Optimal upgrade of a district heating plant into a polygeneration plant using biomass as feedstock

This paper aims at evaluating the possible upgrading of an existing district heating plant for production of electricity and pellets. The evaluation is carried out by optimizing the alternatives from the economic, thermodynamic and environmental point of view. In order to examine how the design can be optimized, a detailed model of the process has been elaborated using ASPEN Utilities and Matlab optimization toolbox. The parameters of the polygeneration plant have then been varied in order to examine how optimal economic benefit can be extracted from the biomass streams whilst still meeting the fundamental process demands of the industries and heat demand of the community. A multi-objective optimization has been used to investigate the Pareto-optimal trade-offs that exist between low electricity costs and investment cost. The resulting polygeneration plant designs conclude that it is feasible to produce 18 and 25 MW of power while at the same time supplying the process steam required by the nearby industries and district heating for the community. The results also shown that it is feasible to operate the plant more hours per year by producing pellets and it could be possible to generate additional district heating (up to 25 ton/h of hot water) to cover the demands of a growing community.