The performance of binary particles mixing and gas-solids contacting

which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds

is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer (

φ

0.1 m×6.5 m). Since binary particles are used in this system

a reasonably validated 3D

transient

multi-fluid model

in which three heat transfer modes relating to the convection

conduction and radiation are considered

is adopted to simulate the flow behavior

temperature profiles as well as volatile contents. The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region. In the further downward section of the downer

the particle flow disperses near the right wall and develops uniformly. Meanwhile

the coal phase is slowly heated in the downer and it is found that most o

f the heat absorbed by the coal is from the convection heat transfer mode. To explore the heat transfer mechanism more quantitatively

two indexes (mixing index and contacting index) are proposed

and it is found that the mixing index initially increased fast and later remained at a relatively flat state. For the contact index

it shows a trend with a first rising and then falling

finally rising continuously. Also

it is found that the convection heat transfer is closely correlated to the contacting status of gas-coal which indicates that the improving of the gas-coal contacting efficiency should be an effective way to strengthen the coal particle heating process.