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  Hydrogen: production
 

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Hydrogen makes up some 90% of the universe by mass; however pure hydrogen, in the form of its diatomic molecule H2, is extremely reactive and is therefore rare in nature: most of the hydrogen present on earth is bound into chemical compounds such as water or hydrocarbons. As a result, the first challenge in the use of hydrogen as a fuel is the production of hydrogen from its compounds.

Molecular hydrogen H2 can be produced through a number of physical and chemical processes by which a compound containing hydrogen is split; in the majority of processes, the source of hydrogen is either water or a hydrocarbon.

The following table presents the main hydrogen production methods.

H2 production from hydrocarbons

Method of production

Source of hydrogen

Description

Steam Reforming

 
Non-renewables:
natural gas
diesel
gasoline
alcohols

Renewables :
biogases
 

The cheapest way of making hydrogen at present is by reforming it from natural gas (mainly methane, CH 4). Most of the hydrogen in use today is made by this method. In order to split the hydrogen from the carbon, the methane is mixed with high temperature steam under pressure and with a catalyst present, producing carbon monoxide (CO) and hydrogen. A second reaction, known as a shift reaction, is then applied to produce more hydrogen and some water from the CO.

Partial oxidation can be used to brake down hydrocarbons such as biomass into char, oils and vapours by reacting them with limited amounts of oxygen to prevent complete combustion. The residues can then be steam reformed.

Gasification


Non-renewables:
coal
oil

Renewables: biomass
 

A gasifier converts the fuel into a synthesis gas (mixture of H2 and CO) by adding steam and oxygen. Normally, gasification is followed by fuel reforming.

Pyrolysis

 
Non-renewables:
coal
oil
methane

Renewables :
biomass
 

Hydrocarbons are converted to hydrogen, without producing carbon dioxide, at a sufficiently high temperature in the absence of oxygen.


H 2 production from water

Method of production

Resources that could be used

Description

Electrolysis

 
Non-renewables:
If electricity is produced from fossil fuels

Renewables:
If hydro, geothermal, wind, solar or nuclear electricity is used
 

Considered as the standard production method, electrolysis on a small scale can be carried out almost anywhere - passing a current through water is sufficient to generate a few bubbles of hydrogen at the cathode and oxygen at the anode. In order to make it efficient an electrolyte is required - an alkali such as potassium hydroxide is often used - and efficiencies of about 90% are standard. Research into high temperature and polymer electrolyte electrolysers is progressing with the hope that these may be cheaper or more efficient than current technology.

Direct heating

 
Non-renewables:
If heat is produced from fossil fuels

Renewables:
If heat is produced from biomass
 

By means of direct heating to a very high temperature, water vapour dissociates into hydrogen and oxygen.

Thermo-chemical reaction

 
Non-renewables:
If heat is produced from fossil fuels

Renewables:
If heat is produced from biomass
 

Thermochemical reactions are complex and multi-stage but show promise for future applications since the temperature needed to split hydrogen-oxygen bonds is about 2,800°C.

Photoche-mical systems

 
Renewables:
Solar light
 

Photochemical systems are similar to thermochemical systems although in this case, the driving force is not thermal energy but solar light.

Photoelectro-chemical systems

 
Renewables :
Solar light
 

In a photoelectrochemical (PEC) system, a photoactive electrode is used: a semiconductor material forming a junction when immersed in aqueous solution. Energy from incident light promotes electrons within the material to form an electron-hole pair, which drive the oxidation and reduction reactions needed for water splitting (catalysts are also used to aid the reaction). PEC cells were first demonstrated by Fujishima and Honda in 1972, using a titanium dioxide electrode in an aqueous electrolyte. Since then, work in this area has aimed to improve the efficiency and stability of the technology, and to reduce the cost.

Biological or biochemical reactions

..

Biological and biochemical routes make use of the ability of microorganisms to produce hydrogen. Such methods have the advantage of producing hydrogen at low temperatures (approximately room temperatures) as opposed to the high temperatures used in direct thermal decomposition or thermochemical hydrogen production.

 

 

For further information contact David Hart (email: firstname.lastname@e4tech.com) from E4tech

 

 
 
 
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