Optimization of rice husk pretreatment for energy production

One of the most widely cultivated crops in the world is rice, leading to millions of tons of rice husks (also known as rice hulls or chaffs). The large amount of this lignocellulosic biowaste has resulted in an extensive search for its utilization. One such usage of this waste is for the production of electricity, such as in combined heat and power or gasification units. However, one of the disadvantages of using rice husks is their high silica content which produces large amounts of undesirable ash upon combustion leading to operation problems such as slagging and clogging. Here, alkali pretreatment for the extraction of silica in the form of sodium silicate has been studied using response surface methodology (RSM) and Analysis of Variance (ANOVA). Three independent variables namely reaction temperature, duration, and alkali concentration were considered using a Box-Behnken design (BBD). The operating conditions were optimized under different scenarios. The first optimization focused on the two goals of high ash removal and high solid yield while the next optimization rounds added the criteria of low NaOH usage and robust design (using propagation of error (POE)). The final treated rice husks can therefore be more suitably used as feed for thermal and/or electric units. The developed empirical predictive models were successfully validated through additional experimentation.