is in the order of €170 per tonne, there is a willingness to pay up to €200 per tonne for biogenic CO 2 from a biogas facility. This new, competing offtake market is pushing up the price of biogenic CO 2 to a level that is forcing merchant CO 2 businesses to reconsider their sourcing options. In a similar arrangement, in February 2024, the Swiss clean-tech start-up Neustark sold 27,600 tonnes of CDR certificates to Microsoft. Neustark mineralises biogenic CO 2 into construction demolition waste to ensure its permanent sequestration. There is now tremendous competition for CO 2 from biogas, driving up prices for commercial CO 2 from this source. Clearly, alternative commercial CO 2 sources must be found. New low-cost CO 2 sources For decades, commercial CO 2 sourcing has been driven by economics and reliable, year-round supply availability. These drivers remain relevant, but now that biogenic CO 2 has additional high- value applications, new low-cost commercial CO 2 sources must be found. Several industries fit the requirements of commercial CO 2 sourcing. However, these are not biogenic sources. The future of commercial CO 2 sourcing will rely heavily on geogenic and fossil CO 2 . Diammonium phosphate (DAP) fertiliser production: The most common phosphate fertiliser is DAP. It is popular because it can be applied as a liquid or granules and introduces both nitrogen and phosphorus to the soil.
DAP is produced from the phosphorus-bearing ore, apatite, which is mixed with sulphuric acid, to yield gypsum as a solid waste material and phosphoric acid. The phosphoric acid is reacted with ammonia, then granulated to produce DAP. However, the apatite ore is not pure because it contains calcite, silica, and clay. During the ore treatment, or beneficiation, clay and silica are easily removed. However, much of the calcite cannot be separated from the apatite and is mixed with the sulphuric acid. During the reaction between calcite and sulphuric acid, CO 2 is released (see Figure 2 ). The flue gas from the sulphuric acid mixing chamber is scrubbed with water to remove hydrogen fluoride (HF). The resultant gas mixture is hot, moist CO 2 . Separation of the CO 2 can be easily achieved using cooling and condensation. The resultant dry, pure CO 2 can be liquefied at low marginal cost. The idea to capture commercial CO 2 from phosphate fertiliser production will be implemented by OCP Nutricrops at its Jorf Lasfar industrial platform in Morocco. Applications for that CO 2 will include pH adjustment at a local reverse osmosis seawater desalination plant. Ethylene oxide production: CO 2 capture is essential in ethylene oxide (EO) production to avoid an accumulation of CO 2 in the reactor gas recycle loop. The equipment to capture CO 2 is generally a twin-tower absorber-stripper system using the hot potassium carbonate (HPC) solvent (see Figure 3 ). The gas leaving the CO 2 stripper column is
Key reactions
Dry CO circa 95%
Sulphuric acid production Calcite acidulation Apatite acidulation
2S + 3O + 2H O 2H SO CaCO + H SO CaSO + CO + H O Ca (PO )F + 5H SO + 10H O 3HPO + 5CaSO 2H O + HF 2HF + SiF H SiF HPO + 2NH (NH ) HPO .
Condensation
Water
Water
Humid CO circa 15%
Scrubbing Ammoniation to DAP
Water
Scrubbing
H SiF (aq)
Sulphuric acid production
Air Sulphur
Sulphuric acid H SO (aq)
O gas
(aq) DAP Di-ammonium Phosphate (NH ) 2 HPO
Acidulation
(aq) Phosphoric acid HPO
DAP granules
Bene ciated Ore, rich in Apatite Ca (PO )F
Water
Drying and sieving
Bene cation
Granulation
Ammoniation
DAP product
Phosphite Ore (Apatite and Calcite
Gypsum CaSO .2H O
Calcite, clay, sand
Recycled DAP slurry
Ammonia NH
Figure 2 Geogenic CO 2 capture from phosphoric acid and DAP fertiliser production
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