Part 2.Briquetting technology

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Chapter 6.An overview of the densification process
Chapter 7.Mechanical piston presses
Chapter 8.Hydraulic piston presses
Chapter 9.Screw presses
Chapter 10.Pellet presses
Chapter 11.Auxiliary equipment

Chapter 6.An overview of the densification process

Densification essentially involves two parts; the compaction under pressure of loose material to reduce its volume and to agglomerate the material so that the product remains in the compressed state. The resulting solid is called a briquette if, roughly, it has a diameter greater than 30 mm. Smaller sizes are normally termed pellets though the distinction is arbitrary. The process of producing pellets is also different from the typical briquetting processes; a more detailed description will be given later in this section.

If the material is compacted with low to moderate pressure (0.2-5 MPa), then the space between particles is reduced. Increasing the pressure will, at a certain stage particular to each material, collapse the cell walls of the cellulose constituent; thus approaching the physical, or dry mass, density of the material. The pressures required to achieve such high densities are typically 100 MPa plus. This process of compaction is entirely related to the pressure exerted on the material and its physical characteristics.

The reduction of material density is the reason for undertaking briquetting as it determines both the savings in transport and handling costs and any improvement in combustion efficiency over the original material. The ultimate density of a briquette will depend to some extent on a range of factors including, most importantly, the nature of the original material and the machine used and its operating condition as well as other minor factors. However, the ultimate apparent density of a briquette from nearly all materials is to a rough approximation constant; it will normally vary between 1 200-1 400 kg/m³ for high pressure processes. Lower densities can result from densification in presses using hydraulic pistons or during the start-up period of mechanical piston presses (which can last several minutes) whilst even higher densities are sometimes achieved in pellet presses. The ultimate limit is for most materials between 1 4501 500 kg/m³. The relation between compression pressure, briquetting process and the resulting density of the briquette is illustrated in figure 9 (Bossel, 1984).

The apparent density of briquette will be higher than its bulk or packing density as the briquettes will not pack perfectly. The usual reduction would be a factor of roughly 2 depending on the size and shape of the briquette; that is bulk densities of 600-700 kg/m³ are usual, sometimes a little less.

The bulk density of the original material may be difficult to define accurately, particularly in the case of materials like straw which are very easy to compress even manually. The lowest bulk densities are around 40 kg/m³ for loose straw and bagasse up to the highest levels of 250 kg/m³ for some wood residues. Thus gains in bulk densities of 2-10 times can be expected from densification. Since the material also will have to be dried in order to facilitate briquetting, the resulting increase in energy content per volume unit can be large compared to the raw materials.

A binding agent is necessary to prevent the compressed material from springing back and eventually returning to its original form. This agent can either be added to the process or, when compressing ligneous material, be part of the material itself in the form of lignin. Lignin, or sulphuric lignin, is a constituent in most agricultural residues. It can be defined as a thermo plastic polymer, which begins to soften at temperatures above 100°C and is flowing at higher temperatures. The softening of lignin and its subsequent cooling while the material is still under pressure, is the key factor in high pressure briquetting. It is a physico-chemical process related largely to the temperature reached in the briquetting process and the amount of lignin in the original material. The temperature in many machines is closely related to the pressure though in some, external heat is applied. There are thus two immediate ways of classifying briquetting processes:

Gains in bulk densities of 210 times can be expected when briquetting biomass.

1) High, intermediate or low pressure: this distinction is, in principle, dependent on the material used but the following rough classification may be adopted:

Figure 9: The Relation between Pressure and Density

Low pressure up to 5 MPa
Intermediate pressure 5-100 MPa
High pressure above 100 MPa

2) Whether or not an external binding agent must be added to agglomerate the compressed material. Usually high pressure processes will release sufficient lignin to agglomerate the briquette though this may not be true for all for all materials. Intermediate pressure machines may or may not require binders, depending upon the material whilst low-pressure machines invariably require binders.

A further classification is based upon the technology used to compress the biomass. This includes:

FIGURE A) Piston presses

In these, pressure is applied discontinuously by the action of a piston on material packed into a cylinder. They may have a mechanical coupling and fly wheel or utilise hydraulic action on the piston.

In these, pressure is applied continuously by passing the material through a screw with diminishing volume. There are cylindrical screws with or without external heating of the die and conical screws. Units with twin screws are also made.

In these, rollers run over a perforated surface and the material is pushed into a hole each time a roller pass over. The dies are either made out of rings or disks though other configurations are possible.

The above list is by no means comprehensive and there exist several other types of briquetting presses, especially in the low-pressure and low-capacity range.

Various types of roller-presses are also utilised to form briquettes, especially in making charcoal briquettes, from carbonized material. A binding agent must be employed in these and the process is more one of agglomeration than densification as there is only a limited reduction of volume.

FIGURE B) Screw extruders

FIGURE C) Pellet presses

FIGURE D) The roller-presses

Chapter 7.Mechanical piston presses

Main features

A reciprocating piston pushes the material into a tapered die where it is compacted and adheres against the material remaining in the die from the previous stroke. A controlled expansion and cooling of the continuous briquette is allowed in a section following the actual die. The briquette leaving this section is still relatively warm and fragile and needs a further length of cooling track before it can be broken into pieces of the desired length.

Figure 10: Piston Briquetter

In mechanical systems, the piston gets its reciprocating action by being mounted eccentrically on a crank-shaft with a flywheel. The shaft, piston rod and the guide for the rod are held in an oil-bath. The moving parts are mounted within a very sturdy frame capable of absorbing the very high forces acting during the compression stroke.

The most common drive of the flywheel is an electric motor geared down through a belt coupling. A direct-drive system using an internal-combustion or steam engine is possible and would not change the basic design of the briquetting machine.

Pressure build-up

The piston top is normally shaped with a protruding half-spherical section in order to get better adherence of the newly compressed material to that formed in the previous stroke. The most common type of briquette press features a cylindric piston and die with a diameter ranging from 40125 mm. The die tapers somewhat towards the middle and then increases again before the end. The exact form of the taper varies between machines and biomass feedstock and is a key factor in determining the functioning of the process and the resulting briquette quality.

The tapering of the dies can, in several designs, be adjusted during operation by means of narrowing a slot in the cylinder. This is achieved by either screw or hydraulic action.

One manufacturer (Krupp) uses a rectangular ram section. This allows for adjustment by narrowing the height or the width of the die.

The optimum tapering, and thus pressure, depends on the material to be compressed.

It can sometimes be enough to choose a nonadjustable die when the material is well defined and known to the operator. Changes in material composition, for example its moisture content, is a reason why it is useful to the operator to be able to adjust the compression continuously. However, in many operating plants in developing countries, machines with fixed dies are used. The main India and Brazilian manufactures produce only fixed die machines though most European suppliers have some die control. In adjustable systems, it is up to the individual machine operator to find the correct setting of the die as no automatic control systems has yet been introduced on the market. Developments are under way towards such automatic systems.

The use of proper taper for a given material feed is an important part of machine operation. It is clear that the attempt to do this on a trial and error basis rather than with the help of the manufacturer has often been a source of poor plant performance in developing countries. The closer liaison possible between user and manufacturer is a reason why domestically produced machines often show better performance than imported.

The pressure in the compression section is in the order of 110 to 140 MPa. This pressure, together with the frictional heat from the die walls, is in most cases enough to bring the material temperature up to levels where the lignin is becoming fluid and can act as a binder to produce a stable briquette. In fact, heat needs to be extracted from the process to prevent overheating. This is done by water-cooling the die.

The closer liason between user and manufacturer is a reason why domestically produced machines often show better performance than imported.

Machine capacity

The capacity of a piston press is defined by the volume of material that can be fed in front of the piston before each stroke and the number of strokes per unit of time. Capacity by weight is then dependent on the density of the material before compression. Thus although the nature of the original material does not markedly alter the physical characteristics of the briquette, it does have a major impact upon the practical output of a machine.

The feed mechanism is crucial and most manufacturers feature a proprietary design. By means of screws or other devices, they try to pre-compress the material in order to get as efficient filling as possible. This is particularly important when using materials whose bulk density is low and which need efficient feeding to achieve reasonable output. Substantial aerating of the "name plate" capacity may occur for agro-residues as most manufacturers rate their machines on a saw-dust feed, one of the denser raw materials.

The feed mechanism can, if badly mismatched with the feedstock, cause serious problems in machine operation. If undersized, voids may occur in front of the piston causing damage to the mechanism. The feeder itself may also jam if it is oversized and tries to move too much material into the piston space.

The need to have an efficient feed-mechanism suitable for a particular residue is the main reason why it is important to buy designs which have actual operating experience with that residue. Most machines can produce some briquettes of acceptable quality with virtually any residue. However the continuous production of briquettes at a reasonable capacity may not be so easy.

The design parameters of piston machines such as flywheel size and speed, crankshaft size and piston stroke length, are highly constrained by material and operating factors. In practice, the output of a machine is closely related its die diameter, as shown in Fig. 12 which contains data drawn from a large number of manufacturers specifications. Part of the variability of Fig. 12 is accounted for by the different bulk densities of the raw material assumed by manufacturers so the relationship for a standard material would be even closer.

Figure 12 shows that for estimating purposes an approximation of 18.5 kg/h/cm² can be assumed achievable during sustained operation of a mechanical piston press when the raw material is wood. With other raw materials of lower bulk density, the actual production capacity can be much less. One manufacturer offers the data shown in Table 3 for the variations between materials.

There is a shortage of data about the capacities of machines when used with different raw materials which reflects the limited experience which has been obtained outside of various wood-wastes. Table 3 shows an estimate, derived from manufacturers data, of how less dense materials have a lower capacity index relative to wood.

It is likely that consumer acceptance of briquettes is partly related to their size. A household user, for example, cooking on a open fire would be unlikely to accept a 10 cm diameter briquette any more than a 10 cm piece of wood. Briquettes can be split or broken but this may not be accepted by the consumer and, with soft briquettes, may lead to crumbling. Industrial customers may, on the other hand, be happy to accept large whole briquettes as these conform to their usual wood sizes. This means that in designing plants to receive certain residue volumes, some attention has to be paid to the intended market in deciding, for example, on the number of machines to be used.

Fig 12: Capacity versus Area of Mechanical Piston Presses

Table 3: Production Capacity Variation between Materials

Capital investments in machinery

Most of the models represented in this study are manufactured in high-cost countries such as Switzerland, West Germany, Sweden and Denmark and are equipped for customers in these countries. This means, in plain language, that they are relatively expensive.

A range of manufacturers estimates for single machines has been obtained, though for fewer examples, and this data, shown in Figure 13 allows for a rough assumption of 1 500 US$/cm² (in 1987 prices). This, together with the capacity estimate above, results in a cost/capacity figure for preliminary estimates of 85 US$/kg/h which is consistent with the figures by other researchers (Kbinsky 1986).

Naturally there are technical differences between the models which ought to reflect the variations in costs between them. However, the 85 US$/kg/h represents the average ex-works price for a European press when applied to wood waste, with appropriate adjustment for aerating using other materials.

Prices quoted by manufacturers in developing countries, in particular, are much lower. The Brazilian manufacturer, Biomax, has a published price-list with machines costing only 3038 US$/kg/h for sizes between 450 and 2 200 kg/in. The price reduction appears to be achieved by a combination of machine simplification and lower labour costs. The main Indian manufacturer, Ameteep, also offer machines at prices lower than European manufacturers though less spectacularly than Biomax.

Figure 13: Cost of Piston Briquetters versus Die Area

The cost reduction achieved by machine simplification are not easy to define but they seem to relate, essentially, to the construction of a unit which requires rather more operator attention than European machines which are often left unattended for several hours. The die size is usually fixed, which means that there is less flexibility in feed variations. However as most plants work with a single feed it is not clear how restrictive this is in practice.

As few plants in developing countries ever need an unattended operation, it is clear that there is likely to be considerable scope for manufacturers to produce cheaper machines for this particular market. However, at the moment the small size of this market and its fragmented nature means that there is very little incentive to do this.

The cost of a basic mechanical piston press can be assumed to 85 US$/kg/h for preliminary estimates.

Maintenance and spare parts

Being robust heavily-built machines, piston presses have long technical lives and they need limited daily service. The main wear parts are the die, piston-head and tapered cylinder. The service lives given by manufacturers for these pars are in the order of 500 to 1000 hours which may be true for clean, non-abrasive materials such as newly chipped wood. Most other materials are less friendly, resulting in shorter service lives of the wear parts.

The prices given by European manufacturers for individual spare parts are given in the Table 4.

This data is not consistent since the various manufacturers have different designs in which each individual piece has different service life. The die itself is subjected to most wear and has often to be exchanged twice as often as the other parts.

Average costs estimates for maintenance offered by the manufacturers also vary widely. One source gives the following figures for cost par annum as a percentage of original investment:

Table 4: Costs of Spare Parts for Piston Machines

Sawdust: 2%
Groundnut shells: 3%
Straw: 5%

Another manufacturer offers the estimate of 3.3 US$/ton product for wood-waste. When briquetting groundnut shells the wear part costs increases to 5.3. $/ton and with waste paper it can go up to 7.9 US$/ton.

Field data of maintenance costs in the case of piston presses have been reported by Overseas Development Natural Resources Institute (ODORI 19887). These confirm the general anticipation that, when operating with an abrasive raw material such as rice husks, the wear of the die piston becomes a severe problem. The service life is reduced to about 70 hours and even if reboring and build-up welding is possible, spare parts will have to be bought several times a year. This became problematic in one of the Indian projects, using a European manufactured press, due to the high prices of the spares and foreign exchange problems plus costs for shipping and import duties.

In the case of locally manufactured machines and spare parts. these problems are not as severe. The price for a 90 mm replacement die is reported to be 2 0003 000 rupees (160-240 US$) which is broadly the same prices reported by European manufacturers but can be paid in local currency and is easily available. One Indian plant has resorted to making its own replacement dies at a local engineering plant following problems with obtaining spare parts. This suggests that the technical problems are not large though the plant owner in this case is a qualified engineer.

An Indian project operating with a mixture of coffee husks and groundnut shells is reported to change die 3 times a year which is equivalent to a service life in the order of 1 300 hours, though intermediate reboring of the die takes place every 200 hours. The resulting maintenance costs (ODNRI 87) are 88 rupees per ton (7 US$/t) for rice husks and 31 R/ton (2.5 US$/t) for the mixture of coffee husks and groundnut shells.

Both manufactures data and operational experience suggest therefore that maintenance costs are likely to be in the range of 3-8 US$/tonne of product with the higher values referring to more abrasive residues such as rice-husks.

Energy costs

Energy costs in briquetting are made up of three separate effects: the losses in the machine moving parts (which are effectively negligible in this context), the frictional losses between the material particles when compressed and the frictional losses between the material and the walls of the press.

The frictional losses between the material particles are essentially constant for a given ultimate density and material and, given that most briquettes have much the same density, is probably roughly constant for all briquettes. Material differences will occur but should not be large.

The major variable element is undoubtedly friction between the briquette-material and the machine and here the larger the diameter the less will be the unit energy losses as the surface areas increase by a lower factor than the die volume.

The standard piston-press is equipped with an electric 3-phase motor which drives the flywheel via a V-belt (sometimes a flat belt). Motor capacity is designed with a safety margin for the promised output. Manufacturers claim that actual power consumption is 60-80% of the installed power of the main drive. To this figure should be added the power consumption of the feeder which for most models lies in a 10-20% range of the main drive motor .Thus for calculations one can assume that the main drive and feeder motor consume power equivalent to the rating of the main motor. This power rate is plotted against the die-area in Figure 14 for a standard woodresidue feed.

This shows a wider spread of data than in the case of capacity vs area (Fig 12) which is not surprising since the manufacturers obviously have the choice of installing as much power as they feel necessary to give an adequate safety margin. It is still possible to derive a curve-fit that could be used for preliminary studies. We suggest the following estimates which are valid for wood-waste"

P(kW) =4*(A)^2/3
where A is the diameter in cm²

This corresponds to:
P(kW) =0.58*(Q)^2/3
Q is capacity in kg/in

When the rate of production (or the diameter) is known it is possible, using this relation, to calculate the energy consumption per ton of output. For example, it takes 58 kWh to produce 1 ton in a 1 000 kg/in machine whilst at 500 kg/in output the same quantity demands 73 kWh. There is thus an overall saving in moving to higher throughputs and to larger briquettes. These formulas are not verified in laboratory tests and should not be used for designing briquetting plants. They can however serve in economic feasibility calculations and when checking if a given data is of the right order of magnitude.

Figure 14: Power Requirements of Mechanical Piston Presses

The variations in energy requirements between different materials can be quite large. One manufacturer gives the estimates presented in Table 5.

Differences in material moisture content can cause even higher variations in energy requirements than those between materials. The drier the material, the higher is the friction loss. This factor limits the lower end of the moisture content range acceptable in presses. With dry materials it can be necessary to condition the material with water or steam prior to the densification, though this is more often seen in pelleting operations.

A complete and fully automated plant will contain a number of electric-motors for disintegrating the raw material and product handling and transportation. The energy demand for these drives will have to be evaluated from case to case. In many overseas plants the handling will be done by hand although there may still be needs for material treatment such as chipping.

Raw material quality demands

The mechanical piston press was developed and has found most widespread use for the briquetting of dry woodwaste. A typical user is a European sawmill or joinery feeding shavings, planings, sawdust or bark to be briquetted either for internal use in solid-fuel boiler or for sale to nearby customers.

Because of the high pressure build-up, the piston press can only density dry material. If it is moist, the steam generated during the compression, will at best crack the surface of the briquette when the pressure is released after the cooling cylinder. At worst a sudden increase of the moisture content of the feed can cause a steam explosion within the cylinder which will expel the briquette violently and damage the machine. An accident in Turkey where the whole machine had to be replaced due to the briquetting of excessively wet material has been reported. (ODNRI 1987)

The moisture limit in most cases is 15% though some material with up to 20% can be densified in a piston press. The ideal operating region in respect of moisture content is 8-12%. With drier material the friction and thus energy demand increases and the lower limit is about 5%.

Table 5: Variations in Energy Needs for Different Materials

These moisture limitations means that before briquetting wet material, it has to be dried. The capital cost of a drier can often double the plant investment required as well as increasing the operating costs.

Another quality aspect is the size of the raw material. Ideally the material should contain both long and short fibres with the maximum particle size depending on material and diameter of the die. Larger machines can accommodate large particle sizes in the range of 8-10 mm with allowance of up to 15 mm for a die diameter of 125 mm.

As has already been discussed, different materials result in fairly large variations in capacity, energy consumption and maintenance costs. In terms of briquette quality however, most ligno-cellulose material within the above moisture and fraction limits can be briquetted with acceptable results in a mechanical piston press. It is the most versatile process available and its use is largely limited by the investment costs.

The main quality problem with piston briquettes is that the material is built up in the form of thin disks corresponding to the volume of residue compacted in each piston stroke. These disks form the natural line of cleavage across the briquette and, if the material does not adhere adequately, the briquette can break up into these smaller disks.

The mechanical piston press can only densify material only densify material with less than 15 % moisture consent.

This is not necessarily a disadvantage. Indeed it may be the best way for large diameter briquettes to be broken up into pieces suitable for household use. This is also the practice in industrial application in Sweden, where the broken briquettes are better suited for automated transportation equipment.

However, if the briquettes are too soft, such splitting can be the first stage of complete disintegration.

Materials known to have been used as raw material in commercial or demonstration projects with mechanical piston presses are listed in Table 6.

Table 6: Materials Used in Mechanical Piston Presses

The normal procedure when studying the detailed feasibility of briquetting project is to send samples of the product to potential vendors who will carry out test pressings. The outcome of these tests will tell both parties something of the product quality they can expect in full scale operation and give a lower limit of the capacity of the machine. As discussed above, material tests should cover not only briquette quality but the ability of the feed mechanism to cope with adequate throughput of residue.

Such testing will not say very much about service life of wear parts and the vendors will, for materials not very well known to them, be unlikely to give any guarantees with respect to maintenance costs.

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