Adsorbents for capturing mercury in coal-fired boiler flue gas

This paper reviews recent advances in the research and development of sorbents used to capture mercury from coal-fired utility boiler flue gas. Mercury emissions are the source of serious health concerns. Worldwide mercury emissions from human activities are estimated to be 1000 to 6000 t/annum. Mercury emissions from coal-fired power plants are believed to be the largest source of anthropogenic mercury emissions. Mercury emissions from coal-fired utility boilers vary in total amount and speciation, depending on coal types, boiler operating conditions, and configurations of air pollution control devices (APCDs). The APCDs, such as fabric filter (FF) bag house, electrostatic precipitator (ESP), and wet flue gas desulfurization (FGD), can remove some particulate-bound and oxidized forms of mercury. Elemental mercury often escapes from these devices. Activated carbon injection upstream of a particulate control device has been shown to have the best potential to remove both elemental and oxidized mercury from the flue gas. For this paper, NORIT FGD activated carbon was extensively studied for its mercury adsorption behavior. Results from bench-, pilot- and field-scale studies, mercury adsorption by coal chars, and a case of lignite-burned mercury control were reviewed. Studies of brominated carbon, sulfur-impregnated carbon and chloride-impregnated carbon were also reviewed. Carbon substitutes, such as calcium sorbents, petroleum coke, zeolites and fly ash were analyzed for their mercury-adsorption performance. At this time, brominated activated carbon appears to be the best-performing mercury sorbent. A non-injection regenerable sorbent technology is briefly introduced herein, and the issue of mercury leachability is briefly covered. Future research directions are suggested.     

Simulation of mercury capture by sorbent injection using a simplified model

Mercury pollution by fossil fuel combustion or solid waste incineration is becoming the worldwide environmental concern. As an effective control technology, powdered sorbent injection (PSI) has been successfully used for mercury capture from flue gas with advantages of low cost and easy operation. In order to predict the mercury capture efficiency for PSI more conveniently, a simplified model, which is based on the theory of mass transfer, isothermal adsorption and mass balance, is developed in this paper. The comparisons between theoretical results of this model and experimental results by Meserole et al. [F.B. Meserole, R. Chang, T.R. Carrey, J. Machac, C.F.J. Richardson, Modeling mercury removal by sorbent injection, J. Air Waste Manage. Assoc. 49 (1999) 694–704] demonstrate that the simplified model is able to provide good predictive accuracy. Moreover, the effects of key parameters including the mass transfer coefficient, sorbent concentration, sorbent physical property and sorbent adsorption capacity on mercury adsorption efficiency are compared and evaluated. Finally, the sensitive analysis of impact factor indicates that the injected sorbent concentration plays most important role for mercury capture efficiency.     

Influence of operating temperature on performance of electrostatic precipitator for pulverized coal combustion boiler

Electrostatic precipitator (ESP) is a major dust collection device used in pulverized coal combustion boilers in Japan. Although the collection efficiency of ESP is over 99%, the performance of ESP is affected by dust properties, especially the electric resistivity of the dust. In Japan, many different kinds of coal are used in pulverized coal combustion boilers. The property of coal ash formed in pulverized coal combustion is different for different coals. It is very important to develop high performance ESPs for different kinds of coal ash. In general, the low temperature ESP operated at about 423 K has been used for a long time, but the electric resistivity of some kinds of coal ash is higher than suitable for the ESP in this temperature range. A high temperature ESP operated at 623 K was developed about 20 years ago and an advanced low temperature ESP operated at 373 K has been used recently for control of the electric resistivity of coal ash.In this paper, we report on the influence of operating temperature from 363 K to 623 K on performance of ESP for a pulverized coal combustion boiler. The influence of coal ash properties including electric resistivity, alkali metal concentration, and operating condition on collection efficiency of an ESP is estimated.     

Synthesis,characterization and evaluation of resin-based carbon spheres modified by oxygen functional groups for gaseous elemental mercury capture

A novel resin-based spherical carbon material was successfully prepared by suspension polymerization of alkyl phenol and formaldehyde and steam activation in combination with surface modification by heat treatment with dry air for enhancing Hg⁰ adsorption. The analysis results demonstrate that the oxidation-modified activated carbon spheres possess better mercury removal performance than untreated sorbents, and the ACS-O300 obtained by oxidation modified at 300 °C is the optimal sorbent at the adsorption temperature range from 25 to 150 °C. The main reason is assigned to the increase of the oxygen functional groups of C=O and C(O)–O–C that play an important role as effective active sites for binding the Hg⁰, even though the C(O)–O–C predominates in mercury removal performance under higher adsorption temperature. The optimum O₂ concentration is 4 vol% at the O₂ concentration range from 0 to 8 vol%. SO₂ and NO are favorable to the mercury adsorption under 4 vol% O₂, while H₂O leads to the inhibition of the mercury adsorption. The TPD results suggest that a strong desorption peak at temperature around 235 °C and a weak peak at 324 °C should generate from mercury desorption of C?=?O and C(O)–O–C, correspondingly. Moreover, the XPS analysis results of the fresh and used sorbent indicates that the C=O and C(O)–O–C serve as strong oxidizer and facilitate electron transfer for converting Hg⁰ to Hg²⁺ in the chemisorption process. These results suggest that the obtained resin-based spherical porous carbon (ACS-O300) is promising for Hg⁰ capture.     

Numerical modeling on simultaneous removal of mercury and particulate matter within an electrostatic precipitator

A computational fluid dynamics (CFD) model coupled with gas-particle mass transfer, electrohydrodynamic (EHD) effect, electric field, particle motion and particle charging is established to advance the understanding of combined particulate matter precipitation and mercury capture within industrial electrostatic precipitators (ESPs). The comparisons between experimental data and numerical results demonstrate that this model can reasonably predict the mercury removal efficiency by powdered sorbent injection (PSI). The mechanism of simultaneous removal of mercury and particulate matter is then discussed in detail by considering the complex interactions among multi-physics. The influences of particle size, mercury concentration, particle injection rate and the EHD effect are investigated. The simulation results indicate that the mercury removal process is primarily controlled by the sorbent particle residence time, surface area and mass transfer rate. Accordingly, reducing the size of sorbent particles (activated carbon) can promote mercury removal efficiency while decreasing the particle collection efficiency. Increasing the initial mercury concentration and adsorbent mass loading also benefit mercury adsorption by influencing the mass transfer rate and the surface area. The EHD effect plays important roles in mercury removal and particle collection by means of altering the flow patterns and particle migration. The two mechanisms of in-flight and wall-bounded mercury adsorption affected by ionic wind are also evaluated and some interesting phenomena are observed.     

A study of Zn-Mn based sorbent for the high-temperature removal of H2S from coal-derived gas

Zn-Mn based sorbents supported on SiO2, gamma-Al(2)O(3) and ZrO2, prepared by the incipient wetness impregnation method with calcination at 973 K were investigated for the removal of H(2)S from coal derived gas at the temperature ranges of 773-973 K. Results reveal that the SiO2 and ZrO2 supports exhibit the better performance because better removal efficiency. The addition of manganese effectually improves the vaporization of zinc. In addition, some operating parameters were also considered in order to understand as well as screen the suitable conditions for the development of Zn-Mn based sorbents on the removal of H(2)S. Over 98% sorbent utilization was established for the use of SiO2 at 873 K. On the other hand, within the 5-15 wt% of Zn-Mn oxides, no significant change in the sorbent utilization was observed. Up to 30 wt% the sorbent utilization decreased slightly compared to lower contents, which may be attributed to the deficient dispersion. With increasing the H2 concentration, the sorbent utilization decreases and an adverse result is observed in the case of increasing CO concentration. The relationship between CO and H2 could be explained via the water-gas shift reaction. Moreover, the apparent activation energy and frequency factor as well as the predicted results were studied with a deactivation model. The results of regression fitting reveal the accurate prediction breakthrough behaviors for the removal of H(2)S.     

Capture of mercury ions by natural and industrial materials

In this paper the technical feasibility of various adsorbents for mercury removal from contaminated waters has been studied. Adsorption isotherms of mercury ions in aqueous solution have been experimentally measured on a granular activated carbon (Aquacarb 207EA), a char, a pozzolana and a yellow tuff. The experimental evidences show that the mercury capture capacity of yellow tuff and char is of few tenths of milligrams per gram of sorbent while for the pozzolana and the activated carbon this value is of the order of 1mg/g of sorbent. Moreover, for a mercury concentration as high as 3000 microg/l the pozzolana shows the highest adsorption capacity. This result seems to be quite interesting, especially in consideration of the extremely low cost of this natural sorbent.     

Application of ESP for gas cleaning in cement industry--with reference to India

Electrostatic precipitators (ESP) are used for gas cleaning in almost every section of cement manufacture. Application of ESP is studied, keeping in view Indian conditions. The characterisation of dust emissions has been done for different units, such as rotary kiln and raw mill, alkali by-pass, clinker cooler, cement and coal mill, in terms of exit gas quantity, temperature, dew point, dust content and particle size. It is seen that all these characteristics have a wide range of variance. The ESP system must effectively deal with these variations. The fundamental analytical expression governing the performance of ESP, i.e. the Deutsch equation, and that for particle migration velocity, were analysed to predict the effect of major operating parameters, namely particle size, temperature and applied voltage. Whereas the migration velocity (and the efficiency) varies directly with the particle size, it is proportional to the square and square root of applied voltage and absolute temperature of the gas, respectively. The increase in efficiency due to temperature is not seen in dc based ESP, perhaps due to more pronounced negative effect on the applied voltage due to the increase in dust resistivity at higher temperatures. The effect of gas and dust characteristics on the collection efficiency of ESP, as seen in the industrial practice, is summarised. Some main process and design improvements effectively dealing with the problem of gas and dust characteristics have been discussed. These are gas conditioning, pulse energization, ESP-fabric filter (FF) combination, improved horizontal flow as well as open top ESP.Generally, gas conditioning entails higher operating and maintenance costs. Pulse energization allows the use of hot gas, besides reducing the dust emission and power consumption. The improved horizontal flow ESP has been successfully used in coal dust cleaning. The open top or vertical flow ESP has a limitation on collection efficiency as it provides for only one electric field.     

High temperature removal of hydrogen sulfide using an N-150 sorbent

In this study, an N-150 sorbent was used as a high temperature desulfurization sorbent for the removal of hydrogen sulfide from coal gas in a fixed bed reactor. The results indicate that the N-150 sorbent could be used for H(2)S removal in the tested temperature ranges. Regeneration test also reveals that utilization of the N-150 sorbent maintains up to 85% compared to the fresh sorbent. No significant degeneration occurs on the N-150 sorbent. In addition, various concentrations of H(2)S, H(2) and CO were also considered in the performance test of the N-150 sorbent. Except for H(2)S, H(2) and CO act the important roles in the high temperature desulfurization. By increasing the H(2) concentration, the sulfur capacity of the sorbent decreases and an adverse result is observed in the case of increasing CO concentration. This can be explained via water-shift reaction. On the basis of the instrument analysis, X-ray powder diffraction determination and SEM images with EDS spectrum characterization, residual sulfur is found in the regenerated N-150 sorbent and this sulfur species is sulfate which resulted by incomplete regeneration. The sulfate formation and sintering effect are major reasons to cause activity loss in the sulfidation/regeneration cycles.     

Catalytic effects of carbon sorbents for mercury capture

Activated carbon sorbents have the potential to be an effective means of mercury control in combustion systems. Reactions of activated carbons in flow systems with mercury and gas stream components were investigated to determine the types of chemical interactions that occur on the sorbent surface. The effects of carbon type, particle size, temperature, and reactive gases were studied. Sorption kinetics and capacities for lignite- and bituminous-based carbons were compared with those for catalytic carbons at temperatures of 107 degrees C, 150 degrees C, and 163 degrees C. In the air and baseline gas studies, the catalytic carbons exhibited far better sorption than the lignite- and bituminous-derived carbons. With the catalytic carbons, the greater sorption kinetics and capacity in an air stream or baseline gas composition compared with nitrogen provides a clear demonstration that O(2) is required in the gas stream for higher reactivities and capacities. Thus, a catalytic chemisorption mechanism predominates for the sorption of mercury at these conditions. The reaction kinetics are inversely proportional to the temperature, indicating that a preliminary physisorption step with mercury associating with a surface site is rate-determining. In synthetic flue gas streams containing HCl (50 ppm), the sorption kinetics of the catalytic carbon are slightly inferior to those of lignite-based carbon. Thus, the reaction is dominated by a different interaction, where HCl reacts with mercury on the carbon surface and the oxidation sites on the catalytic carbon apparently have no advantage. Granular and fine-particle carbons gave similar results in flue gas streams.     

Cadmium adsorption by coal combustion ashes-based sorbents--relationship between sorbent properties and adsorption capacity

A very interesting possibility of coal combustion ashes reutilization is their use as adsorbent materials, that can also take advantage from proper beneficiation techniques. In this work, adsorption of cadmium from aqueous solutions was taken into consideration, with the emphasis on the intertwining among waste properties, beneficiation treatments, properties of the beneficiated materials and adsorption capacity. The characterization of three solid materials used as cadmium sorbents (as-received ash, ash sieved through a 25 μm-size sieve and demineralized ash) was carried out by chemical analysis, infrared spectroscopy, laser granulometry and mercury porosimetry. Cadmium adsorption thermodynamic and kinetic tests were conducted at room temperature, and test solutions were analyzed by atomic absorption spectrophotometry. Maximum specific adsorption capacities resulted in the range 0.5-4.3 mg g(-1). Different existing models were critically considered to find out an interpretation of the controlling mechanism for adsorption kinetics. In particular, it was observed that for lower surface coverage the adsorption rate is governed by a linear driving force while, once surface coverage becomes significant, mechanisms such as the intraparticle micropore diffusion may come into play. Moreover, it was shown that both external fluid-to-particle mass transfer and macropore diffusion hardly affect the adsorption process, which was instead regulated by intraparticle micropore diffusion: characteristic times for this process ranged from 4.1 to 6.1d, and were fully consistent with the experimentally observed equilibrium times. Results were discussed in terms of the relationship among properties of beneficiated materials and cadmium adsorption capacity. Results shed light on interesting correlations among solid properties, cadmium capture rate and maximum cadmium uptake.     

Stabilization/solidification (S/S) of mercury-containing wastes using reactivated carbon and Portland cement

Stabilization/solidification (S/S) of mercury-containing solid wastes using activated carbon and cement was investigated in this study. The activated carbon used in the study was a powder reactivated carbon (PAC). The effect of sulfur-treatment of the PAC was also studied. It was found that PAC was effective in stabilizing Hg in the waste surrogate. Pretreatment of the PAC by soaking it in CS(2) significantly improved the mercury adsorption capacity of the PAC. The adsorption equilibrium was reached within 24h. The optimum pH for the reaction was within the range of 5.0-5.5. After mercury stabilization by adsorption on the reactivated carbon, the Hg waste surrogate was mixed with Portland cement for solidification. Surrogates with up to 1000 mg/kg Hg were stabilized and solidified well enough to pass the TCLP test. The adsorption of mercury by reactivated carbon was in accordance with the Freundlich isotherm. Cement solidification of reactivated carbon-stabilized surrogates, significantly reduced the often-reported interference by chloride ions, by forming a barrier outside of the carbon particles. The S/S process using reactivated carbon and cement is an effective and economical technology for treating and disposing mercury-containing solid wastes.     

Nanosized cation-deficient Fe-Ti spinel: a novel magnetic sorbent for elemental mercury capture from flue gas

Nonstoichiometric Fe-Ti spinel (Fe(3-x)Ti(x))(1-δ)O(4) has a large amount of cation vacancies on the surface, which may provide active sites for pollutant adsorption. Meanwhile, its magnetic property makes it separable from the complex multiphase system for recycling, and for safe disposal of the adsorbed toxin. Therefore, (Fe(3-x)Ti(x))(1-δ)O(4) may be a promising sorbent in environmental applications. Herein, (Fe(3-x)Ti(x))(1-δ)O(4) is used as a magnetically separable sorbent for elemental mercury capture from the flue gas of coal-fired power plants. (Fe(2)Ti)(0.8)O(4) shows a moderate capacity (about 1.0 mg g(-1) at 250 °C) for elemental mercury capture in the presence of 1000 ppmv of SO(2). Meanwhile, the sorbent can be readily separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed mercury.     

Application of genetic algorithm-back propagation for prediction of mercury speciation in combustion flue gas

Coal combustion is one of the main sources of mercury emission. Studies using artificial neural networks (ANNs) to predict mercury emission have shown the feasibility of ANN method. Such analyses aimed to provide guidance for mercury emission control in coal combustion. A mercury emission prediction model was developed by modifying the traditional back propagation (BP) neural networks, and a genetic algorithm (GA) based on global search was used, so called the GA-BP neural networks. In total, six main factors were evaluated and selected as the characteristics parameters. Totally, 20 coal-fired boilers were used as training samples, and three different types of mercury including elemental mercury, oxidized mercury, and particulate mercury were used as outputs. The accuracy of prediction results was analyzed, and source of error was discussed. Results show that correlation efficiency for the training samples was as high as 0.895. Three additional samples were studied to test the predictive model. Results of training and predicting were highly correlated with actual measurement results. It is shown that GA-BP is a promising model for mercury speciation prediction.     

Sorption of Cd(II) and Se(IV) from aqueous solution using modified rice husk

A carbonaceous sorbent was prepared from rice husk via sulfuric acid treatment. Removal of Cd(II) and Se(IV) from aqueous solution was studied varying time, pH, metal concentration, temperature and sorbent status (wet and dry). Cd(II) sorption was found fast reaching equilibrium within approximately 2 h while Se(IV) sorption was slow reaching equilibrium within approximately 200 h with better performance for the wet sorbent than for the dry. Kinetics data for both metals were found to follow pseudo-second order model. Cd(II) sorption was low at low pH values and increased with pH increase, however, Se(IV) sorption was high at low pH values, and decreased with the rise in initial pH until pH 7. A fall in the final pH was noticed with Cd(II) sorption due to the release of protons indicating an ion exchange mechanism. However, for Se(IV) sorption, a rise in the final pH was observed due to protons consumption in the process. For both metals, sorption fit well the Langmuir equation with higher uptake by rising the temperature. Analysis by scanning electron microscope and X-ray powder diffraction for the sorbent after the reaction with acidified Se(IV) confirmed the availability of elemental selenium, Se(0), as particles on the sorbent surface. The reduction process of acidified Se(IV) to Se(0) is accompanied by surface oxidation. Physicochemical tests showed an increase in sorbent acidity, cation exchange capacity and surface functionality after the reaction with acidified Se(IV) indicating that oxidation processes took place on the sorbent surface. On the other hand, no changes in physicochemical tests were found after Cd(II) sorption indicating the absence of redox processes between Cd(II) and the sorbent.     

Adsorption characteristics of trivalent chromium on palm oil fuel ash

The metal uptake characteristics of ash particles obtained from the combustion of oil palm solid waste (referred to as palm oil fuel ash) are evaluated using trivalent chromium as a model adsorbate. The equilibrium and kinetic properties of Cr(III) are studied in batch stirred-tank experiments. The extent of Cr(III) removal increases with an increase in solution pH. The maximum equilibrium uptake capacity at pH 6 is 0.31 mmol/g of sorbent. A Langmuir isotherm model with pH-dependent parameters accounts very well for the measured equilibrium data. Modeling studies using a second order irreversible reaction model and a pseudo-first order kinetic model indicate that transient profiles obtained experimentally for a range of initial metal concentrations and sorbent dosages are in good agreement with calculated curves of both models. The two kinetic models can be employed for a useful summary of the experimental data so long as their rate coefficients are empirically correlated with the two system variables: initial metal concentration and sorbent dosage. Electronic Publication     

Biosorption of nickel from protonated rice bran

In the present study biosorption technique, the accumulation of metals by biomass was used for the removal of nickel from aqueous medium. The rice bran in its acid treated (H(3)PO(4)) form was used as a low cost sorbent. The adsorption characteristics of nickel on protonated rice bran were evaluated as a function of pH, biosorbent size, biosorbent dosage, initial concentration of nickel and time. Within the tested pH range (pH 1-7), the protonated rice bran displayed more resistance to pH variation, retaining up to 102 mg/g of the nickel binding capacity at pH 6. Meanwhile, at lower pH values the uptake capacity decreased. The % age removal of nickel was maximum at 0.25 g of biosorbent dose and 0.25 mm biosorbent size. At the optimal conditions, metal ion uptake was increased as the initial metal ion concentration increased up to 100mg/L. Kinetic and isotherm experiments were carried out at the optimal pH 6.0 for nickel. The metal removal rate was rapid, with 57% of the total adsorption taking place within 15-30 min. The Freundlich and Langmuir models were used to describe the uptake of nickel on protonated rice bran. The Langmuir and Freundlich model parameters were evaluated. The equilibrium adsorption data was better fitted to Langmuir adsorption isotherm model. The adsorption followed pseudo second-order kinetic model. The thermodynamic assessment of the metal ion-rice bran biomass system indicated the feasibility and spontaneous nature of the process and DeltaG degrees values were evaluated as ranging from -22.82 to -24.04 kJ/mol for nickel sorption. The order of magnitude of the DeltaG degrees values indicated an ion-exchange physiochemical sorption process.     

Kinetic studies of elemental mercury adsorption in activated carbon fixed bed reactor

Activated carbons are suitable materials for Hg(0) adsorption in fixed bed operation or in injection process. The fixed bed tests provide good indication of activated carbons effectiveness and service lives, which depend on the rates of Hg(0) adsorption. In order to correlate fixed bed properties and operation conditions, with their adsorptive capacity and saturation time, Hg(0) adsorption tests were realized in a bench-scale unit, consisted of F400 activated carbon fixed bed reactor. Hg(0) adsorption tests were conducted at 50 degrees C, under 0.1 and 0.35 ng/cm(3) Hg(0) initial concentrations and with carbon particle sizes ranging between 75-106 and 150-250 microm. Based on the experimental breakthrough data, kinetic studies were performed to investigate the mechanism of adsorption and the rate controlling steps. Kinetic models evaluated include the Fick's intraparticle diffusion equation, the pseudo-first order model, the pseudo-second order model and Elovich kinetic equation. The obtained experimental results revealed that the increase in particle size resulted in significant decrease of breakthrough time and mercury adsorptive capacity, due to the enhanced internal diffusion limitations and smaller external mass transfer coefficients. Additionally, higher initial mercury concentrations resulted in increased breakthrough time and mercury uptake. From the kinetic studies results it was observed that all the examined models describes efficiently Hg(0) breakthrough curves, from breakpoint up to equilibrium time. The most accurate prediction of the experimental data was achieved by second order model, indicating that the chemisorption rate seems to be the controlling step in the procedure. However, the successful attempt to describe mercury uptake with Fick's diffusion model and the first order kinetic model, reveals that the adsorption mechanism studied was complex and followed both surface adsorption and particle diffusion.     

Methylene blue adsorption by algal biomass based materials: biosorbents characterization and process behaviour

Dead algal biomass is a natural material that serves as a basis for developing a new family of sorbent materials potentially suitable for many industrial applications. In this work an algal industrial waste from agar extraction process, algae Gelidium and a composite material obtained by immobilization of the algal waste with polyacrylonitrile (PAN) were physical characterized and used as biosorbents for dyes removal using methylene blue as model. The apparent and real densities and the porosity of biosorbents particles were determined by mercury porosimetry and helium picnometry. The methylene blue adsorption in the liquid phase was the method chosen to calculate the specific surface area of biosorbent particles as it seems to reproduce better the surface area accessible to metal ions in the biosorption process than the N2 adsorption-desorption dry method. The porous texture of the biosorbents particles was also studied. Equilibrium isotherms are well described by the Langmuir equation, giving maximum uptake capacities of 171, 104 and 74 mg g(-1), respectively for algae, algal waste and composite material. Kinetic experiments at different initial methylene blue concentrations were performed to evaluate the equilibrium time and the importance of the driving force to overcome mass transfer resistances. The pseudo-first-order and pseudo-second-order kinetic models adequately describe the kinetic data. The biosorbents used in this work proved to be promising materials for removing methylene blue from aqueous solutions.     

Multi-functional sorbents for the simultaneous removal of sulfur and lead compounds from hot flue gases

A multi-functional sorbent is developed for the simultaneous removal of PbCl(2) vapor and sulfur dioxide from the combustion gases. The sorbent is tested in a bench-scale reactor at the temperature of 700 degrees C, using simulated flue gas (SFG) containing controlled amounts of PbCl(2) and SO(2) compounds. The removal characteristics of PbCl(2) and SO(2), individually and in combination, are investigated. The results show that the mechanism of capture by the sorbent is not a simple physical adsorption process but seems to involve a chemical reaction between the Ca-based sorbent and the contaminants from the simulated flue gas. The porous product layer in the case of individual SO(2) sorption is in a molten state at the reaction temperature. In contrast, the combined sorption of lead and sulfur compounds generates a flower-shaped polycrystalline product layer.