Week 2 Stove Development

Last week, the stove project began a new prototype mostly based on a design by the Kisangani Smith Group (KSG) in Njombe, Tanzania. After much angle-grinding, hammering, and metal turning, we finished the stove on Monday, July 2nd. Bernard was enormously helpful as a consultant and a master welder throughout the process.
The reasoning behind building a prototype similar KSG’s design is that their stove is similar to our stove in many respects, but has interesting technical differences and is a vastly successful stove thus far.  The hope was that by building a prototype and comparing the burn characteristics of the stoves, we would gain insights about how their design choices affected the end success of their product.
Principally, there are two design characteristics we are interested in. First, the primary air entrance is on the side of the fuel canister rather than the bottom in previous DHE designs. A pipe enters through a side-entrance to the central chimney. We are interested in this pipe’s effect on primary air delivery to the fuel bed. Second, the new model includes a separable heat-transferring lid instead of a basic opening at the top of the stove. We would like to explore the effectiveness of a heat-transferring system versus an open chimney. Furthermore, the small openings in this different type of lid would seem to cause very different airflow from that of our stove. We would like to see whether this causes a more efficient mixing of secondary air with wood gas and possibility eliminate the outer shell of our stove.
Yesterday July 3rd, we conducted our first burn with the new prototype. To prepare the stove, we packed fuel just as we had done in our old model- sawdust around a 2 inch PVC pipe. Packing entailed pressing down on a specially dimensioned disk of metal for evenly distributed pressure.
The design calls for lighting the stove by dropping a match or heat source down the chimney of the fuel bed before placing the lid on the stove. Due to inexperience with this new design, it took several tries to light the stove. We eventually were forced to pour some kerosene down the central chimney of the stove to aid the burning husk we placed through the side air inlet. Until introducing kerosene, the sawdust on the bottom of the pot simply wouldn’t catch, despite the burning corn husks we put in the central chamber. Eventually, we hope to avoid utilizing kerosene as the cost is significant and increasing for Tanzanians.
After we got the fuel to begin combusting and the burn steadied we placed the lid on the stove. A serrated ring of metal on the bottom of the lid (which we termed the “bite” flange) dug into fuel bed and formed some degree of an air tight seal. After placing the lid on the stove, we used mud to seal the slight gaps between the lid and the stove. Ideally, these gaps should not exist as the flange on the lid should fit snuggly with the stove.
For the first twenty minutes, the burn was somewhat dirty. We speculated this might be due to an ineffective bite phalange- much of the fuel had become disorganized at the top from our unsuccessful attempts to light the stove. The flange may not have been properly trapping the wood gas and heat in the center of the stove. More investigation into this will be important in the coming days.
After twenty-five minutes or so, the burn became considerably cleaner. Our pot of water kept heating up, and within little time, wispy vapor began emanating from the surface. We waited with the stove, watching the vapor, for another twenty minutes. When the hot water didn’t reach a boil, at the fifty minute mark, we put a cover on the pot. Ten minutes later, the water began to boil.
Fuel continued to burn for at least another 30 minutes. A healthy flame could be observed at the bottom of the fuel bed from the primary air entrance as well as at the top of the sawdust pack. Presumably, this flame near the top was the combustion of fuel bed. After nearly two hours of watching the stove, we left to get lunch and acquire some cost figures from the hardware store. Unfortunately between leaving and returning, the burn had finished and uncombusted wood gas flowed from the lid.
Removing the lid, we found an interesting sight. All of the fuel opposite to the primary air tube had combusted into white ash. We could see much exposed steel of the inside of the fuel canister. On the other side above the primary air tube, we found much more char and some uncombusted sawdust. It looked somewhat similar to the fuel after the unsuccessful burns in our old design. This side exhibited some degree of hollowing as the uncombusted char was thicker near the top of the fuel bed. Ostensibly, a somewhat circular flow of air developed as it entered the air inlet, collided with one side of the fuel bed and “bounced” back to the other side. Because the air was not directly thrown at this side of the fuel bed there was not the complete combustion of the other half.
Overall, it was a very exciting burn. Despite the difficulty in the beginning, we were able to produce a steady burn lasting between two and three hours using just sawdust and a bit of kerosene. We hope to continue to improve our ability to light this stove, to complete a burn (without grabbing lunch in the middle…), and begin conducting the various Aprovecho testing protocols.

The next steps for us are to modify this design for increased efficiency and user friendliness. We want to produce a product that is easy to use and that is a wholly DHE design. These steps include:

1) Investigating why the first twenty minutes were a dirty burn as well as measuring the emissions from a complete burn
2) Finding ways to start the burn without kerosene. Kerosene is an already expensive commodity of rising cost.
3)  Increasing heat transfer into the pot. We would like to achieve water boil without a lid as some Tanzanian foods require this
4) Looking for ways to burn the fuel that did not combust fully
5) Determining the cost of producing this new stove design and reducing unnecessary components  in construction and in the stove itself.
6) Acquiring coffee husk and various other loose biomass source to perform burns with.



- Aneesh and Kevin