Environmental assessment of a waste-to-energy practice: the pyrolysis of agro-industrial biomass residues

The bio-wastes pyrolysis is a waste to energy strategy that converts bio-wastes into valuable products (bio-char, bio-oil) with wide use in the agri-food sector. However, limited efforts are paid to the investigation of its environmental sustainability: in this context, the study contributes the need towards the assessment of a wide range of environmental impacts for the pyrolysis process of different types of bio-wastes under different operating conditions. The study estimates the potential environmental impacts related to bio-char production from the pyrolysis of several different agro-industrial residues and different temperatures and identifies the process “hot spots”. The analysis is carried out through the life cycle assessment methodology. The functional unit for the analysis is 1 MJ of thermal energy potentially released during the complete combustion of bio-char obtained from the pyrolysis process. The study highlights that, under the examined conditions, the type of biomass affects the environmental impacts of the pyrolysis process more than the peak pyrolysis temperature. Among the biomasses tested, bio-char obtained from orange peels has the lower environmental impacts, with an average percentage difference of about 16% compared to bio-char obtained from olive tree trimmings that has the worst environmental performance. For each biomass, the impacts associated to bio-char obtained with different operational temperatures have percentage differences in general lower than 5%. A contribution analysis shows that the electricity consumed during the operational phase is responsible for the largest impacts in all the examined impact categories, followed by bio-wastes transportation. In detail, the contribution of the electricity to the total impact ranges from minimum values of about 44% (for cumulative energy demand) up to 91% (for terrestrial eutrophication), while transportation contributions range from a minimum of about 4% (for terrestrial and marine eutrophication) to 36% for mineral, fossil and renewable resource depletion. Therefore, the use of more energy efficient processes and technologies and the diffusion of distributed pyrolysis systems near farms can significantly improve the environmental performance of the system examined.