Liquid Hydrogen: Clean Or Polluting Energy Source?

Liquid hydrogen is being investigated as a potential zero-carbon fuel for aircraft. It is already used extensively in industry and as rocket fuel by NASA and the U.S. Air Force. Hydrogen is a clean fuel that burns with oxygen to produce water vapour and no carbon dioxide, soot, or nitrous oxides. However, hydrogen combustion can produce dangerously high levels of nitrogen oxide (NOx), which is a pollutant. Therefore, while liquid hydrogen has the potential to reduce carbon emissions, it may contribute to other forms of pollution.

Does Liquid Hydrogen Pollute?

Hydrogen combustion and NOx emissions

Hydrogen is often touted as the clean fuel of the future. It does not produce carbon dioxide emissions when burnt and instead produces water vapour. However, hydrogen combustion is not entirely pollution-free. It can produce dangerously high levels of nitrogen oxide (NOx), a harmful air pollutant.

NOx emissions from hydrogen combustion can be comparable to or even worse than those from natural gas combustion. This is a significant concern as NOx contributes to the formation of smog and ground-level ozone, which have adverse effects on human health and the environment. The problem is particularly pronounced in aircraft propulsion, where hydrogen combustion can increase pollution in the form of oxides of nitrogen.

The issue of NOx emissions from hydrogen combustion is not new. Similar issues were observed with diesel engines, which produce lower CO2 emissions but higher NOx emissions. To address this problem, most gas turbines today are designed with dry low NOx (DLN) combustion systems to limit NOx emissions from natural gas or methane burning. However, the unique characteristics of hydrogen combustion may require the development of new pollution control technologies.

While hydrogen combustion has the potential to reduce carbon dioxide emissions, the associated NOx emissions cannot be ignored. Independent studies and reviews by air pollution and public health experts are necessary to address this issue comprehensively. The hydrogen industry should embrace these studies and ensure that effective NOx reduction technologies are in place before the widespread adoption of hydrogen combustion technologies.

Hydrogen production and carbon dioxide emissions

Hydrogen is often touted as the clean fuel of the future. When burned, it produces water vapour and no carbon dioxide, nitrous oxides, or soot. However, the process of hydrogen production can emit carbon dioxide. The emissions intensity of hydrogen production depends on the production route. Currently, hydrogen production is dominated by unabated fossil fuels, which emit significant amounts of carbon dioxide. For instance, hydrogen production from unabated natural gas results in emissions of 10-14 kg CO2-eq/kg H2.

There are ongoing efforts to reduce emissions from hydrogen production, such as carbon capture and storage technologies. These technologies can capture and store carbon dioxide, reducing emissions from fossil-based hydrogen production. For example, retrofitting existing assets with carbon dioxide capture can decrease emissions intensity to 5-8 kg CO2-eq/kg H2. Advanced technologies with higher capture rates can further reduce emissions intensity to 0.8-6 kg CO2-eq/kg H2, although these technologies are not yet operational.

Producing hydrogen from renewable energy or nuclear power sources has a significantly lower carbon footprint than using electricity from fossil fuels. However, it is challenging to label renewable hydrogen as entirely carbon-free due to the carbon-intensive manufacturing processes and upstream emissions associated with renewable energy technologies. Biomass provides an interesting case study, with carbon dioxide emissions ranging from negative 48 to nearly 76 kg of CO2e per kg H2.

While hydrogen production from certain renewable sources can result in negative emissions, the overall carbon footprint of hydrogen varies widely. The National Renewable Energy Laboratory (NREL) found that some renewable energy technologies can have carbon dioxide emissions exceeding 200 g of CO2e per kWh over their lifetime. The average emissions intensity of global hydrogen production in 2021 was estimated to be 12-13 kg CO2-eq/kg H2. G7 members, who account for a significant portion of global hydrogen production and demand, are leading the way in decarbonising hydrogen production and promoting the development of low-emission hydrogen technologies.

In conclusion, while hydrogen combustion does not produce carbon dioxide emissions, the production of hydrogen can result in varying levels of carbon dioxide emissions depending on the fuel and technology used, as well as upstream and midstream emissions. Efforts to reduce emissions from hydrogen production, such as carbon capture and storage, are ongoing, and the development of an international hydrogen market will require collaboration between G7 members and emerging economies.

Hydrogen fuel cells and carbon dioxide emissions

Hydrogen fuel cells are an alternative to cars that use internal combustion engines. Hydrogen fuel cells do not produce carbon dioxide emissions when used to power vehicles. However, the process of producing hydrogen does result in carbon dioxide emissions. Hydrogen can be produced from methane, which emits less carbon dioxide than burning fossil fuels. Nevertheless, hydrogen production contributes to carbon dioxide emissions, and the overall environmental impact depends on the method of production.

Hydrogen fuel cells have the potential to reduce carbon dioxide emissions from the transportation sector, which accounts for a significant portion of global energy needs and petroleum consumption. By using hydrogen fuel cells, we can strengthen energy security, conserve petroleum, and diversify transportation energy options. However, the adoption of hydrogen fuel cells faces challenges due to the low energy content of hydrogen by volume, making storage difficult and requiring larger tanks and higher pressures.

The environmental impact of hydrogen fuel cells compared to other alternatives, such as battery electric vehicles (BEVs), is a subject of ongoing research. BEVs have a smaller environmental impact in terms of CO2 emissions, while hydrogen fuel cell electric vehicles (FCEVs) depend on the hydrogen production method. FCEVs supplied with hydrogen from renewable sources have shown reduced emissions, but the production and maintenance of hydrogen stations also contribute to environmental concerns.

While hydrogen fuel cells themselves do not produce carbon dioxide emissions, the process of producing hydrogen and the associated infrastructure can have environmental implications. To fully realize the benefits of hydrogen fuel cells in reducing carbon dioxide emissions, further research and development are needed to optimize hydrogen production methods and address storage challenges. Additionally, the cost of fuel cells needs to become more competitive without compromising performance, and independent studies are required to assess the broader environmental impact of hydrogen combustion technologies.

Hydrogen safety and fire/explosion risks

Hydrogen is a highly flammable gas that must be cooled to around -253°C to become a liquid. Liquid hydrogen is used extensively in industry and as rocket fuel, and its novel applications in cars, vessels, aircraft, and heavy machinery are being explored.

Handling liquid hydrogen demands strict safety measures due to its flammability. Hydrogen is non-corrosive, non-toxic, and odourless, making leaks difficult to detect. Although non-toxic, hydrogen can cause asphyxiation by displacing air.

The fire and explosion risks associated with liquid hydrogen are significant due to its high likelihood of ignition. Upon combustion, liquid hydrogen converts into oxygen and water vapour, and it evaporates almost immediately upon release due to the rapid increase in temperature above its boiling point. Large spills can lead to rainout and pool formation, depending on ground properties, environmental conditions, and the size and duration of the leak.

To mitigate these risks, liquid hydrogen storage requires cryogenic considerations, such as dedicated equipment and insulation, to prevent boil-off. Additionally, liquefying and cooling hydrogen require additional energy and costs compared to hydrogen gas. However, these costs may be offset by increased energy density, space savings, and transport efficiency.

Hydrogen storage and transportation

Hydrogen has nearly three times the energy content of gasoline by mass, but its low ambient temperature density results in a low energy per unit volume. This requires the development of advanced storage methods that have the potential for higher energy density. Hydrogen can be stored physically as either a gas or liquid, with the former requiring high-pressure tanks and the latter requiring cryogenic temperatures.

Storage of hydrogen as a gas typically requires high-pressure tanks (350–700 bar [5,000–10,000 psi] tank pressure). While this is less of an issue for stationary applications, it presents a challenge for vehicles, which require large volumes of hydrogen to provide sufficient driving range. Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption).

Storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one atmosphere pressure is −252.8°C. Liquid hydrogen has a density of 8 MJ/L, compared to 32 MJ/L for gasoline. This makes it challenging to store enough liquid hydrogen to meet the driving range requirements of light-duty vehicle platforms.

Transporting hydrogen also comes with its own set of challenges due to its low volumetric energy density and ability to permeate metal-based materials. Gaseous hydrogen is typically transported by tube trailers or pipelines, while liquid hydrogen is moved by road tankers. Trucking is a flexible option for supplying hydrogen to regions with developing demand, but it is limited by the volume that can be carried on a single truck and the fuel required for transportation.

Pipelines are better suited for delivering hydrogen to end users with large and steady demand over long distances. Hydrogen can also be transported over long distances by railcars, barges, or ships, which are not restricted by the weight requirements encountered on roadways. Additionally, existing domestic natural gas pipelines can be used to transport hydrogen by blending it with natural gas and using separation technologies downstream.

In terms of storage locations, hydrogen can be stored underground in empty salt caverns, depleted aquifers, or retired oil and gas fields. This method, known as "geologic" storage, is ideal for storing hydrogen for long periods of time, such as seasonal energy storage.

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