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Solar-thermal pyrolysis of mallee wood at high temperatures Hongwei Wu, Daniel Gauthier, Yun Yu, Xiangpeng Gao, and Gilles Flamant Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03091 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017
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Solar-thermal pyrolysis of mallee wood at high temperatures Hongwei Wu,*,1 Daniel Gauthier,2 Yun Yu,1 Xiangpeng Gao,1,3 Gilles Flamant2
1
2
Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia
Processes, Materials and Solar Energy laboratory, CNRS-PROMES, 7 Rue du Four Solaire, F-66120 Font Romeu Odeillo, France 3
Discipline of Electrical Engineering, Energy and Physics, School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, WA 6150, Australia
*Corresponding Author. Email:
[email protected]; Tel: +61-8-92667592; Fax: +61-8-92662681
A manuscript submitted to Energy & Fuels for consideration of publication in the special issue for the 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies
October 2017
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ABSTRACT: This study reports solar-thermal pyrolysis of mallee wood powders driven by direct concentrated solar radiation at different temperatures (1540, 1740 and 1930 °C), heating rates (320, 800 and 3200 °C/min) and holding time (0 or 5 mins). 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 because 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% and those of K and Ca are only 13% and 35%, respectively, and the char is less reactive. Therefore, in spite of 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.
KEY WORDS: Biomass; Pyrolysis; Solar-thermal; Char reactivity; Char structure
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1. Introduction The rural and regional Australian communities are facing critical challenges of managing dryland salinity and securing 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 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 At the meantime, Australia as a key sun-belt country has abundant solar energy, available at high solar radiation per square metre. 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” as its production is via photosynthesis that converts and stores solar energy into chemical energy in biomass. Biomass is of low energy density due to high moisture and bulky nature,8 and solar energy is also of extremely low energy density which requires concentration before utilisation at medium and high temperatures.9 Biomass transport is only economic locally due to the low energy density of biomass8 and its long distance transport requires pre-processing 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 stores chemical energy but solar energy is non-storable unless with energy storage media. From application point of view, biomass utilisation is of small-scale and centralised large-scale applications can only be achieved via pre-processing biomass into high-energy-density fuels.4,8 Page 3 of 25
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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 have drawn attentions 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 fluidised bed reactor 16 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 char carbon structure, these catalytic species are also important considerations in determining char reactivity under solarthermal-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 time (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. 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 Page 4 of 25
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analysis. On a dry-ash-free basis, elemental analysis shows that the mallee wood contains 44.6% of C, 8.1% of H, 0.7% of N, and 46.6% of 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. A solar-thermal biomass pyrolysis reactor A solar-thermal pyrolysis reactor, which was 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: 4m × 6m) 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: 2 m) facing the ground that concentrates the direct solar radiation reflected onto it by the heliostat set underneath, and the focal point (area: 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: 200 mm) 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: 10 mm; height: 5mm) in which a biomass sample is loaded. The crucible is located in a graphite foam layer (thickness: 10 mm) for reducing radiation losses from the crucible walls. This foam layer is set on another graphite layer (thickness: 10 mm), which is laid on an alumina support (thickness: 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 set-up 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 fits Page 5 of 25
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perfectly the inside of the crucible. The pyrometer was first calibrated by measuring temperature in a blackbody through exactly the same configuration as the one implemented in the solar experiment (i.e. mirror: CaF2 window; same distances; 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 fibre 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 PID system optimised 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 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 up 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 up at pre-set heating rates of 320–3200 °C min-1 (5.3–53.3 °C s-1) to the pyrolysis temperature (1540, 1730 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 char yield. Each experiment was done for at least five times. Due to the limited sample amounts, selective analyses and characterisation were carried out for selected (not all) char samples subsequently.
2.3. Sample analyses and characterisation Measurement of char reactivity. Char samples produced under various conditions were subjected to isothermal oxidation reactivity measurement, using a thermogravimetric analyser (TGA, model TA SDTQ600). In order to minimise the effect of chemisorption of oxygen on reactivity measurement, a gas mixture Page 6 of 25
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of 5% O2 and N2 was used.24 The chosen temperature of 600 °C was sufficiently low so that the reactivity was measured under kinetic control regime. Other reaction parameters including gas flow rate, particle size and bed depth were also optimised to ensure oxygen consumption through the char bed being 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 levelling-off weight. The char was then further heated to 600 °C, at which the gas was switched from N2 to 5% O2 in nitrogen. 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-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 in order to determine the ash content of the char, being the final mass remaining in the TGA. Repeated measurements were carried out to ensure the reactivity data for all char samples were reproducible. Quantification of AAEM species. The contents of AAEM species (mainly Na, K, Mg and Ca) in mallee wood and selected biochar samples (due to limited amount of samples available) were determined via a procedure reported elsewhere.25 Briefly, ~20 mg 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 a Dionex ICS-3000 ion chromatography with a CS12A column and 20 mM MSA solution as eluent. Char structure characterisation 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 magnification 50× 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 1µm, which is much larger than the carbon microcrystallites in the char samples. Therefore, the Raman microprobe provided averaged information on a Page 7 of 25
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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 G (graphite) band and the D (disordered) 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 represents amorphous or poor organised carbon structures.
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 volatiles yields are ~90 wt% or even higher while the char yield is ~10 wt% or lower. The other observation is that an increase in pyrolysis temperature leads to a decrease in the char yield and an increase in the volatiles 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 volatiles 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 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
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Figure 3 presents the distribution of AAEM species in volatiles and char during mallee wood solarthermal pyrolysis, with at least three important observations. First, under the solar-thermal pyrolysis conditions at such elevated temperatures, the majorities of AAEM species are released as part of volatiles. This is evidenced by the fact that >96% of Na, >75% of K, >93% of Mg, and >53% of Ca are released into gaseous phase, respectively, with
