[Stoves] Rocket stove air supply

[Kevin Chisholm]

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
>