MINLP optimisation model for increased power production in small-scale CHP plants

In this work we present a mixed integer nonlinear programming (MINLP) model for increasing the power production in small-scale (1–20 MWe) CHP plants based on a steam Rankine process and using biomass fuels. Changes that could increase the power production in these plants are, for instance, a steam reheater, a feed water preheater, a two-stage district heat exchanger, and a fuel dryer. In the model we also consider the integration of a gas turbine and a gas engine into the CHP process by using the oxygen remains of the turbine or engine exhaust gases as preheated combustion air in the biomass boiler. The developed MINLP model was tested with four existing small-scale CHP plants. The results showed that there are profitable possibilities to increase the electrical efficiencies and power-to-heat ratios of these plants with the addition of a two-stage district heating exchanger, a feed water preheater, a steam reheater, and a fuel dryer. Furthermore, the integration of a gas engine increased the efficiencies significantly. Overall, the MINLP model gave good results for the example cases, but the model could be still improved by developing its mathematical formulation to a more convex model and by adding the operational changes in the district heating network to the model with multiperiod modeling. The current model gives new possibilities to the design planning and optimisation of the small-scale CHP plants, and it also provides a good basis for the future design modeling of the CHP plants and their optimal integration to the district heating network.