Flue gas condensation heat recovery technology has a good technical and economic performance in industrial exhaust gas waste heat recovery. Thermal efficiency analysis is the traditional analysis method for the flue gas condensation heat recovery system but it cannot reflect the recovered heat degree. Exergy analysis, which can reflect the recovered energy heat degree, was first applied to the evaluation of a flue gas condensation waste heat recovery system in this paper. The calculation method of wet flue gas exergy is more complex as both a heat and mass transfer is presented. Flue gas waste heat exergy efficiency (EE) and the flue gas waste heat exergy utilization rate (EUR) were proposed as the evaluation indexes for exergy analysis. The exergy analysis method was applied to the comparative evaluation of three recovery schemes in a practical project. The results show that when the water vapor content of wet flue gas is less than 10%, the condensed water exergy can be neglected when calculating EE. The EUR could be used as a comprehensive index for comparing different waste heat recovery schemes, and EE could be used to judge whether the energy grade of heat exchange equipment was seriously decreased. Exergy analysis could effectively make up for the deficiency of thermal efficiency analysis that could not reflect the waste heat grade utilization. Exergy analysis and thermal efficiency analysis are recommended to be used simultaneously to make a more comprehensive analysis and evaluation of the system.

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.

Using surrogate model to assist parameter optimization of oil and gas reservoir development can greatly reduce the call times of numerical simulator and accelerate the optimization process. However, for serial simulators or parallel simulators with low speedup ratio, the conventional method is still time-consuming. Firstly, an improved surrogate model assisted particle swarm optimization (PSO) algorithm was proposed in this paper. Then, the performance of the algorithm was analyzed using the Rastrigin function. Finally, the key operation parameters of a gas hydrate reservoir by depressurizationtohotwaterflooding method were optimized with the new method. The results show that the new method only affects the update of the global optimal particle without interfering with the calculation process of the local optimal particles at the early stage of optimization. It realizes the rapid addition of the particle samples through the good parallel features of the PSO algorithm, and therefore, improve the precision of surrogate model in a short time. At the late stage of optimization, it is transformed into a local surrogate model to achieve rapid convergence, when the training time of the surrogate model exceeds the calculation time of the simulator. Both the optimization of Rastrigin function and operation parameters of gas hydrate development reveal that the new algorithm greatly reduces the number of iterations under the same accuracy and thus successfully accelerates the optimization process.

Geothermal resources have become increasingly attractive and promising because of their abundant resource base and environmental protection, especially for hydrothermal resources, which are widely developed for heating in winter. It is indicated that numerical simulation is an important tool for high efficiency geothermal system development. Compared with other methods, numerical simulation is a more comprehensive, scientific, and effective method in the evaluation of recoverable resources and the formulation of development plans. However, there are some problems in the existing software, such as deviation in physical property calculation, incomplete multifields coupling, and poor applicability for low-permeability reservoir and large-scale models. Therefore, based on the optimized multiphysics coupling mathematical model and MPI architecture, a simulator for a multiphysics-coupling geothermal system (SMG) was developed by Sinopec. A case study in the Laoling area, Shandong, China was primarily conducted by using geothermal software SMG, where the effect of well spacing and injection fluid temperature on the production was illustrated. Moreover, a geothermal recoverable resources evaluation method based on numerical simulation is proposed and used for the evaluation of regional geothermal dynamic recoverable resources in Laoling.