Gasifier Stoves

Introduction – the Gasifier Principle

Micro-gasifiers cookstoves are small gasifier devices that create their own gas from solid biomass and are small enough to fit directly under a cook-pot. Gasifiers can separate gas generation from gas combustion in space and time. In a reactor (gas generator) that is optimized for heat-dependent drying and pyrolysis, solid biomass is first converted into gases and vapors. These are guided into a combustion zone (gas burner) where they are burnt with a surplus of oxygen from a secondary air inlet. The following infographic shows this principle.

In a conventional fire the heat is controlled by regulating the fuel supply: the more fuel we add, the more heat we generate, provided sufficient air is available for the combustion. In contrast to this, gasifiers control and optimize both processes separately to achieve efficient and clean utilization of the fuel:

• regulating the heat that is reaching the solid biomass to optimize the drying and pyrolysis process
• controlling the supply of air and regulating the availability of oxygen for optimizing the subsequent steps of wood gas combustion and char gasification. More air to the ‘gas-generator’ produces more gas that can be combusted in the ‘gas-burner’. The regulation, however, is a bit tricky and one of the challenges is to find a better solution to this.
  

Gasifier stoves are currently the cleanest burning option to burn solid biomass in a cook stove. The “gas burner” provides the convenience and efficiency similar to cooking on fossil gas with very little soot and other emissions.

For more information on the process of wood gas generation see also Cooking with Woodgas.

Advantages of Gasifier Stoves

Compared to solid biomass burning improved cookstoves, gasifiers have certain advantages:

• Cleaner burning of solid biomass (considerable reduction of soot, black carbon and indoor/outdoor air pollution).
• More efficient due to more complete combustion (less total biomass consumption).
• Uses a wide variety of small-size biomass residues (no need for wood pieces or charcoal).
• Easy lighting allows for cooking to commence within minutes, much faster compared to lighting charcoal.

There are also advantages compared to stoves operated on alternative fuels like liquid fuels or solar:

Solid biomass fuels are often available locally (affordable access at own convenience), easy to transport and easy to store after gathering.
Creation of gas from dry biomass can be achieved with very simple inexpensive technology directly in the burner unit, which is portable and does not require piping or special burner-heads (in most cases).
Performance similar to biogas (but not dependent on water and bio-digester) and approaching the convenience of fossil gases.

Furthermore, pyrolytic micro-gasifiers can create charcoal, which may be used in a charcoal stove, further processed into charcoal briquettes or used to improve soil productivity as biochar.

Disadvantages and Challenges of Gasifier Stoves

Users may encounter some challenges when using gasifier cookstoves:

• Micro-gasifiers need small-sized fuel. They are only useful if fuel is available in the right size. Firewood in the form of large logs or sticks is not suited and needs other types of improved stoves.
• Most micro-gasifiers are batch-loaded and cannot be refueled during use. Thus, cooking times are pre-determined by the size of the fuel container.
• The heat output of most micro-gasifiers is not easy to regulate unless the stove is operated with a fan for forced convection. In this case the power of the fan can be regulated.
• Micro-gasifiers burn the biomass in two stages: first the gas-generator produces the wood gas, which is a thick whitish ‘smoke’ or fume. This “smoke” is burnt by the gas-burner, which is thus basically a ‘smoke-burner’. If the gas-burner operates well then there is no problem. However, should the flame of the gas-burner extinguish (e.g. blown out by gusty wind), the gas-generator will continue producing wood gas, which will not be burnt and then escape as thick white smoke from the stove.

The Potential of Gasifier Stoves

Gasification has an immense potential to make use of a great variety of biomass residues that are difficult to burn cleanly in conventional stoves. Wherever stick-wood is plentiful and at a low cost available, conventional improved cook stoves (e.g. rocket stoves) are more attractive options. In areas where charcoal and firewood are becoming a scarce or an expensive commodity, micro-gasifiers will be of growing relevance as an option to cleanly burn alternative biomass fuels.

Micro-gasification for household cooking is a relatively young development. The principle was invented in 1985 and the first commercial micro-gasifier was available in 2003. Since 2011, there has been a significant increase in the diversification of gasifier models. New developments come up virtually every day.

Gasifier Stove Diversity

Most gasifier stove models follow the basic TLUD principle. TLUD stands for Top-Lit Up-Draft. This is different from most stoves which are Bottom-Lit Up-Draft. With a TLUD stove, the fuel is loaded all at once into a container and lit from the top of the stove. Slowly, the top fire heats biomass below, and it gasifies. The flame burns the gas emitting from hot biomass below. When all the gas is gone, only charcoal is left and the flame goes out.

 

TLUDs are easy to adapt and replicate within individual projects without patent infringement or copyright issues. The simplest TLUD can be in the form of a single tin can combustion unit with separate entry holes for primary and secondary air.

Basic design features of a TLUD gasifier stove are shown in the following figure.

There is a steadily growing variety of gasifier cookstoves, using either natural draft or forced air by a fan, to improve mixing of flame, gas, and fume and to reduce emissions. The publication Micro-gasification provides a compilation of available models. 

Gasifier Stoves for Chunky Fuels

Chunky fuels such as wood chips, briquettes, pellets, etc. allow for sufficient natural air-flow through a fuel by way of natural draft. Most natural draft stoves are easy to replicate for local tin-smiths in developing countries, even without electricity access. The oldest known Top-Lit Up-Draft (TLUD) concept applied in a developing country is the so called PekoPe design. It was invented by Paal Wendelbo (1929-2014) in Africa in the late 1980s and introduced in Malawi, Tanzania, Uganda, Ghana and Mozambique. It is likely the simplest TLUD design with the longest proven field-experience. It is very simple to make using any type of metal and ideal for replication. Furthermore, the stove can be scaled from household sizes to institutional and commercial sizes.

There are also fan-assisted stoves, which typically have very low emissions and provide a high level of convenience to the user; however, the fan makes them more expensive than natural draft gasifiers.

One example is the Philips stove, which had previously been built in small numbers in India and is now produced in South Africa. In November 2014, Phillips and SNV decided to collaborate in Kenya and Ghana for disseminating the stove.

Another example is the stove ACE 1 produced by African Clean Energy (ACE)LF1] in Lesotho since 2011. The company used to produce the Phillips stove and has now developed its own model. The ACE1 stove is made of a ceramic combustion chamber placed in a stainless steel body. It uses a solar panel to charge the battery, which can also be used to charge a mobile phone or run a LED unit. When fully charged, the battery will power the fan for over 20 hours of cooking. The stove saves up to 70% of fuel compared to traditional stoves. According to the company, 40,000 stoves have been sold up to 2015. The stove is sold for 150 USD.

TChar stoves combine the benefits of a TLUD gasifier stove with the advantages of a charcoal stove. A TChar stove starts with almost any type of dry biomass as fuel for gas-style cooking while making hot charcoal that is subsequently burned to continue the cooking on a charcoal stove that is the base of the TChar unit.