Thermodynamic and Economic Investigation of an Improved Graz Cycle Power Plant for Co2 Capture

Introduction of closed cycle gas turbines with their capability of retaining combustion generated CO2 can offer a valuable contribution to the Kyoto goal and to future power generation. Therefore research and development at Graz University of Technology since the 90's has lead to the Graz Cycle, a zero emission power cycle of highest efficiency. It burns fossil fuels with pure oxygen which enables the cost-effective separation of the combustion CO2 by condensation. The efforts for the oxygen supply in an air separation plant are partly compensated by cycle efficiencies far higher than 60 %. In this work a further development, the S-Graz Cycle is presented, which works with a cycle fluid of high steam content. Thermodynamic investigations show efficiencies up to 70 % and a net efficiency of 60 % including the oxygen supply. For a 100 MW prototype plant the layout of the main turbomachinery is performed to show the feasibility of all components. Finally, an economic analysis of a S-Graz Cycle power plant is performed showing very low CO2 mitigation costs in the range of 10 $/ton CO2 captured, making this zero emission power plant a promising technology in the case of a future CO2 tax. INTRODUCTION In the last hundred years the concentration of some greenhouse gases in the atmosphere has markedly increased. There is a wide consensus in the scientific community that this seems to influence the Earth surface temperature and thus the world climate. Therefore, in 1997 the Kyoto conference has defined the goal of global greenhouse gas emission reduction of about 5 % in the next years compared to the 1990 emission level. CO2 is the main greenhouse gas due to the very high overall amount emitted by human activities. And about one third of the overall human CO2 emissions are produced by the power generation sector. In the EU there is a strong pressure on utilities and industry to reduce the CO2 emissions by power generation. In 2003 the European Parliament passed a directive on emission trading. In 2005 emission allowances will be assigned to about 10 000 companies in 25 countries within the EU which cover about 46 % of the overall EU CO2 emissions. Companies which do not need their full amount can sell it to companies which need more than assigned. As emission allowances become scarce they will have an increasing value, estimates vary between 10 and 20 €/ton CO2 by 2010 and even more by 2015 [1]. So there is a strong driving force to develop commercial solutions for the capture of CO2 from power plants. The main technologies are [2]: post combustion CO2 capture, e.g. by washing of exhaust gases using amines pre-combustion decarbonization of fossil fuels to produce pure hydrogen chemical looping combustion oxy-fuel cycles with internal combustion of fossil fuels with pure oxygen The authors believe that oxy-fuel cycles are a very promising technology and that their Graz Cycle can be the most economic solution for CO2 capture from fossil power generation once the development of the new turbomachinery components needed are done. Oxygen needed in a large amount for this kind of cycles can be generated by air separation plants which are in use worldwide with great outputs in steel making industry and even in enhanced oil recovery. The largest air separation plant already in operation for some years in the Gulf of Mexico produces nitrogen for the injection in the gas dome of a large oil field off-shore [3]. The equivalent amount of this oxygen could feed a Graz Cycle plant of 1300 MW.