Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 69

During Coal Combustion

mercury removal efficiency in the desulfurization tower is strongly dependent on
the mercury species, reaction time (retaining time), other flue gas components,
temperature, etc.

4.5 Modeling Research of Mercury Speciation Transformation
During Coal Combustion

4.5.1 Introduction

Field test studies showed 5% to 95% of the mercury emitted in gaseous form from
coal-fired plants [31,32] . At present, with the inconvenience of sampling mercury at
higher temperature conditions (over 400 °C), it is difficult for us to determine how
mercury is escaping from coal, what is the mercury concentration and speciation in
the emissions, i.e. gaseous Hg 0 or Hg 2+ is not quantitatively known during the coal
combustion process. The field test for mercury formation and oxidation at high
temperature was seldom reported. Sliger et al . [6] observed and measured mercury
oxidation from a furnace operating between 860 °C and 1,171 °C. In this work, the
possible elementary reactions that may lead to oxidation were reviewed and a
chemical kinetic model was proposed as well. As the model indicated, Cl was the
key oxidizing species and the oxidation was limited to a temperature between 700
°C and 400 °C. The oxidation was governed primarily by HCl concentration,
quench rate and other gas composition. The mercury distribution at a higher tem-
perature requires further observation and the mercury oxidation mechanism has also
been a query up till now.
Hou et al . [33] reported that mercury oxidation was simulated in the atmosphere
with different concentrations of Cl 2 and HCl using CHEMKIN3.7 coupled with
FLUENT6.2 software. The three dimensional concentration distribution of mercury
within the cylindrical stack and the impact of the temperature on mercury oxidation
were also obtained. Further, the simulation result showed that even a small amount
of Cl 2 is much more effective in oxidation of Hg 0 than HCl, the temperature for the
higher oxidizing rate of the Hg 0 focused on 950 1,150 K. Although the simulation
trial seemed successful, the simulation of mercury is limited to within the cylin-
drical stack, just a beginning of simulation work. The operating condition has much
greater discrepancy in a real coal-fired power plant, while this simulation work
above indicates that the coupling calculation can solve the problem of the combi-
nation of computational fluid dynamics (CFD) with the complex kinetic mecha-
nism.
With the increasing maturity of the CFD technique and computing power, it is
possible to have a numerical simulation of the field of application, such as distri-
bution of temperature, velocity and formation of pollutants available in 3-D. This
study was employed to demonstrate the mechanism of formation, distribution,