| With the continuous promotion of urbanization and the formation of supercities in China,the treatment of municipal solid waste(MSW)and landfill gas(LFG)pollution have become a major problem in urban management.Landfill is the third-largest source of anthropogenic methane emission,and has an important impact on global warming.Landfill closure cover is an important barrier to control LFG emissions and reduce LFG pollution.Loess is the main cover material in northwestern China.Therefore,an improved understanding of the moisture-gas-heat-methane oxidation interaction law in the loess-gravel capillary barrier cover(CBC)helps to optimize the design of Alternative Earthen Final Covers(AEFCs),thereby reducing emissions of LFG as well as methaneBased on the field methane oxidation experimental facility of loess-gravel CBC,the field-scale methane oxidation test and artificial rainfall test were carried out in different seasons.The distributions of moisture,LFG and heat in the cover were analyzed,and the variations of methane oxidation rates and LFG emissions in different seasons were studied.A fully coupled model for moisture-gas-heat transport considering the effects of methane oxidation,water vapor and temperature gradient in unsaturated soils was established,and the mechanism of moisture-gas-heat transport in the AEFCs and the key influencing factors were studied.Finally,the design method of methane emission reduction in AEFCs is proposed,which provides a reference for the design of the methane emission reduction capacity of the AEFCs.The following conclusions were obtained:(1)A fully coupled model for moisture-gas-heat transport considering the effects of methane oxidation,water vapor and temperature gradient in unsaturated soils was established.The model was verified by the indoor methane oxidation soil column test and the bottom heating soil column to promote water evaporation test.The numerical analysis results of multi-component gas concentrations,matrix suction values,gas pressures and temperature are in good agreement with the measured results of the soil column tests.The model provides a tool for the study of methane transport in AEFCs(2)A newly developed static chamber method with a laser methane detector(LMD)and a biogas analyser was proposed to measure the LFG emissions and methane oxidation rates in landfill covers.The method used a LMD and a biogas analyser to measure concentrations of methane and carbon dioxide in the chamber,avoiding the potential disturbance caused by frequent gas sampling during measurement.The methane oxidation rate was estimated according to the volume fractions of methane and carbon dioxide entering the cover and the measured LFG emissions.Since the LMD could quickly measure the methane concentration,multiple sets of tests could be performed at the same time.(3)The results of artificial rainfall test demonstrated that rainfall caused obvious changes of water and gas migration in the landfill cover,and the "hot spots" of LFG emissions were more obvious after rainfall.Due to the uneven distribution of soil water content and the influence of heterogeneous soil and crack,there were dominant flow channels in the loess cover,and the distribution of LFG emissions showed obvious spatial variability.The cracks can be used as the dominant flow channel to accelerate water infiltration when encountering rainfall.After rainfall,soil water evaporated rapidly through the inner surface of the cracks,resulting in the faster recovery of the air conduction performance of this area compared with other areas.Thus,LFG was preferentially discharged from the area where the dominant flow channel was located.Rainfall enhanced the influence of dominant flow on gas migration in the CBC.The maximum emission of LFG before and after rainfall measured at nine test points increased from 714.98 g m-2 d-1 to 1100.05 g m-2 d-1,and the "hot spot" of LFG emission after rainfall was more significant(4)The performance of the loess-gravel CBC varied greatly from season to season,and the effect of capillary block at loess-gravel interface was more significant in autumn and winter.Cold and wet soil conditions in the CBC were observed in winter,while it was warm and dry in the summer.In summer,the VWC at the bottom of the loess layer was generally lower than the field water capacity(i.e.,32%V/V)due to the influence of high temperature and high evaporation,the effect of the capillary block did not significantly affect the VWC distribution and gas migration in the CBC.In winter,the VWC at bottom of the loess layer was maintained at a level higher than 40%(i.e.,85%degree of saturation)for a long time due to the effect of the capillary block,the gas conduction performance of the CBC was obviously decreased,and the capillary block effect on the gas migration in the CBC became very significant.The maximum methane oxidation rate in sulmer(93.3 g CH4m-2 d-1)was higher than that in winter(57-1 g CH4 m-2 d-1)due to the higher temperature.Hot-spot methane emissions generally appeared near the boundary of the upper-and middle-sections of the CBC and were more significant in summer.This is likely due to(ⅰ)the existence of preferential flow through cracks at the edges of the CBC and(ⅱ)the lower VWC in summer and hence larger gas transfer properties.(5)Based on the numerical back analysis of the field test results,the parameters needed to simulate the moisture-gas-heat transfer and methane oxidation process in the loess-gravel CBC were determined,and then the effect of the water vapor supply,temperature gradient and methane oxidation on the CBC were analyzed.After long-term evaporation,the water volume of the CBC profile with water vapor supply is 19%higher than that of the CBC.without water vapor supply,and the VWC at the bottom of the loess layer with water vapor supply is 35.9%higher than that of the CBC without water vapor supply.The temperature gradient promotes the evaporation of water in the CBC,the evaporation capacity of the CBC with a temperature difference of 15℃ is 75%higher than that of the CBC without temperature difference when the CBC is very dry.In addition,when the temperature gradient of the CBC makes the soil temperature in the methane oxidation area close to the most appropriate temperature,the methane oxidation rate of the cover layer will be increased,otherwise it will be decreased.Due to the heat released by methane oxidation process,the maximum soil temperature in the methane oxidation area can be increased by 1.39℃.The VWC in the shallow part of the CBC(i.e.the main area of methane oxidation)with methane oxidation process is 2.1%higher than that without methane oxidation process.(6)The simplified analysis model of methane emission from AEFCs and the design method of methane emission reduction capacity of AEFCs are provided.As the temperature gradient and water vapor supply have obvious influence on the water-gas transport and methane oxidation in the CBC,the impact of climate changes should be taken into account when designing the methane emission reduction capacity of the CBC.It is suggested that the service performance of the CBC should be simulated according to the meteorological data of the last decade,and the annual LFG influx under safety control pressure should be evaluated according to the simulate results.According to the capacity of LFG production and the annual LFG influx of loess-gravel CBC under safety control pressure,the eighth platform of Xi’an landfill was suggested to start the site closure project 1.5 years after stopping landfill.According to the methane influx at the time of landfill closure,the methane oxidation rate of the CBC required to meet the Australian methane emission standard was suggested to be higher than 177.8 g m-2 d-1.And the required methane oxidation potential of the loess was determined based on the calculation formula for estimating methane oxidation rate of the CBC,which suggested that the amount of compost should not be less than 9.4%. |
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