Different Woods = Different Gases?

[DBoon]

OK, admittedly somewhat of a nerdy question...

If I load hardwoods (sugar maple, hickory, ash, cherry) in my stove I get very visible secondary combustion from the top burn tubes - highly visible purple flames - and not many other flames in the firebox. The stove cruises at 550 to 600 degrees with these woods.

If I load a lot of Colorado Blue Spruce in, I get very few visibly detectable flames from the top burn tubes, but the firebox is very active with more orange flames. If I look closely, I can see some very faint purple flame. And the stove will cruise at 600-650 degrees. The fire looks very different in appearance and I'm curious as to why this is. I have the primary closed down nearly all the way, so some secondary combustion must be occurring, but it looks very different than the first case.

For what it's worth, the Colorado Blue Spruce seems to be as dense as the cherry - it lasts as long in the firebox. Wish I could find more of this (it was an old diseased tree I cut down in my yard).

 

[Backwoods Savage]

It is not uncommon for the flame to be different with various types of wood. One of the best examples I can think of is white birch. Many who burn wood in a fireplace love birch because of the pretty flame. Of course, there are other reasons for the birch, but this just gives one example.

I would definitely expect a different flame from your hardwoods vs. the spruce.

 

[dougstove]

The spruces (in general) have high levels of resins & turpentine related oily compounds (technically, terpenes, built up of repeats of 5 carbon units). Most of these compounds are at least somewhat volatile and will vapourize, and then combust.
The hardwoods have much lower levels of these terpenes, and generally, lower primary levels of volatile organic compounds. Instead, the cellulose & lignin in the hardwood will, under heat, pyrolize into different volatile compounds, that then burn.
Since your spruce is burning a bit hotter, I am guessing the oily terpenes are vapourizing and burning directly and quickly.
White birch, like the softwoods, has fairly high levels of oily compounds in the bark, partly explaining why, like the softwoods, it is good kindling, since the oils vapourize and burn readily.
The purplish colour is probably partly the tinge of blue flame from carbon monoxide combustion, and may be happening in different regions of the firebox with the different wood.
Biochemically yours, Doug

 

[DBoon]

Hi Backwoods Savage and dougstove, you did everything but outline the specific chemical reactions for me. Thanks for satisfying my curiosity.