[Stoves] Rocket stove air supply

Sat Nov 25 20:41:05 CST 2006

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.)

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".

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).

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.

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.)

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.

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.

Paul

-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson

Quoting Thomas Reed <tombreed at comcast.net>:

> Dear Kevin and All:
>
> This is the best analysis of biomass gasification/combustion that I've
> seen and I wonder why it is taking the rocket stove group so long to
> separate these two issues.
>
> Thanks,
>
> TOM REED     BEF
>
> Kevin Chisholm wrote:
>> Dear Hugh
>>
>> Here are some "very basics" that might help you get an answer with which you
>> are comfortable... These are "first principle thoughts", and given that I
>> have never operated a Rocket Stove, it would be very interesting to see how
>> theory compares to practice. :-)
>>
>> PRIMARY AIR (PA) is required to gasify fuel. Primary air is air that "sees"
>> the fuel, usually passing through it. SECONDARY AIR (SA) is air required to
>> burn the fuel gas to completion. Usually, it is introduced after the
>> gasification zone, but "purposeful excess primary air" can serve as
>> secondary air. Basically, PA produces a fuel gas, and SA burns it to some
>> degree of completion. If only primary air is introduced, the device is a
>> gasifier; if there is provision for secondary air, it is a stove
>>
>> PA and SA ratios need to change based on the character of the fuel. Larger,
>> wetter, fuels would require more PA, some of which must function as
>> secondary air, to provide the extra heat required to sustain combustion at
>> the desired rate. On the other hand, smaller, dried fuels require less PA,
>> because less heat release is required to sustain the gasification reaction.
>>
>> The Rocket Stove uses "undergrate air" as PA, and "through the fuel air" as
>> both PA and SA. The air entering the stove through the fuel tends to
>> separate from the fuel before its oxygen is  fully consumed, so it acts
>> mainly as secondary air, to burn to completion the gases that were mainly
>> produced by the "undergrate air."
>>
>> Thus, there seems to be two answers to your question:
>> 1: Larger, wetter stickwood fuel: More undergrate air is required, so pack
>> the fuel port tightly.
>> 2: Smaller drier stickwood fuel: Less undergrate air, and more "over-fire"
>> is required, so pack the smaller, drier fuel lightly.
>>
>> I hope this helps, and I would be very interested in your comments on how
>> theory and practice agree, or disagree.
>>
>> Best wishes,
>>
>> Kevin
>> ----- Original Message -----
>> From: "Burnham-Slipper Hugh" <eaxhb at nottingham.ac.uk>
>> To: <stoves at listserv.repp.org>
>> Sent: Thursday, November 23, 2006 8:42 AM
>> Subject: [Stoves] Rocket stove air supply
>>
>>
>>
>>> Dear Stovers,
>>> The fuel magazine that sticks out the side of a rocket stove is divided
>>> into two: above the shelf is where the fuel goes, and air passes under the
>>> shelf. Am I right in thinking that the fuel should be packed in as much as
>>> possible, to try and minimise the amount of air entering the stove through
>>> the fuel inlet? If so, why? My experience is that char builds up at the
>>> bottom of the elbow, so air is needed to burn the char (which in turn
>>> pyrolises the fuel), and a second air supply is required to burn the
>>> volatile gases. Any pointers would be warmly received.
>>> Confused, Hugh.
>>>
>>> This message has been checked for viruses but the contents of an
>>> attachment
>>> may still contain software viruses, which could damage your computer
>>> system:
>>> you are advised to perform your own checks. Email communications with the
>>> University of Nottingham may be monitored as permitted by UK legislation.
>>>
>>> _______________________________________________
>>> Stoves mailing list
>>> Stoves at listserv.repp.org
>>> http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
>>>
>>>
>>
>>
>>
>> _______________________________________________
>> Stoves mailing list
>> Stoves at listserv.repp.org
>> http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
>>
>>
> _______________________________________________
> Stoves mailing list
> Stoves at listserv.repp.org
> http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
>

----------------------------------------------------------------
This message was sent using Illinois State University Webmail.