Thermogravimetric Analysis: A Study of the Pyrolysis Processes of Coal of Different Particle Sizes Using Photoionisation Mass Spectrometry

Areas of application: Energy conversion; research into coal pyrolysis products;

Introduction‌

As a key initial step in coal conversion and utilisation, the distribution of pyrolysis products has a decisive influence on subsequent processes such as gasification and liquefaction. Particle size, as a crucial parameter, directly affects pyrolysis efficiency and product composition by influencing mass and heat transfer processes and the extent of secondary reactions. Although conventional thermogravimetric analysis (TG) can provide information on weight loss during pyrolysis, it lacks the capability for real-time characterisation of products at the molecular level.

This study utilised bituminous coal from the Huainan coal mine as the research subject. Thermogravimetry (TG) and Py-PI-TOF mass spectrometry were employed to investigate the evolution curves of pyrolysis products and to observe the pyrolysis characteristics of coal with different particle sizes, thereby providing a basis for elucidating the reaction mechanisms and product release mechanisms.

‌Experimental Materials and Methods‌

Samples: Bituminous coal from the Huainan coal mine was selected and ground and sieved into three particle size fractions.

HN-S: <40 μm

HN-M: 224–500 μm

HN-L: 1600–2000 μm

Thermogravimetric analysis (TG)

Heated from room temperature to 800 °C at a heating rate of 50 °C/min, in a helium atmosphere

Py-PI-TOF MS analysis

Sample quantity: 20 mg

Carrier gas: N₂

Flow rate: 200 SCCM

Fixed-temperature mode: Mass spectrometry acquisition for 250 s at 600 °C

Programmed temperature mode: 50 °C/min from room temperature to 800 °C, with a single spectrum of 10 s

‌Results and Conclusions

1. Thermogravimetric Analysis of Coal with Different Particle Sizes

The results of the thermogravimetric analysis (Figure 1) indicate that an increase in particle size significantly enhances the pyrolysis conversion rate. The weight losses for the three coal particle sizes were as follows: HN-S: 23.00 per cent, HN-M: 25.04 per cent, and HN-L: 28.78 per cent.

Figure 1: TG and DTG curves of HN coal of different particle sizes at a heating rate of 50 °C/min

2. Pyrolysis characteristics of coal with different particle sizes at 600 °C

2.1 Distribution of pyrolysis products

The main products detected included alkanes, alkenes, phenols and aromatics (Figure 2); the intensity of volatile products increased with particle size (Figure 3), whilst the yield of inorganic gases such as CH₄, CO₂, CO, H₂O and H₂ also increased correspondingly.

Figure 2: Mass spectrometry of HN-S coal at 600 °C: (a) photon energy of 10.6 eV, (b) photon energy of 14.2 eV

2.2 Time-dependent curves of characteristic pyrolysis products from coal

Figure 4 shows the time-dependent intensity curves of typical pyrolysis products such as propylene (m/z 42), toluene (m/z 92), cresol (m/z 108), phenanthrene/anthracene (m/z 178),   C1-alkylpyrene/anthracene (m/z 192) and C2-alkylpyrene/anthracene (m/z 206). As the coal particle size increased, the appearance of all pyrolysis products was delayed by a few seconds, particularly in the case of polycyclic aromatic hydrocarbons. Polycyclic aromatic hydrocarbons exhibited a multi-stage release process; this phenomenon can be explained by two processes: the release of guest molecules and coal pyrolysis.

Figure 3 Peak areas of major pyrolysis products for HN-S, HN-M and HN-L coals at 600 ℃

Note: H₂O (obtained at 14.2 eV), H₂ (obtained at 15.5 eV)

Figure 4 Time-dependent evolution curves of typical volatile products from the pyrolysis of coal with different particle sizes at 600 °C

3 Curves showing the variation of major pyrolysis products from coal of different particle sizes during the heating process

Figure 5 shows the variation of propene (m/z 42), butene (m/z 56), pentene (m/z 70), phenol (m/z 94), cresol (m/z 108) and C₂-alkylphenol (m/z 122) during the heating process. As shown in the figure, the peak areas of all products increase with increasing coal particle size, which is consistent with the TG/DTG curves. The release of products is mainly concentrated in the 300–600 °C range, and these olefin and phenolic products primarily exhibit single-peak release characteristics during the heating process.

Figure 5  Temperature-dependent curves of major olefin and phenol products from HN coal of different particle sizes

Figure 6 shows the temperature-dependent curves for benzene (m/z 78), toluene (m/z 92), C2-alkylbenzene (m/z 106), naphthalene (m/z 128), C1-alkylnaphthalene (m/z 142), C2-alkylnaphthalene (m/z 156), phenanthrene/ anthracene (m/z 178), C1-alkylphenanthrene/anthracene (m/z 192) and C2-alkylphenanthrene/anthracene (m/z 206). Monocyclic compounds such as benzene, toluene and C2-alkylbenzene primarily exhibit single-peak release, whilst tricyclic compounds (Figures 6g–i) display multi-peak release characteristics. Among these, the low-temperature peaks (150–350 °C): result from the evaporation of unbound guest molecules, whilst the high-temperature peaks (400–600 °C): result from the polymerisation of coal fragments and light pyrolysis products generated by bond breaking, as well as the secondary pyrolysis of tar.

Figure 6  Curves showing the variation of major aromatic products with temperature for HN coal of different particle sizes

4 Schematic diagram of the pyrolysis process for coal of different particle sizes

Figure 7 illustrates the mechanism of volatiles release during coal pyrolysis and compares the differences in pyrolysis behaviour between coarse-grained and fine-grained coal. Both intra-particle and inter-particle processes play important roles in coal pyrolysis. Intra-particle processes primarily influence the timing and yield of volatiles, whilst inter-particle processes mainly affect the yield of volatiles; however, their impact is less significant than that of intra-particle processes. Consequently, larger particles experience delayed release due to impeded heat transfer, whilst more intense secondary reactions enhance the yield of volatiles.

Figure 7: Volatile matter release process during the pyrolysis of coal particles of different sizes

‌

This study utilised a combined TG and Py-PI-TOF MS technique to systematically reveal the significant influence of coal particle size on the distribution of pyrolysis products and their release kinetics; this method provides an effective technical approach for the real-time analysis of the evolution of coal pyrolysis products.

References

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