Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 97

oxide are water-soluble oxidized species, and are more readily removed from flue
gases in scrubber systems. And mercuric sulfate consists of small particle-bounded
species, which would be absorbed and deposited by serous fluid in the desulfuri-
zation tower. Therefore, mercury oxide would be preferred in the stack gases and
eliminated easily.
Modelling and numerical simulation of combustion and mercury species for-
mation, distribution and transformation process in a 410 tons/h coal-fired boiler was
conducted based on the platform supplied by FLUENT6.2 software, integrated with
a sub-model with mercury reaction kinetics. The simulation results were calculated
continuously according to various retention times of the rear flue, which were ob-
tained from the 1-D dynamic model. The simulation data fits well with CEM in-situ
measurement data of mercury species concentrations at a real 250 M. The kind of
CFD model was validated to do the mercury modeling distribution during the
combustion process, and even the 1-D dynamic model for the rear flue worked as
well. The 1-D dynamic model was utilized to do calculation with the platform of
CHEMKIN4.7.
In regard to the 3-D simulation during the combustion process, the mechanism
of mercury species formation, reaction, oxidation, transformation was employed to
study, complicated phenomena of physical or chemical reaction were revealed in
the furnace, esp. mercury species at higher temperatures, divalent mercury oxide
(HgO) exists as well except for Hg 0 , which differed with the previous literature,
which helps to limit mercury emission in an appreciate way efficiently and eco-
nomically. In some cases, activated carbon was injected into the furnace to absorb
the mercury possibly.
In view of the simulation data agreeing well with measurement in-situ data, the
simulation work seems to be accurate and reliable both with the 3-D model in the
furnace and the 1-D dynamic model. In terms of the CFD simulation result, mercury
exists predominantly in gaseous Hg 0 in the furnace, a little amount of HgO and
HgCl 2 coexist. Stemming from three-dimensional integration, Hg 0 is 94.22% of the
total amount, HgO, HgCl 2 are 4.96%, 0.82%, respectively.
It is inferred that HgO formation has a close correlation with O 2 , HgCl 2 forma-
tion has a comparatively strong correlation with Cl 2 . which originates from HCl
existing as a dominant Cl element, over 99%. This is attributed to the fact that Cl 2
has an extremely active impact on mercury oxidation instead of Cl.
With the temperature down along the back-end surface, Hg 0 was oxdized into
HgCl 2, especially in a temperature range from 373540 K, HgCl 2 formed swiftly,
and the 10% ratio at the outlet of the furnace was increased to 40.8% rapidly.
Therefore, this kind of CFD model was validated to do the modeling of mercury
distribution during the combustion process, and even a 1-D dynamic model for the
rear flue worked as well.