Trace element removal from hot gasification flue gases using solid sorbents

Abstract

Coals contain elements which, although usually found in concentrations lower than 1% (trace elements), can give rise to environmental or technological problems. After gasification most of these elements may occur in gas phase in different proportions. In order to avoid the problems that the presence of trace elements in gas phase can originate during coal gasification processes, a suitable technology needs to be developed. The systems currently being studied and developed for gas cleaning in coal gasification, focus on the removal of sulphur, particulate matter, nitrogen, alkali metals and halogens but not on corrosive or toxic trace elements. Nevertheless, the reduction of trace elements using solid sorbents in gas phase at high temperatures appears to be a promising method for combustion systems. The main objectives of this work were to determine the capacity of different solid sorbents for retaining arsenic, selenium, cadmium and zinc species in gases from coal gasification systems at 550 and 750 ºC and to find out how the sorbent characteristics and operational variables (temperature and gas composition) influence retention. To attain these objectives the sorption capacity (mg of element per g of sorbent) and the efficiency (percentage of element retention) were determined. The study was carried out in a laboratory scale reactor, in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. At the end of each experiment, the sorbent bed (mixture of sorbent + sand) was finely ground and dissolved in a microwave oven with HF, HNO3 and H3BO3, and the element in solution was determined by ICP-MS. The results are discussed in the light of the data for combustion conditions reported in the literature, and possible retention mechanisms are proposed. Different amounts of arsenic, selenium, cadmium, and zinc can be retained in solid sorbents at high temperatures. It was observed that, in a coal gasification atmosphere, limestone, fly ashes and metallic oxide mixtures containing spinels, were the best sorbents, though in each case the retention capacity depended on temperature and atmosphere. Retention capacities between 16-24 mg g-1 were obtained using limestone and fly ashes for arsenic retention. For selenium, the maximum retention capacities ranging between 50-56 mg g-1 were attained using limestone. Alumina in a gasification atmosphere containing HCl was the best sorbent for zinc removal (52 mg g-1). The lowest retention capacities were obtained for cadmium, these being <1 mg g-1 for the different sorbents tested. Retention probably proceeds through different mechanisms, but in most cases a chemical reaction is involved.