Report
EXPERIENCE AND RESULTS FROM OPERATION
The operation started up April 3rd 2000. From then on, and to late December, the cleaning plant has gone through a fine tuning phase and most of the occurring problems were solved during the summer and autumn. From late august to mid November the Hg-cleaning facility was out of operation due to a revision/maintenance stop at the thermal power plant. During this period several changes were done. Some of them are mentioned in the following text.
The first Hg measurements showed values as low as 1-5 m g Hg/Nm3. However, it was discovered that these values were on the low side. The Hg analyser, a "Semtech Hg 2000", was only able to analyse Hgo. Hg in ionic form, dissolved in water, was not analysed. Bubbling the gas through an ascorbic acid solution (vitamin C) solved this problem. Doing so, the Hg-ions are reduced to Hg0. The normal level after cleaning is now 5-10 m g Hg/Nm3 for the dry gas. The results with the reduced ions are somewhat higher. The origin of these Hg ions is water droplets that have been drawn off by the fan, which sucks the gas through the plant.
In order to minimise the amount of water mist in the off-gas, a droplet catcher has been installed after the fan and a heat exchanger (cooler) was installed to cool the wash water. In addition the chimney height has been increased from 15 to 42 m.
Clogging of the outlet pipe when adding sodium hypochlorite to the washwater also caused some problems. Cutting the outlet pipe above the washwater level easily solved this problem.
By increasing the chimney height, a new problem occurred. The water mist of the off-gas condensed inside the chimney, resulting in relatively large amounts of condensed water moving up and down. However a new design of the chimney outlet will solve this problem.
Tap-/river water has also been tried for the washing of the gas with similar good results. So far this has been done only once. Only seawater is being used at the moment.
The Hg analyser, a "Semtech Hg 2000", is very well suited to measure the Hg0-content of the gas.
The process removes mercury, but also other metals, and compounds will be removed from the off-gas. The emission of dust is also reduced.
The operational result in Figure 7 shows that the cleaning facility is running well and that the emissions are sufficiently low. Although some "fine tuning" remains, it is felt that the Hg-removal installation is working as expected.
CONCLUSIONS
Tinfos has, since the findings of Mercury in the off-gas system in 1993 worked to find a solution to the emission problem. The signals from the national authorities and from the EU have been a major driving force during this work.
Several existing technologies have been considered. However, none of these were able meet the demands from SFT of some15 kg Hg/year. Therefore, Tinfos looked for alternatives. Together with Miltec, a new technology for the removal of mercury was developed. This technology may be used on a various type of plants (ferroalloy, waste to power, crematories etc.).
Based on the experience of 6 months of operation to date, the cleaning plant have the following features: an off- gas with 5-10m g Hg/Nm3 is obtained, this represents a > 90 % reduction of emissions and is below the demands set by the authorities. The filtrate contains <0,5 m g/l mercury, and can be returned to the primary gas-cleaning circuit. The sludge contains 1,7 % Hg as HgS.
It can be concluded that the plant so far is able to meet the demands from SFT regarding mercury, in addition the emissions of other heavy metals and dust are reduced. This is obtained in a plant with both low investment- and operational costs.