[Stoves] central secondary air

[Crispin Pemberton-Pigott]

‎Dear Ron

There two issues I have seen with the central pipe method of supplying secondary air. I have of course tried it on several designs to see what the effect is for updraft, side draft and downdraft configurations. It is often suggested as a solution to an air supply problem‎ - the problem typically is that of poor mixing. I can describe the most typical case as follows:

Consider a TLUD with a diameter inside the fuel chamber of 200 or 250mm. The designer, having read material on line, places a number of holes above the fuel to let secondary air in. The holes are small and many, copying photos found here and there.

The unit makes gas, has a tall flame, burns clean to the eye and it is claimed to be a success.

Putting a pot on it chills the tall flame, it smokes a lot and the flames come out well past the pot. The heat transfer efficiency is low, maybe 30‎% and it makes a lot of charcoal. The fuel consumption is as high or higher than an open fire. The designer reads that it is "ok" to deduct the char energy and report the energy used in the form of a 'dry fuel equivalent' which is of course interpreted to mean the 'fuel consumption'.

Now we have a stove that in use (not just burning on its own) makes a lot of smoke and CO, saves no fuel, costs more and is much less controllable than an open fire, and needs special preparation of fuel each time it is used. That, Ron is a VERY ‎hard sell, save for one thing: so many claims for superior performance have been made for stoves like this that they get attention, sales and subsidies. I am from time to time asked to 'fix it'.

What to do? Well the obvious problem with these stoves is the tall flame which is directly caused by poor secondary air mixing. ‎I have written about it a lot as you know. If it is not preheated it chills the gasses and the general result is worse. Adding preheating is not enough because the problem was mixing, not always temperature. Adding a 'concentrator disk' is a compensation for poor mixing. It is an extra part which burns out quickly. It does not necessarily reduce the flame length required to burn completely. The target is to reduce the flame length which to get it completed before it hits the pot. Many people assume this is not necessary, assuming that the heat 'gets into the pot' best when the flames are running against it.

To get the flame length down is helped by the concentrator disk but not necessarily if it delays the combustion during the mixing process. What is ideal, everything else remaining the same, is to place a pipe in the centre that feeds air into the centre, passing through the fuel and providing the critically needed air in the centre.

Different methods of doing this are tried: a simple tube with an open bottom and top passing through the fuel as is done by Roger Samson in his rice hull gasifier: the Mayon Turbo Stove  - one of the first available designs. He uses two open tubes‎. They shoot secondary air into the gas stream well below the pot. It is a downdraft gasifier with a moving pyrolytic front. It has continuous char removal, can be refuelled and is very clean burning. It has been manufactured in Third World conditions for a decade, I think, by now. It surprises me that this design is not used more because it is an available product that uses rice hull and doesn't require a fan and can be operated continuously. Anyway...I worked on improving it at the request of Roger and it involved reducing the amount of air passing through and spreading it around and reducing the flame height. It still remained an uncontrolled secondary supply.

A problem introduced by this approach is the pipes are corroded easily because they are in a very hot zone.

A capped pipe is often used as an alternative. ‎Holes are usually drilled in the side and the air is bled into the centre portion of the gas stream. This has the right effect but let's in more secondary air as the fuel uncovers the holes - a significant problem. Because most people can't check the excess air level, they don't know when the conditions are good. The stove is declared clean anyway.

Two problems are introduced by the ‎stove add to the corrosion problem. They are the variable secondary air supply that is technically difficult to control and the creation of a donut-shaped fuel charge. The ring-shaped burn is a continuous source of problems. It is much harder to light 'evenly' as there iis no 'centre' in which to place the ignition material. The fire tends to get going on one side first and it takes time to spread to the other side, and even longer to burn evenly. The tube is a source of cooling right at the place where it should be hottest. When the stove burns wood, it has to be chopped and stacked in such a way that it is evenly spaced around the pipe or else one side finishes well before the other. It makes Everything harder to burn consistently.

The use of a pipe with holes caus‎e the least problems with pelleted or very small fuel largely because the problems with fire distribution tend to be minimal. Lighting is accomplished with a liquid splash and the main issue is only the variable airflow, lack of control and corrosion.

This of course required a processed fuel, pellets or rice hull. Wood, with a central pipe, is hard to get to run properly.

Usually the addition of the pipe is not accompanied by a reduction in the supply from the side. The typical result is a much shorter flame‎ but with higher excess air. This is also declared 'improved' (and it is, I guess).

The best way to use a central pipe is with a flat, elevated top, and no holes in the side. The biggest remaining problem is uneven burning taking place on opposite sides of the pipe.

One could make a plan for controlling the air getting to the pipe which would allow the power to be controlled while maintaining the correct primary/secondary split. Bit is is not easy to do in a way that is cheap and convenient to operate.

What else can be done to overcome these problems - the lighting, uneven burn, variable flow, uncontrolled flow, corrosion and cost?

Put the secondary air in properly in the first place, from the outside!

If the secondary air was fed in correctly, there would be no need for a concentrator ‎disk, no central pipe, easy control, self-balancing primary/secondary split, easy central ignition, self-spreading fires that balance well and quite importantly, a short flame without high excess air.

I described in detail how to alter the design shown to the group a couple of days ago. ‎I estimated the EA% and suggested a target value and hole sizes to achieve it. The approach works for all fuels. One of the biggest advantages is the creation of the same effect as a concentrator disk using only airflow.

The swirl you speak of can be created using this air entry, avoiding if at all possible, any metal in the gas or flame path. Metal chills the flow. End of short story. It makes it harder to burn really clean. Metal corrodes. Metal costs a lot. Metal requires assembly. Why bother?

Paul: message for you. Don't you think the Mayon turbo stove has all the features you are seeking in a char maker? It seems to me to do a lot of things that are oft-stated goals.

For TLUD builders I can report (now that it is public) that there are a couple of models commercially available which are controllable, used wood fuel, have very low emissions, make char and exceed the minimum 25‎% system efficiency requirement of the CSI-Indonesian Pilot program.  Thus when calculated correctly and built well, stoves can still be char-makers and save fuel (compared with an open fire).

Regards
Crispin



Crispin,  cc list

        1.  Yesterday you said this:
        Personally I don’t think a ratio is a good design approach. You need a certain vertical space to burn the gases before you attempt heat transfer. A total of 125-150mm is normally OK assuming you have the fuel deck covered with secondary air and it is burning hot near the fuel bed. If you bleed in secondary air through a lot of small holes, there will be no secondary air in the middle. Some try to compensate for this by putting in a vertical tube. That tube is a sign that he secondary air entrance has not been handled correctly. It is a common ‘solution’ to a problem that should not be there in the first place. Try to avoid it.

        2.  Re the “vertical tube”:   I have been involved in several off-list discussions where central secondary air seemed to be providing turbulence benefits  (benefits that including reducing the height and therefore lowering costs.   A central pipe can also prevent gases from rising in the center, which can be a benefit if one has swirl.   Can you give a cite on where a central secondary pipe was shown to not be of value?

Ron
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20141217/ee38108f/attachment.html>