(PDF) Kinetics of NO Reduction by Coal, Biomass, and Graphitic

This investigation quantifies the effects of char burnout level and conditions on the kinetics of NO reduction by chars made from coals ranging in ran...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/EF

Kinetics of NO Reduction by Coal, Biomass, and Graphitic Chars: Effects of Burnout Level and Conditions Feng Guo, Mark J. Jensen, Larry L. Baxter, and William C. Hecker* Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602, United States ABSTRACT: During the combustion of coal and other carbonaceous materials, the heterogeneous reaction of NO with the evolving char created in the combustion process is important for understanding the formation and reduction of NO. This investigation quantifies the effects of char burnout level and conditions on the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) as well as graphite and coconut char. Kinetic data were measured in a packed-bed reactor at temperatures between 723 and 1173 K. The rate constant for the NO−char reaction was found to depend upon the extent of burnout/conversion and conditions under which the char was burned out. The NO−char rate constant consistently decreases with increasing burnout when the char burnout levels are accomplished in a drop tube reactor at 1800 K and 3−5% O2. However, the NO−char rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3050 ppm of NO in a packed bed at 723−1173 K. For the latter case, the relationship of the NO−char rate constant (on the basis of moisture ash-free char mass) and char burnout is approximately linear with roughly the same upward slope between 20 and 80% burnout for all coal chars studied. The activation energy of the NO−char reaction is apparently independent of both char burnout level and burnout conditions. The CO2/CO ratio in the exhaust stream appears to correlate with the NO−char rate constant for the chars burned out at low-temperature conditions.

■

concentrations.11,12 There appears to be no consensus on the catalytic effect of minerals in the char for the NO−char reaction.13−24 Tullin et al. reported on the decreasing importance of the pores with a decreasing particle size,25 and Sun et al. noted the inaccuracy of a random pore model for high carbon conversions.7,26,27 Biomass chars reportedly behave differently than coal chars with respect to NO−char rate, reaction order, and other trends under otherwise similar conditions.28 In general, biomass chars are more reactive than lignite chars.29 A single particle model, specific to biomass char, has been proposed by Karlstrom et al. to correlate NO−char reaction conditions to NO release.30 Because the carbon in a char reacts during the NO−char reaction, the morphology and reactivity of the remaining char may change and the nature of this change depends upon the conditions under which the burnout occurred.31 These conditions include the heating rate, temperature, and gasphase composition. These changes in morphology, which may or may not be similar to those observed during the O2−char reaction,32−35 according to Garijo et al., may influence both the global and intrinsic kinetics and mechanism of the NO−char reaction.36 Therefore, we believe that a quantitative study of char burnout level and burnout conditions on the kinetics of the NO−char reaction would be beneficial and would aid in the (1) understanding and control of NO formation and reduction during solid fuel combustion, (2) design of low NO emission combustion processes (for example, reburning, air staging, flue gas circulation, etc.), and (3) enhancement of existing models

INTRODUCTION NO is a major pollutant in the atmosphere. It has detrimental effects on human health, vegetation, and ecological systems and contributes significantly to the formation of smog, photochemical oxidants, and acidic precipitation. Although there are important natural sources of NO, the largest contributions come from fossil fuel combustion in both mobile and stationary heat engines. NO forms homogeneously and heterogeneously during solid fuel combustion. The homogeneous kinetics and mechanism of NO formation and destruction during combustion are arguably the single most extensively studied gas-phase reactions and have been reviewed many times, for example by Miller and Bowman1 and more recently by Glarborg et al.2 While some NO can be reduced homogeneously by the volatile material, it is primarily reduced heterogeneously at the char surface.3 The heterogeneous kinetics and mechanism of the NO−char reaction have been the subject of a number of studies, and while there is some consensus on reaction order, some aspects of the reaction are not fully understood.4,5 Among the unresolved issues are the dependence of NO−char rates and rate constants upon the extent of conversion and the reaction context (rate of pyrolysis, temperature, and other species in the gas phase).6−8 Several instructive studies have been performed that shed light on various aspects of the NO−char reaction. A review by Li et al. on the chemistry of nitrogen reduction by chars stated that chemical and physical features of the chars as well as reaction conditions strongly influence NO−char reaction rates, with particular emphasis on pore sizes and size distributions, surface area, and temperature.9 Karlstrom et al. reported that the NO release rate increases significantly with increasing char conversion.10 Wang et al. and Yamashita et al. reported a considerable increase in NO reduction with high CO © 2014 American Chemical Society

Received: January 25, 2014 Revised: May 22, 2014 Published: May 29, 2014 4762

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

carbide (inert to NO reduction) was packed in the reactor and heated in He to the maximum temperature desired using an electric furnace. NO diluted with He (3050 ppm of NO) was then fed downward through the reactor until the outlet NO concentration reached a quasisteady-state value, at which point data were collected. The approximate residence time of NO flow through the reactor was 100−500 ms. Because there was a slow loss of char during a run, the outlet NO concentration increased very slightly with time, and thus, we have used the term “quasi” steady state. In the calculation of NO− char rates and kinetic parameters, the small loss in char mass was accounted for by normalizing to the available char mass. The inlet gas pressure in the reactor was regulated at 300 kPa. The outlet pressure at each run condition was measured to determine the pressure drop across the packed bed. The pressure drop ranged up to 180 kPa. The average pressure was used in calculation of concentrations, and pretests showed that the variation of pressure in this study did not influence the NO−char reaction and kinetic parameters. The feed gas for all experimental runs came from a single gas cylinder with a NO concentration of 3050 ppm of NO in helium (Airgas, Inc.). Kinetic data were continually measured during the duration of the char burnout by determining NO conversion from a comparison of the inlet and outlet NO concentrations. While the inlet NO concentration was somewhat higher than seen in typical industrial applications, it allowed for very precise determination of NO conversion and kinetic parameters, including rate constant values. Because rate constants by their very nature are independent of the concentration, it is presumed that the data obtained here are applicable at lower concentrations as well. Gas Analysis. The composition of the outlet gas was continuously monitored for N2, CO, CO2, N2O, and O2 by a gas chromatograph (GC, PerkinElmer, 3920B) with a thermal conductivity detector (TCD) and two columns (one packed with Chromosorb 106 and the other packed with molecular sieve 5A) and for NO and NO2 by a chemiluminescence NOx analyzer (Thermo Environmental, 42H). Nitrogen and oxygen mass balances were determined between inlet and outlet streams for each run, and variations always fell within ±5%. Kinetic Data Acquisition. The NO−char reaction has been experimentally determined to be fractional or near first-order, as seen in Table 1.10,37,39−42 Also, previous work in our laboratory confirmed that the NO−char reaction order is unity and pore diffusion effects are negligible under the conditions of this study.37

of NO emissions during combustion. Therefore, the objectives of this investigation are to experimentally quantify the effects of char burnout level and burnout conditions on the kinetics of the NO−char reaction for chars derived from a standard set of research coals, coconut, and graphite.

■

EXPERIMENTAL SECTION

Char Preparation. The parent or starting chars used in this study were prepared from 64−75 μm particles of five U.S. standard research coals [Beulah Zap (North Dakota lignite), Dietz (sub-bituminous, subB), Utah Blind Canyon (high-volatile B bituminous, hvBb), Pittsburgh #8 (high-volatile A bituminous, hvAb), and Pocahontas #3 (low-volatile bituminous, lvb)] by devolatilizing samples of each of these coals in a methane/air flat-flame burner (FFB) at a high heating rate (104−105 K/s) and high temperature (peak gas temperature about 1800 K). A portion of the starting char derived from Beulah Zap lignite (dubbed NDL) was washed with HCl to remove mineral matter and was dubbed NDW. Thus, at this point, the starting chars (to be further burned out by two different methods in future procedures) were prepared. Purchased graphite and coconut char samples (Fisher Scientific) were also included in this investigation and were considered starting chars. Proximate analyses and surface area measurements for all of these starting chars were reported in a previous study.37 To investigate the effect of burnout conditions on the NO−char reaction, two methods, one simulating the conditions of practical pulverized coal combustion (method A) and the other simulating the conditions of a post-combustion cleanup process (method B), were employed to further react the starting chars to different burnout levels. The burnout level is defined to be zero for the starting chars. Method A (High Temperature, O2) Char Burnout. Samples of starting char materials (made in the FFB or purchased) were reacted further in a drop-tube reactor (DTR) in 3−5% O2 at high temperature (∼1800 K) to produce chars with different burnout levels up to 90%. Each char sample of a different burnout level and different origin was then placed in a 10 mm diameter VYCOR packed-bed reactor (PBR) to determine its reactivity for the NO−char reaction. The PBR tests for method A chars were fairly short-term to capture the reactivity of the char at its burnout level when it was placed in the PBR. The method A char samples that were tested in the PBR in this study were burned out in the DTR in a previous study by Cope.38 Method B (Low Temperature, NO) Char Burnout. In this method starting char samples were placed directly into the PBR and were continuously burned out at low temperatures (723−1173 K) in an atmosphere of flowing NO (3050 ppm of NO in helium), while simultaneously and continuously measuring kinetic data for the NO− char reaction. The reaction was typically continued until the char sample reached a burnout level of 90% or more. Global rate constants for the NO−char reaction were calculated directly using the integral plug flow equation and depend upon NO conversion and flow rate data. On the basis of a carbon mass balance for the global NO−char reaction

C + (1 + y)NO = (1 − y)CO + yCO2 + 1/2(1 + y)N2

Table 1. Reported NO−Char Reaction Orders for Solid Combustible Fuels char studied graphite, resin, and Wyodak coal pure carbon from phenolic resin spruce bark wheat straw various coals, graphite, and coconut

(1)

where y = [CO2]/([CO] + [CO2]) and [CO] and [CO2] are concentrations of CO and CO2. Char burnout levels were calculated as follows:

BO =

0 12tFNO XNO/(1 + y) W0(1 − ash)

temperature (K)

NO−char order

reference

773−1123

close to 1

40

273−1073

1

41

1073−1323 873−1273 723−1173

0.59, 0.9 0.89, 1.00 1.0

10 and 30 39 37

Therefore, with a NO−char reaction order of one, the integrated rate expression for a PBR is

(2)

where t is time elapsed (s), XNO is average NO conversion in t, F0NO is the inlet molar flow rate of NO (mol/s), W0 is the initial char mass (g), and ash is the ash fraction in the char. The calculated burnout levels were confirmed to be reliable within an error of ±8% using a thermogravimetric analyser (TGA). PBR Operation. The reaction of char samples with NO was carried out at temperatures of 723−1173 K in a vertical PBR with a fritted quartz disc of medium porosity to support each char sample. Each char sample (nominally 64−75 μm particle diameter) mixed with silicon

− ln(1 − XNO) =

0 k1C NO W 0 FNO

(3)

where XNO is the NO conversion, F0NO is the inlet molar flow rate of NO (mol/s), W is the residual char mass (g), C0NO is the inlet NO concentration (mol/L), and k1 is the first-order NO−char rate constant, which has units of L g−1 s−1. The NO−char rate constants were determined using eq 3 in two ways: multi-point determination (MPD) and single-point determination (SPD). 4763

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

MPD. The conversion of NO was measured at five to six different flow rates (from 100 to 500 mL/min). The NO−char rate constant then corresponds to the slope of the line obtained by plotting the data as C0NOW/F0NO versus −ln(1 − XNO). This approach was generally used to determine rate constant values for chars investigated by method A. SPD. Rate constant values could also be obtained directly using eq 3 from continuously measured values of XNO and F0NO. This method was more appropriate for method B chars because the burnout was being accomplished simultaneously in the PBR, and it would have disrupted the burnout process to vary the flow rate as is performed in MPD. A comparison of rate constant values for chars determined by both methods indicates a consistency for the two data determination methods and also that there is no significant effect of the flow rate on the NO−char rate constant.

■

RESULTS AND DISCUSSION Effect of the Char Burnout Condition. When combustion conditions and history differ, the observed NO−char rate dependence upon burnout also differs greatly, as shown in Figure 1 for NDL chars. For method B, the NO−char rate

Figure 2. Dependence of the char surface area (N2 BET) upon the burnout level for both burnout conditions.

Figure 1. Comparison of the NO−char rate variations to the burnout level for different burnout conditions. Burnout levels include the results in the PBR during the transient period.

Figure 3. Dependence of the CaO surface area upon the burnout level for both burnout conditions.

constant increases as the char burnout level increases. Apparently, the milder burnout conditions of method B facilitate the char becoming more active, which is consistent with the observation of the increased Brunauer−Emmett− Teller (BET) surface area seen in Figure 2. The NO−char rate constant appears to go to a maximum at a burnout level between 60 and 70%. The NO−char rate then drops off rapidly with increasing burnout, as would be expected. However, for chars burned out by the severe DTR conditions of method A, the NO−char rate constant consistently decreases as the char burnout level increases; albeit, the points are somewhat scattered. To address the different behaviors for the two different burnout conditions, determination of their total mesopore surface area by standard N2 BET measurements and the CaO surface area of the chars by a CO2 titration technique43 was made for NDL chars as a function of burnout levels achieved by both methods A and B. Results of the measurements are shown in Figures 2 and 3, respectively, for all of the method A chars and only method B chars with 0.1 g) for the measurement of the BET surface area. There is no inherent reason to expect that CaO and BET areas would track one another because the CaO area is only the area of one inorganic

material, while BET measures the total surface area, most of which is organic material. Apparent non-uniform burnout levels were observed for chars burned out over 40%, which probably resulted in the BET surface area of the chars being less than expected for these samples. However, corresponding trends between NO−char rate constants (Figure 1) and CaO surface area (Figure 3) and to some extent BET surface area (Figure 2) can still be observed. The CaO and BET areas decrease by ∼60 and ∼70%, respectively, for method A char, with a corresponding NO−char rate constant decrease of about ∼60%, and thus, for method A chars, both surface areas track rate fairly well. In contrast, for method B chars, the NO−char rate increases by ∼35% (to the 65% burnout level), while the CaO and BET surface areas increase by ∼30 and ∼350%, respectively. Thus, the CaO surface area tracks the rates for both types of char quite well, but the BET area does not do quite as well. The qualitative correspondence seems to indicate that the developments of both the mesopore surface area (roughly measured by the BET surface area) and CaO surface area are important factors in determining the NO−char reactivity for this NDL char. In method A, a high temperature and fast NO− char reaction lead to the loss of the mesopore surface area (BET). Similar results were also observed by Levendis et al.32 in 4764

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

the O2−char reaction and Wang et al.12 in the NO−char reaction. The decrease in the CaO surface area is most likely caused by sintering of fine CaO particles44 and, thus, the loss of catalytic surface sites. However, a low temperature and slow NO−char reaction (method B) favor the development of mesopores, resulting in an increase in the BET surface area.32,34 The CaO surface area increases with the burnout level because of an increase in the CaO content per gram of char as the carbonaceous mass burns away. These results indicate that the CaO surface area is a more reliable basis than the BET surface area for correlating the burnout level with surface area measurements, which agrees with Sun et al.45 Effect of the Method B Burnout Level on the NO− Char Rate Constant. Figures 4 and 5 show the variations of

char mass. On the basis of total char mass, the NO−char rate constants first increase with an increasing char burnout level, then reach a maximum, and begin to decrease. The burnout level corresponding to the maximum rate constant value varies with the char type. The low-rank chars seem to have a maximum NO−char rate at higher burnout levels than the highrank chars. On the basis of a maf char mass, the NO−char rate constants increase almost linearly with the char burnout level up to 90% for all chars, except for the graphite and coconut char. A comparison of mineralized and demineralized chars, NDL and NDW, respectively, indicates that mineral matter has a significant catalytic effect on the NO−char rate constant values up to 90% burnout level, which is consistent with results of a study by Illan-Gomez et al.13 The graphite and coconut char exhibit very different behaviors from coal chars, which may result from their different structures. Coconut char has a very large BET surface area (1050 m2/g), while graphite has a very small BET surface area (4 m2/g). Correlation of the NO−Char Rate Constant with the Method B Burnout Level. As discussed above, the NO−char rate constant based on the maf char mass increases almost linearly with char burnout levels for all method B coal chars. The increasing slopes are roughly the same between 20 and 80% burnout, and a general expression for the NO−char rate constant as a function of the burnout level can be determined k = k50[1 + m(BO − 50)]

(4)

where k50 is the NO−char rate constant at 50% burnout level, m is a single constant for all coal chars representing the slope of the data in Figure 5, and BO is the char burnout level (%). Figure 6 compares the experimental and calculated NO−char rate constants for all method B coal chars of this study between

Figure 4. Variation of the NO−char rate constants with the burnout level (from method B) at 1098 K for different rank chars. NO−char rate constants are based on total char mass (including ash). Different scales are used for different chars (see the legend).

Figure 5. Variation of the NO−char rate constants with the burnout level (from method B) at 1098 K for different rank chars. NO−char rate constants are based on maf char mass. Different scales are used for different chars (see the legend).

Figure 6. Comparison of the experimental and calculated NO−char rate constants for all method B coal chars with 20−80% burnout levels (m = 7.4 × 10−3).

NO−char reaction rates with the char burnout level obtained under method B conditions for biomass, graphite, and six coal chars. The NO−char rates were continuously measured at one flow rate (single-point determination). The figures have the same data with different bases. In Figure 4, the NO−char rate constants are based on total char mass (including ash), whereas in Figure 5, the constants are based on moisture ash-free (maf)

20 and 80% burnout levels. The m value used was 0.0074, and it was determined by minimizing the sum of squared error between the measured and calculated k values for all coal chars. All points fall around the 45° line, with a coefficient of determination (R2) of 0.9977. Changing the m constant value ±10% results in R2 decreasing to 0.9975. 4765

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

MPD. As seen, the MPD activation energy in the lowtemperature regime fits very well with the SPD values, and while the high-temperature value is a little high, it is determined from only three points and the placement of the data points is where one would expect them to be. Additional comparisons of MPD and SPD data were performed for Dietz chars and showed agreement in activation energies within 7.4 and 10.1% in the high- and low-temperature regimes, respectively. Because the agreement was within the experimental error of ±20%, further calculations were primarily performed using the SPD method only. The independence of the activation energy from the burnout level infers that the mechanism of the NO−char reaction may not change significantly with the burnout level of a char. The variation of the NO−char rate with the burnout level, apparently caused by the variations of the surface areas of char and the active mineral matter, is thus attributable to changes in the pre-exponential factor. Therefore, the NO−char rate constant can be expressed by the Arrhenius model

Equation 4 provides a very simple model to correct NO− char rate constant values for the effect of the char burnout level for chars made at a given set of conditions (method B). Effect of the Burnout Level on Activation Energy. Figures 7 and 8 show Arrhenius plots for NDL and Pittsburgh

k = A exp( −E /RT )

(5)

where A is dependent upon the char burnout level but E is not. Both A and E are dependent upon the coal rank and temperature regime. The effect of the temperature on the variation of the NO− char rate with the char burnout level was also investigated. This is shown in Figure 9 for NDL char. It is expected that the

Figure 7. Arrhenius plots of the NO−char rate constants based on maf char mass for NDL char with different burnout levels (from method B). SPD, single-point determination; MPD, multi-point determination.

Figure 9. Dependence of the NO−char rate upon the burnout level (method B) for NDL char at different temperatures. Figure 8. Arrhenius plots of the NO−char rate constants based on maf char mass for Pittsburgh #8 char with different burnout levels (from method B). All data are from SPD.

variation of the NO−char rate with the burnout level is independent of the temperature, because the activation energy of the reaction is independent of the burnout level. Effect of the Burnout Level on the CO2/CO Ratio. While studying the effect of the burnout level on the NO−char reaction, it was found that trends in the CO2/CO product ratio with the char burnout level corresponded with the trends in the NO−char rate constant values for all chars burned out at lowtemperature conditions. Figures 10 and 11 illustrate this point by comparing trends in the CO2/CO ratio to NO−char rate constant values for NDL char at 1023 K (Figure 10) and Pocahontas char at 1098 K (Figure 11). Such a correspondence seemingly suggests that a process of CO converting to CO2 on the char surface may play a role in the overall NO−char rate and that the process depends upon the char structure.

#8 chars for different burnout levels produced by method B. Other chars show similar behaviors. The plots show that the NO−char rates fall into two regimes with different activation energies, as reported previously.46 For each char, the activation energy in each regime remains constant with burnout within experimental error (±20%) in both the high- and lowtemperature regimes. For example, for NDL, the values in the low-temperature regime vary from 21.2 to 24.5 kcal/mol, comparable to measurements of other low-rank coals.26 This suggests that the activation energy of the NO−char reaction is independent of the method B burnout level, which is consistent with the results for the chars produced by method A.37,46 Most of the data included in Figures 7 and 8 were determined using the SPD method, but for comparison and verification, one set of data in Figure 7 was determined using 4766

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

which can be correlated with effective mineral matter sites during the reaction. In addition, a correspondence between the CO2/CO ratio and NO−char reaction rate suggests that a process of CO converting to CO2 may be an important step in the NO−char reaction.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 801-422-6235. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 10. Coincidence of the variations of the CO2/CO ratio and NO−char rate constants on a total char basis with the burnout level (from method B) for NDL char.

ACKNOWLEDGMENTS This work was sponsored by the Advanced Combustion Engineering Research Center at Brigham Young University. Funds for this center are received from the National Science Foundation, the State of Utah, the U.S. Department of Energy, and a number of industrial participants. The authors also thank Richard Cope and Scott Felt for NDL and Dietz char preparation and Xing Li for help in publishing this paper.

■

REFERENCES

(1) Miller, J. A.; Bowman, C. T. Mechanism and modeling of nirogen chemistry in combustion. Prog. Energy Combust. Sci. 1989, 15 (4), 287−338. (2) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Fuel nitrogen conversion in solid fuel fired systems. Prog. Energy Combust. Sci. 2003, 29 (2), 89−113. (3) He, J.; Song, W.; Gao, S.; Dong, L.; Barz, M.; Li, J.; Lin, W. Experimental study of the reduction mechanisms of NO emission in decoupling combustion of coal. Fuel Process. Technol. 2006, 87 (9), 803−810. (4) Aarna, I.; Suuberg, E. M. A review of the kinetics of the nitric oxide carbon reaction. Fuel 1997, 76 (6), 475−491. (5) Molina, A.; Eddings, E. G.; Pershing, D. W.; Sarofim, A. F. Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions. Combust. Flame 2004, 136 (3), 303−312. (6) Bueno-Lopez, A.; Caballero-Suarez, J. A.; Garcia-Garcia, A. Kinetic model for the NOx reduction process by potassium containing coal char pellets at moderate temperature (350−450 °C) in the presence of O2 and H2O. Fuel Process. Technol. 2006, 87 (5), 429−436. (7) Zhang, J. W.; Sun, S. Z.; Hu, X. D.; Sun, R.; Qin, Y. K. Modeling NO−char reaction at high temperature. Energy Fuels 2009, 23, 2376− 2382. (8) De Soete, G. G.; Croiset, E.; Richard, J. R. Heterogeneous formation of nitrous oxide from char-bound nitrogen. Combust. Flame 1999, 117 (1−2), 140−154. (9) Li, Y. H.; Lu, G. Q.; Rudolph, V. The kinetics of NO and N2O reduction over coal chars in fluidised-bed combustion. Chem. Eng. Sci. 1998, 53 (1), 1−26. (10) Karlstrom, O.; Brink, A.; Hupa, M. Time dependent production of NO from combustion of large biomass char particles. Fuel 2013, 103, 524−532. (11) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Influence of char surface chemistry on the reduction of nitric oxide with chars. Energy Fuels 1993, 7 (1), 85−89. (12) Wang, C. A.; Du, Y. B.; Che, D. F. Investigation on the NO reduction with coal char and high concentration CO during oxy-fuel combustion. Energy Fuels 2012, 26 (12), 7367−7377. (13) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. NO reduction by activated carbons. 2. Catalytic effect of potassium. Energy Fuels 1995, 9 (1), 97−103. (14) Yin, Y. S.; Zhang, J.; Sheng, C. D. Effect of pyrolysis temperature on the char micro-structure and reactivity of NO reduction. Korean J. Chem. Eng. 2009, 26 (3), 895−901.

Figure 11. Coincidence of the variations of the CO2/CO ratio and NO−char rate constants with the burnout level (from method B) for Pocahontas char at 1098 K.

■

CONCLUSION The conditions under which char is burned out significantly influence the kinetics of the subsequent NO−char reaction. Burnouts achieved at a high temperature (∼1800 K) in oxygen lead to decreases in BET and CaO surface areas, along with corresponding decreases in the NO−char reaction rate constants with an increased burnout level. On the other hand, burnouts achieved at a low temperature in NO are beneficial to the development of char mesopores and CaO surface area, so that both types of surface area increase as the char burnout level increases. These increases in surface area result in corresponding increases in the NO−char reaction rate up to a 90% burnout level. For chars burned out at 1098 K by reaction with NO (method B), the NO−char rate constant (per gram of maf char) increases almost linearly with the char burnout level up to 90% for all coal chars studied but not for graphite or coconut char. The slope between 20 and 80% burnout is roughly the same for all coal chars studied. Importantly, a single dimensionless value in eq 4 yields a R2 of 0.998 for the char oxidation conditions in this study. The NO−char reaction activation energies for both the highand low-temperature regimes were found to be independent of char burnout level and burnout conditions. This suggests that the NO−char reaction mechanism does not significantly change with an increasing char burnout level, but the variation of the NO−char rate is due to the variations of the surface area, 4767

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768

Energy & Fuels

Article

(15) Zhong, B. J.; Tang, H. Catalytic NO reduction at high temperature by de-ashed chars with catalysts. Combust. Flame 2007, 149 (1−2), 234−243. (16) Illan-Gomez, M. J.; de Lecea, C. S. M.; Linares-Solano, A.; Radovic, L. R. Potassium-containing coal chars as catalysts for NOx reduction in the presence of oxygen. Energy Fuels 1998, 12 (6), 1256− 1264. (17) Kopsel, R. F. W.; Halang, S. Catalytic influence of ash elements on NOx formation in char combustion under fluidized bed conditions. Fuel 1997, 76 (4), 345−351. (18) Zhao, Z. B.; Li, W.; Li, B. Q. Catalytic reduction of NO by coal chars loaded with Ca and Fe in various atmospheres. Fuel 2002, 81 (11−12), 1559−1564. (19) Zhao, Z. B.; Li, W.; Qiu, J. S.; Li, B. Q. Catalytic effect of Na−Fe on NO−char reaction and NO emission during coal char combustion. Fuel 2002, 81 (18), 2343−2348. (20) Zhong, B. J.; Shi, W. W.; Fu, W. B. Effect of catalysts on the NO reduction during the reburning with the coal chars as the fuel. Combust. Sci. Technol. 2001, 164, 239−251. (21) Zhong, B. J.; Tang, H. The catalytic effect of Fe on char−NO reactions at high temperatures. Int. J. Chem. React. Eng. 2008, 6. (22) Zhong, B. J.; Zhang, H. S. Reduction of NO by low-volatile coal chars with or without catalyst addition. Combust. Sci. Technol. 2003, 175 (12), 2181−2199. (23) Zhong, B. J.; Zhang, H. S.; Fu, W. B. Catalytic effect of KOH on the reaction of NO with char. Combust. Flame 2003, 132 (3), 364− 373. (24) Zhang, J. W.; Sun, S. Z.; Zhao, Y. J.; Hu, X. D.; Xu, G. W.; Qin, Y. K. Effects of inherent metals on NO reduction by coal char. Energy Fuels 2011, 25 (12), 5605−5610. (25) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Beer, J. M. NO and N2O formation for coal combustion in a fluidized bedEffect of carbon conversion and bed temperature. Energy Fuels 1993, 7 (6), 796−802. (26) Sun, S. Z.; Zhang, J. W.; Hu, X. D.; Qiu, P. H.; Qian, J.; Qin, Y. K. Kinetic analysis of NO−char reaction. Korean J. Chem. Eng. 2009, 26 (2), 554−559. (27) Fei, J.; Sun, R.; Yu, L. B.; Liao, J. A.; Sun, S.; Kelebopile, L.; Qin, Y. K. NO emission characteristics of low-rank pulverized bituminous coal in the primary combustion zone of a drop-tube furnace. Energy Fuels 2010, 24, 3471−3478. (28) Dong, L.; Gao, S. Q.; Song, W. L.; Xu, G. W. Experimental study of NO reduction over biomass char. Fuel Process. Technol. 2007, 88 (7), 707−715. (29) Chen, W.-Y.; Gathitu, B. B. Kinetics of post-combustion nitric oxide reduction by waste biomass fly ash. Fuel Process. Technol. 2011, 92 (9), 1701−1710. (30) Karlström, O.; Brink, A.; Hupa, M. Biomass char nitrogen oxidationSingle particle model. Energy Fuels 2013, 27 (3), 1410− 1418. (31) Le Manquais, K.; Snape, C.; McRobbie, I.; Barker, J.; Pellegrini, V. Comparison of the combustion reactivity of TGA and drop tube furnace chars from a bituminous coal. Energy Fuels 2009, 23, 4269− 4277. (32) Levendis, Y. A.; Sahu, R.; Flagan, R. C.; Gavalas, G. R. Postignition transients in the combustion of single char particles. Fuel 1989, 68 (7), 849−855. (33) Moghtaderi, B. A study on the char burnout characteristics of coal and biomass blends. Fuel 2007, 86 (15), 2431−2438. (34) Sahu, R.; Levendis, Y. A.; Flagan, R. C.; Gavalas, G. R. Physical properties and oxidation rates of chars from three bituminous coals. Fuel 1988, 67 (2), 275−283. (35) Waters, B. J.; Mitchell, R. E.; Squires, R. G.; Laurendeau, N. M. Overall kinetic parameters for combustion of a highly non-spherical carbon char. Symp. (Int.) Combust., [Proc.] 1989, 22 (1), 17−27. (36) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Reactivity of coal char in reducing NO. Combust. Flame 2004, 136 (1−2), 249−253.

(37) Guo, F.; Hecker, W. C. Kinetics of NO reduction by char: Effects of coal rank. Symp. (Int.) Combust., [Proc.] 1998, 27 (2), 3085− 3092. (38) Cope, R. F. Effects of calcium oxide and burnout level on oxidation of Beulah Zap chars. Ph.D. Thesis, Brigham Young University, Provo, UT, 1995. (39) Wang, X.; Si, J.; Tan, H.; Zhao, Q.; Xu, T. Kinetics investigation on the reduction of NO using straw char based on physicochemical characterization. Bioresour. Technol. 2011, 102 (16), 7401−7406. (40) Aarna, I.; Suuberg, E. M. A study of the reaction order of the NO−carbon gasification reaction. Symp. (Int.) Combust., [Proc.] 1998, 27 (2), 3061−3068. (41) Teng, H. S.; Suuberg, E. M.; Calo, J. M. Studies on the reduction of nitric oxide by carbonThe nitric oxide−carbon gasification reaction. Energy Fuels 1992, 6 (4), 398−406. (42) Dong, L.; Gao, S.; Xu, G. NO reduction over biomass char in the combustion process. Energy Fuels 2009, 24 (1), 446−450. (43) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Importance of catalyst dispersion in the gasification of lignite chars. J. Catal. 1983, 82 (2), 382−394. (44) Cope, R. F.; Arrington, C. B.; Hecker, W. C. Effect of CaO surface area on intrinsic char oxidation rates for Beulah Zap chars. Energy Fuels 1994, 8 (5), 1095−1099. (45) Sun, S. Z.; Zhang, J. W.; Hu, X. D.; Wu, S. H.; Yang, J. C.; Wang, Y.; Qin, Y. K. Studies of NO−char reaction kinetics obtained from drop-tube furnace and thermogravimetric experiments. Energy Fuels 2009, 23 (1), 74−80. (46) Guo, F.; Hecker, W. C. Effects of CaO and burnout on the kinetics of NO reduction by Beulah Zap char. Symp. (Int.) Combust., [Proc.] 1996, 26 (2), 2251−2257.

4768

dx.doi.org/10.1021/ef500242t | Energy Fuels 2014, 28, 4762−4768