Chapter 4.Technical aspects of briquetting
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History of briquetting
The compaction of loose combustible material for fuel making purposes was a technique used by most civilisations in the past, though the methods used were no more than simple bundling, baling or drying.
Industrial methods of briquetting date back to the second part of the 19th century. In 1865, a report was made on a machine used for making fuel briquettes from peat which is a recognisable predecessor of current machines. (A drawing of this machine is reproduced in Fig. 3 by courtesy of the British Institution of Mechanical Engineers.)Since then there has been widespread use of briquettes made from brown coal, peat and coal fines. There are various processes which produce artificial smokeless fuel briquettes from coal fines.
The most common technique used in this type of process is some form of roller press using only moderate pressure and a binder. This type of plant is also used to make all kinds of non-fuel briquettes from inorganic material such as metal ores. Various binders are used; one of the most common is lignin derived from paper-pulp manufacture.
Figure 3: Drawing of 1865 Peat Piston Briquetter
The briquetting of organic materials requires significantly higher pressures as additional force is needed to overcome the natural springiness of these materials. Essentially, this involves the destruction of the cell walls through some combination of pressure and heat. The need for higher pressures means that the briquetting of organic materials is inherently more costly than for inorganic fuels.
The use of various forms of organic briquetting seems to have been common both during World War I and during the '30s depression. The modern mechanical piston briquetting machine was developed in Switzerland based upon German developments in the '30s. Briquetting of sawdust and other waste material became widespread in many countries in Europe and America during World War 11 under the impact of fuel shortages. Parallel needs pushed the Japanese into refining the screw machine discussed below. After the War briquettes were largely squeezed out of the market by cheap hydrocarbon fuels.
The use of organic fuel-briquettes, mainly in industry, was revitalised during the period of high energy prices in the '70s and early '80s, especially in Scandinavia, the USA and Canada.
In Japan, briquetting seems to have been common until recently with widespread use of "Ogalite" fuel briquettes made from sawdust. The Japanese technology has spread to Taiwan and from there to other countries such as Thailand. Japanese, and later Taiwanese, briquetting has been based almost entirely upon the use of screw presses which, although originating in the USA, have been more widely adopted by Asian than European or American manufacturers. Such briquettes were widely used in Japan during the 50s as a substitute for charcoal which was then still a widespread fuel.
It seems quite clear that the development of the modern type of mechanical piston press started in Switzerland during World War II though based upon work done in Germany in the '30s. The Swiss developments were centred around Fred Hausmann and the Glomera press though he was not its original inventor.
Patents and licensing rights changed hands during a period when partnerships were broken up and companies went bankrupt or were bought by others. Whatever the precise situation about original invention, Hausmann undoubtedly played an important role in making the piston-press technology well-known all over the world. In many places the name Hausmann is often equal to a mechanical piston press. Thus the Brazilian industry, certainly the largest outside North America, was begun by a company in which Hausmann was a founding partner, whilst in India one of the main manufacturers began with a licence from Hausmann.
No patents governing the general design of this type of press are effective today and most of the manufacturers of mechanical piston presses identified in this study owe their designs to the original Swiss patent. The nearest current descendant of the first piston press manufacture is, by their own claim, Pawaert-SPM.
The piston press acts in a discontinuous fashion with material being fed into a cylinder which is then compressed by a piston into a slightly-tapering die. The compressed material is heated by frictional forces as it is pushed through the die. The lignins contained in all woody-cellulose materials begin to flow and act as a natural glue to bind the compressed material. When the cylinder of material exits from the die, the lignins solidify and hold it together to form cylindrical briquettes which readily break into pieces 10-30 cm long.
The diameter of the briquette is closely related to the output of the machine. A unit producing 1 t/h of briquettes will have a die 8-10 cm in diameter. This relationship is rather inflexible and may constrain potential markets for the product of bigger machines. Small stoves may not be able to burn such large pieces.
Piston-presses can be driven either by mechanical means from a massive flywheel via a crankshaft or hydraulically. The mechanical machines are usually larger, ranging in size from 0.45 to 0.3 t/h, whilst hydraulic machines normally range up to 0.25 t/h though some models are somewhat larger.
Mechanical presses generally produce hard and dense briquettes from most materials whilst hydraulic presses, which work at lower pressures, give briquettes which are less dense and are sometimes soft and friable.
Piston presses are reliable, once they have been installed properly with dies shaped correctly for the raw materials used. Problems arise if the die has not been shaped correctly or if the feeding mechanism has not been sized for the material to be used. It is normal for machines made in Europe to be designed to operate on wood wastes; the use of agroresidues normally de-rates the throughput and may require some modification to the feeder. Such an output aerating may result in a significant increase in capital charges.
Figure 4: Typical Piston Briquetting Press
It is normal for machines made in Europe to be designed to operate on wood wasted ;the use of agro-residues normally de-rate the throughput and may require some modification to the feeder. Such an output derating may result in a significant increase in capital charges.
Maintenance costs are fairly low amounting mostly to replacing the die every few hundred hours, the precise time depending upon the material. Some feedstuffs, such as rice-husk, may be particularly abrasive on dies. It is important, however, that regular maintenance is undertaken. The heavy, discontinuous action of the piston means that small imbalances and irregularities can quickly become major defects.
Piston presses with hydraulic drives, as distinct from those using mechanical drives with flywheels, are manufactured in the relatively limited geographical region of Western Europe. It is a fairly recent development of the mechanical press for use with light materials where the quality of the product is of less concem. The forces in a hydraulic machine are less violent than in a mechanical unit and they may therefore need less attention.
Typical materials suitable for hydraulic presses are paper, cardboard, manure, etc. though the hydraulic press can in some cases become an alternative to a mechanical press. Since it is normally made with lower capacity than the mechanical press, it is suitable for taking care of waste material from small wood processing industries. Briquettes from hydraulic machines are often used onsite as they may be too soft for much transportation
The earliest development work on screw presses was carried out in the USA in the 30's resulting in the widespread use of the PRES-TO-LOG model which was based on the conical type of extruder currently found in the Belgian Biomat design. During World War 11, a Japanese design which featured a heated die and a prolonged tapered central shaft of the screw resulting in a hollow briquette, was being developed. It was very successful and one of the manufacturers in our study claims to have sold 600 units. The design has been taken up by other manufacturers in Asia and more recently in Europe.
In the screw-presses, material is fed continuously into a screw which forces the material into a cylindrical die; this die is often heated to raise the temperature to the point where lignin flow occurred. Pressure builds up smoothly along the screw rather than discontinuously under the impact of a piston.
Figure 5: The PRES-TO-LOG Briquetter
If the die is not heated then temperatures may not rise sufficiently to cause lignin flow and a binding material may have to be added. This can be molasses, starch or some other cheap organic material. It is also possible to briquette carbonised material in a screw-press and in this, as lignins have been destroyed, a binder has to be employed. Some low-pressure piston machines may also require the use of binders though this is unusual.
If the die is heated then the temperature is normally raised to 250-300 °C, which produces a good quality briquette from virtually all organic feeds provided the initial moisture is below about 15%. The briquettes from screw machines are often of higher quality than from piston units being harder and less likely to break along natural fracture lines.
Screw presses are usually sized in the range 75-250 kg/in though larger machines are available.
The capital costs of screw machines may be a little less than piston units though because of size differences it is difficult to make direct comparisons. However, their maintenance costs are usually much higher because of the considerable wear on the screws which have to be re-built rather frequently. They also have a higher specific energy demand than piston machines.
However, maintenance costs of screw presses are usually much higher because of the considerable wear on the screws which have to be re-built rather frequently.
Figure 6: Typical Screw Briquetting Machine
These operate by extruding small-diameter (10 to 30 mm) pellets through a die which has many holes. The extruding mechanism is often an eccentric roller which moves inside the large cylindrical or conical die.
Such machines were originally developed for the production of animal feedstuffs and mineral-ore pellets. They are expensive and have high through-puts of 5-20 t/h for a single unit.
The smaller product size and high capacity of these types of presses was before the 60's utilised only in the pressing of fodder pellets and similar applications. Since then a limited number of energy applications have materialized in the USA (Woodex), Canada (Bioshell) and in Europe (Sweden, France and West Germany). There have been a few applications of pellet presses in developing countries solely for energy purposes, notably Kenya, Zimbabwe and Zambia. The latter two examples are both defunct however and it is doubtful if the high capital cost and power consumption of this process makes it a viable proposition.
This report concentrates on equipment suitable for industrialized production of briquettes, albeit on a small scale. We are largely omitting the numerous types of hand-driven or animaldriven fuel-forming equipment found in literature and, possibly, used in some parts of the world. Several researchers have proposed schemes for developing equipment suitable for briquetting of agricultural waste on the village level (Scarab 1983). The "green fuel" scheme in Indonesia and work in Thailand by Prof Watna Stienswat is aimed at solving the problem of finding suitable technology for small-scale (<100 kg/in) operations. They operate with wet, i.e., green, material forming, rather than densifying, the material into a briquette that is then solar dried. There has also been work on manually produced briquettes undertaken in Indonesia. (Johannes, 1982)
Another interesting development has been seen in Sri Lanka in which large briquettes are formed from coir dust in a baling press between plates of corrugated steel. Lime is mixed in with the coir dust to make the briquette suitable for handling after solar drying. The method has prospects to offer a relatively inexpensive way of producing briquettes in small as well as larger plants.
It remains unclear, however, whether any manual or semi-manual densification process can ever be commercially viable even in circumstances where labour is very cheap. When allowance is made for their very low throughput, such techniques often require almost as much capital investment as the mechanical processes. The savings achieved are essentially a labour for electricity substitution rather than labour for capital.
Two semi-manual plants making briquettes from semi-rotted bagasse are known to be operational in the Sudan (Paddon, 1987) and are quite successful commercially. Their circumstances are unusual however they are based on Ethiopian refugee labour - and it might be difficult to replicate them elsewhere. However, manual processes do have the great advantage of being able to handle wet wastes which cannot be utilized mechanical processes.
Manual presses cannot be made to generate sufficient pressure to break down cell walls and they cannot, therefore, produce densified briquettes. This means that they cannot realise any significant degree of transport cost savings.
As the application of manual presses is likely to be limited to special cases, one problem with their development is that it is difficult to justify producing specific machines for the job as the initial costs are too high. In the Sudan, manual brick-making presses have been adapted for the purpose. One drawback of this is that the initial chopping, mixing and feeding operations are very dirty and arduous; the actual briquette making part of the process is the easiest.
It remains unclear however whether any manual or semi-manual idensification process can ever be commercially viable even in circumstances where labour is very cheap.
Briquetting and pelletization are justified mainly by the reduction in volume of a bulky waste material. After densification, there are two main quality aspects of the product:
The first aspect, that the product should not crumble and disintegrate when handled, stored and transported, is mainly a function of the quality of the densification process for a given raw material. The second aspect is mainly related to the properties of the raw material and the shape and density of the individual briquette. In the following we will call these factors
The distinction is not always clear and sometimes they interfere with each other. For example, improving the handling characteristics by making a more dense briquette often has a detrimental effect on its combustion behaviour
In principle, in planning a project it is possible to begin with the various demands of transportation, handling, storage and combustion and then to choose a machine which, for a given raw material, produces a briquette which possesses just the required characteristics. In practice, this is seldom possible given that commercial machines tend to have concentrated on a rather limited range of product options. This is particularly important in developing countries as most processes have been developed with other markets than agro-residue briquetting in developing countries in mind. One such example is the hydraulic piston press. It has been developed to work in small wood working industries with waste flows smaller than about 0.1 t/h and where the briquettes are intended for combustion in in-house boilers. Thus it does not matter that the briquettes produced are rather soft. However, such briquettes are likely to be unsuitable in any circumstances where the briquettes are transported
Mechanical piston briquetters, on the other hand, make harder briquettes but they are more sensitive to foreign particles in the material flow. A nail for example is likely to destroy the die and piston top of a mechanical piston machine while it would probably pass through a hydraulic machine unnoticed.
They also produce large briquettes which may not be suitable for the proposed combustion device.
Thus, in practice, some compromise between desired characteristics and what machines are commercially available may be necessary.
Before discussing in more detail the various characteristics of briquettes, it should be pointed out that the briquette handling characteristics are not likely to cause severe problems in a project, other than in cases where there has been a plain mismatch between material, process and purpose. Combustion properties are more critical, especially when trying to introduce briquettes in the household sector, but also when they are intended for combustion in industrial boilers.
Most processes are capable of producing briquettes with densities above 1 000 kg/m³, i.e. the individual briquettes will sink in water. (This is, in fact, a good if crude test for the briquette quality.) The upper limit for the density is set by the physical density of each raw material which, for ligneous material, is about 1 500 kg/m³. The density of individual pieces is termed apparent density. High pressure processes such as mechanical piston presses, pellet presses and some screw extruders, make briquettes in the 1 200 1 400 kg/m³ density range. Hydraulic piston presses make less dense briquettes, sometimes below 1 000 kg/m³.
Briquette handling characteristics are not likely to causes severe problems in a project, other that in cases where has been a plain mismatch between material, process and purpose. Combustion properties are more critical.
There is little point in trying to make even denser briquettes because combustion properties are likely to suffer. The benefits are small because the more important property of briquettes is their bulk density, that is the overall density of many pieces piled together.
The bulk density is a function of both the density of the individual briquette and its geometry. There are differences in bulk densities between large and small briquettes and pellets, but for broad calculations a factor of two between apparent and bulk density can be used (CRA 1987). This means that for briquettes with apparent densities in the 1 200 - 1 400 kg/m³ range, the resulting bulk densities are 600 - 700 kg/m³. For comparisons, the bulk density of the raw material could be as low as 40 kg/m³ for some grades of bagasse to about 150 - 200 kg/m³ for a variety of agro-residues and wood wastes. The higher bulk density of briquettes will significantly increase the distance over which it is economic to transport a residue in order to god a market for it.
In briquetting, the resulting density is affected to a significant degree by the particle size of the raw material. Finely ground material, for example sanding dust from wood plants, will make very dense briquettes but requires high pressures and temperatures to agglomerate without a binder.
The density of the product is also affected by the moisture content. Water in the raw material will prevent the compression of the briquettes and the steam that evaporates from the material due to the high temperatures will leave voids which decreases the apparent density. If the briquettes later pick up humidity from the air, the result is a swelling of the material which also decreases the density. This process can lead to the total disintegration of the briquettes.
This factor is a measurement of the briquette's resistance to mechanical action that will affect them when handled and transported. Tests can be done either in a rotating drum or by repeatedly dropping samples from a specified height. In both methods, the samples are screened (20 mm sieve) and the fraction retained is used as an index of a briquette's friability (CRA 1987).
It is difficult to give a figure for an acceptable friability index as the relationship between test results and reality has never been studied. In the work carried out by CRA some samples received an index of 0, i.e. the briquettes had disintegrated entirely after a certain time, which clearly indicates an inadequate briquette quality.
When the briquettes score higher in tests, say between 0.5 and 1.0, such results are more difficult to interpret. They do have a function though when comparing several processes in order to find the most suitable for a given material.
General observation at a number of operating plants suggests that briquettes produced by mechanical piston-presses and screw-presses are hard enough to be transported by lorry for considerable distances without degradation. No plants using such machines complained about losses due to product disintegration. One or two plants using hydraulic presses did find that the product was too soft for transportation.
Resistance to humidity
Inherent binders (lignin) and most externally added binders are water soluble. This results in one of the weakest points in briquette quality, which is that briquettes must not be subjected to water or humid air. Briquettes and pellets have to be stored under cover and they do have a limited lifetime under humid conditions. The latter problem appears to be only minor even in tropical countries. The dense, hard-surfaced briquettes produced in mechanical piston presses and screwpresses with heated dies have enough resistance to humidity to withstand the rainy season in India, Thailand and Brazil provided they are covered.
The resistance to humidity is traditionally tested in immersion tests, i.e. the briquettes are dunked in water and the elongation or swelling of the briquettes is recorded. Sometimes the time elapsed until the briquette has completely disintegrated
One of the weakest points in briquette quality is that briquettes must not be subjected to water or humid air is taken as a measurement of the quality in this respect. This time can vary from a few minutes up to hours and again it is difficult to give an acceptable value for this parameter. In tests carried out by CRA, it has been found that the rate of elongation is a more precise parameter and they suggest that a figure of less than 50% elongation per minute indicates an acceptable quality.
In other tests, briquettes are subjected to humid air for extended periods and their swelling is recorded. After a period of 21 days in an atmosphere of 20° C and 95% humidity, an elongation of less than 30% is said to be acceptable and less than 20% would be ideal (CRA 1987).
Although resistance to humidity may not be such a crucial factor when storing briquettes, provided they are shielded from direct rain, this factor may be of importance in the combustion and, especially, gasification of briquettes.
Water vapour, driven off from inherent moisture and formed in combustion, creates a saturated climate at high temperatures which is a more fundamental test of a briquette's resistance to humidity. If the briquettes disintegrate too quickly, the loose substance will either elutriate un burned through the boiler or block the airflow to the process, depending on the circumstances. There are no good data on this though it does not usually seem to be a practical problem in combustion. It is possible however that such swelling and disintegration could be a bigger problem in the gasification of briquettes.
One of the most important characteristics of a fuel is its calorific value, that is the amount of energy per kg it gives off when burned. Although briquettes, as with most solid fuels, are priced by weight or volume, market forces will eventually set the price of each fuel according to its energy content. However, the production cost of briquettes is independent of their calorific value as are the transportation and handling costs. The calorific value can thus be used to calculate the competitiveness of a processed fuel in a given market situation. There is a range of other factors, such as ease of handling, burning characteristics etc., which also influence the market value but calorific value is probably the most important factor.
Figure 8: Higher Calorific Value Diagramme
For quick reference, the calorific value of wood and most agro-residues can be calculated using the following formula which although originally derived for wood can be used for most agro-residues with little alteration:
Gross (or higher) calorific value (HCV) = 20.0 x (1 - A - M) MJ/kg where A is the ash content and M the moisture content of the actual fuel.
The lower (or net) calorific value, which takes into account unrecovered energy from the water vapour from inherent moisture and from the oxidation of the hydrogen content, is sometimes used for reference purposes, especially in industrial applications. In wood and most agroresidues, the hydrogen content is about 6% by weight on a dry and ash-free basis, which means that the above formula would be changed as follows:
Lower calorific value (LCV) = 18.7 x (1 A - M) - 2.5 x M
Example: Rice husk with a moisture content of 15% and an ash content of 20% has the following calorific values according to the above formulae:
HCV = 20.0 x (1 - 0.2 - 0.15) = 13.0 MJ/kg
LCV = 18.7 x (1 - 0.2 - 0.15) - 2.5 x 0.15 = 11.8 MJ/kg
For materials with low ash contents and moisture contents between 10% and 15%, that is most briquettes from wood and agroresidues, the resulting calorific values are found in the 17 - 18 MJ/kg range (LCV: 15.4 - 16.5 MJ/kg).
Table 2 (reprinted from Barnard, 85) gives an indication of the variations of ash content and calorific value for a number of agricultural residues. There are discrepancies in the calorific values from different sources, probably due to inaccurate testing procedures. Note that the HCV of an actual fuel has to be adjusted for moisture content using the above formula.
Table 2: Calorific Value and Ash Content of Verious Fuels. (Barnard 85)
|Material||Ash Content %||HCV MJ/kg (oven dry) (oven dry)||Material||Ash Content %||HCV MJ/kg|
|Alfalfa straw||6.0||18.4||Olive pits||3.2||21.4|
|Almond shell||4.8||19.4||Pigeon pea stalks||2.0||18.6|
|Cassava stem||-||18.3||Rice straw||-||15.2|
|Coconut husk||6.0||18.1||Rice husks||-||15.3|
|Groundnut shells||-||19.7||Soybean stalks||-||19.4|
|Maize stalks||6.4||18.2||Sunflower straw||-||21.0|
|Maize cobs||1.5||18.9||Wheat straw||-||18.9|
Combustion in industrial boilers
Experience shows that industrial boilers are usually the most convenient and accessible combustion plants for briquettes. Even so, the range of plants which can utilise briquettes directly are those designed for solid fuels, that is wood or coal. Oil plants can be converted to take solid fuel but only at considerable expense. This means that briquettes can only be readily marketed in the industrial sector in those countries where either coal or wood has an existing base.
The advantages which briquettes possess over the unprocessed residue in ease of handling and transport extend through to the combustion device. This means that most residues can be combusted more efficiently when briquetted even in those cases where the plant can actually handle unprocessed residue. This gain in efficiency may be enough on its own to justify briquetting though it is difficult to obtain accurate data in many circumstances.
The most common problems encountered in burning raw residues are the difficulty of actually feeding material into the plant and that, in the combustion zone, loose residues may blow around and not burn completely. Briquettes avoid both these problems.
There are no good data available on the loss of combustion efficiency in burning raw residue. In India, it was claimed that raw rice-husk showed a 20% drop in efficiency as against rice-husk briquettes, though this was not based on rigourous measurements.
The ease of feeding briquettes is usually an advantage over raw residues. However, in some cases, the raw material can be handled pneumatically (for example, rice husk and jute dust) which although expensive may be advantageous. In practice, the extent to which an industry is prepared to invest in equipment to enable raw residues to be handled and fed into the combustion plant may determine whedher or not briquetting has a role to play.
In Brazil, for example, a number of plants have installed the equipment necessary to combust baled bagasse, which is available in large quantities. There is therefore no incentive to briquette bagasse as it has an immediate outlet.
There is only limited room to generalise about the balance between converting the residue to a convenient form and converting the combustion equipment to burn residues directly. It is probable chat the bigger the plant the more likely chat plant conversion would be economic. However, the exact economics would be very sitespecific.
General experience suggests that briquettes are a good substitute for wood, possessing a consistent quality which can enable a price premium to be obtained over wood.
There are virtually no quantitative data on the combustion characteristics of briquettes in industrial plants whether boilers or various kinds of kilns. General experience suggests that they are a good substitute for wood, possessing a consistent quality which can enable a pricepremium to be obtained over wood. This is evident in Brazil where wood is often sold in variable qualities and quantities. It is also claimed that wood-based briquettes in Ghana are sold at higher prices than wood (World Bank 1987).
It is not dear whether such a premium extends to high-ash residues such as rice-husk. It might be expected that these would have more problems in substituting for wood. However, briquettes based on residues such as coffee-husk and groundnut shells appear to be virtually interchangeable with wood.
The substitution of briquettes for coal may be more problematic though the only source for comparisons at present is India where the usual residue, rice-husk, has an unusually high ash content In this case, briquettes can be burnt satisfactorily only in a limited range of coal appliances, for example step-furnaces. In other types, for example moving grates, the rice-husk briquettes can fall between gratebars before they are completely combusted.
It is also possible that in some coal appliances there could be problems with ash-slagging but no data are known to exist about this.
Combustion in household stoves
It appears that, in practice, briquettes are usually burnt in industry` However, much of the recent interest has been in using briquettes in households in countries where wood shortages and deforestation are problems. In this section, the suitability of briquettes in household stoves is discussed, though based on some very limited data.
Reports of laboratory work carried out in Europe tends to give a rather positive picture of the behaviour of briquettes in household stoves. Tests done at TNO in the Netherlands (Krist-Spit 1985) of six different briquette types in five stoves showed that the substitution of briquettes for woodfuel or charcoal would probably not be restricted by the combustion properties of the briquettes. Some differences between briquettes were observed, however. The Thai bucket stoves performed particularly well and showed thermal efficiencies between 33% and 46%.
They found that the combustion behaviour of briquettes is comparable to wood rather than charcoal The briquettes burn with somewhat higher flames and a little more smoke than charcoal However, the test report states that briquettes from rice-husks and mimosa, if the briquette diameter is small could be competitive to charcoal.
The TNO tests clearly showed that large diameter briquettes, especially when made from raw materials with high ash contents, such as rice-husk and water hyacinth, are unsuitable for domestic cooking purposes because the heat-rate is insufficient and they are difficult to ignite. This is not necessarily a problem of briquettes as such. Large diameter wood logs, comparable to the 8-10 cm diameter briquettes from large piston-presses, are seldom burnt uncut in household stoves.
In the experiments carried out at CRA in Gembloux (CRA 1987), briquettes were subjected to combustion tests in which the elongation during combustion was measured as well as the rate of weight loss. The times during which the combustion resulted in smoke, flames and glow were observed. The overall conclusions of this work can be summarized as follows: hard, dense briquettes swell very little or not at all during combustion, they have a slow rate of weight loss (i.e. they last a long while) and they burn without flames for a longer period as welt thus resembling the performance of charcoal.
For softer briquettes, the opposite is the case, i.e. they swell quickly and by doing so they start to crack up which increases the rate of mass loss and shortens the total combustion period. Such behaviour is particularly characteristic of hydraulic-piston briquettes.
Some of the materials, especially rubber wood, gave off a lot of smoke during the combustion tests which indicates that such briquettes are probably unsuitable for cooking stoves.
This data all comes from laboratory tests which are useful for basic analysis but may not cover all the factors which make a product acceptable in practice.
There are little good data about the potential acceptance of briquettes in household stoves in practice. Some limited market studies in Niger suggest that the laboratory-based results were sound and that briquettes are acceptable to domestic consumers. Recently, in the Sudan, several thousand tonnes of groundnut-shell briquettes, made in a large piston machine, were sold to domestic customers. They were reported as being quite happy with the combustion properties even though they often had to break up the briquettes.
Both the Niger and the Sudanese experience was that piston briquettes can be used in households though possibly in combination with wood fuel. This is corroborated by some limited experience from a plant located in Kigali, Ruanda.
In Thailand, there has been considerable experience of selling screw-briquettes to households made from both rice-husk and from wood residues. The rice-husk product seems problematic because of its ash content but can be burnt whilst wood-based briquettes are quite satisfactory. The problem of acceptability lay with price not quality.
There is also some experience in the use of charcoal-briquettes made by binding charred residue with molasses. The results are contradictory, something which may be related to the different cooking situations in which the briquettes were used.
In India, great problems were found in persuading households or small tea-shops to burn molasses-bound briquettes. There were complaints about smells and about the speed of burning when compared with charcoal or coal-briquettes.
In Sudan, on the other hand, some market research carried out on molasses-bound charcoal briquettes made from cotton stalks were said to be an acceptable charcoal substitute. The high ash content of the Indian briquettes may have been an inhibiting factor but this cannot account for the smell problem.
Gasification of briquettes
The gasification process places higher quality demands on the briquettes than does combustion. The fuel bed is thicker, adding to the weight load on the briquettes at the bottom while residence times are longer, during which the briquettes are subjected to humidity at elevated temperature. In a gasification plant mounted on a vehicle, the vibrations will add additional stresses on the fuel and increase the risk of blocking the gas flow.
There are a number of important potential advantages of using briquettes instead of for example chipped wood for gasification: the briquettes are drier, increasing the efficiency of the process and increasing the calorific value of the produced gas; the bulk density is higher, increasing the residence time in the gasifier and the gas conversion rate and, finally, the size of the briquettes can be chosen to fit together with the size of the gasifier and the gasifier grate.
In tests carried out by CRA, seven gasifiers, both mobile and stationary, were operated with briquettes made from different materials. The overall results were very good, though the high silica content in the rice-husk briquettes caused sintering and blockage of the gas flow.
In general it appears that briquettes could be used to provide a consistent feedstock to most gasification systems. However, very little data based on practical experiences are available.
Chapter 5.Economics of briquetting
The costs of briquetting depend upon a number of operating costs, including labour, maintenance, power, raw material cost and transport, and various other sundry charges, as well as a capital cost component.
Almost all the cost categories discussed above depend to a more or less significant degree either on accounting conventions (this is particularly relevant for the calculation of capital charges) or on-site or country specific factors. We have analysed capital charges using the assumption of a 10% interest charge on a 10-year loan, numbers which fit broadly the kind of loans made to plants which were visited.
Operating costs were analysed on the basis of actual plant experience, design studies and manufacturers' data. However, no allowance has been made for the cost of raw materials over and above transport charges. The country studies make it clear that where briquetting has become commercially viable to a degree there is a tendency for residues to acquire a market price where previously they were free.
The cost ranges derived for a large piston machine are:
|Raw material transport||1-4|
|Other||at least 1|
A simple addition of the least and greatest costs would suggest that a piston-machine briquetting plant would have total factory costs in the range 20-36 US$/t of product.
It would however be misleading to adopt costs in the lower part of this range except under the most favourable circumstances; these might be the use of dry wood-waste drawn from the immediate locality in a country where labour costs and power prices are low and where fairly low-cost machinery is available. A possible location meeting these criteria is Brazil; there a company planning to set up a number of large briquetting plants in the interior has suggested that total costs would be about 26 US$/t including some payment for wood-wastes. This must represent very much the bottom end of the cost range.
In other, less favourably situated countries, it is much more likely that total costs would be towards the upper part of the range. It should be emphasised that these do not include any allowance for residues being priced nor for any profit element.
It would be expected that the unit costs for screw presses would be somewhat higher than for piston machines. They do not appear to offer any significant advantages in investment costs and in some cost categories, notably maintenance and power, they are likely to be more expensive. They also appear to have higher unit labour costs though this is probably a factor relating to scale of production rather than any intrinsic feature of screw presses.
The higher intrinsic costs of the screw machines may however be offset by the fact that their small production levels, and indeed small physical size, means that they can be, literally, squeezed into low-cost situations. These would typically be a small wood-plant able to site a machine right by the waste pile in a building which needs little or no modification.
One user in Kenya who has put a small screw press into such a favourable situation (except that it utilises residue from a nearby sawmill) has assessed total in house costs at about 21 US$/t including depreciation and finance. This includes no allowance for raw material transport costs and may underestimate power and maintenance costs. If these are corrected then it is likely that true factory costs are more like 25 US$/t.
In general, therefore, it would be wise to assume that total costs for briquette production are in excess of 30 US$/t and may be above 35 US$/t if some allowance is made for the cost of raw materials. Only in the most favourable circumstances would costs drop to 25 US$/t.
These numbers are in accordance with the situations in both Brazil and India, the two developing countries where briquetting has managed to establish some kind of commercial basis. In Brazil, it seems possible to survive by selling briquettes somewhere above 30 US$/t whilst in India a marketed price in excess of 40 US$/t is required. In both cases, these prices produce bare commercial survival rather than large profits. In India, it is common to pay up to 15 US$/t for rice-husk; in Brazil, wood-wastes are usually cheaper if charged at all.
These broad cost levels refer only to plants based upon factory residues. The costs for any field-residue plant will be much higher.
The economic viability of briquetting plants depends crucially on whether or not these factory costs are comparable or less than the prices of the main competitive fuel, usually wood but sometimes coal.
It is clear that in many countries the price of fuelwood is well below these levels to an extent that effectively rules out briquettes as commercial propositions. This is often true even if it is assumed that industrial consumers are prepared to pay a premium for briquettes as they are of consistent and reliable quality. Such premiums depend very much on the reliability of the local wood supply. In Kenya, no premium appears to be obtained while in Brazil it may be as much as 40%.
There are exceptions to this. It is reported (World Bank 1986) that fuelwood prices in Addis Ababa reached 83 US$/t in 1985; even allowing for a retail markup this allows considerable scope for briquettes to undercut fuelwood. However, industrial fuelwood prices in, for example, Kenya do not exceed 20 US$/t, a level which briquettes cannot hope to reach. Similar low fuelwood prices in Thailand have effectively destroyed the local briquetting industry despite very low investment costs and good raw material availability.
It is probably true that in most countries, the current level of fuel prices is too low to justify briquetting. Nevertheless the examples of Brazil and India show that it is quite possible for fuel prices to rise to levels where briquetting is viable. Such a rise can, moreover, take place quite rapidly: the fuelwood price in Addis Ababa was reported to be only 9 US$/t in 1973. It is likely that the price of fuelwood in the Sudan has now also reached the point where briquetting is competitive.
The key policy issue in many countries with respect to briquetting must be the judgement as to the extent to which it is worth supporting briquetting activities, in spite of their immediate lack of profits, in expectation that fuel prices will rise in the future. There is no general answer possible to this. There are many countries where fuelwood prices seem set to remain relatively low for the foreseeable future. Indeed in some countries there is genuine optimism about the possibility of stabilising prices indefinitely by the development of fuelwood plantations.
However, there are also countries where it seems likely that deforestation must cause a rise in prices in the not too distant future. In such circumstances briquetting of agro-residues can have a legitimate economic role without any need for subsidy.
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