Characterization of Particulate Matter in the Hot Product Gas from

ARTICLE pubs.acs.org/EF

Characterization of Particulate Matter in the Hot Product Gas from Indirect Steam Bubbling Fluidized Bed Gasification of Wood Pellets Eva Gustafsson,*,† Leteng Lin,† Martin C. Seemann,‡ Jennie Rodin,§ and Michael Strand† †

School of Engineering, Bioenergy Technology, Linnæus University, SE-351 95 V€axj€o, Sweden Chalmers University of Technology, SE-412 96 G€oteborg, Sweden § SEP Scandinavian Energy Project, Bror Nilssons Gata 16, SE-417 55 G€oteborg, Sweden ‡

ABSTRACT: This study characterized the particulate matter (PM) formed during indirect steam bubbling fluidized bed gasification of wood pellets at atmospheric pressure. A system including a dilution probe, a bed of granular activated carbon, and a thermodenuder was used to sample the PM at high temperature with the aim of separating it from condensing inorganic vapors and tars. The particle mass size distribution was bimodal with a fine mode in the 0.5-μm size range. The coarse mode was representatively characterized, but condensing inorganic vapors and tars complicated the evaluation of the results for the fine-mode PM. Morphological analysis of the PM indicated that the char content was low. The inorganic fraction was dominated by potassium and chlorine for fine-mode PM and calcium and silicon for coarse-mode PM.

’ INTRODUCTION To reduce the emissions of fossil carbon dioxide, biomass is increasingly being used for energy purposes. Gasification can be used to convert biomass into gas used for heating, power, and biofuel production. However, compounds in both gas- and particle-phase that may be regarded as contaminants are produced during the gasification process, including fine- and coarsemode particles (e.g., fly ash and fragmented nonvolatilized material) and volatilized components and tars that condense and form particulate matter (PM) during cooling. When producing biofuels through biomass gasification, cleaning and upgrading the product gas is crucial, and various techniques, such as filters and catalysts, can be used. A precise characterization of the various contaminants and their physical states at actual process temperatures would facilitate the design and optimize the function of both filters and catalysts. Although the particle formation mechanisms during biomass combustion have been thoroughly studied, few studies have focused on the particle formation mechanisms during biomass gasification. Even if the same fuel is used in a gasification process, the temperature is usually lower compared to combustion and the environment is reducing affecting both the formation and oxidation of char and the volatilization and formation of inorganic vapors. Several gasification techniques are available, varying in terms of the reactor type used (i.e., fixed bed, fluidized bed, or entrained flow), gasification agent (i.e., air, oxygen, steam, or a combination of these), heat supply (i.e., direct or indirect), and reactor pressure (i.e., atmospheric pressure or pressurized).1 For biofuel production, the nitrogen content of the product gas should be as low as possible, suggesting oxygen gasification or indirect steam gasification in which the gasification and oxidation reactions are physically separated. Gasification is an endothermic process, and in indirect fluidized bed gasifiers the heat required is supplied by the circulating bed material.2 The char produced in the gasifier is oxidized separately and used for heating the bed material. Steam is usually added in the indirect gasifier to promote hydrogen r 2011 American Chemical Society

formation through the water
gas shift reaction and carbon
steam reactions. 3 Indirect gasification generates gas of higher heating value than does air gasification; however, the tar concentration could be high due to the pyrolytic process involved.3 The design of indirect gasifiers varies. In G€ussing (Austria), gasification is performed in a dual fluidized bed reactor where the gasification section is fluidized with steam and the oxidation section is fluidized in air.4 At the Energy Research Centre of The Netherlands (ECN; Petten, The Netherlands) a similar process has been developed, however, the gasification and oxidation sections are combined in a single vessel to facilitate pressurization.5 Typical concentrations of main gas-phase compounds, water vapor content, and tar concentration for the gasifiers at G€ussing and ECN are presented in Table 1. Particle characterization at high temperature is usually problematic since both inorganic and organic vapors may form PM through nucleation and condensation as the gas cools after sample extraction. When sampling PM in the hot product gas produced by biomass gasification, the main concern is tar condensation at temperatures below 400 °C as well as condensation of alkali chloride vapors at temperatures above 500 °C. Various types of probes have been designed to control mainly inorganic vapor condensation when sampling PM produced by biomass combustion.6
8 In these probes, either the vapors are forced to condense and deposit on the probe walls, minimizing particle formation in the probe, or the particles formed are easily distinguishable from the original particles. Various techniques have been used to sample PM produced by biomass gasification at temperatures above the tar dew point. Gabra et al.9 and Yamazaki et al.10 used heated filters to keep the tars in vapor phase while collecting PM produced by biomass gasification. Corresponding measurements were made by Hasler and Received: December 16, 2010 Revised: February 9, 2011 Published: March 21, 2011 1781

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Table 1. Concentrations of Main Gas-Phase Compounds, Water Vapor, and Tar for the Gasifiers at G€ ussing and ECN component (dry volume %, vol %)

a

G€ussing4

ECN5

CO

22
25

24
43

H2 N2

35
45 2
3

16
26 0.4
7

CO2

20
23

10
21

CH4

9
12

11
15

C2H4

2
3

3.7
5

C3H8

n.a.a

n.a.

C2H6

n.a.

0.2
0.9

H2O (wet vol%)

n.a.

25
43

tar (g m
3)

2
10

24
43

n.a. indicates that the concentration was not available.

Nussbaumer11 and van der Nat et al.12 using heated cascade impactors instead of heated filters. Using porous activated carbon for adsorption, tars can be removed before PM collection. In previous studies, a measurement system including a dilution probe and a bed of granular activated carbon was used to sample gas, dilute it with nitrogen, and adsorb the tars at temperatures above the tar dew point.13
15 Instead of separating the condensing vapors before condensation occurs, volatile PM can be vaporized after condensation using a thermodenuder. Denuder operation is based on the fact that the diffusion velocity of gaseous species is higher than the diffusion velocity of particles.16 A thermodenuder consists of an evaporation section where the aerosol is heated, followed by a denuder section where adsorption and/or condensation and cooling take place.17 In the present study, the PM formed during indirect steam bubbling fluidized bed gasification of wood pellets at atmospheric pressure was characterized. The gasifier (2
4 MW fuel) is located at Chalmers Technical University (G€oteborg, Sweden) and is integrated with a circulating fluidized bed (CFB) biomass boiler to supply the hot bed material.18 A measurement system, including a dilution probe, a bed of granular activated carbon, and a thermodenuder, was used for the particle sampling at high temperature with the aim of quenching particle dynamics and adsorbing and removing tars upstream from the particle characterization instruments. Both on- and off-line measurement techniques as well as morphological and elementary analysis techniques were used for the particle characterization.

’ EXPERIMENTAL SECTION Indirect Bubbling Fluidized Bed Gasifier System. The gasifier system used is located at Chalmers Technical University, G€oteborg, Sweden, and is of atmospheric bubbling fluidized bed (BFB) type (Figure 1). The gasifier is an add-on to the existing 12 MWth CFB boiler used to heat the university campus with wood chips as fuel. The principle of the process is similar to that of the indirect gasification process applied, for example, in the G€ussing gasifier.4 Instead of building a stand-alone gasification reactor, the thermal flywheel of the CFB boiler is used as a means of devolatilizing the biomass. As the gasifier is connected to the boiler via two loop seals, the unit can be switched on and off while the CFB boiler remains fully available. Each system, i.e., the gasifier and the CFB boiler, has its own fuel feeding system. The capacity of the gasifier is 2
4 MW fuel and the gasification temperature was 850 °C during the measurements. The silica sand used as bed material enters the gasifier chamber via a gas seal (Figure 1, no.

12) at a temperature of approximately 100 °C above the gasification temperature. The gasifier bed is fluidized using 140 °C steam at a pressure of approximately 1.2 bar and a rate of 200 kg h
1. The biomass and steam are heated by the bed material in the gasifier, forming product gas containing a mixture of gases, particles, tar, char, and ash. The gas phase exits at the top of the gasifier, while the char and ash are recirculated, together with the bed material, to the CFB boiler where the char is combusted. Wood pellets were used as fuel and fed at a rate of 280
300 kg h
1. The wood pellets had a moisture content of 9 weight % (wt %) and an ash content of 0.3
0.5 wt %. Calcium and potassium dominated the wood pellet ash. Both gas and particles were sampled approximately 3 m downstream from the gasifier exit (Figure 1, measurement port). The gas temperature at the gasifier exit was approximately 785 and 770 °C at the sampling point (i.e., tars are present in vapor phase). Downstream from the sampling point, the product gas is fed into the CFB boiler and burned. The heat output of the CFB boiler is balanced by feeding additional wood chips to the required output of 6
8 MW. Further details regarding the gasifier system are presented by Thunman and Seemann.18 Gas and Particle Measurement Systems. To determine the concentration of the main gas-phase components and tars, gas was sampled from the raw gas line via a raw gas port using a 7-m heated tube. Inside the raw gas port, a ceramic filter separated particles from the gas stream. To protect the instruments from water and tars, a gas cleaning system consisting of two water-cooled heat exchangers (15 °C), a Peltier cooler (
5 °C) filled with solvent, a gas pump, a cumulative volumetric flow meter, and a rotameter for instant flow measurement was set up. In the first heat exchanger, the gas was scrubbed using a circulating solvent (2-propanol). The mixture of solvent, water, and tars was collected in a reservoir and used to determine the contents of water vapor and tars in the raw gas. The particle measurement system was previously used to sample and characterize PM produced by biomass gasification.13
15 The main components used for sample extraction and conditioning are a dilution probe and a bed of granular activated carbon. The dilution probe (Figure 2) was used to sample and dilute the gas at high temperature with the aim of quenching particle dynamics, such as particle coagulation and nucleation, and the condensation of inorganic vapors. Nitrogen was used as the dilution gas and was introduced into the dilution section, near the tip of the probe. The dilution ratio (primary dilution) was varied during the measurements by adjusting the nitrogen flow. Downstream from the dilution probe, the gas was passed through a bed of granular activated carbon (Figure 3) to adsorb the tars before cooling the gas, thereby trying to prevent the tars from condensing and contributing to the PM. The bed of granular activated carbon was divided into two compartments, 100 mm and 60 mm long, filled with a total of 70 cm3 of granular activated carbon (activated charcoal Norit, type RB3, diameter 3 mm, surface area 1100 m2 g
1; Norit Nederland B.V., Amersfoort, The Netherlands). The temperature was measured at the bed inlet (T1) and at three positions in the bed (T2
T4), according to Figure 3, using thermocouples. The temperature was controlled using heating tapes, and was as follows during the measurements: T1 = 250
270 °C, T2 = 120
160 °C, T3 = 50
100 °C, and T4 = 40
80 °C. Positions T2
T4 were located near the wall of the bed of granular activated carbon, so the temperatures in the center of the bed were probably higher than indicated by the T2
T4 values. The schematic of the particle measurement system is presented in Figure 4. Downstream from the bed of granular activated carbon, an ejector diluter was used to pull the sample flow through the system (approximately 2.2 dm3 min
1) and further dilute (secondary dilution) the gas with particle-free dry compressed air; the secondary dilution ratio was approximately 10. Downstream from the ejector diluter, the gas could either bypass or pass through a thermodenuder to investigate the volatility of the PM, i.e., to determine whether tars had condensed on the 1782

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Figure 1. Schematic of the circulating fluidized bed (CFB) boiler and bubbling fluidized bed (BFB) gasifier.

Figure 3. Schematic of the bed of granular activated carbon used for tar adsorption. Figure 2. Schematic of the dilution probe. particles. A CO analyzer (Rosemount Analytical, Solon, OH) based on infrared technology was used as a reference for adjusting and determining the dilution ratio. Gas and Particle Characterization. The main gas-phase components were continuously measured using a CP-4900 micro gas chromatograph (Micro-GC; Varian Inc., Palo Alto, CA) with a Molesieve 5A column and a poraPLOT Q-column, both equipped with thermal conductivity detectors. The tar concentration was determined gravimetrically using thermogravimetric analysis (TGA, model 701; Leco, St. Joseph, MI). A mixture of solvent, water, and tars was heated

in crucibles at 30 °C until the solvent was evaporated, 3 h at 105 °C to evaporate the water, and a final 30 min at 800 °C to clean the crucibles and control the weight. The amount of gravimetric tar was determined as the weight difference between the empty crucible and the crucible at the end of heating to 105 °C, and as the weight loss between 105 and 800 °C; these two weight differences should be the same if the analysis is correct. The water vapor content was determined gravimetrically by weighing the mixture of solvent, water, and tars. The water vapor content was calculated using the volume of solvent, the calculated concentration of tars, and the volumetric flow rate of dry gas. A model 3080 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.), 1783

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Figure 4. Schematic of the particle measurement system. was used to determine the number size distribution of particles with mobility equivalent diameters (dB) of 10
420 nm. To study the number size distribution of particles with aerodynamic diameters (dae) of 0.5
20 μm, a model 3321 aerodynamic particle sizer (APS; TSI Inc.) was used. To study the particle mass size distribution, a low-pressure impactor (LPI; Dekati Ltd., Tampere, Finland) with a d50 of 0.030
10.33 μm for stages 1
13 was used. The selected mean diameter of particles collected on a stage was the geometric mean of the d50 of the stage and the d50 of the next higher stage, giving a total of 12 mean dae values between 0.04 and 8.4 μm used for presenting the particle mass size distribution. PM was collected using the LPI on aluminum and polycarbonate substrates (Nucleopore, Whatman Inc., Brentford, UK), greased with Apiezon lowvacuum grease (Apiezon, Manchester, UK), and on ungreased aluminum substrates. All substrates were analyzed gravimetrically, and substrate weight could be determined with a precision of (5 μg. Three ungreased aluminum substrates (dae ≈ 0.08 μm, 0.8 μm, and 2.0 μm) were analyzed using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) to obtain information about particle morphology and qualitative elementary composition. The polycarbonate substrates were analyzed using particle-induced X-ray emission (PIXE) to determine the concentration of elements with atomic numbers (Z) > 12.

Particle Transport Efficiency in the Bed of Granular Activated Carbon and Thermodenuder. Losses due to diffusion

dominate for fine-mode particles (diameter 8 μm was low and, in addition, was difficult to determine due to a low particle number concentration.

’ RESULTS AND DISCUSSION Gas and Tar Analysis. The concentration of the main gasphase compounds is presented in vol % on a dry basis in Table 2; the values are normalized to 100%. In addition, the water vapor content and the tar concentration are presented. In comparison with the compositions of gas produced by the G€ussing and ECN gasifiers (Table 1), the gas compositions at Chalmers and ECN were similar; however, the concentration of N2 was significantly higher at Chalmers due to air leaking in 1784

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Table 2. Concentrations of Main Gas-Phase Compounds, Water Vapor, and Tar component (dry vol%) CO

32.0

H2 N2

22.3 16.0

CO2

14.1

CH4

10.9

C2H4

3.9

C3H8

0.7

C2H6

0.2

H2O (wet vol%) tar (g m
3)

71

Figure 8. Particle mass size distributions using three primary dilution ratios (PDRs) and an effective particle density of 1 g cm
3 measured downstream from the thermodenuder at 400 °C.

8

Figure 7. Particle mass size distributions using three primary dilution ratios (PDRs) and an effective particle density of 1 g cm
3 as measured bypassing the thermodenuder.

through the fuel feeding system, probably leading to additional CO2 and H2O through combustion reactions. In addition, the water vapor content was significantly higher at Chalmers than at ECN. The gas compositions in terms of CO, H2, CO2, and CH4 were similar at Chalmers and ECN, while Chalmers and G€ussing gas compositions differed in terms of CO, H2, and CO2 indicating a stronger contribution from the water
gas shift reaction at G€ussing. The tar concentration at ECN was remarkably higher than at either Chalmers or G€ussing; however, the use of different measurement techniques could explain at least part of the difference. Evaluation of the Operation of the Particle Measurement System Determined Using Online Analysis Techniques. The operation of the particle measurement system was evaluated by varying the primary dilution ratio (PDR). If the condensing vapors are not properly removed and adsorbed in the dilution probe and in the bed of granular activated carbon, the particle size distribution will change when the PDR is altered. The SMPS was used to measure the particle number size distributions using different PDRs when the thermodenuder was bypassed. The particle mass size distributions (Figure 7) were established assuming spherical particles and an effective particle density of 1 g cm
3 (F0) for all particles, and the results were corrected for both primary and secondary dilutions but not for losses in the particle measurement system. The results presented in Figure 7 indicate that the particle mass size distributions were dependent on the PDR. This implies that PM was formed during sampling inside the measurement system through nucleation and condensation of tars and/or inorganic vapors, such as alkali chlorides.

To study the volatility of the PM, the sampled gas was also passed through the thermodenuder at temperatures of 50
850 °C. The particle mass size distributions were determined using the SMPS. The results show that the particle mass size distribution changed radically in the 200
250 °C temperature range, which indicates that tars had condensed to form PM in the particle measurement system. The temperature near the inlet of the bed of granular activated carbon (T1) was above 250 °C throughout all measurements; however, the temperature in the bed (T2) was only approximately 150 °C, and tars may have condensed at the inlet to the bed. Another reason for the incomplete tar adsorption could be the higher total tar concentration than those found in previous studies13,14 using similar particle measurement systems, or a tar composition with a higher concentration of heavy tars. To ensure total evaporation of tars, the thermodenuder was operated at 400 °C. Figure 8 presents the particle mass size distributions for three PDRs downstream from the thermodenuder. The results are corrected for both primary and secondary dilutions but not for losses in the particle measurement system. The particle mass size distributions measured downstream from the thermodenuder were trimodal, in contrast to the unimodal particle mass size distributions obtained when the thermodenuder was bypassed. In addition, the particle mass concentrations were significantly lower. This indicates that a large part of the PM was evaporated and then removed in the thermodenuder, suggesting that tars were not completely adsorbed by the bed of granular activated carbon during sampling. The particle mass size distributions for dB > 70 nm obtained downstream from the thermodenuder were similar and independent of the PDR. This indicates that the PM in this size range originated from particles present in the gasifier, i.e., it was not formed from condensing inorganic vapors in the particle measurement system and the condensed tars were removed using the thermodenuder. The mass concentration and size distribution of PM with dB < 70 nm was affected by changes in the PDR, indicating that part of the PM was formed from nucleation and condensation in the particle measurement system. Studying the particle number concentration offers guidance as to where in the particle measurement system nucleation has occurred. Particle dynamics, such as coagulation, limit the particle number concentrations that may be present in the gas. The number concentration of particles with dB = 10
20 nm, as measured downstream from the thermodenuder, was in the order of 107 particles cm
3, indicating that these particles were formed downstream from both the primary and secondary dilution steps, i.e., by nucleation of tars. The number concentration of particles 1785

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Figure 9. Particle mass size distributions using three primary dilution ratios (PDRs) and an effective particle density of 1 g cm
3; results from bypassing or passing through the thermodenuder at 400 °C are presented.

with dB = 20
50 nm, as measured downstream from the thermodenuder, was lower and in such a range that they may have been formed in the dilution probe by nucleating alkali chlorides, or even in the gasifier when the highest PDR (180) was used. The particle mass size distribution for particles with dae = 0.5
8 μm was established using the particle number size distribution from the APS. Figure 9 presents the particle mass size distributions, using three PDRs and assuming spherical particles and an effective density of 1 g cm
3 (F0) for all particles. In addition, results from either bypassing or passing through the thermodenuder at 400 °C are presented to reveal the content of volatile PM. The results are corrected for both primary and secondary dilutions but not for losses in the particle measurement system. The results show that the particle mass size distribution was independent of the PDR and also unaffected by the thermodenuder, which indicates that the PM in this size range was not substantially affected by the condensation of either alkali or tar in the particle measurement system. This is in accordance with theory, since nucleation produces fine-mode particles and condensing species are enriched in fine-mode particles due to the larger surface area available for condensation. The volatility and reactivity of the PM were studied in detail by successively increasing the temperature in the thermodenuder. No notable change in the coarse-mode particle size distribution as measured using the APS could be observed even at 850 °C, indicating that both the tar and char contents were low. Lin et al. studied the oxidation of biomass char and found that wood char particles were fully oxidized at 750 °C.20 As the particle size distribution found in the present work did not change even at temperatures as high as 850 °C, this indicates that the coarse-mode PM consisted mainly of ash and possibly also of bed material. Particle Mass Size Distributions Determined Using Online Analysis Techniques. Despite the incomplete tar adsorption, the particle mass concentration and size distribution present in the hot product gas can be estimated by combining the results from the SMPS and APS. To compare the mass size distributions as measured using the SMPS and APS, the size distributions must be based on the same equivalent particle diameter; in this case dae was chosen. If spherical particles are assumed, dB can be estimated using the Stokes diameter (dp), and the relationship with dae is given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Fp 3 Cc ðdp Þ ð4Þ dae ¼ dp F0 3 Cc ðdae Þ

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Figure 10. Comparison of particle mass size distributions as measured using the SMPS and APS. The results have been corrected for losses in the bed of granular activated carbon and in the thermodenuder.

where Cc is the Cunningham slip correction factor.21 If Fp = 1 g cm
3, the SMPS and APS results can be compared directly, since dae ≈ dB. Figure 10 presents particle mass size distributions measured downstream from the thermodenuder using both the SMPS and APS as well as PDRs of 180 and 40, respectively. For SMPS, the particle mass size distribution is presented for particles with dB > 70 nm since smaller particles may represent PM formed in the measurement system. For APS, the particle mass size distribution presented is limited to the dae = 0.5
8-μm size range, since the losses of particles with dae > 8 μm are large. The results have been corrected for primary and secondary dilutions and for losses in the bed of granular activated carbon and in the thermodenuder. For the SMPS results, the particle diameter as measured downstream from the thermodenuder was used to calculate the correction function for particle losses. Due to tar evaporation in the thermodenuder, the particle diameter of an individual particle was larger upstream from the thermodenuder when the particle passed the bed of granular activated carbon. However, the correction functions for both the bed of granular activated carbon and the thermodenuder for particles with dB = 70
420 nm were approximately the same, according to Figure 5, so no effort was made to correct this. The mass concentrations (corrected for losses) were 10 mg m
3 for particles in the 70
420-nm size range determined using the SMPS and 130 mg m
3 for particles in the 0.5
8-μm size range determined using the APS. The particle mass concentration is most likely underestimated, since losses other than those in the bed of granular activated carbon and thermodenuder were not taken into account. However, this is the best approximation of the particle mass concentration and size distribution present in the hot product gas at the sampling point. Unlike in previous studies,13
15 the bed of granular activated carbon had to be combined with a thermodenuder in a two-step method to increase the separation of tars in the present work. The reason for this is not known but the incomplete tar adsorption in the bed of granular activated carbon could be due to a too low temperature at the inlet to the bed, the higher total tar concentration compared to previous studies,13,14 and/or a tar composition with a higher concentration of heavy tars. The main part of the tars was removed by the particle measurement system since the mass concentration of PM was considerably lower compared to the total tar concentration. In addition, only the fine-mode PM was affected by nucleation and condensation of inorganic vapors and tars, also downstream the thermodenuder, while the dominating coarse-mode PM was representatively characterized. 1786

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Figure 11. Particle mass size distribution as measured using the LPI when bypassing the thermodenuder.

The particle mass size distribution and concentration as measured using SMPS and APS in this study are similar to results presented from steam and oxygen BFB gasification of wood pellets in a previous study by Gustafsson et al.13 when using a similar particle measurement system. In another study, Gustafsson et al.14 found a significantly higher mass concentration of coarse particles from a CFB gasifier due to a high load of bed material. A comparison of the PM from three different gasifiers representing different gasification technologies was made by Gustafsson et al.14 Particle Mass Size Distribution, Morphology, and Elementary Composition Determined Using Off-Line Analysis Techniques. The particle mass size distribution and concentration were also measured using LPI. No LPI sample was collected downstream from the thermodenuder since the volumetric flow rate to the LPI is 10 Lpm, which would have resulted in too low a residence time in the thermodenuder and the incomplete evaporation of tars. However, the LPI results can be investigated by comparing them with the particle mass size distributions as measured using the SMPS and APS. Figure 11 presents the particle mass size distribution measured using the LPI when bypassing the thermodenuder. Greased aluminum substrates were used for the PM collection and the sampling time was 42 min. The results have been corrected for primary (PDR = 20) and secondary dilution but not for losses in the particle measurement system. Tars seem to have contributed to the PM for LPI stages 1
5 (dae < 0.3 μm), since the mass concentration in this size range is significantly higher, 160 mg m
3, compared with the results obtained using the SMPS downstream from the thermodenuder (10 mg m
3). For LPI stages 6
12, the particle mass size distribution is similar to the APS particle mass size distribution shown in Figure 10, which indicates that the tar content was low in the PM collected on these stages. The particle mass concentration for LPI stages 6
12 was 80 mg m
3 (not corrected for losses in the particle measurement system) versus 130 mg m
3 as measured using the APS. The LPI was also used to collect PM for morphological and elementary analysis. Ungreased aluminum substrates from LPI stages 2, 7, and 9, corresponding to dae ≈ 0.08 μm, 0.8 μm, and 2.0 μm, respectively, were analyzed using SEM-EDS (sampling time for the LPI was 20 min). No individual particles were observed on the substrate from stage 2; instead, the particles were immersed in a smooth layer of substance that seemed to have solidified on the substrate. As the electron beam current was increased in the SEM, the temperature also increased as a consequence, which led to a visible fusing of the PM. This indicates that tars were present in the fine-mode PM, which is in accordance with the results already presented. Figures 12 and 13

Figure 12. SEM image of PM with dae ≈ 0.8 μm.

Figure 13. SEM image of PM with dae ≈ 2 μm.

present typical images, using a magnification of 5000, from stages 7 and 9. Both images show different types of particle structures: flaky, smooth pieces with a physical extension larger than the mean diameter for each stage, as well as more rounded particles with a diameter approximately corresponding to the mean diameter for the stages. Though there are also some agglomerates of smaller particles, the rounded particles are clearly dominant. No visible fusing took place at either substrate 7 or 9 when the electron beam current was increased, indicating that the concentration of tars was low. In the EDS analysis, silicon, calcium, magnesium, potassium, phosphorus, manganese, sodium, sulfur, and chlorine for stage 7 and silicon, calcium, magnesium, potassium, titanium, and phosphorus for stage 9 were detected (in order of occurrence). The elements detected were present in the ash of the wood pellets used as fuel in the gasifier. However, the ash from the wood chips used as fuel in the CFB boiler probably also contributed to the PM, since ash was recirculated with the bed material from the CFB boiler to the gasifier. The presence of silicon also suggests that bed material may have contributed to the PM. Carbon and oxygen were detected using EDS analysis, though the quantification accuracy was low for light elements. 1787

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Figure 14. Mass percentages of dominant elements on LPI polycarbonate substrates determined using PIXE analysis.

Gustafsson et al.13 presented SEM images of PM produced by the steam and oxygen BFB gasification of wood pellets that contained a high number of flaky and smooth pieces that they concluded to be char particles. These results are in line with previous findings in which the content of unconverted char from BFB gasifiers was found to be high.1 The number of flaky and smooth pieces found in the present study was low, indicating a low concentration of char in the PM. The gasifier in the present study is also a BFB gasifier, but of indirect type. This means that the char is recirculated to the boiler with the bed material and is combusted before it is completely gasified. The time available for char attrition is consequently much shorter in an indirect than in a direct gasifier where the char is gasified and partly oxidized in the gasifier reactor. For that reason, the low concentration of char in the PM is possibly a characteristic feature of indirect gasification. Polycarbonate substrates from an LPI corresponding to dae = 0.08
5.25 μm and three blank substrates used as references were analyzed using PIXE (sampling time for the LPI was 44 min). Fifteen elements (i.e., silicon, phosphorus, sulfur, chlorine, potassium, calcium, chromium, manganese, iron, cobalt, copper, zinc, bromine, strontium, and lead) were detected; however, the concentrations of some elements were low and limited to a few substrates. The mass percentages of the dominant elements (i.e., calcium, silicon, potassium, chlorine, manganese, iron, lead, sulfur, and zinc) are presented in Figure 14 (the values are normalized to 100%). The mass of these elements constituted almost 99% of the mass detected by PIXE analysis. In total, 4% of the total mass collected on the polycarbonate substrates was detected in the PIXE analysis. This implies that the mass was dominated by elements not detectable by PIXE (Z < 12), such as hydrogen, carbon, oxygen, sodium, and magnesium. However, the percentage of mass that was detected on each substrate differed greatly; for substrate numbers 2
5 (dae = 0.08
0.32 μm), the percentage was 6% for substrate numbers 6
11, and as high as 22% for substrate number 8 (dae = 1.29 μm). The lower detected mass percentage for stages 2
5 is in agreement with the previous discussion of tars in fine-mode PM. Due to the presence of tars, the elementary analysis of the fine-mode PM was slightly biased since the aerodynamic particle diameter increases as tars condense on the particles. Since the mass of PM collected on each substrate varied, it is of interest to present the particle mass size distributions for separate elements. Figure 15 presents the particle mass size distributions

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Figure 15. Particle mass size distributions for calcium, silicon, potassium, chlorine, manganese, and iron as well as the total particle mass size distributions for the dominant elements as analyzed using PIXE.

for calcium, silicon, potassium, chlorine, manganese, and iron as well as the total particle mass size distributions for the dominant elements (as analyzed using PIXE). The results are corrected for both primary and secondary dilutions but not for losses in particle measurement system. The total particle mass size distributions for the dominant elements were similar to the particle mass size distributions as measured using the SMPS and APS downstream from the thermodenuder (Figure 10); however, the mass concentrations were approximately one order of magnitude less. The elementary analysis clearly demonstrates that the fine-mode PM and coarsemode PM have different elementary compositions and origins. The fine-mode PM was dominated by potassium and chlorine, supporting the presence of alkali chlorides as indicated previously. Potassium and chlorine were also present in the coarsemode PM, though calcium was the dominant element. The coarse-mode PM also contained considerable amounts of silicon as well as manganese and iron, indicating an origin from the ash but also the bed material may have contributed to the PM. Heavy metals such as lead and zinc are usually present in the fine-mode PM since these elements volatilize during gasification. In this study, lead was present in the fine-mode PM in unexpectedly high concentrations compared with those of potassium and chlorine, possibly due to the high volatility of lead. The high concentration of lead could also be due to contamination, either in the gasifier or during analysis. Zinc was present only in small amounts in the coarse-mode PM. Sulfur was present in small amounts in both the finest and coarsest particles. The results of the EDS and PIXE analyses are in agreement. The sodium and magnesium detected using EDS cannot be detected using PIXE. Iron, lead, and zinc were detected using PIXE but not EDS; however, the mass concentrations were low. The elementary composition of the PM suggested that the ash contributed significantly to the PM. However, the elementary compositions of fine- and coarse-mode PM differed distinctly, indicating different types of PM. This is in contrast to the results of a previous study by Gustafsson et al.,13 in which wood pellets were gasified using steam and oxygen in a BFB gasifier. They found that the PM largely consisted of char particles with only a small inorganic fraction dominated by calcium in particles of all sizes. In addition, Gustafsson et al.13 found that the difference in elementary composition between particles of different sizes was small. 1788

dx.doi.org/10.1021/ef101710u |Energy Fuels 2011, 25, 1781–1789

Energy & Fuels

’ CONCLUSION This study characterized the PM formed during the indirect steam bubbling fluidized bed gasification of wood pellets at atmospheric pressure. A system including a dilution probe, a bed of granular activated carbon, and a thermodenuder was used to sample the PM at high temperature with the aim of separating it from condensing inorganic vapors and tars. The particle mass size distribution was bimodal with a fine mode in the 0.5-μm size range. The coarse mode was representatively characterized while condensing inorganic vapors and tars complicated the evaluation of the results for the fine-mode PM. The mass concentration of the fine mode was lower, 10 mg m
3 as measured using the SMPS, than the mass concentration of the coarse mode, 130 mg m
3 as measured using the APS. Morphological analysis of the PM indicated that the char content was low. The inorganic fraction was dominated by potassium and chlorine for fine-mode PM and calcium and silicon for coarse-mode PM. The origin was probably ash (both from the wood pellets used as gasifier fuel and from the wood chips used as CFB boiler fuel) but also the bed material may have contributed to the PM. Indirect steam gasification is advantageous because it allows production of a hydrogen-rich product gas with a low nitrogen content. However, cleaning and upgrading the product gas, for example, using filters and catalysts, is crucial for biofuel production. A precise characterization of the various contaminants and their physical states at actual process temperatures would facilitate the design and optimize the function of both filters and catalysts. The particle mass concentrations and size distributions as well as the morphological and elementary analyses of PM produced by the indirect steam BFB gasification of wood pellets presented here contribute to this characterization. An alternative method for sampling PM was used due to the incomplete tar adsorption in the bed of granular activated carbon. The method could, however, be further developed to optimize the sampling system, including the dilution probe, bed of granular activated carbon, and thermodenuder, and to generate additional results regarding the characteristics of PM produced by indirect biomass gasification.

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’ AUTHOR INFORMATION Corresponding Author

*Tel: þ46 470 70 80 00; fax: þ46 470 832 17; e-mail: eva. [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge financial support from the European Commission sixth Framework Programme (CHRISGAS project SES6-CT-2004-502587) and the Swedish Energy Agency. The operating staff at Akademiska hus, G€oteborg, Sweden, are thanked for operating the gasifier and the CFB boiler. ’ REFERENCES (1) Kaltschmitt, M., R€osch, C., Dinkelbach, L., Eds. Biomass Gasification in Europe; EC Science Research & Development: Brussels, 1998. (2) Umeki, K.; Yamamoto, K.; Namioka, T.; Yoshikawa, K. Appl. Energy 2010, 87, 791–798. (3) Bridgwater, A. V. Fuel 1995, 74, 631–653. 1789

dx.doi.org/10.1021/ef101710u |Energy Fuels 2011, 25, 1781–1789