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In-situ Raman spectroscopy study on catalytic pyrolysis of a bituminous coal Huaili Zhu, Guangsuo Yu, Qinghua Guo, and Xingjun Wang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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Energy & Fuels

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In-situ Raman spectroscopy study on catalytic pyrolysis of a bituminous coal

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Huaili Zhu, Guangsuo Yu*, Qinghua Guo, Xingjun Wang*

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Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education

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East China University of Science and Technology, Shanghai, 200237 (PR China)1

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Abstract: How different catalysts would affect the evolution of char structure with increasing temperature in

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pyrolysis of coal is fundamentally important for coal clean utilization. In this study, we applied in-situ Raman

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spectroscopy and a fixed-bed reactor to characterize the evolution of char structure and product gas formation

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behavior. The results showed that the formation curves of main gases presented two stages and the catalyst

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enhanced the pyrolysis significantly. The Raman spectra were fitted with a combination of 4 Lorentzian bands (D1,

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D2, D4, G) and 1 Gaussian band (D3) in the first-order region. Spectral parameters, such as band position, full

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widths at half maximum (FWHM) and band area ratio, were used to characterize char structure. The D1 and G

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band FWHM, band area ratios ID1/IG, ID3/IG and ID4/IG increased with increasing temperature while the D1 and G

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band positions and IG/IAll decreased indicating a decrease in the ordering of char structure. The addition of catalyst

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led to a lower degree of char structure order. A thermogravimetric analysis (TGA) was also used to measure the

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reactivity of char derived from pyrolysis and the results showed a good correlation between reactivity indexes and

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IG/IAll.

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Keywords: coal pyrolysis; char structure; in-situ Raman spectroscopy; gasification reactivity

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1 Introduction

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Gasification has been an efficient technology for coal clean utilization. The whole process of coal

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gasification involves two main steps: coal pyrolysis and char gasification. As the prior step of char gasification,

*Corresponding authors. Tel.: +86-21-64252974; Fax: +86-21-64251312. E-mail: [email protected](Guangsuo Yu); [email protected](Xingjun Wang)

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pyrolysis or devolatilization of coal is essential and has been extensively studied over the past decades.1-3 Good

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understanding of the mechanism of pyrolysis and decomposition reactions of coal would help clarifying the char

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structure evolution. Pyrolysis conditions, such as heating rate, temperature and catalyst, have dramatically

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influence on the evolution of char structure which affects the gasification reactivity of char.

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In pyrolysis process, coal is devolatilized with the production of char, gas and tar. In initial time, temperature

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is low and the weak non-covalent bonds such as hydrogen bond are cracked and reduced.4 With rising temperature,

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bridge bonds begin to be cleaved, resulting in the formation of free radical groups and a large amount of CO and

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CO2. Meanwhile, the residue char undergoes further condensation reactions, which has been recognized as one of

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the primary reasons responsible for the low reaction rate in the later stage of char gasification.5 Considerable

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literatures have reported that metal-based catalyst, such as alkalis (K),6-8 alkaline earths (Ca)9-11 and transition

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metals (Fe),12 can significantly improve the pyrolysis and gasification reactivity of carbon materials, which is

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partially due to catalytic effect on chemical structure of char in pyrolysis process. Therefore, investigating how a

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catalyst would affect coal pyrolysis behavior and char structure evolution is fundamentally important for coal

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clean utilization technologies.

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In the past decades, considerable works have been done on carbon materials, such as soot, coal and biomass,

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by using different experiment apparatuses combined with Raman spectroscopy analytical technique. Li13-15 and his

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coworkers worked on Victorian brown coal using fluidized-bed/fixed reactor and found correlations between char

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structure and pyrolysis/gasification behaviors. Effect of bio-char structure on its gasification reactivity was also

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studied by using a novel quartz reactor and the results showed that the structure of bio-char played a more

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dominant role in the char intrinsic combustion reactivity.16 Thermo gravity analysis (TGA) combined with Raman

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spectroscopy was applied in investigation of effect of char structure on combustion reactivity and the results

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showed that the increase of char microstructural order under heat treatment could be characterized by Raman


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parameters and the combustion reactivity of the chars from demineralized coals was found to have good

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correlations with band area ratios.17 A gas-fired drum pyrolyser18 and drop tube furnace (DTF)19 were also been

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used as experiment apparatuses to study the evolution of biomass/coal char structure.

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Numerous analytical techniques, such as X-ray diffraction (XRD),20, 25-29

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Fourier transform infrared

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spectroscopy (FTIR),22-24 Raman spectroscopy

and transmission electron microscopy (TEM),19, 30 have been

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used to characterize the evolution of char structure, and good correlations between analytical parameters and char

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structure have been established. XRD technique provides information about stacking structure of aromatic layers

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as well as a size of layers, while TEM illustrates the change of carbon crystal structure. FTIR spectroscopy is used

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to record the chemical information such as C-H bond of aromatic/aliphatic and oxygen-containing groups, but

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some useful information such as molecular structure is still unachievable by FTIR.31 For carbon materials,

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however, Raman spectroscopy is a more suitable technique to illustrate the carbon structure for its sensitive not

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only to crystal structures but also to molecular structures.27

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However, the off-line data collected from different experiments using Raman spectroscopy analytic technique

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cannot directly reflect the changes of coal structure during heating process；While the in-situ Raman spectroscopy

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can achieve the purpose of reflecting the changes of char structure directly. In-situ Raman spectroscopy has not

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been widely used on coal pyrolysis/gasification but has been developed rapidly on graphene32 and other research

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areas.33 In this work, in-situ Raman spectroscopy was used to investigate the catalytic pyrolysis process of a

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Chinese bituminous coal under isothermal heating condition. Three different catalysts, Fe2O3, Ca(OH)2 and

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K2CO3, were chosen in the experiments. A fixed-bed reactor and TGA were also used to characterize the

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microstructure changes and gasification reactivity of char derived from coal pyrolysis. The objective was to apply

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in-situ Raman spectroscopy to characterize the evolution of coal char microstructure directly under different

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temperature in heating process and to investigate the correlation between char chemical structure and pyrolysis


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behavior as well as the gasification reactivity of char.

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2 Experimental

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2.1 Materials

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Raw materials used in the study were a Chinese bituminous coal named Shenfu coal (SF) and three kinds of

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catalysts (K2CO3, Ca(OH)2 and Fe2O3). In this study, all catalysts, including K2CO3 (purity >99.0%), Ca(OH)2

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(purity >99.0%), and Fe2O3 (purity >99.0%), were obtained from Sinopharm Chemical Regent Co., Ltd. Before

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experiments, the raw coal was dried at 105℃ under N2 atmosphere for 24 h and then pulverized and sieved to

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obtain a fraction sample of particle sizes range from 80 to 120 µm. The characteristic analysis data of coal are

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summarized in Table 1 and Table 2.

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The catalyst loading was 10 wt% in this study, 6 which is the weight percent of the metal atoms referenced to

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the amount of dry coal. The method of catalyst loading was a solution impregnation method. The procedure of

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impregnating was as follows: a certain amount of catalyst powders were completely dissolved in 100 ml of

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deionized water to form solution, and then 10 g sample was added into the solution; this mixture was stirred at

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70–80℃ under N2 atmosphere using a magnetic stirrer until the liquid was transformed into a thickened mass.

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Finally, this thickened mass was dried at 105 ℃ in a vacuum oven for 24 h. The dried samples were ground,

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mixed uniformly, and sieved to a size range between 80 - 120 µm before experiments. The samples with different

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catalysts were denoted as SF-Fe, SF-Ca and SF-K, respectively.

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2.2 Product gas release measurement

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A fixed-bed reactor6 was used to pyrolyze coal samples in this study. The system is primarily composed of

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four parts: gas feeding, reactor, controlling units, and analysis units. The reactor (0.05 m i.d. and 1.0 m height) is

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made of a special heat-resistant Inconel 625 alloy, designed to maintain up to 1223 K and 6 MPa. Metal supports

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and quartz sand were first added to the reactor, followed by an alundum tube (50 cm height), and 5 g samples in


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each run were placed in the middle area of the reactor. With the use of quartz and the alundum tube, the samples

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were not in direct contact with the metal tube, avoiding catalytic effect of the metal tube on the pyrolysis of coal

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samples.

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The furnace was heated at a rate of 25 K/min until it reached to predetermined temperature. Meanwhile, the

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flow rate of outlet gas was measured by a flow meter, and the major gaseous products, such as methane, hydrogen,

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carbon monoxide and carbon dioxide, were quantitatively determined using an online non-dispersive infrared flue

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gas analyzer (Gasboard-3100). At the end of each experiment, pure N2 was purged into the reactor until the reactor

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was cooled to room temperature. The remaining chars were collected for further analysis. Each experiment was

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repeated at least three times.

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2.3 Reactivity measurement in TGA

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The reactivity of char reacting with CO2 was measured using a TGA (made by NETZSCH, STA449F3

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Jupiter). The sample (about 15 mg) was placed in an alumina crucible and heated at the rate of 25 K/min to 105℃

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and held for 30 min to drive off the moisture in sample; then heated at the rate of 25 K/min to 1000℃ under a

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stream of N2. Once the temperature reached to predetermined temperature, gasification started isothermally at

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atmospheric pressure by sweeping CO2 into the reactor. The final mass was then taken as the mass of ash. The

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reactivity index (Rs) of char was calculated using the following equation: 34

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ܴ‫= ݏ‬

ଵ

(1)

ఛబ.ఱ

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where ߬଴.ହ is the time required for the conversion to reach 50%.

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2.4 In-situ Raman spectroscopy measurement

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The evolution of char structure in pyrolysis was measured by an in situ Raman spectroscopy system. The

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system is mainly composed of four parts: a Renishaw inVia Raman spectrometer, a heating stage (TS1500,

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Linkam), a temperature controller and an online computer system gathering sample spectrogram during different


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reaction phases continuously. The schematic diagram of the in-situ Raman spectroscopy system is shown in Fig. 1.

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About 2 mg of corresponding sample was placed on the center surface of heating stage which was heated at 25

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K/min to 105 ℃ and held for 30 mins to drive off the moisture in sample. Then the system was heated at the rate

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of 25 K/min to prescribed temperature (800 ℃) under a continuous nitrogen flow of 100 mL/min. The Raman

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signal was collected at 20 ℃, 200 ℃, 400 ℃, 500 ℃, 600 ℃ and 800 ℃, respectively. The ex-situ data

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(spectra of chars derived from pyrolysis at different temperatures) were also collected as a comparison.

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A microscope equipped with a 50*lens was used to focus the excitation laser beam (514.5 nm exciting line of

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a Spectra Physics Ar-laser) on sample and to collect the Raman signal in the backscattered direction. The beam

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was controlled to reach on the surface of char particles with a final laser power of ~2 mW and a spot diameter of 1

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µm. The laser spot was much larger than the size of carbon micro-crystallites, ensuring the collection of the

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average information from a large number of the micro-crystallites of char. The spectra were recorded in the

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wavenumber range of 1000–1800 cm-1 covering the first-order region. The acquisition time for each spectrum was

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30 s. For the accuracy of experimental results, each experiment was repeated three times as well.

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3 Results and discussion

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3.1 Product gases release behavior

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Release behavior of main product gases were investigated in a fixed-bed reactor with an online non

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dispersive infrared flue gas analyzer (Cube, Gasboard-3100) which can record the percentage change of gases

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every second, and then the formation rates of main gases can be calculated. Fig. 2 presents the formation rates of

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main gases of the four samples under the same pyrolysis conditions.

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Compared to raw coal, the catalysts-loading samples show high pyrolysis reactivity. It is found that the main

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gases released in pyrolysis are CO, CO2, CH4 and H2, and the devolatilization does not start until the temperature

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reached to 300 ℃. CO2 and CO release firstly followed by CH4, CnHm and H2, which indicates that oxygen


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containing functional groups crack first to produce CO2 and CO and then the aromatic and aliphatic groups crack

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at higher temperature to produce CH4, CnHm and H2, which is consistent with those literatures.24, 35, 36 The oxygen

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containing functional groups usually contain C=O groups in esters, quinones, carboxylic acids and carbonyl

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compounds. At low temperature, CO and CO2 are mainly derived from the decomposition of carboxylic acids and

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carboxyl compounds. At high temperature, the more stable oxygen-contained groups such as C=O groups in esters

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and quinones decompose with the release of CO and CO2.37 When the temperature rises up to 400℃, a large

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amount of H2 with a few of CH4 and CnHm appear. It can be seen in Fig. 2a that H2 shows high formation rate, a

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wide curve peak starts at approximate 400 ℃ and reaches maximum at 800 ℃. For SF-Fe sample, the curve

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peak of H2 formation rate shows a wider peak width indicating high value of total H2 yield than SF sample (Fig.

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2b). H2 formation rates of SF-Ca and SF-K sample are enhanced quickly and reach maximum values at the

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temperature below 700 ℃ (Fig. 2c and Fig. 2d), which means a better catalytic effect of Ca/K-based catalyst than

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Fe2O3. H2 derives from the decomposition of aromatic structures and heterocyclic compounds at high

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temperature.3 The CH4 formation rate curves show highly similarity for the four samples and start at about 400 ℃

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and then reach a maximum at temperature between 400 ℃ and 500 ℃.

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It is obvious that the pyrolysis process is divided into two stages and the second stage starts at the

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temperature varied from approximate 600 ℃ to 700 ℃, and the gas formation rate is enhanced again. Fig. 2b

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and Fig. 2c show that the second stage is extremely enhanced by catalysts and the Ca-based catalyst has a greater

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catalytic effect than a Fe-based catalyst for H2 and CO production. The CO2 formation rate is enhanced relative to

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that for the other catalyst-impregnated samples while the CO formation is inhibited and there is nearly no CO

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released at the first stage, which may be caused by the high content of carbonate in the initial time. The probable

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reactions are as follows: 6

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K2CO3+C-O→K2O2-C+CO2

(2)


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K2CO3+C=O→K2O2-C+CO2

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(3)

where K2O2-C is an intermediate produced at low temperature.

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For SF-Ca and SF-Fe sample, CO formation rate is dramatically enhanced at the second stage, which is

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probably caused by the improved decomposition of esters and quinones under the effect of Ca/Fe- based catalysts.

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3.2 In-situ Raman spectroscopy study

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Fig. 3 shows the baseline-corrected Raman spectra of samples gathered at different temperatures. It can be

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found that the Raman intensity decreases with increasing temperature for all samples and there are two

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characteristic peaks at 1350 cm-1 (D band or “defect” band) and 1580 cm-1 (G band or “Graphite” band) in each

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spectrum. The intensity of G band is stronger than that of D band. The variations of G and D bands intensity

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reflect the change of char structure in pyrolysis process. It is reported that both Raman scattering ability and the

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light absorptivity of the samples can affect Raman intensity,38 and the structural change affects the light

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absorptivity.

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Furthermore, for getting more spectral information such as peak position, the full width at half maximum

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(FWHM) and integrated area (I) of each band and better understanding of char structure evolution in pyrolysis

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experiments, a curve fitting software, Origin8.5/Peak Fitting Module, was used to resolve Raman spectra into 4

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Lorentzian bands (G, D1, D2 and D4) and 1 Gaussian band (D3 band) (Fig. 4). The D1 band is commonly called

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the defect band and appears at ~ 1350 cm-1 and can be attributed to the in-plane imperfections such as defects and

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heteroatoms and corresponds to a graphitic lattice vibration mode with A1g symmetry.27, 39, 40 The D2 band at

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~1620 cm-1 is always appearing as a shoulder on the G band, which represents the vibration mode of disordered

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graphitic lattices and corresponds to a graphitic lattice mode with E2g symmetry.25 41 The D3 band, at ~ 1500 cm-1,

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is suggested to originate from a sp2 band form of amorphous carbon including organic molecules and fragments

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of functional groups26 and the D4 band at ~ 1200 cm-1 is still under debate for its attribution and is tentatively


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attributed to sp2-sp3 bonds or C–C and C=C stretching vibrations of polyene-like structures.27, 42, 43 The G band or

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‘‘Graphite’’ band (~ 1580 cm-1) corresponds to an ideal graphitic lattice vibration mode with E2g symmetry.25, 44

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3.2.1 FWHM and peak position of bands

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Fig. 4 shows curve-fitted results of SF coal in the first-order region at room temperature. It provides typical

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information about the microstructure of SF bituminous coal. The raw spectrum in the first-order region has two

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obvious bands at ~1350 cm-1 and ~1580 cm-1. It can be found that the G and D2 band are sharp and have narrow

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FWHM, while the D1, D3 and D4 band have wider FWHM indicating the poor order of SF coal.

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The spectral parameters, peak position and FWHM, have been widely studied for characterizing the structure

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of carbon materials and good correlation between Raman spectral parameters and the degree of carbon structure

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order has been discovered.45-47 The variations of D1 and G band FWHM and position with the increasing

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temperature in pyrolysis process are shown in Fig. 5

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The results show that the D1 band FWHM of the four samples increase with increasing temperature, while

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the G band FWHM increases initially and then decreases at 600 ℃. The D1 band FWHM of SF-Fe has the

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maximum increase range and increases from 172 cm-1 at 20℃ to 220 cm-1 at 800 ℃, while the D1 band FWHM

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of SF increases from 161 cm-1 at 20 ℃ to 185 cm-1 at 800 ℃. Fig. 5b reveals that the crystalline components

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increase at first stage and decrease at second stage and the final values of G bands FWHM indicate that the carbon

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structure order decreases at high temperature, which may be caused by the thermal crack of carbon at high

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temperature. The wider G bands FWHM of SF-Ca and SF-K samples also suggest that the Ca/K-based catalysts

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play an important role in cracking carbon at high temperature.

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Fig. 5c shows that D1 band positions of all samples decrease with increasing temperature reflecting the

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fracture of large aromatic ring systems and the formation of heteroatom groups, which is consistent with Fig. 5a.

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The different D1 band positions of four samples in initial time indicate that the catalyst loading process at 105 ℃


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has influence on char structure. The G band positions are shift to low wavenumber with the increase of

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temperature as shown in Fig 5d.

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The band position and band FWHM were often used to characterize char structure order.48 However, because

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Raman measurements only yield relative spectra, the band parameters, particularly the FWHMs, depend to some

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extent on the recorded intensities (i.e. count points) of each spectrum.49 Fig. 5 shows that both band position and

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FWHM vary with temperature and parameters of different chars have no obvious trend. So simply using band

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position or FWHM is not enough to represent the evolution of char structure. The band area ratio, including D1,

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D2, D3 and D4 band to G band (denoted as ID1/IG, ID2/IG, ID3/IG, ID4/IG) and G band to the integrated area (denoted

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as IG/IAll), is a parameter combined two parameters and is widely used to characterize the char structure.

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3.2.2 Band area ratios

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The results of band area ratios variation with increasing temperature are showed in Fig. 6. It can be seen that

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ID1/IG, ID3/IG and ID4/IG increase with increasing temperature significantly while ID2/IG has a decrease trend. As

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reported by Tuinstra in 1970s,50 the ratio ID1/IG is inversely proportional to the microcrystalline planar size, so the

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increase of ID1/IG with increasing temperature represents the decrease of the graphitic microcrystalline planar size,

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as shown in Fig. 6a. It is also can be found that the ID1/IG of SF-Ca and SF-K samples are higher than that of

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SF-Fe and SF samples at high temperature indicating considerable effect of Ca and K on cracking the graphitic

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microcrystalline planar. It is noted that the ID1/IG ratio as well as ID3/IG and ID4/IG of SF-Fe is lower than that of SF

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sample, which may be caused by the comprehensive effects of devolatilization and catalytic. In the

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devolatilization process, the char structure is condensed caused by the release of volatile, thus leading the decrease

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of ID1/IG.17 Meanwhile, the catalyst reacts with carbon with the cracking of the carbon structure leading the

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increase of ID1/IG. For SF-Fe sample, the comprehensive effects of Fe2O3 leading the lower ID1/IG than that of SF.

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The ID2/IG shows an opposite trend that decreases with increasing temperature as shown in Fig. 6b. As stated


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above, the D2 band represents the disordered graphitic lattices at the surface of graphitic crystal. In pyrolysis

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process, those surface structures are broken leading the diminishing of ID2/IG value. The obviously different ID2/IG

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ratios of four samples in initial time suggest that catalyst loading process at 105 ℃ also has significantly impact

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on the surface structures of coal.

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The evolution of the amorphous carbon structure of samples with increasing temperature can be

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characterized by ID3/IG ratio as shown in Fig. 6c. Similar trends of ID3/IG ratio curves are observed in Fig. 6c

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indicating the increase of amorphous carbon structure. As temperature rising, the ID4/IG ratios increase

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simultaneously (Fig. 6d), which is caused by the cracking of larger aromatic rings. The high ID4/IG ratio is

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attributed to the abundance of sp3 or sp2–sp3 mixed structures. It can be found that the SF sample has the highest

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value of ID4/IG ratio at high temperature, which may be caused by the less consumption of carbon and

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oxygen-contained groups in initial time as reported by Chabalala.51

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Fig. 6e shows the variations of IG/IAll ratios of four samples with increasing temperature. For samples of SF

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and SF-Fe, the IG/IAll ratios increase firstly and then decrease, while the IG/IAll ratios of SF-Ca and SF-K samples

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decrease immediately. But the final values of IG/IAll ratios are lower than that in initial time indicating the reducing

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of graphite structure in char. Combined with the results in Fig. 5, it can be inferred that the degree of char

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structure at high temperature is lower than that at normal temperature and catalysts have effects on carbon

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cracking, thus making the catalyst-loading samples obtain lower degree of carbon structure order than raw coal.

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The catalytic effects of the three catalysts can be ranked as K2CO3>Ca(OH)2>Fe2O3.

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3.2.3 Ex-situ Raman spectra

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For comparison, the spectra of chars prepared ex-situ at different temperatures were measured. The results

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(800 ℃ for instance) of in-situ and ex situ spectra are shown in Fig.7. It can be found that ex-situ spectra of all

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samples are different from in-situ spectra significantly. Raman intensities of ex-situ spectra are higher than of


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in-situ spectra for the reason of chemical structural change at high temperature. G and D band position of in-situ

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spectra are shift to low wavenumber meaning the fracture of char structure. For getting more information about

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char structural change, band area ratios of ID1/IG and IG/IAll variations with temperatures are shown in Fig. 8. The

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trends of band area ratios of ID2/IG, ID3/IG, ID4/IG are similar to that of ID1/IG and not shown here.

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Fig. 8 shows that ID1/IG increases and IG/IAll decreases with increase temperature, which means that all chars

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transformed to disordered structure. Compared with in-situ results in Fig. 6, it can be found that ID1/IG ratios are

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lower and IG/IAll ratios are higher than that of in-situ spectra. In heating process, highly ordered char structure is

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partially thermal cracked making char structure order decrease, and the reactions between catalyst and carbon also

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make ordered char structure decrease. Therefore, ID1/IG and IG/IAll ratios of ex-situ spectra at the same condition

250

are significantly different from that of in-situ spectra. It can be inferred that catalyst reacts with carbon at high

251

temperature with producing more disordered char structure which not exist in chars at normal temperature.

252

3.3 Relationship between Raman parameters and char reactivity

253

The gasification reactivities of four different chars derived from pyrolysis experiments are shown in Fig. 9.

254

Fig. 9a shows the variations of char conversion with gasification time and Fig. 9b shows the reactivity indexes of

255

different samples. It is obvious that the SF-K char has the highest reactivity followed by SF-Ca and SF-Fe chars

256

and the SF char has the lowest reactivity, which is consistent with the results of in-situ Raman spectroscopy.

257

In catalytic gasification process, a part of catalyst combines with carbon and transforms to active

258

intermediate which would react with CO2 in the follow step. Through this transformation, the catalyst affects the

259

evolution of char structure and further affects the gasification reactivity of char. Besides, the true densities of chars

260

have been measured and the results are shown in Fig. 10. True density refers to the density of solid matter after

261

removing the internal pore or space between particles and is connected to porosity and represents the physical structure

262

of char. It can be found that true densities of chars with catalysts are higher than that of non-catalyst because of the


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release of more volatiles. SF-Ca char and SF-Fe char, which have higher true densities compared to SF char, have

264

higher reactivities. It can be inferred that the physical structure is not the main factor influencing on the reactivity

265

of char and the different reactivity is attributed to the chemical characteristics.

266

The relationship between IG/IAll ratio at 800 ℃ and reactivity index of char is shown in Fig. 11. It can be

267

seen that good correlation is obtained (correlation coefficients better than 0.9489). With the increase of IG/IAll ratio,

268

the reactivity of char decrease linearly. The good correlation confirms that the strong connection between char

269

structure reflected by the in-situ Raman spectra and gasification reactivity. Therefore, the reactivity of different

270

chars derived from catalytic pyrolysis can be predicted by in-situ Raman spectroscopy technique.

271

4. Conclusion

272

In-situ Raman spectroscopy was used in this work for better understanding the evolution of char structure

273

with increasing temperature directly and clarification of catalytic effects of different catalysts on pyrolysis of a

274

bituminous coal. The conclusions were achieved as follows:

275

(1) The main product gases emission behaviors of different samples were studied in a fixed-bed reactor. The

276

results showed that CO2 and CO released first at the temperature above 300 ℃ followed by CH4, H2

277

and CnHm at the temperature above 400 ℃. With the addition of catalyst, the pyrolysis reactivity was

278

enhanced. The formation rates curves of main gases suggested that the pyrolysis process presented two

279

stages.

280

(2) Raman spectra of four samples at different temperatures were used to characterize the evolution of char

281

structure during pyrolysis. The increased D1 and G band FWHM, ID1/IG, ID3/IG and ID4/IG as well as the

282

decreased D1 and G band positions and IG/IAll at high temperature indicated the decrease of the carbon

283

structure order degree. With the addition of catalysts, the char obtained lower degree of carbon structure

284

than that of raw coal. The rank of the three catalysts is K2CO3>Ca(OH)2>Fe2O3.


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

285

(3) Good correlation between IG/IAll ratios and reactivity indexes of chars derived from pyrolysis was found.

286

With the increase of IG/IAll ratio, the reactivity of char decreased linearly. Therefore, the in-situ Raman

287

spectroscopy could be a useful technique to predict the char reactivity in pyrolysis of coal.

288 289

Acknowledgements This work is partially supported by National Nature Science Foundation of China (Grant 21176078).

290

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406 407 408 409 410 411 412 413 414 415 416 417 418

Figure captions Fig. 1. Schematic diagram of in-situ Raman spectroscopy Fig. 2. Gas formation rates of four samples Fig. 3. Baseline-corrected Raman spectra of four samples at different temperatures. Fig. 4. Curve-fitting for SF coal of Raman spectra in the first-order region. Fig. 5. Variations of D1 and G band FWHM and position with increasing temperature Fig. 6. Variations of band area ratio with increasing temperature Fig. 7. Baseline-corrected Raman spectra of ex-situ and in-situ at 800℃. Fig. 8. Band area ratios of chars ex-situ prepared at different temperatures Fig. 9. a: Variations of char conversion with gasification time; b: Reactivity indexes of different chars Fig. 10. True densities of coal and chars prepared at 800℃ Fig. 11. Correlation between reactivity index and band area ratio of IG/IAll


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419 420 421

Fig. 1. Schematic diagram of in-situ Raman spectroscopy


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422

423 424 425

Fig. 2. Gas formation rates of four samples


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427 428 429

Fig. 3. Baseline-corrected Raman spectra of four samples at different temperatures.


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430 431

Fig. 4. Curve-fitting for SF coal of Raman spectra in the first-order region.

432 433


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435 436 437 438

Fig. 5. Variations of D1 and G band FWHM and position with increasing temperature, a: D1 band FWHM; b: G band FWHM: c: D1 band position; d: G band position


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439

440

441 442 443

Fig. 6. Variations of band area ratio with increasing temperature


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445 446 447

Fig. 7. Baseline-corrected Raman spectra of ex-situ and in-situ at 800℃ ℃.


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448 449 450

Fig. 8. Band area ratios of chars ex-situ prepared at different temperatures


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451 452 453

Energy & Fuels

Fig. 9. a: Variations of char conversion with gasification time; b: Reactivity indexes of different chars


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454 455

456 457 458 459

Fig. 10. True densities of coal and chars prepared at 800℃ ℃


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460 461 462

Energy & Fuels

Fig. 11. Correlation between reactivity index and band area ratio of IG/IAll


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Table captions

463 464

Table 1. Proximate analysis and ultimate analysis of SF coal

465

Table 2 Ash compositions of SF coal

466 467


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468 469

Table 1 Proximate analysis and ultimate analysis of SF coal Proximate analysis wd /%

Ultimate analysis wd /%

Sample

SF

V

FC

A

C

H

O

N

S

33.29

58.65

8.06

74.20

4.21

11.93

0.97

0.63

470 471


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

472

Page 34 of 34

Table 2 Ash compositions of SF coal Constituent

Sample SF

w/%

Al2O3

CaO

Fe2O3

SiO2

SO3

NaO

K2 O

MgO

17.21

13.85

11.78

44.52

6.53

1.89

1.35

1.55

473 474


In Situ Raman Spectroscopy Study on Catalytic ... - ACS Publications

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