Forestry and Wood Processing Wastes
In order to be able to evaluate the sustainability of present consumption and feasibility of introducing modern biomass fuels and bioproducts-based applications, an assessment of the resources and their availability is necessary. Among them different types of residues have to be considered: wood residues from logging and wood processing (saw-milling and manufacturing plywood, particle board, and pulp); wood residues generated by management of forestry (thinning of young stands and removal of dead and dying trees), and perennial crop plantation and replanting of tree; residues resulted in clearing forest lands for agricultural purposes, cutting or lopping trees purely for fuel wood, collecting deadwood, trees growing on agricultural land, communal lands, on waste lands, on private land such as gardens, trees growing along roads, etc. It will be mentioned only gross amounts that may be recommended as raw material for energy and bioproducts (7).
1.4.1.1.1 Logging Residues
Logging residues consist of branches, leaves, lops, tops, damaged, or unwanted stem wood. Such residues are often left in the forests for various reasons of which the low demand for fuel (with high moisture content) in some areas is probably an important one as well as logistics. This is not to suggest that forest-residues recovery is not undertaken. There are countries where there is considerable recovery for use in industries as well as domestic purposes (e.g., as wood chips with bulk density of about 300 kg/m3). Recovery rates vary considerably depending on local conditions. A 50/50 ratio is found in the literature, for example, for every cubic metre of log removed, a cubic metre of waste remains in the forest (including the less commercial species).
1.4.1.1.2 Saw-milling
Recovery rates vary with local practices as well as species. After receiving the logs, about 12% is waste in the form of bark. Slabs, edgings, and trimmings amount to about 34% while sawdust constituents another 12% of the log input. After kiln-drying the wood, further processing may take place resulting in another 8% waste (of log input) in the form of sawdust and trim end (2%) and planer shavings (6%). For calculation purposes a yield factor of 50% has been used (38% solid wood waste and 12% sawdust). Sawmill residues are used for various purposes but much depends on local conditions such as demand centres nearby. Part of the residues is used by sawmills themselves, basically for stem generation for timber drying purposes. However, the bulk remains unused. Where a local demand exists, wood residues are used for various purposes, mainly as a source of energy for brick and lime burning, other small industrial applications as well as a source of raw material such as for parquet making and blackboards, or composite materials. Sawdust sometimes is briquetted and carbonized and solid as a high-grade charcoal, which commands a higher price than normal charcoal. Considerable quantities are also used to cover charcoal mound kilns.
1.4.1.1.3 Plywood Production
Plywood making is a large-scale operation and involves the cutting of the logs to the length required and debarking the logs. After the preparatory operations, sizing, debarking, and cleaning, the logs are sliced, i.e., the logs are rotated in a machine. While rotating, a knife slices or peels off the veneer. Then the sliced veneer is cut into size required and it is dried after which it is ready for further processing. The dry veneer slices are sorted, with sheets having holes or other irregularities being rejected.
The sheets are glued and hot-pressed into plywood sheets. Finally, the plywood sheets are trimmed (cutting into standard sizes), sanded, and graded. Recovery rates vary from 45 to 50% with the main variable being the diameter and quality of the log. Of the log input, the main forms of waste are log ends and trims (7%), bark (5%), log cores (10%), green veneer waste (12%), dry veneer waste (8%), trimmings (4%), and rejected plywood (1%). These form the largest amount of waste while sanding the plywood sheets results in another loss of 5% in the form of sander dust. For calculation purposes a yield factor of 50% has been used, with 45% solid residues and 5% in the form of dust.
1.4.1.1.4 Particle Board Production
Particle board production basically involves size reduction of the wood, drying, screening, mixing with resins and additives, forming of the so-called mat, pressing, and finishing. All types of wood are used for the production of particle board such as solid wood, solid wood residues (off cuts, trimmings), low-grade waste such as hogged sawmill waste, sawdust, planet shavings, etc. During the production process about 17% residues are generated in the form of trimmings. However, this amount is recycled. In addition about 5% screening fines and about 5% sanding dust are generated as residues, which are mainly used as boiler fuel for process steam generation. For calculation purposes a residue factor of 10% has been taken, consisting of screening fines and dust while 17% of the residues are assumed to be recycled.
At present, these residues are used to produce energy for the industry of wood processing, or to supply energy for the needs of the surrounding community, but in function of their accessibility they could be taken into account to be used as a raw material to obtain bioproducts (8).
1.4.1.1.5 Pulp Industry
The same kinds of residues (bark, coarse residues, sawdust) are characteristic also for the pulp industry and they are used as fuel to obtain steam and energy. In the cases when we discuss the challenges to change the industry according to the principles of biorefining, it is possible to reconsider the use of residues as raw materials to recover chemical compounds or to manufacture other bioproducts. These include industrial chemicals, pharmaceuticals, textiles, renewable materials, personal care products, and other manufactured goods. Using biomass in these ways represents a potential to generate higher value returns than when using it primarily to produce energy. The waste product generated during wood pulping, called black liquor, is another example of industrial waste (2, 9).
