Fuel for Thought: Biomass to Briquettes

Scientific American

August 6, 2013

By Rachel Margolese

Dala-dala is the main mode of public transportation in Tanzania. All the dala-dalas seem to operate more or less individually with very little group organization. A dala-dala is, in essence, a retrofitted van. Start with a 15 passenger van. Now rip out the huge cushy seats and replace them with four rows of small, plastic-cushioned chairs. In front of the first row, throw in an extra bench for people to sit facing backwards. Then raise the ceiling by a foot and you’ve got a dala-dala.

Every dala-dala has one driver and one conductor. The conductor handles the payment and advertising by respectively shaking a stack of coins in your face and by beckoning out the window and sometimes forcible pulling you toward the vehicle to encourage you to ride his dala-dala. Then there are the passengers. Fill up the seats. Fill the bench. Squeeze two or three passengers next to the driver. All this extra aisle space? What a waste. Push ‘em in, fill it up. Passengers pile in and stand, bracing themselves on seat-backs, leaning precariously over seated passengers. Babies get passed around depending on whether the mother managed to snag a seat. Bring you chickens along, why not.

And just when the van is stuffed to overflowing and the conductor has left the sliding door open so that he can hang on to the outside, right when the mass of people on board could not possible compress anymore, the dala-dala stops and one more passenger pushes on board. It is a thrilling way to travel.

The record so far: 30 people at one time. Plus a large spider on the roof.

Rachel Margolese
Rachel Margolese ’16 leans on a charcoal briquette press.

The days are far too quickly turning to weeks for our stay in Arusha, filled with many kiln burns, hundreds of briquettes, and perhaps most exciting, meetings with several groups interested in learning about briquetting and charcoal production. One of the major ventures of this summer has been our experiments with our freshly constructed tanuru (charcoal kiln) which have been becoming steadily more successful, with a greater charcoal yield, on each successive run. The charcoal from this kiln, made from sawdust, rice husk, and corn leaves given the season, will supply a clean burning filler for making briquettes. The kiln is designed to bring a large amount to biomass to a high temperature in the presence of limited oxygen. To perhaps make this a bit simpler, but also a little bit more inaccurate, let’s think of three different reactions:

  1. Gasification of Biomass: Biomass + Heat + Air (primary air) = Ash + Syngas + Heat
  2. Pyrolysis of Biomass: Biomass + Heat = Charcoal + Syngas + Heat
  3. Combustion of Syngas: Syngas + Heat + Air (secondary air) = CO2 + H20 + Heat

A bit funny to have heat on all sides of the equations, but the point to be made is that it takes heat to get started, but once going, the reactions can produce excess heat to run to completion. Syngas, essentially biomass that turned into gas, contains several compounds but has a high percentage of hydrogen gas so we can consider it combustible.

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