local resources
safety & health


Other Forms of Biomass

In rural areas of many developing countries it is commonly observed that agricultural/animal residues are used extensively as fuel for many purposes including cooking. While it is not a preferred fuel, it is forced on people due to the nonavailability of wood. In general agricultural residues have competing uses in the form of animal feed and fertilizer. The latter is a significant use for animal waste. Thus it is not wise as a matter of general policy to use these materials as fuels extensively. However in a severe fuel scarcity situation there may be no other alternative. In other words these resources may be considered as a safety net.

Another feature of many of these fuels is that they are available in finely divided form (for example rice and coffee husks). On the average they are difficult, if not impossible, to burn in stove types discussed elsewhere. From time to time special designs have been worked out to handle rice husk (see for example Delepeleire et al. 1981 and Kapoor et al. 1984). A second approach to use these fuels is to upgrade them by some form of densification. Several simple methods have been suggested by Janczak (1981). Many of these require binders of one form or the other, which can alter the original property of the biomass. Nevertheless these methods can have significant applications at times when the residues happen to be the only local biomass resource. These methods will be presented later in this section.

Large scale briquetting

We shall restrict ourselves here to large scale production of machine made high density briquettes. The methods essentially rely upon high pressures and use no binders. Many machines are available now on the market and are described in an extensive work carried out recently by Twente University (1990). Barring a density change this type of densification does not alter any of the properties of the source biomass. A major problem with the use of the machines that are currently on the market is that they are very large and require the collection of residues over a large area. This will add substantially to the cost of the fuel. There are a few exceptions though. There are many large scale rice hulling mills and coffee curing plants which produce enormous wastes by way of rice and coffee husks. These wastes are a source of environmental hazard. Under these circumstances the briquetting machines to convert waste into fuel which can be partially used in house and partially for sale outside can prove quite useful. In the rest of this section we restrict ourselves to a consideration of the properties of briquettes of this type. The reason for this choice is simple. Much more information on these types of briquettes are available than on the type of briquettes suggested by Janczak. Tables 1 and 2 provide the ultimate analysis and the proximate analysis of some typical briquettes (Krist-Spit and Wentink 1985).

Table 1: Typical Ultimate analysis of briquetted fuels by weight (%)
Material H C O Ash Comb. Value
Gross (net)
Rice husk 5.50 40.4 34.5 19.8 15.1(13.8)
Corn stover 6.05 47.1 43.5 3.40 18.6(17.2)
Cotton stalks 5.99 47.1 43.9 3.16 19.0(17.2)
Black coffee husks 5.10 47.8 36.0 8.90 18.6(17.5)
Cow dung 5.18 31.6 37.8 19.3 11.4(10.2)

Note: Dung has considerable Nitrogen and therefore the percentages do not add to 100.

Compared to the wood fuels these materials show a major difference. Their densities on the average are much higher but also are more uniform among the different materials. The calorific values of these fuels are roughly comparable with those of wood fuels. The exceptions are rice husk and cow dung which have much smaller calorific values due to their large ash content.

Table 2: Typical Proximate Analysis of Briquetted Fuels by Weight (%)
ash Density
kg/ m3
Rice husk 64.1 16.1 19.8 1063
Corn stover 77.5 19.0 3.4 990
Cotton stalks 75.8 21.0 3.16 1090
Black coffee husks 65.4 22.7 8.9 1170
Cow dung 56.4 19.3 19.3 1000

Hand-operated briquetting machines

The rural problem is really a serious one. The main concern here is the dilemma of the designer in the face of the energy gatherer's priorities. The latter's interest is in the minimization of the effort in fuel collection rather than the maximization of fuel performance. Many a designer's response to this situation is a mere slogan: the particular design is capable of burning any fuel (see for example Overhaart 1979 and Evans and Wharton 1977). Such a slogan flies in the face of all engineering evidence.

The present practice of using long wood, green wood, ``dry'' wood, any form of twigs, leaves, agricultural residues of all types, and animal waste as and when they are available frustrates the purpose of any design. It is not possible to quote reliable performance figures without tying them to some form of fuel specification. Two of the principal selling points of the newer crop of designs is that they will be able to save on the fuel collection effort. This is hard to sell to an indigent population. The only way out is for the designer and the ``stove seller'' to make a common cause and approach the prospective buyer with circumspection. The whole thing calls for the development of local women's organizations. There is evidence available that many such organizations exist in many countries. What follows in the rest of this discussion assumes the willing participation of such groups in stove propagation among their members.

To handle twigs, all forms of agricultural residues and animal waste essentially requires some form of densification. Modern methods are simply not usable in rural areas (see for example the work of Twente university 1990). A set of alternatives are proposed by Micuta (1985) and Janczak (1981) to prepare bundles out of twigs and to produce briquettes out of animal dung and agricultural residues. All these are hand operated machines and most of the briquetting operations require some form of binder. Figure 1 illustrates these hand operated technologies and Table 3 provides the heating values of typical briquettes produced by such technologies. The briquetting presses are reminiscent of mud block presses that are gaining considerable amount of popularity in India (Jagadish and Shrinivas 1991).

Table 3: Comparison of some biofuels
Composition Gross
Net heat value
Dry beach wood1 19,100 8.0 0.3 17,700
Green beach wood2 13,900 42.9 0.25 12,100
Waste-paper balls made from soaked newspaper)3 17,400 6.9 2.9 16,100
Briquettes made of 30 - 45%
charcoal dust, 30 - 45%
chopped twigs, 15 - 205
9,400 2.4 32.2 18,500
Briquettes made of 25%
charcoal dust; 25%
straw; 30% chopped
twigs; 20% manure5
14,300 7.2 13.7 13,100
Briquettes made of 50%
straw; 50% cow manure6
16,400 5.4 9.5 15,100
Briquettes made of 40%
straw; 40% sawdust;
20% manure7
15,000 9.2 14.0 13,700
Briquettes made from
charcoal dust with
clay as binder8
4,470 - 73.0 4,090

1 Type of wood rarely available in poor rural areas
2 Type of wood commonly found in poor rural areas of developing countries
3 Made by hand; burns better if wood ash is added
4 comparable to medium-quality hard coal; high chopped twigs; 15 - 20content probably from sand
5 Lower percentage of charcoal dust reduces heat output
6 Feasible everywhere, but has high manure content and manure is better used for fertilizer
7 Needs careful drying because of sawdust
8 High mineral content lowers heat value and creates much ash

Presumably with a little bit of design effort the machines shown in the figures could be significantly improved. Needless to say all this requires considerable amount of organizational effort.