Impact of Pyrolysis Temperature on Charcoal Characteristics

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Applied Chemistry

Impact of pyrolysis temperature on charcoal characteristics Johannes Tintner, Christoph Preimesberger, Christoph Pfeifer, Denis Soldo, Franz Ottner, Karin Wriessnig, Harald Rennhofer, Helga C. Lichtenegger, Etelvino H Novotny, and Ena Smidt Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04094 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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Industrial & Engineering Chemistry Research

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Impact of pyrolysis temperature on charcoal characteristics

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Johannes Tintner1*, Christoph Preimesberger1, Christoph Pfeifer1, Denis Soldo1, Franz Ottner2,

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Karin Wriessnig2, Harald Rennhofer1, Helga Lichtenegger1, Etelvino H. Novotny3, Ena Smidt1

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Process Engineering, 1190 Vienna, Austria

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Natural Hazards, 1190 Vienna, Austria

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Embrapa Solos, 22460-000 Rio de Janeiro-RJ, Brazil

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*

corresponding author: [email protected]

University of Natural Resources and Life Sciences, Department of Material Sciences and

University of Natural Resources and Life Sciences, Department of Civil Engineering and

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Abstract

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Charcoals were produced from spruce and beech wood under laboratory conditions at different

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pyrolysis temperatures (300 °C to 1300 °C). Characterization of these charcoals was conducted

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using eight analytical methods. Each method describes specific changes in the temperature range

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till 1300 °C. Therefore the combination of these methods provides comprehensive information on

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different pyrolysis stages. FTIR, NMR spectroscopy, and thermogravimetry display changes till

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700 °C. A prediction model for pyrolysis temperature till 800 °C is presented based on FTIR

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spectra with an R² of 0.98. He-pycnometry resolves the temperature range between 500 °C and

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890 °C. Small angle X-ray scattering (SAXS) describes precisely the evolution of the porous

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structure and completes the set of techniques by a description of the physical properties of the

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charcoal. XRD reveals the crystallographic change of the lignocellulosic structure towards

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precursors of graphite. The formation of calcite out of CaO and CO2 becomes evident.

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Keywords: pyrolysis temperature; spectroscopy; X-ray diffraction; He-pycnometry;

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thermogravimetry

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1 Introduction

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Charcoal production can be assumed to accompany mankind since the Iron Age. It has been

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increased massively at the beginning of industrial revolution in the early 19th century 1. Since at

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least the time, when biochar has become one of the main topics in the discussion about carbon

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sequestration in soils the characterization of its properties and its stability became relevant 2,3.

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Besides chemical characterization especially the physical properties evoke special interest in the

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context of soil amendment. Various analytical tools have already been used (elemental analysis,

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spectroscopic methods, thermal analysis); some of them are still rather rarely in use even if their

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potential is proven (diffractometry, SAXS, pycnometry). Each method elucidates a specific

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property of the material. In terms of information they provide they complement or confirm one

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another. Up to now, the description of physical properties lags behind the description of chemical

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changes 4. Input material, residence time, heating rate, and temperature determine char properties

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5,6.

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temperatures are also favorable in modern slow pyrolysis systems, whereas in fast pyrolysis

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techniques even higher temperatures up to almost 800 °C can be used 8.

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The purpose of this paper is the comparison of different analytical methods that describe

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properties of charcoals produced at different pyrolysis. Besides analytical tools that focus on

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chemical properties (elemental analyses, spectroscopic methods, thermal analysis), He-

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pycnometry and Small Angle X-ray Scattering (SAXS) were applied to provide more insight into

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the physical properties skeletal density and porosity. Especially SAXS is an aspiring technique to

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gain information on the nanostructure of the sample.

Typical temperature of traditional round kilns reached levels between 400 and 600 °C 7. These


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2 Material and Methods

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2.1 Material

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Wood pieces with a diameter between 9 and 15 cm were cut from a single stem of spruce (Picea

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abies) and one of beech (Fagus sylvatica) as the most abundant softwood and hardwood species

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of Middle Europe. Pure graphite (purity, 99.35%) served as a reference.

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2.2 Pyrolysis process and sample preparation

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The stems were cut into pieces with a length between 3 and 5 cm. One piece per species was used

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per temperature step. They were pyrolized separately in a box made of stainless steel (350 x 400

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x 150 mm - width x depth x height = 21 L). The box was purged with nitrogen to set the

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atmosphere around the stem pieces to oxygen free pyrolysis conditions. Right after purging the

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box was put into a Nabertherm ® N41/13 muffle furnace to start preheating followed by the

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pyrolysis process. Pyrolysis temperatures were 300 °C, 400 °C, 600 °C, 700 °C, 800 °C, 1000 °C,

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1200 °C and 1300 °C (one run per temperature). Pyrolysis durations were 4 h not including the

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cooling phase. Temperatures were recorded during the whole process (preheating, pyrolysis and

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cooling phase) both inside the box measuring air temperature and inside the wood with a B&R ®

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PLC (model X20CP1584 with four X20AT6402 modules). Wooden reference samples were dried

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at 70 °C.

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Afterwards, all samples were powdered using a steel disc vibrating mill (Fritsch ® Pulverisette 9)

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and stored at room temperature. Particle size was smaller 20 µm. This particle size ensures

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reproducible analyses 9. All analyses were made with these powdered samples.

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2.3 Elemental analyses



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Five milligram of the samples were analysed on a HekaTech® Euro EA with column sizes of 1 m

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length and 8 mm diameter. Helium was used as carrier gas with 8 ml min-1. For combustion

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10 ml O2 were used additionally. Temperature of the combustion tube was 1000 °C. CO2 and

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H2O were detected using a TCD.

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2.4 Polycyclic Aromatic Hydrocarbons (PAH) analyses

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The procedure applied was based on EN ISO 17993:2003. For calibration PAH mix Agilent

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8500-6035 was used. Dry samples were extracted for 3 h and 15 min in and ultrasonic bath with a

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mixture acetone:acetonitrile:methanol (6:3:1), the ratio was 1:10 (1 g : 10 ml). Certified

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Reference Material BCR - 683 (Beech wood) was used as recovery standard. The HPLC Agilent

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® consisted of the following components: Quaternary pump series 1100 G1311A, Degaser series

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1200 G1379B, Autosampler series 1100 ALS G1313A, Column thermostat series 1100 G1316A,

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column was a Zorbax C18 PAH, 5µ 250 mm, 3 mm. Flow rate was 0.8 ml min-1 and the injection

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volume was 5 µl. Detection was done by means of DAD (G7117C), UV-Vis (G1314A), FLD

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(G1321A). Sample clean-up was done by centrifugation (Mikro 20R Hettich ®, Rotor for

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Eppendorf-cups 1195A, 3000 rpm, 10 min) and full filtration (Whatman ® syringe filter 4 mm,

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0,45µm membrane size PTFE Membrane).

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2.5 Fourier Transform Infrared (FTIR) spectroscopy

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FTIR spectra were recorded in the ATR (attenuated total reflection) mode in the mid infrared

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region (4000 – 400 cm-1) with an optical crystal of a Bruker ® Helios FTIR micro sampler

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(Tensor 27). A total of 32 scans were collected at a spectral resolution of 4 cm-1. Five replicates

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per sample were vector normalized (normalized by the Euclidean norm) and averaged using the

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OPUS © (version 7.2) software.

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2.6 Thermogravimetry (TG) 4 ACS Paragon Plus Environment

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Thermograms were recorded under oxidative conditions on a Netzsch ® Instrument (STA 409

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CD Skimmer). Ten mg of the sample were combusted in an Al2O3 pan with a heating rate of

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10 K min-1 in a temperature range from 30 °C to 950 °C, with a gas flow of 50 ml min-1

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(80% Ar/ 20 % O2). Argon was used as flushing gas (20 ml min-1). A recalcitrance index (R50,x)

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has been calculated as a ratio of the temperatures of 50 % mass loss in the thermogram of a

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certain sample and of graphite 10.

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2.7 Nuclear magnetic resonance (NMR) spectroscopy

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Solid-State 13C NMR spectra of the samples were obtained using a Varian INOVA (11.74 T)

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spectrometer with 13C and 1H frequencies of 125.7 and 500.0 MHz, respectively. A variable-

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amplitude cross-polarization pulse sequence was employed. The experiments were performed

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using magic-angle spinning (MAS) of 15 kHz, a cross-polarization time of 1 ms, an acquisition

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time of 15 ms and a recycle delay of 500 ms. The cross-polarization time was chosen after

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variable contact time experiments, and the recycle delays in CP (cross-polarization) experiments

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were chosen to be five times longer than the longest 1H spin-lattice relaxation time (T1H) as

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determined by inversion-recovery experiments. High-power TPPM (two pulse phase-modulation)

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proton decoupling of 70 kHz was employed 11.

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2.8 He-pycnometry

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The measurements were performed on a micromeritics ® AccuPyc II 1340 gas pycnometer.

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Chamber size was 100 cm³ and He pressure was 1.5 bar. 20 cycles of pressure and evacuation

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were performed for equilibration. An additional further run of 10 measurements was averaged for

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the final value.

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2.9 X-ray diffractometry (XRD)



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The samples were investigated by means of X-ray diffraction (XRD) using a Panalytical ®X´Pert

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Pro MPD diffractometer with automatic divergent slit, Cu LFF tube (45 kV, 40 mA), with an

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X´Celerator detector. The measuring time was 25 s, with a stepsize of 0.017°. Diffractograms

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were recorded from 5° to 70° (2Ө). Semiquantitative mineral composition of the bulk samples

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was estimated using Rietveld refinement with the Panalytical software © X´Pert HighScore Plus.

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2.10 Small angle X-ray scattering (SAXS)

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Measurements were carried out with a 3-pinhole SMAX-3000 SAXS camera (RIGAKU ®)

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equipped with a MM002+ X-ray source (with copper target wavelength of λ = 0.1541 nm). Two-

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dimensional scattering images were recorded with a TRITON ® 200 multi-wire X-ray detector

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(200 mm diameter mapped on 1024 x 1024 pixels). The samples were put between two layers of

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scotch tape. The scattering images were integrated azimuthally to gain information of the

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scattered intensity I(q) in dependence of the scattering vector q, which is related to the scattering

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angle 2θ and the wavelength λ by q=4π/ λ sin θ. The full q range was 0.1 nm-1 to 8 nm-1.The

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obtained data were background corrected and further analyzed with respect to the pore structure,

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following Köhnke et al. 12. Pore radii for spherical pores 13, a value related to the surface

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roughness of structures in the nanometre regime and a value proportional to the specific inner

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surface have been determined

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2.11 Statistics

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Pearson correlation coefficients were calculated by the program IBM SPSS 21®. The PLS-

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Regression model and the PCA were calculated by the program Unscrambler X 10.1 ®. Spectral

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regions included in both models ranged from 3728 to 2422 cm-1 and from 1896 to 400 cm-1.

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NIPALS algorithm was applied, the model was 10-fold cross-validated. Positions of the PLS


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validation results for all samples are given in the figure. Four factors were included in both

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models.

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3 Results and Discussion

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3.1 Elemental analyses

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Main carbon increase and corresponding oxygen and hydrogen decrease occurred between 70 °C

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and 600 °C. At 700 °C carbon content reached values above 90% (figure 1a), hydrogen decreases

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correspondingly (figure 1b). After that level the values did not change strongly anymore and a

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differentiation based on elemental analyses became quite weak. Van Krevelen diagram revealed a

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very high degree of carbonization already at about 600 °C positioning the samples in the left

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down corner (figure 1c).

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The highest correlation between pyrolysis temperature and carbon or hydrogen content was found

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in the temperature range from 70 °C to 600°C resulting in a coefficient of determination R² of

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96.3 % (carbon) and 96.4 % (hydrogen). Including higher temperature steps leads to a decrease of

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the coefficient of determination. A better separation at least till pyrolysis temperatures of 800 °C

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is achieved, when taking the logarithm of values for van Krevelen diagram (figure 1d). For the

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characterization of materials charred at higher temperatures as traditional kiln samples we

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propose to modify the diagram in that way.

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(a)

(b)

(c)

(d)

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Figure 1:(a) Carbon and (b) hydrogen content in relation to pyrolysis temperature; (c) van

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Krevelen diagram; PA-Picea abies, FS-Fagus sylvatica; (d) van Krevelen diagram with log10-

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values

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3.2 Polycyclic Aromatic Hydrocarbons (PAH) analyses

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Results of PAH revealed high concentrations, but comparable with literature 14,15. The European

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Biochar Standard defines limit values of 12 mg/kg for a basic grade and 4 mg/kg for the premium

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grade 16. Limit value for the International Biochar Initiative is set to 6 mg/kg 17. Spruce samples

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contained partly far higher concentrations compared with beech, even at high temperatures. The 8 ACS Paragon Plus Environment

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highest levels were found at pyrolysis temperatures of about 800 °C (figure 2). The ratios

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Phenanthrene/ Anthracene and Naphthalene/ Phenanthrene are used to differentiate biochar types

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18,19.

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processed, unaged char. By such ratios, stability and degradation process of PAH can be

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examined 20. Phe/Ant ratios above 10 indicate rather unstable fractions typical for combustion

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emissions.

The ratios found in our samples are comparable to typical values from traditionally

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(a)

(b)

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Figure 2:(a) Sum of 16 EPA PAHs with limit values of EBC and IBI and (b) the two ratios of

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Phenantrene/ Anthracene and Naphthalene/ Phenanthrene of different pyrolysis temperatures;

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PA-Picea abies, FS-Fagus sylvatica

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3.3 Fourier Transform Infrared (FTIR) spectroscopy

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The spectral characteristics changed considerably especially in the temperature range between

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70 °C and 800 °C. Main changes were found in the broad band of OH-stretching vibrations

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(region between 3600 cm-1 and 3000 cm-1) and the band region of CH stretching vibrations

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derived from methyl-, methylene, and methine groups (between 3000 cm-1 and 2800 cm-1). The 9 ACS Paragon Plus Environment


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specific bands 21 for hemicelluloses (around 1730 cm-1), lignin (e.g. around 1510 cm-1) and

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cellulose (peak maximum at around 1026 cm-1) display the chemical changes of pyrolysis as well.

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Figure 3a displays spectra of Fagus samples at different pyrolysis temperature compared with

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pure graphite. Picea revealed similar characteristics (not shown). From 70 °C to 300°C cellulosic

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compounds decrease and lignin is relatively enriched; as the bands stay more or less the same we

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conclude that the compounds do not change substantially, however. This effect of cellulosic

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decomposition can be monitored also in TG, NMR, XRD, and SAXS. Main changes take place

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between 300 °C and 400 °C. It demonstrates a structural breakdown between these two

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temperature steps. At 400 °C and 600 °C typical bands of aromatic CH bands (1596 cm-1,

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877 cm-1, 820 cm-1, 759 cm-1) occur. Above that temperature all band signals decrease and spare

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just a smooth spectrum due to carbon concentration. Results correspond well to literature 22.

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Based on the spectral pattern PLS1-regression models were established to predict pyrolysis

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temperature. The spectral pattern allows conclusions to be drawn about the production

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temperature of charcoals up to 800 °C. Within the temperature range from 70 °C to 800 °C the

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highest R² with 98 % was reached (figures 3b and 3c). Higher temperatures led to similar spectra

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and their inclusion into the model did not lead to any further increase of the quality criterion.

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According to the loadings plot (not shown) the first component is dominated by the loss of wood

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characteristics especially the decrease of the cellulose peak around 1026 cm-1 till 600 °C, whereas

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the second component reveals aromatic vibrations of an intermediate stage with a maximum at

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about 400 °C indicated by a peak around 1596 cm-1. The increase of pyrolysis temperature is

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paralleled by disappearance of distinct bands in the infrared spectrum. The loss of functional

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groups in the spectral pattern at 400 °C and beyond is caused by the strong relative increase of

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carbon that is equally reflected by elemental analyses 14,23.

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(a)

203 (b)

(c)

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Figure 3:(a) Spectral characteristics of Fagus samples heated up to temperatures from 70 °C till

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800 °C. (b) Scores Plot from a PCA with FTIR spectra of Fagus and Picea samples, samples

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pyrolyzed at 1000 °C, 1200 °C, and 1300 °C are marked by “x” symbols; (c) PLS1-regression

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line including confidence bands (based on a significance level of α =0.05, PLS1 model with 4

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factors)

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The progress of carbonization leads to higher hydrophobicity of charcoals. Oxidation due to

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aging reverses the process and results again in hydrophilic properties. They are favorable for soil

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amendment with regard to water and nutrient storage 24. In FTIR spectra these oxidation 11 ACS Paragon Plus Environment


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processes due to aging can be separated, however, from lower stages of pyrolysis with spectral

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characteristics described in this section 25.

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3.4 Thermogravimetry (TG)

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As long as functional groups are present in spectra of charcoals produced at pyrolysis

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temperatures up to 400 °C a slight mass loss in the thermograms between 80 °C and 130 °C is

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observed. It is caused by water evaporation. With increasing pyrolysis temperature and the

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resulting hydrophobicity this effect disappears. Thermal data of oxidative combustion stress

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again the strong carbonization of our materials. The thermal behavior confirms the results of

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FTIR spectroscopy. Shape and temperature range of the thermograms up to pyrolysis

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temperatures of 400 °C indicate the variety of chemical components. Thermograms of charcoals

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produced at 600 °C and above shift to higher temperatures and feature a more uniform thermal

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behavior according to their chemical composition. Figure 4a displays this pattern for Fagus

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samples. Similar results are obtained for Picea (not shown).

227 (a)

228


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(b)

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Figure 4: (a) Thermograms of beech (Fagus sylvatica) samples pyrolyzed at different

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temperatures (70 °C – 1300 °C) during oxidative combustion, (b) the recalcitrance index R50,x

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Harvey et al. 10 proposed a recalcitrance index R50,x. They distinguished three groups of

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recalcitrance with the limit values of 0.5 and 0.7. Below 0.5 materials are comparably recalcitrant

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as uncharred plant biomass and above 0.7 recalcitrance draws near to soot. According to this

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classification our materials would reach the highest class of carbon sequestration at a pyrolysis

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temperature around 800 °C. In terms of carbon sequestration these pyrolysis products indicate a

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high stability.

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3.5 NMR spectroscopy

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The low pyrolysis temperatures up to 300 °C demonstrate decreasing hemicelluloses and relative

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increases of cellulose and lignin (figure 5a) 26. With higher temperatures progressive

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decomposition of aliphatic remains (alkyl groups) takes place. At 400 °C some O-aryl groups

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remain, but they decrease with increasing pyrolysis temperature (figure 5b). Similar to FTIR

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spectra NMR spectra reveal the chemical change with increasing pyrolysis temperatures and the

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loss of distinct peaks assigned to molecules and molecule groups, respectively. Compared to 13 ACS Paragon Plus Environment


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FTIR spectra NMR spectra provide clear signals of the aryl-group in charcoals produced at

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pyrolysis temperatures above 400 °C. The progressive upfield shift of aryl signal is due to the

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aromatic ring polycondensation 11,27–29. The results are in accordance with elemental analyses.

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Due to the strong carbonization the highest temperature samples do not produce useful spectra.

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As discussed in Novotny et al. 30 this is a result of the high heterogeneity in local magnetic

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susceptibility and/ or chemical shift anisotropy, which is not completely averaged out at the usual

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rates of magic angle sample spinning.

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(a)

(b)

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Figure 5:(a) VACP 13C NMR spectra of heated biomass (until 300 °C). FS and PA are Fagus

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sylvatica and Picea abies respectively with the heating temperature. C: cellulose; H: 14 ACS Paragon Plus Environment

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hemicellulose; L: lignin (including the whole grey box); A: C6 and C4 from amorphous

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cellulose; Cr: C6 and C4 from crystalline cellulose. The dashed ellipses indicate the useful region

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for cellulose crystallinity evaluation. (b) Fagus sylvatica spectra for pyrolysis temperatures above

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300 °C, the dot line indicates the upfield shift of aryl C signal due to the increase of aromatic ring

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poly-condensation.

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3.6 X-ray diffractometry (XRD)

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XRD displays well the transformation of lignocellulosic wood structure into charred organic

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matter between 300 and 400 °C (figure 6). The shoulder at around 16° and the peaks at 21.9° and

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34.7° can be assigned to cellulosic crystallinity 31. At pyrolysis temperatures of 400 °C the

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cellulose peaks disappeared corresponding to the loss of the band in FTIR (1026 cm-1 in figure

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3a) and NMR (indicated with “C” in figure 5a). With increasing pyrolysis temperature the broad

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two peaks of 24° and 43° dominate the diffractograms. Their shift towards the 002 and 011 peaks

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of graphite 32,33, the increase of intensity and sharpness of the peaks indicate the transition

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towards graphite, although it cannot be called “graphite” yet, Finally several sharp peaks can be

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assigned to calcite: 23.0°, the main peak at 29.4°, furthermore 35.9°, 39.3°, 43.0°, 47.6°, 48.4°,

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57.6° 34. Best representation of all peaks is found in the Fagus samples pyrolyzed at 1200 and

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1300 °C. But even at lower temperatures at least the main peak at 29.4° is present. Other peaks

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can be assigned to CaO. Presence of calcite in biochar is well described in literature 35. Our

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results, however, give new insights in its formation. The amount of calcite increases with

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pyrolysis temperature. This can be explained by a relative enrichment. Interestingly, calcite is

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present in materials that reached temperatures > 650 °C at which this mineral is destroyed

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thermally 36. Therefore CaO reacts with CO2 trapped in the pores for carbonation 37 after cooling

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down. CaO might be a relic of Ca-oxalate (in the form of whewellite visible by a small peak at 15 ACS Paragon Plus Environment


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14.9° till 400 °C), a crystal commonly formed in different parts of plants 38. Calcite in biochar

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can influence its behavior as a soil amendment especially in acidic soils 39.

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(b)

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Figure 6: X-ray diffractograms of (a) Picea and (b) Fagus samples; cellulose (cell.), whewellite

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(w.) and calcite (C) peaks are indicated with vertical guiding lines. The shift of graphitic peaks is

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described in the text.

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Main results of this method are: XRD displays the transition of celluloses into precursor phases

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of graphite above 300 °C. Main strength of the method is the elucidation of relative enrichment

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of minerals especially calcite, generated by a de-novo synthesis from CaO and CO2 trapped in the

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porous char structure after cooling down from the pyrolysis process. 16 ACS Paragon Plus Environment

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3.7 He-pycnometry

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Results of pycnometry can be described asymptotically by three lines. Skeletal density

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maintained a level of about 1.4 g/cm³. Then the values increased quite linearly up to a level of

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about 2 g/cm³. The asymptotic constant lines and the inclined one cross each other shortly above

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500 °C at a temperature about 890 °C (figure 7). It can be assumed that polycondesation of

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aromatic rings goes along with prominent physical changes above 400 °C. The density of

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charcoal approximates graphite density of 2.2 and 2.3 g/cm³. This parameter provides

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information in a temperature range that is not resolved well by other methods. It covers perfectly

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the common temperature range of charcoal kilns. The relation between pyrolysis temperature and

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skeletal density was also reported by Brewer et al. 40 and Wiedemeier et al. 41. Wiedemeier et al.

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41

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linear increase from 500 °C till 890 °C is documented by all authors.

found stable values till 400 °C but a further increase even till 1000 °C. At least the comparable

305

306 307

Figure 7: Density of the solid matter measured by means of He-pycnometry. PA-Picea abies,

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FS-Fagus sylvatica

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3.8 Small Angle X-ray Scattering (SAXS) 17 ACS Paragon Plus Environment


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The small-angle x-ray scattering curves of Fagus and Picea both show characteristic features

312

evolving during the temperature treatment. For temperatures up to 300 °C the wood structure is

313

rather intact, featuring a low q signal arising from small voids and cracks in the wood and a

314

shoulder around 1.5 nm-1 attributed to the wood structure, which is destroyed at higher pyrolysis

315

temperatures (figure 8a, 8b and 8d). Degradation of the cellulose around 250°C is reported in

316

Popovski et al. 42. For higher temperatures a clear development of a shoulder attributed to pores

317

in the range of nanometers is visible around 2 nm-1. Starting with a quite wide distribution, a

318

broad shoulder shifting to the left indicates increasing pore size. The pores for both wood types

319

start around 0.3 nm and show a strong increase in pore size at 600 °C and slow further increase in

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pore size with higher temperatures (figure 8d).

321

Both wood types show changing nanostructure with increasing treatment temperature. The

322

change of the power law exponent in the low q regime shows a change in surface roughness of

323

the higher order structure, i.e. cracks and voids (n = 4 smooth surface, 3 < n < 4 surface fractal

324

and 1 < n < 3 volume fractal). While the wood treated with 70 °C features slightly rough surfaces

325

(n = 3.8 and n = 3.9 for Picea and Fagus, respectively). The surfaces are smooth after the 300 °C

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temperature treatment. For temperatures higher than 400 °C the absolute value of the power law

327

exponent decreases again i.e. from n = 4 to n = 2.7 for Fagus and n = 2.3 for Picea, respectively,

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indicating the generation of a fractal pore structure (figure 8c).

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(a)

(b)

330



(d)

(c)

331

(e)

332 333

Figure 8:(a) scattering curves for Fagus sylvatica (FS) samples and (b) for Picea abies (PA)

334

samples, intensities I (q) shifted on the vertical axis, arrows in (a) and (b) indicating the peak shift

335

were included to guide the eye; (c) roughness of the pore structure; (d) pore size given as pore

336

radius; (e) specific inner surface

337 338

The change of the nanostructure is also clearly seen in the specific inner surface. From the SAXS curves a

339

value P/Q can be determined, which is proportional to the specific inner surface 12. The value is increasing

340

over the full temperature rang (Figure 8e) for both wood types.

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The results can be compiled in the following way: SAXS indicates a structural development due

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to heat treatment by the pyrolysis process: With increasing temperature first the wood structure 20 ACS Paragon Plus Environment

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343

decomposed at temperatures above 250 °C. Originally slightly rough surfaces attributed to cracks

344

and voids get smoother. At temperatures higher than 300 °C the surfaces increases in roughness

345

again, featuring also the development of nanopores in the size range of around 0.5 nm, which are

346

increasing in size towards 1 nm for temperatures higher than 400 °C and which onwards only

347

slightly increase. This process is accompanied by an increase of the specific inner surface.

348

4 Conclusions

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Three ranges of pyrolysis temperatures can be roughly distinguished. Chemical changes mainly

350

take place up to temperatures between 400 °C and 600 °C. They are well reflected by elemental

351

analyses, FTIR and NMR spectroscopy, and thermal analyses. With increasing pyrolysis

352

temperatures physical and structural processes dominate till a temperature between 800 °C and

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1000 °C, paralleled by slight further chemical rearrangement. Physical characteristics are

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described by means of pycnometry and SAXS. With a further increase of pyrolysis temperature

355

chemical and physical properties appear constant according to the results obtained except the

356

specific inner surface. Altogether a concise picture of changing charcoal properties depending on

357

pyrolysis temperature can be drawn. This overview is an important pre-requisite for the

358

investigation of biochar functions in soils. Porosity and inner surface are relevant properties in

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soils, as well as the chemical reactivity that corresponds to degree of carbonation.

360

5 Acknowledgments

361

We thank Reinhold Ottner for thermal analysis of graphite, Axel Mentler for PAH analyses and

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Marion Sumetzberger-Hasinger for elemental analyses.

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6 Competing interests, Funding, Data availability



364

The authors declare that there are no competing interests. This research did not receive any

365

specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Data are

366

available upon request.

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PAH C, H, O NMR

STA

FTIR

porosity pycnometry

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Impact of Pyrolysis Temperature on Charcoal Characteristics

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