Gravimetric Feeder Controls

In thermal power plants coal is combusted in a furnace and water is heated to generate steam.Thus in thermal power plants , monitoring and control of coal flow to the furnace is an important process and one of the methods for this process is discussed below.


The basic idea of coal flow control is the speed control of a conveyor belt (approx. 3m long)that is feeding coal to the coal pulveriser. A variable speed drive ( typically a fixed speed induction motor coupled to a magnetic clutch) is a prime mover of the conveyor belt. A roller at mid length of the conveyor belt supports two load-cells suspended vertically from either side of the conveyor belt. The speed of the drive is measured by a tachogenerator.
x = load-cell signal (in mV)
f(x) = mass of coal on per unit length of the conveyor belt (Kg/m)
y = Speed of the drive ( in rpm)
f(y) = speed of the conveyor belt ( in m/s)
Now computation of coal flow measurement is done as

Coal flow = f(x) * f(y)
(Kg/s) (Kg/m) (m/s)

The coal flow is achieved as follows


It is to be understood that inorder to maintain the required the coal-flow, we measure the existing coal-flow and adjust the conveyor belt speed to achieve the desired coal flow.The demand (or required coal-flow) is generally given by the plant Distributed Control System which takes into account the power demand of the grid and other boiler parameters.
The signal conditioning of the measured signals and computation of control voltage to the Variable speed drive (typically a eddy-current clutch) is done as shown below

The signal form load-cells is in mV and it is suitably amplified and converted into a digital signal and read by a microprocessor.The output from the tachogenerator is a ac voltage and this ac signal is converted into pulses and becomes the input for a frequency counter.This frequency is read by the microprocesor.Considering these two signals and the Fuel-flow demand a digital control voltage is generated and given to a Eddy Current clutch (ECC) card.This ECC card converts the digital signal to a analog dc voltage signal. Apart from this , the ECC card incorporates a PI controller which controls the DC voltage to the ECC coil.

Details of Eddy current clutch (ECC)
The ECC is a electromechanical device which uses the principle of magnetic clutch to control the speed of the conveyor belt.
The DC output voltage from the ECC card is given to the magnetizing coil .When the coil magnetizes , it produces eddy currents in the non-magnetic body .These eddy currents produce a magnetic field which is linked with the Rotating cup.The Rotating cup forms the upper assembly whereas the non-magnetic body, the magnetizing coil along with the two tachogenerators form the lower assembly.It is interesting to note that these two assemblies are physically isolated from each other but they are magnetically coupled.The magnetizing coil produces eddy currents in the non-magnetic body (lower assy) .These eddy currents produces magnetic flux which locks the non-magnetic body(lower assy) with the Rotating cup (upper assy) .This is a magnetic coupling of the non-magnetic body with the Rotating cup. A fixed speed induction motor rotates the Rotating cup.This Rotating cup inturn rotates the lower assembly and thus the conveyor belt through the gear system. This is shown in the figure below
As the DC voltage to the magnetizing coil increases, the magnetic coupling becomes strong and the lower assy (non-magnetizing body) catches with the upper assy (Rotating cup) more strongly.This increases the speed of the conveyor belt and hence the coal flow.
Conversely , as the DC voltage to the magnetizing coil decreases, the magnetic coupling between the upper and the lower assy weakens and although the upper assy rotates at a fixed speed , the lower assy lags behind and its speed decreases.This reduces the speed of the conveyor belt and hence the coal flow.



Calibration of Gravimetric Feeders

The Gravimetric coal feeders calculates the coal flow by measuring the instantaneous weight of coal on the coal-belt.For this the load-cell output (in mV) must be correlated with a standard weight.For this purpose we subject the load-cells to a standard weight and run the empty belt.In this condition the tachogenerators are measuring the speed of the belt .Simultaneously the speed of the belt is also measured by a photo-pickup method (another alternative speed measurement method ) .
Imp.note : All these measured quantities are stored in the microprocessor as raw-counts converted into HEX numbers
Now
Reference coal-flow= Load-cell signal for std.weight x speed of belt
(raw-counts) (as measured by photo-pick up method)

Measured coal-flow = Load-cell signal for std.weight x speed of belt
(raw-counts) (as measured by tachogenerator)

Deviation = Reference-Measured
This Deviation has to be less than a user-selectable allowable deviation ( generally 0.01%).If the deviation is more, then belt-tightness,load-cell alignment etc has to be adjusted and calibration has to be re-performed.
Also a raw-count co-relation factor (m) is estimated as
m = Reference coal-flow (Kg/s) / Reference coal-flow (raw-counts)

Also during the calibration process the load-cell signal (in terms of raw counts) is compared for the coal belt empty condition and for the standard weight engaged on the load-cells condition.This gives the tare-weight (in raw-counts ) for the empty belt.Let us call this tare-weight constant as c.

The values of m and c are stored in the microprocessor.

During normal operation of the coal-feeder , the coal-flow is calculated as

y = mx+ (- c)
x= measured coal-flow raw counts
y= measured coal-flow ( Kg/s)

---------- to be continued
Protection and Interlocks of Gravimetric feeders
Problems of Gravimetric feeders