WNA on Hydrogen Production and Uses - 2020

Renewable Energy (Other Energy Resources) / Hydrogen and Its Uses

WNA reported in late 2020 that zero-carbon hydrogen production is decomposition of water by direct use of heat from nuclear energy, using a thermochemical process enabled by high-temperature reactors. Like electricity, hydrogen is an energy carrier (but not a primary energy source). Hydrogen has some potential to replace oil as a transport fuel and in other applications. It is the preferred fuel for fuel cell electric vehicles (FCEVs), though portable storage at vehicle scale is a challenge. Hydrogen can also be used in internal combustion engines.

While hydrogen can replace liquid hydrocarbons, it is never as energy-dense or convenient to store and transport. However, it does compare well with batteries, which is why it and hydrogen-based liquid fuels have a lot of potential. Electricity and hydrogen are convertible one to the other as energy carriers. However, the overall efficiency of electricity-hydrogen-fuel cell-electricity is no more than 40%. One approach to mitigate intermittency of wind and solar electricity is to make hydrogen by electrolysis and feed this into the gas grid, the power-to-gas strategy. It has been suggested that most electricity from wind might be used thus, greatly simplifying electrical grid management.

In future some hydrogen produced for fuel may be converted into ammonia as a more energy-dense carrier medium volumetrically for trade or long-term energy storage. The mass energy density of hydrogen is 120 or 142 MJ/kg*, compared with methane 50 MJ/kg, propane 46 MJ/kg and ammonia 19 MJ/kg. Volumetrically, the energy density of hydrogen is low – 10.8 or 12.75 MJ/Nm3 (or as liquid: 8.5 or 10.0 MJ/L)**.

* Lower (net heat of combustion) and higher (gross heat of combustion) heating values, respectively. Thus 33.3 or 39.4 kWh/kg. The higher heating value includes the latent heat in the vaporized water, the lower heating value (or net calorific value) excludes this.
** Nm3 = normal cubic meter (0 °C temperature and 1.01325 bar (i.e. 1 atmosphere) pressure.

All this points to the fact that while a growing hydrogen economy already exists, linked to the worldwide chemical and refining industry, and potentially for steelmaking, a much greater one is in sight. With new uses for hydrogen as a fuel, the primary energy demand for its production may exceed that for electricity production. Transport is expected to constitute the largest demand for hydrogen in both the EU and South Korea by 2050, reflecting the conversion of the heavy vehicle and large passenger car fleet from diesel to hydrogen FCEVs. Heating for buildings is expected to be the next largest demand, replacing natural gas. Railways can replace diesel fuel with hydrogen.  If hydrogen is to fuel shipping or aviation, refueling needs to be available worldwide, so Europe’s ambitious plans will need much wider take-up.

A September 2020 report by LucidCatalyst said that in addition to deployment of renewables for power generation: “The only known way to address the ‘difficult-to-decarbonize’ economic sectors is with the large-scale use of hydrogen as a clean energy carrier and as a feedstock for synthetic fuels such as ammonia.” But clean hydrogen needs to come from non-fossil sources, at a lower price – under $1/kg – than is prospective today. An abundant supply of low-cost hydrogen would greatly boost world agricultural productivity through increased availability of nitrogen fertilizers, as well as fueling transport.

The Role of Nuclear Power
Nuclear power already produces electricity as a major energy carrier. Operating at very high capacity factors, it is well placed to produce zero-carbon hydrogen. The evolution of nuclear energy's role in hydrogen production over perhaps two decades is seen to be: 

• Cold electrolysis of water, using off-peak capacity (needs 55 kWh/kg). 
• Use of nuclear heat to assist steam reforming of natural gas. 
• High-temperature steam electrolysis, using heat and electricity from nuclear reactors. 
• High-temperature thermochemical production using nuclear heat. 

Steam reforming of methane (from SMRs) requires temperatures of over 700 °C to combine methane and steam to produce hydrogen and carbon monoxide. A nuclear heat source would reduce natural gas consumption by about 30% (i.e. that portion of feed which would simply be for heat), and eliminate flue gas CO₂ emissions. Apart from this, the efficiency of the whole process (primary heat to hydrogen) moves from about 25% with today's reactors driving electrolysis (33% for reactor x 75% for cell) to 36% with more efficient reactors doing so, to 45% for high-temperature electrolysis of steam, to about 50% or more with direct thermochemical production. 

High-temperature steam electrolysis (HTSE, at 600 °C or more) in solid oxide electrolysis (SOEC) to use both heat and electricity has been demonstrated, and shows considerable promise. It is a reverse reaction of the solid oxide fuel cell technology. It requires about one-third less energy than low-temperature electrolysis but has never been commercialized due to the poor durability of the ceramic components in a hot hydrogen environment. US research is at Idaho National Laboratory in conjunction with Ceramatec and FuelCell Energy in a $12.5 million project part-funded by the US Department of Energy. LucidCatalyst shows it producing hydrogen at two-thirds the cost of low-temperature electrolysis across a range of capacity factors.

The IAEA has developed the Hydrogen Economic Evaluation Program (HEEP) to assess the economics of large-scale hydrogen production using nuclear energy. 

Hydrogen Directly from Nuclear Heat
Several direct thermochemical processes are being developed for producing hydrogen from water. For economic production, high temperatures are required to ensure rapid throughput and high conversion efficiencies. They essentially do not use electricity.  In each of the leading thermochemical processes the high-temperature (800-1000°C), low-pressure endothermic (heat absorbing) decomposition of sulfuric acid produces oxygen and sulfur dioxide:

2H₂SO₄ ⇒ 2H₂O + 2SO₂ + O₂

There are then several possibilities. In the iodine-sulfur (IS) process invented by General Atomics in the 1970s, iodine combines with the SO2 and water to produce hydrogen iodide. This is the Bunsen reaction and is exothermic, occurring at low temperature (120 °C): 

I₂ + SO₂ + 2H₂O â‡Œ 2HI + H₂SO₄

The HI then disassociates to hydrogen and iodine at about 350-450°C, endothermically:

2HI â‡Œ H₂ + I₂

This can deliver hydrogen at high pressure.

Combining all this, the net reaction is then:

2H₂O ⇒ 2H₂ + O₂

All the reagents other than water are recycled, there are no effluents, hence it may be called the sulfur-iodine cycle, with zero-carbon hydrogen and oxygen byproducts. 

In February 2010 the Japan Atomic Energy Agency (JAEA) set up the HTGR Hydrogen and Heat Application Research Centre at Oarai to progress operational technology for an IS plant to make hydrogen thermochemically. It has demonstrated laboratory-scale and bench-scale hydrogen production with the IS process, up to 30 litres/h. In parallel with JAEA’s HTTR developments a pilot plant test project producing hydrogen at 30 m³/h from helium heated with 400 kW tested the engineering feasibility of the IS process. An IS plant producing 1000 m³/h (90 kg/h, 2t/day) of hydrogen was to be linked to the HTTR to confirm the performance of an integrated production system, envisaged for the 2020s. In 2014 hydrogen production at up to 20 L/h was demonstrated. In January 2019 it used the HTTR to produce hydrogen using the iodine-sulfur process over 150 hours of continuous operation. JAEA aims to produce hydrogen at less than $3/kg by about 2030 with very high temperature reactors.

The US Nuclear Energy Research Initiative (NERI) launched in 1999 was refocused in 2004 to include the Nuclear Hydrogen Initiative (NHI), allied to the Next Generation Nuclear Plant (NGNP) program established in 2005. NGNP envisaged construction and operation of a prototype high-temperature gas-cooled reactor (HTR) and associated electricity or hydrogen production facilities by 2021, but funding was cut back under the Obama administration and relicensing activities were suspended in 2013.

Under an International NERI agreement, Sandia National Laboratories in the USA and the French CEA with General Atomics in the USA were also developing the IS process with a view to using high-temperature reactors for it. They had built and operated a laboratory-scale loop for thermochemical water-splitting.

South Korea has also demonstrated thermochemical water-splitting at laboratory scale, supported by General Atomics. In December 2008, the ROK Atomic Energy Commission officially approved nuclear hydrogen development as a national program, with the development of key and basic technologies through 2017 and the goal of demonstrating nuclear hydrogen production using the S-I process and a very high-temperature reactor (VHTR) by 2026.

The economics of hydrogen production depend on the efficiency of the method used. The IS cycle coupled to a modular high temperature reactor is expected to produce hydrogen at about $2.00/kg. The oxygen byproduct also has value. General Atomics earlier projected $1.53/kg based on a 2400 MWt HTR operating at 850°C with 42% overall efficiency, and $1.42/kg at 950°C and 52% efficiency (both 10.5% discount rate). Such a plant could produce 800 tonnes of hydrogen per day.  

For thermochemical processes an overall efficiency of greater than 50% is projected.

Production Reactor Requirements for Process Heat
For hydrogen production, high temperature – 750-1000°C, is required, and at 1000°C the conversion efficiency is three times that at 750 °C. The chemical plant needs to be isolated from the nearby reactor, for safety reasons, possibly using an intermediate helium or molten fluoride loop. 

Three potentially-suitable nuclear reactor concepts have been identified, though only the first is sufficiently well-developed to move forward with: 

• High-temperature gas-cooled reactor (HTR), either the pebble bed or hexagonal fuel block type. Modules of up to 285 MWe will operate at 950°C but can be hotter. 
• Advanced high-temperature reactor (AHTR), a modular reactor using a coated-particle graphite-matrix fuel and with molten fluoride salt as primary coolant. This is similar to the HTR but operates at low pressure (less than 1 atmosphere) and higher temperature, and gives better heat transfer. Sizes of 1000 MWe/2000 MWt are envisaged.
• Lead-cooled fast reactor, though these operate at lower temperatures than the HTRs – the most developed is the Russian BREST reactor which runs at only 540 °C. A US project is the STAR-H2 which would deliver 780 °C for hydrogen production and lower temperatures for desalination. 

These are described more fully in the Small Nuclear Power Reactors information page (with coolant characteristics) and the Advanced Nuclear Power Reactors page.  For temperatures beyond 750ºC, molten fluoride salts are a preferred interface fluid between the nuclear heat source and the chemical plant. The aluminum smelting industry provides substantial experience in managing them safely. The hot molten salt can also be used with secondary helium coolant generating power via the Brayton cycle, with thermal efficiencies of 48% at 750°C to 59% at 1000°C. There remain significant challenges in achieving sustained temperatures for commercial hydrogen production in a plant built for the chemical and thermal conditions.