Process safety assessment of thermal technologies for biomass valorization by numerical descriptive approach

Process-based safety assessment of biomass thermal valorization technologies including hydrothermal gasification (HTG), pyrolysis, and conventional gasification has been evaluated based on the Numerical Descriptive Logistics Equation (NuD). Process-related data such as temperature, pressure, heat of reaction, and process inventory were collected from the literature for this purpose. The data has been analyzed by using the NuD equations to obtain the process safety total score (PSTS), and then hierarchy of control (HOC) has been applied to propose the strategies for risk mitigation. According to the NuD results, the HTG process has a lower PSTS (210.2) compared with pyrolysis (226.4) and gasification (228.5), and it reveals that HTG is the safest among these three technologies. HTG has the highest score for pressure (60.48) because it has the high pressure (20–25 MPa) process requirement, but the lowest scores for temperature and heat of reaction due to lower temperature operations (374–400 °C) compared with pyrolysis and gasification. The results determined by the NuD have been compared with subjective safety evaluation methods (Process Safety Index Analysis and Inherent Safety Index), and the results of inherent safety index (ISI) are also consistent with that determined by the NuD. Hierarchy of control has been proposed to control and reduce the process risk. As per results, engineering and administrative measures could be drafted for more effective process safety control.