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ISAT 413 - Module IV:Combustion and Power Generation

Topic 1: Combustion Theory and Fundamentals

Introduction Conversion of Mechanical Energy Conversion of Electrical Energy Conversion of Electromagnetic Energy Conversion of Chemical Energy Combustion Mechanics

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Introduction

Thermal energy is a basic form of energy in that all other energy forms can be completely converted into thermal energy.

Unless the energy stored as some other form, it will eventually be degraded into thermal energy (the term “degraded” is used because the conversion of thermal energy into other energy forms is limited to something less than 100%.)

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The conversion of mechanical energy into thermal energy occurs in any irreversible process and can be summarized in essentially one word ─ friction.

In many processes, friction is considered to be an undesirable phenomenon and every effort is made to reduce or eliminate it. This is particularly true in most thermodynamic or lubrication processes.

Friction is not always bad, however. If it were not for the friction between the soles of your shoes and the floor, it would be impossible to walk. If it were not for the friction between the brake shoes and the drum of the automobile wheel, it would not be possible to convert the kinetic energy of the automobile into thermal energy and stop the car.

Conversion of Mechanical Energy

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Electrical energy can be converted into thermal energy in several processes but the principal energy-conversion process is the joule-heating process. This is the iv or i2R loss encountered when an electrical current of i amperes is passed through a resistance of R ohms as the result of a potential difference of v volts. The resulting power loss or thermal energy production rate is in watts.

In most electrical circuits, the joule-heating loss is an undesirable power loss. However, in some systems, such as electric ranges and furnaces, this process is useful in the production of thermal energy.

Conversion of Electrical Energy

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Another electrical power loss associated with alternating-current systems is the power-factor loss. When electrical energy is passed through a capacitor or an induction coil, part of this energy is stored in the electrostatic and magnetic field associated with each impedance. When the capacitive and inductive impedances are matched (in-phase), there is no power loss. Otherwise, the excess electrical energy generated from the collapse of electrostatic and magnetic fields (when the current reverses) will be converted into thermal energy, which means that the electrical utility must supply more energy to the customers than is actually metered in order to overcome the power-factor, cos(), loss.

cosVIP rmsrmsavg

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The conversion of electromagnetic energy into thermal energy is accomplished in some sort of absorption process. For high-energy electromagnetic radiation such as gamma radiation and x-rays, the absorption process is a volumetric phenomenon. For low-energy radiation, the absorption process is usually a surface phenomenon. Many materials are transparent to some wavelengths of radiation and opaque to other wavelengths. Glass is transparent to the ultraviolet and visible portions of the thermal radiation spectrum but is opaque to the infrared radiation emitted by most surfaces. This leads to the so-called greenhouse effect and is useful in trapping solar energy.

Conversion of Electromagnetic Energy

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The primary source of fuel energy is the production of thermal energy from chemical energy. The most important exothermic chemical reaction in the production of this energy is the combustion of fossil fuels. This reaction is an oxidation reaction in which the three combustible elements found in most of the fossil fuels, i.e., carbon, hydrogen, and sulfur, are respectively converted into carbon dioxide (CO2), water (H2O), and sulfur dioxide (SO2).

The following section on the conversion of chemical energy deals primarily with the mechanics of the combustion reaction as well as some of the principal systems and components needed to accomplish the process.

Conversion of Chemical Energy

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If there is sufficient oxygen, it will be assumed that all the hydrogen burns (582oC ignition temperature) completely to water before any of the carbon or sulfur burns.

Combustion Reactions

]kJ/kg.mol286,470[ 2222 22 HQQOHOH HH

The mass of oxygen requires to completely burn a unit mass of hydrogen is

2

2

2

2

2

2 94701622

32

2

1

H

O.

H.

O

H

O

kg

kg

g

g

mol

mol

The higher heating value of hydrogen (equivalent to QH) is 142,097 kJ/kg and the lower heating value is 120,067 kJ/kg.

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The product of the oxidation of sulfur (243oC ignition temperature) is sulfur dioxide (SO2), is considered to be a major atmospheric pollutant:

]kJ/kg.mol296,774[ SQQSOOS SS 22

The mass of oxygen requires to completely burn a unit mass of sulfur is

S

O.

S.

O

S

O

kg

kg

g

g

mol

mol 222 99800632

32

1

1

The higher and lower heating values of sulfur are the same and equal to 9257 kJ/kg. Because the low ignition temperature, it will be assumed for combustion with incomplete air that all the sulfur burns after the hydrogen burns and before any carbon burns.

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Carbon is one of the most important combustible elements and is an essential part of any hydrocarbon compound. Despite its high ignition temperature (407oC), the oxidation of carbon is slower and more difficult than that of either hydrogen or sulfur.

]kJ/kg.mol393,560[ CQQCOOC CC 22

The mass of oxygen requires to completely burn a unit mass of sulfur is

C

O.

C.

O

C

O

kg

kg

g

g

mol

mol 222 6620112

32

1

1

The higher and lower heating values of carbon are the same and equal to 32,778 kJ/kg.

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Almost all combustion processes rely on air as the source of oxygen. Air is composed of approximately 21% oxygen and 79% nitrogen by volume or mole. These values translate to 23.2% oxygen and 76.8% nitrogen on a gravimetric or mass basis. The molecular weight of air is 28.97 kg/kg.mol (or 28.97 lbm/lbm.mol).

The theoretical or stoichiometric air-fuel ratio gives the minimum air requirements for complete combustion of a fuel.

Theoretical Air-Fuel Ratio

23202

.

O

F

A

D.G.T

fuel kg per air the from needed kg

The factor of 0.232 in the denominator is the mass fraction of oxygen in the air.

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Since most coal analyses are listed as dry, ash-free values, it is often easier and faster to use the following equation for coals:

2320

19980947662 22

.

AMOS.H.C.

F

A

D.G.T

Where the moisture (M) and ash (A) are as-burned values and all other values are dry, ash-free values.

The theoretical, molar, dry air-fuel ratio can be obtained as

Where Zi is the number of moles of the i th element per mole of fuel, and

21024

.

ZZ

ZZ

F

AO

SH

C

D.M.T

fuel of weightmolecularD.M.T

D.G.T

F

A.

F

A

9728

13

gas air/lbm lbm

gas air/mol of mol

gas of H/mol mol

gas of C/mol mol

60% and 40% Given

50155152

10289728

1028210

20042963

00

291060840

6346034010483

..

..

F

A

..

//..

F

A

;.ZZ

...Z

...Z

HCHC

:Solution

D,G,T

D.M.T

SO

H

C

Example 3.2:

Calculate the theoretical, gravimetric, dry air-fuel ratio when burning a LPG composed of 40% propane and 60% butane, in lbm air/lbm.

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The five requisites for good combustion are MATT stands for proper mixing (M) of the reactants; sufficient air (A); a temperature (T) above the ignition temperature; sufficient time (T) for the reaction to occur; and a reactant density () sufficient to propagate the flame.

Since perfect mixing is never attained in the actual combustion process, good combustion can only assured by supplying excess air for the process. Care must be exercised to keep the amount of excess air to a minimum because too much excess air increases the losses in the combustion process and increases NOx emissions.

The exhaust or flue gas includes the products of complete (CO2, H2O, and SO2) and incomplete combustion (CO, hydroxyls, aldehydes, ash particles, …)

Actual Combustion Process

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There are two ways of expressing the amount of air supplied for a given combustion process ─ the dilution coefficient and the percent excess air.

The dilution coefficient is defined as the ratio of the actual to the theoretical air-fuel ratios, or

The percent excess air is defined by the following equations:

Percent Excess Air

theo

act

A/F

A/FDC tcoefficien Dilution

1100

100

DC

A/F

F/AA/F

theo

theoact

air excess Percent

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A typical orsat gas analyzer is shown in the Figure at right is used to determine the volumetric or molar fractions of carbon dioxide, oxygen, and carbon monoxide in the dry exhaust gas.

A 100-cm3 sample of exhaust gas is taken at room temperature in the burette by using the leveling water bottle to collect and transfer the gas sample.

The Orsat Flue-Gas Analyzer

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Since the gas is collected at room temperature over water, it is usually assumed that any water vapor in the exhaust gas will have condensed and that any sulfur dioxide in the exhaust gas will have reacted with the water in the exhaust gas and in the collecting bottle. Consequently, it is assumed that the resulting “dry” gas sample is composed of carbon dioxide, oxygen, carbon monoxide, and nitrogen. The process can be divided into 4 stages (with 3 chemical reactions):

1. An aqueous solution of potassium hydroxide (KOH) CO2

2. A solution of pyrogalic acid in KOH O

3. A solution of cuprous chloride in ammonia CO

4. The remaining Nitrogen

An orsat analysis is required and usually is sufficient to determine the actual air-fuel ratio when burning a gaseous fuel or liquid fuel.

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The refuse analysis is used to evaluate the actual air-fuel ratio when burning a solid fuel such as coal, it is simply an experimental determination of higher heating value, HHV, of the refuse.

The Refuse Analysis

32,778

sekJ/kg.refu HHV,

HHV

HHVecombustibl Percent

carbon

refuse 100100

The refuse mass fraction, R, can be obtained from

rA/R

AR

coal, the in ash of fraction mass

coal of mass

refuse of mass

The mass of unburned carbon in the refuse per mass of fuel consumed, Cr, can be found from either of the following equations:

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ARCR

CRRC r

rr or

100

ecombustibl percent

Thus, the mass of carbon actually burned per unit mass of fuel, Cb can be obtained from the following equation:

rultb CCC

Where Cult is the carbon mass fraction from the as-burned ultimate fuel analysis.

Once the refuse analysis, the orsat analysis of the flue gas, and the ultimate analysis of the coal are known, the actual air-fuel ratio can be evaluated.

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Actual Gravimetric Dry Air-Fuel Ratio

7680

1

0112

01628

7680 2

2

.N

.CO%CO%

C.N%

.

N

F

AF

bA

D,G,A

ratio fuel-airdry c,gravimetri Actual,

Where the number of moles of carbon monoxide and carbon dioxide in the exhaust gas per unit mass of coal is Cb/12.01, NF is the gas-burned nitrogen mass fraction of the fuel (from the ultimate analysis), and NA is the mass of nitrogen from the air per unit mass of coal. And

790

1

22

2

.

Z

CO%CO%

ZN%

F

A NC

D,M,A

ratio fuel-airdry molar, Actual,

Where ZC an ZN are the moles of carbon and nitrogen per mole of fuel, respectively.

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Wet Air-Fuel Ratios

In all of the equations for the air-fuel ratios, the ratio is given for dry air only. In reality, atmospheric air contains water vapor, the “wet” air-fuel ratios can be obtained by multiplying the “dry” gravimetric ratios by 1 + , or multiplying the “dry” molar values by (1 + /0.622) where is the humidity ratio or mass of water vapor per unit mass of dry air, and 0.622 = Ra / Rw. Thus, the ratios become:

D,M,AW,M,AD,G,AW,G,A

D,M,TW,M,TD,G,TW,G,T

F

A

.F

A

F

A

F

A

F

A

.F

A

F

A

F

A

622011

622011

;

;

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Excess Air Theoretically, oxygen and carbon monoxide cannot appear simultaneously in the exhaust gas but they commonly appear in the exhaust gas because of poor mixing. A common rule of thumb states that the approximate percent excess air is 5 times the oxygen percentage in the orsat analysis, if the %CO is small and the %O2 is less than 5. For O2 concentration greater than 5%, the rule of thumb significantly underestimates the percent excess air.

Burning a fuel with insufficient air gives the maximum power from a limited volume as in the internal combustion engine and also produces higher specific power from rockets. Reducing the combustion air reduces NOx, although it does increase the pollution associated with the emission of CO and unburned fuel.

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%....

CO%O%CO%

...

AM

SN

OHC

:Solution

35811509335714100

100

7801001201

1

22

2

22

gas exhaust in N %

analysis burned-as a for Multiplier

kJ/kg. 33,422 HHV and , 5.6% , 1.3%

, 8.4% , 5.5% , 79.2% :E Appendix from analysis coal Ultimate

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Example 3.3:

A certain power plant burns Clay County, Missouri, coal with M = 12% and A = 10%, and an analysis of the refuse pit shows that the higher heating value of the refuse is 4581 kJ/kg. An orsat analysis of the flue gas gives 14.57% CO2, 3.93% O2, and 0.15% CO. Find the dilution coefficient and the percent excess air.

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coal air/kg kg

:ratio fuel-air ltheoretica The

coal C/kg burned kg

coal C/kg unburned kg

coal refuse/kg kg

refuse ash/kg kg

refuse the in ecombustibl

refuse refuse/kg in C kg

analysis, refuse the From

4578

2320

08400560998005509477920662780

60150016307920780

0163010011630

1163086020100

86020139800101

9813

1398077832

4581

.

.

........

F

A

....CCC

...ARC

../.R/A

AR

...R

C.

R

A

%.

.,R

C

D,G,T

rb

r

r

r

r

r

25

19.7%

:air excess percent for thumb of rule the Using

air excess Percent

DC tcoefficien Dilution

coal air/kg kg

:ratio fuel-air Actual

%.

%.DC

..

.

F/A

F/A

..

..../...

.

NCO%CO%/CN%.

F

A

D,G,T

D,G,A

Fb

D,G,A

9335

2191100

19214578

08110

081107680

013078015057146015035813322

7680

3322 22

26

09020450

060105020050

6340304870

985020301870105010050

..Z

...Z

...Z

.....Z

:Solution

N

O

H

C

analysis, gas the From

Example 3.5:

A natural gas with the following molar analysis is burned in a furnace: CO2 = 0.5%, CO = 5.0%, CH4 = 87.0%, C2H4 = 3.0%, and N2 = 4.5%. An orsat analysis gives the following results: 9.39% CO2, 3.88% O2, and 8.3% CO. Calculate the percent excess air and the actual air-fuel ratio in kilogram of air per kilogram of fuel gas.

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%.

...

CO%O%CO%N%

.

....

......

..

.

/./..

.

/ZZ/ZZ

F

A OSHC

D,M,T

985

830883399100

100

67817

0162804500528030

0416870012805001440050

83388338

210

206004639850

210

24

222

:analysis orsat From

gas of kg/kg.mol

gas of (MW) weightMolecular

gas air/ft ft gas air/mol mol 33

28

.

.

..

MW

F/A.

F

A

%..DC

..

.

F/A

F/A

..

/.../..

.

/ZCO%CO%/N%Z

F

A

D,M,A

A,G,D

D,M,T

D,M,A

NC

D,M,A

gas air/kg kg

ratio fuel-airdry c,gravimetri Actual,

air excess Percent

DC tcoefficien Dilution

gas air/mole moles

081767817

423109728

9728

018118011001100

18018338

42310

42310790

20908303999859850

790

222

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The actual combustion process for fossil fuels proceeds in one of two ways:

Blue-Flame Combustion ─ If gaseous hydrocarbon fuel are mixed with some air and heated before the actual combustion takes place, the oxygen reacts with hydrocarbon and formed hydroxylated compounds (aldehydes). The flame resulting from the combustion of these compounds is a “blue” or “nonluminous” flame.

Yellow-Flame Combustion ─ Introduction of fuel and air at the burner with no premixing and heating of the reactants. The flame resulting from this mode of combustion is a “yellow” or “luminous” flame. This is desired in a large power boiler because it increases the radiative heat-transfer rate from the flame to the boiler tubes, reducing the combustion temperature.

Combustion Mechanics

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Three basic physical methods are utilized for the combustion of fossil fuels:

The burning-bed system ─ it is commonly employed in the combustion of solid fuels, the solid fuel is burned in either a stationary bed or a fluidized-bed.

The traveling-Flame-front combustion system ─ it is applicable where the reactant (fuel and oxidant) are completely mixed before combustion occurs. When ignition takes place, the flame front progressively moves through the mixture (for internal combustion engine, etc.)

The gaseous-torch combustion system ─ it is commonly used in large power installations and, in essence, the fuel and air are mixed and burned at the burner. Heavy fuel oil must be atomized, coal must be pulverized to a texture finer than face powder.