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An Investigation of the Interaction between NOx and SOx in Oxy-combustion Nujhat N Choudhury, and Bihter Padak Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02064 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017
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Environmental Science & Technology
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An Investigation of the Interaction between NOx and SOx in Oxy-combustion
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Nujhat N. Choudhurya, Bihter Padakb*
3
Department of Chemical Engineering, University of South Carolina, 541 Main St. Horizon I,
4
Columbia, South Carolina 29201, USA.
5
a
6
b
7
*
Tel: (803) 777-0648, Fax: (803) 777-8142, choudhun@email.sc.edu Tel: (803) 777-7959, Fax: (803) 777-8142, padak@cec.sc.edu
Corresponding Author
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Abstract
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This study focuses on revealing the interaction of sulfur
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oxides (SOx) and nitrogen oxides (NOx) and investigating the
11
application
12
spectroscopy to quantify SOx and NOx emissions from gas-
13
phase oxy-combustion systems. The authors aim to
14
contribute to the current state of knowledge by providing
15
speciation data of NOx and SOx species and it elucidates the influence of nitric oxide (NO) on
16
sulfur trioxide (SO3) generation. Detailed kinetic simulations revealed the influence of
17
combustion parameters and the sensitivity analysis confirmed the dominating influence of
18
hydrocarbon fragments on NO reduction. Accompanying experimental analysis exhibited higher
19
reduction of NO to nitrogen (N2) comparing to the predictions by the kinetic simulations.
20
Moreover, the presence of NO in the system was observed to influence the SO3 generation to a
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variable degree based on the reaction set employed for kinetic simulations. Experimentally,
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slight decrease in SO3 concentration was observed in presence of NO and it can be explained by
23
the radical consumption by NO as SOx and NOx species share the same radical pool. The oxy-
24
combustion mechanisms available in the literature can be improved further to be able to predict
25
this interaction.
of
Fourier
transform
infrared
(FTIR)
26
27
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1. Introduction
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Increasing awareness of the global warming phenomenon has driven the research to develop
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solutions that will lower the carbon dioxide (CO2) emissions from power plants. CCS (carbon
31
capture and storage) technology has been anticipated to contribute to about 1/6th of the carbon
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emission reduction by 2050 1 if fully implemented. Oxy-combustion is a CCS technique that can
33
reduce the carbon footprint of the power plants by providing the option of easier handling and
34
storage of emitted CO2.
35
Oxy-coal combustion has shown promise in reducing emissions of nitrogen oxides (NOx) 2, 3 due
36
to suppression of the Zeldovich mechanism
37
Destruction of the recycled NO in the furnace through the reburn mechanism has been reported
38
to be the dominant factor in NOx reduction 6. Both experimental and kinetic studies have been
39
performed to shed light on the nitrogen chemistry occurring in O2/CO2 environment
40
Combustion parameters such as, temperature, excess O2 amount, recycled NO concentration and
41
the stoichiometric ratio in the system have been reported to influence the reduction of NO 5, 7, 10,
42
15
43
Alongside nitrogen chemistry, sulfur chemistry in oxy-coal combustion has also received much
44
attention 2, 18-26. In traditional air combustion systems, only 0.1-1% of the coal sulfur will convert
45
to sulfur trioxide (SO3) 27 through reactions R(1) - R(3).
4
3, 5-17
.
.
46
SO2 + O (+M) SO3 (+M)
R(1)
47
SO2 + OH (+M) HOSO2 (+M)
R(2)
48
HOSO2 + O2 SO3 + HO2
R(3)
49
and occurrence of the reburn mechanism 5.
The primary formation route R(1) usually occurs at temperatures > 1150K while the secondary
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routes R(2) - R(3) progress below the temperature of 1150K
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medium, coupled with the recycle of the flue gas can contribute to higher SO3 generation in oxy-
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combustion systems. In presence of water vapor, SO3 converts to sulfuric acid (H2SO4) through
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reaction R(4) and the higher acid dew point can cause acid vapor condensation at much higher
54
temperatures leading to severe corrosion 28, 29.
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SO3 + H2O H2SO4
. Change in the combustion
R(4)
56
In addition to nitrogen and sulfur chemistry, interaction between the NOx and SOx species is also
57
of interest, but did not receive much attention in the literature. Earlier studies
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NOx species to have influence on SO2 oxidation. Experimentally, lowered SO2 oxidation was
59
observed with the introduction of NO, and consumption of the available O radicals by reaction
60
R(5) was deduced to be the reason 30.
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NO + O NO2
21, 24, 30
concluded
R(5)
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In a separate study 31, reactions R(6) - R(9) were hypothesised to be influencing SO2 oxidation at
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lower NO concentrations under air combustion conditions.
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NO + O2 NO3
R(6)
65
NO3 + NO 2NO2
R(7)
66
NO3 + SO2 NO2 + SO3
R(8)
67
NO + O2 NO2 + O
R(9) 32
68
But through kinetic simulations, Wendt et al.
concluded the influence to be negligible unless
69
the concentration of NO is above 1000 ppmv. In an oxy-combustion system, as the combustion
70
medium is switched to CO2 and higher concentrations of NO is expected in the boiler due to
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recycle, the scenario can be different. Fleig et al. 24 reported that even a small amount of NO can
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affect the final SO3 concentrations by influencing the radical pool, although direct interaction
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between NOx-SOx species was not included in their simulation. In an attempt to shed light on the
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direct NOx-SOx interactions in oxy-combustion environment, the authors previously conducted a
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detailed kinetic simulation study along with experiments to validate the simulation results
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However, during the experimental analysis, quantification of SO3 concentration in presence of
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NO using the salt method revealed higher levels of variability and no conclusion could be drawn.
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In the current study, the authors aim to provide a clear picture of the NOx-SOx interaction by
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implementing Fourier transform infrared (FTIR) spectroscopy as the quantification technique.
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Although many studies have been performed to investigate NOx reduction in oxy-combustion,
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NOx speciation data collected under a realistic time-temperature boiler profile in presence of SO3
82
is yet to be reported in the literature. Moreover, due to the time consuming nature of the salt
83
method, simultaneously collected temporal profile of SO3 could not be reported in the previous
84
study
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these gaps, the present study aims to provide gas phase NOx and SOx speciation data by
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conducting simultaneous sampling using FTIR spectroscopy. Considering the temperature
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sensitive nature of these species in a combustion system, such data will be valuable to predict the
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emissions from a power plant operating under oxy-combustion mode. For this purpose, oxy-
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combustion experiments have been performed in a lab-scale setup using methane (CH4) as fuel.
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Moreover, parametric study for NOx emissions has been performed via kinetic simulations and
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sensitivity analysis has been conducted to elucidate the reaction pathways of N2 formation from
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recycled NO. The experimentally collected data in comparison with the kinetic simulation
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predictions can contribute to shedding further light on the chemistry of recycled NO and its
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influence on SO3 generation while validating existing reaction mechanisms in the literature.
33
33
.
. Instead, data was collected from experiments conducted on different days. To fill in
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2. Methodology
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2.1. Experimental Setup
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To obtain SOx and NOx
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speciation data, gas phase
99
experiments were performed
100
in a lab-scale combustion
101
setup
102
discussed in details
103
schematic of the setup is
104
shown
105
Experiments were conducted
106
by
107
combustible mixture into the
108
quartz burner to create a
109
premixed laminar flame at the
110
tip
111
running the experiment for an
112
hour to ensure stable condition, flue gas samples were collected and analyzed by using an online
113
Bruker Tensor 27 FTIR spectrometer equipped with MARS variable optical length gas cell. To
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enable the detection of NOx species in the presence of water vapor, multivariate calibration by
115
using GRAMS/AI software was performed for NO-water, N2O-water and nitrogen dioxide
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(NO2)-water. Due to the interference from water, H2SO4 and SO2 in different wave length
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regions, detection of SO3 by using FTIR can be tricky
33-35
in
previously
Figure
introducing
of
the
33
burner.
.
A
1.
the
After
Figure 1. Schematic of the experimental setup
36, 37
. To enable SO3 detection by FTIR,
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multivariate calibration of SO2-CO2 was performed. The inlet SO2 concentration through the
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reactor was checked before the experiments. While analyzing the combustion flue gas, the
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reduction in the SO2 signal was attributed to the conversion of SO2 to SO3. As all the sampling
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lines are heated to 1500C to prevent condensation of water and the presence of hydrogen sulfide
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(H2S) in an oxidizing combustion environment is unlikely, it is safe to assign this reduction in
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the signal to the evolved SO3 concentration. To eliminate any uncertainty due to possible loss of
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SO2 in the reactor, the in-house sulfur calibration files were built by flowing the samples through
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the reactor system and analyzing the gas at the outlet of the reactor. Moreover, the performance
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of the calibration files was always checked by flowing known concentrations of SO2-CO2
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mixtures through the reactor before starting any experiment. If any loss was occurring, the
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calibration procedure and the daily check of the reactor should be sufficient to tackle the issue. A
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list of all the experimental conditions is presented in Table S.1 that is included in supporting
130
information.
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2.2. Kinetic Simulations
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In the current study, detailed gas phase modeling was performed to gain an understanding of the
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reaction pathways of recycled NO destruction and to explore the influence of varied combustion
134
parameters. The kinetic simulation cases revealing the sulfur chemistry in gas-phase oxy-
135
combustion were discussed in a previous study 33 along with the direct NOx-SOx interaction. The
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calculations focusing on the NOx chemistry were conducted by using the plug flow reactor (PFR)
137
module from the CHEMKIN-PRO
138
(CH4/O2/CO2/NO/SO2) was introduced into the PFR while subjecting the reactor to the
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temperature profile presented in Figure 2. This temperature profile was obtained from the
140
experiments conducted in this study and is representative of time-temperature profile prevailing
38
software. A mixture of desired combustibles
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in an actual plant boiler
. Rate of production (ROP) analysis was employed to identify the
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formation and destruction routes of N2 and NOx species. Moreover, to pinpoint the dominating
143
reactions in recycled NOx chemistry, sensitivity analysis was performed.
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The combustion mechanism applied in the current study was drawn from the previous studies 5, 7,
145
14, 15, 40-42
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oxy-combustion environment. The mechanism containing 97 species and 779 reactions was
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applied by Mendiara et al. to study reburn chemistry in CH4 oxy-combustion
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oxidation, nitrogen chemistry and the reburn reactions are included in the mechanism. Moreover,
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the interaction of different hydrocarbon fragments, such as CH, CH2, CH3, HCNO and HCCO,
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with NO are added to this reaction set. NO prediction from this mechanism was reported 15 to be
151
in good agreement with the accompanying experimental analyses 7, 10. Moreover, the mechanism
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performed well in predicting the inlet NO reduction 40 and anticipating the oxidation of HCN 41,
153
42
154
environment. This reaction mechanism coupled with a sulfur subset was utilized previous studies
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investigating sulfur chemistry in oxy-combustion environment
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previous study 33, also employed this mechanism containing both the sulfur and nitrogen reaction
157
sets. To maintain coherence with the previous study, the mechanism used in the current study
158
will be referred to as the Alzueta mechanism in the rest of the narrative.
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In order to study the direct interaction between SOx and NOx species, a reaction subset involving
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28 reactions for S/N/C interaction from the Leeds mechanism
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mechanism (referred to as Alzueta + Leeds(S/N/C) mechanism). Also, a reaction set containing
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four reactions from Wendt et al.
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Alzueta + Wendt) as well as the Alzueta + Leeds (S/N/C) mechanism (referred to as Alzueta +
involving kinetic simulations to elucidate the nitrogen chemistry occurring within the
14
. Hydrocarbon
. This mechanism was chosen for this study due to its proven validity in oxy-combustion
32
26, 43, 44
45
. The authors, in their
was integrated to the Alzueta
was included in the Alzueta mechanism (referred to as
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Wendt + Leeds(S/N/C). Details regarding the sulfur chemistry and direct interaction between
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NOx-SOx species is elaborated in the previous publication
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compare the simulation results to the experimentally collected data.
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3. Results and Discussion
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3.1. Speciation of NOx
169
Figure 2 illustrates the simulated temporal profiles of NO, NO2, N2 and N2O along with the
170
experimental temporal profile of NO for equivalence ratio (φ) of 0.86 and inlet O2 concentration
171
of 32.5%. Experimentally, no NO2 was observed while N2 was not monitored. According to the
172
simulated profile, around 1200K-1300K, reduction of NO occurs due to formation of N2O
173
[reactions R(10)-R(11)] and its subsequent conversion to N2 [reactions R(12)-R(14)], formation
174
of N2 through N, NH and NH2 radical channels [reactions R(15)-R(17)] and interconversion
175
between NO and NO2 [reactions R(18)-R(19)].
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NH + NO N2O + H
R(10)
177
NCO + NO N2O + CO
R(11)
178
N2O + H N2 + OH
R(12)
179
CO + N2O N2 + CO2
R(13)
180
N2O + O N2 + O2
R(14)
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N + NO N2 + O
R(15)
182
NH + NO N2 + CO2
R(16)
183
NH2 + NO N2 + H2O
R(17)
184
NO + HO2 NO2 + OH
R(18)
185
NO + O(+M) NO2(+M)
R(19)
186
33
. The focus of this study is to
NO2 that is formed mostly converts back to NO through reactions R(20)-R(22).
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NO2 + H NO + OH
R(20)
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NO2 + O NO + O2
R(21)
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CH2 + NO2 CH2O + NO
R(22)
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Figure 2. Measured temperature profile and simulated and experimental NO, NO2, N2 and N2O temporal profiles for φ = 0.86, O2 = 32.5% and NO = 1000 ppmv in reactor 190 191
The reduction of recycled NO occurs mostly in the region of 1200K-1300K and the
192
concentration remains constant for lower temperatures. To obtain the temporal profile of NO
193
experimentally, samples were collected from the temperature range of 1016K-598K and the
194
experiments were performed at least twice to check the reproducibility. Similar to the simulated
195
data, with the decreasing temperature from 1016K to 598K, the outlet concentration of NO does
196
not exhibit any significant change with a reasonable day-to-day variability of 0.23-1.96%. The
197
predicted reduction of recycled NO at higher temperatures (above 1200K) cannot be captured in
198
the experiments as the samples are collected downstream of the furnace at lower temperatures.
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However, the reduction percentage (29%-34%) observed experimentally is interestingly higher
200
than the predictions (17%). Moreover, the model predicts 13 ppmv of NO2 to exist at the reactor
201
outlet. However, experimentally, no NO2 was observed. The authors cannot pinpoint the reason
202
for this discrepancy due to the detection limit of the FTIR system.
203
3.1.1. Effect of equivalence ratio
204
To explore the influence of φ on the
205
reduction of recycled NO, gas phase
206
experiments and kinetic simulations
207
were conducted for φ = 0.8 - 0.98, and
208
concentrations of 32.5% O2 in the
209
oxidizer stream and 2000 ppmv of NO in Figure 3. Comparison between the experimental and simulated concentrations of NO, NO2 and N2 for various equivalence ratios at O2 = 32.5% and NO = 2000 ppmv in reactor
210
the reactor. The exit concentrations of
211
NO at various equivalence ratios for
212
both the computational and experimental cases are presented in Figure 3 along with the
213
simulated concentrations of NO2 and N2. For the simulated cases, it can be observed that with the
214
increasing equivalence ratio, the outlet NO concentration decreases from 1679 ppmv to 1581
215
ppmv. In addition to this, NO2 concentration goes down from 14 ppmv to 8 ppmv, and the N2
216
concentration increases from 152 ppmv to 205 ppmv with increasing φ. This increase in the
217
reduction of NO to N2, along with the decrease in NO2, can be explained by the higher
218
availability of hydrocarbon fragments in the richer mixture, which facilitates the reburn
219
mechanism5 to favor more N2
220
concentration slightly decreases when φ is increased from 0.8 to 0.9 and slightly increases going
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from 0.9 to 0.98. An average of 1347 ppmv of NO was obtained for all the equivalence ratios
formation from NOx species. Experimentally, the NO
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investigated with the concentration ranging from 1319 to 1374 ppmv. Since the change in
223
concentration is not significant (only ~3.5% of the NO concentration) within the narrow range of
224
equivalence ratios investigated and the change is in the same order with the fluctuations observed
225
between repeated experiments, it could be due to experimental error and it is hard to depict a
226
definitive trend. Kinetic simulations predicted the reduction in NO to be increasing from 16% to
227
21% with increasing φ from 0.8 to 0.98 while the experimentally observed reduction was much
228
higher and was 33% on average.
229
3.1.2. Effect of NO concentration
230
Similar discrepancies were observed
231
while investigating the influence of NO
232
concentration in the system. Different
233
concentrations of NO (500 ppmv-2000
234
ppmv in the reactor) were introduced
235
into the system while φ and O2
236
concentration were maintained at 0.85
237
and 32.5%, respectively. As observed in
238
Figure 4, both the simulated and
239
experimentally measured outlet NO concentrations increase when the inlet NO concentration
240
varies from 500 ppmv to 2000 ppmv. For the simulated case, the NO reduction increases from
241
8% to 17% as the NO concentration in the reactor increases, which can be due to the increased
242
availability of N radicals at higher NO concentrations facilitating the interaction with fuel
243
fragments to cause higher reduction. Experimentally, as the inlet NO concentration increases
244
from 500 ppm to 2000 ppmv, the outlet NO concentration ranges from 341 ppmv to 1325 ppmv
Figure 4. Comparison between experimental and simulated concentrations of NO, NO2 and N2 for various NO concentrations at φ = 0.86 and O2 = 32.5%
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with a variability of 0.17%-5.38% from experiment to experiment. But the increase in the
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conversion of NO to N2 with increasing inlet NO concentration observed in simulated cases is
247
absent experimentally, and the experimental conversion for NO reduction remains on average at
248
34%, which is again higher than the simulated cases. Figure 4 also demonstrates that the amount
249
of N2 and NO2 generated increases with increasing inlet NO concentration for the simulated
250
cases, but no NO2 was observed experimentally while N2 was not monitored.
251
3.1.3. Effect of O2 concentration
252
In
253
performed to evaluate the effect of O2
254
concentration on NO reduction and the
255
collected data is demonstrated in Figure
256
5.
257
percentage of O2 in the oxidizer exhibits
258
negligible
259
concentration. A very slight increase of
260
outlet NO (by 3 ppmv) and NO2 (by 2
261
ppmv) concentrations occur with the increasing O2 concentration while N2 goes down by 2ppm.
262
A decreasing trend in NO reduction was observed experimentally when the inlet O2
263
concentration is increased from 28% to 32.5%, which can be attributed to the increase in O
264
radicals causing the N radicals to form more NO than N2, thus result in a decrease in the amount
265
of NO reduction, hence more NO. However, for 34% O2 concentration, a deviation from this
266
trend was observed where the NO concentration slightly decreased. A very similar trend was
267
observed previously by Okumura et al. 46 for a coal flame where there is a slight deviation from
addition,
For
the
experiments
simulated
impact
on
were
cases,
outlet
the
NO
Figure 5. Comparison between experimental and simulated concentrations of NO, NO2 and N2 for various inlet O2 concentrations at φ = 0.86 and NO = 2000 ppmv in reactor
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the increasing trend; but overall, NOx concentration was reported to be increasing with the O2
269
concentration. The increase in NOx concentration was attributed to the activation of OH, O and
270
NCO/NH formation reactions when the O2 concentration is increased. Moreover, the reduction in
271
the recycled NO concentration observed experimentally is again higher than the predicted
272
reduction and it is 32% on average compared to 17% predicted by the simulation. Overall, when
273
compared with kinetic modeling results, the discrepancies observed in the extent of NO
274
reduction for all the experimental cases can be due to underestimation of NO to N2 conversion by
275
the kinetic mechanism and the presented data indicates room for more improvement to the
276
existing mechanisms.
277
3.1.4. Sensitivity Analysis
278
Sensitivity analysis was performed using CHEMKIN-PRO to understand the reaction pathways
279
facilitating the formation of N2 from recycled NO to shed light on the NO reduction process. The
280
reactions dominating the formation of N2 are listed below in their decreasing order of influence
281
and the sensitivity coefficient data is presented in Figure S.1 in supporting information.
282
O + OH O2 + H
R(23)
283
CH3 + CH3 (+M) C2H6 (+M)
R(24)
284
CH3 + O2 CH3O + O
R(25)
285
CH3 + O2 CH2O + OH
R(26)
286
CH3 + HO2 CH3O + OH
R(27)
287
NO + HO2 NO2 + OH
R(18)
288
CH2O + O2 HCO + HO2
R(28)
289
HCO + M H + CO + M
R(29)
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CH2O + CH3 HCO + CH4
R(30)
291
C2H4 + O2 CH2HCO + OH
R(31)
292
HCO + O2 HO2 + CO
R(32)
293
CH4 + OH CH3 + H2O
R(33)
294
C2H4 + O CH2HCO + H
R(34)
295
CH3 + NO HCN + H2O
R(35)
296
H + O2(+M) HO2(+M)
R(36)
297
CH4 + H CH3 + H2
R(37)
298
Based on the sensitivity analysis, formation of O and OH radicals from the reverse reaction of
299
R(23) had a positive influence on N2 formation in an oxy-combustion system. As the oxidation
300
of fuel and subsequent formation of hydrocarbon fragments that are required to generate N2 from
301
recycled NO are facilitated by the availability of O and OH radicals, positive influence from this
302
reverse reaction was observed. Formation of C2H6 through reaction R(24) demonstrated a
303
negative effect on N2 generation, which can be explained by the subsequent consumption of
304
radicals by C2H6, which play a role while producing N2. Positive influence was exhibited by
305
reactions R(25) - R(27), R(28) - R(29), R(31), and R(34) - R(35). The hydrocarbon fragments
306
CH3O and CH2O formed through reactions R(25) - R(27) eventually form the HCO radical. The
307
HCO radical can either form CO and contribute to formation of N2 through reaction R(13) or
308
form NCO radical through the intermediate HNCO, which will feed to the NH radical pool and
309
contribute to N2 generation through subsequent reactions. But the ROP analysis revealed that the
310
contribution of HCO to the NH pool is not significant and most of the HCO forms CO through a
311
network of reactions. Since the formation of HCO and CO is beneficial for N2 generation,
312
positive influence was observed from R(28) and R(29). CH2CHO radical produced by the
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reaction channels R(31) and R(34) later breaks down into CH3, CH2O and HCO radicals and thus
314
demonstrated a positive influence on N2 formation. Since R(35) produces HCN, which is an
315
important intermediate for N2 formation, positive influence from this reaction was observed.
316
Negative sensitivity coefficients were obtained for reactions R(18), R(30), R(32), R(33), R(36)
317
and R(37). The negative influence from reaction R(18) can be explained by the consumption of
318
NO to form NO2 instead of facilitating the generation of N2. Through reaction R(30), HCO
319
radical is formed, which is beneficial for the breakdown of NO to N2, but this route consumes
320
two hydrocarbon radicals and forms CH4, which is a stable product. As a result, an overall
321
negative influence on N2 generation from NO was observed from this reaction. Through reaction
322
R(33), even though a CH3 radical is formed, consumption of the OH radical and formation of a
323
stable product, H2O, also occurs, which caused reaction R(33) to exhibit negative influence on
324
the destruction of recycled NO to form N2. Similarly, generation of relatively stable HO2 radicals
325
by consuming H and O2 through reaction R(36) and its subsequent contribution to forming NO2
326
from NO through reaction R(18) can explain the negative impact of reaction R(36) on N2
327
formation. Also, from the sensitivity analysis, negative impact of reaction R(37) was observed
328
and it can be attributed to the formation of stable H2 from radical H.
329
3.2. Interaction Between NOx-SOx Species
330
As it was shown previously33, SO3 formation is influenced by the presence of NO and the direct
331
interaction between NOx and SOx species needs to be investigated. Since the previous SO3
332
measurements conducted by the authors using the salt method was biased by the presence of NO,
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no clear trend was obtained in terms of the effect of NO on SO3 formation experimentally,
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although the kinetic simulations clearly showed an influence. In this study, FTIR spectroscopy
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was employed to measure SO3.
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Before studying the effect of NO, SO3 measurements were conducted first to benchmark the
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FTIR technique. Temporal profile of SO3, presented in Figure 6, was collected for φ = 0.86,
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32.5% O2 in the oxidizer stream and 2500 ppmv SO2 in the reactor through simultaneous
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sampling by FTIR from different temperature points. As seen from Figure 6, with the decline in
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temperature from 1016K to 596K, the evolved SO3 concentration increases from 32 ppmv to 95
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ppmv, which can be attributed to the formation through secondary routes, R(2) - R(3). The
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Alzueta model predicts the SO3 profile to remain constant after 1050K, but experimentally
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significant formation is observed till 600K. A similar trend was observed in the previous study 33
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by the authors where the concentration of SO3 was underestimated at lower temperatures by the
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kinetic mechanism comparing to experimental data obtained using the salt method. The
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experiment was repeated in this study to collect the temporal profile of SO3 using the FTIR
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spectrometer to validate that it is a viable tool to measure SO3. The data obtained by the FTIR
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technique shows good agreement with the salt method data points presented in Figure 6.
Figure 6. Comparison between experimental (FTIR and salt method) and simulated SO3+H2SO4 temporal profile at φ = 0.86, O2 = 32.5% and SO2 = 2500 ppmv in reactor
Figure 7. Comparison between experimental and simulated concentrations of SO3+H2SO4 at the reactor outlet for various NO concentrations at φ = 0.86, O2 = 32.5% and SO2 = 2500 ppmv in reactor
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Figure 7 illustrates how the SO3 concentration changes in presence of NO. These experiments
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were conducted for φ = 0.86, reactor SO2 concentration = 2500 ppmv and inlet O2 concentration
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in oxidizer = 32.5% while changing the NO concentration from 200 ppmv to 1500 ppmv in the
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reactor. As it can be observed from the plot, in absence of NO, 83 ppmv SO3 is present at the
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reactor outlet and with the introduction of NO, the SO3 concentration starts to decline. It should
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be noted that the error bars become larger as the concentration of NO introduced into the system
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increases, except for 1500ppm. The large deviation observed for 1200ppm NO is a result of one
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data point deviating out of the four data points collected, which could be due to experimental
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error. Although, a clear trend was not observed for high NO concentrations, there is a decrease
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when 200ppm NO was introduced comparing to the case where NO was absent. This slight
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decreasing is contrary to the predicted trend by the simulations with different reaction sets,
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Alzueta + Leeds (S/N/C), Alzueta + Wendt and Alzueta + Wendt + Leeds (S/N/C), where the
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SO3 concentration increases when NO is introduced. The Alzueta reaction mechanism alone
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exhibits a small decrease in SO3 concentration at higher NO concentrations; however, it initially
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increases when NO is added comparing to the case when NO is absent. The slight decrease
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observed by the experimental results can be explained by the fact that introduction of NO into
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the system leads to the consumption of O and OH radicals and this has been previously observed
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by earlier experiments conducted under air combustion conditions
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addition of the S/N/C subset from the Leeds mechanism, including direct interaction of NOx and
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SOx species, seems to improve the model predictions for SO3 formation as both the Alzueta +
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Leeds (S/N/C) and the Alzueta + Wendt + Leeds (S/N/C) reaction sets result in a better
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agreement with the experimental data.
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. In presence of NO, the
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In conclusion, FTIR spectroscopy has been employed for measuring SO3 and NO emissions in
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flue gas under gas-phase oxy-combustion conditions and consistent results have been observed
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with the salt method. The data collected to investigate the direct interaction between SOx and
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NOx species show that conversion of SO2 to SO3 is slightly supressed in presence of NO and this
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decline is in contrast with the model predictions. The addition of the S/N/C subset to take into
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account the direct interactions between NOx and SOx species improved the model predictions.
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Future work will involve further improvement of the oxy-combustion mechanisms available in
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the literature. From a simple A-factor analysis, the reactions that play a significant role have
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been determined to narrow down the list of reactions that need to be improved. Quantum
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mechanical calculations will be conducted to calculate the reaction rate parameters and the
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mechanism will be updated accordingly.
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Supporting Information
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Table S1: Test cases for combustion experiments
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Figure S1: Sensitivity coefficients for N2 formation from recycled NO at ϕ=0.86, O2 = 32.5%
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and NO = 2000 ppmv in reactor
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Acknowledgements
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The project was supported by the National Science Foundation under grant number 1236761.
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The authors would also like to thank William Flake Jr. for his help with initial testing of the
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combustion set-up.
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References
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