considered. These aspects are covered through heater impact studies. Retrofit burners for high H₂ firing To utilise high H₂ fuel and remain within legislative NOx emission requirements, it is necessary to utilise ultra-low NOx burner technologies. Diffusion-type burners present different challenges than pre-mix burners when firing high H2 fuel; each type of burner will be discussed separately. Ultra-low NOx diffusion burners The Zeeco Free Jet ultra-low NOx burner utilises internal flue gas recirculation for reconditioning of the fuel gas with inert products of combustion prior to mixing with combustion air. The reconditioned fuel mixture prolongs the combustion reaction, thus reducing peak flame temperature and thermal NOx production (See Figure 1 ). This burner can achieve NOx emissions below 50 mg/Nm³ on 90% H2 fuel without steam injection or post-combustion emissions control. The Free Jet principle relies on the conversion of momentum of fuel gas ejected from the tips to entrain flue gas. To achieve this, discrete high- velocity jets of fuel gas are injected through an array of fuel gas tips on the outside of the tile. The gas jets help to counteract the high flame speed of H 2 flames, ensuring a stable and robust flame over a wide operating range. Ultra-low NOx pre-mixed radiant wall burners Pre-mixed radiant wall burners, as commonly used in ethylene cracking furnaces, present an altogether different challenge when firing high H 2 fuels due to their propensity for flashback. Because of this, burner designers must consider the flammability window and flame speed of each specific fuel composition. The flammability window of pure methane is between 5% and 17%, with a flame speed of approximately 1.3 ft/s. Meaning that when mixed with air at a concentration between 5% and 17%, methane will support combustion at that speed. The flammability window for H2 is between 4% and 74%, with a flame speed of approximately 5.7 ft/s. As the industry pushes H 2 concentration higher, it becomes increasingly difficult to design burners with exit velocities sufficient
Products of combustion (heat is produced)
Inert ue gas
Fuel gas
Fuel gas
Combustion air
Auxilliary gas
Figure 1 Zeeco Free Jet ultra-low NOx burner
materials used in burner construction and the type of burner used. Typical burner construction includes various metal components and a refractory throat or tile. The increased flame temperature of H₂ may require upgrading the steel used for nozzle construction, throat construction, and flame stabilisers to a higher grade stainless or alloy. Refractory used within the burner should be carefully evaluated, and its composition modified to withstand the elevated temperatures characteristic of H₂ firing. The steel used in burners firing H₂ should not be susceptible to H 2 embrittlement and high- temperature H 2 attack. Both phenomena can prematurely degrade an improperly chosen steel, leading to early failure of the burner parts. Increasing H2 content in the fuel gas lowers its specific gravity, causing the fuel gas mass flow rate to decrease. Consequently, it is often necessary to increase the fuel gas pressure to achieve the same burner heat release. Therefore, burner gas tip design and fuel gas piping hydraulics should be assessed and re-sized as necessary. Additionally, existing heater safety interlocks and trip settings need to be reviewed and, as appropriate, amended for high H2 fuel. For example, furnace tunable diode laser (TDL) systems and flame scanners with ultraviolet/ infrared (UR/IR) detection may no longer be suitable, and alternative technologies capable of detecting high H2 flames may need to be
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