Effect of anion concentrations on Hg2+ reduction from simulated desulphurization aqueous solutions

[EN] The influence of different anions on the stability of mercury in absorbing solutions containing sulphite ions was investigated. The re-emission of gaseous mercury in the presence of fluoride, chloride, bromide, nitrate, carbonate and sulphate ions from the scrubbing solution was determined. Fluoride, chloride and bromide ions contribute to mercury stabilization, bromide being the most efficient anion for retaining mercury in sulphite aqueous solutions. High carbonate concentrations prevent the reduction of mercury by sulphite ions due to the stabilization of the complexes formed between oxidised mercury, hydroxide and sulphite ions or through the formation of mercury carbonate species. However, nitrate and sulphate ions do not influence the equilibrium between mercury and sulphite ions and, as a consequence, the re-emission of mercury is not modified. The results of this study suggest that the pH is one of the most important parameters for achieving an efficient control of mercury pollution in desulphurization systems. The re-emission of mercury was observed to increase when the pH value of the slurries was higher than six due to the stabilization of the mercury in the form of View the MathML sourceHg(SO3)22-, Hg(OH)+, Hg(OH)2, View the MathML sourceHgHCO3+, HgCO3 or View the MathML sourceHg(CO3)22-. Since the pH depends on the concentration of SO2 in the flue gas and the type of limestone, the alkalinity that comes from the sorbent used in the scrubber may play an important role in mercury capture in such systems.This work was financed by the projects PI-200780I008 (PI Programme-CSIC) and CTM2011-22921. R.O.G. thanks FICYT (Regional research programme) for funding her PhD work through a fellowship.Peer reviewe

Retention of Elemental Mercury in Fly Ashes in Different Atmospheres

Mercury is an extremely volatile element, which is emitted from coal combustion to the environment mostly in the vapor phase. To avoid the environmental problems that the toxic species of this element may cause, control technologies for the removal of mercury are necessary. Recent research has shown that certain fly ash materials have an affinity for mercury. Moreover, it has been observed that fly ashes may catalyze the oxidation of elemental mercury and facilitate its capture. However, the exact nature of Hg−fly ash interactions is still unknown, and mercury oxidation through fly ash needs to be investigated more thoroughly. In this work, the influence of a gas atmosphere on the retention of elemental mercury on fly ashes of different characteristics was evaluated. The retention capacity was estimated comparatively in inert and two gas atmospheres containing species present in coal gasification and coal combustion. Fly ashes produced in two pulverized coal combustion (PCC) plants, produced from coals of different rank (CTA and CTSR), and a fly ash (CTP) produced in a fluidized bed combustion (FBC) plant were used as raw materials. The mercury retention capacity of these fly ashes was compared to the retention obtained in different activated carbons. Although the capture of mercury is very similar in the gasification atmosphere and N2, it is much more efficient in a coal combustion retention, being greater in fly ashes from PCC than those from FBC plants.This work was carried out with financial support from the Spanish Ministerio de Ciencia y Tecnología PN I+D+I (Project PPQ2001-2359-C02-02) and from the project ECSC (7220-ED/095)Peer reviewe

As and Se interactions with fly ashes

[EN] Arsenic and selenium are toxic element s present in coal in trace concentrations that may be emitted to the environment during coal conversion processes. However, it is possible to retain volatile arsenic and selenium compounds in the fly ashes originated by the process, the proportions retained depending on the characteristics of the ashes and process conditions. This work is focused on the capture of these elements in fly ashes in simulated coal combustion and gasification atmospheres in laboratory scale reactors.[ES] El arsénico y el selenio son elementos tóxicos presentes en el carbón en concentraciones del orden de las trazas que pueden ser emitidos al medio ambiente durante los procesos de conversión del carbón. Sin embargo, los compuestos de arsénico y selenio pueden ser retenidos en las cenizas volantes originadas en el propio proceso en distint as proporciones dependiendo de las características de las cenizas y las condiciones del proceso. Este trabajo se centra en la captura de estos elementos en cenizas volantes en una atmósfera de combustión y gasificación de carbón en reactores a escala de laboratorio.Este trabajo fue financiado por la Comunidad Europea (ECSC Proyectos 7220-ED/069 y 722- ED/095).Peer reviewe

Enrichment of thallium in fly ashes in a Spanish circulating fluidized-bed combustion plant

This work evaluates the behavior of thallium in a 50 MW industrial circulating fluidized-bed combustion plant (CFBC), focusing on the distribution of this element among the bottom and fly ashes separated by the solid retention devices in the plant. The results show that thallium species are mainly retained in the solid by-products and are not emitted to air with flue gases in significant amounts, proving that this technology is a more effective means of preventing thallium emissions than pulverized coal combustion technology (PCC). The mass balance of the thallium content in the solids shows that this element was retained in the ashes separated by the different devices installed in the plant. An evaluation of the ash fractions taken from the strippers, the heat recovery area, the hoppers in the air heater and the electrostatic precipitator, shows that thallium was relatively homogeneously distributed in all the ash samples, independently of their composition, but is slightly related to surface area, which in turn is dependent on particle size and unburned carbon content.The authors acknowledge the collaboration of HUNOSA (Hulleras del Norte S.A.) and the staff of La Pereda Power Plant for their assistance in taking the samples.Peer reviewe

Retention of Arsenic and Selenium during Hot Gas Desulfurization Using Metal Oxide Sorbents

The present work explores the possibility of capturing toxic elements other than sulfur in coal gasification flue gases using metal oxide mixtures. Arsenic and selenium compounds were the elements selected for study because they are toxic species which are present in coal gasification flue gases in different amounts, depending on temperature. Among the regenerable sorbents already developed for hot gas cleaning systems in Integrated Gasification Combined Cycles, metal oxide mixtures based on iron, titanium, or zinc oxides (zinc ferrites and zinc titanates) were tested for arsenic and selenium retention. These sorbents have previously been proved to possess good characteristics for H2S(g) retention. The study was carried out in a laboratory scale reactor, using the sorbent in a fixed bed, at 550 °C. Good retention capacities (56 mg g-1) were obtained in these conditions for selenium in a metal oxide mixture containing zinc titanate. A metal oxide mixture containing zinc ferrite proved to be an appropriate sorbent for both elements, retention capacities being 21 mg g-1 for arsenic and 55 mg g-1 for selenium. The results obtained indicate that arsenic and selenium compounds can be retained together with sulfur compounds in these sorbents and be desorbed in the sorbent regeneration processes.This work was carried out with financial support from ECSC 7220-ED/069Peer reviewe

Effect of Oxy-Combustion Flue Gas on Mercury Oxidation

This study evaluates the effect of the gases present in a typical oxy-coal combustion atmosphere on mercury speciation and compares it with the mercury speciation produced in conventional air combustion atmospheres. The work was performed at laboratory scale at 150 °C. It was found that the minor constituents (SO2, NOx, and HCl) significantly modify the percentages of Hg2+ in the gas. The influence of these species on mercury oxidation was demostrated when they were tested individually and also when they were blended in different gas compositions, although the effect was different to the sum of their individual effects. Of the minor constituents, NOx were the main species involved in oxidation of mercury. Moreover, it was found that a large concentration of H2O vapor also plays an important role in mercury oxidation. Around 50% of the total mercury was oxidized in atmospheres with H2O vapor concentrations typical of oxy-combustion conditions. When the atmospheres have similar concentrations of SO2, NO, NO2, HCl, and H2O, the proportion of Hg0/Hg2+ is similar regardless of whether CO2 (oxy-fuel combustion) or N2 (air combustion) are the main components of the gas.The financial support for this work was provided by the project CTM2011-22921. The authors thank PCTI Asturias for awarding Nuria Fernandez-Miranda a pre-doctoral fellowship.Peer reviewe

Distribution of Trace Elements from a Coal Burned in Two Different Spanish Power Stations

The behavior of trace elements during coal combustion varies, depending on their mode of occurrence in a given coal and the variations in the operational conditions in the power plants. However, the extent to which these variables may influence the behavior of trace elements is not yet clear. The objective of this study is to evaluate the influence of the operational conditions on the behavior of trace elements in two coal-fired power plants when the same coal supply is used. Fractions of the fly ash sampled from the hoppers of the electrostatic precipitators (ESPs) were studied and the relative enrichment (RE) factors of the elements were compared. Moreover, a series of leaching tests were carried out, with the objective of determining the level of risk when the wastes are finally disposed of. The behavior of the trace elements was found to be broadly similar. However, some differences were observed when evaluating their distribution among the different fly ash fractions from the ESPs. These differences are related to (i) the operational variables of the power plants and (ii) the performance of the mills. All of the coal combustion byproducts evaluated in this work may be considered as inert waste when they are disposed of.The authors gratefully acknowledge to FICYT (Regional research programme) for the award of a predoctoral fellowship to Ms Raquel Ochoa-González.Peer reviewe

Arsenic and Selenium Capture by Fly Ashes at Low Temperature

Arsenic and selenium compounds may be emitted to the environment during coal conversion processes, although some compounds are retained in the fly ashes, in different proportions depending on the characteristics of the ashes and process conditions. The possibility of optimizing the conditions to achieve better trace element retention appears to be an attractive, economical option for reducing toxic emissions. This approach requires a good knowledge of fly ash characteristics and a thorough understanding of the capture mechanism involved in the retention. In this work the ability of two fly ashes, one produced in pulverized coal combustion and the other in fluidized bed combustion, to retain arsenic and selenium compounds from the gas phase in coal combustion and coal gasification atmospheres was investigated. To explore the possible simultaneous retention of mercury, the influence of the unburned coal particle content was also evaluated. Retention capacities between 2 and 22 mg g-1 were obtained under different conditions. The unburned coal particle content in the fly ash samples does not significantly modify retention capacities.The results of this work were obtained as part of the project PPQ2001-2359-C02-02 dealing on mercury capture.Peer reviewe

Mercury and selenium retention in fly ashes: Influence of unburned particle content

7 pages, 6 figures, 2 tables.-- Printed version published Sep 2007.-- Issue title: "The 6th European Conference on Coal Research and its Applications".Mercury and selenium are present as trace elements in coal and may be emitted to the environment in gas phase during coal conversion processes or be partially retained on the fly ashes. The present work explores the possibility that selenium may contribute to mercury capture in fly ashes in two different situations: firstly the power station itself, in order to evaluate the influence of typical working conditions, and secondly in a fixed bed of fly ashes enriched with Se, in order to study the capture of mercury in more severe conditions. It was found that the presence of selenium in fly ashes may improve their capacity to capture mercury. However, in the four fly ashes of different origin studied, selenium is not the most important component for mercury retention. In fact, the presence of selenium in fly ash samples enriched in unburned carbon does not have any significant effect on mercury retention.The results of this work were obtained as part of the projects PPQ2001-2359-C02-02 and CTM2004-04252.CO2-02/TECNO.Peer reviewe

Mercury Retention by Fly Ashes from Oxy-fuel Processes

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 reviewe