. Steiner in U.S. Pat. No. 4,082,519, as well as Kertamus, teach the combustion of recycled sulfur compounds to SO.sub.2 prior to introduction to the gasifier. Here, an additional opportunity exists for the formation of SO.sub.3. Although SO.sub.3 would be generated only in minor quantities, it is significant to note that only minor quantities of SO.sub.3 can affect the dewpoint of the gas generated and can furthermore result in formation of dilute sulfuric acid in the sulfur recovery system of prior processes. Unlike many inorganic acids, sulfuric acid is extremely corrosive when in dilute, not concentrated form.
An additional disadvantage to adding SO.sub.2 directly to the gasifier is that organic sulfur compounds such as COS, CS.sub.2, and traces of mercaptans, are easily formed in view of the fact that the sulfur is intimately contacted with carbon. In addition, low temperature operation encourages such formation. Once formed, these organic sulfur compounds are not easily reduced since, as previously mentioned, they are slow to react. Hydrogen is effective in reducing these organic sulfur compounds, particularly at high temperature, but as previously discussed, hydrogen is not easily generated in prior practices because of heat deficiencies. In British Pat. No. 1,390,364, as shown in Example 1, COS in the raw gas accounted for up to 14% of the sulfur fed to the gasifier, while CS.sub.2 accounted for up to about 6% of the sulfur fed to the gasifier. Similar levels of organic sulfur were observed in the aforementioned operations in Trail, British Columbia. In the Kertamus et al. article previously cited, COS was present at up to 47.0 vol. % in the product gas. Although it might first appear that organic sulfur compounds could be recovered from the effluent gas for recycle to the gasifier, that is not believed to be practical since the low temperature and low level of hydrogen in prior processes would not adequately serve to reduce these compounds to more acceptable forms, particularly in the presence of additional coal charged to the gasifier.
With today's increasingly stringent regulations on discharge of sulfur compounds to the atmosphere, the presence of the organic sulfur compounds characteristically produced by coal gasification is presenting serious problems. First of all, these organic sulfur compounds are difficult or expensive to remove from the raw gas by known acid gas removal systems. Separation is enhanced by first catalytically hydrolyzing these compounds to H.sub.2 S, which is much easier to remove. Catalytic processing, however, can be expensive and often impractical. Known sulfur recovery processes additionally have great difficulty in removal of organic sulfur compounds, and in many cases organic sulfur compounds may additionally be formed in some sulfur recovery processes. In the case of the well-known Claus process as reported by Beavon in the aforementioned Chemical Engineering article, COS and CS.sub.2 which form in the Claus reaction furnace are slow to react to form sulfur over the Claus catalyst. Typically 0.25-2.5% of the input sulfur to the Claus process passes untouched in these forms into the Claus tailgas. As of August 1978, at least ten commercially available processes were available for treatment of Claus tailgas to extend sulfur recovery. These processes are characteristically expensive and concentrate on methods, usually catalytic, for reducing organic sulfur compounds to sulfur or to hydrogen sulfide. Claus tailgas is characteristically very lean in sulfur compounds, with over 90% of the gas comprised of nitrogen, carbon dioxide, and water vapor. Typically, within Claus tailgas, organic sulfur compounds account for about 17% of the total sulfur present in said tailgas, with the balance of sulfur compounds being S.sub.2, SO.sub.2, and H.sub.2 S. Recycling of tailgas to the Claus furnace is sometimes practiced, for example, as taught by Hujsak et al. in U.S. Pat. No. 3,681,024, but it is essential to note that even with recycling the net generation of tailgas (stream 60, for example, in the patent of Hujsak et al.) is unavoidable in the Claus process since nitrogen contained in the air charged to the Claus furnace and water formed from H.sub.2 S must somewhere leave the process, and with the tailgas some sulfur compounds are unavoidably lost or must be further treated. Recycling tailgas to the gasifiers employed in prior teachings is believed to be impractical since, as previously mentioned, there would be difficulties in reduction of organic sulfur and, in addition, the nitrogen and CO.sub.2 present in the tailgas would represent a tremendous heat drain to the gasifiers used in prior practices which already are difficult to maintain in thermal balance.
Aside from the Claus process other well-known sulfur recovery processes are available. Examples are the Stretford and Thylox processes, where sulfur is formed by an oxidation-reduction coupling reaction from H.sub.2 S which is absorbed from the raw gas. The practical use of such processes has heretofore been restricted by the fact that organic sulfur compounds such as COS cannot be reduced to sulfur by such sulfur recovery systems.
The invention disclosed herein offers a practical and economical scheme for reducing SO.sub.2 while still being able to keep organic sulfur compounds from accumulating in the system. This is accomplished by use of a two-stage concept, the features of which will be discussed herein.
In the broadest scope of the invention, carbonaceous fuel, oxygen, and steam are fed, preferably by entrainment, to a known gasifier which operates at a temperature of preferably at least 2000.degree. F. to yield a raw gas rich in hydrogen and carbon monoxide, along with a residue ash by-product. The novel features of the invention relate to the use of the raw gas for effecting the reduction of SO.sub.2 without an intolerable buildup of organic sulfur compounds or release of sulfur compounds to the atmosphere.
The raw gas enters the first stage of the disclosed process where a recycle gas containing typically 2-3 volume percent of sulfur compounds, including SO.sub.2, S.sub.2, COS, CS.sub.2, H.sub.2 S, and traces of mercaptans is introduced. Within this first stage these sulfur compounds are converted nearly completely to S.sub.2 and H.sub.2 S. Nitrogen, carbon dioxide, and water vapor contained in the recycle gas serve to cool the gas within the first stage to about 1800.degree. F., thereby coincidentally freezing any molten ash particles which might be entrained in the raw gas exiting the gasifier.
Gas leaves the first stage of the process and then enters a second stage wherein SO.sub.2, or an SO.sub.2 -rich gas, from an external source is added. The SO.sub.2 then reacts to form diatomic and some monoatomic sulfur vapor, along with H.sub.2 S and small quantities of COS and traces of CS.sub.2 and mercaptans. Heat generated by the reaction of SO.sub.2 with hydrogen heats the gas to at least 2000.degree. F. and excess heat is removed preferably by the indirect generation of steam along with simultaneous condensation of a portion of the sulfur vapors.
Gas leaves the second stage and enters a heat recovery unit, such as a waste heat boiler, whereby the gas is cooled to a temperature greater than 340.degree. F., but yet above the temperature at which water vapor begins to condense from the gas. Sulfur condensed from the second stage is combined with sulfur condensed from the heat recovery unit and then frozen for disposal or by-product use.
Gas from the second stage is essentially free of sulfur vapor and is then scrubbed of remaining particulates and cooled, with simultaneous condensation of water vapor. Sulfur compounds present in the cleaned gas may then be removed leaving behind a sulfur-free by-product gas containing CO and H.sub.2, with the sulfur compounds so removed then sent to a sulfur recovery unit, such as the well-known Claus or Stretford processes, for recovery of additional sulfur. Tailgas from the sulfur recovery plant is then sent to the first stage of the process. As an alternative, the known sulfur recovery process can be eliminated and the cleaned gas would instead be treated for removal of CO.sub.2, with the CO.sub.2 -free gas then sent to the first stage of the invention.
For a further understanding of the invention and for features and advantages thereof, reference may be made to the following description and drawings which illustrate preferred embodiments of equipment in accordance with the invention.
US Referenced Citations (3)
Continuation in Parts (3)
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Number |
Date |
Country |
| Parent |
806541 |
Jun 1977 |
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| Parent |
660058 |
Feb 1976 |
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| Parent |
531064 |
Dec 1974 |
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Patent Grant
4275044
