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6.6. Pyrolysis of Decontaminated Wood Decontaminated wood waste can be pyrolyzed to produce biocrude, an alternative energy source, and biochar [106]. Partial liquefaction is a novel system which may offer particle cleaning for recycling into panel products [107]. It is reported that this technology is of interest for handling wood wastes or logging residues [108]. Both pyrolysis oils and solid residues (charcoal or biochar) are typically generated. Solid wastes must be shredded and dried prior to pyrolysis, which is compatible with existing wood waste handling infrastructure. Recent trends appear to favour the use of wood as a co-feedstock when using pyrolysis to dispose of plastics and car tyres [109]. Biochar can be produced from treated wood. This be used for various purposes like improving soil conditions, where it stores the sequestered carbon, and for applications in adsorption, electromagnetic, and energy battery fields [55]. The characteristics and efficiency of biochar depend on factors such as the pyrolysis process, residence time, and reaction temperatures.
7. Challenges and Opportunities 7.1. Competition Effects
The predicted future global shortage of wood and biomass [21] means that much attention has been directed towards the materials circular economy and recycling. This has brought significant progress in reclamation and waste handling, sorting, and cleaning, as reviewed above. It has also supported growth in the use of recycled wood in many applications, but in particular, wood-based panels and biomass energy. It is reported that the wood panels sector is feeling the pressure from the widespread consumption of biomass for energy—including in the UK, where a very large number of installations compete for woody biomass and agri-residues alongside waste wood. It is reported that the proportion of clean (Category A) waste wood entering wood panels has diminished while Categories B and C has increased [110]. Competition for resource is likely to remain a challenge for the future. 7.2. Benefit of Carbon Storage In the context of the competition for biomass, it makes sense to remember that recycling wood contributes substantial benefits to greenhouse gas mitigation, through the storage of biobased carbon in products. The storage of carbon in harvested wood products (HWPs), and, most notably, in long life applications such as construction timber, is increasingly recognised. The European Council recently greenlighted a certification framework for carbon removals, including storage in products [111]. In the UK, the carbon storage potential of wood products is also receiving attention [112]. The potential to store carbon extends into the second, third, or multiple lifespans of the wood, if it can be recycled into new products that are, in turn, recyclable (for more examples, see Figure 2). With this in mind, it is interesting to consider the duration effect for the most common destinations of recycled wood. The Category A recycled timber is highly sought for the animal bedding, landscaping, and play surfaces sector. Yet, in this application, the residence time is relatively short. It would be similarly short for cat litter or other related products. The emerging option of structural products using secondary timber, such as CLT or glulam, would offer a lifespan of decades—depending on building type and design—if this becomes widely adopted. It is already prompting calls to revise our concept of wood cascading to acknowledge that cycling can occur even at the first stage in the sequence [65]. Wood–cement composites from recycled timber could also offer long lifespans, if a product were commercialised.
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