Co-torrefaction of rice straw and waste medium density fiberboard: A process optimization study using response surface methodology

Abstract

Co-torrefaction is a flexible way of improving the properties of various kinds of waste biomass for utilization as a clean solid fuel. Rice straw (RS) and medium density fiberboard (MDF) were used as feedstock for the torrefaction. Three input parameters were evaluated to determine the optimum conditions: rice straw ratio (RSR), torrefaction temperature and residence time. A response surface method based on a Box-Benhken design was used to achieve the optimum conditions (maximizing torrefied heating value and energy yield). Main and interaction effects for the independent variables on the responses were investigated based on analysis of variance (ANOVA). The findings revealed that temperature was the main effect and that there was no interaction effect between the inputs. Thus, a lower temperature optimized co-torrefaction. The optimum conditions for maximizing the heating value (22.13 MJ/kg) and energy yield (99.60%) were an RSR of 25%, a temperature of 208.10 °C and a residence time of 50 min. The experimental values were in good agreement with the corresponding predicted values. These findings should provide guidelines for the thermal pretreatment of mixed waste material for co-firing or co-gasification.