Release of polycyclic aromatic hydrocarbons, carbon monoxide and particulate matter from biomass combustion in a wood-fired boiler under varying boiler conditions

Atmospheric Environment 42 (2008) 8863–8871

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Release of polycyclic aromatic hydrocarbons, carbon monoxide and particulate matter from biomass combustion in a wood-fired boiler under varying boiler conditions Keeley L. Bignal a, Sam Langridge b, John L. Zhou a, * a b

Department of Biology and Environmental Science, University of Sussex, Falmer, Brighton, BN1 9QG, UK Bioenergy Technology Ltd, Pound Lane, Framfield, East Sussex, TN22 5RU, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 April 2008 Received in revised form 3 September 2008 Accepted 4 September 2008

Particulate matter, CO and NO as well as 16 polycyclic aromatic hydrocarbons (PAHs) in both gaseous and particulate phases were measured in the stack of a woodchip-fired 50 kW boiler used for domestic heating. The concentrations of SPAHs in both gas and particle phases varied from 1.3 to 1631.7 mg m3. Mean CO and NO concentrations varied from 96 to 6002 ppm and from 28 to 359 ppm, respectively. The effects of fuel parameters (moisture content (MC) and tree species) and boiler operating conditions on pollutant concentrations were investigated. A relationship was established between SPAHs in gaseous and particulate phases and CO concentrations. The species of tree used for woodchip was less important than MC and boiler operating conditions in affecting pollutant concentrations. It is recommended that in order to minimise PAH release woodchip fuel should have a low MC, and the boiler should be operated with a load demand (high/moderate heat requirement). Slumber modes when the boiler has no load demand and is effectively a smouldering flame should be avoided. This can be achieved by increasing automatic operation capability of wood-fired boilers, for example, by automatically varying fire rates and having auto-start capabilities. The PAH data obtained from this study is particularly useful in contributing to emissions inventories, modelling, and predictions of ambient air quality. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbons Biomass boiler Biomass fuels Combustion Particulate matter Moisture content

1. Introduction A clean, secure and sufficient supply of energy is essential for our sustainable development. Currently, the UK relies heavily on fossil fuels: in 2002 they provided 89% of our energy, with 9% from nuclear, and less than 2% from other sources including renewables (DTI, 2003). The UK Biomass Task Force report (2005) concluded that biomass has great potential for contributing to our renewable energy and climate change objectives, and to diversity of

* Corresponding author. Tel.: þ44 1273 877318; fax: þ44 1273 678937. E-mail addresses: [email protected] (K.L. Bignal), [email protected] (S. Langridge), [email protected] (J.L. Zhou). 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.09.013

supply, in line with energy security objectives. An EU energy policy target is to double renewable source usage from 5.4% in 1997 by 2010 (EC, 1997). Biomass, for heat generation in particular, is a cost-effective efficient way of reducing carbon emissions (Biomass Task Force, 2005). Whilst it is important to replace fossil fuels with ‘carbon-neutral’ fuels, to be truly sustainable and costeffective the fuel must also be clean with minimal impact on the environment and/or human health. There is much concern that increased residential wood combustion has adverse impacts on human health in the local population due to impacts on air quality (Naeher et al., 2007). Aside from CO and CO2, other key pollutants emitted by biomass fuels include particulates, NOx, and organics including volatile organic compounds (VOCs) and polycyclic aromatic

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hydrocarbons (PAHs). Much of the previous research on ambient air quality in relation to residential wood combustion has focussed on particulates, mainly because of the associated health risk (Sexton et al., 1984; Luhar et al., 2006; Naeher et al., 2007). PAHs are emitted in both particulate and gaseous forms and are of particular concern as many are known or suspected carcinogens and mutagens (IARC, 2005). Residential wood combustion is a key source of atmospheric PAHs in many countries (Boman et al., 2003; Lobscheid and McKone, 2004; Lee et al., 2005); for example, in Finland it accounts for over 90% of hydrocarbon and PAH emissions (Hustad et al., 1995). This proportion could increase as biomass fuels are growing in popularity. Data on PAH emissions from this source are therefore critical for emission inventories, predictions of local air quality, and modelling of future emissions. However, these data are lacking or uncertain (Lobscheid and McKone, 2004; Defra, 2006) mainly because studies have concentrated on inorganic pollutants (Mastral and Callen, 2000). This was driven by legislative requirements for emissions testing, but also because they are relatively easy and inexpensive to measure, with a range of equipment available to provide instantaneous, real-time measurements. PAHs, on the other hand, are expensive and difficult to sample and quantify; and only a mean value over several hours can be obtained (Peltonen and Kuljukka, 1995; Mastral and Callen, 2000). Whilst equipment to measure particulate-bound PAHs has become available recently these do not speciate for individual PAH compounds, and cannot measure the gaseous phase PAHs, which can be 95% or more of the total (e.g. Yang et al., 1998). Of the studies on PAH emissions from biomass most are on small residential woodstoves; for example, the US EPA (EPA, 1998) published estimated emission factors of 16 PAHs of 45, 204 and 359 mg kg1 from conventional woodstoves, catalytic and non-catalytic woodstoves, and fireplaces, respectively. Data from larger domestic boilers, such as the one in our study, were not included. Others provide data from uncontrolled open biomass burning (Lemieux et al., 2004) with very different combustion conditions to those in a boiler. Other studies have focussed on ambient concentrations of organics including PAHs and/or particulates and have apportioned these to residential wood burning (e.g. Schauer and Cass, 2000). Of the data that are available on biomass boilers, there are little on the effects on emissions of fuel type in conjunction with moisture content (MC). Study of emissions from the combustion of wet and dry fuels is important, as the MC of biomass varies widely. In addition, there are scarce data on boilers operating in slumber mode, which is crucial in affecting quantity and quality of emissions. One recent study reported PAH concentrations relative to other organic emissions in two boiler modes (flaming and glowing), but without absolute concentrations (Olsson and Kjallstrand, 2006). PAHs and CO are both products of inefficient combustion, and hence dependent on the temperature inside boilers (McGrath et al., 2001). A relationship between CO and PAHs has been found from combustion of coal and waste tyres (Levendis et al., 1998) and for a range of biomass fuels (Vierle et al., 1999); however, this was based

on limited data. Others have reported higher PAH emissions under conditions of inefficient combustion, but have not specifically related PAH to CO levels (Ramdahl et al., 1982; Saez et al., 2003). Some however, have not found a correlation between CO and PAH or other hydrocarbon emissions, and there has been some debate as to whether one exists (Levendis et al., 1998). This study therefore aimed to investigate pollutant release to the atmosphere from a woodchip-fired boiler typical of those used for domestic heating; including investigation of the relationship between CO and PAH concentrations. The effects of fuel MC, the wood species used, and boiler operating conditions on CO, particulate and PAH concentrations were also investigated. 2. Experimental 2.1. The boiler Pollutant release was measured from a 50 kW Arimax 340 fixed retort system biomass boiler, which provides heating for a building at Flimwell, East Sussex. It was tested under two main operating conditions: ‘slumber’, with no load on the boiler and low combustion temperatures; ‘full flame’ , the normal operating mode of the boiler, characterised by 100% load and high combustion temperatures. By drying in an oven at 105  C for 24 h, the MCs of the woodchip fuel were found to be between 17 and 50%. For statistical analysis these were grouped into three categories: low MC (< 25%); medium MC (26–39%); and high MC (> 40%). Different species of wood were also tested (larch, chestnut, and oak, and an unspecified mixture). A summary of the different conditions under which emissions were measured is given in Table 1. 2.2. Gaseous and particulate sampling Sampling was undertaken in October/November 2004 and July 2006. The boiler was run for 2 h following start-up so that conditions could stabilise before emissions testing. The sampling port in the flue was 1.5 m above the boiler. Inorganic flue gas concentrations were recorded every 5 min by two gas analysers: a Xentra 4900 Continuous Emissions Analyser with a 4995C Sample Conditioning Unit (Servomex Group Ltd, UK) for CO, NO and O2; and a Testo 350XL (Testo Ltd, UK) for CO, NO, NO2, SO2 and O2. In addition, discrete samples were taken for PAH analysis in the laboratory using an ‘isokinetic sampling system’ (Clean Air Instrumentation, France); similar sampling equipment/methods are widely used (Ramdahl et al., 1982; Oanh et al., 1999; Li et al., 2001). Gases were drawn from the flue through a heated probe followed by a series of glass components, the first of which contained a heated (125  C) quartz fibre filter (Whatman 934AH, UK) for collecting particulates, including particulate-bound PAHs. The filter is heated so that gaseous PAHs do not condense onto the filter. The gases were subsequently cooled to <20  C by passing through a condenser encased in cooled liquid to a glass holder containing w40 g XAD resin (Supelco, UK), which trapped the gaseous-phase PAHs. This was followed by a series of four glass impingers

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Table 1 Mean concentrations (normalised to 10% O2) of various pollutants from different fuels under different boiler operation (F, full flame; S, slumber, n.m., not measured) MC

Wood type

Boiler mode

No. of samples

CO ppm

NO ppm

% O2

Particulates (mg m3)

Gas-phase PAH (mg m3)

Particulate-phase PAH (mg m3)

Total PAH (mg m3)

Low

Mixed

F S F S F S F S F S F S F S F S

4 2 4 1 3 3 1 1 3 1 1 2 2 1 1 1

677 521 600 2160 234 1048 750 1367 4744 4709 275 2511 2060 1938 96 6002

251 194 159 109 140 80 68 76 196 93 28 357 127 124 117 62

12.8 12.7 14.0 15.1 15.5 16.8 16.6 17.6 15.4 18.7 15.4 19.1 18.5 17.3 11.5 17.9

62.4 56.6 37.7 34.2 23.6 20.7 34.0 160.9 537.6 141.9 119 180.2 400.5 70.8 n.m. 127.8

15.2 47.5 26.1 285.9 17.4 228.5 6.8 1586.3 551.1 1252.3 197.9 190 249.3 357.7 0.2 1324.4

0.4 0.6 0.1 10.2 1.1 8.2 0.03 45.4 46.9 45.5 1.1 51.4 64.9 13 1.2 22.2

15.6 48.1 26.2 296.2 18.5 236.7 6.83 1631.7 598 1297.8 199 241.4 314.2 370.7 1.4 1346.6

Oak Medium

Chestnut Oak

High

Chestnut Larch Oak

n.m.

Mixed

to collect any condensate; the final one contained silica gel to prevent moisture reaching the pump. The volume of air sampled was 0.20–0.52 m3 and the sampling rate was 0.11–0.22 m3 h1 with a total sampling time of 3 h. Glassware was rinsed three times with dichloromethane (DCM) between samples. Filters were heated at 400  C for 4 h prior to sampling to drive off organics. Samples were wrapped in aluminium foil to prevent degradation by UV light then stored at 18  C until analysis. Ambient air, rather than flue gases, was sampled for field blanks. Prior to PAH extraction the mass of particulates on the filter was determined.

250  C. The electron impact mode was used with an ionizing energy of 70 eV and the machine was operated in splitless mode. The target compounds were quantified in selective ion mode. Selected samples were also analysed in full-scan acquisition for compound confirmation. Sixteen PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3cd]pyrene, dibenzo[a,h]anthracene, benzo[g,h,i]perylene) were identified and quantified. 2.4. Quality control

2.3. PAH extraction and analysis The method of extraction and analysis was adapted from a method for sediment samples (King et al., 2004). All glassware was soaked in Decon-90 detergent overnight, rinsed in ultra-pure water followed by DCM and combusted at 400  C for 4 h. Deuterated internal standards (400 ng each of acenaphthene-d10, phenanthrene-d10 and perylene-d12; Supelco, UK) were added to each sample (either XAD resin or filter) to normalise for loss of compounds during analysis. Copper granules and anhydrous Na2SO4 were added to remove sulphurous compounds and water, respectively. PAHs were extracted by ultrasonication for 30 min in 150 ml DCM (glass-distilled grade; Rathburn Chemicals Ltd., Scotland). The XAD resin and solvent was filtered under vacuum to separate the XAD from the solvent. The extracts were reduced to 1.5 ml by rotary evaporation and N2 blow-down, then analysed by GC–MS. The GC–MS is a PolarisQ (Thermoquest) gas chromatograph equipped with an AS2000 Autosampler injector and Ion Trap Detector fitted with an RTX5 MS fused-silica capillary column (30 m  0.25 mm i.d., 0.25 mm film thickness). The injection volume was 1 ml and the helium carrier gas was maintained at a constant pressure of 14.5 psi. The initial oven temperature was 40  C, with an increase of 20  C min1 to 120  C, then 3  C min1 to 300  C where it was held for 3 min. The injector temperature was 250  C, the transfer line was 300  C, and the ion source was

The Servomex gas analyser was calibrated twice-weekly with N2, 500 ppm CO in air, and 500 ppm NO in N2 (BOC, UK) and the drift noted (typically <20 ppm CO, 20 ppm NO, 0.1% O2). The Testo gas analyser was recently calibrated by the manufacturer and readings were regularly checked. Calibration of the samples on the GC–MS was performed with a standard mix of 16 PAHs (Supelco, UK) at concentrations of 267 ng ml1. These were run with samples to check column performance, peak height and resolution. Solvent blanks, laboratory procedure blanks, and field blanks were also run with each batch of samples. The PAH extraction and analysis method was validated in a recovery experiment: recovery efficiencies for individual compounds were 52.5–99.5% for XAD resin and 42.8–87.1% for filters. The relative standard deviation was 2.0–33.7% for XAD and 6.5–43.4% for filters. Laboratory and field blanks showed no contamination. In addition, all pollutant concentrations have been normalised to 10% O2. 3. Results and discussion 3.1. CO, NO and O2 in flue gas Fig. 1 shows the typical cycles of boiler operation with periods of slumber and full flame. Full flame conditions were the standard operating conditions, characterised by efficient combustion, with low CO, high NO and low O2.

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400

O2 NO CO

respectively); however, NO emissions were generally higher in full flame than slumber conditions for fuel of the same MC and type (Table 1). Much of the NO is derived from nitrogen in the fuel so more is released during efficient combustion (Glarborg et al., 2003); although thermal formation of NOx also occurs. These results are similar to those in the literature: NO levels in a 30 kW residential boiler in glowing or flaming phases of operation were w20 and 100 ppm, respectively (Olsson and Kjallstrand, 2006) and total NOx concentrations for a range of automatic boilers from a number of European studies were w50– 130 ppm (Hustad et al., 1995). The range of mean concentrations of CO was higher in slumber (521–6002 ppm) than full flame mode (96– 4744 ppm), as shown in Table 1. These CO levels are relatively high as this is a small, older style boiler with little automated control over combustion conditions. Nonetheless, this type of boiler is common, particularly in Northern Europe, and other studies have found similar levels. For example, w50–500 ppm from spruce wood in a 50 kW boiler (Launhardt and Thoma, 2000); and 172 and 4635 ppm in flaming and glowing modes, respectively, of a 30 kW boiler (Olsson and Kjallstrand, 2006). In contrast, CO and NO concentrations from a 500 kW woodchip boiler were just 20 and 95 ppm, respectively, with a four-fold CO increase in low thermal output (Lundgren et al., 2004). Thus NOx levels were similar to those observed at Flimwell, but CO was much lower due to the more modern and larger boiler. Hustad et al. (1995) report CO levels of over 4000 ppm for a range of poor standard automatic boilers, but less than 200 ppm for those of a high standard.

6000

full flame slumber

5000

300 4000 3000

200

2000 100 1000

Concentration of CO (ppm)

0 0

0

:0 20

0

:0 19

0

:0 18

0

:0 17

0

:0 16

0

:0 15

0

:0 14

0

:0

:0

13

11

:0

0

0 12

Concentrations of O2 (%) and NO (ppm)

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Time Fig. 1. Inorganic gas concentrations in the boiler flue during cycles of slumber and full flame.

There were exceptions however, when high MC wood (larch, oak) was used due to very low combustion efficiency. When no more heat was required for heating the building, boiler operation immediately slowed down. The supply of wood for combustion decreased in frequency and the woodchip embers glowed rather than providing a hot, burning flame. The boiler was ‘ticking over’ until more heat was required. This slumber mode was characterised by inefficient, low temperature combustion with high CO, low NO, and high O2. Stack temperature in slumber mode (typically 60–90  C) was much lower than full flame mode (typically 110–180  C). Nitrogen dioxide was not included in Fig. 1 as its concentrations were generally low (<20 ppm) and typically 5% of total NOx. Sulphur dioxide was below detectable limits (1 ppm). Wood contains little sulphur (usually <0.5% compared to 0.5–7.5% for coal (Demirbas, 2004)) and a large proportion is captured by the ash (Hustad et al., 1995) so little SO2 is formed. The range of mean NO concentrations were similar in slumber and full flame modes (62–357 and 28–251 ppm,

3.2. PAHs All 16 compounds tested for were detected in the emissions from the boiler. Whether the boiler was in slumber or full flame mode, the lighter, more volatile PAHs were dominating in the gas phase. As shown in Table 2, on average, 56% (full flame) and 49% (slumber) of the gaseous emissions comprised naphthalene, followed by phenanthrene (22% and 18% respectively). These two compounds

Table 2 Mean percentage distribution (standard deviation) of individual PAHs during full flame (F) and slumber (S) mode Compound

Naphthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benz[a]anthracene Chrysene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Dibenz[a,h]anthracene Benzo[g,h,i]perylene

Gas phase

Particulate phase

Total

F

S

F

S

F

S

55.6  24.9 6.7  8.0 1.4  4.3 2.7  3.7 22.0  22.8 1.2  1.0 6.2  6.3 3.4  2.7 0.1  0.1 0.2  0.2 0.2  0.7 0.2  0.9 0.1  0.2 0.1  0.3 0.0  0.1 0.0  0.2

48.5  25.5 17.7  7.3 0.7  0.4 5.3  2.6 18.2  15.3 1.6  1.6 4.4  6.6 3.2  5.2 0.2  0.3 0.2  0.4 0.1  0.1 0.0  0 0.0  0.1 0.0  0 0.0  0 0.0  0

27.4  45.4 00 00 1.5  4.2 0.1  0.3 0.2  0.4 5.8  9.0 10.1  20.5 3.0  4.6 5.0  7.0 12.2  19.2 8.3  12.6 4.1  7.7 12.2  18.3 1.5  2.4 8.7  13.1

00 00 00 0.3  0.5 1.4  2.9 0.1  0.2 2.3  1.6 2.8  2.1 2.4  3.0 2.9  2.8 6.5  6.4 4.7  5.8 6.3  9.8 38.3  14.8 2.0  2.0 30.0  12.0

54.0  24.3 6.3  7.6 0.9  2.4 2.3  2.9 16.0  12.0 1.1  0.9 6.0  6.2 3.4  2.7 0.2  0.3 0.3  0.3 2.9  8.8 2.6  8.8 0.6  1.5 2.0  5.4 0.1  0.4 1.3  3.7

47.2  25.2 17.0  7.3 0.6  0.4 5.1  2.6 16.4  13.3 1.4  1.2 3.7  4.3 2.6  3.4 0.2  0.2 0.2  0.3 0.6  1.4 0.2  0.3 0.2  0.4 2.5  4.5 0.2  0.3 1.9  3.5

K.L. Bignal et al. / Atmospheric Environment 42 (2008) 8863–8871

together contributed 78 and 67% to gaseous PAH concentrations in full flame and slumber mode. Conversely, the high molecular weight (HMW) compounds were more abundant in the particulate phase: indeno[1,2,3-cd]pyrene, benzo[ghi]perylene made up 68% of the total particulate PAHs in the slumber mode. However, the total PAH burden was dominated by the gas phase , typically >95% (Table 1). The proportions of different compounds are consistent with the literature: 78–95% of PAH concentrations were in

a

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gaseous phase and 58–92% of total concentrations comprised 2-ring compounds from industrial stacks fuelled by coke, coal, oil or cement (Yang et al., 1998). Naphthalene was also found to be the most prominent PAH in emissions from an eco-labelled wood burning residential boiler (Olsson and Kjallstrand, 2006). A similar pattern of PAH distribution between gas and particulate phases was also recorded from poplar woodchips in a fluidised bed combustor (Saez et al., 2003) and from log combustion in

Gas-phase

100%

80% 6-rings 5-rings

60%

4-rings 3-rings 2-rings

40%

20%

0% F

S

F

mixed

S oak

F

S

chestnut

low MC

F

S oak

F

S

chestnut

medium MC

b

F

S

larch

F

S oak

F

S

mixed

high MC

unknown MC

Particulate-phase

100%

80% 6-rings 5-rings

60%

4-rings 3-rings 2-rings

40%

20%

0% F

S

F

mixed low MC

S oak

F

S

chestnut

F

S oak

medium MC

F

S

chestnut

F

S

larch high MC

F

S oak

F

S

mixed unknown MC

Fig. 2. Relative proportion of each group of PAH compounds in (a) gas phase and (b) particulate phase (F, full flame; S, slumber). Two-ring PAHs include naphthalene; 3-ring PAHs include acenaphthylene, acenaphthene, fluorene, phenanthrene and anthracene; 4-ring PAHs include fluoranthene, pyrene, benzo[a]anthracene and chrysene; 5-ring PAHs include benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene and dibenzo[a,h]anthracene; 6-ring PAHs include indeno[1,2,3-cd]pyrene and benzo[g,h,i]perylene.

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a residential heating boiler (Kakareka et al., 2005). The proportion of total PAH concentrations from log combustion in the gas phase (55 and 69% for damp and dry birch, respectively) was not as high as in our study as logs are likely to emit more particulates. Similarly, Kozinski and Saade (1998) found 7–8 times more PAHs in soot than in gas phase from the combustion of logs in a grate-furnace and this may also be explained by the increased particulate load from log combustion. There was no clear difference in PAH distribution between slumber and full flame modes (Fig. 2) in the gas phase; however, in the particulate phase there was a shift towards HMW compounds (especially 6-ring compounds) in slumber mode. Limited data exist in the literature although one study observed an increase in heavier PAH compounds in smouldering combustion (Ferge et al., 2005). PAH emissions for each fuel were higher in slumber mode than full flame mode with the exception of larch (although this was based on just one sample) (Table 1); this is consistent with inefficient combustion. However, one study on birch logs in a 30 kW residential boiler observed a decrease in organic emissions including PAHs in glowing conditions (Olsson and Kjallstrand, 2006). It is unclear why this is the case although it could be related to differences in the temperature of the combustion chamber. In slumber mode there was a significant positive relationship (p < 0.001) between the concentration of PAHs in the particulate phase and that found in the gas phase (Fig. 3). This suggests that inefficient combustion has significant impact on PAH formation in both phases. Data on the partitioning of PAHs between gas and particulate phases in boiler slumber mode has not been reported elsewhere in the literature. However, there was no clear relationship in full flame mode. Ferge et al. (2005) also observed that in normal boiler operation an increase in gas-phase PAHs was not associated with an increase in particulate-bound PAHs. The range of mean PAH concentrations was higher in slumber mode than full flame boiler mode (48.2–1631.7 and 1.3–641.1 mg m3, respectively). This is within the range reported, e.g. <10 mg m3 for high standard

50

PAH concentration in particulate phase (µg m-3)

45 40 y = 0.03x + 1.67 2 R = 0.84 P < 0.001

35 30 25 20 15 10 5 0 0

200

400

600

800 1000 1200 1400 1600 1800

PAH concentration in gas phase (µg m-3) Fig. 3. Relationship between PAHs in gaseous and particulate phases in boiler slumber mode.

automatic boilers and up to 100 000 mg m3 for manual stoves and wood log boilers (Hustad et al., 1995; Pagger and Theuermann, 1996; Launhardt and Thoma, 2000). Similarly, in tests on a range of stoves and boilers, PAHs were 30 times higher from a domestic furnace employing an old technology relative to a more modern one (Launhardt et al., 1998). The technology employed is obviously critical in affecting PAH concentrations as is the form of the fuel. PAH concentrations from logs are generally much higher than from woodchip, e.g. 5000 mg m3 from dry birch logs (Kakareka et al., 2005), 6318 mg m3 and 1015 mg m3 from dry beech logs on a stove with old and modern technology, respectively, and 22 734 mg m3 and 1391 mg m3 from dry spruce logs on the same stoves (Launhardt et al., 1998). PAH emissions from wood are generally lower than from coal: Lee et al. (2005) found emission factors from a controlled open fire of up to 22 mg kg1 and up to 8 mg kg1 from coal and wood, respectively, for individual PAH compounds. Emissions of PAHs from different fuels in a 30 kW fixed-bed boiler were similar from sawdust briquettes, sawdust–coal briquettes and woodchips (w200– 250 mg kg1); while pure coal produced 7–8 times more PAH (Ross et al., 2002). However, Truesdale and Cleland (1982) found that wood emitted more carcinogenic PAH per energy unit than bituminous coal in a conventional stoker. Similarly, Oanh et al. (1999) found that wood and coal briquettes emitted the same amount of PAH per mass of fuel burnt but that wood produced double the amount of genotoxic PAHs. Data on PAHs emitted by other fuels are lacking. PAH emissions from peat were almost 10-fold higher than from wood combustion in a hot water boiler (Alsberg and Stenberg, 1979). PAH emissions from gas combustion are likely to be low, although there is little data in the literature; and there is no data on domestic oil heating systems (Wild and Jones, 1995). More data are clearly needed to compare different fuels: some studies have concluded that the type of burner has more influence on the emissions than the fuel (Mastral and Callen, 2000) so it is difficult to compare data from different studies where different burners have been tested. 3.3. Relationship between concentrations of PAHs and CO The highest levels of PAHs were associated with low NO, high O2 and high CO (Table 1). These conditions are consistent with inefficient combustion and therefore support our initial hypothesis that higher levels of PAHs will be produced in these conditions. The most useful relationship was between PAHs and CO (Fig. 4), which was particularly significant (p < 0.001) in full flame condition. The slope of the three regression lines is similar with more PAHs being formed in slumber operation over the range of CO concentrations detected. The slope of the relationship is similar to that found by Vierle et al. (1999) who tested a range of biomass material including wood and grass material and pellets in a 50 kW house heating system. Findings by McKenzie et al. (1995) support our data that high CO concentrations are indicative of high emissions of organic compounds. The converse was also found by Launhardt et al. (1998). Individual PAHs showed less clear relationships with boiler conditions than total gas or total particulate-phase PAHs, but generally the HMW

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2000

all data y = 0.7x - 105 2 R = 0.62 P < 0.001

full flame slumber

1800

Total PAH (µg m-3)

1600 1400 1200

full flame y = 0.6x - 108 2 R = 0.64 P < 0.001

slumber y = 0.8x - 111 R2 = 0.59 P = 0.02

1000 800 600 400 200 0 0

1000 2000 3000 4000 5000 6000 7000 8000 9000

CO (ppm) Fig. 4. Relationship between the total PAH emissions and CO during wood combustion. Separate regression lines are presented for emissions from the boiler operating in slumber mode, in full blast and for all the data combined.

compounds only showed a positive relationship with CO in the particulate phase (data not shown). Caution must be used in applying this relationship to all furnaces as PAHs may be present in an oxygen-free environment but no CO will theoretically form as CO is a product of incomplete oxidation (Levendis et al., 1998). For example, during slumber modes, air ingress is minimised and oxygen may be depleted. In addition, in high combustion temperatures the relationship may not hold true: the optimum temperature for PAH formation is 700– 900  C and PAHs are thermally decomposed at higher temperatures (McGrath et al., 2001). Levendis et al. (1998) found that increasing gas temperatures from 1000 to  1300 C increased CO by up to three times, but PAHs dropped below the detection limit. Also the relationship may vary with the form of wood-fuel in terms of logs, briquettes or pellets. Nonetheless, as the model is nonspecific with regard to fuel type (MC and species) and boiler operation (full flame includes a range of conditions from full to low loads on the boiler) it is likely to have broad applications across a range of boilers. The relationship between CO and PAHs will be useful for predicting PAH concentrations from CO concentrations from residential wood-fired boilers where facilities, time and/or cost prevent direct measurement of PAHs. This will aid emissions inventories and modelling work including predictions of ambient air quality.

3.4. Particulates Particle concentrations were between 6 and 4427 mg m3, with an overall mean value of 262 mg m3. This is similar to the range of values reported by Hustad et al. (1995) derived from a number of studies on automatic boilers of 50–150 mg m3. Johansson et al. (2003) measured concentrations of 34–240 mg m3 from wood pellet and briquette combustion. There was no significant difference in the mass of particulates emitted by the boiler under slumber or full flame conditions. This differs from Johnson (2006) who found that ambient particulate

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concentrations in the vicinity (<46 m) of a residential wood boiler were affected by boiler operation with peak levels (416 mg m3) during air intake within 1 h of fuel loading compared to 22–24 h later when air ingress was minimal (115 mg m3). Johansson et al. (2003) observe from literature data that particle mass concentrations increase with excess air so might be expected to increase in full flame operation. They analysed their own data with data from the literature and found no relationship with fuel moisture. Our data are consistent with this observation although two samples with significantly higher particles than the rest were emitted by high MC fuels. There is much concern for public health due to particulates from residential wood combustion; however, emissions are very dependent on the type of device and the technology. For example, emissions of particulates from combustion of wood logs in five old-technology boilers were 16 times higher than from six modern boilers. The best performing of these modern log boilers used dry fuel and had water storage tanks and gave similar emissions to pellet burners (Johansson et al., 2004). There was no clear relationship between the amount of particulate-bound PAH or total PAH and the total mass of particulates. This is consistent with the findings of Hays et al. (2003) who studied particulate emissions from combustion of Douglas fir and white oak logs in a residential wood stove. Consequently, particulate emissions are not a good predictor of particulate-PAH or total PAH concentrations. 3.5. PAHs and CO concentrations in relation to fuel parameters and boiler conditions There was no consistent effect of the species of wood used as fuel across the MCs and boiler modes tested on the concentrations of PAHs and other gases (Table 1), although this could not be tested statistically due to limited replication. PAH and CO concentrations were generally higher in slumber than full flame conditions for each fuel tested. In addition, in full flame conditions there was no real difference between low and medium MCs, and the high MC samples emitted 3 times more PAHs than low and medium MCs. In slumber conditions, however, MC was of little importance as relatively high levels of PAHs were emitted for all fuel types. A similar trend was observed for CO; for example, in full flame high MC oak emitted 2 times more CO than low and medium MCs, and high MC chestnut emitted 19 times more CO than medium MC. However, in slumber CO emissions were variable and high across the fuels tested. For statistical analysis the species data were pooled across the MCs and these differences were found to be significant i.e. in full flame mode, PAH and CO emissions were significantly higher (p < 0.01, and 0.05, respectively) from the high MC woodchip. Therefore, the operation mode of the boiler is the primary factor affecting CO and PAH emissions and MC of the fuel is of importance for MCs over w40% in full flame mode, but has little impact in slumber. Our data are consistent with the literature whether for open burning of biomass (Jenkins et al., 1996a, b) or under controlled combustion in a boiler/furnace (Vierle et al., 1999). Jenkins et al. (1996a) found that although fuel type

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did affect PAHs emissions to some extent, the primary factor was combustion conditions, with emissions varying from 5 to 683 mg kg1 depending on combustion efficiency. Launhardt et al. (1998) tested spruce and beech fuels with a range of combustion technologies (wood furnaces, stoves and a boiler) and found that the largest factors affecting CO, hydrocarbons and PAHs were incineration technique and water content of the fuel. For example, PAH emissions from wet (MC >30%) spruce logs were 5 times higher than from dry logs (MC 15–20%). Dry birch had lower CO and hydrocarbons than wet birch in a fireplace (Dasch, 1982). Gas and particle emissions continue after the flame has gone out ; 20–30% of the hydrocarbon and CO were emitted during smoulder stage in a wood-burning fireplace (Dasch, 1982). Similarly Lundgren et al. (2004) observed that in a comparison between a smaller (50–150 kW) and a larger combustion chamber (150–350 kW) combustion in the smaller chamber was more sensitive to MC of woodchip fuel. 4. Conclusions The combustion of biomass was shown to release a range of PAHs, in both the gas and particulate phases. Average total PAH concentration in slumber mode (494 mg m3) was higher than that in full flame mode (265 mg m3). Overall, approximately 89% of SPAHs were present in the gas phase. In order to maximise combustion efficiency and minimise PAH emissions woodchip fuel should have a low MC, and the boiler should be operated in full flame mode. To minimise slumber modes, users should avoid woodfired boiler use in warmer weather when there is a low demand on the boiler; and install a boiler with the minimum capacity necessary (again to maintain a high load and thereby reduce slumber mode). In terms of boiler design, slumber modes should be avoided by increasing the automatic operation capability of wood-fired boilers, by automatically varying fire rates and having auto-start capabilities. Solid biomass fired combustors potentially have more ‘adjustability’ in optimising their performance to minimise emissions due to the relatively slow burn compared to gas. In principle, harmful emissions can be minimised by improved combustor design, through extending combustion times and modifying combustion chamber temperatures. Acknowledgements This research was funded by EU Interreg IIIA (Grant No. 110/118). We thank Dr Fabrice Cazier at Universite´ du Littoral Coˆte d’Opale, Dunkirk, France for the loan of the isokinetic sampling system. References Alsberg, T., Stenberg, U., 1979. Capillary GC–MS analysis of PAH emissions from combustion of peat and wood in a hot water boiler. Chemosphere 8, 487–496. Biomass Task Force, 2005. Biomass Task Force Report to Government. HMSO, London. Boman, C., Nordin, A., Thaning, L., 2003. Effects of increased biomass pellet combustion on ambient air quality in residential areas –

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