There are several control loops associated with mill, or pulverizer control. These typically include the control of feeder speed, primary air (PA) flow, and mill outlet temperature. Depending on the available instrumentation and the particular type of mill other variables might be included. A non-exhaustive list of these includes mill differential, automatic wheel loading (AWL) pressure, classifier speed, and primary air fan speed. Several other factors are associated with mill control. These include the heat, or BTU, content of the coal, the chemical makeup of the coal, and the hardness or grindability of the coal.
Most recently, we focused on the grindability factor of the coal and its effect on the requirements for PA flow and AWL pressure. The type of mills at this plant are B&W MPS-75’s. The primary air flow is controlled by the PA rating damper; the temperature is controlled by the hot air and the tempering air dampers. There is automatic control of the wheel loading and the plant operates in automatic mode. There is a station for classifier speed but it is a manual loader, that is, there is no automatic control available.
The Hardgrove Grindability Index (HGI) is an index that pertains to the hardness of the coal, or the ease with which it can be pulverized. An HGI of 45 is harder coal, an HGI of 60 is a much softer coal therefore it grinds much faster. The original PA flow set point curve had been developed for an HGI of 45. But after some years had passed, the type of coal that was being burned had been changed to a softer coal. The HGI for the coal now normally runs up around 51 to 55 HGI. The rating dampers had been replaced some years before. The original parallel rating dampers were old and worn, so curves had been manipulated to improve performance at the end of service life with blades damaged or partially missing. The minimum PA flow set point had been increased by the plant in reaction to the condition of the PA dampers and the resulting leakage. The new dampers are opposing in nature and control well. But the PA flow set point curve remained elevated all across the full range of operations. As a result, the operators were forced to use extremely negative bias to get the classifier to reject any pyrites at all. These biases were on the order of -16 to -20 KSCF/HR.
Only with this kind of operator action, was the AWL pressure control able to maintain any sort of coal bed in the pulverizer. Coal was being swept out of the mill at primary air velocities that had been developed for much harder coal. The mills vibrated and rumbled a lot. The mill manufacturer, B&W, was contacted.
Calculations were performed by B&W for coal of varying hardness. The data that was generated showed a substantial difference in the PA flow and AWL pressure requirements for the mills at differing loads.
Let us consider the PA flow. Typically one set point curve is used to generate the primary air flow demand as a function of feeder speed. A second, lower curve is a function of actual coal flow, and is used to cross-limit the PA flow. There is also a minimum demand that is a constant and reflects the minimum amount of air that is necessary to transport coal.
The curve that was being used for one of the mills is shown below:
However, the data that was provided by B&W resulted in a family of curves. Now PA flow was a function not just of feeder speed but also the HGI or grindability factor. What this means is that instead of a single PA flow being associated with a given feeder demand, depending on the grindability factor, the PA flow can vary across a family of curves.
The HGI is provided on the data sheets that are associated with each coal shipment. As a result the operators should be able to estimate when any significant change in hardness is about to occur. The operator is able to enter the grindability factor for each of three coal bunkers at any time. The remote set constant for these is available on each pulverizer sequence page and the AWL control page for each mill. This is labeled “HGI”. The value is rate-limited such that a change from 51 to 52 will take one minute. This will then shift the PA flow set point curve as shown below:
Please note that this drawing is not rigorously correct, but is meant to show the general shape of the new family of set point curves. A second curve, of the same shape but a 3% lower minimum was generated as a function of actual coal flow to serve as the cross-limit for the mill.
The AWL pressure set point was changed in a similar fashion. Note that a higher grindability factor decreases the pressure set point.
This change should allow the operator to operate with little, or no, bias on the primary air flow control with the rating dampers and no bias on the AWL pressure control. There will be an increase in the amount of rejected pyrites from the mill (from none to some). It is important to realize that all of the biasing functionality that was there before this change still exists. This has been done in addition to the controls as they were.
It will take some time to see the long term effects of this change in the PA flow set point.
Most recently, we focused on the grindability factor of the coal and its effect on the requirements for PA flow and AWL pressure. The type of mills at this plant are B&W MPS-75’s. The primary air flow is controlled by the PA rating damper; the temperature is controlled by the hot air and the tempering air dampers. There is automatic control of the wheel loading and the plant operates in automatic mode. There is a station for classifier speed but it is a manual loader, that is, there is no automatic control available.
The Hardgrove Grindability Index (HGI) is an index that pertains to the hardness of the coal, or the ease with which it can be pulverized. An HGI of 45 is harder coal, an HGI of 60 is a much softer coal therefore it grinds much faster. The original PA flow set point curve had been developed for an HGI of 45. But after some years had passed, the type of coal that was being burned had been changed to a softer coal. The HGI for the coal now normally runs up around 51 to 55 HGI. The rating dampers had been replaced some years before. The original parallel rating dampers were old and worn, so curves had been manipulated to improve performance at the end of service life with blades damaged or partially missing. The minimum PA flow set point had been increased by the plant in reaction to the condition of the PA dampers and the resulting leakage. The new dampers are opposing in nature and control well. But the PA flow set point curve remained elevated all across the full range of operations. As a result, the operators were forced to use extremely negative bias to get the classifier to reject any pyrites at all. These biases were on the order of -16 to -20 KSCF/HR.
Only with this kind of operator action, was the AWL pressure control able to maintain any sort of coal bed in the pulverizer. Coal was being swept out of the mill at primary air velocities that had been developed for much harder coal. The mills vibrated and rumbled a lot. The mill manufacturer, B&W, was contacted.
Calculations were performed by B&W for coal of varying hardness. The data that was generated showed a substantial difference in the PA flow and AWL pressure requirements for the mills at differing loads.
Let us consider the PA flow. Typically one set point curve is used to generate the primary air flow demand as a function of feeder speed. A second, lower curve is a function of actual coal flow, and is used to cross-limit the PA flow. There is also a minimum demand that is a constant and reflects the minimum amount of air that is necessary to transport coal.
The curve that was being used for one of the mills is shown below:
However, the data that was provided by B&W resulted in a family of curves. Now PA flow was a function not just of feeder speed but also the HGI or grindability factor. What this means is that instead of a single PA flow being associated with a given feeder demand, depending on the grindability factor, the PA flow can vary across a family of curves.
The HGI is provided on the data sheets that are associated with each coal shipment. As a result the operators should be able to estimate when any significant change in hardness is about to occur. The operator is able to enter the grindability factor for each of three coal bunkers at any time. The remote set constant for these is available on each pulverizer sequence page and the AWL control page for each mill. This is labeled “HGI”. The value is rate-limited such that a change from 51 to 52 will take one minute. This will then shift the PA flow set point curve as shown below:
Please note that this drawing is not rigorously correct, but is meant to show the general shape of the new family of set point curves. A second curve, of the same shape but a 3% lower minimum was generated as a function of actual coal flow to serve as the cross-limit for the mill.
The AWL pressure set point was changed in a similar fashion. Note that a higher grindability factor decreases the pressure set point.
This change should allow the operator to operate with little, or no, bias on the primary air flow control with the rating dampers and no bias on the AWL pressure control. There will be an increase in the amount of rejected pyrites from the mill (from none to some). It is important to realize that all of the biasing functionality that was there before this change still exists. This has been done in addition to the controls as they were.
It will take some time to see the long term effects of this change in the PA flow set point.
