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    June, 2005

Vol 1 Chapter 29: A Comparison of the Efficiences of the Oxy-fuel Power Cycles Water-cycle, Graz-cycle and Matiant-cycle

Olav Bolland, Hanne M. Kvamsdal and John C. Boden

Abstract: One of the technology areas targeted in the CO₂ Capture Project (CCP) has been oxy-fuel combustion. This process generates a flue gas consisting largely of carbon dioxide and water from which carbon dioxide is easily separated. The use of oxy-fuel combustion in gas turbine-based power generation will require new equipment, but also provides an opportunity to develop new cycles which may offer higher efficiencies than current air-based combined cycle systems, thus partially offsetting the additional cost of oxygen production. Three oxy-fuel power generation concepts (Water-cycle, Graz-cycle and Matiant-cycle), based on direct stoichiometric combustion with oxygen, are evaluated in the present study. Considering cycle efficiency and given similar computational assumptions, the Graz-cycle and the latest versions of the Matiant-cycle seem to give rather similar net plant efficiencies (around 45%), while the Water-cycle is 3–5% points behind. When comparing the three cycles with the well-known oxy-fuel gas turbine combined cycle (similar to CC-Matiant-cycle), for which efficiencies in the range 44–48% have been reported, there is no obvious advantage for the three. A challenge for all oxy-fuel cycles is the combustion. Both the fuel and the oxidant are supposed to be consumed simultaneously in the combustion process. This requires very good mixing and sufficient residence time. Incomplete combustion with CO formation may result, or a surplus of oxygen to the combustion process may need to be supplied. Another challenge is the development of turbo machinery capable of working with CO₂/H₂O mixtures at high temperatures and pressures. In general, one can say that oxy-fuel cycles do not exhibit significantly better efficiency compared to postand pre-combustion CO₂ capture methods. One can also question what other advantages oxy-fuel cycles offer compared to other options. A disadvantage with oxy-fuel cycles is that this technology only can be used in plants where CO₂ is to be captured. This means that equipment developed for this purpose may, as it seems today, have a limited market potential, and the motivation for technology development is not that evident. The future for oxy-fuel cycles depends on:

1. Willingness to develop oxy-fuel turbo machinery and combustors, and
2. Future development of oxygen production technology. For the latter, the development of ion transport membranes is vital. In case of oxygen production other than cryogenic distillation, novel cycles like AZEP are very interesting.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO₂ Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources - Volume 1
Edited by: David C. Thomas, Senior Technical Advisor, Advanced Resources International Inc, USA

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