Global Emissions from Fossil Fuelled Power Stations
It is generally accepted that the burning of fossil fuels and the subsequent emission of greenhouse gases, particularly CO2, is leading to climate change and potentially catastrophic increase in the earth's temperature. Hence concern over the emission of greenhouse gases is a key element of energy policy and, in Europe, is recognized through the EU Emissions Trading Scheme which requires major emitters of CO2, such as power stations, to purchase permits to emit CO2. This has the effect of making high carbon generation (particularly from coal) increasingly expensive.
Regional and Local Emissions from Fossil Fuelled Power Stations
Fossil fuelled power plants produce sulphur oxides, particulate matter, and nitrogen oxides. Of the former, sulphur dioxide accounts for about 95% and is a by-product of the combustion of coal or oil. The sulphur content of coal varies from
0.3 to 5%. Coal can only be used for generation in some US states if it is below a certain percentage sulphur.
In the eastern USA this has led to the widespread use of coal from western states because of its lower sulphur content or the use of gas as an alternative fuel. Sulphur dioxide forms sulphuric acid (H2SO4) in the air which causes damage to buildings and vegetation. Sulphate concentrations of 9-10 mg/m3 of air aggravate asthma and lung and heart disease. This level has been frequently exceeded in the past, a notorious episode being the London fog of 1952 (caused by domestic coal burning). It should be noted that although sulphur does not accumulate in the air it does so in the soil.
Sulphur oxide emission can be controlled by:
- • the use of fuel with less than, say, 1% sulphur;
- • the use of chemical reactions to remove the sulphur, in the form of sulphuric acid,
from the combustion products, for example limestone scrubbers, or fluidized bed
• removing the sulphur from the coal by gasification or flotation processes.
European legislation limits the amount of SO2, NOx, and particulate emission, as in the USA. This has led to the retrofitting of flue gas desulphurization (FGD) scrubbers to coal burning plants. Without such equipment coal fired power stations must be retired. Emissions of NOx can be controlled by fitting advanced technology burners which can ensure a more complete combustion process, thereby reducing the oxides going up the stack (chimney).
Particulate matter, particles in the air, is injurious to the respiratory system, in sufficient concentration, and by weakening resistance to infection may well affect the whole body. Apart from settling on the ground or buildings to produce dirt, a further effect is the reduction of the solar radiation entering the polluted area. Reported densities (particulate mass in 1 m3 of air) are 10 mg/m3 in rural areas rising to 2000 mg/m3 in polluted areas. The average value in US cities is about 100 mg/m3.
About one-half of the oxides of nitrogen in the air in populated areas are due to power plants and originate in high-temperature combustion processes. At levels of 25-100 parts per million they can cause acute bronchitis and pneumonia. Increasingly, city pollutants are due to cars and lorries and not power plants.
A 1000 MW(e) coal plant burns approximately 9000t of coal per day. If this has a sulphur content of 3% the amount of SO2 emitted per year is 2 x 105t. Such a plant produces the following pollutants per hour (in kg): CO2 8.5 x 105, CO 0.12 x 105, sulphur oxides 0.15 x 105, nitrogen oxides 3.4 x 103, and ash.
Both SO2 and NOx are reduced considerably by the use of FGD, but at considerable cost and reduction in the efficiency of the generating unit caused by the power used by the scrubber. Gas-fired CCGT plants produce very little NOx or SO2 and their CO2 output is about 55% of an equivalent size coal-fired generator.
The concentration of pollutants can be reduced by dispersal over a wider area by the use of high stacks. If, in the stack, a vertical wire is held at a high negative potential relative to the wall, the expelled electrons from the wire are captured by the gas molecules moving up the stack. Negative ions are formed which accelerate to the wall, collecting particles on the way. When a particle hits the wall the charge is neutralized and the particle drops down the stack and is collected. Precipitators have particle-removing (by weight) efficiencies of up to 99%, but this is misleading as performance is poor for small particles; of, say, less than 0.1 pm in diameter. The efficiency based on number of particles removed is therefore less. Disposal of the resulting fly-ash is expensive, but the ash can be used for industrial purposes, for example, building blocks. Unfortunately, the efficiency of precipitators is enhanced by reasonable sulphur content in the gases. For a given collecting area the efficiency decreases from 99% with 3% sulphur to 83% with 0.5% sulphur at 150 °C. This results in much larger and more expensive precipitator units with low-sulphur coal or the use of fabric filters in 'bag houses' situated before the flue gas enters the stack.