coal grinding plants
Post on 14-Apr-2018
237 Views
Preview:
TRANSCRIPT
-
7/27/2019 Coal Grinding Plants
1/8
Coal grinding plants
-
7/27/2019 Coal Grinding Plants
2/8
Introduction
The equipment used in all FLS coal grinding
plants is designed and arranged so as to comply
with the same high standards of safety. The
plants, nevertheless, differ considerably in
practice due to a number of decisive factors, the
main ones being:
- Coal quality
- Source of hot air for coal drying
- Number of fuel consumption points
- Kiln type and available space, especially when
installing a new grinding plant for an existing kiln
Other aspects to be taken into account when
planning a coal grinding plant are initial cost,
maintenance costs, and operational reliability.
These and other factors form the basis for the
decisions to be made with regard to machinery
selection and operational alternatives, viz.
- Ball mill or roller mill- Direct or indirect firing
- Grinding plant location
- Inert or non-inert operation
- Bag filter or electrostatic precipitator for
dedusting purposes
Mill types
Ball mill - roller mill
The operating principles of these two
types of mill differ in the following ways:
a) A roller mill requires more air to convey the
material than does a ball mill. This means that
only part of the air from a roller mill can be
used as primary air for a rotary kiln of the
modem, high-economy type, as the amount of
primary air would be too great in relation to
kiln combustion requirements, thus increasing
heat consumption. All or some of the air from a
roller mill must therefore be dedusted in a
separate filter. If the raw coal has a low or
moderate moisture content, the amount of air
from a ball mill is generally no greater than can
be used as primary air in the kiln.
b) Grinding plants with roller mills take up less
space and have a lower specific energy
consumption than ball mill plants, the power
saving being 15-25% for the whole grindingplant .
c) The feed to ball mills must be precrushed to
approx. 25 mm lump size, and the drying
capacity is limited to a reduction in moisture
content of 12-15%, whereas a roller mill will
handle material of up to 85 mm feed size,
depending on mill dimensions, and remove up
to 20% moisture.
d) Roller mills are more flexible than ball mills,
since the roller mill can easily be adjusted to
varying throughputs within a range of approx.
40-100% of nominal capacity, as required.
e) Ball mills are usually preferred for grindinghighly abrasive types of coal.
Atox coal mill
Tirax coal mill
-
7/27/2019 Coal Grinding Plants
3/8
Grinding and drying
Grinding
Coal contains varying quantities of vola-
tiles, both combustible and non-combus-tible. All types of coal also contain
hygroscopic (inherent) moisture as well asvarying quantities of impurities which are
released in the form of ash in the com-
bustion process.The degree of grinding required depends
on the type of coal used. Coal with a lowvolatile content requires a high ignition
temperature and must be finely ground.
Coal with a high volatile content, how-ever, must not be ground too fine,
otherwise the volatiles will be expelled tooquickly to be able to mix properly with the
combustion air.
The table specifies the main types of
coal in geological order and the appro-ximate coal meal finenesses recommend-ed.
Drying - Air circulation
There are three factors which are decisive
in determining the amount of air to bedrawn through a coal mill:
- The amount of air at a given tempera-ture must be sufficient to ensure
effective drying of the moist coal.
- The amount of air must be adequatein relation to the evaporated moisture,
so that the dew point of the air leavingthe mill is maintained at a suitably
low level. In practice, the dew pointmust be 15-20C lower than the millexit air temperature to prevent
condensation in the ducts and thededusting installation after the mill.
- In air-swept mills, of both the ball and
roller types, the air velocity must behigh enough to extract the ground
material from the mill.When specifying the moisture of coal, a
distinction is made between surface
moisture, which evaporates at ambienttemperature, and hygroscopic moisture
which is more closely bound to the coal.The inherent moisture content depends
on the geological age of the coal, themoisture ranging from 1-2% in anthraciteto 10-20% in lignite.
The relationship between moisturecontent and dew point as well as the
amount and temperature of the drying air
is shown in the above graphs. As willappear, if the dewpoint is 15C below the
mill outlet temperature of 70C, the dew-point thus being 55C, and the moisture
content 10%, the mill inlet temperature
should not exceed approx. 350C. Thefigure also shows that if the available
drying air has a temperature in the regionof 300C, and the moisture content in the
raw coal is below 10%, the drying air
requirements are less than 1.2 kgair per kg coal. The amount of air to
be extracted from the grinding
plant, including false air and watervapour will in this case be less than
1.5 kg air per kg coal, correspond-ing to approx 17% of the com-
bustion air, based on coal with a
calorific value of 6500 kcal/kg(27,200 kJ/kg).
For a conventional suspensionpreheater kiln this 17% normally
constitutes a suitable amount of
primary air for kiln combustionpurposes.
As to the amount of air necessaryto extract the coal meal from the
mill, operational data has been
collected from a large number ofball mills grinding coal with up to
10% moisture, all with the tempera-ture of the drying air exceeding
300C. On the basis of this data it
can be concluded that the amountof air required to extract the coal
meal from the mill is around 1.5 kgair per kg coal. The corresponding
figure for roller mills is approx. 2.0
kg air per kg coal.The coal must be sufficiently dry
for grinding, storing, and feedingpurposes. Excessive drying should be
avoided, due to the accelerating
process of oxidation which occurs asthe moisture content is reduced, with
the consequent risk of spontaneousignition. In practice, drying off the
surface moisture of the coal and a
small part of the hygroscopic mois-ture will ensure a high degree of
safety combined with good process-ing and handling properties.
In order to maintain a constant
moisture content in the coal mealdischarged from the mill, FLS coal
grinding plants are supplied withfully automatic thermal control
equipment controlling the tempe-
rature of the air at mill outlet.
Air quantities for coal drying in ball mill
Relationship between coal types, composition and grinding fineness
Coal type % volatiles % ash
% hygroscopicmoisture
Normal sieve residueof coal meal
Anthracite
-
7/27/2019 Coal Grinding Plants
4/8
Installations
Direct firing implies a simple and cheap
installation. Regulation of firing in the
kiln is effected by varying the feed to thecoal mill. The coal meal produced is
precipitated in a cyclone, and afterpassing an air sluice, it is blown into the
kiln. Kiln and mill operation are interde-
pendent with direct firing, which meansthat mill throughput must correspond to
kiln requirements.
Indirect firing is the most commonlyapplied method of firing a rotary kiln.
After precipitation in a cyclone, the coalmeal is stored in a bin from which the
burner is fed. With indirect firing the
mill throughput is kept constantirrespective of kiln output fluctuations,
and short interruptions of mill operationdo not affect kiln operation so long as
there is an adequate supply of coal meal
in the bin.
In both systems, depending on milland kiln type, all or part of the drying airmay be used as primary air in the kiln
provided it is passed through a cyclone
to remove most of the coal meal.
Centralised coal grinding plants
In large industrial plants with several
firing units it is often an advantage to
instal a centralised coal grinding systemserving all units. This applies in particu-
lar when existing plants are converted tocoal firing.
Centralised coal grinding plant with Tirax mill arranged for indirect firing in a precalciner kiln
Coal grinding plant with Atox mill arranged for indirect firing
Coal grinding plant with Tirax mill arranged for direct firing
-
7/27/2019 Coal Grinding Plants
5/8
Centralised coal grinding plants
The combination of an Atox mill and a
dedusting bag filter simplifies the instal-
lation and reduces the space require-ments.
Drying with inert kiln gases
Plants using inert kiln gases with an
oxygen content of about 4% for drying
and transport of coal are becoming more
commonplace. They have the advantage
that the risk of fires and explosions is
reduced considerably when the oxygen
level is kept below 14%. Operating condi-
tions and plant maintenance must be
such that this limit is not exceeded during
operation. This is effectively controlled by
an 02-meter placed at dedusting installation
outlet.
In an inert coal grinding plant the
temperature of the drying gas can to some
extent be reduced either by adding cold,
atmospheric air or by recirculating the
mill exit air. The dewpoint of the exit gas
when using cold atmospheric air is lower
than when using recirculation air. In
order to ensure efficient material trans-
port, the air flow rate through the system
must be maintained within a certainrange. It may therefore prove necessary to
inject water into the mill to keep the
system inert while operating with coal
with a low moisture content.
Centralised coal grinding plant with Atox mill arranged for firing in a number of kilns
Coal grinding plant with Atox mill arranged for inert operation
-
7/27/2019 Coal Grinding Plants
6/8
Complete Tirax coal mill Plant
Explosion flaps
Top of bags for jet pulse filter
CO2
units Electrosta tic precipi tator for coal firing plant
Dedusting installations
When all the air passed through themill cannot be utilised as primary airfor kiln firing, an electrostatic precipita-tor or a bag filter must be installed todedust the air. Inert grinding alwaysrequires a dedusting installation.
With a bag filter, usually of the jet-pulse type, the air can be passed direct
to the filter, where the full mill outputis collected and passed on to a coalmeal bin or coal burner. An electro-static precipitator must be preceded bya cyclone for collecting most of the coalmeal so as to reduce the average dustconcentration in the precipitator to alevel at which explosions do not occur.
When electrostatic precipitators areused to dedust air from coal mills, it isadvisable to provide the precipitatorswith the following equipment to ensuresafe operation:a) Electric calorifier for preheating the
precipitator to ensure even heat
distribution during start-up.b) Electric heating elements with ther-
mostatic control to preventcondensation during operation.
c) Vibrators for bottom hoppers
d) CO monitoring device
e) Thermal monitoringf) C0
2or N
2injection equipment for
precipitator inertisationThis equipment is also recommended
for coal mill bag filters, though usuallywithout items a) and b).
The dedusting installation must, of
course, be effectively insulated and sodesigned that coal dust accumulationis avoided. When following theseguidelines, the risk of fire and explo-sions is regarded as negligible.
Safety in FLS coal-grinding
installations
Modern coal-grinding installations aresubject to the highest standards withregard to safeguarding personnel as
well as operational safety.
Everything possible is done to meet
these requirements in the developmentof FLS machinery, while also ensuring
that it represents the latest state of theart.
Plant layouts are designed to verystrict rules and can be made to complywith the provisions of all known safetyregulations and standards.
Electrical alarm and interlockingsystems as well as operational proce-dures aim at also eliminating humanerrors, this explaining why no seriousaccidents have occurred at any FLS coalgrinding installations. This in turncontributes to maintaining a high
runfactor.Among the advantages of FLS equip-
ment are:
l All silos designed for safe mass flow
l Dust collectors and ducts designed toprevent accumulation of dust
l Explosion protection or externalventing according to clientsrequests
l Ample facilities for temperaturemeasuring at all relevant points
l CO monitoringAs fires can occur by spontaneous
ignition, especially during longstandstill periods, modern safetyequipment often includes a systemwhereby inert gas is injected into dustcollectors and coal meal bins as a fire-protective measure.
FLS supply such inertising equip-ment based on C0
2or N
2with
manual or semiautomatic injection asrequired.
-
7/27/2019 Coal Grinding Plants
7/8
A well-planned project and careful
selection of machinery are necessary toensure successful operation of a coalgrinding plant During the planning,
construction, and starting-up phases,F.L.Smidth offer clients the services ofa large staff of mechanical and
electrical engineers, operational
specialists as well as process andcomminution engineers. All these
specialists are capable of incor-porating clients processing require-ments in production plants to ensure
maximum efficiency.As coal grinding plants are exposed
to heat, dust, and mechanical wear,
careful maintenance is an essentialfactor in ensuring durability and highperformance.
A comprehensive manual containingpreventive maintenance procedures isincluded in the supply of all FLS plants.
working condition and productioncapacity.
These visits also give plant manage-
ment and their technical staff anopportunity to consult the visitingspecialist on measures to improve
plant performance and efficiency.Such cooperation between client andF.L.Smidth helps to ensure optimum
and lasting utilisation of a productionplant.
Emission measurement
Also covered are such items as machin-ery lubrication, cleaning, and checkingas well as fault-finding and otherinstructions for the replacement of
component parts.Also supplied are catalogues listing
spare parts for the machinery concerned
and spares recommended to be kept instock.
Efficient maintenance and servicing of
a plant also requires insight on the partof client technical personnel, however,and with this in view, FLS Technical
Services arrange individually adaptedtraining courses so that technicalpersonnel at site can become fully
acquainted with the design and func-tions of the equipment supplied.
Special agreements with FLS Techni-
cal Services can also cover regular sitevisits by engineers who check current Inspecting an electrostatic precipitatorInspection of drying chamber in Tirax mill
Inspection of Tirax mill
Spare roller for Atox mill
-
7/27/2019 Coal Grinding Plants
8/8
DENMARK: F. LSmidth & Co. A/S Vigerslev All 77, DK-2500 Valby CopenhagenBRAZIL: F.L.Smidth S.A., Comrcio e Indstria, Rua Nebraska, 443,Brooklin Paulista- CEP 04560, Caixa Postal 3506, 01000 So Paulo - SPCANADA: F. LSmidth & Co. of Canada, Ltd., 2000,Rue Mansfield, Suite 800, Montreal, Quebec, Canada H3A 2Z5FEDERAL REPUBLIC OF GERMANY: F.L.Smidth & Co. GmbH,Geniner Strasse 133-135, Postfach 1632, D-2400 Lbeck 1FRANCE: F.L.Smidth & Cie-France, S.A.R.L.,55, Rue Ampre, F-75017 ParisHONG KONG: Representative Office of F.L. Smidth & Co. A/S25 Floor Hop Hing Centre, 8-12 Hennessy Road, WanchaiITALY: F.L.Smidth & C. Italiana S.r.l., Via Lucilio, 25,1-00136 RomeJAPAN: F.L. Smidth & Co. (Japan) Ltd., Togeki Building, 1-1, Tsukiji 4-chome,Chuo-ku. Tokyo 104. Postal address: Kyobashi P.O. Box 78, Tokyo 104-91
MEXICO: F.L.Smidth & Cia., S.A. de C.V., Ave. Campos Eliseos, 385,5 piso, Torre B, Chapultepec Polanco, 11560 Mexico City, D.F.PR OF CHINA: F.L.Smidth & Co. A/S, Beijing Representative Office,Yanjing Hotel, Suite 8012/8014, Fuxing Men Wai, BeijingSINGAPORE: Representative Office of F.L.Smidth & Co. A/S,111 North Bridge Road #23-03, Peninsula Plaza, Singapore 0617SOUTH AFRICA: F.L.Smidth & Co. (S.A.) (Pty.) Ltd., Craighall Mews,347 Jan Smuts Ave, Craighall Park, Transvaal. Postal address: P.O. Box 41455,2024 Craighall, TransvaalSPAIN: F.L.Smidth & Cia. Espaola, S.A, Av. General Pern, 38,17 E-28020 MadridUNITED KINGDOM: F.L.Smidth & Co. Limited, 17, Lansdowne Road,Croydon, CR9 2JTUSA: F.L.Smidth & Co.. 300 Knickerbocker Road. Cresskill. NJ. 07626
Data in this brochure is only intended for preliminary project planning. Manufacturer reserves the right to modify equipment details and/or specifications without notice.
top related