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Effect of Oxygen Concentration on NO Formation during Coal Char Combustion Jie Xu, Rui Sun, Tamer Mohamed Ismail, Shaozeng Sun, and Zhuozhi Wang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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Effect of Oxygen Concentration on NO Formation during Coal Char Combustion Jie XUa, Rui SUNa*, Tamer M. Ismailb*, Shaozeng SUNa, Zhuozhi WANGa a

School of Energy Science and Engineering, Harbin Institute of Technology, 92 West

Dazhi Street, Harbin 150001, P. R. China b

Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt

*Corresponding author Rui SUN School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P. R. China Tel.: +86 451 8641 3231 802 Fax: +86 451 8641 2528 E-mail address: [email protected] (R. Sun)

Tamer M. Ismail Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt Tel.: +20 01224745463; fax: +20 0226829366 E-mail address: [email protected], [email protected] (T. M. Ismail).

1

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Effect of Oxygen Concentration on NO Formation during Coal Char Combustion Jie Xua, Rui Suna*, Tamer M. Ismailb*, Shaozeng Suna, Zhuozhi Wanga a

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

b

Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt

Abstract: The effect that the ambient oxygen concentration has on the NO that is released during coal char combustion was studied under particle packed-layer conditions at combustion temperatures of 700 °C~1100 °C. The results show that the addition of quartz sand in the packed-layer mixture can effectively minimize the interaction of the char particles by inhibiting the secondary reaction of char and NO. During the char combustion tests, Char–N/NO conversion decreased with an increasing ambient oxygen concentration at low temperatures (700 °C-900 °C). At this temperature range, with coal char generally burning in Zones I or II, more oxygen penetrated into the pores, and less NO formed because of a more accessible pore surface area and an increased NO reduction time; a weak increasing trend was observed at a high temperature (~1100 °C). At this temperature and above, more char-N/NO conversion was observed because less O2 diffused into the char particles and NO was reduced less during its formation close to the char particle external surface. A change in the oxidization from kinetic to transition or diffusion control affected the conversion of char nitrogen to NO during combustion. A quantified description of the diffusion of oxygen into the pores of char and the effect that the original coal rank has on NO release is presented in this paper. 2


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Keywords: Oxygen concentration; Char-N to NO conversion; Accessible pore surface area; Char combustion

Introduction The emission of nitrogen oxides (NOx, mainly NO and a small amount of NO2) has drawn the attention of researchers and the public because of its serious environmental impacts. In practical coal combustion systems, the nitrogen content in coal (fuel-N) is generally the major contributor to total NOx emissions. Previous studies1-3 provided a general understanding of the conversion of fuel-N in the char (char-N) or volatile matter (volatile-N) to NOx. Nitrogen in volatiles has been shown to form HCN, NH3, and soot-bound nitrogen as intermediate species during the pyrolysis process, which are then oxidized to NOx or can reduce NOx to produce N2 under a low-oxygen atmosphere. In certain low-NOx pulverized-coal (PC) burners and in fluidized-bed (FB) coal combustion, a large fraction of char–N may be oxidized to form NOx directly. In modern low-NOx PC or FB coal combustion systems, volatile-N reacts to produce N2 via the precise control of the reducing atmosphere, and char-N is the main contributor to NOx emissions at the furnace exit.4 The conversion of nitrogen in chars is complicated in PC combustion conditions because the concentrations of both gaseous reactants and products vary with the depth of their penetration into the char pores, which also changes with the extent of char conversion. Some research groups, in their studies of char–N conversion, have reviewed the 3


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understanding of nitrogen conversion. Johnsson5 found that the ratios of char–N to NO and N2O varied between 20 % and 80 % in FB combustion and attributed this finding to wide variations in the experimental techniques and the influence of the devolatilization temperature, combustion temperature, coal type, particle size, and nitrogen content of the char. Thomas3 reviewed the influence of coal and char structural characteristics on the release of NOx during combustion and noted that more developed pore structures produced less NOx. Aarna and Suuberg6 summarized the kinetics of the char–NOx reaction and noted that this may involve the possible initial chemisorption of NOx and reaction of surface complexes. The reduction of pure NO is generally found to be of the first-order with respect to NO concentration. Molina et al.7 focused their research on developing a mechanism and model of nitrogen release from char to the homogeneous phase, its further oxidation to NO, and the reduction of NO on the surface of the char. Based on their analysis of experimental results, the mechanism of NO production from coal proposed by De Soete8 are considered the most comprehensive and useful, and they emphasized the importance of the reaction between NO and char. The conversion of char–N to NO is a very complex process, in which the char nitrogen is converted to NO, and its reduction occurs simultaneously at the inner porous surface and on the outer surface of particles. The complete process involves the adsorption, diffusion, and desorption of reactants and products. Although numerous studies on the production of NO during coal char combustion have enabled the preliminary characterization of this process, there are few studies on the effects 4


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that the ambient oxygen concentration has on char-N conversion immediately after char combustion without the participation of volatiles at temperatures below 1200 °C; at this point, O2 pore diffusion is critical for char-N/NO conversion. The NO produced from the heterogeneous oxidation of char is not amenable to control by, e.g., decreasing the ambient oxygen concentration for volatile-N converted NO abatement. Jensen et al.9 and Park et al.10 found that the ratio of fuel-N to NO is independent of the O2 concentration under the single particle condition and that when a smaller particle size is used, more char–N is converted to NO. Klein and Rotzoll11 reported that the conversion of char–N to NO decreased with increasing inlet oxygen concentration under FB conditions, which may be attributed to the higher char particle temperature leading to a faster NO–C reaction. However, Winter et al.12 found that the conversion of char–N to NO first decreased when the inlet O2 concentration increased from 5 to 10 % but then increased again when the O2 concentration reached 21 %. Both de Soete8 and Ashman et al.13 found that changes in the inlet oxygen concentration had little effect on the char–N conversion to NO in cases of batch coal particles. Tendencies of a decrease in the char–N to NO conversion with increasing fuel equivalence ratios under PC combustion conditions were found by Song,

14

Nelson, 15 and Spinti.16 The effect of increasing O2 concentration on this conversion was greater in the fuel-lean zone than in the fuel-rich zone. In the above experiments, the applied temperatures range from low temperature (FB combustion condition) to the temperature of PC burnout condition and the reaction regime of chars may change from kinetic control to diffusion control. The reaction 5


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regime significantly influences the diffusion of O2 into the pores of chars, which directly affects NO formation and its subsequent reduction on the internal pore surface. In view of these experimental results, it can be concluded that the conversion of char-N will show different behaviors under different oxidation control regimes from kinetic control to diffusion control. Therefore, more experimental studies are needed to further clarify the effect that the oxygen concentration has on the conversion of char-N to NO in kinetic/diffusion and transition control regimes of char. In this work, the effect that the oxygen concentration has on NO formation is investigated in a fixed-bed reactor at applied temperatures ranging from 700 °C to 1100 °C (mainly in the kinetic and transition control regime), and an enhanced explanation is proposed in a quantified description of the O2 diffusion characteristics in the pores of char. Experimental Char sample preparation. Three Chinese pulverized coal (75–100 µm particle size) specimens of different ranks (a lignite coal, denoted as HM; a bituminous coal, YM; and a lean coal, PM) were used. All pyrolysis procedures were carried out in a horizontal tube furnace (HTF) reactor (shown in Figure 1). During coal pyrolysis, coal samples that had been packed in the quartz boat were pushed very quickly into the middle zone of the reactor for 7 min in an inert atmosphere (N2), where the temperature of the zone was kept constant at approximately 900 ℃ with ±5 ℃ variation. When coal pyrolysis finished, the char samples were pushed quickly to the cooling zone of the reactor and were collected after being cooled to room temperature. 6


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The chars are labeled PM900, YM900, and HM900. Proximate and ultimate analyses of all original coal and char samples are listed in Table 1. Brunauer–Emmett–Teller (BET) surface areas and porosity of the chars were determined by the volumetric adsorption method with a Micromeritics ASAP 2010 instrument based on nitrogen adsorption at 77 K. The BET surface areas of the chars of PM900, YM900, and HM900 were 1.78, 3.55, and 89.30 m2/g. Table 1. Proximate and ultimate analyses and average particle sizes of pulverized coal and chars Proximate analysis(wt %,d)

Ultimate analysis(wt %,d)

Samples V

FC

A

C

H

N

PM

13.56

65.58

20.86

70.67

3.53

1.21

YM

23.47

41.84

34.69

49.32

3.28

1.03

HM

29.45

28.68

41.87

40.36

3.29

0.74

PM900a

1.88

76.53

21.59

55.27

0.88

1.21

YM900

3.24

52.82

43.93

52.12

0.76

1.12

HM900

5.71

29.91

64.37

32.75

0.5

0.71

a

900-pyrolysis temperature(°C)

Combustion apparatus and procedures. Char combustion experiments were carried out in an HTF reactor, a schematic of which is shown in Figure 1. The reactor consisted of three parts: a gas distribution system, a quartz reactor with an internal diameter of 34 mm and length of 1200 mm, and a gas analysis system. To maintain a completely inert atmosphere in the reactor during the fast pyrolysis process, 7


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deoxygenation equipment and a seal were used to remove trace oxygen and prevent air from leaking into the furnace. During combustion, 10 mg char with 1 g quartz sand was packed into a quartz boat, which was rapidly pushed into the middle zone of the reactor where temperature of the reactor was kept constant (±5 °C) at the target values 700 °C, 900 °C, or 1100 °C; the inner temperature difference of the reactor, as measured by a thermal couple during the combustion tests, was within 3 °C. Horizontal tube furnace reactor

Gas system

Analyzer system

To vent Mixed

To vent

Switch valve

Thermal couple Gas inlet Gas outlet

Mass flowmeter Deoxidation tube C

C

B

B

Cooling water

Ammeter and Controller Switch Voltmeter

O2

Filter

Gas analyzer

Laptop

Ar

Figure 1. Schematic of horizontal tube furnace reactor and associated components.

The ratio of char to sand was evaluated with respect to its effect on NO release. Argon was used as the carrier gas during char combustion to avoid the production of thermal NOx. The inlet oxygen concentrations were set at 0.5 %, 2 %, 6 %, 10 %, and 20 %. The gas flow rate was 3 L/min. The NO concentrations in the flue gas products at the reactor exit were detected using an infrared (IR) gas analyzer (SIGNAL S4i pulsar NDIR, SIGNAL Group Ltd, UK, accuracy of 1 %). Other nitrogen-containing species, such as HCN and N2O, can also be detected by Fourier Transform IR at low 8


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temperatures, but their quantities were small compared with that of NO and were thus neglected. The conversion of char-N to NO was calculated as follows:

ηNO =

1000M N ×

Q × 60 × 22.4 m ⋅ fN

∫ (C t

0

NO

)

× 10 − 6 dt

,

(1)

where t is the experimental run time (s), Mn is the molar mass of elemental nitrogen (g/mol), Q is the volumetric flow (m3/s) of the reactant gas, CNO is the concentration of NO (ppm), m is the mass of char (mg), and fN is the percentage of nitrogen in char obtained from the ultimate analysis. To eliminate the effects of the reactor and the quartz sand on NO emission, two other experiments were also carried out. The reactor was heated to 1100 °C at 20 % O2 with no sample and with a sample containing quartz sand only. Minimal NO was detected, so the uncertainty caused by the reactor system during combustion was considered to be approximately identical. Char oxidation kinetics. The char intrinsic oxidation rates were measured using a thermogravimetric analyzer (TGA; SDTA851e, Mettler Toledo, Switzerland) at 20 % oxygen in a nitrogen atmosphere in the temperature range of 500 °C to 1000 °C. Approximately 5 mg of char sample was employed, which covered the bottom of the crucible. The gas flow rate was 100 mL/min, and a linear heating rate of 40 °C/min was employed. The sample mass was continuously recorded by a data acquisition system until the char conversion finished. All reported data for mass change are given on a dry, ash-free basis; each measurement was corrected for buoyancy using a blank run under the same heating conditions. 9


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The combustion rate of char was calculated from the relationship between the rate of carbon oxidation and the rate of NO formation during combustion in HTF. The most common approach7 is to consider the carbon oxidation to be proportional to NO formation from the char, with the proportionality constant being the ratio of carbon/nitrogen atoms (C/N) in the parent char: - rC = rNO ⋅ (C/N ) ,

(2)

where –rC is the combustion rate of carbon oxidation and rNO represents the rate of NO formation from char–N oxidation. Results and Discussion Effect of particle interaction on NO formation. For a batch sample, the interaction of particles must be considered because the thickness of the sample in the reaction container will directly affect NO diffusion and reduction on the char surface. Previous experiments9, 10, 17 have shown that more char–N is converted to NO by decreasing the char loading and that almost 100 % of the char–N is evolved as NO under conditions without any secondary reactions.18 This evidence proves that interactions between char particles influence NO emission. Because of its thermal inertia and absence of reaction with char during the combustion process, 8, 9, 19 quartz sand is commonly mixed with a small amount of char to minimize interactions between char particles. Different mixing ratios by mass were obtained by combining 10 mg char with different masses of quartz sand. Figure 2 shows the extent of conversion of char–N to NO, ηNO, as a function of these mixing ratios. The solid points and dashed lines represent the experimental data and fitted 10


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curves, respectively.

Data of PM900 Data of YM900 Data of HM900 Fit of data

0.3

ηΝΟ/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2

0

20

40

60

80

100

120

140

160

Mass Mixing ratio of Quartz sand to char

Figure 2. Dependence of fractional conversion of char–N to NO on mass mixing ratio of quartz sand to char at 900 °C and O2= 10 %.

Similar general trends were observed for the three samples’ fractional conversion of char–N to NO, which first increased sharply with an increasing mixing ratio and then remained constant for sand char ratios exceeding 50:1. Quartz sand mixed with a small amount of char would increase the distance between char particles such that the NO released from one particle had less chance of being reduced on the surface of another char particle. As a result, more char–N evolved as NO when more sand was added to the samples. The selected mass mixing ratio of quartz sand to char of 1000 mg to 10 mg in the combustion tests was greater than 50:1, so the particle interactions were believed to be eliminated. Effect of oxygen concentration on NO formation. Figures 3 to 5 show the fractional 11


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conversion from char–N to NO as a function of inlet O2 concentration at different reaction temperatures. Five ambient O2 concentrations (0.5 %, 2 %, 6 %, 10 %, and 20 %) and three reactor temperatures (700 ℃, 900 ℃, and 1100 ℃) were tested. Figures 3 and 4 clearly show that as the O2 concentration increases, ratios of NO/char–N decrease monotonically for all chars used; however, this is not the case in Figure 5. For the YM900 and HM900 chars at 1100 °C, a similar general trend can be observed in that as the O2 concentration increased, the conversion initially decreased and then increased slightly, but it decreased again at 5 % O2. For the PM900 char, the conversion decreased with increasing O2 concentration from 0.5 % to 20 %. Previous findings on the effect of O2 concentration on NO release have been reported.11-13, 20-22 For low-oxygen concentrations, evidence of an increase in the rate of NO reduction in the presence of O2 was found by Aarna and Suuberg23 for different carbonaceous materials at 1023 K in a packed-bed reactor. Tomita and coworkers22 found a catalytic effect of oxygen in this reaction. The above experimental results indicate that oxygen may increase the rate of heterogeneous reduction of NO on the char surface. This may explain the decrease in conversion at 2 % O2 in Figure 5; however, for the relatively high oxygen concentrations under char combustion conditions, the previous results are inconsistent, and there is no clear explanation.

12


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1.0

PM900 YM900 HM900

0.8

ηNO

0.6

0.4

0.2

0.0 0

5

10

15

20

Oxygen concentration (%)

Figure 3. Dependence of fractional conversion of char–N to NO on inlet O2 concentration at 700 °C.

1.0

PM900 YM900 HM900

0.8

0.6

ηNO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.2

0.0 0

5

10

15

20


Figure 4. Dependence of fractional conversion of char–N to NO on inlet O2 concentration at 900 °C. 13


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1.0

PM900 YM900 HM900

0.8

0.6

ηNO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.2

0.0 0

5

10

15

20


Figure 5. Dependence of fractional conversion of char–N to NO on inlet O2 concentration at 1100 °C.

To interpret these experimental results, the char–N evolution routine should first be considered because competition between NO formation and reduction will ultimately determine the amount of NO released. In fact, the heterogeneous character of the char–oxygen reaction and the complex gaseous environment surrounding the char during combustion make it difficult to explain the pathway for char–N conversion to NO. There is, however, consensus in many studies on the heterogeneous formation of NO via the direct reaction of organic-bound nitrogen and oxygen, immediately followed by heterogeneous reduction by NO and char. The suggested mechanisms of NO production and reduction from coal char proposed by De Soete24 are considered to be the most comprehensive and useful: 14


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O 2 + (− C ) + (− CN ) → (− CO ) + (− CNO );

(− CNO) → NO + C →

(3)

NO + (− C ) ;

(4)

1 N 2 + (− CO ) . 2

(5)

It should be noted that homogeneous intermediates (like HCN and CO) in the formation and reduction of NO may also occur. A small amount of HCN was detected in the combustion tests, but the influence of HCN on NO formation was not considered. The other homogeneous reaction of NO-CO played only a minor role in the reduction of NO because when the oxygen concentration increased, the CO/CO2 ratio quickly decreased (seen in study25), and more CO was converted to CO2. Another possibility to be considered is that at higher oxygen concentrations, the resulting higher particle temperature will lower NO evolution by enhancing the rate of Equation (5). The idea that a limited temperature rise will cause such a large drop in NO evolution at low temperature, as seen in Figures 3 and 4, is unconvincing. The maximum temperature change detected during combustion was less than 3 K; therefore, a change in the oxygen concentration is not expected to significantly influence the rate of NO reduction. The effect of oxygen diffusion in the pores of the char on NO release should be considered. In our experiments, the temperature ranged from 700 °C to 1100 °C, which may cover kinetics control (Zone I), transition (mixed kinetics and diffusion) control (Zone II), and diffusion control (Zone III). The thermogravimetric data are plotted in Figure 6, which shows that there is a clear transition from kinetics control to diffusion control at the applied temperature range. The different reaction regimes would affect O2 15


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diffusion in the pores of the char particles; thus, NO emission may also be influenced.

-4

Kinetic control -5

-6

ln(k)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-7

-8

-9

PM900 YM900 HM900 Fitting line

-10 7.0x10-4 8.0x10-4 9.0x10-4 1.0x10-3 1.1x10-3 1.2x10-3 1.3x10-3

1/T

Figure 6. Arrhenius plots for test chars based on thermogravimetric data.

At lower temperatures (700 °C and 900 °C), the oxidation rate was relatively low, which meant that both O2 and NO may have fully or partly diffused into the pores. Increasing the bulk oxygen concentration yielded higher oxygen partial pressure, which promoted the penetration of O2 into the pores and reactions on internal surfaces of the particle and more NO was reduced by access to a greater surface area for reaction. In addition, the O2 diffusion distance into the pores increased the NO reduction time when NO diffused from the inside to the outside of the particle. This corresponds to an increase in the rate of Equation (5) and, as a consequence, decreased the NO evolution, as shown in Figures 3 and 4. This was not the case at a 16


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high temperature (1100 °C), where the rate-determining step is the diffusion of oxygen and desorption of the NO product close to the external surface of char particles. Under such conditions, reactions should occur near the external surface, where an increasing ambient oxygen concentration may have a small effect on its diffusion into the pores of the char and increasing the oxygen concentration contributed to a faster rate of NO production than NO reduction on the surface. Consequently, greater NO formation was observed for the YM900 and HM900 chars in Figure 5; however, because of the lower reactivity of PM900 char, the downward trend was still observed at 1100 °C. The conversion of char–N to NO did not differ under the low-temperature conditions; NO formed at the char surface may escape the surface rapidly, and the contribution of the second reaction (NO reduction on the char surface) is small. From the above analysis, NO formation can be interpreted in terms of the diffusion of oxygen into the pores. This study investigated the diffusion of oxygen into the pores, including the diffusion depth of oxygen concentration and the consequent reactive area for char–NO reduction in the particle, using a mathematical model to provide possible evidence for the above analysis. A first-order oxidation model was used to fit the intrinsic kinetics data measured in TGA because this model has been widely used in investigations of char intrinsic kinetics. The reaction rate is expressed as r = Ae − E / RT (1 - X )PO 2 ,

(6)

where R is the gas constant and PO2 is the partial pressure of oxygen. Figure 7 shows 17


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the fitting results by the first-order reaction model in the kinetics control zone. The good fitting result indicates the excellent fitting curve of the first-order reaction model for the intrinsic reaction rate in this study.

R=0.97

1.0

R=0.97

R=0.98 0.8

Conversion X

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

Experimental data of PM900 Experimental data of YM900 Experimental data of HM900 Fitting

0.4

0.2

0.0 0

5000

10000

15000

20000

25000

t (s)

Figure 7. Fitting plots by the first-order reaction model at 500 ℃.

The activation energy E and pre-exponential factor A were calculated by multiple linear regressions. The results are presented in Table 2. The intrinsic activation energies E of the three chars decreased in the order PM900 > YM900 > HM900. This result shows that the sensitivity of the oxidation rate to temperature of low-rank coal char was stronger than that of high-rank coal char. Table 2. Intrinsic kinetic parameters of test chars Sample

HM900

YM900 18


PM900

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In zone ℃

E(kJ/mol)

A (s-1/MPa)

E (kJ/mol)

A (s-1/ MPa)

E (kJ/mol)

A (s-1/ MPa)

101.56

5.95×104

109.20

1.10×105

122.92

9.90×104

To establish the conditions for diffusion of oxygen in the pores, the effectiveness factor, ηp, was used to account for the diffusion resistance in the pores during the overall char combustion process.26 The overall reaction rate can be expressed as follows: rm = η p A int e

− E int /RT

(1 - X )P

O2

,

(7)

where the subscripts m and int represent experiments performed with and without diffusional (intrinsic) effects. The overall reaction rate can be considered the rate influenced by pore diffusion. The external mass transfer resistance was calculated to be less than 0.01 % of the total mass resistance, according to a procedure found in the literature, using a correlation that valid for a fixed bed with inert and active particles27. The experimental effectiveness factor, ηp, can be expressed by the ratio of the overall reaction rate (just influenced by pore diffusion) and the intrinsic reaction rate. Once the effectiveness factor is obtained, the Thiele modulus, φp, can be calculated using Equation (8): ηp =

3  1 1  −  , φ p  tanhφ p φ p 

(8)

Here, the char particle is considered using spherical coordinates. With the Thiele modulus, the concentration profile inside the particle, which depends on the Thiele modulus and the normalized distance, λ, can be determined by solving the O2 transport equation:28, 29 19


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n βkρ p R u TC gn 1 d  2 dC g  r = , dr  D eff M c r 2 dr 

(9)

where rp is the radius of a particle (m); ρp is the bulk density of a particle (kg/m3); β is the molar stoichiometric coefficient for the reaction; Ru is the universal gas constant (J/K.mol); n is the reaction order; Deff is the effective diffusion coefficient inside a particle (m2/s); and Mc is the molar mass of carbon (kg/mol). Solving Equation (9) for two boundary conditions,

Cg

r =R

= C g, s and

dC g dr

= 0 r=0

yields

sinh(φ p C g = C g, s

r ) rp

(r/r )sinh (φ ) p

,

(10)

p

Dimensionless parametrization is carried out using Equation (10):

λ =

Cg r ,θ = , rp C g, s

(11)

Then

θ =

sinh(φ p λ)

(λ )sinh (φ ) ,

(12)

p

where r is the specified radius of a particle; Cg is the O2 molar concentration inside the particle; Cg,s is the O2 molar concentration on the particle surface; and λ and θ are dimensionless distance and concentration. Equation (12) describes the concentration profile inside the particle along the radius, which is dependent on the Thiele modulus and dimensionless distance. For convenient comparison between the tested chars, Equation (12) was simplified as follows: 20


Page 21 of 30

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Energy & Fuels

0.5 =

sinh (φ p (1 - H))

(1 − H )sinh (φ ) .

(13)

p

The dimensionless value of H (=1-r0.5/rp) is used to characterize the diffusion depth of oxygen into the char particle, and r0.5 represents the radius of a particle where the concentration θ is equal to 0.5. The larger the value of H is, the smaller effect of pore diffusion is found. Because of the effect of pore diffusion, not all of the BET area can be used to reduce NO on the char surface. Equation (7) can also be in the form of a rate on a basis of the BET surface area, defined by rm′ = η p A int e

− E int /RT

(1 - X )P

O2

S BET ,

(14)

where SBET is the total pore surface area. Then, an accessible pore surface area, Sa, is defined as the accessible reaction surface area for O2:

Sa = SBET ⋅ ηp ,

(15)

Figures 8 and 9 show the changes in the effectiveness factor, ηp, the diffusion depth of oxygen and the accessible pore surface area as a function of oxygen concentration for PM900 char at the three reaction temperatures. It was found that the effectiveness factor, diffusion depth of oxygen, and accessible pore surface area increased with an increasing oxygen concentration, which means that more oxygen could penetrate into the pores, thereby accessing greater pore surface that would subsequently participate in the reaction of char–NO. Figures 10 and 11 summarize the relationship between the conversion of char-N to NO and the accessible pore surface area/diffusion depth of O2. An apparent correlation is found; increasing the accessible surface area and the 21


Energy & Fuels

diffusion depth of O2 resulted in less NO being released with regard to the minimal contribution of the NO reduction reaction on the accessible pore surface for YM900 and HM900 char at 1100 °C. This shows that increasing both the diffusion depth of O2 and the accessible surface area will profoundly promote NO reduction. Reaction temperature also strongly affects oxygen diffusion. The higher the selected temperature is, the lower the oxygen effectiveness factor due to the diffusion effect is; however, Figures 9 and 10 show that the accessible pore surface area at each oxygen concentration is lower at higher temperatures than at lower temperatures. This can be explained by noting that increasing the reaction temperature significantly enhances the reduction rate of char–NO6.

1.0

1.0

700℃ 900℃ 1100℃

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

H

ηp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

0.0

0.0 2

6

10

20

oxygen concentration (%) Figure 8. Effect of oxygen concentration on effectiveness factor and diffusion depth of oxygen at different temperatures of char PM900. White symbol is related to left y-axis and black symbol is related to right y-axis.

22


Page 23 of 30

1.4

700℃ 900℃ 1100℃

1.2

sa(m2/g)

1.0 0.8 0.6 0.4 0.2 0.0 2

6

10

20

Oxygen concentration (%) Figure 9. Dependence of accessible pore surface area on oxygen concentration at different temperatures of char PM900.

1.0 0.8 0.6 0.4 0.2 0.0

ηNO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

700℃ 900℃ 1100℃

PM900

0.2 1.0 0.0 0.8 YM900 0.6 0.4 0.2 0.0 0.1 1.0 0.0 0.8 0.6 0.4 0.2 0.0

0.4

0.6

0.2

0.3

4

6

0.8

0.4

1.0

0.5

1.2

0.6

1.4

0.7

0.8

HM900

0

2

8

10

Sa(m2/g)

12

14

16

18

Figure 10 Dependence of N/NO conversion at different temperatures on accessible pore surface area.

23


Energy & Fuels

1.0 0.8 0.6 0.4 0.2 0.0

ηNO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

700℃ 900℃ 1100℃

PM900

0.0 0.1 0.2 0.3 0.4 1.0 YM900 0.8 0.6 0.4 0.2 0.0 0.00 0.01 0.02 0.03 1.0 HM900 0.8 0.6 0.4 0.2 0.0 0.00 0.01 0.02

0.5

0.04

0.03

0.6

0.7

0.05

0.8

0.06

0.04

0.9

0.07

0.05

1.0

0.08

0.06

H Figure 11 Dependence of N/NO conversion at different temperatures on diffusion depth of oxygen.

In general, at low temperature (under kinetic or transition control), increasing the oxygen concentration will increase both the NO reduction area and time, and less NO is released. At high temperature (close to diffusion control), increasing the oxygen concentration contributes to the NO production rate and may contribute less to NO reduction on less accessible reaction surface areas of particles. Therefore, the change in O2 mass transfer mechanism has an effect on the conversion of char nitrogen to NO during combustion.

Effect of coal rank on NO formation. The effect of coal rank on NO emissions is also shown in Figures 3 to 5. The particular properties of coal have long been known to affect the reactivity of the char, which influences NO emissions. Figure 12 shows the relationship between N/NO conversion and char apparent reactivity at temperature ranges from 700 °C to 1100 °C in HTF. 24


Page 25 of 30

Higher reactivity typically produces a lower conversion of char–N to NO. There is general agreement that the N fractional conversion under combustion conditions is less for chars prepared from low-ranked coals; in this study, the fractional conversion ratios of char-N decreased in the order PM900 > YM900 > HM900. This may have been due to the presence of more catalytically active mineral constituents and larger pore areas for low rank coal and its char. Low-ranked coal chars, such as HM900, have a structure with a higher degree of disorder and thus provide more active sites for high NO reduction and O2 intrinsic reactivity, whereas higher-ranked coal chars have a more ordered carbon lattice structure, which leads to fewer sites on which NO reduction can occur.

1.0

PM900 YM900 HM900

0.8

0.6

ηNO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.4

0.2

0.0 0.0

1.0x10-1

2.0x10-1

3.0x10-1

4.0x10-1

5.0x10-1

6.0x10-1

Reactivity (s-1) Figure 12. Dependence of N/NO conversion on char apparent reactivity at combustion temperature ranged from 700 °C to 1100 °C.

25


Energy & Fuels

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Conclusions The effect of oxygen concentration on the release of NO in the combustion of coal chars was studied under particle-packed conditions. The effect that particle interaction has on NO release was first investigated. The addition of quartz sand effectively minimized the interaction of char particles by inhibiting the secondary reaction of other char particles with NO. Char–N/NO conversion decreased with an increasing oxygen concentration at low temperatures (

Effect of Oxygen Concentration on NO Formation ... - ACS Publications

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