1、 FUNDAMENTALS OF GAS COMBUSTION THIRD EDITION Fundamentals of Gas Combustion Combined Manual and Workbook June 2001 ORIGINALLY PREPARED BY American Gas Association Laboratories FOR American Gas Association 400 N. Capitol Street N.W. Washington, DC 20001 iiCatalog No. XH0105 Third Edition Copyright 2
2、001, Revised June 2001 Registered by American Gas Association Printed in the United States of America iii FOREWARD This manual was originally prepared under the direction of the American Gas Association and Gas Appliance Manufacturers Association Joint Committee for Customer Service. It was written
3、by James C. Griffiths, Senior Research Engineer, and J. Fred Parr, Manager of Engineering Services, American Gas Association Laboratories (Cleveland, Ohio). The 1996 Edition was prepared by Vera Kam, Project Supervisor, and Doug DeWerth, Consultant, A.G.A. Laboratories, Research and Development Divi
4、sion, and updates the 1973 edition. The 2000 Edition was edited and prepared by Mary Smith Carson and James W. Freeman, Consultants, WE propane (C3H8) has three carbon atoms and eight hydrogen atoms; butane (C4H10) has four carbon atoms and ten hydrogen atoms. Figure 1 shows how carbon and hydrogen
5、atoms are combined in fuel gases. The number of carbon and hydrogen atoms in these fuel gases affect the nature of each gas. More atoms in the compound, particularly heavier carbon atoms, make the gas heavier. Larger numbers of carbon and hydrogen atoms in a fuel gas release more heat when the gas i
6、s burned. For this reason, the heat content of butane, for instance, is greater than that of methane. Other characteristics of fuel gases also are related to their chemical make-up. Figure 1. Graphical illustration of four common hydrocarbon fuel gases.4These include the ease with which their flames
7、 can be controlled and utilized. Common fuel gases are not simply one kind of hydrocarbon, but are mixtures of hydrocarbon gases. They may contain other gases as well, such as free hydrogen, carbon dioxide and nitrogen. For example, a natural gas might contain 94 percent methane, 3 percent ethane an
8、d 3 percent of other gases. The presence of each of these gases in the fuel gas has some effect on the nature of the gas. NATURAL GASES The common explanation for the formation of natural gas is called the “organic theory.” During untold millions of years before human life developed on this planet,
9、dead and dying plants and animals washed down into long lost lakes and oceans. These remains were covered by mud and sand. In time, this accumulation exerted high pressures on the buried materials by its own weight. In turn, these pressures created high temperatures. Chemical action took place and c
10、onverted the remains of these once living things into gas and oil. At the same time, the mud and sand changed into rocks, much of it porous. Oil and gas seeped into these tiny holes in the rock, to be stored there under high pressure beneath layers of more solid rock above. Oil and gas also collecte
11、d in dome-like formations, or in traps created by the upthrust of the earths crust. Today, gas and crude oil are sometimes found together, but most of the reserves of gas in the United States are not dissolved in or in contact with oil. Gas from a well may contain small amounts of condensables. Thes
12、e are gases that can be removed as liquids through moderate changes in temperature and pressure, or both. Condensables, which contain sulfur, are important for two practical reasons. First, they can corrode some metals, more so if moisture also is present in the gas. Second, they often have a strong
13、 odor. Water and condensables are usually removed from fuel gases to help prevent corrosion of pipelines through which the gas is transported. The remaining methane, ethane and inert gases have no odor, color or taste. Propane and butane also are nearly odorless. Studies have shown that natural gas
14、is non-toxic and non-poisonous. LIQUEFIED PETROLEUM GASES Liquefied petroleum gases (LPG or LP gas) are propane or butane, or a mixture of the two. These fuel gases are obtained from natural gas, or as a by-product from the refining of oil. LP gases are trans-ported and stored in tanks. They are use
15、d usually in areas in which pipeline gas is not available, as a reserve gas supply in some cities and by some industries, and as the fuel for some engines. Since LPG is stored under pressure, much of the gas is in liquid form in the tanks. As the fuel is drawn from the tank it turns again to gas and
16、 is burned in the same way as other fuel gases. MANUFACTURED AND MIXED GASES As its name states, manufactured gas is man-made. The name applies to a number of gases produced on a large scale for use as fuel gases and some that are produced 5as by-products in other manufacturing operations. For examp
17、le, in iron-making, large amounts of gases are produced that are used as fuel gases. Widespread use of manufactured fuel gases preceded that of natural gas by many years. As early as the 1600s in Europe, it was shown that a usable fuel gas could be produced by heating coal. In 1813, Westminister Bri
18、dge in London was lighted by manufactured gas lamps. Other cities quickly followed suit. Baltimore, in 1816, was the first American city to light its streets with gas. Paris followed in 1820. The use of manufactured fuel gases has declined greatly in the United States. Today, over 99 percent of sale
19、s by gas distribution and transmission companies are natural gas. Some manufactured gases are still used in Europe and other parts of the world, but use of natural gas has risen sharply in these areas also in recent years. Mixed gas, as the name implies, is a man-made mixture of gases. A common exam
20、ple is a mixture of natural gas and manufactured gas. ODORANTS ADDED TO GAS Natural gas processed to remove condensables and moisture has little or no odor and no color. Odorants are added to the gas before distribution to aid in leak detection. Most odorants used are colorless liquids containing su
21、lfur compounds. These compounds give a garlic-like odor that most people associate with a “gassy” smell. Odorants can be put into a gas stream as liquids by various types of odorizers; e.g., by a drip-type odorizer. The odorants vaporize in the pipelines and mix with the fuel as gases. With the abso
22、rption type odorizer, odorants are added to the gas stream in a vapor or gaseous form. The amount of odorant needed to produce a strong odor in the gas is quite small. The amount of sulfur added in this way is too small to create a problem of corrosion either in the pipelines or by flue gases. The s
23、ulfur burns in the gas flame to produce little or no odor or harmful by-products. For sulfur sensitive processes using natural gas, the odorant can be scrubbed out prior to the final process, as is done in glass making. SPECIFIC GRAVITY The density of a substance is the amount of mass (weight) in a
24、given volume. Fresh water, for instance, has a density of 62.4 pounds per cubic foot. Often, it is more useful to refer to the specific gravity of a substance. Specific gravity is the weight of a substance compared to the same volume of some reference substance. Fresh water is used as the reference
25、substance for liquids and solids. If the specific gravity of a liquid is 2.0, one cubic foot of that liquid will weigh twice as much as a cubic foot of water. That is, the liquid will weigh 124.8 pounds per cubic foot (2.0 x 62.4). Gases are quite light, compared to liquids and solids. Specific grav
26、ity of a gas is the weight of one cubic foot of that gas compared to one cubic foot of dry air. Both the air and the gas must be at the same pressure and temperature when the comparison is made. The amount of gas in a given volume varies with pressure. Gases are compressed as pressure increases and
27、expanded as pressure is reduced. Gases expand when heated and 6contract when cooled. In stating the specific gravity of a gas, a pressure and temperature must be clearly stated. Usually, a set of conditions is used that is close to average atmospheric conditions. In the gas industry, “standard” cond
28、itions of pressure and temperature are 30.0 inches of mercury and 60o F. A pressure of 30 inches of mercury will sustain a column of mercury 30 inches high in a tube with a vacuum on top of the column. Since air is used as the reference gas, its specific gravity is always 1.0. Note that this value o
29、f 1.0 has no direct physical meaning with regard to air, such as its density. It is only a relative number or ratio used to express specific gravity of other gases. Natural gases have specific gravities ranging from 0.4 to 0.8, as shown in Figure 2. This means that a cubic foot of natural gas will w
30、eigh only 4/10 to 8/10 that of a cubic foot of air. Again, both must be at the same pressure and temperature when compared. The spread of specific gravities of natural gases reflect the differing amounts of lighter and heavier gases in each mixture. Specific gravity of a gas determines whether that
31、gas will rise or fall when released into the air. Natural gas will rise, having a specific gravity less than 1.0. Usually it will mix readily with the air. As shown in Figure 3, propane has a specific gravity of 1.5; the specific gravity of butane is 2.0. If these gases are released in air they will
32、 fall instead of rising. Figure 2. Specific gravity of natural gas.7Figure 3. Specific gravities of propane and butane. Usually they will not mix readily with the air. In fact, the gases may drift to low spots and collect in “pools,” creating a hazard if open flames are present. Manufactured gases u
33、sually are lighter than air, with specific gravities ranging from 0.4 to 0.7. Specific gravities of mixed gases depend on the gases used and the amounts of each gas in the mixture. A mixture of natural and manufactured gas with a heating value of 800 Btu per cubic foot has a specific gravity typical
34、ly of 0.5. Specific gravity has two other practical aspects. It has an important effect on the flow of gases through orifices, and hence the rating of burners. A gas orifice is a small hole in a fitting (spud) used to limit gas flow to burners. Gas flow rate through an orifice depends on the orifice
35、 size and gas pressure upstream of the orifice. However, more of a lighter gas will flow through a given orifice size than a heavier gas at the same gas pressure. For instance, about 83 percent more natural gas than butane will flow through the same orifice with the same gas pressure. This effect of
36、 specific gravity is taken into account in tables and calculators used to select orifice sizes for burners. Specific gravity also affects gas flow in pipes. A given driving pressure at a pipe inlet will move lighter gas than heavier gas through that pipe. HEATING VALUE Heat energy produced when burn
37、ing a fuel gas is commonly expressed in British thermal units (Btu). One Btu of heat will raise the temperature of one pound of fresh water one degree Fahrenheit, as shown in Figure 4. Burning an ordinary wooden kitchen match produces about 1 Btu of heat. Figure 4. Illustration of British thermal un
38、it. If 1,000 Btu of heat are added to 10 pounds of water, the water temperature will rise 100 degrees Fahrenheit (1,000 10 = 100). Likewise, 5,000 Btu of heat must be added to 100 pounds of water to raise the water temperature 50o F (5,000 100 = 50). 8In equation form: Water temperature rise, degree
39、s F = (Btu of heat added) (pounds of water) (Btu of heat added) = (pounds of water) x (water temperature rise, degrees F) The heating value (or heat value) of a gas is the amount of heat released when one cubic foot of the gas is completely burned. This heating value is expressed in Btu per cubic fo
40、ot of gas at standard pressure and temperature. The more carbon and hydrogen atoms in each molecule of a fuel gas, the higher will be its heating value. Natural gas (which is largely methane) has a heating value of about 950 to 1,150 Btu per cubic foot. The spread of heating values is due to the pre
41、sence of various other substances found in natural gases. More ethane, propane or butane in the gas raises its heating value. Gases such as nitrogen are inert and do not burn. Hence, they do not add to the heating value of the gas. The larger the amount of inert gases present in a natural gas, the l
42、ower the heating value will be. In round numbers, propane has a heating value of 2,500 Btu per cubic foot, and butane about 3,200 Btu per cubic foot. Hourly use of gas varies in different appliances. Input rates can be converted from Btu per hour to cubic feet per hour, or from cubic feet per hour t
43、o Btu per hour, as follows: Input rate, in Btu per hour = (Heating value of gas, in Btu per cubic foot) x (Gas flow rate, in cubic feet per hour) Gas flow rate, in cubic feet per hour = (Input rate, in Btu per hour) (Heating value of gas, in Btu per cubic foot) For example, a typical 30-gallon stora
44、ge water heater might have an input rate of 35,000 Btu per hour. Suppose that a 1,000 Btu per cubic foot gas is used. Using the above formula, gas flow rate is equal to 35,000 Btu 1,000 Btu gas, which figures to be 35 cubic feet of gas per hour. Thus, 35 cubic feet of gas should be supplied to the w
45、ater heater. More examples: A range-top burner burns 1.875 cubic feet of butane per hour. Since butane has a heating value of 3,200 Btu per cubic foot, the burner input rate is 6,000 Btu per hour (1.875 x 3,200 = 6,000). This same range-top burner operating on natural gas with an enlargement of the
46、orifice would need to burn six cubic feet of gas per hour to have the same input (6 x 1,000 = 6,000). A 125,000 Btu per hour furnace uses 2,500 Btu per cubic foot propane. This furnace would use 50 cubic feet of propane per hour (125,000 2,500 = 50). Another unit of heat energy, which is used in the
47、 gas industry, is the therm. A therm of 1,000 Btu per cubic foot natural gas would be 100 cubic feet of gas (100,000 1,000 = 100). A therm of butane is 31.25 cubic feet of that gas (100,000 3,200 = 31.25). Forty cubic feet of propane contain a therm of heat energy (100,000 2,500 = 40). 9CHAPTER REVI
48、EW PROPERTIES AND GENERAL CHARACTERISTICS OF GASES QUESTIONS ANSWERS 6. The major parts of fuel gases are 1._. The chemical symbol for carbon is 2._. The symbol for hydrogen is 3._. (Pg. 3) Nitrogen Hydrocarbons 7. The largest part of natural gas is _. (Pg. 3) Methane Free Hydrogen Heat C 8. More at
49、oms in propane gas tend to make the gas 1._, Larger numbers of carbon and hydrogen atoms in a gas release more 2._ when the gas is burned. (Pg. 4) Butane Heavier Methane H Carbon Dioxide 9. 1._(C4H10) has a higher heat value than 2._(CH4) because it has more carbon and hydrogen atoms. (Pg. 3) 10. Fuel gases may contain other gases as well, such as: 1._ _, 2._ _, and 3._. (Pg. 4) 10 QUESTIONS ANSWERS 11
