Retención de compuestos gaseosos de Hg, As y S en sorbentes sólidos: aplicación a la combustión y gasificación de carbón

Tesis doctoral presentada en el Departamento de Química Orgánica e Inorgánica de la Universidad de Oviedo, 2004.Most elements can be found in coal in different proportions and modes of ocurrence. Although some of them occur in concentrations at the trace element level, they are considered of environmental concern. During coal combustion and gasification, different proportions of mercury, arsenic and selenium toxic compounds, may evaporate or condense on the minute fly ash particles in different proportions. They are then emitted with flue gases into the environment. Mercury and selenium compounds are mostly or entirely emitted in the vapour phase, whereas arsenic is mainly carried out with flue gases in the particulate matter. Although the evaluation of the emissions of these elements from coal utilization processes is at present an issue for discussion, coal combustion may be considered an important antropogenic source of these elements for the environment. However, there is a lack of a similar knowledge concerning the behaviour of these elements at the reducing conditions typical of coal gasification processes. To avoid possible problems from the accumulated emission of toxic trace elements, efficient gas cleaning systems capable of reducing the mercury, arsenic and selenium content in gases produced in coal combustion and gasification need to be developed. The systems currently being studied and developed for gas cleaning, focus on the use of solids sorbents. For this reason the main objective of this project was to develop and ascertain the capability of i) different activated carbons, some of them impregnated with sulphur, and ii) fly ash wastes produced in coal utilization processes, to retain mercury, arsenic and selenium compounds. To attain this objective an experimental study in typical coal combustion and gasification atmospheres was carried out. Different variables were evaluated for the capture. The Hg, Se and As species considered were those theoretically predicted according with thermodynamic equilibrium data. Two commercial activated carbons: Norit RB3 and Norit RBHG3 and three carbons prepared for the activation of a pyrolysed coal (CA, CASU and CAPSU) were evaluated. The fly ashes used came from different sources. The fluidised bed combustion of a bituminous coal (CTP), and a pulverized combustion plant (CTA) fed using with high rank coals. These materials were characterised before and after the experiments by different techniques. The efficiency of the sorbents in different conditions was determined in a laboratory scale reactor, using synthetic gas mixtures, typical of coal combustion and gasification processes. Retention capacity means of was determined by analyzing the solid sorbent after the experiment by means of ICP-MS, ICP-MS/HG, AAS/HG, AAS/CV and an automatic mercury analyser, AMA 254. The results obtained indicated that commercial activated carbons are the best sorbents for elemental mercury retention, while for HgCl2 retention capacity depended on the conditions and type of activated carbon and fly ashes. The retention of arsenic and selenium is always more effective in fly ashes than in activated carbons. In this work it was observed that retention in both coal a combustion and gasification atmosphere is similar, except in case of selenium where retention in a gasification atmosphere is significantly higher than in a combustion atmosphere. Probably, these retentions take place through different mechanisms, but in most of the cases a chemical reaction such as S-Hg, Ca/Fe-As and Ca/Fe-Se occurs.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

The application of regenerable sorbents for mercury capture in gas phase

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.The financial support for this work was provided by the projects CTQ2014-58110-R and GRUPIN14-031. The authors thank PCTI Asturias for awarding N. Fernández Miranda a pre-doctoral fellowship.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

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

Mercury speciation in gypsums produced from flue gas desulfurization by temperature programmed decomposition

[EN] Temperature programmed decomposition was used to identify mercury (Hg) species in gypsum samples produced from flue gas desulfurization in two Spanish power stations (A and B). As stricter emission control/reduction policies, particularly those focusing on Hg, are being implemented, wet flue gas desulfurization (FGD) technologies used for the removal of SO2 can result in the co-removal of highly-soluble oxidized Hg. The amount of Hg retained in FGD products may increase in the future if these units are optimized for co-capture. For this reason, it is important to identify the mercury species in FGD products not only to determine the potential risk when the wastes are finally disposed of, but also to understand the behaviour of mercury during combustion and therefore to improve the technologies for mercury removal. Different mercury species were identified in the gypsum samples. In power station A, HgS were the most probable Hg species, whereas in power station B the main compound was Hg halogenated compounds.This work was supported by the project ABETRAP (RFCR-CT-2006-00006). The authors would like to thank INCAR (CSIC), IJA/IDEA (CSIC) and ENDESA. S.A. for providing the samples employed in this study and the programme of postdoctoral contracts abroad by Ministerio de Ciencia e Innovacion.Peer 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

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

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

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