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Enhanced CO2 capture through reaction with steel-making dust in high salinity water
International Journal of Greenhouse Gas Control 91 (2019) 102819
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Enhanced CO2 capture through reaction with steel-making dust in high salinity water
T
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Mohamed H. Ibrahima, Muftah H. El-Naasa, , Ron Zevenhovenb, Saad A. Al-Sobhic a
Gas Processing Centre, College of Engineering, Qatar University, Doha, Qatar Åbo Akademi University, Thermal and Flow Engineering, Turku, Finland c Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon capture CO2 sequestration Steel-making waste Steel dust Brine
Mineral carbonation (MC) is evolving as a possible technology for sequestering CO2 from medium-sized emission point sources. Industrial wastes have been recently used as an effective source for MC that have higher reactivity than natural minerals; they are also inexpensive and readily available in proximity to CO2 emitters. In this work, accelerated carbonation of electric arc furnace (EAF) baghouse dust (BHD) in a reject brine medium was evaluated in a novel reactor system, specially designed for contacting gases and liquids. This approach is environmentally friendly and eliminates the cost associated with pre-treatment. Experimental design was utilized to determine the effect of the operating parameters (solid to liquid ratio, CO2 gas flowrate and inert particles fraction) on the CO2 uptake. Analysis of the experimental results indicated that the studied factors had a significant impact on CO2 uptake, which was observed to be in the range of 0.1–0.18 gCO2/g BHD. At ambient conditions (24 °C and 1 atm) and at optimum operating parameters, the optimum CO2 uptake was 0.22 g CO2/g BHD. A higher CO2 uptake performance of 1 ± 0.04 gCO2/g BHD was achieved at ambient temperature and pressure of 5 bar. Thermal gravimetric analysis of the solid products revealed that a variety of carbonate products have been produced, particularly, calcium and magnesium carbonates.
1. Introduction Mineral carbonation (MC) is based on the principles of natural weathering (carbonation) of natural rocks where CO2 dissolved in rainwater forms a weak carbonic acid. Consequently, alkali and alkaline earth metals available within the rocks neutralize the acid to form insoluble carbonate minerals (i.e. calcium and magnesium carbonates) (Bobicki et al., 2012; Kunzler et al., 2011). Alkaline earth metals such as pure magnesium and calcium oxides rarely exists in nature; however, they coexist with other elements (i.e. silicon) to form minerals such as olivine (Mg2SiO4), wollastonite (CaSiO3), and serpentine (Mg3Si2O5(OH)4) (Olajire, 2013). The availability of a vast amount of a certain natural mineral is a key factor to sustain the sequestration process. Hence, magnesium-based silicates are preferred due to their abundance globally (Metz et al., 2005; Lackner, 2003). The carbonates formed from the MC reaction are in the solid phase and the amount of heat required to decompose these carbonates and release CO2 is substantial, making them a thermodynamically stable CO2 sink (Power et al., 2013). Once CO2 is mineralized, it is trapped permanently without the need of any additional monitoring contrary to
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other CO2 sequestration options (Mun and Cho, 2013). MC was first conceptualized by Seiftriz (Seifritz, 1990) who suggested introducing high purity CO2 to accelerate the carbonation process, hence the name “accelerated carbonation”. This ensures that carbonation time can shift from geological time scale to hours or even minutes. The literature of MC has expanded significantly since its first conceptualization (Sanna et al., 2014a; Pan et al., 2015; Ibrahim et al., 2019). However, the MC research is still facing several challenges that hinders its viability when deployed on large scale (Olajire, 2013). Overall, the technique possesses several advantages over other sequestration options, such as ocean and geologic sequestration due to concerns over long term stability and leakage (Gunning et al., 2010). In addition, MC produces several stable products that can be economically profitable and these products are typically produced in fewer steps than other techniques. Although naturally occurring minerals (i.e. calcium/magnesium silicates) have the potential to sequester huge amounts of CO2 due to their abundance, it is often considered unfeasible due to the extraction and pretreatment cost of the minerals and the impacts associated with it (Huijgen et al., 2005). This apart from numerous process challenges in terms of carbonation efficiency and energy intensity that led to slow
Corresponding author. E-mail address: [email protected] (M.H. El-Naas).
https://doi.org/10.1016/j.ijggc.2019.102819 Received 2 May 2019; Received in revised form 7 August 2019; Accepted 21 August 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Greenhouse Gas Control 91 (2019) 102819
M.H. Ibrahim, et al.
MC approach. Currently, research has focused on assessing and maximizing the sequestration of CO2 by optimizing: 1) operating conditions including pressure, temperature, solid-to-liquid ratio, CO2 gas flowrate, solid particle size, solid pretreatment and 2) the market value of the solid products (Sanna et al., 2014b). This work presents an experimental evaluation of an innovative direct aqueous carbonation approach of EAF baghouse dust (BHD) in reject brine, using a specially designed reactor system. The work aims to improve the efficiency of the carbonation process and reduce the energy requirement and cost associated with pre-treatment; at the same time, the approach contributes to the management of carbon emissions, steel-making waste and desalination reject brine. To the best of the authors’ knowledge, direct aqueous carbonation of steel-making waste in the presence of reject brine has never been addressed in the open literature.
industrial deployment (Bui et al., 2018). Alkaline industrial wastes rich in magnesium and calcium oxides offer yet another attractive CO2 sequestration route. Although alkaline wastes are available in smaller amounts than natural minerals, they are available at lower cost, has a higher reactivity and uptake capacity and require less pretreatment. Examples of industrial wastes suitable for MC include fly ash such as coal and oil shale ashes, cement industry waste, and iron- and steelmaking slags. These wastes come from fundamentally different industries; however, they share multiple similar intrinsic features:
• The waste is rich in alkali earth metal content. • The waste tends to be less stable than rock minerals and needs to be stabilized. • The manufacturing processes produce substantial amounts of CO . 2
Steel industry contributes approximately 7% of CO2 emissions worldwide (Pan, 2012). In addition, the industry produces several types of wastes that can be utilized. There are four main types of steelmaking wastes, including blast furnace (BF), basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle furnace (LF); each is named after the process steps during which they are produced. The waste out of these steps is a consolidated mix of many compounds, primarily calcium, iron, silicon, aluminum, magnesium, and manganese oxides that are present in different phases. On average, manufacturing 1 ton of steel produces 2.1 ton of CO2 and approximately 170 kg of EAF dust and 180 kg of EAF slags (Said et al., 2016). Desalination plants use large volumes of seawater and discharge concentrated water (reject brine) back to the environment. Brine production is usually equivalent or greater than the total volume of the desalinated water. Brine production in the Middle East and North Africa is estimated to be about 100 million m3/day, accounting for 70.3% of the global brine production (Jones et al., 2019). Another major environmental concern of water desalination is attributed to the release of considerable amounts of CO2, which is mainly due to the use of fossil fuel as the main energy source for desalination plants. CO2 emission in desalination plants depends on the type of the desalination process, whether it is a pressure or thermal driven desalination. It is estimated that carbon emission of desalination plants is about 76 million tons per year (Zhang et al., 2018), and it is expected to reach 218 million tons per year in 2040 (Shahzad et al., 2017). In addition, reject brine contains sufficient concentrations of magnesium and calcium ions that can further add to CO2 capture capacity of the MC process (Bang et al., 2017). In fact, the utilization of brine has been reported to increase the MC process efficiency (Nyambura et al., 2011; Lakmali and Ranjith, 2015; Wang et al., 2013). Hence, taking a combined approach of supplying CO2 from the steel industries or stationary CO2 point sources (i.e. power and desalination plants) and at the same time stabilizing industrial waste by sequestering carbon dioxide in reject brine can greatly contribute to the total CO2 emission reduction globally. There are two main approaches for alkaline waste carbonation: direct and indirect technique. Direct carbonation technique implies that the MC process happens in one single step. Alternatively, indirect carbonation consists of two or more steps that usually include pre-treatment of the utilized waste. Pre-treatment usually involves grinding, sieving and/or extracting techniques such as acid extraction, heat activation or bioleaching (Bobicki et al., 2012). The excessive use of extraction agents is needed to enhance the carbonation kinetics; however, this adds one more step to the overall process hence increasing the total process cost and creating a non-environmentally friendly MC process (Sanna et al., 2014a). Elevated temperature, pressure and certain pretreatments have been reported to improve the carbonation process but inevitably these conditions can increase the carbonation cost up to 4000 €/ ton steel waste (Sanna et al., 2014b). In addition, pre-treatment steps increase the overall MC process carbon footprint (Ncongwane et al., 2018), which negates the value of the CO2 capture process. This necessitates the need for a greener and more sustainable
2. Materials and methods 2.1. Materials characterization EAF BHD samples were collected from open-to-atmosphere storage yard from a local steel factory in the region. Chemical characterization of the samples has been performed using VISTA- MPX CCD inductively coupled plasma-atomic emission spectrometry (ICP-AES) as shown in Table 1 (El-Naas et al., 2015). The ions concentration in the reject brine has been measured by Metrohm ion chromatograph (IC). The range of ion concentrations is shown in Table 2. EMD Millipore system was used to produce deionized water with a conductivity ≤ 4.3 μS/cm. X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of BHD. A mechanical sieve shaker was used for sieving analysis. The sieve analysis of the BHD indicated that the dust particles are in the range of 38 μm to 250 μm with 86.8% of the particles having a diameter less than 150 μm . Thermal gravimetric analysis (TGA) was performed using using a PerkinElmer Pyris 1 thermal gravimetric analyser. Due to temperature limitation of the used TGA equipment, Carbolite furnace (Model RHF 1400) is used to heat certain samples to 1200 °C. Microstructure characterization was carried out using a JOEL JSM-5600 scanning electron microscope. The pH of each BHD-reject brine mixture is measured using a HACH portable pH meter (Model HQ11D) and the conductivity of the mixture was measured using a HACH digital conductivity meter (Model HQ14D). Anton Paar rheometer (Model MCR 302) was used to measure the viscosity of BHDreject brine mixture. 2.2. Aqueous carbonation In each experimental run, a certain BHD mass is mixed with a one liter of reject brine. The reject brine was prepared by evaporating 2 L of sea water to 50% of its volume (1 L) achieving conductivity values in the range of 100–120 ms cm . pH measurements are taken per 10 g of added BHD till the entire dust mass is added to the reject brine at room Table 1 Chemical composition of BHD using an Inductively Coupled Plasma.
2
Component
Composition (Weight %, dry)
FeO/Fe2O3 CaO MgO SiO2 Na2O K2 O MnO Al2O3 SO3 Others
42.8 40.2 4.96 4.49 2.52 1.98 0.68 0.28 1.10 0.99
International Journal of Greenhouse Gas Control 91 (2019) 102819
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2.3. Reactor description
Table 2 Ions concentration range in reject brine using IC.
Concentration range (g/l)
Ca2+
Mg2+
Na+
Cl−
0.32–0.50
0.9–1.5
7.4–12.4
17.8–23.3
The reactor (Fig. 1) is made of stainless steel and comprises of vertical vessel with gas and liquid inlet ports, containing inert particles, with a total volume of the inert particles in the range 3–18% of the total working volume of the vessel. The total volume of the reactor vessel is 3 L; however, only 1 L of the IPSBR volume has been used in this study. The inert particles provide circular motion which enhances the mixing process without compromising the operational aspects of the system such as the need of high mechanical energy. Specifically, the inert particles are dispersed and move within the vessel to promote mixing between the gas and the liquid and to provide a higher gas-liquid interfacial area for effective mass transfer between the two phases. The particles used in this study were made from polyethylene material in the form of a sphere with a density of approximately 0.97 g/l, a diameter of 5.1 mm and surface area of 81.7 mm2 /particle. Generally, the material of the inert particles depends on the liquid and gas used in the system and should not react with any of the liquids or gases or act as a catalyst for the reaction. The movement of the inert particles within the vessel is caused by the gas entering the vessel via the gas inlet. The gas is introduced centrally at the base of the vessel to cause the inert particles to move along a circular or elliptical path up and down the vessel while in operation. The gas feed enters through a single spouting orifice (diameter: 1 mm) at the bottom of the reactor and leaves at the top. It is also worth noting that the IPSBR can be operated in the continuous mode. In such case, the BHD-reject brine mixture can leave through the effluent tube (at bottom of the reactor) by the liquid hydrostatic pressure in the vessel (El-Naas et al., 2017a).
temperature (24 °C). The pH value of BHD and reject brine mixture stabilizes at 11 as an average reading for all prepared solutions. All experiments were conducted in semi-batch mode, in which the prepared solution is placed inside an inert particles spouted bed reactor (IPSBR) system and then CO2 is injected through the bottom of the reactor as a spouting gas. The feed flow rate of the gas mixture is controlled by a mass flow controller (Brooks Instruments SLA 5800, USA). The gas mixture, which contains 10 ± 1.1% (mol/mol) carbon dioxide (CO2) and 90% nitrogen (N2), was provided by Buzwair Scientific and Technical Gasses, Qatar. The mixture serves as the only source of CO2 throughout this work. Carbon dioxide that have been captured inside the IPSBR is calculated by analyzing the CO2 composition of the outlet stream using a continuous CO2 analyzer (Quantek Model 906 acquired from Quantek Instruments, Massachusetts, USA). The length of each experimental run depends on the time it takes for the CO2 concentration at the outlet to be equal to its inlet value, indicating the termination of CO2 capture process. The analyzer readings were regularly confirmed by utilizing a portable CO2 analyzer (Dansensor CO2 CheckPoint, Denmark) and a gas chromatograph (PerkinElmer Model Clarus 580). After the experiment is completed, the IPSBR content is drained for further analysis. The carbonated mixture was left to settle and the clear water of the reject brine was decanted. This water was filtered using PTFE syringe filter (pore size: 0.45μm ) for ions analysis. The remaining solids were dried at 105 °C for 24 h to evaporate any water remaining in the sample.
2.4. Design of experiment Design of experiments (DOE), using Response Surface Methodology (RSM), was utilized to design a set of experiments involving twenty experimental runs. RSM involves developing and executing a fixed number of experiments in specific order to measure how the
Fig. 1. A schematic diagram of the experimental setup which consists of A) Gas cylinder (CO2/N2) B) inert particles spouted bed reactor system C) CO2 gas analyser. 3
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3.1. Effect of gas flow rate
independent variables are affecting the system response. The main idea behind this approach is to maximize the information that can be extracted from conducting a minimum number of experiments (Bezerra et al., 2008). RSM was used to select Central Composite Design (CCD) to create an experimental design space consisting of three operational parameters: gas flowrate (l/min), solid to liquid (S/L) ratio (mass of BHD/ volume of reject brine) (g/l) and inert particles fraction (volume of particles/ volume of reject brine) (%). After several screening experiments, the experimental range of each parameter was chosen to be the following: 0.7–2.3 l/min gas flowrate, 23–57 g/l solid to liquid ratio and 3–18% inert particles fraction. RSM is utilized to generate a set of 20 experiments to analyze the individual and combined effects of the operating parameters on the CO2 uptake. Hereafter, the uptake can be optimized to achieve the best possible results.
The mixture of 10% CO2 and 90% N2 is introduced to the bottom of the reactor to simulate flue gas. Carbon dioxide is converted from the gaseous form to the ionic form according to the following equation: 2− + + CO2(g ) + H2 O(l) ↔ HCO3−(aq) + H(aq ) ↔ CO3 (aq) + 2H(aq)
Subsequently, CO2 reacts with the active alkali metal oxide which is converted to hydroxide form in the presence of reject brine :
M (OH )2 + CO2(aq) → MCO3 + H2 O
(2)
where M is Ca or Mg. The alkalinity of the component governs its CO2 sequestration capacity since carbon dioxide molecules are acidic, hence, more basic compounds (pH > 10) are better suited for MC process. Hereafter, based on surface diffusion phenomenon, CO2 has more affinity to be sequestered, based on oxides basicity, in the following order: basic oxides (CaO, MgO) > amphoteric oxides (FeO, Al2O3) > acidic oxides (SiO2) (Takaba et al., 1995). Hence, the theoretical CO2 uptake can be calculated based on the stoichiometric ratio in Reaction 2. The theoretical uptake is equal to 0.29 g CO2/ g BHD based on the total amount of MgO and CaO in BHD (Table 1). At a relatively low gas flowrate, the gas residence time increases which in turn prolong the contact between CO2 and BHD leading to more uptake. This is demonstrated in Fig. 2 as it shows two experiments with the same solid to liquid ratio (30 g/l) and inert particle fraction (10%), but at different flowrates. An uptake of 0.147 g CO2/g BHD was obtained compared to 0.162 g CO2/g BHD when the gas flow rate changed from 2 l/min to 1 l/min, respectively.
2.5. CO2 uptake calculation The CO2 analyzer readings in volume percent are being converted to equivalent value in the form of CO2 uptake. The uptake is defined by the amount of CO2 that has been reacted over the amount of BHD used in a specific experiment (mass of CO2 sequestered / mass of BHD). Hence, to obtain the uptake value for each experiment the following calculations have been carried out: The area under of the curve CO2 % vs. time represents the volume of CO2 passed through the reactor without being sequestered:
Volume of uncaptured CO2 passed through the system (Vu ) t f CO2 % = F dt 0 100
∫
CO2 %: CO2 composition reading given by the analyzer (dimensionless) F : experiment flowrate (l/min) t f : time that corresponds to the duration of carbonation process (min) dt : time increment (min) Hence, the total volume of CO2 that was fed to the system, (Vt), equals: = F ×
(1)
3.2. Effect of S/L ratio and inert particles volume fraction The CO2 uptake of the carbonation process is expected to increase when the BHD mass is increased, since more dust mass will be available to react with the CO2, while also the pH of the solution is increased. However, in this particular study, this behavior cannot be generalized as it will be explained below by introducing the effect of the inert particles volume fraction. Increasing BHD mass is expected to increase the viscosity of dustreject brine mixture. To confirm this, Fig. 3 shows the viscosity of several S/L ratios at different shear rates measured using rheometer. At a shear rate of 0.1 s−1, 60 g/l slurry has more than nine times the viscosity of 20 g/l slurry. The more viscous a solution is, the more mixing it requires to achieve acceptable CO2 uptake values specially at low shear rate. When the shear rate increases, hence more mixing, the viscosity of mixture significantly decreases. The circular motion
CO2 sat % × tf 100
CO2 sat %: the point at which dust carbonation process completely stops (dimensionless) The volume of CO2 that has been sequestered in the dust carbonation process can be calculated by subtracting the two areas described above:
CO2 sequestered (Vr ) = Vt − Vu The volume of CO2 consumed can then be converted to mass of CO2 through molar volume relationship at the experiment conditions (Temperature and pressure) and then divided by the mass of the dust to obtain the specific uptake value (UCO2 )
Vr UCO2 (g CO2/ g BHD) = ⎛ × MwCO2⎞ ÷ mdust V ⎝ M ⎠ ⎜
⎟
RT
VM = P : molar volume (l/mol) MwCO2 : carbon dioxide molecular weight (g/mol) mdust : mass of BHD (g) 3. Results and discussion The sections below describe the effect of the studied parameters on CO2 uptake and how their interactions affect the MC process. This is discussed in a sequential fashion showing the effect of each parameter on the process individually. After that, the combined effect of the three parameters on the uptake is explained.
Fig. 2. The effect of flowrate on CO2 uptake. F: gas flowrate (l/min), R: solid to liquid ratio (g/l) and P: inert particles volume fraction. 4
International Journal of Greenhouse Gas Control 91 (2019) 102819
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to ensure enough mixing. In addition, the BHD mass does not increase the viscosity of the mixture to a degree that will hinder the mixing process. The figure also shows that uptake increased by 10% by changing the inert particles fraction from 6 to 15%. This behavior signifies that at low BHD mass, the effect of increasing inert particle volume fraction on mixing is minimal compared to Fig. 5. The S/L in Fig. 5 ratio was relatively high, 40 g/L, hence the mixture is more viscous and more inert particles are required. As more particles are present, more CO2 is captured with the same BHD mass. In Fig. 5, increasing particles fraction from 3 to 10.5% resulted in 16% increase in CO2 uptake. Thus, S/L ratio and inert particles fraction are interconnected. Fig. 6 demonstrates this relation by showing how the CO2 uptake changes as a function of the two parameters at a constant gas flowrate (0.7 l/min). The contour plot (Fig. 6b) illustrates that for certain inert particle fraction, an optimum solid to liquid ratio exists. For example, at a particle fraction of 14%, the maximum CO2 uptake is approximately 0.15 g CO2/g BHD. This uptake occurs at S/L ratio of 40 g/L. Increasing the ratio at the same inert particles fraction (14%), will lead to a more viscous solution that hinders the mixing process, hence, lowering the CO2 uptake. This behavior is illustrated in the concave down curve in Fig. 6A. Hence, to achieve higher uptake values, the increasing S/L ratio should be combined with an increase in inert particles fraction as well. This is shown by the linear relationship between the uptake and inert particle volume shown in Fig. 6A. Thus, the role of inert particles is more prominent at high S/L ratios and low gas flowrate at which more mixing is required as opposed to low S/L ratios and high flowrate. Up to this point, the discussion revolved around the individual effect of the three operating parameters on CO2 uptake by selectively choosing similar experiments from the experimental design space and comparing the different effects of these parameters on the uptake. The CO2 uptake results obtained from the 20 conducted experiments generated by CCD range from 0.101 to 0.180 gCO2/g BHD at 24 °C and 1 atm, with an average relative error of 4%. A gas flowrate of 0.7 l/min, S/L ratio of 57 g/l and inert particle volume of 18% were predicted by RSM to be the optimum conditions that would give the highest CO2 uptake. At these conditions, the obtained experimental uptake was 0.22 gCO2/g BHD for a time duration of 920 min, which is comparable to RSM predictions (0.23 gCO2/g BHD) with a relative error of 5%.
Fig. 3. Viscosity of several S/L ratios (20, 40 and 60 g/l) of EAF BHD in reject brine at different shear rates.
Fig. 4. The effect of inert particles volume fraction on CO2 uptake at relatively low S/L ratio (30 g/l) and flow rate of 1 l/min.
3.3. Carbonation at elevated pressure To study the effect of elevated pressure on MC process, an experiment was carried out at the optimum conditions with a process pressure of 5 bar. The pressure inside the reactor is controlled by a valve at the outlet of the reactor. At the end of the experiment, the pressure inside the reactor is decreased gradually to reach 1 bar. This resulted in a CO2 uptake of 1.0 ± 0.04 gCO2/g BHD as an average of two repeated experiments. Compared to the optimum conditions at atmospheric pressure, this is a 480% increase in the uptake value, moreover in 43% less time (400 min compared to 920 min). It is worth noting that in Reaction 2, the formation of CaCO3 is dominant when Ca(OH)2 is present in amounts greater than stoichiometric ratio (El-Naas et al., 2017b). Increasing the pressure forces more CO2 to stay into the solution and shifts the carbonation reaction towards the products side. This makes CO2 available in amounts greater than the stoichiometric ratio. Hence, this situation allows for the formation of sodium bicarbonate: Fig. 5. The effect of inert particles volume fraction on CO2 uptake at relatively high S/L ratio (40 g/l) and flow rate of 1.5 l/min.
Ca (OH )2 + 2CO2(aq) + 2NaCl (aq) → CaCl2 + 2NaHCO3 ΔG = −70.40
provided by the inert particles enhances the mixing process especially when the mixture is viscous (high S/L ratios). Figs. 4 and 5 show the effect of inert particles fraction at different S/L ratios. Two different trends can be demonstrated from the obtained results: First, at relatively low S/L ratio, such as Fig. 4, inert particles volume fraction have no significant effect on the uptake. In this case the flowrate is sufficient
kJ kmol
(3)
The presence of sodium chloride in the reject brine will shift the effective stoichiometric ratio between Ca(OH)2 and CO2 to be 1:2, thereby doubling the uptake capacity of the BHD. As a result, the theoretical CO2 uptake increases to reach 0.55 g CO2/ g BHD. Hence, the theoretical CO2 uptake by the BHD ranges from 0.29 to 0.55 gCO2/g 5
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Fig. 6. Combined effect of solid to liquid ratio and inert particles % on CO2 uptake (a) surface plot (b) contour plot at the optimum flowrate (0.7 l/min).
attributed to the contribution of iron oxide. Approximately 43% of BHD is composed of iron (II) and iron (III) oxides. XPS is used to determine the exact composition of each iron component. The surface analysis of XPS revealed that out of the total iron oxides composition, 70% is iron (II) oxide and the rest (30%) is iron (III) oxide. At room temperature and pressure, iron (II) reacts with water to produce ferrous hydroxide spontaneously in an exothermic reaction, via:
FeO(s) + H2 O(l) → Fe (OH )2 ΔG = − 9.3
kJ kmol
(4)
This is shown in Fig. 8 as 67% of Fe(II) oxide in BHD turned to Fe (OH)2 in brine reject. The adsorbed layer of water on the surface of FeO layers provides a medium for reaction with CO2. Free Fe2+ ions can react with CO32− to form FeCO3 (Kumar et al., 2016):
Fe (OH )2 + CO2(aq) → FeCO3(s) + H2 O(l) ΔG = −25.7
kJ kmol
(5)
With time, the FeCO3 layer increases while the FeO layer shrinks according to the shrinking core model. A detailed explanation for the mechanism for CO2 reaction at the adsorbed iron oxide–water interface has been previously reported (Yamamoto et al., 2010; Wang and Hellman, 2018). Kumar et al. Kumar et al. (2016) demonstrated that FeO can adsorb 0.106 g CO2/g adsorbent in a slightly humid environment. Therefore, it is expected that more CO2 will be adsorbed on the surface of iron oxide in aqueous medium. TGA in Fig. 9 was used to confirm the measured uptake (1.0 ± 0.04 gCO2/g BHD) results at 5 bar. The largest weight loss took place in the temperature range 700–800 °C which represent CaCO3 decomposition temperature, besides the range of 350–650 °C for MgCO3 decomposition temperature (Yan et al., 2015). A total mass reduction of 67% was achieved for the 5 bar experiment. A separate sample of the same experiment was heated up to 1200 °C. The resulted mass reduction was 72% which confirms the TGA results. In other words, 72% of the sample had carbon-bearing compounds, products from the reaction of between CO2 and BHD. The difference between the measured uptake value (1 ± 0.04 gCO2/ g BHD) and the TGA is probably due to the contribution of NaHCO3, which is highly soluble in water, and it is mostly removed when the liquid is decanted and the soluble CO2 in the reject brine within the reactor system. An extra couple of experiments have been conducted as illustrated in Fig. 10. An experiment was conducted in an aqueous medium of deionized water instead of reject brine. The CO2 uptake value was 0.106 gCO2/g BHD, compared to the optimum conditions 1 bar pressure experiment uptake (0.22 gCO2/g BHD). This is a 48% decline in uptake when deionized water was used. This further supports the claim that CO2 reacts with calcium ions in the reject brine hence increasing the CO2 uptake. Finally, to further validate the use of inert particles, an
Fig. 7. SEM image of precipitates of the filtered brine after carbonation showing the formation of sodium bicarbonate confirmed by EDX (the arrows point to the spots for the EDX measurements).
BHD depending on the conversion of calcium and magnesium in the solids to carbonates or bicarbonates. The products of Reaction 5 are calcium chloride and sodium bicarbonate which are both highly soluble in water. However, sodium bicarbonate can precipitate when the water temperature is decreased to 15 °C. SEM analysis of participates from the filtered brine after the carbonation process revealed the formation of sodium bicarbonates taking irregular shapes deposited on precipitated magnesium carbonates as shown in Fig. 7. Thus, the occurrence of this carbonation route can be validated. Although the formation of hydrated magnesium carbonates can be expected, they form as intermediate product that is transformed into anhydrous carbonates while subjected to excess reject brine, and/or longer reaction time (Fricker and Park, 2013). Additionally, analysis of ions available in the brine after the carbonation process revealed that there was a significant reduction in calcium ions compared to its initial concentration as given in Table 2. Throughout the conducted experiments, the calcium was reduced from the original value in Table 2 to be approximately 0.07 ± 0.03 g/l. This indicates that calcium ions in the brine reacted with CO2 and are converted into calcium carbonate contributed to the total CO2 uptake. Again, this contribution depends on the conversion of calcium and magnesium ions to carbonates or bicarbonates. Some of the CO2 captured (1 ± 0.04 gCO2/g BHD) can be 6
International Journal of Greenhouse Gas Control 91 (2019) 102819
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Fig. 10. Optimum experiment comparison at different conditions.
benchmark this particular study. Chang et al. Chang et al. (2011) reported an uptake of 0.27 g of CO2 based on the amount of calcium oxide in the sample. The value is quite high compared to the literature. However, an extraction step was needed to obtain CaO which adds to the process cost. In addition, it was conducted at 70 °C which further adds to the process energy requirement. Pan et al. Pan et al. (2014) used a rotating packed bed to achieve 0.16 g CO2/g CaO at ambient temperature and pressure. Even though these results are only based on calcium oxide, they seem to be comparable to the uptake obtained in the current study before optimization (0.18 g CO2/g BHD). If the results of the current study were only based on the calcium in the sample (Table 2), the uptake value would be 0.45 gCO2/g CaO (0.18/0.4). However, this is not an accurate representation of the results as the uptake in this study occurs due to reaction of CO2 with Ca, Mg and Fe bearing compounds as established above. Polettini et al. Polettini et al. (2016) reported a CO2 uptake of 0.536 g based on the total amount of slag, which is one of the highest reported in the literature. However, compared to the current study, the reported experimental parameters are drastically different. Since the authors utilized a gas stream containing 60% CO2 (compared to 10%), a larger uptake is expected. In addition, at approximately the same pressure (5 bar) and significantly lower temperature, the current study produces double the CO2 uptake (1 ± 0.04 gCO2/g BHD). Therefore, based on the above comparisons, the following can be stated:
Fig. 8. XPS analysis of FeO (a) before carbonation (b) after carbonation.
• At ambient temperature and pressure, this work provides a good •
CO2 uptake based on the amount of dust used compared to the literature. At moderate pressure (5 bar), the uptake results (1 ± 0.04 gCO2/g BHD) is relatively higher than the literature at the same experimental conditions.
It is also important to point out that no pretreatment has been applied to the EAF BHD samples, which greatly contributes to the significance of the results since the additional cost of the pretreatment is eliminated. Also, the carbon footprint of the proposed process is negligible.
Fig. 9. TGA for optimum pressure experiment at 5 bar showing MgCO3 deception at 350 °C and CaCO3 decomposition at 750 °C.
experiment was conducted at optimum conditions without added inert particles. The obtained uptake was 0.17 gCO2/g dust which is 21% lower than the optimum conditions uptake (0.22 gCO2/g BHD).
4. Conclusions Carbon sequestration can be achieved through different techniques that have the potential to capture substantial amounts of CO2. Mineral carbonation is evolving as a possible candidate to sequester CO2 from medium-sized emissions point sources. Industrial wastes, such as ironand steelmaking dust are rich in alkaline compounds especially calcium and magnesium oxides. Electric arc furnace bag house dust exhibits sufficient alkaline properties as it is enriched with calcium and
3.4. Results comparison Table 3 shows a number of different carbon mineralization studies in order to compare it with the current work. These studies concluded with relatively good CO2 uptake result and therefore these are used to 7
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Table 3 Literature comparison in terms of CO2 uptake. L/S: liquid to solid ratio, T: temperature, P: pressure, F: flowrate, RT: rotational speed, PP: partial pressure. Type of waste
Composition
Uptake
Reactor type
Conditions
Ref
BFS
CaO(wt%): 51.1 SiO2(wt%): 11.5 Fe2O3(wt%): 24.1
0.27 g CO/g CaO
Slurry
(Chang et al., 2011)
BOFS
CaO(wt%): 43 Fe2O3(wt%): 28.7
0.16 g CO2/g CaO
Rotating packed bed
BOF
CaO(wt%): 31 MgO(wt%): 7.5 Fe2O3(wt%): 27
0.536 g CO2/g slag
Batch
EAF
CaO(wt%): 40.2 MgO(wt%): 4.96 Fe2O3(wt%): 42.8
0.21 g CO2/g BHD
IPSBR
EAF
CaO(wt%): 40.2 MgO(wt%): 4.96 Fe2O3(wt%): 42.8
1 ± 0.04 g CO2/g BHD
IPSBR
L/S: 10 T: 70 °C CO2 PP: 101.3 kPa CO2 F: 0.1 L/min Time: 720 min RT: 541 rpm T: 25 °C L/S: 10 P: 1 bar Time: N/A T: 83.7 °C P: 5.9 bar L/S: 5 l/kg CO2: 60.6% vol Time: 300 min T: 25 °C P: 1 bar L/S: 17 CO2: 10% vol CO2 F: 0.7 l/min Time: 920 min T: 25 °C P: 5 bar L/S: 17 CO2: 10% vol CO2 F: 0.7 l/min Time: 400 min
(Pan et al., 2014)
(Polettini et al., 2016)
This study
This study
Soliman.
magnesium oxides. In this study, accelerated carbonation of EAF BHD in reject brine was studied in a novel reactor system, specially designed for contacting gases and liquids. RSM was utilized to design an experimental space to evaluate the CO2 uptake capacity of EAF BHD and examining the individual and joint effects of the operating parameters on the uptake. The study examined three main parameters: solid to liquid ratio, CO2 gas flowrate and inert particles volume fraction. Each operational parameter has been varied over a determined range. The experimental results showed that the studied factors had a significant impact on CO2 uptake, which was observed to be in the range 0.10.18 gCO2/g BHD. At ambient conditions (24 °C and 1 atm) and optimum operating parameters, the optimum CO2 uptake was 0.22 g CO2/ g BHD. The best CO2 uptake performance (1 ± 0.04 gCO2/g BHD) was achieved at ambient temperature and pressure of 5 bar. The TGA analysis confirmed the obtained results by showing the decomposition of several carbonation products, such as calcium and magnesium carbonates. Although the results were satisfactory, there is still a room for improvement by targeting higher CO2 uptake. This can be achieved by studying several operational aspects that have not been addressed in this work including temperature, inert particles geometry and size and inlet gas CO2 concentration. In addition, testing and optimizing all operating parameters in continuous mode is essential for large-scale applications. Furthermore, a separate study exploring the kinetics of mineral carbonation in the presence of inert particles can provide helpful insights into MC process. Finally, the crystal structure and product quality of the produced carbonates should also be evaluated.
References Bang, J.-H., Yoo, Y., Lee, S.-W., Song, K., Chae, S., 2017. CO2 mineralization using brine discharged from a seawater desalination plant. Minerals 7 (11), 207. Bezerra, M.A., Santelli, R.E., Oliveira, E.P., Villar, L.S., Escaleira, L.A., 2008. Response surface methodology (RSM) as a tool for optimization in analytical chemistry. Talanta 76 (5), 965–977. Bobicki, E.R., Liu, Q., Xu, Z., Zeng, H., 2012. Carbon capture and storage using alkaline industrial wastes. Prog. Energy Combust. Sci. 38 (2), 302–320. Bui, M., et al., 2018. Carbon capture and storage (CCS): the way forward. Energy Environ. Sci. 1062–1176. Chang, E.E., Chen, C.H., Chen, Y.H., Pan, S.Y., Chiang, P.C., 2011. Performance evaluation for carbonation of steel-making slags in a slurry reactor. J. Hazard. Mater. 186 (1), 558–564. El-Naas, M.H., El Gamal, M., Hameedi, S., Mohamed, A.M.O., 2015. CO2 sequestration using accelerated gas-solid carbonation of pre-treated EAF steel-making bag house dust. J. Environ. Manage. 156, 218–224 Elsevier Ltd. El-Naas, M.H., Mohammad, A.F., Suleiman, M.I., Al Musharfy, M., Al-Marzouqi, A.H., 2017a. Evaluation of a novel gas-liquid contactor/reactor system for natural gas applications. J. Nat. Gas Sci. Eng. 39, 133–142. El-Naas, M.H., Mohammad, A.F., Suleiman, M.I., Al Musharfy, M., Al-Marzouqi, A.H., 2017b. A new process for the capture of CO2 and reduction of water salinity. Desalination 411, 69–75. Fricker, K.J., Park, A.-H.A., 2013. Effect of H2O on Mg(OH)2 carbonation pathways for combined CO2 capture and storage. Chem. Eng. Sci. 100, 332–341. Gunning, P.J., Hills, C.D., Carey, P.J., 2010. Accelerated carbonation treatment of industrial wastes. Waste Manage. 30 (6), 1081–1090. Huijgen, W.J.J., Witkamp, G.J., Comans, R.N.J., 2005. Mineral CO2 sequestration by steel slag carbonation. Environ. Sci. Technol. 39 (24), 9676–9682. Ibrahim, M.H., El-Naas, M.H., Benamor, A., Al-sobhi, S.S., 2019. Carbon mineralization by reaction with steel-making waste: a review. Processes 7 (115), 1–21. Jones, E., Qadir, M., van Vliet, M.T.H., Smakhtin, V., mu Kang, S., 2019. The state of desalination and brine production: a global outlook. Sci. Total Environ. 657, 1343–1356. Kumar, S., Drozd, V., Durygin, A., Saxena, S.K., 2016. Capturing CO2 emissions in the Iron industries using a magnetite – iron mixture. Communication 0495, 560–564. Kunzler, C., et al., 2011. CO2 storage with indirect carbonation using industrial waste. Energy Procedia 4, 1010–1017. Lackner, K.S., 2003. A guide to CO2 sequestration. Science (80-.) 300 (5626), 1677–1678. Lakmali, U.N., Ranjith, P.G., 2015. Effect of brine saturation on carbonation of coal fly ash for mineral sequestration of CO2. Eng. Geol. Soc. Territory 1, 479–482. Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L., 2005. IPCC Special Report on Carbon Dioxide Capture and Storage. Cambridge university press. Mun, M., Cho, H., 2013. Mineral carbonation for carbon sequestration with industrial waste. Energy Procedia 37, 6999–7005. Ncongwane, M.S., Broadhurst, J.L., Petersen, J., 2018. Assessment of the potential carbon
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the Gas Processing Center (Qatar University) staff. In particular, Dr. Abdelbaki Benamor, Nafis Mahmud, Yousef Adel Elhamarnah, Musaab Magzoub, Dan Cortes, and Ahmed 8
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mineral carbonation technologies to sequester CO2. Chem. Soc. Rev. 43 (23), 8049–8080. Sanna, A., Uibu, M., Caramanna, G., Kuusik, R., 2014b. A review of mineral carbonation technologies to sequester CO2. Chem. Soc. Rev. 8049–8080. Seifritz, W., 1990. CO2 Disposal by Means of Silicates. Nature Publishing Group. Shahzad, M.W., Burhan, M., Ang, L., Ng, K.C., 2017. Energy-water-environment nexus underpinning future desalination sustainability. Desalination 413, 52–64. Takaba, H., Katagiri, M., Kubo, M., Vetrivel, R., Brocklawik, E., Miyamoto, A., 1995. Theoretical studies on the affinity of CO2 and N2 molecules to solid surfaces. Energy Convers. Manage. 36 (6–9), 439–442. Wang, R.B., Hellman, A., 2018. Initial water adsorption on hematite (α -Fe2O3) (0001): A DFT + U study. J. Chem. Phys. 148 (9). Wang, W.L., Liu, X., Wang, P., Zheng, Y.L., Wang, M., 2013. Enhancement of CO2 mineralization in Ca2+-/Mg2+-rich aqueous solutions using insoluble amine. Ind. Eng. Chem. Res. 52 (23), 8028–8033. Yamamoto, S., et al., 2010. Water adsorption on r-Fe2O3 (0001) at near ambient conditions. J. Phys. Chem. 3 (0001), 2256–2266. Yan, H., Zhang, J., Zhao, Y., Liu, R., Zheng, C., 2015. CO2 sequestration by direct aqueous mineral carbonation under low-medium pressure conditions. J. Chem. Eng. Jpn. 48 (11), 937–946. Zhang, Z., Borhani, T.N.G., El-Naas, M.H., 2018. Chapter 4.5 – carbon capture. In: Dincer, I., Colpan, C.O., Kizilkan, O. (Eds.), Exergetic, Energetic and Environmental Dimensions. Academic Press, pp. 997–1016.
footprint of engineered processes for the mineral carbonation of PGM tailings. Int. J. Greenh. Gas Control 77 (August), 70–81. Nyambura, M.G., Mugera, G.W., Felicia, P.L., Gathura, N.P., 2011. Carbonation of brine impacted fractionated coal fly ash: implications for CO2 sequestration. J. Environ. Manage. 92 (3), 655–664. Olajire, A.A., 2013. A review of mineral carbonation technology in sequestration of CO2. J. Pet. Sci. Eng. 109, 364–392. Pan, S.-Y., 2012. CO2 capture by accelerated carbonation of alkaline wastes: a review on its principles and applications. Aerosol Air Qual. Res. 770–791. Pan, S.Y., Chiang, A., Chang, E.E., Lin, Y.P., Kim, H., Chiang, P.C., 2015. An innovative approach to integrated carbon mineralization and waste utilization: a review. Aerosol Air Qual. Res. 15 (3), 1072–1091. Pan, S.Y., Chiang, P.C., Chen, Y.H., Chang, E.E., Da Chen, C., Shen, A.L., 2014. Process intensification of steel slag carbonation via a rotating packed Bed: reaction kinetics and mass transfer. Energy Procedia 63, 2255–2260. Polettini, A., Pomi, R., Stramazzo, A., 2016. CO2 sequestration through aqueous accelerated carbonation of BOF slag: a factorial study of parameters effects. J. Environ. Manage. 167, 185–195. Power, I.M., et al., 2013. Carbon mineralization: from natural analogues to engineered systems. Rev. Mineral. Geochem. 77 (1), 305–360. Said, A., Laukkanen, T., Järvinen, M., 2016. Pilot-scale experimental work on carbon dioxide sequestration using steelmaking slag. Appl. Energy 177, 602–611. Sanna, A., Uibu, M., Caramanna, G., Kuusik, R., Maroto-Valer, M.M., 2014a. A review of
9
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Enhanced CO2 capture through reaction with steel-making dust in high salinity water
T
⁎
Mohamed H. Ibrahima, Muftah H. El-Naasa, , Ron Zevenhovenb, Saad A. Al-Sobhic a
Gas Processing Centre, College of Engineering, Qatar University, Doha, Qatar Åbo Akademi University, Thermal and Flow Engineering, Turku, Finland c Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon capture CO2 sequestration Steel-making waste Steel dust Brine
Mineral carbonation (MC) is evolving as a possible technology for sequestering CO2 from medium-sized emission point sources. Industrial wastes have been recently used as an effective source for MC that have higher reactivity than natural minerals; they are also inexpensive and readily available in proximity to CO2 emitters. In this work, accelerated carbonation of electric arc furnace (EAF) baghouse dust (BHD) in a reject brine medium was evaluated in a novel reactor system, specially designed for contacting gases and liquids. This approach is environmentally friendly and eliminates the cost associated with pre-treatment. Experimental design was utilized to determine the effect of the operating parameters (solid to liquid ratio, CO2 gas flowrate and inert particles fraction) on the CO2 uptake. Analysis of the experimental results indicated that the studied factors had a significant impact on CO2 uptake, which was observed to be in the range of 0.1–0.18 gCO2/g BHD. At ambient conditions (24 °C and 1 atm) and at optimum operating parameters, the optimum CO2 uptake was 0.22 g CO2/g BHD. A higher CO2 uptake performance of 1 ± 0.04 gCO2/g BHD was achieved at ambient temperature and pressure of 5 bar. Thermal gravimetric analysis of the solid products revealed that a variety of carbonate products have been produced, particularly, calcium and magnesium carbonates.
1. Introduction Mineral carbonation (MC) is based on the principles of natural weathering (carbonation) of natural rocks where CO2 dissolved in rainwater forms a weak carbonic acid. Consequently, alkali and alkaline earth metals available within the rocks neutralize the acid to form insoluble carbonate minerals (i.e. calcium and magnesium carbonates) (Bobicki et al., 2012; Kunzler et al., 2011). Alkaline earth metals such as pure magnesium and calcium oxides rarely exists in nature; however, they coexist with other elements (i.e. silicon) to form minerals such as olivine (Mg2SiO4), wollastonite (CaSiO3), and serpentine (Mg3Si2O5(OH)4) (Olajire, 2013). The availability of a vast amount of a certain natural mineral is a key factor to sustain the sequestration process. Hence, magnesium-based silicates are preferred due to their abundance globally (Metz et al., 2005; Lackner, 2003). The carbonates formed from the MC reaction are in the solid phase and the amount of heat required to decompose these carbonates and release CO2 is substantial, making them a thermodynamically stable CO2 sink (Power et al., 2013). Once CO2 is mineralized, it is trapped permanently without the need of any additional monitoring contrary to
⁎
other CO2 sequestration options (Mun and Cho, 2013). MC was first conceptualized by Seiftriz (Seifritz, 1990) who suggested introducing high purity CO2 to accelerate the carbonation process, hence the name “accelerated carbonation”. This ensures that carbonation time can shift from geological time scale to hours or even minutes. The literature of MC has expanded significantly since its first conceptualization (Sanna et al., 2014a; Pan et al., 2015; Ibrahim et al., 2019). However, the MC research is still facing several challenges that hinders its viability when deployed on large scale (Olajire, 2013). Overall, the technique possesses several advantages over other sequestration options, such as ocean and geologic sequestration due to concerns over long term stability and leakage (Gunning et al., 2010). In addition, MC produces several stable products that can be economically profitable and these products are typically produced in fewer steps than other techniques. Although naturally occurring minerals (i.e. calcium/magnesium silicates) have the potential to sequester huge amounts of CO2 due to their abundance, it is often considered unfeasible due to the extraction and pretreatment cost of the minerals and the impacts associated with it (Huijgen et al., 2005). This apart from numerous process challenges in terms of carbonation efficiency and energy intensity that led to slow
Corresponding author. E-mail address: [email protected] (M.H. El-Naas).
https://doi.org/10.1016/j.ijggc.2019.102819 Received 2 May 2019; Received in revised form 7 August 2019; Accepted 21 August 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.
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MC approach. Currently, research has focused on assessing and maximizing the sequestration of CO2 by optimizing: 1) operating conditions including pressure, temperature, solid-to-liquid ratio, CO2 gas flowrate, solid particle size, solid pretreatment and 2) the market value of the solid products (Sanna et al., 2014b). This work presents an experimental evaluation of an innovative direct aqueous carbonation approach of EAF baghouse dust (BHD) in reject brine, using a specially designed reactor system. The work aims to improve the efficiency of the carbonation process and reduce the energy requirement and cost associated with pre-treatment; at the same time, the approach contributes to the management of carbon emissions, steel-making waste and desalination reject brine. To the best of the authors’ knowledge, direct aqueous carbonation of steel-making waste in the presence of reject brine has never been addressed in the open literature.
industrial deployment (Bui et al., 2018). Alkaline industrial wastes rich in magnesium and calcium oxides offer yet another attractive CO2 sequestration route. Although alkaline wastes are available in smaller amounts than natural minerals, they are available at lower cost, has a higher reactivity and uptake capacity and require less pretreatment. Examples of industrial wastes suitable for MC include fly ash such as coal and oil shale ashes, cement industry waste, and iron- and steelmaking slags. These wastes come from fundamentally different industries; however, they share multiple similar intrinsic features:
• The waste is rich in alkali earth metal content. • The waste tends to be less stable than rock minerals and needs to be stabilized. • The manufacturing processes produce substantial amounts of CO . 2
Steel industry contributes approximately 7% of CO2 emissions worldwide (Pan, 2012). In addition, the industry produces several types of wastes that can be utilized. There are four main types of steelmaking wastes, including blast furnace (BF), basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle furnace (LF); each is named after the process steps during which they are produced. The waste out of these steps is a consolidated mix of many compounds, primarily calcium, iron, silicon, aluminum, magnesium, and manganese oxides that are present in different phases. On average, manufacturing 1 ton of steel produces 2.1 ton of CO2 and approximately 170 kg of EAF dust and 180 kg of EAF slags (Said et al., 2016). Desalination plants use large volumes of seawater and discharge concentrated water (reject brine) back to the environment. Brine production is usually equivalent or greater than the total volume of the desalinated water. Brine production in the Middle East and North Africa is estimated to be about 100 million m3/day, accounting for 70.3% of the global brine production (Jones et al., 2019). Another major environmental concern of water desalination is attributed to the release of considerable amounts of CO2, which is mainly due to the use of fossil fuel as the main energy source for desalination plants. CO2 emission in desalination plants depends on the type of the desalination process, whether it is a pressure or thermal driven desalination. It is estimated that carbon emission of desalination plants is about 76 million tons per year (Zhang et al., 2018), and it is expected to reach 218 million tons per year in 2040 (Shahzad et al., 2017). In addition, reject brine contains sufficient concentrations of magnesium and calcium ions that can further add to CO2 capture capacity of the MC process (Bang et al., 2017). In fact, the utilization of brine has been reported to increase the MC process efficiency (Nyambura et al., 2011; Lakmali and Ranjith, 2015; Wang et al., 2013). Hence, taking a combined approach of supplying CO2 from the steel industries or stationary CO2 point sources (i.e. power and desalination plants) and at the same time stabilizing industrial waste by sequestering carbon dioxide in reject brine can greatly contribute to the total CO2 emission reduction globally. There are two main approaches for alkaline waste carbonation: direct and indirect technique. Direct carbonation technique implies that the MC process happens in one single step. Alternatively, indirect carbonation consists of two or more steps that usually include pre-treatment of the utilized waste. Pre-treatment usually involves grinding, sieving and/or extracting techniques such as acid extraction, heat activation or bioleaching (Bobicki et al., 2012). The excessive use of extraction agents is needed to enhance the carbonation kinetics; however, this adds one more step to the overall process hence increasing the total process cost and creating a non-environmentally friendly MC process (Sanna et al., 2014a). Elevated temperature, pressure and certain pretreatments have been reported to improve the carbonation process but inevitably these conditions can increase the carbonation cost up to 4000 €/ ton steel waste (Sanna et al., 2014b). In addition, pre-treatment steps increase the overall MC process carbon footprint (Ncongwane et al., 2018), which negates the value of the CO2 capture process. This necessitates the need for a greener and more sustainable
2. Materials and methods 2.1. Materials characterization EAF BHD samples were collected from open-to-atmosphere storage yard from a local steel factory in the region. Chemical characterization of the samples has been performed using VISTA- MPX CCD inductively coupled plasma-atomic emission spectrometry (ICP-AES) as shown in Table 1 (El-Naas et al., 2015). The ions concentration in the reject brine has been measured by Metrohm ion chromatograph (IC). The range of ion concentrations is shown in Table 2. EMD Millipore system was used to produce deionized water with a conductivity ≤ 4.3 μS/cm. X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of BHD. A mechanical sieve shaker was used for sieving analysis. The sieve analysis of the BHD indicated that the dust particles are in the range of 38 μm to 250 μm with 86.8% of the particles having a diameter less than 150 μm . Thermal gravimetric analysis (TGA) was performed using using a PerkinElmer Pyris 1 thermal gravimetric analyser. Due to temperature limitation of the used TGA equipment, Carbolite furnace (Model RHF 1400) is used to heat certain samples to 1200 °C. Microstructure characterization was carried out using a JOEL JSM-5600 scanning electron microscope. The pH of each BHD-reject brine mixture is measured using a HACH portable pH meter (Model HQ11D) and the conductivity of the mixture was measured using a HACH digital conductivity meter (Model HQ14D). Anton Paar rheometer (Model MCR 302) was used to measure the viscosity of BHDreject brine mixture. 2.2. Aqueous carbonation In each experimental run, a certain BHD mass is mixed with a one liter of reject brine. The reject brine was prepared by evaporating 2 L of sea water to 50% of its volume (1 L) achieving conductivity values in the range of 100–120 ms cm . pH measurements are taken per 10 g of added BHD till the entire dust mass is added to the reject brine at room Table 1 Chemical composition of BHD using an Inductively Coupled Plasma.
2
Component
Composition (Weight %, dry)
FeO/Fe2O3 CaO MgO SiO2 Na2O K2 O MnO Al2O3 SO3 Others
42.8 40.2 4.96 4.49 2.52 1.98 0.68 0.28 1.10 0.99
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2.3. Reactor description
Table 2 Ions concentration range in reject brine using IC.
Concentration range (g/l)
Ca2+
Mg2+
Na+
Cl−
0.32–0.50
0.9–1.5
7.4–12.4
17.8–23.3
The reactor (Fig. 1) is made of stainless steel and comprises of vertical vessel with gas and liquid inlet ports, containing inert particles, with a total volume of the inert particles in the range 3–18% of the total working volume of the vessel. The total volume of the reactor vessel is 3 L; however, only 1 L of the IPSBR volume has been used in this study. The inert particles provide circular motion which enhances the mixing process without compromising the operational aspects of the system such as the need of high mechanical energy. Specifically, the inert particles are dispersed and move within the vessel to promote mixing between the gas and the liquid and to provide a higher gas-liquid interfacial area for effective mass transfer between the two phases. The particles used in this study were made from polyethylene material in the form of a sphere with a density of approximately 0.97 g/l, a diameter of 5.1 mm and surface area of 81.7 mm2 /particle. Generally, the material of the inert particles depends on the liquid and gas used in the system and should not react with any of the liquids or gases or act as a catalyst for the reaction. The movement of the inert particles within the vessel is caused by the gas entering the vessel via the gas inlet. The gas is introduced centrally at the base of the vessel to cause the inert particles to move along a circular or elliptical path up and down the vessel while in operation. The gas feed enters through a single spouting orifice (diameter: 1 mm) at the bottom of the reactor and leaves at the top. It is also worth noting that the IPSBR can be operated in the continuous mode. In such case, the BHD-reject brine mixture can leave through the effluent tube (at bottom of the reactor) by the liquid hydrostatic pressure in the vessel (El-Naas et al., 2017a).
temperature (24 °C). The pH value of BHD and reject brine mixture stabilizes at 11 as an average reading for all prepared solutions. All experiments were conducted in semi-batch mode, in which the prepared solution is placed inside an inert particles spouted bed reactor (IPSBR) system and then CO2 is injected through the bottom of the reactor as a spouting gas. The feed flow rate of the gas mixture is controlled by a mass flow controller (Brooks Instruments SLA 5800, USA). The gas mixture, which contains 10 ± 1.1% (mol/mol) carbon dioxide (CO2) and 90% nitrogen (N2), was provided by Buzwair Scientific and Technical Gasses, Qatar. The mixture serves as the only source of CO2 throughout this work. Carbon dioxide that have been captured inside the IPSBR is calculated by analyzing the CO2 composition of the outlet stream using a continuous CO2 analyzer (Quantek Model 906 acquired from Quantek Instruments, Massachusetts, USA). The length of each experimental run depends on the time it takes for the CO2 concentration at the outlet to be equal to its inlet value, indicating the termination of CO2 capture process. The analyzer readings were regularly confirmed by utilizing a portable CO2 analyzer (Dansensor CO2 CheckPoint, Denmark) and a gas chromatograph (PerkinElmer Model Clarus 580). After the experiment is completed, the IPSBR content is drained for further analysis. The carbonated mixture was left to settle and the clear water of the reject brine was decanted. This water was filtered using PTFE syringe filter (pore size: 0.45μm ) for ions analysis. The remaining solids were dried at 105 °C for 24 h to evaporate any water remaining in the sample.
2.4. Design of experiment Design of experiments (DOE), using Response Surface Methodology (RSM), was utilized to design a set of experiments involving twenty experimental runs. RSM involves developing and executing a fixed number of experiments in specific order to measure how the
Fig. 1. A schematic diagram of the experimental setup which consists of A) Gas cylinder (CO2/N2) B) inert particles spouted bed reactor system C) CO2 gas analyser. 3
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3.1. Effect of gas flow rate
independent variables are affecting the system response. The main idea behind this approach is to maximize the information that can be extracted from conducting a minimum number of experiments (Bezerra et al., 2008). RSM was used to select Central Composite Design (CCD) to create an experimental design space consisting of three operational parameters: gas flowrate (l/min), solid to liquid (S/L) ratio (mass of BHD/ volume of reject brine) (g/l) and inert particles fraction (volume of particles/ volume of reject brine) (%). After several screening experiments, the experimental range of each parameter was chosen to be the following: 0.7–2.3 l/min gas flowrate, 23–57 g/l solid to liquid ratio and 3–18% inert particles fraction. RSM is utilized to generate a set of 20 experiments to analyze the individual and combined effects of the operating parameters on the CO2 uptake. Hereafter, the uptake can be optimized to achieve the best possible results.
The mixture of 10% CO2 and 90% N2 is introduced to the bottom of the reactor to simulate flue gas. Carbon dioxide is converted from the gaseous form to the ionic form according to the following equation: 2− + + CO2(g ) + H2 O(l) ↔ HCO3−(aq) + H(aq ) ↔ CO3 (aq) + 2H(aq)
Subsequently, CO2 reacts with the active alkali metal oxide which is converted to hydroxide form in the presence of reject brine :
M (OH )2 + CO2(aq) → MCO3 + H2 O
(2)
where M is Ca or Mg. The alkalinity of the component governs its CO2 sequestration capacity since carbon dioxide molecules are acidic, hence, more basic compounds (pH > 10) are better suited for MC process. Hereafter, based on surface diffusion phenomenon, CO2 has more affinity to be sequestered, based on oxides basicity, in the following order: basic oxides (CaO, MgO) > amphoteric oxides (FeO, Al2O3) > acidic oxides (SiO2) (Takaba et al., 1995). Hence, the theoretical CO2 uptake can be calculated based on the stoichiometric ratio in Reaction 2. The theoretical uptake is equal to 0.29 g CO2/ g BHD based on the total amount of MgO and CaO in BHD (Table 1). At a relatively low gas flowrate, the gas residence time increases which in turn prolong the contact between CO2 and BHD leading to more uptake. This is demonstrated in Fig. 2 as it shows two experiments with the same solid to liquid ratio (30 g/l) and inert particle fraction (10%), but at different flowrates. An uptake of 0.147 g CO2/g BHD was obtained compared to 0.162 g CO2/g BHD when the gas flow rate changed from 2 l/min to 1 l/min, respectively.
2.5. CO2 uptake calculation The CO2 analyzer readings in volume percent are being converted to equivalent value in the form of CO2 uptake. The uptake is defined by the amount of CO2 that has been reacted over the amount of BHD used in a specific experiment (mass of CO2 sequestered / mass of BHD). Hence, to obtain the uptake value for each experiment the following calculations have been carried out: The area under of the curve CO2 % vs. time represents the volume of CO2 passed through the reactor without being sequestered:
Volume of uncaptured CO2 passed through the system (Vu ) t f CO2 % = F dt 0 100
∫
CO2 %: CO2 composition reading given by the analyzer (dimensionless) F : experiment flowrate (l/min) t f : time that corresponds to the duration of carbonation process (min) dt : time increment (min) Hence, the total volume of CO2 that was fed to the system, (Vt), equals: = F ×
(1)
3.2. Effect of S/L ratio and inert particles volume fraction The CO2 uptake of the carbonation process is expected to increase when the BHD mass is increased, since more dust mass will be available to react with the CO2, while also the pH of the solution is increased. However, in this particular study, this behavior cannot be generalized as it will be explained below by introducing the effect of the inert particles volume fraction. Increasing BHD mass is expected to increase the viscosity of dustreject brine mixture. To confirm this, Fig. 3 shows the viscosity of several S/L ratios at different shear rates measured using rheometer. At a shear rate of 0.1 s−1, 60 g/l slurry has more than nine times the viscosity of 20 g/l slurry. The more viscous a solution is, the more mixing it requires to achieve acceptable CO2 uptake values specially at low shear rate. When the shear rate increases, hence more mixing, the viscosity of mixture significantly decreases. The circular motion
CO2 sat % × tf 100
CO2 sat %: the point at which dust carbonation process completely stops (dimensionless) The volume of CO2 that has been sequestered in the dust carbonation process can be calculated by subtracting the two areas described above:
CO2 sequestered (Vr ) = Vt − Vu The volume of CO2 consumed can then be converted to mass of CO2 through molar volume relationship at the experiment conditions (Temperature and pressure) and then divided by the mass of the dust to obtain the specific uptake value (UCO2 )
Vr UCO2 (g CO2/ g BHD) = ⎛ × MwCO2⎞ ÷ mdust V ⎝ M ⎠ ⎜
⎟
RT
VM = P : molar volume (l/mol) MwCO2 : carbon dioxide molecular weight (g/mol) mdust : mass of BHD (g) 3. Results and discussion The sections below describe the effect of the studied parameters on CO2 uptake and how their interactions affect the MC process. This is discussed in a sequential fashion showing the effect of each parameter on the process individually. After that, the combined effect of the three parameters on the uptake is explained.
Fig. 2. The effect of flowrate on CO2 uptake. F: gas flowrate (l/min), R: solid to liquid ratio (g/l) and P: inert particles volume fraction. 4
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to ensure enough mixing. In addition, the BHD mass does not increase the viscosity of the mixture to a degree that will hinder the mixing process. The figure also shows that uptake increased by 10% by changing the inert particles fraction from 6 to 15%. This behavior signifies that at low BHD mass, the effect of increasing inert particle volume fraction on mixing is minimal compared to Fig. 5. The S/L in Fig. 5 ratio was relatively high, 40 g/L, hence the mixture is more viscous and more inert particles are required. As more particles are present, more CO2 is captured with the same BHD mass. In Fig. 5, increasing particles fraction from 3 to 10.5% resulted in 16% increase in CO2 uptake. Thus, S/L ratio and inert particles fraction are interconnected. Fig. 6 demonstrates this relation by showing how the CO2 uptake changes as a function of the two parameters at a constant gas flowrate (0.7 l/min). The contour plot (Fig. 6b) illustrates that for certain inert particle fraction, an optimum solid to liquid ratio exists. For example, at a particle fraction of 14%, the maximum CO2 uptake is approximately 0.15 g CO2/g BHD. This uptake occurs at S/L ratio of 40 g/L. Increasing the ratio at the same inert particles fraction (14%), will lead to a more viscous solution that hinders the mixing process, hence, lowering the CO2 uptake. This behavior is illustrated in the concave down curve in Fig. 6A. Hence, to achieve higher uptake values, the increasing S/L ratio should be combined with an increase in inert particles fraction as well. This is shown by the linear relationship between the uptake and inert particle volume shown in Fig. 6A. Thus, the role of inert particles is more prominent at high S/L ratios and low gas flowrate at which more mixing is required as opposed to low S/L ratios and high flowrate. Up to this point, the discussion revolved around the individual effect of the three operating parameters on CO2 uptake by selectively choosing similar experiments from the experimental design space and comparing the different effects of these parameters on the uptake. The CO2 uptake results obtained from the 20 conducted experiments generated by CCD range from 0.101 to 0.180 gCO2/g BHD at 24 °C and 1 atm, with an average relative error of 4%. A gas flowrate of 0.7 l/min, S/L ratio of 57 g/l and inert particle volume of 18% were predicted by RSM to be the optimum conditions that would give the highest CO2 uptake. At these conditions, the obtained experimental uptake was 0.22 gCO2/g BHD for a time duration of 920 min, which is comparable to RSM predictions (0.23 gCO2/g BHD) with a relative error of 5%.
Fig. 3. Viscosity of several S/L ratios (20, 40 and 60 g/l) of EAF BHD in reject brine at different shear rates.
Fig. 4. The effect of inert particles volume fraction on CO2 uptake at relatively low S/L ratio (30 g/l) and flow rate of 1 l/min.
3.3. Carbonation at elevated pressure To study the effect of elevated pressure on MC process, an experiment was carried out at the optimum conditions with a process pressure of 5 bar. The pressure inside the reactor is controlled by a valve at the outlet of the reactor. At the end of the experiment, the pressure inside the reactor is decreased gradually to reach 1 bar. This resulted in a CO2 uptake of 1.0 ± 0.04 gCO2/g BHD as an average of two repeated experiments. Compared to the optimum conditions at atmospheric pressure, this is a 480% increase in the uptake value, moreover in 43% less time (400 min compared to 920 min). It is worth noting that in Reaction 2, the formation of CaCO3 is dominant when Ca(OH)2 is present in amounts greater than stoichiometric ratio (El-Naas et al., 2017b). Increasing the pressure forces more CO2 to stay into the solution and shifts the carbonation reaction towards the products side. This makes CO2 available in amounts greater than the stoichiometric ratio. Hence, this situation allows for the formation of sodium bicarbonate: Fig. 5. The effect of inert particles volume fraction on CO2 uptake at relatively high S/L ratio (40 g/l) and flow rate of 1.5 l/min.
Ca (OH )2 + 2CO2(aq) + 2NaCl (aq) → CaCl2 + 2NaHCO3 ΔG = −70.40
provided by the inert particles enhances the mixing process especially when the mixture is viscous (high S/L ratios). Figs. 4 and 5 show the effect of inert particles fraction at different S/L ratios. Two different trends can be demonstrated from the obtained results: First, at relatively low S/L ratio, such as Fig. 4, inert particles volume fraction have no significant effect on the uptake. In this case the flowrate is sufficient
kJ kmol
(3)
The presence of sodium chloride in the reject brine will shift the effective stoichiometric ratio between Ca(OH)2 and CO2 to be 1:2, thereby doubling the uptake capacity of the BHD. As a result, the theoretical CO2 uptake increases to reach 0.55 g CO2/ g BHD. Hence, the theoretical CO2 uptake by the BHD ranges from 0.29 to 0.55 gCO2/g 5
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Fig. 6. Combined effect of solid to liquid ratio and inert particles % on CO2 uptake (a) surface plot (b) contour plot at the optimum flowrate (0.7 l/min).
attributed to the contribution of iron oxide. Approximately 43% of BHD is composed of iron (II) and iron (III) oxides. XPS is used to determine the exact composition of each iron component. The surface analysis of XPS revealed that out of the total iron oxides composition, 70% is iron (II) oxide and the rest (30%) is iron (III) oxide. At room temperature and pressure, iron (II) reacts with water to produce ferrous hydroxide spontaneously in an exothermic reaction, via:
FeO(s) + H2 O(l) → Fe (OH )2 ΔG = − 9.3
kJ kmol
(4)
This is shown in Fig. 8 as 67% of Fe(II) oxide in BHD turned to Fe (OH)2 in brine reject. The adsorbed layer of water on the surface of FeO layers provides a medium for reaction with CO2. Free Fe2+ ions can react with CO32− to form FeCO3 (Kumar et al., 2016):
Fe (OH )2 + CO2(aq) → FeCO3(s) + H2 O(l) ΔG = −25.7
kJ kmol
(5)
With time, the FeCO3 layer increases while the FeO layer shrinks according to the shrinking core model. A detailed explanation for the mechanism for CO2 reaction at the adsorbed iron oxide–water interface has been previously reported (Yamamoto et al., 2010; Wang and Hellman, 2018). Kumar et al. Kumar et al. (2016) demonstrated that FeO can adsorb 0.106 g CO2/g adsorbent in a slightly humid environment. Therefore, it is expected that more CO2 will be adsorbed on the surface of iron oxide in aqueous medium. TGA in Fig. 9 was used to confirm the measured uptake (1.0 ± 0.04 gCO2/g BHD) results at 5 bar. The largest weight loss took place in the temperature range 700–800 °C which represent CaCO3 decomposition temperature, besides the range of 350–650 °C for MgCO3 decomposition temperature (Yan et al., 2015). A total mass reduction of 67% was achieved for the 5 bar experiment. A separate sample of the same experiment was heated up to 1200 °C. The resulted mass reduction was 72% which confirms the TGA results. In other words, 72% of the sample had carbon-bearing compounds, products from the reaction of between CO2 and BHD. The difference between the measured uptake value (1 ± 0.04 gCO2/ g BHD) and the TGA is probably due to the contribution of NaHCO3, which is highly soluble in water, and it is mostly removed when the liquid is decanted and the soluble CO2 in the reject brine within the reactor system. An extra couple of experiments have been conducted as illustrated in Fig. 10. An experiment was conducted in an aqueous medium of deionized water instead of reject brine. The CO2 uptake value was 0.106 gCO2/g BHD, compared to the optimum conditions 1 bar pressure experiment uptake (0.22 gCO2/g BHD). This is a 48% decline in uptake when deionized water was used. This further supports the claim that CO2 reacts with calcium ions in the reject brine hence increasing the CO2 uptake. Finally, to further validate the use of inert particles, an
Fig. 7. SEM image of precipitates of the filtered brine after carbonation showing the formation of sodium bicarbonate confirmed by EDX (the arrows point to the spots for the EDX measurements).
BHD depending on the conversion of calcium and magnesium in the solids to carbonates or bicarbonates. The products of Reaction 5 are calcium chloride and sodium bicarbonate which are both highly soluble in water. However, sodium bicarbonate can precipitate when the water temperature is decreased to 15 °C. SEM analysis of participates from the filtered brine after the carbonation process revealed the formation of sodium bicarbonates taking irregular shapes deposited on precipitated magnesium carbonates as shown in Fig. 7. Thus, the occurrence of this carbonation route can be validated. Although the formation of hydrated magnesium carbonates can be expected, they form as intermediate product that is transformed into anhydrous carbonates while subjected to excess reject brine, and/or longer reaction time (Fricker and Park, 2013). Additionally, analysis of ions available in the brine after the carbonation process revealed that there was a significant reduction in calcium ions compared to its initial concentration as given in Table 2. Throughout the conducted experiments, the calcium was reduced from the original value in Table 2 to be approximately 0.07 ± 0.03 g/l. This indicates that calcium ions in the brine reacted with CO2 and are converted into calcium carbonate contributed to the total CO2 uptake. Again, this contribution depends on the conversion of calcium and magnesium ions to carbonates or bicarbonates. Some of the CO2 captured (1 ± 0.04 gCO2/g BHD) can be 6
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Fig. 10. Optimum experiment comparison at different conditions.
benchmark this particular study. Chang et al. Chang et al. (2011) reported an uptake of 0.27 g of CO2 based on the amount of calcium oxide in the sample. The value is quite high compared to the literature. However, an extraction step was needed to obtain CaO which adds to the process cost. In addition, it was conducted at 70 °C which further adds to the process energy requirement. Pan et al. Pan et al. (2014) used a rotating packed bed to achieve 0.16 g CO2/g CaO at ambient temperature and pressure. Even though these results are only based on calcium oxide, they seem to be comparable to the uptake obtained in the current study before optimization (0.18 g CO2/g BHD). If the results of the current study were only based on the calcium in the sample (Table 2), the uptake value would be 0.45 gCO2/g CaO (0.18/0.4). However, this is not an accurate representation of the results as the uptake in this study occurs due to reaction of CO2 with Ca, Mg and Fe bearing compounds as established above. Polettini et al. Polettini et al. (2016) reported a CO2 uptake of 0.536 g based on the total amount of slag, which is one of the highest reported in the literature. However, compared to the current study, the reported experimental parameters are drastically different. Since the authors utilized a gas stream containing 60% CO2 (compared to 10%), a larger uptake is expected. In addition, at approximately the same pressure (5 bar) and significantly lower temperature, the current study produces double the CO2 uptake (1 ± 0.04 gCO2/g BHD). Therefore, based on the above comparisons, the following can be stated:
Fig. 8. XPS analysis of FeO (a) before carbonation (b) after carbonation.
• At ambient temperature and pressure, this work provides a good •
CO2 uptake based on the amount of dust used compared to the literature. At moderate pressure (5 bar), the uptake results (1 ± 0.04 gCO2/g BHD) is relatively higher than the literature at the same experimental conditions.
It is also important to point out that no pretreatment has been applied to the EAF BHD samples, which greatly contributes to the significance of the results since the additional cost of the pretreatment is eliminated. Also, the carbon footprint of the proposed process is negligible.
Fig. 9. TGA for optimum pressure experiment at 5 bar showing MgCO3 deception at 350 °C and CaCO3 decomposition at 750 °C.
experiment was conducted at optimum conditions without added inert particles. The obtained uptake was 0.17 gCO2/g dust which is 21% lower than the optimum conditions uptake (0.22 gCO2/g BHD).
4. Conclusions Carbon sequestration can be achieved through different techniques that have the potential to capture substantial amounts of CO2. Mineral carbonation is evolving as a possible candidate to sequester CO2 from medium-sized emissions point sources. Industrial wastes, such as ironand steelmaking dust are rich in alkaline compounds especially calcium and magnesium oxides. Electric arc furnace bag house dust exhibits sufficient alkaline properties as it is enriched with calcium and
3.4. Results comparison Table 3 shows a number of different carbon mineralization studies in order to compare it with the current work. These studies concluded with relatively good CO2 uptake result and therefore these are used to 7
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Table 3 Literature comparison in terms of CO2 uptake. L/S: liquid to solid ratio, T: temperature, P: pressure, F: flowrate, RT: rotational speed, PP: partial pressure. Type of waste
Composition
Uptake
Reactor type
Conditions
Ref
BFS
CaO(wt%): 51.1 SiO2(wt%): 11.5 Fe2O3(wt%): 24.1
0.27 g CO/g CaO
Slurry
(Chang et al., 2011)
BOFS
CaO(wt%): 43 Fe2O3(wt%): 28.7
0.16 g CO2/g CaO
Rotating packed bed
BOF
CaO(wt%): 31 MgO(wt%): 7.5 Fe2O3(wt%): 27
0.536 g CO2/g slag
Batch
EAF
CaO(wt%): 40.2 MgO(wt%): 4.96 Fe2O3(wt%): 42.8
0.21 g CO2/g BHD
IPSBR
EAF
CaO(wt%): 40.2 MgO(wt%): 4.96 Fe2O3(wt%): 42.8
1 ± 0.04 g CO2/g BHD
IPSBR
L/S: 10 T: 70 °C CO2 PP: 101.3 kPa CO2 F: 0.1 L/min Time: 720 min RT: 541 rpm T: 25 °C L/S: 10 P: 1 bar Time: N/A T: 83.7 °C P: 5.9 bar L/S: 5 l/kg CO2: 60.6% vol Time: 300 min T: 25 °C P: 1 bar L/S: 17 CO2: 10% vol CO2 F: 0.7 l/min Time: 920 min T: 25 °C P: 5 bar L/S: 17 CO2: 10% vol CO2 F: 0.7 l/min Time: 400 min
(Pan et al., 2014)
(Polettini et al., 2016)
This study
This study
Soliman.
magnesium oxides. In this study, accelerated carbonation of EAF BHD in reject brine was studied in a novel reactor system, specially designed for contacting gases and liquids. RSM was utilized to design an experimental space to evaluate the CO2 uptake capacity of EAF BHD and examining the individual and joint effects of the operating parameters on the uptake. The study examined three main parameters: solid to liquid ratio, CO2 gas flowrate and inert particles volume fraction. Each operational parameter has been varied over a determined range. The experimental results showed that the studied factors had a significant impact on CO2 uptake, which was observed to be in the range 0.10.18 gCO2/g BHD. At ambient conditions (24 °C and 1 atm) and optimum operating parameters, the optimum CO2 uptake was 0.22 g CO2/ g BHD. The best CO2 uptake performance (1 ± 0.04 gCO2/g BHD) was achieved at ambient temperature and pressure of 5 bar. The TGA analysis confirmed the obtained results by showing the decomposition of several carbonation products, such as calcium and magnesium carbonates. Although the results were satisfactory, there is still a room for improvement by targeting higher CO2 uptake. This can be achieved by studying several operational aspects that have not been addressed in this work including temperature, inert particles geometry and size and inlet gas CO2 concentration. In addition, testing and optimizing all operating parameters in continuous mode is essential for large-scale applications. Furthermore, a separate study exploring the kinetics of mineral carbonation in the presence of inert particles can provide helpful insights into MC process. Finally, the crystal structure and product quality of the produced carbonates should also be evaluated.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the Gas Processing Center (Qatar University) staff. In particular, Dr. Abdelbaki Benamor, Nafis Mahmud, Yousef Adel Elhamarnah, Musaab Magzoub, Dan Cortes, and Ahmed 8
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