Gasification of Biomass for Second Generation Biofuel Production

Abstract

Gasification of biomass is perceived as one of the most attractive thermochemical processes to produce carbon neutral syngas that can be burned to release energy or used as the building blocks for the production of value-added chemicals, especially liquid fuel in the heavy transport sector. In the present thesis, both experimental and numerical approaches were applied to investigate the behavior of biomass gasification at high temperature and high heating rate conditions. Devolatilization of biomass and conversion of solid char are two most important steps in the gasification process. In order to assess the behavior of the rapid devolatilization of biomass, biomass particles (forest residue, torrefied forest residue, Norwegian spruce, and torrefied Norwegian spruce) were subjected to devolatilization experiments at 1073 K and 1473 K in an electrical heated drop tube reactor (DTR). A computational fluid dynamic (CFD) simulation with a proposed two-competing rate devolatilization model was also performed and compared to the experimental results. The conversion behavior of forest residue char and torrefied forest residue char were further examined under oxidation and gasification conditions at 1473 K and 1573 K in the same DTR. The morphological analysis of the parent biomass and corresponding char were showed. In addition, time-resolved data on compositional transformation of the biomass and char were presented based on the metal tracer technics. In parallel, Eulerian–Lagrangian CFD models were developed to study the overall gasification process in two types of reactors: entrained-flow reactor (EFR) and fluidized-bed reactor (FBR). Comprehensive CFD simulations were conducted to evaluate the performance of biomass gasification in an EFR operating at 1273-1673 K. The model was validated against a wide range of experimental data. Several influential factors including reactor temperature, steam/carbon molar ratio, excess air ratio, biomass type, and particle size were discussed. Regarding the FBR, particle flow pattern, bed expansion, bed pressure drop and fluctuation frequency were compared by using three different well-known inter-phase drag force correlations in a non-reactive condition. Steam gasification in FBRs was analyzed by the CFD model developed from a non-reactive study. Both qualitative and quantitative results were presented to reveal the effects of reactor temperature, steam/biomass ratio, and biomass injection position on gasification of biomass.