Coal Pulverizer Mill

Overview

A coal pulverizer mill grinds raw coal to a fine powder and dries it so it can be blown into a boiler furnace and burned like a gas. The vertical spindle bowl mill, sold under names such as the Babcock MPS and the Foster Wheeler E-mill, is the most common type on utility boilers. Raw coal falls onto a slowly rotating Grinding Bowl, centrifugal force throws it outward into the path of three Grinding Roll units, and the rolls crush it against the Grinding Ring. Hot primary air sweeps up through the bowl throat, dries the coal, and carries the fine particles up to the Classifier, which returns coarse material to the bowl and lets the fines pass to the burners.

One mill typically feeds three to six burners on one elevation of a boiler. A 600 MW unit may run six such mills, with five in service and one as spare. Throughput runs from 20 to 100 t/h depending on mill size, with the largest bowls grinding more than 100 t/h. The mill is sized for a target fineness, usually 70 to 75% of the product passing a 200 mesh (75 μm) sieve, because finer coal burns faster and more completely.

Grinding

The grinding elements are the working core of the mill. The Grinding Bowl turns at 20 to 40 rpm, driven through the Bowl Shaft by the Planetary Gearbox. Three tapered rolls sit on the Grinding Ring and turn freely as the bowl drags coal under them. Each roll carries a replaceable Roll Tire and rides on a sealed Roll Journal with two bearings. The coal forms a bed a few centimeters thick between the tire and the ring, and the rolls crush it by compression rather than impact, which keeps wear and power draw low for a given fineness.

Roll force comes from the Roll Loading Frame. Heavy Coil Spring units on the Loading Pull Rod assemblies set a base load, and on modern mills a Loading Cylinder on each roll adds adjustable force from the Hydraulic Power Unit. Loading is 300 to 700 kN per roll. Operators raise the pressure for hard coal (low Hardgrove index) or for high fineness, and lower it at part load to save power and reduce wear. The Side Wear Liner panels protect the housing wall, since the coal-air stream inside the mill is highly abrasive.

Drying and primary air

Primary air does two jobs: it dries the coal and it lifts the fines. Hot air at 250 to 350 °C enters the Air Plenum under the bowl and accelerates through the vaned Nozzle Ring in the Air Throat Ring. As it passes the grinding zone it evaporates surface moisture from the coal, which can be 12% or more for lignite and sub-bituminous coals. The Primary Air Damper blends hot air from the air heater with cooler tempering air to hold the mill outlet temperature between 65 and 95 °C. Too low and the coal stays wet and pipes plug; too high and there is a risk of a mill fire or, with volatile coal, an explosion.

The throat velocity is set so that ground coal is entrained but heavy material is not. Rock, tramp iron, and pyrite drop through the throat into the Pyrite Reject Spout and collect in a hopper emptied through the Pyrite Reject Gate. Removing this reject protects the rolls and bearings and keeps incombustible material out of the furnace. Primary air flow runs at 1.5 to 2.0 kg of air per kg of coal, and the pressure drop across the mill, 4 to 8 kPa, is one of the main signals the operator watches.

Classification

The Classifier sits above the grinding zone and sets the product fineness. Air carrying ground coal enters the rotating Classifier Cage, whose Classifier Vane units impart a swirl. Particles that are too coarse are thrown outward by centrifugal force, fall down the Classifier Reject Cone, and return to the bowl for regrinding. Fine particles follow the air through the cage and out to the burners. Cage speed, set by the Classifier Drive, is the main fineness control: a faster cage rejects more coarse coal and raises fineness, at the cost of more recirculating load and higher mill differential pressure.

Drive and lubrication

The bowl is turned by the Main Drive, a horizontal Drive Motor of 200 to 700 kW feeding a two-stage Planetary Gearbox that combines a spiral Bevel Gear Set with a planetary Helical Gear Pair. The vertical bowl load and the grinding reaction are carried by the Bowl Thrust Bearing, a tilting-pad bearing in the gearbox housing. The Lube Oil System supplies filtered, cooled oil to the gearbox and thrust bearing through the Lube Oil Pump, Lube Oil Cooler, and Lube Oil Filter. A Lube Oil Heater warms the oil to operating viscosity before a cold start, since the gearbox cannot be run with stiff oil.

Control and protection

The Instrumentation and Controls cabinet runs the mill on a coordinated load loop. Coal feed, primary air flow, and outlet temperature are trimmed together to hold the fuel-air ratio while following boiler demand. The Mill Controller reads mill differential pressure, outlet temperature, bearing temperature, and loading pressure, and drives the feeder, the air dampers, and the classifier. Protection logic trips the mill on high outlet temperature, loss of primary air, loss of lube oil pressure, or high vibration, and it purges the mill with air before any restart to clear settled coal that could ignite. Carbon monoxide monitoring on the outlet gives early warning of a developing mill fire.