Microwave-assisted and conventional hydrothermal carbonization of lignocellulosic waste material: Comparison of the chemical and structural properties of the hydrochars

Accepted Manuscript Title: Microwave-assisted and conventional hydrothermal carbonization of lignocellulosic waste material: Comparison of the chemical and structural properties of the hydrochars Author: Sunday E. Elaigwu Gillian M. Greenway PII: DOI: Reference:

S0165-2370(15)30098-X http://dx.doi.org/doi:10.1016/j.jaap.2015.12.013 JAAP 3630

To appear in:

J. Anal. Appl. Pyrolysis

Received date: Accepted date:

30-7-2015 5-12-2015

Please cite this article as: Sunday E.Elaigwu, Gillian M.Greenway, Microwaveassisted and conventional hydrothermal carbonization of lignocellulosic waste material: Comparison of the chemical and structural properties of the hydrochars, Journal of Analytical and Applied Pyrolysis http://dx.doi.org/10.1016/j.jaap.2015.12.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Microwave-assisted and conventional hydrothermal carbonization of lignocellulosic

2

waste material: Comparison of the chemical and structural properties of the

3

hydrochars

4

Sunday E. Elaigwua,b*, Gillian M. Greenway a

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a

6

b

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Corresponding author. Tel. +44 7733680543; fax: +44 1482466410

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Email addresses: [email protected], [email protected]

Department of Chemistry, University of Hull, Cottingham Road, Hull, HU6 7RX, UK Department of Chemistry, University of Ilorin, PMB 1515, Ilorin, Kwara State, Nigeria

9 10

Abstract

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This study compares the chemical and structural properties of the hydrochars prepared

12

from microwave-assisted and conventional hydrothermal carbonizations of Prosopis

13

africana shell, a waste plant material. The preparation involved heating the raw material

14

in de-ionized water at 200 °C for 5-20 min, and 120-240 min in the microwave and

15

conventional oven respectively. The prepared hydrochars were characterized using the

16

scanning electron microscope, nitrogen sorption measurement, Fourier transform infrared

17

spectroscopy, CHN analyzer, thermogravimetric analysis, and nuclear magnetic

18

resonance. The results showed that the microwave-assisted hydrothermal carbonization

19

process is fast in the carbonization of the Prosopis africana shell as shown by the level of

20

conversion attained within the short time. This study presents new data on the

21

comparison of the hydrochars from microwave-assisted and conventional hydrothermal

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carbonization processes of the same lignocellulosic material in terms of their properties.

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Keywords: Microwave, hydrothermal carbonization, carbonized material, Prosopis africana shell

24 25

1. Introduction

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Various methods exist for the preparation of carbonaceous materials from waste

27

biomasses, with the aim of managing, sequestering carbon, and converting the waste

28

materials into useful products that can find application in the environment, separation

29

science, catalysis, and as electrode material [1]. One of such methods is the

1

30

environmentally friendly hydrothermal carbonization process, which involves the heating

31

of biomass under pressure in water at about 180-200 °C to form char called hydrochar,

32

and water-soluble organic substances [2,3]. Thus, in the hydrothermal carbonization

33

process the stored energy in the biomass is used more efficiently. It is an important

34

process because water acts as both the reagent and reactive environment, and it has been

35

applied in the synthesis of different types of carbonaceous materials [4].

36

Microwave technology has been shown to be more energy efficient than

37

conventional methods of heating in many applications, because it provides selective, fast

38

and homogenous heating, which reduces processing time and costs significantly [5,6]. As

39

such it can be an attractive alternative to the conventional method of hydrothermal

40

carbonization of waste materials. Hydrothermal carbonization process was first reported

41

in 1913 by Bergius and Specht [7], and has been proven to be a promising and a

42

sustainable waste management method. The ultimate goal of the process is to convert

43

waste materials into value-added products, while promoting the integration of the carbon

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formed in the solid-phase. Several other researches have been carried out over the years

45

using conventional and microwave heating to produce valuable products, from different

46

kinds of materials, for various applications [8-17]. Despite all these studies, a direct

47

comparison of the solid product (hydrochar) obtained from the microwave-assisted and

48

conventional hydrothermal carbonization methods of the same lignocellulosic material

49

has not been reported to the best of our knowledge. Therefore, in this study we made

50

comparisons between the hydrochars prepared from a waste plant material (Prosopis

51

africana shell) using these methods, with focus on the similarities between the structural

52

and chemical properties of the hydrochars, in order to provide useful information about

53

the microwave-assisted and conventional oven hydrothermal carbonization processes.

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The Prosopis africana (Mimosaceae) used in this study is a multi-purpose tree of

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great economic value among the rural communities in Nigeria. The shell is one of the

56

widely available waste products obtained from it numerous uses. Detailed information on

57

the Prosopis africana can be found elsewhere [18].

2

58 59 60

2. Materials and Methods The Prosopis africana shell was collected in Benue State, Nigeria. It was crushed and sieved to 2 mm size and stored for use in the preparation of the hydrochars.

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It is important to state that this study was restricted to 200 °C for safety reason

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based on previous report by Titirici et al., [1] that temperatures below 180 °C produces

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only weak or no conversions for cellulose containing materials; while temperatures

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exceeding 220 °C could cause a runaway of the reaction, and hydrothermal reactions at

65

250 °C tend to generate too much heat and pressure, which could burst the safety plate of

66

the autoclave.

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2.1 Preparation of the hydrochars

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2.1.1 Microwave-assisted hydrothermal carbonization

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This experiment was conducted in a 2.45 GHz, and 1600 W microwave oven

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(MARS, CEM, Milton Keynes, UK equipped with XP1500 digestion vessels). The

71

reaction temperature was monitored with an infrared fibre optic sensor fitted into the

72

ceramic sleeve of the reference vessel, while the reaction pressure was monitored with a

73

pressure sensor fitted to the same reference vessel.

74

The procedure has been described in our previous study [18]. Briefly, 2 g of the

75

ground Prosopis africana shell was weighed into a reaction vessel made of Teflon and 30

76

ml of de-ionized water was added to cover the solid. The reaction vessel was sealed and

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placed in the microwave oven. The sample was then heated at 200 °C in the microwave

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oven which was set to ramp to the temperature in 5 min and was held at the temperature

79

for 5-20 min (pressure at 200 °C was about 1.80 MPa). The reaction system was allowed

80

to cool to room temperature, and the carbonized material was filtered off using filter

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paper (Whatman filter paper number 3, ashless 11 cm). The hydrochar obtained was

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washed several times with de-ionized water until a neutral pH was reached. It was then

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dried in a conventional oven at 80 °C for 16 h, and stored in the desiccator for further

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analysis. The hydrochars were denoted as M-PASx-y, where x and y represent temperature

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(°C) and time (min) for the carbonization respectively.

3

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2.1.2 Conventional hydrothermal carbonization

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This experiment was conducted in a Teflon-lined autoclave (Parr Instruments,

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Illinois, USA) placed in a preheated conventional oven. Experiments conducted using the

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same set-up to investigate the time the autoclave will take to reach the set temperature

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showed an average time of about 20 min.

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The preparation was carried based on previous report [19]. 2 g of Prosopis

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africana shell was weighed into a Teflon-lined autoclave and 30 ml of deionized water

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was added. The autoclave was placed in a preheated conventional oven at 200 °C and

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was held for 120–240 min. The oven was allowed to cool down to room temperature and

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the autoclave was removed. The prepared hydrochar was filtered off with filter paper

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(Whatman filter paper number 3, ashless 11 cm), washed several times with deionized

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water until a neutral pH was obtained. It was then dried at 80 °C for 16 h, and stored in

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the desiccator for further analysis. The hydrochars were denoted as C-PASx-y, where x

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and y represent temperature (°C) and time (min) for the carbonization respectively.

100 101 102

2.2 Product yield (%) In each case, the dry mass of the carbonized material was measured and the product yield (dry mass percentage of the raw material) was calculated as follows:

103 104

(Equation 1) 2.3 Ash Content Determination

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2 g of the carbonized material was placed into weighed ceramic crucible. The

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carbonized material and crucible were dried for 16 h at 80 °C and reweighed to obtain the

107

dry carbon weight. The sample and the crucibles were then heated in a furnace at 760 °C

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for 6 hrs. The crucible was removed from the furnace, cooled in a desiccator and the

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remaining solid (ash) was weighed. Percentage ash content was calculated as follows:

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(Equation 2)

4

111

2.4 pH Measurement

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1 % (wt/wt) suspension of carbon material in deionized water was prepared. The

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pH of the suspension was measured with a pH meter (FisherBrand Hydrus 500, Fisher

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Scientific, Loughborough, UK), which has been calibrated with pH 4 and pH 7 buffer

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solutions. The calibration was confirmed by analysis of a pH 7 buffer after every five

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analyses and a recalibration done if the value varied by more than ± 0.1 pH units.

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2.5 Energy properties of the hydrochars

118 119

The higher heating value (HHV) was calculated using Equation 3 as reported by Gao et al. [20]

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(Equation 3)

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The energy densification ratios of the hydrochars were calculated using the

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equation below

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(Equation 4)

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All experiments were carried out in triplicate, and the results are presented in

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Table 1.

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2.5 Characterization Methods

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The characterizations of the materials were carried out using the following

128

instruments. The elemental analysis was carried out using Fisons instruments EA 1108

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CHN analyzer. The surface areas were measured on a Micromeritics Tristar BET-N2

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surface area analyzer, before the analysis the samples were degassed at 120 °C for 3 h.

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FT-IR spectra were recorded using Thermoscientific Nicolet 380 FT-IR (Themo

132

Scientific, Hemel Hempstead, UK), equipped with attenuated total reflectance (ATR).

133

The morphology and particle size were visualized using a ZEISS EVO 60 SEM (Carl

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Zeiss, Cambridge, UK); the samples were coated with gold and platinum alloy and

135

impregnated on a sticky disc before analysis. Thermogravimetic analysis (TGA) were

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carried out under N2 atmosphere at a heating rate of 30 °C min−1 using a Mettler Toledo-

5

13

137

TGA/DSC 1 instrument.

C solid-state magic angle spinning NMR experiments were

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carried out using a Bruker Avance II 500 MHz (11.74T) spectrometer (Bruker, Coventry,

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UK); the samples were packed without further treatment into a 4 mm zirconia rotor

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sample holder spinning at MAS rate νMAS = 8 KHz. Carbon sensitivity was enhanced

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using Proton-to-carbon CP MAS: recycle delay for all CP experiments was 3 s and TPPM

142

decoupling was applied during signal acquisition. Cross polarization transfer was carried

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out under adiabatic tangential ramps to enhance the signal with respect to other known

144

methods. CP time tCP = 500 ms. The number of transients was 200 for all the carbon

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samples.

146 147

3. Results and Discussion

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3.1 Product yields, CHN analysis, ash, and pH content

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The main objective of this work was to compare the similarity between the

150

hydrochars (solid products) from the microwave-assisted and conventional hydrothermal

151

carbonization of a lignocellulosic waste biomass (Prosopis africana shell) in terms of

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their product yields and properties. The results of the product yield are shown in Table 1.

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Despite having a longer processing time, the conventional hydrothermal process

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generated more or almost the same amount of product as the microwave-assisted process,

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which could be attributed to the amorphous cellulose activation under the irradiation with

156

microwaves [21,22]. The product yields for the hydrochars decreased, while the carbon

157

contents increased with increase in the processing time, due to the loss of oxygen during

158

the increasing liquefaction and gasification reactions, which are parallel reactions to the

159

hydrothermal process of the biomass [13,23,24].

160

In contrast to the carbon content, the hydrogen and oxygen contents of the

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hydrochars decreased with increase in the processing time. This observed trend in the

162

hydrothermal carbonization process of biomasses has been attributed to the loss of

163

hydrogen and oxygen due to deoxygenating, dehydration and decarboxylation reactions

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that occur during the hydrothermal carbonization process [25-28]. The ash content was

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low for the M-PAS and C-PAS because most of the inorganic compounds contained in

6

166

the Prosopis africana shell dissolved in water during the hydrothermal carbonization

167

processes, while their low pH values is as a result of the high content of oxygen

168

containing functional groups [18,29].

169 170

Table 1

171 172

The van Krevelen diagram (Figure 1) was also used to provide information about

173

the transformation in the chemical structure of the raw material after carbonization, and

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also to qualitatively assess the carbonization process by plotting atomic ratios of H/C

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against O/C for the raw material and the hydrochars from both processes. The ratios of

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the H/C and O/C of the hydrochars decreased as the carbonization time was increased,

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and were lower than that of the raw material. This suggests that there was transformation

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during the hydrothermal carbonization processes resulting from dehydrogenation,

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decarboxylation, deoxygenation and dehydration reactions [8,30]. The raw Prosopis

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africana shell fell into a region that is typical for biomasses, while the hydrochars have

181

O/C and H/C ratios akin to that of peat [31]. The graph of H/C versus O/C atomic ratios

182

follows at diagonal line from the raw material to the hydrochars, which implies that the

183

reaction that predominates during the hydrothermal process is dehydration reactions [30].

184 185

Figure 1

186 187

3.2 Energy properties of the hydrochars

188

A vital characteristic of biomass and hydrochars is their higher heating values

189

(HHVs). It shows the total amount of energy that is present in a sample. In both

190

processes, the HHVs calculated for the M-PAS and C-PAS increased with increase in

191

residence time (Table 1), which is an indication of the level of carbonization attained.

192

The HHVs of the hydrochar were similar to that of wood and paper due to lignocellulosic

7

193

nature of the raw material [19]. Hydrothermal carbonization process is associated with

194

dehydration, decarboxylation, and condensation reactions leading to the carbonization of

195

the raw biomass and thus resulting in energy densification, which is used to assess the

196

effectiveness of the hydrothermal carbonization processes [19,32]. In this study, the

197

energy densification ratios of the hydrochars are in the range of 1.24 – 1.27 and 1.12 –

198

1.28 for the M-PAS and C-PAS respectively, which is within the range previously

199

reported (19,32,33].

200

The closeness in the properties of the M-PAS200-20 and C-PAS200-240 could mean

201

that other characteristics of these hydrochars may also be comparable. Hence, further

202

characterization will be limited to these hydrochars.

203 204

3.3 Chemical Characteristics

205

ATR-FTIR Analysis

206

ATR-FTIR spectroscopy was used to assess the changes in the Prosopis africana

207

shell during the hydrothermal carbonization process. The FTIR spectra of the hydrochars

208

(M-PAS200-20 and C-PAS200-240) shown in Figure 2a differ in intensity from that of the raw

209

material in Figure 2b, indicating the chemical transformations that occurred when the raw

210

material was hydrothermally carbonized. The strong presence of single bond components,

211

such as, C-O, C-H and O-H is expected since the hydrochars were obtained from

212

lignocellulosic biomass that contains cellulose, hemicelluloses and lignin [34]. The broad

213

peak between 3700-3000 cm-1 (Section 1) comes from the stretching vibration of

214

aliphatic O-H (hydroxyl and carboxyl), while the peaks between 1200-1000 cm-1 (Section

215

6) correspond to C-O stretching vibration from esters, phenols and aliphatic alcohols. The

216

weak intensity of these peaks (Sections 1 and 6) in the hydrochars shows that dehydration

217

and decarboxylation reactions occurred during the hydrothermal carbonization process

218

[32]. The peak between 1800-1650 cm-1 (Section 3) corresponds to the C=O stretching

219

vibration of esters, carboxylic acids or aldehydes from cellulose or lignin. This peak was

220

almost absent in the hydrochars due to the breakdown of cellulose during the

221

hydrothermal carbonization process, while the peak between 1650-1500 cm-1 (Section 4)

8

222

represents the C=C vibrations of the aromatic rings in lignin. This peak was still present

223

after the carbonization process; although with reduced intensity indicating the presence of

224

lignin fragments and intermediate structures in the hydrochars, which implies that under

225

the conditions studied lignin did not totally breakdown [35]. The peaks between 3000-

226

2800 cm-1 (Section 2) are due to the stretching vibration of aliphatic C-H bonds, while the

227

peaks between 1450-1200 cm-1 (Section 5) are due to the bending vibration of C-H bond

228

of the aliphatic carbons, methylene, and methyl groups indicating the presence of

229

aliphatic structures [23]. The notable absence in the hydrochars of peaks below 1000 cm-1

230

(Section 7) due to the deformation of the C-H out of plane bending vibrations is

231

consistent with previous report [25]

232 233

Figure 2

234 235

NMR Analysis

236

The 13C solid state NMR analysis of the M-PAS200-20 and C-PAS200-240 shown in

237

Figure 3 provided further information on their chemical composition, and also used to

238

confirm the functional groups present. It is usually a valuable complementary method to

239

the FT-IR analysis in describing biomass conversion during hydrothermal carbonization

240

process. A close comparison between the NMR spectra of the hydrochars shows very

241

similar features. The peaks between 14–60 ppm shows the presence of aliphatic carbons,

242

while those in the region between 100–160 ppm usually referred to as the aromatic region

243

are all due to C=C double bond from lignin, but between 140–160 ppm are specifically

244

due to the oxygen bound O-C=C (O-aryl) [36,37]. Due to the residual lignin, and the

245

cellulose and hemicellulose content of the raw material, the spectra show carbohydrate

246

resonances (CH2OH groups around 62 ppm, CHOH groups around 72 ppm, and anomeric

247

O-C-O carbons around 90 ppm) in the O-alkyl region between 60 and 100 ppm [14,38].

248

The ATR-FTIR analysis has already indicated the presence of these functional groups.

249

Therefore, the 13C NMR and ATR-FTIR results are in good agreement with each other.

250 9

251

Figure 3

252 253

Thermo-gravimetric analysis (TGA)

254

In order to further assess the extent of conversion of the raw material under the

255

microwave-assisted and conventional hydrothermal carbonization conditions, TGA

256

analysis was carried out on M-PAS200-20 and C-PAS200-240. The TGA curve in Figure 4

257

showed obvious similarities in the behaviour of the hydrochars. The TGA curves can be

258

grouped into three weight loss stages. The first stage below 200 °C where there was no

259

significant weight loss was because the hydrothermal processes have already decomposed

260

the components of the raw materials that would have decomposed at these temperatures.

261

The slight loss in weight at the first stage corresponds to moisture loss and release of

262

volatile compounds [32,39]. The second stage which occurred at about 220–410 °C,

263

represents the temperature range at which chemical bonds begin to break and the lightest

264

volatile compounds are released and it corresponds to the decomposition of cellulose and

265

hemicellulose [32,40]. The third combustion stage from 410–650 °C is usually assigned

266

to thermal degradation of lignin. Among the three components (cellulose, hemicellulose

267

and lignin) of the raw material, lignin is the most difficult to decompose. Its

268

decomposition usually occurs slowly under the whole temperature range used [41]. The

269

TGA result is in agreement with that of ATR-FTIR and NMR.

270 271

Figure 4

272 273

3.4 Structural Characteristics

274

SEM Analysis

275

A close inspection of the raw material and hydrochars by means of scanning

276

electron microscopy technique shown in Figure 5 revealed interesting transformations in

277

the morphologies of the hydrochars vis-a-vis the raw material. The SEM images of the

10

278

M-PAS200-20 and C-PAS200-240 shown in Figures 5 b and c respectively are alike, and are

279

different from that of the raw material (Figure 5 a). The raw material showed a cellular

280

structure that is characteristic of lignocellulosic materials, while the hydrochars have

281

aggregates of spherical microparticles of about 1–10 μm in diameter. These

282

microparticles originate from the decomposition of cellulose, and the subsequent

283

precipitation and growth into spheres [25]. The lignin component of the raw material on

284

the other hand, probably underwent only partial degradation due to its high thermal

285

stability, producing the rough texture observed in the SEM images of hydrochars [32].

286

This result implies that the lignocellulosic structure of the raw material was broken

287

during the hydrothermal carbonization process.

288 289

Figure 5

290 291

Nitrogen Sorption Measurement

292

The nitrogen sorption isotherms for the M-PAS200-20 and C-PAS200-240 are shown

293

in Figure 6. Both hydrochars showed characteristics that are representative of Type II

294

isotherms based on the IUPAC system of classification, which is a typical type of

295

isotherm obtained with non-porous materials [42]. It is therefore not surprising that the

296

hydrochars have relatively small BET surface areas (6.1±2.2 m2/g for M-PAS200-20, and

297

5.9±1.8 m2/g for C-PAS200-240) due to the absence of porosity. In such situation the values

298

of the specific surface area calculated only correspond to the external surface [43]. The

299

hydrothermal carbonization process involves carbonization as well as solubilization of

300

the organics, and the formation of tarry substances. The hydrochar produced becomes

301

contaminated with these substances, which block their pores, and make the apparent BET

302

surface area of the hydrochar to be very small [44]. Therefore, improvements of the

303

porosity and the surface area of hydrochars are usually necessary to enable them fit into

304

specific applications, such as, hydrogen storage or electrical energy storage

305

(supercapacitors) [32].

11

306

Figure 6

307 308

4. Conclusion

309

This article provided some new insights into the similarities between hydrochars

310

prepared from microwave-assisted and conventional hydrothermal carbonization

311

processes of Prosopis africana shell, a waste plant material at a low temperature. It is

312

clear from the study that similar transformation can be achieved at 200 °C. The

313

microwave-assisted process proved to be faster in decomposing the waste material as

314

evidenced by the degree of transformation achieved within a short time when compared

315

to the conventional approach. It can therefore be seen, that hydrochar produced for 20

316

min in the microwave oven exhibited a comparable decomposition pattern to that

317

produced for 240 min in the conventional oven and thus it can be concluded that the

318

materials achieved similar levels of conversion. This work therefore suggests that the

319

microwave-assisted process could have the potential for use in the large-scale production

320

of hydrochars.

321 322

Acknowledgement

323

We wish to thank the Petroleum Technology Development Fund (PTDF), Nigeria

324

for providing the funding for the PhD studentship of Dr. Sunday E. Elaigwu. We also

325

acknowledge Dr. Mark Lorch and Bob Knight of the Department of Chemistry,

326

University of Hull for providing assistance with the NMR, and CEM microwave oven

327

respectively.

328 329

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455 456 457 458 459 460 461 462

17

463

Figure 1: van Krevelen diagram of raw Prosopis africana shell and hydrochars

464

obtained at 200 °C and different residence time for the microwave-assisted (M-PAS)

465

and conventional (C-PAS) hydrothermal carbonization

466 467 468 469

Figure 1

470 471 472 473 474 475 476 477 478 479

Figure 2: ATR-FTIR spectra showing the different sections of the wavelength for

480

the functional groups present (a) ATR-FTIR spectra of M-PAS200-20 (blue) and C-

481

PAS200-240 (red); (b) ATR-FT-IR spectrum of raw Prosopis africana shell

18

482 483 484

Figure 2

a

M-PAS 200-20 (Blue) C-PAS 200-240 (Red) 6 1

2

3 4 7 5

100

b

PASR 95

90

85

2

80

%T

1

6

3

Raw Prosopis africana shell

75

4 7

70

5 65

60

55 350 0

485 486

300 0

2500

2000

Wav enumber s (c m-1)

487

19

1500

1000

488 489 490 491 492 493 494

Figure 3: 13C solid state cross-polarization magic-angle spinning NMR spectra of MPAS200-20 (red) and C-PAS200-240 (black) Figure 3

M-PAS200-20

C-PAS200-240

495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 20

510 511 512 513

Figure 4: TGA curves for of M-PAS200-20, and C-PAS200-240 Figure 4

514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 21

535 536 537 538 539

Figure 5: SEM images of (a) raw Prosopis africana shell; (b) M-PAS200-20, and (c) CPAS200-240 Figure 5

540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 22

561 562 563

Figure 6: N2 sorption isotherms of (a) M-PAS200-20; (b) C-PAS200-240

564 565 566

Figure 6

a

b

567 568

23

Materials

Yields (%)

C (%)

H (%)

N (%)

Oa (%)

O/C*

H/C *

pH

Ash (%)

HHV

Energy

(MJ

densific

−1

kg )

ation ratio

P. africana

46.58±0.01

6.94±0.04

1.57±0.09

44.91±0.10

0.73

1.77

5.34±0.02

3.77±1.13

17.64

shell M-PAS M-PAS200-20

30.60±2.13

58.08±1.12

6.18±0.80

1.61±0.01

34.13±0.31

0.44

1.27

4.05±0.03

9.32±0.50

22.37

1.27

M-PAS200-15

32.21±1.55

55.81±0.06

6.61±0.09

1.60±0.01

35.98±0.14

0.48

1.41

4.21±0.01

9.05±0.45

21.88

1.24

M-PAS200-10

33.01±0.98

55.69±0.00

6.67±0.03

1.57±0.02

36.07±0.05

0.49

1.43

4.37±0.01

9.10±0.10

21.91

1.24

M-PAS200-5

36.37±2.01

55.46±0.08

6.71±0.05

1.48±0.00

36.35±0.06

0.49

1.44

4.51±0.00

8.72±0.85

21.84

1.24

C-PAS200-240

30.85±0.53

57.86±1.10

6.37±0.06

1.46±0.05

34.31±1.21

0.44

1.31

4.34±0.01

9.25±0.45

22.53

1.28

C-PAS200-180

34.78±0.04

53.85±1.02

6.53±0.01

1.47±0.05

38.15±0.96

0.53

1.44

4.41±0.02

9.01±0.32

20.72

1.18

C-PAS200-120

37.87±0.20

51.25±0.90

6.72±0.02

1.65±0.03

40.38±0.95

0.59

1.56

4.57±0.01

8.87±0.75

19.72

1.12

C-PAS

569

a

570

* Atomic ratios

571

Table 1: Product yields, CHN analysis, pH, ash contents and energy properties of

572

the raw Prosopis africana shell and hydrochars from microwave-assisted and

573

conventional hydrothermal carbonization processes

The oxygen content was determined by difference [100% - (C%+H%+N%)] [8]

574 575 576

24

577 578 579 580 581

25