Tuesday, March 8, 2016

Answers to Smoke Questions (part 4 of 3)

If you only light a fire occasionally, for camping or yard debris removal, you may think that smoke is just an inevitable part of burning wood.
(See Part 1, or if you like longer rants, try my earlier post "Wood Burning for Ethical Rich People," .)

Regulators seem to think that where there's fire, there's smoke.
(Notice how that's backwards from the old saying?) 

If you run hot smoke through a cool chimney, the tars in the smoke can condense inside, like steam on cold window panes.  Condensed smoke becomes sticky, flammable creosote, which can cause terrible chimney fires.
In the interests of public safety, it is legally required for wood burning stove chimneys in the USA to be at least 350 F during operation: hot enough that smoke won't condense on the inside.  Modern chimney components are also tested and designed to withstand at least one chimney fire without igniting the house.

It works - if you don't mind the added costs of both the tested-and-approved parts, and the extra fuel to heat the sky. But it's a real limitation on efficiency.

After-market controls to let you 'damp down' a woodstove for slower, steadier heat remain dangerous: they can increase the smoke (by reducing the air to the fire), and reduce the chimney temperature (leading to more creosote buildup.) 

To reduce the unwanted smoke, and the wasted energy to dispose of it safely, there are two key skills:

- Fire tending: Know what the fire needs to burn clean and efficient, and keep it burning well.  Good stoves are a big help, but even in a simple stove or hole in the ground, you can control the fire's efficiency by how you prepare the fuel, lay the fire, and adjust the fuel and air throughout the burn.

- Heat Delivery:  Separate the firebox from the heat delivery system.  Thermal mass heaters, aka masonry stoves, are exempt from the above regulation because they operate in a different way.  A short, bright, clean fire provides heat to a large heat-storage system, which distributes the warmth over hours or days.  Rocket mass heaters are only the most recent (and lower-budget) example in a tradition that has been developed over thousands of years, wherever people cared enough about fuel-efficient heating to build permanent heaters in their homes.

Answer to Smoke or Steam challenge from Part 1:
Is the chimney behind this cat putting out smoke, or steam?

There is definitely some water vapor visible in the plume.  It's a foggy day, so that makes it harder to tell - because the plume doesn't disappear as rapidly.

However, notice the residue in the thinner smoke behind the cat.  As the water-fog in the thickest part of the smoke spreads out and and evaporates, it leaves behind some bluish-white residue, visible over the cat's back and on the far left, as the smoke plume moves away from the chimney. 

There is also a hint of a sort of pearly-yellowish color in the main plume.

I would say this is smoke, but not nearly as dirty as the grey example in Part 1.  Maybe 80% steam, 20% smoke (that's just a guess).
It is coming from a woodstove where the owner has just added some fresh wood; as the fire warms up, the smoke may clear up a bit more and go back to being mostly steam.

The easiest way to tell, not visible in this picture, is to follow the plume downwind until the white fog is completely evaporated. 
Smoke leaves a bluish or yellowish streamer that continues on "forever" into the distance.  
Steam will usually evaporate until it completely disappears, unless it is a really foggy day.

If you like the idea of burning wood without smoke, consider taking a copy of our Art of Fire booklet on your next camping trip, or pre-order our Rocket Mass Heater Builder's Guide coming out in June, 2016.  We're opening our Kickstarter campaign tomorrow for pre-sales, and throwing in a lot of digital bonus materials.