Research On Decoupling Gasification Of Biomass In Fluidized Bed

As a renewable and carbon dioxide neutral energy source, the effective utilization of biomass is of great significance to sustainable development of the environment and resources.Biomass gasification is actually a complex network of series of sub-processes including drying/pyrolysis, gasification, reforming, combustion and so on. The decoupling gasification, which separates biomass drying/pyrolysis from other reactions, and conducts the pyrolysis gas passing through the char layer, can realize lower tar generation and higher gasification efficiency. Based on the above idea, we designed a new U-shape fluidized bed (UFB) gasifier. This article was conducted to clarify the hydrodynamic characteristics of quartz sands in the UFB, and simulated the characteristics of separating fuels' drying/pyrolysis from char gasification and tar reforming reactions in a two-stage fixed bed reactor for a kind of biomass fuel, beer lees. Then compared with traditional coupling gasification, all of these verified the fact that comparing with the coupling gasification, the decoupling gasification has obvious superiority in suppressing tar generation and enhancing gasification efficiency.The major research works and acquired results are just as follows:1. In the optimized UFB apparatus, by changing the gas velocity and bed hold charge, at the optimized fluidization velocity, analyzed fluidized behaviors of quartz sands, through measuring the pressure of pressure points distributed in the bed. And found to guarantee the quartz sand fluidized well, the pressure drop at both side of the inclined perforated plate should maintain at about 1 kPa. At the continuous feeding conditions, using the method of tracer particles, analyzed the particle flowing track and mixed-up behavior, and drew conclusions that the mix behavior of the particles in the UFB near CSTR, the mean particle residence time was 322 s.2. A laboratory two-stage fixed-bed reactor was employed to enable the drying/pyrolysis of the fuel, which is beer lees in this article, to occur in its upper stage and the char gasification and tar reforming/ cracking in its lower stage. Compared the decoupling gasification (DG) with traditional coupling gasification (CG) on the gasification parameters, such as tar yield, carbon conversion rate, gasification efficiency and so on. The results show that:①Under the reaction temperature of 550℃for pyrolysis, 800℃for gasification in the DG and 800℃in the CG, a water content of 40 wt.% in the fuel and a O₂ content of 4 vol.% in the gasification reagent, compared with the CG, the tar yield decreased by 35.3%, the gasification efficiency and gas yield increased by 4.0% and 20%, respectively in the DG. Nonetheless, the lower heating value (LHV) of the produced gas in the DG decreased slightly.②At the pyrolysis temperature of 600℃, the tar yield of the beer lees can achieve maximum, further increased the temperature will lead to secondary cracking reactions. Increasing the gasification temperature, can enhance gas yield and decrease the tar generation. Increasing the oxygen content in the gasification reagent, can shorten the gasification reaction time, promote the carbon conversion rate and increase gas yield, whereas, the lower heating value of the produced gas decreased obviously.③The decoupling gasification makes use of the catalysis founction of char to tar, and fully utilizing the steam in the pyrolysis gas as the gasification reagent to promote the char gasification reaction. So examination of the effects of fuel water content (40～60wt.%), gasification temperature (800～900℃) and oxygen content in the gasification reagent (4%～6 vol.%), all demonstrated that the decoupling gasification led to more considerable influences than the coupling gasification did by causing higher H₂ or CO contents in the produced gas and higher carbon conversion rate. Through compaing with limestone and quartz sand, the char of biomass showed obviously superities on catalysis of char to tar, and which can regulate the component of the produced gas effectively, too.