Safely firing hydrogen in boilers
Hydrogen

Safely firing hydrogen in boilers

By Gerardo Lara, Rentech

Firing existing boilers with hydrogen seems novel and new to many people. However, packaged boilers have been running on nothing but hydrogen for decades. Many boiler manufacturers have experience in dealing with hydrogen. They should be able to advise you on the peculiarities of burning it by itself or in combination with natural gas.

Hydrogen-based boilers are often found in refineries and chemical plants. The reason is simple. Hydrogen is often available in abundance in such facilities as a byproduct of other processes. Why waste hydrogen when you can harness it in boilers and eliminate fuel costs? But interest in hydrogen firing has broadened of late as one way to make progress toward decarbonization goals. Those running their boilers on natural gas, for example, can introduce some hydrogen into the mix to lower carbon emissions. In some cases, that may require a retrofit. Here are some points to consider for those interested in adding hydrogen to existing boilers.

Possible boiler modifications

There are certain technical factors that must be considered related to the combustion of hydrogen and how it compares to other fuels. Natural gas is denser than hydrogen. Hence, facilities will need a lot more of it by area than natural gas. This impacts the size of storage vessels and metering stations as well as diameter of piping and the size of valves. Valves and seals may also have to be replaced to prevent leakage. 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.

Pay attention, too, to impurities and water content in the hydrogen supply as they can shift the Wobbe Index of the fuel. While natural gas and hydrogen can have a similar Wobbe Index, the presence of a small amount of water, natural gas, or carbon dioxide can significantly lower the Wobbe Index.

Further, hydrogen combustion produces larger amounts of water than natural gas as a byproduct. Drainage systems and drying measures should take this factor into account. Coordination with burner manufacturers should help determine if any modifications may be needed to support the combustion of hydrogen.

Environmental systems

Environmental bodies now consider methane as a greenhouse gas as its combustion produces CO2, sulfur dioxide, and nitrous oxide (NOx). By adding hydrogen to natural gas, CO2 emissions can be reduced. However, hydrogen combustion does produce NOx as the peak flame temperature of hydrogen is higher than that of natural gas. Thus, hydrogen blending may lead to issues in meeting NOx targets. Some facilities may need to add flue gas recirculation (FGR) and selective catalytic reduction systems (SCR) to reduce NOx emissions from boilers blending hydrogen with natural gas.

Safety systems

The explosive range of hydrogen is greater than that of coal and natural gas. While methane ranges from a lower explosion limit (LEL) of 5% to an upper explosive limit (UEL) of 15%, hydrogen’s LEL is 4% and its UEL is 70%. Safety systems, therefore, will need to be upgraded to prevent leakage and mitigate the risk of explosion.

Consider, too, that some boiler components materials may have to be upgraded. Engineers should check the superheater and reheater for metal overstress and make any necessary surface adjustments. Depending on the distribution of heat transfer surfaces, higher or lower attemperation will also be required.

To keep retrofit costs down, packaged water tube boilers are probably the easiest type of boiler to gain experience in hydrogen combustion. Their flexibility and straightforward design simplify the addition of hydrogen while keeping risk relatively low.

Anyone moving forward with a feasibility study on a boiler retrofit for hydrogen is advised to conduct an evaluation of the entire boiler system. This should include all combustion equipment. It should examine potential changes in heating surfaces such as superheaters and reheaters, the possible addition of flue gas recirculation, attemperator capacity etc. 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.

Certainty of hydrogen supply

Anyone wishing to fire their boilers with a blend of hydrogen better pay attention to supply. Hydrogen is far from widely available. There are projects that are receiving government funding to produce hydrogen using an electrolysis process. If a hydrogen electrolyzer exists or is being planned in your area, that may be one way to ensure regular supply.

Note that there are already plenty of gas turbines around certified to be able to run on 25% or more hydrogen. However, many of them continue to operate on 100% natural gas. Why? There isn’t any hydrogen available or if it is, the price tag is too high for boiler operation. Therefore, before investing heavily in hydrogen-related modifications to lower your carbon footprint, pay attention to the availability of hydrogen supply.

About the Author: Gerardo Lara is Vice President Fired Boiler Sales at Rentech Boiler Systems, Inc. of Abilene, TX. For more information, visit www.Rentechboilers.com.