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Abstract: The main objective of this paper is to study the effect of operating and design parameters, mainly reactor geometry, air superficial velocity, biomass moisture content, particle size and biomass type (pine bark and sewage sludge), on the performance of the gasification/combustion process of waste biomass in fixed bed downdraft reactors. This experimental approach allows optimizing the dynamic behavior of the thermochemical process. Emphasis was put on interactions between the mentioned parameters with output or response variables, such as biomass consumption rate, fuel/air equivalence ratio, producer gas heating value and composition, tars concentration, and flame front velocity. The effect of the operating and design parameters on response variables was studied by means of multifactorial experimental designs. The statistical R 2 obtained through the data fitting of the multifactorial experimental designs indicated that the factors taken into account explained between 80.8% and 98.8% of the variability of the response variables. For the particular experiments carried out, the optimal gasification conditions were obtained with the following set of inlet conditions: air superficial velocity of 0.06m/s, biomass particle size between 2 and 6mm, and biomass moisture content of 10.62%. The corresponding response variables were: flame front velocity=11mm/min, fuel air equivalence ratio=3.2, biomass consumption rate=125kg/h/m2, lower heating value of the producer gas=2965.6kJ/Nm3, tar concentration=7.73g/Nm3, and an average composition of the producer gas of 8.0% H2, 13.0% CO, 1.4% CH4, 14.9% CO2, and 62.7% N2. [Copyright &y& Elsevier]