Hydrogen

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Content – Other fuels

 


 
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.

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 % karbondioksid by volume) at high temperatures, NOx may be emitted.

Production of hydrogen

The annual global production of hydrogen is around 0,050 Gt (Gigaton – 1 billion ton), most of the hydrogen is used for ammonia (NH3) production.

Most of the hydrogen production at industrial scale is currently done by steam reforming of natural gas.

This process consumes energy and releases carbon dioxide into the atmosphere.

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.

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 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.

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 for methane 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 for methane 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))

FuelWeight Based energy content

(kWh/kg)

Volume Based Energy content

(kWh/Nm3)

Hydrogen (H2)33,333,00
Natural gas (82-93% CH4)10,6 – 13,98,8 – 10,4
Methane (CH4)13,99.94
Propane (C2H5)12,8634,4
Gasoline12,339,1 kWh/l
Diesel12,0610 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

PressurekWh/m3Compared to gasoline (%)
Atmospheric30,03
200 bar5926,51
350 bar103611,38
500 bar148016,26
750 bar222024,4

Comparison of storage methods

TecnologyVolume (L)Weight

(kg)

Density (%H2 by weigth)
 

35 MPa (350 bar) compressed H2

145456,7
70 MPa (700 bar) compressed H2100506,0
Cryogenic liquid H290407,5
Low-temperature metal hydride552151,4

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 hydrocarbon 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.

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