Low-Grade Coals: A Review of Some Prospective Upgrading Technologies^†

Abstract

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.

Report on comparison among current industrial scale lignite drying technologies (A critical review of current technologies)

“…Based on the operating experiences obtained at the plant, the K-Fuel® process was redesigned in 2008 by Bechtel Power Corporation. This enhanced K-Fuel® process design offers significant process, economic, and environmental improvements and it is now the template for Evergreen's business development activities in the USA and abroad [38,39].…”

“…The CRC Lignite found the MTE less expensive than HTD or WTA process and capable of removing more than 70% of the water from the lignite from Victoria and South Australia, resulting in huge CO 2 savings when the dry coal is burnt in a power station [39,4].…”

Modeling particulate matter emissions during mineral loading process under weak wind simulation

“…In the presence of wind, more of the fine particles are separated from the mainstream. They are easily carried away by wind (Etkin, 1980;Katalambula and Gupta, 2009;Wang et al, 2012). This portion of the fine particles is referred to hereafter as "carry-able particles".…”

“…During the mineral matter loading process, fine and coarse mineral particles were separated not only vertically via the wind movement but also horizontally under the convective air and upward impulse (Katalambula and Gupta, 2009;Wang et al, 2012). In other words, there will be particulate matter emitted even if the wind velocity is zero.…”

Lignite Fuel Enhancement

“…This significantly increases the cost of thermal drying, which is the main barrier to large-scale industry acceptance of this technology. A review of thermal drying technology is presented in [2].…”