Rocket Stove Questions and Answers: Rocket Stove Air Supply - Primary and Secondary Air
Hugh Burnham-Slipper (UK) and Kevin Chisholm (CAN), Dean Still (Aprovecho), AD Karve (ARTI, India) November 25-26, 2006
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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.
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.
Use a grate under the fire. Do not put the sticks on the floor of the combustion chamber. Air needs to
pass under the burning sticks, up through the charcoal, and into the fire. A shelf in the stove
opening also lifts up sticks so air can pass underneath them. When burning sticks, it is best to
have them close together and flat on the shelf, with an air space in between each stick. The burning
sticks keep the fire hot, each fire reinforcing the other to burn more completely. It is optimum if the
air passes under the shelf and through the coals so that when it reaches the fire it is preheated to help
the gases reach complete combustion. Air that passes above the sticks is not as helpful because it is colder
and cools the fire. A hot raging fire is clean, but a cold fire can be very dirty.
Design Principles for Wood Burning Cookstoves, Aprovecho Research Center, PCIA, Shell Foundation 2005 p.15
From running stoves under the emissions hood my personal opinion is:
That around 800C combusts CO but that particulate matter may need higher temps?
That mixing is the secret to lowering emissions more than higher temperatures.
That having more air than stoichiometric doesn't hurt if there is mixing.
Levels of Particulate Matter go up when the wood is burning, down when char is being made. Then CO goes up.
Burning 3" of the tip of sticks can be done cleanly in a Rocket but when more than 3" of the stick begins to burn emissions rise.
When the vertical cylinder above the fire is about 10-12" high measured from the floor (sticks are above it burning on a shelf) heat transfer efficiency and combustion efficiency are around optimal.
When the vertical cylinder is taller more air is sucked into the fire by the increased draft which cools gases touching the pot, emissions per wood burned go down but emissions made to cook go up because CE [Combustion Efficiency] goes up but HTE [Heat Transfer Efficiency] goes down.
A cylinder of sheet metal 3" high around the international testing pot (25cm
dia.) with a gap of 12mm reduces fuel used, CO and PM by about one third.
Hope this helps,
The importance of secondary air is immediately understood by laymen if one uses the following analogy. I compare a kerosene lantern with a wick stove.
The fuel in both the cases is kerosene, and both the devices are provided with wicks. In a lantern, the fuel receives only primary air. Supply of secondary air to the flame is prevented in a lantern by the glass housing around the flame. This results into a yellow and sooty flame. In a kerosene wick stove, the flames are surrounded by a perforated cylinder situated above the wicks. This arrangement allows the flame to receive secondary air which burns the gases completely. One therefore gets blue flames without soot.
We have added secondary air to the Rocket several times. It did not reduce emissions. A couple of weeks ago I tried again and added another cylinder around the vertical cylinder of the Rocket combustion chamber so that pre-heated air would rise up and then enter through holes into the top of the fire. It did not produce lower emissions.
I think that there is enough primary air coming into the fire so that more air does not help clean up combustion. Also the velocity of the air coming through the holes is low so it does not significantly add to mixing.
What dramatically reduces emissions in a Rocket stove is mixing of gas, flame, air: just like in Tom Reed's stove and the Philips stove. We'll bring a side-feed Rocket with improved mixing to ETHOS using a fan and another using steam.
In some ways the Rocket is like a kerosene wick stove, the fuel air and flame have a longer time to combust in a vertical chimney. I'm also trying to make a Rocket more like a kerosene stove in which the gases air and flame rise up in an annulus.
See discussion in Stoves List Archives
Heating With Wood: Principles of Combustion Montana University Extension 2003.