Introduction
Controlling nitrogen oxides (NOx) emissions in biomass heating and combined heat and power (CHP) plants remains one of the most complex operational challenges in modern renewable energy systems. Based on WEProS’s experience, even well-designed installations – equipped with selective non-catalytic reduction (SNCR) systems – face significant difficulties in achieving stable and consistent NOx reduction within permissible limits.
The problem does not stem from the SNCR technology itself but from its sensitivity to the dynamics of the combustion process, fuel quality variations, and local thermal conditions in both the combustion chamber and the reduction zone, i.e., the reagent injection area. Excessively high or unstable temperatures lead to a rapid increase in thermal NOx, which is difficult to reduce effectively, while too low a temperature decreases reduction efficiency, resulting in undesired ammonia slip (this phenomenon and its consequences will be addressed in part two of this article).
Three Sources of NOx Formation and Their Control
- Fuel-NOx – formed from nitrogen compounds contained in biomass (dominant share in biomass boilers, 70–85%).
- Prompt-NOx – generated during the early combustion reactions in the presence of hydrocarbon radicals.
- Thermal-NOx – formed by oxidation of atmospheric nitrogen at high temperatures (>1200°C).
What can actually be controlled? The first two mechanisms offer limited leverage:
- Fuel-NOx scales primarily with the nitrogen content of the fuel and the mass flow of fuel (essentially with boiler load). Without changing fuel quality or load, its contribution cannot be reduced.
- Prompt-NOx typically has a minor contribution and is not a key operational variable.
The main operational lever is to limit thermal-NOx through proper air distribution (primary/secondary/tertiary), flue gas recirculation, and control of grate speed, bed length, and fuel layer thickness, keeping reaction temperatures within a safe, predictable range.
The Role of Combustion Process Stability
Combustion stability is the cornerstone of effective NOx reduction. In biomass heating and CHP plants, unstable combustion is the most common cause of NOx exceedances and SNCR inefficiency. Rather than addressing the root cause, operators often attempt to treat the symptoms by considering expensive modifications (e.g., adding an SCR stage), whereas the key lies in controlling and stabilizing the combustion process itself.
Wood-based biomass – even within the same type and supplier – exhibits variable moisture content, density, and calorific value, which directly affect the energy input to the grate and the thermal profile within the combustion chamber, the radiant section, and the convective section of the boiler. In many CHP plants, operators rely on two fixed sets of operating parameters, often determined by experience. Once set, these parameters remain static, which is anything but the kind of continuous regulation required under fluctuating combustion conditions. The fuel itself varies widely, and a typical ordered-year (duration) curve of moisture content shows how strongly biomass deviates from its nominal design value of 40–45%.
Even seemingly small variations in biomass moisture content (5–10 percentage points) can completely change the combustion dynamics. Residence times on the grate, drying, pyrolysis, and burnout zones all shift, along with local temperatures and oxygen concentrations. As a result, even a properly maintained SNCR system loses effectiveness because the temperature profile in the injection zone becomes unpredictable.
Combustion Zones on the Grate
Why the First Sections Are Critical for NOx
Combustion on a moving grate proceeds sequentially: drying → evaporation → pyrolysis/devolatilization → volatile combustion → char burnout. The first sections (drying, pyrolysis, and the start of volatile combustion) have the greatest impact on NOx formation because they define local mixing conditions and temperature peaks. A short ignition front, premature aeration, or excess oxygen in the first zones raises flame temperature and generates additional thermal NOx.
Furthermore, the fuel layer thickness and active bed length are crucial. A layer that is too thin causes uneven combustion, while too short a combustion zone results in incomplete burnout and regions where air passes into the combustion chamber without reacting with fuel. This creates local cooling, stratification of the combustion atmosphere, and ultimately an unstable temperature profile.
Boiler Settings and Nonlinearity of Air Dampers
A practical limitation in many installations is the nonlinearity of air dampers in the under-grate air boxes. The “% opening” does not translate linearly into airflow. Fixed settings (e.g., 30%/50%/70%) are misleading – after mechanical maintenance or changes in tightness, the air distribution may shift entirely, leading to uncontrolled aeration of key zones. Regular flow calibration and air balance measurements are essential, rather than relying on percentage openings alone.
How the SNCR System Works and Why It Is So Sensitive
The SNCR (Selective Non-Catalytic Reduction) system reduces NOx by reacting ammonia or urea with nitrogen oxides within a narrow temperature range – typically 850–1050°C.
Within this window, the reaction occurs:
converting NOx into molecular nitrogen. Below ~850°C the reaction virtually stops, while above ~1050°C ammonia oxidizes and reforms NOx, resulting in the opposite effect.
A key operational issue is the floating temperature profile within the injection zone. Every change in combustion conditions – such as adjustments to primary and secondary air or flue gas recirculation – affects not only the overall combustion temperature but also shifts the entire temperature profile in the region where NOx reduction occurs. As a result, the optimal reaction temperature window (850–1050°C) can move vertically, falling between injection levels.
Biomass boilers typically have two or three injection levels. When the optimal temperature window shifts between them, efficiency drops sharply, as the temperature at active injection points becomes either too low or too high for effective reduction. SNCR systems do not allow continuous adjustment of injection height – the operator can only redistribute reagent between levels, which is often insufficient to maintain optimal conditions.
The condition of injection points is also critical. Deposits of dust, slag, or ash around nozzles distort the spray pattern, impairing reagent dispersion and mixing with flue gas, thus reducing efficiency.
The SNCR performance curve is exceptionally steep – a 100–150 K deviation from the optimum can cut reduction efficiency by up to half. Therefore, maintaining a stable temperature profile in the reaction zone and keeping the injection system clean are crucial.
Another critical factor, particularly in biomass-fired units, is fouling and ash deposition on boiler surfaces. Biomass combustion produces fine particulate matter and alkali compounds that easily adhere to heat exchange surfaces and around the injection lances. This phenomenon leads to the formation of ash deposits and slag bridges that distort flue gas flow, affect local temperature distribution, and block or partially obstruct the reagent jets. In the same way, temperature sensors located in the injection zone often become covered by ash deposits, resulting in incorrect temperature readings. The measured temperature may differ significantly from the actual flue gas temperature, misleading the control system. In practice, this means that not only is the process thermally sensitive, but the instrumentation itself can introduce large measurement errors due to fouling.
SNCR Control Limits: Dosing Saturation and Ammonia Slip
SNCR works effectively only within a narrow temperature window. When the temperature profile in the injection zone drifts, the control system usually increases reagent dosing to the upper limit. Based on WEProS experience, many biomass heating and CHP plants operate at or near the maximum injection capacity. When this occurs:
- at too high temperatures, additional reagent oxidizes back to NOx instead of reducing it,
- at too low temperatures, the risk of ammonia slip (unreacted ammonia) increases,
- even at maximum dosing, emissions remain high because the chemistry and residence time do not support effective reduction.
The SNCR efficiency curve is very steep – a 100–150 K deviation can halve system performance. Simply increasing the reagent flow cannot replace maintaining the correct temperature window or ensuring clean, unobstructed injection points (slag or ash buildup distorts spray and mixing).
Is SCR Really Needed?
Many operators consider installing an SCR (Selective Catalytic Reduction) unit not necessarily to achieve deeper NOx reduction but to comply with tightening emission limits, often without recognizing that the root problem lies in combustion stability rather than SNCR limitations. Before investing in a costly catalytic system, it is crucial to stabilize the combustion process and minimize NOx generation at its source.
According to WEProS’s experience, an SCR system is not always necessary. In many cases, better results are achieved through:
- stabilizing the combustion process,
- improving flue gas recirculation,
- and enhancing control of fuel moisture.
A properly optimized combustion process – focusing on the root cause, namely process stability and combustion control – can maintain emissions below 150 mg/Nm³, and under favorable conditions even around 100 mg/Nm³, without the need for SCR.
Regulatory Limits and Outlook
Current European environmental regulations require biomass plants to maintain NOx emissions at 125–145 mg/Nm³ (6% O₂), but further tightening of these limits is expected. It remains uncertain whether existing biomass heating and CHP plants – especially older grate-fired units – will be able to meet future requirements without major modernization.
The challenge lies not only in deploying new technologies but in understanding the process-level causes of emissions. With stable combustion, proper temperature control within the SNCR reaction window, and consistent fuel quality, low NOx emissions are entirely achievable without costly catalytic systems.
Conclusion
Effective NOx reduction in biomass heating and CHP plants does not begin in the SNCR system – it begins in the furnace. It is combustion stability, consistent fuel quality, and temperature profile control that determine the success of reduction reactions. As WEProS’s experience shows, a well-tuned process with an optimal “temperature window” and properly adjusted injection levels can achieve high NOx reduction efficiency without the need for SCR.
In the next article, we will discuss the importance of water and moisture balance in biomass heating and CHP plants with condensing sections, where water management plays a similarly critical role not only in emission control but also in long-term plant durability.