| The crop straw resources are abundant in china.In 2015,the output was about1.04 billion tons.Converting straw into biogas by anaerobic digestion is an effective way to reuse the resources and reduce environmental pollution.However,the main component of corn straw is lignocellulose,which is difficult to be degraded by microorganism.The slow hydrolysis and acidification during anaerobic digestion caused long retention time and low utilization efficiency of straw.Silage is a common way in the preservation of straw resources.During the process of silage,the microstructure of straw will be destroyed to a certain extent due to fermentation and acidification,thus improving degradability and utilization of straw.In this paper,corn straw silage was used as the raw material in solid-state anaerobic digestion(SS-AD).The effects of silage straw to dry straw ratio,inoculum size(F/I),carbon to nitrogen ratio(C/N),and fermentation temperature on the performance of SS-AD were studied.The mechanism of gas production was preliminarily discussed according to the ultrastructure of the raw materials and the change of physical and chemical properties during the digestion process.The main contents and conclusions are as follows:(1)Effects of silage on the composition and ultrastructure of corn straw.The ultrastructure of the leaves,surface and culm of fresh straw,silage straw and dry straw were analyzed by scanning electron microscopy.The results showed that the microstructure of corn straw was degraded to a certain extent.The cellulose content of silage straw decreased by 34.4% and 16.1% than that of the fresh straw and dry straw,respectively,and the content of hemicellulose decreased by 49.5% and 35.5%respectively.The p H values of fresh straw,silage straw and dry straw were 6.45,4.12 and 8.79 respectively,which indicated that the degradation of organic components in corn straw accumulated a lot of organic acids during silage.(2)Effects of silage on the characteristics of biogas production of corn straw.SS-AD experiments were conducted with fresh straw,silage straw and dry straw as feedstock respectively.The hydrolysis and acidification of silage corn straw was obviously accelerated of during SS-AD.For the reactors started up successfully,the total biogas yield of the silage straw reached to 407.65 L/kg VS,the cumulative methane production was 161.87 L/kg VS and the average methane concentration was37%,which was 28.6%,42.6% and 15.6% higher than that of the dry straw,respectively.However,the rapid hydrolysis and acidification of silage straw might cause excessive acid accumulation,which will result in acid inhibition in the early stage of SS-AD.For the reactors with failed digestion process,the total volatile organic acids accumulated to 38g/kg and the p H decreased to 6.7 quickly in the start-up stage,which showed an obvious acid inhibition.(3)Optimization of SS-AD processing conditions for corn straw silage.Silage and dry straw mixture were used as raw materials for SS-AD.The result showed that the total biogas yield and methane yield of silage were increased by 20.2% and 45.2%compared to dry straw,respectively.The total biogas yield and methane yield of the reactors with silage and dry straw of 3:1 were increased by 1.9% and 13.7%,respectively.Furthermore,the retention time was shortened,the peak time was and the peak value was improved also.Silage and dry straw were mixed with 1:0 and 3:1 as raw materials to optimize the anaerobic dry fermentation conditions.The results showed that the optimal biogas production conditions of 1:0 group were F/I=3,C/N=20 and temperature 37℃,while the optimal gas production conditions of 3:1 group were F/I=3,C/N=30 and temperature 37℃.From the comprehensive consideration of gas production stability and acidification,the 3:1 group can coordinate the relationship between the fast hydrolysis acidification of silage straw and the slow hydrolysis acidification of dry straw.This is beneficial to balance the growth and metabolism of acidogenic bacteria and methanogenic bacteria,and effectively prevent excessive acidification.(4)The mechanism of SS-AD of different single factor fermentation and raw materials with different mixing ratios of silage straw and dry straw were discussed.Finally,the physical and chemical characteristics of the 1:0 and 3:1 digestion materials with the best biogas production after optimization were summarized.The results showed that the degradation rate of VS is 19.1%,the p H of the digestion materials is stable between 7.31-8.28,and the relationship between TVFAs and TA is balanced,and the fermentation effect is good in 1:0 group under the conditions of F/I=3,C/N=20 and 37.According to the analysis of the test results with poor fermentation effect,the main cause of fermentation failure are the low inoculum volume,the decrease in the number and activity of microorganisms in the fermentation material,the content of N in raw materials is too high,The decrease in C/N ratio and its effect on higher ammonia nitrogen concentration may inhibit the inhibition of anaerobic fermentation,which is detrimental to the growth and reproduction of methanogens.The low p H value of fermented materials will make the acid producing bacteria multiply and cause the acidification of the reaction system.Therefore,it is necessary to increase the amount of inoculation and C/N ratio.it can improve the number of microbes in the reactor and the effect of mass transfer between the substrate and the microorganism.At the same time,the control of the p H value in the fermented material can prevent the risk of acid inhibition effectively. |
