TDLAS Applications

In general there are three distinct Measurement Path configurations: Extractive, Open Path, and Standoff. Extractive sampling is usually utilized for control of manufacturing processes in industries including pharmaceuticals, polymers, fine chemicals, pulp and paper, and ceramics. It has also been applied to pollution control in energy production and petrochemical processing. In these applications, target gas concentration is measured within an optical cell (figure 1) installed in a measurement chamber through which the gases of interest are continually drawn. The optical path length within the cell is designed to provide the required sensitivity to the target gas. A multipass optical cell (Herriot cell) may be utilized to provide a long optical path length within a small volume, in many cases yielding sub-ppm sensitivity with one second or faster response. In some applications, the measurement cell is heated or operated at non-atmospheric pressure to maintain the sampled gas in the same chemical state as the process from which it is extracted.

The Open Path configuration shown in Figure 2 is utilized by industry for monitoring emissions of toxic or explosive gases in petrochemical refineries, aluminum smelters, and fertilizer plants. In permanently installed Open Path configurations, the laser beam is transmitted along a line of sight that can be up to few hundreds of meters in length. An optical transceiver is securely mounted at one-end of the path, and a retroreflector is mounted at the opposite end. The laser beam makes a round trip between these two elements, and senses the concentration of target gas along the path. For these applications, the transceivers are designed and certified for permanent outdoor or indoor installation in dusty, corrosive and potentially explosive environments, while the System Console is typically located in a control room or similar benign environment.

Hand-held Standoff TDLAS sensors, shown in Figure 3, were developed recently by Physical Sciences Inc. for inspection of municipal natural gas pipelines. In standoff devices, passive reflectance of a laser beam projected onto walls and other structures enables measurement of path-integrated target gas concentrations over distances up to a few tens of meters. These sensors can be adapted to sense any of the gases listed in Table 1.