A novel multi-stage conversion fluidized bed (MFB) gasifier which coupled an ash agglomerated fluidized bed gasifier (AFB, cold model as jetting fluidized bed, JFB) with a riser has been introduced and studied. The new gasifier aimed to independently control solid and gas retention times and to achieve high solids concentration, which was essential to raise the coal gasification efficiency. The hydrodynamic characteristics of MFB were investigated in a cold model apparatus (JFB 1.5 m in height and 03 m inner diameter, riser 8 m in height and 0.15 m inner diameter) with silica gel particles. Experimental results showed that under suitable operating conditions, MFB could successfully couple JFB with a riser, in which the solids that entered could form three-level step-by-step supplement entrainment and multi-flow regimes formed. A solid distribution ratio was proposed to quantify increased-solid distribution relationship between JFB and the riser, and the transition velocity of the MFB, u(M,tr,) was defined and its physical significance was explained. Finally, a qualitative and quantitative "phase diagram" describing the existence of different flow regimes of MFB as a function of the dimensionless parameters G(s)/(rho(p)v(t)) and Reynolds was proposed and MFB ought to be operated in JF-FF flow regime. This phase diagram was indispensable for the design, operation and scale-up of the MFB gasifier. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.