The energy transition is well underway with hydrogen projects expected to play an important role. io’s Hydrogen Subject Matter Expert, Mary Snowdon explains the considerations to be made when assessing the design of a hydrogen project.
io’s approach to projects is to start with the end in mind, which aligns perfectly with hydrogen projects as the design of the project is largely driven by the offtaker. The hydrogen offtaker dictates the supply profile, level of purification, method of transportation and export specifications. The availability of resources such as power, water or gas determines the production method and colour of hydrogen produced, refer to io’s previous insight to learn more about the colours of hydrogen [https://www.ioconsulting.com/post/what-colour-is-your-hydrogen].
Blue hydrogen projects performed by io found that the conventional natural gas profiles from oil projects, ramp up quickly in the early years and then decline causing a peaked profile that does not align with the stable demand required for most hydrogen offtakers. Modifications to the gas supply profile to produce a more stable profile, buffer storage or a secondary means of hydrogen production, such as an electrolyser, may be required to flatten out the profile and provide a stable supply rate to an offtaker. This is also applicable to green hydrogen projects that are powered directly by renewable sources which are intermittent in nature and require buffer storage to flatten out the production curve, or a secondary power or hydrogen source to ensure security of supply in periods of unavailability.
Three commercially proven technologies are available for blue hydrogen production; Steam Methane Reforming (SMR), Partial Oxidation (PO) and Autothermal Reforming (ATR). Of the three technologies SMR is the most common. The hydrogen yield is similar across all three technologies, however both PO and ATR require oxygen for reforming which requires additional energy for generation. The high operational pressures of the ATR process are beneficial to the carbon capture process enabling a higher yield of CO2 to be recovered compared to SMR. In the SMR process, CO2 can be recovered from the flue gas of the reformer; however, the process is complicated due to the low pressure and high nitrogen concentration. Thermal PO technologies do not use catalysts and are therefore beneficial with feedstocks with high H2S as less pre-treatment is required for the feedstock.
Where a renewable power source is available green hydrogen can be produced using electrolysis. Two commercially proven electrolyser technologies are currently available; Alkaline and Polymer Electrolyte Membranes / Proton Exchange Membrane (PEM) electrolysers. Of the two technologies, alkaline electrolysis is the most mature, however the potential for future cost reduction is higher for PEM electrolysers as there are more opportunities for technical developments. Atmospheric and pressurised solutions are available for both technologies, with pressurised solutions typically discharging the hydrogen at a pressure of 20-30 bara for both electrolyser technologies.
When directly connected to a renewable source, the electrolyser selected must be able to respond to the variable supply of electricity. Power Purchase Agreements (PPAs) where the hydrogen production plant owner purchases a portion of the green electricity from a grid are beneficial as they provide a constant supply of electricity to the plant. However, the electricity demand must be accurately determined, factoring in electrolyser degradation, to ensure that there is sufficient green electricity supplied to the grid to meet the annual demand of the hydrogen production plant. In addition to electricity, electrolysis also requires large volumes of water which must be accounted for in the site selection for a hydrogen production plant.
An agnostic approach to the electrolyser selection in the early phases of a green hydrogen project is recommended as small nuances in operational specifications between electrolyser vendors may significantly impact the balance of plant design. Differences in stack cooling temperatures between vendors in a recent project resulted in the selection of different cooling medium due to the ambient conditions of the project which impacted the CAPEX and OPEX of the project. Therefore, the opportunity should be taken to assess the differences enabling the most appropriate selection to be made.
For the balance of plant design, large, centralised designs are recommended over smaller systems allocated to electrolyser modules as the larger systems typically have lower CAPEX and OPEX, due to the reduction in equipment which reduces the number of inspections, maintenance requirements and reduces the connections for piping, electrical and instrumentation. The modularised design of electrolysers allows them to be scaled up to match future demand. Future requirements should be factored into early design to assess whether there is value in the inclusion in the balance of plant.
If storage is required there are several well-established methods available.
Gaseous hydrogen can be stored in cylinders for smaller volumes or in pipelines using line packing, storage spheres or salt caverns for larger volumes.
Liquid hydrogen can also be stored in cylinders for smaller volumes or liquefied and stored in cryogenic storage tanks for larger volumes.
The method of storing hydrogen is selected based on the storage volume and expected duration of storage both of which are calculated factoring in the offtaker demand, availability, and the feedstock supply.
After production, the hydrogen is purified and compressed to the required specifications and then exported. Hydrogen can be exported by road, rail, pipeline, or ship, or it can be converted into a different vector such as ammonia. Gaseous hydrogen is usually transported by pipeline or trucks and requires compression prior to export. If most of the hydrogen is supplied directly to an industrial process, it is recommended that the production plant be located as close as possible to the offtaker to reduce the export costs, making green hydrogen more cost competitive with grey and blue hydrogen. Liquid export is typically more economical than gaseous export for longer distances as a larger mass can be transported, however, the hydrogen must be liquefied prior to export which requires an energy intensive process and boil-off gas must be factored into the design of the system. The method of transportation for hydrogen export is selected based on the quantity, distance, and local infrastructure.
Oxygen, a by-product from Green Hydrogen production
Oxygen is also produced from electrolysis. The recovery of oxygen and potential oxygen offtakers should be investigated when scoping out a green hydrogen development as results from previous green hydrogen projects by io determined that this can potentially improve the economics.
Energy projects by their nature are interdependent complex systems that encompass technical, economic and strategic elements. The outputs of these systems are often dominated by emergent properties, that is the overarching project has behaviours which emerge only when the parts interact as a single holistic system.
io is a project architect and systems integrator created with this in mind. The application of a systems approach in the early phases of a hydrogen project ensures that all the focus decisions which impact the design can be techno-economically assessed and optimised. This enables the concept selection to be aligned with the project value drivers and to maximise project value.