ANALYSIS AND OPTIMIZATION OF A BIOMASS GASIFICATION AND FISCHER-TROPSCH INTEGRATED PROCESS FOR SYNTHESIS FUEL PRODUCTION

Abstract

The performance analysis of the biomass gasification and Fischer-Tropsch integrated (BG-FT) process of rice straw feedstock are presented in this research. A parametric analysis of the gasification processes utilizing different types of gasifying agent i.e., steam-air and steam-CO2, is firstly performed to investigate the possibility of syngas production with desired H2/CO ratio from a thermal self-sufficient gasifier. The effects of changes in the ratio of gasifying agent on the syngas yield, H2/CO ratio, total energy consumption and cold gas efficiency of the system at different gasifying temperatures are investigated. The syngas yield of both gasification processes significantly increases at low temperature until it reaches a maximum value and is stable at temperatures higher than 700 °C, However the steam-CO2 system offers higher syngas productivity and lower H2/CO ratio; however, the thermal self-sufficient condition is not achieved. The technical, economic and environmental studies of the BG-FT process which gasifier is operated under thermal self-sufficient condition are further performed. The feasibility of FT-offgas recycle to the gasifier is firstly investigated. Regarding to technical aspect, the influence of changing an off-gas recycle fraction at different values of the FT reactor volume on the performance of the syngas processor, the FT synthesis and the energy efficiency is discussed. The production rate of syngas, diesel product and FT off-gas, as well as electricity from the BG-FT process, can be maximized via suitable adjustment of the recycle fraction and selection of the FT reactor volume. The economic analysis using an incremental NPV as an economic indicator implies that the use of the recycle concept in the BG-FT process without the installation of any secondary equipment is less feasible than the once-through concept from an economic point of view. The performance of BG-FT processes with and without tar removal unit based on steam reforming and autothermal reforming (ATR) are compared in term of the produced diesel and electricity, energy consumption, the overall potential environmental impact (PEI) and the combined effect of diesel production rate and PEI. And the BG-FT process with ATR is found to be the most practical configuration, and the process offering maximum internal heat recovery and minimum external utility requirements is proposed. The optimization of the new designed BG-FT process based on the economic objective is performed to determine the optimum operating condition offering the maximum net present value (NPV). The influence of gasifying temperature, FT operating temperature and FT pressure on the diesel production rate and the PEI is investigated. The combined evaluation in term of economic and environmental point of view is further performed using the AHP index, calculated based on the multi-criteria decision analysis (MCDA) method using AHP analysis, as an indicator. The suitable condition offers the best performance from both economic and environmental point of view is finally proposed.