Note: Descriptions are shown in the official language in which they were submitted.
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~0~676
This invention relates to a process for pulverizing
coal to ultrafine size.
While attempts have been made in the past to
pulverize coal to such size, these have not been successful,
either because of the very high consumption of power for
the process, the impurities of the resultant product, or
the insufficiency of the comminution for effective use.
The largest single use of coal is as a fuel in utility
and industrial boilers. In both instances, the larger steam
generation units burn the coal in the pulverized form. Nominal
partlcle sizes of coal as fired are 74 microns for bituminous coal ~`-
and 44 microns for anthracite. These finenesses achieve efficient -
combustion characteristics while still permitting collection of
the ash particles before emission of the flue gas to the atmosphere.
Various types of pulverizers are used to achieve the
finenesses desired and each type has been empirically developed
to a high degree of efficiency for the purpose intended. Pul-
verization to particle sizes finer than stated might improve com-
bustion efficiency slightly but at the expense of pulverizer
2a pawer requirements and difficulty of ash separation. Accordingly
there has been no development
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of pulverizing equipment to produce finer particle 8ize8 in
l large amounts. Some equipment has been developed to produce
; much finer particle sizes, but due to high power con~umption,
~ low output, and high cost, are economical in processing
~ only relatively higher value products, such a~ co~metics,
pharmaceuticals, and foodstuffs. However, a number of
proces~es and use~ are in development which would benefit
~ignificantly through the use of large quantities of coal
of much finer particle size~ than used heretoforeO Pulverizing
coal to a particle size of less than 10 microns would have,
sm~ng many, the following immdiate uses: 1. water and
liquid waste purification; 2. direct burning in combustion
turbines; 30 suspension in liquids to produce colloidal
fuels; 40 as a raw material for the production, by further
processing, of sub-micron particles; and 5. as a feed stock
for gasification processeæ facilitating direct methanation
of carbon.
Besides the direct uses of ultrafine coal, pul~ierizing
to smaller than 10 microns particle size will permit, in the
p~ocess, re val of substantial portions of inorganic ash
an~ pyritic ~ulfur. In combustion processes using the 1 to 10
micron sizes or substantially smaller sub-micron sizes, the
environmental effects of the products of combustlon would
be considerably reduced. ~ith such "cleaner" fuel in fine
particle size~ the way i8 opened for employing combustion
techniques which would minimize formation of N0x products
in the combustion gases emitted into the atmosphereO Use of
these techniques is not now feasible in pulverized fuel
combustionO 3
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4tj76
According to one aspect of the invention there is provided a pro-
cess for pulverizing coal to ultrafine size of less than 10 microns which
comprises drying the coal, subjecting the dried coal to a coarse pulveriza-
tion stage in an atmosphere of inert gas, sub~ecting the coarsely pulverized
coal to a further pulverization stage in the presence of inert gas to obtain
ultrafine coal, separating the ultrafine coal from the inert gas and re-
cycling the separated inert gas to the pulverization stages wherein make up ~ ~
insert gas for supply to the pulverization stages is obtained from flue gas :
from a boiler by cooling the flue gas and scrubbing it to remove particulate --
matter, sulfur dioxide and carbon dioxide. i~
According to a further aspect of the invention there is provided
apparatus far pulverizing coal to ultrafine size, comprising means for drying
coal, a coarse pulverizer for grinding the coal particles to about 74 micron
size, means dissimilar to said coarse pulverizer for reducing said coal
particles to less than 10 micron size, means for introducing an inert gas
into said coarse pulverizer to convey the coal in continuous suspended flow
through said reducing means, means for separating the resultant ultrafine
coal particles from said inert gas, means for cooling and removing condensate
from the separated inert gas and recycling it through said coarse pulverizer, -;
2Q a steam generator, flue gases from which are passed through the said means
for drying coal to dry it before pulverization, a water scrubber for removing
heat and particulate matter from said flue gases, an alkaline scrubber Por
removing sulfur from said flue gases, means for removing the carbon dioxide
from the resultant gases so as to leave essentially only nitrogen, and
means for introducing said nitrogen into the stream of said inert gas stream
to replenish losses thereof.
The invention will be further illustrated by reference to the
accompanying drawings showing, by way of example, an embodiment of the
invention, in which:
3Q Figures lA and lB, taken together show, schematically~ a process
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for pulverizing coal to ultrafine size in accordance with the present
invention.
Referring to Figures lA and lB, lump or run-of-the-mine coal as
received in dumper 1 is crushed in crusher 2 to a nominal maximum size of ;~
approximately 1 inch, then passed through magnetic separator 6 and conveyor
4 and dried with inert boiler flue gases in flash coal driers 5. The dried
coal is stored in coal bunkers 7 from which it is fed, at a controlled rate,
through gas lock feeders 8, to the coarse (74 microns) pulverizer 9. Con-
ditioned inert gas is introduced through pipe 24 into the control feeder,
pressurizing the pulverizer circuits and spaces and causing a flow of inert
gas through the coal from and in the bunker 7, thus purging the spaces be-
tween coal lumps of air. i-
The coal is conveyed from the coarse pulverizer 9, pipe 15, ;
through cyclone separator 11 and through the fine pulverizer and reductor -
mlll 12 by conditioned inert gas 16a to pipe 16 and the ultrafine product
filters 17 where the ultrafine material is placed in storage bunkers for
end use. --
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lO~q~i76
The inert gas separated from the product i~ passed
through pipe 18 and cooled by gas cooler 19, separator 20,
compres~et by compressor 21, and returned through separator 22
and pipe 24 to the pulverizer inlet. A portion of this inert
gas separated from the product is passed through pipe 52,
compressed to a higher pressure by compressor 53 for operation
of the filters 17 and returned to the main inert gas stream
in the filters.
Losses of the inert gas will occur through leakage,
backflow into the bunkers 7 and entrainment in the product.
To replace these losses, inert gas i8 made from the boiler
flue gases used to dry the raw coal. Steps in the preparation
of this iert gas consi~t of water scrubbing in water
scrubber 42 to cool the wet flue gas flowing through pipe 26
and remove particulates, aLkaline scrubbing by scrubber 47
to remove gaseous sulfur compounds and scrubbing with
noethanolami*e in M.E.A. scrubber 32 to remove carbon
dioxide. The remaining gas is essentially pure nitrogen.
Mbre detailed descriptions of the several subsy8tems
~; 20 follv~
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lU ~1676
STORAGE, P Æ ~ARATION AND HANDLING SYSTEM
Coal is recelvèd either directly fro~ a mIne, by
- me~ns of belt ¢onv-yor, or in rail cars if the plant i~
loc-ted r-mote from the mine Even lf the plant iB located
~d~oa~e ~to a mine nd the rotary car dumper l, yard
locomotive and a portion of the car dumper hou~e could be
~ ~ elimln-t d,~ it~m~y be economic-Ily prud-nt to insta1l
T~ ~ompl te rail car facilitie8 Thi8 would improve reliability
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d th`e~raw~m terial suppIy ~nd also f-cllit~te the
opportùoity;for taklng;;adv-otag- of favorable purchasing
10~ ~ oiportunities
Upon rec-ipt, the co-l is t~ken to the~crush r
hou8e 2 where it is crushed to nominal I inch size and
coov yed to~ un ion~poi-t~oere~toe coal c-o be div-rt-d ~-
into~one~of~two~stre _ . ~The first ~3) of the two streams
15~ ~ permit8~tbe~co-1 to~be movet~to~a ground leve1 8to~ge pile
from whicb the~co~ is ~ran-ferr-d by~bulldoz-r8 to~- main
torage~pll-`.~ The~main~-torag-~pile will create p-rmanent
; ~ re-erY-;~cap city vithln~the plant rea~to be u-ed in the
ven~t tbe~r~w~m~téri l ~upply i~ int-rrupt d for any reason.
20- ~ ~ The -econd~;8tre _ 4 c~rr~ies the coal~through fla8h
dry-rs~5 wher-~ext-rn-1 moi-ture i8 re~oved and tbrough
` ~ m-gn tic~-p r-tnr- 6 wh re -t-el part-, ~uch a8 nuts, bolt8,
etc~ ;are re~ov d and fina~lly disc~h~rges it into elevated
r~ 7.
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,` CO:IMIIIUTIO SYSTEM 10~4~i 76
; A controlled amount of the dxy coal is fed by gravity
from the storage bunkers through ~a~ lock feeders 8 to
conventional bowl roller or ball pulverizing mills 9 where
it is reduced to a 74 micron size. In the pulverizing process,
~me of the pyrites are removed through pipe 10. Upon being
dlscharged from the pulverizers, the coal is fed through
pipe lS and cyclone ~eparators 11 into high speed impact type
reductor mills 12 where it is comminuted to some predetermined
size between 1 and 10 microns. Within the reductor mill, the
,~ 10 coal is sub~ected to centrifugal forces which permit removal
~ of additional inorganic ash and pyrites tbrough pipe 13.
3:~ As coal is reduced from a nominal size of 1 inch to
!~ a product of 100% less than some size in the range of 1 to 10
microns, there is a tremendous increase in the exposed
surface area of the co,m~lnuted material. ~long with the
increase in surface area there is a very marked increase
i- in the activlty of the coal and if size reduction takes place
in an air atmosphere, the explosive limits with respect to
temperature and/or park energy may be realized inside the
mechanical grinder. Therefore, to provide for safety of
operations, the coal will be transported under positive
pres~ure in the comminution system by inert gas-~aoh
as nitrogen. The ga~ will be introduced into the ~ystem
,~ ~ at the ga- ~ck feeders 8.
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The micron sized perticles are then pneumatically
carried by means of the inert gas 16a through pipe 16 to
fabric bag-type dust collectors or filter~ 17. Here the
particles continuously separated from the carrying lnert gas
are discharged into product storage hoppers or conveyed to
process equipment depending upon the application.
INERT GAS SYSTEM
To optimize the balancing between investment and
operating costs, the inert gas will be recycled with make-up
added to the system to compensate for losses. The inert gas
-~ which is approximately 99.5Z nitrogen and 0 5X carbon dioxide
~molar composition on a dry basLs) enters the reductor mill 12
from pulverizer 9 at approxi ately 38-C. The entering gas
15 ~ will be water saturated. A relstively small portion of
the gas flowlng to the inlet side of the mill backflows
~rougb the coal feeder line from the coal bunkers and is
vented to the atmo9pbere. 1~13r using inert gas for m~intainiag
a~sligbt positive pressure in the coal bunkers, the quantity
20~ ~ of~aLr entering the system along with the coal feed is
minLmized. Tbe quantity of gas flowing througb the reductor
!,ji" ~ mLlls 12 Ls establLsbed based on the speed and physical
~dimensLons of the mill components, the characteristics
of tbe cla~sifying gas with respect to the den6ity and
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vLscosity, and the characteristics of the comminuted product
with respect to particle size and specific gravity. The
flow rate of gas to each of the mills will be measured and
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controlled to insure proper flow distribution. AB the inert
gas and coaI pa~ses through the mill 12, a considerable
percentage of the horsepower required to drive the mill i8
converted to heat with a resultant rise in temperature of
the proce~sed material. A portion of the heat is expended
in vaporizing inherent moisture from the coal material.
The system i8 designed 80 that the inert gas and comminuted
~; co-l leave the mill through pipe 16 at approximately 130-C.
and, ba~ed on complete drying of the coal which enters the
mill as an assumed inherent water content of ix weight
percent, the dew point o the existing Ka~ is approximately
80-C.
The gas leaving the bag filters 17 i8 pas~ed, by
pipe 18, through a heat exchanger or gas cooler 19 and
is cooled from approximately 130-C. by heat exchange with
cooling water. As the cooling of the gas will result in
partial condensation of water vapor, a phase separator 20
will be provided downstream of the cooler to remove condensate
from the qstem. Subsequently, the effluent gas from the
separ-tor will be boosted in pressure by an electric motor
~driven centrifugal compressor 21 from a suction pressure
; of a few inches water column to a discharge pressure,
downstream of the compressor aftercooler, of approximately
~ 6 psig. Since additional condensstion of water vapor occurs
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in the compressor aftercooler, a phase separator 22 will be
provided for removal of this condensate prior to recycling
the gas to the lnlets of the mills. Supplement~l drying -
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of the compre~sed gas will not be required since the quantity
of water reved from the stream by cooling and partial
condensation will be ~ufficient to reject the entire quantity
of water picked-up by the gas from the comminuted coal in
the reductor mills 12. A portion of the gas stream which
was used for removal of surface moisture in the flash coal
dryers 5 will be taken from the flue gas circuit downstream
of the particulate (42) and sulfur dioxide removal equipment
(47) and will be used to provide the make-up to the inert gas
recycle systemO
Flue gas containing approximately 21 percent carbon
dioxide and 79 percent nitrogen (molar composition on dry basis)
and water saturated will be compressed by compressor 46 from
a few CM. water column to a discharge pressure, downstream
of the compressor aftercooler, of approxim~tely 8 psig.
~ondensate will be removed from the stream, by separator 30,
after leaving the compressor aftercooler 28 and the gas, free
of entrained water, will pass through pipe 31 upwardly
through a column where it will be scrubbed by MEA scrubber 32
by a 15L solution o monoethanolamine (MEA) flQwing downward
in the column. Within the scrubber column, the MEA reacts
with the carbon dioxide at approximately 38-C. to form a
water soluble salt. The gas leaving the MEA scrubber 32
through pipe 33, which will be approximately 99.5 percent
nitrogen and 0.5 percent carbon dio~ide (dry basis), will enter
pipe 24 and the inert gas recycle circuit upstream c the
reductor mills.
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1(~4'1676 ,
The carbon dioxide ri~h M~A solution leaving the
bottom of the MEA tower through pipe 35 is pumped through a
heat exchanger 36 and the eemperature i8 raiged to approximately
lOO-C.. The heated solution i8 sent to the upper section
of a stripping tower 37 and flows downwardly contacting hot
vapors from the reboiler of tower 37, indicated in dash outline.
- Steam in pipe 50 will be used to supply the heat of vaporization
required to generate sufficient boil-up vaporæ to strip the
carbon dioxide from the rich MEA. Condensate passes through
pipe Sl. The carbon dioxide vapor leaving the top of the
tower 37 contains a portion of the vaporzied MEA solution.
This MEA i8 recovered by passing the hot vapor mixture
through a condenser and returning the condensate to the
stripping tower. The carbon dioxide is re~ected to the
atmosphere at 38. The hot carbon dioxide free, lean MEA,
f~o~s from the reboiler section of the stripping column and is
cad}t~ by heat exchange with the rich MEA. The cool lean MEA
-~ solution i8 then pumped back by pump 39 to the absorber tower,
~hus complet$og the cycle.
~ The use of nitrogen produced from this flue gas a~ a
cl-seifying gas in the mill is desirable because it is
eesentially free of oxygen and it provides for safety of
operation. The flow rate of gas required for this purpose
i8 approximately twice that which is available from the flash
dryer system. Therefore, tbe gas flowing througb the mills
will be recycled and only the quantity of gas required for
make-up (5 percent of the recycle stream) will be processed
for carbon dioxide removal for addition to the recycle circuit.
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BO~LER PLANT AND AUXILIARY SYSTEMS
In order to ~upply heat for moisture reval,
inert gas to make up system losses, regenerate MEA solution,
and to heat the building, a pulverized coal-fired steam
boiler 41 i8 installed~ Slightly le~s thsn theoretical air
will be u~ed in the combustion so a~ to exclude oxygen
in the flue gases. The boiler will be equipped to bypass,
variably~ ~ome of its steam generating surface 80 as to vary
the temperature af the flue gas flowing in pipe 62 to the
flash coal dryer 5 to accom~date variations in the moisture
in the incoming raw coal. Flue gas leaving the flash dryers
through pipe 26 will be scrubbed by scrubber~ 42 with
water 43 to reve heat and particulate matter, ehen with
a solution of sodium hydroxide in scrubber 47 to reve
~ulfur gasee. A portion of the flue gases wlll then e~ter
the MEA inert gas system through pipe 31. The carbon dioxide
i8 removed from the flue gas because i~s greater density
would increase windage power in the mills. The remalning
larger portion of the flue gas not needed for the inert
gas sy~tem will be exhausted to the atmosphere. As
previously noted, it has been cleaned of particulate and
~lfur ga~e~ and contains only carbon dioxide and nitrogenO
The flue gases leaving the flash dryer will be
saturated with isture at a temperature approaching 90-C.,
snd it is desirable to reduce thi~ moi~ture before entering
the MEA scrubber 320 Accord$ngly, the liquid from the
water scrubber 42 whlch follows the flash dryer will be
cooled in a mechanical draft cooling tower 45.
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The boiler 41 wLll generate steam for regenerating
MEA ~olution and for space heating and general plant use.
By ad~ustments of the portion o boiler gases bypassing
steam generating surface and the firing rate of the boiler,
S variations o~ steam demand and moisture content of the raw
~ coal can be accomodated.
;~ In the water scrubber 42, the particulate mstter
from the flue gas will be removed. The liquid will pas~
through a settling tank before being pumpted through the
cooling tower 45. The settling sludge consisting of flyash
and fine coal particles will be air-dried and recycled
into the coal entering the boiler's pulverizers. Leaving
the water crubber 42, the flue gas pas-es through the
; sulfur-dioxite removal scrubber 47. In tbis scrubber,
lS the su1ur dioxide is absorbed into a~dilute ~odium hydroxide
solution, converting the ~olution to sodium sulfite, and
-odium bi~ulfite.
The ~ulfite and bisufite are converted back to
odium~hydroxide in a~coagulator reactor~by the additlon
~ calcium hydroxide. The precipitate from the resctor i8
cs1cium sulfite which is ~u~e~ a8 a sludge to settling
beds or dewatered in mechanical equipment for damp hauling
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to disposal are~s.
In an effort to effect additional efficiencies, the
beaviest p-rticles and pyrites from the 74 micron coal in
the product pulverizers re conducted through pipes 10 and 13
and burned in the boil-r 41.
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~ RECIRCULATED COOLING T Æ RS
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In the reductor plant are several souraes of heat
which must be continuously removed. The main source are
water j~ackets of the mills, circulating gas coolers to
remove moisture emanating as inherent moi~ture of the coal,
5 ~ MEA coolers, and bearing oil cooler~ of the mills Therefore,
the~plant includes a cooling water supply 56 and return
;~ -y~tom 60 circulating~through~ hu.r: exchanger 19. Only
clean water clrculates through'this sy8tem. ~
Re3ection of the heat to the stmo~phere is
~ ~ accomplished~by a mechanical draft cooling tower 57.
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The~co~le~d liquid from the cooling tow-r i8 pop-d through
the he-t e-chsnger 19,~_ ntioned abov-, and the heat-d
~cooling~tower water~ from thi- exch-nger i~ retùrned to
the~;cooling~tower~
15~ ~ Thu~ it~will;be ~-en~that I have provided a highly
~eff~cieot~pr~oce~ nd -pp-r-tus for pulv-~rizing coal to
ltr-fine;p-rticlé~ ol 1--- th-n IO microns 80 a~ to ~'
~è~nablé~8ub8equent u8e of the pulverized coal in various
prooe-ses,~;such~ -6 that~involving direct methanation of
20~ ~ ~ coal~when~in powdered fonm, and other application8
: ~ n ~ r-ted~abova.
` ~ ~ Whi}e~ hsve illustrated and described several
~embodiment- of my inv-ntion, it will be under-tood that
thes- are by way of illu8tr-tion only and that various
~ ch~ng~es snd modifictions may be contemplated in my invention
and in the scope of the following claim8.
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