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<div><font style="background-color: transparent;">Very clear. Well said.</font></div>
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<div style="color: black; font-family: arial,helvetica; font-size: 10pt;">-----Original Message-----<br>
From: Crispin Pembert-Pigott <crispinpigott@outlook.com><br>
To: 'Discussion of biomass cooking stoves' <stoves@lists.bioenergylists.org><br>
Sent: Wed, Feb 26, 2014 7:22 pm<br>
Subject: Re: [Stoves] diy TLUD flame<br>
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<div class="MsoPlainText">Dear Juanito</div>
<div class="MsoPlainText"> </div>
<div class="MsoPlainText">>Meanwhile, may I get the impression that this issue is not settled. What is the critical ingredient: fan? chimney? concentrator lid? washers?</div>
<div class="MsoPlainText"> </div>
<div class="MsoPlainText"><span style="color: black;">There is no magical answer - many things 'work'. What is perhaps a source of distraction are claims that 'only this' or 'only that' works. I see that a lot that works and a lot that doesn't. </span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">In order to build a decent stove it is important to first get the air flows under control. By that I mean the primary and secondary and excess air. If you put any more air into a fire than it really needs, there are negative consequences. If you build a stove that already has far too much air passing through the fire, then you will be really misled, or self-misleading, to assume that anything that 'makes it work better' is really doing much if the basics of the combustion are not right to begin with. It is like putting lipstick on a pig.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">So let me say this conditionally: when the primary and secondary air flow are right, it does not take much draft to get a fully mixed and clean burning flame. A rule of thumb is that the diameter can be as large as the draft vertically above the secondary air inlet. But this has to be qualified further by saying that there is an upper limit which is governed by the physical properties of air and hot gases (which are very similar, actually). It is hard to get a 300mm diameter gas path well mixed in a short vertical distance.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">You can use a concentrating ring to produce a smaller diameter gas flow into which you can run secondary air. Unfortunately most advice on the internet has the secondary air entering below that disc, so it is difficult to get the secondary air flow into the centre of the gas column, even if there is a relative small diameter hole in the centre. The result is a fire near the edges and a long thin flame rising from the centre - a diffusion flame with no O<sub>2</sub> in it.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">If the secondary air is admitted above the minimum diameter point, the problem disappears, but nearly no one is advocating this. Jock is doing it right, though perhaps not thinking of it in these terms but that is <i>why</i> his layout works well. Instead, I see people inserting a central air tube that passes through the fuel and bleeds secondary air into the gas stream near the top centre. This helps, but it is helping to solve a problem that should not be there in the first place. It causes all sorts of other problems, particularly relating to creating a controllable fire that can be turned down, and accidentally creating a fire that progressively increases in intensity with time. Many TLUDs suffer from these two problems but there are others issues as well.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">John Davies in South Africa worked with packed bed combustors (gasifiers) which is a special case of TLUD where the airflow is controlled entirely by the fuel. Chopped biomass is not suitable for such an approach. He worked with chipped coal and it seals rather well providing true pyrolysation for 2 hours then char burning for 4 hrs. A lot of TLUD builders have complained about material problems when burning char after pyrolysation is complete, but they are doing this with far too much primary air. If they were going to burn the char, and knew that in advance, they should have used a higher superficial velocity throughout the whole burn.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">It is complicated. The bottom line is that most things said are true part of the time and most TLUD's suffer from one problem or another. One way to get around this is to use a controllable fan as a substitute for auto-balancing air. That is a good solution, but requires a fan and power.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">One of the misleading claims for TLUD's is that they are automatically 'clean burning' because they are creating gases. This is simply not true. It is quite possible that they can burn clean, but I have seen lots that only do that under very particular circumstances. With a slightly different fuel or fuel size, they burn with a lot of smoke and CO. That is why there is so much discussion about fuel preparation. If the airflow through the fuel is a major portion of the control of the fire, instead of the stove architecture, it is fuel-dependent, not fuel <i>independent</i>. In consequence, the variability of the firepower suffers because you would have to vary the fuel to get a different firepower. Or blow air into it with a fan.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">Thanks for your interest.</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
<div class="MsoPlainText"><span style="color: black;">Regards</span></div>
<div class="MsoPlainText"><span style="color: black;">Crispin</span></div>
<div class="MsoPlainText"><span style="color: black;"> </span></div>
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