Investigation on the NO Reduction with Coal Char and High

Nov 8, 2012 - High purity argon (≥99.999%) with a flow rate of 700 mL·min–1 [standard temperature and pressure (STP)] was used to purge the quart...
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Investigation on the NO Reduction with Coal Char and High Concentration CO during Oxy-fuel Combustion Chang’an Wang, Yongbo Du, and Defu Che* State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: Oxy-fuel combustion is one of the near-zero emission clean coal technologies which can realize large-scale CO2 capture, low NOx emission, and easy removal of SO2. This paper focuses on the reduction mechanism of NOx during oxy-fuel combustion. The influence of high concentration CO on the NO reduction with coal char, ash, and various metal materials has been studied in a laboratory-scale quartz reactor. Experimental results indicate that the presence of high concentration CO plays a crucial role in reducing NO on the coal char surfaces. Effect of NO concentration varies with different reaction conditions and different char types, which may be attributed to the different kinetic reaction orders. Moreover, both the reaction temperature and char prepared temperature have a pronounced effect on the char−NO−CO reaction. The active components in coal ash exhibit a clearly catalytic effect on the NO reduction with high concentration CO. Furthermore, the catalytic effect of various metal oxides, as well as coal ash, is significantly improved with the increase of temperature. It is found out that the catalytic activity in NO−CO reaction is Fe2O3 > CaO > MgO > Al2O3 > SiO2. Fixed carbon is more active on the NO reduction than coal ash. Kinetics analysis indicates that the apparent activation energy of the NO reduction with coal char is reduced by the presence of high level CO. In addition, large amounts of active metal oxides can also decrease the apparent activation energy of NO reduction reactions, which is beneficial for further reducing NO emission during oxy-fuel combustion.

1. INTRODUCTION Energy conversion from fossil fuel combustion has the largest contribution to anthropogenic emission of CO2, which is the main composition of greenhouse gases.1 Both the average global temperature and the amount of CO2 in the atmosphere have significantly increased since the beginning of industrialization, and some evidence has testified the relationship between them.2,3 Nowadays, most governments worldwide are aware of the importance of reducing the greenhouse gas emissions, and various measures are being taken actively to reduce CO2 emission. Reduction of CO2 emissions from fossil fuel-fired power plants can be achieved by efficiency improvement, introduction of combined cycles, replacement of hydrocarbon fuels with renewable resources, and CO2 capture and storage (CCS).4−7 Oxy-fuel combustion is one of the technologies to realize large-scale CO2 capture and storage. In recent years, numerous investigations have been conducted on oxy-fuel combustion to realize carbon capture, including the mutivariable optimization of the oxy-fuel combustion process,8 the dynamic stability characteristics of premixed methane/oxygen/ carbon dioxide mixtures,9 the ash particulate formation,10 biomass combustion under oxy-fuel conditions,11,12 CFD modeling of oxy-fuel combustion,13,14 heat transfer behaviors,15,16 and so on. What is more, NOx emission reduction in oxy-fuel combustion has also been extensively studied. The reduction mechanism of NOx during the oxy-fuel combustion process is the subject of the present investigation. Many experimental results from the pilot-scale devices show that the amount of NOx released during oxy-fuel combustion can be approximately one-third of that produced in conventional combustion.1,4,17,18 Several potential mechanisms for the © 2012 American Chemical Society

reduction of NO during oxy-fuel combustion have been proposed, including the inhibition of thermal and prompt NOx due to lower N2 partial pressure, the reburning of the recycled NOx, the change of the flame and fuel/oxidizer mixing pattern, and the resultant effect of the increased CO 2 concentration.11,19−22 The present study focuses on the influence of high concentration CO on the reduction of NOx on the char surfaces during oxy-fuel combustion, which has been seldom investigated. The CO level during oxy-fuel combustion could increase obviously, compared to air combustion, which is likely due to the reduction in the amount of diluent gas, the recycling of CO in the flue gas, and the reactions of CO2 with C atoms and H atoms.23−25 Some existing calculated analyses26,27 and experimental results28−32 have confirmed that the production of CO was raised considerably with increased CO2 concentration under oxyfuel conditions. Moreover, CO has been reported to have a promotion effect on NO heterogeneous reduction with char.33−36 The reactions of NO with carbon-based fossil fuel are of extensive interest due to the prospective technology in reducing nitric oxide emissions from coal-fired power plant.35 Therefore, many researchers have conducted investigations on heterogeneous NO reduction with coal char. Reaction between NO and char is quite complex, including a series of physical and chemical processes, such as chemisorption, desorption of surface complexes, and release of products.37 The heterogeReceived: July 19, 2012 Revised: November 5, 2012 Published: November 8, 2012 7367

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Table 1. Proximate Analysis and Ultimate Analysis of Coal (wt %, ad) proximate analysis

a

ultimate analysis

coal code

w(FC)

w(A)

w(V)

w(M)

w(C)

w(H)

w(O)a

w(N)

w(S)

Qnet,ar (MJ·kg−1)

LX JH

46.09 43.14

18.19 34.66

25.70 21.17

10.02 1.03

57.40 53.76

3.03 3.21

10.10 5.73

0.93 0.93

0.33 0.68

19.76 19.11

w(O) = 100 − w(C) − w(H) − w(N) − w(S) − w(A) − w(M).

Table 2. Composition of Coal Ash (wt %) coal code

SiO2

Al2O3

TiO2

Fe2O3

CaO

MgO

K2O

Na2O

MnO2

SO3

LX JH

56.88 51.05

20.16 41.82

1.24 1.40

8.03 2.21

3.54 1.14

2.35 0.26

1.32 0.20

0.13 0.03

0.07 0.01

2.37 0.77

CO reaction for other materials without C(O) complexes as well as for carbon-based material,35 the pathways for NO−CO reactions R4 and R5 are not widely accepted, and further verification is necessary. Furthermore, insufficient information has been used for comparing the effects of demineralization char, char, coal ash, and various metal oxides contained in coal ash on the NO−CO reaction. Existing work has already suggested a pronounced complexity in the mechanism of NO reduction in conventional combustion systems. High level CO in the oxy-fuel combustion process may also give rise to a significant effect on the NO reduction. Accordingly, the process and mechanism of NO reduction during oxy-fuel conditions may differ considerably and be more complex. Hence, it is of great importance to further study the role of high level CO on the NO reduction in oxy-fuel combustion. In this paper, the influence of high concentration CO on NO reduction with coal char, ash, and various metal materials has been studied in a laboratory-scale quartz reactor. Both experimental and kinetic studies of NO reduction in the presence of high level CO have been performed. The parameters concerned in current experiments includes the char prepared temperature, the reaction temperature, NO concentration, CO concentration, and the char type. In addition, influences of demineralization, various metal materials, and coal ash on the reaction between NO and CO have also been investigated. The catalytic effects of various metal oxides have also been evaluated.

neous reaction of NO with carbon-based fossil fuel can be mainly stoichiometrically represented as follows:35,37 1 N2 + CO 2

(R1)

2NO + C → N2 + CO2

(R2)

NO + C →

Furthermore, several scientists have presented the enhancement of CO on the NO reduction.38−40 Aarna and Suuberg35 pointed out that the product of CO in R1 can react further with NO in turn with catalytic surfaces. The reaction may be represented as follows: catalytic surfaces

NO + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→

1 N2 + CO2 2

(R3)

They also showed that the small amount of CO produced in R1 could mostly be inconsequential in conventional combustion systems. However, compared to conventional combustion, CO level in oxy-fuel condition increases dramatically, which will result in a more profound effect on R3. Besides the direct reaction of NO with CO in the NO− carbon reaction mechanism proposed by Aarna and Suuberg, another hypothesis involves the intercoupling of the reactions through a mechanism, in which the oxides from the surface are stripped by CO,41 shown as follows: CO + C(O) → CO2 + C*

(R4)

1 NO + C* → N2 + C(O) 2

(R5)

2. EXPERIMENTAL SECTION 2.1. Coal Analysis. Two types of coals were selected for this study, including Lingxin bituminous (abbreviated as LX, hereinafter) and Juhui bituminous (JH). Both of these two coals are typical in Northwestern China. Each sample was first ground, sieved to size 100−125 μm, then dried at 105 °C, and kept airproof in a desiccator at room temperature before being used. The coal samples studied in this research were prepared following ISO Standards (ISO 18283:2006(E)). The proximate analysis, ultimate analysis, and net calorific value of coal samples are listed in Table 1, in which all the coals are represented as the corresponding code names. In Table 1, the proximate and ultimate analyses are both on air-dry basis, with subscript “ad” in this paper. Large amounts of material surfaces have certain catalysis on the reaction R3,35 including coal ash and various metal materials contained in coal ash. In the present study, the analysis of mineral matter content of coal ash sample was conducted following the standard method: Test method for analysis of coal ash [GB/T 1574-2007]. The detailed test methods for different mineral matter contents are provided in the Supporting Information. The analysis results are shown in Table 2. 2.2. Sample Preparation. Char preparation was carried out in a tubular furnace. A total of 2 g raw coal was put into a self-made quartz basket, and then, the quartz basket was put into the quartz tube which

where the C(O) represents a surface oxide; the C* represents a free active site. The reaction mechanism above is an indirect reaction mechanism and has also been verified by experiments.42 The absence of N2 during oxy-fuel combustion may play a positive role on reactions R1−R5. However, the actual promotion could be influenced also by the relative concentrations of the other reactants and products. Although sufficient studies have been conducted on the heterogeneous NO reduction with char, less research aims at the influence of high concentration CO on the NO reduction during oxy-fuel combustion. In the previous investigations on the role of CO in the NO−carbon reaction, the CO level is low (≤0.0435 or ≤0.2 vol %,36 for example), which is apparently different from the oxy-fuel condition. In the present study, the concentration of CO varies from 0.1 to 1.5 vol %. Moreover, there are still inconsistencies concerning the NO−carbon reaction mechanism in the presence of CO. Reactions R3, R4, and R5 have all been confirmed by experimental results. Nevertheless, since there is also a catalytic effect on the NO− 7368

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Figure 1. Schematic of char preparation system.

Figure 2. Schematic of the NO reduction experimental system. was in the tubular furnace. The coal chars used in the present experiments were obtained from pyrolysis in a fixed-bed reactor at a heating rate of 10 K·min−1. The schematic of char preparation system is shown in Figure 1. The coal samples were all heated from room temperature to a preset temperature at a heating rate of 10 K·min−1, and the preset temperature is kept for 1 h. During this process, the outlet concentrations of gas species, like CO, CO2, and CH4, were continuously monitored by an FTIR gas analyzer Gasmet DX-4000. If there were no more volatile matter emissions from coal samples, it was believed that there was no more reaction. After the gas analyzer detected no more gas species discharging from the sample, the char sample was naturally cooled down to room temperature. High purity argon (≥99.999%) with a flow rate of 700 mL·min−1 [standard temperature and pressure (STP)] was used to purge the quartz tube during the whole char preparation process. In addition, there is a ring

on the self-made quartz basket, as shown in Figure 1. Therefore, the quartz basket can be conveniently put into and taken out of the reactor using a long hook. No obvious contaminant sticking to the quartz sample holder was observed after the sample heat-treatment was conducted. Specific surface area and pore volume of char samples were determined by the nitrogen Brunauer−Emmett−Teller (BET) surface method. Prior to the BET surface analysis, vacuum degasification at 373 K for at least 6 h was conducted for all the samples. The coal ashes used in the present experiments were prepared following ISO Standards (ISO 1171:2010, solid mineral fuels− determination of ash). In addition, all the metal materials studied in this paper are analytical pure. Samples of the Lingxin coal were also demineralized with a mixture of HCl/HF solutions applying the method adapted by Morgan et al.43 7369

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After acid-washed treatment, samples were prepared in char preparation system to obtain acid-washed char. Then, the samples were dried at 105 °C for 24 h and, subsequently, kept airproof in a glass container at room temperature before being used. 2.3. Experimental System and Procedure. A laboratory-scale fixed bed quartz reactor was employed to carry out NO reduction experiments in this study, as shown in Figure 2. The quartz reactor tube is 30 mm in inner diameter and 640 mm in length, with a pored quartz baffle in the middle of the reactor. During experiments, a layer of quartz wool was tiled on the quartz baffle to hold samples. The reactor was heated by an electrical tube furnace with a maximum temperature of 1473 K and the bed temperature was read by a K-type thermocouple (nickel chromium−nickel silicon thermocouple) with an accuracy of 1.5 K. During the NO reduction experiments, the desired NO and CO concentrations were obtained by controlling the flow rate of NO, CO, and Ar using mass flow meters. The total flow rate of mixture gas is 1225 mL·min−1 (STP). In addition, the outlet concentrations of gas species were continuously analyzed by an FTIR gas analyzer Gasmet DX-4000. During the experiments, both the sampling system and the pipelines were heated to 453 K to ensure accurate sampling. Teflon tube was used to avoid absorption and corrosivity. In Gasmet DX4000, the corrosion resistant sample cell is also heated to 453 K, which ensures that the sample remains gaseous phase even with high concentrations of H2O or corrosive gases. This system utilizes hotand-wet measurement principle (no drying or dilution), which ensures that the analysis is done with a representative sample. Analysis software CALCMET with patent algorithm is employed to deal with the interference of characteristic peak between different N-species. In addition, single component multipoint calibrations have already been carried out for N-species. Before experiments, a total of 0.1 g sample was previously well mixed with 0.9 g of silica sand to maintain a very thin bed of approximate 1.36 mm. The silica sand is necessary to facilitate the introduction of the sample into the reactor and to prevent agglomeration of the char particles.37 The silica sand has been heattreated at 1373 K for 1 h before being used. Therefore, it is believed that there is no obvious off-gas production from the SiO2 bed particles under the present experimental conditions. For each run, valve A was first opened and valve B was closed, the solid mixture material was put into the hopper, the gas mixture was purged into the quartz reactor through mixer, and then valve A was closed. When both the temperature of fixed bed and the effluent gas of the reactor outlet were maintained at the preset value, valve B was opened, the solid sample was dropped onto the quartz wool, and the NO reduction process began. 2.4. Blank Test. For the analysis of current experimental results, the reduction ratio of NO is defined as follows:

ηNO =

(C NO,inlet − C NO,outlet) C NO,inlet

Figure 3. Blank test of quartz catalytic effect, with NO concentration of 964 mg·m−3 and 0.43 vol % CO. concentration CO can play a significant role at 1373 K, with NO reduction ratio of 15%. Mendiara et al.44 found out that the catalytic effect of the quartz is present at 1373 K with initial concentration of CO higher than 1 vol %. Wittler et al.45 also obtained similar experimental results.

3. RESULTS AND DISCUSSION Figure 4 shows the evolution of NO and CO concentrations versus reaction time in the reaction of 0.1 g LX char with NO of 964 mg·m−3 at 1173 K. Compared to Figure 4a, additional CO of 0.43 vol % was introduced in the feed gas in Figure 4b. The coal char went through heat treatment at a maximum temperature of 1273 K. In addition, the samples were added into the reactor at 300 s. It can be obviously seen from Figure 4a and b that the concentration of NO is decreased and CO concentration is increased, whether or not the additional CO is introduced. This confirms that the heterogeneous reaction of NO with char R1 proceed in this investigation. In the present study, repeat experiments were performed for most conditions. The variation extent of NO reduction ratio during these repeat experiments is below 2%. Reaction R2 is the dominant reaction in the low temperature region ( CaO > MgO > Al2O3 > SiO2, which is in accordance with the results obtained by Zhao et al.55,56 They suggested that both calcium (Ca) and iron (Fe) impregnated on char had promotion effect on the char−NO−CO reaction, but the catalytic activity of iron was higher than that of the calcium. Although various materials have catalytic effect on the reaction R3, sometimes it may be relatively difficult to identify the actual catalytic ingredient in the materials which are not pure. 3.4. Comparison between Fixed Carbon and Ash. Both the fixed carbon and ash contained in char have contribution to NO reduction during the NO−CO reaction. In this paper, the respective contribution share of fixed carbon and ash were experimentally studied. The influence of demineralization on the reaction between NO and CO was also investigated. Figure 11 shows the comparison of NO reduction ratio among ash,

Figure 12. Comparison of NO reduction ratio between fixed carbon and ash contained in 0.1g LX char, with 964 mg·m−3 NO and 0.43 vol % CO in the feed gas at different temperatures.

represents the NO reduction ratio of 0.1 g LX char under the same condition as that of 0.07 g acid-washed char. It can be observed from Figure 12 that the fixed carbon can reduce more NO than ash. However, the ratio of NO conversion with ash to that with acid-washed char increases with temperature. The reduction ability of ash increases more significantly with increased temperature than that of fixed carbon. The sum of reduction ratios of 0.03 g ash and 0.07 g acid-washed char is generally higher than that of 0.1 g char. The reduction reactivity of fixed carbon and ash do not satisfy additivity, and an interaction must exist between them during NO reduction. 3.5. Effect of Char Type. Figure 13 presents the effect of char type on NO reduction with char and high level CO. It is Figure 11. Comparison of NO reduction ratio among ash, char, and acid-washed char, with 964 mg·m−3 NO and 0.43 vol % CO in the feed gas at different temperatures.

char, and acid-washed char as all the sample weight was 0.1 g. The NO reduction with acid-washed char is quite similar to that with char. The NO reduction ratio with acid-washed char increases obviously as the temperature is increased, regardless of CO in the feed gas. It can be seen from Figure 11 that for a given temperature, the extent of NO reduction is acid-washed char > char > ash. The results suggest that the fixed carbon is more active on the NO reduction than coal ash. Hence, the carbon contained in char play a major role on reducing NO. Moreover, the active components in coal ash can enhance the char−NO−CO reaction to a certain extent. In addition, experimental results show that 0.43 vol % additional CO results in relative increase of 13%, 11%, and 8% for NO reduction ratio on normal char at 1173, 1273, and 1373 K, respectively. For acid-washed char, 0.43 vol % additional CO only leads to relative increase of 10%, 9%, and 7% for NO reduction ratio at 1173, 1273, and 1373 K respectively. The influence of high concentration CO on the NO reduction with acid-washed char is smaller than that with natural char. This suggests that CO promotes the NO reduction mainly through the catalytic surfaces of various metal oxides. The proximate analysis of LX char shows that the char prepared at 1273 K contains ash of 29.80% and fixed carbon of 70.20%. Thus, the NO reduction ratios of 0.07 g acid-washed char and 0.03 g ash are illustrated in Figure 12. The circle dot

Figure 13. Comparison of NO reduction ratio between LX char and JH char, with 964 mg·m−3 NO and 0.43 vol % CO in the feed gas at different temperatures.

apparent that the NO reduction ratio in a JH char−NO−CO system is considerably lower than that in a LX char−NO−CO system for a given temperature. Three aspects can be responsible for the NO reduction difference between two types of char. First, the ash/fixed carbon ratio for JH char is much greater that that for LX char, as shown in Table 1. Fixed carbon is more active on the NO reduction than coal ash. Second, JH char contains more relatively inert metal oxides and less active metal oxides, such as CaO, MgO, and Fe2O3, as presented in Table 2. Third, the porous structures of LX char 7374

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and JH char are quite different. The nitrogen BET surface area and pore volume of JH char is 8.59 m2·g−1 and 0.0073 cm3·g−1, respectively. Both the two microstructure parameters of JH char are less than those of LX char, which also should be responsible for the difference in NO conversion. Moreover, the diffusion of nitrogen into the JH char is slower than that into LX char, as evidenced by the longer equilibration time for the former char. 3.6. Effect of CO on the Kinetics of NO Reduction. The reaction rate for the heterogeneous NO reduction with char reaction in the fixed bed can be expressed by the following general expression:35,57−60 rNO = −

dC NO = kC NOn dt

(2)

where CNO is the NO concentration inside the sample bed, mg·m−3; t is the residence time of NO inside the sample bed, s; k is the reaction rate constant; and n is the reaction order. The apparent reaction order with respect of NO can be assumed to be to equal unity because the NO reduction ratio is almost independent of the inlet NO concentration for JH char. Comparatively, the apparent reaction order for LX char should be fractional due to the change of NO conversion ratio with inlet NO concentration in the char−NO−CO system. To simplify analysis, JH char is taken for example to analyze the effect of CO on the kinetics of NO reduction in the present study. For JH char, the apparent reaction order in eq 2 is supposed to be unity. The same assumption and kinetics treatment can be found in the literature.36,61 Together with the Arrhenius equation, k = k0 exp(−E/RT), the following equation can be derived from eq 2: −

dC NO ⎛ E ⎞ ⎟C = k 0 exp⎜ − ⎝ RT ⎠ NO dt

Figure 14. Arrhenius plots for JH char−NO reaction with and without additional CO in the feed gas.

observed the break of two temperature regime is at about 873 K, while Li et al.61 found that the transition temperature is 973 K. All the values derived in literature are beyond the temperature region of 1073−1373 K employed in present experiments. Therefore, no two regime Arrhenius behavior has been observed in the present study, as shown in Figure 14. Kinetics analysis results indicate that the apparent activation energy of JH char−NO reaction with high concentration of CO is 114 kJ·mol−1, which is obviously less than 142 kJ·mol−1 when CO is not present in the feed gas. Zevenhoven and Hupa62 suggested the values of apparent activation energies for NO reduction with CO on chars from various fuel types. The values obtained in the present work are consistent with the values reported by these two researchers, which is in the region of 56− 132 kJ·mol−1 for bituminous coal char. Kinetics analysis for LX char exhibits similar behavior (results not shown). The apparent activation energy of the NO reduction with char can be reduced by high level CO, which is beneficial for further reducing NO emission during oxy-fuel combustion. In addition, some active metal oxides, iron (Fe) for example, can also decrease the apparent activation energy of NO reduction reactions, which is consistent with the conclusions drawn by Illan-Gomez et al.51 and Zhang et al.,63 that is, the metal materials can significantly increase the char reactivity by reducing the activation energy.

(3)

where k0 is the pre-exponential factor; E is the apparent activation energy, kJ·kg−1; T is the temperature, K; and R is the gas constant, 8.314 J·mol−1·K−1. Integrating eq 3 along the height of sample bed gives the following equation: ln[−T ln(1 − η)] = −

⎛ T Aδ ⎞ E1 + ln⎜⎜k 0 0 ⎟⎟ RT ⎝ εQ 0 ⎠

(4)

where η is the conversion of NO; T0 is the temperature under standard condition, 273 K; Q0 is the total flow rate of mixture gas under standard condition, mL·min−1; A is the inside crosssection area of the reactor tube, m2; δ is the height of the sample bed, m; and ε is the bed voidage, which is usually between 0.3 and 0.7 and is assumed to be 0.5 in the present study. According to eq 4, a plot of ln[−T ln(1 − η)] versus 1/T would result in a straight line with the slope equal to −E/R. Therefore, the apparent activation energy E and apparent preexponential factor k0 can be calculated according to the above equation. Figure 14 shows the Arrhenius plots for JH char−NO reaction with and without additional CO in the feed gas when the initial NO concentration is 964 mg·m−3. It can be seen from Figure 14 that the linearity of experimental data is quite high. Therefore, the hypothesis of first-order reaction for JH char is acceptable. “Two regime” Arrhenius behavior for NO reduction without CO has been observed by some researchers.33,36 It is reasonable to believe that this behavior is most likely due to a change of NO reduction mechanism. Lopez and Calo36

4. CONCLUSION In this paper, study on the NO reduction with coal char and high concentration CO during oxy-fuel combustion has been conducted in a laboratory-scale quartz reactor. Experimental results indicate that high concentration CO plays a considerable enhancement role in reducing NO on the coal char surfaces during oxy-fuel combustion. The NO reduction ratio first increases significantly with increased initial concentration of CO but becomes asymptotically constant or shows slight decrease at higher levels of CO. The influence of additional CO is bidirectional. The effect of NO concentration varies with reaction conditions and char types, which may be attributed to the different kinetic reaction orders. Moreover, both the reaction temperature and the char prepared temperature have pronounced effects on the char−NO−CO reaction. A noticeable increase in NO reduction ratio with increased temperature can be observed, regardless of CO in the feed gas. But higher char prepared temperature will result in lower NO 7375

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conversion ratio due to the decrease of BET surface area and pore volume. The active components in coal ash, such as MgO, CaO, and Fe2O3, exhibit clearly catalytic effect on the homogeneous reaction between NO and high level CO. By contrast, SiO2 and Al2O3 only exhibit relative weak catalytic activity in the NO reduction reactions at temperatures below 1273 K. The catalytic effect of various metal oxides is significantly improved with the increase of temperature. Experimental results also show that the catalytic activity in NO−CO reaction is Fe2O3 > CaO > MgO > Al2O3 > SiO2. The fixed carbon is more active during the NO reduction than coal ash. The reduction reactivity of fixed carbon and ash do not satisfy additivity and an interaction exists. In addition, different types of char may exhibit significant difference during the NO reduction. Kinetics analysis indicates that the apparent activation energy of the NO reduction with char can be reduced by high level CO. Large amounts of active metal oxides can also decrease the apparent activation energy of NO reduction reactions.

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ASSOCIATED CONTENT

* Supporting Information S

Additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-29- 82665185. Fax: +86-29-82668703. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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