Carbonation of lignite fly ash at ambient T and P in a semi-dry reaction system for CO2 sequestration

Applied Geochemistry 26 (2011) 1502–1512

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Carbonation of lignite fly ash at ambient T and P in a semi-dry reaction system for CO2 sequestration Markus Bauer a,⇑, Niklas Gassen a, Helge Stanjek b, Stefan Peiffer a a b

Department of Hydrology, University of Bayreuth, D-95440 Bayreuth, Germany Clay and Interface Mineralogy, RWTH Aachen University, D-52072 Aachen, Germany

a r t i c l e

i n f o

Article history: Received 18 February 2011 Accepted 31 May 2011 Available online 6 June 2011 Editorial handling by A. Kolker

a b s t r a c t The global rise in atmospheric greenhouse gas concentrations calls for practicable solutions to capture CO2. In this study, a mineral carbonation process was applied in which CO2 reacts with alkaline lignite ash and forms stable carbonate solids. In comparison to previous studies, the assays were conducted at low temperatures and pressures and under semi-dry reaction conditions in an 8 L laboratory mixing device. In order to find optimum process conditions the pCO2 (10–20%), stirring rate (500–3000 rpm) and the liquid to solid ratio (L/S = 0.03–0.36 L kg1) were varied. In all experiments a considerable CO2 uptake from the gas phase was observed. Concurrently the solid phase contents of Ca and Mg (hydr)oxides decreased and CaCO3 and MgCO3 fractions increased throughout the experiments, showing that CO2 was stabilized as a solid carbonate. The carbonation reaction depends on three factors: Dissolution of CO2 in the liquid phase, mobilization of Ca and Mg from the mineral surface and precipitation of the carbonate solids. Those limitations were found to depend strongly on the variation of the process parameters. Optimum reaction conditions could be found for L/S ratios between 0.12 and 0.18, medium stirring velocities and pCO2 between 10% and 20%. Maximum CO2 uptake by the solid phase was 4.8 mmol g1 after 120 min, corresponding to a carbonation efficiency for the alkaline material of 53% of the theoretical CO2 binding capacity. In comparison to previous studies both CO2 uptake and carbonation efficiencies were in a similar range, but the reaction times in the semi-dry process were considerably shorter. The proposed method additionally allows for a more simple carbonation setup due to low T and P, and produces an easier to handle product with low water content. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Dealing with the issue of global climate change is currently one of the biggest challenges for mankind. Carbon dioxide is an important contributor to the earth’s greenhouse effect and anthropogenic use of fossil fuels caused a rise in atmospheric CO2 concentrations (Suess, 1955; Petit et al., 1999; IPCC, 2007). The urgency of decoupling energy production and CO2 emission calls for immediate solutions allowing C capture and storage (CCS). The trapping of CO2 in mineral form represents one possibility in this context. Carbonic acid induced weathering and carbonation of mineral phases, such as silicates (Oelkers et al., 2008) is a natural process. Seifritz (1990) first proposed using weathering processes to sequester CO2 and produce long term stable carbonate minerals. The pool of natural rocks and minerals suitable for carbonation is large in the earth crust and the reaction is thermodynamically favored, but the efficiency of the overall procedure is under debate (Lackner et al., 1995; Huijgen and Comans, 2005; Zevenhoven ⇑ Corresponding author. Tel.: +49 921 552352; fax: +49 921 552366. E-mail address: [email protected] (M. Bauer). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.05.024

et al., 2006; Huijgen et al., 2006, 2007; Oelkers et al., 2008). Due to low reaction rates, heat or chemicals are needed to accelerate the process in a technical system. Moreover, mining and transportation is necessary to provide the mineral material. As an alternative to natural minerals, different alkaline waste materials, such as residues from coal combustion, municipal waste combustion, steel production, cement production or the paper industry, also have a potential for CO2 binding and offer a high reactivity under ambient conditions (Huijgen et al., 2005; Back et al., 2008; Bonenfant et al., 2008a; Perez-Lopez et al., 2008; Huntzinger et al., 2009; Baciocchi et al., 2009, 2010). Lignite fly ash, the type of waste studied here, for instance, is produced in large quantities (5–10 Mt every year in Germany alone) by burning low quality lignite coal for energy production. This amount is small compared to the pool of natural materials potentially suitable for carbonation, but the lignite ashes have the advantage of reacting quickly at ambient conditions without any pretreatment. Lacking a utilization pathway these ashes are today primarily deposited, e.g., in sealed off landfills to prevent reaction or leaching upon contact with water or CO2. A carbonation treatment would not only trap the CO2 and make the handling easier, it even might allow

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the material to be used as a secondary resource in the construction industry. The high reactivity of alkaline wastes with respect to CO2 is based on their often large specific surface area per unit mass and small grain size (Fernandez Bertos et al., 2004; Huijgen et al., 2005; Lekakh et al., 2008). Chemically, most of these materials contain reactive Ca/Mg (hydr)oxides, which are carbonated in the presence of water and CO2. Back et al. (2008) described lignite fly ash interactions with CO2 in an aqueous system as a combination of metal (hydr)oxide dissolution, CO2 transfer/dissociation and carbonate precipitation where ‘‘Me’’ are Ca or Mg (Eq. (1)–(4)).

MeO þ H2 O () MeðOHÞ2 () Me2þ þ 2OH

ðEq:1Þ

pCO2  K H ¼ aðCO2 ðaqÞÞ

ðEq:2Þ

CO2 ðaqÞ þ H2 O () H2 CO3 () HCO3 þ Hþ () CO2 3 þ 2Hþ Me2þ þ CO2 3 () MeCO3

ðEq:3Þ ðEq:4Þ

The carbonation process of lignite fly ash is divided into a time sequence of reactions initiated by the OH and Ca2+ generating dissolution of Ca (hydr)oxides. The high pH leads to a fast CO2 dissolution in water (Usdowski, 1982; Stumm and Morgan, 1996) and results in the precipitation of CaCO3. Consumption of available Ca minerals finally causes a drop in pH, after which Mg (hydr)oxides start to dissolve and the CO2 is stabilized in solution as Mg(HCO 3 )2 (Back et al., 2008). Other waste material minerals, like Ca/Mg silicates, sulfates or Ca–Si/Ca–Fe compounds, may also contribute to CO2 uptake but their role is much less well understood (Huijgen et al., 2005; Baciocchi et al., 2006). The velocity of these reactions depends on the prevailing conditions and each step can be rate limiting for the whole process. Carbon dioxide supply to the reactive sites can limit the total uptake, which can be overcome, e.g., by using higher CO2 partial pressures (Rendek et al., 2006) or by more intensive stirring (Huijgen et al., 2005; Back et al., 2008). Above a threshold value for CO2 supply the system will then be controlled by Ca release from the mineral phase (Huijgen et al., 2005; Back et al., 2008). Reaction temperature controls the carbonation due to its effect on thermodynamic constants, reaction kinetics and diffusion processes (Stumm and Morgan, 1996; Morse et al., 2007). Previously documented optimum temperatures for alkaline waste carbonation in different aqueous reaction systems range from 21 °C to 180 °C (Liu et al., 2001; O’Connor et al., 2002; Huijgen et al., 2005; Li et al., 2007). Finally, the liquid to solid (L/S) ratio in the reaction system is a crucial factor for alkaline waste carbonation. At ambient temperature and pressure the presence of water is required as a mediator for the reaction of mineral components with CO2 (Li et al., 2007). However, the transport of CO2 through the gas/water interface is an important control and thus a high water content can limit the carbonation reaction rate (Lange et al., 1996). The optimum L/S ratio is system specific, depending on physical characteristics and chemical/material properties (mineralogy, grain and pore size), explaining the wide range of reported data (Liu et al., 2001; Fernandez Bertos et al., 2004; Rendek et al., 2006; Costa et al., 2007; Li et al., 2007; Huntzinger et al., 2009). In order to make CO2 binding by alkaline materials technically feasible, treatment pathways are required, in which the potential is utilized to a large extent within short periods of time. Different reaction pathways, reactor types and reaction conditions have been tested to achieve this goal (Park et al., 2003; Huijgen et al., 2005; Baciocchi et al., 2006; Back et al., 2008; Bonenfant et al., 2008b; Prigiobbe et al., 2009). No route has yet emerged as the optimum pathway and it is likely that different routes will be re-

quired for different wastes or similar wastes with different mineral composition. This study examines CO2 binding by lignite fly ash under semi dry conditions, i.e. at water contents between 0.03 and 0.36 L kg1. To the authors’ knowledge, this is the first attempt at mineral carbonation under these conditions in a stirred reactor with continuous CO2 gas flow and at low temperature/pressure. Using such a semi dry approach has several advantages. Compared to systems with high L/S ratios, better CO2 distribution is expected and thus an increase in reaction rate as the aqueous phase is present in thin water films with a large surface area. The low water content also makes material handling and product drying easier and no waste water with high concentrations of mineral ions is produced. The aim was primarily to show that mineral carbonation of lignite fly ash is possible at these low water content conditions. Also an attempt was made to identify important reactions and processes during semi dry ash carbonation and to compare these results with observations from aqueous assays. Furthermore, the efficiency of this carbonation pathway was tested by varying conditions, such as pCO2, mixing intensity and L/S ratio, in order to optimize the procedure with respect to CO2 uptake rates, CO2 uptake capacity and carbonation efficiency. An optimized semi dry carbonation process in the authors’ opinion might be a practicable and promising approach for waste material treatment and CO2 sequestration.

2. Materials and methods 2.1. Reactant material characterization All experiments were conducted with electrostatic precipitator ash material from a lignite power plant near Cologne, Germany. The fresh material was transferred into 60 L plastic barrels directly at the power plant, sealed and stored at low humidity. Of the sample mass 8.5% (w/w) was present in particles >200 lm, which were removed by sieving. They consisted mainly of soot and were thus of no importance for mineral CO2 binding. The grain size distribution <200 lm was measured by laser diffraction (Malvern Mastersizer) and showed 75.7% (w/w) was in the silt fraction (maximum between 6.3 and 63 lm) and 15.1% (w/w) in the clay fraction. Median and 90% quantiles of the distribution were at D(v, 50) = 18 lm and D(v, 90) = 190 lm. Particles <200 lm were analyzed for their chemical and physical properties before use (Table 1). The surface area determined with the BET method was 27.8 m2 g1. The material showed a strongly alkaline pH (12.4; 1 g sample in 10 mL KCl after 24 h) and consisted predominantly of Ca, Mg, Si and Fe solid

Table 1 Initial chemical and mineral properties of the lignite fly ash fraction <200 lm. Element

Solid phase content (mg g1)

Mineral phases

Cinorg1 Cinorg2 Si3 Fe3 Ti3 Al3 Mn3 Mg3 Ca3 Na3 K3 P3 S3

4.0 6.2 33.4 35.4 2.2 10.8 1.3 92.0 284.0 13.2 5.3 0.3 68.5

Calcite Calcite Quartz, silicate hydrate Brownmillerite Brownmillerite Periclase Anhydrite, calcite, lime, brownmillerite

Anhydrite 1

Measurement by elemental analyzer ( ), gas chromatography (2) or XRF (3); mineral phases identified by XRD.

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phases and smaller mass fractions of K, Na and Al solids (XRF analysis). XRD analysis showed the solid phase to contain lime, periclase, brownmillerite, quartz, silicate hydrates and calcite. About 60 mg g1 of the sample was S bound in anhydrite. About 15% (m/m) of the material were XRD amorphous. For details on XRF and XRD analyses see below. The initial inorganic carbonate content was between 4 and 6.2 mg g1. 2.2. The carbonation procedure Carbonation experiments were carried out in an 8 L laboratory mixer produced by Eirich (Hardheim/Germany). The system consisted of a gas tight chamber (stainless steel, rubber seals) containing the rotating mixing vessel with the solid material (Fig. 1). The asymmetrically placed stirrer and the scraper allowed a homogeneous mixing of material and prevented the development of dead spots. For different mixing intensities the stirrer velocity can be regulated (1–3000 rpm) and stirring direction can be varied between highly turbulent counterflow mode (opposed direction of stirrer and vessel) or less turbulent cross flow mode (in line rotation of stirrer and vessel). The gas tight mixing chamber was equipped with sensors for temperature (T) and pressure (P). Gas inlet and outlet tubes allowed a continuous CO2 and N2 gas stream (Q CO2 ;in ; Q N2 ;in ) through the chamber, which was controlled by mass flow controllers (MKS Instruments). This enabled variations of the total gas flow (0.5– 5 L min1) and of the pCO2 (10–20%). Total CO2 concentration in the outflow (pCO2,out) were monitored continuously using an optical CO2 sensor (Vaisala GMP 221 IR sensor). To control gas temperature and remove water vapour a cooling trap was placed between reactor and CO2 sensor. A sluice in the top of the reaction vessel was used for water addition and recovery of solid phase samples. A carbonation experiment consisted of several steps. First dry solid phase material was filled into the mixing vessel. At least 3 kg of solid material were required to reach the minimum filling level of the reactor. In order to reduce the experimental duration and total CO2 consumption the alkaline waste material was diluted

with fine grained quartz sand (0.2 mm diameter). Three kg of inert quartz was added to 0.3 kg of alkaline waste material. After closing the reaction chamber, the sand/waste mixture was homogenized by stirring and equilibrated with the gas flux until pCO2,out was constant. Then the experiment was started by adding water to the reaction chamber. Carbonation experiments were run for between 90 and 400 min. Samples (8–10 g) of the solid/water slurry in the reaction vessel were obtained at intervals of 15 min with a sampling device through the sluice. At the end of the experiments 0.1–1 kg of the carbonated alkaline wastes were recovered. The standard experiment was run with liquid to solid ratio (L/ S) = 0.12 L kg1; 500 rpm stirring rate, pCO2 = 0.15 atm (15% p/po) and crossflow rotation at Qgas = 3 L min1 (residence time of 2– 3 min). Experiments under these conditions were run in triplicate and were analyzed in detail. In order to estimate the influence of different reaction conditions on CO2 uptake rate and capacity, experimental series were carried out in which one parameter was varied at a time. The parameters tested were: Stirring rate (500, 1500 and 3000 rpm), pCO2 (10%, 15% and 20%) and L/S ratio (0.03–0.36 L kg1). 2.3. In situ measurements, slurry sample analyses and calculations T and P in the chamber were recorded manually every 10– 15 min. The amount of CO2 uptake by the slurry was calculated from gas flux and CO2 concentration values, which were recorded every 10 s. For every interval (t = time in sec) the ideal gas law was used to calculate the amount of CO2 retained in the reactor by

nuptake ¼ nin  nout ¼ ððpCO2;in  V in Þ  ðpCO2;out  V out ÞÞ  ðR  TÞ1 where n = molar amount of CO2 (mol) in the gas input/output or bound in the reactor, pCO2 = CO2 partial pressure (atm) in the gas phase, V = gas volume (m3) passing the through the reactor, T = temperature (K) and R = gas constant (8.314 J mol1 K1). By normalization to the initially added alkaline waste amount (m = mass (g)) the CO2 uptake rates (Ruptake in (mol (s g)1)) were

Data recording (logger, manual) QCO2, in QN2, in

T, p, Pmixer

Mixing tool

pCO2, out

Sampling

CO2 Sensor

Rotating mixing vessel

Gas tight casing

Asymetric stirrer Scrapertool T/p Sensor N2

CO 2

Gas flow controller

Mixing vessel

Scraper

Fig. 1. Experimental setup and mixing system: schematic view from the side (left) and from above (right).

M. Bauer et al. / Applied Geochemistry 26 (2011) 1502–1512

obtained and the cumulative CO2 uptake (Upcumulative in (mol g1)) integrates the amount of CO2 uptake of all previous time steps.

nðICsample Þ ¼ nðICwater;vial Þ þ nðICgas;vial Þ nðICgas;vial Þ ¼ ðpCO2gas;vial  V gas;vial Þ  ðR  TÞ1

Ruptake ¼ nuptake  ðt  msolid Þ1 Upcumulative ¼

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X ðnuptake Þ  msolid Þ1

nðICwater;vial Þ ¼ K H  pCO2gas;vial  V gas;vial

Due to the low water content and the resulting absence of a solution phase in situ electrode measurements of pH or EC and the recovery of water samples for the measurement of dissolved species were not possible. In order to differentiate between different time phases of the reaction, 3 g subsamples of the obtained aqueous/solid slurry were eluted with 15 mL deionized water (Fig. 2). After 5 min this suspension was sampled and filtered (0.45 lm, nylon). A filtrate of 0.5 mL was injected into a septumsealed, N2 purged gas chromatography vial (2 mL) and acidified (0.2 mL 1 M HCl sp.) for total dissolved inorganic C (TDIC) mea3+ 3+ surement. Dissolved ions (Ca2+, Mg2+, K+, Na+, SO2 4 , Al , Fe ) in the filtrate were measured after 1:10 dilution and acidification with 0.5% HNO3 (65% sp.) on a Varian Vista-Pro ICP-OES. After shaking the remaining suspension for another 24 h, suspension equilibrium pH values were recorded (Mettler Toledo pH sensor). These pH, TDIC and ion concentrations are representative for the leachates (5 min for TDIC and elements, 24 h for pH) and do not reflect in situ conditions within the reactor. Nonetheless, they provide important qualitative insight into the chemical reaction progress inside the reactor and were  thus used to calculate saturation indices according to SI ¼ log KIAP where SI = saturation index, sp IAP = ion activity product and Ksp = equilibrium constant at 25 °C. Activities were calculated with the Davies equation for ionic strengths between 0.05 and 0.07 mol L1. Values of Ksp from Parkhurst and Appelo (1999) for Ca/Mg (hydr)oxides and Stumm and Morgan (1996) for calcite, were used. For total carbonate measurements (Upcarbonate), 0.5 g of the slurry samples were transferred into 120 mL headspace vials, sealed with a rubber stopper and acidified with 3 mL of 1 M HCl to quantitatively transfer all inorganic C to the gas phase. Only 3–5% of total inorganic C in Upcarbonate was present as TDIC in the slurry water phase. Quantification of both total carbonate and TDIC was carried out by measuring the gas phase CO2 concentration in the headspace with an Agilent GC 6890 gas chromatograph (Carboxen column). Through application of the ideal gas law and the Henry constant, the total inorganic C content of the samples was determined by

where n = molar amount (mol), pCO2 = CO2 partial pressure in (atm), V = volume (L), R = gas constant (8.314 atm L mol1 K1), T = temperature (K) and KH = Henry constant (101.41 mol L1 atm1). About 5 g of each slurry sample were frozen and freeze dried to determine the water content in the reactor for each time point. Select samples of the dried material were submitted to analysis of elemental composition, X-ray diffraction (XRD) and IR absorption spectroscopy (FTIR). X-ray diffraction was done with a Huber 423 goniometer with scan runs from 2° to 110° (2h, Co Ka), increments of 0.018° and 10 s counting time per step. Phases were identified in the diffractograms with the software code EVA and then quantified with the Rietveld program BGMN. The XRD amorphous fraction was estimated by adding a known amount of Fluorite as internal standard. FTIR spectra were measured on a Bruker Vektor 22 with a 0.5–5 mg pestled sample pressed in a KBr (Merck, s.p.) pellet. 3. Results and discussion 3.1. Reaction under standard conditions Immediately after starting the experiment pCO2 in the gas output decreased sharply, showing CO2 uptake by the alkaline waste slurry at a rate of up to 0.98 mmol s1 kg1 (Fig. 3). Over time the uptake rate gradually declined and approached values close to zero (corresponding to pCO2,in = pCO2,out) after 140 min. Almost constant CO2 uptake rates were observed between 5 and 25 min and between 35 and 70 min after starting the experiment, whereas the rate decreased distinctly between 25 and 35 min and steadily after >70 min. The cumulative CO2 uptake (Upcumulative) was 1330 mmol kg1CO2 after 140 min. The pattern over time and the final amount was similar for Upcarbonate, confirming that CO2 removed from the gas phase was indeed transferred into a solid carbonate phase (Fig 3b). The deviations between three replicate assays were small, showing reproducibility of the time course of reaction in the semi dry mixing system (see grey shaded areas in Fig. 3a).

Fig. 2. Workflow for slurry sample chemical analysis.

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Fig. 3. Experimental results for reaction of alkaline waste with CO2 under standard conditions (L/S ratio = 0.12 L kg1; 500 rpm stirring rate, crossflow rotation, pCO2 = 15%; Qgas = 3 L min1): (a) Output pCO2 and uptake rate. (b) Equilibrium pH and total CO2 uptake (from gas phase data and measured as carbonate). (c) Leachate elemental composition. (d) Leachate TDIC concentration.

Pressure in the mixing chamber remained almost constant at atmospheric level throughout the experiment (980–1000 mbar). Temperature increased substantially within the first 30 min starting at 25 °C and reaching 75 °C at the end. The water content decreased during the experiment slowly from 99 mg g1 to 93 mg g1 after 80 min and then more quickly to 59 mg g1 towards the end of the experiment. The equilibrium pH values in the sample suspensions from different time steps decreased with reaction duration from above 12 to 9.5 (Fig. 3b). The strongest pH decline occurred between 60 and 90 min (>1 pH unit). The TDIC concentration in the slurry eluates was negligible at the beginning but increased distinctly after 30–40 min of reaction time, reaching values of more than 20 mmol L1 after >100 min (Fig. 3d). Eluate elemental concentration measurements revealed almost constant values for K+ (1– 2 mmol L1) and Na+ (14–16 mmol L1) over the whole experiment. Calcium in the eluates instead increased at the beginning of the experiment, reaching a maximum of 7 mmol L1 after 40 min. The following decline of Ca2+ concentrations occurred concurrently with an increase in Mg2+ concentration, peaking at 10 mmol L1 after 110 min and slowly decreasing afterwards. The concentrations of S rose within the first 70 min of the experiment to 17 mmol L1 and decreased slightly afterwards (Fig. 3c). As can be expected due to the closed reaction system, the elemental composition of the solid phase did not vary substantially over time (data not shown), so no considerable loss of Ca, Mg, Na, K or S occurred. However substantial changes in solid phase mineralogy were observed by FTIR measurements of samples taken after 5, 55, 95 and 140 min (Fig. 4). The Ca/Mg (hydr)oxide band lines (3643 and 3697 cm1) decreased and became less sharp. Instead, bands related to carbonate phases increased. A substantial rise and wavelength shift could be observed for the general carbon-

ate band area (1400–1480 cm1) and towards the end of the experiment the MgCO3 band at 855 cm1 appeared. The characteristic calcite band at 876 cm1 did not change distinctly. Relative to the carbonate-band area the intensity of the sulfate vibration range (1100–1200 cm1) decreased over reaction time. Due to the high quartz content it was not possible to quantify the phases precisely enough by XRD/Rietveld analysis (Fig. 4). Nevertheless, a comparison of noncarbonated waste and waste carbonated in a preliminary experiment without quartz showed that during reaction lime and periclase contents decreased (from 120 to 63 mg g1 and from 147 to 133 mg g1, respectively) and calcite increased from 27 to 47 mg g1. The substantial increase in the XRD amorphous fraction from 15% to 35% (m/m) during carbonation emphasizes substantial amounts of dissolution/precipitation reactions. Different lines of evidence show that carbonation of alkaline waste took place under the applied semi-dry reaction conditions at low T and P. Gas phase CO2 concentration decreased during passage through the reactor. The solid phase content of carbonate phases increased. The equilibrium pH value in the produced solids was lower than in the reactants. The mineral composition of the solid phase changed during the reaction and the carbonate content increased. Previous studies have shown that the carbonation of alkaline materials is mainly due to the reaction of Ca and Mg (hydr)oxides (Reddy et al., 1994; Fernandez Bertos et al., 2004; Rendek et al., 2006; Huijgen et al., 2005; Baciocchi et al., 2006; Li et al., 2007; Bonenfant et al., 2008b; Huntzinger et al., 2009; Prigiobbe et al., 2009). A detailed analysis of lignite ash carbonation in an aqueous reaction systems (L/S > 10) showed that the process occurs in distinct phases dominated by the dissolution of the metal oxides (Ca, Mg) (Back et al., 2008). Carbonation velocity can be limited either by the mineral dissolution rate, by the precipitation of carbonate minerals or by the supply rate of CO2. For mineral car-

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Ca(OH)2

MgCO3 CaCO3 “SO4” “CO3”

Mg(OH)2

140 min 140 min 95 min 95 min 55 min 55 min 5 min

5 min

Fig. 4. FTIR spectra (top) of solid phase samples after different carbonation times and results of XRD/Rietveld analysis (bottom) of lignite fly ash before/after reaction.

bonation at low water content little was known about these reaction pathways and limitations. The experiments reveal that carbonation also takes place in a semi-dry system in separate phases similar to those described by Back et al. (2008) for an aquatic system. In the initial stage the conversion of Ca (hydr)oxide fractions to Ca carbonates was apparently the dominant process. The high initial CO2 uptake, pH and c(Ca2+) can be interpreted as initially fast Ca (hydr)oxide dissolution leading to saturation with respect to CaCO3 and precipitation. After the water additions this start phase was characterized by a strong increase in CO2 uptake rates followed by 35 min of high CO2 removal from the gas. TDIC concentrations in the eluates were below the detection limit, thus precipitation of carbonates seems the most likely CO2 removal mechanism at that stage. Also, the equilibrium pH values and the Ca2+ mobility were high. The eluate data suggest that Ca (hydr)oxides are thermodynamically highly unstable under the conditions applied. The estimated ion activity products in the eluates from this time period (22 min) indicate undersaturation for lime (SI = 12.0; Ksp = 104.797) and portlandite (SI = 1.9; Ksp = 105.325), but oversaturation with respect to calcite (SI = 1.7; Ksp = 108,42). Compared to the aqueous phase systems (Back et al., 2008) this initial phase lasted longer in the semi dry assays (35 min vs. 15 min), which may be partly due to the higher total amount of reactive material. After the first decline (at 35 min) the CO2 uptake rate in the standard experiment stabilized for almost 30 min on a level of 0.6 mmol kg1 s1. This second reaction phase coincided with the start of the increase in the TDIC and Mg2+ concentrations, and a decrease of Ca2+ concentrations in the sample eluates. The availability of reactive Ca (hydr)oxide phases decreased, which might be due to consumption or, alternatively, due to the formation of carbonate coatings on the grain surfaces, slowing the dissolution or diffusion processes of Ca (Huijgen et al., 2005). The fact that reaching a pH equilibrium in the sampled slurries required a time period of 24 h and that only 53% of the carbonation potential

was used (see section below) support the latter assumption of transport limitation. With declining Ca2+ mobility, the importance of other mineral phases for the C uptake process apparently increased. Due to the relative rise in Mg2+ eluate concentrations it is assumed that Mg (hydr)oxide phases are more important at that point of the process, which is in accordance with observations by Back et al. (2008) using the same lignite fly ashes but at much higher water content. In the present experiments the saturation index of brucite decreased in the eluates from the time period between 35 min and 80 min from SI = 2.6 to SI = 0.55 and even indicated undersaturation at the end of the experiment (SI = 0.25; 142 min; Ksp = 1011.21). This is concurrent with the increase in aqueous Mg2+ and the decrease in equilibrium pH values from >11 to <10. Interestingly, brucite was not detected by XRD, but the amount of amorphous material increased significantly (Fig. 4, bottom). Although the chemical composition of the amorphous fraction was not analyzed, it is speculated that the very high oversaturation leads to the formation of an amorphous Mg (hydr)oxide/carbonate instead of brucite. The presence of such phases is also evident from FTIR (Fig. 4, top). Most previous alkaline waste carbonation studies have dealt with low Mg (<0.3 g kg1) alkaline materials So this study is among the few to give insight into the reaction of Mg phases during carbonation of wastes. The observations are similar to those made by Back et al. (2011) for Mg in an aqueous system. In the last phase of the carbonation process CO2 uptake is slower and controlled by the dissolution of the Mg(OH)2/brucite. Due to the depletion of available Ca (hydr)oxides the system pH dropped and allowed Mg release and slow Mg carbonate precipitation. 3.2. Variation of reaction conditions Changing the process conditions by varying stirring velocity, pCO2 or L/S ratio did not alter the sequence of observed reaction

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phases, e.g., an initially high CO2 uptake or a following decrease in CO2 binding. However, the time of the first occurrence and the duration changed. These effects of varying carbonation conditions help to identify and understand control mechanisms and limitation. 3.2.1. Liquid–solid ratio A low initial water content leads to a very rapid decline of the CO2 uptake rates and results in low cumulative CO2 uptake after 120 min (Fig. 5). With increasing L/S ratio, the reaction periods – characterized by plateaus of more or less constant CO2 uptake rate – were more clearly developed and shifted in time. An exception was the experiment with L/S = 0.36 L kg1 water addition (highest L/S ratio used), in which the uptake rate was low in the beginning and decreased continually. Maximum cumulative CO2 uptake after 120 min, and thus optimum carbonation conditions for the fly ash in the reactor setup, were reached for intermediate moisture level of L/S = 0.12–0.18 L kg1. The equilibrium pH in these product samples dropped from >11 to <10, showing a substantial degree of solid phase carbonation (Huijgen et al., 2005; Li et al., 2007; Bonenfant et al., 2008b). Less CO2 uptake within 120 min was observed for the lower and higher water addition experiments and the carbonation process was clearly not completed, as equilibrium pH values were still >10 (Fig. 5b). Several previous studies have also shown that a fast and complete carbonation occurs at intermediate water contents (Fernandez Bertos et al., 2004; Huijgen et al., 2005; Li et al., 2007; Baciocchi et al., 2009). Optimum conditions for the experiments were at the lower end of literature values (ranging from 0.2 to 2 L kg1). The optimum water content strongly depends on the specific carbonation setup and material. Two contrasting effects, both of which may differ due to experimental design and educt solid used, are responsible for this dependence of the carbonation reaction on the L/S ratio. On the one hand, the reaction of alkaline material with CO2 at ambient temperatures requires the presence of water as a medium for dissolution of solid and gas species. Dry particle or pore surfaces are less reactive with CO2 at ambient temperatures than wet surfaces (Lange et al., 1996). On the other hand, high water content constrains carbonation due to the transport of CO2. A smaller gas/water interface area and longer diffusion pathways to the reactive surfaces result in lower CO2 availability, causing lower uptake rates and cumulative uptake. All the assays initially had enough aqueous phase to allow CO2 uptake, but the evaporative water loss with the continuous gas stream and increasing temperatures caused a decline in CO2 uptake over time, which is especially visible in the low water variant L/S = 0.03. In contrast the slurry for the high water variant L/ S = 0.36 was almost paste-like. The low gas/water interface area

limited CO2 uptake rates and using longer reaction times partly balanced this lack in initial reaction rate and CO2 uptake (Baciocchi et al., 2009). As for a large scale industrial application short residence times are favorable, water loss should be minimized, e.g., by using a H2O saturated input gas, in order to optimize the semi dry carbonation process. 3.2.2. Stirring/agitation and pCO2 Higher stirring and higher pCO2 are expected to cause a faster and more complete carbonation of alkaline residues. Increasing the stirring intensity leads to a better exchange between gas/water or water/solid because of a larger total interface area and a thinner diffusive double layer (Stumm and Morgan, 1996; Huijgen et al., 2005). Additionally, high stirring velocities may have an abrasive effect on particle grains, improving availability of mineral phases and preventing the precipitation of coatings (Huntzinger et al., 2009). Higher pCO2, i.e. a higher CO2 gradient between gas and water, leads to faster CO2 transfer into the slurry (Back et al., 2008; Baciocchi et al., 2009). The results only partly confirm these assumptions. The solid phase carbonate content after 120 min increased in the order 500 rpm  3000 rpm < 1500 rpm, respectively, for the different stirring rates; and 10%  20% < 15% (p/p) for different pCO2 (Fig. 6). This is consistent with previous observations that CO2 uptake increases with mixing intensity or pCO2 only as long as CO2 supply is reaction rate limiting (Huijgen et al., 2005; Baciocchi et al., 2009). The results even suggest other processes to partly prevent the overall carbonation for the highest stirring/pCO2 assays. This might be due to the quick formation of calcite coatings in the high pCO2 experiments, clogging pores and passivating particle surfaces (Huntzinger et al., 2009). Furthermore, it must be pointed out that at the beginning of experiments with high stirring rates or low pCO2 the output pCO2 decreased to zero for up to 40 min, resulting in a maximum possible uptake rate. For different stirring conditions, CO2 uptake (120 min) is substantially different while different pCO2 assays result in almost the same CO2 uptake (120 min), emphasizing that adequate mixing was indeed an important control on the overall reaction. Higher pCO2 has no effect as long as the transport of CO2 towards the reactive surfaces is not rate limiting. 3.2.3. Temperature considerations In contrast to some previously conducted studies on alkaline waste carbonation, the present experimental setup did not allow running experiments at a constant reaction temperature. Instead, T increased from ambient (25 °C) to between 45 and 80 °C depending on the experimental assay. Heat is provided to the system on the one hand by friction (stirring and collision of material

Fig. 5. CO2 uptake rate (left), CO2 uptake (center) and pH (right) as a function of L/S ratio.

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Fig. 6. CO2 uptake rate, CO2 uptake as a function of stirring intensity (left) and pCO2 (right).

grains) and on the other hand by the exothermic reaction (Lackner et al., 1995). Cooling effects are the gas flow through the reaction chamber, the evaporation of water and thermal conduction through the casing of the mixer. For the setup reference experiments with inert quartz it is suggested that friction was the main contributor to the temperature increase. This is consistent with the findings of fastest T increase and highest final T with increasing stirring velocity, little dependence on L/S ratio (except for the highest L/S ratio showing a substantially lower T) and no relation to pCO2. Temperature affects the physical properties in the reaction system, e.g., through evaporation, or changes in viscosity or diffusion rates. Furthermore, thermodynamic equilibrium constants and rate constants for mineral dissolution, mineral precipitation or CO2 dissolution depend on temperature (Stumm and Morgan, 1996; Chen et al., 2006; Atkins and De Paula, 2010). Regarding the CO2 uptake kinetics, temperature has two opposing effects: decreasing equilibrium CO2 concentration and thus lower CO2 dissolution rates with increasing temperature drive the system towards CO2 limitation (Huijgen et al., 2005). Faster mineral dissolution and improved species distribution within the reactor tend to overcome limitation related to provision of Ca2+/Mg2+ ions, thus also driving the system towards CO2 limitation. Increasing temperatures are also known to affect the precipitation equilibria and kinetics of carbonates, especially Mg carbonates (Morse and Arvidson, 2002). It depends on the current limiting factor whether the carbonation reaction rate increases or drops. In the experiments high T, fast stirring and high CO2 uptake coincided in many cases, so it is pointed out that a temperature above 50 °C was positive for overall CO2 binding. This is also supported by other studies (Huijgen et al., 2005; Baciocchi et al., 2009, 2010), which found carbonation to increase with T even in the range from 25 to 100 °C. This observation was attributed to faster Ca dissolution and a higher degree of silicate carbonation and it is assumed this is also true for the present system. Directly allocating causes and effects is not possible with the present dataset, however, the parameters T and stirring are strongly interrelated and both affect CO2 uptake. 3.3. CO2 uptake capacity and carbonation efficiency Considerable amounts of CO2 were bound by the alkaline fly ashes in almost all experiments. In order to elucidate optimum reaction conditions for CO2 uptake CO2 uptake capacities and carbonation efficiencies were calculated for the assays and compared with theoretical and previously published capacity estimates.

3.3.1. Theoretical CO2 uptake capacity Different ways are described in the literature to derive theoretical CO2 uptake capacities for alkaline materials on the basis of chemical data. The capacity on the basis of XRD/Rietveld analysis (CapXRD) takes into account the quantitative contents of the phases lime (CaO) and periclase (MgO) in the educt (Steinour, 1959; Fernandez Bertos et al., 2004; Huntzinger et al., 2009). Alternatively, the elemental content of the educt can be used, either by treating all solid phase Ca and Mg as potentially reactive species (CapELE, Huijgen et al., 2005) or by using ion balance calculations to evaluate the amount of reactive Ca/Mg (CapION, Baciocchi et al., 2006). Finally also the acid neutralization capacity (ANC) obtained by batch titration of ash suspension can be used as a rough estimate for the CO2 binding potential (CapANC, Baciocchi et al., 2006). Most alkaline materials studied previously contained little or no Mg and S, allowing the calculation of the theoretical CO2 uptake capacities on the basis of different Ca species alone. Due to the complex elemental and mineral composition of the lignite fly ash the use of capacity estimates considering mineralogy, Ca and Mg or the ANC (CapXRD, CapION Ca/Mg, CapANC, pH>6) seem to be most appropriate. Consistently, the theoretical CO2 uptake capacities using these methods were in the range from 8.4 to 9.1 mmol CO2 g1. For the calculation of carbonation efficiency the theoretical uptake value of 9.1 mmol g1 was used. This value must be seen as an upper limit for the CO2 uptake potential, as the complete Ca/Mg pool was unlikely to be available for carbonation reaction in the short periods of time used.

3.3.2. Cumulative CO2 uptake The cumulative CO2 uptake represents the amount of CO2 bound by the alkaline waste material at a certain time. This amount was calculated from the pCO2 concentration difference between input and output gas flux (Upcumulative) or by measuring the product carbonate content (Upcarbonate). Table 2 shows the cumulative CO2 uptake reached after 30 min and 120 min (or 90 min, as indicated in Table 2) in the mass normalized form (mmol CO2 g1). The 30 min value represents the CO2 binding predominantly by the fast reaction of CaO/Ca(OH)2 to CaCO3. The 120 min value corresponds to maximum CO2 binding reached. After 30 min between 0.3 and 2.7 mmol g1 of CO2 were bound and the CO2 uptake differed strongly depending on reaction conditions (pCO2, stirring, L/S ratio). On this short time scale also the two different methods of determining the CO2 uptake (Upcumulative vs. Upcarbonate) show large deviations of up to 40%. This observation is attributed to experimental artifacts (inhomogeneous material distribution and sampling inside the reactor).

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Table 2 Cumulative CO2 uptake (Upcumulative) and carbonation efficiency (CE) after 30 min and 120 min, respectively, under different reaction conditions. Experiment

a

Upcumulative Upcumulative CE 30 min (%) CE 120 min (%) 30 min 120 min (mmol g1) (mmol g1)

L/S ratio (g g1) (500 rpm; 15% CO2) 0.03 0.06 0.12 0.18 0.24 0.36

1.5 1.6 2.1 1.9 1.4 0.3

2.2 3.7 4.2 4.2 3.9 1.2

16.0 17.7 22.9 20.4 15.0 3.6

23.7 41.2 45.7 46.4 43.3 13.1

Stirring (rpm) (L/S = 0.12; 15% CO2) 500 1500 3000

1.6 2.1 2.7

3.3 4.8 4.3a

17.8 22.7 29.9

36.2 52.8 47.7

pCO2 in% (L/S = 0.12; 1500 rpm) 10 15 20

1.8 2.1 2.1

4.1 4.8 4.6a

19.4 22.7 23.2

47.7 52.8 50.0

Values for 90 min of carbonation as experiment was stopped after 90 min.

With longer experimental duration (120 min) the variations for different reaction conditions or determination methods decreased and the CO2 uptake ranged from 3.3 to 4.8 mmol g1. Exceptionally little carbonation was achieved, however, for the lowest and highest L/S ratio experiments, where CO2 uptake was limited by lack of either water or CO2. A great difference between calculated and measured CO2 uptake was also found for the high L/S ratio, possibly due to the increasing importance of TDIC in the whole system. The cumulative CO2 uptake values determined for the experiments (0.3–4.8 mmol g1) were within the range of values reported by other authors for a whole variety of different alkaline waste materials, reaction conditions and durations. Similarly high maximum CO2 binding was documented by Bonenfant et al. (2008b) for a steel slag under aqueous conditions (230 mg CO2/g or 5.2 mmol g1) and by Baciocchi et al. (2006) for MSWI waste in semi-dry batch reaction system (24% weight gain, corresponding to 5.5 mmol g1). However, substantially longer reaction times of up to 3 days were required, showing that under optimized reaction conditions semi-dry carbonation is a fast carbonation method compared to others. 3.3.3. Carbonation efficiency (CE) In order to evaluate the semi-dry method, the carbonation efficiency was determined for different time steps and reaction conditions, relating experimentally obtained amounts of carbonation to the calculated theoretical CO2 uptake potential. Up to 53% of the calculated CO2 binding capacity could be realized within a period of 2 h, showing that the samples were not completely carbonated. This observation of incomplete carbonation is consistent with the determined equilibrium pH value, which can be interpreted as a proxy for carbonation efficiency (Huijgen et al., 2005; Bonenfant et al., 2008b). The equilibrium pH in samples taken after 120 min was still between 9 and 10, so some alkaline reacting phases were not carbonated within that period of time. Titration experiments performed with the alkaline waste materials revealed that most of the acid neutralization capacity is released quickly, but equilibrium is reached only after time periods of up to 10 days (results not shown). Possible reasons for that effect are the kinetics of the Ca/Mg dissolution/precipitation reactions. More likely, however, the availability of some fraction of the solid phase was physically limited by slow diffusion of

ions/CO2 through grain pores or by formation of surface coatings (Huijgen et al., 2005; Baciocchi et al., 2006). Reported carbonation efficiencies vary from 10% to 80% of the theoretical capacity for different approaches and the results are well the within the range of these values. Given the relatively short reaction times of 120 min, however, a considerable degree of carbonation was achieved with the stirred, semi-dry assays. Back et al. (2008) (lignite ash in an aqueous system) and Huijgen et al. (2005) (steel slag in a pressurized autoclave) reached similar conversions in short time frames, too. Many other studies, however, required substantially longer reaction durations of >10 h to reach such high values, despite the fact that mostly pure CO2/high pressure/high temperature assays were used. Bonenfant et al. (2008b) carbonated 50–75% of a steel slag under aqueous conditions within 40 h. The carbonation of the combustion residues by Baciocchi et al. (2006) or Fernandez Bertos et al. (2004) required more than 5 h to reach conversions of 50–70%. Huntzinger et al. (2009) got up to 80% carbonation, but their experiments lasted for more than 24 h. Alkaline waste treatment under semi-dry conditions in a mixed reaction system, as applied in the experiments, apparently is an efficient way to perform mineral carbonation within short periods of time. 3.3.4. Implications The proposed method of alkaline waste carbonation at low water content allows for a quick CO2 binding with acceptable conversion yields. The system can be run at low temperature and pressures with gas phase CO2 concentrations typical for power plants. Under these conditions, the pCO2 could be substantially reduced from initially 15% by at least 5% within 2 min of gas residence time. The main part of the carbonation potential of the solid phase could be made available in the first 50 min. The highest Upcumulative and CE within 120 min time were reached for a water content of 0.12–0.18 g g1 (500 rpm, 15% CO2). Even though these values are system specific, especially the stirring parameter, they indicate how to improve the carbonation process. When the proposed L/S ratio are applied and the mixing intensity is raised to system specific threshold of the reaction will mainly depend on CO2 supply which can be increased, e.g., by higher gas flux or pCO2. This might not necessarily increase the cumulative uptake or the carbonation efficiency, but could reduce residence times of the waste in the reactor and thus make the procedure applicable for up scaling and industrial use. Further potential for improvements can be seen in the mineral composition. The chemical data did not allow clear distinction between the reactions of different Ca/Mg containing phase. But the results of XRD and FTIR measurement suggest that Ca phases do react first and that lime is utilized quickly. On longer time scales also Mg minerals were consumed and partly transformed into Mg carbonate phases, as the FTIR results indicate (Fig. 4). Magnesium carbonate precipitation was not always observed in high L/S ratio experiments as the formation of Mg carbonates requires high temperatures (>50 °C) or high Mg oversaturation (low L/S ratios) (Hanchen et al., 2008; Back et al., 2011). Due to the low water content, resulting in high ion concentrations, and the temperatures reached the precipitation of Mg carbonate was possible in the semi dry reactor. This has to be considered as a substantial benefit of the presented carbonation method. However, the role of different Ca and Mg phases (oxides, silicates, sulfates) and other alkaline metal oxide (K2O, Na2O) for the whole process have yet to be elucidated. In order to make alkaline waste carbonation a viable option for CO2 emission reduction not only CO2 uptake characteristics are important but also the properties of the product material. Physical parameters, like grain size distribution and water content, affect product handling and determine the effort required for drying. The utilization possibilities of the carbonated product, e.g., in con-

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struction moreover depend on chemical characteristics like pH and leaching chemistry. There are some benefits of the semi dry method in this context. Firstly no potentially contaminated process water is produced. Secondly no energy intensive dewatering of the product is required. And finally by adjusting water content and stirring in the industrial mixing unit the grain size of the carbonated product can be controlled. Applying a semi-dry carbonation to alkaline waste material could thus make it easier to find an appropriate utilization.

4. Conclusions This study presents the results of carbonation experiments with alkaline waste materials performed at low pressures and temperatures in a continuously mixed and CO2 flushed batch reactor system at low water content. A robust and turbulent mixing system allowed realizing these ‘‘semi dry’’ experiments and provided information on carbonation progress over time, reaction velocity, optimum conditions as well as the CO2 binding potential and carbonation efficiency. The promising results showed rapid CO2 uptake of up to 4.8 mmol g1 and a high carbonation degree of up to 53% in 120 min, although mild process conditions and a simple process were applied. While initially Ca solid phases were the main contributors to carbonation, with increasing experimental duration Mg solid phases also dissolved (due to decreasing pH) and lead to the precipitation of Mg carbonates. The factors limiting CO2 uptake were the CO2 supply to the reaction (low pCO2, high L/S ratio), CO2 supply/solid phase inactivation (low stirring) or lack of water (low L/S ratio). Compared to the standard conditions, CO2 uptake increased when higher pCO2, higher stirring velocity or an intermediate L/S ratio of 0.12–0.18 was used, respectively. Optimum conditions for CO2 uptake were found for 0.12–0.18 g g1 water, for 1500 rpm stirring and for a pCO2 of 15%. Future studies should aim at testing the above described trends for a wider range of reaction conditions and for other alkaline waste materials, which must be expected to have different physical and chemical composition. In particular, the role of Ca/Mg sulfates, K and Na oxides and amorphous silicates for the whole process is unclear, even though materials, such as lignite ashes or steel slags, are known to contain large fractions of these components. The economic application of semi-dry alkaline waste carbonation will strongly depend on the utilization possibilities for the carbonated product, which, in turn, will depend on its physical and chemical properties. This semi dry process route allows controlling grain size and the chemistry of the produced carbonated material, which will help in finding uses for these materials, e.g., in the construction industry. Acknowledgements The research project ALCATRAP was funded by the German Ministry of Education and Research (BMBF) through the GEOTECHNOLOGIEN research and development program (Grant 03G0690A). We thank the Maschinenfabrik Gustav Eirich, Germany, for providing the mixing unit, RWE Power AG, Germany, for the lignite fly ash, and M. Back, U. Kunkel, M. Rohr and D. Kuenkel for their help during writing and in the laboratory. References Atkins, P., De Paula, J., 2010. Atkins Physical Chemistry. Oxford University Press. Baciocchi, R., Polettini, A., Pomi, R., Prigiobbe, V., Von Zedwitz, V.N., Steinfeld, A., 2006. CO2 sequestration by direct gas–solid carbonation of air pollution control (APC) residues. Energy Fuels 20, 1933–1940.

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