Conceptual Gas Turbine Modelling for Oxy-fuel Power Cycles

Licentiatavhandling, 2012

The electricity consumption in the world is growing at an ever increasing rate. Today's electricity generation is highly dependent on combustion of fossil fuel. This leads to emissions of carbon dioxide into the atmosphere. Increased atmospheric CO2 concentration results in higher average surface temperature and climate change. A short- and medium term method to decrease the carbon dioxide emissions is carbon capture and storage.This method captures carbon dioxide from point sources of emissions and then stores the carbon dioxide in geological formations. In this thesis two different types of combined cycles that are well suited for carbon capture and storage are introduced and analysed. The cycles are the Graz cycle and the Semi Closed Oxy-fuel Combustion Cycle (SCOC-CC). The net power output of the conceptual designs analysed here are around 100 MW, which is in the mid-size power output range. The simulation of the two cycles shows promising results and high efficiencies. The Graz cycle net efficiency is around 49% and the SCOC-CC net efficiency is around 46%. The combustion in the cycles takes place using only oxygen as oxidizer instead of air. The combustion products will mainly be steam and carbon dioxide. This influences the properties of the working media in the gas turbines used in the cycles. Traditional design tools for the gas turbine therefore needs modification. The thesis describes the conceptual design tool used to design the compressor part of the gas turbines. The tool is based on a one dimensional model that uses empirical data to compute losses. The thesis also describes the development of a two dimensional compressor design method. Two different layouts of the gas turbine were studied for the SCOC-CC, a one shaft configuration and a two shaft configuration. The one shaft configuration resulted in a compressor design that was relative bulky, with 18 stages. The two shaft configuration resulted in a more favourable compressor design with 14 stages. The design of the gas turbine for the Graz cycle has both benefits and disadvantages originating from the fact the working fluid in the Graz cycle has a higher fraction of steam compared to the SCOC-CC. This difference in the working fluid will result in that the turbomachinery will be smaller for the Graz cycle compared to the SCOC-CC. This however also results in that the heights of the blades in the rear stages of the compressor will be relatively small. This will result in a higher amount of losses generated in the rear stages. The preferred Graz cycle gas turbine configuration is a compressor geared intercooled design.