Steam, Its Generation and Use Part 29

From the nature of mechanical atomizing burners, individual burners have not as large a capacity as the steam atomizing cla.s.s. In some tests on a Babc.o.c.k & Wilc.o.x marine boiler, equipped with mechanical atomizing burners, the maximum horse power developed per burner was approximately 105. Here again the burner capacity is largely one of proper relation between furnace volume and number of burners.

Furnace Design--Too much stress cannot be laid on the importance of furnace design for the use of this cla.s.s of fuel. Provided a good type of burner is adopted the furnace arrangement and the method of introducing air for combustion into the furnace are the all important factors. No matter what the type of burner, satisfactory results cannot be secured in a furnace not suited to the fuel.

The Babc.o.c.k & Wilc.o.x Co. has had much experience with the burning of oil as fuel and an extended series of experiments by Mr. E. H. Peabody led to the development and adoption of the Peabody furnace as being most eminently suited for this cla.s.s of work. Fig. 29 shows such a furnace applied to a Babc.o.c.k & Wilc.o.x boiler, and with slight modification it can be as readily applied to any boiler of The Babc.o.c.k & Wilc.o.x Co.

manufacture. In the description of this furnace, its points of advantage cover the requirements of oil-burning furnaces in general.

The atomized oil is introduced into the furnace in the direction in which it increases in height. This increase in furnace volume in the direction of the flame insures free expansion and a thorough mixture of the oil with the air, and the consequent complete combustion of the gases before they come into contact with the tube heating surfaces. In such a furnace flat flame burners should be used, preferably of the Peabody type, in which the flame spreads outward toward the sides in the form of a fan. There is no tendency of the flames to impinge directly on the heating surfaces, and the furnace can handle any quant.i.ty of flame without danger of tube difficulties. The burners should be so located that the flames from individual burners do not interfere nor impinge to any extent on the side walls of the furnace, an even distribution of heat being secured in this manner. The burners are operated from the boiler front and peepholes are supplied through which the operator may watch the flame while regulating the burners. The burners can be removed, inspected, or cleaned and replaced in a few minutes. Air is admitted through a checkerwork of fire brick supported on the furnace floor, the openings in the checkerwork being so arranged as to give the best economic results in combustion.

[Ill.u.s.tration: Fig. 29. Babc.o.c.k & Wilc.o.x Boiler, Equipped with a Peabody Oil Furnace]

With steam atomizing burners introduced through the front of the boiler in stationary practice, it is usually in the direction in which the furnace decreases in height and it is with such an arrangement that difficulties through the loss of tubes may be expected. With such an arrangement, the flame may impinge directly upon the tube surfaces and tube troubles from this source may arise, particularly where the feed water has a tendency toward rapid scale formation. Such difficulties may be the result of a blowpipe action on the part of the burner, the over heating of the tube due to oil or scale within, or the actual erosion of the metal by particles of oil improperly atomized. Such action need not be antic.i.p.ated, provided the oil is burned with a short flame. The flames from mechanical atomizing burners have a less velocity of projection than those from steam atomizing burners and if introduced into the higher end of the furnace, should not lead to tube difficulties provided they are properly located and operated. This cla.s.s of burner also will give the most satisfactory results if introduced so that the flames travel in the direction of increase in furnace volume. This is perhaps best exemplified by the very good results secured with mechanical atomizing burners and Babc.o.c.k & Wilc.o.x marine boilers in which, due to the fact that the boilers are fired from the low end, the flames from burners introduced through the front are in this direction.

Operation of Burners--When burners are not in use, or when they are being started up, care must be taken to prevent oil from flowing and collecting on the floor of the furnace before it is ignited. In starting a burner, the atomized fuel may be ignited by a burning wad of oil-soaked waste held before it on an iron rod. To insure quick ignition, the steam supply should be cut down. But little practice is required to become an adept at lighting an oil fire. When ignition has taken place and the furnace brought to an even heat, the steam should be cut down to the minimum amount required for atomization. This amount can be determined from the appearance of the flame. If sufficient steam is not supplied, particles of burning oil will drop to the furnace floor, giving a scintillating appearance to the flame. The steam valves should be opened just sufficiently to overcome this scintillating action.

Air Supply--From the nature of the fuel and the method of burning, the quant.i.ty of air for combustion may be minimized. As with other fuels, when the amount of air admitted is the minimum which will completely consume the oil, the results are the best. The excess or deficiency of air can be judged by the appearance of the stack or by observing the gases pa.s.sing through the boiler settings. A perfectly clear stack indicates excess air, whereas smoke indicates a deficiency. With properly designed furnaces the best results are secured by running near the smoking point with a slight haze in the gases. A slight variation in the air supply will affect the furnace conditions in an oil burning boiler more than the same variation where coal is used, and for this reason it is of the utmost importance that flue gas a.n.a.lysis be made frequently on oil-burning boilers. With the air for combustion properly regulated by adjustment of any checkerwork or any other device which may be used, and the dampers carefully set, the flue gas a.n.a.lysis should show, for good furnace conditions, a percentage of CO_{2} between 13 and 14 per cent, with either no CO or but a trace.

In boiler plant operation it is difficult to regulate the steam supply to the burners and the damper position to meet sudden and repeated variations in the load. A device has been patented which automatically regulates by means of the boiler pressure the pressure of the steam to the burners, the oil to the burners and the position of the boiler damper. Such a device has been shown to give good results in plant operation where hand regulation is difficult at best, and in many instances is unfortunately not even attempted.

Efficiency with Oil--As pointed out in enumerating the advantages of oil fuel over coal, higher efficiencies are obtainable with the former. With boilers of approximately 500 horse power equipped with properly designed furnaces and burners, an efficiency of 83 per cent is possible or making an allowance of 2 per cent for steam used by burners, a net efficiency of 81 per cent. The conditions under which such efficiencies are to be secured are distinctly test conditions in which careful operation is a prime requisite. With furnace conditions that are not conductive to the best combustion, this figure may be decreased by from 5 to 10 per cent.

In large properly designed plants, however, the first named efficiency may be approached for uniform running conditions, the nearness to which it is reached depending on the intelligence of the operating crew. It must be remembered that the use of oil fuel presents to the careless operator possibilities for wastefulness much greater than in plants where coal is fired, and it therefore pays to go carefully into this feature.

Table 48 gives some representative tests with oil fuel.

TABLE 48

TESTS OF BABc.o.c.k AND WILc.o.x BOILERS WITH OIL FUEL

_______________________________________________________________________ | | | | | | |Pacific Light|Pacific Light|Miami Copper | | | and Power | and Power | Company | | Plant | Company | Company | | | |Los Angeles, | | Miami, | | | Cal. |Redondo, Cal.| Arizona | |_____________________________|_____________|_____________|_____________| | | | | | | | Rated Capacity | Horse | | | | | of Boiler | Power | 467 | 604 | 600 | |__________________|__________|_____________|_____________|_____________| | | | | | | | | | | Duration of Test | Hours | 10 | 10 | 7 | 7 | 10 | 4 | | | | | | | | | | | Steam Pressure | | | | | | | | | by Gauge | Pounds | 156.4| 156.9| 184.7| 184.9| 183.4| 189.5| | | | | | | | | | | Temperature of | Degrees | | | | | | | | Feed Water | F. | 62.6| 61.1| 93.4| 101.2| 157.7| 156.6| | | | | | | | | | | Degrees of | Degrees | | | | | | | | Superheat | F. | | | 83.7| 144.3| 103.4| 139.6| | | | | | | | | | | Factor of | | | | | | | | | Evaporation | |1.2004|1.2020|1.2227|1.2475|1.1676|1.1886| | | | | | | | | | | Draft in Furnace | Inches | .02 | .05 | .014| .19 | .12 | .22 | | | | | | | | | | | Draft at Damper | Inches | .08 | .15 | .046| .47 | .19 | .67 | | | | | | | | | | | Temperature of | Degrees | | | | | | | | Exit Gases | F. | 438 | 525 | 406 | 537 | 430 | 612 | | _ | | | | | | | | | Flue | CO_{2} | Per Cent | | | 14.3 | 12.1 | | | | Gas | O | Per Cent | | | 3.8 | 6.8 | | | | a.n.a.lysis|_CO | Per Cent | | | 0.0 | 0.0 | | | | | | | | | | | | | Oil Burned | | | | | | | | | per Hour | Pounds | 1147 | 1837 | 1439 | 2869 | 1404 | 3214 | | | | | | | | | | | Water Evaporated | | | | | | | | | per Hour from | | | | | | | | | from and at | Pounds | 18310| 27855| 22639| 40375| 21720| 42863| | 212 Degrees | | | | | | | | | | | | | | | | | | Evaporation from | | | | | | | | | and at 212 | | | | | | | | | Degrees per | Pounds | 15.96| 15.16| 15.73| 14.07| 15.47| 13.34| | Pound of Oil | | | | | | | | | | | | | | | | | | Per Cent of | | | | | | | | | Rated Capacity | Pounds | 113.6| 172.9| 108.6| 193.8| 104.9| 207.1| | Developed | | | | | | | | | | | | | | | | | | B. t. u. per | | | | | | | | | Pound of Oil | B. t. u. | 18626| 18518| 18326| 18096| 18600| 18600| | | | | | | | | | | Efficiency | Per Cent | 83.15| 79.46| 83.29| 76.02| 80.70| 69.6 | |__________________|__________|______|______|______|______|______|______|

Burning Oil in Connection with Other Fuels--Considerable attention has been recently given to the burning of oil in connection with other fuels, and a combination of this sort may be advisable either with the view to increasing the boiler capacity to a.s.sist over peak loads, or to keep the boiler in operation where there is the possibility of a temporary failure of the primary fuel. It would appear from experiments that such a combination gives satisfactory results from the standpoint of both capacity and efficiency, if the two fuels are burned in separate furnaces. Satisfactory results cannot ordinarily be obtained when it is attempted to burn oil fuel in the same furnace as the primary fuel, as it is practically impossible to admit the proper amount of air for combustion for each of the two fuels simultaneously. The Babc.o.c.k & Wilc.o.x boiler lends itself readily to a double furnace arrangement and Fig. 30 shows an installation where oil fuel is burned as an auxiliary to wood.

[Ill.u.s.tration: Fig. 30. Babc.o.c.k & Wilc.o.x Boiler Set with Combination Oil and Wood-burning Furnace]

Water-gas Tar--Water-gas tar, or gas-house tar, is a by-product of the coal used in the manufacture of water gas. It is slightly heavier than crude oil and has a comparatively low flash point. In burning, it should be heated only to a temperature which makes it sufficiently fluid, and any furnace suitable for crude oil is in general suitable for water-gas tar. Care should be taken where this fuel is used to install a suitable apparatus for straining it before it is fed to the burner.

[Ill.u.s.tration: Babc.o.c.k & Wilc.o.x Boilers Fired with Blast Furnace Gas at the Bethlehem Steel Co., Bethlehem, Pa. This Company Operates 12,900 Horse Power of Babc.o.c.k & Wilc.o.x Boilers]

GASEOUS FUELS AND THEIR COMBUSTION

Of the gaseous fuels available for steam generating purposes, the most common are blast furnace gas, natural gas and by-product c.o.ke oven gas.

Blast furnace gas, as implied by its name, is a by-product from the blast furnace of the iron industry. This gasification of the solid fuel in a blast furnace results, 1st, through combustion by the oxygen of the blast; 2nd, through contact with the incandescent ore (Fe_{2}O_{3} + C = 2 FeO + CO and FeO + C = Fe + CO); and 3rd, through the agency of CO_{2} either formed in the process of reduction or driven from the carbonates charged either as ore or flux.

Approximately 90 per cent of the fuel consumed in all of the blast furnaces of the United States is c.o.ke. The consumption of c.o.ke per ton of iron made varies from 1600 to 3600 pounds per ton of 2240 pounds of iron. This consumption depends upon the quality of the coal, the nature of the ore, the quality of the pig iron produced and the equipment and management of the plant. The average consumption, and one which is approximately correct for ordinary conditions, is 2000 pounds of c.o.ke per gross ton (2240 pounds) of pig iron. The gas produced in a gas furnace per ton of pig iron is obtained from the weight of fixed carbon gasified, the weight of the oxygen combined with the material of charge reduced, the weight of the gaseous const.i.tuents of the flux and the weight of air delivered by the blowing engine and the weight of volatile combustible contained in the c.o.ke. Ordinarily, this weight of gas will be found to be approximately five times the weight of the c.o.ke burned, or 10,000 pounds per ton of pig iron produced.

With the exception of the small amount of carbon in combination with hydrogen as methane, and a very small percentage of free hydrogen, ordinarily less than 0.1 per cent, the calorific value of blast furnace gas is due to the CO content which when united with sufficient oxygen when burned under a boiler, burns further to CO_{2}. The heat value of such gas will vary in most cases from 85 to 100 B. t. u. per cubic foot under standard conditions. In modern practice, where the blast is heated by hot blast stoves, approximately 15 per cent of the total amount of gas is used for this purpose, leaving 85 per cent of the total for use under boilers or in gas engines, that is, approximately 8500 pounds of gas per ton of pig iron produced. In a modern blast furnace plant, the gas serves ordinarily as the only fuel required. Table 49 gives the a.n.a.lyses of several samples of blast furnace gas.

TABLE 49

TYPICAL a.n.a.lYSES OF BLAST FURNACE GAS

+----------------------------------------------------------------+ |+-----------------------+------+----+-----+----+------+--------+| || |CO_{2}| O | CO | H |CH_{4}| N || |+-----------------------+------+----+-----+----+------+--------+| ||Bessemer Furnace | 9.85|0.36|32.73|3.14| .. |53.92 || ||Bessemer Furnace | 11.4 | .. |27.7 |1.9 | 0.3 |58.7 || ||Bessemer Furnace | 10.0 | .. |26.2 |3.1 | 0.2 |60.5 || ||Bessemer Furnace | 9.1 | .. |28.7 |2.7 | 0.2 |59.3 || ||Bessemer Furnace | 13.5 | .. |25.2 |1.43| .. |59.87 || ||Bessemer Furnace[47] | 10.9 | .. |27.8 |2.8 | 0.2 |58.3 || ||Ferro Manganese Furnace| 7.1 | .. |30.1 | .. | .. |62.8[48]|| ||Basic Ore Furnace | 16.0 |0.2 |23.6 | .. | .. |60.2[48]|| |+-----------------------+------+----+-----+----+------+--------+| +----------------------------------------------------------------+

Until recently, the important consideration in the burning of blast furnace gas has been the capacity that can be developed with practically no attention given to the aspect of efficiency. This phase of the question is now drawing attention and furnaces especially designed for good efficiency with this cla.s.s of fuel are demanded. The essential feature is ample combustion s.p.a.ce, in which the combustion of gases may be practically completed before striking the heating surfaces. The gases have the power of burning out completely after striking the heating surfaces, provided the initial temperature is sufficiently high, but where the combustion is completed before such time, the results secured are more satisfactory. A furnace volume of approximately 1 to 1.5 cubic feet per rated boiler horse power will give a combustion s.p.a.ce that is ample.

Where there is the possibility of a failure of the gas supply, or where steam is required when the blast furnace is shut down, coal fired grates of sufficient size to get the required capacity should be installed.

Where grates of full size are not required, ignition grates should be installed, which need be only large enough to carry a fire for igniting the gas or for generating a small quant.i.ty of steam when the blast furnace is shut down. The area of such grates has no direct bearing on the size of the boiler. The grates may be placed directly under the gas burners in a standard position or may be placed between two bridge walls back of the gas furnace and fired from the side of the boiler. An advantage is claimed for the standard grate position that it minimizes the danger of explosion on the re-ignition of gas after a temporary stoppage of the supply and also that a considerable amount of dirt, of which there is a good deal with this cla.s.s of fuel and which is difficult to remove, deposits on the fire and is taken out when the fires are cleaned. In any event, regardless of the location of the grates, ample provision should be made for removing this dust, not only from the furnace but from the setting as a whole.

Blast furnace gas burners are of two general types: Those in which the air for combustion is admitted around the burner proper, and those in which this air is admitted through the burner. Whatever the design of burner, provision should be made for the regulation of both the air and the gas supply independently. A gas opening of .8 square inch per rated horse power will enable a boiler to develop its nominal rating with a gas pressure in the main of about 2 inches. This pressure is ordinarily from 6 to 8 inches and in this way openings of the above size will be good for ordinary overloads. The air openings should be from .75 to .85 square inch per rated horse power. Good results are secured by inclining the gas burners slightly downward toward the rear of the furnace. Where the burners are introduced over coal fired grates, they should be set high enough to give headroom for hand firing.

Ordinarily, individual stacks of 130 feet high with diameters as given in Kent's table for corresponding horse power are large enough for this cla.s.s of work. Such a stack will give a draft sufficient to allow a boiler to be operated at 175 per cent of its rated capacity, and beyond this point the capacity will not increase proportionately with the draft. When more than one boiler is connected with a stack, the draft available at the damper should be equivalent to that which an individual stack of 130 feet high would give. The draft from such a stack is necessary to maintain a suction under all conditions throughout all parts of the setting. If the draft is increased above that which such a stack will give, difficulties arise from excess air for combustion with consequent loss in efficiency.

A poor mixing or laneing action in the furnace may result in a pulsating effect of the gases in the setting. This action may at times be remedied by admitting more air to the furnace. On account of the possibility of a pulsating action of the gases under certain conditions and the puffs or explosions, settings for this cla.s.s of work should be carefully constructed and thoroughly buckstayed and tied.

Natural Gas--Natural gas from different localities varies considerably in composition and heating value. In Table 50 there is given a number of a.n.a.lyses and heat values for natural gas from various localities.

This fuel is used for steam generating purposes to a considerable extent in some localities, though such use is apparently decreasing. It is best burned by employing a large number of small burners, each being capable of handling 30 nominal rated horse power. The use of a large number of burners obviates the danger of any laneing or blowpipe action, which might be present where large burners are used. Ordinarily, such a gas, as it enters the burners, is under a pressure of about 8 ounces. For the purpose of comparison, all observations should be based on gas reduced to the standard conditions of temperature and pressure, namely 32 degrees Fahrenheit and 14.7 pounds per square inch. When the temperature and pressure corresponding to meter readings are known, the volume of gas under standard conditions may be obtained by multiplying the meter readings in cubic feet by 33.54 P/T, in which P equals the absolute pressure in pounds per square inch and T equals the absolute temperature of the gas at the meter. In boiler testing work, the evaporation should always be reduced to that per cubic foot of gas under standard conditions.

TABLE 50

TYPICAL a.n.a.lYSES (BY VOLUME) AND CALORIFIC VALUES OF NATURAL GAS FROM VARIOUS LOCALITIES

+----------------+-----+-----+-----+-----+-----+----+-------+------+--------+ |Locality of Well| H |CH_{4}| CO |CO_{2}| N | O | Heavy |H_{2}S|B. t. u.| | | | | | | | |Hydro- | | per | | | | | | | | |carbons| | Cubic | | | | | | | | | | | Foot | | | | | | | | | | |Calcul- | | | | | | | | | | |ated[49]| |----------------+-----+-----+-----+-----+-----+----+-------+------+--------+ |Anderson, Ind. | 1.86|93.07| 0.73| 0.26| 3.02|0.42| 0.47 | 0.15 | 1017 | |Marion, Ind. | 1.20|93.16| 0.60| 0.30| 3.43|0.55| 0.15 | 0.20 | 1009 | |Muncie, Ind. | 2.35|92.67| 0.45| 0.25| 3.53|0.35| 0.25 | 0.15 | 1004 | |Olean, N. Y. | |96.50| 0.50| | |2.00| 1.00 | | 1018 | |Findlay, O. | 1.64|93.35| 0.41| 0.25| 3.41|0.39| 0.35 | 0.20 | 1011 | |St. Ive, Pa. | 6.10|75.54|Trace| 0.34| | | 18.12 | | 1117 | |Cherry Tree, Pa.|22.50|60.27| | 2.28| 7.32|0.83| 6.80 | | 842 | |Grapeville, Pa. |24.56|14.93|Trace|Trace|18.69|1.22| 40.60 | | 925 | |Harvey Well, | | | | | | | | | | | Butler Co., Pa.|13.50|80.00|Trace| 0.66| | | 5.72 | | 998 | |Pittsburgh, Pa. | 9.64|57.85| 1.00| |23.41|2.10| 6.00 | | 748 | |Pittsburgh, Pa. |20.02|72.18| 1.00| 0.80| |1.10| 4.30 | | 917 | |Pittsburgh, Pa. |26.16|65.25| 0.80| 0.60| |0.80| 6.30 | | 899 | +----------------+-----+-----+-----+-----+-----+----+-------+------+--------+

[Ill.u.s.tration: 1600 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters at the Carnegie Natural Gas Co., Underwood, W. Va.

Natural Gas is the Fuel Burned under these Boilers]

When natural gas is the only fuel, the burners should be evenly distributed over the lower portion of the boiler front. If the fuel is used as an auxiliary to coal, the burners may be placed through the fire front. A large combustion s.p.a.ce is essential and a volume of .75 cubic feet per rated horse power will be found to give good results. The burners should be of a design which give the gas and air a rotary motion to insure a proper mixture. A checkerwork wall is sometimes placed in the furnace about 3 feet from the burners to break up the flame, but with a good design of burner this is unnecessary. Where the gas is burned alone and no grates are furnished, good results are secured by inclining the burner downward to the rear at a slight angle.

By-product c.o.ke Oven Gas--By-product c.o.ke oven gas is a product of the destructive distillation of coal in a distilling or by-product c.o.ke oven. In this cla.s.s of apparatus the gases, instead of being burned at the point of their origin, as in a beehive or retort c.o.ke oven, are taken from the oven through an uptake pipe, cooled and yield as by-products tar, ammonia, illuminating and fuel gas. A certain portion of the gas product is burned in the ovens and the remainder used or sold for illuminating or fuel purposes, the methods of utilizing the gas varying with plant operation and locality.

Table 51 gives the a.n.a.lyses and heat value of certain samples of by-product c.o.ke oven gas utilized for fuel purposes.

This gas is nearer to natural gas in its heat value than is blast furnace gas, and in general the remarks as to the proper methods of burning natural gas and the features to be followed in furnace design hold as well for by-product c.o.ke oven gas.

TABLE 51

TYPICAL a.n.a.lYSES OF BY-PRODUCT c.o.kE OVEN GAS

+----------------------------------------------+ |+------+-------------------------------------+| ||CO_{2}| O |CO |CH_{4}| H | N |B.t.u. per|| || | | | | | |Cubic Foot|| |+------+-----+---+------+----+----+----------+| || 0.75 |Trace|6.0|28.15 |53.0|12.1| 505 || || 2.00 |Trace|3.2|18.80 |57.2|18.0| 399 || || 3.20 | 0.4 |6.3|29.60 |41.6|16.1| 551 || || 0.80 | 1.6 |4.9|28.40 |54.2|10.1| 460 || |+------+-----+---+------+----+----+----------+| +----------------------------------------------+

The essential difference in burning the two fuels is the pressure under which it reaches the gas burner. Where this is ordinarily from 4 to 8 ounces in the case of natural gas, it is approximately 4 inches of water in the case of by-product c.o.ke oven gas. This necessitates the use of larger gas openings in the burners for the latter cla.s.s of fuel than for the former.

By-product c.o.ke oven gas comes to the burners saturated with moisture and provision should be made for the blowing out of water of condensation. This gas too, carries a large proportion of tar and hydrocarbons which form a deposit in the burners and provision should be made for cleaning this out. This is best accomplished by an attachment which permits the blowing out of the burners by steam.