Hydrogen Chloride Removal from Flue Gas by Low ... - ACS Publications

Article Cite This: Energy Fuels 2018, 32, 747−756

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Hydrogen Chloride Removal from Flue Gas by Low-Temperature Reaction with Calcium Hydroxide Alessandro Dal Pozzo, Raffaela Moricone, Giacomo Antonioni, Alessandro Tugnoli, and Valerio Cozzani* LISES−Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Alma Mater Studiorum−Università di Bologna, via Terracini n.28, 40131 Bologna, Italy S Supporting Information *

ABSTRACT: Municipal solid waste incineration (MSWI) is a method of waste valorization whose overall sustainability depends on the effective removal of the gaseous contaminants generated. Hydrogen chloride (HCl) is a typical pollutant formed in waste combustion. Dry processes based on its reaction with basic powders such as calcium hydroxide are among the state-of-the-art best available technologies for MSWI flue gas treatment. An experimental investigation of the heterogeneous reaction process between hydrogen chloride and calcium hydroxide in the temperature range between 120 and 180 °C was carried out. A laboratory-scale fixed bed reactor connected to a Fourier transfrom infrared (FTIR) spectrometer was used for the online continuous monitoring of HCl conversion. Solid reaction products were characterized using thermogravimetric analysis and Xray diffractometry. The experimental data collected were used to validate a fundamental kinetic model for the description of the gas−solid reaction between Ca(OH)2 and HCl. A sensitivity analysis was carried out to assess the importance of the different temperature-dependent parameters in the model. The results allow an improved understanding of the heterogeneous reaction process that is applied in acid gas dry removal processes.

1. INTRODUCTION Several air contaminants are generated by waste combustion, but the so-called acid gases (hydrogen halides and sulfur oxides) are specifically related to waste as a fuel.1−3 Halogens, the precursors of halides, are important constituents of wastes, and chlorine is normally the most abundant halogen in wastes, being present in both inorganic salts and organic compounds.4,5 During combustion, most of the organic chlorine is converted to hydrogen chloride (HCl).3 HCl is typically generated in higher amounts than sulfur dioxide (SO2) in municipal solid waste incineration (MSWI)6 and because of its high toxicity, its emission limits are generally lower than those for SO2.7,8 Both of these aspects make HCl the most critical pollutant among acid gases. While currently available flue gas cleaning techniques guarantee high performance in terms of emission control, important opportunities for process optimization are still present, with potential positive returns in cost-effectiveness and overall sustainability. In the past decade, traditional wet processes based on the scrubbing of flue gases have been increasingly challenged by dry sorbent injection (DSI) processes, which show comparable performances with respect to acid gas removal with lower capital and operating costs and simpler layout.9−11 DSI processes consist of the injection of powdery calcium- or sodium-based sorbents in the flue gas, with the resulting reaction products collected downstream in gas−solid separators (typically, fabric filters).12 In particular, calcium hydroxide, Ca(OH)2, is widely used as a sorbent material due to its high availability and low cost. However, it is less reactive than Na-based sorbents,13 thus its use entails the generation of more solid residues.14 In addition, with the aim of ensuring a safe operation, in MSWI plants Ca(OH)2 is usually © 2017 American Chemical Society

injected in high stoichiometric excess, with the consequence of having a considerable quantity of unreacted sorbent in the solid residues. Therefore, a better knowledge of the mechanisms of the gas−solid reaction would be beneficial to the optimization of acid gas dry treatment systems. A wide range of literature is dedicated to the reaction of Cabased compounds with SO2 or HCl at high temperatures.15−21 Less attention has been dedicated to date to the interaction between calcium hydroxide and HCl in the temperature range 150−200 °C, which is the most interesting for MSWI flue gas cleaning applications, where the injection of Ca(OH)2 takes place downstream of the heat recovery section.12 Weinell et al.22 published a comprehensive experimental study on the reactivity of Ca(OH)2 toward HCl under a wide range of operating conditions (temperature, moisture, surface area of the solid reactant). Fonseca et al.23 investigated the role played by relative humidity in the promotion of Ca(OH)2 reactivity at very low temperatures (50−120 °C), while Chisholm and Rochelle24 studied the competitive sorption of HCl and SO2 at 120 °C and a varying degree of moisture. Yan et al.25 performed thermogravimetric experiments to determine the chemical reaction rate constant for HCl sorption by Ca(OH)2 in the range 150−200 °C. In these studies, an abrupt decline in sorbent reactivity leading to an incomplete and temperature-dependent partial solid conversion was always evidenced. However, this phenomenon, also observed in the reaction of Ca-based sorbents with SO226 and CO2,27 cannot be described by Received: October 25, 2017 Revised: December 3, 2017 Published: December 7, 2017 747

DOI: 10.1021/acs.energyfuels.7b03292 Energy Fuels 2018, 32, 747−756

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Energy & Fuels

Figure 1. Layout of the experimental apparatus.

conventional shrinking core28 or random pore29,30 approaches, unless an arbitrary time-decreasing product layer diffusivity is introduced.31,32 Only recently a model based on a crystallization and fracture process33 was proposed to explain the experimental behavior observed.34 Furthermore, in spite of the considerable experimental work carried out, the stoichiometry of the chloridation reaction with respect to chloride in the solid product is still disputed.35,36 While the majority of modeling studies considered calcium chloride (CaCl2) as the sole solid reaction product,22−25,33,37 various experimental reports38−40 suggested the formation of stable calcium hydroxychloride (CaOHCl). Clearly enough, determining whether CaOHCl or CaCl2 is formed in the reaction process is important in order to correctly evaluate sorbent utilization. In the present study, an experimental investigation of the chloridation of Ca(OH)2 was carried out at temperatures between 120 and 180 °C, with the aim of clarifying both the issues of incomplete conversion modeling and solid product identification. An experimental apparatus based on a fixed bed reactor coupled to a Fourier transform infrared (FTIR) spectrometer was used. The FTIR was equipped with a lowvolume gas cell able to provide through cascading IR spectra acquisition the evolution with respect to time of the reaction process, by the online monitoring of gas composition leaving the fixed bed. Thermogravimetric analysis and X-ray diffractometry were applied to provide details on reacted solid samples. The experimental results were used to validate a fundamental model for the kinetics of hydrogen chloride removal from gas streams proposed in a recent study,34 able to describe the decline of reactivity of the sorbent over time. A sensitivity analysis was carried out to investigate the effect of the main model parameters in determining the kinetics of the reaction process. The thorough characterization of the solid reaction product allowed to clarify the speciation of chlorine in the solid phase at different temperatures, while the experimental fixed bed runs, interpreted in light of the adopted model, provided insights on the phenomena determining low sorbent conversion in the temperature range of interest for DSI applications. The understanding of the reaction mechanism of HCl sorption in dry gas lays the foundations for the study of HCl removal in more complex flue gas mixtures.

surface area of 15.1 m2/g, was provided with 96% chemical purity, with 3% of CaCO3 and 1% of other impurities. This reagent grade was chosen for the sake of consistency with previous literature studies in the field,22,24,25,41,42 considering that the presence of CaCO3 impurity due to reaction with atmospheric CO2 does not alter the reaction behavior of Ca(OH)2.35 A brief discussion about selection and characterization of materials is reported in section S1 of the Supporting Information (SI). 2.2. Fixed Bed Reactor. A laboratory-scale fixed bed reactor (FBR) was built to investigate the heterogeneous reaction process among hydrogen chloride and calcium hydroxide. The overall scheme of the experimental setup is shown in Figure 1. Reaction took place in a tubular reactor housed in an oven, where a thin bed of calcium hydroxide and inert material was deposited on a sintered glass frit disk. The experimental conditions tested are summarized in Table 1. The

Table 1. Summary of Experimental Conditions Used in FBR Runs HCl conc (ppm)

temperature (°C)

mass of sorbent (mg)

inert-to-sorbent mass ratio

2500

120 150 180 120 150 180 120 150 180

100

3:1

50

6:1

50

6:1

2500

1250

selected temperature range is representative of the industrial operation of Ca-based DSI systems.9,12 The HCl concentrations selected aim to represent HCl-rich flue gases, such as those generated by the cocombustion of municipal solid waste with relevant fractions of industrial43 or biomedical wastes,44 which is an increasingly common situation in European MSWI.45 The amounts of sorbent and inert in the fixed bed were chosen in order to obtain a bed thickness in the range 1.6−2.2 mm, which is representative of the average depth of the Ca(OH)2 cake adhered on the fabric filter bags in DSI systems.46 A gas flow rate of 180 N mL/min was adopted in order to have a superficial velocity of the gas in the reactor of 0.9 m/min at 180 °C, corresponding to a typical design value for fabric filters.47 Details on the laboratory apparatus, including the experimental procedure followed in FBR runs are reported in section S1 of the SI. 2.3. Thermogravimetry and X-ray Diffractometry. The solid reaction products of FBR runs were analyzed in order to obtain information on their composition. Thermogravimetric (TG) analysis was used to study sorbent conversion thanks to the different decomposition temperatures of the compounds of interest. Simultaneous TG-FTIR measurements were carried out to characterize the evolved gases formed during the thermal decomposition of samples. High-temperature X-ray diffractometry complemented this information by providing solid phase identification during thermal

2. MATERIALS AND METHODS 2.1. Materials. A gas-chromatographic standard gaseous mixture of 3% HCl in nitrogen, supplied by SIAD (Italy), was used to generate the gaseous stream fed to the fixed bed reactor and for FTIR calibration runs. Commercial calcium hydroxide (Sigma-Aldrich), sieved to the range of 45−123 μm, was used as solid reactant, while quartz sand of the same size fraction was used as inert filling material for the sorbent bed. The commercial calcium hydroxide, having a 748


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Figure 2. Integrated absorbance of HCl and H2O for a reference FBR run (reactant gas sent to the reactor at experiment time = 0). Temperature = 180 °C; HCl inlet concentration = 2500 ppm; mass of sorbent = 100 mg. (A−C) IR spectra recorded (A) before reaction start; (B) immediately (250 s) after reaction start; and (C) after 18 min. decomposition. Details on the experimental devices and procedures48 are reported in section S1 of the SI. 2.4. Modeling. In order to better understand the experimental results obtained and to extract kinetic data, the heterogeneous reaction model proposed in a previous work34 was applied. The modeling approach is based on the coupling of a conventional grain model for gas−solid reactions involving porous solids49 with a crystallization and fracture submodel derived from the work of Duo et al.50 The coupled model allows describing the decline in reactivity of the sorbent during the process, which prevents total solid conversion. Further details on the model are provided elsewhere,34 while a brief discussion about the required set of geometrical, morphological and thermodynamic input data and the assumptions introduced to apply the model to the specific geometry used in FBR runs is reported in section S2 of the SI. Pore size distribution of the sorbent sample was used to determine the initial size of sorbent grains. The method of Heesink et al.51 was applied to the purpose, as described in section S3 of the SI. A sensitivity analysis was carried out to determine the effect of variations in the fitting parameters on model results, applying the variance-based method,52 as described in section S4 of the SI.

strongly attenuated and water-related absorption bands appear in the 4000−3500 and 2000−1200 cm−1 wavenumber intervals. The absorption peak around 2350 cm−1 is associated with CO2 generation due to the reaction between HCl and the small fraction of CaCO3 included in the hydrated lime samples. With the progress of time during experimental run, the solid product accumulated at the surface of the calcium hydroxide particles. This generated an increasing diffusional resistance to the sorption of the gaseous reactant, which slowed down the reaction and lead to an increasing HCl penetration through the bed, as confirmed by spectrum C in Figure 2. The experimental run in Figure 2 was stopped when the value of the derivative of the HCl integrated absorbance approached zero. Figure 3 shows the results of experimental runs carried out in different experimental conditions, listed in Table 1. Recorded values of integrated absorbance were converted to concentration values using the Lambert−Beer law and data from calibration runs (see section S1 of the SI). Normalized values were reported in Figure 3, dividing the HCl concentration by the value of concentration in the inlet gas mixture, obtaining experimental breakthrough curves. Figure 3 also reports the cumulative HCl removal with respect to time. The time at which 50% of the inlet HCl concentration is detected in the gas leaving the reactor (defined as breakthrough time, t50) and the overall quantity of HCl removed at the end of each FBR run, as well as the average HCl removal efficiency during the run, are reported in Table 2. The profiles of HCl removal efficiency as a function of reaction time are shown in section S5 of the SI. As shown in Table 2, the main parameter governing HCl sorption in dry gas conditions is temperature, with delayed breakthrough and higher removal efficiency at higher temperatures. Also sorbent quantity has a relevant effect on t50. For an HCl inlet concentration of 2500 ppm, runs with 50 mg Ca(OH)2 show values of t50 and of cumulative HCl

3. RESULTS AND DISCUSSION 3.1. Results of FBR Runs. The experimental output of FTIR monitoring of FBR runs is reported in Figure 2 for a reference run. The figure shows the integrated absorbance of HCl and H2O during an experimental run, along with three illustrative IR spectra. The residence time of the gas in the system was estimated to be of 20 s, and the time reference in the plots was corrected accordingly. As shown in the figure, at first the HCl-containing feed gas is sent to the fixed bed of fresh Ca(OH)2 and an almost complete removal of HCl takes place, with the simultaneous release of water vapor, in agreement with the possible reaction schemes (see section 3.2). The phenomenon is well illustrated by the comparison between spectra A and B, respectively recorded for the outlet gas shortly before and shortly after the start of the experimental run. Actually, spectrum B shows that the HCl absorption peaks are 749


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cumulative HCl sorption, which exhibit a faster initial growth when an HCl inlet concentration of 2500 ppm is used. 3.2. Products of the Heterogeneous Reaction and Solid Conversion. In order to assess sorbent conversion on a molar basis, the identification of the solid reaction product is required. As mentioned in section 1, the determination of the actual solid product of the chloridation reaction is still uncertain. The majority of modeling studies regarding the gas−solid reaction between Ca-based sorbents and HCl assumed calcium chloride (CaCl2) as the solid-phase product, either in anhydrous or dihydrate form:22−25,33,37 Ca(OH)2 (s) + 2·HCl(g) = CaCl 2(s) + 2·H 2O(g)

(R1)

Ca(OH)2 (s) + 2 ·HCl(g) = CaCl 2·2 ·H 2O(s)

(R2)

In contrast, studies focusing on the identification of the reaction product via analytical techniques pointed out the formation of calcium hydroxychloride (CaOHCl) as a stable product:38,40 Ca(OH)2 (s) + HCl(g) = CaOHCl(s) + H 2O(g)

(R3)

In Table 3 the results of previous studies aimed at specifying the products of HCl sorption by calcium hydroxide are summarized. It can be noticed that, for reactions taking place at temperatures around 150−200 °C, in the typical operating range of dry sorbent injection in MSWI applications,54 the dominant product appears to be CaOHCl. In the present study, the sorbent samples after reaction with HCl were analyzed via TGA and XRD to identify the solid product phase obtained in FBR runs. In Figure 4 the results of TG runs carried out on three samples, reacted respectively at 120, 150, and 180 °C with the same inlet concentration of HCl (2500 ppm), are compared with the results of a TG run carried out on unreacted calcium hydroxide in the same experimental conditions. The unreacted material shows two separate weight loss steps approximately in temperature ranges between 350 and 400 and 500−600 °C. The former is due to the release of chemically bound water (dehydration of Ca(OH)2 to CaO), and the latter is due to the release of CO2 (calcination of the CaCO3 impurities to CaO). With respect to the unreacted material, the reacted samples display an additional, well distinguishable weight loss step in the temperature range between 450 and 520 °C. This can be attributed to the thermal degradation of a solid reaction product. The various hydrated forms of CaCl2 lose their water content at T < 200 °C,56 while CaCl2 itself should not decompose (and thus should not undergo weight loss) at temperatures lower than its melting point, 772 °C.57 Similar results were obtained for TG runs

Figure 3. Normalized HCl outlet concentration (points) and cumulative HCl removal (lines) with respect to time and temperature. Temperature: (a) 120; (b) 150; (c) 180 °C. HCl concentration in feed: (●) 2500 ppm of HCl, 100 mg sorbent; (▲) 2500 ppm of HCl, 50 mg sorbent; (⧫) 1250 ppm of HCl, 50 mg sorbent. See Table 1 for other details on experimental conditions.

removal that are almost halved with respect to those obtained using 100 mg Ca(OH)2. Runs carried out with an HCl concentration of 1250 ppm and 50 mg of Ca(OH)2 have the same Ca-to-HCl ratio of runs with an inlet concentration of HCl of 2500 ppm and 100 mg of Ca(OH)2 but show a consistently shorter breakthrough time, thus evidencing a slight positive effect of HCl concentration on bed reactivity. Such effect is also shown by the curves of

Table 2. t50 and Total HCl Removed at the End of the Experiment for the Different FBR Runs temperature (°C)

HCl concentration (ppm)


120

2500 2500 1250 2500 2500 1250 2500 2500 1250

100 50 50 100 50 50 100 50 50

150

180

t50 (min) 3.3 1.6 3.0 7.2 3.5 5.6 15.8 7.3 14.3

± ± ± ± ± ± ± ± ±

0.2 0.1 0.5 1.1 0.3 1.1 0.9 0.4 0.8

total HCl removed (mg) 4.2 2.2 1.9 7.1 4.2 3.3 12.1 6.5 6.0 750

± ± ± ± ± ± ± ± ±

0.3 0.1 0.3 0.6 0.2 0.4 0.7 0.6 0.2

HCl removed per unit mass of sorbent (wt %) 4.2 4.4 3.8 7.1 8.4 6.6 12.1 13.0 12.0

± ± ± ± ± ± ± ± ±

0.3 0.2 0.6 0.6 0.4 0.8 0.7 1.2 0.4

average HCl removal efficiency (%) 19.9 10.4 17.4 33.0 20.4 30.5 55.4 30.6 54.2

± ± ± ± ± ± ± ± ±

1.3 0.5 3.3 3.1 1.0 3.2 3.2 2.7 1.3


Article

Energy & Fuels Table 3. Relevant Studies Dealing with Product Identification for the Reaction between Ca(OH)2 and HCl experimental conditions source Gullett et al.41 Jozewicz and Gullett42 Allal et al.53 Bodenan and Deniard54 Bausach et al.35 Chin et al.55 Partanen et al.40 Bogush et al.38 a

identified product 57% CaCl2·2H2O, 28% unreacted CaO, 10% CaOHCl, 5% Ca(ClO)2 CaCl2·2H2O/CaOHCl CaOHCl

T (°C)

RH (%)

CHCl,in (ppm)

method of identification

500

5000

XRD

500 200

1000 1000

DSC, XRD DSC, XRD

CaOHCl CaOHCl

250 140−170

0−10

1000 naa

XRD XRD

CaOHCl

120

18

240

XRD

CaOHCl

200

500

XRD

650/850

2000

XRD

final product: CaCl2 intermediate: CaOHCl CaOHCl

150−200

0−10

na

XRD, FTIR, EDX

notes the hydrated form of CaCl2 may be due to reaction with ambient air humidity of the highly deliquescent sample during interlaboratory transfer

samples of APC residues from 12 MSWIs across Europe conversion to CaCl2 can only take place if Ca(OH)2 is lacking, at least locally

hydrous CaCl2 detected by XRD, but attributed to hydration after the sample was taken out of the reactor samples of APC residues from six British MSWIs

Not available/not reported.

Figure 4. Weight loss (solid lines) and weight loss rate (dotted lines) curves obtained for unreacted Ca(OH)2 and solid residues obtained from the FBR after runs with 100 mg of sorbent and 2500 ppm initial HCl concentration at temperatures of 120, 150, and 180 °C. TG runs were carried out at 10 °C/min constant heating rate and in 100 mL/ min pure nitrogen flow.

carried out in the other conditions of Table 1, as shown in section S6 of the SI. Thus, TG results strongly suggest that the solid product formed in the reaction is calcium hydroxychloride. Actually, as suggested by Allal et al.53 and observed by Prigiobbe et al.,58 calcium hydroxychloride might undergo thermal decomposition by dehydroxylation: (R4)

Figure 5. Evolved gas analysis for TG-FTIR run performed on solid residue obtained from the FBR after a run with 2500 ppm initial HCl concentration at 180 °C. (a) TG and dTG of sample in pure nitrogen (20 °C/min heating rate). (b) Emission profiles of H2O, CO2, and HCl in nitrogen. The weight loss and vapor release associated with CaOHCl degradation are pinpointed.

The occurrence of reaction R4 might also explain why in previous experimental investigations the product of the Ca(OH)2/HCl reaction was found to be CaOHCl in studies carried out at temperatures between 150 and 200 °C, while at temperatures above 500 °C CaCl2 was reported as the main reaction product (see Table 3). The occurrence of reaction R4 in the temperature interval between 450 and 520 °C is further confirmed by the results of a TG-FTIR experimental run carried out on a sample reacted with 2500 ppm of HCl at 180 °C. The results, shown in Figure 5, evidence that in the temperature range between 450 and 520

°C only water vapor is evolved. Hydrogen chloride is detected in evolved gases only at higher temperatures (higher than 950 °C), where presumably the calcium chloride formed in reaction R4 is decomposed. A quantitative assessment was also carried out, comparing the weight loss experienced by the reacted samples between 450 and 520 °C to the amount of hydrogen chloride converted detected by FTIR analysis. Considering the stoichiometry of reaction R3, based on FTIR results reported in Table 2, the formation of amounts of CaOHCl comprised between 4 and 20

2CaOHCl(s) = CaO·CaCl 2(s) + H 2O(g)

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Figure 6. (a) Breakthrough curve for HCl in FBR (T = 180 °C, CHCl,in = 2500 ppm, bed of 100 mg sorbent) and (b) TG weight loss and weight loss rate curves (N2 atmosphere, 10 °C/min heating rate) of the solid residues collected at the end of the FBR. ω is the amount of CaOHCl in the solid sample (wt %) calculated from experimental data analysis.

Table 5. Temperature-Dependent Parameters Calculated for the Training Set

Table 4. Final Conversion of the Sorbent for the Set of FBR Runs HCl concentration (ppm)


final conversion (%)

120

2500 2500 1250 2500 2500 1250 2500 2500 1250

100 50 50 100 50 50 100 50 50

7.0 5.7 6.2 14.0 14.3 13.5 24.2 24.8 23.3

150

180

120 °C

150 °C

180 °C

3.0 × 10−5 0.38 × 10−13 2.55 × 10−2

3.9 × 10−5 1.87 × 10−13 1.10 × 10−2

5.0 × 10−5 7.51 × 10−13 0.53 × 10−2

100 mg sample of Ca(OH)2, and the corresponding TG analysis. FBR and TG data are in good accordance, showing differences lower than 5% in the calculated amounts of CaOHCl formed. A further investigation of the reaction products present in the solid residue was carried out by XRD analysis. Figure 7 shows the results obtained from the analysis of a solid residue obtained from a FBR run (2500 ppm of HCl, 180 °C) without the addition of inert material in the bed, in order to avoid the strong XRD pattern of quartz (for reference, a XRD spectrum of a sample reacted in the same conditions in the presence of quartz in the bed is shown in section S7 of the SI). At ambient temperature, both unreacted calcium hydroxide and calcium carbonate can be detected in the solid residue, and the only chlorinated phase identified is CaOHCl, in agreement with previous XRD investigations.35,55 Very similar results are recorded when the solid residues are heated up to 300 and 425 °C. At 550 °C, the intensity of CaOHCl peaks appears significantly lowered, and the peaks are no more present at 700 °C. At this temperature, also the peaks of CaCO3 are no more present, due to its high temperature calcination to CaO. Therefore, the analysis of TG, TG-FTIR, and of XRD results strongly suggests that only CaOHCl is formed as a reaction product in the experimental conditions of FBR runs. CaOHCl was assumed as the only reaction product in the calculation of the conversion of the solid reactant in FBR runs, carried out as shown in section S1 of the SI. The final conversion values obtained in the FBR runs carried out are listed in Table 4. Weinell et al.22 reported that final sorbent conversion values in the temperature range 100−200 °C vary between 5.2 and 21.9% for a Ca(OH)2 sample with a surface area of 12.1 m2/g reacting with 1000 ppm of HCl. Chisholm et al.24 reported a total Ca(OH)2 utilization at 120 °C of 5.1%, after reaction with 1000 ppm of HCl. Yan et al.25 obtained an ultimate Ca(OH)2 conversion of 20.1% at 170 °C and of 31.0% at 200 °C, after reaction with 600 ppm of HCl. The asymptotic values of final conversion in dry gas conditions with respect to temperature shown in Table 4 are in fair agreement with results obtained in

Figure 7. XRD analysis of the decomposition of solid residue obtained from the FBR after a run with 2500 ppm initial HCl concentration at 180 °C. Further details are reported in section S7 of the SI.

temperature (°C)

parameter ks (m/s) Ds (m2/s) K (−)

mg was expected, depending on the conversion experienced in the FBR run. The analysis of the weight loss data experienced in the TG runs, based on the stoichiometry of reaction R4 and accounting for the inert material present in the fixed bed confirmed such data. An example is provided in Figure 6, reporting the results of a FBR run performed at 180 °C on a 752


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Figure 8. Training case. Model fitting of breakthrough curves of HCl removal (a) and corresponding Ca(OH)2 conversion (b): mass of sorbent 100 mg, HCl inlet concentration 2500 ppm. model results continuous lines; experimental data (average of 3 runs) dotted lines.

Figure 9. Results of model application to the validation cases. HCl inlet concentration of 2500 ppm of HCl and 50 mg of calcium hydroxide: breakthrough curve (a) and sorbent conversion (b). HCl inlet concentration of 1250 ppm of HCl and 50 mg of calcium hydroxide: breakthrough curve (c) and sorbent conversion (d).

Table 6. Sensitivity Indices for the Main Input Parameters of the Adopted Model (N = 5000) sorbent conversion after 5 min

sorbent conversion after 15 min

sorbent conversion after 30 min

input factor Xi

first-order effect index Si/∑(Si)

total effect index STi/∑(STi)





Ds ks K

78.9% 15.7% 5.4%

91.4% 4.0% 4.6%

65.5% 0.7% 33.8%

66.4% 0.3% 33.3%

45.9% 0.3% 53.8%

46.2% 0.1% 53.7%

only solid product of the heterogeneous reaction. As a first step of model application, the values of ks, the chemical reaction rate constant, Ds, the product layer diffusivity of HCl, and K, the nondimensional numerical coefficient determining the mechanical work associated with nucleation of solid-phase product, were obtained from experimental data fitting. The experimental data set was divided in a training set and a validation set. All the experimental runs carried out at HCl inlet concentration of 2500 ppm and 100 mg of sorbent in the bed (see Table 1) were used as the training set. The other experimental runs (see Table

previous studies for the dry Ca(OH)2/HCl system. This confirms that final conversion is mainly dependent on temperature, while sorbent-to-HCl ratio has only a minor influence. 3.3. Modeling of Experimental Results. For a full understanding of the interplay of kinetic and mass transfer phenomena determining the reaction outcome, the results of FBR runs were interpreted applying the model presented in section 2.4. The input data listed in section S2 of the SI were used for model implementation, considering CaOHCl as the 753


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the model is accurate and succeeds in reproducing the decline in reactivity over time. The model was thus applied to the simulation of the validation data set. As reported in Figure 9, also in the validation stage a satisfactory match between experimental data and model simulations was attained. In particular, the validation case in Figures 9c and d has the same sorbent-to-HCl ratio than the training case, but the inlet HCl concentration is halved. This results in a slower reaction kinetics and in an earlier breakthrough (see also Table 2) that the model is able to capture effectively. This result is of particular value, since previous modeling studies devoted to the Ca(OH)2/HCl system22−24,59 rarely explored the validity of model parameters over different concentrations of HCl and generally required the adoption of concentration-dependent coefficients26 to reproduce the variation of reactivity and breakthrough time obtained for different concentrations of HCl. 3.4. Sensitivity Analysis of Model Output. The model interpretation of experimental data allowed obtaining a set of representative, validated kinetic and mass transfer parameters. A sensitivity analysis was carried out to assess the associated uncertainty. The indicator selected to represent the model output is the sorbent conversion at a given time (after 5, 15, and 30 min). The training case at 180 °C was taken as the reference case. Parameters Ds, ks, and K are the main uncertain input factors whose effect on the model output was investigated. For each variable, the explored uncertainty interval was assumed equal to 20% of the total range of variation that the variable shows in the temperature range 120−180 °C, as reported in Table 5. A total of 5000 combinations resulted appropriate to achieve the convergence of the sensitivity indices, calculated as detailed in section S4 of the SI and reported in Table 6. The results show that the relative importance of the input factors changes as the reaction proceeds. After 5 min of reaction, Ds is the governing variable (SDs = 78.9%), and the influence of the chemical reaction rate constant is still relevant (Sks = 15.7%). After 15 min of reaction, the effect of ks is negligible and the effect of K, linked to the increased energy barrier exerted by the product layer to further nucleation, is increasing (SK = 33.8%). After 30 min of reaction, Ds and K share almost equal relevance in determining the model output. This is in agreement with the role of the product layer in the inhibition of the reaction process, assumed in the model.34 In order to visualize the effect of the main model input factors (Ds and K) on the model output, a set of simulations was carried out setting ks equal to 5 × 10−5 m/s, the value estimated by data fitting for the reaction at 180 °C. Figure 10 shows the surfaces representing sorbent conversion after 5, 15, and 30 min of reaction as a function of Ds and K in the corresponding uncertainty ranges. The sensitivity of sorbent conversion to the variation of input factors increases over reaction time, as expected for a cumulative output value. The plot confirms the interplay of Ds and K in determining the model outcome, with an increasing effect of the latter parameter as reaction proceeds. The results of the sensitivity analysis also show that the summation of the first-order effect indices is close to 1, therefore the effect on sorbent conversion can be expressed as the sum of the single effects. This implies that a low degree of interaction between the three model input parameters is present. Accordingly, the relative values of the first-order and total effects are approximately the same.

Figure 10. Sensitivity of sorbent conversion toward Ds and K, for ks = 5 × 10−5 m/s. (a) Sorbent conversion for the training case at 180 °C, as calculated by the model with the best fitting parameters. Variation of sorbent conversion as a function of variation of Ds and K after 5 (b), 15 (c), and 30 min (d) of reaction. Black dots represent the model output based on best fitting parameters.

1) were used as the validation set. The values of ks, Ds, and K obtained from the fitting of the training set are reported in Table 5. Their trend with temperature is in line with previous literature data. More specifically, the values of Ds and K are within the range of relevant values reported in previous modeling studies of the chloridation reaction in similar conditions. A value of 38 kJ/mol is obtained for the apparent activation energy related to K, in sufficient agreement with the value the 33 kJ/mol obtained by Antonioni et al.34 A more extended comparison of the values obtained for the model parameters to previous literature data is reported in section S8 of the SI. The model fitting of HCl removal and Ca(OH)2 conversion is shown in Figure 8. As expected for a training case, 754


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Energy & Fuels

4. CONCLUSIONS The present study explored the reaction between Ca(OH)2 and HCl in the temperature range of interest for the operation of HCl removal from flue gases originated from the combustion of solid wastes (120−180 °C). In this temperature range, the stable formation of CaOHCl as the solid product of the heterogeneous reaction was observed, as shown by a detailed investigation including TG, TG-FTIR, and XRD analyses. The final sorbent conversion resulted mainly affected by the operating temperature, while the HCl inlet concentration and sorbent-to-HCl ratio only play a minor role. The application of a model for the heterogeneous reaction allowed assessing the temperature dependence of the kinetic and diffusion coefficients, whereas a sensitivity analysis allowed identifying their role in controlling the overall reaction process. While the chemical reaction rate constant has little influence in determining sorbent conversion, the process is mainly controlled by the product layer-related parameters: product layer diffusivity in the initial stage of reaction, and the mechanical resistance of product shell to fracturing, in the later stages. These results highlight that the low final conversion of Ca(OH)2 observed in industrial DSI processes is caused both by the formation of CaOHCl as a stable product, instead of CaCl2 that is formed only at higher temperatures, and by the increasing hindrance to reaction progress exerted by product layer growth. Low sorbent conversion, which negatively affects process economics, in perspective may possibly be improved by design and operating measures aimed at reducing product layer resistance. The comprehension of the reaction mechanism between Ca(OH)2 and HCl in dry gas conditions constitutes the basis for the investigation and modeling of the influence of other flue gas components (moisture, other acid gases) on the Ca(OH)2/HCl system.

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contributions to data analysis; Federica Barontini (University of Pisa, Italy) for performing the TG-FTIR experiment; Prof. Maria C. Bignozzi and Lorenza Carabba (University of Bologna, Italy) for the help with the porosimetry measurements; Prof. Christoph R. Müller and Felix Donat (ETH Zürich, Switzerland) for the help in performing in situ XRD measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03292. Details on experimental apparatus and procedures (section S1), details on modeling (section S2), grain size distribution from porosimetry data (section S3), details on sensitivity analysis (section S4), removal efficiency data (section S5), complementary thermogravimetric analysis of solid residues (section S6), complementary XRD data (section S7), parameters obtained by experimental data fitting (section S8) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +39-051-2090240. Fax: +39-051-2090247. E-mail: [email protected]. ORCID

Valerio Cozzani: 0000-0003-4680-535X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank: Daniele Guglielmi (Termomeccanica Ecologia) for useful discussions and 755


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