Microwave ‐ Assisted Pyrolysis of Pine Wood Sawdust Mixed with Activated Carbon for Bio ‐ Oil and Bio ‐ Char Production

: Pyrolysis of pine wood sawdust was carried out using microwave ‐ heating technology in the presence of activated carbon (AC). Experimental conditions were of 20 min processing time, 10 wt.% of AC, and a microwave power varying from 100 to 800 W. The results obtained showed that the microwave absorber allowed increasing the bio ‐ oil yield up to 2 folds by reducing the charcoal fraction. The maximum temperature reached was 505 °C at 800 W. The higher heating values (HHV) of the solid residues ranged from 17.6 to 30.3 MJ/kg. The highest HHV was obtained for the sample heated at 800 W with 10 wt.% of AC, which was 33% higher than the non ‐ charged sample heated at the same power. Furthermore, the addition of AC allowed showing the probable catalytic effect of the AC in the charged sample pyrolysis bio ‐ oils. of sample before and after heating,


Introduction
Fossil fuels are the most widely used energy sources nowadays and comprise over 80% of the worldʹs energy supply. However, the reservesʹ depletion and the carbon fingerprint impact on the environment require finding alternative energy resources. This includes the use of virgin and waste biomasses due to their widespread availability and low impact on the greenhouse effect.
Lignocellulosic biomass (e.g., wood, crops, agricultural and forestry residues, etc.) would be preferable due to its high density (i.e., high energy content per volume), its low amount of ash, and its very low amount of nitrogen. It consists of 35-50 wt.% cellulose, 25-30 wt.% hemicelluloses, 15-20 wt.% lignin, 2-20 wt.% extractable components, and some minerals [1]. Different lignocellulosic materials were subjected to microwave pyrolysis. They include, among others, the treatment of willow chips and straw [2], oil palm shell [3], pine wood sawdust and corn stover [4], barley dust and macroalgae [5], coffee hulls [6], rice straw and rice husk [7], sugarcane bagasse and sugarcane peel [7], and bamboo leaves [7]. One of the main thermochemical conversion routes for the conversion of biomass resources into value-added products is pyrolysis. This process consists of heating the biomass in the absence of air. It allows the production of three valuable fractions: a bio-oil fraction called "condensable liquid", a volatile fraction, and a carbon-rich solid residue (biochar). Maximizing the yield of one of these fractions requires the optimization of several parameters that include the temperature, the residence time and/or the heating rate [8]. The solid biochar has a wide range of applications such as activated carbon precursors for wastewater treatment [9], soil additive [10], and solid fuel for energy generation [11]. Otherwise, applications as liquid fuel after refining [12] and as valuable chemicals [13] have been considered for bio-oil liquid fraction. Moreover, gas fraction, as the light molecular weight gases, has shown great potential in fuel application [14], electricity generation [15], and as feedstock for H2 production [16].
The application of microwave technology in biomass pyrolysis has increased from the mid-1990s. This processʹs advantages over conventional heating include selective and rapid heating, non-contact and volumetric heating, energy saving, easy to control, and environmentally friendly. The heat transfer in conventional heating is governed by conduction and convection compared to microwave heating, where ionic conduction and dipole polarization occur between feedstock and microwave radiation [17]. Applying microwave radiations in a pyrolysis process is a new concept that could be an interesting way of biomass waste pyrolysis. Microwave action during biomass pyrolysis is reflected by the value of the dielectric loss tangent (tan δ = ε"/ε'), where ɛ" corresponds to the dielectric loss factor, and ɛ' is the dielectric constant. The latter refers to the ratio between the absorbed microwave irradiations and the reflected ones, whereas ɛ" measures the efficiency of the heat energy converted from the received microwave energy [18]. A high value of tan δ is the main condition to convert microwave energy into thermal one. Thus, while some materials are transparent to microwaves, other materials such as carbon and activated biochar are excellent microwave absorbers. Indeed, the loss tangents of coal (tan δ = 0.02 − 0.08) [19], carbon black (tan δ = 0.35 − 0.83) [20], charcoal (tan δ = 0.11 − 0.29) [21], and activated carbon (tan δ = 0.57 − 0.80) [22], are comparable to the loss tangent of distilled water (0.118 at room temperature and 2.45 GHz). Therefore, carbon materials are good microwave absorbers that show a high capacity to absorb and convert microwave energy into heat.
In this study, pine wood sawdust (PWS) biomass was pyrolyzed under microwave irradiation using activated carbon (AC) as a microwave absorber. The effect of AC on the potential energy use of heating fractions, bio-oil composition, and solid characteristics was studied. Results related to temperature profiles, product yields, and pyrolyzed biomass behavior were discussed and compared to conventional pyrolysis described in the literature for the same operating conditions.

Materials
PWS was kindly provided by the Lecopin Company (Sore, France) and was used in this study as the raw material for the thermochemical conversion experiments. Some physicochemical properties of PWS are listed in Table 1. Activated carbon with similar particle density (0.113 g/cm 3 ) than PWS biomass and having a specific surface area of 500 m 2 /g was supplied by CECA S.A. Company (Parentis-en-Born, France) and was used as a microwave radiation absorber. It was then mixed with the biomass before the heat treatment. Relatively high moisture content is a desirable characteristic in microwave biomass pyrolysis as it plays a role in the initial microwave absorption and allows increasing the raw material temperature to the water boiling point [23]. The proximate analysis of biomass is of paramount importance for biomass energy use. Indeed, the high value of volatile matter (81.1%) and the low value of ash content (0.2%) show its higher calorific value potential and better heating rate [24].

Proximate Analysis
The moisture contents (MC) of the samples (untreated and treated PWS biomass) were determined based on the measurements of weight loss after oven-drying at 105 °C for 24 h. "Ash content (AshC)" and "volatile matter content (VMC) were determined according to the ISO 1171 standard procedures. Each measurement was carried out in triplicate, and the mean values are reported. Then, ≈3 g of a sample was weighed, put into a crucible, and then placed in a muffle furnace (Cerhec, Lacroix-Saint-Ouen, France) at 815 °C ± 10 °C to determine the ash content. After 2 h incineration, all organic materials were carbonized, and the ash content was calculated on a percentage dry weight basis. Similarly, VMC was obtained by incinerating ≈7 g of a sample in a muffle furnace at 815 °C ± 10 °C. After 10 min, the sample weight remaining in the crucible was recorded, and the VMC was calculated on a percentage dry weight basis. The "fixed carbon content (FCC)" was then calculated according to Equation (1).

Pyrolysis Process
A 900 W, 2.45 GHz microwave system (NEOS-Milestone, Sorisole, Italy) was used to perform the thermal treatment experiments. Each sample of ≈35 g, introduced in a round-bottom quartz flask, was placed in the microwave system, as shown in Figure 1. A flow of N2 (800 mL/min) was used as an inert gas to carry out the volatile compounds released during biomass heating. The obtained carbonaceous residue (char) and the condensable fraction (bio-oil) were weighed, and the non-condensable vapors (gas) were evacuated. The gas yield was estimated by the difference of weight between bio-oil and char fraction. The mass loss percentages were calculated according to Equation (2). m0 and mf represent the initial mass of the sample before heating and the final mass after heating, respectively.

Mass loss % * 100
(2) Given the low dielectric loss tangent of pine wood sawdust (tan δ = 0.11 for wood and tan δ = 0.18 for pine bark [25]), the feedstock has to be mixed with a suitable microwave absorber (here activated carbon (AC)) to reach high temperatures that are necessary for pyrolysis. 10 wt.% of AC was added to the biomass, mixed, and then exposed to microwave treatment. The desired pyrolysis and torrefaction temperatures were reached by setting the microwave power from 100 to 800 W for 20 min.
The device was switched off for each measurement, and a temperature probe was immediately introduced in the center of the sample to have appropriate and accurate measurements of the temperature during thermal treatment. The duration of each temperature measurement was less than 30 s, which is required to not disturb the temperature increasing and the sampleʹs heating-up. The monitoring was done to see the effects of both microwave power and AC on the temperature evolution during the thermal treatment.
The different samples were labeled as follows: "S (=sample) − microwave power valueactivated carbon mass percentage". For example, "S-300-10" was for the sample heated at 300 W and charged with 10 wt.% of AC, "S-100-N" corresponds to the sample heated at 100 W without AC. Two categories of samples were then treated; the 1st category grouped the samples without AC, heated at 100 W, 300 W, 500 W, and 800 W, and the 2nd category grouped the samples charged with 10 wt.% AC and heated at 100 W, 300 W, 500 W, and 800 W.

Chromatographic Analysis
The compositions of the oil fractions obtained were analyzed using the gas chromatography-mass spectrometry (GC-MS) technique. The analyses were carried out using an Agilent 7820A chromatograph equipped with an Agilent-MS 5975 mass spectrometer (Agilent, Santa Clara, United States). High purity helium was used as carrier gas at a 1 mL/min flow rate with a split ratio of 1:10. The separation was performed using an HP-5 capillary column (5% diphenyl + 95% dimethylpolysiloxane) (30 m length, 250 μm inner diameter, and 0.25 μm film thickness). The initial column temperature set at 45 °C was maintained for 4 min; the temperature was then increased at a rate of 3 °C/min until reaching 280 °C. The sample (1 μL) was injected under high pressure (50 kPa), and the injector temperature was kept at 280 °C. The mass spectra were operated in EI mode at 70 eV and obtained from 20 to 150 (m/z). The chromatographic peaks were identified according to the NIST MS (National Institute of Standards and Technology Mass Spectral) library. Before analysis, the oil fractionʹs moisture was removed by heating for ≈12 min using a heating plate. The organic fraction remaining (≈1 g) was dissolved in 5 mL acetone, put in a GC-MS vial, and then injected. The obtained chromatograms were normalized against the peak of the solvent to allow the quantitative analysis.

Statistical Analysis
Error bars in the graph represent the standard deviation of three biological replicates.

Temperature Profiles
The temperature profiles recorded over the time for the different microwave (MW) powers applied during 20 min of thermal treatment of PWS samples, and with 10 wt.% of AC, are shown in Figure 2.  Figure 2 show that the temperatures tend to increase proportionally to the MW power applied. After approximately 12 min, except for the sample S-100-N, the temperature values almost reached a plateau. Beyond this time, the vaporsʹ release had almost stopped, thereby indicating that the heating process had reached a steady state. Figure 2 shows that for the 1st category of samples, the maximum temperature value reached after 20 min was 210 ± 2 °C (for the sample S-800-N). This temperature corresponds to the torrefaction conditions (200-300 °C). For the 2nd category of samples, pyrolysis temperatures (estimated in the range of 300-700 °C [26]) were reached at 505 ± 2 °C (for the sample S-800-10). These results show that the desired pyrolysis temperature could be obtained using 10 wt.% of MW absorber with a MW power up to 500 W. Usually, temperature values higher than 550 °C lead to the occurrence of secondary reactions and hence it could favor the release of gaseous products [27].

Results in
Macroscopic changes in the solids obtained after the MW treatments could be observed in Figure 3. Gradual changes were noticed in the samplesʹ color, which was mainly due to the uniform heating provided by the addition of the activated carbon and the increase of MW power. In fact, uniform carbonization was observed clearly for all the samples of the 2nd category.

Pyrolysis Products' Yields
Char residue, bio-oil, and gas fraction were the three main fractions of the pyrolysis process. The yields, in mass percentage, obtained using the different process conditions are presented in Figure 4. The increase of MW power leads to enhanced gas production, which implies systematically decreasing the yield of the solid fraction (char). Indeed, the decrease in char yield with the increase of temperature can be explained by removing volatile material during devolatilization of the organic material with the likely presence of partial gasification of the char [6]. The highest oil yield (24 wt.%) was reached at 505 ± 2 °C for the sample heated at 800 W with 10 wt.% of AC. Moreover, the mass loss percentages were equivalent to the yields of volatiles (condensable (oil), non-condensable (gas) fractions, and moisture); it is, therefore, higher in the presence of AC and when increasing the microwave heating power. The addition of 10 wt.% AC (except for the sample S-100-10) involves reaching the highest values of mass loss, which is very likely due to the high temperatures reached in these experiments. Indeed, a temperature beyond 250 °C induces a higher reactivity and a greater degradation of the biomass components (i.e., cellulose, hemicelluloses, and lignin) [28,29]. On the other hand, the mass loss observed below this temperature range is largely due to the evaporation of water and low molecular weight volatile compounds. Gao et al. reported different trends in mass loss percentage for pine sawdust conventional pyrolysis [30]. They reported that 81.3 wt.% of weight loss was noted for the samples of conventional pyrolysis compared to 71.4 and 74.8 wt.% for microwave pyrolysis samples (S-500-10 and S-800-10, respectively) obtained in this work. Moreover, the solid residue that remained was equal to 14.8 wt.% and 17.5 wt.% of the original mass in slow and fast conventional pyrolysis, respectively, compared to 26.9 wt.% on average in microwave pyrolysis. These differences could be related to the difference in the mass loss rate, which is much higher in microwave pyrolysis than that in conventional pyrolysis [30,31].
Microwave pyrolysis produces much larger gas yields than conventional fast pyrolysis over the entire temperature range [31]. The oil yield, however, was lower in the case of microwave heating (33.5 wt.% for conventional fast pyrolysis against 24.0 wt.% for S-800-10 sample heated under MW conditions), which suggests that microwave heating favors secondary cracking reactions leading to obtaining lighter products compared to that of conventional heating [5,31,32].

Characterization of the Solid Fraction
The chemical composition of the obtained chars from PWS microwave thermochemical conversion under various conditions is shown in Table 2. As expected, increasing the temperature values (or microwave power) leads to an increase of the fixed carbon content (FCC%), a decrease of the volatile matter content (VMC%), and a slight increase of the ash content (AshC%). In fact, the increase of FCC by approximately five times was observed for the sample S-800-10, compared to the sample S-100-N. This is the consequence of the significant removal of hydrogen and oxygen atoms from the raw biomass and the low amount of carbon in the form of hydrocarbons in the raw biomass. On the other hand, the char resulting from microwave pyrolysis has slightly lower carbon content compared to that obtained from conventional pyrolysis [5,32]. However, the resulting char from both pyrolysis has the potential to be used as precursor of AC due to its high carbon content (e.g., 78.7, 78.1, and 79.2 wt.% of FCC, respectively, for the S-300-10, S-500-10, and S-800-10 samples). Low ash content in raw pine sawdust is an indicator that this biomass could be a good candidate for fuel production via the thermochemical process. According to De Bhowmick et al. [32], the ash content of pine sawdust char obtained by conventional slow pyrolysis (at 500 °C) was lower than that of the chars obtained in this work by microwave-assisted pyrolysis at the same temperature (chars from the samples of the 2nd category; S-500-10 and S-800-10). These charsʹ relatively high ash content could be related to the increased mineral concentrations and organic combustion residues. For the same reasons, it can be seen that the content of volatile matter decreases as either the temperature or the amount of AC increase. In fact, the heated material has a lower O/C ratio compared to that of the untreated biomass and thereby a higher calorific value [28].
For the 1st category samples, the increase in temperature allowed the increase of higher heating values (HHV) ( Table 2). The same behavior was observed for the samples of the 2nd category. The chars produced in this work had the heating values ranging from 17.7 to 30.3 MJ/kg, which is lower than the transportation grade diesel fuels (≈36.5 MJ/kg) and natural gas (≈42.5 MJ/kg), and approximately similar to that of coal (≈34.1 MJ/kg), lignite (≈26 MJ/kg) or ethanol (≈29 MJ/kg) [33]. Moreover, by comparing the results obtained in the current work to those from conventional slow pyrolysis, similar values of PWS HHV were observed for similar pyrolysis conditions [32].
The addition of AC allowed modifying the quantitative and qualitative composition of bio-oils ( Figure 5B). Variations in the intensity of the product peaks were observed. Moreover, the presence of a MW absorber in the heated samples allowed the formation of new products that were labeled in red color. The analysis of these new compounds allowed the identification of new phenolic compounds (phenol #11″, trans-isoeugenol #29′, and dihydroeugenol #29″) and a new furan compound (5-hydroxymethylfurfural (HMF) #21″). An increase in the content of phenolic compounds might be due to the addition of AC that plays a catalytic role in the mechanism of lignin degradation, whereas the formation of 5-hydroxymethylfurfural could happen due to the presence of continuous secondary degradation reactions with an increase in the temperature. Phenols, derived mainly from ligninʹs decomposition, are widely useful chemicals in pharmacy, synthesis, and the food industry [44,46,47]. One of the pine wood pyrolysis polysaccharide products is HMF. It can also be a catalytic pyrolysis product; for example, zeolites catalyze the dehydration of levoglucosan to 5-hydroxymethylfurfural [48,49]. Unlike conventional pyrolysis, levoglucosan is not observed in the bio-oil of our work [30]. This may be due to the high heating rate in microwave pyrolysis. In fact, the microwave technique is one of the most promising methods of enhancing and accelerating chemical reactions. Thus, this mechanism prevents the appearance of levoglucosan, which breaks down into HMF.

Conclusions
Potential gains from the use of microwave technology, both on the efficiency of pyrolysis process and on the decrease in time analysis, have been illustrated in this work. The main goal was to study the efficiency of activated carbon on the final product properties-the addition of activated carbons allowed unifying biomass heating and increases of heating temperatures. Calorific values of solid residues ranged from 17.6 to 30.3 MJ/kg and were higher in the pyrolysis conditions of 800 W microwave power. Moreover, activated carbonʹs addition allowed the formation of new organic products (e.g., phenolic compounds) and then modifying the quantitative and qualitative composition of the bio-oils generated. The activated carbon probably plays a catalytic role in the pyrolysis of PWS biomass. An increase in microwave irradiation power allowed the intensification of mass loss percentages, the increase of the fixed carbon content, and solidsʹ calorific value. These behaviors were most likely due to the increase of the treatment temperature. Indeed, the temperatures tend to increase proportionally to the MW power applied. Moreover, microwave pyrolysis of PWS produces an abundant solid fraction, in which the amount decreases with the pyrolysis temperature. Additionally, the variation of the MW power did not significantly modify the chemical composition of the resulting bio-oils.
The comparison made between the results obtained in this work and conventional heating has shown that microwave heating allowed obtaining a solid pyrolysis fraction with similar characteristics to solids obtained from conventional heating treatment, but with shorter residence time. This shows the potential of this technology to reduce energy consumption and, therefore, the production cost. Microwave pyrolysis produces much larger gas yields, less oil, and similar char yields than conventional pyrolysis over the entire range of temperatures. The char resulting from microwave pyrolysis has slightly lower carbon content than that obtained from conventional pyrolysis. In general, the main compounds analyzed in microwave pine sawdust pyrolysis oil were found, at higher temperatures, in conventional pyrolysis bio-oil. It can be concluded from this work that microwave technology can reduce the pyrolysis reaction time and increase the quality of value-added products obtained from PWS biomass.