K, Na, Cl

are semi-volatile in combustion processes. That means that a part of these elements vaporises during combustion and escapes the combustion zone with the flue gas while the other part remains and forms bottom ash. The relation between the amount that vaporises and the amount remaining in the ash has in some investigations been related to the relation between water soluble K, Na and Cl compared to the total amount analysed in the fuel.
Solid biomass fuels with high concentrations of these elements are sometimes pre-treated by leaching in water or simply by letting the material be weathered by rain fall e.g. straw. The amount of water soluble gives an idea on the maximum amount of these elements that could be separated from the fuel prior to the combustion by this type of pre-treatment.
The negative effects of Na, K and Cl on combustion equipment are described in main ash forming elements.