Torrefaction of woody waste for use as biofuel

1. Torrefaction of woody waste for use as biofuel. 1. C. M. Grottola. 1. , P. Giudicianni. 1. , J. B. Michel. 2. , R. Ragucci. 1. 2. 1. Istituto di Ri...
4 downloads 0 Views 706KB Size
Subscriber access provided by Kaohsiung Medical University

Biofuels and Biomass

Torrefaction of woody waste for use as biofuel Corinna Maria Grottola, Paola Giudicianni, Jean-Bernard Michel, and Raffaele Ragucci Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01136 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 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 19 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

Torrefaction of woody waste for use as biofuel

2

C. M. Grottola1, P. Giudicianni1, J. B. Michel2, R. Ragucci1

3

1. Istituto di Ricerche sulla Combustione-CNR, P.le Tecchio 80, 80125, Naples, Italy

4

2. HEIG-VD – Haute Ecole d’Ingenierie et de Gestion du Canton de Vaud– Switzerland

5 6

*Corresponding author: Corinna Maria Grottola

7

Email address: [email protected]

8

Postal address: P.le Tecchio 80, 80125, Naples (Italy)

9

Phone: +39 081 768 2245

10 11

12

Abstract

13

Biomass for energy production has been extensively studied in the recent years. To overcome

14

some constraints imposed by the chemical-physical properties of the biomass, several

15

pretreatments have been proposed. Torrefaction is one of the most interesting pretreatments

16

because torrefied biomass holds a wide range of advantages over raw biomass. The

17

devolatilization of water and some oxygenated compounds influences the increase in the

18

calorific value on both a mass and volumetric basis. The increase in the density reduces the

19

transportation costs. Moreover, the decreased moisture content increases the resistance of

20

biomass to biological degradation, thus facilitating its storage for long periods.

21

Under torrefaction conditions, approximately 10-40 wt% of the initial biomass is converted

22

into volatile matter including liquid and non-condensable combustible gases.1,2 The energy

23

efficiency of the process could greatly benefit the exploitation of the energy content of these

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 19

24

products. Recent studies and technological solutions have demonstrated the possibility to

25

realize polygeneration systems that integrate torrefaction/pyrolysis to a combustion process

26

with the aim of obtaining torrefied material/biochar and/or energy from biomass. Some

27

examples include Pyreg, Pyreg-Aactor GT3, TorPlant, and Top Process.4 The identification of

28

the main volatiles produced under torrefaction regime is useful for the optimization of the

29

operating conditions of the integrated system. The integrated process raises some concerns

30

when biomass from phytoremediation and wood from demolition and construction activities

31

are used as feedstock because they could contain potential toxic elements (PTEs). During the

32

torrefaction treatment, the fate of PTEs should be controlled in order to avoid their release in

33

the gas phase and to evaluate the extent of their concentration in the torrefied biomass.

34

The present work aims at studying torrefaction as an eco-sustainable process for the

35

combined production of a solid biofuel with improved characteristics with respect to the

36

starting material and a combustible vapor phase, embedded in the gas carrier flow, to be

37

directly burned for energy recovery. Herein, torrefaction tests on Populus nigra L. branches

38

from phytoremediation, and demolition wood were conducted at three temperatures, 250, 270

39

and 300 °C, at a holding time of 15 min. The energetic content of torrefied materials was

40

determined. At the same time, the fate of the heavy metals (Cd, Pb, and Zn) in the raw

41

biomass at different torrefaction temperatures was studied, and their mobility in the torrefied

42

biomass was investigated and compared to the mobility in the raw biomass.

43 44

Keywords: biomass, phytoremediation, woody waste, torrefaction, heavy metals, acetic acid

45

2 ACS Paragon Plus Environment

Page 3 of 19 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

46

Introduction

47

Energy consumption is increasing progressively with the rapid population growth and

48

economic development. A great interest is oriented to the research of new renewable energy

49

sources and technologies to not only cope with the growing demand for energy but also to

50

facilitate a reduction in greenhouse gases emissions (GHGs). Biomass conversion processes

51

are viewed as a viable option even though it could be advantageous to consider a number of

52

biomass pretreatments in order to fit various chemical and physical characteristics of biomass

53

to the existing combustion technologies.

54

Torrefaction was proposed as a pretreatment process for improving inherent biomass

55

characteristics such as increasing energy density and facilitating storage and handling

56

systems1 through the improvement of the grindability, the reduction in the moisture content,

57

the decrease in microbial degradation, and the sanitization of pest-affected plants.5 The partial

58

decomposition of biomass at low torrefaction temperature generates condensable and non-

59

condensable products and a solid residue rich in carbon, which is referred to as torrefied

60

material; this material can be utilized as high-quality fuel in different applications including

61

cofiring in power plants, entrained flow gasification, and small-scale combustion facilities.6

62

Extensive literature is available on the effect of the two main operating variables, i.e., the

63

final torrefaction temperature and the solid residence time. It was observed that temperature,

64

in the range 250-300 °C, affects the physicochemical characteristics and energy properties of

65

the solid product more than residence time does.1,7-9 A residence time ranging from few

66

minutes to one hour was typically used in torrefaction tests, and a residence time longer than

67

approximately 30 min had only negligible effects.1,7-9

68

However, the environmental sustainability of the torrefaction of lignocellulosic waste such as

69

woody waste and plants grown on contaminated soils must be addressed due to their high

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 19

70

content of potentially toxic elements (PTEs)10,11 that may affect the quality of the gas product

71

and the solid residue, depending on the process temperature.

72

To our knowledge, few studies have addressed this issue. The torrefaction of demolition

73

wood was studied by Edo et al.12 The elemental trace metal analysis suggested that most of

74

the trace metals detected in the raw material remained in the chars at the torrefaction

75

temperature (220 °C) used in the work. Bert et al.13 found that up to 290 °C heavy metals

76

contained in the biomass were retained in the solid matrix. Nevertheless, other authors14,15

77

showed that when some heavy metals are present in the form of chlorides, the devolatilization

78

temperature was greatly reduced, mainly under anoxic conditions. No investigation has been

79

conducted on the mobility of heavy metals retained in the torrefied biomass.

80

In this work, a comprehensive approach is proposed for simultaneously studying the

81

improvement of the energetic characteristics of two kinds of contaminated woody wastes and

82

the environmental aspects related to the presence of contaminants. Contaminations from

83

different sources were considered: Populus nigra L. branches (PN-B) containing heavy

84

metals translocated from the soil to the plant organs during phytoremediation and demolition

85

wood (DW) rich in heavy metals derived from operational activities during the construction

86

and the disposal of woody shipping crates. The aim of the present work was twofold:

87

studying the effect of the torrefaction temperature on the energetic properties of the torrefied

88

biomass; and evaluating the environmental impact of the process by monitoring the release of

89

heavy metals in the vapor phase as well as their mobility in the torrefied materials.

90

Torrefaction tests were conducted under oxygen-limited conditions at a constant heating rate

91

(10 °C/min) and at three final temperatures ranging from light to severe torrefaction regime16,

92

namely, 250, 270, and 300° C, with a residence time of 15 min. The product yields were

93

determined, and the organic and inorganic fractions of the solid products were characterized.

94

The mobility of PTEs in the torrefied materials was also investigated. Finally, condensable

4 ACS Paragon Plus Environment

Page 5 of 19 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

95

volatiles from torrefaction tests were collected separately and analyzed by gas

96

chromatography (GC) in order to understand their potential utilization.

97 98

2. EXPERIMENTAL MATERIALS AND METHODS

99

2.1 Torrefaction system

100

The experimental tests were performed in a small-scale reactor "SOLO furnace", available at

101

HEIG-VD, Switzerland. The cross-section of the furnace is shown in Figure 1. The reactor is

102

divided into two connected and concentric cylindrical zones separated by a perforated plate:

103

the internal one is the torrefaction chamber (diameter 20 cm and height 40 cm), whereas the

104

external cylinder is for gas recirculation (diameter 50 cm and height 80 cm).

105

A cylindrical steel container (diameter 15 cm and height 20 cm) with a perforated plate at the

106

bottom is positioned in the internal section of the reactor and is used to accommodate the

107

feedstock (100 g for each test run) packed in an aluminum paper (15 cm × 5 cm) and closed

108

with a metal ring. The lid is provided with a hole that allows the passage of the gas outlet line

109

and the thermocouple. In the external section, the recirculation of the exhausted gases

110

produced during the torrefaction test occurs. The recirculation is provided through a fan

111

located at the bottom of the external section under a perforated grid (frequency = 30 Hz). No

112

Nitrogen flux was used during the experiments. The fan located at the bottom of the reactor,

113

and the extractor placed on the up-section, are both regulated with the aim to reduce the

114

oxygen content in the reaction environment.

115

The temperature of the sample and the reaction environment was monitored constantly

116

through six K-type thermocouples sketched in Figure 1, connected to a Keysight

117

(Agilent/HP) 34970A Data Acquisition / Data Logger Switch Unit variable drive. The heat

118

flux of the heating coil was used as an adjustable variable in a proportional-integral-

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 19

119

derivative (PID) controller to produce a nominal heating rate equal to 10 °C/min during the

120

tests. The temperature of the sample (TC2) was used as set point temperature. Due to the

121

thermal inertia of the system, the actual heating rate was 5.6 °C/min, up to 220 °C. At higher

122

temperatures, the heating rate increased, probably due to the exothermic decomposition of

123

hemicellulose.17 In any case, this increase was reproducible in all the tests and did not affect

124

the average heating rate that remained equal to 5.6 °C/min. In all the tests, a maximum

125

overshoot of 2 °C was observed.

126

The volatiles produced in the reaction unit entered the condensation device, which consists of

127

two Pyrex condensers in series where condensable volatiles cooled and condensed. At the

128

condenser’s outlet, a Pyrex flask was allocated for the collection of the liquid products. The

129

non-condensing phase was fed to the analytical system for online characterization (Horiba

130

Mexa 7170D). After the process, the aluminum container was quenched by immersing it

131

rapidly in a glass beaker with 5 liters of water at a temperature of 10 °C. The water cooled the

132

container, and any contact between the torrefied material and the water was avoided.

6 ACS Paragon Plus Environment

Page 7 of 19

50 cm

20 cm

GasGas

TC4 TC5

TC6

40 cm

Heating Coils

TC3 TC3

Biomass

TC2

80 cm

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

TC1

Motor Motor

133 134 135 136

Figure 1. Cross Section of the “SOLO” furnace At the end of the test, the sample was immersed in a metal vessel containing 10 °C water.

137

The quenched sample was heated in the furnace for 24 h at 105 °C before final weight

138

measurement in order to remove the water. Solid yields were determined gravimetrically with

139

respect to the fed sample.

140 141

2.2. Material characterization

142

2.2.1 Solid materials

143

PN-B were collected during phytoremediation tests conducted in Litorale Domitio - Agro

144

Aversano NIPS (South Italy, Campania region) in the framework of the European LIFE

145

Project ECOREMED (LIFE11/ENV/IT/275 – ECOREMED, 2016), whereas DW was

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 19

146

obtained from the disposal of shipping crates. The material was ground, and the sieved

147

fraction, in the 400–600 µm size range, was recovered for the torrefaction tests. The samples

148

were oven dried at 105 °C for 24 h and kept in the desiccator before the characterization

149

analyses and torrefaction tests. The moisture content of feedstock and torrefied biomass was

150

measured with a thermobalance (Sartorius Moisture Analyzer - Model MA35) according to

151

the ISO18134-3 procedure. The CHONS content was measured using the elemental analyzer

152

Analyseur Flash 2000 (Thermo Scientific) according to the ISO 16948:2015 procedure.

153

Carbolite AFF 1100 furnace was used for the determination of ash and volatile content

154

according to the ISO 1171/18123:2015 and ISO 18123:2015 procedures, respectively. Fixed

155

carbon was calculated as the amount required to complete the mass balance. The calorific

156

value was determined using a bomb calorimeter (Oxygen Combustion Vessel 1108 - Parr

157

Instrument Company) according to EN14918. Ash composition was determined by dissolving

158

the biomass samples via microwave-assisted acid digestion based on US-EPA Methods 3051

159

and 3052. The digested samples were then analyzed by inductively coupled plasma mass

160

spectrometry (ICP/MS) using an Agilent 7500CE instrument. The results were reported in

161

terms of content of the inorganic species and ion recovery in the torrefied biomass. The first

162

is defined as mass of ion per mass of char and is used to calculate the ion recovery by

163

multiplying it by torrefied yield and then dividing by the mass of ions in the raw biomass.

164

The energy yields of the torrefied materials were calculated on dry basis by equation (1),

165

where “t” stands for torrefied material and “f” for feedstock.

  =

 ∗    ∗ 100 1 

8 ACS Paragon Plus Environment

Page 9 of 19 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

166

Metal mobility was determined through a leaching test on biomass and corresponding

167

torrefied materials using water and an EDTA-NH4 solution, as reported in Gonsalvesh et al.18

168

The amount of heavy metals in the leachate was estimated based on the PTEs recovered in

169

the torrefied biomass. The ion release was the ratio between the amount of PTEs released in

170

the leachate and the amount of PTEs in the torrefied material.

171

2.2.2 Liquid product

172

For the identification and quantification of the main condensable species, liquids obtained

173

from torrefaction tests at 250, 270, and 300 °C were filtered with 0.20-µm microfilters

174

(Millex-FG). Chemical analysis was performed by a gas chromatograph coupled with a flame

175

ionization detector (Agilent Technologies 7820A GC System) and a DB-1701 capillary

176

column (60 m × 0.25 mm i.d., 0.25-mm film thickness). Helium (99.9999%) was used as

177

carrier gas with a constant flow of 1.0 mL/min. The oven temperature was programmed from

178

318 (4 min) to 508 K at a heating rate of 3 K/min and held at 508 K for 30 min. The injector

179

and the FID were kept at 523 K and 573 K, respectively. A sample volume of 1 µL (4.5 wt%

180

of pyrolysis liquid in acetone) was injected.

181

The identification of the main compounds (acetic acid, hydroxyacetone, furfural, 5-methyl

182

furfural, and 5-hydroxy methyl furfural) is based on the match with the retention times of the

183

corresponding standards (Sigma Aldrich 319910) analyzed by GC/FID under the same

184

conditions. The identified compounds were quantified by the internal standard method, using

185

fluoranthene as an internal standard. A calibration curve was prepared by the injection of four

186

standard solutions. The concentration range was determined by successive approximations

187

until it became relatively narrow and encompassed the quantified value. Injections of the

188

liquid samples were made in duplicate, and the maximum relative error observed was ±5% of

189

the average values.

190

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 19

191

3. RESULTS AND DISCUSSION

192

The results of the chemical characterization of PN-B and DW samples are reported in Tables

193

1 and 2. It should be noted that even though the origin of the waste was different, the results

194

of the elemental analysis were comparable except for those of the nitrogen content. The

195

biomass from phytoremediation was richer in ash than the DW was. The higher nitrogen

196

content of DW compared to that of PN-B could be attributed to adhesives used in the

197

production of timber goods (such as particle boards) that ended up in the DW wood waste

198

stream.19 The proximate analysis highlighted a comparatively higher content of volatiles in

199

DW and a higher ash content in PN-B, whereas the fixed carbon content was comparable.

200

The heavy metals present in both samples were Cd, Cu, Pb, and Zn, with the last two being

201

the most abundant, as shown in Table 2. C

H

N

O

wt % daf DW PN-B 202 203 204 205

47.7 (0.3) 47.1 (0.2)

6.1 (0.2) 5.9 (0)

HHV MJ/kg

2.1 (0.4) 0.7 (0.1)

42.6 (0.1) 19.2 (0.3) 41.5 (0.1) 19.0 (0.3)

Table 1. Feedstock characterization: elemental analysis and HHV. The relative error of three replicates is reported in brackets.

moisture

volatiles

wt % as received DW 1.1 (0.1) PN-B 7.0 (0.4)

fixed carbon

ash

Cd

wt % db 80.2 (0.8) 77.0 (0.4)

Cu

Pb

Zn

mg/kg

18.3 (0.7) 1.5 (0) 0.1 (6) 6.4 (6) 30.6 (5.5) 142.4 (6.2) 18.2 (0.4) 4.8 (0) 2.2 (4) 8.2 (1.7) 60.3 (4.8) 50 (8.9)

206 207 208 209

Table 2. Feedstock characterization: proximate analysis and heavy metal content. The relative error of three replicates is reported in brackets.

210

The torrefied biomass yields are shown in Figure 2. As expected, for both the PN-B and DW,

211

the mass yield decreased with the torrefaction temperature. Despite the similar results

212

obtained from the elemental analysis and the higher volatile content of DW, at each

10 ACS Paragon Plus Environment

Page 11 of 19 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

213

temperature, the mass loss was higher for PN-B than that for DW. The mass yield of the

214

torrefied biomass varied between 74.6 and 64.0 wt% for PN-B and between 87.4 and 80.5

215

wt% for DW. Basu et al.20 reported a yield of 78 wt% for poplar wood torrefied at 250 °C,

216

whereas Kim et al.21 obtained solid yields between 92 and 60 wt% for yellow poplar torrefied

217

in the temperature range 240-280 °C. The results on DW mixed with RDF (refuse-derived

218

fuel) are available in the temperature range 220-270 °C12,22 and show yields varying in the

219

range of 94 and 84 wt%. According to the previous findings, in the torrefaction regime,

220

hemicellulose is the main component undergoing devolatilization.23 The higher solid yield

221

observed for DW in this study could be explained by the lower content of hemicellulose in

222

the raw sample. Nevertheless, the results of DW suffer heavily from the inhomogeneity of

223

this type of woody refuse, and as a consequence, this makes it impossible to draw firm

224

conclusions for these materials.

225 226 227 228

Figure 2. Torrefied biomass yields at T = 250, 270, and 300 °C.

229

The evolution of the gas composition during the torrefaction of PN-B at 300 °C, reported in

230

Figure 3a, and the main liquid compounds identified at 300 °C (Figure 3b) confirm that the

231

hemicellulosic fraction of the PN-B sample is decomposed, producing mainly CO2 from the

11 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 12 of 19

232

decomposition of side chains (acetyl groups and carboxylic groups), CO, from the carbonyl

233

end groups left after dehydration of the side chain groups, and condensable compounds.

234

Among the detected liquid compounds, acetic acid is the most abundant, followed by acetol

235

and furan derivatives. However, the torrefaction liquids typically are greatly diluted in

236

water.24

237 238 239 240

Figure 3. Gas species (panel a) and liquid compounds concentration (panel b) obtained from PN-B torrefied at T = 300 °C.

241 242

3.2 Torrefied biomass characterization

243

The results of characterization of torrefied DW and PN-B obtained at different temperatures

244

are reported in Tables 3 and 4.

C

H

N

O

wt % daf DW 250 DW 270 DW 300 PN-B 250 PN-B 270 PN-B 300

50 (1.0) 53 (0.5) 54.4 (0.8) 51.4 (0.9) 51.8 (0.6) 54.2 (0.5)

5.8 (0.2) 5.8 (0.1) 5.8 (0.2) 5.4 (0.1) 5.2 (0.1) 5.1 (0.1)

2.4 (0.3) 3.0 (0.6) 3.4 (0.2) 1.0 (0.1) 0.8 (0.1) 0.9 (0.1)

39.0 (1.3) 36.0 (0.5) 34.5 (0.7) 38.0 (0.6) 37.3 (0.9) 34.0 (0.6)

MJ/kg

Energy yield %

19.3 (0.7) 21.5 (0.2) 22.5 (0.1) 20.8 (0.9) 21.5 (0.7) 23.3 (0.3)

88.1 95.6 94.3 78.1 75.8 75.0

HHV

245

12 ACS Paragon Plus Environment

Page 13 of 19 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

246 247

Energy & Fuels

Table 3. Elemental analysis and energy properties of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

248 moisture

volatiles

wt % as received DW 250 DW 270 DW 300 PN-B 250 PN-B 270 PN-B 300

1.3 (0.5) 1.5 (0.1) 1.3 (0.1) 1.2 (0.7) 1.5 (0.5) 1.6 (0.2)

fixed carbon

ash

Cd

wt % db 71.8 (6) 71.9 (0.8) 70.3 (0.6) 69.1 (0.1) 67.9 (2.8) 62.5 (0.1)

25.4 (5.0) 25.9 (1.2) 27.9 (0.6) 25.7 (0.5) 27.2 (3.2) 31.7 (0.2)

Cu

Pb

Zn

mg/kg 2.7 (1) 2.1 (0.5) 1.8 (0) 5.1 (0.1) 4.9 (2.8) 5.8 (0.1)

0.1 (19) 0.2 (4) 0.2 (2) 2.5 (3.4) 2.6 (1.8) 3.2 (3.5)

8.3 (5) 8.5 (26) 8.9 (7) 9.6 (0.3) 9.7 (4.3) 11.4 (5)

51.7 (10) 185 (7) 52.9 (17) 188 (15) 54.1 (71) 195.4 (7) 70.4 (10) 61 (0.6) 72.3 (3.2) 65.4 (4) 89 (4.6) 73.2 (0.1)

249 250 251

Table 4. Proximate analysis and heavy metal content of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

252 253

With increasing torrefaction temperature, for both the feedstocks, an increase in the amounts

254

of elemental carbon and a decrease in the elemental oxygen and hydrogen amounts were

255

observed, in agreement with the literature1. This result is due to the breaking of the weak C–

256

O and C–H bonds in the hemicellulose matrix responsible for the release of volatile species

257

and permanent gases (mainly CO and CO2)25 that are rich in oxygen and hydrogen, thus

258

causing the deoxygenation of the torrefied biomass. The thermal behavior of elemental

259

nitrogen was different in the two feedstocks, revealing a different chemical nature of the N-

260

compounds in PN-B and DW. The nitrogen content always increased with the torrefaction

261

temperature for the DW sample, whereas for the PN-B sample, the trend was not evident. The

262

O/C and H/C ratios, represented in the Van Krevelen diagram in Figure 4 for both the PN-B

263

and DW torrefied samples, are considered important parameters to characterize solid biofuel

264

composition with respect to coal. Typical H/C and O/C values for torrefied biomass are in the

265

range of 1-1.5 and 0.4-0.65, respectively.26 An increase in the torrefaction temperature

266

reduced both H/C and O/C ratios to values that are within the typical ranges observed for

13 ACS Paragon Plus Environment

Energy & Fuels

267

other torrefied biomasses, even though they were still high in comparison with the

268

characteristic values of coal. At 300 °C, it was observed that the O/C and H/C ratios were

269

greatly decreased to values close to the lignite coal range.27 The lowest torrefaction

270

temperature significantly affected the O/C ratio for the PN-B sample, indicating that

271

significant devolatilization of oxygenated compounds occurred even at low temperature, in

272

agreement with the observed weight loss (Figure 2). At the highest torrefaction temperature,

273

DW and PN-B were characterized by comparable O/C ratios. At each temperature, the H/C

274

ratio, similar in both the feedstocks, was always lower for torrefied PN-B, denoting the

275

devolatilization of a greater amount of compounds containing saturated C-H bonds as well as

276

bound water.

1.6

DW

DW 250

PN-B

1.4 Atomic H/C ratio

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 19

DW 270 DW 300

PN-B 250

1.2 PN-B 270 PN-B 300

1.0

0.8 0.3

277 278

0.4

0.5 0.6 Atomic O/C ratio

0.7

0.8

Figure 4. Van Krevelen diagram for untreated and torrefied PN-B and DW.

279 280

Table 4 shows that, as expected, the volatile content of both torrefied feedstocks decreased

281

with temperature, whereas the fixed carbon content increased. According to the higher mass

282

loss observed for PN-B than for DW, the fixed carbon content was higher and the volatile

283

content was lower for the corresponding material torrefied at 270 °C and 300 °C. The fixed

284

carbon content of torrefied DW and in particular the PN-B sample increased greatly and was

14 ACS Paragon Plus Environment

Page 15 of 19 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

285

comparable to that of coal.1 The HHVs of torrefied solids were remarkably improved at

286

higher torrefaction temperatures and were always slightly higher for PN-B than those for DW

287

across the whole temperature range. However, it should be noted that the energy yield was

288

always lower for PN-B due to the higher devolatilization. Moreover, in the case of DW, the

289

energy yield had a non-monotonous trend with the temperature, showing a maximum at 270

290

°C. In contrast, the energy yield decreased with temperature for PN-B. As temperature

291

increased from 250 °C to 270 °C, the char yields decreased for both PN and DW.

292

Nevertheless, in the case of PN-B, the mass loss is greater than that for DW. In contrast, the

293

decrease in the O/C ratio in the char is smaller for PN-B than that for DW. This result implies

294

that DW released a lower amount of vapors (condensable and permanent gases) with greater

295

oxygen content. It is likely that mainly H2O was produced and released and that most of the

296

energy-containing volatiles were still in the torrefied material.28 As a consequence, with

297

increasing temperature, the increase in the char calorific value is greater than the mass loss,

298

thus determining the increase in the energy yield.

299

The concentration and the ion recovery of the detected heavy metals, namely, Cd, Pb, Cu, and

300

Zn, for the torrefied materials are reported in Tables 4 and 5, respectively. The concentration

301

increased with the torrefaction temperature for both feedstocks. The ion recovery for all

302

torrefied materials is very close to 1, and thus, it can be inferred that the condensable and gas

303

phases evolved from the torrefaction tests were essentially free of heavy metals.

Ion Recovery Cd

Cu

Pb

Zn

gr/gr

DW 250

0.99 (1.5)

0.98 (4.9)

1.12 (9.7)

1.06 (6.8)

DW 270

1.00 (1.5)

1.15 (5)

0.95 (1.75)

0.97 (3)

DW 300

1.15 (6)

1.1 (6)

1.12 (5)

1.00 (4)

PN-B 250

0.99 (3)

0.97 (4.9)

1.05 (2.8)

0.99 (3)

15 ACS Paragon Plus Environment

Energy & Fuels

PN-B 270

1.01 (1.5)

1.07 (5)

1.02 (1.75)

1.00 (3)

PN-B 300

1.02 (7)

1.04 (6)

1.03 (5)

1.06 (7)

304 305

Table 5. Ion recovery of torrefied DW and PB-N obtained at 250 °C, 270 °C, and 300 °C. The relative error of three replicates is reported in brackets.

306

To investigate the effect of torrefaction on the mobility of the heavy metals retained in the

307

torrefied PN-B samples, two leaching tests were performed, in water and in an EDTA-NH4

308

solution. A higher ion release in water was observed for Zn, followed by Cu, Cd, and Pb, and

309

their mobility decreased with increasing torrefaction temperature. This result could be related

310

to the increase in the hydrophobic character of the torrefied biomass with the torrefaction

311

temperature.1 Leaching with EDTA-NH4 was more severe, and all metals were released from

312

the raw materials. Temperature did not have any effect on the PTE mobility up to 300 °C,

313

where part of the metals retained in the char are immobilized in the solid matrix even in more

314

severe leaching conditions. It is likely that in acid conditions, the acid groups, associated with

315

lignin, hemicellulose, and extractives, were easily removed together with the associated

316

inorganic elements.29

317 Water

a) 1.00

PN-B

PN-B 250

PN-B 270

PN-B 300

1.00

ion release g/g

0.60 0.40 0.20

0.80 0.60 0.40 0.20

0.00

0.00 Cd

318 319

EDTA-NH4

b)

0.80 ion release g/g

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 19

Pb

Zn

Cu

Cd

Pb

Zn

Cu

Figure 5. Ion release of heavy metals in water (a) and an EDTA-NH4 leaching solution (b).

320 321

Conclusion

16 ACS Paragon Plus Environment

Page 17 of 19 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

322

The torrefaction of woody waste (demolition wood and biomass from soil phytoremediation)

323

was studied with the aim of evaluating the energetic properties of the torrefied material and

324

the fate of heavy metals during the pretreatment. It was found that with increasing

325

torrefaction temperature the energy properties of both torrefied biomasses were improved. In

326

particular, the study revealed that demolition wood has a high potential in terms of its energy

327

content as well as energy yield. Some concerns arise for the high nitrogen content of DW

328

compared to that of PN-B both in the raw and torrefied materials. For both feedstocks, PTEs

329

were retained in the torrefied biomass up to 300 °C, allowing the production of a heavy

330

metal-free vapor-phase fuel. The higher the torrefaction temperature was, the lower the PTE

331

release by water leaching of torrefied material, thus increasing the safety of the material

332

storage in open areas.

333 334

Acknowledgements:

335

This article is based upon work from COST Action SMARTCATs (CM1404), supported by

336

COST (European Cooperation in Science and Technology, http://www.cost.eu).

337

This work was supported by the European Commission (Project LIFE11/ENV/IT/275-

338

Ecoremed) and the Accordo di Programma CNR-MSE 2013-2014 under the contract

339

‘‘Bioenergia Efficiente”.

340

References

341 342 343

(1) Tumuluru, J. S.; Sokhansanj S.; Hess; J. R.; Wright, C. T.; Boardman, R. D. A review on biomass torrefaction process and product properties for energy applications. Industrial Biotechnol. 2011, 7(5), 384-401.

344 345

(2) Chen, W. H.; Liu, S. H.; Juang, T. T.; Tsai, C. M.; Zhuang, Y. Q. Characterization of solid and liquid products from bamboo torrefaction. Appl. Energ. 2015, 160, 829-835.

346

(3) http://www.pyreg.de last accessed 18.05.2018.

17 ACS Paragon Plus Environment

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

Page 18 of 19

347 348

(4) McCormick, K.; Kautto, N. The bioeconomy in Europe: An overview. Sustainability. 2013, 5(6), 2589-2608.

349 350 351

(5) Correia, R.; Gonçalves, M.; Nobre, C.; Mendes, B. Impact of torrefaction and lowtemperature carbonization on the properties of biomass wastes from Arundo donax L. and Phoenix canariensis. Bioresour. Technol. 2017, 223, 210-218.

352 353

(6) Acharya, B.; Sule, I.; Dutta, A. A review on advances of torrefaction technologies for biomass processing. Biomass Convers. Biorefin. 2012, 2(4), 349-369.

354 355

(7) Bergman, P. C.; Boersma, A. R.; Zwart, R. W. R.; Kiel, J. H. A. Torrefaction for biomass co-firing in existing coal-fired power stations. Proc. ECN reports No. ECN-C-05-013. 2005.

356 357

(8) Sadaka, S.; Sharara, M. A.; Ashworth, A.; Keyser, P.; Allen, F.; Wright, A. Characterization of biochar from switchgrass carbonization. Energies. 2014, 7(2), 548-567.

358 359

(9) Poudel, J.; & Oh, S. C. Effect of Torrefaction on the Properties of Corn Stalk to Enhance Solid Fuel Qualities. Energies. 2014, 7(9), 5586-5600.

360 361

(10) Lievens, C.; Carleer, R.; Cornelissen, T.; Yperman, J. Fast pyrolysis of heavy metal contaminated willow: Influence of the plant part. Fuel. 2009, 88(8), 1417-1425.

362 363

(11) Dilks, R. T.; Monette, F.; Glaus, M. The major parameters on biomass pyrolysis for hyperaccumulative plants – A review. Chemosphere. 2016, 146, 385-395.

364 365 366

(12) Edo, M.; Skoglund, N.; Gao, Q.; Persson, P. E.; Jansson, S. Fate of metals and emissions of organic pollutants from torrefaction of waste wood, MSW, and RDF. Waste Manage. 2017, 68, 646-652.

367 368 369 370

(13) Bert, V.; Allemon, J.; Sajet, P.; Dieu, S.; Papin, A.; Collet, S.; Raventos, C. Torrefaction and pyrolysis of metal-enriched poplars from phytotechnologies: Effect of temperature and biomass chlorine content on metal distribution in end-products and valorization options. Biomass Bioenerg. 2017, 96, 1-11.

371 372

(14) Abanades, S.; Flamant, G.; Gauthier, D. Kinetics of heavy metal vaporization from model wastes in fluidized bed. Environ. Sci. & Technol. 2002, 36(17), 3879-3884.

373 374 375

(15) Yu, J.; Sun, L.; Xiang, J.; Hu, S.; Su, S. Kinetic vaporization of heavy metals during fluidized bed thermal treatment of municipal solid waste. Waste Manage. 2013, 33(2), 340346.

376 377

(16) Matsakas, L.; Gao, Q.; Jansson, S.; Rova, U.; Christakopoulos, P. Green conversion of municipal solid wastes into fuels and chemicals. Electron. J. of Biotech. 2017, 26, 69-83.

378 379

(17) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel. 2007, 86(12-13), 1781-1788.

18 ACS Paragon Plus Environment

Page 19 of 19 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

380 381 382

(18) Gonsalvesh, L.; Yperman, J.; Carleer, R.; Mench, M.; Herzig, R.; & Vangronsveld, J. Valorisation of heavy metals enriched tobacco biomass through slow pyrolysis and steam activation. J Chem. Technol. Biotechnol. 2016, 91(6), 1585-1595.

383 384

(19) Jermer, J.; Ekvall, A.; & Tullin, C. Inventory of contaminants in waste wood. Värmeforsk Service AB, Report. 2001, 732, 6-64.

385 386

(20) Basu, P.; Rao, S.; Dhungana, A. An investigation into the effect of biomass particle size on its torrefaction. The Canadian Journal of Chemical Engineering. 2013, 91(3), 466-474.

387 388 389

(21) Kim, Y. H.; Lee, S. M.; Lee, H. W.; Lee, J. W. Physical and chemical characteristics of products from the torrefaction of yellow poplar (Liriodendron tulipifera). Bioresour. Technol. 2012, 116, 120-125.

390 391 392

(22) Verhoeff, F.; Arnuelos, A. A.; Boersma, A. R.; Pels, J. R.; Lensselink, J.; Kiel, J. H. A.; Schukken, H. Torrefaction Technology for the production of solid bioenergy carriers from biomass and waste. Energy research Centre of the Netherlands. 2011, 75.

393 394 395

(23) Giudicianni, P.; Gargiulo, V.; Grottola, C. M.; Alfè, M.; & Ragucci, R. Effect of alkali metal ions presence on the products of xylan steam assisted slow pyrolysis. Fuel. 2018, 216, 36-43.

396 397 398

(24) Chen, W. H.; & Kuo, P. C. Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy. 2011, 36(2), 803-811.

399 400 401

(25) Giudicianni, P.; Cardone, G.; Sorrentino, G.; Ragucci, R. Hemicellulose, cellulose and lignin interactions on Arundo donax steam assisted pyrolysis. J. Anal. Appl. Pyrol. 2014, 110, 138–146.

402 403 404

(26) Bridgeman, T. G.; Jones, J. M.; Shield, I.; Williams, P. T. Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel. 2008, 87(6), 844-856.

405 406

(27) McKendry, P. Energy production from biomass (part 1): overview of biomass. Bioresour. Technol. 2002, 83(1), 37-46.

407 408

(28) Pahla, G.; Ntuli, F.; Muzenda, E. (2018). Torrefaction of landfill food waste for possible application in biomass co-firing. Waste Manage. 2018, 71, 512-520.

409 410 411

(29) Wigley, T.; Yip, A. C.; & Pang, S. The use of demineralisation and torrefaction to improve the properties of biomass intended as a feedstock for fast pyrolysis. J. Anal. Appl. Pyrol. 2015, 113, 296-306.

19 ACS Paragon Plus Environment