Optimization of Hot Water Pretreatment of Switchgrass for Acidogenic and Anaerobic Digestion
Restricted (Penn State Only)
- Author:
- Li, Zhaoran
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- August 25, 2022
- Committee Members:
- Tom Richard, Thesis Advisor/Co-Advisor
Ali Demirci, Committee Member
Suat Irmak, Professor in Charge/Director of Graduate Studies
Rachel Brennan, Committee Member - Keywords:
- Switchgrass
Liquid Hot Water Pretreatment
Acidogenic Digestion
Anaerobic Digestion - Abstract:
- With the increasing challenges of climate change and worldwide energy security, the world is experiencing a rapid reconfiguration of energy production and resource utilization. These changes are driving rapid growth of alternative and clean energy. Lignocellulosic biomass has a lot of potential as an energy resource. However, the raw material of lignocellulosic biomass is recalcitrant to most downstream biochemical conversion methods due to the existence of lignin and the complicated crystalline structure of cellulose. Liquid hot water pretreatment has previously been applied to increase the conversion of lignocellulosic biomass before ethanol fermentations. In this study, liquid hot water pretreatment was applied to summer-harvested switchgrass prior to acidogenic digestion. Several pretreatment conditions were evaluated with temperatures ranging from 160°C to 190°C, pressure sufficient to maintain the water in the liquid state, and treatment times ranging from 12 min to 17 min. Pretreated switchgrass was then fed to acidogenic digesters at a loading rate of 10 g L-1 in a 10-day batch fermentation at room temperature. The acidogenic digestion inoculum was prepared from rumen fluid and added to the reactors at a feed:inoculum ratio of 10:1 on a volatile solids (VS) basis. Without a pH buffer methanogenesis was inhibited and carboxylic acids accumulated. The results demonstrated a significant increase of carboxylic acid yield from 18% without pretreatment to 33% for switchgrass pretreated at 180°C for 17 min. The optimum pretreatment condition for switchgrass was also tested with anaerobic digestion. In this case, controlled pH was used to keep the reactor neutral (pH 7) so that methanogens could thrive and convert carboxylic acids to biomethane. The feedstock was again switchgrass, but with two levels of recalcitrance represented by both a more recalcitrant winter harvest and a less recalcitrant summer harvest. The pretreatment condition was 180°C for 17 min., which was the best performing pretreatment condition in the prior set of trials. The switchgrass samples were pretreated and added to bioreactors with inoculum at a feed:inoculum ratio of 1:1 on a VS basis, but in this case the pH was controlled at 7. The batch incubation process was maintained at 37°C for 10 days, with both acid production and gas production monitored on Days 5 and 10. Results show that pretreated summer-harvested switchgrass with inoculum converted 31.01% of feedstock chemical oxygen demand (COD) into biogas COD by Day 10. In contrast, the pretreated winter-harvested switchgrass with inoculum demonstrated a lower COD conversion rate of 26.09%. The pretreatment benefits in this study were in the range of 5% to 6% additional biomass conversion. The unpretreated summer-harvested switchgrass with inoculum experienced a conversion rate at 26.25%, while unpretreated winter-harvested switchgrass with inoculum had a COD conversion rate at 20.13%. These results indicate that liquid hot water pretreatment has modest benefits for both summer and winter-harvested switchgrass in both carboxylic acid and biogas forming downstream anaerobic digestion processes.