Ash Formation and Deposition in Oxy-fuel Combustion of Rice Husk, Coal, and Their Blend with 70% Inlet O2

Managing the risk of ash deposition is still a priority for oxy-fuel combustion of biomass–coal blends. This is especially a concern when high inlet O2 concentrations are adopted to reduce the amount of recycled flue gas, which results in higher combustion temperatures and lower gas flow rates. Such changes are expected to affect ash formation and deposition but have little been investigated. Rice husk has been reported to induce no ash-related problems under fluidized bed combustion conditions. However, it is unclear regarding ash formation and deposition behavior during oxy-fuel combustion of a rice husk–coal blend with high O2 concentrations. Particular interests of this work include the effects of rice husk co-firing and high-temperature oxy-fuel combustion on ash transformation and its correlation with ash deposition behavior. Chinese rice husk (RH), bituminous coal (SF), and their blend (the share of rice husk is 16 wt %, SF/RH) were tested in this work. Experiments were carried out on a 100 kW pilot scale combustor under oxy-fuel (O2/CO2 = 70/30 vol %, OXY70) and air combustion conditions. Both the bulk ash and fouling deposits were collected and characterized by techniques, such as computer-controlled scanning electron microscopy and X-ray fluorescence. Results show that co-firing rice husk increases ash particle size distributions (PSDs) and results in higher quartz and complex aluminosilicates in the ash. Coarse ash particles generated from rice husk combustion tend to dilute the deposits and cause its shedding. Consequently, the development of SF/RH deposit weight gain is suppressed. In comparison to air combustion, OXY70 combustion promotes mineral interaction and leads to coarser PSDs. For SF and RH, OXY70 combustion increases ash deposition propensity because of the higher ash impaction efficiency and average capture efficiency. For SF/RH, OXY70 combustion increases ash deposition propensity. The effect of OXY70 combustion on ash deposition propensity is mainly related to the behavior of coarse ash particles from rice husk and the changing of the gas flow rate in the furnace.