Whereas blue hydrogen from methane produces CO2, the by-product of turquoise hydrogen is pure carbon. The obvious advantage is you can make your hydrogen without the need for expensive new infrastructure to transport and store any CO2. Turquoise hydrogen is only at the start-up phase, so Schalk Cloete summarises his co-authored paper that looks at various scenarios to estimate the cost of producing the hydrogen (using molten salt pyrolysis) and, crucially, the carbon. That matters because the market value of the carbon will help pay for the scale-up. With a hydrogen price of €1.6/kg and a levelised cost of carbon at €400/ton to €500/ton, the first plants should be able to operate profitably. Though the global market for pure solid carbon is small (e.g. carbon anodes for the aluminium industry or graphite for batteries) scale-up should bring costs down, opening up new carbon markets (e.g. carbon used as a reducing agent or reactant in various metallurgical and chemical processes). Cloete concludes that this techno-economic assessment shows there is a very strong incentive to accelerate scale-up toward commercial readiness.
The colourful spectrum of hydrogen production options has inspired plenty of discussion within the clean energy community in recent years. Although the green (wind/solar) and blue (fossil fuels with CCS) shades take most of the limelight, a more recent addition also deserves some attention.
What is Turquoise Hydrogen?
Turquoise hydrogen is produced when a hydrocarbon fuel is thermally cracked into hydrogen and carbon. For example, when methane is heated to the required temperature, it spontaneously discomposes as follows: CH4 <=> C + 2H2. The reaction is endothermic (consumes heat) and equilibrium controlled with hydrogen yields increasing with temperature and decreasing with pressure.
Turquoise hydrogen differs from blue hydrogen where methane is reformed and shifted with steam as follows: CH4 + 2H2O <=> CO2 + 4H2. Thus, the key difference is that turquoise hydrogen produces carbon instead of the CO2 produced by blue hydrogen.
Producing Carbon, but no CO2
Carbon formation brings benefits and drawbacks. Most prominently, there is no CO2 stream that needs to be handled, making turquoise hydrogen applicable to regions with no aspirations of building a CO2 transport and storage network. There are also some high-value markets for pure carbon in areas such as anodes, graphite, and sorbents that can enhance process economics. However, these markets are small relative to the projected size of the clean hydrogen market.
Molten Salt Pyrolysis
The main technical challenge with turquoise hydrogen is the handling of the solid carbon. If the reaction occurs over a conventional solid catalyst, carbon rapidly deposits on the catalyst surface, deactivating it. The best solution to this problem is to complete the reaction in a liquid bath of molten metal or salt. Such a bath allows the carbon to float to the surface where it can be more easily removed.
We recently completed a techno-economic assessment of the molten salt pyrolysis route to producing turquoise hydrogen using the process configuration shown in Figure 1. As shown, the process features an electrically heated pyrolysis reactor to split natural gas into carbon (stream 6) and hydrogen (stream 4).
Due to the equilibrium limitations, some methane remains unconverted, so stream 4 is not pure hydrogen. Thus, an additional pressure swing adsorption unit (PSA) is required to produce a pure hydrogen stream for export (stream 8). Most of the methane-containing PSA off-gas is recycled back to the pyrolysis reactor (stream 9) to maximise overall natural gas conversion, but a smaller fraction (stream 10) must be combusted to prevent the traces of nitrogen in the natural gas feed from accumulating in this recycle loop.
One of the main optimisation parameters of the turquoise hydrogen process is the pressure of the pyrolysis reactor. Higher pressures reduce the reactor size but also inhibit the equilibrium conversion, leaving a larger fraction of unconverted methane in the outlet gas. As shown in Figure 2a, this leads to lower hydrogen and carbon conversion efficiencies (η) and lower CO2 avoidance (more carbon is contained in the small fraction of fuel sent to the combustor).The economic assessment shown in Figure 2b indicates that 12 bar is the optimum pressure. In this case, the reactor is reasonably small, resulting in low capital costs, but the conversion is not yet badly affected so the yield remains reasonable. As shown, the economic performance is expressed as the levelised cost of carbon with a revenue (negative cost) for the hydrogen by-product, assumed to be sold at a modest price of 1.6 €/kg.
Simple plant design keeps fixed costs down
Natural gas fuel, here assumed to cost 6.5 €/GJ, is the main cost, with electricity at 60 €/MWh (the primary contributor to variable operating and maintenance, VOM) being the second-largest expense. Due to the simplicity of the plant, capital and fixed operating and maintenance (FOM) costs are relatively low.
Prospects will depend on the future value of Carbon
After subtracting hydrogen revenues, the cost of carbon came in at an attractive 312 €/ton in the 12 bar case. Although little is known about the future potential market for pure carbon, we estimate the relationship between price and volume illustrated in Figure 3. As illustrated, a plant producing carbon at a cost around 300 €/ton would be highly profitable when selling to high value markets such as carbon anodes for the aluminium industry or graphite for batteries. However, the relatively low volume of this market would restrict turquoise hydrogen to only about 1% of the future clean hydrogen market.
Fortunately, a 300 €/ton production cost is low enough to access potentially 20x larger markets when carbon is used as a reducing agent or reactant in various metallurgical and chemical processes. Access to these markets can allow turquoise hydrogen to play a significant role in future energy systems.
There are several uncertain assumptions in this assessment as illustrated in Figure 4. Here, a more pessimistic cost assessment is also included where the carbon is electrically heated to very high temperatures to evaporate any remaining traces of salt. In the base case, it is assumed that all salt can be removed by washing with water heated to 100 °C.
Figure 4 a & b show that natural gas and hydrogen prices are the most influential variables in the assessment. The sensitivity to hydrogen price is particularly interesting as the default assumption of 1.6 €/kg is relatively low. Future low-carbon hydrogen may well cost 2 €/kg or more in Europe, potentially bringing carbon costs below 200 €/ton to secure access to high-volume markets. As shown in Figure 4c, access to relatively cheap low-carbon electricity is also important.
Molten salt pyrolysis is still at the laboratory stage, but this techno-economic assessment indicates a high incentive to accelerate scale-up toward commercial readiness. The availability of high-value-low-volume carbon markets offer an attractive opportunity for more expensive first-of-a-kind plants to be constructed and profitably operated.
As shown in Figure 5, carbon costs remain around 500 €/ton even if the pyrolysis reactor is 3x more costly than our base assessment, an additional 60% contingency is added, and the most expensive salt removal assumptions are applied. This elevated cost remains well within the high-value price range shown in Figure 3. Profitable first-of-a-kind plants can therefore be constructed to drive down the costs of future plants via learning and scale to eventually access high-volume markets.
Turquoise hydrogen has great economic potential and deserves substantial research attention toward scale-up of enabling technologies like molten salt pyrolysis. It offers an attractive low-carbon hydrogen solution to regions without any CO2 storage potential and can economically supply growing markets for high-purity carbon. Such high-value markets also offer a clear path to technological maturity by ensuring the profitability of investments in expensive first-of-a-kind plants.
Schalk Cloete is a research scientist studying different pathways for decoupling economic development from emissions and environmental degradation