Dongjuan Ma's research while affiliated with Taiyuan University of Technology and other places

To improve the fire extinguishing efficiency and reduce the re-combustion risk, a novel approach to fire extinguishing using sodium pyrotechnic aerosol (SPA) was adopted for controlling underground mine fires. Physical simulation experiments of 1 m3 in size show that the extinguishing effect of aerosol consisting of NaNO3 and dicyandiamide is better than that of 0.2 MPa nitrogen, in which the cooling rate peaks at 32.6 °C min−1. Experiments result prove that the N2 and CO2 yield of aerosol reach about 10 mol s−1 m−3 and 5 mol s−1 m−3, respectively. As an additive, phenol–formaldehyde resin embodies advantages compared with agents such as K2C2O4, CH4Mg2O6, and potassium hydrogen phthalate. According to TG/DTG tests, characteristic temperature points of coal with SPA can be increased significantly, and the apparent activation energy of coal also increases to 18.8–80.5%. Theoretically, Na-salt generated by aerosol reaction consumes the O-contained groups in coal and creates an alkaline environment. Abundant N2 and CO2 also carry ultra-fine Na-salt residue continuously suppressing fire. As a new fire extinguishing technique for use in underground coal mines, SPA shows potential for underground fire prevention.

The extinguishing and re-burning of the closed fire area in an underground coal mine were investigated by laboratory-scale physical simulation. Temperatures in the center of the fire source were recorded, and the typical cooling process was observed to include the rapid cooling stage (900-400 °C) and dilatory cooling stage (400-100 °C). With the increase of coal mass from 20 to 80 kg, the rate of cooling decreases and the time required for fire extinguishing increases by 69.5%-193.2%. At temperatures ranging between 500 and 100 °C, yields of CO and H2 show strong correlation with the attenuation of the coal fire, and the trend in the yield of H2 might be used as the optimal indicator considering the different amounts of coal. A significant difference appears in the concentration of H2 released by samples of different dosages of coal in the early stage of cooling, especially when the temperature exceeds 200 °C. During the extinguishing process, micropores in coal fused into mesopores and macropores, while the content of O-containing groups fluctuated significantly. Variations of elemental C and O also indirectly reflect the combustion state in the fire cooling. Taking the experimental reactor as a physical model, the time required for the fire area from closure to safe re-opening is deduced, that is, t = Cm ln (T 1 - T ∞)/(T 2 - T ∞ ). The calculated results were compared with the changes in measured temperatures, providing a theoretical foundation for the re-opening prediction of mine fire areas.

To explore an efficient and sustainable approach to prevent coal spontaneous combustion in underground goaf, chemical deposits produced by a reaction of two-phase aerosols (CO2 and hydroxide) were utilized to prevent the spontaneous combustion of anthracite. A Henan anthracite sample (0.18 mm-0.25 mm) was treated with aerosols continuously. Ba(OH)2-CO2, Ca(OH)2-CO2, Na2SiO3-CO2 and NaAlO2-CO2 aerosols flowed through the surface of the coal sample simultaneously. After 50–55 s, in addition to Ca(OH)2-CO2, white deposits or transparent colloidal deposits appeared and accumulated gradually. After 20 mins, the weight of the produced deposits (dry mass) reached 0.95%-5.10% of the original weight of the samples. According to the results of scanning electron microscope, X-ray diffraction, thermogravimetry and differential scanning calorimetry, physical experiments and quantum chemistry simulations, we inferred that the two-phase aerosols formed dense ultrafine columnar or flocculent coatings on the coal surface. The main components of the deposits were BaCO3, CaCO3, H2SiO3, Al(OH)3, hydrates, etc. The number of residues after the combustion of the coal samples increased by 6.57%-16.12%, and the maximum exothermic peak decreased to 69.0%-86.2% of the raw coal. Ba²⁺ in the aerosol formed two-ligand, three-ligand and four-ligand complexes with the characteristic structure containing S in the coal samples. It can be concluded that the two-phase aerosols exhibit excellent barrier-crossing ability and a higher inhibitory effect on re-ignition for extinguishing fires.

To control underground fire hazards, the principle of a pyrotechnic aerosol was adopted in coal fire-extinguishing experiments. Results show that fire extinguishing efficiency of a pyrotechnic aerosol composed of 63.4% KNO3, 23.6% C2H4N4, 8% C7H6O2, and 2% C8H5KO4 is better than that of nitrogen at 0.25 MPa. The required time to fire extinction is reduced by 41.6% in a 1 m³ control volume. TG/DTG tests support the view that the aerosol and its residues inhibit the oxidation and retards the combustion of coal. According to the EDS tests, the solid residues of the PA are mainly alkaline potassium salts such as K2CO3 and KHCO3. The FTIR results indicate that the K from the pyrotechnic aerosol will interact with the -OH in coal samples and also creates an alkaline environment inhibiting the production of -COOH. The effect on a pyrotechnic aerosol on coal fires is to produce ultra-fine and inert residue particles to extinguish the fire.