Effect Of Rice Straw Pretreatment On Anaerobic Fermentation Performance Of Biogas

The resource utilization of crop straws can not only prevent the waste of biomass resources,but also effectively alleviate the environmental pollution caused by its burning and discarding.Anaerobic digestion biogas production is an important way to utilize crop straw resources.It can not only provide clean bioenergy biogas,but also the biogas slurry and biogas residues obtained after anaerobic fermentation can be returned to the field as organic fertilizers,which is environmentally friendly and efficient.The complex lignocellulose structure of crop straws makes it difficult to be digested and degraded during the anaerobic fermentation process.If it is directly used for fermentation,problems such as slow startup,low gas production efficiency,incomplete fermentation,and long fermentation cycles may occur.Therefore,in order to improve the overall benefits of anaerobic fermentation,it is particularly important to explore appropriate pretreatment techniques to solve these problems.This study compared the effects of two different states of rice straw,dry and fresh,on the performance of anaerobic fermentation.The results show that the biogas output of fresh rice straw is 400.5 m L/g VS,which is 40.9%higher than that of dry rice straw.At the same time,the methane content of fresh rice straw is 56.05%（2.55%higher than dry rice straw）;the volatile solids（VS）degradation rate of fresh rice straw was 72%,which was 7.2%higher than that of dry rice straw.Therefore,fresh rice straw is more conducive to the production of biogas by anaerobic fermentation.The fresh rice straw was used as the fermentation raw material,and it was pretreated by microwave method,ammoniation method and microwave-assisted ammoniation method.With the gas production as the response value,the optimum pretreatment conditions were obtained by single factor and response surface optimization.The microwave method:pretreatment time of 5.66 min,pretreatment material-liquid ratio of 1:5.86（g/g）;the ammoniation method:pretreatment time of 24.3 h,pretreatment ammonia content of 4.05%,pretreatment material-liquid ratio of 1:5.87（g/g）;the microwave-assisted amination method:pretreatment time of 17.87 h,pretreatment ammonia content of 3.94%,and pretreatment material-liquid ratio of 1:3.76（g/g）.Under the best pretreatment conditions,the changes of physicochemical properties of rice straw before and after pretreatment were characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）,fourier transform infrared absorption spectrometer（FTIR）and so on.Through characterization,it was found that the composition and structure of rice straw were greatly changed by the three pretreatment methods.However,in general,the microwave-assisted ammoniation method was the most significant.The dry matter loss rate of rice straw pretreated by it was 27.32±0.37%,7.78%and2.4%higher than the microwave method and ammoniation method,respectively.Under the best pretreatment conditions,the changes of fermentation broth properties,gas production and so on during anaerobic fermentation were monitored,and the effects of different pretreatment methods on the biogas production performance of anaerobic fermentation of rice straw were compared and analyzed.Through comparative analysis,it can be seen that the microwave-assisted ammoniation method is more conducive to the biogas production from anaerobic fermentation of rice straw.The biogas yield of rice straw pretreated by it was 482.2 m L/g VS,which was 20.4%,2%,and 18.04%higher than the control check（CK）,microwave method,and ammoniation method.Although there is little difference between the microwave method and the microwave-assisted ammoniation method in biogas yield,the difference in methane content is significant.The microwave-assisted amination method was58.39%,which was 3.25%higher than the microwave method.The methane content of CK and ammoniation method were 56.05%and 58.32%,respectively.The T₉₀fermentation cycle of the microwave-assisted ammoniation method was6 days,which was 3 days,3 days,and 2 days shorter than those of the CK,microwave,and ammoniation methods,respectively.After pretreatment and anaerobic fermentation,the microwave-assisted ammoniated pretreatment of straw VS degradation rate was 77.81%,which was 5.81%,4.86%and 3.16%higher than CK,microwave method and ammoniated method,respectively;especially lignin,the difference in degradation is most pronounced.