Forests – Forest management and harvesting, both for protection and timber commercialization purposes, are the primary sources of solid biomass fuels, typically as a result of cleaning, pruning, and tree felling operations. The drawbacks and/or concerns associated with professional forest exploitation are: dispersion, difficult accessibility, variety of tree sizes and composition, competition in terms of using these sources for other purposes (wood-based panels, industry or paper mills), the presence of impurities (stones, sand, metals) and high moisture content. These factors have slowed down the widespread use of these products as solid biofuels.
Agriculture – Pruning olive trees, vineyards, and fruit trees are the main sources of solid biomass from agriculture. The main drawbacks to their use, in addition to its seasonality, are collection optimization and their transportation. Herbaceous agricultural residues are obtained from the harvest of some crops, such as cereals (straw) or corn (stover). Again, source availability depends on its seasonality and variation of agricultural production.
Forest and agricultural industries’ residues – Chips, bark or sawdust from primary and secondary industries processing wood, fruit stones, shells and other food industry residues (olive oil, pomace, canning, nuts…) form a significant share of many industrial solid biofuels. In these cases, their seasonality is due to variations in industrial activity.
It must be noted that certain residues or by-products should be treated carefully because they may contain other undesirable materials or substances, such as paint, adhesives, inorganic materials (nails, screws, etc.) which affect the quality and safety of the product obtained and the integrity of the process.
Energy crops – Energy crops are crops specifically formulated for energy use (typically, fast-growing species). There are many experiences related to field experimentation projects, the results of which require some more time to be conclusively assessed. Among the various agricultural herbaceous species likely to become energy crops are a thistle, sorghum and Ethiopian canola, and among tree species, poplars, eucalyptus, and paulownia, the latter two species having lower water demands than poplars.