# Solar-Thermal Pyrolysis of Mallee Wood at High Temperatures

Hongwei Wu, Daniel Gauthier, Yun Yu, Xiangpeng Gao, Gilles Flamant

► To cite this version:

Hongwei Wu, Daniel Gauthier, Yun Yu, Xiangpeng Gao, Gilles Flamant. Solar-Thermal Pyrolysis of Mallee Wood at High Temperatures. Energy & Fuels, 2018, 32 (4), pp.4350 - 4356. 10.1021/acs.energyfuels.7b03091 . hal-01897983

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# Solar-Thermal Pyrolysis of Mallee Wood at High Temperatures

Hongwei Wu,*†‡ Daniel Gauthier,‡ Yun Yu,† Xiangpeng Gao,†,§ and Gilles Flamant‡

†Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia

‡Processes, Materials and Solar Energy laboratory, CNRS-PROMES, 7 Rue du Four Solaire, F-66120 Font Romeu Odeillo, France

§Discipline of Electrical Engineering, Energy and Physics, School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, Western Australia 6150, Australia

**ABSTRACT:** This study reports solar-thermal pyrolysis of mallee wood powders driven by direct concentrated solar radiations at different temperatures (1540, 1740, and 1930 °C), heating rates (320, 800, and 3200 °C/min), and holding times (0 and 5 min). Under such severe pyrolysis conditions, solar-thermal pyrolysis of mallee wood produces predominantly volatiles (≥90 wt %), with very low char yields (≤10 wt %), as reported on a dry basis. Majorities of inherent alkali and alkaline earth metallic (AAEM) species are also released into the gaseous phase. The severe pyrolysis conditions also lead to the low reactivity of char products as a result of not only the char carbon structure becoming ordered and graphitized (as evidenced by the Raman data) but also significant losses of catalytic AAEM species in the char. For example, during the solar-thermal pyrolysis of mallee wood at 1930 °C, 800 °C min⁻¹, and 5 min holding time, the char yield is only ~5 wt %, the retentions of Na and Mg are ~1%, the retentions of K and Ca are only 13 and 35%, respectively, and the char is less reactive. Therefore, despite the low char yield, char conversion is still a critical consideration in the design and operations of solar-thermal reactors under these conditions because the char is very inert.

## 1. INTRODUCTION

The rural and regional Australian communities are facing critical challenges of managing dryland salinity and securing a regional energy supply. Dryland salinity causes extensively adverse environmental impacts to Australia's premium agriculture lands.¹ A key mitigation strategy is to plant mallee trees into the agriculture land via “alley farming” in the form of contours up to 10% of the land occupation.² Therefore, mallee biomass plantation complements to (rather than compete with) food production and leads to abundant biomass production at a large scale for meeting the local energy need.² For example, Western Australia (WA) alone is envisaged to produce up to ~10 million dry tonnes of mallee biomass per annum.¹ Life cycle analysis has demonstrated that mallee biomass production in WA achieves superb economic, energy, and environmental performance.³⁻⁵ In the meantime, Australia as a key sun-belt country has abundant solar energy, available at high solar radiation per square meter. Arguably, Australia has the best solar energy resource in the world,⁶ receiving an average of 58 million PJ per annum of solar radiation (equivalent to over 10 000 times more than Australia's annual primary energy consumption⁷). However, Australia's current use of solar energy is very low (~0.1% of the nation's total primary energy consumption).⁷ Therefore, both biomass and solar energy can potentially make significant contributions to future sustainable development of rural and regional Australia.

Table 1 lists some key features of biomass and solar energy. Both biomass and solar energy are widely distributed. Biomass can be considered as “concentrated solar energy” because its production is via photosynthesis that converts and stores solar energy into chemical energy in biomass. Biomass is of low energy density as a result of its high moisture content and bulky nature,⁸ and solar energy is also of extremely low energy density, which requires concentration before utilization at

<table><caption>Table 1. Features of Biomass and Solar Energy<sup>8–11</sup></caption><thead><tr><th></th><th>biomass</th><th>solar energy</th></tr></thead><tbody><tr><td>distribution</td><td>widely distributed</td><td>widely distributed</td></tr><tr><td>energy density</td><td>low unless preprocessed<sup>4</sup></td><td>extremely low, requiring concentration</td></tr><tr><td>transportability</td><td>transportable but localized transport only unless preprocessed<sup>4</sup></td><td>non-transportable unless converted to transportable solar fuels</td></tr><tr><td>storability</td><td>already in stored chemical energy</td><td>non-storable, requiring storage media</td></tr><tr><td>dispatchability</td><td>dispatchable</td><td>non-dispatchable (i.e., intermittent)</td></tr><tr><td>scale of application</td><td>small scale for distributed applications unless preprocessed<sup>4</sup></td><td>small to large scale</td></tr></tbody></table>

ªAn example of such processes is to convert biomass into high-energy-density biochar and bio-oil via fast pyrolysis at 450–550 °C; biochar and bio-oil can be further mixed to prepare bioslurry fuels that are of high volumetric energy densities and suitable for long-distance transport and large-scale applications.⁴,⁸

medium and high temperatures.⁹ Biomass transport is only economic locally as a result of the low energy density of biomass,⁸ and its long distance transport requires preprocessing to convert biomass into high-energy-density fuels.⁴,⁸ On the other hand, solar energy is non-transportable unless converted into solar fuels.¹⁰ Biomass is dispatchable, but concentrated solar energy is intermittent (non-dispatchable) because biomass

**Special Issue:** 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies

Received: October 12, 2017

Revised: December 19, 2017

Published: December 22, 2017
stores chemical energy but solar energy is non-storable, unless with energy storage media. From an application point of view, biomass utilization is of small scale and centralized large-scale applications can only be achieved via preprocessing biomass into high-energy-density fuels.⁴,⁸ Solar energy can be of both small and large scale.¹¹ Therefore, biomass and solar energy are complementary to each other in many ways, offering good opportunities for developing novel integrated biomass-solar technologies that use concentrated solar energy to drive endothermic biomass thermochemical reactions.

Pyrolysis is the first step of any solar-thermal-chemical processing of biomass. Solar-thermal pyrolysis of biomass via direct concentrated solar radiations has drawn attention in the field.¹²⁻¹⁵ Under direct concentrated solar radiations, the sample may experience high temperatures (1600 °C or above) that are considerably higher than those in conventional reactors (e.g., below 1000 °C in a fluidized bed reactor¹⁶ and below 1600 °C in a drop-tube furnace¹⁷). Biomass contains abundant inorganic species, particularly alkali and alkaline earth metallic (AAEM) species (mainly Na, K, Mg, and Ca), which are important considerations in biomass thermochemical processing.¹⁷⁻²² Therefore, it is important to understand the transformation of these species under solar-thermal pyrolysis at such elevated temperatures. In addition to the char carbon structure, these catalytic species are also important considerations in determining char reactivity under solar-thermal-chemical processing conditions. Unfortunately, such important aspects have not been examined for biomass pyrolysis driven directly by concentrated solar radiations.

Therefore, this study aims to carry out a set of experiments on the pyrolysis of mallee wood powders exposed to direct concentrated solar radiations. A solar-thermal-chemical reactor was deployed for biomass pyrolysis with direct concentrated solar radiations at various pyrolysis temperatures (1540, 1740, and 1930 °C), heating rates (320, 800, and 3200 °C/min), and holding times (0 and 5 min), focusing on the yields of pyrolysis products, transformation of AAEM species, as well as reactivity and carbon structure of char.

## 2. EXPERIMENTAL SECTION

**2.1. Biomass Sample.** Green mallee trees (*Eucalyptus loxophleba*, subspecies *lissophoia*) were harvested from Narrogin, WA, Australia. The wood component was separated from the trees, dried, cut, and then sieved to yield the size fraction of 150–250 µm (hereafter referred to as “mallee wood”) for use in the experimental program. On a dry basis, the mallee wood contains 83.8% volatile matter, 15.6% fixed carbon, and 0.6% ash based on proximate analysis. On a dry and ash-free basis, elemental analysis shows that the mallee wood contains 44.6% C, 8.1% H, 0.7% N, and 46.6% O (by difference). Ash analysis shows that the mallee wood contains abundant AAEM species, with the concentrations of Na, K, Mg, and Ca being 0.035, 0.087, 0.021, and 0.164% on a dry basis, respectively.

**2.2. Solar-Thermal Biomass Pyrolysis Reactor.** A solar-thermal pyrolysis reactor, as reported previously,¹² was employed in the experimental program. As shown in Figure 1, the reactor is an air-proof and transparent Pyrex reactor, which is set at the focus of a 1.5 kW vertical solar furnace. The solar furnace consists of three parts: a heliostat set (area of 4 × 6 m) that sits on the first level of the CNRS-PROMES building and tracks the sun and reflects the solar beam vertically, a parabola concentrator (diameter of 2 m) facing the ground that concentrates the direct solar radiation reflected onto it by the heliostat set underneath, and the focal point (area of about 1 cm²), where experiments are carried out via direct concentrated solar flux (up to 12 MW/m² when the direct incident solar radiation is 1 kW/m²). The transparent pyrex spherical chamber (diameter of 200 mm)

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic view of the solar-thermal pyrolysis reactor system, adapted and modified from a previous study.¹²</figcaption></figure>

can be assembled to the metallic reactor basis. The inert atmosphere of the reactor is controlled via allowing argon flowing through the reactor. At the focus of the solar furnace is a graphite crucible (inner diameter of 10 mm and height of 5 mm), in which a biomass sample is loaded. The crucible is located in a graphite foam layer (thickness of 10 mm) for reducing radiation losses from the crucible walls. This foam layer is set on another graphite layer (thickness of 10 mm), which is laid on an alumina support (thickness of 20 mm) held on a water-cooled sample holder, welded on the reactor metallic basis. This configuration permits to reduce conduction losses, thus the vertical thermal gradient inside the sample thickness.

The sample temperature is measured by a pyrometer (model Kleiber KPE 660/5) that operates in the wavelength range of 4.7–5.2 nm to discard any light reflected by the sample up to 2200 °C, aiming at the sample in the crucible through a CaF₂ window. This window (transparent in the working wavelength range) is located at the upper part of the glass reactor. The pyrometer is firmly held by the frame of the setup that supports the reactor carriage in the focal plane, thus imposing the use of a front-face silvered mirror for sample temperature measurement, with an adjusted position, so that the view beam from the pyrometer perfectly fits the inside of the crucible. The pyrometer was first calibrated by measuring the temperature in a blackbody through exactly the same configuration as the configuration implemented in the solar experiment (i.e., mirror of CaF₂ window, with the same distances and same angles). This permits a slight correction between the pyrometer indication (read temperature) and the sample real temperature (true temperature). The emissivity was set at 0.95 for all pyrolysis experiments, corresponding to that of candle soot.²³ A unique system is implemented for controlling the sample temperature–time history via a flux modulator, which is a shutter made of carbon fiber blades to reduce the device shadow when fully opened and the system inertia, and set on the solar beam reflected from the heliostat, thus before its concentration to avoid cooling requirement. The sample temperature–time history is set via the proportional–integral–derivative (PID) system optimized for the chosen pyrolysis temperature, heating, and holding time. This unique temperature control system permits operations with a flexible combination of multiple steps of heating, holding, and cooling.

In each pyrolysis experiment, ~100 mg of mallee wood was charged into the graphite crucible, which was then loaded into the transparent pyrex chamber and set on the focus of the solar furnace. Before heating the sample, the chamber was repeatedly vacuumed and purged with argon for multiple times to remove air in the system. Then, the sample was heated under argon continuously flowing through the system. Gas inlet and outlet were located in the reactor metal basis, interdependent to the reactor carriage. Finally, the glass reactor was connected to the metal basis by a screwed metal part with an O-ring for ensuring the sealing. The sample was then heated at preset heating rates of 320–
3200 °C min⁻¹ (5.3–53.3 °C s⁻¹) to the pyrolysis temperature (1540, 1740, or 1930 °C) and then held at the temperature for a desired holding time (0 or 5 min). The graphite crucible (with the sample) was weighed before and after the experiments to determine the char yield. Each experiment was performed at least 5 times. As a result of the limited sample amounts, selective analyses and characterization were carried out for selected (not all) char samples subsequently.

### 2.3. Sample Analyses and Characterization.
**2.3.1. Measurement of Char Reactivity.** Char samples produced under various conditions were subjected to isothermal oxidation reactivity measurement, using a thermogravimetric analyzer (TGA, model TA SDT-Q600). To minimize the effect of chemisorption of oxygen on reactivity measurement, a gas mixture of 5% O₂ in N₂ was used.²⁴ The chosen temperature of 600 °C was sufficiently low, so that the reactivity was measured under a kinetic control regime. Other reaction parameters, including the gas flow rate, particle size, and bed depth, were also optimized to ensure that oxygen consumption through the char bed was minimized, so that the reactor can be regarded as a differential reactor with respect to the reactive gas. Briefly, ~5 mg of char (as a thin layer of bed in the crucible) was heated at 20 °C min⁻¹ in pure nitrogen to 120 °C, and the char moisture was taken from the leveling-off weight. The char was then further heated to 600 °C, at which the gas was switched from N₂ to 5% O₂ in N₂. The specific reactivity (R, min⁻¹) of a char at any time was calculated from the differential mass loss data (dW/dt) according to R = -(1/W)(dW/dt), where W is the mass (dry and ash-free) of the char at any time t (min). At the end of a reactivity measurement run, the temperature was further increased to 830 °C at a heating rate of 20 °C min⁻¹, to completely burn off any carbon residue to determine the ash content of the char with the final mass remaining in the TGA. Repeated measurements were carried out to ensure that the reactivity data for all char samples were reproducible.

#### 2.3.2. Quantification of AAEM Species.
The contents of AAEM species (mainly Na, K, Mg, and Ca) in mallee wood and selected biochar samples (as a result of the limited amount of samples available) were determined via a procedure reported elsewhere.²⁵ Briefly, ~20 mg of sample was put in a Pt crucible and then ashed in air following a specially designed heating program to ensure no loss of these species during ashing. The ash sample together with the Pt crucible was then put in a Teflon vial for acid digestion with a mixture of HNO₃/HF (1:1) solution at 120 °C for 12 h. After the evaporation of excessive acids on a hot plate, the digested ash was dissolved in 20 mM methanesulfonic acid (MSA) solution. The AAEM species in the solution was quantified using Dionex ICS-3000 ion chromatography with a CS12A column and 20 mM MSA solution as the eluent.

#### 2.3.3. Char Structure Characterization Using Raman Spectroscopy.
The Raman analysis of char samples was carried out using a Dilor Labram dispersive Raman spectrometer (model 1B). A microscope equipped with a lens of 50× magnification was used to focus the excitation laser beam (HeNe laser with a wavelength of 632.8 nm) on randomly selected areas of each sample and to collect the Raman signal in a backscattered direction. The laser spot diameter reaching the sample was about 1 µm, which is much larger than the carbon microcrystallites in the char samples. Therefore, the Raman microprobe provided averaged information on a large number of randomly distributed microcrystallites. During analysis, ~20 particles were randomly chosen and the spectra were recorded in the range of 800–1800 cm⁻¹, covering the first-order bands, i.e., the graphite (G) band and the disordered (D) band. The acquisition time for each spectrum was ~120 s. As listed in Table 2, each spectrum can be deconvoluted into five bands: G, D₁, D₂, D₃, and D₄ at band positions of 1580 cm⁻¹,²⁶⁻²⁸ 1350 cm⁻¹,²⁹⁻³¹ 1620 cm⁻¹,²⁷,³² 1530 cm⁻¹,³²⁻³⁴ and 1180 cm⁻¹,²⁸,³⁵⁻³⁷ respectively, according to the procedure detailed elsewhere.²⁶ The G band is attributed to a perfect graphite structure, while the D₁ and D₂ bands correspond to the graphite structures with defects, and the bands D₃ and D₄ represent amorphous or poorly organized carbon structures.

<table><caption>**Table 2.** Raman Bands from Spectra Deconvolution and the Corresponding Carbon Structures</caption><thead><tr><th>band</th><th>position (cm<sup>-1</sup>)</th><th>structure</th></tr></thead><tbody><tr><td>G</td><td>1580</td><td>stretching vibration mode with E<sub>2g</sub> symmetry in the aromatic layers of the graphite crystalline<sup>26-28</sup></td></tr><tr><td>D<sub>1</sub></td><td>1350</td><td>graphite lattice vibration mode with A<sub>1g</sub> symmetry as a result of the in-plane imperfections, such as defects and heteroatoms<sup>29-31</sup></td></tr><tr><td>D<sub>2</sub></td><td>1620</td><td>lattice vibration analogous to that of the G band not involving graphene layers, which are not directly localized between other graphene layers<sup>27,32</sup></td></tr><tr><td>D<sub>3</sub></td><td>1530</td><td>amorphous sp<sup>2</sup>-bonded forms of carbon<sup>32-34</sup></td></tr><tr><td>D<sub>4</sub></td><td>1180</td><td>poorly organized carbon structures<sup>28,35-37</sup></td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Distribution of pyrolysis products (volatiles and char) during mallee wood solar-thermal pyrolysis at 1540, 1740, and 1930 °C, 800 °C min⁻¹, and 5 min holding time.</figcaption></figure>

## 3. RESULTS AND DISCUSSION

### 3.1. Distribution of Char and Volatiles during Mallee Wood Solar-Thermal Pyrolysis.
Figure 2 presents the yield distribution of volatiles and char after the mallee wood was processed with direct concentrated solar radiation in the solar-thermal pyrolyser at 1540, 1740, or 1930 °C and 5 min holding time. There are at least two important observations. One is that mallee wood pyrolysis under the conditions produces predominantly gaseous phase products (volatiles). The volatile yields are ~90 wt % or even higher, while the char yield is ~10 wt % or lower. The other observation is that an increase in the pyrolysis temperature leads to a decrease in the char yield and an increase in the volatile yield. Specifically, the char yield decreases from ~10 wt % at 1540 °C to ~8 wt % at 1740 °C and ~5 wt % at 1930 °C, corresponding to the increases in the volatile yield from ~90 wt % at 1540 °C to ~92 wt % at 1740 °C and ~95 wt % at 1930 °C. The results are consistent with previous reports on the trend that an increasing temperature results in a reduction in the char yield during biomass pyrolysis under conventional conditions (typically 400–600 °C with char yields of 36.5–25.9%³⁸). The results in Figure 2 clearly suggest that such reactions are extensive at temperatures as high as those in this study, leading to very low char yields (especially at 1930 °C, only ~5 wt %).

### 3.2. Distribution of AAEM Species in Char and Volatiles during Mallee Wood Solar-Thermal Pyrolysis.
Figure 3 presents the distribution of AAEM species in volatiles and char during mallee wood solar-thermal pyrolysis, with at least three important observations. First, under the solar-thermal pyrolysis conditions at such elevated temperatures, the majority of AAEM species are released as part of volatiles. This is evidenced by the fact that >96% Na, > 75% K, >93% Mg, and
<figure><img src="image_4.png" /><figcaption>Figure 3. Distribution of AAEM species in volatiles and chars during mallee wood solar-thermal pyrolysis at 1540, 1740, and 1930 °C, 800 °C min⁻¹, and 5 min holding time.</figcaption></figure>

>53% Ca are released into the gaseous phase, respectively, with <4% Na, <25% K, <7% Mg, and <47% Ca being retained in char, depending upon the pyrolysis temperature. Second, increasing temperature enhances the release of AAEM species into volatiles. As the pyrolysis temperature increases from 1540 to 1930 °C, the releases of AAEM species are increased from 96 to 99% for Na, from 75 to 88% for K, from 93 to 99% for Mg, and from 53 to 64% for Ca, respectively. Third and last, the releases of Na, K, Mg, and Ca are very different during mallee wood solar-thermal pyrolysis. At 1740 °C, the retentions of Na, K, Mg, and Ca in char are ~1%, ~13%, ~1%, and ~37%, respectively. At 1930 °C, almost all Na and Mg are released into volatiles, so that the retentions of the two elements in the char approach zero, while slight further reductions are observed in the retentions of K (~12%) and Ca (~36%) in the char, respectively.

Nearly complete release of Na is expected because both Na and its oxide (Na₂O) are volatile, with boiling points of 883 and 1132 °C, respectively,³⁹ and prone to be easily released into the gaseous phase,⁴⁰ particularly at high temperatures similar to those in this study. K is also considered as a volatile element, so that it is surprising to see the retention of a small fraction of K in the char even at 1930 °C. While the mechanism responsible for K retention is unknown, the results suggest that, during solar-thermal biomass pyrolysis, there appears to be reactions leading to the retention of thermally stable K in the char, such as possible intercalation.⁴¹ The nearly complete release of Mg into the gaseous phase may be explained by the release of metallic Mg and/or the reduction of MgO by carbon at a high temperature under inert conditions, followed by the release of reduced metallic Mg. The reduction reaction has been reported to take place at 1475–1600 °C.⁴² The release of some Ca can also be attributed to the reduction reaction of CaO into Ca, which can be readily released to the gaseous phase by reducing agents, such as CO and H₂, produced during biomass pyrolysis.⁴³ The high retention of Ca in the char may be attributed to the formation of Ca₃(PO₄)₂ at high temperatures. It is known that Ca₃(PO₄)₂ has a high melting point and can present as either a solid or coexist as a calcium-containing solid–liquid mixture at 1500–2200 °C.⁴⁴

**3.3. Reactivity of Chars from Mallee Wood Solar-Thermal Pyrolysis.** In practice, char with a lower reactivity may result in a lower char conversion rate in a solar-thermal-chemical reactor, leading to the loss of efficiency or the reactor being oversized. Therefore, char reactivity is an important consideration. It is further noted that, in this study, the reactivity measured is intrinsic char reactivity under a kinetic

<figure><img src="image_5.png" /><figcaption>Figure 4. Effect of the pyrolysis temperature on the specific reactivity of chars prepared from mallee wood solar-thermal pyrolysis at 1540, 1740, and 1930 °C and 5 min holding time: (a) 3200 °C min⁻¹ and (b) 800 °C min⁻¹.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5. Effect of the holding time on the specific reactivity of chars prepared from mallee wood solar-thermal pyrolysis at a heating rate of 3200 °C min⁻¹ and temperature of (a) 1540 °C and (b) 1740 °C.</figcaption></figure>

control regime (reactive gas of 5% O₂ in nitrogen and temperature of 600 °C), independent of the gas flow rate, particle size, and bed depth. In other words, it reports the intrinsic char reactivity under optimized conditions of a differential reactor. Figures 4–6 present the effects of the pyrolysis temperature, holding time, and heating rate on char reactivity. Under a kinetic control regime for reactivity measurement, the intrinsic reactivity of char continuously decreases with an increasing char conversion, suggesting that
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Effect of the heating rate on the specific reactivity of chars prepared from mallee wood solar-thermal pyrolysis at 1740 °C and 5 min holding time.</figcaption></figure>

selective oxidation takes place during conversion (i.e., the reactive components consumed earlier with inert components in the char left at increased conversion⁴⁵). This also suggests that the accumulation effect of catalysts is insignificant inside the reacting char as char conversion increases. Otherwise, an increase in char reactivity as a result of increasing catalytic reactivity would be evident at high conversions, as previously observed in brown coal chars prepared at lower temperatures.⁴⁶ Figure 4 further shows that the pyrolysis temperature has a drastic effect on the intrinsic reactivity of char. An increase in the pyrolysis temperature from 1540 to 1740 °C results in a drastic decrease in the intrinsic reactivity of char. Further increasing the pyrolysis temperature from 1740 to 1930 °C leads to the production of very inert char, which is of extremely low reactivity under the reactivity measurement conditions. As shown in Figures 5 and 6, increasing the holding time of the char at the pyrolysis temperature or decreasing the heating rate both reduce char reactivity as a result of the more extensive heat treatment.

The drastic effect of the pyrolysis temperature on char reactivity can be attributed to both the significant reduction of catalytic AAEM species in the char (see Figure 3) and possibly the carbon structure of the char becoming less reactive (see subsequent discussion in section 3.4). The results presented in Figures 4–6 also have some significant implications. While biomass solar-thermal pyrolysis under these severe conditions dominantly produces volatiles, with very low char yields (e.g., ~5 wt % at 1930 °C; see Figure 2), the reactivity of chars is still a critical consideration in the design and operations of solar-thermal reactors for two reasons. One is that the chars retain ~8.2–15.3% of total energy embedded in the mallee wood, depending upon the pyrolysis temperature. The other reason is that the chars have very low reactivity and become less reactive (especially after biomass solar-thermal pyrolysis at 1930 °C). Alternatively, lower temperatures (as suggested in a recent study¹³) should be considered for processing solid fuels, such as biomass, via direct concentrated solar radiations.

**3.4. Carbon Structure of Chars from Mallee Wood Solar-Thermal Pyrolysis.** Further efforts were taken to characterize the carbon structure of char samples, with the results presented in Figures 7–9, taking into consideration examination of two sets of key parameters based on Raman data. The first set of parameters includes the intensity ratio of D₁ and G bands (i.e., *I*<sub>D₁</sub>/*I*<sub>G</sub>) and the full width at half maximum of the G band (i.e., G-FWHM). *I*<sub>D₁</sub>/*I*<sub>G</sub> is an important parameter for the degree of carbon structural order.²⁶,²⁸ A higher *I*<sub>D₁</sub>/*I*<sub>G</sub> ratio indicates that char has less

degree of carbon structural order or, in other words, has higher reactivity. As shown in Figure 7, increasing the pyrolysis temperature and holding time and decreasing the heating rate all broadly lead to reductions in the *I*<sub>D₁</sub>/*I*<sub>G</sub> ratio. The results in Figure 7 clearly suggest that the carbon structure becomes more ordered. G-FWHM is widely reported as another important parameter for the degree of graphitization.⁴⁷ A reduction in G-FWHM reflects a decrease in the interlayer spacing *d*<sub>002</sub>, hence an increase in the graphitization of char.⁴⁷ Again, the results presented in Figure 8 show that G-FWHM of char decreases with an increasing pyrolysis temperature and holding time and a decreasing heating rate, clearly indicating increases in the graphitization of the char. The results on G-FWHM in Figure 8 are consistent with those on the *I*<sub>D₁</sub>/*I*<sub>G</sub> ratio in Figure 7.

The second set of parameters consists of three parameters, including the ratio of the integrated area of the G and D₂ bands to the area of all bands (i.e., S<sub>G+D₁</sub>/S<sub>all</sub>), the ratio of the integrated area of the D₁ band to the area of all bands (i.e., S<sub>D₁</sub>/S<sub>all</sub>), and the ratio of the integrated area of the D₃ and D₄ bands to the area of all bands (i.e., S<sub>D₃+D₁</sub>/S<sub>all</sub>). S<sub>G+D₂</sub>/S<sub>all</sub> is an indication of the contribution of ordered carbon structures, including the turbostratic structure.⁴⁸ S<sub>D₁</sub>/S<sub>all</sub> represents the contribution of ordered carbon structures with defects and heteroatoms, while S<sub>D₃+D₁</sub>/S<sub>all</sub> indicates the contribution of amorphous and poor organized carbon structures. As shown in Figure 9, increasing the pyrolysis temperature and holding time and decreasing the heating rate result in increased S<sub>G+D₂</sub>/S<sub>all</sub>, considerably decreased S<sub>D₃+D₁</sub>/S<sub>all</sub>, but little changes in S<sub>D₁</sub>/S<sub>all</sub>. The results clearly show that more severe thermal treatment

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Variations of the peak intensity ratio of the D₁ and G bands with reaction conditions.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Variations of G-FWHM with reaction conditions.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Variations of the band area ratios with reaction conditions: (a) $S_{G+D_2}/S_{allb}$ (b) $S_{D_1+D_2}/S_{allb}$ and (c) $S_D/S_{all}$.</figcaption></figure>

converts the amorphous and poorly organized carbon structures (that is reduced substantially) into more ordered carbon structures in the char, so that the char becomes less reactive. In addition, the self-gasification effect,⁴⁹ i.e., the reaction between the pyrolyzing biomass/char particles and pyrolytic H₂O and CO₂ produced during pyrolysis, may also contribute to reducing the char yield and reactivity, particularly at high temperatures.

## 4. CONCLUSION

Solar-thermal pyrolysis of mallee wood via direct concentrated solar radiations was conducted under various conditions (pyrolysis temperatures of 1540, 1740, and 1930 °C, heating rates of 320, 800, and 3200 °C/min, holding times of 0 and 5 min). Under such severe conditions, solar-thermal pyrolysis products are predominantly volatiles (90–95 wt %) and the char yields are low (5–10 wt %). Increasing the pyrolysis temperature leads to reductions in both the char yield and the retentions of AAEM species in char. The char samples are less reactive as a result of the char carbon structure becoming more ordered/graphitized and the significant loss of inherent catalytic

AAEM species during pyrolysis. While the yields of chars are low (e.g., 5 wt % at 1930 °C), the chars produced from the solar-thermal pyrolysis of mallee wood at high temperatures are still critically important to the design and operations of solar-thermal-chemical reactors under such severe conditions because these chars are very inert and retain considerable amounts (~8.2–15.3%) of energy embedded in the mallee wood.

## ■ AUTHOR INFORMATION

### Corresponding Author

*Telephone: +61-8-92667592. Fax: +61-8-92662681. E-mail: h.wu@curtin.edu.au.

### ORCID

<figure><img src="image_11.png" /></figure>

Hongwei Wu: 0000-0002-2816-749X

### Notes

The authors declare no competing financial interest.

## ■ ACKNOWLEDGMENTS

This work was initially supported by the French–Australian S&T Cooperation program jointly funded by the Australian and French governments. It was also supported by French “Investments for the Future” programme managed by the National Agency for Research under Contract ANR-10-LABX-22-01 (Labex SOLSTICE).

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