PULVERISED COMBUSTION SYSTEM

PULVERISATION

FUELS

COAL

COAL APPLICATIONS

COAL COMBUSTION

COMBUSTION OF FUELS- SOLID, LIQUID & GAS

EMERGING TRENDS

Nox FORMATION AND CONTROL

Coal dust is a fine powdered form of coal

More surface area per unit weight than lumps of coal

More susceptible to spontaneous combustion

Pulverised coal has significant dust explosion hazard

FUELS

Solid FuelsCoalPeat LigniteBituminousAnthraciteLiquid FuelsHSDLFOHFO

Gaseous FuelsBFGProducer Gas LPGNatural Gas

COAL

CC 40-90%40-90%

HH 2-7%2-7%

OO <18-<18-30%30%

SS <6%<6%

NN 1-5%1-5%

CharChar 20-70%20-70%

AshAsh 5-45%5-45%

HH22OO 2-20%2-20%

VMVM 15-30%15-30%

Heterogeneous organic fuel formed mainly from decomposed plant matter.

Over 1200 coals have been classified.

EvolutionTime,

Pres.&Temp.Coal Rank

Pro

xim

ate

Ana

lysi

s

Ulti

mat

e A

naly

sis

Home – heating and cooking

Transportation – steam engines

Industry – Iron & steel Electricity – power

plants

Oxidation

oxygen combines with other elements and forms oxidies.

Combustion, A special form of oxidation Oxygen combines with fuels; coal, oil, gas substantial amounts of heat is liberated.

The Degree of Flammability depends

convertibility to a gas,-nothing truly burns until it is a gas. Nature of fuel Quantity of the fuel,

Stages of Combustion

Ignition

Combustion Stability

Completion of Combustion

Combustion process, ignition occurs in vapour phase.

Solid and liquid fuels get ignited from their vapours.

Combustion Stability:

Ignited flame shall be sustained further so that the process of combustion would be continuous.

Completion of Combustion:

Fuel staying for the minimum period (residence time)Completely oxidizing the combustible.

Considerations of combustion:

Safety consideration during fuel handling

Generation of Pollutants viz. Nox and Sox .

coal particlep-coal, d=30-70m

devolatilization

volatiles

char

homogeneouscombustion

heterogeneouscombustion

CO2, H2O, …

CO2, H2O, …

tchar=1-2sectvolatiles=50-100mstdevolatile=1-5ms

t

Combustion of Fuels in furnace

Preparing the fuel and air ;

Converting the complex fuel into elementary fuels;

Right fuel and air mixture

Transferring heat from the products of combustion to the boiler or other surfaces.

The physical processes influencing pulverized coal combustion

Turbulent/Swirling flow of air and coal. Turbulent/Convective/molecular diffusion

of gaseous reactants and products. Convective heat transfer through the gas

and between the gas and coal particles. Radiative heat transfer between the gas

and coal particles and between the coal/air mixture and the furnace walls.

COMBUSTION OF DIFFERENT FUELS

Solid Fuel

Volatile matter is released over a temp. of 250 -

900 deg. C.

The volatile matter is first ignited.

The coal particle upon releasing the volatile

matter become a char. The char slowly burns out.Liquid Fuel

Boils and releases volatile matter and gets

ignited

The balance char completes the combustion.

COAL - AIR BALANCING IN FUEL PIPING

Un balance in Coal- air flow into the furnace

results

uneven heat release

uneven distribution of excess air

unpredictable Nox formation

Remedies

Selection of Orifices for Coal air two phase flow regime.

Provision of on -line adjustment dampers in coal air

flow path.

On-line measurement of coal -air flow using microwave

techniques

OIL FIRING SYSTEM FEATURESFuel Oil Preparation

Pumping the oil and heating it are the major preparatory functions.

Filtration of oil to remove any dust, dirt, sediments, sledge etc. This renders long trouble free service life to pumps, valves, atomisers etc

Maintaining the HFO temperature constant, corresponding to the atomizing viscosity of 15 to 20 centistokes, is essential for better fuel oil atomization.

A lower temperature of fuel oil impairs the burner performance and a higher temperature causes oil cracking.

Fuel Oil Atomisation

Atomisation spraying the fuel oil into fine mist Better mixing of the fuel with the combustion air Pressure and viscosity influence atomisation Pressure energy of the steam to velocity energy, which breaks up the oil stream into fine particles

Poorly atomisation results in Bigger spray particlesLonger burning time Carryovers of carbon and flame instability due to low rate of heat liberation Incomplete combustion and smoke

Oil Recirculation

To warm up the oil supply lines

To maintain correct atomising temperature

System Vents

Fuel oil heaters

Oil strainers

oil & steam lines - get rid of air locks.

System Drains

Oil lines with a drain valve at the lowest point.

BURNERS:

To deliver coal , oil and air in a proper proportion

To facilitate ignition energy to the coal air stream To sustain the ignition

To provide a stable flame during the operation

Types of Burners

Tangential Burners

Wall Burners, Ex: Low Nox R burners

Down shot or fan tail burners

Modern Burners are equipped with:

Separate flame envelope ports for coal, oil and gas

Secondary air control to adjust the flame envelops

Ignitors

Flame Scanners - detect the distinct flames in an enclosure

Flame Stabilisers

Flame Analysers

Burner Arrangement

Tangential firing: Four tall windboxes (combustion air boxes) one at each corner of the furnace.

The oil and gas burners are located at different levels or elevations of the windboxes.

The coal , oil and gas burners are sandwiched between air nozzles or air compartments.

That is, air nozzles are arranged between gas spuds, one below the bottom gas spud and one above the top gas spud.

Burner Tilt:

The burners are tiltable +/- 30o about horizontal,

To shifts the flame zone across the furnace height

To control over steam temperature

Combustion Air Distribution

The Combustion air

Primary Air (PA) and Secondary Air (SA)

Secondary Air(SA) provided from FD Fans

Primary Air(PA) provided from PA Fans

Ignitors

Oil and gas are ignited by a pilot flame.

Type of Ignitors

Oil ignitor

Gas ignitor

High Energy Arc ( HEA) ignitor

Flame Sensing

Devices

Flame sensing devices are broadly grouped in to

Infrared flame sensors

UV flame scanners

Visible light scanners

Recent development

Flame analysers for multiple fuels

Emissions of Combustion - Pollutants

1. Nox emission

2. Sox emission

3.CO 2 emission (Green House Gases)

4. CO emission

5. Particulate emission

Emerging Trends

in

combustion system Design

Emerging trends in combustion system Design

Multiple fuel Burners

Low Emission Burner

- Technology development

Longer guarantee period for high ash coals

- Material selection,

- Improved design features

New devices such as thermal analysers.,

Emerging trends in combustion system DesignCont.……

Micro processor based on line measurement and control

Computer simulations using software tools viz., ANSYS, CFD.

Virtual assembly using CAD tools

NOx Formation

and

Control Strategies

NOX FORMATION

Thermal NOx Formation

Nox Formation from Fuel Nitrogen

Control Technique NOx Reduction Potential(%)

Over fire air (OFA) 20-30

Low Nox Burners (LNB) 35-55

LNB + OFA 40-60

Re-burn 50-60

SNCR ( Selective Non Catalytic Reduction)

30-60

SCR (Selective Catalytic Reduction)

75-85

LNB with SCR 50-80

LNB with OFA and SCR 85-95

NOx Control options

Selective Catalytic Reduction (SCR) Reactions

OHNONHNO

OHNONHNO

22catalyst supported OVor TiO

232

22catalyst supported OVor TiO

23

6342

6444522

522

CLOSE COUPLED

OFA

LNCFSW/OFA

CCTFS

PM+NH3/UREA

PM

SCR

0

10

20

30

40

50

60

70

80

90

% N

OX

RE

DU

CTI

ON

FR

OM

BA

SE

*

INCREASING COST

AVAILABLE TECHNIQUES FOR REDUCING NOX FROM TANGENTIAL COAL FIRED UNITS

* BASE IS A TYPICAL PRE 1970 TANGENTIALLY FIRED UNIT. BASE NOX

OVER FIRE AIR (OFA)

Through additional air compartments, to handle 15 percent of total wind box air flow.

Inhibits formation of both fuel Nox and thermal Nox as an Oxygen Deficient environment is established in the primary combustion zone

20 to 30% reduction in Nox formation

NOX DEPENDENCE ON OVERFIRE AIR FLOW

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16

OVERFIRE AIR(%)

NO

(P

PM

@ 3

% O

2)

OFA VS NO

EXCESS O2 (%) VS NO

While Nox emission decrease linearly with

increasing over fire air, Excess air rises (i.e., More Air

Is Needed To Complete The Combustion).

If over fire air is increased beyond 15%. This

decreases boiler efficiency due to the heating of

extra air

Nox Reduction Techniques

STAGED COMBUSTION:

Fuel Bound Nitrogen to be regulated

High-Temperature Formation must be curtailed.

Withholding of Some O2 from primary flame zone

Air staging

LOW NOx BURNERs

Wall burners (oil and gas firing )

Low NOx R-burner is capable of emitting NOx at a level of 150

ppm on oil firing.

Tangential firing ( Coal firing)

CCOFA(Close Coupled Over Fire Air) feature.

Separate Over Fire Air( SOFA ) .

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