This document corresponds to the technical report “Requirement List for the TRL4 Flexby System” (Deliverable 2.3). It presents the study conducted by PMI, with the support of the consortium partners, to define the initial requirements for the FLEXBY system and establish the basis for the process's basic engineering. The report includes steady-state process simulations, heat and mass balances, and a comparison between different feedstocks. It serves as a foundation for lab-scale experimental tests and the subsequent pilot-scale implementation. To begin with, several meetings were held with each consortium partner to address the process's initial requirements and key challenges. The availability of feedstock was assessed in relation to the lab-scale experimental tests at CSIC and US, ensuring alignment with FRIMA's specifications for constructing the microwave pyrolysis reactor. Additionally, GD and IDE outlined their needs regarding the LCA assessment and process optimization, respectively. Following this, PMI conducted a comprehensive State-of-the-Art review of the technological solutions applied in the process, defining operating conditions and identifying key critical parameters. In parallel, a Block Flow Diagram (BFD) of the process was developed as an initial step before carrying out process simulations. Finally, two steady-state process simulations were performed using Feedstock 2 (microalgae-based wastewater residues, referred to as “algae residues” in the document) and Feedstock 6 (oily sludge). The microwave pyrolysis reactor was simulated in Aspen Plus, while the post-processing of the three fractions was modeled in Aspen HYSYS. The report also includes mass and energy balances, along with key parameter estimations, such as the process's energy efficiency.