Physical and Chemical Characterization of Aerosol ParticlesFormed during the Thermochemical Conversion of Wood Pellets

Using a Bubbling Fluidized Bed Gasifier

Eva Gustafsson,*,† Michael Strand,† and Mehri Sanati†,‡

School of Technology and Design–Bioenergy, Växjö UniVersity, SE-351 95 Växjö, Sweden, andDepartment of Design Sciences–Ergonomics and Aerosol Technology, Lund UniVersity,

Box 118, SE-221 00 Lund, Sweden

ReceiVed May 21, 2007. ReVised Manuscript ReceiVed August 19, 2007

Product gas obtained through biomass gasification can be upgraded to hydrogen-rich synthesis gas. Thesynthesis gas can be further converted to liquid or gaseous fuels. However, the raw product gas contains bothgas- and particle-phase impurities that can negatively affect both catalysts and hot-gas filters used for upgradingand cleaning. The present study aimed to characterize, both physically and chemically, aerosol particles formedduring the steam- and oxygen-blown biomass gasification of wood pellets in an atmospheric 20 kW bubblingfluidized bed (BFB) gasifier. The product gas from the gasifier was sampled upstream from the cyclone at500 °C. The particle number size distribution determined using a scanning mobility particle sizer (SMPS) wasbimodal, with modes at 20–30 and 400 nm, mobility equivalent diameters (dB). The total mean numberconcentration of particles with dB ) 15–670 nm was approximately 7 × 105 particles/cm3; however, theconcentration of particles with dB < 80 nm fluctuated. The particle mass size distribution determined using alow-pressure impactor (LPI) was bimodal, and the total mass concentration of particles with aerodynamicdiameters (dae) < 5 µm was 310 mg/m3. Microscopy analysis of particulate matter on the lower LPI stages,expected to sample particles with dae < 0.4 µm, revealed structures approximately 10 µm in diameter. Inaddition, the mass concentration of particles with dae < 0.5 µm determined using a LPI was higher than thatestimated using a SMPS, possibly because of the bounce-off or re-entrainment of coarser particles from higherLPI stages. Elementary analysis of the particulate matter indicated that it was dominated by carbon. The collectedparticulate matter was stable when heated in nitrogen to 500 °C, indicating that the carbon was not present asvolatile tars but more likely as char or soot. The particulate matter collected on all LPI stages contained asmall percentage of ash (noncarbonaceous inorganic material), with calcium as the dominant element.

Introduction

Biomass is used today for the production of heat and powerbut also increasingly to produce fuels, such as ethanol andbiogas, mainly through biochemical conversion. However,thermochemical processing can also be used, and one of themost promising alternatives is to gasify the biomass at hightemperatures. The product gas can be upgraded to synthesis gasrich in H2 and CO using catalytic processes. The synthesis gascan then be converted into a wide variety of liquid and gaseouschemicals, including fuels such as methanol, dimethyl ether,and synthetic diesel. However, the raw product gas containsboth gas- and particle-phase impurities that may seriouslydamage the downstream catalysts. Particulate matter can beremoved using ceramic candle filters at high temperatures. Thefiltration efficiency and the operation and dimensioning ofthe filter depend upon the particle mass concentration and sizedistribution, as well as upon the composition of the particulatematter. Therefore, it is crucial to characterize the particulatematter present in the product gas. The presence of alkali is ofparticular importance, because alkali can form silicates with lowmelting temperatures1 that may negatively affect the filter

operation. Catalysts used for cleaning flue gases from biomasscombustion have been demonstrated to be negatively affectedby particulate matter.2,3 At high filtration temperatures, alkaliwill also be present as vapors that may pass through the filter;they may condense during cooling downstream from the filter,producing new particles or deposits on catalysts or othersurfaces.

Sampling particulate matter from biomass gasification iscomplicated by the presence of tars that condense when the gasis cooled, thereby contributing to the particulate matter.Recently, a technical specification has been developed thatdescribes a method for sampling tars and particles in productgas from biomass gasification.4 This specification defines taras organic compounds present in the gasification product gas(excluding gaseous hydrocarbons, C1–C6). The method specifiesthat the gas is to be sampled isokinetically using a heated probe.The particles are collected on a heated filter to avoid tarcondensation and then analyzed gravimetrically, while the tarsare collected downstream from the filter in an organic liquid

* To whom correspondence should be addressed. Telephone: +46 (0)470-70 81 30. E-mail: eva.gustafsson@vxu.se.

† Växjö University.‡ Lund University.

(1) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne,T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998,54, 47–78.

(2) Moradi, F.; Brandin, J.; Sohrabi, M.; Faghihi, M.; Sanati, M. Appl.Catal., B 2003, 46, 65–76.

(3) Larsson, A.-C.; Einvall, J.; Andersson, A.; Sanati, M. Energy Fuels2006, 20, 1398–1405.

(4) CEN/TC 143 Technical Specification 15439:2006, CEN, 2006.

Energy & Fuels 2007, 21, 3660–36673660

10.1021/ef7002552 CCC: $37.00 2007 American Chemical SocietyPublished on Web 10/02/2007

adsorbent; similar methods have been used by Gabra et al. andYamazaki et al.5,6 Corresponding measurements, using heatedcascade impactors instead of heated filters, have been made byHasler and Nussbaumer and van der Nat et al.7,8 Hasler andNussbaumer characterized particles from wood gasification ina co-current air-blown 270 kW fixed-bed (FB) gasifier and a150 kW bubbling fluidized bed (BFB) gasifier.7 The sizedistribution of particles with aerodynamic diameters (dae) )0.2–10 µm from the FB gasifier was measured downstream froma cyclone at 480 °C using an Andersen cascade impactor. Theparticle mass size distribution was bimodal, with the fine modeat dae < 0.27 µm and a total mass concentration of 130 mg/m3

(dae < 5 µm). The same impactor was used to measure the sizedistribution of particles with dae ) 1.5–40 µm from the BFBgasifier, at the end of the freeboard at 800 °C. The particle masssize distribution was bimodal, with the fine mode at dae ) 1.4µm and a total mass concentration of 500 mg/m3 (dae < 5 µm).To limit tar condensation, the cascade impactor was heated to270 °C during both measurements. van der Nat et al. used acascade impactor to characterize particles with dae ) 0.3–30µm from the steam- and oxygen-blown circulating fluidized bed(CFB) gasification (100 kW) of high- and low-quality woodand miscanthus.8 λ, the ratio between the oxygen input and theoxygen required for total oxidation of the fuel to CO2 and H2O,was approximately 0.5. Particles were sampled downstream fromthe cyclone under isokinetic conditions, and the cascadeimpactor was heated to 220 °C to prevent tar condensation. vander Nat et al. found that different fuels generated somewhatdifferent particle mass size distributions. The concentration offine particles (dae < 1 µm) was higher when using miscanthusas the fuel than when using woody fuels. The mass concentrationincreased with an increasing particle diameter; however, the lastpart of a fine particle mode (dae < 0.5 µm) was present. Whenusing high-quality wood and miscanthus as fuels, a mode wasalso present at dae ≈ 1–2 µm. Agglomerates of 100 nm primaryparticles, mainly consisting of potassium and chlorine, werefound when analyzing particles from the gasification of mis-canthus using scanning electron microscopy (SEM) energy-dispersive X-ray (EDX) analysis. In addition, particles rich incalcium were found when low-quality wood was used as thefuel.

Sampling of particles from biomass gasification at lowertemperatures has also been presented.9 Hindsgaul et al. used acascade impactor with aerodynamic cut-off diameters (d50) of0.22–1.1 µm, as well as quartz-fiber and membrane filters tostudy particles from a two-stage down-draft gasifier fired withwood chips.9 The product gas was sampled isokineticallydownstream from the cooler, upstream from any cleaning device,at 30–50 °C. The gases were then heated to 80–90 °C to avoidwater condensation in the cascade impactor. Particle massconcentrations of 200–400 mg/m3 were found, and particles withdae < 0.22 µm dominated the particle mass size distribution.

Carbon was found to constitute more than 75 wt % of theparticulate matter, and using SEM analysis, primary sootparticles approximately 70 nm in diameter were found.

Selvakumar et al. studied the particle mass size distributionfrom the down-draft gasification (20 kW) of wood and rice husksusing a dilution probe and a cascade impactor (multi-orificeuniform deposit impactor, MOUDI) with d50 of 0.056–18 µm.10

A bimodal particle mass size distribution with modes at dae )0.42 and 13.5 µm was found when using wood as the fuel; thesize distribution was dominated by particles with dae < 1 µm.The particles from both wood and rice husks gasification wereanalyzed using an environmental scanning electron microscope(ESEM) with energy-dispersive analysis of X-rays (EDAX); thecoarser particles were found to be rod- or stick-shaped and richin silicon and carbon, while the finer particles were clusters ofspherical particles not only rich in carbon and silicon but alsoin potassium and chlorine.

While the knowledge of particles from biomass gasificationis limited, particles from biomass combustion have been studiedby several authors.11–13 Fine particles (diameter < 1 µm) areformed via gas–particle conversion and consist of inorganiccompounds, soot, and condensable organic material, whilecoarser particles (diameter > 1 µm) are formed by thefragmentation of both inorganic and organic nonvolatilizedmaterial.14 The concentration, size distribution, and chemicalcomposition of particles from biomass combustion vary con-siderably, depending upon factors such as size and type offurnace or boiler, combustion conditions, and fuel properties.The inorganic part of the fine particles from biomass combustionis usually dominated by potassium salts.11,15 The coarse particlesare usually composed of calcium, iron, aluminum, manganese,silicon, phosphorus, potassium, and sulfur.14,15 The concentrationof particulate matter arising from incomplete combustion (soot,char, and tar) varies depending upon combustion conditions.16,17

Soot, char, and tar are formed in different ways: thermochemicaltreatment of biomass starts with drying and is followed bydevolatilization, when tars are vaporized, leaving a solidremainder of char;18 soot is formed from hydrocarbons in acomplex process at high temperatures.19

There are many similarities between biomass combustion andgasification: both are thermochemical processes; both can usethe same kind of fuel and similar process equipment; and bothprocesses are run at similar temperatures. Therefore, it isreasonable to believe that there may be similarities between theparticle formation mechanisms in both biomass combustion andgasification. However, a major difference between the twoprocesses is that gasification is run at substoichiometric condi-

(5) Gabra, M.; Pettersson, E.; Backman, R.; Kjellstrom, B. BiomassBioenergy 2001, 21, 351–369.

(6) Yamazaki, T.; Kozu, H.; Yamagata, S.; Murao, N.; Ohta, S.; Shiya,S.; Ohba, T. Energy Fuels 2005, 19, 1186–1191.

(7) Hasler, P.; Nussbaumer, T. In Proceedings of the 10th EuropeanConference and Technology Exhibition: Biomass for Energy and Industry,1998, Würzburg, Germany; C.A.R.M.E.N: Rimpar, Germany, 1998; pp1623–1625.

(8) van der Nat, K. V.; Siedlecki, M.; de Jong, W.; Woudstra, N.;Verkooijen, A. H. M. In Proceedings of the 14th European BiomassConference and Technology Exhibition: Biomass for Energy, Industry andClimate Protection, 2005, Paris, France; ETA: Florence, Italy and WIP:Munich, Germany, 2005; pp 642–645.

(9) Hindsgaul, C.; Schramm, J.; Gratz, L.; Henriksen, U.; Bentzen, J. D.Bioresour. Technol. 2000, 73, 147–155.

(10) Selvakumar, N.; Parikh, P. P.; Sethi, V. In Proceedings of EuropeanAerosol Conference, 2005, Ghent, Belgium; Maenhaut, W., Ed.; Ghent,Belgium, 2005; p 60.

(11) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998,29, 421–444.

(12) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris,G.; Revitzer, H. J. Aerosol Sci. 1998, 29, 445–459.

(13) Strand, M.; Pagels, J.; Szpila, A.; Gudmundsson, A.; Swietlicki,E.; Bohgard, M.; Sanati, M. Energy Fuels 2002, 16, 1499–1506.

(14) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. J. Air Waste Manage.Assoc. 2000, 50, 1565–1618.

(15) Pagels, J.; Strand, M.; Rissler, J.; Szpila, A.; Gudmundsson, A.;Bohgard, M.; Lillieblad, L.; Sanati, M.; Swietlicki, E. J. Aerosol Sci. 2003,34, 1043–1059.

(16) Johansson, L. S.; Tullin, C.; Leckner, B.; Sjovall, P. BiomassBioenergy 2003, 25, 435–446.

(17) Wierzbicka, A.; Lillieblad, L.; Pagels, J.; Strand, M.; Gudmundsson,A.; Gharibi, A.; Swietlicki, E.; Sanati, M.; Bohgard, M. Atmos. EnViron.2005, 39, 139–150.

(18) Biomass Handbook; Kitani, O., Hall, C. W., Eds.; 1989; p 372.(19) Kennedy, I. M. Prog. Energy Combust. Sci. 1997, 23, 95–132.

Thermochemical ConVersion of Wood Pellets Energy & Fuels, Vol. 21, No. 6, 2007 3661

tions and the solid biomass is transferred into a product gas,while in combustion, the biomass and gases are completelyoxidized; this difference can have an impact on the particulatematter. The quantity of carbonaceous particulate matter can behigher in gas from biomass gasification than in combustion gas,because of the substoichiometric conditions in the gasificationprocess. Alkali sulfates are usually present in particles frombiomass combustion, but because most of the gaseous sulfur inthe product gas is present as H2S, no sulfates are expected tobe formed during biomass gasification.

The present study aims to characterize aerosol particlesformed during the steam- and oxygen-blown biomass gasifica-tion of wood pellets in an atmospheric 20 kW BFB gasifier.Both on- and off-line measurement techniques were used. Thesampling device, which included a dilution probe and a bed ofactivated carbon, was used to quench particle dynamics andadsorb tars upstream from the particle characterization instruments.

Experimental Section

The Atmospheric BFB Gasifier System. The gasifier systemused is depicted in Figure 1 and is located at TPS TermiskaProcesser AB, Nyköping, Sweden. The atmospheric 20 kW gasifieris of the BFB type, with an inner diameter of 205–260 mm and aheight of 2 m. For particle removal, the system is equipped with acyclone and ceramic filter operating at 360 °C. Wood pellets weregasified at 850 °C in the presence of steam and oxygen, usingmagnesite as the bed material. The water content in the wood pelletswas 8.1 wt %, and the ash content on wet basis was 0.34 wt %.The concentration of both sulfur and chlorine was low, 0.02 and0.01 wt %, respectively, on wet basis. Calcium and potassiumdominated the ash. Further information regarding the gasificationconditions is presented in Table 1. Gas was sampled upstream fromthe cyclone, 20 cm from the top of the gasifier, at a temperature ofapproximately 500 °C. Particle measurements were carried out overthe course of 1 day. Because of the blockage of the fuel feedingsystem, sampling was interrupted for approximately 1 h, and someof the data from before and after the stop are presented separately.

Particle Measurement System. The system used for particlemeasurements, including the dilution probe, is presented in Figures2 and 3. In the particle sampling system, the gas was diluted athigh temperature, and thereby, particle dynamics, such as particlecoagulation and condensation, were quenched. Activated carbon

was used to adsorb the tars downstream from the dilution probebefore cooling the gas and, thereby, preventing the tars fromcondensing and contributing to the particulate matter. The stainless-steel dilution probe was similar to that used by Strand et al. tocharacterize particles from biomass combustion at high tempera-tures.20 The tip of the probe consisted of a 6 mm outer diameter(o.d.) pipe 2.8 cm long; the tip was welded onto a 12 mm o.d. pipewith an 8 mm o.d. pipe inside. Nitrogen was introduced asthe dilution gas into the dilution section, close to the tip of theprobe. During the measurement process, the dilution ratio in theprobe ranged from 10 to 37 (primary dilution) and was controlledby adjusting the flow of nitrogen; the total flow of diluted gasthrough the probe was 7.3 L/min.

Downstream from the dilution probe, the gas was passed througha 16 mm o.d. stainless-steel pipe containing 20 mL of activatedcarbon (Activated Charcoal Norit, type RB3, diameter of 3 mm;Sigma-Aldrich, Stockholm, Sweden). The amount of activatedcarbon was adjusted on the basis of the adsorption capacity ofnaphthalene used as the tar model compound in laboratoryexperiments. The temperature of the bed was controlled usingheating tape and monitored using a thermocouple. The inlettemperature was 220–240 °C, which is sufficiently high for mosttars in the gas to be in vapor phase when entering the carbon bed.To study whether the activated carbon emitted any particles whenheated, nitrogen was passed through a heated bed of activatedcarbon in laboratory experiments. The results indicated that onlynegligible amounts of particles were emitted from the carbon bedunder these conditions. A precyclone with a d50 of approximately5 µm was used to protect downstream equipment and instrumentsfrom clogging and overloading caused by coarse particles. Anejector diluter was used to set the flow through the particlemeasurement system and further dilute (secondary dilution) the gaswith particle-free dry compressed air. In addition, the ejector diluterreduced the pressure to an ambient level, because the gasifier wasoperated slightly above ambient pressure. The dilution ratio in theejector diluter was 4.7–4.9, producing total dilution ratios of 45–180,including both primary and secondary dilution.

Gas and Particle Characterization. The main gas-phasecompounds in the product gas were quantified using a gas

(20) Strand, M.; Bohgard, M.; Swietlicki, E.; Gharibi, A.; Sanati, M.Aerosol Sci. Technol. 2004, 38, 757–765.

Figure 1. Scheme of the atmospheric BFB gasifier system.

Table 1. Gasification Conditions

fuel rate (kg/h) 10steam (as water) (L/h) 0.54air (m3/h) 5.0oxygen (m3/h) 2.1λa 0.35

a λ is the ratio between the oxygen input and the oxygen required fortotal oxidation of the fuel to CO2 and H2O.

Figure 2. Scheme of the dilution probe used for sampling.

Figure 3. High-temperature particle measurement system.

3662 Energy & Fuels, Vol. 21, No. 6, 2007 Gustafsson et al.

chromatograph (GC) equipped with a thermal conductivity detector(TCD). Tars were sampled using a method based on solid-phaseadsorption (SPA)21 and then analyzed using a GC with a flameionization detector (FID). The sampling position for both the maingas-phase compounds and the tars was upstream from the ceramicfilter.

A model 3080 scanning mobility particle sizer (SMPS; TSI, Inc.,Shoreview, MN) including a model 3081 differential mobilityanalyzer (DMA) and a model 3010 condensation particle counter(CPC), was used to determine the number size distribution ofparticles with mobility equivalent diameters (dB) ranging from 15to 670 nm. 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 mean diameterof particles collected on a stage was calculated as the geometricmean 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 usedfor presenting the particle mass size distribution. Particles werecollected on aluminum and polycarbonate (Nucleopore; Whatman,Inc., Brentford, U.K.) substrates greased with Apiezon low-vacuumgrease (Apiezon, Manchester, U.K.), and the substrates wereanalyzed gravimetrically. The weight of the substrates could bedetermined with a precision of (5 µg. Three aluminum substrates(dae ≈ 40 nm, 0.32 µm, and 3.2 µm) were analyzed using a SEMwith energy-dispersive spectroscopy (EDS), to obtain informationabout particle morphology and elementary composition. All poly-carbonate substrates were analyzed using particle-induced X-rayemission (PIXE) to determine the concentration of elements withatomic numbers (Z) > 12. Teflon filters were used to sample thetotal particulate matter with dae < 5 µm. One filter was analyzedusing inductively coupled plasma mass spectrometry/atomic emis-sion spectroscopy (ICP–MS/AES) to determine the concentrationof selected elements. Another filter was analyzed using a model2950 thermogravimetric analyzer (TGA; TA Instruments, NewCastle, DE) to determine the percentage of volatile particulatematter. An electrical low-pressure impactor (ELPI; Dekati Ltd.,Tampere, Finland) and a CO analyzer based on infrared technologywere used as references for adjusting and determining the dilutionratio.

Particle Losses in the Measurement System. Particles can belost in the measurement system because of mechanisms such asimpaction, diffusion, and thermophoresis.22 Impaction is usuallythe most important deposition mechanism for coarse particles, whilediffusion is important for particles with diameter < 0.1 µm. Lossesbecause of thermophoresis are relatively independent of the particlesize. The total losses of particles in the measurement system aredifficult to estimate, because this would require detailed informationabout properties, such as temperature gradients and flow profiles.The losses of particles with dB ) 15–670 nm in the bed of activatedcarbon were determined experimentally using K2SO4 particles. Areference particle number size distribution through the empty pipeused for the bed of activated carbon was determined using a SMPS,and this size distribution was compared to that determined througha 20 mL bed of activated carbon. The greatest particle loss was15% (number concentration), which was found for particles withdB ) 15–50 nm. No correction for losses was made to the particlesize distributions or concentrations.

Results and Discussion

Gas and Tar Analysis. In Figure 4, the concentrations ofmain gas-phase compounds are presented in volume % on drybasis (dry vol %); the values are normalized to 100%. Inaddition, the gas was analyzed for ethane, ethene, and oxygen,only small amounts of which were detected. The water vapor

content was 37 vol % during sampling. The gasification processwas stopped for approximately 1 h because of the blockage ofthe fuel feeding system (see Figure 4).

The gas composition remained stable except during theprocess stop; particle measurements were performed bothbefore and after the stop under steady process conditions. Theconcentration of H2 was high, approximately 30 dry vol %. Thetar concentration was measured 3 times, twice before the stopand once after the stop. Naphthalene, phenanthrene, andacenaphthene were detected and quantified (see Table 2). Theconcentration of tars that could not be identified in the GCanalysis was determined using the response factor for naphtha-lene and is presented in the table as “other tars”. The sampleswere also analyzed for acenaphtylene, benzo(a)anthracene,benzo(a)pyrene, benzo(b)fluoranthene, benzo(g,h,i)perylene, ben-zo(k)fluoranthene, chrysene, dibenzo(a,h)anthracene, fluorene,indeno(1,2,3-cd)pyrene, anthracene, and pyrene, but thesecompounds were not detected in any of the samples.

A model developed by the Energy Research Centre of TheNetherlands (ECN) was used to investigate the probability oftar condensation upstream from the bed of activated carbon inthe dilution probe.23 A worst-case scenario was used in whichthe unidentified tar was assumed to be coronene, the tar modelcompound with the highest condensation temperature. Theinterpretation of the authors was that 220–240 °C was sufficientto avoid tar condensation upstream from the bed of activatedcarbon.

Particle Number Concentration and Size Distribution. Theparticle number size distributions determined using a SMPS,corrected for dilution in the measurement system, are presentedin Figure 5. The two size distributions represent the average of12 and 20 SMPS scans before and after the process stop,respectively.

The particle number size distributions were bimodal, withone mode at 20–30 nm and another at 400 nm. The total meannumber concentrations before and after the process stop were5.2 × 105 and 8.8 × 105 particles/cm3, respectively.

To the best of our knowledge, no previous study hasdetermined particle number concentrations or size distributions(21) Brage, C.; Yu, Q.; Chen, G.; Sjostrom, K. Fuel 1997, 76, 137–

142.(22) Hinds, W. C. Aerosol Technology: Properties, BehaVior, and

Measurement of Airborne Particles, 2nd ed.; John Wiley and Sons: NewYork, 1999; pp 54 and 216.

(23) Energy Research Centre of the Netherlands (ECN). www.thersites.nl(accessed 2007).

Figure 4. Concentration (dry vol %) of main gas-phase compounds inthe product gas, determined using a GC–TCD.

Table 2. Concentration of Tars in the Product Gas SampledUsing SPA and Analyzed Using a GC–FID

timecomponent

10:28 10:37concentration (mg/m3)

12:40

naphthalene 218 172 500phenanthrene 84 11 480acenaphthene 26other tars 922 813 1212benzene 2784 1990 1182

Thermochemical ConVersion of Wood Pellets Energy & Fuels, Vol. 21, No. 6, 2007 3663

from biomass gasification. Strand et al. have, however, used adilution probe to measure particles from the CFB combustion(104 MW) of forest residues mixed with 10 wt % peat at 780°C.20 In that study, the number concentration of particles withdB ) 17–550 nm determined using a SMPS was 1.2–2.2 × 107

particles/cm3, depending upon the dilution ratio, while theparticle number size distribution was unimodal with the modeat dB ≈ 50–80 nm. When using a low dilution ratio, a nucleationmode appeared at dB ≈ 30 nm, which was assumed to consistof particles formed in the dilution probe. In the present study,there was a large difference in the number concentration ofparticles with dB < 80 nm before and after the process stop. Ithas not been determined if this variation was due to anunidentified change in process conditions or if these particleswere formed in the dilution probe.

The results from Strand et al.20 differ from the results obtainedin the present study. Comparable fuels were used, but processconditions, such as λ and temperature, are different for combus-tion compared to gasification and this will affect the particleformation dynamics. The amount of particulate matter origi-nating from incomplete combustion is low from biomass CFBcombustion, while it can be abundant from biomass gasification.During biomass CFB combustion, inorganic vapors, such asalkali, produce high number concentrations of fine particles bynucleation and condensation.11 The role of inorganic vapors inparticle formation during biomass gasification has not beendetermined. The lower total particle number concentration frombiomass BFB gasification could also indicate that particles werelost in the measurement system in the present study.

Particle Mass Concentration and Size Distribution. Thesampling conditions and total amount of collected particulatematter on stages 1–12 using the LPI with aluminum substratesare presented in Table 3, and the corresponding particle masssize distribution is presented in Figure 6. The measurement wasmade after the process stop. The particle mass size distributionhas been corrected for dilution in the measurement system.

The total concentration of particles with dae < 5 µm was 310mg/m3. This is similar to mass concentrations presented byHasler and Nussbaumer and Hindsgaul et al.7,9 Hasler andNussbaumer found mass concentrations of 130 and 500 mg/m3

(dae < 5 µm) from FB and BFB biomass gasification, respec-tively,7 while Hindsgaul et al. found mass concentrations of200–400 mg/m3 from two-stage down-draft biomass gasifica-tion.9 The particle mass size distribution depicted in Figure 6is bimodal, with modes at 0.32 and 3.2 µm. However, the coarse

mode was cut off by the precyclone; therefore, the sizedistribution is valid only for particles smaller than approximately5 µm (dae). The particle mass size distributions presented inprevious studies of particles from biomass gasification varyconsiderably, although a fine mode is usually found between0.2 and 1.4 µm.7–10

The differences between the particle mass size distributionsobtained in the present study and those obtained in the previousstudies could be due to factors such as sampling position andsampling equipment, gasifier type, gasification conditions, andfuel type. The variation in these and other factors makes acomparison between different studies of particles from biomassgasification more difficult. Sampling position and the temper-ature at the sampling position have an impact on the particlecharacteristics. Process devices, such as cyclones and heatexchangers, will reduce the mass of the particulate matter butalso change the particle size distribution. At low temperatures,tars will condense and contribute to the particulate matter. Thetar concentration in this study was around 1–2 g/m3, and becausethe particle concentration was 310 mg/m3, condensed tar couldhave contributed significantly to the particulate matter if notadsorbed before cooling of the gas. The concentrations of bothparticulate matter and tars vary depending upon the gasificationprocess. The concentration of particulate matter is in generalhigher from CFB gasification than from FB gasification; theconcentration of tars can be especially low if a FB cocurrentgasifier is used, because the tars are cracked at high tempera-tures.24 Low tar levels reduce the need for heated samplingequipment and/or a tar adsorption step. The concentration ofchar particles and unburnt particulate matter is dependent uponλ. While λ in this study was 0.35, van der Nat et al. performedmeasurements with λ ≈ 0.5,8 probably resulting in less unburntparticulate matter. The amount of ash is highly dependent uponthe fuel used; while the ash content in wood can be below 2 wt%, in rice hulls, for example, it may exceed 20 wt %,25 whichmay result in particulate matter with both different sizedistribution and elementary composition. van der Nat et al. foundthat the concentration of fine particles was higher when usingmiscanthus as fuel compared to woody fuels.8

A 0.1 mg sample of particulate matter was taken from aTeflon filter and analyzed using TGA. The results indicated thatapproximately 90 wt % of the material was stable up to 500 °Cin nitrogen. It may thus be assumed that the material consistedmainly of char, soot, and ash (noncarbonaceous inorganicmaterial), with only a minor part of volatile tars. Because ofthe small sample size, the result is uncertain; nevertheless, theresult indicates that the tar was adsorbed in the activated carbonbed and did not contribute significantly to the particulate matter.

The particle number size distribution obtained from SMPScan be used to estimate the particle mass size distribution by

(24) Hasler, P.; Nussbaumer, T. Biomass Bioenergy 1999, 16, 385–395.(25) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel

Process. Technol. 1998, 54, 17–46.

Figure 5. Particle number size distribution before and after the processstop, determined using a SMPS.

Table 3. Conditions and Total Amount of Collected ParticulateMatter during LPI Measurement Using Aluminum Substrates

sampling time (min) 30total gas volume (L) 285dilution ratio 45total amount of collected particulate matter

on stages 1–12 (mg)1.96

Figure 6. Particle mass size distribution determined using a LPI.

3664 Energy & Fuels, Vol. 21, No. 6, 2007 Gustafsson et al.

assuming a certain effective particle density distribution. Theestimated particle mass size distributions using SMPS results,assuming that all particles have an effective density of 1 g/cm3

(F0), are presented in Figure 7. The size distributions arecorrected for dilution in the measurement system and representthe average of 12 and 20 SMPS scans before and after theprocess stop, respectively.

In Figure 7, a fine mode at approximately 40 nm is presentin both particle mass size distributions. It is obvious that onlythe tail of a coarse mode is present, with the greater part of themode lying outside the measurement range of the SMPS (i.e.,dB > 670 nm). The total concentration of particles with dB )15–670 nm (Fp ) 1 g/cm3) was 10.0 mg/m3 before the processstop and 9.8 mg/m3 after the process stop.

To compare particle mass size distributions as measured bythe LPI and SMPS, the size distributions must be based on thesame particle diameter, preferably dae. If spherical particles areassumed, dB can be estimated using the Stokes diameter (dp),and the relationship with dae is given by

dae ≈ dp� FpCc(dp)

F0Cc(dae)(1)

where Cc is the Cunningham slip correction factor.22 If Fp ) 1g/cm3, the LPI and SMPS results can be compared directly,because dae ≈ dB (see Figure 8). SMPS results from after theprocess stop were used in the comparison.

The comparison indicates that the particle mass size distribu-tions determined using a LPI and SMPS disagree. The concen-tration of fine particles with dae < 0.5 µm is found to be higherwhen determined using a LPI than with a SMPS. This could bebecause of the bounce-off or re-entrainment of coarse particlesto the lower LPI stages; this would imply that particles werenot properly deposited according to their aerodynamic diametersbut follow the gas to lower LPI stages. A complete comparisonof the LPI and SMPS results requires information about theactual effective particle density distribution. The effectiveparticle density is dependent upon particle morphology andelementary composition. For example, effective particle densities

for soot have been found to vary with dB, ranging from 1.2g/cm3 at dB ) 30 nm to <0.3 g/cm3 at dB ) 300 nm.26 Forparticles from biomass combustion, which are dominated byash, densities of approximately 2 g/cm3 have been used inprevious studies, on the basis of the elementary composition.11,13

Particle Morphology and Elementary Composition. Thealuminum substrates from LPI stages 1, 5, and 10, correspondingto dae ≈ 40 nm, 0.32 µm, and 3.2 µm, respectively, wereanalyzed using SEM–EDS; typical images from these stagesare presented in Figures 9–11, respectively.

Most of the particulate matter on stages 1 and 5 (Figures 9and 10) seems to be agglomerated and partly fused. Theparticulate matter collected on stage 10 (Figure 11) appearsseparated and flaky; however, agglomerated and partly fusedstructures are also present. Unexpectedly, the size of thestructures on stages 1 and 5 is large compared to the geometricmean particle diameters expected for those stages, i.e., dae ≈40 nm and 0.32 µm, respectively. These large structures couldhave resulted from agglomeration on the substrates, interactionswith the grease, bounce-off or re-entrainment of particles fromhigher LPI stages, or a combination of these and other causes.

Agglomerates are often porous and have a low effectivedensity, resulting in a dae smaller than the apparent sizeaccording to eq 1. van Gulijk et al. studied diesel soot sampledon cascade impactor substrates (dae ≈ 0.03–1.5 µm) usingSEM.27 The diesel soot particles found were agglomerates, andthe authors concluded that the apparent size of the agglomerateswas larger than the geometric mean diameter (dae) for theimpactor stage studied. Hindsgaul et al. studied particles frombiomass gasification using SEM and also concluded that largeagglomerates had reached the lower stages of the cascadeimpactor.9 If the agglomerates in the present study had a loweffective density, this could account for at least part of thedifference between the mean dae value of the LPI stage and theapparent size of the structures on that stage.

The EDS analysis revealed that carbon was the dominantelement in all size fractions; other detected elements werecalcium, oxygen, magnesium (probably present as magnesite,which was used as the bed material), aluminum (from thesubstrate material), and bromine. Particles that appeared brighterin the images of stages 1, 5, and 10 were generally rich incalcium, in contrast to the flaky and darker particles that almostcompletely consisted of carbon. Carbon could be present indifferent forms in the particles, for example, as char, soot, andtar or as a mixture of these. While char is fragmentednonvolatilized material, soot and tar particles are formed throughgas–particle conversions. The shape of the flaky particlesindicated that they were char particles. The structure of someof the deposits in the present study appeared similar to the sootagglomerates presented in studies of diesel soot.27 However,the interpretation that the agglomerates found in the presentstudy were soot does not agree with the fact that these particleswere also richer in calcium than the flaky particles. A limitationof the EDS analysis is that the size of the beam is approximately1 µm, which means that individual particles smaller than thiscannot be analyzed with precision. In addition, particles belowthe visible particle layer can affect the elementary compositionas a result of beam penetration.

In previous studies of particles from biomass gasification,Hindsgaul et al. found primary soot particles approximately 70nm in diameter when analyzing particles using SEM,9 while

(26) Maricq, M. M.; Xu, N. J. Aerosol Sci. 2004, 35, 1251–1274.(27) van Gulijk, C.; Marijnissen, J. C. M.; Makkee, M.; Moulijn, J. A.;

Schmidt-Ott, A. J. Aerosol Sci. 2004, 35, 633–655.

Figure 7. Estimated particle mass size distributions before and afterthe process stop, measured using a SMPS (Fp ) 1 g/cm3).

Figure 8. Comparison of particle mass size distributions determinedusing a LPI and SMPS. For SMPS results, Fp ) 1 g/cm3 has been used.

Thermochemical ConVersion of Wood Pellets Energy & Fuels, Vol. 21, No. 6, 2007 3665

van der Nat et al. found agglomerates of 100 nm primaryparticles mainly consisting of potassium and chlorine usingSEM–EDX.8 Selvakumar et al. found larger rod- or stick-shapedparticles rich in silicon and carbon as well as clusters of sphericalparticles rich in carbon, silicon, potassium, and chlorine whenanalyzing particles from biomass gasification usingESEM–EDAX.10

Carbon seems to be present in particulate matter from biomassgasification independent of the gasifier type and samplingposition, but the form in which carbon is present varies. Fineparticles from biomass combustion usually contain potassiumand chlorine,11,15 but these elements could not be detected using

EDS analysis in the present study. Calcium was the onlydetected ash-forming element that was assumed to originate fromthe fuel.

A Teflon filter containing particulate matter with dae <≈ 5 µmwas analyzed using ICP–MS/AES for 24 elements; the elemen-tary composition thus determined is presented in Table 4.

Calcium, sodium, sulfur, potassium, titanium, and irondominated the elementary composition of particles with dae <5 µm. However, the detected elements of the particulate matteronly constituted approximately 9% of the mass, and much ofthe material that was not detected using ICP–MS/AES wasprobably carbon. It is also likely that the sample contained

Figure 9. Examples of SEM images from LPI stage 1, dae ≈ 40 nm.

Figure 10. Examples of SEM images from LPI stage 5, dae ≈ 0.32 µm.

Figure 11. Examples of SEM images from LPI stage 10, dae ≈ 3.2 µm.

3666 Energy & Fuels, Vol. 21, No. 6, 2007 Gustafsson et al.

chlorine, which could not be identified with the analysistechnique used. If all of the elements in Table 4 were presentas their most frequent oxide, the ash percentage would increaseto 14 wt %.

Polycarbonate substrates from a LPI measurement and twoblank substrates used as references were analyzed using PIXE.A total of 11 elements (phosphorus, chlorine, potassium,calcium, titanium, chromium, manganese, iron, copper, zinc,and arsenic) were detected. Six of these elements were detectedin at least half of the substrates, and the mass percentages ofthese elements are presented in Figure 12 (the values arenormalized to 100%). These elements together constituted morethan 98% of the mass detected by PIXE analysis.

Of the elements detected using PIXE analysis, calcium wasdominant in all size fractions, even if its percentage was smalleron the stages collecting coarser particles. The percentage ofmanganese was approximately the same on all stages, whilepotassium and iron were present in higher percentages on thestages collecting coarser particles. The amounts of chlorine andtitanium were very low in all particle size fractions. It isreasonable to believe that the remaining mass that was notdetected using PIXE analysis was mainly composed of carbon-aceous compounds, because carbon was the dominant elementfound in the EDS analysis. In particles from biomass combus-tion, coarse particles are usually rich in calcium, manganese,and iron, while the fine particles are usually richer in potassiumand chlorine.14,15 The size-segregated results of the PIXEanalysis of particles from biomass gasification in the presentstudy differ from the results for biomass combustion presentedin previous studies. This indicates that coarser particles mayhave bounced off or been re-entrained and reached lower stagesof the LPI. However, the elementary composition was not thesame for all size fractions (see Figure 12), which indicates thatdifferent kinds of particles were deposited on the different stages.

A significant difference from previous biomass combustionresults is that the percentage of ash found in the present studywas very low, 2 and 9 wt % as detected using PIXE analysisand ICP–MS/AES, respectively. However, the proportions ofash and carbonaceous compounds also vary considerably inparticulate matter from biomass combustion: the content ofelemental carbon ranged from 0 to 56 wt %, depending uponboth the fuel and operating load in particle samples frombiomass grate combustion.17

Conclusion

The present study characterized aerosol particles formedduring the steam- and oxygen-blown biomass gasification ofwood pellets in an atmospheric 20 kW BFB gasifier.

The particle number size distribution was determined usinga SMPS and was found to be bimodal, with modes at 20–30and 400 nm (dB). The total mean number concentration ofparticles with dB ) 15–670 nm was approximately 7 × 105

particles/cm3; however, the concentration of particles with dB

< 80 nm fluctuated. The particle mass size distributiondetermined using a LPI was bimodal, and the total massconcentration of particles with dae < 5 µm was 310 mg/m3.Analysis of particle morphology indicated agglomerated andpartly fused as well as flaky structures. Unexpectedly largestructures were also found on the lower stages of the LPI. Inaddition, the LPI particle mass size distribution result indicatedthat the concentration of particles with dae < 0.5 µm was higherthan that estimated from SMPS data (with an assumed effectiveparticle density of 1 g/cm3). These large structures could haveresulted from agglomeration on the substrates, interactions withthe grease, bounce-off or re-entrainment of particles from higherLPI stages, or a combination of these and other causes. Themass percentage of ash was low, and carbonaceous compoundsdominated the particulate matter. Carbon was not mainly presentas tars but more likely as char or soot, because the particulatematter was stable when heated in nitrogen to 500 °C. Calciumwas the dominant ash-forming element in all particle-sizefractions. However, the elementary composition differed de-pending upon the particle size: finer particles contained morecalcium and less potassium than coarser particles, in contrastto what is found in measurements of particles from biomasscombustion.

The results of the present study will be valuable for thedesign, construction, and optimization of hot-gas filters andcatalytic processes. However, the impact of parameters, suchas gasifier type, gasification conditions, fuel, sampling position,and measurement system, should be further investigated. Inparticular, the behavior of particles from biomass gasificationin cascade impactors needs to be further addressed, as well asmethods for performing particle measurements at higher tem-peratures than in the present study.

Acknowledgment. Financial support through the EuropeanCommission (EC) 6th Framework Programme (CHRISGAS Projectcontract number SES6-CT-2004-502587) and the Swedish EnergyAgency is gratefully acknowledged. Alberto Becker and LenaNyqvist at TPS Termiska Processer AB are gratefully acknowledgedfor organizing the operation of the gasifier and performing the gasand tar analyses. Dirk Porbatzki and Dr. Michael Müller atForschungszentrum Jülich are also gratefully acknowledged for theanalysis of the wood pellets.

EF7002552

Table 4. Elementary Composition of a Teflon Filter ContainingParticulate Matter with dae <≈ 5 µm, Determined Using

ICP–MS/AES

element wt %

Ca 3.29Na 1.27S 1.16K 0.81Ti 0.81Fe 0.64Mn 0.53Mg 0.33Ba 0.05As 0.04Cr 0.02Sn 0.02

Figure 12. Mass percentages of dominant elements on LPI polycar-bonate substrates determined using PIXE analysis.

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