Article pubs.acs.org/EF
Novel Gas Chromatography−Mass Spectrometry Methods for Characterization of Volatile Organic Compounds and Water in Fast Pyrolysis Liquids Michael Windt,*,† Akeem M. Azeez,‡,§ and Dietrich Meier† †
Thünen-Institute of Wood Research, Leuschnerstrasse 91b, D-21031 Hamburg, Germany Department of Chemistry, University of Ado-Ekiti, PMB 5363, Ado Ekiti, Ekiti, Nigeria § Department of Chemical Engineering, University of KwaZulu-Natal, Durban 4041, South Africa ‡
ABSTRACT: A novel method for the analysis of volatile organic compounds (VOCs) in fast pyrolysis liquids has been developed. Using a full evaporation technique (FET), the total concentrations of major volatiles in old and fresh biocrude oil (BCO) samples ranged between 0.71 and 2.42 wt %. Formaldehyde, methanol, methyl formate, and acetic acid methyl ester (AAM) were the dominant VOCs across all samples. The results revealed that concentrations and types of VOCs in BCOs largely depend upon their feedstock type and storage period. After 24 months, only a quarter of the initial formaldehyde concentration in beech-derived BCO was left, whereas the amount of AAM doubled within the same period. Make-up additives, such as methanol, ethanol, or isopropyl alcohol, often used to homogenize BCO products, were detected in BCOs obtained from commercial and semi-commercial production. Good reproducibility and high correlation of analytical responses of various calibration standards used in the quantitative evaluation of the VOCs illustrate the suitability of a FET-headspace for the analysis of VOCs in BCO samples. There was a close concordance between BCO water contents analytically determined by gas chromatography−mass spectrometry analysis and Karl Fischer titration, an indication of the former suitability as an alternative. Joensuu, Finland.16 Meanwhile, the commercialization of BCO for various applications requires the acceptability of its quality standards to end users.17 With the upward trend in the use of BCO, the evaluation of its technical qualities, such as heating value, viscosity, and pH, as well as its degree of health and explosion hazards will in the future be of great interest and concern to employees or consumers. Monitoring of volatile organic compounds (VOCs) in biological and chemical samples is crucial for determination of flammability and potential health and environmental risks. BCO is a highly odorous chemical product with various lowmolecular-weight organic compounds. Some of these include various volatile organic acids, aldehydes, and ketones. Volatiles, such as formaldehyde and acetaldehyde, both present in BCO, have been observed to pose health hazards to humans.18 The determination of VOCs in BCO is therefore desirable, as a tool for monitoring its potential health and environmental risks. In general, the amount of volatiles in liquid and solid samples can be determined using titrimetric, spectrophotometric, or chromatographic methods. Unlike the chromatographic technique, both titrimetric and spectrophotometric methods lack specificity and sufficient sensitivity.19 In contrast, headspace gas chromatography (HS-GC) is the chromatographic method of choice in the determination of VOCs in complex solid- and liquid-phase samples.20,21 It offers easy sample preparation from a wide range of concentrations and sample types and has reliable response results. The response however is
1. INTRODUCTION Fast pyrolysis is an efficient conversion technology employed in the transformation of renewable lignocellulosic materials to fuel and chemical products. The liquid product obtained from this conversion process is known as fast pyrolysis liquid, bio-oil, or even better, biocrude oil (BCO). BCO is an acidic, viscous, and highly oxygenated complex mixture of chemical components with varying functionality, polarity, and degree of polymerization. A whole bundle of energy-related applications can be envisaged, with its prospective industrial use as fuel in boilers, gas turbines, and diesel engines to generate heat and power.1,2 This organic mixture of several tens of identified chemical products3,4 contains low volatile compounds but also holds significant amounts of non-volatile compounds, such as oligomeric lignins and decomposed sugars. Hence, BCO exhibits potential as a platform material in the production of valuable chemicals, such as flavoring agents, renewable resins, and fertilizers.5−7 However, the limitations of BCO as a direct substitute for petroleum-derived fuels in many internal combustion engines stems from some of the aforementioned properties. To address these deficits in BCO, several upgrading technologies have been developed toward improving its fuel properties. These include various catalytic hydrotreating and hydrocracking options that have been extensively discussed by various authors.8−11 Recent successes and prospects of these upgrading technologies have heightened growing interests in the use of BCO as an energy fuel as well as in the generation of biohydrogen.12−15 Consequently, in addition to a number of medium-scale plants involved in the production of BCO that are located around the world, the first industrial BCO plant has just been established in © 2013 American Chemical Society
Received: July 10, 2013 Revised: October 29, 2013 Published: November 7, 2013 7413
dx.doi.org/10.1021/ef4013104 | Energy Fuels 2013, 27, 7413−7423
Energy & Fuels
Article
Table 1. BCO Samples, Unit, and Year of Productiona production 1 2 3 4 5 6 7 8
ID
wood type
product
capacity
site
year
pyrolysis scale/technology
DYN/BFB-04 REF/CFB-05 BE/BS-10 BE/PU-12 BE/BS-12(R) BE/BS-12(F) BE/BS-12(C) SP/BS-12
hardwood mix pine beech beech beech beech beech spruce
BCO BCO BCO BCO BCO BCO condensate BCO
100 tons/day 20 kg/h 500 g/h 5 kg/h 500 g/h 500 g/h 500 g/h 500 g/h
external external internal internal internal internal internal internal
2004 2005 2010 2012 2012 2012 2012 2012
CP/BFB PDU/CFB BS/BFB PU/BFB BS/BFB BS/BFB BS/BFB BS/BFB
a CP, commercial plant; PDU, process demonstration unit; PU, pilot unit; BS, bench-scale unit; CFB, circulating fluidized bed; BFB, bubbling fluidized bed; (R), stored 6 months at room temperature; (F), stored 6 months in a freezer; and (C), condensate from an intensive cooler.
the co-pyrolysis of plastics and pine biomass comparison of different techniques of HS analyses established excellent precision and good reproducibility for SHE-GC.32 This study describes the development of analytical methods for modest determination of VCOs in BCO. Static HS sampling was carefully designed to obtain representative volatile solutes, and analysis of these VOCs was carried out using GC−flame ionization detector (FID)−MS. The developed method was optimized using a series of standard calibration and different BCO samples. In addition, analytical means of determining associated water content in BCO using GC−FID−MS was also explored and compared to Karl Fisher titration.
affected by the sample matrix, especially when dealing with complex samples.22 HS analysis starts generally with sample conditioning at a selected temperature with subsequent sampling of the vapor phase directly above the liquid or solid samples (HS), sealed in a gastight vial. This procedure ensures analysis of only volatile components, thereby eliminating the matrix effects that several heavy components in the analysis would have induced. The vial-containing sample is isothermally heated until the vapor− liquid equilibration of the sample inside is established. An aliquot of the vapor phase obtained through the use of a gastight syringe is transferred to a gas chromatograph for separation, detection, and quantification. This technique is called static headspace extraction gas chromatography (SHEGC). In another modification, the syringe is replaced with a heated transfer line. The sample vial is pressurized above the capillary column head pressure. This modified technique enables better inert sampling, efficient sample transfer, and complete equilibration.23 Conformably, the vapor phase can be trapped on a sorbent or in some way collected prior to its transformation onto the GC column. This method is called dynamic HS extraction (purge and trap). An example of this is headspace solid-phase microextraction (HS-SPME).23,24 The method has found applications in analysis of VOCs, where high concentrations of analytes of interest are required. Using the same equipment, procedural modification in the analytical step may be introduced, thereby giving rise to another HS method. For instance, a stepwise gas extraction procedure carried out at an equal time interval with the usual HS equipment is termed multiple headspace extraction gas chromatography (MHE-GC). This method was initially developed for the analysis of monomers in polymers and residual solvents in printed films, but it can also be applied to the liquid sample.25 Exhaustive discussions on the theory of HS methods are detailed in many publications and reviews.22−24,26 HS analysis generally has found applications in the analysis of environmental, pharmaceutical, biological, and forensic samples.23,27−30 The SHE-GC technique was employed in the determination of the monoaromatic volatile benzene, toluene, ethyl benzene, and xylene (BTEX) group in olive oil and olives.31 Oil samples were directly introduced into HS vial for analysis, without any sample pretreatment. Results obtained from the mass spectrometry (MS) signals monitored in selected ion monitoring (SIM) mode found overall BTEX concentration levels of 4.2−87 and 23−332 μg kg−1 in olives and olive oil, respectively. In another aspect, the determination of 15 aromatic hydrocarbons from solid residues produced during
2. MATERIALS AND METHODS 2.1. BCO Samples. To ensure comprehensive analysis of volatile products in BCOs, in-house-produced and externally obtained BCO samples were considered. These samples were derived from fast pyrolysis of different raw materials. Beech and spruce sawdust were pyrolyzed on a bench-scale (BS) system to obtain bio-oils. Additionally, BCO from beech was produced from a pilot unit (PU). Both BS and PU are bubbling fluidized-bed (BFB) pyrolysis conversion technologies. They have been described in detail elsewhere.8,33 The external samples were obtained from a circulating fluidized-bed (CFB) process demonstration unit (PDU) and a commercial BFB plant (CP). They were stored for several months and years. In this regard, it is noted that environmental conditions, such as the temperature, humidity, and light, could influence the longterm release of VOCs. In addition, also unfavorable storing conditions, especially deviations of the bunker volume, could impact the release of VOCs. For this work, it was impossible to consider all of these aspects for sample preparation, and therefore, the presented results will only provide a proof of principle. The BCO samples are detailed in Table 1. 2.2. Analytical Methods. 2.2.1. Analyses of Conventional GCDetectable BCO Fractions. The GC-detectable fractions in samples listed in Table 1 were analyzed using GC−MS/FID. In a typical analysis, about 60 mg of the sample was dissolved in 1 mL of acetone, containing a known amount of fluoranthene (∼200 μg/mL) as an internal standard (IS). The analysis was carried out on an Agilent 6890. The following analytical conditions were used: split injection, 1:15; injection temperature, 250 °C; and injection volume, 1 μL. The separation was performed on a 60 m × 0.25 mm VF-1701-MS (Varian) fused-silica column, containing 14% cyanopropyl-phenylpolymethylsiloxane (0.25 μm film thickness). The oven program was as follows: held constant at 45 °C for 3 min, heated to 280 °C with 4 °C/min, and held for 20 min. Helium was used as the carrier gas with a constant flow of 2 mL/min. The system was equipped with a FID and MS detection. Electron impact mass spectra were obtained on a HP 5975b MS unit at 70 eV ionization energy. Various chemical constituents of samples were determined by comparison to mass spectra of authentic compounds and mass spectra in the National 7414
dx.doi.org/10.1021/ef4013104 | Energy Fuels 2013, 27, 7413−7423
Energy & Fuels
Article
The sample was incubated at 145 °C for 5 min, after which 2.5 mL of VOC vapor volume was automatically drawn through a heated HS syringe (150 °C) and injected (split ratio of 1:10). GC was fitted with a Tenax filled quartz liner, and its cryofocus function was enabled starting with an initial temperature of −20 °C, followed by ramping at 12 °C/s to 250 °C, and then held for 2 min. Separation was carried out on a 60 m × 0.25 mm VF-1701MS (Varian) fused-silica column, containing 14% cyanopropyl-phenyl-polymethylsiloxane (0.25 μm film thickness). The oven was held constant at 37 °C for 10 min, followed by a ramp at 20 °C/min until 280 °C. Helium was used as the carrier gas with a constant flow of 2 mL/min. The system was equipped with a parallel FID and MS detection. Electron impact mass spectra were obtained on HP 5975b MS using 70 eV ionization energy. Identification and evaluation of components were performed as previously described above.
Institute of Standards and Technology (NIST) library as detailed in previous works.4,34 Furthermore, to explore the possibility of determining the water content in BCO using GC−MS, a series of analytical trials with varying amounts of deionized water was carried out. In a typical trial, 1 mL of oil was pipetted into a vial, a known amount of deionized water was added, and subsequent mixing was performed with a multi-axle rotating mixer for around 1 day. Analytical preparation for these mixtures was implemented with 60 mg of sample dissolved in 1 mL of acetone with IS fluoranthene (∼200 μg/mL). The GC method used above was largely unchanged but not the heating rate. The oven was heated at 20 °C/min from 20 to 280 °C and held for 20 min to monitor m/z 18 (water) and m/z 202 (fluoranthene). SIM mode was employed for the analysis. 2.2.2. Analyses of Volatile BCO Fractions. All HS analyses of samples were carried out using a Gerstel multipurpose sampler (MPS XL). The robotic device was fixed on an Agilent 6890 GC, fitted with a FID and a HP 5975B MS parallel. Two methods were designed for the HS analysis of VOCs in BCO. The first method (A) involved the dilution of the sample with a known amount of water, followed by the introduction of calibration mixtures, while in the second method (B), standard-spiked pure samples were prepared without water. Detailed descriptions of these methods are outlined as follows: 2.2.2.1. Method A. About 500 mg of the BCO sample was weighed into a 20 mL of HS vial. Three sets of calibration mixtures, a, b, and c (as shown in Table 2), were used for a two-level standard addition
3. RESULTS AND DISCUSSION 3.1. GC Analysis with Liquid Injection of BCO Samples. BCO can easily be characterized by the analysis of its acetone-soluble organic fraction on the GC−FID/MS system. Using this method, up to 40 wt % of the whole BCO sample has been identified and discussed elsewhere.35,36 The chromatogram of BCO sample BE/BS-10 using the aforementioned conventional analytical procedure is shown in Figure 1. Only the retention time section
