The release and transformation of inorganic elements during grate-firing of bran was studied via experiments in a laboratory-scale reactor, analysis of fly ash from a grate-fired plant, and equilibrium modeling. It was found that K, P, S, and to a lesser extent Cl and Na were released to the gas phase during bran combustion. Laboratory-scale experiments showed that S was almost fully vaporized during pyrolysis below 700°C. Sixty to seventy percent of the K and P in bran was released during combustion, in the temperature range 900À1100°C. The release of K and P was presumably attributed to the vaporization of KPO 3 generated from thermal decomposition of inositol phosphates, which were considered to be a major source of P and K in bran. The influence of additives such as CaCO 3 , Ca(OH) 2 , and kaolinite on the release was also investigated. Ca-based additives generally increased the molar ratio of the released K/P, whereas kaolinite showed an opposite effect. Thermodynamic modeling indicated that the fly ash chemistry was sensitive to the molar ratio of the released K/P. When the molar ratio of the released K/P was below 1, KPO 3 and P 4 O 10 (g) were the main stable K and P species at temperatures higher than 500°C. Below 500°C, the KPO 3 and P 4 O 10 (g) may be converted to H 3 PO 4 (l), which may cause severe deposit build-up in the economizers of a grate-fired boiler. By increasing the molar ratio of the released K/P to above 2, the equilibrium distribution of the K and P species was significantly changed and the formation of H 3 PO 4 (l) was not predicted by thermodynamic modeling.
This paper describes a new concept for treatment of air pollution-control (APC) residues from waste incineration and characterises the wastewater and stabilised residues generated by the process. The process involves mixing of APC-residues with a ferrous sulphate solution and subsequent oxidation of the suspension (Ferrox-process 1996). The process results in a significant reduction in the leaching of salts and heavy metals from the residue, by washing out most of the salts and by binding the heavy metals in the iron oxides formed. In the laboratory, a semidry gas-cleaning residue and a fly ash were treated by the process. The generated wastewater contained low concentrations of heavy metals (e.g. Pb: 27-39 microg l(-1) and Cd: 2.6-4.6 microg l(-1)), but high concentrations of salts (e.g. Cl, Na, K, and Ca). The treatment process redUced the leaching of Pb from the residues by more than two orders of magnitude at fixed pH as determined by pH-static leaching tests. Likewise, the leaching of Cd, Zn and Cu was significantly reduced. The effect on elements that form oxyanions (e.g. Cr) is marginal and in the current process there is no reduction in the release of Hg.
Waste incineration on a grate is a well-established thermal treatment technology in Denmark and several other countries. The method is flexible with respect to operation, it allows for recovery of energy, it reduces the volume of solid waste significantly (by a factor of 8-10), and advanced flue gas cleaning technologies ensures very low emissions from modern incineration plants. In 2005, 24% of the total reported Danish waste production was incinerated. However, the presence of inorganic constituents such as alkali metals, Cl, S, and heavy metals in the waste constitute an essential challenge in waste incineration, both with respect to operational issues and due to environmental concerns. Formation of acidic pollutants, high mass loading of aerosols, and deposition of potentially corrosive components on heat transfer surfaces are among the encountered problems caused by volatile alkali compounds and heavy metals during combustion, and the potential leaching of heavy metals from the solid residues upon disposal is another major environmental concern. In this work the partitioning of trace elements in a waste-to-energy (WtE) boiler was studied experimentally, based on results from a fullscale measuring campaign at a Danish WtE plant while applying changes in waste composition and grate operation, respectively. The changes in waste composition were applied by adding (one-by-one): dedicated waste fractions, comprising road salt (NaCl), household batteries, automotive shredder waste, CCA (copper-chromate-arsenate)-impregnated wood, PVC, and, shoes, to a base-load waste. Special focus in the present work was on the partitioning of the elements Pb, Zn, Cl, S, Na, and K, which are all considered problematic elements with respect to deposition and corrosion problems. The partitioning of the elements was found to be influenced by various effects, and there was not necessarily a correlation between the input concentration of an element in the feedstock and the amounts recovered in the fly ash and flue gas fractions. The study indicated that Cl and S played an important role for the partitioning of Pb, and maybe also Zn. When firing Cl-rich waste fractions (PVC, salt, shoes), the partitioning of Pb seemed to shift toward increased vaporization (and subsequent recovery in fly ash and aerosol fractions). The full-scale study also implied that organically bound Cl (as in PVC, shoes) was preferably released (as HCl(g)) to the gas phase, whereas the inorganically bound Cl (salt) was recovered in the bottom ash and fly ash fractions (indicating alkali-chloride bonding). Overall, Cl was concluded to be a critical element with respect to deposition and corrosion problems as it (1) enhanced the vaporization of heavy metals, (2) caused increased mass-load of aerosols if present as alkali-chloride in the waste, and (3) caused increased deposition fluxes and increased concentrations of Cl in the deposits.
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