Miscanthus for local pellet production
Miscanthus is a biofuel that can be efficiently grown in marginal soils with a non-intensive cultivation technique. Partly due to the reduced collection humidity, the biomass obtained is particularly suitable for pellet production
From a multifunctional point of view of the farming activity, in addition to the production of foodstuffs for human and animal consumption, the generation of energy from renewable sources has long been considered an interesting opportunity. From one side, it is possible to increase the sources of income by diversifying risks, and from the other, it contributes to the energy transition underway, reducing the environmental impact of agricultural activities, among other things, often being able to benefit from public incentives specifically allocated.
More and more attention has been paid to electricity generation among energies from renewable sources, maybe because it has been more incentivized. On the other hand, less interest and investment have been dedicated to producing thermal energy or biofuels (wood chips, briquettes, pellets) that can be used in small-scale domestic devices. However, this situation does not appear entirely rational, also considering that the investments required for plants for electricity generation are higher than those for the generation of thermal energy and/or biofuels. The latter can also be cultivated in marginal areas, thus reducing the competition with more "noble" species, but at the same time allowing to make income from areas otherwise difficult to exploit. This is the case, for example, of herbaceous biomass crops, such as standard cane (Arundo Donax), Miscanthus (Miscanthus x Giganteus) and sorghum (Phalaris arundinacea).
It is a species of Asian origin with high photosynthetic efficiency. It can produce between 8 and 44 t/ha per year of dry matter biomass, with a range recorded in central and northern Italy between 16 and 28 t/ha per year dry matter. In addition to the interesting biomass production, miscanthus has the advantage of being easily harvested at very low humidity. It can therefore be easily exploited through combustion to produce direct heat or to be transformed into electricity. If burned, an interesting form of handling it is the pellet, either in purity or mixed with other essences, generally woody. Pellets have the advantage of being suitable to be used in small domestic devices while still ensuring high thermal efficiency. Moreover, thanks to its high volume mass, it can also be profitably used in the urban environment, where space for fuel storage is generally limited. However, whatever the biomass valorization chain is, in addition to the technical and economic feasibility, it is necessary to evaluate the actual environmental sustainability. The life cycle assessment results of miscanthus pellet production are described below, with reference to a company located in the province of Bergamo.
Life cycle assessment
Life Cycle Assessment (LCA) is used to estimate the potential environmental impacts of a product during its entire life cycle. The assessment includes the various ecological effects caused by the consumption of the multiple inputs used and any pollutant emissions into the environment (e.g., exhaust gases from endothermic engines, leaching and surface runoff of nutrients, etc.). Among the several impacts that can be calculated, the most common are carbon footprint, or Climate change, ozone depletion, different forms of toxicity, particulate matter formation and photochemical smog, acidification, terrestrial eutrophication, and consumption of mineral and fossil resources.
This paper reports the results regarding the carbon footprint, which is not only the best-known indicator of environmental sustainability but also because global warming is the environmental effect on which there is the most significant interest. The functional unit of the analysis, i.e., the unit to which all production factors, inputs, outputs, as well as the environmental impact are referred, is one ton of miscanthus biomass for the field phase and one ton of pellets for the pelletization phase. For the definition of the system boundaries, the "from cradle to gate" approach was chosen, which considers all operations, from the extraction of raw materials to the final production of pellets.
The production process
The Miscanthus cultivation involves a growing technique that, if compared to that of the typical arable crops of the Po Valley, is less intensive, both in terms of soil tillage and cultivation care. Since it is a poly annual crop (with an average duration of 15 years), soil tillage is carried out only in the year of planting. It includes subsoiling to a depth of 40 cm, followed by plowing to 35 cm and harrowing. As far as cultivation cares are concerned, the rusticity of miscanthus involves limited needs: it is necessary to intervene with only one irrigation during the year of planting, and usually, neither fertilization nor defense treatments are carried out. Moreover, it must be considered that when the product is harvested, most of the leaves have already fallen to the ground, thus reducing the removal of nutrients and that the fields dedicated to the cultivation of miscanthus in the case examined are located in riparian areas near the Serio river, occasionally involved in the periodic flooding of the river itself.
Harvesting is carried out annually, at the beginning of spring, after the leaves fall, and when the biomass's dry matter content is around 90%. For this purpose, a self-propelled mower-carrier is used, equipped with a standard harvesting header also used for maize chopping. At the end of the 15-year cultivation cycle and before the last chopping, herbicide treatment is carried out to inhibit the reproductive capacity of Miscanthus rhizomes.
All these operations can be carried out with equipment commonly present in the machinery of farms in the area under investigation or, in any case, in the usual availability of contractors. This limits the investments necessary for those farmers who intend to dedicate part of the cultivated area to miscanthus. Still, it also allows a more effective depreciation of the machinery, given that part of the operations (e.g., harvesting) take place at times of the season when activities on arable land are not foreseen.
After harvesting, the collected biomass is temporarily stored and then conveyed via a conveyor belt to a disc separator that removes the coarser foreign material, a magnetic screen for ferrous separation, and finally, a mill for grinding. The dust generated during grinding is intercepted by a cyclone dust collector and bag filter. The milled biomass is then conveyed to the pelletizing press, where a second bag filter also separates the dust generated. The pellets then pass over a bucket elevator with countercurrent air for cooling and are further sieved and finally packaged in big bags. As miscanthus has a moisture content of 10% at harvest, no additional drying is required before pelletizing.
The carbon footprint
For the field phase, the carbon footprint stands at 13.79 kg CO2 eq/t of Miscanthus. As expected, harvesting is the most impacting operation (66% of the total), followed by soil tillage and transport (11 and 8% of the overall impact, respectively). The input of other processes and/or production factors never exceeds 4% of the total impact.
In contrast, the pellet carbon footprint is 121.64 kg CO2 eq/t and depends mainly on electricity consumption (76%), while the field phase for miscanthus production is responsible for 13% of the carbon footprint. Packaging material, corn starch, and pelletizing plant wear play a secondary role.
Thanks to its hardiness, miscanthus cultivation can represent an interesting opportunity in areas with reduced fertility, where the production yields of classic arable crops such as cereals and soybeans are not satisfactory. Given the reduced level of inputs and a non-intensive cultivation technique, the environmental performance is also noteworthy, allowing low-impact pellets to be produced on a regional basis.