-Now one cleaned coal burns for 72 seconds instead of 36
Before this, there was no point in cleaning the coal, since 1 piece of uncleaned coal burns for 30 seconds, and cleaned one for 36 seconds, but you spent 2 pieces of uncleaned coal to get 1 piece of cleaned coal.
Now everything is fine, with a cleaned coal you get 12 seconds more burn time than from two uncleaned ones.