Decarbonisation Technology - May 2023 Issue

Feedstocks and utilities for green hydrogen and e-fuels High purity water, carbon dioxide and nitrogen are essential utilities for green hydrogen generation using electrolysis

Stephen B Harrison sbh4 Consulting

F or several years, attention has focused on green hydrogen as a clean energy vector. When produced on electrolysers using renewable electrical power generated by wind, solar, or hydro schemes, green hydrogen has a very low carbon footprint. Ammonia is derived from hydrogen through reaction with nitrogen, sourced from air, in the Haber-Bosch process. Many of the largest green hydrogen schemes proposed worldwide will convert green hydrogen to green ammonia for cost-effective shipping to international markets. Conversion of hydrogen to ammonia adds cost at the production location but means that ammonia, rather than hydrogen, can be shipped to the end-user destination. Liquid hydrocarbon fuels are incredibly useful energy vectors due to their high energy density and ease of handling. As such, gasoline, diesel, aviation kerosene, and heavy fuel oil have become the fuels of choice for cars, trucks, planes, and shipping. The challenge of the energy transition and decarbonisation is to substitute these refined products with sustainable, convenient, and cost- effective alternatives. Synthetic aviation fuel (SAF) is one such solution. SAF is a broad term meaning the fuel has been derived from non-fossil origins. The largest source of SAF today is biofuel, and more than 300,000 commercial flights operated by more than 40 airlines have used pure SAF or blends with fossil kerosene over the past five years. Thirteen major airports can refuel aircraft with SAF or SAF/kerosene blends. Alternatively, SAF can be produced using renewable electrical power to make green

hydrogen or syngas for conversion to e-fuels through power to liquid (PtL) technology. In this pathway, carbon dioxide (CO 2 ) is required as the source of carbon to build the hydrocarbon molecules. E-methanol burns with almost no emissions of particulates or sulphur dioxide. Methanol, like diesel and heavy fuel oil, does produce CO 2 emissions during combustion. However, since e-methanol is made from CO 2 captured from stack emissions or the air, its use is carbon neutral. Whether the fuel is green hydrogen, green ammonia, e-methanol, or SAF, certification to identify the CO 2 intensity of the production process will be required as a guarantee of origin. In many markets, there are clear requirements emerging that the definitions of renewable fuels must move beyond simple ‘grey’ or ‘green’ labels to a more scientifically valid and environmentally robust classification system. Certification from an independent party to validate the product claims will inevitably be required. Water, air, nitrogen, and CO 2 are the fundamental feedstocks to the above reaction pathways. Water and nitrogen also play key roles as utilities to enable safe and efficient operations (see Figure 1 ). Crystal clear water: natural hydrogen carrier Pure water supply to an electrolyser is essential. Electrolysis splits water molecules into oxygen and hydrogen. Supply of pure water to the electrolyser must be guaranteed. Failure to supply water means the electrolyser scheme must shut down. For a proton exchange