>I always find shells thrown on the fire burn brilliantly (like paper does) but don't last all that long.

With the exception of walnut shells - they do have firepower. Shells actually burn fast only because they're exposed to oxygen on all sides, being thin by nature. Press them into bricks and it's a different matter.

>Pine cones too?

Probably, but the pines are not a native plant in the area. This guy was actually recycling the leftover biomass from the fields and orchards.