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The selection and sizing of a fan to move air through a dryer is very important. The major resistance to the flow of air comes from the grain bed; the pressure drop through the bed support and ducting is of lesser effect, particularly for deep beds. The pressure drop across a grain bed is a function of the depth, the air velocity and the grain itself. Data such as those in Figure 5.4 should be used to evaluate the pressure drop across the grain bed for a given application. It is important to note the major effect of dockage upon the pressure drop generated.
For most situations either axial-flow or centrifugal fans are used. The axial-flow fan moves air parallel to its axis and at right angles to the field of rotation of its blades. With the centrifugal fan the air enters parallel to the drive shaft, moves radially through the blades and is discharged tangentially from the housing surrounding the impeller. Axial-flow fans can be easily mounted in-line in the ducting and are relatively inexpensive but are only capable of operation against pressure drops of less than 1,500 Pa. Compared with axial-flow fans centrifugal fans can operate against higher pressure drops and are quieter in use but are more expensive.
Brooker et al. (1974) provide comprehensive information on the selection and operation of fans. It should be noted, particularly for large-scale dryers containing perhaps hundreds of tonnes of grain, that the risks of mechanical or electrical failure of the fan is likely to result in considerable losses if the fan cannot be repaired within a day or two. Consideration should be given therefore to installation of a back-up fan, particularly in locations where repair facilities are limited.
Heaters can be divided into two types, direct and indirect. In direct heaters the fuel is burnt in situ with the drying air so that the products of combustion pass through the drying bed with the air. Heaters of this type are less expensive and more energy efficient; however, the quality of the grain may be lowered due to contamination with combustion products, particularly if the heater is poorly maintained. In indirect heaters the combustion air does not come into contact with the drying air and a heat exchanger is used to raise the temperature of the latter. Depending on the type of heat exchanger as much as 25 % of the heat may be lost; however, there is no danger of contamination of the grain.
Air for drying can be heated by gas and oil and also solid fuels such as coal, wood and biomass residues. Oil-fired heaters are the most common for use with small on-farm dryers. Oil-fired and gas-fired heaters for all sizes of dryers are commercially available as described by Araullo et al. (1976), Brookeret et al. (1974) and Wimberly (1983). Small heaters are usually transportable and are easily positioned on the suction side of the fan so that the hot air from the heater is drawn into the plenum chamber by the fan together with ambient air.
Use of Biomass
Oil and gas are the conventional fuels employed in heated-air dryers, particularly so for small-scale operations such as the batch-in-bin dryer. The use of these fossil fuels is increasingly expensive and environmentally undesirable. The use of alternative and renewable energy sources is likely become increasingly common as new combustion technologies are developed and conventional fuels increase in cost. In many areas the residues available from grain crops, such as maize cobs and rice husks, are available in large quantities, but are generally under-utilized and present problems of disposal. Depending on the crop production systems employed other agricultural residues may be produced in the vicinity of grain drying plants and may offer alternative fuel options.
Few comprehensive measurements have been made of biomass residue availability. However, estimates have been made from the ratio of crop yield to the residue, data for which is shown in Table 5.6. The estimated world-wide production of agricultural residues (calculated from the crop to residue ratio) is given in Table 5.7. Much of this material has current or potential use for a wide range of applications, but in many places there are underutilized resources that could be used as fuel for grain drying. Table 5.8 provides details of calorific values of a selection of agricultural residues and wood.
There are many different combustion systems that are currently or potentially suitable for combustion of biomass residues. The broad classification of types of combustion systems and their status of development is outlined below (Page 126 et seq.).
Table 5.6. Conversion Ratios for the estimation of Crop Residues.
|Crop||Residue||Crop: Residue Ratio||Source|
|Palm Kernel||Shell||1: 0.35||6|
|Rice Paddy||Husk||1: 0.22||5|
|Soya beans||Straw||1: 1.1||5|
|Sugar Cane||Bagasse||1: 0.2||5|
Sources: 1: Hall & Overend (1987); 2: NRI (unpublished data); 3: Kristoferson & Bokalders (1986); 4: Watson & Ranstad (1987); 5: FAO (1982); 6: Cornelius (1983).
Table 5.7. Estimated Crop Residue Production of Developing Countries (1989).
|Crop||Production '000,000 tonnes||Residue||Production '000,000 tonnes|
Residue Production from Table 5.6. Source: FAO (1990)
Table 5.8. Alternative uses of Crop Residues.
|Material||Gross Calorific Value (daf*) MJ/kg|
|Cotton seed husks||19.4|
|Sugar cane bagasse||19.0|
* dry ash free. Sources: various
Grate Furnaces. The use of grates is probably the most commonly used method world-wide. There are grate systems suitable for burning a wide range of biomass materials, including many particulate residues and straw. The grate is designed to support the biomass fuel and allow air to circulate freely through it. There are many types of this system: flat grates, both static and moving; cone grates; step grates and sloping grates (Sarwar et al., 1992). Flat grates (Figure 5.17. Flat Grate Furnace.) are the simplest type and are found in the majority of log and straw burning systems. Step grates (Figure 5.18. Step Grate Furnace.) are often used to burn rice husks.
Suspension Burners. These are suitable to burn particulate agricultural residues of regular size and shape. The systems typically comprise a cylindrical chamber where the combustion air is introduced tangentially as illustrated in Figure 5.19. These systems have great potential for application in developing countries. A small number of commercial and piloted systems exist (Mahin 1991; Robinson 1991).
Figure 5.19. Sawdust fed Suspension Burner, showing connection between furnace and table feeder.
Fluidized Bed Systems. These systems are especially suited for burning both large and small particulate agricultural residues of relatively high moisture contents. The fluidized bed furnace comprises a combustion chamber containing a sand bed acting as the heat transfer medium. Commercial units are generally large-scale and capital intensive and as such are less suited for application in developing countries.
Under-fed Stokers. These systems are also suitable for particulate biomass residues of relatively high moisture content. The biomass is transported by a screw-feeder through a specially constructed trough into the middle of the furnace. From the sides of the trough primary air is forced through the biomass mound. Secondary air is introduced near the top of the mound allowing complete fuel combustion. There has been relatively little application of under-fed stokers in developing countries.
Gasification Systems. These systems can be designed to burn wood in the form of logs (Figure 5.20. Gasifier.) and also particulate fuels. The biomass is pyrolysed to produce combustible gases and wood tars. These products of pyrolysis are then used as a fuel. Since good combustion control can be obtained with gasifiers the hot gases of combustion can be employed to direct-fired dryers. Gasifications systems are typically at the experimental or adaptive research and development stage although there has been some limited commercial success with wood and charcoal gasifiers (Breag & Chittenden 1979; Hollingdale 1983; Sarwar et al. 1992).
There are handling and combustion advantages in compressing particulate materials into a more compact form, briquettes, for use in existing furnace systems (Smith et al. 1983). Various techniques can be used for converting residues to briquettes; the piston press, the screw press, the pellet press and the manual press. The experience of briquetting agricultural residues has been mixed. Various technical problems have been encountered but the main difficulty has been the fact that, in many places, briquettes are too high in cost to compete with existing woodfuel (Eriksson & Prior 1990).
Details of the commercial availability of equipment for combustion and handling of biomass materials can be found in publications by Eriksson & Prior (1990), Sarwar et al. (1992), and the Biomass Energy Directory (Anon. 1992). A great deal of information and consideration is needed to arrive at any reasonable conclusion on the suitability of a particular combustion system for use in grain drying systems in developing countries. A list of principal organizations involved in research activities on biomass residue combustion is given in Annex 2.
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