During electricity generation, a large amount of low-grade heat is produced as a by-product. In conventional power stations this heat is lost. In combined heat and power (CHP) systems the heat produced during electricity generation is
recycled rather than wasted, thereby increasing the efficiency of the system. CHP is usually only used as a supplement to grid mains electricity supply for boiler produced steam/ heat.
High capital and maintenance costs deter individual users from investing in CHP, and therefore CHP schemes are more likely be used by the public, industrial and commercial sector. The main markets tend to be those requiring a great deal of heat, for example leisure centres, hospitals and industrial sites with process heating requirements, especially the chemical, brewing and paper industries. Sewage treatment works sometimes use CHP, fuelled by biogas.
The increased fuel efficiency of CHPs gives them a potentially useful role in helping to combat global warming, by decreasing carbon dioxide emissions. This reduction can be between 13-57%, depending on the fuel being replaced by CHP. Acid rain can also be reduced by the use of CHP, by cutting emissions of SO2 and NOx. However, one disadvantage of a CHP plant that is close to the point of use is the increased noise level.
The UK Government is promoting CHP through the Energy Efficiency Office's Best Practice Program, and through the Energy Saving Trust. The aim is to doube current CHP capacity by 2010. CHP currently provides about 4% of the UK's electricity.
The term air pollution describes any harmful substances in the air we breathe. The largest sources of air pollution in the UK are transport and industry (including power generation).
Power stations are responsible for releasing large amounts of harmful pollutants into the air. The world emits approximately 69 tonnes of sulphur dioxide (SO2) and 24 million tonnes of nitrogen oxide (NOx) into the atmosphere each year. SO2 and NOx can cause respiratory disorders, with children and asthmatics being particularly vulnerable.
In the past, air pollution was dominated by thick black smoke, which was emitted from the burning of coal by power stations, industries and homes. The famous London Smog of December 1952, which caused thousands of deaths, prompted the introduction of the 1956
Clean Air Act. This created smokeless zones, where no pollution from the burning of coal could be produced. The Act focused on reducing smoke pollution, but the measures taken helped to reduce SO2 levels at the same time. Air pollution in cities dramatically reduced in the following ways:
• domestic emissions reduced due to smokeless zones;
• electricity and gas usage increased and the use of solid fuels decreased;
• cleaner coals were burnt, which had a lower sulphur content;
• taller chimney stacks were used on power stations to help the pollution disperse;
• there was a continuing decline in heavy industry.
Acid rain is the term used to describe the acidity of wet and dry deposition. This includes acidity falling as rain, snow, sleet, hail, mist or fog (wet deposition) and the dry deposition of gases and particles. Rain water is naturally acidic as a result of carbon dioxide (CO2). However, anthropogenic (man-made) emissions of pollutants make acid rain a large problem.
The main pollutants responsible for acid rain are sulphur dioxide (SO2) and nitrogen oxides (NOx). Both are produced from the burning of fossil fuels for power generation. Once emitted, they combine with water in the atmosphere to form sulphuric and nitric acid. The acid then falls to the Earth’s surface, causing damage to plants and animals.
Emissions of SO2 and NOx in the UK contribute significantly to problems of acid deposition in other parts of Europe, e.g. Scandinavia. This occurs because the pollutants remain suspended in the atmosphere, where they can travel long distances before falling back to the ground.
To reduce the problem of acid rain throughout the world, it is necessary for all countries to work together in reducing their pollutant emissions.