Preparation and characterization of sorbents prepared from ash (waste material) for sulfur dioxide (SO 2 ) removal

Transcription

1 J Mater Cycles Waste Manag (2005) 7:16 23 Springer-Verlag 2005 DOI /s ORIGINAL ARTICLE Keat Teong Lee Subhash Bhatia Abdul Rahman Mohamed Preparation and characterization of sorbents prepared from ash (waste material) for sulfur dioxide (SO 2 ) removal Received: September 9, 2003 / Accepted: March 15, 2004 Abstract Sorbents synthesized from various types of ash (coal fly ash, coal bottom ash, oil palm ash, and incinerator ash) for flue gas desulfurization were investigated. The sorbents were prepared by mixing the ashes with calcium oxide and calcium sulfate using the water hydration method. The effects of various sorbent preparation variables, such as the hydration period, the ratio of calcium oxide to ash, and the amount of calcium sulfate, on the Brunauer-Emmett- Teller (BET)-specific surface area of the resulting sorbent were studied using a two-level full factorial design. The surface area of the sorbents obtained range from 15.4 to 122.1m 2 /g. Regression models were developed to correlate the significant variables to the surface area of the sorbents. An analysis of variance (ANOVA) showed that the model was significant at a confidence level of 95%. It was found that apart from all the individual variables studied, interactions between variables also exerted a significant influence on the surface area of the sorbent. From the activity test results, it was found that sorbents prepared from coal fly ash and oil palm ash have the highest SO 2 absorption capacity. Scanning electron microscope (SEM) analysis showed that the sorbent was composed of a compound with a high structural porosity, while an X-ray diffraction spectrum showed that calcium aluminum silicate hydrate compounds are the main products of the hydration reaction. Key words Ash Desulfurization Sorbent Statistical design Surface area Introduction Ash is a waste product left over after the burning of many combustible substances such as paper, rubbish, coal, and K.T. Lee S. Bhatia A.R. Mohamed (*) School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, Nibong Tebal, Pulau Pinang, Malaysia Tel ; Fax chrahman@eng.usm.my many others. Some of the main industries in the world that are producing ash in large quantities are coal-fired power plants (burning coal to generate electricity), palm oil mills (disposing of the remains of the fruit), and incinerators (incinerating domestic rubbish). For instance, a 1400-MW power station burning coal produces 1.3 million tonnes of coal ash per year, which must be disposed of or used in other areas. Energy and environmental forecasts for the coming years point to a greater use of coal as a source of electricity. As more utilities are forced to change from gas and oil to coal as a source of fuel, the availability of quantities of ash (coal ash) will definitely increase. 1 Ash primarily consists of silica (SiO 2 ) and alumina (Al 2 O 3 ). Secondary ingredients are carbon and oxides of iron, calcium, magnesium, and potassium. Although there are many well-developed technologies for ash utilization (particularly coal fly ash) throughout the world (the manufacture of cement, concrete, building bricks, thermal insulation bricks, aggregates in pavements, soil stabilization, and soil conditioning), 1 most of the ash produced still has to be disposed of, in either landfills or ash ponds. For instance, in the USA (in 2000), only 30% of the 57 millions tonnes of ash produced annually is being utilized, while the rest of it is disposed of in landfills. This method of ash disposal requires a lot of land, which is not easily available in urban areas. Apart from that, the utilization of other types of ash, such as oil palm ash and incinerator ash, are still limited. In 1987, Jozewicz et al. 2 reported that when silica and/or alumina (eluted from coal fly ash) is mixed with calcium hydroxide (Ca(OH) 2 ) or calcium oxide (CaO), a sorbent with a high sulfur dioxide (SO 2 ) sorption capacity can be attained. This is due to the pozzolanic reaction between Ca(OH) 2 or CaO and silica and/or alumina that produces highly hydrated compounds (calcium aluminum silicate hydrate compounds) that have a large surface area. These large surface area compounds have a high SO 2 capture capacity. The pozzolanic reaction starts with the elution of silica and/or alumina from the ash by alkaline water, and this step is believed to be rate-limiting. The silica and/or alumina then reacts with Ca(OH) 2 or CaO to form calcium aluminum silicate hydrate compounds. It has also been

2 17 shown that the addition of calcium sulfate (CaSO 4 ) to the preparation mixture can further promote the formation of the hydrated products. 3 This phenomena is because of the role played by CaSO 4, which promotes the formation of hydrated products by suppressing the crystal growth of Ca(OH) 2 to force the reactivity of the Ca(OH) 2 to produce hydrated products. This method of SO 2 removal using a sorbent is known as the dry desulfurization process. This process offers an alternative technology to the wet desulfurization process, which is currently being used in many industries. In the wet process, SO 2 is removed by a limestone gypsum method. Although the wet desulfurization process is suitable for large-scale boilers such as those installed in coal- or oil-fired power plants, it has many disadvantages. Among some of them are the large space required, for installation, the large volume of water required, and the high capital and operating expenses. Owing to the limitations of the wet process, the dry process is a more attractive alternative since it overcomes all the limitations of the wet process. Apart from that, the use of ash as a source of silica and/or alumina for synthesizing the sorbent for the dry desulfurization process is attractive both economically and environmentally, as ash is the most voluminous by-product from combustion systems. 4 After its reaction with sulfur dioxide, the sorbent is converted into an eco-friendly product which can be disposed off easily owing to its multifunctional uses. This includes its application as a fertilizer, a coagulating agent, and a deodorizer for refrigerators, shoes, and even pet excretions. Thus, the preparation of sorbents for flue gas desulfurization using coal fly ash has recently been studied extensively It is well known that the reactivity of the hydrated sorbents depends strongly on their surface area. Fernandez et al. 4 reported that the SO 2 capture activity of these sorbents generally increases with the higher specific surface area of the sorbent. In 2003, Lin et al. 13 reported that the use of calcium in the sorbent increased linearly with the increasing specific surface area of the sorbent. However, most of the sorbents reported so far have been prepared from coal fly ash. In this study, sorbents prepared from different types of ash (coal fly ash, coal bottom ash, oil palm ash, and incinerator ash) using water hydration were investigated. The significance of the influence of various sorbent preparation variables, such as the hydration period (x 1 ), the ratio of calcium oxide (CaO) to ash (x 2 ), and the amount of calcium sulfate (CaSO 4 ) (x 3 ), on the Brunauer-Emmett-Teller (BET) surface area of the sorbent was studied using a two-level full factorial statistical design. Data from the experimental design were then used to develop a multiple linear regression model that correlates the significant experimental variables with the BET specific surface area of the sorbent.the validity of the model was verified using the Analysis of Variance (ANOVA). X-ray diffraction (XRD) was used to detect the various phases present in the sorbent, while scanning electron microscope (SEM) analysis was used to observe the macrostructural properties of the sorbent. The sorbent was also tested for its efficiency in the removal of SO 2. Table 1. BET-specific surface area of various raw materials Raw material Coal fly ash 1.46 Coal bottom ash 2.45 Oil palm ash 1.51 Incinerator ash 3.83 Calcium oxide (CaO) 5.62 Calcium sulfate (CaSO 4 ) 4.89 BET, Brunauer-Emmett-Teller Experimental Sorbent preparation The sorbents were prepared from calcium oxide (CaO), calcium sulfate (CaSO 4 ), and various types of ash. The calcium oxide (laboratory grade) and calcium sulfate (reagent grade) used were supplied by BDH Laboratory Supplies, UK. The coal fly ash and coal bottom ash were supplied by the Kapar Power Plant, Malaysia, of Tenaga Nasional Berhad, while the oil palm ash was supplied by the United Palm Oil Mill, Malaysia, and the incinerator ash was supplied by the Department of Environmental Engineering, Kyoto University, Japan, from one of the incinerator plants operating in Japan. The BET surface areas of these raw materials were determined using an Autosorb 1C Quantachrome analyzer, and the results are given in Table 1. The chemical compositions of the various ashes were analyzed using a Rigaku X-ray Spectrometer RIX 3000, and the results are given in Table 2. The procedure used to prepare the sorbent is as follows. 12 To prepare 13g sorbent, 5g calcium oxide is added into 100 ml water at 65 C. After stirring, the temperature of the slurry will eventually increase to about 80 C, and 5g ash and 3g calcium sulfate are then added to the slurry simultaneously. The slurry is then heated to about 95 C for 10h in order for the hydration process to occur. After the hydration process, the resulting slurry is filtered and dried at 200 C for 2h. The sorbent, in powder form, is then made into pellets and subsequently crushed and sieved into the required particle size range ( µm). Physical and chemical analysis Surface area (m 2 /g) The sorbents were analyzed for their BET-specific surface area (calculated using the BET standard method) and pore-size distribution [calculated using the Barrett-Johner- Halenda (BJH) method] using an Autosorb 1C Quantachrome analyzer. The XRD spectra of the powdered samples were recorded on a Philips PW 1820 system with Cu Kα radiation in a diffraction angle (2θ) range of 5 90 at a sweep rate of 3deg/min. SEM image were taken with a Leica Cambridge S360 camera with 15 kv of accelerating voltage. The chemical composition was determined using a Rigaku X-ray Spectrometer RIX 3000.

3 18 Table 2. Chemical composition of various types of ash Component Coal fly ash Coal bottom ash Oil palm ash Incinerator ash SiO Al 2 O Carbon Fe 2 O CaO MgO Na 2 O P 2 O K 2 O Cl compound S compound Others Table 3. Experimental range and levels of the independent variables Experimental variables Units Coding Range and levels 1 1 Hydration period h x Weight ratio of CaO to ash x 2 1:1 2:1 Amount of CaSO 4 g x Activity test The activity test was carried out in a fixed-bed stainless steel absorber (13 cm outer diameter) under isothermal conditions at 100 C. This reaction temperature was used because at a reaction temperature of 100 C or above, the hydrated water trapped in the sorbent can be released, thus giving moisture and new pores to the sorbent. The generation of new pores will give an additional surface area for the reaction between the sorbent and SO 2 to occur. The sorbent (0.7g) was packed into the center of the absorber and supported by glass wool. The particle size of the sorbent was in the range µm. A gas stream of 2000 p.p.m. of SO 2, with N 2 as the balance, was passed through the sorbent. 14 Before that, the gas stream was passed through a humidification system where the gas was saturated with water vapor. The total flow rate of the gas stream was controlled at 150 ml/min using a mass flow controller. The concentration of SO 2 in the flue gas was measured using the portable flue gas analyzer Enerac 2000E before and after the absorption process. The concentration of SO 2 was recorded continuously until it reached a steady state. A schematic diagram of the experimental setup used for the activity test is given in Fig. 1. Statistical experimental design Fig. 1. Schematic diagram of the experimental rig A two-level full factorial experimental design was used to evaluate the significance of three sorbent preparation variables on the BET-specific surface area of the sorbent. Table 3 lists the range and levels of the three independent variables studied in terms of coded and actual values. The relation between the coded independent variables and the actual values are given in Eq. 1. ( ) Ai A0 xi A (1) where x i is the coded value of the independent variable i, and A i, A 0, and A are the actual value, the actual value at the center point, and the step change of variable i, respectively. For this, a 2 3 full factorial design with four replicates at the central point was employed, which means that 12 experiments are required for each type of ash for this procedure. The complete design matrix and results are shown

4 19 Table 4. Experimental design matrix and results Solid code a Experimental variables BET surface area (m 2 /g) x 1 b x 2 c x 3 d Coal fly ash Coal bottom ash Oil palm ash Incinerator ash 1 4 1: : : : : : : : I 7 1.5: II 7 1.5: III 7 1.5: IV 7 1.5: a The following letters are added in front of the solid code in the text to differentiate between the sorbents prepared by various ashes: F, coal fly ash; B, coal bottom ash; O, oil palm ash; I, incinerator ash. For example, sorbent prepared under condition solid code 1 using coal fly ash is coded as F1 b Hydration period c Weight ratio of CaO to ash d Amount of CaSO 4 in Table 4. All the experimental runs were carried out in random order to minimize personal bias. Data from the design were subjected to a multiple linear regression model (in coded form) as in Eq. 2 using Design Expert software. Y A Ax A x A x Axx Axx Axx Axxx where Y is the measured surface area in m 2 /g, A 0 is the intercept term, and A 1 to A 7 are the coefficients of the effects of variables x i, x i x j, and x i x j x k, respectively. The variable x i x j x k represent the first-order interaction between the variables studied. Results and discussion A two-level full factorial design was employed to evaluate the influence of three experimental variables on the surface area of a sorbent prepared from four types of ash. The complete design matrix and results are shown in Table 4. Experimental runs 9-I to 9-IV were performed at the center point of the experimental design in order to determine the experimental error. As the results of these four runs were consistent, only a single replicate experiment was needed for this study. In relation to the results of the BET-specific surface areas given in Table 4, it was found that the value obtained ranged from 24.2 to 122.1m 2 /g, from 21.3 to 42.2 m 2 /g, from 15.4 to 60.4m 2 /g, and from 15.7 to 31.2m 2 /g for sorbents prepared from coal fly ash, coal bottom ash, oil palm ash, and incinerator ash, respectively. This range is higher than the surface area range of the starting materials, as shown in Table 1 ( m 2 /g). It was also observed that the optimum BET surface area for the sorbents prepared using the various types of ash was obtained with dif- (2) ferent preparation conditions. This might be a result of the varying composition of the various ash used. From the four types of ash used, it was found that the sorbent prepared from coal fly ash gives the highest BET surface area, at a value of 122.1m 2 /g, using preparation conditions of a hydration period of 10h, a CaO to ash ratio of 2:1, and 3g of CaSO 4. The results presented in Table 4 were subjected to analysis using Design Expert software. Regression analysis was used to determine the coefficient A 1 A 7 in Eq. 2, while the significance of the coefficient was determined by using ANOVA. An analysis of variance is the most effective analysis technique in factorial-designed experiments. 15 At a confidence level of 95%, the variance test showed that some of the coefficients were not statistically significant. Therefore, coefficients that were not significant were dropped from the equation leaving only the significant terms. The finalized regression equations (in coded form) that correlate the significant sorbent preparation variables to the sorbent BET surface areas for sorbent prepared from coal fly ash, coal bottom ash, oil palm ash, and incinerator ash are given in Eqs. 3, 4, 5, and 6, respectively. Coal fly ash: S x1 8. 5x x3 83. xx 122. xx 83. xx 58. xxx (3) Coal bottom ash: S x 2. 9x Oil palm ash: S x 7. 3x 65. x x 30. xxx Incinerator ash: S x2x3 1. 7x1x3 42. xxx (6) where the first term in each equation is the average BET surface area for sorbents with a solid code of 1 8. A positive sign in front of the terms indicates a synergistic effect, (4) (5)

5 20 while a negative sign indicates an antagonistic effect. The coefficients of determination (R 2 ) for Eqs. 3 6 were determined as 0.999, 0.845, 0.988, and 0.946, respectively, and the absolute average percentage deviation (AAPD) between calculated and experimental data were found to be 0.1%, 7.3%, 4.5%, and 4.2%, respectively. The high values for the coefficients of determination for all four regression equations indicate that the equations developed are reliable in predicting the sorbent surface area. Also, the low value of the absolute average percentage deviation obtained for the four equations further supports the finding that the model is highly reliable. From these statistical tests, it can be concluded that the regression model equations developed have successfully captured the relation between the significant experimental variables and the surface area of the sorbent. Regression Eqs. 3 5 clearly show that the coefficient for hydration period (x 1 ) is the highest of all the variables.thus, it is possible to conclude that the effect of this variable on the surface area of sorbents prepared from coal fly ash, coal bottom ash, and oil palm ash is the strongest. It has been reported that the dissolution rate of silica and/or alumina present in the ash is the rate-limiting step for the formation of calcium aluminum silicate hydrate compounds. 15 This is in agreement with our results because as the hydration period proceeds, more silica and/or alumina will elute from the ash to react with CaO to form calcium aluminum silicate hydrate compounds and thus have a positive effect on the sorbent surface area. On the effect of the CaO to ash ratio on the surface area of the sorbent, only sorbent prepared from coal fly ash (from Eq. 3) has a significant positive effect, while for sorbent prepared from coal bottom ash (from Eq. 4) and oil palm ash (from Eq. 5), the effect of this variable is negligible. The ratio of CaO to ash generally determines the amount of CaO present in the preparation mixture. It is well known that the amount of CaO and silica and/or alumina dissolved during the hydration reaction determines the amount of active species formed during the pozzolanic reactions. Looking at the composition of the ashes, it was found that coal fly ash contains the highest percentage of silica and alumina at 80%, while coal bottom ash and oil palm ash contain only 57% and 51.3%, respectively. In the case of sorbent prepared from coal fly ash, owing to the high content of silica and alumina in the ash, more CaO is required to form the active species. Thus, a higher ratio of CaO in the preparation mixture will result in the formation of more active species. However, owing to the lower content of silica and alumina in the other two ashes, increasing the amount of CaO in the preparation mixture is not crucial as there is already sufficient CaO to react with the silica and alumina in the preparation mixture even at a CaO to ash ratio of 1:1. The addition of CaSO 4 during the preparation step had a positive effect on the surface area of sorbent prepared form coal fly ash (from Eq. 3), while it had a negative effect on the sorbent prepared from coal bottom ash (from Eq. 4) and oil palm ash (from Eq. 5). The positive effect of CaSO 4 on the surface area of sorbent prepared from coal fly ash was also reported by Ishizuka et al. 3 However, in his work, Ca(OH) 2 is used as the raw material instead of CaO. It was reported that this phenomenon is due to the role played by CaSO 4, since it promotes the formation of calcium silicate by suppressing the crystal growth of Ca(OH) 2, and thus forces the reactivity of the Ca(OH) 2 to produce hydrated products.thus, it can be concluded that CaSO 4 has the same effect on CaO as it has on Ca(OH) 2 for sorbent prepared from coal fly ash. However, this phenomenon was not found to be true for sorbents prepared using the other two types of ash. This is most probably due to the different forms of silica and/or alumina present in the ashes. Unlike the regression models developed for sorbents prepared from coal fly ash, coal bottom ash, and oil palm ash, Eq. 6 shows that there is no significant effect of individual variables on the surface area of sorbent prepared from incinerator ash. However, it is only the interactional effects between the variables that exerts significant effects on the surface area of the sorbent. In another words, if one of the variables is changed with respect to another one, it will have a considerable effect on the total surface area of the sorbent. Interactional effects between the variables is also significant in the sorbent prepared from coal fly ash and oil palm ash, as shown in the regression models in Eqs. 3 and 5. Sorbents with the highest surface area for each type of ash were subjected to an SO 2 -removal activity test in the experimental rig shown in Fig. 1. The breakthrough curves (the ratio of the SO 2 concentration to the initial SO 2 concentration, C/C 0 ) for the prepared sorbents and inert silica sand are shown in Fig. 2. Breakthrough curves for inert silica sand were used as controls. From this figure, it is seen that the sorbents prepared from coal fly ash and oil palm ash give the highest SO 2 absorption activity, and SO 2 was completely removed from the system in the first 10min. From that point, the concentration of SO 2 gradually increases until there is no more SO 2 absorption activity in the sorbent (i.e., when the SO 2 concentration in the outlet flue gas is the same as the inlet concentration). Although the surface area of sorbent prepared from oil palm ash O6 (60.4m 2 /g) is only about half the surface area of sorbent prepared from coal fly ash F4 (122.1m 2 /g), it was found that both of the sorbents exhibited similar desulfurization activity. An XRD spectrum and a SEM micrograph (shown below) revealed that Fig. 2. Breakthrough curves (C/C 0 ) for various sorbents

6 21 Fig. 3. X-ray diffraction spectra for sorbent F4 a before and b after the activity tests the good desulfurization activity by the sorbent prepared from oil palm ash was probably due to the needle-like macrostructure of the potassium calcium aluminum silicate hydrate (K 2 Ca 2 (Al 2 Si) 16 O H 2 O) in the sorbent. In this needle-like macrostructure, it is believed that the calcium (Ca) ions in the compound are arranged in such a way that it is easier for an excess of SO 2 molecules. Thus, the sorbent prepared from oil palm ash can still give very good desulfurization activity even though its surface area is only half that of sorbent prepared from coal fly ash. From Fig. 2, it can also be concluded that the sorbents prepared from coal bottom ash and incinerator ash also have the capacity to absorb SO 2, but their absorption capacity is not as high as that of the sorbents prepared from coal fly ash and oil palm ash. This is probably due to the low surface area of these sorbents. The XRD spectra for sorbent prepared from coal fly ash (F4) before and after it was subjected to the activity test are shown in Fig. 3a and b, respectively. From Figure 3a, it can be deduced that calcium aluminum silicate hydrate (Ca Al 2 Si 4 O 12 2H 2 O), calcium carbonate (CaCO 3 ), and calcium sulfate (CaSO 4 ) are the main phases present in sorbent F4 (prepared from coal fly ash). The formation of Ca Al 2 Si 4 O 12 2H 2 O and CaCO 3 occurs in the hydration process, while CaSO 4 is the raw material which has not reacted. The presence of the Ca Al 2 Si 4 O 12 2H 2 O compound in the sorbent (from the pozzolanic reaction of the raw materials) which has a high surface area is believed to be the main contributor to the high SO 2 absorption capacity of that sorbent. Although the sorbent consists of a complex hydrated compound, it is believed that the active species in the sorbent that reacts with SO 2 is the calcium (Ca) ions only. The role of the high-surface-area hydrated compound is basically to make the Ca ions contained in the sorbent more accessible to SO 2 during the desulfurization reaction. The absence of CaO in sorbent F4 shows that it reacts completely to form Ca Al 2 Si 4 O 12 2H 2 O. Figure 3b shows the XRD spectrum of sorbent F4 after it was subjected to the activity test in the experimental rig shown in Fig. 1. Only two phases are detected in this spectrum, i.e., aluminum silicate hydrate (Al 4 Si 2 O 10 H 2 O) and CaSO 4.This shows that all the calcium ions present in the sorbent have reacted Table 5. Various phases detected in the fresh sorbent using X-ray diffraction analysis Source of ash Coal bottom ash (B8) Incinerator ash (I6) Oil palm ash (O6) Phases Calcium silicate hydrate, Ca 2 SiO 4 H 2 O Calcium hydroxide, Ca(OH) 2 Calcium sulfate, CaSO 4 Calcium silicate hydrate, Ca 2 SiO 4 H 2 O Calcium hydroxide, Ca(OH) 2 Calcium sulfate, CaSO 4 Potassium calcium aluminum silicate hydrate, K 2 Ca 2 (Al 2 Si) 16 O H 2 O Calcium hydroxide, Ca(OH) 2 Calcium sulfate, CaSO 4 with SO 2 in the desulfurization reaction. The Al 4 Si 2 O 10 H 2 O could be from the Ca Al 2 Si 4 O 12 2H 2 O after it has reacted with SO 2. Table 5 lists the various phases detected in the fresh sorbent (before the desulfurization reaction) prepared from coal bottom ash, oil palm ash, and incinerator ash using XRD analysis. The main phases detected in the sorbent prepared from coal bottom ash and incinerator ash were found to be similar. These phases were identified as calcium silicate hydrate, calcium hydroxide, and calcium sulfate. This result shows that part of the calcium oxide used in the preparation of the sorbent is converted into calcium hydroxide only and not into calcium aluminum silicate hydrate compounds. This is most probably due to the low contents of silica and alumina in coal bottom ash and incinerator ash that limit the formation of calcium aluminum silicate hydrate compounds. Tsuchiai et al. 16 reported that the efficiency of SO 2 capture by Ca(OH) 2 is very low due to its low surface area. This explains the low absorption capacity of these two sorbents towards SO 2. Although Ca(OH) 2 is also detected in the sorbent prepared from oil palm ash, this sorbent still manages to show good desulfurization activity. Therefore, it is believed that the good desulfurization activity of the sorbent is due to the presence of a potassium calcium aluminum silicate hydrate phase. SEM micrographs of coal fly ash and various sorbents are shown in Fig. 4. The SEM micrograph of coal fly ash

7 22 Fig. 4. Scanning electron microscope micrographs of a coal fly ash, b F4, c F4 after being subjected to an activity test, and d O6 a b c d Fig. 5. Pore-size distributions of sorbent F4 before and after SO 2 capture shows that it consists mainly of spherical particles of different sizes with smooth surfaces. However, it was observed that in the hydrated coal fly ash (F4), compounds with a higher structural porosity were obtained, as shown in Fig. 4b. This suggests that the spherical coal fly ash reacted extensively with calcium oxide, so that not only the surface layers of the spherical particles but also the inside of the particles could not retain their original shapes. 9 On the other hand, after reacting with SO 2 (Fig. 4c), the porous structure of the sorbent was no longer seen as it was covered by a layer of product, believed to be calcium sulfate. Comparing the SEM micrographs of the sorbents prepared from coal fly ash (F4) and oil palm ash (O6), it was found that in the sorbent prepared using oil palm ash (Fig. 4d), a compound with a needle-like structure was formed. It was concluded that the high SO 2 absorption for sorbent O6 was due to the presence of potassium calcium aluminum silicate hydrate, which has a needle-like structure. The pore-size distribution of sorbent F4 before and after the desulfurization reaction is shown in Fig. 5. It has been reported in the literature that pore sizes between 2 and 100nm have been identified as the effective zone for the sulfation reaction between SO 2 and a sorbent prepared from Ca(OH) 2 /coal fly ash, 6 and therefore special attention has been paid to this region. The BJH procedure, which permits a better characterization of mesoporosity, was applied to obtain the pore-size distribution from nitrogen desorption data. The pore-size distribution is represented by the derivative d(v p )/d(d p ) as a function of pore diameter, where, V p is the pore volume and d p is the pore diameter. For fresh sorbent, mesopores with an average pore size of 38.9nm appeared to be the major contributor to the total pore volume. After desulfurization, a dramatic decrease in the mesopore volume (from 10 to 100nm) was observed, indicating significant pore filling by the reaction product. It can be concluded that the surface of the spent sorbent was covered by a layer of the reaction product, thus reducing its porosity. Conclusions This study has shown that sorbents prepared from coal fly ash, coal bottom ash, oil palm ash, and incinerator ash have the capacity to absorb SO 2. Experimental evidence and mathematical analyses showed that all the variables studied, i.e., hydration period, weight ratio of CaO to ash, and

8 23 amount of CaSO 4, had a significant influence on the surface area of the sorbents. Multiple linear regression models, developed to predict the surface areas of the sorbents prepared using the various ashes, were found to be successful, with high values of the coefficients of determination (R 2 ). The reactivity of sorbents prepared from coal fly ash and oil palm ash was found to be much higher than that from those prepared from coal bottom ash and incinerator ash. The XRD spectra and SEM micrographs showed that the differences in reactivity of the various sorbents are due to the different phases present in the sorbent, and are also due to its macrostructural properties. Acknowledgments The authors thank the ASEAN University Network/Southeast Asia Engineering Education Development Network (AUN/SEED-Net), the JSPS VCC (Program on Environmental Science, Engineering and Ethics), the Ministry of Science, Technology, and Environment (Project No EA001), and the Universiti Sains Malaysia (USM short-term grant) for the funding and support of this project. We also express our deepest gratitude to Kyoto University, Japan, for supplying the incinerator ash. References 1. Torrey S (1978) Coal ash utilization: pollution technology review No. 48. Noyes Data, USA, p Jozewicz W, Chang JCS, Brna T, Sedman C (1987) Reactivation of solids from furnace injection of limestone for SO 2 control. Environ Sci Technol 21: Ishizuka T, Yamamoto T, Murayama T, Tanaka T, Hattori H (2001) Effect of calcium sulfate addition on the activity of the absorbent for dry flue gas desulfurization. Energy Fuels 15: Fernandez J, Renedo J, Garea A, Viguri J, Irabien JA (1997) Preparation and characterization of fly ash/hydrated lime sorbents for SO 2 removal. Powder Technol 94: Fernandez J, Renedo MJ, Pesquera A, Irabien JA (2001) Effect of CaSO 4 on the structure and use of Ca(OH) 2 /fly ash sorbents for SO 2 removal. Powder Technol 119: Garea A, Fernandez I, Viguri JR, Ortiz MI, Fernandez J, Renedo MJ, Irabien JA (1997) Fly-ash/calcium hydroxide mixtures for SO 2 removal: structural properties and maximum yield. Chem Eng J 66: Karatepe N, Mericboyu AE, Kucukbayrak S (1997) Effect of hydration conditions on the physical properties of fly ash Ca(OH) 2 sorbents. Energ Sources 20: Karatepe N, Mericboyu AE, Kucukbayrak S (1998) Preparation of fly Ash Ca(OH) 2 sorbents by pressure hydration for SO 2 removal. Energ Sources 20: Ishizuka T, Tsuchiai H, Murayama T, Tanaka T, Hattori H (2000) Preparation of active absorbent for dry-type flue gas desulfurization from calcium oxide, coal fly ash, and gypsum. Ind Eng Chem Res 39: Renedo MJ, Fernandez J (2002) Preparation, characterization and calcium utilization of fly ash/ca(oh) 2 sorbents for dry desulfurization at low temperature. Ind Eng Chem Res 41: Renedo MJ, Fernandez J, Garea A, Ayerbe A, Irabien JA (1999) Microstructural changes in the desulfurization reaction at low temperature. Ind Eng Chem Res 38: Tsuchiai H, Ishizuka T, Nakamura H, Ueno T, Hattori H (1996) Study of flue gas desulfurization absorbent prepared from coal fly ash: effects of the composition of the absorbent on the activity. Ind Eng Chem Res 35: Lin BL, Shih SM, Liu CF (2003) Structural properties and reactivities of Ca(OH) 2 /fly ash sorbents for flue gas desulfurization. Ind Eng Chem Res 42: Jung GH, Kim H, Kim SG (2000) Utilization of lime-silica solids for flue gas desulfurization. Ind Eng Chem Res 39: Karatepe N, Mericboyu AE, Kucukbayrak S (1999) Activation of Ca(OH) 2 using different siliceous materials. Environ Technol 20: Tsuchiai H, Ishizuka T, Ueno T, Hattori H, Kita H (1995) Highly active absorbent for SO 2 removal prepared from coal fly ash. Ind Eng Chem Res 34: