How to Make Biochar (Charcoal)

Most of you unintentionally make a bit of charcoal in your home fireplaces.  When biomass burns completely, carbon pairs with oxygen to form carbon dioxide in an exothermic (heat-releasing) reaction known as combustion.  The gray, powdery ash left behind consists of incombustible minerals, i.e. sodium, potassium, calcium, etc.  In order to make charcoal, which is essentially the dry carbon skeleton of biological material, you need a different type of thermochemical reaction: pyrolysis.  Pyrolysis is the decomposition of organic material in the presence of heat and the absence of oxygen.  In contrast with combustion, pyrolysis is an endothermic reaction, meaning that it requires energy input.  In any given fire, you may have zones of combustion and pyrolysis occurring simultaneously.  The key to producing charcoal with limited technology is maximizing pyrolysis and minimizing combustion, although some combustion is necessary to supply the energy for pyrolysis.  If you've already sat through a science lecture today, my apologies for boring you further :)

 

Shown above are a few of Eco-Fuel Africa's charcoal kilns.  The farm-scale model on the left is made from a recycled 55-gallon oil drum for about $30 USD.  That's about 70,000 Uganda shillings- the US dollar has some serious clout here, especially with Uganda's massive inflation problems.  For instance, Sanga tells me that the price of traditional charcoal rose 40% last year alone.  Without a commensurate wage increase for the average worker, such price hikes place mounting financial pressure on the vast majority of Ugandans who have zero alternatives to wood charcoal.

Handling the charcoal kilns is hot and somewhat treacherous work.  After the cane waste is packed in and set alight, the cover must be placed on top.  As you can see from the picture on the right, the chimney on one of the kilns is leaning precariously to one side.  Once the waste starts to burn down, the cover often requires readjustment using the attached handles.  I quickly discover that these uninsulated metal fixtures get very hot (duh!), and manage to sear my fingers quite badly upon attempting to fix the top in place.  After I recoil in pain, the shirtless man in the above photo, Hamsa, apologizes for the kiln's temperature, then proceeds to unflinchingly grab the cover with his bare hands and fix it in place.  I guess my soft mizungu skin isn't conditioned for such blistering work.  Feeling useless, I retreat to the shade of the building to take some notes and plan my energy experiment with the kilns.

My experiment is pretty simple- measure the mass of cane waste that goes into the kiln, and measure how much charcoal comes out. Since mass is equivalent to energy (as we all know from Einstien's famous equation), this tells us the efficiency of the charcoal making process, which is the focus of my Environmental Science thesis on the EFA business.  If 15 kg of cane waste produces 5 kg of charcoal, then the process is 33% efficient, and 66% of the embodied biomass energy is lost as waste heat (which are approximately the numbers I calculated in my field experiment).  When I get home, I hope to research ways to improve our kiln technology so we can recover some of that energy for a useful purpose, such as generating electricity, drying briquettes or biomass, etc.  The trick is doing so without adding undue expense or complexity.