Hydrogen is having a moment.

Admittedly, that might sound like a familiar refrain. In his 2002 book The Hydrogen Economy, Jeremy Rifkin envisioned a new hydrogen-powered energy era that would usher in “the next great economic revolution.” Two decades later, former Chemical Processing columnist Alan Rossiter posed the question, “Is Hydrogen the Wonder Fuel?”

Today, global efforts to hit decarbonization targets have renewed interest in hydrogen as a potential energy carrier and alternative fuel. While it has a long history in petroleum refining and chemical processing — primarily as a reactant — hydrogen is being explored for a broader range of applications, from fuel cells to industrial heating to energy storage.

Hydrogen has a lot working in its favor. Perched atop the Periodic Table of the Elements, hydrogen is the lightest element in the universe — and the most abundant. It’s also the simplest element, readily bonding with many other elements.

“Hydrogen’s high-bonding potential makes it the key building block of many items that surround us, from soap and shampoo to paints and building materials and the plastics and packaging we use daily,” the Rocky Mountain Institute explained in a recent article, “How Hydrogen Could Clean Up the Chemicals Industry.”

Hydrogen’s appeal extends beyond its role as a chemical building block. For example, hydrogen fuel cells can produce electricity while only emitting water vapor and a small amount of heat. Hydrogen is also being explored as a potential substitute for coal and other carbon-emitting fuels in the steelmaking process.

“With continued advances in production technologies and expansion of supporting infrastructure, hydrogen has tremendous potential as a lower-emission fuel or feedstock in many industrial, transportation and energy applications,” according to the American Chemistry Council.

However, realizing hydrogen’s full potential also requires understanding and managing the risks.

“Hydrogen isn’t new as a molecule. But for some organizations deploying it at scale, the safety challenges are genuinely unfamiliar,” said Nick Barilo, executive director of AIChE’s Center for Hydrogen Safety and hydrogen safety program manager at Pacific Northwest National Laboratory. “Teams that excel at conventional fuel systems, natural gas or other industrial chemicals may underestimate how differently hydrogen behaves and how different the risk profile truly is.”

When it comes to public opinion toward hydrogen, reactions tend to run the gamut from hype to hysteria. To some, hydrogen is the green fuel of the future — the solution to some of the world’s most pressing energy and environmental challenges. For others, hydrogen’s flammability and other hazardous properties are dealbreakers.

The reality, Barilo emphasized, sits somewhere in the middle.

“I’ve often said in my presentations that hydrogen shouldn’t be feared. It should be respected,” Barilo said. “I have a colleague who often says the difference with hydrogen is that it’s a little less forgiving than some of the other fuels. I think that’s probably a good way to look at it.”

What Makes Hydrogen Different?

On the National Fire Protection Association’s NFPA 704 flammability scale, hydrogen earns a 4 — the highest possible rating.

But hydrogen isn’t alone in that category. Other familiar fuels, including propane and methane, share the same rating. What distinguishes hydrogen is how it behaves when it’s released or ignited.

Hydrogen has a much wider flammability range than conventional fuels. According to the U.S. Department of Energy (DOE), hydrogen can ignite at concentrations ranging from approximately 4%-74% in air, compared with 5%-15% for methane and 2%-10% for propane. In practical terms, that means a hydrogen leak can create a flammable atmosphere across a much broader range of fuel-air mixtures.

Of particular concern is that “a simple static discharge” is all that would be needed to light a hydrogen mixture at its most ignitable concentration, which is 29% in air. However, at concentrations closer to its lower-flammability limit, the minimum energy needed to ignite a hydrogen mixture is similar to that of other fuels, DOE noted in a fact sheet.

Hydrogen is lighter than air, which has “both safety advantages and limitations,” according to Paul Baybutt, founder and CEO of Primatech, a Columbus, Ohio-based process safety and risk management consulting firm.

“In open, well‑ventilated outdoor areas, released hydrogen tends to rise and disperse quickly, which can reduce the duration and extent of a flammable cloud near ground level,” Baybutt explained in a recent technical note published on LinkedIn. “However, that same tendency means hydrogen can accumulate in elevated or confined spaces, such as roofs, ceilings, pockets, enclosures, canopies and poorly ventilated buildings, where it may create a serious fire or explosion hazard.”

Because of hydrogen’s buoyancy, DOE recommends positioning leak-detection devices toward the top of enclosed spaces or ventilation systems, where escaping hydrogen is likely to accumulate.

When hydrogen does ignite, other characteristics — such as burning velocity — come into play.

“Burning velocity is how fast the flame front moves,” Barilo explained. “That burning velocity for hydrogen is about an order of magnitude higher than it is for natural gas. Fuels with higher burning velocity will, with all other things being equal, generate higher pressures and do more damage.”

If a hydrogen release leads to a fire, plant personnel and emergency responders need to be aware that hydrogen — in its pure form — burns with a nearly invisible flame, Barilo noted. Entrained dust, debris and other materials can make hydrogen flames more visible. However, in a laboratory or other cleanroom setting, a hydrogen flame could be difficult to detect, particularly in daylight.

“Hydrogen has a very low radiant heat — about a half of what typical hydrocarbons have,” Barilo added. “When propane is burning, you feel that heat hitting you. You don’t feel it the same with hydrogen. If you’re not careful and you were to walk into the flames, you would be in big trouble.”

That makes reliable flame detection particularly important, along with specialized training programs and emergency response procedures that prepare personnel to recognize and respond to hydrogen fires.

The consequences of a hydrogen release also depend on when ignition occurs. An immediate ignition can produce what Barilo called a “jet fire” — a sustained flame from a pressurized release — while delayed ignition can allow hydrogen to accumulate and form a flammable mixture that could generate damaging overpressure when ignited. Both scenarios need to be evaluated when assessing hydrogen hazards.

“Ultimately, the appropriate safeguards depend on the specific application,” Barilo said. “Introducing hydrogen into an existing space warrants a thorough risk analysis which includes evaluating fire and explosion risks. The hazards need to be mitigated primarily through engineering controls, not administrative controls.”

Assessing Materials Compatibility

One of the fundamental questions chemical processors need to address is whether their existing infrastructure — particularly pipelines designed for natural gas service — can safely accommodate hydrogen. The answer depends on several factors, including the materials involved and the operating conditions.

When hydrogen is introduced into an existing pipeline, hydrogen embrittlement becomes a major concern.

Hydrogen embrittlement occurs when hydrogen molecules come into contact with a metal surface and separate into individual atoms. Those atoms can penetrate the metal and interact with defects in its structure, accelerating the formation and growth of cracks that eventually can cause a component to fail prematurely.

Not all metals are equally vulnerable to hydrogen embrittlement. According to a fact sheet from the Center for Hydrogen Safety, materials such as austenitic stainless steels and aluminum alloys are commonly used for hydrogen service, while high-strength ferritic and martensitic steels are generally avoided.

“A high-strength steel is generally going to be more susceptible to hydrogen embrittlement than a low-strength steel,” explained Chris San Marchi, a distinguished member of the technical staff at Sandia National Laboratories in Livermore, California.

However, metals that are less resistant to hydrogen-induced deterioration can still be used “if analysis of pressure-containing components indicates acceptable mechanical loading,” according to the Center for Hydrogen Safety. The most commonly accepted method for conducting that analysis is fracture mechanics.

“In fracture mechanics, we measure two sets of properties,” San Marchi explained. “We measure something called fatigue crack growth, which is the rate at which a crack grows in fatigue based on the driving force applied to a defect, and [we measure] the fracture toughness, which we often refer to as the fracture resistance of the material.” Both measurements help engineers conduct a fitness-for-service assessment, San Marchi added.

One way to mitigate the risk of hydrogen-induced degradation is to manage the mechanical stresses in a pipeline, according to San Marchi.

“Stress is a very effective variable to mitigate the hydrogen problem,” San Marchi added.

A common measure for evaluating pipeline operating stress is specified minimum yield strength (SMYS). Operating a pipeline at 30% of SMYS, for example, would be a conservative approach that reduces threats to structural integrity, including those associated with hydrogen embrittlement, San Marchi explained. However, he emphasized that the figure was an arbitrary number used for illustrative purposes, not an industry best practice.

Operating stress is only part of the equation. Engineers also need to consider existing defects in a pipeline — which often stem from welds. Older welds — or those produced without proper attention to welding procedures — can contain localized areas known as “hard spots,” where the material is harder than the base steel, San Marchi noted. Those areas can be more vulnerable to hydrogen embrittlement.

“As a general rule, fracture resistance decreases as strength goes up,” San Marchi explained. “That’s true without hydrogen, and it’s even more true with hydrogen.”

Evaluating an existing pipeline also means accounting for variations in the materials used in its construction. A pipeline extending hundreds of miles, San Marchi noted, may contain “a diverse range of steels from different manufacturers, potentially from different eras.”

That makes it difficult to simulate how the pipeline will perform in hydrogen service. As San Marchi explained, engineers need to understand the properties of the materials, the condition of the pipeline itself and how it will be operated. “That’s something industry is currently grappling with,” he added.

The assessment doesn’t stop at the pipeline.

Maryam Davari, a London-based independent energy consultant, emphasized that introducing hydrogen into an existing industrial facility requires evaluating the entire system, including compressors, valves, safety devices and instrumentation.

“Engineering should look at this as a whole,” Davari said.

Putting Safety into Practice

As organizations explore hydrogen for a broader range of applications, they can draw on an established body of codes and standards to guide their safety programs.

Hydrogen safety requirements have been incorporated into fire, building, electrical and mechanical codes, including the International Fire Code, while specialized codes and standards such as NFPA 2 and ASME B31.12 address hydrogen-specific applications.

“Numerous safety standards from organizations like CSA, UL and SAE are also critical,” Barilo added. “There’s been much work done in the last two decades to develop and refine the codes related to hydrogen.”

The Hydrogen Tools portal provides a publicly available collection of hydrogen safety resources, including a searchable Hydrogen and Fuel Cells Codes and Standards Database.

Relevant codes and standards provide an essential foundation for hydrogen safety, but they must be part of a comprehensive safety program that addresses the risks associated with each application. The Hydrogen Safety Panel’s Safety Planning Guide for Hydrogen Projects emphasizes the importance of a strong safety culture and a formal safety plan that addresses hazard identification, risk assessment, management of change, employee training and emergency preparedness.

“Safety culture is very important. I can’t overstate it,” Barilo asserted. “It’s key to ensuring the safety of people and equipment, as well as the industry’s long-term success.”

For organizations considering hydrogen applications, understanding its distinctive properties and associated hazards is essential to managing those risks.

“Hydrogen is a hazardous material,” Barilo concluded. “It has been used relatively safely for about a century, and it can be used safely, but its properties need to be understood and safeguards applied appropriately.”