Potential application of renewable hydrogen and electrofuels
What is hydrogen currently used for?
The US produces and consumes ten million tons of hydrogen each year today. About two-thirds of it is used in oil refineries, and about a fifth of it is used to create ammonia for fertilizer production. It is also used in semiconductor manufacturing and a large number of lesser uses, such as rocket fuel. It is increasingly being looked at and used for transportation in cars, trucks, buses, drayage and material handling vehicles, trains, ships, and even aircraft.
What are the potential uses of renewable hydrogen?
Renewable hydrogen can be used anywhere hydrogen is used today. As we seek to decarbonize the entire energy economy, renewable hydrogen is expected to be used in new applications such as manufacturing steel and concrete. It can also be used in conjunction with sources of carbon to synthesize virtually any fossil fuel in use today. Two promising uses of hydrogen are in producing renewable diesel and synthetic aviation fuel.
Of these, which applications appear most cost-effective and hence most likely to be addressed early on?
Transportation fuels are more expensive on an energy basis (i.e., dollars per BTU) than other fuels and are usually thought to be the lowest hanging fruit, especially in conjunction with low carbon fuel standards that accord and additional financial incentive based on the carbon intensity of hydrogen relative to fossil fuels. Renewable gas standards for pipeline gas, replacement for existing fossil-based hydrogen industrial uses (e.g. fertilizer and refining) can also become cost-effective pending decarbonization regulations placed on those uses.
What current uses of fossil fuels can hydrogen, even in theory, not replace?
Although hydrogen is not a 100% interchangeable replacement for any fossil fuel, it can be used in conjunction with fossil fuels in some cases (e.g., natural gas pipelines, gas turbine co-firing, and diesel engine injection) to reduce their carbon content. It can also be used as a feedstock to synthesize climate-neutral versions of virtually any fossil fuel limited only be the availability of carbon from biological or environmental (e.g., direct air capture) sources. It may be economically impractical to use synthesized hydrocarbons from hydrogen to produce plastics.
Can renewable hydrogen scale up rapidly enough to provide a significant climate benefit?
Yes. In the last two years manufacturers have announced new manufacturing capacity being added to more than double the world capacity for making electrolyzers. Germany has announced a 9 billion euro investment in hydrogen infrastructure, and other countries in Europe and Asia have their own ambitious plans. Whether the industry can scale fast enough to meet the need implied by the IPCC 2030 and 2050 carbon emission reduction targets is an open question, but every day we delay makes meeting those targets a greater challenge. World electrolyzer manufacturing capability is expected to double in 2021, and may do so again in a few short years.
Keywords: climate, scale, scalability, decarbonization, limit, limits, targets.
- ITM’s hydrogen ‘Gigafactory’ moves into manufacturing operation, New Power, Jan 4, 2021.
- Nel to slash cost of electrolysers by 75%, with green hydrogen at same price as fossil H2 by 2025, Collins, Recharge, Jan 21, 2021.
Can existing internal combustion engines be modified to run on hydrogen?
Diesel engines can co-fire hydrogen up to about 30% (by volume) with little modification. Hydrogen is also a feedstock to making renewable diesel, a fuel that is 100% compatible with diesel engines without modification. Modified diesel engines may be able to run on ammonia produced from hydrogen. Hydrogen is not an efficient fuel for spark-initiated (i.e. gasoline) engines.
Can hydrogen be combusted in existing natural gas equipment, such as power-generation turbines, diesel engines, industrial boilers, residential furnaces, and gas ranges?
Yes. Hydrogen can be combusted in each of those applications up to 20-30% (by volume) without modification.
What are the potential applications of renewable hydrogen in transportation?
Hydrogen is being used in virtually every transportation application: light, medium and heavy duty vehicles, locomotives, ships, buses, aircraft, and rockets. We are not aware of any mode of transportation that cannot be fueled by hydrogen. Small drone aircraft are increasingly being fueled by hydrogen in place of batteries for a roughly three-fold increase in range. Airbus is planning to have a long-distance commercial aircraft based on hydrogen by 2035. Heavy duty and aircraft applications of battery technology are challenged by the much greater weight to energy ration of batteries versus hydrogen storage.
Does hydrogen offer a possible pathway to fossil-free drop-in replacement fuels for our current fleet of vehicles and industrial plants?
Hydrogen fuel cell systems can be a drop-in solution to battery electric vehicles (there is a business in converting battery forklifts and battery drones to hydrogen). Other than those, with the possible exception of diesel engines, hydrogen is generally not a drop-in replacement for fossil fuels in vehicles. Hydrogen can be a drop in solution for many industrial uses of fossil fuels, although some retrofit (e.g., boiler tips and piping) may be required.
What current fossil-sourced fuels could be replaced with drop-in replacement fuels derived from renewable hydrogen?
Virtually any fossil fuel can be synthesized from hydrogen plus a source of carbon.
How large are the energetic penalties incurred when renewable hydrogen is used in synthesizing hydrocarbons like methane, propane, and dodecane (kerosene)?
There is about a 20% loss of energy in the process of combining hydrogen and carbon dioxide to make methane.
See: Methanation the Key to the Natural Gas Grid, Schildhauer (web page).
What are the candidate carbon sources should we pursue synthesis of hydrocarbon replacement fuels from renewable hydrogen?
Carbon dioxide is an unwanted effluent from many industrial processes, including all that combust carbon-based fuels. It is also a naturally occurring effluent in biological decomposition and can be found in all biological materials such as agricultural waste. Carbon dioxide is also a significant component of unprocessed landfill gas.
Are there other energy carriers that are under consideration as clean fuels for the future that could be synthesized using renewable hydrogen?
Yes. They include but are not limited to ammonia, methane, methanol, methane, formic acid, methyl-cyclohexane, and renewable diesel.
- Hydrogen Carriers for Bulk Storage and Transport of Hydrogen, Autrey and Ahluwalia, US DOE Energy Efficiency and Renewable Energy Office, December 2016
Given the roundtrip efficiencies of Power to Hydrogen and Power to Methane as storage methods for the electrical power industry, how can it ever make economic sense to use them in lieu of batteries or pumped storage?
Using electricity to make fuels and reconverting the fuels back to electricity will return at best less than 50% of the energy sent to storage back to the electric grid with today’s technology. Battery and pumped hydro storage is in the range of 85-95% efficient. The advantage of creating fuels comes in to play when large amounts of energy need to be stored, as for example in spring when hydro, solar and wind can be available in surplus quantities for weeks at a time. The cost of conventional storage increases with the amount of energy stored, whereas the main cost of making fuels is based more on the cost of the conversion infrastructure.
For example, if the objective is to store 3 weeks of surplus spring energy, a 100 MW electrolyzer costing $200 million would store the equivalent of about 18,000 MWh of electricity (@36% round trip efficiency). Storing that amount of energy with batteries would cost more than a $1.5 billion (at target battery costs!), about 8 times as much. In some bulk energy applications, storage in fuels can be less than a tenth of the cost of battery or pumped hydro storage, despite the lower efficiency. In cases where the energy would otherwise simply be dumped due to lack of demand, the efficiency is hardly a factor—storing even 35% of the energy is better than losing all of it.
The economic advantage is multiplied further if the fuels are used for a more valuable purpose than making electricity such as running buses, heavy duty trucks, locomotives, and even aircraft. In applications where storing short bursts of energy for short periods of time (hours or days) are required, batteries and pumped storage are the better choice.
A convenient-to-produce energy carrier that can be synthesized from renewable hydrogen is ammonia (NH3), as nitrogen can be drawn from ambient air and ammonia is a liquid at ambient temperatures. What are the pros and cons of ammonia as an energy carrier and might it solve some of the storage challenges associated with storing hydrogen?
Ammonia production for fertilizer is the second largest use of industrial hydrogen today after oil refining. About a fifth of the hydrogen produced today ends up as ammonia. Ammonia has advantages over hydrogen as an energy carrier in that it can be stored and transported as a liquid with a higher energy density than hydrogen. Converting hydrogen to ammonia is a technology that is more than a century old, but it does entail additional capital investments and energy losses. Ammonia is also toxic in relatively low concentrations (hydrogen is not). So there are definitely pros and cons. It is definitely a contender for the storage and transportation of hydrogen. Certainly using hydrogen produced using renewable electricity is the only viable means of electrifying the manufacture of ammonia today, so the future of it is relatively bright.
What difficult-to-replace industrial uses of natural gas cannot be replaced with renewable hydrogen?
There may be industrial uses of natural gas where the carbon content of the gas is chemically important to the process. However, since hydrogen can be combined with carbon dioxide to make synthetic natural gas, this is not seen to be an issue.
Fossil-fuel interests point to shipping, aviation, and heavy industrial processes like cement and steel production as sectors of the economy that cannot now be decarbonized. Can renewable hydrogen replace fossil fuels in these applications?
With the possible exception of cement production, there are currently either commercially available or at least pilot project demonstrating hydrogen replacement for each of the sectors named in the question.
Aviation may be the most difficult segment of the transportation industry to decarbonize. How can renewable hydrogen address this challenge?
Although it is an expensive process today, aviation fuels can be synthesized from hydrogen together with carbon (e.g., agricultural waste) feedstocks. It is also true that hydrogen-based small commercial aircraft are already making test flights today with a targeted range of 250 miles. Airbus is planning to have a long-distance commercial aircraft based on hydrogen by 2035.
A significant component of commercial aviation’s climate footprint results from contrails and aviation’s role in cirrus cloud formation. Can renewable hydrogen help in mitigating this problem?
Possibly. Combusting hydrogen in place of aviation fuels would not help the contrail issue. Using renewable hydrogen to synthesize fuels for today’s combustion turbine aircraft would also not help. If the hydrogen is used to create electricity for motorized turbines, it may reduce contrails.
Major international shipping companies like Maersk have committed to decarbonizing their operations. How can renewable hydrogen help in that process? What climate-neutral fuels (or energy carriers) are envisioned for maritime uses?
Hydrogen and ammonia made from hydrogen are definitely under consideration by the shipping industry. Other possibilities exist such as synthesized methanol, formic acid, etc.