Release of Corrosive Species above the Grate in a Waste Boiler and the Implication for Improved Electrical Efficiency

Abstract

A relatively low electrical efficiency of 20-25% is obtained in typical west European waste boilers. Ash species released from the grate combustion zone form boiler deposits with high concentrations of Cl, Na, K, Zn, Pb, and S that cause corrosion of superheater tubes at high temperature. The superheater steam temperature has to be limited to around 425°C, and thereby, the electrical efficiency remains low compared to wood or coal-fired boilers. If a separate part of the flue gas from the grate has a low content of corrosive species, it may be used to superheat steam to a higher temperature, and thereby, the electrical efficiency of the plant can be increased. In this study, the local temperature, the gas concentrations of CO, CO 2 , and O 2 , and the release of the volatile elements Cl, S, Na, K, Pb, Zn, Cu, and Sn were measured above the grate in a waste boiler to investigate if a selected fraction of the flue gas could be applied for increased steam superheating. On a 26 ton/h grate-fired waste boiler, Vestforbraending unit 5 in Denmark, local probe measurements were performed in five ports along the grate and in the top of the boiler chamber. New extraction probe equipment were designed and used to extract a flue gas with high contents of tar. Gas concentration measurements of O 2 , CO, and CO 2 showed that the waste experienced pyrolysis and combustion on grate sections 1 and 2, some char combustion takes place on section 3, and the slag was cooled on sections 4 and 5. The measurements showed that the waste grate combustion process can provide a flue gas with a high energy content and a relatively low concentration of corrosive species. This opens up for the possibility of using an additional superheater section to increase the steam temperature and, thereby, increase electrical efficiency.

Surrogate Reaction Mechanism for Waste Incineration and Pollutant Formation

“…Validation of the predicted release of sulfur-containing species against measurements of SO 2 and sulfur by Bøjer et al …”

“…The results are compared to measurements and the outcome of an empirical model to understand the capabilities of the model. For this comparison, the NO x precursor measurements by Jepsen et al 6 at two operating conditions, corrosive species and NO measurement by Bøjer et al, 7 and the empirical model by Jell et al 9 serve as a reference. Because no UCA analysis is available for the studies by Bøjer et al 7 and Jell et al, 9 MSW1 and MSW3 (UCA analysis from Jepsen et al 6 ) are tested.…”

“…For this comparison, the NO x precursor measurements by Jepsen et al 6 at two operating conditions, corrosive species and NO measurement by Bøjer et al, 7 and the empirical model by Jell et al 9 serve as a reference. Because no UCA analysis is available for the studies by Bøjer et al 7 and Jell et al, 9 MSW1 and MSW3 (UCA analysis from Jepsen et al 6 ) are tested. The compositions are given in Table 7.…”

“…Still, as a result of the large size of waste incineration plants and limited access to the furnaces, species concentration measurements are usually limited to a certain range of the fuel bed and some single selected points in the flue gas. For example, Jepsen et al and Bøjer et al reported species measurements above the grate of full-scale WtE plants using water-cooled probes with a length of up to 6 m. Jepsen et al measured major combustion products and fuel NO x precursors at four points along the grate, and Bøjer et al detailed corrosive species at five incidences. Razmjoo et al presented species concentrations within the fuel bed of a comparable small plant and woody residues.…”

Chemical Model for Thermal Treatment of Sewage Sludge

“…For all thermal treatment processes, the varying feedstock is one of the key challenges. Here, the most relevant properties are the moisture content, due to the high energy demand for drying [6,12], the ratio of volatile components and ash-since this is correlated to the lower heating value of the sewage sludge [13]-and the content of elements involved in emission formation processes, that is, nitrogen, sulfur and chlorine [14][15][16][17][18]. To handle this challenging fuel, often sewage sludge is mixed with other solid fuels, such as biomass wastes, in co-gasification [19] and co-incineration in the cement industry [4,20].…”