ENZYMATIC COAL DESULFURIZATION

Abstract: Coal is desulfurized by treatment with a hydrolase to convert organic sulfur moieties in the coal matrix to sulfates, and by treatment with a sulfatase to cleave the sulfates.

Description: This invention relates to fossil fuel desulfurization, and particularly to coal desulfurization with enzymes such as oxidases and hydrolases. Due largely to environmental concerns, there is an increasing need for low-sulfur emissions from fossil fuels such as coal which contain sulfur. Heretofore, both post-combustion and pre-combustion desulfurization techniques have been available. For example, flue gas desulfurization is a well know post-combustion process. However, it is generally inconvenient, expensive and limited with respect to the amount and types of sulfur combustion products which can be removed. Flue gas treatment also ignores other economic impacts from the handling and processing of fuels containing sulfur, such as corrosion caused by the sulfur in coal to the equipment used to handle the coal. Pre-combustion processes, on the other hand, which result in low-sulfur fuels, can reduce both sulfur emissions and equipment corrosion. Conventional coal desulfurization processes include physical methods such as pyrite flotation or magnetic separation. While these physical methods are convenient and economical, they are capable of removing only inorganic sulfur and generally result in notable energy losses. On the other hand, chemical coal desulfurization processes, such as oxidation with ferric salts, chlorine or ozone, or reduction with solvent-hydrogen mixture, are somewhat more effective in removing organic sulfur, but generally have numerous disadvantages, such as, corrosion problems from reagents, high energy requirements, and costly reagent recovery. Attempts have also been made to remove sulfur from coal by microbiological methods. Early interest in this field focused on microorganisms which were naturally suited for sulfur digestion, such as Thiobacillus found in mine waters and Sulfolobus found in sulfur springs, as reported in Detz et al, American Mining Congress Journal , vol. 65, p. 75 (1979); Kargi et al, Biotechnology and Bioengineering , vol. 24, pp. 2115-2121 (1982). However, such bacteria utilize only inorganic sulfur and have no propensity for organic sulfur removal. More recently, efforts have focused on the adaptation of microorganisms for organic sulfur removal. Such attempts are reported, for example, in Isbister et al, "Microbial Desulfurization of Coal", in Attia (ed), Processing and Utilization of High Sulfur Coal , p. 627 (1985); and Robinson and Finnerty, "Microbial Desulfurization of Fossil Fuels" (University of Georgia). There are, however, numerous obstacles which must be overcome before such techniques become practical. For example, optimal growth conditions in a large scale process are difficult and expensive to maintain, typically requiring expensive growth factors and excessive nutrients or additives. Such additives themselves can be a potential environmental concern and possibly as difficult to remove economically as the sulfur. The growth of the microorganisms can also produce toxic by-products or compounds which may result in mortality or render the microorganisms incapable of catabolizing sulfur. In addition, such fermentation processes are usually plagued with problems such as culture stability, mutation or contamination, reactor upsets, substrate variation, and the like. Thus, there remains an unfilled need for an economical and efficient method for desulfurizing coal and other fossil fuels. According to a first aspect of the present invention there is provided a method for removing sulfur from a fossil fuel substrate containing organic sulfur, comprising the steps of: oxidizing the fossil fuel substrate by contacting the substrate with an acid or with an oxidation enzyme; contacting the substrate with a sulfur-removing enzyme; and recovering a fossil fuel having a reduced sulfur content. The present invention thus involves the biochemical treatment of coal and other fossil fuels to remove sulfur or to desulfurize the fossil fuel. The biochemical treatment comprises contacting the sulfur-containing fossil fuel with an enzyme or enzymes in an amount generally effective to reduce the amount of sulfur in the fuel. The enzymes are added directly to the fossil fuel and need not be produced by microorganisms growing on the fossil fuel as a substrate or growth medium. Thus, the process need not be controlled to maintain the viability of any enzyme-producing microorganisms, but can be optimized to favor enzymatically mediated conversion of the sulfur into a form that can be separated from the fossil fuel. The invention will be described by way of example only with reference to the accompanying drawings in which: Fig.1 is a schematic illustration of an embodiment of the process according to the present invention; Fig.2 is a schematic illustration of an alternate embodiment of the process according to the present invention; Fig.3 is a graphical illustration of spectral data of filtrates of dibenzothiophene (DBT) treated with a peroxidase and a sulfatase as described in Example 1 hereinbelow; Fig.4 is a graphical illustration of spectral data of filtrates of Wyodak coal at various periods of time following treatment with a peroxidase and a sulfatase, as described in Example 2 hereinbelow; and Fig.5 is a graphical illustratiion of spectral data of filtrates of Illinois No. 6 coal at various perods of time following treatment with a peroxidase and a sulfatase as described in Example 3 hereinbelow. The present invention includes a process for treating fossil fuels, and especially fossil fuels containing organic sulfur. Contemplated fossil fuels include petroleum and coal; products of fossil fuel conversion processes, e.g., coal-derived liquids, are also considered. As used herein, coal includes any coalified organic material such as peat, lignite, sub-bituminous coal, bituminous coal and anthracitic coal. The fossil fuel should contain organic sulfur to obtain the most benefit from treatment according to the present invention, although inorganic sulfur could also be removed by this process. By organic sulfur is generally meant organic thiophenes, sulfides and thiols, whereas inorganic sulfur generally refers to sulfates and metallic sulfides such as pyrite. According to the present process, a two-step reaction pathway is generally employed. In such a two-step reaction the organic sulfur is initially converted into a sulfur oxide, e.g., sulfate, by oxidation. However, in some rare instances oxidation may not be necessary, because the organic sulfur may be predominantly in the sulfate form or substantially only the naturally occurring sulfate is to be removed. In this sense, the oxidation can be considered to be an optional reaction. However, for optimal sulfur removal, oxidation is preferred. The oxidation substantially converts the organic sulfur into sulfate. The sulfate is enzymatically removed, for example by hydrolysis induced by a sulfur hydrolase, e.g., a sulfatase. The fossil fuel may be prepared for treatment according to the present method by generally known methods; e.g., solid fossil fuels, such as coal, can be ground and slurried in water. The slurry can be prepared by grinding the solid fossil fuel to an appropriate particle size, typically 10-50 µm, and mixing it with water. For the purpose of illustration only, the invention is described hereinbelow with reference to a ground coal slurry with the understanding that other fossil fuels and media may be analogously employed. For example, in the case of oil, it may be sufficient to prepare an emulsion if an aqueous enzymatic treatment is employed, or to treat the oil neat, with a solvent, or in mixture with another immiscible fluid. The oxidation of the coal slurry may be effected by treatment with an oxidation enzyme, such as, a peroxidase, a laccase, or a like oxidase. As used herein, a peroxidase is any enzyme havng the E.C. number 1.11.1.7, e.g., horseradish peroxidase, and a laccase is any enzyme having E.C. number 1.10.3.2, e.g., Pyricularis oxyzae laccase. For example the oxidation may be effected by contacting the fossil fuel substrate with horseradish peroxidase in the presence of excess oxygen at a temperature of from 0-80°C and a pH from 5-9, and with the amount of the horseradish peroxidase ranging from 0.01 to 10 parts by weight per 100 parts by weight of fossil fuel substrate. Alternatively, partial oxidation may be effected by acidic treatment of the coal particles. The acidic oxidation may be at ambient temperature. This would be done in the conventional oxidative manner of pretreatment of coal prior to desulfurization. The oxidation serves to convert the organic sulfur moieties into sulfur oxide or moieties, such as sulfate. It is desirable to convert the maximum possible amount of organic sulfur to sulfur oxides. On the other hand, full oxidation to sulfur dioxide is generally undesirable, as also is excessive oxidation of the carbon in the coal matrix. Usually the desired degree of oxidation can be achieved by varying the type of oxidase or other oxidant, the oxidant concentration, duration of contact between the coal and the oxidant, and other conditions of treatment, e.g., pH, temperature, oxygen availability. The hydrolysis of the oxidized sulfur moieties is then effected, as mentioned above, by sulfatase treatment. As used herein, sulfatase inclues any enzyme capable of hydrolyzing the sulfur moieties to yield a water-soluble sulfur compound. Specific examples include enzymes having the E.C. number 3.1.6.1, such as limpet sulfatase, Aerobacter aerogenes sulfatase, abalone entrail sulfatase, Helix pomatia sulfatase, and the like. For example, the contacting of the fossil fuel substrate with the sulfatase enzyme may be effected in the presence of excess water at a temperature of 0-80°C and a pH from 5-9 and with the amount of the sulfatase enzyme ranging from 0.01 to 10 parts by weight per 100 parts by weight of substrate. The oxidation and the contact with the sulfur-removing enzyme may be concurrent or consecutive. The oxidation enzyme and the sulfur-removing enzyme may be immobilized on packing prior to contacting the substrate. The coal particles may be treated with the sulfatase and/or oxidation enzymes, with or without additional chemical oxidation. One contemplated process scheme is a fluidized bed reactor as illustrated in Fig. 1. Generally, uniform concentration and temperature are maintained throughout the fluid bed reactor 100, and the enzyme is immobilized on support particles E which are relatively larger in size than the coal particles in the slurry typically fed into the lower portion of the reactor 100. This size difference permits retention of the enzyme support particles E by catalyst retention screen S and gravity separation in the upper portion of the reactor 100 near the effluent port C in the conventional manner of fluid bed operation. Air or other suitable gas is typically supplied to the bottom of the reactor 100 to promote back mixing and CSTR conditions. An alternative processing scheme for a moving bed reactor, which generally follows the format of the Examples set forth below, is illustrated in Fig. 2. The coal slurry is introduced from hold-up/preparation tank 200 generally to the upper end of inclined moving bed 202 and discharged from the lower end thereof. As the coal descends through the reactor 202, it is continuously contacted with enzyme solution in a countercurrent fashion to release the sulfur as sulfate which is soluble in the enzyme solution. The enzyme/sulfate solution effluent from the reactor is recovered by adsorption on a sorbent in enzyme adsorption unit 204. The sulfate solution is readily separated from the sorbent and collected in tank 206 in which, for example, lime or other basic material may be used to precipitate the sulfate prior to disposal. The adsorbed enzyme from unit 204 is then desorbed in unit 208. The desorbed enzyme is then recycled to the reactor 202 along with any makeup enzyme, while the sorbent may be recycled through the enzyme adsorption/desorption cycle. The invention is illustrated by way of the examples which follow. Example 1 A suspension was prepared of 100 mg dibenzothiophene ("DBT") in 3 ml of 0.1 M Tris buffer, pH 7.0. To this suspension at room temperature was added 0.5 ml of horseradish peroxidase (Sigma P 8000) at 2 mg/ml in buffer, and 0.5 ml of Aerobacter aerogenes sulfatase (Sigma S 1629) at 2 mg/ml in buffer. The mixture was maintained at room temperature in an air atmosphere, and reaction samples were periodically removed and filtered. Solids were analyzed for elemental composition and such analyses are presented in Table 1. Filtrates from the peroxidase/sulfatase treated DBT were analyzed for spectral changes and such spectral data are presented in Fig. 3. The spectral data demonstrate a spectral shift in the direction of longer wavelengths indicative of increased polarity which would be expected from conversion of DBT by the peroxidase/sulfatase enzymes. The elemental analysis demonstrates an increase in oxygen content and a decrease in sulfur content. Moreover, it was also observed that starting reaction mixtures were distinctly two-phase liquid-solid mixtures whereas later reaction mixtures were strongly wetted and appeared as milky suspensions. Example 2 The procedure of Example 1 was repeated using 100 mg ball-milled Wyodak coal instead of DBT. The results are presented in Table 2 and Fig. 4. The spectral changes demonstrated in Fig. 4 for Wyodak coal are similar to, although more pronounced than those observed with DBT, indicating more extensive reacting of the Wyodak coal than the DBT, in the presence of the peroxidase and sulfatase. The large drop in sulfur percentage by elemental analysis seen in the data in Table 2 indicates that about 80% of the sulfur was removed. It is believed that the results with the Wyodak coal are better than with DBT because only a fraction of the organic sulfur in coal is aromatic, thiophene-type sulfur which is generally more recalcitrant to chemical conversion than other types of organic sulfur found in coal. The increase in nitrogen percentage is probably due to adherence of the enzymes to the coal particles. Example 3 The procedure of Example 2 was repeated using Illinois No. 6 coal instead of Wyodak coal. The results are presented in Table 3 and Fig. 5. As seen from Table 3 and Fig. 5, the enzyme-mediated treatment of Illinois No. 6 coal desulfurizes the coal in a manner similar to the Wyodak coal.

Claims: A method for removing sulfur from a fossil fuel substrate containing organic sulfur, comprising the steps of: oxidizing the fossil fuel substrate by contacting the substrate with an acid or with an oxidation enzyme; contacting the substrate with a sulfur-removing enzyme; and recovering a fossil fuel having a reduced sulfur content. A method as claimed in claim 1, wherein the substrate is coal, petroleum, or process-derived products thereof. A method as claimed in claim 1 or 2, wherein the oxidation is effected by contacting the substrate with peroxidase or laccase. A method as claimed in claim 3, wherein the oxidation is effected by contacting the substrate with horseradish peroxidase in the presence of excess oxygen at a temperature of from 0-80°C and a pH from 5-9, and with the amount of the horseradish peroxidase ranging from 0.01 to 10 parts by weight per 100 parts by weight of substrate. A method as claimed in any one of claims 1 to 4 wherein the sulfur-removing enzyme is a sulfatase enzyme selected from the group consisting of Aerobacter species sulfatase, lympet sulfatase, abalone entrail sulfatase, and Helix species sulfatase. A method as claimed in claim 5 wherein the contacting of the fossil fuel substrate with the sulfatase enzyme is in the presence of excess water at a temperature of 0-80°C and a pH from 5-9 and with the amount of the sulfatase enzyme ranging from 0.01 to 10 parts by weight per 100 parts by weight of substrate. A method as claimed in any one of claims 1 to 6, wherein the oxidation and the contacting with the sulfur-removing enzyme is consecutive. A method as claimed in any one of claims 1 to 6 wherein the oxidation and the contacting with the sulfur-removing enzyme is concurrent. A method as claimed in any one of claims 1 to 6, wherein said oxidation enzyme and said sulfur-removing enzyme are immobilized on packing prior to contacting the substrate.

Inventors: KERN, ERNEST E., MENGER, WILLIAM M., ODELSON, DAVID A., SINSKEY, ANTHONY S., TRANTOLO, DEBRA J., WISE, DONALD L.
Applicants: HOUSTON INDUSTRIES INCORPORATED
Application year: 1989
Main classification: C10L9
IPC codes: C10L9