Deposit Formation in the FASAN WtE Boiler as a Function of Feedstock Composition and Boiler Operation†

Abstract

The Danish strategy for waste management is to increase recycling while simultaneously reducing the volume of land-filled waste, in order to avoid loss of resources. An important part of this strategy is waste incineration on a grate, which is a well established thermal treatment technology in Denmark and in several other countries in Europe, North America, and Japan. The method is flexible with respect to operation, allows for recovery of energy, and reduces the volume of solid waste significantly (typically by a factor of 8-10). Furthermore, new and advanced flue gas cleaning technologies ensure very low emissions from modem incineration plants. The presence of inorganic constituents such as alkali metals, Cl, S, and heavy metals is a serious challenge in waste incineration, both with respect to operational [slagging, fouling, and corrosion] and environmental issues. Metals, S, Cl, and ash (oxide species) released or entrained from the grate may cause ash deposition of chemically very aggressive species such as chlorides and sulfates, and thereby promote subsequent corrosion of heat transfer surfaces in the plant. As a consequence, the steam temperatures in superheaters of waste-to-energy (WtE) plants, are usually kept relatively low, that is, in the range 400-430 °C, in order to avoid the excessive corrosion problems usually experienced by deposited alkali and heavy metal chlorides at higher steam temperatures. However, low final steam temperatures means low electrical efficiency of the plant. The present work focuses on deposit formation in the 22 MW th FASAN WtE plant, Naestved, Denmark, with particular focus on the influence of feedstock composition (i.e., firing of different waste types) and operation of the plant on ash deposition. Is it possible to affect deposit amount (i.e., flux of deposit), and chemistry, by playing with the fuel feedstock composition and/or the grate operation? Samples of bottom ash, fly ash, aerosols, deposits, and flue gas were taken while firing different waste fractions or while changing the grate operation (air distribution, oxygen content, length of fire on the grate), respectively. The formation of deposits was then quantified by air-cooled probes, and the chemical analyses of the different deposits were compared and related to the feedstock composition and the operation of the grate, through boiler mass balances. The results indicated that the grate operation may indeed affect the chemical composition and the extent [i.e., the flux (g deposit/ area/time)] of deposit formation significantly, whereas the feedstock in the actual campaign did-with shredder waste and PVC as important exceptions-not differ enough from the base waste fired to cause any serious changes in the extent [flux] and chemistry of the deposit.