ME 326 Thermal Power Engineering

Topic : Fuels and Combustion

Dr. Arvind Pattamatta

Heat Transfer and Thermal power Lab

Department of Mechanical Engineering

Indian Institute of Technology Madras

Contents

Fuel types

Coal: its classification, and analysis

Coal properties

Combustion reactions

Mechanism of Coal combustion

Enthalpy of combustion and Flame temperature

Coal firing : Stoker, Pulverized, Cyclone and Fluidized bed.

Coal gasification.

Fuels

Fuels are substances which, when heated, undergo chemical reaction with an oxidizer, typically oxygen, to liberate heat.

Fuels may be solid, liquid or gaseous

Fuels may be fossil (non-renewable) or biomass (renewable)

Fossil fuels may be coal, petroleum-crude derived or natural gas.

Biomass fuels may be wood, refuse or agricultural residues.

Solid fuel embraces a wide variety of combustibles, ranging from wood, peat and lignite, through refuse and other low calorific value substances, to coal and other

solid fuels derived from it.

Coal represents by far the largest component of the worlds fossil fuel reserves.

The carbon: hydrogen ratio of coal is the highest of the fossil fuels, hence the calorific values of coals are principally determined by the carbon in the fuel.

Coal reserves worldwide in BTUs

1 BTU = 1.055 KJ

Physical attributes of Coal

Moisture content

% of moisture in fuel (0.5 10%)

Reduces heating value of fuel

Weight loss from heated and then

cooled powdered raw coal

Volatile matter

Methane, hydrocarbons, hydrogen, CO,

other

Typically 25-35%

Easy ignition with high volatile matter

Weight loss from heated then cooled

crushed coal

Ash

Impurity that will not burn (5-40%)

Important for design of furnace

Ash = residue after combustion

Fixed carbon

Fixed carbon = 100 (moisture +

volatile matter + ash)

Carbon + hydrogen, oxygen, Sulphur,

nitrogen residues

Heat generator during combustion

Coal classification

It is usual to consider coals in terms of their rank: in general, a high ranking coal will have a high carbon content.

The other major coal constituent element, hydrogen, is present in hydrocarbons which are released as volatile matter when the coal is heated.

As the rank of a coal increases, its carbon content increases from 75% to about 93% (by weight), the hydrogen content decreases from 6% to 3%, and the oxygen content

decreases from 20% to 3%.

A useful method for analyzing a coal is the proximate analysis.

Formation of different types of Coal

Coal Analysis

Proximate analysis of coal

Determines only fixed carbon, volatile matter, moisture and ash

Useful to find out heating value (GCV)

Simple analysis equipment

Ultimate analysis of coal

Determines all coal component elements: carbon, hydrogen, oxygen, sulphur,

other

Useful for furnace design (e.g flame temperature, flue duct design)

Laboratory analysis

Coal Analysis

Proximate analysis:

Sample of known mass, to determine:

Moisture dried at 105 to 110oC in an oven

Volatile combustible matter heated to 900oC in a covered crucible Fixed carbon heated to 750oC in an open crucible

Ash the final residue

Coal Ranking

With increasing Rank, the following characteristics are noticed:

1. Age of coal is increased. This increases with increase in depth of deposit.

2. A progressive loss of oxygen, hydrogen and in some cases sulfur, with a corresponding increase in carbon.

3. A progressive decrease in equilibrium moisture content.

4. A progressive loss of volatile matter.

5. Generally, a progressive increase in calorific value.

6. In some cases, a progressive increase of ash content.

Table 9.1 Composition of some typical solid fuels (% by mass)

Fuel Carbon Volatile

matter

Moisture

Ash

Peat

Lignite

Bituminous

Coal

Anthracite

44

57

82

90

65

50

25

4

20

15

2

1

4

4

5

3

Indian

Coal

Indonesian

Coal

South African

Coal

Moisture 5.98 9.43 8.5

Ash 38.63 13.99 17

Volatile

matter

20.70 29.79 23.28

Fixed

Carbon

34.69 46.79 51.22

Typical proximate & Ultimate analysis of various coals

Parameter Indian Coal, % Indonesian Coal, %

Moisture 5.98 9.43

Mineral Matter (1.1 x Ash) 38.63 13.99

Carbon 41.11 58.96

Hydrogen 2.76 4.16

Nitrogen 1.22 1.02

Sulphur 0.41 0.56

Oxygen 9.89 11.88

GCV (kCal/kg) 4000 5500

Coal properties

There are a number of properties which are important in identifying the suitability of a coal for any given application:

Size Some common size groups, together with their rather picturesque names, are given

in Table 1 (next slide).

Heating Value or Calorific value

Coal fuels generally have a range of values from 21 to 33 MJ/kg (gross).

Ash Fusion Temperature

Sulphur Content

Size

Table 1 Size distribution for coals

Name Upper limit (mm) Lower limit (mm)

Large Cobbles

Cobbles

Trebles

Doubles

Singles

>150

100-150

63-100

38-63

25-38

75

50-100

38-63

25-38

13-18

Heating Value

It is the heat transferred when the products of combustion of a sample of coal are cooled to the initial temperature of air and

fuel.

The higher heating value (HHV) assumes that the water vapor in the products condenses and includes the latent heat of

vaporization of the water vapor.

The lower heating value (LHV) assumes that the water vapor formed by combustion leaves as vapor itself.

HHV of anthracite and bituminous coals approximately determined from Dulong and Petit formula:

HHV = 33.83 C + 144.45 (H O/8) + 9.38 S in MJ/Kg;

where C, H, O are mass fractions in coal.

Heating Value

LHV = HHV mwhfg = HHV 2.395mw MJ/Kg Where mw is the mass of water vapor formed given by

mw = M + 9H + gawa

Where M and H are the mass fractions of moisture and hydrogen

in the Coal, ga is the specific humidity of atmospheric air, and

wa is the actual amount of air supplied per Kg of coal.

For Energy balance, HHV is considered in the USA and LHV in Europe.

Coal properties

Ash Fusion Temperature The melting point of the ash left after combustion of the coal is of particular

importance in terms of the combustion and ash disposal equipment.

If the ash fuses it produces a glassy, porous substance known as clinker (slag).

The combustion equipment will be designed to handle either clinker or unfused ash, and use of the wrong type of coal can have dire consequences.

Sulfur Content Many deep-mined coals have a fairly high sulfur content, typically around 1.5% by

weight.

The same consideration apply to coal-fired installations as to oil-fired combustion equipment namely that condensation inside the plant must be avoided and that the

design of the flue must ensure that ground concentration of sulfur oxides are

controlled within acceptable limits.

Principles of combustion

Combustion: rapid oxidation of a fuel

Complete combustion: total oxidation of fuel (adequate supply of oxygen needed)

Air: 20.9% oxygen, 79% nitrogen and other

Nitrogen: (a) reduces the combustion efficiency (b) forms NOx at high

temperatures

Carbon forms (a) CO2 (b) CO resulting in less heat production

Control the 3 Ts to optimize combustion:

Water vapor is a by-product of burning fuel that contains hydrogen and this robs

heat from the flue gases

1T) Temperature

2T) Turbulence

3T) Time

Requirements for Combustion

Good combustion is accomplished by controlling the "three T's"

of combustion which are

(1) Temperature high enough to ignite and maintain ignition of

the fuel,

(2) Turbulence or intimate mixing of the fuel and oxygen, and

(3) Time sufficient for complete combustion.

Requirements for Combustion

Oxygen is the key to combustion

Bureau of Energy Efficiency, India, 2004

Combustion When a solid fuel particle is exposed to a hot gas flowing stream it undergoes

three stages of mass loss

i. Drying

ii. Devolatilization

iii. Char combustion

The relative significance of these three is indicated by proximate analysis of coal

devolatilization

volatiles

char

homogeneous

combustion

heterogeneous

combustion

CO2, H2O,

CO2, H2O,

tchar=1-2sec tvolatiles=50-100ms tdevolatile=1-5ms

t

coal particle p-coal, d=30-70m

Burning of coal particle

i. Drying

The combustible material generally constitutes water e.g. lignites up to 40 % Upon entry into the gas stream, heat is convected and radiated to the particle

surface and conducted into the particle

The drying time of a small pulverized particle is the time required to heat up the particle to the vaporization point and drive off the water

DEVOLATILIZATION Occurs in the temperature range between 400 600 deg C. Devolatilization or pyrolysis is the process where a wide range of gaseous

products are released through the decomposition of fuel.

The volatile matter (VM) comprises a number of hydrocarbons, which are released in steps

Since the volatiles flow out of the solid through the pores, external oxygen cannot penetrate into the particle, hence the devolatilization is referred to as the pyrolysis stage.

A high yield of volatiles produces enough heat to ignite coke particles.

Burning of coal particle CHAR COMBUSTION

The combustion of a char particle generally starts after the evolution of volatiles

from the parent fuel particle, but sometimes the two processes overlap.

The char, being a highly porous substance, has a large number of internal pores of

varying size

Oxygen diffuses into the pores and oxidizes the carbon on the inner walls of the

pores.

The oxygen then undergoes an oxidation reaction with the carbon on the char

surface to produce CO.

The CO then reacts outside the particle to form CO2.

The mechanism of combustion of char is fairly complex.

At about 1200 deg C,

4C + 3 O2 = 2 CO + 2CO2

At 1700 deg C

3C + 2O2 = 2CO + CO2

Burning of coal particle

Burning rate of coke particle

The rate of diffusion of oxygen per unit surface area of the particle is given by Ficks law:

rds = kg(cb cs) Where kg = D/d is the mass transfer coefficient, m/s, D is the diffusion coefficient, m

2/s,

and d is the boundary layer thickness, m.

The diffusion rate of oxygen will be maximum when Cs = 0

rds max = kg cb The burning rate or rate of reaction is given as

rsr = k cs

The maximum reaction rate will occur at cs = cb

rsr max = k cb

For rsd = rsr = rs

b

g

srbgs c

1/k 1/k

1 )

k

r (ck r

Burning rate of coke particle

Temperature has a weak effect on mass transfer by diffusion (kg ~ T0.5).

However, K ~ exp(-E/RT).

At T < 1000 deg C, surface reaction is slower than the oxygen diffusion rate, k > kg

This regime is called diffusion combustion zone where rs = kg cb.

In the diffusion combustion zone, the reduction of CO2/CO occurs resulting in incomplete combustion.

At intermediate temperatures (1000-1400 deg C), transition zone of combustion.

When air fuel mixing is very high, as in fluidized beds, the diffusion resistance becomes negligible, k

Burning rate of coke particle

Combustion Reactions

The basic chemical equations for complete combustion of coal are

TTT and A(Air fuel ratio)

2CO O 2C

oxygen,nt insufficieWith

SO O S

O2H O 2H

CO O C

2

22

222

22

Stoichiometric Air From the ultimate analysis of fuel

C + H + O + N + S + M + A = 1.0

Oxygen required for complete combustion of 1 kg of fuel is

WO2 = 2.67 C + 8 H + S O; where O is the mass of oxygen in the fuel. kg S 2 kg S kg S kg 2 1kg kg 1

kg 64 kg 32 kg 32

SO O S

kg H 9 kg H 8 kg H

kg 9 kg 8 kg 1

kg 36 kg 32 kg 4

O2H O 2H

kg C 3.67 kg C 2.67 kg C

kg 3.67 kg 2.67 1kg

kg 44 kg 32 kg 12

CO O C

22

222

22

Stoichiometric Air

Air contains 23.2 % oxygen by mass. Therefore, stoichiometric air required for complete combustion of 1 kg of fuel is

Wt = WO2 / 0.232 = 11.5 C + 34.5 (H - O/8) + 4.3 S

where C, O, S, H are mass fractions of respective components.

% Excess Air = (Wa-WT) * 100 / WT

The dilution coefficient, d, is given by

d = Wa/WT

The % excess air varies between 15 to 30 % for most large utility boilers.

Combustion equation

Consider a coal having the following ultimate analysis:

C-60 %, H 4 %, S 3.2%, O 4.8 %, N 2 %, M- 5 %, and A 21 %.

The exhaust gas has the following volumetric analysis:

CO2 + SO2 = 12%, CO = 2%, O2 = 4% and N2 = 82 %.

Let a moles of oxygen be supplied for 100 kg fuel. Then, the combustion equation can be written as

OhHgNfOeSOdCObCO

aNaOOSHC

22222

276.3

2232

8.4

32

2.322

4

12

60

By equating the coefficients, Carbon: b+d = 60/12 = 5 Hydrogen: h=2 Sulphur: e = 3.2/32 = 0.1 Oxygen: b+d/2+e+f+h/2 = 4.8/32 + a Nitrogen: g = 3.76a

Actual Air

82.0;04.0

02.0;12.0

gfedb

g

gfedb

f

gfedb

d

gfedb

eb

By solving these equations, the coefficients a,b,d,e,f, g and h are determined and given to be d=0.73, b=4.27, f=1.46, a =7.045 and g = 26.49 Actual air supplied per kg coal = 32a/(0.232X100) = 9.72 kg. The actual amount air supplied per kg fuel can be ascertained from the measured volumetric composition using 1. Orsat analyser, 2) Haldane apparatus, 3) Infra-red gas analyser 4) Gas chromatograph.

Control of Excess Air

EFFECT OF Fuel characteristics ON

COMBUSTION EFFICIENCY

The fuel ratio of a fuel is the ratio of fixed carbon (FC) and VM contents of the fuel.

This ratio has an important effect on the combustion efficiency of coal in a CFB boiler

Higher ratios possibly leading to lower combustion efficiencies

A high rank fuel like anthracite has a higher fuel ratio than a low rank fuel like lignite.

For this reason low-rank fuels (or low fuel ratio) like lignite and

bituminous have higher efficiencies than anthracite.

The fuel ratio is easily computed from the proximate analysis of a fuel

Mass balance of a steam generator

Mass balance of a steam generator

WA + C + H + O + S + N + M + A = Wdfg + 9 H + M + A + C Cab

WA = Wdfg + 8(H-O/8) Cab N S

Mass of dfg produced per kg of coal,

)(3

)7004(

)(12

)32)100(282844(

)(12

)32282844(

2

22

2

2222

2

222

COCO

OCOC

COCO

OOCOCOCOCOC

COCO

ONCOCOCW

ab

ab

abdfg

Volume of flue gases (wet) produced per kg of coal

g

gOH

dfg

dfg

fgp

XT

XXW

M

WV

325.101

273

2734.22

18

2

Energy balance of a steam generator furnace

Energy balance of a steam generator furnace

The 1st law for the steady-state steady-flow (SSSF) for a chemically reactive system, ignoring the changes in K.E and P.E is

HR + Q = HP + Wsf Where HR and HP are the enthalpies of the reactants and products, evaluated at their

respective pressures and temperatures.

T

pf

pTatmKatmKf

P

PR

PRR

dTCh

hhpTh

pTmhHpTmhH

298

,1,2981,298

'

)()(),(

),();,(

Enthalpy of Combustion

For open systems, the heating value is sometimes referred to as enthalpy of combustion

RP

KnMhKnMhHV ))298(())298((

Heating value of fuel For 1 kg of coal containg C kg carbon, the heat released by carbon combustion is

C X 407000 / 12 = 33917 C KJ /Kg.

Heat released by Sulphur = 291000 X S/32 = 9094 S kJ/kg

Mass of hydrogen available for combustion in coal is (H O/8).

Heat released by Hydrogen combustion is (H-O/8) X 286000 /2 = 143,000 (H O/8)

The total heat released by complete combustion of 1 kg of coal is

HHV = 33.917 C + 143 (H O/8) + 9.094 S MJ/Kg.

This equation is very close to Dulongs formula

Adiabatic combustion temperature

A fuel burning with no heat exchange with the surroundings and no work done will result in the adiabatic-combustion

temperature.

It is greater if the fuel is burned in O2 than in Air because of the dilution effect of N2.

It is higher for a stoichiometric mixture as lean mixture has dilution effect, whereas a rich mixture results in incomplete

combustion.

At the adiabatic condition, Q = 0 and

For Coal combustion

R

R

P

ad pTnMhpTnMh ),(),(

P

adTnMh 0)(

Chemical equilibrium

In high temperature combustion processes, the products of combustion are not a simple mixture of ideal products at stoichiometric conditions.

Rather the major species dissociate producing a host of minor species.

For example, the ideal combustion products for burning a hydrocarbon with air are CO2, H2O, O2 and N2.

Dissociation of these species reaction among the dissociated products yield the following species: H2, OH, CO, H, O, N, NO etc.

Therefore we need to develop a methodology to calculate the mole fraction of all of the species at a given temperature and pressure. The final state is not governed

solely by first law considerations, but necessitates invoking the second law.

Consider the combustion reaction

If the temperature is high enough, the CO2 will dissociate.

where a is the fraction of CO2 dissociated. If a = 0, max heat release occurs.

productshottsreaccold

OCOCOOCO

22

tan

22

)1(2

1 aaa

222

1COOCO

Second law consideration

For constant U, V and m, which implies no heat and work interactions (isolated system), the second law requires that the entropy change interior to the system:

The composition of the system will spontaneously shift towards the point of maximum entropy. Hence, the condition for chemical equilibrium can be written as

In order to define the chemical equilibrium for a system at a given pressure, temperature and mass, the Gibbs free energy, G, replaces the entropy as the

important thermodynamic property.

The second law can be expressed as

At Equilibrium

0dS

0,, mVUdS

TSHG

0dG

0,, mPTdG

Gibbs function

For a mixture of ideal gases, the mole specific Gibbs function for the ith species is given by

Where is the Gibbs function of the pure species at the standard-state pressure

(i.e Pi=Po), and Pi is the partial pressure. The standard state pressure is taken to be 1

atm.

The Gibbs function for a mixture of ideal gas species can be expressed as

For a fixed temperature and pressure, the equilibrium condition becomes dGmix = 0.

The second term is zero since changes in partial pressures must sum to zero. Thus

o

iuTi

oTi

P

PTRgg ln,,

Tiog ,

]ln[ ,,

oiuTi

oiTiimix

P

PTRgNgNG

0]ln[]ln[ ,,

oiuTi

oio

iuTi

oi

P

PTRgdN

P

PTRgdN

0]ln[ ,

oiuTi

oimix

P

PTRgdNdG

Equilibrium constant

For a general reaction

The change in the number of moles of each species is directly proportional to its

stoichiometric coefficient i.e

Substituting this into the equation for equilibrium gives

Rearranging and grouping the log terms together

...,... fFeEbBaA

fdN

edN

bdN

adN

F

E

B

A

o

FuTF

o

o

EuTE

o

o

BuTB

o

o

AuTA

o

P

PTRgf

P

PTRge

P

PTRgb

P

PTRga

lnln

lnln

,,

,,

b

oB

a

oA

f

oF

e

oE

uTBo

TAo

TFo

TEo

P

P

P

P

P

P

P

P

TRgbgagfge

ln,,,,

Equilibrium constant The term on the LHS of the above equation is called the standard-state Gibbs

function change

The argument of the natural logarithm is defined as the equilibrium constant

With these definitions, the statement for chemical equilibrium at constant pressure and temperature can be written as

From this relation, we can obtain a qualitative indication of whether a particular reaction favors products or reactants. If is positive, Kp is negative. Hence

reactants will be favored and otherwise.

Also

For Kp to be greater than unity, which favors products, the enthalpy change for reaction, should be negative, i.e, the reaction is exothermic.

TBoTAoTFoTEoTo gbgagfgeG ,,,,

b

oB

a

oA

f

oF

e

oE

p

P

P

P

P

P

P

P

P

K

)/exp( TRGK uTo

p

ToG

)/exp()./exp( uo

uo

p

ooT

o

RSTRHK

STHG

Coal Firing

Since the old days of feeding coal into a furnace by hand, several major advances have been made to improve the

combustion efficiency.

Types of coal firing:

1. mechanical Stoker firing

2. Pulverized firing (1920s:represented a major increase in combustion rates over mechanical stokers)

3. Cyclone Firing (1940s)

4. Fluidized bed firing (1950s)

Proper Size of Coal for Various Types of Firing

System:

S. No. Types of Firing System Size (in mm)

1. Hand Firing

(a) Natural draft

(b) Forced draft

25-75

25-40

2. Stoker Firing

(a) Chain grate

i) Natural draft

ii) Forced draft

(b) Spreader Stoker

25-40

15-25

15-25

3. Pulverized Fuel Fired 75% below 75 micron*

4. Fluidized bed boiler < 10 mm

Mechanical Stoker Firing

A stoker is a power operated fuel feeding mechanism and grate.

Automatic stokers are classified as

1. Overfeed stokers.

2. Underfeed stokers.

The overfeed stokers are mainly classified into two types.

1. Travelling grate stoker

a) Chain grate stoker

b) Bar grate stoker

2. Spreader stoker.

Over feed Vs Under feed stoker

Differences between overfeed and underfeed

stokers:

Sl.No Overfeed stokers Underfeed stokers

1 Suitable for boiler installation where the

coal is burnt with pulverisation.

Suitable for semi-bituminous and

bituminous coals with high

volatile matter.

2 The volatile matter requires longer time

for complete burning and results in

the formation of smoke.

The volatile matter is at higher

temperature before entering the

furnace and hence burns quickly

when mixed with secondary air.

3 The ash is comparatively at lower

temperature.

The ash left at the bottom of the

stoker is at high temperature.

4 The coal is fed into the grate above the

point of air admission.

The coal is admitted into the furnace

below the point of air admission.

5 Both coal and air moves in the opposite

direction.

Both coal and air moves in the same

direction.

Travelling Grate Stoker

The speed of the stoker is 15 cm to 50 cm per minute. non-caking coals are best suited for chain grate stokers. The rate of burning with this stoker is 200 to 300 kg per m2 per hour

Spreader Stoker

Most widely used for stem capacities of 9.5 to 50 kg/s.

It can burn a wide variety of coals from high-rank bituminous to Lignite.

Stoker system characteristics

All kinds of coal can be fired on stokers.

Efficiency is Low.

Stoker firing is limited to low capacities (12.6 kg/s of steam)

These capacities are the result of the practical limitations of stoker physical sizes and relatively low burning rates which require a large furnace width for a given

steam output.

Pulverized Coal Firing

To prepare coal for use in pulverized firing, it is crushed and then ground to such a fine powder that approx 70 % of it will

pass a 200-mesh (0.074 mm) sieve.

Advantages of pulverized coal firing are:

The ability to use any type of coal

A lower requirement for excess air for combustion, resulting in lower fan-power consumption.

Lower carbon loss

Higher combustion temperatures and improved thermal efficiency

Lower operating and maintenance cost

Pulverized Coal Firing

The mechanism of crushing and pulverizing has not been well understood.

Mot accepted law (Rittingers Law) states that the work needed to reduce a material of given size to smaller size is

proportional to the surface area of the reduced size.

To burn pulverized coal in a furnace, two requirements have to be met:

1. the existence of large quantities of very fine particles of coal, usually those that would pass a 200-mesh screen, to

ensure ready ignition because of their large surface to volume

ratios.

2. The existence of a minimum quantity of coarser particles to ensure high combustion efficiency.

Coal Sieve Analysis

Coal Crushers

If the coal is too large, it must go through crushers for being broken into required size (about 3cm), which are part of the

coal-handling system.

To prepare coal for pulverization, the following crushers are preferred:

Ring Crusher

Hammer-mill

BradFord Breaker

Roll crusher

Ring type coal crusher

Hammer-mill coal crusher

Bradford Breaker

Pulverizers

Pulverizing process is composed of the following stages:

Feeding system

Which automatically controls the fuel-feed rate according to

the boiler demand and the air rates required for drying and

transporting fuel to the burner.

Drying

Removal of moisture from coal

Pulverizer or Grinding Mill

Grinding is accomplished by impaction, attrition, crushing, or

combination of these.

Classifier

Separates oversized coal and returns it to the grinders to

maintain the proper fineness for the particular application.

The Pulverized coal system

It comprises of

Pulverizing

Delivery and

Burning equipment.

Classification:

The bin or storage system

The direct-firing system.

Pulverized coal bin system

The bin system is essentially a batch system

The coal is pneumatically conveyed through pipelines to utilization bins near the furnace.

Used mainly in older coal fired plants.

Pulverized coal direct firing system It has greater simplicity, greater safety, lower space requirement, and lower

operating cost.

It continuously processes coal from the storage receiving bunker through a feeder, pulverizer, and primary air fan to the furnace burners.

Large steam generators are provided with more than one pulverizer system, each feeding a number of burners, so that a wide control range is possible by varying the

load on each.

Pulverized coal burner

Similar to an oil burner

It receives dried pulverized coal in suspension in the primary air and mixes with the main combustion air from the steam generator air preheater.

Initial ignition of the burner is accomplished by light-fuel oil jet, spark-ignition.

Excess air requirement

The total air-fuel ratio is greater than stoichiometric but just enough to ensure complete combustion without wasting energy

by adding too much sensible heat to the air.

Cyclone Furnace(1940s) It is widely used to burn poorer grades of coal that contain a high ash content (6-25

%), and a high volatile matter (>15%) to obtain the necessary high rates of

combustion.

Tangential injection of primary and secondary air to impart a centrifugal motion to the coal.

Tertiary air admitted at the center

The whirling motion of air and coal results in large heat rate volumetric densities (4.7-8.3 MW/m3) and high combustion temperatures ( > 1650 deg C)

Cyclone Furnace

Advantages:

The removal of much of the ash, about 60 %, as molten slag is collected on cyclone walls by centrifugal action and drained off the bottom.

Only 40 % ash leaves with the flue gases, compared with 80 % for pulverized coal firing.

Only crushed coal is used and no pulverization equipment is needed and hence boiler size is reduced.

Limitations:

Formation of relatively more Nox in the combustion process.

High forced draft fan pressure and therefore higher power requirements.

Fluidized-bed combustion

It has been under development since 1950s.

In a fluidized bed the turbulent state increases heat and mass transfer and reduces time of reaction, plant size and power

requirement.

Fluidized bed combustion results in high combustion efficiency and low combustion temperatures.

It occurs at lower temperatures, resulting in lower production of Nox as well as the avoidance of slagging problems.

It differs from the cyclone furnace in that sulfur is removed during the combustion process.

Advantages of FB combustion

Major advantage of CFBC is the concurrent removal of SO2.

Desulfurization is accomplished by the addition of limestone directly to the bed together with the crushed coal.

Limestone absorbs the SO2 with the help of some O2 from the excess air

The rate of this reaction is max. at bed temp between 815 to 870 deg C though a practical range of operation of fluidized beds of 750 deg C to 950 deg C is

common.

Other advantages are:

Complete and efficient combustion

Low emissions

Favorable Ash property

Low operating costs and Maintenance

242232

1COCaSOOSOCaCO

Fluidization A fluidized bed is a bed of solid particles which are set into motion by blowing a

gas stream upward through the bed at a sufficient velocity to suspend the particles.

The bed appears like a boiling liquid.

The fluidization occurs when the drag force on the particles in the bed due to the upward flowing gas just equals the weight of the bed.

Fluidization The total pressure drop in a fluidized bed is composed of

P = Pw + Ps + Pf

Pw pressure drop due to friction at the wall

Ps pressure drop due to static weight of solids in bed

Pf pressure drop due to static weight of fluid in bed.

Fluidized beds usually have large wall diameter, so Pw is relatively small.

The average gas density of hot gaseous products is much smaller than that of the solids and Pf therefore is also relatively small

P = Ps = H(1-a)rs g Where a = average porosity or void fraction of bed

Since the voids may be regarded as empty spaces, ms=mb, a = 1-(rb/rs) Where rb is the bulk density of the bed.

Where a0 is porosity in collapsed state = 0.4 for randomly packed beds.

H

H0

01

1

a

a

) m

m

( -1 bed of vol.

solids of vol. bed of vol.

bs

s br

r

Fluidization

The minimum fluid velocity necessary for fluidization may be calculated by equating the drag force on a particle due to the motion of the fluid to the weight of

the particle.

CD Drag coefficient, a function of shape and Re

A Cross sectional area of the particle

rf Density of the fluid

rs Density of the solid particle

V volume of the particle

Vs velocity of the fluid

g gravitational acceleration

For a spherical particle

where rp is the radius of the particle grC

v pf

s

Ds

r

r

3

8

gV 2

vACs

2fD s

rr

Bed pressure drop

Regimes of Fluidization

1. Packed bed (Stoker)

2. Bubbling Fluidized Bed

3. Turbulent Bed

4. Fast Fluidized bed (Circulating Fluidized)

5. Pneumatic Transport (Pulverized)

Regimes of Fluidization

Regimes of Fluidization

Regimes of Fluidization

Packed bed

A packed bed consists of a bed of stationary particles on a perforated grid through which a gas is flowing.

The pressure drop per unit height of a packed bed of uniformly sized particles is given by Erguns equation

Where is the viscosity and rs is the density of the gas, dp is the diameter of the particles and f is the sphericity of particles

p

f

p d

v

d

v

H

p

f

r

a

a

f

a

a2

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particle theas volumesame theof sphere a of area surfacef

Bubbling Fluidized Bed

When the superficial velocity of gas flow through a fixed bed reaches the minimum fluidization velocity, vs, the fixed bed transforms into an incipiently fluidized bed

and the bed starts behaving as a liquid.

The pressure drop across the bed is equal to the weight of the bed, the fluid drag is given as

FD = P A = A H(1-a) (rs rf )g

Also P /H = (1-a) (rs rf )g

The minimum superficial velocity, vmf, may be given in terms of Reynolds number:

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][Re

21

2

3

15.0

22

1

CandC

gdnumberArchimedesAr

CArCCvd

f

pfsf

mfpg

mf

rrr

r

Turbulent Bed

As the velocity of gas through a bubbling fluidized bed is increased, the bed expands, and a point is eventually reached when the bubbles constantly collapse

and reform resulting in a violently active bed.

The bed surface is highly diffused and particles are thrown into the free board above.

The pressure drop fluctuates rapidly.

The amplitude of pressure fluctuation reaches a peak and reduces to a steady state value.

Fast Fluidized Bed

Also referred to as the Circulating Fluidized Bed.

Defined by Basu and Fraser (1991) as follows:

High slip velocity (Ug-Us) between gas and solid, formation and disintegration of particle agglomerates, and a very good gas-solid mixing are the characteristic features of this regime.

The main difference between bubbling beds and CFB lies in the gas velocity used. While bubbling beds normally operate at gas velocities of around 1-3 m/s, CFB typically runs at 5-10 m/s

Fast Fluidized Bed (CFB)

Combustion of fuel particles in a fluidized bed

Fluidized bed combustion of solid fuels

A type of furnace or reactor in which fuel particles are combusted while suspended in a stream of hot gas.

Coal size used 6 to 20 mm.

Types of FBC:

Bubbling fluidized bed combustion

1) Atmospheric

2) Pressurized

Circulating fluidized bed combustion

1) Atmospheric

2) Pressurized

Bubbling fluidized bed (BFB)

A bubbling fluidized bed boiler comprises a fluidizing grate through which primary combustion air passes and a containing vessel, which is either made of (lined with) refractory or heat-absorbing tubes.

The vessel would generally hold bed materials. The open space above this bed, known as freeboard, is enclosed by heat-absorbing tubes.

The secondary combustion air is injected into this section

The boiler can be divided into three sections:

1. Bed

2. Freeboard

3. Back-pass or convective section.

Atmospheric Fluidized bed combustion

Operating Pressure : 1 atm

Temperature : 850 deg C

Fluidizing velocity : 2-4 m/s

Avg. bed material size : 1000 micron

Fuels : Multi fuel

Combustion efficiency : 90 99 %

Pollutant emission control : very good

Application : industrial boilers, power generation.

Atmospheric Fluidized bed combustion

Pressurized Fluidized bed combustion

Operating Pressure : up to 16 atm

Temperature : 850 deg C

Fluidizing velocity : 1-1.5 m/s

Avg. bed material size : 1000 micron

Fuels : Multi fuel

Combustion efficiency : 99 %

Pollutant emission control : Excellent

Application : Combine cycle power generation

Pressurized Fluidized bed combustion

Advantages of PFBC over AFBC

Increase in specific power output and hence potential reduction in capital cost

Increased power generation efficiency

Emissions of oxides of Nitrogen are substantially reduced.

Circulating Fluidized Bed combustion (CFBC)

In a CFB boiler furnace the gas velocity is sufficiently high to blow all the solids out of the furnace.

The majority of the solids leaving the furnace is captured by a gassolid separator, and is recirculated back to the base of the furnace.

A CFB boiler is shown schematically in Figure The primary combustion air (usually substoichiometric in amount) is injected

through the floor or grate of the furnace

The secondary air is injected from the sides at a certain height above the furnace floor.

Fuel is fed into the lower section of the furnace, where it burns to generate heat. A fraction of the combustion heat is absorbed by water- or steam-cooled surfaces

located in the furnace, and the rest is absorbed in the convective section located further downstream, known as the back-pass.

CFBC

Circulating FBC

Operating Pressure : up to 16 atm (if pressurized)

Temperature : 850 deg C

Fluidizing velocity : 4-8 m/s

Avg. bed material size : 200-350 micron

Fuels : Multi fuel

Combustion efficiency : 99 %

Pollutant emission control : Excellent

Application : industrial boilers, combined cycle power generation power generation

Coal Gasification

The gasification of coal for use as a powerplant fuel is being considered as the supply of natural gas diminishes.

Coal gasification existed since 1800s !

Intially gas was manufactured from coal and distributed as town gas for smelting of iron and for burning etc.

The coke was placed in large beds and burned for a period with less than the stoichiometric quantity of air to give

producer gas

Coal gasification process

Low BTU gas:

When the bed is heated to a high temperature, the flow of air is replaced by a flow of steam and water gas is produced.

The resultant mixture also called as synthesis gas or syn gas is a lower quality gas with a heating value of about 10 MJ/m3.

gasproducerAircoke

NCONOC 222 76.32)76.3(2

gaswatersteamcoke

HCOOHC 22

Coal gasification

Coal gasification process

Medium BTU Gas:

If the product desired is a medium or high BTU gas, a synthesis gas shift reaction or conversion is used to produce additional hydrogen by reacting some of the CO

with steam and removing CO2 from the products

High BTU gas:

The final step in producing a pipeline quality gas is called catalytic methanation. The products of the above reaction are reacted over a nickel catalyst at a

temperature of 1100 deg C and pressure of 6.8 bar

The product gas is of a higher quality with heating value of 37.7 MJ/m3 and is a direct substitute for natural gas.

222 HCOOHCO

OHCHHCO 2423

Coal gasifier combined cycle power plant