In solid fuel (such as lignite) chemical looping combustion, solid fuels undergo pyrolysis and gasification. The volatiles from pyrolysis and the gasification product (CO/H-2) react with oxygen carriers. The gas conversion (to CO2/H2O) in the fuel reactor is a key point. However, char particles of different sizes and conversion ratios cause segregation in the fuel reactor, which influences the contact time between fuel gases and the carrier, thereby changing the gas conversion behavior. In order to gain information on obtaining a high gas conversion in the fuel reactor, this work focused on the effect of the char particle segregation on gas conversion. Different factors - the char particle size, the fluidizing gas velocity, and the oxygen carrier reactivity - were taken into account. Smaller char particles with low density would float on top of the fluidized bed, corresponding to a low gas conversion (<80% for 0.6-0.8 mm). Larger char particles with higher density were able to suspend among oxygen carriers or even sink to the bottom. The relevant gas conversion was relatively higher (100% for 5-7 mm). As the gasification continued, the size and density of larger char particles decreased. The gas conversion in the fuel reactor decreased (from 100% to 50%) accordingly. Larger fluidizing gas velocity (>= 1.6 U-mf in the Fe63Al bed) can reduce the segregation effect, resulting in a higher CO conversion. High reactivity carriers can convert CO completely although segregation exists, whereas low reactivity carriers exhibit the segregation effect and thus corresponds to a low CO conversion. (C) 2013 Elsevier Ltd. All rights reserved.