Effects of NOx, α-Fe2O3, γ-Fe2O3, and HCl on mercury transformations in a 7-kW coal combustion system

Bench-scale investigations indicate that NO, NO2, hematite (alpha-Fe2O3) maghemite (gamma-Fe2O3), and HCl promote the conversion of gaseous elemental mercury (Hg-0) to gaseous oxidized mercury (Hg2+) and/or particle-associated mercury (Hg[p]) in simulated coal combustion flue gases. In this investigation, the effects of NOx, alpha-Fe2O3, gamma-Fe2O3, 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 Hg2+-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 (160-195degreesC) and outlet (110-140degreesC) locations of the system. As expected, the baseline Blacksville flue gas was composed predominantly of Hg2+ (Hg2+/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 NO2 (80-190 ppmv) at 440-880degreesC and alpha-Fe2O3 (15 and 6 wt.%) at 450degreesC into Absaloka and Falkirk coal combustion flue gases did not significantly affect Hg speciation. The lack of Hg-0 to Hg2+ conversion suggests that components of Absaloka and Falkirk combustion flue gases and/or fly ashes inhibit heterogeneous Hg-0-NOx-alpha-Fe2O3 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 Hg2+, HCl, and gamma-Fe2O3 in Blacksville flue gas and the inertness of injected alpha-Fe2O3 with respect to heterogeneous Hg-0 oxidation in Absaloka and Falkirk flue gases suggested that gamma-Fe2O3 catalyzes Hg2+ formation and that HCl is an important Hg-0 reactant. The filtration of Absaloka and Falkirk combustion flue gases at 150degreesC through fabric filters with approximate to 60 g/m(2) gamma-Fe2O3 indicated that about 30% of the Hg-0 in Absaloka and Falkirk flue gases was converted to Hg2+ and/or Hg(p). HCl injection (100 ppmv) into the Absaloka combustion flue gas converted most of the Hg-0 to Hg, whereas HCl injection into the Falkirk flue gas converted most of the Hg-0 and Hg2+ to Hg(p). Additions of gamma-Fe2O3 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) gamma-Fe2O3 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. (C) 2004 Elsevier B.V. All rights reserved.