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Reduction of toxic metal emissions from industrial combustion plants-impact of emission control technologies (TOMERED)


Within the project the efficiency of emission control technologies with respect to mercury removal from flue gases will have been evaluated. Specifictelly, the effect of in-furnace NOx control technology on the speciation of mercury out of the boiler was investigated. The influence of the Over Firing Air (OFA) system on mercury speciation at the outlet of the boiler (downstream the economiser)was assessed in order to evaluate the mercury oxidation rate with respect to operational parameters as main combustion stoichiometry, NOx and UBC concentrations in the exhaust gas. Hg measurement were performed along the flue gas duct and the air-preheater as in-furnace control technologies can indirectly affect the Hg oxidation rate by increasing the formation of substrates which promote the mercury oxidation by heterogeneous reactions. Consequently, UBC in fly ash, residence time or temperature cooling rate of flue gas can heavily influence the speciation of mercury. The influence of in-furnace NOx control on trace elemens, is very much linked to the UBC content. If the carbon in ash content is increased, the oxidation can be increased and the sorption and therefore removal in particulate control devises can be increased. The result is available in form of a deliverable report on the impact of NOx control technologies on trace elements.
A cost-effective way to reduce the emission of toxic metals (ToMes) from industrial combustion plants is by primary measures, which do not require (large) process modifications or additional equipment. One such measure involves making use of fuel blends with minimum ToMe emission. In this project fuel blends were tested for interactions, yielding lower ToMe emissions than would be predicted based on a mass-mean ToMe concentration in the blend. Identified interactions were translated into fuel blending rules. The dissemination of the results to the end users - the power producers - is direct and continuous, considering their participation in the project. The potential for use is very high due to the low cost involved with the implementation. The key innovative features of the result lie in new knowledge about interactions between very different fuels such as coal and biomass/waste. The expected benefit flowing from the use of this result is, mainly, a further extension of the envelope of fuels that can be used, especially low quality fuels and residues, without compromising economical or environmental constraints. The result is summarised in a confidential deliverable report "Fuel blending strategies for the enhanced removal of ToMe" which is available for the project consortium on the project website.
Within the project the efficiency of emission control technologies with respect to mercury removal from flue gases was evaluated. At this the impact of SCR-DeNOx catalysts was studied at pilot- as well as full-scale facilities. In order to determine how SCR affects mercury speciation, measurements were carried out on different facilities and the effect of fuel composition, SCR operation conditions (SV, temperature, ammonia injection), SCR catalyst age and type, were investigated. The decrease of elemental mercury through the SCR and then the ESP has been clearly assessed. A clear correlation between the fuel Cl content and the SCR mercury conversion has been assessed meaning that chlorine takes part to the oxidation reactions on the catalyst, although the mechanism is not understood yet. Space velocity strongly affects the mercury oxidation: the larger the SV the lower the mercury oxidation. The effect of SV on mercury oxidation confirms that conversion process is influenced by a catalytic reaction. By running tests at a test facility at different ammonia injection rates resulting in different NOx/NH3 ratios it was seen, that the NOx reduction and the mercury oxidation reaction are competing. From this it was concluded for full-scale plants, that at a new catalyst the NOx reduction reaction takes place at the first catalyst layer. After that the ammonia is consumed and the Hg oxidation can take place at the following catalyst layers. In case of deactivation of the first layer the NOx reduction is moved to the second layer and the mercury oxidation can only take place at the third layer. This means, that catalyst deactivation first affects mercury oxidation before the NOx reduction capacity of a catalyst significantly decreases. Documentation and Information on this result is available in the deliverable reports D07 and D10 which are available on the project website.