| The low content and yield of CH4 in traditional biogas greatly limit the range of biogas application.At the same time,the existing technology can just only increase the content of CH4,which can not increase the yield of CH4,and sometimes even lead to the problem of reducing the yield of CH4.Moreover,the existing technology is liable to cause environmental pollution due to the inadequately use and conversion of CO2.Therefore,the key effects such as intermittent circulating aeration and concentration of ammonia nitrogen were optimized for ex-situ improving the CH4 content and yield of biogas simultaneously by adding extra H2.In addition,the mechanism was further analyzed and explained from the micro-perspective of microbiology and bubble floating law.The main conclusions were as follows:1.The effects of H2 evolution from the corrosion of waste scrap iron and waste sulfuric acid were studied.The results showed that the pH value of waste sulfuric acid had the greatest influence on the H2 evolution from corrosion of waste scrap iron and waste sulfuric acid among the five factors,such as the adding amount of waste scrap iron,particle size of waste scrap iron,pH of waste sulfuric acid,rotational speed,and temperature.When the pH of waste sulfuric acid was 0,the cumulative H2 evolution of waste scrap iron was 1082.5 mL.Under these conditions,rotational speed and temperature had little effects on the corrosion of waste scrap iron and waste sulfuric acid.When the particle size of waste scrap iron was 2.8 mm,the cumulative H2 evolution was the largest,which was 42.9 mL.Therefore,H2 could be produced by the corrosion reaction of waste scrap iron and waste sulfuric acid,and the best condition of H2 evolution could be achieved by directly stationary produced at room temperature.This.method can not only effectively reduce the cost of H2 production,but also protect the environment and improve the utility value of waste products.2.The process of adding extra H2 for ex-situ improving the content and yield of CH4 in biogas simultaneously was studied.The results showed that under the initial operating conditions of 2 times a day and 1 hour for each circulating aeration,the content and yield of CH4 were increased by 45%and 101%,respectively.The utilization rate of H2 and the efficiency of converting H2 into CH4 were 36.4%and 47.3%,respectively.When the intermittent cyclic aeration conditions were further optimized to aeration for 13 times a day,1.5 h for each circulating aeration,the content and yield of CH4 were increased by 83%and 54%,respectively.The utilization rate of H2 increased up to 95.3%.On the basis of best condition of intermittent cyclic aeration,when the concentration of ammonia nitrogen increased to 5500 mg/L,the utilization rate of H2 increased to 98.9%,while the content and yield of CH4 further increased by 89%and 57%,respectively.Therefore,further optimization of intermittent cyclic aeration and ammonia nitrogen concentration could effectively improve the content and yield of CH4 in biogas simultaneously.3.The shifts of microbial community structure for improving content and yield of CH4 in biogas simultaneously by adding extra H2 was carried out.The mechanism of intermittent cyclic aeration and ammonia nitrogen concentration under different operating conditions was analyzed from the view of microbia.The results showed that the main methanogens in the system were Methanosaeta and Methanothermobacter when the number of cycles per day was only 2 and the aeration time was 1 hour per day.The relative abundance of Methanosaeta and Methanothermobacter was 63.4%and 18.3%,respectively.While the total relative abundance of HMs(Methanobacterium and Methanothermobacter)was only 27.5%.When the intermittent cyclic aeration condition was optimized to cyclic aeration for 13 times per day and 1.5 h for each time,the main methanogens in the system were Methanosaeta,Methanothermobacter and Methanobacterium,with relative abundance of 17.6%,45.3%and 25.9%,respectively.The total relative abundance of HMs reached 71.2%.Based on this,the concentration of ammonia nitrogen was adjusted to 5500 mg/L.Methanothermobacter and Methanobacterium were the main methanogens in the system.The relative abundance of Methanothermobacter and Methanobacterium was 13.1%and 60.0%,respectively.The total relative abundance of HMs was 73.1%,while that of Methanosaeta was only 1.3%.Therefore,under the condition of high level of ammonia nitrogen,HMs was the main dominant methanogens,indicating that the way of CH4 production in the system was mainly the conversion of H2 and CO2,which was consistent with the results of CH4 content up to 94.1%under the condition of high concentration of ammonia nitrogen.In terms of bacteria,Clostridium was the dominant bacteria under different operating conditions,which indicated that the operating process had little effect on Clostridium.4.The simulation and experimental study of bubbles of CH4,H2 and CO2 floating in still water were carried out.The changes of morphologic,diameter and retention of bubbles in still water were analyzed.The results showed that when the initial velocity of bubble increasesd from 0 m/s to 1.2 m/s,the jet position of bubbles gradually changed from bottom to top,and the degree of bubble splitting was more thorough.When the initial diameter of the bubble increasesd from 0.02 m to 0.06 m,the transverse force of the bubble increasesd due to the deformation caused by the splitting,and the more right position the bubble rises to the top of the bubble.The morphological changes of single bubble of CH4,H2 and CO2 in still water were basically the same,but were different slightly.Their diameters showed a trend of becoming smaller diameter,which was mainly due to bubble breakup during bubbles floating.Temperature,outlet pressure,initial diameter of bubbles and initial velocity of bubbles had different effects on the retention of single CH4,H2 and CO2 bubbles in still water.The smaller the bubbles and the larger the initial velocity of bubbles,the longer the retention of bubbles in still water.Horizontal spacing had great influence on the floating process of CH4,H2 and CO2 bubbles in still water.When horizontal space increased from 0.01 m to 0.05 m,the retention of bubbles in still water decreased firstly and then increasesd.When the horizontal space was 0.02 m,the retention of these three types of bubbles in water was the shortest.While when the horizontal spacing was less than or greater than 20 mm,the retention was extended.Therefore,the appropriate distance could further extend the residence time of bubbles in still water.The more bubbles there were,the more complex the motion pattern and flow field of bubbles in still water were.During the bubbles of CH4,H2 and CO2 floating up,the middle bubbles rose faster,which causesd the bubbles on both sides to float faster,and the retention of three bubbles in still water was shorter than that of two bubbles floating in still water.Therefore,the optimized hollow fiber membrane could further improve the diffusion conditions of bubbles,which could effectively improve the mass transfer effect of gas in water,and make the gas better utilized microorganisms.Combining with high concentration of ammonia nitrogen,HMs could be effectively enriched.Thus the content and yield of CH4 in biogas could be significantly increased.In summary,in order to further increase the content and yield of CH4 in biogas,micro bubbles can be produced by hollow fiber membrane.The intermittent cyclic aeration can further save the operation costs.On those bases,by increasing the concentration of ammonia nitrogen in the system to enrich HMs can effectively improve the content and yield of CH4 in biogas,and avoid only increasing the content of CH4,leading to the decrease of CH4 yield,which has a better operability and popularization value. |
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