A Cost-effective Integrated Process to Convert Sweet Sorghum Stalks into Biofuels

Abstract

Biofuel is a renewable and sustainable energy as an substitute of fossil fuel. In China, the key challenge to develop biofuels is how to balance the competition of food and bioenergy and supply a sustainable feedstock. Sweet sorghum is highlighted as a multiple platform plant which can supply grain, sugar, cellulose and feed. In addition, sweet sorghum has so good tolerance of drought and sterilize that it can grow on marginal land. However, the loose stalk pith of sweet sorghum results in the infeasibility of liquid fermentation technology due to the considerably high energy consumption.In present research, an integrated process combined advanced solid-state fermentation technology (ASSF) and alkaline pretreatment was presented in this work. Soluble sugars in sweet sorghum stalks were firstly converted into ethanol by ASSF using crushed stalks directly. Then, the operation combining ethanol distillation and alkaline pretreatment was performed in one distillation-reactor simultaneously. The process was optimized according to the yield of sugar-ethanol and amounts of monosaccharide released from SSB. In our novel process, energy consumption of mechanically squeeze and pretreatment of SSB was avoided. Meanwhile, the recalcitrance of lignocellulose was destructed and the biodegradation of lignocellulose into chemical products is feasible to achieve. Cellulosic ethanol as a model product was studied by using this novel integrated process. The mass balance of the overall process was calculated, and 91.9kg was achieved from one tonne of fresh sweet sorghum stalk. Energy consumption for raw materials preparation and pretreatment were reduced or avoided in our process. To produce 1 tonne of ethanol, the energy input in this process was 12,481.2MJ/tonne, while the energy input in other cellulosic ethanol processes is from 17,430 to 33,330MJ/tonne.Based on this technology, the recalcitrance of lignocellulose was destructed via a cost-efficient process and structural carbohydrates in sweet sorghum stalks were hydrolysed into fermentable sugars. Bioconversion of fermentable sugars released from sweet sorghum bagasse into different products except ethanol, such as butanol, biogas, and chemicals was feasible to operate under low energy-consumption conditions. So it is considered as a promising process to produce biofuels at commercial scale.