A microscale electrically heated rotary kiln for slow pyrolysis of biomass and waste was designed and built at the University of Perugia. The reactor is connected to a wet scrubbing section, for tar removal, and to a monitored combustion chamber to evaluate the lower heating value of the syngas. The system allows the evaluation of gas, tar, and char yields for different pyrolysis temperatures and residence times. The feeding screw conveyor and the kiln are rigidly connected; therefore, a modification of the flow rate implies a modification of the inside solid motion and of residence time. The paper provides the theoretical and experimental calculation of the relationships between residence time and flow rate used to determine the working envelope of the reactor as a function of the feedstock bulk density and moisture content, given the actual heat rate of the electric heaters. The methodology is extendable to any rotary kiln reactor with a rigidly connected feeding screw conveyor, given its geometric and energetic specifications. Part II of the paper will extend the energy balance, also introducing the yields of pyrolysis products.