Emissions from domestic solid fuel burning appliances (wood-heaters, open fireplaces)

Technical Report No. 5
J. Gras, C.Meyer, I. Weeks, R. Gillett, I. Galbally, J. Todd, F. Carnovale, R. Joynt, A. Hinwood, H. Berko and S. Brown.
Environment Australia, March 2002
ISBN 0 6425 4867 6


Prior to this study, little quantitative information has been available on the composition of emissions from Australian heaters burning Australian solid fuels, particularly toxic components. The study comprised three main components, a review, a survey and a series of composition measurements. Together this is the most extensive study of its type undertaken in Australia and provides significant advances in the understanding of wood-smoke emission properties in the Australian environment.

The review, which was carried out by Associate Prof. J.J. Todd of the University of Tasmania, draws on a wide range of Australian and international sources (see Technical Report No. 4: Review of Literature on Residential Firewood Use, Wood-Smoke and Air Toxics). It consolidates current knowledge on domestic solid-fuel-burning appliances, including distribution, fuels and emission testing in Australia, together with the historical development in these areas. It also addresses issues of toxic components and health effects drawing more on overseas literature and experience.

The second component of the study is an independent survey that provides new (as of winter of 2000) information on the number and types of wood-burning appliances in use throughout southern Australia, and current fuels. Additional information on operating parameters relevant to emissions was included. The main survey, conducted by Strahan Research, Melbourne, sampled 1007 respondents from 3567 households, giving 95% confidence of responses within 3.1% of national and 6.9% of state population values.

Combustion heaters were used by 78.6% of respondents and 19.3% used open fireplaces. Most heaters were aged, with 57.1% of heaters seven or more years old and thus potentially not meeting any emission standards. Another 38% of heaters should have met the 1993 standard at installation. This suggests scope for emission reduction by encouragement to move to current-standard heaters. Overnight burning was common (34.3% of respondents), 28.3% burnt 24 hours per day. Hardwood was the most common fuel but species varied between states. A survey of wood merchants, by CSIRO, showed that redgum, followed by box species was mostly supplied in Melbourne, Adelaide and Canberra, jarrah was dominant in Perth, and mixed eucalypt species were common for Hobart. Exclusive softwood use was minor (1.2%) and 13.2% of respondents used mixed softwood and hardwood. New estimates of the amount of fuel consumed were obtained. Approximately one half of respondents (49.6%) collected wood from private land, and 41.2% purchased wood from a wood merchant. Most respondents (86%) reported storing fuel in a covered area before burning. The survey implicitly shows a high (but not universal) awareness of the need to not overload heaters and to burn dry fuel.

The main component of the study involved collection and analysis of emissions from 45 controlled burns. This component provides substantial new information on the composition and concentrations of emissions for Australian conditions. Four appliances were tested, two AS4013:1999-compliant combustion heaters (with emission ratings of 0.9 g/kg, and 3.7 g/kg, representing the mass of smoke emitted per kg dry weight of fuel consumed), a 1985-manufacture pre-AS4013 heater and a fireplace insert. Fuels included three eucalypts - jarrah, bluegum and redgum and one softwood (Pinus radiata) with a range of fuel moisture, also a commercial manufactured fuel produced from softwood and hardwood. All tests were conducted in a controlled, AS4013, laboratory setting.

Concentrations of more than 120 gas and aerosol species, comprising toxic species on the Commonwealth's Living Cities Air Toxics priority list, and criteria pollutants were determined with supporting combustion, gas and aerosol parameters. Overnight burns and burns of freshly felled timber and oven-dried low-density pine were included. The time evolution of selected emissions was investigated.

The study was not intended to 'audit' the AS4013 status of the tested heaters with several areas differing from AS4013 by design. But, general conclusions about relative mass emissions from the tested appliances were possible. For seasoned redgum the low emission (0.9 g/kg) compliant heater performed better than 'rated' over three low burns (0.5 ± 0.2 g/kg). The open fireplace insert also performed within the 'nominal' emission standard giving 2.3 ± 0.4 g/kg, as did the non-compliant heater, giving 3.5 ± 0.6 g/kg averaged over three low flow, one high flow and two overloaded low burns. For this appliance the thermal efficiency was low (38 ± 2%) indicating excessive bypass or leakage. The higher emission compliant heater (rated 3.7 g/kg) that was used for most emission tests gave a mean emission rate of 1.8 ± 0.6 g/kg from four high burns and 1.4 ± 0.3 g/kg for all 10 burns of seasoned eucalypt. On low flow this heater gave variable emission factors from 1.8 to 21 g/kg averaging 12.6 ± 5.6 g/kg for three burns of seasoned redgum. Combining the high and low flow burns gives a mean emission rate of 7.1 ± 4 g/kg, which for one standard error falls within the 'nominal' rating. The high variability in mass emissions for the low flow settings is not unusual because burning includes many 'random' or 'chaotic' processes.

Mass emission factors were consistently greater for softwood than eucalypt, expressed either as g/kg or g/MJ; all values for pine exceeded 7 g/kg. No substantial differences in mass emissions were observed between eucalypt species. The emission factor for manufactured logs was intermediate between those of eucalypt and pine.

Mass emissions showed a broad minimum for fuel moisture of about 20–30% although burn parameters were not uniform across the moisture range. The most consistent relationship found was between mass emission factor and combustion efficiency. Greatest mass emissions occurred for burns with the lowest combustion efficiencies and vice versa. No consistent relationship was evident between combustion efficiency and fuel moisture, relative to other causes of variance (such as fuel type and burning condition).

Emission factor summary data were derived on the basis of fuel type, irrespective of other burn conditions with a best estimate weighted average based on burn conditions. These compilations provide extensive new data on mass emission factors for toxic species that can be scaled to commonly measured factors such as mass, CO or total VOCs.

Most of the composition differences between smoke composition from eucalypts and softwood burns can be related to the overall mass emission factors 4.5 g/kg (eucalypt) and 15.8 g/kg (softwood). Exceptions include chloromethane, NO, NO2, NOX, sulfuric and acetic acids, aerosol phthalate, F, S and Cl where emissions relative to aerosol mass were greater for eucalypt than softwood burns. No detectable PCDD/F was detected for the softwood burns to within the background uncertainty (1 ng/kg), whereas significant levels were detected in hardwood burns (overall, 7.5 ng/kg). Polycyclic aromatic hydrocarbons (PAHs) were greater for softwood burns, typically by a factor of about 6.

Many of the emission factors are comparable to literature reports for modern heaters in other jurisdictions. This includes the BTEX species benzene, toluene, ethylbenzene and xylenes for which the best estimate emission factors from this study are comparable with or less than those from modern north-American non-catalyst and catalyst-equipped heaters and significantly less than those from conventional heaters. Carbon monoxide emission factors are about double and the formaldehyde emission factor is typical of recently reported values. For the sixteen US EPA priority PAHs, the combined emission factor is around one half or less than values for modern US non-catalyst and catalyst-equipped heaters and for the seven most toxic species the study best estimate emission factor is typical of reported values. For dioxins the best-estimate emission factor of 4.1 ± 0.9 ng/kg falls within the range of values for wood heaters and open fireplaces (0.2–28.5 ng/kg) but is typically a factor of 2 or more greater than emission factors used for emission inventories in a number of jurisdictions. In part this difference may be because of the unique, wide-ranging combustion parameters in this study, also much of the reported data on dioxins derives from a small number of independent determinations.

Control of aerosol mass emissions through AS4013 should control emissions of most of the species considered. Two clear exceptions are nitrogen oxides and dioxins, where it appears that operation parameters that reduce aerosol mass emissions work to increase the emissions of these species.

The temporal pattern of emissions observed using a series of real-time measurements tended to be reasonably well organized. Smoldering before widespread pyrolysis was observed to be of particular importance for particulate and gaseous emissions (pyrolysis is the thermal breakdown of cellulose and hemicellulose into volatiles and char that provides the immediate fuel sustaining the combustion).

Very high ultrafine particle number concentrations were observed during the entire burn, generally being greatest early in the burn. Particle size distributions appear lognormal with initial geometric mean diameters around 0.2 µm, progressively falling to around 0.05 µm later in the burn.

Emission guidelines and standards worldwide, whilst achieving similar goals of limiting or controlling the emission of smoke from residential wood combustion, have a low degree of compatibility in a quantitative sense. In general, this is due to the local or regional nature of standards and their independent evolution. Selection of emission standards should be rigorously based on a risk-benefit analysis of wood-smoke exposure and the benefits to the community of wood heating. Overall, the existing Australian standards and procedures are relevant tools for reducing exposure to toxic components in wood-smoke. Major deficiencies in the control strategy are the lack of a comprehensive risk analysis of wood-smoke constituents and the absence of any programs to assess the effects, if any, of the introduction or tightening of emission standards on exposure and risk. Several potentially effective control strategies have not been fully implemented. For example there is no current method for auditing the performance of production model heaters and no program for assessing in-service performance of installed heaters. There are no controls on the sale of used heaters and some states have not legislated to enforce the sale of only AS4013-compliant new heaters. Many local jurisdictions could be more effective in dealing with nuisance smoke emissions, and accurate risk assessments would strengthen the case for such actions.