30079_46.pdf

CHAPTER 46

GASEOUS FUELS

Richard J. Reed

North American Manufacturing Company

Cleveland, Ohio

46.1 INTRODUCTION

1505

46.2.7 Net Heating Value 1507

46.2.8 Flame Stability 1509

46.2.9 Gas Gravity 1509

46.2.10 Wobbe Index 1512

46.2.11 Flame Temperature 1512

46.2.12 Minimum Ignition

Temperature 1512

46.2.13 Flammability Limits 1512

46.2 NATURAL GAS 1505

46.2.1 Uses and Distribution 1505

46.2.2 Environmental Impact 1505

46.2.3 Sources, Supply, and

Storage 1507

46.2.4 Types and Composition 1507

46.2.5 Properties

1507

46.2.6 Calorific Value or

Heating Value

46.3 LIQUEFIED PETROLEUM

GASES

1507

1514

46.1 INTRODUCTION

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

fuels include natural gas (primarily methane and ethane) and liquefied petroleum gases (LPG; pri-

marily propane and butane). Gaseous man-made or artificial fuels are mostly derived from liquid or

solid fossil fuels. Liquid fossil fuels have evolved from animal remains through eons of deep under-

ground reaction under temperature and pressure, while solid fuel evolved from vegetable remains.

Figure 46.1, adapted from Ref. 1, shows the ranges of hydrogen/carbon ratios for most fuels.

46.2 NATURAL GAS

46.2.1 Uses and Distribution

Although primarily used for heating, natural gas is also frequently used for power generation (via

steam turbines, gas turbines, diesel engines, and Otto cycle engines) and as feedstock for making

chemicals, fertilizers, carbon-black, and plastics. It is distributed through intra- and intercontinental

pipe lines in a high-pressure gaseous state and via special cryogenic cargo ships in a low-temperature,

high-pressure liquid phase (LNG).

Final street-main distribution for domestic space heating, cooking, water heating, and steam gen-

eration is at regulated pressures on the order of a few inches of water column to a few pounds per

square inch, gage, depending on local facilities and codes. Delivery to commercial establishments

and institutions for the same purposes, plus industrial process heating, power generation, and feed-

stock, may be at pressures as high as 100 or 200 psig (800 or 1500 kPa absolute). A mercaptan

odorant is usually added so that people will be aware of leaks.

Before the construction of cross-country natural gas pipe lines, artificial gases were distributed

through city pipe networks, but gas generators are now usually located adjacent to the point of use.

46.2.2 Environmental Impact

The environmental impact of natural gas combustion is generally less than that of liquid or solid

fuels. Pollutants from natural gas may be (a) particulates, if burners are poorly adjusted or controlled

(too rich, poor mixing, quenching), or (b) nitrogen oxides, in some cases with intense combustion,

preheated air, or oxygen enrichment.

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

Fig. 46.1 Hydrogen/carbon ratios of fossil and synthetic fuels. (Adapted from Ref. 1.)

46.2.3 Sources, Supply, and Storage

Natural gas is found with oil deposits (animal fossils) and coal deposits (plant fossils). As-yet un-

tapped supplies are known to exist (1) near the coast of the Gulf of Mexico in very deep

geopressured/geothermal aquifers and (2) in difficult-to-separate Appalachian shale formations.

Except for these hard-to-extract supplies, U.S. natural gas supplies have been variously predicted

to last 10-20 years, but such predictions are questionable because of the effects of economic and

regulatory variations on consumption, production, and exploration. Except for transoceanic LNG

vessels, distribution is by pipe line, using a small fraction of the fuel in compressors to provide

pumping power.

Storage facilities are maintained by many local gas utilities as a cushion for changing demand.

These may be low-pressure gas holders with floating bell-covers, old wells or mines (medium pres-

sure), or cryogenic vessels for high-pressure liquefied gas.

46.2.4 Types and Composition

Natural gases are classified as "sweet" or "sour," depending on their content of sulfur compounds.

Most such compounds are removed before distribution. Odorants added (so that leaks can be detected)

are usually sulfur compounds, but the amount is so minute that it has no effect on performance or

pollution.

Various geographic sources yield natural gases that may be described as "high methane," "high

Btu," or "high inert."

46.2.5 Properties

Properties that concern most users of natural gases relate to the heat available from their combustion,

flow characteristics, and burnability in a variety of burner types. Strangely, few people pay attention

to the properties of their gas until they are forced to substitute another fuel for it. Some properties

are listed in Table 46.1.2

46.2.6 Calorific Value or Heating Value

The gross or higher heating value (HHV) is usually measured in a steady-state calorimeter, which is

a small fire-tube heat exchanger with a water-cooled surface area so large that it cools the products

of combustion to the temperature at which they entered as fuel and air (usually 60°F). HHV can be

calculated from a volumetric analysis and the calorific values of the pure compounds in the gas (Table

46.2). For example, for a natural gas having the analysis shown in column 2 below, the tabulation

shows how a weighted average method can be used to determine the calorific value of the mixture:

Col. 3, HHV

Col. 1,

Col. 2, from Table 46.2

Col. 4 =

Constituent

% Volume

(Btu/ft3)

(Col. 3 x Col. 2)7100

Methane, CH4

1013

912

Ethane, C2H6

1763

106

Nitrogen, N2

Total

100%

1018 Btu/ft3

It is a convenient coincidence that most solid fossil fuels release about 96-99 gross Btu/ft3 of

standard air; liquid fossil fuels release about 101-104 Btu/ft3; gaseous fossil fuels about 104-108

Btu/ft3.

This would say that the natural gas in the example above should require about 1017 Btu/ft3 gas

divided by 106 Btu/ft3 air = 9.6 ft3 air/ft3 gas. Precise stoichiometric calculations would say

0.909(9.53) + 0.06(16.7) = 9.58 ft3 air/ft3 gas.

46.2.7 Net Heating Value

Because a calorimeter cools the exit gases below their dew point, it retrieves the latent heat of

condensation of any water vapor therein. But that latent heat is not recapturable in most practical

heating equipment because of concern about corrosion; therefore, it is more realistic to subtract the

latent heat from HHV, yielding a net or lower heating value, LHV. This is approximately

LHV

HHV /970 Btu Ib H2O \

I vx £ I

unit of fuel unit of fuel \ Ib H2O unit of fuel/

Values for the latter term are listed in Table 46.2. (Note that available heat was discussed in Chapter

44.)

Table 46.1 a Analyses of Typical Gaseous Fuels2

Analysis in % by Volume

Type of Gas

Acetylene,

commercial

Blast furnace

Blue (water),

bituminous

Butane, commercial,

natural gas

Butane, commercial,

refinery gas

Carbureted blue, low

gravity

Carbureted blue,

heavy oil

Coke oven, by-

product

Mapp

Natural, Alaska

Natural, Algerian

LNG, Canvey

Natural, Gaz de Lacq

Natural, Groningen,

Netherlands

Natural, Libyan LNG

Natural, North Sea,

B acton

Natural,

Birmingham, AL

Natural, Cleveland,

Natural, Kansas City,

Natural, Pittsburgh,

Producer,

Koppers-Totzeka

Producer, LurgP

Producer, W-G,

bituminous^

Producer, Winkler*

Propane, commercial,

natural gas

Propane, commercial,

refinery gas

Sasol, South Africa

Sewage, Decatur

SNG, no

methanation

aO2-blown.

b Air-blown.

CH4 C2H6 C3H8 C4H10 CO H2 CO2 O2 N2

(97.1% C2H2, 2.5% C3H60)

0.084 0.28

— — —

— 27.5 1.0 11.5 — 60.0

4.6 — —

0.7 28.2 32.5 5.5 0.9 27.6

_ _ 6.0 70.7 n-, _____

23.3 iso-

_ _ 5.0 50.1 n-,

(28.3% C4H6)

16.5 iso-

10.9 2.5 —

6.1 21.9 49.6 3.6 0.4 5.0

13.5 — —

8.2 26.8 32.2 6.0 0.9 12.4

32.3 — —

3.2 5.5 51.9 2.0 0.3 4.8

_ _ 15.0 10.0 (66.0% C3H4, 9.0% C3H6)

99.6 — — — — — — — 0.4

87.20 8.61 2.74 1.07 — — — — 0.36

97.38 2.17 0.10 0.05 — — — — 0.30

81.20 2.90 0.36 0.14 — — 0.87 — 14.40

70.0 15.0 10.0 3.5 — — — — 0.90

93.63 3.25 0.69 0.27 — — 0.13 — 1.78

90.0 5.0 —

— — — — — 5.0

82.9 11.9

0.3

— — 0.2 0.3 4.4

84.1 6.7 —

—

— — 0.8 — 8.4

83.4 15.8 —

— — — — — 0.8

0.09 — —

— 55.1 33.7 9.8 — 1.3

5.0 — —

— 16.0 25.0 14.0 — 40.0 .

2.7 — —

— 28.6 15.0 3.4 0 50.3

1 — — — 10 12 22 — 55

— 2.2 97.3 0.5 _____

— 2.0 72.9

0.8

(24.3% C3H6)

980 — _

_ 9.9 0 48 Q

68.0 — —

— — 2.0 22.0 — 6.0

79.9 — —

—

1.2 19.0 0.5 — —

Table 46.1 b Properties of Typical Gaseous Fuels2

Calorific

Value

Btu/ft3 kcal/m3

Gross Net Gross Net

1410 1360 12548 12105

92 91 819 819

260 239 2314 2127

3210 2961 28566 26350

3184 2935 28334 26119

536 461 4770 4102

530 451 4716 4013

569 509 5064 4530

2406 2282 21411 20308

998 906 8879 8063

1122 1014 9985 9024

1011 911 8997 8107

875 789 7787 7021

1345 1223 11969 10883

1023 922 9104 8205

1002 904 8917 8045

1059 959 9424 8534

974 879 8668 7822

1129 1021 10047 9086

288 271 2563 2412

183 167 1629 1486

168 158 1495 1406

117 111 1041 988

2558 2358 22764 20984

2504 2316 22283 20610

500 448 4450 3986

690 621 6140 5526

853 765 7591 6808

Gross

Btu/ft3

Standard

Air

115.4

135.3

126.2

104.9

106.1

101.7

105.0

113.7

104.8

104.3

104.1

104.4

106.1

105.0

106.1

106.2

106.3

135.2

125.3

129.2

188.7

107.5

108.0

114.9

105.3

105.8

Gross

kcal/m3

Standard

Air

1027

1204

1121

932.6

944.2

905.0

934

1011.86

932.6

928.2

927.3

928.2

934.4

945.1

942.4

946.0

945.1

1203

1115

1150

1679

956.6

961.1

1022

936.2

943.3

Gas

Gravity

0.94

1.02

0.70

2.04

2.00

0.54

0.66

0.40

1.48

0.55

0.64

0.57

0.64

0.79

0.59

0.60

0.635

0.63

0.61

0.78

0.80

0.84

0.98

1.55

1.77

0.55

0.79

0.47

Type of Gas

Acetylene, commercial

Blast furnace

Blue (water), bituminous

Butane, commercial, natural gas

Butane, commercial, refinery gas

Carbureted blue, low gravity

Carbureted blue, heavy oil

Coke oven, by-product

Mapp

Natural, Alaska

Natural, Algerian LNG, Canvey

Natural, Gaz de Lacq

Natural, Groningen, Netherlands

Natural, Libyan LNG

Natural, North Sea, Bacton

Natural, Birmingham, AL

Natural, Cleveland, OH

Natural, Kansas City, MO

Natural, Pittsburgh, PA

Producer, Koppers-Totzeka

Producer, LurgP

Producer, W-G, bituminous*7

Producer, Winkler*

Propane, commercial, natural gas

Propane, commercial, refinery gas

Sasol, South Africa

Sewage, Decatur

SNG, no methanation

*O2-blown.

^Air-blown.

46.2.8 Flame Stability

Flame stability is influenced by burner and combustion chamber configuration (aerodynamic and heat

transfer characteristics) and by the fuel properties tabulated in Table 46.3.

46.2.9 Gas Gravity

Gas gravity, G (Table 46.1), is the ratio of the actual gas density relative to the density of dry air at

standard temperature and pressure (0.0765 lb/ft3). This should not be confused with "specific

gravity," which is the ratio of actual density relative to that of water. Gas gravity for natural gases

typically ranges from 0.58 to 0.64, and is used in determination of flow rates and pressure drops

through pipe lines, orifices, burners, and regulators:

flow = flow coefficient X area (ft2) X V2g(psf pressure drop)/p

where g = 32.2 ft/sec2 and p = gas gravity X 0.0765. Unless otherwise emphasized, gas gravity is

measured and specified at standard temperature and pressure (60°F and 29.92 in Hg).

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