Today, our tutorial on power plant technology engages with coal-fired power plants and the utilization of respective combustion technologies.
4. Coal-fired power plants and combustion technology
Coal-fired power plants use the hard coal or lignite. Both fossil fuels occur in large quantities in nature. Coal which is combusted in a boiler is used to drive a steam power process.
Combustible components of coal are mainly carbon and (to a lower extent) hydrogen, as well as sulphur, which is however undesirable. Besides, coal contains a range of other components, such as oxygen, nitrogen, ash and water. Ash denotes the solid, incombustible residues, such as heavy metals.
During the combustion process, the reaction between fuel and atmospheric oxygen releases energy. The amount of energy that is released during the combustion is called calorific or heating value of the respective fuel.
A complete combustion results in the gaseous final products (“flue gas”) carbon dioxide (CO2), water and Sulphur dioxide.
Unwanted gases such as carbon monoxide (CO), hydrogen and methan can be formed if the combustion is incomplete. The formation of these exhaust gases is however prevented as far as possible, as they constitute an “energy loss”.
The combustion relies on atmospheric oxygen. Apart from the oxygen, which is required for the combustion as such, it also contains a considerable amount of nitrogen. In the combustion chamber, the reaction of nitrogen and oxygen leads to the formation of nitrogen oxides – NO and NO2. The power plant operator is caught in a dilemma: While high temperatures increase the efficiency, they also favor the formation of nitrogen oxides.
The flue gases formed during the combustion carry along pollutants, such as the climate-relevant CO2. In order to comply with emission regulations, special measures for combustion technologies and exhaust gas treatment are necessary.
The application of relevant combustion technologies can be a primary measure for emission reduction and thus reduce the formation of pollutants “at the source”. There are three main combustion technologies:
- Fluidized-bed combustion: up to around 300 MWth (combustion capacity)
- Grate firing: up to around 100 MWth
- Pulverized-fuel firing: up to around 2500 MWth
Fluidized-bed combustion
A mixture of ground coal and limestone is inserted into the combustion chamber and thus swirled through the injected air. In case of the fluidized-bed combustion the surface of the fuel is very large and can burn at lower temperatures (up to 900 °C). The formation of nitrogen oxides can thus be reduced. At those lower temperatures limestone and sulphur react to form gypsum, which allows the removal of up to 95 % of the sulphur contained in the coal. Furthermore, gypsum also takes up some part of the ash. Subsequently, only dust separation is necessary. The relatively low temperatures during the fluidized-bed combustion do however result in lower efficiencies.
Grate combustion
During grate combustion, the solid fuels are inserted into the combustion chamber via travelling grates. The grates are realized as continuous belts. In the end the ash drops down and is extracted from the process. The temperature in such a process reaches roughly 1500 °C.
Pulverized-fuel combustion
To be used in a pulverized-fuel combustion process, coal is ground and milled into very small particles. Those are then injected into the combustion chamber on several levels along with the combustion air. When the coal burns and it releases energy in the process. The majority of the ash formed during the process is transported via the flue gas. The fine-grained coal allows for an efficient combustion process and fast power adjustments. The temperatures reach about 1200 °C. Such high temperatures favor the formation of nitrogen oxides, which have to be removed in downstream facilities. The same applies for the sulphur oxides.
Pulverized-fuel combustion is the most common technique in today’s large power stations.
