A Hybrid is Born
Day fifteen in Arusha starts with a quick breakfast of peanut butter and honey on toast and a little one on one football in the backyard with Philbert. The nyama choma (roast beef) from last night has taken Jun out. I admit I feel a little uneasy too but my tough Bombay tummy is holding up well. We are off to EMORG today with a new kiln design and there’s a lot to say about it but first let’s rewind one week to last Thursday.
We spent the morning running our two demonstration kilns (See Olivia’s Busy, Busy for a brief description of their designs) to try to figure out appropriate feedstock “recipes” for the kilns. Fine feedstock must be mixed with coarse feedstock in a proportion that allows for the passage of oxygen through the substrate but that is not so airy that the flame eats through the substrate, turning it to ash.
If a larger proportion of finer and/ or damper feedstock is used, the kiln will take a longer time to produce char. Burn times typically take between 15 and 25 minutes. After packing the kiln, we dribble around 20 ml of petrol on the feedstock (petrol costs 2500 Tsh per liter). The petrol allows for a big flame for the first minute or two of the burn, depending on how densely the feedstock has been packed, this flame disappears after two to five minutes and the kiln begins to emit a thick white gas which is damp to the touch suggesting a high water content. The energy to draw this water vapour out of the kiln comes from the embers in the feedstock and the pressure gradient induced by the natural draft of the chimney. This second phase of the run usually takes between 10 and 15 minutes. After most of the water has escaped from the feedstock, the flame shoots back up with dramatic bravura. At this point the chimney and adapter are removed and a damp wooden stick is used to make sure that the combustion is occurring evenly across the surface of the feedstock. Pockets of raw feedstock are mixed around into the flame to ensure that the action front is descending symmetrically across the face of the drum. After four to five minutes the kiln is quenched with water, the char is removed, inspected, dried out and weighed.
With a better understanding of the recipes that worked well, we set off to meet with the women’s group at EMORG to run a demonstration of the Takachar-inspired kiln with sawdust they had purchased from the local market in Kisongo. The women were impressed with the quality of the char and used it along with cassava flour to press out briquettes. The press that EMORG is currently using to produce briquettes is not able to apply sufficient pressure onto the molds and so although we were hoping to test the briquettes we made using the char we produced last Thursday, we were informed a couple days ago that, in a calamitous turn of events, the briquettes crumbled before they dried.
Our pressing woes disappeared on Monday, with a trip to Vision for Youth. Our buddy Miguel from Johns Hopkins volunteering at V4Y purchased a G clamp to use as a press. The G clamp was tremendously successful in producing well-packed briquettes that showed no indication of crumbling before drying. We are looking forward to testing these briquettes over the next few days, and introducing the concept of using a G clamp to press briquettes to the women at EMORG later today.
Although the retort kiln described in Busy, Busy produced high quality char, it was psychologically off-putting that the material in the outer barrel would burn completely to ash. In response to this, we decided to combine both designs into a hybrid by attempting to produce char both inside and outside the retort. This was achieved by using the retort in the traditional TLUD design (with holes only in the bottom of the outer barrel for air to enter and none on the side to aid combustion) and packing the outer barrel as we otherwise would have with no retort. The little metal tubes shown below were placed in with the feedstock to help the action front descend symmetrically. The thought was that the tubes would serve as reservoirs of heat so that the pyrolysis wasn’t at the behest of the wind—if the flame moved to one side, the metal would retain heat in the other, continuing the pyrolysis until the flame invariably returned.
On Tuesday we built a new cylindrical retort to replace the box-shaped retort we had previously been using. We were hoping that the added dimension of symmetry would help the action front descend more evenly and char the material in the report more consistently. However, we found that the added volume of the cylindrical retort rendered it useless. Having more volume inside the retort means that more energy is required to pyrolyse the material inside. Most of this energy comes from the material in the outer barrel. There simply wasn’t enough material in the outer barrel with this new design to get the reaction off the ground. We thus reverted to using the box retort in the hybrid kiln. We are glad though that we built the cylindrical retort and discovered this restriction on the ratio between the cross sectional areas of the retort and the outer barrel – this is definitely valuable information to document.
We have done some preliminary measurements to calculate the yield of our current hybrid kiln design (how much char is produced for a certain mass of feedstock put into the kiln), we will hold on to that data for the time being, as it probably does not reflect the true potential of the kiln. We get better at running the kiln every day and want to try a few more iterations before we publish yield statistics for our design. At this point, the yield looks promising enough to support a profitable business model. Our main goal over the next few days is to iron out the details of a business model, so that we may present it to our partner organizations instead of presenting a product in isolation.
Finally, we get to yesterday when we met with DHE constact Mr. Sossy, an associate with a women’s group in Tumaini Center in Arusha. We set up a time to meet with the women’s group, which has produced briquettes in the past, to demonstrate the new kiln design. We hope the group shares Mr. Sossy’s enthusiasm to see the new kiln. More on this later!