Numerical Simulation Of Methane Emission Reduction In Biochar Modified Landfill Cover

CH₄(methane)is the second largest greenhouse gas in the world,and its single molecule warming potential is 20?30 times of CO₂(carbon dioxide).As the second largest source of methane emissions,landfill emissions account for 12%?20%of global methane emissions.Therefore,it is of great significance to improve the CH₄ emission reduction capacity of landfill cover to alleviate the global greenhouse effect.The methane oxidation ability of the traditional landfill cover is low,and the new landfill bio-cover uses the natural microbial methane oxidation to achieve the purpose of CH₄ emission reduction.As a porous soil improvement material with high specific surface area,biochar has been proved to be able to promote the CH₄ oxidation of microorganisms in soil,increase the CH₄ adsorption capacity of soil,and has the potential to improve the CH₄ emission reduction of landfill cover.Because the factors that affect the methane emission reduction capacity of landfill cover include climate conditions,soil types,cover design and landfill gas generation rate,the numerical model can be used to effectively evaluate the methane emission reduction capacity of landfill cover,and study and explain the change of its influencing factors with time and space.Most landfill methane oxidation models only focus on the transfer of four components(CO₂,O₂,CH₄ and N₂)without considering the transfer of heat and water.The comprehensive phenomena of water,gas,heat transfer and methane oxidation have not been deeply understood.In this thesis,based on the mass conservation and energy conservation of substances,a coupled numerical model of water-gas-thermal migration including methane oxidation and adsorption was established.Considering the different distribution of pH in landfill cover caused by hydrogen ion migration,the pH correction factor affecting methane oxidation rate was introduced.The model parameters were calibrated by soil column test,and the methane emission reduction performance of two-dimensional biochar modified coating closer to the actual working condition was further studied.The main conclusions are as follows:(1)Exponential pH correction coefficient can better reflect the correlation between pH and methane oxidation rate.Adjusting the initial pH of the modified coating can improve the methane oxidation rate,but the oxidation depth will decrease due to the limitation of O2 diffusion depth in the coating under natural conditions.When the thickness of biochar modified coating exceeds the corresponding threshold of methane oxidation depth,increasing the thickness of biochar modified coating will not improve the efficiency of methane emission reduction.Considering the efficiency and economy of methane oxidation,0.3m thick biochar modified coating can be used as methane oxidation layer.(2)Half saturation constant has a great influence on the methane oxidation rate.The smaller the half saturation constant is,the deeper the methane oxidation can occur.The smaller the half saturation constant is,the stronger the methane oxidation is.For different soils,it is necessary to calibrate the half saturation constant when calculating methane emission reduction efficiency under different conditions.(3)When the landfill gas generation rate is 2.51 mol/m²/day(corresponding to 4-year gas generation rate),the methane emission reduction efficiencies of 0%,5%,10%and 20% biochar modified soil are 30%,90%,95%and 100%respectively.At this landfill gas generation rate,the methane emission reduction of 5%modified soil can reach 90%,and the amount of biochar can be appropriately reduced for the landfill with small gas production.(4)When the permeability coefficient of the oxide layer is less than that of the underlying layer, the water may accumulate in the interface between the two layers,which makes the water content in the oxide layer too high and leads to the decrease of methane oxidation rate in the landfill cover layer.In the design of landfill cover,it is necessary to fully consider the water permeability of each layer.