Development of Geothermally Assisted Process for Production of Liquid Fuels and Chemicals from Wheat Straw

Recently there has been much interest in developing processes for producing liquid fuels from renewable resources. The most logical long term approach in terms of economics derives the carbohydrate substrate for fermentation from the hydrolysis of cellulosic crop and forest residues rather than from grains or other high grade food materials (1,2). Since the presence of lignin is the main barrier to the hydrolysis of cellulose from lignocellulosic materials, delignification processes developed by the wood pulping industry have been considered as possible prehydrolysis treatments. The delignification process under study in our laboratory is envisioned as a synthesis of two recently developed pulping processes. In the first step, called autohydrolysis, hot water is used directly to solubilize hemicellulose and to depolymerize lignin (3). Then, in a second step known as organosolv pulping (4), the autohydrolyzed material is extracted with aqueous alcohol. A s shown in Figure 1, this process can separate the original lignocellulosic material into three streams--hemicellulose in water, lignin in aqueous alcohol, and a cellulose pulp. Without further mechanical milling, delignified cellulose can be enzymatically hydrolyzed at 45-50 C to greater than 80% theoretical yield of glucose using fungal cellulases (5, 6). The resulting glucose syrup can then be fermented by yeast to produce ethanol or by selected bacteria to produce acetone and butanol or acetic and propionic acids (7). One objection to such a process, however, is the large energy input that is required. In order to extend our supplies of liquid fuels and chemicals, it is important that the use of fossil fuels in any lignocellulosic conversion process be minimized. The direct use of geothermal hot water in carrying out the autohydrolysis and extraction operations, therefore, seems especially attractive. On the one hand, it facilitates the conversion of non-food biomass to fuels and chemicals without wasting fossil fuel; and on the other hand, it provides a means for ''exporting'' geothermal energy from the well site. The primary goal of the work discussed in this report was to investigate the effects of variations in autohydrolysis conditions on the production of fermentable sugars from wheat straw. In assessing the relative merits of various sets of conditions, we considered both the direct production of sugar from the autohydrolysis of hemicellulose and the subsequent yield from the enzymatic hydrolysis of cellulose. The principal parameters studied were time, temperature, and water/fiber weight ratio; however, we also investigated the effects of adding minor amounts of phenol and aluminum sulfate to the autohydrolysis charge. Phenol was selected for study because it was reported (8) to be effective in suppressing repolymerization of reactive lignin fragments. Aluminum sulfate, on the other hand, was chosen as a representative of the Lewis acids which, we hoped, would catalyze the delignification reactions