Article pubs.acs.org/EF
Comparison of Emissions from Wood Combustion. Part 2: Impact of Combustion Conditions on Emission Factors and Characteristics of Particle-Bound Organic Species and Polycyclic Aromatic Hydrocarbon (PAH)-Related Toxicological Potential Jürgen Orasche,†,‡,§ Jürgen Schnelle-Kreis,*,‡ Claudia Schön,∥ Hans Hartmann,∥ Hans Ruppert,† Jose M. Arteaga-Salas,‡ 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 Solid Biofuels, Technology and Support Centre (TFZ), D-94315 Straubing, Germany S Supporting Information *
ABSTRACT: The impact of combustion conditions on emission factors and characteristics of log wood combustion was investigated. Two different kinds of log woods (spruce and beech) and one kind of briquette (spruce sawdust) were used to study differences in emission behavior depending upon the wood type. Beech wood was used to examine additionally the impact of different moisture contents and maloperation on emissions of fine particulate matter (PM). Therefore, wood logs with three different levels of moisture content were used. Maloperation was simulated by an overload scenario and an air deficiency scenario. Toxicity equivalent (TEQ) values were calculated for the different combustion conditions. It was found that PM mass varies only by a factor of 8 at a maximum, whereas TEQ values can vary more than a factor of 80 (regular beech wood combustion, 6 μg MJ−1; beech wood combustion in an overloaded combustion chamber, 500 μg MJ−1). In particular, wood with a higher moisture content (19%) released high amounts of intermediate products from lignin and cellulose degradation. The PM emissions in this case were the highest among the tested operation conditions, especially during the initial (cold start) inflaming (660 μg MJ−1), but were not in correspondence with the toxicity potential. The TEQ (37 μg MJ−1) in that case was much lower than during maloperation.
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INTRODUCTION In many countries, wood is one of the main sources for generating heat during the wintertime or for cooking. In most industrial countries, the firing of wood is also used for additional heating beside an oil or gas boiler. Many people use the heat of a fire for purely leisure purposes during the wintertime. In both cases, the firing of wood in the form of logs is the most common. Typical stove types for rooms are masonry heater and log wood stoves of different size and style with or without a glass. In some northern European countries, sauna stoves are widely used. However, log-wood-fired stoves are not easy to operate, and most scientific publications have found huge differences in combustion quality when comparing log wood stoves with modern automatically fired wood combustion appliances.1−7 Users of log wood stoves have to take care about the moisture content of the wood (storage conditions), the right size and shape of logs, the right amount of wood to load in the combustion chamber, sufficient air supply, and the right way to inflame the fuel. Whenever it was possible, in this study, the recommendation of the German Federal Environmental Agency was followed, thus, to inflame the log wood using the top-down method. This implies that © 2013 American Chemical Society
smaller pieces of wood were placed on top of the logs, with the ignition aid (wood wool coated with paraffin) being placed last. In this study, the influences of fuel and combustion conditions on emission factors and characteristics of log wood combustion were investigated under circumstances often happening in households. Therefore, one kind of wood log stove with a nominal heat output of 8 kW was fired with two different kinds of log wood (spruce and beech) and one type of briquette (spruce sawdust). Beech wood was additionally used to examine the impact of moisture content and maloperation. Three levels of moisture content were studied. A very low moisture content of 4 kg instead of 1.6 kg) and twice as high as recommended by the manufacturer of the stove. Air deficiency was implemented by Received: September 13, 2012 Revised: December 20, 2012 Published: January 31, 2013 1482
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Table 1. Fuel Propertiesa combustion experiment
abbreviation
load weight (kg)
C (g kg−1)
H (g kg−1)
O (g kg−1)
N (g kg−1)
S (g kg−1)
ash (g kg−1)
moisture (%)
energy content (MJ kg−1)
log wood, spruce, regular briquettes, spruce, regular log wood, beech, regular log wood, beech, low moisture content log wood, beech, high moisture content log wood, beech, oxygen deficiency log wood, beech, overload
SL SB BL BL2%
1.6 1.2b/1.6 1.6 2.0d/1.6
420 460 410 490
54 55 53 63
370 400 380 430
1.1 1.6 1.9 1.3
0.16 ndc 0.17 nd
5.2 2.1 4.0 6.4
15 8.8 15 1.6
15.7 17.1 15.2 17.6
BL19%
2.4d/2.0
400
52
350
1.1
nd
5.3
19
14.0
BLOD
1.6
410
53
380
1.9
0.17
4.0
15
15.2
BLOL
2.6d/4.3
410
53
380
1.9
0.17
4.0
15
15.2
a
b
c
d
All values refer to raw material. Initial inflaming with spruce wood logs. nd = not determined. Load weight for initial inflaming.
Table 2. Combustion Operation Valuesa
a
combustion experiment
abbreviation
FC (kg h−1)
DR
CO2 (%)
H2O (%)
λ
O2 (%)
TE average (°C)
η (%)
log wood, spruce, regular briquettes, spruce, regular log wood, beech, regular log wood, beech, low moisture content log wood, beech, high moisture content log wood, beech, oxygen deficiency log wood, beech, overload
SL SB BL BL2% BL19% BLOD BLOL
1.5 1.5 2.0 1.9 1.9 1.9 3.7
2.3 3.3 3.7 3.2 3.1 3.5 3.9
5.2 5.3 5.2 6.8 5.8 6.2 10.5
5.6 5.0 5.3 5.8 6.7 6.5 10.8
3.7 5.4 3.8 2.9 3.4 3.2 2.6
16.1 15.8 16.0 14.4 15.5 15.0 11.0
230 260 290 280 260 270 530
71 69 64 70 67 70 61
FC, fuel consumption; DR, dilution ratio; λ, combustion air ratio; TE average, averaged exhaust temperature; and η, efficiency. h, which is described in a separate chapter within the Supporting Information. The exhaust was diluted 2−5-fold in a whole flow dilution tunnel. The dilution tunnel allowed cooling of exhaust below 50 °C, and therefore, organics were able to condense. Determination of particle numbers and size distributions were performed with an electrostatic low-pressure impactor (ELPI, Dekati, Finland). Nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), and organic gaseous carbon (OGC) were measured in the undiluted exhaust. For each batch, the measuring started right after loading when the door was closed. The measurement of a batch was terminated when only 4 wt % of the original mass of the fuel was reached (the stove was mounted on a balance). The total sampling times were between 45 and 60 min. Between the measured batches, small intermediate batches were applied. The batches were repeated 3 times (N = 3), with the exception of the overload experiment and briquette combustion (N = 2). The starting conditions were not repeated because of the requirements for inflaming the fuel. Some combustion conditions described here are not suitable to inflame logs. The major differences were the ignition of briquettes with small pieces of spruce wood and the normal amount of fuel to ignite the fuel for overloading experiments as well as a higher air supply before experiments with oxygen deficiency. The starting conditions are summarized in Table 1. Particle Sampling. Particles were collected out-stack via filter sampling downstream of the dilution tunnel about 7.5 m after dilution and 20 m from the stove outlet. Separation of a partial stream was performed isokinetically. Quartz fiber filters (QFFs) with a diameter of 150 mm were used. A small part of the filter was cut out (1/12 of the whole filter). These filter parts for organic analyses were immediately placed into a freezer after sampling. The samples were stored at −20 °C until analysis. The other part was thermally treated for determination of PM mass (dried). A detailed description of sampling is given elsewhere.8 Organic Analysis. Filter samples were analyzed with in situ derivatization thermal desorption gas chromatography time-of-flight mass spectrometry (IDTD−GC−TOFMS), which was described elsewhere.9 In brief, prior to analysis, filter punches were spiked with isotope-labeled standard compounds for later quantification. Analytes that were not commercially available were semi-quantified by external calibration curves with similar standard compounds and
closing the air hatches (primary and secondary air supplies) to the combustion chamber. With the exception of the air deficiency experiment, all other experiments were conducted at partial load. For all combustion conditions, an estimation of toxicological potential was performed by individual weighting factors for released concentrations of polycyclic aromatic hydrocarbons (PAHs) and assessing emissions of other organic compounds and the particulate matter (PM) mass.
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EXPERIMENTAL SECTION
Combustion Experiments. Combustion experiments were carried out at a test bench of the Technology and Support Centre (TFZ) in Straubing, Germany (see Figure S3 of the Supporting Information). Different types of fuel and combustion conditions were investigated as follows: (i) wood log, spruce (SL), (ii) briquettes, spruce (SB), (iii) wood log, beech (BL), (iv) wood log, beech with low moisture content (BL2%), (v) wood log, beech with a high moisture content (BL19%), (vi) wood log, beech, overload of the stove (BLOL), and (vii) wood log, beech, oxygen deficiency (BLOD). All of the experiments were performed with the same wood log stove under similar conditions. The length of the logs was 25 cm (approximately 7.5 cm thick), and the briquettes had a size of approximately 155 × 65 × 100 mm. The regular logs fired in the stove, both spruce logs and beech logs, had a moisture content of 15%. The same beech wood logs were also used in experiments with overload and oxygen deficiency. For the experiments with logs of low and high moisture contents, logs from beech wood in test fuel shape in accordance with NS 3058-1 (without bark) were used with 1.6 and 19% of moisture, respectively. The applied spruce briquettes had a moisture content of 8.8%. Combustion experiments, fuel properties, and applied fuel amount per batch are summarized in Table 1. Contrary to the study by Orasche et al.,8 where the same stove was used at nominal load, for this study, the flow of primary air through the grate was closed. The secondary air was completely open. Therefore, a comparison of nominal load8 and partial load (this study) was also possible. It has to be expected that most users operate their stoves under partial load; otherwise, the heat output is far too high for ordinary living rooms. One further experiment was initiated to examine a typical leisure evening heating event over a time period of 3 1483
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Figure 1. Percentage of organic compound classes in total PM (wet). Total PM (wet) is illustrated as bubbles. Sum PAH is the sum of PAHs and modified PAHs as far as quantified. Blue bubbles, initial inflaming phase; red bubbles, regular combustion (average). The sizes of bubbles are proportional to total PM mass (mg MJ−1). Batch 1 is always the inflaming phase. Initial inflaming of overload experiments was performed with regular load, followed by two repetitions with overload. The chart of SB combustion visualizes the first inflaming of briquettes (after the initial inflaming with spruce wood pieces) and the following two batches after loading of briquettes on hot embers. All other charts are showing three repetitions after the initial inflaming phase. Inflaming of wood within the air deficiency experiments was performed with some minutes of air supply until sufficient flames were observed. isotope-labeled internal standards (see the Supporting Information). Polar organic substances were treated during thermal desorption with N-methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA, MachereyNagel, Germany) for in situ derivatization. A Pegasus III TOF mass spectrometer (LECO, St. Joseph, MI) was applied for measuring organic compounds. A high acquisition rate (25 spectra per second) and a full mass scan were necessary for peak deconvolution to do consequent compound identification by mass spectra interpretation and mass spectra libraries. Additionally, retention time indices were used for identification.10−15
PM and Organic Gaseous Carbon (OGC). Masses of PM were determined with the wet, unconditioned samples immediately after sampling, as well as with dried samples (according to German VDI 2066), as described by Orasche et al.8 Both values are given for each experiment in Table S1 of the Supporting Information. The loss of weight (removal of water and volatile and semi-volatile organic contents of wood) because of drying at 120 °C for 1 h was between 8 and 71%. The water content of PM was not analyzed, although a part of mass loss by drying was caused by a loss of water. However, the high wet PM mass of experiments BL19% at the inflaming of wood (660 mg MJ−1) was mainly induced by the formation of high amounts of condensable volatile and semi-volatile organic compounds (VOC and SVOC). That was reflected by the observed high values of OGC (990 mg MJ−1) within the undiluted raw exhaust. The higher the OGC values, the higher the amount of potential condensables, which are responsible for the growth or even new formation of particles. Wood with a higher moisture content was not as inflammable as wood with a lower moisture content. However, after a certain time of drying,
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RESULTS AND DISCUSSION The described concentration values are standardized to usable energy. Therefore, efficiencies were calculated, as described within the Supporting Information, taking incomplete combustion and the loss of energy by emission of hot exhaust into consideration. If not otherwise specified, concentration values refer to wet particulate mass. Combustion operation data are summarized in Table 2. Because of different inflaming processes, the phases with initial ignition are just partly discussed. 1484
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increasing the combustion temperatures during a more complete combustion (Figure 1). Generally, the combustion of spruce wood showed higher variability and a higher dynamic range in emissions. Although PM mass did not change significantly during the combustion of spruce briquettes, a strong increase of emissions of organic substances resulting in higher contributions toward PM could be observed. In particular, the formation of resin compounds and anhydrous sugars is forced. This behavior was accompanied by an increase of CO, whereas OGC showed no discernible trend. The ash content of the briquettes was lower compared to the other applied fuels. The embers on the grate seemed to be more compact with less perfusion of air (ash was not removed between batches). Owed to this, the pressed material was obviously not as inflammable as wood logs. Both properties were responsible for increased emissions of lignin and cellulose/hemicellulose degradation products. Resin acids, their methyl esters, and retene showed some differences in concentrations of single compounds, but total emissions were similar, regardless if they were emitted by combustion of spruce log wood or spruce briquettes. Because of the main focus on comparison of combustion conditions and the constraint of the resin compounds on softwood, they are not discussed in detail here. A more detailed discussion is given elsewhere.8 The combustion of wood with a higher moisture content was indicated by higher emissions of particle-bound organic compounds during the inflaming phase, similar to OGC. The total amount of hazardous PAHs and o-PAHs and their contribution to PM were slightly higher compared to regular beech wood combustion. Phenolic compounds contributed one-fourth to the total PM in the cold-start phase and still to 11−15% at regular combustion. The content of anhydrous sugars was 10−16% of the PM, with no significant distinction between initial inflaming and ongoing combustion. Contrary to wood with a high water content, the combustion of wood with a low water content emitted PM with only 1% phenolic compounds and 2% anhydrous sugars. The formation of PAHs was quite high during all burn-offs with overdried wood. Here, 2% of PAHs within PM means about 2 mg MJ−1 in total. The initial inflaming of BLOL was performed with a normal amount of fuel. The emissions of this phase were, as expected, similar to the cold-start experiment of BL. The following reloads with a jammed combustion chamber resulted in concentrations of 3−4% phenols and 2% anhydrous sugars in the PM mass. In particular, batches with overload contained around 10% critical PAHs in the PM compared to SL, BLOD, BL2%, and BL19% experiments, which showed elevated concentrations of ∼2% PAHs within the emitted PM. Significant contributions of 10% alkanes in PM were found only in experiments with oxygen deficiency during the initial inflaming phase. Maxima of concentrations were observed for pentacosane (2.7 mg MJ−1). The occurrence of alkanes might be a result of incomplete combustion of paraffin wax originated from the ignition aid. The lack of oxygen was also visible in organic composition of PM with ongoing burn-offs. Because of increased slagging, contributions of phenol emissions increased over time (from 12 to 22%). The hot embers were not removed; they were used to ignite the reloaded fuel. Therefore, it is possible that the amount of noncombustible material (at these operation conditions) increased, which was also indicated by an increase of carbon monoxide (from 1100 to 1400 mg MJ−1). An increase of alkali oxides and less flammable organic
the combustion of wood begins. Despite lower combustion temperatures at this stage, regular combustion phases of BL19% showed PM concentrations that were similar to good combustion conditions, obtained with optimal dried wood logs (wet PM of 83 mg MJ−1 for BL19% and 82 mg MJ−1 for BL, respectively). Contrary to the former experiments, the combustion of spruce wood released higher amounts of PM than the combustion of beech wood.8 Again, the high PM emissions (wet PM of 410 mg MJ−1 during inflaming and 380 mg MJ−1 during regular combustion) were accompanied by high OGC concentrations (710 and 610 mg MJ −1 , respectively). Contrary to the former experiment,8 where spruce wood logs were combusted under nominal load with a completely opened primary air flow, the combustion quality this time was poor. On the other hand, combustion of beech log wood (BL) showed this time the best results among all experiments with the lowest OGC concentrations (53 mg MJ−1), similar to burning of overdried wood (51 mg MJ−1). Here, the quality of combustion was improved. However, the combustion of briquettes emitted similar amounts of PM-like maloperation by heating with beech wood (approximately 130 mg MJ−1). Particle-Bound Organic Species. A large variety of organic species were determined within this study. Basic works with a broad range of analyzed compounds were published, e.g., by Fine et al., Schauer et al., Simoneit et al., and Rogge et al.16−19 Here, the discussion focuses on comparison to recent studies.1−7 Specific wood combustion tracers were analyzed, such as phenolic compounds from lignin degradation, anhydrous sugars from cellulose and hemicellulose decomposition, and unspecific combustion products, such as alkanes, PAHs, and their oxygenated analogues (o-PAHs). Resin acids, specific tracers from coniferous wood combustion, were also quantified as far as detectable. The contribution of the organic species to PM is highly variable when comparing the different combustion conditions. Nevertheless, the different fuels and combustion conditions were clearly distinguishable by the composition of organic matter within PM. Therefore, Figure 1 visualizes the contribution of the analyzed compound classes to PM, depicting the order of batches performed within each series of experiments. Please note that not every single compound from these compound classes was quantified and some of the organic substances were only semi-quantified. PM with the lowest contributions of all analyzed compound groups was shown by the combustion of beech log wood. Only the initial inflaming showed higher concentrations of organic species (up to 6% of PM), whereas concentrations during regular combustion conditions were significantly lower (
