[Stoves] Secondary air in Rocket Stoves?

[Crispin Pemberton-Pigott]

Dear Marc

 

The primary problem is that side-fed stick burning stoves typically have no
primary air control. Unless the air is brought under control, there is no
easy way to address the issue of excessive excess air (if that is a
permissible phrase).

 

The approach taken with the traditional Keren stove which is being improved
by GERES and YDD in Indonesia (very popular clay product) is to limit the
airflow using the pot-stove clearance. This sets a limit on the total air
flow without making any improvement to the way secondary air is added - and
it literally costs nothing to implement. The pot becomes part of the
combustion chamber, basically.

 

Adding jetted (fan as per U Colorado experiments or natural draft) to a fire
that already has too much air is going to increase PM and CO under some
power conditions. The solution is to limit the ingress of air. 

 

Making a stove taller to create the draft needed for useful secondary
injection only adds to this requirement, though it can create the necessary
mixing. Secondary air should only be added to a stove that is running short
of air, and the designer should deliberately create the conditions in the
chamber where there is such a shortage. That is why the TLUD gasifiers are
so clean - there is almost no way to get excess air into the primary gas
output. In a way it is accidental. When secondary air is added, presto: good
burning conditions with low EA.

 

A fire running with an open door is begging for there to be too much excess
air. The Rocketworx stove (which is a bit of a misnomer because it is not a
true 'rocket stove' as usually defined) addresses this quite well. It has no
air control door, but achieves the purpose anyway by departing significantly
from the recommended Rocket Stove dimension set. It also has preheated
secondary air and a small amount of tertiary or finishing air. And the
result is still a low enough excess air level at high power.

 

As a result, the combustion seems pretty good and the fire-to-pot heat
transfer efficiency can exceed 40%. For an 'uncontrolled' stove (nothing to
adjust) this is quite high. The key is control over the excess air level.

 

That said, I have seen test results for a liquid fuel stove with a 72% heat
transfer efficiency at both high and low power. That is unusually good.  It
too is 'uncontrolled' save for the evaporator handle that sets the power
level. As a result, the CO level is very low at all powers.  A really good
gasifier should be in that range, in my view. 

 

Regards

Crispin

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Marquitusus
Sent: Friday, May 22, 2015 08:54
To: stoves at lists.bioenergylists.org
Subject: [Stoves] Secondary air in Rocket Stoves?

 

Hi to all,

 

In the ETHOS 2015 Dean Still's presentation he states that:

 

"In a Rocket stove only forced air mixing results in almost complete
combustion. Strong jets of air are needed to fully mix the air, flame,
smoke, and gas. Adding a chimney to a Rocket stove doesn't result in the
forceful jets that create adequate mixing. Adding height to the Rocket
combustion  chamber, while giving more time for combustion to occur, also
draws in more cold air that results in more wood being burnt."

 

I was wondering if this "forced air mixing" should came from natural draft.
For example, at some distance above the combustion zone, we can make some
holes in the chimney allowing secondary air to enter and mix with the hot
gases, creating a secondary combustion zone. Maybe we can put a
"concentrator ring" above the holes like in a TLUD to create some
turbulence.

 

This done, we can also increase the chimney height without the problem of
"drawing more cold air that results in more wood being burnt", as part of
the increased draft suction will be used to pull the secondary air inside
the chimney.

 

With this tall chimney, hot gases can have more "mixing, time and
temperature" to achieve the desired complete combustion.

 

Anyone tried something like this?

 

Some questions arising:

- What distance above the primary burning zone should we put the secondary
air?

- What number and size should be secondary air holes? (we can take
experience from TLUDs)

- What size should be the concentrator ring? (we can take experience from
TLUDs)

- What height should be the chimney after the concentrator ring?

 

 

Marc

 

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