Without delving deep into the chemistry, atomic hydrogen is the most abundant element in the universe and is fundamental to the existence of stars and gas planets. However, it rarely exists in earth’s atmosphere as molecular gas H2 due to it’s readiness to form compounds with other molecules, and the fact that it is so light, that it floats above all other heavier gases.
Hydrogen is present on earth in huge amounts as it is a constituent in water, hydrocarbons and other organic compounds. So, hydrogen needs to be extracted by way of chemical reactions before being used.
Experiments by various scientists in the 17th century led to its discovery and since then, the importance of hydrogen in organic and nuclear chemistry has been increasingly understood. The by-product of burning hydrogen is water and this gave rise to its name, Greek for ‘water former’ and it is for this reason that hydrogen forms part of the technology roadmap for vehicles. Using hydrogen as a fuel has a zero emissions reaction at point of use however, the means to produce hydrogen are very energy reliant so sceptics use this argument to counter the ‘zero emissions’ aspect of hydrogen use.
There are many laboratory methods of extracting hydrogen from where it exists as part of a chemical compound but only a relatively small number have been commercialised for production on an industrial scale. This article describes the main techniques for hydrogen production.
The main industrial method for hydrogen production is by steam reforming of hydrocarbon materials such as natural gas, oil and coal. Other methods exist on an industrial scale such as electrolysis of water, however, this only accounts for a very low percentage of overall hydrogen production. Electrolysis could become more important though as it offers a ‘green’ route to hydrogen production if the electricity used is from a renewable source.
Steam reformation of natural gas
This method accounts for roughly half of the global hydrogen production and is possible due to the high methane content of natural gas. High temperature steam at around 1,000 °C is reacted with the methane to produce hydrogen and carbon monoxide. Introduction of steam in a further lower temperature stage oxidises the carbon monoxide to form carbon dioxide and yet more hydrogen.
The by-product of this process is carbon dioxide, however, as the large quantities of CO2 are produced ‘on-site’ it can be captured and dealt with in a number of ways without releasing to the atmosphere.
Gasification of coal
This process is similar to steam reformation where steam and oxygen are combined with coal to produce, amongst other things, syngas, which is a gaseous mixture of hydrogen gas and carbon monoxide. It is important not to fully oxidise or combust the coal. The hydrogen can be stripped out of the syngas directly or the syngas can go through another oxidising process, like in reformation, and release more quantities of hydrogen.
This is a process developed in the 1980s by Kværner, whereby a plasma burner operating around 1,600 °C separates the carbon and hydrogen from a hydrocarbon material, typically methane, natural gas or biogas. The advantages over steam reformation are that the Kværner process is more efficient and doesn’t produce any CO or CO2. The by-product of this process is pure carbon powder which has many industrial uses. It can be argued that it is a ‘green’ process if the energy used in the plasma burner is created renewably.
Developments of this process have led to it being useful for the conversion of municipal solid waste (MSW or landfill) into useful by-products, including hydrogen. The chemical outputs depend on what type of waste is used as the feedstock and this process also has the capability to use and convert hazardous materials, rendering them harmless. Although not yet developed on an industrial scale, this process demonstrates advantages over steam reformation and gasification; with a reduction in energy requirement and the fact that landfill waste products can be the feedstock.
The production of hydrogen and oxygen from the electrolysis of water has been well understood since the 18th century. Two electrodes placed in water are supplied with electricity and hydrogen forms at the negatively charged cathode and oxygen forms at the positively charged anode. Although the principle is very simple, it has not been a popular method on an industrial scale due to the fact that it requires a greater energy input compared with the other methods mentioned and is therefore less efficient. However, it has the potential to become more viable if the energy used comes from a renewable source and, of course, the feedstock is readily available.
Adding an electrolyte to the water will increase its conductivity and speed up the process, however, careful electrolyte selection is required to make the process effective. The choice of electrolyte also affects what gases are produced and if normal salt is used as the electrolyte, chlorine will be produced instead of oxygen. Much of the hydrogen production by electrolysis is actually as a by-product of the industrial production of chlorine.
R&D activities for other methods of production
As mentioned earlier there are many methods for hydrogen production with much going on in the research and development field. Steam reformation of bio-derived liquids (oils, alcohols and sugar based materials) is relatively straightforward and has the advantage of starting off with a renewable feedstock rather than a fossil fuel. The industry infrastructure is already there to produce hydrogen in this way so can be viewed as a near-term solution however, the bio-mass derived fuel industry requires additional development to support the process.
Photoelectrochemical hydrogen production is a straight water-splitting process. Special semiconductors are required that absorb sunlight and split water molecules in a chemical process yielding hydrogen and oxygen. The semiconductors need to be efficient and durable. Efficient in the desire to absorb light over a wide band of wavelengths and durable in that they do not adversely react with any electrolytes. This is a longer-term solution as the materials are still at the R&D stage.
Another long-term prospect is a solar thermochemical process to manufacture hydrogen in a sustainable way. This is a closed-loop process where a temperature of around 2,000 °C is produced by focussing the suns rays by mirrors or lenses. There are hundreds of compounds that can be reacted in such a process but one that has been identified is zinc oxide powder. The focussed solar heat dissociates the zinc and oxygen and then water is added. A further chemical reaction oxidises the zinc, releasing hydrogen from the water molecule. The resulting zinc oxide is then reused and the process starts again.
Specialised micro-organisms can produce hydrogen during their metabolism under direct sunlight. Scientists are researching this area, identifying suitable biological material such as green algae and some types of bacteria. This method of hydrogen production shows some long term merit but it is not without its challenges, one of which is that the oxygen that is also produced impedes the enzymes responsible for hydrogen production.
Industrial processes have been in place for many years to manufacture hydrogen however, they are energy intensive. There is also a challenge to cost-effectively get the hydrogen to its point of use. For these reasons, there are opponents of the idea of a hydrogen economy, citing the energetics of the hydrogen economy just not working out.
Potential solutions include substantial uses of renewable energy although this in itself requires investment. There is also the question of how to transport hydrogen from where it is made to its point of use. A solution for this could be to localise hydrogen production where some of the industrial processes could be scaled down to have small reformation plants on site at vehicle refuelling stations.
ITM Power and Hydrogenics are two companies that produce small scale electrolyser and hydrogen storage and delivery systems that would suit localisation in this way. There are a number of examples of onsite reformers including one based in Nevada by Air Products.
Legislation is driving vehicle emissions to be cleaner than they have ever been before and so car manufacturers are developing solutions that use hydrogen either as a combustible fuel or with fuel cells for electric drive systems. These factors are pushing the hydrogen production industry to come up with solutions that move towards a hydrogen economy although there is still a lot of work to do to make this a viable alternative to liquid fuels.
Writer: Phil Barker