(313a) Low-Temperature Catalytic Gasification of Particulate Waste Materials

Americans generate more than 250 million tons of waste each year. Most of the items discarded go un-recycled, are incinerated, or end up in landfills. With landfill reaching capacity and energetic and environmental impacts of incinerators, waste management and resource utilization have attracted the attention of engineers and scientists worldwide. One such alternative is waste gasification, a process that breaks down carbon-based materials, into synthetic gas.

Common long-chain polymers found in municipal waste, wastewater treatment plants, as well as space exploration waste materials are polyethylene, cellulose, and nylon, among others. The catalytic gasification process is studied in a low-to-mid temperature/high pressure. One of the advantages of this process, typically referred to as Wet-Thermal Catalytic Oxidation (WTCO), is the low temperature and energy footprint requirements as compared to more classical routes such as incineration. Proof-of-concept experiments are reported. These results demonstrate the potential of this technology as a sustainable approach for waste management.

This project is focused on the catalytic gasification of cellulose as it has proven to present different challenges. Indeed, unlike that for low melting temperature substrates, cellulose remains in solid phase. This creates a heterogeneous, a particulate or slurry system, reacting environment where interaction must be promoted between catalyst, substrate, and oxygen. This is not an easy feat, and it requires first a characterization of the fluid dynamics of the particulate system. The fluid dynamics of the WTCO reacting environment in a high pressure stirred reactor presents a formidable problem to assess for design and optimization purposes. A multiphase flow exhibiting stratification of particulates in a reacting environment presents several technical challenges. In this paper, we highlight how experiments and computational fluid dynamics (CFD) can be used as a powerful tool for characterization and process optimization.

An imperative stage in ensuring a successful scale-up and design for this project required characterization of the mixing patterns, particle dynamics, and transport phenomena associated with the degradation (gasification) of cellulosic substrates. Several diagnostic challenges presented by the multiphase flow of the gasification reaction were analyzed. In this paper, the modelling of the gasification of cellulose particles in a pressure stirred reactor using a finite element Multiphysics simulation environment (COMSOL) is used for CFD characterization.

A summary of complementary studies on the kinetics of the Sabatier reaction along with models for the water-gas shift reactions are used to describe catalytic kinetic in the gas phase. A phenomenological model integrating the CFD characterization with transport phenomena, thermodynamics, and gas-phase kinetics is demonstrated. The results clearly demonstrate the potential of catalytic gasification as a sustainable waste management alternative.