Developments in flue gas cleaning

Published on November 9, 2011 by in Energy


On november 7 and 8 2011 the German engineering federation, VDI, organized it’s yearly workshop on flue gas treatment (see program here). In two full days the latest development in the treatment of flue gas are being presented and discussed. Leon Korving of AIFORO presented a lecture (for his current employer SNB) on the optimization of the flue gas treatment with the incineration of sewage sludge. One topic was the optimization of the high temperature removal of SO2 with limestone in the fluidized bed. The other topic was the testing of a mineral based adsorbent for the removal of mercury from the flue gas. See also the full paper: Optimisation of additives in flue gas cleaning of sewage sludge incineration VDI 7 & 8 nov 2011.

But of course a lot more developments were presented by the 15 speakers and discussed during the breaks. This is a summary of the highlights.

The last years has already shown a trend towards dry flue gas treatment and this trend could also be seen at this meeting. Probably the newest development was the introduction of a two stage injection of dry adsorbent. This method has the advantage that either the same removal efficiencies can be achieved with less additives or more efficiency with the same amount of additive. As always the last percentages of removal are the most difficult and this is especially difficult if this has to be done with an adsorbent that has already adsorbed the bulk of the pollutants. Through the addition of a second stage these last percentages are more effectively removed with a fresh adsorbent. Another advantage of the two stage system is that it can more easily cope with peaks in for instance HCl and SO2 loads to the flue gas treatment. The first stage can be used to remove the bulk of the pollutants and while the second stages follows the peaks. Currently the two-stage method seems to be mostly installed in retrofit situations where a semi-dry system with a spray tower cannot meet all the requirements. Nevertheless the system would also be a good idea for new systems.

Another interesting development was the revival of SNCR systems for NOx-removal. With the introduction of dry flue gas treatment systems the slip of ammonia that is normally associated with these systems can pose a problem as the ammonia is not removed in the flue gas treatment. One speaker discussed developments in the injection and control of the ammonia for SNCR and showed that nowadays this slip can be limited to values below 10 mg/Nm3. This is not good enough in all cases as was shown by another speaker for a case in The Netherlands. The Netherlands has very low limits on NOx (<70 mg/Nm3) and in addition the emission of ammonia should be limited to less than 5 mg/Nm3. Nevertheless an SNCR system has recently be installed in combination with a dry flue gas treatment in a waste-to-energy plant that incinerates waste wood. To ensure that the emission limits would be met a final policing, water scrubber was added after the flue gas treatment to remove any residual ammonia and aerosols from the flue gas. To ensure a water-free flue gas treatment the bleed from the scrubber was used as dilution water for the ammonia-dosage in the SNCR.

Further the topic of mercury removal receives increasing attention. Beginning of this year a publication in the German paper `VDI Nachrichten` showed that the coal fired power plants in Germany are large emittants of mercury in Germany. This mercury is present in the coal, but is also present in significant levels in biofuels like sewage sludge that are co-incinerated in the power plants. Currently the mercury emission of these power plants is round 10 ug/Nm3. Apparently future regulations are under discussion in Germany that would require coal fired power plants to comply to very low limits for mercury in the order of 1-2 ug/Nm3. Nevertheless this is not yet announced on the website of the German environmental ministery about the emissions of large power plants, allthough the mercury emission of coal fired power stations is mentioned as an issue and tests are being done to reduce the emission. Already in the United States large emission reductions are going to be imposed this year with new Mercury and Air Toxics Standards (MATS). In a series of world-conferences organized by UNEP the issue of mercury as a pollutant is being discussed (more information here) and this should lead to a worldwide agreement on the reduction of the emission of mercury. In the US solutions for power plants have already been developed in the form of bromated activated carbons that can be dosed at relatively high temperatures before the electrostatic precipitator (ESP).

Another topic discussed several times during the seminar was the use of sodiumbicarbonate versus calcium based additives. Sodiumbicarbonate has the advantage of low stoechometric factors and therefore a lower waste production. Disadvantages are the high costs for the purchase of the additive and the requirement for milling of the sodiumbicarbonate to activate it, before it can be used in the flue gas treatment. Also the disposal of the produced sodiumbicarbonate is an issue. In Germany and the surrounding countries the residues from the flue gas treatment are mostly used to fill old salt mines. These mines are nowadays allowed to use secondary building materials to prevent collapse of the caverns. Several representatives of these salt mines indicated that treatment of the sodiumbicarbonate-residues is problematic for them and that they can therefore only accept a certain small percentage of sodiumcarbonate-residues. The problems arise especially in those cases where a brine is used to transport the residue to the caverns. This brine has to be kept at certain high pH and with sodiumbicarbonate more magnesiumsalts are required to maintain this pH. Solvay, the supplier of the sodiumbicarbonate also offers an option to recycle the sodiumbicarbonate in a plant in the North of France (Resolest), but apparently this option is still too expensive to be of interest for German operators.

Finally another development was the use of low (“homeopathic”) dosages of additives to achieve certain well defined effects in the flue gas treatment. One presentation discussed the injection of sulfurtrioxide just before the elektrofilter to improve the performance of the elektrofilter when burning coals that produce ash with a low electric conductivity and low sulfur content. The sulfurtrioxide is produced by burning sulfur and is then injected just before the elektrofilter. Other examples are magnesium based additives that can be added to the fuel to reduce slagging of the boiler or additives that can be added to fuel oil to reduce NOx, char and formation of SO3.

Those who would like to receive more detailed information on these topics can always contact Leon Korving of AIFORO.

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