Enhancement in Anaerobic Digestion of Corn Stover Pretreated by Freeze–Thaw: Methane Yield, Energy Efficiency, and Economic Analysis

Given the climatic condition in cold regions, freeze–thaw (FT) pretreatment was proposed to enhance the methanogenesis performance of corn stover. The effect of introducing water or alkali into FT pretreatment on anaerobic digestion was investigated. It was found that methane yield improved with the FT transition frequency increasing. Under the condition of 2.5% NaOH and four FT cycles, the NaOH/FT-pretreated sample, with the maximum glucan content (53.29 ± 0.74%) and the minimum lignin content (4.25 ± 0.46%), achieved the highest methane yield of 326.8 ± 9.2 mL/g VS. Meanwhile, the maximum energy conversion efficiency and process energy efficiency were 87.2% and 84.1%, respectively. Methane yield was positively correlated with glucan content and negatively correlated with xylan and lignin content. NaOH/FT pretreatment significantly affected the composition and structure; the combination of chemical modification from NaOH and physical damage from FT enhanced the conversion efficiency of corn stover. Compared to the CK group, the introduction of water and NaOH increased methane yield by 11.9–76.3% and 27.3–96.3%, respectively. However, compared to water, the promoting effect of NaOH on methane production was limited and not economically favorable. Therefore, introducing water into FT pretreatment achieved higher economic benefits. The pretreatment method studied here is efficient and energy-saving, with great potential to improve biomethane yield and economic benefits of other lignocellulosic materials.