thinking about secondary burn

[electrondady1]

i like the idea of a secondary burn chamber on an outside wood stove.

 starting with a conventional stove,

by placing a fan on the end of the exhaust /chimney i can make it a draw through design.

the rate of combustion could be controled by a fan speed control

it would require a controlable secondary air supply with an aperture just downstream from primary combustion chamber.

it would need a balance control on the two air intakes.

i'm thinking that to initiate a fire only the the primary air supply should be open to ensure a good draw.

and once a fire was going the secondary burn chamber could be supplied with more air.

 any thoughts?

 

[smidy]

In my woodstove there is secondary burner, the place where I put the wood is very well insulated (no cooling in that place). Air is coming under the wood.Then over the wood and fire there is a cone whith many small holes where air is forced in whith a fan, so the air in the cone will heat up, so the "smoke" from the fire will ignite and burn almost like a oilburner. From there the gases enter the heatexchanger and to the chimny. Here is a link to the maunfacter http://www.combiheat.se/vedpanna.htm it is in swedish but it is a pretty god picture there telling the "magic"

[electrondady1]

tack vare smidy

ren skorsten?

[smidy]

do you mean if my woodstove dont mess up my chimny whith tar and stuff? yes it burn very clean and completlty smokeless when the fire realy started, and the place where the wood is stay very clean, probably of the heat, when in full action the ceramic walls in the wood place are red of  heat! 3 loads of wood (dry nice wood) bring my 2k liters hotwatertank up from 30 to 90 degrees celsius.  

[Okiezeke]

Hi,

Secondary combustion is a proven concept, but the manufacturers dont want us to know how both because they want to sell us a thousand dollar stove, and because there is no way we could afford to have a DIY UL tested for "legal" installation.

In order to get secondary combustion we need the following:  

A consistently hot primary fire.  To get this a larger woodstove is helpful, something with about 10 cu ft or a little larger.  Full firebrick lining of the combustion chamber is another way to keep it hot.  Two methods used by the commercial guys are pre heated combustion air and non-airtight design.  To preheat primary combustion air, they usually force the air to pass through a foot or longer pipe/passage that is heated by the fire.  Non airtight design means that you cant(or wont) completely shut down the primary air supply.  Always let in enough air so the fire has some healthy flames to it.

A way to introduce heated secondary combustion air to the smoke.  This air must also be heated.  Smoke wont burn much below 500f, so cold air almost never works.  The DIY crowd likes using black steel water/gas pipe like they sell at Lowe's.  This stuff is heavy enough to weld easily and holds a lot of heat to warm the air passing thru.  I use 1/2 in pipe and drill 3/16 holes about every inch to introduce the air to the hot smoke.  Will need some kind of valve to control the amt of air entering the secondary combustion area.

A secondary combustion chamber:  Not all commercial stoves use a separate chamber for secondary combustion, but for us it increases the chances for success.  One easy way is to include a smoke baffle that is sealed at the back and sides, and forces the smoke to travel to the front area of the stove before going up and out the pipe.  The baffle is full width, and 2-4 inches from the stovetop. I make the secondary combustion chamber the width of a firebrick to make it easier to insulate.  Many commercial stoves insulate the inside of the stovetop also, but this is difficult to do and usually just falls off anyway.  If you want to try, ceramic wool is what they use, but there is really no effective glue so you might weld on some kind of wire or mesh to hold it up.  I use 1/4 in. plate for the top so it takes a long time for it to burn through.

The heated secondary combustion air is introduced to the smoke on the bottom of the baffle, with the holes pointing down.  In theory this allows the smoke to be burning by the time it gets to the chamber on top of the baffle and done burning by the time it hits the flue so it doesnt melt the flue pipe.  In my last creation I uses 4 pieces of 1/2 in steel pipe welded to the bottom of the baffle, and passing out the sides of the stove.

A very good article is hearth.com/.../index.php/wiki/convert_your_old_1970_stove_to_secondary_burnHow_To_DIY.  (If I copied this wrong search DIY secondary burn and it should come up.)

I havent been able to install my latest creation, but will soon and I'll let you know how/if it works.

Okiezeke

[electrondady1]

okiezeke, that is very helpful information.

thanks for the link

[Okiezeke]

I finally got my 500 lb monster installed.  According to my laser infrared thermometer the stovetop was about 600 f. and the flue was around 350 f.  This is an indication that combustion is occuring in the secondary burn chamber.  Only ran it for one night and wood consumption was higher than I has expected, but I have 7 different air sources built into the thing and I will need to do some fiddling to get everything balanced.  At least it shows that secondary burn is not out of reach for the DIY crowd.

Zeke

[WoodstoveWizard]

Okiezeke:

Sound like an interesting setup - I presume a stove this size is powering some substantial heat storage of some sort? Hot water tanks or the like?

One tip I have seen, taken from boards where people discuss wood gasification (to power cars for example) is that directing the flue gases through the hottest part of the fire before letting the secondary air in will aid the secondary combustion stage substantially.

You need to arrange it so that flue gases from combustion pass through the bed of embers (the hottest part of the fire) before reaching the secondary burn chamber. This takes the smoke and further disassociates it down to much smaller (and easier to react) molecules and atoms.  By the time the wood gas reaches the burn chamber it should already be completely clear to the naked eye (no large smoke particles) and will be mostly made up of CO, H2, CH4 etc... all of which ignite easily.

I use a similar principal when I make  biochar at home. I load the drum fully with green woody garden waste and then light the top with some dry kindling/twigs/newspaper.

Within minutes the flames spread across the whole top cross section of the oil drum so that any wood gas produced beneath must pass upwards through the hottest part of the fire (the embers on top of the pile of fuel). This arrangement produces a phenomenally hot column of exceptionally clean burning flames.

In essence the "secondary burn chamber" is the open air above the oil drum.

By contrast an arrangement with the embers at the bottom and new fuel put on top burns very smokey, if you can get the wood gas/smoke to combust at all.

So my advice is to arrange the air flow through your primary combustion chamber very carefully before you work with a secondary burn - it will make the seocndary burn much more reliable and effective.

[Okiezeke]

Woodstove wizard,

All sound like good ideas.  This stove was built to test concepts,and I can probably include some of your ideas in my next one.

I am using this stove to heat a poorly insulated 12' by 32' off the grid cabin in the Oklahoma "mountains".  While It missed the coldest weather of the year, it does seem to be up to the task.  I have solar/gas hot water, and solar/wind (w/ diesel backup) electric power and am thinking that wood is a relatively inefficient way to heat water.  Any sunny day the 40 gal solar water tank is plenty warm for household use, but it sure would be nice to get rid of the propane tank!  The old steam railroad engines used wood, but I'll bet they used in a day what I burn in a year.