Can Utility-Scale Power Boilers Safely Fire Hydrogen?
There is plenty of enthusiasm around hydrogen as a possible alternative fuel to natural gas in combustion processes. Some hope it can eventually replace natural gas in applications such as utility boilers. However, any transition to 100% hydrogen has plenty of challenges to overcome due to it having such fundamentally different combustion characteristics. This gives rise to safety, pollution, performance, and operational issues.
Consequently, hydrogen blends are seen as a more realistic objective, at least over the short term. Current burners and boiler infrastructure can successfully accommodate a few percent by volume of hydrogen. For larger blends, modifications may be needed.
Due to the decarbonization benefits that could be realized by switching from natural gas to hydrogen, many utilities, power producers, and industrial sites are investigating the concept of at least blending some hydrogen into their systems. Let’s take a look at the realities of hydrogen firing in boilers and any changes that might have to be made.
Long History
Far from being new and novel, boilers have been running on hydrogen for many decades. Hydrogen-based boilers are often found in refineries and chemical plants where hydrogen is available as a byproduct of other processes.
The logic behind this is simple. If the hydrogen is there, why waste it when it can be used in boilers as a way to reduce the overall fuel bill for the facility? Consequently, best practices already exist for the firing of hydrogen and hydrogen blends in industrial boilers.
“Most manufacturers have experience in dealing with the nuances of hydrogen and can advise you on how best to burn it by itself or in combination with natural gas,” said Gerardo Lara, vice president of Fired Boiler Sales at Rentech Boiler Systems Inc. of Abilene, Texas.
Safety Systems
Perhaps the biggest concern with hydrogen is its explosive potential. Hydrogen represents a greater threat of explosion than natural gas due to its higher explosive range.
The explosive range of methane is about 5% for the lower explosive limit (LEL) to 15% for its upper explosive limit (UEL). However, hydrogen’s LEL ranges from 4% to a UEL of 70%. Thus, there will probably be upgrades needed to safety systems to be able to detect and prevent leakage as well as safety upgrades to address the potential risk of explosions.
Emissions and Environmental Considerations
There are definite environmental and emissions gains to be had by adding hydrogen into industrial boilers. It will lower carbon emissions, but perhaps not as much as would be expected. Adding 20% hydrogen by volume represents about a 7% mass reduction of CO2 per energy unit compared to 100% natural gas. The decarbonization potential rises sharply, of course, as more hydrogen is added.
But there are other emissions to consider. Some environmental bodies now classify methane as a greenhouse gas (GHG) as its combustion produces CO2, sulfur dioxide, and nitrous oxide (NOx). The addition of hydrogen to natural gas brings down CO2 emissions. Be aware, though, that this increases NOx due to the flame temperature of hydrogen being higher than that of natural gas. Anyone considering the addition of hydrogen in boilers should take into consideration how they are going to meet existing NOx targets.
“In some cases, there may be a need to add flue gas recirculation (FGR) and selective catalytic reduction (SCR) systems to mitigate NOx challenges in boilers blending hydrogen with natural gas,” said Lara.
Other Boiler Modifications
Beyond changes due to safety and environmental concerns, there are other areas where modifications may be required. Those considering a retrofit to accommodate hydrogen blending should first appreciate the fact that hydrogen is far less dense than natural gas. That shows up in the size of storage vessels needed for hydrogen, as well as piping diameter, and valve sizing. It is quite likely that certain valves and seals may need to be upgraded to prevent leakage and provide the quantity of hydrogen required.
“Those responsible for the design of the hydrogen supply system should be tasked with providing systems that can accommodate the higher volume of gas needed at the desired pressure and obtain the necessary Btu input for the boiler,” said Lara.
In addition, the purity of hydrogen supply should be investigated. Small amounts of water, natural gas, or carbon dioxide in hydrogen can significantly lower the combustion characteristics of a hydrogen and natural gas blend.
Another factor to consider is that a byproduct of hydrogen combustion is water. Hence, Lara recommended that additional drains and possibly other drying measures be considered as part of any retrofit. Liaise too with burner manufacturers to see if there may be modifications needed to burners to support the hydrogen combustion.
Lara called attention to superheaters and reheaters as areas to assess. Engineers should evaluate such components and other materials to determine if metal overstress may become an issue due to the flame temperature of hydrogen. He also said that higher or lower attemperation could possibly be required depending on the distribution of heat transfer surfaces.
Start with Packaged Boilers
Instead of jumping into an upgrade of multiple boilers simultaneously, Lara said to start small and keep it simple. His advice to keep retrofit costs down is to begin with packaged boilers, as they are most likely to be the easiest type of boiler to gain experience in hydrogen combustion.
“The flexibility and straightforward design of packaged water tube boilers simplifies the addition of hydrogen while keeping risk relatively low,” said Lara.
Where Is the Hydrogen?
Perhaps the biggest challenge facing anyone wishing to switch industrial boilers from natural gas to hydrogen is the supply of hydrogen. Oftentimes, there isn’t much around.
Therefore, anyone wishing to fire their boilers with a blend of hydrogen are advised to favor projects where there already is an abundant supply of affordable hydrogen. Refineries and chemical plants may have excess hydrogen on offer.
Alternatively, plenty of government funding is being invested in hydrogen electrolyzers that produce hydrogen from excess wind power or solar power. Boilers in the vicinity of planned electrolyzers should find hydrogen more readily available.
“Pay attention to certainty of hydrogen supply before you invest in boiler modifications with an aim to lowering your carbon footprint,” cautioned Lara.
Feasibility Study
Lara recommends that anyone considering the switch to hydrogen first conduct a feasibility study on a boiler retrofit that includes an evaluation of the entire boiler system and all combustion equipment. “Examine potential changes in heating surfaces such as superheaters and reheaters, the possible addition of flue gas recirculation and attemperator capacity,” said Lara. “As well as reviewing fans, air heaters, air ducts, and overfire air systems, it should also encompass boiler control and automation systems, including the burner management system as upgrades may be needed there.”
—Drew Robb ([email protected]) has been a full-time freelance writer for more than 25 years specializing in engineering and technology.