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Coupling Pyrolysis and Gasification Processes for Methane-Rich Syngas Production: Fundamental Studies on Pyrolysis Behavior and Kinetics of a Calcium-Rich High Volatile Bituminous Coal Bin Tian, Yingyun Qiao, Jun-feng Fan, Lei Bai, and Yuanyu Tian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01788 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 16, 2017
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Coupling Pyrolysis and Gasification Processes for Methane-Rich Syngas Production: Fundamental Studies on Pyrolysis Behavior and Kinetics of a Calcium-Rich High Volatile Bituminous Coal Bin Tian†, Yingyun Qiao†,*, Junfeng Fan§, Lei Bai‡, Yuanyu Tian†,§,* †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),
Qingdao, 266580, China §
Shandong Provincial Key Laboratory of Universities and Colleges of Low-carbon Energy and
Chemical Technology, Shandong University of Science and Technology, Qingdao, Shandong 266590, China ‡
Department of Chemical and Biomedical Engineering, West Virginia University, West Virginia
26506, United States ABSTRACT: Coupling pyrolysis and gasification (CPG) processes in the fluidized bed reactor to produce methane-rich syngas is an attractive technology in efficient and clean utilization of coals. Pyrolysis plays a leading role in this technology and other relevant processes. Pyrolysis behavior, gaseous product evolution, and kinetics of a calcium-rich high volatile bituminous coal were deeply investigated using thermogravimetry coupled with on-line FTIR. The results showed that inherent minerals in the coal were mainly gypsum, calcite, and quartz. Except for SiO2, CaO is the most abundant species in the coal ash. The first stage mass loss of coal before 654 oC was attributed to functional group cleavage and aromatic ring condensation reactions, during which relative content of CH4 was largest among the hydrocarbon gases. The second stage mass loss was mainly caused by mineral (calcite and gypsum) decomposition. It has demonstrated the major source of CH4 formation was from cracking of aryl methyl rather than the long alkyl side chains on aromatics in the coal. Furthermore, pyrolysis of the coal could be divided into three kinetic stages 1 ACS Paragon Plus Environment
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based on the activation energy variation. The first and last stages for chemical bond cleavage and mineral decomposition in coal were both controlled by internal diffusion mechanism. However, the second stage involving condensation reaction followed second-order reaction mechanism. 1. INTRODUCTION Pyrolysis occupies an exceptional position in thermal conversion of coals and serves as the initial and accompanying reactions during coal hydrogenation, combustion, and gasification.1 Pyrolysis is also a useful tool for correlating coal structures with its reactivity under the accurately controlled conditions.2 Additionally, artificial pyrolysis is often employed to simulate natural coalification process.3 Therefore, coal pyrolysis has been receiving much attention for a long time in some aspects such as, mass loss and volatile release (tar, gases),4 mineral decomposition,5 volatile and char interaction,6 kinetic behavior,7 and cleavage of functional groups.8 Liu et al.9,10 investigated the temperature-dependent release of CH4 and CO during pyrolysis of a superfine pulverized coal in their series works and the specific functional groups for the gas generation were linked. It is commonly recognized that coal pyrolysis is the temperature-dependent process.11 Vaporization of the mobile phase begins near 250 oC, forming the aliphatic tar; emerging of aromatic tar and the organic gases induced by bond scission occurs above 400 oC and condensation reaction becomes dominated after 550 oC, during which large numbers of CO and H2 release. The progress of analytical tools made attempts to investigate the coal pyrolysis in the perspective of bond scission possible. Niu et al.12 investigated pyrolysis behavior of a perhydrous bituminous coal and established a kinetic method based on the direct loss of functional groups by in-situ Fourier transform infrared (FTIR) spectroscopy. Shi et al.13 elaborately analyzed coal pyrolysis process in thermogravimetric (TG) analyzer and proposed that mass loss was mainly 2 ACS Paragon Plus Environment
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caused by cleavage of a specific series of covalent bonds in the coal matrix. Particularly, physicochemical properties of several bituminous coals and their pyrolysis characteristics and kinetics were evaluated by multiple techniques including FTIR and TG/MS. The obtained results showed that these coals exhibited three main pyrolysis stages according to the mass loss rates and released gas species.14 Furthermore, sulfur atoms are thought to enhance the pyrolysis kinetics due to weaker bond energies of aryl and alkyl C‒S compared with that of C–C.15 Besides, the presence of char was proven to impediment of the crystalline mineral decomposition and transformation, which remained the catalytic effect of the minerals at higher temperatures.16 Pyrolysis kinetics is one of the important categories in coal pyrolysis research and it can reflect the pyrolysis mechanism and the reactivity of coal based on the kinetic parameters. It was recommended that pyrolysis kinetics of carbonous materials for each reaction stage should be evaluated with model-free methods and then model-fitting methods as supplementary are used to obtain the activation energy, pre-exponential factor and the fittest mechanism.17 Activation energy of Shenmu bituminous coal was from 293 to 766 kJ/mol in the conversion range of 0.05 to 0.95 based on Friedman method.18 However, activation energy of a low rank Neimeng coal was estimated from 50 to 250 kJ/mol with two model free methods (Friedman and FWO).19 Furthermore, three coals were subjected to thermal decomposition in air atmosphere and the first and second stage of mass loss were controlled by first order reaction and shrinking core mechanisms, respectively based on the Coats-Redfern kinetic method.20 Resent years, cascade conversion of coal in an efficient manner to obtain multiple and value-added products had drawn much attention and some novel technologies emerged, especially in China.21,22 Coupling pyrolysis and gasification (CPG) processes in one fluidized bed reactor using pulverized coal for methane-rich syngas production is one of these efficient coal utilization 3 ACS Paragon Plus Environment
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processes.23 As shown in Figure 1, coal particles are fed into the dilute zone at the bottom of the transport bed and contact with high-temperature gas and particles to perform rapid pyrolysis at oxygen-free atmosphere. The formed char passes through a two-stage cyclone separator to thoroughly strip the particles from the syngas. Larger char particles from the first-stage cyclone are circulated into the gasification zone of the reactor for further gasification. Fine char particles with low reactivity from secondary stage cyclone are injected into the combustion zone to provide heat for gasification and fast pyrolysis. Due to CPG system uses air as fluidizing agent and does not need air separation plant and coal volatile is extracted and maintained in the form of gaseous products prior to combustion and gasification, this technology can produce methane-rich syngas, increase the conversion efficiency, and reduce the investment. However, CPG process is also governed by coal pyrolysis due to most of the CH4 in the syngas is generated from pyrolysis reaction zone rather than the gasification zone at bottom of the reactor.24,25 In principle, medium rank coal, especially high-volatile bituminous coal is the ideal feedstock for CPG system because it has some special features, such as, rich in alkyl side chains, low oxygen content, high volatile matter and hydrogen content, which has potential for producing hydrocarbons.26,27 Furthermore, inherent minerals like calcium-containing species in coal have significant effect on pyrolysis behavior and pyrolysate distribution.28,29 Therefore, understanding the formation mechanism of the gaseous products (especially for methane) and pyrolysis kinetics for calcium-rich high volatile bituminous coal can help design and optimize the CPG process in a cost-effective way. However, the available studies on these issues for calcium-rich high volatile bituminous coal are limited. Importantly, the major source of methane formation during pyrolysis of calcium-rich high volatile bituminous coal is not entirely clear. In this work, TG-FTIR apparatus was employed to investigate the pyrolysis behavior, kinetics, 4 ACS Paragon Plus Environment
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and gaseous product release of a calcium-rich high volatile bituminous coal. The changes of activation energy and the mechanism functions at different pyrolysis stages including the thermal decomposition of the mineral species in coal were determined. The main objectives of this work are to reveal the pyrolysis characteristics, formation mechanism of gaseous products, and kinetic mechanisms during pyrolysis of a calcium-rich high volatile bituminous coal, which is considered to be the most suitable coal type for methane-rich syngas production in CPG system. 2. EXPERIMENTAL 2.1. Coal Sample A typical high-volatile bituminous coal obtained from Yichun coal mine in Heilongjiang province, China was adopted in this work. It was ground and sieved to provide the coal sample with particle size smaller than 75 µm, followed by desiccation in vacuum oven at 80 oC for 12 h prior to use. Particle size distributions of the coal was measured on a Mastersizer 2000 (Malvern, Britain) laser diffraction analyzer with ethanol as dispersion medium. The proximate and ultimate analyses of the coal are determined following the Chinese National Standards of GT/T 212-2008 and GT/T 476-2001, respectively and the results are listed in Table 1. 2.2. Pyrolysis Process and Mineral Identification Pyrolysis behavior and gaseous product evolution patterns of the coal were evaluated on a TG analyzer (STA449 F3, NETZSCH, Germany) coupled with an on-line FTIR spectrometer. Briefly, about 10 mg coal sample was heated from room temperature to 105 oC and maintained for 20 min to remove the absorbed moisture in coal. Then it was heated from 105 to 1000 oC at various heating rates (8, 30, and 100 oC/min) under nitrogen atmosphere of 100 mL/min. The transfer line between the TG and FTIR apparatuses and the FTIR gas cell were maintained at 200 oC to avoid the volatile condensation. FTIR spectra of gaseous products were recorded by collecting 32 scans at a 5 ACS Paragon Plus Environment
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resolution of 4 cm-1 in the wavenumber range of 4000‒400 cm-1. In this work, initial devolatilization temperature (Tin), temperature of maximum decomposition rate (Tmax), maximum decomposition rate (Rmax), and terminal devolatilization temperature (Tf) at different heating rates are adopted to interpret the pyrolysis profiles of the coal. The above four parameters are obtained from TG/DTG curves using a software of Proteus Analysis NETZSCH by setting the same temperature range from 150 to 1000 oC in all cases. Furthermore, devolatilization index (Di) was determined for quantifying the degree of volatile release, as defined in Eq. (1):30 Di =Rmax/Tin Tmax △T1/2
(1)
where Rmax, Tin, and Tmax can be obtained from the TG and DTG curves. △T1/2 is the temperature interval when Rd/Rmax is equal to 0.5. Rd is decomposition rate, which is defined as Eq. (2): Rd = –dmt /dt
(2)
where mt is mass of the coal at time t. Rmax and Rd can be acquired from the DTG curves. Mineral species in the coal and the corresponded char prepared at 720 oC were identified on a Panalytical X’pert Pro MPD type X-ray diffractometer, equipped with a graphite monochromater using Cu Kα radiation (k=1.54060 Å) operating at 45 kV and 40 mA. In order to determine the amorphous minerals in the coal, wavelength dispersive X-ray fluorescence (XRF, Panalytical AXIOS max Petro) analysis was used to quantitatively detect the main inorganic elements in the ash of the raw coal with certified reference materials for calibration. 2.3. Kinetic Methods Non-isothermal pyrolysis of solid-state sample can be recognized as an isothermal process during the infinitely small time range. The pyrolysis reaction can be expressed by the following 6 ACS Paragon Plus Environment
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canonical equation: dα E = kTfα = A exp ‒ RTa fα (3) dt
where t denotes time, α is the degree of conversion, or extent of reaction, A is pre-exponential factor (min-1), Ea is the apparent activation energy of the reaction (kJ/mol), R is the universal gas constant (8.314 J mol-1 K-1), dα/dt signifies the rate of the isothermal process, and f(α) is a conversion function that represents the reaction model and depends on the controlling mechanism. The extent of reaction, α, can be defined either as the mass fraction of sample substrate that has decomposed or as the mass fraction of volatiles evolved and can be expressed as below: α=
m 0 – mt m0 ‒mf
(4)
where mt is the mass of coal presented at any time t, m0 is the initial coal mass, mf is the final mass of coal remaining after the reaction. The combination of A, Ea, and f(α) is often designated as the kinetic triplet, which is used to characterize pyrolysis reactions. Non-isothermal rate expressions, which represent reaction rates as a function of temperature at a linear heating rate, β, can be expressed through an ostensibly superficial transformation of Eq. (5): dα dT
=
dα dt
(5)
dt dT
where dt/dT describes the inverse of the heating rate, 1/β, dα/dt represents the isothermal reaction rate, and dα/dT denotes the non-isothermal reaction rate. An expression of the rate law for non-isothermal conditions can be obtained by substituting Eq. (3) into Eq. (5): dα dT
It is assumed that α = 0
α
=
k T β
f α
=
A β
exp‒
Ea RT
f α
(6)
dα , the equation is expressed as follows: f(α)
gα= 0
α
dα A T E = exp ‒ dT f(α) β T0 RT
2.3.1. Model-free Methods
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(7)
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Model-free methods have caused wide attention to determine variation of activation energy based on an isoconversional basis without any reaction model assumptions.31 Commonly, Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Friedman are the currently popular model-free methods. (i) FWO Method Using Doyle’s approximation (p(x) ≈ ‒5.33‒1.05x) for the temperature integral and taking the logarithms of both sides of Eq. (7) one obtains: A·E
lnβ= ln R ·g(aα) –5.335‒
1.0516Ea (8) RT
In FWO method,32 plots of ln β versus 1/T for different heating rates produce parallel lines for a fixed degree of conversion. The slope (–1.0516Ea/R) of these lines is proportional to the Ea. (ii) KAS method Eq. (7) can also be converted to KAS method of Eq. (9), as following:33 ln
β AR Ea =ln ‒
E ·g(α) RT
(9)
According to the Eq. (9), lnβ/T2 should have a linear relation with 1/T with (–Ea/R) as the slope. (iii) Friedman Method The Friedman method is a differential isoconvensional form that can be expressed (Eq. 10) by taking the logarithms of both sides of Eq. (6).34 lnβ
dα Ea =lnA·f(α)– dt RT
(10)
It is assumed that the value of ln[A·f(α)] remains constant when α is given. The value of ln(β dα/dT) is a function of 1/T at different heating rates with slope of (‒Ea/R), from which Ea at different values of α can be acquired. 2.3.2. Coats-Redfern (C-R) Method 8 ACS Paragon Plus Environment
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An approximate equation with integration by Coats-Redfern method can be obtained as below: g(α) =
ART2 βEa
[1– T ] e‒Ea/RT (11) 2RT
A novel equation correlated ln[g(α)/T2] with 1/T was derived by taking logarithms of both sides of Eq.(11). 2RT Ea g(α) AR ln =ln 1– ‒ βEa E RT
(12)
Owing to 2RT/Ea > C2+ 11 ACS Paragon Plus Environment
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aliphatics >> C2H4 > arenes during pyrolysis. Due to the extinction coefficients of the hydrocarbon gases are almost similar, so that CH4 is the most abundant component in the hydrocarbon species, implying that this type coal is suitable for production of CH4-rich syngas in the CPG system. It is observed that temperatures of maximum release rates for C2+ aliphatics, C2H4, and arenes were identical and corresponded to the Tmax value in DTG curve. However, maximum release rate for CH4 was located at about 510 oC, which was much higher than that of C2+ aliphatics, C2H4, and arenes. Coal pyrolysis was recognized as a temperature-dependent bond scission process and previous studies showed that cleavage of long alkyl side chains was the major source of CH4 for two medium volatile bituminous coals.9 According to the above formation mechanism, thermal cracking of the long alkyl side chains on the aromatic clusters will produce CH4, C2+ aliphatics and C2H4 simultaneously and thus the temperatures corresponded to the maximum release rates should be similar for these species. However, release patterns of CH4 and the additional species (C2+ aliphatics and C2H4,) were asynchronous with increasing the pyrolysis temperature in this work, which meant major release stage of CH4 during pyrolysis of the coal was not related to the cracking of long alkyl side chains on aromatic clusters. In fact, strongly bonded aryl methyl groups are recognized as another source of CH4 and therefore cracking of these groups at higher temperatures mainly contributes to forming large amounts of CH4 at 510 oC. This is also evidenced by the discovery that substituent of –CH3 on the aromatic rings is richer than that of the –CH2– for Yichun high volatile bituminous coal in our previous work.44 Besides hydrocarbon gases, those oxygenated gases including SO2, C‒O and C=O bonds containing species also showed maximum release rate at Tmax, but only the evolution of C–O bond containing species started at higher temperature of about 400 oC, which was ascribed to the breakage of C‒O bond bridges between the aromatic clusters.45 Furthermore, evolution trend of CO crossed a wide temperature range and showed one weak peak at 12 ACS Paragon Plus Environment
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Tmax and another sharp peak near 750 oC. The first stage release of CO is attributed to thermal decomposition of carbonyl groups in the coal and secondary cracking reactions of primary tar. The cleavage of ether/hydroxyl groups with high bonding energies is responsible for release of CO from 500 oC up to 750 oC.10 CO2 and H2O all showed four release stages in the temperature range of 150‒1000 oC, among which temperatures of release peak located at 400, 610, and 710 oC were identical for both CO2 and H2O. Due to pyrolysis water normally generates after 250 oC, the weak release peak for H2O at about 200 oC is attributed to the loss of the hydrated water of gypsum in the coal.46 The sharp release peak for H2O near 400 oC is ascribe to the carboxyl decomposition in coal, while CO2 formation at this temperature derives mainly from intramolecular carboxylic acid anhydrides.47 The release of H2O at 610 oC is produced by the degradation of the hydroxyl group, accompanied by the hydrogenation reaction. However, the degradation of oxygen-containing heterocycles such as, ethers, quinones, and esters also agree with a weak peak, centered at 610 oC in the CO2 evolution profile.48,49 The above results reveal that CO2 and H2O originate from different functional groups in coal structures even though they show same temperature dependence during pyrolysis. Significantly, an obvious release peak for CO2 appears near 710 oC. As mentioned above, TG profile of the coal showed a rapid mass loss centered at about 710 oC and the CO2 release pattern also possessed one sharp peak near 710 oC. Thermal decomposition of the inherent minerals in coal is often recognized to produce CO2 at higher temperatures. As depicted in Figure 2, those decomposable mineral species in the coal identified by X-ray diffraction were principally calcite and gypsum. It is reported that thermal decomposition of calcite is highly influenced by partial pressure of CO2 and its maximum decomposition rate is located at about 720 oC rightly in inert atmosphere.50
Furthermore,
the
reaction
between
carbon
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and
calcium
sulphate
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(CaSO4+2C→CaS+2CO2) in the presence of char matrix and other inherent minerals, such as Fe2O3 can significantly promote gypsum decomposition and reduce its decomposition temperature as low as 700 oC, which also release certain amounts of CO2.46,51 In addition, XRD results showed that mineral species of the char prepared at 720 oC were mainly CaO and CaS except for those undecomposable species such as, quartz (Figure 2 blue line). Therefore, it is confirmed that thermal decomposition of these Ca-containing minerals in the coal matrix is responsible for emergence of rapid mass loss in TG profile and producing large quantities of CO2 at higher temperature of 710 oC. The phenomenon of late CO2 release at temperature higher than 700 oC in this work is different from that of the other bituminous coals reported previously. For example, the release of CO2 was mainly in the temperature range of 350–550 oC during pyrolysis of a Spanish bituminous coal.52 In addition, several typical Chinese bituminous coals including Pingshuo, Lingwu, Shendong, and Hami coals all generated CO2 before 600 oC during pyrolysis based on the thermogravimetry coupled with mass spectrometry.53 3.4. Pyrolysis Kinetics Thermogravimetry can not only provide general information about the overall reaction kinetics but also obtain accurate kinetic parameters based on the experimental data.54 As stated previously, model-free methods that use multiple heating rates can provide more reliable estimation of the activation energy changes in the whole pyrolysis process. Therefore, three model-free kinetic approaches (KAS, FWO, and Friedman methods) based on the isoconventional process are employed to obtain the reasonable activation energy changes during pyrolysis of Yichun calcium-rich high volatile bituminous coal. The plots of these three methods for the coal are shown in Figure 7 and the obtained activation energies at different conversions are listed in Supporting Information of Table S2. In all cases, high correlation coefficients were observed revealing that the 14 ACS Paragon Plus Environment
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applied models could give good fitting effect. The activation energies at each conversion calculated by KAS and FWO methods were almost identical, which were slightly lower than that obtained from Freidman method. As demonstrated in Figure 8, the activation energy did not change much when the pyrolysis conversions were lower than 0.45 and then it showed quick increase in the conversion range of 0.45‒0.70. Differently, the activation energy decreased apparently as conversion proceeded over 0.70 due to the thermal decomposition of calcite and gypsum in the coal, implying that mineral decomposition showed different reaction mechanisms compared with coal organic matters. Variation of activation energy during pyrolysis could indicate the average bond strength of unreacted organic species in coal. In addition, it has also been believed that a change in the activation energy reflects a transformation in the nature of the rate-controlling step.55 Therefore, the overall pyrolysis process of the coal could be divided into three reaction stages according to the activation energy changes. The first stage below 510 oC (i.e. α
