Modeling of energy conversion of biomass: multi-criteria methodology to optimize the design of integrated conversion routes

The energy transition encompasses various subjects, including the efficient use of energy, the adoption of new energy sources, and the production of novel energy carriers. The valorization of biomass for energy production represents a compelling and influential area within this field. The scientific literature on this topic includes experimental and modeling studies, with a particular interest focused on designing and optimizing integrated processes using numerical models to optimize their performance. This doctoral thesis introduces a novel methodology for an early-stage overall process design optimization that considers energy and environmental targets. Unlike traditional approaches where optimization occurs as a separate, ex-post activity, this method integrates optimization directly within the design phase. The methodology is developed based on the Multi-Criteria Decision-Making (MCDM) principles, employing the Technique for Order Preference by Similarity to the Ideal Solution (TOPSIS) as a ranking tool. The methodology is demonstrated through a case study focusing on the conversion of lignocellulosic biomass into energy carriers, specifically hydrogen and drop-in fuels for passenger cars. In this test case, four different design parameters were optimized: the pyrolysis temperature (TPyro), the Steam-to-Char ratio (StoC) in the SEG reactor, the process layout, and the processing capacity of elaborate the input feedstock of the plant. the other parameters of the plants are assumed to be constants. Four process layouts are evaluated, including the Fischer-Tropsch (FT) synthesis and considering the integration of syngas thermal energy: a process without FT synthesis and without syngas thermal integration (NOFT_NOTh), a process without FT synthesis but with syngas thermal integration (NOFT_InTh), a process with FT synthesis but without syngas thermal integration (FT_NOTh), and a process with FT synthesis and with syngas thermal integration (FT_InTh). The analysis is conducted across three plant capacities (1, 15, and 100 tons/hour of biomass input). five different TPyro have been considered, from 300 to 400°C by step of 25°C, so they are: 300, 325, 350, 375, 400 °C. regarding the StoC nine different values have been taken into consideration, from 0.2 to 1.8 by step of 0.2 so they are: 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6 and, 1.8. The description of biomass conversion processes is based on a sequence of chemical transformations occurring under kinetic and equilibrium conditions, simulated using Aspen Plus. To evaluate energy and environmental performance, a Life Cycle Assessment (LCA) is applied using both Well-to-Tank (WtoT) and Well-to-Wheel (WtoW) system boundaries. The key steps of the methodology are six and include the problem definition, the criteria evaluation, the correlation between the operative condition and the evaluating criteria, the criteria weighting, the data analysis and the ranking of the alternatives, and the choice of the best design alternative. The study highlights how the innovative methodology is pragmatic, versatile, and reliable for the resolution of multi-objective optimization problems. Nevertheless, it requires a deep understanding of process variables and their impacts on performance. Defining goals and boundaries is crucial, as boundary choices directly influence system performance and analysis outcomes. The methodology accounts for a wide range of design variable variability, avoiding reliance on pre-determined configurations and enabling a more comprehensive exploration of design possibilities. This integrated approach not only enhances the efficiency of the design process but also provides a practical framework for addressing the complex challenges of sustainable energy production. This is achieved through the capacity to analyze complex, multivariable, and multi-objective problems, providing a simple yet reliable solution to the issue.