How to automatically get combustable items?

My friend made a machine that automatically farms cactuses and I was wondering if there is any combustable items that can be farmed without human intervention?

Solution 1:

There is one way, is using an automated wood farm. It works with a wither boss, you can see here:

The second way is making an semi-automated farm with blaze rods, but blazes needs to be killed by hand. (Blaze farm:

Solution 2:

As @Dan Bron implied in their comment, one other way that would work without having to go to the nether would be using a kelp farm. This one is simple and only requires a piston and observer. There are many possible layouts for kelp farms, but all you have to do is put the kelp into a smelter and make dried kelp, then once you have dried kelp you would craft dried kelp blocks to use as a combustable material. reference youtube video