Conventional recovery boilers employ direct contact between the black liquor and the flue gas in the flue in order to increase the concentration of the black liquor, resulting in sulfur compound and other unpleasant odors. With this the temperature of the generated steam was also limited to approximately 420°C because complete heat recovery was impossible due to the need to avoid internal corrosion by the flue gas. With the help of new a black-liquor concentration technology and employing various innovative designs and materials, a new boiler using concentrated black-liquor as fuel has been developed. With this boiler, concentrated black liquor preheating using steam and an indirect-type black liquor heater before being charged into the boiler. Because it is equipped with a superheater and economizer, the boiler generates steam with a temperature of 500°C and pressure of more than 100 kg/cm2, which is suitable for use in the paper mill or in electric power generation. While also resulting in dramatically improved thermal efficiency, this technology does not produce unpleasant smells as there is no direct contact between black liquor and the flue gases (NEDO, 2008. p.163).