Implications of Fuel Choice and Burner Settings for Combustion

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Implications of fuel choice and burner settings for combustion efficiency and NOx formation in PF-fired iron ore rotary kilns Rikard Edland, Fredrik Normann, Christian Fredriksson, and Klas Andersson Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03205 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Implications of fuel choice and burner settings for

2

combustion efficiency and NOx formation in PF-

3

fired iron ore rotary kilns

4 5

Rikard Edland1*, Fredrik Normann1, Christian Fredriksson2, Klas Andersson1

6

1

7

University of Technology, SE-412 96, Göteborg

8

2

9

*Corresponding author, Tel. +46 31 722 1000. [email protected]

Division of Energy Technology, Department of Energy and Environment, Chalmers

LKAB, Box 952, SE-971 27 Luleå

10

ABSTRACT:

The combustion process applied in the grate-kiln process for iron ore pellet

11

production employs air-to-fuel equivalence ratios in the range of 4–6, typically with coal as

12

fuel and high-temperature air (>1000°C) as oxidant. The NOx emissions from these units are

13

in general significantly higher than those in other combustion systems and the large flows of

14

flue gases make the implementation of secondary measures for NOx control costly. Therefore,

15

it is of importance to investigate NOx formation under combustion conditions relevant for iron

16

ore production, in order to control the emissions from these units. The present work examines

17

NOx formation during the combustion of four pulverized coals, as well as during co-firing

18

with biomass in a pilot-scale kiln (580 kWfuel) based on a two-week experimental campaign.

19

The influence of burner settings was also included in the investigation. Based on the 1 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

20

presented experimental results and the results of previous modelling and experimental studies,

21

we suggest that the NOx emissions are mainly the result of a high conversion of fuel-bound

22

nitrogen (fuel-N) to NO. In particular, char-bound nitrogen (char-N) conversion appears to be

23

higher than in conventional pulverized fuel flames, presumably due to the high levels of

24

oxygen present in the char-burnout region. The temperatures in the kiln varied between the

25

test cases, but thermal NO formation is estimated to be of low importance.

26

1. Introduction

27

Iron ore is typically processed into pellets for easy transportation and handling in steel

28

making. To achieve the high temperatures required for sintering the hematite (Fe2O3) (the

29

pellet temperature should be >1300°C1), a rotary kiln may be used in the so-called ‘grate-kiln

30

process’. The kiln is heated by a flame and large volumes of highly preheated air. In the

31

rotary kiln, the molar air-to-fuel equivalence ratio, λ, is in the range of 4–6, which is high

32

compared to conventional pulverized fuel (PF) burners with λ-values in the range of 1.2–1.5,

33

and the inlet air temperature is around 1000°C (300°C in conventional burners). The process

34

is dependent upon an efficient combustion process, so current operation is often restricted to a

35

specific coal or oil. However, with the increasing volatility of the fossil fuel market and

36

increasing concerns regarding global warming, fuel flexibility and increased use of biomass

37

are of interest. Increasingly stringent regulations designed to reduce NOx emissions have also

38

intensified the desire to mitigate NOx emissions from the iron ore industry. Due to the high-

39

volume flue gas flow from the process, secondary mitigation strategies are costly, so primary

40

measures are preferred. However, when modifying the combustion process it is critical to

41

consider the pellet-making process, given that the sintering of the pellets requires controlled

42

heat transfer conditions and the oxidization of magnetite to hematite requires a high oxygen

43

content in the flue gas. 2 ACS Paragon Plus Environment

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

44

The combustion conditions in the kiln used for iron ore processing are thus considerably

45

different from those in conventional coal flames for heat and power generation, and it is

46

unclear how these conditions influence NOx formation. During the combustion of coal, the

47

main source of NOx is the nitrogen bound to the fuel (fuel-N), as long as the temperature is

48

kept low enough to avoid significant amounts of air-borne nitrogen to react with oxygen

49

(thermal NO formation). Although thermal NO may have some importance for solid fuel

50

combustion, fuel-NO usually contributes with at least 75% of the total NOx formation2. The

51

prompt NO mechanism, whereby hydrocarbon radicals attack the air-bound nitrogen, is

52

considered to play a negligible role in solid fuel combustion3. The properties of the fuel,

53

burner design, and combustion conditions are known to be important factors for controlling

54

NO formation. A simple way to reduce the NOx emissions is to change to a fuel with a lower

55

nitrogen content. In the flame or combustor, a reducing zone can be formed in which N2

56

formation is favored over NO formation. Such a zone may be created by staging the

57

combustion air, either externally or internally. Fuel-NO can further be divided into NO that is

58

formed from the nitrogen in the volatiles (vol-N) and NO that is formed from the nitrogen in

59

the char (char-N). It is mainly the conversion of vol-N that is reduced when air-staging is

60

applied. NO formation from vol-N is strongly dependent upon the local stoichiometry and

61

may reach levels from 10% to 100%4. While the conversion of char-N to NO has been

62

investigated in many studies, the results are complex. For single-particle experiments, char-N

63

conversion may be 100%, whereas when more particles are introduced (approaching a fixed

64

bed system) heterogeneous NO reduction becomes of importance and char-N conversion

65

decreases to around 50%5. Furthermore, the initial NO concentration has a strong influence on

66

the net NO formation during char burnout, in which a higher initial NO concentration

67

decreases the conversion of char-N to NO6, 7. Common NO/char-N ratios found in the

68

literature are in the range of 10%–30%2, 4, 8-10. While the importance of oxygen for char-N 3 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

69

conversion is debated, at high temperatures it appears that the conversion rate increases

70

significantly with increasing amounts of air2, 7, 11. The total conversion of fuel-N to NO is thus

71

mainly a reflection of how the fuel-N is distributed between volatiles and char and the local

72

stoichiometry during the release of vol-N and char-N, of which vol-N conversion is somewhat

73

easier to control. The nitrogen fraction in the volatiles increases with the pyrolysis

74

temperature and the oxygen content of the fuel3, 12. With local sub-stoichiometric conditions

75

(achieved through low levels of excess air, as well as controlled mixing), it is possible to have

76

a total fuel-N conversion of 2000°C29, 30) and the air-to-fuel

105

equivalence ratio is lower, which alters significantly the conditions for NO formation.

106

The present work is a technical-scale experimental investigation of the influences of fuel

107

characteristics and burner settings on the combustion process in general and on NOx

108

formation in particular. The aim is to characterize how the combustion process is affected by

109

a fuel change given the special conditions of iron ore processing, so as to be able to increase

110

fuel flexibility and enable the use of biofuels without risking the efficiency of the iron ore

111

process. We evaluate current combustion praxis (i.e., fuel characteristics, influences of λ-

112

values and preheating temperatures) for low-NOx combustion under conventional combustion

113

to conclude on its applicability to the conditions of iron ore processing.

114 115 116

2.1. Equipment and fuels

117

The kiln is a pilot-scale version of a 40-MWfuel rotary kiln used for iron ore processing. The

118

pilot kiln is designed to explore the features of the combustion process, i.e., the product

2. Experimental

The present investigation was performed with the LKAB test kiln (580 kWfuel) (Figure 1).

5 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

119

stream and the rotation of the kiln are not included. The combustion process is not adiabatic.

120

The burner hood (prior to the kiln inlet) and the first 4 m are scaled with regard to the full

121

scale kiln using constant velocity scaling. The diameter of the scaled kiln part is 0.65 m. The

122

total length of the test facility is 14 m in order to provide further measurement possibilities,

123

e.g. ash extraction. Measurement ports are located along the unit (marked as MH0-MH12 in

124

Figure 1). The burner has six registers (Figure 2), which are denoted N1–N6. Combustion air

125

enters at N1 and N4, while the remaining registers are fuel registers. The air that enters at N4

126

is swirled at an angle of 30°, so it is termed ‘swirl air’, while the air that enters N1 is called

127

‘axial air’. The burner is placed in the center with large volumes of highly preheated air being

128

introduced at two large inlets located above and below the burner (Figure 3). This air will be

129

referred to as ‘secondary air’, and the air flows in the burner (N1, N4 and fuel transport air)

130

will be referred to as ‘primary air’. The key combustion settings are listed in Table 1.

131

Measurements of CO2, CO, NO, NO2, O2, and SO2 were performed at the kiln outlet. The

132

levels of CO, CO2 and SO2 were measured by NDIR (ABB URAS), O2 by paramagnetism

133

(ABB MAGNUS), and NO and NO2 by NDUV (ABB LIMAS). In-flame gas measurements

134

were performed with a filter-equipped cooled probe connected to an FTIR-system (MKS

135

Multigas 2030) and a paramagnetic O2-measuring instrument (Maihak SIDOR). It is

136

estimated that the gas experienced instantaneous quenching due to the excessive cooling in

137

the probe. The probe was traversed horizontally in the kiln at four ports (MH0, MH1, MH3,

138

and MH7 in Figure 1). Seven radial measurement positions were used for each port to map

139

the radial concentration profiles. The central position was not included for MH0 due to the

140

high density of the fuel particles and potential interference with the flame. Temperature

141

measurements were performed using a suction pyrometer with a thermocouple type B at the

142

same locations as the gas measurements. The thermocouple was protected by two ceramic

143

housings, so as to minimize the effects of radiation, and mounted on the tip of a water-cooled 6 ACS Paragon Plus Environment

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

144

probe with a central pipe connected to an ejector, which provided the required suction

145

velocity.

146

Four pulverized coals and two types of biomass were used. Biomass A was a pulverized

147

wood treated with steam explosion, and Biomass B was an untreated but pulverized mixture

148

of fir and pine. The coals were pulverized in a roller mill to the size distribution of a full-scale

149

process. The biomass was instead treated in a hammer mill due to technical constraints of

150

roller mills with respect to the fibers in biomass (it might have been possible to mill Biomass

151

A in the roller mill but this was not tested). During the milling process, the biomass pellets

152

were partly pulverized into their original particle size (prior to pelletization), which was

153

smaller for Biomass A than for Biomass B. The size of the biomass particles was larger than

154

the coals but probably smaller than they would have been in a full scale process. The fuel

155

compositions and volume weighted mean diameter of the particle distribution are presented in

156

Table 2. The fuels were introduced through register N6 during the coal firing. Co-firing of

157

coal and biomass was performed with the burner inserted 220 mm into the kiln and with the

158

coal fed through register N3 and biomass fed through register N2. This was done in order to

159

represent two different burner types seen in full scale. The biomass provided 30% of the total

160

fuel energy input, which corresponded to approximately 40% on a mass basis.

161

For comparison, the outlet NOx concentration is presented as ppm NOx corrected to 6% O2

162

and is calculated according to Eq. (1). The actual outlet concentration was around 15-16% in

163

all cases, which is somewhat lower than the theoretically calculated concentration of 16%

164

(Table 3). Considering the flows that need to be taken into account (secondary and primary air

165

flows as well as the fuel feeding rate), this is an acceptable discrepancy. The ratio of NOx

166

emissions to fuel-N (ߟே ) is calculated according to Eq. (2). This value is equal to or higher

167

than the actual conversion ratio depending on the contribution from the thermal-NO formation

168

from the air born nitrogen. 7 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

169

‫ݔ‬ேைೣ ሺ6%ܱଶ ሻ = ሺ‫ݔ‬ேை + ‫ݔ‬ேைమ ሻ ∗

170

ߟே =

଴.ଶ଴ଽି଴.଴଺ ଴.ଶ଴ଽି௫ೀమ

೛ ൫௫ಿೀ ା௫ಿೀమ ൯௏ሶ೑೗ೠ೐ ቀ బ ቁ

௠ሶ೑ೠ೐೗ ௬ಿ /ெಿ

೅బ ೃ

Page 8 of 28

Eq. (1)

Eq. (2)

171

Where:

172

‫ݔ‬௜

173

ሶ = volumetric flue gas flow (in m3n/h) ܸ௙௟௨௘

174

‫݌‬଴

= normal pressure (101.3 kPa)

175

ܶ଴

= normal temperature (273.15 K)

176

R

= 8314 ௞௠௢௟௄

177

݉ሶ௙௨௘௟ = fuel mass flow (in kg/h)

178

‫ݕ‬ே

= mass fraction nitrogen in the fuel

179

‫ܯ‬ே

= molar mass of nitrogen (14 kmol/kg)

= fraction of component i

௃

180 181

2.2 Fuel test cases

182

into two sections, one for the coal comparison and one for the biomass co-firing. The

183

secondary air temperatures varied somewhat during the experiments and the measured values

184

are displayed in one of the columns.

185 186

2.3 Parameter study

187

involved changing the primary air flow and observing the outlet NOx concentration, although

188

flame measurements were performed on one occasion where the total amount of primary air

189

was significantly lower. The influence of the air-to-fuel equivalence ratio, λ, on the

190

concentration of NOx was investigated by decreasing the oxygen content of the secondary air

191

flow. The effect of secondary air temperature was also examined. Table 4 provides an

192

overview of the ranges and settings for the investigated parameters.

Table 3 presents the combustion settings for each of the different fuels. The table is divided

A study of how the air flows influence NOx emissions was carried out. These tests mainly


Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

193 194

This chapter presents the measurements from the experimental campaign. The results are

195

discussed and interpreted with respect to implications on combustion and NOx formation in

196

direct connection to the results.

197 198

3.1 Coal comparison

199

are presented in Figure 4 in the form of interpolated contour maps, in which the white fields

200

correspond to areas where measurements were not performed due to technical issues.

201

Temperatures of >1500°C were reached for Coal D and >1600°C for Coal C, while the peak

202

temperatures were significantly lower for Coal A and Coal B. The O2 concentrations were

203

high throughout the flame and the CO concentrations were relatively low (usually 2%) relative to the

207

other coals and since it is unlikely that the levels of O2 and CO would be high simultaneously,

208

the O2 levels for Coal C are assumed to be lower than those for the other coals.

3. Results and discussion

The in-flame measurements of temperature and selected gas components for the coal tests

209

It is clear that the oxygen-lean zone in front of the burner is negligible for any of the three

210

coals with O2 data. This means that there is a potential to enhance burner design to decrease

211

mainly the vol-N conversion to NO. From the NO maps, it appears that the concentrations are

212

higher further into the kiln for Coal B than for the other coals, and that the highest

213

concentration is measured for Coal C, although this concentration drops to levels similar to

214

those for Coal A shortly afterwards. NH3 was generally present at around 0–20 ppm, and is

215

therefore not presented; however, an ammonia peak at 200 ppm was observed in the central

216

position in MH1 for Coal C.

217

Figure 5 presents the NOx conversion for each coal. With the exception of small variations

218

in the secondary air temperature, burner settings were kept constant when testing the four 9 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

219

coals. Coal A generated the lowest NOx emissions and Coal B the highest emissions. As

220

anticipated, the level of NOx formation correlates with the amount of fuel-bound nitrogen in

221

the coals. An exception to this is Coal C, which exhibits a comparatively low fuel-N to NOx

222

conversion. However, Coal C has a high volatile content relative to the other coals, which

223

according to literature12 favours a low conversion of fuel-N to NOx (ߟே ). From Figure 6, it

224

can further be seen that the fuel-N to NOx conversion correlates well with the inverse volatiles

225

content.

226

The relative importance of fuel-nitrogen and volatile content is, however, case-specific and

227

difficult to predict without experimentation. Other factors, such as how the nitrogen is bound

228

in the fuel and local temperature profiles, will obviously also affect the formation of NOx in

229

the grate-kiln flames, which, taken together, makes modelling predictions uncertain. Full-

230

scale experiments are thus of great interest for future studies.

231

Although the NOx emissions correlate well with the fuel characteristics, they are

232

substantially higher than the levels seen in conventional PF flames. As a reference point,

233

using the best available technology (BAT) for large combustion plants gives emissions around

234

45-150 ppm (corrected to 6% O2) for pulverized coal using combined primary reduction

235

measures31. Although certain primary measures are not applicable to rotary kilns - due to the

236

rotation - significant improvements should be possible. The emissions seen in the test kiln are

237

also somewhat higher than the ones typically observed in full scale. The high level of

238

emissions (>1000 ppm at 6% O2) may be explained by either higher conversion of the volatile

239

nitrogen or the char-bound nitrogen to NO or thermal NO formation. Thermal NO formation

240

is dependent on O, N, and OH radicals, and is therefore coupled to the fuel combustion,

241

although decoupling is common due to the slower kinetics of thermal-NO formation. To get

242

an estimation of the thermal NO contribution, the nitrogen mechanism by Glarborg and

243

Mendiara32 was used to determine the NO-formation from N2 and O2 at 1700°C, i.e. 100°C 10 ACS Paragon Plus Environment

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

244

above the highest temperature measured during the campaign. Based on the measurements,

245

the high-temperature zone can, as a rough estimate, be assumed cylindrical with a diameter of

246

0.3m and a length of 1m (i.e. a volume of 0.07 m3). In this case, with a constant temperature

247

profile of 1700°C, the thermal NO formation rate becomes 0.0005 mole/s. This should be

248

compared to the total outlet NOx flow, which was about 0.015 mole/s in the case with the

249

highest temperature (Coal C). Thus, according to these rough calculations, the thermal NO

250

formation contributes with maximum 3% of the total NOx emissions.

251

In summary, vol-N conversion to NO appears to be high due to the high oxygen fraction in

252

the proximity of the burner outlet. Furthermore, char-N conversion is likely to be higher than

253

is usually seen in conventional combustion, since the high level of excess air leads to a high

254

level of oxygen in the char-burnout zone. The way in which the secondary air is added

255

probably inhibits external recirculation zones, which further promotes an oxidizing

256

environment.

257 258

3.2 Biomass co-firing

259

reference case with Coal A (all three cases used similar burner settings, see Table 3). Overall,

260

the temperature and concentration maps resemble each other, which means that substituting

261

30% of the coal energy with biomass does not appear to affect significantly (i.e. relative to

262

differences between coals and the requirements of the iron ore pelletization) the combustion

263

process. The oxygen levels were relatively high throughout the kiln for all three cases,

264

although slightly lower levels were found for the co-firing case with Biomass A, which also

265

exhibited a higher CO concentration. The zone with lower oxygen concentration was only

266

slightly longer for the biomass cases, although the diameters of the biomass particles are

267

substantially larger than those of the coal particles and the former are likely to burn later

Figure 7 presents the flame measurements for the two co-firing cases, as compared to a


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

268

within the kiln. The temperatures measured during these experiments never reached more than

269

1500°C, and thermal NO formation was presumably negligible.

270

Figure 8 presents the NOx emissions and ߟே values for the coal plus biomass co-firing

271

cases. Co-firing coal and biomass emitted less NOx than when coal was fired alone, as

272

expected since the biomass have lower nitrogen content. The reduction in NOx emissions due

273

to co-firing (23% relative to Coal A) is of the same magnitude as the reduction in coal flow

274

(30%) in the co-firing cases. This indicates that the reduction in NOx is primarily an effect of

275

reducing the incoming fuel nitrogen, and that there are no significant effects on NO formation

276

owing to interactions between the coal and biomass. The NOx/fuel-N ratios are slightly higher

277

for the biomass cases, which may reflect the fact that the added transport air required for

278

biomass feeding (Table 3) augments the early oxidizing environment for nitrogen released

279

with the volatiles.

280

Since the NO maps in Figure 7 are strikingly similar, and the case without biomass has

281

significantly higher NOx emissions (Figure 8), it is likely that a substantial share of NO is

282

formed from char-N in the coal particles after the last measurement port (MH7). Assuming

283

that the velocity of a coal particle is constant at 9 m/s (the speed of the secondary air), it

284

would take about 240 ms for it to reach MH7, which may be insufficient for complete

285

burnout.

286 287

3.3 Parameter study

288

combustion settings on the combustion process and NO formation.

289 290

The parameter study represents an investigation of the influences of the different

3.3.1 Primary air flow

The influence of different primary air flows in the burner on NOx emissions was

291

investigated by altering the swirl, axial, and transport air flows. It was found that the levels of

292

NOx correlated more with the total amount of primary air (see Figure 9) than with specific

293

swirl/flow settings. The results from the coal experiments are consistent with the finding of 12 ACS Paragon Plus Environment

Page 12 of 28

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

294

Vaccaro24 that the levels of NOx in rotary kilns decrease with decreasing primary air flow.

295

However, the cases with biomass co-firing showed the opposite trend, albeit much more

296

weakly than for the coals.

297

In-flame measurements were performed for one case in which the primary air flow was low,

298

and it is compared to the reference case with Coal A (same as Coal A in Figure 4), in which

299

the primary air flow was high. These two data-points are marked in red in Figure 9, and the

300

specific flame measurements are shown in Figure 10. The measured oxygen concentration

301

right in front of the burner is lower and the CO concentration is higher when the primary air

302

flow is low. Thus, the change in primary air flow has affected the flame and created a

303

reducing zone at the flame root. This flame type should produce less NO, which is in line with

304

the observation that NOx emissions are decreased from 1230 ppm to 930 ppm. We propose

305

that a decrease in vol-N conversion to NO is the most important factor, since less oxygen is

306

available during the release of volatiles. The temperatures are significantly higher when the

307

primary air flow is low, although this does not appear to contribute to any significant level of

308

thermal NO formation.

309 310

3.3.2 Secondary air flow

One of the most pronounced differences between combustion in rotary kilns for iron ore

311

processing and conventional PF combustion units (i.e., those used mainly for utility purposes)

312

is the amount of oxygen that is present during the process. To correlate the increased presence

313

of oxygen to NO formation, parts of the secondary air flow were replaced with a nitrogen

314

flow during one test sequence. Figure 11 shows how the ߟே value relates to the total

315

stoichiometric ratio of oxygen to fuel. The conversion of fuel-bound nitrogen to NO is

316

decreased from around 0.67 at λ=4.2 to around 0.59 at λ=2.8. As expected, the decreased

317

oxygen-to-fuel ratio leads to lower NOx emissions. The reason for this is that less oxygen is

318

available for both vol-N and char-N oxidation. The decrease in NOx emissions is, however, 13 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

319

limited at these levels of excess air, as decreasing the oxygen supply by roughly 30% reduces

320

NOx formation by about 10%. The effect of decreased λ is expected to be significant first

321

when it approaches unity. Such low levels of oxygen are not feasible in rotary kilns for iron

322

ore processing, since oxygen is required to oxidize the product, i.e., in addition to being

323

needed for fuel oxidation.

324 325

3.3.3 Secondary air temperature

326

and the air flow in the upper inlet is on average about 40°C warmer than the air flow in the

327

lower inlet, due to differences in heat loss. The influence of air temperature on NOx emissions

328

was examined by raising the temperatures of the two air flows by about 80°C when Coal A

329

was combusted with low amounts of primary air. The result is displayed in Figure 12. The

330

level of NOx was not affected to a significant degree. If anything, the levels of NOx emissions

331

decreased during the course of the temperature increase. This supports the earlier assessment

332

of the weak importance of thermal NO formation, since the thermal-NO mechanism is

333

sensitive to temperature changes once activated.

The secondary air enters the rotary kiln at inlets above and below the fuel inlet (Figure 3),

334 335

This work analyzes at the implications of fuel choice and burner settings for combustion

336

efficiency and NOx formation in PF-fired rotary kilns used for iron ore production. Four coals

337

and two biomass co-firing cases were assessed. In general, the NOx emissions were

338

substantially higher than those usually detected for pulverized fuel flames. The main reason

339

for this is a high conversion of fuel-N to NO, as the importance of thermal NO is estimated to

340

be limited or even negligible. We show that the primary air flows in the burner have a

341

significant effect on the flame and that low primary air flows lead to a flame with an oxygen-

342

lean zone in front of the burner, thereby decreasing the conversion of volatile nitrogen. The

343

lowest measured NOx emission for such a flame was 900 ppm (corrected to 6% O2), which is

4. Conclusions


Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

344

still exceptionally high. NO that originates from the char is anticipated to be the main

345

contributor to NOx in the studied burner configuration. When the amount of excess air is

346

decreased the NOx emissions are reduced, although this is not expected to be feasible in a full-

347

scale setup due to the importance of oxygen with respect to the product quality. Co-firing coal

348

with biomass reduces the NOx emissions, mainly as a result of the lower nitrogen content in

349

the biomass. Thus, in order to reduce NOx emissions using primary measures in iron ore

350

production rotary kilns, the choice of fuel is critical. An appropriate choice is a fuel with low

351

nitrogen content and high volatile content need to be assessed carefully. If the chosen fuel

352

combusts efficiently with low amounts of primary air, this will facilitate more stringent

353

control of both the thermal and fuel-related reactions routes.

354

Acknowledgment

355

This work was financed by LKAB and the Swedish Energy Agency.

356

Figures

357 358

Figure 1. The test kiln viewed from the side. MH0–MH12 indicate the measurement ports.

359

The distances are given in mm.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

360 361

Figure 2. Schematic of the burner orifice. The registers are denoted N1–N6. N4 is the swirl

362

air register and N1 is the axial air register. The other registers (white) are for fuel feeding.

363 364 365

Figure

3.

Cross-sectional

view

of

the

test

kiln

inlet.


Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Figure 4. Contour maps created by interpolation between measurement points for each test coal. The y-axis in each figure reflects a distance of 0 mm to 650 mm and the black dots represent the measurement points. Blank regions represent sites where no measurements were performed 17

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Figure 5. Nitrogen contents and NOx concentrations for each of the four test coals.


Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6. Ratio of outlet NOx to inlet nitrogen plotted against the percentage of volatile matter for each of the four fuels.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 20 of 28

Figure 7. Contour maps created by interpolation between measurement points for the three cases. The black dots represent measurement points.

20


Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. NOx emission levels and conversion ratios of outlet NOx to inlet nitrogen (ߟே ) for the coal- and biomass-firing cases.

Figure 9. NOx emissions for cases of combustion with different amounts of primary air. 21 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28


Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Figure 10. Contour maps created by interpolation between measurement points for the two cases. The black dots represent measurement points 23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Figure 11. Nitrogen to NOx conversion as a function of global stoichiometry. Coal B was used as fuel and the primary air flow was 60 m3n/h.

Figure 12. Effects of secondary air temperature on concentrations of NOx in a rotary kiln. Coal A was used as fuel and the primary air flow was 60 m3n/h.


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Tables

Table 1. Operating conditions of the LKAB test kiln Fuel input

580 kW

Primary air flow

60–200 m3n/h

Primary air temperature

20°C

Secondary air flow

≈2250 m3n/h

Secondary air temperature

950°–1050°C

Total λ

4–5

Primary air λ

0.24–0.45

Table 2. Compositions and properties of the fuels used. Parameter Unit

Coal A

Coal B

Coal C

Coal D

Biomass A

Biomass B

LHV

MJ/kg, dry

29.7

31.2

28.5

26.7

20.3

19.2

H2 O

Mass-%

0.9

1.3

1.5

2

4.4

7.8

Ash

Mass-% dry

13.3

8.4

9.2

15.2

1.1

0.37

Volatiles

Mass-% dry

21.6

22.6

36.5

26.9

76.2

83.9

C

Mass-% dry

76.1

79.8

72.1

69.6

52.9

50.8

H

Mass-% dry

3.9

3.8

4.7

3.5

5.8

6.2

N

Mass-% dry

1.38

2.09

2.35

1.77

0.11

0.1

O

Mass-% dry

5.1

5.7

11.2

9.4

40

42.5

Diameter

µm (average)

43

44

47

46

210

443

LHV, Lower heating value.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Table 3. Combustion settings for the different fuels used. The air flows are set values and not actual averages.

Fuel(s)

Register

Secondary Axial air T [°C] air flow [m3n/h]

Swirl air flow [m3n/h]

Transport Total λ gas flow (calculated [m3n/h] O2% out)

Coal A

N6

990

60

70

10 (N2)

4.4 (16.1%)

Coal B

N6

965

60

70

10 (N2)

4.5 (16.1%)

Coal C

N6

995

60

70

10 (N2)

4.5 (16.0%)

Coal D

N6

1000

60

70

10 (N2)

4.5 (16.0%)

Coal A

N3

1015

45

30

100 (Air)

4.5 (16.2%)

Coal A + Bio A

N3 + N2

1020

30

30

140 (Air)

4.6 (16.2%)

Coal A + Bio B

N3 + N2

1020

30

30

140 (Air)

4.5 (16.1%)

Table 4. Combustion settings used during the parameter study Investigated parameter

Test range

Primary air

Global stoichiometry


Fuel used

Primary air

60-240 m3n/h

-

≈ 4.5

1050°C

Mixed

Global stoichiometry

2.8-4.2

60 m3n/h

-

1050°C

Coal B


940-1050 °C

60 m3n/h

4.3

-

Coal A

1. Thurlby, J., A dynamic mathematical model of the complete grate/kiln iron–ore pellet induration process. Metallurgical Transactions B 1988, 19, (1), 103-112. 2. Pershing, D.; Wendt, J. In Pulverized coal combustion: The influence of flame temperature and coal composition on thermal and fuel NOx, Symposium (International) on Combustion, 1977; Elsevier: 1977; pp 389-399. 3. Glarborg, P., Fuel nitrogen conversion in solid fuel fired systems. Progress in Energy and Combustion Science 2002, 19, (2), 89-113. 4. Pohl, J. H.; Sarofim, A. F. In Devolatilization and oxidation of coal nitrogen, Symposium (International) on Combustion, 1977; Elsevier: 1977; pp 491-501.


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5. Jensen, L. S.; Jannerup, H. E.; Glarborg, P.; Jensen, A.; Dam-Johansen, K., Experimental investigation of no from pulverized char combustion. Proceedings of the Combustion Institute 2000, 28, (2), 2271-2278. 6. Coda, B.; Kluger, F.; Förtsch, D.; Spliethoff, H.; Hein, K.; Tognotti, L., Coal-nitrogen release and NOx evolution in air-staged combustion. Energy & fuels 1998, 12, (6), 1322-1327. 7. Spinti, J. P.; Pershing, D. W., The fate of char-N at pulverized coal conditions. Combustion and Flame 2003, 135, (3), 299-313. 8. Pershing, D.; Wendt, J., Relative contributions of volatile nitrogen and char nitrogen to NOx emissions from pulverized coal flames. Industrial & Engineering Chemistry Process Design and Development 1979, 18, (1), 60-67. 9. Song, Y. H.; Pohl, J. H.; Beér, J. M.; Sarofim, A. F., Nitric oxide formation during pulverized coal combustion. Combustion science and technology 1982, 28, (1-2), 31-40. 10. Thomas, K. M., The release of nitrogen oxides during char combustion. Fuel 1997, 76, (6), 457-473. 11. Nelson, P. F.; Nancarrow, P. C.; Bus, J.; Prokopiuk, A., Fractional conversion of char N to NO in an entrained flow reactor. Proceedings of the Combustion Institute 2002, 29, (2), 2267-2274. 12. van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K., Influence of process parameters on nitrogen oxide formation in pulverized coal burners. Progress in Energy and Combustion Science 1997, 23, (4), 349-377. 13. Harding, N.; Smoot, L. D.; Hedman, P. O., Nitrogen pollutant formation in a pulverized coal combustor: effect of secondary stream swirl. AIChE Journal 1982, 28, (4), 573-580. 14. Wang, W.; Brown, S. D.; Thomas, K. M.; Crelling, J. C., Nitrogen release from a rank series of coals during temperature programmed combustion. Fuel 1994, 73, (3), 341-347. 15. Harding, A.; Brown, S.; Thomas, K., Release of NO from the combustion of coal chars. Combustion and Flame 1996, 107, (4), 336-350. 16. Wendt, J. O.; Sternling, C.; Matovich, M. In Reduction of sulfur trioxide and nitrogen oxides by secondary fuel injection, Symposium (International) on Combustion, 1973; Elsevier: 1973; pp 897-904. 17. Smoot, L. D.; Hill, S. C.; Xu, H., NOx control through reburning1. Progress in Energy and Combustion Science 1998, 24, (5), 385-408. 18. Kühnemuth, D.; Normann, F.; Andersson, K.; Johnsson, F.; Leckner, B., Reburning of Nitric Oxide in Oxy-Fuel Firing, The Influence of Combustion Conditions. Energy & Fuels 2011, 25, (2), 624-631. 19. Dean, A. M.; Bozzelli, J. W., Combustion chemistry of nitrogen. In Gas-phase combustion chemistry, Springer: 2000; pp 125-341. 20. Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F., Emission control of nitrogen oxides in the oxy-fuel process. Progress in Energy and Combustion Science 2009, 35, (5), 385-397. 21. Davis, R. A., Nitric oxide formation in an iron oxide pellet rotary kiln furnace. Journal of the Air & Waste Management Association 1998, 48, (1), 44-51. 22. Zahl, R. K.; Haas, L. A.; Engesser, J. Formation of NOx in iron oxide pelletizing furnaces; University of Minnesota: 1995. 23. Edland, R.; Normann, F.; Andersson, K.; Fredriksson, C. In Formation of nitrogen oxides in rotary kiln burners: an assessment of pilot scale experiments using gaseous, liquid and solid fuels, INFUB 2015, 2015; 2015. 24. Vaccaro, M. H. In Low NOx rotary kiln burner technology: design principles & case study, Cement Industry Technical Confernece, 2002. IEEE-IAS/PCA 44th, 2002; IEEE: 2002; pp 265270. 25. Nielsen, P.; Jepsen, O. In An overview of the formation of SOx and NOx in various pyroprocessing systems, Cement Industry Technical Conference, 1990., XXXII, Record of Conference Papers., IEEE, 1990; IEEE: 1990; pp 255-276. 26. Nathan, G. J.; Manias, C. G., The role of process and flame interaction in reducing NOx emissions. In The Institute of Energy's Second International Conference on Combustion & Emissions Control, Energy, T. I. o., Ed. Pergamon: 1995; pp 309-318.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

27. Boateng, A. A., Rotary kilns: transport phenomena and transport processes. Butterworth-Heinemann: 2011. 28. Remus, R. Best Available Techniques (BAT) Reference Document for Iron and Steel Production: Industrial Emissions Directive 2010/75/EU Integrated Pollution Prevention and Control; 9279264753; Publications Office: 2013. 29. Mullinger, P. J. J., B. G. , NOx Reduction Techniques for Rotary Kilns American Flame Research Committee, Maui, Hawaii, October 16-20, 1994 1994. 30. Karstensen, K. H., Formation, release and control of dioxins in cement kilns. Chemosphere 2008, 70, (4), 543-560. 31. Reference Document on Best Available Techniques for Large Combustion Plants; JRC, The European Comission's science and knowledge service: 2006. 32. Mendiara, T.; Glarborg, P., Ammonia chemistry in oxy-fuel combustion of methane. Combustion and Flame 2009, 156, (10), 1937-1949.


Implications of Fuel Choice and Burner Settings for Combustion

Recommend Documents