(620a) Development of Solids Flow Sensor Technology for Chemical Looping

Development of Solids Flow Sensor Technology for Chemical Looping
Short Abstract
Chemical looping combustion is being developed by the U.S. Department of Energy as a possible technology for efficient carbon dioxide separation for heat or power generation. Control of them chemical looping process, however, has proven to be a technology challenge. In order to 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 a real-time measurement of solids circulation rate. Initial testing has shown the feasibility of the approach. Testing of the prototype with high temperature solids is the next step.
Introduction
Chemical looping combustion is being developed with support of the U.S. Department of Energy
because it will allow for carbon capture with less efficiency penalty than conventional power cycles with post-combustion capture. 1 2 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 methods very difficult to implement. Chalmers University (Sweden)3, and Southeast University (China)4 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 carriers5, 6. 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.
Commercially available solid mass flow meters fall into several general categories: coriolis, impact plate, diverting or curved chute, momentum probe, differential pressure, optical, capacitance change, microwave, ultrasonic, magnetic, and electrostatic. The maximum process temperature specification identified in the review was 400°C, with most of the sensors designed for near-atmospheric pressure
operation.7 Many low temperature contact methods exist for measuring the flow of low temperature solids, typically bulk materials or foodstuffs. Many are on systems that do not require sealing from the surrounding air. Implementation for the chemical looping combustion environment of 1000 C with a pressurized system of these methods poses considerable technical challenges. Optical methods, while noncontact, suffer from difficulties with window fouling, which are likely to be severe in a chemical looping environment. Microwave technologies appeared promising for modification for higher
temperature use, and have been pursued in this work.
The Chemical Looping Reactor has been designed and constructed at NETL for the purpose of research and development of the chemical looping process. The chemical looping reactor operates at high temperature (~ 1000 °C) and is pressurized. The design of the microwave antenna or launcher must take
these factors 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).
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, helpful for calibration.
The table feeder, while producing a steady flow for calibrations, is limited to low temperature operation. Questions remain on the effect of high temperature on the microwave properties of carrier particles,
and how that will affect their measurement. To address this issue, the High Temperature Solids Flow Verification apparatus has been constructed. A batch of particles, approximately 10 kg, is loaded into the HITSV apparatus and preheated with light fluidization. A slide-gate valve is opened to drop the hot particles through a heated tube past the sensor under test. The hot particles are collected in a container, whose mass is measured continuously with a load cell. The flow rate of the particles can be changed by the size of the slide-gate opening.
To quickly investigate the approach of microwave mass flow sensing, a commercial low temperature microwave Doppler sensor was purchased. It was modified from its standard commercial configuration
for attempted application at the CLR operating temperatures, near 1000 C. In testing, the commercial system calibrated and performed well on the table feeder. At room temperature, on the cold flow model of the CLR, the sensor data trends still appeared reasonable, although in-place calibration was needed. At high temperatures on both the CLR and on the HITSV apparatus, the flow rate data no longer appeared reasonable. The cause of this behavior is still under investigation.
NETL, in collaboration with CMU, is developing a prototype high temperature microwave mass flow sensor specifically for the chemical looping application.8 9 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.10
The
prototype high temperature microwave solids flow launcher is being assembled, and is scheduled for checkout and initial operation on the Chemical Looping Reactor in the summer of 2014.
References

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