4.1 Heater number 2
The result for heater number 2 appeared to be anomalous. On further inspection it was found that the heater had suffered from damage and incorrect operating procedures, which would have led to pellets smouldering in the firebox outside the burn pot and in the ash drawer. It is likely that this would have led to the high emission results observed for this heater. Pellets can not pass directly to the ash drawer from the firebox under normal conditions. It is possible that the pellets were placed in the ash drawer by the householder.
4.2 Emissions factor
Appendix 1 provides the raw data from the 28 test runs, and this data is presented in the form of a histogram in Figure 1. This shows that the data is positively skewed, with a long tale to the right-hand side. In this case, pellet burner number 2 represents all five data points that form the tail in the histogram.
Emission factors are mainly used to calculate emissions inventories. Emissions inventories tend to calculate total emissions as emissions per kilogram of fuel, multiplied by total kilograms of fuel burned per time period, multiplied by the number of such burners in an air shed. This calculation is then summed with the emissions from all other types of solid fuel burners, and other sources.
Although the median may be useful as a descriptive indicator of central tendency, it is not the appropriate figure to report if one wishes to use the results to develop an emission factor. Therefore, only the mean is reported; specifically, the mean of the mean of each individual pellet burner is calculated. This estimation method avoids biasing the overall mean estimate towards the mean of the burner with the most observations.
To account for the affects of burner number 2, two cases are presented and results are presented including and excluding burner number 2. This gives an average of 3.9 g/kg and 1.4 g/kg, respectively. The 95% confidence interval around the mean is 3.9 g/kg ± 7.9 g/kg and 1.4 g/kg ± 0.7 g/kg respectively (see Appendix 2).
Figure 1: Distribution of emissions
Until recently most of the pellet fuel used in New Zealand was produced at a single plant near Christchurch. As a result, the fuel produced was relatively uniform. As with wood stoves, pellet burners tend to be optimised with a given fuel. As noted above, pellets produced recently by a plant in Rotorua appeared to be longer than those produced in Christchurch, with the result that they were fed more slowly by the auger. This had the effect of causing a noticeable decrease in the output of the appliance and is likely to have affected the emissions as well.
Other factors that may affect the performance of a pellet fuel include density, composition and moisture content.