The producer gas composition and the thermochemical conversion process of a small-scale reverse downdraft reactor has been investigated under ten operating conditions with different fuel bed depths and air supply rates. The operating principle of this research reactor is a batch-fed reverse downdraft process, using wood pellets as the solid biomass fuel. The oxygen-limited regime, where the fuel consumption increases nearly linearly with the air supply, has been identified, and four flow rates over the range of this regime have been investigated. The fuel bed depth was varied between one and four reactor diameters (1D (100 mm)-4D (400 mm)). The results demonstrate that increasing the primary air mass flux leads to both greater fuel consumption and higher temperatures as well as heating rates in the reaction front. Greater air supply rates and the resulting higher temperatures lead to a substantial increase in fuel conversion into permanent gases, rather than tars or char, and a rise in the cold gas efficiency (CGE) from 33% to 73%, from the lowest to highest air flow rate at a 4D fuel bed depth. However, the temporal producer gas heating value is similar in all configurations. With increasing depth, it is evident that H-2 production is promoted by the char layer downstream of the reaction front and that a certain layer thickness is necessary to achieve the potential near steady-state product flow at a specific flow rate. Interestingly, a greater fuel bed depth enhances the hydrogen conversion rate to permanent gases by more than 20% and the CGE from 48% to 53%, while the fuel consumption and temperature profiles remain similar. A general trend of increasing performance was identified at the 3D and 4D depths, when compared with the 1D and 2D fuel bed depths. The produced char exhibits a high fixed and elemental carbon content. Therefore, the conversion efficiency of this process can be increased both through increasing the fuel bed depth and, even more, through adjusting the air supply, promoting the yield of permanent gases and the conversion of produced tars.