Converting Cover Crop Biomass to Biogas and Volatile Fatty Acids

Abstract

Cover crops are commonly used for their environmental benefits, including improved soil health and reduced soil erosion. However, there is limited use for the aboveground biomass, except for oilseed crops. The objectives of this study were to (1) characterize the selected winter cover crop biomass at three growth stages, (2) evaluate the biogas and volatile fatty acids (VFAs) yield, and (3) examine the concurrent alkali pretreatment and ensiling effects on biogas yield. Wild pennycress, golden pennycress, CoverCressTM, annual ryegrass, cereal rye, pea, and clover biomass were harvested at three growth stages. Results showed that biogas yield increased with maturity. The third-harvest wild pennycress biomass had a methane yield of 171.80 ± 4.82 L/kg-VS while after 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 5.9-fold increase relative to the untreated biomass. Since the first-harvest of wild pennycress biomass had a lower biogas yield, it was incorporated as a feedstock in AAD to produce VFAs. Gas Chromatography was used to analyze the concentration of acetic, butyric, propionic, hexanoic, and valeric acids. Among the tested cover crops, pea biomass had the highest VFAs yields of 0.304 g/g-VS. VFAs concentration and yield differed by crop and harvest stage, except for pennycress, which showed consistent VFAs trends across stages. VFAs produced from cover crops generated estimated revenues ranging from $92 to$800 per acre. This study demonstrates that the selected winter cover crops have the potential for sustainable bioenergy production