Research On Reaction Characteristics And Structure Evolution During Pyrolysis And Gasification Of Lump Raw Coal And Coal Char

Coal pyrolysis and char gasification are the two main steps in coal gasification.The structure of coal and char will change with the pyrolysis and gasification reaction,which further affects the reaction results.Therefore,an in-depth study on the reaction characteristics and structure evolution during the pyrolysis and gasification of lump raw coal and coal char is of great significance for revealing the reaction mechanism in fixed-bed gasifier and realizing the optimal design of fixed-bed gasifier.In this thesis,lump raw coal and coal char were selected as research object.The reaction characteristics and structure evolution during the pyrolysis and gasification of lump raw coal and coal char were systematically investigated.Furthermore,the heat transfer effect in the pyrolysis process and the diffusion effect in the gasification process were quantitatively evaluated.Main results for this study were as follows:(1)The reaction characteristics and structure evolution during the pyrolysis of lump coal and char were systematically studied.Results showed that during the pyrolysis of Sheumu bituminous coal,the release rate and release amount of CO2 did not increase monotonically with temperature,and decreased when the temperature increasing from 900? to 1000?.Additionally,the change trend of liquid yield at high temperature(1000?)was slightly different from that at low temperature(600?).The pore structure of pyrolysis chars at 800?and 1000? was mainly cylindrical holes.There were obvious differences in aromatic ring structures and cross-linking density between particle surface and particle center,and these differences were significantly affected by pyrolysis temperature.At a certain stage of the pyrolysis process,aromatic ring structures and cross-linking density at surface and center had different change trends.Additionally,the evolution of functional group at surface and center presented a large difference only at 1000?.In the first 2min of pyrolysis,the low-substituted aromatic ring structure at surface decomposed gradually,while the(2H)aromatic structure at center was converted to(1H)aromatic structure.(2)A large particle pyrolysis model was established based on the distributed activation energy model and energy conservation equation,then the heat transfer and reaction characteristics inside the particle were numerically studied.Results showed that the temperature inside the particle can be better predicted by the model.Pyrolysis temperature and particle size had significant effect on the pyrolysis characteristics of lump coal particle.For a lower pyrolysis temperature(600?)and a smaller particle size(6mm),there was only a small gradient in the distribution curves of temperature and volatile yield inside the particle.While for a higher pyrolysis temperature(1000?)and a larger particle size(22mm),the maximum temperature difference between particle surface and particle center exceeded 200?,which resulted in an obvious radial distribution of volatile yield within the particle.(3)The reaction characteristics and structure evolution during the gasification of lump coal char were studied.Results showed that as the gasification reaction progressed,the mole fraction of CO basically showed a linear decreasing trend.Under the studied gasification temperature,CO2 partial pressure and particle size,both the shrinking core model and random pore model could fit the experimental results well.At the gasification temperature of 1300?and 1350?,with the proceeding of gasification reaction,the number of mesopores in the range of 2-50nm first increased gradually,and then decreased.Specific surface area showed a trend of increasing first and then decreasing,average pore size basically increased gradually,while the fractal dimension hardly changed.In the early stage of gasification reaction(carbon conversion less than 20%),the large aromatic ring structure in coal char was consumed significantly,the order degree of particle surface also changed significantly at this stage.(4)The diffusion effect and change of kinetic parameters during coal char-CO2 gasification were investigated.Results showed that the internal effectiveness factor was not constant during the conversion process of coal char.Diffusion effect with different degrees only changed the magnitude of internal effectiveness factor,but it did not change the change law of effectiveness factor.Apparent activation energy and apparent pre-exponential factor changed gradually during the whole conversion process,and showed a certain kinetic compensation effect,the enhancement of diffusion effect resulted in a certain change of kinetic parameters and kinetic compensation effect.