Burning slash piles: big and hot versus small and cool

[Oregon40Acres]

I live in southern Oregon, where our primary concern in small lot forestry management is coping with wildfire. I spend lots of time collecting slash, piling it up, and burning it. While burning a big slash pile, I noticed that at the outset it burned hot, fast, and almost smokeless, but as it cooled, it emitted more smoke. I realized that this is simple chemistry: at high temperatures, the complex hydrocarbons in the wood are completely oxidized to CO2 and H2O. But as the fire cools, we get incomplete combustion and lots of particulates are broken free, risking as smoke.

This led me to conclude that I should burn small slash piles, not big ones, because the small piles burn cooler and emit more smoke. I reason that high-temperature burning releases more CO2, which is not good for reasons of climate change, whereas low-temperature burning releases particulates that will settle to the ground within a few miles, providing nutrients to the forest.

Does this make sense to you?

[rusticretreater]

I would say you are over thinking it.  The growth, death, burning of brush is actually part of a natural life cycle that would occur if we were not around.  Plus it is carbon that is naturally prevalent in nature.  You are simply accelerating the recycling that occurs naturally.  The CO2 debate is for the things like burning fuel that would otherwise not be burned.  Probably why California loves their brush fires. 

[RPF2509]

My thought is that you release equal amounts of carbon regardless of how big your pile is as long as equal amounts of slash are totaly consumed.  Small cool burn may not totally consume all the woody debris so whatever is left as charcoal is carbon not emmitted.  I don't think you should worry about pile size.  Trees and brush capture existing carbon - its the fossil carbon in oil and coal that is adding to the current atmospheric store.  Drive ICE less, heat your house with wood, not oil or natural gas to lower your carbon footprint

[RPF2509]

By the way the particulates in smoke are really bad for everyone's lungs (cancer causing tars and terpenes plus micro particles clog the airsacs) so its best to burn hot and clean, not slow and dirty.  You may have noticed that some areas in Oregon restrict woodburning stoves as well as open burning during certain weather conditions where smoke does not disperse well.

[Don P]

I can't think of any positives to a cool fire air quality wise. If you're gonna break it down with fire, break it down, don't make a bunch of molecular soup. Think about what a catalytic converter is doing in a car or woodstove, burning hot enough to break molecular bonds and leave simpler compounds.

[ID4ster]

I'm looking to have my loggers windrow the slash so that it covers a wider area if there is enough space to do so. My reasoning for this is that there are a lot of nutrients tied up in that slash and I want them dispersed over as wide an area as possible.

Burning the slash raises the ph of the soil underneath the burn pile which makes more nutrients available for tree growth. Trees that are planted in those burned areas grow at a rate 50 - 100% better than trees just outside the burned area. There is also a lot of work being done by USFS forest researcher Paige Demarose on the benefits of biochar and how that is helping soil fertility and water retention. 

The upshot is that I want the slash piles burned at a lower temperature and if charred chunks of slash are left after the burn that is just fine. Those chunks make good micro planting sites and are very good for the soil once they are incorporated into the upper layers. Smaller piles also don't scorch nearby trees nearly as often as large slash piles do. 

Big slash piles are spectacular to see when they are burning at night. But it's not the wisest use of the nutrients or the best for forest health. 

[peakbagger]

Not sure if you are familiar with "burn boxes" they are used in areas that need to get rid of lot of wood. Its an insulated box open at the top with forced draft fan on it. The wood goes in the top and with a forced draft it burns the wood hot and clean. Sort of like a jumbo version of those solo campfire stoves. They even can be equipped to generate power with the waste heat. 

Here is link Agricultural Waste Elimination (airburners.com)

[Joe Hillmann]

Around here you can't just light a pile on fire and get it to burn.  It is too wet here and the fire will hollow out a small part of the pile and eventually burn out, leaving most of the pile unburned.  If you want to get it to burn you need a machine to pile more wood onto the fire once it is started.  So burning long rows would be almost impossible here, any time it would be dry enough for the fire to spread there are burning bans to prevent fires spreading.

If you want charcoal left over.  Start a fire(Ideally in some sort of pit or natural hollow) then keep piling wood on top and keep the fire hot.  The wood will burn from the top down and the charcoal will collect at the bottom and not have enough oxygen to burn.  Once most of the wood is burned up, extinguish the fire by burying it with soil or with water.  That will leave you with lots of charcoal and not much wood.  You can then spread the charcoal wherever you want.

[Don P]

If you do much checking of biochar, cool char is tar filled and pretty much useless as a soil ammendment. Hot charcoal has had the tars consumed and is the dull not shiny, soil quality biochar that the research has been based on. This "cool fire" strategy sounds sort of like "I'm picking it up by the clean end"  .