NO Emission Behavior in Oxy-fuel Combustion Recirculated with

Energy & Fuels 2007, 21, 121-129

121

NO Emission Behavior in Oxy-fuel Combustion Recirculated with Carbon Dioxide Jeong Park,*,† June Sung Park,† Hyun Pyo Kim,† Jeong Soo Kim,† Sung Cho Kim,† Jong Geun Choi,† Han Chang Cho,‡ Kil Won Cho,‡ and Heung Soo Park‡ School of Mechanical & Aerospace Engineering, Sunchon National UniVersity, 315 Maegok, Suncheon, Jeonnam 540-742, Korea, and Energy Team, Research Institute of Industrial Science and Technology, #32 Hyoja-dong, Nam-gu, Pohang 790-330, Kyungbuk, Korea ReceiVed July 4, 2006. ReVised Manuscript ReceiVed October 25, 2006

A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO2 in a CH4-O2/N2/CO2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO2 and is also focused on chemical effects of recirculated CO2. Air-blown combustion and oxy-fuel combustion without recirculated CO2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO2. The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO2, and the role of the recirculated CO2 and its chemical effects are discussed. Importantly contributing reaction steps to the formation and destruction of NO are also estimated in oxy-fuel combustion in comparison to air-blown combustion.

Introduction About 85% of the world’s commercial energy needs are supplied by fossil fuels, and hence, CO2 capture and storage, including its reuse, presents an opportunity to achieve significant reduction in greenhouse gas emissions from fossil energy use. Approximately one-third of all CO2 emissions due to human activity come from fossil fuels used for generating electricity. Currently, there are three main approaches for capturing CO2 from combustion of fossil fuels, namely, precombustion capture, postcombustion capture, and oxy-fuel combustion. In oxy-fuel combustion, nearly pure oxygen (instead of air) is used for combustion that would result in a flue gas composed of mainly CO2 and H2O as well as a small concentration of inert gases and nitrogen (due to air infiltration that happens in practice) and excess O2. Hence, the concentration of CO2 in the flue gas can be greatly increased. As a result, only simple gas purification is required to capture CO2 in this process. In this combustion mode, if fuel is burnt in pure oxygen, the resulting flame temperature would be excessively high. Hence, the CO2 rich flue gas could be recycled to the combustor to reduce the flame temperature and make it similar to that in the air case. This recycle combustion process also has a further benefit in suppressing NOx formation.1,2 The NOx formation in real oxy-fuel burners is due to nitrogen contamination in the fuel stream and/or air leaks.3 While the study covered the

sensitivity of NO production due to effects of air infiltration, fuel contamination, flame radiation, and aerodynamic straining, the recycle combustion process on NOx formation was not provided. From a practical point of view, it may be desirable to have higher intermediate soot without affecting the soot emission level because of the enhancement of the heat transfer by radiation. This is because the enhanced radiant fluxes could decrease the flame temperature and NOx concentrations.4,5 Li and Williams6 found that NOx emission could be decreased in counterflow partially premixed flames by reducing CH concentrations and, hence, the contribution of the prompt mechanism into the NOx formation process. It was also shown in methane-air flames7 and highly preheated H2-air flames8 diluted with CO2 that chemical effects with recirculated CO2 were mainly caused by the reaction CO2 + H f CO + OH, and this affects flame structure and thermal NO considerably, since the reaction also competed with the principal chain branching reaction H + O2 f O + OH for H-atom. It was also recognized that these chemical effects modified reaction pathways, and these could contribute to the formation and destruction of prompt NO. The present study is conducted numerically to grasp the formation and destruction of NO in oxy-fuel combustion recirculated with CO2. The computation covers all the range from air-blown combustion to pure oxygen combustion. An

* To whom correspondence should be addressed. Tel.: +82-61-7503533. Fax: +82-61-750-3530. E-mail : [email protected]. † Sunchon National Universit. ‡ Research Institute of Industrial Science and Technology. (1) Croiset, E.; Thambimuthu, K. Can. J. Chem. Eng. 2000, 78, 402407. (2) Tan, Y.; Douglas, M. A.; Croiset, E.; Thambimuthu, K. Fuel 2002, 81, 1007-1016. (3) Sung, C. J.; Law, C. K. Proc. Combust. Inst. 1998, 27, 1411-1418.

(4) Feese, J. J.; Turns, S. R. Combust. Flame 1997, 109, 266. (5) Beltrame, A.; Porshnev, P.; Merchan, W.; Saveliev, A.; Fridman, A.; Kennedy, L. A.; Petrova, O.; Zhdanok, S.; Amouri, F.; Charon, O. Combust. Flame 2001, 124, 295-310. (6) Li, S. C.; Williams, F. A. Combust. Flame 1999, 118, 399. (7) Hwang, D. J.; Park, J.; Oh, C. B.; Lee, K. H.; Keel, S. I. Int. J. Energy Res. 2005, 29, 107-120. (8) Park, J.; Kim, K. T.; Park, J. S.; Kim, J. S.; Kim, S.; Kim, T. K. Energy Fuels 2005, 19, 2254-2260.

10.1021/ef060309p CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

122 Energy & Fuels, Vol. 21, No. 1, 2007

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respectively. The use of mole fraction is not appropriate to grasp the behavior of CO and NO, since they include the information of convection and diffusion terms, and Nishioka et al.12 suggested the following emission indices of NO and CO.

EIi )

∫0L Wi w•i dx L • - ∫0 WCH wCH 4

Figure 1. Schematic of counterflow configuration showing the location of flame and the stagnation plane.

artificial species,7,8 which displaces the recirculated CO2 in the oxidizer side and has the same thermochemical, transport, and radiation properties to those of recirculated CO2, is introduced to extract pure chemical effects in flame structure and NO emission behavior. Special concerns are given to the different points of NO emission behavior between air-blown and oxyfuel combustions without recirculated CO2, and the impact of chemical effects of recirculated CO2 on flame structure and NO emission is also addressed. Numerical Strategies A laminar opposed jet diffusion flame, shown schematically in Figure 1, offers a convenient geometry for modeling the detailed processes that occur in diffusion flames. The mathematical description near the stagnation point is one-dimensional, and the model adopted in the study is that developed by Kee et al.9 and extended by Lutz et al.10 In this study, we retain the sink term, relevant to the thermal radiation model, and details of the thermal radiation model, based on an optically thin approximation, are found in the previous research studies.11 In the present CH4-O2/N2/CO2 flames, the oxygen mole fraction is changed from 0.21 to 1.0, and mole fractions in the oxidizer side can be defined as follows.

Ri ) Xi/(XO2 + XCO2 + XN2) where i ) O2, CO2, N2 Here, Xi implies volume fraction of a chemical species. The mole fractions of CO2 and N2 vary correspondingly in the allowable range with the fixed oxygen mole fraction. The separation distance of the two opposed jets is 2.0 cm, and the flame zone is located at the position at which momentum flux balances. The global strain rate is obtained considering the balance of momentum flux:7,8

a)

[

2(-VrVF) 1 1+ L (-Vr)

x

]

FF where Vr ) VO/VF FO

Here, the subscripts F and O mean the fuel and oxidizer, (9) Kee, R. J.; Miller, J. A.; Evans, G. H.; Dixon-Lweis, G. Proc. Combust. Inst. 1988, 22, 1479. (10) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; Rupley, F. M. Sandia Report No. SAND96-8243; Sandia National Laboratories: Albuquerque, NM, 1997. (11) Ju, Y.; Guo, H.; Maruta, K.; Liu, F. J. Fluid Mech. 1997, 342, 315.

where i ) CO and NO

dx 4

Here, Wi is the molecular weight of a chemical species i, and w•i is the molar production rate of chemical species i. An artificial species, referred to as X hereafter, is introduced to clearly identify the chemical effects of recirculated CO2 to the oxidizer stream.7,8,13 The artificial species X is defined in such a manner that it has exactly the same thermochemical, transport, and radiation properties as those of recirculated CO2, but it is not allowed to participate in any chemical reaction. Therefore, the X is strictly regarded as a chemical inert species. Numerical calculations are conducted twice with X and the real CO2. The difference between the results calculated with the artificial species and the real CO2 then is wholly attributed to the chemical effects of recirculated CO2. The adopted reaction mechanism is a GRI v-3.0, which consists of 53 species and 325 elementary reaction steps. The governing equations are solved using a CHEMKIN-based code14 and a transport-based one.15 Results and Discussion Figure 2 shows the variation of (a) maximum flame temperature and (b) emission index of CO with mole fraction of recirculated CO2 or artificial species X to oxidizer stream for various mole fractions at the strain rate of 100 s-1. Maximum flame temperature increases with increasing oxygen mole fraction and decreases with increasing mole fraction of recirculated CO2. In the figure, the X implies an artificial species that displaces the recirculated CO2 in the oxidizer side and has the same thermochemical, transport, and radiation properties to those of recirculated CO2, and the difference between the addition cases of CO2 and the artificial species is, thus, due to chemical effects. Flame extinction appears at RCO2 ) 0.32 for RO2 ) 0.21 and RCO2 ) 0.58 for RO2 ) 0.25, respectively. As shown in Figure 2a, maximum flame temperature in all cases decreases as the mole fraction of recirculated CO2 to the oxidizer stream increases. This is because the increase of the mole fraction of recirculated CO2 reduces the reactive species, which participates in the reaction. It is also shown that chemical effects cause the flame temperature to decrease, and the degree of temperature reduction that is due to chemical effects becomes much larger as the mole fraction of recirculated CO2 increases. These chemical effects may be originated from the reverse reaction -R99, CO2 + H f CO + OH,7,8 and this reaction for H radical competes with the principal chain initiation reaction R38, O2 + H f OH + O, which is an indicator of global reaction strength. The intermediate unburned hydrocarbons, which may be originated from reaction -R99, also compete with the principal chain initiation reaction for the H atom. Moreover, the reaction rates of intermediate hydrocarbons and (12) Nishioka, M.; Nakagawa, S.; Ishikawa, Y.; Takeno, T. Combust. Flame 1994, 98, 127. (13) Liu, F.; Guo, H.; Smallwood, G. J.; Gu¨lder, O ¨ . Combust. Flame 2001, 125, 778. (14) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Report No. SAND898009B; Sandia National Laboratories: Albuquerque, NM, 1989. (15) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. Sandia Report No. SAND86-8246; Sandia National Laboratories: Albuquerque, NM, 1994.

NO Emission BehaVior in Oxy-fuel Combustion

Energy & Fuels, Vol. 21, No. 1, 2007 123

Figure 2. (a) Variation of maximum flame temperature with mole fraction of recirculated CO2 for various oxygen mole fractions in the oxidizer stream at the strain rate of 100 s-1. (b) Variation of CO emission index with mole fraction of recirculated CO2 for various oxygen mole fractions in the oxidizer stream at the strain rate of 100 s-1.

H atom are generally much larger than that of the principal chain initiation reaction.16 As a result, the intermediate hydrocarbons and CO, formed by chemical effects, play a decisive role in reducing the flame temperatures as shown in Figure 2a, and this is also confirmed in Figure 2b. That is, the emission index of CO, produced by chemical effects, increases with increasing mole fraction of recirculated CO2 unless the CO2 is excessively recirculated and, thus, dilution effects are prevailing. It is implied in Figure 2 that oxygen displacement significantly modifies the diffusion flame structure from the familiar one obtained with air as the oxidizer, and it is also recognized that chemical effects of recirculated CO2 can make it complicated. Nevertheless the difference among air-blown combustions and oxy-fuel combustions recirculated with carbon dioxide might have not been understood clearly yet. Figure 3 displays fuel oxidation reaction pathways of (a) airblown combustion without CO2 recirculation (RO2 ) 0.21 and RCO2 ) 0), (b) air-blown combustion with CO2 recirculation (RO2 ) 0.21 and RCO2 ) 0.1), and (c) air-blown combustion with X recirculation (RO2 ) 0.21 and RX ) 0.1) at the strain (16) Westbrook, C. K.; Dryer, F. L. Prog. Energy Combust. Sci. 1984, 10, 1.

rate of 100 s-1. In the air-blown combustion without CO2 recirculation of Figure 3a, the reaction pathway typically consists of a C1-branch path (CH3 f CH2O f HCO f CO f CO2 or CH3 f CH2 (s) f CH2 f CH f CH2O f HCO f CO f CO2), which corresponds to a low-temperature oxidation process, and a C2-branch path, which corresponds to a hightemperature oxidation process (CH3 f C2H6 f C2H5 f C2H4 f C2H3 f C2H2 f HCCO f CO). The ratio between the C1branch path and C2-branch path is 1:1.2, and thus, the C2-branch path is a little bit more active. In the air-blown combustion with CO2 recirculation of Figure 3b, the ratio is 1:1.06, and thus, the C1-branch path is relatively dominant compared to that without CO2 recirculation. Moreover, the low-temperature oxidation reaction pathway, such as the pathways of CH2 (s) f CH2O and CH2 f HCO, is also shown to be promoted. Meanwhile, in the air-blown combustion with X recirculation of Figure 3c, the reaction pathway with the ratio of 1:1.05 is similar to that with CO2 recirculation except that the lowtemperature oxidation reaction pathway, such as the pathways of CH2 (s) f CH2O and CH2 f HCO, is relatively shrunk. Figure 4 describes fuel oxidation reaction pathways of (a) oxy-fuel combustion without CO2 recirculation (RO2 ) 0.70 and


Figure 3. Fuel oxidation reaction pathways for air-blown combustion (a) without recirculated CO2, (b) with recirculated CO2 (RCO2 ) 0.1), and (c) with recirculated X (RX ) 0.1) at the strain rate of 100 s-1.

RCO2 ) 0), (b) oxy-fuel combustion with CO2 recirculation (RO2 ) 0.70 and RCO2 ) 0.1), and (c) oxy-fuel combustion with X recirculation (RO2 ) 0.70 and RX ) 0.1) at the strain rate of 100 s-1. As shown in Figure 4a, the ratio between the C1-branch path and C2-branch path is 1:2.50. This means that, in oxy-fuel combustion, the C2-branch reaction pathways, which correspond to a high-temperature oxidation process, become dominant in

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Figure 4. Fuel oxidation reaction pathways for oxy-fuel combustion (RO2 ) 0.7) (a) without recirculated CO2, (b) with recirculated CO2 (RCO2 ) 0.1), and (c) with recirculated X (RX ) 0.1) at the strain rate of 100 s-1.

comparison to the low-temperature oxidation process of the C1branch reaction pathway. Figure 4b implies that the recirculation with the artificial species does not change the ratio so much (1:2.13) in comparison to that in Figure 4a, the case without recirculation of the artificial species. This is because the recirculation quantity (RX ) 0.1) is not sufficient for as much as the high-temperature oxidation process changes. On the other hand, the comparison of the results between parts b and c of



Figure 4 clarifies that the substitution of the recirculated species of the artificial species into the real CO2 produces the dramatic change of the ratio from 1:2.13 to 1.1.05. This represents that, in Figure 4c, the chemical effects of recirculated CO2 diminish the high-temperature oxidation process and, thus, the C2-branch reaction pathway is weakened relatively in comparison to that of Figure 4b. These modifications of oxidation reaction pathways may affect NO emission behavior considerably in both sides of thermal NO and Fenimore NO. Full NO formation is generally known to be attributed to the following four NO mechanisms: thermal NO (Zeldovich mechanism), prompt NO (Fenimore mechanism), N2O mechanism, and NO2 mechanism.12 The Zeldovich mechanism is expressed as follows, and the reaction numbers are based on those in GRI-Mech 3.0.

N + NO ) N2 + O

(R178)

N + O2 ) NO + O

(R179)

N + OH ) NO + H

(R180)

The N2O mechanism is relevant to the following reaction steps and evaluated from the full mechanism calculation.

N2O + O ) NO + NO

(R182)

NH + NO ) N2O + H

(R199)

NCO + NO ) N2O + CO

(R228)

The NO2 mechanism is described by the following four reactions.

HO2 + NO ) NO2 + OH

(R186)

NO + O + M ) NO2 + M

(R187)

NO2 + O ) NO + O2

(R188)

NO2 + H ) NO + OH

(R189)

Thus, NO formation from the Fenimore mechanism is determined by subtracting the contributions of the above three mechanisms (thermal NO, N2O mechanism, and NO2 mechanism) from that of full NO production. However, the contributions from the N2O mechanism and the NO2 mechanism were negligibly small in all the present calculations.12 Figure 5 depicts the variation of NO emission index with oxygen mole fraction in the oxidizer stream (a) without CO2 recirculation (b) with CO2 recirculation. In Figure 5a, NO emission indices from the full mechanism are larger than those from the thermal mechanism in the cases less than RO2 ) 0.3 for which NO emission indices from the Fenimore mechanism are positive. This implies that the major source of NO formation is from the Fenimore mechanism for air-blown combustion and even oxy-fuel combustion of relatively small oxygen mole fraction. It is also noted that the NO formation from the Fenimore mechanism is negative in cases larger than RO2 ) 0.3, and these results are consistent with those of the previous study.3 This implies that thermal NO becomes the dominant source of NO formation as much as the NO formation from thermal mechanism is larger than that from the full mechanism for oxy-fuel combustion. All the tendencies in Figure 5b with recirculated CO2 are similar to those in Figure 5a without CO2 recirculation, except that all the NO emission indices are smaller

Figure 5. Variation of NO emission index with oxygen mole fraction in the oxidizer stream (a) without recirculated CO2 and (b) with recirculated CO2 (RCO2 ) 0.1).

than those in Figure 5a without recirculated CO2. It is also found that chemical effects of recirculated CO2 reduce NO emission indices in all cases. Even though these results stress that simulation of NO production in oxy-fuel combustion by considering thermal mechanism alone could result in overpredictions of the NO emission,3 NO emission with CO2 recirculation and its chemical effects have been seldom clarified. Hence, we examine the behavior of NO emission index with recirculated CO2 and the role of its chemical effects in detail. Figure 6 shows the variation of NO emission index with the mole fraction of recirculated CO2 for (a) RO2 ) 0.3 and (b) RO2 ) 0.7. In Figure 6a, the NO emission index from the thermal mechanism decreases rapidly while that from the Fenimore mechanism decreases mildly with increasing mole fraction of recirculated CO2. Therefore, the NO production from the Fenimore mechanism becomes more dominant than that from the thermal mechanism for mole fractions of recirculated CO2 larger than 0.2. This clarifies that the reduction of NO emission index is mainly due to the rapid reduction of thermal NO for small mole fractions of recirculated CO2 and is due to both the reduction of thermal NO and Fenimore NO for relatively large mole fractions of recirculated CO2. It is found that chemical effects of recirculated CO2 cause the NO emission index to be reduced for all the mole fractions of recirculated CO2. In Figure 6b, the NO emission index from the thermal mechanism, which is larger than that from the full mechanism, decreases rapidly, while the NO emission index from the Fenimore mechanism, which is negative, goes forward to zero rapidly with increasing mole fraction of recirculated CO2. It is also seen in oxygen-


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Figure 6. Variation of NO emission index with mole fraction of recirculated CO2 in the oxidizer stream for (a) RO2 ) 0.3 and (b) RO2 ) 0.7 at the strain rate of 100 s-1.

enriched combustion that chemical effects of recirculated CO2 reduce the NO emission index. Figure 7 demonstrates the spatially resolved profile of temperature and molar production rates of full NO and thermal NO for (a) RCO2 ) 0, (b) RCO2 ) 0.1, and (c) RCO2 ) 0.3 in air-blown combustion. Figure 8 shows the same plots for (a) RCO2 ) 0, (b) RCO2 ) 0.1, and (c) RCO2 ) 0.3 in oxy-fuel combustion of RO2 ) 0.7. The xstag. in Figure 7 denotes the corresponding location of the stagnation plane. In the air-blown combustion of Figure 7, the molar production rate of full NO shows one positive production peak and one negative production peak through the Fenimore mechanism, while that through the thermal mechanism has only one positive production peak. The formation and destruction of NO through the N2O and NO2 mechanisms were negligibly small in all cases, and thus, the formation and destruction through the Fenimore mechanism can be obtained by substracting that through the thermal mechanism from full NO. The formation and destruction of NO mainly occurs near the location of maximum flame temperature on the oxidizer side of the stagnation plane. As was shown in Figure 5, the NO formation through the thermal mechanism is small compared to that through the Fenimore mechanism, and molar production rates of thermal and Fenimore NOs decrease with increasing mole fraction of recirculated CO2. It is, as a result, seen that the Fenimore mechanism serves as the major pathway for NO formation in air-blown combustion. As shown in parts

Figure 7. Spatially resolved profiles of molar production rate of NO and flame temperature for air-blown combustion of (a) RO2 ) 0.21 and RCO2 ) 0, (b) RO2 ) 0.21 and RCO2 ) 0.1, and (c) RO2 ) 0.21 and RCO2 ) 0.3 at the strain rate of 100 s-1.

b and c of Figure 7, the reduction of the molar production rate of NO due to chemical effects of recirculated CO2 becomes larger with increasing mole fraction of recirculated CO2. As shown in the oxy-fuel combustion of Figure 8, the maximum flame temperature increases and the reaction zone is broader compared to those in air-blown combustion. This broader reaction zone represents a longer local residence time favorable for NO formation, despite the same characteristic global residence time due to the fixed strain rate. In the oxyfuel combustion of Figure 8, the molar production rate of full NO indicates dual positive peaks,3 which are respectively dominated by the Fenimore mechanism situated on the relatively fuel-rich side and the thermal mechanism situated on the



Figure 9. Importantly contributing reaction steps to the formation and destruction of NO through Fenimore mechanism for air-blown combustion of (a) RO2 ) 0.21 and RCO2 ) 0 and (b) RO2 ) 0.21 and RCO2 ) 0.1 at the strain rate of 100 s-1.

Figure 8. Spatially resolved profiles of molar production rate of NO and flame temperature with distance from fuel nozzle for oxy-fuel combustion of (a) RO2 ) 0.7 and RCO2 ) 0, (b) RO2 ) 0.7 and RCO2 ) 0.1, and (c) RO2 ) 0.7 and RCO2 ) 0.25 at the strain rate of 100 s-1.

relatively fuel-lean side, while that through the thermal mechanism exhibits one negative peak and one dominant, positivepeak. It should be noted that the negative peak of full NO is more prevalent than the positive peak on the relatively fuelrich side in the oxy-fuel combustion of Figure 8, while the positive peak is more dominant than the negative peak in the air-blown combustion of Figure 7. This is due to the destruction of NO through the thermal mechanism. It is consequently found that, in oxy-fuel combustion, the Fenimore NO is the major pathway of NO destruction while the thermal NO serves the major source of NO formation. Meanwhile, with increasing mole fraction of recirculated CO2 in oxy-fuel combustion, the positive peak through the Fenimore

mechanism is strengthened relatively compared to the negative peak, since the negative contribution through the thermal mechanism disappears gradually, and as shown in Figure 8c, finally the positive peak becomes relatively dominant ,similar to that of air-blown combustion. The destruction of NO through the thermal mechanism disappears, and the formation of thermal NO becomes weak as the mole fraction of recirculated CO2 increases. It is also seen that chemical effects of recirculated CO2 also reduce the formation and destruction of NO through the thermal and Fenimore mechanisms. The important reaction steps through the Fenimore mechanism leading to the formation and destruction of NO have been examined to clearly compare the individual role in NO emission behavior of air-blown combustion with that of oxy-fuel combustion with and without recirculated CO2. Figure 9 displays importantly contributing reaction steps to NO production through the Fenimore mechanism for air-blown combustion with (a) RCO2 ) 0 and (b) RCO2 ) 0.1. Figure 10 illustrates importantly contributing reaction steps to NO production through the Fenimore mechanism for oxy-fuel combustion of RO2 ) 0.7 with (a) RCO2 ) 0 and (b) RCO2 ) 0.1. For air-blown combustion, the NO production through the Fenimore mechanism may be


Park et al.

Figure 10. Importantly contributing reaction steps to the formation and destruction of NO through Fenimore mechanism for oxy-fuel combustion of (a) RO2 ) 0.7 and RCO2 ) 0 and (b) RO2 ) 0.7 and RCO2 ) 0.1 at the strain rate of 100 s-1.

mainly relevant to the following reaction

steps:7

CH + NO ) N + HCO

(R248)

CH2 + NO ) H + HNCO

(R249)

CH2 + NO ) OH + HCN

(R250)

CH2 + NO ) H + HNCO

(R251)

CH2 (s) + NO ) H + HCNO

(R252)

CH3 + NO ) HCN + H2O

(R255)

HCCO + NO ) HCNO + CO

(R274)

It is seen in Figure 9 that the HNO related reaction steps such as R214 and R215 importantly contribute to the NO formation, while reaction step R212 consumes NO.7 Meanwhile, the importantly contributing reation steps to NO destruction are R246, R249, R255, and R274. In addition to them, the reaction steps such as R247 and R248 also play a role of NO destruction. The importance of reaction step R274, known as a HCN recycle route, leading to the consumption of NO has been welldescribed.12,17 The role of reaction steps R246 and R255 as a reburning mechanism was also addressed in the previous study.17 The air-blown combustion of recirculated CO2 in Figure 9b indicates a similar NO emission behavior through the Fenimore mechanism to the above-mentioned one, except that the recirculated CO2 and its chemical effects suppress both the formation and destruction of NO. For oxy-fuel combustion, in addition to R246-R249, R255, and R274, the C-related reaction steps of R244 and R245 importantly contribute to NO destruction. It is, above all, noted that reaction step R212 is the most dominant source of NO formation, while the reaction step contributes to NO consumption in air-blown combustion. Recirculated CO2 and its chemical effects still interrupt both the formation and destruction of NO, even in oxy-fuel combustion. Concluding Remarks

H + NO + M ) HNO + M

(R212)

C + NO ) CN + O

(R244)

C + NO ) CO + N

(R245)

CH + NO ) HCN + O

(R246)

A numerical study on flame structure and NO emission in CH4-O2/N2/CO2 counterflow nonpremixed flames was conducted covering all the ranges from air-blown combustion to pure oxygen combustion, so-called oxy-fuel combustion, for various mole fractions of recirculated CO2, and the following conclusions can be summarized. (1) Oxygen displacement significantly modifies the diffusion flame structure from the familiar one obtained with air as the oxidizer. Chemical effects of recirculated CO2 reduce not only flame temperature but also increase CO production, and these are confirmed by the difference between the results with real CO2 and the artificial species. For air-blown combustion with recirculated CO2, the C1-branch path, known as the lowtemperature oxidation reaction pathway, becomes relatively dominant compared to that without recirculated CO2. From the comparison between the results with real CO2 and the artificial species, chemical effects additively promote the low-temperature oxidation reaction pathway, such as the pathways of CH2 (s) f CH2O and CH2 f HCO. For oxy-fuel combustion, in comparison to air-blown combustion, the C2-branch path, known as the high-temperature oxidation pathway, is much more dominant, and the conversion of the reaction pathways from C1-branch to C2-branch is active. But in the oxy-fuel combustion

CH + NO ) H + NCO

(R247)

(17) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287.

HNO + O ) NO + OH

(R213)

HNO + H ) H2 + NO

(R214)

HNO + OH ) NO + H2O

(R215)

NCO + O ) NO + CO

(R222)

NCO + OH ) NO + H + CO

(R224)

The major NO destruction through the Fenimore mechanism is contributed by7


with recirculated CO2, the reaction pathway of the oxidation process becomes similar to that of air-blown combustion, and its chemical effects result in strengthening low-temperature oxidation processes. (2) The Fenimore mechanism mainly contributes to NO formation for oxy-fuel combustion of mole fractions of recirculated CO2 less than 0.3 including air-blown combustion, while the NO formation through the thermal mechanism is dominant and that through the Fenimore mechanism mainly contributes to NO destruction. Meanwhile, recirculated CO2 and its chemical effects not only reduce the formation and destruction of NO through the Fenimore mechanism but also suppress the NO formation through the thermal mechanism. For oxy-fuel combustion, in addition to reaction steps R246-R249, R255, and


R274, the C-related reaction steps of R244 and R245 importantly contribute to NO destruction. Reaction step R212 is the most dominant source of NO formation in oxy-fuel combustion, while the reaction step contributes to NO consumption in air-blown combustion. It is also seen that recirculated CO2 and its chemical effects still interrupt both the formation and destruction of NO, even in oxy-fuel combustion. Acknowledgment. Financial support of our work by national funds (2005-E-FM02-P-02) of Korea Energy Management Corporation is gratefully acknowledged. This work was also partly supported by the NURI project of the Ministry of Education, Korea, in 2006. EF060309P

NO Emission Behavior in Oxy-fuel Combustion Recirculated with

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