Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 77

During Coal Combustion

concentration in the model increased 100 times, thus avoiding the truncation error.
Correspondingly, Hg concentration distribution was only 1% of the simulation
results.

4.5.2 Three-Dimensional Simulation Results in a Furnace

4.5.2.1 Temperature Fields

The input parameters for simulation (including boiler operational condition) were
selected in accordance with data from the experimental tests of a boiler. The
simulation was conducted as if the boiler had a full load.
Flow and energy processes in the furnace had been simulated by standard CFD
techniques, incorporating a sub-mercury model. Predictions had been validated to a
reasonable degree of agreement against real data taken at the full-scale boiler. The
formation and distribution of gaseous Hg 0 were simulated in the furnace during coal
combustion. In general, mercury belongs to the easily volatile metals. Mercury
mainly exists in elemental form. That is because when the temperature in the fur-
nace is higher than 9731,073 K, mercury composition will decompose into Hg 0 .
Fig. 4.74 shows the temperature field of the longitudinal plan in the boiler. As
shown, the area near the burners had the highest temperature, which could reach
1,618 °C, which also contained alternate temperature bands as a result of the al-
ternate primary air and secondary air. Primary air had low temperature. The com-
bustion was enhanced with the inlet of secondary air, which made the temperature
increase rapidly in some areas. After the incoming tertiary air, the flame was close
to the center, due to the low temperature (60 centigrade) and high momentum (60
m/s) of the tertiary air. The flow was high in rigidity, so it condensed the center of
the flame. All this matched with the facts, and is well shown in CFD. Fig. 4.75 is the
comparison of CFD results and measured results. The figure shows the average
temperature, the maximum temperature and their curves with the height increasing.
The measured data at a height of 3 m was as shown by the stars. From the figure, we
can see that at a height of 25 m, the difference between the temperatures of the CFD
result and measured data is within 3 °C, and it is within 20 °C at a height of 34 m.
The CFD results in a thermal state were extremely credible.