Ash components are normally analysed by combustion of the solid biomass fuel sample under controlled conditions (so called ashing of the sample) followed by digestion and analysis of the prepared ash. During the combustion the ash forming elements in the ash reacts with the oxygen of the combustion air and forms oxides. Thereby the concentrations of these elements can be reported either as concentrations in their oxide form or in the element form.

During the ashing, all organic carbon is oxidised and the major part of it leaves the sample as CO2 –gas. However, depending on the composition of other compounds in the formed ash some CO2 could also be bound in the ash as solid carbonates –CO3.