| Boiling Point No. 01 - Special Edition 1989 |
Residues as a Source of Energy
Seminar 26-27 September 1986
COIR DUST BRIQUETTING IN SRI LANKA
by M I Samarakoon - Polgahawela
The advantages of mass scale agro-industrial production depend largely on the maximum utilisation of all by-products. Available agro-residues are: paddy husk, rice straw, coir dust, and sugarcane bagasse to mention only the most convenient available forms. According to my information, the annual availability of the above mentioned agro-residues is as follows: 320,000 tonnes of paddy husk, 400,000 tonnes of rice straw, 500,000 tonnes of coir dust and 20,000 tonnes of bagasse. This is a factor which has been ignored for decades and only in the recent past has it attracted attention due to sky rocketing of the prices of the fossil fuels. Though thousands of tonnes of agro-residues are turned out annually, none of it has been harnessed with significant importance, except the trunks of the felled rubber trees. In the case of coir dust, at present a small percentage seems to be occasionally used by the coconut industry itself and by small farmers who grow ginger and pineapples etc. because of its capacity to retain moisture but not as a source of energy. Although many workers have endeavoured to utilize these by-products effectively, little headway appears to have been made up to now. The annual accumulation of the vaste amounts of coir dust without adequate or economic methods of disposal, sometimes compel a miller to change the site of production. All the agro residues are bulky or, as in the case of coir dust, have in addition about 85% moisture which make it unattractive as a fuel. Similarly bagasse often has a high moisture content and so is inefficient as a fuel.
Present Methods of Utilization
A small portion of paddy husks and bagasse is used as fuel in mills and factories without any briquetting. Many attempts have been made to briquette coir dust with the conventional briquetting machines with very unfavourable and uneconomical results, as these machines can only be used with coir dust with less than 20% moisture. In addition the machines themselves work at high pressure and very high temperatures, consuming a large amount of expensive electrical energy. The writer has been working in this field for many years and has succeeded in developing an economical process for the manufacture of moulded articles for many domestic and constructional purposes which can also be extended to the manufacture of briquettes. Further, briquetting of agro-residues makes them a more efficient fuel due to the lesser moisture content. It also makes for more economical transportation and storage depending on the form of the briquettes. All these have been considered in the development of my process for the manufacture of briquettes out of coir dust with or without fillers.
The basic principle involved in the process is the use of about 25% of the lignin inherently present in coir dust. In nature all woods, soft and hard, are built up in a similar manner. The lignin originates in the meristematic cells and culminates in the hard and lignified dead cells. In most woods, the walls of young and actively dividing cells are gradually thickened with layers of lignin and this ultimately leads to their death. These in turn are replaced by younger ones, the older ones being crushed in the process and finally a hard wood is formed. This process goes on in the development of the coconut husk and shell too. It is fortunate that coir dust contains a high percentage of this lignin which hardens all woods. The chemical nature of lignin varies from source to source. Nature seems to have used simple reactions from classical chemistry in the building up of the lignin molecule. Although its constitution is not precisely known, it would appear to contain dihydroxy phenyl-glycerene, which as Freudenberg suggests, condenses according to the structural formula given below:
This polymerization should continue at length, building up lignin three dimensionally because an interlinking in all direction is possible. Our problem of finding a use for coir dust could be easily solved if we were in a position to depolymerise the lignin in coir dust and then effect its aggregation once more in the form of one mass, thereby leading to the manufacture of moulded articles, including briquettes. Other less fortunate agro-residues like paddy-husk, straw and bagasse could be incorporated in these briquettes as fillets, the lignin then acting as a binding medium. A series of experiments were carried out by the writer with the view to bringing about this depolymerisation and repolymerisation. The chemicals used can be classified in to the following groups:
a. Delignifying agents including earth alkali hydroxides and oxides
b. Oxidizing agents
As a result of these experiments, it was found that the lignin contained in rested and unrested coir dust could be made to form aggregate units in the presence of one or more of the above mentioned groups of chemicals, when subjected to simultaneous pressure and temperature. The physical properties of the so moulded articles depend upon:
a. Chemicals used
b. Pressure used to mould article
d. The time for which the pressure is applied.
In the case of briquettes, room temperature and a pressure of 85 Kg/sq inch for 10 minutes are sufficient. Briquettes so manufactured can still have a moisture content of about 50-60%. These could then be air dried at room temperature, but if necessary the drying process could be expedited by utilising ventilators to drive a slow moving current of air through the stacked up briquettes. The readers will have noted that an input of energy is required only to operate the presses. The depolymerising chemicals could be chosen in such a way that an exothermic reaction results, which raises the temperature of the briquettes to about 40-60øC on leaving the presses. This too expedites the drying process. Further, the addition of cheap oxidizing agents brings about the complete combustion of the briquettes. Formation of silicium slag by burning briquettes from conventional briquetting machines could be eliminated too by using the proper depolymerising agents.
Briquettes so formed could be dried at room temperature in about 9-14 days. This drying period depends on the room temperature and the relative humidity of air in the drying chamber and outside. So dried, briquettes can have a moisture content below 22%. It may be noted in passing that coir dust briquettes have a calorific value 1.7 times that of rubber wood with a moisture content of about 35% and can be manufactured at a cost of about 40 cents per kilogram (compared to rubber wood), naturally not considering transport and other overheads.
In the course of my experimental work which led to the process under discussion, it was found that a certain percentage of liquid must be present in the coir dust to bring about an aggregation of the lignin units to make it act as a binding material. For the first experiment, rested coir dust was dried at 65øC for 3 hours and 50 grammes of this dust was introduced into a mould 16sq inches in area. This was then subjected to a pressure of 16 tons at 153øC for 15 minutes. The sample board thus obtained was weaker than a similar one produced from air dry dust subjected to the same pressure and temperature. An expansion was observed when the former sample board was removed from the frame of the mould. The thickness of the sample was 0.23 inches when it was removed from the frame but within 48 hours the thickness increased to 0.48 inch. The reversion of the sample board to coir dust was brought about when immersed in water. This clearly shows that there is no binding produced when the rested coir dust is dried at 65øC (or higher temperature) so as to reduce the percentage of moisture present far below the average air dry condition. Hence a certain percentage of water may be said to be essential if the lignin present in coir dust is to be used as a binding material.
Further experiments conducted revealed that water is not the only type of liquid medium that would bring about the aggregation of lignin units. For example, rested coir dust was dried at 110øC for 2 1/2 hours. Four ounces of this dust was mixed with 33cc of coconut oil and then the mixture introduced into a mould of 16sq inches and subjected to a pressure of 1 ton per square inch at 150øC for 8 minutes.
The above experiment was repeated with castor oil, linseed oil, diesel oil, paraffin wax, beeswax, glycerine, benzaldehyde, acetaldehyde, nitrobenzene, acetic acid, stearic acid etc. In all the above cases a satisfactory binding was found in the samples produced. However, liquids of higher boiling points gave better bonding than those with comparatively low boiling points. A peculiar result was obtained with carbon tetrachloride. The sample produced in this case was similar to that from dust dried at 65øC for 22 hours and pressed with no liquid medium added. Hence, the presence of carbon tetrachloride did not bring about an aggegation of lignin units. Since all the substances which gave a satisfactory bonding could not be classified into one general group, the term 'Liquid media' has been used in this paper
(Ed note - Perhaps the liquids were acting as heat transfer media to facilitate the softening of the lignim).
About 65% of the weight of the coconut husk is composed of this ligneous waste material. Other ligneous materials like coconut shell dust and sawdust could also be used provided they were mixed with coir dust in the correct proportions. It is envisaged that the present process will bring about the utilization of these waste products for the manufacture of construction materials and briquettes.
If only 200,000 tons of the 500,000 tons of coir dust falling annually could be converted into briquettes for industrial and domestic use, the result would be a saving of 80,000 tons of industrial fuel oil, bringing about an annual saving of about one third of Sri Lanka's annual requirement. The resulting foreign currency saving would be in the region of Rs 500 million, depending on the world market prices of fuel oil. May I also mention that the above process of manufacturing briquettes out of coir dust is covered by Sri Lankan Patent No 95()2 and could be effectively utilised in all the coconut growing countries in the world. In conclusion it could also be stated that the processes covered by the above patent could be used to manufacture lightweight building blocks, partition boards, floor tiles and even roofing materials.