Coal ball mill working process
The present paper performs the numerical simulation of the gas flow in the coal ball mill with two-way inlet and two-way outlet according to the practical condition of 40.53 type mill. The velocity of inlet gas and the fill ratio of steel balls are studied emphatically. Results obtained provide beneficial reference for milling, drying, choosing and transporting coal powder in the practical product.
Direct combustion coal ball mill with two-way inlet and two-way outlet has a unique gas-solid two-phase field inside, which leads to its efficiency of powder milling, drying higher than that of mill with single-way inlet and single-way outlet. The gas-solid two-phase flow structure and power characteristic of such mill plays an important role in drying, milling and transporting powder. In recent years, the theoretical and experimental research in terms of gas-solid two-phase flow of the mill has not developed as rapidly as that of boiler although investment has been strengthened in both fields, which will greatly limit the development of coal powder combustion technology. The present paper contributes to describe the gas field in the mill by numerical simulation.
Physical model of two-phase flow
Axial flow
Two stream of thermal gas spurt into the mill from the center pipe at its both ends respectively. They will meet at the mill center, dash against each other and then return of opposite direction, flow along the mill wall, and last, go out through the ring-shape outlet. The pre-dried coal enters the mill from the center pipe, and will mix with thermal gas fully under the effect of rolling and milling of the machine. The dried and fined coal particle will rise toward the thermal gas and apart from the steel balls located at the bottom of the roller and develop into gas-solid two-phase flow at axial direction. In stable case, the flow of the two stream of thermal gas at opposite direction is almost equal and the development of the two-phase flow is equal too.
Circular flow
Several spiral ribs are wielded on the wall of thermal-gas-inlet-pipe in order to make both the thermal gas at the mill center and gas-solid flow near the wall move with little spinning. Therefore, two-phase flow stability and its mixing efficiency are improved. Such mill has symmetrical and spherical structure, which will be destroyed at some degree by the steel balls because they occupy 15~20 percent space of the mill. The balls piled up at the bottom may roll and slide along the wall with the mill movement. The cross section of the piling balls appears as crescent moon shape. In the present paper, the steel balls are supposed to fix at the bottom of the mill and their shape and density are assumed to be constant because the rolling speed of the mill is fairly low.
Assumption Gas-solid flow in the mill is practically a three-dimension flow with very complex conditions. In the present paper, assumptions are made as follows :
- No-rotating
The rotating motion of both fluid and mill are ignored in the present research because the circular motion is much weaker than the axial motion, not only for thermal gas but also for gas-solid flow in the mill - Symmetrical gas-field
The thermal gas field and the gas-solid field are assumed to be symmetrical about Y-axis according to the coordinate system set up. The assumption is much close to the practical case when the milling system is operated at stable condition
