Chapter 8.Hydraulic piston presses

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Main features

The principle of operation is basically the same as with the mechanical piston press. The difference is that the energy to the piston is transmitted from an electric motor via a high pressure hydraulic oil system. In this way, the machine can be made very compact and light, since the forces are balanced-out in the press-cylinder and not through the frame. The material is fed in front of the press cylinder by a feeding cylinder (a so called press-dog) which often pre-compacts the material with several strokes before the main cylinder is pressurized. The whole operation is controlled by a programme which can be altered depending on the input material and desired product quality. The speed of the press cylinder is much slower with hydraulic press action than with mechanical which results in markedly lower outputs.

The briquetting pressures are considerably lower with hydraulic presses than with mechanical systems. The reason is the limitations in pressure in the hydraulic system, which is normally limited to 30 MPa. The piston head can exert a higher pressure when it is of a smaller diameter than the hydraulic cylinder, but the gearing up of pressure in commercial applications is modest. The resulting product densities will normally be less than 1 000 kg/m³ and durability and shock resistance will naturally suffer compared to the mechanical press.

Capacity

Capacity data do not show the consistency of the data for the mechanical press. For a given diameter, a manufacturer can specify a range of output depending on the size of the installed motor. With a couple of exceptions, most manufacturers have models with rated capacities in the range of 40 to -135 kg/in, though units can be obtained up to 800 kg/in output. It is this ability to operate at low output levels which is the main attraction of hydraulic piston machines. Essentially they extend the mechanical piston press output down to the levels of the screw press.

Capital investment

It might be expected, on general grounds, that hydraulic units would be cheaper than mechanical presses, given the lower stresses and pressures to which the machines are subject. Comparisons are difficult as the size ranges are rather different but where they overlap it appears that in practice, prices are on an equal level. The machine costs reported by Kubinsky (Kubinsky 1986) also show specific prices in the 100 to 200 US$/kg/h range which makes them as expensive as the mechanical presses for the same sizes.

Raw material quality demands

The use of a lower briquetting pressure means that the hydraulic press can tolerate somewhat hither moisture contents compared to the mechanical press. Figures between 15 and 35 % are given by manufacturers. However, even if the moisture tolerance differs between the raw materials, it seem doubtful that it is possible to successfully produce durable briquettes with raw-material moisture much higher than 20 %.

It is also likely that even if higher moisture contents can tee tolerated in the press, organic material would have to be dried after briquetting if long storage times are planned.

General discussion

Only one project with a hydraulic piston press in developing countries has been identified operating in Kenya on a mixture of coffee husk and sawdust. The experience there is that operation has been riddled with trouble and that the machine required frequent service. Spare parts were originally imported but since this proved too expensive they are now manufactured locally.

These problems maybe a function of particular circumstance but the briquettes produced in the plant were certainly soft and of poorer quality than those from a screw press also installed. In general the product made in hydraulic presses have significantly lower densities than those made in mechanical presses, making it doubtful that they are suitable in developing country projects where the briquettes need to endure long transportation and storage times. They can possibly be considered when looking for a small briquetting machine which makes briquettes for in-house use, though the screw-presses are highly competitive in this size range.

Chapter 9.Screw presses

Overview

These machines operate by continuously forcing material into a die with a feeder screw. Pressure is built up along the screw rather than in a single zone as in the piston machines. Three types of screw presses are found on the market. They are: conical screw presses; cylindrical screw presses with heated dies and ditto without externally heated dies.

Only one manufacturer is currently marketing the conical screw press, Biomass Development Europe (BMD) in Belgium. Cylindrical screw presses are manufactured by 3 manufacturers included in our study while 9 companies have been identified which manufacture the heated die type of extrudes, originally of Japanese design.

ATS in Switzerland manufacture a twin type of screw extruder with a patented design allowing for the direct densification of moist materials. Drying takes place internally in the machine from the frictional heat developed in the process. A system of funnels allows the generated steam to escape from the material and the process can accept raw materials with moisture contents up to 35 %. The energy for the drying will have to be supplied through the mechanical power drive which means that the electric motors are oversized when compared to processes densifying dry material. No presses of this design have so far found utilization in developing countries. In our view, the higher energy costs for drying with electricity compared to fuel or solar drying, plus the difficulties envisioned in installing the large motor drives in weak electricity grids, will make such application unlikely.

Figure 16: screw press with heated die

Conical screw presses

Although this is originally an American design, the only current manufacture appears to be BMD in Europe. They manufacture one model with a claimed capacity of 600 to 1000 kg/in. It features a screw with a compression die-head of a patented design. It is reinforced with hard metal inlays to resist the very high wear experienced with this type of extrudes, especially when briquetting abrasive materials. The die is either a single hole matrix with a diameter of 95 mm or a multiple 28 mm matrix. The briquetting pressure is 60 to 100 MPa and the claimed density of the product is 1 200 -1 400 kg/m³.

The machine is equipped with a 74/100 kW 2-speed motor. Assuming a total of 100 kW for a production rate of 1000 kg/in one arrives at a specific energy consumption of 0.10 kW/kg/h. The manufacturer claims that the actual average energy demand is 0.055-0.075 kW/kg/h. A piston press with the same output demand 0.058kW/kg/h according to the formula derived above.

The price of the machine is US$ 130 000 which corresponds to 130 US$/kg/h using the higher capacity given by the manufacturer.

The mixing and mechanical working of the material in the conical screw press is undoubtedly beneficial to the quality of the product. Continuous operation also aids quality as the briquettes produced do not have the natural cleavage lines as is the case with piston briquettes. This is generally true for all types of screw presses.

The main disadvantage is the severe wear of the die head and die which results in high maintenance costs. The die head of the BMD unit was originally made in hardened steel which gave acceptable results when briquetting wood. Trials with groundnut shells showed alarmingly high wear which led to the redesign of the diehead using carbide inlays. The costs foor the snare parts are:

Figure 17: Conical Screw Press

The service life of the die head is said to be:

• With groundnut shells 100h
• With rice husks (estimated) 300h

Total costs for wear parts when briquetting groundnut shells are said to be 13.8 US$/t. This is two to three times the cost level given for piston presses briquetting the same material. The most serious part of that is that the die head, having to be imported from the manufacturer since the manufacture with carbide inlays requires a special workshop.

The experience with this model, apart from the apparently widespread use of its predecessor in USA, is restricted to two projects, one operating on groundnut shells in USA and another in France operating with wood as raw material. There are some ambitious plans to operate BMD machines in Brazil and Argentina in large multi-unit plants based on saw-mill residues.

Screw extruders without die heating

In this category we have included one European and one Asian manufacturer. Kusters Venlo-Holland of the Netherlands have developed a multiple hole matrix screw extruder for densifying chicken manure. It is a low-pressure process and the manufacturer gives a lower limit of the moisture content at 30 %. The manure is compressed to reduce its volume by half and the product is air dried after compression to be used as a boiler fuel.

The capacity of the machine is 300 kg/in (wet basis) and the price of the unit is US$47 300. The rated power of the installed motor is 22/30 kW. No data is available for maintenance costs.

A similar, though very much simplified machine, is the model developed by Prof Watna Stienswat of Kasetsart University in Thailand and manufactured by Sai Kampangsaen Motors. It is a screw extruder for wet materials (40-50%) with a capacity of 4 ton/day (wet basis) or 500 kg/in. It features a 5 kW motor driving a screw which is rotating with 800 rpm. The cost of the machine is US$730 and a replacement screw costs US$73.

The screw need new threads welded on after about 65 ton of produced briquettes at a cost of US$8. The manufacturer claims to have sold 40 units in two years time and the buyers are said to be farmers and hobby briquette producers. It is handfed and works best with slightly decayed bagasse. The product is very soft and must be placed by hand on the trays used for airdrying. The machine is of limited applicability but, in the right circumstances, may be a low-cost route to briquetting.

Screw extruders with heated dies

This type of extruder has found a number of applications and is manufactured not only in Japan, where it was originally developed, but also in Taiwan, Thailand, Austria and now also in Luxembourg, thanks to technical development work by CRA of Belgium.

The main features of this type of press is a screw which feeds the material from a feeding funnel, compacts it and presses it into a die of a square, hexagonal or octagonal cross-section. The briquettes have a characteristic hole through the centre from the central screw drive. The die is heated, most commonly by an electric resistance heater wired around the die. One press, Wacon of Taiwan, operates with both a 2kW preheater mounted on the chute below the feeding bin plus a 4 kW heated die.

The process can be controlled by altering the temperature. The normal operating temperature is in the order of 250 - 300 °C. The central hole of the briquette will act as a chimney for the steam generated due to the high temperatures in the process. An exhaust is normally mounted above the exit hole from the mould where the briquettes are cut into suitable lengths. A reduction in moisture content is thus achieved during the formation of the briquettes by a couple of percentage points.

The pressure is relatively high which, combined with the high temperatures, limits the moisture content of the raw material to be used. The actual maximum depends of the raw material but is in the order of 15 - 20 %. When briquetting a wet material such as saw-dust, the Japanese manufacturers include a suspension dryer in the system, which brings the moisture content down to 10 %. The resulting product will have a final moisture content of 7 %.

Most models produce a briquette with a diameter of 55 mm and an inner hole diameter of 15 to 25 mm. Variations of the outside diameters between 40 and 75 mm can be found. A common capacity given by the manufacturers of a 55 mm machine is 180 kg/h for wood material and 150 kg/in for rice husk. Variations exist due to differences in screw design and speed.

The energy demand is consistent with these variations in capacity, ranging from 10 kW for a 75 kg/in machine to 15 kW for a 150 kg/in, both based on rice-husk briquettes. To this should be added the 3 to 6 kW of electricity that is used for heating the mould. Assuming the total of 18 kW for a 150 kg/in machine, this results in a specific energy demand of 0.12 kWh/kg. A mechanical piston press would theoretically (such small machines are not made) have about the same energy demand. This level of power consumption is confirmed by the operational results of a plant in Ghana which operates four 150 kg/in screw-presses. Here, the reported power consumption is 0.11 kWh/kg (World Bank 1987).

Both Shimanda and PINI+KAY market machines with larger outputs in the range of 400-800 kg/in. They are equipped with 45 kW motors and PINI+KAY specifies an electric heater of 6 kW. Using an average figure of 600 kg/in and 50 KW the specific consumption is 0.083 kWh/kg for this size of screw press which can be compared with the corresponding figure of 0.069 kWh/kg for a mechanical piston press. The difference is not significant and is not verified in experiments.

Thus although one would expect screw presses to have higher power consumption than piston units, the practical differences do not appear to be large. Screw extruders produce good quality briquettes. The high maintenance costs are a drawback.

The capital cost of the screw presses varies widely with the country of origin. The specific prices range from 20-40 US$/kg/h for a Thai machine to 155 US$/kg/h for a Japanese screw extruder with similar features and output. Though there are large differences between manufacturers capabilities in fulfilling and following up an international contract for delivering such machines, especially when it involves setting up a whole fuel making plant, the figures show that it is possible to manufacture functional machines in a developing country, at least for the local market, at prices well below European or Japanese prices.

An estimate of the total cost for a 2 400 t/a rice husk briquetting plant in Thailand is of the order of 30 000 US$. This sum can be compared with one manufacturers estimate of the equipment cost for a 1 600 t/a briquetting plant delivered from Japan to Africa. Including a rotary drier, two briquetters, sieves, conveyors and spares for 3 years operation, the CIF price came to US$193 000.

The same large differences can be found when looking at maintenance costs, but going in the opposite direction. A Thai briquette factory has one welder for each of the three shifts constantly welding new threads on the screws. The screws need building-up and redressing every 100 hours whilst the dies will have to be rebored every 500 hours and exchanged every 1 000 hours. The Thai manufacturers own estimate of the maintenance cost is equivalent to 14 US$/t, which is the most expensive single item in the production. (Some element of depreciation cost may have been built into this aggregate figure).

On the other end of the scale, the Japanese manufacturer in the above mentioned example estimated that the spare parts necessary for 3 years operation comes to the equivalent of 1.5 US$/t. In the Kenyan project included in our case studies, the maintenance costs for a Taiwanese screw extruder operating on sawdust is given as the equivalent of 4.2 US$/t. This cost is regarded as being excessive by the operators of the plant and a reason for taking it out of service.

The statistical data is limited but it appears that the operation or screw extruders generally results in somewhat higher maintenance costs than with piston presses, when briquetting the same raw material. This means that economic operation in developing countries is, to a higher degree than for piston presses, dependent on local manufacturers of spare parts and availability of skilled service engineers.

The screw extruders produce good quality briquettes which can be handled, transported and stored without any major problems. One example of this is a Thai producer, who, with equipment manufactured by himself, partly from used car parts, makes solid briquettes from rice husks that can withstand a 650 km truck ride to the consumers.

Screw extruders were originally developed and used for briquetting sawdust. The field data verifies that this type of machine also work well when briquetting rice husks, apart from the high wear problems. The African case involved the briquetting of coffee husks in a screw extruder which resulted in reasonable quality of the product when the husk was mixed with sawdust. The following is a list of material which manufacturers claim their machine are not have been briquetting.

Based on claims by manufacturers on the number of machines produced (one reports over 600 units sold) and reported use in Asia, screw extruders probably outnumber piston presses if counting the existing units on the market. They can be manufactured at lower costs than piston presses and at a less advanced engineering level. The main drawback seems to be the high maintenance cost, especially when operating on abrasive materials.

Chapter 10.Pellet presses

Overview

Pellets are the result of a process which is closely related to the briquetting processes described above. The main difference is that the dies have smaller diameters (usually up to approx. 30 mm) and each machine has a number of dies arranged as holes bored in a thick steel disk or ring. The material is forced into the dies by means of rollers (normally two or three) moving over the surface on which the raw material is distributed.

The pressure is built up by the compression of this layer of material as the roller moves perpendicular to the centreline of the dies. Thus the main force applied results in shear stresses in the material which often is favourable to the final quality of the pellets. The velocity of compression is also markedly slower when compared to piston presses which means that air locked into the material is given ample time to escape and that the length of the die (i.e. the thickness of the disk or ring) can be made shorter while still allowing for sufficient retention time under pressure.

The pellets will still be hot when leaving the dies, where they are cut to lengths normally about one or two times the diameter. Successful operation demands that a rather elaborate cooling system is arranged after the densification process.

Main features

There are two main types of pellet presses: flat and ring types.

The flat die type have a circular perforated disk on which two or more rollers rotate with speeds of about 2-3 m/s which means that each individual hole is overrun by a roller several times per second. The disks have diameters ranging from about 300 mm up to 1 500 mm. The rollers have corresponding widths of 75-200 mm resulting in track surfaces (the active area under the rolls) of about 500 to 7 500 cm².

The ring die press features a rotating perforated ring on which rollers (normally two or three) press on to the inner perimeter. Inner diameters of the rings vary from about 250 mm up to 1 000 mm with track surfaces from 500 to 6 000 cm².

Figure 15: Flat Die Type Pellet Press

Machine capacity and energy demand

The capacity of pellet presses is not restricted by the density of the raw material to the same degree as for piston presses. For material with very low density the flat die press performs better since the opening between rolls is larger for the same active surface area. What control the output of the machine is the power from the drive motor which means that there are large differences in machine capacity when materials with different frictional characteristics are densified.

Normally capacities given are valid for densification of animal feed which is much less energy consuming than wood waste, straw and similar raw materials used for fuel production. Often the capacity with the latter materials is a factor of 4 lower than for animal feed production. With this in mind, the output range for ring or flat die presses on the market is likely to be from about 200 kg/in up to 8 ton/in.

In pellet presses, the energy consumption is expressed as drive motor power per active surface area. Normally this surface load is in the 0.03 - 0.07 kW/cm² range.

Power consumption is by manufacturers said to fall within the range of 15 - 40 kWh/ton. We have no field data to verify these figures. One can assume that animal feed production results in power consumption at the lower end of the range while wood waste for example would demand 40 kWh/ton or more. Other waste materials, such as straw or husks, are likely to be even more energy consuming, in line with what is said about piston and screw briquetters. A theoretical comparison with piston presses is difficult since there are two factors, the smaller size of the individual holes and the lower pressing velocity, which counteract.

A common feature in pellet plants is to condition the raw material, often with steam, which decreases the energy consumption. The moisture content of the raw material corresponding to a need for conditioning is not known but is believed to be quite low, or about 5-7- %. Conditioning is also carried out to improve the digestibility of pellets used for fodder. Binders are added which increases the strength of the pellets (and thus chewing time) as well as making the raw material suitable as animal feed.

Capital and operational costs

The rather limited data for this type of press suggests capital costs per unit in the 20-40 US$/kg/h range. This would be for unit sizes from 6 down to 1 t/h when densifying wood waste. Wear of the dies is probably of the same order as for piston briquetters, i.e. a die lasts less than 1 000 hours. Manufacturers data indicate a maintenance cost of 5 US$/t valid for wood waste.

Use of pellet machines

The main application for pellet machines is to produce animal feed from various types of agricultural wastes. Only a very limited number of plants have been set up to produce fuel pellets. In such cases, the motive for choosing a pellet machine has been the large capacity needed and, to some extent, the better handling characteristics of pellets in automated transport systems. To our knowledge, all fuel pellets projects in industrial countries use woodwaste as the main feed.

Some limited experience has been gained with pellet machines for the production of fuel in Africa. It is doubtful that there would be many applications in developing countries given the large output of such a plant. However in cases where an output in excess of 1-2 ton/in is required and where a good market can be identified, pellet presses could be considered

No data is available on the combustion characteristics of pellets. However, given the differences between wood and such small pellets, an industrial market would seem most likely target. One plant in Kenya has been set up to provide fuel for a large wood-burning boiler.

Chapter 11.Auxiliary equipment

Only in a few cases will the briquetting press be the only equipment needed to set up a briquetting plant. Examples of such basic installations are the rice-husk screw briquetters in Thailand where raw material is fed by hand into the machine and collected by hand after pressing.

From this starting point, the complexity of briquetting plants increases up to the fully automated woodwaste briquetting plants found in Europe in which the raw material is fed by a tractor into a hopper from where it is crushed, screened, stored, dryed, stored again, fed into the presses and transported to the product storage in a fully automatic process, supervised by a couple of operators.

It would be far too complex to go into details about the features of each of the equipment types that can be found in densification plants. The following text is a brief description of a few common types and their applications.

Storage

Storage of raw material in developing country projects is likely to be in the form of open storage piles. This demands only a large enough open area adjacent to the process plant. For materials which will have to dried before briquetting there is normally no need to cover the pile in the rainy season. When processing dry material, some kind of coverage is necessary in order to enable operation in periods of heavy raining. For smaller operations, tarpaulins and sheds will offer some storage but the cost of adequate protection in buildings and silos, is likely to be prohibitive except for very large plants.

If a drier is included in the process, a storage bin must be installed for intermediate storage of dried material before briquetting. There are numerous different designs of storage bins with mechanical reclamation of the stored material. They are likely to be quite site specific, taking into account the material flow patterns and the physical characteristics of the stored material.

The briquetted end product will always have to be stored under cover as the briquettes will crumble and disintegrate if rained upon. Storage buildings are quite common but sheds and tarpaulins can offer enough cover, if the briquettes are hard-surfaced.

Handling

Handling normally makes up the largest group of equipment in large, automated plants. The understanding of the best application for each type of conveyor will ensure a cost effective and trouble free conveying system.

Belt conveyors are the most common for transporting the raw material between unit processes. Chain conveyors are normally used for severe operating conditions, or unregulated loading situations. Vibrating conveyors are normally used for feeding hammer mills and chippers, but can also be used for metering and screening the raw material

Bucket elevators are restricted to conveying material with limited particle sizes and are normally not used unless the short distance pre vents the use of inclining elevators to overcome large height differences.

Pneumatic conveyors can be utilised for conveying dry fine material such as bagasse and rice husks The cost of operating such systems is often prohibitive in developing countries, at least in fuel briquetting projects.

The briquetted product can be handled by several of the above unit processes. One simple solution, possible for the hardling of briquettes feaving a mechanical piston press, is to avoid breaking the briquettes and instead letting the machine push the material all the way to the product storage. Turns, even U-turns, are possible since the material is still warm and rather soft.

Comminution

The need for comminution is individual to each raw material Many residues can be briquetted without any preparation at all but for others can the necessary size reduction tee quite costly and sometimes prohibitive. For woody materials such as cotton stalks, a chipper can be used for the fist size reduction. It is necessary to prevent dirt and stones from entering the chipper together with the material or the knifes will get dull quickly

The most common size reduction equipment in briquetting plants are hammer mills. They crush the material into coarse or fine fractions, depending on the type of mill.

Classification

Unless the raw material is guarantied to be clean and contain no oversize particles, there must be a screening operation built into the process In this, oversize material is removed and normally sent back into the hammer mill. Mechanical piston presses and pellet presses are especially sensitive to large particles entering the press. A screen and a metal remover is therefore recommended as minimum auxiliary in such plants.

The different types of screens: disc screens, shaker rolls, drum screens etc have different application depending on the type of material and the quality demand on the product.

Drying

Mechanical dewatering is normally possible only when the moisture content is very high and typically over 50%. Presses are normally very expensive to install and to operate and are unlikely to be used in developing country fuel briquetting projects.

Thermal drying is common and is achieved by bunting some of the product in a hot air furnace. The drying takes place either in a rotary drum or in a cyclone Dryers are normally the most expensive type of auxiliary equipment, both to install and to operate.

The following two cases are included to give an impression of the type of equipment that could be found in operating plants, and their related costs.

Table 7: Capital Costs; Case A

Table 8: Capital Costs; Case B

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