Biomass and bed material particles have been found to exhibit significant density differences during the operation of a circulating fluidized bed boiler. Further studies are required to analyze the mixed flow characteristics of these particles in a furnace. We built a test apparatus for a cold-operation, biomass-circulating fluidized bed and designed a space-storage sampler for furnace particle sampling based on modeling theory. The current study found that adding biomass particles nearly did not affect the flow characteristic and distribution of bed material particles because of density difference. However, biomass-particle enrichment phenomenon unexpectedly occurred in the lower furnace region and at the bottom of the top furnace outlet where biomass particle concentration was extremely low. Analyses and observations revealed that downward flow near the wall can effectively carry biomass particles to the bottom of the furnace, thus leading to the enrichment of biomass particles in the lower furnace region. The inhibition mechanism of dense, inert, bed material particles also limited the flow of light biomass particles into the upper furnace region. Different kinds of biomass exhibited similar particle distribution characteristics. Light biomass particles were enriched at the bottom of the furnace, and could hardly reach the furnace outlet. (C) 2014 Elsevier B.V. All rights reserved.