Development of innovative landfill gas management technologies

Abstract

Climate change is on the political agenda worldwide, and abatement strategies for greenhouse gas emissions are a necessity. One source of greenhouse gas emissions is landfills, as the degradation of organic carbon in landfilled waste generates methane. Landfill gas emission patterns show high spatial and temporal variability, but the development of innovative technologies for both monitoring and mitigation will help in the much-needed conceptual understanding of governing gas transport and emission processes. A methane mass balance can be established based on the individual migration pathways for the generated methane, including methane recovery for energy utilisation or flaring, lateral migration to neighbouring plots, methane oxidation by microorganisms in the cover and emissions seeping into the atmosphere. A methane mass balance forms a good conceptual framework for setting up a mitigation strategy for a landfill. The main part of this PhD project was conducted in relation to a closed Danish landfill (Hedeland landfill, Roskilde, Denmark). Many years of intensive investigations have been conducted at Hedeland landfill to understand better the migration and emission patterns of methane generated at the site. A mitigation strategy has to be established, which takes into consideration both the safety of local residents and the negative impact on global warming from landfill gas migration and emission. A methane mass balance for the landfill could provide a valuable overview and show the individual importance of each migration pathway. As part of this PhD project, a methane mass balance was established for Hedeland landfill based on data from many years of investigation, covering the years 2013-2015. Methane generation was modelled based on a multi-phase, first-order degradation kinetics (Afvalzorg) model, with average methane generation determined at 67±8.6 kg h-1. Methane recovery, emission and lateral migration were found to cover 38% of the modelled methane generation, each accounting for an equal share (9 ±2.9, 8 ±4.1 and 9 ±2.4 kg h-1, respectively). Methane oxidation in the cover was identified as the migration pathway capable of closing the mass balance and accounting for the remaining 62% of the generated methane. Several indications supported a high oxidation rate in the landfill cover at Hedeland, including a low total emission rate, which was determined using the tracer gas dispersion method and a few emission hotspots with elevated methane concentrations at the surface (identified by screening the whole landfill surface, using a flame ionisation detector). Identification of landfill gas emission hotspots is the basis for establishing emission abatement technologies such as biocovers. To overcome the high spatial and temporal variability of landfill gas emissions, a screening tool based on an unmanned aerial system mounted with a thermal infrared (TIR) camera was tested at two Danish landfills (Hedeland and Audebo landfills). The correlation between landfill gas emissions (methane and carbon dioxide), surface temperatures obtained with the TIR camera and soil temperatures at 5- and 10-cm depths was investigated in an established test area at each of the two sites. At Hedeland landfill, no correlation was found between gas emissions and surface temperatures. In addition, identified methane surface fluxes were very limited, with an average for the four measuring campaigns of only 1.3 ±16 g CH4 m-2 d-1. An average methane flux of 371 ±1337 g CH4 m-2 d-1 was found at Audebo landfill for five measuring campaigns. Furthermore, elevated temperatures at both the surface and at 5- and 10-cm depths were found in the same area as where the highest landfill gas surface fluxes were measured, thus indicating that in the right conditions the TIR camera could be used for delineating landfill gas emissions. A minimum flux of 150 g CH4 m-2 d-1 from an area of at least 1 m2 was established as the limit for the TIR camera being able to delineate a landfill gas emission hotspot at a typical Danish landfill. When landfill gas is mixed with air it dilutes, often with a methane content too low for utilisation. However, mitigation is still needed to minimise the negative impacts on the environment, and to ensure human health and safety. Sources to dilute landfill gas could be remediation systems for lateral migration, emissions from leachate and monitoring wells or from air penetrating the cover of the landfill. A cost-efficient mitigation technology for dilute landfill gas could be microbial oxidation in an actively loaded biofilter. This technology was tested in an open-bed pilot-scale compost filter at Hedeland landfill, constructed in a 30 m3 container. The filter was loaded with landfill gas diluted with ambient air to a methane concentration of between 5 and 10 vol.%. The filter was tested in five flow campaigns with the same methane inlet concentration and an increasing methane load between 106 and 794 g CH4 m-2 d-1. The highest observed methane oxidation rate was 460 g CH4 m-2 d-1 with an oxidation efficiency of 58%. Overall, oxidation efficiencies of more than 87% were never achieved, due to substantial preferential flows at the transition point between the compost and container wall despite an attempt to design the container with blockers against preferential flows. However, pore gas profiles showed methane oxidation of 100% in the compost material. These results were supported by tracer gas tests showing an average methane oxidation of almost 86% at 10 cm below the surface of the filter in flow campaign 5, where the load had an average of 701 ±47 g CH4 m-2 d-1. At Hedeland landfill, three remediation systems have been installed to cut off laterally migrating landfill gas from reaching residential houses on neighbouring plots. In 2017, an average methane content of 0.53 ±0.55 vol.% in off-gas from these remediation systems was observed, accompanied by an oxygen content in most cases above 10 vol.%. Treatment of the remediation off-gas in the constructed pilot-scale biofilter would result in a methane load of 717 g CH4 m-2 d-1. Nevertheless, the gas retention time would only be 3 min, due to the high pump flow rate of 80 m3 h-1, which is thought to be below a critical gas retention time. To increase the retention time to 30 min, ten containers similar to the tested filter would be needed. A suggested alternative could be a 111 m2 biofilter embedded in the landfill cover, which would result in the same load as the ten containers. An embedded biofilter is also expected to be able to overcome the challenges of preferential flows experienced in the tested container solution.