The following elements form the bulk of biomass ashes.

They are typically present as oxides and thus the data in the database can be viewed either wt% either as element or as oxides.

Ca

is a main ash forming elements in biomass ashes. Typically the bulk part of ash from woody biomass is a mixture of oxides of the elements Ca, Si, Mg and Al.
The relation between these oxides is relevant for the melting temperature and the viscosity of the ash and thus important for slagging behaviour. CaO increase the melting temperature of ash.  Ca can however in special circumstances be a strongly fouling element forming CaCO3 on superheater tubes.

Si

is the most common bulk ash forming element in biomass. Especially in herbaceous plants, Si is present in significant concentrations as it helps the straw to stay up straight in the wind.
SiO2 forms the main matrix for the ash and slag during biomass combustion. The melting temperature depends strongly on the mixture of other ash forming elements. Especially the alkali metals K, and Na lowers the ash melting temperature of biomass ashes.

Al

One of the most common elements on earth. No really biological function in plants and thus significant concentrations together with high ash content in the fuels indicates that large amounts of contamination such as earth are included. Al has no really affect on the combustion. In some cases it can increase the melting temperature of ash and in others decreases it depending on the mixture.   Al does generally increase the melting temperature of biomass ash and is sometimes also used as an additive for this purpose.

Mg

is a component of chlorophyll, which is a part of all green parts of plants, MgO has relevance for ash melting behaviour similar to CaO.
Green parts of plants are seldom used as fuel and thus the concentrations of Mg are typically relatively low in biomass ashes.

Fe

is very common in earth minerals. Not so biological active and thus does not naturally exist in high concentrations in plant material. High concentrations do therefore suggest that the analysed samples are contaminated by dirt. Fe can in high concentrations be a significant part in slagging at coal fired units, but is seldom problematic while combusting pure biomass fuels.

K

Potassium is a major nutrient and very important for growth of plants. High concentrations are also common in fast growing plants such as straw and other agricultural solid biomass fuels. The concentration does commonly also depend on growing season of the plant i.e. in summer and spring when the growth of the plant takes place the concentrations are higher than in the winter when no growth occurs.
Potassium is volatile during combustion, and due to that typically one of the main components fouling on boilers using solid biomass fuels. For the same reason K is also typically the main component in submicron particle emissions.  Moreover, it reacts with other ash forming elements in the ash and in high concentrations it can significantly lower the melting temperature of ash and thereby cause slagging.

Na

is not a nutrient such as K. High concentrations are typically due to contamination.  Typically, plants growing close to the sea can have significant concentrations of Na.  Na behaves similar to K during combustion and therefore cause the same problems such as fouling, slagging and particle emissions described for K.

S

is relevant as parts of S forms SO2 emissions while the other part remains in the ash. The vaporised S is a major part of particulate emissions forming sulphate particles (mainly K2SO4). Formed sulphates lower the ash melting temperature and can thus be significant contributors to slagging and fouling problems in biomass boilers. However, due to competing reactions with Cl, high S to Cl ratios can in some cases be of advantage, as it can decrease Cl corrosion problems.

Cl

is relevant as it is directly related to emissions of HCl. Formed HCl gas and also chloride particles formed in flue gases are highly corrosive. Cl is typically a major component in particulate emissions from biomass combustion units, mainly in the form of KCl. Chlorides has relatively low melting temperatures and thus Chloride particles can significantly contribute to fouling. Slagging is less common as typically very low concentrations of Cl remain in the bottom ash due to the relatively high volatility of chlorides compared to oxides and sulphates.