Experimental Study And Mechanism Analysis On Integration Of Anaerobic Fermentation And Pyrolytic Gasification From Biomass

The conversion of biomass into gas fuel by anaerobic fermentation and pyrolytic gasification technology has attracted great attention.The sustainable development of anaerobic fermentation technology depends on the appropriate disposal of solid digestate and biogas slurry.The great environmental pollution risk and the limited handling capacity has been existing in the conventional utilization modes of biogas residues,besides,the solid digestate is rich in energy.Meanwhile,biomass pyrolytic gasification technology has been subject to the low heating value of gas product,high risk of tar-derived environmental pollution and safe operation problem,but the conversion efficiency and the handling capacity of this technology are super high.Accordingly,for the sake of high-efficient conversion of all components of biomass into fuel-gas,the integration of anaerobic fermentation and pyrolytic gasification technology was proposed and studied in detail.The main contents were as follows:(1)Air gasification of biogas-derived digestate was performed in a downdraft fixed bed gasifier.The optimal lower heating value 4.78 MJ/Nm³ and cold gas efficiency were achieved at 800°C with ER of 0.28.The optimal tar content of 1.61g/Nm³ was obtained with ER of 0.30.The molar ratio of H₂/CO in gas product reached1.03-1.08.The gasification byproducts(biochar and ash)can be land applied as organic fertilizer.(2)Anaerobic digestion(AD)was applied to pretreat corn straw(CS)before air gasification.The effects and reaction mechanism of AD pretreatment time(7 d,14 d,21 d and 28 d)on physic-chemical properties and gasification characteristics of CS were studied.With AD pretreatment time of 14 d and gasification temperature at 800°C,the optimal LHV and CGE were 6.83 MJ/Nm³ and 73.62%,respectively.Tar content of the anaerobic digested corn straw(ADCS)decreased with AD pretreatment time.Compared with the raw CS,the tar content decreased by 56 wt%in maximum.(3)A new bio-chemical pretreatment method by using biogas slurry(BS)with different moisture contents(40 wt%,60 wt%and 80 wt%)and pretreatment time(1 d,3 d,5 d and 7 d)was advanced for promoting air gasification of corn straw(CS).The effects and reaction mechanism of BS pretreatment on physic-chemical characteristic and the gasification property of CS were comparatively investigated.The higher moisture content and longer pretreatment time contributed to the optimal gasification performance of CS.Considering the time and energy conservation,the intermediate 60wt%moisture content and retention time 5 days was proposed.The optimal LHV was7.15 MJ/Nm³,compared with the raw CS,this value increased by1.15 MJ/Nm³,and the tar content decreased by 21 wt%.(4)Based on a large-scale biogas plant with daily biogas yield of 60000 Nm³,the energy balance was calculated and optimized for the integrated anaerobic fermenetaion and pyrolytic gasification system.In the biomass-to-electricity system,the excess heat from CHP system of AD plant was sufficient for the drying of dewatered solid digestate.Compared with the stand-alone AD plant,the total electricity derived from CHP system in AD process and gas engine during digestate gasification increased by 24.5%.In the biomass-to-gas(biogas+fuel-gas)system,the daily output of biogas and fuel gas were38692 Nm³ and 37548.5 Nm³,respectively.Compared with the biomass-to-electricity system,the daily energy output was nearly doubled.