Additives as a fuel-oriented measure to mitigate inorganic particulate matter (PM) emissions during small-scale combustion of solid biofuels

During combustion of solid biofuels, particulate matter (PM) harmful to humans and the environment is formed and released into ambient air. These aerosols can be distinguished by their origin, as they either originate from inorganic elements (e.g., K, Na, Cl, S, Ca, Mg, Si, P, Zn, Pb) naturally contained in solid biofuels or from the organic matrix (e.g., C, H, O) of the biofuel. While inorganic particles (e.g., KCl, K2SO4, CaO, ZnO) are usually inevitably released during combustion, organic emissions (e.g., PAHs, VOC, soot) are primarily obtained due to the incomplete fuel oxidation. Within this context, this article aims to summarize the basics of inorganic PM formation during small-scale combustion of solid biofuels; this is especially realized for wood fuels. First, some basics on PM emissions are presented; this includes overall definitions, common particle size fractions (e.g., PM10, PM2.5, PM0.1), chemical composition, and physical characteristics. Subsequently, the PM formation during thermochemical conversion of solid biofuels is described for PM of inorganic origin. Then, primary measures to reduce these inorganic PM emissions in small-scale combustion appliances are discussed. This is especially true for an additivation as one option to reduce PM of inorganic origin. Hence, mechanisms and reactions of different additives (e.g., based on alumina silicate, calcium, or phosphorus) are outlined, before conclusions regarding the impact potential of the considered additives are drawn. Finally, the overall information about reactions, products and byproducts are summarized related to the most efficient additive.