A bottom-up energy system optimization model is used to support policy planning for the sustainable use of energy. The methodology of the Finnish EFOM model is presented, including the description of biomass use for energy, power and heat generation, emissions and the end use of energy. The common modelling framework used in many similar models has been enhanced by thoroughly adapting the model to the characteristics of the Finnish energy economy and by some methodological improvements. An important submodel for the energy intensive pulp and paper industries has been incorporated in the model. The model has proven to provide useful results for actual policy planning. The results from scenarios on strategies to reduce CO2 emissions point out that the implementation of effective national policy measures for the abatement of CO2 emissions appear to be difficult in Finland, although technical solutions with reasonable costs exist.