Everything you've ever wanted to know about making and passing gas...
Pressure and Suction Types
1. It is sometimes desirable or necessary to operate a gas engine independently of a central gas plant. In such cases, it is possible to produce the fuel gas required to run the gas engine at a lower cost than when using either illuminating gas or the more volatile grades of liquid fuel, such as gasoline and distillate of petroleum. This has led to the gradual development of an apparatus known as a power-gas producer, which is practically a small gas plant located near the engine, to which it furnishes gas. The process of making gas from coal, coke, or charcoal by means of such a producer is a simple one. The gas-generating and purifying devices are of such size as to be readily installed in power plants using gas engines and operating under ordinary conditions. They occupy but a small amount of space, and the attendance, even in a fairly large plant, requires only a portion of the time of one man.
2. Classification of Producers: Modern gas producers may be divided into two general classes namely, pressure producers and suction producers.
In pressure producers, the gas is generated by forcing a blast of steam from a boiler, or of moistened air from a blower, through a bed of incandescent fuel. The gas is purified in a scrubber, stored in a gas holder of suitable capacity, and supplied to the engine at a pressure of from 2 to 3 ounces. Pressure producers are generally used in large power plants and where more than one engine is supplied from the producer. They are also adapted to the use of different kinds of fuel.
In suction producers, air and water vapor at atmospheric pressure are drawn through the incandescent fuel by the inhaling or suction action of the engine, due to the partial vacuum produced in the engine cylinder during the suction stroke of the piston. The gas so generated is cooled and purified in the same manner as in the pressure producer. The volume of gas in a suction producer is thus never in excess of that required by the engine, and consequently it is not necessary to provide a holder, the gas being drawn directly from the producer to the engine cylinder. The amount of gas generated depends on the force of the suction or the number of inhalations transmitted from the engine cylinder to the producer. The engine governor controls either the volume of the charges or the number of charges required to operate the engine under any load.
3. Power Gas.-The pressure process of manufacturing producer gas for power purposes has not been materially changed since its introduction 20 years ago (1887) by Dowson, in England. Producer gas had previously been used for heating purposes, in which case it was desirable to keep the temperature of the gas as high as possible before being burned. In generating power gas the conditions and requirements are materially different, the desired object being the transformation of the heat in the coal into the chemical energy of cold gas, because it is only in a thoroughly cooled condition that gas can be used efficiently in engines.
In the pressure process of generating gas for heating purposes, the dry air is heated before being introduced into the producer; in the generation of power gas, on the other hand, the object is to reduce the temperature of the gas as much as possible by admitting to the producer a certain amount of air and water vapor, so that, when the mixture is brought in contact with the burning fuel, hydrogen will be liberated. With even this cooling effect, however, the gases leave the producer at a temperature that is generally above 900 degrees F.
The process of manufacturing power gas consists principally in heating some form of fuel to a very high temperature in a vessel from which the atmosphere can be excluded. The vessel in which the heating takes place is called the producer, or generator. The vessel in which water is heated, in order to supply moisture to the air that is admitted to the producer, is called the evaporator, or boiler.
After the gas is made in the producer, it is purified and used directly, or else stored in suitable tanks. The several steps in the process of gas generation will be treated in detail in connection with the various types of producers.
PRESSURE GAS PRODUCERS
4. One of the first types of pressure producers, as introduced by Dowson in England, and of which several plants have been installed in America, is illustrated in Fig. 1. The boiler a generates steam at from 60 to 75 pounds pressure, the steam being conveyed to the injector b through the pipe c. In the injector, the steam is discharged through a small nozzle, and the issuing current draws with it a certain amount of air from the casing d surrounding the pipe e, through which the hot gases leave the producer f. Thus the injector serves merely to deliver a mingled stream of air and steam to the ash-pit g of the producer, beneath the grate.
The producer consists of a cylindrical shell made of steel plates and lined with a highly refractory grade of firebrick. A hopper h, which is closed toward the atmosphere by a removable lid i, and against the interior of the producer by means of the bell j, conducts f uel to the producer while in operation. The bell being tight against its seat, the lid can be removed and the hopper filled with fuel. After closing the hopper, the bell is allowed to drop, permitting the fuel to enter the firepot of the producer, where it descends to the grate and is consumed, the ashes and clinkers being removed through the door k.
5. The steam entering the producer is decomposed into oxygen and hydrogen while passing through the incandescent fuel. This oxygen, together with that which is in the air mixed with the steam, unites with the carbon of the fuel to form carbon dioxide and carbon monoxide. These gases mix with those produced from the fuel by the heat and pass upwards through the port I and the pipe e. The pipe e is provided with fittings having removable handhole covers, for the purpose of giving easy access in case it becomes necessary to clean the pipe. The gas is next forced through the water box m, where it is washed and most of the impurities removed; it then enters the scrubber n, which is filled with coke to within a few inches of the outlet pipe o near the top. As the gas rises in the scrubber, it is met by a descending shower of water distributed over the entire area by means of the sprinkler p. The water cools the gas and carries away some of the impurities, leaving a small portion deposited on the coke. The coke is placed in the scrubber with the larger pieces at the bottom, the sizes gradually diminishing toward the top, and need not be renewed for a period of from I to 2 years, according to the quality of the fuel used in the producer and the amount of tarry matter contained in the fuel. After the coke becomes clogged, the scrubber is emptied and fresh coke provided.
Any dust or other impurities that the gas may contain after leaving the scrubber is removed while passing through the purifier box q - sometimes called a sawdust purifier which consists of a square box with a removable top and contains a series of wooden gratings r, over which are spread layers of sawdust or similar material. The gas is now ready to be stored in a holder, not shown in the illustration, from, which it is supplied to the engine in the same way as illuminating gas, but of course in larger quantity, proportionate to its lower heating or calorific power.
6. The gas generated when the producer is first started is of very poor quality and unfit to be stored in the holder. It is therefore permitted to escape into the atmosphere through the smoke and waste-gas pipe s, until, by a test made at the tube t, the gas shows that it is of the proper quality, by burning with a bright blue flame. As soon as the quality has come up to the desired standard, the valve u in the waste pipe is closed, and the gas is allowed to pass on its way through the scrubber to the holder. The overflow from the water box m passes through the water seal v, which permits the water to flow to the sewer without allowing any gas to escape.
It will be seen, by an examination of Fig. 1, that all pipes between the producer and the holder are provided with fittings having handholes and covers, so that the pipes can easily be cleaned as occasion may require. It must be understood that, especially when using the poorer grades of coal, some of the impurities contained in the gas will adhere to the walls of the pipes, and in time sufficient quantities may ,accumulate to interfere with the free flow of the gas from the producer to the engine.
The pressure of the gas at the point where the injector b connects to the ash-pit of the producer is about 8 inches of water. This pressure gradually diminishes on account of the resistance that the gas encounters during its passage from the producer to the holder, Measured by a water gauge, the pressure in the pipe e between the producer and the water box is equal to about 6 inches; after leaving the, scrubber, the pressure is 4 inches, and before entering the holder it is 2 inches.
SUCTION GAS PRODUCERS
7. Comparison of Suction and Pressure Producers. A comparison of the pressure producer with the suction producer discloses the fact that the chemical changes brought about In both types are practically the same. The difference between the two types is therefore. not in the nature of the product, but in the manner in which the gas is transmitted from the gas apparatus to the engine. The processes of generating and purifying the gas are the same in both cases; but in the pressure producer a pressure above that of the atmosphere is maintained by a forced draft, either from a low- pressure steam boiler or from a blower; while in the suction producer the pressure in any part of the apparatus or its connections is never higher than that of the atmosphere. The draft in the suction producer is furnished. by the engine piston during the suction stroke while the inlet valves are open, and the vacuum created in the cylinder causes the gas from all parts of the producer apparatus to flow toward the engine.
The difference in pressure between the two systems is practically 8 inches of water; so that, in the suction producer, the pressure of the gas as it leaves the scrubber is about 6 inches of water below atmospheric pressure instead of 2 inches above, as in the case of the pressure producer. The relative difference in pressure in the various parts of the apparatus is the same in both systems, and the order of the operations of the process is necessarily alike in both cases. The suction type of producer does not require a large gas holder, a small cast-iron or sheet-metal tank being used instead. This tank is but slightly larger than the customary gas bag or pressure regulator used in connection with engines using illuminating gas.
8. Supply of Air and Moisture.-A suction gas producer of small capacity, in which the evaporator for supplying the necessary moisture to the air is mounted directly above the producer shell, is shown in Fig. 2. The apparatus consists of the producer a with a cast-iron shell; the hand operated blower b, for reviving the fire after a shut-down over night; the evaporator c; the hopper d; the water trap e; the water-seal box f; the scrubber g; and the gas tank or reservoir h. At each suction stroke of the engine, air is drawn into the top of the evaporator c, through the elbow i, which is open to the atmosphere. The evaporator is filled with water from a branch pipe taken from the main supply pipe and kept at a constant level by* an overflow pipe, not shown in the illustration, that carries any surplus supply to the, ash-pit j. The water in the evaporator is heated to about 170' F. by radiation from the burning fuel and by the hot gases that leave the producer through the port k.
The air passing over the surface of the hot water absorbs a quantity of vapor, the amount depending on the temperature of the water; so that the quantity of the water vapor admitted with the air through the pipe I to the space below the grate is greater when the fire is hot than when it is low. The fire is hottest, of course; when the engine is carrying a heavy load. Under heavy load, not only does the increase in the amount of vapor enrich the quality of the gas generated, but also the moistened air has a correspondingly greater cooling effect on the grate and tends to keep the fire at a proper degree of intensity.
9. After entering the ash-pit below the grate, the mixture of air and steam is drawn upwards through the hot bed of fuel, where the steam is decomposed into hydrogen and oxygen, and the formation of carbon monoxide takes place. After transferring a portion of its heat to the water in the evaporator, the gas leaves the producer through the port k, passes through the water trap e, and enters the scrubber at the bottom. The water trap has two pipe connections to the water-seal box f, the lower pipe being provided with a valve m. While the plant is in operation, this valve is open and the water that accumulates in the bottom of the scrubber flows through the lower connection to the seal box f and thence through an overflow funnel n to the sewer. When the plant is shut down, the valve m should be closed, thus causing the water in the trap e to rise well above the lower end of the partition wall o. This closes the gas connection between the producer and the engine. Any excess of water then flowing to the seal box passes through the upper pipe attached to the trap e and thence to the sewer.
10. Passage Of Gas Through Scrubber. While the gas is rising through the coarse coke in the scrubber g, it is met by a descending stream of cold water which is distributed evenly over the area of the scrubber by means of the sprinkler p attached to the top cover plate. In this manner, the gas, from which some of its impurities have been removed while passing through the trap e, is now cooled and washed sufficiently to be delivered to the gas tank h in such condition that it contains no tarry or dusty substances to interfere with the successful running of the engine. When semianthracite or similar fuels containing higher percentages of tarry matter than pure anthracite or charcoal are used, it is necessary to add a sawdust purifier similar to that used in connection with the pressure producer shown in Fig. 1.
11. Supplying the Fuel.-The fuel is supplied to the producer shown in Fig. 2 through the charging device mounted above the hopper d, which consists of the funnel q and a smooth hollow ball r that can be turned, on its ground seat by the hand lever s. The ball has an opening at the top, so that it may be filled with coal through the funnel, after which it is turned over by a quick movement of the hand lever, bringing the opening in the ball in communication with the coal space in the hopper d. As soon as the ball has thus been emptied of its contents, it is turned back and the operations of filling and emptying are repeated until the hopper is filled to the desired height. When not in use for filling the producer, the ball is held tightly on its seat with screws and hand nuts. The quick turning of the ball leaves but a small fraction of a second during which the hopper is open to the atmosphere, and practically no air is admitted to the producer at that point.
12. Removing Ashes and ClInkers.-The removal of clinkers that form in the fire space of the, producer is facilitated by poke holes, with which the hopper is provided, that permit the fire to be stirred from above with suitable poking rods. The clinkers descend to the grate and are removed through the two fire-doors t, t, on opposite sides of the cast iron shell of the producer, while the ashes accumulating in the pit below the grate are drawn out through the ashdoor u.
13. Starting the Producer.-The hand-operated blower b serves to supply the blast necessary to start up the fire after the plant has been shut down for any length of time, say over night. During such a temporary shut-down, the process of gas making is practically stopped, except for the small amount of gas generated by the natural draft caused by the flue pipe v being kept open to the atmosphere by opening the flue valve w. While reviving the fire, the valve w, as well as the valve x in the vent pipe y, is kept open until the gas escaping at the test tubes z, z (one of which is placed in the pipes between the producer and the scrubber, and the other near the inlet to the engine) is of such quality as to burn with a bright blue flame. As soon as this is the case, the valve w, and the valves in z and z are closed and the engine is started in the usual manner. To secure prompt starting, it is found advisable to keep the valve in the vent pipe y open to the atmosphere until a few explosions have taken place in the engine cylinder, and then close it.
14. A suction producer of larger capacity than the one shown in Fig. 2 and equipped with a separate evaporator is shown in Fig. 3. Instead of the cast-iron body shown in Fig. 2 in connection with the smaller type, the producer a Consists of a shell built of steel plate and lined with fire brick; but the hopper d and the coal-feeding device r are made of cast iron, and are essentially of the same construction as in the smaller producer. Instead of a hand blower,a belt-driven pressure blower b furnishes the draft for starting.
The essential difference between the larger and the smaller plant is that the evaporator for heating the water in the smaller plant forms a part of the generator, while in the large plant it is a separate piece of apparatus and is connected to the generator by pipes, In the case of the larger, the evaporator consists of a cylindrical casting c with a hood e having a vertical dividing wall in the center, so that the air entering through the pipe f will be forced over the surface of the hot water in the evaporator before it passes to the ash-pit of the producer through the pipe g. Between the evaporator cylinder c and the hood is clamped a plate h carrying a number of vertical tubes i that are kept full of water, the level of the water being kept constant by an overflow pipe j slightly above the upper surface of the plate h. The hot gases leave the producer through the pipe k, and pass downwards and then upwards in the evaporator, being guided by the vertical partition l, and finally pass on to the scrubber m. In this manner, the water in the tubes is kept at the desired temperature, so that the required amount of vapor is taken up by the air while passing through the hood e to the ash-pit of the producer.
In order to be able to control the amount of moist and dry air used, a regulating plate n is provided in the air pipe, by means of which the air-supply pipe can be opened to any desired extent to the atmosphere in the producer room, thus admitting cool air that has not come in contact with the hot water. The three-way, valve o serves to shut off the air connection to the evaporator when the fire is being revived by the blast from the blower. As soon as the gas has become of good quality, the blower is stopped and the three-way valve set so as to admit air in the regular way through the pipe g.
COMBINED PRODUCER AND EVAPORATOR
15. Another suction gas producer of somewhat different design is shown in Fig. 4. The producer itself consists of a cylindrical steel shell lined with firebrick and fitted with a shaking grate a operated by the hand lever b. The hopper c, through which fuel is supplied to the producer, is sealed by the charging device d, so that any fuel placed in it can be admitted to the hopper without permitting air to enter the producer. From the hopper, the fuel descends into the fire-space through the feeding tube e surrounded by the evaporator f, in which the necessary steam is generated at atmospheric pressure by the heat of the fire and of the gases when leaving the producer.
16. The ashes are removed from the ash-pit g through the door h. A series of poke holes i distributed over the top of the producer permits rods to be inserted for the purpose of poking the fire and removing clinkers from the walls of the lining. Dry air is admitted to the ash-pit through the supply pipe j, while moist air, which is saturated with steam from the evaporator, is supplied to the ash-pit through pipes k, air boxes l, and nipples m. The air enters the top of the evaporator through the valve n. The proportionate amounts of dry and moist air can be regulated as desired by opening or closing the valves n and o. The hand-operated blower p is used for starting or reviving the fire. Handholes q in the top of the evaporator are provided for the purpose of removing any sediment that may accumulate in the bottom of the evaporator. The water supply to the evaporator is automatically regulated by the float r that controls a valve in the water box s. The water rising in the box raises the float and closes the valve, while the lowering of the float opens the water valve. The gas passes from the producer to the scrubber through the pipe t, which is connected to the top of the producer.
17. An outside view of a producer of this type, connected to its scrubber, is shown in Fig. 5. The producer is shown at a and the scrubber at b. The hopper c with the filling device d is located over the producer and is readily accessible by the stairs and the platform around the top of the producer. The fittings e, e, admit the air to the evaporator, and the handles f are connected to the covers of the openings through which the fire is poked. The handle g is provided for the purpose of rocking the grate, and the door h gives access to the fire. The vent pipe is shown at i and the main gas pipe at j, connected to the scrubber at k, with the water trap I extending below the scrubber. There are a number of manholes m, m, on the side of the scrubber, to permit easy access to the interior. The water connections are shown at n, and the gas outlet from the scrubber at o. Producer plants of this style are made in units of from 15 to 250 horsepower, and are very compact and convenient.
18. A gas-producer plant in which the draft is furnished by an exhaust fan operated by a small motor, drawing the gas from one scrubber and forcing it through another into a gas holder, is shown in Fig. 6. This apparatus consists of twc similar generators a and b, an evaporator c, a wet scrubber d, an exhaust fan e, a dry scrubber f, and the gas holder g. The gas generators are of the down-draft type, which is considered especially adapted to the use of fuels containing tarry matter, such as bituminous coal, wood, etc. The gas and tarry substances produced by the fresh fuel in the upper portion of the producer, pass down through the incandes. cent fuel bed, where they are heated to a very high temperature, and a gas free from tar is thus formed.
19. The generators consist of cylindrical steel shells lined with firebrick and provided with firebrick arches h that support the fuel beds. Openings i, i', at the tops of the generators, serve for charging fuel, and for the admission of air, and the usual fire and ash doors are provided for cleaning the arches and for the removal of ashes. Steam jets j, j', one in each generator, are supplied from the boiler c. The boiler is of the vertical type, and is connected by brick-lined flues k, k', to the bottoms of the generators, the passages being controlled by water-cooled valves m, m'. The hot gases leave the generators at the bottom, pass through the evaporator, and impart a portion of their heat to the water contained in the space around the tubes. The steam produced is directed into the top of the fire by the jets j, j'. The hot gases pass up through the tubes to the outlet pipe.
20. The wet scrubber d, consisting of a cylindrical steel shell, contains a number of trays filled with coke moistened by the water sprays n and o. A purifier p filled with excelsior is attached to the top of the scrubber. The gas-inlet pipe I at the bottom of the scrubber is attached to a horizontal perforated diaphragm q submerged in water, so that the gas must pass through the water before rising in the scrubber.
21. The fan or exhauster e maintains the necessary vacuum required to furnish the proper amount of draft and give sufficient pressure to deliver the gas to the holder. The motor that drives the exhauster is connected to the gas holder in such a way that the speed is automatically regulated, by the movement of the holder, to conform to the demand for gas. When the holder is full, the speed of the exhauster is decreased; while in descending, as the gas is consumed, the motor speed is increased, creating a correspondingly stronger draft and a greater production of gas. The direction in which the gas flows after being delivered by the exhauster is controlled by the valves r and s. The valve r is connected to the waste-gas pipe and is kept open to the atmosphere while the fire is being started or revived. As soon as the gas becomes of the proper quality, the valve r is closed and the valve s opened, so that the gas can pass to the dry scrubber f and holder g.
22. The dry scrubber contains two trays t, t', filled with excelsior, sawdust, or shavings. A horizontal partition u divides the scrubber into an upper and a lower chamber. The pipe connections to these chambers are fitted with valves, so that either the upper or the lower chamber can be connected to or shut off from the gas supply, thus making it easy to remove the trays, for the purpose of cleaning and recharging, without interrupting the operation of the apparatus. From the dry scrubber, the gas passes to the gas holder g, which consists of a stationary water tank v, filled with water, and an inverted movable tank w, which fits inside the water tank. The gas enters the holder through the pipe x, whose upper end is slightly above the level of the water in the tank v.
As the amount of gas in the holder increases, the movable tank w rises, giving additional space for the gas between the water surface and the top of the tank w. When the volume of gas in the holder decreases, the tank w descends. The pressure of the gas in the holder is thus kept constant. The lower edge of the tank w is always submerged, forming a water seal that effectually prevents the escape of gas.
23. While the apparatus is in operation, the generators a and b are open at the top, so that the attendant can observe the condition of the fire and add fresh fuel where needed. The condition of the fire may be regulated by occasionally passing a jet of steam up through one and down through the other generator, by means of auxiliary pipes y, y', connected to the bottoms of the generators. The steam is introduced alternately in each generator; and the top door i and the valve in of one generator are closed and steam is blown into the ash-pit through the pipe y. This operation causes an up draft through one generator and a down draft through the other.
Should wood be used as fuel, the generators are filled with coke to a height of 3 or 4 feet above the arches h, and wood in lengths of 2 or 3 feet-or of ordinary cordwood size, 4 feet in length, if the generators are large-is placed on top of the coke. The wood is ignited, and the gas' is delivered to the scrubbers and holders in the usual manner. No steam is admitted at the top, however, as the wood usually contains a sufficient amount of moisture to render the gas of proper quality.
PREPARING PRODUCERS FOR OPERATION
24. Foundations for Producers.-The foundations for producers should be built in accordance with plans furnished by the makers. As a rule, it is necessary to set both the producer and the scrubber on slightly elevated platforms of brick or concrete, to raise the apparatus to a level where it will be easily accessible to the operator and to bring the various parts of the system in proper alinement, so that the pipes and fittings furnished by the maker will connect as intended. Special cases may occur where the conditions are such as to require some deviation from standard plans, and in such instances the manufacturer of the apparatus should be consulted and his recommendations and suggestions followed.
Upon the arrival of the machinery at the place where it is to be installed, it is well to examine all the parts for defects and to clean thoroughly all vessels, castings, tanks, etc., of any packing material, dirt, or sand left accidentally in the castings at the foundry. This suggestion applies to the various parts of the gas producer, as well as to the pipes and fittings.
LINING THE PRODUCER
25. Firebrick.-Where the firebrick lining consists of special shapes, as is the case in most suction producers as well as in small sizes of pressure producers, the bricks should be carefully examined and any that are damaged, broken, or cracked, rejected. As a rule, a few extra bricks of each size are furnished, to allow for possible shortage of material that may be caused by the accidental breaking of some of the bricks while in transit.
Before attempting to place the lining in the producer shell, it is advisable to set the lining up on a floor or any other level place outside the shell, and to make sure that the various bricks fit without leaving excessively large spaces or crevices. If necessary, the bricks should be ground to each other, so as to remove any irregularities in shape and to reduce crevices to not more than 1/8 inch at the joints. It is also necessary to see that the size of the lining is in accordance with the producer shell, that the circle formed by the bricks is not larger in diameter than the shell, and that, when making proper allowance for mortar the total height of the lining will be such as to bring it up to the desired level inside of the producer.
26. Mortar.-After leveling the producer on. its foundation, the laying of the firebrick lining may proceed. In preparing the mortar, care must be taken to use a grade of fireclay that will withstand the heat of the fire. As a rule, the manufacturer of the apparatus supplies the clay to cor. respond with the material used in making the bricks. The mortar is made of fireclay and water, and should be of about the consistency of the cement mortar used in laying bricks for foundations. It is of great importance to work the mortar thoroughly, so as to make it smooth and of uniform composition. There should not be more than a layer, of 1/8 inch in thickness between the various courses of bricks.
Any openings or fissures that show on the inner surface of the lining, and that are therefore exposed to the heat of the fire, must be filled with a smooth pulp made of fireclay, asbestos, and water, of about the consistency of ordinary putty. This pulp will withstand the action of the hot fire, while mortar made of fireclay and water alone would crumble and fall away in a short time. The pulp must be rammed tightly into all fissures, and the whole inside of the lining smoothed up if the irregular shape of the bricks requires it.
It is of the utmost importance to have the inner surface of the lining as smooth as possible, so as to prevent clinkers from adhering to the wall. It also prevents the poking tools from catching in the joints of the brickwork and damaging the lining, when trying to remove the clinkers.
27. Filling Between Lining and Shell.-The lining is usually insulated from the shell of the producer by having the space between the bricks and the metal filled with a suitable material. Sand has been used, and if of the proper grade it will answer the purpose very well. The best sand for this purpose is molders' sand that has been used in the foundry for making iron castings. A much better material, however, although slightly more expensive, is mineral wool, which can be obtained at low cost almost anywhere. Mineral wool is made by subjecting molten slag to a strong air blast the cooled product having a porous, fluffy appearance resembling cotton. Sand has the disadvantage of being liable to run out of any cracks that accidentally develop in the brick lining. This of course would necessitate taking enough of the producer apart to be able to replace the sand lost in this way. Mineral wool will stay in place as long as the lining lasts, and the freedom from danger of a shut-down, such as might occur where sand is used, will more than pay for the additional first cost of the mineral wool.
28. Before filling the space between the lining and the shell, all the fissures around the fire-doors and the annular space around the bricks should be filled first, with a pulp made of fireclay, asbestos, and water, the same as that used for smoothing up the inner surface of the lining. Next the space should be filled with this pulp to a depth of several inches and then the mineral wool used up to within 2 or 3 inches of the top of the lining. The remainder of the space is then filled with pulp like that used in the bottom. This makes the whole space tight against leakage and keeps the insulating material in place, as the pulp will become hard after the fire is started in the producer. When putting in the mineral wool, it should be packed tight with a suitable tool as soon as a small quantity has been applied, and the ramming should be continued until the desired space is filled, so as to form a homogeneous mass of insulating material, After the lining and filling are completed, the top of the producer may be put in place.
FILLING THE SCRUBBER
29. After the scrubber has been placed in position, leveled up, and properly alined with the producer, the coke that is generally used as a purifying agent should be placed in the scrubber. In doing this, care should be taken not to break or grind the coke, and thus make dust and small pieces that will pack the coke tight and interfere with the flow of the gas through the scrubber. When the scrubber is to be entirely filled, the pieces of coke should be selected carefully as to size and the larger pieces placed in the bottom, the size gradually diminishing toward the top. The lower portion of the scrubber may contain pieces of about 4 inches in size, while nothing smaller than 1 1/2 inches should be. used at the top. To avoid breaking the coke in handling, it should be let down into the scrubber by means of a basket, a second rope being fastened to the bottom of the basket, so that it can be tilted and emptied when it has reached the bottom. Another equally good method is for a man to stand on a board in the bottom of the scrubber and distribute the coke after it has been lowered into it. The contents of the scrubber should reach up to within about 6 inches of the lower edge of the gas-outlet pipe connected at the top.
Any coke that may accidentally fall through the scrubber grate should be removed from the space below the grate before the scrubber doors are finally closed. If this is not attended to, some of the small particles of coke may be washed into the pipe connections and cause trouble by clogging them.
30. In making the pipe connections between the various parts of the apparatus, sharp bends should always be avoided, as they produce unnecessary friction and thus retard the flow of the gas in the pipes. Retarded flow is especially objectionable in connection with suction gas producers, and it is of considerable importance to provide long-sweep elbows rather than the ordinary cast-iron fittings. As producer gas always contains some impurities before it passes through the scrubber, it becomes necessary to clean the connecting pipes and fittings regularly. After leaving the scrubber, the gas may still contain a small amount of dust or tarry matter that will accumulate in the pipe connections. To enable the pipes to be cleaned without taking them apart, the fittings should be provided with handholes and removable covers, for the purpose of making their interiors accessible.
31. The flue valve in the waste pipe that branches off from the connection between the producer and scrubber is more liable to become clogged by impurities than any valve beyond the scrubber. This valve must therefore be arranged, so that it can be easily taken apart to be cleaned and lubricated. It is desirable to provide a drip pipe and valve below the flue valve, for draining any water that may collect in the smoke pipe either from the atmosphere or by condensation.
32. In order to have the smoke pipe constructed so as to give a good draft, which is essential in keeping the fire alive over night when the plant is shut down, it should be run in the shortest and most direct way possible. The general arrangement of the smoke pipe is of course governed by local conditions, but it should not have any sharp turns nor run horizontally for any length. If it is necessary to have a short length of horizontal pipe before the stack turns vertically, there should be a drain provided at the bottom of the elbow where the turn is made. The vertical length of the smoke pipe must be sufficient to insure a strong draft, and if there are any buildings in the vicinity the top of the pipe should be carried several feet above the top of such buildings. If this is not done, the gases that will escape from the stack while the fire is being started might cause annoyance to tenants of such buildings.
If the smoke pipe is led into an old chimney that has been used before, it should be carried up through the entire length until it reaches the open air. This is of special importance if any stoves are connected to the same chimney, because, if a fire was lighted in one of the stoves, gas issuing from the smoke pipe into the chimney might be ignited and result in a violent explosion.
TESTING FOR LEAKS
33. Whether the producer is of the pressure or of the suction type, it is equally important that the apparatus itself as well as all pipe connections be made absolutely tight. Neglect in this respect would cause leakage of gas in the pressure producer and result in danger to the health and life of persons in the producer room. While this danger does not exist in the suction producer, owing to the fact that the pressure in this type of apparatus is always below that of the atmosphere, small leaks would cause air to be drawn into the apparatus from the outside and result in weakening the gas and in rendering it of such quality as to prevent good results from its use in the engine. If the leak is very serious, the gas would become so poor as to cut down the power considerably and eventually stop the engine.
34. Before attempting to make gas, all the joints and connections should be tested. A safe method of doing this is to generate pressure in the apparatus by closing the valves and operating the blower provided for reviving the fire. By attaching a small pressure gauge at a convenient point before the pressure is raised, and letting the apparatus stand for a while afterwards it can be determined whether there are any leaks, If the gauge shows a fall in pressure, it is necessary to investigate and locate the place at which the leak occurs. Each part of the apparatus can be shut off from the others, by means of the valves provided, and the point of leakage can thus be accurately determined. When the leak is located, it should be stopped.
The parts most likely to become leaky are the coal-charging device and the fire and ash-doors. In handling the fuel and the ashes, it is almost impossible to prevent impurities from settling upon the surfaces of the doors and charging apparatus. It is therefore advisable to always clean these surfaces after fuel has been admitted or ashes or clinkers have been removed.
OPERATION OF SUCTION PRODUCERS
STARTING THE PRODUCER
35. After it has been ascertained that everything about the apparatus is in good working order in accordance with the directions, the producer is ready to be put in operation. To start the fire, the generator should be filled, to a height of about 18 inches above the grate, with dry, non-resinous wood, or with charcoal. A small quantity of cotton waste soaked in oil and placed upon the grate under the wood will aid in starting the fire. If fat pine-sometimes called pitch pine, on account of the amount of pitch it contains-or a similar wood is used to ignite the coal, a smaller quantity will be sufficient. In case the wood contains much pitch, no gas should be permitted to pass into the scrubber until the wood has been entirely consumed.
36. Before lighting the fire, the evaporator should be filled, and a small amount of water allowed to overflow into the ash-pit. The water-seal box should also be filled, and the water supply turned on in the scrubber as soon as the fire is started. The valve in the smoke pipe must be opened and the top of the hopper closed before lighting the fire. After igniting the wood, the ash-doors, fire-doors, and the pipe supplying moist air from the evaporator to the bottom of the producer must be closed. The connection between the blower and the producer is then opened, and the blower started, turning it either by hand or by power, as the case may be, until the wood is burning freely. Follow this by filling in about 8 to 12 inches of coal and continue blowing for a while until the fire is burning brightly. After this, the producer and hopper should be practically filled to the top with coal. Continue the operation of the blower until the gas at the test pipe between the producer and the scrubber burns steadily with a bright blue flame. Then close the communication between the blower and the producer, and quickly remove any ashes or clinkers that may have been deposited upon the grate. While doing so, the fire-doors through which these ashes are removed should be kept open no longer than is absolutely necessary.
37. Now reestablish communication between the blower and the producer and again operate the blower for a short time until the gas, by burning steadily with a blue flame, proves that it is of the proper quality. As soon as this is the case, all the apparatus, including the pipe connections between the scrubber and the engine, should be filled with gas, thus replacing the air with which they were previously filled. This is accomplished by closing the flue valve and also the vent pipe that branches off from the gas-supply pipe near the engine. The vent pipe is provided for the purpose of making sure that the whole pipe system up to the engine is filled with gas of good quality.
It will generally require from 10 to 15 minutes from the time of starting the fire until all the apparatus is filled with gas. There should also be a test pipe provided in the gas-supply pipe near the throttle valve on the engine. As soon as a trial at this point shows the gas to be of good quality, the, plant is ready for operation and the engine can be started in the usual way.
FIRING THE PRODUCER
38. In order to secure steady and efficient service of the plant, it is necessary for the operator to accustom himself to performing the series of operations carefully and always in the same regular rotation. Experience has shown that the following method of procedure gives the best results: If the fire requires looking after, the first thing to do is to fill in fresh fuel practically up to the top of the hopper, so as to replace any coal that has been consumed during the run. The second operation should be the poking from the top. This is done for the purpose of removing any clinkers that may have begun to adhere to the walls of the brick lining, and also for the purpose of preventing the formation of hollow spaces in the hot bed of fuel known as bridging.
39. The fire should be poked at regular intervals, as determined by the quality of the fuel used and the experience the operator may gain while running the producer under the conditions of load in each particular case. It will not do to neglect removing the clinkers, because, if they should be allowed to accumulate on the walls of the brick lining in any considerable quantity, it would be impossible to remove them while the apparatus is in operation, and consequently it would be necessary to shut down the plant temporarily and interrupt the service.
40. The third operation should be the removal of the ashes from the ash-pit under the grate. This is generally done with a bent scraper. The fourth and last operation consists of poking and removing clinkers from the grate through the fire-doors. With a stationary grate, a bent poker is used for this purpose, after the clinkers have been loosened with a straight bar of suitable shape and length. This removal of clinkers through the fire-door should be done quickly; in order to prevent an excessive amount of air from entering the producer, open one door at a time just enough to permit of the removal of clinkers. If the producer is provided with two doors on opposite sides, close one door while the other is kept open. The whole operation of removing clinkers from the grate should not require more than 20 to 30 seconds.
These operations apply, of course, only to stationary grates. In producers provided with shaking or rotating grates, the cleaning is done by rocking or turning them by means of the hand levers or cranks provided for this purpose.
STOPPING THE PRODUCER PLANT
41. The engine is stopped as usual by simply closing the gas valve and disconnecting the battery. At the same time, in order to stop the producer plant in the proper manner, the valve in the vent pipe must be opened at once, so as to provide an escape for the gases that continue to form in the producer for a short time after the engine has been stopped. Next, the hopper of the producer should be filled with fuel and the flue valve in the smoke pipe opened. As soon as this valve is opened, the valve in the vent pipe near the engine can be closed. The water supply to the scrubber and producer should then be shut off and the valves adjusted that regulate the level of the water in the seal and water trap between the scrubber and producer, so that the gas will be shut off from the scrubber. Experience will show just how far to open the air supply that regulates the draft necessary to keep the fire alive over night without unnecessary waste of fuel while the plant is shut down.
The ashes and clinkers should be removed from the producer, and the fire and ash-doors kept closed. Should it become necessary to remove large quantities of clinkers, it will be found easier to do this immediately after stopping the plant and while the fuel is still incandescent. It is best, in such cases, to draw the fire completely and to remove the clinkers from above after opening the cover of the hopper.
RESTARTING THE PRODUCER
42. To start the plant after it has been shut down over night, it is necessary only to remove from the grate any ashes or clinkers that may be deposited during the night, and to operate the blower until the gas burns with a bright blue flame at the test tube between the scrubber and the engine. Then open the vent and the scrubber valves, see that the hopper is closed tightly, and start the engine in the usual way.
CLEANING THE PIPE CONNECTIONS
43. It is always advisable to attend to the cleaning of pipes and fittings in the day time, so that it will not be necessary to use a light, as a flame brought too close to the apparatus might ignite the gas. It is also advisable, as a matter of precaution, to have more than one person present while the cleaning is being done, so as to guard against accidents.
The building or room in which the producer is located should be well provided with ventilators, so that any escaping gas will be quickly carried away. The gas is very poisonous, and, if it accumulates, is liable to render the workmen unconscious and may cause death. Hence special care should be taken to avoid breathing it. Under ordinary conditions it will be found sufficient to have the pipes examined and cleaned once in 3 months.
44. The contents of the scrubber may last for a year or more before they require renewing. If it becomes necessary to clean the scrubber, the whole producer must, of course, be put out of commission. The manholes of the scrubber should first be opened, so that any gas contained in the scrubber may escape. It may require about 1 hour or more for the gas to stop, after which the coke may be removed. Any sediment that may accumulate in the bottom of the water-seal box, at the bottom of the scrubber, should be cleaned out at least once every other day.
OPERATION OF PRESSURE PRODUCERS
45. The directions already given for the care of producers apply especially to suction producers, but they are almost equally applicable to pressure producers, especially in regard to the firebrick lining, pipe connections, etc. But the arrangement of the fuel bed is different in the pressure type from that used in the suction producer. Instead of having on top of the incandescent fuel a large amount of coal that is not burning, the height of the fuel bed is limited to from 2 1/2 to 3 feet above the ashes when using anthracite, and from 3 1/2 to 4 1/2 feet when using bituminous coal. This will require a pressure for the air blast of from 3 to 4 inches of water.
If the blast is too strong or the coal too fine, the fuel will burn too fast near the walls, and it will be necessary either to reduce the blast or to use a coarser grade of coal. To keep the fuel bed reasonably solid and avoid the formation of bridges or honey combing, a certain amount of poking, or barring, must be done, the frequency of which depends on the character of the fuel or the rate at which the producer is working. A little experience and careful observation will enable the operator to determine just how often the fire needs attention, so as to keep it in the best condition for steady service.
When stopping a pressure producer, no unburnt coal should be left on top of the fuel bed; the top layer should be incandescent. The blast should be decreased just before stopping, the poke-hole caps removed, and the escaping gas lighted at the open holes. Then the blast may be shut off entirely. Air will be drawn into the producer by the receding flame, so that the gas in the producer will burn quietly without any violent puff.
BLAST-FURNACE GAS FOR GAS ENGINES
QUALITY OF GAS FROM BLAST FURNACES
46. The use of blast-furnace gas for gas engines is of recent origin, and cannot yet be said to have passed much beyond the experimental stage. The blast furnace is used for melting iron ores and producing pig iron. The furnace varies from 40 to 100 feet in height, and from 12 to 25 feet in diameter. The fuel employed is coke, and the air blast used to promote combustion produces a temperature sufficiently high to, melt the ore, and has a pressure of from 5 to 15 pounds per square inch above that of the atmosphere. The amount of gas that passes from the blast furnace is about 150,000 cubic feet per ton of pig iron produced. In order that the iron may not combine with oxygen passing through the furnace, the amount of air admitted is insuffi. cient to complete the combustion of the fuel, and hence the gas passing out of the furnace contains a large amount of carbon monoxide. Blast-furnace gas, however, is not as rich in combustible matter as is producer gas, but it contains enough combustible matter to furnish considerable power when used in gas engines of suitable design. The average composition of blast-furnace gas is about as follows:GAS ....................... Per Cent
Carbon dioxide, CO2 .... 08
Carbon monoxide, CO .. 30
Hydrogen, H ................... 02
Nitrogen, N ..................... 60
Total .............................. 100
There is usually present some hydrocarbon that affects these percentages to a slight extent, The thermal value of blast-furnace gas varies from about 90 to 100 British thermal units per cubic foot, depending on the percentage of carbon monoxide present. The fact that the gas is low in hydrogen and rather high in carbon monoxide makes it desirable for gas engines especially designed for its use. It has been found, in practice, that the gas from the blast furnace will furnish about 50 horsepower continuously for each ton of pig iron produced in 24 hours.
47. One of the principal difficulties to be contended with in connection with the use of blast- furnace gas in gas engines is the large amount of gritty dust that the gas contains. This necessitates very careful and thorough cleaning of the gas before it is admitted tothe engine cylinder, The gas should be as nearly free from solid matter as it is possible to make it by any cleaning method now in use. When the gas comes from the blast furnace, it usually contains from 4 to 7 grains, and may contain as much as 12 grains of dust per cubic foot; its temperature is also high, ranging from about 500 to 1,000 degrees F. or more. The amount of dust contained has been reduced by some of the best modern cleaning processes to as low as .01 grain per cubic foot, and even less, which is said to be less than the dust contained in ordinary air.
When the gas from the blast furnace is not sufficiently cleaned before it goes into the gas engine, the dust collects on the inner surface of the cylinder, and, as the piston moves to and fro, the dust is ground between the piston and cylinder, thus causing excessive heating and perhaps cutting of the surfaces. Sometimes, the dust collects in the combustion chamber or valves, becomes incandescent from the heat of the explosions, and causes preignition.
48. To get the greatest efficiency from the combustion of the gas, it should be cool and dry as well as clean. The high temperature of the gas causes it to evaporate some of the water used in the cleaning process and to carry with it a large percentage of moisture. This moisture is detrimental to the combustion, but is a great aid in getting rid of the dust. The particles of dust are moistened, by it, and adhere more readily to any surface with which they come in contact. But the moisture must be removed before the gas enters the engine. This is done by cooling the gas, thus causing the moisture to condense. As it condenses, it falls by gravity to the bottom of the apparatus, carrying with it a considerable amount of dust.
The gas is forced through the cleaning apparatus by a steam jet or by some form of fan or blower.
CLEANING BLAST-FURNACE GAS
49. In the earliest blast-furnace gas-engine plants, the gas was cleaned by about the same process used in a producer plant. It was found, however, that this process did not clean blast-furnace gas sufficiently for use in gas engines. Hence the cleaning process was extended by adding scrubbers and rotary washers to the apparatus already in use. In some cases, the rotary washer was simply a fan with provision, for spraying water into the gas; while in others, it took the form of an enclosed rotating drum or series of disks partly submerged in water. The gas being forced through the washer came in contact with the large wetted surface to which the particles of dust adhered, and as the surface revolved into the water the dust was washed away. A still later development is represented by the centrifugal cleaner, in which the gas is carried around inside of a casing by a revolving drum with projecting vanes, the speed being such that the centrifugal force throws the dust against the casing, from which it is washed by a spray of water.
50. Cleaning Plant With Rotary Washer.-The gas is carried in flues or pipes from the blast furnace to the cleaning apparatus and engines. It is taken from the flue leading downwards from the top of the furnace (known as the down-comer) and conducted to a main, which may also receive the gas from other furnaces. It then passes through a washing process that takes out the larger particles of dust and grit. Next, the gas goes through a long pipe or series of pipes that reduce its temperature and take out considerable dust and moisture. In the first washing process, the gas takes up considerable moisture, and the dust, becoming moist, adheres readily to the surfaces with which it comes in contact. From the long pipes, the gas may pass through a rotary washer consisting of a fan or a drum with vanes on its circumference with a spray of water injected into it. The whirling motion given the gas as it passes through the fan throws the dust against the casing, to which it adheres and from which it is washed into a water outlet by the injection water. From the fan or rotary washer, the gas may pass through other scrubbers or washers similar to the coke scrubber or sawdust cleaner described in connection with the producer-gas plant. Sometimes, two scrubbers or cleaners are used, in which case the gas passes first through a coke scrubber and then through a sawdust cleaner for removing the finer particles of dust. From the last cleaning process, the gas is taken in many plants to a gas holder of considerable size, where the remaining moisture in the form of vapor is condensed by cooling and settles with the remaining dust.
From the gas holder, the gas is conducted directly to the engine cylinder. The gas holder serves the double purpose of a cleaner and a regulator or equalizer of the pressure of the gas delivered to the engine. The pressure of the gas in the holder will usually vary from 1 to 2 or more ounces per square inch above atmospheric pressure.
When the rotary washers are properly designed and installed, the scrubbers between the cleaner and gas holder and the long pipes may be dispensed with, thus reducing the size, first cost, and operating expenses of the cleaner plant. Furthermore, the gas is cleaned much better than in the cleaners used on producer-gas plants.
51. Centrifugal Cleaning Plant.-In the cleaning plants that have given the best results with European blast-furnace gas, a rotary or a centrifugal washer forms the principal part. In some plants, large, slowly turning washers are used; while in others, small and rapidly turning centrifugal washers are employed. In the first water, the cleaning is done by bringing the gas in contact with a long revolving surface that dips into the water at every revolution, thus washing off the collected dirt and wetting the surface. In the second, the gas is driven by centrifugal force against the sides of the casing and the dirt washed out with a spray of water.
A centrifugal plant for cleaning blast-furnace gas is shown in Fig. 7. Before the gas reaches the centrifugal cleaner, it is passed through a dry stationary dust catcher, or cleaner, where larger particles of dust are removed. The stationary cleaner may be simply a large closed drum in which the velocity is low and the dust is allowed to settle out by gravity; or it may be a smaller drum and contain deflectors for changing the direction of motion of the current of gas. From the dust catcher, the gas passes down through the pipe a to the centrifugal cleaner b, The internal drum of the centrifugal cleaner revolves at about 850 revolutions per minute, thus producing sufficient velocity in the gas to cause the particles of dust and dirt to fly outwards against the casing, under the influence of the centrifugal force. The gas usually carries from I to 3 grains of dust per cubic foot as it leaves the dry cleaner, and the centrifugal cleaner reduces this amount to about .01 grain per cubic foot before the gas leaves. The moisture in the gas, however, is increased, and for this reason the use of a dryer or dry filter is found advisable. The centrifugal cleaner b is driven by a motor c, the gas leaves by the outlet d, and passes around the baffle plate in e, where some of the moisture is precipitated. The gas then passes through the pipe f and valve g to the main h, from which it can be drawn off at different points, as desired. The valve i opens to the dry filter j, which contains trays carrying slats covered with mineral wool or other drying substance, through which the gas passes in the direction shown by the arrows. The dry filter is divided into two compartments by the partition k, the gas passing through one tray of mineral wool in each compartment, leaving it comparatively dry. The valve I regulates the opening to the main m, which conducts the gas to a holder or directly to the gas engines.
52. The Centrifugal Cleaner.-A larger view of the centrifugal cleaner is shown in Fig. 8. The blast-furnace gas enters the cleaner through the opening shown at a. The casing b remains stationary, while the drum c is keyed to the shaft d, with which it revolves. The small vanes e are arranged in the form of a spiral around the drum c, so that the gas must pass through a long passage or be thrown against the outer casing as the drum revolves. The gas ]caves the washer through the outlet f, and passes to a dry filter, as previously described. Water is sprayed against the revolving drum through the inlets g, and is carried around by the vanes of the drum with sufficient speed to keep the casing wet and furnish water for washing off the dust as fast as it collects. The dirty water passes out through the water leg h to a drain.
The gas that enters the cleaner is still hot, sometimes having a temperature as high as 300 degrees or even 400 degrees F. It consequently vaporizes some of the water when it first comes in contact with the wet surfaces of the centrifugal cleaner. This vapor mingles with the dust in the gas, and the centrifugal force throws the vapor and dust outwards, causing them to come in contact with the outer casing, to which they adhere. A wire screen is located inside the casing where the injection water enters, so that the water is at once broken up into fine particles, offering more surface to the dust and being more easily vaporized. The temperature of the gas is also reduced by the water, the amount varying from 60 to 260 degrees F., depending on the temperature of the blast-furnace gas, the temperature of the water, and the efficiency of the cleaner. It is customary to keep the water stored in elevated tanks, and feed it to the washers by gravity and to discharge it into clearing ponds. The dust and dirt are allowed to settle out of the water, which is then pumped through cooling coils. back to an elevated tank, from which it again flows to the cleaner by gravity. The water is thus cooled and cleaned, and hence can be used over and over again with the addition of a small amount of fresh water.
Reprinted from the original and modified by Harry Matthews © 2001.
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