Methane is a greenhouse gas (GHG) and the anthropogenic emission of methane to the atmosphere contributes to global warming. There are several anthropogenic methane sources, and the quantification of methane from these emission sources are often based on emission factors and model calculations making reporting uncertain. Reducing the methane emission is an effective way of reducing the overall greenhouse gas emission. Methane reductions can often be difficult to quantify and document, as accurate measurements methods are lacking and not commercial available.
The methane emission from the waste sector is a significant part of the global anthropogenic methane emission, and landfills are responsible for the majority of the GHG emission. Several initiatives have been taken to minimize the methane emission from landfills, e.g. by methane recovery followed by flaring or utilization, or by constructing mitigation installations such as a cover material with enhanced methane oxidizing capability. Due to a series of factors, methane from landfills is emitted very heterogeneous in both time and space, challenging methane quantification. Several methods have been developed to quantify methane emissions from landfills, but none of these have been accepted internationally as the best way to perform emission measurements.
The overall aim of this PhD study was to identify, develop, document and apply an optimal method for quantifying fugitive GHG emissions from waste treatment facilities such as landfills and wastewater treatment plants. The primary objective was to identify a potential measurement method, build the associated analytical platform and document and verify the method. The secondary objective was to apply the method to quantify emissions from Danish landfills and from wastewater treatment plants.
The PhD study reviewed and evaluated previously used methane measurement methods and found the tracer dispersion method promising. The method uses release of tracer gas and the use of mobile equipment with high analytical sensitivity, to measure the downwind plumes of methane and tracer gas. The method was chosen as in enable measurements of the emission from whole landfill areas, including possible hotspot emissions occurring at the landfill.
A fast response and high resolution analytical equipment was purchased and tested. An analytical platform was build, enabling the instrument to be installed in any vehicle and thereby enabling measurements wherever there were roads. The validation of the measurement method was done by releasing a controlled amount of methane and quantifying the emission using the release of tracer gas. The validation test showed that even in areas with large turbulence, such as urban areas, the measured emission could be quantified within a few percent of the released methane. The sensitivity of incorrect location of tracer gas release was also tested, showing the possibility of a significant over-/underestimation of the methane emission by misplacing the tracer gas, and that this error becomes smaller with increasing measurement distance.
A measurement protocol was developed and the methane emission was quantified from a series of landfills with different size, age and gas recovery and mitigation conditions. The landfills were measured between one and four times and the emissions ranged from 2.6 to 60.8 kg methane per hour, with the lowest emissions from the oldest and smallest landfills and the highest emissions from the bigger landfills. It was not possible to correlate the measured emission with a single factor such a landfill age, size or mitigation actions. As an example the highest emission was measured at a landfill with active methane recovery and utilization. Compared with national and European greenhouse gas reporting schemes the measurement showed a large difference, with reporting ranging a factor of 100 above to a factor of 10 below the measured methane emission. The average reporting was three times higher the average measured emission, even when included the two landfills without reporting. The landfills recovering methane for utilization showed a methane recovery efficiency ranging between 41 and 81%, excluding a possible methane oxidation in the top layer of the landfills.
To expand the application of the developed analytical platform to also cover fugitive emissions of other gasses, an additional instrument for measuring nitrous oxide (greenhouse gas) and ammonia (causes eutrophication) was developed and tested in collaboration with the manufacture. The development was done in two stages. First stage was optimization and field testing done during an external research stay at the instrument manufacture in USA. The second stage was field measurements conducted in Denmark with subsequent tuning of the spectroscopy in the instrument. The implementation of nitrous oxide measurements were done by intensive measurements at a Danish wastewater treatment plant. The measurement campaigns showed that the nitrous oxide emission mainly occurred from the aeration tanks during aeration. The nitrous oxide emission showed high temporal variations ranging from vi below quantifiable and up to 10.3 kg nitrous oxide per hour. The methane emission from the wastewater treatment was also quantified and the majority (99%) was emitted from the sludge treatment processes, including anaerobic digestion and open air storage of digested sludge. The methane emission ranged from 10 to 92 kg per hour and was found to change in even short timescales of a few hours. The periods with large emissions correlated with a drop in methane utilization, indicating that emissions came from the digesters tanks or gas storage/use. The measurements indicated that the main emissions occurred in elevated heights, but theoretically calculation showed that this only resulted in a 2% underestimation, although measurement conditions could make the error more significant.
Besides the extensive emission research, the outcome of the PhD study is a mobile analytical platform implementable on any means of transportation able to carry approximately 100 kg, including batteries, inverter, weather station, GPS, pumps, analyzers and screens. The mobile analytical platform can measure real time atmospheric concentrations of methane, nitrous oxide and ammonia and measure concentration changes in parts per billion levels, enabling the use of dynamic tracer dispersion method for quantifying fugitive emissions from various sources. The analytical setup was proven applicable for measuring methane emissions from landfills and methane and nitrous oxide emission from wastewater treatment plants. The flexibility of the analytical platform allows many setups, including short term mobile measurements and long term, stationary measurements, opening up for a large range of applications both for emission quantification and concentration monitoring.