Nitrogen flows in biomass combustion systems – Part II: Options for maximising reactive nitrogen capture and case studies

The release of reactive nitrogen 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. This has led to several legal regulations aiming at reducing the release of reactive nitrogen to the environment.

Up to recently, only little research has been done so far on the quantification of reactive nitrogen flows along the whole biomass combustion cycle, including biomass growth and harvesting.

In 2022, Task 32 (Biomass Combustion) conducted a first scoping study 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.

Now, a second report on the topic of reactive nitrogen (Nr) flows in biomass combustion systems is available.

Download the full report “Nitrogen flows in biomass combustion systems – Part II: Options for maximising reactive nitrogen capture and case studies”

The study aims to deepen the understanding of relevant formation mechanisms for reactive nitrogen during combustion. It explains why NOx emissions from biomass combustion differs fundamentally from fossil fuel combustion: NOx mainly derives from fuel-bound nitrogen rather than atmospheric nitrogen. As a result, only a fraction (≈10–50%) of fuel-N is released as NOx, with the remainder retained in ash or converted to inert N₂. In some cases, biomass combustion can act as a net sink of reactive nitrogen.

The report discussed three case studies that illustrate how design and site choices, as well as control systems, can minimise the impact of biomass combustion in terms of NOx emissions and deposition in nearby sensitive areas.

The study identifies three main factors for the formation of NOx from biomass combustion:

1. the nitrogen content in the fuel,
2. the oxygen content in the combustion chamber, and
3. the degree of combustion conversion in the flue gas.

The report recommends the prioritisation of air staging, fuel staging, and flue-gas recirculation, as primary options to suppress NOx formation at the source.  Post-combustion options remain effective, but costly, and these must be tailored to the specific chemical composition of the biomass to avoid unintended consequences.

For policy and planning, the report calls for regulatory frameworks that recognises the differences between NOx emissions from biomass combustion and fossil combustion. Moreover, the study concludes that adaptive emission-control strategies rather than one-size-fits-all approaches are needed to reflect the variability in NOx formation across biomass types and technologies.

The report urges site-specific assessments to consider the context-dependent impact of biomass combustion and recommends policy makers to include regional nitrogen budgeting into permitting processes.

Finally, the study concludes that biomass combustion, when optimised and governed with system-aware policies, should be viewed as a tool for nitrogen management, while contributing to energy security.