Energy efficiency analysis of natural gas hydrates production method

The decomposition of natural gas hydrates is a phase change process, which involves the consumption and conversion of various forms of energy, such as electrical energy, chemical energy, and thermal energy. In order to evaluate the economy capacity of natural gas hydrates exploitation, an exergy model was established to calculate the energy efficiency ratio (EER) of hydrate production method. The CO₂ replacement method is taken as a case study to introduce the calculation equation and flow chart of energy efficiency ratio in any production period. The amount of CO₂ injection, gas production and mole fraction of methane in produced gas are three key parameters in the process of CO₂ replacement. The ratio between the amount of gas production and CO₂ injection is defined as production injection ratio to eliminate the influence of deposit size. This work studied the influence of production injection ratio and the mole fraction of methane in produced gas on EER. The results show that the EER of gas hydrates production by CO₂ replacement is between 0.31 and 6.4 under the set conditions, and it increases with the increase of production injection ratio. In addition, increasing the mole fraction of methane in produced gas can reduce the energy consumption for gas separation and increase EER. Therefore, there are two effective ways to increase EER of CO₂ replacement through controlling the amount of gas production and the mole fraction of methane in produced gas. The EER model is established to provide guidance for the optimization of gas hydrate mining process.