system optimization and selection of lignite coal drying process for coal mines

Solutions for Coal Drying

System optimization and selection of lignite coal drying process for coal mines

Lignite Coal Misture and Ash

Lignite Coal Production

Country or area Year Quantity，thousand ton

United States 2011 503890

China 2011 450000

Germany 2011 160210

Australia 2012 109730

Russian 2011 75410

Poland 2011 61800

Turkey 2011 60670

Greece 2011 59860

Serbia 2011 39480

Czech Republic 2011 39260

Romania 2011 36920


Canada 2011 36660

Bulgaria 2011 35230

India 2011 32060

Thailand 2011 17160

Mexico 2011 15590

Estonia 2011 14390

Other Asia 2011 13080

Bosnia and

Herzegovina 2011 11790

Philippines 2011 10960

Hungary 2011 9500

Indonesia 2011 8000

1801650

Lignite coal for power

generation


United States 2011 503890

Germany 2011 160210

Australia 2011 109730

China 2012 100000

Russian 2011 75410

Poland 2011 61800

Turkey 2011 60670

Greece 2011 59860

Serbia 2011 39480

Czech Republic 2011 39260

Romania 2011 36920


Canada 2011 36660

Bulgaria 2011 35230

India 2011 32060

Thailand 2011 17160

Mexico 2011 15590

Estonia 2011 14390

Other Asia 2011 13080

Bosnia and

Herzegovina 2011 11790

Philippines 2011 10960

Hungary 2011 9500

Indonesia 2011 8000

1451710

Benefits and issue with

lignite coal drying

• Benefits with lignite coal drying

Reduce moisture

Reduce ash

Increase heating value

Reduce transportation cost

• Issues with lignite coal drying

Spontaneous combustion

Coal dust during handling


lignite coal drying

• Spontaneous combustion of lignite coal

Mechanism of spontaneous combustion of lignite coal

Lignite coal has a large surface area and therefore has a strong ability to absorb oxygen due to its porous structure;

When lignite coal contacting with air, oxygen physisorption of coal occurs first, and physisorption heat is released;

Chemical absorption and chemical reaction happens and chemical reaction and chemical adsorption heat are released afterwards;

It will cause self-heating if heat produced cannot be carried away immediately and coal temperature rises constantly with heat buildup inside coal;

Coal temperature rising causes the oxidation reaction more intensive; intensive reaction would release more heat; and speed of coal temperature rising accelerates further and vicious circle accelerates, spontaneous combustion occurs when coal temperature reaches its ignition temperature.

• Coal dust issue during handling

According to study, coal dust problem is mainly caused by fine coal less than 0.6mm. Solution:

Selecting grinding equipment with minimum fine coal

Separation of fine coal from coarse coal: fine coal can be used as fuel for drying process; the remaining fine coal can be sold nearby; and fine coal briquetting.


lignite coal drying

• Mechanism of inhibition of spontaneous combustion of lignite

By destroying or reducing the structure with lower activation energy;

Coal surface is inerted by covering with binders to reduce contacting chance between coal and oxygen;

By water spraying on the surface of coal pile to absorb heat and reduce heating speed;

• Measures of inhibition of spontaneous combustion of lignite

Dried lignite coal briquetting can destroy or reduce structure with lower activation energy. Many attempts, but no successive story in industry;

Coal pile surface inerted treatment by covering with airtight cloth or scaring formed by binder spraying to prevent contacting between coal and oxygen;

Water spraying on surface of coal pile to cool down. This is widely used in industry;

• Fine coal briquetting (Binderless Coal Briquetting, BCB process; Hot briquetting process, HBP):

(1), Fine coal collection fine coal is collected by cyclone after dryer. Fine coal dusting issue is mainly caused by fines 150 mesh or less; 150 mesh or less fines only accounts for 35% or so from cyclone;

(2), Fine heating Fined coal is heated close to its softening point with vertical indirect heating

(3), Fines screening Fine coal is then screened after heating. According to the mechanism of pressing granulation, occupied state and void ratio of fines are determined by its size distribution; larger bonding interface between fines are necessary for fine briquetting and finer coal would have higher strength of product. Screening before briquetting is necessary: the amount of fines to briquetting process is reduced; strength of product is increased, which is particularly important for lignite coal as it will experience multi loading and unloading for long distance transportation;

(4), Fine briquetting: Fine coal passed screen is briquetted after screening with roll compactor;

Features of BCB / Hot briquetting

Binderless and low cost

Fines are heated up to close its softening temperature and therefore power consumption for briquetting can be reduced to minimum;

Benefits and issue with lignite coal drying

Selection of lignite coal pre-drying technologies

Available drying technologies for lignite coal

Flue gas

fluidized

bed dryer

Flue gas

rotary

dryer

Flue gas

flash

dryer

Superheated

steam fluidized

bed dryer

Steam

rotary

dryer

Microwave

dryer

Size mm 0-50 0-50 0-6 0-6 0-6 0-50

Heating source Flue gas Flue gas Flue gas Steam Steam Electricity

Energy consumption

kw

1 1.25 1.25 1.5 0.8 2

Drying & de-ashing

integrated

Y N N N N N

Water evaporation /

unit

500 30 30 100 30 NA

Investment 1 1,5 1.5 2 2 NA


DDT high temperature flue gas fluidized bed dryer process •High temp flue gas fluidized bed dryer has been developed based on conventional low to medium temp (650 C and below ) to utilize high temp flue gas (900 C to 1000 C ) ; •The inlet flue gas temp was only about 450 C in the original design in the early stage, and it was gradually increased with the increase of production. The highest inlet flue gas was reached 750 C in operation nowadays; •DDT has developed air distribution with independent Intellectual Property Rights which made it possible to increase inlet flue gas to 1000 C;


Features of high temperature flue gas fluidized bed dryer

•Inlet flue gas temperature can be as high as 950C;

•Small equipment dimension, low capital investment and low energy consumption due to higher inlet flue gas temperature;

•Larger capacity: up to 500t/h water evaporation per unit;

•Coarse coal (up to 50mm) can be dried;

•Drying and de-ashing can be integrated;

•Can be designed and operated in slight positive system pressure, oxygen concentration can be controlled to 2.5% or less, which makes sure that drying system is free of coal dust explosion and it is also ideal for coal gasification(for higher yield of tar oil)

•Fine can be separated from coarse coal

•Fine coal can be used as dryer fuel; sold nearby or briquetting for long distance transportation

Flow sheet for high temperature flue gas fluidized bed dryer ( Drying & de-ashing integrated)


LFC process

•Lignite coal is still hydrophilic with drying treatment only, water absorption and spontaneous combustion issues still remain due to no change in structure and property.

•Lignite coal is changed from hydrophilic to hydrophobic with mild pyrolysis treatment, no issue with water absorption and spontaneous combustion as its structure and property are changed.

•LFC is a mild pyrolysis or carbonization process ( moderate temperature and atmospheric pressure):

•The first step is to remove most of water in rotary grate dryer (3% or so);

•The second step is a mild pyrolysis process to remove the remaining water and portion of volatile; to upgrade lignite coal to high-quality solid fuel (Process Derived Fuel, PDF, namely semi-coke) with stable physical and chemical properties. By product, the liquid (Coal Derived Liquid, CDL, namely coal tar), is also produced.

•The third step is purification of the solid product to decrease its activity by passivation treatment.

•For the fines produced in LFC process is briquetted with binder.

Modified LFC process

Drawbacks for LFC process

•Rotary grate dryer and pyrolysis in LFC process are poor in mass and heat transfer;

•Coal to pyrolysis is 3% moisture and at lower temperature requires larger amount of process air flow, which leads to more heating loss and lower heating value of fuel gas from LFC process;

•Briquetting process increases production cost;



Major modifications

Replacement of rotary grate dryer and pyrolyzer with high temp fluidized bed dryer and pyrolyzer due to fluidized bed dryer and pyrolyzer are excellent in heat and mass transfer.

Fluidized bed dryer can be designed as two layers ( drying and heating are integrated ) due to high inlet temp flue gas. Coal to pyrolyzer can dried to 0% moisture and heated up to close its reaction temperature.

Amount of process air flow can be reduced to minimum and loss due to process air can be minumum and heating value of fuel gas from LFC process can be maximum;

Fine briquetting is hot briquetting (Binderless Coal Briquetting, BCB) and production coat can be reduced.


Advantages of modified LFC process over LFC process:

Double layer fluidized bed dryer (drying & heating) vs Rotary grate dryer;

High inlet flue gas temperature (950C) vs low inlet flue gas temperature (455C for dryer and 725C for pyrolyzer);

Low outlet exhaust temperature (75C) vs high outlet exhaust temperature (200C for dryer only, same for pyrolyzer);

Smaller amount of process air flow (1 & 1) vs Larger amount of process air flow 4.5 & 3 ;

Smaller fluidized bed area (2m2& 1m2) vs Larger rotary grate cross area (2m2 & 1m2);

High system energy utilization efficiency vs Low system energy utilization efficiency ;

De-ashing integrated vs no de-ashing integrated ;

Larger capacity per unit (40000t/d) vs Smaller capacity per unit (2000t/d) ;

Low capital investment vs High capital investment

BCB process for fines vs Briquetting process with binder

High tar oil yield due to fast heating speed of coal vs Low tar oil yield due to slow heating speed of coal

High fuel heating value 3 vs Low fuel heating value 1


Factors should be considered for LFC process

•Tar oil ratio from raw coal;

•Tar oil market and price;

•Solid fuel market and price;


system optimization and selection of lignite coal drying process for coal mines

Engineering