[Stoves] A Premix Burner

Sat Mar 16 21:10:34 CDT 2019

Dear Aaron

Thank you for providing the video links and bringing your experiments to this discussion. There is always something to learn from these investigations.

I am making some assumptions so please correct me if this is not the right sequence.

https://youtu.be/p_01Uy6sEl4<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2Fp_01Uy6sEl4&data=02%7C01%7C%7Cb8045918cd0243222f6508d6aa598d18%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636883698924878569&sdata=stl1ni2bcb8Sr6fKw%2FwR5qQ0eq9ktzhbMTXErFxA60g%3D&reserved=0>
FA - Secondary Combustion Chamber

The first video is the short, loud one “making char”. There is a lot of turbulence but still, it is possible to see some of the flame characteristics.  The general appearance is that of translucence. This is a good sign of a clean burn. The orange colour is caused by hot carbon particles with a short lifespan glowing as they evolve. The colour is from the glow of the particle, not the colour of the flame from combustion itself.

At the top can be seem slightly orange flames coming out at high speed. These appear and disappear rapidly as they cook in the ambient air. A judge the flame to be extinguished by cooling before the gases have completed the burn. In that case you can expect there is some particulate matter escaping before they finish oxidizing.  In that case you will also have some CO.  I would like to see the fan turned down then increased slowly to observe how that flame changes as the air supply is increased. It is probably cleaner at lower fan speed.  In general we can notice the flame is short of secondary air because it is still a diffusion flame as it hits the ambient air (still looking for oxygen).

https://youtu.be/25GRrEi9u74<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2F25GRrEi9u74&data=02%7C01%7C%7Cb8045918cd0243222f6508d6aa598d18%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636883698924888568&sdata=i9fsGkJr5lsFhoqR3jqFp3BzGADxbhv%2Ff2om2QYaiEI%3D&reserved=0>
ND-TLUD with Secondary Combustion Chamber

>It’s been difficult to fully prevent backfiring if these stoves by sizing the openings properly but I do have at least 1 that works dependable.  … The forced air version does not have backfire issues cause the primary burn chamber has a bit of flame in it.

There is an important difference between the oxy-acetylene flame and the producer gas flame which is the mixing ratio at which it will burn.  Acetylene has a wide range over which it will burn and the flame varies. Producer gas is more demanding about the air-fuel mix. “Natural gas has a very, very steep curve for flame speed. This is why the smallest primary air change makes so much difference in flame appearance.” Here<https://forum.heatinghelp.com/discussion/86886/flame-speed-question>. “Other factors affecting flame speed include the temp. of the fuel and primary air, burner temp., size/shape of burner or combustion chamber and oxygen % and pressure at the flame front. The charts so often quoted show the curves indicating stoichiometric mixtures at the peak. They don't take these other factors into consideration. If you burn a quantity of NG in the open, it burns rather slowly.”

The diameter of the devices is rather large, and the combustion area much larger than it needs to be. In the second video there is a large open space in the centre. It is likely there is descending air in that centre region. Hold a smoking stick over the centre and see if the smoke wafts downwards.

The gas emerging is largely CO, which is why the fame is blue. That is the colour of the flame, not carbon particles glowing. It is quite a cool flame due to its quick dispersion, so the CO is probably higher than it might be if the flame as more confined.

https://youtu.be/J5jJcpvNuRY<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fyoutu.be%2FJ5jJcpvNuRY&data=02%7C01%7C%7Cb8045918cd0243222f6508d6aa598d18%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636883698924898561&sdata=V8NE%2FGB41mvOEs8CIN6QPA5vlptuJ3cWtN4Op6V3yKI%3D&reserved=0>
ND - TLUD with Secondary Combustion Chamber and a strong vortex

In this video there is a chimney effect which is providing the turbulence and slowly rotating flames around the edge of the cylindrical riser. There is far too much air entering the tube, and the tub mi much too large for the amount of gas being burned. As a result the flame is hugging the wall and the combustion temperature is low. The formation of particles is evident from the orange colour which is in patches that are not attached to the bottom not the top of the flame. That is an important indicator. Look at the flame carefully and see if you can identify where the orange starts and ends.

Part of the flame is blue then it goes orange, then blue again . There is nothing visible in the video in the centre, but in person you may see some translucent flame in the centre because every now and then there s a small amount of blue or orange flame. If there is consistently some barely visible flame in the middle then I withdraw what I wrote about there being far too much air, there is just too much.

The mixing is strong which is good. If the diameter was say, half, flames might occupy the whole tube (1/4 of the cross sectional area). If in that condition the flame emerges from top “looking for air” then the tube is too small.  It appears that there is an air entrance at 10 o’clock where there is a distinctly blue flame all the time.  That is a CO burn at that point.  If you cut the flow of air passing into the hole in half, what happens?  The orange glow training the blue entrance flame indicates the fire is cold – possible because the oxygen on the outside is being used but the central portion of the flow is not – it is carrying forward, cooling and ultimately spoiling the combustion, creating carbonaceous particles in a rapid process of formation and destruction. That is the orange part.

You could reduce the hole diameter, block it partly, or reduce the draft by shortening the vertical tube. The effect would be the same: reduce the incoming air flow and raise the temperature.

If I was with you I would measure the oxygen concentration in the exhaust. If it was below 10% I would concentrate effort on sizing the tube optimally, rather than changing the air-fuel ratio.

> The design of both TLUD's is very similar except the old one has a
> secondary combustion chamber that is made from 20ga stainless steel,
Just a small note to remember: 20gge stainless steel conducts the same amount of heat through its thickness as 10gge mild steel. In effect, stainless steel is an insulator compared with tincanium.
>The pecan shell feedstock was identical in both units with around 13% moisture.
If you had much more moisture, you would see some pinkish colour in the flame, provided there was no orange at the time.
> On the subject of premixing the pyrolysis gas with air it occurs to me
> that the premixer must be separated from the combustion chamber with a
> tube or orifice that is small enough to increase the velocity of the
> cooler mixture and long enough to prevent the premixed from becoming
> too hot, for the purpose of prevent the secondary flame from
> backfiring into the premixed chamber.
I think this idea is misplaced. You never win by cooling the gas between the reactor and the burner in a close-coupled situation. That heat loss is a loss overall. You have to use some of the fuel to bring it back. It is not wrong to have a flame in the region of the top of the reactor. There are some signal advantages to this and only one disadvantage. The main advantage is that the whole flame is far less likely to go out if there is a set of flamelets burning just above the fuel pile as it is possible to keep the upper surface very hot (red) possibly burning some of it in the process.  The single disadvantage is that it might burn some of the char which you may (or may not) be trying to maximise.
All things considered, it is probably better to have a reliable flame than a better yield and a flame that goes out now and then. One of the criticisms of TLUD gasifiers is that the promoters walking around tossing lighted matches into the gagging column of smoke that rises from the burner that just went out. No one wants to have that.
Stoves like the one research by Riaz Ahmad in China (PhD candidate a CAU) have the gas reactor completely separated from the burner by several feet. It is a TLUD but has a start-up phase where gases are vented without burning, then an operating phase, then a shut-down phase. All the “mess” is outside the kitchen.

Please keep posting these videos as you continue experimenting. Please show some failures as well. What doesn’t work is frequently very helpful to others. I had a great comment from Wojciech Treter. After he did some work on a grate and it failed, I told him about earlier work he was unaware of, which had the same result. He said he was “independently recreating the square wheel.” That’s pretty funny. We can learn a lot from failures just by asking. There are a great many things that do not work very well!

Best regards and best hopes

Crispin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20190317/0c2ef13f/attachment.html>