Improvement of Limestone-Based CO2 Sorbents for Ca Looping by

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Improvement of Limestone-Based CO2 Sorbents for Ca Looping by HBr and Other Mineral Acids Mohamad J. Al-Jeboori,† Michaela Nguyen,‡ Charles Dean,† and Paul S. Fennell*,† †

Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, United Kingdom Department of Energy Process Engineering and Chemical Engineering, TU Bergakademie Freiberg, 09596 Freiberg, Germany

‡

S Supporting Information *

ABSTRACT: The effects of mineral-acid doping on the long-term reactivity of limestone-based sorbents for CO2 capture was investigated in this work. Havelock (Canada), Longcliffe (U.K.), and Purbeck (U.K.) limestones were doped with a range of mineral acids (HCl, HBr, HI, and HNO3), and the effects of concentration were also studied. Doped samples were subjected to repeated cycles of carbonation and calcination in a fluidized-bed reactor. The experimental results showed that HBr and HCl as dopants with a 0.167 mol % doping concentration significantly improved the long-term reactivity of Havelock and Longcliffe limestones (doping with HI marginally improved the reactivity); however, doping Havelock limestone with a similar concentration of HNO3 reduced its CO2 uptake. Purbeck limestone was not significantly improved in reactivity by any dopant. Gas adsorption analyses showed that sorbents have a very small surface area: less than 4 m2/g. The pore size distribution appears to change significantly upon doping for those sorbents that are improved by doping, and it is likely that optimizing the pore size distribution upon cycling is one reason for the enhanced reactivity observed. The pore-size distributions of the initially calcined limestones and the changes thereof with cycling and doping explain the differences in the behaviors of the limestones.

1. INTRODUCTION

The main chemical reaction is the reversible reaction of calcium oxide with carbon dioxide

Carbon dioxide capture and storage (CCS) processes aim to reduce CO2 emissions by capturing CO2 generated in industrial processes such as fossil fuel-fired power plants.1−3 Postcombustion CCS processes involve the separation of CO2 from a flue gas before compression of the CO2 for storage. This can be achieved using a calcium-looping cycle4,5 in which calcium oxide is used as a regenerable sorbent for separating CO2 from flue gas at high temperatures (Figure 1).

CaO(s) + CO2 (g) → CaCO3(s)

(1)

The capture of CO2 by reaction with CaO through the reversible carbonation reaction has further industrial uses, for example, sorbent-enhanced water−gas shift reactions.6 CaO has been reported to catalyze the water−gas shift reaction, as well as cracking reactions.7 Upon carbonation and calcination with a CaO-based sorbent, it has been found that the CO2 carrying capacity of the sorbent falls with increasing number of cycles. The carbonation reaction is characterized by two main stages. At first, a fast, kinetically controlled reaction takes place on the surfaces of pores throughout the particles, which is then followed by a much slower, diffusion-controlled step through the deposited product, CaCO3. The calcination reaction, by contrast, is brought to completion in minutes within one reaction step.8 Depletion of the reactivity of the sorbent is affected by sorbent sintering, which causes an increase in the density of sorbent particles; pore closure/loss and reduction of the reacting surface area;9 competing reactions of the sorbent with sulfurous compounds; and ash fouling.10 The simplest way to ensure a high reactivity of the CaObased material circulating in the system is to purge a significant fraction of the material (up to 20%) cycling between the reactors. However, this potentially results in a waste disposal

Figure 1. Flowsheet of the Ca-looping process (all flows in moles). FCO2 is the CO2 flow rate to the carbonator, FN2 is the N2 flow rate to the carbonator, ECO2 is the CO2 capture efficiency at the carbonator, FR is the solids recycle stream (moles), F0 is the replenishment/purge stream (moles), Xave is the maximum average conversion of solids in the system, and μ is the split of fuel to the calciner. © 2013 American Chemical Society

ΔH = −178 kJ/mol

Received: Revised: Accepted: Published: 1426

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2.2. Objective. The aim of this experimental work was to explore inexpensive dopant materials to improve the CO2 carrying capacities of sorbents using a very simple washing procedure. In addition, the focus of this work was to establish how CaO sorbents for CO2 capture perform after doping and the mechanism of such enhancement. The critical benefit of using a small FBR as opposed to a thermogravimetric analyzer (which would allow a greater number of cycles to be performed more easily) was that it allowed sufficient sorbent to be collected for analysis of the pore structure of the material to be investigated. This was important to allow for an understanding of the underlying reason for potential increases in reactivity. 2.3. Fluidized-Bed Reactor. A laboratory-scale atmospheric-pressure FBR (Figure 2) was used. The reactor consisted

issue, unless the emission source is located close to a cement plant, which can use the purged material in place of (part of) its main feedstock of limestone.5,11 Approaches to reduce the impact of the loss of sorbent carrying capacity include thermal preactivation,12 water and steam reactivation,13,14 the production of synthetic sorbents,15−17 and doping.18 Doping can improve the reactivity of natural sorbent materials by reducing the rate of decay of the carrying capacity and/or enhancing the residual carrying capacity. A number of studies have been carried out using different dopants and doping procedures. Potential dopants studied include KMnO4,19 Mg(NO3)2,20 MgCl2, KCl, and K2CO3.21 A previous work related the activation of CaO-based sorbent to the influence of foreign cations in a dopant, in particular, the presence of magnesium ions.20 Recently, we investigated the effects of a range of inorganic salt dopants, including MgCl2, CaCl2, Mg(NO3)2, and the Grignard reagent isopropylmagnesium chloride, on the enhancement of the reactivity of a CaO-based sorbent for CO2 capture.22 Our results showed that the anion of the dopant is the key factor for enhancing the reactivity for doped samples. However, the improvements in sorbent carrying capacity previously achieved have been varied and there remains a need to provide CaO-based sorbents having improved longterm retention of CO2 carrying capacity. In fact, the reduction in capacity does not continue all the way down to zero; there is a residual carrying capacity after a large number of cycles, which is around 0.05−0.1 mol of CO2 per mole of CaO for an undoped limestone. Also, the underlying reasons for enhancement in reactivity are unclear (i.e., whether they are physical, chemical, or a combination). It has been proposed that Cl− was formed and enhanced sintering in the first few cycles of reaction, a critical period in setting the carrying capacity of sorbents in a number of contexts.23,24 In this study, we explored the role of a range of halogen mineral acids (HCl, HBr, HI) and also HNO3 for comparative purposes on the enhancement of the reactivity of Havelock, Longcliffe, and Purbeck limestones. The focus of our study was doping with metal-free dopants and varying the type and size of the anions. The doped samples were subjected to repeated cycles of carbonation and calcination in a fluidized-bed reactor (FBR). The results obtained were compared with similar results for the corresponding undoped limestone. In this work, the concentration of the mineral acid required to significantly enhance the residual activity is very low. Consequently, the concentrations of halide or NOx pollutants released upon cycling into the offgas are relatively low. Halides are a minor component in all solid fuels and biofuels used in power stations and can have significant effects on corrosion. The impact of the quantities of halide released upon cycling on the environment using industrial-scale facilities is also discussed.

Figure 2. Diagram of the FBR experimental apparatus.

of a quartz reaction vessel within an Incoloy (26.25-mm i.d., 33.4-mm o.d., 430-mm length) tube. The Incoloy tube was resistance-heated (1600 A, 2 V), controlled using a pseudoproportional−integral−derivative (PID) controller by the temperature in the bed measured by a type K thermocouple. The fluidized bed of sand and limestone was supported by a small sintered quartz plate 200 mm from the base of a quartz reaction vessel (25.50-mm o.d., 543-mm length). The temperature of both the bed and the outer reactor wall were measured using type K thermocouples. Calibrated rotameters controlled the flows of CO2 and N2 to the bed. All experiments used ∼15% v/v CO2 with a cold flow rate of 47.5 cm3/s to give an approximate U/Umf value of 11.5 at 1173 K.25 The off-gases were filtered using glass wool and a CaCl2 drying agent, with CO2 continuously measured using the FTIR gas analyzer. 2.4. Overview. The experiments were carried out in three stages: (i) doping sorbent with acids, (ii) drying doped sorbent in an oven at 373 K for 1 h, and (iii) cycling of the doped sorbent to establish sorbent performance. 2.5. Doping of Particles. A quantitative wet impregnation method was developed.22 In this method, 4.05 ± 0.02 g of limestone (500−710 μm) was weighed and transferred into a Petri dish. The required concentration of mineral acid to yield a given molar concentration in the limestone, in 2 mL of deionized water, was then poured over the limestone. The Petri dish with the doped sample was dried in an oven at 373 K for 1 h, and the dried sample was stored in a desiccator.

2. EXPERIMENTAL SECTION 2.1. Materials and Physical Measurements. All inorganic mineral acids used were Aristar grade purchased from BDH, except HI, which was analytical grade. A PerkinElmer FTIR-Spectrum100 spectrophotometer equipped with a 100-mm-length gas cell and NaCl plates was used to detect CO2 in the off-gas of the reactor. Semiquantitative XRF analysis for samples was recorded using a Bruker XRF Explorer-S4 analyzer. Brunauer−Emmett−Teller (BET) surface areas and pore volume distributions were determined using a Micromeritics Tristar 3000 N2 sorption analyzer. 1427

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2.6. Experimental Procedure. All experiments were carried out with an inlet gas concentration of ∼15% v/v CO2 at atmospheric pressure (balance N2), a CO2 concentration typical of a coal-fired power station flue gas. The calcination temperature was set to 1173 K, and the carbonation temperature was set to 973 K. After the gas analyzer had stabilized for 30 min, 8 mL of quartz sand of size fraction 355− 425 μm, leveled and weighed (∼12 g), was then added to the reactor. A metered flow of gas (47.5 cm3/s cold) was then applied to fluidize the bed. Once a stable CO2 concentration was achieved, 4.05 ± 0.02 g of limestone of size fraction 500− 710 μm was added to the reactor (to allow easy separation by sieving of the limestone from the sand, for subsequent pore-size distribution analysis). Calcination was then effected for 600 s before cycling between the carbonation and calcination regimes was initiated. Subsequently, both calcination and carbonation were each cyclically conducted for 600 s each. Experiments were performed for 13 cycles of calcination and carbonation and extended to 50 cycles for promising dopants. After the 14th or 51st calcination, the fluidizing gas was switched to N2, and the sample was tipped into a crucible and placed into a desiccator prior to being weighed when cool. Equation 2 was proposed by Grasa and Abanades26 and predicts the experimental carrying capacities at both high and low numbers of cycles XN =

1 1 (1 − X r) + kN

Purbeck, respectively. Purbeck was found to have a higher concentration of silica impurities, with 9.87 mol % SiO2; much of this silica was present as small flint particles. Purbeck was also found to have higher magnesium, iron, and aluminum contents and to contain sulfur and phosphorus. 3.1. Havelock and Longcliffe Limestones Doped with Different Acids. Havelock and Longcliffe limestones were tested with four different dopants at different concentrations (for Havelock, for HCl and HBr, from 0.135 to 0.259 mol % and, for HNO3, from 0.102 to 0.205 mol %; for Longcliffe, for HCl and HBr, from 0.102 to 0.189 mol % and, for HI, from 0.15 to 0.245; Table SI 2, Supporting Information). Undoped Havelock showed a mass loss of ∼20% over 13 cycles of calcination and carbonation. This was the highest propensity for mass loss of the considered limestones. The reactivity of Havelock dropped rapidly to a carrying capacity of ∼10% after 13 cycles. 3.1.1. Variation of Dopant. The residual reactivities of Havelock and Longcliffe limestones, as derived by eq 2, were significantly improved as a result of adding small concentrations of different mineral acids (except HNO3) using quantitative wet impregnation. The greatest increases in residual reactivity achieved for each dopant are shown in Figures 3 and 4 for Havelock and Longcliffe limestones, respectively.

+ Xr (2)

In eq 2, XN describes the carrying capacity in the Nth cycle, Xr is the residual reactivity, and k is the deactivation constant. The residual reactivity describes the carrying capacity of the sorbent after a large number of cycles. We chose this as the main metric to assess the utility of the doping process because this work aimed to reduce the required purge of CaO from the system. In the limit of zero purge (i.e., as the purge rate approaches zero), the reactivity of the material in the system becomes the residual reactivity (after a sufficiently long period of time). However, we also modeled the system (see later in this article) for a number of different purge rates. The carrying capacity determined by integration of the molar rate of reaction (from a mole balance over the CO2 entering and leaving the bed)27 was normalized to account for mass loss, measuring the weight of CaO remaining in the bed at the end of an experiment (i.e., what had not been lost as fines), compared to that initially added (after compensating for mass lost by the calcination reaction).27 2.7. Physical Properties of the Sorbent. The limestone composition was determined by XRF analysis for all three limestones, and the results (in mole percentages) are summarized in Table SI 1 (Supporting Information). A summary of experimental measurements and conditions of the undoped and doped limestones with residual reactivities and concentrations of mineral acids is provided in Table SI 2 (Supporting Information). The Brunauer−Emmett−Teller (BET) surface areas28 and Barrett−Joyner−Halenda (BJH)29 pore volume distributions were determined by N2 adsorption (Micromeritics Tristar 3000) for samples of calcined limestone.

Figure 3. Carrying capacity (normalized) for Havelock limestone, plotted against the number of cycles (line representing the fit by eq 2): (×) undoped, (□) 0.167 mol % HBr, (sideways open triangle) 0.167 mol % HCl, (Δ) 0.164 mol % HNO3, (●) 0.167 mol % HI.

It can be seen that all of the mineral-acid dopants used improved the performance of the limestones to a certain degree, but there were considerable differences in the extent and properties of the improvements. HBr raised the “residual” activity of Havelock from 0.025 to 0.22 and that of Longcliffe from −0.023 to 0.21, whereas HCl raised the residual reactivity of Havelock from 0.025 to 0.16 and that of Longcliffe from −0.023 to 0.17, respectively (see Table SI 2, Supporting Information). (Note: A negative extrapolated value is, of course, physically unfeasible and simply indicates that the longterm reactivity is very low and also that the experiments were not run for a sufficient length of time to enable the equation to accurately model the reactivity. However, to give a qualitative indication of whether the residual reactivity is likely to be high, medium, or low, use of this equation in conjunction with Figures 3 and 4 suffices.) HI marginally enhanced the residual reactivity of the Havelock and Longcliffe limestones from

3. RESULTS Table SI 1 (Supporting Information) indicates that the limestones tested have differences in purity, with Longcliffe showing the highest purity, as the values obtained were 98.89, 96.30, and 87.81 mol % CaCO3 for Longcliffe, Havelock, and 1428

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Figure 4. Carrying capacity (normalized) for Longcliffe limestone, plotted against the number of cycles: (×) undoped, (□) 0.167 mol % HBr, (sideways open triangle) 0.167 mol % HCl, (●) 0.167 mol % HI.

Figure 6. Carrying capacity (normalized) for Longcliffe limestone, plotted against the number of cycles: (×) undoped, (+) 0.102 mol % HBr, (□) 0.167 mol % HBr, (O) 0.189 mol % HBr, (Δ) 0.205 mol % HBr.

0.0254 to 0.0336 and from −0.023 to 0.0469, respectively (within the margin of error). However, HNO3 dopant caused a decrease in the residual reactivity of Havelock by ∼2%. The undoped Havelock limestone and that doped with HI (Figure 3) show similar trends for the reduction in carrying capacity, with the loss in reactivity of the undoped Havelock being slightly higher. The carrying capacities of the HCl- and HBrdoped samples showed similar shapes and slopes. Longcliffe displayed a trend similar to that for Havelock (Figure 4). 3.1.2. Variation of Doping Concentration. Dopant concentration is reported to have a significant effect on the cycling behavior of sorbents.14,18,19 Therefore, in this work, a range of concentrations for each dopant was tested to determine the optimal addition (Figures 5 and 6 and Table SI 2, Supporting Information).

HBr dopant at various concentrations for Havelock and Longcliffe, respectively (see the Supporting Information for other results, specifically, Figure SI 1 for HCl dopant for Havelock and Figures SI 2 and SI 3 for HCl and HI dopants, respectively, for Longcliffe). Concentrations from 0.102 to 0.359 mol % were tested, all of which showed an improvement in reactivity over undoped limestone. Error bars, corresponding to one standard deviation, are shown in Figure 5. It is clear that the reproducibility was high for these experiments and that mass balances closed to within 10%. 3.2. Behavior of Longcliffe after 50 Cycles. The behaviors of undoped Longcliffe limestone and Longcliffe doped with 0.167 mol % HBr over 50 cycles of calcination and carbonation were studied. Longcliffe doped with HBr exhibited an increase in residual reactivity from ∼0.05 to ∼0.13 as derived from eq 2 (Figure 7) (note the more reasonable values for residual reactivity obtained from fitting to a larger number of cycles). Again, comparison with the results in Table SI 2 (Supporting Information) demonstrate that extrapolation by fitting eq 2 to a relatively small number of cycles yielded somewhat inaccurate results.

Figure 5. Carrying capacity (normalized) for Havelock limestone, plotted against the number of cycles: (×) undoped, (+) 0.135 mol % HBr, (□) 0.167 mol % HBr, (O) 0.275 mol % HBr, (*) 0.30 mol % HBr, (Δ) 0.36 mol % HBr. Error bars represent standard deviations between repeated experiments.

Each combination of limestone and dopant had a different optimal concentration of dopant. For the Havelock and Longcliffe limestones, a doping concentration of ∼0.167 mol % yielded the maximal residual reactivity. The amount of dopant has been shown to have a significant effect on the cycling behavior of sorbents. Results in the form of carrying capacity values are summarized in Figures 5 and 6 for

Figure 7. Carrying capacity (normalized) for Longcliffe limestone, plotted against the number of cycles: (×) undoped, (□) 0.167 mol % HBr. 1429

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3.3. Purbeck Doped with Different Acids. Purbeck limestone was tested with the same dopants as Havelock and Longcliffe. For each type of dopant, a range of different concentrations was explored (see the Supporting Information, Figure SI 4). Improvements were realized with HBr and HCl, and a marginal improvement was observed with HNO3 (Figure 8). This improvement was not as impressive as in the Havelock

data showed no iodine in any form left in the limestone after 13 cycles. This indicates that a number of Br− and Cl− ions might incorporate and remain part of the limestone particles during cycling, suggesting that the halogen ion is held in the sorbent either through chemical interaction or diffusion within the structure. The results indicate that doping with HCl and HBr introduced a significant improvement in residual reactivity. Interestingly, these observations indicate that there is an optimal doping concentration limit and that the amount of halogen ions retained within the sorbent cannot be increased further with increasing doping concentration above this optimal level. This is in agreement with the expectation in light of our previous work,22 where it was suggested that the anion of the dopant is the key factor in the enhancement of limestone upon CO2 capture. XRF analysis was conducted after different numbers of cycles for HBr-doped samples of Havelock limestone. Figure 9 and

Figure 8. Carrying capacity (normalized) for Purbeck limestone, plotted against the number of cycles: (×) undoped, (□) 0.167 mol % HBr, (sideways open triangle) 0.167 mol % HCl, (Δ) 0.164 mol % HNO3.

and Longcliffe cases, which could simply be due to the fact that Purbeck has a considerably higher long-term reactivity when undoped, potentially because of its more favorable initial pore size distribution (discussed later). The sequence of improvement for dopants for Purbeck was HBr > HCl > HNO3. Extrapolation using eq 2 and the results for HBr showed a residual reactivity of 0.17, compared to a value of 0.14 for unmodified Purbeck sorbent. 3.4. XRF Analysis. XRF analysis was performed (see Table SI 2, Supporting Information) to determine whether the material doped into the particles of the limestones was retained within the particles after a number of cycles of calcination and carbonation. This is important for both consideration of the mechanism by which the doping increases the reactivity and also assessment of the total amount of volatile halides (i.e., the amount lost from the system), which have the possibility of enhancing corrosion. 3.4.1. XRF Data for Limestones Doped with Mineral Acids. XRF analyses for cycled samples of doped limestones with different mole percentages of mineral acids are presented in Table SI 2 (Supporting Information). These measurements revealed that the composition of the sorbent remained basically unaltered upon cycling, except that, after 13 cycles, ∼0.045− 0.06 and ∼0.08 mol % bromide and chloride (assumed to be present as anions), respectively, were left in the sorbent for Havelock and Longcliffe; for Purbeck, around 0.05 and 0.08 mol % remained for bromide and chloride, respectively (see the Supporting Information for additional data on samples subjected to 13 cycles: Tables SI 3 and SI 4 for samples of Havelock doped with different mole percentages of HBr and HCl, respectively, and Table SI 5 for samples of Purbeck doped with different mole percentages of HBr). However, the XRF

Figure 9. XRF data for Havelock doped with 0.167 mol % HBr: mole ratio of ions versus number of cycles.

Table SI 6 (Supporting Information) present the results. XRF analysis of the sorbent was also carried out after doping but before cycling and showed a value of 0.151 mol % HBr. This indicates that the Br constantly evolves from the system, but that the low concentrations remaining (particularly over the first few cycles) still have significant effects. 3.5. Gas Adsorption Analysis. BJH pore volumes (for pores in the size range of 3−150 nm) were determined for each calcined limestone after 13 cycles. Panels A and B of Figure 10 show the BJH pore volumes of calcined undoped and calcined doped Havelock and Longcliffe limestones, respectively, for doping with 0.167 mol % HBr acid (see the Supporting Information, Figure SI 5A,B for the BJH pore volumes of calcined undoped and calcined doped Havelock and Longcliffe limestones for doping with 0.167 mol % HCl and HI acids and Figure SI 6A,B for the BJH pore volumes of calcined undoped and calcined doped Havelock for doping with different mole percentages of HCl and HBr acids). Figure 10C shows the BJH pore volume of calcined undoped and calcined doped Purbeck for doping with 0.167 mol % HBr acid (see the Supporting Information, Figure SI 5C for the BJH pore volumes of undoped and doped Purbeck for doping with 0.167 mol % HCl and 0.164 mol % HNO3 acids and Figure SI 7 for the BJH pore volumes of calcined undoped and calcined doped Purbeck for 1430

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Figure 10. Differential pore size distribution (V) volume of pores (plotted semi logarithmically) for calcined: (A) Havelock; (B) Longcliffe and (C) Purbeck limestones; undoped and doped with 0.167 mol % of HBr.

Havelock limestone. Interestingly, in the Longcliffe case (Figure 4), doping with HBr and HCl appeared vto cause the carrying capacity to decrease initially, but then led to an improved longterm reactivity thereafter. For the HBr and HCl reagents, values of carrying capacity were lower for the doped samples during the first two and six cycles, respectively, compared to the results for the HI-doped sample. The amounts of bromine retained in the different limestones were roughly the same (Table SI 2, Supporting Information). As supported by the XRF analysis, the amount of Br or Cl did not increase with increasing doping concentration above a certain optimum level (here, optimality is defined as leading to the highest residual activity). These results might support the hypothesis that Br and Cl are present as Br− and Cl− ions that diffuse into the sorbent, which cannot hold more than this number of ions in its ionic structure so that any additional amount has no beneficial effect. The remaining unbound halogen ions could evolve from the system upon cycling as HX, X2, and/or KX (where X is Cl− or Br− anion). Finally, the XRF analysis indicates a decrease in the quantity of potassium present with cycling, which is in agreement with results reported previously for thermal decomposition of KCl at high temperature.30 Doping limestone might incorporate halogen ions (bromide or chloride) into the CaO crystalline matrix, enhancing the ionic diffusion and mobility of the carbonate ions through the CaCO3 by straining the crystalline matrix and encouraging the formation of additional vacancies. This is possible because the apparent ionic radii of Br−, Cl−, and carbonate anion are very close, at 1.89 and 1.81 nm for Br− and Cl−, respectively, compared to 1.85 nm for carbonate anion.31 As the apparent ionic radius of I− and carbonate anion are not close, at 2.16 nm for I− and 1.85 nm for carbonate anion, there is less opportunity for diffusion into the matrix and substitution within it upon doping with HI. 4.1. Mechanism of Reaction Extent Enhancement. Comparison of the pore size distributions in Figure 10 for the undoped calcined Havelock and Longcliffe limestones and their doped samples is informative. The shift of the pore diameter observed during the combination of doping and cycling into a modal pore diameter of ∼60 nm allows the reaction to proceed to the diffusion-limited regime but without excessively reducing the reactive surface area (as is the case when too much dopant is added and the pore size distribution shifts out of the area measurable by BJH analysis). This is in accordance with the work of Abanades and Alvares, 32 who calculated an

doping with different mole percentages of HCl acid). The BET surface area was also derived from the adsorption isotherm for each calcined undoped and doped limestone. Examination of Figure 10A,B and Figure SI 5A,B (Supporting Information) reveals that, after doping with 0.167 mol % HCl or HBr, a large volume in pores of less than 20 nm closes. The pores with diameters greater than 30 nm appear to shift upward in diameter while conserving the same overall volume. After the Havelock limestone had been doped with 0.167 mol % HI, the peak in Figure SI 5A (Supporting Information) for pore diameter showed a distinct channel between 40−80 and 90−210 nm. Longcliffe limestone doped with 0.167 mol % HI showed a similar trend (Figure SI 5B, Supporting Information), in which a distinct channel between 10−45 and 70−170 nm was observed. In contrast to these results, Purbeck limestone doped with 0.167 mol % HBr (Figure 10C) and with 0.167 mol % HCl and 0.164 mol % HNO3 (Figure SI 5C, Supporting Information) exhibited only a mild change in the modal pore diameter. It is clear from Figures 10C and SI 5C (Supporting Information) that, in the undoped sorbent after 13 cycles, a significant proportion of pores around 60 nm already exists; most likely, this is the reason for the high reactivity displayed by this undoped limestone and also the reason why doping fails to significantly improve its reactivity. The measured surface areas for the undoped calcined Havelock, Longcliffe, and Purbeck samples were 2.42, 3.56, and 2.96 m 2 /g, respectively (Table SI 2, Supporting Information). The BET surface areas for limestones doped with HCl, HBr, and HNO 3 were derived for each concentration. Havelock limestone showed increases in surface area with 0.167 and 0.167−0.257 mol % doping and a drop after reaching 0.257 and 0.359 mol % for HCl and HBr, respectively. In addition, limestone doped with 0.167 mol % HI showed an increase in surface area. Doped Longcliffe and Purbeck limestones showed trends similar to those observed for Havelock (Table SI 2, Supporting Information).

4. DISCUSSION Figures 3 and 4 indicate that, for Havelock and Longcliffe limestones, all acids except HNO3 (which caused a decrease) resulted in an increase in the capture capacity after 13 cycles and an improved long-term reactivity. Figure 3 indicates that the values of the carrying capacity increased for the HBr-doped sample after three cycles, compared to the results for undoped 1431

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approximate conversion limit in the fast reaction regime of around 50 nm. This is because, as the reaction proceeds, the thickness of the CaCO3 layer on the wall of a pore becomes sufficiently large for the reaction to become controlled by diffusion through this layer. However, more recent work33 indicates that it might be the coalescence of individual islands of CaCO3 into an impermeable layer that is responsible for the decrease in reaction rate. It is possible that both mechanisms are important. For Purbeck limestone, which, upon calcination (and prior to cycling), contains pores of approximately this diameter (i.e., 60 nm), a marginal improvement is observed upon doping and cycling. It is also clear from the enhanced sintering observed when halogen dopes the limestone that the Cl− and Br− ions might enhance the mobility of carbonate ions through the matrix. These findings relate well with previous works investigating the effects of steam on CaO-based sorbents24 and the effects of thermal preactivation,23,34 where similar effects (enhanced sintering through early cycles leading to a superior long-term reactivity by “locking in” a beneficial pore size distribution) were observed. Further investigation is necessary to determine whether the extension of the “fast” initial reaction period (leading to a higher effective carrying capacity) is purely related to improvement in reactive pore volume or whether there is a significant long-term improvement, for example, in the diffusivity of carbonate ions through the CaCO3 layer building up on the walls of pores. 4.2. Attrition. Aside from sintering, attrition is one of the main mechanisms that can cause a reduction in effective CO2 capture ability. This is due to a particle size reduction by the grinding of fine material from the surface of a particle, which is then elutriated from a fluidized system. A positive effect of doping on the friability of the sorbent is likely to be revealed in a diminishing mass loss with increasing doping concentration. For Havelock limestone, mass loss monotonically decreased with increasing doping concentration. A clear tendency can be seen for dopants that the mass loss decreased as soon as the dopant was introduced and was further reduced when higher amounts of dopant wewre added (see Table SI 2, Supporting Information). During the discussion of mass loss data, it should be pointed out that the determination of mass loss only at the end of an experiment leads to the potential for significant errors in the reactivity change as a function of the number of cycles. Attempts were made to determine the mass loss of the considered limestones after different numbers of cycles, to normalize the data in a less approximate manner. Apart from varying the number of cycles, the experimental procedure was maintained. 4.3. Impact of the Use of HBr-Doped Limestone on Halide Percentage in the Off-gas. An assessment was made to determine the percentage increase in the concentration of halide in the off-gas of an industrial carbonate looping system with HBr-doped limestone in the place of raw limestone. This was done assuming the use of well-characterized coals at the power station: one with low chlorine content and one with high chlorine content (see Table SI 7, Supporting Information). For a description of the modeling method used, please refer to the Supporting Information. The results showed that, if a low-chlorine coal is burned, the overall halide content in the system can increase significantly (there is around two-thirds as much Br as Cl in the carbonator for the low Cl coal, or 1.5 times more Br than Cl in the calciner, for a purge rate of 0.05 mol of CaO purged per mole of CO2

scrubbed). However, there is a marginal increase if a coal with more chlorine is burned (6% and 13% of the halide coming from the bromine). This indicates that enhanced corrosion might be important, but is potentially a minor issue. The effect of the improved residual carrying capacity of the HBr-doped sorbent on the heat requirements at the calciner was also investigated. For a description of the modeling method used, please refer to the Supporting Information. Values for k and Xr derived from 50-cycle experiments (both doped and undoped) were then used. In the undoped case, a minimum calciner heat demand of 33.54% of the coal added to the system to drive the CO2 from the CaO was achieved, with associated F0(min) and Xave(min) values of 0.18 kmol/s and 0.19, respectively (Figure SI 9, Supporting Information). For the doped limestone, it can be seen that the minimum calciner heat demand can be reduced to 29.51% and the same maximum average sorbent conversion achieved (0.19) using a significantly smaller purge rate, F0(min) = 0.064 kmol/s (Figure SI 10, Supporting Information). This indicates that HBr-doped limestone can enable an increase in the efficiency of energy use in the system while significantly reducing the amount of fresh limestone needed.

5. CONCLUSIONS A systematic study has been undertaken to investigate and explore the use of different mineral acids as a preactivation strategy for limestone-derived sorbents for CO2 capture. Three types of limestones were used in this work; Havelock, Longcliffe, and Purbeck. The doped samples were repeatedly cycled between carbonation and calcination in a fluidized-bed reactor (FBR), and the results obtained were compared with those for the corresponding undoped limestone. Several inorganic mineral acids were used in this work (HCl, HBr, HI, and HNO3), and a quantitative wet impregnation doping procedure was employed. Experiments in which particles of limestone were doped with small concentrations of a dopant showed a significant improvement in long-term carrying capacity, whereas doping to a greater extent yielded a marked reduction in capacity. It was proposed that the mechanism whereby the doping works is to shift the pore sizes in the calcined limestone to those of approximately the optimal diameter for repeated reaction. These results support previous studies that indicated that doping could be used as an activation strategy in a fluidized-bed environment, although care is necessary to consider the quantities of mobile halogen species produced.

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ASSOCIATED CONTENT

* Supporting Information S

Chemical composition for limestones in mole percentage, details of experimental measurements and conditions with residual reactivity and concentrations of mineral acids, XRF and BJH data for limestones with HBr and HCl, description of the modeling method, coal characterization, halide released upon cycling at the exhausts, using industrial-scale facilities. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +44 (0) 20 7594 6637. Fax: +44 (0) 20 7594 5638. 1432

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Industrial & Engineering Chemistry Research

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Notes

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The authors declare the following competing financial interest(s): some of the work is in the process of patenting.

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ACKNOWLEDGMENTS This work was supported by the European Community’s Seventh Framework Programme (FP7/2007-2013) under the GA 241302 − CaOling project.

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NOMENCLATURE F0 = fresh make-up of sorbent (kmol/s) FCO2 = molar flow rate of CO2 from the combustor to the carbonator (kmol/s) FN2 = molar flow rate of N2 (kmol/s) FR = molar flow rate of sorbent between the carbonator and the calciner (kmol/s) ECO2 = CO2 capture efficiency Xave = maximum conversion attainable by the average sorbent particle circulating in the carbonation−calcination loop 1 − μ = fuel fraction to the combustor (70%) μ = fuel fraction to the calciner (30%)

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