Analysis of combustion conditions for CO and PM emissions in wood pellet stoves. An experimental study

As pointed out by recent World Health Organization reports, aerosols are the cause of many diseases and premature death and the issue of reducing their production is receiving more and more attention. Besides, concern about greenhouse effect, caused also by the intense exploitation of fossil fuels, is shifting interest towards biomass fuels which have nominally zero-net emissions of greenhouse gases. Unfortunately, a poor exploitation of biomass can cause a significant generation of pollutant and particle emissions and consequently it is important to identify factors that control particulate matters (PM) in order to reduce them. In this study, an extensive set of experiments on four wood pellet stoves (8-11 kW burning power) has been performed to investigate the relations between the main operation parameters of the stove and its emissions, in particular, carbon monoxide (CO) and PM emissions are correlated to the burning-pot geometry. Furthermore, a new burning-pot model which allows to obtain very-low CO emissions is presented. As a matter of fact for optimal inlet air quantities, CO emissions can be reduced close to zero. At the same time, however, PM emissions are reduced but not as much as those of CO. This phenomenon is caused by PM composition that is composed of both organic – related to combustion quality and, as consequence, to CO levels – and inorganic matter. Experiments show also an increment of PM emissions and a wide range of PM colours deposited on filters during tests with the innovative burning-pot: there is an apparent dependence of PM release on the time from ignition. It’s likely that this effect is due to ashes accumulation on the bottom of the burning pot and to their interaction with the primary air stream that flows over them.