Stratification in pipelines with hydrogen into natural gases

Key words:  natural gas pipeline  hydrogen blending  gas cylinder  mixed gas stratification  numerical simulation

Abstract:

Blending hydrogen mixed into the existing natural gas pipelines is an effective way to achieve the large-scale, long-distance and low-cost storage and transportation of hydrogen. However, due to the density difference between hydrogen and natural gases, the addition of hydrogen in the natural gases will cause the non-uniform volume fraction distribution in the pipeline, resulting in the increase of local hydrogen partial pressure and volume fraction in the pipeline, and then leading to the pipe 's failure and leakage. In order to reduce the safety risk of hydrogen doped natural gas pipeline, the computational fluid dynamics (CFD) method was used to establish a hydrogen-natural gas mixture model, to simulate the hydrogen volume fraction changes under the conditions of gas cylinder standing, pipeline being shutdown and pipeline flowing, and then to explore the distribution law of the blended hydrogen-natural gas ' volume fraction. The results show that in the process of static storage, such as gas storage in cylinders and pipeline shutdown, hydrogen gradually floats to the top and methane sinks to the bottom, causing the obvious stratification phenomenon. With the increase of pressure and hydrogen mixing ratio, and also the decrease of temperature and pipe diameter, the hydrogen 's volume fraction gradient in the gravitational direction of the pipeline increases. When the blended hydrogen-natural gas pipeline is running at a very small flow rate, the hydrogen gradually shifts to the pipeline 's top with the flow, resulting in the stratification phenomenon, and it is easier to stratify at the low temperature and the high pressure. The blended hydrogen-natural gas pipeline is suitable in operation under the condition of low pressure and high speed.