IEA Bioenergy published a new report on nitrogen flows from biomass combustion. The key point is that combustion of woody or agricultural waste streams handles nitrogen differently than fossil fuels. With biomass, a smaller portion of the nitrogen in the fuel is released as NOx. A significant portion remains in ash or is converted into harmless N2. Therefore, a well-designed plant can absorb a net amount of reactive nitrogen.
Difference with fossil combustion
NOx from coal or natural gas is primarily formed by thermal transformation at very high flame temperatures, during which atmospheric nitrogen oxidizes. With biomass, the flame temperature is lower, and it is primarily the fuel-bound nitrogen that oxidizes to NO and NO2. In practice, approximately ten to fifty percent of the nitrogen in woody fuels is released as NOx in the flue gases. The remainder is captured in ash or returned as N2. This makes the systemic nitrogen balance of biomass more favorable than that of fossil fuels, according to IEA Bioenergy.
The three buttons in the combustion chamber
According to the researchers, NOx formation depends primarily on three factors: the nitrogen content of the fuel, the amount of oxygen in the combustion chamber, and the degree to which carbon compounds are completely burned out. With air staging, fuel staging, and flue gas recirculation, the operator can reduce NOx formation at the source. Post-treatment remains possible and effective, but requires tailored solutions for each fuel composition to prevent side effects.
From source measures to cleaning
Where necessary, selective non-catalytic or catalytic reduction is considered. The report also emphasizes the importance of proper coordination with wet gas cleaning or condensers, especially when ammonia slip from the deNOx step must be limited. The message is that there is no standard formula. The chemistry in the furnace and aftertreatment varies per boiler, fuel, and location.
Zaandam seeks balance in pH and temperature
In Zaandam, two biomass boilers together generate 3,4 megawatts of thermal power. The flue gases pass through an SNCR and a high-dust SCR, followed by a fabric filter. This is followed by a TerraoSave condenser, which can also function as a wet scrubber. According to the supplier, dust and volatile organic compounds are reduced by approximately 85 percent, NOx by 55 percent, and SO2 by 85 percent. Ammonia slip remains manageable by controlling the pH in the scrubber. At higher pH levels, NO binds to nitrate and water, while NH3 capture works better at lower pH levels. The control switches between condensing and scrubbing modes based on the return temperatures in the heating network.
Small boiler in Marum
In Marum, Groningen, a village network has been running on wood chips from landscape management since 2012. The plant has cyclone dust purification and no deNOx system. By burning residual wood in a controlled manner in the boiler, open combustion in the field has become unnecessary. The boiler demonstrates that cleaner combustion and local chains go hand in hand, but also that the choice between simple or advanced flue gas purification varies by location and permit framework.
Straw in Slagelse
The Danish case study describes a straw-fired heat and power plant. Straw has a different chemistry than wood, including nitrogen and sulfur content, and imposes different requirements for combustion and post-treatment. The report uses the case study to demonstrate that settings in the combustion chamber and flue gas path must be continually adjusted to the fuel.
What this means for policy and permits
The authors call for regulations that recognize the distinction between biomass and fossil fuels. Nitrogen formation from biomass comes primarily from the fuel itself, not from additional nitrogen in the air. They advocate for adaptive emission strategies and regional nitrogen accounting in permits. Site-specific assessments prevent generic limits from unintentionally hindering the best solutions. In this approach, biomass, if properly designed and managed, can contribute to energy security and the management of reactive nitrogen.
Sources IEA Bioenergy publication and accompanying summary December 2025.
