Investigation of the NO Reduction Characteristics of Coal Char at

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Investigation of the NO Reduction Characteristics of Coal Char at Different Conversion Degrees under an NO Atmosphere Zhuo-Zhi Wang,† Jie Xu,† Rui Sun,*,† Ya-Ying Zhao,†,‡ Yu-Peng Li,† and Tamer M. Ismail*,§ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Energy and Power Engineering, Northeast Electric Power University, Jilin, 132012, China § Department of Mechanical Engineering, Suez Canal University, Ismailia, Egypt ‡

S Supporting Information *

ABSTRACT: The effect of preoxidative treatments on the reducing characteristics of ShenHua (SH) coal char and the evolution of surface nitrogen-containing complexes during the reducing process under different temperature conditions were investigated using temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The SH samples were oxidized to different conversion degrees (0.15, 0.22, 0.32, 0.42, 0.52, 0.73, and 0.89) under an O2 atmosphere, and the reductivity of each char sample was considerably enhanced after the preoxidative treatment. This phenomenon could be attributed to the generation of active sites and oxygen-containing complexes on the SH particle surface during the oxidation process. The preoxidized samples were obviously more reactive than the raw char, and per unit mass almost all the preoxidized samples could consume more NO than SH raw char under the same experimental conditions. The entire TPR process could be divided into the following four sections: (a) the reversible physical adsorption stage, (b) the heterogeneous reaction stage, (c) the multireaction stage, and (d) the equilibrium reaction stage. The primary reaction path at each stage could be summarized using the TPR and XPS results. The evolution of the C(N) and the variation in the elemental distribution (C, O, and N) during the TPR process were investigated by XPS. The results showed that N-Q was the most stable organic structure of the nitrogen-containing complexes on the particle, and the decomposition of N-6, N-5, and N-Q occurred when the reaction temperature reached 1173 K. The total amounts of NQ, N-6, and N-5 decreased when the reaction temperature exceeded 1173 K, indicating interaction between the nitrogencontaining complexes occurred. Meanwhile, prior to the attachment of NO molecules to the char particle surface and forming C(N), more NO molecules were consumed by CO at this temperature. The results in this research clarified the effect of the conversion degree on the char reductivity and the primary reduction reaction path under different temperature conditions, providing a technical framework for the reducing process of air-staged combustion technology.

1. INTRODUCTION High-efficiency combustion technology with low pollutant emissions is the primary target of carbonaceous fuel utilization. NOx is one of the most important pollutants that is released during the coal combustion process, and NOx is regarded as a major contributor to the formation of photochemical smog, to acid rain, and to the destruction of ozone in the stratosphere.1 It is generally believed that air-staged combustion is the most sophisticated low NOx combustion technology for reducing NOx release, and therefore it is widely employed in coal-fired power plants.2 Along the coordinate axis of the device, the fuel reaction zone can be divided into the following three sections: the primary combustion section, the reducing section, and the burnout section.3 NO is the primary component of NOx4 during the combustion process, especially under high temperature conditions. The NO that is released from the primary zone is reduced in the reducing section, and the NO concentration in the exhaust gas decreases rapidly after passing through the reducing section. Due to the fact that almost all the O2 is consumed in the primary combustion section, the O2 concentration can reach nearly zero in the reducing section,3 just prior to the injection of the burnout air. Harding5 and Tomita6 conveyed that the burning time of the volatile compound was approximately 10 ms, and the char reaction © XXXX American Chemical Society

period was approximately 300 ms. Due to the apparent difference in the reaction time, the char was considered to be the primary reactant in the NOx reducing process. Therefore, the heterogeneous and homogeneous reactions between coal char and NO will occur in the reducing section, and the mechanism of the reduction reaction and the influencing factors (e.g., the char conversion degree and reaction temperature) for this process are of great significance for the development of low NOx emission technology. In the reducing section of the actual combustion process, char has partly reacted with O2 in the primary section and generated many oxygen-containing complexes on the particle surface. It is generally accepted that there is a significant enhancement in the reducing reactivity of carbonaceous matter in the presence of O2.7−10 The positive effect of O2 was attributed to the variation in the chemical structure of the coal char surface. Oxygen-containing C(O) complexes determined the chemisorption step of NO in the low temperature region, and then they influenced the gasification reaction between char and NO in the high temperature region.10 Suzuki9 stated that an increase in the NO consumption rate resulted from the Received: May 5, 2017 Revised: July 14, 2017 Published: July 18, 2017 A

DOI: 10.1021/acs.energyfuels.7b01291 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Ultimate and Proximate Analyses of the Materials ultimate analysis (dry and ash free)

a

proximate analysis (as received)

sample

C

H

N

S

Oa

moisture (wt %)

volatiles (wt %)

Cfixed (wt %)

ash (wt %)

SH coal SH raw char

79.75 96.81

4.18 0.43

0.71 1.23

0.30 0.35

15.06 1.18

9.78 0.81

28.12 1.05

59.44 93.27

2.66 4.87

Calculated by difference.

Table 2. Ash Content Analysis of SH Raw Coal component percentage (%)

SiO2 32.11

Al2O3 10.33

Fe2O3 11.64

CaO 25.53

MgO 1.26

K2O 0.87

Na2O 1.14

TiO2 0.49

P2O5 0.07

SO3 12.33

samples are shown in Table 1, the percentage of each component in the ash of SH raw coal is summarized in Table 2. To investigate the reducing reactivity and primary reducing pathway of the char samples under different conversion degrees, SH char samples were oxidized with 30% O2 (balanced with Ar) at 1073 K for 30 s, 45 s, 60 s, 75 s, 90 s, 4 min, and 6 min, the abbreviations for the oxidized samples were S1, S2, S3, S4, S5, S6 S7, and S8, respectively. Based on the ultimate analysis of each SH sample and the determination of CO/CO2 released during each preoxidative process, the variation of carbon content in each sample could be calculated; thus the conversion ratio of each SH char sample could be calculated. The conversion degrees of the SH char samples are shown in Table 3.

generation of new active sites upon C(O) decomposition, and that these sites were more reactive than the active sites on the original coal char particle surface. Most of the investigations on the reaction mechanisms between coal and NO were performed under relatively low temperature conditions (usually below 1473 K), and the effect of the char conversion degree on the reducibility was usually neglected. The combustion environment temperature in utility furnaces is actually higher than 1473 K, and the average temperature is generally above 1673 K.2 The residence time determined the coal char conversion degree at the outlet of the primary combustion section, and the surface properties and reduction reactivity of char with different conversion rates will be different. Previous studies have addressed the NO reducing process in a systematic fashion, and the mechanism of the reaction under different reaction temperature conditions has been described comprehensively.11−17 However, the variations in the reactivity and reducing abilities of active sites with the variations in the char conversion degree along with the primary reaction pathways under specific reaction conditions still require more detailed investigation. In this research, a typical bituminous coal known as ShenHua (SH) (SH coal has currently proven reserves of about 18 billion tons, and a market share approaching 10% in China) was employed to investigate the effect of the char conversion degree on the enhancement of the reducing reactivity for a temperature range from 293 to 1723 K, and the dominating reaction path under different temperature conditions was clarified. The experimental procedures consisted of the following three parts: first, raw char samples were preoxidized to different conversion degrees. Then, temperature-programmed-reduction (TPR) experiments were performed for each char sample. In addition, with the application of the XPS method, the evolution of nitrogen-containing complexes during the TPR process could be clarified. Based on the TPR and Xray photoelectron spectroscopy (XPS) results, the reductivity of each coal char sample within different reaction temperature ranges could be clarified, and the primary reaction pathway between NO and char under different temperature conditions could be determined. This research may provide a deep understanding for developing the low NO emission coal combustion technology.

Table 3. Conversion Ratio of SH Samples reaction time and conversion ratioa b

sample

0s

30 s

45 s

60 s

75 s

90 s

4 min

6 min

S1−S8

0

0.15

0.22

0.32

0.42

0.52

0.73

0.89

a b

Calculated by the variation of the elemental content of char sample. Raw char.

2.2. TPR and Partial TPR Methods. The temperatureprogrammed-reduction (TPR) tests were performed in a corundum horizontal fixed bed furnace at a high temperature to clarify the reducing reactivity and characteristics of char samples with different conversion ratios. Prior to the TPR experiment, a blank test was conducted under a 1000 ppm NO stream from room temperature to 1723 K at a heating rate of 5 K/min, and the releasing gas was analyzed by a SIGNAL S4i pulsar NDIR gas analyzer (resolution: CO, 0.1 ppm; CO2, 0.1 ppm; NO, 0.1 ppm). The results of the blank run conveyed that trace amounts of NO were consumed during the blank test process, and thus the experimental system had a negligible effect on the reaction between the coal char and NO. The TPR test procedure was as follows: 40 mg of dried char sample was placed into a corundum boat in the furnace; the reaction temperature was increased from 298 to 1723 K and the heating rate was 5 K/min; the reaction atmosphere consisted of NO (1000 ppm) and Ar and the flow rate was 410 mL/min; the CO and NO concentrations in the exhaust gas after the TPR process were recorded by an NDIR gas analyzer. The experimental TPR system is shown in Figure 1. Experiments were performed to clarify the variations in the elemental distribution on the particle surface during the TPR process, especially the evolution of surface nitrogen-containing complexes. Partial TPR experiments were also conducted on the openable TPR device. The char samples underwent six TPR experiment processes at a heating rate of 5 K/min under 1000 ppm NO; the flow rate of the reaction gas (NO + Ar) was 410 mL/min, and the final TPR temperature of each process was set at 1073, 1173, 1273, 1373, 1473, and 1573 K, respectively. After the specific partial TPR treatment, the char samples in the reactor were quickly cooled to a relatively low temperature (approximately 573 K) under the same reaction atmosphere within a brief cooling period (1−2 min); thus, little char would be consumed during the cooling process. The partial reduction

2. EXPERIMENTAL SECTION 2.1. Material Preparation. SH coal was widely used for power generation in China, and thus it was employed as a sample for this investigation. The raw coal was sieved to uniform distribution within a range from 90 to 120 μm. A horizontal fixed bed furnace was used for coal devolatilization under an Ar atmosphere for 30 min at 1173 K to produce raw char. The proximate and ultimate analysis results of the B


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Figure 1. High temperature TPR experimental system.

Figure 2. Summary of TPR spectra. Sample number: 1, S1; 2, S2; 3, S3; 4, S4; 5, S5; 6, S6; 7, S7; 8, S8. char samples were subsequently collected for surface chemical behavior analysis by high-resolution XPS. 2.3. XPS Analysis. The SH char surface chemical structures were determined by X-ray photoelectron spectroscopy (XPS; PHI 5400 ESCA System, USA), and the instrument was equipped with an Al Kα X-ray source (hv = 1486.6 eV). To ensure sufficient sensitivity, the pass energy was fixed at 93.9 eV. Both the narrow spectra and whole spectra of all the char samples with high resolution were recorded. In wide scan the pass energy was 178.95 eV, and in narrow scan the pass energy was 22.35 eV. The compositions were calculated by the integrated peak areas of C 1s, N 1s, and O 1s from the spectra. PeakFit v4.12 was employed for a deconvolution analysis of the XPS results.

reduction in the concentration of NO consumed by raw char S1 was apparently less than those of S2−S8, which were oxidized before the reduction reaction. Thus, the TPR curves could be summarized to indicate that the preoxidization treatment could enhance the reactivity of the coal char samples. Comparing the TPR curves of each sample showed that the chars with different conversion degrees had different reducing abilities, especially under high temperature conditions. Clearly, the values of the NO concentration for S2 and S3 at the final stage of the curves (1400−1723 K) in Figure 2 were significantly lower than they were for other char samples, indicating that the SH char samples with specific conversion degrees (0.15−0.22) had stronger reducibility. According to previous studies,18−21 the enhancement of the reducing reactivity between char and NO could be attributed to the following two explanations: (1) a reaction with oxygen generated more oxygen-containing complexes (metastable and stable complexes) on the char

3. RESULTS AND DISCUSSION 3.1. Effect of Conversion Degree on the Reaction between NO and Coal Char. 3.1.1. TPR Results of Raw Char and Samples after Preoxidative Treatment. The TPR spectra of samples S1−S8 are shown in Figure 2, and the C


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Figure 3. NO and CO TPR spectra of S1 and S3 char samples.

particle surface18,19 as well as new active sites, and the reduction of NO was promoted by the decomposition of C(O) and the generation of new active sites on the particle surface; (2) the decomposition of C(O) led to the emission of CO. NO could react with CO under high temperature conditions, so the release of CO from the particle surface would accelerate the reduction reaction.20,21 Therefore, when the reaction occurred at a relatively low temperature (T ≤ 800 K), a preoxidative treatment would have little promoting effect on the reducibility of coal char. When the reaction occurred at a relatively high temperature (T ≥ 800 K), the preoxidizing char samples expressed a higher reducing ability, especially S2 and S3, which were oxidized to a low conversion degree (0.15−0.22). The reaction between coal char and NO is a heterogeneous reducing reaction at a low temperature, and the reducing process consists of heterogeneous and homogeneous reactions when the temperature is high enough.20,21 Thus, with the increase in the reaction temperature, the reaction intensity and complexity increased gradually. In an attempt to clarify the reaction characteristics, both the NO TPR spectra and CO emission spectra corresponding to the conversion ratio were employed for the investigation. There were significant differences in the curve profiles between raw char S1 and preoxidized samples S2−S8, and thus the NO TPR spectra and CO emission spectra of S1 and S3 were used as examples to analyze the reducing process of each SH sample. The NO consumption spectra and the CO emission spectra of S1 and S3 during the whole TPR process are shown in Figure 3. As shown in Figure 3a,b, the NO reduction spectra contains four sections, a, b, c, and d. There were three points that could

express the characteristics of the reduction reactions in Figure 3, which were P1, P2, and P3. Section a represents the adsorption and desorption section,2 and the NO molecules decomposed into nitrogen and oxygen atoms before attaching to the char particle surface, forming nitrogen-containing complexes C(N) and oxygen-containing complexes C(O), but most of the NO molecules directly combined with surface active sites, forming C(NO). The metastable structure C(NO) was very prone to being resolved from the particle surface and releasing NO molecules again. There was no significant variation in the NO concentration of section a, so it could be assumed that the reversible physical adsorption dominated this process. When the reaction temperature reached P1, the heterogeneous reducing reaction between the NO and char particles occurred gradually, and the NO molecules began to react with the active sites and C(O) on the char particle surface, releasing CO, CO2 and N2, as reaction products at section b (Figure 3a,b).4 Due to the decomposition of C(O) in each sample, the emission of CO molecules occurred gradually and the NO concentration decreased simultaneously. However, the reaction temperature was not high enough for the occurrence of a homogeneous reaction between NO and CO, and the promoting effect of CO was not a major process under the present reaction conditions.22 In each case, there were apparent inflection points in the TPR curves of the char samples after pretreatment, when the temperature exceeded the value of this point, and the absolute value of the slope of the TPR curves clearly increased. The RMN area is highlighted in Figure 3b,d, and it can be observed from the curve in Figure 3b that the rate D


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Energy & Fuels of NO consumption increased significantly during the front stage of section c. This increase indicated that the variation in the CO curve profile resulted from the acceleration of the NO consumption rate. It could be assumed from this phenomenon that with the increased reaction temperature, CO began to be involved in the reduction of NO and led to the acceleration of the reduction rate in section c. This hypothesis was consistent with the XPS results of S1 and S3 that the total amount of the C(N) decreased significantly when the reaction temperature exceeded 1173 K. After the reaction temperature exceeded P3, the NO concentration remained unchanged and it reached its minimum value, indicating that the NO reduction rate increased to the maximum value for each char in this section. With the further increase in the reaction temperature, the total number of active sites and complexes on the particle surface limited the further increase of the NO reduction reaction rate. The CO emission spectra during the TPR process are shown in Figure 3c,d, and the whole spectra could also be divided into four parts due to the variation in the curve profile. In section A, the attachment of the NO molecule to the surface of the char particles led to the formation of C(N) and C(O), but the temperature in this section was not high enough for the decomposition of surface complexes, especially C(N), which was more stable than C(O), and thus there was no CO tested in section A. With the increase in the reaction temperature, the decomposition of C(O) and the emission of CO began to occur at the starting point (P1′). The amount of CO increased with the increased reaction temperature, and the emission spectra should be smooth, as shown in the TPD results of each preoxidized sample. However, there was a flat zone in the CO emission spectra of all the preoxidized samples, especially S2− S6. This phenomenon was assumed to be the equity of the CO generation rate and the consumption rate in this temperature range (RMN), indicating that CO started to react with the NO when the temperature reached point M (approximately 1200 K), leading to an increase in the NO consumption rate in RMN. The consumption rate of CO became even faster than the generation rate under this condition. Subsequently, the decomposition rate of C(O) increased and became faster than the CO consumption rate when the temperature exceeded point N, and then the CO concentration kept increasing until the end of section B. The third section represented a rapid reaction stage in which NO reacted with both CO and the char samples rapidly, and the consumption rate of CO by NO was obviously faster than the CO formation rate. Thus, the CO concentration decreased rapidly with the increase in the reaction temperature, and the reduction of the NO concentration could be attributed to the reaction between CO and NO. When the temperature was high enough (approximately 1600 K), the TPR process reached an equilibrium stage, indicating that the sample reduction capacity reached saturation, all the CO released from the decomposition of C(O) was consumed by the NO molecules, and the NO reduction rate of each SH sample reached the maximum value at P3′. Based on the TPR results of each sample, the main NO reducing area (sections b and c in Figure 3a,b) were employed for further investigation. The NO TPR curve represented the NO reducing rates at various reaction temperatures. The reduction rates of each sample under 1000, 1200, 1400, and 1600 K (maximum reduction rate) were calculated and are shown in Figure 4, and then the effects of reaction temperature and the conversion ratio on char reduction rate could be

Figure 4. Relationship between conversion ratio and reduction rate under different reaction temperatures (1000, 1200, 1400, and 1600 K).

observed. The reduction rate of raw char S1 was much lower than those of preoxidized samples S2−S8 under the same reaction condition; the absolute values of the reduction rate decreased significantly with the increased conversion degree, showing that preoxidized char samples with an appropriate extent of 0.15−0.22 could enhance the reducibility most significantly. This phenomenon could be attributed to the enhancement of reducing reactivity after oxidative treatment under an oxidizing atmosphere. With the increase in the char conversion degree, more C(O) was generated on particle and C(O) had a positive effect on coal char reducing reactivity;2 thus the reduction rate increased gradually. The further increase in char conversion degree led to mass consumption of fixed carbon content; thus the total number of active sites and complexes on the particle surface decreased significantly and limited the further increase of the NO reduction reaction rate. Clearly, the maximum reduction rate of each sample under different reaction temperatures was different, and the values of S2 and S3 were almost the largest among all the samples, demonstrating that the reducing reactivity of the low conversion degree samples (0.15−0.22) was stronger than it was in raw char and high conversion degree samples under different temperatures. 3.1.2. Characteristic Parameters of TPR Curves. The values of P1, P2, and P3 are summarized in Table 4. The results Table 4. Characteristic Parameters of NO TPR Spectra sample

temp of P1 (K)

S1 S2 S3 S4 S5 S6 S7 S8

1014 ± 5 755 ± 5 850 ± 5 830 ± 5 845 ± 5 840 ± 5 850 ± 5 830 ± 5

temp of P2 (K) 1260 1190 1180 1190 1175 1180 1150 1140

± ± ± ± ± ± ± ±

5 5 5 5 5 5 5 5

temp of P3 (K) 1580 1600 1600 1610 1580 1590 1280 1300

± ± ± ± ± ± ± ±

5 5 5 5 5 5 5 5

showed that the initial reaction temperature of raw char S1 was much higher than those of the preoxidized samples, such as S2−S8. This phenomenon might be attributed to the increase in the amount of Cf and oxygen-containing complexes on the particle surface, making the carbon atoms on the particle surface become more reactive and decreasing the activation energy of the reducing reaction. C(O) and new active sites E


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apparently higher than the values for S2−S8, showing that the C(O) generated from the NO molecule attachment was more stable than the generated oxygen-containing complexes due to the combination between the active sites and O2. Each P2′ value was approximately identical for S2−S6 and higher than that of raw char, and this phenomenon could be attributed to the fact that the char samples that were oxidized under the same atmosphere and temperature might have similar surface chemical structures. However, the P2′ values for S7 and S8 were quite different from those of other samples and lower than those of both raw char and other preoxidizing samples. This phenomenon might have resulted from the enrichment of the ash content, which was a catalyst for the reaction between NO and carbonaceous materials,17 or the surface chemical structure of the sample in the burnout stage was quite different from that of the early and medium stages. Further investigation was necessary to clarify this phenomenon. S2 and S3 had the highest P3′ values among all the samples, indicating that the amount of CO released by S2 and S3 was largest and the reducing abilities of S2 and S3 were better than those of other samples. 3.1.3. Surface Chemical Behavior of Each SH Sample. Surface active sites combined with oxygen molecules generating oxygen-containing complexes on char particle surface, and these complexes were the precursors of CO and CO2. One functional group could release one gaseous molecule (CO or CO2), when the reaction temperature was high enough for the decomposition of C(O). Therefore, the total amount of surface active sites could be calculated from the integration of the CO and CO2 emission spectra during the temperature-programmeddesorption (TPD) process for each char sample,17,24 and the surface active site TPD measurement results of each sample are shown in Figure 5. The overall tendency in Figure 3 conveyed that the preoxidative treatment led to significant enhancements in the NO consumption, implying that the total amount and reactivity of the active site were apparently enhanced after preoxidative treatment. In addition, the results in Figure 5b show that the value of the maximum NO reducing ability demonstrated a linear relationship corresponding to the amount of active sites that is shown as a red line, indicating that the reducing abilities per unit quantity of active sites on the surface oxidized char were almost identical. It could be concluded that the NO consumption per unit mass of char increased significantly after preoxidative treatment, which could be attributed to the enhanced total amount and reducing reactivity of the active sites on the particle surface. Thus, more

were generated by the desorption of some metastable oxygencontaining complexes, while only vacant Cf was present on the S1 surface. Previous studies conveyed that the Cf′ generated by the decomposition of C(O) was more reactive than the original vacant Cf on the surface of carbonaceous materials.23 Most of the active sites on the S1 surface were vacant, while the active sites on the S2−S8 surfaces were combined with oxygen atoms, forming C(O) after pretreatment. The C(O) on the particle surface would decompose during the TPR process and generate new vacant active sites that were more reactive. Thus, the values of P1 for S2−S8 were apparently lower than that for S1, and the reaction activation energy of S1 was larger than those of other samples. When the temperature was high enough, CO started to react with NO and promoted the conversion degree of NO. The increased slope in section c (Figure 3a,b) could be attributed to this phenomenon, and the preoxidative treatment had no significant effect on the occurrence of this homogeneous reaction. Based on the results above, it could be concluded that preoxidized SH char samples with an appropriate extent of 0.15−0.22 could enhance the reducibility and reactivity significantly. The CO emission curve of S3 during the TPD process is also shown in Figure 3d, and the curve profile of preoxidized char was quite different from that of S1. In an attempt to investigate the emission regularity of CO, the specific values of P1′, M, P2′, and P3 are summarized in Table 5. The results in Table 5 Table 5. Specific Value of Data Points in CO Emission Spectra sample S1 S2 S3 S4 S5 S6 S7 S8

temp of P1′ (K) 860 825 820 800 800 760 740 720

± ± ± ± ± ± ± ±

5 5 5 5 5 5 5 5

temp of M (K) 1180 1165 1170 1160 1160 1140 1130

± ± ± ± ± ± ±

5 5 5 5 5 5 5

temp of P2′ (K) 1250 1400 1410 1400 1400 1400 1053 1043

± ± ± ± ± ± ± ±

5 5 5 5 5 5 5 5

temp of P3′ (K) 1580 1610 1600 1560 1570 1590 1373 1273

± ± ± ± ± ± ± ±

5 5 5 5 5 5 5 5

demonstrated that the starting temperature for the CO release from the char surface decreased gradually with the increasing conversion ratio, and thus it could be assumed that the thermal stability of C(O) on the particle surface decreased with the increased conversion degree.17 The value of P1′ for S1 was

Figure 5. Relationship between surface behavior and reducing ability. (a) Total amount of active sites on the surface of each sample; (b) relationship between conversion ratio and the maximum reducing ability of each sample. F


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Figure 6. XPS spectra of each S1 and S3 sample obtained after different partial TPR process. (a) XPS spectra of S1; (b) XPS spectra of S3.

Figure 7. Variation of surface element distribution with the increase of TPR temperature. (a) O 1s/C 1s; (b) N 1s/C 1s.

reducing reactivity of the active sites on the char surface significantly. 3.2. Evolution of Surface Chemical Structure during the TPR Process. 3.2.1. Variations in the Surface Element Content on the Particle Surface during the TPR Process. The XPS spectra of char samples expressed typical asymmetric peaks in the C 1s, O 1s, and N 1s regions, and the evolution of the surface elements (C, O, and N) could be calculated using the XPS spectra. The results shown in Figure 5b show that the reducing activities of Cf on the surface of each preoxidized sample were basically identical, and a previous study on the evolution of surface chemical structures from SH coal char during preoxidative treatment showed that the samples that were oxidized to different conversion degrees under the same experimental condition had a similar surface chemical structure and distribution of active sites (Cstr/Cwea). Therefore, raw char S1 and typical oxidized char S3 were employed as the XPS analysis examples, and the XPS spectra of S1 and S3 are shown in Figure 6. The whole TPR process for S1 and S3 was divided into six parts, and the distribution of surface elements (C, O, and N) of each partial TPR sample is shown in Figure 7. It can be observed from the element content ratio below that, with the increase in the partial TPR temperature, there were no significant variations in the O 1s/C 1s ratios, indicating that the generation rate and decomposition rate of C(O) were approximately identical until the temperature reached 1573 K. When the reaction temperature exceeded 1573 K, the O 1s/C 1s values decreased significantly and this phenomenon was consistent with the CO emission spectra during the TPR process shown in Figure 3. In Figure 3, all the CO molecules or the C(O) precursors were consumed when the reaction

NO molecules could be reduced per unit quantity of active sites on preoxidizing char samples than on raw char. The slope of the linear fitting curve in Figure 5b remained unchanged, showing that the reducing activities of the active sites on the surfaces of S2−S8 were almost identical, and S2 and S3 had the maximum reducing abilities as a result of having the largest amounts of active sites. This phenomenon conveyed that the reaction reactivity of the SH char samples with different conversion ratios could be concluded through the equation shown in Figure 5b, and there was no significant conversion degree effect on the reducing reactivity per unit amount of surface active sites. The differences in the reducing reactivity and ability for each sample could be attributed to the total amount of vacant active sites and specific chemical structures of oxygen-containing complexes on the char particle surface.2 Figure 5 shows that after the activating treatment under the O2 atmosphere, more active sites were generated on the particle surface and the reducing reactivity of new active sites was stronger than the original active sites. With the increased conversion degree, the total number of active sites on the particle surface increased significantly at first, but it apparently decreased when the conversion degree reached burnout stage due to the large consumption of fixed carbon content.15,24−28 The total number of active sites on the surfaces of S2 and S3 was the maximum among all the samples, and this phenomenon was quite consistent with the TPR results. Therefore, it could be concluded that oxidizing char samples to a low conversion ratio (0.15−0.22) would promote the generation of surface active sites and C(O) on the particle surface and enhance the G


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Figure 8. Distribution of nitrogen-containing complexes on char particle surface during TPR process. (a) S1; (b) S3.

summarized in Figure 8. These results expressed the relative amounts of C(N) on the particle surface as a function of the partial TPR temperature for S1 and S3, respectively. The relative amounts of N-6 and N-5 showed an obvious downward trend with the increase in the partial TPR temperature, and the relative amounts of N-Q and N-X had a significantly increasing trend, indicating that N-Q and N-X were more stable than N-5 and N-6 under high temperature conditions. N-X possibly represented inorganic nitrogen-containing materials and other forms of nitrogen on the particle.29 The absolute value of N-X always remains unchanged during the reaction, and then with the increase in the TPR temperature, the relative amount of NX increased, apparently indicating that the total amount of organic complexes (N-6, N-5, and N-Q) decreased on the char particle surface. This phenomenon might result from the reduction of more NO molecules by CO before they are diffused to the particle surface to form C(N), and the interaction between nitrogen-containing complexes forming N2 occurred directly. Therefore, organic C(N) on the particle surface decomposed rapidly under high temperature conditions. Based on the results above, it could be concluded that N-Q was the most stable organic complex on the char surface, and when the temperature exceeded 1173 K, the interaction between C(N) forming N2 and the reaction between CO and NO occurred. 3.3. Primary Reaction Pathway of NO Reduction under Different Temperature Conditions. In an attempt to clarify the primary reaction path of the NO-reducing reaction, the curves were divided into four sections and are shown in Figure 3. When SH samples were placed under an NO atmosphere, the reversible physical adsorption occurred immediately, and with the increase in the ambient temperature around the samples, the chemical adsorption gradually occurred. When the temperature regime was in the first section, the NO concentration decreased slightly starting at approximately 400 K, indicating that an irreversible chemical adsorption of NO on the char particle surface occurred, but the decrease in the NO concentration was not obvious, implying that the reaction rate was very low. It can be observed from the CO emission spectra in Figure 3c,d that almost no CO was released within this temperature range, demonstrating that the temperature was not high enough for the decomposition of both C(N) and C(O) on the particle surface. Thus, it could be assumed that the reversible physical adsorption dominated the reaction process11−13 and the chemical adsorption was unapparent. Therefore, the primary reaction path in the first section might be as follows:

temperature exceeded P3′, and the value of P3′ was approximately equal to 1573 K, indicating that most of the C(O) on each particle would decompose into gaseous products and active sites and then be consumed by NO molecules under high temperature conditions (T ≥ 1573 K). The N 1s/C 1s values apparently decreased when the temperature was higher than 1173 K, and the N 1s/C 1s value reached the maximum value when the temperature of the partial TPR reached 1173 K, indicating that when the temperature was lower than 1173 K, the generation rate of C(N) was higher than its decomposition rate and C(N) gradually accumulated on the surface of char particles. When the reaction temperature was higher than 1173 K, a significant decrease in N 1s/C 1s was observed in Figure 7b, indicating that more C(N) was consumed than was generated on the particle surface. The increase in the temperature was beneficial for the promotion of chemical adsorption, but the XPS results simply expressed the opposite trend, demonstrating that the probability of a combination between NO molecules and char particles was reduced significantly, and a large number of NO molecules were consumed by CO before they spread to the surface of the char particles. The curves in Figure 7 express similar trends, indicating that the distribution and variation of surface elements for both raw char and preoxidized char were similar during the TPR process. In addition, the value of the inflection point (at approximately 1173 K) in Figure 7b was basically consistent with the value of P2 (approximately 1200 K) in Figure 3b and that of section A in Figure 3d, indicating that the variation in the NO consumption rate in Figure 3b resulted from the participation of CO in the reduction of NO and the high consumption of C(N) on the particle surface. Therefore, based on the TPR and XPS results, it could be assumed that 1173 K (second point) was the starting point at which CO participated in the reduction of NO. 3.2.2. Evolution of Nitrogen Functional Groups during the TPR Process. In an attempt to analyze the variations in the nitrogen-containing complexes during the TPR process, PeakFit v4.12 was employed for the deconvolution of the high resolution XPS spectra. There were four peaks in the N 1s spectra29,30 as follows: peak I (398.8 ± 0.1 eV), nitrogen atoms in N-6 groups; peak II (400.4 ± 0.1 eV), nitrogen atoms in N-5 groups; peak III (401.4 ± 0.1 eV), nitrogen atoms in N-Q groups; and peak IV (402.0−404.0 eV), nitrogen atoms in N-X groups. After the deconvolution treatment of each N 1s core-level XPS spectrum, the distribution of C(N) could be calculated by integrating the corresponding curve area, and the results are H


Article

Energy & Fuels NO + Cf ↔ C(NO)

(1)

NO + 2Cf → C(O) + C(N)

(2)

between NO and char under high temperature (≥1600 K) conditions consisted of heterogeneous and homogeneous reactions. Additionally, due to fact that the fixed carbon content of the unit mass sample was different, the reducing ability of each sample with a different conversion degree was different. Based on the analysis of the char reducing reactivity and ability, the samples became the most reductive when the temperature reached 1600 K. Therefore, it could be speculated that the reducing reaction mechanism depended on the reaction temperature, and the primary reaction path of the heterogeneous reaction zone (T ≤ 1200 K) could be summarized as follows:

With the increased reaction temperature, the chemical adsorption rate of NO increased, and more C(O) and C(N) were formed on the particle surface. With the increased reaction temperature, the complexes were gradually involved in the reaction between NO and char. As shown in Figure 3c,d, the decomposition of C(O) occurred when the temperature exceeded P1′. There were vacant active Cf sites on the surface of S1, but the P1′ value for S1 was much higher than those of S2−S8, indicating that new active sites were more reactive than the original ones,16 and the thermal stability of the surface complexes formed by NO chemical adsorption was higher than that formed by O2 chemical adsorption. The spectra in Figure 3a,b show that NO started to react with preoxidative samples at a lower temperature than S1, indicating that the improvement in the char reducing reactivity could be attributed to oxygencontaining complexes after preoxidative treatment and the new vacant active sites generated by the desorption of those complexes. Then, the primary reaction path between NO and preoxidizing char in section b could be assumed as indicated below: C(O) → Cf ′ + CO/CO2

(3)

NO + 2Cf ′ → C′(O) + C′(N)

(4)

C′(N) + NO → N2 + C(O)

(5)

C(N) + NO → N2 + C(O)

(6)

(7)

2C′(N) → 2Cf ″ + N2

(8)

2NO + 2CO → 2CO2 + N2

(9)

(10)

NO + 2Cf → C(O) + C(N)

(11)

C(O) → Cf ′ + CO/CO2

(12)

NO + 2Cf ′ → C′(O) + C′(N)

(13)

C′(N) + NO → N2 + C(O)

(14)

C(N) + NO → N2 + C(O)

(15)

Owing to the fact that the reaction temperature was high enough, the physical adsorption process was inhibited by the chemical adsorption process, and there should be only trace amounts of NO molecules attached to the char surface, with most NO molecules breaking into atoms and forming C(O) and C(N) on the char particle surface. Additionally, CO began to react with NO under this condition. Therefore, the primary reaction mechanism of the multireaction zone (T ≥ 1200 K) might consist of the pathway below:

There were inflection points at the beginning of section c in the TPR curves from Figure 3b, indicating that, with further increases in the reaction temperature, the NO consumption rate increased significantly, and the average reaction rate of section c was larger than that of section b (as calculated through the slope of the curve). There were also inflection points in the CO emission spectra (point M), and the values of point M and P2 were approximately identical, indicating that the consumption rate of CO or C(O) was obviously faster than its generation rate, and the reduction rate of NO was obviously accelerated. Under this temperature condition, NO was consumed rapidly at the expense of the CO, and thus CO might have a promoting effect on the NO reduction reaction. This effect would occur through the direct homogeneous reduction of NO by CO on the particle surface of char samples, where char was the catalyst for this process as shown in eq 9.14,15 It is usually accepted that the desorption of C(N) might lead to the emission of N2 under high temperature conditions,16 and the interactions between C(N) molecules might also promote the NO reduction process as explained below:17 2C(N) → 2Cf ′ + N2

NO + Cf ↔ C(NO)

NO + 2Cf → C(O) + C(N)

(16)

C(O) → Cf ′ + CO/CO2

(17)

NO + 2Cf ′ → C′(O) + C′(N)

(18)

C′(N) + NO → N2 + C(O)

(19)

C(N) + NO → N2 + C(O)

(20)

2C(N) → 2Cf ′ + N2

(21)

2C′(N) → 2Cf ″ + N2

(22)

2NO + 2CO → 2CO2 + N2

(23)

4. CONCLUSIONS The reaction characteristics of SH raw char and preoxidized samples under an NO atmosphere were investigated by TPR and XPS, and the conclusions were as follows: (a) A preoxidative treatment could enhance the reducing reactivity of the SH char samples; S2−S8 were more likely to react with NO than raw S1. When the reaction reached the equilibrium stage, more NO could be consumed per unit mass of S2−S8 than S1. The Cf on S2−S8 expressed an almost identical reducing reactivity, and thus the total amount of Cf determined the reducing ability of each preoxidized sample. (b) There was a critical value for the preoxidized conversion degree for the SH samples (approximately 0.15−0.22), and when the char was preoxidized to the critical conversion degree,

The reduction reaction reached an equilibrium state at the final stage while the reaction temperature was high enough (T ≥ 1600 K). The NO concentration remained unchanged in this section, and the CO was completely consumed by NO under this condition. This phenomenon indicated that the reaction I


Article

Energy & Fuels

(5) Harding, A. W.; Brown, S. D.; Thomas, K. M. Release of NO from the Combustion of Coal Chars. Combust. Flame 1996, 107, 336− 350. (6) Tomita, A. Suppression of nitrogen oxides emission by carbonaceous reductants. Fuel Process. Technol. 2001, 71, 53. (7) Zhang, G.; Yamaguchi, T.; Kawakami, H.; Suzuki, T. Selective reduction of nitric oxide over platinum catalysts in the presence of sulfur dioxide and excess oxygen. Appl. Catal., B 1992, 1, L15. (8) Gupta, H.; Fan, L. S. Reduction of nitric oxide from combustion flue gas by bituminous coal char in the presence of oxygen. Ind. Eng. Chem. Res. 2003, 42, 2536. (9) Suzuki, T.; Kyotani, T.; Tomita, A. Study on the carbon-nitric oxide reaction in the presence of oxygen. Ind. Eng. Chem. Res. 1994, 33, 2840. (10) Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Heterogeneous reduction of nitric oxide on synthetic coal chars. Fuel 2005, 84, 2275− 2279. (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, 85. (12) Rosas, J. M.; Rodriguez-Mirasol, J.; Cordero, T. NO reduction on carbon supported chromium catalysts. Energy Fuels 2010, 24, 3321. (13) Chan, L. K.; Sarofim, A. F.; Beér, J. M. Kinetics of the NO carbon reaction at fluidized bed combustor conditions. Combust. Flame 1983, 52, 37. (14) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. NO and N2O decomposition over coal char at fluidized bed combustion conditions. Combust. Flame 1994, 99, 499. (15) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric oxide reduction by char and carbon monoxide: Fundamental kinetics of nitric oxide reduction in fluidized bed combustion of coal. Fuel 1985, 64, 1306. (16) Yang, J.; Mestl, G.; Herein, D.; Schlögl, R.; Find, J. Reaction of NO with carbonaceous materials: 2. Effect of oxygen on the reaction of NO with ashless carbon black. Carbon 2000, 38 (5), 729−740. (17) Laine, N. R.; Vastola, F. J.; Walker, P. L. The importance of active surface area in the carbon-oxygen reaction. J. Phys. Chem. 1963, 67 (10), 2030−2034. (18) Mühlen, H. J. Factors Influencing the ASA-Determination of Low Temperature Chars: Particle Size, Mineral Matter and Rank. Fuel Process. Technol. 1990, 24, 285−290. (19) Xu, K.; Hu, S.; Su, S.; et al. Study on Char Surface Active Sites and Their Relationship to Gasification Reactivity. Energy Fuels 2013, 27, 118−125. (20) Watanabe, H.; Okazaki, K. Effect of minerals on surface morphologies and competitive reactions during char gasification in mixtures of O2 and CO2. Proc. Combust. Inst. 2015, 35, 2363−2371. (21) Garcia, P.; Molina, A.; Mondragon, F. Desorption activation energy distribution function of nitric oxide chemisorbed on carbonaceous materials at 373K. Carbon 2005, 43, 1445−1452. (22) Zhang, Y. C.; Zhang, J.; Sheng, C. D.; et al. X-ray photoelectron Spectroscopy (XPS) investigation of nitrogen functionalities during coal char combustion in O2/CO2 and O2/Ar atmospheres. Energy Fuels 2011, 25, 240−245. (23) Xiao, B.; Boudou, J. P.; Thomas, K. M. Reactions of nitrogen and oxygen surface groups in Nanoporous Carbons under inert and reducing atmospheres. Langmuir 2005, 21, 3400−3409. (24) Huffman, W. P. The Importance of Active Surface Area in the Heterogeneous Reactions of Carbon. Carbon 1991, 29 (6), 769−776. (25) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of Uncatalyzed and Catalyzed Reaction. Energy Fuels 1996, 10, 158. (26) Teng, H.; Suuberg, E. H. Chemisorption of nitric oxide on char. 2. Irreversible carbon oxide formation. Ind. Eng. Chem. Res. 1993, 32, 416−423. (27) Teng, H.; Suuberg, E. H. Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption. J. Phys. Chem. 1993, 97, 478−483.

the char reached the maximum reducing capacity and reactivity under high temperature conditions (T ≥ 1000 K). (c) With the increase in the reaction temperature, the distribution of N-6, N-5, and N-Q indicated that N-Q was the most stable organic structure of C(N) on the SH char particle surface. The N 1s/C 1s value clearly decreased under high temperatures, showing that reactions 21 and 22 occurred when the reaction temperature reached 1200 K. Prior to the diffusion of the NO molecules to the char surface, massive NO molecules were reduced by CO when the reaction temperature was high enough (T ≥ 1200 K). (d) Based on the TPR and XPS results, the primary reaction pathway under different temperature conditions could be summarized. The heterogeneous reaction between Cf/C(O) and NO was the primary reaction pathway of NO consumption at low temperatures. With the increase in the reaction temperature (T ≥ 1173 K), homogeneous reaction between CO and NO occurred and started to dominate the reducing process. When the reaction temperature reached 1600 K, the reducing ability of all the samples (S1−S8) reached the maximum, and 1600 K was the optimum reaction temperature for NO reduction by SH coal char.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01291. XPS spectra of S1 and S3 after different partial TPR processes (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*(R.S.) E-mail: [email protected]; Tel: +86 451 8641 3231 802; Fax: +86 451 8641 2528. *(T.M.I.) E-mail: [email protected]; Tel: +20 01224745463; Fax: +20 0226829366. ORCID

Rui Sun: 0000-0002-6542-2429 Ya-Ying Zhao: 0000-0002-0387-3988 Tamer M. Ismail: 0000-0001-8324-6759 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the NSFC (No. 51476046 and No. 51536002) in the experimental campaign is gratefully acknowledged. REFERENCES

(1) Yang, J.; Mestl, G.; Herein, D.; Schlögl, R.; Find, J. Reaction of NO with carbonaceous materials: 1 Reaction and adsorption of NO on ashless carbon black. Carbon 2000, 38, 715−727. (2) Fan, W. D.; Li, Y.; Xiao, M. O2 Concentration on the Reduction Reaction of NO by Char at High Temperature. Ind. Eng. Chem. Res. 2013, 52, 6101−6111. (3) Fan, W. D.; Lin, Z. C.; Kuang, J. G.; Li, Y. Y. Impact of air staging along furnace height on NOx emissions from pulverized coal combustion. Fuel Process. Technol. 2010, 91, 625. (4) Yang, J.; Sanchez-Cortezon, E.; Pfänder, N.; Wild, U.; Mestl, G.; Find, J.; Schlögl, R. Reaction of NO with carbonaceous materials: 3 Influence of the structure of the materials. Carbon 2000, 38, 2029− 2039. J


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Investigation of the NO Reduction Characteristics of Coal Char at

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