Decarbonisation Technology November 2026 issue

ZCT’s pilot work has tested a slurry-phase (liquid-suspended) catalyst configuration as an alternative, where fine catalyst particles are suspended in a liquid wax medium inside the reactor. The surrounding liquid acts as a continuous heat sink, absorbing and distributing reaction heat far more evenly than a solid bed can. In ZCT’s experience, this change – moving from solid bed to liquid-

Reactor temperature vs. Product carbon-range selectivity

Target set-point

Light gas/naphtha (below jet range) SAF jet-fuel range (C8–C16) Diesel/biodiesel-range & wax (C17+)

Narrow band held by liquid-suspended catalyst (even heat distribution)

SAF selectivity window

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Fischer-Tropsch reactor temperature (˚C)

Figure 3 Indicative product-selectivity curves vs reactor temperature. The SAF jet-fuel window sits between the lower temperature lighter naphtha/ light-gas regime and the higher temperature diesel/biodiesel-range wax regime, requiring a tightly held mid-range set-point.

suspended catalyst – is the largest single factor for maintaining a narrow, repeatable carbon-chain distribution under continuous operation, because it removes the local overheating that otherwise pushes the product slate off-target (see Figure 2 ). The trade-off is mechanical complexity: liquid- phase systems require catalyst-wax separation downstream and more careful slurry circulation control. ZCT’s view from pilot operation is that this added complexity is worth the selectivity gain, particularly because tighter selectivity directly reduces the cost of downstream upgrading – a cost that otherwise erodes SAF’s economic case against kerosene. Temperature as the decisive process variable Reactor temperature governs the chain- growth probability of the FT reaction – the statistical likelihood that a growing hydrocarbon chain adds another carbon unit rather than terminating. This single parameter is what separates a SAF-range output from a biodiesel- range or heavy-wax output, even when the same syngas, the same H₂ source, and the same catalyst chemistry are used. • Lower reactor temperatures favour longer chain growth, pushing the product slate towards heavier waxes and diesel-range or biodiesel-range hydrocarbons (roughly C16 and above).

• Higher reactor temperatures increase chain termination probability, shifting the slate towards lighter gases and naphtha (below the jet-fuel carbon range), while also increasing methane and CO₂ byproduct formation. • The SAF-relevant window sits between these two extremes: a narrow operating band where chain growth is long enough to meet the jet-fuel carbon range, but controlled enough that the product does not continue growing into diesel- range wax (see Figure 3 ). In ZCT’s cool catalyst configuration, the reactor is deliberately held at the lower end of conventional FT operating temperatures compared to classical high-temperature FT plants, but precisely controlled rather than simply ‘cold’. This is the core distinction in ZCT’s approach: it is not about minimising temperature; it is about holding the narrowest possible temperature band around the set-point that maximises selectivity toward the jet-fuel fraction. The liquid-suspended catalyst makes that narrow band achievable at scale because it prevents local hot spots that would otherwise pull part of the bed above the SAF window and part of it below. This is why ZCT treats temperature control, not just temperature selection, as the defining engineering parameter. A reactor that runs at the correct average temperature but with wide