天然气水合物置换开采的能源效率研究

doi:10.11949/0438-1157.20200360

摘要/Abstract

摘要：

天然气水合物分解是一个相变过程，开采时涉及各种形式能量的消耗和转化，如电能、化学能、热能等。为了科学地评价天然气水合物开采技术的经济性，建立了以有效能（）为核心的能源效率计算方程，并以CO₂置换法开采天然气水合物为例，介绍任意生产周期内能源效率的计算方法和流程框图。在CO₂置换开采天然气水合物的工艺过程中，注气量、产气量和产气中甲烷含量是三个关键参数，将产气量与注气量之比定义为采注比，分析采注比及产气中甲烷含量对能源效率的影响。结果表明：在设定条件下CO₂置换开采天然气水合物的整体能源效率介于0.31～6.4之间；增大采注比，有利于提高能源效率；产气中甲烷的摩尔分率越高，气体分离的能耗越低，能源效率也可显著提高。因此，调控产气量和产气中甲烷摩尔分率是提高CO₂置换法能源效率的主要途径。通过所建立的能效计算方程为天然气水合物开采工艺的优化提供指导。

关键词: 水合物, 生产, 能源效率, 模型, CO₂置换

Abstract:

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.

Key words: hydrate, production, energy efficiency, exergy, model, CO₂ replacement

中图分类号:

TE 53

王晓辉,许强,郑华星,孙长宇,陈光进. 天然气水合物置换开采的能源效率研究[J]. 化工学报, 2020, 71(12): 5754-5762.

WANG Xiaohui,XU Qiang,ZHENG Huaxing,SUN Changyu,CHEN Guangjin. Energy efficiency analysis of natural gas hydrates production method[J]. CIESC Journal, 2020, 71(12): 5754-5762.

图/表 10

表1 大气中气体基准物的组成

Table 1 The standard compositions of air

组分	组成/%
N₂	75.60
O₂	20.34
H₂O	3.12
CO₂	0.03
Ar	0.91
Ne	0.0018
He	0.00052

图1 CO2置换法开采天然气水合物的全流程示意图（修订自Teng等[27]）

Fig.1 The schematic diagram of natural gas hydrates exploitation by CO2 replacement (revised from Teng et al.[27])

图2 二氧化碳船舶运输成本与输送距离的关系

Fig.2 Relationship between shipping cost of CO2 and distance

图3 分离能耗与产气中甲烷含量的关系

Fig.3 The relationship between the energy consumed for gas separation and CH4 mole fraction

图4 CO2置换开采天然气水合物的能源效率计算框图

Fig.4 The flow chart of energy efficiency ratio for the production of natural gas hydrates by CO2 replacement

表2 不同条件下CO2的焓和熵值

Table 2 The enthalpy and entropy of CO2 under different conditions

条件	焓，H/(kJ/mol)	熵, S/(J/(mol·K))
基态(0℃, 0.1 MPa)	8.8	44
运输条件(-20℃, 2 MPa)	6.797	36.653
注入条件(20℃, 15 MPa)	10.406	47.64
回注条件(20℃, 0.1 MPa)	22.076	119.93

图5 CO2置换开采天然气水合物的能源效率与采注比的关系

Fig.5 The relationship between energy efficiency ratio and production injection ratio for the production of natural gas hydrates by CO2 replacement

图6 CO2置换开采天然气水合物过程中各工艺能耗与采注比的关系

Fig.6 The relationship between energy consumption and production injection ratio for the production of natural gas hydrates by CO2 replacement

图7 CO2置换开采天然气水合物的能源效率与产气中甲烷含量的关系

Fig.7 The relationship between energy efficiency ratio and CH4 mole fraction in produced gas for the production of natural gas hydrates by CO2 replacement

图8 CO2置换开采天然气水合物过程中各工艺能耗与产气中甲烷含量的关系

Fig.8 The relationship between energy consumption and CH4 mole fraction in produced gas for the production of natural gas hydrates by CO2 replacement

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