[Stoves] Smokeless transition

Fri Dec 12 13:20:42 CST 2014

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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