Pulsed corona discharge for oxidation of gaseous elemental mercury

Article abstract of DOI:10.1063/1.2952496

Positive pulsed corona discharge has been applied for the oxidation of gaseous elemental mercury (Hg⁰) from a simulated flue gas. The oxidation of Hg0 to HgO and Hg Cl2 can significantly enhance the mercury removal from flue gas. At a gas condition ofO₂ (10%), H2_O (3%), and N₂ (balance), Hg0 oxidation efficiency of 84% was achieved at an input energy density of 45 Jl. The presence of NO, however, hinders Hg0 oxidation due to the preferential reaction of NO with O and O3. On the contrary, SO₂ shows little effect on Hg0 oxidation due to its preferential reaction with OH. It has been also observed that the HCl in gas stream can be dissociated to Cl and Cl₂ and can induce additional Hg0 oxidation to HgCl_2-.

Full text of DOI:10.1063/1.2952496

Pulsed corona discharge for oxidation of gaseous elemental mercury
Kyung Bo Ko, Youngchul Byun, Moohyun Cho, Won Namkung, Ian P. Hamilton, Dong Nam Shin, Dong Jun Koh
,
and Kyoung Tae Kim
Citation: Applied Physics Letters 92, 251503 (2008); doi: 10.1063/1.2952496
View online: http://dx.doi.org/10.1063/1.2952496
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/25?ver=pdfcov
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APPLIED PHYSICS LETTERS 92, 251503 ͑2008͒
Pulsed corona discharge for oxidation of gaseous elemental mercury
1
1
1
1
2
Kyung Bo Ko, Youngchul Byun, Moohyun Cho, Won Namkung, Ian P. Hamilton,
3
,a͒
3
3
Dong Nam Shin,
Dong Jun Koh, and Kyoung Tae Kim
1
School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang,
Kyoungbuk 790-784, Republic of Korea
2
Department of Chemistry, Wilfrid Laurier University, Waterloo, Ontario N2L3C5, Canada
Environmental Research Department, Research Institute of Industrial Science and Technology, Pohang,
3
Kyoungbuk 790-600, Republic of Korea
͑
Received 5 May 2008; accepted 9 June 2008; published online 27 June 2008͒
Positive pulsed corona discharge has been applied for the oxidation of gaseous elemental mercury
0
0
͑
Hg ͒ from a simulated flue gas. The oxidation of Hg to HgO and HgCl can significantly enhance
2
the mercury removal from flue gas. At a gas condition of O ͑10%͒, H O ͑3%͒, and N ͑balance͒,
2
2
2
0
Hg oxidation efficiency of 84% was achieved at an input energy density of 45 J/l. The presence of
0
NO, however, hinders Hg oxidation due to the preferential reaction of NO with O and O . On the
3
0
contrary, SO shows little effect on Hg oxidation due to its preferential reaction with OH. It has
2
been also observed that the HCl in gas stream can be dissociated to Cl and Cl and can induce
2
0
additional Hg oxidation to HgCl . © 2008 American Institute of Physics.
2
͓
DOI: 10.1063/1.2952496͔
There is increasing concern over mercury emission due
to its environmental and neurological health impact. The
largest emission of mercury to the atmosphere occurs from
combustion of fossil fuels, mainly coal combustion, which
accounts for two-thirds of annual total worldwide anthropo-
cobenefit without further energy consumption. For several
years, we have attempted commercialization of a NTP pro-
cess for the removal of NO and SO2 with control of
x
3
50 000 N m /h of flue gas as a basic module, which is typi-
cal of emissions from the sinter plant and industrial waste
incinerator in iron and steel making works. The present
1
5
genic emission of approximately 2190 tons. Although the
mercury is present in coal in only trace amounts, mercury
released into environment can be converted to highly toxic
methyl mercury species by natural occurring biological pro-
cesses. Generally, mercury in flue gas exists in three forms:
study has been initiated to gain insight into the physical
chemistry of mercury mediated by the plasma environment.
0
The effect of NO and SO2 on Hg oxidation has been inves-
0
tigated, and additional Hg oxidation by decomposition of
0
2+
elemental ͑Hg ͒, oxidized ͑Hg ͒, and particle bound ͑Hg ͒.
HCl in PCD has been studied.
p
0
A schematic diagram of the experimental setup for the
During coal combustion, mercury is liberated as Hg . While
0
0
oxidation of Hg is shown in Fig. 1. A wire-plate type of
Hg passes through combustion and postcombustion zones,
0
plasma reactor with 13 tungsten wires of 0.1 mm diameter
some Hg is oxidized, presumably HgCl because of an ex-
2
2
0
2
and two stainless steel plates ͑90ϫ270 mm ͒, which were
cess of chlorine present in coal. Also, some portion of Hg
2
+
used as high voltage and ground electrodes, respectively, was
kept in an oven to maintain the gas temperature at 90 °C.
The distance between the plates is 32 mm, and the tungsten
wires are evenly placed between the two plates at intervals of
and Hg can be bound to fly ash as particles ͑Hg ͒. Most
p
2
+
Hg and Hg species can be effectively removed by conven-
p
tional air pollution control systems ͑wet scrubbers, fabric
0
filters, etc.͒. However, Hg is more difficult to remove due to
1
5 mm. A positive pulse voltage was applied to the reactor
its high vapor pressure and low water solubility. Hence, it is
0
2+
by transferring the stored electric energy at a 1.7 nF capaci-
preferable to oxidize Hg to Hg species, such as HgO and
3
HgCl , for effective mercury emission control.
2
0
A promising technology for Hg oxidation is a nonther-
mal plasma ͑NTP͒ process using pulsed corona discharge
͑
PCD͒. PCD has been studied for several decades and is
recognized as a potential process for the simultaneous re-
moval of NO and SO . Chemically active species such as O,
x
2
OH, HO , and O formed from the electrical discharge can
2
3
induce the oxidation of NO and SO to nitric and sulfuric
x
2
acids. These acids can then be neutralized by the addition of
4
ammonia. Considering that the mercury content in the flue
3
gas is trivial ͑below 20 ␮g/m ͒ compared to the NO and
x
SO content, chemically active species from PCD can be
expected to show good performance on Hg oxidation along
2
0
with the removal of NO and SO . This means that PCD
x
2
could be a desirable method for mercury treatment as a
a͒Author to whom correspondence should be addressed. Tel.: ϩ82-54-279-
FIG. 1. ͑Color online͒ Schematic diagram of the experimental system for
0
6540. FAX: ϩ82-54-279-6239. Electronic mail: jydnshin@rist.re.kr.
Hg oxidation.
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28.143.199.160 On: Sun, 14 Dec 2014 11:04:13
0
1

251503-2
Ko et al.
Appl. Phys. Lett. 92, 251503 ͑2008͒
FIG. 2. ͑Color online͒ Hg0 oxidation and O3 generation as a function of
input energy density in the PCD process.
tor by closing the spark gap switch. The applied peak voltage
was 17 kV and the maximum current was 32 A with a
0
.5 ␮s width. To investigate the effect of input energy den-
0
sity ͑the ratio of the power to gas flow rate͒ on Hg oxida-
tion, the pulse repetition frequency was varied from
5
3
to 40 Hz. The total gas flow rate was maintained at
.5 l/min, and the gas stream after plasma reactor passed
through KCl and NaOH solutions in turn. The KCl solution
captures any oxidized Hg species, while the NaOH solution
removes any traces of acid. As a result, only Hg present in
0
FIG. 3. ͑Color online͒ Effect of NO and SO addition on Hg oxidation: ͑a͒
2
0
0
reduction of Hg oxidation with respect to NO addition and ͑b͒ SO effect
2
0
0
the gas stream can enter the Hg analyzer, which adopts the
on Hg oxidation. Gas conditions: 10% O and 3% H O in N balance with
2
2
2
1
00–300 ppm of NO and SO , respectively.
principle of cold vapor atomic absorption spectrometry. A
2
0
tube furnace ͑at 350 °C͒ was placed just before the Hg ana-
lyzer for the thermal decomposition of O , which would se-
3
0
3
0
times the initial Hg concentration, 45 ␮g/m ͑ϳ7 ppb͒, it
can be inferred that the reaction rate of Hg with O is much
slower than that of NO with O . Also the energy cost for the
oxidation of Hg is about 2.5 MeV/Hg atom, a value which
is much higher than that for the oxidation of NO, which is
several tens of eV/NO molecule.
ergy cost for Hg oxidation, it can be deduced that Hg oxi-
dation is hindered in the presence of NO due to its fast re-
action with oxidant species O and O3.
riously interfere with the Hg measurement by absorbing
53.7 nm of UV light. A moisture trap with a chiller ͑at
°C͒ was installed to prevent H O from condensing in the
0
6
2
0
3
3
2
0
0
0
gas cell of the Hg analyzer.
Pervious reports have concluded that OH, Cl, NO , and
3
0
15,16
especially O are the primary sources for Hg oxidation in
the atmosphere. Although the chemistry of Hg oxidation
in the PCD process is different from that of Hg oxidation in
From such a high en-
3
7
–9
0
0
0
0
atmospheric conditions, O and O species in the PCD pro-
3
0
cess are expected to oxidize Hg into HgO through the fol-
0
Figure 3͑a͒ shows the effect of NO on Hg oxidation. As
lowing reactions:
0
expected, the propensity of Hg concentration to oxidation
appears to be suppressed markedly as an increasing amount
of NO is added to gas flow, and this reduction of Hg oxida-
tion is due to the competitive reaction of NO with O and O .
Additionally, the effect of SO on Hg oxidation is demon-
strated in Fig. 3͑b͒ where an increasing amount of SO2 is
added to the gas flow without addition of NO. It may be seen
that unlike NO, SO shows little impact on Hg oxidation. It
is well known that most SO is oxidized to HSO and SO
3
by OH and O, not O3. In fact, the reaction rate of SO with
OH is about 100 times faster than that of SO with O radical;
thus almost SO is first oxidized by OH to HSO , and then
finally to H SO .
reaction with SO is very minor due to its fast reaction with
OH and, as a result, SO has little effect on Hg oxidation.
0
Hg + O + M → HgO + M,
͑1͒
͑2͒
0
0
Hg + O → HgO + O .
3
2
3
0
0
There is controversy about the effect of OH on Hg oxida-
tion. Some reports claim an important role on Hg oxidation
to HgO through HgOH,
2
0
1
0,11
while other reports devalue its
0
role due to the weak Hg–OH bond ͑ϳ39 kJ/mole͒ and short
2
1
2,13
HgOH lifetime ͑ϳ280 ␮s͒.
Our previous study showed
2
3
0
17
that the reaction of Hg with OH to form HgOH does not
2
0
14
play a significant role in Hg oxidation in NTP conditions.
2
This may be due to the extremely shorter reaction time and
faster decomposition of HgOH in the NTP compared to that
2
3
17,18
Thus, the consumption of O and O by
3
0
2
4
in the atmosphere. Figure 2 shows the reduction in Hg con-
2
centration and the increase in O concentration in the PCD
3
0
2
process as a function of the input energy density, where the
O and H O content were set to 10% and 3% in N . At an
Not only O and O but also Cl and Cl can be strong
3
0
2
2
2
2
0
oxidants for Hg in the PCD process. In the plasma process,
it is expected that HCl in the gas stream will be dissociated
by direct collision with energetic electrons and excited nitro-
input energy density of 45 J/l, the Hg concentration
3
dropped from 45 to 7 ␮g/m , an oxidation efficiency of
0
8
4%. This shows that Hg oxidation is possible in a NTP
0
process. However, because the O concentration at this input
gen molecules, resulting in Hg oxidation to HgCl2 by the
3
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4
energy density is 120 ppm, which is more than 1.7ϫ10
following reactions ͑3͒–͑8͒.
1
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251503-3
Ko et al.
Appl. Phys. Lett. 92, 251503 ͑2008͒
produced by the PCD process is minor due to fast recombi-
nation of Cl with H to HCl, its concentration is still compa-
0
0
rable to that of Hg . Thus, Hg oxidation by Cl can be
2
expected in the PCD process, and some portion of Cl atoms
0
can also contribute the Hg oxidation through reactions ͑3͒
and ͑4͒ rather than recombination into HCl. It is clear from
0
Fig. 4͑b͒ that HCl in the PCD process can cause Hg oxida-
0
tion, where the Hg oxidation efficiency as a function of
input energy density is obtained at four different HCl con-
centrations with NO 100 ppm and SO 200 ppm. Higher
2
concentration of HCl and input energy density result in more
generation of Cl and Cl , and this gives rise to the better
2
0
promotion of Hg oxidation to HgCl . Since the concentraion
2
of HCl in coal-fired flue gas is in the range of 10–100 ppm,
it can be concluded that the PCD process effectively leads to
0
the additional oxidation of Hg even at low temperature.
0
In conclusion, Hg oxidation in the PCD process and the
influence of two major pollutants, NO and SO , on this pro-
2
0
cess have been investigated. Hg oxidation by O and O can
3
be viable, but it is seriously inhibited in the presence of NO
due to the fast reaction of NO with O and O . On the other
3
0
hand, SO shows the little inhibitory effect on Hg oxidation
2
due to the preferential reaction of SO with OH rather than O
2
0
FIG. 4. ͑Color online͒ Effect of HCl addition on Hg oxidation: ͑a͒ trans-
mitted intensity of light near a wavelength of 435 nm for ortho-tolidine
solutions obtained by varying the input energy density and ͑b͒ promotion of
and O . It was also observed that HCl in the PCD process
3
can be dissociated into Cl and Cl by energetic electrons and
2
0
0
Hg oxidation by HCl addition with gas conditions: 10% O and 3% H O in
excited nitrogen species, and this induces the additional Hg
2
2
N balance with NO 100 ppm and SO 200 ppm.
2
2
oxidation to HgCl2.
This work has been supported by the Brain Korea 21
Project and Korean Ministry of Environment as the “ECO-
Technopia 21 Project.”
0
Hg + Cl + M → HgCl + M,
͑3͒
͑4͒
͑5͒
͑6͒
͑7͒
͑8͒
HgCl + Cl + M → HgCl + M,
2
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6
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2
9
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tolidine solution turned yellow, indicating the formation of
Cl , which is a definite evidence for dissociation of HCl in
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1