A Further Step towards a 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 work at Graz University of Technology since the nineties has led to the Graz Cycle, a zero emission power cycle of highest efficiency. It burns fossil fuels with pure oxygen which enables the costeffective 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 for modern combined cycle plants. At the ASME IGTI conference 2004 in Vienna a high steam content S-Graz Cycle power plant was presented showing efficiencies for syngas firing up to 70 % and a net efficiency of 57 % considering oxygen supply and CO2 compression. A first economic analysis gave CO2 mitigation costs of about 10 $/ton CO2 avoided. These favourable data induced the Norwegian oil and gas company Statoil ASA to order a techno-economic evaluation study of the Graz Cycle. In order to allow a benchmarking of the Graz Cycle and a comparison with other CO2 capture concepts, the assumptions of component efficiency and losses are modified to values agreed with Statoil. In this work the new assumptions made and the resulting power cycle for natural gas firing, which is the most likely fuel of a first demonstration plant, are presented. Further modifications of the cycle scheme are discussed and their potential is analyzed. Finally, an economic analysis of the Graz Cycle power plant is performed showing low CO2 mitigation costs in the range of 20 $/ton CO2 avoided, but also the strong dependence of the economics on the investment costs. 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 are 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 (12 and 25 $/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