Agglomeration Characteristics of Silica Sand-Rice Husk Ash Mixtures at Elevated Temperatures

Abstract

Rice husk, a major by-product of the rice milling process, can be a significant energy resource in rice producing countries because of its high energy content. Fluidized bed gasifiers have been proposed for the recovery of energy from rice husk. The major advantage of fluidized bed gasifiers over fixed bed gasifiers is the high mass and heat transfer capability due to very high percentage of inert bed material such as silica sand. In addition, the vigorous mixing and agitation of solid particles in fluidized beds promote a uniform temperature distribution and a high conversion efficiency. However, attempts to utilize rice husk as a feed in fluidized bed gasifiers have been unsuccessful because of the high ash content of rice husk that may result in the agglomeration of inert bed materials at high temperatures. In this work, the effect of rice husk ash content (0.0, 5.0, 10.0, 15.0, 20.0, and 25.0%) on the agglomeration characteristics of silica sand was investigated at various temperatures (750, 850, 900, 950, and 1000°C) using a muffle furnace. A light microscope, an environmental scanning electron microscope, and an energy dispersive X-ray analyzer were used to characterize the structural changes and elemental makeup of the samples. There was no indication of agglomeration below 850°C, but at temperatures of 850–1000°C the silica sand loosely agglomerated in the presence of rice husk ash at all levels of ash content. The effect was more pronounced at 1000°C. The chemical interaction of the Si02 and the low melting temperature mineral oxides (K2O, Na20, and P205), present in notably low concentrations in rice husk ash, appeared to be the mechanism resulting in the formation of the loose agglomerates.