The first generation air system in the 1940s and 1950s consisted of a two level arrangement; primary air for maintaining the reduction zone and secondary air below the liquor guns for final oxidation. The recovery boiler size was 100 – 300 TDS (tons of dry solids) per day. and black liquor concentration 45 – 55%. Frequently to sustain combustion auxiliary fuel needed to be fired. Primary air was 60 – 70% of total air with secondary the rest. In all levels openings were small and design velocities were 40 – 45 m/s. Both air levels were operated at 150 °C. Liquor gun or guns were oscillating. Main problems were high carryover, plugging and low reduction. But the function, combustion of black liquor, could be filled.