Co-gasification of coal, plastic waste, and wood in a fluidized bed reactor: Effect of operative conditions

Over the past decades, the global energy system has largely depended on fossil fuels to meet steadily increasing energy demand. This has had a negative impact on the environment, resulting in increased greenhouse gas emissions and a growing dependence on finite resources. A promising pathway is the development of an energy system based on the use of alternative feedstocks, enabling a reduction or shift away from high-emission fossil fuels and highly efficient and environmentally friendly energy technologies. In this contest, gasification, for producing an alternative gaseous fuel, is gaining increasing attention. Several thermochemical processes—such as hydrothermal carbonization/liquefaction, torrefaction, pyrolysis, gasification, and combustion—have been developed to convert alternative feedstocks into heat and power, energy carriers, and chemicals. Among these, gasification offers significant potential for recovering valuable products from a wide range of feedstocks, from clean fuel gas for energy applications to bulk chemicals. In particular, it enables the use of high-efficiency energy conversion devices, such as gas engines and gas turbines, thereby potentially achieving higher electrical efficiencies. Fluidized bed gasification is among the most promising technologies due to its high operational flexibility. The process can be carried out with different feedstocks, gasifying agents, reactor temperatures, and gas residence times; reagents can be introduced along the reactor freeboard, and operation is possible with or without catalytic bed materials.

In this study, the effects of the gasifying agent and bed material on the performance of the co-gasification of a mixture of coal, plastic waste, and wood were investigated. Experiments were conducted in a lab-scale bubbling fluidized bed reactor using air, oxygen-enriched air, an air/steam mixture, and an oxygen/carbon dioxide mixture as gasifying agents. Silica sand, olivine, and a mixture of olivine and dolomite were employed as bed materials. The results indicate that both the gasifying agent and the bed material strongly influence gas composition and, consequently, process performance. In particular, the use of oxygen-enriched air with silica sand yielded a producer gas with the highest heating value (9.32 MJ/Nm³), whereas the best performance in terms of gas yield (2.98 Nm³/kg) and tar reduction (−94.5%) was achieved using the air/steam mixture with olivine. Regarding tar composition, naphthalene compounds and polycyclic aromatic hydrocarbons (PAHs) were identified as the most abundant and recalcitrant species. In contrast, phenols and furans were found to be the most sensitive to the effects of the gasifying agent and bed material.