New fuel indexes to predict ash behavior for biogenic silica production

•A new fuel index (K + Na + Mg)/P [mol/mol] was defined by a Python code.•New fuel index has high coefficient of determination in the range of 800–1100 °C.•Alkali metals have higher affinity to react with Si and P to form slag.•Sieving and disintegration methods can predict macroscopic slag formation.•Microscopic-slagging initiates at mild temperatures and decreases ash porosity.