PTQ Q3 2023 Issue

10.7 ft/s

1.5 ft/s

at higher H₂ concentrations is thought to be related to a localised cooling effect from the increased water content in the combustion gases. Fortunately, there are several options to counteract the increase in NOx emissions. Burner design modifications (such as retrofit) can be considered first. External flue gas recirculation and/or steam injection may also be imple - mented to lower the flame temperature. If these methods are insufficient, selective catalytic reduction (SCR) may be required. With higher flame temperatures present, the adequacy of refractory materials, burner tiles, and burner internals in contact with the flame should also be considered. High flame speed Flame speed is a critical variable in fired heater design and operation. It describes the rate at which a combustion reac - tion takes place for a given fuel. To use a simplified sce - nario, it can be conceptualised as the speed at which a gas will fully combust if contained in a long tube and ignited at one end. In actual practice, flame speed depends on several factors, including pressure, temperature, fuel composition, excess air, turbulence, and surrounding cooling effects.⁴ However, in an idealised environment consisting of laminar flow at 77°F and 14.7 psia of pressure, a tube filled with H2 and lit at one end will complete the combustion reaction at the opposite end of the tube roughly 7x sooner than a tube filled with methane (see Figure 3 ). 3,4 To ensure reliable, safe combustion in a process burner, it is vital to control the speed of the uncombusted air/fuel mixture so that it is appropriately matched to the combus - tion or flame speed. If the air/fuel speed is lower than the flame speed, the combustion reaction can travel backwards into the burner and upstream equipment (‘flashback’). Flashback potential is especially important to consider for pre-mix burners.2 Therefore, before switching to H2 fuel from hydrocarbon fuel, it is essential to review the burner design to ensure it can accommodate the air/fuel speeds necessary for safe operation. API 535 recommends using no more than 70 mol% H₂ in fuel gas for pre-mix-style burners. 11 However, this will vary based on the specific burner design used. Flame visibility The ability to verify the presence of a stable flame at each burner is paramount to operating a fired heater safely. Usually, this can be done by visual inspection or with con - ventional flame scanners when firing hydrocarbon fuel Figure 3 Approximate laminar flame speed of H₂ vs methane at 77°F and 14.7 psia of pressure. Flame speeds taken from Combustion , Glassman and Yetter, 2008 3

gases. However, when 100% H₂ is used as a fuel source, the flame becomes virtually invisible. This is demonstrated by Figure 4a of a burner firing 100% H₂. In contrast, Figure 4b depicts nearly all H₂ combustion with a small percent - age of natural gas.⁵ It is clear that a small quantity of natural gas in the fuel can dramatically enhance the ability to see the flame. In situations where hydrocarbon fuel gas is not available or allowed, alternate methods of detecting the flame, such as specialised ultraviolet or infrared flame scanners, may be required. Currently, there are commercial flame scanner models available that use different sensing elements to detect either a hydrocarbon or H₂ fuel flame. Radiant/convection duty split As represented in Figure 5 , increasing H₂ content in the fuel can significantly decrease the quantity of flue gas generated. For an existing heater being revamped to run on H₂ fuel, this reduction in thermal mass travelling through the con - vection section decreases the heat absorbed. In a single service heater (same service in the convection and radiant section), this may be counteracted by increasing the heat absorbed in the radiant section, with careful attention paid to the increased radiant flux, bridge wall temperature, and tube metal temperatures. However, if there is a second, independent service in the convection section, the convec - tion coil will need to be reconfigured to maintain the original heat absorbed. The effects on draft and fan operation, as Figure 4a (top) Process burner firing 100% H₂; 4b (bottom) Process burner firing mostly H₂ with a small percentage of natural gas 5

PTQ Q3 2023