Controls in Air-Fuel circuit

- Combustion control
- Furnace draught control
Combustion control
• The primary function of combustion control is to deliver fuel and air
to the burner at a rate that satisfies the firing rate demand and a
mixture (fuel/air ratio) that provides safe and efficient combustion.
• Insufficient air flow wastes fuel due to incomplete combustion and
can cause accumulation of combustible gases that can be ignited by
hot spots in the furnace.
• Too much air flow wastes fuel by heating up the stack excessively.
• Combustion controls are designed to achieve the optimum air/fuel
ratio.
Objectives of combustion control system:
- Regulate input of heat energy to the plant equipment so that it will be
always equal to the plant needs, but not in excess.
- Maintain high efficiency of combustion at all rates
- Only the rate of energy flow should vary, thermal state of the plant
equipment should not fluctuate
- Provide guarding against the hazard caused by insufficient air flow
• For variable loads, the fuel input needs to be adjusted proportional
to the load.
• This must be accompanied by a change in air flow to the combustion
equipment to maintain efficient air-fuel ratio at all loads.
• The primary boiler fuels are gas, oil and coal.
• Steam pressure is the key variable that indicates the state of balance
between the supply and demand for steam.
• If the supply exceeds demand, the pressure will rise- if demand
exceeds supply, the pressure will fall.
• The pressure controller with main header pressure as controlled
variable maniplulates the firing rate demand to control steam
pressure at the desired set point.
• Alternately , steam flow, generator current etc can also be used .
• Plant master
• When more than one boiler supplies a common steam header,there
will be multiple boiler masters but only one plant master.
• Individual boilers receive firing rate demand from plant master
through boiler master control.
• Depending on the load and performance of individual boilers,some
boilers may be shut down, some base loaded(constant firing rate) and
remaining are allowed to swing with the load(variable firing rate).
Combustion control with gaseous fuel
• Fuel control:
It is a simple feed back control loop with firing rate demand (fuel
demand) signal from boiler master controller as set point. The gas flow
is measured , square rooted and given as actual value to the controller.
The controller output is utilized to open/close the flow control valve in
the gas line to increase /decrease the amount of gas to the boiler
satisfying the fuel demand .
• Combustion control – single point positioning
The fuel controller output operates the final control element which in turn
opens or closes the control valve in the fuel line. The mechanical lever/link
between the final control element and the control valve will have further
mechanical link to the combustion air flow damper.
Both fuel valve and air damper are operated by the same controller and the
final control element.
A single point positioning system uses a mechanical linkage to manipulate
the fuel control valve and the combustion air flow damper in a fixed
relationship according to the demands of the controller. The air/fuel ratio
required for proper combustion is fixed through this linkage only.
• Combustion control-parallel positioning: uses a similar strategy as
single-point positioning. But parallel positioning refers to two outputs
used in parallel to control the fuel valve and the air damper.
• Here, air damper valve is operated by a signal generated by a manual
setter. The same signal from the controller is given in two paths.
• The two outputs go to the fuel valve and the air damper.
• The jackshaft in the single-point positioning is replaced by a function
block with a manual air/fuel ratio set.
• Both the methods do not make use of air flow measurement in the
control. These systems can’t take care of air pressure and
temperature variations.
• This weakness can be overcome by including measurements of fuel
and air flow in the control strategy.
• Combustion control- series fuel-air ratio control: flow measurements of
both fuel and air are made use of in this method.
• The fuel flow is controlled as per firing rate demand. The fuel flow signal is
given to air flow control through ratio station where the required air-fuel
ratio is set manually.
• The output of ratio station becomes set point to air flow controller which
takes feedback from air flow transmitter- ‘fuel leading air’ control- fuel
varies first as per firing rate demand and this change causes air to vary as
per ratio set.
• It is possible to have air leading fuel strategy also. Here air varies first as
per the firing rate demand and this change causes fuel to vary according to
ratio set.
• Combustion control:Parallel air-fuel ratio control: Generally, it is
better to control fuel and air in parallel rather than in series. A lag of
few seconds in measurement and transmission will seriously affect
combustion conditions in a series system.
• The effect of interaction and disturbances in the fuel and air control
loops can be minimized by the use of parallel fuel and air control.
• The firing rate demand is directly given as set point to fuel controller
and is given through ratio station to air controller simultaneously. The
fuel and air varies simultaneously as per firing rate demand.
• Combustion control: Metered cross_limited air-fuel ratio control
- Also referred as standard control arrangement
- Includes active safety constraints
Benefits:
- Compensates for fuel and combustion air flow variations
- Provides active safety constraints.
• Three signals are used to balance the fuel-air mixture
- Firing rate demand through steam header pressure
- The fuel flow
- The air flow
The combustion control consists of fuel flow and air flow control loops
that are driven by the firing rate demand signal.
• Maintaining the correct fuel-air ratio also contributes to limiting the fuel
rate to available air and limiting air minimum and maximum to fuel flow, air
leading fuel on load increases and air lagging fuel on load decreases.
• The arrangement also protects against a fan failure or a sticking fuel valve.
Desirable limiting actions:
- Limiting fuel to available air flow
- Minimum limiting of air flow to match minimum fuel flow or to other safe
minimum limit.
- Limiting minimum fuel flow to maintain stable flame.
Metered parallel cross-limited air-fuel ratio control
• If the actual air flow decreases below firing rate demand, then the
actual air flow signal is selected to become the fuel demand by low
selector FY
• If the fuel flow is at a minimum and firing rate demand further
decreases,actual fuel flow becomes air flow demand., because FY-A
will select the fuel signal if it is greater than firing rate demand signal.
• A manual air flow minimum is also available to come into use through
FY-B, such that if fuel flow signal drops below the HIC setting , this
manual setting will become the air flow set point.
• Fuel is minimum limited by separate direct acting pressure or flow
regulator FCV to maintain stable flame.
• These high and low selectors provide a cross limited parallel controlling system.
• When the firing rate demand increases , the high selector provides it as an air flow set
point while the low selector transmits the air flow process variable as the fuel flow set
point.
• So, air flow starts immediately. Fuel flow increase only after the air flow responds.
• When the firing rate demand decreases, the low selector provides it as the fuel flow set
point while the high selector transmits the fuel flow process variable as the air flow set
point.
• Fuel flow starts decreasing immediately, air flow drops only after fuel flow responds.
• It protects from air flow disturbance to lower side, fan failure and fuel valve chokage etc.
• Thus fuel rich condition is always prevented.
• Combustion control with liquid fuel
• Combustion control systems for gaseous fuel hold good for liquid fuel
also.
• Vapourising or atomising of oil is required for proper mixing with
combustion air for complete combustion.
Steam atomisation control:
For oil burners , the fuel has to be prepared for close mixing with air.It
can be done by vapourizing or atomizing. Steam atomising can be done
by mixing the oil with a steam jet in a steam atomizer.
Proper atomization at the burner and hence complete combustion will
be achieved only if oil is kept at constant pressure and viscosity.
When heavy residual oil is burned, it must be continuously circulated
past the burner and back.
Oil flow control for recirculating burner provided with steam atomisation
• The difference between the readings of inlet and return flow meters
can be taken as net oil flow to the burner. This signal is taken for
air/fuel ratio control also.
• The burner back pressure is controlled by the control valve in the
recirculating line.
• The flow controller set point is adjusted by the firing rate demand
signal.
• Atomising steam is ratioed to the net oil flow rate, and the heating
steam is modulated to keep the viscosity constant.
Fuel demand split between oil and gas:
• In multifuel fired boilers more than one fuel will be used.
• Independent air/fuel ratio controls are required for each fuel.
• The biasing stations provide the means of manual control plus
automatic control, with bias of one fuel with respect to the other.
• If gas fuel is at variable pressure, a pressure control valve is installed
upstream to the flow sensor.
• Assuming that both the fuels require the same ratio of air and the
total air is distributed to the burners as required(In practice it is not
so).
• Most of the time the ratio of air required to the fuel will be different
for gas and oil. Design of burners also vary between these two fuels.
So, it is always necessary to have independent air/fuel ratio controls
for each fuel.
Combustion control with solid fuel
The sized coal is transferred from coal storage to coal bunkers located
at an elevation. Coal is fed from bunkers to boiler system by gravity.
There are three basic coal burning methods:
1. On a grate burning
2. Fluidised bed burning
3. In suspension or pulverized coal burning
On a grate burning
For grate type burning, coal needs no further preparation and flows by
gravity to a coal stoker hopper. Usually the amount of coal admitted to the
hopper is weighed at this point. But it can’t be used for control purposes
because of the time lag between the measurement and actual firing in the
boiler. Here, fuel control systems are open loop where a control signal
positions a coal feeding device directly.
A spreader stoker consists of a coal hopper on the boiler front with
air jets or rotating paddles that flip the coal into the furnace, where a
portion burns in suspension and the rest drops to a grate. Fuel control
system is open loop which positions a feeder lever that regulates coal to the
paddles.
Combustion air is admitted under the grate and is adjusted by a single
control device. This amount of air is called ‘Primary air’. To increase the
turbulence and complete the combustion, ‘secondary combustion air’
is added as jets of over fire air above the grate.
Combustion control for metering systems
For ‘metering’ or ‘measured’ types of systems inferential measurement of
the fuel input is necessary. Common form of inferential relationship that
defines combustion conditions is the steam flow/air flow relationship.
Steam flow is the usual measurement of heat output. If the combustion air is
proportional to the steam flow, certain combustion conditions will exist
producing a certain boiler efficiency.
Steam flow heat output can be divided by efficiency to get heat input. Thus
steam flow is an inferred measurement of heat input and when correlated to
combustion air flow, implies a relationship between fuel input and air flow.
Series ratio control systems as well as parallel positioning control systems are
adopted.
Fluidized bed burning
A bed of material is fluffed into a fluid mass by high velocity air applied
from bottom of the bed. An adjustable rate feeder admits the coal to
the fluidised bed.
Fluidised air is the primary combustion air with secondary air added as
required to assure complete combustion.
Types of atmospheric fluidised bed boilers: - Bubbling bed and
circulating bed
a)Bubbling bed fluidised bed boilers:
The bed is located at the bottom of the furnace where a significant
portion of the steam generating tubes are buried in the bed.
The fluidizing air, a portion of the total combustion air from air
preheater is admitted below a high density fluidised bed.
The bed is fluffed to the bubbling condition by the admitted air. The
carryover from the bubbling bed to the combustion chamber is low and
there will be a large amount of unburned fuel in the bed always.
So,it is better to tackle the load changes by controlling combustion air
flow.
The bed temperature is affected by the fuel to combustion air ratio and
by draining or adding of materials to the bed. It can be used for
controlling purpose.
Flue gas analysis can be used to trim the secondary air flow in addition
to the bed temperature trimming.
Net result is a lowering of primary or fluidizing air but no change in
total air flow.
b)Circulating bed fluidised –bed boilers:
We can reduce the density of the fluidised bed by increasing the
velocity of fluidizing air. The bed volume expands when the velocity of
fluidizing air is above bubbling condition. A large amount of bed fuel
leave the bed and are carried over to be collected and circulated back
into the bed.
The primary combustion air enters at the bottom of the combustor as
fluidizing air. After partial combustion, the hot gases and burning
particles transfer heat to the feed water to generate saturated steam.
Then they enter into a hot cyclone particle separator.
The hot gases coming out of the cyclone passes through superheater
and economiser.
The controls are similar to bubbling bed type boiler.
In suspension or pulverised coal burning
Coal is supplied to the pulverizer from coal bunker through adjustable
coal feeder. The primary air stream transports the ground fine powder
coal to the burner. The temperature of the primary air is controlled to
ensure that the coal-air mixture is not carrying moisture to the burner.
Fuel control : The amount of coal to the boiler is controlled as per the
fuel demand. The coal flow rate control is normally done at the inlet of
the mill. Weighing is done by a belt weigher. This weight is multiplied
with the speed of the conveyor measured to get the coal flow rate.
The time delay between feeding point of coal to the conveyor and the
coal mill and the coal mill delay is taken care of by a lag component in
the control system.
Fuel control with parallel pulverisers:
• The fuel demand is shared as per the bias settings among the
pulverisers.
• The flow of the primary air that conveys pulverized coal is
manipulated first. The primary air flow becomes set point of coal feed
controller of the pulveriser.
• They act independently with the ratio set between air and coal.
• Total air flow signal is computed by adding both primary air flow
signals and is used for balancing against fuel demand signal.
Primary air temperature control:
• Primary air temperature control is required as the fuel flows in the
current of primary air and hence to maintain a maximum safe
operating temperature in the pulveriser.Here, cold and hot
combustion air is mixed ahead of the primary air fan to control the
temperature of the coal-air mixture in the pulveriser.
Combustion control:
Similar to that of gaseous fuel
The primary air which carries pulverised coal plus secondary air is
added at the burner to form the total combustion air.
Figure shows a simplified coal –air ratio control where the fuel demand
signal simultaneously controls both fuel and air supply to have faster
response than the case where air leads fuel.
Furnace Draught control
In a boiler, air and fuel flows into the furnace and flue gas flows out. The
force behind this flow is the differential pressure between the gases inside
the furnace and those outside the furnace.
Furnace pressure is referred to as draught or draught pressure.
To move the air through the bed and to produce a flow of gaseous products
of combustion out of the furnace, then through superheater,economiser, air
preheater etc requires a pressure sufficient to accelerate the gases to its final
velocity plus friction losses.
This difference of pressure is called draught whether measured above or
below atmospheric pressure.
Draught inside the furnace is slightly negative.
Normally in a boiler , forced draft fan draws air from atmosphere and
gives a boost to the draft. Liquid fuel and gaseous fuels also add
pressure in the combustion chamber.
After combustion, flue gases travel towards chimney through
economiser, air preheater etc. which are pressure reducing
components.
When chimney is not in a position to develop enough natural draught
to push the complete flue gas , the induced draught fan comes into
picture.Fans and chimney produce pressure in the positive
direction.Friction , turbulence, fuel bed resistance etc will produce
negative pressure.
• Based on the method of movement of air and other gases through
the system, furnaces may be classified as :
- Natural draught
- Forced draught
- Induced draught and
- Balanced draught
Natural draught furnace: uses the stack effect. Gases inside the stack
are less denser compared to the gases outside the chimney.
The gases in the stack will rise creating a vacuum(suction) which will
draw the combustion gases or flue gas out of furnace.
They naturally operate below atmospheric pressure .The draught
produced will be a function of the height of the stack and the flue gas
temperature.
Natural draught is used to supplement the mechanical draught
produced by fans.
Forced draught furnace: uses a fan or blower to force combustion air
through the system by sucking air from atmosphere. Control is
accomplished by regulating the fan speed or damper operation .
-It is operated slightly above atmospheric pressure.
Induced draught furnace: An ID fan takes suction from the boiler flue
gas stream and discharges the flue gas to the stack. It draws gases
through the furnace and combustion air into the furnace – high stacks
are not required.
Control is accomplished by regulating the fan speed or damper
operation.ID fan is operated slightly below atmospheric pressure.
Balanced draught furnace: Equipped with both FD and ID fans. To
control furnace pressure , a balance between in flow and out flow
should be maintained.
It operates at a slightly negative pressures to prevent flue gas leakage
to the surroundings. Very low pressures should be avoided to minimise
air leakage into the furnace and prevent furnace implosion.
FD fan damper is generally manipulated by the air flow controller and
ID fan damper by the furnace pressure controller.
FD fan control(Forced draught control):
Depending on the fuel rate to a particular boiler decided by the master
control, the air requirement and hence the draught to be developed by
FD fan is decided.
Forced draught fan along with the inlet vane control arrangement is
used for combustion air control.Either the furnace inlet air pressure or
total air flow is taken into consideration.
Air flow signal keeping air pressure constant or air pressure signal is an
input to the controller.
The controller output controls the inlet vanes of the FD fan/damper
control. Fan speed control is also possible.
This control takes care of total air requirement demanded by the
master controller.
Furnace pressure(induced draught) : The purpose is to keep the
furnace combustion chamber pressure at a slightly negative value.An ID
fan with damper control is used.
Measurement of furnace draught produced a noisy signal , limiting the
loop gain.
In some cases, it may be required to use only integral control to
stabilize the loop.
Measurement lags will cause stability and interaction problems
Balanced draught control:
In large boilers both FD fan and ID fan are put together in operation.
As FD creates positive pressure, in the beginning of the flow path.
Combustion chamber pressure is slightlty negative to avoid gas leakage
or excessive cold air infiltration- pressure at the top of the furnace is
kept slightly negative.
• FD fan loop is used to satisfy the total air flow requirement and ID fan
loop is used for controlling furnace draught. Care should be taken to
minimise interaction between these two loops.
• In the fig. shown,both FD and ID dampers are operated in parallel.Air
flow controller moves FD fan damper directly and ID fan damper with
a time lag.
• Air flow controller signal to furnace draught controller is a
feedforward signal. Hence whenever the air flow to the furnace is
changed , the out flow from the furnace will also start changing with a
dynamic compensation lag.The mismatch is finally corrected by the
furnace pressure controller .