Ash Formation and Fouling during Combustion of Rice Husk and Its Blends with a High Alkali Xinjiang Coal

Limited data from fluidized bed combustion tests have shown that rice husk, a silicon-rich residual biomass, has the potential to be cofired with coal while not inducing unacceptable ash-related problems. However, there is great concern regarding the behavior of rice husk ash under pulverized fuel combustion conditions, where the temperatures are much higher and expected to facilitate fuel interactions. This work, to the authors' knowledge, is the first to investigate both ash formation and fouling behavior in rice husk firing and its cofiring with coal at a high temperature relevant to pulverized fuel combustion. A Chinese rice husk and a high alkali Xinjiang coal were selected. Combustion tests of individual fuels and their blends (with the share of rice husk being 10% and 20%, respectively) were performed at 1573 K on a laboratory drop tube furnace. Both bulk ash and fouling deposit samples were collected in each test. Techniques including X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and ash fusion temperature testing were used for sample characterization. The results show that the rice husk ash generated at 1573 K is dominated by amorphous silica with only a trace amount of quartz. Partial melting of ash particles is observed and attributed to enhanced formation of potassium silicates, which were not found in fluidized bed combustion. Nevertheless, such differences do not seem to change the nonfouling nature of rice husk ash. The ash from cofiring rice husk and coal possesses similar crystalline structures to the coal ash, but extensive fuel interactions are observed. The high fouling tendency of the coal ash is greatly reduced by cofiring with rice husk. The effects of rice husk are both physical and chemical in nature. The presence of high-fusion-temperature, nonsticky rice husk ash in the deposits and the capture of fouling-inducing species from coal by rich husk ash are considered to play important roles in reducing ash fouling. The nonlinear dependence of ash fouling tendency on the rice husk ratio, as found in fluidized bed combustion, is also observed. This work suggests that rice husk may also be cofired with coal in pulverized fuel boilers while not causing significant ash-related problems, though further investigations are still necessary.