Auto-hydrolysis and Acid-catalyzed Prehydrolysis of Woody Biomass to Produce Value with Fermentable Sugars Prior to Combustion

The main steps of producing bioethanol from the lignocellulosic biomass include pretreatment, enzymatic hydrolysis and fermentation. Biomass pretreatment is a means to remove most of the hemicellulose and a small portion of the cellulose and lignin from the woody matrix into the aqueous phase and to leave a comparatively open structure of the solid for further enzymatic hydrolysis. The overall objective of the present study is to investigate the possible optimal sugar yield by means of either auto-hydrolysis or acid-prehydrolysis pretreatment of woody biomass, and the economic feasibility of the so called "VPC" (value prior to combustion) technology. This concept was established for a process of extracting sugar from lignocellulosic biomass to produce bioethanol, as well as combustion of the solid residue to recover the heating value.;In Chapter 3, mixed hardwood chips were auto-hydrolyzed at different conditions. The tradeoff between fermentable sugar yield and caloric value of the residual solids was studied for the VPC process. Material balances were performed to evaluate the role of temperature and residence time on sugar production and residual solid heating value. The maximum total sugar was obtained from auto-hydrolysis at 160 °C for 2 hours. The solid balance closure became more open as the pretreatment temperature increased and retention time prolonged. Byproducts generation also increased as temperature and retention time increased. The heating values that could be recovered after the hot-water extraction varied from 74% to 95%.;In Chapter 4, a similar auto-hydrolysis study as discussed in Chapter 3 was carried out with mixed softwood chips. For mixed softwood pretreatment, a better solid balance was observed compared with hardwood. The optimal condition for sugar extraction was found to be at 170 °C for 2 hours. A linear relationship between the pH of prehydrolyzate and temperature was demonstrated. The heating value that can be recovered from solid residue was ranging from 87% to 99%.;In Chapter 5, a comparison study of auto-hydrolysis and acid-prehydrolysis of mixed hardwood chips was conducted to understand their sugar yield difference in the liquid extract and their effect on the enzymatic hydrolysis of solid residues. Acetic acid and formic acid were selected for the acid-prehydrolysis. Enzymatic hydrolysis was performed for all the solid residues. A mechanical refining treatment was carried out on all the pulps. As a comparison, enzymatic hydrolysis of pulps without the mechanical refining treatment was also conducted. Auto-hydrolysis at 180 °C resulted in the most open solid balance closure, the highest amount of byproducts generated in the filtrate, and the best enzymatic hydrolysis performance of the solid residue. Acid will facilitate the removal of hemicellulose from the biomass, increase the content of byproducts generated and enhance the enzymatic hydrolysis of solid residues. Formic acid showed better catalytic ability compared with acetic acid. The mechanical refining treatment of the substrate increased the sugar conversion of the residual solids during the enzymatic hydrolysis by at least 10%.;A technical economic evaluation of the VPC technology was established in Chapter 6 to evaluate whether integrating an ethanol production plant to an existing power generation plant will be feasible and profitable. Mass and energy balances of different proposed cases were created by using Excel spreadsheets populated with laboratory data (from the hot-water extraction of mixed hardwood and softwood studies) along with appropriate assumptions. Capital cost, operating costs and income statement and sensitivity analysis were established. It was determined that economic feasibility was highly dependent on the power selling price, ethanol selling price, and ethanol yield. The proposed VPC cases seemed not promising with the current status of this technology.