High ash non-coking coal preparation by tribo-electrostatic dry process

Abstract: Coal is the single largest fossil fuel used world-wide and accounts for more than 60% of the total commercial energy consumed. Between 60 to 80% of this coal is used for electric power generation and most of which through a system of pulverised coal combustion. Major portion of the coal used for such power generation is not clean enough to maintain environmental standards. This problem is attributed to high sulphur content in coal used in most of the western countries or ash as is the case in countries like India. In India at present nearly 260 million tonnes per year of coal is used for power generation and the average ash in coals used is invariably above 40%. A substantial portion of ash is liberated as it enters the boiler from the mill. It is crucial to reduce the amount of ash going from the mill to the boilers not only to improve the performance of power generation and increase the life of the boilers but also became mandatory due to environmental regulations. Thus the main objective of the work is to develop a dry tribo-electrostatic process for the separation of ash forming inorganic matter from coal material with a thorough understanding of the response and behaviour of coal and non-coal matters to contact electrification and in electric field. This work is financially supported by the Department for Research Cooperation of the Swedish International Development Cooperation Agency (SIDA/SAREC). The literature on dry coal preparation processes has been reviewed and the advantages of triboelectric process compared to other processes have been highlighted and further research needs to make it a viable industrial technology are outlined. Three Indian coal samples from three different major coal fields, i.e., Ramagundam, Ib-valley and Talcher, have been collected and characterised for macerals and mineral composition by microscopic and XRD analyses. The beneficiation potential at different size fractions of the coal samples is judged by the washability studies. The charge polarity and magnitude of pure quartz, pyrite kaolinite, illite and carbon after contact electrification with different tribo-charging media have been measured by Faraday cup method using Keithley electrometer. The predicted work functions of the tribo charging material and mineral phases agree closely with the reported values. The electron accepting and donating (acid-base) property of mineral phases determined by Kr¨¹ss Tensiometer through polar and non-polar liquid contact angles on solids also corroborated the acquired charge polarity in contact electrification with copper, aluminium and brass materials underlying their work functions. This methodology can be applied for the choice of organic acidic/basic solvents treatment of coal material to enlarge the difference in work functions between the tribo-charger and mineral phases, and to achieve greater separation efficiency of inorganic matter from coal. The process of tribo-electric coal/ash cleaning is carried out with a newly built cylindrical fluidised-bed tribo-charger with internal baffles, made up of copper metal, and the influence of equipment and process variables have been evaluated. The collecting bins of the material underneath the copper plate electrodes are designed to function as Faraday cups such that the charge polarity and magnitude of particles in each bin can be measured directly. The coal and mineral particles respectively charged with positive and negative polarities with relatively higher magnitude illustrating greater efficiency of contact electrification in the fluidised bed tribo- charger system. The separation results with minus 300 ¦Ìm size fraction of coal containing 43% ash showed that the ash content can be reduced to 18% and 33% with an yield of about 30% and 67% respectively. With a 30% ash coal, a clean coal of about 15% ash is obtained with about 70% yield. These results are comparable to the maximum separation efficiency curves of washability studies on the coal samples. Since the ash percentage of coal particles collected in the bins close to positive and negative electrodes are about 70% and 20%, a better yield with further reduced ash content can be accomplished by recycling the material. Thus, the tribo-electrostatic method observed to be a promising dry coal preparation technique, where the present laboratory size separator needs to be scaled to plant level for commercial application.