[Stoves] Smokeless transition
Paul Anderson
psanders at ilstu.edu
Fri Dec 12 16:05:57 CST 2014
Crispin,
For anyone wanting to burn the char in the device, your explanation is
fine and is much appreciated.
The biggest difference in our positions is that I and some others do NOT
want to burn the char. Reasons can include wanting it for biochar, or
to burn it in a different device later, or to protect the fuel container
for the high heat of char-gasification, or for some reason of simplified
design such as using less expensive materials of controls, and probably
a few more reasons.
For centuries, one objective of scientifically designed combustion
devices was to obtain the maximum of energy possible, and that meant to
leave the minimum of carbon behind in with the ashes. But as you pointed
out, things have been changing.
I got started in 2001 with what has become known as TLUDs by Tom Reed.
Clean combustion was the feature. The TLUD stoves happened to make
charcoal. It was a "by-product" that became a "co-product" that for
some is the "main-product".
Paul
Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
Email: psanders at ilstu.edu
Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 12/12/2014 1:20 PM, Crispin Pemberton-Pigott wrote:
>
> Dear Paul and All the Rest
>
> From Paul: 2. When the MPF (migratory pyrolytic front) approaches the
> bottom, the radiant heat that goes downward has no "next layer" of
> biomass to dry, torrify and pyrolyze. Instead, it radiates to the
> grate and back to the already hot fuel, making it hotter and
> pyrolyzing faster.
>
> That is true but it is not really helping us understand what to do
> about the 'final days' of the wood gas phase and the beginning of the
> charcoal gasification phase.
>
> First things first. Does the designer intend the user intend to burn
> the charcoal, or make charcoal? These are quite different goals.
> Originally the use of wood gas was to overcome the problem of not
> knowing how to burn wood properly. Burning the gases only made a heck
> of a less smoke. In consequence there as charcoal produced. In the
> meantime some people want to make charcoal. Times have changed on
> several fronts so we should be clear what the intention is.
>
> If you are going to burn the char, then you should not try to make a
> lot of it. To optimise char production, you are not going to easily
> turn around and make it burn well. The conditions are different. My
> point is there is no need to overcome problems that shouldn't be there
> in the first place, if in the first place you have avoid those problems.
>
> Second, having decided that you are going to burn the char, you can
> ask if the original reason for making it as because you are avoiding
> PM creation by making and burning wood gas first? If so, then perhaps
> that line of thinking needs review. If you have very low emissions
> does it really matter how you got to that point?
>
> So, first take a lesson from John Davies' packed bed coal chip
> gasifier, the complete plans for which are on this website. He formed
> a secondary injecting cone that directs secondary air at the top of
> the fuel bed. The angle was quite high. One of the effects is to burn
> the char at the top of the fuel bed -- slowly -- which maintains a
> very high temperature at the top of the fuel stack. It is not
> necessary to burn a lot, it is necessary to keep a high temperature.
>
> The effect is that when the pyrolysis is complete and the switch is
> made to char burning, the flame is not maintained only by the gas
> combustion. This is the key point to solving the transition problem.
>
> There are 'among us' two ideas about gasifier building: make gas and
> burn it separately with no thermal connection between the fire and the
> gas generator, and gas burning within close proximity to the fuel bed
> so there is a strong heat exchange between them. The latter case is
> called 'close-coupled'. It is much easier to get the desired result
> (transition) using a close-coupled design.
>
> The problem of the flame going out (making smoke -- smoke being
> condensed combustible volatiles, water droplets and very small bits of
> carbonaceous fuel) is that the gas composition changes at the same
> time as the heat available drops, while the primary/secondary air
> split needs to change by a factor of about 6. Trying to burn the gas
> continuously without thermal support is just asking for problems.
>
> There are several solutions that support the flame. High temperature
> secondary air will limit the problem by keeping the flame hot. Sending
> it downwards into the top of the fuel will keep some of the char
> burning throughout the pyrolysis phase. It also heats the flame from
> below. Having a high mass combustion chamber with enough heat stored
> to support the flame during transition adds free heat at the critical
> moment, but /only/ if it is conductive enough to feed that heat back
> to the wall surface at a rate that stops the wall temperature from
> dropping. This was discussed in a separate post related to whether or
> not it was 'better' to have an insulative or heat conductive
> combustion chamber lining. Dean has always felt that it should be
> insulative so there is an early, fast rise in the combustion
> temperature. I have found that the advantage of that is brief and
> limited. If the combustion chamber has a mass of, say, 1 kg, and it is
> conductive enough to feed heat back into the fire by convection and
> radiation, it will support the combustion of the blast of CO that
> accompanies the transition. If the CO burns, the heat is maintained,
> and the flame remains lit.
>
> While this is still a good idea (a ceramic lining that can support CO
> combustion) it is not as good as close-coupling the combustion.
>
> Because the intention is to burn the char, there is no need to try to
> preserve it in the first place. Thus some 'standard' approaches to
> building the burner can change. The secondary are does not need to be
> fed in above the initial fuel level. The secondary air has to 'be
> there' by it does not have to be wafted over the surface in the
> initial stage. The fuel can remain at the same level and the secondary
> air holes can be below the fuel. This is only going to work if the
> fuel doesn't fall into the holes. See the posts about hole sizing for
> secondary air. If the fuel will fall through the holes , make sure
> there is an easy way to get it out later, and place the air exactly at
> the top of the fuel. If the fuel will not fall through, place the
> secondary air holes below the top of the fuel. As it burns the fuel
> will shrink and sink, uncovering the holes. They will blow secondary
> air across the char burning some of it and maintaining a very hot top
> layer. That was John's intention. In this way it does not have to be
> blown downwards (very simple to make). If you have fuel-in-hole
> problems then you have to use John's cone solution.
>
> You may need two or more rows of secondary air holes -- I use three.
> Some can be below the initial fuel level -- say 1/3 of them. That
> ensures the combustion of some of the char which will later maintain
> the flame during transition.
>
> As Dean has pointed out, there is a moment when the cook might realise
> that the combustor has changed (end of fuel approaching) and they can
> open the primary air. I find this works and in fact if it Is
> anticipated by perhaps 2 minutes there is a smooth transition, but
> there are two problems with this approach (even though people get used
> to it). The first is the cook may be busy and not notice. The second
> is that there is no need to do it. Using the 'burning paper' solution
> is very simple and easy and automatic. The paper covers the extra
> holes needed and as the fire approaches, it burns away allowing air to
> flow. This can be done before the MPF reaches the bottom by having the
> additional primary air holes on the side, above the grate, pushing air
> into the fuel. An advantage of this is that the air does not drive the
> combustion 'at the grate' it drives it into the fuel which is them
> combusted. Thus avoiding melting the grate and accomplishing the goal
> of burning with additional primary air.
>
> Time for a digression -- is the plan to burn cleanly, or to make
> charcoal and then burn it? You have to decide. Always try to avoid
> having to solve problems that should not be there in the first place.
> If the superficial velocity was higher to begin with, there would be
> more combustion of the carbon in the fuel during gasification.
> Close-coupled and having higher CO and a generally higher pyrolysed
> fuel bed temperature is an advantage, not a disadvantage. Don't be
> fanatical. MPF combustion is not magical; you do not get clean
> burning just because of that. You can in fact get very dirty
> combustion with an MPF gasifier, as with anything and any fuel. If
> clean burning is the goal, make the initial conditions much close to
> the final conditions and you will not have such a big transition to
> manage.
>
> Now, back to design if air supply: The primary/secondary air split for
> wood burning is 1:3. For charcoal it is about 3:1. That is a change
> of 9:1 but in practice you can't do that. I mean, you can't increase
> the primary 9x and leave the secondary the same. Won't work well in
> the same physical space because the power and temps will be really
> wrong. You have to cut the secondary and open the primary and it
> should happen automatically. How do you do that? With self-balancing
> air supplies. I can't explain that here -- it needs drawings and math.
> But basically you limit the total air and supply it through a channel
> that /would / feed it to the primary in the right proportion for char
> burning (final configuration) but which can't flow because some
> primary air holes are blocked. Therefore it becomes secondary air
> until the change is made. When the primary holes are unblocked, it
> taps the secondary air flow and changes the proportions automatically.
> Follow?
>
> There are several ways to achieve that so use your imagination. The
> goal is for it to be as automatic as possible. I hope to see at least
> one product using this approach coming to market in the next few weeks.
>
> A warning: if the fuel is itself a significant element of the control
> of the air flow, there are going to be problems -- unless the fuel is
> homogenised and completely predictable like pellets or briquettes.
> "Wood burners" will have to have control of the air by design, not by
> fuel packing, because wood is a lot less predictable.
>
> Final recap: If the desire is to burn the char, set up the initial
> design to make it less sharp a transition by running a higher carbon
> burn rate throughout the cooking session. Make the change in
> primary/secondary split automatic, even if it has to be manually
> initiated.
>
> Regards to all
>
> Crispin
>
>
>
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