[Stoves] A Premix Burner

[Kirk H.]

All,

The stationary fan blade which Chispin is presenting here has a very different function then the stationary fan blade which I presented earlier.  This one is placed below the fuel, whereas the one I presented is placed above the fuel.  This one attempts to create a swirl of primary air through the fuel, whereas the one I presented is intended to direct the flame into a longer path to give the flame more time to burn, for a more complete burn.  

I have questions about swirling the primary air.  Why give the cook the additional task and expense of inserting the paper?  How much swirl gets through the paper?  How much primary air gets through the paper?  How much swirl gets through the fuel which will tend to resist primary air swirl?  If the primary air is still swirling when it gets to the pyrolysis front, does it really have any effect different than vertically moving primary air?  The primary air flow is fairly small so how much swirl can be produced?  Can swirling the relative small quantity of primary air below the fuel help mix the gasses above the fuel?  The focus needs to be on burning the wood gas efficiently, this is the intention behind the pre-mix concept.  Does swirling the primary air somehow make better, cleaner burning wood gas, or in some other way improve combustion?  Would swirling the primary air improve the combustion in a TLUD-ND, TLUD-FA, or TLUD pre-mix stove?

Respectfully,

Kirk H.

From: Crispin Pemberton-Pigott
Sent: Thursday, March 21, 2019 5:25 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] A Premix Burner

Dear Friends

This is a photo of another version of a swirl inducer. I realise it is not something that can be easily hand fabricated but it does work as both a grate and promotor of vortices. Once started, they tend to self-perpetuate vertically, rather like a “dust devil”. 


It is made in three stages: blanking a disk, punching the blade profiles about 2 mm, then forming the outside lip and blade twist.

Installed it looks like this:

It sits on the top of the groove in the cylindrical combustion chamber.

The centre hole is to drop ash out of the middle where there is otherwise no opening. To burn pellets, the blades are covered with a disk of paper to lend support and then the stove is loaded and operated as a TLUD gasifier. When the fire reaches the paper, it burns through and the char drops into a container below, automatically shutting it down.

Regards
Crispin

++++++++

This is a festinating discussion about pre-mixing and flame color.  I do have some things to add here.  I don’t know that I will add anything to pre-mixing technology in this discussion.

Pre-mixing is excellent for burning clean.  It is not necessary, but it is excellent.  I believe it is restricted to TLUD-FA stoves.  My design goal for higher power in TLUD-ND stoves is rapid mixing and burning.  This is fairly easy to achieve.  If the burner is designed to provide lots of surface contact between the wood gas and air as they meet, and yet keeps the gasses concentrated and hot, the result can be a very fast and clean flame.  Adding a little late secondary air (adding secondary air twice) can clean the flame even more.  One must be careful not to add to much secondary air overall.  My thinking is that the first secondary flame uses the heat from the easy to burn gasses to crack the long chain hydrocarbons, and adding a little more late secondary air burns the newly cracked hydrocarbon gasses.  Things I look for in my large burners are; providing lots of immediate surface contact between the wood gas and air, concentrating the heat into a heat reservoir (with high enough temperature to drive the cracking of the hydrocarbons and enough quantity of heat to feed the endothermic cracking reaction), allowing time for the hydrocarbons to crack, adding a little late secondary air to burn the new hydrocarbon gasses, allowing time for these new gasses to burn, using the pressure difference between the atmospheric air and lower pressure wood gas to its fullest potential, and enhancing the pressure difference between the wood gas and air to help drive them into each other.  This arrangement works fast and effectively for my TLUD-ND designs, so the more difficult to achieve pre-mixing is not necessary for clean burning.  

The color of the flame has some barring on my designs.  For a TLUD-ND I look for a yellow flame.  Yellow indicates a flame that is hot enough to burn the carbon particulates, and yet is fairly easy to achieve.  Dean Still at Aprovecho has said that a yellow to orange flame can be very clean burning.  I have found this to be true, and it is much easier to achieve in a hydrocarbon flame than blue.  Attached is a picture of a yellow flame that is very clean.  Note that it finishes burning while it is still hot inside the stove (minimal soot, smell, and smoke).

I am looking forward to learning more about pre-mixing in a TLUD.  Perhaps my views will change.

Kirk H.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20190321/0f668d94/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.png
Type: image/png
Size: 283482 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20190321/0f668d94/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.jpg
Type: image/jpeg
Size: 17882 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20190321/0f668d94/attachment.jpg>