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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.

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 O 2 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 O 2 concentrations. Particular interests of this work include the effects of rice husk co-firing and hightemperature 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 (O 2 /CO 2 = 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.

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Utilization of biomass when combined with carbon capture and sequestration (CCS) is one feasible solution for "bioenergy with carbon capture and storage" (BECCS), which can reduce the CO 2 emissions in the atmosphere. Rice husk is a unique biomass resource because its ash content is typically higher than 15% and more than 88% of this mineral matter is composed of silica. Ash partitioning and deposition mechanisms in rice husk combustion are still under investigation and will be the focus of this paper. Two different rice husk resources were tested in this work: one is from China, and another is from the United States, which are abbreviated as RH_CN and RH_US, respectively. The experimental work was performed in a 100 kW down-fired oxy-fuel combustor (OFC), and both fuels were co-fired with natural gas to achieve combustion conditions that are comparable to coal combustion. Two conditions were tested: (1) air combustion and (2) oxy-combustion with 70% O 2 and 30% CO 2 in oxidant gas (denoted as OXY70). Results suggested that similar sub-micrometer accumulation modes and supermicrometer fragmentation modes appeared for both rice husks. Although the composition of super-micrometer particles was similar and enriched in silica for both rice husks, the sub-micrometer particles from RH_CN are mainly composed of volatile elements (K, Na, Cl, and P), while these from RH_US are mainly composed of Si. This compositional difference in submicrometer aerosols subsequently affects the ash deposit formation; for example, the tightly bound inside deposits from RH_CN have much more volatile elements than those from RH_US. More importantly, apparent ash shedding on deposits occurred after 0.5 h for RH_CN, but this cannot be observed in RH_US until the maximum sampling time (2 h).