Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 61

Transformation After Coal Combustion

The influence of temperature on the conversion was analyzed in the identical
flue gas reaction system. The comparison between calculation and experimental
results was shown, with a temperature range of 300 900 K in Fig. 4.63. Mercury
transformation definitely shifted with temperature decrease. Hg 0 occupied 90%
percent of the total mercury, which means that Hg 0 is the thermal steady state in the
high temperature area of a furnace. Hg 0 converted into Hg 2+ as the temperature
decreased, and the mercury conversion reached 50% at 473 K finally. The reaction
between Hg 0 and oxygen was specifically as follows:

1

Hg + O HgO (4-29)

(g) 2(g) (s,g)

2

The thermodynamic equilibrium simulation results were in accord with the
experimental results in Fig. 4.63, but the simulation curve was mostly lower than
experimental results. The reason stemmed from the fact that the temperature varia-
tion in the heating system was not an isothermal reaction process but a parabola, and
the quenching process of 1 2 s exists in the sampler tube as well. The rapid cooling
of the flue gas assisted the production of Hg 2+ as reported by Wang [25] , and the
production of Hg 2+ increased as the temperature decreased. The thermodynamic
equilibrium model did not take the mixing, dispersion and inhomogeneity of tem-
perature into consideration, thus the simulation results were lower than experi-
mental results, while the existing simulation error coincided with experimental
results considerably. The results demonstrated that chemical thermodynamic equi-
librium analysis could be pretty feasible in research on mercury transformation in
complicated flue gas conditions.

60

Simulation results
Experimental results

50

40

30

20

10

0

200 300 400 500 600 700 800 900

T (K)

Fig. 4.63 Comparison between chemical thermodynamic simulation with experimental results