Lasers in safety

Heaters create reactions between fuel and oxygen (through air), to produce extremely high temperatures, before transferring this energy to process fluids through heat exchangers. The process consumes significant amounts of fuel, generates large quantities of emissions, and creates a potential safety hazard for personnel and plant.

There are considerable benefits to controlling combustion, including reduced fuel costs, lower emissions and a lower risk of explosions. This control is based upon an optimised air-to-fuel ratio, which in turn leads to efficient fuel consumption.

Prior to the development of analyser technologies to measure excess air in the products of combustion, heaters were operated in conditions using high excess air. This avoided the creation of the low-oxygen, fuel-rich conditions that could lead to an explosion, albeit at the cost of inefficient, expensive fuel consumption.

This is because excess air leads to cooler burning, which significantly reduces the combustion efficiency due to increased heat loss to the atmosphere. Excess oxygen (O₂) also combines with nitrogen and sulfur to produce harmful emissions.

Optimisation of the air-to-fuel ratio is dependent on accurate gas analysis of O₂ and carbon monoxide (CO). Traditionally, zirconium oxide cell technologies have been used for the O₂ measurement, combined with a combustibles (COe) sensor such as a thick film calorimeter.

However, in recent years these traditional technologies have been challenged by the introduction of tunable diode laser (TDL) technologies.

The rise of TDL

The first generation of TDL technology used the direct absorption spectroscopy (DAS) technique, providing measurements from a relatively crude approximation of the area under the absorption curve generated by the laser scan. However, this yields a relatively noisy signal, which compromises accuracy.

Wavelength modulated spectroscopy (WMS) is a sophisticated evolution of the DAS technique, which takes a measurement of the second harmonic of the absorption curve.

Written by Rhys Jenkins, Servomex, UK.

This article originally featured in the February 2019 issue of Hydrocarbon Engineering. To read the full article, sign in or register for a free trial subscription.

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Read the article online at: https://www.hydrocarbonengineering.com/special-reports/28012019/lasers-in-safety/