Microbial Conversion Systems for Increasing the Efficiency and Cost Effectiveness of Ethanol Production from Lignocellulose.

摘要

rowing awareness of petroleum's finite existence and negative environmental impacts has accentuated the need for clean, renewable energy sources. There are many energy niches, but the transportation sector is one of the most challenging. Ethanol, currently produced from corn, is a gasoline oxygenate additive that has potential to replace gasoline entirely. Lignocellulosic biomass is a more sustainable substrate than corn, but also more complex. This research investigated two microbial systems for use in ethanol production from lignocellulose: Clostridium and Trichoderma reesei..;The first part of the research centered around four Clostridia capable of producing both glycosyl hydrolase enzymes and fermentation products. The yields of ethanol and other co-producst were measured on a variety of plant-based substrates. Before substrate testing began, the optimal yeast extract concentration was investigated as a nutrient supplement and found to be 2 g/L based on growth of Clostridium cellulolyticum and Clostridium phytofermentans. C. cellulolyticum, C. phytofermentans, Clostridium thermocellum and Clostridium cellulovorans were then grown on xylan, crystalline cellulose and cellobiose. The highest ethanol yield was 0.4 g/g, produced by C. cellulolyticum growing on cellobiose. C. cellulolyticum also had the highest ethanol (0.14 g/g) and acetic acid yields (0.9 g/g) on xylan so it was chosen for the next set of experiments with lignocellulosic substrates.;Rice straw and grape pomace are common lignocellulosic wastes in California so they were investigated as potential substrates for ethanol production. C. cellulolyticum produced ethanol yields of 0.2 g/gVS and 0.08 g/gVS on pretreated rice straw and grape pomace, respectively. Sodium hydroxide pretreatment significantly increased the ethanol yield from rice straw, but not grape pomace. Substrate sterilization by autoclaving increased ethanol yields in raw rice straw, but not for pretreated rice straw, suggesting that sterilization acts as a pretrement for raw rice straw.;Co-culturing of methanogens and C. cellulolyticum was studied to determine the feasibility of co-production of ethanol and methane. Mixed cultures of C. cellulolyticum with Methanoscarcina acetivorans or Methanoscarcina mazei both successfully co-produced methane and ethanol. Co-cultures of C. cellulolyticum with M. acetivorans had the highest methane yield of 19 mL/gVS, in addition to an ethanol yield of 0.06 g/gVS. The addition of methanogens did not increase the ethanol yields, but did decrease the acetic acid concentration in comparison to pure C. cellulolyticum cultures.;The second part of research investigated feasible methods to make glycosyl hydrolase production from lignocellulose using Trichoderma reesei RUT-C30. This approach lends itself to onsite processing since the same lignocellulosic substrate being converted to ethanol can also be used as a substrate for enzyme production. Due to its low lignin content, sugar beet pulp is especially promising production substrate. Several pH methods and media recipes were tested for cellulase, xylanase and pectinase activities. Media recipes were tested to determine the nutrient requiments of Trichoderma reesei RUT-C30. Citrate buffering and replacing ammonium sulfate with urea were both found to be effective pH control methods to maintain the pH above 3.5. One of the lean media, using ammonium bicarbonate as the primary nitrogen source, resulted in comparable enzyme activities to rich media and cost calculations revealed that using this lean media reduced the media costs to one third of the original rich media cost. The final recommendation of enzyme production substrate for hydrolysis of sugar beet pulp depends on the price of sugar beet pulp. If it is greater than around