Like all liquid gases, liquid hydrogen too has to deal with boil-off gas. This by-product of boiling liquid is formed in small or larger quantities in tanks, pipes, and applications. That it will be formed is a given, but the question is what to do with boil-off gas.
In this blog, we review boil-off gas from liquid hydrogen. We explain what it is, how and why it is formed, and the different ways it can be used.
What is boil-off gas?
To liquefy and preserve gases, they are brought to an extremely low temperature. The temperature differs per gas. Oxygen, for example, becomes liquid at -183 °C, nitrogen at -195.8 °C, and hydrogen at -252.9 °C.
All cryogenic infrastructures, such as tanks, piping, and applications, are built to ensure that liquid gases are stored and transported at their characteristic temperature. However, a very small amount of heat inleak is inevitable. After a prolonged period of time, or when transported over long distances, a small percentage of the cryogenic liquid will unavoidably heat up and reach its boiling point. A small amount of gas, known as boil-off gas (BOG), is subsequently produced from the boiling liquid.
The amount of boil-off gas that is produced depends on the design and size of the infrastructure. In general, a large tank will percentage-wise produce less boil-off gas than a smaller tank.
The percentage of boil-off gas is also directly related to the quality of the insulation; the higher the quality of the insulation. The less heat will enter, and the less boil-off gas will be formed.
Compared to other common insulation types, vacuum insulation guarantees the lowest heat inleak. Consequently, most liquid hydrogen infrastructures are insulated with vacuum technology.
Release or capture and use boil-off gas
Liquid hydrogen has a higher volumetric energy density than gaseous hydrogen. This means that more liquid hydrogen can be stored in a tank or application than gaseous hydrogen. Hence, when boil-off gas occurs, the pressure in the infrastructure will increase.
A minor increase in pressure is not necessarily a problem, but should the pressure increase significantly, dangers arise. For example, when the gas accumulates in one place or when the infrastructure comes under too much pressure, the risk of fire and explosion becomes imminent.
Also, gas formation in a tank, pipeline, or application is not good for the quality of the liquid gas. Therefore, most infrastructures are built to discharge boil-off gas when necessary.
But what exactly happens to this discharged gas? Depending on the industry, the infrastructure, and the producer’s preferences. There are, generally speaking, two options: release it into the atmosphere or capture and use it.
Releasing boil-off gas into the atmosphere
The easiest way to dispose of the boil-off gas is to release it into the atmosphere. This method saves energy and eliminates the need for the necessary infrastructure to capture and use the gas. The release of boil-off gas is not harmful to the environment and, in small amounts, is not dangerous.
Unfortunately, this method has some drawbacks. Firstly, the release of hydrogen gas is only harmless in small quantities. An explosion can occur if a large amount of hydrogen gas comes into contact with oxygen gas. Therefore, it is crucial that the release of hydrogen boil-off gas is carefully monitored and done in safe conditions and quantities.
Secondly, releasing hydrogen gas is a waste. Producing hydrogen takes a lot of energy, and it is not efficient to let a useful product go to waste. Many producers, therefore, choose not to release boil-off gas but to collect and use it. The best method to do this depends on the industry and application.
Collecting and using boil-off gas
Boil-off gas from hydrogen can be collected and used in three different ways:
- Under high pressure
- Atmospheric pressure
- In liquid form, after reliquefaction
Use in gaseous form
For use in gaseous form (under both high pressure and atmospheric pressure), a system is built which collects the boil-off gas and may or may not pressurize it directly. Once restored to its optimum form, an application directly reuses the gas.
In this case, boil-off gas is added as a supplement to a larger quantity of liquid hydrogen, which is made gaseous with an evaporator. For an application to function properly, the use of boil-off gas alone is not sufficient.
Use in liquid form (reliquefaction)
To convert hydrogen boil-off gas back into liquid hydrogen, it must be collected and cooled again. This process is called reliquefaction.
For liquid hydrogen-powered applications, this is the best method. However, the equipment to achieve this cooling is costly and requires a relatively large space.
Usually, this method is only used for tanks or applications with sufficient surrounding space.
The best solution
Which solution is preferable depends on the industry and application. To name a few examples:
The refueling station
Refueling stations for gaseous-and in the future, liquid-hydrogen can use boil-off gas in several ways. Boil-off gas can be collected and used directly to supply gaseous hydrogen to cars. However, when also liquid hydrogen will be refueled in the future, boil-off gas will have to be collected and re-liquefied. In this case, reliquefaction is needed.
Fortunately, most gas stations have a relatively large amount of space, so the additional infrastructure required to perform reliquefaction will not be problematic.
Liquid hydrogen transport
Reliquefaction can also be interesting on hydrogen liquid transport vessels. The extent to which this is possible depends on the size and the way the ship is operated. The large equipment required for reliquefaction is often inefficient on a small vessel traveling short distances. However, if a liquid hydrogen tanker travels long distances and docks in ports for long periods, then onboard reliquefaction is often an interesting option.
When boil-off gas is released in, for example, a car, truck, or ship that uses gaseous hydrogen fuel cells. Reliquefaction is not necessary. In this case, the gas can be captured in gaseous form and used directly by the fuel cells.
As illustrated by the examples above, ‘What to do with boil-off gas’ depends entirely on the industry, application, location, and producer’s preferences. And the beauty of hydrogen is: no matter what is done with the gas, the environment is not harmed.
Reducing boil-off gas
Collecting and reusing boil-off gas requires additional material and energy. In most cases, this is worth it; however, it is always more efficient to limit boil-off gas.
Earlier in this blog, we briefly mentioned that insulation plays a vital role in minimizing boil-off gas. Especially for insulating liquid hydrogen, state-of-the-art insulation technology is needed because the temperature of this cryogenic liquid is extremely low.
After extensive comparison of various insulation materials, vacuum insulation has proven to be the most successful in insulating liquid hydrogen. Vacuum insulates 15 times better than other insulation materials (PIR/PUR, or Foamglas, Armaflex, Perlite, and Misselon), and vacuum insulation can be used for transfer lines as well as tanks and applications.
Demaco has been working with vacuum technology since 1989. For both large and small projects, our engineers develop the best solution for liquid hydrogen and various other liquid gases.
Processing boil-off gas is also not new to Demaco. We like to brainstorm with our clients and offer the solution that best fits the specific situation. Will that, for example, be reliquefaction? No problem, we design and build the best infrastructure. No challenge is too big, and for us being able to offer the most efficient solution is very satisfying.
Would you like to know more?
Would you like to know more about our work with liquid hydrogen? Then please take a look at this page or our recent blog about liquid hydrogen. Here you can read all about this versatile cryogenic liquid and the years of experience Demaco has built up working on various advanced hydrogen projects.