An analysis of the impact of storage temperature, moisture content & duration upon the chemical components & bioprocessing of lignocellulosic biomass
The successful utilization of lignocellulosic biomass as a feedstock for fuels and chemicals necessitates storage for 2–6 months. It is correspondingly important to understand the impact of storage parameters—moisture concentration, temperature and duration—on biomass quality. As aerobic storage is the most viable large-scale solution, aerobic storage experiments were carried out with three projected bioenergy feedstocks—sweet sorghum (Sorghum bicolor) bagasse, corn (Zea mays) stover and switchgrass (Panicum virgatum). Stored samples of each were examined for dry matter loss and composition change to develop a material balance around carbohydrates and lignin. A mean dry matter loss of 24% was observed at 8 weeks in sweet sorghum stored at high moisture content (26% w/w). Soluble sugars predominated the dry matter loss in high-moisture sorghum, causing an increase in the mass fraction of lignin in the biomass. In comparison, low-moisture (12% w/w) samples showed negligible loss. High-moisture sorghum dried from 26% to 20% in 8 weeks, and further to 8% in 24 weeks. To control moisture loss in subsequent experiments, switchgrass and corn stover were wetted to specific water activities (aw) and stored under equivalent controlled humidities. Switchgrass stored at water activities of 0.65–0.85 showed no dry matter loss, irrespective of storage temperature or duration. Switchgrass stored at a water activity of 0.99 (∼33% w/w moisture) showed significant dry matter loss at temperatures of 20 and 35 °C, although substantial sample-to-sample variation was observed at 20 °C compared to 35 °C. At 8 and 16 weeks, switchgrass stored at 0.99 aw and 35 °C lost 7% dry matter on average. Corn stover was stored under similar conditions (35 °C, 0.97 aw) for 8 weeks. The stored samples showed 10% dry matter loss on average. The dry matter losses predominantly consisted of cellulose and hemicellulose, resulting in the mass fraction of lignin increasing in switchgrass from 26.6% to 27.0% in 8 weeks and 28.8% in 16 weeks, and in stover from 23% to 25% in 8 weeks. Samples of switchgrass and corn stover that had undergone significant (>5%) dry mater loss were subjected to liquid hot-water pretreatment and washed solids subjected to enzymatic hydrolysis. The glucose yield was calculated and compared to hydrolysis results from samples that had not experienced significant loss to examine the impact of storage losses on carbohydrate extractability. Both degraded and non-degraded switchgrass achieved approximately 8% of theoretical yield from cellulose hydrolysis before pretreatment, and 35% yield after pretreatment. Similarly, irrespective of storage treatment, corn stover samples showed 16% hydrolysis yield before pretreatment and approximately 55% yield after pretreatment. Due to the similar yields, storage losses in the range observed were concluded to not significantly impact carbohydrate extractability in terms of percent yield, although the amount of extractable carbohydrate per dry kg biomass is reduced by 4–16% due to their preferential consumption.
Mosier, Purdue University.
Alternative Energy|Biochemistry|Chemical engineering
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