(782a) The Influence Of Particle Size On Bioconversion Of Hybrid Poplar For Sugar Production

Feedstock logistics, also known as biomass assembly (harvesting, storage, preprocessing and transportation), have not been widely studied by the biofuels community. Consequently, there is still a gap between feedstock suppliers and downstream biomass conversion and processing. In this regard, biomass size reduction has been proposed as an important variable in the biofuel production flow. While smaller particles are thought to be beneficial for flowability and packing, wood size reduction is very energy intensive. Thus, the objective of this work was to determine the influence of particle size on sugar production during bioconversion of hybrid poplar via steam pretreatment and enzymatic hydrolysis.

In this research, chemical and physical properties of solids after steam explosion, enzymatic hydrolysability of solids and overall sugar recovery were analyzed. Fresh 18-year-old hybrid poplar, Populus deltoides x Populus nigra, was processed into six different particles sizes: S1 (0.2 x 0.2 cm), S2 (0.4 x 0.4 cm), S3 (0.4 x 1.2 cm), S4 (0.7 x 0.4 cm), S5 (1.0 x 2.0 cm) and S6 (equal mixture of all the particles). Hybrid poplar particles were impregnated with SO₂ (3% w/w) and steam pretreated at 195°C for 5 minutes. The steam pretreated particles were then chemically and physically characterized, saccharified at 5% (w/v) solids consistency, 5 FPU/g cellulose, and finally a complete mass balance of carbohydrates was determined. In order to find statistical differences among the different groups of particles, one way ANOVA at 5% alpha was performed for each dependent variable: chemical and physical characteristics of steam pretreated solids, enzymatic digestibility (glucan to glucose conversion) and total sugar recovery after pretreatment and enzymatic hydrolysis. In the cases where statistical differences were found (p value<0.05) a multiple comparison of means “Tukey test” was performed to find the specific group or groups of particles that were causing those differences.

The results of the analysis, have shown similar chemical composition of solids after pretreatment for all the particle sizes tested, where the glucan and lignin content ranged from 61 to 65% from and 32 to 36%, respectively. No statistically significant differences were found for both glucan and lignin content for all the particles tested. However, there were statistically significant differences in fiber length and width, where sizes 1, 2 and 3 were shorter and thicker, whereas sizes 4 and 5 were longer and thinner. However, these fiber characteristics did not affect the enzymatic hydrolysis of the solids, since similar cellulose to glucose conversion of 78 to 82% were observed during 72 hours of enzymatic hydrolysis for all the particle sizes. No statistically significant differences in cellulose to glucose conversion were found at 5% alpha level for all the particles tested. In addition, there were not found any statistically significant differences in glucose and xylose recovery after steam pretreatment and enzymatic hydrolysis among the particle sizes investigated. The glucose and xylose recoveries were from 85 to 94% and from 57 to 68%, respectively. Thus, particle size did not influence the overall sugar recovery after pretreatment and enzymatic hydrolysis. This research confirms that steam pretreatment with SO₂ as a catalyst is a robust method that can accommodate particles ranging from 0.2 x 0.2 cm to 1.0 x 2.0 cm without altering downstream bioconversion processes.