New progress in the processing and efficient utilization of coal

Mining Science and Technology( China)21(2011)547-552Contents lists available at Science Direct氵Mining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcNew progress in the processing and efficient utilization of coalZhao Yuemin", Liu Jiongtian, Wei Xianyong, Luo Zhenfu, Chen Qingru, Song ShuleiKey Laboratory of Coal Processing E Efficient Utilization of Ministry of Education, China University of Mining 8 Technology, Xuzhou 221116, ChinaARTICLE INFOA BSTRACTArticle history:Coal accounts for about 70% of the primary energy sources in China. The environmental pollution andReceived 10 December 2010resources waste involved with coal processing and utilization are serious. It is therefore urgent to developReceived in revised form 8 January 2011Accepted 6 February 2011highly-efficient coal resources utilization theory and methods with low-carbon discharge Based on ourAvailable online 14 July 2011long-term basic research and technology development, the progress in beneficiation, cleaning, and trans-formation of coal, which includes dense phase fluidized bed dry beneficiation, deep screening of wet finecoal, micro-bubble flotation column separation, molecular coal chemistry, and transformation and separation of coal and its derivatives into value-added chemicals under mild conditions is discussede 2011 Published by Elsevier B V on behalf of China University of Mining TechnologySeparationCoal conversion1 Introductionimported three representative kinds of advanced technology andequipment during a flurry of research that occurred in theCoal is our country's most abundant fossil energy resource and nineteen-eighties. these failed in the practical application becauseour major source of energy. In 2009 national coal production was of their low efficiency and poor adaptation to the field conditions3.05 billion tons and accounted for 70% of the primary energy sup- in China[4-6)ply. It is expected that this pattern of coal being the major energyIf coal is to be used as a material for production of clean energysource will continue until the middle of this century. Coal is a typ- heteroatoms and inorganic minerals in the coal should beical high-carbon resource. Hence, the most efficient utilization of removed. At the same time, the organic material in the coal shouldcoal, although an arduous task, is an urgent need to fulfill our gov- be degraded into small molecular units and the aromatic functionernments commitment to the intemational community.ality should be hydrogenated. Hydrogenation, pyrolysis, andThe diversity of, and various types of, coal introduce a complex hydrocracking are the steps required for obtaining coal-basedity that mandates many different methods of low-carbon utiliza- clean fuels. Coal is also used as feedstock for high added valuetion. Long term studies by the authors on dry separation, fine chemical products. We must understand the molecular composicoal desulfurization and de-ashing, the direct transformation of tion of coal and its derivatives so that the organic matter in coalcoal under mild conditions into useful derivatives, and separation may be properly decomposed and the reaction mixture so obtainedof mixed fines have been previously reported. These are effective can be cleanly separated into useable products [7]-methods and technologies for low-carbon utilization.Our group has devoted much effort into basic research andMore than two thirds of the coal is located in the northwestern technology development related to these problems over the lastpart of the country or in other areas where water is scarce. Hence 20 years. We have reported a series of innovative results such asseparation of coal by wet cleaning is infeasible there. Many coun- efficient dry separation of coal, depth screening of wet fine coal,ties have agreed on the importance of efficient dry separation of depth desulfurization and de-ashing for the preparation of cleancoal Related research, which has been conducted successfully in coal, and furthered understanding of the composition, structurethe former Soviet Union, the United States, Canada, and Japan, is and value-added utilization of coal and its derivatives [8-21).still in the laboratory stage. Japanese scholars are following the Our group has played an exemplary role in the low-carbon utilizacurrent testing of >13 mm lump coal in a fluidized bed dry separa- tion of coal resources.tion unit located at the CUMT [1-3The flotation column appeared during the nineteen-sixties inCanada as a method suitable for fine coal separation. china2. Dry beneficiation and screening theory of coal and itsapplicationCorresponding author. TeL: +86 516 83590092.E-mail address: ymzhaoecumteducn(Y Zhao).beneficiationH中国煤化工 ration have bec1674-5264/5-see front matter e 2011 Published by Elsevier B.V. on behalf of China University of Mining 8CNMHGdoi:10.1016/mstc201106015Y. Zhao et al /Mining Science and Technology(China)21(2011)547-55increasingly obvious primarily because of a shortage of watersized coal fractions in the fluidized bed was successfully explained.coal cleaning and an increased need for cleaning. Dry screening The core of this theory is that the distribution of the different sizeis widely used in many fields such as coal, metallurgy, chemical, fractions is determined by comparing calculations using the charand building materials. But screening wet, fine materials results acteristic parameters of each size fraction and the dense mediumin the serious problem of pore blocking. It is very difficult to finish In this approach the coal in the fluidized bed is treated as if ithe screening process because of this problem. Therefore, it is of distributes into three size fractions. The first fraction is fine pargreat significance to develop an effective screening technique for cles that move to the upper layer. The second one consists of par-wet fine materialsticles with a sink rate equal to the medium that will intermix withhe medium solids. The last fraction is made up of coarser particles2. 1. Theory of uniform density stabilization of a dense medium airthat stratify according to the bed density because of the upliftingfluidized bedaction of the fluidized bed. This provides a theoretical basis for regulating the bed density as a method of controlling feedstockA necessary condition for forming a uniform density stabilized Separationfluidized bed is the existence of a well distributed air flow. thethe-The solid medium actually consists of the solid working medory of voltage re-regulated air distribution with a twosection lum and the added coal powder. this is because the coal addedto the bed inevitably leaves some material behind during separanumber air flow was proposed. At the same time the view that a tion. Therefore, it is important to study the medium in a gas-solidnarrow size fraction(sieve scale 2)of the solid particles of the fluidized bed to determine the properties of a two-element med-medium hindered back mixing and falling was proposed. The nar-ium. Mathematical models have been proposed for such a system.row size range results in effective grading of the material being The density distribution and separation characteristics were studeparated. A theory describing the uniform density stabilization ied allowing an analysis of the characteristics a two-element solidof a dense medium, air fluidized bed was proposed. A fluidized medium fluidized bed exhibits to be made. A mathematical model,bed layer having a dense phase consisting of high density micro- Eq(2), for a two-element solid medium fluidized bed was estabbubbles was formed. The mechanism of multi-componentseparation was studied by analyzing the forces on the material inPiP2a complex, dense phase fluidized system. the feed had a widea(1+mp1+(P2-p1)00,(1) Fig. 2 shows that when coal powder is transported through aPp (1-2)g when Ur(Ps-p)<0(1) grinder organic matter and inorganic matter develop positive andU=√③where a, Pb, P. g, Cp Un and d are the coal particle acceleration, thebed density, the coal particle density, the acceleration of gravity. therag coefficient, the relative velocity between a coal particle and thedense medium, and the coal particle diameter, respectivelyA distribution law for screening of multi-component particleair fluidized dense medium bed has been studied. the three-grade distribution of different sized coal fractions in the fluidizedbed was described, which allows the lower limit of separation to Flg 1. 40t/h modurH中国煤化工C N MH Gum fluidized bed benebe determined in theory. The observed distribution for different fication system.Y Zhao et aL/Mining Science and Technology(China)21(2011)547-552when a flexible screening surface is present was developed. Theinteraction between the rigidity and the flexibility of the screensurface was studied to optimize moist fine coal screening. Opti-mized design formulas and a dynamic model of a screeningmachine having a flexible screen surface were provided. The designmethod for an optimum flexible and dynamic machine with a fleible screen surface was establishedA statically indeterminate structure was used during the designUnload brushof a large vibrating screen for the first time. a doubly staticallyindeterminate net beam excitation system consisting of a stiffbounding volume and several tubular beams was proposed as away to realize excitation in combination with a net beam volumea plate type statically indeterminate combination bearing consti-tutes the plane beam system and a y-type statically indeterminateFlg 2 Schematic diagram showing fine particle triboelectic separationcombination strengthens the structure of the beams, whichincreases the stability and the reliability of the large vibratingscreen.negative charges due to collision and friction between them, or beThe theory describing the characteristics of moist fine coal weretween the particles and the shell wall. This mass of electrically used to develop a screening machine with a flexible filter surface,charged particles is then fed through a strong electric field. The statically indeterminate net beam excitation, and a large vibratingpositively and negatively charged particles are propelled in oppscreen. the advantage here is that the vibrations are large, the self-site directions by the force from the electric field as they move cleaning ability is great, holes do not become plugged, dynamictoward the plate having the opposite charge. Two material streams stresses are small over-all, and the reliability and screening rateare formed by this action so the separation of organic and inorganic are high. These machines are widely used in many fields includingmatter in the feedstock may be realized. The pyritic sulfur is an the coal, electric power, and chemical industries.important inorganic material that may be so separated. The min-eral dielectric constants and the surface work functions weredetermined by measurement. This established a foundation for 3. Theory of complete desulphurization and de-ashing andinvestigating these electrical properties and provided data useful cleaning of coal, including engineering applicationsfor the further design of triboelectric separators and a practical3.1. Method and theory of swirl-static micro-bubble flotation column2.4. Theory and technology of deep screening of moist fine materialsSwirI-static micro-bubble flotation column separation requiresthe combination of cyclonic separation with static flotation columnThe composition and physico-chemical properties of moist fine techniques Such a design is shown as Fig 3. This design includescoal were used to develop a theory describing the hole-filling and column flotation, cyclonic separation, and pipe flow mineralizationflexibility of the cover film developed during screening of moistcoal particles Adhesion models of moist fine coal were establishedin a process that proceeds from top to bottom.Column flotation occurs at the top of the column and thisEquations describing the flexible vibration of the cover film and affords pre-separation of the feed. As a result high quality concen-other fundamental expressions such as Eqs. (3)-(5)were derived. trate is received at the later stages. Cyclonic separation occurs onCriteria for cover film fracture were given. A high amplitude high the bottom of the flotation column and is used to further separatevibration, screening process was proposed for the realization ofthe qualified tailings. Air is introduced to form bubbles and pipemoist fine coal screening. The theoretical basis for solving the hole flow mineralization is used in cyclonic separation to give furtherfilling problem was establishedseparation. This is linked to the cyclonic separation along the tan82z82zgential direction to form a cycle.卩T(2+5)+ pAc sin atThe study of fine material separation techniques was concernedwith the methods, but not the separating process, for a long time.Nonlinear separation characteristics make materials more difficultOn=丌1(1/a2(1/bCOSS≥SmaxIn Eqs. (3)-(5). T, and A are the mass per unit volume, the surface tension coefficient of the cover film, and amplitude alongColumnthe z axis. a and b are the size of the sieve pores.The nonlinear motion of the coal during high amplitude highvibration conditions were studied. Periodic bifurcations, a period-doubling bifurcation, and chaotic motion are predicted when theCyclonic separatparticles move along the screen. The vibration strength for chaoticmotion of the particles was predicted. The concept of touch-net中国煤化工probability was proposed. A theoretical, and simulation, model ofrandom particle leaking through the screen was established. a theCNMHGoretical basis allowing parameter optimization of the filter process Fig 3. A schematic diagram of a cyclone-static micro-bubble column separator50Y Zhao et aL/Mining Science and Technology( China)21(2011)547-552to separate using the proceeding method. A single separating envi- nents after coal decomposition by destructive analysis provides inronment and process always results in an inefficient process.sight into the coal solids. The possibility exists that newA nonlinear cascade of optimized separating processes is one macromolecular systems are formed from the decompositionwhere as the separation characteristics get poorer the separating products. there is no basis justifying any combination that reflectsenvironment adapts to the changing characteristics. This way an the original composition of the organic matter in coal, itself. Quan-optimized relationship between the separated material and the tum chemical calculations are ineffective and models of coalenvironment in which it exists allows the most efficient separa- molecular composition do not reflect the composition of the organ-tion.An example of a nonlinear cascade would be a single flotation ic matter in the coal. Calculations based on the results of non-followed by a cyclone-static micro-bubble column separation. This separation and/or destructive analysis will provide inappropriatesets a precedent for fine material nonlinear cascade separation and information. In fact, the organic matter in coal is composed ofthis example will accelerate the development of the technology of many kinds of organic compounds and can not be represented byefficient fine separation.any single modelThe technology and equipment for cyclone-static micro-bubbleDifferent organic solvents may be used to divide the organiccolumn separation were developed and are extended and applied matter in the coal into several groups. fractional extraction pro-at hundreds of domestic and foreign enterprises. the economic vides a series of simple compositions that allow an analysis sugbenefits from this method are significant.gesting the underlying composition in the coal. An analysis ofeach fraction using methods such as combined gas-chromatogra3.2. Recycling and clarifying mineral wash waterphy/mass-spectrometry, combined gas-chromatography/infraredspectroscopy, high-performance liquid chromatography coupledIn the process of cleaning coal a ton of coal needs three tons of with mass-spectrometry, or high-performance liquid chromatografor water purification is high and the purification operation is of the soluble organic matter to be elucidated. At the same timeproduction at a coal preparation plant. The principles of pulp solu- sition of coal followed by subsequent extraction provides addi-tion and colloid chemistry have been invoked in proposals for tional information on the structure at the molecular level. Themethods using natural minerals to clean recycled water. The aggre- composition of soluble organic compounds from coal or its reactiongation state of the slime-water system has been changed by the mixtures determined by separable and non-destructive analysis al-addition of slightly soluble high valence natural salts to cause rapid lowed the establishment of a theoretical description of molecularlime-water subsidence. Fig. 4 shows a new way of cleaning fine coal chemistry, both in China and abroad. a series of basic studiesparticles and how the mineralization required for difficulty sepa- have been done in this area [17-19). The composition of raw coalrated slime-water is appliedoxidized coal, and hydrogenated extracted coal has been deter-Pulp colloid chemistry was established by this study, which was mined [20-22]. It appears that Ni removes sulfur, which is precip.then used successfully in the subsidence treatment of slime-water. itated from the process during heating. This is established byThe natural minerals were added into the water system to improve comparing hydrogenation heat treatment of an upper Freeportsubsidence characteristics of the slime-water: This occurs by standard coal sample extract from the USA under heating condi-changing the hardness of the water. This avoids secondary pollu- tions at 300C both with and without the presence of Ni[23).Heattion during the processing of the recycled water.treatment in China at 300C under non- catalytic or Pd/c conditions gave extracts from bituminous coal that showed that Pd/c4.comPosition,decompositionandfineseparationofcoalandpromotestheappearanceofanilineandalkylanilineorganiccomits derivativesto be extracted from the coal via a mechanism that was revealedby this work [24]. Aliphatic amide in carbon bisulfide extract of4.1. Theory of molecular coal chemistryseveral kinds of coal and a series of biomarkers in carbon bisu-fide/ THF mixed solvent extract of Erdos coal were concentratedUnderstanding the composition of organic matter on a molecu- using column chromatography enrichment [25-26). A series oflar level can provide very important theoretical guidance for coal pure organic compounds were separated from extract of Wulinglow-carbon utilizatin. However, the composition of the organic coal (27-28]. It was also found that Fe/S selectively promotesmatter in coal is ccmplex. Some have established models of coal extraction of anthracene from Pingshuo coal. This was done bymolecular composition based on the results from non-separation comparing heat treatment under hydrogen gas of Pingshuo coaland/or destructive analysis and/or quantum chemistry calcula- at 300C where both non-catalytic and Fe/S catalyzed conditionstions.However, the organic matter in coal can not be identified existed(291by specific organic compounds. The composition of volatile compo4.2. Technical route toward a coal based fine chemical industryClay mineralDirect liquefaction of coal is a mild conversion process in termsof the temperatures involved. Coal liquefaction is a process that di-rectly produces liquid fuel from coal as the raw material Catalytichydrogenation and hydrocracking are used to achieve this endfrom a molecular perspective coal liquefaction can be defined asSoluble salt feeda process where the organic matter is decomposed into solubleand easily used lower molecular weight organic compoundsCurrently employed coal liquefaction processes were developed习as a technical中国煤化工fuel. And, hence thmain goal is effr, a cost must be paidfor this efficieCN MH Gion of the inorganicFig 4. Flotation solvent environment.minerals and condensed aromatics allowable in the coal. Some bot-Y. Zhao et aL /Mining Science and Technology( China)21(2011)547-552compounds having hetero-atoms, water, and inorganic solids. Hightemperature coal tar may be divided into light and heavy tarsaccording to the molecular weight of the substances involvedLight tar is measurable by GS/MS analysis. Most of the heavy taris too low in volatility for analysis by GS/MS: It is commonly calledtar asphalt.Tar asphalt amounts to more than 60% of the total weight of thecoal tar. It is used as leak-proofing material for roofing, as pavingglazed beogenasphalt, as an impregnant, or as part of the adhesive used duringPolycycle alapreparation of needle coke and carbon electrodes. The lighter component is composed of two to six ring aromatic hydrocarbons andthe contents of naphthalene, phenanthrene, and anthracene areFig..A technical route for development of a coal based fine chemical industry. greatest. The content of organic compounds containing heteroatoms is less than one percent of the phenol is the most prevalentfollowed by cresol. The third most common species are organictlenecks such as the complexity of the process, the stringent oper- compounds containing nitrogen. Sulfur containing compoundsating requirements, and special equipment make the recycling of are the least prevalent. the content of nitrogen containing com-the catalyst difficult. Furthermore, the coal liquefaction residue pounds is not high but these materials are very important for theconstitutes 30% of the raw coal and is difficult to use efficiently processing of coal tar, especially for catalytic hydrogenation be-and this residue can pollute the environment. At the same time, cause many of these materials can poison the catalyst. Coal tar as-investment and running costs are high. because of the low addi- phalt is used as a leak-proofing material for roofing and for pavingtional value of the product and a lack of competitiveness to the asphalt and is also used for the preparation of high performance fi-process the risk of investing in this technology is very large.ber. It is said that the price of per ton high quality high perforunder modest conditions, completely extract the organic matter performance asphalt carbon fiber P-120 exceeds 2270 S/kg). Coalfrom the coal, and provide easy recycling of any catalyst, water, tar asphalt is an ideal feed for preparation of high performance car-or solvent used. Research is needed to develop such a process. Once bon fibers. the key is that the problems of"leaving out the begin-be developed to take advantage of coal and provide economic ben- matter must be completely removed, as must quinoline insolublefit from itOur group has formed a plan concerning appropriate technol-After nearly 20 years our group has advanced a technical routeogy that is based upon long term basic research. Fig. 5 shows the for innovative coal tar separation, see Fig. 6. The technical core ofprocess being developed, which involves super-critical hot melting this process is distillation. There are still some problems with theytic hycracking products and separation of the organic compounds tion, and coking of the coal tar asphalt during high temperatureformed is another aspect of this approach. Finally, oxidative pyro- distillation. These problems may be avoided by solvent extractionlysis of insoluble, heat resistant material followed by separation is but solvent extraction involves subsequent separation of thea third component. In this technique the highest temperature can extract and extractant the filtration of extractant from extract isnot exceed 350 %c and the solvent and catalyst must be completely very difficult because of the viscous nature of the coal tar Prob-recycled. If 90% of the organic matter in coal can be converted by lems with coal tar coking were solved after many tests in differentthese processes into high performance coal materials this would conditions. Hence, the separation of coal tar can be donerepresent an important step forward. High value added materials successfully.from the coal would include high-performance carbon fibers, con-Light components are separated from the coal tar by choosing adensed aromatic compounds, polycyclic alkanes used for fuel in suitable solvent for extraction. Efficient separation of polycyclichigh-speed fighters and missiles, and other oxygenated organic aromatic hydrocarbons and hetero-organic compounds proceedscompounds used in several important wayssuccessfully after further treatment. polycyclic alkanes are produced by catalytic hydrogenation from polycyclic aromatic hydro-4.3. High temperature coal tar solvent extraction and separationcarbon feedstock. these polycyclic alkanes can be used as a highprocess(30-321quality jet fuel. The catalytic hydrogenation of the polycyclic aro-matic hydrocarbons in the lighter coal tar fraction proceeds easilyHigh temperature coal tar is one coal derivative that is a by- after the hetero-organic compounds are removed. The "leaving outproduct from the proceIt amounts to about five the beginning and the end"is realized by using fractional extrac-to eight percent of thew coal. The main cotion to obtain coal tar asphalt suitable for making high perfornents of this coal tararomatics, a few oimance carbon fibers. the research group currently is cooperating( Fuel for high-speed fighters ad missie)pregnant used for preparing efn中国煤化工asphalt adhesives andCNMHG552Y zhao et aL/ Mining Science and Technology(China)21(2011)547-552with the Zaozhuang Coal Mining Group, Co, Ltd, to develop a pilot [10] Luo ZF. Zhu JF, Fan MM, Zhao YM. Tao X Low density dry coal beneficiationtest process for high temperature solvent extraction as a way tousing an air dense medium fluidized bed. J China Univ Mining Technol2007:17(3):306-9technically support future industrialization.[11] Zhao YM. Liu CC, Fan MM. Research on acceleration of elastic flip-flow screensurface. Int JMiner Process 2000: 59(1): 865 Conclusions[121 Zhao L Liu CS, Yan JX. A virtual experiment showing single para linearly vibrating screen-deck Mining Sci Technol 2010: 20(2): 27([131 Zhao YM. Zhang SG Jiao HG, Tie ZX Simulation of discrete element of(1)A breakthrough in the concept of wet separation theory andmotion on the vibration plane. J China Univ Miningair-solid dilute phase fluidization has been made. The multi2006:35(5)586-91[ in chines[14] Liu m. Zhang M, Li YE, Lu Y. Research on pressure drop performance of thecomponent dense phase fluidized dry separation techniquepacking-flotation column J China Univ Mining Technol 2006: 16(4): 389-92has been studied extensively and a theory describing bed [151 Zhou XH, Lu me residence time in column flotation based on cyclonicaration. J Univ Mining Technol 2007: 17(3): 349-53.uniformity and stability was developed. Triboelectric sepa- [16] Li YF, Zhang M, Lu m. Study of gas content in a sieve packing flotation column.ration of fine particles as an efficient way of using a dry airstream to carry the coal has been reported.17] Wei XY. Zong ZM. Qin ZH. Ji YF, Liu Jz wu L et al. ldeas and prospects of(2)Traditional theory related to dry material, rigid surface, andmolecular coal chemistry In: Yuan QT, Jin Y, editors. Chemical Engineering andlinear particle motion separation has been advanced. A wayMaterials'99.Proceedings of the 2nd Symposium on Chemical, Metallurgicalwet fines may be screened using flexible screen surfaces andmparting a non linear motion to the particles has been[18] Wei XY, Gu XH Zong ZM, Qin ZH, Wu L wH Scientific basis of coal usedcascade of this technique with pipe flow mineralization Applied Che organic matter in coal. In: Proceedings of the Sth Symposium ondemonstratedstructu(3)A theory explaining the action of the cyclone-static micro-bubble column flotation technique has been proposed. 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