Effect Of Mineral On The Particulate Formation And Ash Deposition During Pulverized Coal Combustioin

Zhundong lignite exhibits extremely severe propensity of ash fouling and slagging when fired in practical boilers.It is still a great challenge to predict accurately the deposition behaviors.The cause is,on one hand,the complex physical/chemical nature of the“particle-fluid-wall”interaction,and on the other hand,an often lack of thorough understanding on the fly ash generated in the high temperature atmosphere.This thesis aims to address this problem by the experiments in the 25 kW self-sustained quasi-one-dimensional pulverized coal combustor,which provides a well-controlled condition for simultaneous measuring the particulate formation and ash deposition during coal combustion.More insights can be obtained on the mineral transformation behaviors and heat transfer surface fouling problems.This thesis work has investigated the transformation of volatile minerals during coal combustion.Through a comparison of the PM₁₀ characteristics between Hami and Zhundong lignites,it is found that the formation of ultrafine mode PM_0.2 is controlled by the combined effect of Na,Si and Al,rather than by the sole effect of Na.The size partitioning of Na in the fine particulate generated by a variety of coal samples exibits different modes,and the partitioning behavior can be characterized by a single parameter,l=Na₂O/（SiO₂+Al₂O₃）（mole%）.With a decreasing l,the controlling mechanism of gas-to-particle conversion of Na has been transitted from a nucleation-condensation dominated process to a surface-reaction-dominated one.The binary population balance model has been established to deduce the size-dependent Na partitioning laws rigorously,and also to simulate the Na evolution in the postflame region during the combustion of Zhundong lignite.The modeling work has successfully reproduced the(9⁰-dependent of Na in the ultrafine PM fractions,which is a result of the homogeneous nucleatioin process within the dropping temperature range from 1420 K to 1270 K of the flue gas.This thesis work has also investigated the effect of mineral manipulation on the ash deposition behavior of Zhundong lignite.The ash fouling tendency and PM_0.2 formation ability have been mitigated by either blending Zhundong lignite with a kind of HAF-2bituminous which is rich in Si and Al in ash,or water-leaching the lignite as a pretreatment.The blends are capable of solidify the volatile Na through a surface reaction,and the water-washed coal has reduced amount of volatilized AAEM species.The off-line analyses on the packing density of ash deposits agree reasonably with the in situ temperature measurements of the deposit.The deposition propensities have been significantly promoted in the region with higher flue gas temperatures for the various coal samples,and the ash deposits formed in higher temperatures have increased strength.Finally,a positive correlation has been found between the ash collection efficiency and the formation ability of the ultrafine fraction PM_0.2 for all the coal samples involved in this thesis.A model in the hybrid“Eularian-Lagrangian”scheme has demonstrated that the average sticking efficiency of bulk fly ash PM₁₀₊increases asymptotically to 9.0%for Zhundong lignite and 4.5%for Hami lignite,as the inner layer of PM₁₀ deposit reaches 1.5 g/m² during the initial stage of coal combustion.This thesis work has formulated the impaction efficiency as a function of the Reynolds number and the Stokes number in the 2D flow around a cylinder.It is thus promising to obtain a more quantitative understanding on the ash deposition behaviors in practical boilers based on the experimental results in the 25 kW furnace presented in this thesis work.