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/,,"J _;70 NASA Contractor Report 202306 Potential New Sensor for Use With Conventional Gas Carburizing W.A. de Groot NYMA Inc. Brook Park, Ohio January 1997 Prepared for Lewis Research Center Under Contract NAS3-27186 National Aeronautics and Space Administration Potential New Sensor for Use with Conventional Gas Carburizing W.A. de Groot NYMA Inc., Brook Park, Ohio Abstract The working p'rmciple of most endothermic gas generators is based on passing a mixture of natural gas and air over a heated catalyst. The comb'marion of heat and Diagnostics developed for in-situ monitoring of rocket combustion environments have been adapted for use in heat catalyst converts the air and natural gas into the endothermic treating furnaces. Simultaneous, in-situ monitoring of the gas mixture (a purely catalytic reaction, no combustion). The carbon monoxide, carbon dioxide, methane, water, nitrogen composition of this endothermic gas, and the percentages of carbon monoxide and methane critical for carburization, and hydrogen concentrations in the endothermic gas of a heat varies as a result of fluctuations in composition of the natural treating furnace has been demonsWated under a Space Act gas supply, gas/air ratio, the condition of the catalyst bed, and Agreement between NASA Lewis, the Heat Treating other variables. This in turn leads to a variation in carbon Network, and Akron Steel Treating Company. Equipment installed at the Akron Steel Treating Company showed the potential which could lead to an unacceptable quality of the feasibility of the method. Clear and well-defined spectra of steel product. Adequate monitoring and control of the gas composition carbon monoxide, nitrogen and hydrogen were obtained by means of an optical probe mounted on the endothermic gas of the protective atmosphere is required to ensure product line of a gas generator inside the plant, with the data quality. Because a direct relationship exists between reduction hardware located in the basement laboratory. dewpoint, furnace temperature, and carbon potential, it is possible to determine the carbon potential by measuring the Signals to and from the probe were transmitted via optical other two variables. No accurale determination of the fibers. dewpoint inside the furnace has been available, however, and oxygen probes have replaced dewpoint techniques as a control system to provide a stable endothermic gas atmosphere. Oxygen probes have a solid, but far fIom perfect Background performance record. Problems associated with the probes include their dependence on assuming a constant CO which The carbon potential of a carrier gas in the heat treating might not be true, that soot presence local to the probe will industly is defined as the degree to which a protective distort the bulk reading, that the probe itself acts as a catalyst atmosphere provides carbon for absorption. It depends on the for the decomposition of hydrocarbons, that the reference composition of the gaseous atmosphere and on the atmosphere used is contaminated and as such will distort the temperature of the furnace) _ Most heat treating operations reading, or that the electrode fails. These problems and a use an endothermic gas as protective atmosphere. This number of solutions to atleviate them or make them have less endothermic gas is created in a generator and consists of a of an impact have been described elsewhere. 3 mixture of approximately 20% carbon monoxide (CO), 40% A more accurate, but also significantly more capital nitrogen (N2), and 40% hydrogen (H2). Small amounts of intensive system is based on multiple-gas infrared OR) hydrocarbons or air can be added to this endothermic gas detection systems. Infrared analyzers are very accurate, but upon entering the furnace in order to increase or decrease the have the disadvantage that they are costly and that the available carbon. 2 The composition of the endothermic gas is measurement does not occur in-situ. A sample of the gas critical to the quafity of the finished product. needs to be extracted, filtered, and cooled. Depending on the initial gas composition, changes might occur as the result of Basic Sensor Principle these sample preparation processes. It also increases the time between extraction of the sample and the actual measurement. Raman spectroscopy probes the internal molecular Furthermore, infrared analyzers cannot detect all species rotational/vibrational structure and in doing so recognizes the present. Due to the molecular structure of oxygen, for type of molecule. A detailed explanation of the principles is example, this species does not have an infrared signature and given in refs. 4 and 5. A summary description will be given cannot be measured with infrared techniques. here in the context of the instrument design. An alternate technique, proposed here, is Raman The green light of an Argon Ion laser is focused onto spectroscopy. It is closely related to infrared analysis and has the gas to be analyzed or monitored. The molecules in the gas the capability of detecting and identifying all polyatomic scatter the light in many different wavelengths (colors). Some species, in-situ and real time. 4's The main disadvantage of the of these wavelengths depend on the type of molecules, technique is that the signal generated by gases is very weak. through their internal energy structure. Collecting and Historically this weak signal precluded the use of the analyzing the scattered radiation by wavelength gives technique for process control, but recent developments in information about the type and quantity of the species present. optics and electronics has brought the technique within reach Real time analysis of the intensifies at these wavelengths of being developed for instrumentation and control. In allows quantitative monitoring of the gas composition. combination with high power lasers and fiber optics, this technique can be applied for in-situ and real time 0.05 measurements of gas flows. The advantage over the conventional methods is that several species can be monitored co 2(_ 1285 & 1388 cm"t _" 0.04 simultaneously. A second advantage is that the measurement r- is direct, without the need of assumptions, gas filtering, or in- Ha _ 587 cm-I _ O2 _ 1556 cm-I .'2_ \ __0.03 line calibration. Finally, the probe will not wear or burn off / /I / ¢se57ore"\ due to high temperature exposure, because the probing occurs _ o.02 from a distance. II // \ Nz@2331cm"/ Diagnostics based on Raman spectroscopy were developed for in-situ monitoring of rocket combustion _ O.Ol _= environments. 6"9 A compact instrument was developed, based on this technique, that originally was used to detect hydrogen 0.00 leaks. ]°'1_ With a few minor modifications, this instrument 0 500 1000 1500 2000 2500 3000 3500 4000 4500 was adapted to detect and monitor a number of species. Raman Shift (cm-1) i i i i i i _ i Laboratory experiments with the device are described in 5200 5400 5600 5600 6000 6200 6400 6600 which carbon monoxide, oxygen, nitrogen, and hydrogen are Wavelength (Angstrom) detected with a partial pressure of 0.2 psi, and carbon dioxide and methane with a partial pressure of 0.1 psi. The purpose of the instrument, however, is not to detect, identify and Fig. 1: Computed spectrum of species of interest. quantify the species present in the endogas but rather to monitor the concentrations of selected species. For the heat An example spectrum, in which the wavelengths and intensities are shown where the different molecules will treating applications, the species determined to be important for carburization were carbon monoxide, carbon dioxide, scatter the green light, is displayed in Figure 1. This methane, and water (which directly translates into dewpoint). computed Raman spectrum shows hydrogen (both Results are given of a field test where the system was 'rotational" and 'Vibrational'), carbon dioxide, oxygen, installed and operated on the endothermic gas generator in a carbon monoxide, nitrogen, and water for equal number steel treating plant in order to do in-situ, real time monitoring densities. Computations assumed randomly Iy 'arized incident and collected radiation and a back scatter geometry, where of the composition of the endothermic gas. Initial tests were done to determine the accuracy and response of the device. As the same lens that focuses the laser beam is used to collect the a crucial test it was determined to only monitor the carbon emitted light. The horizontal axis is expressed in monoxide concentration. Future improvements in sensitivity wavenumbers, which are independent of the incident laser and response will allow carbon dioxide and methane to be wavelength (color). A second axis is displayed which monitored and will open the possibility of using this device to indicates the respective wavelengths in the case that an Argon monitor the furnace atmosphere directly. Ion laser with a wavelength of 514.5 nm (green fight) is used. The vertical axis displays the relative intensities of the emitted radiation. The colors of the light emitted by the 2 different species under exposure by the green light from an Laboratory Tests Argon Ion laser are shown at the top of the figure. Each species shows a different intensity even though the Before taking the instrument on a field test, laboratory number densities are equal. Several factors contribute to this tests were conducted to test the instrument under controlled variation. The most important factor is the transition conditions. The optical probe was mounted to a test vessel probability which is the probability that the molecules (Figure 3), with 12 ft optical fibers connecting the probe to undergo a specific transition upon being exposed to incident the main instrument body. Gases analyzed were carbon light. This factor is included in a relative intensity term called dioxide, carbon monoxide, oxygen, nitrogen, methane, and the scattering cross section, which can be obtained hydrogen.
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