Miscanthus conversion to ethanol: Effect of particle size and pretreatment conditions for hot water

Cellulosic biomass is a promising feedstock for ethanol production because it is plentiful and enriched in carbohydrates. While the basic technology for converting biomass into ethanol has been developed, processing biomass still remains relatively expensive, despite lower feedstock costs. The high cost stems in part from the recalcitrance of biomass to enzymatic hydrolysis, which necessitates an expensive pretreatment in combination with a heavy enzyme dosage. The objective of this study was to develop an e fficient process for conversion of Miscanthus x giganteus to ethanol using hammer milling for reduction of particle size followed by a hydrothermal pretreatment. Particle size reduction is crucial for transportation logistics as well as cellulosic conversion. Miscanthus was ground using a hammer mill equipped with screens having 0.08, 2.0 or 6.0 mm sieve openings. Ground samples were subjected to hot water, dilute acid or dilute ammonium hydroxide pretreatments. Sugar yields from enzyme hydrolysis was used to measure pretreatment e ciency. Geometric mean diameters decreased with screen size: 0.08 mm sieve screen (56 um) followed by 2.0 mm (301 um) and 6.0 mm (695 um) screens. Enzymatic sugar yields increased inversely with mean particle size with the best results observed for all pretreatments, using the 0.08 mm sieve screen. Enzyme hydrolysis of unpretreated biomass samples also increased total conversions as particle size decreased, although mean conversions (10 to 20%) were much lower than for pretreated biomass samples (40 to 70%), indicating the need for chemical pretreatments in biomass conversion. Samples ground using the 0.08 mm sieve was used for hot water optimization studies. Hot water pretreatment of Miscanthus was evaluated with respect to pretreatment temperature and retention time. Hot water pretreatments do not require addition of chemicals, lessen the need for expensive reactors, avoid catalyst recycle and overcome neutralization costs. Miscanthus was pretreated at three temperatures (160, 180 and 200 C) for four reaction times (0, 10, 20 and 30 min); the solids loading was kept constant at 15%. Reactions were conducted in mini tubular batch reactors using a fluidized heating bath. Glucose and xylose yields following enzyme hydrolysis of washed pretreated solids were used as a measure of pretreatment e fficacy. Best conditions, among those evaluated, for hot water pretreatment of Miscanthus were 200 C for 10 min. At optimal conditions, 6% glucose and 44% xylose were released into the pretreatment liquor. Enzyme hydrolysis of washed pretreated solids resulted in 77% glucan, 12% xylan and 62% total conversion based upon beginning carbohydrate contents. Pretreated conditions were further evaluated for conversion to ethanol in simultaneous sacchari cation and fermentations (SSF) using native industrial Saccharomyces cerevisiae strain D5A. Ethanol yields were 70% of theoretical based upon beginning glucan content following 72 hr fermentation. Image analysis of solids from three hot water pretreatment conditions resulting in lowest (160 C, 0 min), intermediate (180 C, 10 min) and highest total polysaccharide conversion (200 C, 10 min) were conducted. Pretreated and enzyme hydrolyzed samples were imaged using thick sections for light microscopy, which allowed various plant tissues to be identi ed. The samples were determined to be unsuitable for imaging using atomic force microscopy or negative staining techniques for electron microscopy. Thick sections showed that pretreated and enzymatically hydrolyzed solids from the optimized pretreatment conditions were primarily disintegrated with few intact cell walls. In contrast, at milder pretreatment conditions, cell wall structure was easily identi able even following enzymatic hydrolysis. As such thick section light microscopy can be used to qualitatively judge the success of a pretreatment for Miscanthus