Low-Grade Coals: A Review of Some ... - American Chemical Society

Jun 15, 2009 - of the coal. Low-rank coals include lignite and sub-bituminous coals. These coals have lower energy content because they have lower car...
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Low-Grade Coals: A Review of Some Prospective Upgrading Technologies† Hassan Katalambula and Rajender Gupta* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2H1, Canada ReceiVed December 29, 2008. ReVised Manuscript ReceiVed May 27, 2009

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.

1. Introduction The degree of alteration (or metamorphism) that occurs as coal matures from peat to anthracite is referred to as the “rank” of the coal. Low-rank coals include lignite and sub-bituminous coals. These coals have lower energy content because they have lower carbon content. However, when one talks of low-grade coal, the issue of rank becomes less important. There is no single universally accepted definition of low-grade coal; however, a coal that has only limited use because of undesirable characteristics, e.g., high mineral matter content, can be termed as a low-grade coal.1 All low-rank coals (i.e., sub-bituminous and brown coal) are generally categorized into low-grade coals because of very high moisture contents and low heating values, and these coals need specific technologies for their application to power generation. Anthracites and semianthracites are also classified as low-grade coals because ignition and burnout problems are commonly related to them. For bituminous coal, however, it is a bit difficult to classify which coals are low- or high-grade. However, it can generally be said that bituminous coals with one or more of the following troublesome properties related to their use can be classified as low-grade coals: (i) low heating value, implying low efficiency, (ii) high moisture † Impacts of Fuel Quality on Power Generation and Environment. * To whom correspondence should be addressed. (1) Sheng, C.; Gupta, R.; Wall, T. F. Investigation on Australian lowgrade coals. A report prepared by the Central Research Institute of Electric Power Industry (CRIEPI), Tokyo, Japan, Jan 2004.

content, which also translates into low efficiency, (iii) low volatile matter content, related to flame stability, (iv) high ash content, related to ash problem and efficiency, (v) high sulfur content, implying high SOx emission and high control costs, (vi) low ash fusibility, having potential to slagging, (vii) high alkali/alkaline content, having potential to fouling and slagging, and (viii) low Hardgrove grindability index (HGI), implying high milling power consumption. In summary, a coal is categorized as a low-grade coal if it has one or more troublesome properties related to use in power plants. To avoid upgrading, the vast majority of low-grade coal usage is dedicated to local power stations close to the mine, designed specifically to operate on the characteristics of the particular low-grade coal. Studies and reviews on the fossil fuel resources have been undertaken extensively, some focusing specifically on coals and others going further to include liquid fuels.2-6 Coal deposits (2) Speight, J. Coal. Microsoft Encarta Online Encyclopedia, Microsoft, Redmond, WA, 2007, http://encarta.msn.com. (3) Thielemann, T.; Schmidt, S.; Gerling, J. P. Lignite and hard coal: Energy suppliers for world needs until the year 2100sAn outlook. Int. J. Coal Geol. 2007, 72, 1–14. (4) Tewalt, S. J. T.; Willett, J. C.; Finkelman, R. B. The world coal quality inventory: A status report. Int. J. Coal Geol. 2005, 63, 190–194. (5) Rukes, B.; Taud, R. Status and perspectives of fossil power generation. Energy 2004, 29, 1853–1874. (6) Asif, M.; Muneer, T. Energy supply, its demand and security issues for developed and emerging economies. Renewable Sustainable Energy ReV. 2007, 11, 1388–1413.

10.1021/ef801140t CCC: $40.75  2009 American Chemical Society Published on Web 06/15/2009

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exist in nearly every region of the world; however, commercially significant coal deposits occur in sedimentary rock basins, typically sandwiched as layers called beds or seams between layers of sandstone and shale. A very thorough and recent study on coal resources was performed by Thielemann et al.,3 where an outlook for the energy supply for the needs of the world until the year 2100 was presented. In this work,3 the world’s high- as well as low-rank coal production, consumption, and total potential is presented, as shown in the subsequent sections. There are coal deposits in many countries; therefore, coal has an enormous geostrategic advantage compared to crude oil and natural gas, about 70% of whose reserves are concentrated on a “geostrategic ellipse” between northwest Siberia across the Caspian region to the riparian states of the Persian Gulf.3 These days, we also encounter large amounts of coal fines or wastes ending up in tailings pond of coal preparation plants. These also need processing before we can extract energy from such resources by means of dewatering and/or briquetting. 2. Low-Grade Coal Upgrading Technologies Despite their deficiencies, low-grade coals have been used in different ways while applying some techniques that will improve their performance. The approach toward the upgrading of low-grade coal has been very diverse, but the techniques all aim at removing excess moisture and the unwanted organic and inorganic matter from the coal. Major techniques to upgrade the coals have been (i) blending, (ii) briquetting, (iii) drying, (iv) cleaning (removal of minerals), and (v) chemical upgrading. 2.1. Coal Blending. Coal blends have been an attractive fuel for pulverized fuel power stations. One of the principal reasons for blending of low-grade coals commercially is to meet specifications by blending inferior and superior coal to maximize the volume of saleable coal under a contract. This blending can be to control ash content or other key properties, such as sodium (which influences boiler fouling), sulfur, nitrogen, mercury, etc., any of which may cause problems in the use of a particular coal. Generally, blending provides a way of minimizing costs by making use of several low-grade coals to achieve desired properties. Specific advantages of blending include (a) reducing fuel costs, (b) controlling emission limits, (c) enhancing fuel flexibility and extending the range of acceptable coals, (d) providing a uniform product from coal of varying quality, and (e) solving existing problems, such as poor carbon burnout, slagging, and fouling, and improving boiler performance. A number of researchers have studied the effect of coal blending on various parameters, including ash deposition, combustion behavior, and NOx emission.7-12 Rubiera et al.,7 while studying the modification of combustion behavior and NO emissions by coal blending, reported that the combustion behavior of coal blends in TGA was greatly influenced by coal rank and the proportion of each component in the blend. Higher (7) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J. Modification of combustion behaviour and NO emissions by coal blending. Fuel Process. Technol. 2002, 77-78, 111–117. (8) Nomura, S.; Arima, T.; Kato, K. Coal blending theory for dry coal charging process. Fuel 2004, 83, 1771–1776. (9) Rushdi, A.; Sharma, A.; Gupta, R. An experimental study of the effect of coal blending on ash deposition. Fuel 2004, 83, 495–506. (10) Haas, J.; Tamura, M.; Weber, R. Characterization of coal blends for pulverized fuel combustion. Fuel 2001, 80, 1317–1323. (11) Rubiera, F.; Arenillas, A.; Fuente, E.; Miles, N.; Pis, J. J. Effect of the grinding behaviour of coal blends on coal utilisation for combustion. Powder Technol. 1999, 105, 351–356. (12) Peralta, D.; Paterson, N. P.; Dudgwell, D. R.; Kandiyoti, R. Coal blend performance during pulverized fuel combustion: Estimation of relative reactivities by a bomb calorimeter. Fuel 2001, 80, 1623–1634.

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volatile coals exerted more influence in the low-temperature region and less reactive coals in the char combustion zone. The results in the entrained flow reactor indicated that coal blends burnout and NO emissions show additivity in the case of similar nature coals. When one of the components was a high-rank coal, the burnout of the blend exhibited, in some cases, positive synergistic effects, while a clear deviation from linearity was found in NO emissions. Nomura et al.8 attempted to develop a coal-blending theory for the dry coal-charging process. They also reported the success of Nippon Steel in developing dry coal-charging processes for coke making. The investigation made it clear that, even in cases of high coal bulk density because of dry coal-charging processes, it is possible to control coking pressure by adjusting the blending ratio of a slightly caking and low-rank coal and it is also possible to produce high-quality coke by adjusting the total dilatation of the blended coal at a suitable level. Rushdi et al.9 studied the effect of the coal blending on ash deposition, where deposition experiments with well-characterized samples of Australian black coals and blends were conducted in a laminar drop-tube furnace to assess the behavior of the blends and their potential to form ash deposits. A comparison of the results between the blends and single coals showed that the behavior of the blends was not additive in nature. Some blends developed a thicker ash deposit layer with a maximum thickness higher than 700 mm compared to a thickness of 300 mm for the source coals, whereas other blends had a lower potential to form ash deposits. This non-additive behavior of coal blends results from the interaction between ash particles within the deposit layer. Therefore, the performance of the coal blends may not be the same as that of the source coals with the same bulk composition. Haas et al.10 evaluated the effect of blending on combustion characteristics of pulverized coal through the testing of 6 coals and 15 coal blends in an isothermal plug flow reactor, with the aim of assessing whether the combustion behavior of the blends could be predicted from that of the parent coal. For blends of bituminous coals, an additive behavior was found for ash, volatile matter, and calorific value. Other non-standard fuel properties, such as the high volatile matter, fuel N release, and char combustion, were found to be additive. The blends of the bituminous coals and lignite demonstrated non-additive properties as far as the volatile matter release was concerned. They concluded that the inorganic coal phase is responsible for the non-additive behavior in the blends because the volatile component of the coal mineral matter may undergo reactions with the second blend partner. Overall, from above, it can be seen that coal properties of low-grade coals can be reasonably modified by blending them with different types of coal, and these can be both low-grade coals or one low-grade coal and another high-grade coal. An important conclusion, which can be emphasized, is that the behavior of coal blends, particularly for blends of significantly different coals, is not generally additive of the behavior of the individual coals in the blend. Therefore, blend behavior may not necessarily be predictable from the properties of the individual components. Therefore, testing of blends is essential. 2.2. Coal Briquetting. Coal briquetting has been researched worldwide since the beginning of this century to produce briquettes from coal of various types and with different characteristics for particular uses. There are several reasons for the wide extent of this research. First, all coals are not alike, and often research has been aimed at developing an improved briquetting process for a particular coal. Second, briquetting can

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be performed with or without a binder to help in agglomerating and giving cohesive strength to the briquette. Much research has gone into suitable binders and processes to briquette without a binder. Third, some research has been directed at improving the properties of the briquettes, such as maintaining ignitability while keeping volatile matter low or reducing smoke and sulfur emissions during combustion. Therefore, through briquetting, low-grade coal can be upgraded and used.13-17 In addition, it can be made smokeless for applications where this is necessary, such as cooking. This contrasts to raw coal, which can be highly friable, difficult to handle, and emits offensive and potentially harmful smoke. By briquetting and carbonizing when necessary, the friability and emission problems can be remedied at a cost that is often competitive with traditional fuels.13 La´zaro et al.14 developed a novel method for preparing lowcost carbon-based briquettes. Their method included the briquetting of the carbon material, subsequent activation, and finally an equilibrium adsorption impregnation of the active phase. A local low-rank coal was used for the preparation of the carbon briquettes, while both a model vanadium compound (V2O5) and the ashes of a petroleum coke (PCA) were used as the precursors of the active phase. The catalytic briquettes were tested for NO reduction. The effect of a HNO3 oxidation previous to the impregnation was also evaluated. The reduction tests were carried out in the presence of oxygen and with the addition of ammonia as a reducing agent. The briquettes have shown to be active for NO reduction at low temperatures (100-300 °C). Surface chemistry as well as the porous structure of the support affected the catalyst behavior. Generally, higher NO reduction efficiencies were obtained from the catalysts prepared using the preoxidized briquettes. Changdong et al.15 reported on briquetting plants operating in Australia. Large-scale briquetting of Victorian brown coal commenced at Yallourn in 1924 using the German technology. The original Yallourn plant was however closed in 1970. The Yallourn briquette factory closed in 1970 because of the discovery and reticulation of natural gas in Victoria at that time and the closure of the major Lurgi briquette gasification plant in the Latrobe Valley, which produced town gas. Another plant commenced at Morwell in 1959. This plant is currently owned by Energy Brix Australia, one of the largest cogenerators of energy in the Southern Hemisphere, and is still operating. From its production complex at Morwell in the Latrobe Valley, Victoria, the company has produced over 30 megatons of briquettes, with an annual capability of 1.2 megatons per annum from the region’s vast resources of brown coal (Energy Brix Australia). Along with the national sales, the company has exported over 1 megaton of high quality, environmental friendly brown coal briquettes in recent years. It should be noted that the German binderless extrusion briquetting technology is only applicable to a narrow range of low-ash soft brown coals, generally confined to Germany and Victoria. These coals have however proved difficult to briquette (13) Stevenson, G. G.; Perlack, R. D. The prospects for coal briquetting in the Third World. Energy Policy, June 1989. (14) La´zaro, M. J.; Boyano, A.; Ga´lvez, M. E.; Izquierdo, M. T.; Moliner, R. Low-cost carbon-based briquettes for the reduction of no emissions from medium-small stationary sources. Catal. Today 2007, 119, 175–180. (15) Sheng, C.; Gupta, R.; Wall, T. Investigation on Australian lowgrade coals. A report prepared for Central Research Institute of Electric Power Industry (CRIEPI), Tokyo, Japan, Jan 2004. (16) Tosun, Y. I. Clean fuel-magnesia bonded coal briquetting. Fuel Process. Technol. 2007, 88, 977–981. (17) Mangena, S. J.; du Cann, V. M. Binderless briquetting of some selected South African prime coking, blend coking and weathered bituminous coals and the effect of coal properties on binderless briquetting. Int. J. Coal Geol. 2007, 71, 303–312.

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in lower cost double-roll press briquetting equipment without the addition of a binder. On commercial scale, binderless coal-briquetting (BCB) technology was developed by CSIRO (Australia) and demonstrated in a pilot plant scale in Western Australia by White Energy Technology. For now, The Binderless Coal Briquetting Company (BCBC) is a wholly owned subsidiary of White Energy Technology Limited. It provides a new agglomeration process for coal that has strong economic and practical advantages over current methods for agglomerating coal. The White Energy Technology process brings the fine coal particles into contact with each other under specific conditions (which do not include heating), causing the coal particles to bond together with bonding mechanisms, much the same as what exist within the coal structure itself. The process involves a combination of thermal drying and roll briquetting and produces a strong handleable briquette with moisture levels around 2-5% for bituminous coals.18 White Industries and BHP Billiton are said to be constructing a 1 million ton/year commercial BCB plant in Indonesia. The technology involves binderless hot briquetting of dried subbituminous coals in a double-roll press. It is likely to be applicable to a wider range of low-grade coals than the German technology. Additionally, Kobe Steel’s UBC technology produces binderless briquettes in a double-roll press from lowrank coals predried in a light oil slurry. The energy for evaporating the coal moisture is recovered by vapor recompression. The drying stage was developed as a component of the Japanese brown coal liquefaction pilot plant in the Latrobe Valley in the 1980s. The combined UBC technology was demonstrated in a 6 tons/day pilot plant in Indonesia in 2003-2005, and a 600 tons/day demonstration plant is currently under construction in Indonesia.19 Briquetting and pelletization of fine coals from coal washery tailings worldwide has now become quite important from the point of view of recovering coal as well as water. The pellets or briquettes are made from these coals with or without binders, such as pitch or molasses. Sometimes theses fines are pelletized along with other waste products, such as sawdust or spent mushrooms compost.20 Wallerawang Colliery in Australia constructed a briquetting plant with capacity of 70 000 tons per annum on the banks of a tailings dam using a double-roll press.21 2.3. Drying of Coals. Drying of coal involves removing the moisture from the coal, and this can be achieved through (i) evaporative drying or (ii) non-evaporative drying (dewatering). 2.3.1. EVaporatiVe Drying. Drying techniques in which the moisture is transformed into the gaseous phase in the course of drying are referred to as evaporative drying and, in most cases, involve steam. Generally, in evaporative drying, low-rank coals are dried by evaporating interstitial water in a superheated steam flow countercurrently passed through a sealed rotary cylindrical vessel. A composite steam discharged from the vessel is partially condensed to remove an amount of water substantially equal to the amount of water removed from the coal, with a resultant flow of residual steam reheated and returned to the cylindrical vessel for further drying of coal. Some of the specific techniques (18) http://www.whiteenergyco.com/white-coal-solution/briquettingprocess.php. (19) Otaka, Y.; Endo, H.; Deguchi, T.; Shigehisa, T. Demonstration of low-rank coal upgrading technology in Indonesia. Coal Safety J. 2004, 24. (20) Taulbee, D.; Patil, D. P.; Honaker, R. Q.; Parekh, B. K. Briquetting of coal fines and saw dust: Part I. Binder and briquetting parameter evaluation. Int. J. Coal Prep. Util. 2009, 29 (1), 1–22. (21) Radloff, B.; Kirsten, M.; Anderson, R. Wallerawang colliery rehabilitation: The coal tailings briquetting processes. Miner. Eng. 2004, 17, 153–157.

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Figure 1. Effect of the temperature on the equilibrium moisture content of LY LA steam-dried coal at 1.0 and 2.5 MPa for 20 and 65 h experiments (Bongers et al23).

that are used in evaporative drying are (i) steam drying and (ii) steam fluidized bed drying (SFBD). 2.3.1.1. Steam Drying. Steam drying of coal is a technology that offers, besides reduction of fire and explosion hazards, improved thermal efficiency. In addition, steam-dried coals appear to be less liable to spontaneous combustion because of their decreased reactivity to oxygen in the atmosphere.22 The use of superheated steam as a drying medium is attracting wide interest in several applications, such as processing of paper pulp, fuels, and foodstuff. Chen et al.22 developed a single-particle model to simulate the drying of granular media in superheated steam. The calculations from the model were compared to experimental data reported in the literature on the steam drying of ceramic spheres and water droplets. The validated model was then used to investigate, parametrically, the effects of system variables on the drying behavior of coal in steam. Calculations were presented to illustrate the effects of thermo-physical properties and operating variables on the inversion point temperature, above which the vaporization rate in steam is greater than that in air. Generally, the simulations showed that (i) the permeability and size of coal particles significantly influence the time required for the particles to reach a steady state, (ii) the initial temperature of particles exerts a strong influence on the initial condensation, and (iii) higher steam flow rates and steam temperatures lead to a substantial increase in the vaporization rate. Bongers et al.23 studied the equilibrium moisture contents on pressurized steam-dried Australian low-rank La Trobe Valley coals, namely, Loy Yang and Morwell, and a South Australian Bowmans coal. The study was undertaken using steam pressures of 1 and 2.5 MPa in the temperature range of 180-260 °C in a batch laboratory-scale autoclave. The degree of moisture removal as a function of both temperature and superheat was investigated. The steam equilibrium moisture content of the coals was related to the degree of superheat by a simple equation. The release of organic material only became significant when a major portion of the water had already been removed. The effect of temperature and pressure on the equilibrium moisture content of the LY LA coal was determined for a range of temperatures at both 1.0 and 2.5 MPa. At each drying (22) Chen, Z.; Wu, W.; Agarwal, P. K. Steam-drying of coal. Part 1. Modeling the behavior of a single particle. Fuel 2000, 79, 961–973. (23) Geoffrey, D.; Bongers, W.; Jackson, R.; Woskoboenko, F. Pressurised steam drying of Australian low-rank coals: Part 1. Equilibrium moisture contents. Fuel Process. Technol. 1998, 57, 41–54.

pressure, the equilibrium moisture content of the dried coal decreased from its bed moisture content of 62% to less than 10% moisture with less than 10 °C of superheat, as shown in Figure 1. It has been shown previously that, of the bed moisture in brown coals, about 60% is in the macropores of the bulk water, 30% in the large and small capillaries, and 10% in mono- and multilayers, possibly hydrogen bonded to functional groups in the coal structure. The results show that bulk water is removed with only 2-38 °C of superheat. As the degree of superheat is increased, water is progressively removed from smaller and smaller pores of the capillary water. A further increase in the degree of superheat, even up to 40 °C superheat, has little effect on the strongly bonded mono- and multilayer water. The minimum value of about 5% obtained in these experiments is about 10% of the original bed moisture and probably consists of the mono- and multilayer water, which persists when the drying is carried out under a positive pressure of steam.23 In a separate study, Bongers et al.24 studied the shrinkage and physical properties of steam-dried coals and also looked on the preparation of dried coals with very high porosity. They found that significant amounts of pore collapse and cross-linking occurred during drying even at 105 °C, with a significant hysteresis between desorption and re-adsorption of water. The hysteresis is caused by the limitation of water access to the pore structure of the coal. Cross-linking reactions may have sealed pores or limited the ability of collapsed pores to be expanded on water re-adsorption. 2.3.1.2. Steam Fluidized Bed Drying (SFBD). The SFBD concept was invented by Potter at Monash University and developed to a commercial scale by Lurgi. A 150 000 tons/annum (dry coal) SFBD plant operated for several years at Loy Yang, supplying pneumatically conveyed dry coal dust over 3 km to Loy Yang B power plant for use as a start up and auxiliary fuel. However, the product proved to be too expensive to attract other markets, and the plant has since ceased operation. A study of the Lignite CRC researches (Lignite CRC) also confirmed that, despite the technical advantages of the process, it is unlikely to provide a large reduction of capital cost. Despite the failure of the SFBD plant mentioned above, continued improvements of the technology at RWE led to other (24) Geoffrey, D.; Bongers, W.; Jackson, R.; Woskoboenko, F. Pressurised steam drying of Australian low-rank coals: Part 2. Shrinkage and physical properties of steam dried coals, preparation of dried coals with very high porosity. Fuel Process. Technol. 2000, 64, 13–23.

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successes. Moving from the coarse grain version of the technology on which these initial demonstration plants were based to the finer grain drying technology demonstrated that the capital and operating costs could be considerably reduced. As of now, the “fine grain” (-2 mm) version of the SFBD process is the one being applied in Germany and Australia. One specific example is the RWE-developed fluidized-bed-based WTA process for precombustion drying of brown coal (lignite) for power generation. A large-scale pilot plant has been put up at Niederaussem K in Germany, while a WTA system is to be retrofitted to the Hazelwood plant in Australia. In Germany, several new brown coal-fired power plants were constructed. The 2 × 800 MWe Schwarze Pumpe power plant was commissioned in 1997/1998, with the boilers being the first of a new 800+ MW series with supercritical steam conditions. Today, these units have accumulated about 100 000 operating hours with outstanding performance. The 1000 MW BoA unit located at the Niederaussem power station of RWE is a second example, with BoA being the German abbreviation for brown coal-based power generation using optimized plant engineering. This optimization led to steam parameters of 580 and 600 °C and an advanced flue gas heat recovery system that helped to achieve an overall efficiency in excess of 43%. The unit was commissioned in 2002 and is now in commercial operation. After this, the new generation of BoA plant is currently under construction at Neurath, consisting of two 1100 MWe units.25 The next technological milestone for brown coal use is the drying of the high moisture brown coal with a low-temperature energy source. The most advanced technology for such drying is the WTA technology developed by RWE, with WTA standing for Wirbelschicht-Trocknung mit interner Abwa¨rmenutzung (fluidized bed drying with internal heat recovery). The WTA technology is expected to be offered to new brown coal fueled plants after about 2012, but it may be available as a retrofit option before that. Hazelwood 2030 program in Victoria will be using the WTA technique for drying lignite and reducing the greenhouse gas emissions subsequently.25 Raw brown coal has a water content in excess of 50%. Therefore, in current conventional brown coal boiler technology, a significant part of the combustion energy has to be spent in evaporating this moisture, sapping the steam cycle of efficiency and rendering it less efficient than boilers using drier fuel sources. By predrying with atmospheric, fine grain, fluidized bed technology, the latent heat of evaporation can be recovered and used to continue the evaporation process in lieu of combustion energy, thus boosting the overall efficiency of the power cycle. Features of the WTA process include27 (i) drying in a stationary fluidized bed with superheated steam, (ii) supply of the drying energy via a heat exchanger installed in the drier, (iii) use of drying vapors by means of a heat pump process for drier heating, (iv) use of the vapor condensate for coal or condensate preheating in the power plant, and (v) feed grain size of raw coal input less than 2 mm. 2.3.1.3. Other EVaporatiVe Drying Processes. There are many evaporative drying processes for upgrading low-rank coals, some in widespread commercial use and some under development, and it may not be possible to describe all of them here. However, a brief mention will be given to a few: (i) The (25) http://www.modernpowersystems.com/story.asp?storyCode)2048894. (26) Fei, Y.; Artanto, Y.; Giroux, L.; Marshall, M.; Jackson, W. R.; MacPhee, J. A.; Charland, J.-P.; Chaffee, A. L.; Allardice, D. J. Comparison of some physico-chemical properties of Victorian lignite dewatered under non-evaporative conditions. Fuel 2006, 85, 1987–1991. (27) AdVances in the Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Amsterdam, The Netherlands, 2004.

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rotary drum steam tube dryers in the brown coal-briquetting plants in Germany, Victoria, and India. The steam is in the drum and the coal passes through the tubes, providing enhanced safety from dried coal dust fires and explosions. (ii) Integrated flash mill drying systems. These are used in current pulverized brown coal power stations in Germany, Victoria, and Greece. The brown coal is milled in a “beater” mill and entrained and dried in hot flue gas, recycled from the furnace exit. The same technology has been demonstrated in stand-alone flash mill dryers, where the hot gas is generated from a dedicated coal or natural gas combustor before being fed to the mill. The dried coal product is collected in a cyclone and stored in a silo to prevent explosions and fires. (iii) Hot gas fluidized bed dryers. These have been tried on lignites and sub-bituminous coals in Wyoming and North Dakota but have been prone to fires and explosions. (iv) Entrained flow drying. This is the first stage of the IDGCC process, being developed by HRL in Victoria but which has also been proposed as a low-cost stand-alone dryer. 2.3.2. Non-eVaporatiVe Drying (Dewatering). The earliest non-evaporative thermal dewatering process was developed in Austria in the 1920s and is known as Fleissner drying, after the inventor. Another technology is K-Fuel, formerly known as KFx. In recent years, new technologies working on the same principals have been explored, mainly because of the fact that the latent heat of vaporization in these processes is saved, thus leading to efficiency improvement and reduction in greenhouse gas emission.28 In addition, beneficial removal of some cations, principally Na, present in the lignite occurs. However, the perceived advantage of non-evaporative dewatering, saving the latent heat of evaporation, is no longer limited to dewatering. It can now also be achieved with evaporative drying, as in the SFBD, with energy recovery or vapor recompression. A brief review of what has been done in each of these drying technologies is hereby given. 2.3.2.1. Fleissner Process. The economic drying of lump lignite without forming powder can be attained by the Fleissner process. Fresh lump lignite is heated in a steam atmosphere to 400 pounds of pressure; then the steam is allowed to escape; and the remainder of the moisture is removed with a vacuum pump. This yields a hard lump material containing 10% or less of moisture, and it has a heating power of about 10 000 Btu per pound. It also fires well in the lump state, grinds easily, and forms an excellent fuel powder. As introduced above, the earliest thermal dewatering process was developed in Austria in the 1920s and is named after his inventor, Fleissner. This involves batch autoclave treatment of lump coal in steam at 180-240 °C to produce an upgraded hard lump fuel. The technology was licensed by Voest Alpine, and several plants have operated on harder brown coals in central European countries, with one commissioned in the late 1980s. A continuous version of the process was developed in the 1980s, but it is not clear if it was commercialized. The process was piloted in Victoria and South Australia and was close to commercial application to supply the South Australian Railways before the introduction of diesel locomotives. The difficulty in applying the process to the soft Victorian brown coals arose from the need for the feed to be in lump form and maintain its integrity through the process. Assessments indicated that less than 50% of a Loy Yang feedstock would have sufficient lump size to facilitate drainage of the exposed water from the coal.29 (28) http://www.evgenergy.com/about.shtml. (29) Cooperative Research Centre for Clean Power from Lignite. Report. http://www.crca.asn.au/press_releases/2004/2004-3-08.htm.

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A Japanese version of the Fleissner process (the DK process) with improved heat recovery was piloted in the 1980s. A significant difference between the earlier Fleissner process and the subsequent developments was in the process temperatures. The Fleissner process focused on temperatures around 200 °C, where water exclusion because of shrinkage is maximized but organic contamination of the water, which increases with temperature, is confined to acceptable levels. 2.3.2.2. K-Fuel. K-Fuel is a registered trademark of Evergreen Energy, Inc. (formerly KFx). The K-Fuel process uses heat and pressure to physically and chemically transform high moisture, low BTU coals into a more energy-efficient, lower emission fuel. A co-benefit of the K-Fuel process is the removal of significant amounts of mercury and reductions in the emissions of sulfur dioxide and nitrogen oxides. A pioneering 750 000 ton/year K-Fuel production facility near Gillette, WY, began production at the end of 2005. Production and process development work at this plant, representing a more than $100 million investment, made possible by Bechtel Power Corporation, designed enhanced K-Fuel refinery that offers significant process, economic, and environmental improvements. This enhanced Bechtel design is now the template for Evergreen’s business development activities in the U.S. and abroad. Evergreen Energy plans to develop and operate K-Fuel production plants domestically, either wholly owned or through joint ventures, and through international licensing to third parties.28 2.3.2.3. Mechanical Thermal Expression (MTE). As the name implies, MTE involves squeezing the water out of the lignite using mechanical energy, typically at 13-25 MPa pressure and temperatures in the range of 150-200 °C, and the vessel pressure is generated autogenously, so that water remains in the liquid state and is not evaporated. The Cooperative Research Centre for Clean Power from Lignite (CRC CPL) reported on the successful trial of a revolutionary process for drying brown coal that can reduce greenhouse emissions from power generation by a third or more.29 The CRC’s MTE technology removes more than 70% of the water from the brown coals found in Victoria and South Australia, resulting in huge greenhouse savings when the dry coal is burnt in a power station. Clayton et al.30 extended the use of MTE to other types of materials, especially biomaterials, and reported that the ease of dewatering a given material is dependent upon the physical and chemical properties of the material. For example, in brown coal, which has in the order of 60% water, a significant proportion of the water is contained within the porous coal structure. To express the water from the coal, elevated temperatures and pressures are therefore required. Analogous to brown coal, biomass including sewage sludge and bagasse are notorious for being difficult to dewater, with conventional dewatering methods producing cakes with high moisture contents. In Clayton et al.’s work,30 the MTE dewatering of a range of materials was explored using a laboratoryscale MTE compression-permeability cell. The major focus was on the investigation of the effects of processing parameters, including temperature (20-200 °C) and pressure (1.5-24 MPa) on the final moisture content and the material permeability. These two parameters are related to the extent of dewatering and rate of dewatering, respectively. It was illustrated that, under moderate testing conditions, MTE is effective for dewatering a range of biomaterials. The combined effects of a compression (30) Clayton, S.; Scholes, O.; Hoadley, A. Influencing factors in the dewatering of compressible biomaterials via mechanical thermal expression. Report, Cooperative Research Centre for Clean Power from Lignite, Canberra, Australia, 2004.

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force and elevated temperature collapse and compress the porous structures, resulting in reduced moisture contents and reduced permeability for increases in either pressure or temperature. Mercury porosimetry and SEM micrographs were presented to illustrate the changes in material structure. Their investigation was also extended to a comparison of the direction of water removal. It was shown that, owing to particle alignment, the rate of water removal is greater in the direction perpendicular to the applied force (radial) as compared to the direction of applied force (axial). Bergins31 investigated the kinetics and mechanism during mechanical thermal dewatering of lignites, working on three types of lignites, one each from Germany, Greece, and Australia. His results show that the dewatering kinetics depending upon time, temperature, and pressure and can be described by a new model derived from soil mechanical fundamentals and the rate process theory.32 Because of differences in lignite composition, the experimental determination of some model parameters for each coal was deemed to be necessary. From the activation energy that is determined from experiments concerning dewatering kinetics, it can be deduced that, even during secondary consolidation (creep), the drainage of water is the dominating process. The experiments also provide a clear distinction between the effect of the so-called “thermal dewatering” because of heating of the lignite and the subsequent mechanical expression. Bergins33 further developed a rheological model for consolidation and the creep process, which also shows all of the necessary MTE steps, as presented in Figure 2. Mechanical dewatering of porous materials is a time-dependent volumereducing process. Generally, three phases occur (Figure 2). During the initial consolidation, the gas-filled pore volume is instantly reduced by the external pressure. This stage can be neglected in the present case, because the pore volume of brown coal is completely filled with water. During the primary consolidation, which is classically described by the Terzaghi theory,34,35 the excess pore pressure (pressure of the water flowing out) decreases, while the effective pressure carried by the solid skeleton increases. This stage is completed when the excess pore pressure has vanished everywhere in the porous structure. During the consolidation of soft soils, such as clay and lignite, the secondary consolidation (creep) can be observed as a third phase. The creep of different lignites was analyzed in detail before36 and can be described by a model derived from the rate process theory. For a deeper understanding and use of the MTE process, the creep model has to be extended by a physical modeling of the primary consolidation. Looking on the byproducts from this drying technique, the future development of the MTE will produce large volumes of acidic, salty, and organic-rich product water. The overall viability of the MTE process will in turn rely on the availability of a simple and energy-efficient water remediation strategy. (31) Bergins, C. Kinetics and mechanism during mechanical/thermal dewatering of lignite. Fuel 2003, 82, 355–364. (32) Bergins, C. Mechanismen und kinetik der mechanisch/thermischen entwa¨ sserung von braunkohle. Dissertation, Universita¨ t Dortmund, Shaker, Aachen, 2001. (33) Bergins, C. Mechanical/thermal dewatering of lignite. Part 2: A rheological model for consolidation and creep process. Fuel 2004, 83, 267– 276. (34) Lambe, T. W.; Whitman, R. V. Soil Mechanics, SI Version; Wiley: New York, 1979. (35) Terzaghi, K. Erdbaumechanik auf Bodenphysikalischer Grundlage; Deutike: Wien, Austria, 1925. (36) Bergins, C. Kinetics and mechanism during mechanical/thermal dewatering of lignite. Fuel 2003, 82 (4), 355–364.

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Figure 2. MTE process steps (Bergins32).

Butler et al.37 worked on MTE water remediation using Loy Yang brown coal as a filter bed adsorbent. Water treatment using the feed coal itself as an adsorbent may provide such an option. In this study, a fixed-bed configuration of raw Loy Yang coal was employed. After successful trial of 1 ton/h MTE plant, White Australia has constructed a 15 tons/h pilot plant for drying lignite and intends to progress to a 200 tons/h demonstration plant. In Germany, a further development step toward commercial implementation of the MTE process involved converting the discontinuous pilot press to quasi-continuous fully automatic operations, with a throughput of approximately 1.6 tons of dry brown coal/h. Parallel to testing the MTE plant technology, Rheinbraun and RWE Energie investigated the feeding of the MTE press with coal in quasi-continuous operations, treatment of the raw brown coal, and subsequent treatment of the dry brown coal produced. The various project phases provided evidence of the cost-effectiveness and energy-efficiency of dewatering brown coal using the MTE process. After completion of this development, RWE Energie ordered a MTE demonstration plant with an output of 15 tons of dry brown coal/h, which was constructed at the Niederaussem power station and was successfully in operation at the end of 2001.38 2.3.2.4. Hydrothermal Dewatering (HTD). HTD, the other non-evaporative coal drying technique, has also been studied quite widely. In HTD, higher processing temperatures are required, typically greater than 320 °C, in addition to the vessel pressure that is generated autogenously, so that water remains in the liquid state and is not evaporated. Favas et al.39 investigated the effect of process conditions on the properties of the dried product. This was necessitated by the fact that, although there had been extensive earlier work on this process,40-44 most work had focused on the effects of a relatively small group processing coal variables on the properties of the final product. In this study,41 the intraparticle porosity (50 µm gave products of constant intraparticle porosity, but smaller coal particles gave products with higher intraparticle porosities as a result of agglomeration. Milling of the coal reduced the porosity of the dried coal at a constant particle size. In a different study, Favas et al.45 examined the effect of coal characteristics for a range of Australian and international coals as far as the HTD of low-rank coals were concerned. They dried a range of lower rank coals from Australia, Indonesia, and the U.S.A. under HTD conditions at 320 °C for 30 min in a 500 mL autoclave using a 1:3 dry coal/water mixture. The HTD products were characterized by elemental analysis (both organic and inorganic components), volatile matter determinations, moisture holding capacity, calorific value, and mercury porosimetry. The total organic carbon (TOC) and the concentrations of inorganics in the waste waters were also determined. The coals fell into two broad groups, the lower rank Australian brown coals with gross calorific values in the range of 10-16 MJ/kg (afm ) ash free with moisture content equal to the moisture holding capacity) and the higher rank Indonesian and U.S. coals with gross calorific values in the range of 18-25.5 MJ/kg (afm). Rank was found to be the major factor influencing the properties of the HTD products; however, lithotype was also important. Generally, the change in volatile matter between the dried and raw coal, the change in afm calorific value, as well as the change in moisture holding capacity were all seen to increase with decreasing afm gross calorific value, which, in (40) Fleissner, H. The drying of fuels and the Austrian coal industry. Sonderdruck Spartwirtschaft 1927, 10 and 11. (41) Evans, D. G.; Siemon, S. R. J. Inst. Fuel 1970, 43, 413. (42) Sears R. E., Baker G. G., Maas, D. J.; Potas, T. A.; Patel, R. Technical report DOE/FE/60181-130. Presented at the 13th Biennial Lignite Symposium on Technology and Use of Low-Rank Coal, Bismarck, ND, May 21-23, 1985. (43) Hodges, S.; Woskoboenko, F. Ninth International Conference on Coal Research, WA, 1991. (44) Hashimoto, N.; Katagiri, T.; Shibata, K.; Imai, T. IEA-CLM Workshop ’94 on coal water mixture (CWM) for advanced coal utilization toward the environmentally friendly system, 1994; p 283. (45) Favas, G.; Jackson, W. R. Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals. Fuel 2003, 82, 59–69.

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Figure 3. Relationship between the percentage reduction in MHC of the coal during hydrothermal processing and the afm (ash free with moisture equivalent to the holding capacity) gross calorific value (rank) of the original coal (Favas and Jackson45).

this case, represented the change in coal rank. Figure 3 shows the change in moisture holding capacity mentioned above. TOC values of the wastewater from drying of the low-rank Australian coals were much higher than those of the wastewater from the higher rank coals. Important variations in the amount of leached calcium, magnesium, and iron were noted. Another study on HTD was performed by Racovalis et al.,46 who studied the effect of processing conditions on organics in wastewater from HTD of low-rank coal with a special focus on the nature and amounts of the organics released. Processing conditions examined included temperature, residence time, water/coal ratio in the slurry, slurry loading, and coal lithotype. A high temperature and pressure treatment of low-rank coal slurries was used to simulate the HTD process. It was found that the extent of extraction of organics during the HTD depended upon the coal lithotype and increased with increasing temperature, residence time, and the proportion of steam and water present during processing. For constant residence time, the concentration of organics in the wastewater increased exponentially with temperature over the temperature range of 250-350 °C. The maximum level of approximately 7 g/L (expressed as TOC) of organics was observed following processing at 350 °C. Hot filtration of the coal slurry produced higher organic loading than that found in wastewater obtained at low temperature. To address the problem of the loss of the coal energy that is contained in the leached out organic compounds, Nakagawa et al.47 undertook a dual investigation looking at the HTD of brown coal and, at the same time, they looked at the catalytic hydrothermal gasification of the organic compounds dissolving in the wastewater using a Ni/carbon catalyst. In their study,47 an Australian brown coal (Morwell) was hydrothermally treated for dewatering and upgrading at 250-300 °C. When the coal treated at 300 °C, the water content decreased from 1.31 to 0.59 kg/kg on dry matter and the calorific value increased from 25.8 to 27.8 MJ/kg on dry matter, indicating that the hydrothermal (46) Racovalis, L.; Hobday, M. D.; Hodges, S. Effect of processing conditions on organics in waste water from hydrothermal dewatering of low rank coal. Fuel 2002, 81, 1369–1378. (47) Nakagawa, H.; Namba, A.; Bohlmann, M.; Miura, K. Hydrothermal dewatering of brown coal and catalytic hydrothermal gasification of the organic compounds dissolving in the water using a novel Ni/carbon catalyst. Fuel 2004, 83, 719–725.

Figure 4. Effect of the gasification temperature on the carbon conversion through the CHTG for MW-Water-250 and MW-Water300 (Nakagawa et al.47).

treatment is really effective for dewatering and upgrading the brown coal. The reactivity to oxygen at low temperature was also reduced by the treatment, which will contribute to the suppression of the spontaneous combustion of the coal. On the other hand, the amount of organic compound dissolved in the recovered wastewater increased with an increasing treatment temperature and reached ca. 1.5% on carbon basis at 300 °C. The wastewater was treated using a novel Ni-supported carbon catalyst developed by Nakagawa et al.47 The organic compounds in the wastewater were completely gasified at as low as 350 °C under 20 MPa and at the liquid hourly space velocity of as large as 50, producing combustible gas rich in CH4 and H2. Figure 4 shows the carbon conversions through the catalytic hydrothermal gasification (CHTG) of the Morwel coal processed at 250 °C (MW-Water-250) and that processed at 300 °C (MW-Water-300). The gasification reactivity for MWWater-300 was slightly higher than that for MWWater-250. Despite the fact that hydrothermal drying is of particular promise for low-rank coals, the pumpability of aqueous slurries of these products, as indicated by the intraparticle porosity, remains poor, as reported by Favas et al. and Shibata et al.,48,49 calling for more studies in that direction. (48) Favas, G.; Jackson, W. R.; Marshall, M. Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals. Fuel 2003, 82, 71–79. (49) Shibata, K.; Hashimoto, N.; Sugiyama, T. Proceedings of 21st International Conference on Coal Utilization and Fuel Systems, FL, 1996; p 233.

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Various efforts were made to move toward larger scale plants. HRL and Japan COM built 1 ton/h pilot plants in the mid-1990s, but both abandoned the development because of a range of technical, economic, and environmental issues, including concerns on the effluent water treatment difficulties and costs. These problems are exacerbated by the higher processing temperatures for HTD relative to MTE. As of now, there is no firm indication of any interest in exploring HTD technology further. 2.4. Cleaning of Coals. Cleaning of coal is normally applied to steaming and metallurgical coals for export, where the increase in product value, reduced freight rates, and reduced ash disposal costs justify the cost of cleaning. For local use, high ash steaming coals are generally consumed in local power stations without significant cleaning. There are two main types of coal cleaning, namely, (i) dry cleaning and (ii) wet cleaning. A number of pros and cons to either dry or wet clean the coal have been given, including the limited availability of water in some mines and health problem caused by dry cleaning. However, for in-depth understanding and judgment, each of these cleaning techniques is described. 2.4.1. Dry Cleaning of Coals. Dry coal beneficiation methods, including hand picking, frictional separation, magnetic separation, electric separation, microwave separation, pneumatic oscillating table, air jig, air-dense medium fluidized bed beneficiation, etc., are carried out according to differences in physical properties between coal and refuse, such as density, size, shape, lustrousness, magnetic conductivity, electric conductivity, radioactivity, frictional coefficient, etc. Generally, dry coal cleaning technologies that have been investigated and/or reported by various researchers include (i) aerodynamic classifiers, (ii) electrostatic separators, (iii) magnetic separators, (iv) air-dense medium fluidized bed, and (v) FGX separators. Brief descriptions for each of these technologies are given. 2.4.1.1. Aerodynamic Separators. The aerodynamic separator operates on the principle of separating a feed of sized material according to its relative density by introducing the material into a wind tunnel. The heavy reject particles take a shorter trajectory, and the lighter coal particles take a longer one, allowing them to be collected in separate hoppers. The feed material has to be closely sized, meaning that a number of classifiers are required to operate in parallel on the different sizes. The wind tunnels are arranged in cells to receive the sized material simultaneously. Each of the feed streams is introduced into its appropriate unit, which generates a laminar air flow and velocity commensurate with the particle size. The exhaust air in the finer fractions is cleaned up through a cyclone and dust collector. The problem envisaged with this process would be the screening of the feed coal into multiple sizes and handling this number of closely sized feed and product materials. Dust control, collection, and disposal could also present some difficulties. 2.4.1.2. Electrostatic Separation. Research work was carried out by CSIRO Division of Energy Technology on the use of a high-tension electrodynamic separator (EDS) and an electrostatic separator (ESS), with the former on treating 2 mm fine coal and the latter treating 0.25 mm.50 The EDS machine was a CARPCO high-tension separator, charged by passing through a corona created by an ionizing electrode as it feeds on a 230 mm diameter rotating, earthed, metal drum. Mineral matter is relatively conducting, does not retain an electric charge, and is thrown from the drum. Coal is relatively nonconducting, retains the electric charge imparted to it, and adheres to the drum until swept off by a brush. The EDS process (50) Donnely, J. The Australian coal review. Oct 1999.

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Figure 5. Schematic diagram of the air-dense medium fluidized bed separator (Lei51).

is used commercially in the mineral sands and iron ore industries. The feed size range was generally 0.5 ( 0.25 mm, but some tests were carried out on 2 ( 0.1 and 3 ( 0.1 mm. A range of voltage settings, feed rates, and drum speeds were tested. Sharp separations down to low-density equivalents were achieved for high-vitrinite coals, while moderately efficient separations were obtained for less reactive (thermal) coals. 2.4.1.3. Rare Earth Magnetic Separators (REMSs). The REMSs are capable of handling a wider size range, higher tonnage, and more moist feed than the EDS and ESS. Efficiency of separation was better, and product yields were higher. The capacity of a REMS 40 in. wide roller machine is 4-5 tons/h. While the cleaning efficiency and ash reduction were less than that obtainable by wet cleaning methods, the specific energy of the dry cleaned product was slightly higher because of its low moisture content. A study of capital cost comparisons for a 360 tons/h REMS plant compared to a conventional wet cleaning plant showed that the dry cleaning plant would cost approximately 13% less than the wet plant. However, operating costs would be approximately 50% of the wet plant costs for production of a fine coal product of equivalent energy level.50 2.4.1.4. Air-Dense Medium Fluidized Bed. The dry coal beneficiation with a gas-fluidized bed, referred to as an airdense medium, can be sorted as a dry dense medium concentration. By means of gas-solid suspension as beneficiation medium, the light and heavy materials separate from each other in a fluidized bed according to the difference in their densities. Its capital and operation costs are less than those of wet cleaning plants of the same capacity because of elimination of the complicated coal slurry processing system. The first process for coal was reported by Fraser and Yancey in 1926, and since then, many studies have been carried out with limited success.51 The Mineral Processing Research Center of the China University of Mining and Technology (CUMT) has been developing the dry coal beneficiation technology since 1984. The Qitaihe Coal Dry Preparation Plant, the first commercial plant for this technology in China, was established for beneficiation of 50-6 mm size fraction coal by CUMT in June 1994. Since then, new applications have been found and a 700 000 tons/year dry coal beneficiation plant with air-dense medium fluidized bed has been put into commercial testing. The separator is shown schematically in Figure 5. To obtain an efficient dry separation condition in an air-dense medium fluidized bed, stable dispersion fluidization and microbubbles must be achieved. Its required physical properties are that bed density is identical to the beneficiation density and well-distributed in three-dimensional space and does not change with the time and the fluidized bed as a separation medium is (51) Lei, J. Development of coal dry beneficiation technology in China. Proceedings of the 24th Annual International Pittsburgh Coal Conference, Johannesburg, South Africa, Sept 2007.

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Figure 6. Schematic diagram and operation of the FGX separator unit (Honaker et al.53).

of low viscosity and high fluidity. A homogeneous fluidized bed with a high-density dense phase and a multitude of microbubbles was formed. Advantages of this new dry coal beneficiation technology, as reported by Lei,51 include (i) high precision (It compares favorably to the best existing wet heavy medium beneficiation for effective beneficiation of coal of 50-6 mm size with a probability error, Ep value, of 0.05-0.07), (ii) low investment (Because this technology greatly simplifies the coal beneficiation process and eliminates a complicated and costly slurry treatment system, its capital and operating costs can be reduced to only about half of those of a wet beneficiation plant with the same capacity), (iii) less environmental pollution (This technology requires a small quantity of low-pressure compressed air. Pollution is greatly reduced by the dust removal system. The dust emission in the exhausted air is much lower than that required by environmental protection laws. The separator operates smoothly and steadily with little noise), and (iv) wide ranges of beneficiating density (A stable fluidized bed can be obtained by using mixtures of magnetite powder and fine coal as dense medium to produce a beneficiation density from 1.3 to 2.2 g cm-3. Therefore, this technology can meet the needs of beneficiating different coals for different products. It can either be used to remove gangues at high density or to produce clean coal at low density). Despite the fact that the commercial plant was for the beneficiation of 50-6 mm size coal particles, other efforts are being done to address the beneficiation of other particle size ranges. These include 300-0, >50, 6-1, and