Environmental Engineering Reference
In-Depth Information
3.5 Summary
This chapter focuses on the chemical kinetics and reaction mechanisms of ozone
oxidizing the major flue gas components. In-depth investigations were performed
through theoretic calculations and experiments. The acquired results are as
follows:
(1) The specific chemical reaction mechanisms of ozone oxidizing the major
flue gas components have been built. This relates to 40 flue gas species and 121
sub-reactions, including O
3
, H
2
O, O
2
, NO
x
, SO
2
, Hg, HCl, Cl
2
, HF, H
2
S, CO, and
so on. Sensitivity analysis reveals that the O
3
-induced oxidation of NO
x
is a
stepwise process. At 150 °C, the Hg oxidizer depends mainly on NO
3
, which is
oxidized from NO
x
. The Hg oxidation degree with O
3
is much higher than with Cl.
(2) The ozone decomposition starts at a much lower rate (i.e., the
decomposition percentage of 0.5% per 10 s) at room temperature (298 K) and
gradually increases with the temperature. At 523 K, the lifespan is only 2 s,
whereas at 150 °C the decomposition percentage is 28% per 10 s and the half-life
is 19.2 s. At 150 °C, the dynamics reaction time of ozone and NO
x
is 0.6 s, which
is shorter than the ozone decomposition time.
(3) The reaction mechanism of ozone and NO
x
has been investigated. At
temperatures below 200 °C, the ozone self-decomposition has no effect on the
NO
x
oxidation reaction. However, a relatively high temperature of 300 °C
weakens the oxidation reaction because ozone has decomposed completely. In
addition, no NO
x
oxidation reaction occurs when the temperature reaches 400 °C.
Based on the fact that the dynamics simulation results agree well with those
acquired in experiments, here the established reaction mechanisms can be used in
many other relative studies.
(4) The dynamics simulations of ozone oxidizing Hg
0
show that Hg
0
can be
oxidized completely (i.e., HgO as the main bivalent format) under conditions with
temperatures below 300 °C and the O
3
/NO stoichiometric ratio of 1.5. A high
mercury oxidation rate above 90% necessitates the O
3
/NO stoichiometric ratio
exceeding 1.2.
(5) The experimental results of O
3
oxidizing SO
2
show that the SO
2
oxidation
efficiency is below 30% and the efficient temperature range is 27 - 300 °C. This
means that SO
2
in flue gas is still absorbed mainly in the WFGD system if a
simultaneous multi-pollutants removal system based on ozone is applied.
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