Efficient pH measurement technology in flue gas desulphurisation plants

When organic material is burned in power plants or during various procedures of process engineering, noxious gases form that are not permitted to enter the environment without aftertreatment.

The treatments required include extracting the sulphur dioxide that results during the burning process from the flue gas. In larger plants, a wet process (see box) is generally used for this purpose. Having been used for this method since 1978, Pfaudler probes have proven a long-lasting and low-maintenance solution for measuring pH.

The flue gas desulphurisation (FGD) probe’s purpose is to set the optimum pH value of the scrubbing suspension during wet desulphurisation*. A fundamental problem of measuring pH in FGD is that stubborn scale deposits accumulate on the pH probe in just a short time. These deposits lead to impairment or even failure of the pH measurement. Because of this, conventional pH sensors must be removed or cleaned with acid at short intervals, which results in high operating and maintenance costs. When a Pfaudler pH probe is installed, the smooth enamelled surface and robust design prevents scale deposits from accumulating at the recommended inflow velocity of 1.5-2m/s. Some pH probes have been in constant use for over 25 years. The probe usually only needs to be maintained or calibrated when the entire plant is shut down once a year for maintenance.

* The method used most often for the desulphurisation of exhaust gases is the extremely efficient wet scrubbing method, wherein the acidic-in-solution sulphur dioxide is sequestered by an alkaline scrubbing suspension in a scrubbing tower (absorbing tower). Calcium carbonate is usually used for neutralisation here. The calcium sulphite that results from the process is converted into calcium sulphate (gypsum) in the lower part of the absorbing tower by supplying air into it. It is then used as a building material in the construction industry.

Combined pH/ORP measurement in a power plant

Since early 2000, combined pH/ORP (oxidation-reduction potential) probes have been used in FGD plants to monitor the AOX compounds found in the effluent water. This allows both measurements in the washing suspension to be recorded at the same time with just one measurement probe and one transducer. Pfaudler pH probes that are already in place can be replaced easily, since they are mechanically and electrically compatible. Pfaudler pH/ORP ring probes are generally used with a nominal size of DN 50. They are used directly in the pipeline between a flange connection, which prevents the pipeline’s cross-section from narrowing and achieves a lower probe abrasion.

Enamelled ring probes – The facts

  • Consistent measuring accuracy over the entire service life (no ageing)
  • No scale build-up thanks to smooth enamelled surfaces
  • Minimal maintenance due to lack of cleaning/calibration processes
  • No retractable assembly required
  • Robust against mechanical impacts
  • Extremely resistant to corrosion
  • Resistant to pressure pulses
  • Shock-resistant to ∆T at max. 120°C
  • Can be used with high flow rates
  • Can be stored dry for any length of time
  • Simultaneous measurement of pH and ORP values

Operating instructions for the Pfaudler ring-probe type 03