Biomass burning in obsolete and inefficient small scale combustion appliances can greatly affect air quality, even in mountainous regions (like the Alpine Region) where biomass is strongly used as fuel for domestic heating. The contribution of biomass in terms of primary energy used for residential heating can reach percentages up to 80-90% in the Alpine Region and that is why it is important to know the performance of small scale biomass appliances in terms of their environmental impact, with particular reference to particulate matter (PM) atmospheric emissions.
PM is the most critical pollutant when dealing with the emissions of small scale appliances fuelled by wood biomass. PM is a mixture of solid and liquid particles suspended in air. Particles can present a wide range of granulometries, ranging from a few nanometers (2-3 nm) up to 100 microns (100 µm). We can divide the particles according to three different classes, depending on their diameter Dp :
1) ULTRAFINE PARTICLES (Dp < 100 nm);
2) FINE PARTICLES ( 100 nm < Dp < 1000 nm);
3) COARSE PARTICLES (Dp > 1000 nm).
According to the Directive 2008/50/EC only PM2.5 and PM10 (particles with an aerodynamic diameter lower than 2.5 µm and 10 µm respectively) are regulated and monitored in dedicated air quality networks at EU level in order to verify the compliance of their daily or annual concentrations to specific limit values (in particular, the daily limit concentration value of 50 µg/m3 for PM10 and the annual limit concentrations of 40 and 20 µg/m3 respectively for PM10 and PM2.5).
Since biomass combustion has been scientifically recognized as the major source of PM primary emissions, PM10 concentrations in the Alpine Region can raise up to values that greatly exceed the daily limit value of 50 µg/m3, mainly in the winter season, when residential combustion adds a big PM emissive contribution to the already critical conditions worsened by the occurrence of meteorological phenomena (thermal inversions) that limit the atmospheric dispersion of pollutants during most part of the day.
Some small PM fractions like the polyciclic aromatic hydrocarbons (PAHs, with particular reference to Benzoapyrene, BaP) and the black carbon (BC) (detected through PM chemical analysis or thanks to high precision optical instruments) are markers of biomass burning and are harmful to human health. The smallest particles can actually find their way deep into our lungs and enter the bloodstream, causing cardiovascular and respiratory diseases.
Knowing the performance of our small scale biomass appliances is crucial, starting from their contribution in terms of PM emissions since there are more orders of magnitude of difference in terms of PM emission factors considering all the potential technologies sold on the market. For example, the impact in terms of PM emissions of an open chimney can be almost 100 times higher than the impact of the most efficient pellet boiler on the market. Nevertheless, it is noteworthy to underline that, even when we choose the most efficient small scale biomass appliance, the fuel quality is of primary importance and a bad fuel quality can have a great impact on its performance, increasing PM atmospheric emissions by up to 30%.