Decarbonisation Technology - May 2023 Issue

for these balance of plant items is only 10% of the total electricity consumption for an electrolyser system. CO 2 feedstock purity for PtL and urea A solid oxide electrolysis cell (SOEC), operating in co-electrolysis mode, consumes steam and CO 2 to yield syngas, which can produce e-methanol or liquid hydrocarbons through established chemical pathways, such as methanol synthesis and Fischer-Tropsch conversion. Introduction of steam to the SOEC means that water molecules are delivered to the electrolyser in a highly energised state. Therefore, about 25% less electrical power is required to split them than when using low- temperature electrolysis, such as a PEM or alkaline electrolyser (see Figure 4 ). When e-fuels production is done in traditional refineries, waste heat or excess steam can often be fed to a SOEC. In stand-alone PtL processes for e-fuels production, exothermic reactions such as Fischer–Tropsch or methanol synthesis can provide the heat requirement of the SOEC. This integrated pathway results in high overall efficiency. In co-electrolysis, both the steam and CO 2 must comply with the purity specification

required by the electrolyser. The requirement for CO 2 feedstock purification varies depending on its source. CO 2 captured from the combustion of fossil fuels can contain sulphur compounds that must be removed. The use of captured CO 2 reduces the overall CO 2 impact of SAF and introduces an element of circularity into the value chain. CO 2 derived from direct air capture (DAC) is around three times more expensive than CO 2 captured from stack emissions. However, it is generally free of any potentially harmful impurities, and the purification costs can be avoided. If no waste heat is available, it may be preferable to use an alkaline electrolyser to generate hydrogen. The hydrogen from an alkaline electrolyser can be reacted with captured CO 2 to yield syngas through the reverse water gas shift reaction. In addition to CO 2 being a feedstock for e-fuels, it is required for green urea production. Urea is used to make fertilisers and resins and is formed by reacting CO 2 with ammonia. Thousands of tonnes per year of captured CO 2 could be utilised in this circular way to build carbon-neutral e-fuels, fertiliser and chemical molecules.

Regenerative twin bed dryer for hydrogen purication

Compressor

Electricity

Final product quality gas analysis

Safety gas analysis O in H

Gas holder

Safety gas analysis O in H

Catalytic de-oxo unit

Transformer

Demister

Rectier

Water feed

Phase separator

Purier

Feed water pump

Scrubber

Cooler

Phase separator

Filter

–

Filter

Fresh KOH

Alkaline electrolysis stack

Lye recycle

KOH management

Lye recycle

Lye tank

Figure 5 Low-pressure alkaline water electrolysis process