Combustion Project

Introduction:

Combustion is the phenomenon where rapid chemical combination of oxygen with the

combustible elements of fuel takes place. All of the common fuels like coal, oil, gas, wood, and

there various derivatives have only three elemental constituents – C, H, and S that unite with

oxygen of the air to produce heat energy.

The principles involved in the development of heat by combustion generally accepted as

authoritative were propounded by Berthelot. His “Second Law” as applied to combustion in

furnace practice, is of particular interest and may be stated as follows.

In a boiler furnace (where no mechanical work is done), the heat energy evolved from the union

of combustible elements with oxygen depends upon the ultimate products of combustion and not

upon any intermediate combinations that may occur in reaching the final result.

The rate of union of combustible matters with oxygen rather the rate of chemical reaction in the

combustion process is greatly influenced by the temperature, time, concentration, preparation

and distribution of the reactants and mechanical turbulence. All these factors tend to increase the

contact between the molecules of the reactants.

Combustion Kinetics:

A number of physical and chemical factors determine the combustion of fuel in a furnace.

Physical factors include the process of mixing of fuel and air, the size of fuel particles and the

surface exposed for reaction. The chemical factors are related to the temperature and

concentration of the reactants. The complex fields of velocities, temperatures and concentration

together determine the kinetics of combustion reactions.

Reaction involved in combustion of fuel proceed with evolution of heat i.e. they are exothermic.

This includes the burning of C, H and S in atmospheric air. At high temperatures, sum reactions

may occur with heat absorptions, i.e. they are endothermic,

N2 + O2 = 2NO ------------ (-) 180 kj/mol

CO2 + C = 2CO ------------ (-) 7.25 Mj/kg

These chemical reactions can proceed in either a forward or a reversed direction and are called

reversible. During combustion of fuel in furnaces, the rate of a direct process, say C +

O2 = CO2, is extremely higher than that of the reverse process, i.e. C O2 =

C + O2. The equilibrium of these reactions is shifted towards the formation of the

final products and therefore, these processes are irreversible. The intensity of combustion is

characterized by the rate of reaction involved.

As the combustion here is the heterogeneous process, the concentration of the combustible

substance (coal) is constant and therefore, the rate of reaction depends only on the concentration

of oxygen on the surface of coal (Cs).by law of mass action R=K X Cs

If the concentrations of the reactants do not vary with time, the reaction rate is determined by the

reaction rate constant K, which depends on temperature and nature of reactants, as given by

Arhenious equation

K=K0e-(E/RT)

Where K is a constant, E is the activation energy, KJ /Kg mol, R is the universal gas constant,

8.3143 KJ / Kg mol K and T is the absolute temperature.

Figures below shows the dependants of reaction rate on

temperature

activation energy

concentration of combustible matters

The rate of reaction increases rapidly with temperature. A chemical reaction occurs due to the

collisions of the molecules of reacting substances. If all collisions resulted in a reaction,

combustion would occur at an enormous rate and K would be equal to K0 . The energy that is

sufficient to destroy the molecular bonds of the starting substances is called the activation

energy, E. if the activation energy is high, it is difficult to destroy the molecular bonds of the

original molecules, and so the reaction rate is low. Therefore, as E increases, r decreases. The

activation energy of the reaction C + O2 = CO2 is E CO2 is 140 kj/mol and

that of the reaction C + 1/2 O2 = CO, Eco is 60 kj/mol. Thus it follows

that rate of formation of CO in carbon oxidation is substantially higher than that of CO2, and CO

will be formed predominantly at the surface of burning carbon particles.

Combustion can not take place at any arbitrary concentration of fuel in the mixture, but only in

definite range of its concentration in thee air. There is a lower concentration limit below which

combustion is impossible and an upper concentration limit when any further increase of the

concentration of the fuel prevents combustion. Thus combustion possible only in the

concentration range between these two limits.

Mechanism behind the combustion of coal:

Upon heating a solid fuel particles first undergo a stage of thermal preparation, which consists in

the evaporation of residual moisture and distillation of volatiles. Fuel particles are heated to

temperature at which volatiles are evolved rapidly (400˚C to 600˚C) in a few tens of second. The

volatiles are then ignited, so that the temperature around the coal particles increases sharply and

heating is accelerated. The combustion of volatiles occurs in .2 to .5 sec. A high yield of volatile

produces enough heat to ignite coal particle. When the yield of volatiles is low, the coal particles

must be heated additionally from an external source. The final stage is the combustion of coal

particles at a temperature above 800 -1000 ˚C. This is a heterogeneous reaction (gas –solid), the

rate of which depends on the oxygen supply to the reacting surface. The burning of a coal

particle takes up ½ to 2/3 of the total combustion time which is about 1-2.5 s (for pulverized

coal).

In carbon-oxygen reaction, oxygen is first absorbed form the gas volume on the surface of

particles and reacts chemically with carbon to form complex carbon-oxygen compounds of the

type CxOy, which then dissociate to form CO2 and CO. The resulting reactions at about 1200 ˚C

can be written as

4C + 3 O2=2CO + 2 CO2

The ratio of primary product, CO/ CO2, increases sharply with the increasing temperature of

burning particles. At 1700˚C, the resulting reaction becomes

3C + 2 O2=2CO+ CO2

Where the CO/CO2 ratio is equal to 2.

The primary reactions products are continuously removed from the surface of particles to the

environment. In this process, CO diffusing out encounters the oxygen diffusing into the reacting

surface and reacts with it within the boundary layer of gas to form CO2. Consequently, the

concentration of oxygen decreases sharply as it approaches the reacting surface, while the

concentration of CO2 increases (Fig).At high temperature, CO can consume all the O2 supplied,

which, consequently, will not reach the reacting solid surface (fig) and the endothermic reduction

of CO2 to CO will occur, with the high combustion temperature maintained due to high heat

release.

Optimizing the size and performance of the equipments involved in

combustion:

For combustion optimization firstly the size and performance characteristics of equipments

related to the whole process as like pulverizes, fans, feeders, as well as the arrangement of

burners and SADC are fore mostly required which have influence on

Easy ignition and reliable flame scanning

Maximum heat release

Optimum turn down

Efficient combustion (Minimum unburned)

Optimum temperature

Minimum excess air

Minimum emission

Minimum slag formation

Desired flame shape

Heat release profile matching furnace heat absorption

Arrangement of Burners:

Burners

Burners undertake the task of delivering coal and air in a proper proportion, facilitate ignition

energy to the coal air stream, sustain the ignition and provide a stable flame during the operation,

complete the task of combustion and delivering heat to the intended purpose.

Burners are broadly classified as follows:

(1) Tangential Burners

(2) Wall Burners

(3) Down shot or fantail burners

Modern Burners are equipped with:

(a)Separate flame envelope ports for coal, oil and gas

(b)Secondary air control to adjust the flame envelops.

(c)Ignitors.

(d)Flame Scanners to detect the distinct flames in an

Enclosure.

(e) Flame Stabilizers.

(f) Flame Analyses

Burner Arrangement

In a tangentially fired boiler, four tall wind boxes (combustion air boxes) are arranged, one at

each corner of the furnace. The oil and gas burners are located at different levels or elevations of

the wind boxes.

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.

The fuel and combustion air streams from these burners or compartments are directed

tangentially to an imaginary circle at the centre of the furnace. This creates a turbulent vortex

motion of the fuel, air and hot gases, which promotes mixing, ignition energy availability and

thus combustion efficiency.

The proportioning of air flow is done based on boiler load, individual burner load, by a

series of air dampers. Each of the auxiliary and end air nozzles are provided with louver

type regulating dampers, at the air entry to individual air compartment.

The damper regulates on elevation basis, in unison, at all corners.

Ignitors

A pilot flame ignites oil and gas. This pilot torch may be a

Oil ignitor

Gas ignitor

High-energy arc ignitor

In our boiler, “high energy arc ignitor has been used.

Flame sensing Device

Modern burners are generating flames using multiple fuels. As safety of the furnace and plant are

vital, flame sensing is very important.

Flame sensing devices are broadly grouped in to

1. Infrared flame sensors

2. UV flame scanners

3. Visible light scanners

In our boiler generally UV flame scanners with photo electric diode are used.

Role of FD Fan in Combustion Air Distribution:

The Combustion air, referred to as Secondary Air, is provided from FD Fans. A portion of

secondary air called `Fuel Air’ is admitted immediately around the burners (annular space

around the oil/gas burners) into the furnace. The rest of the secondary air, called ‘Auxiliary

Air' , is admitted through the auxiliary air nozzles and end air nozzles. The quantity of

secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD Fan inlet

guide vane regulation.

Operational optimization of secondary air

75 to 80% of total air distributed at different tiers

Secondary air velocity ~40 m/sec for better momentum and mixing

Secondary air temperature ~ 227 0C

Air distribution in tiers decide combustion efficiency

Fuel air is provided for the twin purposes of cooling nozzles and for positioning flame front-

close the damper if flame front is away and open if flame front is close to nozzle

Other damper openings to be adjusted depending on the operating tiers

Role of PA Fan in Combustion Process:

To dry the moisture in coal and facilitate better grinding in the mill

Transport the pulverized coal from the mill to the furnace at a velocity higher than settling

velocity of pulverized particle and that of flash back

Operational optimization of primary air

P.A / Coal ratio 1.5 to 2.5 (2 for better combustion efficiency)-lower the P.A better the flame

stability. P.A normally 1/fourth (20-25%) of total air

Variable P.A control gives better scope to improve burner performance

Primary air velocity 25 m/sec (to be > 20 m/sec to avoid settling in coal pipe. To be > 15 m/sec

to avoid flash back)

Minimum P.A temperature (at mill outlet) 57 0C to avoid condensation

Maximum P.A temperature (at mill outlet) tested 127 0C to avoid mill fire and softening &

sticking of coal in coal pipe

Normal P.A mix temperature (at mill outlet) is around 80 0C

Role of ID Fan in Combustion:

Induced draft fans are normally located at the foot of the stack. They handle hot combustion

gases. There power requirements are therefore greater then any other fans used in combustion .

In addition, they must cope with corrosive combustion products and fly ash. Induced draft fans

are seldom use alone. They discharge essentially at atmospheric pressure and place the system

upstream (superheater, reheater, economizer, gas side air pre heater, dust collectors and dampers)

under negative gauge pressure. So when mechanical draft is created by both force and induced

draft fans are used in steam generator. ID fan sucks out the flue gas through the heat transfer

surfaces and maintain a balance draft. Actually, it is maintained in slightly negative gauge

pressure to ensure that any leakage could be inward.

Role of Pulveriser:

Pulverized coal firing system

Coal is first ground to dust like size and powdered coal is then carried in a stream of air to be fed

through burners into the furnace. As the entering coal particles get heated in high temperature

flames in the furnace, the volatile matter is distilled off and this reduces the coal particles to

minute sponge like masses of fixed carbon and ash. The volatile gases mix with the oxygen of

the air, get ignited and burn quickly. Oxygen of the hot air reacts with the carbon surface to

release energy. The combustion products form a blanket on carbon particles, which is stripped

off by turbulent mixing of these particles and air. Proper burning of fuel needs the supply of

correct proportion of air, mixing of fuel and air, high temperature, and adequate time to complete

combustion reactions. The ash resulting from combustion

partly falls to the furnace bottom ,the rest is carried in gas stream as fly ash to flue gas outlet, or

is deposited on the boiler heating surfaces.

Modern central station boiler furnaces have water-cooled walls that form part of the heat

absorbing surfaces in steam generation. To burn pulverized coal successfully, the following two

conditions must be satisfied. large quantities of very fine particles of coal, usually those that

would pass a 200 mesh sieve must exist to ensure ready ignition because of their large surface to

volume ratio.

Minimum quantity of coarser particle should be present since these coarser particles causes

slagging and reduce combustion efficiency.

A typical screen analysis of a high volatile bituminous coal sample, pulverized to 80% 200

mesh(0.074 mm opening) 99.5% - 50 mesh, 96.5%-100 mesh,80%- 200 mesh

This represents a surface area roughly 150000 mm 2 /gm with 97% of the surface in the 200-

mesh portion. By over grinding and poor classification it would be possible to have a sample of

the following analysis.

95%- 50 mesh

90%- 100 mesh

80%- 200 mesh

This is not a satisfactory grind because of the high percentage retained on the 50 mesh, even

though the surface area remains the same. Thus classification plays a major rolein matching the

particle size to the reactivity of the fuel.

Greater surface area per unit mass of coal allows the faster combustion reaction because more

carbon becomes exposed to heat and oxygen. This reduces the excess air needed to complete

combustion. This also reduces the dry exhaust loss through chimney and rises the steam

generator efficiency. However, the extra cost of pulverizing equipment and grinding energy party

offset this advantage.

Performance of pulverizer

Hard grove Grindibility index- HGI of the coal indicates its easiness towards pulverization of

coal. It is also related to the power consumption for pulverization of coal. Therefore, the life and

efficiency of the pulverizer depends on the HGI of the coal.

It measures the increase of surface produced by the application of standard amount of work and

express the result as hard grove gridibility index(G) which ranges between 20 and 100 for most

of the coal.

G=13+6.93 ,Where W= gm of coal passing through 200 mesh sieve after 50 gm of coal of size

16-30 mesh are ground in a standard mill for 60 revolution. A high value of G represents soft or

easily grindable coal. The average HGI of Indian coal used in power station 50-80

Advantage of pulverized coal firing

Low excess air requirement.

Less fan power.

Ability to use highly preheated air reducing exhaust losses.

Higher boiler efficiency.

Ability to burn a wide variety of coals.

Fast response to load changes.

Easy of burning alternately with, or in combination with gas and oil.

Ability to release large amounts of heat enabling it to generate about 2000 Tph of steam or more

in one boiler.

Ability to use fly ash for making bricks etc.

Less pressure losses and draught need.

Disadvantage of pulverized coal firing

Added investment in coal preparation unit.

Added power needed for pulverizing coal.

Investment needed to remove fly ash before ID fan.

Large volume of furnaces needed to permit desired heat release and to withstand high gas

temperature.

Role of Feeders:

For more uniform operating condition ,higher burning rate and greater efficiency mechanical

stokers rather feeders are employed to feed the coal as per requirement to the pulverizes. These

may be of the following types:

Traveling grate feeders.

Chain grate feeder.

Spreader feeder.

Vibrating grate feeder.

Under feed feeder

Mainly moving grate or traveling grate stokers are used in our high pressure boiler to maintain

burning rate at an optimum efficiency.

Role of SADC in Modern Combustion Process and its Operation:

SADC system is used in a corner fired, natural circulation, and balanced draft unit.

It controls 6 nos. Fuel (coal) air dampers in A, B, C, D, E & F ELEVATIONS, 3 nos. Fuel ( oil )

air dampers in AB , CD & EF elevations , 6 nos. of auxiliary air dampers in AB , BC , CD , DE ,

EF & FF elevations and 2 nos. of OVER FIRE air dampers . There is a provision for firing light

oil in elev. AB alone

Operation:

1) AUXILIARY AIR DAMPERS :- During the furnace purge period & initial operation of the

unit up to 30% loading , all elevations of aux. air dampers ( AB,CD,DE,EF & FF ) modulate to

maintain a pre-determined ( approx. 40 mm wc ) set point dp, between furnace and windbox . As

the unit loading increases above 30% the set point also ramps up automatically and at pre-

determined breakpoint the slope of the ramp changes. Finally at 60% boiler load the set point

settles at 100 mm wc.

After 30% boiler load, the aux. air elevation associated with the main fuel elevation in service

modulates to maintain the varying dp. Those not associated with any elevation in service are

closed from top to bottom. The closing signal comes from FSSS .

2) FUEL AIR DAMPERS ( COAL ) :- When an elev. of main fuel is started, the associated

coal air dampers open & modulate as a function of feeder rate signal when coal is fired. A coal

air damper is selected to be closed when the respective pulverisers are OFF.

Their operation is independent of boiler load. All fuel air dampers are normally closed. They

open 50 secs. after the associated feeder is started and a particular speed is reached; then it

modulates as a function of feeder speed. 50 secs. after the feeder is removed from service, the

associated fuel air dampers close. They will open fully when both FD fans are OFF.

3) FUEL AIR DAMPERS (OIL):- The aux. air dampers at elev.- AB,CD & EF act as fuel air

dampers when oil firing is taking place and is open to a preset position . These aux. air dampers

are to be closed if there is a back up trip in respective elevation & adjacent pulverisers are off &

there is a “NO BOILER TRIP " signal present.

4) OVER FIRE AIR DAMPERS: - These dampers are positioned as a function of the boiler

load. The lower OFA dampers start opening at 50% boiler load & are fully open when the boiler

load reaches 75% .The upper OFA dampers start opening at 75% boiler load & are fully open

when the boiler load reaches 100% load.

Importance of SADC settings :- Successfully establishing or lighting up of an oil burner and

its stability .-- keeping open of aux. air dampers for initial purging and for air rich furnace

volume at lower loads .-- best ignition stability , distance of ignition point from coal nozzles ,

furnace stability , reliable and constant flame scanner pick-up .

Figure in the below describes how in different boiler load funace to wind box dp varies as

well opening of the dampers at different feed rate.

Funace pressure to W.B dp with different boiler load and damper opening with feed

rate

Burner arrangement and SADC in our tangentially corner fired boiler

Optimizing the quality of coal:

The heat input, the losses, and the burning fuel in the air must be determined and segregated

by combustion calculations in order to establish the efficiency of the heat transfer to the heat

exchanger. Knowing the amounts of each various losses is particularly helpful in deciding

how efficiency may be improved through the possible reduction of certain of the losses.

In this context, we have followed BS-22885(1974)/IS: 8753:1977 method of calculating

boiler efficiency on the basis of coal characteristics and using the data taken from the

laboratory proximate analysis to get various controllable and uncontrollable losses.

Controllable losses:

1) Dry flue gas loss

2) Loss due to solid combustible

3) Carbon monoxide loss

4) Mill rejects loss

Uncontrollable losses:

1)moisture loss

2)radiation loss

3)air moisture loss

4)sensible heat loss

In our calculation of losses we have given focus mainly on

1)solid combustible loss

2)loss due to dry flue gas

3)loss due to moisture and hydrogen in fuel

4)loss due to moisture in air

Solid combustible loss:

Heat loss due to solid combustible

Calorific value of carbon (Kcal/kg) X Combustible matter in ash (%) X Mass of ash per

unit mass of fuel (kg/kg)

==

Calorific value of coal (Kcal/kg)

Trial 1

Considering calorific value of carbon as 8900 Kcal/kg

On 04/03/2010 -- calorific value of coal as 4613 Kcal/kg

Percentage of ash by proximate analysis is 37.9 %

Mass of ash per unit mass of fuel = 0.379


8900 X 11.8 X 0.379

== == 8.69 %

4613

Trial 2



On 04/03/2010 Combustible matter in Ash (%)

Fly ash 2.9%

Bottom ash 8.9 %

Total 11.8 %


Fly ash 2.9%

Bottom ash 6.9%

Total 9.8%




8900 X 9.8 X 0.302

= = 4.9 %

5367

Trial 3






8900 X 11 X 0.351

= = 7%

4905


Fly ash 3.6%

Bottom ash 7.4%

Total 11%

Optimization:

Date Combustible % Ash C.V. of Coal Loss

04/03/2010 11.8 % 37.9 % 4613 8.62 %

06/03/2010 9.8 % 30.2 % 5367 4.9 %

07/03/2010 11.0 % 35.1 % 4905 7.0 %

It is very much clear from above 3 trials that if combustible matter in ash increases and

percentage of ash content as well increases then losses due to combustible get increased.

So for optimize combustion ash content must be held at minimum and secondary combustion

from where unburnt combustible may produce is to be avoided.

Dry flue gas loss:

This refers to the amount of sensible heat loss by dry flue gas leaving the system

Dry flue gas / unit mass of fuel

= (100/ (12 xCO2)) x (C+S/2.67-U) Kg mol/kg

% CO2 in stack exit generally 30 to 35 %

C carbon content in the coal which can be calculated from lab data

As we know

C = 0.97 X F.C. + 0.7 X( V.M. – 0.1 X Ash) – 0.6 X T.M.

F.C. Fixed carbon

V.M. Volatile matter

A Ash percentage

T.M. Total moisture

Date VM FC A TM C

04/03/2010 24.8 34.7 37.9 4.9 45.426

06/03/2010 26.6 40.8 30.2 4.3 58.602

07/03/2010 23.1 39.4 35.1 4.2 56

% S in our coal remains in between 0.2 – 0.5

Date % U % S

04/03/2010 3.9 % 0.2

06/03/2010 2.9 % 0.3

07/03/2010 3.6 % 0.5

Dry flue gas per unit mass of fuel from the above equation we get the following

04/03/2010 – 0.099

06/03/2010—0.155

07/03/2010-- 0.136

Sensible heat loss in dry flue gas per unit mass of fuel fired

= (Dry flue gas/ unit mass of fuel) X 30.6(Tg- Ta)

Dry flue gas loss

(Dry flue gas/ unit mass of fuel) X 30.6(Tg- Ta)

=

CV of Coal

Where Tg = air heater flue gas outlet temperature which is maintained 130-150 0C

Ta = air temperature at the system inlet which varies from 30-40 0C

On 04/03/2010 Dry flue gas loss


= X 100

CV of Coal

0.099 X 30.6(150-40)

= X 100

4613

= 7.22 %



= X 100

CV of Coal

0.155 X 30.6(130-30)

= X 100

5367

= 8.83 %



= X 100

CV of Coal

0.136 X 30.6(140-35)

= X 100

4905

= 8.9 %

Dry flue gas loss :

Date Dry flue gas loss

04/03/2010 7.22 %

06/03/2010 8.83 %

07/03/2010 8.9 %

Date C S U % CO2 Tg Ta CV Loss

04/03/2010 45.426 0.2 3.9 35 150 40 4613 7.22 %

06/03/2010 58.601 0.3 2.9 30 130 30 5367 8.83 %

07/03/2010 56 0.5 3.6 32 140 35 4905 8.9 %

Optimization-

It is very much clear from above 3 trials that when fixed carbon content is higher and

temperature difference between flue gas outlet and air inlet is higher then dry flue gas loss is

higher. So to optimize this flue gas temperature at air heater outlet has to be maintained at an

optimum value.

Heat loss due to moisture and hydrogen in fuel:

The moisture in flue gas per unit mass of fuel

(%M+9 X %H2)

== Kg/ kg of fuel

100

% of H2 is calculated as

H2= 0.036 X FC+0.091(VM-0.1 X A)- 0.05 X M

Heat per unit mass of moisture in flue gases at air inlet temperature

= 1.88 (Tg-25) +2442 + 4.2 (25-Ta) kJ/kg

Date % H % M % of moisture in Heat per unit mass

flue gas per unit

mass of fuel

of moisture in flue

gas (kJ/kg)

04/03/2010 3.041 % 2.4% 0.297% 2614

06/03/2010 2.514% 2.2% 0.248% 2597.4

07/03/2010 2.9% 2.4% 0.285% 2616.2

Heat loss due to moisture and hydrogen in fuel.

(Heat per unit mass of moisture in flue gas) X (% of moisture in flue gas per unit mass of

fuel) X 100

=

CV of Coal X 4.2

Date heat loss due to moisture & hydrogen

in fuel

04/03/2010 4 %

06/03/2010 2.85 %

07/03/2010 3.6 %

Optimization

As loss due to moisture and hydrogen in fuel is a uncontrollable loss when surface moisture

in coal will be increased due to long time stacking of coal in coal yards the loss will be

increased. If coal immediately after unloading is fed to bunkers the loss can be minimized.

Loss due to moisture in combustion air:

Heat loss due to moisture in combustion air

=A x m x 1.88(Tg-Ta)

Where A=total dry air required for combustion

m =water vapour content in combustion air(kg/kg)

considering total dry air,i.e.(primary secondary)=650t/hr in 160MW load

water vapor contenting combustion air =.0132(kg/kg)

and (Tg-Ta)=100 degree cel

Putting these values in the equation we get the loss to be 0.32%

Hence from above analysis we get that

Date Solid

combustible

loss

Dry flue gas

loss

Loss due to

moisture in fuel

Loss due to

moisture in dry air

04/03/2010 8.62% 7.22% 4% 0.32%

06/03/2010 4.9% 8.83% 2.85% 0.32%

07/03/2010 7.0% 8.9% 3.6% 0.32%

Thus total loss

Date Total loss

04/03/2010 20.16%

06/03/2010 16.9%

07/03/2010 19.82%

Coal characteristics generally maintained to optimize the combustion

Coals having FC / VM ratio closer to 1 will have better flame stability

VM less than 13% is not preferable for PC firing

Residence time

110 Mw 1.75 sec, 210 Mw-2.2 sec, 500 Mw-3.5 sec

Crossing point temperature -175 to 250 Deg.C

Flammability temperature – 400 to 600 Deg.

Low ash, high volatile, high moisture, high CV (imported coal)

Flame propagation affected more by moisture than by ash

Priority for drying coal. Hence, PA cannot be reduced below a particular level

Necessary to incorporate split coal nozzle or diverters

Low moisture, low volatile, high ash

Flame propagation affected by high ash

Reduce primary air to minimum extent possible

Increase OFA

Optimization through close control of excess air:

Why excess air?

For complete combustion reaction to take place efficiently, the molecules of O2 must be in

actual physical contact with the atoms of the combustible. It becomes impossible as air

contains almost 4 times the amount of N2 molecules compared to the essential O2 molecules.

The N2 molecules hinder the physical contact between the combustible and O2,

simultaneously non combustible matter in coal, products of combustion do play the same

role.

Inadequate mixing of air and fuel, fluctuating operating and ambient conditions, burner

performance and wear and tear

To ensure that fuel is burned completely or with little combustibles, some amount of excess

air is provided

Why air instead pure oxygen?

Air contains 21% by volume or 23% by weight of Oxygen and is readily available.

Pure oxygen needs processing, the cost of which outweighs the benefit on combustion and

heat release

Excess air calculation:

The ultimate analysis of the fuel is given by

C+H+O+N+S+M+A=1

Oxygen needed for the oxidation processes can be calculated as follows

C + O2 = CO2 + 33940kJ/kg

12 kg 32 kg 44 kg

1 kg 2.67 kg 3.67 kg

C kg 2.67 C kg 3.67 C kg

2H2 + O2 = 2H2O + 142679kJ/kg

4 kg 32 kg 36 kg

1 kg 8 kg 9 kg

H kg 8 H kg 9 H kg

S + O2 = SO2 + 9141kJ/kg

32 kg 32 kg 64 kg

1 kg 1 kg 2 kg

S kg S kg 2 S kg

Oxygen required for complete combustion of 1 kg fuel is

W O2 = 2.67 C+ 8 H + S – O

Where O is the oxygen in the fuel.

Air contains 23.2% oxygen by mass. Therefore theoretical air required for complete

combustion of 1 kg fuel is called stoichiometric air and it is denoted by

Wt= WO2/ 0.232

= 11.5 C + 34.5 (H- O/8) + 4.3 S

Where C, H, O, S is the mass fraction of carbon, hydrogen, oxygen and sulphur in the fuel as

given by the ultimate analysis.

As stated earlier, complete combustion of fuel cannot be achieved if only the theoritical or

stoichiometric air is supplied. Excess air is always needed for complete combustion. It is

expressed as a percentage or by the use of a dilution co-efficient. The percent excess air

supplied is

% excess air ={(Wa - Wt) / Wt} X 100

Where Wa is the actual amount of air supplied for complete combustion of 1 kg fuel . The

dilution co-efficient, d, is given by

D = Wa / Wt

The percentage of excess air varies between 15 and 25 % for most large utility boiler.

Negative aspects of high excess air

Increase in auxiliary power (FD & ID fan)

Increase in furnace temperature and NOx formation

Increase in loss of sensible heat carried away by flue gas

Increase in erosion due to increase in flue gas velocity

Limitation on boiler load due to exhaustion of ID fan capacity

Shift in heat transfer from furnace to convection pass resulting in heating up of down

stream components

Impact of lesser air than stoichiometric requirement

Incomplete combustion leading to

Reduction in energy release

Increase in unburned hydro carbons (CO& CnHm) in flue gas

Increase in unburned carbon level in fly and bottom ashes

Slagging in boiler furnaces

By chemical equation we can easily see that how much heat is lost due to the production of

CO in case of incomplete combustion of carbon

2C+O2 = 2CO + 10120 kJ/kg

Therefore there is a reduction in heat released between burning carbon to carbon di oxide and

carbon to carbon monoxide.

Loss of heat = (33940-10120)

= 23820kJ/kg

Reference curves for Optimum % Oxygen at Economizer outlet for minimum heat rate

Curve to estimate % excess air based % Oxygen

From the above-mentioned points it is very much clear that to maintain the combustion

efficiency rather boiler efficiency at an optimum point controlling the excess air is a

necessity and it is basically controlled by monitoring

oxygen and combustible in flue gas at eco outlet by installing oxygen analyzers.

Unburned carbon level in fly and bottom ash.

In our boiler if we have to maintain excess air within 15 to 25 percent for which oxygen has

to be maintained within 3.5 to 4 percent. With this percentage flue gas outlet temperature at

air heater outlet is maintained within 130 to 140deg cel.

Optimizing the temperature:

Temperature is a prime factor affecting efficiency and losses, particularly the temperature of

the combustion gases finally rejected to the stack. The temperatures throughout the boiler or

heat exchange unit depend closely upon the manner of heat transfer.

However, the adiabatic, or “theoretical”, temperature(without gain or loss of heat), that might

conceivably be attained in burning a fuel under certain conditions, is sometimes calculated.

Even though this temperature never exists in actual practice, it is of value in estimating

furnace temperature and furnace heat absorption.

The theoretical, or adiabatic, temperature is the maximum gas temperature that can be

obtained under certain conditions. With less excess air or higher air preheat temperature, the

gas temperature will be higher. If the gas temperature exceeds about 3200F,a phenomenon

occurs in which the CO2 and H2O constituents of the combustion gases tend to split into

their component parts. This process is called gas dissociation. The effect of this reversal in

the combustion process is to reduce the availability of the heat of combustion and thereby

reduce the heat producing high temperatures. However, since the furnace exit temperatures

are usually not high enough to be affected by this phenomenon, gas dissociation is seldom

considered in combustion optimization calculation.

Here we are discussing how reaction rate varies with different temperature and the

effect of diffusion of gases in the furnace.

Zone I-kinetic region

Zone II-transition region

Zone III-diffusion region of burning

As we know from combustion kinetics that

rs = k x Cs

thus the total reaction rate is controlled by the kinetics of the chemical reaction on the

surface. The temperature region(1) of reactions is called the kinetic combustion zone, and the

reaction is said to be kinetically controlled (which depends on the temperature of the reaction

surface).

At high temperatures, above 1400deg cel, the rate constant of the reaction on the surface

increases rapidly and exceeds the maximum rate of oxygen supplied to the surface, which

varies only slightly with temperature. In this zone, the reaction rate varies slowly inspite of

increasing temperature. Oxygen supplied to the surface by diffusion reacts instantaneously

(at high temperature) and its concentration at the surface becomes zero. This temperature

region is called the diffusion combustion zone (III). With oxygen deficiency at the surface,

the reduction of CO2 to CO occurs at the incandescent coke surface, while the diffusing

oxygen is completely consumed in the gas film in oxidizing CO to CO2. in this zone rate of

combustion increases with the increasing rate of diffusion of gases, i.e. turbulence, and with

the decreasing size of solid particles.

At intermediate temperatures (1000-1400), the rate of reaction at the surface commensurate

with the rate of oxygen supplied to the surface, and both of the process determine the total

rate of the reaction. This is called the transition zone of combustion (II).

Optimization:

It can thus be inferred that when the reaction surface is incandescent and its temperature is

very high, the chemical resistance at the surface is small and the reaction is almost

instantaneous. Solid fuel combustion is most often diffusion controlled. It depends on the rate

at which oxygen diffuses into the reaction surface. If the gas film resistance is reduced by

inducing turbulence, the rate of diffusion of oxygen and hence the rate of reaction get

enhanced. When the air-fuel mixing is very high, as in fluidized beds, the diffusion resistance

becomes negligible and the combustion is then kinetically controlled with the combustion

rate depending on the surface temperature.

Ignition of any fuel begins relatively at a low temperature with ample oxygen supply and it is

essentially in the kinetic zone. As the temperature rises, oxygen consumption in the reaction

zone rapidly increases and the process passes through the transition zone into the diffusion

zone.

The end of the combustion is usually well in the diffusion zone where the residual oxygen

concentration becomes low, thus retarding the combustion of the remaining fuel.

Optimizing SOx and NOx:

NOx and SOx formation and reduction

The formation of nitrogen oxides with the combustible the formation of NOx and Sox with

the combustion of fossil fuel may result from three different reaction mechanism

Thermal NOx SOx formation.

Promt NOx formation.

NOx SOx formation from fuel nitrogen, sulphur

OVERFIRE AIR

Over fire air is introduced into the furnace tangentially through two additional air

compartments, termed as overfire air ports, designed as vertical extentions of the corner

windboxes. The overfire air ports are sized to handle 15 % of total windbox air flow.

The proven success of over fire air as a supplement to the tangential firing in limiting NOx

and Sox formation during coal combustion lies in the fact that this technique inhibits

formation of both fuel NOx and Sox and thermal NOx and Sox as an oxygen deficient

environment is established on primary combustion zone.

At design levels of overfire, a 20 to 30% reduction in NOx and Sox formation is achieved.

.

The previous chart indicates the need for care in identifying optimum NOx control methods.

While NOx control methods. While NOx emission decrease linearly with increasing over fire

air, excess air raises(i.e., more air is needed to complete the combustion)

Application of Low NOx Burners

Wall burners (oil and gas )

A low NOx burner indigenously developed , tested at site and introduced in the contract

GAIL -AURAIYA. This burner is capable of emitting NOx at a level of 150 ppm on oil

firing.

Tangential firing

In the current units CCOFA(close coupled overfire air) feature is incorporated in the

windbox itself. To meet the latest trends in the NOx control measures , provision of a

separate over fire air( SOFA ) is being developed .

Conclusion:

From above analysis of different aspects related to combustion optimization the topic can be

concluded with following points

• Optimization of combustion in Indian high ash coal fired boiler is of special interest

due to the organic and inorganic interaction and the large amount of variation in the

organic.

• The combustion behavior of Indian coal in boiler furnaces needs understanding of the

complex organic and inorganic mix up.

• The high percentage of ash, the low reactive in the organic of coal, the

encapsulation of organic in inorganic, the oxidized coal in many cases and the

blending of many type of coals are some of the reasons for varying behaviour of coals

during combustion.

• Indian high ash coals result in high primary air requirements -primary Combustion

Dilution

• Sec. Air distribution at required elevation is very important

• All unwanted sec. air at any location has to be reduced and should be diverted them

to other needy elevation.

• Mill air flow has to be kept just above settling velocity. Primary air flow is to be

checked thoroughly as reducing primary air can start slight furnace disturbance. In

case of disturbance primary air is to be increased by 1-2 t/hr

• Total air flow with 15 to 25% excess air @ eco out is to be maintained. For fulfilling

the requirement of excess air O2% is kept at around 3.6%-4% at air heater outlet.

• If VM is less than 20 - 22% all fuel air dampers should be closed. After looking flame

front we have to decide for higher VM coal

• Wind box pr is to be kept at around 100 mm for better distribution across elevation.

• Opening of AA damper (Manual Damper) in the range of 40 – 50 %is to be

maintained always irrespective of the mills in service. It should not be closed any

time.

• The FAD of the operating elevation should be closed for the VM is less than 20 %

(Every 1% VM increase, open FAD by 2-3% approx.)

• The FAD & AAD of non-working elevation should be closed always. .

• To get better flame intensity and stability, optimum windbox Dp and reducing the

opening of FAD is suggested.

• Checking and ensuring the same position of SADC’s elevation wise in all the corners

is a regular necessity. Wind box Dp in left and right side should be equal.

• Opening of OFA/top AAD of non-working EL. – Depends on unburned fly ash and

SH/RH spray so, these two have to be monitored for opening OFA.

• Coal fineness at mill outlet has to be maintained as 70% of 200 mesh. If it is more

than that furnace temperature may increase and tend to the formation of slags. If it is

less than that unburned coal particle at bottom ash will increase.

• Mill outlet temperature is maintained at around 800 to850c to dry out the surface

moisture of the coal.

• Soot blowing in air pre heater and different locations of furnace should be given at

regular interval of time for better heat transfer through the convective surface.

Reference:

1. Steam, its generation and use by “The Babcock and Wilcox

company”

2. Power Plant Engineering by P.K.Nug

3. Power Plant Familiarization, WBPDCL(BkTPP) Manual

4. www.google.co.in

5. Reference data collected from chemical laboratory,S.T.P.S

Documents

Combustion Project