Advanced sensors and controllers have been actively sought for enhancing energy efficiency, lowering operation cost, and reducing pollutant emission in fossil fuel power plants. Compared to conventional electrochemical and semiconductor sensors, fiber optic chemical sensors (FOCS) are attractive for applications in the harsh environments involved in fossil energy systems due to their advantages in miniature, immunity to electromagnetic interferences, robustness and capability of remote operation. FOCS has the potential to meet many analytical challenges in high temperature, high pressure, dusty, and corrosive environments associated with the fossil fuel energy processes where current gas sensors may fail to function effectively.
This dissertation aims to develop and demonstrate two types of FOCS fabricated by physically and functionally integrating chemically sensitive nanomaterials with long-period fiber gratings (LPFG): one is the perovskite type doped ceramic thin film coated-LPFG sensor for in situ high temperature H2 detection in fossil fuel derived syngas streams; the other is surface-acidified zeolite thin film integrated-LPFG sensor for potential uses for NH3 detection in biomass-derived syngas and environmental monitoring.
Three representative perovskite-type doped ceramics, i.e. the cerate-based SrCe0.95Tb0.05O3-d (SCTb), the ceria-zirconia solution based SrCe0.8Zr0.1Y0.1O3-d (SCZY), and the zirconate-based SrZr0.95Y0.05O3-d (SZY) have been specifically evaluated to identify the appropriate sensing material for high temperature H2 detection in the harsh environments. The H2 sensing materials have been initially evaluated through the measurement and analysis of the material stability in different atmospheres, the high temperature H2 uptake/sorption, and the thin film total electrical conductivity in H2 with and without CO2. Results of these studies on the material chemical and physical properties suggest that the SCTb has the best sensitivity for H2 detection but lowest selectivity and chemical stability; the SZY exhibits the best stability but lowest sensitivity; and the SCZY has high sensitivity and reasonable selectivity and stability for H2 detection in syngas conditions. The materials’ chemical compositions may be adjusted to achieve balanced H2-sensitivity and chemical stability based on the sensing environment (gas compositions).
The ZSM-5 zeolite films have been synthesized on the LPFG optical fibers (Z-LPFG) by the in situ hydrothermal crystallization method. The Si/Al ratio of ZSM-5 zeolite films is ~23. The shift of resonant wavelength of the Z-LPFG was measured for N2-carried NH3 and other potential interfering gases, including CO2, H2, CH4, CO, H2S and H2O to evaluate the sensing selectivity for NH3. Research has also been conducted to investigate the NH3 sensitivity and speed of response of the Z-LPFG. The zeolite-coated LPFG offers high sensitivity but low selectivity for NH3 gas sensing because of the limited adsorption selectivity for NH3 gas in the ZSM-5 zeolitic pores. In order to improve the NH3 sensing selectivity, the ZSM-5 zeolite was modified through ammonium ion exchange and subsequent calcination to form acidic ZSM-5 (H-ZSM-5). The experimental results have shown that zeolite surface modification combined with temperature control is an effective way to improve the NH3 sensing selectivity and response speed. However, operating at elevated temperature compromises the sensitivity due to reduced amount of gas adsorption.