Coal Firing Methods
The fuel for a coal-fired furnace or boiler may be fed either automatically or by hand. Both methods have certain advantages and disadvantages. For example, automatic (stoker) firing is initially more expensive because it requires the purchase and installation of a suitable mechanical stoker to feed the coal to the furnace or boiler. As a result, stoker firing is a common practice in larger buildings (e. g., stores, hotels) where the initial high cost of the equipment can be more easily absorbed into the total cost of the structure. Despite the relative high cost of stokers, there are some designed for use in single-family residences. These will be considered in detail at a later point in this chapter.
Because there is no need to invest in special and expensive coal — handling equipment, hand-firing the coal has been the traditional method used for firing house heating furnaces and boilers. Although hand-firing coal is less expensive than stoker-firing for these smaller installations, the following objections to the hand — firing method should be noted:
The frequent opening of the furnace or boiler doors allows a large excess of air to enter and chill the flame. The combustion efficiency of the flame therefore tends to fluctuate.
The dumping of a lot of fuel at each firing results in a smoke period until normal combustion conditions are restored. Hand-firing coal is by its nature an intermittent firing method. The flame often reaches a low and inefficient level or is extinguished before new fuel is added.
The amount of draft required for proper combustion is an important consideration, and it depends on a number of different considerations, including the following:
Grate area Fuel size and type Fuel bed thickness Boiler pass resistance
The degree of resistance offered by the boiler passes to the flow of the gases is an important consideration in determining the required amount of draft. These gases must exist at a speed sufficient to prevent them from backing up into the combustion chamber and robbing the fire of necessary oxygen.
The total area of the grate, the type of coal burned (e. g., bituminous, semibituminous), the size of the coal, and the thickness of the coal bed all affect the amount of draft required for proper combustion.
Insufficient draft usually results in the accumulation of excess ashes in the ashpit. Moreover, it necessitates additional attention to the fire including more frequent cleaning. These and other aspects of improper firing contribute to fuel waste and higher operating costs.
Anthracite coal is preferred over other coal for domestic heating purposes because it produces a steadier and cleaner flame. Furthermore, it burns longer and with greater heat than the others. The principal objections to using anthracite coal are that it requires more heat than other coals to start combustion and is slightly more expensive.
Anthracite coal is available in a number of different standardized sizes, each suited to a different size of grate and firepot. The following are some of these coal sizes and their descriptive names:
* Egg Stove Chestnut Pea
Buckwheat-size coal is available in five grades or sizes: Buckwheat No. 1; Buckwheat No. 2, or Rice size; Buckwheat No. 3, or Barley size; Buckwheat No. 4; and Buckwheat No. 5. Buckwheat Anthracite No. 2 (or Rice size) finds the widest use in automatic coal-firing equipment and is used in domestic, commercial, and industrial stokers.
There are certain recommended practices to be followed when firing buckwheat coal. For best results, the following techniques should be employed:
1. Always maintain a uniform low fire. This reduces clinker formation to a minimum and enables those clinkers that do form to be broken up more easily.
2. Use a smaller mesh grate when possible or a domestic stoker. Because of its relatively small size, buckwheat anthracite coal often falls through the openings on ordinary grates when they are shaken.
3. Immediately after coaling, push a poker down through the fresh bed of buckwheat anthracite coal and expose a portion of the hot fire. This tends to prevent delayed ignition and such undesirable accompanying side effects as furnace or boiler doors being blown open.
4. Keep the heating system warm at all times. Allowing it to cool down and then having to warm it up results in burning fuel at a higher rate.
Egg-size coal should be used in large firepots (24-inch grates or larger). This is a deep-firing coal, and the most suitable results are obtained with fuel beds that are at least 16 inches deep.
Stove-size coal was extensively used in heating buildings, although today it has been largely replaced by gas or oil. It is used on grates that are 16 inches or larger. The fuel bed should be at least 12 inches deep.
Chestnut-size coal is used for firepots as large as 20 inches in diameter. Fuel beds for this anthracite coal range in depth from 10 to 15 inches.
With careful firing, pea-size coal can be burned on standard grates. Care should be taken not to overshake the grates (shake only until the first bright coals begin to fall through the grates). After a pea-coal fire has been built, the thickness of the fuel bed should be increased by the addition of small charges until it is at least level with the sill of the fire door. A common method of firing pea coal consists of drawing the red coals toward the front of the firebox and piling fresh fuel toward the back of the firebox.
A strong draft is required when burning pea coal. Figure 3-1 illustrates a satisfactory method of burning this coal size in a boiler. The choke damper is kept open and regulated by means of the cold air check and air inlet dampers. In stoker firing, the air setting is generally kept lower for pea coal because an excess of air under this kind of coal will burn up the retort. Forced draft and small mesh grates are frequently used for burning buckwheat anthracite coal.
PRESSURE REGULATING WEIGHT
Bituminous coal is a broad category encompassing many different burning characteristics and properties. Generally speaking, bituminous coals ignite and burn easily with a relatively long flame. They are also characterized by excess smoke and soot when improperly fired.
The side bank method is commonly recommended for firing bituminous coal. It consists of moving the live coals to one side or the other of the grate and placing a fresh fuel charge on the opposite side. Variations of this firing method call for placing the live coals at the back of the grate or covering the fresh fuel charge with a layer of fine coal. The side bank method results in a slower and more uniform release of volatile gases.
Other recommendations that should be followed when firing bituminous coal include the following:
Fire bituminous coal in small quantities at short intervals. This results in a better combustion because the fuel supply is maintained more nearly proportional to the air supply.
Never fire bituminous coal over the entire fuel bed at one time. A portion of the glowing fuel should always be left exposed to ignite the gases leaving the fresh fuel charge.
Use a stoking bar to break up a fresh charge of coking coal approximately 20 minutes to 1 hour after firing.
Do not bring the stoking bar up to the surface of the fuel. Doing so will bring ash into the high-temperature zone at the top of the fire, where it will melt and form clinkers.
A stoking bar should always be kept as near the grate as possible and should be raised only enough to break up the fuel. The ash will usually be dislodged when stoking, making it unnecessary to shake the grates.
Alternate or checker firing is a bituminous coal firing method in which the fuel is fired alternately on separate sides of the grate. This method tends to decrease the amount of smoke and maintain a higher furnace or boiler temperature.
A similar effect is produced by the coking method of firing bituminous coal. The coal is first fired close to the firing door, and the coke is moved back into the furnace just before firing again.
Semibituminous coal burns with far less smoke than the bituminous type. It ignites with more difficulty than bituminous coal but produces far less smoke.
The central cone method is recommended for firing semibitumi — nous coal. In this method, the coal is heaped onto the center of the bed, forming a cone, the top of which should be level with the middle of the firing door. This allows the larger lumps to fall to the sides and the fine cones to remain in the center and be coked.
The poking should be limited to breaking down the coke without stirring, and gently rocking the grates. It is recommended that the slides in the firing door be kept closed, as the thinner fuel bed around the sides allows enough air to get through.
A stoker is a mechanical device designed and constructed to automatically feed fuel to a furnace. Stokers are used in commercial, industrial, and domestic heating systems. Their use results in more efficient combustion owing to constant instead of intermittent firing.
According to the ASHRAE Guide (1960), coal stokers can be divided on the basis of their coal-burning capacity into the following four classes:
• Class 1 stokers (10 to 100 lbs per hour)
• Class 2 stokers (100 to 300 lbs per hour)
• Class 3 stokers (300 to 1200 lbs per hour)
• Class 4 stokers (over 1200 lbs per hour)
Class 1 stokers are used most commonly in domestic heating installations. The other three classes of stokers are used in commercial and industrial heating systems.
Class 1 stokers are usually the underfeed type and are designed to burn anthracite, bituminous, semibituminous, and lignite coal, and coke. Ash can be removed automatically or manually, with the latter method being the most popular.
Stokers can also be classified on the basis of whether the coal is stored in a hopper or bin. The disadvantage of the hopper design (see Figure 3-2) is that it must be refilled at least once each day. The bin stoker design (see Figure 3-3) eliminates coal handling. The coal is delivered by the supplier and placed directly into the bin.
The underfeed stoker (see Figure 3-4) is generally used for house heating furnaces and boilers. This type of stoker is one in which the
Figure 3-2 Hopper-fed conical grate. Coal is underfed into the furnace and overfed to the fire in a slow movement. (Courtesy Drawz Stoker Mfg. Co.)
Figure 3-3 Bin-fed stoker equipped with conical grate.
(Courtesy Drawz Stoker Mfg. Co.)
TO 117-VOLT SUPPLY LINK
Fuel is fed upward from underneath the furnace or boiler. The action of a screw or worm carries the fuel back through a retort from which it passes upward as the fuel above is being consumed. The ash is generally deposited on dead plates on either side of the retort, from which it can be removed.
Underfeed stokers can be designed for use with either anthracite or bituminous coal, but the individual pieces of coal should be uniform in size and no larger than 1 inch in diameter. As mentioned elsewhere in this chapter, it is desirable to treat the coal with oil in order to eliminate dust. The worm feed mechanism can be regulated to feed coal at variable rates.
Although there are variations in the type and design of domestic stokers, the general features are much the same. An elementary stoker is shown in Figures 3-5 and 3-6, which gives the essentials and the names of parts. These parts may be listed as follows:
5. Air duct.
COAL FEED TUBE
FEED WORM TRANSMISSION
AUTOMATIC AIR CONTROL
Figure 3-6 Domestic underfeed stoker.
8. Feed worm.
The retort is a firepot cast in a round or rectangular troughlike shape in which the coal is burned. It is made of cast iron and is surrounded by the windbox. The retort is provided with a number of air ports, or tuyeres, through which air for combustion is supplied.
The purpose of the fan is to supply forced draft, which is directed to the windbox that surrounds the air ports in the retort. This fan is commonly of the squirrel-cage type.
The air enters the retort through the ports via the air duct from the fan and the windbox that surrounds the retort. The fan is equipped with either manual or automatic control in the form of a damper at either the discharge or intake end. The air supply is controlled by means of these fan controls.
Coal stokers are designed to operate on either high — or low-air — pressure systems. In the high-air-pressure system, the air is forced in small jets into the fire area. A major disadvantage of this type of system is that the coal sometimes tends to fuse, causing clinkers to form and wasting some of the combustible matter of the fuel. A low-air-pressure system tends to produce a more complete circulation of burning gases to all heat-inducting surfaces.
A stoker is usually powered with an electric motor, which operates both the coal feed worm and the fan. The stoker drive consists of a transmission, a shear pin (or clutch throw-out), pulleys, belts, and related components. The purpose of the shear pin is to protect the driving mechanism against damage in case large foreign objects get mixed up with the coal.
The transmission rotates the coal feed worm at the proper speed to feed the amount of coal required. The construction is such that the rate of feed can be changed as desired.
The two kinds of transmission usually employed in stokers are the continuous drive, which is operated by means of reduction gears, and the intermittent drive, which operates with a ratchet. Another drive used in stoker transmissions is the hydraulic (usually referred to as an oil drive), which operates by regulating the oil pressure on the driving mechanism to control the number of revolutions the feed screw makes per minute.
The feed worm (sometimes called the feed screw) carries the coal from the hopper to the retort (firepot). It is geared to the transmission, its rate of revolution depending on the desired feed rate. The feed worm extends from the coal supply in the hopper or bin, through the coal feed tube into the retort, where the coal it carries is discharged.
Ashpits can be constructed so that they are located directly below the furnace or boiler (see Figure 3-7). The ashes are automatically deposited into the pit as the coal is burned. If the pit is designed large enough, the ashes will need to be removed only once or twice a year. It is recommended that the ashpit be constructed so as to permit removal of ashes from outside the house. This will result in a much cleaner and more convenient operation in the long run. Always vent the ashpit to the chimney or outdoors.
Some stokers (e. g., the Drawz stoker illustrated in Figures 3-2 and 3-3) are designed to permit hand-firing in case of power failure. In these situations, a natural draft may be provided by opening the grate and ashpit door.
Figure 3-8 illustrates typical controls for a stoker in a forced warmair heating system. There are approximately three basic automatic controls necessary for satisfactory operation of the stoker. These three controls are as follows:
2. Limit control.
3. Hold-fire control.
Figure 3-7 Bin-fed conical grate stoker.
The purpose of the thermostat is to start the fire when the room temperature falls below a predetermined point and to stop it when the temperature again rises to normal. The thermostat setting should be adjusted to give comfortable room temperature. Usually a setting between 72F and 75F is desirable.
The limit control stops the stoker should the furnace or boiler pressure become greater than the setting of the control. Furnace limit switches on warm-air gravity installations usually require settings above 300F. Hot-water limit switches on hot-water systems usually require settings above 160F. Steam pressure controls on steam pressure installations usually require settings of 2 to 5 lbs.
The purpose of the hold-fire control is to produce a stoker operation at intervals during mild weather in order to maintain fire when the thermostat is not demanding heat. Sometimes the
Figure 3-8 Typical controls for a stoker-fired coal burner in a forced warm-air heating system.
Hold-fire control feeds either too much or too little coal to the retort. The former results in overheating, and the latter may cause the fire to go out. It is best to call a service representative of the stoker manufacturer to adjust the hold-fire control, because this is a complicated mechanism. The two types of hold-fire controls are interval timers and stack temperature control switches.
Timers may be adjusted to give various-length firing periods so that the stoker is operated for a few minutes at preset intervals. This is done to keep the fire alive during cool weather when little heat is required. The cycle of operation may be set for either 30- minute or 1-hour intervals.
Figures 3-9 and 3-10 show two examples of typical timers used on stokers. The combination switching relay and synchronous motor-driven timer shown in Figure 3-9 provides periodic burner operation so that the fire can be maintained during times when the thermostat is not demanding heat. It may be used with any two-wire, 24-volt thermostat or operating controller. This particular timer is adjustable from V2 to 7V2 minutes every 30 or 60 minutes.
The timer shown in Figure 3-10 is designed for line voltage switching. When used with a line voltage controller, it maintains the stoker fire by providing short on periods between the controller off periods. Timing may be adjusted from 1 to 71i2 minutes at 30- or 60-minute intervals.
The stack switch (or stack thermostat) starts the stoker when the stack temperature becomes lower than a predetermined point and operates it until the fire is again kindled to a degree that will guarantee that it will not go out.
Stack switches are not found on all stokers, but they should be required in areas where electric power failures are long enough to let the fire go out. The stack switch will keep the stoker from filling the cold firepot with coal as soon as the electricity goes on again.
Figure 3-9 Honeywell combination switching relay and synchronous motor-driven timer. (Courtesy Honeywell, Inc.)
Sometimes a light-sensitive electronic device (such as an electric eye) is used instead of a stack switch.
The stoker operating instructions found in the paragraphs that follow should be regarded as generalized suggestions or recommendations rather than specific instructions. They may prove useful in those situations in which no operating manual from the manufacturer can be found. When possible, always consult the manufacturer’s operating manual.
A bituminous coal, low in ash (6 percent or less), with an ash fusion temperature of 2200F to 2600F and 1 Vi-inch to 3/8-inch maximum is ideal for stoker operation. Oil treatment of the coal is recommended in order to eliminate dust and add lifetime to the stoker. Generally speaking, in most sections of the country, a high-quality coal is generally most satisfactory and most economical. The annual coal tonnage for domestic stokers is usually low. Convenience and satisfaction are usually the primary factors considered when making the decision to install a stoker; therefore, good coal is recommended. Consult your local coal dealer or the stoker manufacturer for recommendations.
Generally, the procedure involved in starting a fire in a stoker — equipped heating installation includes the following steps:
1. Set the room thermostat above the room temperature.
2. Set the coal feed and air setting to the proper rate.
3. Throw the line switch to the on position so that the stoker starts.
4. Open the hopper lid, and watch the feed screw to make certain that it is turning. Sometimes in shipping or installing the stoker, the feed screw may slip off the shaft on the gear case. Be certain that the feed screw is engaged before putting any coal in the hopper.
5. Fill the hopper with coal.
6. Set the overfire air door on the furnace Vi to V2 open, and lock in this position.
7. Let the stoker run until the retort (inside the furnace or boiler) is filled with coal.
8. Place a quantity of paper, kindling wood, and a small amount of coal on top of the retort and ignite it.
Natural draft has a decided effect on the operating economy of the stoker installation. Check the draft and baffle damper, which should be adjusted to give the lowest possible draft without causing smoking from the fire door.
The check damper in the smoke outlet to the furnace can also be used advantageously when extreme natural draft conditions exist. The ideal arrangement is obtained by limiting draft just to the point at which smoke or fumes are not emitted from the fire door when the stoker is in operation.
As the fuel bed builds up to the desired condition, the air adjustment should be made in the following manner: Open or close the manual air damper to give a yellow and practically smokeless flame (not white-hot) and a fire fed with no intense blasts from air ports in the burner. Sufficient air must be delivered to maintain an even — burning fuel bed with a fairly consistent depth.
Each stoker will usually have some means of automatically controlling the pressure and volume of air delivered by the fan so that the correct amount is supplied to the fire as burning conditions vary. Usually no adjustment is necessary, as the setting made at the factory will enable this control to function properly on most installations.
On some stokers, the coal feed change is easily made by altering the position of the drive belt from the smaller to the larger or from the larger to the smaller pulleys of the motor and transmission.
Follow these instructions:
1. Cut off the stoker line switch.
2. Move the belt-change lever down to reduce the tension on the belt.
3. Move the belt to the pulley desired.
A. Belt on the large pulley of the motor gives maximum feed.
B. Belt on the center pulley of the motor gives intermediate feed.
C. Belt on the small pulley of the motor gives minimum feed.
4. Move the belt-tightening lever up to the original position.
5. Throw in the line switch.
The stoker motor will have a built-in device for protection against excessive motor temperatures. Should the motor become overheated, the protection device on the motor will prevent damage by breaking the electrical circuit. Motor overloads are usually caused by lack of bearing lubrication, low voltage, or excessive belt tension. To reset, push the reset button on motor after the motor has cooled sufficiently.
A stoker transmission will also include an overload protection device that automatically breaks the electrical circuit to the motor in the event that an obstruction should become lodged in the conveying mechanism of the unit. To reset (after removal of the obstruction), push in reset button on the side of the transmission.
Read the manufacturer’s instructions for removing obstructions from the conveying mechanism of the transmission. If these are not available, then you will have to determine the best way to gain access to the obstruction and remove it. There is usually an obstruction cleanout panel located in the back of the hopper. Full access to the feed screw is obtained by removal of this panel. It may be necessary to reverse the rotation of the feed screw manually to relieve the obstruction. To do this, the transmission must be placed in neutral. This can be done by disengaging the transmission from the conveying mechanism.
The electric motor should be lubricated at the beginning of the heating season and twice during the season. Use a good grade of medium engine oil.
The transmission will require approximately one pint of a suitable grade of engine oil. This should be checked once each season. The oil should be removed and replaced at the end of two heating seasons provided there has been no flooding. Should the transmission become submerged in water, it is recommended that it be serviced by a representative of the stoker manufacturer.
It is recommended that the stoker be prepared for the next heating season just after the spring heating has been completed. The stoker should be prepared in the following manner:
1. Remove the coal from the hopper.
2. Paint or grease the inside of the hopper.
3. Open the hopper lid for air circulation.
4. Remove any siftings from the retort base, and remove any ash or clinker formation from the burner.
5. Clean and oil the electric motor and adjust the belts.
6. Oil the stoker screw (or worm).
7. Replace oil in the transmission, if necessary.
8. Run a heavily oiled coal or sawdust through the stoker, leaving the feed screw and coal tube full, over the summer. This prevents corrosion and rusting.
You will find it easier to remove the clinker if you let the fire cool off for 5 to 10 minutes before removing it. Turn the stoker off and open the fire door to cool the fire. Fill the hopper while the clinker is cooling. The clinker normally forms around the retort. Use an iron bar or poker to raise the clinker. Do not dig in the retort. After you have raised the clinker, use the clinker tong to lift it from the furnace. It may be in one piece or several pieces, but remove all of it. Keep the fuel bed clean. Remove clinkers as often as necessary.
Figure 3-11 illustrates the steps involved in a typical coal feed adjustment. Their order (in sequence) is as follows:
1. Select the proper amount of coal feed for the furnace (refer to the coal feed chart provided by the stoker manufacturer).
2. When the proper coal feed is selected, the opposite side of the pointer indicates the proper air setting (see Figure 3-11A).
3. When the coal meter is set on proper coal feed, lock the meter with a wrench at the locknut shown (see Figure 3-11B).
From information on the coal meter, set the air selector knob to the proper point and the automatic damper will furnish the proper amount of air for the amount of coal fed to the furnace. The automatic air damper opens slowly after the stoker starts feeding coal, thus preventing puffbacks out of the fire door, and closes when the stoker stops and automatically banks the fire. When this occurs, the motor stops running.
All mechanical devices occasionally malfunction or operate below a commonly accepted level of efficiency. Coal stokers are no exception to this rule. Table 3-1 lists the conditions that indicate faulty operation:
SETTING COAL FEED METER
Select the proper amount of coal for furnace (Refer to coal feed chart).
Table 3-1 Troubleshooting Coal Stokers
(c) Gear case has been exposed to water.
(d) Blown fuse.
(e) Tripped circuit breaker.
Smoke backed into hopper.
(a) Hopper empty or low in coal.
(b) Clinker obstructing the retort.
(c) Clogged smoke back connection.
(d) Fire burning down in the retort.
Fire is out.
(a) Empty hopper.
(b) Clinkers obstructing the retort.
(c) Switch may be off.
(d) Blown fuse.
(e) Tripped circuit breaker.
(f) Failure in electric controls.
(c) Do not try to operate the stoker. Drain and flush out the gear case immediately and refill with oil.
(d) Replace fuse.
(e) Reset circuit breaker.
(a) Fill the hopper to the proper level.
(b) Remove clinker.
(c) Remove obstruction.
(d) Check air supply (fire may be getting too much) or rate of coal feed (may be too low).
(a) Refill to proper level.
(b) Remove clinkers.
(c) Place in on position.
(d) Replace fuse.
(e) Reset circuit breaker.
(f) Contact manufacturer for a service call.