A key feature of incinerator combustion is the mixing of solid material on the grate. Existing grate systems, however, do not agitate and mix the refuse sufficiently, resulting in poor grate combustion. The objective of this project is to characterise and quantify the grate mixing process. This paper presents the mixing mathematical model that is developed based on the probability for a solid tracer to swap position with an adjacent tracer, in a given time interval. Results from particle tracking experiments that are conducted in parallel with mathematical model development showed good agreement with the model predictions.