For a reliable and efficient process for the recovery of energy and alkali of reed black liquor (BL), the experiments of reed BL combustion in a fluidized bed with limestone as bed material for direct causticization were conducted at temperatures from 700 to 900 degrees C. The products in the direct causticizing process were identified by XRD, and the recovery rate of alkali was measured. In addition, the mechanism of inhibiting bed material agglomeration was studied. The results show that a direct causticizing system successfully recovers NaOH when using limestone as bed material, and the recovery rate of NaOH is more than 87.4%. Na2O center dot 2SiO(2) produced in the combustion process is the main binder of agglomeration. The limestone has a strong effect on inhibiting bed agglomeration, as the decomposition of limestone can provide CaO for the reaction with silica forming high-melting Ca2SiO4 and 3CaO center dot Al2O3 center dot 3SiO(2) instead of low-melting Na2O center dot 2SiO(2).