Can CO₂ from a non-certified fermentation plant earn an “existing-use" (eex-use) credit for RFNBO? 

No. Under DR 2023/1185 Annex A.10, e_ex-use applies only if the CO₂ is captured from a certified biofuel plant meeting RED II/III sustainability and GHG criteria, and the CO₂ has not already received a RED II capture credit. Deliberately produced CO₂ for RFNBO use is also excluded. Without certification, you cannot claim e_ex-use, making it harder to reach the ≥ 70 % GHG-saving threshold.