Journal of Energy Engineering

Abstract

Due to the significant pollution caused by the current extraction technology for deep coalbed methane (CBM), we propose the application of anhydrous cracking technology—liquid nitrogen cyclic freeze–thaw technology—to crack deep coal seams to achieve the release and extraction of coal bed methane. The self-made system for applying confining pressure is employed to simulate the deep coal seam in coal sample testing. Subsequently, the effect of joint structure with different angles of coal surface and bedding on the liquid nitrogen cyclic freeze–thaw process is evaluated. The investigation study identifies the damage patterns of the coal sample against the sole factor and coupling effect of confining pressure and joint angle. The findings reveal a clear impact of the cyclic freeze–thaw of liquid nitrogen on the expansion of deep coal seams. The application of appropriate inclined joints, combined with confining pressure, significantly minimizes the crushing time of coal samples, with the maximum expansion rate occurring at joints with a bedding angle of $45°\pm 5°$. This study provides vital theoretical support for the green and efficient exploitation of deep CBM.

Practical Applications

The results of this study indicate that the waterless hydraulic fracturing technology—liquid nitrogen cyclic freeze–thaw technology—is highly suitable for coalbed methane extraction in deep, low-permeability coal seams, leading to increased gas recovery. The deeper the coal seam, the more pronounced the effects. Moreover, this technology does not cause environmental pollution. It specifically targets the pores, fractures, and joint structures within the coal, preserving the overall framework of the coal and preventing tunnel instability, thereby enhancing coal mining safety. Additionally, under the influence of liquid nitrogen cyclic freeze–thaw, joints at different angles exhibit varying rates of expansion. Joints extending at a 45° angle to the coal’s bedding planes show the most rapid expansion, indicating that this orientation stores the maximum amount of coalbed methane postliquid nitrogen freeze–thaw. This pattern can be scientifically applied in the actual engineering design of extraction wells. The experimental results can provide a theoretical basis for guiding the on-site application of deep coal seam permeability enhancement and green coalbed methane extraction technology, thereby reducing environmental pollution.

Abstract

High salinity gas reservoirs are prone to salt deposition in the middle and late stages of development, resulting in the decrease of porosity and permeability, which seriously affects the development of gas reservoirs. Therefore, it is necessary to understand the changes of reservoir porosity and permeability parameters in time. At present, the relationship between salt deposition, porosity, and permeability parameters is generally determined according to experimental data, which lacks strict theoretical basis. Therefore, based on the phase equilibrium of hydrocarbon-brine and the equation of state of porosity, a model of porosity and permeability variation considering salt deposition is established. The feasibility of the method is verified by matching the experimental data. The reservoir porosity and permeability variation under different salinity were simulated. Combined with the phase distribution, the reasons for the change of salt deposition and pore permeability parameters were analyzed. The results show that the pore permeability parameters are basically unchanged under high pressure, while the pore permeability parameters change obviously under low pressure due to the decrease of liquid water content. According to the downhole pressure and temperature changes, the reservoir porosity and permeability parameters can be monitored in real time, which provides theoretical guidance for the later gas reservoir development.

Practical Applications

During the development of high salinity reservoirs, as the temperature increases and the pressure decreases, the water gradually evaporates and the salt precipitates. It causes blockage of the flow channel, greatly affects the porosity and permeability, and causes the reservoir difficulty in maintaining normal development. Based on the theory of phase equilibrium and mass conservation of salt, the mass of precipitated salt is calculated, and the porosity and permeability change models are established. The variation of underground porosity and permeability can be determined according to the produced fluid components, so as to achieve the purpose of real-time monitoring of the properties of underground reservoirs. The change of reservoir property parameters can be controlled by controlling the production pressure. It can avoid the downhole pollution in the production process in time and effectively improve the production efficiency. It is very important to deeply understand the changing characteristics of underground reservoir properties for the subsequent formulation of a reasonable working system.

Abstract

The porous structures of the catalyst layer (CL) for proton exchange membrane (PEM) fuel cells were reconstructed using the sphere-based simulated annealing (SA) method, in which different carbon sphere diameters, carbon phase fractions, ionomer loadings, and porosities were achieved to simulate different samples. The pore-scale simulation model based on the random walk algorithm was developed to evaluate the tortuosity in the CL. The results show that the carbon phase fraction, particularly the carbon sphere diameter, significantly influences the tortuosities in both pores and ionomer. Larger carbon spheres contribute to tightening the solid phase and enlarging the pore size within the CL, thereby affecting the pore structure and the ionomer distribution. The tortuosity obtained from the random walk algorithm aligns well with those from pore-scale diffusion and flow simulations using the Lattice–Boltzmann method (LBM). The random walk algorithm has the highest efficiency of the three methods in calculating the tortuosity of the CL when the porosity is less than 0.2, demonstrating its wider applicability. This study further proposes formulas for predicting the tortuosities in both pores and ionomer, which are closer to actual CLs than empirical equations. In addition, the present work confirms that the structural randomness resulting from the reconstruction method used in this work has little effect on the tortuosity, indicating that a small number of reconstructed samples suffice for investigating transport processes in the CL. Overall, our research contributes to the prediction of the properties of real porous media in energy engineering.

Practical Applications

This manuscript reports on the reconstruction of the catalyst layer in proton exchange membrane fuel cells using the simulated annealing method under some appropriate assumptions. A pore-scale simulation using a random walk algorithm was performed to compute the tortuosity both in the pores and in the ionomer, which is compared with pore-scale diffusion and flow simulations using the Lattice–Boltzmann method. A parametric study for several geometric parameters was carried out. Some conclusions are drawn from the results of the pore-scale simulations, and several formulas are proposed accordingly. This study helps to predict the tortuosity in the porous structure closer to the actual porous medium than the empirical equations. Meanwhile, the proposed tortuosity formulas improve the accuracy of the macroscopic models compared to using the assumed transmission parameter values. In addition, structural randomness analysis is explored, which will further reduce the workload in actual engineering calculations and improve computational efficiency.

Density is an important indicator for evaluating diesel fuel quality that directly affects the injection timing and injection rule of the engine, and also has a significant impact on the spray broken particle size, the spray penetration distance, the spray cone angle, and so forth, which in turn affects the combustion process and pollutant emission of the compression-ignition engine. Therefore, it is important to accurately predict the diesel fuel mixture density in industrial and compression-ignition engines. However, the mathematical models for predicting the density of diesel fuel mixture with changed temperature are relatively lacking and less accurate, especially for ternary diesel fuel mixtures with different physicochemical properties. This paper proposes a mathematical model including binary and ternary diesel mixtures under changed fuel volume fraction and temperature, and published data were used for verification. The data verification results show that: for the density prediction of binary diesel fuel mixtures at constant temperature, the average relative deviation (ARD) is 0.0245%, the RMS error (RMSE) is 0.000344, and the correlation coefficient ($R$) is 0.9993. For the density prediction of binary diesel fuel mixtures at changed temperature, the ARD is 0.0609%, the RMSE is 0.000695, and $R$ is 0.9980. For the density prediction of ternary diesel fuel mixtures at constant temperature, the ARD is lower than 0.0571%, the RMSE is lower than 0.000610, and $R$ is higher than 0.9861. For the density prediction of ternary diesel fuel mixtures at changed temperature, the ARD is 0.0484%, the RMSE is 0.000513, and $R$ is 0.9996. The diesel mixed fuel density prediction model proposed in this paper has good accuracy and calculation convenience, and provides important reference value for measuring or designing the density of diesel mixed fuel in the field of compression-ignition engines.

Lithium iron phosphate (LFP) batteries are widely utilized in energy storage systems due to their numerous advantages. However, their further development is impeded by the issue of thermal runaway. This paper offers a comparative analysis of gas generation in thermal runaway incidents resulting from two abuse scenarios: thermal abuse and electrical abuse. The study initially focuses on 13-Ah lithium iron phosphate single-cell batteries. Experiments were conducted to induce thermal runaway through both forms of abuse, analyzing the production and dispersion of ${\mathrm{H}}_2$ and CO gases in each case. It was observed that thermal abuse–induced thermal runaway resulted in higher gas concentrations and more pronounced fluctuations, whereas electrical abuse–induced thermal runaway exhibited lower gas concentrations and lower fluctuations. Subsequently, key materials and temperature variations at the positive and negative electrodes were investigated under both types of thermal runaway, revealing distinct differences that are identified as the primary reasons for the significant disparities in ${\mathrm{H}}_2$ and CO gas generation during the two thermal runaway conditions. The conclusions drawn in this paper advance the understanding of the mechanisms underlying ${\mathrm{H}}_2$ and CO gas concentration generation in thermal runaway scenarios.