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Assessing the potential for bioethanol production from oxygen delignified corn stover and wheat straw Pope, Derek Stephen


Bioethanol derived from lignocellulosic biomass offers great potential as a low carbon energy source. Rather than using food resources as a feedstock, abundant non-food lignocellulosic alternatives can be utilized. Process economics represents the greatest barrier facing lignocellulosic bioethanol. An effective pretreatment that increases enzymatic hydrolysis sugar yield and reduces enzyme consumption is crucial for reducing ethanol production cost. Oxygen delignification was studied for its potential as a pretreatment of agricultural residues corn stover and wheat straw. This pretreatment was selected based on its ability to target, disrupt, and solubilise lignin; a component linked to poor enzymatic hydrolysis yields. Oxygen delignification reaction temperature (90-150°C), residence time (15-60 min), and caustic load (2-10%) were studied for their effect on enzymatic hydrolysis. Conditions that increase substrate hydrolysability while limiting sugar solubilisation allowed for a maximum total sugar yield of 81.7% (92.1% of total cellulose and 68.2% of total hemicellulose). Next, the substrate composition of pretreated substrates was analyzed. Results confirmed that reductions in lignin content improved substrate hydrolysability. An improvement to the Zhang et al. [1] model, which predicts sugar concentration during enzymatic hydrolysis, was made by enabling the model to account for changes in substrate lignin content. The addition was based on a hypothesis put forth that lignin reduces the availability of cellulase enzymes. The model (below) was successfully validated. P=So{1-[1+(Ke(Eo-LoLF)/(Ke+(Eo-LoLF))kdt](-k2/(Kekd))}1.11 P = Product (sugar) concentration So = Initial substrate (carbohydrate) concentration Eo = Initial enzyme loading concentration Lo = Initial lignin concentration LF = Lignin factor kd = Enzyme deactivation constant Ke = Equilibrium constant k2 = Reaction rate constant Finally, an economic optimization of bioethanol production was conducted. To perform this analysis, an Aspen Plus simulation of the process was developed. A base case minimum ethanol cost of $0.55/L was observed under pretreatment conditions of 135°C, 8% caustic, and 60 minutes residence time. A sensitivity analysis demonstrated that ethanol cost was vulnerable to increases in the cost of biomass, enzyme, and NaOH. It was also found that changes in the cost of enzyme and biomass affected the optimal pretreatment reactor conditions.

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