Wood Waste Characterization and Reuse Possibilities

Background

The production of wood waste in the world exceeded 234.6 million m³ in 2018 (FAO, 2020). China is the greatest producer of this waste, with 44.3% of the total, followed by Brazil (8.2%) and the United States (6.1%). Different forest-based industries generate this waste due to different factors, such as i) economic: low-cost raw material obtained from reforestation and with defects such as knots, cracks, and growth stresses; ii) legal: laws, regulatory norms or certification that may require the maximum use of native species, as in the Brazilian Amazon; iii) social: needy communities close to sawmills can take advantage of wood waste to produce objects, often with high-added value, or in regions with furniture hubs where a huge number of micro and small companies optimize the use of forest resources; and iv) environmental: strict laws on the destination of liquid, solid and gaseous waste, as well as pressure to value rare native species. Different ways of classifying the timber industries are used, and basically, the wood processing can be divided into mechanical, thermal and chemical.

The main mechanical wood processing industries are sawmills, veneer sheets factories, and wood processing industries such as furniture, molding, and others aiming at high-added value products. The percentage of use of the raw material in these industries, in this case, the logs, boards or veneer sheets, vary according to the final product; in veneer sheet production or log yield, it varies between 35 and 50% (Santos et al., 2015, Silva et al., 2015). On the one hand, sawmills that work with wood from native forests, such as the Amazon, generally use logs above 35%, in many cases reaching 50% of their initial volume (Mendonza et al. 2019). Both the environmental pressure to improve the use of native logs and legal issues contribute to the good use of this wood. On the other, these same companies when using reforestation logs, such as those of the genus Pinus or Eucalyptus, generally use logs close to 30%. In small companies with low automation, these values ​​can be greater than 40% (Monteiro et al. 2017), but in companies with high automation, they can reach values ​​below 20% for products with high-added value. In these industries, large production often prevails at the expense of better use of raw material. In addition, the cost and quality of the logs of these species are inferior when compared to logs of native species.

The residues generated in the sawmills are produced in the different stages of the mechanical processing of wood. These residues have different dimensions and anatomical, chemical and physical composition. In large part, the heterogeneity of the raw material is due to the heterogeneity of the wood present in the tree, which varies between trees and within the same individual, both in the longitudinal and in the radial axes. There is a wide range of use of these residues depending on the location of the sawmill. Countries like Canada and Finland integrate companies from different segments, and sawmill waste is directed to industries such as cellulose, composites, and energy production. They are also commonly used in the sawmill itself, as an energy source for drying boards. In emerging countries, such as Brazil, its use as an energy source is widely adopted in the sawmills themselves or the sale of co-products to ceramic industries or boilers used in agribusiness, such as for drying grains. The energy potential of these residues is high (Monteiro et al., 2017) and there are companies with great potential to be self-sufficient in energy. This fact is ​​very important in countries with a petroleum-based energy matrix (Dowaki & Mori, 2005), mainly in those with policies to reduce the emission of greenhouse gases. The possibility of transforming lignocellulosic waste into new products or even its use as an energy source makes wood advantageous considering the environmental aspects, when compared with other substitute materials such as concrete, steel, aluminum, and plastic, mainly using the life cycle assessment of materials (Ferguson et al., 1996).

One example of thermal processing of wood is pyrolysis, the main technique used to produce charcoal. The raw materials for its production are planted forests and native forests, and the equipment employed to produce coal uses simple techniques such as hot tail ovens or rectangular ovens. They are simple ovens, with varied dimensions, and generally present a yield close to 30% in relation to the wood that starts the process (Fortaleza et al., 2019). Most of this wood is released in the form of gases in the atmosphere, impacting the emission of greenhouse gases. Some companies are using gas burners to reduce this emission and generate electricity (Pereira et al., 2016).