Greetings from my corner of the slowly defrosting eastern wilderness@1140 feet above mean sea level.

When I was a seasonal laborer at the nearby power plant we burned bituminous coal exclusively.

The coal was recieved by rail, unloaded with a tippler and dropped in to a dual apron feeder where the feeders were feeding toward each other and dumping into a hopper with a stationary stainless steel grizzly feeding a fourty eight inch conveyor which was a stationary stacker the coal was conveyed to the pile and the earthover pushed it away from the dump point. The coal was pushed away as far as was possible to allow for additional coal to be moved behind it and piled at the same time.

When additional coal was being delivered to the power plant the dozer operator continued to pile coal as far possible by creating a road/bank of coal to push more coal up on the stock pile beyond the stationary stacker creating a second hill on top of the coal stock pile.

The coal was drawn by down by a vibrating pan feeder fed by a an air powered diverter baffle to allow for feeding from both sides of the stock pile into a crusher/feeder from there it went back into the power plant on a 36 inch coveyor to the coal bunker where it dropped on to one of two conveyors going in opposite dirrections to feed the coal bunkers feeding the the two boilers.

The coal bunkers are elevated above the boilers to allow gravity to do some of the work.

The coal is dropped down into small apron feeders where it is metered into the grinder and ground to 30 mesh or less and then blown into the boiler to be burned.

The coal wastes were pumped out of the boiler and cleaned using a bag house but that changed when the plant was converted to fluidised bed combustion.

The other plant I am familiar with was/is a dual coal stoker where the steam station used by the university used high grade/btu coal to make low pressure steam. they used rom soft steam coal and still do feeding the coal stokers via a common hopper at he rear of the plant.

The waste/slag is conveyed to a holding bin and drop dumped in to an roll of container.

The coal was delivered by rail until the middle seventies until the rail line was abandoned and the coal is trucked there now from a siding 5 miles away to the tune of 86,000 tons per year.

They have coal delivered by rail to the siding and use a car hoe to empty it into an under railcar feeder which loads a waiting truck.

the local scrap yard has a contract to deliver coal and they stock pile it on premises to be delivered as needed as well.

They will be adding two gas turbines to get around the upcoming 2007-09 pollution reduction requirements for coal burners

Most if not all soft coal is burned for power generationin conventional power plants as mentioned above and it is also used to power sugar factories in the sugar beet producing states where they generate thier own power to run the plant, heat the water to diffuse the sugar and convert limestone to CO2 for the winning/refining of sugar.

Anthracite coal is still mined in the east but it is strip mined where the overburden-limestone/shale/slate is blasted away using large diameter rock drills and anfo and the remaining coal left from the original room and pillar mining is removed with front shovels and loaders etc.

The coal-antracite is screened, sized, picked free of rock, washed, oiled and stock piled and or bagged for delivery.

most soft coal strip mines load their coal in to railcars with out screening or washing.

Both soft and hard coal is still sized using the sizes listed below:

the coal sizes are listed in larger to smaller screen size.

coal stokers can burn egg and smaller soft coal due to the induced and forced draft used to ignite it

Anthracite and bituminous coal comes in these standard sizes:

Egg 4-6""

stove 2-4

chestnut 1'-2

pea-thumbnail

rice-little finger size

almost everone is grinding their coal for use to get the most energy out of it by blowing combustion air into the coal to the tune of several million cfm except for the very old plants which use high BTU metallurgical soft stoker coal.

Typically the blower boilers have a 16 blowers feeding ground coal dust into the boiler to be burned with four blowers at each corner at various elevations and an induced draft fan blowing air up the exhaust stack and forced draft fan feeding combustion air into the boiler. these boilers also has a contingent of fuel oil burners to add additional heat to the boiler when needed-especially at a cold start from a complete shut down and compressed air powered soot blowers to remove coal soot freom the boiler walls. the water vapor steam pipesa re refewrred to as harp pipes as they are shaped exactly like the frame on a harp to allow for maximum conversion of heat to steam.

The old Lehigh Valley Railroad was referred to as the road of anthracite as it only used anthracite in its steam locomotives- their be selling point was that you could have your windows open on their passenger cars and not get even a little dirty because it burned clean with no smoke.

Screening is not an issue unless it is being sold to the public as very little soft coal is sold to home owners unless the can burn it in their furnaces providing they have induced draft.

The same goes for hard coal.

The soft coal in most strip mines breaks up easily when it is excavated and sold as is as the end user is the determining factor as to whether it the coal needs additional work since they either use a stoker or a burner that requires coal the consistency of pancake flower. they want the most bang for their buck and soft coal is what they burn because it is plentiful and easier to burn than anthracite and just plain cheap to buy.

Soft coal is handled just like any other commodity using rotary car dumpers, and bucket wheel reclaimers at dock side stock piles to load ships and to stack reclaimed coal from a ships delivery boom and from there it is loaded on to rail cars and moved to its destination which is usually a power plant.

I hope this helped and I hope I did not bore you.

leon Z

as always it depends on the low bidder-case in point-Massey coal is selling coal from Venezuela here in the states -go figure ■

Dear Mr. George Baker,

Coal bulk density is normally considered around 800 kg/m3 and sand around 1600 kg/m3. So, it is obvious that for same tonnes of storage, the volume for coal will be nearly double than for sand.

Regarding screening aspects, the coal which can be screened per square meter (per square feet) value will be different than the same value for the sand. The coal tonne per square meter will be greatly influenced by the coal size and moisture content. In case of sand, it will be mainly influenced by the moisture content. One has to refer to the available tables / charts / past data to decide the required screening area for a particular capacity and particular characteristics of material. Also, the screening frequency would be different particularly while dealing with large size coal.

Regarding stacking, there would be some difference in the repose angle, which is to be taken care to decide the volume of the stockpile. Coal is also sensitive to spontaneous fire, particularly when ambient temperature is high and if the stock pile height is also more. In India, the coal stockpile height is limited to 10 m on this issue. As for sand there is no such issue, so its stockpile height can be higher than this.

For the coal stockpile, if there is a spontaneous fire at the bottom of the stockpile, then one has to also think about removal of the coal from the layer above, to extinguish the fire. Therefore, in India, and possibly in Europe reclaimer machines are used which can remove the coal from the layer above, allowing the extinguishing the fire, which may be deep inside the stockpile. If the coal is being reclaimed from the bottom, then it would be impossible to reach the burning zone of the coal in a running plant.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■