Mechanism and performance of coal spontaneous combustion with a halide carrier inorganic salt inhibitor

This introduction paper is based on the paper "Mechanism and performance of coal spontaneous combustion with a halide carrier inorganic salt inhibitor" published by "Chinese Journal of Engineering".

Fig.1 Test sample SEM: (a) sample 1; (b) sample 2; (c) sample 7; (d) sample 8
Fig.1 Test sample SEM: (a) sample 1; (b) sample 2; (c) sample 7; (d) sample 8

1. Overview:

  • Title: Mechanism and performance of coal spontaneous combustion with a halide carrier inorganic salt inhibitor
  • Author: ZHANG Yan-ni, HOU Yun-chao, LIU Bo, DENG Jun, LIU Chun-hui, YANG Jing-jing, WEN Xin-yu
  • Year of publication: 2021
  • Journal/academic society of publication: Chinese Journal of Engineering
  • Keywords: halide carrier inorganic salt; inhibitor; coal spontaneous combustion; differential scanning calorimetry; apparent activation energy

2. Abstract:

Coal spontaneous combustion seriously restricts the safe production of coal mines, and adding an inhibitor is one of the effective methods to prevent coal spontaneous combustion. To improve the pertinence and high efficiency of the inhibitor, this paper considered the intrinsic properties and external conditions that affect the occurrence of coal spontaneous combustion, combined with the characteristics that the rare earth hydrotalcite can effectively improve the thermal stability, coupling, and flame retardancy of the coal and the halide inhibitor. The halide inhibitor can enhance the permeability, dispersion, and uniformity of the rare earth hydrotalcite as a carrier. The halide carrier inorganic salt inhibitor was prepared. To study the inhibition mechanism and performance of the halide carrier inorganic salt inhibitor on coal spontaneous combustion, differential scanning calorimetry (DSC) was used to test the variation law of parameters, such as stage characteristics, characteristic temperature, thermal effect, and apparent activation energy in the process of coal spontaneous combustion under the action of a rare earth hydrotalcite, MgCl2 and a halide carrier inorganic salt inhibitor. Test results reveal that the OH of the rare earth hydrotalcite laminate can generate a weak hydrogen bond with acidic functional groups such as –COOH in coal molecules so that the activity of the acidic functional groups is weakened. Mg²⁺ complexes with –COO⁻ in coal molecules to form –COOMg–, resulting in the weakening of the C=O activity in –COO⁻, which is the main mechanism of the halide carrier inorganic salts inhibiting coal spontaneous combustion. The endothermic peak of the DSC curve appears as a double peak or multi-peak after the addition of halide carrier inorganic salts to the coal sample. Compared with the raw coal, the peak temperature is shifted back by 50–60 °C, the T₁ temperature is shifted back by 90–100 °C, and the total heat release decreased by 19–27 kJ·g⁻¹. Furthermore, the apparent activation energy of each stage of the coal body is effectively improved. Results revealed that the halide carrier inorganic salt inhibitor could effectively inhibit the reaction process of coal spontaneous combustion.

3. Introduction:

Coal is a primary energy source, and its production and consumption are significant globally. However, approximately 75% of coal seams in China are prone to spontaneous combustion, leading to substantial resource loss and CO2 emissions. Developing effective inhibitors is crucial for preventing thermokinetic disasters in coal mines and ensuring safe production.
Current inhibitors mainly include halide salts, inert gases, and polymer emulsions. These can be classified based on their mechanism into chemical inhibition and physical inhibition. Physical inhibition controls combustibles or ignition sources, while chemical inhibition acts at a microscopic level by destroying or capturing active functional groups in coal molecules, thereby slowing down the coal-oxygen recombination process.
Halide inhibitors are widely used due to their good coating and water-absorbing properties. However, they have disadvantages such as requiring large amounts, causing corrosion to equipment, and having short inhibition times. Rare earth hydrotalcites, as functional materials, offer good thermal stability, ion exchangeability, and tunable layer composition, making them suitable for various applications. Adding rare earth hydrotalcite to halide inhibitors can enhance the thermal stability, coupling, and flame retardancy of the coal-inhibitor system. Conversely, using halide inhibitors as carriers can overcome the poor permeability, dispersion, and uniformity of rare earth hydrotalcites. This study focuses on preparing a halide carrier inorganic salt inhibitor using rare earth hydrotalcite and MgCl2, and investigating its inhibition characteristics and mechanism through theoretical analysis and experimental testing, aiming to provide fundamental data for developing new and efficient coal spontaneous combustion inhibitors.

4. Summary of the study:

Background of the research topic:

Spontaneous combustion of coal is a significant safety hazard in coal mining operations, leading to resource loss and environmental pollution. The application of inhibitors is a key strategy to prevent such incidents.

Status of previous research:

Various inhibitors, including halide salts, inert gases, and polymer emulsions, have been developed. Halide salts are common but suffer from drawbacks like high dosage, corrosivity, and short-term effectiveness. Rare earth hydrotalcites have shown promise in improving thermal stability and flame retardancy. Previous studies have explored individual components like MgCl2 and composite inhibitors, indicating the potential for synergistic effects.

Purpose of the study:

This study aimed to develop a novel halide carrier inorganic salt inhibitor by combining rare earth hydrotalcite with MgCl2. The research focused on understanding the inhibition mechanism and performance of this composite inhibitor on coal spontaneous combustion, with the goal of providing a basis for creating more efficient and targeted inhibitors.

Core study:

The core of the study involved the preparation of a halide carrier inorganic salt inhibitor using rare earth hydrotalcite and MgCl2. The inhibitory effects were evaluated by applying these inhibitors to coal samples. Differential scanning calorimetry (DSC) was employed to analyze the thermal behavior, including characteristic temperatures, heat effects, and apparent activation energies during coal oxidation. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used for microstructural and elemental analysis of the treated coal samples.

5. Research Methodology

Research Design:

The research employed an experimental design. It involved the synthesis of rare earth hydrotalcite, preparation of different inhibitor formulations (rare earth hydrotalcite alone, MgCl2 alone, and their combination as a halide carrier inorganic salt inhibitor), and treatment of coal samples with these inhibitors. The performance of the inhibitors was then evaluated through thermal analysis and microstructural characterization.

Data Collection and Analysis Methods:

  • Materials: Bin-chang non-stick coal was used as the research object. Rare earth hydrotalcite was synthesized using a co-precipitation method. MgCl2 was used as another inhibitor component.
  • Sample Preparation: Coal samples (0.105–0.15 mm) were mixed with different inhibitor solutions (compositions detailed in Table 2 of the original paper) and dried.
  • Differential Scanning Calorimetry (DSC): A NETZSCH DSC200F3 instrument was used. 10 mg samples were tested in an air atmosphere (25 mL·min⁻¹) with a heating rate of 5 °C·min⁻¹ from 30 °C to 450 °C.
  • Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS): A QUANTA FEG-450 SEM with a MAX-50 EDS attachment was used to observe the micro-morphology and elemental composition of the samples.
  • Kinetic Analysis: The apparent activation energy (E) for different stages of coal oxidation was calculated from DSC data using the Arrhenius equation and a derived kinetic model (Equation 5 in the original paper).

Research Topics and Scope:

  • Investigation of the micro-morphology and elemental distribution of coal samples treated with different inhibitors.
  • Analysis of the heat release characteristics (stage characteristics, characteristic temperatures, thermal effects) of coal spontaneous combustion under the influence of rare earth hydrotalcite, MgCl2, and the halide carrier inorganic salt inhibitor.
  • Elucidation of the inhibition mechanism of the halide carrier inorganic salt inhibitor.
  • Determination of the apparent activation energy for different stages of the coal oxidation process with and without inhibitors.

6. Key Results:

Key Results:

  • Microstructure and Composition (SEM/EDS): The chemical co-precipitation method effectively improved the composite degree of the inhibitor with coal. MgCl2, acting as a carrier, enhanced the penetration, dispersion, and uniformity of the rare earth hydrotalcite on the coal surface.
  • Thermal Behavior (DSC):
    • The addition of the halide carrier inorganic salt inhibitor to coal samples resulted in the appearance of double or multiple endothermic peaks in the DSC curves.
    • Compared to raw coal, the peak temperature of the endothermic process was shifted to higher temperatures by 50–60 °C.
    • The initial temperature of significant heat release (T₁ temperature) was shifted to higher temperatures by 90–100 °C.
    • The total heat released during oxidation was reduced by 19–27 kJ·g⁻¹.
  • Inhibition Mechanism:
    • The -OH groups on the laminate of the rare earth hydrotalcite can form weak hydrogen bonds with acidic functional groups (e.g., -COOH) in coal molecules, thereby reducing the activity of these acidic groups.
    • Mg²⁺ ions from MgCl2 can form complexes with -COO⁻ groups in coal (forming -COOMg-), which weakens the C=O bond activity within the -COO⁻ group. This is a primary mechanism for the halide carrier inorganic salt inhibiting coal spontaneous combustion.
    • The halide carrier inorganic salt inhibitor demonstrated both physical (e.g., coating, moisture retention by MgCl2) and chemical inhibition effects, delaying the heat absorption phase, increasing the amount of heat absorbed, and significantly reducing heat release in subsequent oxidation stages.
  • Apparent Activation Energy: The addition of inhibitors, particularly the halide carrier inorganic salt inhibitor, effectively increased the apparent activation energy for both the slow and rapid heat release stages of coal oxidation, indicating a higher energy barrier for combustion and thus better inhibition.

Figure Name List:

Fig.3 Curve of the heat release rate of the test sample: (a) sample 1–6; (b) sample 7–12
Fig.3 Curve of the heat release rate of the test sample: (a) sample 1–6; (b) sample 7–12
Fig.5 Apparent activation energy curve of the test sample during the slow heat release stage: (a) sample 1; (b) sample 7
Fig.5 Apparent activation energy curve of the test sample during the slow heat release stage: (a) sample 1; (b) sample 7
Fig.6 Apparent activation energy curve of the test sample during the rapid heat release stage: (a) sample 1; (b) sample 7
Fig.6 Apparent activation energy curve of the test sample during the rapid heat release stage: (a) sample 1; (b) sample 7
  • Fig.1 Test sample SEM: (a) sample 1; (b) sample 2; (c) sample 7; (d) sample 8
  • Fig.2 Test sample EDS: (a) sample 1; (b) sample 2; (c) sample 7; (d) sample 8
  • Fig.3 Curve of the heat release rate of the test sample: (a) sample 1–6; (b) sample 7–12
  • Fig.4 Heat release rate curve: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5; (f) sample 6; (g) sample 7; (h) sample 8; (i) sample 9; (j) sample 10; (k) sample 11; (l) sample 12
  • Fig.5 Apparent activation energy curve of the test sample during the slow heat release stage: (a) sample 1; (b) sample 7
  • Fig.6 Apparent activation energy curve of the test sample during the rapid heat release stage: (a) sample 1; (b) sample 7

7. Conclusion:

The study yielded the following main conclusions:

  1. Microscopic analysis (SEM/EDS) confirmed that the chemical co-precipitation method enhances the integration of the inhibitor with the coal. MgCl2 acts as an effective carrier, improving the penetration, dispersion, and uniformity of the rare earth hydrotalcite. DSC analysis revealed distinct thermal reaction stages (heat absorption, slow heat release, rapid heat release) and highlighted differences in the thermal reactivity and inhibition mechanisms of various inhibitors.
  2. The halide carrier inorganic salt inhibitor exerts a dual inhibition effect (physical and chemical) on coal spontaneous combustion. This is macroscopically manifested by a delay in characteristic temperatures (peak endothermic temperature and T₁), the appearance of multiple absorption peaks, prolongation of the heat absorption phase with increased heat absorption, and a significant reduction in heat release during the various oxidation stages.
  3. Rare earth hydrotalcite effectively enhances the inhibition efficiency of halide salts, suppresses heat accumulation in coal, delays the onset of spontaneous combustion, and significantly increases the apparent activation energy of coal in different oxidation stages. Therefore, the halide carrier inorganic salt inhibitor can effectively suppress the coal spontaneous combustion process, offering a promising approach for developing more efficient inhibitors.

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9. Copyright:

  • This material is a paper by "ZHANG Yan-ni, HOU Yun-chao, LIU Bo, DENG Jun, LIU Chun-hui, YANG Jing-jing, WEN Xin-yu". Based on "Mechanism and performance of coal spontaneous combustion with a halide carrier inorganic salt inhibitor".
  • Source of the paper: https://doi.org/10.13374/j.issn2095-9389.2020.12.25.001

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