Hydrogen & Carbon Dryness Management

This Network Innovation Allowance (NIA) project investigated dew point management in hydrogen/natural gas blends, pure hydrogen and carbon dioxide transmission pipelines. In the National Transmission System (NTS), which is currently a natural gas network, the purity of the gas is carefully controlled via the network entry specification. Trace components, such as water, nitrogen oxides, sulphur containing compounds, oxygen and carbon dioxide have strict limits on their allowable levels in the network. This is done in part to ensure the gas delivered to end users meets the requirements of the customer, but also to protect transport and storage systems. Purity specifications are being developed for hydrogen, its blends with natural gas, and for carbon dioxide (CO₂). This project focused specifically on the water content within these gases, in what concentrations it is likely to be acceptable, the conditions at which it may condense in the network, its interactions with other trace components and contaminants and the potential detrimental effect on the network.

Limiting moisture content and ensuring gas dryness is important for several reasons:

• Safety & Efficiency: Hydrogen’s efficiency as a fuel can be compromised by moisture. Water in hydrogen can affect the combustion process, leading to a reduced efficiency for applications like gas turbines.
• Corrosion: If dew points aren’t controlled effectively, liquid can drop out of the gas phase, and this moisture can cause corrosion in pipelines and hydrogen embrittlement. For CO₂ pipelines this moisture can react to produce carbonic acid which can further corrode the pipelines.

The outcomes of the project should provide a clearer insight and strategy on how to effectively manage hydrogen and carbon dryness within the NTS, ensuring that the gas remains within the required specifications for current and future demands.

The project was split into three work packages (WP):

1. WP1 focused on hydrogen and its blends, initially reviewing the equations of state (EoS) that model the dew point temperature at varying water content and hydrogen/methane blend ratios. The impact on the network of liquid water formation in hydrogen was examined, including the interaction with other trace components such as CO₂ and H₂S, in particular the effect on welds and pipeline defects. Finally, a summary of international standards for hydrogen purity highlighted the likely water content limits that could be expected by hydrogen users and thus provided by producers.

2. WP2 focused on CO₂, its phase behaviour and the effect impurities have on this behaviour using the most appropriate equations of state. The detrimental effect of CO₂ and liquid water contained within it on pipelines, fittings and other parts of the network was reviewed.

3. WP3 focused on how the water content specifications could be managed on the network, from the point of view of monitoring and controlling water dew point in the gases. The water content expected from various production techniques were reviewed and a high-level costing for the dehydration process for both CO₂ and hydrogen was made.