Zenghua Li

Free radical reaction characteristics of coal low-temperature oxidation and its inhibition method.

Study on the mechanism of coal spontaneous combustion is significant for controlling fire disasters due to coal spontaneous combustion. The free radical reactions can explain the chemical process of coal at low-temperature oxidation. Electron spin resonance (ESR) spectroscopy was used to measure the change rules of the different sorts and different granularity of coal directly; ESR spectroscopy chart of free radicals following the changes of temperatures was compared by the coal samples applying air and blowing nitrogen, original coal samples, dry coal samples, and demineralized coal samples. The fragmentation process was the key factor of producing and initiating free radical reactions. Oxygen, moisture, and mineral accelerated the free radical reactions. Combination of the free radical reaction mechanism, the mechanical fragmentation leaded to the elevated CO concentration, fracturing of coal pillar was more prone to spontaneous combustion, and spontaneous combustion in goaf accounted for a large proportion of the fire in the mine were explained. The method of added diphenylamine can inhibit the self-oxidation of coal effectively, the action mechanism of diphenylamine was analyzed by free radical chain reaction, and this research can offer new method for the development of new flame retardant.

Research on the Composition and Distribution of Organic Sulfur in Coal.

The structure and distribution of organic sulfur in coals of different rank and different sulfur content were studied by combining mild organic solvent extraction with XPS technology. The XPS results have shown that the distribution of organic sulfur in coal is related to the degree of metamorphism of coal. Namely, thiophenic sulfur content is reduced with decreasing metamorphic degree; sulfonic acid content rises with decreasing metamorphic degree; the contents of sulfate sulfur, sulfoxide and sulfone are rarely related with metamorphic degree. The solvent extraction and GC/MS test results have also shown that the composition and structure of free and soluble organic sulfur small molecules in coal is closely related to the metamorphic degree of coal. The free organic sulfur small molecules in coal of low metamorphic degree are mainly composed of aliphatic sulfides, while those in coal of medium and high metamorphic degree are mainly composed of thiophenes. Besides, the degree of aromatization of organic sulfur small molecules rises with increasing degree of coalification.

A review on the mechanism, risk evaluation, and prevention of coal spontaneous combustion in China

In recent years, the ecology, security, and sustainable development of modern mines have become the theme of coal mine development worldwide. However, spontaneous combustion of coal under conditions of oxygen supply and automatic exothermic heating during coal mining lead to coalfield fires. Coal spontaneous combustion (CSC) causes huge economic losses and casualties, with the toxic and harmful gases produced during coal combustion not only polluting the working environment, but also causing great damage to the ecological environment. China is the world’s largest coal producer and consumer; however, coal production in Chinese mines is seriously threatened by the CSC risk. Because deep underground mining methods are commonly adopted in Chinese coal mines, coupling disasters are frequent in these mines with the coalfield fires becoming increasingly serious. Therefore, in this study, we analyzed the development mechanism of CSC. The CSC risk assessment was performed from the aspects of prediction, detection, and determination of the “dangerous area” in a coal mine (i.e., the area most susceptible to fire hazards). A new geophysical method for CSC determination is proposed and analyzed. Furthermore, the main methods for CSC fire prevention and control and their advantages and disadvantages are analyzed. To eventually construct CSC prevention and control integration system, future developmental direction of CSC was given from five aspects. Our results can present a reference for the development of CSC fire prevention and control technology and promote the protection of ecological environment in China.

Evolution of Coal Permeability with Cleat Deformation and Variable Klinkenberg Effect

The characteristics of the gas flow in reservoir have a great impact on exploiting coalbed methane (CBM), so many researchers have carried out the experiments to test the coal sample permeability in the laboratory. The Klinkenberg effect is an important factor in the apparent permeability which is obtained in the laboratory, and it also be recognized as a constant value for a specific gas. From the principle of the Klinkenberg effect, the Klinkenberg coefficient is closely related to the width of the gas flowing path. The coal cleat width changes because of the compressibility and sorption-induced strain features. Therefore, the Klinkenberg coefficient can not be treated as a constant. By using the cubic conceptual model of coal, the deformation behaviors of the coal matrix and fracture are analyzed in this paper, and the influential factors of the Klinkenberg coefficient are obtained. The theoretical equation of methane’s Klinkenberg coefficient was also established. The evolution equation of the cleat width is derived by coupling the effective stress and gas sorption, and the Klinkenberg coefficient model is also rewritten. Using the parameters of the coal sample, some results are obtained. The Klinkenberg coefficient increases with the increase in the pore pressure because of the sorption-induced strain at the constant effective stress; The Klinkenberg coefficient varies with the increase in the pore pressure because of the competition between the stress–strain and sorption-induced strain at the constant mean stress; The Klinkenberg coefficient increases with the increase in the mean stress at a constant pore pressure. The results improve the understanding of the Klinkenberg effect for the gas flow in a coalbed and provide theoretical guidance for CBM exploitation.

Studies on the Low-Temp Oxidation of Coal Containing Organic Sulfur and the Corresponding Model Compounds

This paper selects two typical compounds containing organic sulfur as model compounds. Then, by analyzing the chromatograms of gaseous low-temp oxidation products and GC/MS of the extractable matter of the oxidation residue, we summarizing the mechanism of low-temp sulfur model compound oxidation. The results show that between 30 °C to 80 °C, the interaction between diphenyl sulfide and oxygen is mainly one of physical adsorption. After 80 °C, chemical adsorption and chemical reactions begin. The main reaction mechanism in the low-temp oxidation of the model compound diphenyl sulfide is diphenyl sulfide generates diphenyl sulfoxide, and then this sulfoxide is further oxidized to diphenyl sulphone. A small amount of free radicals is generated in the process. The model compound cysteine behaves differently from diphenyl sulfide. The main reaction low-temp oxidation mechanism involves the thiol being oxidized into a disulphide and finally evolving to sulfonic acid, along with SO2 being released at 130 °C and also a small amount of free radicals. We also conducted an experiment on coal from Xingcheng using X-ray photoelectron spectroscopy (XPS). The results show that the major forms of organic sulfur in the original coal sample are thiophene and sulfone. Therefore, it can be inferred that there is none or little mercaptan and thiophenol in the original coal. After low-temp oxidation, the form of organic sulfur changes. The sulfide sulfur is oxidized to the sulfoxide, and then the sulfoxide is further oxidized to a sulfone, and these steps can be easily carried out under experimental conditions. What’s more, the results illustrate that oxidation promotes sulfur element enrichment on the surface of coal.

The effect of high temperature environment on rock properties—an example of electromagnetic radiation characterization

High temperature causes thermal damage to rock; the macrofracture and microfracture of rock can be produced under the action of temperature treatment. Under the influence of high temperature, the surrounding rock of deep underground engineering will suffer instability damage and cause serious harm to the people. In order to use the electromagnetic radiation (EMR) technology (a non-contact geophysical method) for evaluating the thermal stability of rock in underground thermal engineering applications, we established the EMR testing experimental system of rock under the action of a continuous heat source. The variation of EMR signals of rock under different temperatures was tested, and the EMR signals generates during the process of rock thermal deformation and thermal fracture, which were later analyzed. Under the action of a continuous heat source, the rock materials produced EMR signals with three kinds of frequencies. With the increase of rock temperature, the variation trends of EMR signals varied from the slow growth rate to the rapid growth rate, EMR signals can be divided into five stages. The increase of EMR signals is positively correlated with temperature, the Hurst exponent was higher than 0.7. The thermal stress was responsible for thermal deformation and fracture, thus generating the EMR signals. The research results have important guiding significance for the application of EMR technology to the evaluation of rock thermal stability.

Improved Porosity and Permeability Models with Coal Matrix Block Deformation Effect

Coal permeability is an important parameter in coalbed methane (CBM) exploration and greenhouse gas storage. A reasonable theoretical permeability model is helpful for analysing the influential factors of gas flowing in a coalbed. As an unconventional reservoir, the unique feature of a coal structure deformation determines the state of gas seepage. The matrix block and fracture change at the same time due to changes in the effective stress and adsorption; the porosity and permeability also change. Thus, the matrix block deformation must be ignored in the theoretical model. Based on the cubic model, we analysed the characteristics of matrix block deformation and fracture deformation. The new models were developed with the change in matrix block width a. We compared the new models with other models, such as the Palmer–Manson (P–M) model and the Shi–Durucan (S–D) model, and used a constant confining stress. By matching the experimental data, our model matches quite well and accurately predicts the evolution of permeability. The sorption-induced strain coefficient f differs between the strongly adsorbing gases and weakly adsorbing gases because the matrix block deformation is more sensitive for the weakly adsorbing gases and the coefficient f is larger. The cubic relationship between porosity and permeability overlooks the importance of the matrix block deformation. In our model, the matrix block deformation suppresses the permeability ratio growth. With a constant confining stress, the weight of the matrix block deformation for the strongly adsorbing gases is larger than that for weakly adsorbing gases. The weight values increase as the pore pressure increases. It can be concluded that the matrix block deformation is an important phenomenon for researching coal permeability and can be crucial for the prediction of CBM production due to the change in permeability.

Influence of the Pore Geometry Structure on the Evolution of Gas Permeability

In order to investigate the effect of pore geometry structure on the gas permeability, 3 permeability models with different pore shapes were constructed, considering the sorption-induced deformation, adsorption molecular layer and variable Klinkenberg’s effect. The effect of pore geometry structure on the effective pore radius, Klinkenberg’s factor and permeability was analyzed under 3 different conditions, including constant effective stress conditions, constant pore pressure conditions and constant mean stress conditions. Results showed that, under constant effective stress conditions, the spherical pores show a greater effect on the effective radius, followed by the cylindrical pores and the slit pores. Under the constant pore pressure conditions and the constant mean stress conditions, the effective pore radius is more sensitive to the slit pores, followed by the cylindrical pores and the spherical pores. Under 3 different conditions, Klinkenberg’s factor is more sensitive to the slit pores, followed by the cylindrical pores and the spherical pores. Moreover, the permeability evolution with different pore geometry structures shows similar characteristics with effective pore radius, indicating that the effective radius dominates the permeability difference in different pore geometry structures. Furthermore, a numerical model was proposed to investigate the permeability evolution in the reservoir conditions. In the initial extraction stage, the permeability is increased with the rising slit pores. However, in the later stage, the spherical pores show more notable improvement effect on the permeability. Then, the effective pore radius and Klinkenberg’s factor were analyzed to reveal the influence mechanism of different pore geometry structures, indicating that the effective pore radius dominates the permeability difference, while Klinkenberg’s effect plays more significant role for the permeability trends and shows notable improvement effect on the permeability.

SOM’s Effect on Coal Spontaneous Combustion and Its Inhibition Efficiency

ABSTRACT As an important part of coal, the existence of soluble organic matters (SOMs) has its own impact on the number of active groups within coal and its pore and fracture structure, which have influence on both spontaneous combustion and inhibitory effect of coal. But some in-depth researches are still lacking on this matter. On the basis of testing the composition of SOMs and analyzing the change of coal’s pore and fracture structure, we conducted an oxidation experiment on model compounds stimulating SOMs, and coal’s spontaneous combustion inhibition experiment before and after extraction. By testing characterization parameters like oxygen consumption rate, activation energy, inhibition rate, and the crossing point temperature, we analyzed how SOMs make a difference on spontaneous combustion and inhibitory effect of coal. It shows that, after the extraction of SOMs, both the amount and rate of oxygen consumption of coal goes down, while the activation energy of coal ascends after extraction. Once being added with model compounds stimulating SOMs, coal’s oxidation degree increases. Between 50°C and 140°C, the inhibition rates of ZHCM, ZHYM, and CTM range from 50.7% to 71.2%, 40.1% to 52.0%, and 18.1% to 27.6%, respectively, and residual coals’ inhibition rates surpass raw coals’ inhibition rates. The crossing point temperature of ZHCM reached 122.5°C, 35.2°C higher compared to that of YM. This shows that adding inhibitors after extract can have a strong effect on restraining coal’s spontaneous combustion. Through free radical reaction and oxidation reaction, SOMs would release the heat to hasten the coal’s oxidation process. After extraction, coal’s permeability increases, which makes it easier for an inhibitor to seep into coal, thereby restraining the intensity of coal’s oxidation.

Impacts of long-term water inrush on characteristics of coal gas adsorption and seepage

Frequent coal mine flooding is bound to impact not only the stope stress effects of working face, but also the absorption and seepage characteristics of coal mass. In this paper, we examined the effects of long-term water inrush on coal pore structure as well as gas adsorption and seepage characteristics using water-flooded Taoyuan Coal Mine, Anhui Province, China, as an example. Results showed that: 1) after water inrush, the volume, area, and porosity of coal samples, as well as the proportion of micro-pores decreased significantly with the distance from the water inrush site increasing; 2) the amount of gas adsorption of coal samples near the water inrush site was higher than that away from the site; 3) within 13 m from the site, the seepage capacity of coal gas increased; 4) the water seeped into the coal dissolves a mass of inorganic minerals and organic matter and carries them away. [Received: December 26, 2014; Accepted: June 13, 2015]