| Coal contains minerals in contrast to oil and natural gas, which brings manytroubles during the combustion or gasification. In typical coal combustion process, apart of minerals forms fine particulates through vaporization-nucleation-coagulationmechanisms, whereas a major part of minerals forms coarse fly-ash particles byfragmentation-shedding-coalescence mechanisms. These particles usually deposit on thesurface of heat exchangers, which deteriorate heat transfer and further reduce powergeneration efficiency. Thus, it is significantly essential to study mineral transformationcharacteristics during coal combustion. In this thesis, the characteristics of fineparticulates formation, transformation and deposition are investigated during thecombustion of several typical Chinese coals.First, we explore the formation mechanism of fine particulates. The fineparticulates were sampled from a self-sustained, one-dimensional down fired furnaceand subsequently analyzed by SEM and ICP-OES/MS, in which their emissionproperties are correlated to the difference of coal rank. The results indicate fineparticulate emissions dramatically differ between high-rank Shenhua bituminous coaland low-rank Zhundong lignite. The latter coal produces much more ultra-fine particlesthan the former one. Detailed micro-morphology and chemical components analysisillustrate that the water soluble and ion-exchangable mineral elements are the mainsource of submicron particles during the combustion of low-rank Zhundong coal.Meanwhile, the vaporization of sub-oxide metals from included minerals produces mostof the submicron particles during the combustion of high-rank Shenhua coal. Thecharacteristic time-scale analysis is introduced to clarify the complex parallel-processesduring particle formation. It is found that homogeneous nucleation and heterogeneouscondensation mechanisms of gas-phase mineral elements are prevalent through thewhole combustion duration, whereas the collision-coalescence process amongmicron-size particles rarely happens. As for high-rank Shenhua bituminous coal, theheterogeneous condensation of metal vapors onto the existent particle surface iscontrolled by surface reaction. Comparatively, it behaves more like a diffusion-controlled process for low-rank Zhundong coal. Secondly, we discuss ash deposition mechanisms. It is inferred from ash depositionexperiments that high sodium Zhundong coal exhibits the highest deposition propensity,which is even higher than that of herbaceous biomass. It is mainly because of theformation of thicker inner layer as well as that of stickier fly ash particles duringZhundong coal combustion, as observed from SEM images. The microdynamicsimulation on the basis of discrete element method further proves that the existence ofthe thicker inner layer enhances the energy dissipation during the coarse particlecollision with deposition probe, which results in the higher capture efficiency. Aquantitative prediction based on transport equation indicates that more inner layerdeposits are formed during the early deposition stage of Zhundong coal, compared toshenhua coal. All these above well explain the high propensity of zhundong coal ashdeposition.Finally, we further explore the effect of combustion mode on both particulateformation and ash deposition. The real flue-gas-recycle oxyfuel coal combustionexperiments were successfully fulfilled in the aboved mentioned one-dimensionalfurnace. Fine particulate concentration in furnace under oxyfuel combustion mode ishigher than that under the conventional combustion mode. Theoretical calculationindicates the amount of submicron particles from each gram of ash under oxyfuelcombustion is even less than that under conventional combustion, which means thecontribution of fine particulates from recycled flue gas is unignorable. In addition, theash deposition rate of conventional combustion is higher than that of oxy-fuelcombustion, which is mainly due to a higher impaction efficiency but not depositchemical compositions. |
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