An Investigation of the Interaction between NOx and SOx in Oxy

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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

1

An Investigation of the Interaction between NOx and SOx in Oxy-combustion

2

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, [email protected] Tel: (803) 777-7959, Fax: (803) 777-8142, [email protected]

Corresponding Author

ACS Paragon Plus Environment

1


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Abstract

9

This study focuses on revealing the interaction of sulfur

10

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

21

variable degree based on the reaction set employed for kinetic simulations. Experimentally,

22

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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28

1. Introduction

29

Increasing awareness of the global warming phenomenon has driven the research to develop

30

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

32

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


3


22, 26, 27

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routes R(2) - R(3) progress below the temperature of 1150K

51

medium, coupled with the recycle of the flue gas can contribute to higher SO3 generation in oxy-

52

combustion systems. In presence of water vapor, SO3 converts to sulfuric acid (H2SO4) through

53

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.

55

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

58

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.

61

NO + O NO2

21, 24, 30

concluded

R(5)

62

In a separate study 31, reactions R(6) - R(9) were hypothesised to be influencing SO2 oxidation at

63

lower NO concentrations under air combustion conditions.

64

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

71

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

73

between NOx-SOx species was not included in their simulation. In an attempt to shed light on the

74

direct NOx-SOx interactions in oxy-combustion environment, the authors previously conducted a

75

detailed kinetic simulation study along with experiments to validate the simulation results

76

However, during the experimental analysis, quantification of SO3 concentration in presence of

77

NO using the salt method revealed higher levels of variability and no conclusion could be drawn.

78

In the current study, the authors aim to provide a clear picture of the NOx-SOx interaction by

79

implementing Fourier transform infrared (FTIR) spectroscopy as the quantification technique.

80

Although many studies have been performed to investigate NOx reduction in oxy-combustion,

81

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

85

these gaps, the present study aims to provide gas phase NOx and SOx speciation data by

86

conducting simultaneous sampling using FTIR spectroscopy. Considering the temperature

87

sensitive nature of these species in a combustion system, such data will be valuable to predict the

88

emissions from a power plant operating under oxy-combustion mode. For this purpose, oxy-

89

combustion experiments have been performed in a lab-scale setup using methane (CH4) as fuel.

90

Moreover, parametric study for NOx emissions has been performed via kinetic simulations and

91

sensitivity analysis has been conducted to elucidate the reaction pathways of N2 formation from

92

recycled NO. The experimentally collected data in comparison with the kinetic simulation

93

predictions can contribute to shedding further light on the chemistry of recycled NO and its

94

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

97

To obtain SOx and NOx

98

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

114

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

116

(NO2)-water. Due to the interference from water, H2SO4 and SO2 in different wave length

117

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,


6

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118

multivariate calibration of SO2-CO2 was performed. The inlet SO2 concentration through the

119

reactor was checked before the experiments. While analyzing the combustion flue gas, the

120

reduction in the SO2 signal was attributed to the conversion of SO2 to SO3. As all the sampling

121

lines are heated to 1500C to prevent condensation of water and the presence of hydrogen sulfide

122

(H2S) in an oxidizing combustion environment is unlikely, it is safe to assign this reduction in

123

the signal to the evolved SO3 concentration. To eliminate any uncertainty due to possible loss of

124

SO2 in the reactor, the in-house sulfur calibration files were built by flowing the samples through

125

the reactor system and analyzing the gas at the outlet of the reactor. Moreover, the performance

126

of the calibration files was always checked by flowing known concentrations of SO2-CO2

127

mixtures through the reactor before starting any experiment. If any loss was occurring, the

128

calibration procedure and the daily check of the reactor should be sufficient to tackle the issue. A

129

list of all the experimental conditions is presented in Table S.1 that is included in supporting

130

information.

131

2.2. Kinetic Simulations

132

In the current study, detailed gas phase modeling was performed to gain an understanding of the

133

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

136

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

139

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


7


39

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in an actual plant boiler

. Rate of production (ROP) analysis was employed to identify the

142

formation and destruction routes of N2 and NOx species. Moreover, to pinpoint the dominating

143

reactions in recycled NOx chemistry, sensitivity analysis was performed.

144

The combustion mechanism applied in the current study was drawn from the previous studies 5, 7,

145

14, 15, 40-42

146

oxy-combustion environment. The mechanism containing 97 species and 779 reactions was

147

applied by Mendiara et al. to study reburn chemistry in CH4 oxy-combustion

148

oxidation, nitrogen chemistry and the reburn reactions are included in the mechanism. Moreover,

149

the interaction of different hydrocarbon fragments, such as CH, CH2, CH3, HCNO and HCCO,

150

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

152

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

155

investigating sulfur chemistry in oxy-combustion environment

156

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.

159

In order to study the direct interaction between SOx and NOx species, a reaction subset involving

160

28 reactions for S/N/C interaction from the Leeds mechanism

161

mechanism (referred to as Alzueta + Leeds(S/N/C) mechanism). Also, a reaction set containing

162

four reactions from Wendt et al.

163

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


8

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Wendt + Leeds(S/N/C). Details regarding the sulfur chemistry and direct interaction between

165

NOx-SOx species is elaborated in the previous publication

166

compare the simulation results to the experimentally collected data.

167

3. Results and Discussion

168

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)].

176

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)

181

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).


9


187

NO2 + H NO + OH

R(20)

188

NO2 + O NO + O2

R(21)

189

CH2 + NO2 CH2O + NO

R(22)

Page 10 of 22

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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199

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

221

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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222

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%


12

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with a variability of 0.17%-5.38% from experiment to experiment. But the increase in the

246

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


13


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268

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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290

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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313

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,

333

no clear trend was obtained in terms of the effect of NO on SO3 formation experimentally,

334

although the kinetic simulations clearly showed an influence. In this study, FTIR spectroscopy

335

was employed to measure SO3.


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336

Before studying the effect of NO, SO3 measurements were conducted first to benchmark the

337

FTIR technique. Temporal profile of SO3, presented in Figure 6, was collected for φ = 0.86,

338

32.5% O2 in the oxidizer stream and 2500 ppmv SO2 in the reactor through simultaneous

339

sampling by FTIR from different temperature points. As seen from Figure 6, with the decline in

340

temperature from 1016K to 596K, the evolved SO3 concentration increases from 32 ppmv to 95

341

ppmv, which can be attributed to the formation through secondary routes, R(2) - R(3). The

342

Alzueta model predicts the SO3 profile to remain constant after 1050K, but experimentally

343

significant formation is observed till 600K. A similar trend was observed in the previous study 33

344

by the authors where the concentration of SO3 was underestimated at lower temperatures by the

345

kinetic mechanism comparing to experimental data obtained using the salt method. The

346

experiment was repeated in this study to collect the temporal profile of SO3 using the FTIR

347

spectrometer to validate that it is a viable tool to measure SO3. The data obtained by the FTIR

348

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


17


Page 18 of 22

349 350

Figure 7 illustrates how the SO3 concentration changes in presence of NO. These experiments

351

were conducted for φ = 0.86, reactor SO2 concentration = 2500 ppmv and inlet O2 concentration

352

in oxidizer = 32.5% while changing the NO concentration from 200 ppmv to 1500 ppmv in the

353

reactor. As it can be observed from the plot, in absence of NO, 83 ppmv SO3 is present at the

354

reactor outlet and with the introduction of NO, the SO3 concentration starts to decline. It should

355

be noted that the error bars become larger as the concentration of NO introduced into the system

356

increases, except for 1500ppm. The large deviation observed for 1200ppm NO is a result of one

357

data point deviating out of the four data points collected, which could be due to experimental

358

error. Although, a clear trend was not observed for high NO concentrations, there is a decrease

359

when 200ppm NO was introduced comparing to the case where NO was absent. This slight

360

decreasing is contrary to the predicted trend by the simulations with different reaction sets,

361

Alzueta + Leeds (S/N/C), Alzueta + Wendt and Alzueta + Wendt + Leeds (S/N/C), where the

362

SO3 concentration increases when NO is introduced. The Alzueta reaction mechanism alone

363

exhibits a small decrease in SO3 concentration at higher NO concentrations; however, it initially

364

increases when NO is added comparing to the case when NO is absent. The slight decrease

365

observed by the experimental results can be explained by the fact that introduction of NO into

366

the system leads to the consumption of O and OH radicals and this has been previously observed

367

by earlier experiments conducted under air combustion conditions

368

addition of the S/N/C subset from the Leeds mechanism, including direct interaction of NOx and

369

SOx species, seems to improve the model predictions for SO3 formation as both the Alzueta +

370

Leeds (S/N/C) and the Alzueta + Wendt + Leeds (S/N/C) reaction sets result in a better

371

agreement with the experimental data.


30

. In presence of NO, the

18

Page 19 of 22


372

In conclusion, FTIR spectroscopy has been employed for measuring SO3 and NO emissions in

373

flue gas under gas-phase oxy-combustion conditions and consistent results have been observed

374

with the salt method. The data collected to investigate the direct interaction between SOx and

375

NOx species show that conversion of SO2 to SO3 is slightly supressed in presence of NO and this

376

decline is in contrast with the model predictions. The addition of the S/N/C subset to take into

377

account the direct interactions between NOx and SOx species improved the model predictions.

378

Future work will involve further improvement of the oxy-combustion mechanisms available in

379

the literature. From a simple A-factor analysis, the reactions that play a significant role have

380

been determined to narrow down the list of reactions that need to be improved. Quantum

381

mechanical calculations will be conducted to calculate the reaction rate parameters and the

382

mechanism will be updated accordingly.

383

Supporting Information

384

Table S1: Test cases for combustion experiments

385

Figure S1: Sensitivity coefficients for N2 formation from recycled NO at ϕ=0.86, O2 = 32.5%

386

and NO = 2000 ppmv in reactor

387

Acknowledgements

388

The project was supported by the National Science Foundation under grant number 1236761.

389

The authors would also like to thank William Flake Jr. for his help with initial testing of the

390

combustion set-up.

391


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392

References

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An Investigation of the Interaction between NOx and SOx in Oxy

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