Firewood Cookstoves

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Introduction

Firewood is wood from logs, sticks or twigs. It has been used as a fuel since the beginning of mankind. In principle, it is renewable and relatively easy to produce, transport and store. However, the use of firewood for cooking is commonly associated with deforestation and health problems. This is not an inherent problem of the fuel, but is strongly influenced by the quality and quantity of its correct usage and can be overcome by improving the efficiency of the wood fuel usage.

Two major factors determine if firewood burns clean and efficient: its moisture content and the oxygen supply of the fire. 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 supply 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.


The burning of wood is a sequence of steps:

  1. Moisture is evaporated
  2. Wood decomposes into combustible wood-gas and char
  3. 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 14-15 in the "Manual on Micro-gasification". For more information on the characteristics of firewood as a fuel see Cooking with Firewood.





The Wood-Fuel Cooking System

GIZ woodfuel cooking system 2011.jpg

As shown in the figure beside, in the wood-fuel cooking system, firewood is mixed with air in a reactor. After ignition, a chain reaction is triggered in which heat is generated. This heat is transferred through 3 processes:

Convection: Hot gasses are passing a surface transferring heat into surrounding materials;
Radiation: Red hot embers is radiating heat into surrounding materials;
Conduction: Heat is conducted through materials. Metal is a good heat conductor, whereas air is a poor heat conductor.

The reactor is emitting heat, but also light, gasses and particles. While the emission of heat is wanted, the emission of gasses, particles and light are rather unintended. Good stove designs can reduce the quantity of unwanted emissions in favor of additional heat generation. The heat does not enter automatically into a cooking pot. The design of the heat transfer unit has a big effect on the percentage of the heat transferred into the food to be cooked.



Overall there are two major dimensions for efficiency gains for firewood stoves:

  • Achieve complete combustion (=‘create more heat per unit of fuel used’)
  • Improve heat transfer (=‘get more heat actually into the pot’)



Three-stone Fires or Open Fires

Worldwide, millions of people cook on so-called 3-stone fires or open fires as this is the simplest and cheapest “stove” to create. Only three suitable stones of the same height are needed to balance a pot over a fire. However, the daily use of these 3-stone fires has the following disadvantages:  

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.
  • 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.
  • Smoke:
    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 smoke (= unburned particles).
  • 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.


On the other hand, users welcome some of these inefficiencies due to their positive side effects:

  • Open fires burn slow and do not require frequent attention. This is convenient if other household chores have to be done at the same time.
  • Smoke can chase away mosquitoes, which is especially beneficial 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 favorable in cold areas.



Three-stone Fire versus Improved Cookstove

The development of improved cookstoves is facing a dilemma: the same characteristics are at the same time responsible for both users’ complaints and appreciations of the 3-stone fire. There is no solution which can satisfy all expectations. Any new stove will be a trade-off between different user needs. This dilemma is summarized in the table below. Furthermore, users are used to a specific cooking system and any change in cooking habits needs time.


Common changes of parameters in improved cook stoves Common expectations towards improved stoves "Disadvantages” for associated benefits of open fires

Improve efficiency of heat production

  • More complete combustion = less emission of unburned fuel
  • Shelter the fire against the wind;
  • 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 of the 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
  • Some stoves don’t fit all pot sizes



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.

A 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



Design Principles for Improved Cookstoves

All improved firewood stoves apply at least some of the aspects listed below geared toward increasing efficiency and improving heat transfer.[1]


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) since the hottest point of a fire is a bit above the end of the flame and for the volatiles to burn off high temperatures and enough air supply is needed. 
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 insulation between an inner and an outer wall of the stove. Via a secondary air inlet, air is channeled through this gap and pre-heated before entering the combustion chamber.
  • Rest the wood on a shelf. The air passing under the shelf is preheated.
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.
  • Insert the pot into the stove body.
Make sure that the heat is going into the pot instead of going into the stove body
  • Insulate the fire with lightweight, heat resistant materials.




The Rocket Stove Principle

One of the most successful concepts in stove design is the rocket stove principle, invented by Dr. Larry Winiarsky. Rocket stoves’ characteristics are:[2]


  • An elbow-shaped combustion chamber (1:1.5) with a shelf for the fuel wood, which 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 tall combustion chamber 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, emitting all their heat and discharging mainly carbon dioxide and water vapor.
  • These hot flue gases pass through a well-defined gap between a ‘skirt’, and the pot, 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.
Rocket stove.JPG
The rocket stove principle.


Pic2.JPG
A rocket-type stove in action, and showing insulation of the burning chamber, skirt around pot and support frame




Today, most of the GIZ-promoted wood stoves follow this rocket stove principle (see fact sheets below for examples). Besides household stoves also stoves for institutional or productive purposes can incorporate the rocket stove principle. For example in Malawi, the considerable savings have made institutional rocket stoves very popular among school feeding programs (see also Ashden Award video 2006).

Institutional Stove Compared to an open Fire: 40 instead of 170 kg of firewood
School feeding program Mary`s Meals Blantyre, Malawi

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. Forced convection can reduce emissions of stoves by up to 90 %, thus alleviating Indoor Air Pollution (IAP) levels. See also the article onMicro-Gasifier Cookstoves. Recently, thermo-electric generators (TEG) have been developed to power fans in stoves. They use the temperature differences within the stove to generate electricity, thus eliminating the need for external power supply. TEGs also have great potential to provide power to other applications, such as LEDs or mobile phones. However, the unit makes a stove more expensive and can be destroyed easily, when getting too hot. Pico PV units could also easily provide that little electricity needed for mobile charging without burning firewood.


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.


Fixed Stoves



Portable Stoves

 





Further Information

  • 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).
    Depending on the availability of construction materials, components and production options, three stove options are available for this tool:
    • 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)

 




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.

  1. ↑ Aprovecho Research Center (2005): Design Principles for Wood Burning Cook Stoves. http://www.ewb-usa.org/files/2015/05/PrinciplesWoodBurningCookStoves.pdf
  2. ↑ http://www.ashden.org/files/Aprovecho2006.pdf


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