A three-stage system to remove mercury and dioxins in flue gases

Abstract

Mercury (Hg) from combustion of fossil fuels and waste is the dominant source of anthropogenic Hg emissions, globally amounting to more than 1500 t Hg year −1. These emissions must decrease substantially in order to counteract increasing environmental levels of Hg and reduce future toxic effects. Uppsala Energi AB, nowadays (May, 2002) Vattenfall Värme Uppsala AB, an energy company in Uppsala, Sweden, has invested in equipments for air and water pollution control of their three waste fired steam boilers. The flue gases are cleaned in three stages in series to meet the strict Swedish regulation. Electrostatic precipitators remove most dust in the first stage, wet scrubbers remove most water-soluble gases, and in the last stage a Filsorption unit removes most remaining impurities in particulate as well as gaseous form. The Filsorption process includes additives injection, sorption, and chemical reaction in a reactor and filtration with a fabric filter. The aim with this article is to evaluate the efficiency of the system to recover Hg in flue gases from boilers in routine operation. Flue gases, ashes, and water were sampled yearly for 21 years and analysed for Hg, dioxin, and other potential contaminants received at waste incineration. The results clearly demonstrate the decreasing use of Hg in society the last two decades as influenced by governmental policy regarding Hg. The results also indicate that the equipment efficiently removed Hg and dioxins from the flue gases to a final concentration of approximately 3.5 μg Hg m −3 n and 0.01 ng dioxins m −3 n, corresponding to more than 97 and 99.9% reduction of Hg and dioxins, respectively, by cleaning in three stages. The electrostatic precipitators and Filsorption stages alone, with the scrubber in bypass, removed 90% of Hg in flue gases. Using the scrubber is motivated to remove acid components and additional Hg, but call for water separated after the condensers to be neutralised and cleaned, so that less than 5 μg l −1 Hg and 0.02 ng l −1 dioxins remained, before the water was discharged to a recipient. In conclusion, cleaning flue gases in two or three stages reduced Hg emissions well below Swedish regulations. The strict measures to prevent pollution from waste incineration and simultaneously make use of the energy produced are a good example of a local solution for a global problem.