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[摘要]：Low-grade coals are usually those that are low in specific energy because of high moisture content and/or ash content or produce high emissions of concern. These are commonly lignites or sub-bituminous coals. There is a growing need of using these low-grade coals because of higher quest for power generation. In general, the direct use of the low-grade coals results in higher costs of reducing emissions or in lower efficiency and, consequently, higher greenhouse gas emissions. In the present carbon-constrained environment, there is a need of upgrading these coals in terms of moisture, ash, and/or other trace elements. There are a number of upgrading technologies. The Current paper reviews these technologies mainly categorized as drying for reducing moisture and cleaning (lie coal for reducing mineral content of coal and related harmful Constituents. such as sulfur and mercury. The earliest upgrading of high-moisture lignite involved drying and manufacturing of briquettes. Drying technologies consist of both evaporative and non-evaporative (dewatering) types. The conventional coal cleaning used density separation in water medium. However, with water being a l,cry important resource, conservation of water is pushing toward the development of dry cleaning of coal. There are also highly advanced coal-cleaning technologies that produce ultra-clean coals and produce coals with less that) 0.1% of ash. The paper discusses some of the promising upgrading technologies aimed at improving these coals in terms of their moisture, ash, and other pollutant components. It also attempts to present the current status of the technologies in terms of development toward commercialization and highlights oil problems encountered. One thing that is obvious is the fact that, despite the presence of all these techniques, still the upgrading goal has not been realized adequately. It can therefore be concluded that, because reserves for low-grade coals are quite plentiful, it is important to intensify efforts that will make these coals usable in an acceptable manner in terms of energy efficiency and environmental protection. |
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