H2O combustion process of demineralized coal char

Fuel Processing Technology 195 (2019) 106144

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Effect of reaction conditions on the evolution of surface functional groups in O2/H2O combustion process of demineralized coal char ⁎

T

⁎

Zhuozhi Wanga,b, Yaying Zhaob,c, Rui Sunb, , Yupeng Lib, , Xiaohan Rend, Jie Xue a

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China c School of Energy and Power Engineering, Northeast Electric Power University, Jilin 132012, China d Institute of Thermal Science and Technology, Shandong University, Jinan 250061, China e 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: O2/H2O combustion Coal char Functional group NO Reactivity

O2/H2O combustion is now regarded as a novel and promising technology of coal utilization for next-generation oxy-fuel combustion. The combustion reaction mainly occurs in the mixture of O2 and H2O instead of recycled flue gas (mainly CO2). The purpose of this research is to clarify the evolution characteristics of O/N-containing complexes (C(O)/C(N)) during O2/H2O combustion. Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were employed in this study. The addition of H2O in the atmosphere promoted the generation of NO and HCN in char combustion. There was a good linear relationship between the C(O) amount and char reactivity, as well as the NO conversion ratio, during the O2/H2O combustion process. Moreover, trace amounts of the precursors of CO2 with low thermal stability remained on the particle surface in the O2/H2O combustion. The enhancement of the reaction temperature revealed positive effects on inhibiting NO emissions in the O2/H2O combustion. The relative amount of pyrrole (N-5), which was the precursor of HCN, increased apparently at the expense of pyridine (N-6) and quaternary nitrogen (N-Q) in the early reaction stage. When the char conversion degree exceeded 50%, N-Q and N-6 became the dominant forms of C(N).

1. Introduction

generally low [6,7]. In addition, the H2O molecules added in the primary combustion zone first decompose into mostly %H and %OH rapidly [8]. Then, the %OH attaches to the vacant active sites on the particle surface, generating C(O), promoting the destruction of the original inert and large aromatic ring structures into reactive and small ones [9,10]. Otherwise, the small and mobile %H penetrates into the matrix of carbonaceous material, destroying some cross-linking structures or aromatic ring structures that are initially reactive into inert and condensed structures [11]. Therefore, it can be assumed that the addition of H2O molecules tends to alter the evolution or emission characteristics of nitrogen-containing species in the particles. Researchers such as Thomas [12] and Krammer [13] have already investigated the emission mechanisms of NOx during the combustion processes of coal-based fuels in O2/N2 atmospheres. However, the specific evolution characteristics of nitrogen-containing species in demineralized coal or char during the O2/H2O combustion process are still not very clear. Moreover, knowledge of the behaviours of surface functional groups is of great importance for clarifying the combustion mechanism of coal-

O2/H2O combustion technology is now treated as a novel and potential coal-based fuel utilization technology [1,2]. The combustion of coal particles occurs in an atmosphere consisting of high concentrations of H2O and O2 instead of recycled flue gas (mainly CO2 and H2O) and O2. In the O2/H2O combustion process, both O2 and H2O participate in the oxidation of coal particles in the reaction zone. Due to the differences in the physicochemical behaviours of H2O molecules from those of CO2 and N2, the addition of H2O in the reaction atmosphere might inevitably alter the properties of coal particle combustion in an O2/H2O atmosphere [3]. Thus, prior to industrial application, this technology deserves to be investigated systematically. In the O2/H2O combustion process, the vaporization of additional H2O absorbs a large amount of heat from the primary combustion zone, which is beneficial to control the flame temperatures in a suitable range [4,5]. Due to the removal of circulation of the flue gas system, the whole reaction system becomes simpler, and the operation costs are

⁎

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

https://doi.org/10.1016/j.fuproc.2019.106144 Received 16 January 2019; Received in revised form 5 July 2019; Accepted 6 July 2019 Available online 11 July 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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analysed by FT-IR, TPD and XPS to determine the thermal stability, relative amount and evolution characteristics of C(O)/C(N) during the O2/H2O combustion process. Based on the results above, the relationship between the functional group evolution characteristics and the char conversion ratio/reaction condition are clarified preliminarily.

based fuels [14]. The specific characteristics of C(O) during the combustion process are of great importance for identifying the reactivity of fuels and emission characteristics of gaseous pollutants (such as NOx and SOx) [15]. During the combustion process, massive oxygen atoms attach to the surface of fuel particles, forming oxygen-containing functional groups. The behaviour of C(O) has significant effects on the reactivity of coal or char particles, and C(O) can be regarded as a valid indicator of reactivity [16]. Moreover, the oxygen-containing complexes are the main reactants participating in the reaction between carbonaceous materials and oxidizing reactants (H2O, O2 or NO) [17,18]. Previous researchers have conveyed that gaseous products (CO and CO2) and new active sites that are more reactive than the original ones are the products generated from the decomposition of C(O) under high-temperature conditions [19,20]. Figueiredo et al. [21] revealed that there are six primary chemical structures of C(O) after low-temperature oxidizing treatment in an O2/N2 atmosphere: carboxyl, phenol, ether, anhydride, lactone and quinone. A previous investigation illustrated that when the reaction occurs at relatively high-temperature conditions (1073 K) in an O2/Ar atmosphere, the primary chemical structures of C(O) are also these six types [22]. Additionally, in the progress of O2/H2O combustion, massive %H and %OH are generated from the decomposition of H2O molecules, altering the surface behaviours of carbonaceous material in the combustion process [23,24]. It is expected that the generation and decomposition characteristics of C(O) in the combustion occurred in O2/H2O atmosphere are different from those in O2/N2 atmosphere. However, no relevant studies on the behaviour of C(O) generated during the O2/H2O combustion process have been carried out. C(N) on the surface of carbonaceous material can affect the emission characteristics of nitrogen content in coal or char particles during the oxidation process, and this aspect has been generally investigated [25]. The relative amount of each type of nitrogen-containing complex on the particle surface varies significantly during the combustion process. Stanczyk et al. [26] investigated the evolution of nitrogen-containing complexes during the coal char combustion process (823 K) in an O2/Ar atmosphere (20% O2 and 80% Ar) and conveyed that pyrrolic and pyridinic groups rapidly transform into quaternary nitrogen during the reaction process. Pyridinic and quaternary groups are generally considered to be the structures with the highest thermal stability, and the addition of CO2 in the reaction atmosphere has only a very slight effect on the evolution of C(N) (the effect is sometimes negligible) [27]. Therefore, it can be assumed that C(N) are the precursors of nitrogencontaining gaseous products and that the behaviour of C(N) generally plays a vital role in affecting NOx emission characteristics during the combustion process. However, the effects of the H2O concentration on the evolution characteristics of C(N) during the combustion process have not been fully investigated, and further research is needed. In an attempt to clarify the specific behaviours and evolution characteristics of C(O) and C(N) on the char particle surface during the O2/H2O combustion process, isothermal combustion was carried out in different atmospheres (30% O2 + 1.2/3.5/8.5/15/20 vol% H2O, balanced with Ar) under typical reaction temperatures (1073 K and 1473 K). The whole combustion process is divided into several parts, and the partially oxidized samples with different conversion degrees are

2. Experimental 2.1. Sample preparation The raw coal sample was placed in a drying oven for pre-dewatering, and the coal was dewatered at 378 K for 12 h. By means of the utilization of a mini crusher, the large coal particles were crushed and sieved to a uniform diameter distribution (100–125 μm). Alkali/alkaline ether metallic species (AAEM) normally express catalytic activity in the combustion process of fuel particles, affecting the evolution mechanism of functional groups on particle surfaces, especially at high temperatures [28]. To exclude the influence of AAEM species on the analysis of the reaction mechanism during the O2/H2O combustion process, demineralization treatment was carried out. Additionally, compared with the extraction of organic matter, acid soaking demineralization was easier to operate under laboratory conditions [29]. Therefore, the acid soaking demineralization method was employed for the demineralization of the coal sample, and the treatment was conducted at the in situ temperature. The specific operation procedures are as follows: (a) the coal particles were added in a reagent-grade aqueous solution of HCl (approximately 30%) and stirred at a constant speed for 4 h. (b) After filtration with 4000 mL ultra-pure water, the partially demineralized sample was then added to a reagent-grade aqueous solution of HF (approximately 30%) and stirred at a constant speed for 4 h. (c) After filtration with 4000 mL ultra-pure water, the partially demineralized sample was added to a reagent-grade aqueous solution of HCl (approximately 30%) and stirred at a constant speed for 3 h. (d) After filtration with 4000 mL ultra-pure water, the sample was dewatered at 378 K for 12 h. The devolatilization of the demineralized coal was performed in a fixed-bed horizontal facility (atmosphere: Ar; time: 30 min; temperature: 1173 K; gas flow rate: 1 L/min) [30]. After the devolatilization and demineralization treatment, the size of the pulverized particles remained approximately in the original range (100–125 μm), and almost all of the volatile matter and ash content were removed. The approximate and ultimate results of the demineralized sample are shown in Table 1. 2.2. Combustion experiments In an attempt to clarify the evolution characteristics of each functionality on the surface of demineralized char particles under different conditions, combustion experiments under different temperatures and oxidizing atmospheres were performed in the reaction device, as illustrated in Fig. 1. The specific reaction conditions were as follows: reaction temperature, 1073 K and 1473 K; atmosphere, 30 vol% O2 + 1.2/3.5/8.5/15/20 vol% H2O, balanced with Ar; gas flow rate, 1 L/min. Approximately 20 mg of demineralized char sample was used

Table 1 Ultimate and proximate analyses of demineralized bituminous samples. Bituminous sample

Demineralized coal Demineralized char a b c

Ultimate analysisa

Proximate analysisb

C

H

N

S

Oc

80.03 96.73

4.50 0.55

0.96 1.56

0.26 0.18

14.25 0.98

(wt% dry and ash free). (wt% as received). (calculated by difference). 2

Moisture

Volatiles

Cfixed

Ash

0.19 0.76

32.11 1.56

67.14 97.17

0.56 0.51

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Fig. 1. Horizontal fixed-bed reaction system.

obtained through Eq. (2), and the specific values of Xc are shown in Table 2. Additionally, the emission rate of CO2 at a certain stage in the combustion process could be calculated by Eq. (3), which can be regarded as an equivalent indicator of char reaction rates or char reactivity in the O2/H2O process [9].

for the combustion test each time. The gaseous products released during the O2/H2O combustion process were recorded with an IR gas analyser (GASMET Technologies, Finland, measurement precision ≤2%). No N2 was involved in the combustion atmosphere, implying that NOx was not generated by the “prompt N” or “thermal N” conversion routine. Therefore, the formation of nitrogen-containing gaseous products could be attributed to the emission of nitrogen content in the char particles. Parallel tests were performed to evaluate the repeatability of the experiments. Based on the emission curves of typical gaseous products of the two parallel tests of each sample and the ultimate results in Table 1, the conversion ratio of char-N to each nitrogen-containing gaseous product during the reaction process could be calculated by Eq. (1). The calculation results illustrated that the measurement and experimental errors exhibited negligible effects on the combustion results. Q

ηi =

Xc =

RCO2 =

CCO2 × Qc 22.4

(2)

× 10−2 (3)

mchar

where Xc represents the char conversion degree (%); m0 represents the initial mass (mg) of the sample; mt is the mass (g) of the sample oxidized in the reactor after t seconds; RCO2 is the emission rate of CO2 (mol·g−1·s−1); CCO2 reflects the concentration of CO2 at the time of t (%); mchar represents the mass of char sample placed in the reactor (mg); and Qc is the flow rate of reaction gas during O2/H2O combustion process (L/s).

t

MN ∙ 22.4 ∙ ∫0 (CNO × 10−6)dt m∙fN

m 0 − mt × 100% m0

(1)

where ηi represents conversion ratio of each type nitrogen containing gaseous product (NO, HCN, NH3 and NO2) released during combustion process; t illustrates the reaction runtime of each combustion test (s); MN reflects the molar mass of nitrogen element (g·mol−1); Q illustrates the volumetric flow of the reaction gas (L/s); CNO represents the NO concentration of the flue gas (10−4 vol%); m is the mass of the char sample used for each test (g); fN illustrates the mass fraction of nitrogen in the demineralized char shown in Table 1. To identify the effect of the H2O concentration on the evolution of C(O) and C(N) during the reaction, the samples were oxidized to different burnoff degrees at 1073 K and 1473 K under O2/H2O conditions. Based on the results shown in Table 1, fixed carbon was the main content in the particles (≥97%). Therefore, almost all the mass loss of the samples after the reaction could be attributed to the reduction of fixed carbon. Based on the mass of the sample before and after the reaction, the specific conversion ratios (Xc) of the samples would be

2.3. TPD and partial TPD test To clarify the effects of the H2O concentration in the reaction atmosphere on the evolution characteristics of C(O) in the process of O2/ H2O combustion, the partially oxidized samples were taken as examples for the determination of the surface functional groups. All the partially oxidized samples were de-watered adequately in a drying oven at 378 K for 12 h in advance. The TPD operation procedure is as follows: approximately 20 mg of partial oxidized sample was used for the TPD test each time; the device was heated from approximately 298 K to 1723 K under an Ar atmosphere with a constant heating rate of 5 K/min; the flow rate of Ar was 0.43 L/min; the downstream gas was analysed by an IR gas analyser (SIGNAL S4I, England, 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:

Table 2 Conversion ratios of each demineralized bituminous sample. Samples

Combustion atmospheres

L1-L4 L5-L8 L9-L12 H1-H4 H5-H8 H9-H12

30% 30% 30% 30% 30% 30%

Conversion ratios, Xc (%)

O2 + 1.2 vol% H2O balanced with Ar O2 + 8.5 vol% H2O balanced with Ar O2 + 15 vol% H2O balanced with Ar O2 + 1.2 vol% H2O balanced with Ar O2 + 8.5 vol% H2O balanced with Ar O2 + 15 vol% H2O balanced with Ar

13.1 14.9 12.1 29.1 33.8 32.1

32.9 31.1 32.8 47.3 55.2 51.9

54.1 44.9 45.2 56.1 64.7 60.6

Li and Hi represent the partially oxidized samples obtained from the combustion tests performed at temperatures of 1073 K and 1473 K, respectively. 3

72.1 61.2 61.9 64.9 74.8 68.9

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Fig. 2. Emission characteristics of char-N during the O2/H2O process. (a) and (b): conversion ratio of char-N to the gaseous products at 1073 K and 1473 K, respectively.

2.5. XPS analysis

0.1%). A previous investigation indicated that trace amounts of gaseous products (CO, CO2 or NO) were released during the blank TPD process, illustrating that the corundum boat and reactor expressed a negligible effect on the TPD results [31]. Moreover, parallel tests were conducted to examine the repeatability of the tests, and the emission curves of COx of the two tests were basically identical. Consequently, it could be concluded that the experimental error and measurement error had little effect on the results. The experimental system is shown in Fig. 1, and the COx released by per unit mass char sample were calculated through the following two equations:

nCO =

nCO2 =

Q TPD 22.4

t

× ∫0 (CCO × 10−6)dt m

Q TPD 22.4

To identify the evolution characteristics of C(N) on the particle surface during the O2/H2O combustion process, the partially oxidized bituminous coal samples (D1 - D20) were analysed by X-ray photoelectron spectroscopy (XPS) – PHI 5400 ESCA system, USA. The device was equipped with an Al Kα X-ray source (hv = 1486.6 eV). The pass energy was fixed at 93.9 eV to ensure sufficient sensitivity. The whole spectra and narrow spectra of each sample were recorded with high resolution. Wide scan: the pass energy was 178.95 eV, and narrow scan: the pass energy was 22.35 eV. The deconvolution investigation of the XPS results was performed by the application of PeakFit v4.12 to identify the distribution of each kind of nitrogen-containing complex. Moreover, the relative compositions were approximately calculated by the integrated peak areas of C1s, N1 s and O1s from the XPS spectra.

(4)

t

× ∫0 (CCO2 × 10−6)dt m

(5)

where t is the reaction time of the TPD experiments (s); nCO and nCO2 represent the mole amount of CO and CO2 released from each partially oxidized char sample during the TPD process (μmol/g); QTPD is the volumetric flow rate of the reaction gas (L/s); CCO and CCO2 are the concentration of CO and CO2 released during the TPD process (ppm), respectively; and m is the mass of demineralized char for each test (g). In an attempt to clarify the thermal stability of each type C(O), the partial TPD method was conducted for the investigation. The whole TPD process could be divided into several parts with different final temperatures, and the specific amount of each type of C(O) on the surface of particles that underwent the partial TPD processes could be determined by FT-IR analysis. Based on the variations in the distribution of each kind of functionality, the decomposition temperature range of the complexes with different chemical structures could be identified approximately [31]. Therefore, three typical samples (L3, L7 and L11) were taken as examples and underwent seven partial TPD experiments. The final temperatures of the partial TPD process were 1073, 1173, 1273, 1373, 1473, 1573 and 1673 K. The samples after the partial TPD treatment were cooled to room temperature and collected for FT-IR analysis.

3. Results and discussion 3.1. Emission characteristics of N char during the O2/H2O combustion process Isothermal combustion tests of the demineralized char sample were carried out under different conditions, and the main nitrogen-containing gaseous products released during the reaction process as a function of H2O concentration are summarized in Fig. 2. The results illustrate that NO was the primary nitrogen-containing gaseous product in the downstream gas of the reactor, indicating that most of the nitrogen content in the demineralized char particles was released in the form of NO under O2/H2O conditions in this research. In addition, with the enhancement of the H2O concentration in the reaction atmosphere, the conversion ratio of nitrogen content in char particles to NO initially decreased and then increased, as shown in Fig. 2. This phenomenon might be attributed to the participation of H2O molecules in the combustion reaction of demineralized chars, and massive %H and %OH released from the decomposition of H2O molecules might affect the behaviours and structures of functionalities on the particle surface [8,9]. Generally, the %H could promote the conversion of reactive crosslinking structures or small aromatic ring structures with high reactivity into stable and condensed ones with lower reactivity, leading to the reduction of char reactivity [11]. On the other hand, oxidizing %OH could react with active sites on the particle surface to form C(O) and promote the decomposition of originally inert and large aromatic ring structures into reactive and small ones, increasing the char reactivity [9,10]. Based on the results obtained from the tests in this research, it could be assumed that when the concentration of additional H2O was relatively low, the addition of H2O resulted in positive effects on strengthening the char reactivity/reducibility; when the concentration of additional H2O exceeded the critical value, the addition of H2O expressed negative effects on char reactivity/reducibility.

2.4. FT-IR analysis By means of the application of Fourier transform infrared spectroscopy (FT-IR), the specific distribution of the C(O) with different chemical structures on the surface of the samples (L1−L12) after partial TPD treatment could be qualitatively characterized. The scanning wavenumber range was 400–4000 cm−1. Each sample obtained from the partial TPD process was mixed with KBr at a ratio of 1:400 and was formed into two pellets for the uncertainty analysis of the results. In an attempt to abate the interference of moisture on the FT-IR results, all the pellets were dried at 323 K under an air atmosphere for 48 h in advance. Each pellet was scanned 64 times at a resolution of 4 cm−1. 4

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determined semi-qualitatively. The results illustrated that with the increasing final temperature of the partial TPD test, the distribution of each type C(O) varied significantly, and this phenomenon could be attributed to the differences in the thermal stability of each type of C(O). Based on the results in Fig. 3(e)–(j), the thermal stability of each complex generated in O2/H2O combustion could be approximately identified: carboxyl/ether < phenol < anhydride < lactone < quinone. The type and thermal stability of C(O) generated during both the oxy-fuel and O2/H2O combustion processes were essentially identical [22]. The addition of H2O in the reaction atmosphere changed only the amount and distribution of C(O) on the particle surface. Based on the FT-IR and TPD results (Fig. 4) of char, each sample oxidized at 1073 K and 1473 K, the specific surface behaviour of C(O) generated during the combustion process could be identified. When the demineralized char was oxidized in an O2/Ar atmosphere, precursors of CO2 were the primary form of C(O) on the particle surface [22]. The addition of steam in the reaction atmosphere altered this phenomenon, and the participation of massive %H and %OH generated from the decomposition of H2O molecules could promote the generation of CO precursors on the particle surface. Additionally, the CO2 desorption peaks were very narrow and mainly located in the high-temperature region, indicating that only the precursors of CO2 with high thermal stability (mainly lactone) existed on the surface of demineralized char particles in O2/H2O combustion. The TPD spectra illustrated that trace amounts of CO2 were released when the reaction temperature was lower than 1500 K, indicating that trace amounts of carboxyl or anhydride existed on the surface of demineralized char particles during the O2/H2O combustion process. Thus, it could be assumed that the addition of H2O molecules accelerated the conversion of carboxyl and anhydride into phenol, ether, lactone or quinone, and reaction (10) ought to be predominant. Therefore, it could be implied in terms of the reaction pathways proposed by Zhuang [35] and the results obtained in this study that the evolution pathways of C(O) during the O2/H2O combustion process might be the following Eqs. (6)–(12):

Additionally, the increase in temperature led to an obvious reduction in the conversion ratio of N to NO, and trace amounts of other nitrogen-containing gaseous products were measured in the downstream gas. This phenomenon might be attributed to the following three reasons: 1473 K was high enough for the rapid reduction reaction between char particles and NO in the atmosphere, and massive NO tended to be reduced into N2 by the Cf or C(O) on the particle surface rapidly at high temperatures [17]. A gasification reaction between H2O molecules and carbonaceous materials occurred, generating active reducing agents (H2 and CO) that reduced NO into N2 under high-temperature conditions. When the reaction temperature was high, nitrogen-containing complexes potentially reacted with each other, releasing N2 directly instead of NO. The thermal stability of N-6 and N-Q was very high, and N-6 became the dominant form, followed by N-Q on the particle surface [27]. Therefore, the emission of NO during the O2/H2O combustion process can be attributed to the desorption of N-6 and N-Q. Moreover, HCN was another main product released during the O2/H2O combustion process at 1073 K. Research has revealed that N-5 is the precursor of HCN and that the five-membered ring structure N-5 normally tends to decompose from the particle surface, forming HCN or converting to more stable structures (N-6 or N-Q) at high temperatures [15]. The combustion results indicated that the addition of H2O had positive effects on the generation of N-5 on the char surface at relatively low temperatures. The enhancement of H2O concentration promoted the emission of HCN during the O2/H2O combustion process, indicating that %H or %OH might react with N-6 or N-Q, leading to a portion of N-6 or N-Q converted into N-5. However, trace amounts of HCN were tested during the combustion reactions performed at 1473 K. This phenomenon implied that N-5 tended to convert into thermally stable structures (N-6 or N-Q) under high-temperature conditions instead of remaining on the particle surface. The specific evolution and emission characteristics of C(N) during the O2/H2O combustion processes occurred at 1073 K and 1473 K would be clarified by XPS analysis and discussed in the following sections.

H2 O → ˙H + ˙OH

(6)

3.2. Determination of the behaviour and evolution characteristics of C(O)

˙OH + Cf → Phenol

(7)

3.2.1. Investigation of the behaviour of each type C(O) The type and distribution of C(O) on the surface of samples oxidized under identical conditions are essentially identical [31]. The chars (L1−L12) obtained from three typical experimental conditions (30% O2 + 1.2, 8.5 and 15 vol% H2O) were employed as examples for the FTIR analysis. The FT-IR absorbance spectra of the partially oxidized samples after different partial TPD processes are summarized in Fig. 3(a). There are four apparent wavenumber regions in the FT-IR absorbance spectra of oxidized carbonaceous materials: the aromatic CeH stretching and hydroxyl region (3750–3000 cm−1), the aliphatic CeH stretching region (3000–2800 cm−1), the oxygen-containing functional group region (1800–1000 cm−1) and the aromatic structure region (900–700 cm−1) [32,33]. The results in Fig. 3(a) illustrated that the enhancement of partial TPD final temperature led to variations in the absorbance peak attenuation, indicating that the surface behaviour varied with the increase in the partial TPD final temperature. To determine the relative amount and variation of each type C(O), the 1800–1000 cm−1 wavenumber region of each sample was selected for semi-quantitative investigation in previous studies [31]. The absorbance peaks in the FT-IR spectra could be assigned to different C(O), and there were 17 hidden absorbance peaks located in the selected wavenumber area (1800–1000 cm−1) [24,33]. Deconvolution curve-fitting investigations were conducted by applying OriginPro 9.0 software to the semi-quantitative determination. Carboxyl, phenol, anhydride, ether, lactone and quinone were the primary structures of C(O) generated on the particle surface [19,34]. With the utilization of the normalization method to the deconvolution results, the specific variations in the relative amount of each type C(O) could be

O2 + Cf → Quinone + Ether + CO

(8)

O2 + Cf → Lactone + Anhydride + Carboxyl

(9)

˙H + ˙OH + Anhydride, Carboxyl → Ether + phenol + Lactone + Quinone Phenol,Ether, Quinone → CO +

Lactone → CO2 + Cf,

Cf,

(10) (11) (12)

3.2.2. Relationship between the char reactivity and surface behaviour The reactivity of char generally plays a key role in the whole combustion process, and the surface behaviour of char particles is an important factor that determines the combustion efficiency [9]. Because the O2 in the reaction atmosphere was excessive, CO2 was the primary product after demineralized char combustion, and almost all the mass loss could be attributed to the emission of CO2 during the O2/ H2O combustion process, so the emission rate of CO2 could be regarded as an indicator of the char reactivity. The char combustion reactivity in the early stage was typical and was normally employed as the reactivity index of char reactivity. Thus, the CO2 emission rates of the chars with an approximately identical conversion ratio (30%) under each O2/H2O condition (Tr: 1073 K and 1473 K; O2: 30 vol%; H2O: 1.2, 3.5, 8.5, 15 and 20 vol%) were employed to determine the correlation between the C(O) total amount and char combustion/reduction reactivity [9,36]. The specific results are summarized in Fig. 5. Based on the data obtained from the isothermal combustion tests 5

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Oxygen-containing functional groups Aliphatic structure Hydroxyl/Aromatic C-H

Sample: L3

Absorbance spectra

Absorbance spectra

Aliphatic C-H 1673 K 1573 K 1473 K 1373 K 1273 K 1173 K 1073 K

0

900

1800

2700

3600

4500

Partial TPD to 1173 K R^2=0.969937

1000

1200

1600

0.40

L3

L3 L7

0.10

Ether proportion

0.08 0.06 0.04

1200

1400

1600

L7

0.35

L11

0.02 1000

L11 0.30 0.25 0.20 0.15 1000

1800

1200

1400

1600

(d)

(c) 0.20

0.20

L3

L7

Anhydride proportion

Phenol proportion

L3 L11

0.12

0.08

0.04 1000

1200

1400

1600

L7

0.16

L11

0.12

0.08

0.04 1000

1800

1200

Temperature (K)

1600

1800

(f) 0.32 L3

L3

0.16

Quinone proportion

L7

Lactone proportion

1400

Temperature (K)

(c) 0.20

L11

0.12

0.08

0.04 1000

1800

Temperature (K)

Temperature (K)

0.16

1800

(b)

(a) 0.12

Carboxyl proportion

1400

Wavenumber (cm-1)

Wavenumber (cm-1)

1200

1400

1600

0.28

L11 0.24 0.20 0.16 0.12 1000

1800

L7

1200

1400

1600

1800

Temperature (K)

Temperature (K)

(h)

(g)

Fig. 3. FT-IR results of chars after partial TPD. (a) and (b): example of FT-IR spectroscopy and deconvolution; (c)–(h): correlation between the C(O) relative content and the temperature of the partial TPD.

low concentration range (0–8.5 vol%), the enhancement of H2O concentration promoted the generation of C(O) on the char surface, as well as char reactivity. This phenomenon could be attributed to the attachment of oxidizing hydroxyls to vacant active sites on the particle surface, forming massive reactive functionalities that could accelerate

and TPD tests, the correlations among the char surface chemical structure, reaction condition and char reactivity could be clarified. The total amount of C(O) on the surface of each partially oxidized sample (Xc ≈ 30%) is plotted against the steam concentration in Fig. 5(a). The results illustrated that when the steam concentration was in a relatively 6

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(a)

(b)

Fig. 4. TPD curve of each sample. (a) and (b): TPD profiles of the chars.

temperature remained unchanged, there was also a good linear relationship between the reactivity indicator (amount of functional group on the surface of partially oxidized char with 0.3 conversion degree) and the NO conversion ratio, indicating that C(O) amount could be regarded as a char reducibility indicator [22,31]. Obviously, when the reaction temperature rose from 1073 K to 1473 K, more NO could be consumed per unit mole active site. This phenomenon might be attributed to the participation of CO generated from the decomposition of the phenol and ether structures in NO reduction.

the consumption of carbon content in char particles [10,17]. When the H2O concentration exceeded 8.5 vol%, the inhibition effect of H2O molecules on the combustion reaction started to become dominant, and a further increase in the steam concentration led to a significant reduction in the amount of C(O) on the particle surface. This phenomenon could be attributed to the hydrogen free radical released from the additional H2O molecules reacted with char particles in the combustion process, leading to condensation of small and reactive aromatic ring structures into large ones with lower reactivity [11]. The results obtained from the combustion experiments performed at 1073 K and 1473 K expressed similar trends, illustrating that the specific effects of steam concentration in the atmosphere on the char reactivity might be irrelevant to the reaction temperature. Generally, the reactivity of char with a low conversion degree could be regarded as an indicator reflecting char reactivity [9]. Therefore, the specific consumption rates of carbon content in the char particles under different combustion conditions while the burnoff degree reached approximately 30% are calculated and summarized in Fig. 5(b). It can also be seen from Fig. 5(b) that there was a good linear correlation between the CO2 emission rate and the amount of C(O) on the particle surface, indicating that the amount of C(O) on the surface of partially oxidized char particles obtained from the early combustion stage could be treated as a reactivity index during the combustion process under different reaction conditions. Additionally, C(O) could participate in the reduction of NO directly or indirectly, and the amount of C(O) was generally employed to reflect char reducibility [17]. When the reaction

3.3. Identification of the behaviour and evolution characteristics of C(N) 3.3.1. Distribution of each type of C(N) on the char surface The fate of C(N) on the surface of carbonaceous material has vital effects on the emission characteristics of nitrogen-containing gaseous products (especially NO) during the combustion process and has been widely investigated by XPS spectra [27]. The value of O1s/C1s and N1s/C1s can be employed to illustrate the surface behaviour of each sample. The relative amounts of oxygen, nitrogen and carbon on the surface of each partially oxidized sample (L5-L8, H5-H8) obtained from two typical reaction conditions were identified and are summarized in Fig. 6(a) and (b). The enhancement of char burnoff generally led to an increase in the O1s/C1s value at the early stage of the reaction, and the value of O1s/C1s reached a maximum in the burnoff range of approximately 30–40%. This phenomenon was consistent with that obtained from the TPD results, which illustrated that there were the

Fig. 5. Summary of TPD and combustion results. (a): relationship between the C(O) amount and steam concentration; (b): relationship between the CO2 emission rate/NO conversion ratio and the C(O) amount. 7

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Fig. 6. Evolution characteristics of C(O)/C(N) on char surface during O2/H2O combustion. (a) and (b): variation of the surface element distribution as a function of burnoff; (c): deconvolution example (L6); (d), (e) and (f): relative amount of N-6, N-5 and N-Q on the particle surface.

gaseous products under relatively low-temperature conditions [37]. The XPS results in Fig. 6(b) illustrated that the relative amount of nitrogen content on the particle surface decreased in the early stage of the combustion process. This phenomenon might be attributed to the more rapid attachment of oxygen atoms to the particle surface occupying most of the vacant active sites, as shown in Fig. 6(a). Because the desorption activation energy of nitrogen-containing complexes (N-6 and N-Q) was normally higher than that of the oxygen-containing complexes, the C(N) tended to enrich on the particle surface gradually as the reaction progressed. In contrast to C(O), the relative amount of C(N) increased at the late stage of combustion gradually. Furthermore, as shown in the sub picture in Fig. 6(b), the enhancement of both the H2O concentration and the reaction temperature resulted in positive effects on the emission of the nitrogen content in the char particles. This phenomenon might be attributed to the following reasons: (a) the increase in temperature tended to promote the decomposition of C(N) into N2 directly from char particles [38]; as shown in Fig. 2, the emission rate of nitrogen containing gaseous products (NO and HCN) increased as the H2O concentration increased, so less nitrogen content remained on the particle surface. The XPS results of L1-L12 and H1-H12 were essentially consistent with the combustion results of the char

maximum functionalities on the particle surface in the intermediate reaction stage (30–40%). In contrast, the further increase in the char Xc led to an apparent decrease in the relative amount of oxygen content. This phenomenon might be attributed to the variation of the oxygen bonding capacity of the char particles, and the decomposition rate of C(O) became faster than the generation rate in the late reaction stage. In addition, the results in Fig. 6(a) illustrate that the addition of steam had positive effects on the generation of C(O) when the steam concentration was in a relatively low range (1.2–8.5 vol%) [9]. The participation of H2O molecules (especially %H) inhibited the attachment of oxygen atoms to Cf during the oxidizing process when the steam concentration increased to a relatively high range (8.5–20 vol%) [11]. Moreover, the total amount of Cf on the surface of partially oxidized chars obtained from 1473 K was larger than that obtained from 1073 K, so more oxygen atoms could attach to the particle surface, increasing the value of Ols/Cls. The oxygen element distribution of partially oxidized chars calculated from the XPS data was consistent with that obtained from the TPD results. Additionally, the nitrogen content in the sample generally exists as an organic form, and some of the C(N) has relatively fragile chemical structures that tend to decompose and release nitrogen-containing 8

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samples. There are four main kinds of nitrogen-containing structures on the particle surface: pyridine N-6 (398.8 ± 0.1 eV), pyrrole N-5 (400.4 ± 0.1 eV), quaternary nitrogen N-Q (401.4 ± 0.1 eV) and nitrogen oxide N-X (402–404 eV) [27]. N-X normally represents oxidized nitrogen complexes, inorganic nitrogen, and some other forms of nitrogen in the particles. The relative amount of N-X is relatively small (≤ 10%) and generally remains constant during the combustion process. Due to the negligible effect of N-X on nitrogen element evolution during the reaction process, this aspect is not investigated in the present research. The relative amount of each kind C(N) on the surface of partially oxidized chars as a function of the burnoff under O2/H2O combustion conditions was calculated through the deconvolution of their N1s spectra. The specific values are summarized in Fig. 6(d), (e) and (f). With increasing reaction time, there were apparent variations in the distribution of each type of C(N). N-6 and N-Q were the dominant forms of C(N) initially, and the relative amount of N-5 on the surface of the char sample was small, especially for the samples obtained from 1473 K. Generally, the fivemembered structure of N-5 tended to convert to the more stable forms N-6 and N-Q at high temperatures. Hence, the decomposition of N-Q into N-6 might occur after long-term devolatilization treatment under high-temperature conditions (30 min, 1173 K) [27]. Thus, N-6 was the dominant structure of the nitrogen-containing functionalities on the surface of char particles. Based on the deconvolution results of each type C(N) summarized in Fig. 6(d), (e) and (f), it could be assumed that the whole combustion process could be divided into two stages: (a) (burnoff ≤50%) and (b) (burnoff > 50%). The relative amount of N-5, which was generally treated as the precursor of HCN, expressed an increasing tendency in section (a) [39], mainly at the expense of N-6 and N-Q in the early stage. When the char conversion ratio exceeded 50%, the relative amount of N-5 decreased significantly, and an obvious growth of N-6 and N-Q was observed. The relative amount of N-5 continued to decrease with increasing char burnoff up to 70%. The results in Fig. 6(e) illustrate that the addition of H2O in the reaction atmosphere accelerated the generation of N-5 on the particle surface at the early stage, so more HCN was released from the decomposition of N5. When combustion occurred at 1473 K, only C(N) with high thermal stability (N-6 and N-Q) could exist on the particle surface, and the destruction of N-5 tended to occur immediately when the char particles entered the combustion zone. Consequently, the amount of N-5 on the particle surface became very low, and trace HCN was identified during the O2/H2O combustion process at 1473 K. In an attempt to identify the effect of H2O concentration on the evolution of C(N) during the O2/H2O combustion process, the relative amount of each type C(N) with an identical conversion degree (approximately 0.3) is summarized in Fig. 7. The results in Fig. 7 illustrate that the addition of H2O in the reaction atmosphere alters the evolution characteristics of C(N) during the reaction process. Obviously, the enhancement in the H2O concentration led to an apparent increase of N-5

Fig. 8. Structures of nitrogen containing functionalities on the surface of the demineralized char.

and N-6 at the expense of N-Q, indicating that H2O molecules could accelerate the decomposition of thermally stable N-Q into N-5 or N-6 directly or indirectly. Additionally, due to the massive conversion of stable N-Q into relatively reactive N-5 or N-6, the remission rate of the nitrogen content in the char particles might also be accelerated during the combustion process, and this assumption was consistent with the results shown in Fig. 2. 3.3.2. Evolution characteristics of C(N) during the O2/H2O combustion process Based on previous studies, it could be determined that N-5 and N-6 are located in the five/six-membered ring structures, respectively, as shown in Fig. 8. In addition, N-Q generally lay in aromatic structures or in the chars as part of the graphite layer [40]. Therefore, the reactivity of the nitrogen-containing functional groups should be summarized as follows: N-5 > N-6 > N-Q. The hydroxyls generated from H2O decomposition could attach to the surface of the chars and promote the decomposition of the large aromatic structures that were initially inert into reactive small ones [9,10]. Then, the nitrogen atoms lying in the aromatic structures might be exposed to the outside, and some N-Q would convert to N-6 or N-5, as shown in (13). Subsequently, the small and mobile %H could penetrate into the matrix of the chars and change the structure of the sample. The penetration of %H normally could break the activated aromatic structures and cross-linking structures, resulting in more condensed and stable char structures [11]. Therefore, some N-5 or N-6 structures existing at the edge of aromatic ring structures might be converted into N-Q in the process of aromatic structure restructuring.

O2 + ˙OH + N − Q → N − 5 + N − 6 + C(O)

Fig. 7. Correlation between the H2O concentration and the distribution of the nitrogen-containing functional group. (a): 1073 K; (b): 1473 K. 9

(13)

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O2 + ˙OH + N − 6 → N − 5 + C(O)

(14)

experimental campaign is gratefully acknowledged.

˙H + N − 5 → HCN + Cf

(15)

References

O2 + N − 6 → NO + CO2

(16)

O2 + N − Q → NO + CO2

(17)

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The results shown in Fig. 6(e) show that the relative amount of N-5 increased at the early stage of the reaction (stage a) at the expense of N6 and N-Q, as shown in Eqs. (13) and (14). This phenomenon might be attributed to the destruction of large aromatic ring structures into small ring structures caused by the oxidizing agents (O atoms or %OH). Part of the thermally stable nitrogen-containing functional groups (N-6 and NQ) tended to first convert into N-5 at the early stage of the O2/H2O combustion process and then decompose into the gaseous products HCN or NO. The other N-6 and N-Q might be oxidized into NO by the oxidants (O2 or %OH) and released directly [41]. On the other hand, the thermal stability of N-6 and N-Q was stronger than that of N-5, so when the reaction temperature was constant, the decomposition rate of N-5 was generally faster than that of N-6 and N-Q, leading to a reduction in the relative amount of N-5 as the reaction proceeded. Furthermore, due to the participation of massive %H in the reaction, some reactive small ring structures might change into condensed and stable char structures, leading to the enhancement of the relative amount of N-6 and N-Q on the particle surface. Based on previous studies and the XPS results obtained in this research, the primary evolution pathway of C(N) on the char surface during O2/H2O combustion can be summarized by Eqs. (13)–(17). Based on the results above, it could be assumed that there were two primary NO generation pathways, oxidation of HCN by oxidizing reactants (secondly) and generation from the decomposition of C(N) under high temperatures (primarily). 4. Conclusions Characterization of the properties of C(O) and C(N) on the char surface during the O2/H2O combustion process was carried out. The results showed that the Tr and steam concentration had a direct effect on the generation, conversion and decomposition of surface functional groups. Specific conclusions are shown below: 1. The addition of H2O molecules had positive effects on the emission of nitrogen content in char particles, so the residual nitrogen content on the surface of char particles decreased gradually with increasing H2O concentration in the reaction atmosphere. Meanwhile, the enhancement of steam concentration generally promoted the conversion ratio of nitrogen to NO during the O2/H2O combustion process. 2. A trace amount of carboxyl or anhydride existed on the particle surface during the O2/H2O combustion process, indicating that %H or %OH would promote the conversion of carboxyl and anhydride into ether, phenol, quinone or lactone. The thermal stability of the primary oxygen-containing complexes generated under O2/H2O conditions are ether < phenol < quinone < lactone. 3. The C(O) amount played a vital role in char reactivity during O2/ H2O combustion, and there was a good linear relationship between the combustion/reduction reactivity and the C(O) amount, indicating that the amount of C(O) could be regarded as an indicator of char combustion/reduction reactivity. 4. In the process of O2/H2O combustion, the relative amount of N-5, which was the precursor of HCN, increased at the expense of N-6 and N-Q at the early reaction stage (Xc ≤ 50). When char burnoff exceeded approximately 50%, the relative amount of stable C(N) (N6 and N-Q) increased and became the dominant structures. Acknowledgements The support of the NSFC (No. 51536002) and (No. 51476046) in the 10

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11