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Dynamic tracer dispersion method: A tool for measuring the total methane emission from individual Danish landfills
Mønster, Jacob; Fuglsang, Karsten; Scheutz, Charlotte
Publication date: 2016 Document Version Peer reviewed version Link back to DTU Orbit
Citation (APA): Mønster, J., Fuglsang, K., & Scheutz, C. (2016). Dynamic tracer dispersion method: A tool for measuring the total methane emission from individual Danish landfills. Abstract from Conference and Exhibition on Emissions Monitoring 2016 (CEM), Lissabon, Spain.
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Dynamic tracer dispersion method: A tool for measuring the total methane emission from individual Danish landfills J. Mønster*1,2, K. Fuglsang1, C. Scheutz2 1
FORCE Technology, Park Alle 345, 2605 Brøndby, Denmark
DTU Environment, Technical University of Denmark, Miljøvej, Building 113, 2800 Lyngby, Denmark
1. Introduction Methane is a strong greenhouse gas and is considered 34 times stronger than carbon dioxide on a 100 years scale and 86 times on a 20 year scale (Myhre et al. 2013). There is therefore a growing concern about methane emission sources. Landfills is a significant source of methane emission to the atmosphere and it is estimated that the waste sector in 2004 was responsible for 18% of the anthropogenic methane emission (Bogner et al,. 2008). These emission estimates are often based on mathematical models using waste amount and composition and the models are often poorly validated by actual emission measurements. The methane emission from landfills has a dynamic and diffuse nature, which makes it a challenge to quantify. Numerous measurement methods have been used to measure landfill methane emissions including flux chamber methods, micrometeorological methods and remote measurements methods. One of the most promising methods is a remote measurement method called the dynamic tracer dispersion method. This method has been has been tested and applied on landfills in Denmark (Mønster et al., 2014, 2015) and has subsequently been suggested by the Danish Environmental Protection Agency as the best method among commercial available quantification methods with a operational price feasible for regular measurements. The Danish Environmental Protection Agency recently took actions to support landfill owners installing mitigation initiatives. The Danish EPA will, starting June 2016, financially support the establishment of biocovers on landfills for mitigating the total methane emission. The establishment will only be supported at landfills with an emission larger than 6 kg CH4 h-1, and thus reliable measurements are needed to identify the relevant landfills, and to measure before and after installation of the methane mitigating biocovers. This paper (CEM oral presentation) presents the dynamic tracer dispersion method: The theory and practical application is described using examples from measurements of the methane emission from a number of Danish or Swedish landfills. The method description includes an evaluation of both analytical and site specific uncertainties associated with the measurements. Furthermore, details are given about how the measurements associated with the dynamic tracer dispersion method can be used for obtaining additional information about emission from specific areas of a landfill. Finally, a tool for fast screening is presented. 2. The Dynamic Tracer Dispersion Method Tracer dispersion methods use simultaneous measurements of atmospheric concentrations of a gas of quantification interest and a tracer gas released at the same location as the emission source. The tracer gas is released at a known rate, and assumption is made that the gas for quantification and the tracer gas have the same fate in the atmosphere within the time span of the measurement. Thus, a tracer gas with relatively long atmospheric lifetimes is normally used. Quantification measurements are done downwind from the emission source and tracer gas release and the concentration ratio is used to calculate the emission rate from the emitting source. The tracer dispersion method is generally divided into a stationary and dynamic approach. The stationary approach relies on measurements in a single or multiple stationary
points in the downwind plume (e.g. Czepiel et al. 1996, Galle et al. 2001 and Jacobs et al., 2007), while the dynamic approach is taken by traversing the downwind plume (e.g. Scheutz et al., 2011, Mønster et al., 2014, 2015). The emission of a gas can be calculated from the equation: Plume end 2
E gas Qtracer
gas Plume end 1 Plume end 2
tracer Plume end 1
where Qtracer is the tracer gas release (kg h-1), Cgas and Ctracer is concentrations across the downwind plume (above the background concentration), MW is the molecular weight and x is the distance across the downwind plume. The advantage of the tracer dispersion method is the simplicity of its approach. In the case of methane from landfills, the analysis and calculation are relatively straightforward when the methane and tracer gas plumes are fully mixed. The method insures measurements of the whole downwind plume and possible errors seen during measurements, such as a change in wind direction, can be adjusted for. Additionally, emissions from hotspots, onsite installations or weak landfill structures such as steep slopes, can often be identified using the dynamic approach, and the emission from hotspots can be quantified using a smallscale tracer dispersion method (Fredenslund et al., 2010).
3. Application of the dynamic tracer dispersion method. Applying the method on a landfill requires four steps. 1) Initial preparations using online maps/areal photos, finding possible roads for downwind plume measurements, possible error sources (such as other methane sources in the nearby areas), and useful wind directions. 2) Screening of the landfill (Figure 1) and area around the landfill. The on-site screening is done on accessible roads to find the main emission areas for placing the tracer gas bottles as close to the emission points as possible. The screening of the area around the landfill is to establish the background concentration of methane and the tracer gas, and to find possible additional methane sources that could interfere with the quantification measurements.
Figure 1. The relative methane concentration (red) in two meters height at the roads on and around a Danish landfill. Elevated concentrations revealed two main emission areas.
3) Release tracer gas and perform traverses on the chosen road (Figure 2). For a good statistically quantification, more than ten traverses are advised. 4) Data treatment, including integration of the methane and tracer gas plumes in each of the downwind traverses (Figure 3).
Figure 2. The concentration of methane and tracer gas the downwind plume approximately 700 and 4000 meters from the same landfill shown in Figure 1. Additional methane sources (farms and old landfill) were found in the nearby area. Yellow triangles marks the tracer gas release points.
Figure 3. Data treatment of the downwind plumes shown in Figure 2. The ratio of the integrated plume areas, together with the controlled tracer gas release, results in total landfill methane emission quantification.
4. Uncertainties of the method. The uncertainties of the dynamic tracer dispersion method is a combination of the individual uncertainties of the measurement and data treatment. The analytical instrumentation can measure concentration changes down to sub-ppb level. More specific performance of the instrumentation can be found in Mønster
et al., 2014. The analytical error therefore only becomes an important factor when measured concentration change reaches this level, which is uncommon for landfill emission under average atmospheric conditions. However, the measured concentrations depends on the emission rate at specific site, the atmospheric conditions during the measurements and the distance to the road used for quantification. The uncertainty shown in Table 1 is the combination of the uncertainties during the measurements. This also includes changes in the emission rate, which can be induced by changes in atmospheric conditions and changes in a possible gas handling (e.g. more/less pumping for utilization of the landfill gas). However, the uncertainty does not include possible biased errors, such as a constant error in the tracer gas release or a constant error source of methane or tracer gas. The overall uncertainty of the dynamic tracer dispersion method applied on a landfill is therefore highly depending on the site specific conditions: the size of the landfill part with methane emission, possibility for accurate tracer gas placement at the main emission points/areas, the atmospheric conditions during the measurements, the distance to the measurement road and possible interfering sources. When performing more than ten traverses, the standard error on the mean value is normally below 10%. However, this error can become larger under more difficult measurement conditions.
5. A screening tool The traverses of the downwind methane plume from a landfill using a fast, high resolution methane analyser can be useful as a screening tool for emission estimate. The Gaussian distribution of the methane emission along the wind direction, combined with information about the atmospheric conditions, can be used in a Gaussian plume model to obtain an emission estimate. The topography between the emission source and the measurement road and the variation and uncertainty of the atmospheric conditions and stability makes the uncertainty relatively large, but it can be a useful tool for fast screening for significant emission sources and to estimate their approximate emission rate before determine if a quantification using tracer gas should be performed. Traverses close to a landfill can, together with onsite screening, give information about the nature of the methane emission from the different parts of the landfill. This is a useful tool, not only for determine the location of tracer gas placement for the dynamic tracer dispersion method, but also for locating where possible methane mitigation initiatives could be initiated. Measurement of the methane plume at different distances from the methane source can be used for gaining information about the dilution rate and thus the emission rate, but it can also be used for screening for potential elevated emission sources, such as large waste hills with steep slopes, emission from engine stacks or emission due to incomplete flaring. This approach can also be used for screening for elevated emission sources at other methane emitting facilities such as biogas plants or gas drilling.
6. Conclusion The dynamic tracer dispersion method is a strong tool for quantifying the total methane emission from landfills, due to it’s simple approach and more affordable setup and use than comparable methods. Furthermore, the sensitive and fast response analytical instrumentation allows on and off site screenings for further investigation of the location and relative strength of the emission at different parts of a landfill.
Finally, the same analytical setup can be used for a fast screening downwind of various methane sources, and an emission rate can be estimated using information about the atmospheric conditions.
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