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Stelte W*, Nielsen NPK, Hansen HO, Dahl J, Shang L, Sanadi AR. Pelletizing properties of torrefied wheat straw.

Biomass and Bioenergy, 2013, 49:214-221

DOI:10.1016/j.biombioe.2012.12.025

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Pelletizing properties of torrefied wheat straw

Wolfgang Stelte a*, Niels Peter K. Nielsen a, Hans Ove Hansen a , Jonas Dahl a, Lei Shang b and Anand

R. Sanadi c

*Corresponding author: Phone: +45 7220 1072, E-mail: wst@teknologisk.dk

a Renewable Energy and Transport, Energy and Climate Division, Danish Technological Institute, Gregersensvej 1,

2630 Taastrup, Denmark. b Department of Chemical and Biochemical Engineering, Technical University of Denmark, 2800

Kgs. Lyngby, Denmark. c Biomass and Ecosystem Science, Faculty of Life Sciences, University of Copenhagen, 1958

Frederiksberg, Denmark.

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1

Pelletizing properties of torrefied wheat straw 1

2

Wolfgang Stelte a*

, Niels Peter K. Nielsen a, Hans Ove Hansen

a , Jonas Dahl

a, Lei Shang

b and Anand 3

R. Sanadi c 4

5

6

*Corresponding author: Phone: +45 7220 1072, E-mail: wst@teknologisk.dk 7

a Renewable Energy and Transport, Energy and Climate Division, Danish Technological Institute, Gregersensvej 1, 8

2630 Taastrup, Denmark. b Department of Chemical and Biochemical Engineering, Technical University of Denmark, 2800 9

Kgs. Lyngby, Denmark. c

Biomass and Ecosystem Science, Faculty of Life Sciences, University of Copenhagen, 1958 10

Frederiksberg, Denmark. 11

12

Combined torrefaction and pelletization is used to increase the fuel value of biomass by increasing its 13

energy density and improving its handling and combustion properties. However, pelletization of 14

torrefied biomass can be challenging and in this study the torrefaction and pelletizing properties of 15

wheat straw have been analyzed. Laboratory equipment has been used to investigate the pelletizing 16

properties of wheat straw torrefied at temperatures between 150 and 300 °C. IR spectroscopy and 17

chemical analyses have shown that high torrefaction temperatures change the chemical properties of 18

the wheat straw significantly, and the pelletizing analyses have shown that these changes correlate to 19

changes in the pelletizing properties. Torrefaction increase the friction in the press channel and pellet 20

strength and density decrease with an increase in torrefaction temperature. 21

22

Key words: Biomass, Torrefaction, Wheat straw, Pelletization, Pellet 23

24

1. Introduction 25

26

Biomass from wood and agricultural residues is increasingly important for sustainable heat and power 27

production at industrial scale [1]. Political decision makers have set ambitious goals to push forward 28

*Manuscript

Click here to view linked References

2

renewable energies, and power producers have identified biomass utilization as a potential way to 29

increase their share of renewable energy relatively fast and cost efficient when using it in their existing 30

coal power plants [2]. However the physical properties of biomass have been identified as a challenge 31

for power producers due to its poor handling properties and combustion related problems. Biomass as a 32

material is inhomogeneous in structure and composition and its bulk and energy density is low when 33

compared to conventional fossil fuels. Furthermore is it necessary to bridge long distances between 34

biomass production sites and power plants and that require expensive logistics [3]. There are different 35

technologies to improve the physical properties of solid biomass. Mechanical treatment such as 36

pelletization and briquetting result in a homogeneous biomass product of high density [4]. Thermal 37

treatment i.e. torrefaction removes moisture, volatiles and degrades parts of its carbohydrate fraction, 38

resulting in a product with higher energy density and better moisture resistance [5]. Both processes 39

have been studied extensively and recent efforts have been made to combine both processes where 40

biomass is thermally treated (torrefied) and subsequently pelletized [6-8] The resulting product is a 41

�coal like� product that is expected to be used as replacement and/or supplement for coal in existing 42

heat and power plants [8]. The properties of pellets made from torrefied biomass are in many ways 43

similar to coal as shown in Table 1 . 44

45

Table 1. Benchmark of torrefied pellets with selected bio- and fossil fuels [8-13]. 46

47

Major advantages are their high energy density hydrophobic nature and good grindability, and it is 48

expected that torrefied pellets can be handled similar as coal. Another major advantage of torrefied 49

pellets pointed out by the pellet industry is that they are an ideal fuel to be used in coal fired power 50

plants since only few changes have to be made to the existing power plant design. A recent study has 51

shown that it is possible to fire 100 % torrefied biomass in a pulverized coal boiler without decrease of 52

the boiler efficiency and fluctuation of boiler load [14]. 53

Torrefaction is a mild pyrolysis process at about 200-300 °C in the absence of oxygen that changes the 54

biomass properties significantly [5, 8]. Due to thermal depolymerisation and degradation a large 55

fraction of the biomass carbohydrate fraction is removed from the biomass, especially the 56

hemicelluloses that account for about 25-35 % dry matter of lignocellulose type biomass. Its natural 57

function in biomass is to crosslink the cellulose fibers and the lignin matrix and thereby providing the 58

3

biomass with its tenacious and flexible structure which is crucial for the mechanical stability of each 59

plant. 60

It has been shown that hemicelluloses start to degrade at low torrefaction temperatures of about 230 °C 61

and that the rate of degradation increases strongly with an increasing torrefaction temperature [15]. 62

Cellulose and lignin have been shown to be more resistant to thermal decomposition and significant 63

degradation occurs at higher temperature than for hemicellulose. Chen and Kuo [15] found that at a 64

torrefaction temperature of 260 °C almost 40 % of the hemicellulose have been removed from the 65

biomass compared to less than 5 % of the cellulose and about 3 % of the lignin. 66

The degradation of hemicelluloses results in the loss of the biomass�s tenacious and flexible properties 67

resulting in a brittle material with poor mechanical properties. The heating value increases from about 68

10-17 MJ kg-1

to about 12-24.5 MJ kg-1

depending on species, torrefaction temperature and residence 69

time [5,8,16] . Biomass torrefaction processes have been reviewed in great detail recently by van der 70

Stelte et al. [5] and others [17,18,19] 71

Biomass densification has been an established process to improve the handling properties of biomass 72

for several decades [4]. The raw materials are mainly softwood but also hardwoods are being used and 73

recently agricultural residues i.e. straws and husks from various crops and pulps are gaining interest. 74

Straw is available in large quantities and a low value by product of the agricultural industry. It is 75

already used in large scale heat and power plants [20] but the handling and transportation of straw has 76

remained an issue. Therefore several studies have been made to investigate the pelletizing properties of 77

agricultural residues [21-23]. 78

The pelletizing process consists out of multiple steps that are shown in Figure 1. 79

80

Figure 1. Process stages of biomass pellet production. 81

82

The working principle of a pelletizing process has been explained in detail in one of our earlier studies 83

[24]. In general the process can be subdivided into different steps: compression, flow and friction 84

component as shown in Figure 2. 85

86

Figure 2. Different components of a biomass pelletization process. Figure reproduced from [24] with 87

permission from Society of Wood Science and Technology. 88

4

89

Every time the roller approaches the surface of the die, the biomass is compressed and forms a 90

temporary layer on the die surface (compression component). The flow component represents the 91

energy required to force the compressed layer into the press channels and the friction component stands 92

for the energy required to press the compressed saw dust in the channels. The work required for the 93

three components can be determined experimentally for different raw materials and process parameters. 94

The resulting data provides and estimation for the energy consumption of the pelletizing process that 95

can be used for process design and optimization [24]. 96

97

Turning torrefied biomass into pellets have so far shown to be a challenging step in completing the full 98

production chain for torrefied biomass pellets [6,25]. The problems are two-fold in the sense that both 99

good pellet quality and acceptable pellet production capacities have been difficult to achieve. In terms 100

of quality, the pellet durability and density are often low in comparison to conventional natural wood 101

pellets, and high energy consumption in the pellet mill causes lower production capacities and high 102

costs. 103

Using a standardized single pellet press method has shown that the pelletization of torrefied biomass 104

requires more energy due to the high friction between the torrefied biomass and the metal surface of the 105

press channel within a pellet mill . Furthermore it was shown that the resulting pellets are less stable 106

compared to conventional wood pellets [6,7]. The lower strength has both advantages and 107

disadvantages. On the one hand a lower pellets strength increases the risk for fines and dust formation 108

during handling and transportation but on the other hand it has been shown that torrefied pellets are 109

easier and less energy intense to grind into a fine powder (e.g. prior to co-firing in a coal boiler) [26]. 110

It has been shown that pelletization of torrefied biomass requires higher energy and results in lower 111

crushing strength and density of the pellets compared to untreated biomass. These problematic 112

properties of torrefied material have been verified using a production scale (500 kg h-1

) pellet mill [25]. 113

The small scale trials using a single pellet press offer a number of possibilities to test different 114

parameters, which affect the torrefied material�s behavior in the pelletizing process. The effect of die 115

temperature, pressure, moisture content, degree of torrefaction, and also the effect of additives can 116

therefore be quickly screened before scaling up the trials into medium and large scale. 117

5

The press cannot simulate all aspects of a �real life� pellet mill but can give an indication about how 118

different process parameters affect the product quality and friction (pressure) built up in the press 119

channel. In a production size pellet mill, some process parameters are connected and dependent on each 120

other; for example, friction and temperature depend on the press channel length. Other parameters such 121

as the particle size or moisture content are easier to adjust. The results obtained with a single pellet 122

press tool will become useful when setting up a pilot scale test, i.e. allowing a more efficient testing, 123

since many parameters have been tested already in advance on a small scale. 124

The present study investigates the pelletizing properties of torrefied wheat straw (Triticum aestivum L.) 125

using a single pellet press unit. 126

Wheat is among the most grown crop species in the world with an estimated annual production of 127

about 685 Mt per year [27]. It is therefore considered to be a potential resource for heat and power 128

production. Nevertheless, the utilization of straw is limited due to poor handling properties related to 129

low bulk densities, in the range of 40 kg m-3

for loose straw [28] and about 120 kg m-3

for bales [29]. It 130

is likely that the fuel properties of straw can be improved by torrefaction and subsequent pelletization. 131

The present study investigates the pelletizing properties of torrefied wheat straw using a single pellet 132

press test system. 133

134

2. Methods 135

136

2.1 Materials 137

138

Wheat straw (Triticum aestivum L.) was harvested in the summer season 2011 on the island of 139

Sjælland, Denmark and collected after it has been dried on the field. The wheat straw was chopped into 140

particles < 5mm in diameter using a hammer mill (Model 55, Jensen and Sommer Aps, Denmark). The 141

material was packed in paper bags permeable to air and moisture and stored in a dry storage until use. 142

143

2.2 Torrefaction 144

145

Torrefaction was carried out in a bench scale batch reactor that was designed and built similar as 146

suggested by Pimchuai et al. [30]. A metal box with a volume of about 2 liters and two openings (5 mm 147

6

in diameter) for nitrogen inlet and gas outlet was used as reactor. The box was installed in a 148

programmable muffle furnace (S90, Lyngbyoven, Denmark) and connected to a nitrogen cylinder with 149

pressure and flow regulator, water seal valve and fittings and pipes for gas inlet, outlet and temperature 150

sensors. A temperature sensor (iron-constantan thermocouple) was installed in the center of the metal 151

reactor and connected to a thermometer (52 KJ, John Fluke, USA) and a computer system controlling 152

the heating of the oven. The material was torrefied at 150, 200, 250 and 300 ºC for two hours. 153

154

2.3 Fiber analysis 155

156

Fiber analysis method was based on the standard method used by Sluiter [31]. The moisture content of 157

the raw materials was determined using a moisture analyzer (MA 30, Sartorius, Germany) at 105 °C. 158

The samples were dried in vacuum at 40 °C for 2 days, and milled to <1 mm particle size. The 159

composition of the raw materials was analyzed using a two-step strong acid hydrolysis. 160

Monosaccharides (D-glucose, D-xylose and L-arabinose) were quantified using high-performance 161

liquid chromatography (HPLC) (Summit system, Dionex, USA) equipped with an RI-detector 162

(Shimadzu, Japan). Klason lignin content was determined based on the filter cake, subtracting the ash 163

content after incinerating the residues from the strong acid hydrolysis at 550 °C for 3 h. For extractives 164

analysis, about 6 g per sample were soxhlet extracted in acetone for 3 hours, and subsequent calculation 165

of the dry matter loss. 166

167

2.4 Infrared spectroscopy 168

169

ATR-FTIR spectra of the fracture surface and raw material surface were recorded as described earlier 170

[6] using a Fourier transform infrared spectrometer (Nicolet 6700 FT-IR, Thermo Electron 171

Corporation, USA), equipped with a temperature adjustable ATR accessory (Smart Golden Gate 172

diamond ATR accessory, Thermo Electron Corporation, USA), and equilibrated at 30 °C. Five 173

measurements per sample were performed. To ensure good contact with the diamond surface, all hard, 174

solid samples were pressed against it, using a metal rod and consistent mechanical pressure. All spectra 175

were obtained with 200 scans for the background (air), 100 scans for the sample and with a resolution 176

of 4 cm!1

. 177

7

178

2.3 Single pellet press 179

180

The biomass was pelletized using a single pellet press manufactured and designed at the Danish 181

Technological Institute, Denmark. The units consisted of a cylindrical die made of hardened steel, 182

lagged with heating elements and thermal insulation. The temperature was controllable between 20 and 183

180 °C, using a thermocouple (HSS braid coil, HT 30 regulator, Horst GmbH) connected to a control 184

unit. The end of the die could be closed using a backstop. Force is applied to the press using a universal 185

testing system (AGX, Shimadzu, Japan) and detected using a 200 kN loadcell connected to a detection 186

software (Trapezium X, Shimadzu, Japan). The setup is shown in Figure 3 and its working principle is 187

explained in great detail in an earlier work by Nielsen et al. [24] 188

189

FIGURE 3. Set up of a single pellet testing system. 190

191

Data recorded where the compression force and friction (force that is requited to extrude the pellet from 192

the press channel) 193

194

Compression: 195

An 8 mm press channel die was used at 125 °C temperature. 750 mg sample material was compressed 196

at a rate of 100 mm min-1

until a maximum pressure of 300 MPa was reached. The pressure was held 197

for 10 seconds. 198

199

Friction: 200

Subsequently to compression, the bottom piston was removed from the die, and the pellet was pressed 201

out of the channel at a rate of 100 mm min-1

. A value for the static friction was determined from the 202

force required to initiate the pellet motion. The average value of at least 5 measurements was 203

determined and a 95% confidence interval was calculated. 204

205

2.5 Characterization of pellets 206

207

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Strength and density: 208

The produced pellets were cylindrical with 8 mm diameter and about 10 to 14 mm in length. The 209

pellets were left to cool overnight, and the pellet strength was made by crushing the pellet 210

perpendicular to the length of the pellet. The pellet was laid down on the side and increasing force was 211

applied until crushing. Prior to this, the length, diameter, and mass were measured for density 212

calculation. Twelve replicates were made for each sample. The error bars in the plots refer to the 95 % 213

confidence interval of the mean. The Tukey-Kramer test [32] was used to determine whether the 214

observed differences were significant. 215

216

3. Results and Discussion 217

218

The fiber analysis (Table 2) indicates a chemical change during torrefaction with increasing treatment 219

temperature. The cellulose content of the torrefied wheat straw samples remain stable up to 250 °C but 220

decreases dramatically when increasing the torrefaction temperature to 300 °C. These results match 221

with earlier studies on wood torrefaction indicating severe degradation of cellulose at high 222

temperatures between 250 to 300 °C [6, 15, 16, 33, 34, 35, 36]. Hemicelluloses are more sensitive to 223

thermal degradation than cellulose and degradation starts at temperatures > 200 °C. Di Blasi and 224

Lanzetta [35] have studied the thermal degradation of xylan, one building block of hemicelluloses, and 225

suggested a two-step thermal degradation process starting at about 200 °C. The applied standard 226

method for fiber analysis defines the remaining acid insoluble fraction as Klasson lignin [31]. The 227

insoluble mass fraction remains constant up to 200 °C at about 20 % and the jumps to 80 % and more 228

at 300 °C. This can be explained with different effects. First of all does the volatilization of the 229

carbohydrates result in a relative increase of the remaining components i.e. insolubles and ash. 230

Secondly carbohydrates undergo scission reactions at elevated temperatures resulting in acid insoluble 231

solids and thus an increase of the insoluble fraction [35]. 232

233

Table 2. Fiber analysis of torrefied wheat straw (Data in g kg-1

). 234

235

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The data from fiber analysis was supplemented with infrared spectra obtained from an ATR-FTIR 236

spectrometer (Figure 4). The spectra show the composition of the first few µm of the straw particle 237

surface. The OH-stretching vibration region at 3600-3200 cm-1

gives an indication about the amount of 238

hydroxyl groups and as such potential bonding sites for water. The intensity of the OH stretching 239

vibration decreases with increasing torrefaction temperatures which could be an indication that the 240

concentration of OH groups decreases with cellulose and hemicelluloses dehydration at increasing 241

temperatures. The double peak at about 2850 and 2920 cm-1

has earlier been assigned to CH-stretching 242

vibrations of plant waxes [37]. Against expectations these waxes remain relatively stable to thermal 243

degradation and are still present at elevated temperatures. Major spectral changes occur between 1750-244

1600 cm-1

(carbonyl region) and this has earlier been observed by Gonzalez-Pena et al [38] who studied 245

torrefied wood. Spectral changes were explained with a general increase of absorption in the carbonyl 246

region due to lignin condensation, carboxylation of polysaccharides and opening of aromatic rings. 247

248

Figure 4. ATR-FTIR spectra of straw surface before and after torrefaction at different temperatures. 249

250

The thermal degradation of hemicelluloses and cellulose has a great effect on the ability to absorb 251

water. An increasing torrefaction temperature results in decreased moisture absorption of the wheat 252

straw samples (Figure 5). 253

254

Figure 5. Moisture absorption of torrefied wheat straw. 255

256

This is likely due to the thermal degradation of hemicelluloses and cellulose and thus a smaller amount 257

of available bonding sites for water molecules [33]. However Hakkou et al. [39] have studied the 258

wettability of torrefied beech wood and studied its thermal degradation by means of nuclear magnetic 259

resonance spectroscopy and FTIR. They concluded that the decreased moisture absorption rose from 260

conformational changes of the wood during torrefaction and lignin plasticization. The degradation and 261

volatilization of the biomass constituents results in an increasing energy density of the remaining 262

biomass, and as such an increase of the heating value as shown in Figure 6. The corresponding mass 263

yield is shown in Table 2. The energy density increases by about 40 % when comparing the untreated 264

biomass with the straw torrefied at 300 °C. 265

10

266

Figure 6. Heating value of torrefied wheat straw. 267

268

To increase the density of the biomass further they are pelletized using a single pellet press and 269

characterized according to their density and mechanical stability. All materials, apart from the wheat 270

straw torrefied at 300 °C, result in compact and homogeneous pellets as shown in Figure 7. 271

272

Figure 7. Pellets made from torrefied wheat straw. 273

274

Figure 8. Friction generated in the press channel. 275

276

Figure 9. Compression strength of pellets. 277

278

Figure 10. Unit density of pellets. 279

280

The material torrefied at 300 °C has significantly different pelletizing properties which is shown in 281

Figures 8, 9 and 10. The friction (Figure 8) is increasing moderately when increasing the torrefaction 282

temperature up to 250 °C. There is a big difference between material torrefied at 250 and 300 °C. The 283

friction increases by factor 10 to 30 and the recorded data from repeated trials showed a wide spread, 284

indicated by the error bar (95 % confidence interval). The wide spread is a sign for an instable 285

pelletizing behavior. This observation is also reflected by the compression strength data (Figure 9) and 286

pellet unit density (Figure 10). The material that has been torrefied at 300 °C, does not form stable 287

pellets and disintegrates when pressed out of the pellet press, resulting in a product of poor mechanical 288

stability and low density. In general it can be said that the pellet compression strength tends to decrease 289

with increasing torrefaction temperature, as already observed for wood pellets in earlier studies [6, 7]. 290

The lower compression strength is beneficial when it comes to crushing and milling of pellets prior to 291

firing, since conventional coal mills might be utilized, but certain strength is needed to prevent dust and 292

fines formation during handling and transportation. Earlier studies have shown that torrefied biomass as 293

such has favorable grinding characteristics [40,41] and this is also valid for when wood pellets are used 294

as feedstock for torrefaction processes [26]. The strong decrease of pellet unit density for samples 295

11

torrefied at 300 °C (Figure 10) can be explained with the poor inter-particle bonding between the single 296

biomass particles as seen in Figure 7, and that result in a phenomenon also known as spring back effect 297

[42-44]. This phenomenon has also been observed for pellets made from torrefied spruce [6]. 298

299

Figure 11. Fracture surface of pellets made from a) wheat straw and b) wheat straw torrefied at 300 °C. 300

301

Looking closer at the pellet surface after it has been fractured (Figure 11) reveals some important 302

differences between pellets made from wheat straw and torrefied wheat straw. Figure 11a shows a flat 303

surface, most likely the smooth cuticle of a wheat straw leaf while Figure 11b shows a flaking, brittle 304

surface The harsh torrefaction conditions have made the material brittle and less flexible which makes 305

it crack and disintegrate into smaller particles during pelletization. The relatively flat surface of both 306

samples is indicating weak interaction forces between the individual biomass particles and explains 307

also why the compression strength of both, straw pellets and torrefied straw pellets, is much lower as it 308

has been observed for wood pellets produced and tested under the same conditions [42]. 309

310

311

4. Conclusion 312

313

Fiber analysis has shown that wheat straws polymers are degrading during torrefaction. The thermal 314

degradation of hemicelluloses starts at about 200 °C and at about 250 °C for cellulose. Those results 315

were confirmed by infrared spectroscopy that also indicated structural changes and degradation for 316

lignin at high torrefaction temperature. The thermally induced chemical and physical changes of the 317

wheat straw result in a greater hydrophobicity that is reflected in reduced moisture absorption of the 318

biomass after torrefaction. The mass specific calorific value of the wheat straw increases with 319

torrefaction due to the degradation and evaporation of low molecular weight compounds. Nevertheless 320

the mass loss at high torrefaction temperature and two hours torrefaction time is unacceptable high. 321

Good pelletizing properties are obtained for torrefied biomass heated up to 250 °C. Above that 322

temperature the mechanical properties of the pressed pellets are poor which is also reflected in a low 323

unit density. The severe polymer degradation at high temperatures result also in a high friction between 324

biomass and press channel surface resulting in high pelletizing pressures that will most likely result in 325

12

high energy uptake when up-scaling the process to a pellet mill. Up to 250 °C torrefaction temperature 326

the pelletizing process results in mechanically strong pellets with increased properties i.e. higher 327

heating value and reduced moisture adsorption. The study focused on a broad temperature range 328

between 150 to 300 °C and from the obtained data it can be seen that most changes occur in the range 329

between 250 and 300 °C. Future studies should therefore focus on the temperature range between 250 330

and 300 °C to identify optimum conditions for the torrefaction and pelletization of wheat straw. 331

332

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