parametric studies of the effectiveness of no oxidation process by

parametric studies of the effectiveness of no oxidation process by

JERZY DORA1, MIECZYSŁAW A. GOSTOMCZYK2, MACIEJ JAKUBIAK2, WŁODZIMIERZ KORDYLEWSKI2, WŁODZIMIERZ MISTA3, MONIKA TKACZUK2
PARAMETRIC STUDIES OF THE EFFECTIVENESS OF NO
OXIDATION PROCESS BY OZONE
1
2
DORA Power Systems, Wilczycka 8, 51-361 Wrocław, Poland
Wrocław University of Technology, Institute of Power Engineering and Fluid Mechanics,
W. Wyspiańskiego 27, 50-370 Wrocław, Poland
3
Institute of Low Temperature and Structure Research, Polish Academy of Sciences,
Okólna 2, 50-422 Wrocław, Poland
The process of NO pre-oxidation by ozone was studied in a laboratory apparatus using
air as the carrier gas. Ozone was produced in dry air streams using a dielectric barrier discharge (DBD) nonthermal plasma reactor. The temperature of the process was varied from 17
to 170 C. The stoichiometric ratio of O3/NO was in the range of 0.83.8 and the residence
time varied from 4.3 to 8 s.
Badanie procesu utleniania NO ozonem przeprowadzono w skali laboratoryjnej używając powietrza jako gaz nośny dla NO. Ozon wytwarzano z osuszonego powietrza z wykorzystaniem generatora ozonu typu DBD. Zapewniono możliwość regulacji temperatury w
reaktorze, która była w zakresie od 17 do 170 C. Stosunek molowy O3/NO był w zakresie
0.83.8, a czas przebywania w reaktorze zmieniał się od 4,3 do 8 s.

Corresponding autor, e-mail: [email protected]
1. INTRODUTION
The most abundant gaseous pollutants released to the atmosphere from coal-fired
power plants are sulfur dioxide (SO2) and nitrogen oxides (NOx). It is believed to have a major contribution to acid rains and smog formation. In order to reduce the devastating effects of
release of SO2 and NOx from stationary sources E.U. countries took legislative actions to restrict the emissions of these gases. In 2001 the EU adopted the Directive on the limitation of
certain pollutant from large combustion plants (LCP) to control SO2 and NOx emissions. This
Directive sets also the limit for NOx emission for boilers larger than 200 MWt to 200 mg/m3
of dry flue gas [1].
Following the accession to the EU Poland has agreed to adopt the European Community policy on the environment protection and sustainable development. For the Polish heat
and power generating industry being almost completely coal dependent the most important
consequences result from the EU policy concerning the reduction of emission of certain atmospheric pollutants, among which reduction of NOx emission below 200 mg/m3 (6% O2)
after 2015 is the biggest challenge.
The commercial methods for NOx emission reduction in power plants may be classified into two groups: primary or combustion modification-based technologies (low-NOx combustion systems) and secondary or flue-gas treatment-based technologies (post-combustion
methods). The primary methods are based on modification of the combustion process to prevent generation of NOx above the actual standards [2]. In Poland the primary methods of NOx
control became very common because they have made possible to fulfil domestic standards of
NOx emission (500600 mg/m3) at acceptable costs [3]. Experience of German power plant
industry showed that the limit of NOx emission 200 mg/m3 (6%O2) from coal-fired power
plants could be fulfilled by the primary methods only for the lignite-fired new large boilers
[4]. It is rather unlikely to fulfil this limit for the bituminous-fired plants, therefore additionally the post-combustion methods must be used.
Depending upon the pollutant concentration and the flow rate of the polluted gas, NOx
can be abate by post-combustion methods like selective catalytic reduction (SCR) or recovering (adsorption, and absorption) processes. One major drawback of recovering processes is
that these need a post-treatment to reactivate or clean up waste materials, such as solid adsorbent or liquid waste (water or organic solvent). SCR is regarded as the best available technology (BAT) and actually is the major post-combustion method for control of NOx emission
in coal-fired power plants. In Europe SCR has got commercial status for coal-fired power
plants in the eighties [5]. SCR is capable of achieving the desired reductions, however, high
cost and/or technical limitations caused by unique boiler configurations often make standalone SCR a less than optimum solution. Therefore, Hybrid Post Combustion NOx Control
systems can be used which consist of a combination of selective non-catalytic reduction
(SNCR) with SCR. These Hybrid systems can offer substantial benefit in enhanced performance and they have achieved NOx emission reductions as high as 95%. However, the related
technologies (SCR and SNCR) are considered to be expensive and not environmentally
friendly because they require the use of NH3 as the reducing factor.
An alternative approach to control both the NOx and SO2 emissions are techniques for
simultaneous NOx/SO2 removal in existing wet or semi-dry flue gas desulfurization (FGD)
installations. These techniques are under development mainly in the U.S.A., where the application of SCR technology, particularly in high-sulfur coal-fired power plants, have met some
obstacles [6]. This method consist of two stages: in the first stage NOx is oxidized to highly
soluble NO2, N2O4, N2O3 and N2O5 (further denoted by NOy), and in the second stage products of oxidation are absorbed by caustic scrubbing. Because in Europe, for SO2 removal from
flue gas the most widely process in use is limestone desulfurization in wet scrubbers, an inte-
gration of this process with NOx oxidation to simultaneous control of SO2/NOx/Hg0 could
won commercial acceptance [7].
In the paper a results of preliminary experimental study of NOx oxidation by ozone in
the laboratory apparatus is presented.
2. GAS-PHASE PREOXIDATION OF NO IN FLUE-GAS
The most often used oxidizing agent for NO pre-oxidation is ozone, which is produced
from oxygen or air in non-thermal plasma (NTP) generator (ozonizer). The oxidation of NOx
using ozone is a naturally occurring process in the atmosphere.
For example, the BOC Group, Inc. has developed and patented the low temperature
oxidation technology called LoTOx, in which ozone is generated on demand from gaseous
oxygen in the exact amount required for oxidation of the NOx in the free-standing corona discharge reactor (CDR) [8]. Then the ozone is injected into flue gas stream where it reacts with
NO and NO2. The basic chemical reactions in gas-phase NOx oxidation are:
NO + O3 = NO2 + O2
2NO2 + O3 = N2O5 + O2
These higher nitrogen oxides (N2O5, and/or N2O3) are highly water soluble and are efficiently
scrubbed out with water as nitrous and nitric acids or with caustic solutions as nitrates or nitrite salts. By combining this technology with Wet Scrubbing System, the operator can reduce
also particulates, SOx and NOx all in a single vessel. The LoTOx System is very selective for
NOx removal, oxidizing only the NOx and therefore efficiently using ozone, without causing
any significant SOx oxidation.
Powerspan company proposed new, cost-effective, patented Electro-Catalytic Oxidation technology, known as ECO®, which is designed to simultaneously remove NOx, SO2,
fine particulate matter (PM2.5), acid gases such as hydrogen fluoride (HF), hydrochloric acid
(HCl), and sulfur trioxide (SO3), mercury (Hg), and other metals from the flue gases of coalfired power plants, especially from high-sodium lignite-fired combustion [9]. The core of the
ECO® technology is a dielectric barrier discharge (DBD) reactor composed of cylindrical
quartz electrodes residing in metal tubes. Electrical discharge through the flue gas produces
reactive O and OH radicals,
O2 + e  O + O + e
H2O + e  OH + O + e
O + H2O  2OH
which react with flue gas components (NOx, SO2…) to oxidize and transform it into acid
mist (HNO3, H2SO4…) at the low temperature range of 65150 C. The oxidized compounds
are subsequently removed in a downstream scrubber and wet electrostatic precipitator.
In the both methods highly soluble (Tab. 1) products of NO oxidation (NO2 and higher
oxides) are absorbed and neutralized in caustic scrubbers (e.g. the limestone scrubbing in
LoTOx). Mitsubishi Heavy Industry has proposed improvement of the scrubbing stage by the
wet catalytic absorption process for more efficient NO2 capture in a limestone scrubbing medium [10]. The catalyst was iodine:
2NO2 + 2I-  2NO2- + I2 (liquid)
Iodine ions were regenerated via sulfite ions:
I2 + SO3-2 + H2O  2I- + SO4-2 + 2H+
The efficiency of the oxidation-absorption techniques of flue-gas NOx removal is
strongly dependent on parameters of the oxidation process: temperature, retention time, mixing and water dispersion. Belco Technologies Corporation has developed and patented an
apparatus for controlling NOx emissions [11], in which flue gas is preliminary cooled to
6070 C by water injection (0.6710.7 m3 H2O per 1000 m3 flue gas), NOx is oxidized by
ozone and the products of oxidation are removed by water flowing down the walls of the
scrubber. For the ratio of O3/NO in the range of 0.53.5 mol/mol and the residence time τres
0.510 s NO concentration was reduced below 50 mg/m3 and the efficiency of flue gas cleaning was better than 90%.
Table 1. Selected properties of nitrogen oxides
NO
N2O
NO2
N2O3
30.006
44.013
46.005
76.01
1.3
1.2228
1.443
1.4
1.34
1.8
3.4
- 163.6
- 90.86
- 11.2
-00.1
- 151.7
- 88.48
21.1
3.0
+ 82.05
celeste
Color
colorless colorless
tawn
liquid
Solubility in water 1, g/dm3
0.032
0.111
213.0
500.0
1
3
3
solubility of SO2: 27 g/dm at 50 C and 5.8 g/dm at 90 C
Property
Atomic mass, g/mol
Density at 298
Liquid
K, kg/m3
Vapor
Melting temperature, C
Boiling temperature, C
Enthalpy, kJ/mol
N2O4
92.011
1.443
N2O5
108.01
- 11.2
21.1C
- 35.05
41
decompose
transparent
213.0
white
powder
500.0
The technology for removing NOx from flue gases involving injection of microwave
activated oxidizing compounds such as aqueous H2O2 solution into gas steam was described
by Gravitt et.al. [12]. This method employs microwave energy to produce highly active forms
of oxidizing compounds that react simultaneously with NOx in gaseous streams to form compounds that can be readily removed by conventional pollution abatement processes such as
wet scrubber system. During microwave activation process the temperature of H2O2 droplet is
elevated very quickly (0.053 s) from ambient up to the boiling point of H2O2, which is 152
o
C under standard conditions with fast evaporation and formation of very reactive free radicals like hydroxyl and hydroperoxy radicals (OH and HO2). The proposed process is very
effective and at least about 8090% of NO is oxidized into higher valence nitrogen oxides for
approximately stoichiometric ratio (H2O2: NO ≈ 1:1 mol/mol)
The properties of nitrogen oxides (Tab. 1) became a basis for the development of new
technologies of flue gas cleaning called “Multi-Pollution Control” [10]. For example,
Gostomczyk and Krzyżyńska [13] have examined the effectiveness of simultaneous removal
of NOx, SO2 and Hg0 from the flue gas using combinations of caustic sorbent (Ca(OH)2) and
different oxidizers (O3, H2O2, NaOCl and Ca(OCl)2) in the pilot plant installation. They
showed the ability of simultaneous reduction of NOx and SO2 emissions below 200 mg/m3
and Hg0 <1 μg/m3 at 6%O2 in flue gas.
The review showed that multi-pollutant control appears to be possible using oxidationabsorption processes when the flue-gas pre-oxidation of NO is the first stage. However, the
technologies are still in the developing stage and require further investigations.
3. EXPERIMENAL
3.1. APPARATUS AND PROCEDURE
The experimental studies on the effectiveness of NO oxidation by ozone and absorption of the oxidation products in caustic washers were conducted in the laboratory apparatus.
The scheme of the laboratory set up was shown in Fig. 1. The oxidizing reactor (4) was a spiral reactor made of a cuprum tube having a length of 8.6 m and a diameter of 8 mm. Dried air
was used as the carrier gas (flow rate was 125 dm3/h, the velocity of gas inside the reactor
was 2.5 m/s and the corresponding residence time was τres = 3.44 s). The carrier gas was
doped with NO diluted in N2. Ozone (26 mg/min) was blown into the carrier gas before the
spiral reactor inlet from the ozone generator (12). The products of oxidation were removed
from the carrier gas in the washers (5) with a considerable excess of CaCO3 or NaOH in water
solution. From the washers (5) gas was flowing through the washer (6) (containing the solution of KI for removal of ozone residue) and through the gas dryer (7) (cotton wool) to the gas
analyzer (8). Sulfur dioxide was not added to the carrier gas because the temperature below
230 C ozone practically doesn’t oxidize SO2 to SO3 [14].
Ozone was produced from air in the dielectric barrier discharge (DBD) generator (12).
The ozone concentration was in the range of 26 g of O3 per 1 m3 of air. Nitrogen oxide was
delivered into the carrier gas from a steel bottle (10), where it was stored diluted in N2
(approx. 2.5% of NO from Messer) under pressure 20 MPa. The spiral reactor (4) was put into
a container filled with oil. The oil temperature was adjusted and maintained in the range of
20170 C by an electric heater and a temperature controller Rm combined with a thermocouple PT 100.
The volumetric flow rate of the carrier gas (approx. 125 dm3/h) was measured by a rotameter R1. The volumetric flow rate of air to the ozone generator (12) was controlled by the
mass flow controller ERG 1 NPSb BETA-ERG Sp. z o.o. (11). The volumetric flow rate of
NO (diluted in N2) was controlled by the mass flow controller GFC17 model, AALBORG
INSTRUMENTS & CONTROLS Inc. (9).
1
compressed
air
Z1
P
2
3
4
5
R1
Z2
6
7
8
A
9
Z3
Rm
11
NO
R2
10
12
13
Z4
R3
OA
14
Fig. 1. Scheme of the experimental, 1, 2, 3 and 13 – T-connectors, 4 – spiral reactor, 5 – NO2 absorber, 6 – KI
washer, 7 – gas dryer, 8 – gas analyzer, 9 – mass flow controller, 10 – NO bottle, 11 – mass flow controller, 12 –
O3 generator, 14 – KI washers, Z1 Z4 – valves, R1R3 – flow meters, P – pressure controller, Rm – heater and
temperature controller, OA – ozone analyzer
The NOx concentration in the carrier gas was measured by the gas analyzer (8) GA 40
model, Eljack Electronic, based on electrochemical sensors. The additional washer (6) with
KI solution was used for protecting the gas analyzer against residual ozone in the carrier gas.
An amount of O3 injected into the carrier gas was controlled dividing the flow from
the ozonizer (12) into two flows: the first was directed to the spiral reactor (4) and the second
to the three washers (14) with KI solution to measure the absorbed O3 by the starch-iodine
method [15].
The effectiveness  of the NO oxidation was defined as:
NO = (1 – [NO]red / [NO]ref) · 100%
where [NO]ref and [NO]red denote the reference and reduced NO concentrations in the carrier
gas after the drier (7) measured by the gas analyzer (8) when the ozone generator (12) was
switched off (ref) and switched on (red), respectively.
3.2. OZONE GENERATOR CHARACTERISTICS
A laboratory DBD (Dielectric Barrier Discharge) type ozone generator (12) was used
in the experiment. The generator was fed by dried air at constant volumetric flow rate 20
dm3/h. The electric discharge was ignited by employing AC (60 kHz) high voltage in the
range of 1015 kV. The generator was operating at the ambient temperature (1721 C) and
atmospheric pressure.
Because the ozone generator was a prototype made by a small engineering company
DORA Power Systems some its characteristics were evaluated. Figure 2 shows the ozone
concentration change in air depending on the volumetric airflow through the ozone generator
for its electric power approx. 15 W.
Ozone concentration, g/m 3
8
7
6
5
4
3
2
1
0
0
50
100
150
200
250
3
Volumetric flow of air, dm /h
Fig. 2. Ozone concentration vs. volumetric flow rate of air
300
Dielectric barrier discharge is a low energy electric discharge with non-thermal ionization, however, how long as the plasma reactor is operating in air, the formation of NOx is unavoidable [16]. Because of the purpose of these studies the knowledge of this additional NOx
injection with ozone was essential, therefore a mass spectrometer QMS-200 OmniStar from
Balzers with SEM detectors was used to evaluate the NO and NO2 concentrations. The ozone
concentration in air was measured using the ozone analyzer BMT 964 BT model, BMT
MESSTECHNIK GMBH. The nitrogen oxides concentrations appeared to be two orders
lesser than the concentration of O3 (Tab. 3).
Table 3. Range of O3, NO and NO2 of concentrations in air after the ozone generator
Specie
Concentration, ppm
NO
NO2
O3
200500
100200
30 00070 000
4. RESULTS AND DISCUSSION
4.1. INFLUENCE OF THE STOICHIOMETRY
The effect of the concentration of ozone added on the efficiency of NO oxidation was
studied varying the ozone flow rate and keeping the remaining parameters constant:
- airflow:
95 dm3/h,
- [NO]ref:
180 ppm,
- temperature of the spiral reactor Tr:
18 C.
The results of measurements are shown in Fig. 3.
100
90
Effectiveness, %
80
70
60
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
Stoichiometric ratio of O3/NO, mol/mol
Fig. 3. Effectiveness of NO removal by ozone vs. the stoichiometric ratio of O3/NO
According to the oxidation reaction, NO reacts with ozone in one to one stoichiometry:
NO + O3 = NO2 + O2
The resulted relationship in Fig. 3 is approximately in agreement with the stoichiometry of
NO oxidation due to a relatively long residence time res, which was about 7.5 s.
4.2. INFLUENCE OF THE RESIDENCE TIME
The residence time res is an important factor if the method is going to be used for removal of NO from flue gas in power plants. In the experiment the residence time was varying
by changing the volumetric flow rate of the carrier gas through the spiral reactor in the range:
- velocity of gas in the reactor:
1.072.54 m/s,
- residence time res:
8.0643.385 s.
Flow rates of NO and O3 and the excess of O3 were kept constant:
- flow rate of O3:
1.097 mgO3/min,
- flow rate of NO:
0.584 mgNO/min
- O3/NOref:
1.14 mol/mol,
but the NO concentration varied due to changes of the volumetric flow rate of the carrier gas
(Tab. 4). The influence of the residence time in the spiral reactor on the effectiveness of NO
removal from the carrier gas by ozone was shown in Fig. 4.
Table 4. The effectiveness of NO removal by ozone vs. the residence time res
[NO]ref
[NO]red
Gas velocity
res
NO
ppm
mg/m3
ppm
mg/m3
m/s
s
%
245
328
40
53.6
1.07
8.07
83.67
196
263
40
53.6
1.31
6.55
79.59
169
228
40
53.6
1.56
5.52
75.76
123
165
57
76.4
2.05
4.20
53.66
98
131
67
89.7
2.54
3.40
31.63
100
90
Effectiveness, %
80
70
60
50
40
30
20
10
0
3
4
5
6
7
8
Residence time, s
Fig. 4. Effectiveness of NO removal by ozone vs. the residence time in the reactor
4.3. INFLUENCE OF TEMPERATURE
Because the rate of ozone decay is strongly dependent on the temperature, therefore its
influence on the oxidation process should be examined. The temperature within the spiral
reactor Tr was selected: 17, 50, 150 and 170 C in these studies, (170 C is in the range of the
flue gas temperature from the coal-fired boilers). The flow rate before the spiral reactor was
kept unchanged 125 + 8.5 +1.2 = 134.7 dm3/h. For the mean, reference concentration of NO:
136 mgNO/m3 (208 mgNO2/m3) the ratio O3/NOref was 0.897 mol/mol.
The results are shown in the Fig. 5, where the line with nodes () represents the
effectiveness NO calculated for the [NO]red concentrations as measured. The effectiveness of
NO oxidation by ozone was significantly reduced at elevated temperatures. For the mean reference concentration of NO: 136 mgNO/m3 and for the ratio O3/NOref = 0.897 mol/mol the
effectiveness of NO removal from the carrier gas varied from 57 to 44% in the temperature
range 50150 C, respectively. The best effectiveness of NO removal (approx. 76%) was
achieved at the temperature 17 C and for = 0.887 mol/mol. The least effectiveness (approx.
33%) occurred at the temperature 170 C, when O3/NOref = 0.818 mol/mol.
80
Effectiveness, %
70
60
50
40
30
20
0
20
40
60
80
100
120
140
160
180
Temperature, °C
Fig. 5. Dependence of direct () and corrected (■■■) effectiveness of NO removal by ozone on temperature
A considerable reduction of the effectiveness of NO oxidation by ozone with the increase of the temperature can be explained by decrease of the residence time res in the spiral
reactor due to the volumetric flow rate rise when the temperature Tr increased (Tab. 5). The
second important factor is decay of ozone at an elevated temperature [17].
To compensate the effect of volumetric flow rate increase and reduction of residence
time res on the effectiveness ηNO a correction factor  was introduced:
 = res,17 / res,Tr
where res,17 and res,Tr denote the residence time at the temperature 17 C and Tr, respectively
(Tab. 5).
Table 5. The volumetric flow rate of gas and the residence time in the spiral reactor vs. the temperature
Tr, C
Flow rate,
105, m3/s
Gas velocity, m/s
res, s
Correction factor

17
50
100
150
170
3.742
4.167
4.813
5.458
5.716
1.324
1.475
1.70
1.93
2.02
6.50
5.83
5.06
4.45
4.3
1.0
1.115
1.285
1.461
1.512
Assuming an approximate proportional relationship between [NO]red and the residence
time the correction factor  was used for the correction of [NO]red and recalculation of the
effectiveness ηNO (Tab. 6). The estimated in such a way effectiveness ηNO, corr is presented in
Fig. 5 (line with ■■■ nodes). The mean value of the effectiveness of NO removal in the range
of the temperature of 50150 C was 60% after correction.
Table 6. Effectiveness of NO removal by ozone vs. temperature in the reactor for O3/NO = 0.8180.887 mol/mol
Corrected
[NO]red
Tr
res
°C
s
ppm
mg/m3
ppm
mg/m3
ppm
17
50
100
150
170
6.50
5.83
5.06
4.40
4.30
207
203
209
206
220
277
272
280
276
295
50
88
100
116
148
67
118
134
155
198
79.8
78.5
80.3
97.9
[NO]ref
[NO]red
ηNO
Corrected
ηNO, corr
mg/m3
%
%
107
105
108
131
75.86
56.65
52.15
43.69
32.73
75.68
63.16
67.01
63.83
49.49
4.4. INFLUENCE OF NO CONCENTRATION IN THE CARRIER GAS
Usually, the effectiveness of reduction of NOx emission is better when its concentration is higher, which is also true in this case because the rate of NO oxidation is directly proportional to the concentration of NO. The influence of the NO concentration in the carrier gas
on the effectiveness of NO removal by ozone was investigated at the ambient temperature 19
C. The volumetric flow rate of the carrier gas was 125 dm3/h and the flow rate of ozone (in
air) was 8.5 dm3/h. The content of NO in the carrier gas was varied changing the volumetric
flow rate of NO (diluted in N2) from 0.3 to 1.2 dm3/h (Tab. 4). The results of the measurements are shown in Fig. 6.
Table 4. Influence of NOref concentration in the carrier gas on the effectiveness of NO removal by ozone
[NO]ref
ppm
mg/m3
210
281
164
220
106
142
55
74
[NO]red
ppm
Mg/m3
40
54
39
52
46
62
25
33
O3/NOref
mol/mol
1.00:1
1.28:1
1.97:1
3.79:1
NO
%
81.00
76.22
56.60
55.40
90
Effectiveness, %
85
80
75
70
65
60
55
50
50
100
150
200
NO concentration, mg/m
250
300
3
Fig. 6. Effectiveness of NO removal by ozone vs. NO concentration in the carrier gas
5. CONCLUSIONS
The parametric studies conducted in a laboratory apparatus confirmed great potential
of the process of NO oxidation by ozone to removal of NO from flue gas. However, there are
several factors which influence this process when applied in coal-fired boilers for reduction of
NOx emission. The obtained results lead to the following conclusions:
a) The highest rate of NO oxidation by ozone was observed at the ambient temperature
(1720 C).
b) Increase of the temperature of the carrier gas caused reduction of the effectiveness of
NO removal by ozone due to diminish of the residence time and destruction of ozone
at the elevated temperature.
c) The effectiveness of NO oxidation by ozone appeared to be sensitive to the residence
time; approximately 6 s of the contact time was required to reach the effect of NO removal related to the stoichiometric ratio of O3/NO.
d) The effectiveness of NO removal by ozone diminished when NO concentration in the
carrier gas was decreased.
SYMBOLS
ηNO – effectiveness of the NO oxidation, %
Tr – temperature of the spiral reactor, C
res – residence time in the spiral reactor, s
 - correction factor.
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BADANIA EFEKTYWNOŚCI PROCESU UTLENIANIA NO
OZONEM
JERZY DORA, MIECZYSŁAW A. GOSTOMCZYK, MACIEJ JAKUBIAK, WŁODZIMIERZ KORDYLEWSKI, WŁODZIMIERZ MISTA, MONIKA TKACZUK
Zgodnie z Dyrektywą Unii Europejskiej LCP [1], dotyczącą ograniczenia emisji niektórych zanieczyszczeń do powietrza z dużych źródeł spalania paliw, w roku 2016 zacznie w
Polsce obowiązywać nowy limit na emisję NOx (< 200 mg/m3 dla 6 %O2). Spowoduje to, że
dotychczas stosowane w polskiej energetyce metody pierwotne ograniczania emisji NOx będą
niewystarczające. Z tego powodu większość krajowych elektrowni stanie przed koniecznością
zakupu drogiej inwestycyjnie i eksploatacyjnie technologii SCR lub posiadającej szereg wad
technologii SNCR. Poszukuje się dlatego innych, mniej kosztownych niż SCR, metod ograniczania emisji NOx. Obiecującą alternatywą w stosunku do metody SCR jest grupa metod jednoczesnego usuwania NOx, SO2 i rtęci z zastosowaniem instalacji z mokrym lub pół-suchym
odsiarczaniem spalin (IOS), w których NO utlenia się do wyższych tlenków przed reaktorem
IOS. Konieczność utleniania NO wynika z jego znikomej rozpuszczalności w stosunku do
NO2 i N2O5, które doskonale rozpuszczają się w wodzie. Obecnie badanych jest wiele rodzajów utleniaczy NO, z punktu widzenia ich efektywności i przede wszystkim ze względu na
koszt. Jednym z bardziej obiecujących utleniaczy NO jest ozon, który nie musi być transportowany, ani magazynowany, można go generować bezpośrednio przy kotle.
Celem pracy jest zbadanie wpływu warunków występujących w instalacji kotłowej na
efektywność usuwania NOx ze spalin z zastosowaniem utleniania NO ozonem i wychwytywaniem produktów utleniania w roztworach kaustycznych. Badania nad wpływem parametrów na proces utleniania NO ozonem przeprowadzono w skali laboratoryjnej. Ozon wytwarzano z powietrza z wykorzystaniem generatora ozonu typu DBD (ang. Dielectric Barier Discharge). Gaz nośny dla NO stanowiło powietrze, do którego dodawano tlenek azotu z butli,
żeby zapewnić jego udział około 200 ppm. Reaktorem utleniającym był reaktor typu przepływowego wykonany ze zwiniętego spiralnie przewodu miedzianego. Zapewniono możliwość regulacji temperatury w reaktorze, która była w zakresie 17170 C. Produkty utleniania były usuwane z gazu nośnego za reaktorem w baterii płuczek Drechslera zawierających
wodną zawiesinę CaCO3 lub NaOH. Efektywność procesu utleniania NO ozonem określono
na podstawie wskazań analizatora spalin umieszczonego za płuczkami absorbującymi.
Potwierdzono eksperymentalnie, że dla czasów przebywania res ponad 6 s efektywność ubywania NO z gazu nośnego w wyniku utleniania ozonem jest w przybliżeniu zgodna
ze stechiometrią reakcji NO z O3. Jeżeli jednak skracano czas kontaktu, to obserwowano
znaczne zmniejszenie efektywności usuwania NO z użyciem ozonu, do 50% dla res = 4 s (dla
stosunku molowego: 1,14 mol O3/mol NO). Stwierdzono znaczący wpływ temperatury na
efektywność utleniania NO ozonem: wzrost temperatury procesu powodował obniżenie efektywności utleniania NO, co prawdopodobnie ma związek z szybszym rozkładem ozonu w
wysokich temperaturach oraz ze wzrostem strumienia objętości gazu w reaktorze, czyli krótszym czasem kontaktu. Próbowano wyeliminować wpływ tego drugiego czynnika przez
wprowadzenie współczynnika korekcyjnego. Na tej podstawie oceniono efektywność metody
na 60% w zakresie temperatury 50150 C. Wykazano również, że im wyższa koncentracja
NO w gazie nośnym tym lepsza efektywność procesu utleniania NO ozonem.