Retención de mercurio en cenizas volantes
[EN] Mercury and its compounds are highly toxic species that are characterized by their high volatility, persistence in the environment and an ability to concentrate in organisms and rise up the food chain. Mercury species are released into the environment from natural sources and human activities, coal combustion in power plants being the main source of anthropogenic emissions. Several ways of reducing mercury emissions in coal burning power plants have been studied. However, in spite of the efforts made, no efficient and low-cost technology for the control of mercury has yet been found. Fly ashes are wastes from coal combustion which are separated from the flue gas in the particle control systems. It has been found that fly ashes are able to retain mercury in proportions that vary significantly from one system to another, although the mechanisms by which this retention takes place are unknown. An understanding of these mechanisms would make it possible to optimise the working conditions of the power plants and particle capture systems in order to favour the interaction of mercury and fly ash. This would help to reduce mercury emissions without the need for large investments. The aim of the present study was to identify the factors and mechanisms involved in the retention of mercury in fly ashes and in its possible homogeneous and heterogeneous oxidation. To achieve this objective, fly ashes from different power plants were used in order to have a variety of sorbents with different characteristics with the aim of studying the influence of these characteristics on mercury retention. After the characterization of the fly ashes, gaseous elemental mercury retention and oxidation studies were carried out in a laboratory scale device under different gaseous atmospheres. Mercury retention was evaluated by continuously measuring the mercury unretained in the fly ash using a vapour monitor and by analysing the mercury retained in the fly ash in an automatic mercury analyser. Oxidation was studied by means of the Ontario-Hydro method. The results obtained indicate that fly ash carbonaceous matter plays a significant role in mercury retention, although there are also other factors involved in the process. It was observed that the gaseous species present in the atmosphere affect mercury retention, because they may interact with the surface of the fly ash and mercury, thereby promoting or inhibiting the retention of this element. The gaseous species may also react with the mercury in gas phase and on the surface of the ash, causing it to oxidise. Oxidized mercury may be retained in the ash to a greater or lesser extent, depending on the conditions. The results of mercury oxidation in conjunction with the stability of the species retained in the ashes and the differences in retention capacity point to a chemical – type interaction between mercury and fly ash. Condensation of the mercury oxidised in gas phase cannot be ruled out in those cases where there is homogeneous oxidation.