Bengt Löfstedt from Opsis discusses how a robust technical solution helps monitor sulfur dioxide at high concentrations within a harsh 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 often 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, fertiliser 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 emission 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, and containing high levels of particles, on top of that it is also pressurised.
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 important 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 a lime slurry is injected.
The SO2 concentrations in the raw gas prior to the scrubber is monitored to control the scrubber injection process. Depending on fuel, the SO2 concentrations at this point can reach rather high levels, in the range of 1,000 ppm or more. Also in this application, the gases can be hot, wet, dust laden, and pressurised.
Monitoring techniques
In theory, measuring SO2 concentrations is straightforward. The monitoring devices typically utilise 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 into an analyser where a fraction of the sample is captured in a cell where the absorption or emission of light 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 inside the duct. The received light is led through an optical fibre to an analyser which can be located seperately from the aggressive gases and 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.
An in-situ example
Opsis has supplied a large number of in-situ monitoring solutions to both acid production plants and power generation facilities for high-concentration SO2 monitoring. Thanks to the system design, a single monitoring system can keep track of gas concentrations at several monitoring locations. Additionally, it can also monitor many other process gases of interest, such as SO3, which is another intermediate in sulfuric acid production, and NO, NH3 and HCl since their concentrations are of interest to control flue gas cleaning processes.
The systems allow a wide measurement range, can operate at very high temperatures and at notable overpressures, and can sustain high dust concentrations, all with a minimum of maintenance as evidenced by many references. A typical service interval is between three and six months.
For more information visit: www.opsis.se
