Traditionally, coal has been burned directly as a fuel to provide energy for heating and is still extensively used globally for electricity generation. However, due to its chemical structure and composition, using it in this way is energy inefficient. It is predominantly carbon (75-90%); most of the carbon atoms in its chemical structure are linked to surrounding carbon atoms, some are bonded to hydrogen atoms. There are also small and variable amounts of the elements nitrogen, sulphur, oxygen and various metallic elements e.g. iron. (See Chemistry Explained: Coal.)
Combustion produces a variety of pollutants—acid rain-producing oxides of nitrogen and sulphur, as well as the greenhouse gas CO2 and fine particles of soot (particulates), which are associated with lung damage. Gasification of coal produces synthesis gas (syngas), a cleaner fuel of higher calorific value that can also serve as a feedstock for the manufacture of a variety of chemicals.
Coal gasification to produce syngas
Syngas is of variable composition, but principally carbon monoxide (CO) and hydrogen (H2). It also contains variable amounts of carbon dioxide, methane and other impurities depending on the precise conditions employed in the gasification process.
The main chemical reactions essential to the formation of syngas from coal are described here. Chemical equations are used in the descriptions, but those not familiar with such equations just need follow the accompanying “word equations”.
Gasification occurs in large vessels able to withstand high pressures (gasifiers). The coal is blasted alternately with hot air and steam. Under the conditions in the gasifier, some of the carbon (C) in the coal undergoes oxidation as follows:
Reaction 1 C + O2 → CO2
Carbon in the coal + oxygen in the air → carbon dioxide
Note that the conditions in the gasifier (including the presence of steam and the level of oxygen) mean that only partial oxidation of the carbon content of the coal occurs).
Reaction 1 gives out a lot of heat (i.e. it is highly exothermic), making the temperature about 1400o Celcius. This heat drives out volatile material (including hydrogen, methane, ammonia and tar) from the coal. A solid porous residue is left, referred to as char. (Essentially this is coke, which is commonly used in some solid fuel burners.)
The heat from reaction 1 also drives the following two reactions, which each require energy to make them happen (i.e. they are endothermic reactions).
Reaction 2 between carbon in the char and steam:
C + H2O → CO + H2
Carbon in the char + steam → carbon monoxide + hydrogen
Reaction 3 between CO2 and carbon:
CO2 + C → 2 CO
Carbon dioxide + carbon in the char → carbon monoxide
The outcome of these changes (i.e. the addition of the three equations) is essentially a mixture of carbon monoxide and hydrogen in a volume ratio of 3:1. It should be noted that air contains mostly nitrogen (about 80%) and only about 20% oxygen. The syngas will therefore be diluted with nitrogen. This mixture can be used to manufacture ammonia (NH3) where the nitrogen is combined with hydrogen. Ammonia forms the basis of many fertilisers. However, if such dilution is not required, pure oxygen is used instead of air.
Other sources of syngas
Virtually any organic chemical (i.e. carbon-containing) source can undergo gasification to syngas with the chemistry involved similar to that described for coal. Petroleum fractions and natural gas have been used for many years. The drive towards C-neutrality and energy efficiency has led to the use of biomass (e.g. wood and food waste) and other household and industrial waste for gasification. (See below Developments in the North-east.)
Underground coal gasification
The British Isles has vast coal reserves (available with current technology) and resources (unavailable with current technology). Much is under the sea. Rather than bring the coal to the surface (requiring considerable energy) and then gasifying it, these reserves and resources could be gasified underground and the resultant syngas pumped to the surface.
Figure 1 outlines the process of underground gasification. The chemical reactions involved are essentially the same as described above. Also shown is how syngas can be used in electricity generation and how the CO2 from its combustion may be returned underground for permanent storage. In the interests of C-neutrality it is crucial to capture CO2 (see article Carbon Capture and Storage) from fossil fuel power stations and many manufacturing processes and pump it into suitable underground rock formations for permanent storage.
Outline of syngas production from underground coal, its combustion, capture and storage of the resulting CO2. Steam and hot air are injected under pressure (a) and passed over the coal (b). Gasification to syngas occurs (c), leaving a solid residue (goaf). The high pressure forces the syngas to the surface (d). Syngas may be used in various ways. One is shown here where it is used as a fuel in electricity generation. The products of combustion are water and CO2 (e). The CO2 is separated from the water and pumped back underground (f) where it is absorbed or reacts with the goaf (g) and thus stored.
Underground gasification offers significant advantages for storage of CO2 because firstly, the residue left following gasification—goaf—is very absorbent. It is much more permeable than saline aquifers that have already been used successfully for this purpose. Secondly, an impermeable layer develops above the goaf during gasification, thus sealing off the CO2 storage layer. Thirdly, goaf contains metal oxides that react chemically with CO2 to produce carbonate minerals. The essence of being chemically bound up in this way ensures that CO2 cannot escape even if the seal over the storage layer were breached.
Uses of Syngas
1. As mentioned above, syngas can be used to generate electricity.
2. It can be treated with steam, when the carbon monoxide in the syngas is changed into carbon dioxide and more hydrogen is produced:
CO + H2O → CO2 + H2
Carbon monoxide + steam → carbon dioxide + hydrogen
After capture and storage of CO2 from this product mixture the hydrogen may be used as a fuel (producing only H2O when combusted).
3. It serves as feedstock for the production of a variety of liquid or gaseous fuels and chemicals. Interesting developments in this regard are now taking place involving the use of plants, algae and microbes to synthesise industrial fuels and chemicals. These developments are particularly important in the drive for carbon-neutrality in chemical and fuel manufacture. (See article Use of plants and microbes for production of chemicals and transportation fuels.)
Developments in the North-east
Durham University and Newcastle University are at the forefront of developments in underground coal gasification.
· D Developments are underway for the gasification of undersea coal between the mouth of the Tyne and Alnmouth—as reported in the Newcastle Journal.
· Near Billingham, Teesside, the company Air Products is to develop a 50MW power plant involving the gasification of household, commercial and industrial waste.
References (Links to other websites)
Chemistry of Coal. http://www.chemistryexplained.com/Ce-Co/Coal.html.
Tar: WiseGeek: What is coal tar? http://www.wisegeek.com/what-is-coal-tar.htm
Air Products Tees Valley Renewable Energy Facility: http://www.airproducts.co.uk/teesvalley
Newcastle Journal McCusker P Jul 2 2011: Plans unveiled for a new 'green' industrial era http://www.journallive.co.uk/north-east-news/todays-news/2011/07/02/plans-unveiled-for-a-new-green-industrial-era-61634-28980566/
Durham University: Clean Coal: http://www.dur.ac.uk/cerees/research/cleancoal/
Newcastle University: Younger P, Gluyas J, Cox M, Roddy D (2010) Coal Gasification, INGENIA 43, 42-46.
12 September 2011 (revised 8 July 2013)