Reduction and Oxidation Kinetics of FeâMn-Based Minerals from

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Reduction and oxidation kinetics of Fe-Mn based minerals from South-western Colombia for Chemical Looping Combustion Francisco Javier Velasco-Sarria, Carmen Rosa Forero, Eduardo Arango, and Juan Adanez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02188 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Energy & Fuels

1

Reduction and oxidation kinetics of Fe-Mn based

2

minerals from south-western Colombia for Chemical

3

Looping Combustion

4 Francisco J. Velasco-Sarriaa, Carmen R. Foreroa*, Eduardo Arangoa, Juan Adánezb,.

5 6

a

7 8

Universidad del Valle, Engineering School of Natural and Environmental Resources (EIDENAR), Calle 13 No. 100-00, 760032057 Cali, Colombia.

b

9

Department of Energy and Environment, Instituto de Carboquímica (CSIC), Miguel Luesma

10

Castán 4, 50018, Zaragoza, España.

11

[email protected], [email protected],

12

[email protected] , [email protected]

13 14

*

15

[email protected] (Carmen Rosa Forero). Ciudad Universitaria Meléndez

16

Calle 13 # 100-00. A.A.25360 Cali Colombia.

Corresponding

author:

Tel:

(+57) 3212100

17

1

ACS Paragon Plus Environment

ext

7018.

e-mail

address:

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

18

Reduction and oxidation kinetics of Fe-Mn-based

19

minerals from south-western Colombia for chemical

20

looping combustion

21 Francisco J. Velasco-Sarriaa, Carmen R. Foreroa*, Eduardo Arangoa, Juan Adánezb,.

22 23

a

24 25 26 27

Universidad del Valle, Engineering School of Natural and Environmental Resources (EIDENAR), Calle 13 No. 100-00, 760032057 Cali, Colombia.

b

Department of Energy and Environment, Instituto de Carboquímica (CSIC), Miguel Luesma Castán 4, 50018, Zaragoza, España.

28 29

Keywords: Low-cost oxygen carriers, oxygen carriers in Colombia, carbon capture.

30

2


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Energy & Fuels

Abstract

32

The oxygen carrier (OC) is the main component of the chemical looping combustion (CLC),

33

process. Most OCs have been developed synthetically using an active metal oxide combined with

34

an inert material. When solid fuels are used, the OC becomes mixed with the ashes generated

35

during the CLC process and has to be removed, thereby increasing costs. As a result, there is

36

growing interest in the use of low-cost OCs based on manganese and iron. Given the widespread

37

use of coal to produce energy, there is a trend towards the study of the CLC process using solid

38

fuels, since this process has the lowest energy penalties of all the combustion methods involving

39

CO2 capture.

40

Co-products from the exploitation of Mn and Fe ores have been studied. These materials

41

were selected from a group of eight minerals with Fe and Mn present in their composition,

42

extracted from mines located in south-western Colombia.

43

The material selection process was based on crushing strength analysis and reactivity in

44

thermographic analysis (TGA), using CH4 as fuel. Two materials were selected, one based on Fe

45

and another based on Mn, which presented the best behaviour in their respective group.

46

It was found that the studied two materials were more reactive with H2 and CO than with

47

CH4. This was demonstrated by performing a kinetic study using a shrinking core model (SCM).

48

The selected Mn-based oxide was evaluated to identify whether it had the properties required for

49

chemical looping with oxygen uncoupling (CLOU), commonly found in Mn minerals with a high

50

silica content. However, no evidence to this effect was found in experiments at 1000 °C using N2

51

for OC decomposition and air as an oxidizing gas.

3


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The Mn ore showed the highest reactivity of all the studied materials, with a rate index of

53

11.9%/min in experiments at 950 °C using H2 as the reducing gas. Finally, it can be concluded

54

that the presence of silica improves the reactivity of the Mn ore, making it a promising carrier for

55

use in in-situ gasification chemical looping combustion (iG-CLC) technology.

56 57

Abbreviations

58

ASTM: American Society for Testing and Materials

59

BET: Brunauer-Emmett-Teller surface area analysis

60

bFB: batch fluidized bed

61

CLC: chemical looping combustion

62

CLOU: chemical looping with oxygen uncoupling

63

GHG: greenhouse gas

64

iG-CLC: in-situ gasification chemical looping combustion

65

IPCC: Intergovernmental Panel on Climate Change

66

MeyOx: metal oxide

67

OC: oxygen carrier

68

SCM: shrinking core model

69

TGA: thermogravimetric analysis

70

XRD, X-ray diffraction

71

XRF: X-ray fluorescence

72

∆HRR: enthalpy of reduction reaction

73

∆HRO: enthalpy of oxidation reaction 4


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Energy & Fuels

∆HRG: enthalpy of global reaction.

5


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75

1

76

There has been an evident increase in the concentration of greenhouse gases (GHGs) in the

77

atmosphere, the result of which is rising global temperatures, leading to climate change. CO2

78

capture and storage (CCS) has emerged as a viable option to combat this 1.

79

Among the technologies used to capture CO2 , chemical looping combustion (CLC) technology

80

has emerged as a very attractive option as it inherently separates out the CO2, meaning that this

81

separation requires no additional energy 2.

INTRODUCTION

82 83

This technology was proposed by Richter and Knoche 3, and later Ishida and Jin

4

started

84

research on oxygen carrier (OC) development. The basis of this process is to divide the

85

combustion of a hydrocarbon or a carbonaceous fuel (CnH2n+2) into two separate reactions: a

86

reduction reaction (Eq. ( 1 )) and an oxidation reaction (Eq. ( 2 )) through the introduction of a

87

metal oxide that circulates between two reactors and acts as an OC.

88 3 + 1 + → 3 + 1 + + 1 + ∆

(1)

3 + 1 + 1.5 + 0.5 → 3 + 1 ∆

(2)

+ 1.5 + 0.5 → + 1 + ∆ = ∆ + ∆

(3)

89 90

The reaction between the fuel and oxygen takes place in the reduction reactor. Oxygen is

91

supplied by the metal oxide in the reducing atmosphere generated as the result of the presence of

92

a hydrocarbon fuel, such as coal (Eq. ( 1 )). The removal of oxygen from the air is carried out by

6


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Energy & Fuels

93

fixing the oxygen to the metal oxide in the oxidation reactor (Eq. ( 2 )). The combination of the

94

oxidation and reduction reactions is equal to a conventional combustion reaction (Eq. ( 3 )).

95 96

When screening for materials in CLC, it is essential to find OCs that fulfil the following

97

requirements: high reactivity with the fuel and air, acceptable oxygen transportation capacity,

98

high chemical and mechanical stability to undergo repeated redox cycles, and no agglomeration

99

or carbon deposit formation, among others.

100 101

Potential metal oxides that can be used as OCs are: CuO, NiO, Mn2O3, Fe2O3 and Co3O4, 5-9

102

which have been reported in the literature

103

that serves as a support and enables them to improve properties such as mechanical resistance and

104

resistance to attrition. However, there is a loss of OC material when solid fuels are used, as a

105

result of mixing with the ashes formed during the process. For this reason, there is increasing

106

interest in low-cost OCs. Minerals in their natural state or in the form of industrial waste are very

107

promising alternatives due to their low cost and because they can also have acceptable reactivity

108

10, 11

. These oxides are combined with an inert material

.

109 110

Where Fe is used as an OC, there are several species to which Fe2O3 can be reduced (Fe,

111

FeO, Fe3O4), but owing to thermodynamic limitations, onlythe transformation of haematite

112

(Fe2O3) to magnetite (Fe3O4) can be applied at industrial level in continuous systems 12. Ilmenite

113

is an Fe-Ti-based mineral in which the active system is FeTiO3/Fe2TiO5. Several studies have

114

shown an acceptable performance of ilmenite in CLC at different scales 7


10, 13, 14

. Ilmenite is the

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115

low-cost OC most commonly used with solid fuels and serves as a benchmark material for the

116

study of other OCs 10, 15-18.

117

Mn oxides are becoming important because they are inexpensive and non-toxic. Moreover,

118

their oxygen transport capacity is higher than that of Fe. The highest oxidation state of Mn is

119

MnO2 and it decomposes at 500 °C

120

800 °C

121

applications. Mn2O3 may also be an alternative for the chemical looping with oxygen uncoupling

122

(CLOU) process because it can release 3.4% of its weight in the form of O2 by means of Eq. ( 4 ).

123

However, re-oxidation to Mn2O3 is restricted to comparatively low temperatures at which there

124

are operational difficulties for CLC technology 21.

20

19

. However, only Mn3O4 is stable at temperatures above

. Therefore, only the transformation between Mn3O4 and MnO is considered for CLC

125

6Mn2O3 ↔ 4Mn3O4 + O2

(4)

126 127

Rydén et al.

22

generally reviewed the development of OCs from mixed oxides, i.e. oxides

128

with crystalline structures including several different cations. Since Mn ions have a large number

129

of oxidation states, they can form oxides with different elements, allowing the development of

130

mixed oxide OCs. These may overcome some of the limitations of Mn oxides in order to increase

131

the oxidation temperature to Mn2O3.

132 133 134

Jing et al.

23

studied synthetic combined oxides by mixing Mn3O4 and SiO2 to produce

manganese silicates, such as braunite (Mn7SiO12) and rhodonite (MnSiO3), depending on the

8


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Energy & Fuels

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SiO2 content and the calcination temperature. These materials showed high reactivity and the

136

ability to release gaseous oxygen through the CLOU process.

137 138

Fe and Mn ores are materials that can potentially be used as low-cost OCs and are available

139

in Colombia in the form of significant reserves or in reserves with great potential 24. The aim of

140

this work was to screen different Fe and Mn minerals or mineral wastes and to study the reaction

141

kinetics of those that showed great potential for use as OCs in CLC.

142 143

144

2

EXPERIMENTAL

Figure 1 is a flow diagram of the experiments performed. Each step is detailed below.

145 146

2.1

147

ARL9900 Workstation, equipped with a fluorescence X-ray tube and a rhodium anode on 16

148

samples in order to quantify Fe, Mn and Ti elements associated with potential OCs. Eight

149

samples were selected, each presenting with a concentration above 25%. These were sampled

150

according to the ASTM D2234 norm 25 and subjected to a grinding process until the desired size

151

of 100–300 µm or 300–500 µm was achieved. Crushing strength analysis was subsequently

152

performed. Where this parameter was higher than 2 N, the following step was to perform surface

153

area analysis (BET). Finally, thermogravimetric analysis (TGA) with CH4 was used to determine

154

the conversion and rate index for comparison purposes. The nomenclature used for the samples

155

and the crushing strength, surface area and agglomeration behaviour of the samples are presented

Selection and Characterisation of Materials X-ray fluorescence (XRF) analysis was performed using a Thermo Scientific device, model

9


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156

in Table 1; the results of the XRF analysis are presented in Table 2. X-ray diffraction (XRD)

157

analysis was also performed using a Thermo Scientific device, model ARL9900 Workstation,

158

equipped with an X-ray tube for diffraction that included a copper anode in order to determine

159

the presence of crystalline species reported as active phases of OCs. The semi-quantitative results

160

of the XRD analysis of the materials are presented in Table 3.

161

162 163 164

2.2

Thermogravimetric Testing, TGA

To carry out the thermogravimetric analysis the operating conditions defined by Celaya

26

165

were taken into account for a CI Electronics thermobalance. These conditions were: gas flow 25

166

lN/h, sample weight 50 mg +-1 mg. Three cycles were performed for each experiment in order to

167

verify the accuracy of the results. The gases and temperatures used for reduction and oxidation

168

varied according to each experiment. The OC was heated in an air atmosphere until the operating

169

temperature was reached. Between the oxidation and reduction periods, a purge was carried out

170

with N2 for 2 minutes to prevent contact between the air and the fuel. The time of each period

171

was established once the weight of the sample had stabilized.

172 173

2.3 Data Treatment First, the conversion of the eight minerals was analysed when they reacted with CH4 as the

174

fuel gas, since a lower conversion rate was expected with CH4 than with CO or H2. Then, the Fe

175

ore and Mn ore that showed the highest conversion were selected and their experimental oxygen

176

transport capacities determined using H2. Finally, the reaction kinetics of the selected minerals

177

were determined for CO, H2 and CH4. 10


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Energy & Fuels

2.3.1 Conversion Determination.

179

In order to screen the two minerals with the highest conversion, the eight minerals underwent

180

TGA redox cycles at 950 °C, with CH4 as the fuel gas because it showed a lower conversion rate

181

than either CO or H2

182

the reducing gas, and air was used as the oxidizing gas. The conversion for the reduction and

183

oxidation reaction was determined using Eqs ( 5 ) and ( 6 ), respectively.

$%&' =

27-30

. A mixture comprising 25% CH4, 20% H2O and 55% N2 was used as

(),+, (

(5)

(),+, -,-.,/,0

$1 = 1 − $%&'

(6)

184

Here, $%&' and $1 are the reduction and oxidation conversions, mf,ox and mf,red arethe

185

oxidized and reduced masses in the experiment 3 is the mass at time t, and Ro,OC,exp is the

186

experimental oxygen transport capacity, defined later in Eq. ( 10 ).

187

Comparison of the conversion curves of the Fe (5 in total) and Mn (3 in total) minerals was

188

performed at 950 °C. The observed agglomeration was also taken into account when selecting the

189

materials.

190 191 192

To allow comparisons of reactivity with minerals reported in the literature, the rate index was quantified using Eqs ( 7 ) and ( 8 ).

456 78 9 = 100 ∗ 60 ∗ ;1,1< ∗ = =

'>? '@

A

1%(

=

BC/)

BDEF

∗

'>? '@

A

(7)

1%(

'>G

(8)

'@

11


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193

Here,

Page 12 of 46

Ro,oc is the theoretical oxygen transport capacity of the material, (dXi/dt) is the

194

derivative of the conversion of component i with respect to time, Pref is the reference pressure

195

(0.15 atm) of the fuel gas, and PTGA is the partial fuel pressure for TGA experiments.

196 197

2.3.2 Determination of the Experimental Oxygen Transport Capacity

198

Since natural materials were used and their reactions were not known, it was necessary to

199

define a theoretical oxygen transport capacity (Ro,OC) and an experimental one (Ro,OC,exp). The

200

theoretical oxygen transport capacity was calculated using Eq. ( 9 ), taking into consideration the

201

concentration of the predominant active phases and assuming a possible reaction pathway. The

202

experimental oxygen transport capacity was determined using Eq. ( 10 ) for two selected

203

minerals, one Fe-based and the other Mn-based, using H2 as fuel. A mixture comprising 25% H2,

204

20% H2O and 55% N2 was used as the reducing gas. During the TGA experiment, three cycles

205

were performed with four different concentrations at 950 °C. Variance analysis of one of the

206

factors (concentration) then verified that there were no significant differences between the 12

207

determinations (4 concentrations x 3 cycles). Ro,OC,exp was compared to Ro,OC to corroborate the

208

assumed reactions.

209

;1,H = ;(I ,H ∗ 9J&1 = ;1,H,& L =

(+, (C/K (+,

∗ 9J&1

(9)

(),+, (),C/K

( 10 )

(?

210

12


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Energy & Fuels

211

Here Rmax,OC is the oxygen transport capacity of the pure metal oxide, mox and mred are the

212

masses of the oxidized and reduced active phase, 9J&1 is the fraction of active phase in the OC,

213

3M,1 and 3M,%&' are the final masses in the periods of oxidation and reduction, and 3G is the

214

initial mass of the sample.

215 216

2.3.3 Determination of the Reaction Kinetics of the OCs

217

The kinetic study was performed for the two selected minerals using CO, H2 and CH4 as fuel

218

gases in order to determine the potential applications of the OCs with gaseous or solid fuels. Air

219

was used as the oxidizing gas. Conversion was determined for the two selected materials (an Fe

220

and a Mn material) under the experimental conditions shown in Table 4. When CO was used as

221

fuel, 20% H2O was added to prevent carbon deposition, and when H2 or CH4 were used, 20%

222

H2O and N2 were added as balance. the grain model31 was used for the kinetic study, with kinetic

223

control in the grains for both the reduction and the oxidation of the OC, which reacted according

224

to the shrinking core model (SCM). Eqs ( 11 ) and ( 12 ) describe this model in the case of

225

spherical particles.

226

6 = 1 − 1 − $ PO N

N=

( 11 )

QR ∗%S

T∗UV ∗HSW

( 12 )

227

where $ is the average conversion, 6 is the time, N is the time for a complete conversion,

228

X( is the molar density of the OC, 4Y is the particle radius, Z is the stoichiometric coefficient

229

(moles of solid reagent / moles of gaseous reagent), [\ is the kinetic constant of the reaction, is 13


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230

the reaction order, and Y is the concentration of gas. The logarithmic linearization of the Eq. (

231

12 ) results in Eq. ( 13 ) . When plotting ln(N) against lnCg, the value of n can be determined.

232

X( ∗ 4Y lnN = ln ^ _ + ∗ ln ` Z ∗ [\

( 13 )

233 234 235

The grain radius was calculated using Eq. ( 14 )

26

where aY is the surface area of the OC,

is the molecular weight of the OC and XJ&1 is the molar density of the active phase in the OC.

236

4Y =

3 ∗ 9J&1 aY XJ&1

( 14 )

237 238 239

The reaction kinetic constant, [\ , was determined by experimental data from TGA at different temperaturesusing Eq. ( 15 ) as a function of temperature.

240

[\ = [b e

=

de A fg

( 15 )

241 242

Here, [b is the pre-exponential kinetic factor, hI is the activation energy, and R is the

243

universal gas constant. Both values were obtained through linearization of Arrhenius plot using

244

Eq. ( 16 ).

245

14


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Energy & Fuels

j[\ =

−hI + j[1 Rk

( 16 )

246

3

247 248

3.1

249

shown in Table 2. The presence of Fe was evident in significant concentrations in five samples

250

(between 27.2 wt% and 65.9 wt%), whereas the presence of Mn was evident in 3 samples

251

(between 34.2 wt% and 53.5 wt%). These minerals could be considered potential OCs

252

Si was also found in high concentration in the materials OXMN010A and OXMN010B, a

253

mineral that when mixed with Mn can generate mixed oxides with a good RO,OC and which may

254

display a CLOU effect

255

where it can be noted that all minerals are above 2N, and hence are considered materials with

256

high mechanical resistance 32.

RESULTS AND DISCUSSION Material Characterization The results from XRF analysis of eight of the previously untreated original samples are

22

10, 11, 27-30

.

. The results of the crushing strength analysis are shown in Table 1,

257 258

The results of the BET analysis of eight original samples are shown in Table 1, and it can be

259

noted that the Mn based materials have the largest surface area (between 12.5 m2/g and 25.3

260

m2/g), which indicates a greater contact area between the OC and the reacting gases, favouring

261

CLC. The values found were much higher than those reported in the literature for the Mn ore (0,6

262

m2/g to 7,1 m2/g) 33. For the Fe minerals 34-36, surface area values between 0.1 m2/g and 1.4 m2/g

263

were reported for natural minerals such as bauxite and haematite, while values of 3,7 m2/g were

264

reported for the Fe ore from Carajás 37. The values found (between 0.2 m2/g and 1.1 m2/g) were

265

within the range reported in the literature for the Fe ore. When comparing Fe- based materials to

266

Mn-based materials, the latter were found to have a larger surface area and therefore could

267

present an advantage for use in CLC. 15


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268 269

3.2

270

3. The presence of magnetite in large proportions was evident in the minerals FEMA005 (87.4

271

wt%) and FEMA011 (94.1 wt%), which is referenced as an OC 34, 35, 38, 39, as well as rhodonite in

272

the minerals OXMN010A (85.4 wt%) and OXMN010B (83.2 wt%), also cited as an OC 22, 23, 40.

273

Ilmenite has also been reported as an OC

274

content were ILME007 (90.5 wt%) and FEMA004 (67.5 wt%). The presence of hausmannite in

275

large proportions was also evident in the mineral OXMN009 (95.4 wt%). Studies conducted with

276

Mn3O4 supported on alumina showed this material to have low reactivity with methane

277

however, studies reported that Mn3O4/Mg–ZrO2 particles sintered at 1150 oC proved to be

278

suitable as OCs

279

process and will be the subject of another study.

Ro,OC, Materials Calculation. The semi-quantitative results from the XRD analysis of the materials are presented in Table

42, 43

10, 13, 14

, and the minerals with the highest ilmenite

41

;

. Taking into account Eq. ( 4 ), this mineral could be used in the CLOU

280

With these results in mind, it was possible to use the reactions reported in the literature 22, 40

281

to calculate the theoretical oxygen transport capacity (Ro,OC) using Eq. ( 9 ) and the rate index

282

using Eq. ( 7 ) for reduction with CH4 . Table 1 shows the results for all minerals.

283 284

3.3 Screening of Minerals as Potential OCs An initial screening was performed based on reactivity during combustion with CH4 by means

285

of the rate index, oxygen transport capacity and presence of agglomeration during TGA tests.

286

This screening was performed in order to select one Fe and one Mn ore for further study.

287

Figure 2 shows the conversion of Fe-based and Mn-based minerals in reduction and

288

oxidation, in accordance with the defined experimental conditions and a temperature of 950 °C.

289

When the Fe-based minerals were compared, it was observed that all the materials presented a 16


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Energy & Fuels

290

slow reduction rate and a fast oxidation rate. At 60 s, the reduction conversion of the minerals

291

operation was observed to be between 0.15 and 0.3, and the oxidation conversion was found to be

292

between 0.7 and 0.9. Materials FEMA004, FEMA011 and CRSI003 displayed similar reduction

293

conversions, although the material FEMA011 achieved a higher conversion from 60 s onwards,

294

and it achieved a level of conversion of about 40% in 120 s. Consequently, high inventories may

295

be required when these materials are used in CLC with CH4. Of these three materials, FEMA011

296

displayed the best oxidation behaviour, reaching a conversion of 0.87 at 20 s. Moreover,

297

FEMA011 did not show agglomeration, and it had a high crushing strength (4.6 N) and a

298

relatively high rate index with CH4 (2.8%/min) (Table 1).

299 300

Of the Mn-based minerals, OXMN010A and OXMN010B were the most reactive materials

301

as they did not show agglomeration; they had an appropriate crushing strength (Table 1) and

302

showed similar behaviour (Figure 2). The material OXMN010A showed the best behaviour

303

during oxidation and had a high rate index (12%/min), which is why it was selected for the

304

kinetic study. The presence of Si in the minerals OXMN010A and OXMN010B could lead to the

305

formation of mixed oxides of Mn and Si, which would improve the reactivity of the OC, as

306

reported by some authors

307

mechanical and thermodynamic limitations that can be observed in Mn oxides for re-oxidation to

308

Mn2O3 at high temperatures, as reported in the literature 11, 21, 22.

23, 44

. The formation of mixed oxides could help to overcome the

309 310

When comparing Fe and Mn minerals, OXMN010A and OXMN010B were found to be the

311

most reactive materials since they achieved 80% conversion in 120 s during the TGA reduction

312

(Figure 2). This is also an advantage, since it will allow higher conversion of the fuel with lower 17


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

313

fuel reactor inventories, when compared to the Fe minerals. Additionally, they present the highest

314

rate index values with CH4 (12%/min and 8.8%/min, respectively).

315 316

With regard to oxidation behaviour, the Mn minerals displayed higher reactivity as they

317

reached conversions of up to 80% in 10 s, whereas the Fe minerals only achieved 40%

318

conversion in the same amount of time (Figure 2).

319 320

For comparison purposes, the rate index was calculated not only with CH4 but also with CO

321

and H2 for the minerals selected for the kinetic study (OXMN010A and FEMA011). The

322

comparison was made with values reported in the literature, which are presented in Table 5.

323 324

The rate indexes calculated from TGA in this work show that the reactivity of the mineral

325

OXMN010A was greater than that of the mineral FEMA011. In terms of the reactivity with fuels,

326

this material has a higher reactivity with CO and H2 than with CH4.

327

When comparing this with data reported in the literature, the mineral FEMA011 is observed

328

to be less reactive than the Fe-based minerals presented in Table 5, even though the differences

329

between the bauxite residue and the haematite ore are not significant when CH4 and CO are used

330

as fuels. In addition, the FEMA011 mineral has low reactivity when compared to synthetic and

331

natural OCs using H2 as fuel, which may be a limitation when applied to iG-CLC technology.

332

18


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Energy & Fuels

333

On the other hand, the reactivity of the mineral OXMN010A was greater than that of all the

334

Mn minerals, apart from the reaction of the calcined ore MnBr with CH4. Therefore, the material

335

OXMN010A has a high reactivity compared to natural minerals and with synthetic materials.

336 337

Phase transformations of selected materials: In the case of the mineral OXMN010A, the

338

main species was identified as rhodonite (MnSiO3), which could correspond to the reduced phase

339

(Table 3). From the binary phase diagram Si/(Si +Mn)45 with a molar ratio of 0.41–

340

0.54 (determined using XRF analysis, Table 2), the lower limit was determined by assuming that

341

all Mn would be converted to rhodonite (MnSiO3), and the upper limit was determined by

342

assuming that part of the Mn would be converted to spinel (Mn2AlO4) and a bixbyite (Fe,

343

Mn)2O3) at an oxygen pressure of 0,21 atm; and since the study temperatures were between

344

750 °C and 950 °C, it was possible to determine that the species present in the oxidized phase

345

would be braunite (Mn7SiO12) and tridymite (SiO2), as seen in Figure 3.

346

From the XRD analysis of the sample FEMA011, the main crystalline phase present was

347

identified as magnetite (Fe3O4), which is widely reported in the literature as an active phase in

348

OC 12, 34.

349

Based on the characterization performed, it was possible to use the reactions reported in the

350

literature 22, 40 and summarized in Table 6 to calculate the theoretical oxygen transport capacity

351

(Ro,OC) using Eq. ( 9 ). Moreover, it was also possible to compare the theoretical oxygen transport

352

capacity (Ro,OC) with the experimental transport capacity (Ro,OC,exp) calculated using Eq. ( 10 )

353

from the data reported from TGA for the assumed and verified reactions.

354 19


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

355

For the mineral OXMN010A, Ro,max,OC is equal to 4.2%, given that the active phase has a

356

concentration of 85.4 wt% (Table 3), and Ro,OC would correspond to 3.5%. Ro,OC,exp could be

357

calculated from the experimental data with H2 for the three cycles performed in a thermobalance

358

at 950 °C, at 4 fuel concentrations. A variance analysis was performed and it was found that there

359

were no differences between the estimates of Ro,OC,exp, from which it was determined that for

360

mineral OXMN010A, the value corresponds to 3.4% ( Table 7).

361 362

Given CLC technology can only make use of the reaction corresponding to Fe2O3/Fe3O4 for

363

FEMA011, the calculated Ro,max,OC value is 3.3%, and taking into account that the fraction of the

364

active phase is 94.1% (Table 3), the value calculated for Ro,OC was 3.14%. Similarly, Ro,OC,exp

365

was calculated for FEMA011, giving a value of 3.0% for the reduction to Fe3O4. Therefore, these

366

correlations between the experimental and theoretical results indicate that practically all the metal

367

oxides are active for the redox process and that the assumed reactions are likely to be correct.

368 369

In conclusion, the minerals FEMA011 and OXMN010A were screened and found suitable

370

for the kinetic study owing to their high rates of conversion in comparison to similar materials, as

371

they do not display agglomeration, while having the appropriate crushing strength and presenting

372

high rate index values with CH4.

373 374

To determine the presence of the CLOU effect in the manganese silicates corresponding to

375

the mineral OXMN010A, as reported for similar mixed oxides23, tests were performed at 1000 °C

376

in a thermogravimetric analyser, using N2 for OC decomposition and air as an oxidizing gas. No 20


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Energy & Fuels

377

release of oxygen was observed under those conditions, therefore Eqs ( 11 ) and ( 12 ) apply.

378

These are applicable to solid-gas reactions based on the SCM model for spherical grains.

379

380 381

3.4 Kinetics of Redox Reaction of Selected OCs Determination of the reaction order: To determine the reaction order, the values of N were first

382

calculated using Eq. ( 11 ) at different fuel (CH4, CO and H2) concentrations. This data was fitted

383

using the least squares method to a straight line (Figure 4 and Figure 5). If the reaction followed

384

the SCM model, the slope of the adjusted line would correspond to 1/N. The values of N were

385

used to determine the order of reaction using Eq. ( 13 ). The fitting of the data to a graph is

386

shown in Figure 6, and the results are summarized in the Table 8. The reaction order changed

387

from 0.7 for CH4 and H2 to 1.0 for CO.

388 389

Determination of the activation energy: To determine the activation energies, N first had to be

390

calculated using Eq. ( 11 ) at different temperatures. Using the values of N it was possible to

391

calculate [\ using Eq. ( 12 ). Finally, the activation energy and ko were calculated using Eq. ( 15).

392

These data were fitted using the method of least squares to a straight line (Figure 7).

393 394

Table 8 gives a summary of the quantification of the kinetic parameters for materials

395

OXMN010A and FEMA011, as well as their activation energy, and the reaction constant for the

396

reduction reactions with the fuels (CH4, CO and H2) and oxidation with air. For the mineral

397

OXMN010A, the activation energies ranged between 10.2 kJ/mol for CO and 73.3 kJ/mol for

398

CH4. All values found for the different redox reactions are shown in Table 8. Since no studies 21


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 46

399

with Mn-based natural minerals could be found, a comparison was made with Mn3O4 in Mg-

400

ZrO2, as reported in the literature

401

those reported, which were between 19 kJ/mol and 119 kJ/mol. However, although the reaction

402

of the material OXMN010A with CH4 has a lower temperature dependence than that of Mn3O4

403

with Mg-ZrO2, their rate indexes were found to be very similar (see Table 5). When comparing

404

the OC with different fuels (CH4, H2 and CO), the experimental results of activation energies

405

showed a similar behaviour to that reported in the literature. Activation energies were higher for

406

CH4, than for H2, CO and O2 12, 27. The reaction orders were found to be between 0.7 and 1.0, and

407

they were within the range reported for Mn3O4 ( 0,65 to 1,0), 20.

20

. The activation energies were observed to be lower than

408 409

In the case of FEMA011, the activation energies found for the reduction reaction were

410

between 31.4 kJ/mol and 106.6 kJ/mol. Ilmenite is a mineral based on Fe and Ti for which kinetic

411

studies in a natural state were performed by Abad et al.

412

results of the current study. For the pre-oxidized ilmenite, Abad et al. found values of activation

413

energy in reduction of between 109 kJ/mol and 165 kJ/mol. Moreover, for the same ilmenite after

414

an activation process they obtained values of between 65 kJ/mol and 135 kJ/mol, which indicated

415

that the FEMA011 would have a lower dependence on the temperature in the reduction reactions,

416

despite not being subjected to preliminary thermal treatments. For the oxidation reaction, ilmenite

417

presented activation values of 12 kJ/mol and 25 kJ/mol for pre-oxidized and activated states,

418

respectively. Although these values were lower than those obtained for FEMA011 (28.9 KJ /

419

mol), they were still similar.

46

. These were compared against the

420 22


Page 23 of 46 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

421

Energy & Fuels

3.4.1 Stability Test for the Mineral OXMN010A.

422

Given the good performance of the mineral OXMN010A, it was subjected to further

423

investigation and to 30 cycles in a thermogravimetric analyser in order to verify the behaviour of

424

the material throughout the redox cycles. This evaluation was fundamental and will serve as a

425

starting point for the experiment to be carried out in a batch fluidized bed (bFB), as reported by

426

some authors

427

alternating cycles of oxidation with air and reduction with H2 at 25% at a temperature of 950 °C.

428

Figure 8 shows that the 30 cycles and the stable behaviour of the mineral could be verified

429

throughout the total number of cycles. This permits direct studies in a fluidized bed with this

430

material in order to observe its behaviour with solid fuels.

431 432

11

. The material was tested for stability in a thermogravimetric analyser by

In other studies, it has been found that OXMN010A mineral has a lifetime of 2950 h after 50 cycles in batch fluidized bed using CH4, H2 or CO as fuels 47.

433 434

4

435

This research has resulted in the development of a screening methodology applicable to low-cost

436

OCs (see figure 1), which optimizes available resources and facilitates identification of the best

437

performing materials in CLC and iG-CLC. Screening was performed on eight minerals and they

438

were divided into two groups, Mn-based and Fe-based materials.

439

FEMA011 minerals presented the highest conversion within their groups, therefore showing

440

potential for use in CLC technology.

CONCLUSIONS

23


The OXMN010A and

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

441

From the kinetic study and the determination of the rate index, it was concluded that the

442

OXMN010A mineral presented higher reactivity with H2 and CO than with CH4. This behaviour

443

was similar to that observed for the mineral FEMA011. The mineral OXMN010A was the most

444

reactive of the minerals studied, followed by FEMA011. The higher reactivity of mineral

445

OXMN010A may be due to the formation of mixed Mn and Si oxides.

446

The rate index for Fe minerals was found to be lower than that reported in the literature for

447

similar OCs, and the rate index found for the Mn ore was higher than that reported for similar

448

ores.

449 450 451

Besides not having CLOU properties, the mineral OXMN010A has potential for use in iGCLC as it shows a good reactivity with the main products of the coal gasification, CO and H2.

452 453

Acknowledgements

454 455

This research was conducted with financial support from Unión Temporal Incombustion

456

(Temporary Joint Working Group) and Colciencias through Recoverable Assistance Agreement

457

RC 0852-2012. We are grateful for the collaboration shown by Instituto de Carboquímica,

458

Zaragoza (Project ENE2016-77982-R and European Regional Development Fund (ERDF)).

459

5

460 461

1. IPCC, Climate Change 2014 Synthesis Report, Fifth Assessment Synthesis Report. United Nations Organizations: 2014.

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587

Energy & Fuels

Tables

588

Table 1. Characterisation of work samples.

589

Table 2. XRF analysis results performed on work samples.

590

Table 3. Semi-quantitative results XRD analysis for work samples

591

Table 4. Experimental design for the kinetic study with Fe and Mn minerals

592

Table 5. Rate Index comparison for different Fe-Based and Mn-based materials.

593

Table 6. Theoretical Redox Reactions for selected samples.

594

Table 7. Variance analysis to calculated RO,OC,exp to selected samples, H2, 950 oC

595

Table 8. Kinetic parameters determined with a SCM for selected samples

596

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597

Page 30 of 46

Table 1. Characterisation of work samples. Particle size (µm)

Crushing strength [N]

Surface area (BET) [m2/g]

Agglomeration in TGA

Oxygen transport capacity [Ro,OC ]

Rate index with CH4 [%/min ]

Sample

Origin

CRSI003

Chromite ore – Antioquia

100–300

7.3

0.6

Yes

3.1

2.9

FEMA004

Iron ore - Antioquia

100–300

5.0

1.1

Yes

3.8

2.7

FEMA005

Iron ore - Antioquia

100–300

4.4

0.2

Yes

3.4

1.1

ILME007

Ilmenite ore – Antioquia

100–300

5.9

0.2

No

4.5

1.9

FEMA011

Iron ore - Cauca

100–300

4.6

1.1

No

3.3

2.8

OXMN009

Manganese ore Valle

100–300

5.6

12.5

Yes

6.7

3.6

OXMN010A

Manganese waste – Nariño

100–300

2.3

23.3

No

3.6

12.0

OXMN010B

Manganese waste Nariño

300–500

3.2

25.3

No

3.5

8.8

30


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598

Energy & Fuels

Table 2. XRF analysis results performed on work samples. Sample

599 600 601

Si

Al

Fe

Ca

Mg

Ti

Cr

Mn

LOI

O

CRSI003

0.8

3.5

27.2

0.2

2.8

2.6

28.7

0.5

0.00

33.08

FEMA004

0.0

0.9

41.5

0.3

1.3

12.9

7.0

0.0

0.55

31.73

FEMA005

2.5

0.6

64.5

0.7

0.1

0.00

31.35

ILME007

2.4

0.5

30.7

0.9

0.3

3.53

34.22

FEMA011

1.6

0.4

65.9

0.8

0.2

23.06

18.70

OXMN009

1.8

0.2

1.0

0.5

0.1

53.5

10.32

30.69

OXMN010A

13.0

2.5

3.5

1.9

0.8

0.2

35.7

9.80

31.85

OXMN010B

14.7

2.5

3.2

1.3

0.8

0.2

34.2

0.00

30.98

24.5

Percentages expressed in wt%. Values lower than 0.1 were not considered LOI: Loss on ignition

31


2.5

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

602

603

Page 32 of 46

Table 3. Semi-quantitative results of XRD analysis for working samples SiO2

Mn3O4

MnSiO3

Fe2O3

FeO

Fe3O4

Fe2SiO4

FeTiO3

FeCr2O4

CaO

Sample

Quartz

Hausmannite

Rhodonite

Haematite

Wüstite

Magnetite

Fayalite

Ilmenite

Chromite

Lime

CRSI003

--

--

--

--

--

--

--

--

94.2

--

FEMA004

--

--

--

11.6

--

2.9

--

67.5

7.1

--

FEMA005

--

--

--

--

--

87.4

7.2

--

--

--

ILME007

--

--

--

2.6

--

--

--

90.5

--

5.4

FEMA011

--

--

--

--

--

94.1

2.3

--

--

--

OXMN009

2.0

95.4

--

--

--

--

--

--

--

--

OXMN010A

--

--

85.4

--

6.1

--

--

--

--

--

OXMN010B

--

--

83.2

--

3.2

--

--

--

--

--

Percentages expressed in wt%

32


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604

Energy & Fuels

Table 4. Experimental design for the kinetic study with Fe and Mn minerals Reducing and oxidizing agent

Molar concentration (%)

Temperature (°C)

CH4

25

750-850-900-950

H2

25

750-850-900-950

CO

25

750-850-900-950

O

21

750-850-900-950

CH4

15-20-30

950

H2

15-20-30

950

CO

15-20-30

950

O

5-10-15

950

605

33


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

606

Page 34 of 46

Table 5. Rate Index comparison for different Fe-Based and Mn-based materials. Rate Index (%/min) Material

607 608 609

CH4

CO

H2

FEMA011a

2.6

3.2

3

OXMN010Aa

12

17.9

38.8

Ilmeniteb 48

5.0

2.5

7.9

Bauxite waste 35

3.4

3.9

10.5

Mineral (haematite) 35

3.3

3.4

12,.4

Fe-Syntheticc 49

7.8

6,.6

5.4

MnSA 50

5.0d (0.8)e

5.1 (5.3)

14.2 (11.8)

MnGBHNE 50

9.2 (1.3)

6.4 (1.4)

19.2 (9.0)

MnGBMPB 50

9.3 (2.2)

9.0 (2.,4)

26.4 (14.8)

MnBR 50

12.7 (1.7)

8.2 (3.0)

20.5 (12.5)

Mn ore Åheim Norway 11

7.2f

-

19.8f

Mn ore SINAI-A 51

3.6

-

5.4

Mn ore GUIZHOU 51

0.9

-

1.3

Mn3O4 in Mg-ZrO2 52

11.3

-

-

a

Experiments performed in a thermogravimetric analyser at 950 °C. b After operating in a continuous unit using bituminous coal, c After operating in a continuous unit using methane or PSA off-gas, d calcined material, e used materials and f averages between 900o and 1000 oC.

610

34


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611

Energy & Fuels

Table 6. Theoretical Redox Reactions for selected samples. Sample

OXMN010A22, 23

FEMA01112, 34

Fuel

Reactions

3 3 2m a7 + 12a7 + n → 14a7O + + 3 2 2

CH4

Reduction

CO

Reduction

m a7 + 6a7 + 3 → 7a7O + 3

H2

Reduction

m a7 + 6a7 + 3 → 7a7O + 3

O2

Oxidation

14a7O + 3 → 2m a7 + 12a7

CH4

Reduction

1 1 6p O + n → 4p O n + + 2 2

CO

Reduction

3p O + → 2p O n +

H2

Reduction

3p O + → 2p O n +

O2

Oxidation

4p O n + → 6p O

612

35


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

613

Page 36 of 46

Table 7. Variance analysis to calculated RO,OC,exp to selected samples, H2, 950 oC RO,OC,exp (%) Sample

OXMN010A

Cycles

H2 concentration (%) 1

2

3

15

3.4

3.3

3.4

3.4

20

3.4

3.4

3.6

3.5

25

3.6

3.5

3.5

3.5

30

3.3

3.3

3.3

3.3 3,4

Average

FEMA011

Average

15

3.0

3.1

3.1

3.1

20

2.9

3.0

3.0

3.0

25

3.0

3.1

3.1

3.1

30

3.1

3.1

3.1

3.1

Average

614

36


3.0

Page 37 of 46 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

615

Energy & Fuels

Table 8. Kinetic parameters determined with a SCM for selected samples

Sample

Ea

Ko

Order

Temperature

(KJ/mol)

m/s(mol/m3)^n

(n)

°C

Fuel

Fuel

Oxygen

concentration

concentration

% (v/v)

% (v/v)

CH4

73.3

3.3E-04

0.7

750 - 950

15 - 30

21

H2

16.9

2.0E-05

0.7

750 - 950

15 - 30

21

CO

10.2

2.6E-06

1.0

750 - 900

15 - 30

21

H2

14.2

1.2E-05

0.8

750 - 950

25

5 -21

CH4

106.6

1.6E-02

1.2

750 - 950

15 - 30

21

H2

46.0

2.2E-04

1.1

750 - 950

15 - 30

21

CO

31.4

1.0E-04

0.4

750 - 950

15 - 30

21

H2

28.9

8.9E-06

0.8

750 - 950

25

5 -21

OXMN010A

FEMA011

616

Ea is activation energy and Ko is frequency factor for the sample + fuel/oxygen reaction

37


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

617

Figure captions

618 619

Figure 1. Flow diagram of the experimentation

620

Figure 2. Conversion curves vs time for the reduction and oxidation of Fe and Mn materials,

621 622 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 623 694 695 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 696 767 768 769

Reduction: 25% CH4, 20% H2O and 55% N2, Oxidation: 100% Air, T= 950 °C. Figure 3. Binary phase diagram of (MnySi(1-y))Ox, P(O2)=0,21 atm, Ptotal=1 atm. 1−$13 vs time to determine N for the mineral OXMN010, Reduction: 15–30% H2, CO or CH4, Oxidation: 100% air, T=950 °C (▲15%, ■ 20%, ●25%, ♦30%). 1−$13 vs time to determine N for FEMA011, Reduction: 15–30% H2, CO or CH4, Oxidation: 100% air, T=950 °C (▲15%4, ■ 20%, ●25%, ♦ 30%). Figure 6. Fitting of data Ln(τ) vs. Ln(Cg) to determine the reaction order for the minerals

770

FEMA011 and OXMN010A. Reduction: 15–30% H2, CO or CH4, Oxidation:

771

100% air, T=950 °C to H2 and CH4, T=900 °C to CO. (●H2, ■CO, ▲CH4, ♦ O2).

772

Figure 7. Fitting of data Ln (ks) vs. 1/T to determine the pre-exponential kinetic factor and

773

the activation energy to minerals FEMA011 and OXMN010A. Reduction: 25%

774

H2, CO or CH4, Oxidation: 21% O2, T= 750–950 °C. (●H2, ■CO, ▲CH4, ▼O2).

775 776

Figure 8. Stability, mass vs time curve for 30 redox cycles with the mineral OXMN010A, Reduction: 25% H2 and 75% N2, Oxidation: 100% air, T=950 °C.

38


Page 38 of 46

Page 39 of 46 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

XRF analysis of samples

¿Is the concentration of Yes Fe, Mn or Ti > 25%?

No Discarded

¿Is the crushing strength < 2N?

Yes

Discarded

No BET and XRD analysis

TGA tests with CH4 at 950°C Screening criterial: -Rate Index -Oxygen transport capacity -Agglomeration behavior during TGA tests

Screening for selection of one Fe and one Mn ore

¿CLOU effect is Yes detected in the Mn ore?

No Use equations 11 and 12 in the kinetic study TGA tests with CH4, CO and H2 for the kinetic study

Stability test for the higher performance material.

777 778 779

Figure 1. Flow diagram of the experimentation

39


Use equation 4 in the kinetic study

Energy & Fuels

Fe-based materials

1.0

Reduction

Mn-based materials 1.0

Oxidation

Reduction

CRSI003 FEMA004 FEMA005 ILME007 FEMA011

0.6

Oxidation

0.8 Conversion (X)

0.8 Conversion (X)

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 40 of 46

0.4

0.6

0.4 OXMN009 OXMN010A OXMN010B

0.2

0.2 0.0

0.0 0

30

60

0 90 120

5

10

15

20

0

30

Time (s)

60

0 90 120

5

10

15

20

Time (s)

780 781 782

Figure 2. Conversion curves vs time for the reduction and oxidation of Fe and Mn materials, Reduction: 25% CH4, 20% H2O and 55% N2, Oxidation: 100% Air, T= 950 °C.

40


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Energy & Fuels

783 784

Figure 3. Binary phase diagram of (MnySi(1-y))Ox, P(O2)=0,21 atm, Ptotal=1 atm.

41


Energy & Fuels

H2

0.5

CH4

CO

0.4 1-(1-X)1/3

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 42 of 46

0.3 0.2 0.1 0.0 0

5

10

15

20 0

10

15

20 0

5

10

15

20

Time (s)

785

786 787 788

5

q

Figure 4. Curve adjustment 1 − 1 − $r vs time to determine N for the mineral OXMN010, Reduction: 15–30% H2, CO or CH4, Oxidation: 100% air, T=950 °C (▲15%, ■ 20%, ●25%, ♦30%).

42


Page 43 of 46

CO

H2

0.12

CH4

0.10 1-(1-X)1/3

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

0.08 0.06 0.04 0.02 0.00 0

10 20 30 40 50 60 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (s)

789

790 791

q

Figure 5. Curve adjustment 1 − 1 − $r vs time to determine N for FEMA011, Reduction: 15–30% H2, CO or CH4, Oxidation: 100% air, T=950 °C (▲15%4, ■ 20%, ●25%, ♦ 30%).

43


Energy & Fuels

3.5

1.5

FEMA011 3.0

Ln(ττ)

2.5

0.0

1.5

-0.5 -1.0 -0.6

793 794 795

0.5

2.0

1.0

792

OXMN010A

1.0

Ln(τ)

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 44 of 46

0.0

0.6

1.2

-0.6

0.0

0.6

1.2

Ln (Cg)

Ln (Cg)

Figure 6. Fitting of data Ln(τ) vs. Ln(Cg) to determine the reaction order for the minerals FEMA011 and OXMN010A. Reduction: 15–30% H2, CO or CH4, Oxidation: 100% air, T=950 °C to H2 and CH4, T=900 °C to CO. (●H2, ■CO, ▲CH4, ♦ O2).

44


Page 45 of 46

-12

-12

OXMN010A

FEMA011 -13

-13

-14

-14

Ln(ks)

Ln(ks)

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

-15

-16

-17 8.00e-4

796

797 798 799

-15

-16

8.80e-4

9.60e-4

1.04e-3

-17 8.00e-4

1/T (K-1)

8.80e-4

9.60e-4

1.04e-3

1/T (K-1)

Figure 7. Fitting of data Ln (ks) vs. 1/T to determine the pre-exponential kinetic factor and the activation energy to minerals FEMA011 and OXMN010A. Reduction: 25% H2, CO or CH4, Oxidation: 21% O2, T= 750–950 °C. (●H2, ■CO, ▲CH4, ▼O2).

800

45


Energy & Fuels

801

62.0 61.5

Mass (mg)

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 46 of 46

61.0 60.5 60.0 59.5

OXMN010A

59.0 0

802 803 804

2000

4000

6000

8000

Time (s)

Figure 8. Stability, mass vs time curve for 30 redox cycles with the mineral OXMN010A, Reduction: 25% H2 and 75% N2, Oxidation: 100% air, T=950 °C.

46


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