Firewood Cookstoves
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Firewood
Firewood is the first fuel used for cooking in human history. It is renewable and easy to produce, transport and store.
The burning of wood is a sequence of steps determined by rising temperatures:
- a) Moisture is evaporated
- b) Wood decomposes into combustible wood-gas and char
- c) Char is converted into ash
The main influencing agent for “a)” and “b)” is heat, whereas “c)” is regulated by the supply of oxygen. Find here more information and illustrating figures on pages 8-11 in the manual on micro-gasification (http://www.gtz.de/de/dokumente/giz2011-en-micro-gasification.pdf)
Picture: Malawi-3-stone-fire (GIZ-Feldmann)
Firewood can be used for cooking even in the absence of a “stove”. Even to date, campfires are a popular leisure activity in developed countries. However, they are not favoured for daily cooking. Some disadvantages of the open fire are:
- Smoke (= unburned fuel particles in the air):
The combustion in an open fire tends to be incomplete as oxygen might not reach where it is needed. Low temperatures also contribute to the emission of unburned particles.
- Slow pace of cooking:
Even if a lot of heat might be generated, the heat is not directed to the cooking pot, hence a lot of heat is lost to the environment. This problem is accelerated if there are windy conditions as the flames are not shielded. The cooking pot does not sit in the hottest part of the flames; hence less heat is transferred into the pot than theoretically possible.
- Health risks:
As the flames are not directed or shielded, the cook can easily catch fire when approaching the cooking pot. Sparks pose an additional risk when approaching the fire. Burns are a common effect of open fires. The smoke might cause eye infections.
- High fuel consumption:
The open fire consumes a lot of fuel as (a) not much heat is generated per unit of fuel, (b) only a small proportion of the heat is actually directed to the pot and (c) only a small fraction of the heat directed to the pot is actually transferred into the food.
On the other hand, some of these inefficiencies are also welcomed due to their positive side effects:
- Open fires are burning slow and do not require frequent attention. This is welcome if other household chores have to be done at the same time.
- Smoke can chase away mosquitoes in malaria-infested areas;
- Smoke can be used to preserve food;
- Open flames are emitting light, which is welcome before sunrise or after sunset;
- Open fires are emitting heat, which is welcome in cold areas;
These observations on the open fire can be summarized as follows:
| Parameter | Advantages | Disadvantages |
|---|---|---|
| Small proportion of heat is directed to the pot; Small proportion of heat is transfer into the food |
Slow cooking allows for other household work to be done at the same time | Slow cooking, Inefficient cooking |
| Emission of smoke (unburned fuel particles) | Repellant for mosquitoes, food preservation | Health risk, Inefficient cooking |
| Emission of heat to the environment | Warming of space in cold areas |
Inefficient cooking |
| Emission of light to the environment | Good vision before dawn or after sunset |
Inefficient cooking |
The development of improved cook stoves is therefore facing a dilemma: the same characteristics which are at the same time responsible for complaints against and appreciation of the open fire. There is no solution which can satisfy all the expectations which are expressed in this table. Any new stove will be a trade-off between different user needs.
| Common changes of parameters in improved cook stoves | Common expectation towards an improved stove |
"Disadvantages” for associated benefits of open fires |
|---|---|---|
Improve efficiency of heat production
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Improve heat transfer into the cooking pot
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Households have to prioritize their needs in order to come up with the decision if an improved cook stove is suitable for them. In areas with fuel scarcity, the need for reduced fuel consumption might be ranked higher than the need for space heating or lighting after dark.
Another strategy can be to provide additional solutions to complement the introduction of the improved cook stoves:
- an extra space heater for the cold season;
- a mosquito-repellent net,
- a solar lantern for lighting.
Stoves for firewood have been developed since 3000 years. Overviews on types and models have been developed from various entities, this is just a selection:
- UNESCO(1982): Consolidation of information. Cooking stoves Handbook http://unesdoc.unesco.org/images/0005/000530/053052eb.pdf
- GIZ (1995) by Westhoff/German 'Stove Images - a Documentation of Improved and Traditional Stoves in Africa, Asia and Latin America'. Also in French and Spanish onhttp://www.gtz.de/en/themen/umwelt-infrastruktur/energie/32777.htm and http://www.gtz.de/en/themen/umwelt-infrastruktur/energie/32777.htm]
The Aprovecho Institute in Oregon has analysed the design principles which can help to make firewood stoves more fuel-efficient. If all principles are applied, the result would be called a rocket stove, which was invented by Dr. Larry Winiarsky. For further details see www.aprovecho.org.
The Rocket Stove Principle
One of the most successful new concepts in stove design is the rocket stove principle.
- It has a tall combustion chamber which behaves a bit like a chimney; creating more draught than a standard stove. This assists in mixing the air, fuel particles and volatiles, resulting in a hot flame. The internal walls are insulated, reflecting all the heat back into the chamber rather than losing it to the stove body. The insulation keeps everything very hot so that the chemical reaction is more intense, whilst the tall chamber provides more time in which the gases and particles can be burnt completely, giving out all their heat and discharging mainly carbon dioxide and water vapour.
- These hot flue gases pass through a well defined gap between a ‘skirt’, and the pot, as shown in the illustrations given below, resulting in a large percentage of the heat being forced against the sides of the pot, and being transferred to the pot. Where various sizes of pot are used on the same stove, the skirt can be funnel-shaped to accommodate different pots, although some efficiency will be lost.
- An elbow-shaped combustion chamber, with a shelf for the fuel wood, supports the pre-drying of the firewood and allows a controlled, and sufficient, flow of primary air to be warmed as it passes under the wood to the burning wood tips.
| The rocket stove principle. Source: Aprovecho |
Design principles, which can be used more generally include:
- Insulation around the fire and along the entire heat flow path using lightweight, heat resistant materials
- A well-controlled, uniform draught in the burning chamber during the entire combustion process
- Use of a grate or a shelf under the firewood
- Heat transfer maximised by the insertion of the pot into the stove body or using a skirt around the pot.
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| A rocket-type stove in action, and showing insulation of the burning chamber, skirt around pot and support frame. Source: GIZ / Aprovecho Institute |
However, even all other improved firewood stoves do apply at least some of these principles.
How can we improve the design of the stove to increase the combustion efficiency in a firewood stove?
| Principle | Solutions |
|---|---|
| Increasing the temperature in the combustion chamber (as the burning process is temperature controlled) |
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| Reduce the intake of firewood | By creating a small entrance for the firewood, only the required level of wood can be entered. Excess wood cannot be supplied to the reactor |
| Burn off all the volatiles | Allow enough space in the combustion chamber (increasing the space between pot and fire) |
| Adequate air supply | Air and wood intake into the combustion chamber are regulated and correlated |
| Reduce the inflow of cold air | Regulated air intake (door) |
| Intake of pre-heated air | Air is used as an insulated between an inner and an outer wall of the stove. Air is channeled through this gap before entering the combustion chamber |
| Increasing the draft |
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| Increase the surface of the wood that is in contact with air |
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'How can'we improve the design of the stove to improve the heat transfer in a firewood stove?
| Principles | Solutions |
|---|---|
| Raise the pot to the highest point of the flames |
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| Force the hot air to create turbulences on the surface of the cooking pot | Create a small gap between the cooking pot and the pot rest which is big enough not to choke the fire and small enough to mix the air close to the pot. |
| Increase the surface area for the heat transfer | Create a skirt around the pot which is forcing the hot air to the walls of the pot. It creates turbulences in the air around the pot surface. |
Application of the principles
Clay stove (versus a 3-stone fire)
| Bernier Malawi Clay-stove.jpg |
Increased combustion efficiency:
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Improved heat transfer:
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There are quite a number of improved firewood stoves which – like this simple clay stove – adhere to some of these principles and deliver some improvement compared to the 3-stone fire. They are an entry point for households into the use of improved cook stoves as they are more affordable as compared to the sophisticated rocket stoves. Examples are:
, Chitetezo Mbaula (Malawi)- Jiko Kisasa (Kenya),
- Tulipe (Benin)
- Anagi stove (Sri Lanka)
- VITA (Mauretanien)
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Institutional Stove Compared to an open Fire: 40 instead of 170 kg of firewood |
The considerable savings have made institutional rocket stoves very popular among school feeding programmes in Malawi (see also Ashden Award video 2006, http://www.ashdenawards.org/winners/aprovecho http://www.ashdenawards.org/winners/aprovecho ttp://www.ashdenawards.org/winners/aprovecho)
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School feeding programme Mary`s Meals Blantyre, Malawi |
Fixed stoves from mud, brick or cement
Today most of the GIZ-promoted high-efficiency wood stoves follow this rocket stove principle (see fact sheets for the examples):
One pot, mostly without chimney
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One-Pot Rocket Mud Stove, Shielded Fire, Uganda (2008): |
| Jiko Kisasa Kenya |
| Esperanza stove Malawi (2008) |
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Inkawasi Stoves in Peru (various models adapated for different regions and materials):
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| MIRT stove for injeera baking Tikikil Ehtiopia |
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Rocket Brick Stove, Kenya (2011) GIZ PSDA Stoves Promotion |
Multiple pots with chimney
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Two pot mud-rocket Lorena, Uganda
and construction guide from 2008 featuring air-bypass: http://www.energyandminerals.go.ug/pdf/gtz/HOUSEHOLD%20Stoves%20Construction%20Manual%20August%202008.pdf), available in English and French |
| Jiko Kisasa Kenya, double pots |
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Institutional Rocket Stoves, example with optional chimney: Rocket Institutional Stove, Uganda (2006) |
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Institutional Metal Rocket Stove, Malawi (2008)
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Malawi Institutional Brick Rocket Stove, Malawi (2008)
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Portable / movable rocket stoves
| One pot, mostly without chimney |
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| Institutional Metal Rocket Stove, Malawi (2008) |
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Models developed by Stovetec: http://www.stovetec.net/us/stove-models Models developed by Envirofit: http://www.envirofit.org/ |
| Multiple pots with chimney |
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Inkawasi portatil, Peru (2011) |
Efficient, smoke-free cooking with the Rocket Stove:
Additional information resources
Aprovecho Research Center
For almost 30 years, Aprovecho Research Center (ARC) consultants have been designing and implementing improved biomass cooking and heating technologies in more than 60 countries worldwide. Their website provides a wealth of useful information including construction materials. www.aprovecho.org
Design Tool for Constructing an Institutional Rocket Stove with Chimney
With funding from GIZ HERA, Rocket Stove.org and Prakti Design Lab have developed a new automated tool that allows users to build a customized institutional rocket stove. The tool can be used to instantly design a brick or metal institutional rocket stove with or without chimney for any institutional pot (30 L + capacity).
The stove options are:
- fixed brick stove (w/out chimney)
- portable metal stove with square combustion chamber (w/out chimney)
- portable metal stove with circular combustion chamber (with chimney)
Rocket stove principle
An animation showing the rocket stove principle can be found here: http://vuthisa.files.wordpress.com/2011/03/rocketstoveanimation.gif
New wood fuel stove designs
Two major factors determine if woodfuels burn clean and efficient: its dryness and sufficient ventilation, hence, the right amount of air on the right spot during the process to ensure a complete combustion.
While it depends on the user to make sure that the fuel is dry, the air-flow depends on the stove design. In a natural draught stove, the movement of air is created by the chimney or stack height of the fuel. However, there must be a difference in temperature between the stove and the top of the chimney for generating draught. Natural draught is likely to cause incomplete combustion with higher emissions and energy losses through the chimney. Moreover, it is also difficult to regulate.
Wood fuel stoves with forced convection
Instead of naturally ‘pulling’ air through a stove by stack height, fans or blowers are useful to ‘push’ air into the combustion chamber. This enhances a good air-fuel mix and thus, more complete combustion. Electricity is the most convenient power source to create a forced air-flow. It can be provided by batteries or, if available, through the grid. Recently thermo-electric generators (TEG) are being developed to power fans in stoves. They use the temperature differences within the stove to generate electricity. Though TEGs have great potential to provide also power to other applications (LEDs, cell phone charging), they are still in their infancy. Forced convection can reduce emissions of stoves by up to 90 % and thus alleviating IAP levels. More test results from more widespread use are expected soon.
Additional information: BioLite
BioLite: http://www.biolitestove.com/Technology.html
