The invention discloses a process for treating black liquor comprising pyrolyzing black liquor in a fluidized bed reactor to obtain solid particles and pyrolyzed black liquor gas, heating the solid mass, returning the heated solid mass to the fluidized bed reactor for use in the pyrolysis reaction; condensing the pyrolyzed black liquor gas to obtain a condensate and recover heat and a residual gas released during condensation for heating the solid mass to a predetermined temperature for that is required for the pyrolysis.