FIELD: pulp-and-paper industry. SUBSTANCE: invention aims both at recovering energy in burning process and at reusing chemicals used in cellulose production which are cleaved on burning. Black lye is fed into lower part of boiler and air is introduced at predetermined levels along boiler height in three portions. Gaseous black lye combustion products are reduced in reduction zone. According to invention, a part of air is fed as fourth portion at the level above third air portion level and reduction zone is thus expanded to fourth air portion level resulting in that reduction reaction time becomes at least 3 to 5 s. Boiler construction is presented. EFFECT: enhanced efficiency of process. 9 cl, 2 dwg
