Turbo Stove MS-PITBC FINAL.pdf

TESTING & MODELING THE WOOD-GAS

TURBO STOVE

T. B. Reed a,b , E. Anselmo a and K. Kircher c1

a The Community Power Corporation, 8420 S. Continental Divide Rd., Su 100,

Littleton, CO 80127; b The Biomass Energy Foundation, 1810 Smith Rd., Golden, CO

80401; c The Colorado School of Mines, Golden, CO 80401

ABSTRACT

Through the millennia wood stoves for cooking have been notoriously inefficient,

unhealthy and slow.

A new “wood-gas” cook stove has been developed that has >30% thermal efficiency,

can be started, operated and stopped with very low emissions and can use a wide variety

of biomass fuels. This “Turbo Stove” operates with 3 W of blower power or other air

supply to produce 1-3 kW thermal for cooking. It is simple and inexpensive to build.

Data is presented for this stove on a wide variety of fuels. The stove will bring a liter of

water to boil in 4-10 minutes and can be turned down to the simmer level for longer

cooking and increased efficiency.

The stove operates in several different gasification and combustion modes. In the

“volatile burning” mode, the stove makes 18-25% charcoal from biomass fuels. In the

“charcoal burning mode” the charcoal is gasified to produce a CO flame. If longer

cooking is required, additional fuel can be fed from above, but other modes require more

operator skill.

Presented at the Progresss in Thermochemical Biomass Conversion Conference, Sept.

17-22, 2000, Tyrol, Austria.

For a new stove to be accepted it must fit the fuel supply, cooking practices ,

construction methods and commercial infrastructure of each country. Therefore, it must

be possible to make a variety of stoves and requires understanding of the basic

mechanisms of gasification and combustion of “wood-gas”. A model of the wood-gas

“Turbo Stove” is described based on the measured parameters in this paper.

INTRODUCTION - WOOD COOKING VS WOOD-GAS COOKING

Since the beginning of civilization wood and biomass have been used for cooking. Still,

today over 2 billion people cook badly on slow, inefficient wood stoves that waste

wood, cause health problems and destroy our forests. Electricity, gas or kerosene are

preferred for cooking - when they can be obtained However, they are costly, contribute

to global warming, and depend on having a suitable infrastructure often not available in

developing countries

In the last few decades, many improved wood stoves have been developed (the

Chula, the Hiko, the Maendeleo, the Kuni Mbili, the Wendelbro, etc. 1 These new wood

stoves are often more difficult to manufacture and they do not offer good control of

cooking rate. They are often not accepted by the cooks for whom they are developed.

Since 1850 the preferred means of cooking has been first gas, then electricity.

Gas is still preferred by many cooks. Electric cooking can be 60% electric-efficient, but

power generation and distribution is typically 30% efficient, yielding an overall

efficiency of 18% for electric cooking.

We have developed several simple, inexpensive wood-gas stoves which can

bring the “joy of cooking with gas” to everyone while using a wide variety of renewable

biomass fuels or coal. 2-4

PRINCIPLES OF DOWNDRAFT GASIFICATION FOR COOKING

BIOMASS GASIFICATION

When biomass is burned with insufficient air in a gasifier, it makes a “producer gas”

containing primarily CO, H 2 , CO 2 , H 2 O and CH 4 . Over a million gasifiers powered the

civilian cars and trucks of Europe and Asia during WW II. Downdraft gasifiers are “tar-

burning, char-making” and are most suitable for biomass which contains 80% volatile

material. Updraft, “char-burning, tar-making”, gasifiers are often used for coal which

can be 80% char.

In conventional downdraft gasifiers, air passes down through the fuel mass,

then in the flaming pyrolysis zone burns the volatiles and tars while making charcoal

and pyrolysis gas. The charcoal then further reduces the CO 2 and H 2 O combustion

products back to CO and H 2 fuel.

THE “INVERTED DOWNDRAFT GASIFIER”

In inverted (top burning) downdraft gasification air passes up through the fuel and

meets the flaming pyrolysis zone where the reaction generates charcoal and fuel gas as

shown in Fig. 1. 2,3

Pyrolysis

Gas

Charcoal

Zone

Flaming

Pyrolysis

Zone

Ungasified Wood

Grate

Off

Low

High

Primary

Air Control

Medium

15 CM

Fig. 1-Natural convection gasifier

stove made with 15 cm riser

sleeve 2,3

Fig. 2-Forced convection Turbo

Stove with 3 kW flame from a 3 W

blower 4

NATURAL VS FORCED CONVECTION

Natural convection provides poor mixing of air with fuel gases and can result in

incomplete combustion, soot and emissions in open wood stoves. A chimney can supply

1 mm water pressure per meter of height. Addition of a chimney for cooking can greatly

improve wood combustion in closed models, but also adds complication and requires

wasting heat to operate.

Forced convection provides good mixing and combustion for gas cooking and is widely

used in homes and camping stoves. The 3 W blower used in the Turbo Stove provides

7.5 mm water pressure and makes clean cooking possible.

THE “TURBO STOVE”

The Community Power Corporation and the Biomass Energy Foundation have

developed a new “Turbo wood-gas stove” using forced draft from a 3 Watt blower. One

design is shown in Fig. 2. It consists of an inverted gasifier close coupled to a burner

section to mix air and gas and burn cleanly. A 3 Watt blower generates ~ 7 mm water

column pressure, equivalent to the draft of a 7 meter chimney. We have made it from an

outer 1 gal paint can, an inner burner can and a fuel magazine or with many other

construction methods. 4 Several burners can be assembled to make a cooking “range”.

An oven can be placed on one of the burners for oven heat.

The stove can be started and operated indoors with no exhaust fans and no odor

of burning wood. We have taken the stove to India and the Philippines and cooked with

the Turbo Stove in small villages and on conference room desks with no odor. While the

Turbo Stove currently uses a 12 Volt 3 Watt blower, the power could come from stored

compressed air, bellows, wind-up generators, photovoltaic, thermophotovoltaic, windup

motors, thermoelectric or other sources.

CONSTUCTION AND OPERATING THE TURBO STOVE

THE RESEARCH TURBO STOVE

Water

Calorimeter

Turbo Stove

The research Turbo Stove shown in Fig.

3 consists of

Flames

• An inverted downdraft gasifier and

fuel magazine

Air Jets

Flowmeters

Charcoal

• A combustion section which burns

the gas

Burning

Fuel

Unburned

Fuel

Grate

• Supports for a pot

• Regulated air supply for gasification

and combustion

as shown in Fig. 3. This permits

independent adjustment of the air to the

gasification section and the combustion

section for optimizing cooking

conditions at both high and low levels.

Combustion

Air

Gasification

Air

Digital Balance

Compressor

Fig. 3 - Research Turbo Stove, showing

separate supplies for gasification and

combustion air.

The rate of heating and boiling was used

to measure the heat transfer for cooking.

Draft meters were used to measure the

pressure drop for gasification and for combustion (typically 0.25-0.75 mmw for

gasification and 2.5 mmw water pressure for combustion).

THE PROTOTYPE STOVE

Compressed air and flowmeters do not a practical stove make. We have also built the

prototype stove shown in Fig. 2 that is easier to operate and less expensive. It permits

adjusting the power level by adjusting the gasification air. Some of the data below were

taken on the research and some on the prototype stove.

STARTING AND OPERATING THE STOVES

In a typical run, the stove is filled with weighed pellets of the dry fuel of choice. A

layer of starting chips, (chips, charcoal, or other porous materials soaked in alcohol, fat

or kerosene) is placed on top. The blower is turned on and the starter chips are lit with a

match. For the first few minutes the starter chips ignite the fuel below and make a bed

of charcoal that the gas must pass through. In 1-5 minutes, depending on the fuel, the

main fuel mass is ignited and burns downward regularly in flaming pyrolysis mode until

the reaction zone reaches the grate, making charcoal as it goes. The test variables are

shown in Fig. 4 for the research stove and Fig. 5 for the prototype stove.

600

Weight vs Time, Run 103

500

400

Startup

300

200

High Power burn; 47

l/m Combustion, 15

l/m Gasification air

Medium power,

28 l/m

combustion; 5

l/m gasification

air

L ow power burn; 17

l/m Combustion, 3

l/m Gasificationair

100

Charcoal

Time - min

Fig. 4 – Typical operating data on the research Turbo Stove showing

weight of fuel remaining vs time at high, medium and low power levels

Table 1 – Air Fuel Ratios for gasification and combustion, power levels, turndown

and superficial velocity for research stove

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