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Domestic wood heating appliances with environmental high performance: Chemical composition of emission and correlations between emission factors and operating conditions. Valerie Tschamber, Gwenaelle Trouvé, Gontrand Leyssens, Celine Le-Dreff-Lorimier, Jean-Luc Jaffrezo, Paul Genevray, Dorothée Dewaele, Fabrice Cazier, Stéphane Labbé, and Serge Postel Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00333 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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Domestic wood heating appliances with environmental high performance: Chemical composition of emission and correlations between emission factors and operating conditions. Valérie. Tschamber1,*, Gwenaëlle Trouvé1, Gontrand Leyssens1, Céline Le-Dreff-Lorimier2, JeanLuc Jaffrezo3, Paul Genevray4, Dorothée Dewaele4, Fabrice Cazier4, Stéphane Labbé5, Serge Postel6 1

Laboratoire Gestion des Risques et Environnement - Université de Haute-Alsace, 3b rue A.

WERNER, 68093 Mulhouse Cedex, France 2

Centre Scientifique et Technique du Bâtiment, 11 rue Henri Picherit, BP 82341, 44323 Nantes cedex 3 France

3

Laboratoire de Glaciologie et Géophysique de l’Environnement, Université Grenoble Alpes, 54 Rue Molière, BP 96, 38402 St Martin d'Hères Cedex, France

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Centre Commun de Mesures, Université du Littoral Côte d’Opale, 145 avenue Maurice Schumann, 59140 Dunkerque, France

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LORFLAM, 501 route de Caudan, 56850 Caudan, France

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D2I-INVICTA, 57 rue des Manises, 08440 Vivier-au-court, France

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*Corresponding author : Tel.: (33) (0)3 89 32 61 58; e-mail : [email protected] ABSTRACT If the use of biomass and wood in particular replaces the fossil fuels for the heat production, this has to be made in conditions controlled to minimize the environmental and health impacts. Two recent French domestic appliances presenting high technology of adjustment of different hot air entrances (secondary and post-combustion) were tested regards to their particulate and gaseous pollutants (Total Suspended Particles (TSP), Particulate Matter with diameter below 2.5 µm (PM2.5), carbon monoxide (CO) and Total hydrocarbons compounds (THC) )for different heat output and combustion phases. Characterisation of particulate composition consisted in determining the total carbon (TC) fraction, and its repartition between organic (OC) and elementary (EC) carbon, Polycyclic Aromatic Hydrocarbons (PAH) and wood tracers. Analyses of PAH in the gas phase were also performed. Differences in the proportion of EC/OC in TSP were observed during a wood load: particles are mainly constituted of organic carbon during the inflammation phase. The carbon fraction of the particles at the end of the load decreases to about 20 % with approximately half of organic carbon. Levoglucosan is the major biomass tracer present in the solid phase of TSP. Light PAH are predominant in the gas phase, with the naphthalene representing 75% of the total, whereas heavy PAH with cycle numbers from 5 to 7 are mainly present in the solid phase of TSP. However, considering the toxic equivalent factor, the human health impact of adsorbed and gaseous PAH is almost the same. In these conditions, emission factors of CO and TSP were below the minimal values imposed by the highest level of the environmental French label “Flamme Verte” and future European regulations that should come into force in 2022.

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1. INTRODUCTION To decrease their dependence on fossil fuels, most of European countries encourage the use of renewable energy sources. In this context, development of small-scale wood combustion appliances is particularly promoted resulting from the availability of raw material and its neutral impact with respect to the greenhouse effect. Besides these advantages, it is however well known that residential wood combustion (RWC) contributes to local air pollution with emissions of various gaseous compounds and Total Suspended Particulate matter (TSP) 1–4. Studies devoted to the quantification and speciation of gaseous emissions in the fumes revealed that wood combustion releases considerable amounts of carbon monoxide (CO) and semi-volatile and volatile organic compounds (VOC), including polycyclic aromatic hydrocarbons (PAH), benzene, ethylbenzene, toluene, xylenes (BTEX), light hydrocarbons and oxygenated VOCs 5–9 which have significant impacts on human health 10–12. Evaluations of emissions factors for particles showed that RWC is one of the major sources of atmospheric fine particle (PM2.5) in many parts of the world, and particularly in Europe 1,13–16. In a recent study 1 it was concluded that particles from wood combustion may in winter, at many locations in the Alpine region, add up to 50% or more of the EU daily limit value for PM10. As it is known that health effects caused by atmospheric particles are dependent on their physical and chemical properties 17, several investigations on the characterization of particles were performed 18–22. Regards to their results, particulate matter from wood burning is mainly carbonaceous particles defined as total carbon (TC). TC is constituted by organic carbon (OC) and elemental carbon (EC). When wood burning takes place in poor conditions, EC/OC ratios present low values showing that TC is mainly constituted of a large fraction of organic compounds from incomplete combustion 23,24.

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Ambient air PM levels in the urban environment as well as PM chemical compositions are greatly affected by seasonal effects of emissions patterns. This has detrimental effects on PM toxicity25. Among the myriad of molecular compounds emitted by biomass burning, three isomeric anhydrous sugars levoglucosan, mannosan, and galactosan are formed during pyrolysis of cellulose and hemicellulose26. They are the predominant organic species. Levoglucosan is the most abundant anhydrous sugar among the monosaccharide anhydrides and it is used since the 1980’s as a key marker for the apportionment of biomass burning emissions27. More importantly, it was found that biomass-related PM seems to have a higher PAH contents than the PM emitted from other sources, as shown by chemical analysis. PAH and levoglucosan levels were highly correlated, indicating that particles emitted from biomass combustion are more toxic than PM emitted from other sources28. PAH and nitro-PAH induce lung cancer risks that were estimated by a comprehensive methodology that incorporated human respiratory tract deposition modelling in tissues25,29,30. Besides the effect of particles on human health linked to their significant content in adsorbed PAH and other hazardous organic compounds (constituting the OC part of the particles), including specific tracers of wood burning (sugars as mannosan, levoglucosan, galactosan and phenols as syringol and guaiacol)31–33 , it was demonstrated that EC participates to the complex process involved in climate change 34,35. In order to decrease the environmental and toxicological fingerprint of RWC, numerous studies were performed in the last decade to evaluate emissions factors (EF) of pollutants generated by commercial appliances. As EF are influenced by several factors, most of the investigations have also attempted to compare pollutant emissions as a function of the species of wood, the design of the domestic heating appliance or operating conditions 3,7,8,18,36–39. However, most of these studies have been conducted on appliances devoid

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of new technologies developed to optimize combustion chambers and thus to reduce the emissions of CO and TSP40. CO and TSP are currently the only compounds analyzed in order to define the quality of the combustion by most of the labels assigned to appliances. Several certifications and regulations setting emission measurement methods are currently aiming at significantly stricter emission limits for point source heating applications like wood stoves. The German DIN (Deutsches Institut für Normung) certification and the future Ecodesign regulation also set requirements for organic gaseous compounds (OGC) and nitrogen oxides (NOx). Investigations on EF and characterization of gaseous and particulate emissions from such “new generation” domestic wood heating device are scarce. In this paper, quantitative and qualitative analysis of CO, TSP, PM2.5, and organic compounds (THC, PAH, wood tracers), in gaseous or adsorbed phase, emitted at nominal heat output from an insert and a stove equipped with high technology of adjustment of different hot air entrances are performed. Comparisons with older combustion apparatus from the 80’s and 90’s, which are still working, especially in European countries, are made. The influence of operating conditions on pollutant emissions from two wood domestic appliances was also investigated. 2. EXPERIMENTAL 2.1.Domestic heating wood appliances. One of the two appliances tested is an airtight castiron stove, named WABI, commercialized by D2I/INVICTA (Fig. 1a). The WABI stove features a fresh air intake inlet at the back of the stove. It is equipped with a double wall. For an easy use, the WABI stove features a unique manual air control, on the front. This setting of fresh air arrival is connected to an adjustment control, named “Technology AIR CONTROL”, which distributes the air between three circuits supplying the combustion chamber with primary, secondary and post combustion air. Primary air is directly

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introduced from the air control in the combustion chamber under the bottom grate. Secondary air is a part of the fresh air which is heated in the double wall of the stove before injection in the combustion chamber. Finally, post combustion air corresponds to hot air which is injected to the back of the stove and is used to oxidize the unburned volatile compounds. The second domestic appliance tested is the insert XP54-IN, commercialized by LORFLAM (Fig. 1b). As the WABI stove, the XP54-IN insert features a unique manual setting, connected to the ADS® patented system which is a new technology of air injection. The ADS® system preheats the combustion air and injects it in the heart of the flame. This variable-delivery system automatically distributes the air injected depending on the load of wood and the expected power. At the outlet of the combustion chamber, the flue gas passes through a collector C2-BOX® which ensures post combustion. By extending their residence time and increasing temperature, this technology performs an optimal combustion of gases and fumes. These two wood appliances received the French label “Flamme verte 5 stars”. Table 1 shows the specific characteristics of the two domestic appliances. 2.2. Experimental set-up and tests protocol. Wood combustion experiments were performed using both wood logs of 25 cm in length (charm for WABI stove and beech for XP54-IN insert). Physical and chemical characteristics of the studied woods are given in Table 2. Tests were carried out in natural draft with values of 16 Pa and 12 Pa for WABI stove and XP54-IN insert, respectively. The tested heating appliances were positioned on a scale allowing a continuous follow-up of the mass variations during wood combustion and were connected to an instrumented isolated chimney. This last one was equipped with: -

A gaseous compound analyzer (Anapol EU 5000) composed of IR-absorption cells for

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CO, CO2, Total HydroCarbons (THC) and NO measurements and an electrochemical cell for O2 measurements. This analyzer allows a continuous measurement. The limit of quantification (LOQ) of CO, CO2, THC, NO and O2 are 10ppm, 0.2 %, 9 ppm, 5 ppm and 0.3 % respectively. THC were exclusively measured in the gas phase. They are consequently volatile compounds but they cannot represent the total fraction of VOC as, using IR-absorption cells, only CxHy compounds were measured. It was always mentioned in literature that a Flame Ionization Detector, online or not to chromatographic techniques is needed for detection and quantification of VOC. Also, our THC concentration values are an underestimation of VOC concentrations. -

The measurement of the flue gas temperature is also managed by this analyzer via a thermocouple (temperature transducer). The accuracy of temperature measurement is ±6°C.

-

Two in-stack TSP sampling systems. The first one, placed at 2.0 m above the heating appliance, was used for the measurement of TSP concentration in the fume, according a gravimetric method. It was followed by a heated filter holder at 70 °C containing a PALL glass fibers filter of A/E type (without binder, aerosol retention at 0.3 µm DOP is 99.98 %) allowing the collection of TSP according to the DIN CERTCO standard 41. The fume sample collection flow rate was controlled and maintained at 0.5 Nm3/h. The mass of TSP collected on the filters was calculated by difference of the weight of the filter before and after the experiment using a laboratory balance with an accuracy of 0.1 mg. OC and EC fractions in TSP collected according to this procedure were performed (details given in the following section 2.3.1). The second one, placed at 2.4 m of the heating appliance, consists in a sampling device (CATECO model) provided by CleanAir 7 ACS Paragon Plus Environment

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EUROPE allowing sampling in isokinetic conditions. The sampling probe temperature was set at 150 °C to avoid condensation of the gases and particles. The sampling probe is connected to a heated (150 °C) filter holder in quartz in which glass fibre filter (Whatman 934-AH - diameter: 8.5 cm, aerosol retention at 0.3 µm DOP is 99.98 %) was placed to collect particles whose composition in PAH and biomass tracers were analyzed. A collecting system follows the filter to condense water vapour and collect gaseous PAH, phenolic compounds and organic tracers of biomass combustion on XAD-2 resin (Restek – Ultraclean Resin). -

An Electrostatic Low Pressure Impactor (ELPI, DEKATI), for a continuously measurement of the number concentration of particles and their size distributions in the range 0.03 µm to 10 µm into 12 size fractions, allowing the identification of specific PM fractions as PM0.1, PM1, and PM2.5 . The “zero” procedure of the ELPI was performed, on ambient air, every day. The LOQ varies for each of the 12 stages from 0.2 to 820 particles. cm-3 (appendix). The ELPI was preceded by a Fine Particle Sampler (FPS, DEKATI) composed of two dilution stages (the first occurs at 130 °C and the second one at ambient temperature) which allows a precise control of the dilution ratio. This one varied from 16 to 28 as a function of the wood appliance operating conditions.

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A pressure transducer for flue draught continuous measurement.

For each sampling session, a start-up phase occurred early in the day, with a wood load of 3 kg, which corresponded to the heating of the whole appliance up to the formation of a constant mass of embers. During this phase, the fume temperature increased from room temperature up to the working range of 230-300 °C. Then a preliminary stationary phase (PSP) took place using wood load mass of 1.5 kg in order to roughly maintain constant the combustion chamber temperature and consequently the average temperature of the fumes. This PSP allowed the reach of nominal 8 ACS Paragon Plus Environment

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heat output. Combustion of three to six wood loads at nominal heat output was then performed for 40 to 45 min each. Pollutants (gas and particles) were measured during the nominal heat output, with specific attention to the speciation of organic compounds. During start-up and PSP periods, only CO, THC, TSP and number fraction of PM2.5 were measured. This overall experimental procedure was repeated for three days for each appliance. Two measurements campaigns (2013 and 2014) were performed using the XP54-IN insert because of a failure of the THC analyzer during the first campaign. During the second campaign organic compounds were not analyzed. WABI combustion tests were only performed in 2013 during the first campaign. Details of the campaigns are given in Table 1. 2.3 Chemical characterization of gaseous and particulate compounds 2.3.1. Quantification of organic and elementary carbon in the TSP. Chemical analyses were performed for various elements and components using a range of instrumental techniques on subsampled fractions of the filters. The elemental carbon (EC) and organic carbon (OC) were analyzed using thermo-optical transmission method on a Sunset Lab analyzer 42,43. A punch of 1.5 cm2 was directly analyzed following the EUSAAR-2 protocol 44 and automatic split time was used to differentiate EC and OC accordingly to the EU norm CEN 16243. The EUSAAR protocol was previously tested and adapted to by one of the co-authors as J. L. Jaffrezo for its application at the wood combustion exhaust. On the basis of analyses of ambient air, the LOQ of EC and OC are 5 and 100 ng.m-3 respectively. 2.3.2. Analysis of organic compounds. A half of each filter collected and the XAD-2 resin were dedicated to quantification of PAH (particulate and gaseous phases), phenolic compounds (in the gas phase) and biomass tracers as sugars (in the particulate phase). Quantification of PAH was

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dedicated to the 16 US EPA priority. The analytical method was based on the one described in previous works 45. Filters and resins were submitted separately to a soxhlet extraction by dichloromethane/acetone (50/50) (dichloromethane RS-Plus-For residual pesticides analysis from Carlo Erba / acetone for organic residue analysis from JT Baker) during 24 hours. The other half of the filter was used to quantify the organic tracers (e.g. levoglucosan, mannosan, galactosan) emitted by the appliance. It was sonicated in 30 ml of ethyl acetate (Reagent Grade – Panreac) during one hour and then derivatised thanks to BSTFA + 1 % TMCS (Alltech) and pyridin (Labosi) during three hours46. If needed, the extracts obtained were concentrated under nitrogen flux and analyzed by GC/MS (model VARIAN 3800/1200 TQ). The PAH standards for identification and quantification were prepared from the Polycyclic Aromatic Hydrocarbon mix at 2000 µg/ml in CH2Cl2:C6H6 (1:1) purchased from Sigma-Aldrich. Phenol, guaiacol, syringol, levoglucosan, mannosan and galactosan standards were prepared in ethyl acetate from pure compounds purchased from Acros Organics and Carbosynth. The LOQ of the organic compounds are summarized in the Appendix. 3. RESULTS AND DISCUSSION 3.1. Emissions of gaseous compounds. Mean concentrations of gaseous compounds for a large number of nominal loads (between three and six per day and for two to three days) are given in Table 3 for both wood domestic appliances. Several units are proposed in order to facilitate the comparison with literature data. In the second campaign, PAH and organic tracers were not analyzed. Carbon monoxide mean concentrations given in the Table 3 for the WABI stove are very close to the expected value as shown in Table 1 corresponding to requirements of manufacturers. In the case of the XP-54 IN insert, the same order of magnitude is reached during the second campaign. 10 ACS Paragon Plus Environment

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During the first measurements campaign, mean CO concentration value was twice meaning that optimal operating conditions were not achieved. In both cases, these concentrations comply with the standardized values imposed by the French label “Flamme Verte five stars” (i.e. Table 4) and are very low compare to mean values for oldest wood domestic technologies from the eighties and nineties47, whose a large part are still working. The emission factors of these two appliances expressed in mg per kg of burnt wood are similar to those proposed in Calvo et al. for recent fireplaces and stoves 48 in determined similar conditions. The concentrations of THC’s in the gas phase for both appliances are rather small compared to recent values obtained by Schmidl et al. with a similar recent wood stove of comparable heat output (6.5 kW) 49. For total hydrocarbons concentrations, Schimdl et al. got values ten times higher than the ones measured in this study, with their concentrations being close to 305 mgC.Nm-3 at 13 % of O2. Conventional combustion tests of birch logs in masonry heaters equipped with modern combustion technology were performed by Lamberg et al.18. Results were expressed in OGC and their emission factors were measured in the range of 66 to 250 mg per MJ of dry wood that corresponds to the range 1200-4700 mg per kg of dry wood. Our value of 300 mg of C per kg of dry wood corresponds to the concentration measured by Lamberg only for improved batch combustion in the masonry heaters tested. However, it can be noticed that these authors measured OGC while, in this study, only a THC which is a subset of OGC, were measured. Sippula et al. recently performed wood log combustion in a modern masonry heater and analyzed online total volatile organic matter with six loads in a row per day 50. During the three first loads, concentrations of volatile organic matter were in the range of 100 to 300 mg.Nm-3 at 13 % of O2. The increase in the combustion chamber temperature clearly decreased gaseous organics with a minima value below 50 mg.Nm-3 at 13 % of O2 for the sixth load. The use of a different measurement technique may also explain the variations of the results. 11 ACS Paragon Plus Environment

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Figure 2 presents the correlation between carbon monoxide and THC in the fume including startup, preliminary stationary and nominal phases. It shows a good linear correlation for nominal phases during combustion in the WABI stove (R2 = 0.887 ; n= 11). A tendency to covariation is also observed in the case of the XP54-IN insert but with a larger dispersion (R2 = 0.556 ; n = 12). This was already observed by Ozil et al. 37 and according to Sippula et al.50, the temperature in the combustion chamber is an important factor modulating the correlation. During start-up and PSP phases, the temperature of the chamber is not sufficient to achieve complete thermal degradations, leading high amounts of products of incomplete combustion as hydrocarbons, organic compounds and CO. This point will be discussed in more details below (section 3.3). However it can be already observed that start-up phase measurements show the largest deviation in the case of the WABI whereas the same measurements (and those during the PSP phase) are much closer to that of the nominal phase for the XP54-IN insert. Emission factors of PAH’s in the gas phase are given in Table 5 for both appliances. The XP54IN insert presents a total value twice that of the WABI stove, but the main result is that the emission factors of gaseous PAH are very low for both these appliances compared to stoves and fireplaces from the 1990’s tested with hardwoods in the same conditions 8 : emissions factors were lowered by a factor 20 to 40 compared to similar previous data from the literature 8,51. In Pettersson experiments38, a typical natural-draft wood stove with a nominal heat output of 9 kW was used and fired under different conditions simulating the potential variations for wood stoves in practical use. The stove was a commonly installed in Sweden which very well represents the current Scandinavian market. In Sweden, mainly logs of birch, pine and spruce are burnt for heat production at home. Emissions of total PAH (gas and particles) were recorded and their corresponding mean factors were ranging from 3.5 to 6 mg per MJ in normal combustion conditions (nominal heat output). With values of 0.13 and 0.30 mg per MJ for the total emission 12 ACS Paragon Plus Environment

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factors (sum of gaseous and particulate PAH), WABI and XP54-IN French appliances therefore present high performances compared to recent domestic appliances available on the Scandinavian market. Figure 3 gives the composition of PAHs in the gas phase. A good reproducibility is achieved for the XP 54-IN insert. Conclusions for its reproducibility could not be reached for the WABI stove due to its low emission factors. Similar PAHs composition fingerprint is observed for both appliances. Light molecular weights PAHs with two (naphthalene) and three aromatic rings (phenanthrene, acenaphthylene, fluorene) represent 92 % of the total mass of PAH in the gas phase. This was previously observed during wood combustion in fireplaces and stoves by Khalil et al. and Mac Donald et al. 8,51. Concerning the oxygenated compounds used as wood tracers in the gas phase (phenols, guaiacol, syringol), their fingerprints are difficult to compare with literature data due to their very low emission factors measured in this study, as shown in Figure 4. The emission factor of phenol for the XP54-IN insert presents a value 50 times lower than that observed by Mac Donald et al. during the combustion of hardwood in a fireplace of the 1990’s, and a value 200 times lower in the case of combustion of the same type of wood in older stoves 8. Emission factors of these compounds mainly depend on the nature of the wood, with a very large range (from 170 to 550 mg per kg of dry wood) for fireplaces representing the European market of the 1990’s 9. Related to literature data, only one observation is still the same throughout times: in this family of chemical compounds, the phenol is always the principal component detected in the gas phase 8,9. 3.2 Chemical characterization of particles 3.2.1. TSP and PM2.5 emission factors. Mean concentrations and emission factors of TSP and PM2.5 are given for both appliances in Table 6. PM2.5 mean concentration was measured on the

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whole combustion charge. TSP measurements are lower than those given by the manufacturers, presented in Table 1, especially for the WABI stove. Low TSP concentrations measured in this study, however, are in agreement with the measured CO concentrations. Indeed, in agreement with the literature37, a linear correlation between CO and TSP concentrations can be established for both appliances operating under nominal heat output (Fig. 5). By expressing CO and TSP in the same unit (mg/Nm3 related to 13% O2), TSP concentration equals 0.009 to 0.018 times that of CO for the insert and the stove, respectively. Similarly to the correlation between CO and THC (Fig.2), start-up phase measurements show a largest deviation. Figure 6 presents a typical evolution of CO and PM2.5 concentrations observed during the combustion of nominal loads for both appliances. It can be noted that main CO emissions are evolved during the ignition time for few minutes and drastically be reduced during the flaming phase. Similarly, PM2.5 concentration decreases all along the combustion cycle. As European DIN CERTCO regulation imposes the gravimetric measurement of TSP after the ignition phase (t=3 min) and for 30 min when the concentrations of CO are in this study particularly low, it is reasonable to obtain low concentrations of TSP. These ones are in agreement with the highest level of the French “Label Flamme Verte seven stars” and the future European regulation (i.e. Table 4). Comparison of TSP emission factors in both units as mg. MJ-1 and/or g.kgdw-1 with literature data concerning recent wood and pellet domestic devices available on the European market (Finnish, German, Italian and Portuguese) revealed that our data are much lower (at least by a factor of 10) compared to these measurements. As example, Ozgen et al. got values in the range of 140 to 180 mg. MJ-1 for advanced and traditional wood stove burning beech logs 3. Calvo et al. measured emission factors of PM2.5 from 2 to 14 g. kg-1dw during wood logs combustion in advanced stove with handheld control of combustion air. For identical conditions of experiments, their values 14 ACS Paragon Plus Environment

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were drastically dependent of the nature of the fuel, with the lowest value for beech logs (F. Sylvatica) and the highest value for oak logs (Q. Pyrenaica) 48. Lamberg et al. registered some of the lowest PM1 emissions factor currently presented in the literature, with value of 19 mg. MJ-1 during efficient combustion in a domestic advanced pellet boiler equipped with modern combustion technology as a lambda sensor and microprocessor that control fans to provide sufficient combustion air 18. Mean total number concentrations of the particles in the PM2.5 fraction are given in Table 6. These values are equivalent to previous data obtained with a French advanced fireplace ULYS 700® (13 kW appliance) tested with the European protocol EN 13229 52. In this study, mean number concentration of PM2.5 is of the order of 6. 1012 p.Nm-3 for 13 % of O2 in the fumes. In Table 6 are also given the number size distributions related to three size fractions in the PM2.5. PM2.5 are mainly constituted of PM1 for both devices with a significant difference between the two fractions PM0.1 and PM0.1-1. Ultrafine particles with diameter below 100 nm dominate in the fume for the WABI stove. In the case of the XP54-IN insert, the proportion differs in the PM1 fractions and this appliance slightly generates larger size particles with PM0.1-1 dominating in the fumes. This point was already investigated in previous studies with advanced wood log fireplaces, pellets stove, and masonry heaters 18,52. The size distribution mainly depends on the type of devices and the combustion conditions (nominal or reduced heat output). This difference could be related to the fume temperature. As given in the Table 1, the fume temperature is higher for the WABI stove than for the XP-IN 54 insert with a difference close to 30 °C. It is well known that high temperature favors the volatilization and the transfer of chemical species present in the condensed phase of the particulate matter to the gas phase and smaller particles. Considering the emission factors, only few data for advanced and recent appliances are available in the literature, for the total number of PM2.5 or their number size distributions. Some data are 15 ACS Paragon Plus Environment

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mainly available for studies of older appliances as stoves and boilers of the 1980-2000’s burning logs and / or pellets (based on impactor analyzer for continuous measurement of PM2.5 number distribution) 36,53,54. Most of the main values were ranging between 1013 to 1014 p. MJ-1 with relative high standard deviation depending of various parameters as the nature of the fuel, the moisture content, the type of appliances, the low and high combustion rate, etc36,53,54. In the present study, our emission factors are one order of magnitude lower than literature data, most probably due to the high efficiency of combustion that is reached with both devices. 3.2.2. Carbonaceous content of TSP. TC, EC, and OC contents in TSP collected at 2.0 m of the heating appliance were determined for both appliances. Continously analysis of gaseous and particulate compounds emissions (section 3.1) revealed that higher CO and PM2.5 concentrations are emitted during the ignition phase (Fig. 6) and that correlations between CO and THC as well as CO and TSP could be established for a given appliance in given operating conditions (Fig. 2 and 5). One may also expect that TSP carbon content evolves during the combustion. Figure 7 presents the evolution of fractions of TC, OC and EC in TSP collected at different times of combustion for a same nominal load using the XP54-IN insert. It can be noted that, similarly to CO, TSP concentration is reduced between ignition and flaming phases, a decrease that reaches 57%. TC represents 45 % w/w of particle mass emitted during the ignition phase. This content decreases to about 20 % w/w during the flaming phase. This evolution of the carbon content of TSP is also linked with a modification of the carbon speciation. OC represents more than 80 % of TC during the ignition phase, a fraction decreasing to 72 % and 52% after 16 min and 32 min of combustion, respectively. At the same time, the fraction of EC in TSP increases slightly but is always lower than 10 %. Reduction of both TSP concentration in the fume and OC content of TSP during the combustion process allows an important decrease of OC emission factor from an average of 487 mg.kg-1dw to a minimum of 56 mg.kg-1dw (Figure 8). This result confirms that 16 ACS Paragon Plus Environment

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ignition phase is characterized as a drying phase with emission of volatile hydrocarbons and particles in the fume from wood. Duplication of sampling performed with different wood loads and for various tests days during the ignition phase (3min), 30 minutes after the ignition phase or on the whole combustion cycle lead us to conclude that TC, OC and EC contents of TSP generated by the XP54-IN insert vary in the range 58±19 % w/w, 49±18 % w/w, 8±5 % w/w, respectively. TSP produced from the WABI stove exhibits a less important OC/EC ratio as the values of TC, OC and EC contents measured are 69±16 % w/w, 41±7 % w/w, 28±11 % w/w, respectively. However, taking into account the standard deviation, the same order of magnitude is obtained for carbon content of TSP for the both appliances. These results could be compared with those obtained by Fine et al 55 and Gonçalves et al. 56,57, using a catalyst-equipped wood stove and a cast iron wood stove operated manually with handheld control of combustion air, respectively. TC content of particle emissions during a complete cycle of wood load combustion (from the ignition phase to the end of combustion) ranged, as determined by these authors, between 55 % to 60 %, of which 42 % to 55 % corresponded to OC. These ranges are the same for TC and OC for TSP generated by the WABI stove, but alower of the OC content compared to results obtained with the XP54-IN insert. Figure 8 shows TC, OC and EC emission factors as a function of TSP concentration in the fumes, for particle samplings performed over various periods and at different times of the combustion cycle for the both appliances. In agreement with results discussed above, concerning carbon content of TSP (TC), one may observed that carbon emission factor is particularly important for the ignition phase, which is the most particles emitting phase. Such behavior is observed regardless the appliance. It is also interesting to note that particles sampled after the ignition phase (i.e., t=3 min) during 30 min, present the same emission factor of TC (~100 mg/kgDW) than those sampled at 16 min or 32 min after the beginning of the combustion cycle (Fig. 7a). These 17 ACS Paragon Plus Environment

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three samplings present a OC/EC ratio between 1.1 to 3.2. In comparison, particles sampled during the ignition phase (i.e., the first three minutes of the combustion cycle) exhibit a higher OC/EC ration, close to 7.0. Particle carbon composition appears thus constant throughout the combustion cycle, regardless the duration of sampling, with the exception of the ignition phase for which a larger OC content is observed. 3.2.3. HAP and wood tracers particulate. Emission factors of particulate PAH’s are very low as shown in the Table 5 and only a few of the compounds were detected in our experimental conditions. Data for recent domestic stoves are almost non-existent in the literature. Data concerning older domestic appliances from the 80’s to the 90’s presented values in the range between 14 000 to 34 000 µg per kg of burnt wood 8,9 for individual PAH species. Our data expressed as emission factors are in the range 120 to 244 µg. kgdw-1 in the particulate phase for the WABI and the insert XP54-IN, respectively (as shown in the Table 5). These values are one thousand times lower than those found for wood appliances of the 80’s and 90’s. Nevertheless, this order of magnitude was previously obtained by Boman during the combustion of pellets and birch logs in recent domestic stoves for the Scandinavian market, 27,58. The ratios of ΣPAHgas /ΣPAHpart between the two main phases (PAH gas/particle distribution), as given in the Table 5, are close to 20 for both domestic appliances. This value is higher compared with that of Mac Donald of 3 8. This fact is related to very good operating conditions of use of our domestic appliances that leads the minimization of fine particles in the exhaust. In our conditions, formation of PAH mainly occurs during the combustion in the gas phase with slightly condensation processes on PM.

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Emission factors of PAH phases expressed in equivalent Benzo [a] Pyrene are equivalent in gas and particulate phases for both appliances as shown in Table 5. Considering the toxic equivalent factor, the human health impact of adsorbed and gaseous PAH is almost the same. The mass fractions of the biomass tracers in the particulate phase are presented in Figures 9a and 9b for the insert and the stove, respectively. Fine particulate matter contains traces of sugars and phenols compounds with very low contents compared with their values in the gas phase. A quite good reproducibility is obtained excepted for the syringol that is not always detected in PM from day to day or during loads performed during the same day. While the phenol dominates in the gas phase, it is still detected but in lower amounts in PM for the two heating appliances. Levoglucosan is the only sugar molecule present in significant concentration in the particulate phase for both domestic appliances. Literature data is well documented to demonstrate that levoglucosan is the main sugar compounds present in particles during biomass combustion whatever the experimental conditions 9,55,59–61. In this study, values of levoglucosan are in the range of 0.35 to 0.78 mg per g of TSP. These mass contents are much lower than those measured by Schmidl et al.59 (10.8 mg. g-1 of TSP) during the analysis of particles generated by an insert equipped with a secondary air inlet. Considering the concentrations of TSP in fumes, mean concentrations of levoglucosan are 18.0 and 7.2 µg per Nm3 at 13 % of O2 in the fumes for the XP54-IN insert and the WABI stove, respectively. These values are very close to those of Albinet et al.62, who measured concentrations of levoglucosan ranging from 5 to 20 µg. Nm-3 at the emission sources about 1 m from the exhaust stack (very close field with a dilution factor about 10 to 20). These authors recorded online concentrations of levoglucosan during combustion experiments (at nominal heat outpout with beech wood log at 12 % RH) in a residential wood log stove with the French label 4 stars using a new analytical PILS-LC/ PAD technique. However, it

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can be noted that these concentrations are 50 to 200 times higher than those of levoglucosan measured in ambient air in urban zones in winter close to thermal biomass facilities and domestic appliances63,64. 3.3. The influence of operating conditions on pollutant emissions. The above qualitative and quantitative analyzes of pollutants emitted from wood combustion in domestic appliances presenting high environmental performances, revealed that pollutant emission factors seem to be closely related to the temperature of the chamber and the flue gas temperature. In order to establish correlations between pollutants in the fumes and operating conditions, PM2.5, CO, THC and TSP were measured at various stage of the combustion (start-up, preliminary stationary and nominal phases) during two days of testing for each of the two appliances. Figure 10 shows emissions factors versus temperature of fumes for the different phases of combustion. For the different operating conditions, mean flue gas temperature, at 1.5 m above the heating appliance, evolves in the same range for both appliances. Start-up phases are characterized by a mean temperature of fumes lower than 257 °C (216 °C to 257 °C). An increase of temperature is observed during the PSP phase and then stabilization at nominal phase, in the range 276 °C to 310 °C. A significant decrease of emissions factors, for all the measured pollutants, is observed from start-up to nominal phase. PM2.5 emission factor is thus reduced by about half from the start-up phase to the nominal one. Interestingly, PM2.5 emission factor by number is linearly correlated with the flue gas temperature for all the three operating conditions tested in the XP54IN insert, with a very good value of R2 equal to 0.94. A linear relationship is also observed for the WABI stove when we consider the start-up and nominal phases. PSP phase points are however relatively far from the linear regression. Similarly, excluding PSP phase, exponential correlations between either THC, TSP, or CO emission factors and fume temperature were

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obtained in the WABI stove, with value of R2 higher than 0.846. Similar tendencies seem to be also observed in the case of the XP54-IN insert but with larger deviations of experimental points. Specifically, the link between measurements carried out over the nominal phases and start-up phases is less obvious in the case of this appliance. 4. CONCLUSION Emissions of two domestic wood heating appliances, equipped with high technology of adjustment of different hot air entrances (secondary and post-combustion), towards gaseous and particle emissions were characterized and compared to older combustion apparatus still present in the market. CO, THC, PAH and biomass tracers, including sugars and phenols compounds, were measured in the gas and the particulate phases of the fumes. Regarding the analyzed gaseous pollutant, emission factors obtained with WABI and XP54-IN appliances are very low compared to those from stoves and fireplaces of the 1990’s or even new ones devoid of post-combustion technologies. A linear correlation between CO and THC was established, for the two appliances, when used in given operating conditions allowing a sufficient temperature of the combustion chamber to achieve thermal degradation of organic compounds, i.e. during nominal heat output. Although emission factors of PAH vary depending of the heating appliance, similar fingerprint in the composition was observed. Light molecular weights having two (naphthalene) and three aromatic rings (phenanthrene, acenaphthylene, fluorene) represent 92 % of the total mass of PAH in the gas phase. Similarly to gaseous compounds, mean emissions factor of TSP is low and in agreement with the highest level of the French “Label Flamme Verte 7*” (