Integrated Performance Evaluation Of Liquid Fuel Production From Biomass Pyrolysis Based On Exergy Theory

With global energy crisis and environmental pollution becoming increasingly serious, it is significant to produce high quality liquid fuels as an alternative to traditional transportation fuels from clean and renewable biomass by fast pyrolysis and upgrading. The integrated evaluation on thermodynamic performance and environmental impacts of two processes,i.e., vehicle fuel production from biomass fast pyrolysis by hydroprocessing (Bio-TPF) and Fischer-Tropsch synthesis (Bio-FTL), were carried out based on exergy methodology.Firstly, a thermodynamic exergy analysis model was established based on the the exergy theory. The processes of Bio-TPF and Bio-FTL were simulated using Aspen Plus software. Besides, the influences of main process parameters on product yields as well as exergy efficiencies of the total system and its subsystems were investigated. For Bio-TPF, the results indicate that:(1) the largest exergy loss occurs in pyrolysis subsystem, which is mainly caused by internal irreversibility, followed by the reforming part, furthermore, (2) when mole ratio of steam to carbon (S/Cr) reaches 6 and mass ratio (θ) of the bio-oil employed for reforming and hydrotreating is 0.38/0.62, with reforming at 800℃(TCR) and 1.38MPa (PCR), the exergy efficiency (ηBio-TPF+) of the total system is 53.26% whilst the exergy efficiency (ηBio-TPF)from biomass to transportation fuels (TPF) reaches 31.12%, and the yields of TPF are 122.4 kg/t dry biomass, moreover, (3)ηBio-TPF-and ηBio-TPF-are able to achieve the maximum levels of 56.1% and 35.3% respectively at RCR of 700℃-800℃, PCR of 1.0 MPa-1.5MPa, S/C of 6 and θ of 0.3/0.7.As for Bio-FTL, the results show that the largest exergy loss takes place in pyrolysis subsystem, with the gasification part following, and the exergy loss of the whole system is majorly caused by process irreversibility. Besides, the H2/CO ratio in synthesis gas along with energy consumption of gasification subsystem tends to affect the systematic exergy efficiency significantly, and the best H2/CO ratio for Fischer-Tropsch synthesis is around 2.1. Furthermore, the proper operation parameters for Bio-FTL are suggested to be 800℃-850℃ for gasification temperature,0.1MPa～0.2MPa for gasification pressure and 1.25 for S/Cg, with the temperature and pressure for systhesis at 220℃-230℃ and 2.0MPa-3.0MPa. With the above-described conditions, the total exergy efficiency (ηBtp-FTL+) of Bio-FTL attains 46.7% while the exergy efficiency (ηBio-FTL-) from biomass to C5+ products reaches 33.24%, and the yields of F-T liquid oil (YFTL) and C5+ products (YC5+) are 102.9kg/t dry biomass and 136.9kg/t dry biomass separately. In addition, when S/Cg is larger than 1.5, the product yields and systematic exergy efficiency are enhanced significantly with the employment of recycle gas.Finally, an exergy life cycle assessment model for environmental impacts was established. Three comprehensive evaluation indicators including resource utilization efficiency (y), renewability performance (Ir) and environmental performance (Ic) were proposed to quantify the integrated environmental performances of Bio-TPF and Bio-FTL. Furthermore, sensitivity analysis was conducted by comparing the different influences of main parameters on the three evaluation indicators. The results from life cycle perspective indicate that the integrated performance of Bio-TPF is better than Bio-FTL, whereas neither of the two processes is renewable. The largest contribution to total cumulative exergy consumption (CExC,) is the usage of renewable biomass, which takes up 84.79% and 79.42% for Bio-TPF and Bio-FTL respectively. Power consumed, on the other hand, is in the second place, accounting for 9.95% and 15.29% in the two processes separately. Besides, the main abatement exergy of emissions are generated during biomass cultivation and production stages. And the contributions of abatement exergy for different pollutants are subject to the order CO2>SO2>agricultural wastewater. Moreover, y is more sensitive to the yields of bio-oil and TPF/FTL, while the sensivities caused by electricity consumption in production process, unit abatement exergy of pollutions and the yields of TPF/FTL, as for Ir, and Ie, are more significant. Furthermore, Bio-TPF is more liable to be a renewable system than Bio-FTL with reduced electricity consumption as well as enhanced each product yield. In addition, the reduction of unit abatement exergy is favorable to improve renewability and environmental performance of the system.