CHARACTERIZATION OF FLUE GAS, FLY ASH, AEROSOL AND DEPOSIT COMPOSITIONS AS A FUNCTION OF WASTE COMPOSITION AND GRATE OPERATION

摘要

The Danish strategy for waste management is still to increase recycling and on the same time to reduce the volume of land-filled waste, in order to avoid loss of resources, and waste incineration is an important part of this strategy. In 2004, 26 % of the total reported Danish waste production was incinerated. However, a main environmental concern for waste incineration is the leaching of hazardous elements from the solid residues. In addition, some elements may constitute operational problems, as they may accelerate the deposition and corrosion processes in furnace. In the present work, a full-scale measuring campaign have been conducted at a 22 MWth waste incinerator, in order to investigate the effects of waste composition and plant operation on formation and characteristics of ashes and deposits. The present work focuses on the characterization of flue gas, deposits, fly ash and aerosols, sampled during various test runs. A base-load waste consisting of 80 % household waste and 20 % small combustibles was used as reference fuel in the test runs. Dedicated, well-characterized waste fractions containing high concentrations of potentially harmful elements such as chloride, alkali metals and/or heavy metals, was then mixed with the reference fuel in the individual test runs. The dedicated waste fractions comprised NaCl (road salt), batteries, automotive shredder waste, CCA (Copper-Chromate-Arsenate)-impregnated wood, PVC plastics, and (leather) shoes. Test runs with varying operational parameters, e.g. minimum or maximum oxygen content, were carried out as well. The flue gas main composition (O_2, CO_2, CO, NO, SO_2) was measured on-line during the test runs, and fly ash and aerosols was sampled from the flue gas duct, upstream of the flue gas cleaning system. Deposits were collected on a cooled probe in the top of the furnace, at the entrance to the superheater section. Increased concentrations of certain elements of interest were observed in the fly ash and aerosols when firing dedicated waste fractions. The more pronounced effects were seen for the elements As when firing CCA impregnated waste wood (25 times increase in concentration in fly ash compared to the reference experiments), and for Hg when firing batteries (10 times increase in concentration in fly ash). These observations were in consistence with an increased concentration of the respective elements in the specific waste fractions. The concentration of Cd, Hg, Pb, Zn and Cu was found to increase with decreasing particle size, whereas the concentration of Ni and Cr decreased, indicating surface condensation of the more volatile elements on the smallest particles. A positive correlation between Cl concentration and concentration of the trace elements Pb, Zn, Cd and Cu was found in the fly ash fractions sampled at app. 175 ℃, but not in the deposits sampled at 400 ℃. This indicated condensation of metal chlorides at lower temperatures. It was concluded that the composition of the input waste affected the composition of the fly ash and deposits, whereas no clear trends were seen for changing operational conditions.