DE68905180T2 - ENZYMATIC DESULFURATION OF CARBON. - Google Patents

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 the desulfurization of fossil fuel and particularly to the desulfurization of coal with enzymes such as oxidases and hydrolases.

Mainly due to environmental concerns, there is an increasing need for low-sulfur emissions from sulfur-containing fossil fuels such as coal. To date, desulfurization processes have been available both after combustion and before combustion. For example, flue gas desulfurization is a well-known post-combustion process. However, it is generally impractical, expensive and limited in terms of the amount and types of sulfur combustion products that can be removed. Flue gas treatment also ignores other economic impacts related to handling and processing of sulfur-containing fuels, such as corrosion of the equipment used to handle the coal caused by the sulfur contained in coal. Pre-combustion processes that result in low-sulfur fuels, on the other hand, can reduce both sulfur emissions and corrosion of equipment.

Conventional coal desulfurization processes involve physical processes such as pyrite flotation or magnetic separation. Although these physical processes are practical and economical, they can only remove inorganic sulfur and generally result in significant energy losses. On the other hand, chemical coal desulfurization processes such as oxidation with ferric salts, chlorine or ozone or reduction with a solvent-hydrogen mixture are somewhat more effective in removing organic sulfur, but have In general, there are numerous disadvantages, such as corrosion problems caused by reagents, high energy requirements and costly recovery of reagents.

Attempts have also been made to remove sulfur from coal using microbiological processes. Early interest in this area focused on microorganisms that were naturally adapted to sulfur digestion, such as Thiobacillus found in mine water and Sulfolobus present 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, these bacteria utilize only inorganic sulfur and have no predisposition to remove organic sulfur. Recently, efforts have focused on adapting microorganisms to remove organic sulfur. 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). However, there are numerous obstacles that must be overcome before such techniques become practical. For example, it is difficult and expensive to maintain optimal growth conditions in a large-scale process, which typically requires expensive growth factors and excessive nutrients or additives. Such additives can themselves be potentially environmentally hazardous and may be as difficult to remove economically as the sulfur. The growth of microorganisms can also produce toxic byproducts or compounds that can cause mortality or render the microorganisms unable to catabolize sulfur. In addition, such fermentation processes are usually plagued by problems, such as culture stability, mutation or contamination, reactor disturbances, substrate variation and the like. Consequently, an unfilled gap remains for an economical and efficient process for the desulfurization of coal and other fossil fuels.

According to a first feature of the present invention, there is provided a process for removing sulphur from a fossil fuel substrate containing organic sulphur, comprising the steps of:

oxidizing the fossil fuel substrate by contacting the substrate with an acid or an oxidizing enzyme;

contacting the substrate with a sulfur-removing enzyme; and

Recovery of a fossil fuel with reduced sulfur content.

The present invention thus encompasses the biochemical treatment of coal and other fossil fuels to remove sulfur or to desulfurize fossil fuel. The biochemical treatment involves contacting the sulfur-containing fossil fuel with an enzyme or enzymes in an amount generally effective in reducing the amount of sulfur in the fuel. The enzymes are added directly to the fossil fuel and do not need to be produced by microorganisms growing on the fossil fuel as a substrate or growth medium. Thus, the process does not need to be controlled to maintain the viability of any enzyme-producing microorganisms, but can be optimized to favor the enzymatically mediated conversion of the sulfur to a form that can be separated from the fossil fuel.

The invention will be described purely by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of an embodiment of the process according to the invention;

Fig. 2 is a schematic representation of an alternative embodiment of the process according to the invention;

Figure 3 is a graphical representation of spectral data from filtrates of dibenzothiophene (DBT) treated with a peroxidase and a sulfatase as described in Example 1 below;

Fig. 4 is a graphical representation of spectral data of filtrates of Wyodak coal at various time periods after treatment with a peroxidase and a sulfatase as described in Example 2 below; and

Figure 5 is a graphical representation of spectral data of filtrates of Illinois No. 6 coal at various times after treatment with a peroxidase and a sulfatase as described in Example 3 below.

The present invention comprises a process for treating fossil fuels, and particularly fossil fuels containing organic sulfur. Fossil fuels contemplated include petroleum and coal; products of fossil fuel conversion processes, e.g. coal-derived liquids, are also contemplated. As used herein, coal includes all carbonized organic material such as peat, lignite, subbituminous coal, bituminous coal and anthracite coal. To obtain the greatest benefit from the treatment of the invention, the fossil fuel should contain organic sulfur, although inorganic sulfur could also be removed by this process. By organic sulfur is meant generally organic thiophenes, sulfides and thiols, whereas inorganic sulfur usually refers to sulfates and metal sulfides such as pyrite.

According to the present process, a two-stage reaction route is generally used. In such a two-stage reaction, the organic sulfur is initially converted by oxidation into a sulfur oxide, e.g. sulfate. In some rare cases, however, oxidation may not be necessary because the organic sulfur may be predominantly in the sulfate form or essentially only the naturally occurring sulfate is to be removed. In this sense, oxidation can be considered an optional reaction. However, for optimal sulfur removal, oxidation is preferred. Oxidation essentially converts the organic sulfur into sulfate. The sulfate is removed enzymatically, for example by means of hydrolysis triggered by a sulfur hydrolase, eg a sulfatase.

The fossil fuel may be prepared for treatment according to the present process by well-known methods; for example, solid fossil fuels such as coal may be ground and slurried in water. The slurry may be prepared by grinding the solid fossil fuel to an appropriate particle size, typically 10 - 50 µm, and mixing with water. For purposes of illustration only, the invention described below will be described with reference to a ground coal slurry, with the understanding that other fossil fuels and media may be used analogously. For example, in the case of oil, preparation of an emulsion when an aqueous enzymatic treatment is used, or treatment of the oil neat, with a solvent, or in a mixture with another immiscible liquid may be sufficient.

Oxidation of the coal sludge can be brought about by treatment with an oxidizing enzyme such as a peroxidase, a laccase or a similar oxidase. As used herein, a peroxidase is any enzyme having 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.

The oxidation can be brought about, for example, by contacting fossil fuel substrate with horseradish peroxidase in the presence of excess oxygen at a temperature between 0 - 80ºC and a pH between 5 - 9, the amount of horseradish peroxidase being from 0.01 up to 10 parts by weight per 100 parts by weight of the Fossil fuel substrate.

Alternatively, partial oxidation can be induced by acid treatment of the coal particles. The acid oxidation can take place at ambient temperature. This would be carried out in the conventional oxidative pretreatment of coal prior to desulfurization.

Oxidation is used to convert the organic sulfur moieties into sulfur oxide or moieties such as sulfate. It is desirable to convert the maximum amount of organic sulfur to sulfur oxides. On the other hand, complete oxidation to sulfur dioxide is generally undesirable, as is excessive oxidation of the carbon in the coal matrix. In general, the desired degree of oxidation can be achieved by varying the type of oxidase or other oxidizing agent, the concentration of the oxidizing agent, the duration of contact between the coal and the oxidizing agent, and other treatment conditions such as pH, temperature, and oxygen availability.

Hydrolysis of the oxidized sulfur moieties is then accomplished by sulfatase treatment, as mentioned above. As used herein, sulfatase includes any enzyme capable of hydrolyzing the sulfur moieties to produce a water-soluble sulfur compound. Specific examples include enzymes having E.C. number 3.1.6.1, such as limpet sulfatase, Aerobacter aerogenes sulfatase, sea snail gut sulfatase, Helixpomatia sulfatase, and the like.

Contacting the fossil fuel substrate with the sulfatase enzyme can, for example, be carried out in the presence of excess water at a temperature between 0 - 80ºC and a pH between 5 - 9, with the amount of the sulfatase enzyme ranging from 0.01 up to 10 parts by weight per 100 parts by weight of the substrate.

The oxidation and contact with the sulfur-removing enzyme can be carried out simultaneously or consecutively. The oxidation enzyme and the sulfur-removing enzyme can be mixed during packing before they are brought into contact with the substrate. be immobilized.

The carbon particles can be treated with the sulfatase and/or oxidation enzymes, with or without additional chemical oxidation. One process scheme contemplated is a fluidized bed reactor as illustrated in Figure 1. Generally, a uniform concentration and temperature is maintained throughout the fluidized bed reactor 100, and the enzyme is immobilized on carrier particles E which are relatively larger than the carbon particles in the slurry typically fed into the lower portion of the reactor 100. This size difference allows retention of the enzyme carrier particles E by catalyst retention screen S and gravity separation in the upper portion of the reactor 100 near the discharge opening C in conventional fluidized bed operation. Air or other suitable gas is typically supplied to the bottom of the reactor 100 to promote backmixing and continuous stirred tank reactor conditions.

An alternative processing scheme for a moving bed reactor, generally following the format of the examples set forth below, is illustrated in Figure 2. The coal slurry is fed from the hold/preparation tank 200 generally at the top of the inclined moving bed 202 and discharged at the bottom thereof. As the coal flows downward through the reactor 202, it continuously contacts the enzyme solution in a countercurrent manner 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 to a sorbent in an enzyme adsorption unit 204. The sulfate solution is readily separated from the sorbent and collected in tank 206 where, 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 returned to reactor 202 together with all additional enzyme. while the sorbent can be recycled through the enzyme adsorption/enzyme desorption cycle.

The invention is illustrated by the following examples.

example 1

A suspension was prepared from 100 mg of dibenzothiophene ("DBT") in 3 ml of 0.1 M Tris buffer, pH 7.0. To this suspension were added at room temperature 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. These analytical data are shown in Table 1. TABLE 1 Elemental analysis (weight percent) Sample DBT/Peroxidase DBT/Peroxidase/Sulfatase

Filtrates from the peroxidase/sulfatase treated DBT were analyzed for spectral changes and these spectral data are presented in Figure 3. The spectral data show a spectral shift towards the longer wavelengths, indicating increased polarity that would be expected from conversion of DBT by the peroxidase/sulfatase enzymes. Elemental analysis shows an increase in oxygen content and a decrease in sulfur content. In addition, it was also observed that the starting reaction mixtures were clearly two-phase liquid-solid mixtures, whereas later reaction mixtures were heavily wetted and looked like milky suspensions.

Example 2

The procedure of Example 1 was carried out using 100 mg of ball milled Wyodak coal instead of DBT. The results are presented in Table 2 and Fig. 4. TABLE 2 Elemental Analysis (Wt.%) Sample Hours Wyodak Coal Wyodak Coal/Peroxidase/Sulfatase

The spectral changes detected for Wyodak coal in Fig. 4 are similar to, although more pronounced than, those observed with DBT, suggesting a more extensive response of Wyodak coal than DBT in the presence of peroxidase and sulfatase.

The large percentage decrease in sulfur observed in the data in Table 2 based on elemental analysis indicates that approximately 80% of the sulfur was removed. It is believed that the results are better with the Wyodak coal than with DBT because only a fraction of the organic sulfur in coal is aromatic, thiophene-type sulfur, which is generally more resistant to chemical conversion than other types of organic sulfur found in coal. The percent increase in nitrogen is probably due to the adhesion 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. TABLE 3 Elemental Analysis (Wt. Percent) Sample Hours Illinois No. 6 Coal Illinois No. 6 Coal/Peroxidase/Sulfatase

As can be seen from Table 3 and Fig. 5, the enzyme-mediated treatment of the Illinois No. 6 coal desulfurizes the coal in a similar manner to the Wyodak coal.

Claims (9)

1. A process for removing sulphur from a fossil fuel substrate containing organic sulphur, which comprises the following steps: oxidizing the fossil fuel substrate by contacting the substrate with an acid or an oxidizing enzyme; contacting the substrate with a sulfur-removing enzyme; and Recovery of a fossil fuel with reduced sulfur content. 2. A process according to claim 1, in which the substrate is coal, petroleum or products obtained therefrom by processes. 3. A process according to claim 1 or 2, in which the oxidation is brought about by bringing the substrate into contact with peroxidase or laccase. 4. A process according to claim 3, in which the oxidation is brought about by contacting the substrate with horseradish peroxidase in the presence of excess oxygen at a temperature between 0-80ºC and a pH between 5 and 9, and the amount of horseradish peroxidase ranges from 0.01 up to 10 parts by weight per 100 parts by weight of the substrate. 5. A method according to any one of claims 1 to 4, in which the sulfur removing enzyme is a sulfatase enzyme selected from the group comprising Aerobacter species sulfatase, limpet sulfatase, sea snail viscera sulfatase and Helix species sulfatase. 6. A process according to claim 5, in which contacting the fossil fuel substrate with the sulfatase enzyme takes place in the presence of excess water at a temperature between 0-80ºC and a pH between 5 and 9, and wherein the amount of the sulfatase enzyme ranges from 0.01 up to 10 parts by weight per 100 parts by weight of the substrate. 7. A process according to any one of claims 1 to 6, in which the oxidation and the contacting with the sulfur-removing enzyme are carried out consecutively. 8. A process according to any one of claims 1 to 6, in which the oxidation and the contacting with the sulfur-removing enzyme are carried out simultaneously. 9. A method according to any one of claims 1 to 6, in which said oxidation enzyme and said sulfur removing enzyme are immobilized during packing before being brought into contact with the substrate.