Article pubs.acs.org/EF
Fast Pyrolysis of Heartwood, Sapwood, and Bark: A Complementary Application of Online Photoionization Mass Spectrometry and Conventional Pyrolysis Gas Chromatography/Mass Spectrometry Liangyuan Jia,† Felipe Buendia-Kandia,† Stéphane Dumarcay,‡ Hélène Poirot,† Guillain Mauviel,† Philippe Gérardin,‡ and Anthony Dufour*,† †
Laboratoire Réactions et Génie des Procédés (LRGP), Centre National de la Recherche Scientifique (CNRS), Université de Lorraine, École Nationale Supérieure des Industries Chimiques (ENSIC), 1 Rue Grandville, 54000 Nancy, France ‡ Laboratoire d’Étude et de Recherche sur le Matériau Bois (LERMAB), Faculté des Science et Technologies, Université de Lorraine, Boulevard des Aiguillettes, BP 70239, 54506 Vandoeuvre-les-Nancy, France S Supporting Information *
ABSTRACT: Wood offers important potential for biofuel or chemical production by fast pyrolysis but exhibits variable chemical composition that impacts pyrolysis product composition. Here, fast pyrolysis of heartwood, sapwood, and bark isolated from Douglas fir (softwood) and oak (hardwood) was studied by a microfluidized bed reactor (MFBR) combined with single photoionization mass spectrometry (SPI−MS) to provide insights into the wood zone effects on the composition of pyrolysis volatiles. The difference in pyrolysis volatile composition has been clearly unraveled by principle component analysis (PCA) based on the major ions detected by SPI−MS. Some specific product markers have been defined for each wood zone (heartwood, sapwood, and bark) and related to the chemical composition of wood samples (lignin, carbohydrates, and minerals). The catalytic effect of minerals (notably potassium) has a higher impact on carbohydrate decomposition than on lignin decomposition for a given wood type. Therefore, sapwood and heartwood (for both oak and Douglas fir) can be clearly discriminated by specific markers mainly from carbohydrate pyrolysis. Interestingly, our results show that the wood cylinders exhibit a more marked wood zone effect on product compositions compared to fine powders. SPI−MS results were further compared to those of pyrolysis gas chromatography/mass spectrometry (Py−GC/MS), and many of them are consistent. MFBR combined with SPI−MS is a selective analytical technique to figure out the effect of wood composition on pyrolysis volatiles. ecosystems.10 Another interesting finding indicated that heartwood presents a larger amount of organic extractives, which have an influence on its shrinkage behavior and further affect the quality of household furniture.7 Numerous analytical techniques have been developed to study the differences in physical and chemical composition between different wood zones in various tree species. Wood extractives, such as resin acid, free fatty acid, and phenolic compounds, are considered to be an indicator to detect heartwood because they are usually more concentrated in heartwood compared to sapwood. The distribution of extractives across heartwood and sapwood has been determined by gas chromatography−flame ionization detector/mass spectrometry (GC−FID/MS) and ultraviolet (UV) spectroscopy.11,12 Time-of-flight secondary ion mass spectrometry (TOF−SIMS) was used to discriminate heartwood and sapwood and to further determine their syringyl-to-guaiacyl (S/G) ratios.5,13 Hydrothermolysis products of poplar sapwood, heartwood, and bark were investigated by nuclear magnetic resonance (NMR) spectroscopy, and it was shown that there was a fundamental difference in the behavior of the poplar sapwood and heartwood that might arise from different
1. INTRODUCTION The stem of woody biomass is composed of heartwood, sapwood, and bark, whose differences in properties and chemical compositions are well-known for some tree species.1,2 The heartwood is the inner part of the stem and can often be visually discriminated by its darker color, caused by relatively larger amounts of lignin and extractives, such as flavonoids. In most cases, heartwood is characterized by its higher natural durability, lower porosity, lower moisture content, and less living cells in comparison to the outer part of the stem, named sapwood, which serves as a storage site for water and starch. Sapwood is physiologically active, and its primary role is to conduct water from roots to the crown.2,3 There are few previous studies devoted to bark conversion and its effect on the valorization of wood. Bark can represent a large fraction of tree biomass and is rich in extractives and lignin, which can provide an added value.4 A better understanding of the difference between heartwood, sapwood, and bark makes sense to many fields involving archeology,5 Kraft pulping,6 household articles,7 and biomass valorization to fuels or chemicals.8 The valuable oils extracted from red cedar heartwood, sapwood, and bark exhibit different contents and compositions.9 Studies related to variation ranges and mean values of the carbon concentration in heartwood, sapwood, and bark as a function of the stem height and species are conducive to their commercial or ecological use in forest © XXXX American Chemical Society
Received: January 10, 2017 Revised: March 13, 2017 Published: March 13, 2017 A
DOI: 10.1021/acs.energyfuels.7b00110 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels distributions of the guaiacyl and syringyl units.14 Other techniques, such as Fourier transform infrared spectroscopy (FTIR), UV microscopy, Raman spectroscopy, and scanning electron microscopy (SEM), have also been applied to study some chemical and structural changes in heartwood and sapwood.3,15−17 For inorganic minerals, the distribution of major nutrients in different wood zones was systematically explored by inductively coupled plasma (ICP) spectrophotometry.18 Previous studies demonstrated that pyrolysis behaviors and product quality were strongly dependent upon the biomass type, particle size, and pyrolysis conditions.19−21 Even in the same stem of woody biomass, the different chemical compositions of heartwood, sapwood, and bark may have a significant effect on the product distribution. Therefore, it is important to better understand the effect of wood composition variability on pyrolysis products. Pyrolysis gas chromatography/mass spectrometry (Py−GC/MS) is the most commonly used method to investigate these differences in the wood pyrolysis product. Loureco et al. has performed Py−GC/MS to determine the S/G ratio and the carbohydrate-to-lignin (C/L) ratio of pyrolysis vapors from different tree species.6,22−24 The effects of eastern red cedar wood zones were also investigated by Py−GC/MS. It was shown that heartwood produces different yields of ketones, cedrenes, and lignin-derived products compared to sapwood depending upon pyrolysis conditions.8 Despite all of these interesting studies, there is still an important lack of work that relates the effect of stem wood zone (heartwood, sapwood, and bark) on the composition of primary pyrolysis vapors. Single photoionization mass spectrometry (SPI−MS) is a soft ionization method, and it has been widely used for online analysis of combustion products and solid fuel pyrolysis.25−27 A significant advantage of SPI−MS over other techniques is its ability to monitor the thermochemical processes in real time and to improve the mass spectrometric analysis of fragile compounds, in particular for biomass feedstocks.28−33 Therefore, SPI−MS may be an interesting method to study the effect of the stem wood zone on pyrolysis products. In this study, the product difference in heartwood, sapwood, and bark pyrolysis from Douglas fir and oak was characterized by SPI−MS combined with a homemade microfluidized-bed reactor (MFBR). This method has been proven to be a powerful technique to study fast pyrolysis of different types of biomass particles (powder, lamella, pellet, etc.) under controlled hydrodynamic and mass and heat transfer conditions.21,34,35 Meanwhile, Py−GC/MS experiments were also performed and compared to SPI−MS results. To the best of our knowledge, we present here the first work that combines MFBR−SPI−MS and Py−GC/MS to study the effect of the wood zone on pyrolysis products.
Figure 1. Picture indicating the position of samples from Douglas fir and oak wood disk (ρds, density of Douglas fir sapwood; ρdh, density of Douglas fir heartwood; ρos, density of oak sapwood; and ρoh, density of oak heartwood). for all characterizations and pyrolysis experiments. Cylinder particles (outer diameter of 6 × 20 mm) were also produced from the same wood disks (Douglas fir and oak) and similar positions. Their length is in the direction of fibers as we reported previously.21 The bulk density of oak heartwood (ρoh) is higher than oak sapwood (ρos) (about 0.753 versus 0.693 kg/m3), while the reverse is true for Douglas fir (ρdh/ρds ≈ 0.532/0.632) (for these harvested stems). To provide a more detailed characterization of the biomass samples, the compositional analysis of the six kinds of powder samples (including carbohydrate and lignin contents) was conducted. It was not possible to quantify extractives on these samples by conventional Soxhlet extraction as a result of the low available mass of biomass (sampled locally in wood disks). Biomass chemical composition was analyzed by neutral detergent fiber (NDF) and acid detergent fiber (ADF). NDF and ADF were determined using the Ankom technology methods for fiber analysis in an Ankom 200 fiber analyzer and Ankom fiber filter bags of 25 μm porosity.36 Lignin was determined with the common Klason lignin method by National Renewable Energy Laboratory (NREL) protocols.37 Cellulose was calculated as the difference value between ADF and lignin, whereas hemicellulose content was computed as the difference between NDF and ADF. The mineral element analysis was performed by nitric acid digestion followed by inductively coupled plasma optical emission spectrometry (ICP−OES, iCAP 6000 Series, Thermo Fisher Scientific, Waltham, MA, U.S.A.). The quantification was calibrated with certified references in CNRS, Nancy. 2.2. MFBR Combined with SPI−MS. The design and operation of MFBR combined with SPI−MS have been previously reported.21,28,38 Briefly, the MFBR with an inner diameter of 20 mm was designed for biomass fast pyrolysis. The MFBR is heated by an electric furnace with a quartz window, which allows for real-time visualization of the hydrodynamics in the MFBR. All temperatures were controlled by temperature controllers and measured by K-type thermocouples. Fine wood powders could be stored in a specially designed injection rod and then introduced into the MFBR preheated at 500 °C, and the volatile products were sampled to SPI−MS via a heating transfer line heated at 250 °C. It must be mentioned that the temperature (500 °C) is the one of the fluidized sand and not of the pyrolysis reactions.38 Nitrogen was introduced into the reactor as the fluidizing gas (from bottom) and the sweep gas (from top). Both flow rates were monitored by mass flow controllers. The SPI−MS analysis was performed by a reflectron time-of-flight (RTOF) mass spectrometer equipped with a vacuum ultraviolet (VUV) ionization source based on a laser system (PhotoTOF,
2. MATERIALS AND METHODS 2.1. Preparation and Characterization of Biomass Samples. Douglas fir (softwood) and oak (hardwood) trees were harvested in the Haut-Beaujolais area (southeast France). Heartwood, sapwood, and bark blocks were isolated from the same disk of woods (Douglas fir and oak) as well as on the same radial position, for a total of six types of woody blocks. The position of these six samples is shown in Figure 1. From each block, 250 mg of fine powders with a particle size between 100 and 200 μm were produced and then divided into five even samples (50 mg/sample). Pyrolysis SPI−MS analysis was performed on 30 samples from six types of wood blocks and with five identical samples for each type. The particle size was kept constant B
DOI: 10.1021/acs.energyfuels.7b00110 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Table 1. Biochemical Composition and Mineral Content of Heartwood, Sapwood, and Bark from Douglas Fir and Oak samplea hemicelluloses cellulose lignin Al Ca Cr Cu Fe K Mg Mn Na Ni a
OH
OS
OB
DH
DS
DB
Biochemical Composition (Relative Distribution, wt % on a Dry Ash Extractive-Free Biomass) 27.1 17.5 5.2 21.8 22.2 38.4 50.1 48.8 42.1 47.1 34.5 32.4 46.0 36.1 30.7 Mineral Content (ppm and wt %) 179.1 121.5 152.6 113.8 119.6 491.6 657.9 29527.5 362.8 352.8 1.8 0.4 0.4 15.3 7.8 2.5 1.9 5.2 8.9 3.3 18.4 5.4 31.4 112.6 33.5 429.2 1155.9 1812.4 236.3 310.5 7.7 129.3 583.2 23.1 51.2 12.0 52.7 573.8 37.7 89.2 724.1 893.5 771.1 749.8 807.2 2.5 2.1 2.0 27.3 6.5
16.3 21.3 62.4 292.3 3055.8 0.6 3.8 17.7 2042.4 373.4 574.4 816.5 3.3
OH, oak heartwood; OS, oak sapwood; OB, oak bark; DH, Douglas fir heartwood; DS, Douglas fir sapwood; and DB, Douglas fir bark. temperature for 14 min. The spectra were acquired, and their peak identification was carried out by comparing the mass spectrum to the National Institute of Standards and Technology (NIST) database and those reported in the literature.6,22−24,41 For semi-quantitative analysis, peak areas of identified molecules were normalized by the total areas of integrated peak to give the relative abundance of each species. All measurements for each sample were repeated twice to ensure the reproducibility of the results, and the standard deviation of integrated signals for each compound is indicated by error bars.
Photonion, Schwerin, Germany), which can generate VUV light with photon energy of 10.5 eV (118 nm) via the ninth harmonic of the Nd:YAG fundamental laser frequency.39,40 The repetition rate of the laser was 20 Hz. The TOF mass spectrometer covers the mass range of m/z 10−2000 with a mass resolution of 2000 at m/z 200. Data were recorded by Acqiris AP240 (Agilent Technologies) and displayed by a special software made by Photonion GmbH (Germany). 2.3. Statistical Analysis. Principal component analysis (PCA) is a powerful statistical tool to extract the significant differences in infrared and mass spectra of biomass pyrolysis, which contain dozens of peaks and are difficult to be reliably distinguished solely based on visual observation. PCA allows for extraction of the significant differences between conditions as opposed to noise or meaningless variation contained in the data and transforms the original ordinate system. The new ordinates are named principal components (PCs). The first PC (PC1) corresponds to the highest variance; the second PC (PC2) corresponds to the second highest variance; etc. SPI−MS combined with MFBR has an ability to provide the stable and reproducible pyrolysis results for PCA. More details about this method have been previously reported by our group.21 All raw mass spectra of biomass pyrolysis have been treated with the multiplicative scatter correction (MSC)/extended multiplicative scatter correction (EMSC) method to average the signal intensity of 43 key m/z peaks for each sample before application of PCA, using the Unscrambler software (CAMO software AS, version 10.3, Oslo, Norway). The MSC preprocessing method can remove multiplicative and additive effects from spectra. EMSC preprocessing can avoid removing important variations from the data, and it is an extension to conventional MSC. PCA in this work was performed on all 30 samples to present the difference in product distribution obtained from the three wood zones. 2.4. Py−GC/MS Analysis. Py−GC/MS analysis was performed as a more commonly used method for wood pyrolysis characterization and to supplement SPI−MS analysis. Py−GC/MS was conducted on a pyroprobe 5000 (CDS Analytical) combined with gas chromatography/mass spectrometry (Clarus580 GC/Clarus500 MS, PerkinElmer). The probe has a computer-controlled heating element and can hold the sample in the center of a quartz tube (25 mm length and 1.9 mm inner diameter). Approximately 0.5 mg of wood fine powder was loaded into the quartz tube for each test run. Then, the quartz tube was centered in the pyroprobe and subsequently purged with a continuous helium flow for 15 s. The pyroprobe temperature was set at 1200 °C/s to 500 °C and held for 15 s in a N2 atmosphere. After pyrolysis, the volatile products were sent to the GC injector via a transfer line heated at 250 °C and then separated by an AT-1701 column (60 m × 0.25 mm × 0.25 μm, Grace). The GC injector temperature was held at 250 °C, and the split ratio was set at 10:1. The temperature of the GC oven was set to maintain 45 °C for 3 min and then increase at a rate of 5 °C min−1 to 260 °C, holding the final
3. RESULTS AND DISCUSSION 3.1. Biochemical Composition of Untreated Biomass Samples. The chemical composition and mineral contents of heartwood, sapwood, and bark are presented in Table 1. These results highlight the very high variability of wood composition, even inside the same stem. Both Douglas fir and oak were found to have the highest cellulose content in sapwood (50.1 versus 47.1 wt %) and the highest lignin content in bark (46 versus 62.4 wt %). The total hemicellulose content decreases obviously in the order: oak heartwood, sapwood, and bark (OH, OS, and OB). However, Douglas fir sapwood (DS) shows similar hemicellulose content compared to the Douglas fir heartwood (DH), while Douglas fir bark (DB) contains a lower amount of hemicelluloses. The cellulose content of OS is a little higher than that of OB, while this difference reaches approximately 55% between DS and DB. Generally, the lignin content in heartwood is a little higher than that in sapwood for both Douglas fir and oak, in agreement with previous results obtained for other tree species.6,22 Norway spruce has shown a similar overall trend that heartwood contains more lignin and less cellulose than sapwood.12 Concerning the mineral contents in the various samples, wood and bark samples contain a high concentration in Ca, K, and Na, which have been proven to have catalytic effects on pyrolysis reactions.42−44 Many other inorganic elements (e.g., Cu, Cr, Ni, etc.) are also present in lower amounts (see Table 1). The content of some minerals, such as K, Ca, Mg, etc., is highly variable between wood zones, bark, and tree species (Douglas fir versus oak). Specific to heartwood and sapwood, the Na, K, Mg, and Mn contents of heartwood are noticeably lower than those of sapwood, indicating a potentially higher catalytic effect from these inorganic species during sapwood pyrolysis. Bark presents a very high content in minerals and C
DOI: 10.1021/acs.energyfuels.7b00110 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 2. Representative SPI mass spectra from the fast pyrolysis of Douglas fir and oak powder taken from different positions.
decomposition.30 The peak at m/z 60 in our SPI mass spectra can be mainly assigned to the LG ionization fragment rather than acetic acid (AC) based on its ionization energies. Indeed, the ionization energy of acetic acid is 10.63 eV (Table 2), which is higher than the photon energy of our laser ionization source (10.5 eV). m/z 60 could also be a molecular ion of hydroxyacetaldehyde (HAA), but the work of Holmes et al. suggests that it is easily fragmented by losing CO with an energy barrier of 10.40 eV,46 which is matched by the photon energy (10.50 eV). Therefore, most HAA should be fragmented to CH2OH2+. In comparison to OH, there is an apparent decrease in m/z 60, 114, and 128 signals as well as an increase in m/z 43 for OS pyrolysis products. The peak at m/z 43 (C2H3O+) is a well-known carbohydrate fragment, and it may originate from many pyrolysis products in our SPI mass spectra, including acetone (m/z 58), acetic anhydride (m/z 102), vinyl acetate (m/z 86), and 1-hydroxy-2-propanone (HA, m/z 74) as a result of dissociative photoionization. This is because all of these compounds can produce this same photoionization fragment (m/z 43) with energy barriers lower than 10.50 eV (photon energy of the VUV laser used in this experiment). Our previous study has demonstrated that m/z 128 is an important intermediate compound for cellulose pyrolysis.30 Its chemical structure has been proposed but still needs further evidence. For OB, there is an obvious increase of m/z 110 and 124 signals compared to OH and OS. It should be noticed that the peaks at m/z 142 and 144 are only important in OH. Anyway, the differences for lignin products between OH, OS, and OB are not obvious simply based on a visual observation of SPI mass spectra. For this reason, the PCA method has been conducted on the mass spectra to better identify the significant differences
especially much higher contents in Ca, K, Mg, and Mn than those of heartwood and sapwood for both wood species. 3.2. Wood Zone Effect on Pyrolysis Products from Fine Powder. The mass spectra (m/z 40−280) of primary products from heartwood, sapwood, and bark pyrolysis were obtained by SPI−MS, and all of them were time-averaged to obtain representative spectra (see Figure 2). The assignment of many ions (m/z) has been presented elsewhere,21,30,45 and it has been confirmed by Py−GC/MS analysis in this work. Table 2 presents the main peaks identified by SPI−MS with their molecular assignment and their ionization energy (IE). The IEs of some compounds were obtained by NIST and theoretical calculation results in the literature (marked “T” in Table 2).30 The main compounds identified by Py−GC/MS are also presented in the same Table 2. 3.2.1. Wood Zone Effect on Product Composition of Oak Pyrolysis. Panels a−c of Figure 2 show that wood zones have a higher impact on the pyrolysis products from oak carbohydrates than those from lignin. 4-Hydroxy-5,6-dihydro-2Hpyran-2-one (m/z 114) corresponds to a typical marker for hemicelluloses45 and is a dominant compound for heartwood products. This result is consistent with the chemical composition of oak heartwood (OH), which contains the highest amount in hemicelluloses. Peaks at m/z 60, 70, and 73 exhibit higher relative signals in OH compared to those in OS and OB. These peaks are markers of transglycosylation reactions, which are dominant when the catalytic effects of minerals are low.45 OH has indeed the lowest amount in Ca, K, Mg, and Mn. It should be pointed out that levoglucosan (LG), a typical molecule produced by transglycosylation, is difficult to be detected at m/z 162, owing to its low energy barrier of D
DOI: 10.1021/acs.energyfuels.7b00110 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Assignment of Pyrolysis Products for Douglas Fir and Oaka peak number
retention time (min)
MW (g/mol)
compound
origin
IEs (eV)
1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 19 20 21 23 25 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 48 49 51 52 53 54 55 56 57 58 59 60 61 62 63 65 66 67 68 69 70
5.56 5.80 5.92 6.01 6.72 7.30 8.28 8.98 9.61 10.38 12.69 13.46 13.70 14.61 14.92 15.56 16.92 17.04 17.15 17.95 18.15 19.35 20.07 20.64 20.94 21.45 21.57 21.69 22.43 22.79 23.18 23.77 24.28 25.51 26.05 26.72 26.91 27.59 28.49 29.01 29.68 30.01 30.56 31.15 31.22 31.50 31.72 31.89 32.15 32.59 33.42 33.67 33.89 34.18 34.56 35.87 36.48 37.27 37.55 37.64 38.32
68 56 58* 102* 82 86* 60 70 60 74* 74 104* 67 84 102* 96 98 98 116* 110 98 98 110 96 84 110 112 114 112 128 94 124/128 114* 108 116 138 122 142 152 152 144 144 150 164 166 126 110 154 102* 164 144 124 164 168 152 166/182 166 180 180 194 180
furan 2-propenal acetone acetic anhydride 2-methylfuran vinyl acetate hydroxyacetaldehyde (HAA) (E)-2-butenal acetic acid (AC), not detected by SPI−MS 1-hydroxy-2-propanone (HA) propanoic acid 1,2-ethanediol monoacetate pyrrole 2H-furan-2-one methyl pyruvate furfural 5-methyl-2(3H)-furanone 2-furanmethanol acetol acetate 2-acetylfuran 5,6-dihydro-2H-pyran-2-one 2-hydroxy-2-cyclopenten-1-one 5-methylfurfural 3-methyl-2-cyclopenten-1-one 2(5H)-furanone 6-methyl-4(1H)-pyrimidinone 3-methyl-2,5-furandione, not detected by SPI−MS 4-hydroxy-5,6-dihydro-2H-pyran-2-one cyclotene 2,3-dihydro-5-hydroxy-6-methyl-4H-pyran-4-one phenol guaiacol/furaneol 1,3-dioxol-2-one, 4,5-dimethylp-cresol 5-hydroxymethyldihydrofuran-2-one p-methylguaiacol phenol, 2,3-dimethyl3,5-dihydroxy-2-methyl-4-pyron 3-ethylguaiacol 4-ethylguaiacol 2,3-anhydro-D-mannosan 1,4:3,6-dianhydro-α-D-glucopyranose 2-methoxy-4-vinylphenol eugenol 4-propylguaiacol 5-hydroxymethylfurfural (5-HMF) catechol phenol, 2,6-dimethoxy2-hydroxybutanedial isoeugenol (trans) 2-hydroxymethyl-5-hydroxy-2,3-dihydro-4H-pyran-4-one 4-methylcatechol isoeugenol (cis) 4-methylsyringol vanillin homovanillin/4-ethylsyringol apocynin 4-vinylsyringol guaiacylacetone 4-allylsyringol propioguaiacone
C C C C C C C C C C C C ? C C C C C C C C C C C C ? C C C C H G C H C G H C G G C C G G G C H S C G C H G S G G/S G S G S G
8.88 10.10 9.70/10.30 10.00/10.14 8.38 9.20/10.04
