Ash Formation in Pulverized Biomass Combustion: How Evidence, Mechanism and Model Uncertainty Propagate to Deposit Risk

Pulverized biomass combustion offers a practical route to reduce fossil CO2 emissions while retaining existing coal-fired infrastructure, but its deployment is constrained by ash-related problems—slagging, fouling, high-temperature corrosion, SCR catalyst deactivation, and fine-particle emissions. These problems originate from the high mobility of alkali metals, chlorine, and sulfur in biomass fuels: unlike coal minerals, a considerable fraction of these elements is released into the gas phase and then undergoes homogeneous and heterogeneous reactions, nucleation, condensation, and sulfation. Ash behavior is consequently governed not by bulk ash content alone, but by the occurrence of forms of inorganic elements, their release pathways, the flue-gas cooling history, and the transformation of the resulting particles. This review integrates laboratory, pilot-scale, and industrial evidence to describe the formation of coarse fly ash and submicron particles (PM1), the coupling between the two particle populations through alkali-vapor uptake, and the effects of fuel properties, combustion atmosphere, cooling rate, and mineral additives. Thermodynamic, kinetic-reactor, aerosol-population, and CFD-coupled models are compared in terms of applicability and limitations. Prediction remains limited by fuel variability, uncertain kinetic parameters, and insufficient full-scale validation; progress depends more on constraining measurable inputs through standardized, size-resolved measurements and multi-scale validation than on further increases in model complexity.