30079_44.pdf

CHAPTER 44

COMBUSTION

Richard J. Reed

North American Manufacturing Company

Cleveland, Ohio

44.3 BURNERS 14 39

44.3.1 Burners for Gaseous Fuels 1439

44.3.2 Burners for Liquid Fuels 1441

44.1 FUNDAMENTALS OF

COMBUSTION 14 31

44.1.1 Air-Fuel Ratios 14 31

44.1.2 Fuels

14 33

44.4 SAFETY CONSIDERATIONS 1442

44.2 PURPOSES OF COMBUSTION 1435

44.5 OXY-FUEL FIRING

14 47

44.1 FUNDAMENTALS OF COMBUSTION

44.1.1 Air-Fuel Ratios

Combustion is rapid oxidation, usually for the purpose of changing chemical energy into thermal

energy—heat. This energy usually comes from oxidation of carbon, hydrogen, sulfur, or compounds

containing C, H, and/or S. The oxidant is usually O2—molecular oxygen from the air.

The stoichiometry of basic chemical equation balancing permits determination of the air required

to burn a fuel. For example,

1CH4 + 202 — 1CO2 + 2H2O

where the units are moles or volumes; therefore, 1 ft3 of methane (CH4) produces 1 ft3 of CO2; or

1000 m3 CH4 requires 2000 m3 O2 and produces 2000 m3 H2O. Knowing that the atomic weight of

C is 12, H is 1, N is 14, O is 16, and S is 32, it is possible to use the balanced chemical equation

to predict weight flow rates: 16 Ib/hr CH4 requires 64 Ib/hr O2 to burn to 44 Ib/hr CO2 and 36 lb/

hr H2O.

If the oxygen for combustion comes from air, it is necessary to know that air is 20.99% O2 by

volume and 23.20% O2 by weight, most of the remainder being nitrogen.

It is convenient to remember the following ratios:

air/02 - 100/20.99 = 4.76 by volume

N2/O2 = 3.76 by volume

air/02 - 100/23.20 - 4.31 by weight

N2/O2 - 3.31 by weight

Rewriting the previous formula for combustion of methane,

1CH4 + 2O2 4- 2(3.76)N2 — 1CO2 + 2H2O + 2(3.76)N2

1CH4 + 2(4.76)air —> 1CO2 + 2H2O + 2(3.76)N2

Table 44.1 lists the amounts of air required for stoichiometric (quantitatively and chemically

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Table 44.1 Proper Combining Proportions for Perfect Combustion3

vol air

vol fuel

11.9

35.7

31.0

wtO2

wt fuel

3.08

3.59

2.67

0.571

3.73

8.00

1.41

4.00

3.00

3.51

3.64

3.43

1.00

wt air

wt fuel

13.3

15.5

11.5

2.46

16.1

34.5

6.08

17.2

12.9

15.1

15.7

14.8

4.31

ft3O2

Ib fuel

36.5

42.5

31.6

6.76

44.2

94.7

16.7

47.4

35.5

41.6

43.1

40.6

11.8

ft3 air

Ib fuel

174

203

150

32.2

210

451

79.5

226

169

198

205

193

56.4

vol O2

vol fuel

2.50

7.50

6.50

m2O2

kg fuel

2.28

2.65

1.97

0.422

2.76

5.92

1.04

2.96

2.22

2.60

2.69

2.54

0.74

m3 air

kg fuel

10.8

12.6

9.39

2.01

13.1

28.2

4.97

14.1

10.6

12.4

12.8

12.1

3.52

Fuel

Acetylene, C2H2

Benzene, C6H6

Butane, C4H10

Carbon, C

Carbon monoxide, CO

Ethane, C2H6

Hydrogen, H2

Hydrogen sulfide, H2S

Methane, CH4

Naphthalene, C10H8

Octane, C8H18

Propane, C3H8

Propylene, C3H6

Sulfur, S

a Reproduced with permission from Combustion Handbook.1 (See Ref. 1)

2.38

16.7

2.38

7.15

9.53

0.50

3.50

0.50

1.50

2.00

5.00

4.50

23.8

21.4

correct) combustion of a number of pure fuels, calculated by the above method. (Table 46. Ic lists

similar information for typical fuels that are mixtures of compounds, calculated by the above method,

but weighted for the percentages of the various compounds in the fuels.)

The stoichiometrically correct (perfect, ideal) air/fuel ratio from the above formula is therefore

2 + 2(3.76) = 9.52 volumes of air per volume of the fuel gas. More than that is called a "lean"

ratio, and includes excess air and produces an oxidizing atmosphere. For example, if the actual air/

fuel ratio were 10:1, the %excess air would be

1°^ X 100 - 5.04%

Communications problems sometimes occur because some people think in terms of air/fuel ratios,

others in fuel/air ratios; some in weight ratios, others in volume ratios; and some in mixed metric

units (such as normal cubic meters of air per metric tonne of coal), others in mixed American units

(such as ft3 air/gal of oil). To avoid such confusions, the following method from Ref. 1 is

recommended.

It is more convenient to specify air/fuel ratio in unitless terms such as %air (%aeration), %excess

air, %deficiency of air, or equivalence ratio. Those experienced in this field prefer to converse in

terms of %excess air. The scientific community favors equivalence ratio. The %air is easiest to use

and explain to newcomers to the field: "100% air" is the correct (stoichiometric) amount; 200% air

is twice as much as necessary, or 100% excess air. Equivalence ratio, widely used in combustion

research, is the actual amount of fuel expressed as a fraction percent of the stoichiometrically correct

amount of fuel. The Greek letter phi, 4>, is usually used: 4> = 0.9 is lean; <|> = 1.1 is rich; and 4> —

1.0 is "on-ratio."

Formulas relating %air, 4>, %excess air (%XS), and %deficiency of air (%def) are

%air - 100/cf> = %XS + 100 = 100 - %def

100 1

* ~ %XS + 100 ~ 1 - (%def/100)

%XS - %air - 100 - —— X 100

%def = 100 - %air - ^— x 100

Table 44.2 lists a number of equivalent terms for convenience in converting values from one

"language" to another.

Excess air is undesirable, because, like N2, it passes through the combustion process without

chemical reaction; yet it absorbs heat, which it carries out the flue. The percent available heat (best

possible fuel efficiency) is highest with zero excess air. (See Fig. 44.1.)

Excess fuel is even more undesirable because it means there is a deficiency of air and some of

the fuel cannot be burned. This results in formation of soot and smoke. The accumulation of unburned

fuel or partially burned fuel can represent an explosion hazard.

Enriching the oxygen content of the combustion "air" above the normal 20.9% reduces the

nitrogen and thereby reduces the loss due to heat carried up the stack. This also raises the flame

temperature, improving heat transfer, especially that by radiation.

Vitiated air (containing less than the normal 20.9% oxygen) results in less fuel efficiency, and

may result in flame instability. Vitiated air is sometimes encountered in incineration of fume streams

or in staged combustion, or with flue gas recirculation.

44.1.2 Fuels

Fuels used in practical industrial combustion processes have such a major effect on the combustion

that they must be studied simultaneously with combustion. Fuels are covered in detail in later chap-

ters, so the treatment here is brief, relating only to the aspects having direct bearing on the combustion

process.

Gaseous fuels are generally easier to burn, handle, and control than are liquid or solid fuels.

Molecular mixing of a gaseous fuel with oxygen need not wait for vaporization nor mass transport

within a solid. Burning rates are limited only by mixing rates and the kinetics of the combustion

reactions; therefore, combustion can be compact and intense. Reaction times as short as 0.001 sec

and combustion volumes from 104 to 107 Btu/hr • ft3 are possible at atmospheric pressure.2 Gases

of low calorific value may require such large volumes of air that their combustion rates will be

limited by the mixing time.

Combustion stability means that a flame lights easily and then burns steadily and reliably after

the pilot (or direct spark) is programmed off. Combustion stability depends on burner geometry, plus

Table 44.2 Equivalent Ways to Express Fuel-to-Air or Air-to-Fuel Ratios1

%air

100

105

110

120

130

140

160

180

200

250

300

400

500

600

1100

2100

%def

%xs

2.50

1.67

1.25

1.11

1.05

1.00

0.95

0.91

0.83

0.78

0.71

0.62

0.56

0.50

0.40

0.33

0.25

0.20

0.167

0.091

0.048

Fuel rich

(air lean)

Stoiehiometric

Fuel lean

(air rich)

100

150

200

300

400

500

1000

2000

Fig. 44.1 Percent available heat (best possible efficiency) peaks at Stoiehiometric air/fuel ratio.1

air and fuel flow controls that maintain the point(s) of flame initiation (a) above the fuel's minimum

ignition temperature, (b) within the fuel's flammability limits, and (c) with feed speed equal to flame

speed—throughout the burner's full range of firing rates and conditions. (Fuel properties are discussed

and tabulated in Chapters 46 and 47.)

Liquid fuels are usually not as easily burned, handled, or controlled as are gaseous fuels. Mixing

with oxygen can occur only after the liquid fuel is evaporated; therefore, burning rates are limited

by vaporization rates. In practice, combustion intensities are usually less with liquid fuels than with

high calorific gaseous fuels such as natural gas.

Because vaporization is such an integral part of most liquid fuel burning processes, much of the

emphasis in evaluating liquid fuel properties is on factors that relate to vaporization, including vis-

cosity, which hinders good atomization, the primary method for enhancing vaporization. Much con-

cern is also devoted to properties that affect storage and handling because, unlike gaseous fuels that

usually come through a public utility's mains, liquid fuels must be stored and distributed by the user.

The stability properties (ignition temperature, flammability limits, and flame velocity) are not

readily available for liquid fuels, but flame stability is often less critical with liquid fuels.

Solid fuels are frequently more difficult to burn, handle, and control than liquid or gaseous fuels.

After initial volatilization, the combustion reaction rate depends on diffusion of oxygen into the

remaining char particle, and the diffusion of carbon monoxide back to its surface, where it burns as

a gas. Reaction rates are usually low and required combustion volumes high, even with pulverized

solid fuels burned in suspension. Some fluidized bed and cyclone combustors have been reported to

reach the intensities of gas and oil flames.2

Most commonly measured solid fuel properties apply to handling in stokers or pulverizers. See

Chapter 48.

Wastes, by-product fuels, and gasified solids are being used more as fuel costs rise. Operations

that produce such materials should attempt to consume them as energy sources. Handling problems,

the lack of a steady supply, and pollution problems often complicate such fuel usage.

For the precise temperature control and uniformity required in many industrial heating processes,

the burning of solids, especially the variable quality solids found in wastes, presents a critical problem.

Such fuels are better left to very large combustion chambers, particularly boilers. When solids and

wastes must be used as heat sources in small and accurate heating processes, a better approach is to

convert them to low-Btu (producer) gas, which can be cleaned and then controlled more precisely.

44.2 PURPOSES OF COMBUSTION

The purposes of combustion, for the most part, center around elevating the temperature of something.

This includes the first step in all successive combustion processes—the pilot flame—and, similarly,

the initiation of incineration. Elevating the temperature of something can also make it capable of

transmitting light or thermal energy (radiation and convection heat transfer), or it can cause chemical

dissociation of molecules in the products of combustion to generate a special atmosphere gas for

protection of materials in industrial heat processing.

All of the above functions of combustion are minor in comparison to the heating of air, water

and steam, metals, nonmetallic minerals, and organics for industrial processing, and for space comfort

conditioning. For all of these, it is necessary to have a workable method for evaluating the heat

available from a combustion process.

Available heat is the heat accessible for the load (useful output) and to balance all losses other

than stack losses. (See Fig. 44.2.) The available heat per unit of fuel is

AH = HHV - total stack gas loss = LHV - dry stack gas loss

% available heat - 100(AH/HHV)

where AH = available heat, HHV = higher heating value, and LHV = lower heating value, as

defined in Chapter 47. Figure 44.3 shows values of % available heat for a typical natural gas; Fig.

44.4 for a typical residual oil; and Fig. 53.2 in Chapter 53, for a typical distillate oil.

Example 44.1

A process furnace is to raise the heat content of 10,000 Ib/hr of a load from 0 to 470 Btu/lb in a

continuous furnace (no wall storage) with a flue gas exit temperature of 1400°F. The sum of wall

loss and opening loss is 70,000 Btu/hr. There is no conveyor loss. Estimate the fuel consumption

using 1000 Btu/ft3 natural gas with 10% excess air.

Solution: From Fig. 44.3, % available heat = 58.5%. In other words, the flue losses are

100% - 58.5% = 41.5%. The sum of other losses and useful output = 70,000 + (10,000)(470) =

4,770,000 Btu/hr. This constitutes the "available heat" required. The required gross input is therefore

4,770,000/0.585 - 8,154,000 Btu/hr, of 8154 ft3/hr of natural gas (and about 81.540 ft3/hr of air).

The use of the above precalculated % available heats has proved to be a practical way to avoid

long iterative methods for evaluating stack losses and what is therefore left for useful heat output

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