Effects of NO x, α-Fe 2O 3, γ-Fe 2O 3, and HCl on mercury transformations in a 7-kW coal combustion system

Abstract

Bench-scale investigations indicate that NO, NO 2, hematite (α-Fe 2O 3), maghemite (γ-Fe 2O 3), and HCl promote the conversion of gaseous elemental mercury (Hg 0) to gaseous oxidized mercury (Hg 2+) and/or particle-associated mercury (Hg[p]) in simulated coal combustion flue gases. In this investigation, the effects of NO x , α-Fe 2O 3, γ-Fe 2O 3, and HCl on Hg transformations were evaluated by injecting them into actual coal combustion flue gases produced from burning subbituminous Absaloka and lignitic Falkirk coals in a 7-kW down-fired cylindrical furnace. A bituminous Blacksville coal known to produce an Hg 2+-rich combustion flue gas was also burned in the system. The American Society for Testing and Materials Method D6784-02 (Ontario Hydro method) or an online Hg analyzer equipped to measure Hg 0 and total gaseous mercury (Hg[tot]) was used to monitor Hg speciation at the baghouse inlet (160–195 °C) and outlet (110–140 °C) locations of the system. As expected, the baseline Blacksville flue gas was composed predominantly of Hg 2+ (Hg 2+/Hg[tot]=0.77), whereas Absaloka and Falkirk flue gases contained primarily Hg 0 (Hg 0/Hg[tot]=0.84 and 0.78, respectively). Injections of NO 2 (80–190 ppmv) at 440–880 °C and α-Fe 2O 3 (15 and 6 wt.%) at 450 °C into Absaloka and Falkirk coal combustion flue gases did not significantly affect Hg speciation. The lack of Hg 0 to Hg 2+ conversion suggests that components of Absaloka and Falkirk combustion flue gases and/or fly ashes inhibit heterogeneous Hg 0–NO x –α-Fe 2O 3 reactions or that the flue gas quench rate in the 7-kW system is much different in relation to bench-scale flue gas simulators. An abundance of Hg 2+, HCl, and γ-Fe 2O 3 in Blacksville flue gas and the inertness of injected α-Fe 2O 3 with respect to heterogeneous Hg 0 oxidation in Absaloka and Falkirk flue gases suggested that γ-Fe 2O 3 catalyzes Hg 2+ formation and that HCl is an important Hg 0 reactant. The filtration of Absaloka and Falkirk combustion flue gases at 150 °C through fabric filters with ≈60 g/m 2 γ-Fe 2O 3 indicated that about 30% of the Hg 0 in Absaloka and Falkirk flue gases was converted to Hg 2+ and/or Hg(p). HCl injection (100 ppmv) into the Absaloka combustion flue gas converted most of the Hg 0 to Hg 2+, whereas HCl injection into the Falkirk flue gas converted most of the Hg 0 and Hg 2+ to Hg(p). Additions of γ-Fe 2O 3 and HCl did not have a synergistic effect on Hg 0 oxidation. The filtration of Absaloka and Falkirk flue gases through much greater fabric filter loadings of 475 g/m 2 γ-Fe 2O 3 essentially doubled the baghouse Hg[tot] removal efficiency to about 50%. Results from this investigation demonstrate the importance of evaluating potential Hg 0 reactants and oxidation catalysts in actual coal combustion flue gases.