Calcination Thermokinetics of Carbon Capture Using Coal Fly Ash

Feb 15, 2018 - (5) The reported activation energies of limestone were found to be proportional to CaO content derived from cockle shell sorbent in the...
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Calcination Thermokinetics of Carbon Capture Using Coal Fly Ash Stabilized Sorbent Bolisetty Sreenivasulu,† Inkollu Sreedhar,*,† A. Satyapaul Singh,† and Akula Venugopal‡ †

Department of Chemical Engineering, BITS−Pilani, Hyderabad Campus, Shameerpet, Hyderabad-78, India I&PC Division, IICT, Tarnaka, Hyderabad-500007, India

‡

ABSTRACT: There exists an ever increasing demand for coal fly ash stabilized sorbents which are thermally stable up to multiple carbon capture cycles at high temperatures (up to 950 °C). The industrial scale-up of this sorbent’s regeneration is limited by its inherent kinetics and thermodynamics that need to be maneuvered carefully to make it economically viable. In this study, sorbents regeneration kinetic and thermodynamic parameters have been estimated under nonisothermal conditions using thermogravimetric analysis. The Coats and Redfern equation has been chosen to estimate thermokinetic parameters under nonisothermal conditions. The thermodynamic values of Gibbs free energy, entropy, enthalpy, etc., for the synthesized sorbent (CaO50MgO10FA-C40) were compared with the simulated values (HSC software) with and without fly ash. The experimental data of sorbent regeneration was then fit into a suitable kinetic equation, based on the classic grain model and studied at various isothermal calcination temperatures. Sorbent calcination conversion efficiencies with model fit have been studied in packed-bed reactor under CO2 partial pressures ranging from 0 to 100 kPa and at various calcination cycles of the sorbent. A table of comparison has been presented on sorbent regeneration activation energies of reported and experimentally estimated values in this work.

1. INTRODUCTION Thermokinetic decomposition of mineral sorbents and their thermal properties have gained industrial importance in the calcination process. It is not possible to have faultless scale-up for a large batch of materials at the industrial level without kinetic and thermodynamic data availability.1,2 Sorbents based on CaO for high temperature CC capacities and kinetics of carbonation have been widely studied.3 However, less attention was given to decarbonation or regeneration kinetics of energy intensive CaO sorbent stabilized with coal fly ash (CFA) than the widely studied exothermic carbonation kinetics.2,4,5 Each kinetic datum depends on sorbent material and the contactor configuration that would have an optimized set of process and design parameters.6 The key objective was to have a suitable equilibrium model under which the sorbent properties need to be estimated. For further kinetic estimations, the carbonation− regeneration cycle conditions were needed. Industrial implementation of this technology needs to address cyclic operation of the chemical looping process (CLP) in a continuous chemical reactor operation. Decomposition of a bulky sample is inhibited in the packed-bed due to local high partial pressures of CO2 in the pores or in the interstices of particle bed. The rate of regeneration could be measured under different process conditions with known CO2 concentrations. A decrease in calcination reaction rate was observed with an increase in CO2 partial pressure or total pressure of the system.5 The reported activation energies of limestone were found to be proportional to CaO content derived from cockle shell sorbent in the particle size range of 0.125−4 mm during decomposition or regeneration.7 For magnesite (MgCO3), regeneration activation energies (61−72.2 kcal/mol at 560−680 °C) were found to be in proportion to their particle sizes.8 The aim of the packed-bed reactor design should be to achieve the lowest possible © XXXX American Chemical Society

calcination temperature for CaCO3 so as to have minimum heat requirement during calcination, with sorbent deactivation tending to rise the temperature.9 Increased CC capacity of sorbent has been reported with higher CO2 partial pressures in feed gas mixture.9−13 In our previous studies, the sorbent screening from more than 20 potential candidates has been done at two levels, and the sorbent CaO50MgO10FA-C40 was then selected as the best sorbent in powder form14 that gave the highest capture capacity of more than 11 mmol/gads at the optimal conditions of temperature, time, gas flow rate, sorbent nature, morphology, and amount.14 The positive role played by CFA along with active sorbents of CaO and MgO (CaO50MgO10FA-C40) in enhancing the thermodynamic feasibility and carbon capture rate and capacity was reported by us in our earlier work.15 In this work, calcination kinetic studies were conducted using a synthesized sorbent (CaO50MgO10FA-C40) system in both isothermal and nonisothermal conditions that was found to be the best performing sorbent system from our earlier studies.14 Sorbent calcination studies have been conducted using thermogravimetric analysis (TGA) under nonisothermal conditions for the estimation of thermokinetic parameters. Apparent thermodynamic parameters were estimated using transition state theory (Eyring equation) from the kinetic parameters estimated using the Coats and Redfern equation. Thermodynamic feasibility and spontaneity of the calcination process were verified by estimating ΔG (Gibbs free energy) values which were then compared with those using HSC software for sorbents with and without stabilization with fly ash. Received: December 29, 2017 Revised: February 14, 2018 Published: February 15, 2018 A

DOI: 10.1021/acs.energyfuels.7b04147 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Flow diagram of experimental setup.

Effect of various partial pressures of CO2 from 0.1 to 1 atm pressure has been studied for sorbent regeneration using packed-bed reactor. Experimental data has been validated with suitable model by curve fitting method. The kinetic parameters estimated by us for this sorbent in packed-bed reactor have been compared with the reported sorbents.

− ra =

ΔM dα = = k(1 − α)n Δt dt

(1)

where k is the rate constant, n is the order of reaction, t is the time, and α = ΔM/ΔM∞·ΔM is the loss of mass of the sorbent due to regeneration at time t; ΔM∞ is the loss of mass of all the sorbent assumed to have completely regenerated.The values of α range between 0 and 1 in every cycle. Equation 1 can be written with sorbent decomposition (nonisothermal) heating rate q = dT/dt and rate constant k = k0 exp(−Ea/ RT) as follows.

2. EXPERIMENTAL METHOD AND PROCEDURE 2.1. Experimental Setup and Procedure. Figure 1 shows the packed-bed reactor (PBR) setup that consists of quartz tube of 25.4 mm diameter with 1.5 mm wall thickness. Quartz tube with a length of 55 cm and flange diameter of 48 mm with flange thickness of 4 mm has been used. Simulated N2 and CO2 gas mixture with composition regulated by mass flow controllers was used for CC studies. Silica-gel paste coated onto graphite gasket wrapped with Teflon tape has been used across the flange joints while fixing the quartz tube to avoid gas leakage. The other PBR setup conditions and sorbent synthesis protocol have been discussed in our earlier work along with thermogravimetric analysis (TGA) details.14,15 2.2. Chemicals and Analytical Tools Used. The sorbent CaO50MgO10FA-C40 has been synthesized using active sorbents of CaO (50 wt %), MgO (10 wt %) with stabilizing agent of FA-C (coal fly ash, CFA-C type; 40 wt %). These were blended and mixed with polymer surfactant of poly(ethylene glycol) (PEG-400) using mortar and pestle. The sorbent was sintered in muffle furnace at temperatures of 350 and 550 °C for 2 h each. Later, the temperature was elevated to 1050 °C for 4 h. CaO, MgO, and PEG-400 chemicals were procured from SD Fine-Chem Ltd., Mumbai, India. Shimadzu TG-DTA60 was used for thermogravimetric differential thermal analysis (TG- DTA). All the chemicals were of LR (Laboratory Reagent) grade with purity of >99.7%. Coal Fly Ash-C type (CFA-C) was procured from National Thermal Power Corporation (NTPC), Ramagundam, India. 2.3. Theoretical Background of Thermokinetic Parameter Estimations. 2.3.1. Nonisothermal. The Coats−Redfern method has been used (nonisothermal conditions) to estimate kinetic parameters during regeneration of metal carbonates (CaCO3, MgCO3, and dolomite) employing TG-DTA values.8,16 The kinetic parameters k0 and Ea were obtained from nonisothermal sorbent decomposition. Tp, the peak temperature, could be obtained from the characteristics of the highest rate of sorbent decomposition of DTA curve. These parameters (k0, Ea, and Tp) were then used in the Eyring equation (transition state theory) to estimate thermodynamic parameters of inorganic (metal compounds) sorbent decomposition.17 This methodology was earlier employed for the decomposition of the inorganic sorbents, viz., Ca(OH)2, vanadium oxide(IV) compounds, and magnesium hydrogen phosphate trihydrate (MgHPO4.3H2O) under nonisothermal conditions.17−19 The rate of sorbent regeneration (thermal decomposition) reaction in pure N2 atmosphere could be expressed in terms of the extent of sorbent conversion (α) using TGDTA data as follows.

k dα = 0 (1 − α)n e(−Ea / RT ) dT q

(2)

where k0 is the frequency factor and R is the universal gas constant. Upon integrating eq 2, we get the following with the assumption of k0R/qEa being equal for all values of n and the sorbent heating rate.8

k R⎡ 1 − (1 − α)1 − n 2RT ⎤ −Ea / RT = 0 ⎢1 − ⎥e 2 qEa ⎣ Ea ⎦ T (1 − n)

for n ≠ 1 (3)

Equation 3 reduces as follows by assuming 2RT/Ea ≪ 1.

kR 1 − (1 − α)1 − n = 0 e−Ea / RT qEa T 2(1 − n)

for n ≠ 1

(4)

But, for n = 1, eq 2 with the same assumption could be obtained as

kR − ln(1 − α) = 0 e−Ea / RT 2 qEa T

for n = 1

(5)

The modified form of the Coats and Redfern equation could be obtained from eqs 4 and 5 as follows.

⎡k R ⎤ ⎡ E ⎤ ⎡ f (α) ⎤ ln⎢ 2 ⎥ = ln⎢ 0 ⎥ − ⎢ a ⎥ ⎣ T ⎦ ⎣ qEa ⎦ ⎣ RT ⎦

f (α) =

where

1 − (1 − α)1 − n (1 − n)

f (α) = − ln(1 − α)

(6)

for n ≠ 1

for n = 1

(7) (8)

2

A graph is then drawn for ln[f(α)/T ] vs 1/T that gives a straight line of slope −Ea/R. The intercept of this straight line can be used for estimating the frequency factor, k0 (s−1). From the TG-DTA data based on the first-order decomposition, if eq 8 is substituted into eq 6, the kinetic data could be obtained for heating rate of, say, 20 °C/min. The Eyring equation for thermodynamic parameter estimation can be written as19 k= B

eχk bTp h

eΔS * /R e−Ea / RT

(9) DOI: 10.1021/acs.energyfuels.7b04147 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels where kb is the Boltzmann constant, h is Planck’s constant, Tp is the peak temperature from DTA curve, e = 2.7183 is the Neper number, and χ is the transition factor (its value is unity for monomolecular reactions). The pre-exponential factor k0 (s−1), from the Arrhenius equation and ΔS, can be calculated as

k0 =

eχk bTp h

eΔS * /R

− ra = kCAn =

dα = kc(1 − α)2/3 (Ceq − CCO2) dt

(10)

and

ΔH * = Ea − RTp

ΔG* = ΔH * − TpΔS*

1 1 (1 − X r)

+ kN

−1 −1

where kc is the kinetic constant of CaCO3 calcination (m ·kmol ·s ), CCO2 is the concentration of CO2 in the gas phase (kmol/m3), and Ceq is the equilibrium concentration of CO2 in the gas phase (kmol/m3).

fe =

(11)

10[7.079 − (8308/ T )] P

(17)

PCO2,eq = 10[7.079 − (8308/ T )]

(12)

(18)

where PCO2, is the partial pressure of CO2 and PCO2,eq is the equilibrium PCO

PCO

partial pressure of CO2. Then CCO2 = RT 2 and Ceq = 2,eq where R is RT the gas constant (8.314 J/mol.K) and T is the temperature (K).

PCO2,eq = 0.1333 × 10[10.4022 − (8792.3/ T )]

(19)

Equation 18 was not selected in this work due to its validity between 1 and 300 atm of CO2 and 900−1200 °C.31 However, eq 19 was selected for our kinetic parameter estimations due to its suitable validity between 0 and 1 atm of CO2 pressures at 449−904 °C.32 These values have been employed for our studies. The limestone particle conversion in each calcination cycle number N, calculated from experimental data and measured according to the reaction eq 13, is normalized with maximum achievable carbonated content from the previous cycle. The calcination rate per mole of CaO could be calculated from eq 13, and eq 16 could be written as 2/3 ⎧ ⎡ X − Xcalc ⎤⎫ d(Xcarb − Xcalc) = kc⎨1 − ⎢ carb ⎥⎬ (Ceq − CCO2) dt Xcarb ⎣ ⎦⎭ ⎩ (20) ⎪

⎪

⎪

⎪

kc = kc0 exp(− Eac /RT )

(21)

Eac is the activation energy of CaCO3 calcination, and kc0 is the preexponential factor of CaCO3 calcination. Upon integrating eq 20 and solving for time taken to achieve 100% calcination of carbonated sorbent, we get tc* =

(13)

3Xcarb kc(Ceq − CCO2)

(22)

tc* is the time required to achieve full calcination for a particle of any CaCO3 content (s).

where α is the fraction of CaCO3 calcined to CaO content present in synthesized sorbent (CaO50MgO10FA-C40) with respect to moles of CaCO3 sorbent from former carbonation. Xcalc and Xcarb are the final and initial CaCO3 contents of calciner (both expressed in moles of CaCO3 per mole of CaO), respectively. Xcarb − Xcalc is the CaCO3 content in the sorbent calcined per mole of CaO in the sorbent, and its value ranges from 0 to 1. XN is the conversion of the sorbent calcination in the Nth cycle and has been defined as follows.9,27 XN =

(16) 3

2.3.2. Isothermal Investigations in Packed-Bed Reactor. Calciners need higher calcination efficiencies, and these would not be achieved at low temperature (near carbonation temperatures) kinetics of calcination reaction.21,22 There is no agreement on the general reaction mechanism, and semiempirical models are used for each practical application.23 The small particles (1−90 μm) of limestone sorbent decomposition are chemically controlled, and the increase in particle size is controlled by internal mass transfer.24 The model in which the overall reaction kinetics does not depend on reactant particle size and is controlled by chemical reaction is called uniform conversion model (UCM).25 Another suitable model for gas−solid reactions is the grainy pellet model (GPM).26,22 The shrinking core model (SCM) and UCM are the only two suitable models to describe the sorbent calcination reaction of fine particles.27 While SCM was used to describe the sorbent calcination with very low initial porosity, UCM could be applied for higher initial porosity. The regeneration reaction occurs due to release of CO2 by intergrain and intragrain diffusion. This is considered as being chemical reaction regime controlled in the overall kinetics. RPM (random pore model) was developed for sulfation of lime in which the distributed pore sizes were considered as randomly oriented cylinders of uniform diameters.28 The reaction proceeds through consumption of solid and formation of gaseous product. In this process, increases in pore size and surface area were observed before decreasing to zero. Shrinking core model, or homogeneous model, was considered for radial conversion due to heat and mass transfer within all the small particles.5 The degree of sorbent conversion (α) is determined by the following equation.27,29

moles of CaCO3 (X carb − Xcalc) ΔM α= = = ΔM∞ Xcarb (moles of CaO)(XN − 1)

(15)

Modified random pore model has been used to evaluate pore structure for sorbent calcination reaction.30 It has been assumed that the sorbent regeneration is completely limited by reaction controlling regime and has negligible effect of diffusion control regime.

The value of k0, obtained from the Coats and Redfern method, was substituted into eq 10 to calculate the ΔS* value.17 The Tp value from the DTA curve and the Ea value from the Coats and Redfern method would then be substituted in the following equation to get the value of ΔH*. Enthalpy of activation (ΔH*) has been defined using thermodynamics analogy of arguments.20 since

ΔM Δt

3. RESULTS AND DISCUSSION 3.1. Nonisothermal Thermokinetic Studies. TG-DTA studies have been done to estimate the relevant thermodynamic parameters required for kinetic investigations. Figure 2 shows the nonisothermal TGA data used to estimate the order of the sorbent regeneration using eq 1. This was estimated to be of second-order reaction and is in agreement with reported data.5,33 This graph as such could not be used to estimate kinetic parameters for nonisothermal sorbent regeneration reaction due to the absence of a temperature dependent term. Figure 3 shows the graph drawn of ln( f(α)/T2) vs 1/T to estimate the sorbent regeneration kinetic parameters. These were found to be Ea of 95.12 kJ/mol and k0 of 4.15 × 1018 s−1. The higher value of k0 and the lower value of Ea vis-à-vis

+ Xr (14)

where k is the deactivation constant with a value of 0.52. Xr is the sorbent residual molar concentration with a value of 0.075 at Tcalc < 950 °C. These values were constant for carbonation−calcination cycles higher than 50 cycles.9 The order of reaction could be calculated with eq 15 and is shown in Figure 5. C

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Table 1. Experimental Thermodynamic Parameters of Sorbent Regeneration

Figure 2. Nonisothermal decomposition of sorbent at 100 kPa N2.

reported values shows that our reaction is relatively faster than that reported.17,19,34,35 Table 1 shows the thermodynamic parameters over a temperature range of 0−1000 °C that have been calculated using eq 10 for estimation of ΔS, eq 11 for estimation of ΔH, and eq 12 for estimation of ΔG. The activated thermodynamic parameters were estimated at Tp using the Eyring equation. The activated thermodynamic parameters were derived from kinetic parameters estimated using the Coats and Redfern equation from nonisothermal TGA data.17−19 Over the entire temperature range, a marginal variation in ΔH* and ΔS* values has been observed in Table 1. These values are in good agreement with the reported values.20 The estimated experimental values of ΔG for thermodynamic feasibility from sorbent regeneration data were in agreement with the corresponding data reported by us earlier.15 This experimental ΔG values were compared graphically with simulated values of the sorbent and are shown in Figure 4.

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/K· mol)

log K

0 100 200 300 400 500 600 700 800 900 1000

64.48 54.23 44.21 34.36 24.65 15.07 5.6 −3.77 −13.06 −22.27 −31.41

92.85 92.01 91.18 90.35 89.52 88.69 87.86 87.03 86.19 85.36 84.53

103.88 101.29 99.31 97.72 96.38 95.23 94.22 93.32 92.5 91.76 91.08

−12.3 −7.6 −4.88 −3.13 −1.91 −1.02 −0.335 0.202 0.636 0.992 1.29

K 4.58 2.54 1.31 7.37 1.22 9.57 4.62 1.59 4.32 9.82 1.95

× × × × × × × × × × ×

10−13 10−08 10−05 10−04 10−02 10−02 10−01 1000 1000 1000 1001

regeneration discussed in our earlier paper.15 The active sorbent CaO50MgO10FA-C40 used in this experiment contains molar ratio of CaO:MgO as 3.6:1 (without fly ash) and was reported in our earlier papers.14,15 3.2. Isothermal Kinetic Studies in Packed-Bed Reactor. Sorbent calcination data was obtained from CO2 concentration in the exit flow of the gas mixture and then subsequently used to calculate changes in weight with time. The global rate of decarbonation or regeneration of sorbent is found to follow approximately around second-order reaction (Figure 5) with CO2 concentration in gas mixture. This indicates the uniform thermal decomposition process of CaCO3 which was in agreement with the reported order of reaction that varied in the range of 0.5−2.084 for sorbent regeneration.5,33 The overall intrinsic reaction resistance energy, limited by the formation and rupture of chemical bonds, results in the form of calculated activation energy.27,36,37 Calcination reaction rate depends on calcination temperature and CO2 partial pressure, but it does not depend on CaCO3 content and particle lifetime.27 Figure 6 and Figure 7 show CaCO3 calcined per mole of CaO (Xcarb − Xcalc) vs time for different cycles of sorbent regeneration (at 700 °C) at 0 and 30 kPa of CO2 partial pressures, respectively. These regeneration rates were obtained from the slope of d(Xcarb − Xcalc)/dt for (Xcarb − Xcalc) < XN−1, and these were found to be in agreement with reported limestone sorbent regeneration analyzed with TGA.27 In Figure 6, for sorbent regeneration at PCO2 at 0 kPa (at 700 °C), there

3.6CaO + MgO + 4.6CO2 (g) ↔ CaMg(CO3)2 + 2.6CaCO3

T (°C)

(23)

Equation 23 has been assumed as the main reaction that takes place while representing carbonation of active sorbent.14,15 The reverse of the same reaction has been assumed to occur during regeneration. Figure 4 shows that the regeneration of sorbent starts at 650 °C and coincides with calculated values of sorbent using HSC software. This phenomenon is due to the simultaneous formation of dolomite with CaCO3 which has been in agreement with the reported TGA data of sorbent

Figure 3. Estimation of kinetic parameters from nonisothermal TG-DTA data. D

DOI: 10.1021/acs.energyfuels.7b04147 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. Thermodynamically feasibility of sorbent regeneration reactions using ΔG.

the 1st cycle until the 20th cycle of regeneration. There was a variation of nearly 7 min (from 35 min at 1st cycle to 28 min at 20th cycle) for complete conversion of sorbent, as shown in Figure 7. The difference arises due to capacity loss of sorbent along with other synergetic effects of silicates (with alkaline, acidic, and amphoteric mixed metal oxides present in CFA) due to fly ash doping with CaO and MgO.14,38−44 Figure 8 and Figure 9 compare the conversion curves based on experimental data with those predicted by model obtained from eq 16 which upon integrating yields a polynomial equation. In this model, each particle is a spherical grain of uniformly sized nonporous CaCO3 which upon calcination follows a chemical reaction regime under shrinking core model.27 As depicted by Figure 8, the experimentally measured data (at PCO2 of 0 kPa, 700 °C) is in good agreement with the model equation predicted data in 1st cycle, and marginal deviation of model fit has been observed with increased cycle number, i.e., 20th cycle. Figure 9 shows the degree of sorbent regeneration with model fit from the 1st cycle to the 20th cycle at PCO2 of 30 kPa, 700 °C. The experimental results described

Figure 5. Calculation on reaction order for regeneration reaction experimental data.

is a uniform decrease in the degree of sorbent conversion capacity from 1st cycle until 15th cycle after which the sorbent conversion capacity remained constant up until the 20th cycle. Complete conversion time of sorbent remains constant from

Figure 6. Conversion curves for different cycles at PCO2 of 0 kPa, 700 °C. E

DOI: 10.1021/acs.energyfuels.7b04147 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Conversion curves for different cycles at PCO2 of 30 kPa, 700 °C.

Figure 8. Curve fitting of model and normalized conversion curves at PCO2 of 0 kPa, 700 °C.

Figure 9. Curve fitting of model and normalized conversion curves at PCO2 of 30 kPa, 700 °C.

in Figure 8 and Figure 9 are in accordance with the reported results.5,27

Figure 10 and Figure 11 show the effect of calcination temperature of sorbent (20 g sample size) in the 20th cycle for F

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Figure 10. Effect of temperature on conversion (α) for 20th cycle calcination at PCO2 of 0 kPa.

Figure 11. Effect of temperature on conversion (α) for 20th cycle calcination at PCO2 of 90 kPa.

the two cases, one in the absence of CO2 and the other in the presence of CO2 at 90 kPa. Higher calcination rate was observed with increased calcination temperature and was in accordance with the reported data.27 Presence of CO2 was found to reduce the reaction rate for a given temperature due to increased internal resistance inside the sorbent for CO2 diffusion.27,38 Figure 12 and Figure 13 show the effect of partial pressures of CO2 up to 101 kPa and also the effect of equilibrium partial pressures of CO2 on calcination reaction and at different calcination cycles. When CO2 concentrations were increased to nearly >90 kPa, for sorbent calcination, the calcination rate increased with reaction time. Upon increasing the number of calcination cycles, the reaction time decreased due to decrease in crystallinity of calcined sorbent. Amorphous sorbent is more reactive than crystalline sorbent, and this is in accordance with the reported trends.45,46 CaCO3 formation increased with decrease in crystallinity of CaO present in final sorbent (CaO50MgO10FA-C40) arising from multicycle carbonation− calcinations and shows faster calcination rates, as reported earlier.47 CaCO3 formation is minimal due to higher crystallinity of CaO present in the initial sorbent.14,15,47 This is due to increased changes that occur in sorbent crystalline

Figure 12. Effect of PCO2 on conversion (α) for 20th cycle calcination at 750 °C.

structure from the presence of mixed alkaline metal silicates in CFA stabilized sorbent (CaO50MgO10FA-C40) undergoing multiple carbonation−calcination cycles.14,40,48−55 It has been reported that the presence of clay minerals could reduce the activation energy and the presence of aragonite G

DOI: 10.1021/acs.energyfuels.7b04147 Energy Fuels XXXX, XXX, XXX−XXX

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estimate the rate constant of calcination reaction (kc), 1.6 m3· kmol−1·min−1, using the experimental data generated from the packed-bed reactor. Table 2 shows the comparison of regeneration activation energies of CaO and its stabilized sorbents. The experimental activation energy (Ea), 36 kJ/mol, in PBR was lower than the reported values. But, the Ea value of sorbent regeneration from TGA data in N2 gas environment is 95.12 kJ/mol, which is much higher due to shorter regeneration times. The contradicting difference of kinetic parameter estimations such as Ea in PBR (36 kJ/mol) and in TGA (95.12 kJ/mol) were in line with the reported values.14,29,57 The variation in Ea values shown in Table 2 were due to type of sorbent (with and without stabilized sorbents) regeneration, regeneration gas environment, sorbent quantity, order of the reaction, type of contactor used, operating temperature, and pressure. Reaction controlling regime was reported as rate controlling step in sorbent calcination.2,59 In this study, the chemical reaction controlling regime in sorbent calcination reaction has been observed in both PBR and TG-DTA studies with the corresponding values of Ea, reported in Table 2. The diffusion controlling resistance is negligible for particles of size