HCl Formation from RDF Pyrolysis and Combustion in a Spouting

Spouting-Moving Bed Reactor ... State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, .... on the formation of HCl in a fluidized bed ...
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Energy & Fuels 2002, 16, 608-614

HCl Formation from RDF Pyrolysis and Combustion in a Spouting-Moving Bed Reactor Zhiqi Wang, Haitao Huang, Haibin Li, Chuangzhi Wu, and Yong Chen* Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510070, China

Baoqing Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, China Received July 24, 2001. Revised Manuscript Received November 20, 2001

A reactor with two reaction zones (pyrolysis and combustion) was used to study the formation of HCl from PVC and NaCl and the dechlorination capacity of Ca(OH)2. Several types of RDF (Refused-Derived Fuels) containing PVC or NaCl and with Ca(OH)2 or without Ca(OH)2 were first prepared in our laboratory. Then the formation of HCl from different chlorine resource and the dechlorination capacity of Ca(OH)2 under different operating conditions were studied. The amount of HCl produced from PVC and NaCl was found to be quite different under the same operating condition. High temperature did not favor the dechlorination capacity of Ca(OH)2. Thermogravimetric studies revealed that CaCl2 has a poor thermal stability at high temperature. This may be the reason the dechlorination capacity of Ca(OH)2 decreases with increasing temperature.

1. Introduction The amount of Municipal Solid Wastes (MSW) is increasing dramatically in the world, so its disposal is becoming one of the major environmental issues. Thermal disposal methods including pyrolysis, gasification, and combustion offer distinct advantages over the traditional landfill approach since they provide maximum volume reduction and energy recovery. However, the concern about the pollutants of thermal disposal such as NOx, SOx, CO, HCl, dioxins, furans, and other unburned hydrocarbons has received increased attention from more and more countries around the world. Compared with incineration, a number of studies1-4 have indicated that emission of poisonous matters by suitable pyrolysis or gasification methods might be less than that of direct incineration. Emission controls may be easier for pyrolysis than for incineration due to reduced air flow rate, and lower temperature. Pretreatment of MSW to provide more uniform moisture content and waste composition appears to increase combustion stability and minimize pollutants from combustors.5 Refuse-derived fuels (RDF), made by drying, crushing, * Corresponding author. Telephone: 86-20-87759561. Fax: 86-2087608586. E-mail: [email protected]. (1) Li, A. M.; Li, X. D.; Li, S. Q.; Ren, U.; Shang, N.; Chi, Y.; Yan, J. H.; Cen, K. F. Energy 1999, 24, 209-218. (2) Avenell, C. S.; Sainz-Diaz, C. I.; Griffiths, A. J. Fuel 1996, 75, 1167-1174. (3) Cozzani, V; Nicolella, C.; Petarca, L.; Rovatti, M.; Toguotti, L. Ind. Eng. Chem. Res. 1995, 34, 2006-2020. (4) Buekens, A. G.; Schoeters, J. G. Conservation Recycling 1986, 9, 253-261. (5) Kiligroe, J. D.; Patrick Nelson, L.; Schindler, P. J.; Lanier, W. S. Combust. Sci. Technol. 1990, 74, 223-244.

and then compressing the combustible fraction of MSW into pellets, constitute a good material for pyrolysis or gasification, since they present several advantages of relatively constant density and size, uniform composition, higher heating value, and easy transport. Furthermore, chlorine and sulfur can be removed by adding additives during the molding process in thermal treatment. The chlorine in MSW can not only form HCl but also produce emissions of hazardous chlorinated organic compounds, especially the toxic polychlorinated dibenzop-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) during thermal destruction processing. HCl is a harmful acid gas and is one factor in acid rain formation. Nevertheless, the efficiency of the MSW-fired power generation is less than 15% because the steam condition is limited to avoid the high-temperature corrosion of the boiler tube caused by hydrochloric acid gas.6 This is much lower than the efficiency of coal-fired power generation. To improve the efficiency of power generation, removal of HCl before it reaches the surface of the heat exchanger is an important approach. Cabased additives such as lime or limestone have been considered as suitable materials to capture HCl. In general, most of the HCl is considered to be from organic chlorine thermal destruction, especially poly(vinyl chloride) (PVC) in MSW. Crosato-Arnaldi et al. found that PVC can give rise to HCl gas during the thermal process.7 Boettrner et al. analyzed some vinyl plastics(PVC) homopolymers using DTA and DTG and (6) Piao, G.; Aono, S.; Mori, S.; Deguchi, S.; Fujima, Y.; Kondoh, M.; Yamaguchi, M. Waste Manage. 1998, 18, 509-512.

10.1021/ef0101863 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002

HCl Formation from RDF Pyrolysis

found that polymers can release HCl at about 300 °C.8 Then most studies were concentrated on the dehydrochlorination and its kinetics model of PVC burning.9-11 Inorganic chloride such as NaCl, abundantly present in food trash, has been suggested to be a potential source for the formation of HCl during combustion. Azuma et al.12 classified municipal refuse into six kinds and analyzed the produced gas after burning them. Paper and garbage produced a considerable amount of HCl. Henriksson and Warnqvist13 studied the kinetics of formation of HCl by the reaction between NaCl and SO2, O2, and H2O. They found that the conversion of NaCl into HCl by reaction with SO2, O2, and H2O is rather slow at lower temperature. Uchida and Kamo14 studied the reaction kinetics of formation of HCl in municipal refuse incinerators. They found that some inorganic materials such as Al2O3 and SiO2 have a key role in the formation of HCl from NaCl. Only several engineering data of HCl emission and dehydrochlorination on pilot and full-scale incinerators have been presented. Halonen et al.15 studied the effect of inorganic and organic chlorine on the formation of highly chlorinated organic compounds during incineration. They found that more chlorinated hydrocarbons were formed when organic chlorine was the chlorine source compared with inorganic chlorine. Wey et al.16 studied the impact of organic and inorganic chlorides on the formation of HCl in a fluidized bed incinerator. They found that the amount of HCl generated from NaCl and PVC was quite different under various operating conditions and with different hydrogen sources additives. Liu et al.17 studied the chlorine behavior in a simulated fluidized bed combustion system. Piao et al.6 examined the HCl concentration in flue gas from combustion of RDF in a 30 cm × 30 cm bubbling type fluidized bed combustor. Spouting has been studied as an operation for contacting fluid and coarse particles (dp >1 mm). Spouted beds have been applied to a wide variety of thermal and chemical processes.18,19 A spout-fluid bed is a hybrid fluid-solid contacting bed in which spoutgas is introduced centrally through an orifice accompanied by auxiliary gas from a series of smaller holes in a surrounding distributor. It is believed that a spout-fluid bed might share some characteristics of a spout and a fluid bed and be very useful for handling agglomerating or sticky solids. So a spout-fluid bed may be suitable (7) Crosato-Arnaldi, A.; Palma, G.; Peggion, E.; Talamini, G. J. Polym. Sci. 1964, 8, 747-754. (8) Boettner, E. A.; Ball, G.; Weiss, B. J. Appl. Polym. Sci. 1969, 13, 377-391. (9) Abbas, K. B.; Sorvik, E. M. J. Appl. Polym. Sci. 1973, 19, 29913006. (10) Abbas, K. B.; Sorvic, E. M. J. Appl. Polym. Sci. 1973, 20, 23952406. (11) Carroll, W. F., Jr.; Hirschler, M. M.; Smith, G. F. J. Vinyl Technol. 1988, 10, 106-110. (12) Azuma, H.; Tahara, Y.; Kondo, K. Kogai to Taisaku 1978, 14, 1059-1605. (13) Henriksson, M.; Warnqvist, B. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 249-254. (14) Uchida, S.; Kamo, H.; Kubota, H.; Kanaya, K. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 144-149. (15) Halonen, I.; Tarhanen, J.; Ollikainen, S.; Ruokojarvi, P.; Tuppurainen, K.; Ruuskanen, J. Chemosphere 1994, 28, 2129-2138. (16) Wey, M.-Y.; Fang, T.-J. Environ. Int. 1995, 21, 423-431. (17) Liu, K.; Pan, W.-P.; Riley, J. T. Fuel 2000, 79, 1115-1124. (18) Arbib, H. A.; Levy, A. Can. J. Chem. Eng. 1982, 60, 528-531. (19) Lisboa, A. C. L.; Watkinson, A. P. Can. J. Chem. Eng. 1992, 70, 983-990.

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Figure 1. Schematic diagram of the pyrolysis and combustion reactor.

for RDF pyrolysis or gasification. A study by He et al.20 showed that the minimum total fluid flow rate for spouting with aeration (no fluidization in annulus) always exceeded the minimum spouting flow rate, but was smaller than the minimum fluidization flow rate. And the minimum total fluid flow rate for spoutfluidization was found to be equal to the minimum fluidization flow rate. Considering that a low oxygen level is expected for pyrolysis or gasification of RDF, and the density of RDF is low and not easy moving, horizontal auxiliary gas is introduced to improve the movements of RDF particles in a spouted bed, just like a spout-fluid bed, but no fluidization in the annulus. We call this a “spouting-moving bed”, which is proposed to pyrolyze RDF at lower temperature by partial combustion and then the pyrolysis gas products burn at high temperature in the combustor with secondary air. In this work, four types of RDF containing PVC or NaCl and with Ca(OH)2 or without Ca(OH)2 were prepared in our laboratory. Then the formation of HCl from different chlorine sources such as PVC and NaCl in a pyrolyzer and in flue gas were studied under various operating conditions. The dechlorination efficiency of Ca(OH)2 for PVC and NaCl at different operating conditions was also studied. 2. Experimental Section 2.1. Apparatus. A spouting-moving bed reactor, based on partial combustion to heat reactor and pyrolysis gas combustion was designed and constructed. As shown in Figure 1, the reactor is constituted of two sections. The lower section is a spouting-moving bed pyrolyzer section. The inner face is a ceramic tube (10 mm thickness, i.d. 100 mm ,and 800 mm height) that is not easy to be eroded by acid and alkaline matters. The outer face is a carbon steel tube (10 mm thickness). A 60° and 70 mm height casting conical base distributor in which 72 holes (i.d. 2 mm) are perforated horizontally and an air plenum is attached to the bottom of the cylindrical pyrolysis section, where the auxiliary gas flows into the pyrolyzer. Since the conical distributor is 12 mm thick, (20) He, Y.-L.; Lim, C. J.; Grace, J. R. Can. J. Chem. Eng. 1992, 70, 848-857.

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Table 1. Composition of Feedstock raw material proportion (wt %) paper PE PVC NaCl Ca(OH)2

RDF1

RDF2

RDF3

RDF4

46.0 50.4 3.6

46.0 51.5

46.0 47.2 3.6

46.0 48.3

2.5 3.2

2.5 3.2

Table 2. Properties of Feed RDF Materials materials moisture volatile fixed carbon ash carbon hydrogen oxygen nitrogen sulfur sodium chlorine calcium Ca/0.5Cl

RDF1

RDF2

RDF3

RDF4

Proximate Analysis (wt %) 2.71 3.43 2.67 86.93 85.19 85.01 6.75 6.12 6.44 3.61 5.26 5.88

3.34 82.26 5.61 8.79

Ultimate Analysis (wt %) 66.01 65.26 63.35 10.61 10.49 10.33 21.75 21.65 22.93 0.03 0.02 0.03 0.08 0.08 0.09 0.98 1.52 1.52 1.52 1.75 2

62.5 10.06 23.07 0.04 0.08 0.98 1.52 1.75 2

the holes perforated on it are just like small horizontal tube by 2 mm diameter. The spouting nozzle (Di ) 9 mm) is located in the center of the distributor, in which the spouting gas flows into the pyrolyzer. The upper section is a gas combustor, the inner is also a ceramic tube (i.d. 150 mm and 800 mm height) and the outer is a carbon steel tube. The connector to cascade the upper combustor and the lower pyrolyzer is made up of a truncated 60° cone attached to another 60° inverse truncated cone, where the secondary air is introduced. The connector is 200 mm high and the middle narrow cylinder is 40 mm in diameter and 50 mm in height. The total height of the reactor is 1870 mm (from spouting nozzle to the top of the combustor). Eight K-type thermocouples were set at different heights from the spouting nozzle inlet level (where Z ) 0) to measure the temperature. RDF was fed into the pyrolyzer through a rotary valve feeder, which was connected with a DC motor controller to regulate the RDF feeding rate. At the bottom of the combustor, an electric heater was installed to heat the gas and air. To heat the pyrolyzer to the desired temperature of RDF, another electric heater was employed to preheat the air. At the beginning of the experiment, the pyrolysis bed was first charged with 550 g of silica sand (particle size 0.9-1.6 mm) as bed material, which helped in stable spouting and keeping the bed temperature uniform, then preheated air was fed into the pyrolyzer to heat the pyrolyzer until the bed temperature reached 350-400 °C. During the warm-up phase, the maximum airflow rate was used. When the desired temperature was reached, the RDF was fed and airflow rate was controlled to the desired level. RDF began partial devolatilization and combustion in the pyrolyzer. At the same time, the electric heater at the bottom of the combustor was turned on, and secondary air was fed into combustor. When the mixture gas burned well, the electric heater was turned off. After the reactor operation reached steady state, sample gas was taken from the pyrolyzer and flue tube to detect HCl content. 2.2. Materials. In general, plastics and lignocellulosic materials are two main combustible components in MSW. In the present work, polyethylene and print paper were selected as two main combustible components in MSW to produce model RDF, which can keep the RDF composition constant. Four types of model RDF were produced with a screw extrusion machine equipped with an electric heater and a temperature controller. When the temperature reached about 100-120 °C, crushed paper and PE, and PVC (chlorine content: 42.22 wt%)

Figure 2. The equipment of sampling gas by HCl absorption. or NaCl with or without Ca(OH)2 in different proportions were fed into the machine, the sticky shape of the mixture was extruded from a hole and then cut into the desired length. The RDF size was about 6 mm in diameter and 6-10 mm in length. The composition of feedstock and the properties of the four types of RDF are listed in Tables 1 and 2, respectively. 2.3. HCl Analysis. The equipment of sampling HCl is shown in Figure 2. As shown in Figure 1, two positions (Z ) 78 cm and flue tube) are sampled for analysis of HCl; where Z ) 78 cm is considered as HCl content in pyrolysis gas and flue tube is considered as HCl content in flue gas. The gas sampling tube before gas flow in first silica gel container is kept above 100 °C to prevent condensation of water vapor. The sampling gas flows into two bottles containing 1000 mL of 0.1 N NaOH solution for collecting HCl by bubbling after flowing through the membrane filter. A gas meter, which can give accumulative total gas volume, was employed to record the sampling gas volume. The collected solution was filtered through membrane filter and then was ready to be analyzed by chloride ion selective electrode (96-17B ORION RESEARCH, INC. USA) and ion chromatography (IC) method. Considering that the pyrolysis gas volume is much less compared with the air fed into the reactor, the theoretical concentration of HCl in the pyrolyzer and in the flue gas are calculated, respectively, as follows:

Cp HCl ) Qt HCl/(Qa + Qs)

(1)

Cf HCl ) Qt HCl/(Qa + Qs + Qc)

(2)

where Qt HCl is total chlorine input, Qs is auxiliary air volume input, Qs is spouting air volume input, Qc is secondary air volume input, and the subscripts of p and f are in pyrolyzer and in flue gas, respectively. The HCl conversion rates of total chloride input without Ca(OH)2 in pyrolyzer and in flue gas are defined respectively as follows:

CRp ) (Cp mHCl/Cp HCl) × 100%

(3)

CRf ) (Cf mHCl/Cf HCl ) × 100%

(4)

where Cp mHCl is the measured concentration of HCl in the pyrolyzer without Ca(OH)2; Cf mHCl is the measured concentration of HCl in flue gas without Ca(OH)2. The dechlorination rate of Ca(OH)2 in the pyrolyzer and in flue gas are defined, respectively, as follows:

DCRp ) (Cp mHCl - Cp dHCl)/Cp mHCl × 100%

(5)

DCRf ) (Cf mHCl - Cf dHCl)/Cf mHCl × 100%

(6)

where Cp dHCl is the concentration of HCl in the pyrolyzer with Ca(OH)2, and Cf dHCl is the concentration of HCl in the pyrolyzer with Ca(OH)2.

3. Results and Discussion 3.1. Temperature Control. The temperature distribution has a great effect on the products of pyrolysis

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Table 3. Axial Temperature Distribution of the Reactor at Different Operating Conditions air feed rate (m3/h) Qs Qa Qc

-10

average temperature (°C) at different axial height (Z, cm) 6 10 50 78 115 145

175

RDF1

8 9 10 11 12

2 2 2 2 2

38 37 36 35 34

165 156 158 162 158

598 649 686 724 758

639 675 718 752 777

748 783 807 844 868

653 684 709 721 736

1273 1246 1187 1130 1142

1152 1125 1092 1085 1088

927 899 875 876 881

RDF2

8 9 10 11 12

2 2 2 2 2

37.5 36.5 35.6 34.5 33.5

161 164 159 161 156

592 643 678 705 112

637 671 725 763 288

750 776 804 837 1007

658 672 694 715 945

1259 1223 1159 1185 676

1141 1146 1097 1106 971

900 906 879 881 895

RDF3

8 9 10 11 12

2 2 2 2 2

36 35 34 33 32

162 158 161 164 160

602 644 677 717 756

631 666 713 745 771

743 767 799 838 859

645 669 697 712 725

1236 1238 1171 1206 1198

1115 1140 1023 1100 1087

902 915 864 889 875

RDF4

8 9 10 11

2 2 2 2

35 34 33 32

163 159 162 165

608 648 684 721

637 672 717 746

748 773 808 847

648 652 672 681

1267 1252 1256 1183

1184 1169 1172 1096

921 918 917 892

RDF (feed rate 4.4 kg/h)

Figure 3. Temperature variations at different axial height with the processing time during pyrolysis and combustion of RDF. Feed rate ) 3.0 kg/h, Qs ) 10m3/h, Qa ) 2 m3/h, Qc ) 30 m3/h.

and combustion, and is an important factor in evaluating the performance of the reactor. The temperature is dominated mainly by the amount of air flowing into the reactor and the feed rate of RDF, i.e., the air/fuel ratio. In this experimental device, spouting airflow rate and auxiliary airflow rate, and secondary airflow rate affect the pyrolyzer temperature and combustor temperature. Figure 3 shows the axial temperature distribution in the pyrolyzer and combustor under a typical operating condition with a kind of RDF without chloride.21 From Figure 3, one can see that about 20 min are needed for the temperature to be steady for the spouting-moving bed pyrolyzer from the beginning of RDF feeding. The thermocouple at Z ) -10 cm showed the spouting gas temperature before entering the pyrolyzer. To perform well, the spouting air was controlled by an electric preheater at about 160 °C (Z ) -10 cm) after RDF was fed into the bed and pyrolysis and combustion began. When the temperature of Z ) 78 cm reached about 500 °C, the electric heater fixed at the bottom of the combustor was turned on, and at the same time, secondary air was introduced, we could affirm that the (21) Wang, Z.; Huang, H.; Li, H.; Wu, C.; Chen, Y.; Li, B. Energy Fuels (in press).

Figure 4. Effect of air/fuel ratio on the pyrolyzer temperature. Qa ) 2 m3/h, feed rate ) 4.4 kg/h.

pyrolysis gas burned up by observing the temperature at Z ) 115 cm and Z ) 145 cm, then the electric heater was turned off. About 20 min later, the temperature of the combustor became steady. Table 3 summarizes the data of average temperature under different operating conditions. Adjusting the spouting air volume and tthe auxiliary air volume and the secondary air volume, respectively, can control the temperature profile of the whole reactor. For simplification, the reaction temperature in the pyrolyzer is defined as an average temperature of Z ) 6 cm, 10 cm, and 50 cm. Figure 4 shows the relationship of air/fuel ratio and the pyrolyzer temperature as auxiliary gas is at the same level. From Figure 4, we can see that RDF3 and RDF4 require a higher air/fuel ratio to reach a certain temperature than do RDF1 and RDF2. The possible reason is that RDF3 and RDF4 contain much inorganic material such as Ca(OH)2 and NaCl and those materials may absorb heat to decompose or vaporize. From Table 3, we can find that the temperature distribution of RDF2 containing sodium salt varies largely at high Qs. This phenomenon is caused by sodium salt melting and depositing on the sand particles at high temperature, which results in the sand particle not being able to spout and move. The same result was also found by other studies.22,23 For this reason, the (22) Manzoori, A. R.; Agarwal, P. K. Fuel 1994, 73, 563-568.

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Figure 5. The formation of HCl from PVC at different pyrolyzer temperatures in the pyrolyzer. RDF1: feed rate, 4.4 kg/h.

Figure 7. The HCl conversion rate of total chloride input from PVC at different pyrolyzer temperatures. RDF1: feed rate, 4.4 kg/h.

Figure 6. The formation of HCl from PVC at different pyrolyzer temperatures in flue gas. RDF1: feed rate, 4.4 kg/h.

Figure 8. The formation of HCl from NaCl at different pyrolyzer temperatures in the pyrolyzer. RDF2: feed rate, 4.4 kg/h.

experiment of the entry for Qs ) 12 m3/h for RDF4 was not been carried out. 3.2. Effect of the Pyrolyzer Temperature on HCl Formation from PVC. The profile of HCl formation from PVC at different pyrolyzer temperatures is shown in Figure 5. From Figure 5, we can see that the concentration of HCl in the pyrolyzer decreases slightly with increasing pyrolyzer temperature. But the concentration of HCl in flue gas has little change as shown in Figure 6. Since the temperature in the pyrolyzer was controlled by air/fuel ratio, higher temperature needs more air volume while fuel feed rate is maintained; then the concentration of HCl in the pyrolyzer would decrease with increasing air volume. The total air input (including Qa, Qs, and Qc) is at the same level and there is a little change of the combustor temperature at different experimental conditions as summarized in Table 3, so the concentration of HCl in flue gas changes very little. Figure 7 shows the HCl conversion rates of total chlorine input in the pyrolyzer and flue gas. The HCl conversion rate in pyrolyzer increases slightly with increasing pyrolyzer temperature, which means that relatively more HCl is formed with increasing pyrolysis temperature. The HCl conversion rate in flue gas still has little change under different experimental conditions for the above reason. From Figure 7, we can also find that the HCl conversion rate in flue gas is much higher than that in the pyrolyzer. This can be explained

if some pyrolysis products or some products of incomplete combustion may combined with HCl or Cl to form other chlorinated aromatic hydrocarbons or unknown compounds at relative lower temperature in the pyrolyzer, and those compounds cannot be detected as HCl. When the chlorinated hydrocarbons flow into the combustor, the much higher temperature in the combustor may break them and the HCl can be formed again. And some PVC that has not reacted completely may be further destroyed in the combustor to generate HCl. Figure 7 shows that about 65% of the total fed chlorine forms HCl in the pyrolyzer and eventually about 78% of the total fed chlorine forms HCl in flue gas. 3.3. Effect of the Pyrolyzer Temperature on HCl Formation from NaCl. Figures 8 and 9 display the HCl concentration formed from NaCl in the pyrolyzer and flue gas, respectively. The HCl concentration in the pyrolyzer does not have decreasing tendency with increasing pyrolyzer temperature despite more air volume fed into the pyrolyzer for higher temperature. The HCl concentration in flue gas increases obviously with increasing pyrolyzer temperature. Figure 10 shows the HCl conversion rates of total chloride input from NaCl in the pyrolyzer and flue gas at different pyrolyzer temperature. The HCl conversion rate of total chloride input from NaCl in the pyrolyzer increases sharply with increasing pyrolyzer temperature; and the HCl conversion rate of total chloride input from NaCl in flue gas also increases with increasing pyrolyzer temperatures. It seems that the pyrolyzer temperature has a dominant

(23) Manzoori, A. R.; Agarwal, P. K. Fuel 1993, 72, 1069-1075.

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Figure 9. The formation of HCl from NaCl at different pyrolyzer temperatures in flue gas. RDF2: feed rate, 4.4 kg/ h.

Figure 10. The HCl conversion rate of total chloride imput from NaCl at different pyrolyzer temperatures. RDF2; feed rate, 4.4 kg/h.

role in the formation of HCl from NaCl. The higher the pyrolyzer temperature is, the more HCl is formed. The reaction leading to the HCl formation from NaCl can be described as follows:13

2NaCl + H2O + mSiO2 ) Na2O‚mSiO2 + 2HCl (7) The SiO2 is from the sand in the spouting-moving bed pyrolyzer. Comparing the effect of the pyrolyzer temperature on the HCl conversion rate of total chloride input from PVC and NaCl, it is seen that the pyrolyzer temperature has more influence on the HCl formation from NaCl than that from PVC. And at the very high temperatures concerned, an influence of pyrolyzer temperature on PVC would not be expected. Compared with Figure 7, one can see that the HCl conversion rate of total chloride input from NaCl is much lower than that from PVC. Only about 30-40% of total feed chlorine give rise to HCl in the pyrolyzer and 40-50% of total feed chlorine give rise to HCl in flue gas, respectively. 3.4. HCl Capture by Ca(OH)2 for PVC. Ca-based additives such as calcium hydroxide have been considered as suitable materials to capture HCl by the ollowing reactions:

Ca(OH)2 + HCl ) CaCl2 + H2O

(8)

Ca(OH)2 ) CaO + CO2

(9)

CaO + 2HCl ) CaCl2 + H2O

(10)

Reaction 8 is dominant when the temperature is below 300 °C.24 Reaction 10 is considered to occur when CaO is formed by according to reaction 9. Dehydration of Ca(OH)2 occurs mainly above 400 °C.25 Figures 11 and 12 show the dechlorination rate of Ca(OH)2 and concentration of HCl from PVC in the pyrolyzer and in flue gas at different pyrolyzer temperatures, respectively. As the two figures show, up to 700 °C, the concentration of HCl increases and the dechlorination rate of Ca(OH)2 decreases with increasing pyrolyzer temperature for both in the pyrolyzer and in (24) Karlsson, H. G.; Klingspor, J.; Bjerle, L. J. Air Pollut. Control Assoc. 1981, 31, 1117-1123. (25) Hirabayashi, D.; Saito, Y.; Ozawa, S.; Matsuda, H. Proceedings of the 2nd International Symposium on Advanced Energy Conversion Systems and Related Technologies, December, Nagoya, Japan, 1998, 3-B-3, pp 254-255.

Figure 11. The dechlorination property of Ca(OH)2 for PVC in pyrolyzer. RDF3: feed rate, 4.4 kg/h.

Figure 12. The dechlorination property of Ca(OH)2 for PVC in flue gas. RDF3: feed rate, 4.4 kg/h.

flue gas. About 78% HCl and 68% HCl are removed by Ca(OH)2 in the pyrolyzer and in flue gas when the temperature is below 740 °C, respectively. The dechlorination rate of Ca(OH)2 in the pyrolyzer is rather higher than that in flue gas. It seems that the capture of chlorine by Ca(OH)2 mainly occurs at the pyrolyzer. 3.5. HCl Capture by Ca(OH)2 for NaCl. Figures 13 and 14 show the dechlorination rate of Ca(OH)2 and concentration of HCl from NaCl in the pyrolyzer and in flue gas at different pyrolyzer temperatures, respectively. The same tendency that the dechlorination rate of Ca(OH)2 decreases with increasing pyrolyzer temperature can be seen. The dechlorination rate of Ca(OH)2 in the pyrolyzer is also higher than that in flue gas. Compared with Figure 13 and 14, one can see that

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Figure 13. The dechlorination property of Ca(OH)2 for NaCl in pyrolyzer. RDF4: feed rate, 4.4 kg/h.

Wang et al.

Figure 15. The TG profiles of CaCl2 and fly ash.

10 can be considered as reversible reactions; a high temperature favors the CaCl2 decomposition reaction, which means the capacity of capturing HCl by Ca(OH)2 is lower at high temperature. On the basis of this consideration, we can also explain why the dechlorination rate of Ca(OH)2 in the pyrolyzer is much higher than that in flue gas. The reason possibly is that CaCl2 formed by reactions 8 and 10 in the pyrolyzer or in the combustor would be decomposed when they are in the combustor with very high-temperature just like the reaction of eq 7. 4. Conclusions

Figure 14. The dechlorination property of Ca(OH)2 for NaCl in flue gas. RDF4: feed rate, 4.4 kg/h.

the dechlorination rate of Ca(OH)2 for PVC is rather higher than that for NaCl. A possible reason is that sodium salt may be deposited on the surface of Ca(OH)2 or CaO particle that disturbs the reaction of HCl and Ca(OH)2 or CaO. Although some HCl formed in the pyrolyzer at higher temperature, there still is calcium without capturing HCl. Therefore the results imply that the higher temperature is not beneficial to the dechlorination capacity of Ca(OH)2. And other studies also showed that the capacity of Ca-based additive for capturing HCl is lower at high temperature.16,26 The high temperature may have unfavorably effects on reactions 8 and 10, which means that the CaCl2 has poor stability at high temperature. To test this hypothesis, a thermogravimetric study is carried out using a TG-DTA system (WCT-2, made by Beijing Optical Instrument Factory, China). About 10 mg of CaCl2 or fly ash of RDF3 collected from cyclone is loaded in the sample holder and then the sample is heated at a constant heating rate of 20 °C/ min; at the same time, the computer records the TG profile. The reaction atmospheres are nitrogen or air, and the flow rate is 30 mL/min. Figure 15 shows the results of the thermogravimetric study. From Figure 15, we can see that CaCl2 begins to decompose at about 750 °C in both air and nitrogen. Accordingly, the fly ash containing CaCl2 also has an obvious weight loss at about 750 °C. So reactions 8 and (26) Piao, Guilin.; Hakamada, K.; Kondoh, M.; Yamaguchi, M.; Yamazak, R.; Hatano, S.; Mori, S. Kagaku Kogaku Ronbunshu 2000, 26, 551-556.

The characterization of HCl formation from PVC and NaCl and the dechlorination capacity of Ca(OH)2 in a reactor, which consisted of a spouting-moving bed pyrolyzer and a gas combustor, were carried out. The following conclusions are drawn from this study. 1. Most feed chlorine from PVC (about 60-65%) generates HCl in the pyrolyzer at different temperatures; the pyrolyzer temperature has little effect on the formation of HCl from PVC. About 80% feed chlorine form HCl in flue gas after the combustor with much higher temperature. 2. About 30-40% feed chlorine from NaCl generates HCl in the pyrolyzer at different temperatures. The pyrolyzer temperature has a dominant role in the formation of HCl from NaCl; the HCl conversion rate of total chloride input increases sharply with increasing pyrolyzer temperature. Compared with PVC, a much lower concentration of HCl from NaCl was detected in flue gas. Only about 42-48% feed chlorine form HCl. 3. The reaction temperature plays a key role in the capture of chlorine both for PVC and NaCl. The dechlorination rate decreases rapidly with increasing temperature from 650 to 800 °C. The dechlorination rate of Ca(OH)2 in flue gas is much lower than that in the pyrolyzer. The dechlorination rate of Ca(OH)2 for PVC is much higher than that for NaCl at the same experimental condition. 4. The thermogravimetric study shows that CaCl2 has a poor thermal stability at high temperature, which may be the reason the dechlorination rate of Ca(OH)2 decreases sharply with increasing temperature. Acknowledgment. The authors thank the Chinese National Natural Science Foundation (Project No. 29876041). EF0101863