Accommodating Process Dynamics
Depending on the material being ground or pulverized, maintaining the through-flow of the ground product to its destination is important. Depending on the particular process, it may be of benefit to either maintain a constant flow of the product through the transport lines or you may need to dynamically change the amount of product to complement the dynamics of the downstream process.
We need to look at the equipment that performs the grinding process.
As the material is being ground there is a mass of material in recirculation within the mill. This recirculation contains material of all sizes – that is at the required fineness, near the proper fineness and much coarser material. The magnitude of mass of this recirculating storage will be a function of material through-flow (loading) and air flow. Generally as the material (through-flow) flow increases, the recirculating flow and storage increases. At a fixed material through-flow the amount of storage will increase or decrease with the relative decreasing or increasing of the transport air flow through the mill. We may have an opportunity to utilize this potential change in storage to assist with the dynamics of the overall system process.
If we relied on the normal equilibrium balance of internal storage in the mill to occur when the input loading changes while there is constant air flow, then the time delays to achieve the final material flow equilibrium may be long. In a combustion process where the change in output of pulverized coal to the furnace needs to be as short as possible, this normal balance takes an excessive amount of time to facilitate dynamic load changes in the furnace output. This is a common problem in power generation. One way to minimize this time delay is to actively and temporarily change the equilibrium storage within the mill.
If we wish to immediately increase the delivery of pulverized product out of the mill we can borrow from the recirculated storage in the mill. To do this we step-change the air flow above the new steady-state then lag back desired air flow. This will rob some of the coal in recirculation and deliver it out of the mill. To replace and maintain the normal inventory within the mill, the feed to the mill will also be step-lagged to anticipate and replace the short term dynamic removal of material from the mill. The amount of dynamic removal from the mill must be carefully balanced with the ability of the mill to regrind the additional material fed to the mill to the proper fineness or there may ultimately be an undesirable short term reduction in the material flow out of the mill.
Reduction of the load through the mill is accomplished in the opposite direction.
Problems that may be encountered are few. If load reduction is performed at low or near minimum mill loads, the velocities through the mill cannot be allowed to drop below the manufacturer’s minimums. If they do, there may be additional spillage (rejection) of coarse product since the air velocities may not be able to support the weight of the ground material. The distribution of flow through the air flow plenum to the grinding surfaces may become mal-distributed at these low flows and allow the coarse material to fall through into the reject hopper.
Through all the loads of the mill, these dynamic changes in air flow can change the quality of classification of the delivered product. Momentarily increased air flow can possibly deliver overly coarse product from the mill. Conversely decreased air flow can possibly create overly fine product.
Grinding, drying and transport project – Phase 4
Creating a control arrangement with constraints