| Boiling Point No. 03 - October 1982 |
Biochar may be obtained by burning biomass (leaves, grasses, weeds, or agricultural wastes) in an open or partly closed kiln. The kiln may be a drum, an earthen pot, a trough of brick and mud, or a pit in the ground. Some 10kgs dry biomass are put into the bottom of the open kiln and ignited. When the temperature has reached about 500°C, the kiln should then be filled with biomass and its mouth partly closed with a heat resistant plate. The emerging pyrolitic smoke will expel air from the kiln and prevent air from entering it and burning the glowing char to ash. When all the biomass has turned to char, the smoke will thin out even if a stick is stirred within the kiln. At this stage water should be poured into the kiln to extinguish the fire. The wet biochar is taken out of the kiln and processed directly into brazier briquets, lump briquets, or cartridge briquets.
No glue is needed to make these briquets. They may be moulded with biochar containing 30% sawdust or rice husks. Bamboo or wooden sticks are put through the base-holes of a traditional clay brazier. Wet biochar is pounded in E mortar to a dough and then compressed into the space between the sticks in the brazier. The sticks may be pulled out within an hour leaving canals running through the casted briquets. After drying the brazier briquet for at least 7 days if the sun, it may be burned for cooking or heating.
The brazier briquet is fired by burning biomass underneath it or on its upper surface. When one canal catches fire then eventually all the canals will become incandescent with a temperature of 700°C. The briquets should burn with practically no smoke or odour. The brazier briquet is considered energy efficient because the convection and radiation is in the upward direction only, while heat isolation by the char at the briquet's periphery reduces conduction losses. The calorific value of this biochar is 5,000kcal/kg.
Lump or Cartridge Briquets
Burning 10kg dry biomass will yield 3kg biochar. To make lump or cartridge briquets a dough is pounded from dry biochar and 5%v tapioca glue, or from wet biochar with 2.5% cement, or with 12.5% fresh leaves from a variety of plant families with sticky saps, like legumes, Hibiscus, Euphorbia, etc. The dough is then compressed in a mould to lump briquets of different forms or to cylindrical cartridge briquets with a number of canals.
Spherical biobriquets may be made without any tool by compressing the dough with one's palms and fingers into balls. Cylindrical briquets 3cm in diameter may be made by compressing the dough with one's thumb in a bamboo or plastic tube. A bisquit tin 15cm high and 14.5cm across with 9 holes 10mm wide punched in its bottom, and 8 holes near its mouth may be used as a mould to cast cartridge briquets, or as a brazier for burning them. The mould's wall is lined with a metal or plastic sheet, then sticks are passed through the nine holes in its bottom and dough is compressed into the space between the sticks to a height of 13cm. m e sticks may be pulled out within a few hours and the cartridge dislodged within a few days. The mould without its lining may be used as a brazier when placed on three bricks, loaded with a dry cartridge and fired. Combustion gases will escape through the 8 holes near the mouth of the brazier when a cooking pan is placed on it. A more durable brazier for burning cartridge briquets could be made from cast iron.
Burning dry biomass to biochar reduces its calorific value by about 55%. The energy lost is for a considerable part carried away by the combustible components in the smoke like hydrogen, carbon monoxide, methane, methanol, acetone, acetic acid, tar and carbon particles. A stove burning biomass or wood together with its smoke and char will be an efficient stove, and some traditional stoves like the 'rag' on the islands of Savu and Roti, for cooking palm sugar, do just that. This stove of mud, which is partly underground, commanded the admiration of Captain Cook, who visited those islands when he discovered Australia (1). Combustion in the stove takes place in a closed space. Heat loss through convection, radiation, and conduction is minimal and very little smoke escapes from the stove.
Smoke will also burn in a hybrid stove which comprises a brazier briquet on top of a wood or biomass stove of the same diameter and height (Fig 1). A biobriquet is cast in its upper compartment and fired. When biomass is burned in its ash compartment then the smoke will ascend through the glowing canals of the brazier briquet and flare away. m e heat generated in the hybrid stove is the combined heats of combustion of the biobriquet, the wood or biomass, its smoke, and its char.
(1) Fox, J.J.: Harvest of the Palm, Harvard Univ Press, 1977. p 124,
Energy Studies Centre