ARTICLE pubs.acs.org/EF
Study on Characteristics of Steel Slag for CO2 Capture Juan Yu* and Kaibin Wang Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ABSTRACT: Steel slag is a kind of alkaline mixture and considered to be a potential CO2 adsorbent. In this work, the CO2-trapping characteristics of two types of steel slag, basic oxygen furnace (BOF) steel slag and electric arc furnace (EAF) steel slag, were experimentally investigated. Generally, the higher the temperature, the larger the Ca use of slag. However, the Ca use at 550 °C would be lower than that at 500 °C for a certain CO2 concentration. The CO2 concentration also has an effect on the Ca use. At higher temperatures, a larger Ca use appears at a lower CO2 concentration (75%). As the CO2 concentration decreases, the reaction rate of carbonization increases, regardless of the kind of slag used. With regard to the type of steel slag, EAF steel slag is better than BOF steel slag in reactivity and Ca use. All of the results indicate that the carbonation reaction of steel slag is controlled by not only the reaction kinetics but also the diffusion of the reactive gas CO2. Steel slag has a capacity to capture and permanently sequester CO2. However, what is more important is that it can be used in different flue gases, where the CO2 concentration is typically lower (75%). This provides the steel slag a wide application market.
1. INTRODUCTION Carbon dioxide (CO2), considered one of the main contributors to global warming, is mainly generated by various human activities, such as the combustion of fossil fuels through manufacturing industries, transportation, etc. The steel industry is a main CO2 emission point source. According to the Intergovernmental Panel on Climate Change (IPCC),1 the steel industry accounts for 34% of the total world greenhouse gas emissions. On average, more than 2.0 tons of CO2 is emitted for every 1 ton of steel produced. The global steel production was up to 12.2 billion tons of crude steel in 2009 and is projected to increase in the foreseeable future. Therefore, it is of prime importance to develop practical carbon-capture methods to reduce CO2 emission from the steel industry. Efficient ways to capture CO2 from the flue gas or the atmosphere have been examined and overviewed in the literature.25 For most of the carbon-trapping technologies, the cost of the separation step is much higher than that of the storage step, mainly because of the low concentration of CO2 in the flue gas. It is an unfavorable factor for the iron and steel industry because the CO2 content in flue gas is commonly lower than 20%. Simple and cost-effective methods are more attractive for the huge and complex iron and steel industry. A great deal of steel slag is generated as a byproduct during the steel production process. About 0.130.2 ton of steel slag is produced for per ton of steel. Currently, the steel slag is mainly used for cement production, road building, and phosphate fertilizer production. Steel slag is a kind of alkaline mixture, including many metal compounds, such as calcium, magnesium, iron, aluminum oxides, etc. This confers on it a high basicity (pH ∼ 12) and a potential capacity to capture and permanently sequester CO2 under the form of thermodynamically stable carbonate minerals. The carbonation product does not need to be regenerated because the steel slag is cheap and directly available. Moreover, the carbonation product is more stable than the fresh slag, so that it can be reused in construction and decrease the risk r 2011 American Chemical Society
of the metal leaching from material.6 All of these advantages of steel slag make it a potential feedstock for CO2 capture. Carbonation of alkaline slag can be performed by two different reaction routes: a dry route and a wet route. In the wet gassolid carbonation route, the alkaline elements within slag are leached/ dissolved in aqueous solution and then react with a gas containing CO2 at low temperatures. This aqueous carbonation process takes place in either a single reactor (one step)710 or two inseries reactors (two steps).1113 In the dry gassolid carbonation route, the solid is directly contacted with a gas containing CO2 at high temperatures. Some industrial wastes that are generally composed of alkaline elements and rich in calcium and magnesium have been investigated to sequester CO2. Baciocchi and coworkers14 studied in detail the direct gassolid carbonation of alkaline residues from air pollution control (APC) systems with pure CO2 at reaction temperatures in the range of 200500 °C. Later, they studied the carbonation kinetics of APC residues at different temperatures, CO2 concentrations, and atmospheric pressure to select the best operating conditions.15 Carbonation experiments of hydrated and nonhydrated fluidized-bed combustion ash were carried out on a pressurized thermogravimetric analysis (TGA), achieving CaO conversion rates up to 60% at temperatures above 400 °C and in a 100% CO2 atmosphere.16 However, few investigations were performed on the steel slag using the dry gassolid carbonation route. Thus, to improve the fundamental understanding of the carbonation process within steel slag and provide the basis for pilotand large-scale applications, the present work is to study the CO2 trapping characteristics of steel slag. A series of experiments were carried out first for determining the physiochemical properties of slag samples and the absorption mechanism for CO2. Then, the variations of Ca use of samples with reaction time, temperature, Received: March 20, 2011 Revised: September 25, 2011 Published: September 26, 2011 5483
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Figure 1. Schematic of the experimental system.
Table 1. Major Oxides in Slags concentration (wt %) oxide
BOF steel slag
EAF steel slag
CaO
42.25
30.48
MgO
8.046
4.913
MnO Al2O3
3.110 5.979
3.958 9.261
FeO
20.47
33.91
SiO2
12.36
15.07
CO2 concentration, and slag species were given. Finally, the kinetic behaviors of slags for CO2 capture were analyzed.
2. EXPERIMENTAL SECTION 2.1. Samples. There are two types of steel slags used in the experiments: basic oxygen furnace (BOF) steel slag and electric arc furnace (EAF) steel slag. The raw steel slag has an irregular shape and uneven size distribution. Therefore, the experimental sample was ground and then sieved to 100125 and 200250 μm. The sizes selected are suitable for the needs of the sample container (see Figure 1), whose diameter is 20 mm. The experiments showed that the above two sizes have little effect on the carbonation reaction. Therefore, the following analyses were made only for the sample with a size of 100125 μm. To prevent contact with atmospheric CO2 and H2O, the samples were kept in sealed desiccators. The major oxides in slags were measured by an X-ray fluorescence (XRF) spectrometer and shown in Table 1. The slags were primarily composed of CaO, FeO, and SiO2. The amount of CaO is far greater than that of other alkaline oxides, implying that calcium compounds are the dominant constituents for capturing CO2. The CaO content in BOF steel slag is greater than that in EAF steel slag, owing to the difference of the steelmaking process. Other oxides, such as P2O5, Cr2O5, and TiO2, constitute the remainder of the 100% in the XRF analysis. 2.2. Experimental System. Carbonation experiments were carried out on an electrically heated furnace. Figure 1 is the schematic of the experimental system. It mainly consists of a quartz tube reactor, an automatic sampling apparatus, and gas feeders. The sample was put in the container and reacted with a flow of gas containing CO2 at a certain temperature regulated by an electric heater. The online weight variation
Figure 2. Thermal decomposition of BOF steel slag and EAF steel slag. of the sample with time was measured by an electronic balance and recorded by a computer. The accuracy of weight measuring was (0.1 mg. The gas after the reaction is exhausted from the tube reactor at the bottom of the heater. 2.3. Procedure. To determine the experimental conditions in detail, the thermal decomposition of BOF steel slag and EAF steel slag were performed first in the atmosphere of highly pure N2. From Figure 2, we can see that the present slags are fairly different in the ingredients and contents. There are three distinct weight-loss stages for the BOF steel slag. The first stage is between 40 and 100 °C, owing to the loss of water within the slag. The second stage is in the range of 410450 °C. Some substances, such as Ca(OH)2, are prone to decomposing during this stage. The third stage occurs at the temperatures of more than 600 °C and is mainly attributed to the decomposition of CaCO3. For the decomposition of EAF steel slag, a slow descending slope appears at 40600 °C and no distinct weight loss stages occur. After 600 °C, the weight of slag suddenly drops because of the decomposition of CaCO3. On the whole, the BOF steel slag decomposes faster than EAF steel slag and has a larger weight loss. The complex ingredients of steel slag make the adsorption and carbonation for CO2 difficult to analyze. Considering that CaO is the major part within the present slags and plays the most important role 5484
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upon capturing CO2, emphasis will be put on the carbonation reaction of CaO and CO2. In view of accelerating the reaction rate and avoiding the decomposition of CaCO3, the experimental temperature is set to be 450, 500, and 550 °C. The concentration of CO2 is 10, 25, 50, 75, and 100%. The experimental method is described as follows. The experiment was started by heating the furnace to the required temperature. At the same time, pure N2 gas was passed through the electric heater and exited from the bottom of the reactor. With the ambient hot gas conditions established, the container with a sample of about 500 mg was sent to the reactor and located in the reactor centerline. After heating for 30 min, the gas was switched to the mixture of CO2 and N2 and the carbonation of the sample began. The experiment ended when the weight of the sample changed little. The weighttime curve was obtained through the sampling system, which was used in the data processing and analysis. The feed rate for the hot gas was 400 mL/min.
Table 2. Experimental Conditions of the Carbonation Experiments for BOF Steel Slag and EAF Steel Slaga particle size (μm)
temperature (°C)
100125
450 500 550
a
The balanced gas is N2.
CO2 concentration (%)
10, 25, 50, 75, and 100
The ambient pressure was 1 atm. Table 2 lists experimental conditions for both BOF steel slag and EAF steel slag. Each experiment was repeated twice. The results showed that the error of this method is less than 4% and the reproducibility is good. Some measurements suffering from a low signal-to-noise ratio were processed using the median filtering method provided by data analysis software, Origin V8. This method replaces the signal value at each point by the median value of a group of surrounding points. The points of the window were chosen to be 5 in the present work.
3. RESULTS AND ANALYSIS 3.1. Ca Use. During the experiments, when the sample was preheated for 30 min at the scheduled temperature, the moisture within the sample was lost and the Ca-based compound, such as Ca(OH)2, was transformed into CaO. This can be seen from Figures 3 and 4, which show the scanning electron microscopy (SEM) images of the BOF steel slag and EAF steel slag samples, respectively. These SEM measurements correspond to the raw samples, the preheated samples, and the carbonated samples during an experiment performed in 75% CO2 at 500 °C. As shown in Figures 3a and 4a, the morphology of the raw sample is aniso-rhombic hexagonal, which is the typical shape of CaO. Meanwhile, the layer-like morphology is evident. This indicates
Figure 3. SEM images of the BOF steel slag sample before and after the carbonation at 500 °C: (a) raw sample, (b) preheated sample, and (c) carbonated sample (75% CO2). 5485
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Figure 4. SEM images of the EAF steel slag sample before and after the carbonation at 500 °C: (a) raw sample, (b) preheated sample, and (c) carbonated sample (75% CO2).
that the samples also contain Ca(OH)2. After preheating, shown in Figures 3b and 4b, the layer-like structure disappears and the residual part is CaO. After the carbonation reaction, as shown in Figures 3c and 4c, the samples are more rounded and exhibit the trigonal hexagonal scalenohedral crystal shape, which can be attributed to the formation of CaCO3. The same results can also be observed for samples under other experimental conditions. The raw samples, preheated samples (550 °C), and carbonated samples (75% CO2, 550 °C) for the two slags were also analyzed with X-ray diffraction (XRD). In Figures 5a and 6a, it is noticed that Ca(OH)2, CaO, and CaCO3 are present in the untreated raw samples. At the end of preheating, as shown in Figures 5b and 6b, Ca(OH)2 within the sample decreases, while the characteristic peaks of CaO are distinct. Ca(OH)2 is present in both the preheated sample and the carbonated sample (see Figures 5 and 6), but the amount is little changed, indicating that Ca(OH)2 does not participate in the carbonation reaction. Perhaps, it is difficult for this part of Ca(OH)2 to contact the reactive gas. The peaks of CaCO3 at the end of carbonation are greater than those before carbonation, as seen from Figures 5c and 6c. However, CaO is still present in the carbonated samples, suggesting that only a portion of CaO reacts with CO2. Therefore, XRD analysis showed again that the steel slag is a fairly complicated material in composition and reaction processes.
However, the evidence gained from the SEM qualitative analysis and the XRD analysis still suggests that CaO was the dominating reactant during the carbonation reaction for both the BOF steel slag and EAF steel slag samples. Thus, to facilitate the analysis, the carbonation reaction occurring in the present work is CaO þ CO2 f CaCO3 The CO2-trapping performance of the slag samples can be evaluated by Ca use. The Ca use η is defined herein as the percentage of calcium oxide in the sample that is chemically bound to carbon dioxide. It can be expressed as m00 CaO 100% ð1Þ m0 CaO where m0 CaO is the amount of calcium oxide in the fresh slag sample and m00 CaO is the amount of calcium oxide bound to carbon dioxide. On the basis of the measurements, m0 CaO and m00 CaO were determined from η¼
m0 CaO ¼ CaO M 0 m00 CaO ¼ 5486
WCaO ðm m0 Þ WCO2
ð2Þ ð3Þ
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Figure 5. XRD analysis of the BOF steel slag sample at 550 °C.
where m and m0 are the weight of the sample after and before the onset of the carbonation reaction, respectively, which are supplied by the experiments, CaO denotes the CaO content in the steel slag (see Table 1), M0 is the weight of the unheated raw sample, and WCaO and WCO2 denote the molecular weights of
CaO and CO2, respectively. Certainly, the amount of calcium oxide calculated by eq 2 is less than the actual calcium oxide taking part in the reaction because of the decomposition of calcium hydroxide. However, considering that the amount of calcium hydroxide within the slags is very limited and the dominant 5487
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Figure 6. XRD analysis of the EAF steel slag sample at 550 °C.
Ca-based compound is CaO (see Table 2), it is reasonable to neglect calcium hydroxide in the following analysis on Ca use. Ca use is an important index in evaluating the carbonation capacity of steel slag. In this work, the effects of a number of operating parameters, including the reaction temperature, CO2 concentration, and slag species, on the Ca use of slag were experimentally investigated under isothermal conditions.
3.2. Effect of the Temperature. The temperature has a distinct effect on the Ca use. As shown in Figures 7 and 8, for the two kinds of steel slag tested, generally, the higher the temperature, the larger the Ca use. The Ca use at 550 °C is about 35 times as large as that at 450 °C. Exceptions occur in the cases of 50% CO2 and 75% CO2 for BOF steel slag and 75% CO2 for EAF steel slag, in which the Ca use at 500 °C is/may be 5488
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Figure 7. Variation of Ca use with time and reaction temperature for BOF steel slag.
higher than that at 550 °C. Obviously, the CaO for trapping CO2 is also related to other factors in addition to the reaction temperature. The possible reason is that the carbonation reaction is controlled by not only the reaction kinetics but also the diffusion of reactive gas. Steel slag is a kind of porous material. CO2 can diffuse to the surface of the slag and then diffuse into the pore of the slag to react with CaO. The surface of CaO will be coated by the dense product layer CaCO3, which blocks the further contact of CO2 molecules with CaO. Moreover, because of the different molar volumes between CaCO3 and CaO (CaO, 17 cm3/mol; CaCO3, 37 cm3/mol), the high volume product
CaCO3 will fill the pores inside the slag and reduce the available CaO surface area, making the rate of the reaction decrease further. Another influencing factor is the specific surface area. It can be seen from Table 3 that the specific surface area decreases with an increasing operative temperature, indicating a change of the pore structure toward the formation of larger pores. Therefore, the reaction rate may decrease with the increase of the temperature. A similar phenomenon15 was also obtained for the carbonation of APC residues in 50% CO2 and was explained from the viewpoint of mass transfer by the authors. 5489
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Figure 8. Variation of Ca use with time and reaction temperature for EAF steel slag.
Table 3. Specific Surface Area of Steel Slag Heated to the Operative Temperatures BrunauerEmmettTeller (BET) (m2 g1) temperature (°C)
BOF steel slag
EAF steel slag
450
5.36 ( 0.12
8.57 ( 0.20
500
3.88 ( 0.06
6.21 ( 0.09
550
2.53 ( 0.04
3.92 ( 0.06
3.3. Effect of the CO2 Concentration. The effects of the CO2 concentration are more complex than those of the temperature (see Figure 9). It is generally believed that the higher concentrations of the reactants are more favorable for the reaction. However, in the present work, Ca uses do not increase as the CO2 concentration increases. For the two kinds of steel slag, generally speaking, the Ca use would reach the lowest value at 50% of CO2 at 450 and 550 °C. The higher values may appear when the CO2 concentration is lower (10%) or higher (more than 75%). Similar results were observed in ref 15, where the 5490
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Figure 9. Variation of Ca use with the CO2 concentration for BOF steel slag and EAF steel slag.
Figure 10. Time for attaining certain Ca use at different CO2 concentrations and temperatures for BOF steel slag and EAF steel slag.
maximum Ca conversions were obtained at 10% CO2 at 400 and 450 °C. However, the authors did not give an explanation. As for the present work, reasons for the anomaly are mainly because slag is a complex compound and its reaction is also related to the reaction surface area and the composition of slag. As seen from Figures 7 and 8, the reaction rates at 50% CO2 are obviously lower than those at 10% CO2 and higher CO2 concentrations. That is, it needs to spend more time to reach the same Ca use. However, it is in this period of time that the presence of product CaCO3 may decrease the reaction surface and make the carbonation reaction difficult to proceed. The Ca use at 50% CO2 would ultimately be a lower value. The increase of the reaction rate at lower CO2 concentrations may be related to the complex composition of slag, in which the compounds of Fe, Mn, and Al are present. These metal compounds may have more obvious catalytic effects on the carbonation reaction at lower CO2 contents. It is noticed that this relationship of the reaction rate with the CO2 concentration is inconsistent with the results observed in ref 15. It indicates that there exist different reaction mechanisms for different CO2 adsorbents. The higher Ca use at 10% CO2 may be attractive for industrial applications (for instance, the steel industry, power plant, chemical industry, etc.) because, therein, the CO2 content in flue gas is commonly lower than 20%. If the effects of the reaction temperature are considered further, carbonation at 550 °C is the most favorable choice. For the flue gas with a high CO2 content,
such as that in oxy-fuel combustion (about 8095% CO2),2,4 it appears that the steel slag is also a suitable adsorbent for CO2 trapping. 3.4. Kinetic of Carbonation. Figure 10 shows the reaction time spent for obtaining certain Ca use at different CO2 concentrations and temperatures. The operating conditions under which higher Ca uses could be achieved in the present experiments are chosen and compared. From Figure 10, it can be seen that the reaction time at 10% CO2 is generally the shortest among these cases, regardless of the kind of slag used. For BOF steel slag, it takes 11 min at 10% CO2 to achieve 20% Ca use but 20 min at 75% CO2. The maximum Ca use at 100% CO2 is 18.7%; therefore, the reaction time to achieve 20% Ca use is considered to be infinite. For EAF steel slag, the times to achieve 25% Ca use are 6 min at 10% CO2, 10 min at 75% CO2, and 23 min at 100% CO2. In contrast, the lower the CO2 concentration, the faster the reaction rate. Moreover, it is found that the reaction rate of EAF steel slag is faster than that of BOF steel slag and the ultimate Ca use of EAF steel slag is higher than that of BOF steel slag. It indicates the EAF steel slag is a better adsorbent in reactivity and CO2 storage. In view of a full-scale application, a shorter time or a faster reaction rate would be desired to reduce the reactor size. If the Ca use is considered at the same time, the results above show that the steel slag, especially EAF steel slag, is suitable to capture and sequester CO2 in the typical low-CO2 flue gas or the CO2-rich 5491
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4. CONCLUSION As a potential CO2 adsorbent, the characteristics of steel slags, BOF steel slag and EAF steel slag, were experimentally investigated for CO2 capture. The influence of the reaction temperature on the Ca use has a distinct difference between 450 and 550 °C. Generally, the higher the temperature, the larger the Ca use. However, the Ca use at 550 °C would be lower than that at 500 °C for a certain CO2 concentration. The effect of the CO2 concentration on the Ca use also has a distinct anomaly. At higher temperatures, larger Ca use appears at lower CO2 concentrations (75%). The reaction rate of carbonization increases with the CO2 concentration decreasing. With regard to the type of steel slag, EAF steel slag is better than BOF steel slag in reactivity and Ca use. In view of industrial application, steel slag is a potential feedstock for CO2 capture. It is cheap and does not need regenerating. In addition, what is more important is that it can be used in different flue gases, where the CO2 concentration is typically lower (75%). This provides the steel slag a wide application field. ’ AUTHOR INFORMATION
ARTICLE
(8) Huijgen, W. J. J.; Comans, R. N. J. Carbonation of steel slag for CO2 sequestration: Leaching of products and reaction mechanisms. Environ. Sci. Technol. 2006, 40, 2790–2796. (9) Baciocchi, R.; Costa, G.; Polettini, A.; Pomi, R. Influence of particle size on the carbonation of stainless steel slag for CO2 storage. Energy Procedia 2009, 1, 4859–4866. (10) Bonenfant, D.; Kharoune, L.; Sauve, S.; Hausler, R.; Mimeault, M.; Kharoune, M. CO2 sequestration potential of steel slags at ambient pressure and temperature. Ind. Eng. Chem. Res. 2008, 47, 7610–7616. (11) Stolaroff, J. K.; Lowry, G. V.; Keith, D. W. Using CaO- and MgO-rich industrial waste streams for carbon sequestration. Energy Convers. Manage. 2005, 46, 687–699. (12) Bao, W.; Li, H.; Zhang, Y. Selective leaching of steelmaking slag for indirect CO2 mineral sequestration. Ind. Eng. Chem. Res. 2010, 49, 2055–2063. (13) Lekakh, S. N.; Robertson, D. G. C.; Rawlins, C. H.; Richards, V. L.; Peaslee, K. D. Investigation of a two-stage aqueous reactor design for carbon dioxide sequestration using steelmaking slag. Metall. Mater. Trans. B 2008, 39, 484–492. (14) Baciocchi, R.; Polettini, A.; Pomi, R.; Prigiobbe, V.; Zedwitz, V. N. V.; Steinfeld, A. CO2 sequestration by direct gas-solid carbonation of air pollution control (APC) residues. Energy Fuels 2006, 20, 1933–1940. (15) Prigiobbe, V.; Polettini, A.; Baciocchi, R. Gassolid carbonation kinetics of air pollution control residues for CO2 storage. Chem. Eng. J. 2009, 148, 270–278. (16) Jia, L.; Anthony, E. J. Pacification of FBC ash in a pressurized TGA. Fuel 2000, 79, 1109–1114. (17) Nikulshina, V.; Galvez, E.; Steinfeld, A. Kinetic analysis of the carbonation reactions for the capture of CO2 from air via the Ca(OH)2CaCO3CaO solar thermochemical cycle. Chem. Eng. J. 2007, 129, 75–83.
Corresponding Author
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’ ACKNOWLEDGMENT The authors of this paper gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC) (Grant 51076089). ’ REFERENCES (1) Intergovernmental Panel on Climate Change (IPCC). Climate Change 2007: Mitigation of Climate Change, IPCC Fourth Assessment Report; Cambridge University Press: Cambridge, U.K., 2007. (2) Wall, T. F. Combustion processes for carbon capture. Proc. Combust. Inst. 2007, 31, 31–47. (3) Pennline, H. W.; Luebke, D. R.; Jones, K. L.; Myers, C. R.; Morsi, B. I.; Heintz, Y. J.; Ilconich, J. B. Progress in carbon dioxide capture and separation research for gasification-based power generation point sources. Fuel Process. Technol. 2008, 89, 897–907. (4) Wall, T. F.; Liu, Y.; Sperob, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. An overview on oxyfuel coal combustion—State of the art research and technology development. Chem. Eng. Res. Des. 2009, 87, 1003–1016. (5) Xu, C.; Cang, D. A brief overview of low CO2 emission technologies for iron and steel making. J. Iron Steel Res. Int. 2010, 17, 1–7. (6) Proctor, D. M.; Fehling, K. A.; Shay, E. C.; Wittenborn, J. L.; Green, J. J.; Avent, C.; Bigham, R. D.; Connolly, M.; Lee, B.; Shepker, T. O.; Zak, M. A. Physical and chemical characteristics of blast furnace, basic oxygen furnace, and electric arc furnace steel industry slags. Environ. Sci. Technol. 2000, 34, 1576–1582. (7) Huijgen, W. J. J.; Witkamp, G.; Comans, R. N. J. Mineral CO2 sequestration by steel slag carbonation. Environ. Sci. Technol. 2005, 39, 9676–9682. 5492
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