Effects Of Pyrolysis Pressure And Heating Rate On Structures And Oxidation Reactivities Of Cola Char And Biomass Char

With the rapid development of Chinese economy,the increasing consumption of coal leads to the reduction of high quality coal,utilization of carbon-neutral renewable resources such as biomass can significantly relieve the shortage of fossil fuels.As one of the fundamental process of coal and biomass utilization,pyrolysis plays a crucial role in heating conversion process.As rate controlling step,the structure of char produced from pyrolysis determines its reactivity,and the operating conditions during pyrolysis influence the amount and the nature of volatile products and the physicochemical properties of the produced char,and further influence char reactivity.As key factors,the heating rate and pressure as key factors have great impacts on devolatilization process and release of volatile process,respectively,which will affect the physicochemical properties of char and its reactivity,and eventually the operation condition of the gasifier.The modern gasifier tends to be higher temperature and pressure,so it is necessary to fully understand the pyrolysis mechanisms during pyrolysis process with high heating rate and pressure.Therefore,it is important to study the effect of heating rate and pressure during pyrolysis process on char structure and reactivity for coutilization of coal and biomass.Coal(lignite and bituminous)and biomass(cornstalk)were used to prepare the char samples by a two-step fixed bed reactor with different heating rate and pressure in this paper.The specific surface area,surface morphology,carbon crystallite structure and chemical structure of char samples were characterized.The oxidation reactivities of the chars were also investigated by the isothermal thermogravimetry analyzer.The effect of heating rate and pressure on char structure and reactivity of coal chars and biomass were further discussed.The main results are as following:(1)As increaing heating rate during pyrolysis,the char yields of lignite,bituminous and biomass decrease,the specific surface areas of the lignite char,bituminous char and biomass char increase,the surface morphology of the chars become rougher and the degree of graphitization structure show a decreasing trend,these changing structure finally lead to the improvement of char oxidation reactivity.The change extent of surface and carbon crystallite structure of biomass char with heating rate is more apparent than lignite char and bituminous char,and the amount of oxygen,atomic ratio of H/C and quantities of alkali and alkali earth metals(especially the K)in biomass is higher than lignite and bituminous,as a result,the promotion of biomass char reactivity is more significant(2)As pressure increase during pyrolysis with slow heating rate,the char yields of lignite,bituminous and biomass increase,the specific surface areas of the lignite chars and biomass chars increase,whereas the specific surface areas of the bituminous chars have no obvious change.Moreover,the surface morphology of the chars become smooth and the defective structure of the chars decrease,the degree of graphitization structure show a decreasing trend and the chemical structure tends to be stable with increasing pyrolysis pressure.The diffusion resistance was enhance at high pyrolysis pressure,therefore the second reaction is more intense as pressure increase,leading to the decrease of oxidation reactivity of coal char and biomass char.(3)The pressure has great effect on the release of volatile in fast pyrolysis,which affects the specific surface areas,the surface morphology and carbon crystallite structure of char and then the char reactivity.As pressure increase during fast pyrolysis process,the char yields of lignite,bituminous and biomass increase,the specific surface areas of the lignite char and biomass char decrease,whereas the specific surface areas of the bituminous char increase,the surface morphology of the chars become smooth and the defective structure of the chars decrease,the carbon crystallite structure become orderly,the aromaticity and the degree of graphitization structure of chars show an increasing trend.Therefore,the char produced with high pressure is more inactive than that obtained from atmospheric pressured.