Why Improve Cookstoves?
Current Charcoal Cookstoves:
- Cost families 40% of their income in charcoal
- Are a major cause of Haiti's 98% deforestation
- Release high levels of carbon monoxide
- Are slow to heat up and cumbersome to use
Improved Gasifier Cookstoves:
+ Use 50% less fuel
+ Use waste biomass (no more trees)
+ Dramatically reduce CO and particulate matter emissions
+ Are quick lighting and long burning
How it Works
Gasification is the broad term used for the conversion of a solid fuel into a gaseous fuel. The process to create heat from solid biomass goes in stages: Wood-gasification turns wood to char and gases. It is controlled by heat input and can be slowed by cooling. Char-gasification turns char to ash and gases. It is controlled by oxygen and can be arrested by deprivation of oxygen. Wood-gas is often used as summarizing term for the mixture of combustible gases and pyrolytic vapors from both gasification reactions. It combusts when mixed with oxygen and ignited. In an open fire, all the stages of gasification and combustion occur simultaneously at the same place and with no or little control over the processes.
*Text and image credit: HERA – GIZ Manual Micro-gasification, January 2011
Fundamentally, the challenge in cooking is a question of scale; how to gain control over the pyrolysis, gasification and combustion in a small enough (vertical) space to be used by individual households. Micro-gasification refers to gasifiers small enough in size to fit under a cooking pot at a convenient height. It was conceptualized as a top-lit up-draft (abbreviated TLUD) process in 1985 and developed to laboratory prototype stages by Dr. Thomas B. Reed in the USA. Small-scale micro-gasifiers offer good opportunities for the use in cook stove applications and/or for domestic heating, because they can:
- Cleanly burn the woodgas in mainly smoke-free combustion (unlike conventional burning of solid fuel)
- Provide a steady hot flame shortly after ignition (no waiting, as with charcoal)
- Have high fuel-efficiency due to complete combustion of the fuel (little smoke)
- Be operated batch-fed over extended periods without attention (no tending of fire)
- Utilize a wide variety of solid biomass fuels, even inexpensive often discarded small biomass residues, that other stoves cannot easily handle (no stick-wood)
- Give the user the freedom to decide individually when to use the device, as biomass fuel is often locally available, within reach of most people.
*Text credit: HERA – GIZ Manual Micro-gasification, January 2011
Top-Lit UpDraft (TLUD) Gasifiers
Top-Lit UpDraft (TLUD) is a common variant of micro-gasifier stove. Invented by Tom Reed in 1985, this technology uses two air paths called primary and secondary air, which travel about two concentric cylinders. Fuel is first loaded into the inner cylinder and then lit at the top of the fuel bed (top-lit). For the first few minutes of operation, the fuel is directly combusted producing the glowing char to sustain pyrolysis. As more volatile gases are released, they mix with the now preheated secondary air flowing between the cylinders to inside the inner cylinder. This air provides oxygen, allowing the gases to combust. A flame cap forms around the secondary air holes, which is used for cooking. Pyrolysis proceeds downwards through the fuel in distinct layers of drying, decomposition, glowing char ignition and pyrolysis. The hot gases continue to travel up the inner cylinder to be combusted in the flame cap (up draft). Char ignition consumes the small amount of oxygen in the primary air, providing the heat and zero-oxygen environment for pyrolysis to occur.