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Energy & Fuels 2007, 21, 3036-3043
Quantitative Measurement of Biomass Gasifier Tars Using a Molecular-Beam Mass Spectrometer: Comparison with Traditional Impinger Sampling Daniel L. Carpenter,* Steve P. Deutch, and Richard J. French National Renewable Energy Laboratory (NREL), 1617 Cole BouleVard, Golden, Colorado 80401 ReceiVed April 19, 2007. ReVised Manuscript ReceiVed June 27, 2007
Removal of tars produced during biomass gasification continues to be a technical barrier confronted by developers of commercial thermochemical conversion systems. Quantitative measurement of tar in the synthesis gas (syngas) stream is important to assess the effectiveness of cleanup and conditioning processes and verify the suitability of the cleaned syngas for its intended downstream use (e.g., catalytic conversion to liquid fuels, hydrogen recovery, or electricity production). In an effort to advance the art of gasifier tar measurement and address some limitations of traditional impinger sampling, we have investigated the use of a molecular-beam mass spectrometer (MBMS) sampling system as an alternative method for quantifying real-time tar concentrations in biomass gasifier-derived syngas. The 0.5 ton/day pilot-scale biomass gasification system of the National Renewable Energy Laboratory (NREL) has enabled direct comparisons between MBMS sampling and replicate impinger sampling during continuous operations. The results have shown some systematic differences between the methods, although they do appear correlated. Using a synthetic tar mixture, as well as actual corn-stoverderived syngas, experiments were carried out to compare the accuracy of the two methods. Both methods demonstrated good reproducibility, but the MBMS measurements appear to be more accurate. Tar concentrations determined from impinger sampling averaged 11-21% lower than expected, depending upon the compound. Average MBMS measurements were within 6% of the known values, demonstrating that the MBMS can be used to improve quantitative, continuous, real-time monitoring of gasifier tar.
Introduction Thermochemical conversion or gasification of whole biomass or biomass processing residues to CO and H2-rich synthesis gas (syngas) typically results in the formation of a process gas containing organic and inorganic impurities.1 Some inorganic impurities of concern include hydrogen sulfide, hydrogen chloride, and ammonia, as well as trace amounts of alkali metals. The organic impurities range from light hydrocarbons to highmolecular-weight polynuclear aromatic hydrocarbons (PAHs). The volatile, heavy organic fraction is commonly referred to as “tar”. “Tar” is defined by the European Committee for Standardization (CEN) as “all organic compounds present in the gasification product gas excluding gaseous hydrocarbons (C1 through C6)”.2 In general, the tar is considered a contaminant that must be removed before the syngas can be either processed to higher value products, for instance, by catalytic conversion to liquid fuels, or used as a direct fuel for power production in gas turbines or internal combustion engines. Cleanup and conditioning of syngas derived from thermochemical conversion of biomass has been a research focus of the National Renewable Energy Laboratory (NREL) for several years. Efforts are currently concentrated on high-temperature * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion, NREL/TP-570-25357, 1998 (http://www.nrel.gov/publications). (2) Biomass GasificationsTar and Particles in Product GasessSampling and Analysis, CEN/TS 15439:2006; European Committee for Standardization (CEN): Brussels, Belgium, 2006.
catalytic steam reforming of tars and light hydrocarbons to generate syngas suitable for alcohol fuels production. To this end, the Thermochemical Process Development Unit (TCPDU) of NREL, which includes an integrated tar reformer, is being used to investigate various aspects of biomass thermochemical conversion and catalytic tar reforming. The goal is to develop detailed mechanistic descriptions of these processes that can be incorporated into process engineering models and reactor designs to facilitate the development of economically viable, commercial-scale systems.3 Comprehensive, real-time gas analysis before and after the tar-reforming step is critical for understanding key issues, such as catalyst activity and deactivation kinetics. In addition, quantitative measurement of tar in the gas stream is important to assess the effectiveness of syngas cleanup processes in general and determine what level of residual tar in the cleaned syngas is acceptable for downstream applications. This measurement is difficult, however, because “tar” is a complex mixture of organic compounds, and its composition and properties vary remarkably depending upon the biomass feedstock and gasification conditions.1 Consequently, analytical systems must be sensitive and flexible enough to measure an enormous variety of organic compounds. The analytical system must also be able to obtain the sample at the gasifier process temperature (>400 °C) to avoid tar condensation prior to the measurement. Historically, a gas impinger train has been the method of choice for sampling biomass gasifier gas streams to obtain tar (3) Bain, R. L.; Dayton, D. C.; Carpenter, D. L.; Czernik, S. R.; Feik, C. J.; French, R. J.; Magrini-Bair, K. A.; Phillips, S. D. Ind. Eng. Chem. Res. 2005, 44, 7945-7956.
10.1021/ef070193c CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
Measurement of Biomass Gasifier Tars
samples for detailed analysis.4 The “Method 5” of the Environmental Protection Agency (EPA), originally developed for sampling contaminants in smoke stacks, is the basis for most impinger sampling today.5 The Biomass Gasification Task Working Group of the International Energy Agency (IEA) recently completed an impinger-based, standardized international protocol, CEN technical specification 15439, to assist developers and end-users of biomass gasification technologies in measuring tars in biomass producer gases.2 A gas impinger is a relatively simple device consisting of an inlet tube that extends nearly to the bottom of a gastight reservoir containing a quantity of solvent (2-propanol per the CEN specification) compatible with the tar being collected. The gas is allowed to bubble through the solvent and then exit from the gastight reservoir. The impinger is typically cooled to increase the condensation of tar into the solvent. Optimally, multiple impingers are connected in series to improve the collection efficiency of the overall system by allowing sequential cooling, resulting in staged condensation. Staged condensation also helps prevent aerosol formation, which may occur if the gas is cooled too rapidly. The volumetric flow of the dry, tar-free gas is measured upon exiting the impinger train, so that tar concentrations can be calculated. Advantages of impinger sampling include simple, inexpensive equipment and near-universal applicability to gasifier operating conditions. The impinger sampling train can be engineered to accept the hot sample with minimal preconditioning, ensuring preservation of the tar in the gas stream, and can sample a variety of organic compounds. However, because impingerbased sampling is carried out over some period of time (tens of minutes to an hour), it provides limited information about transient gasifier conditions. This makes it a poor choice for monitoring dynamic systems in real time, such as a catalytic tar destruction reactor that is undergoing deactivation on a relatively short time scale. In addition, the collection efficiency of the sampling train can vary with the gas flow rate through the impingers, the collecting solvent used, and the solubility of the organic tars being collected. The method also requires a high degree of expertise by the operator to ensure reproducible sampling. Xu et al.6 proposed a solvent-free “dry condenser” adaptation of this method. Moersch et al.7 developed an online method for measuring condensable hydrocarbons by difference using a flame ionization detector. These methods both appear suitable for measuring the total tar loading in biomass producer gas and have the important advantage of being simpler to operate than the IEA method. While it is important to know the total tar loading, it has also been suggested that a distinction be made between various tar compound classes (i.e., water-soluble organics, phenols, polyaromatic compounds, and “heavy tars”). Dependent upon the application, the different tars can have unique operational and environmental impacts.8 Although not essential for every application, a detailed chemical characterization of the tars could prove useful for optimizing process conditions to minimize certain types of compounds. To advance the art of tar measurement and address some of the limitations of impinger sampling, (4) Esplin, G. J.; Fung, D. P. C.; Hsu, C. C. Can. J. Chem. Eng. 1985, 63, 946-953. (5) U.S. Environmental Protection Agency (EPA). Method 5, Determination of Particulate Emissions From Stationary Sources; Federal Register, 1971; Vol. 36 (247), pp 24888-24895. (6) Xu, M.; Brown, R. C.; Norton, G.; Smeenk, J. Energy Fuels 2005, 19, 2509-2513. (7) Moersch, O.; Spliethoff, H.; Hein, K. R. G. Biomass Bioenergy 2000, 18, 79-86. (8) Hasler, P.; Nussbaumer, T. Biomass Bioenergy 2000, 18, 61-66.
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molecular-beam mass spectrometry (MBMS) is being investigated as a method for quantifying real-time tar concentrations in gasifier process gas. The technique of MBMS, whereby a molecular beam inlet system is coupled to a mass spectrometer, has been used extensively at NREL for over 2 decades to directly sample the condensable vapors produced during thermochemical conversion of biomass.9-11 This paper reports on a direct comparison of MBMS- and impinger-based methods while sampling both model tar compounds and raw, biomass-derived syngas. Experimental Section Impinger Sampling and Analysis. For this study, the impinger train was arranged according to the CEN technical specification2 with some modifications. Because the interest here was only in volatile organic compounds, no attempt was made at isokinetic sampling of the process gas. Instead, a simple hot sample port was used to conduct the process gas to a heated filter. The physical dimensions of the impingers were similar to the glass units described in the specification but, instead, were fabricated from 316 stainlesssteel using Swagelok fittings to provide a sturdier apparatus. Finally, the arrangement of the impingers was based on an earlier draft of the specification, whereby the first two impinger vessels in the sample train were held between 35 and 40 °C in a water bath, while the remaining four were held from -15 to -20 °C in a dry-ice/ 2-propanol bath. Typically, the collected impinger sample consists of three fractions, each analyzed separately. The first fraction is the aqueous phase from the first impinger; it contains condensed steam, watersoluble organic compounds, and varying amounts of remaining 2-propanol. The second fraction (organic phase) contains the combined condensate and solvent rinses of the remaining impingers. The third fraction consists of an additional 2-propanone washing of all impingers to remove any remaining residue. Water content in the first and second fractions is determined by Karl Fisher titration. All three fractions are then analyzed by gas chromatography/mass spectroscopy (GC/MS) to measure the quantity of individual volatile and semivolatile organic compounds collected. The GC/MS analysis used a 30 m × 250 µm silica column, 0.25 µm film (cross-linked 5% phenyl/95% methyl silicone) with helium carrier gas. The GC oven started at 50 °C for 3 min, ramped 5 °C/min to 150 °C and then 10 °C/min to 300 °C, and was held at 300 °C for 10 min. The MS was scanned from m/z 30 to 450 at 1.75 scans/s. Calibration of the organic compounds analyzed by GC/MS is done by injection of known concentrations of the compounds of interest in the collecting solvent and calculating the response factor for each compound. Aliquots of the three fractions then undergo rotary evaporation to gravimetrically determine residual tar content. The quantities of each organic compound collected, as well as the steam and gravimetrically determined tar, are then related to the cumulative volumetric dry gas measurement to calculate the concentration of each compound in the process gas. Figure 1 shows a flow diagram of the analytical protocol. MBMS Instrument. The MBMS used in these experiments was developed at NREL and has successfully been used to monitor a variety of thermochemical processes, including several offsite campaigns.12-14 The details of the design and operation of the MBMS are described elsewhere;15 a brief description follows. A molecular beam forms as the sampled gases/vapors are drawn through a 300 µm diameter orifice into the first stage of a three(9) Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1, 123-137. (10) Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1, 311-319. (11) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (12) Carpenter, D; Ratcliff, M; Dayton, D. Catalytic Steam Reforming of Gasifier Tars: Online Monitoring of Tars with a Transportable MolecularBeam Mass Spectrometer. NREL/TP-510-31384, 2002 (http://www.nrel.gov/ publications). (13) Gebhard, S. C.; Gratson, D. A.; French, R. J.; Ratcliff, M. A.; Patrick, J. A.; Paisley, M. A.; Zhao, X.; Cowley, S. W. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1994, 39 (4), 1048-1052.
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Figure 1. Flow diagram for analysis of tars collected by impingers.
stage, differentially pumped vacuum system. This free-jet expansion results in an abrupt transition to collisionless flow that quenches chemical reactions and inhibits condensation by rapidly decreasing the internal energy of the sampled gases. The result is that the analyte is preserved in its original state, allowing light gases to be sampled simultaneously with heavier, condensable, and reactive species. The central core of this expansion is extracted with a conical skimmer, located at the entrance of the second stage, and the molecular beam continues into the third stage of the vacuum system. There, components of the molecular beam are ionized using lowenergy electron ionization before passing through an Extrel CMS16 quadrupole mass analyzer. Experience at NREL has shown that, for the type of ionizer used here, 22.5 eV allows for sufficient ionization efficiency while minimizing fragmentation of larger molecules. The ions are detected with an off-axis electron multiplier, and spectra are generated from the measured signal intensity as a function of the ion molecular weight. The mass range of interest (up to m/z 750 with this system) is repeatedly scanned so that the time-resolved behavior of the system under study can be observed. Because the sample is introduced continuously by this technique, quantitative measurement of organic and inorganic constituents in the gasifier process stream can potentially be done once per second. The MBMS system is equipped with several integrated controls that facilitate sampling of a variety of chemical process streams. On-board temperature, pressure, and flow control is achieved with an OPTO 2217 Ethernet-based I/O control system interfaced with a PC. Mass flow controllers allow inert gases to be introduced for sample dilution and internal standards. Liquid standards are injected using two high-pressure liquid chromatography (HPLC) pumps. Data from each of these auxiliary channels are collected for subsequent quantitative analysis. The MBMS enables real-time, continuous monitoring over a large dynamic range [10-6-102 % (v/v)]. It can be used to sample directly from harsh environments, including high-temperature, wet, and particulate-laden gas streams. One limitation of the MBMS is that (14) Ratcliff, M. A.; Onishak, M.; Gratson, D. A.; Patrick, J. A.; French, R. J.; Wiant, B. C. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1995, 40 (3), 681-687. (15) Ratcliff, M. A. Design and Construction of a Transportable Molecular Beam Mass Spectrometer and Its Application to the Analysis of Diesel Exhaust. Masters Thesis T4606, Colorado School of Mines, Golden, CO, 1994. (16) Extrel CMS. Pittsburgh, PA. (17) OPTO 22. Temecula, CA.
Carpenter et al. there is no preseparation of the observed peaks. Although fragmentation is minimized by using low-energy ionizing electrons (22.5 eV), isomers cannot be distinguished, making it difficult to interpret the mass spectra. Complementary analysis, such as impinger sampling, can be important for initial peak identification. Gas Sampling System. To achieve representative sampling, the integrity of the hot product gas must be maintained to a high degree. The gas temperature must be kept high enough to prevent condensation of the vapors but low enough to prevent further thermal decomposition en route to the MBMS instrument. Experience has shown that keeping heated transfer lines as short as possible and in the range of 350-450 °C, depending upon the product gas composition, provides a good compromise that minimizes losses from both condensation and thermal decomposition. In this work, the MBMS was positioned near the sample takeoff point of the main process line in the TCPDU. A heated sample transfer and conditioning system was constructed to filter particles (down to ∼1 µm) and control the temperature, flow, and pressure of the hot product gases. A schematic of the gas sampling system is shown in Figure 2. Process gas arrives at the sampling manifold by means of 0.95 cm (3/8′′) or 0.64 cm (1/4′′) stainlesssteel tubing wrapped with electrical heating tapes and covered in ceramic blanket insulation. The manifold, which houses three inline filters and three high-temperature pneumatic shut-off valves, is constructed of high-temperature block insulation covered with stainless-steel sheet metal. It can accommodate up to three separate heated sample streams as well as a “clean” unheated sample stream, such as scrubber exit gas or calibration gases. The manifold also contains a high-temperature regulating valve that is used for flow control. The sample lines are back-purged with a small amount of nitrogen when not in use to prevent plugging, and the manifold is purged to prevent buildup of explosive gases. The entire sampling manifold, up to and including the critical-flow orifice, was maintained at a temperature of 450 °C for these experiments. Approximately 0.21 N m3 h-1 (0.87 actual m3 h-1) of wet, tarladen gas flowed past the sampling orifice, of which the orifice itself sampled 0.02 N m3 h-1. The unsampled gas passed through a condenser to remove the steam and heavy tars and then through a set of impingers to further clean the gas prior to the downstream metering system. A sample pump was required to maintain adequate flow rates, while a flow control valve and pressure control valve operated in tandem to deliver a constant flow of gas at a constant pressure of 9 psia to the sampling orifice. The sampling pressure and the pressure drop across the orifice-plate flow meter were measured to provide the necessary feedback for pressure and flow control (see Figure 2). MBMS Tar Calibration. Quantitative tar analysis with the MBMS is complicated by several factors that affect its sensitivity to different chemical species. These include differences in electron ionization cross-sections and mass focusing effects during molecular beam formation. Therefore, one must rely on careful calibrations with known standards to obtain accurate concentration data. To accomplish this, a liquid calibration standard containing benzene, toluene, phenol, cresol, naphthalene, and phenanthrene dissolved in methanol is injected into the hot sampling system. Response factors for these tar species in the vapor phase and, hence, their concentrations can then be obtained. Using a HPLC pump, the liquid is metered in through a heated 0.13 mm (0.005′′) stainless-steel capillary immediately upstream of the flow control valve. The OPTO system provides the output control for the HPLC pump and records the weight loss from the standard reservoir using the output signal from an analytical balance. Another factor that must be considered is the effect of the sample matrix, i.e., the bulk gas composition, on instrument sensitivity. It has been observed that sample matrix effects can change the instrument response by 20-30% over the range of gas compositions typically seen in the TCPDU. The method of standard addition, whereby the liquid tar standard is injected directly into the gas stream, is a valid means of correcting for these matrix effects, provided that the quadrupole detector is operated in a linear region.
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Figure 2. Schematic of MBMS gas-sampling system: (1) analytical impingers-TCPDU gas, (2) heated sampling manifold, (3) flow control valve, (4) orifice-plate flow meter, (5) heated sampling orifice, (6) analytical impingers-2FBR study, (7) condenser, (8) cleanup impingers, (9) pressure control valve, (10) sample pump, and (11) dry gas meter. Shaded areas were heated to 450 °C.
However, this requires that the absolute volumetric flow at the sampling point be known to generate concentration data. The total flow is obtained by measuring the rates of condensate accumulation and dry gas flow at the outlet of the flow-by system and adding back the volume taken by the sampling orifice. The standard injection is typically carried out at two levels, and using the mass spectral signals observed from the raw gas and during the injections, a three-point calibration curve is generated for each species in the standard.
Results and Discussion Model Compounds. The experiments to compare the impinger and MBMS methods were carried out using an empty 5.25 cm (2′′) high-temperature, fluid bed reactor system (2FBR).18 A synthetic mixture containing benzene and tars (toluene, phenol, cresol, naphthalene, and phenanthrene) dissolved in methanol was sprayed into the 2FBR by means of a metering pump and vaporized at 350 °C. The resulting vapor was blended with a mixture of steam, nitrogen, hydrogen, and carbon dioxide, modeled after a typical biomass gasification product gas observed in the TCPDU, and transferred to the MBMS by means of a heated sample line. In this case, the sample line was 0.64 cm (1/4′′) in diameter and was maintained at the reactor temperature of 350 °C. The total residence time from the 2FBR inlet nozzle to the MBMS sampling orifice was estimated to be 5.0 s, including 0.6 s in the sample transfer line. Five experiments were performed: three replicates, one with an enhanced steam level, and one with an enhanced tar level. The gas compositions were chosen to represent realistic variations in steam and tar levels seen during typical gasification operations. The limitations of the liquid metering pump, gas delivery systems, the MBMS flow system, and the flow requirements of the impingers were other factors taken into account. Replicate experiments at the base condition enabled the reproducibility of the methods to be evaluated. The enhanced steam and tar levels were included to gauge the ability of each method to accommodate changes in tar and steam concentrations. To allow the impinger and MBMS samples to be obtained simultaneously, the impinger train was located immediately after (18) Czernik, S.; French, R. J. Energy Fuels 2006, 20, 754-758.
the MBMS sampling orifice and before the MBMS sample pump (see Figure 2). The simulated process gas from the 2FBR flowed past the MBMS sampling orifice and into the analytical impingers. Finally, before venting, the noncondensable gas flow rate was measured using a dry gas meter. Because of the system limitations mentioned above, the average wet gas flow rate for these experiments was 0.20 N m3 h-1. This resulted in sampling times of approximately 30 min, depending upon the gas composition, to obtain a sampled volume of 0.10 N m3, the minimum recommended by the CEN specification. Once a sufficient sample was collected, the valve between the MBMS sampling orifice and impingers was closed and the gas was rerouted through the MBMS cleanup train. The impingers were then removed, and the collected samples were analyzed offline. The resulting impinger and MBMS values were then compared with the actual concentrations in the synthetic gas mixture from the 2FBR. Table 1 summarizes the average composition of the simulated process gas supplied by the 2FBR. The tar concentrations were calculated on the basis of the known composition of the liquid standard, the feed rate of the 2FBR liquid metering pump, and the total measured flow (steam/condensate, gas, and tar) from the 2FBR. The concentration calculation assumes that one-fourth of the methanol thermally decomposes in the 2FBR to form hydrogen and carbon monoxide (CH3OH f CO + 2H2). This preliminary estimate is based on the analysis of these gases by 2FBR instrumentation while feeding methanol into the heated reactor with N2 and/or steam. Figure 3 shows the mass spectrum of the 2FBR synthetic mixture measured using the MBMS. The spectrum contains peaks from benzene and the five quantified tar species, as well as the methanol fragment ion peak at m/z 31 and a peak at m/z 40 from argon, added as an internal standard gas. Several peaks were not scanned over concerns of saturating the detector, the sensitivity of which was increased for improved detection of the tar species. The avoided peaks included m/z 18 (water), m/z 28 (N2 and CO), m/z 32 (CH3OH and O2), and m/z 44 (CO2). Other peaks observed in the spectrum were attributed to fragment ions formed during ionization. Once the flow from the 2FBR was stable, the liquid standard was injected into the MBMS sampling manifold at two flow
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Table 1. Composition of the Synthetic Tar Standard and 2FBR Simulated Gas component steam hydrogen CO2 CO nitrogen methanol benzene toluene phenol cresol naphthalene phenanthrene total
concentration in liquid standard (wt %)
experiment 1
76.268 9.747 6.382 3.235 1.284 2.384 0.699 99.999
45.9 18.5 12.0 0.88 19.5 2.51 0.180 0.100 0.049 0.017 0.027 0.006 99.7
rates: 0.25 and 0.50 mL/min. A second HPLC pump provided 0.25 mL/min of makeup methanol for the low level to maintain equivalent total flow rates of 0.5 mL/min. Using the average values of the signals observed in the raw gas and those observed during the injections, a three-point calibration curve was generated for each species in the standard. Figure 4 shows typical MBMS response curves for each compound in the liquid standard along with the linear least-squares fits (dashed lines).
Figure 3. Mass spectrum showing components of the synthetic gas mixture produced in the 2FBR.
Figure 4. Response of the MBMS for various tar species during a typical liquid standard injection.
concentrations in simulated process gas (vol %) experiment 2 experiment 3 experiment 4 43.4 19.0 11.4 1.50 18.6 4.87 0.341 0.189 0.094 0.032 0.051 0.011 99.5
45.7 18.5 12.0 0.89 19.7 2.51 0.178 0.099 0.049 0.017 0.026 0.006 99.7
62.8 12.5 7.8 0.80 13.0 2.43 0.180 0.100 0.049 0.017 0.027 0.006 99.7
experiment 5 46.2 18.4 12.0 0.85 19.4 2.47 0.185 0.103 0.051 0.018 0.028 0.006 99.7
The change in concentration for each level was calculated on the basis of the mass flow of each component (from the weight loss measurement of the balance) and the total gas flow as discussed earlier. The error bars, reported to (2s, represent the estimated uncertainty of the observed MBMS signals used in the averages and are typical of scatter in raw MBMS data. The response factors, derived from the slopes of the linear leastsquares fits to the calibration data, were then applied to the observed signals of interest. This calibration procedure was carried out separately for each of the five experiments to eliminate the possibility of introducing errors by changes in gas composition. Figure 5 shows the averaged results for the three replicate experiments, where the actual gas-phase concentrations of benzene and the five tar compounds from the 2FBR are compared with the concentrations measured by both the MBMS and the impingers. The average concentrations and uncertainties ((2s) for the three experiments are reported in mg N-1 m-3 (dry basis). Table 2 summarizes these results as a percentage difference from the known standard. As shown, the concentrations measured by the impingers were consistently lower than the actual concentrations. The MBMS measurements were closer to the actual values, within 10% in most cases, but were on average somewhat higher than the actual values. Both techniques showed roughly the same variability, ranging from 2 to 11% for the MBMS and 5 to 15% for the impingers, depending upon the compound (see Figure 5). The results from the high tar (experiment 2) and high steam (experiment 4) fall mostly within
Figure 5. Comparison of actual tar concentrations to average MBMS and impinger measurements for replicate model tar compound experiments.
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Table 2. MBMS and Impinger Measurements, Percent Difference from the Known Standard experiment 1
experiment 2
experiment 3
experiment 4
experiment 5
compound
MBMS
impinger
MBMS
impinger
MBMS
impinger
MBMS
impinger
MBMS
impinger
benzene toluene phenol cresol naphthalene phenanthrene
-2.8 5.0 -3.9 -12.4 1.0 -4.8
-16.3 -12.7 -18.7 -21.9 -19.6 -6.8
-4.8 2.1 -8.6 -3.8 -3.6 -5.6
-29.8 -11.7 -0.9 10.0 -9.9 -17.6
-0.2 8.8 4.7 -1.1 5.4 -9.1
-17.8 -15.0 -14.2 -13.5 -16.0 -11.0
-2.3 0.3 6.2 1.9 -0.1 1.1
-24.9 -22.8 -7.6 -11.1 -9.3 -12.5
0.9 5.4 5.7 0.7 -0.7 4.7
-14.1 -12.2 -12.6 -16.4 -12.7 -33.6
this variability range, indicating that both the MBMS and impinger measurements can accommodate this change of compositions during actual gasifier sampling. The notable exceptions were the impinger measurements of benzene and toluene during the elevated steam and tar conditions, which showed larger differences and will be discussed in the following sections. Corn Stover Gasification. In addition to the model compound studies, a comparison was carried out while sampling gas produced from corn stover gasification during TCPDU operation. The syngas was produced by co-feeding 20 kg h-1 each of crushed corn stover pellets and steam into a fluidized bed reactor operating at 650 °C. The resulting pyrolysis vapors were then directed to a thermal cracker operating at 800 °C, effectively simulating indirect gasification. The residence time from the TCPDU process line to the MBMS sampling orifice was estimated to be 5.3 s. Figure 6 shows an averaged mass spectrum of the gas sampled by the MBMS during corn stover gasification. Predominant peaks include methane (m/z 16), benzene (m/z 78), and argon (m/z 40), which is typically added as a tracer gas in the process. Other selected peaks, typical of those encountered in previous biomass gasification studies,1,3 are identified in the inset of the figure. As expected, the spectrum is comprised largely of tertiary tar compounds, characterized by the polycyclic aromatic hydrocarbon series of peaks and, to a lesser extent, some alkylated aromatic derivatives (toluene, indene, methylnaphthalenes, etc.) and secondary tars (phenolics). Concurrent to the MBMS monitoring, impinger sampling of the raw syngas was carried out. In this case, the sampling point
for the impingers was before the MBMS sampling manifold (see Figure 2); this configuration was used to enable subsequent MBMS sampling of the impinger train effluent. For this experiment, the wet gas flow rate was approximately 0.5 N m3 h-1, a more typical sampling rate. The residence time from the TCPDU process line to the impinger sampling point was estimated to be 0.7 s. Table 3 summarizes the GC/MS results from the impinger measurements of tar in the raw, corn-stover-derived syngas. The GC calibration included 33 individual compounds, 32 of which were found in the gas sample. Also included in the table are the corresponding MBMS measurements for the six quantified species and estimates of the “total tar” measured by each method. The impinger “total tar” figure was obtained by adding the combined gravimetric tar amounts, determined as per the specification by rotary evaporation, to the total of GC-measured tars. The MBMS “total tar” concentration estimate was obtained by adding together the following contributions: the concentration from applying the response factor for naphthalene (m/z 128) to the mass spectral intensities summed from m/z 80 to 176, the concentration from applying the response factor for phenanthrene (m/z 178) to the even-numbered mass spectral intensities summed from m/z 180 to 400, and the measured concentrations of toluene, phenol, cresol, naphthalene, and phenanthrene/ anthracene, each determined separately from the calibration. Qualitatively, the results of the two methods differ in a consistent manner with that observed in the 2FBR model compound study. Both methods showed good reproducibility for samples collected under similar operating conditions. However, relative to MBMS results, the impinger results from
Figure 6. Averaged mass spectrum of raw syngas derived from indirect gasification of corn stover.
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Table 3. Tar Concentrations Measured in Raw, Corn-Stover-Derived Syngas by the Impingers and the MBMS, Where Available (g N-1 m-3, Dry Basis) component
molecular weight
impinger (MBMS)
benzene pyridine toluene phenol styrene ethylbenzene m,p-xylene o-xylene o-cresol m,p-cresol indene indan 1,3,5-trimethylbenzene 1,2,4-trimethylbenzene 2-methoxyphenol naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benzo(a)anthracene chrysene benzo(b)fluoranthene benzo(k)fluoranthene benzo(a)pyrene indeno(1,2,3-c,d)pyrene benzo(g,h,i)perylene dibenzo(a,h)anthracene “total tar”
78 79 92 94 104 106 106 106 108 108 116 118 120 120 124 128 142 142 152 154 166 178 178 202 202 228 228 252 252 252 276 276 278
7.35 (17.0) 4.94 5.32 (7.38) 9.12 (7.92) 3.04 0.05 1.18 0.41 0.20 (1.62)a 1.10 3.97 0.04 0.03 0.08 0.00 3.32 (4.78) 1.20 0.83 1.47 0.10 0.73 0.81 (1.49)b 0.34 0.27 0.25 0.11 0.10 0.05 0.02 0.48 0.01 0.01
