Abstract
Abnormally accelerated thickness reduction in the water-wall tube made of plain carbon steel has taken place in the secondary combustion zone of many domestic municipal solid waste (MSW) incinerators since introducing the low air-ratio combustion in order to suppress the emission of high concentration NOx. In the present study, corrosive failure analyses were conducted for the water-wall tube operated for a long period under the low air-ratio condition in actual MSW incineration plants in Tokyo Metropolitan, with relation to the environmental analyses and with comparing with the case of high O2 combustion of 10% O2. Low air-ratio combustion was found to cause the significant temperature increment in water-wall tube at the secondary combustion zone together with an increased thermal fluctuation so as to enhance various modes of the scale degradation such as exfoliation, adhesion reduction, and/or cracking, which result in the much increased thinkness of the multiple-layered and non-protective scale. On the other hand, no correlation was observed between the much enhanced water-wall tube corrosion and concentration of the specific corrodants possible for forming the low-melting eutectics such as chlorides. Such an increased tube temperature also could bring about the promotion of the complicated corrosion by both gaseous species and molten salt of mainly chlorides as compared with the case of high O2 combustion. Possible corrosive failure mechanism of water-wall tube induced by the low air-ratio combustion was discussed to some detail.
