These transactions have a null gas price because they are sent by the miner itself.
If we look at the transaction history of the address 0xBbed1c61b6E68c397b021C1274080A2005042C08 we see one incoming transaction of 100 ETH on 13 April 2021 coming from...F2Pool. This leads to think, without too much doubt, that this address belongs to F2Pool.
Why some transactions come from the same person and get mined by 2 different miners ?
Your post mentions two addresses sending these free transactions :
Transactions sent by this address are only mined by F2Pool, which makes sense because, as seen above, the address belongs to F2Pool.
Transactions sent by this address are in fact mined by two miners : SparkPool and BabelPool.
However, by doing a quick research we learn these two miners are closely related "partners".
From the BabelPool website :
Babel Finance’s Ethereum mining pool is technically supported by Spark
Pool, which is a technology-leading comprehensive mining pool and the
world's largest Ethereum mining pool.
Babel Finance will work closely with Spark Pool and mining partners to serve miners better through more financial innovations.
Spark Pool and Babel Finance are partnering together and looking
forward to more collaborations in mining finance to protect the future
What's the purpose of these transactions ?
These transactions are very expensive in gas to the point that an entire block is reserved for them. That's because they mint a gas token named CHI which creates a large number of smart contract at the same time. On the other hand, when CHI tokens are burnt, the previously created contracts are destroyed (using the
selfdestruct OPCODE) which frees storage and rewards the sender. This process allows to reduce transactions gas costs.
These miners are therefore accumulating gas through this CHI gas token, may be to sell it if the gas price increases.
Miners want to maximize their profit. Thus, they have developed some strategies. One of them is to mine blocks containing no transactions. In fact, when they receive a new proposed block, honest miners :
- download and validate the block
- remove the transactions that have been included in the block from their transaction pool.
This is time consuming.
This is why, in some cases, a miner only collects the hash of the newly broadcasted block before starting to mine the next block. Since the miner has not verified the transactions of the candidate block and has not updated its transaction pool, its new block must not include any transaction at the risk of containing transactions already mined which would invalidate the block.
Now what about those 1-transaction blocks ?
That's basically the same strategy which is applied, except the miner includes its own transaction which mints for free a large amount of CHI gas token. These ones can be viewed as an investment and sold if the average gas price spikes. Another important point to mention : it does not seem miners share their CHI tokens with the pool members (who are therefore losers on these blocks). So these blocks could be somehow attractive to the miner.
This type of mining strategy is however doomed to disappear because the gradual decrease in the block reward over the long term encourages miners to build full blocks to benefit from the transaction fees.