Zone Combustion of High Sulfur Coal to Reduce SO.sub.x Emission

This invention relates to a method of reducing the SO.sub.x emission in the combustion of coal which contains sulfur compounds and more particularly, to a three zone furnace for burning high sulfur coal.

The use of coal for the generation of power and the like is increasingly important as supplies of hydrocarbons become scarce. The use of coal has been limited by the high sulfur content of much coal. When burned, high sulfur coal produces SO.sub.3 and SO.sub.2 which have an adverse environmental impact.

Attempts to minimize this impact have included extensive cleaning of the sulfur coals prior to burning. For example, in U.S. Pat. No. 4,052,170-Yan, paramagnetic impurities including sulfur compounds are magnetically separated from the coal before burning. U.S. Pat. Nos. 4,077,866 Owen, Venuto and Yan and 4,118,201-Yan show other techniques for producing low sulfur fuel from coal.

Another approach is to remove SO.sub.3 and SO.sub.2 from the flue gas emitted from the furnace. Some of the techniques for accomplishing this are described in "SULFUR DIOXIDE EMISSION CONTROL BY HYDROGEN SULFIDE REACTION IN AQUEOUS SOLUTION," Bureau of Mines Report of Investigations/1973, R. I. 7774, United States Department of the Interior.

Desulfurization has also been practiced during the conversion of coal to its volatile components, an example of which is shown in U.S. Pat. Nos. 3,736,233 Sass et al., in 4,253,409 Warmser and 3,727,562 Bauer.

The equipment which is required to separate the sulfur from the coal prior to burning and to collect SO.sub.x emissions from the flue gas is very expensive. This has limited the use of these approaches to controlling SO.sub.x emission.

Another approach which does not require the use of expensive equipment is to add scavengers to the coal. See U.S. Pat. Nos. 4,245,573-Dixit et al, 4,256,703-Dixit et al and 4,322,218-Nozaki. It is known that a significant amount of sulfur will be retained in the ash if the coal contains basic minerals, such as calcium carbonate, dolomite, or soda ash. These basic minerals act as scavengers which trap the SO.sub.x, preventing its emission, and facilitating removal with the ash.

It is an object of the present invention to improve the trapping efficiency of sulfur scavengers, both indigenous in the coal or added to it.

In accordance with the present invention a, multiple stage combustion furnace is operated in a manner which facilitates the removal of ash containing sulfur from the combustion process.

In accordance with the present invention, the coal is pyrolized in a reducing atmosphere to convert the sulfur compounds to combustible gases including H.sub.2 S. The H.sub.2 S is effectively trapped in the basic minerals contained in the indigenous coal or in the added scavengers. I have found that the trapping of sulfur compounds in the ash is performed more efficiently by my invention than in the direct combustion of coal.

A three zone furnace is used to practice the invention. In the first, lower zone, char is combusted in an oxidizing atmosphere, at high temperature to achieve complete combustion and high carbon utilization. Primary air supplied to the first zone is restricted so that little or no excess oxygen reaches the second zone, which is above the first. In the second zone, the coal is pyrolized to form char and combustible gases in a reducing atmosphere. In this zone the sulfur compounds in the coal are converted to highly reactive H.sub.2 S. This H.sub.2 S reacts with the basic minerals in the coal which is added to the second stage. It also reacts with the char produced in the first stage to produce ash in which the sulfur compounds are trapped. Basic mineral scavengers such as limestone and dolomite are added to the second stage to further trap the hydrogen sulfide. The furnace has a third zone, above the second zone, in which the combustible gas from the second zone is completely combusted by the introduction of secondary air. Any H.sub.2 S which escapes from the second zone is converted into SO.sub.x and is emitted as flue gas.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims.

The drawing shows a three zone furnace which is suitable for practicing the invention.

The drawing shows a furnace of the type shown in FIG. 20.52 of "Chemistry of Coal Utilization", National Research Council Committee on Chemical Utilization of Coal, H. H. Lowry, Chairman and Editor, New York, Wiley, 1945.

It includes a first zone 11 in which char is combusted at the highest temperature to achieve complete combustion and high carbon utilization. In accordance with the present invention, the flow of primary air to the first zone is controlled so that very little, preferably no excess oxygen reaches the second zone 12. In zone 12 the coal is pyrolized to form char and combustible gas in a reducing atmosphere. The temperature in this zone is above 500.degree. C. and preferably above 600.degree. C. Coal is added to the top of zone 12. The sulfur compounds in the coal, e.g., FeS.sub.2 and disulfide, are converted to highly reactive H.sub.2 S in accordance with:

In the foregoing R and R' are alkyl groups which are present in the coal.

To enhance this reaction, a small amount of steam is added at the bottom of zone 11 to produce some hydrogen according to the following reaction:

In zone 12, the H.sub.2 S reacts with basic minerals in the coal, CaCO.sub.3 or MgCO.sub.3, as follows:

Alternately, limestone added as a scavenger to second zone 12 is decomposed to active lime before reaction.

At least part of the CaS or MgS formed in the char is further oxidized to sulfates in the first zone 11:

In this way, the sulfur is effectively trapped. The ash containing the trapped sulfur is easily removed from the slag quench tank 14 at the bottom of zone 11. This process is more efficient than direct combustion of coal in which the following typical reactions occur:

The difference in efficiency is shown in the examples in the next section.

The combustible gas from zone 12 is completely combusted in the third zone 13 by introducing secondary air. Any H.sub.2 S which survived through zone 12 due to insufficient or unavailability of scavenger is converted into SO.sub.x in zone 13 and emitted. To circumvent this problem, additional scavengers such as limestone or dolomite are introduced along with the coal into the top of zone 12.

To demonstrate the validity of the process, coal was pyrolized in H.sub.2 or He gas at various temperatures to obtain the char. The char was combusted and compared with direct combustion of the coal for SO.sub.2 emission.

The coal analysis is shown in Tables 1, 2 and 3. Table 1 shows proximate and ultimate analyses. Table 2 shows analysis of mineral matter. It is noted that this coal is quite rich in CaCO.sub.3 and MgCO.sub.3. Table 3 shows sulfur type distribution.

The experimental conditions, sulfur contents of the products and the SO.sub.2 emission from combustion of char are shown in Table 4. It is noted that, in spite of the high sulfur content of the char, very low sulfur was emitted upon combustion. The pyrolysis can be conducted in H.sub.2 or He with the same result; apparently there is sufficient H.sub.2 in the pyrolysis zone from this particular coal.

While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. The appended claims are, therefore, intended to cover all such modifications.