Content – Other fuels
You may also be interested in reading about: “The biofuel controversy”
Hydrogen is the first element of the periodic system and is the lightest of all molecules.
At ambient conditions on a volume basis hydrogen stores the least amount of energy compared to other energy carriers such as natural gas and gasoline. However on a mass basis hydrogen stores almost three times the energy of gasoline.
Hydrogen is a colorless, odorless non toxic but higly flammeable gas (at atmospheric pressure) and the most common element in the universe. However hydrogen is not freely available and needs to be generated from other compounds and to be seen as a energy carrier rather than an energy source.
More than 90% of the hydrogen currently generated (in total around 70 million tons annually, by mid 2019) is used for industrial purposes such as ammonia (NH3) and fertiliser production, in oil refining, steel production and other industrial processes.
Hydrogen can be generated from many types of energy sources or carriers, including natural gas.
Hydrogen burns in oxygen, including open air, forming water.
Chemical reaction:
2 H2 + O2 → 2 H2O
CO2 emission
There are no CO2 emmissions related to combustion of hydrogen only H2O if burning in pure oxygen. If however, hydrogen is burned in air (78,08 % nitrogen, 20,95 % okxygen, 0,94 % argon, 0,04 % carbon dioxide by volume) at high temperatures, NOx may be emitted.
More than 97% of the hydrogen currently generated is from fossil sources (6% of global natural gas use and 2% of global coal use is spent on hydrogen production annually, (2019)).
Steam reforming of Natural Gas
The reaction occurs at a temperature of 900℃, with nickel as the catalyst.
Steam reforming of Natural gas have two main steps:
- Reformation of Natural Gas
The first step of the SMR process involves methane reacting with steam at 750-800℃ to produce a synthesis gas (syngas), a mixture primarily made up of hydrogen and carbon monoxide (CO). - Shift Reaction
In the second step, the water gas shift (WGS) reaction, the carbon monoxide produced in the first reaction is reacted with steam over a catalyst to form hydrogen and carbon dioxide (CO2). This process occurs in two stages, consisting of a high temperature shift (HTS) at 350℃ and a low temperature shift (LTS) at 190-210℃ .
Chemical reaction: CH4 + 2H2O → 4H2 + CO2
This method requires around 45 kwh per kg hydrogen produced (this includes the energy carried by the methane used in this process)
Another method broadly used to generate industrial hydrogen is electrolysis from water. This method consumes a relatively large amount of electrical power (close to 50 kwh/kg H2 and another 15 kwh/kg H2 if compression is required), which currently is a limiting factor for its application. The process itself does not release carbon dioxide into the atmosphere.
On the other hand, If the electricity used for hydrogen production is “surplus” electricity that could not have been used for direct consumption, the efficiency issue would of course come out differently. This could be the case if the electricity is produced from an intermittent source, such as wind power that otherwise would have to be shut down due to lack of current consumers for the electrical power produced. In this case the hydrogen would serve as one viable solution for storage of energy.
Steam reforming of natural gas
The reaction occurs at a temperature of 900 oC, with nickel as the catalyst.
Chemical reaction: CH4 + 2H2O → 4H2 + CO2
Electrolysis from water
Electrolysis at ambient temperature and ambient pressure requires a minimum voltage of 1.481 volt. The theoretical voltage required at ambient temperature and pressure is only 1,23 volt but due to various process losses the actual voltage is 1,48 volt.
The theoretical power needed to split water into hydrogen and oxygen is 39.4 kilowatt-hours per kilogram (142 MJ/kg)
The power consumtion for industrial electrolysers is 3.8 – 4.4 kWh/Nm3 H2 or 41,8 – 48,4 kWh/kg hydrogen. One kilogram of hydrogen represents about 11Nm3.
To produce 1 kg of H2 around 9 litres of water is needed. This process also produces 8 kg of oxygen as a by-product.
Freshwater or desalinated water is needed to produce hydrogen due to issues related to corrosion and chlorine production if untreated seawater is used.
Desalination of seawater requires an additional 4 kWh per m3 of water to be treated.
Methanol cracking
Mathanol and steam is by the presence of a copper-zinc cathalyst split into H2, CO, CO2, CH4 and H2O (water vapour). The reaction occurs at a temperature of around 300 oC and at a pressure between 10 – 25 bar.
Chemical reaction: 2CH3OH + 2H2O → 5H2 + CO2 + CO + H2O
Partial oxidation of hydrocarbons
Oxygen is reacted with hydrocarbon in a sub stoichiometric process (less oxygen than what is needed for a complete combustion of the hydrocarbon). The chemical reaction when methane is used is:
CH4 + ½ O2 + H2O → CO2 + 3H2
Pyrolysis
This method is an endothermic process that decomposes hydrocarbon fuels into hydrogen and carbon (“carbon black”).
Hydrogen can be produced by pyrolytic decomposition of natural gas. When formed, elemental solid carbon (C) is formed. When splitting of the hydrocarbon molecules occurs pyrolytically in an electric arc (plasma), carbon and hydrogen are the only products.
This electrical power consumption of this process is around 14 kwh/kg hydrogen and it produces 3kg of carbon.
The chemical reaction if methane is used is:
CH4 → C (carbon black) + 2H2
For more information about plasma assisted decomposition please go here: “How to turn oil and gas into low emission fuel”
Energy content
Weight Based energy content & Volume Based Energy content for some fuels (based on lower heating value (LHV))
| Fuel | Weight Based energy content
(kWh/kg) |
Volume Based Energy content
(kWh/Nm3) |
| Hydrogen (H2) | 33,33 | 3,00 |
| Natural gas (82-93% CH4) | 10,6 – 13,9 | 8,8 – 10,4 |
| Methane (CH4) | 13,9 | 9.94 |
| Propane (C2H5) | 12,86 | 34,4 |
| Gasoline | 12,33 | 9,1 kWh/l |
| Diesel | 12,06 | 10 kWh/l |
| Ammonia (NH3) | 5,2 | 3,92 kWh/l |
* The higher heating value (HHV) is 18,8% higher than the lower heating value
Storage of hydrogen
Current methods being developed include compressed gas, cryogenic liquid and absorbed solid.
Hydrogen is a gas that occupies a large volume under ambient conditions,
for storage (11 m3/kg).
Energy content of hydrogen at various conditions
| Pressure | kWh/m3 | Compared to gasoline (%) |
| Atmospheric | 3 | 0,03 |
| 200 bar | 592 | 6,51 |
| 350 bar | 1036 | 11,38 |
| 500 bar | 1480 | 16,26 |
| 750 bar | 2220 | 24,4 |
Comparison of storage methods
| Tecnology | Volume (L) | Weight
(kg) |
Density (%H2 by weigth) |
|
35 MPa (350 bar) compressed H2 |
145 | 45 | 6,7 |
| 70 MPa (700 bar) compressed H2 | 100 | 50 | 6,0 |
| Cryogenic liquid H2 | 90 | 40 | 7,5 |
| Low-temperature metal hydride | 55 | 215 | 1,4 |
700 bar compressed hydrogen storage is the most common solution for lighter vehicles while 350 bar compressed hydrogen is more often used for heavier vehickles.
Storage of pressurized hydrogen
The current practical limitation for storage of pressurized hydrogen is around 750 bar due material and design limitations of pressure vessels.
Storage of liquified hydrogen
Liquifaction of hydrogen reqires large amounts of energy, around 1/3 of the energy content of the hydrogen itself. To liquify hydrogen the temperatire needs to be -253oC or less and is therefore called kyrogen storage (20K above absolute zero). The density of liquid hydrogen is 71 kg/m3
Storage (solid) of hydrogen within metal hydrides
Hydrogen can also be stored in metal hydrides.
These are chemical compounds that are formed when gaseous hydrogen reacts with a metal.
The metal is normally in the form of powder, whose particles are only a few micrometers in diameter.
The most interesting metal hydrides can take up hydrogen at room temperature and at moderate pressure (eg. 5 bar)
The hydrogen is compressed more closely in the metal hydride than in pure liquid form. This is due to forces in the metal lattice.
Metal hydrid storage requires the least amount of energy to operate compared to other methods. The main disadvantage, currently being the weíght since the metal hydrids only can hold less than 2% of hydrogen by weight.
Storage of hydrogen in hydrogen-rich fluids
Hydrogen stored in hydrogen-rich liquids such as ammonia (NH3) or methanol (CH3OH) significantly redusces the volume.
Used for vehicular purposes the fluid is reformed and pure hydrogen reacts in a PEM fuel cell to generate elecgtricity.
For methanol reformation the chemical reaction is:
CH3OH + H2O → CO2 + 3H2
Forming 0.188 kg H2 per. kg methanol reformed.
Hydrogen as an energy carrier.
Currently the use of hydrogen as a source of energy or as an energy carrier is very limited. However, finding “cleaner” sources of energy to replace fossil fuels is becoming more and more important from an environmental and community health standpoint as well as from supply security reasons.
Several car and truck manufacturers have over the last decades and are still exploring the possibility to use hydrogen as an energy carrier.
Within transportation hydrogen may be used to produce electricity via fuel cells or it may be combusted in internal combustion engines similar to compressed or liquefied natural gas.
Provided the hydrogen has been produced from non or low polluting sources the emissions of harmful substances would be very low.
In the case of using hydrogen in fuel cells the local emissions would only be water (H2O).
If the hydrogen is used for combustion, depending on the combustion temperature, NOx also might be released in addition to the water.
The total efficiency would be low since the solution would suffer from a relatively low efficiency of hydrogen generation in addition to the rather low efficiency of an internal combustion engine (25% – 35%).
In addition to automotive and transportation, hydrogen is also considered as a replacement for natural gas within national gas grids providing gas for domestic and industrial heating.
Both full and partial (partial in this connection means mixing hydrogen with natural gas) replacement is considered.
Using hydrogen within a natural gas grid has several challenges related to replacement or modification of appliances and also related to the grid itself. This is to a large extent due to the small size of the hydrogen molecules, potentially causing leakage and issues related to the grid material.