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

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, a global leader in combustion technology, 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.

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

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.

When it comes to cost, Riello’s new ULX range offers a route to low-NOx compliance. For large-scale installations, it presents a viable alternative – due to a lower capital outlay – to modular boiler systems or heat pumps, making it one of the most cost-effective solutions for achieving ultra-low emissions.

What’s Driving Innovation?

Regulatory pressure is the major motivator. The EU Medium Combustion Plant Directive (MCPD) and BREF BAT-AELs set increasingly stringent limits, particularly for plants between 1–50 MW thermal input. Meanwhile, California has set the benchmark with a 9 ppm NOx limit under Rule 1146.2.

Burner manufacturers must now design products that meet varying regional requirements while maintaining efficiency and reliability. Riello’s global footprint, including production in Europe and distribution networks across the UK and North America, positions it well to meet these regulatory demands.

What’s Next: Future-Proofing for Tomorrow’s Fuels

As the energy mix evolves, so must burner design. Hydrogen is one of the most promising decarbonised fuels, but it also burns hotter and faster, increasing NOx risks. 

Digitalisation is another frontier. Advanced burner models now incorporate IoT-enabled sensors and digital combustion control, allowing real-time optimisation for emissions, efficiency, and safety. In the near future, AI may play a role in adaptive combustion tuning, automatically adjusting flame characteristics to maintain ultra-low NOx across changing conditions.

There’s also growing interest in biofuels and synthetic gases, which bring unpredictable NOx profiles. Riello’s engineering teams are actively testing burner adaptations for these fuels to ensure compliance and performance.

Conclusion: So, How Low Can You Go?

Technically, burners can now achieve NOx levels once thought impossible, even below 10 ppm. But the true limits aren’t just chemical or thermal, they’re also practical and economic. Achieving these ultra-low figures reliably and affordably, across a range of fuels and load conditions, is the real challenge.

That’s why the future of burner technology lies not only in emissions reduction, but in smart, flexible systems that balance environmental performance with operational demands. With companies like Riello continuing to push the boundaries of innovation, the industry is well placed to meet tomorrow’s clean air goals, one flame at a time.

www.rielloburners.co.uk