Methane cracking for turquoise hydrogen
About 70 percent of the hydrogen that is produced in the world today is derived from natural gas using thermochemical processes. Gas-phase reforming processes are fed with methane, naphtha, or refinery gas. In these processes syngas is produced which is a mixture of hydrogen and carbon monoxide. If hydrogen is the target gas, carbon monoxide may be converted to carbon dioxide and hydrogen in a subsequent water-gas shift reactor.
These thermochemical processes generate about 10 kg of carbon dioxide (CO2) for each kg of hydrogen produced. If the CO2 is not mitigated with CCUS, the resulting hydrogen is referred to as grey or black hydrogen. Blue or purple hydrogen are produced if carbon capture is implemented.
As an alternative to the above processes, methane from natural gas can be ‘cracked’ or split through pyrolysis to form hydrogen gas and solid carbon. Methane cracking/pyrolysis has now added to the rainbow nomenclature for hydrogen production options and is known as turquoise hydrogen. Methane pyrolysis is endothermic, meaning that it requires heat to drive the reaction forward.
There are different options for the external heat supply. Indirect heating using burners fuelled by hydrogen or natural gas as a fuel is one option. Indirect electrical heating or direct heating with an electrical plasma is also possible. These heating modes could use renewable electricity, biomethane or low carbon hydrogen to minimise CO2 emissions from the process.
Research into methane pyrolysis has been undertaken since the 1960’s. In the past 10 years methane pyrolysis has picked up momentum and several technologies have been piloted. Each project has sought to overcome some of the challenges inherent in this process. It is only recently that commercial operations based on methane pyrolysis have emerged.
A question that arises from methane pyrolysis is: what happens with the solid carbon? Graphite, carbon black and activated carbon are functional materials, used in various applications. Carbon black powder is a filler material in tyres and black consumer goods such as cell phones. Activated carbon is a filter for gases and water to remove harmful pollutants. Graphite is used to produce electrodes for gold, silver, steel, and aluminium processing.
If methane cracking for turquoise hydrogen production becomes an established pathway to hydrogen, the amount of solid carbon produced will greatly exceed demand from current applications. If the carbon becomes a low-cost by-product it could be used as a soil-improver or as a substitute for coke in metallurgical processes.
One of the attractions of turquoise hydrogen production is that the process has the potential to be carbon negative if it is fed with biomethane and if renewable power is used for indirect heating or to generate the plasma.
To determine whether methane pyrolysis has a net positive effect on decarbonisation, greenhouse gas emissions and climate change, a complete Life Cycle Analysis (LCA) is required. This is a detailed investigation into the holistic carbon cycle for a process, ranging from production though usage to subsequent product recycling or disposal.
NexantECA Training, insights and services
Find out more about black, grey, green, blue, purple and turquoise hydrogen at the new NexantECA Training course: Hydrogen – Clean and powerful energy, helping you to understand hydrogen’s role in the energy transition. Keep an eye for our next scheuled Hydrogen training course by visiting our 2021 training calendar
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Stephen B. Harrison, Principal