| Using crop stover as raw material to produce biogas and biogas fertilizer by anaerobic fermentation is an effective way to solve energy shortage,environmental pollution and ecological restoration,which has important economic and social benefits.The main components of straw are cellulose,hemicellulose and lignin.Due to the composition and structure of lignin and hemicelluloses degrading very slowly during anaerobic fermentation,which is not only affected the volumetric efficiency of the reactor,but also reduced the sedimentation performance of the straw.The floated straw could scum on the liquid surface of the anaerobic fermentation reactor and caused fermentation failure.Nowadays,stirring was used to destroy the scum layer and prevent the scum from encrusting in some research and factory productions,but stirring coluld?only promote the release of biogas.It was difficult to remove the influence of the scum layer fundamentally once the scum still existed while the mixing stopped.In order to solve the problem,aerobic hydrolysis and anaerobic fermentation two-phase processes were applied to various parts of corn stover(pith,rind,leaf).The effect of the aerobic hydrolysis on the various parts of corn stover in water absorption,sedimentation and anaerobic fermentation characteristics had been analyzed.Determined the appropriate hydrolysis time and the optimal straw pulverization particle size,explored the ways to eliminate the anaerobic fermentation scum layer,and maximize the volumetric efficiency of the fermentation system.Based on the results of single factor test,the interaction of hydrolysis time,temperature and particle size on the sedimentation ratio of various parts of straw was investigated by response surface methodology,and the aerobic hydrolysis process parameters were optimized.By using of the Gompertz model,first-order model and Design-Expert were used to analyze the data,the conclusion of which provided theoretical and technical supports for the development of straw biogas.The main findings and conclusions were obtained as follows.(1)In the process of hydrolysis,water absorption rate and dissolution rate of pith,rind and leaf was obvious differences,so the sedimentation performance of different parts were different while at the same time.At 16 h,the water absorption rates reached 679%,347%,and 160%,respectively.The dissolution rate of the material was related to the hydrolysis time(y=a-bcx exponential function),and the dissolution of the materials at 24 h is 20.74%,17.97%,and 7.83%,respectively.(2)In the process of single-phase anaerobic fermentation,the substrate concentration of corn stover and each part that could normally start but slightly different,and the thickness of scum layer showed a gradual thickening trend with the increase of substrate concentration,and gradually thin with the extension of fermentation time.Corn stover,rind and leaf could start normal when the concentration of volatile substrate was 3.6%and the volatile substrate concentration of the pith was 2.7%.When the volatile substance concentration of pith was 3%and the volatile substance concentration of rind and leaf were 4%,there was no biogas.On the 10th day after rind fermentation,the thickness of the scum layer reached the lowest value.On the 18th day of pith and leaf fermentation,the thickness of the scum layer reached the lowest value,and the thickness of the scum layer decreased by 64.3%,73.7%,65.3%and 59%,respectively.The volume ratio of pith,rind and leaf in the initial scum layer of corn stover fermentation was about 48%,18%and 29%,respectively.(3)Aerobic hydrolysis can damage straw lignin structure significantly,and increase the degrade rate of lignin and cellulose.The degradation rates of lignin in pith,rind and leaf were 4.20%,3.91%and 4.90%,respectively.The degradation rates of cellulose were 14.49%,17.69%and 22.59%,respectively.It could be seen that the different tissue structure and composition of various parts of corn straw directly affect the degradation of cellulose.At the same time,acetic acid accounted for more than 60%of total volatile fatty acids(VFAs)and ethanol.(4)In the aerobic and anaerobic two-phase methane production fermentation process of corn straw,aerobic hydrolysis could effectively improve the methane production rate of each part of the straw,reduce the thickness of the scum layer and shorten the fermentation cycle.After 12 h of aerobic hydrolysis of pith and rind,the cumulative methane yield of volatile substance reached the maximum.After 8 h of aerobic treatment of leaf,the cumulative methane yield of volatile substance reached the maximum,323 mL/g,251 mL/g and 264 mL/g,respectively,there were 35.0%,30.1%and 8%higher than the control group.The cumulative methane production up to 90%was 8?9 days,4?5 days shorter than that of the control group.Aerobic hydrolysis could increase the porosity and water absorption capacity of straw in microstructure,improve its sedimentation performance,and effectively reduce the thickness of scum layer in the process of anaerobic fermentation.The thickest scum layer in the 12 h aerobic treatment group was only 45.8%,7.1%and 13.6%of that in the control group.After 12 h of aerobic hydrolysis,the main component of the scum layer in the initial anaerobic fermentation was pith,with a volume ratio of more than 90%.(5)Different particle sizes of various parts of corn stover had good sedimentation performance through aerobic hydrolysis.Compared with the untreated group,the rind,leaf and pith reached the optimal sedimentation state on the 7th,14th and 14th day after anaerobic fermentation,9,4 and 6 days earlier respectively.At the same time,the straw particles below 40 mesh settled 100%.The sedimentation rates of pith,rind and leaf of 5?10 mesh were 32.7%?76.6%,82.6%?100%,66.2%?84.0%,respectively,which increased by 5.8%?31.9%,39.1%?50%,35.5%?24%respectively.The initial sedimentation rate of 20?30 mesh granules increased by 80.3%?87.4%,and the sedimentation rate of other parts reached 100%.The cumulative methane yield,maximum daily methane production,lag phase and hydrolysis constant of the aerobic hydrolysis treatment group were significantly better than the untreated group.The cumulative methane yield generally increased with the increase of the number of particles.The cumulative methane production in the pith,rind and leaf fluctuated at 287.56?360.80 mL/gVS,194.80?298.94 mL/gVS,210.43?259.90 mL/gVS,respectively.They increased by 37.37%?45.30%,18.27%?36.20%,and 2.50%?9.26%respectively.(6)The mathematical model was established between the sedimentation ratio of various parts of corn stover and hydrolysis time,temperature and particle size parameters by regression analysis.And it is better to predict the sedimentation state of aerobic hydrolysis treatment in various parts of corn stover in the initial stage of anaerobic fermentation,and finally determine the optimal hydrolysis process parameters:time at 12.25 h,temperature at 42.7?,and particle size was 24 mesh.Under the optimal conditions,the results were consistent with the predicted values.The average sedimentation ratios of the pith,rind and leaf were 68.6%,89.1%and 95.8%,respectively. |
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