H2O combustion process

Fuel 253 (2019) 1424–1435

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Full Length Article

Effects of reaction condition on NO emission characteristic, surface behavior and microstructure of demineralized char during O2/H2O combustion process ⁎

T

⁎

Zhuozhi Wanga, Yaying Zhaoa,b, Rui Suna, , Yupeng Lia, , Xiaohan Renc, Jie Xud a

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 c Institute of Thermal Science and Technology, Shandong University, Jinan 250061, China d School of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063009, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coal char O2/H2O combustion Surface active site Microstructure NO

Surface behavior and structural characteristics of char particles generally play a vital role in affecting char NO emission characteristics during combustion process. A typical bituminous Shenhua from northwest China was employed for demineralization in this research. The demineralized coal without catalytic interference of alkali metal salts was employed as the experimental sample. Moreover, by means of the utilization of isothermal combustion test, Temperature Programmed Desorption (TPD) and Raman spectrometer, the conversion ratio of nitrogen content in char to NO, as well as the evolution of char surface behavior and micro-structure in the process of O2/H2O combustion under different conditions were determined. With the increase of reaction temperature (Tr) and O2 concentration, more surface active sites (Cf), oxygen containing functional groups (C(O)) and small aromatic ring structures generated on char particle surface inhibiting the emission of NO during O2/H2O combustion process. Under identical reaction conditions (Tr, O2 and H2O concentration), the partially oxidized chars with moderate burnout degree (Xc ≈ 0.3) had the largest amount of Cf on particle surface. On the other hand, the amount of Cf and small aromatic ring structures first increased and then decreased as the H2O concentration enhanced. When the Cf amount and the value of I(Gr+VL+Vr)/ID reached the maximum at the H2O concentration of approximately 8.5 vol%, the conversion ratio of char-N to NO reached the minimum value. Meanwhile, the increase of H2O concentration initially accelerated the NO reduction rate and this positive effects gradually diminished when the H2O concentration exceeded the critical value (8.5 vol%). Thus, the optimal combustion condition for char combustion with the lowest NO conversion ratio in this study was: 1473 K, 30% O2 and 8.5 vol% H2O.

1. Introduction

combustion mechanisms and the emission characteristics of NO during combustion process [5]. Devolatilization occurs immediately when the pulverized coal particles enter the reaction zone, generating volatiles and char particles. However, the combustion rate of char particles is apparently lower than that of volatile materials, so the reactions between char particles and oxidizing agents are generally the limiting step in coal combustion [6]. The reactivity of coal char plays a crucial role in investigating coal utilization efficiency, and NOx emission characteristics during coal combustion process [2]. Because of the longer contact time between char particles and flue gas, a large amount of NOx in the flue gas could be reduced by the char particles through secondary reactions [7]. Previous researches indicated that, the reducibility of coal char

Nowadays, oxy-fuel combustion is considered to be a promising technology for the reduction of CO2 emission while producing electricity [1]. Subsequently, the combustion of coal particles occurs in a mixture of O2 separated from air and a portion of recycled flue gas (primary H2O and CO2) instead of air in the coal boilers. Moreover, O2/ H2O combustion is considered to be a novel and promising coal combustion technology for the next generation oxy-fuel combustion [2]. The combustion of coal particles occurs in the atmosphere consisting of O2 and H2O during O2/H2O combustion process instead of the mixture of recycled flue gas (mainly CO2 and H2O) and O2 [3,4]. The variations in reaction atmosphere would lead to significant variations of the

⁎

Corresponding authors. E-mail addresses: [email protected] (R. Sun), [email protected] (Y. Li).

https://doi.org/10.1016/j.fuel.2019.05.119 Received 27 January 2019; Received in revised form 24 April 2019; Accepted 22 May 2019 Available online 29 May 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 Ultimate and Proximate analyses of the sample. Ultimate analysisa

Sample

Raw coal Raw coal char Demineralized coal Demineralized char a b c

Proximate analysisb

C

H

N

S

Oc

Moisture

Volatiles

Cfixed

Ash

79.75 96.81 80.03 96.73

4.18 0.43 4.50 0.55

0.71 1.23 0.96 1.56

0.30 0.35 0.26 0.18

15.06 1.18 14.25 0.98

9.78 0.81 0.19 0.76

28.12 1.05 32.11 1.56

59.44 93.27 67.14 97.17

2.66 4.87 0.56 0.51

wt% dry and ash free. wt% as received. calculated by difference.

the char combustion was achieved by means of the application of a high resolution gas analyzer. Subsequently, the partially oxidized chars with different conversion degrees (Xc) obtained from different reaction conditions were collected for subsequent TPD and Raman analysis to clarify the evolution characteristics of C(O) and microstructure during O2/H2O combustion. In summary, the correlation among NO emission characteristic, reaction condition (Tr, O2 and H2O concentration), char surface property and structural characteristic could be preliminarily determined.

particles was generally determined by the surface behavior and microstructural characteristic [8,9]. Reaction between coal char and H2O could lead to significant variations of char microstructure as well as surface behaviors [10,11]. Previous researchers conveyed that H2O molecules could decompose into hydrogen free radicals and hydroxyl groups under high temperature conditions, the attachment of hydrogen free radicals and hydroxyl groups would alter char reactivity [2]. The oxidizing hydroxyl groups could combine with vacant active sites forming C(O) on char particle surface. Because the hydrogen free radicals preferentially penetrated into char matrix and elevate the char aromatization, massive amorphous carbon (small and reactive aromatic ring structures) in char particles would be consumed [12]. This phenomenon indicated that the NOx emission mechanisms obtained from the combustion occurred under O2/inert gas atmosphere might be changed for the participation H2O. However, few researchers have conducted systemic investigations on the effect of reaction condition on NO emission characteristics of demineralized coal char during O2/H2O combustion process. NO is normally the primary component of NOx released in the combustion process of coal based fuel, and the nitrogen content in char particles is one source of nitrogen containing gaseous pollutants generated from coal combustion, accounting for more than 30% of the whole amount of NO released during O2/H2O combustion process [13]. Previous researchers conveyed that addition of H2O could alter NO emission characteristics during coal based fuel combustion process, and the contribution ratio of nitrogen in char to the total NO was promoted [14]. When the combustion occurred in a high O2 concentration atmosphere, the participation of H2O molecules could limit the NO emission [10]. This phenomenon might be attributed to the generation of reducing gaseous products which could inhibit the conversion of char-N to NO [2]. Previous research reported that the increase of Tr would promote the decomposition of H2O molecules and expressing negative effects on NO reduction, more NO would be released into the exhaust gas [15]. Consequently, the effect of reaction condition (H2O concentration, Tr and O2 concentration) on char surface behaviors and structural characteristics ought to be investigated systematically for better understanding of char reactivity and NO emission characteristics during O2/H2O combustion processes. Aiming to clarify and set up the correlation among NO emission characteristic, surface behavior and structural characteristic of the char during O2/H2O combustion process under different conditions, combustion reactions under different conditions (Tr: 1073, 1273 and 1473 K; O2 concentration: 2, 5, 10, 20 and 30%; H2O concentration: 1.2, 3.5, 8.5, 15 and 20 vol%) were carried out. On line measurement of the main gaseous products (e. g., CO2, CO, CH4 and NO) released from

2. Experimental 2.1. Char preparation Besides reaction condition, the reactivity and behavior of char is greatly influenced by alkali/alkaline ether metallic species (AAEM) that could be dispersed to atomic scales by functionalities in coal or char particles [10,16,17]. In order to eliminate the interferences of AAEM with the determination of the evolution characteristics of char surface behavior and microstructure during the combustion process, demineralization treatment of the raw pulverized coal (typical bituminous Shenhua coal) with a uniform particle size distribution (100–125 μm) was carried out. The demineralization procedures were as follows: the raw coal particles were first mixed with the reagent-grade aqueous solution of HCl (about 30%) and stirred for 4 h; after rinsing with ultrapure water, and the partially demineralized coal was subsequently mixed with the reagent-grade aqueous solution of HF (about 30%) and stirred for 4 h; after rinsing with ultra-pure water, and the partially demineralized coal was subsequently mixed with the reagent-grade aqueous solution of HCl (about 30%) and then stirred for 3 h; finally rinsing with ultra-pure water for 1 h. After the demineralization treatment, the demineralized coal was collected and dried at 378 K for 12 h in a drying oven. Thorough devolatilization of the demineralized coal particles was carried out in a horizontal fixed-bed device under inert atmosphere (Ar) for 30 min at 1173 K [18]. The particle size of the sample basically remained approximately unchanged (100–125 μm) after the demineralization and pyrolysis treatment. Meanwhile, the approximate and ultimate analysis of the demineralized coal/char are shown in Table 1, the ash content determination of the raw coal is shown in Table 2. 2.2. Combustion test 2.2.1. NO emission characteristic and char reactivity The combustion experiments of the demineralized char sample were

Table 2 Ash content identification of the raw coal. Component

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

TiO2

P2O5

SO3

Percentage (%)

32.11

10.33

11.64

25.53

1.26

0.87

1.14

0.49

0.07

12.33

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Gas system

Horizontal reactor

Analyzer system

To vent Switch valve

Mixed

Heat Gas inlet tracing

To vent

Heat tracing

Thermal couple

Filter

Gas outlet

Mass flowmeter Deoxidation tube

Gas analyzer

Cooling water Ammeter and Controller Switch Voltmeter

O2

Laptop

Steam generator

Ar

Fig. 1. Schematic diagram of horizontal fixed-bed reaction system.

the higher reactivity, and the value of r0.3 could be calculated by Eq. (2):

conducted under different reaction conditions (reaction temperature: 1073, 1273 and 1473 K; O2 concentration: 2, 5, 10, 20 and 30%; H2O concentration: 1.2, 3.5, 8.5, 15 and 20 vol%), a total of 75 combustion tests were carried out in the reaction system shown in Fig. 1. The liquid ultra-pure water was injected by the injection pump into the heat tracing tube for vaporization, and then mixed with the mixture of O2 and Ar evenly. Subsequently, the reaction gas (O2, Ar and water vapor) was injected into the reactor at a flow rate of 1 L/min. Approximately 20 mg demineralized char sample was first mixed with 2 g quartz sand to minimize interactions between the char particles [19], then employed for the combustion test each time. By means of a Fourier transform infrared spectroscopy gas analyzer (accuracy: 0.01%; resolution of Gigar interferometer: 8 cm−1; respond time: 0.1 s−1), the typical gaseous products (NO, CO, CO2 and CH4) generated during the reaction were recorded online for further investigation. The specific conversion ratios of ηNO, ηCO, and ηCH4 were clarified by the equation below:

ηi =

1000 × M ×

Q 60 × 22.4

r0.3 =

Q 60 × 22.4

× (CCO2 × 10−2 + CCO × 10−6) m×f

(2)

where CCO2 reflects the instantaneous reaction rate of CO2 when approximate 30% conversion ratio of char reached (%); CCO represents the instantaneous reaction rate of CO when approximate 30% conversion ratio of char reached (ppm). 2.2.2. Partially oxidized char preparation In order to identify the relationship among amount of C(O), Xc and reaction condition, the demineralized char was oxidized to different Xc under different conditions, and the specific Xc of the partially chars obtained from the typical conditions (O2: 30%; Tr: 1073, 1273 and 1473 K; H2O: 1.2, 8.5 and 15 vol%) are as shown in Table 3. After thorough demineralization and devolatilization treatment, trace amounts of ash content or volatile materials were remained and the total amount of carbon content in the char sample was very high (≥97%). Consequently, almost all the mass loss of char during the combustion process could be attributed to the generation of C containing gaseous products (CO2, CO and CH4). Then, the conversion ratio of each partially oxidized sample could be approximately calculated through the difference in sample weight before and after the reaction.

t

× ∫0 (Ci × 10−6)dt

m×f

1000 × M ×

(1)

where t reflects the reaction time (s); M represents the molar mass of nitrogen/carbon element (g/mol); Q is the volumetric flow rate of the reaction gas (L/min); Ci reflects the specific concentration of each type gaseous product (i: NO, CO or CH4) in the downstream gas recorded by the gas analyzer (ppm); m is the specific mass of the char sample employed for the test each time (mg); f reflects the percentage of N and C content in the char sample obtained from the ultimate analysis. Samples at initial stage of the reaction are generally more representative, so the consumption rate of carbon content (calculated from the emission rate of CO and CO2) in the chars with 30% conversion degree was employed as the reactivity index (r0.3) [2]. The greater value of r0.3 means the rapider reaction rate of char particles illustrating

2.3. TPD test Oxygen containing functional group generated from the attachment of oxygen atoms to vacant active sites on the surface of char particles, and the total amount of C(O) could be calculated quantitatively through the desorption amount of the gaseous products (CO and CO2) generated

Table 3 Xc of partially oxidized chars obtained from typical conditions. Tr

H2O concentration: 1.2 vol%

1073 K 1273 K 1473 K

0.13 0.3 0.29

0.33 0.41 0.47

H2O concentration: 8.5 vol% 0.64 0.53 0.56

0.72 0.62 0.65

0.15 0.31 0.34

0.31 0.4 0.55

1426

H2O concentration: 15 vol% 0.46 0.54 0.65

0.57 0.66 0.75

0.12 0.28 0.32

0.33 0.39 0.52

0.45 0.55 0.31

0.62 0.69 0.68

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reactive aromatic structures, as well as an indicator of char reactivity.

during TPD process [20]. Meanwhile, TPD method is an effective way to clarify the surface behavior of carbonaceous material [21,22], and the partially oxidized chars obtained from different reaction conditions were placed in the horizontal reactor in O2 atmosphere at 573 K for 24 h to ensure that all the vacant Cf were adsorbed to saturate [20]. Subsequently, the fully adsorbed chars were employed for the TPD test, so the total amount of Cf and C(O) on the surface of each sample could be identified. Approximately 20 mg of dried partially oxidized char sample was used for the TPD test each time, and the reactor was heated from the room temperature to 1723 K under argon (99.999%) atmosphere at a heating rate of 5 K/min. The gaseous products (CO and CO2) released from char particles was recorded online by a SIGNAL IR gas analyzer (CO, measure range: 0–1000 ppm, measure precision: 0.1%; CO2, measure range: 0–1000 ppm, measure precision: 0.1%; NO, measure range: 0–1000 ppm, measure precision: 0.1%). Trace amount of gaseous products were tested during the Blank TPD test, indicating that the reaction system had a negligible interference with the TPD results. Additionally, the parallel test of each sample was carried out to examine the experimental repeatability, and the results of two repeated experiments of each char sample were approximately identical indicating that the measurement error and experimental uncertainty was acceptable. CO/CO2 amount of per unit mass partially oxidized char sample could be determined by the equations below:

nCO =

nCO2 =

1000 ×

Q 60 × 22.4

Q 60 × 22.4

3.1. NO emission characteristics under different reaction conditions Aiming to clarify the effects of reaction condition (Tr, H2O concentration and O2 concentration) on NO emission characteristics during the utilization of coal char, the combustion tests of the demineralized coal char were carried out. The conversion ratio of nitrogen content in char to NO as a function of H2O concentration and O2 concentration at different combustion temperatures (1073, 1273 and 1473 K) are summarized in Fig. 2(a), (b) and (c) respectively. When H2O concentration was remained unchanged at a typical H2O concentration (about 8.5 vol %), the relationship between NO emission characteristics and O2 concentration at different temperatures are investigated and illustrated in Fig. 2(d). 3.1.1. NO emission changing with reaction atmosphere (O2, H2O concentration) The NO emission characteristics from the combustion of demineralized char particles under different O2 concentrations are illustrated in Fig. 2. In the O2 concentration range of 2–30%, the conversion ratio of NO decreased gradually with the increasing O2 concentration. It could be concluded that, when H2O was added into the gas flow, the O2 addition is likely to enhance the char surface reactivity and inhibit the conversion of Char-N to NO. Previous researches reported that carbonaceous materials were more likely to react with O2 molecules than hydrogen free radicals and hydroxyl groups generated from H2O molecules, especially under low temperature conditions [15]. Hence, the enhancement of oxygen concentration might lead to the generation of more C(O) on char particle surface. The amount of C(O) was an indicator of char reactivity, and generally had positive effects on reducing NO into N2 by char particles under high temperature conditions [8,9]. The results summarized in Fig. 2(d) illustrated that, under relatively low oxygen concentration conditions (0–10%), the enhancement of O2 concentration could obviously promote the reduction of NO emission by char; when the O2 concentration exceeded 10%, the accelerating effects of further O2 addition on the NO consumption were not as significant as that in the low concentrations. This phenomenon could be attributed to the influence of oxygen concentration on the char surface behaviors in the process of combustion. Previous researchers revealed that the higher O2 concentration in the reaction zone, the larger amount of active site or oxygen containing functionality could be formed on the surface of char particles [9]. However, the enhancement of O2 concentration could generally strengthen the consumption of carbon content and functional groups in char particles. So it could be implied that the impact of the O2 concentration virtually illustrates a competition between the generation of functional groups on particle surface and the consumption of carbon content in char particles during the combustion process. Thus, in low oxygen concentrations, increasing the oxygen

t

× ∫0 (CCO × 10−6)dt (3)

m 1000 ×

3. Results and discussion

t

× ∫0 (CCO2 × 10−6)dt m

(4)

where nCO and nCO2 reflect the total amount of CO and CO2 released by per unit mass sample during the TPD process (μmol/g); Q represents the volumetric flow rate (L/min) of the Ar; t represents the reaction time (s) of the TPD tests; CCO and CCO2 reflect the specific concentration of CO and CO2 released during the TPD process (ppm); m is the mass of the sample employed for each test (mg). 2.4. Raman spectroscopy The Raman spectra of each sample was recorded by means of a Raman microscope system and the excitation source of the device was a 632.8 nm laser line (1.96 eV). All the Raman spectra were recorded in the range of 100–4000 cm−1. Previous researches reported that the Raman spectra range 800–1800 cm−1 could be divided into 10 peaks (see Table 4) [11,23]. The G, Gr, VL, Vr, D and S bands were necessary for identifying the structural characteristics of the char samples [24]. G and D bands represent the aromatic quadrant ring breathing and large aromatic ring structures (≥6), respectively [25]. Meanwhile, the Gr, VL and Vr bands generally reflect amorphous carbon structures with small aromatic ring structures (3–5 rings) [26]. Consequently, the ratio of area sum of Gr, VL and Vr to the area of D (I(Gr+VL+Vr)/ID) could be regarded as an index illustrating the relative amount of small and Table 4 The specific assignments of each Raman band [23,26]. Name

Position (cm−1)

Description

G1 G

1700 1590

Carbonyl group, C]O

Gr VL Vr D SL S Sr R

1540 1465 1380 1300 1230 1185 1060 960–800

3–5 rings aromatic ring structures, amorphous carbon Semi-circle breathing of aromatic ring structures, amorphous carbon Methyl group, semi-circle breathing of aromatic ring structures, amorphous carbon D band on highly ordered carbonaceous materials, CeC between aromatic ring structures, aromatic ring structures ≥ 6 rings Aryl-alkyl ether, para-aromatics Caromatic-Calkyl, aromatic (aliphatic) ethers, CeC on hydro-aromatic ring structures, hexagonal diamond carbon sp3, CeH on aromatic ring structures CeH on aromatic ring structures, benzene (ortho-disubstituted) ring CeC on alkanes and cyclic alkanes, CeH on aromatic ring structures

Graphite E22g , aromatic ring quadrant breathing, alkene C]C

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(a)

(b)

50

Tr = 1073 K Tr = 1273 K

Char N to NO (%)

40

Tr = 1473 K H2O concentration = 8.5 vol.%

30 20 10

0

5

10

15

20

25

30

35

O2 concentration (%)

(c)

(d)

Fig. 2. NO emission characteristics during O2/H2O combustion process. (a)–(c): ηNO with varying O2 and H2O concentration at reaction temperatures of 1073, 1273 and 1473 K; (d): ηNO with varying O2 concentration and temperature at H2O concentration of 8.5 vol%.

3.1.2. NO emission changing with reaction temperature The variation trends of the conversion ratio of NO with changing the oxygen and H2O concentration at the reaction temperatures of 1073, 1273 and 1473 K are summarized in Fig. 2. In all cases, with the increase of reaction temperature, less NO was measured in the downstream gas of the reactor. This phenomenon demonstrated that the enhancement of reaction temperature had a positive effect on inhibiting the nitrogen content in char particles converting into NO during the combustion process [28]. Additionally, the reduction of NO over demineralized char particles in the reactor would be kinetically intensified with elevating the reaction temperature, because the enhancement of reaction rate was generally larger than the decrease resulted from the shortened gas residence time at high temperatures [15]. Reactions between coal char and H2O molecules would occur at elevated temperatures. The gaseous products released in the gasification reaction generally consists of H2, CO and CH4, which might homogeneously reduce NO in the combustion zone [8]. Without H2O addition into the reaction atmosphere, there was no CH4 detected in the char combustion process at all the temperatures (1073, 1273 and 1473 K). When H2O participated in the combustion reaction under high temperature conditions, CH4 started to be detected in the downstream gas indicating the participation of H2O molecules in char combustion

concentration could enhance the reducibility of demineralized coal char more apparently, and the trend of experimental results under the three temperature conditions (1073, 1273 and 1473 K) was basically identical. Besides oxygen atoms, hydrogen free radicals and hydroxyl groups generated through the destruction of H2O molecules could also attach to particle surface, changing char surface behaviors [10,12]. The hydroxyl groups could combine with Cf on char surface forming C(O) and accelerating the destructing of some large aromatic ring structures initially with low reactivity into more reactive structures [27]. Moreover, the addition of external H2O might also reduce the contact probability between O2 and char particles, leading to a kinetically slower generation of C(O) which could subsequently participate and accelerate the consumption of NO in the atmosphere. Thus, the externally added H2O in the process of char combustion had both positive and negative effects on char reactivity, and there ought to be a critical value of the concentration of additional vapor during combustion process. The results in Fig. 2 illustrated that, with the increase of H2O concentration, the conversion ratio of NO initially decreased and then increased apparently. The critical concentration of additional H2O was about 8.5 vol% in this research.

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surface [29]. Oxygen containing complex could decompose into gaseous products (CO or CO2) and new active sites which were generally more reactive than the original ones and were easier to adsorb oxidizing agents [32]. Thus, it could be assumed that more C(O) or Cf could be generated on particle surface in the process of char combustion under high Tr and O2 concentration conditions. When the O2 concentration and Tr remained constant, in Fig. 3(e)–(g), the enhancement of H2O concentration promoted the generation of C(O) in the relatively low concentration range (< 8.5 vol %) and inhabited its formation in the relatively high concentration range (> 8.5%). This phenomenon can be attributed to the particularity of the decomposition products (hydrogen free radicals and hydroxyl groups) of H2O molecules. Compared with hydroxyl groups and oxygen atoms, the hydrogen free radicals tended to penetrate into the matrix of char particles consuming amorphous structures which were reactive and had positive effects on the formation of functional groups [12]. On the other hand, the hydroxyl groups could react with active sites on char particle surface forming C(O) which could react directly or indirectly with the oxidants (e. g., NO, O2 or H2O), so the reaction between hydroxyl groups and char could strengthen the reactivity of coal char. Meanwhile, the results in Fig. 3(e)–(g) illustrated that there existed a critical concentration of H2O (8.5 vol%) during the combustion process, and the experimental results obtained from other conditions also expressed identical tendency. There was a positive correlation between the total amount of Cf on char surface and its NO reduction rate [8,31], so the largest amount of NO might be consumed by the particles in the combustion process at the vapor concentration of approximately 8.5 vol%. The TPD results were basically consistent with the NO emission results shown in Fig. 2.

rapidly [29]. Thus, CH4 could be regarded as an indicator for illustrating the start of H2O-char reaction, and the main generation pathways could be summarized as (5) and (6). When the reaction temperature was 1073 K, trace amount of CH4 was tested in the process of combustion at different oxygen concentrations and H2O concentrations, indicating that the reaction temperature (1073 K) was not high enough for the occurrence of the H2O-char reaction. When the reaction reached 1273 K, the concentration of CH4 in the downstream gas became testable, illustrating that the increase of Tr could strengthen the intensity of H2O-char reaction and promote the generation of reducing gaseous products (especially, CO and H2) which could react with NO.

H2 O + C→ CO + H2

(5)

2H2 + C→ CH 4

(6)

When H2O and O2 concentration were remained constant, more reducing gaseous products (mainly CO and H2) would be generated and released into the atmosphere at higher temperatures, and reduced more NO into N2 [8]. Due to the homogeneous and heterogeneous reduction reactions caused by the reducing gaseous agents (CO and H2) and the C(O) or Cf on particle surface, the conversion ratio of N in char to NO during high temperature combustion process was obviously lower than that of combustion occurred at low temperatures. Therefore, the occurrence of H2O-char reaction at high temperatures might also be a reason for the decrease of NO conversion ratio. 3.2. Effect of reaction condition on char surface behavior Trace amount of volatile matters (≤1.56%) and alkali metal salts (≤0.51%) residue in the demineralized char particles after thorough devolatilization and demineralization treatment, so there were almost no gaseous products (NOx or COx) released during the TPD test of demineralized raw char particles [30]. Thus, it could be concluded that basically all the gaseous products (CO and CO2) released during the TPD process of the partially oxidized chars were resulted from the decomposition of C(O). Based on the amounts of CO2 and CO generated during each TPD process, the total amount of C(O) on the surface of per unit mass of each partially oxidized char could be determined approximately through the integration of the TPD curves. In order to clarify the evolution of char surface behavior during combustion process, the TPD results of the chars oxidized under typical H2O concentrations (1.2, 8.5 and 15 vol%) at three temperatures (1073, 1273 and 1473 K), while the O2 concentration was remained in 30% were taken as examples and summarized in Fig. 3(a)–(c). The chars at low conversion degree normally could be employed for setting up the relationship between char reactivity and reaction conditions, so samples with approximately 30% conversion degree were selected for the investigation in this study [2]. Therefore, the TPD results of the partially oxidized chars (Xc ≈ 0.3) obtained from the early stage of the combustion tests are summarized in Fig. 3(d)–(g). Subsequently, the correlation between reaction condition and char surface behavior could be clarified. The results in Fig. 3(a)–(d) illustrated that the total amount of Cf on particle surface increased first and then decreased gradually. Due to the consumption of carbon content in char particles, the total amount of Cf decreased apparently in the middle and later combustion stage [10,31]. With the increase of O2 concentration and Tr, more Cf were generated in the process of char combustion under identical H2O concentrations. Due to the increase of O2 partial pressure, more oxygen atoms could diffuse to the surface of char particles rapidly forming more C(O) and participated in the activation of some inert structures in char particles subsequently, so the increased O2 concentration was conductive to the increase of char surface reactivity as well as Cf amount. While the Tr was high enough (1273 or 1473 K), massive H2O molecules would also be involved in the reaction rapidly, generating a large amount reducing gaseous products (e. g., H2 and CO) and massive new C(O) on char

3.3. Determination of structural characteristics of partially oxidized chars 3.3.1. Effect of reaction condition on the intensity of Raman spectrum of chars Good correlations were identified between the reactivity of coal char and the structural features [33]. Xu et al. [2] revealed that the properties of char sample with proper conversion degree could be regarded as the reactivity index demonstrating the reactivity of the char. Thus, the partially oxidized chars with approximately identical conversion degree (Xc ≈ 0.3) which were obtained from the process of char combustion under different conditions could be employed for clarifying the correlation between char structural features and the reaction condition. Additionally, the intensity of Raman spectrum was affected by the light absorptivity, as well as the Raman scattering ability of the char particles [34]. The existence of oxygen element on the surface of char particles could increase the Raman intensity due to the resonance effect between oxygen atoms and the aromatic structures they are attached to [23]. Consequently, the total Raman area could reflect the electron-rich structures (oxygen containing functionalities) on particle surface, and the intensity of Raman spectrum could be regarded as an indicator illustrating the relative amount of the functionalities [35]. The results in Fig. 4(a)–(c) describe the relationship among the total Raman area, conversion ratio, Tr and H2O concentration during the O2/ H2O combustion process carried out under a typical O2 concentration (30%). It could be assumed that the Raman spectrum intensity of the chars reached the maximum value while the conversion ratio was around 0.3, indicating that there was normally the largest amount of oxygen element on the surface of char particles in the early stage of combustion process. Further enhancement of the conversion degree generally led to a significant reduction of the total Raman areas. This phenomenon could be attributed to the attachment of massive oxygen atoms to the vacant Cf on particle surface, leading to the rapid accumulation of C(O) on particle surface in the early stage of char combustion. The vacant Cf absorbed oxygen atoms to saturation gradually at this stage. As the reaction proceeded, massive oxygen containing gaseous products (CO or CO2) were released rapidly at the expense of 1429

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18000

20000

H2O concentration: 1.2 vol.%

T=1073 K O2: 30%

H2O concentration: 8.5 vol.%

Total amount of Cf ( mol/g)

Total amount of Cf ( mol/g)

20000

H2O concentration: 15 vol.%

16000 14000 12000 10000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

18000

H2O concentration: 1.2 vol.%

T=1273 K O2: 30%

H2O concentration: 8.5 vol.% H2O concentration: 15 vol.%

16000 14000 12000 10000

0.1

0.2

Char conversion ratio

0.3

14000

0.1

0.7

0.8

20000

Total amount of Cf ( mol/g)

Total amount of Cf ( mol/g)

T=1473 K O2: 30%

16000

10000

0.6

(b)

20000

12000

0.5

Char conversion ratio

(a)

18000

0.4

16000 12000

8.5 vol.% H2O

8000 4000 0

2

5

10

20

30

H2O concentration: 1.2 vol.%

1073 (Xc 0.3

10023

11587

12379

14447

15550

H2O concentration: 8.5 vol.%

1273 (Xc 0.3

11447

12697

13744

15078

16242

H2O concentration: 15 vol.%

1473 (Xc 0.3

13822

14455

15789

17811

18730

0.2

0.3

0.4

0.5

0.6

0.7

O2 concentration (%)

0.8

Char conversion ratio

5% O2

16000

R² = 0.6058 R² = 0.6004 R² = 0.7252

12000 8000 4000 0

1073 (Xc 0.3)

1.2 9514

3.5 9978

8.5 11587

15 7483

20 6781

1273 (Xc 0.3)

10089

10413

12197

9987

1473 (Xc 0.3)

12411

12984

14455

12014

H2O concentration (vol.%)

20000

20% O2

16000

R² = 0.4905 R² = 0.6516 R² = 0.7272

12000 8000 4000 0

1073 (Xc 0.3)

1.2 12437

3.5 12998

8.5 14447

15 10389

20 9774

9437

1273 (Xc 0.3)

13004

13711

15078

12467

11719

1473 (Xc 0.3)

15701

15877

17811

15114

H2O concentration (vol. %)

(e)

(f)

Total amount of Cf ( mol/g)

20000

(d) Total amount of Cf ( mol/g)

Total amount of Cf ( mol/g)

(c)

30% O2

20000 16000

R² = 0.8104 R² = 0.8431 R² = 0.8084

12000 8000 4000 0

1073 (Xc 0.3)

1.2 13571

3.5 13779

8.5 15550

15 11489

20 10004

12098

1273 (Xc 0.3)

13946

14656

16242

13741

12117

14931

1473 (Xc 0.3)

16758

17333

18730

16073

14894

H2O concentration (vol.%)

(g)

Fig. 3. TPD results of the chars. (a)–(c): variation of C(O) during combustion process at 30% O2; (d): C(O) as a function of O2 concentration and Tr at 8.5 vol% H2O; (e)–(g): relationship among C(O), Tr and H2O concentration at the O2 concentrations of 5, 20 and 30%.

Fig. 4(d)–(f) illustrated that the enhancement of O2 concentration led to the increase of total Raman areas of the chars under all the conditions of reaction temperature and H2O concentration. Meanwhile, reaction temperature also played a positive role in the combustion process, the increase of Tr generally led to apparent enhancements of the total Raman areas implying that more oxygen atoms would attach to particle surface under higher temperature conditions [36]. The primary pathways of char reaction with H2O addition could be summarized as the following reactions (7)–(10). It could be assumed that the enhancement of O2 concentration and Tr could promote the adsorption of reactants forming C(O) on particle surface, as well as the decomposition of these functionalities [8]. The new active sites generated from the

reactive structures or C(O), and the structures with low reactivity and low oxygen binding capacity were largely remained on particle surface, leading to the reduction of reactivity [23,30]. Aiming to clarify the effect of oxygen concentration on the evolution of char structure during H2O containing combustion process, the total Raman areas of the partially oxidized chars with approximately identical conversion ratio obtained from the combustion occurred in different O2/H2O concentrations at the temperatures of 1073, 1273 and 1473 K were determined, and the results are summarized in Fig. 4(d)–(f). It was believed that more oxygen atoms would connect with the aromatic ring structures in the presence of higher O2 concentration increasing the total Raman area [23]. The results in 1430

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400

1073 K H2O: 1.2 vol.%, O2: 30% 1273 K H2O: 1.2 vol.%, O2: 30%

350

Total area of Raman spectra

Total area of Raman spectra

400

1473 K H2O: 1.2 vol.%, O2: 30%

300 250 200 150 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

350 300 250 200 1073 K H2O: 8.5 vol.%, O2: 30%

150 100

1273 K H2O: 8.5 vol.%, O2: 30% 1473 K H2O: 8.5 vol.%, O2: 30%

0.1

0.2

Char conversion ratio

0.3

Total area of Raman spectra

Total area of Raman spectra

1473 K H2O: 15 vol.%, O2: 30%

250 200 150

0.1

0.2

0.3

0.4

0.5

0.6

300

200

100

0

0.7

1073 K 1273 K 1473 K 1.2 vol.% H2O Xc 5

10

(c)

20

25

30

35

(d)

400

400

Total area of Raman spectra

Total area of Raman spectra

15

0.3

O2 concentration (%)

Char conversion ratio

300

200 1073 K 1273 K 1473 K 8.5 vol.% H2O

100

0

0.7

400

1273 K H2O: 15 vol.%, O2: 30%

300

100

0.6

(b)

1073 K H2O: 15 vol.%, O2: 30%

350

0.5

Char conversion ratio

(a)

400

0.4

Xc 5

10

15

20

25

0.3 30

35

300

200 1073 K 1273 K 1473 K 15 vol.% H2O

100

0

Xc 5

10

15

20

25

0.3 30

35

O2 concentration (%)

O2 concentration (%)

(e)

( f)

Fig. 4. Total Raman areas of the chars collected from different conditions. (a)–(c): total areas of each sample as a function of Xc and Tr at H2O concentrations of 1.2, 8.5 and 15 vol%; (d)–(f): total areas of each sample as a function of O2 concentration and Tr at the H2O concentrations of 1.2, 8.5 and 15 vol%.

decomposition of functional groups were generally more reactive than the original ones and had a stronger combining ability with oxidant [37]. Thus, with the increase of reaction temperature and oxygen

concentration, the total amount of oxygen containing functional groups and the total Raman area of the chars would increase significantly.

H2 O + Cf → C(O) + C(H) 1431

(7)

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O2 + Cf → C(O)

(8)

C(O) → C'f + CO/CO2

(9)

C(H) → C'f + H2

had positive effects on char reactivity. The results in Fig. 5(b)–(d) illustrated that, when the reaction temperature and H2O concentration was remained constant, the enhancement of O2 concentration would increase the relative amount of small aromatic ring structures in char particles. Previous researches reported that the reactions between oxygen and char could create some reactive structures (e. g., small aromatic ring structures); some initially condensed, inert and large aromatic ring structures would be broken into reactive and small structures during the reaction process [10]. Owing to the special properties of H2O molecules, with the increase of the concentration of additional H2O, the relative amount of small aromatic ring structures in the chars initially increased and then decreased at a certain reaction temperature and O2 concentration. The oxidizing hydroxyl groups might react with the char particles and promote the destruction of some originally large aromatic structures with low reactivity into small ones with higher reactivity under high temperature conditions [10]. However, massive hydrogen free radicals would be generated at high H2O concentration conditions, then the small and mobile %H were preferential to penetrate into the matrix of char particles consuming small aromatic ring structures with high reactivity and increasing the degree of structural condensation at the expense of reactive structures [38]. As shown by the data in the sub-figs of Fig. 5, the relative amount of small aromatic ring structures reached the maximum at the H2O concentrations of about 8.5 vol%.

(10)

3.3.2. Evolution of char structural features during H2O containing combustion process In an attempt to clarify the structural feature evolution of the demineralized char particles in the process of combustion under different conditions, the Raman spectrum of each char sample between 800 and 1800 cm−1 was deconvoluted into 10 peaks [11,23]. The ratio between some typical bands could be used as an index to identify the char structure characteristics. For instance, I(Gr+VL+Vr) represents the total Raman areas of small aromatic ring structures (< 6 rings) in amorphous carbon, and ID reflects the area of D band denoting large aromatic ring structures (≥6 rings) [11]. Meanwhile, the ratio I(Gr+VL+Vr)/ID could illustrate the relative amounts of small aromatic ring systems in the char particles [24]. The typical partially oxidized chars (X c ≈ 0.3) were employed as examples, and the specific values of the ratio I(Gr+VL+Vr)/ID of the chars are summarized in Fig. 5. Compared with the large aromatic ring structures, the small ones were generally more likely to react with the oxidizing agents [23]. This phenomenon indicated that the existence of small aromatic structures

3.6

10000 8000

D: Mainly aromatic ring structures ( 6)

6000 4000

3.2

Gr, VL, Vr: Mainly small ring structures ( 6) and amorphous structures

S: Mainly links

2000

H2O: 1.2 vol.%

Tr = 1073 K

H2O: 8.5 vol.%

Xc

H2O: 15 vol.%

2.8 2.4

3.6

Data point Polynomial fit R^2 = 0.78144

3.4

2.0 1.6

3.2 3.0 2.8 2.6 2.4

0

4

8

12

16

20

Steam concentration (vol.%)

0 800

1000

1200

1400

1600

1800

0

5

-1

Wavenumber (cm )

10

15

3.2

H2O: 1.2 vol.%

Tr = 1273 K

H2O: 8.5 vol.%

Xc

H2O: 15 vol.%

3.6

0.3

3.2

2.4

I(Gr+VL+Vr)/ID

2.8 3.6

I(Gr+VL+Vr)/ID

3.4

2.0 1.6

3.2 3.0

Data point Polynomial fit R^2 = 0.69779

2.8 2.6 2.4

0

4

8

12

16

H2O: 1.2 vol.%

Tr = 1473 K

H2O: 8.5 vol.%

Xc

H2O: 15 vol.%

5

10

30

35

0.3

2.8 2.4

3.6 3.4

2.0 1.6

20

3.2 3.0 2.8

Data point Polynomial fit R^2 = 0.9976

2.6 2.4

0

Steam concentration (vol.%)

0

25

(b)

I(Gr+VL+Vr)/ID

3.6

20

O2 concentration (%)

(a)

I(Gr+VL+Vr)/ID

0.3

I(Gr+VL+Vr)/ID

12000

G: Mainly aromatic ring quadrant breathing

I(Gr+VL+Vr)/ID

Raman intensity, arb

14000

15

20

25

30

4

8

12

16

20

Steam concentration (vol.%)

35

O2 concentration (%)

0

5

10

15

20

25

30

35

O2 concentration (%)

(c)

(d)

Fig. 5. Ratio of I(Gr+VL+Vr)/ID of chars oxidized under different conditions. (a): an deconvolution example; (b)–(d): relationship among the ratio, atmosphere and reaction temperature. 1432

Fuel 253 (2019) 1424–1435

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both Cf amount and value of I(Gr+VL+Vr)/ID, the value of r0.3 increased apparently. This phenomenon indicated that fracture of aromatic ring structures especially the decomposition of large ring structures into small ones would promote the generation of defect structures, leading to the generation of new active sites on particle surface [23,42]. These variations occurred in char particles expressed a significantly positive effect on char reactivity, and the gasification rate of carbon content in char particles was accelerated obviously in the process of O2/H2O combustion. The r0.3 value of each sample is also plotted against the Cf amount/the value of I(Gr+VL+Vr)/ID in Fig. 6. There was a good linear relationship between r0.3 value and the Cf amount/the value of I(Gr+VL+Vr)/ID, illustrating that both Cf amount and the I(Gr+VL+Vr)/ID value of chars with low conversion degree could be used as a good reactivity index reflecting char reactivity during O2/H2O combustion process. Moreover, the char reactivity plays a vital role in determining NO emission characteristic of coal char during the combustion process. The partially oxidized char samples (Xc ≈ 0.3) obtained from the typical conditions were employed for the investigation in this research. Subsequently, the relationship between the NO conversion ratios in the typical combustion processes and the Cf amounts/I(Gr+VL+Vr)/ID values of the selected chars are summarized in Fig. 7 for setting up the correlation among Cf amount, char microstructure and NO conversion ratio in the process of O2/H2O char combustion. The results in Fig. 7(a) illustrated that, there was a good linear relationship between the NO conversion ratio and the total amount of Cf on the surface of chars prepared from the corresponding combustion processes. It could be assumed from the results in Fig. 7(a) that, the surface behaviors of char particles played a dominant role in affecting the NO emission in the process of char combustion under all the reaction conditions in this research [41]. Obviously, the enhancement of the total amount of Cf on particle surface would enhance the reducibility of char particles, so more NO could be reduced into N2 by Cf or C(O) rather than released into the flue gas directly [8]. Thus, the conversion ratio of nitrogen content in char particles to NO during char combustion process decreased apparently. There were generally more defective sites at the edge of small aromatic ring structures, which were more likely to generate Cf or combine with oxygen atoms forming C(O) on particle surface during reaction process [23]. Meanwhile, there was also a linear relationship between the I(Gr+VL+Vr)/ID value of each partially oxidized char sample (Xc ≈ 0.3) and NO conversion ratio. It further confirmed that the enhancement of NO consumption rate could also be attributed to the variation of char surface behavior and microstructure [40]. Both the Cf amount and I(Gr+VL+Vr)/ID value could be regarded as effective

3.4. Effect of surface behavior and structural feature on the combustion characteristics of demineralized coal char under O2/H2O reaction conditions Generally, the surface behavior and microstructure characteristic are two factors which could greatly affect the char reactivity [39]. Based on the results obtained from this study and other researchers, the main reaction pathways of char particles during O2/H2O combustion process could be concluded as (11)–(17) [8,40]. Obviously, the reaction between oxidizing agents and char particles tended to first form surface oxygen/nitrogen/hydrogen containing complexes during O2/H2O combustion process. Then, the decomposition of these complexes led to the formation of gaseous products. Compared to the (13), (14), (16) and (17), the (11), (12) and (15) were the rate determining reactions [41]. Therefore, both the combustion reaction of char particles and the NO reduction reaction were mainly determined by the surface chemical behavior of char particles. Additionally, the total amount of active sites on the surface particles was closely related to the microstructural characteristics. The active sites mainly exist at the edges of amorphous carbon structure and graphite defect structure, these structures could be characterized by the degree of ordering and condensation of aromatic ring structures [42]. Consequently, the microstructural characteristic could be employed for illustrating the total amount of surface active site, as well as, the reactivity of char particles. Cf + H2O → C(O) + C(H)

(11)

Cf + O2 → C(O)

(12)

C(O) → Cf′+ CO/CO2

(13)

C(H) → Cf′ + H2

(14)

Cf + NO → C(O) + C(N)

(15)

C(N) → Cf′ + N2

(16)

CO + NO → CO2 + N2

(17)

The combustion curves could be obtained by means of the application of online FT-IR gas analyzer, the instantaneous reaction rate of each char at the conversion ratio of approximately 0.3 could be calculated approximately (r0.3) by Eq. (2) and regarded as a reactivity index illustrating char reactivity under different conditions [2]. The consumption rates (r0.3) of carbon content in the typical combustion processes (Tr: 1073, 1273 and 1473 K; O2 concentration: 5 and 30%; H2O concentration: 1.2, 8.5 and 15 vol%) were calculated and employed for the investigation in this research. The TPD, Raman and combustion results are summarized in Fig. 6. As illustrated in Fig. 6, it could be assumed that with the increase of

1.4

1073 K 30% O2 1.2 vol.% H2O

1.2

1273 K 30% O2 8.5 vol.% H2O

1.0

1073 K 5% O2 1.2 vol.% H2O

0.8

1473 K 5% O2 15 vol.% H2O

1473 K 30% O2 15 vol.% H2O

r0.3 (mmol.g-1.s-1)

r0.3 (mmol.g-1.s-1)

1.4

1273 K 5% O2 8.5 vol.% H2O

0.6 R^2 = 0.7023

0.4 0.2 0.0

0

3000

6000

1273 K 30% O2 8.5 vol.% H2O

1.0

1073 K 5% O2 1.2 vol.% H2O

0.8

1473 K 5% O2 15 vol.% H2O

Total amount of Cf on particle surface ( mol.g-1)

1473 K 30% O2 15 vol.% H2O 1273 K 5% O2 8.5 vol.% H2O

0.6 0.4 R^2 = 0.7224 0.2 0.0 0.0

9000 12000 15000 18000 21000

1073 K 30% O2 1.2 vol.% H2O

1.2

0.5

1.0

1.5

2.0

2.5

3.0

Value of I(Gr+VL+Vr)/ID

(a)

(b)

Fig. 6. Relationship among r0.3, Cf amount and the value of I(Cr+VL+Vr)/ID. 1433

3.5

4.0

Fuel 253 (2019) 1424–1435

Conversion ratio of char-N to NO (%)

Conversion ratio of char-N to NO (%)

Z. Wang, et al.

55 1073 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

50

1273 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

45

1473 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

40 35 30

y=-0.0041x+74.317 R^2=0.8272

25 20 15 10

1073 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

5

1273 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

0 6000

8000

1473 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

10000 12000 14000 16000 18000 20000

Total amount of Cf on char particle ( mol/g)

55

1073 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

50

1273 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

45

1473 K, O2: 30%, H2O: 1.2, 8.5, 15 vol.%

40 35 30

y = -27.412x + 94.723 R^2 = 0.7841

25 20 15 10

1073 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

5

1273 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

0 1.8

1473 K, O2: 5%, H2O: 1.2, 8.5, 15 vol.%

2.0

2.2

(a)

2.4

2.6

2.8

3.0

3.2

3.4

3.6

Value of I(Gr+VL+Vr)/ID

(b)

Fig. 7. Relationship among NO conversion ratio, Cf amount and the value of I(Cr+VL+Vr)/ID.

indicators reflecting the NO emission characteristics in the process of char O2/H2O combustion, as well as char reactivity [43,44]. Based on the results above, it could be concluded that combustion occurred in the mixture of 30% O2 and around 8.5 vol% H2O (balanced with Ar) at the temperature of 1473 K was the optimal combustion condition with the highest combustion reactivity and the lowest NO emission in this study.

Acknowledgements

4. Conclusion

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

The effects of Tr and reaction atmosphere on the NO conversion ratio, Cf generation/decomposition and evolution of char microstructure during combustion process were investigated for setting up the correlation among the NO emission characteristic, surface behavior and structural characteristic under different combustion conditions: 1. The enhancement of Tr and O2 concentration would inhibit the conversion of nitrogen content in char into NO in the process of char combustion with additional H2O. Meanwhile, the addition of H2O would accelerate the consumption of NO during char combustion process in low H2O concentration range (< 8.5 vol%), but promote the release of NO in high H2O concentration range (> 8.5 vol%). 2. The TPD results illustrated that raising the Tr and O2 concentration was conducive to the generation of Cf, and the chars with moderate burnout degree (Xc ≈ 0.3) had the largest amount of C(O) on particle surface. With the increase of H2O concentration in reaction atmosphere, the Cf amount first increased and then decreased, and the critical H2O concentration was approximately 8.5 vol%. 3. While the H2O concentration was relatively low (< 8.5 vol%), addition of H2O molecules preferentially to promote the destruction of some large aromatic structures into small ones, leading to the increase of char reactivity. However, the additional H2O molecules would accelerate the removal of small aromatic ring systems, and big aromatic ring structures started to be dominant in high H2O concentration range (> 8.5 vol%). 4. There was a good linear relationship between the NO conversion ratio/char reactivity and the Cf amount/I(Gr+VL+Vr)/ID value, indicating that surface behavior and structural characteristic of char could affect the NO emission and char reactivity. Additionally, both the Cf amount and I(Gr+VL+Vr)/ID value of chars obtained from early combustion stage could be used as an index to reflect the NO emission characteristics and char reactivity in the process of char combustion.

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