Energy Fuels 2010, 24, 2042–2051 Published on Web 02/16/2010
: DOI:10.1021/ef9012196
Method for High-Temperature Particle Sampling in Tar-Rich Gases from the Thermochemical Conversion of Biomass Eva Gustafsson* and Michael Strand School of Engineering, Bioenergy Technology, Linnaeus University, SE-351 95 V€ axj€ o, Sweden Received October 26, 2009. Revised Manuscript Received January 18, 2010
The thermochemical conversion of biomass produces compounds in both gas and particle phases that may be regarded as contaminants. These contaminants include both particulate matter (e.g., fly ash, soot, and fragmented nonvolatilized material) and volatilized metals and tars that condense and form particulate matter during cooling. In this study a method for high-temperature particle sampling in tar-rich gases from the thermochemical conversion of biomass was developed and tested. Both a bed of granular activated carbon and a denuder were used for tar adsorption. First, the transport efficiency of particles was determined both theoretically and experimentally using a K2SO4 reference aerosol, and the losses were found to be smaller in the denuder than in the bed of granular activated carbon. The adsorption capacity was then tested using a model aerosol of K2SO4 and diethyl-hexyl-sebacate (DEHS). The adsorption capacity of the bed of granular activated carbon was found to be higher than that of the denuder. The adsorption capacity was also tested using a model aerosol of K2SO4 particles and tar-rich gas from a laboratory-scale gasifier. As for DEHS, the result indicated that the capacity of the bed of granular activated carbon was higher than that of the denuder; it was also found that the adsorption was incomplete when the tar concentrations increased. In addition, the bed of granular activated carbon was successfully tested during experiments using a 100 kW circulating fluidized bed gasifier. The results indicate that the tar adsorption capacity is dependent not only on the total tar concentration but also on the tar composition.
(>500 °C).1-3 At temperatures under 400 °C, tars, produced abundantly during gasification and pyrolysis, condense. Tars are generally defined as organic compounds present in the gas, excluding gaseous hydrocarbons, C1-C6.4 Tars are formed from the main components of biomass, that is, cellulose, hemicellulose, and lignin, which first form primary pyrolysis products.5 These primary products are converted into secondary products, such as phenols and olefins, as the temperature is increased. A further increase in temperature generates aromatic compounds with aliphatic substituents. Finally, polyaromatic hydrocarbons (PAHs), such as naphthalene and pyrene, are formed. The concentration and distribution of tars varies considerably depending on process parameters, such as temperature and process design. While most of the tars are in vapor phase at normal process temperatures, nucleation and condensation to particulate matter take place if the temperature declines below the tar dew point. Particle formation during biomass combustion has been studied using different types of probes designed to control vapor condensation, especially of inorganic vapors. Strand et al. designed a high-temperature dilution probe, testing it at 780 °C using a circulating fluidized bed (CFB) boiler fired with wood chips.6 At high dilution ratios the alkali vapor deposited on the probe walls, while at lower dilution ratios a nucleation mode was generated in the probe. A similar dilution probe was
Introduction To reduce the emissions of fossil carbon dioxide, biomass is increasingly used for energy purposes. Various thermochemical processes, such as combustion, pyrolysis and gasification, can be used to convert the biomass into heat, power, and biofuels. The thermochemical conversion of biomass produces compounds in both gas and particle phases that may be regarded as contaminants. These contaminants include particulate matter (e.g., fly ash, soot, and fragmented nonvolatilized material), as well as volatilized metals and tars that condense and form particulate matter during cooling. Filters and catalysts may be used to clean and upgrade the product gas at high temperatures. Precise descriptions of the various contaminants and their physical states at actual process temperatures facilitate the design and optimization of both filters and catalysts and, in addition, help explain the mechanisms of contaminant formation and conversion. The characterization of contaminants through high temperature extraction is usually problematic due to the physical and chemical transformation of the sample. A main concern is the transformation of both inorganic and organic vapors to particulate matter through nucleation and condensation as the gas cools. Various compounds form particulate matter as the temperature of the product gas gradually decreases. In biomass combustion, for example, alkali sulfates and chlorides form particulate matter at high temperatures
(3) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. J. Aerosol Sci. 1998, 29, 445–459. (4) CEN/TC 143 Technical Specification 15439:2006; CEN: 2006. (5) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier Tars: Their Nature, Formation and Conversion; National Renewable Energy Laboratory: Golden, CO, 1998. (6) Strand, M.; Bohgard, M.; Swietlicki, E.; Gharibi, A.; Sanati, M. Aerosol Sci. Technol. 2004, 38, 757–765.
*To whom correspondence should be addressed. E-mail: eva.gustafsson@ lnu.se. Telephone: þ46 470 70 80 00. (1) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421–444. (2) Jimenez, S.; Ballester, J. Combust. Sci. Technol. 2006, 178, 655– 683. r 2010 American Chemical Society
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used by Wiinikka et al. to investigate the particle formation at high temperatures during fixed bed combustion of wood pellets.7 Jimenez and Ballester tested and compared three methods for high-temperature sampling of particles from biomass combustion: an aerodynamic-quenching particlesampling probe, a dilution probe, and a thermophoretic sampling probe.8 All three methods gave rise to particle formation in the probes, but in the cases of the aerodynamicquenching particle-sampling probe and the thermophoretic sampling probe, the particles formed in the probe could be distinguished from the original particles. Heated cascade impactors and filters have also been used to sample aerosols at temperatures above the dew point of condensing vapors. For biomass combustion, the focus has been on inorganic vapors, while for biomass gasification tars have been a major concern. Valmari et al. studied the size distribution of particles from CFB combustion of willow using a Berner low-pressure impactor at 650 °C.3 In another study, Valmari et al. used a system including a quartz fiber filter to study particle-forming compounds at 810-850 °C during CFB combustion of forest residue and willow.9 Gabra et al. and Yamazaki et al. used heated filters to keep the tars in the vapor phase while collecting particulate matter from biomass gasification.10,11 Corresponding measurements were made by Hasler and Nussbaumer and van der Nat et al. using heated cascade impactors instead of heated filters.12,13 Tars can also be removed before particle collection, for example, using porous activated carbon to adsorb tars in vapor phase. A porous material contains pores of different sizes, usually classified according to their diameter: micropores have diameters less than 2 nm, mesopores diameters of 2-50 nm, and macropores diameters greater than 50 nm.14 In the case of physical adsorption, molecules in the gas phase interact with the solid adsorbent surface by van der Waals forces.15 The process of adsorption is described by the adsorption isotherm, the relationship between the amount of adsorbed gas and the partial pressure of the gas or vapor at a certain temperature. Several characteristics of activated carbon, including pore size distribution, micropore and mesopore volume, specific surface area, surface functional groups, and adsorbent particle size, have been identified as affecting the adsorption removal efficiency of organic compounds.16-18 The relative impact of these characteristics is partly dependent on the organic compound studied17 and on competition from other
compounds, such as low-molecular-weight hydrocarbons and water.19 Mastral et al. tested the adsorption of naphthalene, phenanthrene, and pyrene using different kinds of activated carbon and found that adsorption increased with increasing molecular size and decreasing volatility;17 this result was confirmed in later studies.20,21 Mastral et al. studied the adsorption of binary mixtures of different PAHs on activated carbon; they observed competition between the different compounds, adsorption efficiency being highest for the PAH with the lowest volatility.22 Zhen-Shu studied the effect of temperature on the adsorption of PAH on activated carbon fibers.18 The result was that removal efficiency decreased with increasing temperature, since the process is exothermic; at lower temperatures, however, there is a risk of condensation and pore blockage that could reduce adsorption. In a previous study, particles from the steam and oxygenblown gasification of wood pellets were characterized physically and chemically.23 A dilution probe was used to sample and dilute the gas with nitrogen at approximately 500 °C. Then, a bed of granular activated carbon with an inlet temperature of approximately 220-240 °C was used to adsorb the tars before the gas was further cooled, thereby preventing particle formation. In a later study, bed operation was further investigated, especially regarding the inlet temperature, using a laboratory-scale gasifier.24 The results indicated that a bed inlet temperature of 300 °C was preferable for keeping the tars in the vapor phase and adsorbing them in the bed. A disadvantage of using a bed of granular activated carbon for tar adsorption is particle losses. Gustafsson et al. determined the particle losses in a 20 mL bed of granular activated carbon in a 16 mm outer diameter pipe to be 15% for particles with a mobility equivalent diameter (dB) of 1550 nm.23 There is a potential for less particle losses if a denuder at 200-300 °C is used for tar removal instead of a bed of granular activated carbon. Denuder operation is based on the fact that the diffusion velocity of gaseous species is higher than the diffusion velocity of particles.25 Denuders often consist of either a cylindrical tube or an annular tube comprising two coaxial tubes. Compounds in the gas phase will diffuse to the walls of the denuder while the particles pass through. Prerequisites for efficient denuder operation are laminar flow, large collection capacity, and the absence of chemical reactions inside the denuder that change the particles. The collection capacity of the denuder is usually increased by coating of the walls with a reagent that reacts with the gases to be removed. A thermodenuder is a special case of a denuder that could be used to remove volatile matter from particles. The thermodenuder consists of a section where the aerosol is heated, followed by a denuder section where adsorption and
(7) Wiinikka, H.; Gebart, R.; Boman, C.; Bostr€ om, D.; Nordin, A.; € Ohman, M. Combust. Flame 2006, 147, 278–293. (8) Jimenez, S.; Ballester, J. Aerosol Sci. Technol. 2005, 39, 811–821. (9) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 379–389. (10) Gabra, M.; Pettersson, E.; Backman, R.; Kjellstrom, B. Biomass Bioenergy 2001, 21, 351–369. (11) Yamazaki, T.; Kozu, H.; Yamagata, S.; Murao, N.; Ohta, S.; Shiya, S.; Ohba, T. Energy Fuels 2005, 19, 1186–1191. (12) Hasler, P.; Nussbaumer, T. In Biomass for Energy and Industry; European Conference and Technology Exhibition; Rimpar: W€urzburg Germany, 1998; pp 1623-1625. (13) van der Nat, K. V.; Siedlecki, M.; de Jong, W.; Woudstra, N.; Verkooijen, A. H. M. In Biomass for Energy, Industry and Climate Protection; European Biomass Conference and Technology Exhibition; Paris, France, 2005; pp 642-645. (14) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press: London, 1998. (15) Cheremisinoff, N. P.; Cheremisinoff, P. N. Carbon Adsorption for Pollution Control; PTR Prentice Hall: Englewood Cliffs, N.J., 1993. (16) Hu, X.; Hanaoka, T.; Sakanishi, K.; Shinagawa, T.; Matsui, S.; Tada, M.; Iwasaki, T. J. Jpn. Inst. Energy 2007, 86, 707–711. (17) Mastral, A. M.; Garcia, T.; Callen, M. S.; Navarro, M. V.; Galban, J. Environ. Sci. Technol. 2001, 35, 2395–2400. (18) Zhen-Shu, L. J. Env. Eng. 2006, 132, 463–469.
(19) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels 2004, 18, 202–208. (20) Mastral, A. M.; Garcı´ a, T.; Callen, M. S.; Murillo, R.; Navarro, M. V.; L opez, J. M. Fuel Process. Technol. 2002, 77-78, 373–379. (21) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Ind. Eng. Chem. Res. 2003, 42, 155–161. (22) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V.; Galban, J. Energy Fuels 2003, 17, 669–676. (23) Gustafsson, E.; Strand, M.; Sanati, M. Energy Fuels 2007, 21, 3660–3667. (24) Gustafsson, E.; Strand, M. In 16th European Biomass Conference and Exhibition: From Research to Industry and Markets; Valencia, Spain, 2008; pp 1037-1040. (25) Spurny, K. R. Analytical Chemistry of Aerosols; Lewis: Boca Raton, FL, 1999.
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cooling take place. First, the volatile material is evaporated and then it is removed from the gas and adsorbed. In the application presented in this study, the gas is not cooled before entering the denuder section; for that reason, its operation resembles that of a thermodenuder. Burtscher et al. presented a thermodenuder that could be used to separate the volatile and nonvolatile particle fractions.27 The adsorption and cooling section consisted of cylindrical stainless steel gauze, and activated carbon used for adsorption surrounded the gauze. Either air or water could be used in actively cooling the adsorption section to prevent nucleation and recondensation on the particles. The losses in the thermodenuder due to sedimentation, diffusion, and thermophoresis were determined both theoretically and experimentally. The losses were found to increase with increasing temperature in the heating section and with decreasing particle size. Burtscher et al. successfully tested the thermodenuder on atmospheric particles, on combustion particles, and on the adsorption and desorption of PAH on NaCl and combustion particles. Wehner et al. and Fierz et al. further improved the thermodenuder developed by Burtscher et al.26,28 Wehner et al. increased the exit temperature from the heating section, preventing the recondensation of vapors on the particles before adsorption occurred. Fierz et al. used a heated adsorption section instead of a cooled one to avoid the same problem. In the present study a method for high-temperature particle sampling in tar-rich gases from the thermochemical conversion of biomass is developed and tested. Both a bed of granular activated carbon and a denuder are used for tar adsorption. Important characteristics studied are particle transport efficiency and tar adsorption capacity. To determine particle losses, a K2SO4 reference aerosol is used. The adsorption capacity is studied using K2SO4 particles and diethylhexyl-sebacate (DEHS, C26H50O4) vapor as well as real product gas from both a laboratory-scale gasifier and a 100 kW CFB gasifier. A major difference between the study conditions and those of general thermodenuder applications is the high concentration of condensable material present in the raw product gas from the biomass gasifier. The concentration of tars from a fixed-bed gasifier could be as high as 150 g/m3 while the concentration from a CFB gasifier is normally 2-30 g/m3.29
Figure 1. Schematic of the bed of granular activated carbon.
Figure 2. Schematic of the denuder.
The denuder consists of a concentric stainless steel tube within which a concentric tube of stainless steel gauze is mounted (see Figure 2). The outer stainless steel tube has an inner diameter of 45 mm while the inner diameter of the inner tube of stainless steel gauze is 30 mm. The total length of the denuder is 455 mm. Approximately 400 mL of granular activated carbon (type RB3 activated charcoal, diameter 3 mm; Norit Nederland B.V.) was used to fill the cavity between the two tubes. The temperature was measured near the inlet of the denuder. Particle Transport Efficiency in the Bed of Granular Activated Carbon and in the Denuder. Experimental Determination of Particle Transport Efficiency. The experimental system for determining particle transport efficiency in the bed of granular activated carbon and in the denuder, using a K2SO4 reference aerosol, is depicted in Figure 3. The size-dependent particle transport efficiency was determined at room temperature at different volumetric flow rates (0.4-4.4 L/min) by analyzing the change in the particle number size distributions as the aerosol was alternately bypassed and fed through either a 100 mm bed or a denuder. The K2SO4 reference aerosol was generated using an AGK 2000 liquid nebulizer (Palas GmbH, Karlsruhe, Germany). A solution of 0.5 g/L K2SO4 (reagent grade; Scharlau Chemie S.A., Barcelona, Spain) was sprayed using air at a volumetric flow rate of 4.4 L/min. The aerosol was then dried using 2.56 mm silica gel with a humidity indicator (orange) (Scharlau Chemie S.A.). Downstream from the drier, the volumetric flow rate through the bed or denuder was set by withdrawing a flow of 0-4 L/min to a pump. An ejector diluter was used downstream from the bed or denuder to dilute the gas with pressurized particle-free air. The ejector diluter was allowed to withdraw a volumetric flow of approximately 2.9 L/min irrespective of the volumetric flow rate through the bed or denuder. This was accomplished by balancing the flow from the bed or denuder to the ejector diluter, by either supplying additional air or allowing excess aerosol to flow out of the system. A scanning mobility particle sizer (SMPS) (TSI Inc., Shoreview, MN), including a model 3081 differential mobility analyzer and a model 3010 condensation particle counter (both from TSI Inc.) was used to determine the number size distribution and total number concentration of particles with dB of 15-670 nm. Theoretical Determination of Particle Transport Efficiency. Several mechanisms cause particle losses in the bed of granular activated carbon and denuder, thereby reducing the transport efficiency. For small particles, losses due to diffusion dominate, whereas inertial losses dominate for large particles. Losses due
Experimental Section Bed of Granular Activated Carbon and Denuder. The bed of granular activated carbon is contained in a stainless steel tube with an inner diameter of 23 mm (see Figure 1). Temperature measurements could be made at four positions along the bed. The filling of the tube (i.e., the axial position and length of the bed) can be adjusted by moving perforated steel plates on a threaded bar. This study used lengths of 100-160 mm, corresponding to approximately 40-70 mL of granular activated carbon (type RB3 activated charcoal, diameter 3 mm, BET surface area 1100 m2/g; Norit Nederland B.V., Amersfoort, The Netherlands). In the experiment using the 100 kW CFB gasifier, the bed was split into two parts, totaling 160 mm in length, to attain adsorption sections of different temperatures. (26) Wehner, B.; Philippin, S.; Wiedensohler, A. J. Aerosol Sci. 2002, 33, 1087–1093. (27) Burtscher, H.; Baltensperger, U.; Bukowiecki, N.; Cohn, P.; Huglin, C.; Mohr, M.; Matter, U.; Nyeki, S.; Schmatloch, V.; Streit, N.; Weingartner, E. J. Aerosol Sci. 2001, 32, 427–442. (28) Fierz, M.; Vernooij, M. G. C.; Burtscher, H. J. Aerosol Sci. 2007, 38, 1163–1168. (29) Hasler, P.; Nussbaumer, T. Biomass Bioenergy 1999, 16, 385–395.
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Figure 3. Experimental system for determining particle transport efficiency using a K2SO4 reference aerosol.
to thermophoresis are fairly independent of particle size.30 The calculation of the theoretical losses due to diffusion will be described and the results compared with the experimentally determined losses. Particles diffuse to the surfaces of the granules of activated carbon and to the walls of the bed and denuder, where they are permanently lost. Diffusion is caused by the collisions between particles and gas molecules, which cause random Brownian particle motion. The net particle movement of the particles is always from a high- to a low-concentration region (i.e., surfaces and walls). The aerosol particle diffusivity and, consequently, the particle losses increase with decreasing particle size. The losses also increase as the volumetric flow rate decreases due to increased residence time. The losses due to diffusion in the bed and denuder were calculated according to Willeke and Baron using eqs 1-3, where η is the transport efficiency, d is the inside diameter of the tube, L is the length of the tube, Vdiff is the diffusive deposition velocity, Q is the volumetric flow rate, Sh is the Sherwood number, and D is the diffusion coefficient.31 η ¼e
-πdLVdiff =Q
mately 170 °C, similar to tar compounds. It was used by Fierz et al. in testing the removal capacity of a thermodenuder.28 K2SO4 particles were generated using a liquid nebulizer, as described previously. A solution of 0.5 g/L of K2SO4 was sprayed using 4.8 L/min of nitrogen. To supply DEHS, the dried aerosol was passed through oven (1) via a stainless steel tube containing pieces of fiberglass filter impregnated with DEHS (purum; Sigma-Aldrich Sweden AB, Stockholm, Sweden). Downstream from oven (1), the aerosol was cooled in order to coat the K2SO4 particles with DEHS. The mass concentrations of K2SO4 and DEHS were determined gravimetrically for two cases using polypropylene-backed PTFE membrane filters (pore size 1.0 μm, diameter 25 mm; Micro Filtration Systems, Dublin, CA). The mass concentration of pure K2SO4 particles was determined at 100 °C in oven (1), as no substantial amounts of DEHS were expected to evaporate from the impregnated filter at this temperature. The mass concentration of DEHS was calculated as the change in mass concentration as the temperature in oven (1) was increased from 100 to 150 °C and 170 °C. The mass concentrations were also estimated from reference measurements of the particle number size distributions made using the SMPS. Spherical particles and an effective density of 2700 kg/m3 for K2SO4 and 914 kg/m3 for DEHS were assumed. The mass concentrations determined using filter measurements as well as the corresponding mass concentrations given by the SMPS reference measurement are presented in Table 1. In subsequent measurements, temperatures of 150-200 °C in oven (1) were used in determining the adsorption capacity of the bed of granular activated carbon and denuder. The mass concentration of DEHS was estimated using the SMPS reference measurements only, since there was sufficient agreement with the gravimetrically determined mass concentrations as presented in Table 1. Having established the reference, the aerosol of K2SO4 and DEHS was fed to oven (2) where the DEHS was re-evaporated at 300 °C, producing the model aerosol consisting of K2SO4 particles and DEHS vapor. The temperature, T1, was either 200 or 300 °C, in order to study the effect on adsorption capacity of different temperature profiles in the bed and denuder. The volumetric flow rate through the bed and denuder was approximately 3 L/min as determined by the ejector diluter.
ð1Þ
πDL Q
ð2Þ
Vdiff d 0:2672 ¼ 3:66 þ D ξ þ 0:10079ξ1=3
ð3Þ
ξ ¼
Sh ¼
¼e
-ξSh
Figure 4. System for determining the adsorption capacity of the bed of granular activated carbon and the denuder using a model aerosol of K2SO4 and DEHS.
These equations are valid assuming that the flow is laminar, the cross-section of the tube is circular, and the particles that diffuse to surfaces and walls are deposited. The particle transport efficiency determined experimentally was compared with the calculated theoretical transport efficiency, taking only losses due to diffusion into account. For the bed of granular activated carbon, it was assumed that a large number of channels were formed between the granules. The theoretically determined particle transport efficiency was adjusted to match the experimentally determined efficiency by adjusting the tube length. For the denuder the best approximation of the tube length is the length of the stainless steel gauze, though gas may also pass through the gauze into the activated carbon resulting in larger losses due to diffusion. Adsorption Capacity of the Bed of Granular Activated Carbon and of the Denuder Determined Using K2SO4 Particles and Vapor of DEHS. It is difficult to determine the general tar adsorption capacity of the bed of granular activated carbon and denuder, since the tar composition varies depending on the process conditions. In this study, K2SO4 and DEHS were used as model compounds to investigate the adsorption capacity of a 100 mm bed and denuder and to simulate particle growth by tar condensation (see Figure 4). DEHS is a heavy organic molecule (molecular mass 427 g/mol) with a boiling point of approxi(30) Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, 2nd ed.; Wiley: New York, 1999. (31) Willeke, K.; Baron, P. A. Aerosol Measurement: Principles, Techniques and Applications; Van Nostrand Reinhold: New York, 1993.
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Table 1. Mass Concentrations of K2SO4 and DEHS Determined Gravimetrically Using Filter Measurements and the Corresponding Mass Concentrations Given by the SMPS Reference Measurement
K2SO4 (100 °C) DEHS (150 °C) K2SO4 (100 °C) DEHS (170 °C)
concentration (filter) [mg/m3]
concentration (SMPS) [mg/m3]
1.10 1.36 1.05 7.02
1.29 1.07 1.22 8.14
Figure 6. Schematic of the dilution probe. Table 2. Gasification Parameters gasification temperature [°C] fuel [kg/h] bed material [kg/h] steam [kg/h] oxygen [kg/h] air [kg/h] nitrogen [kg/h]
830-840 10.6 2.2 11.7 4.0 1.2 1.9
Gas was sampled from the top of the gasifier using a stainless steel dilution probe (see Figure 6). The tar-rich gas was filtered using a quartz filter on the dilution probe tip to produce a particle-free gas. The gas was then mixed with heated nitrogen, containing K2SO4 particles (total concentration 1.4106 particles/cm3 and geometrical mean dB = 55 nm), near the probe tip. The mixing ratio in the probe was varied from 1:2 (one part tar-rich gas and two parts nitrogen containing K2SO4 particles) to 1:17 in the experiments when the bed of granular activated carbon was used for tar adsorption and from 1:10 to 1:47 when the denuder was used. Downstream from the dilution probe, the gas was fed through either a 100 mm bed or a denuder to adsorb the tars. The temperature at the inlet to the bed and denuder was approximately 290-300 °C, while the outlet temperature was approximately 25 °C. A NGA 2000 gas analyzer (Rosemount Analytical, Solon, OH) was used to measure the CO concentration, to verify the mixing ratio in the dilution probe. An ejector diluter was used to dilute the gas with particle-free dry compressed air. The dilution ratio in the ejector diluter was between 11 and 12 and the temperature was approximately 25 °C, thus the particulate matter was not considered to be affected by the dilution. Downstream from the ejector diluter, the particle size distribution was analyzed using the SMPS. The volumetric flow rates through the bed and the denuder were approximately 3.6 and 3.8 L/min, respectively. Tar Adsorption Capacity of the Bed of Granular Activated Carbon Using 100 kW CFB Gasifier Experiments. The bed of granular activated carbon was also tested while measuring particles from a 100 kW steam- and oxygen-blown CFB gasifier during an experimental campaign performed at Delft University of Technology. Further details regarding the gasifier setup and gasification tests are presented in Siedlecki et al.32 Miscanthus was used as fuel and magnesite as bed material. The raw product gas was analyzed using a nondispersive infrared (NDIR) instrument and a CP-4900 micro GC (Varian Inc.). The gasification parameters are presented in Table 2. The system used for particle measurement is depicted in Figure 7. A dilution probe, similar to that depicted in Figure 6 but without a quartz filter, was used to sample the raw product gas from the gasifier, downstream from the cyclone. The probe was installed perpendicular to the flow direction, and nitrogen was used for dilution (primary dilution) at a primary dilution ratio of approximately 15. The gas was then fed through the bed of granular activated carbon in order to adsorb the tars. The
Figure 5. System for determining tar adsorption capacity using a laboratory-scale gasifier.
The particle size distributions downstream oven (2) were measured using the SMPS. By comparing the particle size distributions and the total number concentrations from reference measurements and measurements through the bed and denuder conclusions could be drawn about the adsorption capacity. In additional experiments, the bed or denuder was replaced with an empty stainless steel tube or a 100 mm bed of inert pieces of glass, to investigate the mechanisms involved in removing the DEHS. The stainless steel tube had an inner diameter of 23 mm and a length of 508 mm. The glass pieces were irregular with an approximate size of 5-10 mm and were placed in the same stainless steel tube that contained the bed of granular activated carbon. Tar Adsorption Capacity of the Bed of Granular Activated Carbon and of the Denuder Determined Using Laboratory-Scale Gasifier Experiments. The tar adsorption capacity and particle growth by tar condensation were also studied using a laboratory-scale bubbling fluidized bed gasifier, previously described by Gustafsson and Strand.24 In the present case, a combination of K2SO4 particles and tar-rich gas from the gasifier was used as the model aerosol. Any change in the particle size distribution or total concentration was due to new particle formation in the sampling system, especially from condensing tars. The laboratory-scale gasifier, including the system for generating K2SO4 particles and for particle characterization, is depicted in Figure 5. Pulverized wood pellets with a particle size of 0.4-0.63 mm were gasified at 800 °C, using air as the oxidizing agent and nitrogen as the carrier gas. The total gas flow was 5.1 L/min with an oxygen concentration of 1%. The fuel was fed at a rate of approximately 0.1 g/min and the residence time in the gasifier was approximately 4 s. Alumina (Al2O3), 0.4-0.6 mm in size, was used as the bed material. Raw product gas was sampled twice during the experiments using the denuder for tar removal. The concentrations of CO2, CO, CH4, and H2 were analyzed using a CP-4900 micro gas chromatograph (micro GC; Varian Inc., Palo Alto, CA). The mass concentration of tars that condensed above 60 °C was determined gravimetrically using a filter mounted downstream from a sintered steel tube.
(32) Siedlecki, M; Nieuwstraten, R.; Simeone, E.; de Jong, W.; Verkooijen, A. H. M.; Energy Fuels 2009, 23, 5643-5654.
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Figure 7. System for particle measurements, including the bed of granular activated carbon.
Figure 9. Transport efficiency of K2SO4 particles through the 100 mm bed of granular activated carbon at room temperature and volumetric flow rates of 3.4 and 4.4 L/min, experimentally and theoretically determined.
Figure 8. A typical K2SO4 particle number size distribution measured using the SMPS.
total length of the bed was 160 mm, corresponding to approximately 70 mL of granular activated carbon divided into one 100 mm section and one 60 mm section. The temperature was measured at the bed inlet and at three positions in the bed. The volumetric flow rate through the bed was approximately 5 L/min. An ejector diluter was used to dilute the gas further (secondary dilution) with particle-free dry compressed air, generating total dilution ratios of approximately 100. The temperature in the ejector diluter was approximately 25 °C, thus the particulate matter was not considered to be affected by the dilution. Downstream from the ejector diluter, the gas was alternately bypassed and fed through an oven and then to the instruments used for gas and particle characterization. The temperature in the oven was varied between room temperature and 300 °C to investigate the content of volatile particulate matter, that is, to determine whether tars had condensed on the particles. The SMPS and a model 3321 aerodynamic particle sizer (APS) (TSI Inc.) were used for the particle characterization. The SMPS measured particles with dB of 10-470 nm, while the APS measured the number size distribution and total number concentration of particles with aerodynamic diameters (dae) of 0.5-20 μm. A gas analyzer (Rosemount Analytical) was used to measure the CO concentration to verify the total dilution ratio.
Figure 10. Transport efficiency of K2SO4 particles through the 100 mm bed of granular activated carbon at room temperature and volumetric flow rates of 0.4, 1.4, and 2.4 L/min, experimentally and theoretically determined.
rates are presented in Figures 9 and 10. Spherical particles were assumed, which means that the mobility equivalent diameter approximately equals the particle diameter. Particle transport efficiencies for dB = 15-200 nm are presented, since the number of particles with dB >200 nm was low in the reference aerosol (see Figure 8). A total channel length of 165 m was used in the theoretical calculations since it produced the best agreement with experimental results. The total length should not, however, be considered to have a precise physical correspondence to the channels in the bed due to the approximations made. The total length used in eqs 1 and 2 should instead be considered as an experimentally determined parameter. This means that the particle transport efficiency for volumetric flow rates other than those used in the experimental determination could be approximated. In Figures 11 and 12, the experimentally and theoretically determined diffusion losses in the denuder are presented at room temperature and various volumetric flow rates. For the denuder, the theoretically determined transport efficiency is remarkably higher than the experimentally determined transport efficiency. This indicates that approximating the denuder as a tube is not accurate, possibly due to the diffusion of particles through the stainless steel gauze. This is not in accordance with the results presented by Fierz et al., where the losses in the denuder were found to be similar to the theoretically determined losses in a tube.28 The results presented in Figures 9-12 indicate that the losses are higher in the bed of granular activated carbon than in the denuder. The losses increase as the particle size decreases
Results and Discussion Particle Transport Efficiency in the Bed of Granular Activated Carbon and in the Denuder. A K2SO4 reference aerosol was used to determine the particle transport efficiency in the bed of granular activated carbon and in the denuder. The total particle concentration was approximately 2.5 106 particles/cm3 and the geometric mean dB was 50-60 nm. Figure 8 presents a typical particle number size distribution measured using the SMPS. The experimentally and theoretically determined sizedependent transport efficiencies of particles through a 100 mm bed of granular activated carbon at different volumetric flow 2047
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Figure 13. Particle number size distributions for K2SO4 and K2SO4 þ DEHS during experiments using a 100 mm bed of granular activated carbon, 34.2 mg/m3 DEHS, and T1 = 300 °C; reference measurement and measurements through the bed, complete adsorption.
Figure 11. Experimental transport efficiency of K2SO4 particles through the denuder at room temperature and various volumetric flow rates.
Figure 14. Particle number size distributions for K2SO4 and K2SO4 þ DEHS during experiments using the denuder, 5.9 mg/m3 DEHS, and T1 = 300 °C; reference measurement and measurements through the denuder, incomplete adsorption.
Figure 12. Theoretical transport efficiency of K2SO4 particles through the denuder at room temperature and various volumetric flow rates, taking only losses due to diffusion into account.
for both the bed and the denuder, indicating that diffusion is the dominant loss mechanism. The losses in the bed decrease as the volumetric flow rate increases, in accordance with theory, while the losses in the denuder seem to be fairly independent of flow rate at flow rates above 1.4 L/min. Losses due to inertial forces are not an appreciable loss mechanism for particles of the size used for experimental determination of losses in this study; thermophoretic losses are not appreciable either, since the experiments took place at room temperature. However, these loss mechanisms could be of great importance when using the bed of granular activated carbon and the denuder in applications in which the particle size is larger and the temperature in the bed or denuder is higher, generating temperature gradients. Determination of Adsorption Capacity of the Bed of Granular Activated Carbon and of the Denuder Using K2SO4 Particles and Vapor of DEHS. The adsorption capacity was tested using an aerosol of K2SO4 particles and DEHS vapor with a 100 mm bed of granular activated carbon, a denuder, a steel tube, and a bed of glass pieces. To determine whether the adsorption was complete, the particle number size distributions and total concentrations from reference measurements and measurements through the carbon bed, denuder, steel tube or glass bed were compared. Figures 13 and 14 present comparisons of particle number size distributions where the adsorption was complete and incomplete respectively. The results have not been corrected for dilution in the ejector diluter, and in addition, the dilution ratio varies depending on whether a reference measurement or a measurement through the bed and denuder was performed. Figure 13 shows the particle number size distributions for K2SO4 and K2SO4 þ DEHS during one experiment using the
bed of granular activated carbon for adsorption and a DEHS concentration of 34.2 mg/m3. Since the particle number size distributions, as measured through the bed, were not appreciably affected by adding DEHS vapor, this indicates that DEHS did not form particulate matter in any substantial amount, instead being completely adsorbed by the bed. Figure 14 shows the particle number size distributions for K2SO4 and K2SO4 þ DEHS during one experiment using the denuder for adsorption and a DEHS concentration of 5.9 mg/m3. The particle number size distribution, as measured through the denuder, shifted toward larger particles when DEHS was added; this indicates that DEHS formed new particulate matter and was incompletely adsorbed by the denuder. The reference particle size distribution of K2SO4 and DEHS was found to be bimodal when a high concentration of DEHS was used (Figure 13) and unimodal when a low concentration of DEHS was used (Figure 14). However, this is not considered to affect the result of the experiments since the DEHS re-evaporates in oven (2). Table 3 presents the maximum mass concentrations of DEHS (which were used in this study) at which complete DEHS adsorption was achieved for the bed of granular activated carbon, denuder, steel tube, and glass bed, as well as the minimum mass concentrations of DEHS at which the DEHS adsorption was incomplete. The DEHS mass concentration was estimated using the SMPS and are corrected for dilution in the ejector diluter (dilution ratio 10). Results are presented for inlet temperatures of 200 and 300 °C. The results presented in Table 3 indicate that the adsorption capacity of the bed of granular activated carbon was at least 34 mg/m3 DEHS if T1 = 300 °C, while the adsorption 2048
Energy Fuels 2010, 24, 2042–2051
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Table 3. Mass Concentrations of DEHS at Which Adsorption in the Bed of Granular Activated Carbon, Denuder, Steel Tube and Bed of Glass Pieces Was Complete and Incompletea T1 [°C]
200
Table 4. Concentrations of CO2, CO, CH4, and H2 in the Gasifier
300
complete incomplete complete incomplete [mg DEHS/m3] adsorption adsorption adsorption adsorption carbon bed denuder steel tube glass bed a
3.6 0.5 0.5 4.1
27.5 3.7 3.9 a
34.2 0.2 0.6 a
a
component
concentration A [dry volume %]
concentration B [dry volume %]
CO2 CO CH4 H2
0.66 0.65 0.20 0.14
0.61 0.47 0.12 0.10
5.9 4.2 84.4
The experiment has not been performed.
capacity of the denuder was at least 0.5 mg/m3 DEHS, at volumetric flow rates of approximately 3 L/min. The capacity of the steel tube was similar to that of the denuder while the capacity of the glass bed was higher, indicating that the important property is the surface area of the adsorption section. This also implies that only a small part of the granular activated carbon in the denuder was available for adsorption, possibly due to mass transport limitations through the stainless steel gauze. In addition, the temperature at the inlet of the carbon bed or denuder also affected the adsorption capacity. For the carbon bed, using DEHS mass concentrations of approximately 30 mg/m3, particle formation took place at a bed inlet temperature of 200 °C but not at 300 °C. It is therefore important to keep the gas well above the dew point until it enters the adsorption section. It is known that adsorption decreases with increasing temperature since it is an exothermic process. However, it is also important to keep the compounds to be adsorbed in vapor phase until the adsorption section since condensation on the adsorbent surface could block the pores, especially, the micropores, and reduce the surface area available for adsorption.18 Tar Adsorption Capacity of the Bed of Granular Activated Carbon and of the Denuder Determined Using LaboratoryScale Gasifier Experiments. The tar adsorption capacities of the bed of granular activated carbon and the denuder were studied using a model aerosol of K2SO4 particles and tar-rich gas from a laboratory-scale gasifier. Two samples (A and B) of concentrated raw product gas from the gasifier were extracted and analyzed using a micro GC during one experiment where the denuder was used for tar adsorption. The concentrations of CO2, CO, CH4, and H2 in the gasifier are presented in Table 4 and are mean values of three analyses. Due to the large amount of nitrogen as carrier gas relative to the fuel feed, the concentration of all gases is low. Since no water is added to the process and the fuel is dried before use, the concentration of H2 is low compared with those of CO and CO2. Apart from this, the ratios between the components presented in Table 4 are in agreement with the results of other studies of air biomass gasification (e.g., Campoy et al. and Narvaez et al.33,34). The mass concentration of tars that condensed above 60 °C was approximately 30 mg/m3, as determined gravimetrically using a filter mounted downstream from a sintered steel tube. The tar concentration, when the dilution effect in the gasifier due to the large amount of nitrogen as carrier gas has been taken into account, is comparable to concentrations presented in previous studies (e.g., Hasler
Figure 15. Particle number size distributions using the 100 mm bed of granular activated carbon for tar adsorption using various mixing ratios in the dilution probe.
Figure 16. Particle number size distributions using the denuder for tar adsorption using various mixing ratios in the dilution probe.
and Nussbaumer29). The tar concentration however, is also dependent on the measurement technique, making comparisons of the results of different studies difficult. The results of the experiments using the bed of granular activated carbon and the denuder for tar adsorption are presented in Figures 15 and 16. The particle number size distributions determined using different mixing ratios in the dilution probe are compared with the particle number size distribution determined when no tar-rich gas was added in the dilution probe. In the experiments in which the bed of granular activated carbon was used for tar adsorption, the particle number size distributions were similar to the K2SO4 reference (no tar-rich gas) when mixing ratios of 1:8 and 1:17 were used; however, when mixing ratios of 1:2 and 1:4 were used, large numbers of fine particles (dB < 50 nm) were generated. This indicates that the tars were adsorbed by the bed when high mixing ratios and lower tar concentrations (6 mg/m3), at a volumetric flow rate of approximately 3.6 L/min. In the experiments in which the denuder was used for tar adsorption, fine particles (dB 30 mg/m3) than for tars produced in the laboratory-scale gasifier (3 mg/m3). The adsorption capacity of the denuder was approximately the same for both DEHS and tars, that is, approximately 0.5-0.6 mg/m3. However, the volumetric flow rates through the bed and denuder were slightly lower in the experiments using DEHS, which could explain part of the difference. Tar Adsorption Capacity of a Bed of Granular Activated Carbon Using 100 kW CFB Gasifier Experiments. The adsorption capacity of the bed of granular activated carbon was also tested when sampling particles in a tar-rich gas from a 100 kW CFB gasifier using miscanthus as fuel. The results of the raw product gas analysis are presented in Table 5. The water content of the raw gas was 52%, as determined gravimetrically. The mass concentrations of tars and phenols, determined using solid phase adsorption (SPA) and gas chromatography with flame ionization detector (GC-FID), were approximately 1.5 and 0.3 g/m3 (dry gas), respectively. Tars of molecular size up to pyrene (C16H10) could be quantified. In this case, only the bed of granular activated carbon was used for tar adsorption, since the results of the experiments with DEHS and the laboratory-scale gasifier indicated that the adsorption capacity of the denuder was too low to adsorb the tars efficiently at the volumetric flow rates used. The temperature in the bed was approximately 300 °C at the bed inlet; the temperatures in the bed are presented in Figure 17. To investigate the content of volatile particulate matter, that is, to determine whether tars had condensed on the particles during sampling, the particle number size distributions and total concentrations measured using the SMPS and APS, when the gas was bypassed and fed through the oven at 300 °C, were compared. The results presented in Figure 18 are corrected for losses in the oven determined at room temperature. A primary dilution ratio of approximately 15 was used since it was enough to adsorb the tars. Spherical particles and an effective particle density of 1 g/cm3 were assumed, which results in dB and dae being represented at the same scale.
Figure 17. Schematic of the bed of granular activated carbon used in the 100 kW CFB gasifier experiments.
Figure 18. Comparison of particle number size distribution, measured bypassed and fed through the oven.
The particle number size distributions presented in Figure 18 did not change when the gas was fed through the oven at 300 °C. This implies that the content of volatile particulate matter is low, that is, tars have been adsorbed in the bed of granular activated carbon and have not substantially contributed to the particulate matter. If tars had nucleated or condensed on the particles the tars would have reevaporated when heated to 300 °C and the geometric mean diameter of the particle number size distribution would have changed. The tar concentration in the gas entering the bed was approximately 100 mg/m3, much higher than the corresponding mass concentrations of DEHS (30 mg/m3) and of tars from the laboratory-scale gasifier (3 mg/m3). However, different measurement techniques were used to determine the tar concentrations; and naphthalene, a tar compound with a relatively low condensation temperature, dominated the tars from the CFB gasifier. Naphthalene likely did not condense above 60 °C in the experiments using the laboratory-scale gasifier and was consequently not included in the estimated tar concentration. In addition, the laboratoryscale gasifier and the CFB gasifier are of different types and scales. Probably, both the tar concentration and tar composition are different. As well, slightly different volumetric flow rates through the bed and denuder were used during the experiments, which could explain part of the difference. However, the results indicate that the tar adsorption capacity of the bed of granular activated carbon is dependent not only on the total tar concentration but also on the tar composition, since the dew point varies with both concentration and composition. Conclusions In the present study a method for high-temperature particle sampling in tar-rich gases from the thermochemical conversion of biomass was developed and tested. Both a bed of 2050
Energy Fuels 2010, 24, 2042–2051
: DOI:10.1021/ef9012196
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granular activated carbon and a denuder were used for tar adsorption. First, the transport efficiency of particles was determined both theoretically and experimentally using a K2SO4 reference aerosol, and the losses were found to be smaller in the denuder than in the bed. The adsorption capacity was then tested using a model aerosol of K2SO4 and DEHS. The adsorption capacity of the bed of granular activated carbon was found to be higher than that of the denuder. It was found that the adsorption capacity was dependent on the adsorption temperature profile; it is important to keep the compounds to be adsorbed in vapor phase until the adsorption section. The adsorption capacity was also dependent on the available surface area, and probably only a small part of the granular activated carbon in the denuder was available for adsorption. The adsorption capacity was also tested using a model aerosol of K2SO4 particles and tar-rich gas from a laboratory-scale gasifier. The results also indicated that the adsorption capacity of the bed of granular activated carbon was higher than that of the denuder; however, it was also found that at low mixing ratios tars were not completely adsorbed by the bed due to the higher tar concentrations. Only the bed of granular activated carbon was tested during experiments using a 100 kW CFB gasifier. The particles were heated to 300 °C after sampling to investigate the volatility of the particles, that is, to determine whether tar condensation had taken place. There was, however, no change in particle size distribution, indicating that the bed had adsorbed the tars present in the product gas. The mass concentration of tars from the CFB gasifier was considerably higher than those of DEHS and tars from the laboratory-scale gasifier. This indicates that the tar adsorption capacity is dependent not only on the total tar concentration but also on the tar composition, since the dew point varies with both concentration and composition. The conclusion is that the bed of granular activated carbon is suitable for tar adsorption during high-temperature particle sampling in tar-rich gases from the thermochemical conversion of biomass. Particle losses were considerably lower in the denuder than in the bed; however,
the denuder adsorption capacity was too low for the tar concentrations present in product gas at the volumetric flow rates used. However, the tar adsorption capacity seems to be dependent on both the total mass concentration and the composition of tars. The method developed in the present study enables the use of online instruments for particle characterization in tar-rich gases. This is an advantage compared to previous studies where heated filters and cascade impactors were used.10-13 However; a disadvantage with the method is that it is difficult to determine the particle losses in the measurement system. Also, a high and complex tar composition with a mixture of light and heavy tars could complicate the use of the method. The present study demonstrates the use of a bed of granular activated carbon in combination with a dilution probe for high-temperature particle sampling in tar-rich gases from the thermochemical conversion of biomass. However, some parameters need further investigation: for example, the transport efficiency in the bed of granular activated carbon needs a description, the amount of granular activated carbon needed for adsorption, the volumetric flow rate through the bed, and the adsorption characteristics using different temperatures in the bed of granular activated carbon all need optimization. The study has also demonstrated that the operation of the bed of granular activated carbon is dependent on the mass concentration and composition of tars, which is another matter that needs further investigation. Acknowledgment. Financial support through the European Commission (EC) 6th Framework Programme (CHRISGAS Project SES6-CT-2004-502587) and the Swedish Energy Agency is gratefully acknowledged. Charlotte Parsland at V€ axj€ o University is gratefully acknowledged for performing the gas analysis and tar quantification for the laboratory-scale gasifier. Marcin Siedlecki, Eleonora Simeone, and Dr. Wiebren de Jong at Delft University of Technology are gratefully acknowledged for organizing the operation of the CFB gasifier and performing the gas and tar analyses.
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