| dc.description.abstract | Fuels and chemicals from lignocellulosic biomass, a renewable energy source, is an attractive solution to meet ever-increasing global energy needs and reduce global climate change. In the biochemical conversion of lignocellulose, the first and the most expensive step is pretreatment. This study focuses on the efficacy of shock pretreatment, a mechanical process that uses a shockwave to alter the biomass structure.
Corn stover was pretreated with lime and shock. The two pretreatments (lime-only and lime + shock) were evaluated using enzymatic hydrolysis, batch mixed-culture fermentations, and continuous countercurrent mixed-culture fermentation. In a 120-h enzymatic hydrolysis, shock pretreatment increased the glucan digestibility of SLP (submerged lime pretreatment) corn stover by 3.5% and OLP (oxidative lime pretreatment) corn stover by 2.5%. The continuum particle distribution model (CPDM) was used to simulate a four-stage continuous countercurrent mixed-culture fermentation using empirical rate models obtained from simple batch experiments. The CPDM model determined that lime + shock pretreatment increased the total carboxylic acids yield by 28.5% over lime-only pretreatment in a countercurrent fermentation with a VSLR (volatile solids loading rate) of 12 g/(L·day) and LRT (liquid retention time) of 30 days. In a semi-continuous countercurrent fermentation performed in the laboratory for 112 days with a VSLR of 1.875 g/(L·day) and LRT of 16 days, lime + shock pretreatment increased the total carboxylic acids yield by 14.8%. The experimental results matched closely with CPDM models predictions (4.05% error).
Calcium carbonate and magnesium carbonate were compared as buffers for mixed-culture fermentations of lime and lime + shock pretreated corn stover. Batch fermentations at five different substrate loadings of lime and lime + shock pretreated corn stover were performed with MgCO3 and CaCO3 buffer. In batch fermentations with 100 g/L substrate, the carboxylic acid production more than doubled (2.7 times for lime and 2.6 times for lime + shock corn stover) when MgCO3 buffer was used. In addition, CPDM was used to simulate and predict the performance of a four-stage countercurrent fermentation using MgCO3 and CaCO3 buffer. CPDM predicts that in a four-stage countercurrent fermentation with a high volatile solids loading rate (VSLR 12 g/(L·day)) and low liquid residence time (LRT 10 day), using MgCO3 buffer will yield a carboxylic acid concentration of 26.1 g/L, a 22.5% increase over CaCO3 buffer. Adding shock to lime pretreatment increased the yields at all substrate loadings in both batch fermentations and CPDM model predictions.
The effect of hydrogen and carbon dioxide gas concentrations in the headspace of mixed-culture fermentations was studied. Using H2:CO2 (1:1) at 1 atm in the fermenter headspace increased the total carboxylic acids by 37%. Using CO2-only in the headspace reduced the total acids by 4%, but shifted the acid spectrum toward high-molecular-weight acids. | en |
