Investigation on the Control of Agglomeration during Fluidized-Bed

However, when the river sand was used as the bed material, the agglomeration hardly occurs at 773−873 K, but the bed failures at 973 K when salts mi...
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Energy Fuels 2009, 23, 4304–4310 Published on Web 08/25/2009

: DOI:10.1021/ef900340a

Investigation on the Control of Agglomeration during Fluidized-Bed Incineration of Wastewater Containing Alkali Metal Salts Rushan Bie,† Shiyuan Li,*,‡ Ying Zhao,† and Lidan Yang† †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China, and ‡Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China Received April 16, 2009. Revised Manuscript Received July 2, 2009

The role of alkali metal salts in causing agglomeration and defluidization during incineration of wastewater, which contains NaCl, Na2CO3, Na2SO4, or their mixtures, was investigated at 773-973 K in a fluidized bed. Experimental results indicated that the reactions between Na2CO3 or Na2SO4 and quartz sand (or river sand) do not occur at 773-973 K, while NaCl begins to react with quartz sand to produce Na2SiO3 at 823 K. At a bed temperature greater than or equal to 873 K, bed agglomeration is formed when wastewater contains NaCl or its mixture with other alkali metal salts using quartz sand as bed material. However, when the river sand was used as the bed material, the agglomeration hardly occurs at 773-873 K, but the bed failures at 973 K when salts mixtures (NaCl-Na2CO3, NaCl-Na2SO4, or NaClNa2CO3-Na2SO4) are formed in bed material. The degree of agglomeration increases with the increase of temperature and decreases with the increase of melting points of salts mixtures. It has been found that bed agglomeration and defluidization can be eliminated efficiently at 873-973 K by adding additives such as kaolin or CaO.

In addition, the mixtures of these alkali metal salts will form low-melting point eutectics, leading the particles to cohere together when the operating temperature exceeds their melting points. Agglomeration changes the fluidization characteristics, such as minimum fluidization velocity, bubble size, and bubble frequency.4,5 In the worst cases, defluidization will occur, leading to unscheduled shutdown. The investigations of agglomeration have tended to focus on coal, solid waste, sewage sludge, and biomass fuels combustion.6-8 Many research works have been carried out on the prediction of FBC failure due to ash-related problems. It is generally believed that the elements that affect agglomeration and defluidization are alkalis, earth alkali metals, silicon, sulfur, vanadium, chlorine, and nickel.6-11 When the alkali metal compounds with low-melting points form liquid bridges, they cause agglomeration of particles in the bed

1. Introduction Fluidized-bed combustion (FBC) technology has been widely used to dispose municipal solid waste, biomass, sewage sludge, and organic wastewater. It offers advantages such as high combustion efficiency, low emission levels, and good fuel flexibility.1 However, these wastes have high alkali content in general, and bed agglomeration becomes one of the operational problems. The wastewater from chemical industries usually contains sodium compounds, which will be neutralized by acid before incineration, forming sodium salts such as NaCl and Na2SO4 or their mixtures, or forming Na2CO3 after incineration. These alkali metal salts can react with SiO2 (using quartz sand as a bed material) to produce the viscous sodium silicates with low-melting points in a fluidized bed. These molten matters function as glue for the bed material to stick together, forming bed material agglomeration causing bed defluidization. The reactions between NaCl, Na2CO3, Na2SO4, and SiO2 are as follows:2,3 2NaClþnSiO2 þH2 O ¼ Na2 O 3 nSiO2 þ2HCl

ð1Þ

Na2 CO3 þnSiO2 ¼ Na2 O 3 nSiO2 þCO2

ð2Þ

Na2 SO4 þnSiO2 ¼ Na2 O 3 nSiO2 þSO2 þ0:5O2

ð3Þ

(4) Tardos, G.; Pfeffer, R. Chemical reaction induced agglomeration and defluidization of fluidized beds. Powder Technol. 1995, 85, 29–35. (5) Lin, C. L.; Wey, M. Y.; You, S. D. The effect of particle size distribution on minimum fluidization velocity at high temperature. Powder Technol. 2002, 126, 297–301. (6) Yan, R.; Liang, D. T.; Laursen, K.; Li, Y.; Tsen, L.; Tay, J. H. Formation of bed agglomeration in a fluidized multi-waste incinerator. Fuel 2003, 82, 843–851. (7) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Agglomeration problems during fluidized bed gasification of olive-oil residue: Evaluation of fractionation and leaching as pre-treatment. Fuel 2003, 82, 1261–1270. (8) Ninomiya, Y.; Zhang, L.; Sakano, T.; Kannaoka, C.; Masui, M. Tranformation of mineral and emission of particulate matters during cocombustion of coal with sewage sludge. Fuel 2004, 83, 751–764. (9) Conn, R. E. Laboratory techniques for evaluating ash agglomeration potential in petroleum coke fired circulating fluidized bed combustors. Fuel Process. Technol. 1995, 44, 95–103. (10) Moilanen, A., Nieminen, M., Sipila, K., Kurkela, E. Ash behaviour in thermal fluidized-bed conversion processes of woody and herbaceous biomass; VTT Energy: Espoo, Finland, 1996. (11) Lin, C. L.; Wey, M. Y. The effect of mineral compositions of waste and operating conditions on particle agglomeration/defluidization during incineration. Fuel 2004, 84, 2335–2343.

*Corresponding author. Tel.: þ861082543055. Fax: þ861082543119. E-mail address: [email protected]. (1) Linak, W. P.; Wendt, J. Toxic metal emissions from incineration: Mechanisms and control. Prog. Energy Combust. Sci. 1993, 19, 145–185. (2) Acharya, P. Process challenges and evaluation of bed agglomeration in a circulating bed combustion system incinerating red water. Environ. Prog. 1997, 16, 54–64. (3) Becker, K. P.; Wall, C. J. Fluid bed incineration of wastes. Chem. Eng. Prog. 1976, 72, 61–68. r 2009 American Chemical Society

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Table 1. Chemical Composition of the Organic Wastewaters no. 1 2 3 4 5 6

salts composition (molar ratio) Na2SO4 Na2CO3 NaCl 38% NaCl þ 62% Na2CO3 35% NaCl þ 65% Na2SO4 50% NaCl þ 26% Na2SO4 þ 24% Na2CO3

weight of salts (%)

weight of acetone in wastewater (%)

5 5 5 NaCl:5 NaCl:5 NaCl:5

10% C3H6O 10% C3H6O 10% C3H6O 10% C3H6O 10% C3H6O 10% C3H6O

salts melting point (K) 1157 1124 1074 906 896 885

Table 2. XRF Analyses of the Three Materials

materials or they are released to the gas phase in which they condense and deposit on the heat- transferring surface or the flue gas channel. The control methodologies for particle agglomeration and bed defluidization during fluidized-bed combustion include the use of mineral additives, alternative bed materials, pretreatment of fuels, and the use of lower bed temperatures. Vuthaluru et al.12 evaluated the effectiveness of three control techniques and their control mechanisms in the remediation of ash-related problems during fluidized bed combustion of low quality coals. Acharya2 studied the bed agglomeration in a circulating fluidized bed combustion system incinerating red water. Results indicated that the bed agglomeration was caused by the low melting point of salts and could be controlled by additives. The use of additives to control agglomeration and defluidization was investigated by other researchers.13-15 Bie et al.16 put forward a new incineration system to solve the bed agglomeration by using a low temperature (around 773-973 K) fluidized bed together with a high temperature (around 1123-1223 K) circulating fluidized bed (CFB) in series. The wastewater containing alkali metal salts are first incinerated in a low temperature fluidized bed, in which the bed temperature is less than the melting points of the salts and the reaction temperature of salts with bed material. This assures that the bed agglomeration and defluidization problems are solved. In addition, the operating temperature around 773-973 K is beneficial to reduce HCl, which is an emission produced from organic chlorine, efficiently by using sorbent CaO.17 The combustible substances from the low temperature fluidized bed enter the dilute phase of the high temperature CFB in order to be burned out completely. Coal can be used as auxiliary fuel which will reduce the operating cost, as this currently is the most cost efficient fuel resource in China. In this paper, we will test and verify the new incineration system by whether the bed agglomeration can be suppressed or not in the low temperature fluidized bed. If bed agglomeration would still occur, we will investigate the mechanism of

elemental composition (wt %) O Na Mg Al Si P Fe S Cl K Ca Ti Mn

river sand

quartz sand

kaolin

52.207 2.246 0.060 6.087 35.477 0.023 0.561 0.006 0.014 2.450 0.679 0.057 0.019

55.901 0.025

52.948

0.187 43.813 0.032 0.003 0.030 0.005

0.192 14.903 15.033 0.110 0.004 0.027 14.714 0.161

agglomeration caused by NaCl, Na2CO3, Na2SO4, or their mixtures by X-ray diffraction (XRD) and scanning electron microscopy energy dispersive X-ray (SEM/EDX). Four kinds of additives will be applied to eliminate the bed agglomeration, and the evaluation of the additives will be discussed. 2. Experimental Apparatus and Procedure The work described in this paper was performed in a benchscale bubbling fluidized bed, which consists of an electrically heated column with a height of 600 mm and an inside diameter of 40 mm. The experimental apparatus is similar to that in our previous work.18 An air compressor supplies combustion air at a feeding rate of 0.19 N 3 m3 3 h-1. The organic wastewater containing sodium salts are sprayed into a dense bed by a precision pump with an average feeding rate of 6.4 g 3 min-1. To simulate the real situation of wastewater that contains alkali metal salts and at the same time to avoid the influence of other elements, six kinds of organic solution containing sodium salts were used in the experiments, their compositions are shown in Table 1 as well as the melting points of the salts and their mixtures. Two kinds of bed material with a size of 0.2-0.355 mm and four kinds of additives, namely CaO, Fe2O3, Al2O3, and kaolin, were adopted in the experiments for studying the effect of the additives on the bed agglomeration. Among them, CaO, Fe2O3, and Al2O3 are in analytical purity with diameters below 0.125 mm, and kaolin’s composition is shown in Table 2. The analysis of XRD on kaolin is shown in Figure 1; it shows that kaolin consists of Al2O3 3 2SiO2 3 2H2O and CaCO3. The X-ray fluorescence (XRF) analyses of the two bed materials are given in Table 2. The main component of the quartz sand is silicon dioxide (SiO2), while the components of the river sand are SiO2, Al2O3 3 2SiO2 3 2H2O, and NaSi3AlO8 according to the analyses of XRD as shown in Figure 2. The experiment was divided into two sections. One focused on the formation of bed agglomeration during the incineration of six kinds of organic wastewaters using two different bed materials at 773, 823, 873, and 973 K. The other focused on the control of bed agglomeration by adding additives and evaluating the capacities of different additives. In the first part test run, the experiment stopped after the fluidization was kept stable for 3 h. However,

(12) Vuthaluru, H. B.; Linjewile, T. M.; Zhang, D. K.; Manzoori, A. R. Investigations into the control of agglomeration and defluidisation during fluidised-bed combustion of low-rank coals. Fuel 1999, 78, 419–425. (13) Linjewile, T. M., Manzoori, A. K. Role of additives in controlling agglomeration and defluidisation during fluidized bed combustion of high-sodium, high-sulphur low-rank coal. Proceedings of the Engineering Foundation Conference “Impact of Mineral Impurities in solid fuel combustion”, Hawaii, November 1997. (14) Li, X.; Lv, H.; Xu, M.; Ma, J.; Yan, J.; Cen, K. Agglomeration characteristics in fluidized bed incineration of organic condensed wastewater. J. Chem. Ind. Eng. (China) 2005, 56, 2166–2171. (15) Bie, R. S.; Yang, L. D.; Zhou, D. Influence of additives on bed sintering in fluidized bed incinerator treating waste salty liquid. J. Chem. Ind. Eng. (China) 2002, 53, 1253–1259. (16) Bie, R. S., Li, S. Y., Fan, Q. X., Lv, X. R. The Fluidized Bed Incinerator with Double Temperatures and Double Beds on Gasification and Oxidation for Organic Wastewater with High Concentration. Chinese invention patent, code ZL 2004 1 0044168.9, 2006. (17) Li, S. Y.; Bie, R. S.; Lv, X. R. Reaction kinetics of Ca-based sorbent with HCl in fluidized bed. J. Chem. Ind. Eng. (China) 2005, 56, 318–323.

(18) Bie, R. S.; Li, S. Y.; Yang, L. D. Reaction mechanism of CaO with HCl in incineration of wastewater in fluidized bed. Chem. Eng. Sci. 2005, 60, 609–616.

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823, 873, and 973 K, the gaseous HCl was detected in the flue gas, and the HCl concentration increased when the temperature increased. This means that the reaction in eq 1 takes place and is accelerated by the increasing of temperature. At a bed temperature equal to or greater than 873 K, agglomeration

once the defluidization caused by bed agglomeration occurred during the test, the fluidized bed would be shut down immediately. We define the time of defluidization occurring as when the bed temperature increases and the differential pressure decreases suddenly. The flue gas from the furnace passed through the distilled water, and the pH meter was used to obtain acid gas concentration change trend. Several instrumental approaches (i.e., XRF, XRD, and SEM/EDX) were applied to identify the bed materials and agglomerates formed in the tests.

3. Results and Discussions 3.1. XRD Analysis of the Agglomerates or Bed Material. No agglomerate was observed when the organic wastewaters, containing Na2SO4 or Na2CO3 (wastewater sample nos. 1 and 2), were incinerated in a temperature range of 773-973 K, using quartz sand as a bed material. The results of XRD analyses of the bed material at 973 K are shown in Figures 3 and 4. From the analyses, it can de deduced that Na2SO4 and Na2CO3 cannot react with SiO2, and the temperature range is also far below their melting points; therefore, the agglomeration does not occur at 773-973 K for the incineration of organic wastewater containing only Na2SO4 or Na2CO3. The wastewaters containing NaCl (sample nos. 3-6) were incinerated at 773-973 K with quartz sand as the bed material. The results show that at 773 K, the bed is fluidized very well and no HCl emission was detected. However, at

Figure 3. XRD of bed material sampled at 973 K (Na2SO4; quartz sand as bed material).

Figure 4. XRD of bed material sampled at 973 K (Na2CO3; quartz sand as bed material).

Figure 1. XRD of the kaolin.

Figure 2. XRD of two bed materials: (a) quartz sand as bed material; (b) river sand as bed material.

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Figure 5. XRD of agglomerate sampled at 973 K (quartz sand as bed material).

Al2 O3 3 2SiO2 þNaClþH2 O f 2HClþNaSi3 AlO8

was observed. From XRD analyses of agglomerates formed at 973 K, the new substance Na2SiO3 is found as shown in Figure 5. It implies that NaCl begins to react with SiO2 to produce Na2SiO3, which is different from Na2Si3O7 obtained by Li et al.14 in the high temperature range of 1123-1223 K. It is necessary to emphasize in particular that Na2SiO3 was not detected by XRD when the test run was carried out with sample no. 3. The reason for this was, perhaps, that the amount of reaction product of eq 1 was too small to be detected by XRD. The results of using river sand as the bed material are similar to the quartz sand for sample nos. 1 and 2 at 773, 873, and 973 K, but it is quite different for sample Nos. 3-6. First, the gaseous HCl was found at a temperature greater than or equal to 773 K. The foregoing experimental results have shown that NaCl cannot react with SiO2 at 773 K, and from the XRD analyses results at 773-973 K. We find that Al2O3 3 2SiO2 3 2H2O which is a component of river sand was not detected in the bed material. XRD of the agglomerate sampled at 973 K is shown in Figure 6. A logical explanation for this result is that Al2O3 3 2SiO2 3 2H2O lost crystal water during a period of increasing temperature at first, then NaCl reacted with Al2O3 3 2SiO2 to produce HCl and NaSi3AlO8 as shown in eqs 4 and 5. When the temperature is greater than or equal to 773 K, reactions 1 and 6 also take place. ð4Þ Al2 O3 3 2SiO2 3 2H2 O f Al2 O3 3 2SiO2 þ2H2 O

Al2 O3 3 2SiO2 þNa2 O 3 SiO2 f NaSi3 AlO8

ð5Þ ð6Þ

For the incineration of sample no. 3 with river sand as the bed material, the bed was in good fluidization at 773-973 K. For sample nos. 4-6, the bed was fluidized very well at 773-873 K, whereas it failed at 973 K. The agglomerates with a size of 1-3 mm were found in the bed material in all test runs; the XRD analyses of agglomerates sampled at 973 K are shown in Figure 6. The results show that the detected products include original salts and NaSi3AlO8, excluding Na2SiO3. These indicate that the reaction rate in eq 1 is less than that in eq 5, and at the same time, reaction 6 also occurs at the same time. Good fluidization is attributed to the reaction explained in eqs 5 and 6 which suppresses agglomeration, because the product albite (NaSi3AlO8) is a sodium-aluminum-silicate with a melting point of 1381 K. However, the bed agglomeration occurs for sample nos. 4-6 at 973 K due to the formation of sodium salt eutectics whose melting points are lower than 973 K. In the real case, the wastewater contains other elements such as Mg, Ca, K, etc. besides Na, and the melting point of salt eutectics are perhaps even lower. Therefore, the influence of agglomeration in the first fluidized bed is more serious. 4307

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Figure 6. XRD of agglomerate sampled at 973 K (river sand as bed material).

3.2. SEM/EDX Analysis of the Agglomerates. From the foregoing conclusions, the incineration of the organic wastewaters does not result in agglomeration (sample nos. 1-6) at 773 K, regardless of bed material choice (quartz sand or river sand). However, at a temperature greater than or equal to 873 K, agglomeration occurred when quartz sand was used as the bed material. It was found that some bed material continuously covered the furnace wall during the experiment. Moreover, agglomeration was facilitated by the increase of temperature. Figure 7 shows the results of SEM/ EDX analysis of the agglomerate sampled from the incineration of wastewater no. 3, which only contains NaCl with quartz sand as bed material at 973 K. The SEM shows that the bonding between the quartz particles is loose and porous. However, it is clear that the reaction takes place between quartz particles and NaCl to form a bridge. EDX analysis shows that the elements of the bridge are Na, Si, and Cl, which suggests that the bridge consists of Na2SiO3 and some crystallized NaCl. Nevertheless, Na2SiO3 has not been found in the XRD analysis of the agglomerate; probably, the amount of Na2SiO3 is too small to be detected at low temperature. In addition, the test temperature does not reach the melting point of NaCl. Therefore, the bonding between the particles is not strong. With the melting points of the sodium salts decreasing, the defluidization times decrease at 973 K. When the operation temperature exceeds their melting points, these sodium salts start to change into a liquid phase, easily flowing to the

Figure 7. SEM/EDX results of agglomerate sampled at 973 K (NaCl; quartz sand): (a) 150, (b) 300.

surface of particles to cause bed agglomeration. Figure 8 shows the results of SEM/EDX analysis of the agglomerate produced from the incineration of no. 4 wastewater at 973 K. From the photographs, it is very clear that melting eutectics 4308

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Figure 10. SEM/EDX results of agglomerate sample at 973 K with kaolin added (NaCl/Na2CO3/Na2SO4 = 0.5/0.24/0.26; quartz sand).

3.3. Evaluation of Various Additives. The additives are broadly used to control bed agglomeration and defluidization by many researchers. The basic principle is to limit the formation of low melting point matter by modifying the characteristics of the ash deposited on bed particles. From foregoing experimental results, agglomeration and defluidization were found for the incineration of wastewaters containing NaCl with quartz sand as the bed material at temperatures exceeding 873 K. Therefore, in the second section of experiments, four kinds of additives, i.e. CaO, Fe2O3, Al2O3, and kaolin, were added in the bed material (quartz sand) respectively and mixed evenly before each test run. Incineration of wastewater containing NaCl (no. 6) was carried out at a bed temperature of 973 K with additives of CaO, Fe2O3, Al2O3, and kaolin. The additives were very effective in eliminating bed agglomeration and defluidization. Figure 10 showed the results of SEM/EDX analysis of the bed material for adding the kaolin. No agglomeration was found for any additives during the test run, and the quartz sand is wrapped up by a coating. However, a Na-Al-silicate with a high melting point has not been found in the XRD analysis. The function of other additives is similar. This phenomenon can be explained by the following reasons. One is the amount of sodium silicate produced by the reaction between quartz sand and NaCl which is very small at low temperature, so the high melting point substance produced by the reaction of additive with sodium silicate is also small. The amount is probably below the detection limit of XRD, which is 7% (weight percentage). The other is attributed to the “physics effect”, the surface of quartz sand is coated by additives preventing the formation of a liquid bridge. The evaluation of the four kinds of additives is carried out at 973 K. The term p is defined as the molar ratio of the minimum required quantity of the additives to the total amount of Na in the wastewater. The four additives with the same amount are mixed evenly with bed material respectively. The value of p can be computed by the amount of the additive and the amount of Na which is obtained from the defluidization time and the feeding rate of sodium salt solution. The comparison of p for different additives is shown in Figure 11. From the experimental results, it can be concluded that the value of p increases with a decreasing melting point of the sodium salts eutectic. However, the maximum value of p is only about 1.2 when the melting point of the eutectic of three sodium salts (no. 6 in Table.1) is the

Figure 8. SEM/EDX results of agglomerate sample at 973 K (NaCl/ Na2CO3 = 0.38/0.62; quartz sand): (a) 300, (b) 2000.

Figure 9. SEM/EDX results of agglomerate sample at 973 K (NaCl/ Na2CO3/Na2SO4 = 0.5/0.24/0.26): (a) quartz sand, (b) river sand.

have covered the surface of quartz sand, and the bonding between particles is very strong. It is suggested that the low melting point eutectics of sodium salts might be the main route leading to the agglomerate formation in the fluidized bed, since a quite low operation temperature was used (773973 K). The results of SEM/EDX analysis of the agglomerate for wastewater no. 6 also show the surface of quartz sand covered by molten eutectics, as shown in Figure 9. Because the melting point of the salts mixture is only 885 K, as shown in Table 1, which is less than the operation temperature 973 K, therefore, a large amount of liquid-phase salts eutectics migrate easily to the surrounding particles making the bed material adhesive on the one hand. On the other hand, it reacts with SiO2 to generate Na2SiO3 to form a gluelike material. Even though the river sand was used for bed material, bed agglomeration was also found during the experiments. 4309

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Figure 11. Comparison of p for four additives when the organic contains different alkali metal salts (973 K; quartz sand).

lowest. Therefore, at 973 K, the additives will eliminate bed agglomeration efficiently. Among the additives investigated, CaO and kaolin are the first choice in practice because of their low cost. Considering the capacity of CaO for binding gaseous HCl produced from incineration with a relatively high percentage in the range of 773-973 K,19 CaO is the preferred additive for being used as a sorbent to control HCl emission.

when the wastewater contains NaCl or its mixture with other sodium salts using quartz sand as bed material. As the river sand was used as bed material, the agglomeration does not happen at 773-873 K for wastewater containing six kinds of salts, but the bed fails at 973 K when salts mixtures (NaCl-Na2CO3, NaCl-Na2SO4, or NaCl-Na2CO3-Na2SO4) are formed in bed material. Increasing the temperature facilitates agglomeration. In addition, the extent of agglomeration increases with the decreasing melting point of the salts mixture. From the SEM/EDX analysis, it has been concluded that agglomeration and defluidization during incineration at lower temperatures is mainly ascribed to the lower melting point of the eutectic of alkali metal salts. Four kinds of additives are effective in controlling agglomeration at a temperature of 773-973 K. CaO is more suitable in practice considering the low costs and the binding capacity with gaseous HCl.

4. Conclusion The mechanism of agglomeration and defluidization, caused by sodium salts in wastewater, has been investigated in the low temperature range of 773-973 K in a fluidized bed. Experimental results indicate that the reactions between Na2CO3 or Na2SO4 and quartz sand (or river sand) do not occur at 773-973 K, while NaCl begins to react with quartz sand (SiO2) to produce Na2SiO3 at 823 K. At the temperature greater than or equal to 873 K, bed agglomeration is found

Acknowledgment. The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Project Code: 50706055) and the Natural Science Foundation of Heilongjiang Province (B0317) of China.

(19) Li, S. Y.; Bie, R. S. Modeling the reaction of gaseous HCl with CaO in fluidized bed. Chem. Eng. Sci. 2006, 61, 5468–5475.

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