Characterization of Inorganic Particulate Matter from

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Energy & Fuels 2004, 18, 338-348

Characterization of Inorganic Particulate Matter from Residential Combustion of Pelletized Biomass Fuels Christoffer Boman,* Anders Nordin, Dan Bostro¨m, and Marcus O ¨ hman Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden Received July 7, 2003. Revised Manuscript Received October 31, 2003

The increased focus on potential adverse health effects associated with exposure to ambient particulate matter (PM) motivates a careful characterization of particle emissions from different sources. Combustion is a major anthropogenic source of fine PM, and, in urban areas, traditional residential wood combustion can be a major contributor. New and upgraded biomass fuels have become more common, and fuel pellets are especially well-suited for the residential market. The objective of the present work was to determine the mass size distributions, elemental distributions, and inorganic-phase distributions of PM from different residential combustion appliances and pelletized biomass fuels. In addition, chemical equilibrium model calculations of the combustion process were used to interpret the experimental findings. Six different typical pellet fuels were combusted in three different commercial pellet burners (10-15 kW). The experiments were performed in a newly designed experimental setup that enables constant-volume sampling. TotalPM mass concentrations were measured using conventional filters, and the fractions of products of incomplete combustion and inorganic material were thermally determined. Particle mass size distributions were determined using a 13-step low-pressure cascade impactor with a precyclone. The PM was analyzed for morphology (using environmental scanning electron microscopy, ESEM), elemental composition (using energy-dispersive spectroscopy, EDS), and crystalline phases (using X-ray diffractometry, XRD). For complementary chemical structural characterization, time-offlight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectroscopy were also used. The emitted particles were mainly found in the fine (1 µm) were present primarily in the experiments with bark and logging residues. Relatively large and varying amounts (28%-92%) were determined to be products of incomplete combustion. The inorganic elemental compositions of the fine particles were dominated by potassium, chlorine, and sulfur, with minor amounts of sodium and zinc. The dominating alkali phase was KCl, with minor but varying amounts of K3Na(SO4)2 and, in some cases, also K2SO4. The results showed that zinc is almost fully volatilized, subsequently and presumably forming a more complex solid phase than that previously suggested (ZnO). However, the formation mechanism and exact phase identification remain to be elucidated. With some constrains, the results also showed that the amounts and speciation of the inorganic PM seemed to be quite similar to that predicted by chemical equilibrium calculations.

Introduction Globally, an increasing interest for sustainable energy production can be observed, and biofuels, as a renewable and CO2-neutral energy source, are expected to be a major contributor. New and upgraded biomass fuels (i.e., pellets, briquettes, and powder) have become more common, and fuel pellets are especially well-suited for the residential biomass market, providing possibilities of more automated and optimized systems with higher combustion efficiency and less products of incomplete combustion (PIC). The raw materials presently used for production of fuel pellets are generally stem-wood assortments (∼95%), such as sawdust, planer shavings, or dried chips from sawmills and the wood working * Author to whom correspondence should be addressed. E-mail: [email protected].

industry, whereas the use of bark, agricultural residues, and other forest fuels occurs only occasionally.1 However, in a future with an increasing utilization of forest resources, other types of wood forest-based materials (e.g., bark and logging residues) might also be used for pellet production. To accomplish a substantially increased use of biomass fuels, careful evaluations must therefore be made concerning the consequences for the environment and human health. In this extensive area of multidisciplinary issues, detailed characterization of the emissions from different sources is a fundamental part. The emissions of particulate air pollution are presently an intensely debated issue from the perspective of both global warming and human health. Anthro(1) Hillring, B.; Vinterba¨ck, J. For. Prod. J. 1998, 48 (5), 67-72.

10.1021/ef034028i CCC: $27.50 © 2004 American Chemical Society Published on Web 12/06/2003

Inorganic PM from Biomass Pellet Combustion

pogenic-derived atmospheric aerosols are important for the climate on a global and regional scale. The emissions of carbonaceous aerosols are especially considered as an important contributor to the heating of the global atmosphere.2 Furthermore, combustion processes in the urban environment are major anthropogenic sources of fine particulate matter (PM) pollution, and the present residential wood-log combustion is a significant reason for the deterioration of ambient air quality in residential areas.3 PM in the ambient air today is generally considered to be an important indicator of air pollution that causes adverse health effects. Epidemiological evidence exists that links increases in PM mass concentrations with cardiopulmonary disease4 and mortality.5 No specific particle property or component responsible for the toxicological effects has yet been identified. However, the importance of particle properties other than mass concentration, such as chemical composition, particle size, and number concentration, have been emphasized.6,7 The present knowledge of combustion particles is mainly based on the research and development (R&D) work performed within coal and internal engine combustion and has recently been reviewed.7 One concluding remark was that more characterization studies of combustion aerosols, including transient emissions, detailed chemical speciation, and number of ultrafine particles are needed, given the basis for further work with atmospheric particle transformation and healthrelated hypothesis. From studies of coal combustion, it is known that the particle mass size distribution most often consists of two modes: one fine mode in the submicrometer range (particle diameters of 1 µm).7 Fine-mode particles are formed by nucleation and/or condensation processes of volatilized gas phase precursors, which subsequently coagulate and form agglomerates. The coarse mode can consist of a wide variety of mechanically generated particles, which are normally ash aggregates that have been entrained with the flue gases from the fuel bed during combustion. Important differences between biomass and coal regarding ash transformation and PM formation are the low mineral, high volatile, and high alkali contents in biomass fuels.8 PM from biomass combustion can be classified either as inorganic ash material, soot, or organic material, and the distribution varies with the combustion conditions for different fuels in different appliances. The amount of carbonaceous material can be substantially reduced relatively easily by primary measures that facilitate increased combustion efficiency, which is illustrated by the ongoing technical development of residential biomass appliances. In large and medium-sized combustion (2) Ramanathan, V.; Crutzen, P. J.; Kiehl, J. T.; Rosenfeld, D. Science 2001, 294, 2119-2124. (3) Larson, T. V.; Koenig, J. Q. Annu. Rev. Public Health 1994, 15, 133-156. (4) Pope, C. A., III.; Dockery, D. W. In Air Pollution and Health. Holgate, S. T., Samet, J. M., Koren, H. S., Maynard, R., Eds; Academic Press: London, 1999; pp 673-705. (5) Pope, C. A., III.; Burnett, R. T.; Thun, M. J.; Calle, E. E.; Krewski, D.; Ito, K.; Thurston, G. D. JAMA 2002, 287 (9), 1132-1141. (6) Harrison, R. M.; Yin, J. Sci. Total Environ. 2000, 249, 85-101. (7) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. J Air Waste Manage Assoc. 2000, 50 (9), 1565-1618. (8) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process Technol. 1998, 54, 17-46.

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plants, the conditions are more easily optimized and stable, and, therefore, the PM is dominated by inorganic (ash) constituents that contain mainly potassium, sodium, sulfur, and chlorine in the fine mode with a minor but varying fraction of coarse fly-ash particles that contain refractory elements such as calcium, silicon, magnesium, and aluminum.9-11 If trace elements such as cadmium, lead, and zinc are present in the fuel, they can be volatilized during combustion in all types of devices and be found as condensed species of fine PM.12 Although the principal PM formation and transformation mechanisms are more or less the same, independent of the size of the combustion appliance, an important difference between biomass combustion in large plants and residential appliances is that the coarse fraction is often limited in the latter. The emission characterization work conducted thus far that concerns PM from residential biomass combustion comprises mass concentration, mass/number size distribution, and elemental composition.13-17 Some work that involves speciation of the particle-bound organic fraction has also been performed.18-20 However, few studies have been devoted to the actual inorganic-phase composition of the PM from biomass combustion,17,21,22 and the effects of different combustion techniques and fuels have never been considered. Today, the testing and certification of these types of appliances only comprise the emissions of carbon monoxide (CO), organic gaseous carbon (OGC), and total PM (dust). To increase the possibilities to link specific toxicological effects with exposure to combustion-related PM and also to make health impact assessments of future combustion technologies, detailed physical and chemical characterization of the PM is needed for different fuels and appliances. (9) Chriestensen, K. A. Dissertation. Technical University of Denmark: Lyngby, Denmark, 1995. (10) Valmari, T.; Lind, T.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13 (2), 390-395. (11) Brunner, T.; Dahl, J.; Obernberger, I.; Po¨lt, P. In Proceedings of the 1st World Conference and Exhibition Biomass for Energy and Industry, Sevilla, Spain, 2000; Kyritsu, S., Beenackers, A. A. C. M., Helm, P., Grassi, A., Chiaramonti, D., Eds.; James and James, Ltd.: London, 2001; pp 1991-1994. (12) Nordin, A.; Backman, R. In Ashes and Particulate Emissions from Biomass CombustionsFormation, Characterization, Evaluation, Treatment. Obernberger, I., Ed.; dbv-Verlag: Graz, Austria, 1998; pp 119-134. (13) Rau, J. A. Aerosol Sci. Technol. 1989, 10, 181-192. (14) Purvis, C. R.; McCrillis, R. C.; Kariher, P. H. Environ. Sci. Technol. 2000, 34, 1653-1658. (15) Hueglin, C.; Gaegauf, C.; Ku¨nzler, S.; Burtscher, H. Environ. Sci. Technol. 1997, 31, 3439-3447. (16) Wieser, U.; Gaegauf, C. K. In Proceedings of the 1st World Conference and Exhibition Biomass for Energy and Industry, Sevilla, Spain, 2000; Kyritsu, S., Beenackers, A. A. C. M., Helm, P., Grassi, A., Chiaramonti, D., Eds.; James and James, Ltd.: London, 2001; pp 805-808. (17) Johansson, L. S.; Tullin, C.; Leckner, B.; Sjo¨vall, P. Biomass Bioenergy 2003, 25 (4), 435-446. (18) Kleeman, M. J.; Schauer, J. J.; Cass, G. R. Environ. Sci. Technol. 1999, 33, 3516-3523. (19) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 2001, 35, 1716-1728. (20) McDonald, J. D.; Zielinska, B.; Fujita, E. M.; Sagebiel, J. C.; Chow, J. C.; Watson, J. G. Environ. Sci. Technol. 2000, 34, 20802091. (21) Lind, T.; Valmari, T.; Kauppinen, E.; Maenhaut, W.; Huggins, F. In Ashes and Particulate Emissions from Biomass Combustions Formation, Characterization, Evaluation, Treatment. Obernberger, I., Ed.; dbv-Verlag: Graz, Austria, 1998; pp 155-169. (22) Kaufmann, H.; Nussbaumer, T. In Proceedings of the 10th European Conference and Technology Exhibition on Biomass for Energy and Industry. Kopetz, H., Weber, T., Palz, W., Chartier, P., Ferrero, G. L., Eds.; Wu¨rzburg, Germany, 1998. C. A. R. M. E. N: Rimpar, 1998; pp 1326-1329.

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Table 1. Characteristics of the Studied Pellet Fuelsa Sawdust fresh (S0) effective heating value (MJ kg-1 d.wt.) moisture content (% d.wt.) ash content (% d.wt.) elemental compositionb Cc Hc Oc Nc S Cl Si Al Fe Mg Ca Na K P Mn Cr Co Cu Sr V Zn Zr a

Logging Residues stored (S6)

fresh (L0)

stored (L6)

Bark fresh (B0)

stored (B6)

18.7 13.1 0.49

18.9 13.3 0.61

19.4 10.2 2.44

19.5 11.4 2.53

18.8 11.2 3.35

18.2 10.1 7.89

51.9 6.0 41.8 0.12 0.02 0.004 0.06 0.01 0.02 0.01 0.10 0.01 0.05 0.01 0.01 0.82 ppm 0.0703 ppm 1.36 ppm 7.11 ppm 0.112 ppm 14.0 ppm 0.352 ppm




52.5 5.7 39.3 0.40 0.03 0.03 0.37 0.07 0.04 0.08 0.81 0.03 0.26 0.07 0.06 2.66 ppm 0.462 ppm 1.51 ppm 42.2 ppm 0.567 ppm 131 ppm 1.86 ppm

52.5 5.7 39.3 0.40 0.04 0.02 1.86 0.39 0.27 0.11 1.11 0.14 0.37 0.07 0.07 27.3 ppm 1.28 ppm 5.37 ppm 56.9 ppm 4.47 ppm 136 ppm 17.3 ppm

% d.wt. ) percentage based on dry weight.24

b

Values are in % d.wt., unless noted otherwise. c Average values taken from Nordin.24

Therefore, the objective of the present work was to determine the mass size distributions, elemental distributions, and inorganic-phase distributions of PM from different residential combustion appliances and pelletized biomass fuels. In addition, chemical equilibrium models were used to interpret the experimental findings.

Appliances and Fuels. The emissions from the combustion of six different fuels were characterized from three different commercial pellet burners (with nominal heat outputs of 1015 kW) installed in a reference boiler that is presently used for national certification tests of residential pellet burners in Sweden. The three burners, all of which had been certified according to the national certification criteria “P-marking” at the Swedish National Testing and Research Institute,23 were representative for the present European market. They also represented different classes of burner constructions with overfeeding, horizontal feeding, and underfeeding of the fuel, respectively. Fresh and stored (for six months) material from softwood sawdust (S0/S6), logging residues (L0/L6) and bark (B0/B6) were used as raw materials, and the raw material originated from the same felling district, with 60% Norway spruce and 40% Scots pine. These raw materials are all either presently used or are potential new biomass resources aimed at the pellets market. All six pellet assortments were produced in the same plant, and the raw materials were dried with the flue gases from an oil burner before milling and pelletizing. The characteristics of the pelletized fuels are given in Table 1. The ash compositions of the fuels corresponded closely to previously reported data24 for wood, logging residues, and bark, except for the silicon content, which was relatively high for sawdust (wood) and stored bark. The increased silicon con-

centrations probably originate from typical contamination in the form of quartz or silicates during the production, handling, and/or storage of the fuels. More detailed information about the quality properties of the different pellet fuels is given by Lehtikangas.25 Experimental Procedure. The boiler was connected to a water-based heat exchanger system, where the heat output was set to a constant value of 3 kW, which corresponds to a typical situation for small houses during wintertime. The fact that the nominal output of the pellet burners exceeded 3 kW during operation resulted in intermittent operation conditions, which are also typical for residential use. Each emission measurement started after 2-3 initial cycles when the system was in thermal balance and continued over 4-6 combustion cycles, corresponding to 3-6 h for burners A and C and 6-9 h for burner B. Temperature measurements with thermocouples (type K) were performed at different locations in the combustion zone in the burners. To be able to determine the total gas and PM concentrations and allow for representative isokinetic sampling during the intermittent operation, the experiments were performed using a previously designed dilution setup that enables constant volume sampling (CVS). The presently used dilution tunnel sampling system has been more carefully described and evaluated by Boman.26 Continuous gas measurements of oxygen gas (O2), carbon dioxide (CO2), CO, and total gaseous hydrocarbons (THC) were performed with conventional instruments in the dilution tunnel, as well as O2 and CO2 in the raw flue gas before dilution. The dilution ratios were calculated as the ratios between CO2 in the diluted gas and CO2 in the undiluted gas. In the present study, the intent was to dilute the flue gas just enough to maintain a constant flow during the intermittent burner operation. The dilution ratios varied within the range of 1.5-2.0 during the combustion periods, depending on combustion intensity, and the flue-gas temperature at the sampling points was consistently controlled to 60 ( 5 °C.

(23) Swedish National Testing and Research Institute, Report SPCR 028, 1999. (24) Nordin, A. Biomass Bioenergy 1994, 6 (5), 339-347.

(25) Lehtikangas, P. Biomass Bioenergy 2001, 20 (5), 351-360. (26) Boman, C. Licentiate Thesis. Umeå University, Umeå, Sweden, 2003.

Methods and Materials



Table 2. Elements and Solution Models Used in the Chemical Equilibrium Model Calculationsa Modeling Stage 1: “Volatilization” elements solution models

C, H, O, N, S, Cl, P, K, Na, Ca, Mg, Si, Al, Zn Slagg: ASlag-liq Salt: Salt-liq (-SALT) (Ca, Mg): liq-K,Ca/CO3,SO4 (-LCSO) s-K,Ca/CO3,SO4 (-SCSO) s-Ca(SO4), Mg(SO4) (SCMO) liq-Ca,Mg,Na/(SO4) (-LSUL) s-Ca,Mg,Na/(SO4) (-SSUL) CaCl2 (-CACL)

a

Modeling Stage 2: “Condensation” C, H, O, N, S, Cl, K, Na, Zn Salt: Salt-liq (-SALT) Alk-Cl (-ACLA) Na,K/OH (-AOH) CO3,SO4/Li,Na,K (-CSOB) Na,K/SO4 (-NKSO) K2SO4-solid (-KSO) K2CO3 (-KCO) Na,K/CO3 (-NKCA) Na,K/CO3 (-NKCB) K3Na(SO4)2 (-KNSO) A Na,K/OH,F,Cl (-NKXA) B Na,K/OH,F,Cl (-NKXB) Na/Cl,OH (-NCOA) Na/Cl,OH (-NCOB)

Designations of the solution models were taken from FACT-Win 3.05.

Sampling of Particulate Matter. Total-PM concentrations were determined by isokinetic sampling, using conventional equipment with glass fiber filters in the experiments with burners A and B and quartz fiber filters for burner C. The particle mass size distributions were determined by isokinetic sampling, using a 13-step low-pressure cascade impactor (Dekati, Ltd.) with a precyclone and a downstream Teflon filter. The cyclone cutoff was ∼16 µm, and the impactor separates the particles by aerodynamic diameter in a range of ∼0.03-10 µm. An ejector dilutor (Dekati, Ltd.) was used before the impactor, to dilute the sample gas (∼7.64 times) with clean dried air, thereby allowing for integrated sampling during several combustion cycles without overloading the substrates. To allow for extensive phase and elemental analyses, aluminum foils were used as impactor substrates for burners A and B and quartz filters were used for burner C. The aluminum substrates were greased with Apiezon L vacuum grease, to prevent particle bounce, and heated for 2 h at 105 °C, to avoid mass losses during sampling. The sampling probes, filter holders, precyclone, ejector dilution air, and impactor were all kept at 60 ( 5 °C. After at least 8 h of conditioning in a desiccator, total-PM filters and impactor substrates were analyzed gravimetrically with an analytical balance (0.01 mg) before and after sampling. To separate the total-PM samples into different fractions, a standard thermal method was used. Subsequent heating and weighing one of the two equivalent filters per sample in oxidizing atmosphere (air) for 2 h at different temperature intervals resulted in content of water (105 °C), unburned material (105-550 °C), and residual ash (550 °C). Chemical Analysis of Particulate Matter. Total-PM samples and impactor samples were analyzed for morphology and elemental composition, using a Philips model XL30 environmental scanning electron microscopy-field emission gun (ESEM-FEG) that was equipped with an EDAX energydispersive spectroscopy (EDS) detector. Selected impactor samples were further analyzed by X-ray diffractometry (XRD) for identification of crystalline phases. The XRD investigations were performed using a Bruker d8Advance instrument in θ-θ mode, with an optical configuration that involved primary and secondary Go¨bel mirrors. The sample foils were mounted on a rotating, low-background, silicon single-crystal sample holder. Continuous scans at a rate of 1 °/min were applied. By adding repeated scans, the total data-collection time for each sample amounted to at least 15 h. A Fourier smoothing was applied to the scans and the background was removed. Analyses of the diffraction patterns were performed, using Bruker software together with the PDF2 databank. In addition, a small number (1-3) of impactor samples were analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) spectroscopy. These analysis methods can provide chemical structural information on a molecular level, and, in

this work, they were used first and foremost as complementary tools to ESEM/EDS and XRD for possible further detailed chemical characterization. The TOF-SIMS analyses were performed at the Swedish National Testing and Research Institute in Borås, using model TOF-SIMS IV, Ion-TOF GmbH. The XPS analyses were performed using a Kratos Axis Ultra spectrometer. Wide spectra (pass energy 160 eV) and spectra of individual photoelectron lines were acquired using a monochromatized Al KR source that was operated at 225 W. To compensate for surface charging, a low-energy electron gun was used. Spectra were processed with Kratos software, and the binding energy scale was referenced to the C 1s line of aliphatic carbon contamination set at 285.0 eV. Zn K-edge extended XAFS (EXAFS) data were measured at the Stanford Synchrotron Radiation Laboratory (Stanford, California) on beam line 4-1. The ring energy, was 3.0 GeV with ring current between 60 and 100 mA. A Si(220) double-crystal monochromator was used and detuned 50% to eliminate higher harmonics. The data were measured at room temperature in the fluorescence mode, with a Lytle detector27 that was filled with argon gas. A Ni-6 filter and a Soller slit setup were used to reduce Kβ fluorescence and scattering contributions to the signal. Internal calibration was performed by simultaneously measuring spectra from a Zn foil in transmission mode, throughout the duration of all scans. Chemical Equilibrium Model Calculations. To help interpret the experimental findings, chemical equilibrium model calculations were performed using the software program FACT-Win 3.05. The program uses the method of minimization of the total Gibb’s free energy of the system. Thermodynamic data were taken from the FACT database28 including stoichiometric data as well as nonideal solid and liquid solution models. Fuel characteristics were taken from Table 1. The calculations were performed using both local (i.e., reducing atmospheres within fuel particles) and global (i.e. oxidizing) approaches under atmospheric pressure (1 bar). An air-to-fuel ratio (λ) of 2.0 was used for the global calculations, which corresponds to the typical average conditions present in the burners during operation. The process temperature in the combustion zone over the burner grates varied over a range of 900-1100 °C, and the particle sampling was performed downstream the boiler at 60 ( 5 °C; therefore, the high temperature and cooling stages were studied separately: (1) The degree of volatilization of ash elements was determined in the temperature range of 900-1100 °C, using fuel data from Table 1. Two liquid phases were assumed to coexist: i.e., an oxide/silicate (slag) melt and an alkali salt melt. In (27) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, D. R.; Wong, J.; Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods Phys. Res., Sect. A 1984, 226 (2-3), 542-548. (28) Bale, C. W.; Pelton, A. D. FACT-database of FACT-Win version 3.05, CRCT E Ä cole Polytechnique de Montre´al: Montreal, Quebec, Canada, 1999.

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Table 3. Summary of Average Gas and Particle Measurement Resultsa fuel

O2b (%)

CO2b (%)

CO (mg/MJ)

THC (CH4-equ) (mg/MJ)

1000 970 1840 1630 1850 1480

361 375 650 556 418 322

PM mass concentration (mg/MJ)

PM mass median diameter (µm)

PM ash contentc (%)

0.37 0.38 0.36 0.33 0.23

27 4 10 14 48 15

79 57 88 88 101 157

0.21 0.20 0.22 0.21 0.23

54 15 60 37 63 3

94 64 178 134 183 192

0.21 0.39 0.25 0.24 0.28 0.39

64 46 37 47 49 21

Burner A S0 S6 L0 L6 B0 B6

12.5 10.7

8.0 9.7

S0 S6 L0 L6 B0 B6

7.2 6.6 6.3 4.8 4.5 6.2

13.2 13.7 13.9 15.2 15.4 14.1

1610 1350 1090 2340 2010

61 38 44 65 60

S0 S6 L0 L6 B0 B6

14.1 13.5 12.7 11.0 13.5 12.4

6.4 7.0 7.8 9.3 7.1 8.1

940 1310 1430 1680 990 580

66 94 159 111 169 189

241 300 377 296 140 114 Burner B

Burner C

a S0, fresh sawdust; S6, stored sawdust; L0, fresh logging residues; L6, stored logging residues; B0, fresh bark; B6, stored bark. b Average values during burner operation in the nondiluted gas. c Standard ashing method at 550 °C.

Figure 1. SEM images of total PM samples on quartz fiber filters, showing three different typical particle types that could be identified: fine submicrometer-sized particles/aggregates (Type 1), spherical coarse particles (Type 2), and irregular aggregated coarse particles (Type 3). addition, all relevant binary solid and liquid solutions with calcium and magnesium were included. The elements and solution models used in the calculations for the first modeling stage are shown in Table 2. The local conditions that prevail within a burning fuel particle were obtained by assuming dry particles without access to O2 from air. (2) The chemical equilibrium condensation behavior of the volatilized material at 900 and 1100 °C, respectively, during cooling downstream from the boiler were then determined by calculations, down to a temperature of 50 °C. Because all slag is left as bottom ash or deposits on the grate, only the relevant alkali salt models were used for the post-combustion zone (i.e., the liquid salt solution model and relevant binary solid alkali salt solutions models). The elements and solution models used in the calculations for modeling stage 2 are shown in Table 2. The elements carbon, hydrogen, oxygen, and nitrogen were included as the equilibrium levels of CO2, water (H2O), O2, and nitrogen gas (N2), nitrogen monoxide (NO), and nitrogen dioxide (NO2) at 900 and 1100 °C, respectively.

Results and Discussion Experimental Results. The results from the gas and particle measurements are summarized in Table 3. The emission levels of CO were relatively similar between the different burners and fuels, whereas the emissions of THC and total PM were significantly higher for

Figure 2. Particle mass size distributions for the different biomass fuels combusted in pellet burner (burner B). (No data are given for S0.)

burner A, compared to the other two, because of differences in burner construction and operation. Compared to typical emission levels for residential pellet burners, the concentrations of CO, THC, and PM determined in the present study were relatively high. This was probably caused by the nature of intermittent operation



Figure 3. Elemental composition of the fine PM for the different fuels combusted in burner B (left) and burner C (right). Standard deviations within the fine mode (i.e., composition at different impactor substrates) are shown as error bars. For burner B (using aluminum foils), aluminum was excluded from the analysis; for burner C (using quartz fiber filters), silicon was excluded.

typically that is used, which creates less-stable combustion situations with increased average and total emissions. This has also been shown in previous studies29,30 and illustrates some of the benefits with using constant volume sampling (CVS) or other similar methods with controlled gas flows and sampling conditions (e.g., temperature and dilution) for testing and development of appliances during these types of intermittent operation at typical low heat outputs. Three different typical particle types could be identified by the ESEM analysis, in regard to morphology (Figure 1): (1) fine submicrometer particles/agglomerates, (2) spherical “larger” particles, and (3) irregular “large” agglomerated particles. The PM was dominated by the fine mode (type 1), by mass, and, therefore, even more by number. The particles of type 2 and 3 were least common in PM from sawdust and most common in PM from bark, although the PM varied relatively much in size and was limited in mass, which is consistent with the results from the impactor measurements. The particle mass size distributions were, in all cases, dominated by a fine mode with mass median (aerodynamic) diameters (MMD) of 0.20-0.39 µm. In Figure 2, the results from the impactor measurements with burner B are shown to illustrate this observation. Minor fractions of the PM mass were also found in the coarse mode fraction, primarily from the experiments with bark and logging residues, although they varied relatively much and was indistinct. From the impactor results and the fact that no significant mass (

Characterization of Inorganic Particulate Matter from

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