Assessment of utilization options for MSW gasification bottom ash

Bottom ash (BA) is the residue produced in greatest amount from incineration of municipal solid wastes. Several countries allow reuse of BA for civil engineering applications; however, potential leaching of heavy metals from BA is one of the main constraints to this opportunity. Therefore, usually BA need to be processed (e.g. by inertization) before landfilling or reuse, in order to avoid pollution arising from heavy metals leaching (Sabbas et al., 2003; Reijnders, 2005, Ferraris et al., 2009). This requirement increases the costs of the entire combustion treatment process (Ferraris et al., 2009). This paper presents the first results of a wider research activity which aims to investigate the characteristics of BA from two municipal Refuse Derived Fuel (RDF) gasification plants. For instance, both technical and environmental parameters were determined, including: grain size distribution, water content and major anions contents, elemental composition. Besides, leaching characteristics were also determined by means of the EN 12457-2 batch leaching test, the TS 14429 acid neutralization capacity (ANC) test and the Availability test (NEN 7371). Data obtained from the analysis of chemical composition, EN 12457-2 batch leaching test and the percentage by weight of each particle size were used to predict a mean metals content of the raw bottom ash, by means of the Monte Carlo statistical method. A great number of hypothetical samples were generated considering 5% of uncertainty for each grain size class fraction by weight; the minimum and maximum concentration of each metal was determined for every sample. The lowest and the highest concentration values obtained for each element represent the limits of the expected ranges. Results from EN-12457 batch leaching test were compared with the Italian law limits for either landfilling (as inert or not hazardous waste) or reuse. One of the two analyzed BA samples (named A) showed levels of heavy metals always below the Italian regulatory limits for reuse (except for Cu in the fraction >1mm), the other BA sample (named B) displayed concentrations of Pb (only for the grain size in the range 1÷2 mm), Cu, Cr and Ni above the limits for reuse and of Pb, Cu and Cr above the limits for inert waste landfill. The different behaviour of these residues was attributed to the characteristics of the RDFs delivered to the gasification plants; also the wear of the furnace refractories might have exerted some influence (Ecke et al., 2001, Kwak et al., 2006). The vitrified structure of the tested residues, as a consequence of the high operating temperature of the gasification plants, reduced availability of metals for leaching. The formation of a glassy material decreases the surface area and reduces the mobility of metals, which are entrapped within the amorphous matrix (Ferraris et al., 2009). Results from ANC test confirmed the glassy structure of the residues: a sharped decrease of pH with the addition of acid indicated that the tested BA were made up of a larger proportion of glassy materials with hydrophobic characteristics (Johnson et al., 1995). Tests results demonstrated that some kind of pre-treatment could be necessary for the residues to fulfill heavy metals leaching law limits. The utilization of size separation as pre-treatment was also discussed.