Concentrated non-condensable gases (CNCG) are collected and lead to a collection tank, from which they are taken to a separate incinerator. In the separate incinerator, total reduced sulfur compounds in the CNCG are oxidized into SO2. The flue gas from the boiler is led to a catalytic reaction vessel. The catalyst oxidizes SO2 by using excess oxygen supplied through combustion air into SO3. Flue gases from the catalytic converter are passed into a condensing tower. The temperature of the flue gases is reduced with the cooling liquid and the SO3 in the flue gases reacts with H2O to produce sulfuric acid.