Fuel Processing Technology (v.72, #3)
Effect of bed materials and additives on the sintering of coal ashes relevant to agglomeration in fluidized bed combustion by C Tangsathitkulchai; M Tangsathitkulchai (163-183).
Agglomeration propensity of Thai low-rank coal ashes was determined by measuring the compressive strength of sintered ash pellets over the temperature range of fluidized bed combustion. Physical and chemical changes of the sintered products were ascertained from scanning electron microscope-energy dispersive X-ray detection (SEM-EDX) and X-ray diffractometry (XRD). A clear difference existed in the strength–temperature relationship between these ashes. This difference was attributed to the role and relative amounts of clays and anhydrite components that form the low temperature melting eutectics. The bed materials (sand, CaO, CaCO3, and CaSO4) and additives (gibbsite and andalusite) when combined with the ashes caused a strength reduction due to the inert dilution effect that prevented the interaction of anhydrite and clays. To comprehend the mechanism of sintering and bed agglomeration more clearly, modified ashes which produced extra amount of amorphous silicate materials were prepared and tested. The bed materials and additives, when sintered with these modified ashes, gave reduction of pellet strength by varying extents based on three possible mechanisms namely, a pure inert effect, an inert/reaction effect and an inert/adsorption effect, with gibbsite being the most effective. Of the four test ashes, Lanna ash was the only ash that exhibited almost no strength development under all conditions, due principally to its very low clays content and relatively stable forms of mineralogical compositions.
Keywords: Sintering; Coal ash; Bed agglomeration; Bed materials;
Dry beneficiation of Slovakian coal by Y Soong; T.A Link; M.R Schoffstall; M.L Gray; D.J Fauth; J.P Knoer; J.R Jones; I.K Gamwo (185-198).
The dry beneficiation of three types of Slovakian brown coal, namely Ci'gel, Handlova', and Nova'ky coal was conducted via triboelectrostatic separation. Three different types of separators—parallel plate, cylindrical and louvered plate—were used for this study. It was found that a parallel plate separator could reduce the ash contents of Ci'gel and Handlova' coals. The poor quality of separation for the Nova'ky coal studied is probably due to the particle–particle interactions and surface oxidation states of the coal.
Keywords: Beneficiation; Triboelectrostatics; Slovakian coal;
Improvement and characterization of an impregnated iron-based catalyst for direct coal liquefaction by Jisheng Zhu; Jianli Yang; Zhenyu Liu; Dady B Dadyburjor; Bing Zhong; Baoqing Li (199-214).
An impregnation method to prepare an active iron-based catalyst for direct coal liquefaction was improved. With the same catalytic activity, the water usage in the improved method is only 1% of that used in the unmodified method. The improved method not only simplifies the impregnation procedure and reduces cost, but also generates small catalyst particle size on coal surface. Water in the coal promotes thermal liquefaction, but deactivates the impregnated catalyst (possibly due to the adsorption of H2O molecular on the catalyst surface). Electron probe microanalysis (EPMA), X-ray diffraction (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS) analyses show that the catalyst precursors prepared by both methods are in nanometer size and highly dispersed on coal surface. The irons deposited on coal surface are not only in sulfide forms, but also coordinate with oxygen from moisture- and oxygen-containing groups of coal. The impregnated iron may be composed of FeOOH and FeS or in the forms of Fe–O–S or Fe–S–O. The iron transforms to crystalline pyrrhotite in coal liquefaction.
Keywords: Impregnation; Iron-based catalyst; Direct coal liquefaction;
Comments on the use of molybdenum components for slag viscosity measurements by D. French; H.J. Hurst; P. Marvig (215-225).
The interest in coal gasification for power generation has led to the increased use of high temperature viscosity measurements for the characterisation of slag properties. This communication reports on the slag reactivity with molybdenum crucibles for a large range of slag compositions.
Keywords: Viscosity measurement; Slag reactivity; Molybdenum;
Combustion of fly-ash carbon by P. Pranda; K. Prandová; V. Hlavacek; F. Yang (227-233).
Thermodynamic aspects of carbon combustion are discussed. At lower temperatures (<400°C), carbon dioxide is the dominant species in the reacting system. Thermodynamic calculations indicate that at low temperatures methane can be produced. Based on literature data, the reaction rate of methane generation is low and therefore the methane combustion represents probably only a marginal contribution to the overall energy balance. Calorimetric experiments proved that impregnation of pelletized carbon fly-ash samples by sodium and/or potassium carbonates have beneficial effect on conversion of CO to CO2. The value of the effective heat of combustion, calculated from CO and CO2 outlet concentrations, increased for sample modified by Na2CO3 by 5% and for sample modified with K2CO3 almost by 20%.
Keywords: Coal combustion; Cone calorimeter; Carbon gasification thermodynamics;
Author Index (237).
Subject Index (239).
Contents of Volume 72 (2001) (241-242).