Coal Pulverising in Boilers 1

8/13/2019 Coal Pulverising in Boilers 1

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Coal Pulverising in Boilers

written by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 6/1/2011

Pulverizing coal for a boiler is very important factor in overall cycle efficiency. There are many

types of pulverizers available, but proper selection will ensure consistent boiler and cycle

efficiency. This helps in reduction of carbon-dioxide emission per million units of electricity

generated.

Boilers for steam generation in power plants and process industries use coal as fuel. The

percentage of boilers operating with coal as fuel outnumbers the boilers using all other fuels

combined. Coal is pulverized before firing for achieving a stable and efficient combustion. Many

types of pulverizers are used in boilers by different designers.

History of pulverization

The history of pulverization dates back as early as 1824 and was envisioned by Carnot in a coal

fired engine. In 1890 Diesel made use of pulverized coal in his diesel engine. Pulverized coal

firing was first developed in the cement industry and then migrated to the power and process

industries. Actually Thomas Alva Edison and the Niepce brothers of France were pioneers in

pulverized coal firing. This technology gained momentum after World War I in the powergenerating industry. It was John Anderson, chief engineer of power plants at the WisconsinElectric Power Company who introduced pulverized coal firing in power stations.

Pulverized coal is the most efficient way of using coal in a steam generator. The coal is groundso that about 70 % will pass through 200 mesh (0.075 mm) and 99 % will pass through 50 mesh

(0.300 mm). A pulverized coal boiler can be easily adapted for other fuels like gas if required

later without much difficulty. However, during the design stage it is possible to make boilers

firing multiple fuels. With pulverization technology, large size boilers could be designed,

manufactured, erected, and run much more efficiently.

Types of pulverizers

Mainly there are three types of pulverizer used in industry: the slow speed mills like ball tubemills, the medium speed mills like bowl, ball and race, roller mills fall in this category, and the

third type is the high speed impact mill. The slow speed and medium speed mills are selected forcoals ranging from sub-bituminous to anthracite. The high speed mills are used mainly for

lignite.

The purpose of a pulverizer in a coal fired boiler

o To supply pulverized coal to the boiler as per requirement of steam generation

o Transport the pulverized coal from pulverizer to the burners in the boiler

o To remove moisture in coal to an acceptable level for firing in boiler

o To remove high density inorganics from coal during pulverization


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o To classify coal particles to the required level of fineness, normally 70 % through 200

mesh and less than 2% on 50 mesh

Coal parameters affecting pulverizer output

While selecting a pulverizer, the coal characteristics play an important role. The Hardgrove

index, total moisture, input coal size, output fineness, and mill wear have direct impact on the

mill output.

o The Hardgrove index of coal tells us about the ease with which it can be pulverized. A

higher Hardgrove index indicates the coal is easier to grind. 50 HGI normally is taken for

calculating the base capacity of the mill. When coal with HGI higher than 50 is fed to the

pulverizer, the output will be higher than base capacity, and below 50 HGI, the output

will be lower.

o The total moisture in coal has a high effect on mill output. The higher the moisture, thelower the output.

o Higher pulverized coal fineness increases the recirculation in the mill and the outputreduces.

o The inlet size of the coal also affects the mill output directly.

o Mill air flow variations result in changes in mill outlet temperature and fineness as well

as capacity.

Ball tube mill

Ball tube mills are either pressurized or suction type. In the pressurized type, the hot primary airis used for drying the coal and to transport the milled coal to the furnace. In this type, leakage in

the mill area is high.

In the suction type, the exhauster is used for lifting the milled coal from the pulverizer to the

furnace through a cyclone. The tube mills have a large circular drum, with adequate ball charge,which is rotated at about 70% of the speed at which the ball charge would be held against the

inner surface by centrifugal force. In this mill the grinding balls can be replenished on the line.

Normally the ball mill designers use three types of balls with three different diameters. These

balls reduce in size as the mills operate and so the highest size ball is normally used for

recharging. In earlier days, most of the ball mills had a single inlet and outlet, but now designers

use both ends to feed coal and also for taking out pulverized coal. The control systems are well

made to understand the requirement of ball charge and the output from the mill. Ball mills arealways preferred to be operated at full capacity because the power consumption of this type ofmill is very high at lower loads when compared with other types. Ball mills can be designed for a

very high capacity like 75 tons per hour output for a specific coal.

Vertical spindle mill

There are many different varieties of vertical mills. Designers use large steel balls ranging from 2to 6 or more between two grinding rings for pulverizing. There are also other types like conical

rollers with shallow bowl; deep bowl, etc. where load is applied on the rollers and the bowl


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rotates while pulverizing. These types of mill are designed normally up to 60 tons per hour for a

specific coal; however there are vertical mills with 90 tons per hour output. A vertical spindlemill is also designed for pressurized and suction type requirements. Boiler designers use this type

of mill for poor quality coal as this type of mill rejects foreign materials like stones and otherhigh density materials. The power consumed by the mill per ton of coal ground is only two-thirds

of the ball mills. However if the primary air fan power is also taken into account, in the case of a

pressurized mill the power consumption is lower only by about 15%.

High speed impact mill

This type of mill uses a central horizontal shaft which has a number of arms, and a beater of

different design is attached to these arms to beat the coal to be pulverized. High speed impact

mills are mainly used in pulverizing lignite. Today all boiler designers opt to use ball or vertical

spindle mill for coal other than lignite.

While selecting the type of mill boiler designers must clearly understand the coal characteristics,

the overall system being used, and the maintenance requirement. It is always seen that if theadvantage of the mill alone is considered, then the overall boiler economics can prove to be

different.


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Fly Ash Erosion in Boilers Firing High Ash Coals


Coal is one of the the main fuels for power production. Coal quality deterioration over the years

has created challenges for boiler designers the world over to compact and minimize erosion in

pressure parts. Fly ash erosion is a major factor for pressure parts damage in high ash coal fired

boilers.

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In high ash coal fired boilers, fly ash erosion is a major concern and the tube failures due to flyash erosion are almost 35% of the total tube failures. The amount of ash in coal and its velocity

are major factors in the rate of pressure part erosion. Fly ash erosion is experienced in the

economizer, primary SH, and inlet section of steam reheater tubes. When non-uniform flue gas

flow distribution occur in these areas, the rate of erosion increases multifold.

Factors influencing fly ash erosion in coal fired boilers are

o The velocity of flue gas

o The temperature of flue gas

o The mineral content in coal

o The change in direction of flue gaso The arrangement of pressure parts and

o The operation above the maximum conditions design rating or with excess airflow above

design rate.

Of these factors, the velocity of flue gas, the temperature of flue gas (ash), and mineral matter in

coal are the main influencing factors.

The velocity of flue gas

For low ash coals, the weight loss in pressure parts due to erosion is proportional to flue gas

velocity to the power of 1.99. However for high ash Gondwana coals the erosion rate is velocityto the power of 3 to 5. The power depends upon the percentage of ash in coal, the percentage of

silica in coal ash, the percentage of quartz in this silica, the percentage of alpha quartz in this

quartz, and the structure of alpha quartz.

Temperature of flue gas

Higher temperature softens the minerals in the ash as well as reduces the strength properties of

the material of pressure parts; due to this ash erosion is not predominant in high temperature

zones like furnaces, final superheaters, exit reheaters, etc. The ash erosion mainly starts in the

conventional two-pass boilers from the area where gas temperature is around 700 750 deg.C.


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The low temperature superheater (LTSH) and economizer are the areas where ash erosion is

severe in a conventional two-pass boiler. The temperature of flue gas entry to LTSH can bearound 650 to 700 degree C and leaving, the economizer can be around 350 300 degree C. The

minerals, which mainly constitute the ash in flue gas at these temperatures, become hard andattain its full abrasiveness.

Mineral matter in coal

The proportion and composition of the mineral matter in coal will determine the extent of fly ash

erosion that can take place. All the mineral matter undergoes phase transformation during the

process of combustion of coal in furnace. The phase transformation of the mineral matter is

dependent on various factors like the presence of oxygen (oxidizing or reducing atmosphere) in

the localized area of furnace, the temperature of the flame / furnace, the retention time, thecomposition of the minerals in question, etc.

Measures to Reduce Flyash Erosion

The following are the areas in boiler where coal ash erosion is normally experienced.

(i) economizer bends and tubes

(ii) LTSH bends and tubes

(iii) screen tubes

(iv) goose neck portion at furnace top

(v) soot blower openings in the water walls

(vi) wind box opening in the furnace

(vii) bottom hopper tubes

In the case of (i), (ii) and (iii) the erosion is due to ash in the flue gas stream directly impact and

flow over the tube. In the case of (iv) and (vi) it is more due to ash collected in this region sliding

over the tubes. In the bottom hopper impact of the water wall deposit is predominant. In the caseof (v) and (vi) it is more due to entrained ash / fuel causing erosion due to eddies formed in thisarea.

To reduce the erosion in these areas

(a) Reduced gas velocity in second pass

(b) Use Inline arrangement for all second pass heat transfer surface


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(c) Provide shield in places prone for higher erosion

(d) Provide cassette baffles for LTSH and economizer bends

(e) Go for refractory lining in areas of high erosion where shields cannot be provided.

As low grade coals are now emerging to be used in large quantity in boilers for power generation

and process steam requirement, it has become necessary to protect the pressure parts from ash

erosion. It can be said with confidence that in the case of high ash coals, erosion cannot beavoided; it can be only minimized to an optimum level. However data show that the boiler

pressure parts in the second pass like LTSH and Economizer may need replacement in full from

about 10-15 years of operation depending upon the nature of the ash, the type of operating

regime maintained, etc.


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Coal Formation Theories


Coal may be defined as a compact stratified mass of plant debris which has been modified

chemically and physically by natural agencies, interspersed with smaller amounts of inorganic

matter. In situ and Drift are the two major theories of coal formation.

Theories of Coal Formation

The natural agencies causing the observed chemical and physical changes include the action of

bacteria and fungi, oxidation, reduction, hydrolysis and condensation - the effect of heat and

pressure in the presence of water.

Many factors determine the composition of coal.

o Mode of accumulation and burial of the plant debris forming the deposits.

o Age of the deposits and the geographical distribution.

o Structure of the coal forming plants, particularly details of structure that affect chemicalcomposition or resistance to decay.

o Chemical composition of the coal forming debris and its resistance to decay.

o Nature and intensity of the peat decaying agencies.

o Subsequent geological history of the residual products of decay of the plant debris

forming the deposits.

The In situ Theory of Coal Formation

Major in situ coal fields generally appear to have been formed either in brackish or fresh water,

from massive plant life growing in swamps, or in swampland interspersed with shallow lakes.

The development of substantial in situ coal measures thus requires extensive accumulations of

vegetable matter that is subjected to widespread submersion by sedimentary deposits.

Accumulations of vegetable matter and associated mineral matter, generally clays and sands, are

balanced by the subsidence, or motion of the Earths surface, in the area on which these materials

are accumulating. Hence, coal formed like this has bands of coal and inorganic sedimentary

rocks arranged in a sequence.

The Drift Theory of Coal formation

It was the difference in coal properties of Gondwana coals that led to the formation of the drift

theory. The mode of deposition of coal forming can be explained as said below:

o Coal is formed largely from terrestrial plant material growing on dry land and not in

swamps or bogs.

o The original plant debris was transported by water and deposited under water in lakes or

in the sea.


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o There will not be a true soil found below the seam of coal.

o The transported plant debris, by its relative low density even when water logged, wassorted from inorganic sediment and drifted to a greater distance in open water. The

sediments, inorganic and organic, settled down in regular succession.

o The process of sedimentation of the organic and inorganic materials continues until the

currents can deposit the transported vegetation in the locations.

o These deposits are covered subsequently by mineral matters, sand, etc. and results in coal

seams.

o The depositions can also stop for a particular period and again begin to happen depending

upon the tidal and current conditions.

o The coal properties vary widely due to the varied types of vegetation deposited.


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Coal Analysis for Boiler Designers


Hydrocarbon fuels are the major source of energy for power and process steam generation, and

coal takes a major share in this. Boiler furnace design will depend more on fuel characteristics,

and further heat transfer surface sizing will depend on furnace outlet temperature.

The world's thermal power is mainly dependent on coal as its fuel. When designing a boiler, fuelanalysis plays a major role. The performance of the boiler, and ultimately the entire unit, can

change considerably if the coal being used is substantially different from that for which the

boiler was designed.

What is fuel?

o Any combination of organics and inorganic material which during chemical reaction or

transformation gives out large amount of heat is called fuel

o Fuel can be hydrocarbon fuel and non-hydrocarbon

o Industrial fuels have heat values from as low as 500 kcal/kg to as high as 11000 kcals/kg

Heat generated by fuel is used a boiler to generate steam for process, power generation, and avariety of other applications. The chemical characteristics of the fuel decide many aspects ofboiler design. The boiler designed for gas fuel will have the smallest furnace size, and the boiler

designed for coal will have the biggest size.

Why is coal different?

o All fuels are hydrocarbons

o Gas and oil have defined hydrocarbons and structure, which means C & H in fuel does

not vary much

o Coal is a heterogeneous fuel and has only an assumed structure C & H vary highly

o C & H in Coal can be only be known if you do an ultimate analysis, and the way in which

these hydrocarbon behave can be different from one coal to the other. This will depend

on its reactivity and formation.o Formation of coal has vast impact on boiler design; the Gondwana coals are different

from American and European. (Coal formation theories).

Why is consistency in hydrocarbon important for boiler performance?

o It starts from combustion air calculation; the carbon hydrogen ratio decides the quantity

of combustion air.

o Flame temperature is dictated by the chemical composition of the fuel, and this changes

the furnace behavior.

o The completion of combustion is another very important factor in boiler design. This will

depend on how the hydrocarbon rings are formed and bonded. If the coal burns slowly

requiring more residence time then the SH and RH (superheater and reheater) behavior


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Predicting Combustibles in Pulverized Coal Fired Boilers -

Fly Ash and Bottom Ash


Predicting the percentage of combustibles in fly ash in a tangential fired boiler using proximateanalysis of coal gives boiler designers an edge during the proposal and contract stages. Here is

how to predict fly ash and bottom ash combustibles in order to compute carbon loss in a boiler.

In boilers with pulverized firing systems, about 80% of the ash in coal being fired is carried as

fly ash. The other about 20% get collected as bottom ash. During the combustion of coal, some

portion of the hydrocarbon, mainly char, leaves the furnace as unburned particles. The amount of

such unburned particles leaving the furnace depends on many factors like the coal property, the

type of burning system, the resident time available in the furnace, the ash percentage in coal, thecalorific value of coal, the fuel ratio, the operating conditions, etc. The existence of unburned

carbon in ash decreases not only the combustion efficiency, but also the grade of fly ash forcommercial sale.

Carbon loss is influenced by the following: (1) coal preparation and grinding, such as changes in

ash and maceral content ; mean, standard deviation, and higher moments of the particle size

distribution; moisture remaining in the pulverized coal, (2) properties of the pulverized coal andits char like heating value, char yield on pyrolysis, char structure, char reactivity, ash content and

composition, and characteristics, and (3) adjustments of the burners and furnace such as airpreheat, excess air, mixing, residence time, and furnace temperature.

Hottel and Stewert (1940) were the first to consider the interaction between furnace design and

coal properties in the determination of carbon conversion, analyzing the effects of grind,

reactivity, temperature, excess air, and residence time on unburned carbon loss.

With the estimated values of percentage combustibles in fly ash as well as bottom ash, the

carbon loss can be calculated by using the formula given in BS_EN_12952, ASTM, PTC 4 and

any other International Standards.

Boiler designers during the design stage have only proximate analysis, ultimate analysis and ash

composition of coal. Carbon loss calculation involves calculating the carbon loss in fly ash and

bottom ash. This article provides a tool for the designers and others to predict the percentage ofcombustibles in fly ash and bottom ash in a tangential fired boiler using proximate analysis of

coal and the residence time in the boiler furnace. Based on combustibles in flyash and bottom

ash, it is possible to compute the carbon loss in a boiler.

Fly ash unburned prediction

The major portion of carbon loss in a boiler is from unburned carbon in fly ash. A method was

developed by me after a large volume of data was subjected to analysis and validation. It is seen

from the analysis and literatures that the fuel ratio i.e. the ratio of fixed carbon and volatile


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matter in coal has a very significant effect. The ash in coal is a burden for combustion and can

cause large problems during and after combustion. Deposits and slagging in boiler furnaces usinghigh amounts of medium slagging and slagging coals are common. After combustion they can

foul the heat transfer surface in the convection region. So it is seen that log of ash % correlateswell with fly ash combustibles. Coal calorific value indicates the heat value of the coal being

fired hence has to be taken into account when we want to predict the fly ash combustibles. The

calorific value of the coal in question divided by the calorific value of carbon gives meaning to

the factor. This indicates the relative stage where the coal in question lies with respect to its

ultimate transformation and also is an indirect indicator of the difficulty to ignite and burn. I

would not like to call it as reactivity as the same has not been studied / understood much with

respect to this ratio of coal. Inverse of residence time is another major factor which affects the fly

ash combustibles. As boilers are operated within a close range of excess air and fineness of coal,these variables do not affect the unburned to any significant level.

A factor combining all the parameters is evolved which is used for fitting a curve withpercentage combustibles in fly ash. The factor is defined as

[{(FC/VM)+(HVV/8080)*100+Log(A)}/Res^2]

The equation governing the curve fitted on a fourth order polynomial is

Y = -3E-06 X4

+ 0.0004 X3- 0.0161 X

2+ 0.2969 X - 0.9438

with a R square value of 0.8824.

As this predictive equation is only made for a pulverized coal tangentially fired boiler, this has tobe verified for pulverized coal wall firing, down shot firing, opposed firing, etc. However, more

than 50% of the pulverized coal fired boilers in the world are equipped with tangential firingsystem.

Bottom ash unburned prediction

The single most independent variable affecting the bottom ash combustibles is the plus 50 mesh

size of pulverized coal. A plot of percentage bottom ash combustible plotted against percentage

plus 50 particle sizes has a fourth order polynomial curve with an R2value of 0.9412. The

equation governing this fit is

Y1= 0.0233X14- 0.3925X1

3+ 1.9277X1

2- 0.1593X1+ 0.2357

where, Y1is percentage combustibles in bottom ash and X1is plus 50 mesh particle percentage

in the pulverized coal.

It is seen that this percentage plus 50 in the pulverized fuel should be retained below 2% to

minimize the percentage combustible in bottom ash. This is generally recommended by boiler

manufacturers.


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Flyash & Bottom ash Combustible

- See more at: http://www.brighthubengineering.com/power-plants/37958-predicting-combustibles-in-pulverized-coal-fired-boilers-fly-ash-and-bottom-ash/#sthash.VFDN9Fhs.dpuf


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Importance of Boiler Water Treatment

written by: johnzactruba edited by: Swagatam updated: 5/18/2011

Maintaining water quality is high on the agenda of all boiler operators. What makes it so

important? Why is it all the more important in a Supercritical unit?

Boiler water quality has long been an important factor in the operation of boilers. As the powerplant operating pressures increase, water quality requirements also become stricter. With thecurrent units operating at Supercritical pressures, the requirements are tough. Continuous

improvements and changes in the methods of maintaining water quality, understanding the

corrosion mechanisms, and the development of new chemicals have resulted in a more

economical and efficient water regime management.

Four Reasons Why Boiler Water Treatment is Important

There are four main reasons why water quality is so important. Impurities in water form scales.

o Water contains dissolved salts, which upon evaporation of water forms scales on the heat

transfer surfaces. Scales have much lower heat transfer capacity than steel: the heat

transfer coefficient of the scales is 1 kcal/m/C/hr against 15 kcal/m/C/hr for steel . Thisleads to overheating and failure of the boiler tubes. Scale also reduces flow area, which

increases pressure drop in boiler tubes and piping.

o Low pH or dissolved oxygen in the water attacks the steel. This causes pitting or

lowering the thickness of the steel tubes, leading to rupture of the boiler tubes.

Contaminants like chlorides, a problem in seawater cooled power plants, also behave in asimilar way.

o Flow assisted corrosion occurs in the carbon steel pipes due to the continuous removal of

the protective oxide layer at high flows.

o Impurities carried over in the steam, causing deposits on turbine blades leading to

reduced turbine efficiency, high vibrations, and blade failure. These contaminants canalso cause erosion of turbine blades. Silica at higher operating pressures volatilizes and

carries over to the turbine blades.

The first step is to get the make-up water to the steam cycle as pure as possible. The correct

operation of the De-Mineralisation (DM) plants ensures this.

The second step is to form a magnetite layer on the inside surface of the tubes which protects themetal surface from any further corrosion attacks.

The third step is to maintain this magnetite layer throughout the life of the plant.

If the water quality goes down, this protective layer will be destroyed and corrosion startsdamaging the tubes.

In a 500 MW power thermal plant around 1300 Tons of water is circulating per hour

continuously in the water steam cycle through the boiler, turbine, condenser and heaters. As the


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water circulates, there is an increase in contaminant level and a change in water quality. This is

due to many reasons like:o contact with almost 25000 m area of wetted steel in the tubes, piping and heat

exchangers

o the residue of chemicals added

o entrapped oxygen and other gases especially in the vacuum area

o returning condensate from traps, glands, vents and drains

o impurities in the DM water make-up

Major parameters that require monitoring for water treatment are:

1. The dissolved solids.

2. The pH of the boiler feed water.

3. Dissolved Oxygen in the feed water entering the boiler.4. Silica in boiler water.

Water Steam Circuit- Sub Critical vs Super Critcal


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Differences in a Supercritical Unit

The Water Steam circuit in a Supercriticalunit is different from that of a sub-critical unit. This

makes the water quality requirement more stringent in a Supercritical unit.

In a subcritical unit, water steam separation takes place in the drum. Any contaminants remain in

the water. As their concentration level increases, continuous blow down removes these. The

drum, water walls, and down comers act as a reservoir and an internal circulation circuit and helpin concentrating and removal.

o In supercritical once-through boilers this is not possible, which means any contaminants

will adhere to the tubes or caries out through the steam. To prevent this, purity of the

water entering should be very good. This makes it mandatory to have a full condensate

polishing unit before the water enters the heating sections.

PH control is by the addition of chemicals like Tri-Sodium Phosphate in the boiler water or the

caustic treatment. This helps in maintaining the pH levels in the range of 9.0, slightly alkaline.


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The chemical reactions result in the formation of salts, which increases the dissolved solids level.

In subcritical unit, blow down removes this.o This is not possible in supercritical units. So an all volatile treatment (AVT) is used.

This method uses amines whose reaction products are volatile, leaving behind no solids.This passes along with steam and removal takes place in the de-aerator or polishing

systems. AVT is also the new method in subcritical drum type units.

o Super critical units also use the oxygenated treatment (OT) system, which involves

injecting a known quantity of oxygen in the feed water. This helps in maintaining the

magnetite or hematite layer, which provides the barrier to prevent any further corrosion in

the piping and tubes.

o During start-ups and at lower loads where the water chemistry regimes are fluctuating,

boiler water control is by the AVT method.

.

Dissolved oxygen removal is in the deaerator where at saturation temperatures oxygen strippingis easier. Addition of hydrazine at the deaertaor outlet also removes the dissolved oxygen if any

in the feed water.

o Supercritical units also use deaerators. But some plants using only OT operate without adeaerator.

Silicacontrol can is by blow down in a subcritical unit.

o In Supercritical units the only way is to ensure very low Silica in incoming DM water and

good removal in the condensate polishing unit.

As the thermal plant operating pressures increase and become supercritical, water chemistry

management also becomes critical. Along with adopting the correct water treatment method, a

high quality DM plant and precision analytical instruments for monitoring online waterchemistry is a must to eliminate outages of the plant.


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Low Drum Level in Boilers - A Major Cause of Concern


Drum level in a boiler indicates the level of water in the drum available for steam generation.

Low drum level operation leads to availability loss of the boiler. Low-level trips in a boiler must

always be responded to quickly and correctly.

The drum level in a boiler is maintained near the previously defined normal water level, whichgenerally is below the geometric center of the drum. Maintaining a high drum level has its own

problems like carryover of salt to superheaters etc., but low drum level operation has much more

serious effect on the boiler tubes. Whenever this happens, the operator is warned by alarm to take

corrective action. If this is not responded to and the drum level goes further down to a

dangerously low level, the boiler trips on auto to protect the boiler. The specific causes for adrum level trip, the boiler response, and the immediate action of the boiler control room operator

and the local operator are given in outline form below.

Specific causes

o One feed pump trips

o Mal-operation of feed control autoo Mal-operation of feed pump scoop

o Mal-operation of feed control / regulating valve

o Sudden reduction in load

o Sudden tripping of one or more millso Tube failure in water wall with large opening

o Mal-operation of emergency drain valve

o Mal-operation of low point drain

Plant response

o Low drum level alarm

o Very low drum level trip

Immediate boiler desk operator action

o Start reserve feed pump if needed

o Change to manual feed water control if required

o Never by-pass the very low drum level trip

o Trip the unit if the visible level goes out even if auto did not act

Immediate local operator action

o Check the tripped pump, rectify cause and inform boiler desk operator that it is ready for

restarto Check for possible tube leak in furnace first and other areas if needed


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o Check drum drain and low point drain for possible opening

o Check the feed water auto controller in local and inform condition, also arrange forrectification

o Check feed controller for any link failure

The effect of very low drum level operation is very severe that it can cause the water wall tubes

to get overheated; it can cause snaking of water wall tubes leading to lot of projection in and out

of furnace. It can cause instant short term fish mouth tube failures in water wall. This kind of

failure has led to furnace explosion under certain specific conditions and based on the location of

the failure. If the failure of the tube is in such a location in furnace that the steam coming out of

the tube mixes with the coal particles to form producer gas, then an explosive mixture forms andthe boiler furnace explodes. The overheating is seen to happen in many water wall tubes rather

than a single tube as experienced in many cases.

Rectification of the furnace wall becomes more difficult and time consuming as it requiresextensive checking both by an NDT (non-destructive testing) method as well as a samplingmethod. Leaving tubes that have snaked may not lead to any adverse effect in performance of the

boiler except in certain specific locations, like near the burner where fuel impingement can occur

and cause fuel ash erosion.

The reason for such major failure of water wall tubes when the drum level goes to lower than the

very low limit is due to the fact that the steam in the drum gets entrained in the down comers of

the circulation system, and this upsets the whole natural circulation in the boiler. When the

circulation in furnace tubes is upset, the cooling of tubes does not take place effectively, which

leads to failure of tubes due to short term overheating.

Low Drum Level


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Coal Power - the Backbone of Power Generation


Coal fired power plants form the major percentage of boilers used for power generation across

the globe. The main reason is coal availability and its economics for power generation. Looking

only from a technical aspect, gas fired boilers will be preferred when compared to oil and coal

boilers.

Any power plant will have fuel storage and handling equipment and systems, the fuel preparation

system, water treatment system, the boiler, the ash handling system, compressed air system,

lighting system, firefighting system, turbine, generator and electric power gridding system. The

coal power plant is no exception. The pros and cons of these systems individually has to be

looked into to understand the total value of a coal fired power plant. Here the discussion will bemainly on the fuel handling, fuel preparation, ash handling system, and boiler point of view.

Fuel storage and handling

o The calorific value of coal varies very widely across the globe. It can be as low as 2800

kcals/kg to as high as 7000 kcals/kg on an air dried basis. This means the quantity to be

stored, and handled per million kilo calories, will also vary widely.o Oil and gas, which are classified under fossil fuels, do not have such large variation.

Hence storage and handling does not differ much.

o Coal is stored in the open generally; however closed storage is also adopted in a limited

basis. Storage of oil and gas need special tankers and equipment.o Coal requires large and heavy equipment to handle, however oil and gas requires much

simpler and compact equipment to handle.

o Coal can result in lot of dust problems, but gas and oil can result in leakage which can

explode.

o Coal requires large bunkers near boiler area; oil can be stored in day tanks and gas can bepressure reduced and needs, in most cases, no storage.

Fuel preparation system

The fuel preparation system in the case of oil is a simple heating and pumping unit. In the case of

gas it is also a simple pressure reducing and water removing system. In the case of coal firedboiler it is an elaborate system starting from a crushing unit to a grinding unit with a rejectshandling system, if the mills in the grinding system have rejects. Optimizing the milling system

for optimum performance requires knowledge about the coal being fired and the combustion

characteristics, which involves large laboratory tests. In the case of oil, simple laboratory tests

estimating the temperature for firing viscosity will be enough. The need to know the chemical

composition is for all fuels.


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Ash handling system

This is unique to the coal fired boiler. There are two ash handling systems to be provided: the flyash and bottom ash handling system. This size of this system will depend upon the ash

percentage in the coal. However no such system is required for an oil or gas fired boiler.

Boiler

Boiler design depends on the type of fuel being fired; gas fired boilers have the smallest furnaceand the coal fired the biggest. The coal fired boiler furnace size further changes with respect to

the amount and type of inorganics (ash) the coal contains. When we compare two boilers firing

with the same percentage ash coal- one slagging type and the other non-slagging- the one with

the slagging nature will have to be designed with a larger furnace size. Except for the economics

of coal fired boiler for steam generation, in all other aspects it has a lesser preference with the

users. Coal fired boilers have many more auxiliaries, are cumbersome to operate, theuncertainties in fuel characteristics are high, higher pollutants have to be addressed, etc. There

are many types of design available for all fuels, and more so for coal firing with combinations ofauxiliaries. The amount of auxiliary power consumption is the highest for coal fired boilers.

Other systems

All other systems like the compressed air system, lighting system, firefighting system, turbine,

generator and electric power gridding system can have the same features for any type of boilerused with any fuel as they all are independent of boiler type and fuel.


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Understanding Flame Quality in Tangential Firing Boilerswritten by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 5/18/2011

In a tangential coal fired boiler, the furnace act as a single burner and so it is required to look atand understand the quality of the flame. It is necessary to start from the control room of the

boiler, then go to the mill, furnace, bottom ash and fly ash areas and study all in detail.

Understanding the quality of flame in any boiler furnaceis very important in tuning the boiler to

the optimal level of performance. The aspects of combustion tuninginvolve looking at the boiler

furnace and making sure the quality of flame is acceptable and good.The gas and oil fired boilers

do not pose much problem in establishing a good flame in the furnace. The available instrumentslike flame scanners, CO monitors and oxygen indicators, along with the exit gas temperature,

give a good indication to perceive if the quality of the flame is good. In coal fired boilers andmainly in tangential fired boilers, the furnace acts as a single burner, so it is required to look at

the flame and understand the quality of the flame.

It is necessary to start from the control room of boiler then go to the mill area, to the furnace, and

then to the bottom ash and fly ash area to fully make sure of combustion quality in furnace.

The control room of the boiler

o Look at the load at which the boiler is operating, availability of support fuel, SH & RHparameter

o Look at the number of mills operating

o Note the load, air flow, and outlet temperature on each mill

o Check the oxygen level at Eco / APH outleto Check the furnace pressure, scanner performance- watch for a few minutes for any

fluctuations

o Look at the coal proximate analysis within 8 hours- if not available then at the max 24 hrs

o Check the PC fineness reading of each running mill if available

o Keep a note of those mills which have plus 50 more than 2% and minus 200 below 65%

Check each running mill in the mills area

o Bowl mills

Check each spring loading by feeling the bumping of the pressure spring shaft

Regular bump indicate the springs are loaded - how much cannot be estimated byfeel - low minus 200 & or mill reject can be an indicator

Watch for any abnormal sound

Check the level of mill reject - look for coal in rejects - if nil or very low then ok

Look at classifier vane position check if they are equal in each mill. Close

further to improve fineness - if needed


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o Ball mills

Check for ball noise in the mill area - high noise indicates low coal levels or low

speed

Check the mill speed and by-pass air

Check classifier settings

Look for any gear box noise

Understanding the quality of flame in any boiler furnace is very important to tune the boiler to

the optimal level of performance. The aspects of combustion tuning involve looking at the boiler

furnace and making sure the quality of flame is acceptable and good. The gas and oil firedboilers do not pose much problem in establishing a good flame in furnace. The available

instruments like flame scanners, CO monitors and Oxygen indicators along with the exit gas

temperature give a good indication to perceive the quality of flame is good. In coal fired boiler

mainly in tangential fired boiler, as the furnace act as a single burner, it is required to look at theflame and understand the quality of the flame.

Checking the furnace and flame conditiono Look at the flame front in each corner and elevation- stand in front of the peephole at the

corner and see if you can find a tip of flame moving to and fro (visible and out of sight)

o If it is not visible note the fuel air damper position in the windbox and close it to less than

5% open condition

o Look at the flame in the furnace from just above wind box and count the number of

flickers. Go to the furnace outlet plane elevation - normally at this elevation more peepholes are provided. Check the flicker at this elevation- if less than 10 per minute and

bright orange red in colour, the flame is good

o Open all peep holes one by one in the bottom hopper level - normally around 10 Mt level

o Watch the particles falling each peep hole on both side wallso If large number of shooting star-like particles of pebble sizes or above is seen with

frequent intervals or continuous this indicate deposition on the waterwalls

o If this is experienced, then look at the bottom ash collection for any glazed lumps - this

confirms deposition tending to slag

o Open the other peep holes and see if they are loaded with ash particles or bridged by

fused particles or glowing ash in with a narrow opening or black colour in center. If loose

ash of some amount is found, this indicates friable ash and is normally seen in goodoperation / furnace condition. Higher fused ash and bridging indicates that furnace

deposits are high and/or high temperature combustion

o In the high heat flux region - just about 1 Mt above the wind box / top burner level - one

will see fused lumps, but not bridging or flowing slag - if seen then higher furnacedeposition is indicated

o Look through the top most peep holes in the front wall - generally given at furnace outlet

plane, apart from seeing the flame flicker level as said above, look at the Platen / Panel /

Final SH which is provided


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Watch for deposit on these. Large volume deposit seen indicate lower frequency

of soot blower operation or high deposition levels You may also see a swing in super heater panel / platen / final in this region. This

is not uncommon and not to worry.

o Look through the peep hole in the side walls near the reheater region.

If flame seen licking this area indicate the burning is getting completed at much

higher elevation than envisaged.

This can be due to low reactive coal or improper air distribution or very high

primary air

Watching carefully the flame / bright spots coming into this region with respect to

width and intensity can give an indication on the elevation of combustion

completion

Checking the bottom ash and fly ash

o Bottom ash Look at bottom ash, if you find glazed lumps then suspect clinkers

The higher the size and quantity, the higher the intensity of clinkering

A large amount of loading black particles indicates higher unburned in bottom ash

A higher quantity of friable ash along with coal particles indicates a higher plus

50 size % in PC or wall deposits shedding at frequent intervals

o Fly ash

While collecting flyash from ESP field hopper, allow it to flow freely for some

time so that locked up ash falls off

Look at the fly ash from the first working field of ESP - not to worry about the

dummy field If fly ash has higher blackish colour, this indicate higher unburneds

If higher black particles are seen, this indicate very low reactive constituents in

coal


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Advantages and Disadvantages of Coal for Power Plants

written by: jaychris edited by: Lamar Stonecypher updated: 4/27/2011

Fossil fuels are indeed the top fuels used all over the world for generating power and electricity.

Among the fossil fuels, coal is the most widely used fuel in power plants. Coal fired plants use

different kinds of machinery that convert heat energy produced from combustion into mechanical

energy.

Coal, gas, and oil are the fossil fuels responsible for most of the world's electricity and energy

demands. Coal, which is readily available in most of the developing and developed world, has

been used as a major source of fuel even in ancient human civilizations. It also found its use in

historic steam engines at the dawn of the industrial revolution.

Advantages of Coal as Power Plant Fuel

Today, advances in technology have allowed coal to improve living conditions with its currentrole in meeting mans fuel needs. Coal has been used extensively in power generation where

better technology is employed to ensure that there is a balance between ecology and economics

in producing sustainable and affordable energy. But, is coal really the answer to affordableand

sustainableenergy? To find answers for this question, it is best to learn about the advantages-

and disadvantages- of coal fired plants. Some of its advantages include reliability, affordability,

abundance, known technologies, safety, and efficiency.

Reliability.One of the greatest advantages of coal fired plants is reliability. Coals ability to

supply power during peak power demand either as base power or as off-peak power is greatlyvalued as a power plant fuel. It is with this fact that advanced pulverized coal fired power plants

are designed to support the grid system in avoiding blackouts.

Affordability.Energy produced from coal fired plants is cheaper and more affordable than other

energy sources. Since coal is abundant, it is definitely cheap to produce power using this fuel.Moreover, it is not expensive to extract and mine from coal deposits. Consequently, its price

remains low compared to other fuel and energy sources.

Abundance.There are approximately over 300 years of economic coal deposits still accessible.With this great amount of coal available for use, coal fired plants can be continuously fueled in

many years to come.

Known technologies.The production and use of coal as a fuel are well understood, and thetechnology required in producing it is constantly advancing. Moreover, coal-mining techniques

are continuously enhanced to ensure that there is a constant supply of coal for the production of

power and energy.

Safety.Generally, coal fired plants are considered safer than nuclear power plants. A coal power

plant's failure is certainly not likely to cause catastrophic events such as a nuclear meltdown


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would. Additionally, the welfare and productivity of coal industry employees has greatly

improved over the years. In fact, injuries, time lost, and fatalities have decreased significantly inthe past years.

Fig.1. 2009 US Electricity Source Generation

Source: http://en.wikipedia.org/wiki/Fossil_fuel_power_station

Disadvantages of Coal-Fired Power Plants

On the other hand, there are also some significant disadvantages of coal fired plants including

Greenhouse Gas (GHG) Emissions, mining destruction, generation of millions of tons of waste,

and emission of harmful substances.

Greenhouse gas emissions.It cannot be denied that coal leaves behind harmful byproducts upon

combustion. These byproducts cause a lot of pollution and contribute to global warming. The

increased carbon emissions brought about by coal fired plants has led to further global warmingwhich results in climate changes.

Mining destruction.Mining of coal not only results in the destruction of habitat and scenery,

but it also displaces humans as well. In many countries where coal is actively mined, manypeople are displaced in huge numbers due to the pitting of the earth brought about by

underground mining. Places near coal mines are unsafe for human habitation as the land could

cave in at anytime.

Generation of millions of tons of waste.Millions of tons of waste products which can no longer

be reused are generated from coal fired plants. Aside from the fact that these waste products

contribute to waste disposal problems, these also contain harmful substances.

Emission of harmful substances.Thermal plants like coal fired plants emit harmful substances

to the environment. These include mercury, sulfur dioxide, carbon monoxide, mercury, selenium,and arsenic. These harmful substances not only cause acid rain but also are very harmful to

humans as well.


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Economizer and Air Pre-Heaters are Provided for Heat

Recovery


Boilers are provided with economizer and air pre-heaters to recover heat from the flue gases. Anincrease of about 20% in boiler efficiency is achieved by providing both economizer and air pre-

heaters. Providing economizer alone gives only 8% efficiency increase and so designers provide

both.

Most of the high capacity boilers firing coal operate with an efficiency of around 86% on the

Higher Heat Value basis. Loss of around 14% can be attributed to various losses of which the dry

gas loss is about 35% of the total. When both economizers and air pre-heaters are not provided

the boiler efficiency drops to around 66% from 86%. When air pre-heater is not provided theboiler efficiency will be around 74 % only. Thus we can conserve about 20% extra fuel when we

provide both economizers and air pre-heaters in boilers.

Economizer

The feed water from the high pressure heaters enters the economizer and picks up heat from the

flue gases after the low temperature superheater. Many types of economizer are designed for

picking up heat from the flue gas. These can be classified as an inline or staggered arrangementbased on the type of tube arrangement. The staggered arrangement is compact and occupies less

volume for the same amount of heat transfer when compared to the inline arrangement.

Economizers are also designed with plain tube and fined tubes. The fins can be longitudinal orspiral. All these types are suitable for clean fuels like gas, oil, and low ash coals. For high ash

coals, only the plain tube inline arrangement is used. This is mainly to reduce ash erosion and

thus reduce erosion failures. These economizers pick up about 50 to 55 degrees centigrade in alarge capacity boiler, which will reduce the flue gas temperature by about 150 to 170 degree

centigrade. The boiler designers always keep the economizer water outlet temperature to about25 to 35 degrees below the drum saturation temperature. This is done to mainly avoid steaming

in the economizer. A steaming economizer generally is less reliable. As a rule of thumb, for

every one degree pick up of economizer water temperature, there will be a drop of about 3 to 3.5

degrees.

Air pre heaters

Air pre-heaters are provided in boilers to preheat the combustion air. There are two main types:

recuperative and regenerative air heaters.

Tubular or recuperative air pre-heaters are provided in boilers of medium and small range ofsteam generation. This type of air pre-heater becomes very large in size if they have to be used in

very high capacity boilers like 600 tons/hr of steam production and above. In these casesregenerative air pre-heaters are used. The arrangement of all these air pre-heaters differ with the

design and, in large, the way they are combined for very high capacity boilers. Regenerative air


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per-heaters are compact and can have a stationary or rotating hood. A combination of tubular and

regenerative type of air pre-heaters is used in very high capacity boilers. The tubular being usedfor primary air heating and the regenerative used for the secondary air heating. In case the boiler

designers do not want to go for a combination of tubular and regenerative air pre-heater, thenthey have a choice of tri-sector regenerative air heater. Normally the ambient air is heated to

about 300 to 350 degree centigrade. This results in a flue gas temperature drop of around 230 to

250 degree centigrade. So for each degree pick up in air temperature, roughly 0.8 degree drop in

flue gas temperature is achieved.

Steam coil air pre-heaters are another type. These are used only during start up of the boiler to

prevent low temperature corrosion. This air heater does not contribute to improving theefficiency of boilers, but are provided to improve availability. It is seen that during start up the

chances of low temperature corrosion is high, and hence the need to provide the steam coil air

heaters is evident.

Both economizer and air pre-heaters are called heat recovery systems in a boiler. Were it not forthese heat recovery systems, present day boilers would be operating at much lower efficiency

levels.


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Low Furnace Pressure and Boiler Operation


Low furnace pressure can cause equal damage like furnace pressurization or a explosion. Low

furnace pressure leads to a situation called furnace implosion if not corrected at the right time.

To correct a imploded boiler requires a very long outage and work to be carried out.

Balanced draft boilers are prone to a very low pressure condition inside the furnace. Thiscondition creates a large amount of force on the waterwalls of the furnace. The buckstays which

are provided in the boiler furnace are designed to handle both high and low pressure conditions

that can damage the furnace. However if the low pressure continues, and if it exceeds the limit

the buckstays can withstand, then the furnace is subject to implosion. Implosion happens when a

large pressure acting on the walls of the furnace from outside overcomes the very low pressureprevailing inside the furnace. This leads to a high differential pressure between the atmospheric

pressure outside and the pressure inside the furnace.

Some of the furnace implosions have led to replacements of the furnace walls and have caused

large financial losses to the owners. It is also possible to get into a furnace implosion condition in

positive pressure boilers. In fact, in balanced draft furnaces the availability of openings softens

the effect to some extent when they are near breakeven points. The damage to the furnace is

nearly the same if implosion happens.

The reasons of furnace implosion are many.

o Induced draft fan control failure is one of the common causes in a balanced draft boiler

o Induced draft fan vane control failure also leads to furnace low pressure condition in

balanced draft furnace.

o Sudden load throw off leading to a large fuel cutting is another common cause of low

furnace pressure in both positive and balanced draft boilers

o Sudden reduction in air flow

o Sudden tripping of one forced draft fan, leading to a large reduction in air flow

The plant responds to these conditions by a large reduction in furnace pressure and leads to

unstable furnace flame conditions.

Depending on the indication and the reason for the cause of the problem the boiler operator will

have to respond.

o Switch to induced draft fan control from auto mode to manual mode if the control system

is the reason leading to low furnace pressure and take corrective action. Ask the controls

engineer to rectify the fault before again turning on to the auto mode.

o Check the induced draft fan vane control system for proper functioning by changing the

set point and finding the response. Rectify if required.

o Check the air flow condition and ensure the correct air flow to the boiler as per the load

condition


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o If one of the forced draft fan has tripped then after establishing the reason for trip and

correcting the same, restart the forced draft fan.

The local operator in the field invariably has to check the induced draft fan vane control if this

was the reason causing the low furnace pressure. If one of the forced draft fan has tripped, then

he has to make ready the fan for restart after ascertaining the reason for the trip.

Low Furnace Pressure Flowsheet


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High Furnace Pressure Problems in Boilerswritten by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 8/15/2010

High furnace pressure is a direct indication of a disturbance in combustion performance. The rateof change in the furnace pressure can vary very much, that is from a simple sudden surge due to

fuel variations to a huge furnace explosion.

Combustion in a boiler furnace is normally considered a controlled explosion, as all boilers

operate at a regime where explosion pressure is the maximum. This is mainly due to the fact that

it is at this regime we get the maximum efficiency of the boiler. Furnace pressure in boilers is

one of the most critical and important parameter to be maintained and monitored. Furnacepressure in a balanced draft furnace is always kept negative, and in pressurized furnace it is

positive. Present days designers adopt balanced draft furnace for all solid fuel fired boiler,however there are installations of pressurized furnace even in the case of solid fuel fired boilers.

In the case of gas and liquid fuel, generally designers adopt pressurized furnaces. It should be

understood that the selection of pressurized or balanced draft furnace depends upon many other

factors.

Present day boilers have online monitoring of furnace pressure and are controlled on auto. There

are also alarms and trips provided for furnace pressure to alert the operator and also to protect the

boiler from very high pressure surges. In the case of furnace explosion, if the explosion process

is triggered then the boiler cannot be protected by any of these devices. Only the good and safeoperating procedures can prevent explosions. There are many reasons why furnace pressure goes

high, and the plant responds to these changes in furnace pressure like any other boiler parameter.

Specific causes

o Tripping of induced draft fan

o Mal-operation of regulating vanes of the fans

o Unstable flame

Low wind box pressure

Improper burner operation

Sudden starting of mills with fuel in the mill Sudden fuel input in to the furnace

Loss of ignition energy

o Gradual buildup of fouling in air pre-heaters

o Tripping of air pre-heater

o Furnace water seal broken

o High excess air levels

o Large amount of air ingress in the second pass of boiler

The causes given are all applicable for balanced draft furnace, but in the case of forced draft

furnace some of the causes are not applicable.


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Plant response

o Furnace pressure high alarm appears for corrective action by operator

o Furnace trip activates if pressure goes above trip limit

o Boiler furnace pressure surge

o Can lead to explosion

Immediate operator action

o Check draft reading for any damper closure

o Check vane control mechanism of fans for any mal-operation

o If induced draft fan tripped, reduce load and stabilize boiler parameters

o Stabilize combustion if combustion is unstable

o Restart air pre-heater if tripped

o Check furnace seal and establish if brokeno Check induced draft fan for any mal-operation


o Check for any opening of man holes in second pass

o Check air pre-heater rotation

o Check dampers in flue gas path for proper position

o Check induced draft fan fully to ensure it is in proper operating condition

o Check and ensure furnace bottom seal water flow

o Check combustion condition and inform control room of any disturbance

It is a good practice for the local operator to get clearance from the control room before making

any change (unless it is a standard procedure to take corrective action by the local operator). Asfurnace pressure increase is one of the indications for boiler explosion boiler operating engineers

and all concerned must view increase in furnace pressure very critical and take corrective actionand ensure safety.

High Furnace Pressure in Boiler


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Effect of Low Fuel Oil Temperature on Boiler Operation


Low fuel temperature can lead to a large amount of unburned fuel accumulation in the furnace.

This can result in a very high probability for furnace explosion and air pre-heater fires.

Maintaining correct fuel oil temperature ensures good combustion of fuel and boiler efficiency.

Fuel Oil Temperature and Boiler Operation

Fuel oil is one of the fossil fuesl used in boilers to generate steam. The hydrocarbons in fuel oil

need heating for reducing the viscosity to a favorable level for handling and firing. Depending

upon the composition of the oil, the effect on performance of boiler can be judged. All oils

should have a viscosity of 80 Redwood No 1 at the burner inlet to result in an effectivecombustion. The temperature required can be determined by doing a simple viscosity test in a

laboratory. It is a good practice to check the temperature required to get 80 Redwood No 1viscosity for each consignment of fuel oil received.

The fuel oil is heated in a fuel oil heater using a condensing type heater. The heated oil is then

transported though pipe line to the boiler front and distributed to each burner through a set ofcontrol, trip, and isolation valves. These pipe lines are generally insulated and heat traced using

electrical heating or steam heating. The temperature of the fuel oil at the outlet of the heater is

maintained in such a way to take care of the loss of heat in the piping and system so that the

viscosity at the gun tip or gun inlet will be 80 Redwood No 1 and below.

When the fuel oil temperature goes down the operator will have to understand the specific

causes, how the plant will respond, what he has to take as an immediate action and how the localoperator must respond.

Specific causes

o Insufficient steam flow to fuel oil heater

o Temperature controller of the heater system faulty

o Excessive condensate in the fuel oil heater lowering heat transfero Faulty steam trap not evacuating the condensate as required

o Fuel oil heater can be dirty thus reducing heat transfer

o Too high recirculation of fuel oil from heater outlet to the tank

o Higher firing rate than design

Plant response

o The drop in temperature of fuel oil increases the viscosity leading to poor combustion

o Black smoke from chimney due to unburned carbon soot carryover

o Oil particles carryover to air pre-heater and threat of air pre-heater fire


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Immediate operator action

o Check the pressure and temperature of steam at the inlet of fuel oil heater

o Restore the steam parameter if required

o Open steam trap by-pass valve to check for excessive condensate

o Remove excessive condensate from the heater

o Reduce the oil recirculation if required

o Reduce firing rate if needed

o Bring online additional fuel oil heater to ensure fuel oil temperature with the oil

consumption

o Discontinue oil firing if fuel oil temperature does not come to the required level


o Check the fuel oil heater for all its controls and parameterso Check steam pressure and temperature to the heater

o Check the steam trap drains and ensure proper condensate level

o Inform boiler control room all local findings.

Irrespective of whether fuel oil is used for load carrying or as support for fuel in a boiler, it is

very important to maintain the fuel oil temperature as the amount of unburned fuel collection

over a period of time can explode the boiler or lead to air pre-heater fires. Both of this lead to along outage of the boiler and a loss of production.


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Poor Oil Gun Performance in Boilers


Oil gun performance can be affected by factors like design, fuel, air distribution, etc. Coal fired

boilers have oil guns for start-up and warm-up requirements. Oil fired boilers use them for start-

up, warm-up, and load carrying.

Oil guns are in coal and oil fired boilers. In coal fired boilers the oil guns are used for start-upand warm-up purpose. These oil guns are designed with a capacity of around thirty percent of

boiler maximum continuous rating. They are used for starting coal mills and as coal flame

stabilization fuel. In oil fired boilers, with the main fuel being oil, they do all the functions of

start-up, warm-up and load carrying. The oil gun performance can be poor due to oil quality,

combustion, mechanical parts of the oil gun, and air distribution for the oil guns.

Causes for improper combustion

o Burner design

Type of burner

Axial

Velocity Swirl

Swirl number

Velocity

o Flame retainer Bluff body

Vane diffuser

Others

o Proportioning of air

Primary

Secondary

Tertiary

o Type of atomizer

Pressure atomization

Air atomization

Steam atomization Air distribution

Multiple burner ducts

Distribution between burnerso Windbox design

Distribution between sides

Distribution between elevation

Fuel preparation

Distribution

High differential pressure

Low differential pressure


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o Viscosity

o Asphalt contento Burner disposition

Tangential firing system

Wall fired system

Front wall fired

Side wall fired

Opposed wall fired

Fuel property

Chemical composition

Percentage residue

Viscosity

Flash point temperature

Fire point temperature

There are many types of problems that can happen in oil gun performance which can be due to

the influence of one of the factors above or a combination of the above. Normally experiencedproblems in oil guns in a boiler are (a) Flame fluctuation, (b) Fire puffs on starting, (c) Smoky

flame and smoky stack, (d) Improper flame condition like flame smoky, flame front unstable,high flame noise etc.

Flame Fluctuations can be caused by problems on the air side, fuel side, and atomizer side.

Air side

o Low air Inadequate air

Inadequate mixing

Fluctuation in air flow

Forced air fan damper flap problem

Air flow control mal function

Burner air distribution

Poor draft

Fuel side

o Too high a firing viscosityo Water in oil

o Fluctuation in oil pressure

o Atomizing steam pressure variation

Atomizer side

o Improper location

o Improper alignment

o Worm out atomizer


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Fire puffs on starting can happen when the condition for starting is not as per requirement. Water

in oil, low oil pressure, pilot flame not stable or adequate, incorrect windbox pressure, low airflow, and a worn-out atomizer are some the reasons for this.

Smoky flame and smoky stack is experienced by many while operating the boiler or during

starting. The burner, fuel, and air can contribute to this problem.

Burner

o Atomizer

Damaged oil gum tip

Carbon deposit on tip

Different tip in oil burner

Dirty oil gun

Oil gun position Damaged diffuser

Improper alignment of oil gun

Burner swirler

Air register swirler

Air side

o Low excess air level

o Improper air distribution

Inter burner

Intra burner

Fuel side

o Low oil temperature

o High atomizing viscosity inlet to oil gun

o Low oil pressure

o Oil pressure fluctuation


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Causes for Improper Oil Combustion


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Handle Both Forced Draft Fans Trip in Running Boilerwritten by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 5/3/2010

All forced draft fan trip in a boiler will cause a boiler trip. Putting the boiler back in operationand giving steam at the required parameter to the consumer is very important. Delay in this can

cause a very high loss to the plant.

Boilers have forced draft (FD) fans to supply air for the combustion of fuel. In addition to this

fan, there can be many other fans such as induced draft (ID), primary air fans (PA), seal air fans,

scanner air fans, etc. In these the FD, ID, and PA fans are large capacity fans, while the others

are smaller fans. In the case of a pressurized boiler, only an FD fan is present. Here the FD fanhandles the full pressure drop of the whole boiler air and flue gas system. In the case of a

balanced draft system, the ID fan evacuates the flue gas from the furnace and handles thepressure drop in the flue gas section. The PA fan is used in the case of solid fuels to carry the

fuel to the furnace and give the primary air requirement to the fuel.

Trip in a running boiler

Any trip of major equipment in the boiler causes the boiler parameters to vary widely before it

stabilizes depending upon the action taken by the operator. Boiler tripping can also be caused if

some vital equipment trips or if some unsafe condition appears. The important factors to be

understood in a trip of equipment in a boiler are mainly four.

1. The specific cause of trip

2. The plant response to the trip

3. The immediate operator action required4. The immediate local operator action requirement

In the case of both FD fans tripping, the boiler will go for trip as the air supply to the fuel is cut

off in full or excluding the primary air depending upon the system design. The PA fan also trip

as soon as all the FD fans trip. Depending upon whether the boiler is for a process unit or powerstation, the action on the steam consumer end will vary.

The specific cause of the trip

For both FD fans to trip the main reasons can be as below.

o 6.6 KV supply failure

o 0.4 KV supply failureo Cooling water to motors failure

Plant response to the trip


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Once Through and Drum Type Boiler Designs Compared


As carbon emission is a major concern today, more super critical pressure units are bound to be

preferred due to the increase in plant cycle efficiency. This will make once through type boilers

take over from drum type boilers. Fuel conservation is an important factor for energy security

presently.

Drum Type and Once-through Boilers

The major proportion of boilers operating in the world today are drum type boilers. These boilers

have certain restrictions during start-up due to the presence of a high thickness component- the

drum. The once through design mainly avoids this, along with a few more advantages. Theconcept of once through boilers dates back to 1824, referenced through patents in the United

States. It was in 1923 that the first commercial 4 tons/hr once-through boiler was made by MarkBenson, a Czechoslovakian, and supplied to English Electric Company Ltd at Rugby, England.

When we try to analyze two types of boiler design which can cater to the same requirement, it is

necessary to look at certain specific factors. These key factors generally include:

1. How the boiler will respond to load changes

2. How the efficiency will change

3. How the auxiliary power consumption will vary in each design

4. The availability of additional systems or equipment5. How the control system for each will vary

6. What are the water chemistry requirements?7. Suitability for cyclic and two shift operation

8. The operation and maintenance aspects of these designs

9. The cycle time needed for each design, and10.The overall economics of each

Once-through Boiler Characteristicso The once through boiler has high load response characteristics due to the fact that it does

not have a drum and has a much lower water inventory.

o In the once through boiler, many times the load change response is dictated by the firingsystem and its controls rather than the boiler, per-say.

o Once through boilers of super-critical pressure boilers have higher efficiency. However inthe sub-critical range there is no difference in efficiency when compared to the drum type

design.

o Generally the power consumption is higher by 5 to 8 % for the same capacity boilers of

drum type.

o Once through boilers have a separate start-up loop along with all its controls.


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o As the load demand is met by varying both fuel and feed water flow simultaneously, the

controls are more sophisticated and have to be more reliable. More redundancies are builtin.

o The water quality level is much more stringent than drum type boilers. Normally acondensate polishing unit is employed in once through units.

o In once through boilers the superheater headers are subjected to both fatigue and creep

when cyclic or two shift operation is resorted to. Hence these boilers are more preferred

for base load operation. However, the load change rate that theses boilers can take is

higher due to the absence of the drum.

o A closer regime of operation is expected in once through boilers.

o The absence of the drum makes it possible to reduce the overall cycle time for the once

through boiler. However, the overall plant cycle time may not vary only marginally.

o Once through boilers life time cost is expected to be more than the drum type units.

Drum Type Boiler Characteristicso Cold start-up takes much more time in drum type units as the metal temperature

difference across the thickness dictates this.

o Drum type units do not find application in super-critical pressure power plants.

o The drum type boiler is more adaptable to cyclic and two shifting operation.

o The water chemistry is maintained within a bandand can accommodate some variations

when compared to once through type boilers.

o The control system is more simplified when compared to the once through type as loadvariations are done by fuel control and feed water is controlled by drum level.


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Low Steam Pressure in Boilerwritten by: Dr V T Sathyanathan edited by: Lamar Stonecypher updated: 9/25/2010

Operation of boiler with low steam pressure makes power generation less competitive. Steamenthalpy depends on both pressure and temperature. It is the heat energy which is converted to

mechanical energy in the turbine, so low pressure of steam reduces the work done by the turbine

and load.

Steam pressure and temperature are the key parameters of power generation for thermal power

plants and also for process application. Steam pressure and temperature decide the amount of

heat available, in the form of enthalpy, to do work in turbine. A drop in both will reduce the plantheat rate in different magnitude. Many boiler operators prefer to operate the boiler at slightly

lower pressure, when they have load variation, fuel quality variations, etc, as they get more

margins between the safety valve set pressure and the operating pressure. Main steam pressure is

used as the input to master pressure controller to change the fuel input to the boiler.

The main reasons for variation in steam pressure when the boiler operates with steady conditions

are:

o Sudden increase o !enerator "oad

o #a"$operation o steam pressure contro""er

o%oa" han! up in mi""s

o Disturbed combustion condition in urnace

o Trippin! o one or more mi""s

Sudden increase in load can be due to grid demand or due to process requirement in the plant.

Coal hang up in coal mills is is not uncommon due to the presence of foreign material in coal oreven due to high moisture coal. Disturbed furnace condition can be due to many reasons.

The plant responses due to these conditions give indication to the boiler desk operator and local

operator to take immediate action. Normally the turbine will slow down due to the braking effect

of the generator. During this time the steam demand signal will go up reducing the pressure

further if the fuel input does not increase. As the main steam pressure drops, there is a goodreason for the auxiliary steam pressure to drop. This will depend on many other operatingconditions from the turbine and the boiler side.

Immediate operator action will depend upon the cause that resulted in the reduction of steam

pressure. The first action of the operator will be to check and reduce the turbine generator load so

that the steam pressure does not drop to a very low level. The operator will switch to manual

mode for pressure control if he suspects the pressure controller is misbehaving. This he can infer

from the erratic way in which the pressure control is behaving even before the main steampressure drops continuously or otherwise. Starting of the spare mill will be required if any of the

operating mills have tripped. He first will reduce the load if possible and bring in the new mill


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immediately. During this disturbance the boiler opreator will have to keep vigil on the water

level in the boiler drum. If this level drops more and reaches the trip level, then he will lose theunit and will have to line-up for start-up again.

The local operator will have to quickly check if the coal mill feeder is delivering the coal in all

the operating mills. Check for any coal hang up in any of the operating mills. He has to also

check the furnace conditions and inform the desk operator. If he has to line up another mill to

start, he has to inform the boiler control room and prepare the mill for starting.

Low Steam Pressure in Boiler


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What are the types of HRSG's?

written by: johnzactruba edited by: Lamar Stonecypher updated: 9/20/2010

Heat Recovery Steam Generators (HRSG's) are a critical part of a Combined Cycle Power Plant.As with any other product or process, there are different types. Here we take a look at the

different types.

The main application of the (Heat Recovery Steam Generators) HRSG is in Combined Cycle

power plants. In these plants the power generation from the Rankine cycle part, ie. the steamturbine, is around one third of the total power generated . HRSG's produce the steam for this.

Classification of HRSGs is on application, design, or operation. Some of the types are described

below.

Fired and Unfired

One way is to classify it based on the heat input.

Normally HRSG's do not have any additional heat input. The performance and output of the

HRSG is dependent on the exhaust heat of the gas turbine. At part loads, this leads to reducedoutput from the HRSG's. In addition, ambient conditions also affect the Gas Turbineperformance. This could affect the downstream process were the steam is used. To avoid such

situations supplemental firing of oil or gas takes place. Even though this may not be an efficient

process, it avoids costly production disturbances.

Supplemental Firing takes place in burners in the gas duct at HRSG inlet. Oil or gas is the

supplementary fuel. Since the flue gas at exhaust of a Gas Turbine is high in Oxygen content,additional air is not required for combustion. This eliminates the need of Forced draft or Induced

Draft fans.

Vertical and Horizontal Types

Another classification is on the construction or design of the HRSG. Based on the gas flow it can

be vertical or horizontal.

o Vertical types have gas flow vertically upward with coils placed horizontally.

o Horizontal types have gas flows horizontal with coils placed vertically.


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From the performance and cost point of view both are the same. More than the technical issues it

is a proprietary design of individual manufacturers or client preferences. Some of the differencesare :

o Horizontal types require a 30 % larger footprint area.

o More expansion joints are required in horizontal units.

o Structural requirements are higher in vertical types.

o Horizontal types are more difficult for maintenance and inspections.

o Overall cost may be same in both types.

Multiple Pressure Operation.

Yet another classification is on the operation pressure.

o Smaller HRSG units operate on single pressure. The water to steam conversion takes

place in one single pressure circuit. This is similar to conventional combustion boilers.

o In larger units, for optimizing the performance of the HRSG, steam generation takes

place in multi pressure circuits. The current optimum is to use three pressure levels.

(Details in the next article)

How is the HRSG different from a fossil fuel fired boiler?

written by: johnzactruba edited by: Lamar Stonecypher updated: 7/13/2009

Heat Recovery Steam Generators (HRSG's) absorb heat from the exhaust of gas turbines toproduce steam. Functionally they produce steam like any other boiler but with some differences.

What is an HRSG?

Heat Recovery Steam Generator's (HRSG's) are waste heat boilers. The steam turbine or adownstream process uses the steam. The term HRSG refers to the waste heat boiler in a

Combined Cycle Power Plant. In its basic form these are bundles of water or steam carryingtubes paced in the hot gas flow path. These recover the heat from the gas to generate superheated

steam, hence the name Heat Recovery Steam Generator.

The water steam circuit of an HRSG consists of an economizer, an evaporator, and a Super-

heater placed in the flue gas duct. The evaporator section consists of a drum to which the coils

are connected to create the circulation.

The Differenceso HRSG is only a heat transfer area. There is no furnace even though the sections like

economizer, evaporator, and super heaters are present. An exception is the

supplementary-fired HRSG .

Coal Pulverising in Boilers 1

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