In-situ observation of the combustion of air-dried and wet Victorian brown coal

Abstract

This work investigates the fundamental and practical implications of the application of drying technologies to Victorian brown coal combustion. The base case of 60% moisture content coal preheated prior to combustion is compared with partially dried coal (with or without pre-heating) and coal dried to equilibrium moisture content (10–15%). Pulverised coal was combusted in a drop tube furnace and in-situ observations of combustion phenomena, particle temperature and gas temperature were made. An ignition delay was found to occur when partially dried coal was combusted without pre-heating. Flame stability was also decreased when wet coal was combusted without pre-heating. No ignition delay was observed when the water in coal was heated prior to entering the furnace, as in current boilers. The peak particle temperature was found to be higher than the wall temperature by around 130 °C for dried coal, 80 °C for preheated wet coal and 40 °C for non-preheated partially dried coal. The gas temperature profile in the furnace was measured and found to lag behind the particle temperature peak. It was concluded that the evolution and evaporation of water in the wet case lead to an ignition delay, cooler peak particle temperatures and prolonged char combustion. The difference in particle temperatures between preheated wet coal and dried coal and the gas temperature behaviour was attributed to the steam gasification reaction, although studies to elucidate reasons for the differences are ongoing. The quantified results on ignition delay and particle temperatures have important implications for the design of new technologies, in particular the boilers and feed size preparation, for power generation from high-moisture brown coals.