I knew both would be the answer when I asked the question. I actually think it's more of their binders. I don't think they make pellets out of pineBoth. Pellet length can cause problems with your feed system. Too long and they cause jams. Too short they tend to have more dust and can degrade more rapidly. The type of binder being used is also a factor. What wood the pellets are made out of is the main factor in how hot and clean they burn.