Controlling Combustion of Hydrogen Fuel Mixtures

Hydrogen is a great, clean-burning fuel source. Just look at its combustion equation:

2H2 + O2 -> 2H2O + Heat

Water and heat are created from the combustion of hydrogen and oxygen.

Emerson's Andrew VerdouwI mention all this because Emerson’s Andrew Verdouw had the opportunity to be the keynote speaker at the National Association of Power Engineers (NAPE) monthly meeting in Indianapolis. His presentation was on the combustion of hydrogen with hydrocarbon fuels using vibrating element technology.

He opened noting that in addition to being clean burning, the burner technology is mature and, on a mass basis, it is the most energetic of the chemical fuels. You might suspect that seeing the hydrogen-fueled rocket boosters that propel them into space.

Andrew highlighted the sources of hydrogen as a fuel. It is typically produced as an end-process stream, both intended and as a byproduct in cracking operations, petroleum processing, gasification, steam reforming, and water shift reactions.

As a fuel, it’s challenging since hydrogen production may not always match the fuel demand of the boiler or heater. Natural gas is often introduced into a common fuel header to maintain a target pressure to the burner fuel train. Now, the single fuel source is a mixture of hydrogen and natural gas. The objective is to consume the hydrogen efficiently and minimize the natural gas required to meet the fuel demand.

Andrew highlighted the control challenge in meeting these objectives. It boils down to the combustion air requirements. For natural gas, one standard cubic foot (SCF) requires approximately 9.7 SCF of air. For hydrogen, the ratio drops to 2.4 SCF of air per 1 SCF H2. A change in fuel mixture has a big impact on combustion air requirements.

As the mixture becomes natural gas rich, sub-stoichiometric conditions can appear quickly bringing unstable combustion and high levels of carbon monoxide as the combustion process is starved for air. If the mixture becomes hydrogen rich, the air required quickly falls, which means excessive air to the burner, lower thermal efficiency, and the possibility of a burner flameout.

Also, Andrew noted that the heating value changes as the fuel mixture changes from ~290BTU/SCF for 100% hydrogen to ~983BTU/SCF for 100% natural gas. Traditional cross-limited combustion control can’t deal with this large degree of change very well and is designed for a constant air-to-fuel (A/F) ratio. When the stoichiometric A/F ratio changes, the air demand for combustion changes. The excess air calculation in traditional combustion control cannot adjust or allow for changes.

Operators do their best to work around this by manually adjusting the fuel and air loop set points and bias the air demand high enough to keep the flame going. This results in high excess air, which reduces the thermal efficiency and requires constant attention to do the manual adjustments.

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Technology has come to aid of this difficult control problem. Recent advancements in Coriolis measurement, Micro Motion 3098 specific gravity meters, provide real-time specific gravity (density) measurements for gases. This means that these instruments can report real-time heating values for the fuel mixture. In a fuel mixture involving the two components—hydrogen and natural gas—a unique density (specific gravity) exists for every concentration of the two gases.

Also, for every unique concentration of hydrogen, a unique F/A can be calculated.

O2 setpoint curves can be determined empirically through combustion testing, since they are a function of the burner/air capability. Andrew conceded that this is likely the most challenging part. Excess air curves are a function of the burner air system capability. Logic in the control system interpolates these curves at the various levels of hydrogen concentration as required.

The control strategy reads the measured lower heating value (LHV) of the fuel mixture, which is applied directly to the fuel and air flow-to-BTU calculations and fed into the cross-limits.

Hydrogen-Natural Gas Combustion Control Strategy

This approach is more reliable, repeatable, and energy efficient than the manually operated alternative. Andrew cautioned that there are limitations to this approach. It is confined to binary or pseudo-binary (natural gas compositions) fuel mixtures. Multi-component gases can be tolerated if they are constant and only the hydrogen content varies. Finally, Coriolis meters, used in this application do not handle inert components such as N2 and CO2 well if these components vary.

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