Methodology for Sampling and Characterizing Internally Mixed Soot

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Methodology for Sampling and Characterizing Internally Mixed Soot-Tar Particles Suspended in the Product Gas from Biomass Gasification Processes A. Malik,* P. T. Nilsson, J. Pagels, M. Lindskog, J. Rissler, A. Gudmundsson, M. Bohgard, and M. Sanati Ergonomics & Aerosol Technology, Lund University, P.O. Box 118, SE-22100, Lund, Sweden ABSTRACT: When biomass is used to produce fuels and green products by thermochemical conversion, the ability to handle or remove the fine particle phase in the product gas is crucial. The product gas from biomass gasification contains relatively volatile organic compounds (“tar”) condensed on nonvolatile cores of, for example, aggregated soot particles and char. The problems are, for example, that particles will poison catalysts used for upgrading of the gas and loss of thermal energy occurs when carbonaceous particles are being formed. The aim of the work is to design and use novel methodologies to characterize the particles in the product gas stream. A methodology has been developed to sample and characterize fine particles by a sampling probe connected to either a denuder or a packed bed device. The system was designed to avoid condensation of organic compounds when diluting the sample and decreasing the temperature. A flame soot generator connected to a condensation
evaporation unit was used to produce internally mixed model particles, i.e., particles consisting of a core of soot with an outer layer of condensed volatile compounds. A scanning mobility particle sizer (SMPS) and a differential mobility analyzer followed by an aerosol particle mass analyzer (APM) were used to characterize the particles. Because of the agglomerated structure of soot, the SMPS system was not adequate to fully characterize the mass of volatiles condensed onto the soot core, and therefore the DMA-heater-APM technique was used to determine the mass fraction of the condensed phase on the soot particles. The two different configurations were studied, and the sampling system was shown to work at a high load of organic mass. In both cases, the organic removal efficiency was >99.5%. Minor condensation of organics on the sampled soot was found for the denuder but not the packed bed. On the other hand, the particle losses were substantially higher for the packed bed compared to the denuder. The results showed that the tested sampling methodology can be used to get sufficient characterization of particles in the product gas and to evaluate the performance of biomass product gas cleaning systems at high temperature.

’ INTRODUCTION The formation of tar, char, and soot from hydrocarbons in biomass gasification/combustion implies loss of chemical energy in these products resulting in diminishing energy efficiency for the whole process chain.1 The presence of such particulates also undermines the cleaning system for produced syngas and downstream catalytic processing units.2
6 To preserve the heating value in the produced gas and manage the product gas cleaning system efficiently, online detection of undesired byproduct such as tar and soot in the gas stream is required. With the evolution of advanced uses of biomass derived syngas, it becomes necessary to develop progressively more stringent gas cleaning systems. However, the performance of cleaning systems such as by filtration and scrubbing of the producer gas will be determined based on the characteristics of particulate matter in the product gas up- and downstream of the cleaning device. Understanding and identifying the composition and size distribution of the existing particles in the gasifier producer gas will provide better knowledge needed for improved design of product gas cleaning systems. In thermochemical conversion of biomass, fine particle formation takes place under harsh process conditions. It is assumed to comprise a heterogeneous phase, that is, a nonvolatile core (made up of, for example, soot, char, and potassium salts) as nuclei coated with condensable compounds (mostly organics). Many previous investigations have dealt with tar as the particle phase in producer gas from biomass gasifier.7
15 In the majority of these investigations, r 2011 American Chemical Society

the particles were sampled by filter samplers and were thus neither real time nor size dependent measurements. The fine particles from combustion (conditions of excess oxygen) have been extensively studied by several research groups,2,16
22 while only a few studies in our knowledge have investigated the fine particles from biomass gasification (substoichiometric conditions with respect to oxygen).4,5,23
25 The dominant portion of emitted fine particles in efficient combustion processes is inorganic compounds17
19 of similar composition as ashes while in gasification producer gas, particles consist of predominantly carbonaceous compounds such as soot, tar, char, and inorganics.24,25 It is worth mentioning that the organic composition in biomass thermochemical conversion varies from light hydrocarbons to large polyaromatic hydrocarbons (PAHs). All organics boiling at temperatures above benzene could be considered as tars.26 Among those, a complex mixture of condensable hydrocarbons is formed, depending on the process conditions, which condenses in different parts of the process and pose problems for the overall conversion process. Beside composition, the concentration of tars in a biomass producer gas has been shown to vary with gasifier technology. It was reported to be in the order of 100 g/N m3 for updraft gasifiers, 10 g/N m3 for fluidized bed gasifiers, Received: October 19, 2010 Revised: February 21, 2011 Published: March 30, 2011 1751

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Energy & Fuels and 1 g/N m3 for downdraft gasifiers.8 The tar compounds have also been classified into primary, secondary, and tertiary tars based on the process conditions in which the compounds were formed. It was asserted that the primary tars were formed by thermal decomposition of the building blocks of biomass and contained oxygen in significant amounts. Secondary and tertiary tars are formed by destruction of primary tar compounds and recombination of fragments. In these processes, oxygen and some hydrogen is removed.7 A comparative measurement of tar carried out by traditional impinger sampling and online molecular beam mass spectroscopy (MBMS) proved the latter to be suitable for real-time monitoring of gasifier tar.11 Another effort by using a lab-scale two stage reactor for studying the release and destruction of tar has been shown to reduce tar levels. However, complete tar removal is difficult to achieve due to the complex dependence of process conditions and fuel types;12 furthermore, in another study it was shown that catalytic ceramic candle filters operated at 675
840 °C only converted about 60% of the tars.13 Real-time analysis of characteristics of such volatile compounds condensed on particles is critical to address the efficiency of the high-temperature cleaning devices and to aid in keeping the level of impurities in the cleaned product gas within acceptable limits for downstream application.16,17 To collect and characterize submicrometer particulates in this high temperature range in a reducing environment is a challenging issue, and only a few studies have addressed the particles from gasification.23
25 Recently an investigation incorporating either a packed bed or a denuder to collect the tars allowing measurement of the particle size distribution “as is” at high temperature sampling was reported.25 The study represented a major step toward developing a setup for high-temperature sampling of fine particles from biomass gasification. However, the criterion to identify during what conditions tar condensation had occurred onto the nonvolatile core particles was only qualitatively given and hence no quantitative tar removal efficiency was reported. Moreover, the nonvolatile core in biomass gasification is expected to be dominated by char and soot with agglomerated structure, unlike the compact K2SO4 model particles used in the research mentioned. Because of structure, agglomerated particles are inherently difficult to investigate on a mass basis with a scanning mobility particle sizer (SMPS). The differential mobility analyzer-aerosol particle mass analyzer (DMA-APM) method allows a primary determination of the mass of agglomerated particles and the relation between mobility diameter and mass.26
28 It can be used with a heater for size dependent online measurements of the volatile mass fraction of complex soot aerosols at a given temperature.29 The present investigation involves the development of a sampling system and a model particle generation system for production of soot and organic compounds internally mixed in the same particles that allow simulation of particles produced in a biomass gasifier. The knowledge gained from this research will help to understand conditions in gasifiers in order to reduce formation of fine particles in the gasification process and also to evaluate the performance of an efficient cleaning system downstream gasifier. Two sampling configurations were used, of which configuration 1 consists of a dilution probe and a downstream denuder while in configuration 2 a packed bed was used instead of denuder. The labscale sampling setup has been characterized and calibrated for different loading of organics onto nonvolatile soot cores. The particle characterization was performed using SMPS and DMA-heater-APM systems, which are examples of online size

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selective particle measurement techniques. This measurement was complementary to SMPS size distribution data and allowed quantification of organic compounds condensed on the soot agglomerates. The constructed sampling system will be utilized in an EC granted FP7 project GREENSYNGAS in order to measure the performance of a novel high-temperature product gas cleaning system.

’ EXPERIMENTAL METHODS Development of Sampling Configuration. The basic idea of the sampling method is to keep the sample at a temperature above the dew point of the tars throughout the system and then in the final part of the system slowly reduce the temperature in either a denuder or packed bed where activated carbon is used to capture the tars. The slow cooling of the diluted sample ensures that the supersaturation of the tars is kept low with the ambition to suppress nucleation of pure tar particles and tar condensation onto existing particles formed at higher temperature. The design of the high temperature sampling probe with denuder/packed bed and aerosol generation setup is shown in Figures 1 and 2. The model particles generation system consisted of a soot generator followed by an evaporation
condensation aerosol generator (SLG 270, TOPAS GmbH) to coat the soot with organics. The tar model compound used in this paper was dioctyl sebacate (DOS) also known as diethylhexyl sebacate (DEHS) with the chemical formula C26H50O4, boiling point 212 °C, and MW = 426 g mol
1. The vapor pressure of DOS (2.4  10
5 Pa at 37 °C) is sufficiently low to be almost exclusively present in the particle phase at room temperature. DOS has also been chosen as a model compound in previous investigations.4,5,25 The generated mixed particles were heated to 200 °C to fully vaporize the DOS. The sample consisting of the tar model (DOS) in the vapor phase and particle phase soot was then sampled by the probe and diluted 1:10. The temperature inside the probe and the inlet line was held at 200 °C. This was achieved by using a heating tape. The same heating tape was used to preheat the dilution air to 200 °C to achieve a smooth and stable temperature profile after mixing with N2. This is a simple and flexible way of heating the probe and will facilitate measurements in real conditions, i.e., at gasifiers. A description of the sampling probe, denuder, and packed bed can be seen in Figure 2. The dilution probe consisted of an inner tube (i.d. 8 mm) centered in a larger tube (i.d. 12 mm). The dilution took place at the tip of the inner tube where the dilution air was mixed with the sample. The total length of the probe was 350 mm, and the sample inlet was 6.35 mm (i.d). The total sample and dilution air flow rates were 0.3 and 3 lpm, respectively, to maintain 10 times dilution. The diluted flow after the probe was split into two lines; a variable portion was drawn through the denuder/packed bed and then delivered to the particle characterization system. A larger portion of the flow was drawn through air- and water-coolers and connected to vacuum. The denuder (or packed bed) contained activated carbon and was installed to capture gas-phase organic compounds present in the diluted aerosol particle sample flow. The connecting metal tube between the probe and the denuder or packed bed had a 6.35 mm inner diameter and was about 300 mm long. The tube was heated with a second heating tape to 200 °C in order to make sure that no condensation took place before the sample entered the section with activated carbon. The adsorption of DOS was achieved by letting the diluted flow cool down in the denuder or packed bed. The temperature was kept at 200 °C about 100 mm into the denuder and was then allowed to drop down toward a temperature of about 30 °C. The denuder consisted of an outer stainless steel tube and inner stainless steel net shaped like a tube. The inner tube (i.d. 10 mm) was centered in the outer tube (i.d. 31 mm) with the annular space between the two tubes filled with activated charcoal (Norit RB4) of 4 mm diameter pellets, Figure 2. Gustafsson et al.25 have reported an increased absorption capacity if a packed bed of activated 1752

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Figure 1. Experimental setup for overall particle generation, sampling, and characterization.

Figure 2. Sketch of dilution probe, denuder, and packed bed. carbon is used instead of a denuder. The packed bed tested here consisted of a 100 mm long stainless steel pipe, filled with activated carbon (Norit RB4). The inner diameter of the tube was 25 mm. Both the inlet and the outlet of the denuder and bed were 6.35 mm (i.d). The inlet temperature of the bed was 200 °C, and the outlet temperature was about 28 °C and no active cooling was used. The system design provides the possibility to measure before and after the probe and denuder/packed bed with an SMPS without influencing the flow through the device.

Generation of Model Aerosols Relevant for Biomass Gasification. In order to generate stable and reproducible soot particles of known size and concentration, a coflow diffusion flame soot generator fuelled by propane and filtered compressed air as an oxidant was used.30,31 With the flame quenched at a well-defined flame height using a horizontal flow of compressed air and variation of the ratio of fuel-to-air, the characteristics of soot can be changed. The soot generated (2 lpm flow rate) was further diluted (1:10) and delivered to the sampling setup using an ejector diluter (DI-1000, Dekati Inc.). The fuel and air flow rates were controlled by mass flow controllers (Bronkhorst Inc.). The quenching airflow was made symmetric by the use of ceramic honeycomb monoliths, which resulted in a stable and well-defined flame. The generated soot is highly agglomerated with a primary particle size of about 15 ( 2 nm and effective densities slightly lower than those from diesel engines (about 0.55 g/cm3 at 100 nm for particles from the soot generator). The details of the soot generator working principle and operation can be found elsewhere.30 The evaporation
condensation aerosol generator (model SLG 270, TOPAS) essentially consists of a particle generator, a screen, a saturator, a reheater, and a condensation chimney outlet in-series (Figure 3). The temperature through saturator and reheater together with flows/bypass through the screen and saturator can be controlled. The atomizer present in the commercial unit was replaced in this study by the external soot generator. A higher fraction of the particle flow that passes through the saturator increases the amount of organics available for condensation. The reheater is used to make sure that the DOS is in the gas phase and to provide controlled condensations in the laminar flow regime in the chimney. The soot particles were coated with about 28 wt % organics when originating from the evaporation
condensation aerosol generator during tests with no intentional addition of DOS (case 1-D and case 1-PB in Table 1). The aim during tests of case 1 was not to introduce any

Figure 3. A section of condensation aerosol generation system (model SLG270, TOPAS Inc.).

Table 1. Generated Model Compounds with Soot As Nonvolatile Core Coated with DOS for Tests with the Denuder or Packed Bed Connected to the Probe DOS þ soot-core

soot-core mass

mass median

test

mass concentrationa

concentrationa

diameter

cases

(mg/m3)

(mg/m3)

(nm)

1-D

0.5

0.36

126

2-D

65

0.39

475

3-D

122

0.52

550

4-D

158

0.49

645

1-PB

0.37

0.26

142

2-PB 3-PB

40 141

0.19 0.17

480 680

Denuder

Packed Bed

a Mass concentrations corrected for effective densities obtained by DMA-APM.

DOS through the system, but since lines in the aerosol generation system contained traces of DOS, coating on the soot particles was unavoidable. In different cases of DOS coating as mentioned in Table 1, the size of particles and concentration of DOS were changed while the particle numbers concentration was maintained at 1.3  106 to 1.4  106 particles/cm3. 1753

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During configuration 1, there were four cases of soot with similar soot mass concentration but strongly varying DOS concentration which were used to test the DOS collection efficiency of the sampling system. In configuration 2, three cases of soot with varying amounts of DOS were tested with the packed bed as shown in Table 1. Instrumentation. Scanning Mobility Particle Sizer. The number and mass size distribution of particles produced from the particle generation system were characterized by a scanning mobility particle sizer (SMPS 3934, TSI Inc.).17
20 It mainly consisted of a Differential Mobility Analyzer (Long column DMA, TSI Inc.) and a Condensation Particle Counter (CPC 3022, TSI Inc.) operating in a closed loop at a sample flow rate of 0.3 L/min and a sheath air flow rate of 3 L/min allowing a measurement range of 15
800 nm with scan time of 240 s up and 60 s down. A 63Ni bipolar charger (charge neutralizer) was placed upstream of the DMA to impose a bipolar equilibrium charge distribution of the soot particles. The flow rate measurements were performed using a bubble flow meter (Gillian Inc.). Aerosol Particle Mass Analyzer (APM). The APM measures the fundamental mass properties of the aerosol particles online. Coupled in series with a DMA (DMA-APM), the measured mass is related to a selected mobility diameter of the particles (Figure 4). The system can be used for the determination of the particles effective density and massmobility relationship.32 The APM consists of an outer (r2) and an inner (r1) cylinder rotating at the same rotational speed ω. The aerosol is introduced in the gap between the cylinders and a voltage (VAPM) is applied to the inner cylinder while keeping the outer cylinder grounded. Thus, the force keeping the charged particles in orbit is the electrical force, balanced by the centrifugal force. Since the centrifugal force is mass dependent, the particle mass can be determined according to m¼

qE qVAPM ¼ 2 2 r ω lnðr2 =r1 Þ rω2

ð1Þ

where r is the average radial distance to the gap between the cylinders from the axis of rotation ((r2
r1)/2), q the particle charge, and E the electrical field. The APM is described in more detail elsewhere.33 In the system, a DMA and an APM are coupled in tandem where the DMA selects particles of one mobility diameter at a time and the mass distribution of the selected particles is determined by stepping the APM voltage.33 As shown in Figure 4, a heater was introduced between the DMA and the APM which could be bypassed and a comparison of the particle mass with and without the heater could be made to quantify the degree of organic coating on the soot particles.27,28,32 The peak APM voltage (VAPM) was fitted from the raw data using a simple lognormal fitting procedure. From the fitted voltage the particle mass was calculated using Eq. 1. A long DMA (TSI, St. Paul, MN) operated at a sheath flow rate of 6 lpm and an aerosol flow rate of 1 L/min was used, followed by the APM (model 3600, Kanomax, Japan) and a CPC (model 3010, TSI Inc.). The heater consisted of a 400 mm long, 31 mm inner diameter stainless steel pipe, heated to 275 °C. No activated carbon was used in the cooling section. Pagels et al. have showed that condensed material can be removed with a similar heater with recondensation onto the particles being less than 2% in the low concentration range occurring downstream the DMA.34

’ RESULTS AND DISCUSSION The temperature profile plays an important role when optimizing the adsorption of volatile material on the active charcoal surfaces in the setup. The temperature profile in configuration 1 has been depicted in Figure 5. It shows that the temperature was 200 ((5) °C at the inlet of the denuder, the same as inside the dilution probe. This ensured that the DOS was fully vaporized before entering the denuder and that the condensation of DOS took place inside the denuder. In configuration 2, the temperature within the probe and the inlet of the packed bed was maintained at

Figure 4. DMA-heater-APM setup.

Figure 5. Temperature profile of the probe with denuder with particle flow rate of 1 lpm (arrows pointing at the sample inlet to the probe and the hot inlet and cold outlet of the denuder).

200 °C and the particle flow was allowed to passively cool down to 28 °C at the packed bed exit to make sure that the condensation of DOS took place on the activated carbon inside the packed bed. Loss of Fine Particles in the Setup. In the denuder and packed bed, tar gas molecules should be collected with as high efficiency as possible, but at the same time the particle penetration should be maximized. There are particle losses in the system due mainly to diffusion and thermophoresis which have also been investigated in order to allow quantitative particle characterization with the probe. To be able to determine the losses in the sampling setup, soot was used in configurations 1 and 2. Ratios of SMPS measurement with pure soot particles (evaporation
condensation generator bypassed) upstream and downstream the probe
denuder/ packed bed system at 25 and 200 °C are presented in Figure 6. The measured size dependent penetration efficiency of the particles through the denuder and packed bed is shown in Figure 6. The losses of particles were as expected higher in the packed bed compared to the denuder.25 The difference in penetration between measurements at room temperature and 200 °C was on average about 4% suggesting that thermophoretic losses are about 3
5% in the denuder. The results are compared with a simple theoretical penetration model using diffusion and thermophoresis at 200 °C with assumption of laminar flow. The following expression for the combined theoretical penetration of the particles under diffusion, Pdiff, and thermophoresis, Pthermo, has been used for the calculation. ð2Þ

P ¼ Pdiff Pthermo Penetration due to diffusion alone is given as:

35

Pdiff ¼ nout =nin ¼ 1
5:50μ2=3 þ 3:77μ 1754

for

μ < 0:009

ð3Þ

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Figure 6. Penetration of soot particles through the system consisting of the probe and either the denuder or the packed bed setup at 25 and 200 °C at a flow rate of 1.0 lpm. No dilution was used to improve the precision in the loss quantification.

Pdiff ¼ 0:819 expð
11:5μÞ þ 0:0975 expð
70:1μÞ

for

μ g 0:009

ð4Þ

and μ ¼ DL=Q

ð5Þ

where μ is the deposition parameter, D is the diffusion coefficient of the particles, L is the length of the tube, and Q is the volumetric flow rate through the tube. Penetration due to thermophoresis has been represented by the following relation Pthermo ¼ 1
k dT

ð6Þ

where k represents the correction for losses per degree C due to thermophoresis. From comparison between losses found in experiments at 200 °C and room temperature, k was estimated to be 0.00023 and 0.0002 per degree change in temperature for the cases of the denuder and packed bed, respectively. An “effective” tube length of 20 m was fitted for the denuder, and a channel length of 80 m was fitted for the packed bed. The theoretical losses shown in Figure 6 fit reasonably well with actual size dependent measured losses. Characterization of Model Particles from the Particle Generation System. The number size distributions of the model particles measured before entering the probe
denuder/packed bed setup are shown in Figure 7a. The higher the DOS concentration, the more the particle size is shifted toward larger particles due to condensational growth. Note that the aerosol dynamics causes a very narrow particle size distribution upon condensational growth. The mass concentration of condensed DOS on soot upstream the probe (Table 1) was calculated from the SMPS volume concentration. The calculation was based on the density of 0.91 g/cm3 for the DOS fraction and 1.8 g/cm3 for the soot mass fraction in denuder cases 2-D
4-D. This is equivalent with assuming that the agglomerated soot particles are fully coated with DOS and can be considered spherical. In both configurations for case 1 when the particles were still nonspherical, the measured effective densities with the DMA-APM setup were used to

Figure 7. Soot coated with 0.5 (case 1-D), 65 (case 2-D), 122 (case 3-D), and 158 (case 4-D) mg/m3 DOS (a) before entering the sampling probe and (b) after the probe and denuder in configuration 1.

determine the mass concentration from SMPS measurements. Although for case 1 the aerosol generator was held at room temperature, the DMA-heater-APM measurements showed that the soot picked up traces of DOS (Table 1). Note that SMPS measurements alone were inconclusive (due to the agglomerated particle shape) to deduce the soot and DOS concentration in this case. Configuration 1: Denuder Connected to the Sampling Probe. The particles were heated to 200 °C to transfer the DOS to the gas phase as would be the case during measurements in a real gasifier. In order to investigate the efficiency of the setup and influence of the probe on the size distribution, the sample through dilution probe was passed through denuder. In Figure 7b, the number size distributions downstream the probe and denuder are shown (corrected for probe dilution ratio 1:10). The SMPS results show that the denuder captures a majority of the DOS and that condensation of DOS on the soot particles is minor even at the highest DOS concentration (case 4-D). Measurements of the pure soot downstream the soot generator shows that the geometric mean diameter (GMD) of the soot unintentionally shifted from 70 nm during case 1-D to 90 nm during cases 2-D
4-D. Therefore the size-shift downstream the probe between cases 1-D and 2-D is most likely caused by a shift in the soot characteristics from the soot generator during these experiments. Between other experimental series, the variations in the soot GMD was less than 5 nm. Figure 8 illustrates the mass based characteristics of soot upstream and downstream the probe and denuder. The total mass concentration varied between 0.5 and 158 mg/m3 upstream the probe due to the strongly varying DOS mass concentration. There is also a strong variation in mass median diameter. The 1755

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Figure 8. Mass weighted GMD and total mass concentration of particles in tests with DOS concentration of 0.5, 65, 122, and 158 mg/m3 (cases 1-D
4-D) before and after the sampling probe þ denuder calculated from SMPS data.

total mass concentration of the samples downstream the probe and denuder increases slightly at higher coating of DOS. It appears that this is caused by condensation of DOS due to incomplete denuder absorption. Also the geometric mean diameter shows a slight increase with increasing original DOS concentration. However, it should be pointed out that the variations in soot output mentioned above between the test cases could also affect this result. Thus, while it can be shown from the SMPS measurements that the majority of the DOS is captured in the denuder, the DMA-heater-APM data is crucial to quantify the fraction of the DOS that is collected on the particles. Configuration 2: Packed Bed Connected to Sampling Probe. The denuder present in configuration 1 was replaced by a packed bed (shown in Figure 2) in order to further enhance the removal capacity of the sampling system. This configuration was tested for cases of soot coated with 40 mg/m3 DOS and 141 mg/m3 DOS at conditions similar to configuration 1 (cases 2-PB and 3-PB in Table 1). The number size distribution of particles before and after the probe-packed bed setup is given in Figure 9a,b. The obtained data show clearly that the packed bed removes the majority of DOS. Collection Efficiency of DOS for the Denuder and Packed Bed Using the DMA-Heater-APM Technique. The DMAheater-APM technique was used to fully quantify the removal of DOS from the particles and to test the capacity of the sampling probe coupled with both the denuder and packed bed. Soot particles sizes of 87 and 150 nm were selected with the DMA for the heater-APM tests. The larger size is close to the mass weighted GMD of the soot aerosol and thus gives a good estimate relevant for the whole polydisperse size distribution. A smaller size (87 nm) was chosen to investigate whether smaller particles are enriched in DOS to a higher degree than larger particles. In the packed bed (configuration 2) essentially no organics had condensed on the soot particles as shown in Figure 10, when a sample containing 40 mg/m3 of DOS entered the probe. At 141 mg/m3 DOS, a slight mass fraction was evaporated in the DMA-heater-APM setup at 275 °C. However it was still less than 10%. This clearly confirms that when the sample is cooled in the packed bed, essentially all DOS is collected in the bed and the particles leaving the system are close to pure soot particles. A different result was obtained when using the denuder. During case 1-D (0.35 mg/m3 DOS entering the probe), essentially pure soot particles were sampled after the denuder. However, the higher the initial DOS concentration, the higher the volatile mass fraction in the particles. This indicates that a fraction of the DOS had

Figure 9. Number concentration of soot with added 0.5 mg/m3 DOS (case 1-PB), 40 mg/m3 DOS (case 2-PB), and 141 mg/m3 DOS (case 3-PB) (a) before and (b) after sampling probe-packed bed setup (configuration 2).

Figure 10. Mass fraction of DOS in the particles exiting the denuder/ packed bed as a function of DOS concentration entering the dilution probe. The DMA-heater-APM setup was used for 150 and 87 nm particles.

condensed on the soot particles. At 158 mg/m3, about half of the mass of sampled particles consisted of DOS (150 nm particles) compared to 23
33% of DOS with 65 mg/m3 DOS. A slightly higher DOS mass fraction was found for the smaller (87 nm) particles compared to the larger (150 nm) particles. The performance of the setup to collect DOS was quantified in terms of the collection efficiency (CE) defined as CE % ¼ DOS entering denuder or packed bed
DOS Leaving denuder or packed bed DOS entering denuder or packed bed

ð7Þ 1756

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Figure 11. Collection efficiency of denuder and packed bed setup as calculated by SMPS.

With the use of the DOS concentration data upstream and downstream the probe given above, the collection efficiency could be quantified with high precision. The method to calculate the DOS collection efficiency consisted of the following three steps: (1) determining the dilution corrected total volume concentration from the SMPS data28 upstream and downstream the probe, (2) using the effective density from the DMA-APM system to calculate the total mass concentration (soot þ DOS) upstream and downstream the probe, and (3) calculating the DOS mass concentration upstream and downstream the probe from the determined mass fraction of DOS in the particles (Figure 10). As can be seen from Figure 11, the mass based DOS collection efficiency was found above 99.6% for all tested cases of the denuder and the packed bed. The condensation of DOS in the case of the denuder led to a slightly lower collection efficiency (about 99.7 for the denuder compared to 99.98% for the packed bed). The remarkably high precision in the determination of the collection efficiency of DOS is a result of a high precision (∼5% relative error) in the DMA-APM measurement of the (rather small fraction of) DOS that is condensed on the particles exiting the sampling configuration. In this investigation, moderate soot concentrations were used in all experiments, while up to very high DOS concentrations were used (it is well-known that tar concentrations from gasification can be very high). The experiments at the highest DOS concentrations therefore gave very high collection efficiencies (>99.6%). Even though only a very small fraction of the DOS entering the system condensed onto the soot particles, about 50% of the mass of the particles still consisted of DOS. The small fraction of DOS ending up on the soot particles for the denuder is negligible for most applications. One example of an application where it may matter is in health effect studies where the particle surface chemistry can be of importance for the toxicological response. Future studies should investigate the separation efficiency with substantially higher soot concentrations combined with the high DOS concentrations. Gustafsson and Strand (2010) also tested the capacity of a denuder and a packed bed to adsorb gas phase DOS from a sampling system designed for biomass gasification studies.25 Their method involved assessing the DOS collection by comparing particle size distributions of spherical/compact K2SO4 particles upstream and downstream the denuder/packed-bed. They found complete DOS removal up to a concentration of 0.5 mg/ m3 for the denuder and 3.6 mg/m3 for the carbon bed when an inlet temperature of 200 °C was used. This is slightly lower than for our experiments in the case of the denuder. Perhaps it can be

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explained by differences in temperature profiles of the systems between the two studies. The axial (and radial) temperature profile in the cooling section is a key to understand the collection efficiency of the denuder. Previous studies in our lab with a less controlled temperature profile showed significantly reduced collection efficiency at 2 mg/m3 of DOS coated on soot.4 Our results are also of relevance for thermal denuders commonly used to characterize the volatility of atmospheric and combustion aerosols, where the aim is to remove volatile compounds at a given temperature from a sample originally at room temperature. Recondensation of volatilized molecules is often discussed as a potential artifact in such devices.36 We found that for the configurations used in our study, recondensation is negligible at an initial DOS concentration of about 0.5 mg/m3 entering the denuder (case 1-D), while evidence for minor recondensation (∼0.3% of entering DOS) was found in the concentration range of 60
160 mg/m3 (cases 2-D
4-D) entering the denuder. Another related application that could benefit from our results is the volatile particle remover used in the recent European PMP standard for number concentration measurement of exhaust emissions from cars. In that setup, a first heated dilution stage is followed by a heated tube (300 °C) followed by a second ejector diluter where the sample is cooled to ambient conditions and further diluted.37 Future studies should focus on comparing the risk for condensation of volatile organics (such as tars and organics from engine exhaust) using our denuder setup and the ejector dilution setup commonly used in the PMP standard.

’ SUMMARY AND CONCLUSIONS Two configurations, which can be used to investigate the formation of fine particles in the product gas from biomass gasification, have been tested. The configurations consisted of a dilution sampling probe connected to either a denuder or a packed bed device. A two component test aerosol was used which consisted of agglomerated nonvolatile soot cores and dioctyl sebacate (DOS) to simulate tars in real gasifiers. In a first stage, a scanning mobility particle sizer (SMPS) was used to evaluate the removal capacity of the denuder and the packed bed. Because of the agglomerated structure of the soot, the SMPS system was insufficient to fully quantify the condensation of volatile material on soot. Therefore, an aerosol particle mass analyzer (APM) connected downstream a DMA and a heater was used as a complement to quantitatively study the collection efficiency in the devices regarding organics condensed on the soot particles. Concentrations of up to 150 mg/m3 could be sampled with less than 0.4% and 0.02% of the DOS condensing onto the soot core for the denuder and the packed bed, respectively. While the gas-phase collection efficiency in the packed bed is higher, so are also the particle losses. When quantitative measurements are to be performed for small particle sizes, it may be advantageous to use the denuder. The systems were tested with up to very high DOS concentrations and moderate soot concentration levels. Therefore, the DOS mass fraction of the sampled particles was significant (up to 50%) when tested with the highest DOS concentrations for the denuder. The detailed investigation of particle characteristics and separation of organic material by the dilution probe connected with the denuder or packed bed has shown to provide a novel method to characterize fine particles in the product gas from 1757

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Energy & Fuels gasified biomass. The obtained results of this work provide methodology to be able to fully characterize such particles and to aid future development of advanced technology in hightemperature cleaning devices. The sampling techniques will be demonstrated on a slipstream of a real gasifier at higher temperatures in order to provide quantitative and real time monitoring of the performance of the gas cleaning system.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: þ46-46222 3284. Fax: þ46-462224431.

’ ACKNOWLEDGMENT The authors would like to acknowledge the financial support by the Swedish Energy Agency (STEM), the Swedish Research Council (FORMAS), and the European Commission (EC) 7th Framework Programme (GREENSYNGAS Project, Contract Number 213628). ’ REFERENCES (1) Cao, Y.; Wang, Y.; Riley, J. T.; Pan, W.-P. Fuel Process. Technol. 2006, 87, 343–353. (2) Strand, M.; Bohgard, M.; Swietlicki, E.; Gharibi, A.; Sanati, M. Aerosol Sci. Technol. 2004, 38 (8), 757–765. (3) Larsson, A. C.; Einvall, J.; Sanati, M. Aerosol Sci. Technol. 2007, 41 (4), 369–379. (4) Lindskog, M.; Malik, A.; Pagels, J.; Rissler, J.; Wierzbicka, A.; Sanati, M. In Proceedings of NOSA, Nordic Aerosol Conference, Lund, Sweden, November 12
13, 2009; P 25, p 51. (5) Lindskog, M.; Malik, A.; Pagels, J.; Sanati, M. The Fifth International Conference on Thermal Engineering Theory and Applications, Marrakesh, Morocco, May 10
14, 2010; Paper ID 30, pp 28. (6) Moradi, F.; Brandin, J.; Sohrabi, M.; Faghihi, M.; Sanati, M. Appl. Catal., B 2003, 46 (1), 65–76. (7) Technical Specification: Biomass Gasification-tar and Particles in Produced Gas-Sampling and Analysis, CEN/TS 15439, 2006; E. (8) Rabou, L. P. L. M.; Zwart, R. W. R.; Vreugdenhil, B. J.; Bos, L. Energy Fuels 2009, 23, 6189–6198. (9) Carpenter, D. L.; Deutch, S. P.; French, R. J. Energy Fuels 2007, 21, 3036–3043. (10) Chen, Y.; Luo, Y.-H.; Wu, W.-G.; Su, Y. Energy Fuels 2009, 23, 4659–4667. (11) Rapagn
a, S.; Gallucci, K.; Di Marcello, M.; Foscolo, P. U.; Nacken, M.; Heidenreich, S. Energy Fuels 2009, 23, 3804–3809. (12) Heidenreich, S.; Nacken, M.; Salinger, M.; Foscolo, P. U.; Rapagn
a, S. Presented at GCHT-7, Newcastle, Australia, June 23
25, 2008. (13) Toledo, J. M.; Corella, J.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 1389–1396. (14) Diaz-Somoano, M.; Martinez-Tarazona, M. R. Energy Fuels 2005, 19, 442–446. (15) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 1535–1543. (16) Rissler, J.; Pagels, J.; Swietlicki, E.; Wierzbicka, A.; Strand, M.; Lillieblad, L.; Sanati, M.; Bohgard, M. Aerosol Sci. Technol. 2005, 39 (10), 919–930. (17) Lillieblad, L.; Szpila, A.; Strand, M.; Pagels, J.; Rupar-Gadd, K.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Energy Fuels 2004, 18 (2), 410–417. (18) Strand, M.; Pagels, J.; Szpila, A:; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Energy Fuels 2002, 16, 1499–1506.

ARTICLE

(19) Wierzbicka, A.; Lillieblad, L.; Pagels, J.; Strand, M.; Gudmundsson, A.; Gharibi, A.; Swietlicki, E.; Sanati, M.; Bohgard, M. Atmos. Environ. 2005, 39 (1), 139–150. (20) Johansson, L. S.; Leckner, B.; Gusstavsson, L.; Cooper, D.; Tullin, C.; Potter, A. Atmos. Environ. 2004, 38, 4183–4195. (21) Hasler, P.; Nussbaumer, T. In Proceedings of the 10th European Conference and Technology Exhibition, Murzburg, Germany, June 8
11, 1998; pp 1330
1333. (22) Obernberger, I.; Brunner, T.; Joller, M. In Aerosols from Biomass Combustion; International Seminar at 27 June 2001 in Zurich (Switzerland) by IEA Bioenergy Task 32 and Swiss Federal Office of Energy, Nussbaumer, T., Ed.; 2001; pp 69
74. (23) Gustafsson, E.; Strand, M.; Sanati, M. In Proceedings of the 15th European Biomass Conference & Exhibition, Berlin, Germany, May 7
11, 2007; pp 1128
1130. (24) Gustafsson, E.; Strand, M.; Sanati, M. Energy Fuels 2007, 21, 3660–3667. (25) Gustafsson, E.; Strand, M. Energy Fuels 2010, 24, 2042–2051. (26) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion, NREL/TP570
25357, National Renewable Energy Laboratory (NREL): Golden, CO, 1998. (27) Sakurai, H.; Tobias, H. J.; Park, K.; Zarling, D.; Docherty, K. S.; Kittelson, D. B.; McMurry, P. H.; Ziemann, P. J. Atmos. Environ. 2003, 37, 1199–1210. (28) Pagels, J.; Wierzbicka, A.; Nilsson, E.; Isaxon, C.; Dahl, A.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M. J. Aerosol Sci. 2009, 40, 193–208. (29) Park, K.; Dutcher, D.; Emery, M.; Pagels, J.; Sakurai, H.; Scheckman, J.; Qian, S.; Stolzenburg, M. R.; Wang, X.; Yang, J.; McMurry, P. H. Aerosol Sci. Technol. 2008, 42, 801–816. (30) Malik, A.; Abdulhamid, H.; Pagels, J.; Rissler, J.; Lindskog, M.; Nilsson, P.; Bjorklund, R.; Jozsa, P.; Visser, J.; Spetz, A.; Sanati, M. Aerosol Sci. Technol. 2011, 45 (2), 284–294. (31) Lutic, D.; Pagels, J.; Bjorklund, R.; Josza, P.; Visser, J. H.; Grant, A. W.; Johansson, M. L.; Passo, J.; F€agerman, P. E.; Sanati, M.; Spetz, A. L. J. Sens. 2010, 2010, 1–6. (32) Park, K.; Cao, F.; Kittelson, D. B.; McMurry, P. H. Environ. Sci. Technol. 2003, 37, 577–583. (33) Ehara, K.; Hagwood, C.; Coakley, K. J. J. Aerosol Sci. 1996, 27 (2), 217–234. (34) Pagels, J.; Khalizov, A. F.; McMurry, P. H.; Zhang, R. Aerosol Sci. Technol. 2009, 43, 629–640. (35) Hinds, W. C. Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles, 2nd ed.; John Wiley & Sons, Inc.: New York, 1999; p 163. (36) 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. (37) Giechaskiel, B.; Chirico, R.; DeCarlo, P. F.; Clairotte, M.; Adam, T.; Martini, G.; Heringa, M. F.; Prevot, A. S. H.; Baltensperger, U.; Astorga, C. Sci. Total Environ. 2010, 408, 5106–5116.

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