|Clean Coal Technologies (CCTs) are those which facilitate the use of coal in an environmentally satisfactory and economically viable way. Among other aspects, they meet various regulations covering emissions, effluents, and residues. In some situations, CCTs offer the possibility of satisfying even more stringent standards, at an acceptable cost..
A basic approach to the cleaner use of coal is to reduce emissions by reducing the formation of pollutants such as NOx and/or cleaning the flue gases after combustion. A parallel approach is to develop more thermally efficient systems so that less coal is used to generate the same amount of power, together with improved techniques for flue gas cleaning, for effluent treatment and for residues use or disposal. Thermal efficiency may be increased by using a higher grade coal.
Various methods for coal-fired power generation are well established and widely used, and different steps are being taken to ensure that environmental requirements are satisfied. The methods include:
- pulverised coal combustion (PCC) with subcritical steam driving a steam turbine, with various levels of flue gas cleaning to meet local requirements;
- cyclone fired wet bottom boilers with subcritical steam driving a steam turbine, again with various levels of flue gas cleaning to meet local requirements;
- stoker boilers for small applications, with subcritical steam, possibly using a low sulphur content coal.
Various technologies are undergoing development in order to provide an environmentally satisfactory method of using coal as a basic fuel for power production in new plants. All methods which meet environmental standards in different parts of the world are included. Some are now commercially available, backed by large-scale operating experience in a number of countries. Others are still at the demonstration stage. These technologies include:
In considering each CCT, key factors to consider are:
- the overall economics of plant construction and operation;
- the nature and cost of the coal to be used;
- the load pattern to be met, and flexibility required. For CHP units, the pattern of the demand for heat;
- the ease with which new units can meet environmental requirements, and the operational constraints arising;
- the thermal efficiency of generation, which impacts directly on CO2 emissions;
- the state of development of the technology.
The efficiency levels achievable with PCC compare favourably with the possible efficiencies achievable using other methods. Care must be taken in assessing the effects of increased efficiencies. Units which load follow may spend a considerable proportion of their time operating below maximum output. Comparative efficiency figures such as the ones in the diagram, are nearly all quoted at full load output, whereas realistic economic comparison should be based on normal or typical operating patterns. It is important to note that units operate more efficiently with colder air temperatures, and with lower temperature cooling water.
An overview of the technologies used throughout the world to control emissions of particulate matter, SO2 and NOx from pulverized coal combustion is also presented on this website. These technologies include: