Investigation of Tunable Diode Spectroscopy for Monitoring Gases in Geothermal Plants

Abstract

The results of an investigation directed at the development of instrumenttation for the real-time monitoring of gases, such as hydrogen sulfide (H 2 S) and chloride (HCl), in geothermal process streams are described. The geothermal power industry has an interest in the development of new low maintenance techniques since improved capabilities could lead to considerable cost savings through the optimization of various gas abatement processes.Tunable diode laser spectroscopy was identified as a candidate technology for this application and a commercial instrument was specified and procured for testing. The measurement principle involved the use of solid state diode lasers and frequency modulation techniques. The gallium arsenide diode lasers employed emit light in the 0.7 to 2.0 micron region of the electromagnetic spectrum. This region contains the overtone and combination absorption bands of a number of species of industrial interest, including H 2 S and HCl. A particular device can be tuned over a small range to match the absorption line by changing its applied temperature and current. The diode current can also be sinusoidally modulated in frequency as it is tuned across the line. This modulation allows measurements to be conducted at frequencies where the laser intensity noise is minimal; and therefore, very high signal-to-noise measurements are possible.The feasibility of using this technology in various types of geothermal process streams has been explored. The results of laboratory and field studies are presented along with new advances in laser technology that could allow more sensitive and selective measurements to be performed. iv v ACKNOWLEDGMENTSThe author would like to thank the operations personnel at the Northern California Power Agency; and in particular, Managers Murray Grande and Steve Miller, for their support of the H 2 S field trials. She would also like to thank President Paul Hirtz, Russell Kunzman, and the staff at Thermochem, Inc. for their help with the HCl steam experiments.The contributions of her colleagues, Charles Jeffery, Greg Lancaster, and Greg Mines are also acknowledged. Charles Jeffery assisted with the laboratory experiments. Greg Lancaster participated in the untreated vent gas field experiment. Greg Mines transported and installed the systems at the field sites; and in addition, provided valuable insight into geothermal operations.