Promoting Effect And Mechanism Of Carbon-based Materials On Anaerobic Digestion Of Organic Matter

Biomass energy is becoming more and more important in the entire energy industry because people value environmental issues.The methane gas obtained by anaerobic digestion(AD)of organic matter is a typical representative.However,the traditional anaerobic digestion has a long stagnation period,the reaction system is not stable enough,the degradation rate of the substrate is low,and the rate of methanogenesis is slow.In recent years,researchers have found that the addition of carbon materials can promote AD to a certain extent and improve the methanogenic properties of the fermentation substrate.Based on the research status of AD and methanogenesis,this paper designs experiments and explores the effects of several carbon materials(carbon dots(CDs),acetylene black(AB),hydrochar(HC),pyrolytic carbon(PC))on AD.First,a single raw material(glucose,cellulose and ethanol)was used as a substrate,and CDs were used as additives to study its effect on the AD of a single raw material.Then,a complex raw material(vinegar residue,VR)was used as a substrate,CDs,HC,AB as additives to study the effects of these additives on the AD of methane by complex raw materials.Finally,studying the effect of AB and PC in dry AD.According to the final gas production results of each batch of experiments,combined with the characteristics of raw materials and carbon materials,the gas production promoting effect and mechanism were analyzed.The main conclusions obtained are as follows:(1)The biochemical methane potential(BMP)experiment was carried out using glucose,cellulose and ethanol as substrates and CDs as additives.The results showed that compared with the control group(without CDs),adding 1.0 g/L of nitrogen-doped carbon dots(N-CDs)to glucose increased the final methane yield by 6.6%;Adding 1.0g/L of N-CDs can increase the final methane yield by 15%.When ethanol is used as the starting material,the addition of 5.0 g/L of Zn-doped carbon dots(Zn-CDs)can increase the final methane yield by 24.6%.CDs have good water solubility and can be evenly distributed in the experimental system,and the conductivity of CDs is much higher than that of Pili on the surface of microorganisms,which can enhance the electron transfer between microorganisms.In addition,these added CDs can enrich some of the micro-organisms that can participate in interspecies electronic transfer(such as Pseudomonadaceae,Methanosarcinaceae)or directly metabolize methanogenic microorganisms(such as Methanobacteriaceae),which can enhance the effect of anaerobic digestion.(2)Further study on the use of VR as a substrate,N-CDs,HC,AB as an added carbon-based material,for BMP experiment.The results showed that 1.0 g/L of HC can increase the methane yield by 50.8%,1.0 g/L of AB can increase the methane yield by232%,5.0 g/L of N-CDs can increase the methane yield by 229%.HC is a lamellar structure,which is much larger than that of microorganism,and it can provide a plurality of growth sites and a good living environment for the fermenting microorganism in the AD system.The high specific surface area(74.31 m~2/g and 31.6m~2/g)and good electrical conductivity of AB and N-CDs(2.06 S/cm and 890μS/cm)enable them to fully contact the syntrophic microorganisms and optimize the rate of electron transfer during methanogenesis.From the results of microbial analysis of the biogas residue samples,compared to the control group without added carbon-based materials,these three carbon materials added could enrich cellulose-decomposing microorganisms,such as Ruminococcaceae(5.1%to 8.2%).The addition of acetylene black enriched Methanosarcinaceae(78.8%to 84.4%),and the addition of N-CDs enriched Methanobacteriaceae(4.0%to 7.5%).The enrichment of these microorganisms increases the rate of hydrolysis of the vine residue,the rate of electron transfer between microorganisms,and the rate of methane production.(3)In order to fully expand the application of carbon-based materials in different anaerobic systems,the experiment used complex raw materials(straw)as a substrate to study the effect of acetylene black(1%～20%,based on straw TS)on the production of methane by dry anaerobic digestion(without leachate spraying).The results showed that addition of 5%AB can increase the final methane yield by 8.8%.In order to optimize the problem of unbalanced C/N of substrate,the experiment was carried out by using a spray-type dry AD reactor using straw and pig manure as substrates.The results showed that when the mass ratio of straw to pig manure was 2:1,the methanogenesis was the best.The addition of 5%PC to substrate could increase the final methane yield by 63%and increased methane production rate by 43%.The promoting effect of AB and PC on dry AD for methane yield is not only affected by the types of substrates and reactors,but also related to the specific surface area,conductivity and particle size of the carbon-based materials.