Sulfite oxidation in wet flue gas scrubbers

Abstract

Reducing emissions of sulfur dioxide from heat and power generation is highly important in order to mitigate environmental concerns such as acidification and particulate matter concentrations in the atmosphere. Wet flue gas desulfurization (WGFD) is a key technology to achieve this.

This thesis focuses on the oxidation of sulfite (SO ) which is formed when sulfur dioxide is absorbed in WFGD units. The sulfite oxidises to sulfate (SO ). This thesis aims to model the process of wet flue gas desulfuriation and validate the results by measuring sulfite concentrations in full scale scrubber units.

The modelling was conducted using the flowsheeting software Aspen PLUS. Modelling of Renova’s scrubber units at the S¨aven¨as waste-to-energy (WTE) plant in Gothenburg, Sweden and Fjernvarme Fyn’s scrubber units at their WTE plant in Odense, Denmark was conducted. A sensitivity analysis for the model comparing different operating conditions and sulfite oxidation kinetics was carried out. Measurements of sulfite concentrations were taken at these two sites using a portable field spectrophotometer in order to validate the results from the modelling. The measurements of sulfite were found to fluctuate to a high extent between measurement occasions.

The modelling results predict sulfite concentrations in the same order of magnitude as the measurements, for example, the base case scenario of the simulations provided sulfite concentrations close to some of the measurements taken, especially for Fjernvarme Fyn’s case where simulations gave a result of approximately 3700 mg/l of sulfite versus a measured 3000 mg/l. However, the large variations in measured concentrations remained unexplained.

The spectrophotometer provided results that overestimated the concentration of sulfite by a factor of approximately 5 when compared to measurements using iodometry.

In the simulations, a rate constant for the sulfite oxidation reaction of 1.2 ∗ 10–5 s-1 was used. A test using the spectrophotometer measuring the decay of sulfite over time was conducted to estimate the apparent rate constant, which yielded the result 7.21 ∗ 10–5 s-1 .