These elements are classified as the main elements as they together typically comprise about 99 % of the dry and ash free part of the biomass sample.
Typically about 50% of plant material (d.b.) consists of carbon. Carbon is the major part that combust and thereby also the main part of the heating value.
High H-concentrations increases the heating value. However due the organic nature of biomass fuels the ratio between H and C varies very little between biomass fuels of the same specie.
Relevant for emissions of NOx from combustion. Due to the typically relatively low flame temperature during biomass combustion, formation of high temperature NOx is negligible and thereby fuel derived NOx become very relevant in biomass combustion systems. N is a major nutrient in plants and therefore fast growing plants (e.g. wheat) typically contains higher concentrations of N compared to slow growing plants (trees).
Oxygen is present in all biomass fuel mainly in form of different sugar molecules. Due to the organic nature the concentration of O is in relatively constant relations to C in all biomass fuels. High O concentration lowers the heating value of the biomass fuel compared to fossil fuels, which have relatively much lower concentrations of O. O is seldom measured and instead calculated from the analyses of other elements in the fuel.