Process parameter optimization of urban biowaste carbonization

Abstract

About 75% of Municipal solid waste (MSW) collected around the country is organic biomass which mainly includes food waste, wood, paper, saw dust and paddy husk. Urban councils in Colombo city and nearby suburbs collect biowaste separately which has created a huge potential in converting urban biowaste into value-added component like biochar, thus resolving the problems associated with MSW management and mitigating socio-economic and environmental issues related to MSW. In this study, torrefaction is identified as the most viable technology available for the conversion of organic MSW into biochar and the study mainly focuses on developing a three dimensional computational fluid dynamics (CFD) model of a continuous packed-bed torrefaction reactor for organic MSW and then optimizing the process variables and the geometry. A mathematical model including all heat, mass and energy transfers, and heterogeneous & homogeneous reactions is firstly developed and then converted to a numerical model and simulated using OpenFOAM for an insulated cylindrical reactor in which hot gas at elevated temperatures (473 – 623K) is provided from the bottom while solid at ambient conditions is fed from the top. The torrefaction reactor is optimized for gas inlet temperature and residence time and then the geometry of the reactor is optimized for the optimum gas inlet temperature and residence time. Four reaction zones are identified in the reactor domain; i.e. drying, softening & depolymerization, limited devolatilization & carbonization and extensive devolatilization and carbonization. The optimum inlet gas temperature, residence time and D/L ratio are 573K, 13000s and 0.24 respectively. For the optimum conditions, biochar yield is 55.7% while ash content is 19.1%. Further In dry basis, 95.9% of biomass is decomposed and the total weight loss based on the initial wet biomass is 86.6%.