DESCRIPTION OPERATION AND MAINTENANCE PULVERIZED COAL SYSTEM
Bharat Heavy Electricals Limited
Pub No: 2764
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PULVERIZED COAL SYSTEM. GENERAL
The system for direct firing of pulverized coal utilizes Bowl Mill to pulverize the coal and a Tilting Tangential Wind box to admit the pulverized coal together with the air required for combustion (secondary air) to the furnace. As crushed coal is fed to each pulverize by its feeder (at a rate to suitthe load demand), primary air is supplied from the primary air fans. The primary air-dries the coal, as it is being pulverized, and transports the pulverized coal through the pulverized coal piping system to the coal nozzles in the windbox assemblies. A portion of the primary air is pre-heated in the air heater (which also preheats the secondary air for combustion). The hot and cold primary air are proportionally mixed, prior to admission to the pulverizer to provide the required drying as indicated by the pulverizer outlet temperature. The total primary airflow is measured in the air inlet duct and controlled to maintain the velocities required to transport the coal through the pulverizer and coal piping. The total primary airflow may constitute from approximately 20 to 30% of the total unit combustion air requirement. The pulverized coal and air discharged from the coal nozzles is directed towards the center of the furnace to form a firing circle. Fully pre-heated secondary air for combustion enters the furnace around the coal nozzles and through the auxiliary air compartments directly adjacent to the coal nozzle compartments. The pulverized coal and air streams entering the furnace are initially ignited by a suitable ignition source* at the nozzle exit. Above a predictable minimum loading condition the ignition becomes self sustaining Combustion is completed as the gases spiral up in the furnace. A large portion of the ash is carried out of the furnace with the flue gas; the remainder is discharged through the furnace bottom into the ash pit. NOTE: -Refer to Publication No. 2502 "Tilting Tangential Firing System" for further details. COMBUSTION OF PULVERIZED COAL IN TANGENTIALLY FIRED FURNACES
The velocity of the primary air and coal mixture within the fuel nozzle tip exceeds the speed of flame propagation. Upon leaving the nozzle tip the stream of coal and air rapidly spreads out with a corresponding decrease in velocity, especially at the outer fringes where eddies form as mixture occurs with the secondary air. Here flame propagation and fuel speeds equalize, resulting in ignition. As the stream advances in the furnace; ignition spreads until the entire mass is burned completely. The speed which the air and coal mixture ignites after leaving the windbox nozzles depends large, on the amount of volatile matter in the fuel. Heat released by oxidizing the volatile components in the coal, accelerates the heating of the fixed carbon to its ignition temperature. The key to complete combustion consists of bringing a successive stream of oxygen molecules into contact with carbon particles, the smallest of which are relatively large by comparison with the oxygen molecules. As combustion of the carbon progresses it becomes increasingly difficult to bring about contact with the diminishing oxygen supply in the limited time available, which for this type of firing is in effect greater due to the longer travel taken by the gases. The cyclonic mixing action that is characteristic of this type of firing is most effective in turbulently mixing the burning coal particles in a constantly changing air and gas atmosphere. As the main part of the gases spiral upward in the furnace, the relatively dense solid particles are subjected to a sustained turbulence, which is effective in removing the products of combustion from the particles, and in assisting the natural diffusion of oxygen through the gas film that surrounds the particles. 2
PULVERIZERS (See Figures 1 and 2)
The pulverizer, exclusive of its feeder, consists essentially of a grinding chamber with a classifier mounted above it. The pulverizing takes place in a rotating bowl in which centrifugal force is utilized to move the coal, delivered by the feeder, outward against the grinding ring (bull ring). Rolls revolving on journals that are attached to the mill housing pulverize the coal sufficiently to enable the air stream through the pulverizer to pick it up. Heavy springs, acting through the journal saddles, provide the necessary pressure between the grinding surfaces and the coal. The rolls do not touch the grinding rings, even when the pulverizer is empty. Tramp iron and other foreign material is discharged through a suitable spout. The air and coal mixture passes upward into the classifier with its deflector blades where the direction of the flow is changed abruptly, causing the coarser particles to be returned to the bowl for further grinding. The fine particles, remaining in suspension, leaves the classifier and pass through the coal piping to the windbox nozzles. For details of construction, lubrication recommendations, controls and operating adjustments refer to the Publication No. 4001/4002/4003 - "BOWL MILL". FEEDERS.
The raw crushed coal is delivered from the bunkers to the feeders, which in turn feed the coal at a controlled rate to the bowl mills, each feeder supplying an individual bowl mill. The feeder can be either a rotary type of feeder, chain feeder or a Gravimetric Feeder. If rotary feeder is provided refer for details the instructions on RAW COAL FEEDER -ROTARY TYPE, Publication No. 4004. If Chain Feeder is provided refer for details the instructions RAW COAL FEEDER - DRAG LINK CHAIN TYPE, Publication No. 4005. For a Gravimetric Feeder refer to manufacturer's instructions. The feeder is driven by PIV drive unit the speed of which is varied, but for the Gravimetric Feeder depending on the feed rate required. Refer to the instruction on Feeder for details. In order to avoid overloading the mill motor due to over-feeding, a motor current sensing device should be provided to reduce the coal feed to a minimum if the bowl mill becomes overloaded and to restore the coal feed when the load of coal in the bowl mill becomes normal again. Refer to Bowl Mill instructions Publication No. 4001/4002/4003 for further details. PULVERIZED COAL DRYING
Far satisfactory pulverizer performance, the temperature of the primary air and coal mixture leaving the classifier should be kept at approximately 82°C for low volatile coals and approximately 75°C for high volatile coals. Too low a temperature may not dry the coal sufficiently; too high a temperature may lead to fires in the pulverizer. The outlet temperature must not exceed 95°C in any case. The moisture content of coals varies considerably. Therefore the best operating conditions for a particular installation must be determined by experience. Figure 3 shows the location of dampers, shutoff gates and valves generally utilized. The hot air control damper (No.1) and the cold air control damper (No. 2) regulate the temperature of the air entering the pulverizer, by proportioning the air flow from the hot air and cold air supply ducts. These dampers also regulate the total primary airflow to the pulverizer. The hot air shutoff gate (No. 3) is used to shutoff the hot air to the pulverizer. The hot air gate drive must be interlocked with the pulverizer motor circuit se that the gate will be; closed any time the pulverizer is not in service. It must also be interlocked with the 3
temperature contro er to e ect c os ng o temperature exceed 95°C.
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The mill discharges valves (No. 5), the cold air shutoff gate (No. 4) and the seal air shutoff valves (No. 8) are always kept wide open. They are closed only when isolation of a bowl mill or feeder is required for maintenance. An adequate air supply of clean air for the bowl milltrunnion shaft bearings etc., is assured by installing two numbers of booster fans, one always standby, and two filters, with the interconnections for cross operation during maintenance. One-fan runs continuously giving required seal air for the operating Pulverizers. The filter or booster fan can be isolated for maintenance by closing the valves before and after the filter or booster fan suitably. An auto transfer damper is provided at the seal air fan outlet unless there is a specific requirement for butterfly damper. The coal pipe seal air valve (No. 6) is utilized to admit seal air to the coal pipes for cooling when the bowl mill is isolated. The seal air valve is open whenever the bowl mill discharge valves are closed and vice versa. Primary air velocity requirements in the bowl mills and coal piping variations in system airflows. Therefore a constant airflow is maintained up feeder speed and then the airflow is varied proportional to the load. This is to instability at the lower load and to avoid settling and drifting in the pulverized excessive spillage of coal from the bowl mill through the tramp iron spout.
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NOTE: - Coal spillage may also be caused by overfeeding, insufficient heat input for drying, too low a spring pressure on the rolls or excessive wear of the grinding elements. PULVERIZED COAL PIPING
The arrangement of the pulverized coal piping from the Bowl Mills to the windbox nozzles is shown in the Mill and Fuel Piping Arrangement drawings. Each bowl mill supplies an entire elevation of windbox nozzles. By distributing the fuel in this fashion a balanced fire is maintained regardless of which bowl mills are out of service. Orifice plates are installed in the vertical run of the coal piping leaving the Pulverizer Discharge Valves, to compensate for unequal resistance to flow due to different length of piping to the wind boxes. The fuel pipings are connected at places by means of fuel pipe couplings. For assembly details refer Publication No. 2501 Pipe Coupling assembly instructions.
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