COMMERCIAL HEATING I


n today’s decarbonising economy, the pressure is on to reduce more than just CO2. Nitrogen oxides (NOx), the pollutants behind acid rain, smog, and serious respiratory problems, are increasingly in the spotlight. Tightening legislation across the UK, Europe, and globally is driving burner manufacturers to rethink combustion design and emissions control.


The current landscape: what’s achievable today


Modern burner technology has made significant progress in reducing NOx emissions. Traditional atmospheric gas burners typically emit between 80–120 mg/kWh. In contrast, modern low NOx models can reduce emissions to below 80 mg/ kWh, while advanced systems now achieve levels consistently under 30 mg/kWh, equivalent to approximately 9–15 ppm at 3% O2 in optimal conditions. Riello, offers a range of burners designed to meet these demanding performance standards. ¡ The RX Series is developed specifically for low- emission applications, achieving NOx levels below 30 mg/kWh. ¡ The RS/E Series incorporates modulating control to maintain a precise fuel-to-air ratio, supporting efficient combustion and reduced NOx output even during load variations. Building on this, Riello’s RS ULX Series offers further emissions reduction without the need for external flue gas recirculation (FGR). Using Riello’s patented ULX combustion technology, the RS ULX consistently achieves NOx levels below 40 mg/ Nm³ at 3% O2, and as low as 30 mg/Nm³ at 3% O2 in optimised installations. This performance is made possible through


enhanced internal gas recirculation and an advanced burner head design. These features make the RS ULX Series a strong option for high- temperature applications, including steam and superheated water boilers. The RS ULX is fully compliant with Class 4 of EN 676 and aligns with key regional and sector- specific NOx emissions regulations. Offering low emissions, ease of integration, and reliable performance, it provides a practical solution for sites working to reduce environmental impact within recognised industry frameworks.


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How low can you go? the race to cut NOx emissions in burner technology


Nathan Chance, National C&I technical & sales manager at Riello Burners in the UK discusses how low we can realistically go with NOx. Or are we approaching a natural limit defined by physics, chemistry and cost?


The Science of NOx: why it’s hard to eliminate


NOx is a by-product of high-temperature combustion, especially when burning in air (which is 78% nitrogen). The main mechanism, thermal NOx, forms when nitrogen and oxygen in the air combine at flame temperatures above 1,300°C. Other types of NOx, such as prompt NOx or fuel NOx, are less common in gaseous fuels but become relevant when burning oil or biomass. Lowering flame temperature is the most direct


way to reduce NOx, but this comes with trade-offs. Go too low, and you risk flame instability, incomplete combustion, and carbon monoxide (CO) formation, a different, but equally problematic pollutant.


Technology in action: how low-nox burners work


Above: Nathan Chance, national c&i technical & sales manager at Riello Burners


8 November 2025


To navigate these challenges, modern burners use a combination of techniques: ¡ Staged combustion, splitting the fuel and air into phases, reducing peak flame temperatures.


¡ Flue Gas Recirculation (FGR), recycling a portion of exhaust gas back into the flame, further cooling combustion. ¡ Lean premix technology, ensuring excess air is mixed thoroughly with fuel before ignition. ¡ Surface and radiant burners, spreading the flame across a larger area to reduce hotspots. The trade-offs: practical and economic limits While the technology exists to push NOx into single


digits, doing so can complicate burner operation. As NOx decreases, the risk of CO and unburned hydrocarbons increases. Highly lean flames can also lead to flashback or blow-off.


Flame instability, incomplete combustion


Achieving ultra-low NOx can be complex, but Riello simplifies the process by standardising control components across its burner range. While real-time monitoring isn’t built into the burners themselves, Riello units are compatible with MODBUS systems, allowing end users to integrate monitoring as required.


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