Effect of CO2 Reactivity on NOx Formation and Reduction

Article pubs.acs.org/EF

Effect of CO2 Reactivity on NOx Formation and Reduction Mechanisms in O2/CO2 Combustion Hirotatsu Watanabe,* Takashi Marumo, and Ken Okazaki Department of Mechanical and Control Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, I6-7, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: In this study, the effect of CO2 reactivity on low NOx combustion by varying CO2 mole fraction in inflow gas was experimentally and numerically investigated. A flat CH4 flame doped with NH3 for fuel N was formed in a reactor allowed secondary gas injection to simulate the fuel-rich region in a low NOx burner. The primary relative O2/CH4 ratio (λprimary) was 0.6 or 0.7, and the total relative O2/CH4 ratio was set to 0.8 by injecting a secondary gas. Measurement showed excited OH radical increased with increasing inlet CO2 mole fraction, and calculation showed that OH radical formation increased with increasing inlet CO2 mole fraction through the CO2 + H → CO + OH. N2 formation provided useful information to discuss low NOx combustion because an increase in the N2 yield indicated low NOx combustion. At λprimary = 0.7, the N2 yield decreased with increasing inlet CO2 mole fraction. Meanwhile, the N2 yield increased with increasing inlet CO2 mole fraction at λprimary = 0.6, regardless of the gas temperature. Sensitivity analysis showed that rate-limiting reactions for N2 formation were changed as λprimary varied. In fact, NH increased the rate-limiting reaction for N2 production at λprimary = 0.7, while not only NH but NH2 increased the rate-limiting reactions for N2 production at λprimary = 0.6. At λprimary = 0.7, most of the NH3 and HCN were decomposed; however, at λprimary = 0.6, some amounts of NH3, HCN, and CH4 remained. When minimal amounts of NH3 and HCN remained, the dominant role of OH radicals was to oxidize NH, which was an important NO reducing agent. However, when greater amounts of NH3 and HCN remained, the OH radicals produced NH2 by oxidizing NH3. Moreover, sensitivity analysis showed that H radical formation plays an important role in N2 formation under fuel-rich conditions. Hydrocarbon decomposition is needed to produce the H radical. The OH radical was active in CH4 decomposition, and CO2 could react with hydrocarbon radicals; thereby, with an increase in inlet CO2 mole fraction, exhaust CH4 mole fraction decreased, and the H radical increased. This resulted in the enhancement of N2 formation.

1. INTRODUCTION The competitiveness of coal relies on its wide availability and stability in supply and cost, compared to fuels such as natural gas. Coal contributes to 39% of worldwide electricity generation and plays an important role in global energy supply.1 For coal to serve as a source of the global energy supply, the greenhouse gases that are emitted from its utilization must be reduced. Carbon capture and storage (CCS) technologies have recently attracted considerable attention, because they have the potential to drastically reduce CO2 emissions. O2/CO2 combustion systems use O2 instead of air for combustion to produce a flue gas that consists of mainly water vapor and CO2, thus easily removing CO2. CO2 capture from O2/CO2 combustion is considered to be more economical than that from conventional air-fired combustion with amine scrubbing of dilute CO2 in the flue gas.2 O2/CO2 combustion has some other advantages over conventional coal-air combustion, such as lower NOx emission and easier CO2 removal. Some studies3,4 have shown that O2/ CO2 combustion has a high potential for reducing NOx emissions, because of its recycling process. A high concentration of CO2 may affect the combustion characteristics, including NOx formation and reduction through a direct chemical reaction of CO2. Several studies5−8 have reported that CO2 is not inert but participates in chemical reactions primarily through the reaction CO2 + H = CO + OH. Watanabe et al.9 © 2012 American Chemical Society

recently showed that staged combustion is effective in O2/CO2 combustion to reduce NOx emissions, and the primary relative O2/CH4 ratio, which produced minimum fixed N compounds in O2/CO2 combustion, was lower than that in air combustion. Meanwhile, different burner geometries to achieve staging in boilers, termed low NOx burners, have been developed.10,11 It is important to investigate the NOx formation mechanisms in O2/ CO2 combustion by applying a low NOx burner. In O2/CO2 combustion, O2 mixes with the recycled gas before entering the furnace. Therefore, one of the important features of O2/CO2 combustion is the ability to change the O2 concentration of the burner inlet gas by adjusting the ratio of the recycled flue gas. However, the adiabatic flame temperature of coal combustion in 21% O2 (same as that in air combustion) with CO2 dilution is lower than that of air combustion, because of the higher heat capacity of CO2. By adjusting the ratio of the recycled flue gas to give ∼30% O2 concentration of the burner inlet gas, an adiabatic flame temperature similar to that of air combustion can be achieved.12 It has also been suggested that the O2 concentration in the recycled gas should be ∼30% to achieve stable combustion with conventional burners.2 However, Toporov et al.13 demonstrated a stable oxy-fired Received: November 1, 2011 Revised: January 3, 2012 Published: January 20, 2012 938

dx.doi.org/10.1021/ef201702g | Energy Fuels 2012, 26, 938−951

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Figure 1. Schematic diagram of experimental apparatus.

Figure 2. Overview of mixing area: (a) x−r plane (at secondary gas inlet nozzle), (b) r−θ plane (at secondary gas inlet nozzle).

during O2/CO2 combustion of pulverized coal under three different O2 concentrations at inflow gas (12%, 24%, and 36%). However, the effect of CO2 concentration in the inlet O2/CO2 mixtures on the NOx formation and reduction mechanisms has not been well discussed, from the viewpoint of CO2 reactivity. In this study, the effect of CO2 concentration in the inlet O2/ CO 2 mixtures on the NO x formation and reduction mechanisms are investigated, and the relationships between CO2 reactivity and NOx formation are discussed in the fieldsimulated low NOx burner. The fuel-rich region plays an important role in determining performance of low-NOx combustion; thereby, the fuel-rich region is the focus in this

pulverized coal swirl flame with an O2 concentration of 30 mm were measured with a suction pyrometer. Gas temperature profiles from x = −20 mm to x = 30 mm were measured with a bare thermocouple (dTC = ϕ 0.5 mm), because gas aspiration of the suction pyrometer affected the flame shape and combustion characteristics. The conduction and radiation loss appear, but latter one is dominant in this work, as shown in the Supporting Information. Therefore, the measured temperature was modified using the radiation loss equation, where x = 0−30 mm. A detailed equation is given in the Supporting Information.

where F denotes the volume flow rate.

XCO2,inlet = 0.77

(2)

3. MODEL SIMULATION The NOx formation mechanisms were investigated with detailed chemical reaction kinetics. The conservation equations for steady plug flow were solved using CHEMKIN-PRO.19 The adopted reaction mechanisms were obtained from a modified GRI-Mech 3.0. It is well-known that GRI-Mech 3.0 is welloptimized for methane combustion.20 However, GRI-Mech 3.0 was verified under specific conditions, and the debate on NOx chemistry of this mechanism has continued, even in non-

The CO2 mole fraction in the recycled gas is an important parameter in O2/CO2 combustion. In this experiment, the inlet CO2 mole fraction (XCO2,inlet) in the inflow gas was defined as 940


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Figure 3. Experimental apparatus and model reactors used in CHEMKIN-PRO simulation: (a) experimental appartatus at secondary gas inlet nozzle, and (b) reactor configuration used in CHEMKIN simulation.

hydrocarbon reactions, which have been well-studied for decades. Although further discussions are required to use GRI-Mech 3.0 for O2/CO2 combustion, some researchers have used original GRI-Mech 3.0 in O2/CO2 combustion.6,21 Meanwhile, the following reactions used in GRI-Mech 3.0 have significant problems under the fuel-rich conditions that are the focus of this work.22−24 Therefore, the modified GRI-Mech 3.0 was used in this work.

N + CO2 → NO + CO

(R1)

NH + CO2 → HNO + CO

(R2)

H2 + HNO → NH + H2O

(R3)

Table 2. Fraction of Flow to Rapid Mixing and Nonmixing Part in Calculation Calc. One (split: half)

Calc. Two (split: optimum)

XCO2,inlet

fraction of flow to rapid mixing part

fraction of flow to nonmixing part

fraction of flow to rapid mixing part

fraction of flow to nonmixing part

0.5 0.6 0.7

0.50 0.50 0.50

0.50 0.50 0.50

0.44 0.46 0.44

0.56 0.54 0.56

combustion8,25 but nonhydrocarbon reactions, which have been well-studied.22,23,26 First, Fernandez et al. have shown reaction R1 to be quite slow by measurements,27 and this step is insignificant, which agrees with the other results in an O2/CO2 atmosphere.25 Therefore, similar to Mendiara and Glarborg,8 we used the reaction rate of reaction R1 proposed by Glarborg,26 which agrees with the upper limit of the measurement results27 and also with the shock tube measurements.28

When an original GRI-Mech 3.0, which consisted of 53 species and 325 elementary reaction steps, was applied, the above reactions played an important role in NOx formation and reduction under these experimental conditions. However, reactions R1−R3 are doubtful in not only O 2 /CO 2 941


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Figure 4. Measured temperature profiles: (a) λprimary = 0.6, λtotal = 0.8, and (b) λprimary = 0.7, λtotal = 0.8.

Reactions R2 and R3 were studied in a shock tube,29 and their proposed reaction rates are used in the original GRI-Mech 3.0; however, these reaction rates are significantly fast and the products of these reactions are questionable, as noted by Skreiberg.24 The flow reactor and jet-stirred reactor experiments indicated that NH formation via the reaction of HNO and CO or H2 was insignificant.23,26 In addition, Fontijn et al. have shown that HNO + CO cannot be the products of reaction R2, and reaction R3 was also in question by theoretical work.30 The significant problem is that reactions R2 and R3 cause the considerable discrepancy between calculation and experiment under fuel-rich conditions.22,23 Dagaut et al. have shown that reaction mechanisms including reactions R2 and R3 grossly overpredicted the reduction of NO under fuel-rich conditions;23 however, their calculations were in good agreement when reactions R2 and R3 were excluded. Therefore, for these reasons, we omitted reactions R2 and R3, as Dagaut et al.23 and Glarborg et al.26 did. Figure 3 shows the experimental setup near the mixing part and the model reactors used in CHEMKIN-PRO simulation. For simplification, a plug flow model was used, because it was easy to discuss the reaction paths in a plug-flow model in which, unlike a premixed flame model, gas diffusion is not considered. Calculations using a premixed flame model are shown in the Supporting Information. The mixing part in the CHEMKIN-PRO simulation was difficult to express because only flash mixing was available and the actual spatial distribution around the mixing part was expected to be nonuniform, although the 1.0-mm diameter of the injection hole of the secondary allowed rapid mixing. Therefore, speculative assumptions were needed to calculate the mixing area. Aside from this work, we performed CFD calculation for the mixing part and showed that some parts were well-mixed, whereas others were not mixed. A nonuniform area only appeared around the mixing area. Based on these results, it was assumed that the mixing part was expressed as a combination of rapid mixing and nonmixing parts, as shown in Figure 3b. The length of the mixing part was set to 1 mm, and it separated the mixing part into the same two plug-flow reactors. Only one reactor had a flash mixer. The internal diameter of these two reactors was 18.4 mm, and the sum of the two reactor volumes was equal to the volume of a main reactor having a diameter of 26.0 mm. The degree of mixing was dependent on the split flow rate to the mixer; however, it was very difficult to determine the

split flow rate. A 50/50 split was easy to use in the calculation. First, the effect of a split flow rate on nitrogen compounds formation was investigated, and the validity of a 50/50 split was confirmed. Table 2 shows calculation conditions showing the fraction of flow to the rapid mixing and nonmixing parts. In one calculation, the flow rate was split 50/50. In the other calculation, a split flow rate was determined so that the error between the calculation and the experiment becomes small. Therefore, the fraction of flow varied along with XCO2,inlet in Calc. Two (optimum), shown in Table 2. For simplification, the gas species fractions at each inlet for the mixing part were the same.

4. RESULTS AND DISCUSSION Figure 4 shows the measured temperature profiles. Because the flat flame is formed around x = 0 mm, the gas temperature rapidly increases at this point. The gas temperature increases at the secondary O2 injection point (x = 20 mm) at λprimary = 0.6, because a considerable amount of the unburnt hydrocarbon remains before mixing. With an increase in the inlet CO2 mole fraction (YCO2,inlet), the gas temperature at λprimary = 0.6 and 0.7 decreases, because of the heat capacity of CO2. Figure 5 shows the measured the sum of NOx, HCN, and NH3 normalized by inflowing NH3. It is important to reduce

Figure 5. Measured sum of NOx, HCN, and NH3 conversion ratio. 942


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Figure 6. Emission characteristics of NOx, HCN, and NH3 under λprimary = 0.6 and λtotal = 0.8 (Calc. 1, split: half; Calc. 2, split: optimum): (a) NOx conversion ratio, (b) sum of NOx, HCN, and NH3 conversion ratio.

Figure 7. Emission characteristics of N2 (Calc. 1: the gas temperature used in the calculation is that at each inlet CO2 mole fraction, Calc. 2: the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction shown in each panel): (a) λprimary = 0.6, λtotal = 0.8, and (b) λprimary = 0.7, λtotal = 0.8.

the sum of NOx, NH3, and HCN in the fuel-rich regions in the low NOx burner, because the remaining amounts of HCN and NH3 in the primary stage are easily converted to NO by adding diluted O2. Therefore, not only the NO conversion ratio but also the sum of NOx, HCN, and NH3 is investigated in the suppression of NOx formation, because the major nitrogencontaining species at this temperature are NO, HCN, NH3, and N2. An important finding here is that the sum of NOx, HCN, and NH3 decreases with increasing CO2 at λprimary = 0.6, while it increases with increasing CO2 at λprimary = 0.7. Figure 6 shows the emission characteristics of NOx, HCN, and NH3 under λprimary = 0.6 and λtotal = 0.8. Both calculations, using different split flow rates, show the same tendency as the experiment: the sum of NOx, HCN, and NH3 decreases as the CO 2 increases, although the discrepancy between the calculation (Calc. 1) and the experiment is significant. However, the calculations are becoming more similar to the experiments by changing the fraction of flow to mixing area; the

calculation (Calc. 2) almost agrees with experiment. This indicates that the dominant reason for discrepancies between the data and model results is mixing modeling. At λprimary = 0.7, both the calculation and the experiment show that (i) the sum of NOx, HCN, and NH3 increases as CO2 increases and (ii) most of the NH3 and HCN were decomposed, as shown in the Supporting Information. The most important feature is that the sum of NOx, HCN, and NH3 (almost NOx) increases as the inlet CO2 mole fraction at λprimary = 0.7 increases, which contradicts the results at λprimary = 0.6. Moreover, when the plug flow model is changed to a premixed flame model in the CHEMKIN calculation, this tendency does not change, as shown in the Supporting Information. Regardless of the degree of the discrepancy caused by a split flow rate, both the calculation and the experiment show the same tendency about the effect of CO2 on the formation of nitrogen components. Besides, the optimum splitting fraction varies with conditions. 943


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Figure 8. Reaction path diagram illustrating the major reaction mechanism focusing on N2 and NO formation (each arrow size denotes time-integral reaction rate): (a) λprimary = 0.7, λtotal = 0.8 (XCO2,inlet = 0.70, primary area), and (b) λprimary = 0.6, λtotal = 0.8 (XCO2,inlet = 0.70, after mixing).

Therefore, a 50/50 splitting is used in all of the following calculations. Fuel-N should preferably convert to N2 rather than NOx, NH3, or HCN, from the viewpoint of low NOx combustion. The N2 yield from NH3 (N2 CR) defined in eq 4 represents the effect of low NOx combustion on O2/CO2 combustion.

N2 yield = 2 ×

exhaust N2 flow rate inflow NH3 flow rate

(4)

In O2/CO2 combustion, N2 originates from fuel-N because it does not exist in the inflow gas. In low NOx combustion, the maximum production of N2 in the fuel-rich region is very important. Figure 7 shows the calculated N2 yield, which increases linearly with increasing inlet CO2 mole fraction at λprimary = 0.6, whereas it decreases linearly with increasing CO2 at λprimary = 0.7. By changing the inlet CO2 mole fraction, not only does the CO2 concentration vary, but also the gas temperature. Therefore, two calculations were performed. In Calc. 1, the gas temperature used in the calculation is that at the setting

Figure 9. Measured profiles of OH* chemiluminescence (λprimary = 0.7, λtotal = 0.8). 944


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Figure 10. Calculated OH profiles at primary region (λprimary = 0.7, λtotal = 0.8): (a) profiles using each temperature distribution and (b) profiles using the same temperature distribution under XCO2,inlet = 0.77.

Figure 11. Calculated N2 profile and sensitivity plots under λprimary = 0.7 and λtotal = 0.8 (the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction of 0.77): (a) N2 mole fraction and (b) N2 sensitivity (XCO2,inlet = 0.60).

where R, r, and t are the time-integrated reaction rate, each reaction rate, and residence time, respectively. The timeintegrated reaction rate (R) provides useful information about the dominant reaction paths from NH3 to N2. First, the area in which significant reactions progress is determined when the parameter is calculated. Then, its length is converted to the residence time using gas velocity profiles. N2 formation is wellprogressed in the primary area under λprimary = 0.7. Meanwhile, it is progressed after the mixing area under λprimary = 0.6. Therefore, reaction paths of primary area under λprimary = 0.7 and these after mixing area under λprimary = 0.6 are shown in Figures 8a and 8b, respectively. The major reactions are obtained through CHEMKIN-PRO calculations, and based on Miller and Bowman.31 In particular, the radicals OH, O, and H

inlet CO2 mole fraction. In other words, the gas temperature varies with inlet CO2 concentration. In Calc. 2, the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction of 0.70 to investigate the effect of CO2 concentration only. Results show whether CO2 reactivity contributes to low NOx combustion, which is dependent on λprimary, regardless of the gas temperature. Figure 8 shows the reaction path diagram illustrating the major reaction mechanism focusing on N2 and NO formation. Reaction paths also show the reaction strength, which is expressed by a time-integral reaction rate that is defined as

R=

∫0

t

r dt

(5) 945


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Figure 12. Calculated NO profile and sensitivity plots under λprimary = 0.7 and λtotal = 0.8 (the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction of 0.77): (a) NO mole fraction and (b) NO sensitivity (XCO2,inlet = 0.60).

Figure 13. Time-integrated reaction rates related to N2 formation (λprimary = 0.7, λtotal = 0.8): (a) NH + NO → N2O + H (reaction R5) and (b) NH + OH → HNO + H (reaction R7).

Figure 10 shows the calculated OH concentration profiles. In Figure 10a, the gas temperature used in the calculation is that at each inlet CO2 mole fraction. In Figure 10b, the gas temperature distribution is fixed to that at the inlet CO2 mole fraction of 0.77. Although the same temperature is used in the calculations, the calculated OH concentration is similar to the measured OH* concentration, as shown in Figure 9. The calculation shows that the reaction

play an important role in determining N2 and NO formation. Reaction rates related to the OH, O, and H radicals and O2 under λprimary = 0.7 are higher than those under λprimary = 0.6. NH3 decomposition by OH and O radical is not well progressed under λprimary = 0.6, compared to that under λprimary = 0.7. An important point in low-NOx combustion is to produce N2 in the fuel-rich region. NH3 decomposition rate and reaction rates to produce N2 need to increase for low-NOx combustion in the fuel-rich region, where H and OH radicals are generally low. Figure 9 shows the measured profiles of OH* chemiluminescence at λprimary = 0.7. The OH* concentration increases as the inlet CO 2 mole fraction increases. While OH* chemiluminescence measurements generally cannot be used to determine concentrations of ground-state species, they can be obtained along with numerical modeling.

CO2 + H → CO + OH

(R4)

is most strongly influenced by the inlet CO2 mole fraction, compared with other reactions related to the formation of the OH radicals; one of the important roles in CO2 reactivity is related to the formation of the OH radicals. Therefore, in this study, the role of the OH radicals in the low NOx combustion mechanisms is discussed. 946


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Figure 14. N2 profile and sensitivity plots under λprimary = 0.6 and λtotal = 0.8 (the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction of 0.70): (a) N2 mole fraction and (b) N2 sensitivity (YCO2,inlet = 0.70).

Figure 13a shows the time-integrated reaction rate of reaction R5. The time-integrated reaction rate of reaction R5 decreases as XCO2,inlet increases. This indicates that an increase in inlet CO2 concentration inhibits the rate-limiting reaction for N2 formation. NH is an important NO reduction agent, and the reason why the reaction rate of reaction R5 is decreased with increasing XCO2,inlet is that the following reaction (reaction R7) consumes NH.

Before the effect of CO2 on low-NOx combustion is discussed, first, rate-limiting reactions for N2 formation are selected by using sensitivity analysis. Sensitivity results at specific inlet CO2 mole fractions are shown because ratelimiting reactions vary slightly by XCO2,inlet. Figure 11 shows calculated N2 profile and sensitivity plots for N2 formation under λprimary = 0.7 and λtotal = 0.8. The gas temperature used in this calculation is fixed at the inlet CO2 mole fraction of 0.77 to discuss the CO2 reactivity only. The N2 mole fraction decreases as XCO2,inlet increases. Reactions shown in Figure 11b are the rate-limiting process for N2 formation. Positive sensitivity of the reaction indicates an increase in the corresponding reaction rate in a forward direction, and negative sensitivity indicates a corresponding reaction rate in the opposite direction. Figure 12 shows calculated NO profile and sensitivity plots for NO formation under λprimary = 0.7 and λtotal = 0.8. In contrast to N2 formation, NO increases with increasing XCO2,inlet. Comparing the sensitivity of NO formation (Figure 12b) with that of N2 formation (Figure 11b), the rate-limiting reactions for N2 formation are almost the same as those for NO formation. Different reactions, rather than the same reactions, can explain the reason why NO and N2 formation show opposite tendencies as XCO2,inlet varies. The reaction exhibiting positive sensitivity for N2 formation and not having positive sensitivity for NO formation is

NH + NO → N2O + H

NH + OH → HNO + H

Figure 13b shows the time-integrated reaction rate of reaction R7. The time-integrated reaction rate of reaction R7 increases as XCO2,inlet increases, because the amount of OH radical increases as XCO2,inlet increases, because of reaction R4. The OH radical is a strong oxidizer; thereby, CO2 reactivity deteriorates low NOx combustion at λprimary = 0.7. However, opposite resultsthat CO2 enhances low-NOx combustionare obtained at λprimary = 0.6, as shown in Figure 14a. The N2 mole fraction increases with increasing XCO2,inlet. Figure 14b shows sensitivity plots for N2 formation under λprimary = 0.6. The following reactions exhibit positive sensitivity for N2 formation:

(R5)

H + O2 → O + OH

(R8)

NH2 + H → NH + H2

(R9)

NH + NO → N2O + H

(R5)

CH2 + CO2 → CO + CH2O

Reaction R5 is an important rate-limiting reaction for N2 formation. Because the gas temperature is >1500 K in this area, almost the entire amount of N2O is rapidly converted to N2 through the following reaction:32,33

N2O + H → N2 + OH

(R7)

(R10)

The following reaction has negative sensitivity for N2 formation:

H + CH3 → CH4 (R11) Figure 15 shows time-integrated reaction rates of reactions R5, R8, R9, R10, and R11. The time-integrated reaction rates at XCO2,inlet = 0.7 are higher than those at XCO2,inlet = 0.6 in positive

(R6)

Reaction R6 is the most important reaction for N2O destruction. 947


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Figure 15. Time-integrated reaction rates for dominant N2 formation under λprimary = 0.6 and λtotal = 0.8: (a) positive sensitivity and (b) negative sensitivity.

948


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Figure 16. H profile and sensitivity plots under λprimary = 0.6 and λtotal = 0.8 (the gas temperature used in the calculation is fixed at the inlet CO2 mole fraction of 0.70).

Figure 17. Time-integrated reaction rates related with H formation under λprimary = 0.6 and λtotal = 0.8: (a) OH + CH3 → CH2 + H2O (reaction R12) and (b) OH + CH4 → CH3 + H2O (reaction R13).

Besides, the amount of H radical increases in reactions R8 and R9. Figure 16 shows the calculated H profile and H formation sensitivity under λprimary = 0.6 and λtotal = 0.8. The H mole fraction increases as the inlet CO2 mole fraction increases, although H radicals are consumed by the reaction (CO2 + H → CO + OH). H sensitivity plot shown in Figure 16b provides this reason. Hydrocarbon decomposition is needed to produce the H radical in the fuel-rich region, because hydrocarbon is the source of the H radical. Hydrocarbon reacts with the OH radical (reactions R12 and R13) and produces more light hydrocarbon radicals. These reactions have a positive sensitivity coefficient for H production. In addition, CO2 could be

sensitivity reactions (see Figure 15a). As for reaction R11, the time-integrated reaction rate varies slightly by XCO2,inlet (see Figure 15b). Especially, reactions R8 and R9 play an important role in N2 formation, because these reactions have a high positive sensitivity coefficient and these reaction rates vary markedly by XCO2,inlet. In particular, the time-integrated reaction rate of reaction R9 at XCO2,inlet = 0.7 is more than twice that at XCO2,inlet = 0.6. Contrary to the λprimary = 0.7 case, some amount of NH3 and HCN remains at λprimary = 0.6, as shown in Figure 6, because of the lack of O2. Therefore, NH3 decomposition due to OH radical becomes important to produce NH2. NH2 formation caused an increase in the reaction rate of reaction R9. 949


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increased with increasing inlet CO2 mole fraction because the reaction rate, CO2 + H → CO + OH, increased with increasing inlet CO2 mole fraction. In addition, even when the same temperature was used, the OH radicals increased with increasing inlet CO2 mole fraction. N2 formation provided useful information to study low NOx combustion, because N2 was only formed from fuel-N in O2/CO2 combustion. At λprimary = 0.7, the N2 yield decreased as the inlet CO2 mole fraction increased, whereas at λprimary = 0.6, the N2 yield increased as the inlet CO2 mole fraction increased, regardless of the gas temperature. In fact, CO2 reactivity enhanced low NOx combustion at λprimary = 0.6, whereas it worsens low NOx combustion at λprimary = 0.7. Whether CO2 reactivity enhances low NOx combustion was shown to be strongly dependent on λprimary. At λprimary = 0.7, most of the NH3 and HCN amounts were decomposed; however, at λprimary = 0.6, some amounts of NH3 and HCN remained. Sensitivity analysis showed that ratelimiting reactions for N2 formation were changed as λprimary varied. In fact, NH increased the rate-limiting reaction for N2 production at λprimary = 0.7, while not only NH but NH2 increased the rate-limiting reactions for N2 production at λprimary = 0.6. When minimal amounts of NH3 and HCN remained, the OH-radical-oxidized NH was an important NO reducing agent. However, when greater amounts of NH3 and HCN remained, the OH radical produced NH2 by decomposing NH3. This indicated that NH3 decomposition via the OH radical was more important than NH destruction to produce N2. In addition, sensitivity analysis showed that the formation of H radicals played an important role in N2 formation under fuel-rich conditions. Hydrocarbon radical reactions by OH radical and CO2 were the rate-limiting reactions for H radical formation. Besides, the OH radical was active in the decomposition of hydrocarbon. Therefore, as the inlet CO2 mole fraction increased, the exhaust CH4 mole fraction decreased, and H radicals increased. This resulted in the enhancement of N2 formation.

Figure 18. Measured exhaust CH4 mole fraction under λprimary = 0.6.

expected to react with hydrocarbon radicals.34 In the reaction schemes used in this research, CO2 reacts with the CH2 radical (reaction R10). Reaction R10 has positive sensitivity for H radical formation. Reactions R12, R13, and R10 are the limiting process in producing the H radical.

OH + CH4 → CH3 + H2O

(R12)

OH + CH3 → CH2 + H2O

(R13)

CH2 + CO2 → CH2O + CO

(R10)

Therefore, OH and CO2 play an important role in producing the H radical in the fuel-rich region. Figure 17 shows time-integrated reaction rates of reactions R12 and R13 under λprimary = 0.6 and λtotal = 0.8. These reaction rates increase as the inlet CO2 mole fraction increases. This is because CO2 increases the amount of OH radicals, through reaction R4. Figure 18 shows measured exhaust CH4 mole fraction under λprimary = 0.6. With an increase in inlet CO2 mole fraction from 0.60 to 0.70, the exhaust CH4 mole fraction decreases by approximately one-half, even though the gas temperature decreases. This is because the reaction rate of reaction R12 increases as the inlet CO2 concentration increases. CH4 measurement supports the above reaction mechanisms. In addition, the OH radical is more active than the H radical, with regard to CH4 decomposition in the temperature range of 300−1500 K, as shown in the Supporting Information. As a result, CO2 reactivity enhances low NOx combustion at λprimary = 0.6, whereas it worsens low NOx combustion at λprimary = 0.7. λprimary is a dominant factor in determining whether CO2 reactivity enhances low NOx combustion. These results support our previous result,9 which shows that the NOx conversion ratio in O2/CO2 combustion is lower than that in air combustion at low λprimary, whereas it is higher than that in air combustion at high λprimary. Throughout this work, CO2 reactivity, represented by reactions R4 and R10, has a significant effect on the N2 formation process in O2/CO2 combustion under fuel-rich conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Reactor model, comparison between the plug flow model and the premixed flame model, CH4 decomposition rate, and temperature measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-3-5734-2179. Fax: +81-3-5734-2179. E-mail: [email protected].

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ACKNOWLEDGMENTS This study was partly supported by a JSPS Grant-in-Aid for Scientific Research (A) (21246035), JST Strategic International Cooperative Program and J-Power.

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5. CONCLUSION Effect of CO2 reactivity on NO reduction was investigated by varying the CO2 mole fraction in the inflow gas, which is a variable parameter in O2/CO2 combustion, experimentally and numerically. Calculations have shown that the OH radical

NOMENCLATURE dTC = thermocouple diameter (m) F = volume flow rate (m3 h−1) X = mole fraction

Greek Symbols

λ = relative O2/CH4 ratio, defined by eq 1 950


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