<P> The condensation of water releases significant amounts of low temperature heat (more than 2 gigajoules (560 kWh) per ton of water), that can be recovered by the cooler for e.g. district heating purposes . </P> <P> Excess condensed water must continuously be removed from the circulating water . </P> <P> The gas leaves the scrubber at its dew point, so even though significant amounts of water may have been removed from the cooled gas, it is likely to leave a visible stack plume of water vapor . </P> <P> A dry or semi-dry scrubbing system, unlike the wet scrubber, does not saturate the flue gas stream that is being treated with moisture . In some cases no moisture is added, while in others only the amount of moisture that can be evaporated in the flue gas without condensing is added . Therefore, dry scrubbers generally do not have a stack steam plume or wastewater handling / disposal requirements . Dry scrubbing systems are used to remove acid gases (such as SO and HCl) primarily from combustion sources . </P>

Scrubbers in smokestacks remove large amounts of what major air pollutant