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Charcoal Cookstoves

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Introduction

Charcoal is charred wood, which has lost most volatiles in the production process and has a low moisture content. Thus it emits far less particles when burnt as compared to wood-fire. It is an energy-dense light-weight (on energy value per weight), easy-to-handle, and therefore convenient fuel, which burns without producing much smoke other than during lighting. These properties make it a preferred fuel especially in urban and peri-urban areas. For more information on charcoal as a fuel see Cooking with Charcoal.

Few people make their own charcoal. It is mostly produced in rural areas in portable or fixed kilns as an income generating activity. It is then usually sold into the more urban areas where firewood collection is less feasible and people have more purchasing power to buy fuel.

The main disadvantage of charcoal is its highly inefficient production process: more than half of the wood’s energy content is lost without further use during the charring process.[1] To produce 1 kg of charcoal in a traditional kiln 8-12 kg of wood are needed. See also the article and illustrations on charcoal production.


Charcoal Stoves

Charcoal stoves are mostly lightweight, portable and have generally one fire per pot. They are batch-fed, which means that all fuel required is filled in a container and lit at the start of the cooking process. Heat from charcoal is mainly transferred through radiation. This requires that the cooking pot sits close to the surface of the fuel, making the combustion chamber rather shallow.

Most traditional charcoal stoves are made of scrap metal with no option to regulate the burn-rate of the fuel and often without pot-rests, so that the pot sits directly on the charcoal. This causes high emissions of potentially lethal carbon monoxide and wastes a lot of fuel, as carbon monoxide is unburnt fuel with a high energy value. The inability to regulate the air supply and turn down the heat during the simmer-phase also leads to unnecessary waste of fuel.

The Cookswell East African Jiko museum - 20+ years of cookstoves.jpg

In Africa, the most widespread example of an improved charcoal stove is the Kenyan Ceramic Jiko (KCJ). This stove type has a ceramic liner in a metal cladding. The ceramic liner protects the outer metal structure from deterioration by the fire. It also provides improved insulation, leading to a hotter flame and thus to a better efficiency. It has pot-rests creating a small gap between the charcoal and the pot, to allow some of the produced carbon-monoxide to burn off. Combustion is thus improved and less dangerous smokes emitted. Due to these design characteristics and depending on proper use and maintenance the stove can save between 30-50 % of charcoal compared to traditional charcoal stoves and reduce toxic emissions.[2] In Kenya, improved charcoal stoves are found in about 47% of all households, in Nairobi and Mombasa penetration rate is around 80% (2004 estimate).[3] As charcoal is usually purchased, users see the monetary benefit of saving fuel, which has made this stove model an economic success.

Similar designs have been replicated in many parts of Africa in the last two decades, sometimes with such success that in Eastern Africa, for example, they can be considered the new baseline technology.


Original KCJ's being distributed through a regional supermarket chain (Nakumatt):

Jikos going to Kigali
Jikos going to Kigali


Recent efforts to design even more efficient charcoal stoves than the KCJ include light-weight designs made of metal only. These are relevant for areas where suitable clay is lacking or where it is found challenging to combine metal and ceramic workers to produce one common product.


Design Improvements of Charcoal Stoves

Charcoal stoves can be improved in terms of durability and better heat use. The main agent which controls the burning of charcoal is air. To regulate the heat output, the airflow must be adjustable.

Charcoal radiates to all directions, not only upwards towards the pot. Thus it is crucial to minimize heat loss to the bottom and the sides of the stove. Cleverly designed air circulation in the stoves can "re-use” heat and create draft needed for efficient combustion. This can be achieved for example by adding a deflector plate between charcoal chamber and the stove bottom to radiate heat back upwards.

Although radiation decreases exponentially with the distance, the pot must not be put directly on the glowing char. A gap should be allowed where ideally fresh or even pre-heated secondary air can be directed above the charcoal to ensure a complete combustion of the carbon monoxide. This reduces emissions and increases fuel efficiency.


Summary of desired features:

  • durability
  • insulation or re-use of the radiated heat
  • high and adjustable airflow to maximize combustion
  • low emissions of carbon monoxide


New generation charcoal stoves were a major topic at Stove Camp 2010 held at Aprovecho Research Institute. A summary of Christa Roth is provided below:[4]


Observation Necessary Action Derived Design Principles

Charcoal radiates heat to all
sides: as much can radiate
towards the bottom of the stoves
as can radiate upwards towards
the pot.

Avoid loss of radiating and
conducting heat from charcoal that is not directed towards the pot.

  • Add space between the charcoal grate and other stove parts: Lift the charcoal grate slightly off the bottom of the stove and increase the space to the sides of the stove.
  • Limit the places where the hot grate can conduct heat to other stove parts.
  • Add a deflector plate between charcoal chamber and the stove bottom to radiate heat back upwards.
  • Insulate the stove bottom to prevent heat loss through the bottom.
  • Insulate sides of the stove.

Regain heat through air circulation (air cooling of stove) by passing air through heated stove parts thus preheating air entering the combustion system. This can be by passing primary air through the deflector plate below the grate and/or secondary air through a gap between double side walls of the stove.

Charcoal combusts in function of the available oxygen. Thus heat generation is a function of
air supply to the charcoal grate.

Get the right amount of air to the charcoal grate. Too little will choke the combustion, too much will cool the flue gases.

If power of the stove is too low, increase air supply by

  • making more holes in the grate.
  • adding a ‘Henson pig-tail’ vertical air-pass through the charcoal bed.

Do not pile the charcoal up too high, as this will restrict air flow through the charcoal bed (this is influenced as well by the shape and particle size of the charcoal chunks).

The combustion of charcoal goes from oxidizing C to CO, then in
a subsequent step from CO to CO2.

CO is a toxic gas and has still considerable energy value. Ensuring a complete combustion
will increase energy output and reduce toxic emissions.

Avoid CO emissions.

Charcoal radiates heat but there is also considerable convection of hot flue gases.

Optimize transfer of created heat into the pot.

Avoid obstructions between the radiating charcoal bed and the bottom of the pot (increase the view factor of the charcoal seeing the pot).




Stove Factsheets and Manuals

The stove factsheets are a series of technical information sheets on different stoves promoted by GIZ. Wherever available, additional information such as construction manuals and user guidelines for the respective stoves is also provided.

 

Further Information


References

This article was originally published by GIZ HERA. It is basically based on experiences, lessons learned and information gathered by GIZ cook stove projects. You can find more information about the authors and experts of the original “Cooking Energy Compendium” in the Imprint.



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