Silicon oxide forms the main component of the ash with trace amounts of Al₂O₃, Fe₂O₃, CaO, MgO, K₂O, and Na₂O. Ash’s physical and chemical properties depend on the combustion process’s components, such as combustion type, feeding type, temperature, residence time, and availability of oxygen (aerobic or anaerobic).

The characteristics of rice husk compared with other solid fuels can be summarized as follows:

– Its high silica content causes excessive wear to parts of processing machines, such as conveyors or grinders, and hampers digestibility in livestock. The content of volatile matter in the rice husk is higher than in wood. It’s much higher than in coal; whereas, fixed carbon is much lower than in coal. Ash content in the rice hull is much higher than in wood and coal, which causes barriers to energy conversion.

– The high content of ash, alkali, and potassium causes agglomeration, fouling, and melting of the parts of combustors or boilers.

Utilization of the rice husk

Rice husk was long considered a waste from the rice milling process and was often dumped and/or burned. But because it can be easily collected and is cheap, some rice hull has always been used as an energy source for small applications, such as brick production, steam engines, and gasifiers used to power rice mills, and for generating heat for rice dryers. The high silica content of ash makes it a good additive for the steel and concrete industries.

To a lesser degree, rice hull ash is used as a soil conditioner, activated carbon, insulator, and others.