The Forcing of Mercury Oxidation as a Means of Promoting Low-Cost Capture

Abstract

Trace amounts of mercury are found in all coals. During combustion this mercury is vaporized and can be released to the atmosphere. This has been a cause for concern for a number of years, and has resulted in a determination by the EPA to regulate and control these emissions. Present technology does not, however, provide inexpensive ways to capture or remove mercury from flue gases.The mercury that exits the furnace in the oxidized form (HgCl2) is known to much more easily captured in existing wet pollution control equipment (e.g., wet FGD for SO2), principally due to its high solubility in water. Until recently, however, nobody knew what caused this oxidation, or how to promote it. Recent DOE-funded research in our group, along with work by others, has identified the gas phase mechanism responsible for this oxidation. The scenario is as follows. In the flame the mercury is quantitatively vaporized as elemental mercury. Also, the chlorine in the fuel is released as HCl. The direct reaction Hg+HCl is, however, far too slow to be of practical consequence in oxidation. The high temperature region does supports a small concentration of atomic chlorine due to disassociation of HCl. As the gases cool (either in the furnace convective passes, in the quench prior to cold gas cleanup, or within a sample probe), the decay in Cl atom is constrained by the slowness of the principal recombination reaction, Cl+Cl+M  Cl2+M. This allows chlorine atom to hold a temporary, local superequilibrium concentration. Once the gases drop below about 550°C, the mercury equilibrium shifts to favor HgCl2 over Hg, and this superequilibrium chlorine atom promotes oxidation via the fast reactions Hg+Cl+M  HgCl+M, HgCl+Cl+M  HgCl2+M, and HgCl+Cl 2  HgCl2+Cl. Thus, the high temperature region provides the Cl needed for the reaction, while the quench region allows the Cl to persist and oxidize the mercury in the absence of decomposition reactions that would destroy the HgCl2.Promoting mercury oxidation is one means of getting high-efficiency, "free" mercury capture when wet gas cleanup systems are already in place. The chemical kinetic model we developed to describe the oxidation process suggests that oxidation can be promoted by introducing trace amounts of H2 and/or CO within the quench region. The reaction of these fuels leads to free radicals that promote the selective conversion of HCl to Cl, which can then subsequently react with Hg.The work reported here from our Phase I Innovative Concept grant demonstrated this phenomenon, but it also showed that the process must be applied carefully to avoid promoting the recombination of Cl back to HCl. For example, addition of H2 at too high a temperature is predicted to actually decrease Cl concentrations via Cl+H2HCl+H. At lower temperatures this reaction is slowed due to its activation energy. Thus, within the correct window, the process becomes selective for Cl promotion. Key parameters are the injection temperature of the promoter, the amount of the fuel added.A successful process ba...

Oxidation of Mercury in Products of Coal Combustion

“…Krishnakumar and J. J. Helble, personal communication, 2008). Based on the simulations of the homogeneous system by Dr. Krishnakumar, CO is not expected to have a significant effect on the gas-phase oxidation of mercury over the entire post-combustion temperature range in coal-fired electric utility boilers, from the furnace exit to the stack (1177 to 77 o C), consistent with the experimental measurements reported by Kramlich and Castiglone (2004). The effect of CO, in the homogeneous system, is expected to be a slight promotion of mercury oxidation, rather than the inhibition of mercury oxidation observed in the present experiments with SCR catalyst.…”