Optimisation of torrefaction conditions of groundnut shell and maize stalk

Abstract

The yield and properties of torrefaction products are affected by both feedstock characteristics and torrefaction conditions. This study aimed at optimising torrefaction process conditions i.e. heating rate (5–15 °C/min), holding time (10–120 min) and temperature (200–300 °C) for groundnut shell and maize stalk. Response surface methodology based on Box Behnken design and Derringer’s desirability function approach was applied in the optimisation of torrefaction conditions. The optimum heating rate, holding time and temperature were 5 °C/min, 120 min and 200 °C for groundnut shell and 10 °C/min, 70 min, 200 °C for maize stalk, respectively. The optimum biochar and energy yields were 90.3 and 96.7% for groundnut shell and 88.4 and 97.6% for maize stalk, respectively. The effect of torrefaction process parameters on biochar and energy yields for both groundnut shell and maize stalk followed the order, temperature > holding time > heating rate. Second order regression models were developed to predict biochar and energy yields of groundnut shell and maize stalk. The modeled and experimental results were in agreement. Pearson correlation coefficients for the linear relationship between the experimental and predicted biochar and energy yields of the two feedstocks were > 0.95. The coefficient of determination (R2) for each model was > 0.95. The greatest percentage enhancement in the higher heating value (HHV) within the ranges of torrefaction conditions used was 46.3% for maize stalk and 39.2% for groundnut shell.