Magnetic properties of ash produced during combustion of various types of biomass pellets

• Type of biomass does not affect the magnetic properties of solid combustion products. • Conditions of combustion influence magnetic properties of solid combustion products. • Magnetite-like phases with different morphology are the main carriers of magnetic signal. • Different magnetic characteristic are result from different state of magnetic domains. The present study examines the magnetic properties and morphological forms of magnetic particles produced during the combustion of different biomass pellets. A variety of biomass pellets available on the market (pine, spruce, oak, beech, cereal straw, sunflower husks, and miscanthus) were combusted in real conditions using a heating boiler designed for solid fuel combustion, commonly used in domestic heating installations. Following the combustion process, the resulting fractions—bottom ash, dust collected from the upper shelf, and the fine fraction obtained from the exhaust gas duct—were collected from three distinct sections of the installation. The results demonstrated that the type of biomass does not significantly affect the magnetic properties of the combustion products. Instead, the fractions formed during the combustion of the same biomass had a much greater influence on their magnetic properties. The primary magnetic phase produced during combustion was magnetite, as evidenced by the thermomagnetic and isothermal remanent magnetization acquisition curves. The individual fractions of combustion products exhibited distinct variations in grain size, with differing proportions of single-domain grains relative to multi-domain grains. Microscopic analyses revealed the presence of various morphological structures, especially in dust samples collected from the upper shelf (e.g., spherules, plates, needle-like particles, and various forms of crusts), which also exhibited the highest values of magnetic susceptibility. This investigation of the combustion products from different biomasses is significant due to the increasing use of biomass pellets in individual and local heating systems, and these parameters can aid in the identification of combustion products.