UNIT 4

Control Loops in Boiler

Control LoopsDrum Level Control– Single element– 2 element– 3 element control

steam Temperature Control– Attemperators– Desuperheaters

Deareator control

Combustion control– Air/Fuel Ratio control– Furnace draft control

Super Heaters

• Heating the steam further from saturation temperature is called superheating

• A superheater is a device used to convert saturated steam or wet steam into dry steam used in steam engines or in processes, such as steam reforming.

• Super heater is generally employed between boiler and turbine unit.

• It is used to increase the temperature and pressure of the working steam

• It is also used to increase the power generation in steam power plants.

Need of Superheater temperature control

• To Minimize thermal stress on boiler• To Minimize thermal stress on turbine

Challenging in steam temperature control

• Temperature is a slow process• Has time lag• Has long dead time • Nonlinear properties

Types of control

• Fire side control• Water side control

EI1002 Power Plant Instrumentation 6

DeSuperHeater

Fire Side Control Strategies

Water side control strategies

Spray nozzle

Spray nozzle

Fire side control

EI1002 Power Plant Instrumentation 17

Burner position

317-St.Joseph's College of Engineering 19

Control loops

• Single stage steam temperature control

• Simple feedback loop-reject the disturbance after it affects the process

Fb control loop-Plant model

• Cooling water from condenser• Non-return value to retrict the steam flow into water tube

Two stage control loop

Plant model

Cascade steam temperature control

CASCADE CONTROL SYSTEM

Feed forward plus feedback steam temperature control

Steam pressure control

• Balance between the heat inflow and heat outflow of a boiler indicated by drum pressure.

• Change in firing rate – change the steam pressure after some delay.

• Steam pressure in bus is control by firing rate in the boiler

Steam header

Steam pressure control loop

• Controller controls the firing rate demand.

• Some boiler works constantly ,some works by controlling,some boiler for

emergency(open/close)

Air-fuel circuit control

• 1)combustion control• 2)furnace draft control

Combustion control

Types:• Oil or gas fuel to air control• Solid fuel to air control

Oil or gas fuel to air controlCombustion control

• Simple fuel control by firing demand

Combustion control – single point positioning(jack shaft control)

• Controller controls the control value of fuel.

• Mechanical link established between control value and combustion air damper.

• But it is very difficult to linearize the movement of control value and damper due to different flow characteristics

Combustion control-parallel positioning

• Single controller gives control signal to both control values and air damper

Combustion control:series fuel –air ratio control

Combustion control:parallel fuel –air ratio control

• Problem if the Combustion Air Loop is Slow• blower that responds slowly to control commands, COc, relative to the valve (the time

constant of the blower "process" is where the firing demand, SPw, suddenly increases. The fuel flow valve responds quickly, increasing fuel feed to the burner. The ratio relay will receive SPw and raise the set point of the combustion air mass flow rate, SPc, so the two streams can remain in ratio.If the air blower response is slow, however, a fuel rich environment can temporarily develop. That is, there will be a period of time when we are below the desired 5% to 20% of excess air (below the 105% to 120% of theoretical or stoichiometric air) as we wait for the blower to ramp up and deliver more air to the burner.If there is insufficient air for complete combustion, then carbon monoxide and partially burned fuel will appear in the exhaust stack. As such, we have a situation where we are wasting expensive fuel and violating environmental regulationsmuch larger than that of the valve)

OVERRIDE CONTROL• The second ratio relay receives the actual measured combustion air mass flow

rate, PVc, and computes a matching fuel flow rate based on the design air/fuel ratio

Combustion control:metered cross limited fuel –air ratio control

Metered parallel –cross limited air-fuel ration control

Steam automisation control

fuel demand split between oil and gas

Bubbling bed fluidised-bed boiler

Stack temperature optimisation

Optimisation using stack gas analysers

Combustion control

OVERFEED –TRAVELING GRADE

UNDER FEED• Underfeed stokers can be designed for use

with either anthracite or bituminous coal,

FLUIDZED• A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding

vessel) is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids; such as the ability to free-flow under gravity, or to be pumped using fluid type technologies.

• The resulting phenomenon is called fluidization.• A fluidized bed consists of fluid-solid mixture that exhibits fluid-like properties. As such, the upper surface

of the bed is relatively horizontal, which is analogous to hydrostatic behavior. The bed can be considered to be a heterogeneous mixture of fluid and solid that can be represented by a single bulk density.

• Furthermore, an object with a higher density than the bed will sink, whereas an object with a lower density than the bed will float, thus the bed can be considered to exhibit the fluid behavior expected of Archimedes' principle. As the "density", (actually the solid volume fraction of the suspension), of the bed can be altered by changing the fluid fraction, objects with different densities comparative to the bed can, by altering either the fluid or solid fraction, be caused to sink or float. This is illustrated in the following video:https://www.youtube.com/watch?v=3BqVFGCUviY.

• In fluidized beds, the contact of the solid particles with the fluidization medium (a gas or a liquid) is greatly enhanced when compared to packed beds. This behavior in fluidized combustion beds enables good thermal transport inside the system and good heat transfer between the bed and its container. Similarly to the good heat transfer, which enables thermal uniformity analogous to that of a well mixed gas, the bed can have a significant heat-capacity whilst maintaining a homogeneous temperature field.

• Application[edit]• Fluidized beds are used as a technical process which has the ability to promote high levels of contact

between gases and solids. In a fluidized bed a characteristic set of basic properties can be utilised, indispensable to modern process and chemical engineering, these properties include:

• Extremely high surface area contact between fluid and solid per unit bed volume• High relative velocities between the fluid and the dispersed solid phase.• High levels of intermixing of the particulate phase.• Frequent particle-particle and particle-wall collisions.

1. Fuel Input - Fuel and limestone are fed into the combustion chamber of the boiler while air (primary and secondary) is blown in to “fluidize” the mixture. The fluidized mixture burns at a relatively low temperature and produces heat. The limestone absorbs sulfur dioxide (SO2), and the low-burning temperature limits the formation of nitrogen oxide (NOX ) – two gases associated with the combustion of solid fuels.2. CFB Boiler - Heat from the combustion process boils the water in the water tubes turning it into high-energy steam. Ammonia is injected into the boiler outlet to further reduce NOX emissions.3. Cyclone Collector - The cyclone is used to return ash and unburned fuel to the combustion chamber for re-burning, making the process more efficient.4. State-of-the-Art Air Quality Control System - After combustion, lime is injected into the “polishing scrubber” to capture more of the SO2. A “baghouse” (particulate control device) collects dust particles (particulate matter) that escape during the combustion process.5. Steam Turbine - The high-pressure steam spins the turbine connected to the generator, which converts mechanical energy into electricity.6. Transmission Lines - The electricity produced from the steam turbine/generator is routed through substations along transmission lines and delivered to distribution systems for customer use.

Direct firing system for pulverized coal

FD fan control(Forced draft control)

Balanced draft control