For most commercial biomass gasifiers the cold gas efficiency makes up 50-70%. However, thermodynamic modeling demonstrates the possibility of its increase to 80-85%. Thermodynamic models predict an optimal composition of flows coming to the gasifier, and a temperature. These parameters are hard to reproduce at gasifier since they often depend on the operating parameters. This paper proposes a semi-empirical approach which makes it possible to carry out a thermodynamic analysis of operating parameters and optimization of gasifier operation. To test the approach we did experiments on charcoal and biomass gasification in a downdraft gasifier. Modeling was done on a non-stoichiometric model maximizing the reaction system entropy. The semi-empirical approach reveals three limitations of the cold gas efficiency of the experimentally observed process. The first limitation is related to the attainment of a carbon boundary line ( which is estimated thermodynamically) by the reaction system. This line corresponds to the maximum cold gas efficiency of the process. The second limitation deals with a shift along the carbon boundary line. The third limitation is a stoichiometric limitation on the formation of combustible gas components. The process of wood gasification is characterized by a number of phenomena which are untypical of the downdraft process. These phenomena underlie the hypothesis about the wood gasification mechanism. According to this hypothesis the process of gasification runs in the layers of individual particles. At the same time there is either no fuel bed stratification or it does not manifest itself. (C) 2015 Elsevier Ltd. All rights reserved.