Hydrogen fuel is a zero-emission fuel burned with oxygen. It can be used in fuel cells or internal combustion engines. It has begun to be used in commercial fuel cell vehicles, such as passenger cars, and has been used in fuel cell buses for many years. It is also used as a fuel for spacecraft propulsion.
As of 2018, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming or partial oxidation of methane and coal gasification with only a small quantity by alternative routes such as biomass gasification or electrolysis of water or solar thermochemistry, a solar fuel with no carbon emissions.
Hydrogen is found in the first group and first period in the periodic table, i.e. it is the lightest and first element of all. Since the weight of hydrogen is less than air, it rises in the atmosphere and is therefore rarely found in its pure form, H2. In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy.
- 2H2 (g) + O2 (g) → 2H2O (g) + energy
If carried out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.
The energy released enables hydrogen to act as a fuel. In an electrochemical cell, that energy can be used with relatively high efficiency. If it is used simply for heat, the usual thermodynamics limits on the thermal efficiency apply.
Hydrogen is usually considered an energy carrier, like electricity, as it must be produced from a primary energy source such as solar energy, biomass, electricity (e.g. in the form of solar PV or via wind turbines), or hydrocarbons such as natural gas or coal. Conventional hydrogen production using natural gas induces significant environmental impacts; as with the use of any hydrocarbon, carbon dioxide is emitted. At the same time, the addition of 20% of hydrogen (an optimal share that does not affect gas pipes and appliances) to natural gas can reduce CO2 emissions caused by heating and cooking.
Because pure hydrogen does not occur naturally on Earth in large quantities, it usually requires a primary energy input to produce on an industrial scale. Hydrogen fuel can be produced from methane or by electrolysis of water. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming or partial oxidation of methane and coal gasification with only a small quantity by other routes such as biomass gasification or electrolysis of water.
Steam-methane reforming, the current leading technology for producing hydrogen in large quantities, extracts hydrogen from methane. However, this reaction releases fossil carbon dioxide and carbon monoxide into the atmosphere which are greenhouse gases exogenous to the natural carbon cycle, and thus contribute to climate change. In electrolysis, electricity is run through water to separate the hydrogen and oxygen atoms. This method can use wind, solar, geothermal, hydro, fossil fuels, biomass, nuclear, and many other energy sources. Obtaining hydrogen from this process is being studied as a viable way to produce it domestically at a low cost.
The world's largest facility for producing hydrogen fuel is claimed to be the Fukushima Hydrogen Energy Research Field (FH2R), a 10MW-class hydrogen production unit, inaugurated on 7 March 2020, in Namie, Fukushima Prefecture. The site occupies 180,000 square meters of land, much of which is occupied by a solar array; but power from the grid is also used to conduct electrolysis of water to produce hydrogen fuel. 
Hydrogen is locked up in enormous quantities in water, hydrocarbons, and other organic matter. One of the challenges of using hydrogen as a fuel comes from being able to extract hydrogen efficiently from these compounds. Now, steam reforming, which combines high-temperature steam with natural gas, accounts for the majority of the hydrogen produced. This method of hydrogen production occurs at temperatures between 700-1100°C, and has a resultant efficiency of between 60-75%. Hydrogen can also be produced from water through electrolysis, which is less carbon intensive if the electricity used to drive the reaction does not come from fossil-fuel power plants but rather renewable or nuclear energy instead. The efficiency of water electrolysis is between about 70-80%, with a goal set to reach 82-86% efficiency by 2030 using proton exchange membrane (PEM) electrolyzers. Once produced, hydrogen can be used in much the same way as natural gas - it can be delivered to fuel cells to generate electricity and heat, used in a combined cycle gas turbine to produce larger quantities of centrally produced electricity or burned to run a combustion engine; all methods producing no carbon or methane emissions. In each case hydrogen is combined with oxygen to form water. This is also one of its most important advantages as hydrogen fuel is environmentally friendly. The heat in a hydrogen flame is a radiant emission from the newly formed water molecules. The water molecules are in an excited state on initial formation and then transition to a ground state; the transition releasing thermal radiation. When burning in air, the temperature is roughly 2000 °C (the same as natural gas). Historically, carbon has been the most practical carrier of energy, as hydrogen and carbon combined are more volumetrically dense, although hydrogen itself has three times the energy density per mass as methane or gasoline. Although hydrogen is the smallest element and thus has a slightly higher propensity to leak from venerable natural gas pipes such as those made from iron, leakage from plastic (polyethylene PE100) pipes is expected to be very low at about 0.001%.
The reason steam methane reforming has traditionally been favoured over electrolysis is because whereas methane reforming directly uses natural gas, electrolysis requires electricity. As the cost of producing electricity (via wind turbines and solar PV) falls below the cost of natural gas, electrolysis becomes cheaper than SMR.
Hydrogen fuel can provide motive power for liquid-propellant rockets, cars, trains, boats and airplanes, portable fuel cell applications or stationary fuel cell applications, which can power an electric motor. The problems of using hydrogen fuel in cars arise from the fact that hydrogen is difficult to store in either a high pressure tank or a cryogenic tank.
Internal combustion engine conversions to hydrogen
Combustion engines in commercial vehicles have been converted to run on a hydrogen-diesel mix in the UK,[when?] where up to 70% of emissions have been reduced during normal driving conditions. This eliminates range anxiety as the vehicles can fill up on diesel. Minor modifications are needed to the engines, as well as the addition of hydrogen tanks at a compression of 350 bars. Trials are now underway to test the efficiency of the 100% conversion of a Volvo FH16 heavy-duty truck to use only hydrogen. The range is expected to be 300km/17kg; which means an efficiency better than a standard diesel engine (where the embodied energy of 1 gallon of gasoline is equal to 1 kilogram of hydrogen). At a low cost price for hydrogen (€5/kg), significant fuel savings could be made via such a conversion in Europe or the UK. A lower price would be needed to compete with gasoline in the US, as gasoline is not exposed to high taxes at the pump.
Hydrogen fuel is hazardous because of the low ignition energy and high combustion energy of hydrogen, and because it tends to leak easily from tanks. Explosions at hydrogen filling stations have been reported. Hydrogen fuelling stations generally receive deliveries of hydrogen by truck from hydrogen suppliers. An interruption at a hydrogen supply facility can shut down multiple hydrogen fuelling stations.
- Fuel cell vehicle
- Hydrogen compressor
- Hydrogen safety
- Hydrogen storage
- Hydrogen technologies
- Hydrogen vehicle
- Oxyhydrogen flame
- Photocatalytic water splitting to isolate hydrogen
- Synthetic fuel
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