Lo scarto dello scarto ha un valore? (Do by-products of by-products have commercial value?)

When dealing with production of biofuels it is very important to include a number of factors in the cost equation, such as the energy necessary for the production, transformation, transportation of the biomasses. Moreover the production process is known to produce by-products which may need to be disposed of. Possible re-use or valorization of these by-products may have a prominent impact on the economical viability of these processes. In the previous issue of Green we have analysed the production of biodiesel from biomasses. We have seen how the by-products are a solid residue and glycerin. We will deal with the valorization of glycerin in the next issue and we will focus now our attention on the solid residue. The optimal crops for the production of biodiesel are soy, sunflower and rape. After oil extraction the residues are rich in proteins, lignin and cellulose. These can be used directly as animal feeds, fertilizers, inert materials (for the construction industry) and combustion. The heat of combustion of these residues is comparable to that of wood and roughly 44% compared to that of oil, which makes them also a very good alternative for co-firing. However it is important to also consider what the products of the combustion are. Next to CO2, H2O and ashes there are harmful substances such as CO, nicotine, benzene and many more. Thin powders are also produced, which normally contains heavy metals and aromatic polycyclic hydrocarbons. Ashes represent around 30% of the burned residues and therefore their utilization is advisable. Being rich in Phosphorus and Potassium they are utilized as ferilisers if the content in heavy metals allows so. Otherwise they can be used as inert material in the production of concrete. The residues can also undergo a transformation process instead of being used directly. For the production of combustibles both thermochemical (pyrolysis) and biochemical (biogases via fermentation) conversions are common practices. Another useful transformation of these residues foresees the separation of their components thanks to acidic or enzymatic hydrolysis, which however turn out to be often rather costly. One innovative pre-treatment for the abatement of the costs is called “Steam Explosion”, which separates vegetable substrates in their three main components (emicellulose, cellulose and lignin). The use of the SE technology allows an easier dissolution and separation of the components (sugars in the acidic medium and lignin in the basic medium) which are in turn transformed in product with added value, such as resins, solvents, sweeteners, and so on.