Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 89

During Coal Combustion

chlorination pathways to date as well as a pathway involving HgO, with model
predictions in good accord with experimental data for higher reactor temperatures
but drastic under-prediction of mercury chlorination for lower temperatures.
Niksa [7] developed and evaluated an elementary reaction mechanism for homoge-
neous Hg 0 oxidation with an emphasis on major interactions among Cl-species and
other pollutants in coal-derived exhausts. Xu et al . [37] also developed the kinetic
model consisting of 107 reactions and 30 species. This kinetic model included the
oxidation and chlorination of key flue-gas components, as well as six mercury
reactions involving HgO with new reaction rate constants calculated neither from
experimental data nor by estimation, which was commonly used by other investi-
gators before, but directly from transition state theory (TST). Furthermore, ap-
proximately 10% of the mercury was observed to be present as HgO at lower
temperature as seen by Gullett et al . [38] . Senior et al . [1] suggested that HgO pro-
duction might be significant for coals with low chlorine content. Therefore a me-
chanism for HgO should be involved, especially under conditions of high oxygen
concentration and low chlorine concentration at low temperature.
Consequently, based on the achievements and efforts beforehand, the homoge-
neous mercury speciation in combustion-generated flue gases was modeled by a
detailed kinetic model consisting of 94 reactions and 31 species which involved free
radicals i.e.O 3 , OH, O and other reaction agents. This kinetic model included the
oxidation and chlorination of key flue-gas components from the NIST database
which were well recognized and verified by the experimental data by Sliger et
al . [6,28,34] .
Furthermore, 14 mercury reactions were proposed, involving Hg chlorination
(HgCl, HgCl 2 ) which were built by Widmer and West [3] (Reactions from Nos. 1 8

as tabulated in Table 4.17) and also commonly used by other investigators before.
And the important and previously unrecognized pathway of homogeneous the Hg
oxidation mechanism concerning Hg reactions involving HgO was developed as
well. HgO reactions are listed from Nos. 913 in Table 4.17. The detailed kinetics
model consists of 31 species, i.e. C, CO, CO 2 , Cl, Cl 2 , ClO, ClO 2 , H, H 2 , HCl, H 2 O,
HO 2 , O 3 , H 2 O 2 , HOCl, O, O 2 , OH, N 2 , Hg, HgCl, HgCl 2 , HgO, SO 2 , NO, H 2 S, S,
NO 2 , NO 3 , HNO and HNO 3.
The formulation of the homogeneous mercury reaction mechanism started with
the kinetic framework of Widmer et al ., the key flue-gas components and together
with our work with HgO reaction as tabulated in Table 4.17 [3] . The rate coefficients
are in the modified Arrhenius form, k = AT exp( E a /( RT )). The external factors
considered involved a temperature from 400 K to 1800 K and constant pressure of
1.0 atm. And the sub-mechanisms were taken directly from the literature and used
without modification. Sub-mechanisms involving other key gas components typi-
cally found in flue gas included Cl/H/OH reaction systems developed by Baulch et
al. [39] . They were adopted here, and other sub-mechanisms include those of At-
kinson et al . [40] for Cl/HO 2 /HOCl systems, Tsang et al . for H/HO 2 /HNO/OH sys-
tems, Boughton et al . for HNO 3 /H/NO reaction systems and other reaction systems
in Table 4.17 in their entirety. In total, 94 elementary reactions and 31 species were
involved in this present kinetic model.