CMA

43 MSW Incineration ash 3.1 Types and generation volumes The bottom ash fraction is continually discharged from the combustion chamber and is then cooled (often by water spray). This fraction is classified as non-hazardous waste. The bottom ash typically represents around 20-30 per cent of the original waste feed by weight, only about 10 per cent by volume. The amount of ash will depend on the level of waste pre-treatment prior to entering the incinerator and will also contain metals that can be recovered for recycling. The use of bottom ash in road constructions is a recovery application area for this waste material. However, in some countries, such application is not accepted by the environment authorities (e.g., Norway), for which the remaining option is landfilling. Flue gas cleaning is often a significant contributor to overall incineration costs (approximately 15% to 35 % of the total capital investment). The emissions limits for specific pollutants that are present in the combustion products (flue gases) are defined by national standards (see table 5). The clean-up of the flue gases will produce solid residues comprising fly-ash, lime/ bicarbonate, and carbon. These residues are usually combined (although several systems may separate fly ash and other components), often referred to as Air Pollution Control (APC) residues, and are classified as hazardous waste; therefore, their disposal must be undertaken in accordance with relevant regulations and guidance. Typically, the weight of APC residues produced will be around 2-6 per cent of the weight of the waste entering the incinerator. In fluidized bed furnace, the lower (but more even) operational temperatures, nature of the fuel and process in fluidized beds mean that a greater proportion of volatile heavy metals remain in the bottom ash. Consequently, concentrations of heavy metals in the flue-gas residues are reduced. However, sometimes there are problems with Cr (VI) levels in the soluble part of the bottom ash. Furthermore, the degree of vitrification of the ash may be reduced and the burnout may be improved. Majority of solid residue from fluidized bed incineration is fly ash, which, according to conditions and applied fluidized bed technology, can form up to 90 % of the total ash residue. The bottom ash is also mixed with fluidized bed material (e.g. sand, additives for desulphurization etc.). When waste or RDF is burnt in a rotating fluidized bed the ratio of bottom ash to fly ash is about 50:50. 3.2 Chemical composition MSWI ash compositions are expected to be variable based on geographic location and variables such as waste stream characteristics and combustion technologies. Chlorides and alkalis in MSW ash are primarily contributed by food waste, plastic, ceramics and glass, and these constituents tend to form volatile organic and inorganic salts at MSWI combustion temperatures; airborne salts then condense in the cooler portions of the APC equipment and are thus concentrated in the fly ash stream. Table 6. Concentration range (µg/kg) of organic species in MSWI residues (Vehlow, 2002) Substance Bottom ash Boiler ash Filter ash PCDD/F (I-TEQ) <0.001 to 0.01 0.02 to 0.5 0.2 to 10 PCB <5 to 50 4 to 50 10 to 250 PCBz <2 to 20 200 to 1000 100 to 4000 PCPh <2 to 50 20 to 500 50 to 10000 PAH <5 to 10 10 to 300 50 to 2000 PCBz: Poly Chlorinated Benzenes; PCPh: Poly Chlorinated Phenyls Typical concentrations of organic compounds in the various solid residues are compiled in Table 6. The data are collected from ash residues generated in modern facilities. It is evident that volatiles are accumulating in the ash residues at increasing distance from the furnace. This is due to the decreasing temperature which increases desorption of volatile substances onto the surface of the flue gas particles. Typical concentration range of metals for the fly ash collected from a modern Czech MSWI plant is given in Table 7. The concentrations for some of the toxic metals are sometimes significantly higher than the limits for hazardous waste (e.g. Norwegian limits).