Using hydrogen in fuel to eliminate CO 2 emissions in fired heaters
The potential to reduce CO 2 emissions in fired heaters is unlimited if 100% hydrogen fuel is utilised
Luke Glashan Wood
H ydrogen (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 decar- bonisation 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 chal- lenges 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 mol- ecule 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 H 2 con- tent 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 phenom- enon where NOx begins to decrease above a certain H₂ level (approximately 80-90 vol.%). This decrease in NOx
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Figure 1 Yearly CO₂ emissions in tons per year vs vol% H₂ in fuel for a 100 MMBtu/hr (fired duty) heater. Assumes balance of fuel is methane and 15% excess air
Figure 2 Adiabatic flame temperature (AFT) in °F vs vol% H₂ in fuel. Assumes balance of fuel is methane and 15% excess air
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PTQ Q3 2023
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