Studies On The Key Technologies Of Cellulosic Etanol Production Processes Using Lignocellulosic Feedstock

Cellulosic ethanol production from abundant and renewable lignocellulosic biomass is the leading and dominant technology trend in the current global bioenergy industry. This technology provided a practical option for resolving energy crisis, global warming, and environmental pollution etc. The major processing steps in the bioconversion pathway from lignocellulose to ethanol through the sugar platform technology includes pretreatment, inhibitor removal (or detoxification), enzymatic hydrolysis, ethanol fermentation, and product recovery. Although numbers of cellulosic ethanol pilot or demonstration plants have been established in the world, the high cost of cellulosic ethanol is significantly blocking its commercialization steps. The high cost comes from various aspects, including the high energy consumption during pretreatment and distillation process; the massive environmental cost of waste water treatment from the pretreatment, detoxification and fermentation steps; the expensive cellulase cost and the low ethanol yield of bioconversion etc. In this PhD dissertation, the overall cellulosic ethanol processes were revolutionized by targeting the energy-saving and waste water reduction problems, and a key process technology strategy in the process was proposed with a novel resource-conservation concept. The major novelties and breakthroughs of the dissertation were summaried as follows:Firstly, two very important but rarely concerned parameters, the feedstock filling ratio to the pretreatment reactor and the solid/liquid presoaking ratio were investigated during pretreatment. Based on maintaining the pretreatment efficiency at a satisfactory level, the solid/liquid ratio during pretreatment was reduced to 1:1 (the total water usage was less than the dry weight of the lignocellulose, and either the dilute-acid usage or the steam consumption was no more than half of the dry lignocellulose weight) by regulating the above two parameters and the water concentration of the materials after pretreatment was less than 50% (w/w). There was no free water released from the current pretreatment. Comparing with the conventional dilute-acid pretreatment, the fresh water usage and the steam consumption were decreased at least by 80% and 50%, respectively. This "dry" pretreatment process realized the low steam consumption and approximately zero waste water discharge.Secondly, a newly isolated kerosene fungus, Amorphotheca resinae ZN1, was applied to the biodetoxification unit. During the solid-state biodetoxification, the toxic products in the pretreated lignocellulose, including formic acid, acetic acid, furfural and HMF etc. were degraded thoroughly and quickly without cellulose and hemicellulose degradation. The biodetoxification process was conducted at ambient temperature and static conditions, so there was no need for water and energy use. Furthermore, the water content of the detoxified materials was less than 45%(w/w) and the A. resinae ZN1 fungus ceased to growth spontaneously in the subsequent simultaneous saccharification and fermentation (SSF) process without a specific sterilization treatment.Thirdly, a new helical stirring bioreactor was designed and operated for the simultaneous saccharification and fermentation of biodetoxified feedstock at an extremely high solid loading up to 40%(w/w). The fermentation slurry contained 8.0%(v/v) of ethanol and the cellulose conversion rate of 70% were obtained with relatively low cellulase dosage. No fresh-water was added into the extremely high solid loading SSF process besides the liquid cellulase and the yeast seeds. The stirring energy consumption of the helical stirring bioreactor during the SSF process was decreased by 80-90% comparing to the conventional bioreactor.Finally, over 90% of the ethanol in the fermentation broth was recovered directly by a simple vacuum distillation at 37?and the ethanol was also concentrated by 2-3 folds to 23% (v/v) in the distillate, which was greater than that of starch or sugar-based ethanol. At the same time, over 80% of the cellulase activity in the distillation stillage was preserved. Both the high ethanol recovery and the high cellulase recovery could greatly decrease the subsequent distillation energy demand and the cellulase usage. The vacuum distillation provides a practical method for continuous or semi-continuous ethanol fermentation from lignocellulose at high solids loading.Conclusively, in this dissertation, the resource-saving cellulosic ethanol production process proposed realized the goals of significantly energy-saving and waste water reduction while maintaining the ethanol yield and production efficiency. The cellulosic ethanol cost was significantly redeuced using the proposed processing technology by cutting the energy consumption and waste water emission to a large extent and paved the way of fundamental and engineering researches for the future commercialization of cellulosic ethanol industry.