CIEC Promoting Science at the University of York, York, UK

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Basic chemicals Hydrogen
Hydrogen

Hydrogen is one of the key starting materials used in the chemical industry.  It is a fundamental building block for the manufacture of ammonia, and hence fertilizers, and of methanol, used in the manufacture of many polymers.

 

Uses of hydrogen

Hydrogen is used in the manufacture of two of the most important chemical compounds made industrially, ammonia and methanol. It is also used in the refining of oil, for example in reforming, one of the processes for obtaining high grade petrol and in removing sulfur compounds from petroleum which would otherwise poison the catalytic converters fitted to cars.

In years to come, hydrogen itself may become one of the most important fuels for cars as on burning it does not produce carbon dioxide, but there are major problems to be overcome before it can be used in this way, including its manufacture, storage and distribution.

A pie chart illustrating the uses of hydrogen, the largst one being in the manufacture of ammonia for fertilizers

Figure 1  Uses of hydrogen.

Annual production of hydrogen

World 50 million tonnes

Manufacture of hydrogen

By far the most important process for making hydrogen is by steam reforming.

The key parts of the process are the conversion of a carbon-containing material to a mixture of carbon monoxide and hydrogen followed by the conversion of carbon monoxide to carbon dioxide.

At present, the hydrocarbon used is generally methane or other light hydrocarbons obtained from natural gas or oil.

The gas or vapour is mixed with a large excess of steam and passed through pipes containing nickel oxide (which is reduced to nickel during the reaction), supported on alumina, in a furnace which operates at high temperatures:

The reaction is endothermic and accompanied by an increase in volume.  It is thus favoured by high temperatures and by low partial pressures.  The reaction is also favoured by a high steam:hydrocarbon ratio.  This increases the yield but increases operating (energy) costs.  The high ratio also helps to reduce the amount of carbon deposited which reduces the efficiency of the catalyst.  The most effective way to reduce carbon deposition has been found to be impregnation of the catalyst with potassium carbonate.
In the second part of the process, the shift reaction, carbon monoxide is converted to carbon dioxide:

This reaction is significantly exothermic, and so high conversions to carbon dioxide and hydrogen are favoured by low temperatures.  This is difficult to control due to the heat evolved, and it has been common practice to separate the shift reaction into two stages, the bulk of the reaction being carried out at around 650 K over an iron catalyst, and the 'polishing' reaction carried out around 450 K over a copper/zinc/alumina catalyst.

The carbon dioxide and any remaining carbon monoxide are then removed by passing the gases through a zeolite sieve. From time to time, the vessel containing the sieve is taken out of the gas stream and flushed with hydrogen to displace carbon dioxide and regenerate the sieve.
To obtain a sustainable (i.e. zero emission of greenhouse gases) production of hydrogen, the carbon dioxide is captured and stored or used.

Thus, overall, one mole of methane and two moles of steam are theoretically converted into four moles of hydrogen, although this theoretical yield is not achieved as the reactions do not go to completion:

There is a significant change in the choice of fuel used in the reformer. Instead of a hydrocarbon gas, coal is more available than methane in some countries (notably China). Methane and other gases based on oil are in short supply and need to be imported.

Research is being undertaken to see whether biomass instead of coal or oil can be used effectively to manufacture hydrogen.  The issue is that the energy used in collecting it, transporting it to the place of use can be high relative to the savings in switching to biomass.

The future

Hydrogen is potentially an environmentally attractive fuel for the future. When it burns to produce energy, the only product is water.

The obvious route to produce the gas is by the reverse process, the electrolysis of water. The overall equation is:

However, this needs electricity from power stations.  If the station uses fossil fuels, it defeats the purpose, namely to produce a fuel without the production of carbon dioxide. Other forms of generating power, such as nuclear, wind and geothermal, do not have this disadvantage.

Much research is being undertaken to produce fuel cells where about half of the energy from the reaction between hydrogen and oxygen to produce water is released as an electrical potential. One such fuel cell is the PEM cell, where PEM stands for Polymer Electrolyte Membrane or Proton Exchange Membrane.  Both describe the action of the cell.

A stream of hydrogen is delivered to the anode on one side of a membrane (a fluorinated polymer based on poly(tetrafluoroethene) (PTFE) or carbon fibre.

The protons permeate the membrane and react with oxygen at the cathode:

Both reactions are catalysed by platinum, which is in the form of nanoparticles embedded in the electrodes.

A diagram illustrating the structure of a membrane cell for the production of hydrogen from water

Figure 2  Structure of a membrane cell.

The hydrogen must be very pure.  If it contains even small amounts of carbon monoxide from its manufacture, the catalyst is rapidly poisoned.

Much research is being conducted to find ways of improving the lifetime of the catalysts used.

Although fuel cells are being demonstrated in some cars and other vehicles, there are practical difficulties in the distribution and storage of hydrogen.  One approach would be to convert a liquid fuel into hydrogen, in situ in the car. For example, methanol is being used in experimental cars.  The vapour is converted into hydrogen and carbon dioxide by a reforming reaction, similar to the process described above for the large scale manufacture of hydrogen. This demands a very high level of engineering skills to produce conversion units which are light enough for a car but strong enough to withstand all the problems caused by continuous vibrations.  Projects are underway around the world to convert natural gas to hydrogen in mini-reformers.

An enormous amount of research is also being undertaken on using sunlight as the energy source, one being via biophotolysis.

This involves the production of algae in water through photosynthesis, followed by bacterial decomposition of the algae to produce hydrogen.  An important discovery was that by depriving the algae of sulfur, normal photosynthesis is inhibited and instead an enzyme is activated and hydrogen, not oxygen is produced in light.  Present research is concerned with making these processes more efficient.

 

Date last amended: 18th March 2013