Thermodynamics and Turbomachinery of the Oxyfuel Naki Cycles

Hoeftberger, D. (2010). Thermodynamics and Turbomachinery of the Oxyfuel Naki Cycles. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-10-005

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Abstract

The increasing amount of carbon dioxide (CO2) in the atmosphere causes continuing climate change. The Kyoto Protocol was agreed by a majority of governments around the world to address this challenge. Its aim is to reduce the emission of greenhouse gases by a substantial amount compared to the 1990 emissions. One promising way is to reduce the emissions of CO2 using CCS (carbon dioxide capture and storage) in power generation and in some industrial plants which produce high quantities of this greenhouse gas. The capture of CO2 can be accomplished by different methods like oxyfuel cycles, precombustion systems, and postcombustion system.

To realize one alternative to capture CO2 Prof. Nebojsa Nakicenovic of IIASA (International Institutefor Applied Systems Analysis) proposed an oxyfuel cycle. This so-called Naki cycle which is, in principle, a closed cycle gas turbine with recuperative heat exchanger uses CO2 as the working fluid. The pressure rise is accomplished in liquid state by a pump. Hence, the working fluid has to be condensed after being cooled down in the recuperative heat exchanger. To investigate the thermodynamic cycle efficiency and feasibility of turbomachinery this work was carried out.

In this work three different variants of the so-called Naki cycle (Naki I, Naki II, and Naki III) wer studied. The cycles were modeled in the simulation software IPSEpro in which the thermodynamic invetigation of these cycles was conducted. The thermodynamic evaluation of Naki I was performed using coal dust (pure carbon) as fuel. In the investigation of Naki II, two different fuels were compared. Thse are methane and syngas from coal gasification. The evaluation of Naki III was carried out using methane. With the thermodynamic data, a first dimensioning of the turbomachinery was possible. In the termodynamic evaluation some parameters (for example turbine inlet temperature and pressure or mass flows) were chosen so that feasible turbomachinery dimensions could be expected.

For the most promising cycle, Naki II, a possible turbomachinery design was described in detail (espeially the high-pressure turbine).

Lastly a rough economic evaluation of all three variants of the Naki cycle was carried out. The results of this evaluation give an overview of costs related to the capture of CO2. Each variant of the Naki cycle was compared with a reference plant without CO2 capture. These comparisons lead to mitigation costs (i.e., the costs for one tonne CO2 avoided by a Naki power plant in comparison to a reference plant).

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