Impacts of Harvest Date and Concurrent Alkali Pretreatment and Ensiling on Anaerobic Digestion of Pennycress Biomass

Abstract

Pennycress (Thlaspi arvense L.) is an annual cover crop known for its exceptional cold tolerance and high oil and protein yields. Pennycress can be integrated into a corn–soybean rotation in the U.S. However, the utilization of pennycress biomass remains largely unexplored, including assessing compositional changes through its growth and organic matter digestibility. This study harvested pennycress at three growth stages, characterized the biomass for anaerobic digestion (AD), and tested the effects of concurrent alkali pretreatment and ensiling on the biomass methane yield. Results showed that the biomass harvested when the plants were undergoing senescence (“third-harvest”) had higher contents of acid detergent fiber, neutral detergent fiber, and lignin, while the biomass harvested when 80–90% of the pods were fully-sized (“second-harvest”) had the highest protein content. The AD experiments showed that the first-harvest biomass (90% of flowers opened) failed to produce biogas due to a drop in the pH and alkalinity, the second-harvest biomass was inhibited for methane production (45.74 ± 0.20 L/kg-VS), and the third-harvest biomass had a methane yield of 171.80 ± 4.82 L/kg-VS. After the alkali pretreatment and ensiling, a methane yield of 270.4 ± 3.10 L/kg-VS was obtained from the second-harvest biomass, representing a significant 4.5-fold increase (adjusted for the organic matter loss) relative to the untreated second-harvest biomass.