The increasing interest in small-scale renewable-energy plants for distributed power generation has led to a demand for suitable software tools to study and develop control systems able to manage advanced integrated systems. Biomass, as a renewable energy resource, needs to be processed if it is to be exploited in small CHP units and pyrolysis is one of the options available for transforming solid biomass into useful liquid and gaseous fuels. This work is concerned with the development of a time-dependent model of a rotary-kiln pyrolyser for biomass: the model is intended for the de- velopment of control systems and the simulation of integrated energy systems, where the pyrolyser is connected to a power-generation package. The model was developed within the TRANSEO environment and based on a quasi 2-D nu- merical discretisation of the rotary kiln. Results are shown for a real case that is currently under construction: the model is able to predict the impact of different operating conditions on fuel yields, as well as capturing the main transient phenom- ena occurring during changes in the pyrolyser operating conditions. Kewords: Biomass, pyrolysis, distributed generation, transient modelling.