This webinar addressed the growing regulatory and technical challenges associated with nitrogen oxide (NOx) emissions from commercial and industrial combustion equipment. The session focused on Weishaupt’s new ULN (Ultra-Low NOx) burner technology, examining how advanced burner design enables compliance with increasingly stringent NOx limits while maintaining operational reliability, efficiency, and flexibility across a wide firing range.
The presenter began with a brief overview of evolving NOx regulations, including Colorado House Bill 23-1161, which introduces approximately 20 ppm NOx limits for smaller furnaces and water heaters beginning January 1, 2025. While this legislation primarily affects equipment under 2 MMBtu/h, it illustrates a broader national trend toward tighter emissions requirements.
Across the United States, NOx standards vary significantly by state and application. Examples cited include:
- Texas: 30 ppm NOx for boilers under 1,000 horsepower and 9 ppm for larger units
- Utah: 9 ppm NOx requirement for gas-only applications
- Other states without formal mandates still see demand for low-NOx solutions driven by owner sustainability goals
This regulatory variability necessitates burner platforms capable of meeting a wide range of emissions targets without extensive redesign.
The Weishaupt ULN burner series is offered in two primary configurations: natural gas only and dual fuel (natural gas and No. 2 fuel oil)
On natural gas, ULN burners are designed to achieve sub-7 ppm NOx without the use of flue gas recirculation (FGR). This is a significant technical distinction, as traditional ultra-low NOx burners often rely heavily on FGR to suppress flame temperature and reduce thermal NOx formation.
For oil firing, the largest ULN model (WK series) can achieve ≤90 ppm NOx on No. 2 oil without FGR. Smaller WM (“Monarch” or red series) burners require FGR to consistently meet 90 ppm NOx on oil across the full firing range. Their turndown ratios are:
- Up to 6:1 on natural gas
- Up to 4:1 on No. 2 oil
These turndown capabilities support stable operation under part-load conditions while maintaining emissions compliance.
The ULN burner lineup spans a wide range of capacities:
- WM20/3 and WM30/1: up to ~200 horsepower
- WM50 (forthcoming): up to 600 horsepower
- WK ULN series: up to 1,000 horsepower, requiring a remote combustion air blower
Mechanically, the ULN burners share several design elements with other Weishaupt platforms, including:
- Motor-mounted Nord variable frequency drives (VFDs)
- Burner-mounted oil pumps with magnetic clutches
- Fully modulating operation on both gas and oil for dual-fuel models
Fuel switching on dual-fuel units is simplified through the magnetic clutch system, allowing changeover with minimal mechanical intervention.
A key technical focus of the webinar was the elongated, multi-stage mixing head used in ULN burners, particularly in the WM series. This design enables staged combustion, which is critical for achieving single-digit NOx levels. Two primary NOx formation mechanisms were discussed:
- Thermal NOx – formed at high flame temperatures after combustion
- Prompt NOx – formed early in the combustion process, prior to complete flame development
For NOx limits in the 30 ppm range, reducing thermal NOx alone is often sufficient. However, achieving single-digit NOx requires suppression of both thermal and prompt NOx.
The ULN mixing head addresses this by:
- Splitting combustion into multiple zones
- Introducing fuel gradually across stages
- Lowering local flame temperatures
- Reducing oxygen availability during early combustion phases
This staged approach minimizes prompt NOx formation, enabling ultra-low emissions at the exhaust without reliance on external dilution methods.
The WK series employs a different mixing head geometry than the WM series, allowing it to achieve oil NOx targets without FGR, while smaller WM burners use FGR to supplement staged combustion on oil firing.
Additional safety features are integrated into the ULN burner design to address the higher complexity of staged combustion systems:
- Dual flame detection using both UV sensors and ionization electrodes, ensuring reliable flame monitoring during ignition and steady-state operation
- Temperature monitoring in the second premix zone of the mixing head
- A temperature limiter device interlocked with the burner safety circuit to shut down the burner in the event of excessive premix temperatures
These measures are intended to mitigate risks associated with elevated internal temperatures and ensure stable combustion across the firing range.