COAL PREPARATION AND BURNING TECHNOLOGY IN CEMENT INDUSTRY

IMPACT OF COAL ASH AND VOLATILE MATTER• The chemical composition of coal ashes is usually broadly similar to that of ,tie

argillaceous (clay or shale) components in the cement kiln feed raw mix, i.e .they are rich in the acidic oxides Si02, Al 203 and Fe203 while their cao contents are usually low.

• Therefore, compared with gas or oil fuels, the kiln feed raw mix must be adjusted with. the proportion of clay or shale reduced, to maintain the required cement clinker composition. As a rough guide 12%ash content will reduce the LSF of the kiln feed material by about 1%.

• Hence, the coal ash content should be maintained reasonably constant preferably to within about +/- 2%.

• For coals above about 15% ash content a stockpile-reclaim blending system is usually required.

• Also, the coal must be ground sufficiently finely that the ash can be incorporated homogeneously into and react with the raw material feed in the kiln. For coals up to about 15 % ash content a coal fineness of about 20% residue on 90 micron is usually sufficient, but very high ash coals or low volatile coals need to be more finely ground.

• A second guide to fineness requirements, for flame characteristics is a 90µm residue equal to 50% of the volatile matter content. It can be seen that this would indicate lower residue requirements particularly for the high rank bituminous coals and the anthracitic coals.

• The primary volatile constituents that can effect are chlorides,sulphur and alkalis. These constituents can have an important effect on the chemical composition of the clinker and the quality of the cement, and on the formation of rings or build-ups in the rotary kiln or the riser pipe of a suspension preheater or precalciner.

• It is the total input to the kiln of these constituents which matters, and the extent to which they can escape from the kiln system via the kiln exhaust gases and kiln dust. Therefore, the tolerable levels of these constituents in the kiln fuel is determined by:-

• the amounts of them which are also entering the kiln via the Raw materials feed. the type of kiln system.

COAL AS RECEIVED FINENESS AND FREE MOISTURE CONTENT

• If the coal has a high proportion of fines and ahigh free moisture content, it will tend to stick onto conveyor belts and it will not discharge regularly from hoppers and feeders.

• This can cause irregularities in the operation of the coal mill and sometimes the kiln.

• Hoppers need to be properly designed to suit the flow properties of the reals which will be employed -steep cone angles, avoidance of corners, generously sized coal discharge aperture, and smooth internal surf aces.

• Coals with a <1 mm size fraction of 30% or more can prove extremely difficult to handle through a poorly designed system. This is made worse if free moisture content exceeds 7-8%.

COAL RECEIVING AND STORAGE• Coal is received in wagons/trucks from coal mines and is stored in yards as

received coal.

• The quantity of coal to be stored depends on the works consumption and the method of supply.

• CRUSHING

• the received coal is of large size and if it is to be ground in mills

• the received coal is crushed to desired size by jaw/roller crusher.

• crusher or impactor used for crushing the coal to match coal mill feed size.

• below 25 mm for ball mill• below 50 mm for VRM

COAL BLENDING (STACKING & RECLAIMING)STACKING

The crushed coal is stored as piles through stacker.the coal is of a higher and variable ash content, or if a number of very different coals are to be used, then a windrow blending stockpile with end or side-scraper type reclaim will be required. This type of reclaim is more expensive on operating costs than a front-end loader.Whether or not the stockpile should be covered depends on the climate and the environmental requirements. The heavier the rainfall the more worthwhile it is to cover the coal stock, also if the climate is dry and windy a cover will avoid the nuisance and loss of coal by windblown dust.coal is stored in piles in two methods for proper homogenisation 1) windrow method 2) chevron method

PRECAUTIONS DURING STORAGE OF COAL

FLOOR SHOULD BE CLEARED OF VEGETATION, FLATTENED ; A CONCRETE FLOOR WOULD BE IDEAL

CARE SHOULD BE TAKEN TO ENSURE THAT LUMPS AND FINES ARE DISTRIBUTIED AS EVENLY AS POSSIBLE

AIR POCKETS SHOULD BE AVOIDED BY PILING COAL, LAYER BY LAYER

BLANKETING THE HEAP WITH FINE COAL /POLYTHENE SHEETS SPRAYING OF WATER ONCE IN A WEEK OR 10 DAYS ADEQUATE METHOD OF STACKING CHECKING THE TEMPERATURE OF COAL STACK FREQUENTLY

IMPROPER STORAGE OF COAL CAUSES

SPONTANEOUS COMBUSTION

REDUCTION IN SIZE BY DISINTEGRATION

LOSS OF CALORIFIC VALUE DUE TO OXIDATION

COAL RECLAIMING AND STORAGE IN HOPPER

• Coal stacked in piles is reclaimed by a reclaimer and is stored in raw coal hopper.

• In the raw coal hopper there is chance of material jamming in the hopper due to moisture content in it.

• This problem can be overcome by lining the hopper with polymers.

• The coal from raw coal hopper is sent to mill for grinding of coal.

PRACTICAL EFFECTS OF COAL PROPERTIES ON GRINDING

Coal is ground in either in ball mills or vertical mills the grindability of coal depends upon its properties.

1) Types of Coal

2) Types of Associated Minerals

3) Safety

TYPE OF COAL

• High rank coals tend to be hard, relatively difficult to grind, to be unreactive and therefore possibly giving rise to ignition and flame stability difficult. As indicated earlier finer grinding may be necessary if volatile contents are low.

• If a high-speed mill is to be used, wear problems will also be encountered, possibly severe. Drying is not likely to be difficult.

• Conversely low rank coals (brown coals ,lignites) are reactive, have high water contents, and may be difficult to dry adequately. They may be difficult to grind through halving a fibrous nature and also by inadequate drying. Wear due to the coal substance should not be a problem.

• Medium rank coals are of course intermediate in properties except for grindability, which is generally easier, and the possible presence of a greater proportion of fine material holding surface water and giving problems of drying and transport.

TYPES OF ASSOCIATED MINERALS:

• The principal problem here is mill wear, arising mainly from the presence of more than a few percent of hard and/or corrosive minerals. Some minerals may also give rise to problems associated with coating of the grinding elements.

• For a given type of mill, wear increases with fineness of grind.

SAFETY

• The high surface area of finely ground products renders the self-heating problem more serious in coal grinding installations, danger spots being hoppers horizontal or slightly inclined pip-, and filter bags.

• All hoppers used for pulverised coal should be fitted with temperature sensors and means for flooding with C02 or water.

• Conveying pipe gas velocities must be designed to ensure that coal deposition does not occur in horizontal sections of pipe, typical velocities being around 30 m/see, with a minimum of at least 24 m/sec.

• Although the high rank coals are also less reactive in the finely ground state the above design considerations should be applied rigorously in any installation handling fine coal.

COAL GRINDING IN MILL

• COAL MILLS

• Two types of mill are commonly used for coal grinding in the cement industry these are:-

• Vertical roller mills

• Ball mills

COAL MILL

DRYING OF COAL

• IF THE MOISTURE IN THE RAW COAL BELOW 15%, IT IS DRIED IN THE DRYING-CUM-GRINDING UNIT.

• MAXIMUM TEMPERATURE OF HOT GASES AT MILL INLET IS ABOUT300- 350 0C AND EXIT GASES TEMPERATURE IS AT 65-80 0C.

• COAL MAY BE DRIED TO A RESIDUAL MOISTURE OF ABOUT 1-1.5%DURING GRINDING IN THE MILL

FIRING SYSTEMS IN CEMENT KILNS

INDIRECT FIRING

KILN BURNER PIPE

CLINKER COOLER

MULTI-CHANNEL BURNER

FIRING OF COAL INSIDE KILN

Thank you