| In winter,the fermentation temperature is low and the "sick" operation of the system has become a bottleneck problem restricting the development of small and medium-sized biogas projects in China.Discharging and taking away a large amount of heat is one of the main reasons leading to this problem.Therefore,the use of biogas slurry waste heat to heat the feed and enhance the fermentation temperature is of great significance to the biogas project.At present,domestic and foreign scholars in biogas slurry waste heat utilization related research,mainly through "biogas slurry-water-feed" way for waste heat utilization.The heat loss of this method is large and the operation process is complex,and there is still room for further improvement.In the early stage,the research group put forward the theory of heat and mass recovery and developed a heat and mass recovery device for waste heat utilization through "biogas slurry and feed" direct heat exchange.The equipment can improve the utilization efficiency of waste heat in biogas slurry to some extent.In addition,there is no relevant research on the optimal return mass ratio.The optimal recycling ratio can not only reduce the amount of biogas produced by anaerobic fermentation engineering,recover the waste heat of biogas and improve the temperature of fermentation raw materials,but also improve the efficiency of anaerobic fermentation gas production.In order to determine the optimal recycling ratio,the effects of long-term reflux biogas slurry on the anaerobic fermentation of cattle manure were studied in a wet anaerobic fermentation plant with 50 L continuous feed.The daily gas production,methane volume fraction,pH value,SCOD content and ammonia nitrogen content under different reflux ratios of biogas slurry(0%,30%,40%,50%,60%)were studied at moderate temperature(37±1)?.The dynamic changes of methane volume fraction,SCOD and ammonia nitrogen content were also studied The mechanism of reflux promoting methane production was analyzed by mechanical analysis,and the optimum proportion of Biogas Engineering with cow dung as fermentation raw material was determined.In order to reduce heat loss and improve the utilization rate of residual heat in biogas slurry,an integrated device for discharging solid-liquid separation and reheating and reconditioning was developed.On this basis,the annual heat load,the effective heat recovery,the proportion of heat recovery to the total heat load and the heat collection area of the solar thermostatic anaerobic fermentation system with heat recovery and mass recovery are calculated theoretically.In order to verify the theoretical results,a heat and mass recovery test platform was built.The experimental results of heat and mass recovery are consistent with the theoretical calculation results,which not only verify the accuracy of the theoretical calculation,but also show that the new heat and mass recovery device has certain optimization effect on improving the utilization rate of biogas slurry waste heat and simplifying the system operation.Energy value analysis was used to evaluate the benefits of the system.The conclusions of this thesis are as follows:(1)The efficiency of the anaerobic fermentation of cow manure was improved the most by the proportion of 40%.In the test group with a return to mass ratio of 40%,the daily gas production gradually increased and tended to be stable during the return to mass phase.During the test cycle,the maximum daily gas production reached 62.45 L,and the volume gas production rate was 1.39L/L.Compared with the test group with a return to mass ratio of 0%,the maximum daily gas production increased by 25.46%,and 9.22%,respectively.Back to mass ratio of 50%,60% of the experimental group while nissan in back to the stage of mass volume is back to the mass ratio of 30%,40% of the nissan group faster the highest volume of 62.29 L and 62.40 L,but the experimental group at the beginning of the 78 d and 95 d nissan volume,respectively,to the end of the test cycle,nissan volume is below the 40% group and continue to reduce.(2)The high proportion of recycle mass has an inhibitory effect on anaerobic fermentation.The pH value and SCOD content of the test group with 50% and 60% return to mass ratio increased with the increase of return to mass operation days,but they were still within the appropriate concentration range of anaerobic fermentation and had no inhibitory effect on gas production.Ammonia nitrogen content increased continuously with the increase of recycling operation days.After reaching 1500mg/L,the gas production in the test group was inhibited,and the gas production gradually decreased.Therefore,when using biogas slurry as the raw material in the biogas project,the ratio of the slurry to the slurry should be 40% or the ammonia nitrogen content in the slurry should be reduced before the slurry is slurry.(3)Kinetic model analysis was carried out on the anaerobic fermentation process to verify the methane production rate of the substrate under the condition of different proportion of return to mass.It was concluded that 40% of return to mass ratio had the greatest improvement on the methane production rate per unit matrix and gas production rate per unit volume of the fermentation system.This is because a large number of methanogens are reintroduced into the anaerobic fermentation system in the recycled biogas slurry,which improves the conversion efficiency of the substrate.(4)Under the condition of good insulation measures,the heating feed load of the fermentation tank accounts for 89.3%?93.2% of the total heat load of the system.The effective heat recovery in different months of the year was calculated theoretically under the condition of 40% recycle mass ratio.By returning the thermal mass test,under the condition of the environment temperature of 18?,returning the thermal mass can be feed heating temperature is 16.2? to 26.6?,accounting for 51.94% of the heat necessary to sustain the 37? constant temperature fermentation.Under the condition of winter,returning the thermal mass can be feeding heating to 18.9? temperature of 2.5?,37? system to maintain constant temperature fermentation of winter 47.23% of calories.The experimental results are consistent with the theoretical analysis on the recovery of residual heat efficiency of biogas slurry in summer and winter.(5)System to maintain constant temperature fermentation calories by recycling collection of biogas slurry waste heat and 14 set of hot area of 53.9 m2 solar array to provide,this system has realized the 100% renewable energy heating,can ensure that 37 ? constant temperature fermentation biogas engineering in winter.Compared with the traditional coal-fired boiler heating,the initial investment increased by 61%,with an annual net benefit of 13,0287 yuan.Compared with the combined heating of solar and coal-fired boilers,the initial investment decreased by 7.28%,with an annual net benefit of 116,077 yuan.Therefore,it has good ecological and economic benefits. |
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