Study On Bulk And Flow Properties Of Pulverized Coal Systems And Their Relationship Between Inter-particle Forces

Cohesive powders show a number of problematic flow behaviors and their flow properties significantly affected by strong gas-particle interaction and inter-particle forces.Thus,the trouble-free handling of these cohesive powders is a serious problem as well as presenting a scientific and technological challenge.This doctoral dissertation mainly focused on the study of powder flow properties with the background of pressurized entrained-flow gasification technology.Typical fuel powders and fly ash were selected as research materials.The bulk and flow properties and influence factors in terms of particle size,particle size distribution,surface characteristics and particle mechanical properties were investigated.The research results will offer great support for the development of powder flow theory and industrial operations.Main research content is shown as the following:1.Pulverized coal from industry was used as the experimental material.In order to study the effect of particle size,the pulverized coal was sieved into seven samples with different particle size.Firstly,the material properties as packing,incipient flow and wall friction properties were evaluated.Meanwhile,a transparent Perspex hopper was used to monitor the change of discharge behavior of pulverized coal caused by the increase of particle size.From the visual observation of discharge test,a progressive transition from blocking to unstable flow and to mass flow was found as the particle size increased.Hence,a complete set of physically based equations for compressibility,flowability and powder consolidation has been derived.Then,a new method,taking into account the stress state in the hopper,is introduced to predict the discharge behaviors of pulverized coal.The validity of this new method has been confirmed by comparing theoretical and experimental discharge behavior for pulverized coals.The theoretical approach based on stress state analysis,is able to provide results which correlate well with the experiments.2.The influence of fines(below 40 μm)on the bulk and flow behavior of the pulverized coal was analyzed and a comparison between the influence of two size-cuts(<20 μm,20-40μm)was further given.Experimental results were obtained based on bulk and flow tests,including compressibility,permeability and shear tests etc.In parallel,a transparent Perspex hopper and an outlet uniaxial stress tester apparatus were fabricated to determine the discharge behavior and the collapse phenomena in the hopper,respectively.The stress at the outlet of the hopper was decreased gradually until reached a stable value and the more fine particles pulverized coal contains,the more time required for it to stabilize.Accordingly,the pulverized coal performed different discharge behaviors at different packing time.Finally,a packing model was proposed to quantitatively describe the packing process of pulverized coal in the hopper and explain the various discharge behaviors at different packing times.3.The mechanical properties of a series of industrial fuel powders,including Bituminite,Lignite and Petroleum coke,were studied.Flow properties for all the powder samples were used to calculate the tensile strength as a function of the consolidation stress.Furthermore,the BET surface area and dispersive surface characteristics were estimated by using a surface energy analyzer.In order to analyze the bulk flow properties of fuel powders in the light of the micro contacts mechanics,the fundamentals of fuel powder adhesion and consolidation were re-proposed according to the model "stiff particles with soft contacts".In our case,a multi-contact concept was introduced to account for the particle irregular shape.The modified model was applied to describe the contact between rough particles and estimate the relevant inter-particle forces.These results were used together with the Rumpf approach to relate the isostatic tensile strength to the material consolidation.Acceptable agreement was found between the predicted and measured tensile strengths.4.Two groups of lignite powder,characterized by different particle size distributions were prepared and selected as research materials.The first group was prepared by an air classfier which allowed obtaining samples characterized by narrow particle size distributions(PSD).The second group contains two kinds of industrial grade pulverized lignite,associated with relatively wider particle size distribution.The packing properties and flow behavior of pulverized coal was characterized in terms of compressibility and flow properties using FT4 powder flow rheometer.Furthermore,BET surface area and dispersive surface energy were determined using a surface energy analyzer.In order to understand the effect of particle size distribution on the bulk flow properties of these powders,a micro-scale approach inspired to the Rumpf and Molerus approach is proposed.According to this approach,an estimate of the tensile strength of industrial lignite powders is sought starting from the data relative to the narrow sized samples.The validity of the approach is demonstrated by the acceptable agreement between the values of the isostatic tensile strength estimated with the application of the model and the values of the same properties directly estimated from the measured flow properties.Finally,based on the model results,bulk flow properties of the industrial grade pulverized lignite were investigated in the light of the micro-scale particle contact.5.Four industrial fly ashes,with similar chemical compositions but characterized by different particle size distributions were selected as a research material.The bulk flow properties of these powders were measured at ambient temperature and at 500℃ with the High Temperature Annular-Shear Cell(HT-ASC).In order to better understand the effect of temperature on the bulk flow properties of fly ashes,the model proposed by us,which combines a continuum approach and a particle-particle interaction description was used to correlate the powder isostatic tensile strength with the inter-particle forces and micro-scale particle contact structure.The dependence of the tensile strength on consolidation and temperature is correctly described by the model.Based on the modeling results,the effect of temperature on bulk flow properties of fly ashes were investigated in the light of the micro-scale inter-particle contact.