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Investigation of gaseous elemental mercury oxidation by non-thermal plasma injection method Jinjing Luo, Qiang Niu, Youxian Xia, Yinan Cao, Rupeng Du, Shiqiang Sun, and Changyi Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01405 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 6, 2017
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Energy & Fuels
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Investigation of gaseous elemental mercury oxidation by non-thermal plasma
2
injection method
3 4
Jinjing Luo1,*, Qiang Niu1, Youxian Xia, Yinan Cao, Rupeng Du, Shiqiang Sun,
5
Changyi Lu
6
College of the Environment & Ecology, Xiamen University, Xiamen, Fujian, P.R.
7
China
8
1
9
authors
These authors contributed equally to this work and should be considered co-first
10
*
11
address:
[email protected] (J.Luo)
Corresponding author. Tel. +86-0592-2188119.Fax. +86-0592-2180655. E-mail
12 13
ABSTRACT
14
Non-thermal plasma (NTP) injection method was used to oxidize elemental
15
mercury (Hg0) in this study. Mixture of water vapor and oxygen was selected as the
16
discharge gases. Active species generated by a dielectric barrier discharge (DBD)
17
plasma reactor were introduced into the flue gas duct, where they reacted with Hg0.
18
Different parameters including active particles, supply voltage, flowrate of injection
19
gas,
20
considered. Experimental results indicated that it produced high yield of oxidative
21
species than using single discharge gas, and the reason was believed to be the
22
dissociation and
23
by electron impact. And it was assumed that active radicals includes O, O3, water
system
temperature
and
typical
excitation of water molecule
flue
gas
and
1
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components
oxygen
were
molecule
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cluster ions (O2+·H2O) and OH·. Increasing plasma injection volume led to higher
25
content of reactive species in the system, which promoted Hg0 oxidation.
26
Approximately 98.3% of Hg0 was oxidized at 4 kV of voltage with 20 ml/min of
27
plasma injection flowrate. When the supply voltage increased, the temperature in
28
discharge region increased accordingly, which resulted in the decomposition of
29
reactive species, and Hg0 oxidation process was restrained as a consequence.
30
Existence of NO and SO2 in the system respectively indicated negative effects on Hg0
31
oxidation process, which was believed to be the result of the competitive consumption
32
of oxidative radicals, since both the reaction rate coefficient of SO2 with ·OH and NO
33
with ·OH are faster than that of Hg0 with ·OH.
34
Keywords:
35
Elemental mercury, Non-thermal plasma, Plasma injection, Oxidation, Active species
36
1. Introduction
37
Mercury pollution has attracted considerable public concern due to its high
38
toxicity, bioaccumulation and detrimental effects on human health and ecosystem.1,2
39
Coal-fired power plants are considered as the primary source of anthropogenic
40
mercury emission.3,4 Depending on combustion conditions and flue gas chemistry,
41
mercury in coal-fired flue gas exists mainly in three forms: elemental mercury (Hg0),
42
oxidized mercury (Hg2+) and particulate-bound mercury (HgP).5,6 Both Hg2+ and HgP
43
could be removed effectively by typical air pollution control devices (APCD) such as
44
wet flue gas desulphurization (WFGD), electrostatic precipitator (ESP) and fabric
45
filter (FF).7,8 However, Hg0, the major mercury species in flue gas, can hardly be 2
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removed due to its water insolubility and chemical inertness.9,10 Consequently, the
47
oxidation of elemental mercury to water soluble oxidized species followed by
48
removal in downstream APCDs is a promising way to reduce mercury emissions from
49
coal-fired plants.
50
Recently, a number of technologies have been used to oxidize Hg0 in flue gas,
51
such as oxidant injection (Cl2, NaClO2, H2O2, etc),11,12 catalytic oxidation13 and
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photochemical oxidation14. Although these oxidation technologies have shown good
53
prospects in laboratory studies, they are still unable to obtain commercial applications
54
due to a variety of unsolved problems such as high cost, low stability and removal
55
efficiency or secondary pollution.
56
Compared with the above oxidation methods, the NTP technologies show the
57
advantages of efficient oxidation process, no chemical additives, operating over a
58
wide range of temperature and pressure, and multi-pollutants simultaneous
59
removal.15-17 During the discharge process, Hg0 can be converted into Hg2+ by excited
60
molecules or free radicals, like O3, O and OH, etc. which generated from background
61
gas bombed by high energy electrons.18 Although, activated carbon injection is the
62
most commonly employed technique for Hg0 removal19, activated carbons are
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expensive, ranging in the price from $500 to $3000 per ton, the resulting annual cost
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of ACI for mercury removal is around one million dollars for a 500-MWe coal-fired
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power plant20. The cost of NTP technologies were reported to be 10%-20% less than
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ACI under the similar removal performance.21 Therefore, the NTP technologies are
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considered to be one of the most promising methods for Hg0 removal in coal-fired 3
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power plants. Among a variety of NTP technologies, dielectric barrier discharge
69
(DBD) has raised wide attention due to its stable discharge features, high degree of
70
technological maturity and effective production of active species.22,23
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Currently, the process of Hg0 oxidation using NTP involves two strategies: direct
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oxidation and plasma injection. Fig.1 shows the schematic diagram of direct oxidation
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method and plasma injection method. For direct oxidation method, flue gas directly
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pass through the plasma reactor and Hg0 can be converted into Hg2+ by excited
75
molecules or free radicals like O3, O and OH·, etc. The Powerspan Co. has developed
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an electro-catalytic technology, in which DBD plasma has been used to oxidize NOx，
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SOx and Hg0. A variety of studies have been done to investigate the performance of
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Hg0 passing through DBD reactor.24,25 The deficiencies of this strategy are the
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potential discharge spaces being blocked by particulate matters and electrodes being
80
corroded by the flue gas components, and creepage is another serious obstacle.
81
For plasma injection method, NTP reactor is placed on the outside of the flue
82
duct or inserted into the flue duct and used to generate active species. When the
83
discharge begins, various active radicals (such as OH·, O and ·HO2) are formed via
84
the collision between high energy electrons and discharge gases. Subsequently these
85
species are injected into flue duct to oxidize Hg0. Chang et al.26 tested the
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simultaneous removal performance of NO and SO2 using corona radical shower
87
system and achieved 99% removal efficiency of SO2 and 75% removal efficiency of
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NO, respectively. An et al.27 utilized a surface discharge plasma reactor (SDPR) to
89
generate free radicals and evaluated the performance of Hg0 removal by injection 4
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method. Compared with direction oxidation, the investment cost and system size are
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reduced because large plasma reactors are not required. In addition, plasma injection
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would effectively avoid clogging, scouring and corrosion problems. What’s more, the
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energy consumption of this process was much lower than that of direct oxidation
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methods, with good prospects for industrial applications.
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Duo to the high utilization of energy and removal efficiency, plasma injection
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method was a potential alternative for purification Hg0 from flue gas.28,29 Most
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research choose O2 or air as discharge gas, and reactive oxygen radicals (O2+ and O3)
98
were produced. In dry air, O2+ might be quenched due to its short lifetime, while with
99
the presence of water vapor, O2+ could react with H2O to form water cluster ions.
100
Therefore, damp O2 was selected as discharge gas in this study to make better use of
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oxygen radicals, and the Hg0 oxidation process was investigated by plasma injection
102
method. In addition, the effects of operation parameter and the influence of the
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coexistence of NO and SO2 were also discussed.
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2. Experiment setup
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Experimental apparatus is shown in Fig.2. It mainly consists of a simulated flue
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gas unit, a plasma injection system and analytical instruments. Simulated flue gas in
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this study was prepared with introducing N2, NO, SO2 and Hg0 into the system, in
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which, N2, NO, and SO2 were inlet from gas cylinders. Flowrate of the gase was
109
controlled by mass flow controller (Metron Inc., China). A temperature-controlled
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mercury permeation device (VICI Metronic Inc., USA) was used to obtain the desired
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Hg0 concentration (50 ± 2 µg/m3). Flowrate of total simulated flue gas was kept at 1 5
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L/min. The diameter and length of the reaction pipe were 6 mm and 1.5 m,
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respectively. The flow velocity and reaction time were about 0.59 m/s and 2.54 s.
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Pipelines wrapped with a heating belt and were heated up to 140 oC to prevent the
115
deposition of mercury species and the condensation of moisture.
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Active species were generated using a DBD reactor, consisting of a high voltage
117
electrode, a grounding electrode and the insulating medium (inner and outer quartz
118
tube).30 The central stainless steel rod was connected to the high voltage power supply.
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The outer quartz tube was wrapped with copper wires and served as the ground
120
electrode. The discharging gas, consisting of oxygen or/and water vapor, was
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introduced into the hollow cavity between inner and outer quartz tube. Water vapor
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generated by an evaporator was introduced into the discharge zone using O2 (100
123
ml/min) as the carrier gas. Flowrate of discharge gas was kept at 500 ml/min, and the
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plasma injection flow volume was controlled by the pump.
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A Testo 350 flue gas analyzer (Testo, Germany) was used to monitor the
126
concentrations of NOx and SO2, and Hg0 concentration was measured continuously by
127
the EMP2-WLE8 mercury analyzer (Nippo Instrument Corporation, Japan). Initially
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the sample gases passed a NaOH scrubber and chiller to remove acid gases and
129
moisture. Afterwards, the detector (cold vapor atomic absorption spectrometry) made
130
the measurement of Hg0 concentrations in the sample gases. Since ozone has a similar
131
absorption wavelength as Hg0 at 253.7 nm, a tube furnace heated to 350 oC was
132
connected upstream of mercury analyzer to decompose the redundant ozone and
133
eliminate its interference on mercury measurement. The oxidant concentrations were 6
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136
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determined using the total residual oxidant (TRO) method.31 Hg0 oxidation efficiency was calculated by following equation: Mercury oxidation Rate (%) = 0
[Hg 0 ]off − [Hg 0 ]on × 100 [Hg 0 ]off
0
Where [Hg ]off (µg/m3) and [Hg ]on (µg/m3) are the concentrations of Hg0 when
138
the plasma was turned off and on, respectively.
139
3. Results and discussion
140
3.1 Effect of water vapor in the discharge gas
141
To precisely evaluate the performance of Hg0 oxidation during plasma injection
142
process, Hg0 was presented under pure N2 atmosphere. The original flue gas
143
composition was kept as: Hg0 (50 µg/m3 ) and N2 (as balance gas). Discharge gas is an
144
important factor for reactive particles formation in plasma discharging process. In this
145
study, we used damp O2 as discharge gas and H2O concentration was adjusted in the
146
range of 0 and 9%. The effects of H2O concentrations on Hg0 oxidation efficiency
147
were shown in Fig.3. It illustrated that the existence of H2O in discharge gas enhanced
148
Hg0 oxidation. The Hg0 oxidation efficiency increased from 81 ± 1.2% to 93 ± 0.8%
149
with the H2O concentrations increasing from 0 to 9% in discharge system. It was
150
believed that in the course of the discharge, reactive oxygen radicals (O2+ and O3)
151
were produced with the existence of O2, meanwhile, with the presence of water vapor
152
in the discharge zone, O2+ further reacted with H2O to form water cluster ions
153
(O2+·H2O), and these species might dissociate to ·OH as shown in R1 and R2.32
154
Besides, the electron-impact dissociation of H2O might lead to the production of ·OH
155
and H radicals as shown in R3.33 It was assumed that ·OH, along with oxygen radicals, 7
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reacted with Hg0, and enhanced the Hg0 oxidation efficiency in this study (R4-R7).34
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O2+ + H2O + M → O2+·H2O + M
(R1)
158
O2+·H2O + H2O → O2 + H3O+ + ·OH
(R2)
159
e + H2O → e + ·OH + H
(R3)
160
O3 + Hg0 → HgO + O2
(R4)
161
O + Hg → HgO
(R5)
162
·OH + Hg → HgOH
(R6)
163
HgOH + O2 → HgO + HO2
(R7)
164
3.2 Effect of applied voltage
165
Energy input is regarded as a key parameter for the performance of a barrier
166
discharge reactor, since it has a strong effect on reactions inside the reactor.35 The
167
efficiency of Hg0 oxidation as a function of applied power voltage was obtained with
168
two different plasma injection flowrate (Fig.4). With the plasma injection flowrate of
169
40 ml/min, the Hg0 oxidation efficiency decreased from 98.5 ± 0.6% to 87.6 ± 1.1%
170
with increasing the applied voltage from 4 to 7 kV. Same tendency can also be
171
observed with the injection flowrate of 20 ml/min. Yang et al.36 and An et al.27
172
reported a contrary phenomenon. Their research showed that with increase of supplied
173
voltage, oxidation of Hg0 promoted. They believed that with the increase of supplied
174
voltage, the number of generated high-energy electrons increased, which caused to the
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yield of active substances, and thereby promoting the oxidation reaction. However,
176
energy consumption in DBD process is ultimately transformed into heat energy,
177
which resulted in the increase of the temperature of both the dielectric material and 8
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the discharge region in DBD reactor.37 Temperatures of the dielectric medium with
179
various applied voltage in this work were listed in Table 1. It indicated that with the
180
increase of voltage from 4 kV to 7 kV, the temperature of the dielectric medium
181
increased accordingly. The temperature rise in the discharge region might play an
182
essential role in the reactive species dissociation. As can be seen from Fig.4, with the
183
injection flowrate of 40 ml/min, the total oxidant concentrations in the system
184
decreased from 31.31 µg/L to 7.83 µg/L with the increase of voltage from 4 kV to 7
185
kV. Hg0 oxidation efficiency was hindered due to the dissociation of reactive species.
186
Therefore, in order to obtain a higher mercury oxidation efficiency, a cooling system
187
to reduce heat released by electrodes is suggested to the system. While in this study,
188
considering both energy consumption and reactive species productivity, a low applied
189
voltage that could maintain the stability of the gas discharge is selected.
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3.3 Effect of plasma injection flowrate
191
The influence of plasma injection flowrate on Hg0 oxidation was investigated
192
within the range of 20 to 100 ml/min and the results are shown in Fig.5. As observed,
193
when the injection flowrate was increased from 20 to 80 ml/min, the Hg0 oxidation
194
efficiency increased from 89.9 ± 2% to 98.4 ± 0.8% accordingly. Reactive species
195
such as O3 and ·OH generated by DBD reactor were responsible for Hg0 oxidation.
196
Fig.5 indicated that increasing plasma injection flowrate introduced more active
197
species into flue gas duct and thus promoted Hg0 oxidation reaction. It was noted that
198
the oxidation efficiency remained unchanged when injection flowrate was increased
199
from 80 to 100 ml/min. Considering the Hg0 concentration (ng/L) in simulated gas 9
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200
was trivial compared to that of active species (µg/L), reactive species injected with 80
201
ml/min of flowrate are adequate to oxidize Hg0. Hence increasing injection flowrate
202
did not further improve Hg0 oxidation efficiency.
203
3.4 Effect of flue gas components
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3.4.1 Effect of SO2
205
SO2 is one of the typical components in flue gas. The influence of SO2 on Hg0
206
oxidation process was investigated in this work with concentrations varied from 0 to
207
300 ppm. Fig.6 showed that the presence of SO2 inhibited Hg0 oxidation reaction. In
208
the absence of SO2, Hg0 oxidation efficiency of 96.6 ± 2.2% was obtained with active
209
particle injection flowrate of 50 ml/min. When 100 ppm of SO2 was added into flue
210
duct, the oxidation efficiency slightly decreased to 93.9 ± 1.3%. And when SO2
211
concentration in the system increased from 100 to 200 ppm, Hg0 oxidation efficiency
212
further dropped to 48.4 ± 1.5%. At the time of SO2 content increased to 300 ppm, the
213
oxidation efficiency reduced to 43.7 ± 1.2%. While for the experiments with the
214
injection flowrate of 80 ml/min, the tendency of the curve showed same as that of 50
215
ml/min, except that the decline range of Y-axis was much smoother. Contrasted the
216
two curves, it was found that the one with lower injection flowrate showed break at
217
100 ppm, while the one with higher injection flowrate showed break at 200 ppm.
218
Quantities of injected reactive species was assumed to be the reason. With no
219
exceeding 100 ppm of SO2 in the system, 50 ml/min of reactive species being injected
220
into flue duct was believed to be adequate for the simultaneous oxidation reactions for
221
both SO2 and Hg0. When more than 100 ppm of SO2 in the system and injection 10
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flowrate kept at 50 ml/min, the preferential reactions of SO2 with reactive species, esp.
223
with ·OH will dominate the reaction process, since the reaction rate coefficient of SO2
224
with ·OH (8 ×10-12 cm3 molecule-1·s-1) is faster than that of Hg0 with ·OH (9.0×10-14
225
cm3 molecule-1·s-1). As a consequence, the consumption of reactive species by SO2
226
resulted in the decrease of Hg0 oxidation efficiency. And for experiments with 80
227
ml/min of reactive species being injected into flue duct, no exceeding 200 ppm of SO2
228
in the system would not noticeably change both oxidation reactions, since more
229
reactive species supplied more than twice amount of oxidants for both SO2 and Hg0,
230
as shown in Fig. 5.
231
An et al.27 believed that increasing SO2 concentration did not change Hg0
232
oxidation efficiency substantially. The inconsistency with this study was believed to
233
attribute to different discharge gas selected and the difference of reactive species
234
produced. An’s group used O2 as discharge gas, and the majority of reactive species
235
produced are ·O and O3. From Table 2, it showed that the reaction rate of SO2 with O3
236
is much lower than that of Hg0 with ·O and O3, as a result, SO2 showed little effects
237
on Hg0 oxidation. In this study, wet oxygen gas was selected as the discharge gas,
238
which means O2 together with H2O being introduced into the discharging zone,
239
and ·OH was formed through the reaction of H2O and ·O, O3 (R8 and R9). Since
240
reaction rate coefficient of SO2 with ·OH is faster than that of Hg0 with ·OH (Table 2),
241
Hg0 oxidation reaction was hindered by the existence of SO2.
242
H2O + ·O → 2 ·OH
(R8)
243
H2O + O3 → H2O2 + O2
(R9)
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3.4.2 Effect of NO
245
NO is ubiquitous in flue gas environment and has significant impacts on Hg0
246
oxidation.38 Four different concentrations of NO were studied for their effects on Hg0
247
oxidation with 5 kV voltage and injection flowrate of 80 and 60 ml/min, respectively.
248
As shown in Fig.7, NO distinctly hindered the Hg0 oxidation reaction. In the absence
249
of NO, Hg0 oxidation efficiency was 98.4 ± 0.8% with plasma injection flowrate of 80
250
ml/min. When 200 ppm NO was introduced into flue gas, Hg0 oxidation efficiency
251
dramatically decreased to 62.3 ± 1.8%, and with NO concentration increased to 300
252
ppm, Hg0 oxidation efficiency further dropped to 59.5 ± 1.6%. The inhibitory effect of
253
NO on Hg0 oxidation was due to the consumption of active species (O, ·OH and O3)
254
by NO. From Table 2, it listed that the reaction rate coefficients between NO and
255
O, ·OH, O3 are much larger than those between Hg0 and O, ·OH, O3. Therefore, with
256
the existence of NO in Hg0-containing system, active species are preferentially
257
reacted with NO, and resulted in the decrease of Hg0 reaction.
258
The two curves in Fig.7 indicated the same tendency, and both of them showed
259
break at 100 ppm. It was inferred that the reactive species introduced into the system
260
was sufficient for the oxidation reactions of both NO and Hg0, if no more than 100
261
ppm of NO in the system. If more NO in the system, the introduced amount of
262
reactive species would not supply enough oxidants for NO and resulted in the
263
reduction of Hg oxidation efficiency.
264
3.5 Effect of flue gas temperature
265
Flue gas temperature was adjusted by controlling heating belt. The flue gas 12
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temperature ranged from 100 oC to 200oC was investigated in this study for its
267
influence on Hg0 oxidation, and two injection flowrates of 20 and 60 ml/min were
268
selected. Experimental results are shown in Fig.8. It indicated that the system
269
temperature had an insignificant effect on Hg0 oxidation for both injection flowrates.
270
And the oxidation efficiency remained invariant within the temperature range.
271
Although an increase in temperature would result in the decomposition of reactive
272
species, it is believed that the quick reaction rates between the chemical active species
273
and Hg0 (Table 2) weakened the effect of flue gas temperature on reaction process.
274
4. Conclusion
275
Injection of plasma for Hg0 oxidation in coal-fired flue gas was investigated in
276
this work. The reaction between O2+ and H2O in discharge gas produced additional
277
reactive species, which promoted the Hg0 oxidation. Increasing plasma injection
278
flowrate favored the Hg0 oxidation due to the enhancement of active species injection
279
concentrations. The consumed power converted into heat energy played an essential
280
role in the re-dissociation of reactive species, and thus further increasing applied
281
voltage would resulted in the decrease of Hg0 oxidation efficiency. The flue gas
282
temperature has little impact on Hg0 oxidation within the test temperature range due
283
to the short reaction time between highly active species and Hg0. The inhibitory
284
effects of NO and SO2 on Hg0 oxidation were observed, which was attributed to their
285
preferential consumption of reactive species.
286
Acknowledgements
287
This project was supported by Science and Technology Guidance Project of 13
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Fujian Province (2016H0033).
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(a)
(b)
Fig.1 Schematic diagrams of direct oxidation (a) and plasma injection (b).
367
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DBD reactor
High voltage supply O2
Water bubbler
Pump
Flue gas analyzer Furnace Mercury analyzer
NO
SO2
N2
Mercury generator
Exhaust gas treatment
368 369
Fig.2 Schematic diagram of the experimental setup
370
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Fig.3 Effect of H2O concentrations in discharge gas on Hg0 oxidation efficiency. The
373
plasma injection flowrate: 40 ml/min, the applied voltage: 5 kV.
374
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Fig.4 Effect of applied voltage on Hg0 oxidation efficiency. The discharging gas: O2
377
and 9% H2O, the plasma injection flowrate: 20 and 40 ml/min.
378
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Fig.5 Effect of plasma injection flowrates on Hg0 oxidation efficiency. The
381
discharging gas: O2 and 9% H2O, the applied voltage: 5 kV.
382
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383 384
Fig.6 Effect of SO2 concentrations on Hg0 oxidation. The discharging gas: O2 and 9%
385
H2O, the plasma injection flowrate: 80 and 50 ml/min, the applied voltage: 5 kV.
386
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Fig.7 Effect of NO concentrations on Hg0 oxidation. The discharging gas: O2 and 9%
389
H2O, the plasma injection flowrate: 80 and 60 ml/min, the applied voltage: 5 kV.
390
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391 392 393
Fig.8 Effect of flue gas temperature on Hg0 oxidation. The discharging gas: O2 and 9% H2O, the plasma injection flowrate: 20 and 60 ml/min, the applied voltage: 5 kV.
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Table 1 Temperatures of the medium with different applied voltage Voltage (kV)
4
5
6
7
145
186
206
231
Temperature of the dielectric o
medium ( C) 396
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397
Table 2
398
Reaction rate coefficients of NO, SO2 and Hg0 with various reactive species Reaction rate (cm3/
Reference
Elementary reaction molecule·s) NO + O → NO2
3.00×10-11
[38]
NO + O3 → NO2 + O2
1.73×10-14
[39]
NO + OH → HNO2
3.30×10-11
[40]
Hg0 + O → HgO
5.6×10-15
[41]
Hg0 + O3 → HgO + O2
7.5×10-19
[41]
Hg0 + OH → HgOH
9.0×10-14
[41]
SO2 + O3 → SO3 + O2
2.7 ×10-23
[35]
SO2 + OH → OHSO2
8 ×10-12
[42]
399
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