1. Background

Mercury emitted from coal combustion is oxidised at relatively lower temperatures in the flue gas stream by the chlorine and bromine species that are also released from the coal combustion process. Oxygen-containing radicals such as O and OH do not play a significant role in mercury oxidation other than to influence the radical pool distribution. The approach to model the oxidation processes is to take the base reaction mechanisms of mercury interaction with chlorine, and the analogous reactions with bromine, then combine these with a simple gas phase chemical kinetic mechanism that describes the chemistry of the flue gas composition, along with mechanisms that define the remaining halogen chemistry. Barring an experimentally determined initial composition, the initial conditions of the flue gas stream in terms of the chlorine, bromine, carbon, hydrogen, nitrogen and oxygen partitioning amongst the possible species, can be estimated based on an equilibrium distribution amongst the constituent species at a defined temperature, pressure, and major stable species composition.

2. Mercury – chlorine – bromine kinetic mechanism

An 8 step mercury – chlorine reaction mechanism as originally proposed by Widmer at al. [1] is given in the following reactions 1 to 8. An analogous set of reactions defines the mercury – bromine chemistry in reactions 9 to 16.

Hg + Cl + M ⇌ HgCl + M        (1)      Hg + Br + M ⇌ HgBr + M         (9)

Hg + Cl₂ ⇌ HgCl + Cl             (2)      Hg + Br₂ ⇌ HgBr + Br            (10)

Hg + HCl ⇌ HgCl + H            (3)      Hg + HBr ⇌ HgBr + H             (11)

Hg + HOCl ⇌ HgCl + OH       (4)       Hg + HOBr ⇌ HgBr + OH       (12)

HgCl + Cl + M ⇌ HgCl₂ + M    (5)      HgBr + Br + M ⇌ HgBr₂ + M   (13)

HgCl + Cl₂ ⇌ HgCl₂ + Cl         (6)      HgBr + Br₂ ⇌ HgBr₂ + Br       (14)

HgCl + HCl ⇌ HgCl₂ + H        (7)      HgBr + HBr ⇌ HgBr₂ + H       (15)

HgCl + HOCl ⇌ HgCl₂ + OH   (8)      HgBr + HOBr ⇌ HgBr₂ + OH  (16)

Rate coefficient parameters

For the mercury-chlorine chemistry, reaction 1, and also the rate of chlorine atom recombination are given by Donohue [2]. Rate coefficients for Reactions 2 to 8 are taken from the theoretical study by Krishnakumar and Helble [3], and the remaining reactions of the mechanism from Roesler et al. [4] are used to describe the chlorine chemistry. For the mercury-bromine chemistry, reaction 9 and bromine atom recombination are again given by Donohue [2]. Niksa et al. [5] give the rate coefficient of reaction 12, Wilcox and Okano [6] provide rate coefficient values for 10, 11, 13 to 16, while Molina et al. [7] define the remaining bromine reactions.

There is much uncertainty concerning the rate coefficients of many of these reactions. Therefore, a valid approach is to apply an optimisation procedure in order to identify both those reactions that are especially important and also to provide optimal values of the reaction Arrhenius parameters. From the work of Clements et al. [8], a [GLOSS]genetic algorithm[/GLOSS] optimisation methodology against lab-based target datasets of mercury oxidation as a function of HCl and HBr concentration is used to optimise the most sensitive mercury and halogen reactions. Those reactions modified from their literature sources are given in the following table.

The base C/H/N/O oxidation scheme with which these halogen chemistry reactions are amalgamated is the [GLOSS]GRI-Mech 3.0[/GLOSS][9].

3. Validation

The combination of the reactions described in Section 2, based on GRI-Mech 3.0, with the general halogen chemistry and specific halogen-mercury chemistry, has been validated by Clements et al [8] against experimental datasets produced by Preciado et al. [10]. The capability of the optimised model to predict the experimental results in the case of both HCl and HBr addition, in both air and oxygen (27%) enriched cases is illustrated in the following two figures, showing in general an excellent agreement.