Reduction of Recycled NOx by Simulated Coal Volatiles in Oxy-Fuel

ARTICLE pubs.acs.org/EF

Reduction of Recycled NOx by Simulated Coal Volatiles in Oxy-Fuel Combustion Yongchun Zhang, Jun Zhang,* Changdong Sheng, Liang Zhao, Qizhong Ding, and Kun Wang School of Energy and Environment, Southeast University, Nanjing, 210096, PR China ABSTRACT: The reduction of recycled NO by simulated coal volatiles (CH4 and NH3) during oxy-fuel combustion has been studied in a flow reactor. Special emphasis was given to the effects of different variables on the reduction of recycled NO, and to the comparison with results obtained in air-fired combustion. The experiments were conducted in a temperature range of 873
1273 K. The stoichiometries varied from fuel-rich to fuel-lean. The results showed higher temperature favored the reduction of recycled NO under fuel-rich conditions, and the recycled NO reduction obtained in CO2 was much lower than that obtained in N2. However, under stoichiometric and fuel-lean conditions, the recycled NO reduction decreased with increasing temperature, and a slightly higher recycled NO reduction was obtained in CO2 compared to that in N2. Higher recycled NO concentration accelerated its reduction. The recycled NO reduction decreased with increasing CO2 concentration under fuel-rich conditions, while it increased under fuel-lean conditions. Higher CO maximum concentration was obtained in the CO2 case due to the CO2 þ H S CO þ OH reaction. But a decrease in the CO concentration was observed when the CO2 concentration reached a certain level. This indicates there are other important reactions responsible for the effect of a high concentration of CO2 besides CO2 þ H reaction.

1. INTRODUCTION Recently, carbon dioxide (CO2) emissions have received considerable attention as a greenhouse gas responsible for global warming. Globally, fossil-fueled power plants are the primary sources of CO2 emission. Oxy-fuel (O2/CO2) combustion technology is one the most promising ways to capture CO2 from large coal-fired power plants.1,2 In this technology, coal is combusted in a mixture of oxygen and recycled flue gas (mostly CO2 and H2O). So a high concentration of CO2 (>95%) in the flue gas is obtained which makes separation and capture of CO2 from the flue gas easier and more economical. This technology is not only suitable for new-build power plants but also for retrofit to existing power plants. During oxy-fuel combustion, fuels are burned in an environment of very high CO2 and H2O concentrations. This leads to significant differences between oxy- and air-combustion,3
8 such as formation of emissions (NOx, SOx).9
11 Studies indicate that NOx emission in oxy-fuel combustion is less than one-third of that in air combustion.2 The lower NOx emission is dominantly attributed to the reduction of recycled NO in furnace.12,13 However, knowledge on the recycled NO reduction by volatiles and char from coal is still very scarce at present. There are some studies on the oxidation of methane,14
16 ammonia,17 and hydrogen cyanide18 under oxy-fuel combustion. It was reported that CO2 þ H S CO þ OH reaction is mainly responsible for the chemical effect of a high CO2 concentration. The present work is focused on the reaction of recycled NO with volatile matters during oxy-fuel combustion. The objective of this work is to study the effects of different variables on the reduction of recycled NO by simulated coal volatiles (CH4 and NH3) under oxy-fuel combustion conditions in a laboratory flow reactor. The effect of high CO2 concentration on the recycled NO reduction mechanism is also discussed. r 2011 American Chemical Society

2. EXPERIMENTAL APPARATUS AND PROCEDURES A schematic diagram of the experimental setup used in this study is shown in Figure 1. The setup comprises basically a feeding system, a reaction system, and a gas analysis system. The reaction system includes a quartz tube reactor (inner diameter 0.9 cm, length 80 cm), which is heated by silicon carbide rod elements (SiC). A Pt
Pt/Rh (S-type) thermocouple is located at the middle of the tube. Temperature profiles of the reactor are measured at nonreacting conditions (1.3 L/min air) with a type S thermocouple. The length of the isothermal zone of the reactor is about 40 cm as shown in Figure 2. At the outlet of the reactor the product gas is rapidly quenched by means of a double-pipe water cooler and dried in a dryer for the removal of moisture. The experiments were conducted in a temperature range of 873
1273 K. In the experiments, the standard gas mixture containing 2 vol % NH3 in CH4 was used to simulate the volatile hydrocarbon and volatile nitrogen. It was mixed with NO, O2, and CO2 (or N2) in a mixer. All of these feed gases were supplied from gas cylinders and regulated by mass flow controllers. A constant total flow rate of 1.3 L/min (at normal temperature and pressure) was used. The methane inlet concentration was fixed at around 2%, accordingly the concentration of NH3 was about 400 ppm. The oxygen excess ratio (λ) was defined as the quotient between the oxygen/methane relation available for reaction and the stoichiometric oxygen/methane relation, which was varied between 0.5 and 1.4. The amount of O2 was varied depending on the oxygen excess ratio considered for each experiment. The concentration of recycled NO was ranged from 400 to 1200 ppm. When the influence of CO2 concentration was investigated, the CO2 concentration was changed from 20% to 90%, and argon was used as balance gas. For a comparison, experiments were conducted in both CO2 and N2 as bulk gas. In the N2 experiments, the conditions were exactly the same as for oxy-fuel combustion conditions except that CO2 was replaced by N2. In both Received: March 9, 2011 Revised: May 17, 2011 Published: May 18, 2011 2608

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This reaction, on one hand, increases the formation of CO by converting CO2 to CO, on the other hand, it alters the composition of the O/H radical pool, since it competes for atomic hydrogen with reaction O2 þ H S OH þ O

ðR2Þ

which is the main chain-branching reaction in combustion process. This would conceivably affect the NO reduction chemistry. The hydrocarbon radicals are the most important reducing agents for NO when hydrocarbon is used as reburn fuel.20 At high temperatures NO is primarily reduced by smaller hydrocarbon radicals, mainly CH2, CH, and C.25,26 However, these hydrocarbon radicals may be formed only in small quantities and the CHi þ NO reactions (i = 0, 1, 2) may be less significant in the present work because of the low reaction temperatures (873
1273 K). Mendiara and Glarborg14 suggested that the interaction of methane and NO under oxy-fuel conditions occurs primarily through reaction of NO with CHi (i = 2, 3) and HCCO radicals

Figure 1. Schematic diagram of experimental apparatus.

CH3 þ NO S HCN þ H2 O

ðR3Þ

CH3 þ NO S H2 CN þ HO

ðR4Þ

CH2 þ NO S HCNO þ H

ðR5Þ

HCCO þ NO S HCNO þ CO

ðR6Þ

3

Subsequently, both H2CN and HCNO are mainly converted thermally to hydrogen cyanide

Figure 2. Measured temperature profiles across the reaction zone. cases, because the mixture was highly diluted, low heat was released during the reaction and isothermal conditions could be assumed. The product gas from the reactor was monitored for NO, O2, CO, CH4, and CO2 by a flue gas analyzer (MRU, Vario Plus). This flue gas analyzer uses electrochemical sensors for NO and O2, and NDIR detection for CO, CH4, and CO2 measurements. The uncertainty of these measurements is (3% and the standard detection ranges for NO, O2, CO, CH4, and CO2 are 0
4000 ppm, 0
20.9%, 0
10.0%, 0
2.5%, and 0
100.0%, respectively.

3. REACTION MECHANISM The comprehensive reaction mechanism for the reduction of recycled NO by CH4 and NH3 under oxy-fuel conditions is still unclear. However, the reaction mechanism for oxidation of hydrocarbons, NH3 and HCN in air-fired conditions, as well as interactions of these components, has been extensively investigated in previous studies.19
24 In addition, some recent work on oxidation of CH4, NH3, and HCN under oxy-fuel combustion conditions has also been reported.14
18 Glarborg and Bentzen15 studied the chemical effects of a high CO2 concentration in oxy-fuel combustion of methane. They reported that the main reaction responsible for the effects is CO2 þ H S CO þ OH

H2 CN S HCN þ H

ðR7Þ

HCNO S HCN þ O

ðR8Þ

Depending on the stoichiometry and the temperature, hydrogen cyanide is eventually converted to N2 or NO. The ammonia chemistry in oxy-fuel combustion of methane has been studied by Mendiara and Glarborg.17 They reported that the oxidation pathways for NH3 vary significantly with stoichiometry and temperature. Ammonia is converted to NH2 mainly through NH3 þ OH S NH2 þ H2 O

ðR9Þ

The subsequent reactions of NH2 largely determine the fate of the nitrogen atom. Formation of N2 takes place mainly through reactions of amine radicals with NO, involving NH2 NH2 þ NO S NNH þ HO

ðR10Þ

NH2 þ NO S N2 þ H2 O

ðR11Þ

NNH S N2 þ H

ðR12Þ

NNH þ O2 S N2 þ HO2

ðR13Þ

NH þ NO S N2 O þ H

ðR14Þ

N2 O þ H S N2 þ OH

ðR15Þ

as well as NH

ðR1Þ 2609

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Figure 3. Effect of oxygen excess ratio on the reduction of recycled NO by CH4 and NH3 at 1273 K. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments (recycled NO concentration = 600 ppm).

Formation of NO occurs mostly with HNO as an intermediate. The major source of HNO is the reaction of NH2 with atomic oxygen NH2 þ O S HNO þ H

ðR16Þ

Subsequently HNO is converted to NO mainly through HNO þ M S NO þ H þ M

ðR17Þ

HNO þ O2 S NO þ HO2

ðR18Þ

HNO þ H S NO þ H2

ðR19Þ

HNO þ OH S NO þ H2 O

ðR20Þ

A minor reaction pathway to NO formation involving NH is NH2 þ H S NH þ H2

ðR21Þ

NH þ O2 S HNO þ O

ðR22Þ

NH þ O2 S NO þ OH

ðR23Þ

maximum (69%) at λ = 0.8. In CO2 experiments, η peaks at λ = 0.7 (33%). It is noteworthy that the η in N2 decreases to less than zero when λ > 1.1. A similar result is observed in the CO2 case when λ > 1. This indicates the amount of NO formed from NH3 oxidation exceeds that of reduced recycled NO. Higher η is obtained in the N2 case in λ range of 0.5
1.0, but it starts to become lower than that in CO2 when λ exceeds 1.1. As mentioned above, the hydrocarbon radicals are the major reducing agents for NO reaction. One of the major sources of hydrocarbon radicals is the reaction of methane with O/H radicals or O2. Under reducing conditions, the initial increase of λ increases the hydrocarbon radical generation, thereby enhancing the NO reduction. With the further increase in λ, hydrocarbon radicals are consumed quickly by additional O/H radicals and O2, leading to a decrease in NO reduction, and at the same time, NO formation from NH3 oxidation is enhanced, which results in a negative value of η (as shown in Figure 3). It has been concluded in the previous studies27 that atomic hydrogen would enhance conversion of nitrogen species to N2, while hydroxyl radical (OH) promotes oxidation of nitrogen intermediates to NO. Therefore, reaction R1 would be expected to promote formation of NO at high CO2 levels. This is confirmed in Figure 3 under fuel-rich conditions. However, with the conversion of a reducing atmosphere to an oxidizing atmosphere, the destruction of NO by hydrocarbon radicals is decreased significantly while the conversion of NH3 to NO is enhanced. Reaction R1 suppresses the O/H radical pool, in particular of atomic oxygen. One of the main pathways to NO formation from NH3 is the reaction of NH2 with O radical to form HNO17 NH2 þ O S HNO þ H

This step is inhibited in CO2 due to the reduced O atom concentration, thereby limiting the formation of NO from HNO

3. RESULTS AND DISCUSSION 3.1. Calculation of the Recycled NO Reduction Efficiency.

where Nout is the nitrogen atom number of NO discarded into atmosphere, Nin is the nitrogen atom number of recycled NO. It should be noted that a negative value of η may be obtained under oxidizing conditions due to the formation of NO from NH3 oxidation. 3.2. Effect of Oxygen Excess Ratio. To identify the differences between oxy-fuel and air-fired combustion, the reduction of recycled NO by CH4 and NH3 was investigated in both CO2 and N2 as bulk gas. Figure 3 shows the effect of oxygen excess ratio, λ, on the reduction of recycled NO at 1273 K. The oxygen excess ratio is changed from 0.5 to 1.4. The profile of η is similar in both bulk gases, increasing with λ until reaching maximum and subsequently decreasing. In experiments with N2, η reaches a

HNO þ M S NO þ H þ M

ðR17Þ

HNO þ O2 S NO þ HO2

ðR18Þ

HNO þ H S NO þ H2

ðR19Þ

HNO þ OH S NO þ H2 O

ðR20Þ

Also the conversion of NH2 to NH through the reaction

To determine the reduction extent of recycled NO, the NO reduction efficiency, η, is used in the present paper. It is defined as follows: η ¼ ð1
Nout =Nin Þ  100%

ðR16Þ

NH2 þ H S NH þ H2

ðR21Þ

is inhibited since the H concentration is lower in CO2, thereby limiting the formation of NO through the NH þ O2 reaction (R22, R23) NH þ O2 S HNO þ O

ðR22Þ

NH þ O2 S NO þ OH

ðR23Þ

Figure 4 shows the CO concentration as a function of oxygen excess ratio at 1273 K. In experiments with N2, the CO concentration reaches a maximum at λ = 0.8, and subsequently decreases with λ. A similar profile of the CO concentration is observed in the CO2 experiments, and it peaks at λ = 0.7. The CO maximum concentration coincides with the η peak in the both cases. The oxidation of CH4 is the source of CO. With the increase of λ more hydrocarbons are initially oxidized to CO, 2610

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Figure 4. CO concentration versus oxygen excess ratio at 1273 K. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments (recycled NO concentration = 600 ppm).

however CO will be further oxidized to CO2 with λ further increasing. Higher CO maximum concentration is attained in the CO2 case. This is mainly due to the effect of the CO2 þ H reaction (R1) at high CO2 levels, converting CO2 to CO. 3.3. Effect of Temperature. Figure 5 shows the effect of temperature on the reduction of recycled NO at different oxygen excess ratios (λ = 0.8, 1.0, and 1.2) in both CO2 and N2 as bulk gas. The temperature is changed from 873 to 1273 K. Figure 5a shows the results for fuel-rich conditions (λ = 0.8). In experiments with N2, η decreases initially with temperature, and then increases dramatically as temperature approaches and exceedes 1173 K. In CO2, a similar trend of η is observed. The minimum of η is exhibited at around 1073 K. Lower η is attained at any temperature in the CO2 case. As shown in Figure 5b, the profiles of η in the two bulk gases are somewhat different under stoichiometric conditions (λ = 1.0). In experiments with N2, η decreases initially and then increases when temperature increases from 873 to 1173 K, and it startes to decrease again with the further increasing temperature. In the CO2 case, η decreases initially with temperature and startes to increase above 1173 K. The η in CO2 is lower than that in N2, except at temperature of 1273 K at which they are close to each other. The results of η for fuel-lean conditions (λ = 1.2) are shown in Figure 5c. It is observed that η decreases with temperature in both N2 and CO2. The η in CO2 is higher than that in N2 only above 1173 K. Under fuel-rich conditions, the hydrocarbon radicals as the major reducing agents for NO reaction are generated mainly by dissociation/recombination reactions of hydrocarbon, and their concentrations are low in low-temperature conditions. With the temperature increasing, the consumption of hydrocarbon radicals by the O/H radical pool and by O2 is promoted. So the reduction of recycled NO initially decreases with temperature. However, with the temperature further increasing, the generation of hydrocarbon radicals is greatly enhanced. Therefore, an increase in the recycled NO reduction is observed with the increasing temperature in both bulk gases, especially in the N2 case. Under stoichiometric and fuel-lean conditions, the production of hydrocarbon radicals through the reaction of methane with O/H radicals plays an important role in low-temperature conditions because of the abundance of O2. More hydrocarbon radicals are produced at low temperature compared to that under

Figure 5. Effect of temperature on the reduction of recycled NO by CH4 and NH3. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments: (a) λ = 0.8; (b) λ = 1.0; (c) λ = 1.2 (recycled NO concentration = 600 ppm).

fuel-rich conditions, leading to a higher recycled NO reduction. But with the temperature increasing, the consumption of hydrocarbon radicals is greatly enhanced. On the other hand, the conversion of NH3 to NO is also promoted. Therefore the recycled NO reduction decreases with temperature increasing. It can be seen from Figure 5 that the recycled NO reduction is closely related to both λ and temperature. Changes in temperature have a smaller impact on the reduction of recycled NO in oxy-fuel combustion than in air combustion. This is due to the strong temperature dependence of CH3 formation. At low temperature, CH3 may recombine with H atoms to form CH4.20 This reaction is suppressed in oxy-fuel combustion because H atoms are consumed by reaction R1. So the formation of CH3 becomes less sensitive to temperature under oxy-fuel combustion conditions, and the NO reduction process also becomes less sensitive to temperature. Figure 6 shows the CO concentration as a function of temperature at different oxygen excess ratios (λ = 0.8, 1.0, and 1.2). It can be seen from Figure 6a that the profiles of CO in the two bulk gases are somewhat different under fuel-rich conditions (λ = 0.8). In the experiments with N2, the CO concentration represents a nonmonotonic trend, increasing initially with temperature and then decreasing until reaching a minimum at 1073 K and subsequently increasing. However, when CO2 replaces N2, the CO concentration increases with the temperature throughout the entire temperature range. Figure 6b shows that the shapes of the CO concentration profiles are complex in the two bulk gases under stoichiometric conditions (λ = 1.0). The CO maximum concentrations in N2 and in CO2 are observed at temperatures of 1173 and 1073 K, respectively. Figure 6c shows the results for lean conditions (λ = 1.2). The profile of the CO concentration is 2611

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Energy & Fuels similar in both cases, increasing its concentration with temperature until reaching a maximum at 1023 K and then decreasing dramatically to zero. The CO maximum concentration obtained in the CO2 case is higher due to the conversion of CO2 to CO through reaction R1 at high CO2 levels. The oxidation of CH4 is the source of CO. Figure 7 shows the CH4 concentration as a function of temperature at different oxygen excess ratios (λ = 0.8, 1.0, and 1.2). It can be seen from Figures 7 and 6 that the decrease of CH4 coincides with the increase of CO. CH4 is first dissociated to CH3 radical. In fact, the formation of CO is controlled by the fate of the CH3 radical. On one hand, the CH3 radical is further converted into CxHy (x = 1, 2, y = 1
6) and then CxHy is oxidized into CO; on the other hand, CH4 is reproduced through reaction of CH3 with H. Under

Figure 6. CO concentration versus temperature. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments: (a) λ = 0.8; (b) λ = 1.0; (c) λ = 1.2 (recycled NO concentration = 600 ppm).

ARTICLE

fuel-rich and stoichiometric conditions, in the experiments with N2, the reaction of CH3 with H to form CH4 is competitive at low temperature because of the abundance of hydrogen atom, and thus the CH4 concentration increases with temperature in the temperature range of 923
1073 K and 1023
1073 K (as shown in Figure 7a
b), coinciding with the decrease of CO. Whereas in the experiments with CO2, H atoms are consumed by CO2 through reaction R1, so the reproduction of CH4 becomes less important, therefore the CH4 concentration decreases monotonously with temperature. Under fuel-lean conditions, the reproduction of CH4 is always less important because of the low H concentration in both bulk gases, therefore the CH4 concentration decreases with temperature. 3.4. Effect of NO Concentration. Figure 8 shows the effect of NO concentration on the reduction of recycled NO at 1273 K for different oxygen excess ratios (λ = 0.8, 1.0, and 1.2). It can be seen that higher recycled NO concentration promotes its reduction in both bulk gases. Under fuel-rich conditions (λ = 0.8), higher increase in η with rising recycled NO concentration is observed in the CO2 case compared to that in N2, although lower recycled NO reduction is achieved (in Figure 8a). However, under stoichiometric (in Figure 8b) and fuel-lean conditions (in Figure 8c), a slightly higher recycled NO reduction is attained in the CO2 experiments. This indicates that the presence of high CO2 concentration suppresses recycled NO reduction under fuel-rich conditions, while it promotes recycled NO reduction under stoichiometric and fuel-lean conditions. The explanation has been described in the previous section. Figure 9 shows the CO concentration as a function of recycled NO concentration at 1273 K for different oxygen excess ratios (λ = 0.8, 1.0, and 1.2). It appears that the change in recycled NO concentration has little effect on the formation of CO. Under fuel-rich conditions (λ = 0.8), the CO concentration in the CO2 experiments is much higher than that in N2, while, under stoichiometric conditions, lower CO concentrations are obtained in the CO2 case. Under fuel-lean conditions (λ = 1.2), the CO concentrations in the two bulk gases are both close to zero. The CO2 þ H reaction (R1) plays an important role in formation of CO at high CO2 levels. This effect is especially important under fuel-rich conditions due to an easy availability of atomic hydrogen. With the increase in oxygen excess ratio, the concentration of atomic hydrogen would be reduced, thereby decreasing the formation of CO from CO2 through reaction R1. On the other hand, the ability of CO2 to act as an oxidizing agent may be increased, which indirectly increases the oxidation of CO.

Figure 7. CH4 concentration versus temperature. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments: (a) λ = 0.8; (b) λ = 1.0; (c) λ = 1.2 (recycled NO concentration = 600 ppm). 2612

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Figure 8. Effect of NO concentration on the reduction of recycled NO by CH4 and NH3 at 1273 K. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments: (a) λ = 0.8; (b) λ = 1.0; (c) λ = 1.2.

Therefore, under stoichiometric conditions (λ = 1.0), the lower CO concentration is observed in the CO2 experiments compared to that in N2 (in Figure 9b). 3.5. Effect of CO2 Concentration. Figure 10 shows the effect of initial CO2 concentration on the reduction of recycled NO at 1273 K for different oxygen excess ratios (λ = 0.8, 1.0, and 1.2). The initial CO2 concentration is changed from 20% to 90%. Under fuel-rich conditions (λ = 0.8), the reduction of recycled NO decreases monotonously with increasing CO2 concentration. However, an opposite trend is observed for fuel-lean conditions (λ = 1.2). Under stoichiometric conditions (λ = 1.0), the recycled NO reduction increases initially and then decreases as CO2 concentration reaches and exceeds 40%. Normann et al.16 pointed out that the relative order of the quantity of radicals was changed from H > OH > O in air combustion to OH > O > H in oxy-fuel fuel due to the CO2 þ H reaction (R1). It could be expected that the same change would occur with the CO2 concentration increasing from 20% to 90% in our experiments. Thus, the reduction of recycled NO is decreased with the increase in CO2 concentration under fuel-rich conditions. However, under fuellean conditions, the reduction effect of hydrocarbons is greatly decreased, and the formation of NO from NH3 is enhanced. So a negative value of η at λ = 1.2 is observed in Figure 10. The suppression effect of reaction R1 on atomic oxygen formation is highlighted in this condition, which limits conversion of NH3 to

ARTICLE

Figure 9. CO concentration versus NO concentration at 1273 K. The experimental data are shown as closed symbols for N2 and open symbols for CO2 experiments: (a) λ = 0.8; (b) λ = 1.0; (c) λ = 1.2.

Figure 10. Effect of initial CO2 concentration on the reduction efficiency of recycled NO at 1273 K (recycled NO concentration = 600 ppm).

NO through the pathways involving reaction of NH2 with O radical to form HNO as mentioned previously. This effect is enhanced at higher CO2 levels. Therefore, η increases with CO2 concentration under fuel-lean conditions (λ = 1.2). Under stoichiometric conditions, the impact of the changes in O/H radicals induced by increase in CO2 concentration on NO reduction is complex. It seems that there is a certain H/OH/O ratio that results 2613

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Figure 11. CO concentration versus initial CO2 concentration at 1273 K (recycled NO concentration = 600 ppm).

in maximum recycled NO reduction. More work is needed to understand the effect mechanism of O/H radical pool on the reduction of NO. The CO concentration as a function of CO2 concentration at 1273 K for different oxygen excess ratios (λ = 0.8, 1.0, and 1.2) is shown in Figure 11. It can be seen that, under fuel-rich conditions (λ = 0.8), the CO concentration increases with CO2 concentration until reaching a maximum at 50% CO2 concentration, and subsequently decreases. Under stoichiometric condition (λ = 1.0), the CO concentration reached a maximum at 30% CO2 concentration then decreased with CO2 concentration and reached zero at 60% CO2 concentration. Under fuel-lean conditions (λ = 1.2), almost no CO is observed. These results indicate that the CO formation is not monotonously promoted by a higher CO2 concentration. It seems to be that CO2 would consume CO when the CO2 concentration reaches a certain concentration. But it is not through direct reaction with CO. There may be other important reactions responsible for the effect of a high concentration of CO2 besides reaction R1.

4. CONCLUSION An experimental study of the reduction of recycled NO by simulated coal volatiles (CH4 and NH3) during oxy-fuel combustion has been conducted in a flow reactor. Special attention was paid to the effects of different variables on the reduction of recycled NO, and to the comparison with traditional air-fired combustion results performed in the same experimental setup. Results have been interpreted based on the results of previous related theoretical studies. Both stoichiometry and temperature had a significant impact on the recycled NO reduction. Under fuel-rich conditions, higher temperature favored the reduction of recycled NO, and the recycled NO reduction obtained in CO2 was much lower than that obtained in N2. However, under stoichiometric and fuel-lean conditions, the recycled NO reduction decreased with increasing temperature, and a slightly higher recycled NO reduction was obtained in CO2 compared to that in N2. Higher recycled NO concentration accelerated its reduction, especially for the CO2 case under fuel-rich conditions. The recycled NO reduction decreased with increasing CO2 concentration under fuel-rich conditions, while it increased under fuel-lean conditions. Higher CO maximum concentration was obtained in the CO2 case due to the CO2 þ H S CO þ OH reaction, converting CO2 to CO.

ARTICLE

But besides this reaction, there may be other important reactions responsible for the effect of a high concentration of CO2 because a decrease in the CO concentration was observed when the CO2 concentration reached certain levels. More work is needed to better understand the chemical effects of a high CO2 concentration in oxy-fuel combustion. Based on the results of the present work, it can be expected that during oxy-fuel combustion more volatile N will be converted to NO in reducing parts of the flame compared to that during air-fired combustion. In the oxidizing parts, the conversion of volatile N to NO is suppressed but this effect is small. In addition, increasing the recycled NO concentration favors lowering the NO emission during oxy-fuel combustion. Our previous studies12 indicate that the contribution of a high CO2 concentration to the reduction of total NO in oxy-fuel combustion accounted for about 18
33%. Combining the results of the present study, it can be expected that this reduction may be mainly attributed to heterogeneous reduction reactions, e.g., CO/NO/char reaction, rather than to homogeneous reduction reactions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge the financial support of Projects of International Cooperation and Exchanges National Science Foundation of China under Project 507211460649, the Scientific Research Foundation of Graduate School of Southeast University, and the support of part of the present work by Opening Foundation of State Key Laboratory of Coal Combustion (FSKLCC1003) at Huazhong University of Science and Technology, Wuhan. ’ REFERENCES (1) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Prog. Energy Combust. Sci. 2010, 36, 581–625. (2) Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. Chem. Eng. Res. Des. 2009, 87, 1003–1016. (3) Tan, Y.; Croiset, E.; Douglas, M. A.; Thambimuthu, K. V. Fuel 2006, 85, 507–512. (4) Nozaki, T.; Takano, S.; Kiga, T.; Omata, K.; Kimura, N. Energy 1997, 22, 199–205. (5) Shaddix, C. R.; Molina, A. Proc. Combust. Inst. 2009, 32, 2091–2098. (6) Sheng, C.; Li, Y. Fuel 2008, 87, 1297–1305. (7) Sheng, C.; Lu, Y.; Gao, X.; Yao, H. Energy Fuels 2007, 21, 435–440. (8) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Fuel Process. Technol. 2009, 90, 797–802. (9) Shaddix, C. R.; Molina, A. Proc. Combust. Inst. 2011, 33, 1723–1730. (10) Hu, Y. Q.; Naito, S.; Kobayashi, N.; Hasatani, M. Fuel 2000, 79, 1925–1932. (11) Croiset, E.; Thambimuthu, K. V. Fuel 2001, 80, 2117–2121. (12) Zhang, Y.; Zhang, J.; Sheng, C.; Liu, Y.; Zhao, L.; Ding, Q. Energy Fuels 2011, 25, 1146–1152. (13) Okazaki, K.; Ando, T. Energy 1997, 22, 207–215. (14) Mendiara, T.; Glarborg, P. Energy Fuels 2009, 23, 3565–3572. (15) Glarborg, P.; Bentzen, L. L. B. Energy Fuels 2008, 22, 291–296. (16) Normann, F.; Andersson, K.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2010, 49, 9088–9094. (17) Mendiara, T.; Glarborg, P. Combust. Flame 2009, 156, 1937–1949. 2614

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dx.doi.org/10.1021/ef200368v |Energy Fuels 2011, 25, 2608–2615