Dilute Sulfuric Acid Pretreatment of Switchgrass in Microwave Reactor for Biofuel Conversion: An Investigation of Yields, Kinetics,

Lignocellulosic materials provide a raw material source for biofuel conversion and offer several advantages over fossil fuels- usage of a renewable resource, reduced greenhouse emissions, a decreased dependence on foreign oil, and stimulation of the agricultural sector. However, a primary technological challenge in converting lignocellulosic biomass into fuel is overcoming the recalcitrance of its matrix to enzymatic hydrolysis. To overcome these problems for chemical processing, naturally occurring cellulose biomass must be pretreated before it can be further processed using enzymatic hydrolysis or bioconversion. The goal of this work was to develop a model that predicts the glucose yield (pretreatment and enzymatic digestibility) of dilute acid pretreated switchgrass as a function of pretreatment process conditions (acid loading, 0-1.5 vol%, temperature, 165-195oC, and residence time, 1-10 min). This project was the first study that used a multivariable design experimental series to directly compare the pretreatment effectiveness (product yield, biomass composition and appearance, pH, etc) of using conventional and microwave heated reactors. Microwave-pretreated switchgrass afforded up to a 100% higher total glucose yield (combined pretreatment and enzymatic-hydrolysis liquor yields) at equivalent pretreatment severity and at one tenth of the reaction time, relative to conventional pretreatment. Under best pretreatment conditions of 0.75 vol% acid, 195oC, 1 min residence time, 99% glucose yield and 99% hemicellulose removal were achieved. Kinetic parameters were estimated for the cellulose and xylan hydrolysis reactions in the pretreatment liquor and the solid residue. The kinetic model gave an average correlation coefficient of 0.93 for all reactions. In addition, the combined severity factors (CSF) were also determined for each experiment. Highest observed enzymatic glucose yield corresponded to a CSF of 1.7. A mass and energy balance, and economic analysis based on production scale was developed for both reactor systems. The microwave pretreatment process theoretically yielded 48% more ethanol relative to the conventional process. For microwave pretreatment to be commercially viable, two criteria must be met. One, the cost for largescale continuous microwave reactors would need to be significantly lower than current estimates. And second, higher solids content must be used (\u3e20 wt% in the slurry) to maximize output