Chemical looping combustion is being developed by the U.S. Department of Energy as a possible technology for heat or power generation with integrated carbon dioxide separation. Improved sensors and controls are among the technical challenges that must be overcome for the commercial success of chemical looping. For improved control the chemical looping process, accurate and rapid measurement of key process parameters is required. A high temperature solids-flow sensor, based on microwave Doppler technology, is being developed to provide online measurement of solids circulation rate. This paper reports on progress in the development and testing of this sensor technology for CLC conditions.
Chemical looping combustion is being developed with support of the U.S. Department of Energy because it has potential for carbon capture with less efficiency penalty than conventional power cycles with post-combustion capture.  Chemical looping uses an intermediary, such as a metal oxide, to collect oxygen from air in the air reactor. The oxidized particles are then transported and to the fuel reactor, where the oxygen carrier particles are reduced. The product gas stream contains mostly carbon dioxide and water vapor, which be readily separated for carbon capture, utilization and storage. An inert buffering gas is injected in the process between the reactors to help separate the oxidation and reduction processes.
One of the challenges for operation of a chemical looping combustion process is the measurement and control of the solids circulation rate. The circulation rate is important for controlling both heat transfer and chemical reaction processes. It has been observed to have large variations on a short time scale, in cold flow systems used to model the hydrodynamics. While the circulation rate may be estimated from pressure drops at various points in the process, this measurement approach is indirect, and has had considerable uncertainty.
While there are many methods employed for measurement of air conveyed solid materials, the operating temperature and pressure parameters used or examined for use in lab- or pilot-scale chemical looping systems make most measurement methods for conveyed solids very difficult to implement. Chalmers University (Sweden), and Southeast University (China) presently have lab-scale reactors using iron at 1000°C and nickel at 1040 °C. Proposed chemical-looping gas turbine systems have the highest pressure requirements (up to 21 bar) with temperatures as high as 1050°C for iron-based carriers,. In contrast, most solids flow measurement technology has been developed for atmospheric pressure, low temperature applications such as conveyance of food ingredients, such as grains and sugar, or raw materials such as plastic pellets.
The Chemical Looping Reactor has been designed and constructed at NETL for the purpose of research and development of the chemical looping combustion process. The chemical looping reactor operates at positive pressure at high temperature (~ 1000 °C). The design of the microwave antenna and waveguide must take these conditions into account.
Microwave Solids Flow Sensor Development
A high temperature solids flow sensor based on microwave Doppler technology is being developed for chemical looping. To meet the special requirements of this application, microwave sensing provides several advantages. First, it is a non-contact method which allows a pressure boundary. Second, it is less sensitive to the surface quality of the pressure boundary, or microwave window, than an optical (visible wavelength) method. Third, it has already been commercially implemented for slightly elevated temperatures (up to 200 C).
NETL, in collaboration with CMU, is developing a prototype high temperature microwave mass flow sensor specifically for the chemical looping application. This approach allows adjustment of the launcher design, electronics, and data processing. The electronics and a first prototype microwave launcher have been tested at room temperature, and data processing has been developed. The prototype high temperature microwave solids flow launcher has been assembled, and is undergoing testing for chemical looping operations.
The system is calibrated using a rotating table feeder. Particulate material is fed through a drop tube to the surface of a small round disk (the table), near its edge. The particles dropped onto the rotating table are brushed off into a funnel. The rotation of the disk and the height of the gap between the drop tube and the disk control the particle feed rate. The particles drop from the funnel, gain velocity, and pass by the sensor under calibration. The particles are collected at the bottom of the drop tube and continuously monitored with a load cell to determine the actual mass vs time (solid flow rate) for the calibration. Within a limited range, the solids flow rate produced is very steady.
Testing of the microwave Doppler sensor is being performed on several different experimental systems at NETL. In addition to the table feeder, the sensor is also being applied on the cold flow model of the CLR. While being low temperature, it does have the benefit of allowing visual observation of gas-solids flow behavior. Another apparatus for use in testing the flow sensor is the High Temperature Solids Flow Verification apparatus. It provides a short period of gravity driven flow of high temperature particles, which have been preheated to CLC temperature conditions. Lastly, testing on the CLR assures the construction of the sensor is suitable for CLC application conditions. Results of development and testing are reported.
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