EXPERIMENTAL EVALUATION OF THE COMPRESSIBILITY OF LANDFILL GAS DURING EXTRACTION

摘要

It is commonly believed that the flow rates of landfill gas extraction wells are infiuenced by the natural variation of barometric pressure. However, based on the experimental data developed as part of this paper, it appears that the compressible characteristic of landfill gas plays a more important and predominant role. Landfill gas system operators have long observed variations of certain operating parameters that appear to have daily and seasonal cycles. The variations that are observed by operators includes: (1) increased consumption of supplemental fuel by fiares during night time hours; (2) increased oxygen concentrations in the extracted landfill gas during night time hours ; (3) increased surface emissions during day time hours as well as during low atmospheric pressure conditions. These changes appear to indicate that the extraction flow rate of the collection system may be changing in some way to cause a cycle of over and under extraction of landflll gas. Since these observed variations appear to be systematic, it suggests that improvements to the current method of landfill gas extraction may be possible. This may lead to a higher level of control over the extraction process which would provide the ability to extract only that amount of gas that is necessary to attain the goals of collection. The phenomenon of diurnal fiow rate variation was investigated experimentally. Time series measurements of fiow rates, temperature, pressure and gas quality of individual wells as well as the main gas mover, a nine-stage centrifugal blower, were monitored. In addition, weather conditions were also monitored and recorded. The experimental results confirm that fiow rates at both the extraction wells and blower unit have a diurnal rhythm. In addition, the experimental results demonstrate that landfill gas is diluted following a specific lag time following periods of higher flow rates. The conclusion of this study is that the observed flow rate fluctuations are highly correlated with the heat loss associated with landfill gas extraction. The gas temperatures within the waste column will range up to 140 degrees Fahrenheit and remain fairly constant. However, upon extraction, the gas will cool to near ambient temperatures. This temperature loss is not constant but varies throughout the day and night along with ambient temperature conditions. In regards to the influence of barometric pressure variation, this study did not find a clear correlation with the diurnal fiow rate fluctuations of the blower unit and extraction wells. Considering the compressible nature of landfill gas as a parameter in designing collection systems has the potential for optimizing landfill gas collection efficiency. This increased collection efficiency may reduce operational cost, increase energy recovery and achieve a higher level of emission control.