Response Surface Methodology Applied to the Evaluation of the SO2

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Response surface methodology applied to evaluate the SO2 sorption process by two Brazilian limestones Daniela Andresa Mortari, Ivonete Ávila, and Paula Manoel Crnkovic Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400049z • Publication Date (Web): 16 Apr 2013 Downloaded from http://pubs.acs.org on April 23, 2013

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1

Response surface methodology applied to the evaluation of the SO2 sorption process in two Brazilian limestones

Daniela A. Mortaria∗, Ivonete Ávilab, Paula M. Crnkovica,c,

a

School of Engineering of São Carlos, University of São Paulo - USP, São Carlos CEP

13566-590, Brazil. b

Department of Energy, Faculty of Engineering at Guaratinguetá, Univ Estadual Paulista –

UNESP, Guaratinguetá CEP 12516-410, Brazil. c

Institute of Chemistry, Univ Estadual Paulista – UNESP, Araraquara CEP 14801-970,

Brazil.

∗

Corresponding author at: School of Engineering of São Carlos, University of São Paulo, São Carlos CEP 13566590,

Brazil. Tel.: +55 16 33738603; fax: +55 16 33739402. Email address: [email protected] (D. A. Mortari).

ACS Paragon Plus Environment

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2

Response surface methodology applied to the evaluation of the SO2 sorption process in two Brazilian limestones

Abstract This paper proposes a response surface methodology to evaluate the influence of the particle size and temperature as variables and their interaction on the sulfation process using two Brazilian limestones, a calcite (ICB) and a dolomite (DP). Experiments were performed according to an experimental design (Central Composite Rotatable Design - CCRD) carried out on a thermogravimetric balance and a nitrogen adsorption porosimeter. In the SO2 sorption process, DP showed more efficient than ICB. The best results for both limestones in relation to conversion and BET surface area were obtained under central point conditions (545 µm and 850 oC for DP and 274 µm and 815 oC for ICB). The optimal values for conversion were 52% for DP and 37% for ICB. For BET surface area, the optimal values were 35 m2 g-1 for DP and 45 m2 g-1 for ICB. A relationship between conversion and pore size distribution has been established. The experiments that showed higher conversions also exhibited more pores in the region between 20 and 150 Å and larger BET surface area, indicating that the amount of smaller pores may be an important factor in the reactivity of limestones.


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3 1. Introduction The combustion of fossil fuels is the major source of SO2 emissions.

1-4

The Brazilian

bituminous coals are characterized by high levels of ash (20 - 50 wt. %) and sulfur (1 – 6 wt.%), which can lead to problems in their combustion in conventional plants.5-6 The possibility of using fluidized bed reactors has become promising once they allow the addition of limestones, which are efficient SO2 sorbents, reducing the emissions of pollutants.7 At typical temperatures of bed combustion, 800-900 oC,8 limestones (CaMg(CO3)2) are completely calcined, producing calcium oxide (CaO) and magnesium oxide (MgO) (Equation 1). In this process, CO2 is released making the physical structure of the calcined limestone more porous and, consequently, increasing the surface area.9-11 The calcined limestone reacts with the SO2 generated during the coal combustion, producing calcium sulfate (CaSO4) and magnesium sulfate (MgSO4) by means of a sulfation reaction (Equation 2). CaMg(CO3)2 (s) → CaO (s) + MgO(s) + 2CO2(g)

(1)

CaO(s) + MgO(s) + 2SO2(g) + O2(g) → CaSO4(s) +MgSO4(s)

(2)

The volume of sulfated solid products (CaSO4 and MgSO4) is approximately three times larger than that of the respective reagents (CaO and MgO). During their formation, a partial or total pore blockage occurs in the interior of the particles, avoiding their complete conversion.12 The reactivity of MgO as an SO2 sorbent is controversial and some authors consider MgO remains inert in relation to the sulfation reaction,8, 13-14 whereas others claim that SO2 can be removed by MgO at temperatures between 450-500 oC and 800 oC.15-16 The literature has shown evidences on the occurrence of MgSO4 as a product of the SO2 sorption by limestones.17-19 The understanding of the conversions, mechanism and limiting factors in the reaction between limestones and SO2 has been the subject of previous researches. Dennis and Hayhurst 20 observed that the presence of a small amount of O2 under fluidized bed conditions accelerates the sulfation of limestone causing no significant decrease in the conversion and reaction rate coefficients [20]. The effect of atmosphere on the conversion of SO2 sorption by limestones under thermogravimetry conditions was studied by Ávila et al. 21 The results show that the value of maximum conversion is 29% higher in nitrogen atmosphere than in air


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4 atmosphere. The influence of the internal structure of the particles on the sulfation reaction is very strong and the effect of sorbent porosity on this process has been studied.

22-25

The SO2

sorption by limestone is affected by pore size and pore size distribution, intra-particle superficial area,22-28 porosity and density of particles.29 Previous researches have showed that sorbents with a distribution of pores larger than 100 Å have higher ability as SO2 sorbents, whereas those with small pores (smaller than 80 Å) have lower efficiency due to the rapid blockage of the pores during sulfation. For larger particles, the sulfation is limited by diffusion through the layers of the product, whereas for smaller particles (1 – 90 µm) it is controlled by the kinetics of the chemical reaction. 24-25 In a more recent study, Ávila et al. 30 showed that the mesoporous region was the most affected region by the pores blockage during the SO2 sorption by a dolomite. Thermal analysis has been widely used in the characterization and kinetic investigation of the limestones calcination,8, 10, 18-19, 21, 31 and sulfation.9, 11, 19-20 Studies about the effect of the calcination temperature on the removal of SO2 by limestone have showed that diffusion is the limiting factor in the reaction of SO2 with CaO and the reaction rate is inversely proportional to the particle size.32 The pore size distribution of the calcined CaO is considerably affected by the calcination temperature and the samples calcined at 950 °C show less resistance to diffusion and a higher activity for the SO2 removal. The maximum rate of conversion of CaO to CaSO4 decreases with increasing temperature due to the rapid blockage of the pores. The constant rate of reaction decreases exponentially with time due to the formation of CaSO4 layers on the surface of CaO.17, 30-31. Several studies have reported on variables that influence the sulfation process conversion.17, 22, 24, 27-28, 31, 33 To our knowledge, no investigation has been concerned with the response surface methodology applied to the SO2 sorption by limestones and the correlation with the changing on the sorbent surface. It is clear that the limestone’s particle sizes and temperature play a crucial role in the sulfation processes, but the literature still lacks of studies which show the interaction between these variables on the conversion, and BET surface area. In addition, the use of statistical tools is advantageous in order to optimize measurable parameters, reduce the number of experiments, save time and evaluate the interaction of all parameters. From this perspective, the present study provides mathematical


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5 models to evaluate the interactions between temperature and particle sizes on the conversion, and BET surface area applied in SO2 sorption by limestones. Through the analysis of response surfaces obtained it is possible to predict the conversion process and BET surface area for different values of temperature and particle sizes. This study proposes a response surface methodology to evaluate the influence of the particle size and temperature as variables and their interaction on the sulfation process using two Brazilian limestones, a calcite and a dolomite. Nitrogen adsorption porosimetry and thermogravimetric experiments were performed according to an experimental design (Central Composite Rotatable Design - CCRD). A relation among conversion, pores development, and morphology of the sorbent particles has been established. 2. Material and methods Two Brazilian limestones were studied as sorbents of SO2: a dolomite, from São Paulo State, called DP, and a calcite, from Rio Grande do Sul State, called ICB. Before being submitted to the TG and porosimetry experiments, these limestones were crushed and sieved and particles sizes between 97 and 775 µm were selected. 2.1 Thermogravimetric experiments Thermogravimetric analysis (TG) and derivative thermogravimetry (DTG) were performed in a Shimadzu TGA-51 analyzer using a sample mass of 10 ± 0.5 mg and an alumina crucible. Prior to the experiments, the thermogravimetric equipment was calibrated by following the Shimadzu´s Instruction Manual. The procedure comprised two steps: calcination and sulfation. For the calcination, the natural sample was heated at 30oC min-1 from room temperature up to the desired temperature under a flow rate of 80 mL min-1 of synthetic air as a carrier gas. An isothermal condition of 30 minutes at the desired temperature was applied. The desired temperature depends on the conditions proposed in the experimental design. The sulfation process started after the 5 first minutes of the isothermal condition introducing SO2 (100% v/v) with a 20 mL min-1 flow in the furnace to obtain a concentration of 20% of this reactant gas. Figure 1 shows the TGA and DTG curves of calcination followed by sulfation under conditions of particle size of 545 µm and heating rates of 30 oC min -1 from room temperature to 850 oC.


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6 Indication [1] refers to the start of the carbon dioxide (CO2) release, which occurs between 650 oC and 850 oC. Indication [2] refers to the calcium oxide (CaO) and magnesium oxide (MgO) produced after calcination (Equation 1). Indication [3] shows a mass gain due to the formation of calcium sulfate and magnesium (CaSO4 and MgSO4) (Equation 2). The reactivity of the limestone was evaluated by means of the TG curves obtained in the sulfation reaction and the conversion of this step was calculated according to Equation 3.

X=

M − MC WSO2 + 12 WO2 Y YMg M A  Ca +  WCa WMg 

   

× 100

(3)

where MA is the initial mass of the sample, MC is the mass after calcination, MF is the final mass after sulfation, YCa and YMg are the mass fractions of calcium and magnesium, respectively, and W is the molar mass species. MA, MC and M were taken at points [1], [2] and [3] respectively, indicated in Figure 1. For calcite (ICB), YCa is 0.35 and YMg is 0.02 and for dolomite (DP), YCa is 0.17 and YMg is 0.12. The values of mole mass species adopted were 64.04, 31.98, 40.07, 24.30 for WSO2 , WO2 , WCa and WMg , respectively. 2.2 Porosimetry experiments The BET surface area, pore diameter and pore volume of the limestones were obtained from nitrogen adsorption and desorption isotherms conducted at 77 K in an ASAP 2020 analyzer. In order to provide enough samples for porosimetry tests, calcinations tests for both ICB and DP limestones were carried out in a vertical tubular furnace, considering the same experimental conditions of heating rate, gas flow rate and temperature adopted in TG tests. This methodology was described in details elsewhere. 30, 34 The weights of the samples were 0.600 ± 0.010 g for calcined limestone and 1.000 ± 0.010 g for natural limestone. Before the porosimetry experiments both calcined and natural samples were dried in an oven for 12 h at 150 ° C and degassed (heated and evacuated) for the removal of gases adsorbed inside the particles. The sample was conditioned for evacuation (restricted vacuum) at a rate of 5 mmHg s-1 until 5 mmHg and moved to unrestricted vacuum at the same rate until 10 µmHg, remaining


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7 under this condition for 6 minutes. A P/Po (ratio of applied pressure (P) and saturation vapor pressure of the adsorbed gas (Po), in this case N2) was applied to 37 points (24 in the adsorption and 13 in the desorption). 2.3 Scanning electron microscopy Scanning Electron Microscopy (SEM) was performed in a ZEISS LEO 440 (Cambridge, England), OXFORD detector, operating with electron beam of 20kV to complete the morphological analysis of the particles. 2.4 Experimental design Parameters, such as sulfation temperatures, particle sizes and limestone type directly affect the sorption process, which can be measured from the reaction conversion. Several conditions can influence the sulfation process and, in order to optimize measurable parameters, reduce the number of experiments, save time and evaluate the interaction of all parameters, the Central composite rotatable design (CCRD) was used. CCRD is a statistical technique employed for multiple regression analysis and describes the interactive, cumulative and individual effects of the test variables on the process yield.35 This tool provides a second-order model for the production of a response surface. Replicates at the central point, four axial points and 22 plus start configuration were performed randomly. In this study, sulfation temperature (T) and particle size (ϕ) are the variables evaluated at five different levels, which were selected on the basis of prior studies.

20-22

The design

matrix with the coded and real values of these variables is shown in Table 1. In the CCRD, the usual levels are 0, +1, -1,

2 (1.41) and - 2 (-1.41), which were

applied for temperature. However, these levels could not be used for particle size due to the unavailability of selected screens starting from the central point (0) of 545 µm. The levels used for the particle size were 0, +1, -0.81, +2.20 and -1.52 for the DP and 0, +1, -0.96, +1.68 and -1.59 for the ICB. In the first study, an experimental design was developed using the same variables and levels for both ICB and DP. As only for ICB, the higher conversions tended to a smaller particle size and lower temperatures, a second experimental design with smaller particle sizes


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8 and lower temperatures for ICB was performed, as shown in Table 1. The values obtained from CCRD (Table 1) were fitted in the quadratic polynomial model related to the response by a multiple regression procedure represented by Equation 4. k

k

Y = β 0 + ∑ β j x j + ∑∑ β ij xi x j + ∑ β jj x 2j + ε j =1

i< j

(4)

j =1

where Y is the predicted response, β0, βj, βij and βjj are constant coefficients, i and j take value from 1 to the number of variables, xi and xj are the coded independent variables or factors and ε is a random error. The statistical significance was determined by both ANOVA (analysis of variance) and Student test (t). 3. Results and discussion 3.1 Thermogravimetry experiments

The conversion curves for all experiments obtained from TGA according to CCRD, for both materials are shown in Figure 2. For ICB, the conversion processes starting at 30% and stops very quickly. The values are the same until the end of the process, while the values for DP continue to increase. Table 1 shows the conversions values obtained through Equation 3 and TG curves according to CCRD, considering MA, MC and MF values, temperature and particle size as variables. A second-order model equation was obtained for the conversion as a function of temperature (x1) and particle size (x2) for each limestone, as shown in Equation 5 for DP and Equation 6 for ICB. The equations were obtained using a regression analysis and considering the statistically significant parameter (α) 95%. Conversion = 52.028 + 1.60x1 + 1.55 x2 – 4.58x12 – 1.46x22 + 1.46x1x2

(5)

Conversion = 37.04 + 1.97x1 + 0.45 x2 – 2.11x12 – 2.87x22 – 1.30x1x2

(6)

The ANOVA for these models is expressed in Tables 2 and 3 for DP and ICB, respectively. For both limestones, t test showed that only β0, x12 and x22 are significant.


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9 The models were verified by R2. This parameter indicated that 80% and 72% of the response variability for DP and ICB, respectively, could be explained by the model. The predictive values, shown in Tables 2 and 3, were good enough because in this case, both limestones are natural and heterogeneous material. This characteristic interferes with the responses repeatability. The mean squares regression and waste were statistically different, indicating that the models are significant. The F test value (5.17) was higher than the critical value (3.97), presenting significance with 95% confidence. For both models, the comparison between the calculated F test (0.23 for DP and 4.58 for ICB) and the critical F (6.59) for pure errors and lack of fit shows there were model fits. Figures 3 and 4 show the response surface (a) and contour diagram (b) for DP and ICB, respectively, generated by the models. The response surface and contour diagram analysis allowed verifying that in both cases the maximum conversions were obtained in the central point condition– 52% for DP and 37% for ICB – whose values are related to the average value of five replicates in the central point. This difference in the particle size between the limestones is an indication that the structure may exert some influence on the sorption behavior of the limestones. As observed in Figures 3 and 4, the temperature and particle size produce a negative effect on the extreme values of the selected ranges. The lowest conversions were obtained at 800 oC and 750 µm for DP (approximately 20%) and at 745 oC and 97 µm for ICB (approximately 18%). Lower conversions were also found for the DP at temperatures above 900 oC for all particle sizes. The decrease in the conversion can be described as a consequence of several factors. According to Dogu

33

, above 950 ° C, there occurs a pore blockage due to the

sintering of the particles, which tend to collapse and consequently lose surface area available for reaction. According to Yan et al.

36

, in gas-solid reactions, smaller particles have higher

conversion due to the reduction in the importance of the diffusive limitation of gas into the particles. For ICB, the lowest conversions were obtained for a smaller particle size (97 µm). This result can be attributed to the effect of the distribution of particles on the alumina support used during the thermogravimetric tests, i. e., as the particles are smaller and ensure that the


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10 mass of the sample is approximately 10 mg, multiple layers of limestone are formed on the support. These layers possibly lead to barriers for both release of CO2 in the calcination process and incorporation of SO2 during the sulfation. 3.2 Porosimetry experiments

In order to understand the role of the particles porosity in the sulfation process, we also evaluated both porosity and morphological structures of the particles for each experiment shown in Table 1. Parameters, such as BET surface area, pore volume, and pore diameter for DP and ICB in their natural and calcined forms, are shown in Table 4, in which the evolution of the porous structure can be clearly distinguished. Comparing the values of natural and calcined forms at central points (9, 10 and 11 experiments), whereas the BET surface area for the DP, under the central point conditions, increased 11 times after calcination. For the ICB, this increase was approximately 68 times, on average. Moreover, after calcination, the pore volume increased, on average, 13 times for DP and 50 times for ICB. A second-order model equation was also obtained for the BET surface area of the calcined samples as a function of temperature and particle size. The response surfaces for both limestones were obtained by the model presented in Equations 7 and 8 for DP and ICB, respectively. BETsurface area = 31.89 + 3.10x1 + 1.49 x2 –5.93x12 –1.53x22 – 0.13x1x2

(7)

BETsurface area = 41.91 + 7.49x1 – 2.33 x2 – 12.56x12 – 3.90x22 + 0.41x1x2

(8)

where x1 and x2 are the coded values for temperature and particle size, respectively. The surface responses and contour diagram related to the BET surface area as a function of temperature and particle size are shown in Figures 5 and 6. The ANOVA for the models expressed in Equations 7 and 8 is shown in Tables 5 and 6. The variations explained by the model were 82% for DP and 77% for ICB, respectively. The comparison between the F test (0.59 for DP and 0.32 for ICB) and the critical F value (19.16)


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11 for pure errors and lack of fit show that there is model fitting for the BET surface area. T test shows that β0 and x12 are significant for DP and β0, x1 and x12 are significant for ICB. These results indicate that the BET surface area is affected by the temperature and the particle size. The comparison of the two response surfaces (Figures 5 and 6) shows that after calcination under all conditions the ICB exhibits a larger development of the porous structure than DP, in percentage terms. The contour diagram shows that the best results (45 m2 g-1 for ICB and 35 m2 g-1 for DP) were obtained under conditions of 320 µm and 820 oC for ICB and 620 µm and 880 oC for DP. Besides the BET surface area, pore size is another important characteristic in the sulfation process.25-26 The pore diameter distributions of the calcined and natural forms are shown in Figure 7 for both limestones. An additional assessment of the porous structures can be understood in terms of nitrogen adsorption isotherms. As observed in Figure 8, the DP adsorption is, approximately, 2.6 times higher than that of the ICB. The changes related to the porosity of the limestone before and after calcination can be seen qualitatively in the images obtained by Scanning Electron Microscopy (SEM) (Figure 9). Figure 9 (f) shows that the surface of the sulfated ICB presents a trend to be blocked due to the filling of the superficial pores. However, in Figure 9 (c) for DP, this behavior is not observed and the outer layer remains accessible to the reactant gas SO2 during the sulfation process. For DP, the variation in the particle size in the range studied did not exert any influence on the change of parameters of the porous structure in comparison to the effect of the temperature variation. However, for ICB, the pore area increased with the decrease in the particle size. These results indicate that for this limestone the variation in the particle size influences the development of the porosity of the material. A relationship between conversion and pore size distribution has been established. The


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12 experiments show that both limestones presented higher pore distribution at 20-150 Å region. However, it was identified narrower pore size in a greater proportion for the ICB (Figure 7). Possibly, this characteristic led to higher pore blockage, and hence the conversion values were lower for the ICB than the DP (Figure 2). After calcination, the ICB showed a large increase in the BET area and a decrease in the pore diameter. All these changes in the physical structure favor the conversion process. However, the DP showed higher conversions. These results may be explained by the physical characteristics of the material in its natural form. The BET surface area and pore volume of the natural DP are approximately 5 times larger than those of the natural ICB and, according to Dam-Johansen and Ostergaard,32 porous limestones are more reactive due to the ease of CO2 release, which determines the rate of calcination.37 The complete calcination entails the best sorption of SO2 and, consequently, the highest conversions are obtained. Moreover, after calcination there appear cracks in the structure of the DP (Figure 10). These cracks probably allow the SO2 to diffuse into the particles, causing a gas-solid reaction on their surface and also inside of them. This fact explains the increased N2 adsorption during the porosimetry tests and higher sulfation conversions in the tests. It is noteworthy that Ca and Mg contents present in the limestones may affect the SO2 sorption performance, and these values are quite different in the studied limestones. However, there are controversies in the literature concerning this issue, for instance, WieczorekCiurowa

38

and Fuertes et al.

39

state that MgSO4 is instable at high temperature; Borgwardt

17

and Harvey state that the MgO + SO2 reaction is slower than CaO + SO2 reaction, which is an argument that there is no significant reaction between MgO and SO2. On the other hand, Borgward40 say that there are evidences of the MgSO4 among the products of the reaction. In addition, considering that total fraction of Ca and Mg (YCa+Mg) are 0.37 and 0.29 for the ICB and DP, respectively, It was expected that the conversion values were higher for the ICB than DP, but, an opposite behavior occurred, indicating that the difference in the pore size distribution for the ICP and DP was crucial in the conversion results. The results obtained in this study are important because the literature lacks studies on the interaction between particle size and temperature. To our knowledge, no investigation has been concerned with the response surface methodology applied to the SO2 sorption by limestones and the correlation with the changing on the sorbent surface. Such knowledge


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13 favors the identification and evaluation of appropriate conditions involved in reactive processes, as desulfurization, minimizing empiricism into a scientific criterion. These results can be applied on larger scales, favoring a rational use of raw materials, saving time and operational cost and, in general, increasing efficiency. 4. Conclusions The influence of the particle size and temperature as variables and their interaction on the sulfation process using two Brazilian limestones, a calcite and a dolomite has been studied. The statistical methodology showed optimal conditions for the conversion process and porous development after calcination. The highest conversions for DP (52 %) were found at 850 oC and particle size of 545 µm; the lowest conversions were obtained for temperatures over 900 °C and below 780 oC. The low conversion above 900 °C was attributed to a possible effect of sintering. The low conversion below 800 ° C was attributed to kinetic limitations of the sulfation reaction. The best results for porous development (35 m2 g-1) were obtained under conditions of 620 µm and 880 oC for DP. For ICB, the highest conversions (36.7 %) were found at 815 oC and particle size of 274 µm. The lowest conversions were obtained for a smaller particle size (97 µm). The best results for the development of pores (45 m2 g-1) were obtained under conditions of 320 µm and 820 oC for ICB and. In the SO2 sorption process, DP showed more efficient than ICB. By establishing a correlation between the response surface and the parameters of the physical structure it can be observed that at maximum conversion condition, both limestones presented higher pore distribution at 20-150 Å region. The different conversion between both calcined limestones may be related to the narrower pore size, which was identified in a greater proportion for the ICB than for the DP. This characteristic leads to higher pore blockage and consequently slows down the diffusivity of SO2 to the inner layers of the particles. ACKNOWLEDGEMENTS The authors are indebted to the Research Foundation of São Paulo State (FAPESP –


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14 Projects 2011/00183-2 and 2011/11321-7) and Coordination for the Improvement of Higher Education Personnel (CAPES – Projects PNPD 0034088 and BEX 1149/10-5) for the financial support given to this research. The assistance from Prof. Angela P. Giampedro and Prof. Dr. Edenir Rodrigues Pereira Filho are also acknowledged. REFERENCES [1] Nazari, S.; Shahhoseini, O.; Sohrabi-Kashani, A.; Davari, S.; Paydar, R.

Delavar-

Moghadam, Z. Energy, 2010, 37 (7), 2992-2998. [2] Glarborg, P. P Combust Inst., 2007, 31 (1), 77-98. [3] Theodosiou, G.; Koroneos, C.; Moussiopoulos, N. Build Environ., 2007, 32 (3), 1522– 1530. [4] Maciás-Pérez, M. C.; Bueno-López, A.; Lili-Ródenas,M. A.; Salinas-Martínez De Lecea, C.; Linares-Solano, A. Fuel, 2007, 8, 677-683. [5] Depoi, F. S.; Pozebon, D.; Kalkreuth, W. D. Int J Coal Geol., 2008, 76, 227-236. [6] Süffert, T. Carvão nos Estados do Rio Grande do Sul e Santa Catarina, (1997) CPRM: Porto Alegre. [7] Jacobs, J. P. Chem. Eng. Sci., 1999, 54, 5559-5563. [8] Fuertes,A. B.; Marban, G.; Rubiera, F. Chem. Eng. Res. Des., 1993, 71, 423-428. [9] Hartman, M.; Couchlin, R. W. AIChE J., 1976, 22, 490-498. [10] Simon, G. A.; Garman, A. R.; Boni, A. A. AIChE J., 1987, 33, 211-217. [11] Anthony, E. J.; Bulewicz, E. M.; Jia, L. Prog. Energy Combust. Sci., 2007, 33, 171–210. [12] Zarkanitis, S.; Sotirchos, S. V. AIChE J., 1989, 35, 821-830. [13] Hsia, C.; Pierre, G. R. S.; Raghunatahn, K.; Fan, L. S. AIChE J., 1993, 39, 698-700. [14] Khraisha, Y. H.; Dugwell, D. R. Chem. Eng. Res. Des., 1991, 69, 76-78. [15] Silcox, G. D.; Kramlich, J. C.; Pershing, D. W. Ind. Eng. Chem. Res., 1989, 28, 155-160.


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Energy & Fuels

15 [16] Borgwardt, R. H. Chem. Eng. Sci., 1989, 44, 53-60. [17] Borgwardt, R. H.; Harvey, R. D. Environ. Sci. Technol., 1972, 6, 350-360. [18] Dennis, J. F.; Hayhurst, A. N. Chem. Eng. Sci., 1987, 42, 2361-2372. [19] Crnkovic, P. M.; Milioli, F. E.; Pagliuso, J. P. Thermochim. Acta, 2006, 447, 161-166. [20] Dennis, J. S.; Hayhurst, A. N. Chem. Eng. Sci., 1990, 45, 1175-1187. [21] Ávila, I.; Crnkovic, P. M.; Milioli, F.E. Quim. Nova, 2006, 29(6), 1244-1249. [22] Borgwardt, R. H.; Bruce, K. R. AIChE J., 1986, 32, 239-246. [23] Bhatia, S. K.; Perlmutter, D. D. AIChE J., 1981, 27, 226-234. [24] Adánez, J. Fierro, V.; Diego, J. A.; Diego, L. F.; Garcia-Labiano, F. Thermchim Acta., 1996, 277, 151-164. [25] Iisa, K.; Hupa, M. J I Energy., 1992, 65, 201-205. [26] Adánez, J.; Garciá-Labiano, F.; Gayán, P. Fuel Process. Technol., 1993, 36, 73-79. [27] Li, Y.; Sadakata, M. Fuel, 1999, 78, 1089-1095. [28] Davini, P. Fuel, 2002, 81, 763-770. [29] Yrjas, P.; Lisa, K.; Hupa, M. Fuel, 1995, 74, 395-400. [30] Ávila, I.; Crnkovic, P. M.; Milioli, F. E.; Luo, K. H. Appl. Surf. Sci., 2012, 258 (8), 35323539. [31] Kök, M. V.; Smykatz-Kloss, W. J Therm Anal Calorim, 2008, 91 (2), 565-568. [32] Dam-Johansen, K.; Ostergaard, K. Chem. Eng. Sci., 1991, 46, 827-859. [33] Dogu, T. Chem. Eng. J., 1981, 21, 213-222. [34] Ávila, I.; Crnkovic, P. M.; Milioli, F. E. Quim. Nova, 2010, 33 (8), 1732-1738.


Energy & Fuels

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

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16 [35] Colla, E.; Pereira, A. B.; Hernalsteens, S.; Maugeri, F.; Rodrigues, M. I. Food Bioprocess Technol., 2010, 3, 265-275.

[36] Yan, C-F.; Grace, J.R.; Lim. C.J. Fuel Process. Technol., 2010, 91 (11), 1678-1686. [37] Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by powders and porous solids, Academic Press: London, 1999. [38] Wieczorek-Ciurowa, K.; J. Therm. Anal. Calorim. 1992, 38, 523. [39] Fuertes, A. B.; Velasco, G., Fuente, E.; Alvarez, T.; Fuel Process. Technol., 1994, 38, 181-192. [40] Borgwardt, R. H.; Environ. Sci. Technol. 1970, 4, 59-63.


Page 17 of 33

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

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17 Figure Captions Figure 1. TG and DTG curves of calcination followed by sulfation. The temperature of the isothermal step is 850 oC. Figure 2. Conversion curves for all experiments obtained from TGA according to CCRD, for (a) DP and (b) ICB. Figure 3. Response surface and contour diagram obtained for the DP conversion. Figure 4. Response surface and contour diagram obtained for the ICB conversion. Figure 5. Response surface and contour diagram obtained for the DP BET surface area. Figure 6. Response surface and contour diagram obtained for the ICB BET surface area. Figure 7. Pore diameter distribution of DP and ICB. Figure 8. Comparison of the adsorption and desorption isotherms of calcined DP and ICB. Figure 9. Change in the physical structure of limestone. (a) Natural DP , (b) calcined DP under central point conditions (c) sulfated DP under central point conditions, (d) natural ICB, (e) calcined ICB under central point conditions and (f) sulfated ICB. (Images obtained by SEM with 10000X increase). Figure 10. Changes in the external porous structure after calcinations (a) calcined ICB (b) calcined DP (500X increased).


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

Page 18 of 33

18

Table 1. Central composite rotatable design (CCRD) matrix with coded, real values for the variables and responses to conversion (X). TG results for DP Run

MA

MC

MF

Real and coded values for DP T o

Φ

X

TG resuls for ICP MA

MC

MF

Real and coded values for ICP T o

Φ

X

(mg)

(mg)

(mg)

( C)

(µm)

(%)

(mg)

(mg)

(mg)

( C)

(µm)

(%)

1

10.40

7.16

10.54

-1 (800)

-0.81 (460)

45.31

9.57

5.88

7.91

-1 (765)

-0.96 (168)

29.60

2

10.05

6.78

9.93

+1 (900)

-0.81 (460)

43.70

10.07

6.21

8.92

1 (865)

-0.96 (168)

37.50

3

10.02

6.47

9.77

-1 (800)

+1 (650)

45.92

10.33

6.50

8.58

-1 (765)

+1 (385)

33.50

4

10.09

6.68

10.22

+1 (900)

+1 (650)

48.92

10.35

6.24

8.98

1 (865)

+1 (385)

36.90

5

9.97

6.14

9.40

-1.41 (780)

0 (545)

45.59

9.54

5.41

7.40

-1.41 (745)

0 (274)

29.10

6

10.31

7.29

10.76

1.41 (920)

0 (545)

46.93

9.87

5.96

8.24

1.41 (885)

0 (274)

32.20

7

9.97

6.59

9.97

0 (850)

-1.52 (385)

47.27

9.75

5.79

7.76

0 (815)

-1.59 (97)

28.20

8

10.15

7.02

10.21

0 (850)

2.20 (775)

49.16

10.28

6.32

8.35

0 (815)

+1.68 (460)

27.50

9

10.35

6.83

10.85

0 (850)

0 (545)

54.16

9.85

5.59

8.08

0 (815)

0 (274)

35.20

10

9.95

6.23

10.12

0 (850)

0 (545)

54.51

9.28

5.40

7.74

0 (815)

0 (274)

35.20

11

9.70

6.42

9.87

0 (850)

0 (545)

49.89

10.15

6.19

9.04

0 (815)

0 (274)

39.20

12

10.27

7.18

10.84

0 (850)

0 (545)

49.69

9.14

5.39

7.77

0 (815)

0 (274)

36.30

13

10.35

6.83

10.85

0 (850)

0 (545)

54.16

9.86

5.80

8.49

0 (815)

0 (274)

38.00


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

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19

Table 2. ANOVA of the quadratic model for DP conversion. Source of variation

Sum of squares

Degree of freedom

Mean squares

F test

Regression

123.73

5

54.02

5.7

Residual

30,36

7

4.60

Total

154.36

12

Pure error

25.85

4

6.46

Lack of fit

4.51

3

1.50

Coefficient of determination (R2) = 0.80 F0.05; 5;7(Ftabulated) = 3.97 F0.05; 4;3(Ftabulated) = 6.59


0.23

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Page 20 of 33

20

Table 3. ANOVA of the quadratic model for ICB conversion. Source of

Sum of

Degree of

Mean

variation

squares

freedom

squares

Regression

137.87

5

27.57

Residual

55.75

7

7.96

Total

193.62

12

Pure error

12.57

4

3.14

Lack of fit

43.18

3

14.39

Coefficient of determination (R2) = 0.71 F0.05; 5;7(Ftabulated) = 3.97 F0.05; 4;3(Ftabulated) = 6.59


F test

3.46

4.58

Page 21 of 33

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

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21

Table 4: Pore characteristics for both limestones DP and ICB in the natural and calcined forms. SP (m2/g) - BET

V P. ds (cm3/g) - BJH

D P. ds (Å) - BJH

Q Ads (cm3 g-1)

Run DP

ICB

DP

ICB

DP

ICB

DP

ICB

1

17.2

20.3

0.141

0.090

201.8

110.4

91.37

58.31

2

24.7

45.3

0.094

0.227

96.9

121.3

60.71

146.21

3

23.0

12.4

0.161

0.062

174.9

127.5

103.81

39.73

4

29.8

38.6

0.143

0.216

121.8

134.4

91.94

139.37

5

17.5

10.1

0.142

0.058

196.2

148.3

91.47

37.12

6

24.9

16.3

0.095

0.07

95.0

106.1

61.19

43.82

7

28.7

31.7

0.170

0.151

151.6

116.1

109.76

97.51

8

27.1

25.8

0.205

0.130

187.4

124.9

132.31

83.97

9

27.6

37.1

0.168

0.169

152.3

113.9

108.22

108.78

10

35.3

48.1

0.207

0.270

150.5

123.2

133.77

155.19

11

33.2

38.6

0.187

0.175

142.8

108.5

120.90

113.11

Natural

2.9

0.6

0.019

0.004

159.6

208.4

12.40

2.69

Sp = superficial area; Vp ads = pores volume in the adsorption; Vp ds = pores volume in the desorption; Dp ads = pores diameter in the adsorption and Dp ds = pores diameter in the desorption; Q Ads =

adsorbed.


quantity

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Page 22 of 33

22

Table 5. ANOVA of the quadratic model for DP BET surface area. Source of

Sum of

Degree of

Mean

variation

squares

freedom

squares

Regression

264.83

5

52.96

Residual

59.76

5

11.95

Total

324.60

10

Pure error

31.72

2

15.86

Lack of fit

28.05

3

9.35

Coefficient of determination (R2) = 0.82 F0.05; 5;5 (Ftabulated) = 5.05 F0.05; 2;3 (Ftabulated) = 19.16


F test

4.43

0.59

Page 23 of 33

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

Energy & Fuels

23

Table 6. ANOVA of the quadratic model for ICB BET surface area. Source of

Sum of

Degree of

Mean

variation

squares

freedom

squares

Regression

1377.15

5

275.43

Residual

400.42

5

80.08

Total

1777.57

10

Pure error

71.14

2

35.57

Lack of fit

329.28

3

109.76

Coefficient of determination (R2) = 0.77 F0.05; 5;5(Ftabulated) = 5.05 F0.05; 2;3 (Ftabulated) = 19.16


F test

3.44

0.32

Energy & Fuels

24

Figure 1

0,04

12 Air + SO2

Air

[3]

11 [1]

0,02

9

0,00

8 -0,02

[2] 7

TGA DTG

6 0

500

1000

1500

2000

2500

3000

t (s)


-0,04 3500

DTG (mg/s)

10 Mass (mg)

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

Page 24 of 33

Page 25 of 33

25

Figure 2

60 ICB

DP

50 40 run 1 run 2 run 3 run 4 run 5 run 6 run 7 run 8 run 9 run 10

X (%)

X (%)

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

Energy & Fuels

30 20 10 0 0

4

8

12

16

20

4

t (min)


8

12 t (min)

16

20

Energy & Fuels

26

Figure 3 770 52,00 50,38 48,75 47,13 45,50 43,88 42,25 40,63 39,00

50 48 46 44 42

660 605 550 495

820

840

T (oC)

860

φ

800

770 715 660 605 550 495 440 385

(µ m )

40 780

715

φ (µm)

52

Conversion (%)

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

Page 26 of 33

880

900

920

440 385 780

800

(a)

820

840

860 o T ( C)

(b)


880

900

920

Page 27 of 33

27 Figure 4 450 52,00 48,50 45,00 41,50 38,00 34,50 31,00 27,50 24,00

48 44 40 36 32

400 350

φ (µm)

52

300 250 200

28

780

800

T(C ) o

820

(µ m

760

)

24

450 400 350 300 250 200 150 100

840

φ

Conversion (%)

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

Energy & Fuels

860

880

150 100 760

(a)

780

800

820 o

T ( C)

(b)


840

860

880

Energy & Fuels

28

Figure 5

35

-1

2 SP BET (m g )

30 25 20 15

900 870 840 810

0 -5

860 840 820

) (C

5

880

o

10

900

o

40

920

T ( C)

35,00 31,88 28,75 25,63 22,50 19,38 16,25 13,13 10,00

45

T

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

Page 28 of 33

780 385 440 495 550 605 660 715 770

800 780 400

450

φ (µm)

500

550

600

φ (µm)

(a)

(b)


650

700

750

Page 29 of 33

29

Figure 6 880 45

45,00

860

40

38,75

35

26,25

20

20,00

15

o T ( C)

25

820 800

13,75

10 880

5

780 7,500

840

0

800

-5 150

225 φ (µm

300

760 375

)

T ( oC)

-1

840

32,50

30 2 ) S P BET (m g

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

Energy & Fuels

1,250 -5,000

760 100

450

150

200

250 φ (µm)

(b)

(a)


300

350

400

450

Energy & Fuels

30

Figure 7 90 75

-1

-1

Natural run 1 run 2 run 3 run 4 run 5 run 6 run 7 run 8 run 9 run 10 run 11

DP

ICB dA/dlog(D) Pore Area (m . g .Е )

60

2

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

Page 30 of 33

45 30 15 0 10

100

1000

10

Pore Diameter (Å)


100

1000

Page 31 of 33

31 Figure 8 Quantity Adsorbed /desorbed (cm³/g STP)

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

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160 140 120

Ads DP Des DP Ads ICB Des ICB

100 80 60 40 20 0 0,0

0,2

0,4

0,6

0,8

1,0

Relative Pressure (P/Po)


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Page 32 of 33

32

Figure 9

(a)

(b)

(c)

(d)

(e)

(f)


Page 33 of 33

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

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33

Figura 10

(a)

(b)


Response Surface Methodology Applied to the Evaluation of the SO2

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