Relationship between fuel quality and gaseous and particulate matter emissions in a domestic pellet-fired boiler

The general purpose of this work is to evaluate the relationship between the fuel quality and the gaseous and particulate matter (PM) emissions in a domestic boiler fired with five different types of pellets, namely, two commercial pellets from pine residues, commercial pellets from cork residues, and in-house made pellets from olive wood and olive pruning residues. Initially, to evaluate if the pellets fulfill the requirements established in the European standards, a detailed physical and chemical characterization of the five types of pellets was performed. Subsequently, to examine the impact on pollutant emissions of the fuel type, all pellets were burned in the domestic boiler, each for three boiler thermal loads, and their gaseous and PM emissions were measured. Finally, to better understand the impact of the fuel ash composition on PM emissions, a number of selected PM samples were morphologically and chemically characterized. All pellets fulfilled the physical and mechanical requirements for non-industrial pellets, except in the case of one of the commercial pine pellets, which showed bulk density and durability values lower than the limits established in the European standard. In addition, pellets made from olive pruning showed higher nitrogen and sulfur contents, and cork pellets showed significantly higher sulfur content than those allowed for their use in domestic appliances by the European standard. Gaseous and PM emissions are significantly affected by the fuel type. The high CO emissions of the olive based pellets were attributed to their high particle densities and high length to diameter ratios, while the high NOx emissions of the olive pruning pellets were attributed to its very high nitrogen content. Among the agro-pellets, the cork pellets originated PM emissions lower than those from the olive wood pellets and much lower than those from the olive pruning pellets. All PM emissions were dominated by particles with sizes below 2.5 μm, which are significantly affected by the fuel ash composition, namely by the amount of volatile inorganic elements K and Na.