Ash Deposition Behavior of Upgraded Brown Coal and Bituminous Coal

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on the heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The ash composition in coal and operational conditions in boilers such as heat load greatly affect the ash deposition behavior. This study focuses on the relationship between ash deposition characteristics and ash melting characteristics. Upgraded brown coal (UBC) and two types of bituminous coal with different melting temperatures and ash compositions were used as samples. The deposition tests were conducted using a refractory furnace. A water-cooled tube was inserted into the furnace to make the ash adhere to it. The molten slag fraction in ash was estimated by means of chemical equilibrium calculations. The results showed that the ash deposition characteristics have a close relationship to the ash melting characteristics. UBC with a low ash melting point shows a relatively high rate of ash deposition. The molten slag fraction in ash obtained by the chemical equilibrium calculation correlates with the deposition fraction of ash obtained by experiments, even under coal blending conditions. Therefore, the molten slag fraction in ash obtained by the chemical equilibrium calculations is one of useful indices to predict the blending method with UBC to reduce the deposition fraction of ash.