Cellulosic ethanol: Optimization of dilute acid and enzymatic hydrolysis processing of forest resources and switchgrass

In recent years, growing attention has been focused on the use of lignocellulosic biomass as a feedstock to produce renewable alternatives to fossil fuels, such as ethanol. Cellulosic ethanol technology is nearing commercialization with dozens of planned projects in the United States to be completed over the next several years. However, the production of ethanol from lignocellulosic biomass still has challenges to overcome due to the recalcitrance of the biomass, prior to becoming economically feasible. This study focused on three areas of importance to the development and deployment of cellulosic ethanol the effect of pretreating mixtures of forest feedstocks (hardwood and softwood) and switchgrass, optimizing dilute acid hydrolysis conditions for maximum total sugar yields post enzymatic hydrolysis, and U.S. federal energy and agricultural policies and their potential roles in the diffusion of innovation for cellulosic ethanol.The effects of woody biomass mixtures were investigated on the rates of hemicellulose hydrolysis by dilute sulfuric acid. Very good agreement between the model predictions and single species acid hydrolysis data confirmed the validity of a pseudo first order model approach. This model was then utilized to predict monomer sugar concentrations for mixtures of hardwood (aspen, basswood, and red maple), a softwood (balsam), and the energy crop switchgrass, with very good agreement. The results of this study show that there are not significant synergistic or antagonistic effects by mixtures of woody biomass species on the kinetics of hemicellulose hydrolysis by dilute acid. Kinetic parameters were developed for each woody biomass species with xylose formation activation energies ranging from 76.19--171.20 kJ/mol and pre-exponential factors ranging from 2.19x108--7.73x1019 min -1. Overall xylose yields for pure biomass species ranged from approximately 66--88% with balsam having the lowest yield and switchgrass producing the highest yield. Kinetics of hemicellulose hydrolysis for each of the five biomass species was very similar such that maximum sugar concentrations were achieved at almost identical reaction times. This observation further suggests that mixtures can be effectively processed together without compromising sugar yield. These mixture results have practical applications to commercial production of cellulosic ethanol.The effects of dilute acid hydrolysis conditions were investigated on total sugar (glucose and xylose) yields after enzymatic hydrolysis with additional analyses on glucose and xylose monomer and oligomer yields from the individual hydrolysis steps for aspen, balsam, and switchgrass. The results of this study, in the form of measured versus theoretical yields and a severity analysis, show that for aspen and balsam, high dilute acid hydrolysis xylose yields were obtainable at all acid concentrations (0.25--0.75 wt.%) and temperatures (150--175°C) studied as long as reaction time was optimized. Switchgrass shows a relatively stronger dependence on dilute acid hydrolysis acid concentration due to its higher neutralizing mineral and ash content. Maximum total sugar (xylose and glucose monomer plus oligomer) yields post-enzymatic hydrolysis for aspen, balsam, and switchgrass, were 88.3%, 21.2%, and 97.6%, respectively. In general, the highest yields of total sugars (xylose and glucose monomer plus oligomer) were achieved at combined severity parameter values (log CS) between 2.20 and 2.40 for the biomass species studied.The role that U.S. federal energy and agricultural policy may be playing in fostering or impeding the development of cellulosic ethanol in the U.S. was assessed. One of the major ways of understanding technological development is the theory of the diffusion of innovation. This theory identifies factors that can impede and facilitate this diffusion. The degree to which three of the main U.S. federal energy policies aim at addressing three key aspects of innovation diffusion: time risk and communication was addressed. It was determined that these policies focus more on the producer stage of the cellulosic ethanol lifecycle than the landowner or consumer stages. In addition, they contain many provisions aimed at overcoming risk- and communication-related impediments to adoption, but fewer aimed at speeding up the process. Finally, they contain at least one provision likely to be a serious impediment to adoption.