steam generator part 3

STEAM GENERATORBOILER

Prepared by:Mohammad Shoeb SiddiquiSr. Shift SupervisorSaba Power Plant

STEAM GENERATORBOILER


Part 3


Steam Generator (Boiler)Hello,I am trying to explain about Steam Generator (Boiler) in this session, due to length of said presentation, I am deciding to divide it in three parts.Part 1 cover the “Introduction & Types of Steam Generator”Part 2 cover about the “Parts of Steam Generator and Its Accessories & Auxiliaries” and ThisPart 3 cover the “Design, Efficiency, Performance & Protection”

Design, Efficiency, Performance & Protection”

Classification Of Boiler, Fundamental Of Boiler Design, Efficiency, Safety & Environment Control

Prepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor

Stean Generator (Boiler)


1. Mode of circulation of working fluid.2. Type of fuel.3. Mode of firing.4. Nature of heat source.5. Nature of working fluid.6. Position of the furnace.7. Type of furnace.8. Boiler size.9. Materials of construction.10. Shapes of tubes and their spatial position.11. Content of the tubes.12. Steam pressure.13. Specific purpose of utilization.14. General shape.15. Manufacturers trade name.16. Special features. Prepared by:

Mohammad Shoeb SiddiquiSr. Shift Supervisor

Boilers are classified on the basis of Fundamental Of Boiler Design

Mode of classification TypeCirculation 1. Natural Circulation Boiler,

2. Forced CirculationTube shape and position (Depending on the form of tubular heating surface)

1. Straight tube boiler. 2. Bent tube boiler.

Tube shape and position (Depending on the inclination of tubular heating surface)

1. Horizontal Boilers, 2. Vertical Boilers, 3. Inclined Boilers

Furnace position 1. Externally fired furnace 2. Internally fired furnace

Tube Contents 1. Fire tube boilers, 2. Water tube boilers

Steam pressure 1. Lowe pressure Boilers, 2. Power Boilers, 3. Miniature Boilers

Mode of firing 1. Fired boilers, 2. Non-fired boilers

Heat source 1. Fuel fired boiler, 2. Waste heat boiler3. Electrical powered boiler, 4. Nuclear powered boilers

Nature of fuel 1. Coal fired, 2. Gas fired, 3. Oil fired,4. Wood fired, 5. Bio gas fired

Type of Furnace 1. Dutch oven boiler, 2. Open boiler, 3. Scutch boiler, 4. Screened boiler, 5. Twin boiler

Fundamental Of Boiler Design

Fundamental of Boiler Design

In A Steam Generating Unit Two Distinct Fundamental Processes Take Place:-

1. Conversion of the potential energy of the fuel into thermal energy.

2. Transfer of this liberated thermal energy to the working fluid to generate steam for useful purpose


Following factors must be taken into account in the design consideration of a boiler:

1. Service requirements.2. Load characteristics.3. Fuel characteristics.4. Mode of fuels burning.5. Hydrodynamics of gas flow.6. Feed water quality7. Furnace size, shape and material of constructio

n.8. Type of furnace bottom.9. Boiler proper.10. Boiler operation.11. Capital investment. Prepared by:



Load Characteristic:The boiler designer should essentially

consider the following load characteristics:

1. Maximum load, normal load and minimum load.

2. Load factor.3. Nature of load – constant or fluctuating

.4. Duration time of each load rate.



Fuel Characteristics: From these very characteristics, a boiler

designer gets the knowledge of the heat value available from the fuel as well as its specific properties such as:-

a). Ash content and the percent of volatile matter.b). Nature of ash and its fusion point.c). The presence of corrosive agents as sulfur and

vanadium that will dictate the flue gas exit temp as well as the material of construction of the heating surfaces of the boiler to avoid the problem of corrosion and slugging.



Mode of Fuel Burning It is the capacity of the fuel

burning device that controls the rate of fuel input which in turn determines the furnace volume and its design specification



Gas Flow Characteristic The gas flow through the boiler is

affected by the differential pressure between the combustion products in the furnace core and the flue gases at the boiler exit. This pressure difference, called draught (draft) may be affected by natural means or by mechanical means to supply the necessary primary and secondary air to sustain and control fuel combustion



Feed Water Quality The presence of dissolved solids and

gases, suspended matter and organic contaminants in feed water cause corrosion, scaling, priming and foaming that effectively impair the performance of a boiler.

Feed water quality, together with other factors, influences the design of drum internals, steam separator and steam washer etc.



Furnace

The furnace volume must be sufficient to maintain the necessary heat release rate and further temperature while the combustion space should be sufficient to contain the flame so that it does not directly hit the water walls.



Boiler ProperThe factors control the design of boiler proper

are:- The operating pressure and temperature. The quality of steam – whether the steam

required should be wet, dry or superheated. If wet steam is required, the designer may do away with the separators and super-heaters.

Layout of heating surface – The prime aim of boiler designer is to obtain the best disposal of heating absorbing surface within the limitations of space as dictated by the furnace and other components.



Boiler Proper

Heating surface requirements – These depend upon the duty of the element heat exchangers such as primary evaporators, secondary evaporators, super-heaters radiant and connective re-heater, economizer and air pre-heater.

Circulation of steam and water – Natural or forced.

Provision of continuous blow drum. The capacity of Boiler drum.



Operation and MaintenanceAccessibility for operation, maintenance and

repairing must be easy and quick to ensure higher operating efficiency and offset the long outage time.

Adequate provision must be made for: Soot blowing Tube cleaning – chemically / mechanically Washing economizer and air pre-heater

surfaces. Automation should be injected wherever it leads

to higher reliability and greater ease in boiler operation.



Capital InvestmentThe following factors are involved in determining the overall

capital investment in designing a boiler:

Cost of equipment.

Cost of fuel.

Cost of labour and materials for operation, maintenance and repairing.

Cost of the auxiliaries, e.g., cost of running pumps, fans, ash disposal systems, etc.

Expected life of the equipmentPrepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor


BOILER EFFICIENCY AND ITS CALCULATION

Combustion:Combustion refers to the rapid oxidation of fuel accompanied by the production of heat, or heat & light. Complete combustion of a fuel is possible only in the presence of adequate supply of oxygen.Oxygen (O2) is one of the most common elements on earth making up 20.9% of air. Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to a gas before they burn. Usually heat is required to change solid or liquid fuels into gases. Fuel gases burn in their normal state if enough air is present.



Nitrogen is considered to be a temperature reducing dilutant that must be present to obtain the oxygen required for combustion.Most of 79% of air is nitrogen, with traces of other elements.Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the fuel gases. This reduces the heat available for transfer through the heat exchanger surfaces. It also increases the volume of combustion by products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel air mixture. This nitrogen also can combine with oxygen(particularly at high flame temperatures) to produce oxides of nitrogen (Nox) , which are toxic pollutants. Prepared by:



Carbon, Hydrogen and sulphur in the fuel combine with oxygen in the air to form CO2 ,water vapour , and SO2 releasing 8084 Kcals , 28922 Kcals and 2224 K.cals of heat respectively. Under certain conditions, carbon may also combine with oxygen to form CO, which results in the release of a smaller quantity of heat 2430 K.cals / kg of carbon. Carbon burnt to CO2 will produce more heat per kg of fuel than when CO or smoke is produced.



C + O2 CO2 + 8084 Kcals/kg of carbon2C + O2 2CO + 2430 Kcals/kg of

carbon2H2 + O2 2H2O + 28922 Kcals / kg of

HydrogenS + O2 SO2 + 2234 Kcals/kg of

sulpher

Each kg of CO formed means loss of (8084-2460 ) Kcals of heat Prepared by:


BOILER EFFICIENCY AND ITS CALCULATION3 T’s of Combustion:

The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the “three Ts” of combustion, which are:

Temperature high enough to ignite and maintain ignition of fuel

Turbulence or intimate mixing of the fuel and oxygen and Sufficient

Time for complete combustion.



STOICHIOMETRIC COMBUSTIONThe efficiency of a boiler depends on efficiency

of the combustion system. The amount of air required for complete combustion of the fuel depends or the elemental constituents of fuel i.e. , Carbon , Hydrogen and sulphur etc. This amount is called stoichiometric air.

In general a certain amount of air more than that of Stoichiometric air is required for complete combustion and ensure that release of the entire heat contained in fuel oil.



FUEL ANALYSIS:There are two methods: the ultimate

analysis splits up the fuel into all its component elements, solid or gaseous, and the proximate analysis determines only the fixed carbon, volatile matter, moisture, and ash percentages. The ultimate analysis must be carried out in a properly equipped laboratory by a skilled chemist, but proximate analysis can be made with fairly simple apparatus



PROXIMATE ANALYSIS:Proximate analysis indicates the percentage by

weight of fixed carbon, volatiles, ash and moisture content in coal. The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator during burning. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume, pollution control equipment and ash handling systems of a furnace.



ULTIMATE ANALYSIS:The ultimate analysis indicates that

various elemental chemical constituents such as carbon, hydrogen, oxygen sulphur etc. It is useful in determining the quantity of air required for combustion and the volume and composition of the combustion gases. This information is required for the calculation of flame temperature and the free duct design.



CALORIFIC VALUE:The calorific value is the measurement of heat

or energy produced, and is measured either as gross calorific value (GCV) or net calorific value (NCV). The difference being the latent heat of condensation of the water vapour produced during the combustion process.

GCV assumes all water vapour produced during combustion process is fully condensed.

NCV assumes water leaves with the combustion products without fully being condensed.



BOILER EFFICIENCYThermal efficiency of boiler is defined as the

percentage of heat input i.e. effectively utilized to generate steam. There are two methods of assessing boiler efficiency:

Direct Method:Where the energy gain of the working fluid

(water and steam) is compared with the energy content of the boiler fuel.

Indirect Method:Where efficiency is the difference between

the losses and energy input. Prepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor

Direct Method:This is also known as “input - output method” due to the fact that it need only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency.



Boiler efficiencyBoiler efficiency =Heat

output / Heat inputParameters to be monitored for the calculation of boiler efficiency by direct method are: Quantity of steam generated per hour (Q) kg/hr. Quantity of fuel used per hour (q) in kg/hr. The working in pressure [in kg/cm²(g)] and superheat temperature (°C), if any. The temperature of feed water (°C)Type of fuel and gross calorific value of the fuel (GCV) in Kcal/kg of fuel.


Heat Input

Radiation Loss

Steam Output

ESP

Second Pass

Furnace



Boiler efficiency (η) = (Q x (hg-hf) /q x GCV) x 100 %

where hg - Enthalpy of saturated steam in Kcal / kg of

steam hf - Enthalpy of feed water in Kcal/kg of waterq - quantity of fuel.

Disadvantages of direct method Does not give clues to the operator as why the

efficiency of the system is lower. Does not calculate various losses accountable for various efficiency levels. Prepared by:



Indirect method:Indirect method is also called as heat loss method.

Subtracting the heat loss fractions from 1000 can arrive at the efficiency. The standard do not include blow down loss in the efficiency determination process.

The principle losses that occur in a Boiler are: Loss of heat due to dry flue gas. Loss of heat due to moisture in fuel and combustion. Air loss of heat due to combustion of hydrogen. Loss of heat due to radiation Loss of heat due to un burnt carbon.

In the above, loss of heat due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design Prepared by:


BOILER EFFICIENCY AND ITS CALCULATIONIndirect method:The data required for calculation of boiler

efficiency using indirect method are: Ultimate analysis of fuel (H2, O2, S, C,

moisture content, ash content). Percentage of oxygen or CO2 in flue gas Flue gas outlet temperature in °C (Tf) Ambient temperature in °C (Ta) and

humidity of air in kg/kg of dry air. GCV of fuel in Kcal/kg. Percentage combustible in ash GCV of ash in Kcal/kg


BOILER EFFICIENCY AND ITS CALCULATIONIndirect method:1. Percentage of heat loss due to dry flue gas =(m x Cp x (Tf-Ta) / GCV of fuel ) x 100Where is:m = mass of dry flue gas in kg/kg of fuel.Total mass of flue gas (m) = mass of actual air

supplied + mass of fuel suppliedCp = specific heat of super heated steam (0.45 K

cal / kg).Tf = Flue gas outlet temperature in °CTa = Ambient temperature in °C

Loss due to dry flue gas = 4.928%Prepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor


Furnace

Second Pass

ESPHeat Input

1. Loss due to dry flue gas = 4.928%

Radiation Loss



Indirect method:

2. Percentage heat loss due to evaporation of water formed due to H2 in fuel =

9 x H2[ 584 +Cp (Tp-Ta)] / GCV of fuel X 100 Where is: H2 = Percentage of H2 in kg of fuel584 = Latent heat corresponding to partial

pressure of water vapour ( 584 K cal / kg).

Loss due to Hydrogen in Fuel = 5.537%Prepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor


Indirect method:3. Percentage of heat loss due to moisture present

in fuel = M{ 584+CP(Tf – Ta) }X 100 / GCV of fuelWhere , M = % of moisture in 1 kg of fuelLoss due to Moisture in Fuel = 1.263%4. Percentage of heat loss due to moisture present

in air = AAS x humidity factor x CP X (Tf-Ta) X 100 / GCV

of fuelWhere, AAS = actual air supplied ( 0.45 K cals / kg)Loss due to Moisture in Air = 0.074%



Indirect method:5. Percentage of heat

loss due to un-burnt in fly ash

= [(Total ash collected /kg if fuel burnt X GCV of fly ash) / GCV of fuel ] X 100

6. Percentage of heat loss due to un-burnt in bottom ash

= [{Total ash collected /kg of fuel burnt X GCV of bottom ash} / GCV of fuel ] X 100

Due to Sen. Heat of Fly Ash = 0.102%Loss due to Unburnt Carbon = 0.331%Due to Sen. Heat of Bottom Ash = 0.071%4) Due to Sen. Heat of Fly Ash = 0.102%

Radiation Loss



Indirect method:7. Percentage of heat loss due to radiation and other

unaccounted loss.Unaccounted Losses = 1.327%Total Losses = 13.83%

Thus, Boiler efficiency (η) = 100 – (1+2+3+4+5+6+7)


Data required for Boiler Efficiency CalculationsUnit load MW FW Flow at Econ inlet T/hr Wet bulb Temp 0C Dry bulb Temp 0C Barometric Pressure mmHg Total Coal Flow T/hr Unburnt C in BA (Bottom Ash) % Unburnt C in FA (Fly Ash) % Radiation & Unaccounted Losses % % Fly ash to Total Ash % % Bottom ash to Total ash %



Data required for Boiler Efficiency CalculationsProximate Analysis of Coal Air Dry As firedMoisture % %

Ash % %

Volatile Matter % %

Fixed Carbon % %

Gross Cal. Value Kcal/kg Kcal/kg

Ave FG O2 APH in Ave FG O2 APH Out

Ave FG CO2 APH in Ave FG CO2 APH Out

Ave FG CO APH in Ave FG CO APH Out

Ave. FG Temp APH in Ave. FG Temp APH Out

Air to APH in Air APH out

Total Primary Flow

Total Air Flow L

Total Air Flow R

Design Ambient / Ref air Temp




Controllable lossesFollowing losses can be controlled1. Loss due to dry flue gasThe designer gives this loss at the flue gas APH outlet temp of 1400CAny increase in the FGT more than 1400C will

be resulting in more losses. This temp has to be controlled by proper cleaning of the furnace

2. Losses due to the unburnt coal in bottom and fly ash.


BOILER EFFICIENCY AND ITS CALCULATIONControllable losses2. Losses due to the unburnt coal in bottom

and fly ash.3. Loss due to unburnt in bottom ashThe designer gives this %age as max 4.8 %

any increase in this percentage beyond this will result in more losses.

If unburnt in bottom ash is more, the culprit is the coal mill, check the fineness of pulverised coal. Check the % retention on 50 mesh. It shall not exceed 1%.


BOILER EFFICIENCY AND ITS CALCULATIONControllable lossesCheck the Unburnt in fly ash sample taken

from the first hopper of ESP/BF. As per the designer it shall not exceed 0.8%.

If Unburnt in fly ash exceeds 0.8% it indicates incomplete combustion due to less amount of air.

Check for O2 % at the APH Flue Gas inlet for 2.8%, increase if necessary to 3.2%. Again check for Unburnts in fly ash. Simultaneously check for air leakages/ingress in the second pass.



Controllable losses

Assumptions:- Fly Ash is 80% of Total Ash.Bottom Ash is 20% of Total AshSulphur is 0.4% in Coal


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency1. Stack Temperature -The stack temperature should be low as possible.

However, it should not be so low that water vapour in the exhaust condenses on the stack walls. This is important for fuels containing significant sulphur , a low temperature can lead to sulphur dew point corrosion. Stack temperature greater than 140°C indicates potential for recovery of waste heat. It also indicate the scaling of heat transfer / recovery equipment and hence the urgency of taking on early S/D for water / flue side cleaning


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency2. Incomplete Combustion

In complete combustion can arise from a shortage of air or sulphur of fuel or poor distribution of fuel. It is usually obvious from the colour or smoke and must be corrected at the earliest. With coal firing, unburned carbon can comprise a big loss. It occurs a gift carry over or carbon in ash and may amount to more than 2% of the heat supplied to the boiler. Non-uniform fuel size could be one of the reasons for incomplete combustion. Increase in the fines in pulverized coal also increases carbon loss as because finer coal particle may fall through grate bars or carried away with furnace draught.


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency3. Excess Air Control:

Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion and to ensure satisfactory stack condition for some fuels. The optimum excess air level for maximum boiler efficiency occurs when the sum of the losses due to incomplete combustion and loss due to heat in flue gases is minimum. This level varies with furnace design, type of burner , fuel and process variables . It can be determined by conducting tests with different air fuel ratio. Controlling excess air to an optimum level always results in reduction in flue gas losses; for every 1%reduction in losses 0.6% rise in efficiency.


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors

influencing a boiler efficiency4. Reduction of Scaling and Soot

loses :In oil and coal fired boilers , soot build up on tubes acts as an insulator against heat transfer. Any such deposits should be removed or regular basis. Elevated stack temperatures may indicate excessive soot build up. Also same result will occur due to scaling on water side for not maintaining proper water Chemistry.


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency4. Reduction of Scaling and Soot loses :

Higher exit gas temperatures at normal excess air indicate poor heat transfer performance. This condition can result from a gradual build up of gas side or waterside deposits. Waterside deposits require a review of water treatment procedures and tube cleaning to remove deposits. An estimated 1% loss with every 22°C increase in stack temperature stack temperature should be cheeked and recorded regularly as an indicator of shoot deposits. When flue gas temperature rises about 20°C above the temperature, it is time to remove shoot deposits.


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency4. Reduction of Scaling and Soot loses :

It has been estimated that about 3mm of shoot can cause an increase in fuel consumption by 2.5%. Thus soot /slag deposition on the surface of the water wall tube, super heater, Re-heater, Economizer tubes reduces the boiler efficiency considerably and increases the flue gas outlet temperature. Choking due to ash deposition on the heating element of air pre-heater also reduces the combustion air temperature and there reduces efficiency and increases flue gas temperature.


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency5. Blow Down Control:

Boiler blow down is necessary for controlling the dissolved solids contained in the boiler water, and this is achieved by a certain amount of water is blown off and replaced by feed water. Thus maintaining optimum level of total dissolved solids (TDS) in boiler water. Uncontrolled continuous blow down is very wasteful. By monitoring boiler water conductivity and PH Blow down can be controlled and these reducing the efficiency losses


BOILER EFFICIENCY AND ITS CALCULATIONThe following are the factors influencing a boiler efficiency6. Quality of Fuel:

Quality of fuels influences the efficiency of Boiler to a large extent. It depends upon fixed carbon percentage, which gives a rough estimate of heating value of coal. Volatile matter content in the coal is an index of gaseous fuels present in coal. It proportionately increases flame length, and helps in easier ignition of coal. It also influences secondary air requirement. Ash content is coal affects the combustion efficiency and thus boiler efficiency also causes clinkering and slugging. Moisture content in fuel increases heat loss due to evaporation and superheating of vapour, helps to a limit, in binding fines, aids radiation heat transfer.

Sulphur affects clinkering and slagging tendencies. Limits exit fuel gas temperature


BOILER EFFICIENCY AND ITS CALCULATIONTYPICAL INSTRUMENTS


BOILER SAFETY EQUIPMENTSBoiler ProtectionsBoiler safety equipments includes

• BMS (Burner Management System)• Control System• Inter lock System.• Safety valve.• Gauge Glass• Pressure & Temperature Measurements

(Gauges, Transmitters, Switches & Recorders)


BOILER SAFETY EQUIPMENTS

BMS (Burner Management System)

A burner management system is responsible for the safe start-up, operation and shutdown of a boiler. It monitors and controls igniters and main burners; utilizes flame scanners to detect and discriminate between the igniter and main flames; employs safety shut-off valves, pressure, temperature, flow and valve position limit switches and uses blowers to cool the scanners and/or provide combustion air for the igniters. Its proper operation is crucial to the safety of a boiler.




Background:

In the past, most boilers have operated on temperature control from the outlet temperature to a control valve in the main fuel line, with a single shut-off valve upstream on the Control valve. This shut-off valve was either automatic with the plant DCS or operated manually with an Operator action, with each company having various additions to this basic concept.



BMS (Burner Management System)Operating philosophy:The main function of the BMS is to allow and ensure the safe start-up, operation, and shutdown of the Fired Boiler. Once the logic is configured and the system properly commissioned the BMS will provide a safe and consistent operating sequence. The human interface will guide the operator so that the heater may be safely operated, and if needed, be quickly and safely restarted.




BOILER SAFETY EQUIPMENTSBoiler Protections

BOILER SAFETY EQUIPMENTS Boiler Protections

BMS (Burner Management System)Operating philosophy:The following sequence of operation is typical for most Fired Boilers.1) Initially, the PLC will check that all the permissives and interlock are in place to allow start up.2) Start Purge. The PLC will check that the permissives are at their correct status. The system will typically wait for the operator to request the boiler to start, although all permissives are met and the boiler is ready to purge. Once the boiler start/purge is requested a pre-set timer will commence. Assuming the timing is not interrupted by an Interlock activation, it will continue until complete. Once finished, it will notify the operator that “Purge Complete” has been accomplished.



BMS (Burner Management System)Operating philosophy:3) Ignite Pilots. Once the purge is completed, the operator will be notified that the system is ready to start the pilots. The pilot header double block and bleed valves will energize. Instantaneously, the individual local pilot firing valves will open and the ignition transformers will be energized. The pilot valves and the ignition transformers will only be energized for a maximum of 10 seconds. If the pilot flame is not detected within this time the individual pilot isolation valve will close.4) Prove Pilots. Each pilot has its’ own dedicated flame detector, which in most cases is via a flame rod. Once proven, the individual pilot valve will hold in and continue to burn, in the event a pilot is not lit.


BMS (Burner Management System)Operating philosophy:5) Light Main Burners. Before the main burners are lit, the PLC will continue to check the permissives to ensure it is safe to light the main burners. The two main permissives are that there is sufficient flow in the process coils and the pilot burners are proven. The system then proceeds to energize the main header vent and shut-off valves. The first burner will light at the minimum fire rate. A five second trial for ignition is provided from the time the individual isolation valve is opened until the detection of the flame. If the flame is not detected, the individual main burner isolation valve is de-energized. Prepared by:



BMS (Burner Management System)Operating philosophy:6) Confirm Main Burner Status. Once this is achieved the system is ready to be ramped up to operating conditions. This is usually performed manually until the process variable is close to the operating set point, then the temperature and gas flow/pressure controllers can then be switched to auto mode.



BMS (Burner Management System)The following is a list of new requirements that are common to various boiler and BMS codes. 1) Mandatory Purging. 2) Permissive Interlocks 3) Double Block and Bleed systems 4) Pilots and Ignition Systems 5) Dedicated Flame Monitoring Systems 6) High / Low Pressure, Temperatures and Flow. 7) Combustion Air and Draft Pressure Alarms and Controls 8) Dedicated Logic Solver

Each of these requirements have been developed due to incidents that have occurred because of the lack of them.



1) Mandatory Purging: The most important function of the boiler control system is to prevent the possibility of an accumulation of combustible fuel followed by accidental or improper ignition sequence resulting in an explosion. Correct pre-ignition purging of the heater is crucial to the safe operation of the heater.

A time to achieve the four volume changes may be calculated from the required flow rate. In the event the purge does not continue for the prescribed time or required purge, permissives are no longer met and a re-purge is required. Only when a successful purge is completed can any ignition source be introduced.



1) Mandatory Purging: (Safety Rule)• CSA B194.3-05 Section 9.2 Pre-purge 9.2.1:

When either an intermittent or interrupted pilot or a direct transformer spark igniter is used to light the main burner and the combustion air supply is by mechanical means, the appliance control system shall provide a proven purge period prior to the ignition cycle. This purge period shall provide at least four air changes of the combustion zone and flue passages at an airflow not less than 60% of that required at maximum input.

• NFPA 86 Section 5-4.1 Pre-ignition (Pre-purge, Purging Cycle) 5-4.1.2: A timed pre-ignition purge shall be provided. At least 4 standard cubic feet (scf) of fresh air or inert gas per cubic foot (4m3/m3) of heating chamber volume shall be introduced during the purge cycle. Prepared by:



BMS (Burner Management System)Permissives & Interlocks:These can be numerous and may include all of the items on the BMS. The interlocks are usually designated on the P&ID with the symbol “I”. These are used in the configuration of the operating procedure and logic so as to ensure the safe sequential operation of the boiler particularly in start-up and during operation. The permissives are an integral part of the start-up and light-off procedure. The main importance of the interlocks is to detect the need for partial or total shut down of the boiler. Prepared by:



BMS (Burner Management System)Permissives & Interlocks:Typical interlocks would include high/low fuel and pilot gas pressure, high/low stack temperature, loss of flame, high/low firebox pressure and loss of combustion air. In addition to these general trips each boiler must be examined individually to determine if additional trips may be required due to the boiler design or configuration. These non-standard trips are designed to protect the boiler and personnel from tube and structural damage that may occur from improper operation for a long duration.



BMS (Burner Management System)Shut-off Valves.The safety shut-off valves are the key component in the BMS to prevent the accumulation of an explosive mixture in the boiler. Standard practice is to provide automated safety shutoff valves installed in a double block and bleed configuration on the main and pilot headers. An additional safety shut-off valve should be located on each individual burner for multiple burner systems. These shut-off valves must also be certified for use safety shut-off valves. In accordance with CSA requirements these valves need to be certified to CSA 6.5 certified and per NFPA requirements these valves need to be FM-7400 listed



BMS (Burner Management System)Shut-off Valves.


BOILER SAFETY EQUIPMENTSBMS (Burner Management System)Pilots and Ignition Systems:


Unfortunately the majority of boilers still in use within the industry rely on a manual ignition system. This means the heater is manually lit by the operator inserting an oily rag oran ignition torch to the base of the burner. With today’s technology and easy accessibility to the electronic ignition system it is no longer necessary to manually light the heaters and potentially place the operator in harms way.The electric ignition system consists of an ignition rod provided with the burner and a high voltage ignition transformer. When all interlocks are cleared and permissives are in place the ignitor lights the pilot burner which then in turn lights the main burner.


BMS (Burner Management System)Pilots and Ignition Systems:


An electronic ignition system, controlled by the BMS, eliminates the need for the operator to provide an ignition source for the burners; which is considered to be the most dangerous part of the startup procedure. As the BMS controls the ignition of the burners the heater purge cannot be so easily bypassed.Typically each burner is provided with its own pilot and ignition system.


BMS (Burner Management System)Dedicated Flame Monitoring Systems:


Each burner is equipped with its own flame monitoring devices, independent detectors are required to supervise the pilot and the main flame. Exceptions may be made depending on the type of pilot and burner. In most installations two different detection techniques are used, a flame (ionisation) rod to monitor the pilot flame and an UV (Ultraviolet) or IR (Infrared) Scanner to ‘see’ the main flame. Flame rods are typically used on the pilot flame as they are more cost effective, however the same detectors should not be used on the main flames as these typically run at a higher temperature and the flame rod would burn out in a short amount of time.



Dedicated Flame Monitoring Systems:



BMS (Burner Management System)Alarms and Shutdowns:Alarms and shutdowns provided by the boiler’s instrumentation provide added safety and permissives to allow the system to transition from different stages of boiler operation in a proper and safe manner. There is some instrumentation typical to all systems and others that are dependent on the size, purpose, and configuration of the boiler. The instrumentation is located on the fuel gas trains and the heater itself. In the past, switches have been widely used for these applications for various reasons, however it is strongly recommended that transmitters be used in lieu of switches. Switches may fail in an unsafe position without providing any indication until the unit is required to operate. Transmitters report back dynamic information and have built-in diagnostics, so they are able to provide indication of failures. Considering the losses involved if an incident was to occur, it is far more cost efficient to use the more reliable transmitters.



BMS (Burner Management System)Combustion Draft / Pressure Alarms and ControlsBoiler draft is the motive force that will ensure proper air and flue gas flow through the boiler. The most critical draft point is at the arch as it is the controlling variable in the design of the burner and boiler itself. This measurement is a critical indicator in the operation of a fired boiler and alerts the operator to any tendency to go positive.Although this is normally not a shut-down device, it is closely linked to opening the stack damper blade. The pressure control loop consists of a pressure transmitter located at the boiler arch and an actuator on the stack damper. A low draft pressure (high firebox pressure) alarm should be added to this transmitter. The control loop is to be configured such that the stack damper opens when there is a low boiler draft. In the event that the boiler draft is still too low but the stack damper is 100% open the burner firing rate must be reduced.



BMS (Burner Management System)Dedicated Logic Solver: Traditionally most BMS’s have been implemented using a discrete series of relays and switches as the logic solver. When designed correctly these hardwired panels may prove to be very reliable. Unfortunately the permissives and interlocks on such systems are easily bypassed. With the advent of the microprocessor, unitized flame safeguard controllers have been introduced. These units provide the required logic to safely start and stop the heater. The draw back of these units is that they are only designed to control single burner systems.



BMS (Burner Management System)Dedicated Logic Solver:The area of biggest change in BMS systems as of late is the ability to use a Programmable Logic Controller (PLC) as the primary safeguard and logic solver. Certain conditions must be adhered to in order to validate their use, which vary between different codes. Common requirements are that a power or hardware failure will not prevent the system from reverting to a safe condition and an external Watch Dog Timer must be used to monitor a dedicated output channel. Any failures will cause the system to revert to a safe condition. As technology advances and the introduction of true safety rated systems progresses, these requirements and guidelines will inevitably change.

PLC Panel with Lights and Push Buttons



EMERGENCIES ON STEAM GENERATION SYSTEM

LL Level Of Steam Drum LL Flow Of Water HH Pressure Of Furnace LL Fuel Pressure HH Fuel Pressure Instrument Air Loss Power Loss



EMERGENCIES OF STEAM GENERATION SYSTEM

Water Loss Fuel Loss Electrical Failure Instrument Air Failure Vacuum Loss Of Surface Condenser High Pressure Of Steam Drum



Boiler Logs Are ImportantThe majority of boiler accidents can be prevented. One of the most effective tools is the proper use of operating and maintenance logs. Boiler logs are the best method to assure a boiler is receiving the required attention and provide a continuous record of the boiler's operation, maintenance and testing. Because a boiler's operating conditions change slowly over time, a log is the best way to detect significant changes that may otherwise go unnoticed.


INSTRUMENTATION FOR MONITORING & TESTING

Level Measurement Temperature Measurement Flue Gas Analysis Furnace Pressure Fuel Control Air Flow Control



LOCAL MEASUREMENT

Level Measurement By Using Level Glass

Pressure Measurement By Using Pressure Gauge

Temperature Measurement By Using Temperature Measuring Element

Composition AnalyzerPrepared by:Mohammad Shoeb SiddiquiSr. Shift Supervisor


Water Level Control and Low Water Fuel CutoffsThese devices perform two separate functions, but are often combined into a single unit. This method is economical, providing both a water level control function and the safety feature of a low water fuel cutoff device. We recommend, however, that both steam and hot water boilers always have two separate devices — a primary and a secondary low water fuel cutoff. Many jurisdictions require two such devices on steam boilers.

Steam Drum Low Low Level Low Low Flow Of Fuel



Water LevelThe importance of proper cleaning and maintenance of the water gage glass, or sight glass, cannot be stressed enough. The gage glass on a steam boiler enables the operator to visually observe and verify the actual water level in the boiler. But water stains and clogged connections can result in false readings. The glass also may break or leak. Take the time to replace the glass, even if the boiler must be shut down. That inconvenience is nothing compared to the damage that may result from operating a boiler without a gage glass.

The Normal Operating Water Level (NOWL) should be approximately in the middle of the gauge glass.



ELECTRONIC MEASURING DEVICES Level Transmitter Pressure Transmitter Temperature Transmitter Level Switch Pressure Switch Flow Transmitter



STEAM BOILER VARIABLES Major Variable

Steam Pressure Steam Temperature Water Level Feed Water Pressure Furnace Draft



STEAM BOILER VARIABLES Steam Flow Feed Water Flow Combustion Air Flow System Drafts Or Pressure Feed Water Temperature Flue Gas Temperature Fuel Flow Fuel Pressure Fuel Temperature



COMBUSTION CONTROL SYSTEMS Fixed Positioning Positioning With Operator Trim Pressure Ratio Fuel And Air Metering Cross Limited Metering Oxygen Trim Control



Safety ValveOften considered the primary safety feature on a boiler, the safety valve should really be thought of as the last line of defense. If something goes wrong, the safety valve is designed to relieve all the pressure that can be generated within the boiler. Keep in mind that the same conditions that make other safety devices malfunction can also affect the safety valve. Don't let testing and maintenance schedules slide.

The spring-loaded pop-off safety valve pops open when steam pressure exceeds the MAWP.Safety valves are routinely tested to ensure proper operation and must be serviced by an authorized manufacturer representative.



Water TreatmentWater must be treated for safety. Minerals can cause a build up of deposits and cause overheating of boiler parts.

Carryover occurs when a high boiler water level causes water particles to be carried into steam lines.

Bottom blowdown, the boiler should be under light load and the water level should be at the NOWL.



The purpose of the drum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum, thus reducing boiler efficiency and carrying moisture into the process or turbine. The functions of this control module can be broken down into the following: ● Operator adjustment of the setpoint for drum level ● Compensation for the shrink & swell effects ● Automatic control of drum level ● Manual control of the feedwater valve ● Bumpless transfer between auto and manual modes ● Indication of drum level and steam flow ● Indication of feedwater valve position and feedwater flow ● Absolute/deviation alarms for drum level The three main options available for drum level control are: Prepared by:



Single-element drum level control The simplest but least effective form of drum level control.This consists of a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the difference is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional output. The output is then passed to the boiler feedwater valve, which then adjusts the level of feedwater flowNotes: ● Only one analogue input and one analogue output required ● Can only be applied to single boiler / single feedpump configurations with relatively stable loads since there is no relationship between drum level and steam- or feedwater flow ● Possible inadequate control option because of the swell effect into the boiler drum.



Single-element drum level control



Two-element drum level controlThe two-element drum level controller can best be applied to a single drum boiler where the feedwater is at a constant pressure.The two elements are made up of the following: Level Element: a proportional signal or process variable (PV) coming from the drum level transmitter. This signal is compared to a setpoint and the resultant is a deviation value. This signal is acted upon by the controller which generates corrective action in the form of a proportional value. Steam Flow Element: a mass flow rate signal (corrected for density) is used to control the feedwater flow, giving immediate corrections to feedwater demand in response to load changes. Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller. This imbalance can arise from: ● Blowdown variations due to changes in dissolved solids ● Variations in feedwater supply pressure ● Leaks in the steam circuits constant pressure.

Notes: ● Tighter control of drum level than with only one element ● Steam flow acts as feed forward signal to allow faster level adjustments ● Can best be applied to single boiler / single feedpump configurations with a constant feedwater pressure



Two-element drum level control.



Three-element drum level controlThe three-element drum level control is ideally suited

where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow.The three-elements are made up of the following: Level Element and Steam Flow Element: corrects for unmeasured disturbances within the system such as: ● Boiler blowdown ● Boiler and superheater tube leaks Feedwater Flow Element: responds rapidly to variations in feedwater demand, either from the ● Steam flow rate feedforward signal ● Feedwater pressure or flow fluctuations In order to achieve optimum control, both steam and feedwater flow values should be corrected for density.

Notes: ● The three-element system provides tighter control for drum level with fluctuating steam load. Ideal where a system suffers from fluctuating feedwater pressure or flow ● More sophisticated level of control required ● Additional input for feedwater flow required




Three-element drum level control


Enhanced Three-element drum level controlThe enhanced three-element drum level control module incorporates the standard three element level components with the following improvements: ● The three-element mode is used during high steam demand. The two-element mode is used if the steam flow measurement fails and the module falls back to single element level control if the feedwater flow measurement should fail or if there is a low steam demand.● The drum level can be derived from up to three independent transmitters and is density compensated for pressure within the boiler drum.

Notes: ● Tighter control through a choice of control schemes. Drum level maintained on failure of steam or feedwater flow measurements ● This module introduces an additional level control loop Boiler drum level control



The Stack Temperature GageA stack temperature gage is normally

installed on a boiler to indicate the temperature of the flue gas leaving the boiler. A high stack temperature indicates that the tubes may be getting a buildup of soot or scale. Also, the baffling inside the boiler may have deteriorated or burned through, allowing gases to bypass heat transfer surfaces in the boiler.



Environmental protection and the control of solid, liquid and gaseous effluents or emissions are key elements in the design of all steam generating systems.The emissions from combustion systems are tightly regulated by local and federal governments, and specific rules and requirements are constantly changing. At present, the most significant of these emissions are sulfur dioxide (SO2), oxides of nitrogen (NOx), and fine airborne particulate. All of these require specialized equipment for control.A key element in a successful emissions control program is measurement and monitoring.


Environmental Protection


Environmental ProtectionControl approaches One or more of the following measures have typically

been adopted to control emissions:1. Emission standards These limit the mass of SO2, NOx,

or other pollutant emitted by volume, by heat input, by electric energy output, or by unit of time (hourly, daily, annually).

2. Percent removal requirements These specify the portion of the uncontrolled emissions that must be removed from the flue gas.

3. Fuel requirements Primarily aimed at SO2 control, these either limit the type of fuel that can be burned or the fuel sulfur content.

4. Technology requirements These typically indicate the type of control technology specifically required or indicate the use of the best available control technology or reasonably available control technology at the time of installation. These requirements depend in many cases on some level of economic feasibility.



Kinds of Pollutants, Sources and ImpactsThe focus of these slides are stationary emission sources, particularly fired utility and industrial boiler systems. Key pollutants from these sources are SO2, SO3, NOx, CO and particulate matter. Another class of emissions is called air toxics. These are potentially hazardous pollutants that generally occur in only trace quantities in the effluents from fired processes.However, they are undergoing more intense examination because of their potential health effects.


Environmental ProtectionKinds of Pollutants, Sources and ImpactsAir pollutants are contaminants in the atmosphere which, because of their quantity or characteristics, have deleterious effects on human health and/or the environment. The sources of these pollutants are classified as stationary, mobile or fugitive. Stationary sources generally include large individual point sources of emissions such as electric utility power plants and industrial furnaces where emissions are discharged through a stack. Mobile sources are those associated with transportation activities. Fugitive emissions generally include discharges to the atmosphere from conveyors, pumps, valves, seals and other process points not vented through a stack. They also include emissions from area sources such as coal piles, landfills, ponds and lagoons. They most often consist of particulates and occur in industry-related activities in which the emissions are not collected.



Sulfur oxidesSulfur oxides have been related to irritation of the human respiratory system, reduced visibility, materials corrosion and varying effects on vegetation. The reaction of sulfur oxides with moisture in the atmosphere has been identified as contributing to acid rain. Wood and bark typically contain 0.0 to 0.1% elemental sulfur on a dry basis. During the combustion process some of this sulfur can be converted to flue gas SO2, but the conversion ratio is typically low (10 to 30%).Sulfur dioxide (SO2)Most of the sulfur in fuel converts to SO2 with small quantities of sulfur trioxide (SO3). The main source of sulfur oxides is from the combustion of coal, with lesser amounts from other fuels such as residual fuel oil.


Environmental ProtectionSulfur trioxide (SO3 )Some of the sulfur dioxide that forms converts to sulfur trioxide (SO3). The typical conversion rate is 1% or less in the boiler. However, the catalytic process that is frequently used to control NOx levels has the undesirable side effect of converting additional SO2 to SO3, which can range from 0.5 to 2% additional conversion. The SO3 readily combines with water to form sulfuric acid (H2SO4) at flue gas temperatures less than 500 oF (260 oC). This acid can create extremely corrosive conditions. The sulfuric acid condenses to form a fine mist when the flue gas passes through a wet flue gas desulfurization system that is used to remove sulfur dioxide (SO2). This sulfuric acid mist contributes to the total stack particulate loading. Such mist is extremely fine, less than 0.5 micron, and very small amounts of this mist (5 ppm or less) can cause visible, bluish stack plumes, even in the absence of solid particulate.


Environmental ProtectionNitrogen oxides (NOx) This category includes numerous species comprised of nitrogen and oxygen, although nitric oxide (NO) and nitrogen dioxide (NO2) are the most significant in terms of quantity released to the atmosphere. NO is the primary nitrogen compound formed in high temperature combustion processes where nitrogen present in the fuel and/or combustion air combines with oxygen. The quantity of NOx formed during combustion depends on the quantity of nitrogen and oxygen available, the temperature, the intensity of mixing and the time for reaction. Control of these parameters has formed the basis for a number of control strategies involving combustion process control and burner design.


Environmental ProtectionNitrogen oxides (NOx) Based on the most recent EPA emissions inventory, utilities account for 22% of NOx emitted, with the transportation sector emitting 56%. Of the total utility NOx emissions, approximately 90% comes from coal-fired boilers.The most deleterious effects come from NO2 which forms from the reaction of NO and oxygen. NO2 also absorbs the full visible spectrum and can reduce visibility. NOx has been associated with respiratory disorders, corrosion and degradation of materials, and damage to vegetation. NOx has also been identified as a precursor to ozone and smog formation.



Carbon monoxide This colorless, odorless gas is formed from incomplete combustion of carbonaceous fuels. CO emissions from properly designed and operated utility boilers are a relatively small percentage of total combustion source CO emissions, most of which come from the internal combustion engine in the transportation sector. The primary environmental significance of CO is its effect on human and animal health. It is absorbed by the lungs and reduces the oxygen carrying capacity of the blood. Depending on the concentration and exposure time, it can cause impaired motor skills and physiological stress.



Particulate matter Solid and liquid matter of organic or inorganic composition which is suspended in flue gas or the atmosphere is generally referred to as particulate. Particle sizes from combustion sources are in the 1 to 100 μm range, although particles smaller than 1 μm can occur through condensation processes.

Among the effects of particulate emissions are impaired visibility, soiling of surrounding areas, aggravation of adverse effects of SO2, and human respiratory problems.


Environmental ProtectionPM10 and PM2.5 Subsets of particulate matter, PM10 is particulate matter 10 μm and finer and PM2.5 is particulate matter 2.5 μm and finer. Fine particles are emitted from industrial and residential oil combustion and from vehicle exhaust. Fine particles are also formed in the atmosphere when gases such as SO2, NOx and VOCs, emitted by combustion processes, are transformed into fine particulate by chemical reactions in the air (i.e., sulfuric acid, nitric acid, and photochemical smog). PM2.5 is considered to have more deleterious health affects than coarser particulate.Others types of pollutions are: VOC, (Volatile organic compounds) represent a wide range of organic substances. Toxic air pollutants, Mercury, & Carbon dioxide


Environmental ProtectionOthers Types of Pollutions are: VOC, (Volatile organic compounds) represent a wide range of organic substances. Toxic air pollutants, Mercury, & Carbon dioxideVolatile Organic Compounds (VOC)Volatile organic compounds (VOCs) are also gaseous products of incomplete combustion. Its represent a wide range of organic substances. These compounds consist of molecules containing carbon and hydrogen, and include aromatics, olefins and paraffinsAs such, the emission of VOCs during wood firing is influenced by the same factors affecting CO. Typically, VOC emissions while stoker-firing wood and bark fuels do not exceed 0.05 lb/106 Btu (0.02 g/ MJ) heat input, expressed as methane.


Environmental ProtectionToxic Air Pollutants This is a large category of air pollutants that could have hazardous effects. The EPA had only promulgated standards for arsenic, asbestos, benzene, beryllium, mercury, radio nuclides and vinyl chlorides for certain defined industries. Toxic pollutants for which emissions are to be regulated. The list includes a wide range of simple and complex industrial organic chemicals and a small number of inorganic+, particularly heavy metals. The EPA has identified hundreds of categories of air toxics sources, among which are municipal solid waste combustors, industrial boilers, and electric utility boilers. For these combustion sources, mercury has been the primary focus.



MercuryMercury Present in only trace amounts in coal, mercury is released during the combustion process as elemental mercury, and is predominantly in the vapor phase at the exit of the furnace. Emissions from utility plants are extremely low. Mercury in some chemical forms is very toxic. From whatever source, mercury can find its way into water sources where it can be converted into water soluble species such as methyl-mercury by microorganisms and accumulate in the fatty tissues of fish. Consumption of contaminated fish is the main identified risk to humans.


Environmental ProtectionCarbon Dioxide (CO2)CO2 is one of several so-called greenhouse gases which may impact the climate and contribute to global warming. CO2 is emitted from a variety of naturally occurring and manmade sources including the combustion of all fossil and hydrocarbon based fuels.Improving the power cycle efficiency (more power from less fuel) and the use of fuels with less carbon content are potential methods to address CO2 emissions from any combustion source. Another option is separation and capture followed by sequestration.



Air pollution control technologiesThe strategies for control of all emissions from a utility or industrial boiler are formulated by considering design fuels, kind and extent of emission reduction mandated, and economic factors such as boiler design, location, new or existing equipment, age and remaining life.SO2 control strategies and technologiesSO2 emissions from coal-fired boilers can be reduced using pre-combustion techniques, combustion modifications and post-combustion methods.Pre-combustion These techniques include the use of natural gas or low sulfur oil in new units or the use of cleaned (beneficiated) coal or fuel switching in existing units.


Environmental ProtectionCombustion modifications These techniques are primarily used to reduce NOx emissions but can also be used to control SO2 emissions in fluidized-bed combustion where limestone is used as the bed material. The limestone can absorb up to 90% of the sulfur released during the combustion process. Sorbent injection technologies Sorbent injection, while not involving modification of the combustion process, is applied in temperature regions ranging from those just outside the combustion zone in the upper furnace to those at the economizer and flue work following the air heater. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the SO2. Typical calcium sorbents include limestone [calcium carbonate (CaCO3)], lime (CaO), hydrated lime [Ca(OH)2] and modifications of these compounds with special additives. Sodium or magnesium based compounds are also used.


Environmental ProtectionWet and dry scrubbing technology Worldwide, wet and dry scrubbing or flue gas desulfurization (FGD) systems are the most commonly used technologies in the coal-fired electric utility industry. In the wet scrubbing process, a sorbent slurry consisting of water mixed with limestone, lime, magnesium promoted lime or sodium carbonate (Na2CO3) is contacted with flue gas in a reactor vessel. Wet scrubbing is a highly efficient (> 97% removal at calcium/ sulfur molar ratios close to 1.0), well established technology which can produce usable byproducts.Dry scrubbing involves spraying an aqueous sorbent slurry into a reactor vessel so that the slurry droplets dry as they contact the hot flue gas [~300 oF (~149 oC)]. The SO2 reaction occurs during the drying process and results in a dry particulate containing reaction products and un-reacted sorbent entrained in the flue gas along with fly-ash. These materials are captured downstream in the particulate control equipment.Dry scrubbing is a well established technology with considerable operational flexibility. The waste residue is dry.



NOx control technologies NOx emissions from fossil fuel-fired industrial and utility boilers arise from the nitrogen compounds in the fuel and molecular nitrogen in the air supplied for combustion. Conversion of molecular and fuel nitrogen into NOx is promoted by high temperatures and high volumetric heat release rates found in boilers. The main strategies for reducing NOx emissions take two forms: 1)modification of the combustion process to control fuel and air mixing, and reduce flame temperatures, and 2)post-combustion treatment of the flue gas to remove NOx.


Environmental ProtectionCombustion modification This approach to NOx reduction can include the use of low NOx burners, combustion staging, gas recirculation or reburn technology.Post-combustion The two main post-combustion techniques for NOx control are selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). In SNCR, ammonia or other compounds such as urea (which thermally decomposes to produce ammonia) are injected downstream of the combustion zone in a temperature region of 1400 to 2000F (760 to 1093C). SCR systems remove NOx from flue gases by reaction with ammonia in the presence of a catalyst (see Chapter 34). SCR is being used worldwide where high NOx removal efficiencies are required in gas-, oil- or coal-fired industrial and utility boilers.


Environmental ProtectionParticulate control technologies Particulate emissions from boilers arise from the noncombustible, ash forming mineral matter in the fuel that is released during the combustion process and is carried by the flue gas. Another source of particulate is the incomplete combustion of the fuel which results in unburned carbon particles. Coal cleaning Historically, physical coal cleaning has been applied to reduce mineral matter, increase energy content and provide a more uniform boiler feed. Although reduction in flue gas particulate loading is one of the potential benefits, coal cleaning has been driven by the many other boiler performance benefits related to improved boiler maintenance and availability and, more recently, the reduction in SO2 emissions.


Environmental ProtectionMechanical collectors These are generally cyclone collectors and have been widely used on small boilers when less stringent particulate emission limits apply. Cyclones are low-cost, simple, compact and rugged devices. However, conventional cyclones are limited to collection efficiencies of about 90% and are poor at collecting the smallest particles. Improvements in small particle collection are accompanied by high pressure drops.Dust collector Mechanical dust collectors are used after the last heat trap on the boiler to collect the larger size flyash particulate, sometimes as protection for the ID fan. They typically consist of multi-cyclone tubes enclosed in a casing structure. The tubes consist of outer inlet tubes with spin vanes and inner tubes used without recovery vanes. The dust collector efficiency is in the range of 65 to 75% at an optimum draft loss of 2.5 to 3.0 in. wg (0.62 to 0.75 kPa). Due to the abrasive nature of the flyash, the outer collection tubes and cones are made of high hardness (450 Brinell) abrasion resistant material.


Environmental ProtectionPrecipitator Electrostatic Precipitators (ESP) Precipitator Electrostatic Precipitators (ESP) are typically used after the mechanical collector to reduce the particulate concentration in the flue gas and to meet environmental requirements. Due to the high carbon content in the fly-ash, it is important to reduce the fire potential in the precipitator. It is necessary to ensure no tramp air enters the precipitator and that the fly-ash is continuously removed from the hoppers. Hopper level detectors and temperature detectors alert the operator. Installations can be equipped with fire fighting apparatus such as steam inerting. Other suppliers recommend de-energizing the precipitator if a predetermined oxygen content in the flue gas is exceeded.ESPs are available in a broad range of sizes for utility and industrial applications. Collecting efficiency can be expected to


Environmental ProtectionElectrostatic precipitators (ESP) be 99.8% or greater of the inlet dust loading. ESPs are considered to be less sensitive to plant upsets than fabric filters because their materials are not as sensitive to maximum temperatures. They also have a very low pressure drop. ESP performance is sensitive to fly-ash loading, ash resistivity and coal sulfur content. Lower sulfur concentrations in the flue gas can lead to lower ESP collection efficiency. ESPs tend to collect coarser particulate more easily, whereas a fabric filter tends to have a more uniform collection efficiency across the particle size range. Therefore, a fabric filter has higher collection efficiency of fine particulate than an ESP. The desire to further control sulfuric acid mist emissions and very fine fly-ash has led to the utilization of wet ESPs downstream of wet flue gas desulfurization systems.


Environmental ProtectionFabric filters These filters, also commonly referred to as bag-houses, are available in a number of designs (reverse air, pulse jet, and shake/deflate), each having advantages and disadvantages in various applications. Applications include industrial and utility power plants firing coal or solid wastes, plants using sorbent injection and spray dryer FGD, and fluidized bed combustors. Collection efficiency can be expected to be at least 99.9% or greater. Fabric filters have the potential for enhancing SO2 capture in installations downstream of sorbent injection and dry scrubbing systems


Environmental ProtectionReferences:

1.Steam its Generation & Use (Edition 41) by The Babcock & Wilcox Company.

2.Wikipedia

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Prepared by:Mohammad Shoeb SiddiquiSr. Shift SupervisorSaba Power [email protected][email protected]/shoebsiddiqui

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