Turquoise hydrogen production by methane pyrolysis
Technologies for methane pyrolysis are at different levels of maturity up to early-stage commercial operations.
Stephen B. Harrison
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Hydrogen is in general regarded as a clean energy vector. But whether or not hydrogen has a positive impact on mitigation of greenhouse gas (GHG) emissions depends heavily on its mode of generation. Nowadays, hydrogen is predominantly produced with a hefty fossil CO2 footprint, while costs for fossil CO2 are externalised. Within this article, hydrogen production by means of methane pyrolysis is examined. Different technical approaches to methane pyrolysis are presented, and their benefits and drawbacks are highlighted (see Figure 1 and Table 1).
A major question in the whole value chain of hydrogen production through methane pyrolysis is the downstream utilisation of the produced solid carbon. If natural gas, shale gas or fracking gas is used in methane pyrolysis, fossil CO2 emissions are unavoidable in downstream processes, which eventually result in downstream emissions similar to state-of-the-art, coke based processes. To overcome this intrinsic obstacle, the use of upgraded biogas and synthetic e-methane are presented. In both ways, the carbon is derived from the atmosphere, either via a biological pathway in terms of biogas, or via direct air capture (DAC) of CO2. If atmospheric CO2 is used as the feedstock in renewable methane production, then methane pyrolysis could provide a viable pathway to the supply of sustainable solid carbon or graphite for various industrial applications.
At present, about 95% of the hydrogen that is produced worldwide is derived from fossil fuels using various thermochemical processes. Gasification consumes solids such as petcoke and coal. Other gas-phase processes are fed with methane, naphtha, or refinery gas.
Autothermal reforming (ATR), steam methane reforming (SMR) and partial oxidation (POX) are the main thermochemical hydrogen production processes in use today (see Figure 2 and Table 2). In these processes, syngas is produced; this is a mixture of hydrogen and carbon monoxide. Sometimes, two out of the three processes may be combined in series to achieve the desired ratio of carbon monoxide to hydrogen in the resultant syngas. If hydrogen is the target gas, carbon monoxide may be converted to carbon dioxide and hydrogen in a subsequent water gas shift reactor.
As a rule of thumb, these thermochemical processes produce about 10 kg of CO2 per kg of hydrogen. If CO2 is not captured, the resulting hydrogen is referred to as grey or black hydrogen.
This grey hydrogen may be used in fuel cell powered cars with a consumption of about 1 kg of hydrogen per 100 km. This would result in emissions of 100g CO2/km if upstream emissions are also considered. This exceeds current European fleet emissions of 95g CO2/km. The direct use of natural gas as CNG in internal combustion engines would emit less CO2 from a total system perspective.
Through this example, it is clear that hydrogen must be produced at scale as a clean energy vector to mitigate greenhouse gas (GHG) emissions and combat climate change. One option is to use carbon capture, utilisation and storage (CCUS) to reduce CO2 emissions from grey or black hydrogen production in order to produce blue or purple hydrogen. Green hydrogen produced in electrolysers fed with renewable electrical power is another. Turquoise hydrogen produced by methane pyrolysis, also known as methane splitting or cracking, is a potential third pathway to low-carbon hydrogen production at scale.
Chemistry of methane pyrolysis
Methane pyrolysis is endothermic, meaning that it requires heat energy to drive the conversion of methane to hydrogen and solid carbon. The reaction is represented by:
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. It is also possible to use indirect electrical heating or direct heating with an electrical plasma. 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 1960s but the technology was not implemented at scale for many decades. In the past 10 years methane pyrolysis has picked up momentum and several companies have piloted various technologies. Each project has sought to overcome some of the challenges inherent in this process. It is only in the past few years that we have seen commercial operations based on methane pyrolysis emerge.
Plasma pyrolysis from renewable power
Monolith Materials started the development of its methane pyrolysis process in 2012. In 2016 construction started on the Olive Creek 1 plant in Lincoln, Nebraska. It was commissioned in 2020 and has a production capacity of 14 000 t/y of carbon black and around 2500 t/y of hydrogen. A second, larger plant named Olive Creek 2 is planned to have a capacity of 194000 t/y of carbon black and will produce close to 40000 t/y of hydrogen which will be converted to ammonia for potential use in the local corn belt as a fertilizer.
In this process, which was initially developed by KvÃ¦rner,1 methane is heated to 1650°C using an argon plasma generated by electrodes powered by renewable energy sources. At this temperature, the methane molecule splits. This eventually leads to the formation of carbon black, while the protons split off from the methane molecule and recombine to form hydrogen molecules. The graphite electrodes may provide some catalytic effect and the initially formed carbon black granules catalyse the production of additional carbon black. In various papers related to methane pyrolysis, various additives to the methane have been identified that can either stimulate the reaction2 or enhance the physical properties of the carbon black.3 The reaction takes place without the need for an additional solid catalyst.
The prospect of producing turquoise hydrogen from renewable electricity means that this will be a carbon neutral hydrogen generation process, if the process can cope with the volatility of wind and solar electricity supply. If the electricity is sourced via a PPA then the operation would need to follow the electricity supply of the associated wind and solar farms, which seems to be challenging. Otherwise, green certificates can be purchased to cover for the electricity input, but then this process plant is just another constant load in the electricity grid, like an aluminium smelter.
Moving carbon bed thermal process
BASF has been conducting methane pyrolysis development since 2010. From 2013-2017, within a project funded by the Federal Ministry of Education and Research (BMBF), a lab-scale reactor was built and operated at Ludwigshafen, Germany to identify key process parameters.4,5,6 A follow-up project, also funded by the BMBF, involves a larger pilot plant and construction began in 2019.
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