DCLR from the direct liquefaction product of the Shenhua Group was used as a sample, whose proximate and ultimate analyses are shown in Table 1. Results suggested that DCLR contained highly volatile matter, carbon, and ash content.

Table 1

Table 1

Table 1 Proximate and ultimate analyses of DCLR (%) Sample Proximate analysis Ultimate analysis Mad Aad FCad Vad Cad Had Oad Nad St, ad DCLR 0.14 17.74 48.37 33.75 75.00 4.22 17.83 0.79 2.16

Table 1 Proximate and ultimate analyses of DCLR (%)

Microwave and conventional pyrolysis methods were carried out in a MKX-J2B microwave oven and a GXL1400X vacuum-tube furnace respectively. About 100 g of DCLR was added to a quartz reactor, which was then placed inside the furnace. After eliminating the entrapped air in the reactor by using an inert gas, the reactor was heated and maintained at the desired power (800 W) or temperature (900 ℃) for 40 min. Tar was collected as the pyrolysis gas passed through the two-stage condensation system with a cooling medium temperature of 0 ℃, and then the gas was analyzed using an online gas analysis meter after drying. The power was turned off, and the coke samples were collected for subsequent analysis after naturally cooling at room temperature.

Four-component analyses of DCLR and coke were carried out according to the national standard of coking products. Toluene-insoluble content was determined in a Soxhlet apparatus with an Allihn condenser. Gas composition was analyzed using a Gasboard-3100 online infrared analyzer. The structure and composition of coke and tar were characterized using an IR Prestige-21 Fourier transform infrared (FTIR) spectrometer and a gas chromatography (6890)-mass spectrometry (5973) system respectively.

As shown in Fig.1, DCLR was rapidly pyrolyzed up to 700 ℃ in 8 min in a microwave field with the maximum heating rate of 329 ℃· min^-1, which was then slightly decreased, although 900 ℃ was still reached in 20 min. No obvious change in temperature was observed afterwards. By contrast, the temperature of DCLR in a conventional field reached 900 ℃ in about 45 min at an almost constant heating rate. Microwave heating is a kind of volumetric heating induced by the dielectric loss of a material in an electromagnetic field. This heating process is closely related to internal molecular polarization. Heat transfers from an internal to an external surface with high transfer efficiency and heating rate. Moreover, DCLR contains plenty of pyrrhotite, which has excellent microwave absorbing properties and can reach 955 K in 42s. Moreover, DCLR could form “ hot spots” during pyrolysis, resulting in increased heating rate.

Fig.1

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	Fig.1 Elevated temperature curves of DCLR under different heating methods (a): Heating curves; (b): Heating rate

As shown in Fig.2, coke obtained after DCLR pyrolysis in a microwave field decreased by 2.85% compared with that in a conventional field, whereas tar and gas yields increased by 0.66% and 2.19% respectively. Material temperature can reach 900 ℃ in 40 min regardless of heating method (Fig.1). Meanwhile, abundant volatiles were released accompanying DCLR pyrolysis. The increased heating rate plays a significant role in improving DCLR pyrolysis product yield.

Fig.2

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	Fig.2 Pyrolysis product yields under different heating methods

The heating rate of microwave pyrolysis was faster than that of conventional pyrolysis. Consequently, the produced gas products rapidly released, resulting in more thorough volatile evolution, decreased solid product yield, and increased gas product yield.

As shown in Table 2, gas compositions roughly had the same proportion under conventional and microwave pyrolysis conditions, except for H₂ content, which increased by 3.61% in a microwave field. Temperature variations can significantly affect pyrolysis gas composition and yield according to the results shown in Fig.1, in which the terminal pyrolysis temperature reached 900 ℃ under both pyrolysis conditions. Consequently, the gas compositions aligned well. DCLR is a hydrogen-rich substance, and H radicals were mainly the result of the poly-condensation reaction of the aromatic compounds during pyrolysis, especially the hydrogenated aromatic compounds that were involved in condensation, dehydrogenation, and poly-condensation reactions. During the microwave field, the fast pyrolysis promoted the cracking of dehydrogenation reaction and increased the content of hydrogen gases such as H₂, CH₄, C_nH_m.

Table 2

Table 2

Table 2 Gas compositions under different heating methods (V/%) CO2 CO CH4 CnHm H2 Microwave 0.89 3.13 13.26 1.95 66.25 Conventional 0.75 3.52 11.67 1.90 62.64

Table 2 Gas compositions under different heating methods (V/%)

Table 3

Table 3

Table 3 Contents of char and DCLR compositions under different heating methods (%) HS/% A/% PA/% THFIS/% Microwave 6.70 0.19 0 93.11 Conventional 7.95 0.19 0 91.86 DCLR 26.01 30.08 9.92 34.00 Note: HS: Heavy oil; A: Asphaltene; PA: Pre-asphaltene; THFIS: Tetrahydrofuran insoluble

Table 3 Contents of char and DCLR compositions under different heating methods (%)

Results in Table 3 suggested that asphaltene, pre-asphaltene and most of the heavy oil were pyrolyzed during microwave and conventional pyrolysis, in which heavy oil content decreased from 26.01% to 6.70% (microwave pyrolysis) and 7.95% (conventional pyrolysis), respectively. Preasphaltene was completely pyrolyzed, and asphaltene content decreased from 30.08% in DCLR to 0.19% in the coke, whereas the tetrahydrofuran-iosoluble content of coke was still > 90%, indicating that recovering oil first through pyrolysis was a reasonable way to upgrade DCLR.

The FTIR spectra of DCLR and coke are shown in Fig.3. The broad and intense characteristic absorption peak near 3 437.6 cm^-1 was assigned to the — OH functional group and used to measure alcohol or phenol content existing in the form of dimer or polymer. The bands between 3 000 and 2 500 cm^-1 were assigned to saturated — C— H stretching vibration mode. The bands near 1 632.0, 1 436.78, and 1 079.99 cm^-1 were assigned to — C=C— functional group, saturated and unsaturated — C— H bending vibration mode, and — C— O stretching vibration mode, respectively. The bands between 1 300 and 300 cm^-1 were fingerprint region attributed to — C— X bending and stretching vibration and decomposition of inorganic mineral substance. As is shown in Fig.3, the intensities of absorption peaks near 3 437.6, 1 632.0, and 1 079.99 cm^-1 significantly decreased, whereas the absorption peaks that varied from 3 000 to 2 500 cm^-1 and near 1 436.78 cm^-1 almost disappeared during microwave and conventional pyrolysis, implying that all corresponding functional groups were involved. The intensities of the absorption peaks at 3 437.6, 1 632.0, and 1 079.99 cm^-1 in the FTIR spectra obtained under microwave pyrolysis condition were lower than that under conventional condition, indicating that DCLR was more thoroughly pyrolyzed in the microwave field.

Fig.3

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	Fig.3 FTIR spectra of DCLR and coke under different heating methods

Fig.4 shows the FTIR spectra of tar obtained microwave and conventional pyrolysis methods. The bands between 3 768.41 and 2 879.34 cm^-1 were assigned to — OH, unsaturated C— H, and saturated — C— H stretching vibration mode. The absorption peak near 1 637.35 cm^-1 was assigned to aromatic — C=C— , whereas the bands from 1 300 to 300 cm^-1 were assigned to — C— X stretching and bending vibration. The intensities of absorption peaks in the FTIR spectra obtained under microwave pyrolysis condition were higher than those under conventional condition. The wide absorption peaks between 3 768.41 and 2 879.34 cm^-1 were attributed to the interference of characteristic vibrations with the close wavelength and vibration coupling of certain absorbance, produced double peaks that looked like a widened mono-peak at low resolution. This result suggested that tar contained plenty of hydroxyl groups, as well as saturated — C— H and unsaturated C— H groups. The heating rate in the conventional pyrolysis was low, hence the relatively long residence time for tar. This long residence time prompted the secondary cracking of tar that led to decreased intensity of the corresponding absorption peaks. These results suggested that the content of unsaturated functional groups in the tar can be efficiently improved under optimum pyrolysis conditions in microwave and conventional fields, thereby improving the utilization value of tar.

Fig.4

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	Fig.4 FTIR spectra of tar obtained under different heating methods

Only organic compounds with < 300 ℃ boiling point can be detected using GC/MS. Thus, tar was first extracted using petroleum ether before analysis, and insoluble substances after extraction were mainly asphaltene. Results of extraction experiments showed that the extraction rates of tar obtained by microwave and conventional pyrolysis methods were 69.7% and 62.0% respectively, indicating that the asphaltene of tar obtained by microwave pyrolysis was lower than that by conventional pyrolysis. Data in Tables 4 and 5 shows that the contents of fatty hydrocarbons, aromatic hydrocarbons, and phenols in tar after extraction were approximately similar, as well as the contents of C_1~5, C_11~20, and above C₂₀ in the tar obtained under the two pyrolysis conditions. The petroleum extraction rate of microwave pyrolysis products was 69.7%, which was higher than that of conventional pyrolysis products, indicating a higher light fraction in tar after microwave pyrolysis. In addition, the GC/MS chromatograms of tar in Fig.5, where the horizontal ordinate representing the mass-to-charge ratio, depicted that quasi-molecular ions were positioned forward and intensity increased under microwave heating method. It was suggested that the light fraction accounted for a high proportion of tar and microwave pyrolysis contributed to tar conversion to light fraction. This was because some of the molecules thermally decomposed (thermal effect), and another part vibrated to break the molecular chains (non-thermal effect) in the microwave field, there by prompting the decomposition of most of heavy oil and asphaltene. Asphaltene and polycyclic aromatic hydrocarbons in DCLR absorbing microwaves can reduce heavy oil and asphaltene system viscosity. With increased system temperature, the internal energy of molecules increased until it was higher than the C— C and C— H bond energy, and then the organic molecules bonds broke. Consequently, the absorption microwave ability, rate, and efficiency of DCLR pyrolysis improved.

Table 4

Table 4

Table 4 Tar compositions under different heating methods (%) Pyrolysis methods Aliphatics(CH) Aromatics (benzene) Phenols Alcohols Ketone Others Alkanes Alkenes Others Microwave 11.13 1.32 1.45 78.25 5.39 0.80 0.31 1.36 Conventional 10.87 0.96 0.75 79.13 5.43 0.43 0.98 1.45

Table 4 Tar compositions under different heating methods (%)

Table 5

Table 5

Table 5 Distribution of different carbon numbers in tarunder different heating methods (%) Carbon number Microwave Conventional C6~10 23.94 23.02 C11~19 51.53 53.56 C> 20 24.39 23.18

Table 5 Distribution of different carbon numbers in tarunder different heating methods (%)

Fig.5

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	Fig.5 GC/MS of tar under (a) microwave and (b) conventional heating methods