Holey Briquette Gassifier Stove Development:

Richard Stanley, Kobus Venter 14 August 2003 

Fuel Type
Stove Design


Dear stovers,

We (Kobus Venter and Richard Stanley ) have been developing a gasifier stove tailored to the holey briquette over the past four months or so. We have been doing this to see if using a holey briquette with its potential of offering a standardised fuel load (with control over the blend and shape) would allow us to make gasification more feasible and less costly in terms of the stove design. We want to see if it will help to popularise the briquette technology and product. While I have lent my experience in extending the briquette technology and its practical applications, Kobus has more interest in the stove per se and provided most of the technical design content and conducted the actual tests. Both of us are taking our experience from the stoves group, especially Lanny Hensen, Crispen, Ron Larson, Tom Reed, AD Karve, Paul Anderson, but also from the comments of others.

We are attaching our first progress report for your review, observations and suggestions. We can now say that after some eight controlled experiments, each involving modifications and combustion tests, we have achieved some success in actually gasifying the briquette. We have learned a lot about required design parameters about the stove and its required performance but we have a lot more to learn to finalise the design.

Like the briquette technology, the resulting stove will not be patented but rather diffused as widely as possible to a whole score of local manufacturers each responding to their local markets. In as much as any of you are interested in such, we say go for it. Let it flourish under free local competition in your respective local markets. This is not naive or foolish from a business perspective because of the consideration of the market: The stove is designed for use primarily by the rural or urban poor. It is unlikely to ever penetrate these mass markets if one attempts to manufacture and distribute from one central point over a wide spread area. Each of us will have better local resources for making and reaching our respective local markets.

The report follows:

Holey briquette Gasifier Stove Development:

14th Aug 2003

Kobus Venter / Richard Stanley


The intention in this effort is to develop a gasifier stove which will use a biomass based briquette made by the low pressure, manual process by rural /urban poor producers.

Our rational in designing a gasifier around the briquette is that it allows us a consistent shape, optimises the biomass resource and will enable postproduction of carbon-enriched briquettes from the char by-product. This will entice more users into the briquette technology as an income generating activity, entice the population away from fuelwood and charcoal production and at the same time improve domestic health conditions.

As seen from the perspective of the user, the use of the gasifier stove - in conjunction with holey briquettes - will provide a fuel which is more convenient, less expensive, cleaner, and more easy to handle and store. The gasifier stove as proposed at least will provide a clean and “modern burning” appliance.

As seen from the perspective of the producer, the production of char enriched briquettes (as with the conventional use of charcoal fines in briquettes) will utilise the same production equipment and processes, and the product will command higher selling prices in the market while costing less to produce than the conventional briquettes made of 100% biomass.

We have been contemplating several different design options, but will concentrate here on only the most promising one with the list for now. While we have had several trials and have successfully gasified the briquette with a useable char by-product, it is far from optimised:

At this stage we are sorting out the A/F ratios, and optimising the timing and positioning of entry ports for primary and secondary air etc. What we have learned thus far is that whatever the final design, it must fit the following technical criteria:

• The burn must be from the top down

• Primary and secondary air must be fed into the reaction at different locations

• The blue flame must be generated ABOVE the briquette, not along its sides

• The blue flame should be prolonged, consistent and replicable with subsequent and similar briquette fuel loads.-and,

• the success of gasification and pyrolysis of the briquette will likely be measured by the quality and quantity of heat output as well as the quality (consistency, density and solidity) of the charcoal by-product.

On to the specifics:


Briquette Specifications:

Material: Non-woody biomass: a mix of Guava/Mango leaves, waste paper and some carton board.

Dimension: Diameter: 10cm, Height: 7.5 - 8.5 cm, Central hole dia. : 20 - 30 mm diameter,

Density and weight: Density: 0.22 grams per cubic cm (g/cm3), Weight: 180 - 210 grams dry weight,

Production process: Produced by a compound lever manual press, under low pressure (13 – 15 kg/sq cm) out of a water slurry.


Combustion Chamber configuration

KV conducted eight (8) official and several more unofficial test burns each with varying configurations, varying the position, insulation and direction of the primary- and secondary air supply; Other variables were the combustion chamber heights and the pot/chamber gas exit gap.

The configuration of Test#6 is used below as it yielded the best results thus far.

Material: Refractory ceramics (riser sleeve):

Dimensions: Height: 23 cm, Diameter (IDD): 14 cm, Wall thickness: 25mm

KV reviewed the fuelbed height calculation given by BTG at www.cookstove.net to determine the fuelbed height prior to experimentation. He decided to make the fuelbed height equal to the diameter of the combustion chamber on that basis. The chamber length above the fuelbed was determined through trial and error, but a length of 165% greater than the fuelbed height has ultimately resulted in the best configuration up until now.

Primary air supply:

The combustion chamber rests on top of a stainless steel grate. A 100mm diam. wide plate (tin can base) was used to limit the primary air and placed on top of the grate. The air then enters through the space between the chamber and the plate as well as through a 26mm diam. hole cut into the center aligned with the hole in the briquette.

Cross-sectional area of air entering around the outside of the plate is approximately 76cm2. Cross-sectional area of air entering through the center hole is estimated at 4cm2, giving a total of 80cm2. The overall cross sectional area was reduced by approximately 50% to 40cm2 when the blue flame started to appear in this test. In subsequent unofficial testing KV has reduced the primary air supply down to almost nothing (0), with no visible negative influence on flame holding or behaviour.

Insulation/preheating of Primary air: A steel plate was placed below the chamber and a liberal amount of ash was spread on top to insulate the cold air before entering the chamber. Heat was subsequently trapped above the ash layer and below the chamber.

Secondary air supply:

Dimension, quantity, position & direction of secondary air holes: 10 x 10mm thick holes, drilled into the chamber radially and spread more or less evenly around it, at a point 45mm up from the bottom of the chamber, and at a 45° angle off direct radial direction (much as the vanes on a simple turbine for example). The angle was chosen in an attempt to create a vortex or swirl, to allow for better mixing of volatile gases and air. Total cross-sectional area of secondary air: 9cm2 .

Insulation/pre-heating of Secondary air: The ceramic refractory material insulates the fire, but does get hot throughout. The secondary air is pre-heated travelling through the holes in the chamber before entering the reaction and the distance is extended by the 45° angle of the holes. (seen as 45 deg off a radial direction). This method of preheating was suggested by stover, Crispin Pemperton-piggot.


Testing Format

Calculation format courtesy of Piet Visser from BTG (Biomass Technology Group) at www.btgworld.com "Testing Protocol - Test procedure for simple water boiling tests - Fuel consumption calculation" (These were used as guidelines only, as gasification requires careful interpretation of the calculations).

Testing Procedure

Most procedures are also courtesy of BTG, but KV has added other procedures due to briquettes posing unusual burning conditions and characteristics. We have made certain presumptions about data required for calculating the power and efficiency of a gasifying stove and would appreciate the list clearing up some of our misconceptions.

•Note down weight of briquette

•Assemble and prepare stopwatch, pot and water, a balance and thermometer (digital, with 1 gram / 1° C accuracy respectively, (we regrettably did not have this, but instead used an ordinary kitchen-variety scale and merely estimated the water temperatures)

•Note the method of providing primary and secondary air and the corresponding TOTAL

cross-sectional area of each

•Note height and diameter of chamber, as well as thickness

•Note the height at which secondary air enters the chamber and at what angle on a vertical and/or horizontal plane the holes are directed

•Weigh empty pot and note the type of material it is constructed from (i.e Stainless)

•Weigh empty combustion chamber

•Fill pot with water (1 litre) and weigh the pot AND water. Estimate the initial temperature of the water

•Place briquette inside stove and weigh stove AND briquette

•Pour measured amount of paraffin over top of the briquette

• Ensure the primary air openings are unobscured and fully open

•Place a thick metal base plate underneath the stove with liberal sprinkling of ash to insulate the incoming air

•Ignite the briquette on top with a single match

•Start stopwatch

•Note the PPM concentration of smoke release and the time period from high to low concentration

•Place pot on top of chamber 2 -3 minutes after torching. Pot is to be elevated above the chamber at 1 cm increments for each test allowing emission gases to scrape against the pot


•Note down the time when blue flames become more prominent (This normally occurs 2 - 4 minutes after paraffin or kindling has burnt off)

•Drop the stove closer to base plate to limit primary air inflow. Alternatively turn down primary air

•Note the time when water first boils

•Note flame length above briquette

•Note colour and location of flames

•Note the turbulence of flames

•Note the time and location of any flames going out

•Note the time for the start of the carbon burn i.e. when blue flame goes out

•Note the time the water stops boiling rapidly


•Weigh the pot AND water a second time

•Weigh the stove and now pyrolysed briquette, place back on plate and place water back on

•Note the colour, concentration and time period of the SMOKE release until very little or no smoke is further observed

•Note the time when heat from the stove proceeds to decline, i.e. at a point where slight water simmering or water temperature holding can no longer be maintained. This can also effectively ascertained with a thermometer.


•At this time weigh the pot and water a third and final time together

•Also weigh the stove and charred remainder of the briquette together

•Next morning weigh the ash separately and note it down to calculate ash %

•Determine the calorific value (kJ/Kg) of the biomass used.

TEST RESULTS: --for test # 6 (est. configuration thus far)--

The burn profile

Ignition: Used starter fluid, 80 ml paraffin with instantaneous ignition.

Flame description:

Blue/Yellow flame appearance for first six minutes with copious amounts of smoke

From 7th minute through to 28th minute: Blue Flame, with flames extinguished in 28th minute. Duration of blue flame burn/gasification: 21 minutes

Thermal performance

Type of pot: Stainless steel (0.5mm thick)

Time water (1 Litre) put on: 2nd minute

Time water boiled: 7th minute

Time taken to come to boil: 5 minutes

Initial temperature of water: 20°C

Time water stopped boiling rapidly: 36th minute

Time on high power: 34 minutes

Mass of fuel used during high power: 125 grams

Mass of water boiled on high power: 220 grams

Time low power burn ended: 46th minute

Charcoal content: 24 grams (13%)

Time on low power: 10 minutes

Mass of fuel used during low power: 24 Grams

Mass of water boiled on low power: 10 grams

Ash content: 17% (31 grams)

Energy and efficiency calculations

Formulas used (Simple water boiling tests):

1 - Max/Min Power (kW) =

(Mass of fuel used during high/low power period in kg) X (Calorific value of the fuel kJ/kg)

Duration of the high/low power (s)

2 - Efficiency at high Power (%) =

(Initial mass of water X spec. heat of water (4.2kJ/kg.K) X ((Boiling temp. of water (K) – Initial temp. of water (K)) + (Mass of evaporated water X Latent heat of water (2257 kJ/kg)

Mass of fuel used during high power (kg) X Calorific value of the fuel (kJ/kg)

3 – Efficiency at low Power (kW) =

(Mass of evaporated water X Latent heat of water (2257 kJ/kg)

Mass of fuel used during low power (kg) X Calorific value of the fuel (kJ/kg)


Calorific value of biomass: Estimated at 18 000 kJ/kg

Calorific value of charcoal: Estimated at 28 000 kJ/kg

Specific heat of water: 4.2 kJ/kg.

Latent heat of water: 2257 kJ/kg

Boiling temperature of water: 90°C

High power estimation: 1.1 kW

Low power estimation: 1.1 kW

High power efficiency: 35.2 %

Low power efficiency: 3.3%


In earlier tests KV experimented with different air supplies for primary and secondary air. The best results were obtained by having the secondary air enter just below the top level of the briquette at a 45° horizontal angle. This created a vortex + turbulence and caused better mixing of air and volatiles. The first two tests involved very high chambers (37 and 28 cm) and the location of the secondary air supply (27cm and 18cm respectively) was too high. The primary air opening was too small in the 1st and too far open in the 2nd test. KV attempted to gasify two stacked briquettes in the first test, but this turned out to be near impossible, as is explained in the conclusion. In the 3rd and 4th test the chamber was too low (14 cm and 10cm), even though he did supply more than enough secondary air (18 cm2). In either case, the draft was insufficient due to the short combustion chamber height, to draw this air in. The air holes were located lower down, adjacent to the briquette during the test.

In test number 5, KV carried on with the small chamber height, re-drilled 10 (instead of 20) holes at a 45° upward angle and this caused more turbulence and better mixing. KV also started placing a tin can base below the briquette to test the effect more closely of reduced air supply, and also to more accurately measure the incoming primary air. In test number 6, KV added an extra 9 cm high piece to increase the chamber height from 14 to 23cm. The secondary air was this time directed downward. When gasification started KV dropped down the stove to restrict approximately 50% of primary air and immediately the blue flame started burning ABOVE the briquette and not on the sides. The increase in secondary air created a definite vortex and swirls, accompanied by sparks and turbulent air. The flame was also markedly higher. The briquette slowly pyrolysed downward for 21 minutes, before extinguishing. Very little smoke was released during this time.

In the remaining two tests, KV experimented with less primary air at start-up (but still prefer more air for start-up as was done in test number 6) and directed the secondary air horizontally (not down or up), but at a 45° angle off the line of a radius, to increase the speed of the swirl. The clearance between the pot and the chamber was brought down from 4 to 2cm. We feel that this clearance should be reduced even further, bearing in mind Dean Still’s recommendation of maintaining the cross-sectional area of the incoming air at the exit point.

Another area of interest was the continued prominence of a blue and yellow flame in the middle of the briquette, which also went out when the briquette was completely pyrolysed. Exactly how this flame impacts on conditions inside the combustion zone is still unclear, but it does appear to not only help maintain high temperatures inside the refractory ceramics, but might also aid in the burning of excess gases that are not fully consumed by the blue flame above and adjacent to the downward moving flaming pyrolysis layer. The hole in the briquette also adds approximately 53cm2 (11.5%) to the surface area, even after subtracting the loss of top and bottom surface areas made by the hole.

KV also mentioned using starter fluid (paraffin) in igniting the briquette on top. In subsequent testing (unofficial) it was found that less such fluid or even lower value kindling can be used instead. The thinking in earlier tests was to drive up the combustion temperatures during start-up and even though this occurred it was countered by too much excess air, which cooled the reaction again. In subsequent tests consistent flame holding was achieved more rapidly with much less starter fluid requirement due to less excess air (especially primary air) being drawn in. In future we will employ digital thermometers to establish more precisely the influence of excess air on combustion chamber and flame temperatures. We feel confident that paraffin can be substituted with kindling commonly used by local communities.

The extremely low power efficiency reported may be due to the misjudgement of the calorific value of the char or it may be due to the fact that an ash layer often forms over the char and as it is emitting primarily radiant heat, this ash can have a dramatically restrict actual heat output.


1) The high draft (created by the extra length) has the potential to suck in more secondary air if we limit the amount of primary air. This could result in a blue flame forming only on the top of the briquette and slowly pyrolysing downward (flaming pyrolysis layer). This is in comparison to earlier tests where the blue flames were forming on the sides, pyrolysing the sides and blocking further releases of volatiles. The added draft increased the flame length for licking the pots, but the best heat transfer took place when the clearance between the pot and chamber was decreased from 4 cm down to 2 cm. We calculated a high power increase of more than 13% (from 22% to 35%).

2) We now know, more or less, how much primary and secondary air is required, at what position/angles and at what times. It would be appreciated if list members could assist us in calculating the airflow velocity through the stove.

Problem areas:

1) Creating an extended feed capacity

We have speculated about refuelling options: KV's efforts of introducing a 2nd briquette at various stages have yielded disappointing results. The 2nd briquette did not pyrolyse properly, probably due to the low position of the secondary air inlets.

In contemplating removing the pyrolysed briquette for further blending and recompressing as a charcoal enriched briquette, the removal of the char briquette residue poses a problem in the stove. The flame does appear to need a space to swirl above the fuel bed as predicted by BTG. Refuelling of the stove with a fresh briquette alters this ‘gas swirling’ space, posing problems relating to the design and possible increase in cost of the stove.

We had followed an earlier idea of an inclined feed tube, which would be fitted with an airtight cap (courtesy of Lanny Hensen). The idea was that the feed tube would contain two or three additional briquettes which would simply slide (by gravity) into the chamber as the main briquette was pyrolysed, and reduced in size but it proves far more complex to build and alters the ideal air feed location which we have found is so essential for proper gasification.

2) Also much needed, is the determination of the height at which pots need to be placed without interfering with airflows in the reaction: IE., the height which will result in optimum heat transfer to the pot. Our intention is also to attach an Internally notched funnel shaped device to divert hot air around the pot surface of any size pot, which has to be borne in mind as well.

These two problem areas, in addition to the earlier mentioned concern about the testing procedure and lack of proper testing equipment, constitute the state of our progress thus far.

Work continues but we most welcome at this point any input or comment from interested list members.


Kobus Venter/ Richard Stanley