Material analysis of Bottom ash from waste-to-energy plants.

Bottom ash (BA) from waste-to-energy (WtE) plants contains valuable components, particularly ferrous (Fe) and non-ferrous (NFe) metals, which can be recovered. To assess the resource recovery potential of BA in the Czech Republic, it was necessary to obtain its detailed material composition. This paper presents the material composition of BA samples from all three Czech WtE plants. It was found that the BA contained 9.2-22.7% glass, 1.8-5.1% ceramics and porcelain, 0.2-1.0% unburnt organic matter, 10.2-16.3% magnetic fraction, 6.1-11.0% Fe scrap, and 1.3-2.8% NFe metals (in dry matter). The contents of individual components were also studied with respect to the BA granulometry and character of the WtE waste collection area.

Possibilities of mercury removal in the dry flue gas cleaning lines of solid waste incineration units.

Dry methods of the flue gas cleaning (for HCl and SO2 removal) are useful particularly in smaller solid waste incineration units. The amount and forms of mercury emissions depend on waste (fuel) composition, content of mercury and chlorine and on the entire process of the flue gas cleaning. In the case of high HCl/total Hg molar ratio in the flue gas, the majority (usually 70-90%) of mercury is present in the form of HgCl2 and a smaller amount in the form of mercury vapors at higher temperatures. Removal of both main forms of mercury from the flue gas is dependent on chemical reactions and sorption processes at the temperatures below approx. 340 °C. Significant part of HgCl2 and a small part of elemental Hg vapors can be adsorbed on fly ash and solid particle in the air pollution control (APC) processes, which are removed in dust filters. Injection of non-impregnated active carbon (AC) or activated lignite coke particles is able to remove mainly the oxidized Hg(2+) compounds. Vapors of metallic Hg(o) are adsorbed relatively weakly. Much better chemisorption of Hg(o) together with higher sorbent capacity is achieved by AC-based sorbents impregnated with sulfur, alkali poly-sulfides, ferric chloride, etc. Inorganic sorbents with the same or similar chemical impregnation are also applicable for deeper Hg(o) removal (over 85%). SCR catalysts convert part of Hg(o) into oxidized compounds (HgO, HgCl2, etc.) contributing to more efficient Hg removal, but excess of NH3 has a negative effect. Both forms, elemental Hg(o) and HgCl2, can be converted into HgS particles by reacting with droplets/aerosol of poly-sulfides solutions/solids in flue gas. Mercury captured in the form of water insoluble HgS is more advantageous in the disposal of solid waste from APC processes. Four selected options of the dry flue gas cleaning with mercury removal are analyzed, assessed and compared (in terms of efficiency of Hg-emission reduction and costs) with wet methods and retrofits for more efficient Hg-removal. Overall mercury removal efficiencies from flue gas can attain 80-95%, depending on sorbent type/impregnation, sorbent surplus and operating conditions.