Effects of carbonization on the combustion of rice husks briquettes in a fixed bed

Carbonization of raw materials for making briquettes has been identified as a possible way of improving calorific values and fixed carbon contents of briquettes. In spite of these benefits, the main challenges of carbonization are increased percentage of ash content and energy required to process the fuel. In this study, fuels with mixtures of carbonized and raw rice husks were combusted in a fixed bed to examine the effect of carbonization on their combustion properties with an aim of establishing an optimum ratio. Bed temperature distribution, ignition, flame propagation speed, combustion rates and reaction zone thickness were investigated. All experiments were performed in a fixed bed operated in a reverse counter-current flame propagation mode. Four mixtures consisting of different ratios of carbonized rice husks were used to process briquettes; namely, 25 wt%, 50 wt%, 75 wt% and 100 wt% biochar. Air-mass flux for combustion of each mixture was varied between 0.02 and 0.6 kg/m2s. It was established that carbonization increases ignition time and peak bed temperature but lowers flame propagation speed, reaction zone thickness, and combustion rates. Briquettes with lower percentage of biochar had higher volatiles and higher devolatilization rate that shortened combustion period. Briquettes with carbonized rice husk of between 50 wt% and 75 wt% were found to have optimum combustion properties. Processing of briquettes should be done within these carbonization ratios in order to achieve desirable combustion properties.