The effect of turbulence on heterogeneous reactions on the surface of char particles embedded in a turbulent oxidizer, consisting of oxygen and carbon-dioxide, is in this work studied numerically. It is shown that for a small Damkohler number (Da), which is the ratio between a turbulent and a chemical time scale, the char conversion rates are somewhat increased by the turbulence. This is found to be due to the increased mass transfer rate to the char particle surface that is caused by the turbulence-induced relative velocity between the char and the oxidizer. For large DamkOhler numbers, however, the char conversion rate is strongly reduced due to particle clustering. This reduction is explained by the fact that when particles are clustered in densely populated particle clusters, the transfer of oxygen to the particles in the centre of the clusters is reduced since the oxygen is consumed by the particles closer to the external surface of the cluster. At the same time, high concentrations of oxygen exist in the voids between the particle clusters. This oxygen cannot take part in the conversion of the char until it is transported to the char surface. The effects of turbulence on the heterogeneous reaction rates are furthermore modelled based on Direct Numerical Simulation (DNS) data for a simplified reacting gas particle system. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.