Determination of emission factors of nitrous oxide from fluidized bed sewage sludge incinerators by long-term continuous monitoring

This paper describes the results of field measurements of nitrous oxide (N2O) emissions from fluidized bed sewage sludge incinerators. N2O was monitored continuously at six incinerators for 7-14 days. To study possible seasonal differences in N2O emissions, measurements were done at two incinerators in both summer and winter. N2O emissions varied with time, with large fluctuations in all incinerators. This result suggests that N2O emission factors determined by a grab sampling or short-term monitoring are not reliable. The average emission factors obtained were 1,520-6,400 g-N2O/t-dry sludge, which is much higher than the values of 800-1,500 g-N2O/t-dry sludge published in "Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories" by the IPCC. N2O emissions were mostly determined by the highest freeboard temperature of the incinerator. N2O emissions were drastically lower at higher values of highest freeboard temperature. The relation of N2O emissions to the highest freeboard temperatures was nearly the same in all incinerators. This suggests that one can estimate the N2O emission factor of an incinerator by analyzing the freeboard temperatures for several weeks. Seasonal differences in N2O emissions from a small-capacity incinerator seemed to be due to the change in freeboard temperature. A large-capacity incinerator had the same N2O emissions in the summer and winter, because the change in freeboard temperature was small. Incinerators having larger incineration capacities tended to have higher N2O emission factors. This means that large capacity incinerators are operated with relatively low freeboard temperature.