A profound understanding of the fuel conversion on a grate boiler is crucial for an optimised boiler operation with a minimised pollutant emission. This work presents gas measurements and numeric simulations of a 150 kW moving grate boiler. The fuel bed model considers drying, pyrolysis and gasification. In the experiments, temperature profiles above the fuel bed and the fuel bed height are measured. Pyrolysis gas is sampled with a cooled probe and analysed on its CO, CO2, CH4, VOC, H2O, H-2 and O-2 composition. The experiments show that drying occurs in the first third of the grate with a relatively constant release of water vapour from the fuel bed. The temperature of the dried fuel further increases on the grate. The release of volatiles from the fuel pyrolysis starts towards the end of the drying zone and occurs in a relatively narrow zone. The fuel bed simulations are validated, discussed and used to analyse the influence of parameters on the fuel conversion such as the moisture content of the fuel and the primary air distribution. The model forms a basis to calculate the entry conditions for subsequent gas phase simulations. (C) 2019 Elsevier Ltd. All rights reserved.