Fuel control schemes in boiler

Measurable Fuels :

  • · The control of gas and oil fuels tends to be straightforward, because they are easily measured and can be regulated with a control valve in the fuel line.

  • · If closed-loop control of flow is not available, then a valve positioner capable of providing a linear relationship between flow and control signal is desirable, as shown in the following figure:

  • · A flow control loop (preferred) is the more usual means of control, providing more precise linearization and immediate response to flow disturbances, as shown in the following figure:

  • · When it is desired to fire the fuels in a predetermined ratio to each other regardless of load, a manually adjustable signal splitter can be used, as shown in following figure:

Waste or Auxiliary Fuel Controls

  • · The master controller should be designed and used to respond to total load demands only and not to correct for fuel upsets. A typical fuel control system for accommodating variations in auxiliary fuel without upsetting the master is shown in following figure:

  • · Following figure describes a slightly more advanced system, in which the allowable maximum percentage of waste fuel that can be burned is set on the ratio relay FY-1.

  • For proper operation, the subtractor FY-2 must be scaled with the flowmeter ranges taken into consideration, and further scaling is required if the heat flow range of the total heat demand does not match that of the waste fuel flow-meter.

  • · When waste fuel gas availability becomes limited, waste fuel gas pressure will drop and PY-3 will select it for control, thereby overriding the waste flow controller.

  • FY-2 will respond to this by increasing the supplemental fuel flow.

Unmeasured Fuels

  • Coal can be an unmeasured fuel. In such cases coal control systems are open loop, wherein a control signal positions a coal-feeding device directly.

  • One control arrangement is shown in the following figure. Here, the primary air comes from a pressure fan that blows through the pulverizer, picking up the coal
    and transporting it to the furnace.

  • The improved scheme is shown here, those improvement are added for the metered fuel and for highly variable fuels as will be explained later on.

  • The system utilizes the mill power level (kilo-watts), the mill sound level (decibels), and the operator-selected set point (kilowatts) as the principal inputs to the mill process controller (MIC).
  • Boiler fuel demand (or steam flow) can be used by the control system as a feedforward signal to anticipate major changes in mill load demand, and to preposition the mill feeder(s) at the proper steady-state speed.

Metered Coal Controls :

  • When the flow of coal is controlled, it is done at the inlet to the mill, as illustrated in following figure:

  • The capacity of the pulverizer contributes a delay of a few seconds to several minutes, If the delay is less than 1 min, a corresponding delay can be inserted into the coal flow transmitter output (FY-1), which will enable the loop to overcome this problem. If the mill delay exceeds 1 min, the flow of the primary air that conveys the pulverized coal must be manipulated as a function of coal demand.

  • Because the coal loading of the air is not uniform, Shinskey recommends the improvements noted in control scheme of the previous section (unmeasured fuels).

  • These include the determination of the actual heat release based on steam reading, and the slow integral correction of the total flow of primary air until the estimated and actual heat release are matched.

Highly Variable Fuels

  • Solid fuels tend to vary significantly in heating calorific value. This variation can be detected and compensated for by comparing the apparent heat release of the boiler process to the apparent heat input and adjusting the fuel flow accordingly to achieve the desired heat input.
  • The improvements noted in control scheme of the previous section (unmeasured fuels) show the use of steam flow (FT) for load. PT and PY are the steam drum pressure and its rate of change. In effect, this is summing the rate of energy out with the rate of change of energy storage (FY-1). This heat-release measure is then com-pared to the current compensated fuel flow and the difference, considered to be the error between the two due to heating value variation in the fuel, is integrated slowly and multiplied by the fuel flow, producing an adjusted process variable for the fuel controller.