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As to whether they will adopt PoW or POS consus, i may not know what they will implement in this regards but one thing is sure it is going to be highly regulated.
It will work like the fixed exchange rate, which will make them to eliminate volatality of such digital currency.
What i also think is this ; can a digital currency exist without blockchain technology?
Central banks would most likely not adopt PoW or PoS consensus. That is useless in a fiat digital currency implementation. There is no volatility either because that is fiat pegged. The value and everything else are that of a fiat currency. If you know how the fiat system works today, it's everything that will work in a central bank digital currencies. The only difference is that it is made digital. It might also not built on a blockchain. If it is, then it is certainly a centralized blockchain.