a simple method to prevent air pollution from the particulate and gaseous pollutants from power plants , by recuperating the heat in the flue gases down to atmospheric temperature and pre - heat the air needed from combustion in the boilers . most fly ash is removed at the face of the regenerators . condensed moisture from the flue gases dissolves so 2 and forms a smog , with remaining fly ash particles acting as condensation nuclei , which are caught on the wetted surfaces within the regenerator , washed therefrom to vessels to settle out solids , and the liquid is then treated by vacuum to recover the dissolved so 2 , which is compressed to liquid so 2 and stored in refrigerated tank cars , to be sold as a valuable byproduct . the bulk of the fly ash , which settles on the face of sloping &# 34 ; snow fences &# 34 ; built across the top of the pebble bed , slides like snow off of a sloping roof and into gutters with perforated false bottoms , up which flows a little gas and carry it to downspouts and bins . the very fine fly ash travels with the flue gas downwardly to become smog when combining with the water of condensation and absorbed so 2 . cold wash is periodically used to flood to top of the bed and wash out any accumulations of fly ash .