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Reduction of calcination temperature in the Calcium Looping process for CO2 capture by using Helium: In-situ XRD analysis Jose Manuel Valverde, and Santiago Medina ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01966 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016
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ACS Sustainable Chemistry & Engineering
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Reduction of calcination temperature in the Calcium Looping
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process for CO2 capture by using Helium:
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In-situ XRD analysis
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Jose Manuel Valverdea∗ , Santiago Medinab
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6
a
Faculty of Physics. University of Seville. Avenida Reina Mercedes s/n, 41012 Sevilla, Spain b
X-Ray Laboratory (CITIUS), University of Seville,
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Avenida Reina Mercedes, 4B. 41012 Sevilla, Spain
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∗
Corresponding author: Email:
[email protected] 9
Keywords: Global warming; Greenhouse gases; Carbon Capture;
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Chemical Looping; Energy penalty; Sorbent regeneration
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Abstract
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Limestone (CaCO3 ) calcination to yield CaO plays a central role on a myriad of natural and
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industrial processes among which the recently emerged Calcium Looping (CaL) process to
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capture CO2 is gaining a great relevance in the last years. A main drawback of this process
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however is that calcination to regenerate the CaO sorbent particles must be necessarily car-
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ried out in short residence times and under high CO2 partial pressure in order to extract a
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highly concentrated CO2 stream from the calciner reactor. This requires rising up the calciner
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temperature typically over 930◦ C , which brings about an important energy penalty to the
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technology. Calcination can be speeded up by superheated steam through a chemical action
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involving H2 O adsorption but this catalytic effect leads also to excessively friable CaO solids.
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This poses an inconvenient for their transport in practice using circulating fluidized bed re-
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actors since very fine particles that result from fracturing cannot be recovered by commercial
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cyclones. The in-situ XRD analysis reported in this work shows that by using Helium even at
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relatively small concentration, the calcination rate of limestone is notably enhanced even at
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high CO2 partial pressures, as corresponds to CaL conditions, whereas the structure and reac-
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tivity of the generated CaO remains unchanged as compared to calcination using other balance
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gases such as N2 or O2 . This would allow reducing significantly the calciner temperature at
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CaL conditions, thus mitigating the energy penalty for CO2 capture.
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I.
INTRODUCTION
Limestone (CaCO3 ) calcination:
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CaCO3 CaO + CO2 (g)
∆r H 0 = +177.8kJ/mol
(1)
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to obtain lime (CaO) is of great relevance for a large number of natural and industrial
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processes. In the last years, it has gained a renewed interest as it plays a main role on the
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Ca-looping (CaL) process for CO2 capture, which has been successfully demonstrated at
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pilot-scale level [1–5]. In this process, CaO particles become carbonated at contact with the
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flue gas in a gas-solid circulating fluidized bed (CFB) reactor (carbonator) operated under
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atmospheric pressure and typically at temperatures around 650◦ C . The carbonated solids
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are then driven into a second CFB reactor (calciner) wherein CaO particles are regenerated
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by calcination for their use in a new cycle. The final goal of this process is to retrieve
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a highly concentrated CO2 stream from the calciner to be compressed and stored, which
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implies that calcination must be necessarily performed under high CO2 partial pressures
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(typically under 70-90% CO2 vol. concentration at absolute atmospheric pressure [6, 7]).
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This is currently achieved in 1-2 MWth pilot-scale plants by burning fossil fuel in the calciner
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using O2 (oxy-combustion). However, oxy-combustion brings about a notable energy penalty
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to the process, additional CO2 is released and the regenerated CaO particles rapidly loose
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reactivity towards carbonation due to promoted sintering and deactivation by ashes and
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irreversible sulphation [8–12].
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Thermochemical data [13–15] shows that the CO2 partial pressure for the calcina-
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tion/carbonation reaction to be at equilibrium (Peq ) at a given temperature T is given
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by 3
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Peq (atm) ≈ 4.083 × 107 exp(−20474/T )
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(2)
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Thus, the minimum temperature for calcination under a CO2 partial pressure P = 0.8 atm
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(as typical of the calcination environment) would be Tcal ≃ 880◦ C . Nevertheless, a further
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constraint of the CaL process is that calcination must be attained in short residence times
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(on the order of a few minutes). Lab-scale experimental observations and pilot tests show
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that this is only feasible if the temperature in the calciner is increased up to ≈ 950◦ C [4, 5],
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which raises significantly the specific energy consumption for CO2 avoided (SPECCA) [16].
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Thus, understanding the underlying physicochemical mechanisms that govern the kinetics
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of limestone calcination becomes specially important for the large-scale deployment of the
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CaL technology.
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If the CO2 partial pressure in the calcination environment P is much smaller than Peq it is
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generally accepted that the rate of CaCO3 conversion is ruled by the calcination temperature
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T and can be well fitted by an Arrhenius law
dα = r(T, P ) f (α) dt
( )γ P r(T, P ) = A exp(−E1 /RT ) 1 − Peq
(3) (4)
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Here α is the conversion degree (ratio of mass of CaCO3 calcined to the initial mass), r is the
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surface reaction rate, f (α) is a mechanistic-rate function [17], A is a pre-exponential term, γ
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is an empirical exponent of order unity, R = 8.3145 J/mol-K is the ideal gas constant and E1
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is a positive activation energy, which is around the calcination enthalpy change E1 ≈ ∆r H 0
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[18]. However, a through understanding of the kinetics of limestone calcination is still
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lacking [19–24]. Recent in-situ XRD and 2D-XRD analysis coupled to thermogravimetry 4
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and in-situ transmission electron microscopy indicate that calcination takes place through
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a CaO crystallographic transformation [22, 24, 25]. Accordingly, calcite crystals would
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decompose into a metastable CaO∗ form and CO2 , which remains adsorbed to the solid.
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In a second step, CO2 desorption is accompanied by the structural transformation of CaO∗
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into the stable CaO structure, which is an exothermic step despite the overall calcination
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reaction is endothermic. In the limit of low CO2 partial pressures (P/Peq ≪ 1), the reaction
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is just limited by chemical decomposition since CO2 desorption and the CaO structural
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transformation occur extremely fast [22]. On the other hand, this step becomes severely
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hindered as the CO2 partial pressure approaches the equilibrium pressure. Consequently,
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calcination under partial CO2 pressures close to the equilibrium pressure is characterized by
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very long nucleation periods [24, 25]. Experimental observations show that for a fixed value
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of P/Peq close to unity the reaction rate reaches a maximum at a certain critical temperature
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Tc above which the nucleation period is prolonged as the temperature is increased while the
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apparent activation energy becomes negative due to the exothermicity of the structural
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transformation. Thus, if calcination has to be carried out under high CO2 partial pressure
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(as is the case of the CaL process) the calcination temperature has to be increased well
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beyond the equilibrium temperature in order to decrease the value of P/Peq below typically
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0.6 for the reaction to be completed in a few minutes [24, 25]. In practice, this means that
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the temperature of the calciner reactor has to be kept around ≈ 950◦ C [4, 5, 26].
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A subject of active discussion concerning limestone calcination is why the presence of
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certain gases in the calcination environment such as superheated steam or Helium acceler-
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ates calcination and by which mechanisms these gases might have an effect, if any, on the
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mechanical strength and reactivity of the formed CaO [27, 28]. Early experimental observa-
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tions dating back to the beginning of the 20th century [27] pointed out towards a relevant 5
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role of the heat conductivity of the gases present in the calciner environment on limestone
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decomposition. Thus, heat transfer would be promoted for calcination under superheated
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steam or Helium with high thermal conductivities thereby causing an effective acceleration
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of the reaction. On the other hand, more recent in-situ XRD studies have shown that the
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enhancement effect of steam would be related to adsorption of H2 O molecules which is faster
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and more significant than CO2 adsorption thus facilitating desorption of the latter as a nec-
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essary step for decarbonation [28]. A side effect of this chemical action is the production of
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small CaO nanocrystals highly reactive towards carbonation, which would be in principle
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beneficial for CO2 capture. A further consequence is that CaO particles formed by calcina-
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tion under superheated steam are quite friable [29, 30], which can be an added advantage
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for cement and fertilizers production. Yet excessive fracturing of the solids poses a serious
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inconvenient to the CaL process since very fine particles entrained by the gas in circulating
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fluidized bed reactors cannot be recovered by commercial cyclones (typically with a cut off
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particle diameter around 10 µm) [1].
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The purpose of the present work is to investigate the structural transformation that takes
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place during calcination of limestone under different gases such as N2 , O2 , and He by means
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of in-situ XRD analysis. These gases are commonly employed indistinctively in thermo-
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gravimetric tests to study the capture performance of CaO based sorbents during multicycle
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carbonation/calcination cycles [4]. Yet, any effect of these gases on the calcination kinetics
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and/or structural transformation so far dismissed could have also a significant influence on
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the CO2 capture performance of the regenerated CaO as it is seen to occur for example
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when superheated steam is injected in the calciner reactor [31, 32]. 6
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II.
MATERIAL AND METHODS
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A powdered natural limestone of high purity (99.6% CaCO3 ) from Matagallar quarry
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(Pedrera, Spain) has been used. Volume weighted mean particle size is 9.5 µm as measured
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by laser diffractometry using a Malvern Mastersizer 2000 instrument. Figure 1 shows a
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schematic layout of the experimental setup employed for studying calcination of this lime-
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stone by means of in-situ XRD analysis. A controlled gas flow at atmospheric pressure
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is passed downwards across a thin layer of the powder sample (150 mg), which is evenly
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distributed over a porous ceramic plate (1 cm dia.). In this way, the gas is homogeneously
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distributed across the powder, which promotes the gas-solid contacting efficiency thus facil-
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itating mass and heat transfer.
MFC
MFC
N2,O2,He
CO2
FIG. 1: Schematics of the experimental setup used for the in-situ XRD calcination tests. The gas mixture is achieved by means of a pair of mass flow controllers (MFC) duly calibrated for the type of gas used (either CO2 , N2, O2 or He).
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The gas flow rate is controlled by means of a pair of MKS thermal based mass flow con-
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trollers (100 sccm full range), which allows us testing the calcination behavior of limestone
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under environments of different gas mixtures at accurately controlled proportions. Calci7
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nation tests have been carried out under pure CO2 , N2 , O2 and He as well as mixtures of
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these gases in which the CO2 volume concentration was fixed to 80% as representative of
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the calcination environment in the CaL process for CO2 capture. The total gas flow rate
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was fixed to 100 sccm in all the tests. One of the MKS flow controllers was calibrated for
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CO2 as received while calibration for the other one was originally made for N2 . Correction
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factors for this controller were taken into account when other gases (He and O2 ) were used
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according to their specific heat and density.
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A Bruker D8 Advance powder diffractometer has been employed equipped with an Anton
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Paar XRK 900 high temperature chamber and a fast response/high sensitivity detector
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(Bruker Vantec 1) with radial Soller slits. This reactor chamber is specifically designed for
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the kinetic analysis of gas-solid reactions at high temperatures avoiding any dead volumes
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to ensure homogeneous filling with the reaction gas. The furnace heater has been carefully
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designed for avoiding temperature gradients across the sample. Accurate measurement and
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control of temperature is achieved by means of NiCr/NiAl thermocouples placed close to
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the sample holder. 60 mm Gobel mirrors (Bruker, Germany) for Cu Kα radiation (0.15405
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nm wavelength) with parallel Johansson geometry in the incident beam were employed.
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Instrumental contribution for structural adjustments and resolution were checked in a wide
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range of diffraction angles by using Corundum, LaB6 and silicon standards.
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In a typical calcination test the temperature is increased at 10◦ C /min from ambient
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temperature while XRD scans of duration ∆t = 295 s in the range 20◦ < 2θ 880◦ C . Yet, the kinetics of limestone
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calcination under high CO2 partial pressure becomes extraordinarily slow nearby the equi-
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librium as CO2 desorption and the CaO structural transformation are hindered. Thus,
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the calcination temperature has to be increased up to ≈950◦ C for calcination to be fully
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achieved in short residence times (typically below 10 minutes), as required in practice, by
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means of oxy-combustion, which imposes an important energy penalty to the technology.
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An effective method to catalyze the calcination reaction is to inject superheated steam in
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the calciner, but the CaO particles regenerated in the presence of steam are characterized
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by a rather small crystallite size and very high friability, which is a serious drawback for 17
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the technology. This work reports an in-situ XRD analysis of limestone calcination under
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diverse atmospheres. Calcination tests were carried out under pure gases (CO2 , N2 , O2 ,
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He) and mixtures of these gases in which the CO2 concentration was fixed to 80% vol. as
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representative of the calcination atmosphere in the CaL process for CO2 capture. The re-
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sults obtained show that the presence of Helium in the calcination environment leads to a
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significant acceleration of the reaction, which is arguably due to the enhancement of heat
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transfer and CO2 diffusivity. Thus calcination under a 80% CO2 /20% He vol/vol mixture is
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fully achieved in a few minutes (< 10 min) at temperatures just slightly above 900◦ C while
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it takes more than 30 minutes when O2 or N2 are used as balance gases instead of He. At the
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same time, the crystal structure and reactivity of the CaO formed remains unchanged when
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using Helium as balance gas as compared to O2 or N2 , which indicates that CaO friability
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is not promoted as is the case when using superheated steam to catalyze the reaction. The
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results of the present study suggest that using He in the calcination environment of the
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CaL process would lead to a significant decrease of the calcination temperature while the
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mechanical strength of the particles is not affected.
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V.
ACKNOWLEDGEMENTS
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This work was supported by the Spanish Government Agency Ministerio de Economia y
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Competitividad (contract CTQ2014-52763-C2-2-R). The X-ray the Functional Characteriza-
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tion services of the Innovation, Technology and Research Center of the University of Seville
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(CITIUS) are gratefully acknowledged. 18
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For Tables of Contents Use Only
High Temperature chamber
X-ray tube
Detector
O2 N2
He
Limestone sample
CO2 Gas output
Reducon of calcinaon temperature in the Calcium Looping process for CO2 capture by using Helium: In-situ XRD analysis Jose Manuel Valverde, San ago Medina
XRD analysis shows that the Calcium-Looping process for CO2 capture is enhanced by using Helium in the calcinaon environment without compromising sorbent resistance to a ri on
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