This article concerns itself with vertical grinding mills used for coal pulverization only (coal pulverizers), although vertical grinding mills can -and are- used for other purposes.
The 3D model in the saVRee database represents a vertical grinding bowl mill. Other grinding mill types include the ball tube mill, hammer mill, ball and race mill, and , roll and ring mill.
Coal Pulverizer Purpose
Irrespective of the machine used to pulverize the coal, all coal pulverizers have a similar purpose. Coal pulverizers are used to dry, grind and classify.
Drying is achieved as hot gas passes through the pulverizer and removes moisture from the pulverized coal. The hot gases used should have a low oxygen content, this reduces the likelihood of accidental combustion within the mill. It is possible for hot gases to be taken directly from the furnace exhaust manifold, providing safety interlocks and valves are used.
Grinding is achieved using the impact, crush or attrition methods (see below). Grinding increases the cross sectional area of the coal and allows it to dry quicker whilst also making it easier to ignite. Grinding is often referred to as 'size reduction'.
Classifying occurs in order that pulverized coal reaches the desired fineness prior to entering the furnace. Larger particles are returned to the grinding area where they are further reduced in size until they are small enough to pass through the classifier.
Coal pulverizers increase the cross-sectional area and dryness fraction of coal prior to it entering a furnace. Pulverized coal ignites more efficiently than raw lumps of coal, thus the overall efficiency of the furnace increases if it is fired on pulverized coal. Coal pulverizers are used extensively within industries that operate coal fired furnaces, this includes the power generation and steel industries.
Note: Vertical grinding mills are used extensively within the cement industry.
Vertical Grinding Mill Components
A vertical grinding mill consists of the following components:
- Electric Motor - to rotate the grinding table. The electric motor is often a three phase variable speed drive (VSD) induction motor, but designs vary.
- Reduction Gearbox - to reduce the rotational speed of the incoming motor shaft prior to a secondary shaft connecting to the grinding table.
- Grinding Table - a rotating round surface where the coal rests. Coal is crushed between the grinding table and grinding rollers. Grinding table surfaces are coated to resist erosion. If the grinding table is only a horizontal surface, it is a roll and race type grinding table. If the grinding table has a sloped surface, it is a roll and bowl type grinding table.
- Grinding Rollers - fixed rollers that grind the coal.
Grinding Rollers and Grinding Table
- Rockers - used to retract the rollers from the mill interior.
- Classifier - used to return over-sized coal to the grinding table. Correct sized coal particles travel through the classifier to the furnace. It is also possible to use a cyclone separator and/or separator to classify the coal particles.
Classifier Inside Mill
- Hot Gas Inlet - pulverized coal is dried by hot gases. The hot gases are usually heated by the exhaust gas of the furnace, or, are the exhaust gas from the furnace; this increases the overall plant efficiency due to recapturing of heat from the exhaust gas system.
- Coal Inlet - coal is fed to the centre of the grinding table. Coal fed to the mill may be raw or pre-treated.
- Pulverized Coal Outlet/Discharge - pulverized coal flows out of an outlet duct.
How Vertical Grinding Mills Work
The below video is an extract from our Mechanical and Electrical Engineering Explained Online Video Course.
Raw coal (sometimes pre-treated) is fed into the centre of the grinding table. The grinding table rotates which causes the coal to move towards the outer periphery of the grinding table. The coal is ground between the rollers and grinding table. When enough coal has been reduced in size, it will flow over the top of the dam ring (retention ring).
Coal Pulverizer Inlets and Outlets
Louvres (nozzle ring) around the outer perimeter of the grinding table allow hot gas to flow into the grinding mill through the mill base. Some of the pulverized coal is transported from the base of the mill up to the classifier by the hot gas stream i.e. the coal is transported pneumatically. Regulating the amount of gas that passes through the nozzle ring also regulates the amount of pulverized coal that is delivered to the furnace.
Oversized pulverized coal impacts with the rotating classifier and drops back onto the grinding table for further grinding. Correctly sized coal particles pass through the classifier and exit the mill through the discharge duct. Pulverized coal that has passed through the classifier is pneumatically conveyed to the furnace.
Coal Pulverizer Types (Impact, Crush and Attrition)
The type of pulverizer employed depends upon requirements. Pulverizers typically utilise three main types of grinding:
Impact – an external force impacts with the coal. This type of grinding can be compared to when a hammer hits an object e.g. a piece of coal.
Crush – the coal is forced between two bodies. This type of grinding has the same effect as a rolling pin when rolling across salt granules on a table.
Attrition – the coal is ground against itself. Rubbing two sugar cubes against themselves represents attrition grinding.
Coal Pulverizer Types (Low, Medium and High Speed)
Another defining factor for pulverizers is their operational speed. Pulverizers are split into low, medium and high speed groups. The type of grinding method employed is usually linked to the pulverizer’s operational speed:
Low Speed = Attrition
Medium Speed = Crushing
High Speed = Impact