Coal cleaning recovery

Although froth flotation is recognized as the best available fine coal cleaning technique, it becomes iueffective when the particle size is much smaller than 0.1 mm or when the feed contains a large amount of clay, resulting iu low coal recovery or poor selectivity. A solution to these problems is the use of modified flotation devices. 

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Table VI. Sulfur Reduction and Btu Recovery by Coal Cleaning...

Standard Conditions. The results for several cleaning experiments on the Pittsburgh No. 8 (PT8) and Illinois No. 6 (IL6 or IL6A) coals are presented in Table II. For the Pittsburgh coal cleaned under standard conditions (runs 3 and 16), 77-82% ash removal and 65-72% sulfur removal were achieved with recoveries around 85-90%. For the Illinois coal, (IL6 6, IL6A 1, and IL6A 2), the ash and sulfur removal was better at 87-89% and 72-78%, respectively, while recovery was lower at 75-79% on a moisture- and ash-free (MAF) basis. 

Fig. 19.6. Different levels of coal cleaning and the effect on coal recovery and economics.

Various coals cleaned in the Otisca T-Process contained in most cases below 1% ash, with the carbonaceous material recovery claimed to be almost 100%. Such a low ash content in the product indicates that very fine grinding liberates even micromineral matter (the third level of heterogeneity) it also shows Freon-113 to be an exceptionally selective agglomerant. 

The energy content of coal is related to its rank (degree of coalification) (Table 6.1) which is influenced by the content of nonfuel components (e.g., minerals and moisture) (Chapters 7 through 9). Thus, a primary objective of coal cleaning is to maximize the recovery of the heat value of the coal, consistent with achieving standard specifications for ash, moistme, and sulfur contents. 

In more general terms, coal cleaning is a balance between recovery of the maximum energy content of the coal and rejection of extraneous mineral matter. Indeed, it is difficult (perhaps impossible) not to reject some coal with the rock and slate. Thus, every coal refuse pile in existence will contain some measure of recoverable energy. One major challenge is to reduce this potential loss of energy to the minimum practical amount. 

Conventional coal cleaning plants are quite efficient for Btu recovery, as well as for ash and pyritic sulfur reduction. Btu recovery is generally between 85% and 90% and the ash reductions on a lb of ash/MM Btu basis are usually in the 70%-80% range for Pittsburgh seam coals, and in the 85%-90% range for Illinois and central Appalachian coals (Rosendale et al., 1993). 

Emissions control systems play an important role at most coal-fired power plants. For example, PC-fired plants sited in the United States require some type of sulfur dioxide control system to meet the regulations set forth in the Clean Air Act Amendments of 1990, unless the boiler bums low sulfur coal or benefits from offsets from other highly controlled boilers within a given utiUty system. Flue-gas desulfurization (FGD) is most commonly accomphshed by the appHcation of either dry- or wet-limestone systems. Wet FGD systems, also referred to as wet scmbbers, are the most effective solution for large faciUties. Modem scmbbers can typically produce a saleable waUboard-quaUty gypsum as a by-product of the SO2 control process (see SULFURREMOVAL AND RECOVERY). 

Sulfur Dioxide Emissions and Control. A substantial part of the sulfur dioxide in the atmosphere is the result of burning sulfur-containing fuel, notably coal, and smelting sulfide ores. Methods for controlling sulfur dioxide emissions have been reviewed (312—314) (see also Air POLLUTION CONTROL PffiTHODS COAL CONVERSION PROCESSES, CLEANING AND DESULFURIZATION EXHAUST CONTROL, INDUSTRIAL SULFURREMOVAL AND RECOVERY). 

The increased mining requirement is due to a decrease in clean coal recovery at lower specific gravities of separation. Although the specific gravity of coal is less than its mineral impurities, some coal is lost with the refuse material (mineral matter) as the specific gravity at which the coal is cleaned decreases. The rela-... 

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