Jul-2023

The potential to reduce CO2 emissions in fired heaters is unlimited if 100% hydrogen fuel is utilised

Using hydrogen in fuel to eliminate CO2 emissions in fired heaters.

Luke Glashan
Wood

Article Summary

Hydrogen (H₂) has long been considered a high-value product and thus not typically considered a fuel for fired equipment. However, as the world searches for cleaner, more sustainable solutions, H₂’s unique property of producing zero carbon dioxide (CO₂) when combusted has been gaining attention. In conjunction with other decarbonisation technologies (such as carbon capture), H₂ is expected to play a major role in eliminating CO2 emissions in fired heaters.

Challenges
Figure 1 represents the potential to reduce yearly CO₂ emissions in a 100 MMBtu/hr (fired duty) heater. However, making the switch does come with challenges.

Hydrogen is a unique molecule and poses unique challenges as a fuel source. It is a basic building block of life  and the most abundant element in the universe.1 However, without attachment to carbon, it is a vastly different molecule with distinctive properties when combusted. The following are some of the potential challenges associated with switching to all or partial H₂ firing:
• Higher flame temperature
• Higher flame speed
• Flame visibility (or lack thereof)
• Radiant/convection duty split
• Impact on radiant heat transfer
• Corrosion mechanisms
• Changes to safety and control methods
• Burner piping and fuel gas skid resizing
• Stack plume visibility.

High-temperature flame
Increasing the amount of H2 in fuel gas has a significant impact on the flame temperature, as shown in Figure 2.

While Figure 2 is based on the adiabatic (theoretical) flame temperature, it highlights that a 100% H₂ flame can be hundreds of degrees Fahrenheit hotter than a flame from hydrocarbon fuels. The primary issue with this effect is that the formation of thermally generated nitrous oxides (NOx) increases proportionally with flame temperature.2 API 535 Figure 10 offers some generic guidance on the potential NOx increase associated with increased H2 content in fuel gas (+55% from 0 to 100% H₂).11 However, this general guideline will vary, depending on specific details of the burner design and fuels used. It is also of interest to note that many in the industry have observed a phenomenon where NOx begins to decrease above a certain H₂ level (approximately 80-90 vol.%). This decrease in NOx 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 implemented 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 reaction takes place for a given fuel. To use a simplified scenario, 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 combustion 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 conventional flame scanners when firing hydrocarbon fuel 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 percentage 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 convection 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 convection coil will need to be reconfigured to maintain the original heat absorbed. The effects on draft and fan operation, as applicable, should be considered as well. Excess air may be increased to generate more thermal mass, but doing so will also increase NOx emissions and reduce efficiency.