The spontaneous combustion behavior of some low rank coals and a range of dried products

Experimental and Quantum Chemical Study on the Inhibition Characteristics of Glutathione to Coal Oxidation at Low Temperature

In response to the frequent occurrence of coal spontaneous combustion accidents, this paper proposes to use glutathione (GSH) as an inhibitor to inhibit the coal oxidation at low temperature. Based on the gas production of oxidation, thermogravimetric analysis, electron spin resonance, and in situ Fourier infrared transform spectroscopy experiments, it is known that GSH has a good inhibiting effect on lignite, long-flame coal, and fatty coal. The optimal action temperature of GSH is 60-150 °C, which can effectively slow down the weight loss and exothermic process and reduce the gas production of CO and CO₂. Compared with the raw coal, the GSH-treated coal samples possess higher crossing point temperature and lower reactive group content. Subsequently, quantum chemical calculations are performed using density functional theory. The results demonstrate that the inhibiting mechanism of GSH is inerting the reactive radicals in coal and converting them into more stable compounds. Meanwhile, the activation energy of the reaction between GSH and each reactive radical is small, and all of them can occur at room temperature and pressure. This study lays the groundwork for future development of inhibitors.

Study of Butylated Hydroxytoluene Inhibiting the Coal Oxidation at Low Temperature: Combining Experiments and Quantum Chemical Calculations

In order to cut off the chain reaction in the process of coal oxidation at low temperature (COLT), butylated hydroxytoluene (BHT) was used as an inhibitor to explore its inhibition effect and mechanism. In this paper, in situ Fourier transform infrared spectroscopy, electron paramagnetic resonance, and gas production of COLT experiments were conducted to compare the inhibited coal sample (BHT-Coal) with the raw coal. The results showed that BHT can effectively inhibit the formation of active free radicals, reduce the content of active alkoxy, carbonyl, and hydroxyl groups, increase the production temperature of CO, CO₂, and C₂H₄, and reduce the concentration. The crossing point temperature increased from 132.3 to 157.4 °C, indicating that BHT can reduce the spontaneous combustion tendency of the raw coal. To explore the inhibition mechanism of BHT on COLT, five typical active free-radical models were established, and their active sites, active bonds, and thermodynamic parameters were calculated according to the density functional theory. The results showed that the highly active H atoms of the phenolic hydroxyl group in BHT can combine with active free radicals to generate stable compounds, and the activation energy of each reaction is small, which can occur under normal temperature and pressure. The inhibition mechanism of BHT is to reduce the concentration of the free radicals, so as to weaken the chain reaction strength during the COLT. This study provides a reference for the development and utilization of inhibitors.

Quantum Chemistry Calculation Study on Chain Reaction Mechanisms and Thermodynamic Characteristics of Coal Spontaneous Combustion at Low Temperatures

The coal spontaneous combustion phenomenon seriously affects the safety production of coal mines. Aiming at the problem of complex coal molecular structure and incomplete reaction sequences at present, the mechanisms and thermodynamic parameters of coal spontaneous combustion chain reactions were explored by combining experimental detections and molecular simulations. First, the active groups on the surface of coal were obtained by Fourier transform infrared spectroscopy (FTIR), mainly including methyl (-CH₃), methylene (-CH₂), methyne (-CH), phenolic hydroxyl (-ArOH), alcohol hydroxyl (-ROH), carboxyl (-COOH), aldehyde (-CHO), and ether (-O-), and the coal molecular models containing functional groups and radicals were established. According to the charge density, electrostatic potential, and frontier orbital theories, the active sites and active bonds were obtained, and a series of reactions were given. The thermodynamic and structural parameters of each reaction were explored. In the chain initiation reaction stage, O₂ chemisorption and the self-reaction of radicals play a leading role. In this stage, heat gradually accumulates and various radicals begin to generate, where the intramolecular hydrogen transfer reaction of a peroxide radical (-C-O-O·) can produce the key hydroxyl radical (-O·). In the chain propagation reaction stage, O₂ and -O· continuously consume active sites to accelerate the reaction sequences and increase the temperature of coal, and index gases such as CO and CO₂ generate, causing the chain cycle reactions to gradually form. The chain termination reaction stage is the formation of stable compounds such as ethers, esters, and quinones, which can inhibit the development of chain reactions. The results can further explain the reaction mechanism of coal spontaneous combustion and provide references for the development and utilization of chemical inhibitors.

Effects of Water-Soluble Sodium Compounds on the Microstructure and Combustion Performance of Shengli Lignite

Different water-soluble sodium compounds (NaCl, Na2CO3, and NaOH) were used to treat Shengli lignite, and the resulting effects on the microstructure and combustion performance of the coal were investigated. The results showed that Na2CO3 and NaOH had a significant impact on combustion performance of lignite, while NaCl did not. The Na2CO3-treated lignite showed two distinct weight-loss temperature regions, and after NaOH treatment, the main combustion peak of the sample moved to the high temperature. This indicates that both Na2CO3 and NaOH can inhibit the combustion of lignite, with the latter showing a greater effect. The FT-IR/XPS results revealed that Na+ interacted with the oxygen-containing functional groups in lignite to form a "-COONa" structure during the Na2CO3 and NaOH treatments. It is deduced that the inhibitory effect on combustion of lignite may be attributed to the stability of the "-COONa" structure, and the relative amount is directly correlated with the inhibitory effect. The XRD/Raman analysis indicated that the stability of the aromatic structure containing "-COOH" increased with the number of "-COONa" structures formed. Additionally, experiments with carboxyl-containing compounds further demonstrated that the number of oxygen-containing functional groups combined with Na was the main reason for the differences in the combustion performance of treated lignite.

Pore structure and its impact on susceptibility to coal spontaneous combustion based on multiscale and multifractal analysis

The relationship between the properties of coal and its tendency to spontaneous combustion is critical for the environment, safety concerns, and economy. In this study, to eliminate the complex influence of moisture; the samples having similar moisture content were selected from Shanxi and Henan provinces. The chemical properties, physical properties, and tendency of coal samples to spontaneous combustion were characterized based on the conventional analysis, mercury intrusion porosimetry, fractal dimensions, and crossing point temperature (CPT). The results confirmed that the coal rank, volatile matter, oxygen contents, and fixed carbon content had a good linear relationship with the CPT. The relationship between the ash content and CPT presented a “U-shaped” non-linear correlation. For the pore size distribution, the total pore volume also possessed a linear positive correlation with the CPT. The fractal curves could be distinctly divided into two stages: low-pressure (<20 MPa) and high-pressure (>20 MPa), from which the fractal dimensions were obtained using the Sponge and Sierpinski models. The relationship between the fractal dimensions (D_s1, D_s2, and D_g1) and CPT could be divided into two distinct stages: a decrease in the CPT with increasing fractal dimensions (2.6–2.85), and then an in increase in the CPT. CPT decreased with increasing parameters of D₁, D₂, H, and D₁₀, and it gradually increased with increasing D_-10-D₁₀, D_-10-D₀, and D₀-D₁₀. The above characteristics are important to comprehensively and systematically reveal the mechanism of spontaneous combustion.

Effects of alkali-oxygen oxidation temperature on the structures and combustion properties of Shengli lignite

Spontaneous combustion has become a critical limiting factor for the safe and efficient utilization of low rank coals.

A field study of lignite as a drying aid in the superheated steam drying of anaerobically digested sludge

Dried sludge is preferred when the sludge is either to be incinerated or used as a soil amendment. This paper focuses on superheated steam drying which has many benefits, because the system is totally enclosed, thereby minimising odours and particulate emissions. This work reports on field trials at a wastewater treatment plant where anaerobically digested sludge is dried immediately after being dewatered by belt press. The trials showed that unlike previous off-site tests, the sludge could be dried without the addition of a filter aid at a low production rate. However, the trials also confirmed that the addition of the lignite (brown coal) into the anaerobically digested sludge led to a more productive drying process, improved product quality and a greater fraction of the product being in the desired product size range. It is concluded that these results were achieved because the lignite helped to control the granule size in the dryer. Furthermore neither Salmonella spp or E coli were detected in the dried samples. Tests on spontaneous combustion show that this risk is increased in proportion to the amount of lignite used as a drying aid.