Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 39

Transformation in Flue Gas

istics of mercury emissions were very similar. However, there were some differ-
ences. The proportion of Hg 2+ in the total Hg (g) was from 17% to 54%, and the
average proportion was 69%. The proportion of Hg 0 was from 46% to 83%, and the
average proportion was 31%.
When pulverized coal burned in a state of suspension, the proportion of Hg 2+ in
the total Hg (g) was from 40% to 50%, and the average was 44%. That of Hg 0 was
from 50% to 60%, and the average was 56%. The total Hg (g) concentrations in flue
gas were in the range of 10 - 15 g/(N·m 3 ).
In the fluidized-bed combustion, the proportion of Hg 2+ in the total Hg (g) was
from 30% to 41%, and the average was 35%. The proportion of Hg 0 in the total
Hg (g) was from 59% to 70%, and the average was 65%. The Hg (g) concentrations in
all conditions of flue gas were in the range of 6.8 - 9.3 g/(N·m 3 ), and less than 10
g/(N·m 3 ).
Given the conditions of the three different combustion methods, the speciation
distributions of Hg 2+ and Hg 0 were similar. Coal burning in a fixed bed increased
the Hg 0 concentration, which may be attributed to the shorter gas cooling process
and the shorter reaction time in the experiment. The concentration of Hg (g) , when
using the fluidized-bed combustion method, was slightly lower than that when
using the suspension combustion method. This may be attributed to the high pro-
portion of burning stone coal, the addition of limestone in some conditions, and the
high concentration of fly ash. These factors enhanced the mercury adsorption by
particles, thus reducing the concentration of Hg (g) .

4.3 Research on Reaction Kinetics of Mercury Speciation
Transformation in Flue Gas

Mercury is an extremely volatile element. Most mercury compounds are thermally
unstable and tend to decompose into Hg 0 when the temperature is higher than 700
°C. At temperatures of coal combustion, almost all the mercury is converted into
Hg 0 in the boiler. Therefore, during the coal burning, mercury is converted into
gaseous Hg 0 and flows into the flue gas regardless of its original classification
(organically or inorganically combining state). During the cooling process, the
temperature of flue gas gradually decreased when the gas passed through the heat
transmission equipment. Gaseous mercury underwent a more complicated physical
and chemical reaction with other flue gas components . Finally, Hg 0 usually existed
in all three forms, namely, Hg 0 , Hg 2+ and Hg p .
The Hg 2+ in flue gas was mainly the result of the reaction between chlo-
rine-containing substances (e.g., Cl 2 , HCl, and Cl) and mercury. Therefore, the
mercury speciation in flue gas mainly was HgCl 2 . And mercury also reacted with
O 2 , SO x , and NO x to produce HgO, HgSO 4 , and so on. The research on the oxidation
process of mercury in flue gas must consider the effect of the gas components, such