Study of the Reaction of Lime with HCl under Simulated Flue Gas

Publication Date (Web): October 14, 2005 ... Dry scrubbing systems are the cheapest and easiest to use; they have been widely employed .... extra care...
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Ind. Eng. Chem. Res. 2005, 44, 8730-8738

APPLIED CHEMISTRY Study of the Reaction of Lime with HCl under Simulated Flue Gas Conditions Using X-ray Diffraction Characterization and Thermodynamic Prediction Terence Chin, Rong Yan,* and David Tee Liang Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Centre, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723

Hydrochloric acid (HCl) is commonly removed from flue gases using hydrated lime (Ca(OH)2) as a sorbent in the dry scrubbing process. Although this method is relatively easy to operate and the lime sorbent is cheap, this process is highly inefficient, as only ∼25% of the lime used is converted, due to the short contact time and the lack of understanding of the reaction chemistry of lime in a complex flue gas system. A comprehensive X-ray diffraction (XRD) characterization was carried out on the product of a hydrated lime reaction with HCl in a simulated flue gas system containing SO2, CO2, O2, and moisture. Together with thermodynamic calculations, the chemical nature of the multiple reactions that happened in this system was investigated. It was confirmed that the products of hydrated lime with HCl under the simulated flue gas conditions were chiefly Ca(OH)Cl, CaCl2, CaCO3, CaSO3‚1/2H2O, CaSO4‚2H2O, and anhydrous CaSO4. Previous results suggested that the presence of CO2 and SO2 in the system formed products with a high product diffusion resistance, leading to pore blockage and the premature termination of the reaction. However, in this study, experimental proofs showed that HCl was able to break down the CaCO3 and CaSO3‚1/2H2O layer, allowing the reaction to continue. In the presence of excess O2 (and moisture), part of the CaSO3‚1/2H2O was oxidized to CaSO4‚2H2O and CaSO4. The presence of O2 is, therefore, counterproductive to the full utilization of hydrated lime, as sulfur in its S(VI) form is more stable and resistant to HCl attack, thereby preventing blocked pores from being cleared. Introduction Hydrochloric acid (HCl) is formed when halogencontaining waste components are combusted in municipal and hazardous waste incinerators (MWI and HWI), when coal with high chlorine content is burned, and also when various industrial processes are performed. The pretreatment HCl concentration generated from these facilities can exceed 1000 ppmv at times and poses serious environmental problems when released into the atmosphere. The methods to treat this pollutant include wet , semidry, and dry scrubbing.1 Although wet scrubbing comes first in terms of efficiency, it produces wastewater that requires further treatment and disposal, has corrosion problems, and tends to generate a visible plume in the flue gas. Semidry scrubbing systems, where lime slurry is injected into the flue gas, also have a relatively high efficiency, but they are more complicated to operate and require additional investment in equipment compared to dry scrubbing. Dry scrubbing systems are the cheapest and easiest to use; they have been widely employed in industrial processes. To compensate for dry scrubbing’s low sorbent utilization efficiency, large quantities * To whom correspondence should be addressed. Tel.: 6567943244. Fax: 65-67921291. E-mail: [email protected].

of sorbent relative to the theoretical minimum for acid neutralization are used (usually more than 5 times). This is mainly due to the short contact time between the sorbent and the pollutant gas, typically in the range of a few seconds. This results in a large amount of solid waste generated.1 The limitations of the dry and semidry scrubbing systems using various forms of lime as a sorbent led to much research work done to improve them. Nevertheless, the majority of the works focused on the reaction of lime and HCl itself without considering the effects of other constituents that are normally present in flue gas.2-8 So far, there are few published works that consider the combined effects of HCl and other influencing species (CO2, SO2, and water)9-13 and fewer still that consider all the components present in flue gas reacting with lime simultaneously.14,15 The common issue featured throughout most previous works is the incomplete conversion of lime when reacted with HCl, SO2, and CO2 (sometimes as low as a 10% conversion). Some authors attribute this to the phenomenon of pore-clogging in lime particles, where the product layer formation on the surface of the sorbent acts as a barrier for further reaction.2,13-14,16 Duo et al.15,17 presented a complex “crystallization and fracture” model, explaining how crystal formation on the lime surface and its subsequent fracture affects the lime conversion.

10.1021/ie058021v CCC: $30.25 © 2005 American Chemical Society Published on Web 10/14/2005

Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8731 Table 1. Properties of the Hydrated Lime Sample property

value

average particle diameter (µm) BET surface area (m2/g) micropore area (m2/g) external surface area (m2/g) pore volume (cm3/g) average pore diameter (Å)

4.50 38.1 2.46 35.7 0.119 125.3

While studying the lime reaction with various acid gases, most people assumed that standard and conventional reactions are taking place without characterizing the real products. Those who went on to characterize the products with XRD were often rewarded with interesting and unexpected results.2,8,18 This showed that the reaction of lime with the various reactive constituents in the flue gas is not as straightforward as expected. To improve the efficiency of using lime as a sorbent in the dry scrubbing of HCl, the real reactions going on in the system have to be known clearly. Thermodynamic calculations have been widely applied to predict the predominant reactions and species in a complicated system containing multicomponents and multiphases. Yet, their application is still rarely found in lime/HCl reactions together with other reactive gases. Verdone and Filipis19 did some theoretical thermodynamic calculations for HCl, NOx, and SOx removal in a typical flue gas dry scrubbing system for incinerators and concluded that sodium based sorbents are more efficient than calcium based ones over the temperature range of 100-600 °C. This paper aims to investigate the actual reactions taking place when hydrated lime is reacted with the main flue gas reactive species (HCl, SO2, CO2, and water) under simulated flue gas conditions. To achieve this, a thermodynamic calculation was performed to know the theoretical limits of the system. Extensive X-ray diffraction (XRD) characterizations (and also quantification, where possible) of the end products were carried out. An in-depth discussion of the related reactions provided a clearer insight into the chemistry involved in the dry scrubbing system. Also, the results here lend further support to our previous observations reported in an earlier publication.20 Experimental Section Materials and Apparatus. The details of the raw materials and instruments used were described previously,2 including the diagram of the whole setup. The advantage of this setup was that it allowed us to change the reactive gases or to stop the reaction at any time and remove the reacted sample from the thermogravimetric analyzer (TGA) for XRD identification, facilitating our study on the reaction of hydrated lime with various acid gases. The reactive gases used here, CO2 (100%) and HCl (2094 ppmv), were purchased from Messer Singapore, while the SO2 gas (5000 ppm) was provided by Linde Gas. The properties of the hydrated lime sample used here (known as lime 3 in our previous work2) are shown in Table 1. Reaction in TGA. A thin layer of hydrated lime sample (about 10 mg) was spread evenly on the platinum pan of the TGA. The pan was then loaded into the furnace and heated to the desired temperature with a ramp rate of 10 °C/min in a N2 environment. The simulated flue gas was then introduced into the furnace by the switching of a four-way valve. An immediate

weight increase was seen when the reaction began. To end the reaction at any time, the four-way valve was simply switched back to the original position, allowing N2 to purge the whole system. The rate of weight gain in the TGA was an indication of the reaction rate of hydrated lime with the acidic species. The total flow rate was kept at 150 mL/min. At that flow rate, the external mass transfer effect was minimal and was assumed to be negligible2. Also, because the hydrated lime sample was spread on the TGA sample pan as a very thin layer, the internal diffusion at the initial reaction stage was also assumed to be negligible. In all the results presented here, the O2 concentration in the simulated flue gas was kept at 9% and the temperature at 200 °C, similar to real flue gas conditions. Lime-HCl systems refer to hydrated lime reacting with HCl as the only reactive species in the gas mixture, and lime-CO2 refers to the hydrated lime reaction with CO2 as the only reactive species, and so forth. Lime-HCl/CO2 or lime-HCl/SO2 refers to the hydrated lime reaction with a mixture of HCl and CO2 or of HCl and SO2, respectively, as the reactive species in the flue gas, and so forth. Besides these straightforward reactions mentioned above, two sets of sequential reactions were also carried out. In the sequential lime reaction with CO2 and HCl, hydrated lime was first reacted with CO2. When the conversion of hydrated lime reached the limit, CO2 was replaced with HCl for some time, and then HCl was replaced by CO2. The CO2 and HCl concentrations used were 10% and 540 ppm, respectively. Between each switching, the reaction chamber was purged with N2 to clear out any residual species. In the same manner, the sequential hydrated lime reaction with SO2 and HCl was carried out. The SO2 concentration used was 1000 ppm. The other conditions remained the same. Thermodynamic Calculations. The thermodynamic calculations were carried out using the Outokumpu HSC Chemistry version 4.0 software. The thermodynamic data for calcium hydroxychloride (Ca(OH)Cl) was obtained from the work of Allal et al.,21 since it was not present in the database. The GIBBS equation solver using the Gibbs energy minimization method was selected. XRD Characterization. Owing to the sensitive nature of the reacted products taken from the TGA reactor, extra care had to be taken for the storage of products before the XRD analysis could be carried out. After each reaction, the product inside the reactor was cooled with dry N2 to room temperature and placed immediately inside an airtight bottle. This was done to minimize its contact with water vapor from the surroundings. The powder XRD patterns were collected using a Siemens D5005 diffractometer with Cu KR radiation and step-scanned over a 2θ range of 5-80° at intervals of 0.04 s, with a step time of 20 s. Phase identifications were conducted using the Bruker software Diffrac-plus EVA supported by the powder diffraction file (PDF-2). To quantitatively analyze the species in the product, the Rietveld quantitative phase analysis was carried out using a fundamental parameter procedure as implemented by TOPAS R (version 2.1). In each case, a background polynomial, scale factor, cell parameters, zero point correction, and sample displacement were refined. Atom positions, site occupancy factors, and isotropic thermal parameters were fixed. The weight

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Table 2. Summary of the Thermodynamic Results for the Reaction of Lime with HCl and CO2 reaction 1 Ca(OH)2 + 2HCl(g) f CaCl2 + 2H2O(g) 2 Ca(OH)2 + HCl(g) f Ca(OH)Cl + H2O(g) 3 Ca(OH)2 + CO2(g) f CaCO3 + H2O(g) 4 CaCO3 + HCl(g) f Ca(OH)Cl + CO2(g) 5 CaCO3 + 2HCl(g) f CaCl2 + CO2(g) + H2O(g) a

Table 3. Summary of the Thermodynamic Results for the Reaction of Lime with SO2 under Various Conditions

temp (°C) ∆G (kJ/mol) log10(K)a 100 200 300 100 200 300 100 200 300 100 200 300 100 200 300

-118.0 -120.2 -122.1 -78.6 -80.0 -81.3 -63.3 -61.5 -59.5 -15.3 -18.5 -21.8 -54.7 -58.7 -62.5

16.5 13.2 11.1 11.0 8.8 7.4 8.9 6.8 5.4 2.1 2.0 2.0 7.7 6.5 5.7

K: equilibrium constant.

reaction 8 9 10 11 12 13

percent (wt %) of each Bragg diffracting phase was calculated from the refined scale factors.

14

Results and Discussion

15

Thermodynamic Considerations. There are quite a number of chemical reactions going on in a real system when hydrated lime is exposed to various types of reactive contaminants in the flue gas. In this section, the various possible reactions were first examined individually and then as a whole from a thermodynamic point of view. While the thermodynamic calculations help in predicting the different types of chemical reactions that take place in this complex system, the results were used with caution, bearing in mind that the kinetic aspects play a large role in determining whether a predicted reaction actually takes place. The calculations here merely allowed us to know the theoretical boundaries to work within. Table 2 summarizes the thermodynamic results for hydrated lime reactions with HCl and CO2 at varied temperatures (100, 200, and 300 °C). Reactions 1 and 2 in Table 2 are reactions of hydrated lime with HCl. Although both reactions are thermodynamically possible, as indicated by the decrease of the Gibbs free energy (∆G° < 0), it has been shown that the reaction with HCl to form Ca(OH)Cl has a stronger tendency to occur in both laboratory and real-life (municipal waste incinerator) conditions.2,8,18 Reactions 3-5 are for systems containing hydrated lime together with CO2 and HCl. Hydrated lime when reacted with CO2 has a tendency to form CaCO3. However, the CaCO3 that is formed can subsequently be attacked by the presence of HCl (reactions 4 and 5). The reactions present in Table 3 are for systems containing hydrated lime together with SO2 and HCl. Hydrated lime could react with SO2 to form CaSO3‚1/ 2H2O. In the presence of excess O2 (and H2O), CaSO4‚ 2H2O (gypsum) and CaSO4 can also be formed either directly or with CaSO3‚1/2H2O as an intermediate (reactions 8-12 in Table 3). The CaSO3‚1/2H2O layer that is formed can subsequently react with HCl to form CaCl2 (reaction 13) but not Ca(OH)Cl, a reaction with a positive Gibbs free energy generated (∆G° > 0) at the temperature range of relevance here (reaction 14). The oxides of sulfur with +6 valence S(VI) are more stable than their S(IV) counterparts. It is thermodynamically impossible for HCl to react with either gypsum or CaSO4 (as given by the positive Gibbs free energy for reactions 15-18 in Table 3).

16 17 18

Ca(OH)2 + SO2(g) f CaSO3‚1/2H2O + 1/2H2O(g)

temp ∆G° (°C) (kJ/mol) log10(K)

100 200 300 Ca(OH)2 + SO2(g) + 1/2O2 f 100 CaSO4 + H2O(g) 200 300 Ca(OH)2 + SO2(g) + 1/2O2 + H2O(g) f 100 CaSO4‚2H2O 200 300 CaSO3‚1/2H2O + 1/2O2(g) + 3/2H2O(g) f 100 CaSO4‚2H2O 200 300 CaSO3‚1/2H2O + 1/2O2(g) f 100 CaSO4 + 1/2H2O 200 300 CaSO3‚1/2H2O + 2HCl(g) f 100 3 CaCl2 + SO2(g) + /2H2O(g) 200 300 CaSO3‚1/2H2O + HCl(g) f 100 Ca(OH)Cl + SO2(g) + 1/2H2O(g) 200 300 CaSO4‚2H2O + 2HCl(g) f 100 CaCl2 + SO2(g) + 3H2O + 1/2O2(g) 200 300 CaSO4‚2H2O + HCl(g) f 100 Ca(OH)Cl + SO2(g) + 2H2O + 1/2O2(g) 200 300 CaSO4 + 2HCl(g) f 100 CaCl2 + SO2(g) + H2O + 1/2O2(g) 200 300 CaSO4 + HCl(g) f 100 Ca(OH)Cl + SO2(g) + 1/2O2(g) 200 300

-106.5 -94.7 -82.8 -341.2 -326.9 -312.4 -338.0 -295.8 -254.5 -231.5 -201.1 -171.7 -234.7 -232.2 -229.6 -11.6 -25.5 -39.3 27.8 14.8 1.4 52.6 42.0 31.6 62.0 51.6 41.4 53.3 49.4 45.5 62.8 59.0 55.2

14.9 10.5 7.5 47.8 36.1 28.5 47.3 32.7 23.2 32.4 22.2 15.7 32.9 25.6 20.9 1.6 2.8 3.6 -3.9 -1.6 -0.1 -30.8 -19.4 -12.1 -36.3 -23.8 -15.8 -31.2 -22.8 -17.3 -36.8 -27.3 -21.1

After examining the reaction equations individually, the “equilibrium composition” module of the software was used to observe the trend with all of the reactants present in the system. The method of calculation by the software is the same as before, using the Gibbs equations solver. In Figure 1 is the equilibrium speciation of a system containing equal starting amounts of HCl, SO2, and CO2 (1 kmol) under excess O2 (∼33.3%) and moisture (∼16.7%) at 200 °C and 1 atm. The starting amount of hydrated lime is limited at 4 kmol. While varying the HCl amount from 1 to 4 kmol, the production of CaSO4 and CaCO3 (each at 1 kmol) is thermodynamically stable. The formation of Ca(OH)Cl takes priority over CaCl2 when hydrated lime is first reacted with HCl. At larger concentrations of HCl (>2 kmol), when hydrated lime becomes insufficient, the HCl continues to react with the Ca(OH)Cl formed to produce CaCl2. When subsequent increments of HCl are added

Figure 1. Predicted equilibrium speciation at varied amounts of HCl.

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Figure 2. Predicted equilibrium speciation at varied amounts of SO2.

to the reaction mixture, after all the Ca(OH)Cl has become converted, the HCl reacts with CaCO3 instead (according to reactions 4 and 5 in Table 3), causing the decrease of the amount of CaCO3 for quantities of HCl larger than 4 kmol. There is a possibility that the reduction of CaCO3 could also be due to HCl competing directly with CO2 to react with hydrated lime rather than HCl reacting with the CaCO3 that was formed, as the compositions given by the thermodynamic calculation were at equilibrium states. However, this can only be determined by the appropriate kinetic data and is not discussed here. It is noteworthy that in Figure 1 a slight decrease of CaCO3 from 1 kmol is observed when HCl varys from 2 to 4 kmol, which corresponds to a minor increase of Ca(OH)Cl for the same quantity of HCl. This phenomenon accounts potentially for the deviation of the manually-inputted database of CaClOH.21 Yet, the overall tendency of the prediction is satisfied. In Figure 2, the amount of SO2 is varied from 1 to 5 kmol instead, under the same conditions discussed above, while the amount of HCl is fixed at 1 kmol. It can be seen that equal amounts (1 kmol) of CaSO4, CaCO3, and Ca(OH)Cl were formed with 1 kmol of SO2

input. With the increase of the amount of SO2 from 1 to 5 kmol, the formation of CaSO4 took priority over that of Ca(OH)Cl, CaCl2, and CaCO3. The decrease of the amount of CaCO3 and the conversion of Ca(OH)Cl to CaCl2 at relatively high SO2 input are most likely due to the limited amount of calcium and an insufficient oxygen supply in the system. Further calculations with variation of the temperature from 100 to 600 °C have been performed, and the results show very little effect on the overall equilibrium speciation (details not shown here). XRD Characterization. The PDF number and the characteristic peaks of the species identified in the product samples can be seen in Table 4, for reference. The strongest peak of each compound is highlighted in bold. Owing to the existence of two unidentified peaks (as mentioned in our previous work20), not all the reacted products could be quantified properly. Furthermore, the product coming from the TGA was not free from impurities, making the background noisy. Those that could be quantified are shown in Table 5, with the error being within the 5-10% range. Any crystalline phase with less than 5% of weight percentage may be undetected, as this is the usual lower limit of XRD, judging from previous experiences. Note that, in the table, the composition may not always add up to 100%, as some of the species with smaller quantities (such as CaSO4) were left out. The results here will be quoted throughout the discussion in this section. There is also likely to be a significant amount of amorphous phase present in the product sample, contributing to the noise in the background. The “amorphous phase” is composed of both crystal structures that were too small to be detected and the truly amorphous portion. Bode´nan and Deniard18 estimated that this amorphous portion accounts for more than 30% of some reacted lime samples taken from MWIs. The reaction chemistry derived hereafter is based on the XRD spectrum and complimented by the thermodynamic predictions from the previous section. (1) Reaction with HCl. For the sake of completion, some of the important results regarding the hydrated

Table 4. PDF Numbers and 2-Theta Locations of the Main Peaks Belonging to the Compounds of Interest from the XRD Analysisa

a

compound

PDF no.

2-theta (θ)

Ca(OH)2, portlandite, syn Ca(OH)Cl, calcium hydroxide chloride CaCl2, calcium chloride Ca(CO3), calcilte, syn CaSO4‚2H2O, gypsum, syn CaSO3‚1/2H2O, calcium sulfite hydrate CaSO4, calcium sulfate

72-0156 73-1885 24-0223 81-2027 33-0311 84-0962 45-0157

18.1, 28.7, 34.2, 47.3, 50.9, 54.5, 62.7, 64.5, 72.0 17.9, 26.6, 28.1, 32.3, 36.3, 38.3, 45.5, 47.0, 50.7, 60.8, 62.1 19.8, 29.3, 31.2, 38.6 23.0, 29.4, 36.0, 40.0, 43.1, 47.5, 48.5 11.6, 20.7, 29.1, 31.1, 33.3 15.9, 23.4, 28.2, 34.1 14.6, 25.5, 32.0

The strongest peak of each compound (where applicable) is highlighted in bold.

Table 5. Compositions of the Reaction Products Obtained by Quantitative XRD Analysis (wt %)a reaction 3b

1. unreacted lime 2. lime 3 + HCl (800 mg/m3) + CO2 (2%) 3. lime 3 + HCl (800 mg/m3) + CO2 (10%) 4. lime 3 + CO2 (8%) 5. lime 3 + CO2 (10%) 6. lime 3 + HCl (800 mg/m3) + SO2 (1000 ppm) 7. lime 3 + HCl (800 mg/m3) + SO2 (1600 ppm)b 8. lime 3 + SO2 (1000 ppm)b

portlandite Ca(OH)2

calcite CaCO3

Ca(OH)Cl

gypsum CaSO4‚2H2O

CaSO3‚1/2H2O

95 8 5 58 42 21 85

5 49 14 42 58 16 35 7

43 83 55 -

8 59 1

5 5

a The absence of a species above the 5% range (the usual detection limit of the XRD) is indicated by this symbol, -. b Refers to estimated values.

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Figure 3. XRD spectrum of the products from the reaction of lime 3 with 8 and 10% of CO2, respectively, in the absence of HCl.

Figure 4. XRD spectrum of the products from the reaction of lime 3 with HCl (800 mg/m3) and CO2 at concentrations of 2% (bottom) and 10% (top), respectively. Conditions were as follows: flow rate ) 150 mL/min, temperature ) 200 °C, and [O2] ) 9%.

lime reaction with HCl previously published in our earlier work2 are summarized here. According to eqs 1 and 2, the reaction of hydrated lime with HCl can be written as follows:

Ca(OH)2(s) + HCl(g) f Ca(OH)Cl(s) + H2O(g)

expected to have taken place, judging from the XRD spectrum that was obtained:

Ca(OH)2(s) + CO2(g) f CaCO3(s) + H2O(g) (3)

(1)

CaCO3(s) + HCl(g) f Ca(OH)Cl(s) + CO2(g) (4)

Ca(OH)2(s) + 2HCl(g) f CaCl2(s) + 2H2O(g) (2)

The reaction in eq 4 could have taken place when the highly reactive HCl attacked the CaCO3 layer after it was formed (in agreement with the thermodynamic prediction made earlier). Of course, for the lime-CO2 system, only the reaction in eq 3 would have taken place. In Figure 3 are the XRD results corresponding to the hydrated lime reactions with CO2 (at 8% and 10%) in the absence of HCl (lime-CO2 system). A large amount of unreacted hydrated lime (∼50%, Table 5, cases 4 and 5) can be observed in the reaction product. Note that

It was observed from the XRD spectrum that, for some hydrated lime samples used (lime 3 in particular), the Ca(OH)Cl formed was subsequently converted to CaCl2 when the reaction was allowed to continue for a long time (800 min). This is in agreement with the thermodynamic predictions shown earlier in Table 2 and Figure 1. (2) Reaction with CO2. In the lime-HCl/CO2 system, other than eqs 1 and 2, the following reactions were

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Figure 5. XRD spectra of the products from the reaction of lime 3 with CO2, HCl, and then CO2 again in sequence.

there is even a minor portion of CaCO3 (∼5%) which is actually from the raw sample (see Table 4, case 1). As discussed in our previous work,20 this was probably due to the high resistance from the product layer that tends to form when hydrated lime is reacted with CO2. This layer prevented the subsequent CO2 from reaching the unreacted hydrated lime below the product layer. In Figure 4 are the spectra of the products from two selected reactions in the lime-HCl/CO2 system (with 2 and 10% of CO2, respectively). The main products observed were Ca(OH)Cl and CaCO3. Only a small amount of unreacted Ca(OH)2 can be seen (Table 5, cases 2 and 3). When the concentration of CO2 was increased from 2 to 10%, the amount of CaCO3 in the product decreased from 49 to 14% with respect to the increase of Ca(OH)Cl from 43 to 83% instead of the opposite. This could be due to the HCl reacting with the CaCO3 that was formed, according to eq 4. It was previously observed20 in experiments that both CO2 and SO2 (compared to HCl) demonstrated a very much higher reaction rate with Ca(OH)2 during the first minutes. This means eq 1 cannot compete with eq 3 in terms of reaction kinetics, particularly at increasing CO2 concentrations from 2 to 10%. The generated CO2 from eq 4 tends to react with the excessive Ca(OH)2 available there to form more CaCO3 until the “equilibrium” is reached, i.e., a certain amount of Ca(OH)Cl is formed. The increased CO2 concentration from 2 to 10% might initially enhance the formation of CaCO3, but eventually, it increases the generation of Ca(OH)Cl. As for the sequential CO2-HCl-CO2 reaction, the strongest peak (apart from the unidentified peak) belonged to CaCO3, followed by the peaks for the unreacted Ca(OH)2 (Figure 5). The unidentified peak could be due to the presence of a new Ca-based crystalline form that is not present in currently available databases (for details refer to ref 2). The amount of unreacted hydrated lime in the product after reaction was not very much, given that the CaCO3 peaks were higher than the Ca(OH)2 peaks. This was despite the fact that the hydrated lime was exposed to 10% CO2 for only about 170 min (plus about 10 min of HCl). Previously in the lime-CO2 system (cases 4 and 5 in

Table 5 and Figure 3), the hydrated lime was exposed to the same concentration of CO2 for about 800 min, yet there was still a substantial amount of unreacted hydrated lime left in the product. In Figure 3, the Ca(OH)2 peaks were almost comparable in height with the CaCO3 peaks. The introduction of HCl midway through the reaction, though for a short time, has enabled CO2 to continue reacting with the hydrated lime under the product layer; that is, the introduction of HCl for a short period of time enabled more hydrated lime to be utilized by CO2. (3) Reaction with SO2. The following reaction was expected to have taken place in a system with SO2:

Ca(OH)2(s) + SO2(g) f CaSO3‚1/2H2O(s) + 1/2H2O(g) (5) In the presence of excess O2, the CaSO3‚1/2H2O formed can also be converted to either CaSO4‚2H2O (gypsum) or CaSO4:

CaSO3‚1/2H2O(s) + 1/2O2(g) + 3/2H2O(g) f CaSO4‚2H2O (6) CaSO3‚1/2H2O(s) + 1/2O2(g) f CaSO4 + 1/2H2O(g) (7) With HCl also present in the system, the reactions in eqs 1 and 2 would also have taken place. In addition to that, the HCl gas could have attacked the CaSO3‚1/2H2O layer in the following manner:

CaSO3‚1/2H2O(s) + 2HCl(g) f CaCl2(s) + SO2(g) + 3/2H2O(g) (8) For the lime-SO2 (1000 ppm) system, a large amount (∼85%) of the hydrated lime remained unreacted, as expected (case 8 in Table 5 and Figure 6). Similar to the case of CO2, this was due to the formation of the product layer that prevented the SO2 from reaching the unreacted hydrated lime layer below. Also seen in the

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Figure 6. XRD spectrum of the products from the reaction of lime 3 with SO2 at a concentration of 1000 ppm.

Figure 7. XRD spectra of the products from the reaction of lime 3 with HCl (800 mg/m3) and SO2 (1000 and 1600 ppm).

spectrum (Figure 6) were small amounts of CaSO3‚1/ 2H2O, CaSO4, CaSO4‚2H2O, and CaCO3. The CaCO3 should be from the raw sample, which contained around 5% of it (Table 5, case 1). Two concentrations of SO2 (1000 and 1600 ppm) were chosen for the lime-HCl/SO2 system, and the results are shown in Figure 7. Some amount of the unreacted hydrated lime sample (∼21%) can still be seen in the spectrum containing 1000 ppm of SO2 (case 6, Table 5). The majority of the product here is Ca(OH)Cl (∼55%), with a small amount of gypsum (∼8%). When the amount of SO2 was increased to 1600 ppm (case 7, Table 5), no unreacted hydrated lime was detected and Ca(OH)Cl had also disappeared. The majority of the product formed was gypsum (∼59%) with small amounts of CaSO3‚1/2H2O (∼5%). However, a surprisingly high

content of CaCO3 was detected in cases 6 and 7, which might be due to the sample’s exposure to the atmosphere. At high concentrations of SO2 and with insufficient hydrated lime reactant, the absence of Ca(OH)Cl agreed well with the thermodynamic prediction (see Table 3). Gibbs free energies are all increased for those reactions relative to reactions generating Ca(OH)Cl. Also from the thermodynamic predictions, it was known that HCl could only react with CaSO3‚1/2H2O (reaction 13 in Table 3) but not with the stable gypsum layer (reactions 1518 in Table 3). Therefore, the accumulation of gypsum was seen in the product as the SO2 concentration was increased from 1000 to 1600 ppm. This was unlike the case for CO2, where the product with hydrated lime,

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Figure 8. XRD spectra of the products from the reaction of lime 3 with SO2, HCl, and then SO2 again in sequence.

CaCO3, was susceptible to reaction with HCl (reactions 4 and 5 in Table 2). In Chisholm and Rochelle’s work,14 the authors also reported that the CaSO3‚1/2H2O layer was attacked by HCl gas to form CaCl2‚2H2O and released SO2 in a reversible reaction. This is almost similar to the reaction in eq 8, except that the product was CaCl2 and there was no evidence of reversibility in the reaction. In their work, the sulfur becomes stable and resistant to the HCl reaction when it is oxidized from the S(IV) form to the S(VI) form in the presence of excess O2, in agreement with the results here. Anhydrous CaSO4 was detected by XRD in instances where there were no simultaneous reactions with HCl (lime-SO2 systems and sequential SO2 to HCl reactions) (Figures 6 and 8). This could be due to the fact that there were less water molecules present in the system, as the lime-HCl reaction that formed water molecules was absent. Conclusions In conclusion, the combined thermodynamic calculations and the XRD characterization allowed an in-depth investigation on the actual reaction chemistry behind the complex system of dry scrubbing of flue gas containing HCl, SO2, CO2, O2, and moisture using hydrated lime. When hydrated lime came into contact with HCl, Ca(OH)Cl had a higher tendency to be formed than CaCl2. The hydrated lime reaction with CO2 formed CaCO3. From previous results, it was known that this species had a high product layer diffusion resistance and caused the reaction to terminate prematurely by preventing the unreacted hydrated lime below the product layer from contacting the reactive gas. When HCl was present, it attacked the CaCO3 layer, forming either Ca(OH)Cl or CaCl2, and exposed the unreacted hydrated lime to further reaction. The reaction of hydrated lime with SO2 formed the thermodynamically favorable species, CaSO3‚1/2H2O. In the presence of excess O2, CaSO4‚2H2O was formed, and under dryer conditions, the anhydrous CaSO4 was formed instead. Similar to the case of CO2, the product

of the reaction of hydrated lime with SO2 also has a high product layer resistance. When HCl was present, it attacked the CaSO3‚1/2H2O layer to form CaCl2, as predicted by the thermodynamic calculation. However, both CaSO4‚2H2O and CaSO4 were resistant to reaction by HCl. In this sense, the presence of O2 in the flue gas was counterproductive for the scrubbing using hydrated lime, as it caused the sulfur in the S(IV) form to be oxidized to the more stable S(VI) form, thereby reducing the efficiency of the hydrated lime. The presence of HCl helped with the removal of SO2 and CO2 using hydrated lime. From the point of view of thermodynamics, varying the temperature from 100 to 600 °C had minimal effect on the results. Further studies comparing the kinetics of the reactions of hydrated lime with HCl, SO2, and CO2 under industrial conditions would be helpful for gaining a better understanding of the system. Literature Cited (1) Niessen, W. R. Combustion and Incineration Processes, 3rd ed.; Marcel Dekker: New York, 2002. (2) Yan, R.; Chin, T.; Liang, D. T.; Laursen, K.; Ong, W. Y.; Yao. K. W.; Tay, J. H. Kinetic Study of Hydrated Lime Reaction with HCl. Environ. Sci Technol. 2003, 37, 2556. (3) Daoudi, M.; Walters J. K. A Thermogravimetric Study of the Reaction of Hydrogen Chloride Gas with Calcined Limestone: Determination of Kinetic Parameters. Chem. Eng. J. 1991, 47, 1. (4) Daoudi, M.; Walters J. K. The Reaction of HCl Gas With Calcined Commercial Limestone Particles: The Effect of Particle Size. Chem. Eng. J. 1991, 47, 11. (5) Mura, G.; Lallai, A. On The Kinetics of Dry Reaction Between Calcium Oxide and Gas Hydrochloric Acid. Chem. Eng. Sci. 1992, 47, 2407. (6) Li, M.; Shaw, H.; Yang, C. L. Reaction Kinetics of Hydrogen Chloride with Calcium Oxide by Fourier Transform Infrared Spectroscopy. Ind. Eng. Chem. Res. 2000, 39, 1898. (7) Gullet, B. K.; Jozewicz, W.; Stefanski, L. A. Reaction Kinetics of Ca-Based Sorbents with HCl. Ind. Eng. Chem. Res. 1992, 31, 2437. (8) Jozewicz, W.; Gullet, B. K. Reaction Mechanisms of Dry CaBased Sorbents with Gaseous HCl. Ind. Eng. Chem. Res. 1995, 34, 607.

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(9) Stein, J.; Kind, M.; Schlu¨nder, E. U. The Influence of HCl on SO2 Absorption in the Spray Dry Scrubbing Process. Chem. Eng. J. 2002, 86, 17. (10) Karlsson, H. T.; Klingspor, J.; Bjerle, I. Adsorption of Hydrochloric Acid on Solid Slaked Lime for Flue Gas Clean Up. J. Air Pollut. Control Assoc. 1981, 31 (11), 1177. (11) Fonseca, A. M.; O Ä rfa˜o, J. J.; Salcedo, R. L. Kinetic Modeling of the Reaction of HCl and Solid Lime at Temperatures. Ind. Eng. Chem. Res. 1998, 37, 4750. (12) Weinell, C. E., Jensen, P. I., Dam-Johansen, K.; Livbjerg, H. Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity. Ind. Eng. Chem. Res. 1992, 31, 164. (13) Matsukata, M.; Miyatani, T.; Ueyama, K.; Matsui, S.; Iwasaki, T. HCl and SO2 Simultaneous Absorption at 1023 K with Calcined Limestone. Proceedings of the 14th International ASME Conference on Fluidized Bed Combustion, Vancouver, Canada, May 11-14, 1997; p 397. (14) Chisholm, P. N.; Rochelle, G. T. Dry Absorption of HCl and SO2 with Hydrated Lime from Humidified Flue Gas. Ind. Eng. Chem. Res. 1999, 38, 4068. (15) Duo, W.; Seville, J. P. K.; Kirkby, N. F.; Clift, R. Formation of Product Layers in Solid-Gas Reactions for Removal of Acid Gas. Chem. Eng. Sci. 1994, 49, 4429. (16) Simons, G. A Predictions of CMA Utilization for In-situ SO2 Removal in Utility Boilers. Resour., Conserv. Recycl. 1992, 7, 161.

(17) Duo, W.; Kirkby, N. F.; Seville, J. P. K.; Clift, R. Alteration with Reaction Progress of the Rate-Limiting Step for Solid-Gas Reactions of Ca-Compounds with HCl. Chem. Eng. Sci. 1995, 50 (13), 2017. (18) Bode´nan, F.; Deniard, Ph. Characterization of Flue Gas Cleaning Residues From European Solid Waste Incinerators: Assessment of Various Ca-based sorbent processes. Chemosphere 2003, 51, 335. (19) Verdone, N.; Filippis, P. D. Thermodynamic Behaviour of Sodium and Calcium Based Sorbents in the Emission Control of Waste Incinerators. Chemosphere 2004, 54, 975. (20) Chin, T.; Yan, R.; Liang, D. T.; Tay, J. H. Hydrated Lime Reaction with HCl Under Simulated Flue Gas Conditions. Ind. Eng. Chem. Res. 2005, 44, 3742. (21) Allal, K. M.; Dolignier, J. C.; Martin, G. Determination of Thermodynamical Data of Calcium Hydroxichloride. Rev. Inst. Fr. Pet. 1997, 52 (3), 361.

Received for review February 21, 2005 Revised manuscript received August 31, 2005 Accepted September 2, 2005 IE058021V