(571o) Waste Valorization: High-Purity Syngas Generation from Co-Gasification of Waste Plastics and Biomass Via Chemical Looping Process

The co-gasification of plastics and biomass is a process that combines the two feedstocks to generate several benefits. When plastics and biomass are co-fed into the gasifier, the high H/C ratio of plastics helps counterbalance the higher oxygen concentration in biomass, resulting in increased hydrogen yield, reduced tar generation, and improved syngas quality. The use of biomass, which is renewable and carbon-neutral, complements the energy content of plastics in co-gasification, resulting in a higher calorific value of the feedstock mixture and greater energy yields per unit of input. This process supports sustainability goals by decreasing dependence on fossil fuels and mitigating greenhouse gas emissions.

One of the most promising technologies for co-gasification is the chemical looping process, a two-reactor system that uses iron-based oxygen carriers to generate high-purity syngas. The feedstock is introduced into the first reactor, a moving bed reducer, where the iron-based oxygen carriers move concurrently with the feedstock. Steam is added along with feedstock for char gasification, and the oxygen carriers donate their lattice oxygen to carbon from the feedstock during this process, forming carbon monoxide (CO) and hydrogen (H₂). The result is high-purity syngas, i.e., CO+H₂, which can be upgraded to valuable chemicals in downstream processing. The second reactor, a fluidized bed combustor, regenerates the oxygen carriers using air and recirculates them back to the reducer. This process eliminates the need for expensive units like tar reformers, air separation, and acid gas removers.

The gasification process is highly endothermic, requiring a lot of energy. Several approaches have been explored to counteract this and achieve an autothermal system, i.e., no external heat is required. One of these methods involves burning biomass in the combustor to supply heat to the process, while another involves co-feeding natural gas, waste plastics, and biomass into the system. This serves two purposes: creating an autothermal system due to the high heating value of natural gas and using natural gas as a hydrogen source to produce higher-quality syngas. In the proposed strategies, the syngas purity is higher than 90%, and H₂:CO is approximately 2, which is required for downstream processing of valuable chemicals such as methanol.

Compared to the existing indirect gasification of mixed plastic waste, the proposed strategies outperformed them. The indirect gasification process relies on oxygen from an air separation unit to burn natural gas, generating heat for plastic gasification while producing carbon emissions, either treated by a highly energy-intensive acid gas removal process or left untreated. Therefore, the proposed strategies that use co-gasification of plastics and biomass appear to be a more sustainable and cleaner alternative.

Finally, the feasibility of this process was investigated by conducting experiments on a 2.5 kW thermal bench scale unit and simulating it using Aspen Plus software. The coherence between the experimental and simulated data was verified, indicating that the process is feasible and can be scaled up for commercial applications.