Study on the Simultaneous Measurement of O₂ and CO Concentrations in the Exhaust Gas of a Methane/Air Flame Using Tunable Diode Laser Absorption Spectroscopy

Abstract

Oxygen (O2) and carbon monoxide (CO) are produced in combustion reactions and are important gas species, which are closely related to combustion efficiency and production of air pollutants. O2 generates thermal nitrogen oxide (NOx) by reacting with nitrogen in the air at high temperatures, and the presence of excess O2 also affects the energy efficiency as a result of increasing exhaust heat energy. CO is generally well-known for being a toxic gas and is a crucial gas species produced by incomplete combustion. However, it is difficult to measure O2 and CO level variations in a huge combustion system, such as used in steel annealing and power plant boilers, because of their harsh environment. Therefore, as a multi-species measurement technique, which is non-intrusive and has a high sensitivity and high response, tunable diode laser absorption spectroscopy was chosen as the optical method used to measure O2 and CO concentrations in exhaust gases. In this study, experiments were carried out to measure the O2 and CO concentrations in an electrical furnace and a combustion system using direct absorption spectroscopy and wavelength modulation spectroscopy. The measured results were compared to those of a gas analyzer of the electrochemical sensor type. The O2 and CO concentrations were measured in the exhaust gas produced from methane/air flame through adjustment of the equivalence ratio to form both fuel-lean and fuel-rich conditions. To measure the O2 and CO concentrations precisely and without interference from other combustion products, visible and near-infrared wavelength regions at 760.8 and 2325.2 nm were selected, respectively.