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Student engineers re-introduce coffee husk stoves in Tanzania as time runs out on their project

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Dartmouth, TanzaniaEditor’s Note: Students from Dartmouth’s Thayer School of Engineering are working in Tanzania to help improve sanitation and energy technologies in local villages. This series chronicles work being done by the student-led group, known as Humanitarian Engineering Leadership Projects (HELP), to design "rocket stoves" in the village of Mwamgongo and top-light updraft design (TLUD) gasification stoves in the village of Kalinzi. The goal is to create a healthier, more energy-efficient cooking apparatus that these villagers will accept and use. HELP students are filing these dispatches from the field during their trip. This is their 15th blog post for Scientific American.

At the beginning of August, Zach visited the Kalinzi team and suggested that rocket stoves might be a viable option for Kalinzi and the peripheral towns. We decided to focus all of our attention on developing a stove that burned solely coffee husks. We knew that this would be a challenge, as it would require a redesign of the stove from the ground up. It was a challenge we found worth tackling because a stove able to burn waste coffee husks instead of wood would have a significant economic impact on the families in the region, as well as have the potential to reduce the rate and severity of deforestation in the Kigoma region.

People in the Kalinzi area of the Kigoma region already have access to a type of coffee husk-burning stove, although its operation leaves much to be desired. This stove design consists of a metal cylinder with a hole in the side of the body, near the base of the stove, and a hole in the top directly below the pot. To use the stove, it is packed with husks and the bed of fuel is compressed around wooden sticks placed vertically from the top to bottom of the bed and horizontally from the bottom hole to create a channel for airflow. The stove is then lit and burnt for a batch time of about five-to-six hours, depending on the dimensions of each particular stove.Dartmouth, Tanzania

However, users complained that the stove produces a flame so hot that it quickly destroys the aluminum cookware commonly used. In addition, this traditional coffee husk stove produces a great deal of insidious smoke, causing illness in users and their children. Finally, the prolonged burn time of five-to-six hours, which might seem advantageous under certain circumstances, actually leads to wasted biomass, inconvenient cooking practices and significant quantities of dangerous smoke laden with fine particulates and carbon monoxide. In fact, this batch time is far too long, as the typical Tanzanian meal (with the exception of beans, of course) takes about 45 minutes to prepare. The waste of energy is currently not an issue, as the coffee husks are abundant enough to sustain the current users, but it inhibits the widespread propagation of the stove.

Our stove must be superior in all of these aspects to propagate the use of coffee husks as a fuel source. The main flaw in the traditional coffee husk stove is that the gasification—or the release of flammable gases from the cellulosic material—of the coffee husks is uncontrolled. In addition, the air only has one channel to flow through, causing an inconsistent gasification pattern throughout the fuel bed.

We started off with a metal cylinder that had holes at the top and bottom. The first big modification was adding a perforated metal cylinder in the center. This would allow air to flow better through the stove and control the gasification throughout each batch’s burn by keeping the center air channel from collapsing as used husks slough away. We also added holes around the center and side of the outer cylinder. We put vertical stabilizing bars, which we dubbed "ugali handles" onto the stove to hold it steady during the aggressive stirring required for preparing ugali.

After a week of testing that included varying the dimensions of the body and center column as well as the number of holes in each, we took our design to the metalworkers at Mwanga market to build prototypes out of stronger metal. The metalworkers were enthused over the design and produced a few models for us. Our final design burned 1.5 kilograms of husks for about 50 minutes per batch.

We took our design back to Kalinzi and demonstrated the stove to all of the six women who we had performed bean tests with. Our culminating experience was successfully cooking ugali with one of the women in her kitchen in front of an audience of about 30 people. The women were all impressed with the design, especially with the strength of the flame.

Still, the stove emitted smoke midway through the batch cycle, and we measured moderately high levels of carbon monoxide emissions. We decided that we needed to do further research to quantify these emissions before they could be safely placed in people’s homes. We are currently partnering with students at the University of Dar es Salaam to develop the design further and optimize the fuel chamber shape to reduce these emissions. Fortunately, we were able to leave two stoves at an Anglican mission and the local coffee cooperative’s processing mill, which both do their cooking in open-air environments and will allow us to get long-term user feedback when we return.

Unfortunately, our eight weeks went very quickly, and we had to put off the launch of the new stove design until our trip to Tanzania next summer. Leaving Kalinzi was difficult, as it meant leaving a community in which we had worked and built relationships for almost two months. We were overwhelmed by the hospitality shown to us, and we were strengthened in our resolve to help these communities. We arrived with a willingness to help but did not expect how much we would take away, personally and as a group.

It is hard to sufficiently describe the silent stillness and awe of our final Kalinzi sunset, with fading orange hues casting shadows over hills of banana trees and the rust-tinged roofs of the village houses. Hopefully, with the work we have done—and will continue to do with the Tanzanian students—we will be able to curtail deforestation and preserve this beautiful environment for future generations to thrive upon in a sustainable manner.



Photos courtesy of Kevin McGregor





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  1. 1. PaulOlivier 9:14 pm 07/1/2011

    It is very easy to gasify coffee bean husks in a top-lit updraft gasifier. But a small fan is needed to force air through the husks. I use a small 40 x 40 mm fan, and the speed of the fan is controlled by a speed regulator. The amount of power consumed by this fan in the gasification of coffee bean husks is generally less than one watt.
    Virtually no smoke is created during the gasification of coffee bean husks.
    Here are two pictures of coffee bean husk being gasified:
    http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0467.JPG
    http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0468.JPG
    Of course I get a much better result gasifying rice hulls:
    http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0444.JPG
    http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0445.JPG

    Here are some of the latest designs of my 150 mm gasifier:
    http://dl.dropbox.com/u/22013094/150%20Gasifier%20Jpegs/001.jpg
    http://dl.dropbox.com/u/22013094/150%20Gasifier%20Jpegs/003.jpg
    http://dl.dropbox.com/u/22013094/150%20Gasifier%20Jpegs/004.jpg
    All of these gasifiers are fabricated out of stainless steel so that they can be used over many years.
    My 100 gasifier sells for about $30 US.
    The 150 gasifier sell for about $50 US.
    This is a totally non-profit effort.
    I also design larger gasifiers of a 100 kW capacity.
    Here equipment price drop to less than $5.00/kW.

    For the larger waste management concept that I am promoting in developing countries, see:
    http://dl.dropbox.com/u/22013094/Paper/composting.pdf

    Many thanks.
    Paul Olivier
    Dalat, Vietnam
    US telephone that rings in Vietnam: 1-337-447-4124

    Link to this

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