UNIT - IV BOILER CONTROL II PULVERISED FUELS

Prepared By: U.Rajkanna, AP/EIE Page 1

UNIT - IV

BOILER CONTROL – II Burners for liquid and solid fuels – Burner management system – Furnace safety interlocks –

Coal pulverizer control– Combustion control for liquid and solid fuel fired boilers– air/fuel ratio

control – fluidized bed boiler – Cyclone furnace

PULVERISED FUELS: ❖ Pulverised fuel systems are used for large capacity plants and using low cost fuel (low

grade) as it gives higher thermal efficiency and better control as per the load demand.

❖ In a pulverised fuel firing system the coal is reduced to a fine powder with the help of

grinding mill and then projected into combustion chamber with help of hot air.

❖ The amount of air required (Secondary air) to complete the combustion is supplied

separately to the combustion chamber.

❖ The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and

air through combustion.

❖ The amount of air which is used to carry the coal and to dry it before entering into

combustion chamber is known as primary air.

❖ The efficiency of pulverised fuel firing system mostly depends on the size of the powder.

BURNERS FOR LIQUID AND SOLID FUELS:

❖ The efficient utilization of pulverised coal depends on a large extent upon the ability of

the burners to produce uniform mixing of coal and air and turbulence within the furnace.

❖ The air which carries the pulverised fuel in the furnace through the burner is primary air

and remaining secondary air required for complete combustion is admitted separately

around the burner or elsewhere in the furnace.

❖ The pulverised coal burners should satisfy the following requirements

o It should mix the coal and primary air thoroughly and project the same in the

furnace properly with secondary air which is generally added around the burner.

o It should create proper turbulence and maintain stable combustion of coal and air

throughout the operating range of the plant

o It should control the flame shape and its travel in the furnace. This is generally

done by the secondary vanes and other control adjustments provided in the

burner.

o The mixture of coal and air should move away from the burner at a rate equal to

flame travel to avoid the flash back with the burner.

o The burner should also be provided with adequate projection against

overheating, internal fires and excessive abrasive wear.

❖ The factors affect the performance of the pulverised fuel burners are

o The characteristics of the fuel used

o Fitness of the powdered coal

o Volatile matter

o The geometry of the burner

o Place of mixing the fuel and air


o Proportions of primary and secondary airs

o Furnace design

o Pattern of load changes

❖ The classification of burners is made on the rapidity of burning the coal and air in the

furnace

o Long flame or U flame or Streamlined burner

o Short flame or turbulent burner

o Tangential burner

o Cyclone burner

Long flame or U flame or Streamlined burner

❖ The arrangement of primary air and coal flow and supply of secondary air is shown in

figure 1.

Figure 1. Long Flame Burner

❖ Tertiary air is supplied around the burner to form an envelope around primary air and

fuel to provide better mixing

❖ The burner discharges air and fuel mixture vertically in thin flat streams with practically

no turbulence and produces a long flame.

❖ Heated secondary air is introduced at right angles to the flame which provides necessary

mixing for better and rapid combustion.

❖ Furnaces for low volatile coal are equipped with such burners to give a long flame path

for slower burning of coal particles.

❖ The longer path provides more time to burn and it is necessary to control the velocity in

this zone (tip velocity is limited to 25m/s).

❖ Less heat of ignition is available due to low volatile content and it is necessary to reduce

the cooling effect from the wall tubes in the ignition zone by using a refractory belt

around the furnace or by refractory front wall.


❖ Generally low volatile coals have higher fusion temperature than bituminous coal and

therefore higher furnace ratings are permissible.

Short flame or turbulent burner

❖ The turbulent burners are usually set into furnace walls and project the flame horizontally

into furnace as shown in figure 2.

❖ The fuel air mixture and secondary hot air are arranged to pass through the burner in

such a way that there is good mixing and the mixture is projected in highly turbulent form

in the furnace.

❖ Due to high turbulence created before entering the furnace, the mixture burns intensely

and combustion is completed in a short distance.

❖ This burner gives high rate of combustion compared with other types.

❖ The velocity at the burner tip is as high as 50m/s

❖ The bituminous coal is successfully used with this burner.

❖ By proper adjustments a long penetrating flame or short intensely hot flame can be

produced.

❖ All modern plants use this type of burner

❖ This is generally preferred for high volatile coals.

Figure 2. Turbulent Burner Figure 3. Tangential Burner

Tangential burner

❖ Tangential burners are set in the furnace as shown in figure 3 and discharge the fuel-air

mixture tangentially to an imaginary circle in the centre of the furnace.

❖ The swirling action produces sufficient turbulence in the furnace to complete the

combustion in a short period and avoids the necessity of producing high turbulence at

the burner itself.

❖ High heat release rates are possible with this method of firing.

❖ This type of burner is sometimes constructed with tips that can be angled through a

small vertical arc (±30) so as to raise or lower the position of the turbulent combustion

region in the furnace.

❖ This arrangement controls the temperature of the gases at the furnace aperture and

maintains constant superheat temperature of the steam as the load varies.

❖ When the burners are tilted downward, the furnaces gets filled completely with the flame

and the furnace exit gas temperature is reduced as the furnace absorption is greater.


❖ This reduces the heat given to the superheater.

❖ The reverse is also true when the burners are tilted upwards.

❖ The usual limit of tilt ±30 ͦis sufficient to provide 100ᵒC difference in the furnace gas exit

temperature.

❖ In a pulverised fuel firing system, the fluid ash carried in the gas stream is likely to be

deposited on exposed metal surfaces and solidify thereon as slag.

❖ Sufficient heat absorbing surface must be provided to cool the molten ash by radiation

below its softening temperature before it comes into contact with metallic surfaces.

❖ Thus, in the large units, a tall furnace is used and in very large units, it becomes

economical to sub-divide the furnace either by water cooled division walls or by

arranging side by side separately cased furnaces.

Cyclone burner

❖ The major disadvantages of utilising pulverised coal as fuel are:

o The capital and running cost of pulverised mills are considerable

o Nearly 70% of the ash in coal goes with exhaust gases in form of ‘fly ash’ and it

requires expensive dust collectors in the gas circuit

❖ The cyclone burner is a new method of burning coal particles in suspension which

overcomes the disadvantages associated with pulverised fuel system as mentioned

above.

❖ Basically, this burner was designed to burn crushed low grade bituminous coal that

normally have a high content of low fusing temperature ash.

❖ The first commercial cyclone furnace boiler was designed to burn bituminous coal.

❖ The cyclone burner is horizontal cylinder of water cooled construction 2 to 3 meters in

diameter and 2.5 m in length as shown in figure 4.


Figure 4. Cyclone Burner

❖ The inside part of the cyclone cylinder is lined with chrome ore.

❖ The horizontal axis of the burner is slightly deflected towards the boiler.

❖ It is externally arranged to the boiler furnace and equipped with a single scroll type inlet

at one end and a gas discharge throat into the boiler at the other end.

❖ The coal used in this burner is cherished to 6mm maximum size and blown into a

cylindrical cyclone furnace.

❖ Air at 80cm water pressure and coal admitted tangentially to the cylinder at outer end

creates strong and highly turbulent vortex.

❖ As the coal with air moves from the front to rear, secondary air is introduced tangentially

as shown in figure to complete combustion

❖ Extremely high heat liberation rate and the use of preheated air cause high temperature

(2000 ᵒC) in the cyclone.

❖ The fuel supplied is quickly consumed and liberated ash forms a molten film flowing over

the inner wall of the cylinder.

❖ The molten ash flows to an appropriate disposal system as the horizontal axis of the

burner is tilted.

❖ Advantages:

o Simplified coal crushing equipments can be used instead of costly pulverised

mills

o All the incombustible are retained in cyclone burner

o As the forced draught is used with this type of burner it can be operated with less

excess air. (Excess air required reduced to 15%)

o Easy control of combustion

o It can be used for low grade fuels, reduces the size of steam generatoe and limits

fly ash emission.

BURNER MANAGEMENT SYSTEM (BMS):

Safety Instrumented System ❖ Takes a process to a safe state when predetermined (dangerous) conditions are violated

(e.g. ESD - Emergency Shutdown)

❖ Permits a process to move forward in a safe manner when specified conditions allow

(e.g. BMS)


❖ Takes action to mitigate the consequences of an industrial hazard (e.g. FGS - Fire &

Gas System)

Purpose of BMS:

❖ To inhibit startup when unsafe conditions exist.

❖ To protect against the unsafe operating conditions and admission of improper quantities

of fuel to the furnace.

❖ To provide the operator with status information – operator assistance

❖ To initiate a safe operating condition or shutdown interlock if unsafe condition exists.

❖ As per NFPA 85, “the BMS is a control system dedicated to boiler furnace safety and

operator assistance”.

Why implement BMS in an SIS (Safety Instrumented System)

❖ Increased safety

❖ Increased system availability

❖ Regulatory compliance

Is BMS a SIS?

❖ Burners, furnaces and boilers are very critical and complex systems.

❖ There is evidence that OEMs and end users who wish to comply with standards

(IEC/NFPA), or to meet certain insurance requirements, will have to classify burner

management systems as safety instrumented systems, to achieve certification by a third-

party agency.

❖ In the process industry, a BMS is included in the IEC 61511 definition, although not by

direct reference. There is also no exclusionary clause.

❖ BMS are defined as SIS if they contain sensors, a logic solver and a final control

element according to IEC 61511.

❖ All safety critical processes must be analyzed and their potential risk determined.

❖ By considering a BMS as a SIS, companies can ensure that these systems are

designed, maintained, inspected and tested per both the applicable prescriptive

standards (API, NFPA, etc.) as well as the latest SIS performance-based standards

(ANSI/ISA, and IEC)

Hardware Design of BMS (PLC Based):

❖ Design Features:

o Each PLC based BMS should incorporate a number of design techniques which

help detect and act upon unsafe failure modes which can occur in any

microprocessor based system

o These design features include

▪ Critical Input Checking

▪ Critical output channel monitoring

▪ Electro mechanical master fuel trip relay

▪ Redundant Watchdog timers

▪ Low water cut-out monitoring during blow down


Single Burner Management System

❖ Provision for Multiple fuel firing

▪ Capped gas input during curtailment

▪ Changeover from gas to oil at any load

▪ Simultaneous firing of waste and fossil fuels

❖ Redundant scanners, change scanner with fuel

❖ Single or multiple burner applications

❖ Integration of BMS with SCADA

❖ For Operators

▪ Clear written messages to indicate status, required operator interaction, trip/alarm

indication

▪ High visibility of display

▪ Ease Troubleshooting

▪ Priority messages

Software Design:

3 Main Logic Part to a BMS System

❖ 3 fundamental items in BMS are

o States & Transitions – When to move from one to another

o Outputs – Valve Positions defined for each State

o Trips – Including which is active during each State


BMS States:

States of BMS

BMS State Transition Diagram


Interlocks & Trip Logics in Burner:

Purge Interlock

Igniter Interlock


Main Flame Interlock

Single Burner Main Fuel Trip


Outputs– Defined Per State:

Advantages of the System:

❖ Flexibility

❖ Reliability

❖ Flame scanners

❖ Application specific

❖ Not restricted to fuel

FURNACE SAFETY INTERLOCKS: ❖ Interlocks:

o Interlock is a feature that makes state of two mechanisms or functions mutually dependent.

o To prevent the system from undesired state. ❖ Interlocks are mainly used in boiler operation for the following:

o Safety of men and machine o Startup and shut down of boiler o Sequencing of burner

Safety Interlocks: ❖ Fuel Admission Interlock:

o Fuel is not supposed to be admitted initially unless the furnace is thoroughly air purged.

❖ Low Air Flow: o Due to any reason if the air flow goes down below some minimum level, the fuel

has to be shut off. ❖ Lower Fuel Supply:


o If the quantity of fuel flow is vey less or pressure is below the safe level, the fuel has to be shut off to avoid unstable flame or backfire.

❖ Loss of Flame: o Fuel has to be totally shut off if loss of flame is detected either in the furnace or in

the individual burners. ❖ Fan Interlock:

o For normal operation of the boiler both ID and FD fans are to run normally. Stoppage of one may require the stoppage of the other one also.

❖ Low Drum Level: o Low drum level requires the stoppage of fuel. This is normally an optional one.

❖ High Combustibles: o If the high combustibles are found in the waste gas, it may be required to shut

down fuels and check for the reasons.

COAL PULVERIZER CONTROL: ❖ Fuel Control:

o As per the fuel demand, the amount of coal to the boiler is controlled. o The coal flow rate control is normally done at the inlet of the mill. o Weighment is done by using any one of the familiar belt weighers. o By measuring the speed of the conveyor and multiplying it with the weight per

meter measured by belt weigher, the coal flow rate is calculated. o The time delay occurring between the feeding point of coal to the conveyor and

that of coal mill plus coal mill delay is taken care of by introducing a lag component in the control system as shown in below figure 5.

Figure 5. Pulverised Coal Fuel Control

❖ Fuel Control with Parallel Pulverisers: o A simplified control schematic in case of two pulverisers operating in parallel is

shown in below figure 6. o The fuel demand is shared as per the bias settings among pulverisers. o The flow of primary air that conveys pulverised coal is manipulated first. o The primary air flow becomes set point of coal feed controller of the pulveriser. o They act independently with ratio set between air and coal.


o The total air flow signal is computed by adding both primary air flow signals and is used for balancing against fuel demand signal.

Figure 6. Fuel Control with Parallel Pulverisers

COMBUSTION CONTROL FOR LIQUID FUEL FIRED BOILERS:

Steam Atomisation Control: ❖ The main difference in oil burners is the method of preparing the fuel for close mixing

with air. ❖ This can be done by vapourising or by atomizing. ❖ The commonly used way is steam atomizing. ❖ This can be accomplished by mixing the oil with a steam jet in a steam atomizer. ❖ Proper atomization at the burner and therefore complete combustion will be achieved

only if oil is kept at constant pressure and velocity. ❖ When heavy residual oil is burned, it must be continuously circulated past the burner

back. ❖ 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. ❖ Figure 7 shows the oil flow control for recirculating burner provided with steam

atomization.


Figure 7. Oil Flow Control for Recirculating Burner Provided with Steam Atomisation

❖ The burner back pressure is controlled by the control valve in the recirculating line, whereas the flow controller set point is adjusted by firing rate demand signal.

❖ The firing rate is controlled by an alteration in the opening of the burner orifice. ❖ Atomising steam is ratioed to the net oil flow rate, and the heating steam is modulated

to keep the fuel oil viscosity constant.

Fuel Demand Split Between Oil and Gas: ❖ In multifuel fired boilers more than one fuel will be used. ❖ Figure 8 illustrates the controls required when the fuel demand is split between gas and

oil on a closed loop basis.

Figure 8. Fuel Demand Split Between Oil and Gas


❖ The biasing stations provide the means of manual control plus the ability of automatic control, with bias of one fuel with respect to the other.

❖ If gas fuel is at variable pressure, a pressure control valve is frequently installed upstream to the flow sensor.

❖ The total fuel to the air flow controller is shown simply assuming that both the fuels require the same ratio of air and the total air is distributed to the burners as required.

❖ But in actual practice it is not so. ❖ Since one of the requirements ultimately is to have fuels ratioed to combustion air, any

totalisation of fuel for control purposes should be on ‘air required for combustion’ basis. ❖ Hence even if splitting is done as shown in the figure between gas and oil, the

combustion controls can be different for gas and oil. ❖ Most of the time the ratio of air required to the fuel will be different for gas and oil. ❖ Also the designs of burners vary between these two fuels. ❖ Hence it is always necessary to have independent air/fuel ratio controls for each fuel.

COMBUSTION CONTROL FOR SOLID FUEL FIRED BOILERS: ❖ Three basic coal burning methods are

o On a grate burning o Fluidised bed burning o 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 stoker

hopper. ❖ In many cases a weighing system is installed to weigh the amount of coal that is

admitted to the stoker hopper. ❖ This measurement of coal cannot be used for controlling purposes because of the time

lag involved between measurement and actual firing in the boiler. ❖ Hence coal has to be treated here as an unmeasured fuel, ❖ In such cases fuel control system is open loop, wherein 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 furnace, where a portion burns in suspension and the rest drops to a grate.

❖ Combustion air is admitted under the grate. ❖ There is no way to control fuel to a separate stoker except in an open loop manner by

positioning a feeder lever that regulates coal to the paddles. ❖ Combustion air is admitted through fuel bed on the grate from underneath and is

adjusted by a single control device. ❖ This amount of air is sometimes called primary air. ❖ In-order to increase the turbulence and complete the combustion, secondary combustion

air is added as jets of over fire air above the grate. ❖ Stokers are of three general types having basic subtypes as shown in below table 1

Table 1. Stoker Types

S.No General Types Basic Subtypes

1 Spreader stoker i)Dump grate ii)Travelling grate iii)Vibrating grate

2 Underfeed stoker i)Single-retort


ii)Multiple-retort

3 Overfeed stoker i)Chain grate ii) Vibrating grate

Fluidised bed burning: ❖ In this process, 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. ❖ For fluidised bed combustion, the fluidised air is the primary combustion air with

secondary air added as required to assure complete combustion. ❖ Normally the process operates at atmospheric pressure. ❖ Two types of atmospheric fluidised bed boilers are

o Bubbling Bed Fluidised Bed Boiler o Circulating Bed Fluidised Bed Boiler

❖ Bubbling Bed Fluidised Bed Boiler: o Following Figure 9 shows simplified schematic diagram of a bubbling bed

fluidised bed boiler. o The bed is located at the bottom of the furnace where a significant portion of the

steam generating tubes is buried in the bed. o The fluidizing air, a portion of total combustion air from air pre-heater, is admitted

below a high density fluidised bed. o The bed is fluffed to the bubbling condition by the admitted air. o As because the bed is only bubbling, the carryover from the bed to the

combustion chamber is low. o There will be a large amount of unburned fuel inventory in the bed always. o Hence it is better to tackle the load changes by controlling combustion air flow. o The bed temperature, which gets affected by the fuel to combustion air ratio and

by draining or adding of materials to the bed, can be used for controlling purpose. o The key differences from other boilers lie in the control of bed temperature and

fuel to combustion air ratio.

Figure 9. Bubbling Bed Fluidised Bed Boiler

o Combustion Control: o Following figure 10 shows a combustion control for fluidised bed boiler with bed

temperature trimming.


Figure 10. Bed Temperature Trim of Air Flow Control

o In addition, it is suggested that the flue gas analysis can be used to trim the secondary air flow.

o The net result is that there will be no change in total air flow, but with a lowered amount of primary or fluidizing air.

❖ Circulating Bed Fluidised Bed Boiler o The high density fluidised bed can be made low density one by increasing the

velocity of the fluidizing air. o The bed volume expands when the velocity of fluidizing air is above that of the

bubbling condition. o A large amount of bed fuel leave the bed and are carried over to be collected and

circulated back into the bed. o Following figure 11 shows a circulating fluidised bed boiler.

Figure 11. Circulating Bed Fluidised Bed Boiler

o The primary combustion air enters at the bottom of the combustor as fluidizing air.


o After partial combustion, the hot gases and burning particles transfer heat to the feed water to generate saturated steam.

o Then they enter into a hot cyclone passes through superheater and economiser afterwards.

o The control is similar to that of bubbling bed type boiler.

In suspension or pulverized coal burning: ❖ Coal has to be pulverized for burning it in suspension. ❖ Coal is ground to a fine powder with the help of coal mill in the pulveriser unit. ❖ Different types of coal mills are in operation. ❖ There may be two or more pulverisers operating in parallel per boiler. ❖ Figure 12 demonstrates a direct firing system for pulverized coal. ❖ Coal is supplied to the pulverizer from raw 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 so as to ensure that the coal-air mixture

is not carrying moisture to the burner.

Figure 12. Direct Firing System for Pulverised Coal

AIR/FUEL RATIO CONTROL: ❖ Normally Air/Fuel ratio control is known as series control ❖ This means the change in air flow rate occurs as per ratio set only after the change has

occurred in fuel flow rate. ❖ But in boilers used in power plants, fuel and air should be controlled in parallel rather

than in series for safety reason ❖ This is necessary because a lag of only one or two seconds in measurement or

transmission will seriously upset combustion condition in a series system ❖ This can result in alternating periods of excess and deficient combustion air


❖ One such parallel fuel ratio control used in boilers is shown in below figure 13 ❖ Firing rate demand from the master control of the boiler is given as set point value to the

fuel controller. ❖ Fuel flow rate changes as per the requirement of the master control. ❖ The same firing rate demand signal is simultaneously given to the air controller through

a ratio setter so that the air flow rate is also controlled along with fuel flowrate maintain the proper ratio required for efficient combustion.

Figure 13. Parallel Fuel Air Ratio Control

REFERENCE:

❖ Burner Management System:

http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/Delta

V%20SIS/SIS%20Presentations/Burner%20Management_straightforward%20approach.

pdf

❖ Krishnaswamy.K and Ponnibala.M., “Power Plant Instrumentation”, PHI Learning Pvt.

Ltd., New Delhi, 2011

❖ Arora.S.C, Domkundwar.S, Domkundwar.A.V, “A course in Power Plant Engineering”,

DhanpatRai & Co.(P) Ltd, Fifth revised and enlarged edition, 2006.

http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/DeltaV%20SIS/SIS%20Presentations/Burner%20Management_straightforward%20approach.pdf

