COMBINED SULFUR AND NITROGEN OXIDE REMOVAL
There are a number of processes that are capable, in principle at least, of removing both sulfur dioxide and nitrogen oxide in the same process. This ought to be more economical since only one process is needed instead of two. The use of activated charcoal has already been discussed as capable of treating both. Another is to use electron irradiation of the flue gases. Electrical discharge techniques have also been tested. However, none of these has yet been deployed widely commercially and most power plants rely on the well-tested processes described earlier.
PARTICULATE (DUST) REMOVAL
There are two principal systems for removing particulates from the flue gas of a coal-fired power station: electrostatic precipitators (ESPs) and fabric (bag house) filters.
Invented by the American scientist Frederick Cottrell, the ESP is well established and the technology has been widely exploited. It utilizes a system of plates and wires to apply a large voltage across the flue gas as it passes through the precipitator chamber. This causes an electrostatic charge to build up on the solid particles in the flue gas; as a result they are attracted to the oppositely charged plates of the ESP where they collect. Rapping the plates caused the deposits to fall to the bottom of the ESP where they are collected and removed. A new ESP will remove between 99% and 99.7% of the particulates from flue gas. However, it must be tuned to the particular coal being burned in the power plant. Where coals of different types and from various sources are to be burned, the alternative may be more effective.
Bag filters, or bag houses, are tube-shaped filter bags through which the flue gas passes on its way to the power plant stack. Particles in the gas stream are trapped in the fabric of the bags from which they are removed using one of a variety of bag-cleaning procedures. These include using supersonic blasts of air to dislodge particles so that they fall to the base of the unit and can be removed. These filters can be extremely effective, removing over 99% of particulate material. They are generally less cost effective than ESPs for collection efficiencies up to 99.5%. Above this, they are more cost effective. A system that combines a bag house–style filtration system with an ESP is under development too. This aims to provide a cost-effective high removal efficiency system, but has not yet been extensively demonstrated.