CaO

Environ. Sci. Technol. 2008, 42, 1499–1504

Analysis of SO2 Sorption Capacity of Rice Husk Ash (RHA)/CaO/NaOH Sorbents Using Response Surface Methodology (RSM): Untreated and Pretreated RHA IRVAN DAHLAN, KEAT TEONG LEE, AZLINA HARUN KAMARUDDIN, AND ABDUL RAHMAN MOHAMED* School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

Received September 6, 2007. Revised manuscript received December 5, 2007. Accepted December 17, 2007.

The SO2 sorption capacity (SSC) of sorbents prepared from rice husk ash (RHA) with NaOH as additive was studied in a fixedbed reactor. The sorbents were prepared using a water hydration method by slurrying RHA, CaO, and NaOH. Response surface methodology (RSM) based on four-variable central composite face centered design (CCFCD) was employed in the synthesis of the sorbents. The correlation between the sorbent SSC (as response) with four independent sorbent preparation variables, i.e. hydration period, RHA/CaO ratio, NaOH amount, and drying temperature, were presented as empirical mathematical models. Among all the variables studied, the amount of NaOH used was found to be the most significant variable affecting the SSC of the sorbents prepared. The SSC for sorbent prepared with the addition of NaOH was found to be significantly higher than sorbents prepared without NaOH. This is probably because NaOH is a deliquescent material, and its existence increases the amount of water collected on the surface of the sorbent, a condition required for sorbent-SO2 reactiontooccuratlowtemperature.Theeffectoffurthertreatment of RHA at 600 °C was also investigated. Although pretreated RHA sorbents demonstrated higher SSC as compared to untreated RHA sorbents, nevertheless, at optimum conditions, sorbents prepared from untreated RHA was found to be more favorable due to practical and economic concerns.

Introduction Sulfur dioxide (SO2) and its derivative pollutants are ubiquitous threats and it has negative effect on aquatic ecosystems, crops, manmade materials, and poses risks to forests and even to human health (1). Most of SO2 emissions are attributed to fossil fuel-burning (i.e., oil, gas, and coal) mainly in thermal power station. Currently, world primary energy consumption is still based on these fossil fuels (2). For example, Malaysia, as one of the fastest developing countries in the southeast Asian region, is still strongly dependent on fossil fuels, i.e., fossil fules account for about 94% of Malaysia’s energy requirements in 2003 (3). * Corresponding author phone: +604-5996410; fax: +604-5941013; e-mail: [email protected]. 10.1021/es702232k CCC: $40.75

Published on Web 02/02/2008

 2008 American Chemical Society

As energy consumption increases, especially resulting from the burning of coal (which has the highest content of sulfur among fossil fuels), total SO2 emission will also increase particularly in the Asian region whereby an increase from 56.4 million tons in 1995 to 61 million tons in 2000 was reported (4). Further increase in SO2 emission is estimated to occur in this region with China being the highest SO2 emitter (25.5 million tons) compared to any country in the world in 2005 (5). Therefore, there is an urgent need and importance to reduce SO2 emission using efficient yet economical desulfurization technology, typically by means of wet or dry methods. Although wet methods are currently the most widely adopted in industry, dry technologies, however, have fewer problems of corrosion, scaling-up, easier byproduct handling, and they require no wastewater treatment (6, 7). Nevertheless, acid-gas sorption capacities of dry technologies are generally low. Consequently, enhancing the sorption capacities of dry sorbent toward SO2 has been a challenging issue. Previously, we have reported that sorbents prepared from coal fly ash, CaO, and CaSO4 (8, 9) using hydration process have higher SO2 sorption capacity (SSC) due to higher surface areas. Coal fly ash (mainly consist of SiO2/Al2O3) which is a pozzolanic material, can react with Ca-based compounds to form calcium silicate hydrates in the presence of water. These pozzolanic products usually are poorly crystallized and have relatively high surface areas which can enhance SSC (10, 11). In our more recent study using other siliceous materials, i.e. oil palm ash (12–14) and rice husk ash (15, 16), we have shown that these ashes can also be utilized to prepare sorbents that are highly reactive toward SO2 sorption. The successful utilization of sorbents prepared from various waste-derived siliceous materials can not only further cut down the operating cost of the desulfurization process but also help to utilize these waste-derived siliceous materials, which otherwise have no commercial value, from industries. Therefore, more studies are required to further increase the SSC of these kind of sorbents. Ruiz-Alsop and Rochelle (17) studied the use of various salt additives during the preparation of sorbent using Ca(OH)2. Most of the additives tested were found to increase the sorption capacity of the sorbent toward SO2, especially sodium salts and chlorides. Kind et al. (18) synthesized fly ash and CaO sorbents with CaCl2 as an additive, which exhibited higher reactivity than those sorbent without CaCl2. The addition of CaCl2 was also reported by Brodnax and Rochelle (19) with similar results. On the other hand, Peterson and Rochelle (20) examined the SSC of fly ash/Ca(OH)2 sorbents with the addition of NaOH. The SSC of the sorbent was greatly enhanced, possibility due to the increasing dissolution of silica from fly ash. These results were also supported by other researchers (21, 22). Much research has been carried out and reported the various effects of sorbent preparation variables on the sorbent SSC. However, the general practice in determining the optimal conditions of these variables is by varying one parameter at one time while keeping the others constant. Such work is extremely laboratorious and time-consuming. In this study, response surface methodology (RSM) coupled with central composite face centered design (CCFCD) was utilized with an attempt to further enhance the SSC of sorbents prepared from rice husk ash (RHA). RHA, which is produced from the burning of rice husk at about 300 °C, has been chosen as the source of siliceous material in this study and is available abundantly in Malaysia, i.e., (81 thousand metric tons of RHA is generated annually (23). Apart from that, it was reported that the silica in RHA undergoes VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. CCFCD Matrix of the Four Variables in Coded Units with the Experimental and Predicted Values of SSC Y3 (RHA300 °C), mg/g

Y6 (RHA600 °C), mg/g

run

x1

x2

x3

x4

experimental

predicted

experimental

predicted

D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17 D18 D19 D20 D21

1 1 1 -1 1 -1 -1 -1 -1 1 0 0 0 0 0 0 0 0 0 0 0

1 1 -1 1 -1 -1 1 -1 0 0 -1 1 0 0 0 0 0 0 0 0 0

1 -1 1 -1 -1 1 1 -1 0 0 0 0 -1 1 0 0 0 0 0 0 0

-1 -1 1 1 1 -1 1 -1 0 0 0 0 0 0 -1 1 0 0 0 0 0

13.73 11.15 12.01 5.15 10.30 9.44 14.59 3.43 14.59 18.88 12.87 19.73 6.01 11.15 15.44 15.44 15.44 16.30 15.44 16.30 15.44

13.74 11.12 12.02 5.12 10.27 9.45 14.60 3.40 14.62 18.91 12.91 19.77 6.13 11.10 15.48 15.48 15.75 15.75 15.75 15.75 15.75

15.44 9.44 12.87 6.86 7.72 10.30 13.73 8.58 18.02 24.02 18.88 21.45 9.44 12.87 19.73 18.88 18.88 19.73 19.73 19.73 18.88

15.39 9.68 12.81 7.11 7.96 10.24 13.67 8.82 17.66 23.66 18.51 21.09 8.47 13.11 19.37 18.51 19.83 19.83 19.83 19.83 19.83

structural transformations depending on the combustion conditions of rice husk. However, generally, the RHA that results from the combustion process contains both crystalline and amorphous silica. Silica in the amorphous phase is more reactive than the crystalline phase (24), therefore the former phase predominates in the pozzolanic reaction. In our preliminary studies, the reactivity of untreated RHA with Ca-based materials as a sorbents toward SO2 was investigated (15, 16). In this work, the effect of pretreating RHA by combusting at 600 °C was also investigated. The untreated and pretreated RHA hereinafter is referred to as RHA300 °C and RHA600 °C, respectively. The ashes were then mixed with CaO and NaOH for the preparation of the sorbents. The sorbent preparation variables were varied according to CCFCD to demonstrate the effects of sorbent preparation variables on the SSC. A mathematical model was then developed to correlate the SSC to the sorbent preparation variables that can be used for optimization purposes.

Materials and Methods Sorbent Preparation. The sorbents were prepared using CaO (BDH Laboratories, England), NaOH (Sigma-Aldrich (M) Sdn.Bhd.) and RHA. The RHA was collected from Kilang Beras and Minyak Sin Guan Hup Sdn.Bhd., Nibong Tebal, Malaysia. Prior to use, the RHA was sieved to produce less than 200 µm particle size. In order to obtain RHA600 °C from RHA300 °C, the latter was further combusted in a furnace (Carbolite RWF 1200) at a temperature of 600 °C for 4 h with a heating rate of 5 °C/min. The chemical composition of RHA300 °C is 68.0% SiO2, 2.30% K2O, 1.20% P2O5, 0.71% MgO, 0.59% CaO, 0.32% SO3, 0.32% Cl2O, 0.16% Al2O3, 0.40% others and 26.0% LOI. Whereas RHA600 °C had the following composition: 89.0% SiO2, 2.60% K2O, 1.50% P2O5, 0.86% MgO, 0.68% CaO, 0.40% SO3, 0.26% Cl2O, 0.21% Al2O3, 0.29% others, and 4.20% LOI. The preparation of sorbents (24) is described as follows. A specified amount of RHA and CaO were added into 100 mL deionized water which was previously added with (or without) a certain amount of NaOH (according to experimental designs described below). Then the conical flask which contains the mixture was inserted into a water bath shaker at 65 °C with a rotating stirring rate of 150 rpm for a specified time. The slurry was then filtered and dried at 150 °C for 2 h. The sorbents were then pelletized, crushed, and sieved to obtain the particle size range of 250–300 µm. Hydration time, RHA/ 1500

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CaO ratio, amount of NaOH, and drying temperature were varied according to the statistical experimental design. The sorbent prepared was then tested for the sorption capacity toward SO2 by subjecting it to simulated flue gas. Statistical Experimental Design, Analysis, and Model Fitting. Design Expert software version 6.0.6 (STAT-EASE Inc., Minneapolis, MN) was used to generate the experimental designs, statistical analysis, and regression model. The central composite face centered design (CCFCD) with a quadratic model and R ) 1 was chosen in this study (25). Four independent variables namely hydration period (x1), RHA/ CaO ratio (x2), amount of NaOH (x3) and drying temperature (x4) were studied for each type of RHA. Each independent variable was varied at three levels represented by -1, 0, and +1 (Table A1 of the Supporting Information). A total of 21 different combinations were performed for each type of RHA prepared in random order according to the CCFCD configuration shown in Table 1. Three replications were carried out for all design points except the center point. The resulting sorbents were tested for SSC and the results are also presented in Table 1. The SSC of sorbents prepared from RHA300 °C and RHA600 °C were represented by Y3 and Y6, respectively. The data obtained were then fitted to the following second-order polynomial equation. n

Y ) b0 +

∑ i)1

n

bixi +

∑ i)1

n-1

biixi2 +

n

∑ ∑ b xx

ij i j

(1)

i)1 j)i+1

where Y is the predicted SO2 sorption capacity (SSC, mg/g), b0 is the offset term (constant), bi are the linear coefficients, bii are the quadratic coefficients, bij are the interaction coefficients, and xi, xj are the coded values of the sorbent preparation variables. The significance of the second-order model as shown in eq . was evaluated by analysis of variance (ANOVA). The insignificant coefficient was eliminated after the F (Fisher) test and the final model was obtained. The model was then used to optimize the sorbent SSC for all the variables studied. Additional experiments were also carried out to verify the predicted optimize conditions. SO2 Sorption Study. The SSC of the sorbents was examined in a laboratory-scale stainless steel fixed-bed reactor (0.8 cm ID) fitted in a furnace for isothermal operation. During the study, 0.5 g of sorbent was filled in the center of the reactor (supported by 0.05 g of borosilicate glass wool).

FIGURE 1. Morphological details of RHA: (a) RHA300 °C; (b) RHA600 °C. Simulated flue gas containing SO2 (2000 ppm), NO (500 ppm), O2 (10%), water vapor (50% humidity), and N2 as the balance was subsequently passed through the sorbent at a reaction temperature of 87 °C. The total flow rate of the gas stream was controlled at 150 mL/min using mass flow controllers. The concentration of SO2 in the flue gas was measured using a portable flue gas analyzer IMR 2800P before and after the sorption process. The schematic diagram and details of the experimental rig is presented in our previous study (8, 9). The SSC of the sorbent is expressed by the weight of SO2 captured from the flue gas per gram sorbent (20). The amount of SO2 captured by the sorbent was evaluated from the time the sorbent could maintain 100% removal of SO2 until it shows negligible activity. Every experimental run was repeated at least three times to increase the precision of the results, and only the average value was reported. The reproducibility of the experimental data was found to be sufficiently high with relative error between repeated runs was less than 5%. Chemical and Physical Analysis. The chemical composition of RHA was analyzed using Rigaku RIX 3000 X-ray fluorescence (XRF) spectrometer. The scanning electron microscopy (SEM) examinations were performed with Leo Supra 35 VP scanning electron microscope. X-ray diffraction (XRD) spectrum were recorded on a Siemens D5000 X-ray diffractometer for the powdered samples with Cu KR radiation in the range of diffraction angle (2θ) 10–70° at a sweep rate of 1°/min to determine the phases present in the sorbent.

Results and Discussion Characteristics of Untreated and Pretreated RHA. Based on our previous study (16), it was shown that SSC of sorbent prepared from RHA did not show any correlation with the BET specific surface area and/or pore size distribution of the sorbent. Therefore, in this study neither BET specific surface area nor pore size distribution were reported. In contrast, from the previous study, it was reported that the amount of RHA used in the preparation mixture is the main controlling factor for producing sorbent with high SSC whereby the SSC increases with higher content of RHA (silica). Therefore, in this study the characteristics of amorphous silica in RHA was further investigated by pretreating RHA with heating at 600 °C with the aim to remove the organic matter and leaving behind a silica-rich residue that can be used to synthesize the sorbent. Figure 1 shows the surface morphologies of RHA300 °C and RHA600 °C, respectively. Both the samples were found to have uniform structure, the former with a dome-shaped structure and the latter with a skeletal structure. The presence of amorphous silica in both RHA was confirmed from the X-ray diffraction patterns (Figure A1 of the Supporting Information). It was observed that there are no defined peaks, only a broad “hump” between (2θ) 15 and 30° diffraction angle. However, an indistinct peak corresponding to R-cristobalite silica was detected at 2θ ) 22°, a similar trend reported elsewhere (26, 27). Moreover, these patterns are similar to X-ray spectra of silicic acid reported in the literature (28) and are characteristics of amorphous substances. On the other hand, the crystalline

structure was detected when rice husk was heated at 1000 °C (shown in the inset of Figure A1 of the Supporting Information). Apart from that, heating RHA at 600 °C changes the color of the ash from black to whitish-gray, an indication of higher content of silica. Effect of Sorbent Preparation Variables on the Sorbent Sorption Capacity. The effects of sorbent preparation variables on the sorbent SSC were evaluated using a CCFCD to determine the optimal sorbent preparation variable that results in maximum SSC. The data in Table 1 were then fitted to eq 1. using multiple regression analysis technique. The resulting empirical mathematical models of the SSCs (Y3 and Y6) are given in eqs 2. and 3 in term of actual factor, respectively. Y3 ) 11.614 - 0.985x1 + 0.714x2 + 67.58x3 - 0.039x4 + 0.0159x21 + 0.325x22 - 114.068x23 - 0.000106x24 - 0.0496x1x2 0.697x1x3 + 0.00563x1x4 + 1.589x2x3 + 0.00477x2x4 + 0.0257x3x4 (2) Y6 ) 11.266 - 0.273x1 - 6.325x2 + 70.192x3 + 0.0319x4 + 0.0148x21 + 0.0509x22 - 149.526x23 - 0.0003062 x24 0.00993x1x2 + 0.161x1x3 + 0.00134x1x4 + 2.2245x2x3 + 0.0334x2x4 + 0.0429x3x4 (3) The predicted values for SSC responses (Y3 and Y6) were calculated using these models and the resulting values are also tabulated in Table 1. The significance of each model was assessed from the coefficient of determination (R2) which was found to be 0.997 and 0.995 for Y3 and Y6 responses, respectively. These values imply that only 0.3 and 0.5% of the variation in Y3 and Y6 responses can not be explain by the models. The models were also verified by using analysis of variance (ANOVA) for each response (see Table A2 of the Supporting Information). The calculated F-value corresponding to SSC for each model are 170.25 and 91.03, respectively and both of this value exceeds the tabulated F-value (F0.05 (14,6) ) 3.96) based on 95% confidence level. The significance of each term in the model was also evaluated by looking at the P value (Prob > F). A P value lower than 0.05 indicates that the term is significant at 95% confidence level. Referring to the ANOVA analysis (Table A2 of the Supporting Information), it was observed that most of the terms in the model were significant with P < 0.05. In usual cases, the term with P > 0.05 can be omitted from the model. However, in this study, the insignificant terms were still maintained to support the hierarchical nature of this model. Both the models were also found to have in-significant lack of fit. Based on these statistical tests, both the models were found to be adequate in representing the actual correlation between the SSC and the sorbent preparation variables. Apart from that, all the linear terms of the sorbent preparation variables studied except for x4 (drying temperature) have significant effect on the SSCs for both types of sorbents prepared (using untreated and pretreated RHA). This finding is in agreement with the result reported in our previous study (16), whereby hydration period and the amount of RHA/CaO were found to be the most important variables that effect the preparation of sorbent that is highly reactive toward SO2. The insignificant effect of drying temperature indicates that neither increasing nor decreasing the drying temperature will produce sorbent with higher SSC. This finding is clearly supported by the data trend shown in Table 1, whereby the sorbents prepared with the same preparation variables but at different drying temperature (D15-D17) had almost similar SSC for both types of sorbents prepared. In addition, the amount of additive (x3) used in the preparation mixture had the largest effect on the SSC (highest F-value). The effect of this variable VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. XRD pattern of sorbent D10 (prepared from RHA600 °C) and D12 (prepared from RHA300 °C). XRD label: (A) SiO2; (B) CaCO3; (C) Ca(OH)2; (D) Ca2Mg5Si8O22(OH)2.

FIGURE 2. Surface plot of SSC as a function of RHA/CaO ratio and additive amount: (a) sorbent prepared from RHA300 °C, (b) sorbent prepared from RHA600 °C. will be discussed in detail in the subsequent section. For the quadratic term, only variable x1 and x3 in model Y3 and variable x3 in model Y6 shows significant effect. On the other hand, for the effect of interaction between variables, only the interaction between variables x2 (RHA/ CaO ratio) and x3 (NaOH amount) was found to give significant effect for both the models. Figure 2 shows the effect of varying RHA/CaO ratio and amount of NaOH on SSC at constant hydration period (12 h) and drying temperature (200 °C) for sorbents prepared using the untreated and pretreated RHA, respectively. The interaction characteristic for both types of sorbent appears to be similar with a elongated U-shaped plots, i.e. in one axis, there is a linear increase in SSC, however, on the other axis, the SSC increase up to only a certain point, then decreases thereafter. To be specific, when the amount of NaOH was held constant, an increase in the RHA/CaO ratio significantly increases the SSC of both types of sorbents prepared. Conversely, when the RHA/CaO ratio was maintained constant, the SSC of both types of sorbent prepared increase with increasing amount of NaOH up to about 0.25 mol/L. At higher concentration of NaOH, the SSC gradually decreases. This result reflects that 1502

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the optimum concentration of NaOH to be used in the preparation mixture that favors high sorbent SSC is about 0.25 mol/lL. At this optimum value, the maximum SSC of sorbents prepared using RHA300 °C and RHA600 °C was obtained at 19.73 and 24.02 mg SO2/g sorbent, respectively (Table 1). This result is in concurrence with the work reported by previous researcher (20, 22). Effect of the Addition of NaOH as Additive. Based on the data tabulated in Table 1, the inclusion of NaOH as additive in the preparation mixture resulted in sorbents with significantly higher SSC as compared to sorbents prepared without the addition of NaOH. For sorbents prepared without the addition of NaOH, the SSC obtained ranges from 3.43 to 11.15 mg SO2/g sorbent while for sorbents prepared with the addition of NaOH, the SSC obtained ranges from 9.44 to 24.02 mg SO2/g sorbent. The SO2 were absorb into the sorbent in the form of sulfate salts (CaSO4) as confirm by the presence of SO42- peaks using Fourier Transform Infrared (FTIR) analysis. This results clearly indicates that the addition of NaOH in the sorbent significantly increases the SSC of the resulting sorbent with an optimum NaOH concentration of about 0.25 mol/L. In order to study the role of NaOH, we have attempted to test the desulfurization capacity of using NaOH alone, but with no success as NaOH liquefied easily at high reaction temperature. Nevertheless, we could conclude that the increase in the sorbent SSC with the addition of NaOH is not significantly due to the direct reaction between NaOH (strong base) and SO2 (acidic gas) because if this were to be the case, higher concentration of NaOH would have resulted in even higher sorbent SSC. However, as mentioned earlier, the addition of NaOH with concentration higher than 0.25 mol/L resulted in a decrease in the sorbent SSC. Nevertheless, a small amount of NaOH could still directly react with SO2 to form Na2SO3. Previously, it was reported that the presence of NaOH as additive at optimum value might increase the dissolution rate of silica from the ash (20). The increase in the dissolution of silica will eventually increase the formation of Si-Ca complex that has a high SSC (7, 12, 13, 16, 29). Nevertheless in this study, the X-ray diffraction pattern of sorbents prepared with the addition of NaOH as shown in Figure 3 exhibited different results as compared with our earlier examination of RHA sorbent without NaOH addition (16). Only a weak peak corresponding to calcium magnesium silicate hydroxide is detected in sorbent D12 prepared from RHA300 °C, while the main phases were identified as CaCO3 and Ca(OH)2. For sorbent D10 prepared using RHA600 °C, only weak peaks of CaCO3 was detected. Apart from that, the amount of amorphous silica was found to reduce in both the sorbents indicating that it has reacted to form highly amorphous compounds that cannot be detected by XRD. The insignificant amount of Ca-Si complex in the sorbent indicates that the reaction between CaO and silica during the hydration process has somehow been inhibited by the

presence of NaOH. One possible reason is due to the fact that NaOH is a strong alkaline and thus the addition of NaOH limits the dissolution of CaO as both are basic compounds (common ion effect) (20). Since we can now rule out that the high SSC of sorbents prepared with the addition of NaOH is not due to the formation of Ca-Si complex, thus the next step to further understand the role of NaOH in increasing the sorbent SS is by studying the surface morphology of the sorbent. The surface morphology of the sorbent prepared using RHA300 °C and RHA600 °C before and after reacting with SO2 was studied through Scanning Electron Micrograph (SEM) analysis (Figures A2 and A3 of the Supporting Information, respectively). It was observed that the skeletal structure of RHA was no longer visible after the hydration process (see Figures A2b and A3b of the Supporting Information). Instead, the sorbents were found to have needle like appearance structure. The particle size of the sorbent significantly increase after the reaction with SO2 (see Figure A2a and c or Figure A3a and c of the Supporting Information for comparison). The larger particle size of the sorbent could be due to the agglomerating of smaller particles during the desulfurization reaction. The extensive agglomeration could also be because of higher content of water vapor present on the surface of the sorbent during the desulfurization reaction. The source of water vapor during the desulfurization reaction is from the humidified flue gas. It has been reported by Liu et al. (30) and Ho et al. (31) that the reaction of SO2 with sorbents prepared from siliceous material at low temperature requires the presence of water in several molecular layers thick on the surface of the sorbent particles. The extent of reaction increases with increasing amount of water adsorbed. Thus, with the addition of NaOH in the sorbent that can act as a deliquescent material, more water vapor can be retain on the surface of the sorbent leading to higher desulfurization activity. However, the addition of NaOH higher than 0.25 mol/L caused the SSC to decrease. As described earlier, addition of higher concentration of NaOH limits the dissolution of CaO as both are basic compounds (common ion effect). Closer scrutiny at higher magnification (5000×) reveals coarse surface protuberances on the sorbent after reacting with SO2 for both types of sorbents (Figures A2d and A3d of the Supporting Information). The surface of the sorbent was covered by lumps of uneven shapes particles, which is most probably the product of the desulfurization reaction. Based on this SEM analysis, it can be concluded that the sorbents prepared with the addition NaOH have a tendency to agglomerate together as compared with sorbent prepared without the addition of NaOH which are more loose. To further strengthen this result, the effect of water vapor pressure in the simulated flue gas was studied in terms of relative humidity (RH). Form the experimental results obtained, it was found that RH exerted significant influence on the sorbent SSC (see Figure A4 of the Supporting Information). When the reaction was carried out under dry condition (0% RH), the sorbent SSC was only about 10.30 mg/g and 7.72 mg/g for sorbent D10 prepared using RHA300 °C and RHA600 °C, respectively. However, with the introduction of water vapor by humidifying the flue gas, the sorbent SSC increases significantly with higher relative humidity. This result is in agreement with those reported by other researchers (30–33). Effect of Untreated vs Pretreated RHA and Optimization. The results on sorbent preparation variables appraised by CCFCD until this point has demonstrated that both the sorbents prepared from untreated and pretreated RHA shows higher sorbent SSC as compared to our previous study (8, 9, 12-16). However, as mentioned earlier, one of the aims of this study is to determine if the untreated RHA or pretreated RHA is more favorable in producing sorbent with a higher SSC. One way of doing this is first by optimizing the sorbent

preparation variables to obtain sorbent with the highest SSC. Again, this optimization process is carried out by using the optimization point prediction function in Design Expert Software, whereby three different combinations of sorbent preparation variables that can result in sorbent with the highest SSC were obtained for both types of sorbent. The prediction is given along with their optimization criteria, predicted and experimental values of SSC (see Table A3 of the Supporting Information). Based on the data presented, it was found that the software gave a very accurate prediction as the error between predicted and experimental value is less than 10%. The results also showed that the sorbents prepared from RHA 600 °C resulted in sorbent with a higher SSC possibility due to higher content of amorphous silica as confirmed by the XRD analysis. However, a lengthy hydration time of 20 h is needed for the preparation of the sorbent.The reason for the elongated hydration period required for the preparation of RHA600 °C sorbents might be due to the presence of NaOH which reduces the dissolution of CaO. Since RHA600 °C already contains a very high amount of reactive silica, the positive effect of NaOH on the sorbent SSC is no longer significant. This result indirectly depicted that the presence of additive (NaOH) will only give significant effect on the sorbent SSC when the availability of raw reactive species, in this case amorphous silica, is less/limited. Nevertheless, such a long hydration period is unlikely to be used in the industry due to practical constraints and unfavorable economic concerns. Thus, we attempt to optimize the system again by now fixing the hydration period at 4 h (see also Table A3 of the Supporting Information). It was observed that both the different types of sorbents now shows almost similar sorbent SSC. Therefore, for practical and cost-effective considerations, sorbents prepared from RHA300 °C is more favorable than sorbents prepared using RHA600 °C. The activity of the sorbent prepared in this study were compared with raw materials alone and as well as other sorbents made from other siliceous materials (see Table A4 of the Supporting Information). It was clear that the sorbent synthesized from RHA with the addition of NaOH was far superior than the other sorbents. Therefore, this result demonstrates the positive effect of NaOH addition.

Acknowledgments We acknowledge the Ministry of Science, Technology and Innovation (MOSTI) Malaysia, Yayasan Felda, and Universiti Sains Malaysia for funding this project.

Supporting Information Available Table A1 provides the experimental design in the coded and actual levels of variables; Table A2 contains ANOVA analysis for the regression model equation and coefficients; Table A3 provides the optimization criteria along with their predicted and experimental sorbent SSC values; while Table A4 shows the comparison of various sorbent SSC. Figure A1 shows the X-ray diffraction pattern of rice husk heated at 300 °C, 600 and 1000 °C; Figure A2a-d shows the surface morphology of sorbent D12 (RHA300 °C) before and after reaction with SO2; Figure A3a-d shows the surface morphology of sorbent D10 (RHA600 °C) before and after reaction with SO2; Figure A4 shows the effect of water vapor content (RH) on the sorbent SSC. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Burtraw, D.; Evans, D. A.; Krupnick, A.; Palmer, K.; Toth, R. Economics of pollution trading for SO2 and NOx. Annu. Rev. Environ. Resour. 2005, 30, 253–289. (2) International Energy Outlook. 2007 (IEO2007); http://www. eia.doe.gov/oiaf/ieo/index.html. VOL. 42, NO. 5, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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