coal grinding plants

7/27/2019 Coal Grinding Plants

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Coal grinding plants


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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


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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


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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


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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


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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.


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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


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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.

coal grinding plants

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