Modern Thermal Power Plants - Aspects on Modelling and Evaluation

Sammanfattning: The European Union (EU) has laid down very clear objectives for the reduction of greenhouse gases in the hope that it will prevent or mitigate climate change. Political incentives are used to make the power industry adopt changes in order to reach these EU targets. In this thesis, some solutions that could help power companies meet the EU objectives are evaluated. Thermodynamic models have been developed to evaluate the proposed methods. A description of the models and the way in which they are used to model power plant cycles in off-design mode is included in this thesis. The focus was on combined-cycle power plants, which have the highest efficiency among commercial power plants today. Three ways of adapting power plants so as to meet the EU targets were formulated: • reduced CO2 emissions • increased use of biofuels • improved part-load abilities A new method based on using low-grade heat when implementing carbon capture in a combined cycle power plant is presented. The results show that the method can increase the total efficiency and reduce the initial cost of the power plant. The method is applicable for both retrofitting to existing plants and for new plants. The effect of using low-calorific bio-fuels in a combined-cycle power plant was investigated. The results show that below a heating value of about 20-25 MJ/kg the plant quickly departs from its design point. The supply of power to the national grid is expected to be fluctuate more in the future due to the uneven availability of wind and solar power. Therefore, two part-load operation strategies were evaluated. The first involves a strategy that entails less wear on the gas turbine, which could extend the maintenance interval of the unit. The second method combines two well-established part-load strategies for part-load operation of steam-cycle power plants. The combination of the two methods will increase the part-load efficiency of the power plant.