Burning Coal : How much Air is required ?

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

The most important requirement to burn coal is the correct quantity of air. This article explains how to calculate the correct quantity of air .

Coal is the most widely used fuel source used to produce Electricity. Air provides the necessary Oxygen for burning coal. It is the chemical reaction between Carbon in coal and the Oxygen in the air that produces the heat energy.

Elements in Coal

There are 3 elements in coal that combine with Oxygen in the air duing the combustion process. The main and important element is Carbon which could be around 30% - 60 % .Then there is Hydrogen in the range of 1 % to 3 % and Sulphur in the range of 0.3 % to 3 % . Apart from this three there is Nitrogen 1% to 2 % and Oxygen from 5 % to 12 % . The Oxygen is also used up in the Combustion process. The exact amounts of each element depends on the type and rank of coal, how the coal was formed millions of years ago and the location or mines from which the coal is sourced.

Stoichiometric or theoretical air quantity.

This is calculated based on the chemical reaction between the elements and oxygen.

Carbon combines with Oxygen to form Carbon-dioxide and heat.

C+ O₂ > CO₂

1 C +32/12 O > 44/12 CO₂

1 kg Carbon +2.67 kg Oxygen > 3.67 kg Carbon Dioxide

Hydrogen Combines with Oxygen to form Water and heat

2 H₂+ O₂> 2H₂O

1 H +32/4 O >36/4 H₂O

1 kg Hydrogen +8 kg Oxygen >9 kg Water

Sulphur Combines with Oxygen to form Suphur Dioxide

S + O₂ > SO₂

1 S +32 /32 O >64 /32 SO₂

1 kg Sulphur +1 kg Oxygen >2 kg Sulphur Dioxide.

For the purpose of calculation we consider a coal having 57.2 % Carbon, 2.2 % Hydrogen , 0.5 % Sulphur and 6.9 % Oxygen.

The theoretical Oxygen required to burn this coal is then

2.67 x C % + 8 x H% + 1 x S % - O% = 1.64 kg of Oxygen for 1 kg of Coal.

Air contains 23.2 % by weight of Oxygen.

The theoretical Air required to burn the coal is

= 1.64 / 23.2% = 7.1 kg of Air for 1 kg of Coal.

This is the theoretical Air required to burn the coal.

Quick Calculation of Theoretical Air.

The Heating value of Coal also depends on the elemental Carbon and Hydrogen. This means that the air required and the heating value have an almost fixed relationship. The theoretical air required for a unit heating value is practically a fixed value. This is around 0.332 kg of air for one MJ of heat input. This is true for a wide range of coals used in power plants.

By knowing the calorific value of coal , the theoretical air quantity can be directly calculated using this factor.

Next Article »

Burning Coal in Power Plants

Coal is the most widely used fuel source used to produce Electricity. The most important requirement to burn coal is the correct quantity of air. This article series explains how to calculate the correct quantity of air, the effects of less or more air.

1. Burning Coal : How much Air is required ?
2. Burning Coal : Effects of Too Much Air or Less Air.

Comments

Showing all 27 comments

vivek Jul 11, 2011 5:11 AM

RE: Burning Coal : How much Air is required ?

How much air is required to burn 100kg of carbon?

pretty komana Jun 15, 2011 5:57 AM

RE: Burning Coal : How much Air is required ?

thank you johnzactruba

Pamdi Jun 6, 2011 2:28 AM

Producer Gas

Dear Mr. John,
I am a new member and would like to know how much Carbon Monoxide is produced from combustion of 1 ton of coal.As you would know that the main constituent of producer gas is Carbon Monoxide. My contention is ,since Carbon Monoxide is the result of incomplete combustion, what should be the quantity of air for this incomplete combustion.

Pamdi

johnzactruba Nov 8, 2010 6:18 PM

reply. Mohamed Thyeeb

The teoritical air required for 1 kg of coal depends on the coal charecterstics and not on the type of boiler.
The excess air that is required will change from boiler to boiler depending on the burner design, type of coal, operator requirements etc

Mohamed Thyeeb Nov 6, 2010 1:43 AM

Air reequired for 1 kg of coal

Dear sir,
I want to know, weather amount of air required for 1 kg of coal will vary from boiler to boiler. Or not?
If yes , what are the paameter to be considerd and how to calculate it.

Please reply me as soon as possible.
Regards,
Mohamed Thyeeb-TCO

johnzactruba Aug 9, 2010 6:47 PM

reply ummed

Lower ph ( acidic) will attack the iron, wheras higher ph ( alkaline ) will from scales.
The chances of ingress of chemicals that reduce the ph is more in a power plant. So when the water evaporates the concentration of chemicals tends to reduce the ph. So maintaining a higher ph is a warranty against fluctuations so that pH is never in the acidic region.

ummed Aug 8, 2010 5:25 AM

Alkanity

what is role of alkanity in the boiler why we maintain some alkanity in boiler.

How did it will react when excess &what will be held whenever less of it in boiler.

ummed Aug 8, 2010 5:23 AM

lime stone calculation

How much limeston required for 1.0 Kg sulphure removing in coal combution in boiler.

What is the chemical reaction & mathematical calculation .

johnzactruba Aug 7, 2010 8:53 AM

reply lfox

The combustion itself is a complicated process with many reactions taking place. But most of these are unstable intermediate reactions. The end result is the formation of CO2 and H2O. The caculations take into account only the end result.
Since coal is not the same in form as the oils or gas, to assure that full combustion takes place, air is provided in excess of the stoichiometric requirement.

lfox Aug 6, 2010 10:50 PM

stoichiometric burning of coal

How do you calculate for the fact that coal does not burn readily with oxygen in the same manner as liquid fuels. Do you calculate for the unit mass of combustible vapor liberated from unit of mass of coal per unit time, at combustion temp/press, and following the mass analysis of the vapor determine the mass of air required for stoichiometric combustion? also do you account for the fact that the vapor does indeed have a certain amount of oxygen as you say which no doubt is NOT in the free form but is part of a molecule of either CO2 or H2O most likely H2O as hydrogen is more reactive with O2 than carbon? or is it factored in the percentages of the mass analysis considered for the stoichiometric calculation as free hydrogen?

johnzactruba May 6, 2010 9:02 AM

reply asit konar

VM refers to volatile Matter in Coal. Some part of the Carbon in Coal is in the from of HydroCarbon chains. When heated up this is released immediately. This is called the Volatile Matter.
Main component of ASh is Silica.

asit konar May 3, 2010 10:48 PM

coal

What is VM and what is the main component of ash?

ummed Apr 11, 2010 1:09 PM

Heat Value of coal

R/Sir,

How did we can calculate Heating value of coal or any fuel , what is the dew point of Sulphur,OXigen,Nitrogen, CO2 etc.

regards

ummed Apr 11, 2010 1:04 PM

Ash coloure goes to red

R/Sir,

After coal combution We get ash colour red or pinkish.what would be possible causes of it

regards

johnzactruba Apr 10, 2010 7:42 PM

reply UMMED

Lower excess air means some of the coal particles do not get sufficient air to burn. These coal particles will be seen as unburnt carbon in the ash. Higher unburnt carbon means the boiler efficiency is less.
This will also inncrease the Carbon dioxide content in the flue gas , which is also a loss.

In some coals where the ash has a lower melting point lower excess air will result in slagging.

UMMED Apr 9, 2010 3:05 PM

O2 Calculation for Coal fire

R/Sir,

What will be effect on boiler of Excess Or Less O2 used in boiler for cobnt.
& how did we can calculate required o2 for coal burning.
What will effect on LOI of coal due to O2 FLACTUATION IN BOILER.

pretty Mar 16, 2010 11:24 AM

hello

hello.i was asked to calculate the theoretical air and actual air when a pyrite is roasted with an exess of air to give FeO3 and SO2.the roast gas contains 6.3 volume %SO2,the balance being N2 and O2.

Aashish Gupta Dec 24, 2009 7:31 AM

Air equivalent

Hi, I wanted to know how to i convert the mass of air into volume and then into pressure at which it should be combusted.

Michalis Nov 15, 2009 3:24 PM

burning coal

Thank you very much for your help John..

johnzactruba Nov 14, 2009 9:52 PM

reply : Michalis Burning Coal

Yes this is for 1 kg of Sulphur or 1 Kg of Coal. But kg of coal has only S% of Sulphur so you have to consider this.
To find air required you should find out using the above
for 1 kg S how much O2
fo 1 kg C how mush O2
for 1 kg H2 how much o2 is required. add all this. Since O2 is only 23.2 % in Air you have to divide the calculated O2 quantity by 0.232 to get the air required.

Michalis Nov 12, 2009 8:12 PM

Burning Coal

John these numbers is for 1kg only right?
1 kg of S gives 64/32=2 SO2 : this is 2 x22.4 = 44.8 litres of SO2
Same way for Carbon 1 kg Carbon require
32/12 x 22.4 litres of Oxygen and gives out
44/12 x 22.4 litres of CO2. The volumes are at STP.
If i want for a tonne combustion i have to multiplied the numbers by 1000? Also to find the volume of air i have to plus the Oxygen and the CO2 ? Thank you again.

johnzactruba Nov 12, 2009 6:16 PM

reply Michalis

Sorry there was mistake. In the article it is correctly written.
1 kg of S gives 64/32=2 SO2 : this is 2 x22.4 = 44.8 litres of SO2
Same way for Carbon 1 kg Carbon require
32/12 x 22.4 litres of Oxygen and gives out
44/12 x 22.4 litres of CO2. The volumes are at STP.

Michalis Nov 11, 2009 2:03 PM

Burning coal.

Hello John. Thank you for your help but can you check agoin about the solve ---1 kg of S gives 64/2*22.4 = 11.2 litres of SO2--- what is the 2 which is multiplied with 22.4? Can you make the same with the carbon because i don't get the correct answer.
Cheers Michalis

johnzactruba Nov 10, 2009 11:06 PM

correction

it is 22.4 litres at STP not NTP

johnzactruba Nov 10, 2009 11:06 PM

correction

it is 22.4 litres at STP not NTP

johnzactruba Nov 10, 2009 11:02 PM

reply : Michalis-Burning coal.

The theoritical air required in kg per kg of coal can be found from th equation given in the aboive article.
The mass diivded by the density of air at STP can be used to get the Volume of air required.
For SO2.
I kg mol of S gives 64/32 kg mol of SO2.
For any gas 1 kg mol occupies 22.4 litres at NTP
then 1 kg of S gives 64/2*22.4 = 11.2 litres of SO2. If coal contains 2.5 % S then 2.5 % x 11.2 litres of SO2 will be produced.
The air requirement also can be calculated like this.

Michalis Nov 9, 2009 3:12 PM

Burning coal.

Hello. I want to burn coal which has an average composition ,in mass (%) of : Carbon:87.5%, sulphur:2.5%,innert mineral matter: 10%. I want the volume of air(at stp) required to completely combust 1tonne of coal? Also i want the volume percentage of sulphur dioxide, SO2, in the product gas following combustion? . If you know how this solved showed me partially to understant what happens. if you can help me please answer me on my e-mail : mikex24@gmail.com Thank you.