Monitoring Sulfur Dioxide at High Concentrations

Online Editor

Bengt Löfstedt discusses the needs for SO2 monitoring at high concentrations, and a robust technical solution that withstands the aggressive monitoring environment.

Sulfur dioxide (chemical formula SO2) is a commodity in many chemical production processes. It is also a product of combustion of fuels containing sulfur. In either case, the levels of SO2 need to be monitored in order to control scrubbers and other gas treatment processes. Let’s take a brief look at two areas where high or very high SO2 concentrations can be found: sulfuric acid production and power generation.

Sulfuric Acid Plants

Sulfuric acid (H2SO4) is used in a multitude of industrial processes such as mineral processing, fertilizer manufacturing, oil refining, wastewater processing, and pulp and paper production. It’s a base commodity in the industrial world.

SO2 is one of the main intermediate chemicals in a sulfuric acid production process, and the monitoring of its concentration is central for process control. Due to SO2 slip in the process, it is often also of interest to keep track of SO2 in the tail gas in order to control emissions reduction processes.

In the early stages of the production process, the SO2 concentration can reach very high levels, in the order of 10% or more. The gas mixture is often hot, wet, dust laden, and on top of that also pressurized.

Power Generation Facilities

Fossil fuels often contain sulfur, and high levels of SO2 can be generated in the combustion processes. SO2 emitted through the stack of a power plant is eventually converted to sulfuric acid which can cause severe damage to health and the environment in general. It is therefore literally vital to reduce the SO2 emissions as far as possible. This can be achieved by various types of gas scrubbers, often wet scrubbers where water, or even better a lime slurry is injected.

In order to control the scrubber injection process, the SO2 concentrations in the raw gas prior to the scrubber can be monitored. Depending on fuel, the SO2 concentrations at this point can reach rather high levels, in the range of 1,000 ppm or even higher, and also in this application, the gases can be hot, wet, dust laden, and pressurized.

Monitoring Techniques

In theory, it is fairly easy to measure SO2 concentrations. The monitoring devices typically utilize the optical properties of the SO2 molecule, either by measuring the absorption of certain wavelengths or the emission of certain wavelengths. This signal then gives the concentration.

There are some different approaches to the practical design of an SO2 monitor. One method is to extract a gas sample from the duct via a heated tube and lead it to an analyser, where a fraction of the sample is captured in a measurement cell where the light absorption and/or emission can be measured. Gas dilution and/or miniature filters, scrubbers, converters, etc. along the path of the sample are used to manage the aggressiveness of the primary gas. However, this makes the systems rather complex with high maintenance needs and they are often prone to breakdowns.

A better solution is to measure the concentrations directly in the duct. This is called in-situ monitoring. A beam of light is sent from an emitter to a receiver, straight through the gas mixture in the duct. The received light is lead through an optical fibre to an analyser which can be located well separated from the aggressive gases and also the potentially aggressive ambient environment. The emitter and receiver are protected from the process gases by purge air. No active component of the monitoring system is exposed to the gases.

Bengt Löfstedt is with Opsis AB

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