Application of Sapphire-Fiber-Bragg-Grating-Based Multi-Point Temperature Sensor in Boilers at a Commercial Power Plant

Abstract

Readily available temperature sensing in boilers is necessary to improve efficiencies, minimize downtime, and reduce toxic emissions for a power plant. The current techniques are typically deployed as a single-point measurement and are primarily used for detection and prevention of catastrophic events due to the harsh environment. In this work, a multi-point temperature sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (SFBGs) were fabricated via the point-by-point method with a femtosecond laser. The sensor was packaged and calibrated in the lab, including thermally equilibrating at 1200 °C, followed by a 110-h, 1000 °C stability test. After laboratory testing, the sensor system was deployed in both a commercial coal-fired and a gas-fired boiler for 42 days and 48 days, respectively. The performance of the sensor was consistent during the entire test duration, over the course of which it measured temperatures up to 950 °C (with some excursions over 1000 °C), showing the survivability of the sensor in a field environment. The sensor has a demonstrated measurement range from room temperature to 1200 °C, but the maximum temperature limit is expected to be up to 1900 °C, based on previous work with other sapphire based temperature sensors.

Review of optical fibre sensors for electrical equipment characteristic state parameters detection

“…Deng et al [144] demonstrated an FBG sensing system with 14 optical fibres in total of 218 FBGs for winding temperature monitoring in a 35 kV/4000 kVA oil-immersed power transformer, and the hot-spot was accurately located in a temperature-rise test. Yang et al [145] fabricated multipoint temperature sensors based on sapphire FBGs and applied them in boilers at a commercial power plant for more than 40 days, the performance was consistent with measurement range between room temperature and 1200°C. However, the distance of distributed sensing for FBG-based fibre temperature sensors is limited by the number of FBGs because signal demodulation is hard to realise for too many FBGs with different λ B , and the peak shift is difficult to be determined if the temperature to be measured is not uniform.…”

“…Deng et al [144] demonstrated an FBG sensing system with 14 optical fibres in total of 218 FBGs for winding temperature monitoring in a 35 kV/4000 kVA oil‐immersed power transformer, and the hot‐spot was accurately located in a temperature‐rise test. Yang et al [145] fabricated multipoint temperature sensors based on sapphire FBGs and applied them in boilers at a commercial power plant for more than 40 days, the performance was consistent with measurement range between room temperature and 1200°C.…”

“…The FBGbased fibre temperature sensors have been well applied in electrical equipment for abnormal temperature rise sensing [143][144][145][146][147][148] due to the quasi-distributed capability and low sensitivity. The sensor structure is similar to that shown in Fig.…”

Strain Transfer Characteristics of Multi-Layer Optical Fiber Sensors with Temperature-Dependent Properties at Low Temperature

“…By the way of the Bragg grating written into silica with femtosecond lasers using either the phase mask method or the point-by-point method, fiber Bragg gratings (FBGs) could be used for sensing strain and/or temperature in environment at temperatures less than 1000 °C [ 10 ]. Consider the most widely used optical fiber material, fused silica, being incapable of withstanding the chemically corrosive environments, the sapphire-FBG based temperature sensor was fabricated and packaged to show great linearity of temperature response from room temperature to elevated temperature [ 11 ]. For structural health monitoring of the next-generation of nuclear reactors, different technologies for realizing temperature resistant FBGs were developed for temperature and strain measurements especially for components exposed to high temperature and radiation levels [ 12 ].…”

“…The performances of conventional room-temperature sensors, including sensitivities, response times, and lifetime, degrade rapidly when temperature gets lower. A few applications using optical fiber sensors at cryogenic temperatures have been developed lately, such as FBGs embedded in or bonded to substrates (e.g., PMMA, Teflon) with larger thermal expansion coefficients for overcoming their low temperature sensitivity, a continuous liquid level sensing system for liquid nitrogen and helium tanks [ 11 , 17 ].And some optical fiber sensors including FBG, Raman-scattering, Rayleigh-scattering and Brillouin-scattering for monitoring cryogenic temperature of high-temperature superconducting tapes at 77 K or even lower temperature have been attempted [ 20 , 21 , 22 , 23 ]. In these investigations, optical fiber sensors were mainly developed to measure cryogenic temperature in which the deformation of the materials and structures commonly were not considered.…”