Physical properties of hydrogen and electrofuels

Isn’t hydrogen highly flammable?  Can it be used safely?

All fuels contain energy and can cause a hazard if not properly handled. While hydrogen is flammable it can be used safely, and has been for more than a century by many industries. Just as with other flammable materials such as gasoline, diesel, and natural gas, it is important to know the properties and follow established rules to safely handle hydrogen.

Project developers and engineering firms are encouraged to contact the Center for Hydrogen Safety in developing new projects and for training on how to handle hydrogen safely.

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Are there properties of hydrogen that make it more or less hazardous than other combustible fuels that we use routinely, like natural gas, propane, and gasoline?

All fuels have unique properties that need to be considered and addressed, and hydrogen is no exception. For example, hydrogen is 14 times lighter than air, so it rises and disperses rapidly in open areas. This phenomenon is very different from other common fuels such as gasoline or propane. The vapors/gases of these materials will pool near the ground and remain a hazard for extended periods of time. However, other properties, such as its small molecule size and ability to embrittle some materials mean that it is more prone to leaks and has to be handled appropriately.

Hydrogen has other unique properties, therefore, project developers and engineering firms are encouraged to contact the Center for Hydrogen Safety in developing new projects and for training on how to handle hydrogen safely.

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How much water is consumed in producing hydrogen through electrolysis?

Production of renewable hydrogen uses amounts of water comparable with producing conventional fuels. Roughly five gallons of water are needed to make a kilogram of hydrogen with electrolysis. From one-half to one-third the water requirement is rejection water from the purification process and is suitable for agricultural purposes. This puts the water usage a bit less than that for gasoline in transportation applications (see Figure 1).

 

Figure 1 Transportation Water Use and Emissions Comparison [RHA Fact Sheet based on data from Life-Cycle Analysis of Water Consumption for Hydrogen Production, Elgowainy, Han, Lee, et al, Argonne National Laboratory, June 8, 2016 Annual Merit Review, Slide 15, based on electrolysis with wind power), ]

 

 

 

Figure 1 Transportation Water Use and Emissions Comparison [RHA Fact Sheet based on data from Life-Cycle Analysis of Water Consumption for Hydrogen Production, Elgowainy, Han, Lee, et al, Argonne National Laboratory, June 8, 2016 Annual Merit Review, Slide 15, based on electrolysis with wind power), ]

 

See: Life-Cycle Analysis of Water Consumption for Hydrogen Productions, Elowiny, Lampert, et al, Argonne National Laboratory, June 2015.

Is water always consumed in envisioned uses of renewable hydrogen, or are there applications where the water can be recycled and reused in a closed system?

The electrolyzers used to produce hydrogen from electricity and water need very pure water and typically start with city-quality water, but need to further purify it. Membraneless (e.g., alkaline) electrolyzer technologies may be more accommodating of impurities. Water is rejected in the purification process amounting to half to two-thirds of the water “consumed” in the process. It typically takes roughly 10 liters of water to make a kilogram of hydrogen.

The rejection water is clean enough to use for agricultural purposes, as purification does not add impurities to the inlet water. The one-third to one-half of the water that is split to release hydrogen could theoretically be collected in fuel cell applications where the hydrogen is reconstituted as pure water. Keep in mind that each kilogram of hydrogen (with roughly the energy equivalence to a gallon of gasoline) will produce nine kilograms of water once consumed, so recovery may often be difficult or infeasible.

See: Life-Cycle Analysis of Water Consumption for Hydrogen Productions, Elowiny, Lampert, et al, Argonne National Laboratory, June 2015.

 

Is fresh water essential for electrolysis or might desalinized water be used in arid, water-short regions?

Today’s technology requires pure water and much of the water “requirement” involves wastewater from the purification process. Saline or polluted water can be used, but it must be purified before going to the electrolyzer cell. The less pure the inlet water, typically the more water and energy it takes to make hydrogen due to the need to purify the water. There is work on electrolyzers that can take saline or brackish water, but these are not yet (2021) commercially available.

 

What substances are produced when hydrogen is consumed?  Are there conditions under which harmful pollutants are emitted?

Today, most hydrogen is used as a feedstock in petroleum refining and fertilizer manufacture. These industries have a number of harmful emissions. Many of the envisioned new uses of hydrogen for transportation involve consuming hydrogen in fuel cells to produce electricity, emitting only pure water as the effluent.

In hydrogen applications that involve combustion in air, most, but not all, criteria air pollutants are eliminated.[1] When hydrogen is combusted in air, oxides of nitrogen (NOx) can form in addition to water vapor.  Because hydrogen burns hotter than methane and other hydrocarbons, greater attention is required to limit NOx formation during combustion than with most other fuels. NOx (principally nitrogen oxide [NO] and nitrogen dioxide [NO2]) affect tropospheric ozone and contribute to acid rain and photochemical smog, which has known adverse health effects including asthma.

Use of hydrogen combustion is likely in the future in a range of new applications in order to reduce carbonize emissions. These applications may include industrial process heat, power generation in peaker plants (where fuel cells are deemed uneconomic due to low capacity factors), and possibly in transportation applications, such as ocean shipping, trains, and aviation.

 

How does renewable hydrogen compare with fossil methane (natural gas) as a fuel?  Are there uses of fossil methane that renewable hydrogen cannot replace?

Natural gas is 70-90% methane. Hydrogen can be combined with environmental sources of carbon dioxide (e.g., biogas effluents) to make methane. Doing so makes it entirely compatible with every use of natural gas. The main advantage of hydrogen as a fuel in place of fossil fuels is that its use releases no carbon dioxide.

Hydrogen itself has chemical and energetic properties that differ from methane/natural gas. Perhaps the most salient difference is that it has a lower heat content per volume (e.g. cubic foot) of natural gas. As a result, beyond 20-30% mixture of hydrogen and natural gas, natural gas appliances may function noticeably differently with hydrogen. With adjustments, most uses of natural gas can be met with hydrogen. Major gas turbine manufacturers have said their existing gas turbines can run on a combination of natural gas and up to 30% hydrogen (by volume) without modification. Gas turbines capable of running on 100% hydrogen are planned by manufacturers for the 2030-2035 timeframe.

Manufacturer hydrogen power plant links:

General Electric

Mitsubishi

Siemens

Solar Turbines

Wärtsilä

 

How does hydrogen compare with methane (natural gas) as a chemical feedstock?  Can it replace the use of oil, natural gas, and coal in chemical industries?

There are uses of fossil fuels for which extracting the hydrogen is the main objective (e.g. oil refining, and fertilizer production). Other uses of natural gas as a chemical feedstock generally make use of the carbon content of the natural gas. Hydrogen can be combined with carbon dioxide to form methane, which is completely interchangeable with natural gas as a feedstock. Using hydrogen in those applications would require an additional, not necessarily petroleum-based, source of carbon.

Candidate non-fossil sources for carbon for both chemical feedstock and fuel uses include biomass from forestry, agricultural, municipal, and sewage sludge wastes; industrial processes, such as from cement manufacture; and direct-air capture.

 

Can you clarify the many different terms now being used around hydrogen?

  • Blue hydrogen
  • Brown hydrogen
  • Electrofuels
  • Green hydrogen
  • Grey hydrogen
  • Methanation
  • Power to fuel
  • Power to hydrogen
  • Power to methane
  • Renewable hydrogen
  • Renewable natural gas

 

Image: Frontier Economics

Blue hydrogen

Almost all hydrogen today is derived through a process called steam methane reforming (SMR) in which the primary feedstock is natural gas. In stripping out the hydrogen from the hydrocarbons (mostly methane) carbon dioxide is a by-product. If the carbon dioxide is captured and sequestered out of the atmosphere, the resulting hydrogen is called “blue hydrogen.” It has a lower carbon emissions footprint than natural gas based SMR hydrogen, which is also called “grey hydrogen.”

Brown hydrogen

Prior to “natural gas,” gas pipelines carried a toxic mixture of gases variously called “town gas,” “city gas,” “coal gas,” or “synthetic gas.” It was produced by heating coal to a high temperature in the presence of steam. The resulting gas was 30-50% hydrogen, with a large carbon monoxide component. There is a new technology based on pumping high temperature steam into coal seams and extracting the hydrogen while leaving the toxic gases behind. Hydrogen derived in this was is sometimes referred to as “brown hydrogen.”

Electrofuels

The process for disassociating water into hydrogen and oxygen with an electric current has been known for more than two centuries. Fuels derived from electricity in this way are sometimes called “electrofuels” or “electric fuels.” Hydrogen is the primary one, but other fuels such as ammonia, methane, methanol, and formic acid can be derived.

Green hydrogen

The term “green hydrogen” generally refers to non-fossil derived hydrogen.  Hydrogen generated through electrolysis using renewable electricity is expected to be the primary production route. Hydrogen can  also be produced through processing organic matter and would generally be considered green hydrogen. An open question is whether electrolytic hydrogen (hydrogen based on splitting water with electricity) should be called green hydrogen if the electricity used to supply the power comes from legacy large hydroelectric facilities, and nuclear power. Other colors are beginning to be used to distinguish the electricity source of electrolytic hydrogen– pink for nuclear, and yellow for generic grid power.

Grey (or gray) hydrogen

Almost all hydrogen today is derived through a process called steam reformation in which the primary feedstock is natural gas. In stripping out the hydrogen from the hydrocarbons (mostly methane) carbon dioxide is a by-product that is routinely released to the atmosphere. Hydrogen produced in this way is called “grey hydrogen.”

Methanation

Methanation is the process of combining hydrogen with carbon dioxide to make methane, the principle constituent of natural gas. If the carbon dioxide is taken directly from ambient air or intercepted from some other pathway leading to the atmosphere, then recycling it into methane is carbon neutral, assuming the resulting methane is fully combusted (i.e., not leaked to the atmosphere).

Power-to-fuel

This is a general term that captures the concept of using electricity to create fuels (aka “electrofuels” or “electric fuels”). The first step in that process is usually creating hydrogen– processes to create ammonia and methane directly are under development. Through additional well-established processing, the hydrogen can be processed into a large number of other substances, including alkanes (methane, ethane, propane, butane, octane, etc.), which make up the bulk of conventional hydrocarbon fuels.

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Power to hydrogen

Power to hydrogen refers to using electricity to disassociate water into its component hydrogen and oxygen and collecting the hydrogen (usually not the oxygen) for useful purposes.

Power-to-methane

Power to methane means using the energy in electricity to produce methane, a synthetic form of natural gas. The process entails splitting water with electricity (electrolysis) to release hydrogen and subsequently combining hydrogen with carbon dioxide to create methane. The resulting methane is completely interchangeable with natural gas

Renewable hydrogen

The term “renewable hydrogen” is not uniquely defined, but generally relates to sources of hydrogen that derive from renewable sources. The focus of this document is producing hydrogen from renewable electricity (and water), but what qualifies as renewable electricity and what provenance for the power need be shown, and whether only 100% renewable electricity can qualify are all openly at issue.

As an example of the ambiguity, “renewable hydrogen” defined under Washington State statute includes making hydrogen with electricity produced by legacy hydropower plants. This does not qualify as “renewable hydrogen” under California statutes, which exclude legacy hydropower as a qualifying source of power. California does allow for renewable hydrogen from natural gas if the gas has been matched by an equivalent amount of biogas injections into the pipeline system– a policy not without controversy.

Renewable natural gas

Renewable natural gas is a term used generally for non-fossil sources of gas that can substitute in at least some cases for “natural gas,” which comes from gas and oil wells. Renewable natural gas includes gas from electrolytic sources as well as biogenic sources, such as methane from landfills or bio-digesters.

 

[1] Criteria air pollutants are particulate matter, photochemical oxidants (including ozone), carbon monoxide, sulfur oxides, nitrogen oxides, and lead.  EPA named these pollutants “criteria air pollutants” because it sets national ambient air quality standards for them based on criteria that capture the latest scientific information on their impact of human health and welfare.