Mitigation of methane emissions in a pilot-scale biocover system at the AV Miljø Landfill, Denmark: 2. Methane oxidation

摘要

Greenhouse gas mitigation at landfills by methane (CH_4) oxidation in engineered biocover systems is believed to be a cost effective technology but so far a full quantitative evaluation of the efficiency of the technology in full scale has only been carried out in a few cases. A third generation semi-passive biocover system was constructed at the AV Miljø Landfill, Denmark. The biocover was fed by landfill gas pumped out of three leachate collection wells. An innovative gas distribution system was used to overcome the often observed uneven gas distribution to the active CH_4 oxidation layer resulting in overloaded areas causing CH_4 emission hot spot areas in the biocover surface. The whole biocover CH_4 oxidation efficiency was determined by measuring the CH_4 inlet load and CH_4 surface fluxes. In addition, CH_4 oxidation was determined for single points in the biocover using two different methods; the carbon mass balance method (based on CH_4 and carbon dioxide (CO_2) concentrations in the deeper part of the cover and CH_4 and CO_2 surface flux measurements) and a new-developed tracer gas mass balance method (based on CH_4 and tracer inlet fluxes and CH_4 and tracer surface flux measurements). Overall, the CH_4 oxidation efficiency of the whole biocover varied between 81 and 100% and showed that the pilot plant biocover system installed at AV Milj0 landfill was very efficient in oxidizing the landfill CH_4. The average CH_4 oxidation rate measured at nine campaigns was approximately 13 g m~(-2) d~(-1). Extrapolating laboratory measured CH_4 oxidation rates to the field showed that the biocover system had a much larger CH_4 oxidation potential in comparison to the tested CH_4 load. The carbon mass balance approach compared reasonably well with the tracer gas mass balance approach when applied for quantification of CH_4 oxidation in single points at the biofilter giving CH_4 oxidation efficiencies in the range of 84 to a 100%. CH_4 oxidation rates where however much higher using the tracer gas balance method giving CH_4 oxidation rates between 7 and 124gm~2d~(-1) compared to the carbon mass balance, which gave CH_4 oxidation rates -0.06 and 40 g m~2 d~(-1) The study also revealed that the compost respiration contributed significantly to the measured CO_2 surface emission, and that the contribution of the compost respiration decreased significantly with time probably due to further maturation of the compost material.