Energy Fuels 2010, 24, 940–944 Published on Web 12/09/2009
: DOI:10.1021/ef901122d
Experimental Research and Numerical Simulation on NOx Release Characteristics along the Boiler during Pulverized Coal Combustion Jing Jin,* Junjie Fan, Yongtao Sha, and Bo Song College of Power Engineering, University of Shanghai for Science & Technology, Shanghai 200093, China Received October 2, 2009. Revised Manuscript Received November 20, 2009
Numerical simulation and experimental study on release along the boiler during pulverized coal combustion have been conducted. With the increase of temperature, the peak value of NOx release moved forward, the NOx emission increased. But when the temperature increased to a certain degree, NOx emission began to reduce. NOx emission increased with the increase of nitrogen content of coal. The peak value of NOx release moved backward with the increase of coal rank. NOx emission increased obviously with the increase of stoichiometric air ratio. There exists a critical average diameter (dc). If d e dc, NOx emission reduced with the decrease of pulverized coal size. If d > dc, NOx emission reduced with the increase of the pulverized coal size. The results showed that the simulation results are in agreement with the experimental results for concentration distribution of NOx along the axis of the furnace.
model with two-equation pyrogenation model.8 NOx release is not only related to the physicochemical characteristics of coal powder, but also to the environmental factors during coal burning process. In this paper, the numerical simulation is studied to give qualitative guidance for the experimental research on the NOx release during coal burning. Then, effects of furnace chamber temperature, coal types, stoichiometric air ratio, and coal particle sizes on NOx release along the furnace chamber are studied.
1. Introduction Controlling NOx emission from coal-burning boilers of power plants is a worldwide concern since NOx is one of the main sources of air pollution. The formation mechanisms of NOx are extremely complicated because the production of NOx is influenced by many factors in the operation of boilers. Researches on it will cost a great deal of manpower and material resources, and the reliability of the results are well influenced by factors such as experimental condition, operating model, and the accuracy of test equipment. In contrast to experiments, numerical simulation is a new approach that is economical and time-efficient. Moreover, the data gained by this approach are comprehensive. Therefore, it has good instructional and complementary function on experimental research. Many researchers have been conducted both at home and abroad on the release of NOx during coal burning.1-6 The method of “post treatment” was introduced to simulate NOx release behaviors during coal burning in boilers, mainly to study the distribution of temperature field and pollutant concentration.7 The model of volatile N releasing process in single particle coal was used in numerical simulation and experimental research on the volatile N releasing process of different coal types, mainly to compare the general pyrogenation
2. Methods of Numerical Simulation and Experimental Research Both numerical simulation and experimental research are conducted on the one-dimensional coal boiler in the lab, and the experimental system is shown in Figure 1. Fuel is supplied by an electromagnetic-vibrating feeder. The furnace chamber is 200 mm in inner diameter and 2520 mm in height. There are five holes on one side of the furnace, which can be used for measuring, sampling, observing, and so on. The first hole is 540 mm away from the fuel entrance. The thermocouples are installed on the other side of the furnace. The temperature in the furnace is heated up by silicon carbide rods. The pulverized
*Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10 (1), 188–196. (2) Van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23 (4), 349–357. (3) Weng, A. X.; Zhou, H.; Zhang, L.; Cen, K. F. J. Eng. Thermal Energy and Power 2004, 19 (3), 242–245. (4) Li, Y. H.; Li, S. G.; Feng, Z. X.; Li, Z. Z. Proc. CSEE 2001, 21(8): 34-41. (5) Wendt, J. O. L. Combust. Sci. Technol. 1995, 108 (4), 323–344. (6) Guo, X. M.; Hui, S. E.; Che, D. F.; Xu, T. M. Power Engineering 2003, 23 (1), 2164–2167. (7) Xiang, J.; Xiong, Y. H.; Zheng, C. G.; Sun, X. X. Proc. CESS 2002, 22(6): 156-160. (8) Zhou, H.; Weng, A. X.; Cen, K. F.; Fan, J. R. J. Eng. Thermal Energy Power 2004, 19 (2), 127–130. r 2009 American Chemical Society
Figure 1. Scheme of experimental system.
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Energy Fuels 2010, 24, 940–944
: DOI:10.1021/ef901122d
Jin et al.
Table 1. Proximate and Ultimate Analysis of Coalsa proximate analysis (%)
ultimate analysis (%)
coal types
Mad
Aad
Vad
FCad
Cad
Had
Oad
Sad
Nad
Longkou lignite Shenfu bituminous Xintai lean coal Jincheng anthracite
8.40 1.76 0.50 2.81
22.10 30.11 23.90 19.44
39.70 23.06 12.80 7.00
29.80 45.07 62.80 70.75
49.90 53.68 65.60 71.53
3.40 3.31 3.18 1.94
14.22 9.66 3.02 2.49
0.60 0.51 1.85 0.93
1.38 0.97 1.95 0.85
a
ad-air dried basis; Mad-moisture content; Aad-ash content; Vad-volatile content; FCad-fixed carbon; Cad-carbon content; Had-hydrogen content; Oadoxygen content; Sad-sulphur content; Nad-nitrogen content.
Figure 2. Effects of temperature on NOx release characteristics.
Here, YHCN and YNO are mass proportions separately for HCN and NO. The source items of transmission equations can be shown as SHCN ¼ Spvc, HCN þ SHCN -1 þ SHCN -2 ð3Þ
coal is produced by the supersonic and ultramicro gas flow pulverizing machine, and the pulverized coal particle size is measured by laser particle size analyzer. Research conditions are 800-1200 °C in furnace chamber temperature, 0.8-1.1 in stoichiometric air ratio, 15-80 μm in coal particle size, and 2.2 kg/h in fuel supply velocity. The proximate and ultimate analysis of coals are shown in Table 1. In the numerical calculations, the k-ε turbulent model is used for gas phase turbulent calculation, the part balanceable PDF (probabilistic density function) model is used for turbulent combustion calculation, the particle random orbit model is used for simulating the movement of coal particle group, the two equation parallel competing reaction model is used for pyrogenation calculation, the kinetic/diffusion control reaction rate model is used for coke combustion calculation, and the P1 model is used for radiative heat-transfer calculation. When the temperature of furnace chamber is below 1200 °C, most released NOx are formed from fuel N. As the temperature goes up, the nitrogenous compounds in the fuel are pyrolyzed into intermediate products (HCN, NH3, etc.) in the reacting zone. Some of the intermediate products are oxidized into NOx, and the others are deoxidized into N2. Recent researches indicate that HCN is the main intermediate product for NOx release during coal combustion.9 Thus, when fuel N transforms into NOx, the mainly intermediate product is HCN, and some of HCN transform into NOx. The component transmission equation for solving NOx is D ðFYΝΟ Þ þ r 3 ðFvBYΝΟ Þ ¼ r 3 ðFDrYΝΟ Þ þ SΝΟ Dt
SNO ¼ SNO -1 þ SNO -2 þ SNO -3
In these equations, SHCN-1, SHCN-2, SNO-1, and SNO-2 represent generation and consumption rate (kg(m3 3 s)-1) of HCN and NO. And SNO-3 is a heterogeneous deoxidation reaction source item. Spvc,HCN is a generation source item of HCN, which equals the sum of volatile Svol,HCN and coke Schar,HCN.
3. Research Results and Analyses 3.1. Effects of Furnace Chamber Temperature on NOx Release. Figure 2 is the distribution of NOx release of Shenfu bituminite burning under different temperature conditions along the furnace chamber. NOx release can be divided into three stages. The amount of NOx generated in the first stage is small. Because of the inadequate mix of fuel and oxidant in the original combustion period, an amount of prompt NOx is generated. But, with fast release of the volatile portion, the oxygen concentration is instantly lowered, which leads to immediate deoxidation of the NOx fraction, thus resulting in the first peak of NOx release. As the measurement point is behind the 0.5 m inlet, the first peak value was not obtained in the experiment. A large number of NOx is released by volatile combustion in the second stage. As the volatile portion is released largely and burned, the temperature is high, which causes N compounds in coal to decompose and fuel N is largely released, resulting in fast generation of NOx and a fast increase of NOx concentration. Thus, NOx concentration reaches a maximum value along with the highest furnace temperature. In the third stage, the oxygen in flue gas is gradually decreased
ð1Þ
and for intermediate product HCN is D ðFYHCN Þ þ r 3 ðFvBYHCN Þ ¼ r 3 ðFDrYHCN Þ þ SHCΝ Dt
ð4Þ
ð2Þ
(9) Zhao, K.; Tan, H. Z.; Zhou, Q. L.; Xu, T. M. J. Eng. Thermal Energy Power 2004, 24 (3), 246–248.
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: DOI:10.1021/ef901122d
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Figure 3. Effects of different coal types on NOx release characteristics.
Figure 4. Effects of stoichiometric air ratio on NOx release characteristics.
with the pulverized coal burns. Although coke NOx generates gradually, the generated NOx in flue gas would be reduced by the nitrogen compounds from coke and volatile decomposition. So the overall NOx concentration is decreased. Generally, N elements in coal mainly exists in the form of pyrrole N and pyridine N. Research results show that only 10% pyrrole N and pyridine N are pyrolyzed as the temperature reaches 800 °C, almost all pyrrole N and 90% of pyridine N are pyrolyzed as the temperature reaches 900 °C, and almost all pyrrole N and pyridine N are pyrolyzed as the temperature goes up to 1050 °C.10 Thus, when the temperature is below 800 °C, the N in coal is relatively thermo-stable and the release is incomplete. As the temperature increase, the NOx release increases and the release peak value moves ahead. As shown in Figure 2, when the temperature goes up to 1160 °C, the peak value of NOx release is higher than the value of 1060 °C, but the amount of NOx at the outlet is less than that of the later. The main reason is that when the temperature continues to rise over 1060 °C, NOx release does not increase (fuel N keeps constant and thermal N increases slightly), but the higher temperature is helpful for the deoxidization and decomposition of NOx, which makes the NOx decomposable velocity higher than producible velocity.
3.2. Effects of Different Coal Types on NOx Release. Figure 3 shows the distribution of the NOx release along the axis of furnace during combustion with different types of pulverized coal. It can be seen that there exists a great difference in NOx release for different coal types when the boiler temperature and the stoichiometric air ratio are the same. The volatile and nitrogen contents of different types of coal are different to some extent, thus the NOx release is also different. Generally, the higher volatile and nitrogen in the coal, the larger amount of NOx release. This is because most of nitrogen existing in coal is aromatic rings, and the release of NOx is greatly related with volatile.11 Higher temperature is needed for low volatile pulverized coals to breaking the heterocyclic chains of aromatic rings. That means larger amount of NOx can be releases under higher temperature. On the other hand, because the igniting temperature for the pulverized coals with high volatile is lower, and the peak value of combustion temperature increases, the thermal type NOx and fuel type NOx increase with local high temperature. Therefore, the peak value of NOx release moves forward and NOx release increases. The effect of fuel N content on NOx is greatly obvious. With the coal nitrogen content increasing, the total NOx emissions increase is approximately linear. In Figure 3, the amount of NOx release from Xintai lean coal is largest,
(10) Tan, H. Z.; Liao, X. W.; Zhao, K.; Xu, T. M. Power Eng. 2004, 24 (1), 121–124.
(11) Cen, K. F.; Yao, Q.; Luo, Z. Y. Advanced Combustion Dynamics; Zhejiang University Press: Hangzhou, 2002; pp 587-590.
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Figure 5. Effects of coal particle size on NOx release characteristics.
because the nitrogen content in Xintai lean coal is higher than the others types of coals. 3.3. Effects of Stoichiometric Air Ratio on NOx Release. Effect of stoichiometric air ratio on NOx release characteristics is shown in Figure 4. It can be seen that NOx emission increases remarkably with the increases of stoichiometric air ratio. The N in coal can be transformed into NOx only through oxidation reaction. With the increase of stoichiometric air ratio, oxygen concentration in the burning area is increased, and the amount of volatile NOx increases accordingly. Thus, the concentration of NOx emission is relatively higher. In contrast, when the stoichiometric air ratio is small, oxygen concentration is also lower, so it is hard for volatile N to transform into NOx. The mutual multiple reactions in volatile N enhance the deoxidizing reaction of NOx, resulting in less NOx release. Furthermore, at stoichiometric air ratios less than 1, the volatile NOx release is affected due to its incomplete combustion. The reaction activation energy of coke N into NO is bigger than C. Normally, coke N generates in the coke combustion zone of the flame tail. The oxygen concentration in this zone is much lower than that in volatile combustion zone, and coke particles are easily melted, which result in interstitial close and reduction of reaction surface. Thus, the coke NOx is reduced. 3.4. Effect of Pulverized Coal Size on NOx Release. Figure 5 shows the relation curves of coal particle size and NOx release concentration of Shenfu bituminite, Longkou lignite, and Xintai lean coal along the furnace axis under the same temperature and stoichiometric air ratio conditions,
respectively. It can be seen that the position of NOx release peak value moves ahead as the pulverized coal particle size is diminished. When coal is pulverized, the specific area of particle enormously increases and the thermal resistance greatly reduces. Thus, the temperature rising velocity of coal powder airflow accelerates and the ignition point moves ahead. Simulation calculation results shown in Figure 5a indicates that when the particle size changes from 15 to 60 μm, NOx emission reduces with the decrease of pulverized coal size. On the contrary, if the coal size is bigger than 60 μm, NOx emission reduces with the increase of the pulverized coal size. Figure 5b shows that the peak value of NOx release and the emission concentration at the outlet reach their maximum values of 899 and 673 mg 3 Nm3-, respectively, as the average particle size of Shenfu bituminite is 60.8 μm, whereas the corresponding value are 870 and 660 mg 3 Nm3-, and 886 and 667 mg 3 Nm3- when the average particle sizes are 49.0 and 68.3 μm, respectively. The experimental results of Longkou lignite and Xintai lean coal also indicate that there exists the critical phenomenon for particle size shown in Figure 5c and Figure 5d, but different coal types correspond differently to coal critical particle size dc. For example, dc is about 50 μm for Longkou lignite, but it is 70 μm for Xintai lean coal. The results coincide with Abbas’s conclusion in ref 12. When coal was superfine pulverized, coal burning velocity (12) Abbas, T.; Costen, P.; Lockwood, F. C.; Romo-Millares, C. A. Combust. Flame 1993, 93 (2), 316–326.
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: DOI:10.1021/ef901122d
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might be enhanced and the release velocity of volatile material can be accelerated. Volatile material releases from the interior to exterior of the particle in an injection way, which would enhance the concave, convex, hole, and size of the coal coke surface. This is favorable for heterogeneous deoxidizing reaction. The increase of specific area taking part in chemical reaction results in the increase of contact area with NOx, reducing the needed reaction activation energy and enhancing the efficiency of NOx deoxidization. Coal surface can provide the conditions for secondary reactions. At the center of coal particles, produced thermal decompositions migrate and escape to the outside. During the migration they may crack, coagulate, and aggregate. Then the deposition of carbon occurs. As the size of pulverized coal increasing, the deposition rate of carbon increased, burning velocity reduced, volatile emission decreased, and the temperature peak value descended remarkably. Furthermore, the mixed efficiency of volatile material and air is worse, which causes lower NOx emission. Comparing to the simulation calculation with experimental research results, there still exist definite differences. The main reasons are analyzed as follows. First, the method of “post treatment” used in numerical simulation on NOx release during coal burning cause the calculating result to be not precise. Second, in the simulation calculation, both the front index factor and activation energy in the reaction rate laws are all assumed to be constant for the same coal rank. When the results are used in other reaction conditions,
they might create a certain error. Finally, the instability of the combustion might also cause some error for experimental measurement. Although simulation results do not coincide well with experimental results in quantity, the changing trends of the two results are completely in agreement. Consequently, the research results can guide the design and application of low NOx combustion technology. 4. Conclusions (1) With the increase of the temperature, the peak value of NOx release moves forward and NOx emissions increases, but when the temperature increases to a certain degree, NOx emission reduces with the increase of the temperature. (2) Different coal types have different NOx release. Generally, NOx emission increases with the increase of nitrogen content of coal. The peak value of NOx release moves backward with increase of coal rank. (3) NOx emission increases obviously with the increase of stoichiometric air ratio. (4) There exists a critical average diameter (dc). If d e dc, NOx emissions reduce with the decrease of pulverized coal size. If d > dc, the NOx emissions reduce with the increase of the pulverized coal size, and the critical value increases with the increase of carbonized degree of coal. Acknowledgment. This work was supported by The High Technology Research and Development of China (863 Program) (2007AA05Z340), and the Foundation for Person with Science and Technology Ability of Shanghai Bai Yu Lan (2009B101).
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