[Stoves] gasifier rocket stove

[Dean Still]

See: http://www.bioenergylists.org/en/aprogasrocket

Hello Stovers,

In the first part of June the Aprovecho lab was contacted by USAID
concerning stoves used in Darfur. I had thought for a long time that a VITA
stove, designed by Sam Baldwin in the early 1980's, would be a good refugee
stove. Generally speaking, increasing heat transfer reduces fuel use.
Increasing combustion efficiency reduces unhealthy emissions. THE VITA stove
excels at increasing heat transfer efficiency.

The VITA stove is a simple cylinder of sheet metal (with grate) that
surrounds a dedicated pot. The cylinder is slightly larger than the pot,
creating a small channel (6mm to 14mm depending on pot size) all around the
pot. Hot flue gases are forced to scrape against the sides of the pot after
touching the bottom. The increased heat transfer efficiency improves fuel
used, time to boil, etc. compared to the three stone fire.

In Water Boiling tests done at Aprovecho, the three stone fire used 1118g of
kiln dried Douglas fir to boil and simmer (for 45 minutes) 5 liters of
water. The VITA stove used 689g. 

The three stone fire boiled the 5 liters in 26.7 minutes. The VITA stove
boiled the water in 14.0 minutes.

However, the VITA stove produced high amounts of CO (43g during the WBT) and
PM (2150mg during the WBT). The benchmarks of performance that Aprovecho
uses to help define an improved stove without a chimney are 20g of CO and
1500mg of PM made during the WBT. (850g of dry wood)

THE VITA saved fuel, was fast to boil but was not clean burning. However, in
refugee situations perhaps folks would be more concerned with wood use? If
so, then the low cost VITA stove, might be an appropriate technology in that
situation?

EXPERIMENTS WITH A VITA TYPE STOVE

We spent several weeks experimenting with the VITA stove concept. As can be
seen in the photograph on the STOVES list, Aprovecho staff ended up
simplifying the VITA stove, only cutting a rectangular fuel entrance in the
cylinder. The VITA stove has more air holes under the grate. 

We made a four post pot support that is hammered into the ground so it can't
move around too much. The pot support is 12cm high above the ground. The
posts are 12cm apart so the structure is square. The sticks of fire wood are
supported on a bar between the posts holding the sticks 5cm above the
ground. There is no grate in the last prototype tested. Sticks of wood burn
above the ground, or above an insulating layer of ash.

An adjustable cylinder is made that establishes an 11mm gap between various
sized pots. There are 11mm spacers inside the cylinder. Two wing nuts are
loosened to adjust the cylinder until the spacers touch the pot. Then the
wing nuts are tightened. 

The rectangular fuel entrance, which is 12cm long by 9cm high, lines up with
the posts and bar of the pot support inside the cylinder. The sticks of wood
burn inside the pot support. The air flow is regulated by the size of the
fuel entrance and the 11mm channel between the pot and the cylinder.  The
controlled air flow seems to create a slower burn, resulting in less smoke
while maintaining low fuel use.

A metal shelf holds the sticks in position.

Three metal stakes are driven into the ground, attached to the outside of
the cylinder. These stakes hold the metal cylinder so that the cook
hopefully does not need to touch the hot exterior when vigorously stirring.

We have now run 7 Water Boiling tests of the stove prototypes under the
emissions hood. In each test the fuel use was as expected but PM was
surprisingly low. The ends of the horizontal sticks of wood burn slowly with
a darker yellow color compared to the brighter yellow in a rocket stove. The
wood seems to be burning in a 'gasification' mode.

The last experiment on the modified VITA stove resulted in the following:

556 g of kiln dried wood used during the WBT. 23g of CO made, 415mg of PM
emitted, time to boil was 25.7 minutes. PM was low in all experiments. A
photo and table showing results from the 7 experiments will be posted on the
STOVES list.

Air flow into the open combustion chamber is horizontal forcing the smaller
flames to flow together, also horizontally, before becoming vertical. The
slower rate of combustion seems to have a beneficial effect on the emissions
of Particulate Matter. 

Lowering the pot in experiment 5, and to some degree in experiment 6, had a
detrimental effect on the production of PM and CO. Raising the pot in
experiment 6 lowered both PM and CO. Further testing is planned. 

CONCLUSION

It seems possible to create low PM and low fuel use in a modified,
adjustable VITA stove (a Gasifier Rocket stove?). The combustion is
reminiscent of Gasifier stoves because the rate of burn is slower and flame
color is darker. This type of slower combustion seems to produce lowered
emissions of PM. However, CO is relatively high in all 7 experiments. The
fuel use is lower than a standard VITA type stove, as measured during the
WBT.

Best,

Dean


_______________________________________________
Stoves mailing list
Stoves at listserv.repp.org
http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
http://stoves.bioenergylists.org
http://info.bioenergylists.org