Article pubs.acs.org/EF
Application of Solid-Phase Microextraction (SPME) as a Tar Sampling Method Mozhgan Ahmadi,*,† Erik Elm Svensson,‡ and Klas Engvall† †
Chemical Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden Nynas AB, Raffinaderivägen, SE-149 82 Nynäshamn, Sweden
‡
ABSTRACT: This paper presents the result of an investigation of the potential use of solid-phase microextraction (SPME) as a tar sampling method. The SPME stationary phase used was 50 μm of polydimethylsiloxane (PDMS) coated on a fused silica fiber. Tar model compounds normally present in a producer gas from gasifiers, benzene, toluene, indane, indene, naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene, were used in the investigation. The adsorbed compounds were analyzed by injection into gas chromatography coupled to a flame ionization detector (GC− FID). The amount of adsorbed tar on the SPME fiber determined the detection and quantification limits for the method. The results showed that adsorption of tar model compounds on the SPME fiber increased with decreasing polarity. The adsorption of compounds increased with a decreasing temperature, enabling a possibility to tune the sensitivity of the method by changing the sampling temperature. Conclusively, SPME has a very high potential as a tar sampling method and, in combination with GC− FID trace analysis of tar, is a feasible application.
1. INTRODUCTION Gasification of biomass to produce a producer gas is an effective first step in the conversion of renewable feedstocks into chemicals or other useful energy forms. The possibility to use essentially all forms of biomass and also waste will most likely make gasification an important part of creating a future sustainable society. Nevertheless, during biomass gasification, undesirable products, such as tar, represents one of the major limitations for full commercialization of the technology. The amount of tar in the gas varies depending upon the type of biomass, gasifier, and gas cleaning. If present in a high enough concentration, tar may cause serious technical problems, such as clogging of pipes and fouling of sensitive equipment.1,2 In the case of syngas applications, trace amounts of tar also challenge the downstream gas upgrading to, e.g., motor fuels and chemicals.3 Techniques for removing tar have therefore been the focus of many publications in recent years.4 The different end user applications of the produced gas put different demands on the level of tar and also other impurities that can be tolerated, as shown in Table 1.5 These low levels of tar are very demanding in terms of chemical analysis of the final synthesis gas, and today, no viable tar sampling method is available. The standard tar sampling method6 is based on solvent extraction and final determination by weight and identification
by gas chromatography (GC). This method therefore requires very long sampling times to give accurate results for low tar levels, which is very impractical for process monitoring. Another method, the solid-phase adsorption (SPA) method7 is more rapid (45 min/sample) and, therefore, more suitable for process monitoring. Very little information regarding the detection and quantification limits of SPA has been reported in the literature, but a detection limit of 2.5 mg/Nm3 has been mentioned previously.8 To compensate for low tar concentrations in the SPA method, it is possible to increase the sample volume (>100 mL). However, with the increase of the sample volume, there is a risk to overload the SPA column, which may saturate the SPA column and result in a breakthrough of tar compounds present in the gas.9 Hence, there is a need for a reliable tar sampling method to measure tar concentrations at levels below 0.1 mg/Nm3, which in this context can be regarded as trace amounts. Furthermore, to be useful for process control, the tar sampling method also needs to be as rapid as the SPA method. Solid-phase microextraction (SPME) has previously been successfully used for trace analysis of polyaromatic hydrocarbons (PAHs) in water and exhaust gases from diesel engines.10 This study presents results from an investigation with the aim at developing a tar analysis method based on SPME as a trace tar sampling method for syngas applications from biomass gasification. The tar model compounds used in the study were benzene, toluene, indane, indene, naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene. Benzene is generally not defined as a tar but will in this work be termed as such for simplicity.
Table 1. Concentration Limit of Tar for Different End User Applications5 application
tar (mg/Nm3)
gas engine gas turbine syngas methanol synthesis fuel cell
8.33 1.39 1.12 0.115 0.00838 0.00269 0.000798 0.000845
b
LOD
c
1.95 0.664 0.199 0.0887 0.0194 0.00695 0.00556 0.00100 0.000194 0.0000821 0.000147 0.000134
concentration at 125 °C (mg/Nm3) LOQ
d
6.49 2.21 0.662 0.296 0.0645 0.0232 0.0185 0.00335 0.000646 0.000274 0.000489 0.000445
condensation limitb
LODc
LOQd
>8.33 >8.33 >8.33 >8.33 >8.33 >8.33 >8.33 >8.33 >8.33 4.50 3.73 3.41
11.4 5.37 1.69 0.694 0.120 0.105 0.0637 0.0228 0.0108 0.0149 0.0324 0.0363
38.0 17.9 5.64 2.31 0.399 0.351 0.212 0.0762 0.0361 0.0496 0.108 0.121
a
The upper concentration boundary for the analysis is set by the condensation limit, which is also included in the table. bThe maximum concentration under which no condensation was observed. cLOD = level of detection. dLOQ = level of quantification.
compounds but would simultaneously make it harder to detect the lighter compounds. Therefore, in the case of process monitoring, several parallel analysis vessels at different temperatures are needed to cover for all tar compounds simultaneously. The number of sampling points and their temperatures can be adjusted to match the specific needs. In the present work, the split ratio in the GC analyses was 1:100. Important to note is that a decrease of the split ratio down to splitless would further lower the detection and quantification limits. 4.3.5. Analysis Time. To be useful for process monitoring, the analysis method needs to be relatively rapid. The SPA method is regarded as the most rapid off-line method available today, where the sampling, elution, and analysis take around 45 min. The SPME method in the present work requires a slightly longer sampling time, no elution, and a similar GC program, resulting in a total analysis time of similar magnitude as for the SPA method.
major constituent. Other compounds will be present in minor amounts, such as light hydrocarbons. The formation of hydrocarbons with a lower number of carbons is more favored in gasification. For the sample matrix to interfere with the SPME analysis, using PDMS as the stationary phase, the major matrix compounds need to be able to compete with the tar compounds for the adsorption sites. Therefore, the major matrix compounds should be nonpolar and have a boiling point close to or above the sampling temperature. Most of the matrix compounds are nonpolar, but few have a boiling point close to the lowest sampling temperature considered in this work. It is only C5+ hydrocarbons that can be considered to be close to fulfilling these requirements. The amount of C5+ in producer gas is generally considered to be minute if aromatic hydrocarbons are excluded. Thus, compounds of the major matrix are unlikely to affect the adsorption of tar compounds. 4.3.4. Detection and Quantification Limits. The detection and quantification limits were determined by calculating the standard deviation of the noise observed from five blank 30 min runs. The detection and quantification limits in terms of peak height from the chromatograms were set to 3 and 10 times the standard deviation, respectively. The peak heights were correlated to the peak area for each compound to express the detection and quantification limits in terms of concentration. In Table 3, the condensation, detection, and quantification limits are shown for the analysis temperatures of 60 and 125 °C. The condensation limit was calculated by comparing the measured response to the calculated response. The calculated response is the response of the specific concentration at the specific temperature assuming no condensation. The analysis will not be accurate if the concentration of a compound is exceeding the condensation limit, because condensation will limit the concentration of the compound in the gas phase and, thereby, limit the access to the SPME fiber. At 60 °C, the detection limits are well below the limit previously reported for SPA (2.5 mg/Nm3) for all compounds. At 125 °C, this is satisfied for all compounds heavier than toluene. Analysis at trace levels below 0.1 mg/Nm3 (e.g., syngas production) will be possible at 60 °C for all compounds heavier than naphthalene. For compounds lighter than naphthalene, analysis of trace levels will be possible if the temperature at the sampling point is lower than 60 °C. In other words, a higher temperature at the sampling point would make it possible to analyze higher contents of the heavier
5. CONCLUSION The potential of applying SPME as a tar sampling method for tars have been verified using tar model compounds. The affinity between the stationary phase of the SPME fiber and tar compounds is the parameter controlling the adsorbed amount. The affinity is determined by the polarity of the stationary phase and the tar compounds. Because of employment of a nonpolar stationary phase in the present study, affinity was increased with a decreasing polarity. The polarity was mainly determined by the molecular weight of the tar compounds. A difference of several orders of magnitude was observed between the adsorbed amount of the lightest model compound (benzene) and the heaviest model compound (pyrene) in the investigated temperature range (20−160 °C). The temperature was shown to have a strong impact on the adsorption on the SPME fiber. The temperature may be used to tune the sensitivity of the SPME tar analysis method. An optimal temperature exists for each compound, where maximum sensitivity is achieved and condensation is avoided. Tar sampling by SPME was shown to be highly sensitive. The study demonstrates that the tar sampling with SPME and analysis with GC−FID is a suitable tar analysis method, with very low detection and quantification limits. The method is therefore identified as especially suitable for high-purity 3859
dx.doi.org/10.1021/ef400694d | Energy Fuels 2013, 27, 3853−3860
Energy & Fuels
Article
applications with trace amounts of tar, such as in the production of syngas.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +46-0-8-7906602. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work has been carried out within the Swedish Gasification Centre consortium. Funding from the Swedish Energy Agency and the academic and industrial partners is gratefully acknowledged.
■
REFERENCES
(1) Knoef, H. A. M.; Koele, H. J. Tar measurement protocol. Realisation of a Standard Procedure for Tar and Particle Content Determination in Producer Gas from Biomass Gasifiers; Biomass Technology Group (BTG): Enschede, The Netherlands, 1998; Novem-EWAB 9832. (2) Knoef, H. A. M.; Stassen, H. E. M. UNDP/WB Small Scale Biomass Gasifier Monitoring Programme, Vol. II: Detailed Performance of the Gasifier Installations; Danish Energy Agency: Copenhagen, Denmark, 1997. (3) Han, J.; Kim, H. Renewable Sustainable Energy Rev. 2008, 12, 397−416. (4) Hasler, P.; Salzmann, R.; Kaufmann, H. P.; Nussbaumer, T. Method for the sampling and analysis of particles and tars from biomass gasifiers. In Biomass for Energy and Industry; Kopetz, H., et al., Eds.; C.A.R.M.E.N.: Rimpar, Germany, 1998; pp 1600−1603. (5) Knoef, H. A. M. Handbook Biomass Gasification; Biomass Technology Group (BTG): Enschede, The Netherlands, 2005. (6) Neeft, J. P. A.; Knoef, H. A. M.; Zielke, U.; Sjöström, K. Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases, Version 3.1; International Energy Agency: Paris, France, 1999; Energy Project EEN5-1999-00507. (7) Brage, C.; Yu, Q.; Chen, G.; Sjöström, K. Fuel 1997, 76, 137− 142. (8) Brage, C. Personal communication; Chemical Engineering and Technology, KTH Royal Institute of Technology, Stockholm, Sweden, 2012. (9) Bielicka-Daszkiewicz, K.; Voelkel, A. Talanta 2009, 80, 614−621. (10) Ballesteros, R.; Hernandez, J. J.; Lyons, L. L. Atmos. Environ. 2009, 43, 655−662. (11) Pawliszyn, B.; Pawliszyn, M. Solid Phase Microextraction (SPME); Springer-Verlag: New York, 1997. (12) Chen, J.; Pawliszyn, J. B. Anal. Chem. 1995, 67, 2530−2533. (13) Yang, J.; Pawliszyn, J. Anal. Chem. 1994, 66, 844A−853A. (14) Martos, P. A.; Pawliszyn, J. Anal. Chem. 1997, 69, 206−215.
3860
dx.doi.org/10.1021/ef400694d | Energy Fuels 2013, 27, 3853−3860
