Combustion Characteristics and Nitric Oxide ... - ACS Publications

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Combustion Characteristics and Nitric Oxide Release of the Pulverized Coals under Oxy-enrich Conditions Chunbo Wang,* Ming Lei, Huimin Liu, and Hongyu Lu School of Energy and Power Engineering, North China Electric Power University, Baoding City, Hebei Province, People’s Republic of China ABSTRACT: Instead of using pure oxygen in oxy-fuel combustion, oxy-enrich combustion uses air to replace part of pure oxygen for coal combustion. Compared with oxy-fuel combustion, oxy-enrich combustion uses much less pure oxygen, which may reduce the cost for CO2 capture due to the reduction of pure oxygen requirement. The thermo-gravimetric technique was used to study the combustion characteristics of three pulverized coals under oxy-enrich atmospheres. The results show that both the ignition and burnout temperatures of pulverized coals decrease with the rise of oxygen concentration, while the combustibility index S increases gradually. Under the same oxygen concentration, the ignition and burnout temperatures in oxyenrich combustion are lower than those in oxy-fuel combustion due to the different properties between N2 and CO2. A fixed-bed reactor (FBR) was used to investigate the NO emissions of a coal sample at different atmospheres. It is found that temperature has played an important role in NO releasing. With the increase of furnace temperature, the peaks of NO release appear to occur early. For the tested coal, the conversion rates of fuel-N to NO in air are higher than those in oxy-fuel or oxy-enrich atmospheres, which may be due to the effect of CO2 gasification. The conversion of fuel-N to NO appears to have some complicated tendencies in oxy-enrich combustion, and some possible reasons about this were discussed.

1. INTRODUCTION Combustion of fossil fuel in a O2/CO2 environment (oxy-fuel combustion) can enrich CO2 in flue gas to a high level (even more than 90%) for CO2 capture. However, the significant capital and operation costs of the air separation unit (ASU) of oxygen production to realize oxy-fuel combustion are one of the major barriers for its wide application in coal-fired power plants. Zanganeh and Shafeen1 have proposed a new approach to partially use air in the oxy-fuel combustion, and the simulation results showed that the process could be optimized to capture CO2 while minimizing the overall energy demand of the ASU and CO2 compression unit. The economic performance of the improved oxy-fuel combustion was estimated by Huang et al.,2 who indicated that the increasing air usage could reduce not only the capital cost, but also the electricity production cost. Doukelis et al.3 performed a techno-economic analysis of the improved oxy-fuel combustion assuming the use of MEA solvent for CO2 capture and found the electric power generation cost could be reduced compared with conventional oxy-fuel combustion. A new combustion mode, “oxy-enrich combustion” is suggested in this paper. For this combustion mode, the CO2 volume concentration at exhaust flue gas stream is controlled between 30% and 40%, while both air and pure oxygen are used for coal combustion. Because of the high efficiency and low energy consumption, pressure swing adsorption (PSA) technology, which completes the collection and purification of CO2 through a periodic pressure transformation process, is becoming quite competitive in the field of CO2 collection.4,5 PSA technology is widely applied in the synthesis ammonia facilities, where Japan is the vanguard of using PSA technology in CO2 sequestration.6,7 It has been demonstated that the PSA process can capture CO2 from the exhaust gas of synthesis © 2012 American Chemical Society

ammonia production on a large industry scale when the CO2 concentration is 30%−40%. The CO2 concentration of oxyenrich has been set to the same level (30%−40%) so it can also employ the PSA process to capture CO2. In this investigation, the combustion characteristics and NO emissions of the pulverized coals in oxy-enrich combustion were studied.

2. EXPERIMENTAL SECTION Materials. Three typical Chinese coals, Shenhua (SH) coal, Jiang Jiahe (JJH) coal, and Yangquan (YQ) coal were selected for testing. Coal particles between 74 μm and 90 μm were used in the tests. The properties of samples are listed in Table 1. Apparatus and Procedure. Thermogravimetric analyzer (TGA) was used for testing the combustion characteristics of the pulverized coals. The heating rate was set to 20 °C/min while the gas flow rate was controlled to 100 mL/min. The gas compositions for the tests are listed in Table 2. Fixed-bed reactor (FBR) was used for testing the NO emission of pulverized coal. The experimental system is shown in Figure 1. During tests, coal samples of ∼100 mg were loaded on a ceramic boat and sent into a furnace. Gas streams at the flow rate of 1 L/min with different compositions were injected into the furnace and provided the oxygen for combustion while passing the coal samples. The furnace is electrically heated and a thermal couple is installed at the furnace center upstream of the coal samples. The furnace temperature is measured by the thermal couple. During tests, four furnace temperatures, 800, Received: Revised: Accepted: Published: 14355

April 27, 2012 September 13, 2012 October 3, 2012 October 3, 2012 dx.doi.org/10.1021/ie301097c | Ind. Eng. Chem. Res. 2012, 51, 14355−14360

Industrial & Engineering Chemistry Research

Article

Table 1. The Ultimate and Proximate Analysis of Coal Samples proximate analysis (%)

a

ultimate analysis (%)

coals

Mad

Vad

Aad

FCad

Cad

Had

Oada

Nad

Sad

YQ SH JJH

1.02 2.15 3.23

9.35 25.97 28.87

26.14 21.12 25.27

63.49 50.76 42.63

56.19 54.95 57.1

1.58 3.72 0.81

12.46 16.57 9.77

0.94 0.88 0.46

1.67 0.61 3.36

Values by difference.

Table 2. Test Conditions atmosphere

O2 (%)

CO2 (%)

N2 (%)

air oxy-fuel 1 oxy-enrich 1 oxy-fuel 2 oxy-enrich 2

21 30 30 40 40

0 70 30 60 40

79 0 40 0 20

Figure 2. TG and DTG curves of YQ pulverized coal under different atmospheres.

and heat capacity of gas mixture (also called oxy-enrich atmosphere here) decrease, resulting in higher adiabatic flame temperatures in oxy-enrich combustion. In addition, since O2 has a higher diffusion rate in an oxy-enrich atmosphere than in oxy-fuel atmosphere (due to the different diffusivity of O2 in N2 and in CO2), O2 may diffuse to the coal particle surface more easily in the oxy-enrich atmosphere, so the burning rate of pulverized coal is expected to be faster in a oxy-enrich atmosphere.9 To investigate the combustion characteristics of the different coals under oxy-enrich atmosphere further, another two coals, SH and JJH coals, were tested and the results are shown in Figure 3 and Figure 4.

Figure 1. Schematic diagram of fixed-bed reactor.

900, 1000, and 1100 °C were set for the tests and the electrical heating system was automatically controlled to maintain that temperature. Delta 2000CD-IV gas analyzer was connected with the furnace outlet to measure the gases produced online.

3. RESULTS AND DISCUSSIONS 3.1. TGA Analysis. 3.1.1. Effects of Gas Compositions on Combustion Characteristics of the Pulverized Coals. The TG and DTG curves of YQ coal under different atmospheres are present in Figure 2. As shown in Figure 2, the combustion profiles of YQ coal apparently shift to a lower temperature region with the increase of oxygen concentration, which indicates that higher oxygen concentration could improve the combustion characteristics of the pulverized coal.8 Also in Figure 2, the TG and DTG curves of oxy-enrich combustion slightly move toward a lower temperature region when compared to oxy-fuel combustion at the same oxygen concentration. It implies that the combustion performances of coal in an oxy-enrich combustion may be better than those in oxy-fuel combustion (at the same oxygen concentration). These effects can be explained by differences in gas properties between CO2 and N2. CO2 has different properties from N2. For example, the molecular weight of CO2 is 44, compared to 28 for N2, thus the density of CO2 is higher than N2. Besides, the heat capacity of CO2 is higher than N2.9 When CO2 in oxyfuel atmosphere is partially replaced by N2, both the density

Figure 3. TG and DTG curves of SH coal under different atmospheres.

Similar to YQ coal, the combustion profiles of SH coal and JJH coal also shift to lower temperature zones as the oxygen concentration increases. Also, at the same oxygen concentration SH coal and JJH coal under oxy-enrich atmosphere have better combustion characteristics compared with oxy-fuel combustion due to the different physical properties of N2 and CO2, as discussed for YQ coal. 14356

dx.doi.org/10.1021/ie301097c | Ind. Eng. Chem. Res. 2012, 51, 14355−14360


Article

S= =

(dw/dt )max (dw/dt )mean R d ⎛⎜ dw ⎞⎟ E dT ⎝ dt ⎠T = Ti (dw/dt )T = Ti Tb (dw/dt )max (dw/d t)mean Ti 2Tb

where (dw/dt)max is the maximum burning rate, (dw/dt)T=Ti is the burning rate at Ti, (dw/dt)mean is the mean burning rate. Generally speaking, the higher the index S is, the faster the pulverized coals can burn.10 Figure 6 shows the combustibility index S of the three coal samples under different atmospheres. With the increase of

Figure 4. TG and DTG curves of JJH pulverized coal under different atmospheres.

3.1.2. Ignition and Burnout Temperatures. The ignition temperature and burnout temperature in TGA tests can be used to evaluate the ignition characteristics and burnout characteristics of the pulverized coals. In this work, the ignition temperature (Ti) is defined by TG/DTG curves that are detailed in the literature8 and the burnout temperature (Tb) which represents the temperature where coal oxidation is completed, is defined as the temperature at which the DTG profile reaches 1%/min at the end of the profile.8,10 Figure 5 shows the ignition and burnout temperatures of the three coal samples at different atmospheres.

Figure 6. The combustibility index S of coal samples at different atmospheres.

oxygen concentration, the indices S of the three coal samples increase gradually, which indicates that the combustion performance of pulverized coals can be improved by increasing oxygen concentration. Additionally, the combustibility index S at oxy-enrich atmosphere is higher than that at oxy-fuel atmosphere under the same oxygen concentration, suggesting that the combustion performance of pulverized coal is improved. Compared to YQ coal, the combustibility indexes S of SH and JJH are higher (Figure 5), probably because of their higher volatile content when compared to YQ coal (refer to Table 1) 3.1.4. Kinetics Analysis. The reaction kinetics equation of coal combustion can be defined as below:11 ⎛ −E ⎞ dα dα A ⎟ = kf (α) → = (1 − α)n exp⎜ ⎝ RT ⎠ dt dT β

Figure 5. Ti and Tb of coal samples at different atmospheres.

(1)

where α is the degree of reaction, α = ((m0 − mt)/(m0 − m∞)) (mt is the mass of the sample at time t, m0 and m∞ are the mass of the sample at the beginning and at the end of the mass loss reaction, respectively), k is the rate constant described by the Arrhenius equation, namely, k = A exp(−E/(RT) (A is the preexponential Arrhenius factor, E is the activation energy and R is the gas constant), f(α) = (1−α)n (n is the reaction order), β is the heating rate (β = dT/dt). By using the Coats−Redfern11 method for kinetics analysis, eq 1 can be transformed into

As shown in Figure 5, the ignition and burnout temperatures of the tested coals decrease at higher oxygen concentration. In addition, the ignition and burnout temperatures of coal under an oxy-enrich atmosphere are lower than those under an oxyfuel atmosphere at the same oxygen concentration. As discussed in 3.1.1, N2 is more favorable for coal combustion, which leads to lower ignition and burnout temperatures under the oxy-enrich atmosphere. It also can be found, under the same experimental conditions, the ignition and burnout temperatures of YQ coal are higher than those of SH coal and JJH coal, mainly due to its low volatile content and the relative high fixed carbon content (refer to Table 1). 3.1.3. Combustibility Index S. A comprehensive combustibility index S is introduced to evaluate the combustion characteristics of pulverized coal, and it is defined as follows:10

⎛ AR ⎞ ⎡ −ln(1 − α) ⎤ E ln⎢ ⎟− ⎥ = ln⎜ 2 ⎣ ⎦ ⎝ βE ⎠ RT T

(2)

2

By plotting ln[−ln(1−α)/T ] vs 1/T obtained from a test, the data fall on a straight line with a small amount scatter. The activation energy and the pre-exponential factor can be 14357



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obtained through the slope −E/R and the intercept of the straight line, respectively. The kinetic parameters of coal samples under different atmospheres are listed in Table 4. Table 4. Kinetic Parameters of Coal Samples under Different Atmospheres coal

condition

E (kJ/mol)

YQ


131.84 137.14 143.43 150.91 158.87

SH


JJH


A (min−1)

temp range (°C)

R(%)

5.13 1.62 4.95 2.02 9.01

× × × × ×

107 108 108 109 109

515−682 455−619 443−593 447−592 422−528

99.92 99.84 99.95 99.72 99.87

69.09 70.03 72.35 90.31 94.99

3.15 4.04 7.22 4.08 1.06

× × × × ×

104 104 104 106 107

372−514 363−492 360−477 346−460 339−431

99.37 99.21 98.62 99.59 98.60

104.16 107.27 110.69 112.44 127.53

1.67 5.29 1.12 1.66 2.63

× × × × ×

106 106 107 107 108

461−627 455−551 443−562 447−528 422−502

97.03 99.98 98.79 99.97 99.65

Figure 7. NO release profiles of YQ at different temperatures.

The volatile content in YQ coal is relatively low (refer to Table 1). When the furnace temperature is not high (e.g., 800 or 900 °C), volatile release may be neglectable at an early stage. The volatile release and combustion together with char oxidations probably start at the same time, so the volatile-N and char-N released in the forms of nitrogen-containing gases in the reactions may result in the single large peaks on the curves. At higher furnace temperature (e.g., 1000 or 1100 °C), the release rate of volatile matter may accelerate, volatile matter can accumulate and ignite easily before the char burning. The peaks at the beginning may be related to conversions of volatile-N to NO during volatile releasing and oxidation processes, the “shoulder” nearby the peaks on the NO release curves are likely the release of char-N during the combustion process of char. 3.2.2. Fuel Nitrogen Conversion. Tests have shown that the NO release tendencies in oxy-fuel and oxy-enrich atmospheres are very similar to that in air. So only the final fuel nitrogen conversions are discussed here. The NO yield is defined as the molar ratio of nitrogen released as NO to the nitrogen in the orginal coals. NO yield is calculated by the integral of its concentration along the time using the NO release curves in Figure 7. NO yields of YQ at different atmospheres and furnace temperatures are shown in Figure 8. From the Figure it can be

Niu et al.11 found that with the increase of oxygen concentration, the combustion characteristics of pulverized coal improved. He also found that the activation energy E would increase at higher oxygen concentration, which implied that the coal combustion rate is more sensitive to temperature at a higher oxygen concentration environment. Similar results have been found in this study as shown in Table 4: the activation energy E for all three tested coals increase with the increase of oxygen concentration. At same oxygen concentration, although there is a slight increase of activation energy at oxy-enrich atmosphere, the pre-exponential Arrhenius factor is much higher at the oxy-enrich atmosphere than at oxy-fuel atmosphere, suggesting an enhanced combustion rate at the oxy-enrich environment. 3.1.5. Summary. According to TGA analysis, it can be found that the ignition and burnout temperatures of the pulverized coal decrease while the combustibility index S increases gradually with the increase of oxygen concentration, which indicates the improvement of the ignition, burnout, and combustion rate at high oxygen concentration. At the same oxygen concentration, the oxy-enrich combustion has better combustion performances than oxy-fuel combustion due to the different properties between N2 and CO2. 3.2. FBR Analysis. 3.2.1. NO release. Figure 7 shows the NO release of YQ coal in air at four furnace temperatures during the FBR experiments, which demonstrates the significant influence of the furnace temperature on the release of NO. As shown in Figure 7, NO begin to release almost simultaneously at four different furnace temperatures, but with the increase of the furnace temperature, the peak of NO release appears to occur earlier. At low furnace temperatures (800 and 900 °C), there is only an obvious peak on the NO release curve, but at 1000 and 1100 °C, an “shoulder” appears nearby the peak on the curves.

Figure 8. Fuel-N to NO conversion of YQ.

found that the total conversions of fuel-N to NO in air from 800 to 1100 °C are all higher than those in oxy-fuel or oxyenrich atmospheres. This can be explained by the gasification effect. As suggested by Jamil et al.,12 the initial CO2 gasification of nascent char occurred at 600 °C with a considerable reaction rate. Because of the high concentration of CO2 in oxy-fuel and 14358



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enrich environment, probably due to the lower specific heat of N2 compared to that of CO2 and the higher diffusivity of O2 in N2 than in CO2. According to the FBR test results, with the increase of furnace temperature, the peaks of NO release occur earlier. The NO yields of coal samples in air are all higher than those in oxyfuel or oxy-enrich atmospheres, which can be explained by the effect of CO2 gasification. Temperature has an important influence on the competing reactions of the oxidation of fuel-N and NO/CO/char reaction in the process of NO formation. Since the reaction temperatures of pulverized coal change at the different atmospheres, the NO yield of coal samples exhibited some complicated tendencies during the experiments.

oxy-enrich atmospheres, the CO2 gasification reaction can be enhanced with the production of CO during the reaction process, which could promote the NO reduction catalyzed by the carbon surface. As shown in Figure 8, with the increase of oxygen concentration, the conversion rates of fuel-N to NO for YQ coal decline gradually at same furnace temperature. And, the conversion rates of NO in oxy-enrich combustion are lower than those at oxy-fuel combustion at the same oxygen concentration. Figure 9 plots the NO conversion value via

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (51276064). Figure 9. NO conversion via furnace temperature.

REFERENCES

(1) Zanganeh, K. E.; Shafeen., A. A novel process integration, optimization and design approach for large-scale implementation of oxy-fired coal power plants with CO2 capture. Int. J. Greenhouse Gas Control 2007, 1 (1), 47−54. (2) Huang, Y.; Wang, M.; Stephenson, P.; Rezvani, S.; McIlveenWright, D.; Minchener, A.; Hewitt, N.; Dave, A.; Fleche, A. Hybrid coal-fired power plants with CO2 capture: A technical and economic evaluation based on computational simulations. Fuel 2011, DOI: 10.1016/j.fuel.2010.12.012. (3) Doukelis, A; Vorrias, I.; Grammelis, P.; Kakaras, E.; Whitehouse, M.; Riley, G. Partial O2-fired coal power plant with post-combustion CO2 capture: A retrofitting option for CO2 capture ready plants. Fuel 2009, 88 (12), 2428−2436. (4) Yong, Z.; Mata, V.; Rodrigues, A. E. Adsorption of carbon dioxide at high temperature-A review. Sep. Purif. Technol. 2002, 26 (2), 195−205. (5) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: MI, 1997; pp 237−247. (6) Chou, C.; Wu, C.; Chiang, A. S.T. A complementary pressure swing adsorption process configuration for air separation. Sep. Technol. 1994, 4 (2), 93−103. (7) Alpay, E; Chatsiriwech, D.; Kershenbaum, L. S. Combined reaction and separation in pressure swing processes. Chem. Eng. Sci. 1994, 49 (24), 5845−5864. (8) Li, Q; Zhao, C.; Chen, X.; Wu, W.; Li, Y. Comparison of pulverized coal combustion in air and in O2/CO2 mixtures by thermogravimetric analysis. Jo. Anal. Appl. Pyrol. 2009, 85 (1−2), 521−528. (9) Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Kharea, S.; Rathnama, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. An overview on oxyfuel coal combustionState of the art research and technology development. Chem. Eng. Res. Des. 2009, 87 (8), 1003−1016. (10) Huang, X.; Jiang, X.; Han, X.; Wang, H. Combustion characteristics of fine- and micro-pulverized coal in the mixture of O2/CO2. Energy Fuels 2008, 22 (6), 3756−3762. (11) Niu, S.; Lu, C.; Han, K.; Zhao, J. Thermogravimetric analysis of combustion characteristics and kinetic parameters of pulverized coals in oxy-fuel atmosphere. J. Anal. Appl. Pyrol. 2009, 98 (1), 267−274. (12) Jamil, K.; Hayashi, J.; Li, C. Pyrolysis of a Victorian brown coal and gasification of nascent char in CO2 atmosphere in a wire-mesh reactor. Fuel 2004, 83 (7−8), 833−843.

the furnace temperature for the four atmospheres. It can also be found that all the atompsheres' conversion rates of fuel-N to NO for YQ first increase as the temperature increases from 800 to 900 °C. The conversion rates all reach their maximum at 900 °C and then decrease as the temperature continues to rise. Sun et al.13 found that temperature played an important role in the oxidation of the following fuel-N and NO/CO/char reactions: char

2NO + 2CO ⎯⎯⎯→ N2 + 2CO2

In other words, whether the fuel-N oxidizes to NO or reduces to N2 during the combustion process is closely related to the reaction temperature. Therefore, the nonmonotonic change of NO yield at different atmospheres and temperatures might be the result of two competing processes, the oxidation of fuel-N and the NO/CO/char reactions. 3.2.3. Summary. According to FBR analysis, it can be found that the peak of NO release occurs earlier with the increase of furnace temperature (see Figure 7). Temperature has an important influence on the oxidation of fuel-N and NO/CO/ char reactions which compete with each other in the process of NO formation. Since the reaction temperatures of pulverized coal probably change under different atmospheres, the NO yield appeared to have some complicated tendencies during the experiments.

4. CONCLUSION On the basis of the TGA results at oxy-fuel and oxy-enrich atmospheres, it was found that with the increase of oxygen concentration, the ignition and burnout temperatures of the pulverized coal decrease while the combustibility index S increases gradually, which indicates the improvement of the combustion characteristics at high oxygen concentration environments. At the same oxygen concentration, the reduced ignition temperature and burnout temperature in the oxyenrich environment compared to the oxy-fuel environment suggest a better ignition and burnout performances in the oxy14359



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(13) Sun, S.; Gao, H.; Chen, H.; Wang, X.; Qian, J.; Wall, T. Experimental study of influence of temperature on fuel-N conversion and recycle NO reduction in oxyfuel combustion. Proc. Combust., Inst. 2011, 33 (2), 1731−1738.

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Combustion Characteristics and Nitric Oxide ... - ACS Publications

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