Environ. Sci. Technol. 2004, 38, 899-903
Retention of Arsenic and Selenium Compounds Using Limestone in a Coal Gasification Flue Gas MERCEDES DIAZ-SOMOANO AND M. ROSA MARTINEZ-TARAZONA* Instituto Nacional del Carbo´n (CSIC), Francisco Pintado Fe, 26, 33011-Oviedo, Spain
Volatile arsenic and selenium compounds present in coals may cause environmental problems during coal combustion and gasification. A possible way to avoid such problems may be the use of solid sorbents capable of retaining these elements from flue gases in gas cleaning systems. Lime and limestone are materials that are extensively employed for the capture of sulfur during coal processing. Moreover, they have also proven to have good retention characteristics for arsenic and selenium during combustion. The aim of this work was to ascertain whether this sorbent is also useful for retaining arsenic and selenium species in gases produced in coal gasification. The study was carried out in a laboratory-scale reactor in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. In these conditions, retention capacities for arsenic may reach 17 mg g-1 in a gasification atmosphere free of H2S, whereas the presence of H2S implies a significant decrease in arsenic retention. In the case of selenium, H2S does not influence retention which may reach 65 mg g-1. Post-retention sorbent characterization, thermal stability, and water solubility tests have shown that chemical reaction is one of the mechanisms responsible for the capture of arsenic and selenium, with Ca(AsO2)2 and CaSe being the main compounds formed.
gases reaching the gas turbine and the toxics released into the atmosphere are below defined limits. Several gas cleaning systems have been developed for use in IGCC plants (21), although mainly for sulfur retention. In some systems, the flue gas is cleaned after being cooled to low temperatures (500 °C). The main case for developing hot gas cleaning systems for IGCC is that they enable the energy retained as heat in the hot combustion gas to be used in the gas turbine cycle. Thus, there is a need to investigate the possibility of removing trace compounds from flue gases produced by coal gasification, and their retention at high temperatures, using solid sorbents, appears to be a promising technology. The capacity of solid sorbents for retaining arsenic and selenium compounds from coal combustion and waste incineration has been studied previously (22-26). Among these sorbents Ca-based materials are effective in reducing arsenic and selenium in coal combustion by the addition of the sorbent to the bed in fluidized bed combustion, or by sorbent injection into the gases (22-23). It has been observed that the mechanism of interaction between arsenic oxide and lime is dependent on temperature. Ca3As2O8 was found to be the reaction product below 600 °C, and Ca2As2O7 was the product in the 700-900 °C range (23-24). In the case of the interaction of calcium and selenium, the mechanism suggested during combustion involves the formation of Ca(SeO3)2, which is produced in very small amounts in the presence of SO2 (25-26). In view of the possible reactions between arsenic and selenium compounds in gas phase and solid calcium sorbents, the main objective of this work was to determine the capacity of limestone for retaining arsenic and selenium species in gases from coal gasification systems and to find out how operational variables (temperature and gas composition) influence retention. The results are based on a laboratory-scale study which was aimed at understanding the interactions between limestone/lime and arsenic and selenium compounds in gasification atmospheres.
Experimental Section Introduction Coals contain arsenic and selenium in different concentrations ranging from 0.5 to 80 µg g-1 and 0.2 to 1.4 µg g-1, respectively. During coal combustion, arsenic and selenium species, together with other volatile trace element compounds, may evaporate and either condense on fly ashes, preferment in the small fly ash particles, or remain in gas phase. Both types of species are totally or partially emitted into the environment (1-16). Although trace element behavior during coal combustion processes has been widely studied (1-11), little is known about the partitioning of trace elements during coal gasification (10-16). Though it is thought to be similar, some differences can be expected. For instance, it has been suggested that in gasification processes arsenic and selenium may form hydrides, which would increase their volatility against combustion processes (14). The integrated gasification combined cycle (IGCC) is a relatively clean way of using coal for power generation because of its high efficiency and minimal environmental impact (17-20). In this process, the gas is cleaned before it is combusted to ensure that the corrosive compounds in the * Corresponding author phone: +34 985118988; fax: +34 985 297662; e-mail:
[email protected]. 10.1021/es034344b CCC: $27.50 Published on Web 12/20/2003
2004 American Chemical Society
The limestone used in this work was taken from a deposit in Asturias, Spain, and was characterized using different techniques. X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICPMS) were employed to determine the elemental composition. The crystalline species were identified by X-ray diffraction (XRD) and the morphological study was carried out by scanning electron microscopy (SEM). The particle size was determined in a Coulter counter apparatus and the surface area was determined by volumetric adsorption of nitrogen at 77 K. Most of these determinations were carried out in the solid before and after thermal treatment at 900 °C in the different gasification atmospheres. The laboratory-scale apparatus used for the sorption experiments consisted of a quartz reactor fitted with an internal and external tube and heated by two furnaces (Figure 1). The sorbent and element source (As2O3 or Se), were placed inside the internal tube but heated separately in the two furnaces. Synthetic gas mixtures, typical of coal gasification processes (Table 1), were passed through the reactor. These gas mixtures carried the element compound in vapor phase through the sorbent bed at a flow rate of 0.5 L min-1. The element that was not retained in the limestone was captured in impingers containing HNO3 (0.5 N). The solid compound was evaporated to obtain the desired trace element in gas phase, 1 µg mL-1 of the element being obtained in the gas VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Experimental device.
TABLE 1. Gas Composition (%V/V) of the Atmospheres Used in This Study gas composition (% V/V)
CO
CO2
H2
N2
H2O
H2S
mixture I mixture III
57.6 57.6
3.30 3.30
18.8 18.8
16.3 15.4
4.00 4.00
0.90
mixture. As2O3 was evaporated at 260 °C and Se was evaporated at 470 °C. To avoid trace element condensation on the limestone, the experiments were carried out at temperatures higher than the evaporation temperatures: 350, 550, and 750 °C for arsenic and 550 and 750 °C for selenium. The sorbent bed was prepared by mixing 1 g of limestone with 3 g of sand. Before the sorption experiments, the limestone was subjected to thermal treatment in the same gas composition at 900 °C to avoid sorbent transformations by the high temperatures employed during the retention experiments. The quantity of trace element retained in the sorbent was analyzed by ICPMS after dissolution of the arsenic and selenium compounds in a microwave oven. Simultaneous sulfur retention was also evaluated in some of the experiments. The sulfur content in the sorbent was determined in a LECO S144-DR apparatus. The composition of the arsenic and selenium species in gas phase was predicted using thermodynamic equilibrium models (HSC Chemistry 4.0 software). The sorption capacity (mg of element per g of limestone) and efficiency (percentage of retention of the element) were evaluated. To determine maximum retention capacity (MRC), a sequence of experiments was conducted in which the quantity of the elements was increased until the sorbent was saturated. Water solubility tests and thermal stability studies of sorbent post-retention were also carried out. The watersoluble fraction was determined in an ultrasonic bath with Milli-Q water at 40 °C for 2 h (13-14, 23). Desorption was evaluated in a gasification atmosphere free of trace elements at the same temperature as for the retention experiment and after 3 h.
Results and Discussion It is not possible by experimental means to identify arsenic and selenium compounds in the coal gasification atmospheres employed in the 350-750 °C temperature range. To predict the possible species of elements that might be present in these conditions an approximation was made, assuming that thermodynamic equilibrium had been reached. Thermodynamic calculations using HSC-Chemistry 4.0 software had already been performed in previous studies (16, 27) in the same conditions as those used in this work. The equilibrium composition obtained had shown that arsenic and selenium are present in gas phase between 350 and 750 °C. The composition of arsenic depends on temperature, with As4(g), As2(g), and AsO(g) being the gaseous species expected, and not on the presence of H2S(g) in the gas 900
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FIGURE 2. SEM micrographs (×10 000) for limestone (a) before thermal treatment, (b) after thermal treatment in mixture I, and (c) after thermal treatment in mixture III.
TABLE 2. Elemental Composition of the Limestone Used as Sorbent in This Work compound
wt %
element
ppm
Al2O3 SiO2 CaO MnO MgO Fe2O3 Na2O P2O5 TiO2
0.07 0.53 99.6 0.01 0.31 0.04 0.01 0.01 0.01
As Cd Cl Co Cu Pb Se Sr Zn
0.53 1.73 14 1 9 1.77 23.7 1286
