Experimental Investigation of the Reactivity of Sodium Bicarbonate

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Kinetics, Catalysis, and Reaction Engineering

Experimental Investigation of the Reactivity of Sodium Bicarbonate towards Hydrogen Chloride and Sulfur Dioxide at Low Temperatures Alessandro Dal Pozzo, Raffaela Moricone, Alessandro Tugnoli, and Valerio Cozzani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00610 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Industrial & Engineering Chemistry Research

Experimental Investigation of the Reactivity of Sodium Bicarbonate Towards Hydrogen Chloride and Sulfur Dioxide at Low Temperatures Alessandro Dal Pozzo, Raffaela Moricone, Alessandro Tugnoli, 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 (*) corresponding author, Tel. +39-051-2090240, Fax +39-051-2090247 e-mail: [email protected]

Abstract The use of sodium bicarbonate (NaHCO3) as a solid reactant for the removal of acid pollutants in industrial flue gas streams is a simple and effective process solution. Nonetheless, despite its technological maturity, the industrial application of NaHCO3-based flue gas treatment is still highly empirical. A better knowledge of the heterogeneous reaction process could allow process optimization, resulting in a reduction both in the consumption of reactants and in the generation of solid waste products. In the present study, the reactivity of NaHCO3 towards HCl and SO2 was investigated in the temperature range between 120 and 300°C. The key role of thermal activation in determining the reactivity of the sorbent was confirmed. The choice of the optimal temperature for acid gas sorption results from a trade-off: higher temperatures increase the reaction kinetics, but induce the sintering of the activated sodium carbonate. The occurrence of sintering is particularly detrimental for high removal efficiency towards SO2, possibly due to the role of the sodium sulfite layer originated by SO2 sorption. As a consequence, the optimal operating temperature resulted 150°C for SO2 and 210°C for HCl. The choice of operating temperature in industrial dry sorbent injection units for acid gas abatement is discussed in view of the present findings.

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1 - Introduction The removal of acid pollutants from flue gases is required in several industrial combustion processes. Hydrogen chloride (HCl) and sulfur dioxide (SO2) are formed in waste incineration and in a number of industrial processes involving combustion (steelmaking, cement production, glassmaking ceramics manufacturing), whenever the fuel feed contains respectively chlorine or sulfur.1-5 Among the different dry, semi-dry and wet techniques available for acid gas removal from flue gas,6 dry sorbent injection (DSI) has seized an increasing market share in recent years.7 DSI consists in the direct introduction in the flue gas ductwork of a pulverized basic sorbent, which reacts with acid pollutants via a heterogeneous reaction process based on an acid-base neutralization reaction. The usual choice for the DSI sorbent in industrial practice is between calcium-based and sodium-based materials. Sodium-based sorbents, mainly represented by sodium bicarbonate (IUPAC name: sodium hydrogen carbonate, NaHCO3), are valued for their high reactivity towards acid pollutants and the possibility to recycle their reaction residues.8 When introduced in the flue gas at a temperature higher than 100°C,9 sodium bicarbonate undergoes thermal decomposition to sodium carbonate: 2⋅NaHCO3 (s) = Na2CO3 (s) + CO2 (g) + H2O (g)

(R1)

The nascent sodium carbonate reacts with the acid pollutants to form sodium chloride and sodium sulfite according to the following reactions: Na2CO3 (s) + 2⋅HCl (g) = 2⋅NaCl (s) + CO2 (g) + H2O (g) Na2CO3 (s) + SO2 (g) = Na2SO3 (s) + CO2 (g)

(R2) (R3)

Then, in the presence of oxygen, sodium sulfite can turn to sodium sulfate:10 Na2SO3 (s) + ½⋅O2 (g) = Na2SO4 (s)

(R4)

In industrial applications, DSI is performed injecting the sorbent upstream of a particulate collection device (in general nowadays a fabric filter is used), as shown in Fig. 1. Therefore, the acid pollutants in the flue gas can react with the sorbent both during the entrained flow in the flue gas pipe after the injection point, and on the cake formed on the bags of the fabric filter.

Figure 1. Typical layout of a NaHCO3-based dry sorbent injection system.


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Despite the commercial success of sodium bicarbonate injection as a simple and effective acid gas removal solution, open issues remain concerning the reaction process. While NaHCO3-based treatment systems always guarantee a high efficiency in the control of acid gas emissions, operational experience across European waste-to-energy plants shows that the same acid gas removal performance is obtained with NaHCO3 feed rates ranging from 1.1 to 1.4 times the stoichiometric feed rate.11 This wide variability may be ascribed to a limited understanding of the underlying chemistry of sodium-based acid gas sorption. In particular, scarce information is available on the limits of the operating temperature range in which NaHCO3-based DSI is effective. This is a relevant issue in industrial applications, since specific design constraints (e.g. the need to perform mercury removal with activated carbon in the same DSI unit) might require the system to operate at slightly different temperatures than those recommended by the sorbent supplier, or the inherently fluctuating conditions of the process (e.g. due to fouling of the upstream ducts) demand some flexibility in terms of operating temperature. Limited attention to date was dedicated in the literature to investigate the heterogeneous reaction process of sodium bicarbonate with acid gases, at least compared to the vast literature on acid gas removal with calcium-based sorbents at low, medium and high temperature. Bares et al.12 were probably among the first researchers to investigate the reaction between sodium carbonate and SO2, by analyzing the sorption of 0.5-1 vol% SO2 on a fixed bed of Na2CO3 at 150°C, reporting an influence of SO2 concentration on reaction rate. Keener and Davis13 explored the effect of temperature on the reaction between NaHCO3 and SO2 (2.45 vol%), finding a reverse temperature relationship between 150 and 400 °C. Conversely, Kimura and Smith14 focused on the reaction between NaHCO3 and SO2 (0.25-5 vol%) in the temperature range 80-140 °C, showing a positive effect of temperature. With respect to reactivity with HCl, a pioneering study by Mocek et al.15 documented that Na2CO3 from reaction R1 has a reactivity towards HCl (1000 ppm) at 150 °C that is orders of magnitude higher than that of non-thermally activated Na2CO3. Fellows and Pilat16 investigated the sorption of HCl (415 or 760 ppm) in a broad temperature range (100-280 °C). They found a positive effect of temperature on the reactivity of activated Na2CO3, while particle size and concentration of the gaseous reactant had minimal influence on the sorption process. Verdone and De Filippis17 extended the study of HCl sorption at higher HCl inlet concentrations (3000-6000 ppm), identifying 400 °C as the optimal reaction temperature for achieving maximum sorbent conversion. Duo et al.18 and, more recently, Hartman et al.19 explored the kinetics of the reaction between thermally decomposed Na2CO3 and HCl at even higher temperatures (respectively, 300-600 °C with 900 ppm HCl and 500 °C with 100-700 ppm HCl), demonstrating the possibility to achieve deep removal of HCl with Nabased sorbents even at elevated temperature. Lastly, Ren and coworkers examined the performance of Na2CO3 in the abatement of HCl emissions generated from biomass combustion20 or torrefaction, 21 both in entrained flow and fixed bed configurations, showing HCl removal efficiencies higher than 50% in the range 300-1100 °C for a molar ratio of Na to Cl in the biomass around 5. In the present study, an experimental investigation of the reaction between activated sodium carbonate and acid gases at low concentrations of HCl and SO2 was carried out in the temperature range of interest for waste-to-energy flue gas treatment applications (120-300 °C).7,22 Although in most applications HCl and SO2 are simultaneously present in flue gas, the reaction process was considered separately for the two species, in order to obtain fundamental data needed to approach the analysis of the actual simultaneous reaction process. Experimental runs were carried out in a tubular fixed bed reactor, with the aim of simulating the conditions of a cake of sorbent on the bags of a fabric filter. Goal of the study was to clarify the temperature dependence of the reactivity of activated sodium carbonate in the range of interest, overcoming the fragmented and in part contradictory evidences present in literature. The results obtained from the laboratory-scale system allowed the investigation of the role of thermal activation and sintering on the reactivity and overall 3 ACS Paragon Plus Environment


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sorbent conversion in HCl and SO2 heterogeneous reactions with thermally activated sodium carbonate. Implications for the operation of NaHCO3-based DSI systems are eventually discussed. 2 – Experimental section 2.1 – Materials Gas-chromatographic standard gas mixtures respectively 3% SO2 in nitrogen and 3% HCl in nitrogen, provided by SIAD (Italy), were used to supply the gaseous reactants. Sodium bicarbonate (reagent grade, VWR), sieved to the range of 63−120 μm, was used as solid reactant. A specific surface area of 0.8 m2/g was determined by nitrogen adsorption (Flow Sorb II 2300, Micromeritics, USA). Commercial sodium carbonate (reagent grade, VWR), surface area 0.7 m2/g, was used as a benchmark to compare the reactivity of sodium carbonate obtained by the thermal decomposition of sodium bicarbonate in experimental runs. Quartz sand sieved to the range of 63−120 μm as the sorbent was used as inert filling material for the fixed bed. 2.2 – Fixed bed reactor: set-up and protocol The reaction between sodium bicarbonate and the acid gases was investigated in a laboratory-scale fixed bed reactor, sketched in Fig. 2. Full details on the experimental apparatus are reported elsewhere.23

Figure 2. Layout of the experimental apparatus for the fixed bed reactor (FBR) runs. The fixed bed reactor was loaded with 50 mg of sodium bicarbonate, diluted in 300 mg of quartz sand of the same size range to avoid gas channeling (bed thickness: ~ 2 mm). The reactor was positioned in a heating chamber with forced convection and the reactor temperature was monitored by a type K thermocouple. At the beginning of an experimental run, a purge flow of pure nitrogen was sent to the fixed bed. The reactor temperature was raised to the desired value (120, 150, 180, 210 or 300 °C) and was allowed to stabilize for 40 min. The acid gas was then sent to the fixed bed, modifying the position of the bypass valve in Figure 1. A flowrate of 10 NL/h of HCl or SO2 in nitrogen, with concentrations of 1000 or 2500 ppm, was fed to the tubular reactor. The desired acid gas concentration was obtained mixing the 3% gaschromatographic mixture with pure nitrogen using a mass flowmeter. 4 ACS Paragon Plus Environment

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The acid gas flowed on the sample until a complete breakthrough was observed (90 min or longer of reaction time, depending on the run). The gas stream leaving the reactor was continuously analyzed by means of a Fourier transform infrared (FTIR) spectrometer (TENSOR 27, Bruker, USA) equipped with a gas cell, allowing the collection of cascading IR spectra of gas composition. The FTIR spectra were recorded at a resolution of 4 cm-1. Each spectrum was obtained as average of 16 consecutive scans. This resulted in a time resolution of 3.7 s. The FTIR spectra were recorded and elaborated using the Bruker OPUS/IR software. The concentration of HCl and SO2 in the outlet gas as a function of time was followed by monitoring the time evolution of the integrated absorbance on characteristic wavenumber ranges. The selected ranges were 3150-2500 cm-1 for HCl and 1425-1280 cm-1 for SO2. The profiles of CO2 and H2O generated during the reaction were also followed, by considering respectively the intervals 24002200 cm-1 and 4000-3700 cm-1. Each fixed bed run was repeated 3 times and the average of measurements is reported. 2.3 – Characterization of samples A thermogravimetric analyzer (TGA-Q500, TA Instruments-Water, USA) was used to study the isothermal decomposition of sodium bicarbonate. The thermogravimetric (TG) analysis was carried out on samples of about 20 mg, positioned on a platinum pan. Isothermal decomposition was performed in pure nitrogen flow (60 mL/min) by jumping to the desired temperature at maximum heat rate and then maintaining the sample at the temperature for 90 min. Temperature equilibrium was achieved within 5 min after the start of the jump. The crystalline species present in the samples before and after reaction were determined using Xray powder diffraction (XRD). The XRD analysis was performed by means of a diffractometer Empyrean (PANalytical, Netherlands) operated at 45 kV and 40 mA using CuKα radiation (λ = 1.5418 nm). Each sample was scanned in the 2θ range of 20−80°. The step size was 0.016°, and the scan time per step was 0.68 s. The morphological features of the samples were visualized via scanning electron microscopy (Ultra 55 Plus, Zeiss, Germany). Prior to imaging, the samples, placed on a conductive carbon tape, were sputter-coated by gold and palladium for 90s under Ar plasma to improve their conductivity.



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3 – Results and Discussion 3.1 – Thermal activation and effect on sorbent reactivity The first step of the study consisted in the analysis of the thermal activation process. The thermal activation of sodium bicarbonate, i.e. its decomposition to sodium carbonate according to reaction R1, was analyzed by isothermal TG runs in pure nitrogen in the temperature range between 120 and 210 °C (Fig. 3). Appreciable weight loss can be already observed at 120 and 150°C. The results show that the complete conversion to sodium carbonate (corresponding to a sample weight loss of 63% according to the stoichiometry of reaction R1) requires 90 and 25 min, respectively. The reaction occurs at a considerably faster rate at 180 and 210 °C, with complete conversion obtained after 3 and 1 min, respectively.

Figure 3. Results of isothermal TG runs on NaHCO3 samples (pure nitrogen, 60 mL/min). The most important outcome of thermal activation is its effect on sorbent morphology. As shown in the micrographs reported in Fig. 4, sodium bicarbonate (panel a) is a non-porous solid. During the decomposition reaction, the release of ca. 37% of the mass of the pristine sodium bicarbonate in the form of H2O and CO2 causes the formation of a porous structure in the nascent sodium carbonate. This porous structure is clearly visible in panels b and c, respectively showing the sample obtained after the 90 min TG run at 150 and 180 °C. In contrast, the sample obtained from a TG run carried out at 300 °C, shown in panel d, presents a much less open morphology, due to the occurrence of sintering of the porous grain of sodium carbonate formed by thermal activation. As already noticed by Hartman et al.,9 significant sintering in activated sodium carbonate takes place at temperatures remarkably lower than the Tammann temperature of the compound (~ 425 °C, i.e. half its melting point)24, which is typically considered as a proxy of the onset temperature of lattice mobility.25 Fig. 4 also reports the XRD patterns of the samples. Regardless of the activation temperature (in this case, the temperature of the isothermal TG run), the chemical species originated from the decomposition of sodium bicarbonate is sodium carbonate. The only noticeable difference between the samples activated at different temperatures, which underwent the same storage conditions in the time between their preparation and the XRD analysis, is a higher intensity of the peaks associated to the hydrated form of sodium carbonate for the samples obtained at a lower activation temperature. This is again indicative of the higher surface area and, hence, of a higher adsorption capacity of sodium carbonate activated at lower temperature. 6 ACS Paragon Plus Environment

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Figure 4. SEM micrographs and XRD pattern of a) reagent-grade NaHCO3, and Na2CO3 from NaHCO3 activation at: b) 150 °C, c) 180 °C, d) 300 °C.



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To highlight the role of thermal activation in enhancing the reactivity of the sodium-based sorbent towards acid gases, a dedicated fixed bed experimental run in ramp of temperature was carried out. A sample of 50 mg of NaHCO3 was exposed to a flow rate of 1000 ppm SO2 in N2, ramping temperature in the heating chamber from ambient temperature to 180°C (the profile of temperature increase with time was followed by the type K thermocouple). Fig. 5 reports the concentration of SO2 and H2O in the offgas leaving the reactor, as detected by FTIR. Thermal activation appears crucial in triggering fast desulfurization of the gas stream. At temperatures lower than 130 °C, no variation in the SO2 concentration of the gas stream is observed. As soon as thermal decomposition starts (evidenced by the presence of water vapor in the reaction stream, produced by release of water vapor according to reaction R1), the concentration of SO2 in the gas leaving the sorbent bed falls to less than 10% of the inlet value. Thus, the onset of significant SO2 sorption coincides with the onset of thermal activation. The promoting effect of the thermal activation is related to the increase of the porosity and surface area of the sorbent caused by the release of H2O and CO2, and not to the chemical nature of the product of activation, Na2CO3. This was confirmed comparing the reactivity of Na2CO3 obtained from NaHCO3 thermal activation versus reagent-grade Na2CO3 in isothermal runs carried out at 150 °C feeding a 1000 ppm SO2 mixture to the reaction bed (Fig. 6). As visible in the SEM micrographs, reagent-grade Na2CO3 is basically non-porous, while Na2CO3 from NaHCO3 decomposition, as also shown in Fig. 4, developed an extensive pore structure during activation. The BET surface area measured for Na2CO3 from NaHCO3 thermal decomposition at 180 °C was 3.8 m2/g, compared to the 0.7 m2/g of reagent-grade Na2CO3.An 80% SO2 breakthrough through the bed takes place in less than 1 min for reagent-grade Na2CO3, while the same value is recorded only after 13.5 min for thermally activated Na2CO3, hence demonstrating the key role of the porous structure developed during activation in enhancing acid gas sorption by sodium carbonate.

Figure 5. Reaction between SO2 (1000 ppm) and NaHCO3 (50 mg) in the fixed bed reactor under a temperature ramp. The normalized outlet concentrations (actual value divided by the maximum value recorded during the experimental run) of SO2 and H2O is shown alongside the thermal profile.


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Figure 6. Comparison of SO2 breakthrough curves for reaction at 150 °C with reagent-grade Na2CO3 and Na2CO3 from NaHCO3 activation. SEM micrographs show the sorbent samples in the insets.

3.2 – Reaction with HCl and SO2 at different temperatures The reactivity of activated sodium carbonate towards HCl and SO2 was tested in isothermal sorption runs. All the runs were carried out heating the sorbent to the desired reaction temperature in pure nitrogen and providing an additional activation time of 40 min before starting the reaction. The duration of this stabilization time was chosen to facilitate the thermal decomposition of NaHCO3 to Na2CO3, while avoiding the exposure of the sample to pre-reaction sintering for an unrealistically high time. The results reported in Fig. 3 for the isothermal runs at 120 °C, in contrast to the other temperatures tested, show that complete decomposition of NaHCO3 is not obtained within this time interval. Therefore, control tests at 120 °C were run using a longer stabilization time (2 h), showing little changes in the HCl and SO2 sorption capacity of the sample, within the range of variability of the 3 runs at 120 °C conducted with a stabilization time of 40 min. Fig. 7a shows the breakthrough curves obtained for HCl for runs carried out at different temperatures with a gaseous mixture having an inlet HCl concentration of 2500 ppm. Fig.7b shows the corresponding results obtained with a gas mixture having an inlet SO2 concentration of 1000 ppm. These concentration values were selected as representative of the HCl and SO2 content in flue gas generated by municipal or hazardous waste incinerators.6,26,27 In the case of HCl, the reactivity of activated Na2CO3 increases with temperature in the range 120210°C, which is a typical operating range for DSI systems in the waste to energy (WtE) sector. As shown in the inset of Fig. 7a, t50, the time at which there is 50% HCl breakthrough, and t80, the time at which there is 80% HCl breakthrough, shift from 1.5 to 20.7 min and from 3.0 to 48.9 min, respectively, when temperature changes from 120 to 210°C. Conversely, at 300 °C the sorption capacity of activated Na2CO3 markedly decreases, with t50 occurring after 3.3 min of reaction. In light of the considerations of section 3.1, this abrupt decline of reactivity can be attributed to the loss of surface area in the activated sorbent at 300 °C compared to that at lower temperatures (see again the micrographs of Fig. 4). In the case of SO2, the reactivity of activated Na2CO3 shows a maximum at 150 °C, for which t50 is 21.8 min. At temperatures higher than 150°C, the breakthrough of SO2 occurs noticeably earlier, with t50 ranging from 6.0 min at 180 °C to 1.4 min at 300 °C. The breakthrough curves obtained can be used to calculate the cumulated conversion of the solid reactant Xs, according to the relationship: 9 ACS Paragon Plus Environment


𝜒𝑠 =

𝑏 𝑛𝑠

∫

𝑡

(

𝑉 ∙ 𝐶𝑖𝑛 1 ― 0

𝐶𝑜𝑢𝑡 𝐶𝑖𝑛

)

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(1)

𝑑𝑡

where b is the stoichiometric coefficient of the sorbent (1 for HCl sorption, considering the occurrence of R1 and R2; 2 for SO2 sorption, considering the occurrence of R1 and R3), ns the moles of NaHCO3 initially present in the reactor, 𝑉 the flow rate of the gas stream, Cin and Cout respectively the inlet and outlet molar concentration of the acid pollutant in the gas stream. Fig. 7c reports the calculated sorbent conversion after 3 hours. Even if in all experimental conditions after 3 h the solid does not show noticeable reactivity towards the acid gas (complete breakthrough conditions), the actual solid conversions are far from complete, and, in particular, for SO2, only at 150 °C a sorbent conversion higher than 40% is observed.


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Figure 7. Reaction of activated Na2CO3 with acid gases. a) Normalized HCl outlet concentration at different reaction temperatures (CHCl,in = 2500 ppm, mass of sorbent = 50 mg). b) Normalized SO2 outlet concentration at different reaction temperatures (CSO2,in = 1000 ppm, mass of sorbent = 50 mg). Insets: times at which 50% and 80% breakthrough are reached for the different temperatures. c) Final sorbent conversion after 3 h of reaction with HCl or SO2 at different temperatures.



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3.3 – Investigation of the different reactivity of NaHCO3 as a function of temperature Further tests were conducted to cast light on the dependency of HCl and SO2 removal by activated Na2CO3 on temperature, i.e. on the presence of a maximum of sorbent conversion in the temperature interval 120-300 °C. It is well known that in this range there are no thermodynamic limitations to acid gas abatement: from a thermodynamic point of view, reactions R2 and R3 should produce the almost complete removal of HCl and SO2.28 Hence, the explanation for the different behavior of the sodiumbased sorbent at different temperatures should stem from kinetic and mass transfer considerations in the heterogeneous system of interest. The low conversions calculated at the higher temperatures might suggest that physical adsorption is actually taking place instead of chemical reaction. The investigation of the reaction via FTIR spectrometry allows clarifying this aspect by collecting spectra of the gaseous species leaving the reactor. With reference to the reaction with SO2, Fig. 8a shows that CO2 released by reaction is detected at all the temperatures tested, in an amount that is closely related to the solid conversion shown in Fig. 7c, confirming the occurrence of reaction R3. For example, the SO2 sorption capacity of activated Na2CO3 at 210 °C is equal to the 84% of that at 180 °C (see Fig. 7c) and the amount of CO2 released by the reacting sorbent at 210 °C is equal to the 79% of that at 180 °C (area underlying the curves in Fig. 8a). In addition, the composition of the solid samples after the reaction with SO2, probed by XRD, also confirms the occurrence of reaction R3. As shown in Fig. 8b, the XRD spectra of sorbent samples after the reaction with SO2 at 150 °C and at 180 °C demonstrate that SO2 sorption at both temperatures generates sodium sulfite. The sample reacted at 180 °C (conversion equal to 9%, see Fig. 7c) exhibits an XRD pattern equally populated by Na2CO3 and Na2SO3 peaks, while for the sample reacted at 150°C (conversion equal to 44%, see Fig. 7c), Na2SO3 peaks are clearly dominant, due to the higher presence of the reaction product in the external layers of sorbent particles. Thus, only a weak scattering from Na2CO3 is present in this case. Therefore, both the analyses of evolved gas species and resulting solid phases clarify that the sorption of acid gases is a chemical process across the entire range of temperatures tested.

Figure 8. Reaction products detected for SO2 capture: a) normalized CO2 outlet concentration during SO2 sorption at different temperatures. b) XRD patterns of: activated sodium carbonate before reaction (1), and after reaction with 1000 ppm SO2 at 150°C (2) and 180°C (3). Peaks identifying Na2CO3 and Na2SO3 are highlighted. Normalization by highest intensity peak.


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Earlier investigations attributed the decrease of the reaction rate in the temperature range 150-250 °C to the oxidation of the primary reaction product, sodium sulfite, to sodium sulfate,29 which may block the active sites for sulfation. However, this is certainly not the case in the experiments of Fig. 7b, which were carried out with nitrogen as carrier gas, thus not allowing reaction R4 to occur. In order to observe the effect of oxygen addition in the experimental set-up of the present study, Fig. 9a compares the SO2 removal obtained at 150 °C and 180 °C using air as carrier gas, instead of nitrogen. The presence of oxygen is clearly shown to slow down the overall reaction rate of SO2. This is likely due to the additional diffusional resistance in the transport of SO2 to the reaction interface provided by the formation of Na2SO4.30 Owing to the occurrence of reaction R4, as mentioned before, the diffusion of oxygen towards the layer of solid products competes with the diffusion of SO2 towards the reaction interface. In addition, Na2SO4 has a higher molar volume than Na2SO3. As such, it tends to form a rather compact barrier layer, impervious to diffusion.31 Nonetheless, as shown in the inset of Fig. 9a, the final sorbent conversion appears unaffected by the presence of oxygen and, therefore, it can be inferred that the presence of oxygen slows down but does not limit solid conversion. Fig. 9b shows the effect of a different particle size of sorbent on SO2 removal. The use of sodium bicarbonate of a coarser size class (125-250 μm) does not influence significantly neither reaction kinetics nor final conversion. Similarly, Fig. 9c shows the effect on SO2 removal of a higher SO2 inlet concentration (2500 ppm). Clearly enough, breakthrough takes place earlier than for a SO2 inlet concentration of 1000 ppm, since the amount of sorbent in the bed was unchanged, but the final sorbent conversion and its dependence on temperature are not affected by this variation. Excluding effects of particle size and concentration of gaseous species, the declining reactivity of activated Na2CO3 with reaction temperature has to be ascribed to morphological changes in the sorbent. While temperatures higher than 120 °C are required for fast thermal activation, any excess of temperature becomes detrimental for reactivity once the sintering of the solid reactant starts occurring. Fig. 4 in section 3.1 already evidenced that the porous structure developed by thermal activation at 300 °C is reduced compared to that observed at 150-180 °C. In the range 150-300 °C, the porosity of the activated Na2CO3 decreases when increasing the activation temperature. The loss of available surface area for reaction has an evident effect on the acid gas removal performance. This is in agreement with previous observations by Güldür et al.,32 which documented that Na2CO3 originated from the thermal activation of trona, a Na-containing mineral, exhibits a decreasing surface area and a decreasing sorption capacity towards SO2 with increasing activation temperature. The decrease of surface area and, thus, of the reactivity of the sorbent might be also accentuated by the nature of the reaction product formed by acid gas sorption, hence explaining the different behavior of SO2 and HCl. While sodium chloride does not melt up to 801 °C,33,34 sodium sulfite has a melting point of 500 °C35,36 and, according to the Tammann’s rule,25 it should be subject to sintering in the temperature interval tested in the present study. The sintering of the product layer of sodium sulfite could then increase the diffusional resistance of the gaseous reactant and induce/worsen the sintering of the Na2CO3 reactive interface, producing the observed adverse effect of temperature on sorbent conversion. 3.4 – Relevance to the operation of NaHCO3-based DSI systems The present results evidenced that the temperature range at which sodium bicarbonate injection is highly effective is narrow, comprised between the lower limit of thermal decomposition to sodium carbonate and an upper limit deriving from the sintering of the newly-formed sodium carbonate. Operating far from the optimal temperature implies an increasing need of excess sorbent. Considering the results of Fig. 7c on the final sorbent conversion at different temperatures, it can be estimated that the removal of 1 kg of HCl would require 2.8 kg of sodium bicarbonate at 210 °C, 3.7 13 ACS Paragon Plus Environment


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kg at 180 °C, and 4.9 kg at 150 °C. Hence, assuming that the sorbent deposited on the filter bags of a DSI system behaves similarly to the sorbent deposited on the fixed bed reactor, operating a DSI system at 150 °C instead of 210 °C would imply a 75% extra consumption of reactant to achieve the same removal performance. For SO2 sorption, even more so, the reaction at 180 °C would require 1.8 times the amount of sodium bicarbonate needed at 150 °C. Given that the optimal reaction temperature is different for HCl and for SO2, the choice of the operating temperature in an actual DSI system should take into due consideration the expected HCl/SO2 ratio in the raw flue gas. Pursuing the maximum utilization of the sorbent is a key aspect in the optimization of flue gas cleaning operation. Feeding excess sodium bicarbonate to the DSI system not only results in a higher reactant cost per unit of acid gas removed, but also causes an increase in the generation of process residues, whose disposal is a significant environmental drawback of dry acid gas removal systems.37,38 Nonetheless, in some situations, the acid gas removal unit in a flue gas treatment line might be required to deviate from the optimal operating temperature, due to design constraints: e.g. if a flue gas treatment line using selective catalytic reduction (SCR) for the abatement of nitrogen oxides (NOX) is considered, a typical SCR unit requires an inlet temperature of at least 200 °C and the catalyst is subject to poisoning by the SO2 content in the flue gas.39 Therefore, the acid gas removal unit has to be upstream of the SCR to reduce the concentration of SO2 in the flue gas. In this case, the quantitative information on the relationship between reaction temperature and sorbent performance obtained in the present study may contribute to assess whether it is more cost-effective to operate the unit at an optimal temperature in the range 150-180 °C and then reheat the flue gas before the inlet of the SCR unit, or to inject sodium bicarbonate at temperatures higher than 200 °C, accepting a lower utilization of the sorbent but avoiding flue gas reheat. In most cases, a further constraint to the operating temperature of DSI systems is given by mercury (Hg) removal, which is typically performed by means of activated carbon, injected in DSI together with the acid gas sorbent. Both activated carbon and novel alternative mineral-based adsorbent materials ensure acceptable Hg removal efficiencies at temperatures lower than 190 °C.40,41 This constraint does not penalize the NaHCO3-based abatement of acid gases: as already discussed, on the basis of the present results, if both HCl and SO2 are present in the gas stream, the optimal operating temperature would be in the range between 150-180 °C.


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Figure 9. Reaction of activated Na2CO3 with SO2 in different conditions. a) Normalized SO2 outlet concentration at 150 and 180 °C after reaction in different carrier gas: nitrogen or air (CSO2,in = 1000 ppm, mass of sorbent = 50 mg). b) Normalized SO2 outlet concentration at 150 and 180 °C after reaction with fine-sized (63-125 μm) or coarse-sized (125-250 μm) Na2CO3 (CSO2,in = 1000 ppm, mass of sorbent = 50 mg). c) Normalized SO2 outlet concentration at 150 and 180 °C for an inlet concentration of 1000 or 2500 ppm SO2 (mass of sorbent = 50 mg). Insets: final sorbent conversion after 3 h of reaction in the different cases. 15 ACS Paragon Plus Environment


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4 – Conclusions The present study explored the reaction between activated sodium carbonate from sodium bicarbonate decomposition and the two main acid pollutants found in industrial flue gases (HCl and SO2) in a temperature range of practical interest (120-300 °C) for WtE flue gas treatment. Thermal decomposition is necessary to form the porous structure in activated sodium carbonate that promotes sorbent reactivity. Yet, significant sintering of nascent sodium carbonate is detected even at temperatures markedly lower than the Tammann temperature of the material. Sintering reduces the sorption capacity of activated Na2CO3 towards acid pollutants. In particular, while for the reaction with HCl the maximum sorbent conversion was observed at 210 °C, for SO2 the reactivity of activated Na2CO3 is reduced significantly even between 150 and 180°C (a 65% decrease of final sorbent conversion was observed in this temperature range). From the viewpoint of process optimization, this information is useful for the identification of optimal operating conditions in flue gas treatment systems. From the viewpoint of sorbent optimization, the inherent limitations of natural sodium bicarbonate emerged in this study suggest the potential for synthetic approaches. Sorbent modification methods aimed at improving the resistance of activated Na2CO3 to sintering could be envisaged to harness the benefits of improved kinetics at higher temperatures, while avoiding the insurgence of adverse morphological changes.

Acknowledgments Prof. C.R. Müller and the FIRST Center for Micro- and Nanoscience at ETH Zürich (Switzerland) are acknowledged for kindly providing access to the instruments for XRD and SEM measurements. A. Armutlulu and M. Rekhtina (ETH Zürich) are gratefully acknowledged for the support in XRD and SEM measurements.


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Experimental Investigation of the Reactivity of Sodium Bicarbonate

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