Rice hull stoves

Characteristics of the fuel:

Rice hulls are a difficult fuel to burn; if piled in a heap and lit, they will smolder but not burn. However, if enough oxygen can be provided, they will ignite. Two solutions to this problem have been developed in Indonesia: Packed stoves and natural draft stoves.

Packed stoves

Background

Packed stoves are hollow containers, usually built of brick and mud, or fired clay. There is a door at the base and a pothole, slightly smaller than the cookpot to be used, at the top (Fig. 7-89).

Fig. 7-89: Packed Stoves

A common type of packed stove for-use in households and small restaurants in Bali is made of brick, holds about 1½ kg of rice hulls and burns for about 6 hours on one filling of fuel. In central Java, similar stoves are used when boiling coconut sap to make sugar.

There is a wide range of stoves that burn packed rice hulls. Probably the simplest is a fired clay bucket, a type that has been used in the city of Jogjakarta and the areas of Indonesia surrounding it for at least 30 years. It holds, when packed, about 1/2 kg of rice hulls and burns about 2 hours. Multiple pot packed stoves are usually several separate stoves combined into a unit. Villages in Bali reportedly prepare food for festivals on larger versions of the household and restaurant stove described above. These hold between 1 and 2 gunny sacks of rice hulls (10 to 25 kg) and burn continuously for 4 to 7 days; they are disassembled after the festivals.

Packing the stoves as shown in Figure 7-90 makes a flue up the center which provides the necessary flow of oxygen for combustion. The stoves are lit by shoving 2 or 3 burning sticks in through the door at the base. As long as the burning sticks are inside, the stove burns with a vigorous, smoky flame; if the sticks are removed, the flame goes out and the rice hulls smolder until they are completely consumed. A packed stove burning rice hulls will use about 20% of the wood used by an open fire. Small sticks and twigs are good enough for this purpose.

Fig. 7-90: Packing the Stoves

Construction

Construction is simple and involves, at most, basic masonry work. It should be possible to make packed stoves out of fired bricks, unfired bricks, fired clay, sand/clay mixture, concrete, or any other material capable of withstanding heat and the slight amount of pressure exerted on the walls by the packing. Cans, oil drums, and other already existing containers can be used for packed stoves.

Technical assessment

Advantages:
- Packed stoves can be made very quickly and inexpensively.
- A wide variety of containers is suitable, without major alteration, for use as packed stoves.
- Construction, if necessary, is extremely simple. A mason or other craftsperson will probably not be necessary, Packed stoves are suitable for owner-building.
- Rice hulls are cheap; they may be available without cost in some areas.
- Small packed stoves (e.g. terra cotta bucket) are easily portable.
- Small packed stoves are suited to mass production and distribution.
- A packed stove can, if rice hulls are temporarily unavailable, burn wood by itself.
- Rice hulls surrounding the fire insulate it and retain heat.

Disadvantages:
- Packed rice hulls, once lit, cannot be extinguished, but continue to smolder until consumed.
- Packed stoves smoke a great deal.
- Removing the ashes and repacking the stove can be very messy.
- Rice hulls pack best if they are of a powdery consistency. Old mills, previously used in Indonesia and other parts of Asia, ground the hulls off rice into suitably fine fragments. Unfortunately, modern milling equipment passes rice between rubber rollers and cracks the hulls off as large half shells. These large pieces do not pack well in stoves and the mass of packed hulls collapses into the bottom of the stove before burning completely. Because of this, it is necessary to add equal amounts of expensive rice polishings to the rice hulls in order to make them pack well. This adds to the cost, but the resulting fuel mixture is still much cheaper than wood.

Ideas for variations
- Other agricultural wastes and lowgrade fuels may be burnable in packed stoves. Sawdust, wood shavings, straw, and chaff are all possibilities. Try mixtures of fuels.
- It may be possible to find inexpensive binders for fuels that do not pack well. This would eliminate the need in some areas to use rice polishings as a binder. On the other hand, it might be better not to pack such fuels, but to consider using, instead, a natural draft stove similar to those described later in this section.
- Dampers would improve control of the fire.
- It should be possible to easily fit a chimney to packed stoves. It would be permanently attached, as shown in Figure 7-91, above the packed fuel and below the pot. A chimney would improve the draft and also diminish the smoke problem common with these stoves.
- The ideas incorporated in the Louga stove should be very useful in packed rice hull stoves. Try sinking the pot into the surface of the stove to improve heat transfer, gain wind shelter, and make better use of residual heat. The Louga stove, itself, might be directly adaptable to burning rice hulls if packed as shown in Figure 7-92.

Fig. 7-91

Fig. 7-92

Natural draft stoves

Background

Smoke and hot gases rising through a flue create a strong suction that draws air through a slanted grate located at the front of this type of stove. Rice hulls burned in this fashion burn rapidly with a clean, hot flame producing almost no smoke.

Natural draft stoves in Indonesia are generally large and used for commercial processes that require long, vigorous boiling. They incorporate a hopper that feeds a steady supply of rice hulls to the grate. The pots are usually fixed and sealed in place; pots that are removable must be carefully resealed with a mixture of mud and rice hull ash, or the draft will be spoiled and the stove will not operate. The grates are usually fiat iron bars set in as steps, though one stove seen used a slanted sheet of perforated iron for a grate that seems to work well. They are commonly made of fired bricks, cement mortar and plaster.

Natural draft stoves have been in use in some parts of Java for more than 20 years and are a well developed technology. It is generally accepted there that tie following design criteria are important:
- The wider the grate, the hotter the fire.
- The grate should be steeper than 45°.
- The taller the flue, the stronger the draw, meaning a longer flame, more heat, and greater fuel consumption.
- On stoves employing more than one cookpot, the floor should slant slightly upwards towards the stack end.

"Tahu" (soy cord) stove (Fig. 7-93) of a type commonly used in Jogjakarta and Central and East Java. This stove is used to boil between 800 and 1400 lifers of soy milk daily, in 150 lifer batches. This consumes about 10 gunny sacks (125 kg) of rice hulls. During cooking, the hulls in the hopper are stirred and pushed down onto the grate about every 10 minutes. The 4 meter stack produces a strong draft, resulting in a powerful, yellow flame. The stove has been in use for 15 years and the only maintenance has been to replace the iron grate after 3 to 5 years of constant use.

"Gula Jawa" (coconut sugar) stove (Hi". 7-94) used in Blitar, East Java. This stove boils about 50 lifers of coconut sap down to 7 kg of sugar cake daily. This requires 4 hours cooking time and uses half a gunny sack of rice hulls (6 kg). The process of boiling the sap makes full use of the range of temperature in the three pots; all three pots boil, and, as the sap is reduced in volume, it is transferred to the slower boiling pots. Clay mortar is used in this stove rather than cement. This stove was inexpensive, costing only Rp. 3500 (about U.S. $ 8).

"Ipa" stove. A 3 pot natural draft stove called the Ipa stove is being disseminated throughout the Philippines. It is constructed from adobe blocks, except for the top, which is cement.

Technical assessment

Advantages:
- Natural draft stoves make efficient use of a low-grade fuel that is difficult to burn, but very cheap and readily available in the region where they are employed.
- They are capable of producing a great deal of heat for extended periods.
- Natural draft stoves are well-suited to burning the hulls left by modern rice milling machines. They do this better than do the packed stoves.
- They can be built from inexpensive materials. Although most of the stoves are built from fired bricks, both unfired bricks and sand/clay mixture should be suitable materials.

Disadvantages:
- Natural draft stoves usually require a mason for their construction.
- Natural draft stoves are relatively complex stoves and take a long time to construct.
- Of the two types of rice hull stoves natural draft stoves are by far the more expensive.
- Natural draft stoves require the use of metal for the grates. This can, in some places, be expensive.

Fig. 7-93: "Tahu" Stove

Fig. 7-94: "Gula Jawa" Stove

Ideas for variations
- Dampers to adjust the burning rate would probably make stepped grate stoves more versatile and efficient.
- Centralized manufacture and distribution of any components requiring critical accuracy in manufacture would expedite dissemination.
- Cost of natural draft stoves would be reduced and constructions simplified if, in their construction, .sand/clay mixture or unfired bricks were substituted for fired bricks. Use of sand/clay mixture, however, would make measuring and layout crucial, use of standard-sized bricks is one way to assure dimensional accuracy in construction.
- Natural draft stoves are probably capable of burning such other fuels as chaff, sawdust, and wood shavings. Consider them for use with fuels to which it is difficult to supply oxygen.
- Tunnel design principles from Lorenatype stoves are, to some extent, applicable to these stoves. Baffles under the pots to direct the hot gases against the pots are a good thing to consider. A winding flue with potholes situated at the turns is also possible. One might easily end up with a stove looking like a Guatemalan Lorena or Nepali Chulo, but with a stepped grate built into the firebox.

For more information about rice hull stoves see

Technologies for the Utilization of Rice Hulls as a Fuel in Java and Bali, report from a survey conducted by the Development Technology Center of the Institute of Technology, Bandung, for the ASEAN Food-handling Subcommittee. The drawings were originally by Craig Thorburn. This report is available from

Volunteers in Asia, Box 4543 Stanford, CA 94305 U.S.A.