Minimization of inorganic particulate matter emissions with a novel multi-fuel combustion technology that enhances inorganic retention in a compact updraft fixed-bed

•Fuel-flexible biomass combustion technology with low emissions developed.•Compact updraft fixed bed served as an efficient filter for inorganic particles.•Temperature and minor element distribution measured across fixed bed height.•Different potassium retention mechanisms identified for woody and herbaceous fuels.•Conflicting literature on potassium release in fixed beds for wood clarified.