INTEGRATED TORREFACTION-EXTRUSION SYSTEM FOR CONVERSION OF FIBER-PLASTIC WASTES INTO SOLID FUELS

Abstract

Waste generation is increasing, and a significant portion is being landfilled. In parallel, we are constantly pursuing cleaner fuels due to environmental and regulatory factors. To address both these challenges, torrefaction of wastes to produce clean fuels and feedstock for other thermochemical processes is one of the potential solutions. This work focuses on (a) fundamental understanding of the properties of un-torrefied and torrefied wastes and (b) development of a pilot-scale integrated torrefaction-extrusion system for converting fiber-plastic wastes to solid fuels. In this study, a 60:40 fiber-plastic waste blend was used for performing extensive experiments on densified and un-densified wastes to understand the effect of torrefaction. Wastes were torrefied at 300°C and mass loss between 0% and 51%. The product was characterized for moisture content, particle size distribution, energy content, grinding behavior, and chlorine. This was followed by a study to understand the effect of extrusion. The torrefied feedstock was extruded into rods, and products were characterized using thermomechanical, rheological, flexural, water absorption, size distribution, heat content, and combustion tests. It was shown that plastic in the feedstock acts as an enabler and improves properties like binding, water resistance, heat content, and increased degradation rate. Overall benefits of both torrefaction and extrusion in the production of clean and high-calorific value fuel were observed. An integrated pilot-scale torrefaction-extrusion reactor (70 kg/h throughput) has been developed to demonstrate a continuous process close to industry setting and produce large quantities that potential end users can use for combustion. We experimentally measured the thermo-mechanical properties of the torrefaction-extrusion reactor and the produced pellets. We present thermal dynamics, the effect of shaft configuration on residence time, specific mechanical energy, heat transfer coefficient, the specific heat of mixed wastes, and properties of pellets. The residence time was studied using different screw configurations; with cuts in the flighting, the residence time increased by a factor of 3.7. The overall reactor heat transfer coefficient was measured to yield 52.5 W/m2°C. The specific mechanical energy for each mechanical component was measured as a function of mass flow rate; generally, the specific mechanical energy showed a threefold decrease in specific energy from ~10 to 50 kg/h. Lastly, we present the complete pilot-scale system, including the pre- and post- processing equipment. The techno-economic analysis (TEA) and life cycle assessment (LCA) showed baseline cost of producing uniform pellets is $55.28/dry tonne (2020$) and that the torrefied product has net negative cradle-to-gate embodied greenhouse