The only free bitcoin accelerator is https://www.viabtc.com/tools/txaccelerator/. Other bitcoin accelerators are either not free or they are scam.

If your transaction is not having more than 500 bytes in size and the transaction fee paid is not less than 10 sat/byte, you can use ViaBTC free accelerator.

Others that call themselves free accelerators will keep rebroadcasting your transaction and not accelerating anything.