The incombustible components of the coal partly fall to the bottom of the boiler and are hauled away from there. Partly they are carried off by the flue gas. To prevent this fly ash from getting into the environment, it must be removed from the flue gas. To achieve this the flue gas is directed through electrostatic precipitators where the particulates are negatively charged and then attracted by the positively charged electrode. The fly ash or dust collects there and is regularly rapped off. This way more than 99.9 percent of the dust can be removed from the flue gas.