Measuring pollutants from wood fuelled heaters
Measuring the emissions from a biomass stove is actually a discipline in its own right, and for this reason even quite large stove manufacturers tend to use external laboratories which are equipped for this purpose.
One of the complications is that for repeatable emissions measurements it is necessary to know the thermal output of the stove during testing, and quantifying this is quite difficult.
Also, measuring the organic gaseous compounds (OGC) requires the use of an instrument called a flame-ionisation detector, which is expensive to buy or hire and requires a trained person to operate.
Different testing procedures are used around the World, meaning that it is difficult to compare the emissions figures of stoves from one regulatory block with those from another.
The main difference arises from the way that particulates are defined. In the EU, particulates are quantified by the hot filter method meaning that they only include substances which are in the solid or liquid phase at the elevated temperature of the filter (usually 180 OC).
Wood tars which are in the gas phase at this temperature are defined as ‘OGC’ (organic gaseous compounds), even though when they cool down to ambient temperature they will condense and become particulates. In North America by contrast, particulates are quantified using the dilution tunnel method, which involves cooling the flue gas down to ambient temperature before the measurement is taken.
This results in a definition of particulates which corelates more closely with that used by scientists in the air quality sector.
At present most regulations still include limits for particulate emissions on a gravimetric basis i.e. the mass contained in a cubic metre of flue gas.
However, since the health effects of particulates depend on their size, with the smallest being the most dangerous, attempts are being made to introduce regulations containing limits of the number of particulates permitted.