Effect of Temperature on NO Release during the Combustion of Coals

Feb 10, 2010 - Peng Tan , Lun Ma , Qingyan Fang , Cheng Zhang , and Gang Chen. Energy & Fuels ... Shujun Zhu , Qinggang Lyu , Jianguo Zhu , Chen Liang...
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Energy Fuels 2010, 24, 1573–1583 Published on Web 02/10/2010

: DOI:10.1021/ef901198j

Effect of Temperature on NO Release during the Combustion of Coals with Different Ranks Weidong Fan,* Zhengchun Lin, Youyi Li, and Yu Li School of Mechanical and Power Engineering, Shanghai Jiao Tong University, No. 800, Dongchuan Road, Minhang District, Shanghai, P.R. China 20024 Received October 20, 2009. Revised Manuscript Received December 29, 2009

It is highly important to study the effect of the actual combustion environment temperature in a boiler on NOx formation and its types during the coal combustion. In this paper, a horizontal high temperature tube furnace system was established. Four typical Chinese coals with different ranks including a lignite coal, a high volatile bituminous coal, a high ash bituminous coal, and a low-volatile anthracite coal were employed in a series of isothermal combustion experiments in order to find out the combustion environment temperature in a boiler ranging from 1000 to 1600 °C affecting total NO transient release, as well as thermal NO transient release characteristics under the condition of fast heating. The results show, with the combustion environment temperature increasing, NO generates gradually in advance, the peak of NO appears earlier, and the peak value of NO increases. The effect of the combustion environment temperature on NO release amounts of per unit mass coal samples exhibited the change trend of “W”-type curve. At low temperature (1000 °C) and high temperature (1600 °C), the four test coals have higher emissions of NO. NO release amounts from four types of coal samples completely support the relative nitrogen contents in them. The NO release amount for the coal sample with high nitrogen content is relatively more. About 30-60% of coal nitrogen of four types of coals can be converted to NO during their combustion. The conversion rate of bituminous coal is generally the largest but that of anthracite is the smallest. At lower temperature (below 1200 °C), low volatile content or high ash content in coal will lead to the longer release process of NO. However, at high temperature (above 1300 °C), the effect of volatile content and ash content in coal on the NO release time will greatly be reduced. The release of fuel nitrogen is not in accordance with the fuel burnout degree. The impact of the combustion environment temperature on the release extent of nitrogen is very significant. As the combustion environment temperature increases, the NO formation proportions at all stages of combustion will be closer to the burnout proportion. High ash content in coal will weaken the improvement of the uniform release of final char nitrogen that results from the increase of combustion environment temperature. During the coal samples combustion, the fuel NO formation will produce a certain inhibiting effect on the thermal NO formation. In the actual furnace of the boiler with the region of high temperature of 1600 °C, the main part of all NO generation from anthracite combustion is still fuel NO. In a word, these results provide a theoretical foundation for developing the technique of reducing NO emission in coal combustion engineering.

is very complex, so many scholars are interested in performing a great deal of basic research for these mechanisms.1-19

1. Introduction For boilers burning pulverized coal, the main nitrogen oxide released during the coal combustion process is NO. After it is exhausted from the boilers, NO will be gradually oxidized to other nitrogen oxides and form different forms of nitrogen oxides. They are collectively referred to as NOx. The generation of NOx mainly comes from three sources: fuel NOx, thermal NOx, and prompt NOx. Nitrogen content in coal is oxidized to generate fuel NOx, and thermal NOx is generated from the oxidation of nitrogen in high-temperature air. Prompt NOx usually forms in fuel-rich conditions. The NOx formation mechanism in the process of fuel combustion

(5) Niksa, S. Twenty-Fifth (International) Symposium on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, 1993. (6) Solomon, P. R.; Colket, M. B. Fuel 1978, 57 (12), 749–755. (7) Visona, S. P.; Stanmore, B. R. Combust. Flame 1996, 105 (1), 92– 103. (8) Song, Y. H.; Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1981, 25 (5), 237–240. (9) Song, Y. H.; Pohl, J. H.; Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1982, 28 (1), 31–40. (10) Kyung, S. J.; Tim, C. K.; Soon-Jai, K. Fuel Process. Technol. 2001, 74 (1), 49–61. (11) Jones, J. M.; Patterson, P. M.; Pourkashanian, M.; Williams, A. Carbon 1999, 37 (10), 1545–1552. (12) Visona, S. P.; Stanmore, B. R. Combust. Flame 1996, 106 (3), 207– 218. (13) Harding, A. W.; Brown, S. D.; Thomas, K. M. Combust. Flame 1996, 107 (4), 336–350. (14) Jones, J. M.; Harding, A. W.; Brown, S. D.; Thomas, K. M. Carbon 1995, 33 (6), 833–843. (15) Thomas, K. M.; Grant, K.; Tate, K. Fuel 1993, 72 (7), 941–947. (16) Jones, J. M.; Thomas, K. M. Carbon 1995, 33 (8), 1129–1139. (17) Brown, S. D.; Thomas, K. M. Fuel 1993, 72 (3), 359–365. (18) Yao, M. Y.; Che, D. F.; Liu, Y. H.; Liu, Y. H. Environ. Sci. Technol. 2008, 42 (13), 4771–4776. (19) Yan, X.; Che, D. F.; Xu, T. M. Effect of rank. Fuel Process. Technol. 2005, 86 (7), 739–756.

*To whom correspondence should be addressed. E-mail: wdfan@ sjtu.edu.cn. Telephone: þ86-21-34206049. Fax: þ86-21-34206115. (1) Chaiklangmuang, S.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 2002, 81 (18), 2363–2369. (2) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Nineteenth Symposium (International) on Combustion, The Combustion Institute: Pittsburgh, 1982, 1271-1280. (3) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10 (1), 188–196. (4) Asay, B. W.; Smoot, L. D.; Hedman, P. O. Combust. Sci. Technol. 1983, 35 (1), 15–31. r 2010 American Chemical Society

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Their work has led to many results that guided the development of low NOx combustion technology.20-31 In the actual combustion process of pulverized coal in boilers, prompt NOx usually is generated by a very small percentage, so its amount can be negligible. Thermal NOx can be generated typically at more than 1500 °C. The amount of fuel NOx is the most or is about 80% of the total amount of all types of NOx formation in boilers.32 Therefore, the studies of NOx generation mechanisms of coal-fired boilers are mostly focused on finding out the generation details of thermal NOx and fuel NOx. In particular, the study of fuel NOx is more attractive and challenging. However, most experimental research in the mechanisms of fuel nitrogen release and conversion were carried out under the condition of a relatively low temperature (usually below 1200 °C). However, the combustion environment temperature in utility boilers is much higher. Average combustion temperature in a furnace usually is above 1400 °C. Under the condition of such high temperature, the trends of fuel nitrogen release maybe show a lot of differences. The higher the combustion temperature is, the shorter is the time of NOx release.33 The higher temperature results in the ratio of volatile nitrogen and char nitrogen converted from fuel nitrogen is different from that at low temperature. Some studies showed when the combustion temperature was low, the majority of fuel nitrogen remained in the coke, while the most fuel nitrogen released by volatiles form at the higher temperature.33-37 The increase of the combustion environment temperature will also affect the reduction of NO by volatile and char in the combustion process. As the combustion environment temperature increases, the homogeneous reduction degree of NO by pyrolysis gas from coal is enhanced, and the heterogeneous reduction degree of NO by char is also enhanced.38 Therefore, some past works about fuel nitrogen release mechanisms are not representative of the pulverized coal combustion in utility boilers. On the other hand, the combustion environment temperature is so high that it might lead to more fuel NO formation. At the same time, thermal

NO formation will also appear highlighted. Under the combustion temperature in utility boilers, there are still some research on the generation relationships of fuel and thermal NO. Especially for different coal, the generation relationships of fuel NO and thermal NO may determine the different options of NOx control methods in engineering. For high volatile bituminous coal and lignite, it may be appropriate to reduce the furnace combustion temperature and reduce the oxygen concentration in high-temperature zones to achieve low NOx combustion. However, for anthracite, lean coal, and other low volatile coal, using such methods will affect their stable combustion and burnout. Therefore, in order to ensure the combustion stability and burnout of anthracite, lean coal, and other low-volatile coal, the high combustion environment temperature is needed in the furnace combustion zone, but the high combustion environment temperature maybe will lead to more NOx formation. Thus, the related investigations are carried out and found to be appropriate to reduce the high combustion environment temperature in engineering.30 However, there is a controversial view for NOx formation trend during the anthracite combustion. Namely, it is possible for a too high combustion environment temperature to result in more NOx emission from the anthracite combustion. That result is considered to be a reason that thermal NOx produced in the high combustion environment temperature accounts for a large proportion of total NOx yield. Even thermal NOx proportion of total NOx yield may be more than 60% and much higher than that of the high volatile bituminous coal. If this conclusion is right, the decrease of the combustion environment temperature of anthracite combustion leads to a better effect of controlling NOx emission, but this would undermine the ongoing technical development for the overall staged combustion of anthracite to achieve its low NOx emission.30,31 It is highly important to study the effects of the actual combustion environment temperature in a boiler on NOx formation and its types, but little research has been done. Therefore, in this paper, four typical coals including a lignite coal, a high volatile bituminous coal, a high ash bituminous coal, and a low-volatile anthracite coal were employed for the sake of finding out the combustion environment temperature ranging from 1000 to 1600 °C affecting total NO transient release, as well as thermal NO transient release characteristics under the condition of fast-heating. This provides a theoretical foundation for developing the technique of reducing NO emission in coal combustion engineering.

(20) Staiger, B.; Unterberger, S.; Berger, R.; Hein, K. R. G. Energy 2005, 30 (8), 1429–1438. (21) Chae, J. O.; Chun, Y. N. Fuel 1991, 70 (6), 703–707. (22) Coda, B.; Kluger, F.; F€ ortsch, D.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1998, 12 (6), 1322–1327. (23) Spliethoff, H.; Greul, U.; R€ udiger, H.; Hein, K. R. G. Fuel 1996, 75 (5), 560–564. (24) Costa, M.; Azevedo, J. L. T. Combust. Sci. Technol. 2007, 179 (9), 1923–1935. (25) Ribeirete, A.; Costa, M. Fuel 2009, 88 (1), 40–45. (26) Ribeirete, A.; Costa, M. Proc. Combust. Inst. 2009, 32 (2), 2667– 2673. (27) Vander Lam, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23 (4), 349–377. (28) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. J. Energy Inst. 2007, 80 (3), 123–130. (29) Huang, L. K.; Li, Z. Q.; Sun, R.; Zhou, J. Fuel Process. Technol. 2006, 87 (4), 363–371. (30) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86 (15), 2457–2462. (31) Fan, W. D.; Lin, Z. C.; Li, Y. Y.; Kuang, J. G.; Zhang, M. C. Energy Fuels 2009, 23 (1), 111–120. (32) Jones, J. M.; Patterson, P. M.; Pourkashanian, M.; Williams, A. Carbon 1999, 37 (10), 1545–1552. (33) Li, X. T.; Ni, M. J.; Cen, K. F. J. Eng. Thermophys. (Chinese) 1990, 11 (3), 338–341. (34) Fan, J. J.; Jin, J.; Zhong, H. G.; Chen, Z. J. Boiler Technol. (Chinese) 2005, 36 (3), 38–41. (35) Ying, L. Q. Master Thesis, Zhe Jiang University, China, 2004. (36) Thomas, K. M. Fuel 1997, 76 (6), 457–473. (37) Jan, E. J. Fuel 1994, 73 (9), 1398–1415. (38) Gou, X.; Zhou, J. H.; Zhou, Z. J.; Yang, W. J.; Liu, J. Z.; Cen, K. F. Proc. CSEE (Chinese) 2007, 27 (23), 12–17.

2. Experimental Instruments and Methods 2.1. Experimental Instruments. The experiments were carried out in a tube reactor. A diagram of the system is shown in Figure 1. The experimental apparatus consisted of a gas mixing section, a tube reactor, and a gas analysis section. The heating device is a rotary tube-type electric resistance furnace with a power-handling capability of 4 kW. Temperature programming can be performed using an intelligent temperature controller, and the highest achievable temperature is 1700 °C. A double platinum-rhodium thermocouple was employed in the experiment. The reactor tube was made from refractory alundum and has an inner diameter of 20 mm. It can withstand the greatest temperature at 1650 °C. A ceramic boat as a carrier of the coal sample was placed in the middle of alundum tube. The N2, O2, or Ar from different steel cylinders or air from the oil-less air compression engine enters the air distribution box through a reducing valve, and each gas distribution is controlled by a flow meter. Air would react with the coal samples in the ceramic boat 1574

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actual combustion of the pulverized coal in boilers, such as low temperature (1000 °C), middle temperature (1300 °C), and high temperature (1600 °C). The burnout rate of the pulverized coal is very close to 100% for each run, which can be supposed to be completed combustion.

3. Results and Discussion 3.1. NO Distribution Trend as a Function of Time during the Pulverized Coal Samples Combustion. Figure 2 shows the NO emissions trends with combustion time increasing in the combustion processes of four coal samples at different constant temperatures from 1000 to 1600 °C. It can be seen that NO release trends from four coal samples are almost the same. The coal samples start to ignite and NO also generates immediately with its combustion after being pushed into the alundum tube with the ceramic boat. With the continuation of the combustion process, along with the burning rate increasing, NO concentration for each coal sample begins to rise and reach its peak. Then, as the combustible substances in the coal samples gradually burn out, NO concentrations rapidly decrease and gradually approach zero. As the constant combustion environment temperature increases, NO generates gradually in advance, and the peak of NO appears earlier. For example, the endurance period of the peak concentration of NO at 1300 °C is shorter than that at 1000 °C, and the NO peak at 1300 °C also increases greatly. It can be seen that the coal sample fires earlier and the fuel nitrogen also releases and oxidizes earlier as the combustion environment temperature increases. Since the increase of combustion environment temperature causes a more intense combustion, the materials in the coal sample releases and burns more quickly in a certain period of time, and at the same time, the amount of the coal sample burned is more. Therefore, the peak of NO emission is higher. In addition, the more fuel nitrogen as the volatile nitrogen form is released and oxidized at the higher temperature, while the majority of fuel nitrogen remains in the char at the lower temperature. Because the release of char nitrogen is slower than that of volatile nitrogen, the lower temperature may cause the peak of NO emission to reduce down to zero and cause a longer endurance period. The generation of NO below 1400 °C can be considered to come entirely from oxidation of fuel nitrogen. 3.2. Conversion Trends of Fuel Nitrogen into NO from the Coal Samples at Different Temperatures. Figure 3 shows the impact of the combustion environment temperatures on NO release amount of per unit mass coal samples for four coal samples. The NO release amount from the coal sample is calculated by the expression: Z to þ Δt F  CNO  30  dt=22:4 ð1Þ MNO ¼

Figure 1. Scheme of the experimental system: (1) bottle of N2, (2) bottle of O2, (3) bottle of Ar, (4) flow regulating valve, (5) rotameter, (6) mixing gases box, (7) alundum tube, (8) ceramic boat, (9) tube furnace, (10) temperature controller, (11) cooling apparatus, (12) heating tape, (13) temperature controller, (14) gas analyzer, and (15) computer.

after entering the alundum tube. Carrying flue gas generated by a reaction was still at a high-temperature state. It was necessary to immediately enter the spray cooler, and it was cooled to about 130 °C in order to avoid the further reaction of various components of flue gas and affect the stability of gas composition. Then, it entered the flue gas analyzer through a pipeline wrapped with the heating belt (to ensure that the flue gas temperature in pipe was fixed at 130 °C to avoid its condensation). The outlet gas was continuously analyzed using a TESTO 350-Pro gas analyzer. The error for NO measurements is less than (5%. The error for O2 measurements is less than (0.8%. The analyzer was examined using standard gases with constant concentration before each test. If its errors were more than above the error ranges, the analyzer would be sent to the instrument company to be checked or calibrated to achieve the measurement accuracy requirements. 2.2. Experimental Coals. Four Chinese coals with different ranks were used for experiments. These included one anthracite, two bituminous coals from different mines, and one lignite. They were Yangquan anthracite, Shenhua bituminous, Linfen bituminous, and Baorixile lignite, respectively, which are denoted, respectively, as YQ, SH, LF, and BRXL hereinafter. All the coal samples were air-dried at 105 °C for 24 h, then ground, and sieved to a size of less than 60 μm. The ultimate and proximate analyses of coals are listed in Table 1. 2.3. Experimental Methods. For each run, 50 mg of pulverized coal sample diluted with 1 g of R-Al2O3 was employed to model the suspension firing state of the pulverized coal. At the same time, the purpose of coal sample diluted with R-Al2O3 is also for avoiding its concentrated heat release. Otherwise, it will result in a great increase of temperature in coal samples, while the condition of constant combustion temperature controlled by the electric resistance furnace cannot be guaranteed. R-Al2O3 exerts little influence on the combustion process and flue gas compositions because it is an inert medium. At first, the mixture sample of pulverized coal and R-Al2O3 was packed into the ceramic boat. Then, the ceramic boat was packed into the alundum tube by means of pushing into it. The sample ignite mode is similar to the quick ignite process of the pulverized coal jet into the furnace of the boiler. Two kinds of mixture gases of O2/N2 or O2/Ar with a constant O2 volume concentration of 21% were used as combustion gases. For each experiment, the combustion gases were fixed at a constant flow rate of 1 L/min. NO produced during the combustion in a O2/N2 atmosphere includes fuel NO and thermal NO, but NO produced during the combustion in a O2/Ar atmosphere is only recognized as fuel NO. The furnace temperatures were kept at a series of constant values for different runs, individually. The isothermal combustion experiments were conducted at seven temperatures of 1000, 1100, 1200, 1300, 1400, 1500, and 1600 °C. These temperatures represent different temperature conditions during

to

Here, MNO is the NO release amount from the coal sample, mg; F is the volume rate of the combustion gas, L/s; t0 is the start moment of NO measurement, s; Δt is the full time of NO measurement, s; Nd is the nitrogen content in coal, %; and CNO is NO concentration in flue gas, μL/L. The calculation of NO release amount of per unit mass coal samples is just MNO/M. Where, M is the total mass of coal sample, g. As shown in Figure 3, NO release amounts from four types of coal samples indicated nearly a consistent change trend with the combustion environment temperature increasing. NO release amounts of per unit mass 1575

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Fan et al. Table 1. Characteristics of Coals

proximate analysis, wt %(as air-dried)

ultimate analysis, wt %(as air-dried)

coal

volatile matter

ash

moisture

fixed carbon

C

H

S

N

O

LCV (KJ/kg)

Yangquan Shenhua Linfen Baorixile

7.42 24.22 18.00 35.69

17.29 10.70 41.50 10.64

1.07 11.50 2.60 14.72

74.22 53.58 37.81 38.95

75.10 63.13 50.44 54.82

3.18 3.62 3.27 4.39

0.84 0.41 0.74 0.22

1.24 0.70 0.63 0.63

1.28 9.94 1.36 1.00

28020 24136 18000 22370

Figure 2. NO emissions trends from four pulverized coal samples vs combustion time at different temperatures.

reaction resistance. The consumption of oxygen on the surface of coal particles diffused from far field is relatively smaller. Thus, oxygen concentration around coal particles is relatively higher. As a result, fuel nitrogen will have more opportunities to combine with oxygen to form NO. At the same time, the amounts of NO release and oxidation are also more due to the combustion process becoming longer. This is same as the volatile release trend during coal pyrolysis. In addition, the reduction degree of NO by char is also relatively weak at low temperature. These factors may lead to the maximum NO release during the coal samples combustion at low temperature (1000 °C). At high temperature, thermal NO generation brings the increase of total NO generation. Moreover, from the comparison of NO release amounts of per unit mass coal samples, it can be seen that the NO release amount of Yangquan anthracite is the most and next is that of Shenhua bituminous. The NO release amount of Linfen bituminous coal is closer to that of Baorixile lignite coal. As shown in Table 1, this result completely reflects the relative nitrogen content in coal samples. The NO release amount for the coal sample with high nitrogen content is relatively more, such as Yangquan anthracite. This experimental result illustrates

Figure 3. Effects of the combustion environment temperatures on NO release amount of per unit mass coal samples.

coal samples did not change monotonously but exhibited “W”-type curves. At low temperature (1000 °C) and high temperature (1600 °C), the four test coals have higher NO emissions. Especially at low temperature, NO emission achieves the maximum. At low temperature, the burning rate of the coal sample may be lower, and its burning is closer to dynamic combustion mode. That is, in its burning, the diffusion resistance is significantly smaller than the chemical 1576

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monotonously by the combustion environment temperature. Namely, it does not show that the higher the combustion environment temperature is, the greater is the corresponding conversion rate of coal nitrogen into NO. This suggests that for air-staged low-NOx combustion technology, the combustion enhanced measure and air staging technical measure are usually used together. In engineering, raising combustion temperature will enhance the combustion of the pulverized coal, and more fuel NO will not be produced because of the combustion enhanced. However, it is possible to accelerate the release process of fuel nitrogen, and then is beneficial to control the conversion process of fuel nitrogen into NO with sufficient time. On the other hand, although air staging in the furnace can reduce the temperature of the initial combustion stage or combustion temperature in the primary combustion zone, the decrease of the combustion temperature cannot reduce fuel NO formation or the conversion rate of fuel N into NO. In fact, poor O2 supplement at the initial combustion stage or in the primary combustion zone resulting from air staging can just achieve the purpose of reducing the conversion rate of fuel N into NO. As shown in Figure 5, for Shenhua bituminous coal, Linfen bituminous coal, and Baorixile lignite coal, the impacts of the combustion environment temperatures on the conversion rates of coal nitrogen into NO exhibit “W”-type curves. At low temperature (1000 °C) and high temperature (1600 °C), for this three types of coal, their conversion rates of coal nitrogen into NO coal are much higher, but the actual conversion rate at high temperature (1600 °C) does not completely originate from fuel nitrogen, which also includes part of thermal NO formation. Additionally, it can also be seen that coal nitrogen cannot be completely converted to NO, and their conversion rates are all less than 60%. For these four types of coals, 30-60% of coal nitrogen can be converted to NO. Through comparing coal nitrogen conversion rates, we can see that the conversion rate of Shenhua bituminous coal is generally the largest, but the conversion rate of Yangquan anthracite is the smallest although its nitrogen content is the highest. For Shenhua bituminous, Linfen bituminous, and Baorixile lignite coals, their conversion rates increase with a large extent at high temperature (1600 °C). It indicates that the combustion environment temperature is relatively more sensitive to their thermal NO formation. However, the combustion environment temperature exerts a little influence on thermal NO formation during Yangquan anthracite combustion, which is not a consistent viewpoint that low volatile anthracite is generally considered to produce thermal NO easily when it burns at high temperature. 3.3. NO Release Rate Trends during the Pulverized Coal Samples Combustion. The effect of the temperature on the NO release rate is shown in Figure 6. The NO release rate can reflect the NO release properties in a certain period of time. It is a total of the effect of the temperature on NO release amount of per unit mass coal and the effect of the temperature on NO release time. It also is able to fully indicate how coal rank or quality affects combustion characteristics and the NO release process of coal at different temperatures for a certain mass coal sample. As shown in Figure 6, the higher the temperature is, the greater is NO release rate. Below 1500 °C, the NO release rate of Linfen bituminous coal is at a minimum and that of Yangquan anthracite is greater. One reason is that Linfen bituminous coal contains so much ash content that it burns significantly more slowly at low temperature. As shown in Table 1, ash content on a dry basis of

Figure 4. Effects of the combustion environment temperatures on the amount of NO release per unit heat of coal sample.

Figure 5. Effects of combustion environment temperatures on the conversion rate of fuel nitrogen into NO.

that the higher the nitrogen content of coal is, the more corresponding NO generation may be after it is burned. However, for coal-fired boilers, when operating at a certain thermal power, the consumptions of coal with different heat values are not the same. Therefore, it is more appropriate to compare their differences of NOx emissions, when different coal is combusting in the same boiler, by the use of the amount of NO generation per unit heat of the coal sample.39 Figure 4 shows the effects of oxygen concentration on the amount of NO generation per unit heat of coal samples from the combustion of four types of test coals under different combustion temperatures. When Linfen bituminous coal combusts at middle temperature (1200 °C) and high temperature (1600 °C), its amount of NO generation per unit heat of coal sample is larger than that of the other three types of test coals. Because Linfen bituminous coal is a typical low quality coal with high ash content and low heat value, it illustrates that NOx emission is usually higher when the boiler is firing low heat value coal. Figure 5 shows the influence of the combustion environment temperatures on the conversion rate of fuel nitrogen into NO. The conversion rate of fuel nitrogen into NO is calculated by the expression: MNO  100% ð2Þ XNO ¼ M  Nd  30=14 As shown in Figure 5, the conversion rate of coal nitrogen into NO is not a constant under the conditions of combustion environment temperatures. In the meantime, the conversion rate of coal nitrogen into NO has not been changed (39) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2008, 47 (6), 1835–1845.

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Linfen bituminous coal is 41.5 wt %, which is much higher than Shenhua bituminous coal with ash content of 10.7 wt %. A special study, about the impact of ash content in raw coal on its combustion process and NO release characteristics, has been conducted in the literature.40 In this literature, the three coals with different ash contents of 36, 44, and 53 wt % were tested under a staged combustion condition in a type of water-cooled horizontal and cylindrical furnace in order to investigate the influences of ash content on their combustion characteristics. The results show that, as the ash content increases, gas temperature decreases and O2 consumption and NOx formation becomes slow near the burner. Also, the increase of the ash content leads to the increase in NOx concentration and unburned carbon fraction at the furnace exit. Authors believe that, the reason being, for the high ash coal, the large heat capacity of the ash and the covering of combustible matter suppress combustibility with

ash during the char oxidization. Of course, as shown in Figure 3, this is also a reason why NO release amount of per unit mass Linfen bituminous coal is less under the temperature range of 1300-1500 °C. However, that the temperature is higher will also narrow the gap of NO release rate of Linfen bituminous coal with other rank coals. Particularly at 1600 °C, the NO release rate of Linfen bituminous coal is more than those of the other three types of coal, while the NO release rates of the other three coal samples are very close. As shown in Figure 5, that the NO release rate of Linfen bituminous coal is greater is mainly due to its NO release time is even less than those of the other three types of coal. In addition, there is also a large amount of thermal NO formation in its combustion process. This problem will be analyzed in following section. 3.4. Release Rate Trends of Nitrogen Relative to Total Burnout Rate during the Four Types of Coal Combustion. Figure 7 displays the cumulative fraction of released nitrogen as a function of overall burnout for four types of coal samples. The smoothed lines represent the different temperature cases and are used to quantify the differential results presented later. Also shown is a straight line labeled parity with a slope of unity, which represents elemental loss in direct proportion to burnout rate of coal samples. For the calculation of the burnout rate, it is obtained by the ratio of the combustion amount from the moment of fuel firing to the end of a certain combustion stage and the total combustion amount. As one of the combustion reactants, O2 consumption can objectively indicate the integrated and average combustion behavior of all combustible matters in fuel. Thus, the combustion amount at this stage is obtained by the integration with respect to O2 consumption record curve

Figure 6. Effects of combustion environment temperatures on NO release rates during the coal samples combustion.

Figure 7. Cumulative nitrogen released as a function of normalized burnout degree for four coals samples.

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from the moment of fuel firing to the end of a certain combustion stage. The total combustion amount is obtained by the integration with respect to the whole O2 consumption record curve. The data in Figure 7 illustrate how nitrogen released is significantly less than the burnout rate during the most combustion stages for four coal samples. This illustrates that the release of fuel nitrogen is not in accordance with the fuel burnout degree, and it almost always lags behind the fuel burnout degree. Of course, it can also be obtained from the figure, at the stage of fuel firing and initial combustion (burnout rate of 5-8% or less), and under the conditions of higher than certain experimental temperature (for different coal samples, this temperature is different but most of them are more than 1300 °C), that the release degree of nitrogen is higher than the burnout degree. This is because at the early stage of coal firing, with a lot of volatile released quickly, volatile nitrogen occupying a large proportion of nitrogen fuel will be oxidized to generate a large number of NO in the oxygen-rich environment. The amount of NO generated at this time is more than the average level of nitrogen release in the whole coal combustion process. For high-volatile bituminous coal, it is particularly obvious, such as the curves of Shenhua bituminous coal in Figure 7. Its curves representing a temperature being more than 1200 °C illustrate that the release degree of nitrogen is higher than the degree of its burnout. For Baorixile lignite coal with a higher volatile content, the temperature when the release degree of nitrogen is higher than the degree of burnout is above 1300 °C. This is because high moisture contained in Baorixile lignite coal affected the degree of nitrogen oxidized. When the combustion environment temperature is at or above 1500 °C, a large number of thermal NO generating at the early stage of coal burning brings a clear contribution to total NO production. The results from these four types of coal samples all showed this trend. So compared to the ratio of NO release degree only from fuel nitrogen to the degree of burnout in this early stage of coal burning, it will show a certain degree of error. When coal combustion is at the middle and late stage, with generation proportion of thermal NO decreasing (the reason will be analyzed in the following section), this error will be reduced. When the combustion stage of coal samples from the early stages reaches the midstage, the degree of NO formation lagging behind the burning process will gradually increase, especially at the lower temperature and is more obvious. On the one hand, from the stage of volatile combustion transiting to the stage of the char combustion, the release amount of char nitrogen is generally less than that of volatile nitrogen, and char nitrogen releases more slowly. Yih H. Song8 found that volatilized nitrogen compounds accounted for a major fraction of NOx produced from coal-nitrogen, especially at high temperatures and low fuel/oxygen equivalence ratios. It can be seen that after coal samples combustion entered the stage of char burning, as volatile nitrogen completely released, the NO formation process slowed down. On the other hand, the role of NO reduced by char has also reduced NO generation. As a result, it demonstrates that the lagging degree of NO formation to burnout also increased. At the final stage of coal combustion, that is, at the end stage of its complete burnout, NO yield will rapidly increase and is close to the proportion of burnout at this stage. It indicates that

there is a larger release of char nitrogen at the final stage of coal combustion. The effect trends of combustion environment temperature on the NO release proportion can also be seen in Figure 7, as well as the differences resulting from coal ranks. The impact of the combustion environment temperature on the release extent of nitrogen is more significant, especially for Shenhua bituminous and Baorixile lignite coals with higher volatile contents. With the increase of the combustion environment temperature, in the entire combustion stage, the nitrogen release degree at all stages will be more close to the burnout degree. This impact trend basically presents a monotonous relationship with temperature increasing. The general influence trend of the combustion environment temperature on the relations of the nitrogen release degree and the burnout degree for Yangquan coal and Linfen bituminous coal is same as that of Shenhua bituminous coal but is relatively weaker. For Yangquan anthracite coal, under the conditions of temperature of 1100-1600 °C, the nitrogen release curves at the middle stage of its combustion are almost concurrent with each other. For Linfen bituminous coal, under the conditions of temperature of 1000-1500 °C, the nitrogen release curves at the middle and final stages of its combustion almost have such an interval with little difference. The increase of the combustion environment temperature will actually make burning strengthened in favor of more and rapid release of volatile. The corresponding fuel nitrogen will be released quickly due to burning being strengthened. The amount of NO generated at all stages will increase; thereby, the lagging degree of NO formation to burn out will decrease. For Shenhua bituminous and Baorixile lignite coals with higher volatile contents, the effect that promoting combustion environment temperature accelerates more volatile released quickly is more significant. For Yangquan anthracite coal with low volatile content and Linfen bituminous coal with high ash content, promoting combustion environment temperature plays a small role on accelerating more volatile released quickly. Thus, the combustion environment temperature exerted a relatively weaker influence on the relations of the nitrogen release degree and the burnout degree for Yangquan anthracite and Linfen bituminous coals. It illustrates that controlling NO generation through burning strengthened resulting from the increase of the combustion environment temperature will achieve a better effect for the coal with low ash and high volatile contents. In addition, at the final stage of the coal samples combustion, as the combustion environment temperature increases, the NO formation proportion will be closer to the burnout proportion. This is beneficial to the uniform release of final char nitrogen and avoiding its integrated and rapid release. For Shenhua bituminous and lignite coals with high volatile, the increase of combustion environment temperature can more significantly improve the uniform release of final char nitrogen. Generally, for the late burning of anthracite, the integrated and rapid release of much char nitrogen will lead to higher generation of NO, and NO produced is more difficult to be controlled so that final NO emission is higher.31 However, as shown in Figure 7, for the late burning of Linfen bituminous coal, the increase of combustion environment temperature does not show an obvious improvement of the uniform release of final char nitrogen. It shows that the final release of char nitrogen is integrated and rapid. This illustrates that high ash content in coal makes the control of the final release of char nitrogen and final NO

(40) Kurose, R.; IKeda, M.; Makino, H. Fuel 2001, 80 (10), 1447– 1455.

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emission become very difficult, which is even more difficult than anthracite. Baxter et al.3 previously proposed the ratio (dN/N/dC/C) by the nitrogen release rate which is viewed as a selective removal versus carbon and the differential basis gives a more direct indication of the relative reaction rate of carbon and nitrogen loss. Values of the ratio (dN/N/dC/C) > 1 indicate that nitrogen is more reactive than carbon, i.e., more prone to devolatilization, to oxidation, or in general to leave the solid particle. Using this analysis by B. Coda,22 he obtained some useful parameters for optimizing the nitrogen kinetic paths under reburning or air-staged conditions. Baxter et al. performed experiments in entrained flow reactors to resolve the rank dependence of nitrogen release for 15 U.S. coals ranging from lignite, subbituminous coal, and high volatile bituminous coal to low-volatile bituminous coal by use of (dN/N/dC/C) and (dN/N/dm/m). m represents the daf mass loss of coal. Some important conclusions he made include the following. (1) Rates of nitrogen release during pulverized coal combustion vary with coal type and also change significantly over the course of the combustion process. The relative rates of release of elemental nitrogen and total sample mass show clear and consistent trends with parent coal rank that can be rationalized mechanistically in terms of the mode of occurrence of nitrogen in coal. (2) For lignites and subbituminous coals in the early phases of devolatilization, fractional nitrogen release rates are much slower than the fractional release of carbon or overall mass. The early volatiles released from lignites and subbituminous coals are dominated by light gases formed from functional groups in the coal containing little, if any, nitrogen. (3) For the lowrank coals, the preferential release of nitrogen continues in the early stages of char combustion. After the disappearance of the visible volatiles flame, a slower evolution of volatiles continues, in which nitrogen is released preferentially to carbon. In the young char in the early stages of combustion, nitrogen-containing aromatic structures are less stable thermally and may also be more susceptible to heterogeneous oxidative attack. As can be seen from these conclusions, for the coal with higher volatile content, at the early stage of its pyrolysis, the release of nitrogen is much slower than the lose of the total mass of carbon. Only for the coal with low volatile content, the coal nitrogen can be released in advance during its early pyrolysis. These conclusions are inconsistent with the trends shown in Figure 7 of this Article. This is because the abscissa of Figure 7 represents the burnout percentage of the whole sample (burnout rate), while its vertical ordinate represents the percentage of NO generation (the ratio of certain instantaneous generation and total generation). The analysis means of this Article is focused on better knowledge to the relationship between the process of combustion and the NO generation from nitrogen release, which is more suitable to understand the trends of the speed of fuel nitrogen converted to NO resulting from the fuel combustion process. Because most studies have already shown that a large number of nitrogens will be released and also generate a large number of NO with the great release of volatile matter at the early stage of combustion of the coal with high volatile content,3 in such conditions, the generation of NO can be controlled through changing the oxygen supply, such as the use of an air-staged combustion measure. However, if the (dN/N/dC/C) and (dN/N/dm/m) are employed to show the ratio of the release rate of nitrogen and the loss rate of carbon and the ratio of

the release rate of nitrogen and the loss rate of total mass of the coal sample, it cannot report the relationship between the speed of fuel nitrogen converted to NO and the speed of overall combustion of the fuel. This is because the loss rate of carbon or the loss rate of total mass of the fuel cannot strictly report information about the process of combustion. On the one hand, the moment of the volatile release at the early pyrolysis of the fuel does not mean consumption of the combustion simultaneously accompanied by its pyrolysis. On the other hand, the fuel mass does not necessarily show a linear relationship with the combustible mass in the fuel, i.e., that the proportion of a certain stage weight loss to the fuel total mass is not necessarily equal to the proportion of a certain stage combustion amount to the total combustion amount. Therefore, according to the analysis measure used by Baxter, it may be obtained that for the coal with higher volatile content, at its early stage of pyrolysis and combustion, the release rate of nitrogen is much less than the release rate of a large amount of volatile. Then, it resulted in the release rate of nitrogen lagging the loss rate of the carbon or total mass. For the coal with low volatile content, such as low volatile bituminous coal used in Baxter0 s experiment, and even extended to anthracite, at its early stage of pyrolysis and combustion, the release rate of nitrogen is instead higher than the loss rate of the carbon or total mass and indicates the nitrogen release in advance. Clearly, this is opposite to the viewpoint that the lower the volatile content of coal is, the slower is the release of nitrogen. In order to indicate more truly the relationship between NO generation rate and the combustion rate at the various stages of coal combustion, in this paper, the change curve of the ratio of the generation rate of NO (represented by the dNO/NO) and the overall burning rate of the fuel (still represented by the dM/M) versus the burnout rate is employed to indicate the release characteristics of coal nitrogen. The generation rate of NO is obtained by the differentiation with respect to NO generation record curve. The overall burning rate is obtained by the differentiation with respect to O2 consumption record curve. Thus, this method can better indicate the relationship between the generation of NO and the combustion process of coal. It is easy to judge the change of this relationship under different conditions for the different coals. There will be great help for control of NO generation during the process of coal combustion if this method is used to analyze the release characteristics of coal nitrogen. Figure 8 displays the ratio of the two functions (dNO/NO)/ (dM/M) as a function of overall burnout for four types of coal samples. It showed the same trends as Figure 7. At the early stage of firing and combustion of fuel (burnout rate of 5 to 8% or less), the curves representing higher than a certain temperature shows (dNO/NO)/(dM/M) is more than 1. That is, the generation rate of NO is higher than the combustion rate. This is because at the early ignition of coal, combined with the integrated release of much volatile, the volatile nitrogen occupying a large proportion of nitrogen fuel will be rapidly oxidized to NO in the oxygen-rich environment. As the combustion process continues, NO generation rate is less than the combustion rate. NO generation lags behind the fuel burning, and for the lower combustion environment temperature condition, at the majority of stages from the initial combustion to close to burnout, (dNO/NO)/(dM/M) are all less than 1. For Shenhua bituminous and Baorixile lignite coals with the higher volatile contents, although their (dNO/NO)/(dM/M) is less than 1, their values are still more than those of Yangquan 1580

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Figure 8. Release rates of nitrogen relative to mass loss rate as a function of burnout degree for four coals samples.

anthracite and Linfen bituminous coals. It shows that under the oxygen-rich conditions, for the coal with high volatile content, NO will generate faster, although still lag behind the burning. Figure 8 also shows the influence of the combustion environment temperature on the relationship between NO formation rate and the combustion rate. With the increase of the combustion environment temperature, at the majority of stages from the initial combustion to close to burnout, (dNO/NO)/(dM/M) curves will gradually shift up, which indicate that as a function of temperature increase, the increase extent of the generation rate of NO is more than that of the burning rate. At the middle and late stage of the combustion, when the combustion environment temperature is higher than a certain value, (dNO/NO)/(dM/M) will be more than 1. Then, it will decrease below 1 again at the stage of close to burnout. This indicated the same trends as curves from Baxter’s experiments.3 Of course, for these four types of coals, the impacts of the combustion environment temperature of on their (dNO/NO)/(dM/M) are different. The impacts are more significant for Shenhua bituminous and Baorixile lignite coals with higher volatile contents. The impact degree is smaller for Yangquan anthracite coal with the low volatile content, but there is still significant effect for it. For Linfen bituminous coal with high ash content, the impact degree is relatively weak. Especially at 10001500 °C, the peak region with (dNO/NO)/(dM/M) > 1 does not appear on the (dNO/NO)/(dM/M) curve sections at the middle and late stage of the combustion, and these curves basically coincide at a larger stage of combustion. Certainly, at 1600 °C, the curve section with (dNO/NO)/(dM/M) > 1 is

partly resulted from the rapid generation of a large number of thermal NO at this temperature. It can also be seen from Figure 8 that (dNO/NO)/(dM/M) jumped again from less than 1 to more than 1 at the end stage of combustion. This illustrated that the residual char nitrogen released at integrative mode and rapidly at the stage of close to burnout so that NO production rate increased. Of course, at this stage, the burning rate may also have been reduced to a greater extent. For Yangquan anthracite and Linfen bituminous coals, at the end stage of combustion, the increase of (dNO/NO)/(dM/M) is more significant, so it indicates that for the coal with low volatile content or high ash content, at its final stage of combustion, its char nitrogen is very easy to release at integrative mode and rapidly to generate NO so that the final NO emission is difficult to be controlled. 3.5. Conversion Rate Trends of Nitrogen into NO during the Coal Samples Combustion Process in a O2/N2 Atmosphere or a O2/Ar Atmosphere. To obtain more specific details of the formation relationship between fuel NO and thermal NO, the experiments of transient NO release properties during the coal samples combustion in an O2/Ar atmosphere were carried out. Oxygen concentration in an O2/Ar atmosphere is the same as that in an air atmosphere, which is equivalent to that in which the argon gas replaced nitrogen in the air as the balance gas. Thus, NO formation in this experiment may be considered to be complete fuel NO. Figure 2 shows that at 1600 °C, there was obvious NO formation before the coal samples burned. Therefore, the comparison experiments of O2/N2 atmosphere and O2/Ar atmosphere were only carried 1581

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Figure 9. In an air and argon atmosphere, NO release characteristics during the coal combustion process.

out under the temperatures of 1550 and 1600 °C. Figure 9 shows that the comparisons of NO transient release characteristics during the coal combustion in the air and argon atmosphere at 1550 and 1600 °C. The main difference between the combustions in this two atmospheres exhibits before beginning combustion and after the end of combustion. In an argon atmosphere, for the four coal samples, the generated NO concentrations before beginning combustion and after the end of combustion are all zero, while these generated NO concentrations in an air atmosphere at 1550 and 1600 °C are about 25 and 75 ppm, respectively. They are just slightly lower for Baorixile coal. After the coal samples burned out, the generated NO concentrations gradually rose to 25 and 75 ppm again. It indicates that at 25 ppm, 75 ppm thermal NO concentrations are generated before or after fuel combustion at 1550 and 1600 °C, and the higher the combustion environment temperature is, the greater is generated thermal NO concentration. The shapes of NO generation curves of the entire combustion process of coal samples under these two atmospheres are essentially same, such as the curves showing the same width. It indicates that the argon gas atmosphere does not basically change the coal samples combustion processes and NO formation process. That indicates that the trends of fuel NO formation under argon gas atmosphere can completely display that under an air atmosphere. The peak value of NO formation under air atmosphere is higher, while the values at the two sides of the peak value are closer. It indicates that, when fuel NO and thermal NO simultaneously are produced during the coal samples combustion, the fuel NO formation will produce a certain inhibiting effect on the thermal NO formation. Through comparison of NO production curves under two atmospheres, the thermal NO production concentration at the two sides of the peak value is less than the thermal NO production concentration before the coal samples

Figure 10. Effects of combustion environment temperatures on the thermal NO production proportions of four coal samples.

combustion under argon gas atmosphere. Especially for each curve under the air atmosphere, the NO value indicated by its right part always decreases to 0 and, after a period of time, gradually rises back to the thermal NO concentration level before the coal sample was sent into the reactor. It illustrates that thermal NO formation during coal combustion is not independent and gradually returns to the prelevel of thermal NO production without the coal sample after the coal sample burned out. To explain this trend, further studies on the mechanism are necessary in the future. Figure 10 shows that the effects of combustion environment temperatures on the thermal NO production proportions of total NO formation amounts (represented by the TNO). As the combustion environment temperature increases from 1550 to 1600 °C, the thermal NO formation amounts of Shenhua bituminous, Linfen bituminous, and Baorixile lignite coals will all significantly increase, with the increase in the degree of Linfen bituminous coal being the largest. The thermal NO formation amount of Yangquan 1582

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anthracite instead decreases under 1600 °C, which might be due to a greater extent of increase of its fuel NO formation that has played certain inhibition on thermal-NO formation. In general, with the exception of the result of Linfen bituminous coal at 1600 °C, the ratios of the thermal NO formation amounts of four coal samples to their total NO formation amounts are basically below 30%. In other words, the ratios of fuel NO formation amounts to total NO formation amounts are about 70%, which is consistent with a number of studies. At the same time, it can be known that the ratio of the thermal NO formation amount of anthracite to its total NO formation amount is not more than 50% and is basically the same as the result of coal with high volatile content. It clearly proves that under 1600 °C, the main part of all NO generation from Yangquan anthracite combustion is still fuel NO. Of course, if there is a higher temperature region in the actual boiler, it is possible that the ratio of the thermal NO formation amount to total NO formation amount will increase. However, the thermal NO formation amount basically will not supply the main part of total NO formation amount, so it can be seen that air-staged combustion is suitable for anthracite to control the generation of NO and can achieve good controlling results for NO generation of anthracite.31 When the experimental curves of four types of coal samples are compared, as the combustion environment temperature increases from 1550 to 1600 °C, the thermal NO formation proportions in total NO formation for Shenhua bituminous, Linfen bituminous, and Baorixile lignite coals will all significantly increase, with the increase in the degree of Linfen bituminous coal being the largest. The increase of combustion environment temperature may result in the ratio of thermal NO generation to total NO formation continuously increasing, even for the coal with high volatile content.

test coals have higher emissions of NO. 30-60% of coal nitrogen of four types of coals can be converted to NO during their combustion. The conversion rate of Shenhua bituminous coal is generally the largest, but the conversion rate of Yangquan anthracite coal is the smallest, although its nitrogen content is the highest. Low volatile content or high ash content in coal will lead to the release process of NO longer at low temperature. However, the effect of volatile content and ash content in coal on the NO release time will greatly be reduced at high temperature (above 1300 °C). The release of fuel nitrogen is not in accordance with the fuel burnout degree. The impact of the combustion environment temperature on the release extent of nitrogen is more significant. At all stages of combustion, as the combustion environment temperature increases, the NO formation proportion will be closer to the burnout proportion, but high ash content in coal will weaken the improvement of the uniform release of final char nitrogen that results from the increase of combustion environment temperature. Thus, for coals with low volatile or high ash content, its char nitrogen is easier to release integrally and rapidly at the final stage of combustion. This makes the control of the final release of char nitrogen and final NO emission very difficult. The fuel NO formation will produce a certain inhibiting effect on the thermal NO formation. At high combustion environment temperature, for bituminous and lignite with high volatile contents, the conversion rates of fuel nitrogen will be stabilized and changed little. However, for anthracite, it still markedly increases, which may be related to the greater release of its fuel nitrogen during the final combustion. Higher combustion environment temperature will lead to more thermal NO generation. At 1600 °C, the ratios of the thermal NO formation amounts of four coal samples to their total NO formation amounts are basically below 30%.

4. Conclusions Acknowledgment. This work was supported by the Hi-Tech Research and Development Program of China (863 Program) (Contract Nos.: 2006AA05Z321, 2007AA05Z305). This work was also supported by the National Natural Science Foundation of China (Grant No. 50876061).

As the constant combustion environment temperature increases, NO release amounts of per unit mass coal samples exhibited the change trend of “W”-type curve. At low temperature (1000 °C) and high temperature (1600 °C), the four

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