Effect of catalytic particle clustering on methane steam reforming in a circulating, fluidized bed (CFB) reformer is analyzed numerically using a two-dimensional hydrodynamic model coupling with methane steam reforming reaction kinetics. Gas flow fields and distributions of temperature and gas species in a Haldor Topsoe Ni/Mg Al2O4 spinel catalytic particle cluster are predicted. The computed results indicate that the rates of methane steam reaction of a catalyst particle in the cluster are less than that of an isolated catalyst particle out side the cluster. The yields of H-2, CO and H2O in the cluster increase with the increase of operating temperature and inlet gas velocity but decrease with the increase of reactor pressure and feed steam. (C) 2007 Elsevier B.V. All rights reserved.