Material of the month: Coal
Keeping us warm on winter nights for centuries, coal’s heyday may have passed but its use is still widespread, particularly in developing countries. Maria Felice examines the history, properties and future of this dark material.
Despite its significant contributions to the economy over the past 400 years, coal is often associated with dirt and poverty – the tale of children finding a piece of coal in their stocking at Christmas is particularly appropriate at this time of year. Coal, like oil, is a fossil fuel, but the images its name conveys couldn’t be more different to those of wealth and luck that are associated with the latter.
To understand what coal is, it is instructive to look at two more primitive fuels, namely wood and charcoal. Plants capture solar energy and use this to produce chemical energy during photosynthesis. This energy is released when dead plants decompose and it can also be released by burning dead plants. Indeed, wood was the first fuel known to man and enabled him to cook food, create warmth and clear land for crops. Later, charcoal began to be used – this is wood that has already been partially burned in an oxygen-poor environment so that almost pure carbon is left behind. It burns hotter and more cleanly than wood, but a lot of the original fuel content is lost in the process.
Coal is formed from buried plant matter that experienced extreme pressures and temperatures that transformed it into peat and then into coal. The source of all fossil fuels is living matter that became trapped deep in Earth’s surface and underwent physical and chemical changes. Coal has a much higher energy density than wood and charcoal. Once it began being mined in large quantities it transformed society. Indeed, many believe the Industrial Revolution was kick-started in the UK because there was so much coal that could easily be mined.
Types of coal are ranked depending on how much of their plant matter has been transformed into carbon. The coal with the highest amount of carbon is anthracite. Different types of coal have different uses, for example steam coal (thermal coal) is mainly used in power generation and coking coal (metallurgical coal) is mainly used in steel production. These are two types of coal towards the anthracite end of the carbon content scale.
In the steel industry, carbon is used to reduce ores of iron oxide into iron, and coal is the primary source of this. In 2010, the world crude steel production was 1.4 billion tonnes and more than 700 million tonnes of coking coal was used in its production. It is actually coke that is used to reduce the iron oxide and this is obtained from coking coal by heating it to approximately 1,000°C in the absence of oxygen to drive off volatile compounds – a process known as pyrolysis.
The trends in coal production and use have changed tremendously over the years. In the UK, the amount being produced annually has reduced from more than 200 million tonnes in the early 20th Century to less than 20 million tonnes nowadays. Similarly, the amount being used has reduced from 200 million tonnes to less than 70 million tonnes. However, since 2000, the global consumption of coal has grown faster than any other fuel. Nowadays, 76% of the world’s coal is used by five countries – namely China, the USA, India, Russia and Japan. The biggest reserves are in the first four of these countries and globally there are reserves to last around 112 years at the current rates of production. The reserves included in this value are proven ones, that is reserves from which coal can be recovered economically. This means that if the price of coal is lowered, these values will decrease. There are proven reserves of oil and gas of 46 and 54 years respectively.
It seems that the extraction and use of coal is here to stay, at least for the next century or so. It is therefore important that the environmental impact of coal is minimised. With regards to mining, problems such as land disturbance and noise pollution can be controlled using modern techniques and technology. A more serious problem, which is also under control, is that of methane gas that is released during coal extraction. This gas can become highly explosive when mixed with air, so recovering it is essential for safety reasons. Furthermore, methane is a greenhouse gas with a global warming potential 23 times that of carbon dioxide, so minimising the amount that is released into the atmosphere is also essential. Finally, methane is an energy resource in its own right, so the captured methane is used for energy generation. Methane can be recovered from mines that are currently being worked on as well as abandoned and unmined projects.
With regards to reducing the environmental impact of burning coal in power plants, several advances have been made and only a few are mentioned here. Flue gas desulphurisation (FGD) is used to control the amount of SOx gases (sulphur dioxide and trioxide) released into the atmosphere. Reburning is a technique that can be used to reduce NOx (nitric oxide) emissions. The process includes injecting fuel downstream from the primary combustion so that a reducing zone is formed. Supercritical plants – plants where the steam temperature is increased – are being selected for new coal-fired power stations. These plants offer higher efficiency and, therefore, reduced costs and harmful emissions per unit of electricity generated. However, they require advanced materials that can withstand the more severe conditions, and so the research workload increases.
Ralph Waldo Emerson, the American who led the Transcendalist movement of the mid 19th Century, glorified coal when he described it as follows: ‘Every basket [of coal] is power and civilisation. For coal is a portable climate. It carries the heat of the tropics to Labrador and the polar circle, and it is the means of transporting itself whithersoever it is wanted. Watt and Stephenson whispered in the ear of mankind their secret, that a half-ounce of coal will draw two tons a mile, and coal carries coal, by rail and by boat, to make Canada as warm as Calcutta, and with its comfort brings its industrial power.’
While coal continues to play a massive role in society and industry across the world, the risks associated with working in coal mining and the environmental damage caused by the processing and use of coal must not be forgotten. Electricity generation by burning coal releases more CO2 per unit of electricity than any other method. Using carbon capture and storage technology to extract this gas from power plant exhaust and prevent it from being released into the atmosphere is a possibility, but it is still years away from being widely implemented.
Read more about CCS in coal-fired power stations in the news article 'Cleaning up coal' in the current issue of Materials World