Abstract: A method of cleaning flue gases from gaseous pollutants formed during combustion of oil during start-up of a boiler and/or operational disturbances in the boiler, wherein the boiler is for combustion of fuel including one of fossil fuel, coal, biomass fuel and waste. Flue gases are conducted from the boiler through a flue duct to a barrier filter for seperation of particulate pollutants. Finely-dispersed particles are introduced into and mixed with the flue gases in the flue duct in at least one of upstream of and in the barrier filter. The finely-dispersed particles are separated in the barrier filter while forming a dust cake. Condensed gaseous pollutants and droplets are taken up on the surface of the finely-dispersed particles in the flue gases and in the dust cake.