Comparison of Emissions from Wood Combustion. Part 1: Emission

Article pubs.acs.org/EF

Comparison of Emissions from Wood Combustion. Part 1: Emission Factors and Characteristics from Different Small-Scale Residential Heating Appliances Considering Particulate Matter and Polycyclic Aromatic Hydrocarbon (PAH)-Related Toxicological Potential of Particle-Bound Organic Species Jürgen Orasche,†,‡,§ Torben Seidel,† Hans Hartmann,∥ Jürgen Schnelle-Kreis,*,‡ Judith C. Chow,⊥ Hans Ruppert,† and Ralf Zimmermann‡,§ †

Department of Sedimentology and Environmental Geology and Interdisciplinary Center for Sustainable Development, Georg-August-University, D-37073 Göttingen, Germany ‡ Joint Mass Spectrometry Centre, Cooperation Group “Comprehensive Molecular Analytics”, Helmholtz Zentrum München, D-85764 Neuherberg, Germany § Joint Mass Spectrometry Centre, Institute of Chemistry, Division of Analytical and Technical Chemistry, University of Rostock, D-18057 Rostock, Germany ∥ Department of Renewable Raw Materials, Technology and Support Centre (TFZ), D-94315 Straubing, Germany ⊥ Desert Research Institute, Reno 89512, Nevada, United States S Supporting Information *

ABSTRACT: An investigation was performed to study the emissions of state of the art small-scale residential heating appliances. The different combustion systems were compared at optimal combustion conditions. A comprehensive characterization of released organic species of all combustion systems was performed. An approach was performed to estimate the toxicity of the emitted particulate matter by the content of polycyclic aromatic hydrocarbons (PAHs). It is based on the proposal of the German Research Foundation (DFG) that the health risk is proportionally summarized by different PAHs with different health risk potentials. This approach allows for a rough but fast comparison of different furnaces by the calculation of the toxic equivalent (TEQ) value in addition to the emission of particulate matter (PM). Best results were obtained by combusting wood as pellets in a modern pellet boiler (PM = 11−13 mg MJ−1 and TEQ = 0.12−0.75 μg MJ−1). On the opposite of the emission scale, the toxic potentials of the typical log wood stove were found to be about 2 orders of magnitude higher (PM = 67−119 mg MJ−1 and TEQ = 14−28 μg MJ−1) compared to the pellet boiler, despite optimized combustion conditions.

■

INTRODUCTION In recent decades, oil boilers have become the main heating source for residential buildings in many North American and European countries, next to heating with natural gas. The technique of oil combustion is highly developed in the sense that fine dust emissions from these sources are low in comparison to other sources.1,2 Because of the rising costs of oil on the international markets, the application of alternative heating with wood has had a renaissance, among others, such as geothermal heating and solar heating. The positive side effect of this is the use of renewable energies, which reduce the release of greenhouse gases into the atmosphere. However, in the case of wood, there are also negative drawbacks, which are directly visible and can also be smelt. Wood smoke has become one of the main sources of fine particulate matter (PM) in the ambient atmosphere, especially during winter times. A recent emission modeling study showed that up to 9 μg m−3 additional PM10 load from wood combustion in residential areas has to be expected during the winter time.3 One main reason is the large number of installed log wood stoves and tiled stoves in many countries, which are also used for additional heat comfort. © 2012 American Chemical Society

Log wood stoves are known to generate high concentrations of organic matter with a high potential of formation of secondary organic aerosols (SOAs), which are responsible for additional PM mass.4−10 Automatically fired pellet boilers with optimized combustion were found to emit only traces of possible precursors of SOAs.11−15 The question is what impact on health effects do particles from modern wood combustion boilers have. The emissions from those boilers show very low contents of hazardous organic components, such as polycyclic aromatic hydrocarbons (PAHs), which are mainly formed at incomplete combustion. On the other hand, hydrophilic salts are enriched in particles originating from modern boilers. However, particle sizes are still in the fine and ultrafine range and, therefore, can penetrate the deeper lung regions.11−14,16,17 Pellet boilers represent the most progressive way for the combustion of wood, reducing such incomplete combustion conditions. However, there are also other techniques that Received: August 2, 2012 Revised: October 3, 2012 Published: October 4, 2012 6695

dx.doi.org/10.1021/ef301295k | Energy Fuels 2012, 26, 6695−6704

Energy & Fuels

Article

spruce pellets (PB), (ii) multi-fuel chip/pellet boiler (nominal load of 30 kW) fired with wood chips from spruce (CB), (iii) log wood boiler (nominal load of 30 kW) fired with logs from beech or spruce (LB), (iv) pellet stove (nominal load of 13 kW) fired with spruce pellets (PS), and (v) log wood stove (nominal load of 8 kW) fired with logs from beech or spruce (LS). With the exception of the log wood boiler, all boilers were automatically fired small-scale combustion furnaces. All boilers were equipped with an automatic control system for combustion via a λ probe. The pellet stove uses a simplified control system, as described within the Supporting Information. The pellets that were used were in accordance to the norms DIN plus (Germany) and Ö NORM M 7135 (Austria), which are now replaced by the European norm EN 14961-2. The moisture content was 8%, and the ash content was 0.3%. The applied wood chips had an ash content of 0.5% and a moisture content of 23% and were in accordance with the EN 14961 and the older Ö NORM M7133. They were acquired mainly from stem wood. The wood logs were split logs with a length of about 25 cm (stove) and 50 cm (boiler). The logs originated from spruce and beech. Both types of wood are common in Germany. Beech is used even more than birch wood and is, next to oak wood, the most common hardwood. Spruce, which is softwood, was the most cultivated tree in the last few centuries in German forestry. Other softwoods are of minor relevance in Germany. The logs fired in the stove or boiler, both from spruce and beech, had an optimal moisture content of 13−16.5% (Table 1). Sampling. Sampling of PM was performed out-stack after dilution of the exhaust in a dilution tunnel. Separation of a partial stream was performed by the help of an elbow and nozzles with a specific diameter for isokinetically sampling of particles. The unheated plane filter holder made from polytetrafluoroethylene (PTFE) casing for a 150 mm (diameter) QFF (T293, Munktell, Grycksbo, Sweden) ensured enough sample material for the inorganic and organic analyses with good detection limits and low background concentrations of the elements and enabled us to sample the entire combustion cycle on a single filter. PM masses were determined in two different ways. The first value was weighed without conditioning the wet filter. The second value was determined after drying at a constant temperature. Therefore, 11/12 of the filter (the part that was not used for organic analysis) was dried at 120 °C for 8 h. Dried filters were conditioned at room temperature and, thereafter, weighed. Subsequently, 1/12 of the filter was immediately put into a freezer after sampling. These sample parts were stored at −20 °C until analyses of organic compounds and and elemental carbon (EC)/organic carbon (OC). Experiments were always carried out in the same manner. The first sampling was started at cold conditions. This means that both the boiler/stove and chimney tubes were cold at the beginning (room temperature). The ignition of log wood (stove and boiler) was performed by the use of ignition aids made out of wood wool coated with paraffin wax. All other systems were equipped with an automatic heater for ignition. The collection of particles started immediately at the time of ignition. This means that start time was the first contact of wood with the flame or when the controlling unit of the automatically fired systems displayed that ignition took place. Online measuring systems were started before the sampling. The collection of particles at experiments with a log wood stove was performed over the whole batch. One batch was defined as a complete fuel depletion ( 95% in the case of wood log combustion in the stove. As shown later, the amount of SVOCs decreased considerably with more complete combustion in the wood boilers and XP is shifted to lower values (see eq S3 of the Supporting Information). The mass concentrations of PM (wet and dried values) are shown in Table 2. The drying of filters at 120 °C for 8 h, followed by conditioning and weighing, is a compromise caused by the easier determination of a stable weight. This strategy leads to an underestimation of PM in most cases. The higher the content of SVOCs, the higher the losses of organic matter during drying and the higher the deviation of PM. This is still a problem of most standardized calculations, which are based on national norms. For this reason also, the PM masses shortly after sampling are presented. Even conditioning of filters was avoided to prevent more volatile compounds from evaporation and interaction with other samples and laboratory air. The filters were weighed immediately after sampling. As a result, the losses of collected PM were 2−21% when drying the sampled filters from boiler emissions, which ranged between 14 mg MJ−1 (PB) and 32 mg MJ−1 (CB) before drying. The losses of mass increased up to 41% because of drying when samples of the initial inflaming phases were treated. The mass loss was predominant for samples from operation of the LS, especially during cold-start inflaming [410 mg MJ−1 (beech) and 250 mg MJ−1 (spruce) before drying], indicating high amounts of SVOCs. In total, the corresponding filters lost between 58% (spruce) and 71% (beech) of their original weight because of drying (Table 2). In the following, only wet PM mass is considered. High values of PM were found especially for log-wood-fired systems during cold-start conditions. PM emitted from log wood boilers were increased up to 210 mg MJ−1 considering cold-start inflaming of beech wood and 110 mg MJ−1 for spruce wood. The inflaming of logs needs significantly more time and produces more emissions, owing to complicated drying of large wood pieces during combustion and following distillation of inflammable gases. This is more complex because of less homogeneity and less area for impact of flames. The critical amount of inflammable gases to supply the flame and keep it burning is affecting emitted concentrations of PM and organic substances. OGC and CO. During cold-start inflaming of log wood combustion systems, high concentrations of OGC were also observed. OGC as an indicator of volatile organic compounds (VOCs) showed averaged concentrations during the inflaming of log wood in the stove from 430 mg MJ−1 (spruce) to 1200 mg MJ−1 (beech). These values were reduced to 160 mg MJ−1 when regular combustion conditions were reached. This is still 2 orders of magnitude higher than concentrations of OGC (1−

■

RESULTS AND DISCUSSION All values presented were calculated regarding the different efficiencies of the investigated systems (see the Supporting Information). PM. A long drift distance and a moderate dilution ratio of up to 10 allowed for condensing of semivolatile organic 6697


Energy & Fuels

Article

Table 2. Summary of the Emissions of Gaseous Analytes, PM, TEQ Values, and Major Inorganic and Organic Constituentsa

6698


Energy & Fuels

Article

Table 2. continued

All values are calculated by considering the efficiencies of the different combustion systems. Values are distinguished by “cold start” (initial inflaming of wood in the cold stove) and batches at “nominal load”. Because of chromatographic co-elution of some of the PAHs with mass weight of 216 units (“PAH 216”) and 302 units (“PAH 302”) and also the benzofluoranthenes, summarized values are provided. The concentration of 1-methylpyrene is itemized (sufficient chromatographic separation) because of its impact on TEQ values. a

2 mg MJ−1) emitted by boilers under nominal load. Although similar to the other boilers at nominal load, the log wood boiler emitted high amounts of OGC with 530 mg MJ−1 on average during initial inflaming of beech wood logs. In this experiment, the OGC was accompanied with the highest emissions of CO on average (5200 mg MJ−1) too. CO, often used as an indicator for incomplete combustion, showed consistent high values when log wood was combusted. Despite of good burning conditions with high temperatures and good air supply, concentrations of CO never dropped below an average of 1500 mg MJ−1 during the combustion of logs in the stove. Peak values reached more than 10 000 mg MJ−1. Although the CO values of the LB were comparable to the log wood stove during ignition, the further process of batch produced only from 15 mg MJ−1 (beech) to 27 mg MJ−1 (spruce) CO on average. These values were even below the results of the CB and within the range of values of the PB (17 mg MJ−1). Particle-Bound Inorganic Elements. In emissions from automated heating appliances, 30−40% of PM was found to originate from potassium. More volatile salts, such as K2O and KOH, are formed at higher temperatures, which were common in the used boilers (PB, CB, and LB in Figure 2), followed by the formation of K2CO3 after particle formation. Values of potassium were usually significantly higher than 1 mg MJ−1, with the exception of the pellet stove. The low concentrations of potassium within the emitted PM are possibly due to the fact that the temperature within the combustion chamber of the PS was considerably lower than for other combustion systems. Therefore, the amount of emitted particular potassium salts was reduced. The emissions of hazardous elements, such as zinc, were strongly influenced by the used wood. The emissions of zinc were up to 10 times higher when using spruce wood in comparison to the results obtained when using beech wood for LB and LS (Table 2). Particle-Bound Organic Species. In Table 2 and Tables S3 and S4 of the Supporting Information, the results of all measured organic species are summarized. In Figure 3, the averaged contributions of organic substance classes are visualized. Because of the importance of potassium as a wood

Figure 2. Concentrations of potassium (calculated as K2CO3), PAHs/ o-PAHs, levoglucosan, DHA, and phenols/lignans within PM of the collected fly ash (weight percent). Top chart, inflaming phase; bottom chart, regular combustion. In the background, total PM (wet).

combustion marker, it is also considered. Other inorganic elements are of minor importance in the flue ash. All combustion systems showed increased contributions of all organic compound groups during cold-start inflaming. Significant amounts of resin compounds were only observed in the starting phase. At higher temperatures, resins were nearly completely combusted. Considerable amounts of lignin and cellulose degradation products were only found during coldstart inflaming, when regarding the automatically fired systems. Among these modern boilers, significant contributions of PAHs were found only during the inflaming within the log wood boiler. PM generated by the log wood stove still showed considerable amounts of all regarded organic species, including PAHs (>1% of PM), even during normal combustion phases, with the exception of resin compounds. The contents of measured organic species do not give a full explanation of the losses of PM by drying of filter samples, but a correlation of measured SVOCs and PM is clearly visible. Moreover, high 6699


Energy & Fuels

Article

With the exception of retene, all measured resin-originated compounds were only slightly modified during the cold-start inflaming. This fact and the fact of delay of residues of tarry consistence on the inner walls of the sampling line are an indication that resin is quickly transferred as particles to the exhaust. Because of increasing temperatures in the combustion chamber, resin bubbles are forced to burst, and thereby, resin particles are released. Resin bubbles on the bark and resin capillaries within the wood structure are common for the used coniferous wood from spruce (Picea abies). Because combustion experiments with beech wood were always conducted after respective experiments with spruce wood, the carry-over of resin compounds was clearly visible at the inflaming phase and the first phase of regular combustion. The acids appeared mainly in the inflaming phase with a following decrease down to near or below the detection limit. On the other hand, experiments with the log wood stove showed a constant background level of retene and the methyl esters during all repetitions (Figures S3 and S4 of the Supporting Information). Hence, a formation out of adsorbed precursors within the dilution tunnel can not be excluded. Log wood stove combustion experiments with spruce wood showed that the release of free resin acids and their methyl esters occurs mainly during the cold-start inflaming. Because of higher temperatures at regular combustion conditions, the decay of resin compounds is forced. The highest value of a resin compound measured within this study was 5.6 mg MJ−1 DHA during inflaming of spruce logs in the LB (Table S3 of the Supporting Information). In that case, the concentration of DHA was nearly identical to that of levoglucosan. The ratio of DHA/levoglucosan remained at 1 during regular combustion, but the total concentration of both substances decreased by a factor of 40. With the exception of pellet combustion, the ratio of DHA to the respective methyl ester (DHA−ME) was approximately 60:1 when combusting spruce wood. The release of DHA and DHA−ME in emissions of the PB or PS were low compared to CB and log wood burners. The lower resin content and the better combustion conditions reduced the emissions of these compounds to a minimum. The ratio of DHA/DHA−ME of ∼3:1 was found only at regular combustion conditions. The values of PS in Table S3 of the Supporting Information are calculated with all batches, wherefore values of a batch with an automatically cleaning procedure (at each 4 h, the grate slewed to release ash to the ash chamber) were involved in the calculation of averaged values. In this case and in cases of cold-start conditions, the ratio of DHA/DHA−ME was much higher because of higher concentrations of DHA. More interesting is the fact that concentrations of retene were found to be very similar to those of DHA−ME (with similar ratios of DHA/retene); the concentration range of both substances was 0.4−10 μg MJ−1. Retene and DHA−ME seem to have similar reaction pathways or at least the same reaction rate of modifying of precursors, such as DHA. Resin acids of minor relevance showed the highest concentrations in the phases of initial inflaming, whereas values of regular combustion conditions were often below the limit of quantitation (LOQ). Lignin Degradation Products. Especially during the inflaming, high values of emitted phenols and lignans were observed, up to 19% of particle mass (LS, beech in Figure 2). With the exception of emissions of the log wood stove, the fractions of phenols/lignans emitted at regular combustion conditions were reduced far below 1% of PM. Results of

amounts of SVOCs were accompanied by higher values of VOCs, indicated by the OGC measurements. The higher moisture content (23%) of used wood chips was indicated by high CO concentrations during the initial inflaming phase. Moisture contents of >20% are common for wood chips. The higher emissions of PM and CO during the inflaming were a result of lower combustion temperatures because of higher losses of energy for drying of chips. This was indicated by a strongly reduced boiler capacity (QB) of 14.4 kW during the inflaming phase in comparison to the 30 kW at least, which were reached under nominal load (see Table S2 of the Supporting Information). In contrast, the pellet boiler reaches almost 80% (19.5 kW) of its nominal load during the first 45 min. This is reflected by ambivalent results for organic substances. Concentrations of the higher volatile organic species were found to be even higher within emissions from the pellet boiler, whereas concentrations of less volatile compounds were found to be higher within emissions from the wood chip boiler. These findings agree well with the higher exhaust temperatures of the CB. SVOCs have less time to condensate, and the sampling temperatures at the experiments with the PB were somewhat lower. Anhydrous Sugars. Similar to most of the organic substances, levoglucosan showed very high concentrations during cold-start inflaming of the combustion process. For example, up to 4% levoglucosan of total PM was observed (emitted during inflaming of beech wood logs within the stove). Of more importance is that levoglucosan is an organic tracer for biomass combustion and potassium is an inorganic tracer and how the ratio of both is. During inflaming, the concentrations of levoglucosan were found to be 7−10 times higher than those of potassium (with regard to log wood stove and pellet stove). With increasing combustion temperatures, the concentrations of levoglucosan decreased significantly, whereas emissions of potassium were relatively constant. When burning spruce wood logs, the ratio of levoglucosan/potassium was decreased to 2:1. Other stove experiments showed that the ratio of both substances shifted toward higher values of potassium than levoglucosan. Concentrations of potassium were twice (log wood stove, beech) and 5 times (pellet stove) higher than those of levoglucosan. In all other cases (boiler), the emission values of potassium were found to be at least 10 times higher than that of levoglucosan. Contrary to potassium, which is relatively independent of combustion conditions, the concentrations of levoglucosan strongly depend upon the completeness of combustion. Therefore, the ratio of levoglucosan/potassium is affected by combustion conditions. Resin Compounds. Resin mainly consists of low volatile tricyclic diterpenic acids, so-called resin acids, and highly volatile aromatic compounds. The acids, such as dehydroabietic acid (DHA), are often used as tracer compounds for source apportionment as specific markers of coniferous wood combustion emissions. However, such resins have shown to be a problem during experiments with a dilution tunnel. A carry-over of resins in the dilution system was found. This drawback is due to the moderate dilution and the long drift distance within the tubes of the applied whole flow dilution tunnel. Although tubes of the test bed were cleaned mechanically after each sequence of each experiment, a delay of the resin acids could not be avoided. Such carry-overs by wall losses can be reduced, e.g., by the use of a porous tube diluter,14,31−33 which avoids the contact of particles with the inner wall of the tube instead of a dilution tunnel. 6700


Energy & Fuels

Article

OGC values were roughly identical (1 and 1.5 mg MJ−1, respectively), the emission of CO from the pellet boiler used in this study (15 mg MJ−1) was found to be more than 5 times lower on average than of that used by Lamberg et al. (80 mg MJ−1). Of course, in both studies, the log wood furnaces emitted much more CO of 580 mg MJ−1 (masonry heater) and 1290−1360 mg MJ−1 (log wood stove). Lamberg et al. determined 16 mg MJ−1 OGC (masonry heater), whereas in this study, 110 mg MJ−1 was found. Bari et al.35 studied PM from wood-fired heating in residential areas. Therefore, they also compared the emissions from a pellet stove and a log wood boiler. Although their approach is rather different, some aspects should be mentioned. Bari et al. did sampling according to the German VDI guideline 2066 (VDI 2066, part one). This means sampling on a heated filter was performed at 150 °C. Using a different fuel for the pellet stove (beech wood pellets instead of spruce pellets), they found 31 mg m−3 PM (pellet stove) and 48 mg m−3 PM (log wood boiler) at complete combustion conditions. Transferring the results of the current study to 13% O2, significantly smaller amounts of PM were found [18 mg m−3 (PB) and 20 mg m−3 (LB)]. Whereas values of BaP in emissions of a log wood boiler are comparable within the scope, the values of a pellet oven were quite different. The results for the LB were 0.78 μg m−3 (Bari et al.) and 0.16 μg m−3 (this study). The results for the pellet stove were 7.74 μg m−3 (Bari et al.) and 0.04 μg m−3 (this study). Toxicity. The German Research Foundation (DFG) has developed an approach for an assessment scale for health risk from the exposure by PAHs36 (see Table S1 of the Supporting Information). Similar to polychlorinated dibenzodioxines and dibenzofuranes, the health risk is evaluated by weighing of compounds with different health risk potentials. The concentrations of the single compounds multiplied with the corresponding toxic equivalence factors (TEFs) are added to the toxic equivalent (TEQ) values. The TEF of BaP is set to 1. The other PAHs, which are related to BaP, have a higher or lower health risk potential. Although this approach is developed for workplace exposure, it is applied here for direct emission aerosols. Nevertheless, this approach can lead to an over- or underestimation of the health-affecting potential of PM caused by the rough toxic classification and the non-consideration of other potential hazardous compounds beside those of PAHs. Hence, further toxic relevant characterization parameters have to be discussed, such as PM, oxygenated PAHs, and resin compounds. High concentrations of particle-bound PAHs emitted during initial inflaming of wood logs are responsible for the highest values of TEQ found in this study. TEQ values of emissions of cold-start inflaming of the log wood boiler (27−76 mg MJ−1) were within the range or beyond of experiments with the log wood stove (14−28 mg MJ−1), whereas TEQ values at regular combustion conditions (0.30 mg MJ−1) were found to be within the range of the other boilers (PB, 0.12−0.75 mg MJ−1; CB, 0.35−3.0 mg MJ−1). Technically, improvements, such as an automated ignition procedure, may reduce the emissions of the inflaming phase of the logs within the log wood boiler. The lowest emissions, as observed during cold-start inflaming of pellet boilers or chip boilers might not be reachable. Little advantages of spruce wood, when compared to beech wood, during cold-start inflaming were observable when comparing emitted PM and TEQ values. These values showed no

phenols and lignans are given within Table S3 of the Supporting Information. The guaiacolic compounds vanillin, acetovanillone, methylvanillate, vanillic acid, coniferaldehyde, and 3-guaiacylpropanol appeared in most collected samples because of their origin in both hardwood and softwood. Concentrations were high enough to detect them even in emissions from boilers, such as PB and CB. Syringyl aldehyde and sinapyl aldehyde are syringolic compounds and should be emitted specifically by hardwood combustion. However, low concentrations were also detectable at combustion of pellets and chips. A weak carry-over of phenols originating from the lignin structure seems to be possible, as was observed for resin acid. On the other hand, residuals of hardwood within the pellet and chip materials can not be excluded. In comparison to real hardwood combustion, the emitted amounts of syringoles by pellet and chip combustion were negligible. There are rather specific tracers (similar to resin acids as tracers for coniferous wood combustion) for hardwood combustion, e.g., 4methylsyringole, 4-vinylsyringole, acetosyringone, and syringylacetone. Up to 9 mg MJ−1 4-methylsyringole was observed during initial inflaming (LS, beech). Similar concentrations were determined for coniferaldehyde during the cold start of LS [9.5 mg MJ−1 (spruce) and 8.8 mg MJ−1 (beech)]. PAHs. Several studies investigated the composition of PAHs in PM. Studies from Boman et al. and Pettersson et al.12,34 have focused on emissions from log wood stoves and pellet stoves. They involved gas-phase sampling for detailed determination of OGC and also for volatile PAHs. The values of phenanthrene in the current study were much lower than those found by Boman et al. Concentrations of PAHs with four rings (perylene or fluoranthene) or higher are comparable. However, generally, values of PAHs that were determined by Boman et al. were somewhat higher compared to this study. By example, the concentrations of BaP were 12 μg MJ−1 (spruce) and 11 μg MJ−1 (beech). Pettersson et al. found 30−100 μg MJ−1 (birch), 300 μg MJ−1 (spruce), and 38 μg MJ−1 (pine). The situation is similar for CO and OGC. Here, values of 1900 mg MJ−1 CO (OGC of 160 mg MJ−1) on average were detected. Pettersson et al. found 2400 mg MJ−1 CO (OGC of 410 mg MJ−1). The results of the pellet stoves are quite comparable, with somewhat higher values within this study. The American pellet stove, tested by Boman et al., produced similar results at regular combustion conditions, such as the German stove, which were used in this study: 35 ng MJ−1 BaP (Boman et al.) and 48 ng MJ−1 BaP (this study), with OGC of 4.1 and 10 mg MJ−1 and CO of 100 and 413 mg MJ−1 for Boman et al. and this study, respectively. When the results were normalized to fuel energy input, Lamberg et al.14 found 1.6 μg MJ−1 BaP in the emissions of a modern masonry heater operated with birch wood. The comparable combustion principle of the log wood stove tested in this study emitted 8.3 μg MJ−1 (spruce wood) and 7.9 μg MJ−1 (beech wood), respectively. A total of 51 mg MJ−1 PM1 was emitted by combustion of birch wood. In the presented study, the combustion of spruce wood emitted 47 mg MJ−1 PM and the emission of beech wood was 65 mg MJ−1 PM. Because of a better storage of heat within a masonry heater, the results of the masonry heater would be even better when considering efficiency. Low values of PM (dried, 8 mg MJ−1; wet, 12 mg MJ−1) were emitted by the pellet boiler in this study. Lamberg et al. found 19.7 mg MJ−1 PM1 emissions from a modern pellet boiler. Furthermore, the boilers emitted 2 ng MJ−1 BaP (Lamberg et al.) and 26 ng MJ−1 BaP (this study). Where the 6701


Energy & Fuels

Article

Figure 3. Comparison of PM (wet), PM (dried), TEQ values, and summarized values of o-PAHs emitted by wood combustion systems. For each system, initial inflaming phases (blue bars; N = 1) and the regular combustion process (red bars; N = 3) are visualized. (Right side) Toxicity scale of the wood combustion systems by the toxicity of released PM, distinguishing the inflaming phase (blue) and regular combustion (red).

significant difference when comparing spruce and beech wood combustion at nominal load. In general, the TEQ values of automatically fired systems were lower than those generated by the stove by far. The combustion system with the lowest TEQ values in emissions was found to be the pellet boiler. The TEQ values in emissions from this appliance were about 200 times lower than those of the stove. The ratio of TEQ values between a log wood stove and a pellet boiler might be fundamentally higher. It has to be expected that the toxic potential of emissions of a log wood stove is more than a factor of 1000 higher on average than emissions of a pellet boiler. It can be assumed that most of the stoves are of less quality compared to the high-quality stove used in this study. It also has to be assumed that many of the installed stoves are not operated within optimum conditions as performed in the presented experiments, whereas automatically fired pellet boilers are easy to operate at optimum conditions. The importance of weighing the PAH composition can be demonstrated by the comparison of TEQ values with the summarized concentrations of the measured PAHs. The ratio of the sum of PAHs (LS, spruce)/sum of PAHs (PB) at regular combustion is only 50 compared to 200 of the same ratio using TEQ values. Beside the PAHs, the oxygenated PAHs (o-PAHs) are playing a major role for the toxic potential of PM from wood combustion. An assessment of o-PAH-related health effects caused by the exhaust of different wood combustion systems can only be performed by comparing the total sums of the concentrations of these hazardous compounds. For the oxygenated polycyclic aromatics, the ratio of the sum of o-PAHs (LS, spruce)/sum of o-PAHs (PB) at regular combustion was 70:1. A more detailed discussion of toxicity of this compound class and others will be given in a further publication by the authors. The differences within the results of emitted PM were not as high as for TEQ values (Figure 3). On the basis of the same

example used before, ratios of PM (LS, spruce)/PM (PB) were only about 6:1, calculated with values of either wet or dried PM. The results imply once more that the usual way of determining PM emissions does not take into account possible health effects. Health effects caused by emissions of log wood stoves are methodically underestimated. Improvements in combustion technologies are not sufficiently appreciated. From the calculation of PM emissions after drying of filters, the results of the pellet boiler were only 3−23 times lower than PM emissions of the log wood stove. In Figure 3, a toxicity scale is shown, which gives a rough overview of the PAH-related toxicological potential of emissions from the different combustion systems. When the starting phase is taken into account, the emissions of the PB have the lowest toxicity potential, followed by the CB and PS. Because of the high emissions of hazardous material during the cold start, the LB is not as environmentally friendly as the other boilers. The log wood stove emits during all combustion phases high amounts of contaminated material. The toxicity potential of emitted PM is far beyond modern combustion appliances. Additional toxicity tests are essential for assessment of composition of PM from wood combustion. At the actual state of knowledge, the weighing of all hazardous organic compounds by their toxicity is not possible because of the missing database for many compounds of the complex mixtures of wood smoke. Also, a weighing of hazardous elements is not yet possible concerning the comparability of health effects of organic compounds and inorganic elements. In particular, the role of particle-bound zinc salts has to be assessed in the future. With regard to the results of this study, the use of wood with low concentrations of zinc avoids higher values of zinc within emitted PM. Also, the use of boilers with lower combustion temperatures seems to reduce the content of inorganic elements, such as potassium, zinc, or cadmium. 6702


Energy & Fuels

Article

characterization of their physicochemical properties, and identification of the physicochemical properties that influence the biological effects of wood smoke particles. The question of health effects of emissions of modern boilers could not be answered within this study. The chemical composition of the exhaust changes dramatically compared to log wood stoves. The effects of highly inorganic composition of particles emitted by pellet or chip boiler have to be investigated in more detail. Health effects can be studied by the exposure of lung cells with wood combustion particles via the air−liquid interface (ALI).40 Last, atmospheric dilution changes gas particle partitioning of SVOCs considerably. Thereby, online measuring techniques, such as an aerosol mass spectrometer (AMS) and a single-photon ionization/resonance-enhanced multiphoton ionization time of flight mass spectrometer (SPI−/REMPI−TOFMS), will be helpful tools.41−46 For answers to these questions, efforts are already running within the Helmholtz Virtual Institute of Complex Molecular Systems in Environmental Health (HICE).47

When alternative heating fuels are taken into consideration, natural gas boilers emit nearly no particles. Hence, no TEQ values could be calculated. Because of a scarce number of publications considering the emissions of oil boilers, only an estimation of TEQ values of such appliances is possible. The results of two studies from Miller et al.37 and Rogge et al.38 were used to estimate the TEQ values of oil combustion, although these appliances were quite different compared to novel boilers. Emissions of PM can be assumed to be around 1.7 mg MJ−1.1 Rogge et al. found somewhat higher values from 5 to 13 mg MJ−1 (referred to the net calorific value of fuel, QF). The results of PAHs from Miller et al. deliver a TEQ value of 0.025−0.28 mg MJ−1, and the results from Rogge et al. deliver a TEQ value of 0.0003−0.03 mg MJ−1, depending upon combustion conditions and used fuel. Please note that not every single value of PAHs, which was used for calculation of TEQ values of emissions from wood boilers, is available from these oil combustion studies. It has to be assumed that modern wood boilers can achieve similar emission values as oil boilers when comparing the average of actually installed residential oil boilers (estimated lifetimes of >20 years) to new pellet boilers (most of them less than 10 years of age). On the other hand, there might still be a gap when comparing the newest technologies. In that case, TEQ values of oil boilers might be around 100 times lower than those of wood boilers.

■

ASSOCIATED CONTENT

S Supporting Information *

Technical descriptions of the used combustion appliances, lists of hazardous PAHs, equations and graphics of the absorptive partitioning model, equations for calculation of efficiencies, and a complete table of all results of conducted experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

CONCLUSION It was found that combustion within modern boilers reduces the PM emissions significantly (up to 10×). However, characteristics of emissions change by far more; the calculated TEQ values were found to be up to 200 times lower compared to log wood stoves. Therefore, a professional installation of boilers is necessary concerning a sufficient heat reservoir to reduce inflaming cycles. Best results with lowest values of PM and TEQ were obtained by the application of a pellet boiler and a chip boiler. The emission properties of the chip boiler might be even improved using dried wood chips with a moisture content less than 20%. The cold-start inflaming process of the used log wood boiler is improvable. In this case, an automatic ignition system is recommendable. The ongoing batch of the log wood boiler was rather good, even when compared to the other boiler types. The emission behavior of a simple constructed log wood stove is not comparable in the least to modern heating appliances. A further publication will show that this difference in TEQ easily increases up to a factor of 1000 in the case of worse combustion conditions. This study showed that measuring only the PM mass does not reflect either the variability of toxicological potential of emitted dust or the improvements in combustion technologies. A better assessment is possible by the quantification of PAHs and calculation of the resulting TEQ values. A weighted evaluation of emissions of wood combustion appliances by analyzing of hazardous PAHs can give a more widespread view on toxicity of wood smoke. However, the summarizing effects of complex mixtures of inorganic salts, PAHs, their modified substitutes, CO, and many other volatile and semivolatile compounds have to be validated properly. A serious discussion will only be possible by studying the health effects with an integrated view on PM from wood combustion by the help of further studies observing atmospheric behavior and biological impacts of PM. Kocbach Bølling et al.39 already highlighted in a review in 2009 the major gaps in the knowledge concerning the characterization of the atmospheric transformations of wood smoke particles, the

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was kindly supported by the Federal Ministry of Science and Culture, Lower Saxony, Germany, and the Helmholtz Virtual Institute of Complex Molecular Systems in Environmental Health (HICE), www.hice-vi.eu.

■

REFERENCES

(1) Federal Environmental Agency (UBA). Hintergrundpapier: Die Nebenwirkungen der Behaglichkeit: Feinstaub aus Kamin und Holzofen; UBA: Dessau, Germany, 2006. (2) Hays, M. D.; Beck, L.; Barfield, P.; Lavrich, R. J.; Dong, Y.; Vander Wal, R. L. Environ. Sci. Technol. 2008, 42, 2496−2502. (3) Brandt, C.; Kunde, R.; Dobmeier, B.; Schnelle-Kreis, J.; Orasche, J.; Schmoeckel, G.; Diemer, J.; Zimmermann, R.; Gaderer, M. Atmos. Environ. 2011, 45, 3466−3474. (4) Aiken, A. C.; DeCarlo, P. F.; Kroll, J. H.; Worsnop, D. R.; Huffman, J. A.; Docherty, K. S.; Ulbrich, I. M.; Mohr, C.; Kimmel, J. R.; Sueper, D.; Sun, Y.; Zhang, Q.; Trimborn, A.; Northway, M.; Ziemann, P. J.; Canagaratna, M. R.; Onasch, T. B.; Alfarra, M. R.; Prevot, A. S. H.; Dommen, J.; Duplissy, J.; Metzger, A.; Baltensperger, U.; Jimenez, J. L. Environ. Sci. Technol. 2008, 42, 4478−4485. (5) Donahue, N. M.; Robinson, A. L.; Pandis, S. N. Atmos. Environ. 2009, 43, 94−106. (6) Elsasser, M.; Crippa, M.; Orasche, J.; DeCarlo, P. F.; Oster, M.; Pitz, M.; Cyrys, J.; Gustafson, T. L.; Pettersson, J. B. C.; Schnelle-Kreis, J.; Prévôt, A. S. H.; Zimmermann, R. Atmos. Chem. Phys. 2012, 12, 6113−6128. (7) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.;

6703


Energy & Fuels

Article

(36) Greim, H. Gesundheitsschädliche ArbeitsstoffeToxikologischarbeitsmedizinische Begründungen von MAK-Werten und Einstufungen; DFG, Wiley-VCH Verlag GmbH and Co. KGaA: Weinheim, Germany, 2008; Vol. 45. Lieferung. (37) Miller, C. A.; Ryan, J. V.; Lombardo, T. J. Air Waste Manage. Assoc. 1996, 46, 742−748. (38) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1997, 31, 2731−2737. (39) Kocbach Bølling, A.; Pagels, J.; Yttri, K.; Barregard, L.; Sallsten, G.; Schwarze, P.; Boman, C. Part. Fibre Toxicol. 2009, 6, 29. (40) Paur, H.-R.; Cassee, F. R.; Teeguarden, J.; Fissan, H.; Diabate, S.; Aufderheide, M.; Kreyling, W. G.; Hanninen, O.; Kasper, G.; Riediker, M.; Rothen-Rutishauser, B.; Schmid, O. J. Aerosol Sci. 2011, 42, 668−692. (41) Bente, M.; Sklorz, M.; Streibel, T.; Zimmermann, R. Anal. Chem. 2008, 80, 8991−9004. (42) Bente, M.; Sklorz, M.; Streibel, T.; Zimmermann, R. Anal. Chem. 2009, 81, 2525−2536. (43) Fendt, A.; Streibel, T.; Sklorz, M.; Richter, D.; Dahmen, N.; Zimmermann, R. Energy Fuels 2012, 26, 701−711. (44) Grabowsky, J.; Streibel, T.; Sklorz, M.; Chow, J. C.; Watson, J. G.; Mamakos, A.; Zimmermann, R. Anal. Bioanal. Chem. 2011, 401, 3153−3164. (45) Oster, M.; Elsasser, M.; Schnelle-Kreis, J.; Zimmermann, R. Anal. Bioanal. Chem. 2011, 401, 3173−3182. (46) Streibel, T.; Hafner, K.; Mü h lberger, F.; Adam, T.; Zimmermann, R. Appl. Spectrosc. 2006, 60, 72−79. (47) Zimmermann, R. Anal. Bioanal. Chem. 2011, 401, 3041−3044.

Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, T. F.; Monod, A.; Prévôt, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J. Atmos. Chem. Phys. 2009, 9, 5155−5236. (8) Heringa, M. F.; DeCarlo, P. F.; Chirico, R.; Tritscher, T.; Dommen, J.; Weingartner, E.; Richter, R.; Wehrle, G.; Prévôt, A. S. H.; Baltensperger, U. Atmos. Chem. Phys. 2011, 11, 5945−5957. (9) Iinuma, Y.; Böge, O.; Gräfe, R.; Herrmann, H. Environ. Sci. Technol. 2010, 44, 8453−8459. (10) Nopmongcol, U.; Khamwichit, W.; Fraser, M. P.; Allen, D. T. Atmos. Environ. 2007, 41, 3057−3070. (11) Bäfver, L. S.; Leckner, B.; Tullin, C.; Berntsen, M. Biomass Bioenergy 2011, 35, 3648−3655. (12) Boman, C.; Pettersson, E.; Westerholm, R.; Boström, D.; Nordin, A. Energy Fuels 2011, 25, 307−314. (13) Johansson, L. S.; Leckner, B.; Gustavsson, L.; Cooper, D.; Tullin, C.; Potter, A. Atmos. Environ. 2004, 38, 4183−4195. (14) Lamberg, H.; Nuutinen, K.; Tissari, J.; Ruusunen, J.; Yli-Pirilä, P.; Sippula, O.; Tapanainen, M.; Jalava, P.; Makkonen, U.; Teinilä, K.; Saarnio, K.; Hillamo, R.; Hirvonen, M.-R.; Jokiniemi, J. Atmos. Environ. 2011, 45, 7635−7643. (15) Schmidl, C.; Luisser, M.; Padouvas, E.; Lasselsberger, L.; Rzaca, M.; Ramirez-Santa Cruz, C.; Handler, M.; Peng, G.; Bauer, H.; Puxbaum, H. Atmos. Environ. 2011, 45, 7443−7454. (16) Atkins, A.; Bignal, K. L.; Zhou, J. L.; Cazier, F. Chemosphere 2010, 78, 1385−1392. (17) Jalava, P. I.; Happo, M. S.; Kelz, J.; Brunner, T.; Hakulinen, P.; Mäki-Paakkanen, J.; Hukkanen, A.; Jokiniemi, J.; Obernberger, I.; Hirvonen, M.-R. Atmos. Environ. 2012, 50, 24−35. (18) Orasche, J.; Schnelle-Kreis, J.; Abbaszade, G.; Zimmermann, R. Atmos. Chem. Phys. 2011, 11, 8977−8993. (19) Allen, J. O.; Durant, J. L.; Dookeran, N. M.; Taghizadeh, K.; Plummer, E. F.; Lafleur, A. L.; Sarofim, A. F.; Smith, K. A. Environ. Sci. Technol. 1998, 32, 1928−1932. (20) Graham, B.; Mayol-Bracero, O. L.; Guyon, P.; Roberts, G. C.; Decesari, S.; Facchini, M. C.; Artaxo, P.; Maenhaut, W.; Köll, P.; Andreae, M. O. J. Geophys. Res. 2002, 107, 8047. (21) Isidorov, V. A.; Kotowska, U.; Vinogorova, V. T. Anal. Sci. 2005, 21, 1483−1489. (22) Lundstedt, S.; Haglund, P.; Oberg, L. Environ. Toxicol. Chem. 2003, 22, 1413−1420. (23) Sauvain, J.-J.; Vu Duc, T. J. Sep. Sci. 2004, 27, 78−88. (24) Wang, Z.; Li, K.; Lambert, P.; Yang, C. J. Chromatogr., A 2007, 1139, 14−26. (25) Chow, J. C.; Watson, J. G.; Chen, L. W. A.; Chang, M. C. O.; Robinson, N. F.; Trimble, D.; Kohl, S. J. Air Waste Manage. Assoc. 2007, 57, 1014−1023. (26) Chow, J. C.; Watson, J. G.; Robles, J.; Wang, X. L.; Chen, L. W. A.; Trimble, D. L.; Kohl, S. D.; Tropp, R. J.; Fung, K. K. Anal. Bioanal. Chem. 2011, 401, 3141−3152. (27) Heinrichs, H.; Herrmann, A. G. Praktikum der Analytischen Geochemie; Springer-Verlag: Berlin, Germany, 1990. (28) Donahue, N. M.; Robinson, A. L.; Stanier, C. O.; Pandis, S. N. Environ. Sci. Technol. 2006, 40, 2635−2643. (29) Shrivastava, M. K.; Lipsky, E. M.; Stanier, C. O.; Robinson, A. L. Environ. Sci. Technol. 2006, 40, 2671−2677. (30) Lipsky, E. M.; Robinson, A. L. Environ. Sci. Technol. 2006, 40, 155−162. (31) Sippula, O.; Hokkinen, J.; Puustinen, H.; Yli-Pirilä, P.; Jokiniemi, J. Atmos. Environ. 2009, 43, 4855−4864. (32) Sippula, O.; Hokkinen, J.; Puustinen, H.; Yli-PirilaI,̂̀ P.; Jokiniemi, J. Energy Fuels 2009, 23, 2974−2982. (33) Tissari, J.; Sippula, O.; Kouki, J.; Vuorio, K.; Jokiniemi, J. Energy Fuels 2008, 22, 2033−2042. (34) Pettersson, E.; Boman, C.; Westerholm, R.; Boström, D.; Nordin, A. Energy Fuels 2011, 25, 315−323. (35) Bari, M. A.; Baumbach, G.; Brodbeck, J.; Struschka, M.; Kuch, B.; Dreher, W.; Scheffknecht, G. Atmos. Environ. 2011, 45, 7627−7634. 6704


Comparison of Emissions from Wood Combustion. Part 1: Emission

Recommend Documents