Bioshredders and chippers: economic management of biomass residues
The need to manage agricultural and forestry biomass and the maintenance of green areas lead to increasing greater attention focused on such machinery as bioshredders and chippers. In this field, manufacturers offer a wide range of technologies suitable for all requirements but those who operate these machines must gain a better understanding of the technical features involved for making the correct purchase and using them in the best way possible
The function of bioshredders and chippers is, on the one hand, to reduce the volume of trunks, branches, brush, twigs, leaves and grass as well as organic residues to facilitate transport and, on the other, enable the recovery of these residues which would otherwise be broken down and discarded in another form. Chips can be used as combustible fuel in suitable wood burning furnaces, as mulch or for the cover of paths. According to the composition of the materials used, bioshredders produce compost, pellets or, in the field of industry, for the manufacture of chipboard or are simply discarded.
Shredding and chipping
The work of the bioshredder is to reduce materials to fragments in shapes which are not uniform by driving the product against hammers, teeth or knives inserted in drums or mounted on discs with the aid of a hole screen. The materials treated are of a nature different from that of wood.
The chipper reduces materials which are more or less ligneous to chips in variable sizes and thicknesses, a few centimeters, resulting from sharp cuts by one or more knives set on one or more rotor discs. Independent of type, the essential components of the machine are the hopper for feeding and the shredding or chipping chamber to which must be added, for the medium and big models, the apparatus for feeding in the materials, a conveyer belt or rollers, and a chute for the discharge of the product. Power is provided by a gasoline or diesel internal combustion engine, an electric motor or the machine is driven by the tractor engine.
As stated above, the difference between a bioshredder and a chipper is the type of apparatus for breaking down the materials. In brief, this is performed by a disc system or a drum system. In the machine with a simple disc, one or more knives are mounted on this disc to do the cutting. During rotation at more than 2,000 rpm/min a rear fan platform provides a stream of air to transport the wood chips. There are versions with a disc carrying knives and counterknives and others with more than one knife-mounted disc.
The machine for breaking up materials with a drum consists of a rotor on a horizontal axis in which are mounted hinged steel flail hammers designed specifically for shredding or pulverizing the wood or the drum may carry knives or blades depending on the type of material to treat and the product to produce.
Some models mount two hoppers, one for feeding the shredder hammer device and another for chipping the biggest branches using a knife-mounted disc.
The size of the product is determined by the hole screen located at the exit of the shredding chamber. These screens have different size holes so if the intention is to produce a finely shredded product for making pellets, for example, a screen with small holes will be used.
Classification
The machines in table are classified according to transport system: manual, with or without wheels; drawn by a three-point tractor hitch; mounted on wheels or a wheel chassis and towed by a tractor or off-road vehicle. Excluded are big bioshredders or big chippers mounted on a trailer or multi-axle truck or equipped with track. The power these machines require exceeds 150 kW and the investment needed is more than 200,000 euro. For the same reason, left out of consideration are stationary versions of bioshredders used on worksites or in saw mills for breaking up wood or other scraps for demolition, similar industrial wastes or organice refuse. The machines requiring manual transport are generally used by private parties for breaking up materials produced by the maintenance of household gardens.
Motorization is provided by electric motors rated at 2000 to 3000 W. For the lower power models the maximum size of branches that can be handled is no more than 25 mmÆ whereas bigger models can take in diameters of up to 40 mmÆ with performance ranging from 1 to 2 m3/h and beyond, depending on the material fed in. Internal combustion gasoline or diesel engines mounted on manually transported machines reach power rating of up to 10 kW for treating branches of up to 60-80 mmÆ and work performance reaching 6 m3/h. Some higher capacity professional models include a turbine for discharding the product through a chute which can be oriented while creating suction which facilitates the feeding of fine materials. There are some machines which combine a collection system, usually a vacuum, for picking up leaves and twigs manually through the hopper located on the side.
Machines towed by the three-point hitch of a tractor are powered off a universal joint linked to the tractor PTO. According to the model, these machines require power ranging from 8-10 kW up to 50 kW and beyond. This means that higher capacity machines must be hitched to tractors with power rating of more than 80 kW. The machines mounted on wheels or a single axis tractor and towed by a tractor or an off-road vehicle can be powered by the tractor PTO but are usually equipped with their own power plants, electric motors or internal combustion engines, for power of 6-15 kW and up to more than 70 kW. Their work capacity ranges from 3-5 m3/h and up to 30 m3/h. Their performance depends on the diameter of the branches and trunks fed in which, in turn, is linked to the diameter or the disc or drum as well as the degree of shredding or chipping desired. The diameter of the knife disc extends from 300 to 1200 mm and the drum diameter varies from 500 to 1500 mm.
With conditions being equal, influencing the quality and uniformity of the product are the working components as well as the hole screen mounted at the exit of the fragmentation chamber where various screens with differing hole diameters can be installed on the same model.
Rollers or conveyer belts, usually operated by hydraulics, feed the hopper. The loading of some models is performed by a crane.
For increasing versatility, some bioshredders mount two hoppers, one for branches of 20/30 mmÆ fed to hammers and the other for bigger branches of up to 120 mmÆ sent to the knives on a lateral disc. The power requirement of the smaller models with capacities of 5-6 m3/h comes to 6-9 kW for an electric motor or 8-10 kW for an internal combustion engine whereas for the bigger machine with work capacities of up to 12 m3/h the power needed is 20-22 kW for electric motors and 26-28 kW for diesel engines.
Production chains based on combining a bioshredder and pellet mill are becoming more widespread. Unlike the bioshredder, to make chips suitable for pellet milling must be dried down to a 15% humidity content and be refined by bioshredder processing using screens with holes of 6.8 mmÆ. This production chain works well for home use. The way it works is by combining a biosharedder of a type described above with a work capacity of 5-6 m3/h and a pellet mill with a capacity of 40 a 80 kg/h for the production of 6 mmÆ pellets with a three-phase electric power requirement of 4 kW or tractor power of 15-20 kW.
Also increasing in popularity is a no stress system which involves an electronic device for the automatic regulation of feeding the material to avoid overloading. To reduce fuel consumption and CO² emissions machines are marketed with special sensors for regulating operations in relation to the work load. With a pause in feeding, the power regime is reduced to the minimum. For limiting noise emissions some machines powered by internal combustion engines mount soundproofing panels in polyester around the impellers and the fragmentation chamber. The product is discharged via a chute which can be oriented or on a conveyer belt for loading directly into a trailer.
Criteria for selection and safety measures
Though the information in table is schematic and incomplete, it provides an idea of the types of machines discussed above and now on the market. The offers are numerous and differentiated for hobbyists as well as professionals. The wide range of the prices on the market is an indicator of the importance of making the correct choice which responds to the real requirements of the end-user. This choice must be made according to specific criteria. Beginning with the quantities and qualities of the materials to be treated, the main factors to consider in making this choice are: the admissible diameter for feeding along with the length of the drum or diameter of the disc; the feeding system; the flexibility of operation if differing types and dimensions of materials are to be treated; the bulk of the machine and accessibility to the work site in relation to the area of operations; work capacity and quality also in relation to the equipment of the machine and its level of technology evolution; the type and toughness of materials and frequency of maintenance required.
Models for hobbyests are built mainly for materials derived from the maintenance of home gardens. The lower power machines are advisable for areas of less than 500 m2; the higher power models with trolleys and gasoline or diesel engines can be considered semi-professional, are suitable for medium to large tracts and can handle large diameter pruning residues with a good work capacity.
Professional machines, the worksite, work capacity and type along with the end-use of the product are factors for determining the choice of a machine. In any case, these are dangerous machines which require training for their correct use as well as suitable garments, footwear and hard hats.
