The Experimental And Simulation Study Of Generation And Oxidation Of Coal-derived Soot

Coal resources is one of the most important energy sources and strategic material in China for a long time,thus,the clean and effective utilization of coal is national demand.Soot is usually generated as the byproduct during coal utilization because the soot particles are readily formed from the small gas molecules and tar in fuel rich condition.The generation of soot will decrease the carbon conversion efficiency.Besides,the soot particles are easily discharged into atmosphere due to the low reactivity.Those soot particles are harmful for human health and natural environmental because the size of soot particle is extremely small.The investigation of the generation of soot during rapid pyrolysis of coal and the study of gasification/oxidation mechanism of coal-derived soot are important for clean utilization of coal.Those studies can provide data support for inhibiting soot generation and promoting soot consumption during coal utilization.In this work,the evolution of physical and chemical structure of Shenfu bituminous coal during rapid pyrolysis was characterized.The effects of pyrolysis atmosphere(N2/CO2),pyrolysis temperature and residence time on coal pyrolysis were investigated.The gasification/oxidation modes and structure evolution of coal-derived soot were explored by analyzing the soot with multiple kinds of analytical instruments(HRTEM,FT-IR XRD,and Raman,etc.).Here an atomistic representation of soot particle was generated by using a novel shell-by-shell construction from the HRTEM image.A simplified oxidation simulation approach was applied based on this soot model.(1)The rapid pyrolysis of coal was studied,and the effect of pyrolysis atmosphere on pyrolysis product was investigated.A novel equipment which can change residence time(0～774 ms)was constructed to compare the rapid pyrolysis of coal in N2 and CO2 atmosphere.The loss of char and the yield of pyrolysis gas increased with temperature and residence time in both N2 and CO2 atmosphere.The effect of CO2 atmosphere on pyrolysis was related with temperature.The CO2 atmosphere inhibited the release of volatile at 1073 K,while promoted the decompose of functional group and release of small gas molecules at high temperature.Compared with N2 atmosphere,the CO2 atmosphere was favorable for the release of hydrogen and surfer element from char,while the migration of nitrogen elements was inhibited with presence of CO2.More CO and less H2 and CH4 were generated during pyrolysis in CO2 atmosphere.The evolution of micro-structure was investigated with FT-IR during coal pyrolysis.The aliphatic compound was more easily consumed than aromatic compounds.The cleavage of methyl and cyclization was enhanced by substitute N2 with CO2,besides,the CO2 atmosphere promoted the break-up of aliphatic hydrogen.More H-containing free radicals generated in CO2 atmosphere during the polycondensation of aromatic ring units.(2)The formation of soot during coal rapid pyrolysis was investigated,and the effect of pyrolysis atmosphere on soot particles was studied.The yield of soot,maturity and C/H atomic ratio of soot increased with temperature and residence time.The properities of soot were related pyrolysis atmosphere.Compared with N2 atmosphere,the CO2 atmosphere increased the soot yield,structure ordering and C/H atom ratio.The CO2 atmosphere enhanced the dehydrogenation and polymerization of tar during soot formation,significantly promoted the lateral and longitudinal growth of carbon nanostructures.The CO2 atmosphere improved the order of the internal carbon lattices,besides,the contribution of CO2 to reduce basic structure units(BSU)defects was also found.Thus,the CO2 derived soot was less reactive than those N2 derived soot.(3)The gasification of coal-derived soot with CO2 was studied.The evolution of morphology and nanostructure was first comprehensive characterized by using HRTEM,XRD,surface analyzer and et al.The soot gasification modes were obtained from soot structure evolution.The gasification occurred on both the surface and internal of soot particle because much pores generated during gasification.The gasification of SF-soot,N330 and N990 at 1273 K followed a hybrid of Shrinking Core Model(SCM)and Homogeneous Reaction Model(HRM).A unique gasification mode forming concentric spherical structures was first observed during the gasification of SF-soot at 1473 K.The internal region of soot particles was preferentially consumed during gasification of N330 and N990.The gasification of the soot in CO2 was mainly a disordering process,accompanied by a slight thermal annealing-derived maturing at high temperature.The crystallites were preferentially consumed along the graphitic edges for SF-soot and N330,while the crystallites were preferentially consumed along the longitudinal orientation during the gasification of N990.(4)The oxidation of coal-derived soot with 02 was studied,and the structure evolution and oxidation model were also investigated.The oxidation mode and structure evolution of SF-soot are similar with gasification.The interior of soot particle was preferentially consumed during oxidation of SF-soot at 1273 K,and the oxidation at 1473 K followed concentric spherical mode(CSM).The oxidation mode of Printex was also dependent on temperature.The Printex oxidized in hybrid mode of SCM and HRM at 1273 K,while the interior of soot particle was preferentially consumed during oxidation of Printex at 1273 K.The true density of SF-soot and Printex frist increased and then decreased during oxidation.Most carbon nanostructures transformed to disordered structures during oxidation.The crystallites were preferentially consumed along the longitudinal orientation during conversion 0～0.2,and consumed simultaneously along the longitudinal and horizontal orientation during conversion 0.2～0.4,while mainly consumed along horizontal orientation during conversion 0.4～0.8.(5)Generation and oxidation simulation of soot model.A large scale atomistic representation of coal-derived soot was rapidly generated after the analysis of HRTEM image with several scripts.The resulting core-multiple shell atomistic representation has～1 million atoms within 3014 molecules.The generation of outer shells included the analysis of HRTEM image,the analysis of lattice fringe,the generation of shell in molecule by molecule method,the assembly of shell,the addition of heteroatoms,and the evaluation of soot model.A simplified oxidation approach including molecular dynamics diffusion and evaluation of the distance between oxygen molecules and reactive sites was applied.The reactivity profiles of this representation were similar with experimental results with a peak rate during conversion 0.2～0.3.