A preliminary study on torrefaction pretreatment of hemp stem for waste-toenergy valorization of woody biomass from flower hemp cultivation

Hemp cultivation gained an important role in recent years. Two major production chains can be defined, one dedicated to the cultivation of hemp for the production of fiber and wood and the other relating to the production of inflorescences for the market of derivative products containing cannabidiol, better known as CBD. The problem that will be highlighted and addressed in this article is related to the enhancement of cannabis production waste for CBD. In particular, since flowers are the only useful part of crops dedicated to this purpose, to date the stems of plants are considered a difficult waste to exploit. The alternatives are few and one of the simplest and most immediate is certainly the waste-to-energy process of this material; in fact it is not possible to obtain fiber and wood of good quality from the CBD production chain, because these crops are optimized for flower production. Waste-to-energy, on the other hand, requires mechanical pre-treatments in order to use the raw material within the typical thermal conversion systems (e.g. biomass boilers, pellet stoves, gasifiers). The great obstacle to being able to efficiently transform these wastes into fuel, lies in the very nature of the hemp stems as the fibers constituting the outermost part of the plant stems are well known for their good mechanical resistance. These properties however become a problem when the material has to be treated with standard machinery for pelleting and briquetting, as they hind the mechanical components suitable for the purpose giving rise to maintenance and breakdown issues. To solve the problem, the possibility of decreasing the mechanical properties of hemp fibers has to be investigated, so as to permit the transformation of this material into an economically sustainable fuel. It was therefore decided to pre-treat the stems of the plants with a torrefaction process, to sufficiently weaken the fibers to be mechanically treated. Shear resistance was tested with a “piston device” to obtain a qualitative estimate of the mechanical behaviour of the hemp fibers only; in fact it is assumed that to solve the problem, it is sufficient to concentrate on the weakening of the fiber instead of the entire stem of the plant. In conclusion, a first estimate is defined of the minimum energy necessary for the roasting of the plant stems, sufficient to weaken the fibers to make the mechanical transformation of the stems of hemp plants into fuel, possible and economically sustainable.