Boiler DesignJuly 2010

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Boiler Design Fundamentals1

PRABHAT KR. GUPTA

MECHANICAL ENGG. SACHDEVA INST. MATHURA

UPTU LUCKNOW


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STEAM-GENERATOR FUNCTIONS

Evaporating water to steam at high pressure

Produce steam at exceptionally high purity

Superheat the steam in the unit to a specified

temperature, and maintain that temperature Reheat the steam which is returned to the boiler and

maintain that reheat temperature constant

Reduce the gas temperature to a level that satisfies

the requirement for high thermal efficiency and atthe same time is suitable for processing in theemission-control equipment downstream of theboiler

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Understanding Combustion

Understanding of Coal

Coal Petrography

Steps in Combustion of coal

Char Burning

Volatile matter content

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What is Coal?

The macromolecular network consistsof aromatic clusters that are linked andcross linked by brides. Most bridges are aliphatic in nature,but may also include other atoms suchas oxygen and sulfur.

Bridges that contain oxygen as ethersare relatively weak in nature. A mobile phase also exists. Themobile phase consist of smallermolecular group that are not stronglybonded to the macromolecule. The percentage of aromatic carbon

usually increases with coal rank. Other important elements, a smallfraction, in coal are sulfur andnitrogen. They account for almost allthe pollutants formed during coalcombustion.

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Steps of Coal Combustion

Coal is ground to size ofbetween about 5 and400 m in diameter, andcarried by the combustion

air to the furnace burners.Combustion takes place attemperatures from 1300Cto 1700C, dependinglargely on the rank of the

coal. The steps are: Drying

Devolatalization

Volatile Combustion

Char Burning


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Combustion of pulverized coal particles

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The drying Process

Evaporation of surface moisture and, subsequently, theloss of inherent moisture, starts at temp of 100110C,complete dehydration at about 350C.

Heat is driven from the furnace environment to theparticle surface by radiation and convection.

Heat transfer in the process is influenced by the furnacetemperature, coal-particle size, particle moisture content

and particle porosity

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Volatile Matter

Volatile matter is an important parameter, providing arough indication of the reactivity or combustibility of acoal, the ease of ignition, and hence the flame stability.

However, while char reactivity is one of the main

factors determining combustion efficiency, there is nostandard test for its determination; Standard Methods do not take into account the actual

firing conditions. Volatile matter contains both combustible and non-

combustible (for example, carbon dioxide and water).So the calorific value of volatiles can be significantlydifferent for coals with the same proximate volatile-matter yield.

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Devolatilisation

Generally, devolatilisation starts at the particle surface andthen proceeds toward the centre.

The devolatilisation initiation temperature varies with thetype of fuel, and typically ranges from 450C to 500C forcoal particles. Heating rate also has a significant influence.

Generally, the weaker carboxyl, hydroxyl and aliphaticbonds break up at lower temperatures, while the strongerheterocyclic components decompose at higher temperatures.

After the weak bonds break up, the functional groupsdecompose to release gases, mainly CO

2, light aliphatic

gases and some CH4 and H2O. Devolatilisation is affected by coal rank, macerals presents,

Coal density, heating rate and the gas environment.

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Devolatilisation Products

Bituminous coals yield comparatively larger amounts of tar thanother coals, whilst lower- rank coals release less tar but largeramounts of light gases.

The light volatile gases may include species such asCH4, C2H4, C2H6, CO, CO2, H2 and H2O.

Typically, bituminous coals pyrolysed at 1300 K produce 50%CH4, 13% H2 and 27% CO and CO2when pyrolised in a vacuum,against 8% CH4, 59% H2 and 27% CO and CO2when pyrolysisoccurs in air.

The tar produced is a heavy hydrocarbon-like substance with anatomic H/C ratio >1.0, consisting of hundreds to thousands oforganic species .

Devolatilisation of a coal includes a rapid initial release of about8090% of the volatiles followed by the slow release of theremaining 1020%.

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Devolatilisation Model

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TANGENTIALLY FIRED SYSTEMS

Based on the concept of a single flame envelope

Fuel and combustion air are projected from the corners ofthe furnace along a line tangent to a small circle, lying in ahorizontal plane, at the center of the furnace.

Turbulence and mixing that take place along its path arelow compared to horizontally fired systems

Significance of this factor on the production of oxides ofnitrogen

Possible to vary the velocities of the air streams and changethe mixing rate of fuel and air, and control the distancefrom the nozzle at which the coal ignites

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Tangentially Firing System

Provides great flexibility formultiple-fuel firing

Fuel and air nozzles tilt inunison to raise and lower theflame to control furnace heatabsorption and S/H & R/HTemp

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Tangentially-Fired Furnaces

Advantages: The efficient mixing, due to vortex, rapid contact between fuel and

air, and flames interaction, that would ensure a reliable combustionwith uniform temperature distribution.

Uniform heat flux to the furnace walls; consequently failures due tohigh thermal stresses have been avoided.

The air and fuel streams can be admitted inclined either upward ordownward from the horizontal, a feature that is used to vary theamount of heat absorbed by the furnace walls and to control thesuperheater temperature.

Vortex motion at the furnace center prevents or minimizes sluggingof the furnace walls, erosion due to impingement and local over-heating.

NO, in tangentially fired unit is lower than other firing types. NO,

emissions from TF boilers are about half the values from wall firingsystems. Tangential-fring technique is characterized by lower carbon losses

(do not exceed 1%), and greater adaptability for the combustion of"difficult" fuels (e.g. fuels with low calorific value, high melting-pointash, or low volatile content)

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VERTICALLY FIRED SYSTEMS

Used principally to fire coals with moisture and-ash-free volatilematter between 9 and 13 percent

Require less stabilizing fuel than horizontal or tangential systems

Have more complex firing equipment and more complex operatingcharacteristics.

Portion of the heated combustion air is introduced around the fuelnozzles and through adjacent auxiliary ports

High pressure jets are used to avoid, short-circuiting the fuel/airstreams to the furnace discharge

Tertiary air ports are located in a row along the front and rear walls

of the lower furnace Firing system produces a long, looping flame in the lower furnace,

with the hot gases discharging up the center

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Flow pattern of vertical firing

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LIMITS OF STEAM TEMPERATURE AND PRESSURE

Materials of superheater govern the practical limitsof steam temperature and pressure.

The large majority is in the 400 to 565C

temperature range Problems do arise during sustained elevated

temperature operation because of the adverse effectsof certain fuel constituent on unit availability.

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Principles of design

Components arranged to efficiently absorb heat and providesteam at the rated parameters .

Components : boiler surface, superheater, reheater,economizer, and air heater.

Supplemented by steam-water separation and the control ofsteam outlet temperature.

The steam generator : furnace and the convection pass.

The furnace : water-cooled enclosure walls inside of whichcombustion takes place, combustion products are cooled to

an appropriate (FEGT). The convection pass contains tube bundles, which compose

the superheater, reheater, boiler bank, and economizer. Theair heater usually follows the convection pass.

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Boiler configurations

Combustion system, fuel, ash

characteristics, operatingpressure, and total capacitylargely determine the boilerconfiguration.

All utility boilers feature gas-

tight, fully watercooled furnaceenclosure walls and floorsmade of all welded membranepanel construction.

Each design normally includes

a single reheat section,although the supercriticalonce-through boiler has alsobeen supplied as a doublereheat unit.

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Boiler design

Boiler design driven by four key factors:1. efficiency (boiler and cycle),

2. reliability,3. capital and operating cost, and4. environmental protection.

Temperature- enthalpy diagram providesimportant design information about theunit configuration.

At subcritical pressures, constanttemperature boiling water cools thefurnace enclosure, and the flow circuitsmust be designed to accommodate thetwo-phase steam-water flow and boilingphenomena.

At supercritical pressures, the water actsas a single-phase fluid with a continuousincrease in temperature as it passesthrough the boiler.

Fluid circulation systems: naturalcirculation and once-through

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Design criteria

1. Define the energy input2. Evaluate the energy absorption in the boiler and other heat transfer

components.

3. Perform combustion calculations

4. Determine the size and shape of the furnace,

5. Determine the placement and configuration of surfaces. Placesuperheater and reheater, where the gas temperature is high enough,avoid excessive tube metal temperatures or ash fouling. Minimize theimpact of slag

6. Design pressure parts in accordance with applicable codes usingapproved materials.

7. Provide a gas-tight boiler setting or enclosure around the furnace,boiler, superheater, reheater, and economizer.

8. Design pressure part supports and the setting for expansion and localconditions, including wind and earthquake loading.

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Radiation controls heat transfer to the furnaceenclosure walls. These walls are cooled with boilingwater

The convection pass enclosure contains most of thesuperheater, reheater, and economizer surfaces.These enclosure surfaces can be water or steamcooled.

Besides providing the volume necessary forcomplete combustion and a means to cool the gasto an acceptable FEGT,

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Relative heat absorption

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Superheaters and reheaters

For relatively low final outlet temperaturessuperheaters solely of the convection type aregenerally used.

For higher temperatures, surface requirements arelarger: superheater elements located in very high gas-temperature zones.

Metallurgy: selection of materials for strength and

oxidation resistance, the use of high steam pressurerequires very thick walls in all tubing subject to steampressure.

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Design of superheater

A radiant superheater receivesenergy primarily by thermalradiation with little energy fromconvective heat transfer.

Radiant absorption does not

increase as rapidly as boileroutput.

Series combination of radiant andconvection superheaterscoordinates the two oppositesloping curves to give a flat

superheat curve over a wide loadrange.

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Parameters considered for Super Heater/Reheater Design

Outlet steam temperature

Range of boiler load

Superheater surface required

Gas temperature zone in which the surface is to be located, Type of steel, alloy or other material best suited for the

surface and supports

Rate of steam flow ( control over tube metal temperatures)

Arrangement of surface to meet the characteristics of thefuels, (spacing)

Physical design and type of superheater as a structure orcomponent.

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Flow in Superheaters and Reheaters27


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ECONOMIZERS

Help to improve boiler efficiency by extracting heatfrom flue gases.

Generally, economizers are arranged for downward

flow of gas and upward flow of water. Upward flow of water helps avoid water hammer,

which may occur under some operating conditions

Material is generally low-carbon steel

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IMPACT OF FUEL ONBOILER DESIGN

Three very important parametric influences on furnacesizing are: Fuel reactivity Gaseous-emission limitations (particularly those

concerning oxides of nitrogen), and Fuel-ash properties.Ash properties particularly important for designing ofcoal-fired furnaces: Ash fusibility temperatures Ratio of basic to acidic ash constituents Iron/Calcium ratio Fuel-ash content in terms of kg of ash/million joules Ash friability

Coal and Ash Properties

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Effect of coal rank on sizing of apulverized-fuel furnace

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Pendent PanelPendent Panel

ARRANGEMENT OFUPPER-FURNACE HEATING SURFACE

Horizontal arrangementHorizontal arrangement

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ADVANTAGES OF THE PENDANT PANEL


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ADVANTAGES OF THE PENDANT PANELDESIGN

The support elements are out of the gas stream above thefurnace roof. Superheater and reheater are free to expanddownward. and have only simple alignment devices in the gasstream

No relative motion between the furnace tubing and thesuperheater or reheater tubes

The above support and sealing arrangement favors shopmodularization of tubes, headers, attachments and supports

Field erection, major pressure-part construction can be carriedout in several areas simultaneously.

Widely spaced panels along with steam cooled wall sections inthe upper furnace. Have high radiant-heat absorption.resulting improved steam-temperature control range.

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ADVANTAGES OF THE HORIZONTAL


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ADVANTAGES OF THE HORIZONTAL-SURFACE DESIGN

Vertical gas flow through the superheater and reheater surfaceminimizes the potential for localized tube erosion in a 90turn into the rear gas pass.

Horizontal tubing facilitates designing for drainability, whichsimplifies freeze-protection procedures, boilout, and

hydrostatic testing Large fused ash deposits that are removed by sootblowers will

usually drop through wider spaced tube sections below,directly to the furnace bottom.

Horizontal arrangement requires that in start-up there isadequate cooling flow through the vertical hanger tubes that

support and align the horizontal tube bundles. Thermocouplesshould be used to monitor hanger-tube temperatures on start-up, especially in tubes with downward flow.

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CONVECTION-PASS DESIGN

For designing a proper balance is required to maintain athermal head with which to transfer heat from gas to steam asthe heating-surface use is optimized and undesirably highmetal temperatures are avoided

To limit pressure-part erosion from flyash, the flue-gasvelocity must not exceed reasonable limits.

It is impractical to propose a steam generator capable ofburning any kind of coal.

Certain coals need wide transverse tube spacing to reduce thefouling rate and possible bridging of ash deposits. Thisarrangement minimizes serious fouling problems.

The transverse spacing of the convection - pass tube banks isreduced as the gas temperature is reduced along its flow path

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BURNING CHARACTERISTICS OFCOALS

The combustion has anumber of overlapping steps,including heating, ignition,devolatization and charburnout

Flammability Index is theignition temperature of asuspension of pulverized fuel

CHAR REACTIVITY:Quantity and rate of volatilesreleased Swelling andagglomeration, porestructure influencing theburning characteristics ofchar. Char Characterization

and Thermo-GravimetricAnalysis are the indicators.

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Thermo-grammatric Analysis37


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PULVERIZING PROPERTIES OF COAL

GRINDABILlTY.-measures the easeof pulverization. It should not beconfused with hardness of coal. It isaffected by Moisture in the Coal.

MOISTURE: comprised ofequilibrium moisture and surface or

free moisture. Surface moistureadversely affects both pulverizerperformance and the combustionprocess. The surface moistureproduces agglomeration of the finesin the pulverizing zone, and reducespulverizer drying capacity because ofthe inability to remove the finesefficiently and as quickly as they areproduced.

Sufficient hot air at adequatetemperature is necessary in theMilling System

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COAL-ASH SLAGGING AND DEPOSITION

Parameters for Coal ash Behaviour

ash-fusibility temperatures

base/acid ratio

iron/calcium ratio silica/alumina ratio

iron/dolomite ratio

dolomite percentage

ferric percentage

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ASH-FUSIBILITY Initial Deformation Temperature (IT): The temperature at which the

tip of the ash pyramid begins to show any evidence of deformation.

Shrinkage of the cone is ignored if the tip remains sharp. Softening Temperature (ST), H = W: The temperature at which the ash

sample has fused into a spherical shape i);l which the height is equal to thewidth at the base. The H = W softening temperature in a reducingatmosphere frequently is referred to as the "fusion temperature. "

Hemispherical Temperature (HT), H = 1/2W: The temperature at

which the ash sample has fused into a hemispherical shape where the heightis equal to 1/2 the width at the base.

Fluid Temperature (FT): The temperature at which the ash sample hasfused down into a nearly flat layer with a maximum height of 1/16 inch.

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BASE/ACID RATIO

Base/acid ratio reflects a potential for ash-containingmetals to combine in the combustion process to producelow melting salts

Extremes at either end indicate a minimum potential for

forming combinations with low fusibility temperatures. Alkaline metals, sodium and potassium, are exceptions

A base/acid ratio in the 0.4 to 0.7 range manifests lowash-fusibility temperatures and a higher slaggingpotential

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SILICA/ALUMINA RATIO

The silica/alumina ratiocan provide additionalinformation relating to ash

fusibility For two coals having equal

base/acid ratios, the onewith a higher

silica/alumina ratio shouldhave lower fusibilitytemperatures

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IRON/CALCIUM RATIO

Fe203/CaO ratios between 10 and 0.2 have amarked effect on lowering the fusibilitytemperatures

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Other Parameter

IRON/DOLOMITE RATIO : Has similar propertiesas Iron/Calcium ratio, use of this ratio is recommendedwhen the MgO content of the ash is high.

DOLOMITE PERCENTAGE (DP): a higher DP usually

results in higher fusion temperatures and higher slagviscosities.

EQUIV ALENT Fe203 AND FERRIC PERCENTAGE (FP):describe the degree of iron oxidation in coal-ash slags.

SILICA PERCENTAGE (SP):As SP increases, the slagviscosity increases.

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Boiler DesignJuly 2010

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