Reducing Greenhouse Emissions from Lignite Power Generation by Improving Current Drying Technologies

Abstract

The reduction of greenhouse emissions, the reduction of ash fouling and the improvement of thermal efficiency are major issues facing electricity generating companies, particularly those using coal. The high water content of lignites is a major disadvantage with regard to power generation efficiency, as current technology uses energy to evaporate the water directly. Two pretreatment processes, hydrothermal dewatering (HTD) and mechanical thermal expression (MTE) have been evaluated to remove water as a liquid from coals, thereby avoiding the need to supply latent heat for vaporisation. This can improve efficiency of power generation from lignites and reduce greenhouse emissions per unit of electricity generated. In both processes the fluid pressure is maintained greater than saturation pressure to prevent evaporation. In HTD, dewatering is related to irreversible pore collapse following structural changes on heating. In MTE water is removed as pores collapse through the application of mechanical force. Furthermore, inorganics, particularly Na, are leached from the coal into the wastewater, decreasing the fouling potential of the product during combustion. Raw Loy Yang brown coal, 62% water (by weight), was processed in small batch MTE and HTD units at varying temperatures. The raw coal was also subjected to different mechanical pressing pressures in the MTE unit. The coal products were analysed by ultimate and proximate analysis, helium pycnometry, CO2 surface area and mercury porosimetry. The treated coal was found to lose only a small amount of volatile material at processing temperatures up to 250°C. However, an increase in processing temperature to 320°C (HTD) significantly increased the loss of volatile material, reduced the moisture content of the filter cake, decreased the large-pore (0.03-1µm radius) volume of the coal and increased the concentration of organic material in the wastewater. An increase in mechanical pressure in the MTE unit from 6–12MPa was found to increase the density and decrease the porosity, decrease the moisture content and also increase the carbon content of the product. The amount of water remaining in the coal products correlated strongly with the residual pore volume.