|Boiling Point No. 39 - Using Biomass Residues for Energy (ITDG - ITDG, 1997, 44 p.)|
by Widad Osman Ahmed, IT Sudan, PO Box 4172, Khartoum, Sudan
Production de briquettes au Soudan
L'expence du Soudan en mati de briquettes date de la fin des ann 80. Des briquettes sont produites artir de coques d'arachide, de tiges de coton et de bagasse. L'auteur examine les diffntes technologies associ haque rdu. Les briquettes de coques d'arachide sont accessibles es prix comptifs par rapport au bois de feu. Les briquettes artir de tiges de coton sont plis par le prix de cette mati premi. Les briquettes de bagasse sont produites artir de presses manuelles dont l'investissement est relativement faible.
The reason for briquetting
Biomass densification means the use of some form of mechanical press to reduce the volume of agricultural waste or residue and to convert it to a solid form. This is easier to handle and store than the original material and is known as briquetting. The briquetting process also improves the burning characteristic of the waste. There is around thirty million tonnes of agro-waste which could be utilized in Sudan, and this represents 20% of the country's current needs for conventional energy (petroleum and electricity).
Sudan's experience of briquetting
Sudan's experience in the field of briquetting started in the late eighties, to alleviate desertification and forest degradation. Briquettes made from groundnut shells, cotton stalks and sugar-cane waste (bagasse) are produced using different technologies. The briquettes' quality depends on:
· the volume of raw material,
· seed variety,
· the binding material,
· the moisture content of the shells.
Briquetting of groundnut shells
Direct briquetting using a high-pressure piston machine without a binder is used to produce groundnut briquettes at En Nuhud of Kordafan, where there is suitable sandy soil for growing groundnut plants. Each season, an average of 5655 tonnes of residues is always available. A project of UNSO (UN Sudano-Sahilian Office) and the Ministry of Energy and Mining started there, producing on average an annual 3210 tonnes of briquettes, manufactured over a seven month period.
The machine, with an installed capacity of 1.5 tonnes/hour, is composed of a piston, silo, hammer mill and chipper, and in case of cotton stalks a dryer is also used. The machine can take the raw material using an automated feed, or it is pushed along the conveyor by hand. Water is sprayed from nozzles when needed, the raw materials are ground and mixed together, the mixture moves into a cylinder, and a piston pressure of 1 tonne/cm2 applied. The moisture content ranges between 10-15%, and the temperature inside the cylinder may reach 260°C. This melts the leguinen on the shell which forms a binding substance for the compressed materials. The 5cm diameter cylindrical briquettes pass over the product line for cooling down and are then collected in sacks; each sack weighs 50-55kg.
The briquettes provide a long continuous fire although the smoke immediately after the fire is lit is a considerable disadvantage of the product. This smoke, due to the oil content of the shells, can be alleviated by using paper to aid ignition.
The major problem facing briquette production is the machine spare parts; the piston tip and press sleeve deteriorate very fast, lasting only 90 hours instead of 500 hours, due to the presence of sand from the areas where the groundnuts are grown. The problem is being solved by manufacturing these spare parts locally in a railway maintenance yard in Atbara. The briquettes can be used instead of wood in households in the rural areas and in small industries and restaurants as cheap fuel.
The calorific value of groundnut briquettes is 5233kcal/kg, compared to 4800kcal/kg for firewood. In terms of cost, a 55kg sack of briquettes costs Ls. 1500 (1515Ls. = $1US) while one kantar (49.9kg) of fuel wood costs Ls. 2500. This competitive price may allow briquettes to replace wood and charcoal in some situations, and thus may contribute to rural development.
Cotton stalk briquetting
Fuel briquettes are produced from cotton stalks in the Gezira area using molasses as a binder. In this method, which is based on the experience of the Sudan Council of Research - Energy Institute and GTZ for producing cotton stalk charcoal in a scheme in Ruhad, a second technology is required. Cotton stalks are reduced to carbon in the field using mobile metal kilns. The carbon product is ground into powder, a little molasses is added as a binding material and a rounded black briquette is produced. (Figure 1)
Figure 1: Cotton stalk briquettes
One example of the use of this technology is in a bakery in the town of Wadmanain. The result of using cotton stalk briquettes as a fuel is that the cost of bread has been reduced by 30% from that produced using wood fuel. One problem inhibiting this technology is that the price of cotton stalk as a raw material is not cheap compared to other wastes and so the profit margin does not encourage the private sector to invest.
Bagasse/molasses fuel block briquetting
The manufacture of bagasse/molasses fuel blocks in Sudan is a low-capital labour-intensive technology, which depends on using molasses as a binder. It is as estimated that more than half a million tonnes of waste bagasse is available at New Halfa Sugar Mill in the eastern region. The bagasse is of two types:
· older bagasse which has been standing in a heap for two to three years, can be used for compressed briquettes
· more recently produced bagasse which is more fibrous and springy, has to be carbonised before briquetting or left to rot further.
The technology to make compressed briquettes uses manual presses to compress the bagasse after the addition of a molasses binder. The raw materials are rotted bagasse, fresh bagasse, and pit molasses and the mix ratio is 65%, 15% and 20% respectively. A Terteram block-making press is used and the materials are mixed by hand (Figure 2).
Figure 2: Terteram block-making press
The mixture is put into the press mould, and machine handles are operated to press the mixture to a dense block (Figure 3).
Figure 3: Bagasse/molasses briquettes
The daily output of 600 blocks are unloaded and stacked in the sun to dry. The maximum com-pressive force is 48kg/cm2 producing a block with dimensions 295mm x 140mm x 80mm and weighing 2.3kg. The daily output of six tonnes of fuel blocks is equivalent to energy released from clearance of 590 hectares (1400 fedans) of woodland each year. The quantity of smoke released from the briquettes and their quality are considered satisfactory.
FAO is providing some technical financial assistance for some government and non-government organizations to establish this work on a commercial basis. All the technologies which use molasses as a binder mix it with bagasse by hand. However, there is some scope for improving the mixing and pressing techniques with existing fuel-block production factories. The current methods of rubbing the molasses into the bagasse by hand is time-consuming and inefficient. One fuel-block factory at New Halfa has tried to use a cement mixer as a simple inexpensive mechanical method.
The cheap fuel blocks of bagasse provide a chance for small enterprises to expand local industries e.g. brick-making, lime-making etc. and will reduce the harmful effects of bagasse in the area, such as the cost of security against fire and the transportation costs to taking the bagasse away.
Many studies are now running to develop these technologies to compete with other alternative sources in conserving fuel wood consumption.