Coal Preparation in the United States Status Report
Energy consumption in the United States during 2005 was 2 352 million tonnes oil equivalent (oil, natural gas, coal, nuclear, and hydroelectric) with coal representing 25% of that consumption. It is anticipated that coal’s contribution to the total US energy consumption will increase displacing a percentage of the oil consumption. The United States has 111 338 000 000 tonnes of recoverable anthracite and bituminous reserves and 135 305 000 000 tonnes of recoverable subbituminous and lignite reserves. This is the largest coal reserve base of any country in the world. At the current rate of consumption this represents a 240 year reserve. The United States is self sufficient in coal reserves and is a slight net exporter of coal (net importer from South America with coal shipped both directions across the Canadian border).
Current US coal production is approximately 1. 1 billion tonnes per year with this rate having fairly consistently increased at about 14 000 000 tonnes per year during the past 25 years (Fig. 1). The increase in coal production is primarily from increased production of the low sulfur Wyoming Powder River Basin subbituminous coal which is not beneficiated. 2005 production from the 17 mines in the Powder River Basin of Wyoming and Montana was 391 000 000 tonnes representing about 35% of US coal production (average of 23 000 000 tonnes production per mine in the basin) (Fig. 2). Production of bituminous coal, the majority of which is beneficiated, has been fairly constant during the majority of the past 25 years but has increased during the past two years.
Coal beneficiation is defined as removing inerts / non-combustibles and sulfur from run of mine coal to increase the energy content per unit mass of the fuel and to reduce the sulfur oxide emissions. With the higher rank bituminous (and anthracitic) coals containing <10% inherent moisture this is normally a water based specific gravity separation reducing the ash and pyritic sulfur content (higher specific gravity than the coal) with the resultant moisture removed mechanically (screens and centrifuges) and, with finer size distribution coals, thermally.
North American low rank coals have inherent moisture contents up to 35% by weight. With the increasing utilization of low rank coals in the Untied States there is an ongoing interest in reducing the inherent moisture of these coals with the retained moisture not only being an “inert”, but also having a high Latent Heat of Evaporation. In addition to the removal of ash, pyritic sulfur, and moisture from coal, the United States and Canada are instituting mercury emission standards which can also be addressed through beneficiation processes. The following sections will address the current status of (1) bituminous coal beneficiation, (2) low rank coal beneficiation, and (3) mercury reduction.