Regularities in the course of conversion process in the reaction zone of a fixed bed reactor are studied. The efficiency of gasification process is shown to be determined by its thermal mode and in particular by the extent of heat exchange between the reaction zone and the environment. The influence of air consumption, insulation thickness, and draught preheating on the conversion efficiency is mediated by the thermal mode. The reacting system tends to the carbon boundary point (CBP) which corresponds to the maximum efficiency at a given set of external conditions. These conclusions have made it possible to generalize the knowledge on the conversion process and infer the three limitations on the gasification efficiency. The found regularities are insensitive to the fuel combustibles composition and are typical both of a wood char and woody biomass.