Nitrogen flows in biomass combustion systems – Full cycle perspective

Feb 2023

A parametric scoping study aimed at optimising nitrogen flows in biomass combustion


Nitrogen flows in biomass combustion systems – Full cycle perspective


The release of reactive nitrogen (Nr) in the form of ammonia and nitrogen oxides from a wide variety of sources in society such as combustion processes and agricultural activities rises concerns due to its direct and indirect (as precursor) harm to environment and public health. It negatively impacts ambient air quality and can lead to unwanted deposition of reactive nitrogen in nature conservation areas, the latter leading to unwanted shifts in vegetation type and disturbing eco-balances. This has led to several legal regulations aiming at reducing the release of reactive nitrogen to the environment.

For the thermal use of biomass intensive debates are ongoing on the level of emissions of NOx at the stack, however only little research has been done on the quantification of reactive nitrogen flows along the whole biomass combustion cycle, including biomass growth and harvesting. It is important to address the system perspective here, since there is a fundamental difference between biomass combustion and other sources of reactive nitrogen like engines or fertilizers. During their growth cycle, plants and trees absorb nitrogen from their environment and store it in cell tissue. In most plant species, this nitrogen in lignocellulosic cell tissue solely originates from reactive nitrogen in the form of fertilizer, NOx or NH3 induced acid rain.

While the NOx emission from solid biomass combustion is typically only related to the nitrogen contained in the fuel and in practice never exceeds that level due to the relatively low combustion temperatures, NOx emissions formed from fossil sources typically results in additional reactive nitrogen that was not in our ecosystem before. The full cycle with biomass growth, harvesting and combustion can thus act as a net removal of reactive nitrogen, while combustion of fossil fuels leads to an addition of reactive nitrogen to the atmosphere.

Results from an Austrian study on small-scale combustion systems show that approximately half of the nitrogen contained in wood fuels is emitted as NOx while the other 50% are converted to harmless N2. For other biomass types or combustion systems this may be different. As almost all tree species take up the nitrogen in form of reactive nitrogen species from the soil, the energetic wood use results in a net negative Nr balance. This mechanism however depends on various parameters, such as the type of biomass being used and its ability to absorb Nr, the possible use of fertilizer for some biomass types in the growth cycle, the performance of the deNOx system in place and the possible release of ammonia slip, alternative uses for the residues, siting of a combustion plant in relation to nearby nature conservations areas, etc. Such factors should be taken into account when evaluating the possible impact of biomass combustion systems on Nr balances at a system level.

This scoping study – produced in the frame of IEA Bioenergy Task 32 (biomass combustion) – aims to identify the key parameters that influence nitrogen balances for a broad diversity of biomass combustion plants based on different technologies and fuels with varying nitrogen contents. In a subsequent study to be carried out in 2022-2024, the influence of the mentioned parameters will be studied in more detail and quantified to provide policy recommendations for optimal use of various biomass combustion systems under various conditions.


Illustration: Nitrogen flows in managed forests with high deposition and leaching. (Callesen et al, 2010)
Most of the nitrogen is contained in the residue fractions (bark, branches). Removal of such forest residues means that reactive nitrogen is directly removed from the forest, which can help avoid unwanted eutrophication.