Subscriber access provided by University of South Dakota
Article
Influence of wood species and burning conditions on particle emission characteristics in a residential wood stove Robin Nyström, Robert Lindgren, Rozanna Avagyan, Roger Westerholm, Staffan Lundstedt, and Christoffer Boman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02751 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Influence of wood species and burning conditions on particle emission characteristics in a residential wood stove
Robin Nyström1, Robert Lindgren1, Rozanna Avagyan2, Roger Westerholm2, Staffan Lundstedt3, Christoffer Boman1* 1
Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics,
Umeå University, SE-901 87 Umeå, Sweden 2
Department of Environmental Science and Analytical Chemistry, Arrhenius Laboratory, Stockholm
University, SE-106 91 Stockholm, Sweden 3
Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
1 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 31
Abstract Emissions from small scale residential biomass combustion are a major source of indoor and outdoor particulate matter (PM) air pollution, and the performance of stoves, boilers and fireplaces have been shown to be influenced both by fuel properties, technology and user behaviour (firing procedures). Still, rather scarce information is available regarding the relative importance of these variables for the particle characteristics and emissions of different particulate components, e.g. soot, PAH, oxy-PAH, and metals. In particular, the behaviour of different wood fuels under varying firing procedures and combustion conditions, has not been studied thoroughly. The objective of this work was therefore to elucidate the influence of wood species and combustion conditions on particle emission characteristics in a typical Nordic residential wood stove. The emissions from four different wood species were investigated at two controlled combustion conditions including nominal and high burn rates, with focus on physical and chemical properties of the fine particulate matter. Considerably elevated carbonaceous particle emissions (soot and organics) was found during high burn rate conditions, associated with a shift in particle number size distribution towards a higher fraction of larger particles. In some cases, as here seen for pine, the specific fuel properties can affect the combustion performance and thereby also influence particle and PAH emissions. For the inorganic ash particles, the content in the fuel, and not burning condition, was found to be the main determining factor as seen by the increased emissions of alkali salts for aspen. Wood stove emission data on 11 specific oxyPAHs together with 45 PAH was combined with controlled variations of burning conditions and fuels. The oxy-PAH/PAH ratio during high burn rate was found to increase, suggesting an enrichment of particulate oxy-PAH. Accordingly, the main influence on emission performance and particle characteristics was seen between different burn rates, and this study clearly illustrates the major importance of proper operation to avoid unfavorable burning condition regardless of the wood species used.
2 ACS Paragon Plus Environment
Page 3 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1. Introduction Woody biomass in different forms is used all over the world for residential heating and cooking. It is also common in the developed parts of the world to use batch fired wood stoves and open fireplaces for aesthetic combustion as well as an extra source of heating, even where other sources of heating (e.g. gas, oil, district heating) is already installed. It has been estimated that around 65 million domestic wood burning installations exist within Europe1, these kind of stoves are most commonly designed for burning of wood logs and are often older technology without proper combustion control, and therefore with large variation in emission performance caused by user behavior. Emissions from this kind of small scale biomass combustion appliances are a major source of outdoor particulate matter (PM) air pollution2, especially during the winter season, when these emissions have been estimated to be a significant contributor to the total PM levels in ambient air3, 4. In rural parts of Europe as much as 70% of all organic PM in the ambient air during wintertime have been proposed to originate from residential wood combustion5. Furthermore, PM air pollution concentrations from wood burning in rural areas, with limited traffic, have been found to be in the same range as those from traffic in major cities6, 7. Since the use of domestic wood burning in Europe is expected to increase even further, at the same time as other important sources are decreasing emissions (e.g. the traffic sector), this sector is estimated to become the dominant source of fine (PM2.5) primary particle air pollution in Europe from 20201. Combustion of wood and other biomass in general is a heterogeneous process with both emissions levels and particle characteristics varying with applied combustion technologies. For residential appliances large differences in combustion efficiency and emission performance have been seen between old and new models of the same type of appliance (e.g. stoves, boilers, masonry heaters)8, 9. In addition, the type of fuel (e.g. wood logs, pellets), the amount of fuel load, the source (e.g. wood species) and fuel quality (e.g. moisture content, size, contaminants) also affects the burning conditions 10-15
. Furthermore, it has been shown that the emissions of carbonaceous matter, i.e. soot and organic
compounds, from small scale biomass combustion is strongly linked to combustion conditions and that emissions have also been shown to vary with different wood species8, 16, 17.The detailed combustion 3 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
conditions in a stove has also been shown to significantly influence the emission profiles of contaminants such as polycyclic aromatic hydrocarbons (PAHs) during different burning phases under batch wise combustion of wood18. Some effort has been put into categorizing particle types emitted from residential biomass emissions, and a conceptual model were previously proposed based on existing literature, including three main particle types; soot (elemental carbon agglomerates), organic spherical carbon particles, and inorganic ash particles 13. This model was recently revised within a position paper that added more complete information of particle size distributions by type as well as new findings in biomass combustion particle formation and dynamics19. It is today well known that PM emissions from residential wood combustion is totally dominated by particles in the sub micrometer size range (< 1 µm) 13, 20. Based on the well-known gas-to-particle formation mechanisms for these particles, the fine mode for these combustion derived particles is in this perspective defined as PM120. Particle size is an important property influencing particle behavior, for example since changes in size can lead to different deposition behavior in the respiratory system during inhalation and therefore also potentially different health effects21. Another important contributor to the observed adverse health effects caused by PM exposure in the ambient air is the chemical composition13, 22. For example, the content of different metals, mainly transition metals with redox potential, has been discussed and correlated with PM toxicity23, 24. Furthermore, organic species such as polycyclic aromatic compounds (PACs) are also influencing the toxicity of PM. Among these, PAHs have been most widely studied, as they are known to be both toxic, mutagenic and carcinogenic25, 26. However, other less studied PACs may also be of toxicological importance. For example, the oxygenated PAHs (oxy-PAHs) are suspected to be significant contributors to the oxidative stress caused by air PM, and they are also known to have other toxic effects27, 28. In some cases, the oxy-PAHs show higher toxicity than the PAHs they have been derived from29, 30 . In Sweden wood combustion and vehicular traffic have been regarded as the major local sources of PAHs/PACs into the environment31.
4 ACS Paragon Plus Environment
Page 5 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Accordingly, the emission performance in wood stoves and other domestic fireplaces for biomass, can be influenced both by the technology used, the fuel properties and the user behaviour (firing procedures). Still, rather little information is available regarding the importance of these variables for the emissions of different particulate emission components, e.g. soot, PAHs, oxy-PAHs, and metals, and the behaviour of different fuels under varying combustion conditions. The objective of this work was therefore to elucidate the influence of wood species and combustion conditions on particle emission characteristics in a typical Nordic residential wood stove. The emissions from four different wood species were investigated at two controlled combustion conditions including different burn rates, with focus on physical and chemical properties of the fine particulate matter. 2. Experimental section Combustion setup and fuels used. Four different wood species were combusted in a typical Nordic 9 kW wood stove commonly installed in the 1990´s, operated in laboratory setup, enabling controlled combustion and sampling conditions. The stove is a natural draft stove with inside lining of five soapstone tiles of 25 mm thickness. The stove has been used in several previous studies, e.g. Avagyan et al 2016, Eriksson et al 2014, and Pettersson et al 201117, 18, 32. The four wood log fuels used were; silver birch (Betula Pendula), quaking aspen (Populus Tremula), Norway spruce (Picea Abies) and scots pine (Pinus Sylvestris). All wood fuels included the original bark content and the general characteristics of the wood fuels including also elemental analysis is shown in Table 1. The air supply to the stove was measured on-line by a mass flow meter (Kimo Instruments, Montpon, France), enabling monitoring of the flow rate of air throughout the combustion cycle. When the combustion air enters the stove it is redirected either through a regulating grate in the bottom of the combustion chamber, or through slits in the walls of the combustion chamber. In this study, the normal firing procedure for the stove, as defined by the manufacturer, was used which stated that during the startup procedure the regulating grate should be kept open, and as soon as the fuel ignites properly the grate should be closed and the air is thereby redirected through the slits in the walls instead of through the bottom grate. In the present study, no large variations between the different tests could be observed, and the grate was in all cases closed after 3-4 minutes. 5 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 31
During typical household firing in this kind of stoves, the air flow through the combustion chamber is determined by the natural draught in the chimney, where the draft depends mainly on flue gas temperature, mass flow and the ambient air temperature. As this can cause variability on the combustion conditions, by uncontrollable outside factors as the weather, the setup used in this research utilizes a flue gas fan to regulate the chimney draft. By this measure, a constant draft of 10 Pa was maintained throughout the study, mimicking the chimney draught found in real life residential applications. Thus, ensuring more controlled testing conditions, preferably giving also more stable combustion conditions and higher repeatability of the experiments. Definition of burn rate. The stove was operated with two different combustion cases (burn rate modes); nominal burn rate (NB) and high burn rate (HB). For the nominal burn rate mode, the operation recommendations from the manufacturer was applied stating that each batch should consist of three wood logs with a combined weight of 2.5 kg. In our study, the batches during NB therefore consisted of three logs with a total weight of 2.6 (±0.07) kg, where each log had weights in the range of 0.6-1.1 kg. The high burn rate mode was achieved by following the same recommendations as NB, but slightly overloading the stove with more and smaller finer cut logs, without exceeding the realistic authentic range. The total weight for the batches during HB was 3.5 (±0.05) kg divided over five smaller wood logs, each weighing around 0.5-1 kg. The NB can be considered as a reference case for normal recommended operation of this kind of wood stoves, while the HB was applied for comparison and evaluation of the influence of a different firing scenario with excess burn rates. The phenomenon of excess burn rates in this kind of closed chimney wood stoves is well known and have been discussed in the literature for a long time 33. In principal, the combustion procedure resembles the NB mode, with the difference that short episodes of hot and air-starved combustion occur in the beginning of the flaming phase. This is normally caused by the supply of larger fuel batches (“overload”), but also by using dryer wood as well as finer cut wood, which causes a faster ignition of the whole fuel batch and therefore an “overshoot” in burn rate, before the conditions in the stove are stabilizing. This mode of operation may cause elevated emissions of products of incomplete combustion such as soot and organic matters including PAHs18, 33, 34.
6 ACS Paragon Plus Environment
Page 7 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Batch wise combustion and sampling approach. The scattered emission data from wood stove presented in different studies is not only caused by the inherent variations of batch combustion process, but also influenced by parameters such as stove operation procedures and included part of the batch cycle. This makes it somewhat difficult to compare results from different studies, particularly since whole batch emissions are scarcely reported. In addition, there are also differences between batches, not least because cold starts differ from fuel additions to glowing embers, but also influenced by the placement, size and physical properties of the logs used for each batch. In this paper, each batch was therefore added to glowing embers in a similar manner in all cases, with at least one previous batch of the same fuel combusted beforehand, to avoid cold start conditions and any possible influence of remaining residues from the former experiment. The sampling periods for both particulate and gaseous emissions consisted of one whole batch, starting slightly before the addition of logs to the glowing embers and ending when the last flame had extinguished. This procedure resulted in varying sampling times for the different combustion situations tested, i.e. combustion modes and fuel used, as the burnout times differed. For each combustion case, triplicate particle samples were collected during three succeeding fuel batches. Gas analysis, filter sampling and online particle measurements. A schematic illustration of the measurement set-up, focusing on the sampling lines, is shown in Figure 1. The gaseous components excess oxygen (O2), carbon monoxide (CO), and nitrogen monoxide (NO) in the flue gases, was monitored continuously in the undiluted flue gases by a flue gas analyzer using electrochemical sensors (Testo model 350, Testo AG, Lenzkirch, Germany). In addition, the flue gas temperature was measured with a thermocouple in the flue gas analyzer probe. Dilution and sampling of aerosols was performed in three sampling points in the flue gas channel approximately 2-3 meters from the top of the stove. The flue gas temperature at the sampling points was approximately 250−300 °C. In one sampling line the flue gases were diluted in two steps using ejector dilutors (Dekati Ltd., Tampere, Finland) with a dilution ratio (DR) of ~10 times in each step, i.e. ~100 in total. After dilution in this line, online measurement of particle mobility size distribution and number concentration was performed using a scanning mobility particle size system (SMPS 3937, 7 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 31
TSI, Shoreview, MN, USA) with a size range set to 15-385 nm and a total scan time of 40 seconds including 10 seconds purge. Furthermore, collection of material to be used for analysis of carbon fractions and PAHs was also performed in this sampling line. The DR in this line was determined by measuring NO concentrations in the undiluted flue gases, measured by the Testo 350, and compared to corresponding NO values in the diluted gases using a chemiluminescence based instrument (>0.001 ppm, ECO Physics CLD 700 AL Med, Switzerland). The flue gases in the two additional sampling lines were diluted by the use of porous tube diluters (PTD)35 followed by traditional filter sampling for collection of particles for measurement of total dust (PMtot) and subsequent analysis of oxy-PAHs. DR was here kept as low as possible (4-70 times) and was controlled by mass flow controllers supplying the PTD with dilution air while the same time controlling the total suction flow of the pump with a regulating valve. All filters were stored in a desiccator for approximately 24 hours before weighing, both prior and after the sampling. In addition, the filter samples were wrapped in Al-foil and stored in a freezer until subsequent chemical analysis. Analysis of PAHs and oxy-PAHs. In the following, a brief description of the analytical methods and procedures used for PAHs is given. A more detailed description of the analysis system setup, operation and parameters can be found elsewhere36-39. To summarize, the PAH analysis, on the three sampled filters per test, was performed using an liquid chromatograph coupled to gas chromatograph mass spectrometer (LC–GC/MS) system, consisting of a CMA/200 micro sampler (CMA Microdialysis AB, Sweden), an LC pump (Varian Inc., Palo Alto, CA, USA), a UV detector (SPD-6A, Shimadzu, Japan), with a normal phase LC column (Nucleosil 100-5NO2 124 × 4.6 mm, 5 µm) and a GC/MS system consisting of an Agilent 6890N GC (Agilent Technologies, Palo Alto, CA, USA) with an Agilent 5973N MSD (Agilent Technologies). In total, 45 PAHs were determined, i.e. PAHs with three-rings and larger. The validation procedure has been reported in detail in other studies 38-40. The limits of detection (LOD) ranged from 2.1 – 14 pg and limits of quantification (LOQ) 7.1 – 49 pg, with the coefficients of determination (R2) of all calibration curves ≥ 0.98. The extraction efficiency has been determined using standard reference materials (SRM); SRM 1649a, SRM 1650b and SRM 2975 39, from the National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). 8 ACS Paragon Plus Environment
Page 9 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Furthermore, the analytical methods and procedures for oxy-PAHs have been described in detail previously by Arp et al41. To summarize, only one filter per test was analyzed for oxy-PAH, and the samples were extracted by pressurized liquid extraction (PLE) using n-hexane/acetone (1:1) at 120 °C and 14 MPa in an ASE 350-system. The extracts were then purified on columns with KOHimpregnated silica gel eluted with dichloromethane, after which they were analyzed with gas chromatography (GC) high resolution mass spectrometry (HRMS) using an HP 5890 GC coupled to a Waters Autospec Ultima HRMS. The GC was equipped with a DB5ms capillary column (60 m × 0.25 mm × 0.25 µm; J&W Scientific, Folsom, CA, USA) and the MS operated in electron ionization mode. Target compounds were identified by comparing GC retention data for the molecular ions in the samples and the reference standards. In total, 11 oxy-PAHs were determined in this study. Carbon fractioning analysis. Sampling for carbon fractionation analysis was performed using a setup consisting of two parallel filter lines. In the first line, a 47 mm (dia.) PTFE Membrane filter (Capitol Scientific Inc., Austin, TX, USA) was followed by a 47 mm (dia.) quartz filter (Pall Corporation, Port Washington, NY, USA), while in the parallel line, a single 47 mm (dia.) quartz filter was used. This sampling methodology follows a well-established procedure, and a more detailed description is found in Turpin et al42. The fraction of organic and elemental carbon in the wood combustion PM samples was analyzed by a thermal-optical carbon analysis method as extensively applied in previous studies and described elsewhere42 . The carbonaceous matter on the quartz filters were analyzed using the EUSAAR 2 thermal protocol, a protocol designed to minimize potential positive and negative biases and increases the accuracy of the OC/EC fractioning. The EUSAAR 2 protocol is defined with four steps in Helium atmosphere followed by four steps in Helium/Oxygen atmosphere, each step with different temperatures and residence times. The method and its advantages have been described more in detail by Cavalli et al43. Ion chromatography analysis. The PTFE filters used in the carbon fractioning sampling line, as described above, were used for ion chromatography analysis of major inorganic ions. In this method, the filter material was initially extracted in water using an ultrasonic method. Anions (Cl-, SO42 and 9 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 31
NO3-) were analyzed according to SS-EN ISO 10 304-1 :2009 while cations (Na+, K+, Mg2+, Mn2+ and Ca2+) were analyzed according to SS-EN ISO 14911:2000, using an anion- and cation Dionex ion chromatography, respectively (Dionex Corporation, Sunnyvale, CA, USA). 3. Results and discussion Gaseous emissions and burn time. In Table 2, the average emissions of gases and particulates as well as PM properties are given based on three replicates for each fuel during both NB and HB. Since the flue gas analyzer had a maximum detection limit for CO of 5000 ppm the percentage of the burn cycle exceeding this value is also reported. For NB the CO peaks above the maximum value mainly occurred during the startup, while for HB it was protracted to larger parts of the burn cycle. In Figure 2 examples of burn cycles for NB and HB during birch combustion is shown, to illustrate typical differences in combustion behavior between NB and HB, with regards to excess oxygen and CO concentrations. The same type of figure for the three other fuels used can also be found in Figure S1 in the supplementary information. With the same stove settings, the start-up behavior differed between the fuels, as seen by differences in the O2, CO and PM emissions, as well as for the total burn time. Burn time is here defined as the time from the addition of fuel to glowing embers until the time point when the last flame was extinguished. Thus, the aimed differences in burning conditions were confirmed by the differences in both excess O2 in flue gases and observed burn times. The average O2 in flue gases for NB and HB was 11.9% and 7.0%, respectively, with the lower average O2 indicating a higher burn rate with higher overall oxygen deficiency during the HB conditions. As also seen in Figure 2, the HB mode was associated with shorter episodes of very low O2 (5000 ppm). Furthermore, Table 2 also shows the burn times for the different experiments. In this study, birch and aspen showed similar burn time during both NB, whereas some deviations for spruce and pine could
10 ACS Paragon Plus Environment
Page 11 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
be observed. During NB, pine and aspen had the lowest and highest burn time with 40.3 min and 60.3 min, respectively. For HB, no clear difference was discernable since all fuels almost had the same burn time. In this study four different fuels were combusted using the same stove applying the same firing procedure and burning conditions, designed to mimic normal household usage (i.e. without proper combustion control). As a result, the combustion conditions were not optimized for all wood species used, caused by the different physical properties of them. While pine and spruce are conifers, birch and aspen are deciduous tree species with normally lower carbon content and therefore lower heating value than conifer species. On the other hand, the often denser wood from deciduous tree species tends to burn longer44. All these factors may therefore influence the wood properties and thereby their combustion. As an example of variations between the wood species used in this study, previous reported density values are; birch 640 kg/m3, aspen 415 kg/m3, pine 550 kg/m3, and spruce 405 kg/m3 45
. The density values reported in the literature can be considered as rough estimates since the variation
between trees in different parts of the world, terrain type etc. largely influence the tree characteristics. Still, the values given can be used as indications of the difference between the used wood species. PM emission factors and major chemical composition. As seen in Table 2 the PMtot emissions varied greatly between the different fuels. As it has been shown that PM emissions from residential biomass combustion systems are dominated by particles below 1 µm (less than 10% of the total mass are from coarse particles)11, 13, it is assumed in this study that PMtot equals fine particles
