coal gentutorial1

Coal Quality Analysis&

Impact on Power Plant Performance

Central Power Research InstituteBangalore

COAL FLOW PROFILE

COMBUSTION SEQUENCE OF COAL

RELEASE OF MOISTURE

RELEASE OF VOLATILES (Devolatilisation)

COMBUSTION OF CHAR (Char oxidation)

COAL

ASH

EMISSIONS

FLY ASH

BOTTOM ASH

(Flue gas)

What is coal?

Coal is a readily combustible “rock” -whose composition consists of more than 50 % by

weight of carbonaceous material

How is coal formed?

Coal is formed by the physical and chemical alteration of “peat” by processes involving bacterial decay, compaction

heat, and time- (coalification)

Peat is an unconsolidated deposit of plant remains from a water-saturated environment (Marshy land)

Rank % Carbon (daf)

% Volatile Matter (daf)

Specific Energy (Gross in MJ/kg, daf)

Wood 50 >65-

Peat 60 >60 14.7

Brown Coal(Lignite)

71 52 23

Sub-Bituminous 80 40 33.5

High Volatile Bituminous Coal

86 31 35.6

Medium Volatile Bituminous Coal

90 22 36

Low Volatile Bituminous Coal

91 14 36.4

Semi-Anthracite 92 8 36

Anthracite 95 2 35.2

* daf -Dry Ash Free Basis

Rank of Coals

COMPOSTION OF COAL

COAL

CHEMICAL OPTICAL / PETROLOGICAL

MOISTURE NON COMBUSTIBLES

COMBUSTIBLES MACERALS

MINERAL MATTER 1. VOLATILE

MATTER2. CHAR

1.VITRINITE2.EXINITE3.INERTINITE

EXINITE

VITRINITE

INERTINITE

VITRINITE- Grey in colour( More reactive )

INERTINITE- White in colour( Less reactive )

EXINITE- Grey-black in colour(Reactive in the presence of catalysts)

PETROGRAPHIC CLASSIFICATION OF COALS

PETROGRAPHIC CLASSIFICATION OF COALS

Proximate Analysis parameters and impact on power plant performance

1. MOISTURE CONTENT OF COAL

Three forms :

I) Surface Moisture : Acquired during spraying water & coal washing

II) Inherent Moisture: Water absorbed in the capillary pores of coal

III) Combined Moisure: Water molecules in the form of hydrates

1. MOISTURE CONTENT OF COAL

•1% moisture reduces 0.07% of flame stability

• High moisture content leads to poor coal flow properties & Mill clogging

• Very Low Moisture content leads to “dusty” environment

• Reduction of heat energy derived from coal

Preferred moisture content for power plant applications= 4-6%


2. VOLATILE CONTENT OF COAL

Contains essentially three compounds

i) Gases: Combustible gases like hydrogen, carbon monoxide, methane, ethane and hydrogen sulphide

ii) Tar: Complex mixture of hydrocarbons and other organics includes benzene, toluene, phenols, cresols,naphthalene, anthracene and free carbon

iii) Ammonia liquor: Contains nitrogen and suphur compounds with cyanides in aqueous condensate


2. VOLATILE CONTENT OF COAL

• Responsible for coal ignition • Very low volatile content leads to poor ignitability and flame

• Very high volatile content leads to spontaneous combustion of coal while storage and smoke formation (leads to forced excess air levels to carry off the smoke)

Preferred volatile content for power plant applications= 25% (daf)


3. ASH CONTENT OF COAL Two forms :

I) Inherent Ash :

Present in the original vegetable material from which the coal was formed consists of oxides like potassium, sodium, magnesium, calcium, silicon and makes up to 0.7-1.2%. This is inseparable form coal

II) Extraneous Ash :

Impurities picked up from adjacent earthy or stony bands in the coal seam. This consists of various materials like shales, clay, pyrites,etc


3. ASH CONTENT OF COAL

• Higher Ash content leads to poor flame stability and ash disposal problems

• Ash with low melting temperatures cause sintered ash depositions (slagging and fouling) over water walls and affects heat transfer.

• Higher ash content leads to poor heat input to the boiler

Preferred ash content for power plant applications= <40% (d) For Indian boilers


4. CHAR (Fixed Carbon) CONTENT OF COAL

• Strong hydrocarbon matrix of coal

• Contains about 90% elemental carbon

• Needs thermal energy with oxygen environment for decomposition


4. CHAR (Fixed Carbon) CONTENT OF COAL

• Most of the heat energy is contributed by char

• Char fixes the combustion rate of coal (Char oxidation is the slowest process in the total combustion kinetics)

• Lesser the combustion rate (coal reactivity) lesser the boiler efficiency


Proximate Analysis

Proximate AnalysisProximate Analysis Surface moisture: Percentage loss in weight when exposing

coal to the atmosphere in the laboratory condition for 24hrs

Total moisture: Percentage loss in weight when the coal is heated to 110oC and kept in the same temperature till it attains constant weight.

Volatile matter: Percentage loss in weight minus Total moisture when the coal is heated to 950oC in inert atmosphere and kept in the same temperature for 7 minutes.

Ash content: Percentage weight remained when the coal is heated to 815oC in oxygen environment and kept in the same temperature till it attains constant weight.

Ultimate Analysis parameters and impact on power plant performance

1. CARBON CONTENT OF COAL

• Amount of carbon depends on the maturity of coal and it increases with coal rank

• Most of the heat energy is contributed by carbon in coal (1kg of carbon gives 8200kcal of heat)

• Knowledge of carbon content is vital in combustion calculations and theoretical air requirement.


2. HYDROGEN CONTENT OF COAL

Though hydrogen releases energy, it combines with oxygen to form water which reduces the available heat content of coal.

(The available heat energy is utilized for the latent heat of vaporization of water formed by the combination of hydrogen with oxygen)

3. NITROGEN CONTENT OF COAL

• Nitrogen is responsible for NOx formation

• NOx is a environmental pollutant which causes acid rain


4. SULPHUR CONTENT OF COAL

Three forms :

I) Pyrites : Iron Sulphide (FeS2)

II) Organic Sulphur : Organic compounds in which sulphur combined with carbon in coal

III) Gypsum : Calcium Sulphate (CaSO4)


4. SULPHUR CONTENT OF COAL

• Sulphur is responsible for SOx formation

• SOx is a environmental pollutant which causes acid rain

• SOx deposits a film of corrosive sulphuric acid on the cooler parts of the plants

• The presence of high quantity of pyrites (Fe2S) causes wear and erosion of mill components and transport pipes.


4. OXYGEN CONTENT OF COAL

• Percentage of oxygen decreases as the rank of coal increases

• The greater the percentage of oxygen in coal, the lower is the tendency to form a high-grade coke

• Presence of oxygen reduces the available hydrogen by combining with the hydrogen to form water


Other Coal parametersOther Coal parameters

Coal parameter About the Parameter Applications in Power plant

Gross Calorific Value

Heat Content of Coal Energy Calculations

Elemental Ash Analysis

Elemental constituents in ash (Si,Al,Ti,Ca

Fe,,K,Na,Mg,Mn,S,P,etc)

Slagging and Fouling tendency of coal

Ash Utilization Heavy metal

pollution Abrasion Index Wear Potential of Coal Wear and erosion of

crushers, pulverizers, coal flow ducts and bends

Ash Fusion Temperature

Melting tendency of ash Slagging and fouling tendency of coal

Parameter Typical Limit

Total moisture(% ) 5-10 15 max

Volatile Matter (%) –air dried basis 18-30 19 min

Ash (%) – air dried basis 30-45 40 max

Sulphur (%)- air dried basis 0.3-0.7 1.5 max

Gross calorific value (kcal/kg) – air dried basis

3000-5000 3000 min

Hard Grove Grindability Index 45-80 45 min

Ash Fusion Temperature(Initial Deformation Temperature oC)

>1600 1150 min

Fouling Index Low Med/High

Slagging Index Low Med/High

Size distribution Maximum 38mmMinimum 3mm

Maximum 50 mmMinimum 0.5mm

Recommended coal quality for pulverized coal power generation

Thanks a lot for your patience

coal gentutorial1

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