Reutilization of MgO Byproducts from the Calcination of Natural

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Reutilization of MgO by-products from the calcination of natural magnesite in dry desulfurization: A closed-loop process R. del Valle-Zermeno, Jesús de Montiano Redondo, Joan Formosa, Josep Ma. Chimenos, M. Josefina Renedo, and Josefa Fernández Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00717 • Publication Date (Web): 23 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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Reutilization of MgO by-products from the calcination of natural magnesite in dry desulfurization: A closed-loop process R. del Valle-Zermeño*,1, J. de Montiano-Redondo2, J.Formosa1, J.M. Chimenos1, M.J. Renedo2, J. Fernández2 1

Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat de Barcelona, Martí I Franquès, 1, E-08028 Barcelona, Spain

2

Departamento de Química e Ingeniería de Procesos y Recursos (QuIPRe), Universidad de Cantabria, Avda. de los Castros s/n, E-39005 Santander, Spain

* Corresponding author: Tel.: +34 93 402 13 16; Fax: +34 93 403 54 38. E-mail address: [email protected]

1 ACS Paragon Plus Environment

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Abstract Dry desulfurization is an attractive alternative to wet flue gas desulfurization (WFGD) since it avoids wastewater management and requires less energy inputs and lower operation costs. In this research, the by-products from a process generating the SO2 emissions were tested for dry desulfurization. Thus, the same by-products from the calcination of natural magnesite (two cyclone dusts from the air pollution control system –LG-MgO and LG-D- and one fraction obtained after calcination – LG-F- ) that were already studied in a WFGD were assessed in a dry desulfurization process. The by-products were tested raw and modified by two hydration methods and in semi-dry conditions. The SO2 sorption performance was evaluated by means of breakthrough curves (ppm of SO2 at the outlet vs. time) and desulfurization potential (L of SO2 adsorbed per kg of by-product). Accordingly, the by-products showed a good performance with respect the lime used as absorbent in conventional dry FGD. The breakthrough curves showed that the process was characterized by a first stage controlled by surface reaction rate and a subsequent product layer diffusion controlled stage. During the former, the three MgO by-products showed the same behaviour although those with a high CaO content presented an enhancement of the latter mechanism. The dust material LG-MgO presented the best desulfurization potential, up to 25.2 L·kg-1, while LG-D and LG-F achieved 16.5 and 15.9 L·kg-1 respectively. The modification of the by-products significantly improved the adsorption capacity, especially for LG-MgO in semi-dry conditions, to up to 35.0 L·kg-1. The use of these by-products in a dry process avoids generating wastewater effluents and allows obtaining a solid mixture basically made of magnesium and calcium sulfite, which could be further recovered or reutilized. This is, to the knowledge of the authors, the first study considering a closed-loop dry desulfurization process.


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Keywords: SO2 removal; dry desulfurization; MgO by-products; magnesium sulphite; adsorption.


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1. Introduction The Industrial Emissions Directive of the European Union (2010/75/EU) has been demanding the cement, lime and magnesium oxide industries to reduce their SO2 emissions to limits as low as 50 mg·Nm-3 by means of sustainable methods. Therefore, in the case of the calcination process for obtaining magnesium oxide from natural magnesite, a sustainable wet flue gas desulfurization (WFGD) method with a 100% removal efficiency in a closed loop process was proposed by the authors

1,2

: the by-

products from the MgO obtaining were reused as desulfurization agents. In other words, the by-products from the calcination process that is generating the SO2 emissions were sustainably reused for desulfurization. Even though WFGD has been the most widely applied method for SO2 removal, the large amounts of wastewater effluents are the main drawback. In this sense, dry desulfurization has been presented as an attractive alternative since it avoids wastewater managing problems and requires less energy inputs and lower operation costs 3,4. At industrial scale, dry desulfurization relies on the atomization of an alkaline sorbent in a reaction chamber upstream of a particulate collection device 5. Like in WFGD, CaO-based components have been preferably used as sorbents 4,6,7. There are some important issues concerning dry desulfurization: the sorbent and the reaction product utilization

5,7

. The chemical reaction between the sorbent and the SO2

is a gas-solid reaction that strongly depends on diffusion mechanisms and hence on the contact efficiency and contact residence time 8. In lime-based sorbents, calcium utilization is limited by the precipitation of calcium sulfite formed on the sorbent surface 9. These issues can be addressed by modifying the CaO particle characteristics, including the surface area, pore size and distribution, and volume 6. Other enhancing approaches include the improvement of the sorbent’s reactivity by blending it with


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additives, in order to favor the formation of compounds with a larger specific surface area. In this sense, Sanders et al. (1995) early reported that mixing quicklime slurries with coal fly ashes resulted in an enhancement of reactivity due to the formation of a calcium silica hydrate, which possess a high specific surface area 10. A similar mixture was studied by Tsuchuai et al., (1995), where the activity was reported to be closely related to the progress of the hydration reaction and the drying temperature of the hydration products during the preparation procedures 11. Other similar studies have also aimed at improving hydration conditions of CaO by varying time and temperature 12–14, or by using other additives

9,15,16

. The use of other pozzolanic additives has also been

reported, as in the study of Maina and Mbarawa (2011), who examined lime reactivity by adding fly ash and bottom ash from coal power plants and zeolite 17. The lime-zeolite blends were found to have the highest reactivity due to surface area improvement while temperature had the largest effect on reactivity. Bueno-López et al. (2002) studied other alternatives, such as the addition of Al2O3, SiO2, MgO, TiO2 or SiC as dispersants in Ca(OH)2/CaO regenerative sorbents

18

. After regeneration, the sorbents prepared from

carbonaceous dispersants exhibited a lower activity loss for a second SO2 retention due to the avoiding of the sinterisation–agglomeration of the calcium oxide particles

18

.

Even though CaO-based sorbents have attracted much of the research, bagasse ash and ammonium acetate were used as additives by Dube et al., (2015) on a Mg-based sorbent. The effects of hydration temperature, hydration time, amount of ammonium acetate and amount of bagasse ash on the surface area of the sorbent were analyzed and a removal efficiency of up to 99.9% at reaction time of 2 h was achieved. As in the case of calcium based sorbents, the nonporous coating product layer that forms during the process stops any further reaction and becomes the rate limiting step 19.


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The objectives of developing high-active sorbents for SO2 removal also relay in producing valuable byproducts. Thus, when using CaO-based sorbents, CaSO4 becomes the main reaction product, which makes the process very attractive 8. Gypsum formation during SO2 removal becomes an important target as well as increasing the conversion rate of CaO 7. Taking into account all the above mentioned, the aim of this study is to assess the desulfurization performance of the by-products from the calcination of natural magnesite in a dry desulfurization process. The dry desulfurization experiments were carried out at low temperatures and using a flue gas with a composition similar to petroleum coke (petcoke) or coal-burning kilns. For the effects of comparison, the same by-products from the WFGD studies were taken under consideration

1,2

. These by-

products are characterized for being a mixture of mainly magnesium (hydr)oxides and different proportions of calcium (hydr)oxides, dolomite, siliceous materials and other minor elements such as Fe and Al, whose origin relies in the mineral ore 20. Therefore, unlike other studies, the use of additives was not necessary given the nature of the MgO by-products. In order to improve the reactivity, a trial of experiments included the hydration of each by-product by means of hydration methods that were previously reported by the authors and the results of desulfurization obtained are herein presented 21

. 2. Methods and materials 2.1. Raw by-products Three different kinds of by-products were considered. These by-products were

carefully already analysed in a WFGD at laboratory scale

1,20

. Selected samples of

around 25 kg each were supplied by Magnesitas Navarras S.A. located in Navarra (Spain). All samples were collected during the calcination process of natural magnesite.


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Once the magnesite ore is extracted from the natural deposits and selected, it is fed into two rotary kilns at 1200 and 1600°C for calcination. The gases generated from these two kilns are taken to an air pollution control system where fly particles are collected in fabric filters. This dust material is classified as Low-grade MgO (LG-MgO) and was the first by-product considered for study. The second by-product was the corresponding dust material also collected in fabric filters when the natural ore has been enriched in dolomite -CaMg(CO3)2- and was catalogued as Low-Grade Dolomite (LG-D). At the outlet of the rotary kilns the calcined product is subsequently air quenched and physically classified with respect the particle size. The smaller fraction of caustic calcined magnesia is separated and collected as fine MgO (LG-F), being the third byproduct under consideration. All gases used in the dry desulfurization experiments were provided by Air Liquide S.A. Three types of hydration methods (HM) were applied, following the previous experience of the authors

9,21

. Two of them, HM1 and HM2,

consisted in placing the by-product under stirring (700 rpm) to a 1/4 and 1/0.6 solid-towater ratio (S/W) respectively at room temperature for 2 h. The samples were subsequently dried at 110°C for 24 h. The HM3 was considered representative of semidry conditions since the S/W was kept at 1/0.25 before being tested without prior drying. 2.2.Physicochemical characterization of the by-products and reacted sorbents For chemical and physical characterization, a representative subsample (about 5 kg) of each by-product was obtained by quartering each of the initial samples with a riffletype sample splitter. The three representative subsamples were analysed by X-ray fluorescence (XRF) using a Philips PW2400 X-ray sequential spectrophotometer to elucidate major and minor elements. The crystalline phases were determined by X-ray 7 ACS Paragon Plus Environment

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diffraction (XRD) in a Bragg–Brentano Siemens D-500 powder diffractometer with CuKα radiation. A TA Instruments SDT Q600 simultaneous TG-DSC was used to perform the thermogravimetic analyses (TGA) in an inert atmosphere of the raw materials (30-1400 °C, 10°C·min-1 and a sample mass of approximately 25 mg). The citric acid activity test is designed for determining the reactivity of MgO by acid neutralization using phenolphthalein as indicator. The magnesia activity time is the time elapsed between the addition of acid and the formation of a reddish colour

22

. Acid

neutralization values of less than 60 s are used to define highly reactive (soft-burnt) MgO. Medium reactive MgO gives a measure between 180 and 300 s, while a low reactivity MgO (hard-burnt) and a dead-burnt MgO give values greater than 600 s and 900 s respectively

23

. The particle size distribution (PSD) was obtained by using a

Beckman Coulter LS 13 320 laser analyzer in polar media to avoid hydration and the specific surface by BET single point method with a Micrometrics Tristar 3000 apparatus. Some of the reacted sorbents were analysed by a Setaram thermal analyzer model SETSYS-1700 coupled to a Balzers Thermostar/Omnistro mass spectrometer (30-1350 ºC at 10 ºC·min-1 in N2 and synthetic air atmosphere and a sample mass of approximately 10 mg). The number of m/z (mass-to-charge ratio) signals selected corresponds to H2O (18), SO2 (64), SO3 (80) and CO2 (44). Acid test was performed in order to evaluate MgO reactivity. 2.3.Desulfurization experiments The desulfurization experiments were carried out using the laboratory experimental assembly shown in Figure 1. A gas flow of approximately 1000 mL·min-1 was mixed and prepared prior entering the desulfurization reactor. The gas mixture containing 785, 120 and 95 mL·min-1 of N2, CO2, and synthetic air respectively was humidified to a 55% of relative humidity before being mixed with 2.5 mL· min-1 of SO2 for obtaining 8 ACS Paragon Plus Environment

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the desirable gas composition (12% CO2, 9.5% air, 0.25% SO2 and 78.5% N2). The mix of gases were flowed and humidified until obtaining a stable composition. After humidification and stabilization, the gas mixture was flowed through the reactor (upper inlet) containing each by-product dispersed in inert silica sand (Fontaineblau) inside a furnace at 56ºC. This point was considered the start of the experiment. The ratio of byproduct to inert silica sand (B/SiO2) varied depending on the amount of sample tested, 1 g of by-product per 30 g silica or 3 g of by-product per 50 g of silica. By this manner, over pressure in the system was avoided. The desulfurization potential of hydrated byproducts was also evaluated. Table 1 presents a summary of the experimental trials. The time of experiment was 1 hour for all trials. The gas flow at the outlet of the desulfurization reactor was conducted to a water condensation system before being continuously analysed with respect to the concentration of SO2 and CO2 using an Emerson MLT-1 NGA 2000 gas analyser. The data was compiled using NGA win control software. In order to separate them from the inert silica sand, the reaction products were sieved to a particle size below 60 µm and subsequently characterised using XRF and TGA. Each experiment was performed by triplicate and the results are presented as the average. 2.4. Desulfurization assessment The dry desulfurization potential was assessed by means of two parameters based on the breakthrough curves: Reactivity Index (RI) and desulfurization potential. These two approaches allowed comparing the results obtained in this research to those from previous studies 1,2. Breakthrough curves were directly obtained from the data recorded by the gas analyser by representing the concentration of SO2 (mL SO2·m-3; ppmv) at the outlet at a given time (SO2,t) as a function of that time (t). The RI was calculated by


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taking into account the lowest concentration of SO2 achieved (SO2,t) and the initial SO2 concentration (SO2,0), as in Equation 1 24: =

, , ,

× 100 (Eq. 1)

For determining the desulfurization potential, the cumulative volume of SO2 absorbed (L) was calculated by taking into account the data obtained in the breakthrough curves using the Equation 2 () ()

= ∑'%#() *%

, ×!×⧍ #×$%&

(Eq.2)

where f is the flow of gases (1 L·min-1), ∆t is the incremental time considered in the breakthrough curve (min), and m is the mass of sample of by-product (kg). Values obtained by means of Eq. 2 are the accumulated values and are represented as a function of time. 3. Results 3.1. Characterization of the by-products The main chemical composition obtained by XRF analysis, given as most stable oxides of the elements, for the three by-products is shown in Table 2. As it can be seen, magnesium was the main element in the three by-products and the calcium content was also important, especially in LG-D. The results from XRF and XRD (not shown) were used along TGA for determining the chemical composition (including carbonates and hydroxides, which can be identified by XRD). Thus, the three techniques were used complementary. Figure 2 shows the TG curves (mass loss (%) vs. temperature) of the three by-products. The mass loss steps can be generally ascribed to moisture and water of crystallization loss (below 200 ºC), Mg(OH)2 decomposition to MgO releasing H2O (from 200 to 450 ºC), MgCO3 decomposition to MgO and CO2 (between 450 and 625 10 ACS Paragon Plus Environment

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ºC), CaMg(CO3)2 decomposition to MgO, CaO and CO2 (between 625 and 750 ºC), CaCO3 decomposition to CaO and CO2 (up to 1000 ºC), and decomposition of MgSO4 and CaSO4 at around 1100 and 1200 °C respectively. Hence, the mass loss percentages for each reaction of decomposition were used to characterize the carbonates and hydroxides phases of each by-product according to previous studies

21

. The chemical

characterization results are presented in Table 3. Magnesium was mainly present as periclase (crystalline MgO) in all by-products, although its presence as uncalcined dolomite was significant in LG-D and to a lesser extent in LG-MgO. The main forms of calcium occurrence were dolomite, calcite, anhydrite, and lime. However, calcium in LG-MgO was only present in the CaCO3 form while in LG-F was presented as CaO. As expected, LG-D presented high amounts of dolomite and a lower content of CaCO3 and CaSO4. The citric acid activity test (see Table 2) showed neutralization times greater than 900 s for both LG-D and LG-F and times greater than 810 for LG-MgO. According to these values, LG-D and LG-F should be termed “dead-burned” magnesia and LGMgO as “hard-burned” magnesia. It should be noted that this test is defined to categorize highly pure magnesia, therefore the content of CaO in each by-product, as well as the presence of other alkalis, could have also influenced the neutralization of the added acid, resulting in an even higher reactivity. Therefore, it is expected that the reactivity of the magnesia is even lower than that predicted by the citric acid test, as in the case of LG-MgO. Respect to the physical properties (Table 2), LG-MgO and LG-D presented the largest specific surface area, calculated using the BET absorption model, 6.6 and 4.6 m2·g-1 respectively, while LG-F presented 2.6 m2·g-1. The dust materials (LG-MgO and LG-D) also presented a mean particle size (d50) lower than LG-F (23.1, 37.5 and 141.4 µm respectively). Therefore, the by-products differed not only in their chemical compositions but also in their physical characteristics.


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3.2. Desulfurization Results 3.2.1. Breakthrough curves (SO2 at the outlet vs. time): effect of mass sample Figure 3 presents the concentration of SO2 at the outlet of the desulfurization reactor as a function of experimental time for the three raw by-products and for the two mass samples under consideration (1 and 3 g). As expected, a greater amount of by-product increased the SO2 capture, as it can be seen by the broader peak in the adsorption curves. Breakthrough curves were characterized by a first steep controlled by the surface reaction rate followed by a gradual decrease in adsorption. Thus, surface reaction rate dominated initially before diffusion resistance became the limiting step as the generation of the product layer gradually increased. Therefore, the chemical reaction rate is important initially, while the product layer diffusion controls the reaction in the end

25

. The comparison of the modified by-products by HM1, HM2 and HM3 and the

RI are addressed in subsequent sections. 3.2.2. Volume of SO2 adsorbed per mass of by-product (L·kg-1) In order to compare the performance among each raw by-product (LG-MgO, LG-D and LG-F), Figures 4 and 5 present the volume of SO2 adsorbed per mass of by-product for 1 and 3 g respectively. As it can be seen in Figure 4, LG-MgO presented the best SO2 sorption capacity, up to 25.2 L of SO2 per kg of by-product. Taking into account that commercial calcium hydroxide with a purity of 89%, tested in the same conditions, has a sorption capacity of approximately 50 L of SO2 per kg, the by-product showed a good performance with respect the widely used lime, as in WFGD 1,2,26. The adsorption capacity of LG-D and LG-F was lower, achieving 16.5 and 15.9 L·kg-1 respectively. The amount of by-product had no significant effect on the adsorption capacity as the values obtained with 3 g were 21.4, 15.8 and 15.3 L·kg-1 for LG-MgO, LG-D, and LG-F 12 ACS Paragon Plus Environment

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respectively (Figure 5). It must be noted that the curve of LG-MgO obtained with 1 g of by-product (Figure 4) showed a higher slope that the corresponding with 3 g (Figure 5), indicating that, as the amount of sorbent decreased, the surface reaction rate increased. Thus, the slightly higher SO2 capture for 1 g of by-product can be attributed to a lower diffusional influence. As figures show, all samples presented a gradual reduction of the sorption rate as desulfurization proceeded in agreement with the stated in section 3.2.1. The straight line section in all the curves presented in Figure 4 and 5 (for times < 500 s) described the surface reaction control mechanism while the curves tending to stabilization where characteristic of the diffusion created by the layers of reaction products. As it can be observed in both figures, at the start of the surface reaction controlled stage (time < 250 s), the composition, reactivity, specific surface area and particle size didn’t have a significant influence, since all the MgO by-products showed the same slope. However, as desulfurization proceeded, the high content of CaO in both LG-D and LG-F raw materials and the subsequent fast formation of calcium products (e.g. gypsum) over the particle surface promoted a control of diffusion mechanisms over the overall process with these two by-products. As for LG-MgO, its greater specific surface area, higher Mg(OH)2 content and lower particle size were characteristic of its surface reaction control mechanism. Once the layer of the reaction products covered the particles surface and allowed the process to be controlled by diffusion, no differences in the desulphurization rate were observed for 1 g of raw material. Nevertheless, a slightly higher diffusion rate was observed for 3 g of LG-MgO (Figure 5), which could be related to its lower particle size. 3.2.3. Effect of hydration Taking into account that the desulfurization potential was not significantly influenced by the mass of by-product, the effect of hydration was evaluated for 1 g of 13 ACS Paragon Plus Environment

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material. A comparison of the desulfurization potential of each by-product modified by the three hydration methods is presented in Figures 6, 7, and 8 for LG-MgO, LG-D, and LG-F respectively. The corresponding results of the raw materials (1 g of solid) are included for the sake of comparison. As it can be seen, all HM significantly improved the adsorption capacity, especially in semi-dry conditions (HM3) with a gas-liquid-solid process. However, each by-product presented a particular tendency on the sorption enhancement by hydration. LG-MgO (Figure 6) presented the highest adsorption capacity (35.0 L·kg-1) with HM3, 27% higher than HM2 and 29% higher than HM1 (27.5 and 27.1 L·kg-1 respectively). The improvement in SO2 sorption is also attributed to the Mg(OH)2 formed in the hydration, which significantly increased the sorption capacity during the surface reaction controlled stage (Reaction 1)27: +,- + /0 - → +,(-/)0 (1) According to the chemical characterization of the by-products (Table 3), LG-MgO presented 50 % of MgO that to some extent is available to be hydrated. Given the low reactivity of LG-MgO (Table 2, catalogued as “hard-burned”)

21

, minimum hydration

conditions (hydration time and S/W) are needed in order to increase the sorption capacity by the formation of Mg(OH)2. Thus, both HM1 and HM2 did not achieve these minimum hydration conditions and had a similar effect on raw LG-MgO. LG-D (Figure 7) achieved a SO2 capture of 34.5 L·kg-1 while LG-F presented the lowest value (26.8 L·kg-1), both also in semi-dry conditions (HM3). HM2 and HM1 had a similar effect on LG-D sorption capacity since both hydration conditions mainly favoured the hydration of Ca phases (22.4 and 20.9 L·kg-1 respectively). LG-F, though it contained 76.7 wt% of MgO, was obtained at the end of the calcination kiln and therefore was likely to be partially sintered (see Table 2: lowest BET value, larger d50 and catalogued as deadburnt)

20

. However, LG-F showed a significant increase in the controlled surface 14 ACS Paragon Plus Environment

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reaction period because its higher CaO content promoted the formation of a hydration product -Ca(OH)2- with a high sorption capacity. Therefore, HM1 and HM2 had a significant effect on LG-F and LG-D and a lower effect on LG-MgO because of its negligible CaO content. Taking all these results into account, semi-dry conditions improved the sorption capacity of the three by-products because the SO2 can rapidly react with the hydrated alkaline phases when it is dissolved in water, increasing the controlled surface reaction stage

28

. Thereby, the gas-liquid-solid reaction mechanism

promoted by semi-dry conditions was complemented by the hydration reactions for enhancing SO2 sorption. 3.2.4. Characteristics of the spent sorbents The XRF analysis of the spent sorbents showed that all samples presented SiO2 content in the 30-40% range, due to an incomplete sieving of the reacted samples. This fact influenced thermal behaviour and the interpretation of the mass loss steps since silica remained unaltered during TGA. Taking this into account, the spent sorbents were analysed qualitatively in order to elucidate the desulfurization mechanism and propose a further potential reutilization of the reaction products formed. According to XRF and XRD, the main reaction products were MgSO3 and MgSO4. The formation of CaSO4 and CaSO3 was limited by the availability of Ca in each by-product. Calcium was partially presented in LG-D and LG-F as CaO and its reaction with water vapour to form Ca(OH)2 prior sulfation was considered the desulfurization route. However, calcium was only present as CaCO3 in LG-MgO. Thus, in order to elucidate the desulfurization route of LG-MgO, which was the most efficient by-product, Figure 9a and 9b present the TG-MS results of raw LG-MgO (3 g) after desulfurization in N2 and air atmosphere respectively. Along with the mass loss steps, the MS signal (63.96/A) in Figure 9a showed a SO2 release at about 400ºC and 950ºC. The water and the CO2 15 ACS Paragon Plus Environment

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emissions were also confirmed by mass spectrometry (figures not shown). Thus, the mass loss steps were ascribed as: (i) water loss (< 200°C), (ii) magnesium sulphite decomposition to MgO and SO2 (200-530°C), (iii) CO2 release from MgCO3 decomposition (500-650°C), (iv) CO2 release from CaMg(CO3)2 decomposition (600900°C) and (v) CaCO3 decomposition along with the desulfation of calcium sulphite (900-1000 °C), as reported elsewhere 21,29–31. All the mass loss steps corresponded to the compounds of the raw by product that remained in the spent sorbent. The decomposition temperature of magnesium sulphite (maximum peak at 340°C) was lowered by the reducing conditions promoted by the CO emitted from the incomplete combustion of the remaining petcoke, the fuel used during the industrial calcination process

30,32

. The final amount of CaCO3 was a balance between the CaCO3 present in

the raw sorbent, the CaCO3 formed by reaction of calcium hydroxide and CO2 and the CaCO3 that disappeared after reacting with SO2, as reported elsewhere for a desulfurization process with calcium hydroxide

26,33

. The preceding was confirmed by

Figure 9b, where all the mass loss steps before 800ºC could be described as in Figure 9a although the mass loss step attributed to the decomposition of calcium sulphite was not detected and a mass loss step at 1300ºC with SO2 release (63.96/A) was observed instead; this mass loss was attributed to the SO3 (SO2 + ½ O2) released from the calcium sulphate formed by oxidation of the CaSO3 (or partially by the reaction of CaCO3 and the SO2 released) in the TG process in air atmosphere. Therefore, magnesium and calcium sulfite were the main reaction products obtained after desulfurization. Taking this into account, the limestone contained in LG-MgO acted as desulfurization agent along with the MgO phases. Accordingly, given the chemical composition of the reaction products obtained after the desulfurization, these could be further recovered or


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reutilized, as the magnesium sulphite contained could be reused for regenerating the sorbent or as fertilizer 27. 3.2.5. RI and comparison with WFGD performance Table 4 presents a summary of the average RI for each experimental trial. This parameter took into account the minimum value observed for SO2 concentration at the outlet of the process (i.e. maximum SO2 adsorbed). Therefore, it is only related to the surface reaction controlled stage. As it was shown in Figure 4, LG-MgO presented the longest surface reaction controlled period, as reflected in the RI values. The same increase in the reaction control period is shown in Figure 6, 7 and 8 for HM3 conditions. In this aspect, HM3 yielded the highest reactivity index in all by-products, a maximum 99% (98 ± 1%) for LG-D. The lowest RI was observed for raw LG-F (79 ± 6%). In accordance to the past sections, the low reactivity of LG-F diminished the chemical reaction rate and therefore its reactivity index. Especially for LG-MgO and LG-D, HMs effectively increase SO2 removal capacity in the stage controlled by surface reaction rate, allowing to obtain lower SO2,t before diffusion became the rate limiting step. The average desulfurization potential (L of SO2 per kg) of LG-MgO, LG-D and LG-F for a 100% removal efficiency in a WFGD process were reported to be in the 344256 L·kg-1 range (2.9-3.9 kg·m-3)

1,2

. In the same study, the authors compared the by-

products performance with that of the widely used lime (834 L·kg-1 or 1.2 kg·m-3). Therefore, the use of these by-products in a wet process is significantly more effective at totally neutralizing SO2. However, a dry desulfurization process avoids generating wastewater effluents and allows obtaining a solid mixture basically made of MgSO3/MgSO4, CaSO3/CaSO4 and unreacted compounds (e.g. MgCO3, CaMg(CO3)2, MgO and CaO). Therefore, the compromise between efficiency, consumption of by-


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product and generation of reaction products has to be analysed and taken into consideration before extrapolating to the industry. 4. Conclusions As an alternative to wet flue gas desulfurization, a sustainable dry desulfurization process was studied by assessing the performance of the by-products obtained during the calcination process of natural magnesite. Thus, the by-products from the calcination process that is producing the SO2 emissions could be used as desulfurization agents in a closed-loop process. The three by-products under consideration, LG-MgO and LG-D (dust materials from fabric filters) and LG-F (coarse fraction of the final magnesia product) presented different chemical and physical characteristics. In order to improve the by-products reactivity towards SO2, three different hydration methods were also applied to the raw by-products. Raw LG-MgO (68.1 wt.%) presented the best desulfurization potential (L of SO2 per kg of by-product), up to 25 L·kg-1, very convenient compared to the performance of the widely used lime in the same conditions (50 L·kg-1). The adsorption capacity of LG-D and LG-F was lower, achieving 16.5 and 15.9 L·kg-1 respectively. The analysis of desulfurization curves (accumulative L·kg-1 vs. time) showed that the greater specific surface area, the higher Mg(OH)2 content and lower particle size of LG-MgO were not of significant influence on the surface reaction control period. However, the high content of CaO in both LG-D and LG-F raw materials and the fast formation of product covering the particle surface influenced their behaviour in the diffusive control period. Hydrating the raw by-products allowed to increase the SO2 sorption capacity to up to 30-35 L·kg-1. Thus, for all by-products, semi-dry conditions had the greatest influence among the hydration methods applied. The use of the same by-products in a wet process was reported to be more effective than the performance in dry state. However, dry desulfurization avoids generating 18 ACS Paragon Plus Environment

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wastewater effluents while obtaining a solid mixture basically made of magnesium and calcium sulphite. These solids could be reused for other applications, allowing to further extend the sustainability criteria of the overall process. Acknowledgements The authors would like to thank Magnesitas Navarras S.A for their financial support. Ricardo del Valle Zermeño is grateful to the Government of Catalonia (Departament d'Universitats, Recerca i Societat de la Informació) for the research grant (FI-DGR 2014) and to the Fundació Montcelimar (Barcelona, Spain) for funding. References

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Del Valle-Zermeño, R.; Niubó, M.; Formosa, J.; Guembe, M.; Aparicio, J. A.; Chimenos, J. M. Chem. Eng. J. 2015, 262, 268–277.

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Mathieu, Y.; Tzanis, L.; Soulard, M.; Patarin, J.; Vierling, M.; Molière, M. Fuel Process. Technol. 2013, 114 (0), 81–100.

(4)

Scala, F.; D’Ascenzo, M.; Lancia, A. Sep. Purif. Technol. 2004, 34 (1-3), 143– 153.

(5)

Nolan, P. S. Flue Gas Desulfurization Technologies for Coal-Fired Power Plants, The Babcock & Wilcox Company U.S., in: Coal-Tech 200 International Conference, November, 2000, Jakarta, Indonesia, 1-13.

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Sanders, J. F.; Keener, T. C.; Wang, J. Ind. Eng. Chem. Res. 1995, 34 (1), 302– 307 .


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Table 1. Summary of the desulfurization experimental trials By-product B/SiO2

By-product Conditions Raw

3/50

1/30

HM1 LG-MgO 1/30

HM2 HM3 Raw

3/50

1/30

HM1 LG-D 1/30

HM2 HM3 Raw

3/50

1/30

HM1 LG-F HM2

1/30

HM3 2 3


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4 5

Table 2. Chemical composition determined by XRF analysis, acid test results and physical properties for the three by-products. LG-MgO (%)

LG-D (%)

LG-F (%)

MgO

68.1

41.5

78.7

CaO

8.9

22.1

9.9

SO3

8.1

8.3

2.1

Fe2O3

2.6

3.5

2.9

SiO2

1.5

0.7

2.8

K2 O

1.5

0.4

-

LOI

8.7

23.1

3.6

Acid test

Hard-burned

Dead-burned

Dead-burned

BET (m2·g-1)

6.6

4.6

2.6

d50 (µm)

23.1

37.5

141.4

6 7


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Table 3. By-products chemical characterization using XRF, XRD and TGA. LG-MgO (%)

LG-D (%)

LG-F (%)

Mg(OH)2

4.2

3.9

3.4

MgCO3

17.2

11.1

-

CaMg(CO3)2

6.3

33.6

1.7

CaCO3

15.1

10.1

0.7

MgSO4

-

4.6

-

MgO

50.1

23.5

76.7

CaO

-

5.9

8.6

CaSO4

1.7

3.7

-

Rest

5.6

4.6

11.1


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Table 4. Summary of the average reactivity index (RI) obtained for each experimental trial.

By-product

RI (%)

By-product Conditions Raw

88 ± 3

HM1

90 ± 6

HM2

91 ± 5

HM3

93 ± 4

Raw

83 ± 3

HM1

89 ± 3

HM2

96 ± 1

HM3

98 ± 1

Raw

79 ± 6

HM1

79 ± 6

HM2

82 ± 4

HM3

96 ± 1

LG-MgO

LG-D

LG-F


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Figures Figure 1. Experimental scheme of the dry desulfurization experiments. Figure 2. TGA of the raw by-products (LG-MgO, LG-D and LG-F). Figure 3. Concentration of SO2 at the outlet (ppm) as a function of time (s) for the three by-products. Effect of mass sample. Figure 4. Desulfurization potential (volume of SO2 absorbed per mass of by-product) of 1 g of each of the by-products (L·kg-1). Figure 5. Desulfurization potential (volume of SO2 absorbed per mass of by-product) of 3 g of each of the by-products (L·kg-1). Figure 6. Desulfurization potential (L·kg-1) of raw and modified (HM1, HM2 and HM3) LG-MgO. Figure 7. Desulfurization potential (L·kg-1) of raw and modified (HM1, HM2 and HM3) LG-D. Figure 8. Desulfurization potential (L·kg-1) of raw and modified (HM1, HM2 and HM3) LG-F. Figure 9. TG, DTG and MS curve (M+ 64, corresponding to SO2) of raw LG-MgO after desulfurization using N2 as carrier gas (9a) and synthetic air (9b).


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Figure 1 Del Valle-Zermeño et al. (2015)


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Page 28 of 35

100 LG-F

90

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

LG-MgO

70

LG-D

60 0

200

400

600

800

Temperature (°C)

1000

1200

1400 Universal V4.5A TA Instruments

Figure 2 Del Valle-Zermeño et al. (2015)


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4000

LG-D

3500

3000

3000 2500

2500 2000

2000 1500

1500 1000

1000 1g

1g 500

3g

500

3g

0

0 0

500

1000

1500

2000 2500 Time (s) 4000

3000

3500

4000

0

500

1000

1500

2000 Time (s)

2500

3000

3500

4000

LG-F 3500 3000

SO2 at the outlet (ppm)

SO2 at the outlet (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

3500

LG-MgO

2500 2000 1500 1000 1g 3g

500 0 0

500

1000

1500

2000

2500

3000

3500

4000

Time (s)

Figure 3. Del Valle-Zermeño et al., (2015)

29

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30 SO2 adsorbed per mass of by-product (L·kg-1)

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LG-MgO LG-D LG-F

25

20

15

10

5

0 0

500

1000

1500

2000 Time (s)

2500

3000

3500

Figure 4 Del Valle-Zermeño et al., (2015)


4000

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25 SO2 adsorbed per mass of by-product (L·kg-1)

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LG-MgO LG-D LG-F 20

15

10

5

0 0

500

1000

1500

2000 Time (s)

2500

3000

3500



4000

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40

SO2 adsorbed per mass of by-product (L·kg-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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35 30 25 20 15 LG-MgO: HM1

10

LG-MgO: HM2 LG-MgO: HM3

5

LG-MgO: Raw 0 0

500

1000

1500

2000 Time (s)

2500

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3500



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35 30 25 20 15 LG-D: HM1

10

LG-D: HM2 LG-D: HM3

5

LG-D: Raw 0 0

500

1000

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2000 Time (s)

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4000

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25

20

15

10 LG-F: HM1 LG-F: HM2 5

LG-F: HM3 LG-F: Raw

0 0

500

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2000 Time (s)

2500

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Reutilization of MgO Byproducts from the Calcination of Natural

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