Characteristics of smoldering on moist rice husk for silica production
In order to assess the possibility of silica production via smoldering of moist rice husk, experiments of washed (moist) rice husk (7 kg with moisture content of 51%) in a newly designed smoldering apparatus was performed. The temperature inside the fuel bed during smoldering was recorded, and characteristics of ash were analyzed. Results showed that the highest temperature in the middle of the naturally piled fuel bed was about 560.0◦ C, lower than those in most of combustors. Some volatiles from the lower part of the fuel bed adhere to its upper ash during piled smoldering. Silica content and specific surface area of ash from smoldering of washed (moist) rice husk were 86.4% and 84.9 m² /g, respectively. Compared to our experiments, they are close to smoldering of unwashed rice husk (89.0%, 67.7 m² /g); different from muffle furnace burning (600◦ C, 2 h) of washed (93.4%, 164.9 m² /g) and un-washed (90.2%, 45.7 m² /g) rice husk. The specific surface area is higher than those from most industrial methods (from 11.4 to 39.3 m² /g). After some improvements, the smoldering process has great potential in mass product of high quality silica directly from moist rice husk.