Fiber Optic Sensors Based on Fiber Bragg Gratings for Methanol Steam Reforming Temperature Monitoring

Abstract

This work describes the use of sensors based on fiber Bragg gratings (FBGs) to monitor the variation in the temperature gradients in a catalyst plate in a steam reforming metal plate test reactor. Steam reforming consists of a series of chemical reactions between steam, a hydrocarbon fuel (such as methane) and carbon monoxide taking place over a catalyst layer with the ultimate goal of producing hydrogen. Methane steam reforming is currently the principal technique used for hydrogen generation and is likely to increase in terms of production volume in the future as commercial fuel cell applications become more widespread. The amount of hydrogen obtained from these reactions is inherently linked to the process temperature which is in turn influenced by the heat transfer characteristics of the reformer. Steam reforming is a very endothermic process and it is extremely challenging to achieve fast response and good load following of the fuel cell power demand. FBGs can be used to obtain critical information to improve the load response and maximize the power density of compact heat-exchange reformers. Currently, non-contact instruments, such as single point radiometers and thermal imagers, are used to measure the temperature in methane steam reforming reactors. Many issues are linked with these measurement tools such as inaccuracies in the results due to difficulties associated with calibration which depends on the surface characteristics and operating conditions. Moreover, temperature measurements are localised and in commercial reformer design, access to some sections of the reformer is not possible during operation. Consequently, obtaining a temperature profile across the reformer is challenging. In this work, a fiber-optic sensor based on FBG technology is used to monitor temperature directly on the catalyst plate of a small-scale experimental reactor. An in-fibre Bragg grating occupies a short length (typically 2 mm-10 mm) of an optical fibre and is comprised of regions of modified refractive index photo-inscribed at regular intervals along the length of the fibre core. When light propagating in an optical fibre encounters a grating, a narrow band of wavelength is preferentially reflected. The center-wavelength of this reflected band (i.e., the Bragg wavelength) is a function of the refractive index of the fibre core and of the grating spacing. When the fibre is subjected to mechanical or thermal strain, both the refractive index of the fibre core and the grating spacing change and there is a corresponding shift in the Bragg wavelength that can be related to the magnitude of the applied strain or temperature change. FBGs are well suited for temperature measurements in steam reforming reactors. Multiple FBGs can be located on a single fibre and interrogated independently. This allows multiple measurements to determine the temperature gradient across a reformer tube while requiring only a single access port for the fiber into the reformer. Furthermore, the small sensor size allows high spatial resolution of the temperature gradient. In addition, optical fiber is able to withstand the harsh chemical conditions and high temperatures under which steam reforming takes place. The insertion of a fiber-optic sensor directly in the reactor and located under the catalyst plate aims to resolve the issues currently associated with temperature monitoring in methane steam reforming reactors and provide an accurate measurement of the temperature profile along the length of the catalyst plate using multiplexed Bragg gratings. Experiments were conducted with regenerated type-I FBGs written in standard SMF-28e germanium doped fiber using UV exposure. Type-I gratings erase when exposed to temperatures of the order of 400 °C for extended periods of time. Regeneration is a high temperature (>900 °C) annealing process which leads to the formation of gratings that have been shown to be stable at temperatures as high as 1295 °C. Due to the fragility of the fiber following the annealing process, the FBG is first positioned in the custom-modified reactor and the gratings are regenerated in-situ. Multiplexed gratings are used to obtain a temperature profile, which is generated by electric heaters, in the metal plate test reactor. The temperature profiles acquired with the regenerated FBGs during reforming are reported and compared with the reference temperature profiles obtained with K-Type thermocouples, also embedded in the reactor.