Study On The Influence Of Key Parameters Of Landfill Gas Vertical Well Collevtion Systerm On Gas Collection Rate

With the continuous improvement of urbanization in China,the production of municipal solid waste is increasing.Sanitary landfill is widely used in China and other countries because of its low cost and easy management.A large amount of landfill gas(LFG)mainly composed of methane and carbon dioxide will be generated after waste degradation.The random discharge of LFG will cause environmental pollution and cause safety hazards such as explosions and fires.Failure to discharge LFG will increase the air pressure in the landfill and affect the stability of the landfill.The methane content in the LFG is high,and it can be used for heating and power generation after being collected and processed.At present,LFG is mainly collected and processed through the vertical well collection system.The collection rate of the LFG vertical well collection system is affected by many factors.At present,many achievements have been made in this research,but there are still shortcomings.Therefore,further research on the landfill gas collection system has important theoretical significance and practical value.The method of numerical simulation adopted to study the key factors of the vertical well collection system affecting the collection rate in this paper.Then the calculation formula of the collection rate of the shaft collection system was proposed.The main work and research results are as follows:(1)Based on the fluid theory of porous media and Darcy’s law,the COMSOL finite element software was used to develop a two-dimensional axisymmetric numerical model of the LFG vertical well collection system.In the model,the solid unit is used to simulate the migration of landfill gas in the pumping well,and the gas permeability of the solid unit is used to simulate the friction effect of the well wall on the fluid,which avoids the error caused by the boundary of the normal pressure well on the research results.Compared with the existing research results,the feasibility of the model is proved.The variation law of the collection rate of LFG vertical well collection system with the gas permeability of the pumping well unit is studied,and it is proposed that the frictional effect of the well wall on the fluid under different conditions can be simulated with different gas permeability of the pumping well unit.The influence of the geomembrane covering layer on the top of the landfill on the collection of landfill gas is studied,and the results show that: when the geomembrane covering layer is used,the collection rate of the shaft collection system is significantly improved.At the same time,the air pressure in the landfill increases because the landfill gas that is not collected by the well is difficult to diffuse through the geomembrane covering layer on the top of the landfill.(2)Two curves of gas permeability of garbage dumps with depth were obtained through summarizing the variation data of waste gas permeability with depth measured by domestic and foreign scholars through laboratory tests and field tests.The effect of waste gas permeability on collection rate is studied.The results show that the collection rate decreases with the decrease of waste gas permeability.(3)Considering the variation of waste gas permeability along the depth with the curve that conforms to the variation in Chinese landfill,the numerical model was used to study the effect of the characteristic of collection well,gravel layer and waste on collection rate of LFG vertical well collection system under conditions with and without geomembrane layer.(4)The corresponding second-order response surface calculation formula is established based on the relationship between key influencing factors and the collection rate of active LFG vertical well collection.The response surface formula is used to calculate the collection rate of two cases,and the calculated results are in good agreement with the numerical simulation results.Therefore,the response surface calculation formula can be used to optimize the design of the shaft collection system within the scope of parameter research in this paper.