Simple techniques for basic
bio-fuels

Jacek Janczak

JACEK JANCZAK is a Polish engineer. He wrote this article originally as an
FAO study for the United Nations Conference on New and Renewable Sources of
Energy, Nairobi, August 1981.

All kinds of familiar agricultural fuels will burn much more efficiently
with proper processing. The methods are elementary and the costs are small.

1. Adding a piece
of wood helps combustion

2. Tying the bundle

3. Forming the
bundle

Simple fuel technologies for developing countries are based on available
resources and the "do it yourself" principle. While the tools for processing
fuel may be manufactured locally, what is needed is a well-organized
demonstration and training programme covering large areas of these countries.

Although simple processing increases the heat value of big-residues
considerably, every kind of fuel calls for a specific type of stove
or furnace to provide optimal combustion. The term "fuel" means a substance
which, during chemical reaction with oxygen, produces a heat or exothermical
reaction. Most of the oxygen in the reaction derives from the air, which
consists of 21 percent oxygen by volume and 79 percent nitrogen. Fuel can be
divided into two main parts: the combustible part, which includes volatiles and
solids; and the non-combustible part, including ash and moisture.

4. Easily
constructed hand-press

4a. Compressing
the bundle

5. Another type of
simple hand-press

To achieve the optimal use of fuels it is necessary to control the speed of
combustion from one piece of fuel to another. This is achieved by the controlled
introduction of air into the combustion chamber. The most important factor
determining good combustion is the complete consumption of the fuel used.
Agricultural waste usually has poor combustion, but it can be
improved by agglomeration, densification or drying.

Moisture decreases the heat value of fuel because evaporation of water
consumes heat. For example, the evaporation of 1 litre of water consumes 5.5
times more heat than heating this water from ambient to boiling temperature.
In the case of firewood, the differences in heat value between green and
seasoned wood are considerable, as is shown in Table 1.

Processing of bio-fuel

Firewood. Twigs, straw, hay and dry leaves are often the
only fuels available to rural people. Round firewood is often either too
expensive or simply not available. But the material that is available burns so
rapidly that it is difficult to maintain a steady fire for cooking.

Given these constraints, the easiest method of improving combustion is by
pressing the material into bundles, i.e., by "agglomerating" and "densifying"
the biomass volume. This restricts the access of air and thus slows down
combustion. Well-pressed bundles should contain a mixture of twigs, biomass
(cellulose) materials and, if possible, a piece of wood in the middle. The
bundles may be compressed by simple presses (Figures 3, 4, 4a and 5).
They should be fastened by a band of straw, home-made cord or wire (Figure
2). Wire is best because it is fire-resistant. Before being used, the bundles
should be dried for a long time in a well-ventilated place protected from the
rain.

The bundles can be made by hand, but to increase pressure and thereby
heat value a number of implements can be used. Those illustrated in
Figures I through 5 can be made at home at practically no cost or
manufactured by local smiths.

Briquetting. Neither wood nor agricultural wastes such as wood dust,
sawdust and coffee husks can be burned directly in domestic stoves. They burn
with difficulty, produce considerable smoke and are unsuitable for
cooking. The same is true of charcoal dust.

One of the best methods of handling such residues is by the agglomeration of
small particles into briquettes. There are two principal methods of briquetting,
with or without a binder. Doing without the binder is more convenient, but it
requires sophisticated and costly presses and drying equipment, and is
energy-intensive.

Briquetting with a binder such as tar, pitch or asphalt calls for
high-pressure industrial processes to produce briquettes from hard coal, coke
dust and, to a lesser extent, coal dust.

6. Simple briquetting press

7. Another simple
hand-press for making briquettes

Such processes are also sophisticated and expensive, and while they are not
appropriate for villages they might be very useful for central or local
government projects, especially in countries having an abundance of wood and
agricultural waste.

For rural populations the most suitable briquetting methods are those which
are based on available waste and building materials. The manufacturing should be
done in locally made hand-operated presses. The hand-made briquette is held
together mainly by the binding material. Depending upon local availability, the
most suitable organic combustible materials for binding are resin, tar, animal
manure, sewage mud (sludge) and fish waste.

If combustible binding materials are not available or the waste needs a
stronger binding material as, for instance, in the case of charcoal dust, then
it is necessary to apply noncombustible binders such as slime, clay or mud.

Although the non-combustible binder lowers the heat value of the briquette
and increases the ash content, it does make possible the use of materials which
otherwise would be valueless as fuel.

Presses, Design varies from the simplest, technologically primitive
home-made hand-presses to very advanced automatic units. Among the illustrations
is a simple hand-press (Figure 6) which can be manufactured by any village
smith. The lower arm of the press is fixed to a wooden board. The pressure
ranges from about 50 to 100 kg per briquette, or 5 to 15 kg/cm2. The
briquettes produced are spherically shaped, preventing the fracture of edges
which is a weak point in home-made briquettes.

The materials used are soaked newspaper or any waste paper. The addition of
wood ash makes the balls harder and prolongs combustion. The briquettes are
pressed into balls of about 5-cm diameter, and the weight of a dry briquette is
about 30 g. The size of a briquette may vary, but larger balls are more
difficult to press. The cost of making one press would entail 3-4 man-hours and
1-2 kg of iron. The working time can be considerably reduced if a number of
presses are manufactured together and work is well organized.

Another illustration is of a simple metal briquetting press (Figure 7). It
produces a pressure of 200 kg per briquette or about 3
kg/cm2. The square form is fixed to the base by the hinge, which
makes it mobile and facilitates removal of the briquette. The press can be
modified by stronger construction and a longer lever. The base should be kept
above the ground. Underneath the container there should be an opening with a
metal cover: this plate is opened and the finished briquette is removed by means
of the hand lever.

Another press which has been developed and tested consists of a square
container in which waste is pressed by a screw similar to that in a wine press.
Pressure is up to 2 tons per briquette or about 40 kg/cm2. The rate
of production, however, is slower than with the lever press.

Good results have been obtained by adapting these presses for bricks or earth
blocks. They are now well known in developing countries under such names as
"Cinva-Ram", "Terstaram" and "Combustaram" (Figure 8). To produce briquettes it
is necessary to insert moulds of the required forms (Figure 9).

In Europe during World War I screw-type briquette presses worked on the same
principle as meat-cutting machines. They produced a continuous briquette mass
for cutting into sections. The screw press has much lower pressure than the
piston press and consequently has limited application for waste which is easily
convertible into briquettes resistant to crumbling.

Table 1. IGNITION TEMPERATURES OF SOLID MATERIALS

Material

Ignition temperature (°C)

Lignite dust

150-170

Hard coal dust

150-220

Coke from lignite coal

300-400

Coke from hard coal

350-700

Graphite

700-850

Charcoal (soft)

250-300

Charcoal (hard)

300-450

Peat

200-450

Paper

300

Wood dust

315-460

Source: Author's measurements.

Table 2 NET HEAT VALUE OF FIREWOODS

Heat value

Specific weight of dry wood

Seasoned

Green

kcal/kg

kg/dm3

Pine (Pinus)

4658

3870

0.49

Weymouth pine (Pinus strobus)

4 876

4 055

0.37

Birch (Betula)

4 658

3 869

0.61

Spruce (Picea)

4622

3 589

0.43

Larch (Larix)

4 597

3 818

0.55

Fir (Abies)

4 559

3 785

0.41

Locust (Robinia)

4 527

3 760

0.73

Beech (Fagus)

4478

3 705

0.69

Linden (Tilia)

4 474

3 713

0.49

Elm (Ulmus)

4 419

3 810

0.64

Oak (Quercus)

4329

3742

0.65

Ash (Fraxinus)

4329

3593

0.68

Alder (Alnus)

4 288

3 555

0.49

Sycamore (Acer pseudoplatanus)

4 245

3 845

0.59

Black poplar (Populus nigra)

4 205

3 518

0.41

Source: T. Wojciechowski, Nauka o drewnie, PWRIL, Warsaw, 1961.

Table 3. MATERIALS USED FOR THE MANUFACTURE OF BRIQUETTES

Paper waste
Municipal waste
Wood waste
Coconut
fibre
Peat
Tundra dust
Cotton waste
Sawdust Straw
Groundnut
shells
Hazelnut shells
Olive-oil residue
Sugar cane
Bark
Rice
husks
Sunflower husks
Leather waste
Hemp
Manure
Used motor-oil
(as an additive to solids)
Hay
Shrubs
Fish waste
Food-processing
waste

Advanced industrial presses for wood and agricultural waste exert much higher
pressures, ranging from 1000 to 1200 kg/cm2. At these pressures the
temperature is very high. Both high temperature and high pressure destroy the
elasticity of the wood, making it possible to form the briquettes without a
binder.

An example of an advanced piston press is one manufactured by Fred Hausman
Ltd. of Basel, Switzerland (Figure 10). Its capacity ranges from 100 to 3 000
kg/in. The materials for use in such a press include wood waste, bagasse, tree
bark, olive residue, leather waste, nutshells, waste paper, peat, the husks of
rice or sunflower seeds and other agricultural waste material. The briquette
press is only one of the elements of production, which include a grinder, a
suspension drier, a press, and a packaging machine. Also illustrated is a
production line design (Figure 11) for making briquettes from wood waste.

An installation similar to this is used in the Polish timber industry with
wood bark as the source of material. The basic equipment includes a grinder, a
suspension dryer and a briquetting press.

Waste preparation. The first operation, common to all briquette-making
processes, is the chopping and/or grinding of combustible waste. In simple
village technology, where the materials to be prepared are branches, twigs and
straw, the best tool for chopping is a hatchet or broad axe. For production on a
large scale, different tools are required such as the straw chopper commonly
used by European farmers.

The second operation is the drying of the main combustible ingredients. In a
simple technology, drying should be performed without the use of fuel. The only
real option, therefore, is natural drying in well-ventilated places protected
from rain. Such drying takes a long time, so the basic ingredients for
briquetting should be prepared well in advance.

Third comes the mixing of various kinds of waste and binding materials in
order to produce the optimum consistency and heat value. If used motor-oil is
available it may be added to the waste, but, although it increases the heat
value of the briquettes, it makes them crumbly.

8. Lever press
adapted for making briquettes, with mould inserted

9. Mould for
briquettes

The fourth operation is compressing the briquettes, and the fifth is drying
them in open air, under shelter. Depending upon the binding material and drying
conditions, it will take between a few and several months to produce dry
briquettes. Drying significantly increases the heat value of the briquettes and
thus saves fuel. They should therefore be produced well in advance of intended
use.

New types of fuel

Experience has shown that the successful implementation of new technical
projects depends on socio-cultural, historical and economic factors as well as
on purely technical ones. Many projects involving obvious technical improvements
have failed be cause the non-technical factors were neglected.

When starting a new project, preliminary work on organization and research is
essential before proceeding with technical design. The most effective way of
achieving success is to assess the situation from several viewpoints, namely
those of forestry administrations, women's associations, contractors and
technicians. These groups should be invited to express their views regarding the
availability of combustible waste, the social climate, the attitudes of local
people to innovation, and economic and other relevant factors.

On the basis of this inquiry, targets for the promotion of new fuels should
be established. Costly and time-consuming surveys are not necessary. The
environmental aspects should also be taken into consideration because the
combustion of biomass waste in stoves eliminates one of the important elements
in the ecological chain. Under normal conditions biomass waste is
recirculated to the soil.

If this delicate equilibrium is broken, the consequences may be observed in
decreasing productivity of the soil and, in extreme conditions, in its
desertification.

Owing to the interdependence of fuel and stoves, it is recommended that stove
and fuel improvement programmes be undertaken concurrently. A combined programme
might consist of improvements to existing stove designs and fuel types to suit
local cooking practices, and the design and construction of new stove models,
together with the introduction of new types of fuel.

10. Piston press
can produce up to 3 000 kg/in

11. Production
line for waste-fuelwood briquettes

1. Conveyor - 2. Grinder - 3. Fan - 4 Cyclone - 5. Bin (wet material) - 6.
Bin discharge - 7. Furnace - 8. Fan - 9. Suspension dryer - 10. Insulation - 11.
Cyclone - 12. Briquetting press - 13. Cooling line - 14. Ground wet material -
15. Hot air.

New briquettes should be laboratory tested. This is the only way to collect
and compare the technical data of the various options available locally. Testing
equipment is not too costly, about US$5000, and it can often be found in the
laboratories of universities and technical schools. It is important to develop a
comprehensive test procedure, especially concerning measurement of net heat
value, humidity and ash content. The fuel should be tested both in the
laboratory and in everyday cooking.

Table 4. COMPARISON OF SAMPLE BID-FUELS

Composition

Gross heat value

Humidity

Ash

Net

Comment

kcal/kg

Percent

kcal/kg

Dry beech wood

4 554

8,0

0,3

4 224

Type of wood rarely available in poor rural areas

Green birch wood

3 308

42,9

0,25

2 889

Type of wood commonly found in poor rural areas of developing countries

Waster-paper balls (made from soaked newspaper)

4 143

6,9

2,9

3 825

Made by hand; burn better if wood ash is added

Briquettes made of
30-45% charcoal dust
30-45% chopped
twigs
15-20% manure

4626

2,4

32,2

4 408

Comparable to medium-quality hard coal: high ash content probably from
sand

Briquettes made of
25% charcoal dust
25% straw
30% chopped
twigs
20% manure

3 397

7,2

13,7

3 109

Lower percentage of charcoal dust reduces heat output

Briquettes made of
50% straw
50% cow manure

3 898

5,4

9,5

3 599

Feasible everywhere, but has high manure content -and manure is better
used for fertilizer

Briquettes made of
40% straw
40% sawdust
20% manure

3 561

9,2

14,0

3 266

Needs careful drying because of sawdust

Briquettes made from charcoal dust with clay as binder

1 064

-

73,0

975

High mineral content lowers heat value and creates much ash

Field testing should also be applied, both to develop or improve new fuels
and to analyse their heat values. Groups of people from different backgrounds
could be organized to investigate, promote, and later use the fuels in their
households on an experimental basis. They should be invited to carry out further
investigation on the composition of locally available biomass waste for
briquetting and binding materials.

In most developing countries there are considerable quantities of biomass
waste which could be processed by means of agglomeration and densification. The
best results are usually achieved if several kinds of material are mixed with a
binder. The heat value of well-dried, compressed biofuel waste is similar to
that of fuelwood. However, the uncontrolled use of biomass waste could have
adverse ecological consequences on the soil, such as reduced fertility. Careful
evaluation of how much and which biomass can be used as fuel in specific regions
should be undertaken and monitored by competent officials.

References

HAUSMANN. F. 1974. Briquetting wood waste by the Hausmann method.
Modern sawmill techniques t. 3. E. Miller Freeman Publications. Inc., San
Francisco, California, USA.

RUDHARDT, PAUL. 1919. Les combustibles suisses et leur utilisation.
Sonor S.A., Genève.

FAO. 1979. Problems and priorities in developing
wood stove programmes. Intermediate Technology
Development Group. December 1979. Stephen Joseph.