Hard-Target Reflection Laser Spectroscopy of Carbon Monoxide Gas Concentration for the Early Detection of Spontaneous Combustion of Coal

Abstract

We report on the hard-target reflection spectroscopy of carbon monoxide (CO) gas based on the technique of infrared tunable diode laser absorption spectroscopy aiming at developing a low-cost yet sensitive sensor for the early detection of spontaneous coal combustion. A narrow-band distributed feedback laser emitting around 2333.7 nm is used to monitor CO gas molecules contained in a 5 cm gas cell. The light diffusely backscattered from the surface of a lump of coal placed at the end of a 50 cm light path is detected with a photodiode in the coaxial transmitter/receiver setup. From the variation of the detected signal profile with the CO partial pressure in the cell, the detection limit of the current system is estimated to be about 30 parts per million per meter (ppm·m), which meets the sensitivity required for monitoring the self-heating of coal in mines, silos, or stockpiles.