| China’s municipal solid waste has been disposed of in landfill for a long time,and waste in MSW landfills exceeds billions of tons,so it is urgent to solve the problem of landfill gas pollution caused by landfilled waste.Landfill methane is the third largest source of anthropogenic methane emissions,and the reduction of landfill methane is of great significance for the implementation of carbon peak and carbon neutrality strategy in China.Therefore,it is of great significance to study the gas production and emission laws of methane in landfills and choose proper measures to control methane emissions and reduce carbon emissions during the treatment of landfill closure.Based on the gas production law of methane in landfills,this paper verified the applicability of the modified two-stage gas production model of high kitchen waste to the gas production law of landfills in China,clarified the gas production law of domestic high kitchen waste landfills.and calculated the landfill gas production and the corresponding carbon emission equivalent.Field experiments were carried out to test the surface methane flux of soil cover at different landfilling ages,and the calculation methods of methane oxidation rate and oxidation fraction of soil cover were modified based on the conversion ratio of cell biomass.The methane oxidation capacity and carbon emission reduction effect of loess cover of municipal solid waste landfill in Northwest China were studied.The CALMIM model was used to simulate the methane emission of the existing covering structure of the landfill in Chapter 3,and the field test data were compared and analyzed.The sensitivity analysis of the coverage rate of gas collection system,the maximum methane oxidation rate of cover materials and the coverage rate of vegetation were carried out for the standard cover and soil cover in a landfill in Shenzhen.Finally,the relationship between methane concentration distribution and vegetation restoration at different landfilling ages was studied.Considering this,plant species were recommended for vegetation restoration at different periods after landfill closure.The main work content and relevant conclusions of this paper are as follows:(1)High kitchen waste landfill would quickly reach the peak gas production period after landfilling,but the peak duration was short,and the gas production rate after the peak was much lower than that of low kitchen waste in foreign countries.The comparison results showed that the Land GEM and Scholl-Canyon models could underestimate the gas production at peak gas production period and overestimate gas production after the peak.It was recommended to use the modified two-stage gas production model to predict the methane gas production law of landfill waste in China.This model was used to predict the national landfill gas production.The production of landfill gas peaked at 16.7 billion m~3 in 2019,the carbon emission equivalent of that would reach 178 million tons in 2020,and the total carbon emissions from 2021 to 2030would exceed 1 billion tons.(2)Considering that it was difficult to quantitatively test the methane oxidation capacity of landfill cover,the calculation methods of methane oxidation rate and oxidation fraction of cover were modified based on the conversion ratio of cell biomass,so that the methane oxidation capacity of soil cover could be quantitatively evaluated directly.Results showed that surface methane fluxes at 80%of the test sites were below the Australian CFI limit of 60 g/(m~2?d).The surface methane flux was correlated with the landfilling age of the underlying waste body,which decreased from 1.13 g/(m~2?d)to 0.53 g/(m~2?d)at a landfilling age of 3 to 7 years,and remained stable after 7 years.The variation trend of methane oxidation rate in the upper cover of different landfilling ages was basically consistent with the surface methane flux,and the methane oxidation rate had a positive correlation with the methane flux at the bottom of the cover.The highest methane oxidation rate was 63.30 g/(m~2?d)when the highest methane flux at the bottom was 75.95 g/(m~2?d),corresponding to the methane oxidation rate of 83.3%.The average methane oxidation rate of the soil cover of each age measured in the field was more than 90%,which was significantly higher than that of the soil cover of European and American countries.It proved that excellent methane oxidation ability of ecological soil cover.(3)The CALMIM simulation results were higher than the field test results,but basically maintain within an order of magnitude,and the methane emission trend was consistent with the increase of landfilling age.The reason why the simulation results were generally larger was that the CALMIM model did not consider the spatial variability of the soil cover,such as surface cracks,etc.At the same time,the model did not take into account the effect that the soil compaction in landfill would reduce gas diffusion.Increasing the coverage rate of the gas collection system and the maximum methane oxidation rate of the cover structure could effectively reduce the methane emission from the standard cover and soil cover.The restriction conditions of CALMIM model were analyzed,and the application prospect of CALMIM model in China was clarified.It was mainly suitable for simulating the methane emission of soil cover in the slow degradation stage of high food waste landfills in China.(4)Methane concentration survey and vegetation restoration survey were carried out in Jiangcungou landfill in Xi’an.The results showed that the methane concentration gradually decreased with the increase of landfilling age,and the methane concentration at the distance of 1.5 m from the surface was significantly lower than that close to the surface.The greater the concentration,the more significant the difference.The number of plant species and genera increased first and then decreased with the increase of age,and reached the highest value at the 7th year of age.Within 3 to 7 years of age,the number of plant species increased from 5 to 30,from 4 to 14 of families,and from 5 to26 of genus.When the landfilling age reached more than 7 years,the number of plant species and families began to decline,but the average plant height continued to rise.At the same time,the transition from herbaceous plants to shrubs began.Vegetation restored following the"herbaceous-shrub-tree"sequence after landfill closure.The vegetation restoration at the site proved that the ecological soil cover has excellent ecological recovery performance after landfill closure. |
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