Physical coal cleaning

Concern over the release of hazardous trace elements from the burning of coal has been highlighted by the 1990 Clean Air Act Amendments. Most toxic elements are associated with ash-forming minerals in coal (5). As shown in Table 1, levels of many of these toxic metals can be significantly reduced by physical coal cleaning (6). 

Conventional coal cleaning processes can remove about 50% of pyritic sulfur and 30% of total sulfur. For northern Appalachian region coals it has been shown that a greater sulfur reduction can be achieved by applying physical coal cleaning to finer size coals (Table 3) (8). 

Disseminated pyrite grains are associated mainly with the microlithotypes such as vitrite, bimacerite, trimacerite, and carbominerite (9-11). The size distribution of the various forms of pyrite in coal has been studied by a number of researchers, primarily for the purpose of assessing the removability of pyrite by physical coal cleaning (9-10.12-17). 

FEELEY AND BLAUSTEIN Advanced Physical Coal Cleaning... 

There have been several assessments of the potential of physical coal cleaning for reducing SO2 emissions (29-31). Maronde and Deurbrouck ( 1) estimated the maximum potential sulfur reduction that could be achieved through extensive cleaning of coals from the Northern Appalachian Region Maryland, Ohio,... 

In any development of advanced physical coal-cleaning (PCC) techniques, an important consideration is the heterogeneous nature of coal and, in particular, the variable manner in which pyrite occurs in coal. This variability influences the behavior of coal with regard to cleaning (26). In some coals, pyrite is distributed throughout the coal matrix as particles only microns in size. Thus, to separate pyrite from these coals, the coal must be crushed to very fine size in order to "liberate the pyrite from the coal particles. However, conventional commercial PCC techniques cannot... 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

Computer Economics of Physical Coal Cleaning and Flue Gas Desulfurization, C.R. Wright, T.W. Tarkington, and J.D. Kilgroe, Presented at EPA/EPRI Symp. on Flue Gas Desulfurization, New Orleans, LA, November 1-4, 1983. 

Physical Coal Cleaning for Utility Boiler SO2 Emission Control," EPA- 600/7-78-034, February 1978. 

Growing concern over environmental effects of acid rain has resulted in increased Interest in development of pre-combustion removal of sulfur from coal. Physical coal cleaning processes are effective for pyritic sulfur removal but do little to reduce the organic sulfur content of coal This paper reports the removal of organic sulfur from coal, employing ethyl or methyl alcohols as the solvent/ reactant. The process is based on the observation that, under supercritical conditions, reactions occur that selectively remove organic sulfur from the coal matrix. 

The scheme used in physical coal cleaning processes varies among coal cleaning plants but can generally be divided into four basic phases initial preparation, fine coal processing, coarse coal processing, and final preparation (Figure 6.1). 

The determination of h forms of sulfur forms (ASTM, 201 Ij) measures the quantity of sulfate sulfur, pyritic sulfur, and organically bound sulfur in the coal. The sulfur forms test measures sulfate and pyritic sulfur and determines organic sulfur by difference. Pyritic sulfur is an indicator of potential coal abrasiveness and is used for assessing coal cleaning because sulfur in the pyritic form is readily removed using physical coal-cleaning methods. 

Beneficiation The treatment of mined material, making it more concentrated or richer see Physical coal cleaning. 

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