• Energy Research

In order to make further progress in the field of reducing mercury emissions to the atmosphere, it is necessary to develop efficient and economically viable technologies. Low-cost solid sorbents are a candidate technology for mercury capture. However, kinetic models are required to predict the adsorption mechanism and to optimize the design of the process. In this study, several low-cost materials (biomass chars) were evaluated for the removal of gas-phase elemental mercury and kinetic studies were performed to investigate the mechanism of mercury adsorption. These kinetic studies were also used to predict the behavior of a fixed-bed column. The models applied were pseudo-first-order and pseudo-second-order equations, Fick’s intraparticle diffusion model, and the Yoon–Nelson model. The chars obtained from the gasification of plastic-paper waste demonstrated the best behavior for mercury capture because of their high Brunauer–Emmett–Teller surface area, large total pore volume (mainly micropore volume), and high chlorine content. The Yoon–Nelson model provided a better fitting for the samples with low mercury retention capacities, while in the case of the plastic-paper chars, all of the models provided relatively accurate predictions because their highly microporous structure retarded the internal diffusion process and their increased chlorine content enhanced chemisorption on their surface. The authors thank the project CTM2011-22921, the Energy Research Centre of the Netherlands (ECN) for supplying the chars employed in this study and the Spanish Research Council (CSIC) for awarding Ms. Aida Fuente-Cuesta a pre-doctoral fellowship and for financing her a stay at the Aristotle University of Thessaloniki (Greece). Peer reviewed

The objective of this study is to determine the mechanism of mercury retention in fly ashes, the main solid waste from coal combustion power plants, and to evaluate the interactions between the type of mercury and fly ashes. The work was based on the results of mercury speciation in the gas and the solid fly ash before and after mercury retention. The identification of the mercury species in the gas was performed using previously validated methods, but the speciation of the mercury retained in the fly ashes was carried out using a mercury temperature-programmed desorption technique (HgTPD) still under development. The fly ashes were sampled from conventional coal combustion in air and oxy-combustion power plants. The main mercury species identified in the raw fly ashes and after they were subjected to an oxy-combustion atmosphere were mercury bound to organic matter and HgS, the ratio of these species depending on the characteristics of the ashes. The results obtained indicate that fly ashes are the route of mercury oxidation in an oxy-combustion atmosphere, although they hardly retain any mercury unless the unburned carbon content is high. HgTPD analysis shows that the main mechanism for mercury retention in the fly ashes is via the carbon matter. The financial support for this work was provided by the National Research Program under project CTM2011–22921. The authors thank CIEMAT (Department of Energy) and CIUDEN for supplying the fly ashes employed in this study, PCTI Asturias for awarding Ms. Nuria Fernandez-Miranda a pre-doctoral fellowship and the Spanish Research Council (CSIC) for awarding Ms. Marta Rumayor a JAE-predoc fellowship. Peer reviewed

This work investigates the scope of a mercury temperature programmed desorption (HgTPD) technique for identifying mercury species in solids. The specific objective of this study was to clarify the mechanism of mercury retention by chars used as sorbents in coal combustion in air and oxy-combustion atmospheres based on the identification of the mercury species retained. Different mercury species were identified by HgTPD depending on the flue gas composition and the type of char. The results led to the conclusion that depending on these conditions the main mechanism of mercury retention will be the interaction of mercury with organic matter, or the interaction of mercury with sulfur to form HgS. In a few particular cases Hg2(NO3)22H2O was produced on the char surface. It was found that HgTPD is a highly useful technique for investigating the different mechanisms of mercury/char/gas interactions. The financial support for this work was provided by the National Research Program under project CTM2011–22921. The authors thank the Energy Research Centre of the Netherlands for supplying the chars employed in this study, the Spanish Research Council (CSIC) for awarding Ms. Marta Rumayor a JAE-predoc fellowship and PCTI Asturias for awarding Ms. Nuria Fernandez-Miranda a pre-doctoral fellowship. Peer reviewed

[EN] Mercury displays a different reactivity and behaviour depending on its speciation. Determination of the mercury species present in combustion flue gases is important for proposing effective control technologies. The Ontario Hydro (OH) method is accepted as the only wet-chemical method suitable for measuring total and speciated mercury in flue gases. However, the continuous development of combustion technologies has the effect of modifying the operational variables and the composition of the resulting flue gases, leading to measurement biases in mercury determination. In this work, an alternative method based on the use of an ionic exchanger resin is proposed for the determination of gaseous oxidised mercury. The results show that for sampling flue gases containing reactive gases over long periods of time, the use of this resin is a suitable method for determining gaseous elemental and oxidised mercury, even in the presence of large amounts SO2 or NO2. Application of the OH method when elevated amounts of SO2 or NO2 are present in the gas composition leads to an overestimation of the amount of oxidised mercury due to the oxidation of the mercury in the KCl impinger solution. The financial support for this work was provided by the project CTM2011–22921. Peer reviewed

Mercury is a well-known toxic element, and flue gas streams emitted from coal-fired utilities are one of the largest anthropogenic sources of this element. This study briefly reviews the proposed technologies for reducing mercury emissions from coal combustion, focusing on an emerging process which involves the use of regenerable sorbents and especially those loaded with noble metals. Among the mercury species formed during coal combustion, elemental mercury is the most difficult to remove from the flue gases due to its low reactivity and insolubility in water. The widespread interest in using regenerable sorbents with metals is due to their ability to retain elemental mercury. With this technology, not only can efficiencies of 100 % be reached in the retention of elemental mercury but also a way to avoid the generation of new wastes loaded with mercury. This study considers the main aspects that must be taken into account when developing effective regenerable sorbents for mercury capture, with special attention to sorbents containing noble metals. The characteristics of this process are compared with those of other processes in a more advanced state of development.

Identification of the chemical form of mercury in coals and during the entire combustion process is essential for successfully controlling mercury emissions. However, the speciation of mercury in coal and, in general, in carbonaceous samples has so far proved elusive because of the limitations of current analytical techniques as a consequence of the detection limits of the equipment or interferences originating from the matrix. In this work a new device based on temperature programmed desorption (HgTPD) has been developed to identify mercury species in carbonaceous materials. The samples studied were coals of different rank obtained from around the world and activated carbons that had previously been used as mercury sorbents in simulated coal combustion atmospheres. The mercury species present in both types of sample were identified without any interference from the matrix. This achievement is extremely important as it will contribute to a better understanding of (i) the behavior of mercury species in coal utilization processes and (ii) the retention mechanisms and subsequent stability of the mercury species captured by carbon sorbents. The authors are grateful to the National Research Program under project CTM2011–22921 and thank the Spanish Research Council (CSIC) for awarding Ms. Marta Rumayor a JAE-predoc fellowship Peer reviewed

This study evaluates the behaviour of total Cr and Cr (VI) during coal combustion in two Spanish power stations. The content and distribution of Cr in the feed coal and combustion wastes was determined and the Cr contents were normalized using enrichment factor indexes. The speciation of Cr in the fly ash fractions from the different hoppers of the electrostatic precipitators was established and the possibility that the Cr (VI) might lixiviate when ashes are disposed of at landfill sites was assessed. Differences in the distribution and behavior of Cr in the two power stations were observed. According to European directive 1999/31/CEE, soluble Cr(VI) in the fly ashes studied would be unlikely to pose an environmental or health risk when the ash is disposed of.