Ind. Eng. Chem. Res. 2008, 47, 9871–9877
9871
Desulfurization Characteristics of Waste Cement Particles as a Sorbent in Dry Desulfurization Jiawei Wu,† Atsushi Iizuka,† Kazukiyo Kumagai,† Akihiro Yamasaki,*,‡ and Yukio Yanagisawa† Department of EnVironment Systems, Graduate School of Frontier Science, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba, Japan 277-8563, and Department of Materials and Life Science, Seikei UniVersity, 3-3-1Kichijoji-kitamachi, Musashino, Tokyo, Japan 180-8633
Dry desulfurization characteristics of waste cement particles were examined with laboratory-scale experimental apparatus based on the weight change of the sample exposed to a gas flow containing SO2. The waste cement particles, a byproduct of recycling aggregates from waste concrete, are fine particles with diameters ranging from 10 to 200 µm and an average diameter of 105 µm. The effects of the operation parameters, i.e., the reaction temperature (650-950 °C), SO2 concentration (61-1543 ppm), oxygen concentration (0-10%), NO2 concentration (0-500 ppm), absolute humidity (0-15000 ppm), and particle size (10-200 µm), on the desulfurization performance were investigated. The desulfurization rates were found to depend on the 1.26th order of the SO2 concentration and to slightly depend on the absolute humidity and the particle size, but they were almost independent of the concentrations of oxygen and NO2 in the gas flow. Arrhenius type temperature dependence was observed up to 850 °C with activation energy of 12 kJ/mol. The observed dry desulfurization rates of the waste cement particle were almost equivalent to those of the conventional sorbents such as limestone and calcium hydroxide. Therefore, it is confirmed that the waste cement particles could be applicable in dry desulfurization as an inexpensive sorbent derived from wastes. Introduction Coal will be used continuously for at least several tens of decades because of its abundance as a primary energy resource. It has been estimated that coal consumption in China will increase from 1.2 billion tons/year at present to 2.3 billion tons in 2020,1 although the percentage use of coal as a primary energy resource will drop from 76 to 67% during the same period. The most important environmental concern in using coal is the emission of SOx during combustion derived from the sulfur contents in coal. In developed countries such as Japan, the emission of SOx is under strict control with effective desulfurization facilities equipped in most combustion facilities and the use of coals with low sulfur contents.2 On the other hand, the average coal used in China contains as much as 3 wt % of sulfur,1 and combustion facilities burning coal may not necessarily have desulfurization facilities. As a result, China is now the world’s largest emitter of sulfur dioxide, and the annual emission rate was recorded as 23.46 million tons in 1997. The deployment of low-cost and effective desulfurization processes is of primary importance for environmental protection, not only for China but also for neighboring countries. Two types of desulfurization methods, wet and dry desulfurization, can be applicable for the furnace or flue gas desulfurization. Wet desulfurization is more effective in terms of SO2 removal rates compared with the dry desulfurization, but the wet process is costly due to the requirement of larger amounts of water and wastewater treatment. Considering the application and deployment of desulfurization in inland China, where water resources are insufficient, dry desulfurization would be more appropriate. Limestone is generally used for the dry desulfurization processes, and various * To whom correspondence should be addressed. Tel. and Fax: +81422-37-3887. E-mail:
[email protected]. † The University of Tokyo. ‡ Seikei University.
studies on its reaction mechanism and kinetics have been reported.3-5 Dry desulfurization with direct limestone injection is less costly, but the desulfurization efficiency is lower than that for wet desulfurization. Thus, an improvement in desulfurization efficiency is needed for practical application. A great deal of effort has been devoted to improve sorbent reactivity in dry desulfurization. Sorbents obtained by the hydration of calcium hydroxide and different sources of silica have led to a significantly higher conversion of calcium compared to the conversion obtained using hydrated lime.6-10 A sorbent prepared by mixing fly ash with hydrated lime slurries was found to significantly enhance the calcium utilization rate in desulfurization.11 In this case, calcium silicate formed by the hydration reaction between calcium hydroxide and fly ash is attributed to the enhancement of desulfurization rates. Li et al.12 developed a new method for preparing a low-cost and highly active sorbent for desulfurization by mixing calcium hydroxide with fly ash in water at an ambient temperature with a short preparation time. The calcium utilization rate of the sorbent so prepared was found to be 2 or 3 times that of the original calcium hydroxide particles. In all cases where fly ash or other silica sources and calcium hydroxide are present in hydration, a pozzolanic reaction takes place and hydrated calcium silicates with a general composition of (CaO)x-(SiO2)y-(H2O)z are formed. Therefore, the key substances for effective desulfurization are calcium silicate hydrate and calcium hydroxide. We have paid attention to waste cement particles as a potential sorbent for dry desulfurization. The waste cement can be obtained as a byproduct in aggregate recycling from waste concrete. The weight portion of the waste cement particles could be as large as about one-third the total weight of the waste concrete. Because the main components of the waste cement are calcium silicate hydrate gel (e.g., 3CaO · 2SiO2 · 4H2O) and calcium hydroxide (Ca(OH)2), it is expected the waste cement particles will show high activities as a sorbent for dry desulfu-
10.1021/ie8009654 CCC: $40.75 2008 American Chemical Society Published on Web 11/13/2008
9872 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 1. Schematic drawing of the thermogravimetric analyzer (TGA) system: 1, pressure gauge; 2, mass flow controller; 3, valve; 4, bubbling bath.
rization. The dry desulfurization reaction by waste cement particles may involve the following schemes. Calcium hydroxide: Ca(OH)2 f CaO + H2O
(1)
CaO + SO2 f CaSO3
(2)
2SO2 + O2 f 2SO3
(3)
CaO + SO3 f CaSO4
(4)
1 CaSO3 + O2 f CaSO4 2 Calcium silicate hydrate gel:
(5)
3CaO · 2SiO2 · 4H2O + 3SO2 f 3CaSO3 + 2SiO2 + 4H2O
(6)
3CaO · 2SiO2 · 4H2O + 3SO3 f 3CaSO4 + 2SiO2 + 4H2O
(7)
1 (5) CaSO3 + O2 f CaSO4 2 The emission rate of waste concrete is rapidly increasing both in China and Japan. In particular, it is estimated that the quantity of concrete debris in China will reach 1.5 billion metric tons in 2030 and 5.37 billion metric tons in 2050.13 Because of the resource depletion of aggregates, aggregate recycling from waste concrete has been developed, and some processes have already been commercialized. However, most waste cement is presently disposed of without being reused effectively. Thus, the use of the waste cement as a sorbent for dry desulfurization would also be very attractive both economically and environmentally. However, little information is available on the desulfurization characteristics of the waste cement particles despite its high potential for a sorbent. In this study, experimental studies on the desulfurization characteristics of the waste cement particles using laboratory-scale experimental apparatus were conducted. And the desulfurization performance was compared with that of the conventional sorbent including calcium hydroxide and limestone to examine the feasibility of waste cement being applied in dry desulfurization. Experimental Section Physical and Chemical Analyses for the Waste Cement Sample. The waste cement sample, which was supplied by Tateishi Construction Corp., was generated as a byproduct in a
commercial plant recycling aggregates from waste concrete. The waste cement is obtained in the form of fine particles mainly composed of hydrated cement. The morphology of the sample was observed with a scanning electron microscope (SEM), JSM-5600. The particle size distribution of the sample was measured by a laser diffraction method using a SALD-2100 (Shimadzu) analyzer. The specific surface area (BET surface area) and pore size distribution of the sample were determined with nitrogen adsorption equilibrium measurements under liquid nitrogen temperature using a BELSORP 18 (BEL Japan) analyzer. The chemical composition of the sample was determined using an energy dispersive X-ray fluorescence spectrometer (JEOL JSX-3220) and inductively coupled plasma atomic emission spectrometer (Shimadzu ICPS7510). Characterization of the sample was conducted by a classified sample sieved by stainless steel meshes. Desulfurization Experiments. The desulfurization experiments were carried out with a thermogravimetric analyzer (Shimadzu DTG-60H) equipped with a gas flow system. The schematic flow diagram for the experimental system is shown in Figure 1. A known amount of the waste cement sample was mounted in a sample cell of the thermogravimetric analyzer. The sample was heat-treated with increasing temperature from room temperature to 850 °C at a rate of 20 °C /min under a nitrogen flow rate of 100 mL/min. After confirming no further change of the sample weight, the flow was switched to the test gas flow containing SO2, and the weight change of the sample was monitored. The total flow rate of the test gas was 100 mL/ min, which enables the reaction system to have differential conditions. The SO2 concentration in the gas flow was changed in the range of 61-1543 ppm. The oxygen concentration was changed in the range of 0-10%. The nitrogen dioxide was changed in the range of 0-500 ppm. The absolute humidity was changed in the range of 0-15000 ppm. The test gas flow was balanced with nitrogen. The absolute humidity was changed in the range of 0-15000 ppm, which was controlled by adjusting flow rates of the dry gas line and wet gas line. The wet gas flow was a nitrogen flow with bubbling in a water bath. The humidity of the feed gas was measured with a humidity meter (Onset, HOBO). The reaction temperature was changed in the range of 650-950 °C. The effect of the particle size was studied with classified samples by sieving the original sample. Five kinds of metal sieving meshes with the sizes of 10, 20, 53, 106,
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9873
Figure 2. SEM image of the waste cement sample.
Figure 3. Particle size distribution (surface area based) of the waste cement sample.
and 153 µm were used for the classification. In this study, sieved waste cement samples were used only for studying the effect of the particle size; unsieved waste cement samples were used for all other experiments. Comparison of the Desulfurization Performance of Limestone and Calcium Hydroxide. Commercial limestone (JFE Mineral Co., Ltd.) and reagent grade calcium hydroxide (Wako Pure Chemical Industries, Ltd.) were used without further treatments for comparison with waste cement particles. Results and Discussion Characterization of the Waste Cement Sample. Figure 2 is an SEM image of the waste cement sample. The waste cement particles have a porous structure; some parts have a more open structure, while others are more compact with many small granular particles attached on the surface. The surface area based size distribution of the waste cement particles is shown in Figure 3. The diameters of particles were distributed in the range of 10-200 µm.The peak was located near 10-20 µm, but the mean diameter was 105 µm. The BET surface area of the waste cement sample was 6.15 m2/g, and the specific pore volume was 2.74 × 10-2 cm3/g. The chemical composition (element based) of the waste cement particles is given in the second column of Table 1. Calcium was the main component of the waste cement with a calcium oxide equivalent of about 38 wt %. Stoichiometrically 1 metric ton of waste cement particles could capture 0.44 ton of SO2. The iron content had a Fe2O3 equivalent of about 14 wt %, and the silicon content had a SiO2 equivalent of about
12 wt %. The total chemical composition was about 70 wt % as shown in the second column of Table 1, and the remainder was mostly water needed for concrete production. The original sample was classified into four groups by sieving to examine the effect of the particle sizes on the desulfurization performance. The properties of each group are summarized in the third to sixth columns of Table 1. Despite the large difference in the mean diameter, the specific surface areas of the samples for the different groups were in the same range of 5-6 m2/g, and the chemical compositions were almost the same. The small difference in the specific surface area among the samples with different particle diameters indicates that the apparent surface area of the waste cement particles are mainly determined by the internal structure of the particles, not by the external surface area. Desulfurization Performance of the Waste Cement Particles. Time Variation of the Weight Change of the Waste Cement due to the SO2 Capture. Figure 4 shows the time variation of the weight change of the unsieved waste cement sample due to exposure to the gas flow containing 1013 ppm of SO2. The feed gas rate was 100 mL/min and was balanced with nitrogen without containing water vapor, oxygen, or nitrogen oxides. The temperature was changed in the range of 650-950 °C. For a given temperature, the weight of the waste cement particles increased with SO2 exposure time. At the initial stage of the reaction up to about 30 min, the weight increase was almost proportional to the exposure time for all temperature conditions. The weight increases are due to the formation of solid products of CaSO3 and CaSO4 by the reaction of the calcium contents in the waste cement with SO2. The calcium contents in the waste cement involved in the desulfurization reaction would be calcium oxide (CaO) and calcium silicate hydrate (e.g., 3CaO · 2SiO2 · 4H2O) and their derivatives, and the reaction would proceed according to the scheme shown in eqs 1-7. The formation of CaSO3 and CaSO4 was confirmed from the results of X-ray diffraction (XRD) analysis (Rigaku, Miniflex) for the waste cement sample after the desulfurization experiments. The weight change leveled off after 30 min. This could be attributed to the blocking of the direct contact of the calcium contents with the gas-phase SO2 by the solid products of CaSO3 and CaSO4 at the surface of the waste cement particles. The weight increasing rate was found to increase with increasing temperature in the range of 650-850 °C. However, the rate at 950 °C was almost equivalent to that at 850 °C, which can be explained in terms of the sintering of the calcium contents in the waste cement sample.14 Influencing Factors of the Desulfurization Rate. The effect of the operation parameters, i.e., reaction temperature, SO2 concentration, oxygen concentration, NO2 concentration, absolute humidity, and particle size, on the desulfurization rate expressed by the calcium utilization was investigated. The desulfurization performance could be expressed in terms of the calcium utilization, U, which is defined as U (%) )
(∆W ⁄ MSO3)MCa W0wCa
× 100
(8)
where ∆W is the weight increase of the sample, MSO3 is the molar weight of SO3, MCa is the molar weight of calcium, and W0 and wCa are the weight and the weight fraction of calcium in the sample after heat treatment, respectively. In the definition of eq 8, it is assumed that only solid calcium sulfate (CaSO4) will be produced in the desulfurization. It was observed that calcium sulfite (CaSO3) was also produced by the desulfurization reaction. However, the calcium utilization based on the calcium
9874 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 1. Chemical Composition of the Waste Cement Particlesa waste cement particles after sieving
Physical Properties BET surface area (m2/g) specific pore volume (cm3/g) Chemical Composition (wt %) CaO SiO2 Fe2O3 Al2O3 SO3 K2O MnO ZnO P2O5 TiO2 total a
original waste cement particle
mesh size ) 10-20 µm
mesh size ) 20-53 µm
mesh size ) 53-106 µm
mesh size ) 106-153 µm
6.15 2.74×10-2
5.37 1.72×10-2
6.18 2.03×10-2
5.98 2.75×10-2
5.21 1.25×10-2
38 12 14 4 0.7 1 0.09 0.1 0.5 0.6 71
39 11 13 3 0.6 1 0.1 0.1 0.5 0.5 70
39 12 14 3 0.6 1 0.08 0.09 0.7 0.5 70
39 12 14 4 0.6 1 0.1 0.09 0.6 70
39 12 13 3 0.6 1 0.1 0.1 0.6 0.6 70
Data shown for various mesh sizes for sieving.
sulfate production can be used as a convenient index for the comparison of the desulfurization efficiencies among sorbents. In fact, when the calcium sulfite is formed, the net calcium utilization would be larger than the calcium utilization based on eq 8. Thus, the calcium utilization based on eq 8 is the lowest limit of the net calcium utilization for the desulfurization. The desulfurization rate is given by the changing rate of the calcium utilization against the reaction time. In Figure 5, the results in Figure 4 were replotted as the calcium utilization. From the
experimental results, about 8-14% of the calcium content was used for the desulfurization reaction in 1 h. Figure 5 shows the effect of the reaction temperature on the time variation of the calcium utilization based on the weight change shown in Figure 4. The reaction temperature was changed in the range of 650-950 °C. The calcium utilization for each temperature increased linearly for the initial several tens of minutes and leveled off after that. The calcium utilization for a given reaction time increased with increasing temperature in the range of 650-850 °C. However, the desulfurization activity at 950 °C was almost equivalent to that at 850 °C, which can be explained in terms of the sintering of the calcium contents in the waste cement sample. From the experimental results, about 8-14% of the calcium content was used for the desulfurization reaction in 1 h. Figure 6 shows the effect of the temperature on the initial desulfurization rate for the linear region. The temperature effect of the initial desulfurization rate, r, can be expressed by the Arrhenius-type equation,
( )
r ) A exp -
Figure 4. Time variation of the weight change for the original waste cement sample: feed gas, 1013 ppm SO2 with nitrogen balanced, dry, without oxygen.
Figure 5. Effect of the reaction temperature on desulfurization reactivity for the original waste cement sample: feed gas, 1013 ppm SO2 with nitrogen balanced, dry, without oxygen.
Ea RT
(9)
where Ea is the activation energy, R is the gas constant, T is the absolute temperature, and A is the frequency factor, which is a
Figure 6. Temperature dependence of the initial desulfurization rates for the original waste cement sample: feed gas, 1013 ppm SO2 with nitrogen balanced, dry, without oxygen.
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9875
Figure 7. Effect of the SO2 concentration on the calcium utilization for original waste cement sample: temperature ) 850 °C; feed gas, SO2 with nitrogen balanced, dry, without oxygen.
Figure 9. Effect of O2 concentration on the calcium utilization for the original waste cement sample: temperature ) 850 °C; feed gas, 1013 ppm SO2 with nitrogen balanced, dry.
Figure 10. Effect of NO2 concentration on the calcium utilization for original waste cement sample: temperature ) 850 °C; feed gas, 1013 ppm SO2 with nitrogen balanced, without oxygen, dry. Figure 8. Effect of the SO2 concentration on the initial reaction-changing rate for the original waste cement sample: temperature ) 850 °C; feed gas, SO2 with nitrogen balanced, dry, without oxygen.
parameter independent of the temperature. The activation energy for the weight-changing rate was found to be 12 kJ/mol on the basis of the experimental results for the temperature range from 650 to 850 °C. The calculated activation energy was much smaller than those observed for desulfurization reactions by conventional sorbents3 but was closer to those observed for physical processes such as diffusion. This result suggests that the desulfurization reaction is controlled by the diffusion of the reactive species (e.g., SO2) in the waste cement particles. Figure 7 shows the effect of the SO2 concentration on the calcium utilization. The concentration of SO2 was changed in the range of 61-1543 ppm. The results indicate the desulfurization rate was an increasing function of the SO2 concentration. The order of the dependency of the desulfurization rate was found to be about 1.26 on the basis of the initial desulfurization rate for the linear region as shown in Figure 8. Considering the experimental errors, the order of SO2 concentration can be recognized as 1 practically, which has been often observed for limestone.3 Figure 9 shows the effect of the oxygen concentration on the calcium utilization. The oxygen concentration was changed in the range of 0-10%. The apparent desulfurization reaction rate slightly depended on the oxygen concentration, and the order of the dependency of the desulfurization rate was about 0.08 on the basis of the experimental results. Although the desulfurization product could depend on the oxygen concentra-
Figure 11. Effect of absolute humidity on the calcium utilization for the original waste cement sample: temperature ) 850 °C; feed gas, 1013 ppm SO2 with nitrogen balanced, without oxygen.
tion, the observed desulfurization rate was almost independent of oxygen concentration. The result is consistent with the previous results observed for limestone.3 Figure 10 shows the effect of the nitrogen dioxide concentration on the calcium utilization. The nitrogen dioxide concentration was changed in the range of 0-500 ppm. No noticeable effect was observed on the desulfurization reaction activity of the waste cement particles. Figure 11 shows the effect of absolute humidity on the calcium utilization. The absolute humidity was changed in the range of 0-15000 ppm. The results indicate the desulfurization
9876 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 12. Effect of the particle size on the calcium utilization for the sieved waste cement sample: temperature ) 850 °C; feed gas, 1013 ppm SO2 with nitrogen balanced, dry, without oxygen.
rate was an increasing function of the absolute humidity, and the order of the dependency was found to be 0.24. This result is different from limestone, where no effect of humidity has been reported for experimental conditions similar to those of the present study.3 The effect of the particle size is shown in Figure 12 by demonstrating the calcium utilization for each classified group. The calcium utilization increased with a decrease in the particle size, but the effect is not remarkable. The order of the dependency of the initial desulfurization rate was found to be 0.13. The calcium content was independent of the particle size of the waste cement particles as shown in Table 1. The specific surface areas for the samples are almost equal irrespective of the diameters. The above results suggest, therefore, that the desulfurization rate is mainly determined by the surface area, which can be attributed to the porous structure inside the particles where active calcium content is distributed. This result is consistent with the observed low activation energy for the reaction (Figure 6), which suggests that the desulfurization rate is mainly controlled by a diffusion process in the porous structure inside the particles. From the above results, the desulfurization rate can be summarized by the following empirical rate equation:
(
)
1.2 × 104 × RT [SO2]1.26[O2]0.08[H2O]0.24(particle size)0.13 (10)
r (mol s-1 m-2) ) 5.88 × 10-5 exp -
where [SO2] (ppm) is the concentration of SO2, [O2] (%) is the concentration of O2, and [H2O] (%) is the absolute humidity in the gas flow. The particle size term is given in meters. Here the unit of the desulfurization rate was converted to (mol s-1 m-2) from the specific surface area of the sorbent (Table 1) and based on the assumption that the product is exclusively CaSO4. These dependencies in eq 10 are consistent with the previous works summarized in ref 3. Apparent reaction orders of SO2 in previous works are about 1 and 0 for oxygen and humidity. Comparison of the Desulfurization Performances of Limestone and Calcium Hydroxide. The desulfurization characteristics of the conventional sorbents, calcium hydroxide and limestone particles, were investigated with the same procedure for the waste cement samples. In Table 2 properties of these sorbents are shown for comparison. The limestone sample is a fine powder with the mean diameter of 8 µm, while the calcium hydroxide sample is coarse particles with diameter larger than 150 µm. However, the BET surface area of the limestone sample is much smaller than other samples, which may have porous structures.
Figure 13. Comparison of reaction rate (initial 5 min, denoted by bars) and calcium utilization after 60 min reaction (denoted by circles) among various sorbents: temperature ) 850 °C; feed gas, 1013 ppm SO2 balanced with nitrogen, dry, without oxygen. Table 2. Comparison of Physical and Chemical Properties among Sorbents sorbent
limestone
calcium hydroxide
waste cement
particle size (µm) BET surface area (m2/g) calcium content (wt %)
8 0.6 39.4
>150 11.82 54.1
15 6.15 27.3
Similar weight-increasing characteristics due to SO2 exposure were observed for both samples of limestone and calcium hydroxide. Figure 13 shows the comparison of the reaction rate and calcium utilization among the sorbents. The reaction rates shown are determined from the initial weight changes up to 5 min of the reaction time. The utilization ratios of calcium shown are the ones after 60 min reaction. The results indicate the desulfurization rate of the waste cement particle was superior to those of limestone or calcium hydroxide. This is due to the fact that the calcium silicate hydrate gel, which is more active for the desulfurization than calcium oxide, is included in the waste cement. In addition, iron contained in the waste cement (Table 1) may contribute the desulfurization activity as a catalyst.15 It is confirmed, therefore, that the waste cement particles have a desulfurization performance almost equivalent to those of limestone and calcium hydroxide. Considering the prices of limestone or calcium hydroxide, the waste cement particle, a waste, could be a promising sorbent for inexpensive dry desulfurization processes. On the basis of the experimental results, a case study was conducted for the desulfurization process. The following assumptions were made. A coal-fired thermal plant of 100 MW was used, and the sulfur content is 1.0%. The generation rate of sulfur dioxide is 6.1 × 103 metric tons/yr. The calcium utilization is 10%. The necessary amount of waste cement is 1.4 × 105 metric tons/yr. Conclusions The following conclusions can be drawn from the present research work. (1) It is possible to use waste cement particles as a dry desulfurization sorbent. From the experimental results, the desulfurization reaction rate is expressed by the following empirical equation,
(
)
1.2 × 104 × RT [SO2]1.26[O2]0.08[H2O]0.24(particle size)0.13
r (mol s-1 m-2) ) 5.88 × 10-5 exp -
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(2) The observed desulfurization rate of the waste cement particle was almost equivalent to those of limestone and calcium hydroxide. Therefore, the waste cement particle could be applicable in dry desulfurization as an inexpensive but effective sorbent derived from wastes. Acknowledgment The authors acknowledge the financial support of a Grantin-aid for Scientific Research (17-11378) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the kind supply of a waste concrete sample by Tateishi Construction Co. The authors also thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, The University of Tokyo, and Dr. Noriko Yoshizawa of AIST for use of their facilities. Literature Cited (1) Xu, X.; Chen, C.; Qi, H.; He, R.; You, C.; Xiang, G. Development of coal combustion pollution control for SO2 and NOx in China. Fuel Process. Technol. 2000, 62, 153. (2) China Coal Industry Yearbook 2002; China Coal Information Research Institute: China, 2002. (3) Hu, G.; Dam-Johansen, K.; Wedel, S.; Hansen, P. J. Review of the direct sulfation reaction of limestone. Prog. Energy Combust. Sci. 2006, 32, 386. (4) Åmand, L.-E.; Leckner, B.; Dam-Johansen, K. Influence of SO2 on the NO/N2O chemistry in fluidized bed combustion. Fuel 1993, 72, 557.
(5) Dam-Johansen, K.; Østergaard, K. High-temperature reaction between sulphur dioxide and limestonesII. An improved experimental basis for a mathematical model. Chem. Eng. Sci. 1991, 46, 839. (6) Renedo, J. M.; Fernandez, J. Preparation, characterization, and calcium utilization of fly ash/Ca(OH) 2 sorbents for dry desulfurization at low temperature. Ind. Eng. Chem. Res. 2002, 41, 2412. (7) Jozewicz, W.; Chang, J. S.; Brna, T. G.; Sedman, C. B. Reactivation of solids from furnace injection of limestone for SO2 control. EnViron. Sci. Technol. 1987, 21, 664. (8) Ho, S. C.; Shih, M. S. Ca(OH) 2/fly ash sorbents for SO2 removal. Ind. Eng. Chem. Res. 1992, 31, 1130. (9) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Highly active absorbent for SO2 removal prepared form coal fly ash. Ind. Eng. Chem. Res. 1995, 34, 1404. (10) Davini, P. Investigation of the SO2 adsorption properties of Ca(OH) 2-fly ash systems. Fuel 1996, 75, 713. (11) Sanders, F. J.; Keener, C. T.; Wang, J. Heated fly ash/hydrated lime slurries for SO2 removal in spray dryer absorbers. Ind. Eng. Chem. Res. 1995, 34, 302. (12) Li, Y.; Nishioka, M.; Sadakata, M. High calcium utilization and gypsum formation for dry desulfurization process. Energy Fuels 1999, 13, 1015. (13) Shi, J.; Xu, Y. Estimation and forecasting of concrete debris amount in China. Resour., ConserV. Recycl. 2006, 49, 147. (14) Borgwardt, H. R. Sintering of nascent calcium oxide. Chem. Eng. Sci. 1989, 44, 53. (15) Yang, T. R.; Shen, S. M.; Steinberg, M. Fluidized-bed combustion of coal with lime additives: Catalytic sulfation of lime with iron compounds and coal ash. EnViron. Sci. Technol. 1978, 12, 915.
ReceiVed for reView June 19, 2008 ReVised manuscript receiVed August 30, 2008 Accepted September 05, 2008 IE8009654
