I would strongly recommend against home-built steam pressure boilers. Pressurized steam contains ENORMOUS amounts of energy. It kills the unwary, the uninformed, and innocent bystanders.

This is roughly the equivalent of filling a vessel with black powder and lighting a fuse of unknown length. Please don't do it.

If you want to convert wood to mechanical energy, set up a gasification system and a piston engine.

From the website of one of the regulators of pressure equipment:

"The boiler explosion at a 2001 county fair in Medina, Ohio,
tragically demonstrates the importance of proper inspection and
maintenance. In that instance, the crown sheets on an antique
steam-tractor boiler had been reduced to 23% of their original
thickness. The mechanical failure of the boiler occurred at a
pressure below that of the installed safety devices, instantaneously releasing an estimated 28,000,000 ft-lbs of force, and lifting the 20-ton steam tractor some twelve feet in the air. Five fatalities and forty-eight injuries resulted from the blast."