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
7. Differences between test burns and common practice
Burns conducted for limited studies, such as this study, cannot represent the full range of burning conditions that may be experienced across the community. In particular the tests reported here followed the Australian Standard procedure that attempts to minimise variance due to largely-controllable variables. The AS4013 standard test calls for the loading of a single uniform charge of fuel, onto a prepared coal bed, with emission factors determined as an integral over combustion of this fuel charge. For burns on low flow settings an initial part of the fuel is used to establish sustainable combustion before filter collections are initiated. Whilst this mode of operation of a combustion heater is fairly intuitive, and the Standard test method probably reflects general sustained heater operation, in practice the rigorous timing in the Standard method will be absent and many more random variables such as fuel size, stacking, moisture content, volume of combustion chamber filled, etc, will be experienced. Nevertheless, the practice of refilling into an existing coal bed is likely to be repeated several times per burning session (evening) for heaters operating in homes. Low density fuels, such as dry pine, burn very rapidly requiring much more frequent refuelling (one reason for their low popularity, Shelton 1983). Aspects that are less-well simulated by the standard method are the initial kindling phase, in which the heater temperature is initially brought up to several hundred degrees and also heater overloading for extended late-night burns. Across southern Australia, the practice of overnight burning is observed by approximately 34% of the households that use wood combustion for heating (see Appendix 2). For any heater, a repeated burning pattern represented by the Standard test method is likely to result in a series of impulses of relatively short duration with extended intervening periods of low aerosol mass emissions. Integrated across an air-shed such impulses probably smooth out. But, if the assumption is made that early in the evening most heaters go through a kindling phase and are then operated on a high flow setting, the integrated mass emissions across the air-shed may initially be significantly greater than the mean emission factor would suggest. Similarly, late in the evening if heaters are turned to a low flow setting and particularly if the heater is reloaded, or possibly overloaded to extend combustion, a second, late, peak in mass emissions might be expected. This is also likely to be larger than would be indicated by the mean emission factors and number of heaters in the air-shed. This pattern of variation has already been reported by Gras et al. (2001) for Launceston, based on measured light scattering data. The derived source function for Launceston had one emission peak in the early evening, around 17:00 and a later stronger peak at around 01:00; there is also some evidence for another weak morning peak around 05:00–06:00 consistent with early morning reloading.