Mechanism Of Microwave Pretreatment Affecting Switchgrass For Methane Production

Fossil fuels, which are nonrenewable resources, are now becoming scarce, therefore, using biofuels as an alternative energy source can reduce our dependency on nonrenewable energy resources. Lignocellulosic biomass is an abundant renewable resource which is more available. Currently, it has been the hot issue that the lignocellulosic feedstock was high-efficiently translated to energy products such as biogas. Therefore, pretreatment technology of lignocellulose becomes the focus of the improving measures of biomass translating efficiency. The objective of this study was to understand the mechanism of microwave pretreatment affecting switchgrass for methane production. Matter solubilisation, lignocellulosic structure and accessibility were analyzed, and hydrolysis kinetic models were evaluated to investigate the effect of microwave pretreatment on the effective utilization of lignocellulose. And then understand the mechanism of microwave pretreatment affecting lignocelluloses.There were the following three parts in this study.(1) The characteristics of switchgrass after microwave pretreatment.Alamo switchgrass stem was used as the experiment materials. The pretreatments were conducted in a serial of microwave final temperature (90℃,120℃,150℃and180℃) and holding time (0,5,10,15,20min) at150℃. The results showed that microwave pretreatment improved the matter solubilisation of switchgrass, and the highest increase (130.75%) occurred at180℃. Microwave pretreatment changed the structure of switchgrass, and some caves emerged. Specific surface area of cell wall increased, and60.61%of specific surface area was enhanced atl80℃, thus the accessibility of lignocellulose was improved. But cellulose crystallinity was increased by microwave pretreatment.(2) The effect of microwave pretreatment on effective utilization of lignocellulose.The anaerobic digestion of solid fraction of switchgrass after microwave pretreatment was conducted. Dynamic samples of each treatment were obtained, depending on the digestion time. Then the degradation of volatile solids and cellulose was determined. The first-order kinetic models were established, and then compared the hydrolysis rate of volatile solids and cellulose among treatments according to kinetic constant. The results showed that the hydrolysis rates of volatile solids and cellulose were improved by microwave pretreatment, and the degradation of volatile solids and cellulose was enhanced. The correlation analysis indicated that the change of switchgrass structure and the increase of specific surface area were the main reasons for the improvement of hydrolysis ratios of volatile solids and cellulose.(3) The effect of microwave pretreatment on anaerobic digestion for methane production.Solid and liquid mixture and solid fraction of switchgrass after microwave pretreatment were used to conduct the anaerobic digestion test, and then methane production and the degradation efficiency of volatile solids and cellulose were determined. Results showed that microwave pretreatment enhanced methane yield. The highest methane production (143.15mL/g VS) from solid and liquid mixture occurred at180℃, and19.57%higher than control. Methane yield of solid fraction of switchgrass pretreated by microwave was higher than that of soaked treatment. With the exception of180℃ treatment, methane production from solid and liquid mixture was higher than solid fraction. Except the solid and liquid mixture of switchgtass pretreated180℃, the degradation of volatile and cellulose from solid and liquid mixture and solid fraction was significantly improved by microwave pretreatment during anaerobic digestion. The correlation analysis indicated that the enhancement of solubilization and the improcement of hydrolysis ratio of lignocellulose were the main reasons for the increase of methane production.It was concluded that the effect of microwave pretreatment on the characteristics of lignocellulose was reflected by the following three aspects:the improvement of solubilization, the change of structure, the increase of specific surface area of cell wall and the enhancement of the lignocellulosic accessibility. Microwave pretreatment improved the hydrolysis of ligcellulose during anaerobic digestion by the change of structure and enhancement of accessibility. Methane production was increased by microwave pretreatment through the improvement of matter solubilisation of switchgrass and the enhancement of lignocelluloses hydrolysis.