Measurements of methane emissions, methane oxidation and VSCs at Swedish landfills

1. Measurements of methane emissions, methane oxidation and VSCs at Swedish landfills using FTIR spectroscopy Jerker Samuelsson Bo Galle Chalmers University of Technology Department of Inorganic Environmental Chemistry S-412 96 Göteborg Sweden Gunnar Börjesson Bo Svensson Department of water and Environmental studies Linköping University S-581 83 Linköping Sweden During the year 1998/99 total methane emission measurements have been successfully conducted on a Swedish landfill, measuring every second month. The method used is based on tracer gas release from the landfill combined with time resolved concentration measurements downwind the landfill using FTIR absorption spectroscopy. A project has been initiated, aiming at improving the Swedish landfill methane budget, including studies of methane oxidation in the top soil layer of the landfills and presence of VSCs (Volatile Sulphur Compounds). Below, the methodology used is described along with obtained results and a presentation of the initiated Swedish landfill methane project. Introduction Methane is an important climate gas contributing to the global warming.The concentration of CH4 in the atmosphere increases globally by 0.6-0.8% annually. The IPCC has estimated that more than 10% of the anthropogenic methane emissions originate from landfills. Besides developing and implementing new waste treatment practises such as incineration and separation at source, it is important to find methods to reduce the methane emissions from existing and future landfills. The organic carbon available in a landfill is biogenically degraded to CH4 and CO2. As CH4 has a GWP (Global Warming Potential) that is 20 times stronger than for CO2, a considerable improvement would be made if a larger fraction of the carbon was emitted as CO2. This can be achieved either by pumping the gas out for incineration or by bacterial methane oxidation in the top soil layer of the landfill. To validate the effect of different methods there is a need to be able to quantify both the total emissions from landfills and the degree of methane oxidation in the cover layer. Due to the spatial variability of the emissions, integrating methods are needed. A method called the TCT (Time Correlated Tracer) method has been successfully applied at a Swedish landfill, estimating the total methane emission. Total methane emission measurements During one year the total methane emission from a landfill serving a small Swedish town, Falköping, has been monitored using FTIR absorption spectroscopy. Measurements were carried out over 1-3 days every second month. The observed average emission over the year was 37.5 kg/h, with an estimated accuracy of about ±15% in respectively measurement. In December a strong rise in the emission, starting from a low level, was observed, coinciding with thawing of the ground. The methodology used, called the TCT method (Time Correlated Tracer), is based on tracer release at the landfill, accompanied by time resolved concentration measurements in the plume downwind the landfill. The emitted tracer mixes with the leaking methane, and analysis of the ratio between the methane and the tracer concentrations in the landfill plume, yields the methane emission as, If the tracer simulates the methane emission well, indicated by the degree of correlation between the concentrations, the total emission from the landfill is derived. The experimentalset up used is based on a medium resolution FTIR spectrometer coupled to an optical White cell. Infrared light is transmitted through the gas cell over a pathlength of total 56 meters, and an absorption spectrum of the gas is recorded. The spectra are analysed using multiregression techniques (CLS), fitting pre stored, synthetic calibration spectra of known concentrations. The system offers sensitive, real time analysis of a number of interesting gases (CH4, CO2, CO, N2O, H2O, NH3, hydrocarbons etc.) simultaneously, and with high time resolution. Mixing ratios down to a few ppb is detectable. The Swedish landfill methane project A project has been initiated aiming at improving the Swedish landfill methane budget. Every second month during the year 2000/2001 ten representative landfills, located from the south to the very north of Sweden, will be measured upon. The study includes total CH4 emission measurements, estimation of the methane oxidation in the landfill top soil layer, and presence of VSCs (Volatile Sulphur Compounds) such as CS2 and CH3SH. The project also comprise studies of the influence of VSCs on the oxidation of methane. VSCs show an inhibitory effect on the oxidation process. Additionally to this, measures of the efficiency of the landfill gas extraction systems at the different sites will also be generated. Total fluxmeasurements are conducted by means of tracer release at the landfill and time resolved concentration analysis, as described by the TCT-method. In order to enable in situ, real time, sensitive analysis, a temperated, medium resolution FTIR-system with a multireflection optical gas cell is used. The system is compact and stable, allowing rough field conditions and enabling quick and comfortable transporting between different measurement locations. Particularly attractive is the possibility to observe the quality of the measurement on line, enabling correction of the measurement, due to for instance changing meteorological conditions. Compared to the traditional, manual gas sampling methods with subsequently laboratory analysis, this methodology saves a lot of valuable time and limits the tracer gas emissions. The methane oxidationis studied by analysis of the change in isotopic signature between the emitted methane and CH4 originating from the anoxic zone. The methane oxidizing organisms in the landfill top soil cap, have a preference for lighter isotopes, resulting in an enrichment in both D (deuterium) and 13C for methane emitted from the soil cap. In this project mainly the shift in the 13C ratio is used for the oxidation quantification, but also deuterium (DCH3) is studied. The isotopic analysis is carried out by means of high resolution FTIR. This set up is quite unique and it allows online incubation studies, that can be highly resolved in time. Incubation studies are conducted on soil samples from the different sites, generating the preference of 12CH4 over 13CH4 for the soil methane oxidizing bacteria, needed to quantify the methane oxidation potential. High resolution FTIR is also used to study the inhibitory effect of VSCs on the methane oxidation, and to determine the presence of VSCs in the landfill gas at the different sites. To enhance the spectral signal of interest, a cryo gas sampling equipment is used. This is based on condensation of the sampled gas by cooling in liquid nitrogen (T=77 K). Utilizing the difference in vapour pressure between the different gases, the oxygen and nitrogen may be removed from the sample by decreasing the pressure in a controlled way. In the remaining sample a concentration enhancement of about thousand times is thus obtained . Figure 1. Correlating concentrations of methane and tracer gas, N2O, from a total emission measurement made at a Swedish landfill, Falköping, using the TCT-method. Figure 3. The emission of methane from the landfill in Falköping, Sweden, during one year, derived with the TCT-methodology. Figure 4. Methane emission from the landfill in Falköping during three consecutive days in December 1998. An emission increase coinciding with thawing of the ground, but also with an atmospheric pressure decrease, is observed. Figure 2. Correlation plot for determination of the ratio between the leaking methane and the tracer gas from the TCT measurement in figure 1, yielding a total methane emission of 44 kg/h. Figure 6. Absorption spectrum of methane measured in a gas cell with 56 m optical path, shown along with a CLS fitted, synthetic calibration spectrum, corresponding to 2.60 ppm CH4. Figure 5. Schematic overview of the FTIR multireflection cell system, which may be attached to an incubation study device. Figure 8. Absorption spectrum of methanethiol (CH3SH), corresponding to 100 ppm-meters. Figure 9. Lineweaver-Burke-plot, showing the effect of methanethiol on the methane consumption rate in a mineral soil, at different concentrations of methane respectively methanetiol. A competitive inhibition by CH3SH is observed. Figure 7. Map indicating the location of landfills included in the Swedish landfill methane project, where total methane emission measurements will be conducted every second month during 2000/2001.