240321 SHV Brief Gemeenten en NVG Adviescommissie bijlage 1 -Science Nanoparticle emissions from residential wood combustation 2019

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Table 3 Summary of average particle size from cited sources. Equipment Summary: average particle sizes are given unless otherwise specified References

Cordwood Stoves Start up: 180–260 nm [56,61–65,67] Steady state: 30; 107; 175–180 nm Burn out: 30–60 nm; 180–200 nm Burn out or start up showing the highest concentrations of NPs and steady state having the lowest concentrations of NPs In some studies, aggregations of NPs with diameters of 313–662 nms Low MW PAHs favored small NP diameters & High MW PAHs favored coarse particles Increase in oxygen in combustion chamber caused for smaller particles Decrease in oxygen caused PAH levels to increase and bind to NPs Image: high temperature combustion produced carbon aggregations with single particles having a single nuclei and turbostratic structure; low temperature combustion produced carbon dominated spherical particles Pellet Stoves 7–30 nm [67,68] Steady state showed lowest concentrations of NPs Cordwood Boilers 60–180 nm; but as small as 12 nm to as large as 400 nm [31,58] Highest concentrations seen during start up Advanced boilers produced smaller particles and lower concentrations than traditional boilers PAHs higher in older boiler technology Pellet Boilers 60–130 nm (no bark pellets) [9,18,31,56,61,69] 220 nm (bark pellets) No obvious trend was observed between full and intermittent output in terms of particle size, only a very slight increase in mass concentration.

Particles were likely inorganic salts Chip Boilers Steady state: 25–75 nm; lowest particle concentrations [21,56,63,70] Modulating: 65 nm Smolder: 160–200 nm; highest particle concentrations Lower excess air values led to lower particle counts and larger particles, however another group found greater concentrations with increased oxygen SEM & TEM images indicated different types of particles; agglomerates of spherical compact particles, perfectly spherical representing coal fly ash or irregular shaped crystalline platelets. Efficient was typically salts and crystalline while intermediate was a variety of soot agglomerates and ash; smolder was large agglomerates of soot PAHs bound to NPs lowest during efficient burns and highest during smoldering conditions Fireplaces and Masonry 10–20 nm; 50–65 nm; 170 nm [2,55,71–73] Heaters 70–250 nm during smoldering 100–326 nm range assumed to be agglomerates owed to cooling Start-up had highest concentrations; steady state had lowest Decrease in oxygen corresponded to decrease in NPs Newer technology stoves seemed to produce more NPs Agglomerates with individual particles spherical in shape Normal combustion showed spherical and irregular shapes while smoldering combustion showed more irregular and sintered-like structures Some salts but a presences of heavy organic compounds as well

found the start-up and flaming phase were characterized by small from both normal and smoldering wood combustion periods in a con- particle diameters whereas smoldering conditions had larger particle ventional masonry heater made of soapstone using birch cordwood at diameters. Number concentrations were greatest during the start-up 7% moisture content. The results from the study showed a higher phases and especially for cold start-up phases but were lower for particle emission number measured in a dilution tunnel by the ELPI and smoldering phases. FMPS during the normal combustion and less than half the particle A group in France looked at the mass and number distributions of emission number during smoldering state with geometric mean dia- wood combustion particles emitted from beech cordwood (12% meters of 56 and 65 nm by FMPS and ELPI measurements, respectively, moisture content) during normal and reduced operation from two dif- for normal combustion, and 118 and 160 nm values for smoldering ferent fireplaces; one a conventional fireplace (13 kW) with only pri- combustion [55]. Surprisingly for cordwood combustion, the repeat- mary air intake from the front and the other was a more compact ‘next ability was good. Even if all operating parameters are kept constant, generation’ fireplace (4 kW) with primary and secondary air intake.

manual fed cordwood appliances are very variable [58]. The study Measurements were taken from a dilution tunnel using a gravimetric found the average particle number size distribution for normal com- based filter measurement and ELPI. Surprisingly, the study found the bustion to be unimodal at 65 nm and the smolder combustion to pro- opposite trend of most, in that the older technology was more favorable duce a bimodal distribution with peaks at 70 and 250 nm [55]. In ad- to less particulates but ultimately showed agreement that more NPs dition, SEM and TEM analysis from the hot stack showed both spherical were produced in the newer technology. The study found the next and irregular shaped particles making up larger agglomerates and fine generation fireplace produced much higher particle emission numbers ash particles remained as separate particles. Normal combustion for than the conventional fireplace; specifically, for the reduced con- showed spherical and irregular shapes while smoldering combustion ditions, the emission numbers were 50 times greater in the more showed more irregular and sintered-like structures. The elemental modern fireplace than the older conventional unit [73].

While 80% and analysis showed single particles were composed mainly of carbon, but 60% (total number of particles) of the total PM consisted of particles also contained K, S, Zn, and less commonly O, Ca, Fe, Mg, Cl, P and Na with a diameter less than 100 nm, for the new and old fireplaces, re- were found [55]. Similar to studies mentioned above, particle material spectively, it only contributed to a very small extent of the total mass of volatized under the beam indicating the presence of heavy organic the aerosol; 5.5–5.8% and 2.6–0.3% for the new and old fireplaces, compounds. The group was able to suggest the shifting of particle size respectively during normal and reduced operations [73]. during the different combustion conditions seemed to be determined by Another group in Finland looked at the PM and gaseous emissions the amount of condensed organic vapor in the flue gas [55].

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In addition to the work done by Lamberg et al. [9] on a pellet boiler, the old-type wood boiler could be reduced if it was connected to a the group also studied three conventional masonry heaters and one storage tank, similar to advanced modern units, higher quality fuel modern masonry heater made of soapstone. All masonry heaters burned (lower moisture content) was used, or if fuel amount was decreased and birch logs with a low moisture content (10–13% on a dry basis). The small batches of fuel were loaded instead of a full charge. The study group explored continuous combustion, steady-state periods, cyclic found poorer combustion conditions increased the amount of particles operation, and ignition phases. Overall, GMD sizes ranged from 63 nm smaller than 100 nm. Finally, the group compared the wood combus- to 142 nm. The largest diameters were found during incomplete com- tion devices to oil burners finding pellet systems rivaled oil burners but bustion phases and also produced bimodal number and size distribu- old-type wood boilers would have low efficiencies, high PM emissions, tions; the ignition phase also produced a larger GMD [9]. Similar to the and unoxidized gaseous compounds [31].

pellet boiler discussed above, the most abundant PAHs were pyrene and While advanced systems that employ multiple control devices may fluoranthene in addition to phenanthrene and cyclopenta[c,d]pyrene decrease the amount of PM emission concentrations as well as reduce for one of the masonry heaters. Total PAH emissions from PM1 emis- the amount of harmful PAHs produced, they tend to be expensive. In sions (PAH/PM1) were higher than that of the pellet boiler but still 2006, the International Energy Agency commented on using ESPs to reasonably low with a range of 0.15–2.8%. reduce submicron particles but such devices were very expensive and not affordable for residential use [27]. Another review suggested the 7. Discussion easiest way to control emissions is to use a high quality fuel with low ash content, low moisture, and constant piece size whose options may This literature review revealed two earlier review papers and about also not cost competitive, or boilers must use proper combustion eighty research articles and reports. An earlier review by Kumar et al. chamber designs and adequate air pollution control technology [54]. reports only limited information on NP emissions from non-vehicles Therefore, there is a need to develop cost sensitive, yet effective, sub- exhaust sources [74]; the authors reported only two studies associated micron PM reduction technologies.

One interesting technology dis- with NP emissions in RWC. The current review identified and reviewed cussed in a review by Lim et al. [77], is the use of a miniature pipe many more studies that explicitly or implicitly provide information bundle heat exchanger (a technology used in the automobile industry) regarding NP emission data from RWC. However, measuring and where particles migrate to the lower temperature surfaces and deposit quantifying the presence of NP in emissions, and how these emissions on the tube walls to remove particles from the flue gas. Other options vary based on fuel, appliance type, and combustion period are not al- may also include better combustion designs or controls, such as lambda ways well investigated making correlations hard to draw. The second probes. Lambda probes can detect the oxygen levels and effectively major review by Bølling et al. (2009) pointed out there has been a lot of modulate the air to fuel ratio. Several studies above indicated as op- work done in steady-state operating conditions but there is limited data timal oxygen levels were reached, less particles were formed and a shift on particles from conventional cordwood boilers, modern wood stoves, towards larger particle sizes was achieved [2,56,63,78]. Perhaps and modern cordwood boilers [10].

The current review shows that this lambda sensors could be used to favor a combustion condition which statement still holds true eight years later. Table 3 below summarizes reduces the amount of harmful NPs produced. the information collected from the papers above. In addition, cold Incomplete combustion, smoldering, and transition conditions are starts, modulation periods, and in-efficient burns exist and there may be known to favor PAH and black carbon production. Further, when an underestimation of real-life emissions which contain particle frac- temperatures are low, PAHs can condense or adsorb onto particles [77]. tions that can be associated with health impacts in local areas [56]. As Smaller NPs are known to carry larger PAH molecules and are difficult reported by Tom et al. [75], the majority of published studies were to remove, while larger more coarse particles carry lower molecular associated with laboratory experiments and field performance mea- weight PAHs [77]. Likewise, Ghiassi et al. found the most reactive soot surements were lacking. had an amorphous nanostructure which was composed of short in- There is a significant need to characterize the particle types in dividual layer planes with no orientation relative to each other [79]. biomass smoke, since the regional and global climate effects of aerosols The laboratory study by Hata el al.

indicated biomass type and the from biomass burning depend so strongly on particle properties [17]. heating rate had a significant effect on the water soluble OC fraction However, manually fed cordwood stoves and boilers have a tendency to and the smaller the NP, the more water soluble OC the particle con- vary in their burn conditions even if fueled exactly the same way. tained [22]. Very few studies have looked at the microstructure of NPs Schmatloch and Brenn directly observed this when fueling a cordwood but this could also provide important information on the NPs reactivity. furnace and trying to reproduce the size distribution in particles; burn The microstructure of NPs can give information about particle forma- one had much fewer concentrations of NPs less than 140 nm and greater tion, condensation, coagulation [65,80,81] and the ability to accumu- amounts of NPs larger than 220 nm than burn two which showed op- late reactive substances [65,82]. posite trends [58]. This is owed to the nature of wood- how it burns and Ideally, a NP with a low mobility is desired to reduce inhalation or falls within the combustion chamber. This makes reproducibility very atmospheric risks.

Ultimately, it is important to understand the rate of difficult and particle size distributions to have a larger variation than environmentally induced changes and how the migration of particles advanced automatic feed boilers [58]. This statement compliments the changes their properties [76]. Understanding the aggregation behavior findings by Gaegauf et al. [56]. This also makes it difficult to compare under both atmospheric and aquatic conditions may help further pre- particle size distribution among studies. To complicate matters, Baer dict their potential interactions with the ambient environment [26]. indicates that nanoparticles are inadequately characterized since they Mavrocordatos et al. [18] investigated the affect salt content within the change with time and can be altered or damaged during analysis [76], particles play and found their morphology was significantly affected in so precautions must be taken to not bias research. Some key factors aqueous environments. Perhaps if a sufficient amount of information which may cause some uncertainty are sampling locations, sampling was gathered on NPs soot emissions from RWC which included explicit temperatures, and dilution ratios [74]. Ozgen et al.

point out that the details on fuel type, operating parameters, and sampling conditions, variability of the results concerning the number emissions and the size manufacturers and operators could be informed with a ‘best practices’ distribution during different stages of the wood burning cycle suggests procedure. that emissions are strongly connected to the combustion conditions As shown in many of the studies reviewed herein, NPs account for a depending on the appliance design and operation [72]. substantial amount of the particle emissions associated with biomass The study by Johansson et al. [31] emissions from pellet boilers combustion. However, standards enforced by regulatory agencies are were closer to those of an oil burner while old-type wood boilers had mass concentration based, focusing on size ranges less than or equal to the highest. An important piece from the study was that emissions from 10 µm (total PM; PMT) or less than or equal to 2.5 µm (PM2.5). Due to

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NPs small size, current measures may not be appropriate for particles in more efficient and “cleaner” systems, an increase in released NPs may the nano and ultrafine size fraction since they contribute very little to follow. However, with more efficient boiler systems PAH levels are the overall mass. However, they contribute significantly to the overall being decreased; causing one to argue what is more important NPs or number of the airborne particles present [11,83] and, potential adverse PAHs? health effects posed by NPs may go unknown [19]. Lim et al. [77], Particle distribution of batch-wise fired appliances (wood stoves) reported that dilution sampling affects particle emissions by promoting varied significantly during a burn cycle, while wood log and continuous nucleation but results in an increase in the formation of ultrafine par- fed boilers showed a fairly constant particle size distribution. These ticles which are better represented by particle number measurements. differences are likely to influence the biological effects induced by Torvela et al.

investigated the intermittent period when switching from wood smoke particles and therefore it has been recommended to ex- one condition to another, finding the properties of the PM emissions plore how combustion conditions influence the particle properties, their varied significantly even within a short period suggesting that an eva- possible health risks, and reactivity within the environment. Although a luation of environmental and adverse health effects based on average lot of work has been done to understand the health effects associated particle property values can be misleading [21]. Further, Lamberg et al. with RWC NPs, but little is known about the environmental fate of RWC concluded from their study that PM1 mass does not reflect all of the NPs and their effect on climate. variation affecting possible toxic properties of fine particles originating In addition, NP emission data from wood burning processes under from combustion [9]. real-world operating conditions are lacking. More data are needed From the Gaegauf et al. [56] study in particular, the highest con- about source, expected quantities or concentrations, and environmental centration of total suspended particles (TSP) (mg/m3) correlated nicely pathways to help identify situations which may be more favorable than to the highest total number of nanoparticles ( < 600 nm) collected per others.

Several studies show results of size characterization but mor- MJ of output. Masonry stoves had the highest TSP concentration along phology and chemical composition data are limited. Predicting or with the highest emission factor of NPs per MJ. In comparison, auto- studying the aggregation, agglomeration, dispersion, size, solubility, matic feed pellet boilers had the lowest TSP and the lowest total surface area, charge, and composition are all necessary parameters to number emission factor of NPs. However, when looking at the emission better predict the environmental fate and any health concerns of bio- factor (mg/MJ) for both TSP and NPs data in detail, the group noticed mass combustion NPs. While biomass is often considered and en- while each appliance type typically showed good correlation between couraged as a renewable energy source, it is important to not ignore the two values, individual cases such as the cordwood boiler showed consequences of poor combustion practices and NP production. roughly a 40% reduction in the NP emission factor from the TSP Several articles have suggested pathways to reduce NPs such as emission factor. This supports the suggestion to modify existing test emission control strategies or higher quality fuel sources.

Emission method regulations or add to them a measurement involving particle control devices suggested for large biomass boilers include ESPs and count, size, or surface area. condensing heat exchangers but their effectiveness in controlling NP One positive note from a study which compared vehicle emissions to has not been assessed. RWC emissions suggests wood smoke may be associated with less ne- gative health effects as diesel NPs were smaller in diameter and have Acknowledgement 41% more surface area [64]. Unfortunately another study found PAH content was higher for combustion particles from wood smoke com- Rebecca Trojanowski is grateful to New York State Energy Research pared to vehicle exhaust, causing risk for a high mutagenic and carci- and Development Authority (NYSERDA) for partial support of this re- nogenic potential [65]. search through contracts # 29697 and # 63038, Dr. Barbara Panessa- Warren (Guest Scientist at Brookhaven National Laboratory), Dr. 8. Conclusions and recommendations Thomas Butcher (Group Leader of the Energy Conversion Group at Brookhaven National Laboratory), and Dr. John B. Warren (Scientist in Current emission regulations from wood combustion devices in the the Instrumentation Division at Brookhaven National Laboratory).

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