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

Abstract

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.