Fate and abatement of mercury and other trace elements in a coal fluidised bed oxy combustion pilot plant

Abstract The fate of trace elements and the properties of oxy-combustion residues were evaluated in a 90 kWth bubbling fluidized bed (BFB) oxy-combustion pilot plant fed with coal and limestone (bed material). Sampling of solid streams (coal, limestone, bottom ash, cyclone fly ash, and bag-filter fly ash) and exhaust gas (SOx, NOx, CO2, CO, and Hg) was undertaken through 4 h operating at 70% load, 70:30 CO2/O2 ratio of raw gas and 800–820 °C in a single (without re-circulation) oxy-combustion test run. The retention of SO2 (98.2 %), NOx (99.9%), and Hg (92.5%) and remaining elements (99.9%) attained high efficiencies while low retention for Cl is achieved (19%). Speciation of gaseous Hg revealed that most of the gaseous Hg occurs in the exhaust gas as Hg0. The proportions of Cl and Hg in the exhaust gas may be considered of concern with respect to subsequent CO2-rich gas treatment and oxy-combustion cycles by partial re-circulation of exhaust gas. The high abatement capacity for most of elements (Hg, S, NH 4 + , F, Se, As, B, Se, Cd, and Sn) is mostly due to condensation in bag filters. The low gas temperature (45 °C), and high-Ca content promotes condensation on fly ash collected in the bag filters. The Hg gaseous speciation and the enrichment in bag filters is indicative that most of the Hg retained is Hg2+, most probably as Hg-sulphate species. Calcite and lime are the main crystalline phases in bottom ash, quartz, illite, and calcite in cyclone fly ash and gypsum–bassanite in bag-filter fly ash. The dissolution of these species produces a moderate alkalinity (pH = 10.7–11.1) in bottom ash and cyclone fly ash leachates and slight (pH = 8) in bag-filter fly ash leachates. The leaching potential of most of elements in oxy-combustion residues is relatively low, falling in the range of non-hazardous materials according to Decision 2003/33/EC. Nevertheless, the high condensation of elements of environmental concern coupled with the use of a fuel rich in the aforementioned elements would increase the leachable potential, and consequently, may affect the disposal of this type of fly ash.