CLEANING PROCESSES IN POWER STATIONS

In thermal power stations used for electricity generation, the coal fuel is carried by an air stream into the furnace combustion zone in the form of a finely crushed powder. The heat released by carbon oxidation converts the water into steam, which then drives turbines that are used to generate the electricity. Various processes take place to remove pollutants from the flue gases before they enter the atmosphere. 

Air admission to the combustion zone is controlled so that sulfur is oxidized to SO2 only, thereby minimizing SO3 formation. This is because the sulfates that arise from reactions with metals present (such as Mg, Ca and Fe) may melt to form slag. Slag is an accumulation of non-volatile materials that can adhere to the furnace walls, thereby opposing the transfer of heat to the water beyond. The presence of SO3 can also result in boiler corrosion. 

Note The admission of excess air, above that required for combustion, is undesirable because the heating of this air would be a waste of thermal energy in hot, but unused, expelled gases. 

The flow of gases leaving the hot combustion zone is principally composed of nitrogen (from the air), a small amount of residual oxygen, carbon dioxide, sulfur dioxide and much of the non-volatile particulate matter that is referred to as fly ash. To meet emission standards, now mandatory in many countries, these gases are subjected to two cleaning processes  

The sulfur removed during coal combustion generates gypsum (see below) and slag material. Slag has no commercial value. By contrast, the sulfur removed from oil and gas can often be recovered in the form of useful materials, including elemental sulfur. NO , is not usually removed, but a small proportion may be retained during FGD. 

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