Calorific Value Of Coal.pdf 1k2o2z

Calorific Value of Coal

Saurabh Priyadarshi M.Sc. (Geology), MMEAI, MGMI, QCI Chief Geologist

IPL. [email protected]

________________________________________________________________________________  

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Calorific Value of Coal

Abstract Coal is generally analysed to determine its physical, chemical, thermal, mechanical properties like size consist, proximate values, ultimate values, calorific value and HGI. Calorific Value (CV) is an important thermal property of coal which indicates the amount of heat energy that could be liberated on combustion of a specific quantity. The other important thermal properties of coal are plastic, agglutinating and agglomerating indices. These analyses enable determination of suitable end use industry like metallurgical, thermal or cement. This knowledge piece is a scientific compendium ranging from energy or calorie entrapment in coal to its variation with rank, definition and determination of CV, units of expression and interunit conversion of CV, understanding effect of analyses bases of CV , role of CV in evolution of coal grading system ,impact of moisture on CV and impact of CV on coal quantity required for power generation and cost. Key Words: Coal, Calorific Value, Rank, kcal/kg, UHV, GCV, AR, AD GCV, UHV, Calorimeters.

_________________________________________________________________________________ 1. Energy Entrapment. Coal originates from plant debris buried some 280-350 millions years ago in swamps. The contemporaneous solar energy irradiated the plants causing photosynthesis which promoted cellular entrapment of energy in form of carbon, hydrogen, nitrogen and oxygen.

content, hardness descending order.

and

Calorific

Each coal composition.

has

specific

type

value

in

chemical

Low rank coal is low in carbon and high in oxygen while high rank coal is high in carbon and low in oxygen.

Prolonged burial of plant debris under dynamic heat and pressure metamorphosed them into different ranks of coal which release heat energy on combustion.

High carbon coal has high heat energy or calorific value.

This heat energy is called calorific value.

3. Definition of Calorific Value (CV)

2 Types of Coal and Rank

Calorific Value/ CV / heat of combustion (∆Hc0) is defined as the amount of heat energy released

The plant debris under varying regimes of heat and pressure is metamorphosed and transforms into following four types. a. b. c. d.

on burning a unit mass of coal in a bomb calorimeter with oxygen under specific conditions.

The Fuel Content, Heating Value, Heat of Combustion can be used interchangeably with Calorific Value.

Lignite, Sub-bituminous, Bituminous and Anthracite.

4. Determination of CV Each of the above coal type has a specific degree of coalification / rank, maturity, carbon

CV of coal is determined by two methods. 1. Direct Method.

 

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Calorific Value of Coal 2. Indirect Method.

The major difference between GCV/HHV and UHV/LHV/NCV is the water vapour or latent heat produced in the former.

4.1. Direct Method 1 A Bomb Calorimeter is used under static (isothermal) or adiabatic conditions in a lab or a plant to determine CV.

4.1.3. Why GCV/HHV?

4.2. Indirect Method 2 In this method the proximate and ultimate analysis of coal is carried out to determine the radicals listed in (a) and (b) and inputs of (b) are used in formula (i) to calculate approximate CV.

b. Ignite coal sample through a cotton thread connected to an ignition wire inside the steel container. c. During combustion, ensure that core temperature in the Bomb Calorimeter goes upto 1000oC and pressure upto approximately 200 bars for few milliseconds.

a. Proximate analysis: moisture %, ash%,

volatile matter % and fixed carbon % by difference. Use : Fixed carbon gives a rough estimate of coal.

d. Measure the heat and express the heat units as described in Table 1.

Plant

To measure heat units produced on burning unit mass of the coal at constant volume in oxygen excluding water vapor.

4.1.2. Difference UHV/LHV/NCV

between

of CV

b. Ultimate analysis: Carbon %, Hydrogen %, Nitrogen %, Oxygen % & Sulphur %.

Table 1: Expression of Heat

Laboratory.

than

b. GCV/HHV assumes that the all water vapors or latent produced during combustion are condensed and can be recovered.

a. Take about 1g of coal sample in a crucible and place inside a Bomb Calorimeter filled with 30 bar of oxygen (Quality: technical oxygen 99.98%).

To measure heat units produced on burning unit mass of the coal at constant volume in oxygen including the heat of condensation of steam/ water vapour.

less

a. UHV/LHV/NCV assumes that all water vapors or latent heat produced during combustion leave uncondensed and can not be recovered.

4.1.1. Procedure.

Objective of Test

UHV/LHV/NCV

Because

NB: The term Bomb is misleading but is generally used for such equipment. Technically, the term decomposition vessel is more appropriate. CV can also be determined by other apparatus like Solution Calorimeter, Differential Scanning Calorimeter, Titration Calorimeter, Gas Calorimeter and Reaction Calorimeter.

Test Facility

is

Dulong Equation: CV = (144.4 %[C]) + (610.2 % [H])(65.9 % [O]) + (0.39 % [O]2)

Expression of Heat (CV/Heat Units )

(i)

4.3. Does CV vary with Analyses Methods? The CV determined directly is greater and closer to the real figure as compared to indirect method since direct method s for all vapors or heat of condensation.

GCV/HHV

5. Expression of CV. NCV/LHV/UHV

GCV/HHV

The CV of a substance is generally expressed by the units- calories/ per gram. In Asia, Australia and New Zealand, SI system of kcal/kg is used.

and

 

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Calorific Value of Coal In Europe and the Americas following units are in vogue. (Table 2).

The bases are suffixed to the analyses either as a superscript or within braces.

Table 2: Expression of CV. SN

System

Eg:

Unit

Acronym

1

f-p-s

Btu/lb

British thermal unit per pound.

2.

MKS

MJ/kg

Mega Joule per kilogram.

3.

MKS

kcal/kg

kilocalorie per kilogram

4.

MKS

kJ/kg

kilo Joule per kilogram

kcal/kg

MJ/kg

0.004187 kcal/kg

MJ/kg

1.8

kJ/kg

4.187

kcal/kg

238.8

Btu/lb

429.9

kcal/kg

Braces

GCV (adb)/UHV (arb)

:

d. MAF (Moisture and Ash Free) or DAF (Dry and Ash Free): The coal samples exclude Total Moisture and ash forming minerals.

MJ/kg Btu/lb

MAF

c. DB (Dry Basis): The coal sample is exclusive of Total Moisture (surface and inherent) but contains ash forming minerals.

By Factor

Btu/lb

DB

b. AD (Air Dry) or MF (Moisture Free): The coal sample contains only Inherent Moisture and ash forming minerals.

Table 3: Interunit Conversion Factors. Multiply

MF

a. AR (As received): The coal sample is analyzed as received. It contains Total Moisture (surface and inherent) and ash forming minerals.

The CV of coal expressed in different units is interconvertibel (Table 3).

To

AD

GCV /GCV /GCV /GCV /GCV / DAF AR MF AD GCV or UHV /UHV /UHV DB MAF DAF. /UHV /UHV / UHV

6.1. Meaning of Bases.

5.1. Interunit Conversion 3

From

AR

Superscripts:

0.5556 Btu/lb

MJ/kg

7. Evolution of Coal Grading in India

0.002326

kJ/kg

Kcal/kg

kJ/kg

.23884

GCV (Kcal/kg)

NCV (Kcal/kg)

-

Equation: GCV=NCV+53H (ii) Where H = % of hydrogen present in the coal sample including hydrogen of moisture and of water of constitution.

5

Indian coals have been graded on basis of ash and moisture content using several methods since 1924. As the methods were dated, Indian government advised CFRI in 1962 to study the prevalent grading system and recommend a suitable method commensurate with times. 7.1. CFRI Study CFRI undertook an extensive study of Indian coal Industry and practices and came out with their recommendations. Some of the aspects of the study are as follows.

6. Bases of Coal Analyses. Coal analyses is reported under different conditions or bases like AR, AD, MF, DB, MAF and DAF.

7.1.1. Distribution: Indian coals are distributed mostly along the Central and Eastern basins (Fig 14).

 

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Calorific Value of Coal Fig 14: Distribution of Indian Coal

Table 5: Pre 1962 Mining Scenario. Scenario Mining Method

2

Quality

3

Average quality

4 5

6

Key Consumers Key Parameter monitored by Consumers Derivation 1

7

Derivation 2

Study Observations Approximately 85% of coal came from underground and rest from open cast mines. Qualities of Underground coals were superior. Ash%=:20-25; Moisture%=2-5; GCV (kcal/kg) = 5000-5550. Railways(steam locomotives) Loss of heat by steam engines through ash in coal. The constant 8900 came from the highest dmmf CV of coking coals of Jharia coalfield. The factor 138 comes from low moisture and ash level.

7.2. CFRI Recommendation

7.1.2. Characteristics: Indian coals are of drift origin which causes widespread variation in quality. Indian coals range from sub-bituminous to bituminous and lignite rank. Some other characteristics are mentioned Table 4.

SN 1

5

Based on the study of Indian Coal Industry and associated aspects, CFRI recommended UHV system for coal grading for Indian coal (specific formula for high and low moisture coals)

in

High moisture coals UHV = 8900 – 138(A+M)

Table 4: Coal Characteristics.

(iii)

Where A: Ash % and M: Moisture %

SN

Characteristics

1

Drift origin.

2

Drift origin ensures inclusion of mineral matter during coalification.

3

Due to mineral matter, ash content is high.

4

Ash content ranges from 35% to 50%.

5

The inherent ash content is intricately embedded in coal and can’t be removed normally.

6

Removal of inherent ash content in coal requires coal beneficiation.

7

The washability curves of coal show that reduction of ash below a certain level results in generation of proportionate rejects, reduction of yield due to NGM and is costly.

Low moisture low volatile coal UHV = 8900 – 138(A+M) –150(19.0- VM)

(iv)

Where A: Ash % and M: Moisture %, VM: Volatile Matter %

Where ash and moisture % are determined as per IS: 1350-1959 standards after equilibrating coal sample at 60% relative humidity and 40°C temperature. NB: 1.In case of coal with moisture less than 2% and volatile content less than 19%, the UHV shall be the value arrived as above, reduced by 150 kcal/kg for each 1% reduction in volatile content below 19% fraction pro-rata.

8. Correlation between UHV and GCV 5 ? Indian government advised MoC in 2001-2003 to determine whether any correlation exists between GCV and UHV of Indian Coal in order to firm up a valid scientific reason to migrate from prevalent CFRI recommended UHV system to the international practice of GCV system of coal grading.

NGM: Near Gravity Material

7.1.3. Mining Scenario The pre 1962 mining industry scenarios are listed in Table 5.

CFRI undertook study of six major coal fields of India, i.e. MCL, SECL, WCL, CCL, NCL and

 

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Calorific Value of Coal SCCL.In the process 103 ROM samples were drawn for analyses and study.

To nullify the effect CFRI developed a co relation between GCV and UHV GCV=2111+0.6812x UHV

The analyses of coalfield wise samples are presented in (Table 67/2) Table 67/2: Analyses. Coalfields 

Coalfield

Weighted

NB: India continued with the UHV system even after the GOI instituted two major studies well up to December 2011.

Average 9. UHV System Coal Grading System in India.

Moist % 

Ash% 

Ash+Moist % 

GCV 

 60% RH 

 60% RH 

 60% RH 

kcal/kg 

UHV

Government of India classified Indian Non Coking (NC) coals into 7 grades on basis of UHV for marketing in India through notifications. (Table 7)

kcal/kg 

Min  Max  Avg  Min  Max  Avg  Min  Max  Avg  Min  Max  Avg 

Min  Max  Avg 

WCL 

1.5 

9.1  5.3  18.9  60.8  39.9  27.4  65.7  46.6  2104  5379  3742  ‐167  5119  2476 

SECL 

2.2 

9.1  5.7  15.0  43.2  29.1  21.0  50.0  35.5  3555  6345  4950  2000  6002  4001 

NCL 

5.2 

10  7.6  17.4  42.4  29.9  24.6  48.0  36.3  3541  5545  4543  2276  5505  3891 

MCL 

2.5 

8.3  5.4  30.0  52.8  41.4  34.8  57.8  46.3  2585  4460  3523 

CCL 

0.8 

8.4  4.6  23.5  46.5  35.0  27.1  52.0  39.6  3280  5635  4458  1724  5160  3442 

SCCL 

4.5 

8.9  6.7  17.1  46.5  31.8  24.9  41.6  33.3  3311  5681  4496  1779  5464  3622 

This was in vogue till December 2011

924  3380  2152 

Table 7: UHV System Grade

Coalfield wise analyses were studied by CFRI .From the study, CFRI inferred that intra coal field scatters between GCV and UHV is far less than that in inter coal field. (Fig 2 7/2)

A B

Fig 2 7/2: Scatter Pattern of GCV and UHV C Scatter :GCV and UHV in Indian Coalfileds

D

6000 5000

4950 4543 3742

4001

2476

3622

F

SECL

NCL

874.5

WCL

1015.5

2152 1370.5

0

3442

652.5

1000

E

3523

3891

949

2000

4496

4458

3000

1265.5

kcal/kg

4000

MCL

CCL

SCCL

G

CoalMines Intra Mine Diff between GCV and UHV(kcal/kg)

Exceeding 5600 but not exceeding 6200 Exceeding 4940 but not exceeding 5600 Exceeding 4200 but not exceeding 4940 Exceeding 3360 but not exceeding 4200 Exceeding 2400 but not exceeding 3360 Exceeding 1300 but not exceeding 2400

Corresponding Ash% + Moisture % at (60% RH & O 40 C) Not exceeding 19.5 19.6 to 23.8

23.9 to 28.6

28.7 to 34.0

34.1 to 40.0

40.1 to 47.0

47.1 to 55.0

Gross Calorific Value GCV (Kcal/ Kg) (at 5% moisture level) Exceeding 6454 Exceeding 6049 but not exceeding 6454 Exceeding 5597 but not exceeding. 6049 Exceeding 5089 but not Exceeding 5597 Exceeding 4324 but not exceeding 5089 Exceeding 3865 but not exceeding. 4324 Exceeding 3113 but not exceeding 3865

NB: 1.Coal from Assam, Arunachal Pradesh, Meghalaya, and Nagaland are not graded. 2. Coking coal used for steel industry grade-I (ash<15%) and grade-II (ash 15-18%).3. Further classification for semi-coking, weakly coking coal is done on ash %age and UHV.

CFRI further evaluated their inference and attributed the scatters to the prevalent (UHV) grading system, wherein;

10. Conversion Under UHV system UHV-GCV- NCV can be converted from one to another using the following equations:

UHV system assumes that Ash and Moisture values are mostly similar

intracoalfiled but varies inter coalfields. •

Useful Heat Value (UHV) (Kcal/Kg) UHV= 8900138(A+M) Exceeding 6200

Inter Mine GCV(kcal/kg)

Inter Mine UHV(kcal/kg)

•

(v)

UHV= 8900 - 138(A+M) GCV= (UHV + 3645 -75.4 M)/1.466 NCV = GCV - 10.02M

GCV system considers organic and petrographic composition of coal is almost similar intracoalfiled but varies with intercoalfiled.

(vi) (vii) (viii)

(UHV, GCV and NCV are in kcal/kg, (“A” is % Ash; “M” is % Moisture).

 

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Calorific Value of Coal 11. GCV System of Coal Grading in India.

Comparative standout differences between UHV regime and Present Mining Practices presented (Table 9.) .The appreciation of comparatives will facilitate resolution of dilemma.

From January 20126 Coal India Limited (CIL) migrated from Useful Heat Value (UHV) system of grading, marketing and pricing non-coking (NC) coal in vogue since 1979 to Gross Calorific Value (GCV) system from 1st January 2012.

Table 9: Differences. SN 1

Under GCV system the NC coals have been classified into 17 bands of 300 KCal/Kg each ranging from 7000 kcal/kg to 2200 kcal/kg (Table 8 7/1).vis-à-vis 7 grades in the erstwhile UHV system.

2

Table 8 7/1 GCV System. SN

Grade

2.1 2.2 2.3

Kcal/kg GCV( Min)

GCV(Max)

3

1

G1

7000

2

G2

6701

7000

3

G3

6401

6700

4

G4

6101

6400

5

G5

5801

6100

6

G6

5501

5800

7

G7

5201

5500

8

G8

4901

5200

9

G9

4601

4900

10

G10

4301

4600

11

G11

4001

4300

12

G12

3700

4000

13

G13

3400

3700

14

G14

3101

3400

15

G15

2801

3100

16

G16

2501

2800

17

G17

2201

2500

the are the the

4

5

6

UHV Regime The figure 8900 in the formula {UHV= 8900 138(A+M)} came from highest dmmf CV of coking coals of Jharia coalfield only. Coal quality presumably consistent. Ash%: 20-25 Moisture%: 2-5 GCV(kcal/kg): 50005550 Figure 138 in the formula {UHV= 8900 - 138(A+M)} applicable to low range of moisture and ash. Coal primarily consumed by steam engine of railways-one of the most inefficient steam generation equipment. UHV determined indirectly by deducting ash and moisture. UHV employed in India only for coal grading.

Present Mining Practice This figure 8900 is irrelevant today as coal is excavated from several coalfields, i.e. Mahanadi, Ib valley, North Karanpura etc.. Coal quality varies. Ash%: Moisture%: GCV(kcal/kg):

>20-45 6-11 35005000 Weightage factor of 138 is not applicable to present day high range of moisture and ash. Coal consumed by modern industries which use more efficient stoker fired system.

GCV determined directly by a bomb calorimeter. GCV employed almost globally for coal grading.

12.2 .Comparison: UHV vs. GCV •

In UHV system the coal is classified into 7 grades ranging between 600 to 1100 kcal/kg while in GCV system coal has been classified into 17 bands of 300 kcal/kg each; which is much narrower than UHV system.

•

Due to narrow calorific bandwidth in GCV system the variation in coal quality shall be far less than coals received under UHV system

•

Due to narrow GCV bandwidth consumers can chose coals of desired grade and not end up paying for a coal in large grade band.

•

The graphic differences between GCV and UHV grading system is presented in Fig 3 7/1 .

12. Grading Dilemma: GCV or UHV? Coal is a complex substance. An understanding of coal, its exploitation and usage is necessary to resolve the dilemma as the DNA of any mineral grading system is encrypted in its origin, geology petrography, quality, mining technology and end use etc. 12.1. UHV Regime vs. Present Mining Practice 5

 

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Calorific Value of Coal Fig 3 7/1: Difference: GCV vs. UHV Comparison: GCV vs.UHV Grade System 8000.00

The combustible material in coal remains unchanged right from the point of release (mines/stockyard/purchase point) to the point of consumption (power plant/steel plant/cement plant etc) except for Total Moisture (TM) %.

Rs/MT Grade

7000.00

GCV(New ) Min GCV(New ) Max

6000.00

UHV(Old) Max UHV(Old) Min

kcal/kg

5000.00

GCV(Old) Min GCV(Old) Max

4000.00 3000.00

TM in coal comprises Inherent Moisture (IM) and Surface Moisture(SM). IM is locked up in the molecular structure of coal and can be removed only by heating to temperature over 1000C. SM lies on surface of coal and can be removed by evaporation/prolonged exposure to atmosphere.

2000.00 1000.00 0.00 G1

G2

G3

G4

G5

G6

G7

G8

G9 G10 G11 G12 G13 G14 G15 G16 G17 Bands

Presence of moisture in any form reduces the CV of coal as shown in Table 10.

12.3. Dilemma Resolved: It’s GCV From discussions in SN 11.1 and 11.2 it is concluded that UHV system is dated and out of sync with present day mining practices. Figures and tables highlight GCV as an unbiased and contemporary system in of geology and consumer.

Table 10: Effect of Moisture on CV of Coal. Metrics

Total Moisture(TM) % Inherent Moisture (%) Surface Moisture (%) Calorific Value GCV(kcal/kg)

GCV system is relevant, scientific, ensures conservation of mineral and provides value for money to consumer as well as the supplier. 12.4 India: Globally Aligned Coal Grading. India embraced GCV grading system in 2012 after 33 long years and aligned itself with global coal grading system

Remarks

Point of Release (PoR) 12.0 2.0 10.0 4000

1. TM and GCV are determined in lab. 2. TM and GCV are reported on AR basis.

12.5. Should India Continue with GCV?

Point of Consumption (PoC) 8.0 2.0 6.0 PoC

{(100-TM )/(100-TM PoR PoR }*GCV (ix) ={(100-8)/(10012)}*4000 =4181.81 1. TM and GCV are determined in lab. Coal is spread out and air dried to remove Surface Moisture. 2. GCV is determined by applying Total Moisture Correction and reported on AD basis.

From the above example it is observed that CV of coal at point of release with high moisture is lower than at the point of consumption due to reduction in moisture which causes increase in combustibles per kilogram.

GCV considers the potential energy /intrinsic energy for expression of energy content in coal which is an important physical principle. India should hold on to the GCV system and not relent to cartels. India shall do well to voice its scientific temper by seeking coal supplies for industry specific use based on Ash, Moisture, Sulphur, HGI and HGI values.

14. Effect of Calorific Value on Coal Quantity and Unit Cost of electricity.

Such secondary parameters should remain contemporaneous with mining practice and end usage.

Coal provides energy to heat up boilers in a power plant. The energy comes from CV of coal. Hence CV of coal plays an important role in design of boiler-turbine-generator and in firming up quantity and quality requirements and in

13. Effect of Moisture on Calorific Value.  

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Calorific Value of Coal estimation of generation.

cost

per

unit

of

10765/20000=.538 kg /hr

electricity

(xii)

Hence for 1000 MW/hr, coal quantity will be

Eg: Sample calculation for a 1000MW coal fired power plant (All figures in 11.1 to 11.7 are assumed)

.538 x1000 x1000 = 538000 kg/hr = 538000/1000=530.8 T/hr. (xiii)

14.5. Coal Cost.

14.1. CV of Coal / Energy Content

Coal cost includes all expenses starting from mining to delivery of coal to the power plant.

20000 kJ/kg* or 4760 kcal/kg GCV (*Where 1kJ=.238 kcal).

Coal of CV 20,000 kJ or 4760 GCV (which is G9 as per GCV grading system introduced in India from January 2012) is marketed at Rs 1844/MT

14.2. Power Plant Efficiency. In a power plant, energy conversion takes place in 2 stages

Hence the cost of coal required for 1000 MW Power Plant/hr will be

Ist Stage: Boiler and Combustion Efficiency:

530.8 t x Rs 1844 /hr = Rs 978795.20

Efficiency of 88% on GCV basis. (This is the

(xiv)

normal value for a well optimized power plant.)

14.6 Unit cost of electricity generation.

2nd Stage: Steam Cycle Efficiency:

A 1000 MW Power Plant produces 10, 00,000 units of electricity. Hence the cost of coal per unit will be

Modern Rankine cycle adopted in coal fired power plants have efficiencies ranging between 32 % to 42%.This depends on steam parameters like pressure and temperature. Higher steam pressures and temperatures in the range of 230 bars and 6000C respectively have efficiencies around 42%. For calculation let’s consider

Rs 978795.2/10, 00,000 = Rs 0.9787. (xv)

15. Conclusion This KP is a one stop shop for all those who wish to know and those who need to specifics of mineral exploration, mine planning, geology, quality control and HSE. The present article is yet another contribution to the KYC (KYC (KNOW YOUR COAL) series.

Efficiency of 38%.

Hence overall Power Plant Efficiency 38% of 88%=33.44%

(x)

14.3. Heat Rate.

_______________________________________

Heat rate is the amount of heat required to produce one unit of electricity. One KW is equivalent to 3600 / kJ/ hr.If energy conversion is 100% efficient then 3600 kJ will be required to produce one unit of electricity.

References. 1. IKA Working of Bomb Calorimeter. 2. Properties of Coal. 3. Coal Conversion facts. 4. Coal Initiative Report. 5. CFRI Diamond Jubilee Conference. 6.2nd Ver. Coal Price 2012. 7. Tables and Graphs. 8. Revised Coal Price.

Considering a Power Plant Efficiency of 33.44 % the heat rate would be 3600/33.44%=10765 kJ / kW / hr (xi)

14.4. Coal Quantity For producing 1 KW / hr with coal of CV 20,000 kJ/kg, coal quantity will be

 

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Calorific Value of Coal Acronyms Term

Expression

CV

Calorific Value

HGI

Hard grove Index

1g

one gram

oC

Zero degree centigrade

%

percent

GCV

Gross Calorific Value

HHV

Higher Heating Value

NCV

Net Calorific Value

LHV

Lower Calorific Value

UHV

Ultimate Heating Value

(f-p-s

foot-pound-second

MKS

Meter, Kilogram, Second

Btu

British thermal unit

MJ

Mega Joule

Lb

pound

Kg

kilogram

kJ

kilo Joule

SI

System Internationale

_

Released by email on 21st September 2012-09-2012 To: All BD Team CC: CEO, VP (ATPP), VP (NTPP),

NB: All readers are requested to email their s.

 

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