Numerical Modeling of Oxy-fuel Combustion in a Refractory-Lined

Oct 28, 2013 - designed and constructed to certify our previously explored fuel nitrogen transportation mechanism based on a fixed-bed reaction platfo...
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Numerical Modeling of Oxy-fuel Combustion in a Refractory-Lined Down Flame Furnace Yu Li, Weidong Fan,* Qinghong Guo, and Xiaowei Zhang School of Mechanical and Power Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, People’s Republic of China ABSTRACT: In this paper, a refractory-lined down flame furnace (DFF) with a kind of swirl burner installing at the top is designed and constructed to certify our previously explored fuel nitrogen transportation mechanism based on a fixed-bed reaction platform. In our previous research based on two different kinds of experiments, there are several conclusions about oxy-fuel combustion technology: (1) In oxy-fuel combustion, the preact of fuel NO release is more obvious than that in O2/N2 and the amount of NO released is the lowest when at the highest temperature. (2) Fuel NO is redistributed that the proportion of volatile NO is increased from 50% at 1000 °C to nearly 100% at 1600 °C for bituminous coal, and it is the same trend for lignite and anthracite. (3) The combustion characteristics of oxy-fuel are different from those in traditional conditions because the combustion duration of particles in O2/CO2 at the temperature ranging from 1000 to 1300 °C is longer. On the basis of numerical simulation, the work herein is forwarded to a pilot-scale experiment applying oxidant-staged technology and the conclusions are qualitatively embedded into computational models.

1. INTRODUCTION The threat of carbon dioxide on the ecosphere of the Earth gives a thrust on the development of carbon capture technology. Among a variety of technologies, the oxy-fuel technology is more applicable for its wide adaptation on the retrofit of a conventional boiler and is outstanding for its advantages in pollutant control.1,2 Recently, oxy-fuel combustion technology has a profound progress based on the laboratory experiments and pilot-scale study.3−8 The topics are mainly concentrated on the combustion characteristics, the detailed fuel N distribution between volatile and char, the conversion ratio of fuel N to NO in various conditions, and the radiative properties in the oxyfuel combustion process. On the basis of a drop-tube furnace (DTF), Sun et al. carried out a series of experiments to study the conversion of char N and volatile N to NO and the reduction of recycled NO through homogeneous and heterogeneous reactions during the fuel-rich or fuel-lean combustion of pulverized coal in O2/CO2.3 They found that a higher conversion ratio of fuel N to NO happened in fuel-lean conditions and the effect of the temperature on the char N and volatile N conversion ratio is contrary when the temperature increases from 1000 to 1300 °C. Rathnam et al.4 carried out their research on DTF and thermogravimetric analysis (TGA) to study the combustion characteristics, such as the reactivity, volatile yield, and burnout of pulverized coal in various O2 concentrations by comparing the results from O2/N2 and O2/ CO2. They concluded that the volatile yield and burnout in O2/ CO2 is higher than that in O2/N2. A higher temperature may enhance these trends. Mackrory et al.5 studied the parameter profile along the furnace when adopting an oxidant-staged strategy. On the basis of their findings, they highlighted that the oxidant-staged combustion technology would be more efficient in NO reduction, which is in accordance with our previous work.9 Miklaszewski et al.7 did their work on a laboratory-scale equipment featuring cloud combustion and a 8.79 MWth oxy© 2013 American Chemical Society

fuel boiler featuring high temperatures. On the basis of both, a kind of high-speed camera and a kind of process named inverse radiation interpretation were employed to study the combustion speed, radiative properties, and temperature profile. The work all mentioned above enabled us to acknowledge directly the up-to-date progress on the basic research of the oxy-fuel combustion technology. Beside the experimental research, many numerical simulations concerning oxy-fuel combustion technology are carried out to study both the plausibility of the oxy-fuel technology and the variety numerical model concerning combustion behavior, radiation model, and algorithms, which enable a comprehensive and long-term foundation for the development.10−14 Edge et al.11 adopted the large eddy simulation (LES) model for the simulation of a pulverized fuel (PF) swirl burner to capture the flame flicker while improving the radiation calculation by adding full-spectrum k-distribution (FSK) into the numerical modeling. Chen et al.12 simulated the swirl burner using four viscous models and coal char and volatile submodels. Yin14 mainly focused on improving the radiation model, a refined weighted sum of gray gas model (WSGGM), in O2/CO2 for a more accurate prediction of the radiation in a real furnace of both air and oxy-fuel combustion. Finally, the simulation works gave us a strong and accurate tool to facilitate the research of oxy-fuel technology. Some researchers not only carried basic experiments but also tried to give a mathematical description about experimental results, aiming to build connections between experiments and numerical modeling.15,16 This research method is quite Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 31, 2013 Revised: October 25, 2013 Published: October 28, 2013 155

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the section view) can be pushed into the heated reaction zone in a uniform speed, which further enhanced the repeatability of the experiments by excluding the human factor. The corundum crucible is a cylindrical container with evenly distributed pores at the bottom, which similarly realize the real combustion situation of pulverized coal by allowing flue gas to penetrate into the coal sample packed in the crucible. The coal sample in the crucible is packed in a “sandwich” way that pulverized coal is placed between two layers of refractory cotton, avoiding the leakage of the coal sample and char particles. The power of the electrical heating device is 4 kW, and the heating process can be performed using an intelligent temperature controller. A platinum− rhodium thermocouple is employed in the experiment. The reactor tube was made from refractory alumina, which can withstand the highest temperature of 1650 °C. Because of the heat capacity of the reaction tube and crucible, the heat sink of devolatilization and evaporation and the heat source of combustion could be ignored and the combustion environment temperature could be recognized as constant as designated. The flue gas was analyzed by a gas analyzer. According to the repeatability certification, the fluctuation of NO measurements is less than ±5%. As shown in Figure 2, two types of experiments were designed to (1) clarify the relative contributions of char N and volatile N to NO emission and the effect of the volatile−char interaction on fuel N conversion in O2/CO2 and O2/Ar atmospheres and (2) research combustion characteristics and NO release of pulverized coal or char in terms of the particle size, temperature, atmosphere, and oxygen concentration. In the first type of experiment, the combustion of volatile and char is separated. First, the coal sample undergoes the pyrolysis process in a pure CO2 or Ar atmosphere, and the released volatile is entrained in the flowing gas, with char remaining in the crucible. Downstream from the crucible about 10 cm away, an amount of oxygen is injected into the reaction zone to maintain a certain oxygen concentration, and the released volatile entrained in the flowing gas is consumed by the injected oxygen. Meanwhile, the reaction products are analyzed by a flue gas analyzer. Until the concentration of oxygen in flue gas resumes to the designated value (such as 21, 24, and 27 vol %) after the combustion of volatile, O2/CO2 or O2/Ar of the same oxygen concentration is then injected into the reaction tube from the left end to complete the combustion of the remaining char in the crucible. What is described here is the so-called “separation of volatile and char combustion”. This set of tests is mainly used to check the redistribution of fuel NO conversed from volatile N and char N and the effect of recycled NO on both volatile NO and char NO release. However, limited to the topics contained in this paper, only the closest results are introduced. In the second type of experiment, the combustion is completed without separation, just once through. This set of experiments is mainly used to do a series of tests to check the effect of the combustion environment temperature, oxygen concentration, and reaction atmosphere on the combustion characteristics and overall NO release.

appreciated for the inner consistence, integrity, and well exhibition of the work. Inspired by this method, we tried to qualitatively incorporate what we found in experimentation into the numerical simulation and to establish a set of parameters necessary in the numerical model, pursuing consistence between experimentation and simulation while expecting the further application of the research results on a larger scale or industrial operation. On the basis of this preliminary work and the references presented in this paper, we are expecting to further our research into the more detailed and complicated NO mechanism [eddy dissipation concept (EDC), eddy dissipation model (EDM), and finite rate and eddy dissipation (FRED)17] in the next phase. In addition, the simulation results were expected to give a general guide on the research direction on the refractory-lined down flame furnace (DFF).

2. EXPERIMENTAL SECTION 2.1. Fixed-Bed Reaction Platform. During the past several years, the laboratory investigation about oxy-fuel combustion was carried out mainly on a fixed-bed reaction platform, as shown in Figure 1.

Figure 1. 1, Volatile combustion O2 gas cylinder; 2, CO2 (Ar or CO2 + recycled-NO) gas cylinder; 3, Char combustion O2 gas cylinder; 4, Pressure relief valve; 5, Pressure gauge; 6, Mass flow meter; 7, Flow meter indicator; 8, Gas mixing box; 9, Electrical heated tube furnace; 10, Front portion sealing device; 11, Tail sealing device; 12, Corundum crucible; 13, Electric motor; 14, Drive screw; 15, Push rod; 16, Flue gas cooler; 17, Flue gas analyzer; 18, Computer. The gas-mixing section is consisted of gas cylinders, gas pipeline, pressure gauge, mass flow meters, gas-mixing tank, and intake pipe. In the system, the gas pipelines are strictly sealed to improve the experimental accuracy. Through the automatic motor feeding system, the corundum crucible installed at the end of the thin tube (shown in

Figure 2. Diagram of two sets of experiments for combustion of pulverized coal. 156

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Figure 3. Geometry of refractory-lined DFF. (Left) Three-dimensional (3D) picture, (middle) computational domain: (1) primary stream with pulverized coal, (2) secondary swirl stream, (3) staged stream, and (4) outflow, and (right) pilot-scale furnace under commission.

Table 1. Proximate Analysis and Ultimate Analysis of SH Coal ultimate analysis (wt %, as received)

proximate analysis (wt %, as received)

coal species

Car

Har

Nar

Sar

Oar

fixed C

ash

M

V

Qnet.v (MJ/kg)

SH

63.13

3.62

0.70

0.41

9.94

53.58

10.70

11.50

24.22

24.136

Those main conclusions closely relative to the theme of this paper are illustrated in section 3.1. 2.2. Pilot-Scale DFF. On the basis of previous works, we carried out on fixed-bed reaction platform. It is quite necessary to forward the research into pilot-scale application and numerical simulation. As shown in Figure 3, this refractory-lined DFF is constructed with rate capacity as 15 kW and staged oxidant stream ports are evenly set along the furnace height. Along the flue gas flow direction, the thermal couples are set to monitor the temperature profile. According to previous work on air-staged combustion technology by Coda et al.,18 Fan et al.,19 and Mackrory et al.,5 it is believed that the optimumstaged position is located between 700 and 900 mm downstream from the burner. As a preliminary work, a numerical simulation of this furnace is carried out to predict the operating results, with the findings on the fixed-bed reaction platform embedded into the combustion and NO model. To focus on the certification of combustion characteristics and redistribution of fuel N, the staged stream port at the optimum location x = 794 mm is idealized to radially inject the oxidant stream into the flue gas perpendicular to the axis according to the design by Ribeirete et al.,20 and the size of the cylinder injector port is 20 mm in height and 30 mm in diameter. Because of the axial symmetry characteristic of the combustion zone, the 1/8 periodic zone of the furnace space is constructed to be the computational domain for a higher mesh density but less computational amount. 2.3. Coal Information. Shenhua (SH) bituminous coal is a coal widely used in China power generation. This kind of coal, with its basic information listed in Table 1, is the main testing coal of a series of experiments in our research, and hence, there is a lot of knowledge about its combustion behavior. Therefore, it was applied in our simulation with its combustion characteristics and redistribution mechanism of fuel N embedded into the combustion and NO pollutant models.

3. EXPERIMENTAL RESULTS AND NUMERICAL SIMULATION 3.1. Experimental Results. 3.1.1. Combustion Characteristics. As shown in Figure 4, the upper row is the ignition time versus the combustion environment temperature and the down row is the combustion duration ratio with the 30 μm pulverized coal combusted in this condition (O2/N2, 21 vol % oxygen concentration, and 1000 °C) as a reference value. With the second type of experiment shown in Figure 2 as an illustration, the reaction tube is set at a certain temperature and the parameters of the atmosphere are also maintained at a certain value. The coal sample filled into the crucible is placed at the cool left end. The crucible is then pushed into the reaction tube at time = 0 s, and meanwhile, the gas analyzer is started to record. Thus, in the tests, the ignition time is defined as the moment when the chemical species (the product of combustion of volatile) start to fluctuate. Because the ignition is directly related to the devolatilization process, the comparison of the ignition time between O2/CO2 and O2/N2 can be concluded and extracted to guide the numerical simulation qualitatively at the preliminary stage. The laboratory results show that the volatile released in O2/CO2 is earlier than O2/N2 at a temperature ranging from 1000 to 1300 °C. There are two factors that determine this phenomenon. First, it is believed that a higher heat capacity of O2/CO2 enables a relatively larger temperature difference than that in O2/N2, although an amount of heat is transferred from the atmosphere to pulverized particles. Second, the radiation characteristics of O2/CO2 enhanced the heating process of pulverized coal particles. Both of the two factors enhance the heating process of volatile and, thus, guarantee the earlier ignition time. From the down row in Figure 4, the relationship between combustion duration and combustion environment temper157

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Figure 4. Combustion characteristics of pulverized coal combusted under different atmospheres (the upper row is the ignition time, and the lower row is the combustion duration).

Figure 5. NO cumulative percentage versus O2 cumulative percentage at 1000, 1300, and 1600 °C in a N2 atmosphere.

a low temperature range of 1000−1300 °C, the former one plays the main role, and at a high temperature range of 1300− 1600 °C, the latter one rules the combustion process. According to Figure 4, a high temperature is suitable for oxyfuel combustion technology. However, it is tough to give a clear conclusion between the oxygen concentration and combustion duration based on the data shown in Figure 4 because the differences between

ature proved that the high combustion temperature ranging from 1300 to 1600 °C in O2/CO2 can maintain an equivalent reaction rate as that in O2/N2. Essentially, the combustion duration, reaction rate, or flame propagation is the speed of the generation, storage, and transfer of energy from particle to particle. The heat capacity and radiation characteristics are the main difference between O2/CO2 and O2/N2. The former one slows the speed, and the latter one increases the speed. When at 158

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conditions are hard to tell apart. In many previous studies, it is commonly acknowledged that, to compensate for the decreased reaction rate and obtain a similar furnace parameter profile in O2/CO2, the O2 concentration should be increased to around 28 vol %.1,9,21−23 In addition, important is that a higher oxygen concentration leads to less gas volume and a higher gas temperature, which could enhance the reaction rate, thus increasing the flame stability. 3.1.2. Evolvement of the Combustion Process. What we show in Figure 5 is the NO cumulative percentage versus the O2 cumulative percentage (i.e., combustion process) in O2/N2. The cumulative percentage is defined as the ratio of the species already released (NO) or consumed (O2) to each total amount during the proceeding of the combustion. The value of “shenhua30-o21-n2-1000” means a group of pulverized coal with 30 μm in diameter combusted in an O2/N2 atmosphere of 21 vol % oxygen concentration at 1000 °C. It is quite clear that the NO release process is more rapid as the combustion reaction temperature increases and the fraction of NO released at the beginning of combustion becomes larger. On the basis of the other experimental results in O2/Ar and O2/CO2, this phenomenon is also of the same trend but has a little difference. As shown in Figure 6, the three curves of O2/N2, O2/Ar, and

Figure 7. Fuel NO yield per O2 consumption (mg/mg) under different atmospheres.

O2/CO2 is around 0.010−0.020 mg of NO/mg of O2, which is much smaller than that generated in O2/N2. NO shown in the O2/N2 column at 1600 °C ranging from 0.015 to 0.025 mg of NO/mg of O2 is thermal NO removed. Its total NO is around 0.055 mg of NO/mg of O2, almost 3 times the yield in O2/ CO2. This information is meaningful to facilitate the simulation in section 3.2. In addition, the less NO emission in O2/CO2 combined with the oxidant-staged combustion technology make the oxy-fuel combustion technology more promising. 3.1.4. Redistribution of Fuel N between Volatile and Char. There is a very important variate “fraction of N in char [dry and ash free (DAF)]” which is used in calculating the split of atomic nitrogen for the fuel NO model. Before, without a quantitative reference, this variate is often determined empirically.10,24 According to what is shown in Figure 8, the volatile NO/fuel

Figure 6. Comparison of NO release divergence from the diagonal line between N2, Ar, and CO2.

O2/CO2 show that the NO cumulative percentage at an early period of combustion in O2/CO2 is larger than those in other two atmospheres. Specially, this trend is aggravated at a high temperature ranging from 1300 to 1600 °C. On the basis of these results and the NO reduction theory in air-staged combustion technology, we are assured that the oxidant-staged combustion technology in O2/CO2 is quite promising. That is why we have the confidence to incorporate oxidant-staged stream into the numerical simulation of DFF. 3.1.3. NO Release Amount. The advantages of oxy-fuel combustion technology in NO control are widely acknowledged. We tried to obtain concrete data to describe these advantages. As shown in Figure 7, the NO release distribution in three atmospheres is drawn to show the influence of various factors. It is quite obvious that the NO released in an O2/CO2 atmosphere is the lowest. When the temperature ranges from 1000 to 1300 °C, the amount of NO is normally half of that in O2/N2. When the temperature is as high as 1600 °C, the advantage is very outstanding, because the amount of NO in

Figure 8. Effect of the particle size on the proportion of volatile NO to fuel NO during separated combustion.

NO value of bituminous coal in O2/CO2 is increased from 0.50 to 1.0 when the temperature increases from 1000 to 1600 °C. The value of bituminous coal in O2/Ar is around 0.65 during the temperature range. Because the devolatilization at the beginning of the combustion process is often immersed in a comparatively lower temperature zone, such as 1000−1300 °C, the volatile NO/fuel NO value is specified as 0.7 on average, i.e., with fraction of N in char as 0.3. As a preliminary extraction 159

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of the parameters from Figure 8, the dependence of volatile NO/fuel NO upon the temperature and atmosphere is temporarily ignored and the complicated UDF models are expected to embed into the computational fluid dynamics (CFD) in the next phase. The larger redistribution of fuel N in volatile enables the oxidant combustion technology to be more efficient, because a larger portion of volatile NO is reduced in the following reduction zone. The reason why oxy-fuel combustion technology has such an advantage in low NO emissions may also partly rely on the redistribution of fuel N. 3.2. Numerical Modeling. 3.2.1. Parameters of Computational Cases. As mentioned in section 2.2, the computational simulation of the pilot scale refractory-lined DFF is the target into which we want to embed those conclusions found on fixed bed reaction platform. What is shown in Table 2 is the boundary conditions of the two cases.

Figure 9. Specification of redistribution of fuel N according to the experiments, ignoring its dependence upon the atmosphere and temperature. CR = conversion ratio.

is chosen for the combustion model. The discrete ordinates (DO) are employed for the radiation calculation, and under the materials menu, the “wsggm-domain-based” option is selected to calculate the absorption coefficient and the scattering coefficient is set as 0.1. The discrete random walk model is enabled to increase the accuracy of the discrete particle behavior. As for the computational strategy, the flow and turbulence equations are activated to iterate 300 times, while the temperature is patched as 1000 K to obtain a better initial value for the full calculation (i.e., all equations are activated). The temperature profile of the wall along the furnace height is set as 1373 K in the upper furnace and 1173 K in the down furnace as measured in the commissioning. 3.2.2. Comparison of Simulation. 3.2.2.1. Comparison of Combustion Characteristics. With the oxygen concentration in O2/CO2 as 28 vol % and activation energy of first rate in the “two-competing-rates” model as 3.56 × 107 J kg mol−1 in O2/ CO2, 5.06 × 107 J kg mol−1 in O2/N2, the results show a quite reasonable profile of various parameters. The interactive relationship of the same parameter [such as the high temperature zone, volatile release zone, and main discrete particle model (DPM) burnout zone] between O2/N2 and O2/ CO2 is largely in accordance with those conclusions illustrated in section 3.1. When the fractions of volatile (i.e., devolatilization or ignition characteristics) and DPM burnout (indirectly related to combustion duration) are taken as an example, it is mentioned in a previous section that the 28 vol % oxygen concentration and temperature around 1300 °C are appropriate for the oxyfuel combustion technology to achieve the same operating condition, such as the ignition and flame propagation of pulverized coal. In Figure 10, we can see that the fraction of volatile in O2/CO2 is about the same position and distribution range as that in O2/N2. The distribution of DPM burnout (i.e., pulverized coal combustion process) is also of a similar shape as that in O2/N2. The parameter values at outflow are listed in Table 3. 3.2.2.2. NO Formation and Distribution. As mentioned in the previous section, the fraction of N in char is critical in determining the NO released during the pulverized combustion. Some researchers also found that the fraction of N between volatile and char and the conversion ratio to NO are temperature-dependent.3,25 Before the deeper exploration of Figure 8, the dependence is temporarily not considered at the preliminary stage. According to the split combustion of the pulverized coal on the fixed-bed reaction platform, the fraction of N in char is approximately set as 0.3 according to Figure 8. In addition, in accordance with the NO profile obtained on the

Table 2. Boundary Conditions of O2/CO2 and O2/N2 Cases boundary conditions capacity (kW) coal mass flow rate (kg/h) oxygen concentration (vol %) theoretical oxygen demand (m3/kg) access oxidant ratio oxidant stream (m3/h) primary stream volume ratio primary stream volume flow rate (m3/h) primary stream temperature (K) primary stream velocity (m/s) secondary stream volume ratio secondary stream volume flow rate (m3/h) secondary stream temperature (K) swirl angle (deg) secondary stream velocity (m/s) staged stream volume ratio staged stream volume flow rate (m3/h) staged stream temperature (K) staged stream velocity (m/s) upper furnace temperature (K) down furnace temperature (K)

O2/CO2

O2/N2 15 2.27

28

21 1.31 1.2

12.77

17.03 0.1

1.28

1.70 353

2.28

3.04 0.6

7.66

10.22 473 60

9.78

13.04 0.3

3.83

5.11 474

1.00

1.35 1375 1175

The 1/8 periodic computational domain is meshed with the mesh number as 40 053 cells, with a rotation angle of 45°. The swirl angle of 60° of the secondary stream is selected according to a numerical simulation of an industrial swirl burner.9 The renormalization group (RNG) k−ε turbulence model with a SIMPLEC scheme is chosen for pressure−velocity coupling. Species transport is chose for the species model, and in the coal calculator menu, the fraction of N in char (DAF) is set as 0.3 for both O2/CO2 and O2/N2. The model of the redistribution of N is illustrated in Figure 9, which is simplified by ignoring the dependence of fuel N upon the temperature and combustion atmosphere. As illustrated in section 3.1.1, the devolatilization in O2/CO2 (28 vol %) is close to that in an O2/ N2 (21 vol %) atmosphere and the flame shape of both is similar to an axial-stretched horn. With the analysis mentioned above under consideration, the “two-competing-rates” for the devolatilization model is selected and the activation energy of the first rate is empirically and accordingly set as 5.06 × 107 J kg mol−1 for O2/N2, 3.56 × 107 J kg mol−1 for O2/CO2 after trial computation. The kinetics/diffusion limited with a default value 160

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Figure 10. Comparison of the contours of various parameters between O2/N2 and O2/CO2: (left column) O2/N2 and (right column) O2/CO2.

Table 3. Parameters of Flue Gas at Outflow outflow

O2/CO2

O2/N2

mole fraction of O2 (vol %) mole fraction of CO2 (vol %) mole fraction of H2O (vol %) mole fraction of N2 (vol %) temperature (K)

5.6 87.7 6.6 not extracted 1021

5.0 13.6 4.9 76.4 1188

DTF,19 the conversion fractions of volatile N and char N under fuel submenu in the NO model in O2/N2 are set as 1 to achieve a similar NO profile along the furnace. As for thermal NO, the [O] and [OH] models are set as partial equilibrium. In O2/CO2, the conversion fraction of char N under formation model parameter submenu contained in the NO model is very important to adjust the NO emissions. The conversion fraction of char N, which would directly determine the NO level at the outflow, is set as 0.4 to approximately keep accordance with those conclusions derived from experiments on the fixed-bed reaction platform: (1) a larger amount of NO is released at the first half of the combustion process in O2/ CO2, and (2) total NO released in O2/CO2 is half or a third of NO released in O2/N2. According to the simulation, the final results show that the mole fraction of NO at the outflow is 139 ppm in O2/N2 and 66 ppm in O2/CO2. In addition, the final emission of NO in O2/N2 is of the same level when compared to previous works on air-staged combustion technology.19,26 What is shown in Figure 11 is the contour of NO in O2/CO2 and O2/N2. For more details about the distribution of the NO profile along the furnace height, Figures 12 and 13 are drawn to give readers a clear understanding on the NO formation and reduction processes, which embody a one-dimension feature.

Figure 11. Comparison of NOx formation between the two cases.

According to the oxygen distribution in Figure 10 and the NO distribution in Figure 11, the distribution of NO in O2/ CO2 gives us inspiration. The oxidant-staged combustion strategy with the oxidant ratio as 0.7 produced a large reducing zone, which is enough for the total reduction of NO released in the fuel-rich zone. Consequently, NO contained in the exhaust is mainly determined by the conversion of char N. According to the above analysis, the amount of char combusted in the burnout zone (i.e., the oxidant stage ratio) should be carefully considered because the final NO level is determined by char N contained in char. Because of the total reduction of NO in the fuel-rich zone, the recycle of NO-contained flue gas into the fuel-rich zone or as balance stream is expected to bring more benefits on the reduction of NO. 161

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-34206049. Fax: +86-21-34206115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283−307. (2) Kanniche, M.; Gros-Bonnivard, R.; Jaud, P.; Valle-Marcos, J.; Amann, J.-M. Appl. Therm. Eng. 2010, 30, 53−62. (3) Sun, S. Z.; Cao, H. L.; Chen, H.; Wang, X. Y.; Qian, J.; Wall, T. Proc. Combust. Inst. 2011, 33, 1731−1738. (4) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y. H.; Moghtaderi, B. Fuel Process. Technol. 2009, 90, 797−802. (5) Mackrory, A. J.; Tree, D. R. Fuel 2012, 93, 298−304. (6) Ndibe, C.; Spörl, R.; Maier, J.; Scheffknecht, G. Fuel 2013, 107, 749−756. (7) Miklaszewski, E. J.; Zheng, Y.; Son, S. F. Fuel 2013, 104, 452− 461. (8) Watanabe, H.; Marumo, T.; Okazaki, K. Energy Fuels 2012, 26, 938−951. (9) Li, Y.; Fan, W. D.; Ren, P.; Wang, K.; Wang, J. C. Proceedings of the 7th International Symposium on Coal Combustion; Harbin, China, July 17−20, 2011, pp 629−636, ISBN: 10 3-540-35606-1. (10) Cao, H. L.; Sun, S. Z.; Liu, Y. H.; Wall, T. F. Energy Fuels 2010, 24, 131−135. (11) Edge, P.; Gubba, S. R.; Ma, L.; Porter, R.; Pourkashanian, M.; Williams, A. Proc. Combust. Inst. 2011, 3, 2709−2716. (12) Chen, L.; Ghoniem, A. F. Energy Fuels 2012, 26, 4783−4798. (13) Smart, J. P.; O’Nions, P.; Riley, G. S. Fuel 2010, 89, 2468−2476. (14) Yin, C. G. Energy Fuels 2012, 26, 3349−3356. (15) Murphy, J. J.; Shaddix, C. R. Combust. Flame 2006, 144, 710− 729. (16) Zhang, J.; Prationo, W.; Zhang, L.; Zhang, Z. X. Energy Fuels 2013, 27, 4258−4269. (17) Liu, J. Z.; Chen, S.; Liu, Z. H.; Peng, K.; Zhou, N.; Huang, X. H.; Zhang, T.; Zheng, C. G. Ind. Eng. Chem. Res. 2012, 51, 691−703. (18) Coda, B.; Kluger, F.; Fortsch, D.; Spliethoff, H.; Hein, R. G. Energy Fuels 1998, 12, 1322−1327. (19) Fan, W. D.; Lin, Z. C.; Li, Y. Y.; Kuang, J. G.; Zhang, M. C. Energy Fuels 2009, 23, 111−120. (20) Ribeirete, A.; Costa, M. Fuel 2009, 88, 40−45. (21) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kato, M. Energy Convers. Manage. 1997, 38, S129−S134. (22) Okazaki, K.; Ando, T. Energy 1997, 22, 207−215. (23) Li, X. C.; Rathnam, R. K.; Yu, J. L; Wang, Q.; Wall, T.; Meesri, C. Energy Fuels 2010, 24, 160−164. (24) Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F. Fuel 2008, 87, 3579−3585. (25) Shaddix, C. R.; Alejandro, M. Proc. Combust. Inst. 2011, 33, 1723−1730. (26) Konttinen, J.; Kallio, S.; Hupa, M.; Winter, F. Fuel 2013, 108, 238−246.

Figure 12. Cloud distribution of the NOx concentration in O2/N2 in the computational domain.

Figure 13. Cloud distribution of the NOx concentration in O2/CO2 in the computational domain.

4. CONCLUSION Finally, on the basis of the laboratory investigation of oxy-fuel combustion technology and previous air-staged combustion technology, we are confident to claim that the qualitatively numerical modeling of those conclusions extracted from experimental results is successful. The related parameters are ready to be adopted in the model setup when applied to other normal cases. Of course, more accurate modeling should embrace those works mentioned in the Introduction. In other words, the devolatilization process in oxy-fuel combustion technology with 28 vol % oxygen concentration is of the same level when compared to that in O2/N2. The combustion process (i.e., combustion duration or reaction rate) of pulverized coal keeps the same level as that in O2/N2. A larger proportion of NO is released at the first half of the combustion process. NO released is almost half or a third of NO released in O2/N2. In addition, according to the simulation results, the final NO emission level may be largely dependent upon the burnout of char N. The redistribution of N that we explored is of critical importance to the NO simulation. 162

dx.doi.org/10.1021/ef401499g | Energy Fuels 2014, 28, 155−162