As the world is moving towards a more sustainable future, the energy sector is at the forefront of the transition. The shift from traditional fossil fuels towards renewable sources of energy is well underway, and this has put natural gas and hydrogen in the spotlight. Both of these energy sources are gaining attention, but how do they compare?
Size of the Molecules
When comparing natural gas and hydrogen, one key difference is the size of their molecules. In terms of molecular weight, hydrogen (H₂) is much lighter than methane (CH₄), the primary component of natural gas. The molecular weight of hydrogen is 2 grams per mole, while the molecular weight of methane is 16 grams per mole. This means that the hydrogen molecule is eight times lighter than the methane molecule.
In terms of diameter, the van der Waals radius of a hydrogen molecule is about 120 picometers, while the van der Waals radius of a methane molecule is about 200 picometers. This means that the hydrogen molecule is about 40% smaller in diameter than the methane molecule.
Lower and Upper Explosive Limits
The lower explosive limit (LEL) is the lowest concentration of a gas or vapour in the air that can catch fire or explode if there is an ignition source. The LEL is an important safety consideration when handling and transporting flammable gases like natural gas or hydrogen.
The LEL of hydrogen is 4%. This means that if the concentration of hydrogen in the air is less than 4%, it is too lean to ignite and support combustion. However, if the concentration of hydrogen in the air is between 4% and 75%, it is within the explosive range and can ignite and cause a fire or explosion if a spark or flame is present.
The LEL of natural gas can vary depending on the specific composition of the gas mixture. Typically, the LEL of natural gas is 4-5% by volume in air, and the upper limit is around 15-15.5%. In short, the explosive range of pure hydrogen is significantly greater than that of natural gas, which is why safety precautions are so critical when handling hydrogen.
How Natural Gas and Hydrogen Combust to Create Heat Energy
When hydrogen gas is burned, it reacts with oxygen gas in the air to create water vapour and releases heat energy. Methane, the primary component of natural gas, combusts with oxygen in the air to produce carbon dioxide, water vapour, and heat. The combustion of natural gas releases CO₂ emissions, which is why hydrogen as an energy source is so appealing – it produces no CO₂ emissions.
Energy Density
Energy density is a measure of the amount of energy that can be stored in a given volume or mass of fuel. The energy density of hydrogen is higher in its liquid phase compared to its gas phase. At standard temperature and pressure (STP), the energy density of hydrogen gas is about 0.010 MJ per litre (L). However, when hydrogen is cooled to its boiling point of -252.87°C and compressed to its liquid phase, its energy density increases to about 8.5 MJ/L. This means that liquid hydrogen contains significantly more energy per unit volume compared to hydrogen gas.
The energy density of natural gas can vary depending on its composition and pressure, but on average, the energy density of natural gas is about 38 MJ/m³ or 38,000 MJ/L. If you consider the energy density in MJ/kg, then the combustion of hydrogen produces about 141.8 MJ of energy per kg of hydrogen. A common range for the energy density of natural gas is around 50-55 MJ/kg.
Technically, hydrogen has a much higher energy density than natural gas; however, the global distribution systems and our household appliances are built for gas, meaning that the apples-to-apples comparison of hydrogen energy density to natural gas energy density is in the gas form and as such, the energy density of natural gas is much greater.
What Are the Types of Hydrogen and Why Is It Important?
Hydrogen has long been touted as a potential solution to our energy challenges, with its ability to produce electricity cleanly and efficiently without harmful emissions. However, not all hydrogen is created equal, and understanding the different types of hydrogen is crucial to unlocking its full potential.
One type of hydrogen that has been gaining attention in recent years is green hydrogen, which is produced using renewable energy sources like wind, solar, or hydropower. Unlike other types of hydrogen, green hydrogen production does not emit greenhouse gases (GHGs), making it a truly sustainable source of energy. While it is currently more expensive to produce than other types of hydrogen, the promise of this clean energy source has spurred efforts to reduce its cost and increase its adoption.
Another type of hydrogen is blue hydrogen, which is produced using fossil fuels like natural gas, coal, or oil. However, to mitigate the environmental impact of this production method, the carbon dioxide emissions are captured and stored underground, a process known as carbon capture and storage (CCS). Blue hydrogen offers a cleaner alternative to grey hydrogen, the most commonly produced type of hydrogen worldwide, which does not include CCS and therefore contributes to GHG emissions.
Understanding the different types of hydrogen and their environmental impact is crucial to developing a sustainable energy future. While grey hydrogen remains the most affordable option for now, the development and adoption of green and blue hydrogen technologies will be instrumental in reducing carbon emissions and mitigating climate change. As we move towards a more sustainable future, the classification of hydrogen will play a critical role in determining whether or not burning hydrogen as fuel can truly reduce CO₂ emissions.
Hydrogen as an Alternative to Natural Gases
Hydrogen is often seen as a promising alternative to natural gas, such as methane, due to its high energy density per kilogram, combined with the fact that it can be transported as a gas. At first glance, it appears to be an excellent alternative to the current CO₂-emitting energy source of natural gas, assuming the hydrogen source is green or blue. However, the feasibility of hydrogen as a primary energy source is currently hindered by various factors.
Transporting hydrogen gas through existing natural gas transmission and distribution systems to homes for use as a primary energy source seems like an excellent solution in an energy utopia. Unfortunately, the size of the hydrogen molecule, explosive limits, and the energy density of the gas phase indicates that it is not feasible. The molecule is so small that it leaks more from existing infrastructure than natural gas does, and if it does leak in a home and is trapped, it is much more likely to explode than natural gas. It’s been a while but I’m sure folks can remember the Hindenburg Blimp; we want to avoid that at all costs in our homes. The energy density in the gas form is also much less than natural gas, requiring a lot more of it at higher pressure (or in liquid) to achieve the same results for heating homes or cooking. Blending in hydrogen gas is an option that has been proven to be safely transported in existing natural gas systems up to 10-15%, but this raises the question of whether it is reducing carbon emissions at the endpoint where the energy is created.
As a thought exercise to think through, if adding 10-15% hydrogen into existing natural gas systems (assuming its green or blue hydrogen), imagine yourself as a hydrogen molecule and there are only 10 of you for every 100 natural gas molecules, you’re already at a disadvantage. On top of that, you’re 40% smaller than the natural gas molecules, so you tend to escape more easily through valves and other fittings. By the time you reach your destination to be burned on the stove, there may be only one of you left, and you’ll need to be backfilled by natural gas molecules to produce enough heat to boil water for dinner. As a result, the CO₂ emissions reduction you thought you were getting by adding 10-15% hydrogen at the injection point isn’t what you’ll actually get by the time the water is boiled. With most of the hydrogen molecules escaping and the low energy density of hydrogen in gas form, it won’t effectively help mitigate climate change or contribute significantly to cooking dinner when utilised in our existing natural gas systems.
While green or blue hydrogen may have promise in other applications, unfortunately, it doesn’t seem to be a viable option as a reliable energy source for our homes through existing natural gas transmission and distribution systems. It’s unlikely to produce the desired results for reducing CO₂ emissions when blended into natural gas, except perhaps to give a false sense of contribution to mitigating climate change to the people blending it at the source.
This article was originally published in the World Pipelines June 2023 issue.
