VARIATION IN THE NOx COMPOSITIONRATIO OF EMISSIONS FROM LANDFILLGAS ENGINES

This paper reports on two studies, of the long-term variation in NOx emissions and the variation in the NOx composition ratio of emissions from landfill gas engines. Long term trends (either increasing or decreasing) in total NOx and CO emission concentrations were observed to correspond to an inverse trend in HC emission concentration. The ratio of NO2:NOx stack emissions varied significantly throughout the monitoring periods, displaying a roughly uniform frequency distribution. The majority of values of this ratio ranged between 0.20 to 0.79 at one site, and between 0.30 and 0.59 at the other. Such variation in the ratios would not be detected by intermittent sampling. In the second study, from ambient air measurements, the results highlighted that NO2 diffusion tube measurements consistently over estimate the NOx concentrations recorded by the fixed monitoring points. There are clear peaks in NOx, NO2 and NO for all of the monitoring units for wind directions between 210 and 240 degrees which accords with the prevailing south-westerly wind direction. The NOx ratio downwind (NO2:NOx) varies between 10 and 95% over the full data set although the mean value is approximately 80%. Current modelling protocol is to assume 50% NO2 for short term impacts and 100% for long term air quality impacts. This study shows that the real situation is far more complex