OK, I'll make a little demo .tgd for volumetric fire.
The basics are, a dense cloud layer with some 'Y' stretching in the fractal for height, some wispiness etc. and some extreme fake internal scattering.
The cloud colour is the orange, the scattering colour is the yellow...
The image I posted does have a layer of smoke but I disabled it for quickness for that sneak-peek. The fire and the smoke(another cloud layer) use the same density shader set to different heights, the trees are populated using that same density shader but inverted. The burnt trees use it too but not inverted, so they only appear where there's fire. Also using the fire density fractal is a layer of blackened ground in the surface shaders, you can't really see it here.
Once I put the basics back together, I'll upload it in a new thread. I don't have access to this very project file at the moment though because my other hard drive isn't connected to this computer for now. I made this about 6 months ago, when I was too intimidated to post at what I thought of as 'The Planetside Mathematitians Forum'. Now though, I know you're all just as screwy as me!