The sap in woods like cedar, juniper and pine are a potential problem in the tank. Oils? One reason cedar is used for outdoor things like decks, fences is that it is famous for holding the sap for longer than lots of other wood.
But the saving grace of those woods is that it is very easy to tell when they are totally free of the sap that causes problems. I never try to ID the wood that I use as it is totally unimportant when it is totally dry.
So some thoughts on your wood? When does wood burn? When the heat is intense which dries the wood or when the wood is already dry! So you have a piece of wood that has been in a fire and it almost burned. That is a very important clue that it is dry. Second is when you look at the cut end, you see that the color is almost totally uniform from outer layers to inside. Since the moisture is what colors the wood and it dissipates from outside to inside, seeing it uniform says it has dried all the way through!
Downside to totally dry wood is that it will be very light and float easily. Since it has been burned, it will likely be very hard and take a long time to soak up, so expect to weight it down in some way. I screw pieces of floor tile slate to make it stand the way I want as well as hold it down.
It looks like a winner to me!