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
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For 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 (CO2) 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 CO2 emissions during combustion. However, since e-methanol is made from CO2 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 CO2 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 CO2 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 membrane (PEM) system, that will probably not be a major issue, but for an alkaline system an unplanned shutdown may result in corrosion of the electrodes and a reduction in electrolysis efficiency during future operation (see Figure 2).
The capital and operating costs of pure water supply are low, but the costs of failure are high: reliability is key. Water supply for a typical green hydrogen scheme will generally be only 1 or 2% of the total operating cost. Impurities such as calcium ions in the water will rapidly damage a PEM electrolyser membrane due to the interaction with the catalyst coating. Alkaline electrolysers also have sensitivities to poisons in the water. The consequences of an impure water supply are unacceptable.
The main quality parameter associated with the pure water supply to an electrolyser is conductivity. As a rule of thumb, a conductivity of less than 2 μS/cm (0.2 mS/m) should be the target. Ions, such as calcium or sodium, that are dissolved in the water will increase conductivity. So, measurement of this parameter will confirm that damaging dissolved salts are not present. Electrolyser manufacturers will provide a more detailed specification for the feed water, and most will provide the necessary deionisation equipment as part of a complete package.
Purity standards for electrolyser feed water
Two internationally recognised standards refer to demineralised water purity for electrolysis. The US-based ASTM D1193-06(2018) Standard Specification for Reagent Water identifies three grades of purity. Many electrolyser producers will request supply of Type 2 water as a minimum purity. It has a maximum permissible conductivity of 1 μS/cm (0.1 mS/m).
ISO 3696:1987 is an alternative to the ASTM document. It is titled ‘Specifications for Water for Analytical Laboratory Use’. As with the ASTM document, the ISO Standard also includes three grades of purity, and the typical feed for an electrolyser would be Grade 2 with a maximum conductivity of 0.1 mS/m.
In addition to conductivity, the total organic content and total silica are important parameters for electrolysis feedwater. Maximum concentrations of these impurities are also specified in the above standards. Other contaminants to be avoided include carbonate and sulphate ions, as well as silicon and aluminium oxides.Water desalination and purification
The main source of renewable power generation globally at present is hydropower. Where electrolysers are used in proximity to a hydro dam, there will always be access to fresh water to create hydrogen. However, the main ramp-up in renewable power generation is from wind and solar power. The optimum location for wind power generation is often offshore, in salt water. The best places to generate low-cost solar power are generally in arid desert locations with limited access to fresh water. Hence there will often be the need to desalinate water.
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