Methane Production And Mass Transfer Performance During Digestate Recirculation Of An Innovative Batch Fixed-bed Reactor

As a large agricultural country,the annual output of agricultural waste is huge,and biogas utilization is considered to be one of the economical and feasible ways.The crust problems of straw biogas project limit the efficiency of straw utilization.Based on straw biogas project that has been reported in the literature,this paper optimizes the design of the bed fermentation reactor.Study on the mass transfer process of digestate recirculation between rice straw in anaerobic digestion fixed-bed reactor.Based on the study of mass transfer process,a batch test was conducted to investigate on biogas production during digestate recirculation.The analysis of the energy consumption and net energy output of the biogas project is carried out according to the test results.The main findings are as follows:(1)The design of batch anaerobic digestion fixed-bed reactor is optimized by treating the lignocellulosic biomass.In the optimized reactor,a mesh-type anaerobic digestion fixed-bed is arranged to fix the rice straw.Hydraulic circulation stirring is set.The internal mass transfer process of the fixed-bed was analyzed with the rice straw porous materials,and the gas production performance of the fermentation process was studied.(2)Rice straw as a plant porous media,the viscous drag coefficient and the inertia drag coefficient were tested by flow rate-pressure drop measuring device.The results show that the higher density of rice straw,the harder migration it is in internal liquid of fixed-bed reactor.The viscous drag coefficient and the inertia drag coefficient are increased by 76.40%and 56.53%from the straw density of0.075g/cm3 to 0.12g/cm3.The viscous drag coefficient and the inertia drag coefficient both increase as the velocity increased.At the same time,the test results are more inaccurate with the calculated results of the Ergun equation.After fitting the experimental results,the equation coefficients a=787.32 and b=0.21 are obtained,which can guide the digestate recirculation process of the bed reactor.(3)Mesophilic fermentation condition was carried out in an optimized fixed-bed fermentation reactor for 50 days.Under low reflux conditions(50%),VFA accumulation wasn't observed under each density,the pH value ranged from 7.0 to 7.5,and the average methane content exceeded 50%,the fermentation system was stable.The highest degradation of lignocellulose of anaerobic digestion fixed-bed in the surface layer and internal rice straw occurred at TS=12%,the degradation value of cellulose and hemicellulose was 58.57%,62.23%,49.29%,and 46.73%,while the highest cumulative gas production was 200.03 mLCH4/g?VS at TS=10%,which indicates that it is difficult to achieve the migration of substances in the fixed-bed with 50%reflux flow.(4)Increase the digestate recirculation flow of the fermentation system and explore the fermentation performance of rice straw(TS=11%).The fermentation system was stable,the total VFA was less than 500mg/L during the whole fermentation process,and pH value was in the range of6.90~7.23.Under the conditions of 100%and 150%reflux flow,the lignocellulose degradation value of rice straw in fixed-bed was similar.The highest cumulative gas production was 119.88 mLCH4/g?VS under 100%reflux conditions,so it has higher biogas production performance under 100%reflux conditions.Maintaining the same reflux conditions,the total solid of rice straw in fixed-bed was raised to 13%,and the average methane content exceeded 50%,the fermentation process was stable.Under150%reflux condition,the surface and internal cellulose and hemicellulose degradation amounts of rice straw in fixed-bed were up to 56.01%,36.31%and 74.51%,63.68%,and the cumulative biogas production was130.82 mLCH4/g?VS.(5)Taking a full scale anaerobic digester with a working volume of 500 m~3 as an example,,hydraulic retention time is set at 50 days,set the same reflow conditions as(4),the energy consumption and net energy output of treating rice straw per ton are calculated.The calculation results show that the net energy output under the conditions of 50%,100%,150%and 200%reflow is 4747.82 MJ/t,4405.27MJ/t,52623 MJ/t,2990.43 MJ/t,of which 150%return flow net energy output is the highest.