Research On Primary Fragmentation Mechanism And Chemical Structure Variations Of Coal/biomass During Rapid Pyrolysis

Rapid pyrolysis is a key procedure of gasification in entrained-flow gasifier.During the rapid pyrolysis,the physical and chemical structure of the fuel particles would sharply reform,which accordingly have a great impact on the gasification efficiency.In this process,primary fragmentation is a physical phenomenon of the breakage of raw fuel particle into smaller pieces.At the rapid heating condition,the coal/biomass pyrolyze quickly and release a large amount of gases,which contributes to the occurrence of primary fragmentation.Fragmentation would strongly reform the particle size and morphology,which has an apparent influence on gasification efficiency and ash formation.On the other hand,the chemical structure of the char would also change,which may fundamentally change the gasification reactivity.Therefore,it is necessary to investigate the variations of physical and chemical structure.It would also help to better understand the thermal behavior of rapid pyrolysis,and to predict the gasification characteristic of pyrolyzed char.In this research,the physical and chemical structure of coal/biomass was characterized.The effect of pyrolysis time,pyrolysis temperature and solid properties were investigated.The rapid pyrolysis experiment was conducted in an apparatus with a variable residence time,to study the thermal behavior at the primary stage of rapid pyrolysis.The fragmental behavior of coal/biomass was quantified based on the measurement of particle size distribution,morphology and densities.A fragmentation model was established on the basis of the analyses of the pressure distribution inside particles during rapid devolatization.The variation of the char chemical structure was researched by analyzing the elemental composition and infrared spectra.(1)Researches on the rapid pyrolysis behavior showed that,the pyrolysis extent is higher when residence time is longer and temperature is higher,which displays as the larger value of mass loss and gases released meanwhile the less remaining volatiles.The primary fragmentation is actually a phenomenon of hundreds milliseconds.The fragmental characteristics of different coals are diverse,which the overall order is NM lignite?SF bituminous coal>JS anthracite.At low temperature,the char maintained the morphology of coal,while at high temperature,the structure of char damaged intensely due to the quick release of gases.(2)Researches of primary fragmental behavior of coal showed that,primary fragmentation is quite sensitive to the time and temperature.Basically,longer duration and higher temperature would enhance the fragmentation,and at high temperature,coarse fragmentation took place quickly and completed in a short time.The order of primary fragmentation was NM?SF>JS.During the fragmentation,the morphology of coal rarely changed,indicating the core of the particles did not fragment sharply.Therefore,major fragmentation occurs at the outer zone of particle in the case of NM and SF,and the exfoliation dominates the pyrolysis of JS.In addition,swelling occurred in the rapid pyrolysis of SF.(3)Researches of primary fragmental behavior of biomass showed that,the pyrolysis of biomass is more apparent than coal.Similarly,pyrolysis temperature and time have positive contribution to the rapid pyrolysis and primary fragmentation of biomass.However the morphological parameters of biomass significantly changed,displaying as the increases in sphericity and aspect ratio.During rapid pyrolysis,fine particles tended to be more regular while fragmentation occurred at outer region of coarse particles.The order of primary fragmentation was sawdust>wheat straw>rice straw,which was thought to be connected with their ash contents.(4)The mechanism of primary fragmentation was discussed during rapid pyrolysis.The pressure due to devolatilization got maximum in the center of particle and the difference in pressure provided the driving force of the primary fragmentation.The fragmental rate got higher when pressure difference rose,furthermore,they were found to be linear related.The resistance was diverse with the particle properties.The Pressure difference and the Resistive stress impulse increases with the ash and carbon content of coal,and also grow monotonously with the ash content in biomass.(5)The chemical structure of coal and char was studied during rapid pyrolysis.The H/C and O/C atomic ratio of char increased with the growth of pyrolysis temperature and time.After pyrolysis,the fraction of graphite structure inside char grow with the average layer of carbon crystallite decreasing.The chemical structure parameters(CSPs)increases,which were attributed to the increase in thermal stability of coal with the increase in coal rank.