Impacto ambiental y tecnológico del mercurio en los procesos de oxicombustión del carbón

Abstract

[EN] The growing demand for energy has led to increasing greenhouse gas emissions (GHG) resulting in significant climate changes with implications at global scale. Within the energy sector, CO2 from coal combustion is the main cause of total GHG emissions. Of the available options for reducing CO2 emissions, carbon capture and storage after oxy-fuel combustion processes, have gained ground as a promising alternative to conventional combustion in air. Oxy-coal combustion technologies ensure a relatively high level of development and exhibit a high versatility when they are implemented in existing facilities. Nevertheless, oxy-fuel processes must still overcome relevant environmental and technological challenges before their operation comes into effect at industrial scale. Of these challenges, a lack of knowledge of the behavior of mercury species during the CO2 capture and purification stages is one of the most notable. The change of the flue gas composition from oxy-coal combustion, as a result of the partial recycling of the stream and the injection of oxygen as comburent may affect the mercury speciation which in turn could have a negative effect on the mercury retention efficiency of the gas cleaning systems. Although from a qualitative point of view, environmental problems originating from the presence of mercury in the flue gas and combustion by-products in oxy-fuel facilities are similar to those produced in a conventional air combustion plant, the presence of trace amounts of elemental mercury (Hg0) causes significant operational problems during the compression of CO2 in that it can accumulate in quantities sufficient to cause a severe attack on and failure of the cryogenic aluminium heat exchangers. In view of the complexity and importance of mercury in these processes, the aim of the present work has been, on the one hand, to study and assess the behavior of mercury in the main gas cleaning systems of coal-fired power plants, and, on the other hand, to evaluate the possibility of retaining persistent Hg0 at the end of the cycle of oxy-combustion using a regenerable sorbent. In addition, the effect of mercury on the CO2 retention capacity of some solid sorbents used specifically for the retention of CO2 is analyzed. For this purpose, experimental devices at lab scale were set up by means of which mercury oxidation and retention could be evaluated under the simulated oxy-coal flue gas composition and operational conditions typical of Selective Catalytic Reduction (SCR) systems, Particle Control devices and Wet Flue Gas Desulfuration systems (WFGD). Furthermore, an additional experimental device was set up to evaluate the retention of Hg0 in a regenerable carbonaceous sorbent impregnated with gold and to analyze the influence of mercury on the CO2 retention capacity of different solid sorbents used to CO2 capture.

The results showed that flue gas composition had a significant influence on the speciation of mercury. The homogeneous oxidation of mercury was affected by the presence of NO and NO2, whose capacity to oxidize mercury depended on the concentration of CO2. The presence of a high amount of H2O, which is characteristic of oxy-coal atmospheres, also influenced mercury oxidation. Assessment of the mercury retention capacity of SCR systems showed there was a high degree of oxidation when an enriched-CO2 stream was employed. As regards mercury retention by the catalysts, a high concentration of H2O and CO2 was found to decrease the number of active sites available for Hg adsorption. Under oxycombustion conditions, fly ashes favored mercury oxidation though its retention predominantly depended on the amount of unburned carbon in the fly ashes. The main mercury species identified in fly ashes were mercury bound to organic matter (Hg-OM) and HgS, whose stability in water confirmed their low degree of lixiviation. The capture of mercury in the WFDG device was significantly affected by the composition of the flue gas. High concentrations of CO2 produced in the oxy-coal combustion processes reduced the pH of the gypsum slurry, thereby decreasing the transformation of oxidized mercury (Hg2+) to Hg0. Mercury retention occurred mainly in the liquid fraction as Hg2+, although the characteristics and composition of the limestones may modify the partitioning of mercury in the by-products of the scrubber. A new sorbent based on a carbonaceous support impregnated with gold nanoparticles demonstrated a 100% efficiency in capturing elemental mercury under oxy-combustion conditions at the end of the cycle. The presence of SO2 and HCl in the gas stream did not have a negative effect on the sorbent. In fact, its retention capacity was improved. The presence of Hg0 in the flue gas decreased the CO2 retention capacity of some solid sorbents employed for CO2 capture, suggesting that competitive interactions may be taking place between CO2 and mercury for the same active sites on the surface of the material. This was especially noticeable in the sorbents with a high microporous volume.