NOx and N2O emission characteristics from fluidised bed combustion of semi-dried municipal sewage sludge

Incineration is one of the major methods for the disposal of sewage sludge. Currently, several plants are incinerating mechanically dewatered (wet) sl udge (20-40 wt.% d.m.) or semi-dried sewage sludge (30-55 wt.% d.m.), altho ugh some plants burn dry sludge (with more than 80 wt.% d.m.). Whereas sign ificant information is available on NOx and N2O emissions characteristics o f wet and dry sludge, not much has been reported on semi-dried sludge. This paper presents some of the results obtained from the combustion of semi-dr ied sludge in a semi-pilot scale fluidised bed combustor (150 mm in diamete r and 9 m high) together with some measurements from a large-scale FBC inci neration plant (7 m(2) bed area, 9 m high and a capacity of 3 t/h dry sludg e). The investigations have shown that semi-dried sludge exhibit emission c haracteristics which are similar to those of wet sludge. NOx decreases slig htly whereas N2O remains more or less the same with increase in oxygen conc entrations. Just like wet sludge, staged combustion was not effective for t he reduction of NOx and N2O. However, increasing the freeboard temperature led to rapid reduction of N2O and some NOx reduction was achieved using flu e gas recycling technique. Comparison shows that the results from the test rig a ere more or less similar to those obtained from the large-scale plant s. (C) 2000 Elsevier Science Ltd. All rights reserved.