Coal and primary air flows in ancient pulverizers

Over the last month I have been working at a plant site in Alabama. The boiler controls were built in 1995. Some interesting quirks have come to light.
The plant engineers were somewhat apologetic about their coal flow measurement when I began working on their boilers. There are five units at this site. There are four B&W boilers that I have begun tuning. On all four of these boilers the coal flow is inferred from pulverizer differential. My reply to the engineers was that although this is not necessarily the most accurate method of developing the boiler coal flow, it was used for quite a long time in the power industry. Controls do not depend on accuracy, but rather, repeatability.
One of the first issues that were brought up by the operators was the propensity of the mills to “plug up” on load changes. They were absolutely white-knuckled whenever the mills began to unload, watching the mill current, ready to slap the controls into manual mode to prevent a high amp condition. These are B&W table top pulverizers. This seemed odd to me.
On investigating the pulverizer controls I realized that the controls were not typical. The fuel master provided the pulverizer demand. This went directly to the primary air (PA) flow control where it became the PA flow demand. The measured PA flow then became the pulverizer differential pressure set point. This means that whenever the PA flow moved, the feeder moved. This added noise to an already noisy system. Not only that, there was no cross-limiting of the air and fuel supply to the mill. Normally the fuel master demand goes to the feeder controls, and the feeder control output is then used to determine the PA flow demand through a minimum PA flow curve, that is provided by the mill manufacturer.
The reason for the mill plugging issue when the mills were unloading lay in the control configuration. As the load increased the PA flow came up ahead of the coal flow. But when decreasing, the PA flow decreased ahead of the coal flow. This drop in the transport medium (PA flow). If the demand moved fast enough, coal would build up in the mill, increasing the differential, and eventually causing high amps and a mill trip.
We modified the feeder demand to look directly at the fuel master demand. In addition we added cross-limiting on the mill fuel and air controls. This allows the air to lead the coal on an increase in load, and the coal to lead the air on a decrease in load. The plugging issue was substantially reduced and the mill controls were rendered more stable and easily controlled.