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 the 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 very 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 than 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 on 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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Co-gasification of oil sand fluid coke with sub-bituminous coal was performed in an entrained-flow gasifier. The underlying objective of this work was to assess the combined effects of the operating variables (i.e., temperature and oxygen and steam concentrations) and coal/coke blending ratio in an entrained-flow gasification process, where the focus was to quantify the relationships between the response variables and vital operating factors. With a view to the shortcomings of the classical "onefactor-at-a-time" method in identification of the effect of experimental factors and their interactions, a statistical design of the experiment based on response surface methodology (RSM) was used. The response variables used in this work were H 2 , CO, H 2 /CO ratio, gasification efficiency, and carbon conversion. Experiments were conducted over a temperature range of 1000À1400°C, using steam and oxygen to carbon weight ratios of 0.9À4.3 and 0À0.4, respectively, equivalent to 15À50 vol % steam and 0À3 vol % oxygen in N 2 carrier gas. All of the response variables were successfully fitted to either a two-factor interaction or quadratic model. Using RSM, the effects of individual operating factors and their interactions were categorically determined, which were not otherwise possible by the classical design of experiment methodology. Using the resultant response variable correlations, H 2 production was optimized as a function of the temperature, steam and oxygen concentration, and different blending ratios. The full potentiality of Canadian oil sand coke for entrained-flow gasification was successfully investigated via RSM. The results of this work, however, are only applicable to entrained-flow gasification systems.

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