Energetic and Environmental Performances of a Domestic Hot Water Condensing Boiler Fired by Wood Pellets

In order to assess the performances of the domestic heating boilers with a CE mark, a wood pellets fired condensing boiler has been purchased and installed in the Laboratory for tests. The objective of the study was not only to check the announced performances, but mainly to investigate the influence of the different parameters affecting the functioning of the boiler, the thermal efficiency and the pollutants (NOx and CO) emission being aimed at. Tests have been performed to study the influence of the cycling frequency of the burner functioning, the overall excess air, the return water temperature, and the output power of the boiler. As announced by the manufacturer, the tested boiler fulfils the requirements of the Blue Angel Label, but the recommended value for the excess air (1.2) can be reduced to a value of about 0.4 without excessive emission of CO. The boiler could perform better if the ON period of the cycle of the burner is increased, even better if the OFF period is suppressed. The NOx are mainly formed from the nitrogen content of the pellets and their emission can only be reduced by a lower nitrogen content of the pellets than the limit stated in the standard. Random peaks observed in the evolution of CO, the boiler being operating in steady state regime, are attributed to the combustion process of pellets which is a non-continuous process by nature.