Chemical-looping combustion (CLC) is an emerging carbon capture technology that is characterized by a low energy penalty, low carbon dioxide capture costs, and low environmental impact. To prevent the contact between fuel and air, an oxygen carrier is used to transport the oxygen needed for fuel conversion. In comparison to a classic oxyfuel process, no air separation unit is required to provide the oxygen needed to burn the fuel. The solid fuel, such as coal, is gasified in the fuel reactor (FR), and the products from gasification are oxidized by the oxygen carrier. There are promising results from the electrically heated 100 kWth unit at Chalmers University of Technology (Sweden) or the 1 MWth pilot at Technische Universität Darmstadt (Germany) with partial chemical-looping conditions. The 1 MWth CLC pilot consists of two interconnected circulating fluidized bed reactors. It is possible to investigate this process without electrically heating due to refractory-lined reactors and coupling elements. This work presents the first results of autothermal operation of a metal oxide CLC unit worldwide using ilmenite as oxygen carrier and coarse hard coal as fuel. The FR was fluidized with steam. The results show that the oxygen demand of the FR required for a complete conversion of unconverted gases was in the range of 25%. At the same time, the carbon dioxide capture efficiency was low in the present configuration of the 1 MWth pilot. This means that unconverted char left the FR and burned in the air reactor (AR). The reason for this is that no carbon stripper unit was used during these investigations. A carbon stripper could significantly enhance the carbon dioxide capture efficiency.
Chemical Looping Combustion (CLC) has been extensively investigated up to pilot scale. Despite promising results, this CO2 capture technology has to be further developed towards a potential industrial application. In order to accelerate the scale up of the process, it is essential to simulate CLC with process simulation software (Aspen Plus® V8.8). In this study, a process simulation model of the CLC pilot plant at Technische Universität Darmstadt is developed. The air reactor and the fuel reactor are integrated in the process model to calculate the conversion of the oxygen carrier and gases. The solids distribution and also the chemical conversion of gases and solids are implemented by user-defined functions programmed with FORTRAN. These codes are based on mathematical equations for fluidized beds and kinetic data of the chemical reactions taken into consideration. For this reason, dedicated thermal gravimetric analysis (TGA) experiments were carried out to determine both kinetics of redox reactions of oxygen carrier and gasification kinetics of the coal used in the 1 MWth CLC unit. The onedimensional solids distributions and gas/solid mixing are calculated from empirical correlations according to Kunii and Levenspiel. The models are validated by means of experimental data obtained from the the 1 MWth CLC pilot plant with ilmenite as oxygen carrier and Russian hard coal ("Taldinsky") as fuel. Among others, pressure profiles and gas concentrations at reactor exits are calculated for various operational points with autothermal operation. The results show good agreement with the experimental data. The process model was applied to optimize the operational conditions of the pilot plant.
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