Decompression and pressure regulation of pure hydrogen and hydrogen-enriched compressed natural gas pipelines

[Objective] Pressure regulation systems, serving as one of the vital links between long-distance pipelines and urban gas pipeline networks, play a significant role in the successful implementation of the “Hydrogen Society” Program. However, the contrasting physical properties of hydrogen and natural gas impose challenges to the effectiveness of process control in pressure regulation. [Methods] This study employed a combined approach involving decompression and pressure regulation experiments conducted on pure hydrogen and hydrogen-enriched compressed natural gas(HCNG), along with dynamic simulations of pressure regulation. By establishing a foundation for assessing the pressure stabilization effect of the decompression and pressure regulation system, considering factors such as pressure stabilization accuracy, response time, and fitness function, a sensitivity analysis was performed to investigate the influence of various parameters, including hydrogen blending ratios, flow fluctuation cycles, downstream flow change ranges, pipeline transmission pressures, as well as proportional, integral, and derivative parameters(PID). [Results] The following results were obtained:(1) With more frequent disturbances, gas flow rates increased, leading to larger system fluctuation ranges. Consequently, achieving pressure stabilization became more challenging for the decompression and pressure regulation system. This finding highlighted the importance of restraining the movement of pure hydrogen and HCNG at high flow rates in transmission pipelines.(2) The flow rate of the pressure regulation system carried sinusoidal fluctuations. Setting the pressure stabilization accuracy at ±1.5% and the proportional and integral parameters within the range of 1 to 2, the decompression and pressure regulation experiments showed that the decompression and pressure regulation of pure hydrogen and HCNG could be substantially achieved within the required pressure range of urban gas pipelines.(3) Pure hydrogen exhibited greater instantaneous fluctuations compared to natural gas when flowing through pipelines at identical rates of transmission.As a consequence, the response time and adaptability function of the control system gradually increased with higher hydrogen blending ratios. Moreover, the instantaneous pressure fluctuation of pure hydrogen reached 1.15 times that of pure methane, resulting in a response time and adaptability function of the control system that increased to 1.13 and 2.68 times those of pure methane, respectively. Under the same proportional and integral parameters, it was more challenging to maintain the pressure stabilization for the hydrogen-containing gas. Hence, decreasing the integral parameter or increasing the proportional parameter proportionally to the hydrogen blending ratio is recommended, to attain the same or even better pressure stabilization effect as observed in natural gas scenarios. [Conclusion] The research findings provide theoretical insights to guide the pressure regulation practices of pipelines carrying pure hydrogen and HCNG.