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

Abstract

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.

Experimental Study of SO2 Emission and Sulfur Conversion Characteristics of Pressurized Oxy-Fuel Co-combustion of Coal and Biomass

“…The above analysis results confirm that the self-desulfurization capacity of fuel ash is largest at 50% M b and the co-combustion of coal and biomass has a synergistic effect under pressurized oxy-fuel condition, which is probably because the alkali metal and alkaline earth metal oxides in biomass fuel such as CaO and K 2 O react with gaseous SO 2 . Different from the experimental result in this study, the content of SO 3 in the fuel ash in other studies increased or decreased with the increase of M b . − − The results of the XRF and XRD analyses point out that there are lower contents of K 2 O and Na 2 O and no MgSO 4 and Na 2 SO 4 in the fuel ash, which may be because the fuel has a relatively low content of Mg and Na.…”

Behavior of Metal Impurities on Surface and Bulk of Biogenic Silica from Rice Husk Combustion and the Impact on Ash-Melting Tendency

“…They reported that different slagging levels can be observed in different biomasses due to their distinctive concentrations of K, Si, and Ca in their fuel compositions . Reducing the concentrations of alkali metals, K in particular, prior to combustion can considerably decrease the ash-melting tendency and slag formation. , Thus, a selective pretreatment can effectively mitigate the ash-melting tendency of biomass fuels. There are few investigations in the generation of biogenic silica from RH using surface-sensitive methods to confirm the high purity of the generated silica .…”

Experimental and kinetic studies on ash fusion behavior: a high‐precision acquisition method for ash fusion temperatures

“…A large volume of studies has been carried out on ash fusion characteristics of coal and biomass ash, mainly involving AFTs, 14–16 mineral phases, 17,18 and their interactions of coal/biomass ash, 19,20 as well as the evolution of troublesome elements 3,21,22 . The research on ash fusion characteristics is usually associated with the analysis of AFTs obtained from high‐temperature fusion test.…”