Numerical Study On Maldistribution Of Gas-solid Flow In The Multiple-branching Conveying Pipelines Of Limestone System

Pneumatic conveying technology is widely applied in many fields ofindustry, and one of them is the limestone desulfurization system of circulatingfluidized bed （CFB） boilers in the power plants. As one of the most importantauxiliary system of CFB boiler, limestone pneumatic conveying system is ofgreat significance to the operation of CFB. However, the maldistribution ofpowder in the different injecting ports seriously limits the inner-furnacedesulfurization performance due to a inefficient mixing process of limestonepowder and SO2. Therefore, the maldistribution caused by a multiple-branchingconveying pipeline, leads to the desulfurization efficiency down obviously.The flow characteristics of limestone powder in an industrial scalemultiple-branching conveying pipeline system is studied numerically with a newcommercial software Barracuda which using a Computational Particle FluidDynamics （CPFD） methods in this dissertation, which is belong to3D dilutephase and wide particle size distribution （PSD）flow. Four main results areobtained.（1）The reason and behavior of maldistribution were investigated. Theserious maldistribution are presented by both particle mass distribution andparticle size distribution in two branch pipelines. The solid mass in the right branch pipe is more than the left branch pipe obviously caused by90oduct atincoming flow direction at a "one-to-two" pipeline.The PSD nonuniformcharacteristics exists simultaneously that bigger particles in the left and finerparticles in the right at the same time.（2）The maldistribution characteristics is influenced by different operatingparameters. The gas-solid flow structure and its maldistribution phenomenon arecarefully analyzed under four different operating parameters, including solidconcentration （Np）, air velocity（Ug）, air pressure （P）and the characteristicof different PSD.Maldistribution is not a monotonous trend with different solidconcentration. There is a specific solid concentration area corresponding to asignificant maldistribution phenomenon in a different air velocity. With theincrease of air velocity, the degree of maldistribution is reduced, but thedecreasing slope is obviously different according to different concentration areas.When the pressure is less than0.1MPa, there is a serious maldistribution,otherwise the pressure is greater than0.1MPa, solid mass distribution tends touniform in two branches. So0.1MPa is the critical value. PSD influences theparticle size range and the d0.5when a maldistribution phenomenon appears.（3）The optimization schemes of distributor structure is carried out. Toreveal maldistribution problem, optimize scheme on distributor structure isproposed, that is, adding a deflector to balance the powder mass in two branchpipeline. Its performance is demonstrated to be reasonable from the simulationresults. Even more, reasonable deflector angle can also improve nonuniformphenomenon of particle size distribution.（4） The design criterion of distributor structure with deflector wasobtained. A geometry parameter H, associated with branch position in thedistributor is found vital in controlling the deflector to realize balanced mass flow.