Research and Development: The 'Turbo' wood-gas stove

by T. B. Reed^a, R. Walt^b

^a The Biomass Energy Foundation, 1810 Smith Rd., Golden, CO 80401;
^b Community Power Corporation, Aurora, CO

A new 'turbo wood-gas stove' is currently being developed that cooks quickly and efficiently, with no significant emissions, in a closed space.

The inverted down-draught gasifier

The inverted down-draught gasifier develops 3 kW of heat using a 3W electric blower. It can be started, operated and stopped indoors with no odour of burning wood and can boil 500 ml of water in 3-5 minutes, comparable to an electric or gas stove.

In operation, air is drawn down through a bed of burning wood, consuming the volatile gases from the wood, creating charcoal. The resulting gas then passes over this charcoal and is reduced to a gas that typically provides 4.5-5 MJ/Nm³ of useful energy; this gas is called producer gas. However, since hot gases naturally rise, it is necessary to supply power to draw the gases down through the gasifier.

Figure 1: Inverted down-draught gasifier

In 1985 we developed the inverted down-draught gasifier shown in Fig. 1. The fuel charge is lit on the top, forming:

· a layer of charcoal on the top
· a flaming pyrolysis zone below
· a layer of unburned fuel on the bottom of the pile

Primary air for pyrolytic gasification enters at the bottom and moves up, forming gas in the flaming pyrolysis zone. This is shown in Figure 1. This system can operate on either natural or forced draught (1,2,3).

The first gasifier we made used only natural convection (1,2). The rate of gas production and heating was controlled by the primary air supply to the gasifier. As an option, the gasifier could make charcoal with a 20-25% yield. While this stove burned cleanly, natural draught does not easily provide good mixing. The heating rate was low, due to the use of natural draught.

Turbo stove construction

We have now developed a new 'Turbo gasifier' stove using a forced draught. One model is shown in Figure 2. It consists of an inverted down-draught gasifier plus a burner to mix the air and gas and burn cleanly. It uses a 3W blower to generate about 0.7 mm wg pressure, equivalent to the draught available from a 10m chimney.

The stove can be started and operated indoors with no exhaust fans and no odour of burning wood. We have taken the stove to India and the Philippines and demonstrated the turbo stove in small villages and on NGO desks. There was a great deal of interest.

The laboratory model of the stove was made from two scrap metal cans, but stoves can easily be constructed from a wide variety of materials by local craftsmen. While currently it uses a 12V, 3W blower, the power could come from bellows, wind-up generators, photovoltaic cells etc.

Operation of Turbo stove

In a typical test, 153.7 g of wood chips were loaded in the fuel-holder and 5g of starter chips were placed on top. The starter chips were lit with a match and the 12V blower was switched on, using a DC power supply. A highly turbulent clean fire resulted. After one minute, a 2 quart copper-bottomed pot containing 500 ml of water was place on the pot supports. A thermocouple was inserted 7 cm below the top of the bed. The water boiled in 145 seconds and the experiment continued for ten minutes. Results are shown in Table 1 and Figure 3.

Figure 2: Forced draught 3 kW Turbo stove

Table 1 - Typical run in Turbo Stove using 153 g of wood chips

Figure 3: Time and temperature curve for typical Turbo Stove run

The fuel-holder was immediately removed from the burner chamber and sealed to exclude the air. The resulting charcoal weighed 37.7 g, a yield of 24.5%. The water which had not boiled away was measured at 299 ml, so 201 ml boiled away.

The energy content of the fuel (volatiles plus charcoal) was approximately 18 kJ/g or a total of 2766.6 kJ. A mass of 37.7g of charcoal containing approximately 24 kJ/g energy, or 904.8 kJ remained unburned, so 1861 kJ of energy was released by the 116 g of burning volatiles. (The energy from the volatile gases alone was l6 kJ/g with charcoal as a byproduct.) It took one minute to light the fire before adding the pot, so the energy released for cooking during the remaining 9 minutes was 1675 kJ, or 3.10 kW. Where h = efficiency:

The water boiled in 145 seconds compared to 236 seconds for a typical burner on an electric stove, reflecting the higher intensity of forced draught wood-gas cooking. The bottom of the pot was clean, indicating near complete combustion of the wood-gas.

Wood-gas Stove Fuels

We have successfully gasified:

· 1 cm - 3 cm softwood chips
· 1 cm - 2 cm hardwood sticks (10 cm long)
· 5 mm diameter canes from bushes; 6 mm wood pellets
· 9 mm peanut hull pellets
· dung
· coal.

The burn time is directly proportional to the bulk density of the fuel, but all of these fuels would be useful candidates for cooking.

We operated with wood chips with 0, 10, 20, 25 and 30% moisture content (wet basis). The stove operated equally well at all levels, but the charcoal production decreased from 25% to 3%.

Wood-gas stove emissions

Wood-gas stoves have low emissions compared to wood stoves and can be operated indoors with no smell. In the experiment above, a Nighthawk CO meter was mounted 38 cm directly over the top of the burner and over the pot. It registered about 60 ppm carbon monoxide (CO) during the run, but the average CO in the room was probably less than 10 ppm CO. More work on measuring and improving emissions needs to be done.

Conclusions

A forced convection wood-gas stove has been developed that produces a 3 kW output using a 2 W blower. It can be started, operated and stopped indoors with no odour of burning wood. In order for the stove to be commercially successful, there is still work to be done in:

· optimising the stove for various available fuels
· determining the effect of moisture
· adapting the stove to various cooking situations
· adapting the stove for local manufacture
· developing other uses for this simple clean heat source

References

1. La Fontaine, H. and Reed, T.B., 'An Inverted Downdraught Wood-Gas Stove and Charcoal Producer', Energy from Biomass and Wastes XV, (ed. D. Klass), Washington, D.C., 1993.

2. Reed, T.B. and Larson. R., 'A wood-Gas Stove for Developing Countries', Developments in Thermochemical Biomass Conversion. (Ed. A. V. Bridgwater), Blackie Academic Press, 1996.

3. Brand, Stephen. 'Multi-Fuel Gasification for Energy Production in Rural Areas', Proceedings, 2nd Biomass Conference of the Americas, Aug. 21, Portland Ore., the National Renewable Energy Laboratory, NREL/CP-200-8098, 1995.