Retention of mercury in activated carbons in coal combustion and gasification flue gases

Fuel Processing Technology 77 – 78 (2002) 353 – 358 www.elsevier.com/locate/fuproc

Retention of mercury in activated carbons in coal combustion and gasification f lue gases M. Antonia Lopez-Anto´n, Juan M.D. Tasco´n, M. Rosa Martı´nez-Tarazona * Instituto Nacional del Carbo´n, CSIC, C/Francisco Pintado Fe, No. 26, 33011, Oviedo, Spain Received 29 January 2002; received in revised form 27 March 2002; accepted 28 March 2002

Abstract To avoid the emission of toxic mercury compounds from coal combustion and gasification, efficient gas cleaning systems need to be developed. In this work, the effectiveness of activated carbons for retaining mercury in gases from coal gasification was evaluated and contrasted with the results obtained in a coal combustion atmosphere. The performance of a sulphur-loaded carbon (RBHG3) was compared with that of the same carbon without sulphur (RB3). Minor differences were observed in the two atmospheres studied. The retention of mercury at 120 jC was close to 30% in RB3 and up to 70% in RBHG3. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mercury; Coal combustion; Coal gasification; Activated carbon

1. Introduction The mercury content in coal [1,2] commonly ranges between 0.02 and 0.1 Ag g 1. During coal combustion and gasification, mercury compounds are mostly or entirely emitted into the environment in the vapour phase [3]. To avoid possible problems from the accumulated emission of toxic mercury compounds, efficient gas cleaning systems capable of reducing the mercury content in gases produced in coal combustion and gasification need to be developed [4]. Several solid materials such as activated carbons, calcium-based sorbents, fly ashes and zeolites have been considered for mercury control in flue gases from coal combustion [5 –9]. Some experience has also been gained from solid waste

*

Corresponding author. Fax: +34-98-5297662. E-mail address: [email protected] (M.R. Martı´nez-Tarazona).

0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 5 4 - 1

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incinerators, in which mercury species in gases are typically removed by using Ca(OH)2 and activated carbons as sorbents [10]. In general, in coal combustion and waste incineration atmospheres, Ca(OH)2 can be considered as a good sorbent [11] for the retention of Hg(II), but not for the retention of Hg0, for which sulphur- or iodineimpregnated activated carbons [12 –14] seem to be the best option. The effectiveness of these sorbents for the retention of mercury species depends on their chemical properties, process conditions, mercury speciation, etc. [15,16], and in order to achieve maximum efficiency, the influence of these variables should be controlled. In spite of the data on the retention of mercury in coal combustion and waste incineration, there is a lack of a similar knowledge concerning the behaviour of mercury species and their retention in solid sorbents at the reducing conditions typical of coal gasification. This knowledge will be of great interest in near future if the Integrated Gasification Combined Cycle (IGCC) becomes a cleaner way to use coal for power generation. For this reason, and with the final objective of contributing to the selection of sorbents for their use in coal gasification systems, the aim of this work was to compare the behaviour of two activated carbons of similar characteristics, one of which was impregnated with sulphur, for retaining mercury compounds from coal combustion and coal gasification processes, and to evaluate the influence of the gas atmosphere on retention.

2. Experimental Two commercially activated carbons were used as sorbents: Norit RB3 and Norit RBHG3. Norit RBHG3 was prepared by the impregnation of Norit RB3 (a peat-based, steam-activated carbon) with sulphur compounds (the nature of which was not specified by the producer). These activated carbons were employed in their original form of extruded cylinders of 3-mm diameter. The laboratory scale apparatus used for the sorption experiments consisted of a quartz reactor holding an internal and an external tube and heated by two furnaces. The sorbent and the mercury source (Hg0) were placed inside the internal tube but heated separately in the two furnaces. Synthetic gas mixtures, typical of coal combustion and gasification processes (Table 1), were passed through the reactor. These gas mixtures carried the mercury compound in vapour phase through the sorbent bed, at a flow of 0.5 l min 1. The element that was not retained in the activated carbon was captured in two impingers containing 4% KMnO4 + 10% H2SO4 and 0.5 N HNO3. The evaporation of the sources was carried out at 190 jC to obtain 0.4 Ag ml 1 of mercury in gas phase. To calculate the velocity of evaporation and concentration of mercury in gas phase, 20 mg of the source of element was weighed and then heated in each of the two gas atmospheres for 1 min. After this period, it was cooled and weighed, this operation being Table 1 Gas compositions (% by volume)

Combustion Gasification

%CO

%CO2

%H2

%O2

%SO2

%H2O

%H2S

%N2

– 64

15 3.7

– 20.9

9.2 –

0.2 –

6.6 4.0

– 1.0

69 6.4

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repeated until complete evaporation. Under these conditions, solid Hg0 was evaporated at 0.2 mg min 1. Mercury retention was evaluated by analysing its content in the sorbents after the experiments, using an Automatic Mercury Analyser (AMA). Thermodynamic equilibrium models were used to predict the composition of the chemical species in gas phase with HSC-Chemistry 4.0 software. In order to improve our understanding of the interactions between the mercury and sorbents, the latter were characterized by Fourier transform infrared spectroscopy (FTIR) and physical adsorption of gases.

3. Results and discussion The theoretical assessment of the equilibrium composition of Hg-containing species over the 100 – 1600 jC range was performed in the two gas atmospheres from Table 1. The results obtained indicate that in the coal combustion atmosphere, Hg(g) is the most abundant species in gas phase together with a small amount of HgO(g), the proportion depending on temperature. In the gasification atmosphere, Hg0(g) and Hg(CH3)2(g) would be the stable species expected according to thermodynamic equilibrium data. The porous texture characteristics of RBHG3 and RB3 carbons were studied comparatively using adsorption –desorption isotherms of nitrogen (at 77 K) and carbon dioxide (at 273 K). The adsorption isotherms of nitrogen belonged to type I of the BDDT classification (typical of microporous solids) with only the minor participation of type IV. They had a narrow type H4 hysteresis loop, indicating the occurrence of slit-shaped mesopores. The sulphur-impregnated sample exhibited some minor low-pressure hysteresis, attributable to the irreversible uptake of adsorptive molecules in pores with about the same width as those of the adsorbate molecules, and/or swelling of the nonrigid pore walls. The nitrogen adsorption isotherms yielded BET surface area values of 1183 m2 g 1 (RB3) and 868 m2 g 1 (RBHG3). Fitting the Dubinin-Radushkevich (DR) equation to the CO2 adsorption isotherms at 273 K yielded micropore volumes of 0.37 cm3 g 1 (RB3) and 0.30 cm3 g 1 (RBHG3), and equivalent micropore surface areas of 1009 m2 g 1 (RB3) and 830 m2 g 1 (RBHG3). In the case of RB3, the similarity between N2 and CO2 surface areas suggests that this carbon had been activated to a medium-to-high burn-off degree. In comparing the data for both carbons, the parallel decrease in N2 and CO2 surface areas upon impregnation suggests that sulphur loading occurs in a relatively uniform way, with the partial filling of pores of various sizes, especially the larger ones, accessible to N2 at 77 K and high relative pressures. To check the effect of temperature on both the retention of the element and stability of the sorbent, a preliminary study was carried out in which retention experiments for Hg0 were performed in the combustion atmosphere at four different temperatures: 120, 190, 230 and 270 jC. The combustion atmosphere (Table 1) was used as the reference atmosphere and experiments in the gasification atmosphere were studied comparatively under the optimum conditions observed in combustion. Fig. 1 shows the amount of mercury retained in the experiments against the quantity of mercury passing through the bed (Hg evaporated) with the two activated carbons. The retention of mercury species in RBHG3 carbon did not significantly vary when the temperature was increased from 120 to 270 jC. However, mercury retention in RB3 carbon was considerably lower and was more

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Fig. 1. Mercury retention in RBHG3 and RB3 activated carbons as a function of temperature (combustion atmosphere).

influenced by temperature, maximum retention being obtained at the lowest temperatures. Furthermore, the activated carbon impregnated with 6.07 wt.% sulphur (RBHG3) loses some of this sulphur at temperatures higher than 120 jC. After various sorption experiments for 30 min at 120, 190, 230 and 270 jC, the content of sulphur in the RBHG3

Fig. 2. FTIR spectra of carbons before and after mercury retention experiments (combustion atmosphere).

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Table 2 Retention of mercury from Hg0 evaporation in combustion and gasification atmospheres at 120 jC Activated carbon

% Hg retained Combustion

Gasification

RB3 RBHG3

34 F 10 74 F 12

24 F 6 81 F 11

activated carbon was 7.04, 5.61, 3.62 and 2.47 wt.%, respectively. For this reason, the temperature chosen for the study was 120 jC. Moreover, an enhancement of the sulphur content in RB3 from 0.43 wt.% in the original carbon to 1.47 wt.% after sorption experiments of 30 min at 120 jC was also observed in both atmospheres. FTIR spectra of both activated carbons before and after the sorption experiments in a combustion atmosphere (Fig. 2) show a band at 1100– 1200 cm 1 corresponding to a CUS bond that increases in the sorbent post-retention, probably due to the adsorption of (and/or reaction with) the SO2 present in the gas atmosphere. CUS bond formation in RB3 indicates that sulphur incorporation to the RB3 activated carbon occurs as the same time as the retention of mercury. Table 2 gives data for the retention of mercury. The percentages of Hg retained have been calculated as the slope of the lines obtained in a consecutive series of experiments, in which the quantity of mercury passing through the sorbent bed was increased. The confidence limit was determined as F S.D. (standard deviation). From the results in Table 2, it can be observed that when Hg0 was evaporated in both atmospheres, retention in the activated carbon RBHG3 was considerably higher than in RB3, as might be expected in an activated carbon impregnated with sulphur, in which chemisorption and/or reaction between sulphur compounds and mercury may occur. In fact, Hg0 chemisorption on activated carbons prevails over physical adsorption even in the absence of sulphur additives [17]. Moreover, the retention percentages were similar in both combustion and gasification atmospheres. According to thermodynamic data, when Hg0 is the compound evaporated in either of the gas atmospheres, different mercury species in gas phase may be formed, the common compound in gas phase in the equilibrium being Hg(g).

4. Conclusions Surface chemistry is more important than porous texture in controlling Hg retention in activated carbons. The retention is of the same order in typical coal combustion and gasification atmospheres, there being only minor differences. Although these results need to be confirmed, these small differences could be due not only to the different mercury species present in the gas phase, but also to chemical surface modification, such as simultaneous sulphur retention, during the sorption experiments.

Acknowledgements This work was carried out with the financial support of ECSC (7220-ED/095).

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