Since the beginning of civilization wood and biomass have been used for cooking. Over 2 billion people cook badly on inefficient wood stoves that waste wood, cause health problems and destroy their/our forests. Electricity, gas or kerosene are preferred for cooking - when they can be obtained However, they are costly and depend on having a suitable infrastructure and are 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, etc.), but the new wood stoves are often more difficult to manufacture, more heat goes to heat the stove than heat the food, and they do not offer good control of cooking rate. They are not often accepted by the cooks for whom they are developed.
These problems could be solved if it were possible to make a low cost, gasifier-combustor that could be operated as easily as electric/gas/kerosene stoves. We have developed the "inverted downdraft gasifier" to solve these problems.
Biomass gasifiers have many confusing names. They can be divided into three main categories depending on the source of heat for the initial pyrolysis stage: In Charcoal burning gasifiers, the air (or oxygen) first encounters charcoal (as in updraft, counter-flow gasifiers). They produce very tar levels of 5-20%. In tar burning gasifiers (downdraft, inverted downdraft, crossdraft, co-flow) the air first encounters fresh fuel and burns the volatiles in a process called "flaming pyrolysis". They produce 0.1-1%tar. In mixed burning gasifiers some air reacts with the fuel, some with the charcoal (as in fluidized beds and many varieties of fixed bed gasifiers), producing intermediate levels of tar.
Downdraft gasifiers in the 5-100 kW level were widely used in World War II for operating vehicles and trucks because of the relatively low tar levels. In operation, air is drawn down through a bed of burning wood, consuming the volatiles. The resulting gas then passes over the resulting charcoal and is reduced to a gas with typically 4.5-5 MJ/Nm3 called producer gas.
However, since hot gases naturally rise, it is necessary to supply power to draw the gases DOWN through the gasifier. In 1985 we developed the "inverted downdraft gasifier" (also called "upside downdraft", top lighted, or charcoal making gasifier) shown in Fig. 1. The name comes from the fact that the fuel charge is lit ON THE TOP, and forms a layer of charcoal there; the flaming pyrolysis zone is below that; the unburned fuel is on the bottom of the pile, and primary air for pyrolytic gasification enters at the bottom and moves UP, forming gas in the flaming pyrolysis zone, as shown in Fig. 1. It can operate on either natural or forced draft.1,2,3
Fig. 1 - Inverted downdraft gasifier made from "riser sleeve", showing primary air inlet, fuel zone, flaming pyrolysis zone and charcoal zone.
The inverted downdraft gasifier can be operated in batch mode which is suitable for cooking meals. (The gasifier can also be operated continuously by addition of an auger feed for the fuel at the bottom and an auger to remove charcoal at the top. However, this complicates construction.)
We previously developed a natural draft cook stove which uses only natural convection.1,2 The rate of gas production and heating is controlled by the primary air supply to the gasifier. As an option, the gasifier can make charcoal with a 20-25% yield. While this stove burned cleanly, natural draft does not easily provide good mixing. The heating rate was low, due to the use of natural draft.
We have now developed a new "Turbo gasifier" stove using forced draft and one model is shown in Fig. 2. It consists of an "inverted downdraft gasifier" plus a burner to mix air and gas and burn cleanly. It uses a 3 Watt (Radio Shack) blower to generate ~ 0.7 mm wg pressure, equivalent to the draft of a 10 meter chimney.
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 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 a 1 gallon paint can and a 1 quart tomato can, but stoves can easily be constructed from a wide variety of materials by local craftsmen. While currently it uses a 12 Volt 3 Watt blower, the power could come from stored compressed air, bellows, wind-up generators, photovoltaic, thermoelectric or other sources.
Dozens of runs have been made on various models of the Turbo Stove. As a sample, 153.7g of wood chips were loaded in the fuel magazine and 5 g of starter chips were placed on top. The starter chips were lit with a match and the blower started at 12.0 V from a DC power supply. A highly turbulent clean fire resulted. After 1 minute a 2 quart copper bottomed pot containing 500 ml of water was place on the pot supports. A thermocouple was inserted 3 inches below the top of the bed. The data in Table 1 were taken and are shown in Fig. 3.
Table 1 – Typical run in Turbo Stove on 153 g of wood chips
|
Time min |
Water temp -C |
Bed temp -C |
Comments |
|
0 |
Startup 1-min |
||
|
1 |
17 |
22 |
Pot on |
|
2 |
42 |
22 |
Heating |
|
3 |
74 |
26 |
Heating |
|
3.5 |
93 |
41 |
Boiling |
|
4 |
94 |
400 |
Vigorous |
|
4.5 |
94 |
680 |
Vigorous |
|
5 |
94 |
720 |
Vigorous |
|
5.5 |
94 |
620 |
Vigorous |
|
6 |
94 |
702 |
Vigorous |
|
6.5 |
94 |
697 |
Vigorous |
|
7 |
94 |
688 |
Vigorous |
|
8 |
94 |
653 |
Vigorous |
|
9 |
94 |
660 |
decreasing |
|
10 |
93 |
668 |
OFF |
The fuel magazine was immediately removed from the burner chamber and air excluded. The resulting charcoal weighed 37.7 g, a yield of 24.5%. The remaining water was poured in the measuring cup and measured 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. 37.7 g of charcoal containing approximately 24kJ/g energy, or 904.8 kJ remained unburned, so 1861 kJ of energy was released by the 116 g of burning volatiles. (The volatile energy alone was 16kJ/g, a useful figure for calculations when charcoal is a byproduct.) 1 min was consumed in lighting the fire before adding the pot, so the energy released for cooking during the remaining 9 minutes was 1675 kJ, or 3.10 kW.
The efficiency of water boiling was
Eff = (Heat to water + heat for vaporization)/heat released = (161.2 + 460.5)/1675 = 37.1%
Charcoal production Efficiency = 37.7/153.7 = 24.5%
The water boiled in 145 s as compared to 236 s on the large burner of our electric stove, reflecting the higher intensity of forced draft wood-gas cooking. The gasification front took 4 minutes to reach the thermocouple, so is travelling 1.9 cm/min. The bottom of the pot was clean, indicating near complete combustion of the wood-gas.
Figure 3 – Time and temperature for typical Turbo Stove run
.
- Wood-gas Stove Fuels
We have successfully gasified: 1-3 cm softwood chips; 1-2 X 10 cm hardwood sticks; 5 mm diameter canes from bushes; ¼ inch wood pellets; 3/8 inch peanut hull pellets; dung and 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.
Many of the tests made on the stove used 2x1x½ cm air dry (about 7% moisture in Denver) mixed chips in order to have reproducibility. However, we find the stove operates at least as well on sticks standing vertically if they are well packed.
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 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 15" directly over top of the burner and over the pot. It registered about 60 ppm 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.