Category: Coal Firing

Troubleshooting Coal Stokers

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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:

Table 3-1 Troubleshooting Coal Stokers

Symptom and Possible Cause Possible Remedy
Abnormal noises.
(a) Loose pulleys or belt. (a) Tighten or replace.
(b) Dry motor bearings. (b) Oil the bearings.
(c) Worn gears. (c) Oil or replace.
(d) Gears lack oil. (d) Oil the gears.
Motor will not start.
(a) Hard clinkers over or on retort. (a) Remove the clinkers.
(b) Foreign matter caught in the feed screw. (b) Remove the foreign matter.
(c) Packing of coal in the retort caused by the end of the feed screw being worn. (c) Remove packed coal from the retort and replace feed screw.
Stoker operates continuously.
(a) Controls out of adjustment. (a) Contact manufacturer for a service call.
(b) Dirty fire. (b) Rebuild or clean fire.
(c) Fire out. (c) Rebuild fire.
(d) Dirty furnace or boiler. (d) Clean furnace or boiler.
Furnace filled with unburned coal.
(a) Clinkers clogging the retort. (a) Remove the clinkers.
(b) Coal feed set too high. (b) Reduce coal feed setting.
(c) Insufficient air getting to the fire. (c) Open manual damper. If this does not help, check air ports for clogging. Also, check the windbox. If it is full of siftings, they should be removed.
(d) Windbox filled with siftings. (d) Empty windbox.
Stoker will not run.
(a) Limit control has shut off furnace or boiler due to overheating. (a) Allow limit control time to cool off.
(b) Low-water cutoff has shut down the boiler. (b) Check water level in boiler and correct.
(b) Low-water cutoff has shut down the boiler. (b) Check water level in boiler and correct.
(c) Gear case has been exposed to water. (c) Do not try to operate the stoker. Drain and flush out the gear case immediately and refill with oil.
(d) Blown fuse. (d) Replace fuse.
(e) Tripped circuit breaker. (e) Reset circuit breaker.
Smoke backed into hopper.
(a) Hopper empty or low in coal (a) Fill the hopper to the proper level.
(b) Clinker obstructing the retort. (b) Remove clinker.
(c) Clogged smoke back connection. (c) Remove obstruction.
(d) Fire burning down in the retort. (d) Check air supply (fire may be getting too much) or rate of coal feed (may be too low).
Fire is out.
(a) Empty hopper. (a) Refill to proper level.
(b) Clinkers obstructing the retort. (b) Remove clinkers.
(c) Switch may be off. (c) Place in on position.
(d) Blown fuse. (d) Replace fuse.
(e) Tripped circuit breaker. (e) Reset circuit breaker.
(f) Failure in electric controls. (f) Contact manufacturer for a service call.

How to Adjust Coal Feed

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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).

adjustment coal feed How to Adjust Coal Feed

How to Remove Clinkers

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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.

Starting Coal Firing

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Generally, the procedure involved in starting a fire in a stokerequipped 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 1?4 to 1?2 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.

Coal Selection

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A bituminous coal, low in ash (6 percent or less), with an ash fusion temperature of 2200°F to 2600°F and 1 1?4-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.

Domestic Coal Stoker Automatic Controls

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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:

1. Thermostat.
2. Limit control.
3. Hold-fire control.

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 72°F and 75°F 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 300°F . Hot-water limit switches on hot-water systems usually require settings above 160°F . 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 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 1?2 to 71?2 minutes every 30 or 60 minutes.

honeywell relay Domestic Coal Stoker Automatic Controls

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 71?2 minutes at 30- or 60-minute intervals.

honeywell stoker timer Domestic Coal Stoker Automatic Controls

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. Sometimes a light-sensitive electronic device (such as an electric eye) is used instead of a stack switch.

Domestic Coal Stoker Construction

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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:

1. Retort.
2. Fan.
3. Motor.
4. Transmission.
5. Air duct.
6. Air control.
7. Hopper.
8. Feed worm.
9. Bin.

underfeed stoker 2 Domestic Coal Stoker Construction

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.

domestic underfeed stoker 2 Domestic Coal Stoker Construction

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-airpressure 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 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.

Stoker Firing

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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.

hopper fed conical grate Stoker Firing

conical grate Stoker Firing

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 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 stoker Stoker Firing

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.

Firing Semibituminous Coal

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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 semibituminous 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.

Firing Bituminous Coal

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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.