Speciation of combustion-derived particulate phase arsenic

The speciation of inorganic arsenic compounds produced in pulverized coal combustion was investigated in a bench-scale study conducted under fuel-lean combustion conditions. Results suggest that at temperatures of 850-1100 K, and in the absence of particulate, arsenic initially condenses as As2O3 rather than the thermodynamically favored As2O5 Synthetic fly ash in the form of uniform 0.25 mum spherical silica particles produced by sol-gel methods, added to the experimental system to simulate the presence of combustion-derived fly ash, did not affect arsenic speciation, nor did doping of the silica particles with small amounts of calcium. As(V) was only observed at these temperatures when calcium was added to the system as calcium acetate, producing a reactive calcium oxide fume and leading to the formation of calcium arsenate. From these results, it was concluded that the formation of arsenic(V) in combustion systems requires either the oxidation of As2O3, which appears to be a kinetically limited process, or the presence of reactive calcium compounds. Additional experiments conducted with calcium silicate in an isothermal tube furnace suggested that calcium in silicates is capable of reacting with arsenic under long (order minutes) reaction time conditions. The apparent ability of calcium silicate particles to capture arsenic under idealized conditions suggests that calcium embedded in silicate fly ash may be capable of forming calcium arsenate,but the extent of this reaction on the time scales of a pulverized coal combustor or waste incinerator remains unclear.