“When considering what a global energy system on a 1.5°C or 2°C [2.7°F or 3.6°F] pathway will look like by 2050, hydrogen consistently plays a critical role as a low-carbon fuel,” according to a report from Rocky Mountain Institute authored by Thomas Koch Blank and Patrick Molloy. Here are some highlights from that report.
When considering the carbon emission savings for using hydrogen, a complete view must take into account, both the carbon “cost” of producing the hydrogen, as well as the reduction in carbon emissions made possible by using hydrogen instead of fossil fuel at the point of use.
CARBON DIOXIDE INTENSITY OF HYDROGEN PRODUCTION
The different commercial methods of hydrogen production have advantages and disadvantages.
Electrolysis uses electricity to split water molecules to produce hydrogen and oxygen. It takes about 5 kWh of electricity to produce 1 kilogram (2.2 lb) of hydrogen. The carbon cost of this process depends on the carbon emissions from generating the electricity. This averages globally around 0.48 kgCO2/kWh, but varies widely, the report said, from around 0.67 kgCO2/kWh in India and 0.02 kgCO2/kWh in Sweden. It can be even lower for combined heat and power plants, depending on how the emissions are allocated between heat and power output.
According to the report, hydrogen production by electrolysis with grid power will be at parity with SMR within the next five years, the U.S. and the European Union should exclusively focus on electrolysis until carbon capture and storage is a viable and scalable technology.
Fossil fuel-based processes provide the majority of hydrogen currently in use.
The most widely used technology is steam methane reform (SMR). This process starts with a source of methane (CH4), typically natural gas. Methane reacts with high-temperature steam under pressure in the presence of a catalyst to produce hydrogen, carbon monoxide and a small amount of CO2.
Coal gasification is also used. This process reacts coal with oxygen and water at high temperature and pressure to produce a mixture of gases, primarily carbon monoxide and hydrogen. A subsequent step reacts this mixture with steam to produce CO2 and additional hydrogen.
Both these processes use a negligible amount of electricity, but produce CO2. An SMR plant emits 8 to 12 kg of CO2 for each kg of hydrogen produced, while coal gasification results in 18 to 20 kg CO2 per kg hydrogen.
The overall reduction in CO2 for any given application depends not only on the reduction at the point of use going from fossil fuels to hydrogen, but needs to take into account the carbon emitted in producing the hydrogen. In the future, as carbon capture and storage becomes economically feasible, this will enable reduction of the carbon emissions relating from production of hydrogen. The growing availability of renewable, non-carbon electric generation technologies, such as solar and wind power, also will reduce the hydrogen-production carbon emissions. Areas with large availability of hydropower already enjoy this reduction.
The report presented some of the opportunities for using hydrogen to reduce overall CO2 emissions.
The steel industry has been considered one of the most difficult in which to reduce carbon emissions, but using hydrogen changes the game. This industry offers perhaps the highest decarbonization impact for replacing fossil fuels (mainly coal) with hydrogen. In addition to supplying heat, hydrogen acts as a catalyst in the process. Even in areas with high-carbon-emissions electricity, changing from the traditional oxygen blast furnace technology to hydrogen direct reduction of iron can immediately start decarbonizing the steel industry. “By the time new steel-making technologies are available at commercial scale around 2030, the immediate impact for each replaced blast furnace will be immediate and significant,” according to the report.
For use in transportation, the picture of changing to hydrogen includes delivering the hydrogen fuel to the vehicle. Current options are to use hydrogen as fuel for internal combustion engines and as fuel for fuel cells.
An alternative to using molecular hydrogen is to use anhydrous ammonia (NH3), which is made from hydrogen and considered a hydrogen-based fuel. For shipping applications, ammonia may be “a more practical carrier of hydrogen, since it is easier to transport and store, with a significantly higher energy volume density than hydrogen in its molecular form,” said the report.
For shipping and municipal bus transportation fleets the achieved greenhouse gas reductions were about the same, 13 and 14 kg CO2 per kg H2, respectively. (Note that the shipping number is for ships using ammonia fuel.)
Burning hydrogen is one of the few low-carbon ways to achieve the high temperatures needed in many industrial processes. Coal and natural gas are commonly used. To produce 1 million BTUs, it requires 8.07 kg (17.8 lb) of hydrogen.
The actual adoption of hydrogen as a fuel in different industries in different locations will depend on many factors, including natural gas and electricity prices in each market and any subsidies or incentives offered by governments. Hydrogen as a fuel is likely to continue to show growth. According to a report from the International Renewable Energy Agency, as much as 18% of final consumption may be provided by hydrogen by 2050.