Removal of sulfur dioxide from flue gas

Comparison of calculated and experimental adsorption energies of SO2 in a micropore. Reprinted with permission form Ref. [35], Copyright (1998) American Chemical Society 

Since the molecular size of SO2 is around 0.43 nm (LJ parameter o i = 0.429 nm) the presence of pmes smaller than 0.8 nm in the structure of carbon should be crucial for physical adsfflption of this molecule. Indeed the importance of such pores in the proces of SO2 removal was pointed out in the literature. Raymundo-Pinero and coworkers studied the dependence of the amount adsorbed on various carbons on the porosity measured using Dubinin-Radushkevich method, CO2 adsorption and the total pore volume calculated from nitrogen adsorption [26]. The results obtained show the relatively px)d correlation for the volume of micnopotes calculated form carbon dioxide adsorption. It has to be pointed out here that it is believed that CO2 at experimental conditions chosen in that research adsorbs only in pores smaller than 0.7 nm. Such correlation is found only when oxygen is present in the system. Lack of oxygen decreases the amount adsorbed by a factor of two to six depending on the type of carbon. 

The evidence on adsorption of sulfur dioxide in micropores in the absence of oxygen was found by Molina-Sabio and co workers [27]. While calculating the micropore volumes of various carbons using CO2, N2, and SO2, a relatively good agreement in the values was obtained, A small discrepancy found in the case of SO2 was explained by the polarity effect. The strraig adsorptive -adsorptive interaction in the gas phase caused weaker adsorbent-adsorbate interaction than in the case of N2 and CO2. 

A significant effret of very small micropores on SO2 adsorption was also noticed by Bagreev and coworkers [34]. Fig. 9 shows the dependence of Sft adsorption capacity on the volume of pores with widths between 0.679 and 0.858 nm. Two slopes distinguished in this figure suggest two different steps/mechanisms of adsorption. It was further concluded that the adsorption capacity is governed by two surfece features porosity and sur ce chemistry. Their contributions are impossible to separate and the combined effect is addressed in section 3.3. 

A well pronounced effect of the volume of pores on the capacity for SO2 removal was noticed for carbonaceous adsorbents derived from sewage sludge [37]. In spite of the low surfece area, the capacity of such adsorbents is significant and when experiments were run at diy conditions where catalytic function of the adsorbent was not fully activated, good correlation between the capacity and total pore volume was found. 

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Tsuchiai. H.. et al.. Removal of sulfur dioxide from flue gas by the absorbent prepared from coal ash Effects of nitrogen oxide and water vapor. Ind. Eng. Chem. Res.. 35(3). 851-855 (1996). 

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