Burning Coal in Power Plants – Calorific Value and Moisture

Written by: johnzactruba • Edited by: Lamar Stonecypher
Updated Oct 20, 2009

Coal is the primary fuel for producing Electricity. Some of the characteristics of coal have profound influence on the day to day working and economics of the power plant. This article discus two of the important characteristics – Calorific Value and Moisture.

Calorific Value or Heating Value

This is the most important parameter that determines the economics of the power plant operation.

It indicates the amount of heat that is released when the coal is burned. The Calorific Value varies on the geographical age, formation, ranking and location of the coal mines. It is expressed as kJ/kg in the SI unit system. Power plant coals have a Calorific Value in the range of 9500 kJ/kg to 27000 kJ/ kg.

The calorific value is expressed in two different ways on account the moisture in the coal. Coal contains moisture. When coal burns the moisture in coal evaporates taking away some heat of combustion which is not available for our use.

When we say Gross Calorific Value or Higher Heating Value it is the total heat released when burning the coal.

When we say Nett Calorific Value or Lower Heating Value it is the heat energy available after reducing the loss due to moisture.

The Heating Value determines how much fuel is required in the power plant. Higher the Calorific Value lesser the amount of the coal required per unit of Electricity. Higher Calorific value also means the cost of the coal is higher but is offset by the lower cost of logistics, storage and ash disposal.

Moisture

The coal when mined contains moisture. The moisture is in two forms. First is the inherent moisture which is entrapped within the structure of the coal. Second is the external moisture that is outside of the coal structure. The amount of moisture depends again on the geographical age, location and condition in the mines. A part of this moisture can easily evaporate in atmospheric conditions during its transfer from the mines, storage at the power plant and finally feeding to the boiler in the power plant. Depending on where and when you determine the moisture, values will be different for the same of coal.

The amount of moisture determines how much of heating is to be done to dry the coal before it is burned in the boiler.

Reporting Coal Properties

Moisture in coal is expressed as % by weight. So the change in the moisture content changes the proportion of the other coal constituents and the Calorific Value.

‘As Received’ coal, is the coal received in the power plant premises. The payment to the coal companies are normally made based on the ‘As Received’ coal properties.

‘As Fired’ coal is the coal entering the boiler system. The performance of the boiler and power plant is based on the ‘As Fired’ coal properties.

‘Air Dried’ coal is what is used in the laboratory for analysis. This coal is dried in atmosphere and has the lowest amount of moisture. Laboratory results are reported as ‘Air Dried’ coal properties.

The difference between the above three conditions is the proportion of the Moisture. The Calorific Value and other coal constituents analysed in the laboratory on ‘Air Dried’ basis is converted to ‘As received’ or ‘As Fired’ basis proportional to the moisture content.

Mah_samy1984 Feb 14, 2012 11:07 PM

RE: Burning Coal in Power Plants – Calorific Value and Moisture

I find this topic is very interesting but I was searching for the temperature in the furnace of the coal, how much degree Co it reaches. If anyone can answer my question I will be thankful for him and I would like to know what his reference is for his temp. Degree

avinashjee Feb 3, 2012 5:34 PM

There is third type of Moisture called "Standardized Moisture" or Moisture at 60% RH at 40 degree C.This became necessary for Indian Power-grade Coal having high moisture contents.The Air drying in Laboratory was not a standard because of seasonal variation in Air dried Moisture.All results of Proximate Analysis are converted on Standardized Moisture so that these can be reproduced in Laboratories in all seasons.

avinashjee Feb 1, 2012 6:53 PM

During rains the water adds to the net weight at the powerhouse end but this free moisture gets evaporated during stacking and while pulverizing for boiler feed.

avinashjee Jan 30, 2012 6:10 PM

Most of the Indian power grade coal is of High-moisture category(>2%). There is seasonal variation in Air dried Moisture of laboratory Coal samples, as such these samples are kept under standardized conditions of "60% Relative Humidity at 40 C" for 48 hours. The Moisture % so determined is termed as "Moisture at 60%RH at 40C". This Moisture value is used as a basis for standardized Ash% and VM% calculations and GCV is obtained by a Formula based on these three parameters.

RD Patel Jan 17, 2012 3:59 PM

can not reduce the total moisture of indonesian coal because this coal is very hygroscopic only 10% moisture reduse if such type of coal spread in sun heat

johnzactruba Jan 10, 2012 11:39 PM

Yes 4200 is kcal/kg and 2400 is kcal/kwhr.

Raj Jan 10, 2012 12:04 PM

Whatever you are saying is in Kcal/kg not in KJ/kg

Ashok Pradhan Dec 17, 2011 6:14 AM

Is it advisable to cover wagons/trucks carrying coal to destination by plastic sheets in the rainy season to avoid excess of water getting in to coal to reduce the surface moisture.Higher the moisture higher the energy required to drive away the same.? Ashok Pradhan

roshan Jun 30, 2011 1:28 AM

Clarification on GCV ( ARB)

One of my client asked for GCV ( ARB) but all the testing agencies give GCV (ADB) and NCV ( ARB). Is he right in asking what he is asking and why is he asking that GCV(ARB) and not GCV (ADB).

Apu Mazumder Jun 15, 2011 8:31 AM

Power Plant

Wheather Station Heat Rate of a coal based Thermal Power Plant is linearly proportional to the GCV of Coal.

johnzactruba Mar 27, 2011 7:24 AM

reply lokesh

the units that you are mentioning is 4200 kcal/kg and 2400 kj/kwhr.

lokesh Mar 25, 2011 6:22 AM

how to calculate coal quantity for powerplant

Dear sir,
I have doubt that GCV value mentioned by your article,because u mentioned that 10500 kj/kg to 25000 kj/kg depending upon the coal type.But in our plant we are using Indonesian coal,it has GCV of 4200 kj/kg.and heat rate is 2400 and whether that is correct r not.our plant capacity is 2 x 150MW.please tell me Calculations sir,

johnzactruba Mar 12, 2011 10:02 AM

reply Julius Anyega

The briquettee has moisture around 10 % or lesser. This gives roughly 4700 kwh/tonne. Logs on the other hand have much higher moisture. For dried logs it can be 30 %. This giives approximately 3200 kwh / tonne. For the same heat output you require only 70 % by weight of briqutettes. Briquettes may have also lesser ash content.

Julius Anyega Mar 12, 2011 7:47 AM

Calorific Value and Heating

I need help to understand what the following illustration means
_______________________________
A tea Factory uses 1 Cubic metre of wood logs (approximately 450kgs) which as a total calorific value of 5170. 1 Kg of Wood briquettes from China has a calorific value of 4300 - 4800.What is the impact of using wood logs and wood briquettes on heating, and amount of density required.

Ery Dec 27, 2010 10:44 PM

Panamax Vessel

Dear Sir,

May i check with you, for panamax vessel, who shall bear the cost for the use of floating cranes to load the coal into the vessel? would it be the buyer or seller.

Thank you

johnzactruba Dec 17, 2010 10:36 PM

reply FORY

Gross CV include the heat relaesed by combustionof Carbon and Hydrogen in the coal.
Hydrogen combustion produces moisture. This moisture and the moisture already present in the coal evaporates during the combustion. This takes away some of the heat produced. So the Nett heat available is less than what is produced . This is called Nett Calorific value.

ery Dec 15, 2010 10:43 PM

Difference between GAR and ARB

Dear SIr, May i check with you if there is any difference between GAR and ARB or can it be used interchangeably? Because I have a coal specs that for instance is for GAR 4700 and i notice the ARB fro calorific value is also 4700. Does this meant hat GAR refers to GCV for ARB? Thank you!

FORY Dec 15, 2010 6:01 AM

DEAR SIR

CAN YOU PLEASE TELL ME WHAT IS GROSS CALORIFIC VALUE (NAR) AND WHAT SI NET CALORIFIC VALUE (ARB)... HOW DO I GET NCV (ARB) FROM GCV( NAR)

johnzactruba Sep 1, 2010 7:01 PM

reply reddy

when the moisture increases the cv willr educe.

reddy Sep 1, 2010 6:26 AM

while moisture increse what abt the GCV, it will increase or decrease or remains same

johnzactruba Jul 31, 2010 11:06 AM

reply Sunil K

The most cost effective way to dry coal is by air drying. This requires the coal to be spread thin to have maximum contact with air. This is possible in a laboratory for finding the air dried moisture where the sample size is only 1 kg or less.
But in a mine this simply not possible since your dealing with millions of tons of coal. If there was system the energy requirement would be so high making the coal cost prohibitive.
The maximum moisure that gets removed may be in the order of 5 % to 10 %, again depending on duration of storage , method of transportation, ambient temperature and humidity, size of coal and other reaasons ;.

Sunil K Jul 29, 2010 5:34 AM

% of Total Moisture in Coal

I would like to Know any article / report as to how much and how moisture can be reduced from Total Moisture (%) .If coal from Indonesian mine is having 45 % Total moisture ( 17% Inherent Moisture) ..how much maximum moisture can be reduced before actual loading into the Ship.

Prasad Jul 10, 2010 11:52 AM

coal

Pls send me the info on the various parameters that are requiredwhen coal is required to be used in thermal power sector and also steel industries please.
what arethe specs and their levels or percentage thats required to be?

v.j.patel Jul 10, 2010 12:11 AM

coal drying process

i am need of drying process of lignite coal of r.o.m. of 47 % and would like to take advantage of drying before shippong to port. coai is mined from open deposite and stored at coal yard, waiting for arrival of ship to load at jetty.

high moisture is my concern. and would like to mini mise as possible with economical viable process of drying it.

Prabir Ghosh Jul 3, 2010 12:09 AM

To Calculate NCV.

if i have 6000 GCV(ADB) coal on an air dried basis, what formula would i use to calculate the net calorific value of the same product, and what factors are involved

Prabir Ghosh

Tom Jun 30, 2010 4:03 AM

Issues in transportation

Taking into account real world issues,

What is normal for transit loss in calorific value (in percentage terms)? What would be considered outside normal?

For example, I have to send coal that is 6300-6100 ADB from Indonesia to China, will it arrive as it was sent out? Or will the product change during transport?

danny Jun 29, 2010 2:36 AM

NAR Formula

I have a future buyer from Korea. They asked me about NAR (Nett As Received). Their country use NAR measure instead of calorie. I believe it contains how much calories when they received my coal. All i need to know what is NAR Formula. Thank you, Sir.

johnzactruba May 26, 2010 8:08 AM

reply sandeep

For coal firing it is the normal practice to use GCV.

sandeep May 26, 2010 6:06 AM

Global practice to calculate bolier efficency GCV

while calculating Boiler efficency we have to used the GCV or LCV .....logically which is better..

salsabil May 16, 2010 6:58 PM

::: inquiries regarding the coal and please reply as soon as possible:::

Dear Sir,

1. What are the differences between lignite, bituminous and anthracite coals from a furnace design perspective ?

2. What is the main difference in furnace design requirements for gas, oil and coal fuels ?

3. why erosion is an important issue in coal fired furnaces(What are the main reasons), the reason for its occurrence and in what boiler components it is usually encountered ?

4. state the role of superheaters and reheaters in the design of boilers and also what is their effect on plant efficiency ?

with the kindest regards.

salsabil.

Dr. R. S. Sharma May 12, 2010 10:27 AM

GCV(ADB) AND GCV(DMF)

I would like to know , how can iwe change GCV (DMF) TO GCV(ADB) kindly explain the GCV(DMF).

johnzactruba Apr 21, 2010 9:34 AM

reply Moftah

When you say NCV ( Nett Calorific Value) it means the effect moisture ( both Moisture in coal and the moisture formed due to Hydrogen is removed).
If you take 1 kg of coal on dry basis it means the sample of coal has no moisture. It Can be ultimate analysis or it can be proximate analysis.

MOFTAH Apr 21, 2010 8:54 AM

the NCV OF COAL IN dry basis

Dear Sir,

I am woundering about the formula which we can use to calculat the NCV OF COAL IN dry basis.

(dry basis means excluding the all moisture)
secondly, does the dry basis mean the ultimate anylsis of the coal OR NOT????????

please reply as soon as possible

kind regards ,

Moftah

Prabir Ghosh Mar 19, 2010 5:29 AM

GCV on as Fired Basis

Refer to your article "COAL ANALYSIS – Effects and Corrections due to Moisture" at
http://www.brighthub.com/engineering/mechanical/articles/61239.aspx, I am very much thankful to you that clear my doubt and it has been more informatic side which you clear all doubt, in future I would like to know from you more.
Once again thank to you.

Prabir

Read more: http://www.brighthub.com/engineering/mechanical/articles/22202/comments.aspx?cp=2#ixzz0ic4QFGKA

Irfan Afzal Mirza Mar 18, 2010 1:47 AM

Excellent Article

Sir Thanks,

After reading the article and comments, i think i have more clear basic concepts now.

Thank you for helping all of us.

Personal Regards,

Irfan Mirza

johnzactruba Feb 20, 2010 6:50 PM

reply Ranoliya Laxman

Please read the following article which explains the correction calculation.
http://www.brighthub.com/engineering/mechanical/articles/61239.aspx

Ranoliya Laxman Feb 19, 2010 12:12 AM

How calculate FIRE BASIS GCV of COAL

We want to know the calculation of GCV to Fire basis

johnzactruba Feb 18, 2010 8:14 AM

reply BHAGVAT

It is not possible to find Sulphur and Phosphorus in Coal from Proximate analysis.
Proximate anlaysis basically indicates the combustible contents and the inert contents.

BHAGVAT Feb 17, 2010 3:02 AM

DEAR SIR
HOW TO FIND OUT SULPHUR AND PHOSPHORUS CONTENT IN THE COAL ON THE BASE OF PROXIMATE ANALYSIS.

BHAGVAT Feb 17, 2010 2:58 AM

GCV OF COAL (DRY BASIS)

dear sir,
please show me formula for gcv of coal on theDRY BASIS.

johnzactruba Jan 18, 2010 5:38 AM

reply antonius

You can use the formula if you know the Moisture %
NCV = GCV - 10.2 x M
where GCV is in kcal/kg an M is Moisture %. This a good estimate for indian coals. - sub bituminous.
Actually this will depend on the Hydrogen content in the coal, which is normally available only in an ultimate analysis

antonius Jan 18, 2010 2:45 AM

Calculating Nett Calorific Value

if i have 6300 GCV coal on an air dried basis, what formula would i use to calculate the net calorific value of the same product, and what factors are involved?

Prabir Ghosh Jan 13, 2010 1:33 AM

TM & GAR Penalty

Dear Sir,
We intent to know that can we penalise in TM & GAR on Coal product ?

johnzactruba Jan 12, 2010 8:35 AM

reply IMRAN MULANI

Sampling is very important in coal analysis because of the nature of the coal. Proper procedure has to be followed in taking the sample. becasue what is analysed is only a few grams from thousands of tons of coal. The same also applies to BCM.

johnzactruba Jan 12, 2010 8:30 AM

reply Prabir Ghosh

Please read my article "COAL ANALYSIS – Effects and Corrections due to Moisture" at
http://www.brighthub.com/engineering/mechanical/articles/61239.aspx

johnzactruba Jan 10, 2010 9:56 AM

reply jani

GAR is Gross As Received basis indicating the Gross Calorific value and analysis.
ARB is as received basis analysis and is the same.

jani Jan 10, 2010 4:31 AM

GAR and arb

Dear sir,

I need to know is that the same between GAR and arb? Thank you

Prabir Ghosh Jan 5, 2010 1:09 AM

I would like to know how to calculate the CV on as fired basis if we have GAR & GAD of Coal.

Read more: http://www.brighthub.com/engineering/mechanical/articles/22202.aspx#ixzz0biSTiiTL

Milind Nerlekar Dec 28, 2009 7:22 PM

CV on as fired basis

I would like to know how to calculate the CV on as fired basis

IMRAN MULANI Dec 27, 2009 6:56 AM

Calculation of GCV based on Ash & Moisture ARB

is there any recognised formla for calculating GCV based on ASH & Moisture (ARB), as we have installed BCM, but the results are varying very much for ex. one sample has the ASH 35 % Moisture 14% GCV for this has come out to 4726 in other case there is slight difference in the ASH and Moisture and GCV has come out to 3300. sir is it possible. Sir as far as i know GCV is taken out only when the moisture is fully exhausted or with some nominal moisture, while making the payment to the vendor if we show him our test GCV which is LOW as that of TESTED Lab, then in that case how we should go further, Sir can you help me by giving your suggestion.

Mubeen Dec 15, 2009 11:43 PM

Fuel Ratio

Dear Sir,

Is South African coal suitable for using in CFBC Boilers as it has higher fuel ratio.
Please explain.

Regards
Mubeen

johnzactruba Dec 7, 2009 8:40 PM

reply mubeen

Please read my article "Basic Calculations for a Power Plant- Calculating the Coal Quantity" for details on calculating the specific coal consumption.

http://www.brighthub.com/engineering/mechanical/articles/22202.aspx

Using lower hating value or higher heating value is alright as long as you use the corresponding efficiency or heat rate . To avoid confusion always use higher heating value.

Mubeen Dec 7, 2009 5:25 AM

specific fuel consumption

Dear Sir,
Please let meknow how to calculate the specific fuel consumption of coal in kg/kwh. Do we have to take net calorific value or gross calorific value (arb)
for calculation.
Regards
Mubeen

johnzactruba Nov 26, 2009 5:56 PM

reply -Manjul Chawla

When you recieve the coal in the coal yard from the mines the the moisture is high. This called 'AS recieved basis- AR'.
When you fire the coal in the boiler it is 'as fired basis', the moisture is different.
In the lab before testing the coal is dried in air for 24 hours, after that it is ' air dried basis'.
Since CV is reported on per kg basis, 1 kg of the sample depending on from where you collect it , contains varying amount of dry portion which actually gives the heat. This is why there is the difference.

Manjul Chawla Nov 25, 2009 3:52 AM

Meanings

Dear Sir,

While going through some reading material re Coal, I came across terms like GAR and ADB with reference to the calorific value with the calorific value (GAR) lower than the calorific value (ADB).

Shall appreciate if you could let me know the meaning of the same.

How can the calorific value of the same coal be different unless the same are being used in terms of Gross and Nett basis.

regards

johnzactruba Nov 24, 2009 12:35 AM

reply - darwin

for calculating boiler performance the coal has to be 'as fired basis' .
'As recieved' is when you get the coal in the coal yard
" air dried basis' is normally used in the laboratory.

darwin Nov 17, 2009 11:43 PM

coal consumption

dear sir,
pls advise whether we calculate the amount of coal coming to boiler based
on air dried basis or as received?tks

hemant dohan Oct 28, 2009 7:25 AM

thanks

Dear Sir

thanks a lot for the help.
regards

johnzactruba Oct 28, 2009 6:32 AM

reply to-HEMANT DOHAN-how to calculate coal gcv as recd. base and adb

You can use this correction method.
GCV_ar = GCV_adb x [100 - M_ar] / [100- M_ad]
ar- as recd
ad- air dried
M - moisture %

HEMANT DOHAN Oct 27, 2009 8:09 AM

how to calculate coal gcv as recd. base and adb

Dear Sir,
please let us know the formula to calculate the coal gross heat value on ARB and ADB

regards

johnzactruba Oct 21, 2009 8:18 AM

Reply to N Mariotti-Coal Moisture data

The moisture in coal is in two parts. Onethat is on the surface of the coal particles. The other is entrapped among the molecular structure of the coal.
One is that is on the surface easily evapoartes when kept in dry air. The other require the coal to be heated to greater than 100 deg c for removal.
inherent moisture is the second category.

N Mariotti Oct 20, 2009 11:49 AM

Coal Moisture data

Dear Sir
could you please let me know is my interpretation is correct about moisture content data.

If I receive a coal with 40% moisture AS RECEIVED, with 25% inherent moisture (Air dry basis),what does represent the difference (15%)?
Is this free moisture AR?
Many thanks for your support.

Regards
Mariotti

johnzactruba Aug 17, 2009 6:44 AM

Please be more specific

C.KUMARESAN Aug 17, 2009 6:33 AM

how effectivily use the high moisture coal

Sir,
I would like to know that how effectivily use the high moisture (40%)and high HGI(85%)coal for power plant. Kindly explain the difficulties of using this coal.

mahendra shivhare Jul 22, 2009 2:13 PM

about infoamatiom thermel poer plant

sir.
i want to know infoarmation about boiler effficency, how can iwe calculate.
coal testing. water testing , and also some techanical points about FBC boiler

Burning Coal in Power Plants – Calorific Value and Moisture

Calorific Value or Heating Value

Moisture

Reporting Coal Properties

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