(Invited) Refractory Ceramic Sensors for Process and Health Monitoring of Slagging Gasifiers

Current sensor technologies for measuring the temperature and health of slagging gasifiers are primarily based on thermocouples that are inserted into the gasifier through open access ports within the refractory.  The major disadvantages of this technique include the intrusive nature of the access port that provides a path for slag penetration, and the severe temperature and corrosion limitations of the sensor materials.  The current work presents the development of a series of ceramic-based thermistors, thermocouples and strain sensors, which may be used within a variety of refractory brick for the gasifier application.

The key aspect of this technology is that these sensors are incorporated and interconnected through the volume of the refractory brick and do not significantly impact the intrinsic properties of the refractory.  This technology circumvents the need to insert an isolated monolithic, stand-alone sensor into the refractory via an access port.  This insures the integrity of the sensor within the harsh environment and does not introduce flaws or slag penetration pathways within the refractory.  These sensors can be used to in situ monitor reaction conditions and degradation within slagging gasifiers and will provide better understanding of the health of the refractory (which is key for planning maintenance schedules and reducing the refractory degradation).  The development of such a refractory sensor system concept can be also implemented into other applications, such as conventional coal-fired boiler technology, biomass gasification, and steel and glass manufacturing.

As stated above, the objective of our work is to develop high-temperature sensors such as thermocouples, thermistors and strain/stress sensors composed of refractory composite materials that are electrically conductive and chemically stable at high temperatures (750°-1500°C) and high pressures (up to 1000 psi). The high-temperature sensors investigated in this work were composed of intermetallic composites directly embedded into refractory oxides. The composites used for this work were synthesized by a mixed-oxide route. Metal silicides (such as MoSi₂, WSi₂, ZrSi₂, etc.) were inserted within a matrix material composed of refractory oxides (Al₂O₃, ZrO₂, Y₂O₃, etc.) to form a 3-3 ceramic composite.  The physical and electrical properties were specifically manipulated by altering the level of percolation of the conductive species (silicide) within the refractory constituent (refractory oxide). 

Prior to the development of the high-temperature sensors, the silicide-oxide composites developed in this study were sintered up to 1600°C under argon atmosphere in order to investigate densification, microstructural evolution, phase development, and their thermal and electrical performance as a function of the composition (fraction of silicides) and processing. The 4-point DC conductivity and coefficient of thermal expansion (CTE) measurements were performed between 1000°-1200°C. All results were compared in detail to select compositions that would work as high-temperature sensor materials.  The sensors were fabricated from the composite materials by screen-printing, pressing, or casting the sensor design into a monolithic preform and inserting this preform into the refractory brick during the consolidation process.

An example of one of these embedded ceramic-based sensors is a thermocouple composed of two separate silicide-alumina composite compositions which were patterned to produce a couple within the interior of a refractory matrix.  The thermocouple successfully displayed a predictable thermoelectric voltage trend (as a function of temperature), and the voltage was 11.56 mV at 1000°C.  For reference, a high-temperature type-B thermocouple (Pt-Rh) produces a voltage of 4.83 mV at the same temperature.    

Acknowledgements:

This research was funded by the US Department of Energy, National Energy Technology Laboratory under contract no. DE-FE0012383. The authors would like to thank the project monitor, Maria Reidpath, for her insightful discussions and guidance. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.