Pipeline sharing: Remaining capacity evaluation for liquid ammonia transportation in multi-product pipelines under variable flow rate

As an effective carrier for hydrogen storage and transportation, ammonia energy emerges as a crucial solution to address the spatial imbalance between renewable energy supply and demand, driving the anticipated large-scale, long-distance transportation needs for liquid ammonia. While pipelines represent an economically viable and safe mode for continuous transportation of liquid energy, current multi-product pipelines predominantly operate at low capacity, leading to severe infrastructure underutilization. This study proposes utilizing pipeline remaining capacity for transporting liquid ammonia. We develop a spatiotemporal-capable evaluation methodology applicable to pipelines with sequential transportation. However, ammonia presents challenges related to gasification and electrostatic safety during pipeline transport, particularly concerning flow rates. Our methodology addresses these challenges by incorporating variable flow rate requirements to accommodate operational boundary disparities between liquid ammonia and oil products, thereby ensuring pipeline safety. A hybrid temporal representation method is innovatively proposed, embedding continuous time windows within discrete temporal frameworks to accurately differentiate ammonia batches from oil batches in the model. This approach preserves computational efficiency while enabling precise scheduling and safety velocity constraints for ammonia batches. The evaluation model also integrates two contract-guided parameters to facilitate scenario-based exploration of remaining capacity utilization. Through controlled variable analysis based on the case study of China's Lanzhou-Changsha Pipeline, we conduct horizontal comparisons of ammonia transportation volumes in different scenarios and vertical investigations into influencing mechanisms of remaining capacity. The results indicate that: i) significant utilization variations arise from batch positioning, which is governed by the temporal-spatial matching between injection timing and pipeline idle windows; ii) an increased number of batches enhances operational flexibility, boosting capacity utilization by 47.40 % in our case; and iii) ammonia transportation volume demonstrates a positive correlation with operational tolerance and an inverse correlation with contractual stringency, reaching scenario-specific trade-off points (5 % and 5 × in our case). This research provides theoretical guidance for the development of "ammonia-hydrogen" energy systems and liquid ammonia transportation via multi-product pipelines, offering practical insights to advance energy transition.