1950
Energy & Fuels 2009, 23, 1950–1955
New Approach to Understanding NO Emission during Bubbling Fluidized Bed Coal Combustion: Separation of NO Formation and Reduction Processes in the Bed Takahiro Murakami* and Yoshizo Suzuki National Institute of AdVanced Industrial Science and Technology (AIST) 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
Abdullah Khan Durrani Institute of Chemical Engineering and Technology, UniVersity of the Punjab, Lahore 54590, Pakistan ReceiVed October 1, 2008. ReVised Manuscript ReceiVed February 5, 2009
A new approach for separately evaluating NO formation and NO reduction during coal combustion in a fluidized bed was investigated. Sixteen different coals were burned in a laboratory-scale bubbling fluidized bed combustor with three different char loadings, which were established by changing the inlet oxygen concentrations. NO conversion ratios (XNO ) the ratio of the mass of N atoms in the emitted NO to the mass of N atoms in the coal) were determined for the coals, and then “ultimate NO conversion ratios” (XNO*) were obtained by extrapolation of the lines describing the dependence of XNO on O2 consumption rate to an O2 consumption rate of zero. The XNO* values correspond to the values that would be obtained for a single coal particle burning in the absence of char particles. Using this technique, the contribution of char particles to NO reduction was evaluated. The XNO* values depended on the N content and the O/N molar ratio of the coals. The bed temperature dependences of the sensitivity values (the slopes of the lines describing the dependence of XNO on O2 consumption rate), which include the contribution of NO reduction by char particles, differed substantially for the different coals. However, char loading had no clear effect on N2O emission. This result suggests that the reduction by the char particles involves the competitive reaction of NO and N2O, and the effect of the N2O reduction by burning char particles was less than NO reduction. Our results indicate the importance of taking into account the effects of NO reduction by char particles on overall NOx emissions during fluidized bed combustion of coal.
Introduction Fluidized bed combustion (FBC) is an excellent technique for coal combustion because the low operating temperature (∼1123 K) and in-bed desufurization with limestone result in low NOx and SOx emissions. Therefore, FBC has been used for coals with a wide variety of properties. In previous studies, the relationships between NOx emissions and FBC operating parameters such as temperature, pressure, air ratio, and bias ratio and coal properties such as N content, fuel ratio, C/N, O/N, and H/C have been experimentally elucidated.1-6 In addition, the effect of limestone on NOx formation has been studied.7,8 Several attempts have been made to predict NOx emissions by using correlations obtained from fundamental studies. * Corresponding author. Tel.: +81-298-61-8624. Fax: +81-298-61-8209. E-mail:
[email protected]. (1) Åmand, L. E.; Leckner, B. Combust. Flame 1991, 84, 181–196. (2) Johnsson, J. E. Fuel 1994, 73, 1398–1415. (3) Liu, D. C.; Shen, B. X.; Feng, B.; Lin, Z. J.; Lu, J. D. Energy Fuels 1999, 13, 1111–1113. (4) Suganal, D. D.; Hosoda, H.; Hirama, T. An Evaluation and Comparison of NO2 and NOx with Char in a Fluidized Bed Combustor. Proceedings of Coal Tech 2000, Jakarta, November 13-14, 2000; pp 281289. (5) Svoboda, K.; Pohorely, M. Fuel 2004, 83, 1095–1103. (6) Janvijitsakul, K.; Kuprianov, V. I. Fuel 2008, 87, 1574–1584. (7) Liu, H.; Gibbs, B. M. Fuel 2001, 80, 1211–1215. (8) Shimizu, T.; Satoh, M.; Sato, K.; Tonsho, M.; Inagaki, M. Energy Fuels 2002, 16, 161–165.
However, the predictions based on these correlations are not sufficiently accurate. The predictions for FBC of coal are usually less accurate than the predictions for pulverized coal combustion, and coals with the same analytical values frequent give very different NOx emissions in FBC.4 Therefore, combustion tests are still required for the accurate determination of NOx emissions. For more accurate and reliable predictions, a better understanding of the factors relating to NOx emissions under FBC conditions is required. Kunii et al. pointed out that char particles can reduce NO to N2 under FBC conditions.9 Since their pioneering study, many studies of the mechanisms of NO reduction by char have been done.10-13 The activity of char for NO reduction varies markedly because char characteristics such as porosity have a strong effect. In addition, minerals in char particles promote the reaction between NO and char. In particular, sodium-containing minerals have noticeable effects on the reduction reaction.13 Figure 1 shows the reaction pathway for the formation and destruction (9) Kunii, D.; Wu, K. T.; Furusawa, T. Chem. Eng. Sci. 1980, 35, 170– 177. (10) Jensen, A.; Johnson, J. E.; Andries, J.; Laughlin, K.; Read, G.; Mayer, M.; Baumann, H.; Bonn, B. Fuel 1995, 74, 1555–1569. (11) Lin, S.; Suzuki, Y.; Hatano, H. Energy Fuels 2002, 16, 634–639. (12) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteljn, F.; Moulijn, J. A. Combust. Flame 1994, 99, 499–507. (13) Zhao, Z.; Qiu, J.; Li, W.; Chen, H.; Li, B. Fuel 2003, 82, 949– 957.
10.1021/ef800833m CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
NO Emission during Coal Combustion
Energy & Fuels, Vol. 23, 2009 1951
Figure 1. Reaction pathways for the formation and reduction of NO and N2O during devolatilization, pyrolysis, and char combustion.
Figure 2. Schematic diagram of a laboratory-scale bubbling fluidized bed combustor.
of NO and N2O under FBC conditions.2 Researchers have developed mathematical models for these pathways and carried out kinetic calculations.8-10
bed could be separately evaluated and the effect of different coals on NO emissions could be explained.
Previous studies on the effect of different coals on NOx emissions have dealt mainly with overall NOx emissions. However, because of the existence of a dense particle phase containing char particles, which strongly reduce NOx, the overall NOx emissions will reflect both NOx formation and NOx reduction by char. Therefore, it is important that the contributions of these two processes be evaluated separately. In particular, more information about the contribution of char to NO reduction is required.
Experimental Details
The objective of the current study was to evaluate the effect of different coals not on the overall NO emission but on NO formation and NO reduction individually. To separate the formation and reduction processes, a new experimental technique was employed. We carried out combustion experiments using 16 different coals in a laboratory-scale bubbling fluidized bed combustor with different char loadings in the bed. By using this methodology, the formation and reduction processes in the
The main components of the laboratory-scale bubbling fluidized bed combustor were a screw feeder, a reactor, an electric furnace, a cyclone, an ash pot, and filters for dust and moisture removal (Figure 2). The transparent quartz reactor was 32 mm in internal diameter and 450 mm in height, and the central part of the reactor was heated by the electric furnace. A sintered porous silica plate placed in the quartz tube served as a gas distributor. The bed temperature was measured with a type-K thermocouple immersed in the bed. Silica sand (mean diameter, 200 µm) was employed as the bed material; the static bed height was 30 mm. Fresh bed material was used in every experiment, to minimize the effect of the minerals in the ash. Combustion air (a mixture of N2 and O2 in proportions controlled by means of mass flow controllers) was supplied to the bed through the gas distributor. Coal was pneumatically injected into the bed continuously by means of the screw feeder after the bed temperature exceeded a predetermined temperature. The coal supply point was a few millimeters above the gas distributor. The coal feed rate
1952 Energy & Fuels, Vol. 23, 2009
Murakami et al. Table 1. Coal Properties
proximate analysis [wt %]
ultimate analysis [dry, wt %]
sample no.
mois.
VM
FC
ash
C
H
N
S
O
ash
HHV [MJ/kg (d.b.)]
SS088 SS033 SS075 SS090 SS099 SS078 SS085 SS100 SS087 SS083 SS071 SS074 SS049 SS084 SS086 SS076
3.24 10.84 6.25 2.58 4.86 6.11 1.45 4.23 6.73 3.20 9.30 6.26 2.01 2.92 2.23 5.68
31.58 43.57 29.36 45.95 31.62 40.03 24.16 32.22 43.26 39.86 39.87 27.91 24.98 33.10 40.52 33.81
58.39 44.71 53.40 42.68 51.72 47.64 60.09 52.00 45.40 44.92 45.02 57.67 59.82 50.77 47.99 52.27
6.79 0.88 10.99 8.79 11.80 6.22 14.30 11.55 4.61 12.02 5.81 8.16 13.19 13.21 9.26 8.24
76.02 71.98 72.62 71.78 71.29 72.36 72.44 71.35 70.92 68.80 71.29 75.10 71.20 68.21 72.59 73.12
4.36 5.31 4.20 5.71 4.27 5.23 4.10 4.25 4.82 4.85 5.08 4.18 4.02 4.16 5.39 4.84
0.90 1.03 1.04 1.14 1.24 1.28 1.46 1.59 1.65 1.67 1.71 1.72 1.77 1.81 1.87 2.10
0.70 0.05 0.43 1.16 0.78 0.17 0.31 0.57 0.41 0.59 0.50 0.30 0.34 0.41 0.14 0.27
11.00 20.64 9.99 11.19 10.02 14.34 7.18 10.18 17.26 11.67 15.01 9.99 9.21 11.80 10.54 10.93
7.02 0.99 11.72 9.02 12.40 6.62 14.51 12.06 4.94 12.42 6.41 8.71 13.46 13.61 9.47 8.74
30.67 25.56 27.32 30.88 29.41 27.74 29.33 28.95 29.08 28.91 26.61 28.03 28.16 27.68 30.55 28.07
was maintained at 0.05-0.35 g/min, and the inlet gas flow rate was fixed at 2.50 LN/min. Thus, the superficial gas velocity (Uo) was about 0.21 m/s, and the minimum fluidization velocity (Umf) was 0.014 m/s at 1123 K, respectively. Therefore, Uo/Umf is about 15, which is higher than that of the usual combustor. Experiments were conducted at three bed temperatures: 1093, 1123, and 1153 K. The O2 concentration in the combustion air was 20, 15, or 10% by volume, and the O2 concentration in the flue gas was adjusted to approximately 3.5% by controlling the coal feed rate. Because the char loading in the bed was determined by the balance between the combustion rate and the coal feed rate, the char loading should decreased when the O2 concentration difference between the inlet and the outlet of the bed becomes small, assuming that the average combustion rate of char was constant. Therefore, the experimental conditions described in this paragraph resulted in three different char loads in the bed. The flue gas was analyzed after passing through the cyclone and the filters. The concentrations of O2 and NO in the flue gas were analyzed with a continuous gas analyzer (NOA-7000 Shimadzu Corp., Kyoto, Japan), and the N2O concentration was measured with a micro-gas chromatograph (Q30L gas chromatograph, MTI Analytical Instruments, CA). Bed temperature, O2, and NO concentrations in the flue gas were recorded at 5-s intervals, and the N2O concentration was analyzed every 4 min. Steady operation was continued for at least 15 min for each experimental condition. The analytical values for the 16 different coals are listed in Table 1. The coals were selected to obtain a wide range of nitrogen contents (0.9-2.1 wt %) and a fixed carbon content of 42-60 wt %. The coals (particle diameter,
