Advancements in ultra-low NOx burner technology
Next-generation burner for hydrogen firing can be easily retrofitted while also meeting strict NOx emissions regulations
Eric Pratchard and Todd Grubb Zeeco Hector Ayala, Aloke Sarkar, and HS Lee ExxonMobil Technology and Engineering Company
M eeting global net-zero commitments requires extensive decarbonisation of the oil and gas indus- tries and other heavy industries. One effective way to reach these goals is to use hydrogen (H₂) as a process burner fuel at nearly 100% by volume instead of hydrocar- bon fuels. Current ultra-low NOx burners (ULNBs) and new burner designs often struggle with high hydrogen concen- trations due to increased flashback risk and higher costs for NOx emissions control, since hydrogen’s higher flame temperature results in more adiabatic NOx formation. As the demand increases for better solutions with lower carbon and NOx emissions, industry needs a next-gener- ation ULNB capable of 100% hydrogen firing while also producing significantly lower NOx emissions. Furthermore, it is essential that this burner can be easily retrofitted into existing fired heaters to reduce the capital costs associated with decarbonising key refinery processes. To meet that demand, Zeeco and ExxonMobil 1 col- laborated to design, develop, test, and implement a new next-generation ULNB design that can fire 100% hydrogen in addition to a wide range of fuel gas compositions, while significantly reducing NOx emissions without relying on complicated or costly additional control systems or emis - sions solutions. The new burner does not use external flue gas recirculation or lean-pre-mix technology and meets tar - gets in both natural and forced draft systems with ambient or preheated combustion air. The companies jointly conducted burner testing for both single and multi-burner setups across various pro - cess conditions. The test results demonstrated excellent flame stability, performance, and emissions reduction, with flame dimensions similar to current-design ULNBs (see ‘Performance test results’). ExxonMobil installed the new proprietary burners, called Free Jet Gen 3, in a pro - cess heater at its Baytown, Texas, facility. Early operational results aligned with burner performance tests, and the burners are achieving emissions reduction and operational flexibility as anticipated. Current emerging technologies and selective catalytic reduction (SCR) systems, considered possible alternatives to this new burner design, can be complex, expensive, and require additional protective systems or operational requirements.
Burning hydrocarbon-based fuel containing as much as 80% hydrogen only cuts CO₂ emissions in half. To achieve significant carbon reductions, a higher concentration of hydrogen, likely close to 95%, is needed to reach net-zero emissions targets. Thus, to meet industry goals for decar- bonisation, a process burner design must be commercially available that can safely, reliably, and cost-effectively com- bust close to 100% hydrogen. Most fired heaters and process furnaces today were designed for firing natural gas or refinery fuel gases that contain a high proportion of hydrocarbons plus hydrogen, inert gases, and traces of other compounds. Hydrogen con - Current emerging technologies and selective catalytic reduction systems can be complex, expensive, and require additional protective systems or operational requirements tent for typical refinery fuel gas may vary between 20% and 40%. When converting burners to fire high hydrogen, concentrations of 90% to 100% are probable, and that changes burner operating parameters, requiring adapta - tions to the design to ensure optimal burner and heater operation. Hydrogen flame speed is significantly higher than that of typical hydrocarbon fuels, resulting in faster combus - tion and increased heat release per unit volume. The flame speed of hydrogen combustion is approximately 1.7 m/s (5.6 ft/s),2 while the flame speed of natural gas is significantly slower at only 0.4 m/s (1.3 ft/s). Additionally, the stoichio - metric adiabatic flame temperature of hydrogen (2,182°C or 3,960°F) is higher than that of natural gas (1,937°C or 3,520°F). Hydrogen’s high flame speed causes combustion to occur more rapidly than when firing natural gas. This rapid combustion process releases the combustion energy in a smaller volume, leading to localised elevated near-flame temperatures, which compound the effect of the inherently high adiabatic peak flame temperatures on NOx emission rates. Any region with elevated temperatures above 760°C
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