Clean Energy Technology

Hydrogen has a lower molar mass and a higher volumetric flow than natural gas, which requires higher compression effort, and its smaller molecular size also poses an additional sealing challenge to minimise external leakage. For relatively large volumetric flows and moderate pressure lifts (

The construction of inland hydrogen transmission pipelines is regulated by the ASME B31.12 standard and although it is a mature technology, the characteristics of existing hydrogen pipelines differ from the features required for new pipelines. Currently, the largest hydrogen pipelines are 18 inches in diameter, whereas new hydrogen pipelines could be up to 36-48 inches in diameter; low steel grades are used (generally below X52), whereas higher steel grades may be preferred in new pipelines to reduce the amount of steel required without compromising integrity; and existing pipelines operate under static loads, whereas future pipelines should be able to withstand pressure variations due to cyclic loading and linepack. In addition, there is no standard for the construction of offshore hydrogen pipelines and research is underway to identify criteria that will ensure the highest level of safety while reducing costs.

The direct use of ammonia has been successfully demonstrated in micro gas turbines with a power capacity of up to 50 kW. In larger gas turbines, the slow reaction kinetics of ammonia with air, flame stability and the NOx emissions are issues being investigated in ongoing research activities.

Ammonia can be cracked into hydrogen and nitrogen (by a thermal and catalytic decomposition), so that the produced mixture of hydrogen and nitrogen is burnt in the combustor of the gas turbine. The heat required for decomposing (or cracking) the ammonia at temperature levels of 600-1000°C, depending on the catalyst, can be partially supplied by the hot gas turbine exhaust gases (550-650°C) when it is operated in simple cycle. In combined cycle mode, the energy supplied for decomposition would slightly reduce the electricity generation efficiency of the overall process. To minimize the energy demand for cracking, partial cracking of ammonia is possible where the fuel mixture is composed of hydrogen, nitrogen and ammonia. Already small amounts (few %) of residual ammonia in the cracking product gas can lead to excessively high NOx emissions (few hundred ppm) if current state-of-the-art lean premixed combustion technology is applied.

A liquefied hydrogen tanker is a ship designed to transport liquefied hydrogen (LH2). Shipping LH2 is similar to liquefied natural gas (LNG), but as the boiling point of hydrogen (-253 °C) is much lower than that of natural gas (-162 °C), special thermal insulation is needed to minimise high boil-off gas rates, for example, using double-shell vacuum insulation tanks or membrane-based insulation systems. In addition, LH2 ships aim to use hydrogen boil-off gas as fuel for the loaded leg of the journey, providing a low emission shipping fuel and at the same time preventing venting it.

Repurposing implies converting an existing natural gas pipeline into a dedicated hydrogen pipeline. The main elements of the conversion process include nitrogen purging to remove undesirable parts, replacement of compressors, a thorough inspection of the pipeline and the integrity of its components, and replacements of valves and other leak-prone parts, and reconfiguring or replacing gas meters. Due to differences in chemical properties, hydrogen can accelerate pipe degradation through a process known as hydrogen embrittlement, whereby hydrogen induces cracks in the steel. A range of solutions exists to combat this: regularly monitor the integrity of the pipeline, e.g. through in-line inspections (ILI) and pigging; apply a hydrogen barrier coating to protect the pipeline; lower the pipeline pressure until the required threshold value for safe operation is met; and minimise pressure swings. The optimal approach will depend on transport capacity requirements, status of the existing pipeline (e.g. existing fractures) and trade-offs between capital and operating expenditure. There are still challenges on the repurposing of offshore gas pipelines, as the monitoring of the pipeline with the current technology is difficult, and sometimes there is no detailed documentation on the pipeline operation over past years. There is no standard for offshore hydrogen pipelines, unlike the ASME B31.12 for onshore hydrogen pipelines.

Co-firing of ammonia in existing coal power plants can be an option to reduce the CO2 emissions impact of these plants in the near term. Blending shares of up to 20% in energy terms are considered feasible with only minor adjustments to a coal power plant. In smaller furnaces with a capacity of 1 MWth, blending shares of 20% have been achieved without any problems, in particular no ammonia slip.

At a biomass-fired power plant with post-combustion capture using chemical absorption, the CO2 is separated from the combustion flue gas by using a chemical solvent (e.g. amine-based). The CO2 is released at high temperature and the solvent regenerated for further operation.

Liquid organic hydrogen carriers (LOHCs) can be transported using existing ships and port infrastructure. LOHC can be transported in chemical tankers, whose tanks are specially coated, for example, with phenolic epoxy, stainless steel or zinc paint, and may have dedicated piping arrangements to carry different cargoes. The type of coating may determine the chemical that can be transported. Product tankers, which are a type of oil tanker, carry refined oil and are often designed to carry chemical cargoes as well, and may be capable of transporting LOHCs. Chemical tankers typically range in size from 5 000 to 35 000 deadweight tonnage (dwt), while product tankers range in size from 35 000 to 120 000 dwt. Depending on the chemicals used as LOHCs, the type of tanker that can be used may differ and there may also be some size restrictions at ports due to safety.

Hydrogen can be transported to consumers with relatively small demands in multi-element gas container trailers, such as steel high-pressure tubes and in lighter composite pressure vessels (types II and III). Trucks that haul gaseous hydrogen in steel tubes compress it to pressures of around 180-250 bar, carrying approximately 380 kg onboard and limited by the weight of the tubes. However, recently light-weight composite storage vessels are increasingly used, with capacities of 560-900 kg of hydrogen per trailer (350-500 bar), increasing considerably the hauling efficiency per trip. In addition, larger volumes of hydrogen can also be transported in cryogenic vessel trailers, which can carry around 1 500-3 000 kg of hydrogen per trip. Liquid hydrogen trailers are thermo-insulated to minimise hydrogen boil-off rate.

High shares of ammonia can be sprayed liquid directly into the combustor of natural gas turbines to reduce CO2 emissions generated in the combustion process, although this can result in higher NOx emissions than those produced without ammonia, which require control technologies

In solid adsorption, the carbon dioxide is separated from the combustion flue gas by using a solid sorbent (e.g. zeolites, metal organic framework). Compared to amine-based chemical absorption, solid adsorbents can have lower regeneration energy and greater adsorption selectivity.