Pulverized Sponge Iron, a Zero-Carbon and Clean Substitute for

Subscriber access provided by ST FRANCIS XAVIER UNIV

Combustion

Pulverized sponge iron, a zero-carbon and clean substitute for fossil coal in energy applications Henrik Wiinikka, Therese Vikström, Jonas Wennebro, Pal Toth, and Alexey Sepman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02270 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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 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 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.

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 27 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

Pulverized sponge iron, a zero-carbon and clean substitute for fossil coal in

2

energy applications

3

Henrik Wiinikka,ab* Therese Vikström,a Jonas Wennebro,a Pal Toth,ac and Alexey Sepmana

4

a

5

RISE Energy Technology Center AB, Division of Bioeconomy, RISE Research Institutes of Sweden, Box 726, SE-941 28, Piteå, Sweden

6 b

7 8

c

Division of Energy Science, Luleå University of Technlogy, SE-971 87, Luleå, Sweden

Department of Combustion and Thermal Energy, University of Miskolc, Miskolc-Egyetemvaros,

9

H3515 Hungary

10

*Corresponding author: [email protected]

11 12

ABSTRACT:

13

The direct combustion of recyclable metals has the potential to become a zero-carbon energy

14

production alternative, much needed to alleviate the effects of global climate change caused by

15

the increased emissions of the greenhouse gas CO2. In this work, we show that the emission of

16

CO2 is insignificant during the combustion of pulverized sponge iron, compared to that of

17

pulverized coal combustion. The emissions of the other harmful pollutants NOx and SO2 were 25

18

and over 30 times lower, respectively, than in the case of pulverized coal combustion.

19

Furthermore, 96 %wt. of the solid combustion products consisted of micron-sized, solid or

20

hollow hematite (α-Fe2O3) spheres. The remaining 4 %wt. of products was maghemite (γ-Fe2O3)

21

nanoparticles. According to thermodynamic calculations, this product composition implies near-

22

complete combustion, with a conversion above 98%. The results presented in this work strongly

23

suggest that sponge iron is a clean energy carrier and may become a substitute to pulverized coal

24

as fuel in existing or newly designed industrial systems.

25 26

Keywords: metal combustion, sponge iron, maghemite, coal combustion, 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 27

27

1. INTRODUCTION

28

Transitioning to clean, zero-carbon, renewable-based energy production technologies is essential

29

in order to reduce greenhouse gas emissions from fossil fuel use in fulfilment of the Paris

30

Agreement goals in order to alleviate the effects of global climate change.1-3 Solar-, wind- and

31

hydropower can eliminate the need for fossil fuels in electricity generation;4 however, the storage

32

and transportation of the renewable-based energy from the production site to the end-users

33

remain challenging. Recyclable metals have the potential to become storage and carrier materials

34

of renewable-based energy .5-7 Fig. 1A shows a proposed cycle of metals in energy production.

35

The metal powder is combusted in air, producing heat and power in-situ in areas with high energy

36

demand. In principle, no combustion products are formed apart from the solid metal oxides. Once

37

combusted, the metal powder can be regenerated by direct reduction using electrolytic hydrogen

38

(H2) in regions where renewable electricity is abundant. With respect to energy density (MJ/dm3),

39

metal powder energy carriers outperform batteries and liquid H2 (Fig. 1B, Table S1), higher

40

energy density (MJ/dm3) of the metal fuels will which facilitate long distance transportations by,

41

for example merchant ships (tankers and cargo vessels) or railway. Benefits of metal powders as

42

energy carriers over liquid H2 can include secure handling and less stringent safety requirements.6

43

Among possible metal fuel candidates (e.g. aluminum,9 magnesium,10 lithium11), iron has been

44

suggested as the optimal carrier since it is thought to combust in solid state without forming any

45

metal vapor, gaseous oxides or nanoparticles;6 hence, the resulting combustion products are

46

easily separable and a micron-sized oxide fraction can be recovered from the product stream.

47

Another significant benefit of using iron as an energy carrier is that reduction technology is

48

already commercially available: for example, the direct reduction (DR) process that converts iron

49

ore in the form of fines, pellets or sinter into sponge iron at a temperature well below the melting

50

point of the iron itself,12 held a share of 73 Mt of global iron production in 2016.13 In general, the 2 ACS Paragon Plus Environment

Page 3 of 27 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

51

DR process uses carbon monoxide (CO) and H2 from natural gas reforming as reducing agents.12

52

However there are also technologies that use H2 exclusively as a reducing agent.14 In the future,

53

the direct reduction of iron by using H2 from solar-powered water electrolysis will likely become

54

feasible with prices similar to that of steam-reformed H2 produced from natural gas15 – in this

55

case, the reduction process in the metal fuel cycle will also be zero-carbon. Preliminary

56

theoretical investigation has shown that the sponge iron system can be more efficient in storing

57

and transporting renewable-based energy than systems of other energy storage media, such as

58

liquid H2.16 One of the challenges in implementing an iron-based, zero-carbon energy system is

59

the lack of industrial devices that can combust the iron fuel.6 Assuming that existing combustors

60

can be retrofitted or used without modification for the combustion of pulverized sponge iron

61

(PSI), the scale of benefits of transitioning to an iron-based energy system can be estimated: for

62

example, the wide-scale replacement of pulverized coal (PC) with PSI in existing combustion

63

facilities can significantly reduce global CO2 emission, considering that in 2014, 46% of global

64

CO2 emissions originated from coal combustion.17 The combustion of micron-sized iron particles

65

(3-27 µm) has been demonstrated, showing that flame speeds in PSI flames are similar to those in

66

hydrocarbon flames.18 Combustion of iron nanoparticles (25-85 nm) has also been investigated in

67

enginelike conditions.19 However, the combustion of PSI with technically relevant particle sizes,

68

in the context of becoming a practical substitute for PC, has not been investigated before. Known

69

results regarding iron-based oxygen carriers20,21 from the chemical-looping combustion (CLC)

70

and chemical-looping reforming (CLR) fields, obtained using fluidized bed reactors are not

71

directly applicable to pulverized fuel combustion due to the significantly higher temperatures and

72

heating rates of powder combustion. The objective of this work was therefore to assess if PSI can

73

replace PC in large-scale energy applications by studying the combustion behavior of PSI and

74

comparing it with that of PC. The typical combustion environment inside a PC boiler was 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 27

75

modeled by using a small-scale entrained flow reactor. Similar reactors have been routinely used

76

before to investigate many aspects of PC combustion.22-29 Combustion behavior and emissions

77

were compared to those of PC.

78 79

2. EXPERIMENTAL

80

2.1. Combustion experiments. The combustion experiments were performed in a small scale

81

entrained flow reactor (EFR), see Fig. 2. The EFR consisted of a 2 m long alumina (Al2O3) tube

82

with an inner diameter of 50 mm installed in an electrically heated (SiC heating element) vertical

83

tubular furnace (Entech Energiteknik). The EFR was designed to operate at temperatures up to

84

1400°C using five individually controlled heating zones (each 354 mm long). The powder was

85

injected pneumatically into the EFR with a syringe particle feeder30 installed above the EFR. The

86

powder transport gas (N2) flow rate (1 NL/min) was controlled by a mass flow controller (MFC).

87

The particle injection tube, which was installed in the center of the EFR had an inner diameter of

88

6 mm and was encased in a water-cooled jacket with an outer diameter of 16 mm. The oxidizer (a

89

mixture of N2 and O2) was injected into the EFR outside the particle injection tube. Before

90

entering the EFR, the oxidizer passed a flow straightener made of a sintered porous disk (1.6 mm

91

thick). The amount of N2 and O2 was controlled by two MFC’s that supplied gas in the ranges of

92

0-5 NL/min and 0-50 NL/min for for O2 and N2, respectively.

93

After the EFR, the flue gas passed through a pre-cyclone (Dekati) which separated particles

94

with an aerodynamic diameter above 10 µm from the flue gas. The particle separated in the pre-

95

cyclone is hereafter referred as solid residues. After the pre-cyclone, a small portion of the flue

96

gas flow was withdrawn from the flue gas channel and analyzed with respect to CO2, CO, H2O,

97

NO and SO2 by a Fourier Transform Infrared Spectroscopy (FTIR) instrument (MKS 2030-HS).

98

The O2-content in the same slip stream was analyzed by using a lambda sensor. Another 4 ACS Paragon Plus Environment

Page 5 of 27 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

99

representative portion of the flue gas flow was sampled from the same location in the flue gas

100

channel, diluted approximately 50 times with N2 in two steps (Dekati) and analyzed with respect

101

to particle concentration and particle mass size distribution with a 30 L/min multi-stage, low

102

pressure impactor (LPI). The LPI (Dekati) was designed according to the principle of inertial

103

impaction.31 The LPI separated particles from the gas according to their aerodynamic diameters

104

in 13 stages from 0.03 to 10.7 µm.32 The particles, collected in the LPI is hereafter referred to

105

fumes since most of them are most like produced from condensation of inorganic vapor as

106

discussed later in this paper.

107 108

2.2. Fuels and experimental conditions. Two commercially available sponge iron powders,

109

delivered by Höganäs (Iron A) and Alibaba (Iron B), were used as fuel in this work. A

110

bituminous coal was used as a reference fuel. The morphology (SEM micrographs) and chemical

111

composition of the fuels are presented in Table 1.

112

Operational parameters of the EFR are summarized in Table 2. The amount of O2 supplied

113

with the combustion air approximately corresponded to an excess air ratio (λ) of 2 for all studied

114

fuels. Theoretically, in stoichiometric combustion (λ=1), all Fe is oxidized to the most stable iron

115

oxide form, Fe2O3. The same thermal power, ~310 W, was supplied to the EFR independent of

116

the fuel. The EFR was operated at a temperature of 1200°C during all experiments. The λ used in

117

this experiment is slightly larger than what would be expected during industrial combustion in of

118

PSI and PC performed in a highly turbulent flow. The reason for the larger λ is the low

119

turbulence level in this small scale experiment (Reynold number ~ 70) and therefore more O2 is

120

made available in order to secure that oxidation rate in the EFR is not limited by mixing and

121

instead controlled by reaction kinetics.

122 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 27

123

2.3. X-ray powder diffraction (XRD). X-ray powder diffraction (XRD) was used for the

124

identification of possible crystalline phases in the iron oxide solid residue and fumes. Both Umeå

125

University (UmU) and ALS Global (ALS) independently analyzed the solid residue from PSI

126

combustion with an XRD instrument equipped with a CuKα radiation source. The fumes from

127

PSI combustion were only analyzed by UmU. Particulate matter samples collected on the

128

impactor plates 4-6 with a d50 size cut of 0.160-0.400 µm were analyzed together in order to

129

obtain a sufficient sample size. The quantification (wt-%) of the identified crystalline phases was

130

carried out by using the Rietveld method.33

131 132

2.4. Wet chemistry analysis. Complementary quantification of the amount of Fe, FeO, Fe3O4

133

and Fe2O3 compounds in the solid residue samples were also determined by an inhouse wet

134

chemistry analytic method (titration). The analysis was performed by Luossavaara-Kiirunavaara

135

AB (LKAB), the largest supplier of iron ore pellets in Europe. The average results and the

136

standard deviation from these three independent measurements (2 from XRD and 1 from

137

titration) are presented in the paper.

138 139

2.5. Scanning Electron Microscopy (SEM). The fuels (iron and coal powders), the solid

140

particles collected in the pre-cyclone and the particulate matter collected on the impactor plates

141

were characterized with respect to morphology and elemental composition by Scanning Electron

142

Microscopy (SEM, model Hitachi TM3030 plus) equipped with an energy disperse spectroscopy

143

(EDS) detector (Bruker Quantax 70).

144 145

2.6. Transmission Electron Microscopy (TEM). Iron oxide nanoparticles were imaged and

146

analyzed in a transmission electron microscope (TEM, Jeol JEM 2100F) equipped with an energy 6 ACS Paragon Plus Environment

Page 7 of 27 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

147

dispersive X-ray spectrometer (EDS, JED 2300). Images were recorded by using a bottom

148

mounted CCD camera (Gatan, Ultra Scan) operated using the Digital Micrograph program

149

package. The chemical composition of observed particles was determined by spot measurements

150

by using a focused beam. The diffraction mode was used to confirm the appearance of crystallites

151

in the materials which were then imaged in dark field mode.

152 153

2.7. Thermodynamic equilibrium calculations (TEC’s). In order to interpret the experimental

154

results, thermodynamic equilibrium calculations (TEC’s) were performed by using the

155

equilibrium module (Gibbs energy minimization) in FactSage™ 6.4.34 In general, the

156

thermodynamic model calculates the composition of the product gas and inorganic components

157

as a function of temperature. However, in FactSage there is also an option to specify the standard

158

enthalpy of formation, ∆hf (J/mol) for the reactants, and solve directly for temperature (i.e., to

159

include the energy equation). This option was used in this work for some calculations. The

160

thermodynamic databases used in the calculations were FACTPS that includes data for pure

161

stoichiometric gas, liquid and solid phases and FToxide that contains data for pure oxides and

162

oxide solutions. By using the oxide solution models SLAGA (oxide melt: FexOy), SPINA

163

(spinel: Fe3O4) and MeO_A (monoxide: FeO, Fe2O3) it is possible to calculate the complex phase

164

diagram of Fe-Fe2O3.35

165

Three sets of TEC’s were performed in order to understand the combustion behavior of the

166

iron particles in the EFR. In all TEC’s, λ was varied from sub-stoichiometric conditions (λ=0.17)

167

to conditions representing an excess of the oxidizer (λ=2.68). In the first set of TEC’s, the energy

168

equation was included in the calculations. Adiabatic combustion was assumed. The initial

169

temperature of the iron particle and the combustion air was 25°C. The first set of calculations can

170

be used to in the interpretation of the initial oxidation of the iron ore particles close to the burner 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

Page 8 of 27

171

(i.e., the iron dust flame). In the second set of TEC’s, the process temperature was fixed to the

172

process temperature of the EFR. This calculation can be used in the interpretation of the

173

combustion products when the iron ore particle temperature equals to that of the EFR. Finally, in

174

order to investigate the influence of preheating on the amount of gaseous iron species (Fe(g) and

175

FeO(g)) the first set of TEC’s was repeated with a varying degree of air preheating (200, 400, and

176

600°C). This set of TEC’s was motivated by the fact that combustion air can be preheated in the

177

EFR before it reacts with an iron ore particle.

178 179

3. RESULTS AND DISCUSSIONS

180

3.1. Flue gas emissions. The emission of CO2, minor gaseous components (CO, NOx, and SO2)

181

and SR during the combustion of two types of PSI and coal were measured. Solid residues (SR)

182

and fumes were defined as particulate matter with an aerodynamic diameter above and below 10

183

µm, respectively. The flue gas composition (Table S1) and emissions (Fig. 3) showed significant

184

differences between PSI and PC combustion at similar experimental conditions. As expected, the

185

emission of CO2 was insignificant in PSI combustion, verifying that the combustion stage in the

186

metal fuel cycle is a zero-carbon process. In addition to being the most significant contributor to

187

anthropogenic CO2 emissions, PC combustion is a significant emitter of the controlled pollutants

188

NOx and SO2,36,37 responsible for approximately 15% and 50% of global emissions,

189

respectively.38 Our tests showed that substituting PSI for PC has a significant positive impact on

190

NOx and SO2 emissions: the emission of NOx was reduced from approximately 500 mg/MJfuel to

191

20 mg/MJfuel, while the emission of SO2 was reduced from 230 mg/MJfuel to 1-7 mg/MJfuel,

192

depending on the type of PSI. The NOx emissions for PSI are also significantly lower compared

193

to reported values from industrial PC boilers,39,40 see Table 3. Furthermore, the observed NOx

194

emissions from PC combustion is similar compared industrial emissions verifying that the EFR 8 ACS Paragon Plus Environment

Page 9 of 27 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

195

and the experimentally conditions used in this work is relevant. This observation suggests that

196

implementing a retrofit metal fuel cycle might also alleviate adverse environmental effects

197

typically attributed to NOx and SO2 emissions, such as acid rain, atmospheric particulate matter,

198

and the reduced concentration of tropospheric ozone.41 Furthermore, the emissions of NOx from

199

PSI combustion was much lower compared to the NOx emission of 125±16 mg/MJ recalculated

200

by us from the reported value of 1100±140 mg/Nm3 (O2 % in flue gas ~9 %), for magnesium

201

combustion in air,10 indicating that PSI also is a superior fuel with respect to NOx emissions.

202

The large reduction in NOx emission can be explained by the fact that different formation

203

mechanisms dominate in PSI and PC combustion. In coal combustion, NOx are formed by the

204

thermal-, Fenimore- and fuel-bound nitrogen mechanisms.41 Both the Fenimore- and fuel-bound

205

nitrogen mechanisms require interaction with either hydrocarbon radicals or nitrogen from the

206

fuel itself. Compared to PC, PSI does not contain a significant amount of nitrogen (see Table 1),

207

and no hydrocarbon intermediates are formed during its combustion; therefore, in PSI

208

combustion, the NOx are only produced by the thermal mechanism. This mechanism involves the

209

dissociation of N2 and O2 and is initiated at temperatures above 1500°C. At stoichiometric

210

conditions, the adiabatic flame temperature of PSI combustion almost reaches 2000°C;6

211

therefore, one can conclude that during PSI combustion, most of the NOx are formed by the

212

thermal mechanism. The much higher NOx emissions observed during magnesium/air

213

combustion9 can again be attributed to the dominance of the thermal mechanism and the

214

significantly higher temperatures of magnesium flames,42 compared to those observed in flames

215

of PSI. Similarly to hydrocarbon combustion, optimization through e.g., oxidant staging or flue

216

gas recirculation41 can most likely further reduce NOx emissions from PSI combustion. The

217

pollutant SO2 is formed when sulfur inherent to the fuel is oxidized in the combustion process. As

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 27

218

the types of PSI studied here had significantly lower sulfur content compared to that of typical

219

PC (see Table 1), SO2 emissions were also lower.

220 221

3.2. Solid products after combustion. The solid residues (SR) of PSI combustion consisted of

222

micron-sized oxide particles and a fine fume fraction of nanoparticles. The micron-sized particles

223

were separated from the flue gas by using a cyclone. The chemical composition and morphology

224

of the separated SR from PSI combustion are shown in Fig. 4A-B and Figs. S1-S3. The amount

225

of hematite (α-Fe2O3, 88-92 wt%), the most oxidized form of the three relevant iron oxides (FeO,

226

Fe3O4 and Fe2O3), indicated efficient combustion and near-complete oxidation. The rest of the

227

micron-sized particles consisted mostly of magnetite (Fe3O4). The morphology of the PSI

228

particles changed significantly during combustion: from an irregular shape to a nearly spherical

229

or slightly ellipsoidal shape (Fig. 4B). During combustion, the size of the reacting particles

230

increased and small cracks and holes appeared on their surface. Most of the produced particles

231

were solid; however, many appeared to have large internal cavities (Fig. 4B and Fig. S3). Small

232

voids inside iron oxide particles (10-60 µm) has been observed before during the solid state

233

oxidation of iron at 700 °C20 and explained by the Kirkendall effect.43 In this case, with the

234

temperature of the iron oxide particles being above the melting point, we instead suggest that the

235

observed changes in size and shape can be explained by a combination of oxidation and melting,

236

as well as by the evaporation of a small portion of the iron from the center of the particles in a

237

way similar to what has been observed in aluminum combustion.44

238

Thermodynamic equilibrium calculations (TEC’s) predicted the presence of both an iron oxide

239

melt and gaseous iron in the early stage of the oxidation process as well as particle temperatures

240

exceeding 1500°C (Fig. S4). The oxidation of iron to wustite (FeO) causes rapid heating – the

241

ejection of rapidly expanding, gaseous iron formed in the center of a molten iron oxide particle 10 ACS Paragon Plus Environment

Page 11 of 27 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

242

can result in the hollow structures. Further oxidation of FeO to Fe3O4 and Fe2O3 increases

243

particle temperature, approaching the adiabatic temperature of approximately 2000°C (Fig. S4).

244

Simultaneously, particle size increases due to mass uptake from oxygen and decreasing density

245

(7.9 g/cm3 to 5.2 g/cm3). For example, the diameter of a spherical, homogeneous iron particle can

246

increase by approximately 30% when it is converted to Fe2O3 – the diameter of hollow particles

247

might increase even more. As seen from the TEC’s (Fig. S5), at a reactor temperature of 1200°C,

248

solid Fe3O4 can be fully oxidized to solid Fe2O3, supporting the experimental observation

249

regarding the Fe2O3 and Fe3O4 content of the produced iron oxide particles.

250

Experimental results indicated that approximately 4 wt% of the iron evaporated and formed

251

fumes after condensation (Fig. 5) – this observation is in reasonable agreement with the

252

predictions of TEC’s (Fig. S6); however, it contradicts the results of previous studies that

253

reported the fully solid state combustion of PSI, without forming any metal fumes.6 The fumes

254

consisted of aggregates of iron oxide nanoparticles in the form of maghemite (γ-Fe2O3) and α-

255

Fe2O3 (Figs. 6A and 6B and Figs. S7 and S8). Nanoparticles form when gaseous iron (Fig S6) in

256

the form of Fe(g) or FeO(g) reacts with oxygen. Due to the extremely low vapor pressure of iron

257

oxides, nuclei form by homogeneous condensation and grow by heterogeneous condensation

258

until all Fe(g)/FeO(g) have been absorbed, resulting in primary particle sizes between 30 and 50

259

nm. The aerodynamic diameters of the vast majority of aggregates formed by the coagulation of

260

primary particles were below 5 µm (Fig. 5). The observation regarding the formation of γ-Fe2O3

261

nanoparticles is interesting, due to the known superparamagnetic and ferromagnetic properties of

262

these particles and their potential applications in magnetic data storage, sensors, biosensing, drug

263

delivery, catalysis and cancer treatment.45-47 Flame synthesis is one possible production method

264

of γ-Fe2O3 nanoparticles.48,49 Although considered as material loss in the metal fuel cycle, a small


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 12 of 27

265

amount of the fumes produced during PSI combustion may therefore become a high value by-

266

product.

267 268

3.3. Practical implications. The amount of produced SR and fumes during PSI combustion was

269

significantly higher compared to that observed during PC combustion (Fig. 3), suggesting that

270

handling the residues might become the first technical challenge in implementing PSI

271

combustion. However, due to the high melting point of the iron oxides (above 1550°C) and the

272

absence of impurities, one can expect that PSI can be combusted without any ash-related

273

operational problems that often occur during coal combustion.50 Therefore, steam pressure can

274

potentially be increased relative to that typically achievable in PC combustion, allowing for

275

increasing power efficiency. Due to the coarse granularity of the SR, iron oxide particles can be

276

separated from the flue gas by using ordinary cyclones, in a manner similar to the separation of

277

coal combustion fly ash. A subsequent cleaning device such as an electrostatic precipitator or

278

baghouse filter might be necessary for the separation of fumes and nanoparticles from the flue

279

gas. Compared to PC combustion, the mass load of fumes is higher during PSI combustion;

280

however, the iron oxide nanoparticles were non-adhering at relevant flue gas temperatures

281

(150°C), in contrast with fumes from coal combustion (Fig. S9). Maintaining low flue gas

282

temperature is of significant interest in order to maximize energy recovery from the combustion

283

system. During PC combustion, the dew point of sulfuric acid (H2SO4) sets a practical limit on

284

the minimum achievable flue gas temperature. At the dew point, H2SO4, formed from SO2 and

285

H2O, condenses from the flue gas forming a corrosive deposit, leading to the low temperature

286

corrosion and plugging of economizers and air preheaters.51 Since the flue gas of PSI combustion

287

is practically free from H2O and SO2, the flue gas temperature can be reduced without risking the

288

condensation of H2SO4 (Fig. S10A). Based on our experiments, when substituting PC with PSI, 12 ACS Paragon Plus Environment

Page 13 of 27 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

289

we estimated that the heat losses through the flue gas can be reduced from 6.5% to below 2%

290

(Fig. S10B).

291 292

4. CONCLUSIONS

293

In this work, pulverized sponge iron (PSI) and pulverized coal (PC) were combusted under

294

similar conditions in an entrained flow reactor. The following conclusions can be drawn from the

295

results of the study:

296

•

substitute for PC in furnaces and boilers.

297 298

•

The emissions of harmful pollutants, NOx and SO2 were insignificant compared to that of PC, indicating that PSI can become an environmentally friendly substitute for PC.

299 300

The combustion of PSI was almost complete (>98%) indicating that PSI can be used as a

•

In contrast with results of previous studies, a small part of the iron (~4 wt%) actually

301

vaporized during the combustion process and iron oxide nanoparticles (or fumes) formed

302

from the gaseous iron through condensation.

303

•

Because of its many advantageous combustion characteristics, we believe that PSI can

304

become a zero-carbon fuel in a recyclable metal fuel cycle substituting PC in existing or

305

newly designed combustion devices in the future and the results presented in this paper

306

should encourage more studies relevant for using PSI as an energy carrier such as cyclic

307

studies of the reduction-oxidation behavior of PSI, pilot scale combustion trials with PSI

308

as fuel and techno-economic analysis of the concept.

309 310

Acknowledgment


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 27

311

The authors gratefully acknowledge LKAB for assistance in wet chemistry analysis of the iron

312

oxides and B. Lindblom (LKAB) for helpful discussions. We also thank K. Jansson (Stockholm

313

University) for carrying out TEM analysis, N. Skoglund (Umeå University) for carrying out XRD

314

measurements and H. Sefidari (Luleå University of Technology) for advice on TEC’s. This work

315

was financed by the Swedish Government via the strategic-competence model for RISE ETC.

316 317 318 319 320 321

References 1. Nakata, T.; Rodinov, M. Application of energy system models for design a low-carbon society. Prog. Energy Combust. Sci. 2011, 27, 462-502 2. Rockström, J.; Gaffney, O.; Rogelj, J.; Meinshausen, M.; Nakicenovic, N.; Schellnhuber, H.J. A roadmap for rapid decarbonization. Science 2017, 355, 1269-1271

322

3. Paris agreement, United Nations Framework Convention on Climate Change, 2015

323

4. Jacobson, M.Z.; Delucchi, M.A. Providing all global energy with wind, water, and solar

324

power, Part I: Technologies, energy resources, quantities and area of infrastructure, and

325

materials. Energy Policy 2011, 39, 1154-1169

326 327

5. Wen, D. Nanofuel as a potential secondary energy carrier. Energy Environ. Sci. 2010, 3, 591-600

328

6. Bergthorson, J.M.; Goroshin, S.; Soo, M.J.; Palecka, J.; Frost, D.L. Direct combustion of

329

recyclable metal fuels for zero-carbon heat and power. Applied Energy 2015, 160, 368-

330

382

331

7. Bergthorson, J.M.; Yavor, Y.; Palecka, J.; Georges, W.; Soo, M.; Vickery, J.; Goroshin,

332

S.; Frost, D.L.; Higgins, A.J. Metal-water combustion for clean propulsion and power

333

generation. Applied Energy 2017, 186, 13-27


Page 15 of 27 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

334 335

Energy & Fuels

8. Bergthorson, J.M. Recyclable metal fuels for clean and compact zero-carbon power. Prog. Energy Combust. Sci. 2018, 68, 169-196

336

9. Shkolnikov, E.I.; Zhuk, A.Z.; Vlaskin, M.S. Aluminium as energy carrier: Feasibility

337

analysis and current technologies overview. Renew. Sustain. Energy Rev. 2011, 15, 4611-

338

4623

339

10. Garra, P.; Leyssens, G.; Allgaier, O.; Schönnenbeck, C.; Tscamber, V.; Brilhac, J.F.;

340

Tahtouh, T.; Guezet, O.; Allano, S. Magnesium/air combustion at pilot scale and

341

subsequent PM and NOx emissions. Applied Energy 2017, 189, 578-587

342

11. Maas, P.; Schiemann, M.; Scherer, V.; Fisher, P.; Taroata, D.; Schmid, G. CFD

343

Simulation of a 100 MWth lithium combustion slag tap furnace as a basis for an energy

344

storage process. Energy Procedia 2017, 105, 3978-3983

345

12. Zervas, T.; McMullan, J.T.; Williams, B.C. Gas-based direct reduction process for iron

346

and steel production. Int. J. Energy Research 1996, 20, 157-185

347

13. 2016 world direct reduction statistics. Midrex Technology Inc, 2017

348

14. Nuber, D.; Eichberger, H.; Rollinger, B. Circored fine ore direct reduction. Millen. Steel

349 350 351

2006, 2006, 37-40 15. Rodriguez, C.A.; Modestino, M.A.; Psaltis, D.; Moser, C. Design and cost consideration for practical solar-hydrogen generators. Energy Environ. Sci. 2014, 7, 3828-3835

352

16. Mignard, D.; Pritchard, C. A review of the sponge iron process for the storage and

353

transmission of remotely generated marine energy. Int. J. Hydrogen Energy 2007, 32,

354

5039-5049

355

17. Key world energy statistics, International Energy Agency, 2016

356

18. Tang, F.D.; Goroshin, S.; Higgins, A., Lee, J. Flame propagation and quenching in iron

357

dust clouds. Proc. Combust. Inst. 2009, 32, 1905-1912 15 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 16 of 27

358

19. Mandilas, C.; Karagiannakis, G.; Konstandopoulos, A.G.; Beatrice, C.; Lazzaro, M.;

359

DiBlasio, G.; Molina, S.; Pastor, J.V.; Gil, A. Study of Oxidation and Combustion

360

Characteristics of Iron Nanoparticles under Idealized and Enginelike Conditions. Energy

361

Fuels 2016, 30, 4318-4330

362

20. Qin, L.; Majumder, A., Fan, J.A.; Kopechek, D., Fan, L.S. Evolution of nanoscale

363

morphology in single and binary metal oxide microparticles during reduction and

364

oxidation processes. J. Mater. Chem. A 2014, 2, 17511-17520

365 366

21. Tang, M., Xu, L.; Fan, M. Progress in oxygen carrier development of methane-based chemical-looping reforming: A review. Applied Energy 2015, 151, 143-156

367

22. Timothy, L.D.; Froelich, D.; Sarofim, A.F.; Béer, J.M. Soot formation and burnout

368

during the combustion of dispersed pulverized coal particles. Proc. Comb. Inst. 1986, 21,

369

1141-1148

370 371 372 373 374 375 376 377 378 379 380 381

23. Helble, J.; Neville, M., Sarofim, A.F. Aggregate formation from vaporized ash during pulverized coal combustion. Proc. Comb. Inst. 1986, 21, 411-417 24. Baxter, L.L.; Mitchell, R.E.; Fletcher, T.H.; Hurt, R.H. Nitrogen release during coal combustion. Energy Fuels 1996, 10, 188-196 25. Baxter, L.L.; Mitchell, R.E.; Fletcher, T.H. Release of inorganic material during coal devolatilization. Combust. Flame 1997, 108, 494-502 26. Hu, Y:, Naito, S., Kobayashi, N., Hasatani, M. CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases. Fuel 2000, 79, 1926-1932 27. Terame, T.; Takarada, T.; Fine ash formation during pulverized coal combustion. Energy Fuels 2009, 23, 2018-2024 28. Zang, Y.; Zhang, J.; Sheng, C.; Liu, Y.; Zhao, L.; Ding, Q. Quantitative analysis of NOx reduction in oxy-coal combustion. Energy Fuels 2011, 25, 1146-1152 16 ACS Paragon Plus Environment

Page 17 of 27 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

382 383 384 385

Energy & Fuels

29. Ma, H.; Zhou, L.; Ma, S.; Wang, Z.; Cui, Z.; Zhang, W.; Li, J. Reaction mechanism for sulfur species during pulverized coal combustion. Energy Fuels 2018, 32, 3958-3966 30. Molinder, R.; Wiinikka, H. Feeding small biomass particles at low rates. Powder Technol. 2015, 269, 240-246

386

31. Solomon, P.A.; Fraser, M.P.; Herckes, P. Methods for chemical analysis of atmospheric

387

aerosols in Aerosol measurements: principles, techniques, and applications. P. Kulkarni,

388

Baron, P.A.; Willeke, K. Eds, Wiley, New Jersey, ed. 3, 2011, part. 2, chap. 8

389 390 391 392

32. Marjamäki, M.; Keskinen, J.; Chen, D.R.; Pui, D.Y.H. Performance evaluation of the electrical low-pressure impactor (ELPI). J. Aerosol Sci. 2000, 31, 249-261 33. Rietveld, H.M. A profile and refinement method for nuclear and magnetic structures. J.Appl. Cryst. 1969, 2, 65-71

393

34. Bale, C.; Bélisle, E.; Chartrand, P.; Decterov, S.; Eriksson, G., Hack, K.; Jung, I., Kang,

394

Y., Melançon, J:; Pelton, A., Robelin, C.; Petersen, S. FactSage thermochemical software

395

and databases – recent developments. Calphad 2009, 33, 295-311

396

35. Eisenhüttenleute, V.D., Allibert, M. Slag atlas, Verlag Stahleisen, ed. 2, 2008

397

36. On the limitation of emissions of certain pollutants into the air from large combustion

398 399 400

plants, European Parliament and of the Council, Directive 2001/80/EC, 2001 37. 2014 Program Progress – Clean Air Interstate Rule, Acid Rain Program, and Former NOx Budget Trading Program, United State Environmental Protection Agency, 2014

401

38. Energy and air pollution, IEA Word Energy Outlook Special Report, 2016

402

39. Chui, E.H.; Gao, H. Estimation of NOx emissions from coal-fired utility boilers. Fuel

403

2010, 89, 2977-2984


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 27

404

40. Wang, Q.; Chen, Z.; Liu, T.; Zeng, L.; Zhang, X.; Du, H.; Li, Z. Industrial experiments on

405

anthracite combustion and NOx emissions with respect to swirling secondary air for a 300

406

MWe deep-air-staged down-fired utility boiler, Energy Fuels 2018, 32, 7878-7887

407

41. Warnatz, J.; Mass, U.; Dibble, R.W. Combustion, Springer, Berlin, ed. 4, 2010

408

42. Wang, S.; Corcoran, A.L.; Dreizin, E.L. Combustion of magnesium powders in products

409 410 411 412 413

of an air/acetylene flame. Combust. Flame 2015, 162, 1316-1325 43. Smigelskas, A.D.; Kirkendall, O.E. Zinc diffusion in alpha brass. Trans. AIMS. 1947, 171, 130-142 44. RW Barlett, JN Ong, WM Fassell, CA Papp. Estimating aluminium particle combustion kinetics. Combust. Flame 1963;7:227-234

414

45. Perez. J.M. Iron oxide nanoparticles: hidden talent, Nat. Nanotech. 2007, 2, 535-536

415

46. Laurent, S.; Forge, D.; Roch, M.; Robic, C.; Vander Elst, L.; Muller, R.N. Magnetic iron

416

oxide

nanoparticles:

synthesis,

stabilization,

vectorization,

physicochemical

417

characterizations, and biological application. Chem. Rev. 2008, 108, 2064-2110

418

47. Zanganeh, S.; Hutter, G.; Spitler, R.; Lenkov, O.; Mahmoudi, M.; Shaw, A.; Pajarinen,

419

J.S.; Nejadnik, H.; Goodman, S.; Moseley, M.; Coussens, L.M.; Daldrup-Link, H.E. Iron

420

oxide nanoparticles inhibit tumor growth by inducing pro-inflammatory macrophage

421

polarization in tumor tissues. Nat. Nanotech. 2016, 11, 986-994

422

48. Li, D.; Teoh, W.Y.; Selomulya, C.; Woodward, R.C.; Munroe, P.; Amal, R. Insight into

423

microstructural and magnetic properties of flame-made γ-Fe2O3 nanoparticles. J. Mater.

424

Chem. 2007, 17, 4876-4884

425

49. Kumfer, B.M.; Shinoda, K.; Jeyadevan, B.; Kennedy, I.M. Gas-phase flame synthesis and

426

properties of magnetic iron oxide nanoparticles with reduced oxidation state. J. Aerosol.

427

Sci. 2010, 41, 257-265 18 ACS Paragon Plus Environment

Page 19 of 27 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

428

50. Bryers, R.W. Fireside slagging, fouling and high-temperature corrosion of heat transfer

429

surface due to impurities in steam-rising fuels. Prog. Energy Combust. Sci. 1996, 22, 29-

430

120

431 432

51. Srivastava, R.K.; Miller, C.A.; Erickson, C.; Jambhekar, R. Emissions of sulfur trioxide from coal-fired power plants. J. Air Waste Manage. Assoc. 2004, 54, 750-762

433


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 20 of 27

434

Tables

435

Table 1. Physical and chemical composition of the fuels. Iron A

Iron B

Coal

3190

3000

780

-

-

74.2

-

-

3.9

99

99

1.33 0.372 0.01 -

7.39

7.39

29.6

Morphology

Bulk density (kg/m3) Elemental comp. (wt% dry) Carbon (C) Hydrogen (H) Nitrogen (N) Sulfur (S) Chlorine (Cl) Iron (Fe) Effective heating value (MJ/kg dry)

436 437

Table 2. Operating conditions of the EFR for the different fuels Powder mass flow (g/min) Thermal power (W) N2 in transport gas (g/min) N2 in combustion gas (g/min) O2 in combustion gas (g/min) Reactor temperature (°C)

Iron A 2.50 310 1.25 5.75 2.14 1200

Iron B 2.50 310 1.25 5.75 2.14 1200

Coal 0.63 310 1.25 8.13 2.86 1200

438 439


Page 21 of 27 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

440 441 442 443

Energy & Fuels

Table 3: NO emissions from industrial combustion of coal. The table compares literature data regarding NO emissions from seven industrial PC boilers39,40 with the data obtained in this study. Some information regarding the boiler, burner arrangement, NOx controlling technology and fuel is also given in the table. Facility Literature data 300 MWe, sub-critical, W-fired, no NOx controlling strategy39 360 MWe, sub-critical, W-fired, no NOx controlling strategy39 200 MWe, sub-critical, T-fired, Proprietary NOx controlling strategy39 1000 MWe, ultra supercritical, T-fired and twin furnace, low NOx burners and overfire air NOx39 600 MWe, sub-critical, T-fired, overfire air39 600 MWe, sub-critical, T-fired, overfire air39 300 MWe, down-fired boiler with deep-airstaged and low-NOx technology40 This study Entrained flow reactor Entrained flow reactor Entrained flow reactor

Fuel

NO emissions (mg/Nm3 at 6 % O2)

Anthracite, 1.6 wt-% N

1290 – 1500

Anthracite, 0.94 wt-% N

990 – 1080

Bituminous, 0.79 wt-% N

360 – 450

Bituminous, 1.12 wt-% N

320 – 400

Bituminous, 1.07 wt-% N Lignite, 0.47 wt-% N Anthracite, 0.78 wt-% N

375 – 410 556 – 686 674-836

Iron A Iron B Bituminous, 1.33 wt-% N

33 ± 1.5 36 ± 0.2 684 ± 2.6

444 445


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

446

Page 22 of 27

Figures

447 448

Fig. 1. An overview of the metal fuel cycle and energy densities of current and

449

potential future energy carriers. (A) The meal fuel cycle for iron. (B) Energy density

450

and specific energy of iron and iron powder compared to those of fossil fuels (coal, diesel,

451

gasoline, and LNG), biomass (forest residue, bio oil, MeOH), H2 compressed to 700 bar

452

(CH2), liquefied H2 (LH2) and batteries.

453


Page 23 of 27 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

454 455

Fig. 2. Experimental facility. A schematic of the experimental setup used to combust the fuels.

456

The setup was based on an EFR and different flue gas analysis systems including a pre-cyclone,

457

FTIR and LPI.

458


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 27

459 460

Fig. 3. The results of combustion tests. Emission of CO2, minor gaseous components (CO,

461

NOx, and SO2) and SR during the combustion of two types of PSI and coal. Solid residues (SR)

462

and fumes were defined as particulate matter with an aerodynamic diameter above and below 10

463

µm, respectively. Note the different vertical scales for different emitted species.

464


Page 25 of 27 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

465 466

Fig. 4. Chemical composition and morphology of iron oxide SR. (A) XRD pattern and

467

chemical composition of the SR particles. (B) SEM images (500x) of PSI before (left plate) and

468

after the combustion experiments (right plate) together with a crushed hollow particle under

469

higher magnification (1500x).

470


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 27

471 472

Fig. 5. Particle concentration and aggregate size distribution. Particle concentrations and

473

aggregate size distributions of the fumes measured by DLPI for both sponge iron powders and the

474

pulverized coal. The particulate matter for which the analysis results are shown here included the

475

part of mineral matter injected to the EFR with the fuel that was emitted in the fume fraction.

476

Note the logarithmic scale of the x-, and y-axes. Aggregates with a particle diameter below 1.0

477

µm (i.e., submicron aggregates) dominated the aggregate size distribution. Particulate emission

478

during coal combustion was much lower compared to that of PSI combustion.

479


Page 27 of 27 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

480 481

Fig. 6. Chemical composition and morphology of iron oxide fumes. (A) XRD pattern and

482

chemical composition of fume particles. (B) TEM and HRTEM images illustrating the primary

483

particle size in the aggregates and the microstructure of individual nanoparticles.


Pulverized Sponge Iron, a Zero-Carbon and Clean Substitute for

Recommend Documents