fire hazards associated with the spontaneous combustion of coal are eliminated in the superheated steam drying process, the target moisture content can be achieved in a short time by using higher steam temperature.
3.7 Desulfurization
The total sulfur content of a raw coal is distributed throughout the macerals and minerals present and may occur as elementary sulfur, as sulfates, as sulfides, or in organic combination in the coal (Speight, 2013).
It has been customary to classify the forms of sulfur as inorganic, pyritic, and organic; inorganic sulfur comprising the sulfates, pyritic the sulfides, and organic the remainder, including any elemental sulfur that may be present. Standard methods of analysis have been devised for direct determination of total sulfur, sulfate sulfur, and pyritic sulfur, the organic sulfur being reported as the difference between the total sulfur content and the sum of sulfate content plus pyritic sulfur content. On a more localized basis such as in the United States (Chapter 1), the sulfur content will vary significantly by region (Table 3.7) thereby creating issues arising from sales to regional power producers because of the various laws relating to emission of sulfur oxides.
Even when coal has been prepared to meet the specifications for size, mineral (ash), and moisture contents, it may still be dirty by environmental standards. In this case, the important contaminant is sulfur, which is converted, during combustion, to gaseous product(s):
Table 3.7 Sulfur distribution in selected US Coals (Speight, 2013).
| Region | Total sulfur (%) | Inorganic sulfur (%) | Organic sulfur (%) | Inorganic/total sulfur (%) |
| Northern Appalachia | 3.01 | 2.01 | 1.00 | 67 |
| Southern Appalachia | 1.04 | 0.37 | 0.67 | 36 |
| Alabama | 1.33 | 0.69 | 0.64 | 52 |
| Eastern Midwest | 3.92 | 2.29 | 1.63 | 58 |
| Western Midwest | 5.25 | 3.58 | 1.67 | 68 |
| Western | 0.68 | 0.23 | 0.45 | 34 |
| Total United States | 3.02 | 1.91 | 1.11 | 63 |
Unless removed, the sulfur oxides end up as stack gas emissions.
In recent years, deliberate attempts been made to achieve coal desulfurization on an industrial scale by modification of the coal preparation practices. In part, this has been due to development of more precise methods for the separation of coal and minerals. However, these practices are usually very dependent upon the coal properties. Thus, where low-sulfur coal feedstock is a necessity, the deliberate selection of naturally occurring low-sulfur coal has been the most effective solution and has been the practice followed in producing metallurgical coals, political aspects notwithstanding as evidenced by the selection of higher sulfur (and inappropriate) coal for politically sensitive, rather than market, satisfaction.
There are strong incentives to develop processes for removing sulfur from coal before combustion (precombustion cleaning), during combustion or after combustion (post combustion cleaning) (Chapters 12, 13, 14).
Indeed, since the passage of the original Clean Air Act of 1970, subsequently amended in November 1990, coal preparation efforts in the United States have emphasized development of technology for the reduction of sulfur.
Coal desulfurization can be achieved on a commercial scale by means of physical or physicochemical methods which generally use the principal of density separation techniques or other techniques that exploit the surface properties of coals and minerals. For example, the methods exploit the difference in properties that exist between the various forms of inorganic sulfur [pyrite and/or marcasite (FeS2) and occasionally including galena (PbS)] and sulfur in the organic matrix of the coal.
Desulfurization by chemical techniques is somewhat less well developed than desulfurization by physical methods. However, a number of methods are under serious consideration and they can be divided into three general groups: (i) those which remove pyritic sulfur; (ii) those which remove organic sulfur; and (iii) those methods which remove either the pyritic sulfur or the organic sulfur or both (Couch, 1991; Ali et al., 1992).
Therefore, effective desulfurization requires that three criteria should be satisfied: (i) the reagent must be highly selective to either pyritic or organic sulfur for both) and not significantly reactive with other coal components, (ii) the reagent must be regenerable so that once-through reagent cost is not a major factor, and (iii) the reagent should be either soluble or volatile in both its unreacted and reacted form so that it can be near totally recovered from the coal matrix.
The use of strong bases (alkali, caustic) appears to offer some solution to the problem of organic sulfur removal and this approach continues to be investigated (Chatterjee and Stock, 1991).
3.8 Transportation
There are many occasions when coal is transported by rail, road, and water in its journey from mine to market. In some mining areas near the coast the coal was taken by conveyors directly from the mine to the holds of large coastal vessels. For example, in Britain, much of the coal from the northern coalfields is taken to the south in coastal cargo vessels called colliers. Large-scale haulage of coal by truck is normally economic only over relatively short distances.
There has, however, been the tendency during recent years to construct large industrial (chemical or power) plants close to the mine site in order to reduce coal-hauling costs and the coal is carried directly to the plant either by high-capacity truck on conveyor belts. In fact, the oil sand processing plants in northern Alberta employ this concept and transport the sand on several miles of conveyor belt to the processing plant (Speight, 1990).
It
