[Stoves] Controlling the Primary / Secondary Air Split in ND-TLUDs

Crispin Pemberton-Pigott crispinpigott at outlook.com
Fri Jan 29 17:35:28 CST 2016


Dear Julien and All

 

I think I can group your observations into two topics: First, the fuel sets the need for the air split, and it operates under a certain set of conditions and temperatures, largely. Second, varying conditions require a different split for different conditions, even when the fuel is the same.

 

Is that agreeable? So we can answer them one by one – first to establish how to create a split that works well enough to maintain an excess air level between, say, 80% and120% which I think are good targets for simple stoves. There is no magic number, but I have seen a very good set of down-drafting parts that maintained an EA of 105% across the full range of power on the stove, using a single controller. So in principle it is possible.

 

The change in fuel ‘need’ between a char making and char burning state makes a really interesting challenge. Two things come to mind. One is that holes with air being passed through have very different reactions to a change in flow rate it he edge is treated differently. A rounded edge and pass much more than a sharp edge at high flow rate, but at a low flow rate they are very similar.  So in principle by selecting hole and forming the edges (tapers etc) one can have a split that changes with a change in draft.

 

I have proposed before how to make a large change in the air flow rate to convert from char making to char burning.

 

Here is one implementation (Java)

 



 

It is a piece of paper that blocks most of the air to the fuel bed, which burns away when the fire gets down there. It then opens up the airflow to a maximum determined by the fuel packing.

 

That doesn’t address the issue of you manually change the burn rate, it just deals with the conversion of fuel types.

 

To make a single control change the flow rates of primary and secondary is an ideal that starts with ‘something better than having one side open’. That has to be done by channel restriction, either in the channel size or a hole somewhere. I plan to use a single sheet of metal with two holes in it pushed into a received that is a gross control over air entering the coal stoves in Tajikistan. It would be the same for a wood stove.

 

The idea is to have a controllable entry channel, and a ‘splitter’ that has to orifices which set the air split for upper and lower. That at least starts to offer the ability to change the splitter depending on what the fuel is, and also to have gross control over the power without having to tune two different regulators.

 

If one went from wood ships to pellets to coal pellets to char pellets to wood sticks to dung chunks, the appropriate splitter would be inserted into the channel. Then a controller would operate in front of the splitter.

 

Lastly, the use of weighted draft controller mechanisms offers a way to use weights and flaps to change the airflow according to the draft at that point. 

 

I will toss in one more picture from Bishkek. I found this on a Russian coal stove (very advanced combustion). It is a wax-powered mechanical device. When the wax expands (like inside an engine thermostat) it has enough power and movement to operate a lever that opens a large steel flap on the bottom of the stove to let in more air. When the water temperature drops, it opens the fire air door mechanically using the chain. Really impressive and simple.

 

Regards

Crispin 

 





Dial the temp you want as a minimum, adjust the chain.

 

 

 

Hi Crispin and all;

In an earlier posting on “Riser Height and a 'Counter-Current' Woodgas Burner - YouTube Vid”:
 <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2016-January/011227.html> 
http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2016-January/011227.html

Crispin said:

“...

The split between primary and secondary is uncontrolled, for all intents and purposes, and should not be. In the same manner that you advocate at the end that reducing the area of holes through which the primary air can enter, this should be applied to the secondary air as well, arranged in such a way that the incoming has to go to one or the other. It should not be controlling one and then the other is free to flow more or less depending on the riser height. In short, don't use a macro structure as a fine tuning air control mechanism.

There are several products that work in that 'controlled split' manner though they are hard to find. The point is to decide in advance what the pri/sec split should be, create it, test it to be sure it is correct, then scale the draft accordingly. If you were to conduct this test on a SeTAR type real time test bench, it would provide a very useful set of excess air and CO/CO2 ratio charts the quickly identify the ideal conditions for the pri/sec split. This will be independent of the riser height. In such a case the riser height will affect the power, not the split, then the power can be regulated by the gross control of the air entering. This is a different approach taken be nearly all the TLUD designers so far which is to regulate the primary and let the secondary run free according to flame temperature and architecture.

If you have control over the entry and the correct split by construction, the riser height is used for total available draft, and for directing that energy (a risen has 'power') to the correct mixing of the air and gas, not the flow of secondary air.

...”

Having a single user control valve for air entry into a stove, then having the design of the determine the proportion of secondary/primary air is an interesting idea.  I can see how this could be obtained in part by the relative magnitude of buoyancy along the path of secondary air vs. primary air, with the possibility of some constricting holes to help guide the split to the best ratio.

Clearly, the method you propose has had some success.  However, I still wonder how a mechanism could handle a few challenges.

===============

1) Turning down a ND-TLUD is very sensitive to small amounts of primary air, because it doesn’t take very much primary air to support a flaming pyrolytic front in a small fuel pieces like pellets or nut shells.   Would it be difficult to get precise, and responsive control with a general air regulator?

2) As the stove is turned up, the proportional demand for oxygen in the reactor increases and the gas burner decreases.  Tom Reed reported for the “Turbo Stove” burning pellets that the ratio secondary:primary air changes from 6.2 to 3.1   (Reed, TB; Anselmo, E; Kircher, K.  2000.  Testing and modeling the wood-gas turbo stove.  Presented at the Progresss in Thermochemical Biomass Conversion Conference, Sept. 17-22, 2000, Tyrol, Austria).   Can changes in relative magnitude of buoyancy in the gas burner vs. reactor be used to determine this?

3) Although #2, above talks of a forced draft TLUD, we can still get large differences in the requirement for primary depending on the fuel we are burning.   Wood chips are more reactive than pellets, even though they are the same thickness.  

The big effect, I believe comes as the thickness of fuel particles goes from 5 mm pellets and chips to 3-5 cm thick pieces of wood.  The nature of the gasification reactions changes, and increase the demand for oxygen.  Pellets produce an ideal migratory pyrolytic front.   That is because the heat generated from partial oxidation of pyrolytic gases is sufficient to pyrolyze the centre of the pellets.   The thickness of the migratory pyrolytic front is not much bigger than a pellet.  However, as fuel thickness increases, it takes longer for the centre of pieces of wood to reach pyrolysis temperatures.  The outside of a piece of wood can be 800 °C when the centre is still around ambient temperatures, because wood, and especially char, are not great conductors of heat.  As a consequence, more primary air is needed to generate more heat to complete pyrolysis.  That means more combustion of char, which demands a much higher flow of air than woodgas.   

In short, when the fuel is pellets we have an ideal TLUD; when the fuel is large pieces of wood, the TLUD reactor becomes closer to a typical combustion stove, having char and woodgas combustion occurring side by side.

Some of what I am talking about can be seen in graph I have attached, from an experiment I conducted in 2014.  The reactor temperature range for pellets was between 550 and 750 °C, so there was not too much char combustion happening there.  However for 3-5 cm pieces of wood, the reactor temperature was between 800 and 1200 °C; those are char combustion temperatures.  The diagram shows that the fuels used in my experiment fall into to groups (1) pellets and wood chips, and (2) large pieces of wood.

Char combustion generates a lot of heat, so that would increase buoyancy to draw primary air.   The question is, would a single air regulator and primary/secondary air splitter be able to accommodate the changes in combustion reactions was we go from 5 mm to 5 cm fuel?

4) There are differences in resistance to gas flow in the fuel beds composed of different materials.  That means that the more of the buoyant force generated in one fuel bed could be expended in overcoming friction than in another fuel bed.  

5) Sometimes the fire in the reactor has reached the grate well before all the fuel above has pyrolyzed.  This happens ignition front follows channels in the fuel bed, or has reached the grate via falling embers.   With woodchips, unless there are big spaces between the chips, the ignition front will almost always channel down the sidewalls of the reactor.   The resultant woodgas is hard to ignite, and we get a lot of smoke.  

The operator may be able to resolve the smoke problem by a large increase in primary air.  Could this be expeditiously done by a single air regulator and a primary/secondary air split?

===============

So there are the challenges.   I suspect that part of the solution is in having ND-TLUD stoves designed for specific fuels; namely, (a) pellets and nut shells,  or (b) pieces of wood.   This is a reasonable thing to do, because the reactor for pieces has to be wider and twice as long as for pellets.  Not many people like to wait around for 2-3 hours while 30 cm of pellets burn.

Cheers,
Julien.

-- 

Julien Winter
Cobourg, ON, CANADA

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