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N2O and NO emissions from co-firing MSW with coals in pilot scale CFBC
Fuel Processing Technology 85 (2004) 1539 – 1549 www.elsevier.com/locate/fuproc
N2O and NO emissions from co-firing MSW with coals in pilot scale CFBC Zhiwei Li *, Qinggang Lu, Yongjie Na Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100080, PR China Accepted 1 October 2003
Abstract Co-firing municipal solid waste (MSW) with bituminous coal and anthracite was conducted using pilot scale circulating fluidized-bed combustion. Both N2O and NO emissions from co-firing MSW with bituminous coal are higher than those obtained from co-firing with anthracite. N2O decreases significantly, whereas NO rises with the increase of Ca/(S + 0.5Cl) molar ratios. Increasing the cofiring rates leads to the reduction of N2O emission, but an increase of NO emission. Raising furnace temperature is an effective way to control N2O emission, but NO emission is scarcely affected. D 2004 Elsevier B.V. All rights reserved. Keywords: N2O and NO; Municipal solid waste; Coal; Circulating fluidized-bed combustion
1. Introduction Municipal solid waste (MSW) incineration is projected to become the dominant disposal option with the advantage of energy recycling, weight and volume reduction. However, MSW incineration has been shown to produce PCDD/Fs, which has led to emission limits for 1 TE value as low as 0.1 ng/Nm3 [1,2] in many countries. Circulating fluidized-bed combustion (CFBC), as an established technology, is feasible for MSW disposal with its advantages of burning a wide variety of solid fuels and low emissions [3]. In addition, hydrogen chloride released from MSW incineration can be captured through lime addition, and it also promotes sulfur dioxide capture by bed material and calcined limestone [4 –6]. So, co-firing MSW with coal can suppress PCDD/F formation, with low SO2 and HCl emissions, which has been reported by Lu [7]. * Corresponding author. Tel.: +86-10-6253-8778; fax: +86-10-6257-5913. E-mail address: [email protected] (Z. Li). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.10.025
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Because of its low combustion temperature (850 – 950 jC), coal-fired CFBC emits substantially more N2O (40 – 500 mg/Nm3) than conventional boilers (0– 10 mg/Nm3). N2O is a sharp greenhouse gas and stratospheric ozone depleting substance. NO is known to play an important role in photochemical smog and acid rain formation. As a result, much work has been carried out on N2O and NO reduction in fluidized-bed combustion of coal and chars. The final N2O and NO emissions from combustion of coal and char are the result of homogeneous and heterogeneous formation and in situ destruction reactions [8]. The formation and reduction of N2O and NO in coal and charfired fluidized-bed combustion are a very complicated process, and much work has been carried out on this subject. It is known that there exists a trade-off between N2O and NO reduction [8,9]. Addition of limestone to coal and char combustion led to a significant decrease in N2O emission and an increase in NO emission [4,10 – 14]. Temperature is another important parameter influencing N2O and NO emissions from coal and char combustion, and an increase in temperature often leads to significant N2O reduction [14,15] and NO increase [16]. Available information about N2O and NO control is mainly obtained from coal and char combustion, but the information about the control of N2O and NO from co-firing MSW with coals is limited. Composition of MSW is different from coal, with much higher ash, volatile and nitrogen contents. It is known that fuel-nitrogen is the main source of NO, and N2O emissions and bed material more or less catalytically affects the formation and destruction of N2O and NO in fluidizedbed combustion [10]. Therefore, the characteristics of N2O and NO emissions from cofiring MSW and coals with CFBC might be different from those of coal and char combustion. It is the purpose of this paper to study the influence of temperature, co-firing rates and Ca/(S + 0.5Cl) molar ratios on N2O and NO emissions during co-firing MSW with bituminous and anthracite using pilot scale CFBC.
2. Experiment 2.1. Pilot scale CFBC plant The investigations were conducted in a pilot scale CFBC plant (Fig. 1). It consists of combustion system, control system, flue-gas sampling and analysis system. The combustion system is composed of a furnace with an inside diameter of 300 mm and a height of 6000 mm, a cyclone and a U-valve. The refractory-lined bottom of the furnace is 800 mm in height. The other parts are all made of high-temperature alloy covered with heat insulation material on the outside. Forty kilograms of quartz sand with the size of 0.5– 1.0 mm was fed into the furnace as bed material during start-up. The primary air was fed into the furnace through the air distributor in the flow range 200– 250 m3/h. Considering the height of the furnace, no secondary air was added. Thermal input to the furnace with coal and MSW was between 0.17 and 0.22 MWth. MSW and coal were separately added into the furnace through two screw feeders from the holes 890 mm above the air distributor, in the range 0– 80 and 6– 30 kg/h, respectively, depending on the cofiring rates.
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Fig. 1. Test facilities of the pilot scale CFBC plant—1: furnace; 2: water tube; 3: cyclone; 4: gas analyzer; 5: coal and MSW screws; 6: U-valve; 7: primary air meter; 8: forced draft fan; 9: air compressor; 10: bag filter; 11: induced draft fan; 12: economizer; 13: stack; T1 – T6: thermocouples; P1 – P6: pressure taps.
The temperature profiles in the furnace were maintained by adjusting the depth of a water tube penetrated into the furnace from the top. The water tube was 4000 mm in length and 51 mm in outside diameter. Concentrations of H2O, SO2, HCl, CO, CO2, NO and N2O in flue gas were continuously analyzed via an on-line infrared type gas analyzer. The gas-sampling nozzle was at the exit of the cyclone. Oxygen concentration in flue gas was analyzed with a ZrTable 1 Properties of the fuels Bituminous
Anthracite
Waste
Proximate analysis (wt.%, as received basis) Water 0.42 Ash 27 Volatiles 13.2 Fixed carbon 59.38 LHV (MJ/kg) 23.5
3.32 20.6 4.52 71.56 24.2
11.7 57.5 26.6 3.2 6.4
Ultimate analysis (wt.%, water ash free) Carbon 85.09 Hydrogen 3.92 Oxygen 7.24 Nitrogen 1.29 Sulfur 2.46 Chlorine 0 Calcium 4.56
94.62 0.96 4.01 0.11 0.3 0 1.33
73.46 7.88 15.13 3.15 0.38 0.47 9.77
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Table 2 Compositions of the coal ash and limestone Composition
Bituminous ash (wt.%)
Anthracite ash (wt.%)
Limestone (wt.%)
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O Total
35.75 22.86 9.08 17.23 1.18 10.26 0.8 1.04 0.34 0.25 98.79
53.26 18.7 6.8 6.84 1.58 2.1 1.1 2.77 1.01 0.52 94.68
1.32 0.75 0.47 91.84 0.97 0 0 0.28 0.01 0.08 95.72
type probe. Main operation parameters were recorded in a computer at the interval of 15 s. Gas analysis data were stored at an interval of 60 s. 2.2. Characteristics of coals and MSW The fuels used in the experiments included two kinds of coal—anthracite and high sulfur bituminous—and MSW. The ultimate and proximate analyses of the fuels are given in Table 1, and ash contents in Table 2. It is interesting that the calcium content in MSW is much higher than that in coals, which leads to very high Ca/(S + 0.5Cl) molar ratios with co-firing MSW and coals even without limestone addition. The size distributions of coal, MSW and limestone are all presented in Fig. 2. Most coal particles are smaller than 6 mm, with 50% cut size of about 1 mm. Most of the MSW is crashed to less than 10 mm. The size of limestone particles mainly range from 0.1 to 1 mm. 2.3. Experimental conditions In the experiments, the highest furnace temperatures were observed at 820 or 1520 mm above the air distributor, and were defined as the furnace temperature in this paper. To
Fig. 2. Size distribution of fuels and sorbent under investigation. Stars, triangles, diamonds and squares represent bituminous, anthracite, MSW and limestone, respectively.
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control the furnace temperature, fuel feeding rates and the depth of the water tube inserted into the furnace were adjusted. Average oxygen concentrations were controlled between 7.5% and 8.5%. Since the calcium content in the MSW was very high and plastic waste Table 3 Experimental conditions and results No.
Coal type
Temperature (jC)
Co-firing rate (%)
PVC, S addition
Limestone
Ca/ (S + 0.5Cl)
N2O (mg/nm3)
NO (mg/nm3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad
850 868 855 838 850 856 856 854 853 859 849 831 848 852 850 858 844 843 831 853 850 830 849 835 801 807 865 855 859 902 845 851 844 845 850 850 889 884 873
0 0 0 0 0 0 0 0 32 34.6 25 39 30.9 36.6 44 51 53.6 54 56 40 57.7 53 76 73 37 43 44 53 54 50 51 56 53 57 54 54 27.4 26.4 32.2
Nb Nb Nb Nb Nb PVC PVC PVC Nb PVC Nb Nb PVC PVC Nb Nb PVC Nb Nb PVC PVC PVC Nb Nb Nb Nb Nb Nb Nb Nb Se Se PVC + Se Se Se PVC + Se PVC PVC PVC
Nb Nb Nb Yc Yc Nb Nb Nb Nb Nb Yc Yc Yc Yc Nb Nb Nb Yc Yc Yc Yc Yc Nb Yc Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb Yb Yc N Yc Yc
1.5 1.5 1.5 3.5 3.5 1.3 1.3 1.39 3.3 2.24 4.8 5.8 4.43 7.46 4.2 4.1 3.08 7.1 6.3 7.53 7.62 7.48 7.6 8.5 3.6 10.3 10.4 11 11.1 10.8 4 3.1 2.6 2.7 2.5 2.6 4.17 6.9 7.2
331 315 298 330 313 325 338 314 296 214 177 153 248 113 271 207 242 129 139 68 103 67 87 61 416 116 75 113 108 48 154 171 159 134 122 117 31 35 33
384 408 434 410 416 369 297 301 592 391 682 661 503 788 665 644 285 749 792 728 576 736 647 613 559 328 386 543 375 323 411 341 337 253 242 247 237 336 310
a b c d e
Bituminous. Without addition. With addition. Anthracite. Sulfur.
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Fig. 3. Furnace temperature. Bolt solid lines, broken thin lines and solid thin lines represent the temperatures at 820, 2520 and 6000 mm above the air distributor, respectively.
content in the MSW was comparatively low, some PVC powder, sulfur powder and limestone were mixed with coal before being added to the furnace, in some cases, to control the Ca/(S + 0.5Cl) molar ratios. Some experiment parameters, N2O and NO emissions are listed in Table 3. The co-firing rates were between 0% and 76% for the different test runs with bituminous, and the co-firing rates were between 27.4% and 57% for anthracite. Air superficial velocities in the furnace were between 3.3 and 3.8 m/s. Co-firing rate is the heat input with the MSW divided by the total heat input with coals and MSW. All gas emissions presented in this paper are normalized to dry gas with an oxygen concentration of 6%, at 273 K and 101.3 kPa. 2.4. Combustion stability Co-firing MSW with coals was stable in the experiments. Furnace temperatures of cofiring MSW with bituminous are shown in Fig. 3. In the experiments, co-firing rates were 0, 25%, 54% and 74%, respectively, and the furnace temperatures were kept quite stable,
Fig. 4. N2O and NO emissions. Blank diamonds, blank squares and blank triangles represent the N2O emissions in test1, in test2 and in test3, respectively; solid diamonds, solid squares and solid triangles represent the NO emissions in test1, in test2 and in test3, respectively.
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except those of changing co-firing rates. The furnace temperatures in the other test runs were more or less the same as those shown in this one. The emissions of N2O and NO from coal combustion at 850 jC, without any MSW and sorbent addition, are presented in Fig. 4. The emissions were observed from three different tests conducted in 3 days and it is obvious that the results are reproducible.
3. Results and discussion 3.1. Influence of furnace temperature on N2O and NO emissions It is known that rising furnace temperature leads to an increase in the rate of N2O destruction. Observations of thermal decomposition of N2O were reported with combustion of sludge [21,22], coal and char [14,15]. N2O emission from co-firing MSW with coals is shown in Fig. 5. As can be seen, raising the furnace temperature leads to N2O reduction. But in Fig. 6, there is no significant NO emission increase when furnace temperature rises from 800 to 900 jC, which is in contrast to a previous report [16]. Ca/ (S + 0.5Cl) molar ratios might be the dominant parameter influencing NO emission within the investigated temperature range, as discussed below. It is clear that NO and N2O emissions from co-firing with bituminous are higher than those observed from co-firing with anthracite, as presented in Figs. 5 and 6. 3.2. Influence of Ca/(S+0.5Cl) molar ratios and co-firing rates on N2O and NO emissions The influences of Ca/(S + 0.5Cl) molar ratios on N2O and NO emissions are shown in Figs. 7 and 8, respectively. An increase in Ca/(S + 0.5Cl) molar ratios generally leads to a decrease in N2O emission and an increase in NO emission. Co-firing of MSW with bituminous at about 850 jC, N2O emission decreases from 300 mg/nm3 to less than 100 mg/nm3, and NO increases from 350 to 700 mg/nm3, at an average, when Ca/(S + 0.5Cl) molar ratios increase from 2 to 8.
Fig. 5. Influence of furnace temperature on N2O emission. Diamonds, triangles and squares represent anthracite, with the Ca/(S + 0.5Cl) molar ratios of 10 – 11, 2.5 – 4 and 4.2 – 7.2, respectively; circles represent the bituminous, with the Ca/(S + 0.5 Cl) molar ratios of 3.1 – 3.3.
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Fig. 6. Influence of furnace temperature on NO emission. Diamonds, triangles and squares represent anthracite, with the Ca/(S + 0.5Cl) molar ratios of 10 – 11, 2.5 – 4 and 4.2 – 7.2, respectively; circles represent the bituminous, with the Ca/(S + 0.5Cl) molar ratios of 3.1 – 3.3.
Fig. 7. Influence of Ca/(S + 0.5Cl) molar ratios on N2O emission. Triangles and circles represent anthracite and bituminous, respectively.
Fig. 8. Influence of Ca/(S + 0.5Cl) molar ratios on NO emission. Triangles and circles represent anthracite and bituminous, respectively.
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Fig. 9. Influence of co-firing rates on N2O emission. Triangles and circles represent anthracite and bituminous, respectively.
The influences of co-firing rates on N2O and NO emissions at about 850 jC are shown in Figs. 9 and 10, respectively. In cases with co-firing MSW and bituminous, an increase in NO emission is observed with MSW addition, but N2O emission decreases from 340 to 80 mg/Nm3 with an increase of co-firing rates from 0% to 76%, as shown in Fig. 9. In this investigation, the increase of co-firing rate, which means higher nitrogen content in the mixed fuel and presents a greater potential for N2O emission [10], does not lead to N2O increase but in reduction. As shown in Table 4, the Ca/(S + 0.5Cl) molar ratios increase from 1.5 to 7.6 as the co-firing rates rise from 0% to 76%, and the increased Ca/ (S + 0.5Cl) molar ratios lead to N2O decomposition, as shown in Fig. 7. It is clear that the reduction of N2O emission can be attributed to the catalytic reaction of N2O decomposition over calcined limestone [13,17,18]. In addition, the ash concentration in the furnace also increases with the rise of the co-firing rates, and it further enhances the catalytic reactions of N2O decomposition over calcined limestone with stronger mixing of particles and gas. Due to the high contents of nitrogen and calcium in the MSW, both nitrogen contents and Ca/(S + 0.5Cl) molar ratios increase with higher co-firing rates, as shown in Table 4, and they both lead to the increase of NO emission. With higher Ca/(S + 0.5Cl) molar
Fig. 10. Influence of co-firing rates on NO emissions. Triangles and circles represent anthracite and bituminous, respectively.
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Table 4 Ca/(S + 0.5Cl) molar ratio and ash concentration in the furnace (co-firing bituminous and MSW) Co-firing rate (%)
0
32
51
76
Ca/(S + 0.5Cl) molar ratio Ash concentration (kg/m3)
1.5 7.2
3.3 16.1
4.1 19.7
7.6 25.6
ratios, NO emission increases through the oxidation of volatile nitrogen as well as char nitrogen with the catalytic activity of calcined limestone [10,13,17]. It is also shown in Figs. 7 and 8 that N2O and NO emissions from co-firing MSW with anthracite are lower than those from co-firing with bituminous at lower Ca/(S + 0.5Cl) molar ratios. Char concentration in the furnace in co-firing with anthracite is higher than that in co-firing with bituminous, and char takes part in reactions in reducing both N2O and NO formation from volatile N and char N oxidation [9,19,20]. Thus the higher char concentration is attributable to low N2O and NO emissions from co-firing MSW with anthracite. Moreover, attention should be paid to the fact that the volatile content in MSW is higher, and more volatile nitrogen, especially HCN, is released during devolatilisation, and its oxidation over bed material in the furnace has a high selectivity for NO formation and low selectivity for N2O formation [10].
4. Conclusion Through co-firing MSW with bituminous and anthracite using pilot scale CFBC, the influence of operating parameters on N2O and NO emissions was investigated and the main results are concluded as follows and compared with results found in literature. (1) There is a strong dependence of N2O and NO emissions on Ca/(S + 0.5Cl) molar ratios, and calcined limestone can significantly catalyze the reactions of N2O reduction and NO formation. (2) Increasing co-firing rates leads to higher Ca/(S + 0.5Cl) molar ratios of the mixed fuel and higher ash concentrations in the furnace, which enhances the catalytic reactions of N2O decomposition and NO formation over the surface of bed material particles containing calcium. (3) Furnace temperature is an important parameter influencing N2O emission and increasing furnace temperature results in N2O reduction, but there is no strong dependence of NO emission on furnace temperature in the investigated range.
References [1] M. Frankenhaeuser, H. Manninen, I. Kojo, J. Ruuskanen, T. Vartiainen, R. Vesterinen, J. Virrki, Chemosphere 27 (1 – 3) (1993) 309. [2] R.L. Lindbauer, Chemosphere 25 (7 – 10) (1992) 1409. [3] M.Y. Tsai, K.T. Wu, C.C. Huang, H.T. Lee, Waste Manag. 22 (2002) 439.
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[4] A.D. Lawrence, J. Bu, P. Gokulakrishnan, J. Inst. Energy 72 (1999) 34. [5] M. Matsukata, T. Miyatani, K. Ueyama, S. Mutsui, T. Lwasakit, in: F.D.S. Preto (Ed.), Proceeding of the 14th International Conference on Fluidized Bed Combustion, ASME, Vancouver, 1997, p. 397. [6] W. Xie, K. Liu, W.P. Pan, J.T. Riley, Fuel 78 (1999) 1425. [7] Q. Lu, et al., J. Eng. Thermophys. 24 (2) (2003) 347(In Chinese). [8] Y.H. Li, G.Q. Lu, V. Rudolph, Chem. Eng. Sci. 53 (1) (1998) 1. [9] J.R. Pels, M.A. Wojtowicz, F. Kapteijn, J.A. Moulijn, Energy Fuels 9 (1995) 473. [10] H. Liu, B.M. Gibbs, Fuel 80 (2001) 1211. [11] C. Philippek, J. Werther, J. Inst. Energy 70 (1997) 141. [12] P.F.B. Hansen, K. Dam-Johansen, in: L. Rudow, G. Commonwealth (Eds.), Proceeding of the 12th International Conference on Fluidized Bed Combustion, ASME, San Diego, CA, 1993, p. 779. [13] T. Shimizu, M. Satoh, T. Fujikawa, et al., in: J.R. Grace, J.X. Zhu, H.D. Lasa (Eds.), Proceeding of the 7th International Conference on Circulating Fluidized Beds, Canadian Society for Chemical Engineering, Niagara Falls, Ontario, Canada, 2002, p. 781. [14] M.E. Collings, M.D. Mann, B.C. Young, P.E. Botros, in: L. Rudow, G. Commonwealth (Eds.), Proceeding of the 12th International Conference on Fluidized Bed Combustion, ASME, San Diego, CA, 1993, p. 619. [15] A. Boemer, A. Braun, U. Renz, in: L. Rudow, G. Commonwealth (Eds.), Proceeding of the 12th International Conference on Fluidized Bed Combustion, ASME, San Diego, CA, 1993, p. 585. [16] S.K. Goel, J.M. Bee´r, A.F. Sarofim, in: K.J. Heinschel (Ed.), Proceeding of the 13th International Conference on Fluidized Bed Combustion, Orlando, Florida, ASME, New York, 1995, p. 887. [17] T. Shimizu, M. Inagaki, Energy Fuels 7 (1993) 648. [18] D.G. Gavin, A. Dorrington, Proceedings 1991 International Conference on Coal Science (Newcastle, UK), International Energy Agency Coal Research, Butterworth-Heinemann, Oxford, UK, 1991, p. 347. [19] J.E. Johnsson, K. Dan-Johanson, in: K.J. Heinschel (Ed.), Proceeding of the 13th International Conference on Fluidized Bed Combustion, Orlando, Florida, ASME, New York, 1995, p. 859. [20] J.E. Johnsson, Fuel 73 (1994) 1398. [21] J. Werther, T. Ogada, Prog. Energy Combust. Sci. 25 (1999) 55. [22] M. Sa¨nger, J. Werther, T. Ogada, Fuel 80 (2001) 167.
N2O and NO emissions from co-firing MSW with coals in pilot scale CFBC Zhiwei Li *, Qinggang Lu, Yongjie Na Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100080, PR China Accepted 1 October 2003
Abstract Co-firing municipal solid waste (MSW) with bituminous coal and anthracite was conducted using pilot scale circulating fluidized-bed combustion. Both N2O and NO emissions from co-firing MSW with bituminous coal are higher than those obtained from co-firing with anthracite. N2O decreases significantly, whereas NO rises with the increase of Ca/(S + 0.5Cl) molar ratios. Increasing the cofiring rates leads to the reduction of N2O emission, but an increase of NO emission. Raising furnace temperature is an effective way to control N2O emission, but NO emission is scarcely affected. D 2004 Elsevier B.V. All rights reserved. Keywords: N2O and NO; Municipal solid waste; Coal; Circulating fluidized-bed combustion
1. Introduction Municipal solid waste (MSW) incineration is projected to become the dominant disposal option with the advantage of energy recycling, weight and volume reduction. However, MSW incineration has been shown to produce PCDD/Fs, which has led to emission limits for 1 TE value as low as 0.1 ng/Nm3 [1,2] in many countries. Circulating fluidized-bed combustion (CFBC), as an established technology, is feasible for MSW disposal with its advantages of burning a wide variety of solid fuels and low emissions [3]. In addition, hydrogen chloride released from MSW incineration can be captured through lime addition, and it also promotes sulfur dioxide capture by bed material and calcined limestone [4 –6]. So, co-firing MSW with coal can suppress PCDD/F formation, with low SO2 and HCl emissions, which has been reported by Lu [7]. * Corresponding author. Tel.: +86-10-6253-8778; fax: +86-10-6257-5913. E-mail address: [email protected] (Z. Li). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.10.025
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Because of its low combustion temperature (850 – 950 jC), coal-fired CFBC emits substantially more N2O (40 – 500 mg/Nm3) than conventional boilers (0– 10 mg/Nm3). N2O is a sharp greenhouse gas and stratospheric ozone depleting substance. NO is known to play an important role in photochemical smog and acid rain formation. As a result, much work has been carried out on N2O and NO reduction in fluidized-bed combustion of coal and chars. The final N2O and NO emissions from combustion of coal and char are the result of homogeneous and heterogeneous formation and in situ destruction reactions [8]. The formation and reduction of N2O and NO in coal and charfired fluidized-bed combustion are a very complicated process, and much work has been carried out on this subject. It is known that there exists a trade-off between N2O and NO reduction [8,9]. Addition of limestone to coal and char combustion led to a significant decrease in N2O emission and an increase in NO emission [4,10 – 14]. Temperature is another important parameter influencing N2O and NO emissions from coal and char combustion, and an increase in temperature often leads to significant N2O reduction [14,15] and NO increase [16]. Available information about N2O and NO control is mainly obtained from coal and char combustion, but the information about the control of N2O and NO from co-firing MSW with coals is limited. Composition of MSW is different from coal, with much higher ash, volatile and nitrogen contents. It is known that fuel-nitrogen is the main source of NO, and N2O emissions and bed material more or less catalytically affects the formation and destruction of N2O and NO in fluidizedbed combustion [10]. Therefore, the characteristics of N2O and NO emissions from cofiring MSW and coals with CFBC might be different from those of coal and char combustion. It is the purpose of this paper to study the influence of temperature, co-firing rates and Ca/(S + 0.5Cl) molar ratios on N2O and NO emissions during co-firing MSW with bituminous and anthracite using pilot scale CFBC.
2. Experiment 2.1. Pilot scale CFBC plant The investigations were conducted in a pilot scale CFBC plant (Fig. 1). It consists of combustion system, control system, flue-gas sampling and analysis system. The combustion system is composed of a furnace with an inside diameter of 300 mm and a height of 6000 mm, a cyclone and a U-valve. The refractory-lined bottom of the furnace is 800 mm in height. The other parts are all made of high-temperature alloy covered with heat insulation material on the outside. Forty kilograms of quartz sand with the size of 0.5– 1.0 mm was fed into the furnace as bed material during start-up. The primary air was fed into the furnace through the air distributor in the flow range 200– 250 m3/h. Considering the height of the furnace, no secondary air was added. Thermal input to the furnace with coal and MSW was between 0.17 and 0.22 MWth. MSW and coal were separately added into the furnace through two screw feeders from the holes 890 mm above the air distributor, in the range 0– 80 and 6– 30 kg/h, respectively, depending on the cofiring rates.
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Fig. 1. Test facilities of the pilot scale CFBC plant—1: furnace; 2: water tube; 3: cyclone; 4: gas analyzer; 5: coal and MSW screws; 6: U-valve; 7: primary air meter; 8: forced draft fan; 9: air compressor; 10: bag filter; 11: induced draft fan; 12: economizer; 13: stack; T1 – T6: thermocouples; P1 – P6: pressure taps.
The temperature profiles in the furnace were maintained by adjusting the depth of a water tube penetrated into the furnace from the top. The water tube was 4000 mm in length and 51 mm in outside diameter. Concentrations of H2O, SO2, HCl, CO, CO2, NO and N2O in flue gas were continuously analyzed via an on-line infrared type gas analyzer. The gas-sampling nozzle was at the exit of the cyclone. Oxygen concentration in flue gas was analyzed with a ZrTable 1 Properties of the fuels Bituminous
Anthracite
Waste
Proximate analysis (wt.%, as received basis) Water 0.42 Ash 27 Volatiles 13.2 Fixed carbon 59.38 LHV (MJ/kg) 23.5
3.32 20.6 4.52 71.56 24.2
11.7 57.5 26.6 3.2 6.4
Ultimate analysis (wt.%, water ash free) Carbon 85.09 Hydrogen 3.92 Oxygen 7.24 Nitrogen 1.29 Sulfur 2.46 Chlorine 0 Calcium 4.56
94.62 0.96 4.01 0.11 0.3 0 1.33
73.46 7.88 15.13 3.15 0.38 0.47 9.77
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Table 2 Compositions of the coal ash and limestone Composition
Bituminous ash (wt.%)
Anthracite ash (wt.%)
Limestone (wt.%)
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O Total
35.75 22.86 9.08 17.23 1.18 10.26 0.8 1.04 0.34 0.25 98.79
53.26 18.7 6.8 6.84 1.58 2.1 1.1 2.77 1.01 0.52 94.68
1.32 0.75 0.47 91.84 0.97 0 0 0.28 0.01 0.08 95.72
type probe. Main operation parameters were recorded in a computer at the interval of 15 s. Gas analysis data were stored at an interval of 60 s. 2.2. Characteristics of coals and MSW The fuels used in the experiments included two kinds of coal—anthracite and high sulfur bituminous—and MSW. The ultimate and proximate analyses of the fuels are given in Table 1, and ash contents in Table 2. It is interesting that the calcium content in MSW is much higher than that in coals, which leads to very high Ca/(S + 0.5Cl) molar ratios with co-firing MSW and coals even without limestone addition. The size distributions of coal, MSW and limestone are all presented in Fig. 2. Most coal particles are smaller than 6 mm, with 50% cut size of about 1 mm. Most of the MSW is crashed to less than 10 mm. The size of limestone particles mainly range from 0.1 to 1 mm. 2.3. Experimental conditions In the experiments, the highest furnace temperatures were observed at 820 or 1520 mm above the air distributor, and were defined as the furnace temperature in this paper. To
Fig. 2. Size distribution of fuels and sorbent under investigation. Stars, triangles, diamonds and squares represent bituminous, anthracite, MSW and limestone, respectively.
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control the furnace temperature, fuel feeding rates and the depth of the water tube inserted into the furnace were adjusted. Average oxygen concentrations were controlled between 7.5% and 8.5%. Since the calcium content in the MSW was very high and plastic waste Table 3 Experimental conditions and results No.
Coal type
Temperature (jC)
Co-firing rate (%)
PVC, S addition
Limestone
Ca/ (S + 0.5Cl)
N2O (mg/nm3)
NO (mg/nm3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ba Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad Ad
850 868 855 838 850 856 856 854 853 859 849 831 848 852 850 858 844 843 831 853 850 830 849 835 801 807 865 855 859 902 845 851 844 845 850 850 889 884 873
0 0 0 0 0 0 0 0 32 34.6 25 39 30.9 36.6 44 51 53.6 54 56 40 57.7 53 76 73 37 43 44 53 54 50 51 56 53 57 54 54 27.4 26.4 32.2
Nb Nb Nb Nb Nb PVC PVC PVC Nb PVC Nb Nb PVC PVC Nb Nb PVC Nb Nb PVC PVC PVC Nb Nb Nb Nb Nb Nb Nb Nb Se Se PVC + Se Se Se PVC + Se PVC PVC PVC
Nb Nb Nb Yc Yc Nb Nb Nb Nb Nb Yc Yc Yc Yc Nb Nb Nb Yc Yc Yc Yc Yc Nb Yc Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb Yb Yc N Yc Yc
1.5 1.5 1.5 3.5 3.5 1.3 1.3 1.39 3.3 2.24 4.8 5.8 4.43 7.46 4.2 4.1 3.08 7.1 6.3 7.53 7.62 7.48 7.6 8.5 3.6 10.3 10.4 11 11.1 10.8 4 3.1 2.6 2.7 2.5 2.6 4.17 6.9 7.2
331 315 298 330 313 325 338 314 296 214 177 153 248 113 271 207 242 129 139 68 103 67 87 61 416 116 75 113 108 48 154 171 159 134 122 117 31 35 33
384 408 434 410 416 369 297 301 592 391 682 661 503 788 665 644 285 749 792 728 576 736 647 613 559 328 386 543 375 323 411 341 337 253 242 247 237 336 310
a b c d e
Bituminous. Without addition. With addition. Anthracite. Sulfur.
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Fig. 3. Furnace temperature. Bolt solid lines, broken thin lines and solid thin lines represent the temperatures at 820, 2520 and 6000 mm above the air distributor, respectively.
content in the MSW was comparatively low, some PVC powder, sulfur powder and limestone were mixed with coal before being added to the furnace, in some cases, to control the Ca/(S + 0.5Cl) molar ratios. Some experiment parameters, N2O and NO emissions are listed in Table 3. The co-firing rates were between 0% and 76% for the different test runs with bituminous, and the co-firing rates were between 27.4% and 57% for anthracite. Air superficial velocities in the furnace were between 3.3 and 3.8 m/s. Co-firing rate is the heat input with the MSW divided by the total heat input with coals and MSW. All gas emissions presented in this paper are normalized to dry gas with an oxygen concentration of 6%, at 273 K and 101.3 kPa. 2.4. Combustion stability Co-firing MSW with coals was stable in the experiments. Furnace temperatures of cofiring MSW with bituminous are shown in Fig. 3. In the experiments, co-firing rates were 0, 25%, 54% and 74%, respectively, and the furnace temperatures were kept quite stable,
Fig. 4. N2O and NO emissions. Blank diamonds, blank squares and blank triangles represent the N2O emissions in test1, in test2 and in test3, respectively; solid diamonds, solid squares and solid triangles represent the NO emissions in test1, in test2 and in test3, respectively.
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except those of changing co-firing rates. The furnace temperatures in the other test runs were more or less the same as those shown in this one. The emissions of N2O and NO from coal combustion at 850 jC, without any MSW and sorbent addition, are presented in Fig. 4. The emissions were observed from three different tests conducted in 3 days and it is obvious that the results are reproducible.
3. Results and discussion 3.1. Influence of furnace temperature on N2O and NO emissions It is known that rising furnace temperature leads to an increase in the rate of N2O destruction. Observations of thermal decomposition of N2O were reported with combustion of sludge [21,22], coal and char [14,15]. N2O emission from co-firing MSW with coals is shown in Fig. 5. As can be seen, raising the furnace temperature leads to N2O reduction. But in Fig. 6, there is no significant NO emission increase when furnace temperature rises from 800 to 900 jC, which is in contrast to a previous report [16]. Ca/ (S + 0.5Cl) molar ratios might be the dominant parameter influencing NO emission within the investigated temperature range, as discussed below. It is clear that NO and N2O emissions from co-firing with bituminous are higher than those observed from co-firing with anthracite, as presented in Figs. 5 and 6. 3.2. Influence of Ca/(S+0.5Cl) molar ratios and co-firing rates on N2O and NO emissions The influences of Ca/(S + 0.5Cl) molar ratios on N2O and NO emissions are shown in Figs. 7 and 8, respectively. An increase in Ca/(S + 0.5Cl) molar ratios generally leads to a decrease in N2O emission and an increase in NO emission. Co-firing of MSW with bituminous at about 850 jC, N2O emission decreases from 300 mg/nm3 to less than 100 mg/nm3, and NO increases from 350 to 700 mg/nm3, at an average, when Ca/(S + 0.5Cl) molar ratios increase from 2 to 8.
Fig. 5. Influence of furnace temperature on N2O emission. Diamonds, triangles and squares represent anthracite, with the Ca/(S + 0.5Cl) molar ratios of 10 – 11, 2.5 – 4 and 4.2 – 7.2, respectively; circles represent the bituminous, with the Ca/(S + 0.5 Cl) molar ratios of 3.1 – 3.3.
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Fig. 6. Influence of furnace temperature on NO emission. Diamonds, triangles and squares represent anthracite, with the Ca/(S + 0.5Cl) molar ratios of 10 – 11, 2.5 – 4 and 4.2 – 7.2, respectively; circles represent the bituminous, with the Ca/(S + 0.5Cl) molar ratios of 3.1 – 3.3.
Fig. 7. Influence of Ca/(S + 0.5Cl) molar ratios on N2O emission. Triangles and circles represent anthracite and bituminous, respectively.
Fig. 8. Influence of Ca/(S + 0.5Cl) molar ratios on NO emission. Triangles and circles represent anthracite and bituminous, respectively.
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Fig. 9. Influence of co-firing rates on N2O emission. Triangles and circles represent anthracite and bituminous, respectively.
The influences of co-firing rates on N2O and NO emissions at about 850 jC are shown in Figs. 9 and 10, respectively. In cases with co-firing MSW and bituminous, an increase in NO emission is observed with MSW addition, but N2O emission decreases from 340 to 80 mg/Nm3 with an increase of co-firing rates from 0% to 76%, as shown in Fig. 9. In this investigation, the increase of co-firing rate, which means higher nitrogen content in the mixed fuel and presents a greater potential for N2O emission [10], does not lead to N2O increase but in reduction. As shown in Table 4, the Ca/(S + 0.5Cl) molar ratios increase from 1.5 to 7.6 as the co-firing rates rise from 0% to 76%, and the increased Ca/ (S + 0.5Cl) molar ratios lead to N2O decomposition, as shown in Fig. 7. It is clear that the reduction of N2O emission can be attributed to the catalytic reaction of N2O decomposition over calcined limestone [13,17,18]. In addition, the ash concentration in the furnace also increases with the rise of the co-firing rates, and it further enhances the catalytic reactions of N2O decomposition over calcined limestone with stronger mixing of particles and gas. Due to the high contents of nitrogen and calcium in the MSW, both nitrogen contents and Ca/(S + 0.5Cl) molar ratios increase with higher co-firing rates, as shown in Table 4, and they both lead to the increase of NO emission. With higher Ca/(S + 0.5Cl) molar
Fig. 10. Influence of co-firing rates on NO emissions. Triangles and circles represent anthracite and bituminous, respectively.
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Table 4 Ca/(S + 0.5Cl) molar ratio and ash concentration in the furnace (co-firing bituminous and MSW) Co-firing rate (%)
0
32
51
76
Ca/(S + 0.5Cl) molar ratio Ash concentration (kg/m3)
1.5 7.2
3.3 16.1
4.1 19.7
7.6 25.6
ratios, NO emission increases through the oxidation of volatile nitrogen as well as char nitrogen with the catalytic activity of calcined limestone [10,13,17]. It is also shown in Figs. 7 and 8 that N2O and NO emissions from co-firing MSW with anthracite are lower than those from co-firing with bituminous at lower Ca/(S + 0.5Cl) molar ratios. Char concentration in the furnace in co-firing with anthracite is higher than that in co-firing with bituminous, and char takes part in reactions in reducing both N2O and NO formation from volatile N and char N oxidation [9,19,20]. Thus the higher char concentration is attributable to low N2O and NO emissions from co-firing MSW with anthracite. Moreover, attention should be paid to the fact that the volatile content in MSW is higher, and more volatile nitrogen, especially HCN, is released during devolatilisation, and its oxidation over bed material in the furnace has a high selectivity for NO formation and low selectivity for N2O formation [10].
4. Conclusion Through co-firing MSW with bituminous and anthracite using pilot scale CFBC, the influence of operating parameters on N2O and NO emissions was investigated and the main results are concluded as follows and compared with results found in literature. (1) There is a strong dependence of N2O and NO emissions on Ca/(S + 0.5Cl) molar ratios, and calcined limestone can significantly catalyze the reactions of N2O reduction and NO formation. (2) Increasing co-firing rates leads to higher Ca/(S + 0.5Cl) molar ratios of the mixed fuel and higher ash concentrations in the furnace, which enhances the catalytic reactions of N2O decomposition and NO formation over the surface of bed material particles containing calcium. (3) Furnace temperature is an important parameter influencing N2O emission and increasing furnace temperature results in N2O reduction, but there is no strong dependence of NO emission on furnace temperature in the investigated range.
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