Experimental Study On Thermal-chemical Conversion Characteristics Of Wood In Fluidized Bed Using CT Nondestructive Testing Technology

The issue of environmental degradation caused by the extensive use of fossil fuels has aroused phenomenal concern.Biomass is environmentally friendly renewable energy,and its proportion among China's energy structure is increasing.However,the large-scale utilization of conventional biomass is constrained by high costs of collection and transportation,low energy density and so on.The use of large size artificial forest wood is expected to solve these problems.Thermo-chemical conversion is one of the most common forms in biomass utilization.The fluidized-bed combustor is an attractive option for the large-scale utilization of biomass on the basis of its several advantages over other conventional methods,such as greater fuel flexibility,uniform temperature distribution,high thermal-chemical conversion efficiency and low pollutant emissions.Therefore,it is of great theoretical significance and application value to study the thermo-chemical conversion of large size wood in a fluidized bed combustor and the characteristics of its solid product biochar.Experimental study of thermal-chemical conversion of wood were carried out by using a laboratory scale turbulent fluidized bed combustor built from our own designs.Based on CT detection technology,this thesis focuses on three aspects: shrinkage characteristics and devolatilization time of wood under air atmosphere,pore structure and fractal characteristics of Chinese fir semi coke during pyrolysis,pyrolysis characteristics and kinetics of Chinese fir sawdust and its semi coke.The study of shrinkage characteristics of wood in different directions during devolatilization was carried out under an air atmosphere.The effect of the fuel physical properties and external operating parameters on final mass conversion,shrinkage characteristics and devolatilization time were also investigated.It is proved that the use of CT non-destructive testing method does not destroy fragile semi-coke and it can get the development trend of internal carbon contours.Results show that,all shrinkages and mass conversion increase with increasing residence time.Devolatilization time,final tangential and radial shrinkages increase with increasing particle size,while decrease with increasing bed temperature.Final longitudinal shrinkage shows an inverse trend compared with other two shrinkages.The tangential shrinkage is slightly greater than the radial one and much greater than the longitudinal one for all tests.Different woods show different shrinkage characteristics,and poplar has the maximum final shrinkage and mass conversion.Based on the predicted results of CT detection on the devolatilization time,the preparation of biochar was carried out in the fluidized bed under a nitrogen atmosphere.The isothermal nitrogen adsorption / desorption method was used to study the pore characteristics of Chinese fir semi coke during pyrolysis process and the pore surface morphology and spatial structure were analyzed using fractal methods.Results show that,The specific surface area of Chinese fir semi-coke obtained at 500 oC is the smallest and its average pore size is the largest.The number of micropore and macropore in semi coke is very small and most of the pores belong to mesopores and the size is too small.SEM analysis showed that semi coke is mainly composed of carbon particles.Its surface is a stripe fibrous structure and is covered with a deposited adsorbent layer,also the pore structure is irregularly cracked.The semi coke porosity interface appears to have obvious fractal characteristics.The thermo gravimetric analysis was used to analyze whether the volatiles in semi coke is completely removed,and to verify the CT test results.The pyrolysis characteristics and kinetic parameters of Chinese fir sawdust and its semi coke were studied under different heating rates.Results show that,there was a significant difference in the rapid pyrolysis stage of Chinese fir sawdust and its semi coke.With increasing heating rate,the initial and final temperature of pyrolysis increased,and the TG-DTG curves move toward the high temperature side.Kinetic analysis shows that the pyrolysis process can be described by a three-dimensional diffusion model.During semi-coke devolatilization process,the activation energy of heavy volatile was significantly higher than that of light volatile.Heating rate has little effect on the activation energy of Chinese fir sawdust and light volatile,but in most cases,the increase in the heating rate increases the activation energy of the heavy volatile.