Control of coal flow in pressurized vertical spindle mills with four outlet pipes

Being able to achieve balanced coal and air flows at individual burners in a pulverized coal fired power plant is important for achieving efficient combustion and low emissions. Deviations from the required air-to-fuel ratios at individual burners result in greater quantities of unburned carbon in fly ash, and unstable combustion with higher levels of carbon monoxide (CO) emissions. Control of pipe-to-pipe distribution of coal in vertical spindle pulverizers with multiple outlet pipes is a challenging task, and a control mechanism that has been proven to be capable of accurately accomplishing this task is still not available in actual utility boiler applications.;In an earlier study by Energy Research Center (ERC), the capabilities of controlling and balancing coal flow distribution between the pulverizer model outlet pipes by using mechanical flow control elements (FCEs) positioned inside the discharge turret of a laboratory scale pulverizer model were investigated [3]. Laboratory tests showed that by adjusting the FCEs' positions while coal was flowing through the system, it was possible to reduce the coal imbalance to less than +/- 2.0 percent. No measurable effect of coal flow balancing on the primary air flows in the outlet pipes was found in the experiments.;In this current study, a total of six flow control element (FCE) designs were tested at two different axial locations inside the discharge turret of the laboratory scale four outlet pulverizer model. At Location 1, FCEs were positioned immediately before the inlets of the outlet pipes, against the outer wall of the discharge turret. At Location 2, they were positioned in the annulus region of the inlet of the discharge turret, against the raw coal inlet pipe. At each axial location, four identical FCEs were installed half-way between the outlet pipes. The objective of the laboratory tests was to determine the optimal FCE geometry and the best location inside the discharge turret for the most effective coal flow control.;At Location 1, FCE U-1.52 was found to be the most effective design. By operating FCE U-1.52 in its effective range, coal flow imbalances in the two neighboring pipes were changed by +/- 36 percent, while the coal flow imbalances in the other two non-neighboring pipes were changed by +/- 7 percent. At Location 2, FCE L-0.67 was found to be the most effective design. By operating FCE L-0.67 in its effective range, coal flow imbalances in the two neighboring pipes were changed by +/- 58 percent, while the coal flow imbalances in the other two non-neighboring pipes were changed by +/- 16 percent.;Among the six FCE designs tested in this study, FCE L-0.67 at Location 2 was found to be the most effective design for controlling the coal flow distributions between the outlet pipes. A number of trials were made with FCE L-0.67 to balance the coal flow distribution between the outlet pipes using all four FCEs installed inside the discharge turret. Initial coal flow imbalance at the base case with no FCEs installed was +/- 28 percent. Consecutive adjustments made to the FCEs' positions reduced the coal flow imbalance to less than +/- 8 percent.;In the second stage of the current research, effects of primary air flow rate, classifier vane angle, and air-to-fuel ratio on the coal flow distributions between the outlet pipes were investigated on a six outlet pulverizer model. In addition, the sensitivity of coal and primary air flow rates in each outlet pipe to orificing was investigated on the existing 4 outlet pulverizer model. The results revealed that coal flow in any outlet pipe varied as much as 30 percent as a function of pulverizer operating parameters. Use of orifices was found to be an ineffective method for adjusting coal flow distributions among the outlet pipes. Primary air flow was much more sensitive to the changes in the orifice area than the coal flow.