| Methane is the second largest greenhouse gas in the world,and a large number of methane gases will be produced in landfills,which accounts for 4%of the total global greenhouse gas emissions,and shows an upward trend year by year.However,there is no gas collection device in small and medium-sized landfills in China,if the methane in the landfills is discharged into the atmosphere at will.It will accelerate the greenhouse effect,and high methane content will cause explosions.Many scholars have studied the methods of methane emission reduction in small and medium-sized landfills.They believe that adding the substances containing methane oxidizing bacteria in clay layer will have a good methane emission reduction effect.In this paper,the methane oxidation ability and gas migration law of the bio-covered layer of Taizhou Shabei landfill and the modified clay composed of the stale garbage of Hangzhou Tianziling landfill with a landfill time of 1.5 years were studied by means of laboratory tests and numerical simulation.The optimum conditions of methane oxidation in the overburden are discussed and studied.The main work in this paper is as follows:(1)The methane oxidation rate of aged refuse modified clay material was studied by culture flask test.The effects of the content of aged refuse,moisture content,temperature and carbon dioxide in landfill gas on methane oxidation rate were studied by controlling a single variable.The results show that the methane oxidation ability of the sample does not increase further when the content of the aged garbage reaches 50%.The optimum moisture content of the sample ranges from 25%to 35%,the optimum temperature is 25 C,and the methane oxidation ability of the sample is obviously enhanced by carbon dioxide.(2)A self-made soil column device was used to simulate the landfill site environment,and field simulation tests were carried out on the bio-covered layer of aged refuse modified clay.The water content of the test was controlled to 10%and 20%respectively.The effects of water content on gas migration and methane oxidation in the bio-covered layer were studied.The results show that the sample with 10%moisture content has no methane oxidation ability,while the sample with 20%moisture content has strong methane oxidation ability,which can oxidize all methane in the soil column.The reaction mainly occurred at 20cm?40cm and reached its maximum on the 24th day.The air pressure in the soil column increases linearly with the increase of height,and the permeability coefficient of the material decreases with the increase of water content.During the test,the temperature in the soil column increases with the increase of methane oxidation rate.(3)A one-dimensional gas migration model of methane in landfill cover system was established,and the semi-analytical solution of methane migration in landfill cover system was obtained by table lookup method and Laplace transform.After fitting with the results of soil column test,it is found that the error is within the controllable range.The porosity and diffusion coefficient in the model are modified to study the sensitivity of parameters in the model.Thalculation results show that the sensitivity of diffusion coefficient is greater than that of soil sample,and the porosity sensitivity of soil sample is smaller.By establishing methane in the overburden layer,the sensitivity of diffusion coefficient is higher than that of soil sample.The one-diaensional transport model and sensitivity analysis can guide the design parameters of the actual landfill site and predict the methane concentration at different heights of the overburden. |
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