Forecasting slagging and fouling behavior in Biomass Heat and Power Plants (BMHP) is a main interest for operators and manufacturers. Several forecasting tools already exist but most of them work with idealized boundary conditions. Depending on the biofuel results of these tools give unrealistic settings, for instance to high sintering temperatures using the ash melting behavior test CEN/TS 15370-1. A new approach using a high-temperature-XRD allows observing mineral reaction within the biomass ash during heating processes. This can be used to predict actual critical temperatures and regions in the combustion chamber, where critical slagging might occur.