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Energy & Fuels 2001, 15, 1100-1102
Combustion Characteristics of Super Fine Pulverized Coal Particles Jiang Xiumin,* Zheng Chuguang, Qiu Jianrong, Li Jubin, and Liu Dechang National Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China Received December 18, 2000. Revised Manuscript Received March 30, 2001
Combustion of super fine particles is a new pulverized coal combustion technique. Compared with conventional combustion techniques, it has the advantages of good flame stability, high burnout, low NOx release, and low comprehensive cost. Using NETZSCH thermobalance, STA 409C, and maintaining the test conditions for all the samples at the heating rate of 20 °C/min, the paper investigated the combustion properties of Heshan (Hs) subbituminous coal and Jincheng (Jc) lean coal, each in four different average sizes. Significant improvement of combustion properties was observed when particle size was reduced down to 0-20 µm.
Introduction The effect of pulverized coal particle size on combustion has long been a research subject. Recently the proposal of super fine pulverized coal particle combustion provided a new way to understand particle size effect.1,2 Through experiments it is found that NOx and SO2 release from super fine pulverized coal particle combustion process were significantly reduced.3 The operation cost of a 700 MW unit could be reduced at the rate of 1.7 U.S. dollar/kW. Year, when using super fine pulverized coal particles.4 This suggests that the technique of super fine pulverized coal particle combustion has more advantages, such as better stability, higher combustion efficiency, lower NOx and SO2 emission, and higher comprehensive efficiency, than that using conventional particle sizes. In this paper MALVERN laser grainulometric analyzer, MAM5004, was used to detect particle size distribution and NETZSCH thermobalance, STA 409C, was used to do the combustion property test on four different size samples of Hs subbituminous coal and Jc lean coal. Results have proved that the combustion property of super fine pulverized coal particle has been perfected obviously. Experiment The ultimate analysis data were obtained on LECO CHN 600 and a sulfur analyzer, then oxygen content was obtained by difference. The proximate analysis was done on LECO MAC (1) Xiumin, J.; Jubin, L.; Jianrong, Q. The Influence of Particle Size on Combustion Analyzing and Combustion Characteristics of Pulverized Coal. J. Coal (in Chinese) 1999, 24 (6), 643-647. (2) Xiumin, J.; Jubin, L.; Jianrong, Q.; et al. Physical Properties of Micro-Pulverized Coal and Their Influence on Combustion Characteristics. J. Fuel Chem. Technol. (in Chinese) 2000, 28 (2), 170-174. (3) Xiumin, J.; Jianrong, Q.; Jubin, L.; et al. A Study of NOx, SO2 Emission of Ultrafine Coal Powder in Lower Temperature Combustion. Acta Scientiae Circumstantiae (in Chinese) 2000, 20 (4), 431-434. (4) Nakamura, M.; Takashi, K.; Kuwahara, M.; et al. Demonstration Test and Practical Studies on Combustion Technology of MicroPulverized Coal. International Conference on Power Engineering-97, Tokyo, 1997; Vol. 2, 453-458.
Table 1. Analysis of Coal Samples items
Hs
Jc
carbon (as air dried basis, %) hydrogen (as air dried basis, %) oxygen (as air dried basis, %) nitrogen (as air dried basis, %) sulfur (as air dried basis, %) ash (as air dried basis, %) moisture (as air dried basis, %) volatile (as air dried basis, %) fixed carbon (as air dried basis, %)
33.45 1.97 4.64 0.64 4.79 51.56 2.95 14.49 31.00
77.73 2.33 1.11 0.99 0.25 14.78 2.81 11.31 71.10
Table 2. Mean Particle Size and Mass Hs
Jc
M, mg
d, µm
M, mg
d, µm
23.3 23.1 23.2 23.3
10.90 23.92 30.35 57.40
23.2 23.2 23.3 23.2
19.30 31.45 48.85 83.77
50. Four different size samples of each coal were prepared and the mean particle sizes were analyzed on MALVERN laser grainulometric analyzer MAM 5004. Coal combustion property was analyzed on NETZSCH thermobalance STA 409C. The heating rate used for all the tests was 20 °C/min. The tests were done in oxygen at the flow rate of 70 mL/min. The ultimate and proximate analysis data of two coals are list in Table 1. The sample mass and mean particle size for thermobalance test are in Table 2.
Results and Discussion Thermogravimetric Curves (TG). The weight loss curves are shown in Figure 1 and Figure 2. The TG curves show the resemblance between Hs (10.90) and Jc (19.30), that the separation points from easy-burn materials to not-easy-burn materials could be observed clearly, which were marked by point A in Figure 1 and point B in Figure 2. This feature reveals that the burning rate of easy-burn materials was raised because of the early release of volatile and easy ignition. As a result, the separation points, A and B, could be observed clearly. Particle Size vs Ignition Property. Figure 3 shows the plot of the ignition temperature vs particle size.
10.1021/ef000283g CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001
Combustion of Super Fine Pulverized Coal
Energy & Fuels, Vol. 15, No. 5, 2001 1101
Figure 1. TG curve of Hs coal samples.
Figure 3. Ti vs particle size.
Figure 2. TG curve of Jc coal samples.
Figure 4. Tmax vs particle size.
Figure 4 is Tmax, the temperature of relative maximum combustion rate on DTG curves. The ignition temperature was calculated from formula 1:
Ti ) T(TG,i + TDTG,i)/2
(1)
where Ti is the ignition temperature, TTG,i is the ignition temperature on TG curves,and TDTG,i is the ignition temperature on DTG curves. There are a lot of arguments about the effect of particle size on ignition temperature because of the differences in test facilities and test conditions. The current widely accepted opinion is that fine single particles have higher ignition temperature than coarse ones. In contrast, the ignition temperature of dense particle flow decreases with particle size. Figure 3 clearly shows the similar trend that ignition time and temperature decrease with particle size, especially the two super fine samples, Hs (10.90) and Jc (19.30). This trend is because the relative coal particle specific surface area is enlarged, volatile is easier to be released ,and char ignition is easier to occur. The result of the tests on lump coal samples shows similar trend to that of the tests on particle flows. The paper found out a good correlation between ignition temperature and mean particle size, which was shown in Figure 3 and Figure 4. As in Figure 3 and Figure 4, particle size seems to have more significant effect on Hs coal. This might be because this coal has higher volatile content. Particle Size vs Burnout Temperature. Based on the TG and DTG curves, the relationship between burnout temperature and particle size, was plotted in Figure 5 and the similar conclusion could be drawn. The result shows that reducing particle size can speed up the burning process under lower burnout temperature. As in Figure 5, Hs coal shows better burnout property than Jc coal. This might be also because Hs coal has higher volatile content. But we can find that two kinds of coal have a similar trend.
Figure 5. Burnout temperature vs particle size.
Figure 6. DSC curve of Hs coal samples.
The paper also found out a good correlation between particle size and burnout temperature, which was shown in Figure 5. Differential Scanning Calorimetry Curves (DSC). The DSC is shown in Figure 6 and Figure 7. The properties revealed by DSC curves are agreeable with those by TG curves, i.e., Hs (10.90) and Jc (19.30) show clear separation between easy-burn materials and noteasy-burn materials. The integral of the area under the DSC curve is in direct proportion to the heat change of samples as shown by5
Sarea ) K ∆H m
(2)
(5) Jinghong, C.; Chuanlu, L. Thermal Analysis and Application (in Chinese); Science Press: P. R. China Beijing, 1985.
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Xiumin et al.
Figure 8. S vs particle size of Hs and Jc coal.
Figure 7. DSC curve of Jc coal samples. Table 3. Characteristic Value of DSC Curve sample
Sarea
peak at min
height
Hs10.90 Hs23.92 Hs30.35 Hs57.40 Jc19.30 Jc31.45 Jc48.85 Jc83.77
-103.43285 -103.10800 -101.78824 -109.88759 -166.51977 -151.63020 -140.67029 -158.10790
24.86287 27.20383 27.52552 28.10055 27.75553 28.45058 28.66308 28.65058
-14.28880 -14.59883 -14.58776 -14.57568 -14.62595 -14.80710 -14.77974 -14.84421
where K is the function of test facility and m is the mass of sample. Since weight is nearly constant for all the tests, the Sarea can directly show the heat change ∆H. The data suggest that the smaller the particle size, the shorter the reacting time and the bigger the heat release is. This is agreeable with Figure 4. Characteristic value of DSC curve was listed in Table 3. Comprehensive Evaluation. At low heating rate, the ignition can be considered as chemical reaction control process and can be described by Arrhenius formula:6
(dW/dt)c ) A exp[-E/(RT)]
(3)
where (dW/dt)c is the burning rate, A is the frequency factor. After differential and algebraic transformation on formula 3, the expression can be written as
dW c 1 R d dW c ) E dT dt dt T2
( ) ( )
(4)
at ignition point,
dW c 1 R d dW c ) E dT dt T)Ti dt T)Ti T 2
( )
( )
(5)
i
or,
(dW/dt)cmax (dW/dt)cmean R d dW c ) E dT dt T)Ti (dW/dt)c Th
( )
T)Ti
(dW/dt)cmax(dW/dt)cmean Ti2Th
(6)
c where (dW/dt)max is the maximum burning rate, c (dW/dt)T)Ti is the burning rate corresponding to the c is the mean burning ignition temperature, (dW/dt)mean rate, Th is the burnout temperature, and Ti is the ignition temperature.
(6) Xuexin, S.; Jianyuan, C. Physics and Chemistry Basis for Pulverized Coal Combustion (in Chinese); Huazhong University of Science and Technology Press: P. R. China Wuhan, 1991.
The left side of formula 6 could be explained as follows. R/E represents the reactivity of coal and the smaller the E, the higher the coal reactivity. (d/dT) c means the turning rate of burning rate at (dW/dt)T)T i ignition point and the higher it is, the faster the ignition c c process. (dW/dt)max /(dW/dt)T)T is the burning rate rai c tio. (dW/dt)mean/Th is the ratio of mean combustion rate with burnout temperature and the bigger the c (dW/dt)mean /Th is, the faster the coal will burnout. The formula reflects comprehensive combustion properties of pulverized coal particle. Formula 6 can be expressed as
S ) (dW/dt)cmax(dW/dt)cmean/(Ti2Th)
(7)
where S is defined as comprehensive combustion property index and the higher the S, the better the coal’s combustion property. The relationship between S and particle size is shown in Figure 8. S increases with decreasing particle size. This means that super fine particle can improve combustion properties significantly. Conclusion Coal particle size has great influence on coal’s combustion property. Ignition time and temperature decreased when coal particle size was reduced. The data got from STA 409C thermobalance reveal that ignition temperature, burnout temperature, and maximum burning rate are all exponential functions of the particle size. In addition, it is easier to achieve higher burnout, and demanded burnout time is also reduced. The relationship between heat value of samples and particle size from DSC curve suggests that combustible materials in super fine particle are easier to be burnt out. From S we can know that combustion property of super fine coal particle was perfected obviously. The two coals used are regarded as poor coal in terms of ignition and burnout in power station. The results in this paper indicate that their combustion properties can be improved by using super fine particles. So using super fine particles can help power stations that burn poor coals to achieve higher combustion efficiency. Application of this technique will potentially reduce oil consumption to start the boiler, stabilize the combustion at lower load, reduce NOx and SO2 release,and raise combustion efficiency. Acknowledgment. The authors acknowledge the financial support of the Northeast Company of National Electric Power Company and National Laboratory of Coal Combustion of P. R. China. EF000283G
