as the combustion flue gas exits the boiler it is routed through a rotating flat basket of metal mesh which picks up heat and returns it to incoming fresh air as the basket rotates. this is called the air preheater. the gas exiting the boiler is laden with fly ash, which are tiny spherical ash particles. the flue gas contains nitrogen along with combustion products carbon dioxide, sulfur dioxide, and nitrogen oxides. the fly ash is removed by fabric bag filters in baghouses or electrostatic precipitators. once removed, the fly ash byproduct can sometimes be used in the manufacturing of concrete. this cleaning up of flue gases, however, only occurs in plants that are fitted with the appropriate technology. still, the majority of coal-fired power stations in the world do not have these facilities. legislation in europe has been efficient to reduce flue gas pollution. japan has been using flue gas cleaning technology for over 30 years and the us has been doing the same for over 25 years. china is now beginning to grapple with the pollution caused by coal-fired power stations.