High-Temperature Cleaning for Chlorine-Containing Coal Gas by Supported Manganese Oxide Sorbent

This research aimed to achieve HCl removal for chlorine-containing hot coal gas by using supported oxide sorbents in a fixedbed reactor at 673–873 K. Mn2O3/SiO2 was chosen as the optimal sorbent to eliminate chlorine species, after thermodynamic screening of the dechlorination potential of sorbents based upon various metals. The dechlorination experiments and results of the ICP, BET, XRD, XPS, and FTIR analyses provided in-depth views of the reaction chemistry behind the complex system of HCl removal in a simulated syngas containing 3,000 ppm HCl, 25 vol% CO, 15 vol% H2, and N2. When the sorbent composed of 23 wt% Mn2O3/SiO2 came into contact with HCl, CO, and H2, the reaction mechanism contained two paths. At lower temperatures Mn2O3 tended to react with HCl, while at higher temperatures it might first be reduced into Mn3O4 and then react with HCl. The probable products from the reaction (Mn2O3 and HCl or Mn3O4 and HCl) are MnCl2, Cl2, and H2O. That is, as the reaction temperature increased, the second path started to become more important. The final product of this reaction might also include metallic manganese in addition to MnCl2. Furthermore, when the temperature increased, the equilibrium constants of all the reactions reduced, and subsequently resulted in the decreasing sorbent performance.


INTRODUCTION
With the increasing population and the burgeoning industrial development, the growing demand for energy resources has been a matter of world concern.Coal, as currently the most abundant fossil fuel with relatively low and constant price, is a popular option to fulfill the future energy demand.Therefore, technology of clean energy production from coal becomes one of society's greatest needs.
Integrated gasification combined cycle (IGCC), one of the most advanced and efficient clean coal technologies, has recently gripped the world's attention.IGCC generates electricity by integrating coal gasification and combinedcycle power generation technology.However, sulfur and chlorine contained in coal will be emitted mostly in the form of H 2 S (0.2-3.0 vol%) and HCl (50-2,000 ppm) (Davidson, 1996;Bakker, 2004;Yang et al., 2006;Wang et al., 2009;Lin et al., 2010), during the process of gasification (Tu et al., 2011).Distinguished for low pollution to the environment, IGCC requires the removal of these deleterious contaminants prior to the combustion of syngas for electricity production.Furthermore, long-term exposure to these sulfur and chloride species will put the components of IGCC system under the threat of severe corrosion attack (Krishnan et al., 1988).Coal gas treatment processes vary in quality and effectiveness.Among them, absorption by sorbents provides a relatively direct and convenient way to remove the contaminants resulted from coal gasification.Popular candidates for desulfurization include Ca-based sorbent (Li et al., 2007), Fe-based sorbent (Ko et al., 2006), Mn-based sorbent (Bakker et al., 2003;Ko et al., 2005), Zn-based sorbent (Bu et al., 2008), and mixed metal oxide sorbent (Tseng et al., 2008;Lou et al., 2009;Chang et al., 2012).As for dechlorination, absorption by Ca-based sorbents has always been a prevalent choice (Yan et al., 2003;Partanen et al., 2005).Others like Na-based sorbents (Nunokawa et al., 2008) and industrial catalysts mixed with multiple components (Dou et al., 2007) were also developed.Traditionally, research activities for clean coal technology still predominately focus on desulfurization, while relatively little attention has been paid to the studies on chloride elimination due to the smaller amounts of chlorine contained in syngas.Nevertheless, some studies have revealed the influence of the presence of HCl that cannot be ignored.HCl can influence the capacity of sulfur removal negatively (Kiil et al., 2002), or positively (Gupta et al., 2000).Taking the possibility of simultaneous sulfur removal into consideration, sorbents capable of eliminating both species will be better options.
Consequently, in this study results of thermodynamic screening of dechlorination potential on sorbents based upon various metals were reported.Their previous performances to remove H 2 S were also considered.Moreover, laboratory study was carried out to evaluate the feasibility and capacity of the sorption of HCl by selected metal oxide sorbent.A fixed-bed reactor was designed for relatively convenient operation, and the experiments were conducted at high temperature ranging from 673 to 873 K to reach high thermal efficiency for IGCC system.

Sorbent Preparation
In this study, experiments at laboratory scale were conducted using sorbents prepared by incipient wetness impregnation on support materials at room temperature.The support materials were pure commercial products, SiO 2 (Alfa Aesar, stock #44740).Prior to the impregnation, SiO 2 powder was crushed and sieved to 30-50 mesh, and then was dried in an oven at 393 K for 12 hours to remove the impurities and remaining water.Impregnation was proceeded with an aqueous manganese nitrate solution (Mn(NO 3 ) 2 •4H 2 O) sprayed with appropriate amounts of deionized water on SiO 2 .After drying at room temperature for 12 hours, the sorbent was dried in an oven at 393 K for 24 hours.Finally, the impregnated sorbents were calcined in airflow conditions at 973 K for 8 hours in a fixed-bed quartz reactor.The weight loading of manganese oxide on SiO 2 was 20 wt%, which was corrected to approximately 23 wt% by later sorbent characterization analysis.Sorbents were dried in an oven for 2 hours every time before use to remove the remaining water vapor.

Thermodynamic Screening
In this study, thermodynamic screening was carried out through the calculation of equilibrium constant (K) at 400-1,500 K. Twenty-two chemical formula were developed for the selection, and the results are tabulated in Table 1.It should be noted that the experiments utilizing Mn 2 O 3 /SiO 2 sorbent were carried out beforehand and the experimental results suggested that the reaction cannot do without the involvement of H 2 , in which case H 2 was not excluded from reaction (1).The thermodynamic criteria only provide a thermodynamic viewpoint on the dechlorination feasibility, and further laboratory scale experiments were conducted for deeper investigations.

Sorbent Characterization
Inductivity coupled plasma-mass spectrometer (ICP-MS) analysis was carried out on a Hewlett Packard 4500 instrument.The fresh solid sorbents were dissolved by an aqua regia solution and digested in a microwave oven for 40 minutes.The solution then was filtered and diluted with de-ionized water to a constant volume before analysis.Specific surface area and pore volume of the fresh and chlorinated sorbents were measured by Brunauer-Emmett-Teller (BET) surface area on Micrometrics ASAP 2000 instrument through adsorption of nitrogen.Crystalline structures of the fresh and chlorinated samples were recorded by an X-ray powder diffractometer (XRD) (Rigaku, Model D/Max III-V) employing Cu Kα (λ = 1.54056Å) radiation at room temperature.The scan rate was 3 °/min with 30 mA current and 20 kV voltage.The diffraction patterns were analyzed with the use of MDI Jade 5.0 software and Powder Diffraction File (PDF) database published by International Centre for Diffraction Data (ICDD).The chlorinated powder sample sorbents were pressed into disks for X-ray photoelectron spectroscopy (XPS) measurements on a VersaProbe PHI 5000 spectrometer.The core level spectra were analyzed using a XPS peak fitting software, XPSPEAK 4.0.

Dechlorination Experiment
The HCl removal experiments were carried out in a laboratory scale fixed-bed reactor at atmospheric pressure.The apparatus consisted of a simulated syngas system, a hightemperature reaction system, and a gas analyzing system.
Simulated syngas, with a typical composition of 3,000 ppm HCl, 25 vol% CO, 15 vol% H 2 , and N 2 as a balance gas, was supplied from gas cylinders, mixed in the mixer to ensure the gas mixtures were turbulent, and then introduced to the reaction system.Flow rate of HCl were monitored through a mass flow meter (Protec, PC-540), and that of the rest were monitored by IR soap bubble meters (Gilian, Gilibrator-2).It is worth mentioning that experiments with 500, 1,000, and 3,000 ppm HCl were conducted in advance and the best sorbent capacity was achieved when the inlet HCl concentration was 3,000 ppm.Hence in this study the reaction system was fed with 3,000 ppm HCl.The hightemperature reaction system included a bench-scale fixedbed reactor (a 1.5 cm i. d., 1.8 cm o. d., and 85 cm long quartz tube), an electrical furnace, a temperature controller (TTE, TM-4800), and a K-type thermocouple (0.35 cm o. d., 60 cm long).The quartz tube was housed in the furnace, and set with a 200 mesh frit quartz disk 45 cm below the top of the tube to support the sorbents.The thermocouple was inserted into the tube with its tip set above where the sorbents were for accurate temperature measurement through temperature controller.The gas analyzing system was composed of a HCl analyzer and a FTIR.The outlet HCl concentration was recorded every twelve minute by an online gas filter correlation HCl analyzer (Thermo Electron Corporation, MODEL 15C), and other outlet gases were analyzed by a Fourier Transform Infrared Spectroscopy (FTIR).The FTIR spectrum of the outlet gases were obtained on a Perkin Elmer Spectrum One B spectrometer by recording the amount of absorbed light as a function of the scanning wavenumber (cm -1 ).The recorded IR spectra were processed at a resolution of 4 cm -1 with the use of Time Base 2.0 software.
In every experiment the reaction tube was packed with 1.0 g sorbent (1.0 cm thick).The frit quartz disk was well paved with a layer of glass wool between sorbents and itself prior to every experiment to avoid unstable pressure and the leakage of sorbents.The weight hourly space velocity (WHSV) was set at 6,000 mL/hr/g.Chlorinated samples were obtained after the experiments and were characterized by BET, XRD, and XPS.

Thermodynamic Screening
Novel technologies like IGCC are demanding the development of effective sorbents for contaminant removal.Since solid sorbents are usually based upon certain metals, eleven candidate solids on metals Mn, Ca, Fe, Zn, Cu, Co, Mo, V, Sr, Ba, and W showing thermodynamic feasibility for high-temperature desulfurization were determined by Westmoreland and Harrison (Westmoreland and Harrison, 1976).On the basis of this result, thermodynamic screening of dechlorination potential on sorbents based upon metal Mn, Ca, Fe, Zn, Cu, Co, Na, Mg, Pb, Cd, Ni was performed in this study.These elements were selected because they are not toxic, radioactive, and expensive.
Thermodynamics of the dechlorination reactions (listed in Table 1) were evaluated through the calculation of equilibrium constant (K) in Fig. 1, where the dechlorination potentials of manganese compounds, especially Mn 2 O 3 , to be more precise, were better than that of iron compounds (Fig. 1(a)).Since the equilibrium constant (K) is higher, the de-chlorination potential is better.Additionally, Mn 2 O 3 still has the highest equilibrium constant compared with other sorbent candidates at temperature below 1,000 K (Figs. 1(b) and (c)).In previous studies, it was found that solid sorbents based upon Mn have promising desulfurization potential (Westmoreland and Harrison, 1976;Atimtay, 2001).Laboratory scale experiments have also been conducted, and Mn-based sorbents were proved to have high desulfurization performance (Bakker et al., 2003;Ko et al., 2005).Therefore, through this thermodynamic screening Mn 2 O 3 was selected to be the sorbent for the following HCl elimination experiments in this study.

Dechlorination Experiment
Nominal loading of the sorbent was 20 wt% Mn 2 O 3 /SiO 2 .By the use of ICP, the actual weight loading of the manganese oxide on supported SiO 2 was determined.After three times of ICP analyses, the average Mn 2 O 3 recovery was 115.4%, suggesting that the sorbent preparation process overloaded the amounts of Mn 2 O 3 on SiO 2 .The corrected loading manganese oxide on SiO 2 was approximately 23 wt%.Besides, the surface area and pore volume of fresh sorbent were 152.04 m 2 /g and 0.59 cm 3 /g, respectively, obtained from the BET analysis results (Table 2).
Research into the feasibility of manganese oxide sorbent for HCl removal was carried out (Fig. 2).The influence of temperature on the reaction was studied at 573-873 K. Sorbents were reacted with 3,000 ppm HCl in the presence of 25 vol% CO, and 15 vol% H 2 with N 2 as the balance gas.According to Taiwan Stationary Pollution Source Air Pollutant Emissions Standards, the HCl emissions are regulated within 80 ppm in pipe.As a result, the breakthrough time was set as the outlet concentration of HCl reached 80 ppm.The theoretical breakthrough time is defined in reaction (23).Performances of the selected sorbents were represented as sorbent utilization showed in reaction (24).This equation is valid on the premise that the concentration of HCl remains zero before theoretical breakthrough time, or the sorbent utilization should be calculated by integrating the area above the curve before breakthrough and dividing it by the multiplication of inlet HCl concentration (ppm) and theoretical breakthrough time (hour).The sorbent utilization equals to the experimental breakthrough time divided by the theoretical breakthrough time.Fig. 2. Breakthrough curves for HCl removal at 573-873 K using Mn 2 O 3 /SiO 2 sorbent (inlet HCl = 3,000 ppm, CO = 25 vol%, and H 2 = 15 vol% with N 2 as the balance gas, WHSV = 6,000 mL/hr/g). * where F is mole flow rate of inlet HCl concentration (mL/min), M is molecular weight of the metal oxide (g/mole), X is proportion of metal oxide to sorbent (g/g), W is weight of sorbent placed in the reactor (g), A is mole of HCl that every metal oxide can absorb (mole/mole), τ is sorbent utilization (%), t is experimental breakthrough time (hr), t* is theoretical breakthrough time (hr).The influence of temperature in the reaction of Mn 2 O 3 /SiO 2 sorbent with HCl was studied at temperature ranging from 673 to 873 K, as shown in Fig. 2. Sorbents were reacted with 3000 ppm HCl in the presence of CO, and H 2 .CO and H 2 concentrations were kept at 25% and 15%, respectively, with N 2 as the balance gas.The space velocity was maintained at 6000 mL/hr/g.The theoretical breakthrough time calculated by reaction ( 23) is 10.3 hour.The experimental breakthrough time at 673, 773, and 873 K were 55.8, 17.7, 8.4 minute, and the sorbent utilization were 10.0, 3.0, 2.2% at 673, 773, 873 K, respectively, as presented in Table 2. Sorbent utilizations were calculated through the integration of the area above HCl concentration curve before breakthrough (when HCl concentration reached 80 ppm), and division of it by the multiplication of inlet HCl concentration (ppm) and theoretical breakthrough time (hour), as mentioned before.All of the breakthrough curves display the same pattern of progressively rising concentration of HCl as the experiment proceeded.Significant effect of temperature on the reaction was discovered.Progressively decreasing sorbent utilization was observed with increasing temperature.The first possible explanation is that the rising temperature could intensify the reducing atmosphere provided by H 2 and CO.Thus Mn 2 O 3 contained in the sorbent was reduced to manganese oxides that have lower reactivity for HCl removal, resulting in the diminishment of sorbent utilization with increasing temperature.However, the above explanation is not in agreement with the phenomenon observed in the previous thermodynamic calculations and the following XRD results.Though the reducing atmosphere provided by H 2 and CO could reduce Mn 2 O 3 to manganese oxides having lower reactivity for this system, it was found that the reacted sorbent contained only a slight of remaining Mn 2 O 3 and Mn 3 O 4 reduced from Mn 2 O 3 .The decreasing sorbent utilization as temperature rose motivated special emphasis on the HCl removal performance at lower temperature, 573 K as shown in Fig. 2. The experimental results indicated that there was only a slight utilization increase observed at 573 K. Thus, optimal temperature of 673 K was implied to the following experiments in consideration of IGCC thermal efficiency rather than lower temperature, 573 K.
The surface area and pore volume of the reacted sorbents were assessed by BET equation (Table 2).The fresh sorbent had the highest surface area (152.04 m 2 /g) and pore volume (0.59 cm 3 /g), indicating the fact that the dechlorination process decreased the surface area and enclosed the pore volume of fresh sorbents.In addition, the surface area and pore volume decreased with increasing reaction temperature.Under high temperatures it is plausible to cause sintering effect.To understand the extent of sintering effect, surface area measurement of all the cases were analyzed based on the BET equation and tabulated in Table 2.For all samples, a substantial loss of surface area is observed after dechlorination processes.The reacted sorbent (reacted at 673 K and 773 K) do not appear a huge loss in surface area and pore volume which reacted at low temperature.Due to reacting at high temperature, the reacted sorbent (reacted at 873 K) has the huge loss in surface area.
As can be seen in Fig. 2, XRD analyses are given for six samples: one pure SiO 2 , one fresh Mn 2 O 3 /SiO 2 sorbent, and four sets of reacted sorbents.These samples were collected from the previous experiments (Fig. 2).The first XRD patterns were obtained from a pure SiO 2 sorbent.According to the PDF database published by ICDD, the patterns present a clear view of SiO 2 (42-1401) which can benefit the identification of SiO 2 contained in the following patterns.The second XRD patterns were fresh sorbent confirming that the sorbents initially contained mainly Mn 2 O 3 (65-7467) supported by SiO 2 .This successfully proved the validity of the sorbent preparation method.The rest of the XRD results were obtained from previous experiments which displayed compounds that were not expected.Typically in a high-temperature dechlorination process, the inlet simulated syngas contacted the solid sorbents with which the chlorine species reacted to form chlorine compounds, and thus the end product of the chlorination was expected to be MnCl 2 (22-0720), as shown in reaction (1) and reaction (2).Nevertheless, MnCl 2 was not detected in the XRD patterns at 573-773 K, and clear Mn 3 O 4 (24-0734) patterns were discovered instead.In the patterns of sorbents reacting at 573 K, a great amount of Mn 2 O 3 could still be observed, while at 773 K, almost all of the Mn 2 O 3 were transformed into Mn 3 O 4 , meaning that the Mn 2 O 3 was gradually transformed into Mn 3 O 4 when the temperature increased.This could be explained by the effect of temperature on the rate of a chemical reaction, that is, the reaction rate increased with rising temperature, and intensified the reduction of Mn 2 O 3 to Mn 3 O 4 .Special observation was carried out for sorbents that had reacted at 873 K.The peaks of Mn 3 O 4 (c) and MnCl 2 (d) observed in Fig. 3 were not clear enough for characterization.Amorphous phase might have formed in the reacted sorbents after chlorination process and the XRD patterns of those compounds probably failed to be detected.
Additional information on the chemical state of sorbents can be obtained from the following XPS results.The XPS spectra of Mn 2p3/2, Cl 2p, and O 1s together with their fitting curves are depicted in Figs.4(a)-(c).Samples for XPS analysis were taken from the previous dechlorination experiments (Fig. 2).Firstly, Fig. 4(a) gives the Mn 2p3/2 XPS spectra and its peak position of Mn 2p3/2 can include the possibility of the presence of Mn compounds with oxidation state of +0, +2, +8/3, and +3.The peak located at around 641.8 eV indicates the existence of Mn as a mixed Mn 3+ and Mn 8/3+ phase from Mn 2 O 3 and Mn 3 O 4 in the reacted sorbents.Traditionally the exact identification of Mn 2 O 3 and Mn 3 O 4 is challenging due to the overlap of their binding energies and their close values.The fact that the binding energy for Mn 2p3/2 in Mn 2 O 3 is situated between 641.3-641.9eV (Chiganez and Ishikawa, 2000;Álvarez-Galván et al., 2004;Han et al., 2006;Kang et al., 2007;Delimaris and Ioannides, 2008;Davar et al., 2009) and that for Mn 2p3/2 in Mn 3 O 4 is situated between 641.6-641.8eV (Chiganez and Ishikawa, 2000;Apte et al., 2006;Han et al., 2006) led to difficulty in peak deconvolution.However, it is still reasonable to infer that the sorbent did contain Mn 2 O 3 and Mn 3 O 4 because of previous XRD characterization (Fig. 3), in which case the binding energy at about 641.8 eV was characterized to Mn as a mixture of Mn 3+ and Mn 8/3+ .Fig. 4(a) also gives the binding energy of Mn 2p3/2 to be 640.6 eV which was assigned to Mn 2+ in MnCl 2 in accord with the data in the literature (Srivastava and Kalam, 2005), and this along with the later Cl 2p results can confirm the existence of MnCl 2 in all chlorinated sorbent samples that failed to be done by previous XRD experiments.Furthermore, Fig. 4(a) reveals the presence of metallic Mn located at around 639 eV in the reacted sorbents (Wagner et al., 1979;Moulder et al., 1995).It might be formed after the dechlorination experiments due to the reduction of Mn 2 O 3 , and its amorphous phase was not successfully detected by previous XRD characterization.Secondly,Fig. 4(b) shows the XPS characteristic peaks of Cl 2p core level of chlorides in the sorbent samples.In conformity with literature (Capan et al., 1998;Srivastava and Kalam, 2005), the binding energy of Cl 2p centered around 198-199 eV corresponded to Cl -in MnCl 2 .The XPS atomic distribution analysis results in Table 3 also indicate the presence of Cl 2p to be 2.1% based on atomic ratio in the reacted sorbent.Recalling the previous XRD results, clear patterns of Mn 2 O 3 and Mn 3 O 4 were found in the reacted sorbents instead of that of MnCl 2 .These XPS results could compensate for insufficiency of the XRD analysis.Thirdly, Fig. 4(c) displays the XPS patterns of O 1s formed by two peaks.Peaks at approximately 529.5 eV can be attributed to the bonded oxygen in manganese compounds as a mixed O 1s for both Mn 2 O 3 and Mn 3 O 4 due to their close binding energy value (Audi and Sherwood, 2002), while given the literatures (Atrens and Jin, 1987;Atrens and Lim, 1990;Briggs and Seah, 1993;Costa et al., 1994) showing that the binding energy of water ranges from 532.2 to 532.9 eV, peaks lying in this range are

2-Theta
Fig. 4. XPS spectra of sorbents that had reacted with 3,000 ppm HCl, 25 vol% CO, 15 vol% H 2 with N 2 as the balance gas, and 6,000 mL/hr/g WHSV at 673-873 K ((a) Mn 2p3/2, (b) Cl 2p, (c) O 1s).It was assumed that the missing chlorides might have presented in the sorbents in the form of an amorphous phase, and the XRD failed to detect them.Moreover, for the sorbents that had reacted with HCl at 873 K the existence of Mn species and chlorine compounds that had not been discovered in XRD results was found to contain in the sorbent samples (probably in amorphous phase as well) thanks to XPS analysis.The XRD characterization revealed the species contained in the sorbents after the dechlorination process transforming from fresh Mn 2 O 3 to Mn 3 O 4 , while the XPS analysis contributed to the identification of MnCl 2 formed by the process at 673-873 K and the confirmation of their amorphous phase at 873 K on the sorbent surface.

Reaction Mechanism
A number of chemical reactions were expected to take place in the system when Mn 2 O 3 /SiO 2 sorbent made contact with the inlet HCl and other components contained in the simulated syngas.Taking above experimental results into consideration, the following equations can be invited to the reaction system.27) has the highest equilibrium constant among all of the reactions presented in Fig. 5.
In favor of the possibility to simultaneously eliminate both H 2 S and HCl in the future, optimal temperature of 673 K was considered a better option for the following experiments since desulfurization reaction usually takes place at no lower than 573 K (Ko et al., 2006;Tseng et al., 2008).HCl was achieved, and more and more HCl was detected by FTIR.This observation was in agreement with the previous experimental results (Fig. 2).
In conclusion, the rising temperature intensified the formation of Mn 3 O 4 from Mn 2 O 3 contained in the sorbents (Fig. 3), assumed to be associated with the chemical reaction rate accelerated by the increasing temperature.Thus as the temperature went up, extent of the involvement of reaction (25) in the reaction system was increased as well.In this case, at lower temperature, Mn 2 O 3 tended to react with HCl in the atmosphere of CO and H 2 to produce MnCl 2 , CO 2 , and H 2 O (reaction (26)), while at higher temperature, instead of reacting with HCl, Mn 2 O 3 might first be reduced into Mn 3 O 4 , and then reacted with HCl in the atmosphere of CO and H 2 to produce MnCl 2 , CO 2 , and H 2 O (reaction (25) and reaction ( 27)).In other words, in this study the reaction mechanism contained two paths, one is reaction (26) where Mn 2 O 3 reacted with HCl, and the other is reaction (25) to reaction ( 27) where Mn 2 O 3 was firstly reduced into Mn 3 O 4 and then reacted with HCl.When the reaction temperature got higher, the second path (reactions ( 25)-( 27)) started to have a growing tendency.The XPS results also indicated the possibility of the reduction of Mn 2 O 3 to Mn metal, inferring that the final product of the reaction might include amorphous Mn metal besides MnCl 2 .Additionally, though the rising temperature seemed at first glance to be favorable for HCl elimination since it motivated the production of Mn 3 O 4 that had the highest equilibrium constant (Fig. 5), in comparison with the whole thermodynamic trend this alone according to Fig. 5 contributes only a smaller proportion to the reaction system.As the temperature reached a certain stage and then motivated the reactions to reach equilibrium, thermodynamics became predominant.In the thermodynamic point of view (Fig. 5), as the temperature increased, the equilibrium constants of reactions ( 25)-( 27) all decreased.This can explain the phenomenon observed in Fig. 2 where the sorbent capacity to remove HCl decreased with increasing temperature.

CONCLUSIONS
In this study, experiments were conducted in a fixed-bed reactor at 573-873 K to evaluate the feasibility and reaction mechanism of the elimination of 3,000 ppm HCl in the presence of CO and H 2 by selected metal oxide sorbent, Mn 2 O 3 /SiO 2 for the development of IGCC.
It was found the rising temperature intensified the formation of Mn 3 O 4 from Mn 2 O 3 contained in the sorbent, assumed to be associated with the chemical reaction rate accelerated by the increasing temperature.In this case, at lower temperature, Mn 2 O 3 tended to react with HCl in the atmosphere of CO and H 2 to produce MnCl 2 , CO 2 , and H 2 O, while at higher temperature, instead of reacting with HCl, Mn 2 O 3 might first be reduced into Mn 3 O 4 , and then reacted with HCl in the atmosphere of CO and H 2 to produce MnCl 2 , CO 2 , and H 2 O.The XPS results also indicated the possibility of the reduction of Mn 2 O 3 to Mn metal, inferring that the final product of the reaction might probably include amorphous Mn metal besides MnCl 2 .
Additionally, though the rising temperature seemed at first glance to be favorable for HCl elimination since it motivated the production of Mn 3 O 4 that had the highest equilibrium constant, in comparison with the whole thermodynamic trend this alone contributes only a small proportion to the reaction system.As the temperature reached a certain stage and then motivated the reactions to reach equilibrium, thermodynamics became predominant.In the thermodynamic point of view, as the temperature increased, the equilibrium constants of major reactions all decreased.This can explain the phenomenon observed where the sorbent capacity to remove HCl decreased with increasing temperature.

Table 1 .
Reaction equations for thermodynamic screening.

Table 2 .
BET analyses and Sorbent utilization of Mn 2 O 3 /SiO 2 sorbent for HCl removal with 3000 ppm HCl at various temperatures.

Table 3 .
Atomic distribution in fresh and reacted sorbent.
Reaction of Mn 2 O 3 with CO and H 2 in reaction (25) indicates the possibility of the reduction of Mn 2 O 3 in the sorbent to form Mn 3 O 4 accompanied by the production of CO 2 and H 2 O, while reaction (26) shows the reactions of Mn 2 O 3 with HCl accompanied by CO and H 2 to produce MnCl 2 , CO 2 and H 2 O.In addition, Mn 3 O 4 formed from the reduction reaction can subsequently be attacked by HCl (reaction (27)), showing that Mn 3 O 4 could also react with HCl, CO, and H 2 to form MnCl 2 , CO 2 , and H 2 O.To be noticed, reaction (