Article pubs.acs.org/EF
Effects of Microwave Treatment on the Chemical Structure of Lignocarbohydrate Matrix of Softwood and Hardwood Alexandr Arshanitsa,* Tatiana Dizhbite, Oskars Bikovens, Gunars Pavlovich, Anna Andersone, and Galina Telysheva Latvian State Institute of Wood Chemistry, 27 Dzerbenes Street, LV-1006 Riga, Latvia S Supporting Information *
ABSTRACT: A comparative study of the modification of chemical composition of softwood and hardwood by torrefaction at different temperatures (150−300 °C) was carried out using microwave treatment and convective heating. The nonthermal effect was established for softwood, revealing increased aromaticity of the samples obtained under microwave treatment. A difference of the effects of microwave treatment on softwood and hardwood was revealed. In the case of softwood, both carbonization processes and unsaturation development of the matrix structure were promoted, whereas for hardwood only promotion of the carbonization processes was observed. Formation of extractable compounds as the result of wood microwave treatment was monitored, and the significant increase in formation of hydrophilic extractable compounds in the range of 210−280 °C (hardwood) and 250−300 °C (softwood) was found. The destructive changes of lignin macromolecules at relatively low temperatures (beginning with 150 °C) were confirmed by decreasing Klason lignin content and simultaneous significant increase in the yield of acid-soluble lignin from both wood species. The structure of the matrix formed in the result of softwood microwave treatment at 230−280 °C was much more thermostable than both the original wood and the solid material obtained by convective heating at the respective temperatures.
1. INTRODUCTION Wood pellets are the most common fuel made from lignocellulosic biomass that is a widely available renewable energy resource. The global market for wood pellets is constantly growing.1 One of the possibilities for enhancing the quality of wood pellets is increasing their energy density using a torrefaction process. Torrefaction is a mild thermal pretreatment of biomass at in general 200−300 °C under an inert atmosphere. The higher heating value (HHV) of torrefied biomass is 10−30% higher in comparison with that of fresh biomass.2 The transformations of the major constituents of woodhemicellulose, cellulose, and ligninduring traditional thermal treatment in the same temperature range as torrefaction have been extensively studied for decades. For the range of temperatures usually applied in the torrefaction processes, it is known that after the elimination of moisture, extractives start to degrade3 and splitting of acetyl groups from hemicelluloses results in formation of acetic acid that could catalyze decomposition of less ordered carbohydrates.4,5 At about 200 °C, thermal decomposition of hemicelluloses occurs with formation of oligomeric compounds.3 The first decomposition processes in cellulose, namely, low-temperature dehydration reactions, begin at 210−220 °C and occur in the less ordered (amorphous) regions of cellulose.6,7 For lignin, the complex of parallel, successive, and competitive reactions of destruction and condensation develop depending on the wood species and thermal treatment conditions (temperature, heating rate, atmosphere, etc.). Lignin decomposes over a broader temperature range (200−500 °C) than cellulose and the hemicellulose components of biomass.8,9 Torrefaction of biomass can be done using not only conventional heating3 but also microwave treatment.10 The © 2015 American Chemical Society
main advantages of microwave-based heating versus conventional heating include the selectivity of biomass heating and the ability to start and stop the process, instantaneously improving the control processing.11 Our recent research showed that microwave treatment drastically increases hydrophobicity and water resistance of treated pellets.12 Those are very important operating characteristics responsible for fuel quality of wood pellets during transporting and storage. The aim of the present work was to study modification of softwood and hardwood lignocellulosic matrices after thermal treatment of wood under inert atmosphere at the temperatures within the 120−300 °C range using microwave treatment (frequency of 2.45 GHz) as the source of thermal energy, with emphasis on wood component composition and lignin component chemical structure. The realization of this task will provide a better understanding of the mechanism of biomass structure transformations necessary for microwave treatment optimization. Taking into account that microwave treatment is considered to be a prospective tool for enhancement efficiency of differing lignocellulosics processing, the knowledge obtained could be useful not only for biofuels pellet production but also for the development of new and/or improved stages for biorefineryoriented biomass transformation or self-sufficient processes (e.g., extraction of biologically active products3). In the present study, to determine the chemical composition of the biomass after microwave treatment and to achieve a better understanding of the mechanisms governing the biomass components transformation at microwave treatment, analytical Received: October 19, 2015 Revised: December 18, 2015 Published: December 20, 2015 457
DOI: 10.1021/acs.energyfuels.5b02462 Energy Fuels 2016, 30, 457−464
Article
Energy & Fuels
Table 1. Mass Recovery Yields of Hardwood and Softwood after Microwave Treatment at Various Temperatures and Chemical Composition of Solid Residues (%) treatment temperature, °C
yield
cellulose
Klason lignin
nontreated 150 170 190 210 250 280 300
100 100 99.8 99.6 98.6 85.6 78.7 78.0
54.0 53.8 52.1 53.8 55.6 56.4 54.5 58.1
20.5 19.2 19.0 19.8 19.9 29.8 36.3 38.1
nontreated 150 170 190 210 250 280 300
100 99.3 99.0 97.9 92.9 85.5 77.7 70.0
49.0 51.2 49.7 47.4 48.0 47.8 46.8 48.0
29.6 27.5 28.2 28.2 32.4 39.2 47.0 53.1
lipophilic extractivesa
hydrophilic extractivesb
4.6 11.4 10.8 10.7 10.6 9.7 8.0 7.4
0.40 0.52 0.53 0.33 0.34 0.15 0.16 0.07
3.4 3.4 3.3 3.5 4.4 7.2 7.6 6.8
0.1 3.9 4.2 4.8 4.6 3.1 3.5 2.9
1.08 1.09 0.95 0.92 1.02 0.62 0.41 0.57
2.9 2.7 2.9 2.7 3.1 4.7 5.0 5.7
acid-soluble lignin
Hardwood
Softwood
a
Compounds extractable by hexane. bCompounds extractable by 80% aqueous ethanol. equipped with automatic temperature controller to raise the pellets temperature to 230, 260, 280, and 300 °C. Argon flow rate into reactor was 0.015 m3·min−1. After the required temperature of the pellets was reached, the isothermal heating was prolonged for 10 min. 2.4. Wood Component Composition. Before analyses, the pellets, ground in a knife type crusher Retsch 100 (Retsch GmbH, Germany) using sieve (0.5 mm), were additionally fined in a Retsch Mixer Ball Mill MM200 (Retsch GmbH, Germany) at the frequency of 30 s−1 for 30 min, and the fraction with particle size d < 0.05 mm was used for the analyses. The samples were dried at 50 °C in vacuum for 24 h and then were kept in a desiccator until analysis. 2.4.1. C, H, and N Elemental and Ash Content. The C, H, and N contents were measured according to LVS EN 15104:2011 using a Vario MACRO elemental analyzer (ELEMENTAR Analysensysteme, Germany). Ash content was measured as a residue after ignition at 550 °C in a Carbolite ELF 11/6B furnace (Turkey). 2.4.2. Content of Extractable Lipophilic and Hydrophilic Compounds. Sequential extraction with hexane (Soxhlet apparatus with average number of percolation 5−6 per hour) followed by 80% aqueous ethanol (batch extraction) was performed on the samples of parent and microwave-treated wood. The yield of lipophilic and hydrophilic extractives (hexane and 80% ethanol extractable compounds, respectively) was determined gravimetrically after evaporating the solvent to dryness at 50 °C under vacuum. 2.4.3. Lignin Content. The acid-insoluble lignin content in parent and microwave-treated extractive free wood samples, known as Klason lignin, was determined by the Tappi Standard method, T 222 om-98, while the acid-soluble lignin content was quantified according to Tappi Standard Method UM 250 as described in ref 15. 2.4.4. Cellulose Content. The cellulose content was measured in extractive free wood samples using the Kürschner and Hoffer method (treating the wood with the concentrated nitric acid in ethanol medium).16 Pentosane composition was determined by an alditol acetate procedure17 after treatment with 72% sulfuric acid for 1 h at room temperature; the acid was diluted with water to give 4% sulfuric acid and heated at 121 °C for 1 h. Alditol acetates were quantified by gas chromatography (GC, Agilent 6850 Series GC System, column TC17) using myo-inositol as an internal standard. The results are expressed as arabinan and xylane content. All analyses were done in triplicate at least. The measured values are shown as an average with a confidence interval at level of significance α
pyrolysis (Py-GC/MS/FID), Fourier transform infrared (FTIR) spectroscopy, and routine chemical analysis were used. In addition, total yield of lipophilic and hydrophilic wood extractives was monitored depending on the microwave treatment temperature. For estimation of the nonthermal effect13,14 on the process of biomass thermal transformation at microwave treatment, the characteristics of wood lignocellulosic matrix after microwave and convective heating were compared.
2. MATERIALS AND METHODS 2.1. Wood Pellets. Commercial softwood pellets obtained from the mix of debarked pine and spruce wood (50:50 w/w ratio; diameter, 8 mm; length, 20−25 mm) as well as hardwood pellets from aspen wood (diameter, 6 mm; length, 15−25 mm) produced using a laboratory flat die pellet mill KAHL 14-175 (Amandus Kahl GmbH, Germany) were used for the investigation. The moisture content of the softwood and hardwood pellets was 7.5 and 7.4%, whereas the ash content was 0.32 and 0.51%, respectively. 2.2. Microwave Treatment. A microwave device of original design (Latvian State Institute of Wood Chemistry, Riga) was used that consisted of a tubular resonator (V = 0.04 m3), coated inside with a thermally insulating mineral blanket Cerablanket AC1 (Morgan Thermal Ceramic, Czech Republic), which is transparent toward microwave radiation. The coaxial waveguide was attached to the bottom of the resonator along it lengthwise axis. The rotated reactor from silica glass (V = 1300 cm3) with a polytetrafluorethylene frame was radially placed in the resonator. The device was equipped with a magnetron (microwave power, 0.9 kW at 2.45 GHz). The K-type thermocouple IP-67 (d = 6.4 mm), connected with a controller (Pixsys ATR142-ABC; ALLTRONICS PERU S.A.C), was attached inside the reactor, and the temperature of the pellets was measured. The pellets were heated in the argon atmosphere in the reactor placed inside of resonator up to a temperature varied in the range 120−300 °C at constant power rate. After the desired temperature was reached, the sample was maintained at this temperature in the reactor for 10 min isothermally. The reactor was rotated with a speed of 10 rpm throughout the treatment. 2.3. Treatment by Convective Heating. For the treatment of pellets using convective heating, they were placed on an aluminum pan then loaded into a stainless reactor and heated in an electrical furnace NABERTHEM L40 (Naberthem, Germany) with power rate of 6 kW 458
DOI: 10.1021/acs.energyfuels.5b02462 Energy Fuels 2016, 30, 457−464
Article
Energy & Fuels = 0.05. For the results obtained, the variation coefficient did not exceed 3%. All results are expressed on a dry-weight and ash-free basis. 2.5. FTIR Spectroscopy. Fourier transform infrared spectra of the samples were recorded in KBr pellets by a Spectrum One (PerkinElmer, UK) FTIR spectrometer in the range of 4000−450 cm−1 (resolution, 4 cm−1; number of scans, 64). The resulting spectra were normalized to the intensity of maximum at 1510 cm−1, which is assigned to the aromatic skeletal vibrations in lignins.18 2.6. Analytical Pyrolysis. To determine the changes in lignin structure, pyrolysis-gas chromatography−mass spectrometry (Py-GC/ MS/FID) was applied. The Py-GC/MS analysis was performed using a Frontier Lab (Japan) Micro Double-shot Pyrolyser Py-2020iD (pyrolysis temperature, 500 °C; heating rate, 600 °C/s) directly coupled with the Shimadzu GC/MS-QP 2010 apparatus (Japan) with capillary column RTX-1701 (Restec, US), 60 m × 0.25 mm × 0.25 μm film (injector temperature, 250 °C; ion source with EI of 70 eV; MS scan range ,m/z 15−350; carrier gas helium at the flow rate of 1 mL/ min and the split ratio 1:30). The mass of a sample probe (residual moisture content
