Oxy-Steam Fluidized Bed Gasification of Southern Pine Biomass, Lignite Coal, Plastic Wastes, and their Blends for Hydrogen Production

Abstract

Rising energy demand, climate change, and waste management issues drive the need to convert waste into energy via thermochemical conversion processes such as gasification. This study explores the potential of producing syngas from the oxy-steam fluidized bed gasification of various feedstocks, including southern pine biomass, municipal plastic wastes, lignite coal, and their blends. Twelve distinct blends were analyzed, focusing on combustion reaction kinetics, stability, and comprehensive combustion indices. It was found that increasing the biomass content in the blends improves both combustion stability and performance. The optimal blend, consisting of 60% biomass, 10% coal, and 30% municipal waste plastic, exhibited the lowest activation energy of 110 kJ/mol. Following the characterization and optimization of these blends, oxy-steam fluidized bed gasification experiments were conducted for various feedstocks. Notably, gasification of municipal plastic waste at 950°C, with a steam-to-carbon ratio of 3 and an equivalence ratio of 0.2, yielded syngas with the highest hydrogen concentration of 52.54 vol.%, followed by coal at 49.10 vol.%. The syngas compositions from these experiments were used to develop gasification kinetics models for biomass, coal, and plastics by implementing inversion problem of chemical kinetics with genetic algorithm as an optimization tool. The developed genetic algorithm-based gasification kinetic model outperformed conventional models, achieving low average absolute errors in predicting syngas compositions. Additionally, the developed kinetic models for biomass, coal, and plastics were integrated into a 2D Eulerian-Eulerian computational fluid dynamics (CFD) model using ANSYS Fluent, which reliably predicted syngas compositions. Moreover, low-temperature oxy-steam fluidized bed co-gasification was conducted at temperatures ranging from 715 to 745 °C on 50/50 blends of biomass and various plastic wastes including polyethylene terephthalate, high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. The syngas compositions obtained from these gasification experiments were used to develop kinetic models for each type of plastic waste. These kinetic models were then integrated into a 2D Eulerian-Eulerian CFD model using ANSYS Fluent. The model accurately predicted the syngas compositions from the gasification of different plastics, achieving average root mean squared errors of 2.57%, 3.12%, 3.76%, 4.54%, and 4.91% for C₂-C₃, CH₄, CO, H₂, and CO₂ gases, respectively.