System optimization and selection of lignite coal drying process for power plants

Solutions for Coal Drying

System optimization and selection of lignite coal pre-drying process for power plants

Delta Drying Technologies Ltd Add: Suite 219, Block B ，Qilu Software Park, No. 1 Shunhua Rd, Jinan, China Email: [email protected]; [email protected] Website www:coaldryingtech.com Tel 0086-13953108165; 001-610-829-9317

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Lignite Coal Moisture 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

How moisture affects boiler

Raw lignite coal entering to furnace without pre-drying leads to

•Increasing volume of flue gas

•Lowering furnace temperature

Larger boiler furnace being needed

Increasing capital cost for boiler;

Increasing operational and maintenance cost;

Influence mechanism and degree of moisture on boiler efficiency

Two major influence factors:

•The evaporated moisture from lignite coal drying immediately prior to combustion is carried by the flue gas through the boiler furnace, which increases the humidity of flue gas greatly and specific heat of vapor is one time higher than dried flue gas;

•Flue gas temperature is raised due to the increase of acid dew point temperature due to higher flue gas humidity;


Raw coal @ 65% TM Raw coal dry to 10% TM

Dried flue gas kg/h 1215000 1035000

Flue gas humidity g/kg dried air 206 13

Vapor in flue gas kg/h 263000 13000

Vapor / dried air % v/v 26 2

Exhaust temp / acid dew point

temperature C *

170 140

Loss due to exhaust % 26.7 5.5

Raw coal @ 40% TM Raw coal dry to 10% TM

Dried flue gas kg/h 1089000 1035000

Flue gas humidity g/kg dried air 74 13

Vapor in flue gas kg/h 85000 13000

Vapor / dried air % v/v 11 2

Exhaust temp / acid dew point

temperature C *

155 140

Loss due to exhaust % 12.4 5.5


• Relationship between heating value and moisture of lignite coal and boiler efficiency

How pre-drying improves boiler efficiency

Major factors why pre-drying can improve boiler efficiency

•Moisture evaporated from lignite coal in the pre-drying process is discharged out of drying system, not carried to boiler furnace;

•Humidity of flue gas is reduced as a result and acid dew point temperature and flue gas temperatures are decreased as well;

•Exhaust temperature from pre-drying process is even lower than from boiler (it is 70-75C for flue gas as drying medium; and 105C for superheated steam as drying medium)

•Acid corrosion problems caused by lower drying exhaust can be fixed easily (locations for acid corrosion are inner wall of drying chamber, cyclone and duct)

•It is low grade energy and work is done for low pressure stem from turbines in the superheated steam drying system.

Lignite coal pre-drying

technology

A，RWE WTA technology •Dryer from WTA process is fluidized bed dryer with immersed steam heat exchanger, that is heat required for drying process is supplied by immersed steam heat exchanger; •Drying system is close loop system with superheated steam as fluidized medium, not air for conventional fluidized bed dryer; •Steam pressure for immersed heating exchanger is extracted from turbine at 4 bar; •After separated, part of exhaust is recycled as fluidized medium, the rest of exhaust is discharged out of drying system; •Fine raw with 0-3mm is fed to dryer from top and discharged at bottom;


technology

B，GRE DryFining technology •DryFining is drying and beneficiation integrated process; •Dryer from DryFining process is fluidized bed dryer with immersed hot water heat exchanger, that is heat required for drying process is supplied by immersed hot water heat exchanger; •Hot water for immersed heating exchanger is heated by cooling water first, then by low temp flue gas; •Drying system is open loop system with hot air as fluidized medium heated by low temp flue gas; •Coal with high ash content is settled down at bottom of drying chamber, then is discharged out of drying chamber and is separated by dry separation process. Coal gangue is discharged out of drying system and clean coal is returned to dryer for further drying;


technology

B，GRE DryFining process •Advantages of DryFining technology: Energy consumption is lower in term of quality of energy utilized as waste energy utilized coming from cooling water and flue gas after economizer; Ash can be removed partially; •Discharges of DryFining technology: Dependent on power plant; Only partial water can be removed due to limitation of available waste energy; Larger capital investment due to very low fluidized medium temp and hot water;


technology

C，DDT fluidized bed dryer with immersed steam heat exchanger process •Delta Drying Technologies (DDT) developed fluidized bed dryer with immersed steam heat exchanger process for lignite coal drying for power plants. It is slightly different from RWE WTA process: Close loop superheated steam as fluidized medium in WTA process vs Semi close loop moist air as fluidized medium; Higher purity of exhaust (steam content >95% v/v) vs lower purity of exhaust (steam content >85% v/v, O2 concentration <3.5%); Higher requirement for system sealing vs lower requirement for system sealing; Higher capital investment vs lower capital investment; Lower cost for exhaust utilization vs higher cost for exhaust utilization;


technology

D，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;


technology

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;

Selection of Lignite coal pre-drying technology

Fluidized bed dryer with immersed steam heat exchanger for lignite coal pre-drying


High temp flue gas fluidized bed drying process


Comparison between lignite coal pre drying processes As only partial of water can be removed with DryFining technology due to its limitation of available waste energy. The percentage of water can be removed for different lignite coal is shown in the following table:

30% - 18.5% / 30% - 10% = 63% 38% - 29.0% / 38% - 10% = 40% 40% - 31.5% / 40% - 10% = 37% 50% - 44.3% / 50% - 10% = 23% 60% - 56.4% / 60% - 10% = 15% 65% - 62.3% / 65% - 10% = 12%

For lignite coal with 40% of moisture, it can only be reduced to 31.5%, its water evaporation is only 37% of water evaporation from 40% moisture to 10% moisture. Therefore, DryFining technology is excluded in comparison. Comparison is only conducted among fluidized bed dryer with steam heat exchanger and flue gas fluidized bed dryer;


• Comparison between technologies Fluidized bed dryer with immersed steam heat exchanger and flue gas fluidized bed dryer ;

• Raw coal moisture, heating value, have effects on capital investment, CO2 reduction, power output; • Pre-drying improve boiler efficiency; reduce fan power consumption due to reduction of flue gas;

reduce mill power due to tonnage reduction; • Pre-drying also occurs power consumption and steam or coal consumption for drying process;

• The following table is for reference only, please contact with Delta Drying Technologies Ltd for project argumentation;

Thermal value db kcal/kg

Plant efficiency with wet coal @ 60% moisture

Plant efficiency with dried coal

@ 10% moisture

Water evaporation for 300mw capacity

t/h

Investment for dryer

m$

CO2 reduction

Power output

reduction

5000 (WTA) 31.40% 36.60% 180 36.00 14.2% 10.2%

5000 DDT Steam dryer 31.40% 36.60% 180 27.00 14.2% 10.2%

5000 High temp dryer 31.40% 34.20% 180 18.00 8.2% 2.0%

Thermal value db kcal/kg

Plant efficiency with wet coal @ 40% moisture

Plant efficiency with dried coal

@ 10% moisture

Water evaporation for 300mw capacity

t/h

Investment for dryer

m$

CO2 reduction

power output

reduction

5000 (WTA) 36.30% 38.60% 67 13.40 6.0% 4.6%

5000 DDT Steam dryer 36.30% 38.60% 67 10.05 6.0% 4.6%

5000 High temp dryer 36.30% 37.60% 67 6.70 3.5% 1.6%


• The following factors should be taken into consideration: Dust emission permit; Flow sheet for existing power plant (pulverized system, boiler, Air pre-heater, Econimizer,

ESP or bag house, FGD, WESP)


Economic Evaluation of power plant lignite coal pre-drying technology • Lignite coal pre-drying projects can save

energy, but not save money under certain conditions due to varied mining cost, moisture and heating value;

Mining cost is about few dollars per ton in Victoria Australia;

Mining cost is from 7 EU to over 50EU in Europe with highest in Turkey;


Economic Evaluation of power plant lignite coal pre-drying technology •Its IRR is very low, even negative for DCF analysis for power plant lignite coal pre-drying project without consideration for CO2 emission in Australia (carbon tax was repealed in 2014); •Its IRR is over 25% for DCF analysis for power plant lignite coal pre-drying project without consideration for CO2 emission in Turkey; •Its IRR is over 25%-35% to 46%-76% for DCF analysis for power plant lignite coal pre-drying project without consideration for CO2 emission in China depending on distance from coal mines, far from coal mines, higher IRR. Its IRR even higher for power plants to convert from hard coal to change to lignite coal;

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.

LFC Process for Power Plants

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 lower coal 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;


Modified LFC process

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


Advantages of modified LFC process for power plants •Only produce liquor fuel (CDL), solid and gas fuel are used for power generation; •Shorter flow sheet and lower capital cost; •Ideal for power plants with lower mining cost in Australia, Greece, Germany, etc; Economic Evaluation of modified LFC process for power plants •For power plants in Australia and Greece, its IRR could be up to 50%; •For power plants in Germany, its IRR could be up to 35%; * Its IRR is also depend on ratio of tar oil from raw coal and tar oil price;


Flow sheet for modified LFC process


System optimization and selection of lignite coal drying process for power plants

Engineering