Tunable Diode laser Sensor for Multiple Species Monitoring in Harsh Atmospheres

摘要

Energy intensive industries such as steel, aluminum, and glass require combustion processes that are characteristically at high temperature with high levels of particulate matter. Monitoring and control of these processes for improved efficiency, pollutant reduction, and product quality requires a sensor adaptable for such harsh environments. Traditional industrial monitoring relies on extractive sampling that requires frequent maintenance due to probe plugging or corrosion and routine calibration. In addition, capturing the temporal behavior of the process can be problematic with extractive sampling systems because of the slow response time associated with the sampling line lengths and slow response analyzers. To meet the demands of these harsh combustion processes the ideal sensor would perform in-situ process monitoring, require little or no maintenance and provide real-time process information The use of tunable diode lasers based on absorption monitoring overcomes many of the problems associated with conventional extractive sampling. However, the majority of industrial combustion processes will undergo temperature variations along with changes in the atmosphere oxidation or reducing state during normal operation. Therefore, temperature monitoring along with key combustion species monitoring that describes the atmosphere e.g., O_2 and CO, is often necessary for optimal process control. The temperature is not only a useful parameter describing the state of the process, but is needed to accurately determine the species concentration since the absorption measured is dependent on temperature. To monitor both reducing and oxidizing combustion atmospheres in addition to gas temperature requires a diode laser system capable of multiple species monitoring. Here we describe an industrial prototype system operating in the near-infrared for simultaneous monitoring of O_2 (.76 μm), CO (1.5 μm), H_2O (1.5μm) and gas temperature. The prototype system addresses the issues of added complexity with multiple species monitoring by using only two diode lasers and a beam launch and receiver optical design to discriminate the vastly different laser wavelengths while suppressing background radiation noise and beam steering from thermal gradients. Measurement results using the system for industrial process monitoring on a 100-ton/hr steel reheat furnace are presented. The measurements in this test were conducted at different zones in the furnace and at different heights relative to the processed material. The results show dynamic variations in concentration and temperature that could aid in improved atmosphere control.