ABSTRACT
The largest point sources of carbon dioxide emissions are fossil fuelled power stations. Atmospheric emissions of carbon dioxide are considered one of the major reasons for global warming. In the medium and long run an efficient and inexpensive integration of CCS (carbon capture and storage) to power cycles is a key challenge in energy technologies.
The similarity for processes with pre-combustion and integrated CO2 capture is the necessity of technically pure oxygen. In case of pre-combustion cycles the oxygen is required to achieve a high carbon conversion rate. In oxyfuel cycles (integrated CO₂ capture) the combustion takes place in an oxygen rich atmosphere to reduce the amount of non-condensable gases in the exhaust gases. The non-condensable gases would remain in the captured carbon dioxide which is separated by condensing water out. The production of oxygen is conventionally supplied by a cryogenic air separation unit. The idea is to replace the ASU by an oxygen transport membrane (OTM).
This paper deals with modelling of an OTM and the integration of the membrane model into power cycle models. Two different power production processes are discussed in this work where an OTM reactor is integrated in. First an IGCC process, where CO₂ can be separated from a syngas before combustion, is proposed. Hard coal is gasified in presence of oxygen and steam. After gas treatment CO₂ capture takes place before the synthesis gas is supplied to the burner. Besides the IGCC process, a lignite fired oxy-fuel boiler cycle is proposed with an integrated OTM reactor where CO₂ is captured by cooling the cleaned exhaust gases. In this way water is condensed out. A part of the carbon dioxide rich stream is recirculated to the boiler to control temperature; the excess carbon dioxide is compressed for sequestration.
The model of the OTM considers heat and mass transfer through the membrane for different operating loads regarding pressure, temperature, pressure difference, and partial pressure difference of permeate and retentate side of the membrane. The membrane is modelled in MS ExcelTM by using the finite difference method. Necessary thermodynamic properties are also calculated in MS ExcelTM via Aspen Properties. The performance of the membrane and its impact on different power cycles is presented. Therefore various configurations of the membrane are analysed. Advantages and disadvantages for the IGCC and the lignite fired oxyfuel boiler of oxygen transport membranes are discussed.