Effect of Silica and Alumina Ratio on Bed Agglomeration During Fluidized Bed Gasification of Rice Straw

Bed agglomeration is one operational challenge during fluidized bed gasification when rice straw is used as raw materials. Rice straw contains high potassium and other components which may lower ash melting point causing bed agglomeration. This study focused on the effect of alumina and silica ratio of bed materials in fluidized bed gasifier. The ratio of alumina bed was 0, 0.25, 0.50, 0.75 and 1.0. The experiments were performed at 700, 800 and 900°C with equivalence ratio (ER) of 0.2. Rice straw size 425–850 μm was used as raw materials. The result showed that the high ratio of alumina deceased bed agglomeration at 700°C. However, similar alumina ratio increased bed agglomeration at 800 and 900°C. In addition, the effect of temperature on defluidization time was significant. As the operating temperature increased, the defluidization time decreased. Although the ratio of the bed material was different but the result of defluidization time show a similar trend. As a result, high operating temperature may not suitable for fluidized bed gasification with this particular biomass. The SEM/EDS analyzed showed that potassium, calcium and silicate are major element in a linkage between bed particles. As a result, 75% of alumina bed ratio at 700°C was sufficient to avoid bed agglomeration during fluidized bed gasification of rice straw. In conclusion, specific ratio of alumina and silica can prevent agglomeration in fluidized bed gasification of rice straw when operating temperature lower than apparent eutectic melting point of involved alkalis from rice straw. The result from this investigation may lead to options on mitigating the problem of bed agglomeration in fluidized bed gasifier of rice straw.