Hydrogen production from nuclear power
This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
Introduction: implication for road transport
Nuclear power is relevant to road transport and motor vehicles in three respects: (1) hybrid vehicles potentially use off-peak power from the grid for recharging; (2) Nuclear heat can be used for production of liquid hydrocarbon fuels from coal; and (3) hydrogen for oil refining and for fuel cell vehicles may be made electrolytically, and, in the future, thermochemically using high-temperature nuclear reactors.
Hybrid vehicles are powered by batteries and an internal combustion engine. Higher capital cost is offset by lower running costs and lower emissions. Better batteries will allow greater use of electricity in driving, and mean that charging them can be done from mains power, as well as from the motor and regenerative braking. These plug-in electric hybrid vehicles (PHEV) are on the verge of being practical and economic today. See also paper on Electricity and Cars.
Widespread use of PHEVs that get much of their energy from the electricity grid overnight, at off-peak rates, would increase electricity demand and mean that a greater proportion of a country's electricity could be generated by base-load plants and hence at lower cost. Where the plant is nuclear, it will also be emission-free.
While thermochemical production of hydrogen is a long-term objective, it will require dedicated nuclear plants running at high temperatures. Electrolytic hydrogen production however can use off-peak capacity of conventional nuclear reactors.
Coal to liquid hydrocarbon fuels
The Fischer-Tropsch process was originally developed in Germany in the 1920s, and provided much of the fuel for Germany during the Second World War. It then became the basis for much oil production in South Africa by Sasol, which now supplies about 30% of that country's gasoline and diesel fuel. However, it is a significant user of hydrogen, catalyzing a reaction with carbon monoxide. The hydrogen is now produced with the CO by coal gasification, part of the gas stream undergoing the water shift reaction. A nuclear source of hydrogen coupled with nuclear process heat would more than double the amount of liquid hydrocarbons from the coal and eliminate most CO2 emissions from the process.
Using simply black coal, 14,600 tonnes produces 25,000 barrels of synfuel "oil" (with 25,000 tonnes of CO2).
The hybrid system uses nuclear electricity to electrolyse water for the hydrogen. Some 4400 tonnes of coal is gasified using oxygen from the electrolysis to produce carbon monoxide which is fed to the Fischer-Tropsch plant with the hydrogen to produce 25,000 barrels of synfuel "oil". Very little CO2 results, and this is recycled to the gasifier.
Hydrogen is already a significant chemical product, chiefly used in making nitrogen fertilizers and, increasingly, to convert low-grade crude oils into transport fuels (e.g., (CH)n tar sands or (CH1.5)n heavy crude to (CH2)n transport fuel). Some hydrogen is used for other chemical processes. World consumption is 50 million tonnes per year, growing at about 10% per year.
There is a lot of experience handling it on a large scale. Virtually all hydrogen is made from natural gas, giving rise tosignificant quantities of carbon dioxide (CO2) emissions.
Like electricity, hydrogen is an energy carrier (but not a primary energy source). As oil becomes more expensive, hydrogen may eventually replace it as a transport fuel and in other applications. This development becomes more likely as fuel cells are developed, with hydrogen as the preferred fuel, though storage at vehicle scale is a major challenge. Meanwhile hydrogen can be used in internal combustion engines.
If gas also becomes expensive, or constraints are put on carbon dioxide emissions, non-fossil sources of hydrogen will become necessary. Sun, wind, hydro and nuclear are all possible.
Like electricity, hydrogen for transport use will tend to be produced near where it is to be used. This will have major geo-political implications as industrialized countries become less dependent on oil and gas from distant parts of the world.
Electricity and hydrogen are convertible one to the other as energy carriers.
In the short-term, hydrogen can be produced economically by electrolysis of water in off-peak periods, enabling much greater utilization of base-load generating plants, including nuclear power plants. In the future, a major possibility is direct use of heat from nuclear energy, using a chemical process enabled by high-temperature reactors.
World oil refineries and chemical plants today have a demand for hydrogen (in thermal terms at 121 MJ/kg: 6050 PJ, equivalent to 70% of US nuclear electricity) which is drawing close to the US nuclear output in thermal terms. The rapidly-growing demand for hydrogen favors technologies with low fuel costs, and the scale of hydrogen demand is appropriate relative to its production by nuclear reactors. Limited hydrogen pipeline networks already exist, allowing production facilities to be some way from users.
In the USA, 11 Mt/yr of hydrogen production has thermal energy of 48 GWt, and constitutes 5% of US natural gas consumption, releasing 77 Mt CO2. The use of hydrogen for all US transport would require some 200 Mt/yr of hydrogen, though this scenario is a long way off. (89,88g hydrogen occupies 1 m3 at standard temperature and pressure (STP); 1 tonne hydrogen occupies 11,126 m3 at STP. Each tonne of hydrogen from natural gas gives rise to 11 tonnes of carbon dioxide emissions.)
All this points to the fact that while a growing hydrogen economy already exists, linked to the worldwide chemical and refining industry, a much greater one is in sight if storage and handling problems at automotive scale can be overcome. The development of affordable fuel cells will also be vital. With new uses for hydrogen as a fuel, the primary energy demand for its production may approach that for electricity production.
Meanwhile, a significant increase in electricity demand is likely due to wider adoption of hybrid vehicles. Charging the batteries of these from mains power will be cheaper than using their internal combustion engines. This demand is well within the planning horizons for new generating plants.
Nuclear energy and hydrogen production
Nuclear power already produces electricity as a major energy carrier. It is well-placed to produce hydrogen if this becomes a major energy carrier also.
The evolution of nuclear energy's role in hydrogen production over perhaps three decades is seen to be:
- electrolysis of water, using off-peak capacity;
- use of nuclear heat to assist steam reforming of natural gas up to 900 degrees C;
- high-temperature electrolysis of steam, using heat and electricity from nuclear reactors; then
- high-temperature thermochemical production using nuclear heat.
The first three are essentially cogeneration.
Efficiency of the whole process (from primary heat to hydrogen) then 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. From hydrogen to electric drive is only 30-40% efficient at this stage, giving 15-20% overall primary heat to wheels, compared with 25-30% for PHEV.
Low-temperature electrolysis using nuclear electricity is undertaken on a fairly small scale today, but the cost of hydrogen from it is higher (one source says $4-6 per kg, compared with $1.00-1.50 from natural gas, while another source says cost will be comparable to electricity at 4c/kWh when natural gas is US$ 9.50/GJ—cf $7 in July 2005).
High-temperature electrolysis (at 800°C or more) has been demonstrated, and shows considerable promise. US research is taking place at the Idaho National Laboratory in conjunction with Ceramatec.
Hydrogen 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.
There are then several possibilities. In the iodine-sulfur (IS) process, iodine combines with the SO2 and water to produce hydrogen iodide which then dissociates to hydrogen and iodine. This is the Bunsen reaction and is exothermic, occurring at low temperature (120°C):
This can deliver hydrogen at high pressure.
The net reaction is then:
All the reagents other than water are recycled; there are no effluents.
The Japan Atomic Energy Authority (JAEA) has demonstrated laboratory-scale and bench-scale hydrogen production with the IS process, up to 30 liters/hr.
The Sandia National Laboratory in the USA and the French CEA are also developing the IS process with a view to using high-temperature reactors for it.
General Atomics' preliminary laboratory work on thermochemical production should be complete by 2006. A 10MW pilot hydrogen plant using fossil heat would then be built, followed by nuclear thermochemical production by 2015.
The economics of hydrogen production depend on the efficiency of the method used. The IS cycle coupled with a modular high temperature reactor is expected to produce hydrogen at $1.50 to $2.00 per kg. The oxygen by-product also has value.
For thermochemical processes, an overall efficiency of greater than 50% is projected. Combined cycle plants producing both H2 and electricity may reach efficiencies of 60%.
Production reactor requirements
High temperature—750-1000°C—is required, though 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 reactor concepts have been identified, though only the first is sufficiently well-developed to move forward:
- High-temperature gas-cooled reactor (HTGR), 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 HTGR but operates at low pressure
- Lead-cooled fast reactor, though these operate at lower temperatures than the HTGRs—the best developed is the Russian BREST reactor which runs at only 540°C. A US project is the STAR-H2 which will deliver 780°C for hydrogen production and lower temperatures for desalination.
Each 600 MWt module would produce about 200 tonnes of hydrogen per day, which is well matched to the scale of current industrial demand for hydrogen.
The Korean Atomic Energy Research Institute (KAERI) has submitted a Very High Temperature Reactor (VHTR) design to the Generation IV International Forum, hoping that it can be used for hydrogen production. This is envisaged as 300 MWt modules each producing 30,000 tonnes of hydrogen per year. KAERI expects the design concept to be ready in 2008, engineering design in 2014, construction to start 2016 and operation in 2020.
KAERI also has a research partnership with China's Tsinghua University focused on hydrogen production, based on China's HTR-10 nuclear reactor. A South Korea-US Nuclear Hydrogen Joint Development Center involving General Atomics was set up in 2005.
Molten fluoride salts are a preferred interface fluid between the nuclear heat source and the chemical plant. The aluminium smelting industry provides substantial experience in managing these materials 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.
A 2004 evaluation by the Japan Atomic Energy Agency (JAEA) has indicated that by 2010 it expects to confirm the safety of high-temperature reactors and establish operational technology for an IS plant to make hydrogen thermochemically. In April 2004, a coolant outlet temperature of 950°C was achieved in its High-Temperature Engineering Test Reactor (HTTR)—a world first, opening the door for direct thermochemical hydrogen production.
Meanwhile, a pilot plant test project producing hydrogen at 30 m3/hr from helium heated with 400 kW is underway to test the engineering feasibility of the IS process. After 2010, an IS plant producing 1000 m3/hr (90 kg/hr, 2t/day) of hydrogen should be linked to the HTTR to confirm the performance of an integrated production system, envisaged for the 2020s.
JAEA plans a 600 MW GTHTR300C unit for hydrogen cogeneration using a direct cycle gas turbine for electricity production and the IS process for hydrogen production, deploying the first units after 2020. This could produce hydrogen at 60,000 m3/hr (130 t/day)—"enough for about a million fuel cell vehicles" (at 1 t/day for 7700 cars).
The economics of thermochemical hydrogen production seem sound. General Atomics projects US$1.53/kg based on a 2400 MWt HTGR operating at 850°C, with 42% ovrall efficiency, and $1.42/kg at 950°C and 52% efficiency (both 10.5% discount rate). At 2003 prices, steam reforming of natural gas yields hydrogen at US$1.40/kg, and sequestration of the CO2 would push this to $1.60/kg. Such a plant could produce 800 tonnes of hydrogen per day—"enough for 1.5 million fuel cell cars" (at 1 t/day for 1800 cars).
In the meantime, hydrogen can be produced by electrolysis of water, using electricity from any source. Non-fossil sources, including intermittent ones such as wind energy and solar energy, are important possibilities (thereby solving a problem of not being able to store the electricity from those sources). However, the greater efficiency of electrolysis at high temperatures favors a nuclear source for both heat and electricity.
Use of hydrogen as fuel for transport
Combustion of hydrogen produces only water vapor; there are no emissions of carbon dioxide or carbon monoxide. However, it is far from being an energy-dense fuel, and this limits its potential use for motor vehicles.
Hydrogen can be burned in a normal internal combustion engine, and some test cars are thus equipped. Trials in aircraft have also been carried out. In the immediate future the internal combustion engine is the only affordable technology available for using hydrogen. Several car companies have announced hydrogen-powered passenger vehicles. For instance, one hundred BMW Hydrogen 7s have been built, and 25 are used in test programs in the USA. The cars have already covered more than 2 million kilometres in test programs around the globe. BMW is currently the only car manufacturer using hydrogen stored in its liquid state.
However, eventually hydrogen's main use is likely to be in fuel cells. A fuel cell is conceptually a refuelable battery, making electricity as a direct product of a chemical reaction. But where the normal battery has all the active ingredients built in at the factory, fuel cells are supplied with fuel from an external source. They catalyze the oxidation of hydrogen directly to electricity at relatively low temperatures, and the claimed theoretical efficiency of converting chemical to electrical energy to drive the wheels is about 60% (or more). However, in practice, about half that has been achieved, except for the higher-temperature solid oxide fuel cells that have reached 46% efficiency.
On-board storage is the principal problem for hydrogen as an automotive fuel - it is impossible to store it as simply and compactly as gasoline or LNG fuel. The options are to store it at very low temperature (cryogenically), at high pressure, or chemically as hydrides. The last is seen to have most potential, though refuelling a vehicle is less straightforward. Pressurised storage is the main technology available now and this means that at 345 times atmospheric pressure (34.5 MPa, 5000 psi), ten times the volume is required than for an equivalent amount of petrol/gasoline. This disadvantage is coupled with a weight penalty due to the storage system, which is about 50 times heavier than the hydrogen it stores - the target is to get it down to 20 times as heavy by 2010, and perhaps ten times as heavy one day.
One promising hydride storage system utilizes sodium borohydride as the energy carrier, with high energy density. The NaBH4 is catalyzed to yield its hydrogen, leaving a borate (NaBO2) to be reprocessed.
Fuel cells are currently being trialled in electric forklift trucks and this use is expected to increase steadily. They apparently cost about three times as much as batteries but last twice as long and have less downtime. The first fuel cell electric cars running on hydrogen are expected to be on the fleet market soon after 2010. Japan has a goal of 5 million fuel cell vehicles on the road by 2020. (Current electric car technology relies on heavy storage batteries, and the vehicles have limited endurance before slow recharge.)
Current fuel cell design consists of bipolar plates in a frame, and the developer of the proton exchange membrane type, Dr Ballard, suggests that a new geometry is required to bring the cost down and make the technology more widely available to a mass market. The automotive division of Ballard is being sold to a Daimler-Ford joint venture (50.1% Daimler, 30% Ford, 19.9% Ballard), allowing Ballard to concentrate on stationary applications. Other reviews point out that fuel cells are intrinsically not simple and there are no obvious reasons to expect them to become inexpensive.
Fuel cells using hydrogen can also be used for stand-alone small-scale stationary generating plants—where higher temperature operation (e.g., of solid oxide fuel cells) and hydrogen storage may be less of a problem or where it is reticulated like natural gas. Cogeneration fuel cell units for domestic power and heat are being deployed in Japan under a subsidy scheme which terminates in 2012, by which time unit costs will need to drop from US$50,000 to $6000 and the units will need to last for a decade.
At present, fuel cells are much more expensive to make than internal combustion engines (burning petrol/gasoline, natural gas or hydrogen). Figures of over $1000 per kilowatt are quoted, compared with $100/kW for a conventional internal combustion engine.
The webpage H2Mobility provides information on some 400 current and past developed prototype cars, buses, trucks, bikes, ships and aircraft as well as speciality vehicles.
The initial use of hydrogen for transport is likely to be municipal bus and truck fleets, and prototypes are already on the road in many parts of the world. These are centrally-fuelled, so avoid the need for a retail network, and onboard storage of hydrogen is less of a problem than in cars.
Other large-scale hydrogen uses
A peak electricity nuclear system would produce hydrogen at a steady rate and store it underground so that it was used in large banks of fuel cells (e.g., 1000 MWe) at peak demand periods each day. Efficiency would be enhanced if by-product oxygen instead of air were used in the fuel cells.
As the scale of hydrogen production increases, more uses in the oil industry become feasible, particularly the extraction of oil from tar sands. Current practice uses natural gas to produce steam to recover the hydrocarbons, but projected increased production in Canada will exceed available gas supplies. Nuclear-produced hydrogen could thus be used for both heat and hydrogenation of the very heavy crude oil.
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