Microwave-assisted conversion of low rank coal under methane

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Microwave-assisted conversion of low rank coal under methane environment Victor Abdelsayed, Dushyant Shekhawat, Mark Smith, and Sonia Hammache Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03805 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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Microwave-assisted conversion of low rank coal under methane environment Victor Abdelsayed1,2*, Dushyant Shekhawat1, Mark Smith1, Sonia Hammache1,2 1National

Energy Technology Laboratory, U.S. Department of Energy, Morgantown, WV 26507-0880 2AECOM, 3610 Collins Ferry Rd., Morgantown, WV 26507-0880

ABSTRACT Addition of methane during microwave coal pyrolysis could greatly affect the product distribution to valuable products including the formation of char. In this work, low rank coal was exposed to microwave energy in the presence of different methane concentrations (0, 25, 50, and 90%) at 980 ℃ for 2 hours. Increasing methane concentration was found to increase both the char and tar yields mainly due to carbon deposition during methane decomposition and hydrogenation of trapped carbon into tars. Analysis of the gas composition suggested that some of the methane was activated in the presence of the coal minerals, which could act as a catalyst, forming light hydrocarbons C2-C7 which accounts for up to 5% of the gaseous products at a methane concentration of 90%; they accounted for less than 0.5% in the absence of methane. Methyl groups may also have been substituted into aromatic compounds as observed in the tar analysis where the number of methyl substitutions in detected parent phenol and naphthalene increased with methane concentration. Tar yield could also increase indirectly with methane addition through hydrogenation reactions with unsaturated coal compounds. The formation of char under different methane concentrations was examined by many characterization tools including Raman, dielectric properties, XRD, BET, SEM, and EDS. The results suggest that the addition of methane did not help in forming ordered carbon chars, not only due to amorphous carbon deposition on the char surface, but also due to hydrogenation or alkylation reactions with char. The presence of methane enhanced the formation of C2s and benzene; a possible correlation between the formation rate of benzene and hydrogen is proposed during pyrolysis.

Keywords: Microwave pyrolysis, Methane addition, Dehydroaromatization, Tar yield, Char properties, Low rank coal

*Corresponding author. Tel:+13042855273; fax: +13042854850. E-mail address: [email protected]

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1. Introduction Coal is still one of the most abundant fossil fuels that accounts for almost 30% of the world power generation. High-rank coals are usually the main source for these coal-fired power plants due to their high heating value. Utilization of low-rank coals, which accounts for almost half of the world’s coal reserve 1, as a feed stock for chemical industries would be demanding. Coal conversion processes such as gasification, combustion, or coke-generation usually start with pyrolysis 2, which is a thermochemical pre-treatment mainly intended to devolatilize coal and to reduce its sulfur and heavy tar content. In general, pyrolysis conditions greatly affect the structure-reactivity relationship of the char produced 2-3, which can impact the overall coal conversion process. Pyrolysis of coal begins with cleaving of some of the covalent bonds and the formation of free radicals. A two-step radical mechanism was proposed 4 during coal pyrolysis: the first is to generate volatile radicals from coal; the second is recombination of these volatile radicals. Recombination could be either in the gas phase to form gaseous products or on the char surfaces to form heavy, polyaromatic carbon through condensation or polymerization reactions into some carbon trapped within the char structure. Controlling or influencing these recombination steps could maximize the output of value-added chemicals from this thermal decomposition process. The heating source, whether it is a traditional, thermal source or a non-traditional energy source, like microwaves, may influence the radical recombination and the product distribution generated during coal conversion. Further, the selective heating of microwaves could influence the decomposition pattern of coal. Microwaves have been used recently as a non-conventional energy source for the pyrolysis of coal 5. Due to its low heating value, low rank coal (LRC) is less favorable for utilization in conventional, thermal-based coal-to-liquid technologies. Microwave may provide a special advantage in converting LRC into chemicals due to the nature of its composition, which contain high levels of inorganic minerals, moisture and sulfur, all of which have higher dielectric properties than the hydrocarbon content in coal. These could help initially with coal heating until the coal starts to cross link and absorb microwave energy more efficiently on their own 6. Additionally, the high ash content in this LRC could contribute


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catalytically, under microwave reaction conditions, in converting added gases, such as methane, into useful chemicals. From an economic point of view, due to its abundance and relatively low cost, both LRC and natural gas could be combined in a microwave-assisted pyrolysis reaction to upgrade coal tars as well as forming useful chemicals. Coal is a hydrogen deficient carbon source and the addition of methane could act as a hydrogen source to upgrade its pyrolysis into chemical products via hydrocracking of tars. The presence of inorganic minerals in the coal matrix may be advantageous by allowing the microwave selective heating to exploit any catalytic activity towards methane conversion into chemicals. Hydropyrolysis of coal by conventional methods is usually a catalytic process that requires high temperature and pressure to breakdown coal into methane7. Recently, microwave-based technology has been used in several coal-related studies5c, 8. Kamei et al have studied the effect of methane addition during coal pyrolysis8b. Wan et al. 9 reported on the catalytic conversion of methane to aromatic hydrocarbons under high power pulsed microwave (2.4 GHz) and radio frequency (40 MHz) energy on different supported transition metal catalysts. They proposed that the reaction could be initiated by the formation of methane free radicals which then proceed to the formation of acetylene as an intermediate. In this paper, the effect of methane addition during microwave-assisted coal pyrolysis was studied. The char formation under different methane concentrations was investigated by different pre- and post-char characterizations. The goal was to study the impact of methane concentration on both the product distributions (gases and tars) and char properties and determine their relationship.

2. Experimental 2.1 Pyrolysis Studies


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The coal powder was obtained from the U.S. Department of Energy’s National Carbon Capture Center managed by Southern Company. To ensure homogeneity, the coal powder was ball milled before being pressed and sieved into particles with diameters between 300 and 600 m. The proximate, ultimate, and ash mineral analyses are shown in Table 1.

Table 1. Ultimate, proximate, and ash composition of Mississippi lignite coal Ultimate analysis (wt%, db*) Proximate analysis (wt%, db*) C H N S O FC VM Ash 51.75 3.57 1.27 0.73 16.97 30.53 43.76 25.71 Ash mineral analysis (wt%) Al Ba Ca Fe Mg Mn P K Si 18.80 0.17 19.00 3.74 3.57 0.34 0.07 1.05 45.3 *db: Dry basis

Na 0.47

Sr 0.41

S 5.61

Ti 1.15

Microwave-assisted pyrolysis experiments were performed using a 2.45 GHz 2 kW magnetron with a single mode microwave cavity from Sairem, (model GMP20K). The microwave reactor was operated in a continuous mode under a fixed forward power of 1.0 kW corresponding to an average coal bed temperature of about 980 ± 25 ℃. A quartz reactor tube with a 1.0-inch outer diameter was placed inside the microwave cavity. The cavity was equipped with a viewing port used for remote temperature measurement using a laser-aiming IR pyrometer from Micro-Epsilon (model CTLM3). This pyrometer has a temperature range from 200 to 1500 ℃ and a detector spectral range of 2.3 m, below the cutoff transmittance wavelength of quartz, allowing for the temperature reading from the sample surface rather than the reactor quartz surface. Typically, a 10g sample of untreated coal was loaded into the reactor and irradiated for two hours under pure Ar (UHP, 99.999 %) or methane concentrations from 25, 50 or 90% balanced in Ar gas in a fixed flow rate of 200 sccm and 0.1 MPa pressure. The experimental setup diagram is shown in Figure 1.


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Figure 1. Experimental setup for microwave reactor

2.2 Product Collection and Analysis Reactants and products exiting the reactor were first conditioned by two cold traps maintained at -10 ℃ and 25 ℃ to separate condensates and wax tars, respectively. Light gases (H2, N2, CO, CO2 and CH4) were analyzed online using a 200 amu scanning magnetic sector mass spectrometer from Thermo, model Prima BT Benchtop MS. Light hydrocarbons (ethane, ethylene, acetylene, propylene, 1,3-butadiene, benzene, and toluene) were analyzed using an Agilent 3000A Micro GC equipped with a TCD detector and four columns: molecular sieve 5˚A, Plot Q, Plot U, and Alumina. The total weight of each of these light gases and light hydrocarbons were calculated by integrating over the duration of the reaction. At the end of each reaction, the tars were collected from both traps and weighed. The wax tars were dissolved in 2 ml hexane and a 2l sample of each was injected into a GC-MS from Perkin Elmer (model Clarus 500) equipped with a capillary column Restek Rtx-5MS, 30 meters, 0.25 mm ID, 0.25 mm film thickness and a quadrupole analyzer operating in electron impact (70 eV) mode. The oven was first


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maintained at 33 ℃ for 0.5 min and then heated at a rate of 4 ℃/min to 350 ℃. Helium was used as a carrier gas with a constant flow rate of 1.2 mL/min and the split ratio was 50:1. The data were acquired and processed using the TurboMass software. The compounds were identified by comparing their mass spectra to spectral data in the instrument database. A semi-quantitative analysis was made to compare the distribution of the compounds in different oils. The tar, char and gas product yields were calculated using equation 1, 2, and 3, respectively, and are given as:

𝑌𝑡𝑎𝑟 = 100 ×

(𝑤𝑡. 𝑜𝑓 𝑜𝑖𝑙 𝑡𝑎𝑟) + (𝑤𝑡. 𝑜𝑓 𝑤𝑎𝑥 𝑡𝑎𝑟) 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑡. 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒

𝑌𝑐ℎ𝑎𝑟 = 100 × 𝑌𝑔𝑎𝑠 = 100 ×

𝑤𝑡. 𝑜𝑓 𝑐ℎ𝑎𝑟 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑡. 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒

∑𝑤𝑡. 𝐻2 + ∑𝑤𝑡.𝐶𝑂 + ∑𝑤𝑡.𝐶𝑂2 + ∑𝑤𝑡.𝐶𝐻4 + ∑𝑤𝑡.𝐿𝑖𝑔ℎ𝑡 ℎ𝑦𝑑𝑟𝑜𝑐𝑎𝑟𝑏𝑜𝑛𝑠 (𝐶2 ― 𝐶7) 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑤𝑡. 𝑜𝑓 𝑐𝑜𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒

(1) (2) (3)

2.3 Char Characterization 2.3.1 Surface area and micropore analysis Nitrogen adsorption experiments were performed at 77 K using a Micromeritics ASAP-2020 unit. Char samples were vacuum-degassed at 300 ℃ for 10 hours to remove the surface humidity and pre-adsorbed gases before exposure to the adsorption gas. The surface area was calculated from the N2 isotherm data using the Brunauer-Emmett-Teller (BET) model 10. The micropore volumes and areas were measured using t-plot analysis 11. Pore size distributions were calculated using Barrett-Joyner-Halenda (BJH) model from the desorption isotherm. 2.3.2 SEM and elemental mapping analysis Scanning electron imaging and microanalysis were obtained utilizing a JEOL FE-7600 scanning electron microscope (SEM) interfaced to a Thermo-Electron Noran System Seven (NSS) X-ray microanalysis system. The detector utilized in the X-ray microanalysis was a Thermo-Electron Ultradry Energy dispersive spectrometer, which was calibrated utilizing the Cu Kα line at 8.041 kV.


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2.3.3 Dielectric properties A microwave network vector analyzer from Keysight (model N5231A PNA-L) was used to measure the dielectric properties of raw and char samples. The measurements were made with a 7 mm diameter coaxial airline (HP model no. 85051-60010). Typically, the powders were grinded and loaded in the airline sample holder such as the samples density are the same to be able to obtain comparative results. The complex permittivity of raw coal and prepared chars were measured in the frequency range between 1.0 GHz and 5.0 GHz at room temperature. The permittivity 𝜀 of a dielectric material is defined in equation 412 as: 𝜀 (𝜔) = 𝜀′(𝜔) ―𝑖𝜀′′(𝜔) 𝜀′′

tan 𝛿 = 𝜀′

(4) (5)

Where ' is the real part and represents the ability of the dielectrics to store the microwave electrical energy, and " is the imaginary part and represents the loss of microwave electrical energy in dielectrics. The loss tangent (tan 𝛿), defined in equation 5 12, quantifies the magnitude of the microwave electric field loss (and therefore heating) in the process.

2.3.4 X-ray diffraction (XRD) Powder X-ray diffraction analysis was performed on a PANalytical X’pert Pro (PW3040) X-ray diffraction system utilizing Cu K radiation. Samples were placed on a zero diffraction Si holder and were scanned from 5 to 70 ° (2θ). Analysis was carried out using Highscore Plus Analysis software equipped with a standard ICDD X-ray diffraction database supplied by PANalytical.

2.3.5 Raman Spectroscopy


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Raman spectra of the raw and prepared chars were collected in air at room temperature using a Jobin Yvon Labram HR Evolution spectrometer. The char sample was spread on a glass slide and placed under a microscope equipped with a 100 X lens, which was used to focus the excitation laser beam (Nd:YAG: 532 nm, 25mW) on the sample, and the back-scattered Raman signal was collected using a CCD detector and a 1800 gratings/mm. The spectra were recorded in the range of 800 - 2000 cm-1, covering the first order bands region. At least three different spots were measured for each char sample. The spectral parameters were determined through the deconvolution of the Raman spectra collected was performed using OriginPro v8.1 software, with multipeak fitting package. The carbon microcrystalline planner size (La) was determined using the modified Knight formula 13 and is given in equation 6 as: 𝐿𝑎 =

𝐶𝑜 + 𝐶1𝜆𝐿 𝐼𝐷/𝐼𝐺

(6)

where Co and C1 are equal to 12.6 nm and 0.033, respectively. These constants are valid for the excitation laser wavelength (L) between 400 and 700 nm. ID/IG is the band area ratio between the carbon D and G band, respectively.

3. Results 3.1 Microwave coal pyrolysis 3.1.1- Yields and gas composition analysis Figure 2a shows the normalized tar, char, and gas yields obtained during microwave coal pyrolysis at 980 ℃ as a function of methane concentration. Both the char and tar yields increased as the methane concentration increased to 50% and then decreased at 90%. Decomposition of methane 14 or tars 15 into carbon or polyaromatic coke could increase the char yield. Similarly, the tar formation increased initially with methane, possibly due to alkylation reactions with organic coal molecules 16. This increase could also be due to coal hydrocracking reactions 17 enhanced by the rich hydrogen environment present during


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coal pyrolysis under methane. Under a rich methane environment (90%), excessive carbon deposition could cause some diffusion limitations that prevent methane-char interaction a concomitant slowing of the tar formation rate via hydrocracking reactions. The overall gas yield decreased with increasing methane concentration to 50% before showing an increase at 90% methane, possibly due to a decline in tar formation while other gaseous light hydrocarbons (C2-C7) increase. This could suggest that the complete decomposition of methane into carbon is not the only reaction of methane with char since other light hydrocarbons (C2-C7) were observed. This agrees with the decrease in char yield observed at 90% methane and could indicate that the methane added continued to contribute in light hydrocarbon formation during the coal pyrolysis reaction even in a 90% methane environment. The average compositions of the gases exiting the reactor were analyzed for pyrolysis reactions. Both the light gases (H2, CO, CO2) and light hydrocarbons (ethylene, acetylene, ethane, propylene, 1,3-butadiene, benzene, and toluene) compositions as a function of methane concentration are displayed in Figure 2b and 2c, respectively. The addition of methane has increased the formation of these valuable hydrocarbons, especially C2s and benzene. In the presence of methane, the gas compositions mainly contain H2 accounting for 90 and 85% by volume as methane concentration increased from 25 to 90%, respectively. This decrease in H2 production correlates with the increase in light hydrocarbons observed in Figure 2c and suggests a decrease in methane decomposition into carbon and an increase in the formation of these light hydrocarbons. The percentage of produced hydrocarbons in the gas compositions increased as the methane concentration increased, indicating that some of the methane could get activated and coupled either in the gas phase as methoxy groups 18 or on the char surfaces. The char surfaces contain some inorganic minerals as shown in Table 1, which could possibly act as catalytically active microwave sites to activate methane and break the first CH bond, without leading to complete dehydrogenation, and formation of the observed C2 to C7 molecules.


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It is also important to note that these organic molecules continue to be formed even after the complete devolatization of coal, under microwave elevated temperature conditions, and in the presence of methane. The gas product distribution obtained under a pure Ar environment showed relatively small amounts of hydrocarbons and only for a short reaction time of 10 to 15 min before they completely disappeared. On the other hand, coal pyrolysis under methane shows a continuous formation of light hydrocarbons. Figure 2b shows that the concentration of CO is always higher than CO2 at all methane concentrations, similar to what we reported in our previous work, which compared microwave and conventional coal pyrolysis under N2 5c. Under the complex chemical structure of coal, many secondary reactions, such as: reverse water-gas -shift (CO2 + H2 = CO + H2O), reverse Boudouard (C + CO2 = 2CO), or methane reforming (CH4 + CO2 = 2H2 + 2CO) reactions could took place during microwave pyrolysis of coal to increase the CO/CO2 ratio. In this work, the CO/CO2 ratio is always more than 1 and independent of the presence of methane.


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Figure 2. (a) Normalized tar, char, and gas yields, and (b-c) average gas composition for light gases and light hydrocarbons, respectively, as a function of methane concentration added during microwave coal pyrolysis at 980 ℃ for 120 min.

Figure 3 shows the concentrations of H2, benzene, and ethylene formed as a function of coal pyrolysis time under different methane concentrations. Insignificant amounts of H2, ethylene, and benzene were


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formed under pure Ar due to thermal decomposition and devolatilization of coal. Addition of methane resulted in an increase in ethylene and benzene concentrations possibly via methane coupling followed by the dehydroaromatization (DHA) reaction to form benzene. These complex reactions could be catalyzed by the inorganic minerals in coal char, which are mainly composed of Si and Al with other transition metals, such as Fe and Mo. These metal oxides most likely resemble the best-known elemental composition of Mo/HZSM-5 catalyst for DHA reaction 19. Additionally, a recent report 18 suggested that methane could be activated at 1090 ℃ on single iron sites embedded in silica matrix to form CHx groups followed by gas phase aromatization to form benzene without the need of these zeolite pore channel systems found in HZSM-5 catalyst supports 20. Microwave-assisted hydrocracking of coal tars under methane environment is also a possible pathway to produce benzene. A separate comprehensive reaction mechanistic study in currently under way to understand benzene formation. Another possible correlation was observed between benzene and H2; the benzene concentration increased at almost the same time H2 concentration decreased. These results suggest that the methane reaction towards decomposition to carbon and H2 decreased (H2 concentration goes down) and the DHA reaction became relatively favored to produce more benzene. The decrease in H2 concentration could be due to coal hydrogenation or DHA to form aromatics. Ethylene and benzene concentrations follow the same trend confirming the existence of some correlation in the reaction mechanism. Ethylene is an essential building block in the benzene formation mechanism where it first oligomerizes on acidic sites of zeolite catalyst support followed by dehydrogenation and aromatization steps to form benzene. To confirm that this benzene formation is not due to a homogeneous pyrolysis of methane to C2+, methane was conventionally heated to the same reaction temperature (980 ℃) homogeneously, in the absence of coal, in a fixed bed reactor under pure Ar gas. Only trace amounts of benzene and C2s were observed, which indicated the importance of coal and microwave irradiation in this reaction. Pyrolysis of methane to C2+ hydrocarbons were reported in literature but required high reaction temperatures (>1727 ℃) to obtain a reasonable methane conversion since it is a highly endothermic reaction 21.


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Figure 3. (a) hydrogen and benzene concentrations in solid and open square symbols, respectively and (b) ethylene concentration (solid circle symbols) as a function of reaction time during microwave coal pyrolysis at 980 ℃ and under different methane concentrations (0, 25, 50, and 90%) balanced in Ar for 120 min.

3.1.2- Tar analysis The GC-MS analysis of wax tars generated during microwave coal pyrolysis under different methane concentrations are displayed in Figure 4. In the absence of methane, the products concentration and distribution are very low and narrow, respectively. It only shows the presence of phenols, substituted phenols, and naphthalene compounds; while most of the tars may have been converted into gaseous products, agreeing with the higher gas yield observed under pure Ar. Our recent work showed that microwave pyrolysis of coal under nitrogen produced high gas and low tar yields compared to conventional thermal pyrolysis 5c. The addition of methane increased the concentration of these products, mentioned above, and new products started to be observed. Under 25% methane, aliphatic hydrocarbons chain series from C20 to C44 were observed. A doublet peak was observed at each of these aliphatic compounds indicating the presence of saturated alkane and unsaturated alkene in the tar produced. The


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presence of these aliphatic hydrocarbons chain series only in presence of methane could indicate a possible direct C-C coupling between methane molecules or indirectly via hydrocracking reactions between coal and H2 formed by methane decomposition leading to release of more tars. As the methane concentration increased, the concentration of indirect H2 increased which could facilitate further microwave assisted hydrocracking reactions 22 with coal constituents leading to more aromatic and substituted aromatic compounds in tars as observed in Figure 4. Substituted phenols and naphthalene, with one or more methyl groups, were observed under high methane concentration. Tar products became rich in hydrogen at higher methane concentrations with detected aromatic compounds that have high H/C ratios. For example: Naphthalene C10H8: H/C = 0.80, Biphenylene C12H8: H/C = 0.67, Anthracene C14H10: H/C = 0.71, and Pyrene C16H10: H/C = 0.62 became higher in intensity with methane, which could indicate that some coal hydrogenation is possible under microwave reaction conditions.

Ar

5

25 %

TIC (a.u.) x 10 5

10

15

20

25

30

35

Fluoranthene Pyrene Eicosane

Heptadecane 3-tetradecene Anthracene 1-nonadecene Octadecane

1-methyl naphthalene 2-ethyl naphthalene 1-dodecane 1-tetradecane Biphenylene Nonadecane

50 % Phenol 2 methyl phenol 4 methyl phenol 3,5 dimethyl phenol 4 ethyl phenol Naphthalene

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

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Aliphatic Hydrocarbons

C20 - C44

90 % 40

45

Time (min)


50

55

60

65

70

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Figure 4. GC/MS total ion chromatograms for wax tars collected at 25 ℃ during microwave coal pyrolysis runs at 980 ℃ for 120 min under different methane concentrations (0, 25, 50, and 90%) balanced in Ar.

3.2- Char characterization 3.2.1- Surface area properties Figure 5a shows the N2 isotherms for generated chars. All isotherms are of type I, which indicates the presence of microporous structures. The BET surface area, micropore surface area and volume, and total pore volume are reported in Table 2. In general, coal pyrolysis at elevated temperatures, results in pore collapsing and restructuring to produce chars of low surface area 23. Additionally, the microwavegenerated chars had higher surface areas and pore volumes than the corresponding conventional chars under the same pyrolysis temperature 5c. The surface area and the pore volume of the chars decreased as the concentration of methane added during coal pyrolysis increased. In absence of methane, the coal char has a BET surface area of 52 m2/g, which gradually decreased to 9 m2/g under 90 % methane. This decrease in surface properties could be due to microwave-induced reactions during pyrolysis, which involved the decomposition to carbon and hydrogen of methane and generated tars. It is reported 24 that the decrease of specific surface area and pore volume of coal char is caused by coke deposition from the decomposition of organic volatiles, especially aromatic compounds, over the char surface. The presence of catalytically active mineral species on the char surface could also increase tar decomposition. These results agree with the observed increase in char yield in Figure 2a upon addition of methane to the pyrolysis reaction of coal. The hysteresis loops observed in Figure 5a are due to capillary condensation of adsorbed N2 in meso- and macropores of char structures. The hysteresis loops became broader as the methane concentration decreased, suggesting a decrease in the formation of mesopores as more carbon was deposited in the char pores with increasing methane concentration during coal pyrolysis. Figure 5b shows the distribution of pore volume with respect to pore size, which can represent pore size distribution 25.


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According to IUPAC classification of porous materials, pores can be divided into groups depending on their diameter: micropores are those with pores less than 2 nm, mesopores with 2 to 50 nm, and macropores with larger than 50 nm. The pore size distribution had a tri-modal pore distribution. Two pore size modes were in the mesopore size regime and were observed at 3.8 nm and 2.7 nm, the latter is with a lower intensity, both had narrow pore size distributions. The third is a broad pore size distribution, in the macropore size regime, between 60.8-76.3 nm. In general, the intensities of the pore size distributions decreased significantly as soon as methane was added during coal pyrolysis and then decreased slowly as methane increased from 25 to 90%, likely due to carbon deposition. The relative intensity between meso to macro pore size distributions, within the same char, decreased as the methane concentration increased, which could indicate that carbon started to deposit on the external char surfaces at higher methane concentrations.

Figure 5. (a) N2 adsorption isotherms and (b) pore size distribution for Mississippi coal char generated during microwave pyrolysis at 980 ℃ under different methane concentrations: 0, 25, 50, and 90% balanced in Ar.


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Table 2: Surface properties of Mississippi chars generated under microwave pyrolysis at 980 ℃ at different methane concentrations. Methane Conc. (%) 0 25 50 90

SBET 2

Smicro a

(m /g) 52 19 14 9

2

Vmicro b

(m /g) 21 12 8 2

3

Vtotal b

(cm /g) 0.010 0.005 0.004 0.001

3

c

(cm /g) 0.083 0.022 0.020 0.019

Calculated using BET method in the range up to P/Po = 0.2. Calculated using t-plot method. c Total pore volume up to P/Po = 0.985. a

b

3.2.2- Char surface morphology and elemental mapping The SEM images for char samples prepared in the absence and presence of methane are displayed in Figure 6. Large clusters of aggregated particles with low porosity were observed for char prepared under 90% methane (Figure 6c and 6d), likely due to carbon deposition on the char surfaces. These results agree with the observed surface properties of the prepared char samples, as discussed in Section 3.2.1, where a lower surface area and porosity were observed under methane compared to pure Ar. The presence of polar groups in the coal structure could result in the formation of hot spots during microwave pyrolysis due to polarization under a microwave field. These hotspots could act as microwave active sites helping decompose some of the methane added or the tars generated during pyrolysis into amorphous carbon or polyaromatic coke, respectively. Figure 6b shows the presence of highly symmetrical microspheres in the char prepared under Ar that resemble fly ash morphology. This could indicate that the char was exposed to elevated temperatures and experienced localized heating during microwave pyrolysis under pure Ar. Our previous work on Mississippi coal pyrolysis under N2 environment showed the formation of microspheres within the char matrix under microwave pyrolysis at 500 W similar to what was observed in this work with 1000 W. The


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elemental microanalysis showed that these microspheres were mineral-rich in Ca, Al, Si but low in C compared to elsewhere in the char sample 5c.

Figure 6. SEM images of Mississippi coal char prepared under pure Ar (a, b) and under 90 % methane balanced Ar (c, d) at 980 ℃ for 120 min.

To further examine the char formed under these reaction conditions, elemental mapping analysis was conducted, and the results are shown in Figure 7. In the absence of methane, carbon is uniformly distributed on the char particles with some silica/alumina rich regions. No carbon aggregates were observed under pure Ar environment. In the presence of 90% methane, the elemental maps of C, Si, Al, and Ca confirm the presence of carbon clusters aggregated and deposited on the char surface. These carbon rich regions show almost no Si or Al signal during elemental mapping scans, where these two elements comprise most of the ash composition, as shown in the ICP analysis of ash shown in Table 1.


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Figure 7. Elemental mapping analysis for Mississippi coal char after microwave pyrolysis under (a) pure Ar and (b) 90% methane balanced Ar at 980 ℃ for 120 min.

3.2.3 Dielectric properties of char The complex permittivity, also known as dielectric properties, of coal chars was studied to assist in understanding the extent of interaction between the microwaves and the carbon structures in the char samples. Different polarization mechanisms could come into play when a material interacts with the alternating electric and magnetic fields of microwaves, depending on its chemical and physical properties, temperature, and frequency used 26. Solids-microwave polarization could be due to one or more losses such as conduction, space-charge, dipolar, or electron-ion shift 27. Figure 8a shows the real part of this complex permittivity (') as a function of frequency. The results showed that the raw coal has a dielectric constant that slightly decrease with frequency, from 2.37 at 1 GHz to 2.25 at 5 GHz. A significant increase was observed for all prepared chars after being pyrolyzed under microwave conditions, likely due to carbon cross-linking and condensation reactions of char-constituent molecules with a subsequent higher conducting carbon network compared to raw coal. Similar results were observed for conventionally-prepared coal chars prepared under a N2 atmosphere and at different pyrolysis temperatures 28. Increasing the pyrolysis temperature of coal increased the dielectric properties of the char.


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The loss tangent (tan 𝛿), which measures the loss in the magnitude of the microwave electric field in the char samples, is shown in Figure 8b as a function of applied frequency. The change in loss tangent with frequency was observed to be higher for chars compared to raw coal within the studied frequency range. The results showed that the loss tangent of char decreased gradually with increasing methane concentration during pyrolysis. Under pure Ar the high loss tangent could indicate that the carbon structure in char is relatively more ordered compared to the chars formed under methane. Relatively high ordered carbon chars would have more sp2-hybridized carbon domains with delocalized pi electrons present in the planar aromatic carbon structures. Higher conduction losses are expected to occur with relatively high ordered carbon structure under the alternating electric field of microwave, which will oscillate these conjugating pi electrons and polarize the char material 29. The addition of methane reduced the carbon ordering mechanism during microwave coal pyrolysis and decreased the graphitic nature in the char samples, possibly due to hydrogenation and alkylation reactions, as well as amorphous carbon formation due to some methane decomposition.

Figure 8. (a) Real part of the complex permittivity and (b) the dielectric loss tangent as a function of frequency for coal chars generated under microwave at 980 ℃ and 1000 W under 0, 25, 50, and 90% methane feed balanced in Ar.


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3.2.4- X-ray diffraction analysis Figure 9 displays the diffraction patterns of fresh coal and chars generated by microwave pyrolysis at 980℃ under 0% and 90% methane. Fresh coal did not show any ordered carbon structures, and the only sharp crystalline diffraction peaks observed were assigned to SiO2 crystalline phase. The coal ash diffraction pattern (Figure 9 upper layer) collected after combusting the coal in air at 1100℃ revealed the main crystalline inorganic minerals present in coal are SiO2 and CaS, in agreement with the elemental analysis of coal ash listed in Table 1. In general, the amorphous nature of coal carbon decreases with higher pyrolysis temperature 30. As the pyrolysis temperature increases, coal starts to devolatilize, and its molecules start to cross-link via dehydration and decarboxylation reactions. At even higher pyrolysis temperatures, dehydrogenation and aromatization reactions increase its stacking and ordered structure. The presence of ordered graphitic-like carbon domains within char structures were indicated by the appearance of two distinct, broad carbon diffraction peaks, corresponding to the (002) and (100) reflections of graphite at 2-theta between 20 and 30o and between 40 and 50o, respectively 31. The carbon black X-ray diffraction pattern was measured and is displayed in Figure 9 for comparison with generated chars and to account for the carbon ordering development. In the absence of methane, the SiO2 phase could no longer be discerned in the char while new crystalline SiC diffraction peaks were observed. This result could indicate that during microwave coal pyrolysis under Ar localized hot spots or micro-plasmas may be occurring, resulting in the carburization of SiO2 into SiC, which usually requires higher reaction temperatures (1500-1600℃ )32 than the bulk temperature observed in this experiment (980℃). Similarly, the formation of new crystalline CaS diffraction peaks was observed in all generated chars; however, higher peak intensities were observed for char formed under Ar. This is also consistent with the formation of localized hot spots that could induce the carbothermic reduction of CaSO4 to CaS and CO2 33. The higher loss tangent observed for char generated under Ar compared to methane suggests a higher tendency to absorb microwave energy and convert its electric energy into thermal energy producing


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localized hot spots on the char surface. Also, the hot spots density could be correlated to the number of ordered carbon domains and how well it is stacked in the char structure. This would also lead to an increase in the carbon stacking of aromatic rings in char samples, as indicated by the higher diffraction intensity of the carbon peaks (002) and (100) observed under pure Ar, compared to raw coal and chars generated under 90% methane. According to the literature 32b, higher stacking structure of carbon in char,



CaS

SiO2

leads to higher dielectric properties, which results in better microwave absorption and heating.







Ash



C C

Carbon Black

 





90 %

 

SiC

Intensity (a.u.)

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

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

 



0%

 

20

30

Raw coal 40

50

60

70

2  (deg.) Figure 9. X-ray diffractions patterns for fresh coal, microwaved coal chars collected after pyrolysis at 980℃ under pure Ar and 90% methane balanced Ar, carbon black powder, and coal ash (from bottom to top).


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3.2.5- Raman analysis Raman spectroscopy was used to analyze the evolution of carbon structures in the first-order band region between 1000 and 1800 cm-1 34 for coal chars generated under microwave irradiation. The Raman spectrum of highly ordered polycrystalline carbon in graphite sheets was measured as a reference material and is shown in Figure 10a. The graphite spectrum exhibits a sharp narrow with high intensity G-band around 1580 cm-1 corresponding to the lattice vibration mode with E2g symmetry and a very weak and broad defect D1-band at 1350 cm-1 corresponding to the graphitic lattice vibration with A1g symmetry. In general, the presence of additional bands in the first order region with the G-band are characteristics for the presence of disordered or defects in graphite carbon sp2 structure and are known as D or defect bands. Figure 10b shows the Raman spectra of Mississippi coal chars collected after microwave-enhanced pyrolysis under different methane concentrations. The spectra of these chars generally exhibit two broad and strongly overlapping peaks with intensity maxima at about 1350 cm-1 and at about 1580 cm-1. The first order spectra were best fitted with four Lorentzian-shaped bands: G, D1, D2, and D4 at about 1580, 1350, 1610, and 1200 cm-1, respectively and one Gaussian-shaped band D3 at about 1500 cm-1. The Raman peak deconvolution may reflect the size of aromatic rings in coal char, the nature of substituted groups and the extent of cross linking in chars 35. Presence of Ca and Na ions in LRC was found to affect char formation reactions during coal pyrolysis 35. D2 band is another first-order band at 1620 cm-1 that corresponds to structure disorder and can be observed as a shoulder in the G-band and with a graphitic vibration mode of E2g symmetry. The high signal intensity between G and D1 could be assigned to D3 band at about 1500 cm-1 which corresponds to amorphous carbon (mixed sp2-sp3) 36. As the methane concentration increased the D3 band increased, which could be due to the continuous increase in amorphous carbon buildup at high methane concentrations clearly observed at 50% and 90% methane. Chars with high D3 band are reported to be highly active 34.


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The spectral parameters: band positions, vibrational modes, and band area ratios were listed in Table 3. Figure 11 shows the relationships between these integrated band area ratios as a function of methane concentration. During microwave coal pyrolysis under pure Ar, the char generated has the highest observed ordered carbon domains with the highest IG/Iall ratio where Iall is equal to all vibration band areas (IG+ID1+ID2+ID3+ID4). As the methane concentration increased, the IG band decreased and the IDi (where i = 1, 2, 3, and 4) increased. The IG/Iall ratio decrease with increasing methane concentration, as shown in Figure 11, which indicates that the addition of methane did not help in ordering the char carbon or increase its graphitic nature. During microwave coal pyrolysis some of the methane will thermally decompose into amorphous carbon and H2 which could impact the ordering mechanism of char carbon via hydrogenation reactions 37, deposited amorphous carbon on char surface 14. Additionally, some of the methane may participate in some alkylation reactions with the char molecules during microwave pyrolysis, thereby reducing its graphitic nature. Similarly, the concentration of carbon defects in char structure increased with methane concentration indicating some SP2 could be converted into SP3 in the presence of methane via indirect hydrogenation or alkylation reactions, which could be reflected in Table 3, where the ratio of IDi/IG decreased as the methane concentration increased, the ratio of IDi/IG decreased. These results are consistent with the low loss tangent observed in chars in present of methane. Also, they are in agreement with the observed enhanced concentrations of light hydrocarbons observed in the presence of methane which could suggest that the addition of methane upgraded the formed coal tars, increased the hydrogenated products, and changed the nature of carbon structure in chars by lowering its tendency to graphitize and became more orderly structured in chars. Xu el al. 28 have measured the dielectric and Raman properties of char at different temperatures and correlated them with the degree of carbon ordering in char structure. The microcrystalline planer size (La) of ordered carbon in these chars were calculated according to Matthews et al. 13 and are listed in Table 3. The degree of carbon ordering in the lattice structure can be estimated by measuring the microcrystalline planer size, if it is more than 100 nm, only G band is


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observed while when it is less than 100 nm, D1 and D2 bands starts to appear 38. La is ranging from about 5.8 nm for char generated in absence of methane and gradually decrease by increasing methane concentration down to 1.5 nm. For graphite sheets La was calculated to be 33.4 nm in agreement with [ref]. In general, a quick change in the spectral parameter and La was observed as the methane concertation increased from 0 to 50 % then slowly changed up to 90% as observed in Figure 11.

Figure 10. Raman spectra of (a) graphite sheet, (b) coal chars generated under microwave at 980℃ and 1000 W under 0, 25, 50, and 90% methane feed balanced in Ar.

Table 3. First-order Raman bands and vibration modes obtained after curve fittings of microwave coal chars generated under different methane concentrations. Band

Peak position  (cm-1)

Vibration mode

G D1 D2 D3 D4

~1580 ~1350 ~1610 ~1500 ~1200

Stretching w E2g symmetry Lattice w A1g symmetry Lattice w E2g symmetry Amorphous SP2carbon periphery mixed SP2-SP3

Band area ratio Char under# Graphite* Ar 25% CH4 25% CH4 90% CH4


IG/IAll 0.87 0.43 0.30 0.18 0.16

ID1/IG 0.15 0.85 1.43 3.15 3.23

ID2/IG 0.13 0.21 1.02 0.45

La (nm) ID3/IG 0.00 0.05 0.42 0.61 0.76

ID4/IG 0.17 0.25 0.33 0.69

33.4 5.8 3.5 1.6 1.5

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4 0.2

La (nm)

6

0.4

IG / Iall

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

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2 0.0

0

20

40

60

80

100

Methane % Figure 11. Variations of band area ratios and the corresponding carbon microcrystalline size as a function of methane concentration added during microwave coal pyrolysis at 980 ℃.

4. Conclusion The addition of methane during microwave-assisted coal pyrolysis had an impact on char formation and other product distribution. In this work, low rank coal was reacted under microwave power in the presence of different methane concentrations (0, 25, 50, and 90%) at 980℃ for 2 hours. Increasing methane concentration increased both the char and tar yields due to carbon deposition from methane decomposition and hydrogenation of trapped carbon via hydrocracking reactions. In addition to methane decomposition into carbon during coal pyrolysis as confirmed from the low surface area of prepared chars and carbon-rich batches observed from elemental mapping analysis on char surface, methane can interact in the presence of coal char and form other valuable hydrocarbons. Analyses of the gas compositions suggested that some of the methane could be activated by the minerals in the coal and form light hydrocarbons C2-C7, which accounted for up to 5% of the gaseous products at 90% methane and less than 0.5% under pure Ar. Methyl groups could also get integrated into aromatic coal compounds as seen in the increase in the amount of methyl substituted phenols and naphthalene with methane. Tar yield also


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increases from methane decomposition through hydrogenation of various coal compounds. The results suggest that the addition of methane did not help in forming ordered carbon chars, due to amorphous carbon deposition on the char surface, hydrogenation of char and alkylation reactions. The presence of methane enhanced the formation of C2s and benzene; a possible correlation between benzene and hydrogen was proposed. The char characterization also confirmed that as the concentration of methane, added during coal pyrolysis, increased the graphitic nature of carbon in these chars decreased possibly due to coal hydrocracking and methane decomposition reactions associated with the reductive atmosphere of coal pyrolysis under methane. The high D to G carbon vibration band ratio observed in Raman and the low tangent loss observed in dielectric measurements of prepared chars suggested that the addition of methane decrease the carbon ordering and lower the electric conductivity of generated chars. The X-ray diffraction results revealed that the carbon diffraction peaks are more pronounced for char prepared in absence of methane compared to that prepared under methane. 5. Acknowledgement This work was performed in support of the US Department of Energy’s Fossil Energy Advanced Gasification Program. The Research was executed through NETL Research and Innovation Center’s Advanced Gasification effort. Research performed by AECOM Staff was conducted under the RES contract DEFE0004000. Authors acknowledge Mr. Richard Bergen, project activity manager, for guidance and valuable discussions, and Mr. John Johnson for running the reactors and the associated analytical instruments. 6. Disclaimer This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, nor AECOM, nor any of their


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

employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 7. References

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Microwave-assisted conversion of low rank coal under methane

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