Species Release and Transformation Mechanism during Pyrolysis and

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Detailed investigation on sodium (Na) species release and transformation mechanism during pyrolysis and char gasification of high-Na Zhundong coal Rongbin Li, Qun Chen, and Haixia Zhang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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Detailed investigation on sodium (Na) species release and transformation mechanism during pyrolysis and char gasification of high-Na Zhundong coal †

†

Rongbin Li*, , Qun Chen , Haixia Zhang

‡

†Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China ‡Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China KEYWORDS: Sodium; Occurrence mode; Pyrolysis; Gasification; Mechanism

ABSTRACT: A typical type of Zhundong coal with high-sodium (Na) concentration was pyrolyzed and the pyrolyzed char was gasified at temperature 850℃-950℃ in a laboratory-scale electrically heated tube-quartz fluidized bed reactor, to study the releasing characteristics and transformation mechanism of Na species in various occurrence modes including the H2O-soluble, NH4Ac-soluble, HCl-soluble and insoluble Na species. The various Na species in collected residues were quantified using a method of sequential chemical extraction (SCE) combined with inductive coupled plasma equipped atomic emission spectrometer (ICP-AES). The crystalline

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phases in residues were identified through the X-ray diffraction (XRD) analysis. The releasing characteristics and transformation mechanism of various Na species was in detail discussed through the qualitative and quantitative analyses on the experimental results. It was shown that the H2O-soluble and NH4Ac-soluble Na species mainly contributed to release of total Na species during pyrolysis while the H2O-soluble and insoluble Na species did that during char gasification, and a larger quantity of Na species was released at the stage of char gasification. And, different occurrence Na species behaved different releasing and transformation characteristics, showing a strong relationship with processes of pyrolysis and gasification. A chart describing the releasing characteristics and transformation mechanism of various Na species was given. It was speculated that the H2O-soluble Na species in the form of Na2SO4 and NaCl were more likely to be released into gas in the form of ultrafine particulates or at molecular/atomic scales through the particle pores along with the gas flow, with the NaCl partially being transformed to the NH4Ac-soluble Na species during pyrolysis and char gasification. The NH4Ac-soluble Na species was more likely to be transformed to the insoluble Na species after series of reactions between radicals from volatiles and Na species during pyrolysis, and was released accompanying with the consumption of char substrate during gasification. Besides, various Na species showed enrichment in residue particle during pyrolysis and char gasification. This work can provide a comprehensive understanding of various Na species release and transformation mechanism for the Zhundong coal gasification, which is essentially beneficial to optimize the gasification process in industry for controlling the ashrelated problems.

1. INTRODUCTION


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Coal, as an important fossil fuel, meets persistently energy demands in many aspects for industrial production and life of human being in the past and will continue to do in future. Lowrank coal such as brown coal or lignite and sub-bituminous coal, that accounts for a large proportion in the coal family, can provide to some extent a more economical energy compared to the high-rank coals. Low-rank coal is often characterized by nature of high aliphatic structural units and low aromatic structural units, and by special features such as high oxygen content, high moisture content, high volatile content, low ash, and abundant minerals and metallic elements including Mn, Ba, Sr, Fe, Al, Si, Ca, Mg, K, Na, and etc.. When the low-rank coals are converted through the traditional techniques such as direct combustion or gasification, a great number of operating and environmental problems arise to restrict their application, which may be attributed to the nature and special features of the raw coals. Therefore, cautions must be taken and basic understanding of behaviors on the low-rank coal conversion becomes urgent and of significance to achieve effective and environmentally friendly utilization. Recently, a coalfield with huge low-rank coal deposit was ascertained in the east of Junggar Basin, Xinjiang Province in Western China, which is often called briefly as Zhundong coalfield1. The Zhundong coal is of high quality with very low ash and very low levels of impurities such as Fe, S and trace element contents. However, when the Zhundong coal is directly combusted or gasified, serious ash-related problems of fouling, slagging, corrosion and bed agglomeration (in circulating fluidized bed) are encountered, that cause serious economic and safety problems due to unscheduled shut-down and frequent operations and plague engineers in the coal-fired power plants or in the coal gasifiers2-4. Investigations found that the Zhundong coal contains high contents of alkali metallic species, especially Na species. Analyses on more than 9 types of Zhundong coal5-7 demonstrated that the content of Na is widely higher than 2%, even reaches to


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10% in Na2O according to the ash composition analysis, which is distinctly different from that in the other type of coals. Further, the Na species in Zhundong coal exist in various occurrence modes or chemical forms, and they can be classified as “H2O-soluble”, “CH3COONH4 (NH4Ac)soluble”, “HCl-soluble” and “insoluble” determined by a sequential chemical extraction method of successive leaching with H2O, NH4Ac, and HCl solutions2, 6, 8, respectively. And, the H2Osoluble Na species and NH4Ac-soluble Na species often account totally for 60%-90% relative to the other type of Na species in Zhundong coal. The high content of Na is considered to be mainly responsible for the ash-related problems. The physical release and chemical transformation of Na species present during the high-Na lowrank coal conversion. A number of excellent researches on release and transformation of the Na species can be found in the literature. For example, based on the experimental measurements of atomic Na release and a theoretical equilibrium model in which single Loy Yang brown coal particles were burned in a flat flame environment, Van Eyk et al.9 found that approximately 67% of H2O-soluble Na species (mainly NaCl) and approximately 100% of the NH4Ac-soluble Na species were released into the gas following devolatilisation. The incomplete release of H2Osoluble Na species during devolatilisation was considered to be the reaction of H2O-soluble Na species within the coal structure whereby the char retained Na. Quyn et al.10, 11 found that the H2O-soluble Na species in the form of NaCl in the coal substrate was mainly released as Na and Cl separately, and the H2O-soluble Na species in the form of NaCl volatilized more easily than the NH4Ac-soluble Na species in the form of carboxylate, from the pyrolysis of the NaCl-loaded and Na-exchanged Loy Yang brown coals in a thermogravimetric analyzer (TGA) and in a fluidized-bed/fixed-bed reactor. Kosminski et al.12 found that more Na species were released during gasification in steam or carbon dioxide than for pyrolysis in nitrogen. An intermediate


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transformation product of sodium carbonate was identified in the residual pyrolyzed char. Wei et al.13 found that part of H2O-soluble Na species and insoluble Na species were transformed into HCl-soluble Na species during steam gasification of the NaCl-loaded and Na-exchanged coals. In practice, the Na vapors released from the coal/char particles may adhere to the cold wall in the form of fine particulates14, or further react with sulfur dioxide (SO2), sulfur trioxide (SO3) in the gas to form sodium sulphates15-18. And, the transformation products may participate the reactions with ash constituents such as silica (SiO2), aluminum oxide (Al2O3), ferric oxide (Fe2O3), calcium oxide (CaO) or other metal oxides to form the low-melting point alloys19. These reaction products may cause the ash-related problems. In addition, the release and transformation of Na species are affected by varieties of factors such as temperature10,

12, 13

, Na species

concentration13, operating conditions including pressure and atmosphere20-23, fuel additives24, coal chemical element compositions21, 25, 26. These extensive investigations provided an intensive and comprehensive understanding of releasing characteristics and transformation mechanism of the Na species for controlling the ash-related problems. For the Zhundong coal, Li et al.2 believed that the tendency of ash deposition during the Zhundong coal combustion is closely related to the form of alkali metals except for its content. He et al.27 combusted the sequentially washed coal with H2O, NH4Ac, and HCl solutions and found that the various Na species were released in a decreasing order of H2O-soluble Na, NH4Ac-soluble Na, HCl-soluble and insoluble Na, and over 64% of the total Na species released came from the H2O-soluble Na species. They hereby suggested a possible way of washing the coal with water before combustion for limiting the Na species release. Compared to combustion, Song et al.28 found that more Na species was retained in the ash under gasification conditions, and they also found that there was no slagging, agglomeration occurred when the Zhundong coal


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was gasified under the oxygen-rich condition with steam water in a circulating fluidized bed (CFB) reactor with a fuel treatment rate of 0.4 t/d29. They concluded that the high-melting point sodium silicates of NaAlSiO4 markedly formed in ash inhibited the slagging and agglomeration. It seems that the gasification process can inhibit to some extent the ash-related problems of utilizing the Zhundong coal. Wang et al.30 carried out experiments on devolatilization of two types of Zhundong coal in a laboratory-scale electrically heated tube-quartz reactor, and focused on effects of factors including occurrence modes of Na species, temperature, atmosphere of N2 and CO2, duration time, and coal particle size on the release and transformation of Na species in the four forms. They found that the release of Na species behaved a nonmonotonic variation with the coal particle size, and the duration time had an insignificant influence on release and transformation of Na species at low temperature, while it did significantly at high temperature. No more Na species was obviously released from the residual chars of Zhundong coal with the extension of duration time at high temperature, but more H2O-soluble Na species was still transformed into insoluble Na species. And, devolatilization under CO2 atmosphere can inhibit the release of Na species and formation of insoluble Na species compared to that under N2 condition. However, little attentions were paid on the releasing characteristics and transformation mechanism of Na species. And up to date, the comprehensive mechanism of Na species during the Zhundong coal thermal conversion (e.g. devolatilization, combustion, gasification) has not been clearly elucidated. To gain the underlying mechanism of release and transformation of Na species in various occurrence modes and provide a comprehensive understanding, basic knowledge for controlling and solving the ash-related problems, the coal thermal conversion (e.g. combustion, gasification) was artificially individed into two main parts, that is coal devolatilization/pyrolysis and char


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combustion/gasification, and experiments including pyrolysis of a typical type of Zhundong coal and gasification of the pyolyzed char were carried out at temperature 850℃ -950 ℃ in an electrically heated fluidized bed reactor in this paper. The Na species in various occurrence modes retained in resultant residues from pyrolysis and char gasification were quantified by the method of sequential chemical extraction combined with inductive coupled plasma equipped atomic emission spectrometer (SCE+ICP-AES). The releasing characteristics and transformation mechanism of Na species in various occurrence mode during pyrolysis and char gasification are in detail discussed according to the qualitative and quantitative analyses. And, a chart describing the releasing characteristics and transformation mechanism of Na species in various occurrence modes is given.

2. EXPERIMENTAL 2.1 Sample preparation A sample of “as-mined” coal (marked as TCML) obtained from the coal field in the Mori Kazak Autonomous County in east of Junggar basin was used as raw materials in this study. The coal sample was firstly air-dried in an oven at temperature 105℃ to eliminate the external moisture, and then pulverized and sieved to obtain a size range of 180 µm-355 µm. Table 1 shows the proximate and ultimate analyses of the coal sample. Table 2 shows the ash composition analysis. According to the ash composition analysis, the content of Na element in coal was calculated to be 1993.1µg/g coal, which is lower than 2587.2µg/g coal that was determined by the ICP-AES method. Because the ash temperature in ash composition analysis is


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815℃ according to the Standards GB/T1574-2007, a quantity of Na species might volatilize. Herewith, the results by ICP-AES determination were used throughout this study. 2.2 Experimental setup and procedure The whole experiment covers pyrolysis of the coal and gasification of the pyrolyzed char. The coal was firstly pyrolyzed under N2 atmosphere at temperatures of 850℃, 900℃ and 950℃and the pyrolyzed char was gasified under CO2 atmosphere at corresponding temperatures of 850℃, 900℃ and 950℃, respectively. All of the experiments were carried out in a laboratory-scale electrically heated tube-quartz fluidized bed reactor, in which a porous quartz frit is installed on the bottom as the gas distributor. The inner diameter of reactor is 16 mm and the total heating length is 580 mm. The influence of tube wall on behaviors of Na species during experiments was considered to be negligible because the reactor was manufactured by the material of high density alumina quartz. Figure 1 shows the schematic diagram of experimental system. The system is mainly consisted of the reactor, carrier gas cylinder, electric heater, flowmeters, absorption bottles and temperature controller. The reactor is heated by the electric heater with a maximum temperature of 1200℃ measured by thermocouples with an accuracy of ±1℃. The gas flow is controlled by mass flow meters with an accuracy of 2%. A typical experimental run steps as follows. For the pyrolysis, the reactor was firstly heated up to a pre-set temperature and was kept at the temperature for a period of time under the N2 atmosphere. Then, the prepared coal sample of around 2g was directly loaded into the reactor from top, and the reactor was plugged with a stopper immediately. When the experiment finished, the resultant residue was cooled down in the reactor to room temperature under N2 atmosphere, and the residue was withdrawn from the reactor and collected and weighed. Finally,


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the Na species in various occurrence modes in residues was determined using the SCE+ICP-AES method. The experimental temperature was set to be 850℃, 900℃ and 950℃, and the holding times were set to be 30s, 2min, 5min, 10min, 20min, 30min and 45min. And, the char from pyrolysis at time of 45min was used for CO2 gasification. The char-CO2 gasification operated the same as pyrolysis, except for the atmosphere and samples’ amounts used. The reactor was under CO2 atmosphere during gasification experiment and under N2 atmosphere during cooling residue. Table 3 gives the amounts of samples used in the pyrolysis and gasification experiments. Considering that the volatile-char interactions could play an important role in the behavior of Na species during pyrolysis31, a close to 2g coal sample was used under all pyrolysis conditions. And, considering the higher carbon conversion rate, larger quantities of char samples were used under the high temperature gasification conditions. For both pyrolysis and gasification, the flow of carrier gas was around 360 mL⋅min-1 during process of loading sample, and was around 480 mL⋅min-1 during processes of heating up, experiments and cooling down. Lowering the gas flow rate when loading samples kept the reactor under N2 or CO2 atmosphere and meanwhile made the sample be not carried out of the reactor along with the flow. Both carrier gases had a purity of 99.999%. 2.3 Analysis and characterization Despite that amount of Na species released into the hot gas product can be measured on-line by using excimer laser-induced fluorescence (ELIF)9, 22, 32, molecular beam mass spectrometry (MBMS)33, the releasing amount and behaviors of Na species in various occurrence modes cannot be clearly obtained. The SCE combined with ICP-AES is an off-line measurement method that can distinguish the Na species in various occurrence modes in residue during


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coal/char conversion. In this study, all the samples/residues were sequentially extracted with deionized water, ammonium acetate (NH4Ac, 1.0mol⋅L-1), hydrochloric acid (HCl, 1.0mol⋅L-1) at temperature of 60℃ for 24h, and the solid residue after HCl extraction was digested with solution of nitric acid (HNO3) and hydrofluoric acid (HF) and hydrogen peroxide (H2O2) using a microwave digestion. The amount of Na species in various occurrence modes in the extracted and digested solutions were determined by the ICP-AES. Besides, the X-ray diffraction (S2, Rigaku, Japan) was employed to determine the crystalline phases in the coal sample, chars and residues. The Surface Area and Porosity Analyzer (QUADRASORB SI-MP, Quantachrome, America) was employed to determine the parameters including specific surface area, total pore volume and average pore diameter of the coal sample, chars and residues. For the parameter analysis, nitrogen (N2) at temperature 77.35K was used as the adsorbate. And, the specific surface area was characterized using the standard multipoint Brunauer-Emmett-Teller (BET) method34, 35, the total pore volume and average pore diameter were characterized using the Barrett-Joyner-Halenda (BJH) method35, 36.

3. RESULTS AND DISCUSSION 3.1 Occurrence modes of Na species in the coal and pyrolyzed chars The occurrence modes of Na species in the coal and pyrolyzed chars were distinguished by “H2O-soluble”, “NH4Ac-soluble”, “HCl-soluble” and “insoluble”. In chemical form, the H2Osoluble Na species dissolves within inherent moisture in coal in the form of sodium chloride (NaCl) and sodium sulphate (Na2SO4), because the inorganic anions of Cl-, SO42- were detected


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in extracted solutions by using the ion chromatograph (IC-900, Thermo Fisher, America). The NH4Ac-soluble Na species is organically bound to coal/char matrix as cations in the form of carboxylic groups (-COONa). The HCl-soluble Na species is organically bound to coal/char matrix as coordinate covalent bonds to -N or –O containing groups, and the insoluble Na species mainly exists in coal/char substrate as silicate or aluminosilicate. The insoluble Na species is much more stable than soluble Na species and not like to volatilize. Figure 2(a) shows the content of Na species in various occurrence modes in the coal and chars pyrolyzed at temperatures of 850℃, 900℃and 950℃ and at time of 45min, respectively, and Figure 2(b) shows the relative content of various Na species in the coal and chars. For the raw coal, the content of total Na species determined was 2587.2µg/g coal. The H2O-soluble Na species and NH4Ac-soluble Na species are predominant, which take up ~70% and ~27% of the total Na species, respectively, and the proportion of HCl-soluble Na species and insoluble Na species is less than 3%. Different from the raw coal, the contents of total Na species in chars are obviously higher than the content of raw coal, in quantities of 3646.0µg/g char, 3807.3µg/g char and 3869.7µg/g char, suggesting that the Na species is enriched when the coal was converted to char. In addition, although the H2O-soluble Na species and NH4Ac-soluble Na species in chars still take up a large proportion compared to the other two type of Na species, the proportion becomes smaller when the coal was converted to char. The proportion of H2O-soluble Na species and NH4Ac-soluble Na species decreases from ~70% to ~60% and from ~27% to ~15%, respectively. While the HCl-soluble Na species and insoluble Na species are largely increased by 4-10 times and 10-20 times, respectively. 3.2 Release and transformation of Na species during pyrolysis of the coal


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Volatile matters and solid residues are formed through complex reactions involving bond breaking, vaporization, condensation and cross-linking during pyrolysis37. The Na species in various occurrence modes will be released and transformed, accompanied by the diffusion of volatile gases and the pyrolysis reactions. Figure 3 shows the variation in content of Na species in various occurrence modes with the holding time and temperature of pyrolysis. In order to facilitate understanding, all the quantities of Na species determined were multiplied by the residue yield rate to convert to the content based on 1g coal. General speaking from Figure 3, the total content of Na species, the content of H2O-soluble Na species and NH4Ac-soluble Na species have a sharp drop at first period of 5min, while the HCl-soluble and insoluble Na species follow an inverse trend at this period. All of the contents of Na species change slowly after 5min. Quantitatively, it can be obtained that the total amounts of Na species released were 381.3µg⋅g-1, 372.9µg⋅g-1, and 304.5µg⋅g-1 at temperatures of 850℃, 900℃ and 950℃ and at time of 45min, accounting for 14.7%, 14.4% and 11.8% of the content of total Na species in raw coal, respectively. Individually, the decrement of H2O-soluble Na species was 491.3µg⋅g-1 (27.2% of the total H2O-soluble Na species in the coal), 368.8µg⋅g-1 (20.4%) and 491.5µg⋅g-1 (27.2%) at temperatures of 850℃, 900℃ and 950℃, and that of NH4Ac-soluble Na species was 353.4µg⋅g-1 (19.6% of the total NH4Ac-soluble Na species in the coal), 451.1µg⋅g-1 (25.0%) and 402.4µg⋅g-1 (22.3%), respectively. The increment of HCl-soluble Na species was 190.7µg⋅g-1 (10.6% of the total HCl-soluble Na species in the coal), 59.6µg⋅g-1 (3.3%) and 75.3µg⋅g-1 (4.2%), and that of insoluble Na species was 272.7µg⋅g-1 (15.1% of the total insoluble Na species in the coal), 387.5µg⋅g-1 (21.4%) and 514.1µg⋅g-1 (28.4%), respectively. The variation of total Na species is


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comparable to those in Wang et al.30, and the quantitative results are in general agreement with Van Eyk et al.9 who found that ~19% of total Na species and ~33% of the H2O-soluble Na species from the Loy Yang brown coals was released during pyrolysis, and with Kosminski et al.12 who found that 10%-12% of the total Na species from Lochiel coal loaded with NaCl was released in the first 2min of pyrolysis. The deviation between our quantitative results and those in Van Eyk et al.9 and Kosminski et al.12 may be due to the difference from the coal sample used. From the variation and quantitative data, we can draw that the release of H2O-soluble and NH4Ac-soluble Na species contribute mainly to the decrease of total Na species. A part of H2Osoluble and NH4Ac-soluble Na species were transformed to the HCl-soluble or insoluble Na species during pyrolysis, resulting in the increase of the HCl-soluble and insoluble Na species. And, the higher pyrolysis temperature favors the transformation of Na species among different modes, especially formation of insoluble Na species, but the temperature has little influences on the release of Na species under this investigation. Additionally, it should be noted that there is a rapid releasing and transformation of various Na species in the initial time of pyrolysis. This behavior is considered to be closely related to the pyrolysis process. For the coal particle used in this investigation, the pyrolysis proceeded on the entire particle and volatiles were released from the coal within much short time because the temperature at center of coal particles can reach to the ambient temperature in less than 1s as the coal was loaded into the reactor, according to the heat transfer calculation38. A mechanism of release and transformation of Na species in various occurrence modes and the relation between the behaviors of Na species and pyrolysis process are in detail analyzed in the following. For the H2O-soluble Na species, it is speculated that a large quantity of crystallized Na species would be formed and deposited attached dispersedly to the surfaces of macro-, meso-


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and micropores of coal/char substrate when the external moisture, inherent moisture and crystal water were evaporated. The crystallized Na species may exist in the coal/char substrate as one of (or a combination of) three possible forms: ultrafine particulates less than 0.1µm39, molecular/atomic scales40, or liquid when temperature is higher than their melting point, which can however not be exactly ascertained under this investigation. During pyrolysis, the crystallized Na species can volatilize in the form of vapor at high temperature, or be carried out of coal/char particle through the pores with gas flow of the vapor and volatiles, resulting into the release of the H2O-soluble Na species. However, most of the H2O-soluble Na species were not easy to be released due to being millions of pores in the coal/char particle, and were retained within the particle. Particularly in the later time of pyrolysis, the amount of H2O-soluble Na species kept almost unchanged because no more volatiles were released from the coal. The carrying effect of gas flow can also be confirmed from the results of Zhang et al.14 who observed the fine NaCl phase in condensed volatile matter. The XRD analysis in Figure 4 identifies the existence of the species of NaCl and Na2SO4 in the pyrolyzed chars, the chemical reactions to describe this process can therefore include: crystallization NaCl(aq) or Na 2SO 4( aq )  → NaCl(s) or Na 2SO 4( s )

(1)

volatilization NaCl(l) or Na 2SO4 ( l) → NaCl(g) or Na 2SO 4( g )

(2)

Besides the release, transformation from the H2O-soluble Na species to NH4Ac-soluble Na species may occur as described in the reaction (3). In reaction (3), the R represents radicals from benzene, naphthalene or phenantherene, etc.

RCOOH( s ) + NaCl(s )  → RCOONa (s ) + HCl( g )

(3)


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For the NH4Ac-soluble Na species, they exist in the coal/char matrix as carboxylic groups (RCOONa). Two possible pathways are that the R-COONa groups was decomposed as Reaction (4), leading to the release of CO2 and formation of R–Na group41, and fragmentation of Nacontaining carboxylates like R-COONa was formed with breaking of bridge bonds during pyrolysis. The R-Na group was still attached to the matrix and retained within the coal/char substrate, as can be catalysts for the subsequent char gasification42-45. In the other one, the fragmentation of Na-containing carboxylic groups was liberated due to C-C bonds breaking and was changed into Na-containing free radicals, as described in reaction (5). The resultant radicals were not stable and possibly reacted with the oxygen-containing radicals such as -OH and -O in emitted volatile gas to their oxides or hydroxides, and then with Al2O3, SiO2 in ash to form sodium silicates or aluminosilicates (the insoluble Na species), as described in reactions (6)-(8). The sodium hydroxide (NaOH) was easy to volatilize due to a lower melting point of 318.4℃, and easy to react with CO to Na2CO3 which was identified by XRD analysis, as shown in Figure 4. Therefore, it can be concluded that the decrease of NH4Ac-soluble Na species is mainly attributed to reactions between the Na-containing groups and radicals formed from volatiles. decompose R − COONa  → R − Na + CO 2

(4)

heat R − COONa → R + ( −COONa )

(5)

→ NaOH + CO2(g) ( −COONa ) + ( −OH ) 

(6)

2 ( −COONa ) + ( −O )  → Na 2O + 2CO2(g)

(7)

Na 2 O + Al2 O 3 + SiO 2 + L  → sodium silicates/aluminosilicates

(8)


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The increase of the HCl-soluble Na species is relatively small, which is speculated to be mainly due to the reactions between Na-containing radicals and coal/char matrix. The increase of insoluble Na species is mainly due to the transformation from the H2O-soluble Na species and NH4Ac-soluble Na species. In addition to formation of the insoluble Na species from reaction (8), reactions between the H2O-soluble Na species in the form of Na2SO4 and SiO2 or Al2O3 in ash are another source of the insoluble Na species formation, as described in reactions (9) and (10)46. Na 2SO 4 + 3SiO2  → Na 2 O ⋅ 3SiO 2 + SO2(g) + 1 O 2(g) 2

(9)

Na 2 O ⋅ 3SiO 2 + Al 2 O 3 + 3SiO 2  → Na 2 O ⋅ Al2 O 3 ⋅ 6SiO 2

(10)

3.3 Release and transformation of Na species during CO2 gasification of the pyrolyzed char The chars pyrolyzed at temperatures of 850℃, 900℃ and 950℃ were gasified under CO2 atmosphere at the corresponding temperatures of 850℃, 900℃ and 950℃, respectively. Figure 5 shows the variation in the carbon conversion rate with the holding time and temperature of gasification. The carbon conversion rate is defined as the ratio of weight of the consumed char to that of the initial char. From Figure 5, the carbon conversion rate increases with increasing time and temperature, and reaches its maximum of 88.7% at temperature of 950℃and at time of 45min under this investigation. Figure 6 shows the variation in content of Na species in various occurrence modes with the carbon conversion rate during CO2 gasification of pyrolyzed char at temperatures of 850℃, 900


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℃ and 950℃, respectively, based on 1g coal. The values at carbon conversion 0 are equal to ones in chars pyrolyzed at time of 45min. It can be seen from Figure 6 that the content of total Na species in gasified residues decreases gradually in cases of temperature 850℃ and 900℃. While in the case of temperature 950℃, the content of total Na species has a sharp drop, especially when the carbon conversion rate is higher than 70%. The decrease of the total Na species indicates that more Na species was continued to be released into the gas following pyrolysis. And, the higher temperature leads to a larger releasing rate. Quantitatively, 24.1%, 25.6% and 64.4% of the total Na species in the coal were released into gas at the stage of char gasification at temperatures of 850℃, 900℃ and 950℃ after time 45min. A cumulative 38.8%, 40.0% and 76.2% of the total Na species in the coal were released combining the pyrolysis and char gasification under this investigation. Individually, the content of H2O-soluble Na species and insoluble Na species has a drop while the content of HCl-soluble Na species and the NH4Ac-solube Na species changes gently. Therefore, it can be concluded that both of H2O-soluble Na species and insoluble Na species are mainly responsible for the decrease of total Na species during char gasification. We speculate that the H2O-soluble Na species and insoluble Na species exist within the chars/gasified residues as ultrafine particulates on the surfaces of pores or at molecular/atomic scales in char matrix after pyrolysis. When the char was gasified, a part of the H2O-soluble Na species and insoluble Na species would detach physically from the surfaces or matrix with the consumption of char and be carried out of the char particle along with the gas flow, resulting into the release of these two Na species. Particularly in the case of temperature of 950℃, the decrease of H2O-soluble Na species is greatly accelerated because that the char substrate is quickly consumed. For NH4Ac-soluble


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Na species, it should be noted that there is a sharp drop when the carbon conversion rate is higher than 70% in the case of temperature 950℃, which can also be attributed to the quick consumption of the char substrate. To provide a further understanding of releasing characteristics and transformation mechanism of Na species in various occurrence modes during char gasification, Figure 7 shows the variation in content of various Na species with the carbon conversion rate during CO2 gasification of pyrolyzed chars at temperatures of 850℃, 900℃ and 950℃, respectively, based on 1g gasified residue. Different from the results in Figure 6, it can be seen from Figure 7 that all the other types of Na species show an increase in content except the insoluble Na species. And, the higher carbon conversion rate is, the higher content of Na species. These results indicate that the releasing rate of Na species is much slower than consumption rate of carbon, and large quantities of Na species (mainly the H2O-soluble and insoluble Na species) should be captured by the macro-, meso- and micropores formed during gasification, leading to the enrichment of Na species in residue substrate with consumption of char. And, it appears to suggest that the behavior of each Na species is closely related to the char properties such as the specific surface area and pore size during the gasification process. Table 4 gives the surface and pore structure parameters of the coal, char pyrolyzed at temperature 900℃, and residues gasified at temperature 900℃ and at various carbon conversion rates 26%, 38%, 51%, and 62% (corresponding time 10min, 20min, 30min and 45min, respectively). It can be seen that the specific surface area and the total pore volume in gasified residues increase with increasing conversion rate, and the specific surface area is particularly about hundred times of that in the coal and chars, and the total pore volume is about ten times of that in the coal and chars. The average pore diameter in the residue is much smaller than that in the coal and chars. Therefore, the fine particulates or


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Energy & Fuels

molecular/atmoic forms can be again captured and retained on the newly formed surfaces of pores, and they can only be detached when the adjacent char substrate was consumed. Additionally, the high temperature favors the enrichment. Because the char substrate was quickly consumed, it can be seen from Figure 7(c), that the growth of contents of the total and H2Osoluble Na species is approximately exponential with the carbon conversion rate in case of temperature 950℃.

Besides the release, the various Na species can transform each other during char gasification. Figure 8 shows the crystalline phases determined by XRD in the gasified residues at temperature of 900℃ with holding times of 10min, 20min, 30min and 45min. The Na2SO4 and NaCl are two forms of the H2O-soluble Na species in the coal. However, the NaCl can almost not be identified in the gasified residues from the XRD patterns in Figure 8. The presence of Na2SO4 and absence of NaCl indicate that they behavior differently during char gasification. A possible mechanism for release and transformation of Na2SO4 and NaCl is that Na2SO4 was mainly carried out of the char particle via the pores along with the gas flow, while NaCl was more likely to be transformed to NH4Ac-soluble Na species as reaction (3), due to slight increase of NH4Ac-soluble Na species. Moreover, the insoluble Na species such as NaAlSiO4 or Na2Si2O5 are identified from the XRD patterns, which might be transformed from the H2O-soluble Na species in the form of NaCl and the NH4Ac-soluble Na species. In conclusion, a description of releasing characteristics and transformation mechanism of Na species in various occurrence modes during the Zhundong coal pyrolysis and the pyrolyzed char gasification processes is unified and illustrated in Figure 9.


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4. CONCLUSIONS The experimental results on the release and transformation of Na species in various occurrence modes from pyrolysis of a high-Na Zhundong coal and gasification of the pyrolyzed char in a fluidized-bed reactor, and analyses on the releasing characteristics and transformation mechanism indicate: 1) For the Zhundong coal used in this study, most of Na species were released following pyrolysis. Experimental results showed that a cumulative 38.8%, 40.0% and 76.2% of the total Na species in the coal were released at experimental temperatures of 850℃, 900℃ and 950℃respectively, with a corresponding release of 24.1%, 25.6% and 64.4% at the stage of char gasification. 2) The H2O-soluble and NH4Ac-soluble Na species contributed mainly to release of total Na species during pyrolysis while the H2O-soluble and insoluble Na species did that during char gasification. 3) Releasing characteristics and transformation mechanism of various Na species are different at stages of pyrolysis and char gasification. The H2O-soluble Na species and insoluble Na species were more likely to be released into the gas in the form of ultrafine particulates or at molecular/atomic scales through the pores of particle along with the gas flow during pyrolysis and char gasification. The NH4Ac-soluble Na species and HCl-soluble Na species were more likely to be released and transformed through reactions between the Na species and free radicals from volatile during pyrolysis, and these two types of Na species showed a gentle change during char gasification.


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4) The H2O-soluble Na species in the form of Na2SO4 and NaCl had different releasing characteristics and transformation mechanism. Na2SO4 was mainly carried out of the char particle via the pores along with the gas flow, while NaCl was more likely to be transformed to form the NH4Ac-soluble Na species. 5) In addition to release and transformation, an enrichment process of H2O-soluble, HCl-soluble and insoluble Na species during pyrolysis, and enrichment of H2O-soluble, HCl-soluble and NH4Ac-soluble Na species during char gasification were observed under the investigation. AUTHOR INFORMATION Corresponding Author *Phone: +86 10 62797933. E-mail: [email protected] ORCID Rongbin Li: 0000-0002-3605-6570 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51506199). REFERENCES (1) Zhou, J. B.; Zhuang, X. G.; Alastuey, A.; Querol, X.; Li, J. H. Geochemistry and mineralogy of coal in the recently explored Zhundong large coal field in the Junggar basin, Xinjiang province, China. Int. J. Coal. Geol. 2010, 82 (1-2), 51-67. (2) Li, G. Y.; Wang, C. A.; Yan, Y.; Jin, X.; Liu, Y. H.; Che, D. F. Release and transformation of sodium during combustion of Zhundong coals. J. Energy Inst. 2016, 89 (1), 48-56.


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(3) van Eyk, P. J.; Kosminski, A.; Ashman, P. J. Control of Agglomeration and Defluidization during Fluidized-Bed Combustion of South Australian Low-Rank Coals. Energy Fuels 2012, 26 (1), 118-129. (4) Song, G. L.; Qi, X. B.; Song, W. J.; Lu, Q. G. Slagging Characteristics of Zhundong Coal during Circulating Fluidized Bed Gasification. Energy Fuels 2016, 30 (5), 3967-3974. (5) Weng, Q. S.; Wang, C. A.; Che, D. F.; Fu, Z. W. Alkali metal occurrence mode and its influence on combustion characterisitics in Zhundong coals. J. Combust. Sci. Technol. 2014, 20 (03), 216-221. (6) Liu, J.; Wang, Z. H.; Xiang, F. P.; Huang, Z. Y.; Liu, J. Z.; Zhou, J. H.; Cen, K. F. Modes of occurrence and transformation of alkali metals in Zhundong coal during combustion. J. Fuel Chem. Technol. 2014, 42 (3), 316-322. (7) Chen, C.; Zhang, S. Y.; Liu, D. H.; Guo, X.; Dong, A. X.; Xiong, S. W.; Shi, D. Z.; Lv, J. F. Existence form of sodium in high sodium coals from Xinjiang and its effect on combustion process. J. Fuel Chem. Technol. 2013, 41 (7), 832-838. (8) Smeda, A.; Zyrnicki, W. Application of sequential extraction and the ICP-AES method for study of the partitioning of metals in fly ashes. Microchem. J. 2002, 72 (1), 9-16. (9) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J. The release of water-bound and organic sodium from Loy Yang coal during the combustion of single particles in a flat flame. Combust. Flame 2011, 158 (6), 1181-1192. (10) Quyn, D. M.; Wu, H. W.; Li, C. Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples. Fuel 2002, 81 (2), 143-149. (11) Quyn, D. M.; Wu, H. W.; Bhattacharya, S. P.; Li, C. Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence. Fuel 2002, 81 (2), 151-158. (12) Kosminski, A.; Ross, D. P.; Agnew, J. B. Transformations of sodium during gasification of low-rank coal. Fuel Process. Technol. 2006, 87 (11), 943-952. (13) Wei, X. F.; Huang, J. J.; Fang, Y. T.; Wang, Y. Transformation of sodium during gasification of a lignite with addition of NaCl and NaAc. J. Fuel Chem. Technol. 2009, 37 (1), 6-10. (14) Zhang, L.; Ninomiya, Y.; Yamashita, T. Occurrence of inorganic elements in condensed volatile matter emitted from coal pyrolysis and their contributions to the formation of ultrafine particulates during coal combustion. Energy Fuels 2006, 20 (4), 1482-1489. (15) Glarborg, P.; Marshall, P. Mechanism and modeling of the formation of gaseous alkali sulfates. Combust. Flame 2005, 141 (1-2), 22-39. (16) Niksa, S.; Helble, J.; Harada, M.; Ando, T.; Shigeta, J.; Kajigaya, I. Coal quality impacts on alkali vapor emissions from pressurized fluidized bed coal combustors. Combust. Sci. Technol. 2001, 165, 229-247. (17) Steinberg, M.; Schofield, K. The chemistry of sodium with sulfur in flames. Prog. Energy Combust. Sci. 1990, 16 (4), 311-317.


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(18) Srinivasachar, S.; Helble, J. J.; Ham, D. O.; Domazetis, G. A kinetic description of vapor-phase alkali transformations in combustion systems. Prog. Energy Combust. Sci. 1990, 16 (4), 303-309. (19) Kosminski, A.; Ross, D. P.; Agnew, J. B. Reactions between sodium and kaolin during gasification of a low-rank coal. Fuel Process. Technol. 2006, 87 (12), 1051-1062. (20) Schurmann, H.; Monkhouse, P. B.; Unterberger, S.; Hein, K. R. G. In situ parametric study of alkali release in pulverized coal combustion: Effects of operating conditions and gas composition. Proc. Combust. Inst. 2007, 31, 1913-1920. (21) Oleschko, H.; Muller, M. Influence of coal composition and operating conditions on the release of alkali species during combustion of hard coal. Energy Fuels 2007, 21 (6), 3240-3248. (22) Gottwald, U.; Monkhouse, P.; Wulgaris, N.; Bonn, B. In-situ study of the effect of operating conditions and additives on alkali emissions in fluidised bed combustion. Fuel Process. Technol. 2002, 75 (3), 215226. (23) Jin, H.; Chen, Y. N.; Ge, Z. W.; Liu, S. K.; Ren, C. S.; Guo, L. J. Hydrogen production by Zhundong coal gasification in supercritical water. Int. J. Hydrog. Energy 2015, 40 (46), 16096-16103. (24) Schurmann, H.; Unterberger, S.; Hein, K. R. G.; Monkhouse, P. B.; Gottwald, U. The influence of fuel additives on the behaviour of gaseous alkali-metal compounds during pulverised coal combustion. Faraday Discuss. 2001, 119, 433-444. (25) Oleschko, H.; Schimrosczyk, A.; Lippert, H.; Muller, M. Influence of coal composition on the release of Na-, K-, Cl-, and S-species during the combustion of brown coal. Fuel 2007, 86 (15), 2275-2282. (26) Matsuoka, K.; Yamashita, T.; Kuramoto, K.; Suzuki, Y.; Takaya, A.; Tomita, A. Transformation of alkali and alkaline earth metals in low rank coal during gasification. Fuel 2008, 87 (6), 885-893. (27) He, Y.; Qiu, K. Z.; Whiddon, R.; Wang, Z. H.; Zhu, Y. Q.; Liu, Y. Z.; Li, Z. S.; Cen, K. F. Release characteristic of different classes of sodium during combustion of Zhun-Dong coal investigated by laserinduced breakdown spectroscopy. Sci. Bull. 2015, 60 (22), 1927-1934. (28) Song, G. L.; Song, W. J.; Qi, X. B.; Lu, Q. G. Transformation Characteristics of Sodium of Zhundong Coal Combustion/Gasification in Circulating Fluidized Bed. Energy Fuels 2016, 30 (4), 3473-3478. (29) Song, W. J.; Song, G. L.; Qi, X. B.; Lu, Q. G. Transformation characteristics of sodium in Zhundong coal under circulating fluidized bed gasification. Fuel 2016, 182, 660-667. (30) Wang, C. A.; Jin, X.; Wang, Y. K.; Yan, Y.; Cui, J.; Liu, Y. H.; Che, D. F. Release and Transformation of Sodium during Pyrolysis of Zhundong Coals. Energy Fuels 2015, 29 (1), 78-85. (31) Li, C. Z. Importance of volatile-char interactions during the pyrolysis and gasification of low-rank fuels A review. Fuel 2013, 112, 609-623. (32) Glazer, M. P.; Khan, N. A.; de Jong, W.; Spliethoff, H.; Schurmann, H.; Monkhouse, P. Alkali metals in circulating fluidized bed combustion of biomass and coal: Measurements and chemical equilibrium analysis. Energy Fuels 2005, 19 (5), 1889-1897.


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(33) Blasing, M.; Muller, M. Mass spectrometric investigations on the release of inorganic species during gasification and combustion of German hard coals. Combust. Flame 2010, 157 (7), 1374-1381. (34) Foo, K. Y.; Hameed, B. H. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 2010, 156 (1), 2-10. (35) Lowell, S.; Shields, J. E.; Thomas, M. A.; Thommes, M., Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density. Springer Netherlands: 2004. (36) Sing, K. S. W.; Williams, R. T. Physisorption hysteresis loops and the characterization of nanoporous materials. Adsorpt. Sci. Technol. 2004, 22 (10), 773-782. (37) Yu, J.; Lucas, J. A.; Wall, T. F. Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: A review. Prog. Energy Combust. Sci. 2007, 33 (2), 135-170. (38) Li, H. B.; Bai, X. G.; Hu, Z.; Wang, Y.; Zhang, B. J. Pyrolysis of Coal in a Fluidized Bed: Experimental and Mathematical Simulation. Coal Chem. Ind. 1998, (83), 28-33. (39) Zhang, L.; Masui, M.; Mizukoshi, H.; Ninomiya, Y.; Kanaoka, C. Formation of submicron particulates (PM1) from the oxygen-enriched combustion of dried sewage sludge and their properties. Energy Fuels 2007, 21 (1), 88-98. (40) Li, C. Z. Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal. Fuel 2007, 86 (12-13), 1664-1683. (41) Schafer, H. N. S., The Science of Victorian Brown Coal: Structure, Properties, and Conseuences for Utilization. Butterworth-Heinemann: Oxford, United Kingdom, 1991. (42) Delannay, F.; Tysoe, W. T.; Heinemann, H.; Somorjai, G. A. The role of KOH in the steam gasification of graphite - indentification of the reaction steps. Carbon 1984, 22 (4-5), 401-407. (43) Kuang, J. P.; Zhou, J. H.; Zhou, Z. J.; Liu, J. Z.; Cen, K. F. Catalytic mechanism of sodium compounds in black liquor during gasification of coal black liquor slurry. Energy Conv. Manag. 2008, 49 (2), 247-256. (44) Suzuki, T.; Ohme, H.; Watanabe, Y. A mechanism of sodium-catalyzed CO2 gasification of carbon investigation by pulse and TPD techniques. Energy Fuels 1992, 6 (4), 336-343. (45) Mims, C. A.; Pabst, J. K. Role of surface salt complexes in alkali-catalyzed carbon gasification. Fuel 1983, 62 (2), 176-179.

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Table Captions Table 1. Proximate and ultimate analyses of the TCML coal sample (wt%) Table 2. Ash composition analysis of the TCML coal sample (wt% in ash) Table 3. Amounts of coal/char samples used in the experiments of pyrolysis and char-CO2 gasification (g) Table 4. Surface and pore structure parameters of the coal, pyrolyzed char, and gasified residue at different carbon conversion in case of temperature 900℃


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Table 1. Proximate and ultimate analyses of the TCML coal sample (wt%) Ultimate analysis

Proximate analysis Mad

Ad

Vd

FCd

Vdaf

C

H

N

O

St

Cl

14.34

3.69

31.54

64.77

32.75

75.34

3.53

0.61

16.31

0.53

0.065

Table 2. Ash composition analysis of the TCML coal sample (wt% in ash) SiO2 3.73

Al2O3 Fe2O3 6.16

5.37

CaO

MgO TiO2

33.45

5.42

0.41

SO3 29.34

P2O5 K2O Na2O 0.00

0.45

7.28


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Table 3. Amounts of coal/char samples used in the experiments of pyrolysis and char-CO2 gasification (g) Items

30s

2min

5min

10min

20min

30min

45min

850℃ 2.0320 1.9619 1.9334 2.0689 2.0509 2.0601 2.0449 pyrolysis

900℃ 2.0125 2.0364 2.019

2.0222 2.0164 2.0138 2.0405

950℃ 2.2036 1.9557 2.0014 2.1825 2.3148 2.361 Char-CO2

2.3894

850℃ 0.9302 0.9582 1.0911 1.2827 1.8748 1.8263 2.437

gasification 900℃ 1.4632 1.3294 2.0642 1.5806 1.5679 2.1602 2.3797 950℃ 1.8696 1.8312 1.894

2.0454 2.3041 3.0386 1.5742

Table 4. Surface and pore structure parameters of the coal, pyrolyzed char, and gasified residue at different carbon conversion in case of temperature 900℃ Items

the coal

char

26%

38%

51%

62%

Specific surface area, m2⋅g-1

5.214

8.095

471.4

571.1

665.2

741.5

Total pore volume, cm3⋅g-1

0.01652

0.02045

0.1413

0.1874

0.2195

0.2387

Average pore diameter, nm

11.53

8.864

2.254

2.343

2.341

2.321


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Figure Captions Figure 1. Schematic diagram of the electrical heated tube-quartz fluidized bed reactor system Figure 2. Occurrence modes of Na species in the coal and chars pyrolyzed at temperatures of 850℃, 900℃ and 950℃ and time of 45min, respectively, (a) content of Na species (b) relative content of Na species Figure 3. Variation in content of Na species in various occurrence modes with the holding time during the coal pyrolysis at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g coal Figure 4. XRD patterns of the chars pyrolyzed at temperatures 850℃, 900℃ and 950℃ Figure 5. Variation in the carbon conversion rate with the holding time during char-CO2 gasification at temperature 850℃, 900℃ and 950℃ Figure 6. Variation in content of Na species in various occurrence modes with carbon conversion rate during char-CO2 gasification at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g coal Figure 7. Variation in content of Na species in various occurrence modes with carbon conversion rate during char-CO2 gasification at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g gasified residue Figure 8. XRD patterns of the residues gasified at temperature 900℃ and holding times 10min, 20min, 30min and 45min


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Energy & Fuels

Figure 9. Illustration on releasing characteristics and transformation mechanism during the coal pyrolysis and char gasification

2

3

4

6 1

5

7

1. N2/CO2 cylinder; 2. Valves; 3. Flowmeters; 4. Temperature controller; 5. Electric heater; 6. Reactor; 7. Absorption bottles Figure 1. Schematic diagram of the electrical heated tube-quartz fluidized bed reactor system


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(a)

Page 30 of 38

(b)

Figure 2. Occurrence modes of Na species in the coal and chars pyrolyzed at temperatures of 850℃, 900℃and 950℃ and time of 45min, respectively, (a) content of Na species (b) relative content of Na species


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Energy & Fuels

(a)

(b)

(c)

Figure 3. Variation in content of Na species in various occurrence modes with the holding time during the coal pyrolysis at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g coal


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a-Na2SO4; b-Na2CO3; c-SiO2; d-Na2SO4/Na2S5/CaS; e-NaCl; f-Na2Si2O5/Na8Ca3Si5O17 Figure 4. XRD patterns of the chars pyrolyzed at temperatures 850℃, 900℃ and 950℃


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Figure 5. Variation in the carbon conversion rate with the holding time during char-CO2 gasification at temperature 850℃, 900℃ and 950℃


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(a)

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(b)

(c)

Figure 6. Variation in content of Na species in various occurrence modes with carbon conversion rate during char-CO2 gasification at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g coal


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Energy & Fuels

(a)

(b)

(c)

Figure 7. Variation in content of Na species in various occurrence modes with carbon conversion rate during char-CO2 gasification at temperatures (a) 850℃, (b) 900℃ and (c) 950℃, based on 1g gasified residue


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a-Na2SO4; b-Na2CO3; c- CaCO3; d-NaAlSiO4/Na2Si2O5; e-Na; f-NaCl Figure 8. XRD patterns of the residues gasified at temperature 900℃ and holding times 10min, 20min, 30min and 45min


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Energy & Fuels


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Figure 9. Illustration on releasing characteristics and transformation mechanism during the coal pyrolysis and char gasification


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Species Release and Transformation Mechanism during Pyrolysis and

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