Development of robust TDLAS sensors for combustion products at high pressure and temperature in energy systems [electronic resource]
Keywords: Mechanical Engineering.

Energy extraction by combustion of fossil fuels leads to the release of greenhouse gases and other harmful emissions. It has been a priority for combustion research in the recent few decades to mitigate these emissions and harness the energy contained in these fuels more efficiently. Coal is among the most abundant and widely distributed sources of fossil fuel in the world. Integrated gasification combined cycle (IGCC) is one of the cleanest methods of extracting energy from coal, when combined with carbon capture and storage. The gasifier, the cornerstone of this technology, produces synthesis gas (also known as syngas) via partial oxidation of coal. Continuous monitoring of the syngas composition is imperative to the success of this technology as it indicates the extent of reaction in the gasifier, the heating value of the output syngas and hence the overall health of the gasification system. The primary objective of the research presented in this dissertation is the development of robust, in-situ sensors that can reliably monitor concentrations of CO, CO2, CH4 and H2O in gasifiers. Tunable diode laser absorption spectroscopy (TDLAS) sensors for detection of CO, CO2, CH4 and H2O at elevated pressures in mixtures of syngas were developed and tested. Wavelength modulation spectroscopy (WMS) with 1f-normalized 2f detection was employed. Fiber-coupled DFB diode lasers operating at 2325, 2017, 2290 and 1352 nm were used for simultaneously measuring CO, CO2, CH4 and H2O, respectively. Criteria for the selection of transitions were developed, and transitions were selected to optimize the signal and minimize interference from other species. To enable quantitative WMS measurements, the collision-broadening coefficients of the selected transitions were determined for collisions with possible syngas components, namely CO, CO2, CH4, H2O, N2 and H2. Sample measurements were performed for each species in gas cells at a temperature of 25 °C and pressures up to 20 atm. To validate the sensor performance, the composition of synthetic syngas was determined by the absorption sensor and compared to the known values. A method of estimating the lower heating value and Wobbe index of the syngas mixture from these measurements was also demonstrated. The sensors demonstrated in a sample cell were then deployed in a pilot-scale (1 ton/day), high-pressure (up to 18 atm), entrained-flow, oxygen-blown, slagging coal gasifier at the University of Utah. Measurements of species mole fraction with 3-second time resolution were taken at the pre- and post-filtration stages of the gasifier synthesis gas output flow. Although particulate scattering makes pre-filter measurements more difficult, this location avoids the time delay of flow through the filtration devices. With the measured species and known N2 concentrations, the H2 content was obtained via balance. The lower heating value and the Wobbe index of the gas mixture were estimated using the measured gas composition. The sensors demonstrated here show promise for monitoring and control of the gasification process. The sensors were further improved using a scanned-wavelength modulation spectroscopy technique and was demonstrated for the first time in the product stream of an engineering-scale (50 tons/day coal throughput) transport reactor gasifier (15 ton/hr syngas production). A robust optical design was created to counter various challenges including beam steering, loss in beam intensity due to particulate scattering and wide dynamic range in transmission in a typical gasifier environment. In addition, due to the unavailability of low-loss, high-strength fibers and combiners at the wide range of operating wavelengths, an extensive optical design was required for enabling such a group of sensors to operate simultaneously. The results from the measurements during the gasification process, starting from the propane heat-up till the full-scale gasification process, reveals interesting dynamic behavior not observable by extractive measurement techniques. These sensors show high-bandwidth detection in a gasifier and thereby eliminate the need for surrogate indicators that can monitor the transient performance of the gasifier. Methane, a more potent greenhouse gas than CO2, is often produced as an intermediate product of hydrocarbon combustion processes. Hence, a more sensitive CW laser absorption diagnostic (50 times stronger than the fiber-coupled CH4 sensor at 2290 nm, which was designed for detection of higher concentrations at high pressure) for in-situ measurement of methane mole fraction at high temperatures was also developed. The selected transitions for the diagnostic are a cluster of lines near 3148.8 cm-1 from the R-branch of the [nu] 3 band of the CH4 absorption spectrum. The selected transitions have 2-3 times more sensitivity to CH4 concentration than the P-branch in the 3.3 micron region, lower interference from major interfering intermediate species in most hydrocarbon reactions, and applicability over a wide range of pressures and temperatures. Absorption cross-sections for a broad collection of hydrocarbons were simulated to evaluate interference absorption, and were generally found to be negligible near 3148.8 cm-1. However, minor interferences from hot bands of C2H2 and C2H4 were observed and characterized experimentally, revealing a weak dependence on wavelength. To eliminate such interferences, a two-color on-line and off-line measurement scheme was proposed to determine CH4 concentration. The colors selected, i.e. for on-line (3148.81 cm-1) and off-line (3148.66 cm-1), were characterized between 0.2 to 4 atm and 500 K to 2100 K by absorption coefficient measurements in a shock tube. Minimum detectable levels of CH4 in shock tube experiments were reported for this range of temperatures and pressures. An example measurement was shown for sensitive detection of CH4 in a shock tube chemical kinetics experiment.