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Energy & Fuels 2002, 16, 136-142
Pyrolysis and Combustion of Refuse-Derived Fuels 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 April 23, 2001. Revised Manuscript Received August 20, 2001
A reactor with two reaction zones (pyrolyzer and combustor) is developed, which is proposed for the thermal disposal of solid wastes, especially RDFs (refuse-derived fuels). A model RDF was used to evaluate the performance of the reactor. The RDF is continuously fed to the spoutingmoving bed pyrolyzer in which the RDF partially combusts to supply heat for pyrolysis of the RDF at low temperature, then the pyrolysis product burns with secondary air at high temperature in the upper combustor. A special connector cascades the two parts to a vertical configuration, where secondary air is introduced. The effect of operating parameters, such as spouting air and auxiliary air, as well as secondary air flow rate on the pyrolyzer temperature, pyrolysis gas composition at different positions, combustor temperature, and emissions of CO and NOx in flue gas, are studied.
1. Introduction Municipal solid waste (MSW) disposal is a serious global environmental issue. Thermal disposal methods, such as incineration, pyrolysis, and gasification, offer some benefits over conventional means of landfill: energy recovery and reduction of the volume and quantity of waste. 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 are 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 (RDFs), made by drying, crushing, 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. * To whom correspondence should be addressed. 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. Conserv. Recycl. 1986, 9, 253261. (5) Kiligroe, J. D.; Patrick Nelson, L.; Schindler, P. J.; Steven Lanier, W. Combust. Sci. Technol. 1990, 74, 223-244.
Much work has been done concerning the gasification and pyrolysis of solid waste. A study on the steam gasification of solid waste in a bench scale installation was done by Braekman-Danheux et al.6 They found that steam gasification of solid waste produced a synthesis gas with good calorific value, and some toxic species can be trapped in the ashes in a nonleachable form if kaolinite is added. Li et al.1 have studied the characteristics of MSW pyrolysis in a rotary kiln by external electric heat; Avenell et al.2 have studied solid waste pyrolysis in a so-called flaming pyrolyzer which was autothermal and a batch fixed-bed pyrolyzer. The volatiles’ release rates and temperatures and products of single-particle RDF pyrolysis were studied by Lai et al.7 Piao et al.8 have investigated the combustion of RDF in a bubble fluidized bed; their results were based on the RDF being combusted directly. 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.9-11 A spout-fluid bed is a hybrid fluid-solid contacting bed in which spoutgas is introduced centrally through orifice accompanied by auxil(6) Braekman-Danheux, C.; D’haeyere, A.; Fontana, A.; Laurent, P. Fuel 1998, 77, 55-59. (7) Lai, W.; Krieger-Brockett, B. Combust. Sci. Technol. 1992, 85, 133-149. (8) Piao, G.; Aono, S.; Kondoh, M.; Yamazaki, R.; Mori, S. Waste Manage. 2000, 20, 443-447. (9) Sue-A-Quan, T. A.; Watkinson, A. P.; Gaikwad, R. P.; Lim, C. J.; Ferris, B. R. Fuel Process. Technol. 1991, 27, 67-81. (10) Arbib, H. A.; Levy, A. Can. J. Chem. Eng. 1982, 60, 528-531. (11) Lisboa, A. C. L.; Watkinson, A. P. Can. J. Chem. Eng. 1992, 70, 983-990.
10.1021/ef0101006 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/16/2001
Pyrolysis and Combustion of RDFs
Figure 1. Schematic diagram of pyrolysis and combustion reactor.
iary 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. Therefore, a spout-fluid bed may be suitable for RDF pyrolysis or gasification. A study by He et al.12 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 low oxygen level is expected for pyrolysis or gasification of RDF, and that the density of RDF is low and that RDF particles are not easily moved, a horizontal auxiliary gas is introduced to improve the movements of RDF particles in a spouted bed, just as in a spout-fluid bed, but with no fluidization in the annulus. We call this apparatus a “spouting-moving bed”, which is proposed to pyrolyze the RDF by partial combustion. At present work, a reactor with a spouting-moving bed to pyrolyze RDF, based on gas combustion and autothermal regulation, was developed. The model RDF produced in our laboratory was used to evaluate the performance of the reactor. The temperature distribution, gas composition distribution, as well as emission of NOx and CO at different operating conditions were 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 Figure 1 shows, the reactor is constituted of two sections. The lower section is spouting-moving bed pyrolyzer section. The inner face is ceramic tube (10 mm thickness, i.d. 100 mm, and 800 mm in height) which is not easily eroded by acid and alkaline materials. The outer face is 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 are attached to the bottom of the cylindrical pyrolysis section, where the auxiliary gas flows into the (12) He, Y.-L.; Lim, C. J.; Grace, J. R. Can. J. Chem. Eng. 1992, 70, 848-857.
Energy & Fuels, Vol. 16, No. 1, 2002 137 pyrolyzer. Since the conical distributor is 12 mm thick, 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 the gas combustor, the inner side of which is also a ceramic tube (i.d. 150 mm and 800 mm height) and the outer is 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. A 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 product gas to ignition temperature as soon as possible. To heat the pyrolyzer to ignition 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 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 pyrolyzer to heat pyrolyzer until the bed temperature reached 350-400 °C. During the warm-up phase, the maximum air flow rate was used. When the desired temperature is reached, the RDF was fed and the air flow rate was controlled to the desired level. A RDF began partial devolatilization and combustion in the pyrolyzer. At the same time, the electric heater at the bottom of combustor was turned on, and secondary air was fed into combustor. When the mixture gas burned well, the electric heater was turned off. When the reactor operation reached steady state, the product gases taken at the pyrolyzer and combustor as well as flue gas were analyzed. 2.2. Gas Analysis. A gas chromatography (GC-20B, Shimadzu, Japan) was employed to determine the O2, CO, CO2, N2, H2, CH4, C2H4, and C3H6 contents in the product gas from pyrolyzer and combustor. A portable combustion analyzer (KM-9003(II), Nanjing Analytical Instruments Factory, China) was used to detect the CO, O2, and CO2 in the flue gas. Another portable commercial NOx analyzer (Nonoxor II, Bacharach, Inc., Pittsburgh, PA) was used to determine the NOx emission of the flue gas. Because of the content of sulfur in the materials used in the present work is very low, SOx was not considered. 2.3. Materials. In general, plastics and lignocellulosic materials are two main combustible components in any MSW. In the present work, polyethylene and print paper were selected as two main combustible components of any MSW to produce the model RDF, which can keep the RDF composition constant. The model RDF was 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 (polyethylene) in 5.5:4.5 proportion were fed into the machine, and the sticky shape of the mixture was extruded from a hole and cut into the desired length. The RDF size was about 6 mm in diameter and 6-10 mm in length. The properties of PE and print paper, and those of the RDF, are listed in Table 1.
3. Results and Discussion 3.1. Temperature Distribution. The temperature distribution has a large effect on the production of the products of pyrolysis and of combustion, and is an important factor to evaluate the performance of the
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Table 1. Properties of Feed Materials proximate analysis (wt %) materials PE prints RDF
ultimate analysis (wt % dry basis)
moisture
volatile
fixed carbon
ash
C
H
O
N
S
0.02 6.81 2.95
99.89 72.92 88.18
0.08 12.47 5.95
0.01 7.8 2.92
85.71 45.88 70.09
14.28 6.84 10.92
0 47.06 18.9
0 0.04 0.04
0.01 0.18 0.06
ture in the pyrolyzer while the auxiliary gas is at the same level. The air/fuel ratio is defined as follows:
AR ) (Qs + Qa)/theoretical air quantity
Figure 2. Temperature variations at different axial height with the processing time during pyrolysis and combustion of RDF. Feed rate 3.0 kg/h, Qs ) 10 m3/h, Qa ) 2 m3/h, Qc ) 30 m3/h.
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 air flow rate and auxiliary air flow rate, and secondary air flow rate, affect the pyrolyzer temperature and combustor temperature. Figure 2 shows the axial temperature distribution in the pyrolyzer and combustor under a typical operating condition. From Figure 2, one can see that about 20 min were needed for the temperature to be steady for the spouting-moving bed pyrolyzer from the beginning of RDF feeding. The thermocouple at Z ) -10 showed the spouting gas temperature before entering pyrolyzer. To perform well, sometimes the spouting air was controlled by an electric preheater at about 160 °C (Z ) -10) after the RDF was fed into the bed and pyrolysis and combustion began. In fact, when the performance of the reactor was steady, spouting gas at room temperature can also keep the pyrolyzer working well. The temperatures of thermocouples at Z ) 6 cm and 10 cm are for the bed temperature, and the bed temperature is uniform as shown in Figure 2 and Table 2. When the pyrolyzer temperature reached about 450 °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 pyrolysis gas burned up by observing the temperature at Z ) 115 cm and Z ) 145 cm, then electric heater was turned off. About 20 min later, the temperature of combustor became steady. 3.1.1. Effect of Air/Fuel Ratio on Temperature Distribution. Considering the pyrolyzer heated by partial combustion of the RDF, the amount of air for the RDF in the pyrolyzer plays a key role in the bed temperature control, while the reactor itself and the amount of bed material (sand) were fixed. Figure 3 shows the relationship of the air/fuel ratio and the average bed tempera-
The combustion air fed into pyrolyzer includes the spouting air and the auxiliary air. As Figure 3 shows, at the same operation conditions, the bed temperature only increases slightly with increasing RDF feed rate when the air/fuel ratio is fixed at different feed rates of RDF. Table 2 summarizes the data of bed temperature under different operating conditions. Since the air fed into pyrolyzer to heat the bed includes spouting air and auxiliary air, they both have influences on the temperature distribution in the pyrolyzer. From Table 2, we can see that the bed temperature (Z ) 6 cm and Z ) 10 cm) is more close to the increasing warm spouting air (about 160 °C) as the auxiliary air is fixed. And freeboard temperature (Z ) 50 cm) also increases with increasing spouting air flow rate. But the gradient of the temperature between Z ) 6 cm and Z ) 10 cm is larger with increasing auxiliary air as the spouting air is fixed. Furthermore, the gradient of the temperature between Z ) 6 cm and Z ) 10 cm is much clearer with increasing auxiliary air as the warm spouting air is fixed, and the temperature of Z ) 6 cm only increases slightly with increasing cool auxiliary air volume. The reason causing this result is that the auxiliary air fed into the pyrolyzer is at room temperature, which would cool the bed material while it is fed into the bed. 3.1.2. Effect of Auxiliary Air on Temperature Distribution. To investigate the effect of auxiliary air volume on the distribution of temperature of pyrolyzer, both spouting air and auxiliary air are fed at room temperature. Figure 4 shows the effect of auxiliary air volume on the temperature of the pyrolyzer as spouting air volume fixed. The average bed temperature increases with auxiliary air volume increasing and the gradient of temperature between Z ) 6 cm and Z ) 10 cm is also much larger. The more auxiliary air fed into pyrolyzer, the more RDF burn with oxygen and more heat give off, so the temperature increases. Nevertheless, the more cool air introduced, the more heat absorbed by the cool air; as a result, compared with Table 2, we can see that the degree of temperature increasing with increasing cool auxiliary air is much lower than that with increasing the same warm spouting air volume. When much air is fed into pyrolyzer, the air superficial velocity increases, the entrainment of ground unburned carbon occurs, which burns in the freeboard zone, so the temperature of freeboard zone (Z ) 50 cm) is higher. Figure 5 exhibits the effect of different spouting air/ auxiliary air ratio on the temperature of pyrolyzer when keeping the total air fed into pyrolyzer constant. From Figure 5, we can see that the axial gradient of temperature in the pyrolyzer becomes larger with increasing spouting air volume than that with increasing auxiliary air flow volume for a fixed total air fed into the
Pyrolysis and Combustion of RDFs
Energy & Fuels, Vol. 16, No. 1, 2002 139
Table 2. Axial Temperature Distribution of the Reactor at Different Operating Conditions air feed rate (m3/h)
average temp (°C) at different axial height (Z, cm)
RDF feed rate (kg/h)
Qs
Qa
Qc
-10
6
10
50
78
115
145
175
3 3 3 3 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2
8 9 10 11 9 9.5 10 12 14 15 9 9
2.4 2 2 2.2 2 2 2 2 2 2.6 3 4
31 31 30 36 40 40 37 36 35 33 38 37
162 155 156 158 162 158 161 164 159 161 172 175
702 739 786 822 685 709 755 796 840 936 697 726
714 747 798 825 699 732 765 793 856 950 821 895
737 768 832 867 738 783 821 871 956 863 698 789
611 613 684 743 611 647 679 732 794 625 570 644
1275 1227 1144 978 1043 1069 1209 1223 1095 962 1186 1256
1232 1152 1135 962 1009 1016 1141 1146 973 888 1184 1249
1077 997 1028 797 921 935 955 976 869 811 946 1073
Figure 3. Effect of air/fuel ratio on the bed temperature in the pyrolyzer. Qa ) 2 m3/h.
Figure 4. Effect of horizontal auxiliary air on the temperature at different axial height in the pyrolyzer. RDF feed rate: 4.2 kg/h.
pyrolyzer. Increasing spouting air flow volume can increase the fountain height for a fixed total air volume, actual gas velocity in the pyrolyzer is increased, so the same elutriation of ground unburned carbon particle occurs. Therefore, exothermic reaction of combustion occurs mostly in the upper zone of pyrolyzer. Auxiliary air is introduced dispersedly through 72 horizontal channels (i.d. 2 mm), so the RDF and oxygen gas contact well. Increasing auxiliary air volume can decrease fountain height,13 actual air velocity is relatively de(13) Sutanto, W.; Epstein, N.; Grace, J. R. Powder Technol. 1985, 44, 205-212.
Figure 5. Effect of horizontal auxiliary air and spouting air on the temperature at different axial height in the pyrolyzer for a fixed total air volume. RDF feed rate: 4.2 kg/h.
creased, so less ground unburned carbon particle is entrained into the upper zone in pyrolyzer. This reason also can explain the results that the temperature of Z ) 6 is lower but the temperature of Z ) 50 cm is much higher while the spouting air flow volume increased for a fixed total air flow volume. 3.2. Gaseous Product. When RDF is heated, decomposition takes place and a large amount of organic products both large molecular (such as heavy liquid residual defined under the generic name of tar) and small molecular (light gases such as methane and hydrogen) matters are obtained. In our experimental setup, heavy products are not separated and burn with light gas in the upper combustor. In this study, N2, O2, CO, CO2, H2, and C1-C2 are analyzed to evaluate the performance of the reactor. The profile of the gas production was another factor with which to evaluate the performance of the reactor. The gases produced in the trials were analyzed mainly by gas chromatography. The axial profiles of the different gas components are showed in Table 3. 3.2.1. Gaseous Product in the Pyrolyzer. The gas product in the pyrolyzer can show if the aim of the partial combustion and the partial pyrolysis of RDF is reached. From Table 3, one can see that the content of O2 in the product gas in the pyrolyzer is very low, in the range of about 0.8-2.0 vol % along different axial height at different runs, which implies that the O2 introduced to the pyrolyzer react with fuel well. At the same time, the contents of pyrolysis gas, such as H2, CO, CH4, C2H4, and C3H6, are much higher than that of oxygen in pyrolyzer, which suggests that partial
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Table 3. Gas Composition Distribution in the Reactor at Different Operating Conditions feed rate RDF (kg/h)
3
3
4.2
4.2
Qs
8
9
9
10
air (m3/h) Qa Qc
2.4
2
2
2
31
31
40
37
height (cm) Z
temp (°C) T
20 50 78 145 175 20 50 78 145 175 20 50 78 145 175 20 50 78 145 175
714 727 611 1232 1077 747 769 613 1152 997 699 739 611 1009 921 765 821 679 1141 955
H2
O2
N2
3.86 4.56 4.70 0.09
1.57 0.95 0.82 8.09 6.12 1.75 1.12 0.97 8.26 6.25 1.18 0.96 0.87 8.79 10.63 1.32 0.85 0.76 8.03 5.95
69.23 69.02 69.07 80.07 81.22 67.77 67.54 67.00 78.50 80.00 67.96 68.72 67.92 80.30 80.32 65.42 66.32 66.25 81.31 82.18
4.13 4.77 4.84 0.06 4.58 4.63 4.80 0.07 5.63 5.61 5.73 0.11
combustion and pyrolysis of RDF occur in the pyrolyzer successfully. The content of oxygen in pyrolyzer decreases with increasing bed height from Z ) 20 cm to Z ) 78 cm. Although the corresponding carbon dioxide concentration also drops clearly, one can consider that there is still some combustion reaction occurring at the middle part of pyrolyzer but not occurring in the upper of pyrolyzer because the temperature also first increases then decreases along the bed height. The carbon dioxide concentration drops because more pyrolysis gas is simultaneously produced, but the content of nitrogen that is the highest concentration in the product gas changes little along the bed height. The contents of hydrogen, methane, and carbon monoxide all increase with increasing axial bed height, especially from Z ) 20 cm to Z ) 50 cm. These results show that pyrolysis reactions mainly occur at the lower section of pyrolyzer. Large molecule products may further cracked into small molecule products such as hydrogen and methane while flowing up to the upper section of pyrolyzer; they can also react with the remaining small amount of oxygen to form carbon monoxide. For ethylene, because RDF has a high content of polyethylene, the content is higher than usual fuels. The content of ethylene increases slightly with increasing bed height at lower temperature, but decreases slightly in the upper section of pyrolyzer at higher temperature; whether ethylene at this higher temperature can further cracked into smaller molecular gas could not be ascertained. Figure 6 shows the pyrolysis gas composition profiles of H2, CO, CH4, and C2H4 at different air/fuel ratios. The gas composition at Z ) 78 cm is considered the final pyrolysis gas product in the pyrolyzer. It can be seen in Figure 6 that the contents of H2 and CH4 increase with increasing air/fuel ratio, which also occurs from increasing the temperature. A higher reaction temperature leads to faster decomposition reactions and more gas production. The C2H4 concentration has a decline tendency with increasing air/fuel ratio; a possible reason is that a higher temperature and a faster decomposition rate are not favorable for C2H4 formation, but favorable for H2 and CH4. The content of CO has little change at
gas composition (vol %, dry basis) CH4 CO 1.92 2.29 2.69 0.05
6.83 8.41 8.47 0.09
3.10 3.46 3.96 0.04
7.66 9.01 9.31 0.07
2.82 2.96 3.16 0.02
8.96 9.46 9.70 0.02
3.58 3.55 3.93 0.06
8.20 9.39 9.71 0.12
CO2
C2H4
C2H6
13.80 11.59 10.68 11.58 12.66 12.61 9.55 10.00 13.06 13.75 10.96 9.07 9.30 10.33 9.05 11.90 9.67 9.92 10.34 11.87
2.53 2.83 3.21 0.02 2.89 4.29 3.99 0.01
0.27 0.35 0.38 0.18 0.25 0.10
3.81 4.04 4.07 0.01
0.27 0.16 0.18
3.88 4.50 3.62 0.03
0.08 0.12 0.08
Figure 6. Effect of air/fuel ratio on the gas composition. Feed rate: 4.2 kg/h.
low air/fuel ratios. Since the production of CO can be from both pyrolysis and carbon reacting with insufficient O2, increasing air feed rate is not sure to decrease the content of CO; and carbon could react with H2O produced by combustion as coal-water conversion reaction at higher temperature (800-1000 °C) to form CO. All those reasons can result in that the content of CO has little change at different air/fuel ratios. The profile of pyrolysis gas heating value at different air/fuel ratios is showed in Figure 7. One can see that the heating value of pyrolysis gas increase with increasing air/fuel ratio below the air/fuel ratio value of 0.33. When the value of air/fuel ratio is higher than 0.33, the gas heating value decreases due to the collected content of >C2 gases decrease and the N2 content increases. 3.2.2. Gaseous Product in the Combustor. From Table 3, we can see that the pyrolysis gas burns well with secondary air introduced from the 60° inverse truncated connector. Though the pyrolysis gas, such as H2, CO, CH4, C2H4, and C3H6, at the middle part (Z ) 145 cm) of the combustor can be detected by the gas chromatography, the content of those gases is very small. At the upper part (Z ) 175 cm) of the combustor, the pyrolysis gas cannot be checked out by the chromato-
Pyrolysis and Combustion of RDFs
Figure 7. Effect of air/fuel ratio on the calorific value of pyrolysis gas. Feed rate: 4.2 kg/h.
Figure 8. Effect of excess air ratio on the combustor temperature. Feed rate: 3 kg/h.
graphy employed in this study: only O2, N2, and CO2 are detected. 3.3. Emissions of CO and NOx in Flue Gas. To further evaluate the combustor performance, a commercial combustion analyzer and a portable NOx analyzer are employed to detect the content of CO and NOx in the flue gas at the ppm level. Secondary air is used to burn with pyrolysis gas in the combustor, which can enhance the oxidation and improve combustion efficiency, so the amount of secondary air has a key role in the emissions of CO and NOx in the flue gas. Excess air ratio of the secondary air is defined as follows:
EAR ) (Qs + Qa + Qc)/ theoretical air quantity Figure 8 shows the effect of excess air ratio of secondary air on the temperature of combustor (average value of the temperature at Z ) 145 cm and Z ) 175 cm) while spouting air volume and auxiliary air volume is kept at 10 m3/h and 2 m3/h, respectively. The temperature of the combustor increases to a maximum value with the excess air ratio increasing from 1.25 to 1.32 and then begins to decrease. More cool excess air can absorb the heat in the combustor, so the combustor temperature can be controlled by the amount of secondary air. The content of CO in the flue gas reveals the combustion characteristics of the combustor. The effect of an excess air ratio of secondary air on the emissions of CO
Energy & Fuels, Vol. 16, No. 1, 2002 141
Figure 9. Effect of Excess air ratio on the CO emissions. Feed rate: 3 kg/h.
Figure 10. Effect of excess air ratio on the NOx emissions. Feed rate: 3 kg/h.
is shown in Figure 9. The CO contents in the flue gas decrease to a minimum value of 72 ppm upon increasing the excess air ratio from 1.25 to 1.82 and then increase sharply to about 110 ppm when excess air ratio reaches 1.93. The CO emissions should have become lower when more air is fed into the combustor because the pyrolysis gas should combust more completely with more oxygen. However, a high excess air ratio decreases the temperature of combustor as shown in Figure 10: the temperature of the combustor at EAR ) 1.93 is much lower than that of the other EAR values. Lower temperature decreases the oxidation reaction of pyrolysis gas with oxygen; furthermore, much air fed into the combustor accelerates the gas velocity and shortens the pyrolysis gas stay time in the combustor. These factors would result in a higher CO emission. NOx formation has been studied under a large range of experimental conditions and is fairly well understood.14-18 The contribution to the final NOx emissions in coal combustion mainly comes from fuel-bound nitrogen in most circumstances. However, RDF used as fuel in this study has a very low nitrogen content as Table 1 shows, thus the thermal-NOx formation and (14) Tullin, C. J.; Sarofim, A. F.; Bee´r, J. M. Energy Fuels 1993, 7, 796-802. (15) Winter, F.; Lo¨efler, G.; Wartha, C.; Hofbauer, H.; Preto, F.; Anthony, E. J. Can. J. Chem. Eng. 1999, 77, 275-283. (16) Li, Y. H.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1998, 53, 1-26. (17) Lyngfelt, A.; Lechner, B. Fuel 1999, 78, 1065-1072. (18) Gasnot, L.; Desgroux, P.; Pauwels, J. F.; Scochet, L. R. Combust. Flame 1999, 117, 291-306.
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prompt-NOx formation from nitrogen in the air18 are main sources of NOx emissions. Figure 10 exhibits the effect of an excess air ratio on the NOx emissions. Compared with Figure 8, it seems that the combustion temperature dominates the NOx emissions. Generally speaking, the lower temperature is, the less NOx is emitted. When the temperature is 850 °C, the NOx level is as low as 42 ppm. Considering the effect of excess air ratio on CO and NOx together, the excess air ratio should be taken as 1.5-1.7 to achieve both reasonably low CO and NOx emissions. 4. Advantages of the Two-reaction Zone (Pyrolyzer and Combustor) Apparatus The reactor developed in this experiment was expected to reach following aims: The reactor includes the pyrolyzer and the combustor, the RDF can pyrolyze with partial combusiton at low temperature in pyrolyzer; the products including light gas and heavy molecular organic matters can combust at a high temperature in the combustor. Horizontal auxiliary air was introduced into the spouting-moving bed pyrolyzer, and the auxiliary air’s level can easily be controlled to improve mass and heat transfer and pyrolysis temperature. A low temperature of the pyrolyzer would reduce trace volatile pollutant matters such as heavy metals. Gas combustion in combustor can be complete, and emissions of CO and trace organic pollutants, such as chlorinated dibenzo-p-dioxins and -furans and polyaromatic hydrocarbons, might be reduced as low as possible by controlling the secondary air. The heat to pyrolyze fuel is supplied by itself; for a low heating value for the fuel, coal can be mixed in with the fuel to supply heat. 5. Conclusions RDF pyrolysis and combustion in a two-part reactor including a spouting-moving bed pyrolyzer and a gas combustor have been investigated. The reactor was run under a wide range of conditions to assess the system, and the reactor performed well under these operating
Wang et al.
conditions. Experiments on the bench scale process have shown that (1) Temperature control in the spouting-moving bed pyrolyzer can be achieved by changing the air/fuel ratio. The bed temperature is relatively constant with preheated spouting air (about 160 °C), and the increase of warm spouting air increases the temperature gradient between the bed temperature and the freeboard temperature in the pyrolyzer. (2) The effect of spouting air/auxiliary air was studied without preheating. The axial gradient of temperature in the pyrolyzer with increasing spouting air flow volume is larger than that with increasing auxiliary air flow volume for a fixed total air supply to the pyrolyzer. (3) Pyrolysis gas containing less than 1 vol % oxygen can be obtained from the pyrolyzer. The heating value of the pyrolysis gas increases when the fuel/air ratio increases below 0.33 then begins to decline at a fuel/air ratio >0.33. (4) The pyrolysis gas can combust completely with secondary air fed from the connector in the combustor. A gas chromatograph hardly distinguished among out the pyrolysis gas such as H2, CH4, CO, C2H4, and C3H6. (5) Secondary air supply can influence the combustor temperature as well as the emissions of CO and NOx in the flue gas. The CO emissions decrease from 298 ppm to 72 ppm with increasing the excess air ratio from 1.25 to 1.82; the NOx emissions decrease from 75 to 43 ppm with increasing the excess air ratio from 1.25 to 1.93. Acknowledgment. This work was financially supported by the National Natural Science Foundation of P. R. China (Project 29876041). Nomenclature AR ) air/fuel ratio EAR ) excess air ratio Qs ) spouting air volume, m3/h Qa ) auxiliary air volume, m3/h Qc ) secondary air volume, m3/h Z ) vertical distance from spouting gas inlet, cm EF0101006
