I can't tell you how to burn tree piles, but I am an expert on how not to do it.
We had about 8-10 acres worth of trees piled up from clearing the area for the pond. I had been trying to burn them for about a week without success. Like Eddie said, diesel just burns off. So I figured it would take more diesel and several tires. So I filled about 20 five gallons cans of diesel, about 20 tires, and a can of gas for the generator. Spent a whole day pouring and lighting diesel without getting the pile to light. Tired and ready to go home and take a shower, I poured out the last can and as I lit it, the voice in my head, smarter than I am, said wait, that's not diesel, that's gas. Well, about that time BOOM! Once I determined that I was still alive, I looked to see where the kids were. My 8 year old was standing there, eyes like saucers, said "Daddy, your hair is on fire". Sure enough.
The skin peeled off like the fat off a baked ham. I healed. No damage done.
Moral of story: Be careful.
