The residence time of the fluid in a reactor can be analysed with at least three different computational methods: (a) Eulerian simulation of the residence time measurements; (b) solution of the Eulerian transport equation for residence time; (c) Lagrangian particle tracking. Methods (a) and (c) are compared with analytical and experimental data from a pilot lagoon for validation, and the superiority of the Eulerian approach is demonstrated. Method (b), which has been validated in earlier studies, is applied to study the flow in the secondary combustion chamber of a biomass grate furnace. An inefficiently exploited zone of the furnace is identified, and a change in operating conditions, aimed at improving the reactor utilisation, is discussed.