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Understanding of Halogen Impacts in Fluidized Bed Combustion Dennis Y. Lu,* Edward Anthony, and Ruth Talbot CETC-Natural Resources Canada, Ottawa, Canada
Franz Winter, Gerhard Lo¨ffler, and Christian Wartha Vienna University of Technology, Vienna, Austria Received May 15, 2000. Revised Manuscript Received January 8, 2001
There is growing interest in cofiring coal with industrial wastes, some of which have an elevated halogen content. This study looks at the effects of adding halogens in a CFBC system. Experimental work was carried out on a pilot plant miniscale circulating FBC unit to which NaCl and I2 were added during the combustion of a high-sulfur coke and a low-sulfur bituminous coal at typical FBC temperatures. Further, the effects of limestone addition and cofiring with natural gas in conjunction with halogen addition were also investigated. Results showed that the halogen species inhibited CO and suppressed NO reduction and N2O formation. The distribution of halogen-containing products was predicted by the FACT thermodynamic database package for a wide range of combustion temperatures and other operating parameters. Results indicated that fuel type and combustion conditions have a pronounced effect on the amount of halogen or halide released. Dramatic changes in halogen products and their distribution were produced by changing the fuel from coal to petroleum coke, by adding limestone, and by cofiring with natural gas. A CFBC NO/N2O model has been employed which is based on the general kinetic model and a single particle NO/N2O formation model. The model uses the semi-theoretical approach with some measured parameters as inputs. It is capable of describing the NO, N2O, and HCN concentration histories satisfactorily even in the case of iodine addition.
Introduction Recent studies on the effects of halogens in fluidized bed combustion (FBC) systems suggest that FBC is dominated by superequilibrium free radical processes, as are high-temperature flame systems,1-5 where halogens act as catalysts for free radical recombination.6 This hypothesis explains the potential extreme increase in CO and hydrocarbons concentration in flue gas following the addition of halides3-5,7-9 such as when using NaCl or other inorganic salts as catalysts to improve sorbent utilization in FBC systems.10 (1) Bulewicz, E. M.; Janicka, E.; Kandefer, S. 10th International Conference on Fluidized Bed Combustion; ASME: New York, 1989; pp 163-168. (2) Liang, D.; Anthony, E. J.; Loewen, B. K.; Yates, D. J. 11th International Conference on Fluidized Bed Combustion; ASME: New York, 1991; pp 917-922. (3) Becker, H. A.; Code, R. K.; Gogolek, P. E. G.; Poirer, D. J. Report QFBC. TR.91.2; CANMET, 1991. (4) Julien, S.; Brereton, C. M. H.; Lim, C. J.; Grace. J. R.; Anthony E. J. Fuel 1996, 75, 1655-1663. (5) Anthony, E. J.; Bulewicz, E. M.; Janicka, E.; Kandefer, S. Fuel 1998, 77, (7), 713-728. (6) Gaydon, A. G.; Wolfhard, H. G. Flames, 4th ed.; Chapman and Hall: New York, 1978. (7) Winter, F.; Lo¨ffler, G.; Wartha, C.; Hofbauer, H.; Preto, F.; Anthony, E. J., Can. J. Chem. Eng. 1999, 77, 275-283. (8) Mueller, C.; Kilpinen, P.; Hupa, M. Combust. Flame 1998, 113, 579-588. (9) Gokulakrishnan, P.; Lawrence A. D., Combust. Flame 1999, 116, 640-652.
The growing interest in cofiring coal with industrial wastes and the high halogen content of some fossil fuels has motivated the investigation of the effect of halogens on combustion characteristics and gaseous emissions from FBC systems.11 Waste products such as municipal and hazardous wastes and spent liquors from wood pulping processes often have a high chlorine content. Fuel halogens, except for F, will normally be found in the gas phase as HX (where X ) Cl, Br, and I), or as an atom/molecule of X depending on the concentration of H. The removal of the reactive radicals can thus take place via5
H + X2 f HX + X
(R1)
H + X + M f HX + M
(R2)
where M is any stable molecule. These are followed by reaction 3 and equivalent reaction 4:
HX + OH (H) f X + H2O (H2)
(R3a, R3b)
H + OH (H) f H2O (H2)
(R4a, R4b)
(10) Smith, G. W.; Lenc, J. F.; Shearer, J. A.; Chopara, O. K.; Myles, K. M.; Johnson, I. 9th International Conference on Fluidized Bed Combustion; ASME: New York, 1987; pp 891-897. (11) Anthony, E. J.; Bulewicz, E. M.; Preto, F. 49th Industrial Waste Conference; West Lafayette, IN, 1994; pp 673-680.
10.1021/ef0000983 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/29/2001
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It is not particularly important to identify whether reaction 4a or 4b is faster since the concentrations of radicals H and OH are balanced via the fast equilibrium reaction of H2O + H f H2 + OH. The net result will always be a lowering of all active free radical concentrations in the gas phase. This must lead to an increase in the CO level since CO is the primary product of carbon oxidation and OH oxidizes CO almost exclusively via reaction 5:
CO + OH f CO2 + H
(R5)
Several experimental studies of FBC units have confirmed that halide-containing species inhibit CO oxidation through their interaction with the hydrogenoxygen radicals pool.1-5 Winter et al.7 have shown that halogens suppress the concentration of the radicals pool, and an increase in NO and a decrease in N2O were noted for different feedstocks in both lab- and pilot-scale units. Becker et al.3 observed an increase of up to 30% in NO and a small decrease in N2O upon Cl addition via CaCl2 and polyvinyl chloride (PVC) pellets to a pilotscale bubbling FBC unit. Anthony et al.11 introduced iodine in a circulating FBC unit and found an increase in NO and a steady decrease in N2O. These data are supported by the work on iodine addition in single particle tests12 where it was concluded that NO is mainly heterogeneously formed by char-nitrogen oxidation, while HCN is released in low concentrations, then homogeneously oxidized to NCO that further reacts with NO to form N2O. Lawrence and Gokulakrishnan9 added HCl, and saw a decreased NO concentration which they attributed to the inhibition of NO formation from oxidation of HCN and NCO and increased N2O emissions by inhibiting N2O destruction via the suppression of H and OH radicals toward equilibrium levels. However, it should be noted that the effect of HCl on the chemistry of NO and N2O is strongly temperature dependent. Julien4 indicated that the addition of Cl or Br to the fuel decreased NO, and doubled SO2 emissions, but had no effect on N2O emissions. The increase in SO2 and decrease in NO with Cl addition can be explained by surface modification of CaO particles due to the formation of a liquid CaCl2 phase favored by high HCl near the feed point (since the coal they used, Highvale, has a high natural Ca:S molar ratio). Formation of CaCl2 at high levels has the potential to make the CaO surface less available, thereby reducing sulfur capture and catalytic oxidation of volatile nitrogen to NO. Fuel type and combustion conditions affect the amount of each fuel halogen released and there is a significant database on this for pulverized coal combustion. However, less is known about FBC systems in terms of both net releases of halogens and the effect of halogens on the homogeneous reactions. This paper describes detailed work on the effects of halogen addition on gaseous emissions from a miniscale pilot CFBC unit to modeling work based on single fuel particle burning under welldefined conditions. (12) Winter, F.; Wartha, C.; Lo¨ffler, G.; Hofbauer, H., 26th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1996; pp 3325-3334.
Table 1. The Properties of Fuels fuel moisture proximate analysis (wt % dry basis) volatile matter fixed carbon ash ultimate analysis (wt % dry basis) carbon hydrogen nitrogen sulfur oxygen (by difference) HV (MJ/kg dry basis)
coal (coal valley)
coke (pine bend)
9.61
2.2
32.2 56.5 11.3
10.87 88.31 0.82
69.5 4.0 0.9 0.2 14.1 27.2
86.77 3.1 1.5 5.65 2.16 34.97
Experimental Setup Materials. For this study, two fuels frequently used in FBC, namely petroleum coke and coal, were used. These were Pine Bend coke, which is low in volatiles and ash, and high in sulfur, and bituminous Coal Valley coal, which is low in sulfur and high in volatiles. The proximate and ultimate analyses of these two fuels is given in Table 1. Pine Bend coke was also used to determine the kinetic constants in the modeling work. The sorbent employed was Nova Scotia limestone (CO2 ) 43 wt %, CaO ) 55 wt %). Pure salt (NaCl ) 99.9 wt %) was either added with the fuel in coal trials or injected in a water solution (10-20%) via a secondary air port. Only water injection was employed in the case of the coke trials. Iodine was added with the fuel in the solid phase. Apparatus. The CANMET pilot mini-CFBC unit consists of a refractory-lined combustor, a hot cyclone, an inclined L-valve return leg, air feed system, and an automated “loss in weight” weight feeder with computer control. The unit is equipped with comprehensive instrumentation for the measurement of variables such as temperature, pressure and gas concentrations. Stack gases are extracted from the combustor through a sintered metal filter located at the top of the cyclone for sampling and analysis. For further details of the unit, measurements, and methodology, see elsewhere.13
Equilibrium Halogen Products It is essential to know the products that may result from halogen/halide addition under FBC operating conditions to correctly understand their impacts on the combustion process. These were determined based on the calculation of a general elemental balance for bituminous coal generated using the FACT (Facility for the Analysis of Chemical Thermodynamics) database package. In an earlier work, Liang et al.2 indicated that halogen (Cl, Br, or I) products are predominantly in the gas phase at typical FBC temperatures (850 °C). However, as shown here, fuel type and combustion conditions can affect the amount of each halogen released. Here, the possible forms of halogen release have been predicted by a new updated FACT package14 for both fuels at different conditions in this study. The fuel was expressed in elemental form in order to be acceptable to the FACT package and all the possible products were extracted. The equilibrium queries of fuel combustion with air (E1 and E2) were given using the appropriate (13) Anthony E. J.; Lu, Y., 27th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 3093-3101. (14) http://www.crct.polymtl.ca.
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molar coefficients for our experimental work (see later).
for coal: C + 0.6844H + 0.0111N + 0.0011S + 0.1523O + 0.000025Cl + 0.0036Ca + 5.0745N2 + 1.1807O2 + 0.0131NaCl/0.003I2 (E1) for coke: C + 0.4248H + 0.0148N + 0.0244S + 0.0187O + 0.00013Ca + 5.1183N2 + 1.1909O2 + 0.014NaCl/0.0032I2 (E2) The elemental balances were determined for each halogen/halide between 700 and 1000 °C, at 50 °C intervals, for atmospheric pressure conditions. Calculations were performed for solid fuel combustion alone, cofiring with natural gas (∼12% of total heat input) and with limestone addition. Modeling The effect of the halogens Cl and I was studied under simple flow reactor conditions applying the program PFRCalc V 2.0.15 An inlet gas composition was calculated assuming that the ultimate analyses of the two fuels (Table 1) were constant for fuels. It was assumed that the volatiles products are C2H4, CO, H2O, H2S, and HCN, and the products of char combustion are CO, H2O, and SO2. The char nitrogen is assumed to be released as HCN (20 mol %), NO (30 mol %), and N2 (50%). The expected gas composition was then calculated employing the mass balance for C, H, O, N, and S for the coke and coal, respectively, and adding the H2O for the moisture content (Table 2). This gas mixed with air reacts in a flow reactor at an air-to-fuel ratio of 1.3, a temperature of 850 °C, and a residence time of 1.5 s. Finally, the effects of two different levels of halogen addition, i.e., HCl (2500 ppm, 5000 ppm) and I2 (500 ppm, 1000 ppm), were compared with the model results for the case without halogen addition. The detailed chemical kinetic reaction scheme was developed from a number of different reaction schemes. The base mechanism describing the C/H/O/N chemistry is taken from Glarborg et al.16,17 The sulfur chemistry at intermediate temperatures was modeled by a mechanism published by Glarborg et al.18 Finally, the reaction mechanisms of Babushok et al.19 and Roesler et al.20 were added to describe the effect of iodine and chorine addition, respectively. The actual rates of the reverse reactions were then calculated using the thermodynamic data taken from Sandia Thermodynamic Database21 with changes as recommended by Glarborg et al.16,18 The (15) Lo¨ffler, G.; Winter, F.; Hofbauer, H.; Report No. VTWS-99-FB02; Institute of Chemical Engineering, Fuel Technology and Environmental Technology, Vienna University of Technology: Vienna, Austria 1999. (16) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27. (17) Glarborg, P.; Ostberg, M.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. 27th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 219-226. (18) Glarborg P.; Kubel, D.; Dam-Johansen, K.; Chiang, H. M.; Bozzelli, J. W. Int. J. Chem. Kinet. 1996, 28, 773-790. (19) Babushok, V.; Noto, T.; Burgess, D. R. F.; Hamins, A.; Tsang, W. Combust. Flame 1996, 107, 351-367. (20) Roesler, J. F.; Yetter, R. A.; Dryer, F. L. Combust. Flame 1995, 100, 495-504. (21) Kee, R. F.; Rupley, F. M.; Miller, J. A.; Sandia National Laboratories Report SAND87-8215B; Sandia National Laboratories: Livermore, CA 1993.
Table 2. Assumed Gas Composition from Devolatilization and Char Combustion coke C2H4 CO H2O H2S SO2 HCN NO N2
1.02 79.17 17.13 0.25 1.56 0.30 0.21 0.35
coal vol % vol % vol % vol % vol % vol % vol % vol %
5.47 68.65 25.09 0.03 0.05 0.44 0.10 0.17
vol % vol % vol % vol % vol % vol % vol % vol %
thermodynamic data for the iodine species and NOCl were taken from Burgess et al.22 and Mueller et al.,8 respectively. Results Equilibrium Calculations. The equilibrium halogen products over a temperature range of 700° to 1000 °C were calculated using (E1) and (E2) as inputs to FACT (Tables 3-6). A list of possible products contains more than 45 species for chloride and more than 20 species for iodine. The tables present only those halogencontaining products which are greater than 1 × 10-4 mol % in any of the possible species phases. The results from Tables 3 and 4 indicate that the possible halogen products, particularly with the chloride species, change dramatically depending on the temperature and fuel type. While the melting and boiling points of NaCl are 810 °C and 1465 °C, respectively, FACT predicts that gas-phase NaCl may appear at temperatures even lower than 810 °C. With increasing temperature, more solid and liquid NaCl is converted to the vapor phase in the form of either NaCl or {NaCl}2, while some of the NaCl is converted to HCl. However, this conversion is predicted to be less than 7% in the temperature range examined here. The clear implication here is that, if NaCl is added to the system at these levels, any effect on CO emissions is likely to be due not to HCl but NaCl directly. In fact, such an effect is evidently possible via the following reaction:
NaCl + H f Na + HCl
(R6)
At the typical FBC temperature of 850 °C, FACT predicts a chloride distribution of 12.6 mol % in liquid NaCl, 81.6 mol % in gas NaCl/{NaCl}2, and 5.8 mol % gas HCl. All calculations show a negligible level of CaCl2 either in liquid or vapor phase although the coal ash contains some Ca. However, it is interesting to note that, over the entire temperature range examined here, when NaCl is added in the combustion of coke (a fuel with low volatile and ash contents (Table 1)), chlorine products are always predicted to be predominantly in the vapor phase (mainly as HCl). The formation of a small amount of Cl and Cl2 is also predicted. The iodine-containing species are shown to be present entirely in the gas phase in the temperature range investigated. The I products are mainly associated with the formation of I and I2, as well as a very small amount of HI. The concentration of CaI2 is negligible and is not shown in the overall summary. Increasing the temperature leads to a higher conversion of I2 to I. The gas(22) Burgess, D. R. F., Jr.; Zachariah, M. R.; Tsang, W.; Westmoreland, P. R. Technical Note 1412; NIST: Washington, DC, 1995.
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Table 3. The Predicted Chloride/Iodide Distribution in Products at Different Temperatures for Coal temperature (°C) products
700
750
800
850
900
950
1000
HCl Cl NaCl {NaCl}2 NaCl (solid) total HI I I2 HIO total
0.960 0.002 1.459 1.065 96.515 100.00 0.032 59.332 38.318 2.317 100.00
1.429 0.001 5.388 4.384 88.799 100.00 0.052 74.287 23.715 1.946 100.00
3.156 0.002 17.497 15.641 63.704 100.00 0.074 85.704 12.640 1.563 99.98
5.787 0.007 44.252 37.316 12.637 (liquid) 100.00 0.097 91.946 6.740 1.205 99.99
6.626 0.011 62.991 30.735
5.998 0.016 74.927 19.057
5.480 0.021 83.434 11.063
100.00 0.137 95.294 3.523 1.040 99.99
100.00 0.146 97.189 1.948 0.713 100.00
100.00 0.173 98.174 1.094 0.557 100.00
Table 4. The Predicted Chloride/Iodide Distribution in Products at Different Temperatures for Coke temperature (°C) products
700
750
800
850
900
950
1000
HCl Cl Cl2 NaCl {NaCl}2 total HI I I2 HIO total
99.395 0.029 0.560 0.014 0.000 100.00 0.027 59.403 38.819 1.751 100.00
99.461 0.051 0.349 0.089 0.001 100.00 0.043 74.426 24.054 1.477 100.00
99.147 0.086 0.283 0.468 0.012 100.00 0.061 85.202 13.557 1.179 100.00
97.484 0.137 0.204 2.083 0.090 100.00 0.080 91.727 7.279 0.913 100.00
91.622 0.198 0.138 7.560 0.479 100.00 0.100 95.313 3.885 0.702 100.00
77.260 0.249 0.076 20.820 1.592 100.00 0.121 97.222 2.115 0.542 100.00
54.176 0.252 0.030 42.443 3.096 100.00 0.143 98.245 1.188 0.424 100.00
Table 5. The Predicted Chloride Distribution in Products at Different Operating Conditions products
basea
HCl NaCl {NaCl}2 NaCl (liquid) total
5.787 44.252 37.316 12.637 100.00
HCl Cl Cl2 NaCl {NaCl}2 NaCl (liquid) total
97.484 0.137 0.204 2.083 0.090 100.00
L.A. + N.G.
solutiond
Coal with Addition of NaCl, mol % 6.179 5.800 0.467 51.301 44.413 44.878 42.508 37.452 37.844 12.328 16.811 99.99 99.99 100.00
0.472 45.019 37.963 16.547 100.00
8.947 47.162 39.770 4.112 99.99
Coke with Addition of NaCl, mol % 97.546 4.236 0.773 0.193 0.006 0.000 0.373 0.000 0.000 1.819 40.907 41.460 0.059 34.495 34.961 20.356 22.806 100.00 100.00 100.00
0.387 0.000 0.000 41.713 35.175 22.725 100.00
98.543 0.078 0.063 1.282 0.032
@ 6% O2
L.A.b
N.G.c
100.00
Base operation: O2 ) 3%, Tb ) 850 °C with addition of halogen as solid. b Limestone addition (L.A.) : Ca/S ) 18 for coal and 1.89 for coke. c Cofiring with natural gas (N.G.) at 11-12% of total input heat. d NaCl addition as 10-20% solution. a
phase distribution of I products at 850 °C is predicted to be 92 mol % of I and 7 mol % of I2. In contrast to NaCl addition, there is no difference in the I product distribution between the two types of fuel. Tables 5 and 6 summarize the effects of operating conditions on the distribution of halogen-containing species at typical FBC temperatures (850 °C). With coal combustion, under all operating conditions with NaCl addition, the Cl products are mostly in the vapor phase as NaCl and {NaCl}2. However, some changes in HCl were predicted, i.e., HCl decreases when cofiring natural gas and increases when NaCl is injected as a water solution. In the case of coke combustion with halide addition, there is a dramatic change in Cl product distribution when limestone is added and/or the coke is co-fired with natural gas. Under these conditions, the Cl product was predominated by NaCl and {NaCl}2 instead of HCl, and a significant amount of NaCl (∼20 mol %) is retained in the liquid phase. It should be noted that co-firing with natural gas is the only operating condition that significantly alters the I product distribu-
Table 6. The Predicted Iodide Distribution in Products at Different Operating Conditions products
basea
@ 6% O2
L.A.b
N.G.c
L.A. + N.G.
HI I I2 HIO CaI2 total
Coal with Addition of I2, mol % 0.097 0.068 0.097 65.528 91.946 92.272 91.555 33.318 6.740 6.180 7.152 0.938 1.205 1.480 1.197 0.003 0.000 0.000 0.000 0.216 100.00 100.00 100.00 100.00
65.266 33.571 0.949 0.000 0.215 100.00
HI I I2 HIO CaI2 total
Coke with Addition of I2, mol % 0.080 0.056 0.084 57.098 91.727 92.388 91.766 41.430 7.279 6.410 7.278 1.464 0.913 1.146 0.872 0.000 0.000 0.000 0.000 0.009 99.99 100.00 100.00 100.00
51.132 46.825 1.859 0.000 0.184 100.00
a Base operation: O ) 3%, Tb ) 850 °C with addition of halogen 2 as solid. b Limestone addition (L.A.): Ca/S ) 18 for coal and 1.89 for coke. c Cofiring with natural gas (N.G.) at 11-12% of total input heat.
tion in the gas phase. When natural gas is introduced at around 10-12% of the total heat input, the I products
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Table 7. Operating Parameters and Emission Results from Coal Trials trial
halogen (g/h)
SO2a (ppm)
NOxa (ppm)
N2Oa (ppm)
841 545 764 469
170 121 159 86
170 161 192 326
94 74 86 98
Addition of NaCl 872 2.48 878 3.06 890 2.67 879 3.01 884 2.65
1210 1322 3453 3575 7201
188 161 147 140 102
164 141 127 107 229
70 47 27 25 18
Addition of I2 866 3.06 870 2.44 882 2.21 871 2.75 853 5.18
766 5821 10157 8182 4965
91 115 161 157 85
153 248 239 212 112
40 27 14 56 36
Vf (m/s)
Tb (°C)
Tf (°C)
O2 (%)
0 0 0 0
3.37 3.35 3.06 3.17
852 851 856 852
870 846 862 856
3.09 6.09 2.99 2.82
coal5, base coal6, NaCl (s) coal7, NaCl (s) coal8, NaCl (s) with N.G. coal9, NaCl (s) with L.A.
0 100 200 200 200
2.78 2.83 2.94 3.05 3.01
855 853 849 851 851
coal10, base coal11, I2 (s) coal12, I2 (s) coal13, I2 (s) with N.G. coal14, I2 (s) at high O2
0 100 200 200 200
2.72 2.72 2.80 2.88 2.93
855 855 854 855 855
coal1, base coal2, high O2 coal3, with N.G. coal4, with L.A.
a
COa (ppm)
Corrected at 3% oxygen dry basis. Table 8. Operating Parameters and Emission Results from Coke Trials trial
halogen (g/h)
Vf (m/s)
Tb (°C)
Tf (°C)
O2 (%)
COa (ppm)
SO2a (ppm)
NOxa (ppm)
N2Oa (ppm)
850 859 854
2.60 3.57 3.77
1564 1151 1109
3539 915 767
256 349 311
184 190 175
Addition of NaCl 863 3.54
872
745
238
124
coke1, base coke2, with L.A. coke3, with L.A. and N.G.
0 0 0
2.99 2.77 3.18
849 857 856
coke4, base with L. A. coke5, with water and L. A. coke6, NaCl (liquid) with L. A. coke7, NaCl (liquid) with L.A.
0
2.26
860
0
2.60
859
859
2.29
881
842
213
144
100
3.00
857
860
3.46
1407
285
316
46
200
2.85
861
864
2.52
3054
160
569
9
0 200 200
3.14 2.88 2.86
856 857 858
Addition of I2 847 2.86 833 2.86 840 2.48
1118 12066 9494
2013 2698 953
144 255 270
178 N/A N/A
coke8, base coke9, I2 (s) coke10, I2 (s) with L.A. a
Corrected at 3% oxygen dry basis.
are mainly found in the form of HI (50-65 mol %), thus lowering the distribution of I and I2. There is also a small amount of CaI2 in the gas phase (∼0.2 mol %). This result helps to explain some earlier confusing results where I addition showed a relatively large effect on coal combustion but very little when added to a CFBC burning wood, which has an elevated volatile production.11 Gas Emissions. Tables 7 and 8 give the operating conditions and gas emissions associated with the addition of halogens. The effects of limestone addition and cofiring with natural gas on CO, SO2, NOx and N2O emissions can also be found in these tables. Under base conditions (i.e., without the addition of halogen), decreases in CO and SO2 emissions occurred upon increasing oxygen, cofiring with natural gas, and limestone addition. It should be noted that the use of limestone produces the most significant effects on the reduction of both CO and SO2 emissions. However, a significant increase in NOx concentration was also found in the case of limestone addition. By contrast, cofiring with natural gas and increasing oxygen have a minor effect on NOx emissions. The influence of changing operating parameters on N2O emissions is less clear. Except for NOx, all measured gas concentrations are higher for petroleum coke
than for coal. NOx emissions from coke combustion and coal combustion are comparable. For petroleum coke, which normally has higher fuel nitrogen than coal, char loadings can also be expected to be higher. In consequence, there is greater NO reduction by CO, as NO reduction occurs on the surface of char particles. Higher CO levels also result from the catalytic effect of SO2 on radical recombination reactions.13 Halogen addition produced significant effects on gas emissions in this study (Tables 7 and 8). A dramatic increase in CO concentration was observed with the addition of NaCl or I2. CO emissions increased at the lower NaCl (100 g/h) addition rate with NaCl introduced as either a solid or in water solution. CO emissions increased even more sharply upon I2 addition. For coal, the CO concentration of around 800 ppm for the base run increased to about 5000 ppm with the addition of 100 g/h I2 and to around 1% when I2 rate was doubled to 200 g/h. By contrast to CO emissions, N2O concentrations decrease with the addition of halogen, although these changes might also be caused by minor changes in the operating parameters, more specifically the effect of variation in the combustor temperatures. However, the effect of halogen addition on SO2 and NOx emissions shows no general trend. Depending on the type of
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Figure 1. Calculated concentrations of O, H, OH, and CO during the ignition period. Temperature 850 °C, air-to-fuel ratio 1.3. Bold lines indicate the results without halogen addition, while thin lines present the concentrations with HCl or I2 addition. (a-b) Coke: inlet concentrations are CO 22.47 vol %, C2H4 0.29 vol %, H2O 4.86 vol %, HCN 865 ppm-vol, NO 589 ppm-vol, SO2 4440 ppm-vol, H2S 711 ppm-vol, O2 15.17 vol %, HCl 5000 ppm (a) and I2 500 ppm (b). (c-d) Coal: inlet concentrations are CO 16.57 vol %, C2H4 1.32 vol %, H2O 6.05 vol %, HCN 1071 ppm-vol, NO 245 ppm-vol, SO2 109 ppm-vol, H2S 81 ppm-vol, O2 16.38 vol %, HCl 5000 ppm (c) and I2 500 ppm (d).
halogen, either an increase or a decrease in SO2 and NOx concentration was observed. NaCl was found to reduce SO2. This effect was clearer when NaCl was added in conjunction with the sorbent since sulfur capture by limestone can be promoted by salt addition in FBC.1,10 For high-sulfur coke, sulfur capture of 80% was obtained with limestone addition at Ca:S molar ratio of 1.89. Remarkably high sulfur reductions of 92 and 96% were achieved when NaCl was added at rates of 100 and 200 g/h, respectively. As a consequence of the dramatic SO2 reduction, NOx also increased with NaCl and limestone addition. However, it was observed that there is no significant effect on SO2 and NOx emissions when NaCl is added by itself in the combustion of coke (Tables 7 and 8). On burning coal it was noted that the NaCl itself acted to slightly decrease NOx concentration instead of increasing it in conjunction with limestone addition. The change in SO2 concentration upon the addition of NaCl is most likely attributable to the fluctuation of operating parameters, such as oxygen concentration. Increases in SO2 and NOx emissions were also observed with the addition of I2 to the system. The only
reducing trend found with the addition of iodine was on N2O emissions. Less than half of the base case N2O emissions were observed at the high I2 addition rate (200 g/h). Unfortunately, the FTIR analyzer was not available for the trials for iodine addition in coke combustion. However, following earlier work performed on the CANMET 0.8 MWth CFBC unit, it can be assumed that the same type of reduction of N2O would occur for coke. In earlier work a steady decrease in N2O emissions was obtained with increased iodine concentration in the bed and an even sharper reduction of N2O was observed for coke (the same as in this study) than for coal (DEVCO Prince).7 Model Calculations. As expected the calculated emissions from the flow reactor are not equal to the CFBC emissions. This arises because of no heterogeneous effects as SO2 captured by limestone, no heterogeneous catalyzed oxidation of HCN or reduction of NO and N2O on the bed material. Similarly such an approach fails to consider secondary CO formation from the Boudouard reaction, and the quenching of radicals on the surface of the bed material. Moreover devolatilization and char combustion occur along the whole riser
Fluidized Bed Combustion
Energy & Fuels, Vol. 15, No. 3, 2001 539
so that the released species may have significantly lower residence times than would be calculated based on gas flow rates. Nevertheless the calculations demonstrate the effect of the halogen species on the radicals pool and consequently on homogeneous reactions in the combustion chamber. The general trends observed experimentally can thus be confirmed and explained. In Figure 1 the effect of halogens on free radical concentrations and the rate of CO oxidation during the ignition can be seen. In the presence of HCl or I2, ignition is delayed and the concentration of O, H, and OH are significantly reduced. The latter results in a decreased CO oxidation rate. Thus, in the case of coke (refer to Table 2), CO emissions increase from 5 to 86 and 262 ppm-vol for 2500 and 5000 ppm-vol HCl addition, respectively (Figure 1a). The effect of iodine is even stronger (compare to Tables 7 and 8). Thus, the CO outlet concentration increased to 242 ppm-vol when 500 ppm I2 was present in the inlet gas (Figure 1b). For 1000 ppm-vol I2, CO oxidation is almost completely suppressed (84% CO not converted). The results for coal are similar. The calculations also show that HCl acts as homogeneous catalyst for the recombination of free radicals (i.e., O, H, OH) via the following route:
HCl + O f Cl + OH
(R7)
HCl + OH f Cl + H2O
(R8)
HCl is regenerated directly by
Cl + H2 f HCl + H
(R9)
Cl + HO2 f HCl + O2
(R10)
Cl + Cl + M f Cl2 + M
(R11)
NO + Cl + M f NOCl + M
(R12)
NOCl + Cl f NO + Cl2
(R13)
Cl2 + H f HCl + Cl
(R14)
and via Cl2
Iodine acts via the reaction sequence R2 and R3. At an I2 concentration level of 500 ppm in the inlet gas, the I concentration is too small to significantly affect H atom concentration produced via reaction R5 and the chain branching reaction of O2 + H f H + OH. Thus, I2 addition causes a significant ignition delay and decreases the radical level for most of the reaction time, but does not affect the maximum radical concentrations greatly (Figure 1). For higher I2 concentrations, however, the oxidation of CO is almost completely suppressed. Discussion Inhibition of CO Oxidation and Sulfation. Inhibition of CO oxidation was always observed regardless of the type of fuel or halogen or halides addition, which is as expected given that reaction 5 is the major route for CO oxidation in FBC as in the flame combustion process. The effect of I2 is much more powerful than that of NaCl in terms of increasing the CO concentration. This is in agreement with the findings in flame studies,
which indicate that the magnitude of the influences of halogens on radical recombination is F, Cl
4000 ppm under their conditions), a liquid CaCl2 phase is transiently formed on CaO particles passing through the halogen injection zone. They hypothesized that the liquid-phase calcium chloride has the potential to prevent sulfur capture by making the CaO surface less available and decomposing CaSO4 formed during earlier sulfation reactions. However, the chloride and iodine distribution in the products predicted by FACT suggests that there should be very little in the way of liquid calcium halogens in the experiments carried out in this study (Tables 3-6). The fact that iodine addition did not decrease sulfur capture with limestone addition when burning petroleum coke (Table 8), implied that any transient CaI2 production should not be expected in FBC conditions. Effect on NO/N2O Formation. The CFBC NO/N2O formation model is capable of describing the NO, N2O, and HCN concentration histories satisfactorily even in the case of iodine addition.7 Without addition of iodine, the model predicts that the high release of HCN and NO in the bottom region of the riser causes high concentrations of HCN and NO in the dense region. Because of the low volatile matter content of coke, NO is mainly formed during char combustion, whereas HCN
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Figure 2. Calculated outlet concentrations of NO and N2O versus halogen concentration. Temperature 850 °C, Air-to-fuel ratio 1.3, residence time 1.5 s. Coke: inlet concentrations: CO 22.47 vol %, C2H4 0.29 vol %, H2O 4.86 vol %, HCN 865 ppm-vol, NO 589 ppm-vol, SO2 4440 ppm-vol, H2S 711 ppm-vol, O2 15.17 vol %. (a) HCl addition, (b) I2 addition.
originates from devolatilization and char combustion. The formation of N2O is at a maximum in the bottom part of the riser. But the N2O destruction reactions are also very strongly affected by the high concentrations of radicals in the gas phase and the high solids and char load in the emulsion phase.23 The char as well as the bed material are catalysts supporting N2O decomposition. In the upper part of the riser the conversions of carbon and fuel-nitrogen are small. Therefore, the rates of the heterogeneous as well as the radical reduction reactions of NO and N2O are low.12 Only at 900 °C does the N2O concentration decrease with height. At this temperature the thermal decomposition becomes more important.7 The lower radical level produced because of the presence of halogen also changes the selectivity in fuel-N conversion. HCN is converted to NCO mainly via R15, and NCO either reacts with O to convert to NO (R16) or reacts with NO to N2 (R17) and N2O (R18), respectively,
HCN + O f NCO + H
(R15)
NCO + O f NO + CO
(R16)
NCO + NO f N2 + CO2
(R17)
NCO + NO f N2O + CO
(R18)
The decreased O radical level in the presence of halogens decreases the selectivity for NCO toward NO. Moreover, the destruction of N2O mainly by the H radical is decelerated via
N2O + H f N2 + OH
(R19)
Thus, the halogens cause higher N2O emissions and reduce the NO levels as shown in Figure 2 for the gas composition representing coke combustion. In contrast, the measurements show increased NO and decreased N2O emissions at least for the iodine addition. In the CFBC riser, volatiles and CO are released over the whole height of the reactor. Thus, the radical concentra(23) Hulgaard, T. Nitrous Oxide from Combustion. Ph.D. Thesis, Technical University of Denmark, Lyngby, Denmark, 1991.
tions are lower than they would be if the reactions were localized and iodine addition can cause total suppression of the HCN conversion. Thus, N2O formation via reaction R18 is hindered and the reduction of NO, which is formed heterogeneously from char combustion via reactions R17 and R18, is lower. Conclusions The following conclusions concerning the impacts of halogen addition on FBC emissions, may be made based on the work performed in this study: 1. Iodine addition causes the inhibition of CO oxidation by reduction of radical concentrations, as well as the suppression of NO reduction and N2O formation. No evidence has been produced which supports a role for iodine in SO2 capture. 2. NaCl addition is found to enhance sulfur capture and act as a catalyst for the sulfation process. As a consequence of significant SO2 reduction upon addition of NaCl and limestone, the concentration of NOx may increase significantly. 3. On the basis of results from the FACT database package, possible halogen products and their distribution appear to vary dramatically according to fuel properties and operating conditions at typical FBC temperatures. 4. Modeling has been successfully employed to describe the NO/N2O chemistry under CFBC conditions, which includes formation and destruction reactions even in the case of iodine addition. 5. Under the conditions tested for coke, the model indicates that NO is mainly heterogeneously formed, and N2O is homogeneously formed, even during char combustion. Thermal destruction seems to be of minor importance at temperatures below 900 °C. Acknowledgment. The authors acknowledge the contributions of G. Lett from CANMET Energy Technology Centre-FBC group for operating the pilot-scale CFBC. The comments and fruitful discussions with Professor Bulewicz from Cracow University of Technology, Poland, are also highly appreciated. EF0000983
