Charcoal gas stove

By Vuthisa

Background

This blog post aims to provide the reader with some background on charcoal stove usage as well as charcoal burn characteristics.  Vuthisa developed a unique charcoal stove, able to extract and burn harmful Carbon Monoxide gases inherent to all types of charcoal.

Jiko2
Jiko charcoal stove

We discovered that charcoal use by households is not a new concept and constitutes the primary urban fuel in most of Africa and it is estimated (2003) that approximately 250 million people cook with charcoal. The prevailing tendency in household fuel usage has been to move away from wood fuel towards charcoal for reasons ranging from smokeless burn, ease of use, easy storage, no insect infestation, no need to air-dry, cost effective to transport and high temperature burn.

Rural village in the Eastern Cape provinceSeveral intervention studies have also shown that switching from wood to charcoal can substantially reduce respiratory infections, which may also account for the move over to charcoal. Much of the world’s charcoal feedstock is not plantation wood and the unsustainable harvesting of biomass result in widespread deforestation, thereby handing charcoal its bad reputation. In our context, countries like South Africa (and most developing countries) with well managed commercial plantations and exotic invaders however can provide a sustainable supply of charcoal for household end-use applications. In our opinion improved charcoal stoves should only be sold to communities if the charcoals are produced from carbonised invasive alien vegetation or managed commercial plantations. See our Welcome post in this regard. The manufacturing process is also very polluting and wasteful and there is a serious need of improved charcoal kilns. Join the Portable Kiln Google Group, which I started in order to improve the efficiency of this design or for more information visit this information page and join our design challenge.

More modern fuels such as paraffin- and LP Gas are becoming more popular than fuel wood for cooking and space-heating, but have led to several deaths due to accidental shack fires or lethal gas leaks. Burning charcoal conventionally inside a home is a hazardous and potentially fateful undertaking due to the dangers of Carbon Monoxide (CO) poisoning. I highly recommend Carbon Monoxide Poisoning – A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References.

Some experts agree however that if issues of concern related to CO such as ventilation and education can be successfully addressed, the widespread implementation of improved charcoal cook stoves should be considered. Traditional charcoal burning stoves such as the metal and ceramic Jiko and Loketto were designed to retail cheaply and provide durability during extended usage, which they do, but failed to address the problem of Carbon Monoxide (CO) poisoning once the occupants go to sleep.

Vuthisa decided to investigate charcoal burning by developing and field testing many prototypes. Feedback from stove users indicated that charcoal can be very abrasive on metal surfaces in direct contact with flames and due to the rapid heating and cooling tendencies of a charcoal fire. Charcoal burned in coal or fuelwood stoves reduced the lifetime of these stoves significantly. Refractory ceramics (and low-density clay bricks) offered the best results as it insulates the fuel from the stove body, but also allows combustion temperatures to rise above 600°C for complete combustion of volatiles.

We discovered that the volatile Carbon Monoxide (CO), a fuel in its own right should instead of being vented off be re-combined with pre-heated Oxygen (O2) from the air in a process termed Gasification. This results in a LP gas-like flame, venting harmless CO2. The process occurs spontaneously and the charcoal fuel batch will burn out in its entirety within 90 minutes from lighting the stove. Depletion of Oxygen levels in a poorly ventilated room can therefore not occur and CO issuance build-up remains below Health and Safety limits.

Best Burn Method

We found it is possible to light the stove indoors with minimal particulate issuance by top lighting the stove using kindling (visit woodheat.org to see the advantages of the top down lighting technique). We monitored CO build-up inside the room with a Draeger X-am 5000 CO monitor, maintaining adequate ventilation. The maximum CO ppm (parts per million) recorded was 30 ppm over a 1.5 hour period. Once the occupants gets exposed to levels of around 200 ppm for extended periods they will experience nausea, headaches and vomiting. Higher levels of around 1000 ppm for example, have far more serious consequences, including falling into a coma and never waking up.

Back to charcoal burning: After the stove is lit, temperatures will continue to rise inside the chamber and more coals will start to combust, although the top layer will be protected by a so-called “pyrolysis wind”. Primary air is drawn up the combustion chamber by virtue of the internal chimney. The stack length has been optimized to control the draft to combine 6 parts air to 1 part fuel, with the secondary air supplying the remaining 6 parts air to resultant 1 part producer gas. Pre-heated secondary air (drawn in along with the primary air before splitting off) will spontaneously interject into the area above the charcoal particles and mix with a constantly escalating supply of producer gas (CO and other volatiles) from the coals. Stoichiometric air/fuel ratio is achieved after approximately 8 to 10 minutes following start-up and gasification commences, the visual clue being a blue flame front forming off the burn plate (catalyst). Most charcoals contain approximately 20 to 30% producer gas, i.e. is not vented during the charcoal manufacturing process and the stove will deplete these supplies in approximately 30 to 40 minutes. The resultant ‘coke’ will also partially gasify and burn to ash over the remaining 30 to 45 minutes. The stove can be operated on a maximum fuel load of 500 grams of charcoal for approximately 1.5 hours. It can use as little as 150 grams of charcoal, sufficient to cook a small meal or for boiling approximately 1 L of water. A full patent was granted in 2006 and is still in force. Double click on video image below to see charcoal stove in action.

We’re not the only ones thinking charcoal gasification has potential in the developing world. See this publication by Ulrich Graf called Low Cost Charcoal Gasifiers for Rural Energy Supply (GTZ, 1994, 49 p.). The publication demonstrates to interested laypersons and experts the conditions and applications under which small charcoal gasifiers can be one option for development within a range of simple energy technologies.

It is also possible to “pipe off” volatiles produced by a charcoal gasifier to run a generator as can be seen in this example:

Or watch this video below showing some of our Bioenergylist discussion group members experimenting with a burner attachment. Double click on the still image below to start video.

I also converted a barbecook® into a charcoal gasifier.  Just to prove the concept as I think flame grilling with charcoal has a lot of potential.

Durability

Remainder of mix poured over mold

Bricks extracted from mould and ready to be firedVuthisa partnered up with the University of KwaZulu-Natal’s Ceramic Department in Pietermaritzburg to develop insulated fire bricks. We have made significant progress with our insulated fire bricks, with the mixture consisting of mainly Al2 O3, Fe2 O3, SiO2, CaO, grog and a light-colored refractory clay. An external supplier was contracted to supply the pre-mixed clay in powdered form. The cost per cubic metre is low and lends itself perfectly for this application. The density is lower than what can be achieved with clay mixes that contained sawdust. Our most recent bricks achieved a density of approximately 0.5 g/cm3 which qualifies it as an extremely lightweight yet durable refractory brick. The bricks and the ash filtering base are then joined together via a specially formulated paste-like cement that can withstand temperatures of up to 1300°C to form a hex shaped combustion chamber.

We also have a monolithic, precast, pre-fired, silicon-hardened, fibrous, refractory ceramic sleeve (not replicable outside South Africa) with a density factor of around 0.5 g/cm3, which is being used in our outdoor camping stove.

Dissemination

Stove programs in South Africa have a poor track record (Wood as a source of fuel in South Africa, MV Gandar, 1983). Attention is all too often focused on fuel efficiency, economics and ease of construction at the expense of the socio-cultural environment. It is therefore important to slowly introduce a new stove concept to the target community to test their acceptance of it, but also to demonstrate the advantages that owning such a stove offers. It will be vital to do product acceptance trials in the proposed project implementation area and adopting a successful dissemination technique will take the stove project towards success through inception to maturation. Extension workers should record cooking technique and fuel usage information. The information must be interpreted to fine tune the design specific to the needs of the community.

In summary

It is hoped that by informing a wider audience, strategies can be formulated to improve charcoal making and charcoal burning technologies.

Camping stove to create awareness

Our prototype portable charcoal camping stove aims to highlight the plight of millions of people cooking on inefficient and unsafe charcoal stoves. It is not for sale at this time. Kindly donate (on the sidebar to the right) to help us bring the stove to the market!

In line with creating awareness around issues concerning indoor air pollution, we also promote the clean-burning StoveTec Rocket stove. Click here for more information.

Enquiries welcome via our Contact us page or visit the News link from time to time for updates.

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