Firewood Cookstoves

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Firewood

Firewood has been used as a fuel since the beginning of mankind. It is renewable and relatively easy to produce, transport and store.

The burning of wood is a sequence of steps:

• 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 today, 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:
  The cooking pot does not sit in the hottest part of the flames; hence less heat is transferred to the pot than theoretically possible. Even if a lot of heat is generated, the heat is not directed to the cooking pot and heat is lost to the environment. This problem is accelerated if there are windy conditions as the flames are not shielded.
• 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 also 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 that is 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 burn 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 emit light, which is welcome before sunrise or after sunset;
• Open fires emit 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 transfered 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 are at the same time responsible for both complaints and appreciations of the open fire. There is no solution which can satisfy all the expectations. Any new stove will be a trade-off between different user needs. This dilemma is summarized in the table below.

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 More complete combustion = less emission of unburned fuel Shelter the fire Direct flames to the pot	Consume less fuel Emit less smoke Be safer Cook faster	Faster and more efficient stoves require often more attention by the cook No mosquito repellent, no food preservation Less space heating No lighting after dark
Improve heat transfer into the cooking pot Position cooking pot at the hottest place of the flame; Hot gasses are passing close to the pot to maximize heat transfer;	Consume less fuel Cook faster Be safer	Stove is sometimes higher than the open fire

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 for over 3000 years. Overviews on types and models have been developed from various entities. Below is just a selection of these:

• 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.

A rocket-type stove in action, and showing insulation of the burning chamber, skirt around pot and support frame. Source: GIZ / Aprovecho Institute

All improved firewood stoves apply at least some of these aspects (listed below) geared toward increasing efficiency and improving heat transfer.  

How can we improve the design of the stove to increase the combustion efficiency in a firewood stove?

Aspect	How to achieve
Increasing the temperature in the combustion chamber (as the burning process is temperature controlled)	Shelter the fire against wind; Use isolative materials to reduce heat losses to the side and to the bottom;
Reduce the intake of firewood	Create a small entrance for the firewood. Then 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	Regulate air and wood intake into the combustion chamber and ensure both the amount of air and wood intake are correlated
Reduce the inflow of cold air	Regulate air intake (door)
Intake of pre-heated air	Use air 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	A vertical combustion chamber can increase the natural draft in the stove; A ventilator (battery, grid driven) can force the air through the stove
Increase the surface of the wood that is in contact with air	Small door allows only smaller pieces of wood to be entered into the stove; Rest the wood on a shelf (wood and air enters the stove through the same entrance, the firewood above the air); Firewood and air enter the combustion chamber through different entrances; the tips of the firewood hang free in the chamber with the air being supplied from below.

How can we improve the design of the stove to improve the heat transfer in a firewood stove?

Aspect	How to Achieve
Raise the pot to the highest point of the flames	Create a pot rest at the hottest point above the flames ; Direct the hot flue gasses directly against the cooking pot (= pot sits ‘on top of a chimney’)
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 forces the hot air to the walls of the pot. This 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: Fire is shielded against the wind; Door is reducing the amount and size of wood used; More space to burn off the volatiles; Less intake of cold air;
	Improved heat transfer: Pot sits higher above the flame; Most flue gasses have to pass the small gap between the potrest and the pot;

Quite a number of improved firewood stoves which – like this simple clay stove – adhere to many of the principles mentioned above and deliver some improvements 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 than the sophisticated rocket stoves. Examples are: (see below for Factsheets)

• Chitetezo Mbaula (Malawi)
• Jiko Kisasa (Kenya)
• Tulipe (Benin)
• Anagi stove (Sri Lanka)
• VITA (Mauretanien)

Institutional Stove compared to an Open Fire

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)

School feeding programme Mary`s Meals Blantyre, Malawi

Fixed Stoves

Today most of the GIZ-promoted high-efficiency wood stoves follow this rocket stove principle (see fact sheets for examples):

MIRT stove for injeera baking Tikikil, Ehtiopia
Jiko Kisasa, Kenya (2011)
Rocket Brick Stove, Kenya (2011)		GIZ PSDA Stoves Promotion Rocket Brick Stove: Builder’s Manual & User’s Guide
Two pot mud-rocket Lorena with Air Bypass, Uganda (2011)		Construction guide from 2008 featuring air-bypass available in English: http://www.energyandminerals.go.ug/pdf/gtz/HOUSEHOLD%20Stoves%20Construction%20Manual%20August%202008.pdf available in French:
One-Pot Shielded Fire Stove with air bypass, Uganda (2011)
Fixed One-Pot Rocket Mud Stove, Benin, Uganda (2011)
Esperanza stove, Malawi
Malawi Institutional Brick Rocket Stove, Malawi (2008)
Inkawasi Stoves in Peru (various models adapated for different regions and materials): Inkawasi UK for firewood and dung Inkawasi Tawa Inkawasi Tres hornillas Inkawasi Pichqa Inkawasi Sujta Inkawasi Plancha de Fierro
Manual Uso y Mantenimiento de la Eco-Estufa Justa, Honduras (2011)
Manual Construyendo la Eco-Estufa Justa 16 x 24, Honduras (2011)

Portable Stoves

Jambar Stove Firewood, Benin, Kenya, Senegal (2011)

Tulipe-Céramique, Benin, Burkina Faso (2011)

Burkina Mixte, Burkina Faso (2011)

Ouaga Métallique, Burkina Faso (2011)

Multimarmite Stove, Burkina Faso (2011)

Sakkanal, Senegal (2011)

Chitetezo Mbaula, Malawi (2008)

Institutional Metal Rocket Stove, Malawi (2008)

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 design a brick or metal institutional rocket stove with or without chimney for any institutional pot (30 L + capacity).

The stove options for this tool are:

• a fixed brick stove (w/out chimney)
• a portable metal stove with square combustion chamber (w/out chimney)
• a portable metal stove with circular combustion chamber (with chimney)

http://www.rocketstove.org/

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 ventilation. Hence, the right amount of air on the right spot is necessary 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 to generate 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) have been developed to power fans in stoves. They use the temperature differences within the stove to generate electricity. Though TEGs have great potential to provide power to other applications (LEDs, cell phone charging) as well, they are still in their infancy. Forced convection can reduce emissions of stoves by up to 90 %, thus alleviating IAP levels. More test results from more widespread use are expected soon.

Models developed by Others

Stovetec: http://www.stovetec.net/us/stove-models

Envirofit: http://www.envirofit.org/

BioLite: http://www.biolitestove.com/Technology.html

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