[Stoves] Rocket stove air supply

[Kevin Chisholm]

Dear Paul
----- Original Message ----- 
From: "Paul S. Anderson" <psanders at ilstu.edu>
To: "Discussion of biomass cooking stoves" <stoves at listserv.repp.org>; 
"Thomas Reed" <tombreed at comcast.net>
Sent: Saturday, November 25, 2006 10:41 PM
Subject: Re: [Stoves] Rocket stove air supply


> Dear Hugh, Kevin, Tom and all,
>
> First, I will agree with Kevin (at least in part).  In order to avoid the
> problems of definitions, I will simplify the issue of "burning of dry 
> biomass"
> into two parts:
>
> Smoke making (gasification)
> Smoke burning or smoke combustion (combustion of gases, tars, etc.)
>
> In agreement with Kevin, a device that only makes the smoke (gases) can be
> called a smoke-maker (gasifier).  And something that covers the whole 
> process
> from making the gases through to the combustion of those gases could be 
> called
> a stove (or furnace or some other name that relates to an actual final
> usage of
> the full burning process.)
>
Agreed so far...

> For Kevin:  some stoves have the capability to make the smoke quite 
> separtely
> from the burning of those gases.  They are stoves (I agree with you.)
> But they
> can be correctly classified as "gasifier stoves".

I would suggest that in the phrase "gasifier stove", "gasifier" is 
redundant. As far as I can see, it is impossible to have a biomass stove 
that does not first gasify the fuel. Since ALL biomass stoves are "gasifier 
stoves", using the term "gasifier stove" does nothing to differentiate one 
stove style from another stove style.The Rocket is a gasifier stove. A three 
stone fire is a gasifier stove. Dr. Reddy's charcoal stove is a gasifier 
stove. All biomass stoves have in common the characteristic that they gasify 
the biomass.
>
> To Hugh:  In the case of the Rocket stoves, there is no (or very limited)
> distinct, intentional, or controllable separation of smoke making from 
> smoke
> burning.  Kevin labelled the air that enters under the grate (from below 
> the
> fuel tray) as primary air.  Some, perhaps most, of that air is primary
> air. But some of that air can pass through the area of the fuel and
> function as
> secondary air.  And there is no control of the separation of the
> "from-below-grate-air" into what is primary in function (smoke making)
> and what
> is secondary in function (smoke burning).

There can indeed be control over the primary and secondary air in a Rocket 
style stove:
Primary air can be controlled by the degree to which the ashpit door is left 
open. If there is no "slider" or door per se, the opening can be constricted 
by partially blocking with a stone or series of stones.
Secondary air can be controlled by the density of packing of the fuel 
sticks.

No doubt, there are "shape variations" in construction of a Rocket style 
stove that can also effectively change the tendancy for air to be consumed 
as primary air or secondary air.
>
> In fact, consider the traditional "burning of fuels" (a small stack of 
> fuel,
> which is appropriately NOT the norm in Rocket stoves, hence part of 
> Rocket's
> advantage over the stack of fuel in many other stoves).  In the stack, 
> heat at
> the bottom can cause smoke making (primary air), and that smoke rises ever 
> so
> slightly and finds that additional air (from under the grate) is still
> present,
> so it picks up the oxygen and yields a flame of secondary combustion
> WHILE STILL
> BELOW THE OTHER FUEL THAT IS HIGHER IN THE STACK.  That flame helps drive 
> the
> primary-air reaction (smoke making) that occurs a centimeter or two higher 
> in
> the stack (thus causing more smoke making).  The lack of much control over
> these events greatly contributes to the undesirable "spikes" in heat and
> emissions that come from traditional burning of dry biomass.

Excess primary air that functions as secondary air within the fuel bed 
simply accelerates the process of gasifying the fuel above it. Simply add 
nore secondary air, and burn the extra gas to completion.
>
> Eventually, some (perhaps most) of the "smoke or gases" is above the
> top of the
> fuel stack.  THEN is when the major SECONDARY air is needed.  And it is 
> needed
> in a way that promotes good mixing of the new oxygen in with the smokey 
> gases
> if there is to be a clean burning.  And if the mixing is done well, the 
> TOTAL
> zone of the secondary combustion (smoke burning) will be intense and
> limited to
> a rather short vertical zone, not being flames that shoot high (which
> is a sign
> of the combustible gases going so far away from the fuel before the oxygen
> actually gets to the gases.)

Agreed.
>
> An excellent illustration that shows clean burning in a short zone of
> secondary
> combustion is in Reed Woodgas Campstove.  But that is a T-LUD gasifier and 
> it
> is constructed to function in that way.

The Reed Campstove is a very well designed special purpose stove. The Rocket 
and Dr. Reddy's stove are very well designed general purpose stoves. As 
noted above, they are all gasifier stoves.) The Reed Camp stove does one 
kind of job very well, while the other two stoves operate under a range of 
conditions reasonably well. These other stoves sacrifice maximum performance 
for versatility. That is what "Optimal Stove Design" is all about... maximum 
performance in one area is sacrificed for good performance in a number of 
areas. One stove can be designed to do one job very well under specific 
conditions, and another stove can be designed to do a range of jobs under a 
variety of conditions reasonably well.
>
> Must close now.  I am in Cambodia with a gasifier cookstove project until 
> 19
> Dec.  I will try to respond when I get to the computers here.
>
Best wishes with your project.

Kevin