Abstract
Chemical looping combustion (CLC) has unique potential for avoiding the large costs and energy penalties of existing CO2 capture technologies. Oxygen is transferred to the fuel using an oxygen carrier, thus avoiding contact between air and fuel. Consequently, the combustion products, CO2 and H2O, come in a separate stream, and more or less pure CO2 is obtained after condensation of H2O. CLC is normally conceived as a dual fluidized bed process, with high gas velocities in an air reactor driving the circulation, similar to circulating fluidized beds (CFBs), except that the material is led to a fuel reactor before being returned to the air reactor. Crucial for the process is the properties of the oxygen carrier and that circulation is sufficient to transfer needed oxygen and heat to the fuel reactor. Comprehensive literature shows successful use of many oxygen carriers in sustained pilot operation. In contrast, the need for reaching adequate circulation in an industrial-scale system has been given little consideration. Normally, a system similar to CFB boilers is assumed to give sufficient circulation. However, literature data indicate that circulation in CFB boilers is 5–50% of what is needed. Measures to provide sufficient circulation may cause difficulties, such as erosion or bed material loss in the cyclone. Here, a circulation system based on collection of the downflow of particles along the walls is proposed, and a design of a 200 MWth combined CLC–CFB boiler based on this principle is presented. Further, operational strategies and the need for flexibility are discussed. The design is focused on making an industrial-scale demonstration boiler, which can be used in CLC operation with different oxygen carriers and different fuels and that can explore different operational strategies to find optimal conditions. It is recommended that the upscaling of the technology aims directly at the industrial scale.
Highlights
AND BACKGROUNDThe key technologies currently being evaluated and developed for CO2 capture, e.g., pre-combustion, post-combustion, and oxy-fuel, all suffer from the need for gas separation
The data presented from large-scale circulating fluidized beds (CFBs) boilers are uncertain and differing, and there seems to be a large spread in the estimated backflow ratio, even for data relating to the same CFB boiler
There are good reasons to be cautious with such a strategy: (1) A design would not be based on existing operational experiences with the needed flows but rather on the extrapolation of results from actual operation, such as the data from the MWth CFB with a m riser, which is well below the 30−50 m height of industrial-scale boilers
Summary
The key technologies currently being evaluated and developed for CO2 capture, e.g., pre-combustion, post-combustion, and oxy-fuel, all suffer from the need for gas separation. These gas separation steps involve significant operational costs as well as large energy penalties, estimated to the order of about 10 percentage points of power plant efficiency, leading to a substantial increase, of around 30% or more, in fuel consumption and plant size. Gas separation technology is generally a mature technology, and no major technology breakthrough is foreseen. This is in great contrast to chemical looping combustion (CLC), where, ideally, no gas separation is needed. The exhaust gas stream ideally consists of only CO2
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