Article pubs.acs.org/EF
Desulfurization Performance of MgO Byproducts as a Function of Particle Size R. del Valle-Zermeño,* J. Formosa, J. Gómez-Manrique, and J.M. Chimenos Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat de Barcelona, Martí i Franquès, 1, E-08028 Barcelona, Spain ABSTRACT: The reuse of MgO byproducts as SO2 absorbents in a sustainable closed-loop process follows the guidelines of legislation and economic optimization. The aim of the present study was to enhance the desulfurization performance of an MgO byproduct by sieving to different size fractions and relating them with their reactivity and physical characteristics. The byproduct presented a rather irregular size distribution, with Mg presented as MgO, Mg(OH)2, and MgCO3 and Ca as CaO, Ca(OH)2, and CaSO4. The time of saturation (tS) was used for evaluating the desulfurization performance of each fraction with 100% removal efficiency. Thus, two different conditions for enhancing the desulfurization performance were described. A chemically established condition where the sorption capacity is improved by adding more solids and a second one dependent only on physical parameters. Accordingly, sieving to the finest size fraction could improve the desulfurization capacity close to an optimum value (2.9 kg of solids can totally neutralize 1 m3 of SO2). Taking into account that the average grinding cost to a particle size below 0.075 mm is 11.54 euro per ton, adding an extra operational unit prior to desulfurization might be a feasible alternative for attaining 100% removal efficiency. Moreover, CaSO3 and CaCO3 were the main reaction products from desulfurization, which could be reused as construction aggregates.The procedure for finding a valuable methodology for improving the SO2 removal capacity of this kind of byproduct could be extended to other wastes and residues.
1. INTRODUCTION Flue gas desulfurization (FGD) is recognized as the most effective technology for removing SO2 emissions derived from fossil-fuel combustion and metallurgical and oil-refining industries.1−3 In turn, wet FGD using Ca-based slurries as absorbents (limestone and lime) is the most widely applied method because of the abundance of the components, the reliability of the system, and the well-known chemistry that takes place during the process.4−8 The literature has already highlighted the virtues of substituting lime and limestone with magnesium compounds because of the greater solubility of magnesium sulfite/sulfate products with respect to their calcium equivalents.9−11 This fact allows maintaining an alkaline pH during longer periods of desulfurization and hence results in an improved SO2 removal efficiency. However, although this enhancement effect is widely known, the higher costs of magnesium compounds has resulted in only slight additions (3−8 wt %) to the Ca-based conventional methods, modifying them to become magnesium-enhanced limestone/ lime processes.12,13 Fostered by the Industrial Emissions Directive of the European Union (2010/75/EU) and the 2013 Best Available Techniques (BAT) reference documents, the potential reutilization of residues as desulfurization agents have been the topic of several researches.10,11,14 In this regard, the authors have reported that the reutilization of several byproducts from the calcination of natural magnesite is a feasible technology that can attain a 100% removal efficiency by fully making profit of the magnesium enhancement effect.10 By this manner, a sustainable closed-loop processin which the byproducts from the SO2 emission process itself are reused as absorbentscould be adapted to the overall production process, following the guidelines of legislation and economic optimization. © XXXX American Chemical Society
Taking into account the heterogeneous nature of these byproducts and the interest of extrapolating any enhancement method to all kinds of industrial residues and wastes, the search of alternatives for improving SO2 removal capacity is of great interest. Among the parameters affecting absorbents reactivity, particle size distribution (PSD), and dissolution of solids stand out for their significant influence.1,15−21 Ramachandran and Sharma (1969) were the first to study the effect of particle size over gas absorption, and they reported that the cations supplied by the dissolving particles increase the diffusional gradients of the dissolved gas and therefore enhanced gas absorption.22 Furthermore, Sada et al. (1977−1984) extended these findings to the use of magnesium and calcium hydroxide suspensions.23−27 On the basis of these studies, subsequent attempts for improving SO2 removal rate with limestone slurries were addressed by using fine PSD that could favor a faster dissolution of the solids.5,28,29 In the framework of this issue, the dissolution process plays a very important role in SO2 absorption.1,7,18,19,29,30 It takes place both at the scrubber and the feed tank of the FGD system. In the case of limestone, the rate of dissolution is controlled by mass transfer of hydrogen ions with chemical reaction in the liquid film surrounding the particles and subsequently controlled by diffusive phenomena.31−33 The pH media also exerts influence differently: at low pH values (pH < 5) the dissolution is mass-transfer controlled, whereas at high pH ranges, the surface kinetics becomes more important.32,34 Thus, higher sulfite ion concentrations from the dissociation of SO2 may act as inhibitors of the dissolution Received: January 7, 2016 Revised: February 15, 2016
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DOI: 10.1021/acs.energyfuels.6b00041 Energy Fuels XXXX, XXX, XXX−XXX
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was used to perform the thermogravimetic analyses (TGA) of all size fractions (30−1400 °C, 10 °C·min−1 in a nitrogen atmosphere and a sample mass of approximately 25 mg). Taking into account the semiquantitative values obtained from XRF analysis and the stoichiometry of the decomposition reactions in TGA, the average chemical composition of each size fraction was estimated. Besides obtaining the granulometric curve from sieving, particle size distribution (PSD) was determined by means of a Beckman Coulter LS 13 320 laser analyzer in nonpolar media to avoid hydration (acetone) and the specific surface area by BET single point method was obtained using a Micrometrics Tristar 3000 porosimeter. 2.3. Reactivity and Neutralization Capacity. The reactivity of each size fraction was determined by means of the Citric Acid Test. This test is designed for defining the reactivity of MgO by the neutralization with citric acid and using phenolphthalein as indicator. The magnesia activity time is the time elapsed between the addition of the acid and the formation of a reddish color.38 Acid neutralization values of less than 60 s are used to define highly reactive (soft-burnt) MgO. Medium-reactivity MgO gives a measure between 180 and 300 s, whereas a low-reactivity MgO (hard-burnt) and a dead-burnt MgO give values greater than 600 and 900 s, respectively.39 As for the neutralization capacity, the Acid Neutralization Capacity (ANC) test (CEN 14997) was used for determining the buffering capacity of each size fraction when HNO3 is added. Only the first part of the standard was performed, as it was considered to be the most interesting for predicting desulfurization potential. It allowed obtaining a curve of pH versus hydronium (H3O+) per mass of solid. The second part of the test was already studied for LG-F as a whole in a previous research.40 2.4. Desulfurization Experiments. The desulfurization experiments were carried out by using the same experimental apparatus described in del Valle-Zermeño et al. (2014, 2015).10,11 For each size fraction, different slurries with different solid-to-water (S/W) ratios (0.5, 1.25, 2.5, 3.75, and 5 wt %) were placed in a discontinuous stirred glass batch reactor (0.2 L). The agitation was provided by a vertical agitator using a propeller (500 rpm). The gas phase was injected and bubbled from the bottom of the bubbling reactor by means of a ceramic gas diffusor. A flowmeter was used for fixing the gas flow (4.5 L min−1). The concentration of SO2 at the outlet was measured by a XTREAM2 flue gas analyzer from EMERSON. Once the solid entered total suspension, the gas was injected, and the time until detecting SO2 at the outlet (±10 ppm) was measured. This condition represented 100% removal efficiency, and the time recorded was considered as saturation time (tS). The tS was used for evaluating the desulfurization performance of each solid fraction as it allowed determining the consumption of byproduct per volume of SO2 with a 100% removal efficiency, as described in del Valle Zermeño et al.10 All experimental trials were performed at room temperature (25 °C ± 2 °C). The effects of slight variations in room temperature over the analysis were considered not significant. After each experimental trial, the slurry was filtered. Selected samples of the residual solids obtained after filtration were characterized by means of TGA using a temperature program reported in the literature for sulfate reaction products:41,42 50−1000 °C (10 °C·min−1), isothermal (30 min), and 1000−1400 °C (1 °C·min−1). The mass of each sample was 25 ± 0.5 mg using nitrogen as the carrier gas (50 mL·min−1).
process because of the crystallization reaction that takes place over the surface of the reactants.21 Apart from enhancing absorption, dissolution also plays a relevant role in determining the quality of the reaction products (e.g., gypsum).32 Many studies have aimed at increasing limestone dissolution by correlating the chemical composition, apparent porosity, dissolution kinetics, and PSD.21,35−37 The early studies carried out by the authors highlighted that PSD and reactivity were of significant influence to the performance of a kind of MgO byproduct obtained at the end of the calcination process of natural magnesite (known as low-grade fine MgO − LG-F). Although LG-F presented the highest MgO content among byproducts (∼75.5 wt %) and the greatest acid neutralization capacity, its longest residence time inside the calcination kiln and its irregular PSD affected the mass-transfer nature of SO2 absorption. Due to its high production volume (5600 t per year), increasing its desulfurization capacity by finding industrial alternatives is of great interest. Therefore, the aim of the present study was to enhance the desulfurization performance of LG-F by determining the desulfurization potential of different size fractions and relate it to their chemical composition, reactivity, and physical characteristics. The procedure for finding a valuable methodology for improving the SO2 removal capacity of this kind of byproduct could be extended to other wastes and residues.
2. MATERIALS AND METHODS 2.1. Materials. The byproduct under consideration was supplied (∼25 kg) by Magnesitas Navarras S.A. located in Navarra (Spain) and was earlier studied as a whole by the authors.10 The sample was collected at the end of the calcination process of natural magnesite during the obtaining of magnesia. At the outlet of the rotary kiln (1100 °C), the calcined commercial product is subsequently air quenched and physically classified with respect the particle size. This caustic magnesia is commonly used in the agriculture and livestock sector as magnesium supply. A certain fraction of caustic calcined magnesia (particle size 1000 °C. As expected, LG-F presented a mass-loss pattern equidistant to those from the rest of size fractions. Taking into account the stoichiometry of the TGA decomposition reactions and the semiquantitative amounts of XRF, the average chemical composition of each size fraction was estimated (Table 3). Hence, Mg was uniformly presented as MgO, Mg(OH)2, and MgCO3 in the 0−0.355 mm range. The same behavior could be accounted for Ca, whose main phases were CaO, Ca(OH)2, and CaSO4. A different composition was estimated for the 0.355−0.500 mm size fraction, in which the calcium content as Ca(OH)2 increased to almost 16 wt % at the expense of lowering the MgO content. A greatest difference among size fractions was observed for the specific surface area determined by the BET method (Table 4). The finest fraction presented the highest specific surface area, which tended to decrease as the size fraction increased. However, above 0.355 mm, the specific surface area value increased, which suggested an increase in porosity due to a greater sintering effect over the small particles. Like the coarsest fraction, the 0.355−0.500 mm size fraction was not taken into consideration for the ANC and desulfurization tests because it represented less than 1 wt % of the total LG-F content.
Figure 1. Granulometric curve of LG-F: percent passing (%) vs sieve size (mm).
Table 1. Particle Size Distribution (PSD) of LG-Fa LG-F (μm) d10 d25 d50 d75 d96 mean
15.4 55.2 125.3 205.1 339.0 141.4
a Note: dX denote the X percentage of particles with a size below the indicated. Mean diameter calculated as the average.
The latter obtained by laser dispersion is addressed to fine fractions while sieving allows separating the coarse particles in broad ranges. As it can be seen, the sample presented a rather irregular size distribution, with more than 90 wt % of the total content passing through the 0.355 mm mesh screen. Thus, a very small amount of sample with a particle size above 355 μm could be obtained, which hampered the subsequent analysis made on the 0.355−0.500 mm. Due to the low quantity obtained, the >0.500 mm size fraction was discarded for further analysis. The majority of the sample was retained in the 0.160− 0.250 mm mesh screen, followed by the sieve sizes over 0.075 and 0.250 mm. Less than 10 wt % of the sample presented a size below 0.075 mm.
Table 2. Chemical Composition Determined by XRF Analysis for Each Size Fraction
MgO SiO2 CaO Fe2O3 MnO SO3 LOI
0−0.075 mm
0.075−0.160 mm
0.160−0.250 mm
0.250−0.355 mm
0.355−0.500 mm
80.65 2.93 10.24 2.02 0.16 0.55 3.30
81.89 2.92 8.81 2.80 0.17 0.60 2.19
84.53 3.29 8.09 1.99 0.16 0.40 1.46
85.59 3.07 5.85 2.11 0.17 0.48 2.13
73.74 2.73 14.87 1.78 0.16 0.65 6.51
C
DOI: 10.1021/acs.energyfuels.6b00041 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. TGA curves (weight loss vs temperature) of the size fractions under consideration: 0−0.075, 0.075−0.160, 0.160−0.250, 0.250−0.355, 0.355−0.500, and >0.500 mm.
Table 3. Average Chemical Composition of Each Size Fraction Estimated from the Three Characterization Techniques: XRF, XRD, and TGA size fractions (mm) MgO Mg(OH)2 MgCO3 CaSO4 CaO Ca(OH)2 Fe2O3 SiO2 MnO
0−0.075
0.075−0.160
0.160−0.250
0.250−0.355
0.355−0.500
77.9 2.4 2.3 0.9 4.8 6.7 2.0 2.9 0.2
81.5 1.7 1.2 1.0 4.8 4.0 2.8 2.9 0.2
83.3 0.9 1.2 0.7 4.3 4.0 2.0 3.3 0.2
83.9 1.2 1.2 0.8 2.4 4.5 2.1 3.1 0.2
71.7 1.5 4.2 1.5 1.5 15.7 1.8 2.7 0.2
0.355−0.500 mm size fraction, not considered in the ANC test but identified by TGA (see again Figure 2). 3.2. Desulfurization Performance. The relationship between tS versus S/W for each size fraction is presented in Figure 4. As it can be seen, the desulfurization capacity increased along the S/W. The decrease in size fraction also favored SO2 removal, because the highest tS (and hence longest period of SO2 neutralization) was obtained for the finest fraction under consideration (0−0.075 mm). It presented the best desulfurization performance (76 min for 5% S/W), which accounted to be 6 times better than the 0.075−0.160 mm fraction and LG-F as a whole. Thus, lower particle sizes increased the rate of dissolution of alkaline phases and hence improved the SO2 absorption. This improvement is further enhanced if the quantity of solid that is available for dissolution is increased. The fractions with a size above 0.075 mm presented a very similar behavior, with a steady SO2 absorption capacity in the whole S/W range under consideration. Therefore, two different approaches for analyzing the enhancement in the desulfurization performance were considered. On the one hand, for a certain particle size fraction, improving the
Table 4. BET Results for the Different Size Fractions size fraction (mm) 0−0.075 0.075−0.160 0.160−0.250 0.250−0.355 0.355−0.500
−1
specific surface area (m ·g ) 2
5.98 2.87 2.69 3.62 4.03
± ± ± ± ±
0.03 0.01 0.01 0.02 0.02
The ANC, expressed as the curve of pH versus H3O+ consumption, is presented in Figure 3 for the different size fractions. As it can be seen, a common profile in the acid neutralization curve is shared by all size fractions, with an initial and rapid decrease in the 12−10 pH range due to CaO/ Ca(OH)2 before attaining a long period of pH stabilization due to the equilibrium of MgO/Mg(OH)2.40 Due to the lack of carbonates, and their buffering effect ascribed by the equilibrium of HCO3−/CO32−, a sharp decrease below pH 7.5 was observed for all size fractions except the “LG-F as a whole”.43 This is attributed to the content of MgCO3 in the D
DOI: 10.1021/acs.energyfuels.6b00041 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. pH as a function of H3O+ consumption (acid neutralization capacity).
Figure 4. Desulfurization performance (tS vs S/W) of each size fraction.
because of the mass-transfer-control nature of the process.11 Although 0.250−0.355 mm presented a higher specific surface area (please see Table 4) than 0.075−0.160 mm, the latter might have allowed adding a larger amount of particles for the same mass of solid, which is a possible explanation for its greater tS. Taking into account the results from section 3.1, the ANC test was not capable of predicting the differences in desulfurization performance among different size fractions, because the same profile of neutralization curves was obtained for all size fractions (Figure 3). This is attributed to the different conditions in which ANC and desulfurization tests take place. The ANC test, because it is almost 8 h long, is controlled by the thermodynamic equilibrium among phases as the acid is gradually added and hence depends on the chemical composition. In the case of desulfurization, tS is related to the kinetic process of SO2 dissociation in the liquid and hence on gas- and liquid-side mass-transfer coefficients.33
SO2 sorption capacity is only attributed to the addition of more solids. This condition was considered to be chemically established, as it depended on the availability of the alkaline phases. On the other hand, at a fix S/W and different particle size, the desulfurization enhancement was considered to be solely dependent on physical parameters (specific surface area). As the S/W increased, the difference between the tS of the finest fraction and the rest of size fractions became more evident. For S/W < 1.25% (diluted conditions), the desulfurization potential was primarily affected by the absorption potential of water, and hence, only slight differences were observed among the different size fractions. However, for S/W > 2.5%, the effect of adding more solid became significant, and the desulfurization potential was to a large extent influenced by the dissolution capacity of the solids. This is in accordance with the results of a previous study, where the role of water in wet FGD was reported to be dependent on the S/W E
DOI: 10.1021/acs.energyfuels.6b00041 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 5. Desulfurization potential (kg·m−3) for 100% removal efficiency of each size fraction as a function of S/W.
Figure 6. TGA curves the final residues obtained after desulfurization as a function of S/W ratio for the 0−0.075 mm size fraction.
Taking into account the tS, the gas flow, and the S/W, the consumption of byproduct for completely neutralize a certain volume of SO2 (kg·m−3) was calculated for each size fraction (Figure 5). In the case of the coarse fractions, the consumption of byproduct per volume of SO2 increased along with the S/W until 54.8 and 69.1 kg·m−3 for 0.160−0.250 and 0.250−0.355 mm, respectively. This increase became steady after reaching a 2.5% S/W. The behavior of LG-F as a whole and 0.075−0.160 mm was very similar, which shows that the desulfurization capacity of LG-F mainly depends on this size fraction. As for the finest fraction, the results showed a steady trend in the 6.1− 3.2 kg·m−3 range for the entire S/W range under study. Accordingly, improving the LG-F desulfurization performance by sieving to the finest size fraction (0−0.075 mm) allowed improving the SO2 sorption capacity close to that of the
optimum byproduct (2.9 kg of solids per m3 of SO2).10 This is approximately three times the consumption of widely used high-grade lime.10 Taking into account the average grinding cost to
