| Effect of addition of biomass on the flowability of pulverized coal has been studied experimentally and theoretically on the basis of co-gasification of biomass and coal in an entrained flow gasifier. With a powder comprehensive characteristics analyzer and a powder rheometer, the static mechanical properties of blends were investigated. The dynamic properties, such as gravity discharge, have been analyzed with the related equipments in order to keep insight into the fundamental issues of granular flow. The main contents are summarized as follows:1. Effects of addition of secondary particle on the flow parameters of host particle have been studied by a powder comprehensive characteristics analyzer. With theoretically analysis of the variation of inter-particle force, the quantitative relation of granular physical properties and angle of repose (ΦR) has been obtained which have a good consistency with the experimental results. Results showed that the variation of angle of repose increases linearly with the increase of w, and inversely proportional to particle size dm. When the mass fraction of secondary particle is lower, the increment of tangent of ΦR has the following relation with related physical properties:△tanΦR＝kρb-ρs/ρbρs w/dm2. With a powder rheometer, the influence of particle shape on angle of internal friction has been investigated, and there exists a linear relation between the variation of angle of internal friction and the product of w and the difference of the tangent friction angle of particles, that is tan Φb,-tanΦh∝w(tanΦg-tanΦh). On this basis, an integrated model has been proposed, that is tan Φb-tanΦh=k0w(tanΦg-tanΦh) According to the magnitude of granular aspect ratio, three regions have been divided for various binary granular systems. In addition, the relationship between the k0and granular shape factor has been described as follows: 3. The effects of biomass particle on the gravity discharge behavior of coal have been studied in a silo, including the particle size, mass fraction and storage time, etc. Our experimental study revealed that the addition of needle-shaped particles into pulverized coal actually improved the flowability of the blend, which is named "needle particle effect". A revised model based on Beverloo correlation and Crewdson’s equation was proposed to predict the gravity discharge rate of biomass-coal blends, which agreed well with the experimental data with the error kept below10%, just as follow: W=W0(1-0254μg(wA1.60+0.650)/ρmgdm2)1/2During the investigation of the effect of storage time on gravity discharge, granular "time effect" was observed. Based on the relation between the change of porosity and time, and a mathematical model has been developed as follows: Wg＝k1e(-t/τ1)+W∞4. The influences of needle particle on the cohesive powder flow fluctuation and granular flow behavior in the free fall process have been studied. The result showed that the addition of elongated particle significantly reduce the flow fluctuation of fine granules and produce a stable linear flow like hourglass. Based on the experimental evidence and previous reports of flow dynamics, we find that the linear flow is driven flow orientation, reduced-agglomeration and local perturbation. Transition from a clustering state to a continuum fluid state in free falling mixed granular stream have been observed and illuminated by diminished of granular effective surface tension and agglomeration strength. Based on the theoretical analysis, the criterion of cluster foemation was proposed, and it further proved that the cohesive strehgth is closely related with the cluster formation. Experiment images strengthen the conclusion that granular stream process the transition from contractive to dilated with the addition of rice straw.5. The effects of addition of biomass on the coal fluidization and rheological behavior were studied, mainly focused on the influence of biomass mass fraction on the minimum fluidization velocity and segregation. When the content of biomass is less than10wt%, the blends can obtain the better flowability. For biomass-coal blends, after fully fluidization indicates, sawdust can effectively improve the fluidized quality instead of rice straw due to the lower segregation behavior. The results show that the minimum fluidization velocity mainly distributes in10-15mm/s, and increases with the increase of biomass mass fraction and biomass particle size. In addition, a modified form of the model has been developed to predict the minimum fluidization velocity of biomass-coal blends which agrees well with the experimental results. |
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