Article pubs.acs.org/EF
Impact of Torrefaction on the Chemical Structure of Birch Wood Tooran Khazraie Shoulaifar,*,† Nikolai DeMartini,† Stefan Willför,† Andrey Pranovich,† Annika I. Smeds,† Tommi Antti Petteri Virtanen,‡ Sirkka-Liisa Maunu,‡ Fred Verhoeff,§ Jacop H. A. Kiel,§ and Mikko Hupa† †
Process Chemistry Centre, Åbo Akademi University, FI-20500, Turku, Finland Laboratory of Polymer Chemistry, Helsinki University, FI-00014, Helsinki, Finland § ECN Biomass, Energy Research Centre of The Netherlands (ECN), 1755 ZG Petten, The Netherlands ‡
ABSTRACT: Torrefaction is the thermal pretreatment of biomass at temperatures of 200−300 °C in an inert atmosphere with the objectives of improving resistance to biodegradation, reducing hydrophilicity, improving grindability and increasing energy density. In this work, we studied the effect of torrefaction temperature (240−280 °C) on the chemistry of birch wood. The samples were from a pilot plant at ECN, and in that way, they were representative of industrially produced samples. We have measured the concentration of hemicellulose and cellulose; changes in the extractives content and composition; and in the lignin structure. We used acid methanolysis and acid hydrolysis for hemicellulose and cellulose analysis, respectively; Klason lignin method, 13C CP-MAS NMR, Dipolar Dephasing NMR, and Py-GC-MS analysis for lignin characterization; and acetone extraction, HPSEC, GC-FID, and GC-MS analysis for extractives characterization. The results provide a more complete picture of the chemical changes in wood by torrefaction.
1. INTRODUCTION
torrefied biomass will be composed predominantly of cellulose and lignin and their degradation products. A limitation of TGA analysis is that it does not measure the composition of the residue after heat treatment. Therefore, other chemical methods should be applied to determine the chemical composition of the biomass after torrefaction. Accordingly, a better understanding of the torrefaction mechanism can be achieved. Several methods have been applied to determine the chemical changes in the course of the torrefaction. Sugar analysis has been utilized to study the carbohydrate degradation at mild heat treatment.1,7−9 Phuong7 and Brito8 observed a decrease in sugar content after heat treatment at temperatures as low as up to 200 °C. The Klason lignin method and thioacidolysis have been applied to determine the chemical changes in lignin.9−12 Some studies also focused on the extractive contents for low-temperature thermal treatment.13,14 In addition, different spectroscopic methods have been used to reveal the chemical changes during torrefaction.11,15,16 For instance, Shang16 observed a decrease in the concentration of carboxylic groups with FTIR after torrefaction. Likewise, Khazraie17 found the decrease in carboxylic group using quantitative wet chemistry. Melikor18 used NMR spectroscopy to study the deacetylation of hemicelluoses, demethoxylation of lignin and cellulose crystallinity. In the present study, the thermal decomposition during torrefaction of carbohydrates in birch wood, namely hemicelluloses and cellulose, was determined by analyzing the sugars using acid methanolysis19,20 (for hemicelluloses) and acid hydrolysis (for cellulose). The Klason lignin method was applied for the lignin content of raw birch, while Pyrolysis Gas Chromatography (PyGC-MS) and Cross-Polarization Magic Angle Spinning Nuclear
Torrefaction of biomass is a thermochemical pretreatment in the temperature range of about 200−300 °C under an inert atmosphere.1 This method improves the final characteristics of the fuel, including resistance to biodegradation, reduced hydrophilicity, and higher energy density. These features make the storage and handling of biomass and pellet production more convenient. In addition, better grindability and ignitability are obtained, which are beneficial in pulverized fuel combustion.2 During the torrefaction process, both condensable and noncondensable gases, with high oxygen content, are formed. This results in a char with a slightly lower O/C1 and, accordingly, an increase in the heating value of the resulting torrefied biomass. The increased heating value coupled with a decrease in the mass results in a higher energy density.2 The biomass major constituents including hemicellulose, cellulose, and lignin are relatively sensitive to the torrefaction temperature. Thermogravimetric analysis (TGA) of wood and its components indicates that thermal degradation of each component is independent of the other.3 TGA studies have been carried out with individual wood components to understand their stability at different temperatures. After evaporation of the water in the biomass, extractives start to degrade.4 At about 200 °C, hemicellulose and lignin start to degrade, but only to a small extent.4,5 The macromolecules of the hemicelluloses start to decompose to corresponding oligomeric fragments, and then, short chains are depolymerized to monomer units and subsequently decomposed to volatiles.6 The extent of hemicellulose degradation increases significantly at about 250 °C, while lignin degradation is slight at this temperature. Cellulose degradation has been shown to be severe at 290 °C. Consequently, in the lower torrefaction temperature range, biomass degradation is mainly due to hemicellulose degradation, and to some extent, extractives can be partly evaporated or degraded. As a result, the © 2014 American Chemical Society
Received: February 26, 2014 Revised: May 2, 2014 Published: May 14, 2014 3863
dx.doi.org/10.1021/ef5004683 | Energy Fuels 2014, 28, 3863−3872
Energy & Fuels
Article
Magnetic Resonance (13C CP-MAS NMR) spectroscopy were utilized to analyze changes in the lignin structure. The latter method also revealed changes in carboxylic and acetyl groups. The formation/destruction of extractable materials is of interest for biorefinery concepts around fine chemicals. The total acetone-extractable content was measured gravimetrically, molar mass distribution was analyzed by High Performance Size Exclusion Chromatography (HP-SEC), and the chemical composition of the low-molar-mass extractable materials was determined by GC-MS. The results provide a better understanding of the chemical changes in the biomass during torrefaction, which is important both for controlling the torrefaction process and for better understanding the possibilities of using torrefaction in biorefinery concepts beyond mere fuel production.
DD NMR was applied to observe the changes in lignin during heat treatment. In DD NMR, the high power proton decoupling is switched off in a short period of time, allowing the signal from protonated carbons, excluding methyl carbons, to become dephased. Only the signal from quaternary and methyl carbons are then detected, making interpretation of the spectra from the lignin aromatic region more reliable. In this method, all spectra were measured using a Bruker Avance III 500 NMR spectrometer with a magnetic flux density of 11.7 T, using a 4 mm double resonance 1H(BB) VTN CP-MAS standard bore probehead. Samples were packed into ZrO2 rotors and plugged with KEL-F end-caps. The spinning rate was set to 10 kHz. A variable amplitude cross-polarization was used, with 55 kHz rf-field on the carbon channel, and an amplitude ramp going from 50% to a maximum of 70 kHz rf-field on the proton channel. The length of the contact time was 3.5 ms, and the spectral width was 35 kHz. During the acquisition period, the protons were decoupled using swept-frequency TPPM decoupling with 80 kHz rf-field strength. The length of the acquisition period was 30 ms. The dephasing delay in the dipolar dephased spectra was 120 μs. Sixteen thousand scans with 3.5 s delay between successive scans were collected for the dipolar dephased spectra. All spectra were referenced externally via adamantine by setting the low field resonance at 38.48 ppm. Rf-field strengths, decoupler offset, and Hartmann−Hahn match were calibrated using α-glycine as a standard. Py-GC-MS Method. Py-GC-MS was used to determine the changes in the content of p-hydroxyphenyl, guaiacyl, and syringyl units of lignin. Acetone extraction pretreatment was performed on the samples prior to Py-GC-MS. The details of the method have been described by Smeds et al.23 2.4. Extractives Analysis. The biomass was extracted using acetone/ water (v/v: 95/5) as solvent in an Accelerated Solvent Extractor (ASE-300). Accurately weighed freeze-dried samples of about 2 g were placed in the extraction vials and the rest of the vials were filled with carefully washed inert quartz sand. The operating temperature was 100 °C and the operating pressure was 10.34 MPa. Three static cycles were applied with each cycle lasting 5 min. The extracted liquids were collected in a glass tube, and finally, the levels of all extraction liquids were adjusted to 50 mL with acetone. The extracted materials were quantified by three different methods. In the first step, the total extracted materials were quantified by weighing the sample after evaporating the solvent to dryness at 50 °C in nitrogen atmosphere; and in the second step, the low-molar-mass compounds were analyzed by GC-FID and GC-MS. In the third step, the molar mass distribution of the extractable materials was analyzed by HPSEC (High Performance Size Exclusion Chromatography). In the first method, about 10 mL of the acetone solution, which was first adjusted to 50 mL by acetone, accurately weighed, was dried first under nitrogen flow at 50 °C until nearly dry, and then in a vacuum desiccator at 7 Pa and 40 °C for 2 h. The residue was weighed several times during drying until a constant weight was obtained. The result was the concentration of acetone-extractable material. In the second method, the samples were analyzed by GC-FID according to the method first described by Ö rså and Holmbom.24 GC-FID was used for quantification and GC-MS for identification of the extractives. In the third method, HPSEC, about 1 mL of acetylation mixture (acetic anhydrate/pyridine 50/50 v/v) was added to 5 mg of the dried extracts. The mixture was left in dark place at room temperature, while it was shaken once a day. After 5 days, 2 mL of ethanol was added to the solution and the solution was dried in a flow of nitrogen at 40 °C. Then, 5 mL of tetrahydrofuran (THF) was added to the dried acetylated extractable material. The result was a clear transparent solution. After filtration of the solution into a vial, the HPSEC analysis was performed. The details of the method have been described in another study.25 Shimadzu GPC for CLASS VP software was used to process the HPSEC data. The peak area of the chromatograms was calibrated by analyzing triglyceride, steryl esters, fatty acids, and resin acids, each at 0.5 mg/mL. It should also be noted that the molar mass scale is calibrated based on different molecular weight polystyrene and, thus, not very accurate for the unknown complex mixtures of extractable materials; however,
2. EXPERIMENTAL SECTION 2.1. Torrefaction and Sample Preparation. Torrefaction was performed in a pilot plant at ECN, the Energy research Center of The Netherlands. The feed stock of this work was commercial birch wood which was supplied to ECN in The Netherlands. The dimensions of the birch wood chips were smaller than 40 × 40 × 2 mm, while dusts (particles
