Dispersed-phase flow control in boilers with pressurized vertical spindle mills.

摘要

Control of the pipe-to-pipe distribution of pulverized fuel (PF) has been difficult to achieve in pulverizers that have multiple outlet pipes connected directly to the discharge turret at the top of the pulverizer. The current research has focused on developing a new technology for balancing the PF flow among the outlet pipes of vertical pulverizers without affecting the primary air (PA) flow distribution. Possible PF flow balancing approaches were evaluated using CFD simulations, followed by experimental testing of the most promising approaches in a 1:7 scale four-outlet pulverizer model. Each outlet pipe was connected to a separate cyclone to accurately measure the PF and air flows in each outlet pipe. A five-hole prism probe and a fiber optic probe were used to measure the 3D velocity components of air and the particle concentration, respectively.;The mill internals were modified by adding flow control elements (FCEs) at strategic locations. These FCEs can be easily adjusted from outside the pulverizer model while the PF is flowing through the system. The initial pipe-to-pipe PF flow imbalance was measured to be within +/- 16 percent. By adjusting the FCEs, it was possible to reduce the PF imbalance to +/- 1.6 percent. The tests showed there was no measurable effect of PF flow balancing on the outlet distribution of the primary flows. The PF flow distribution was found to be insensitive to the PF feed rate through the mill. The FCEs were designed to produce negligible pressure drop across the pulverizer.;Various turbulence models were evaluated against experimental data obtained from the literature and from the measurement results of this study. The RSM turbulence model gave the most accurate predictions for the highly turbulent, highly swirling, 3D flow in the complex geometry encountered in the current research. While the standard k-&egr; and realizable k-&egr; models also gave acceptable predictions, the RNG k-&egr; failed to give satisfactory results. Two-way coupling between the air and the particulate phase was used in the CFD simulations because one-way coupling failed to give satisfactory predictions of the flow inside the vertical spindle mill.