Previous in Forum: Liquid Argon Conversion   Next in Forum: Question on Piping Designengineer
Close
Close
Close
14 comments
Member

Join Date: Dec 2012
Posts: 8

Energy Released in Combustion

12/06/2012 12:44 AM

I am working in as an Engineer in coal handling plant. I read one book of Thermal Engineering in which following reactions were mentioned :

C + O2 CO2 + 8084 kcal/kg of Carbon

2C + O2 2CO + 2430 kcal/kg of Carbon

2H2 + O2 2H2O + 28922 kcal/kg of Hydrogen

S + O2 SO2 + 2244 kcal/kg of Sulphur

Is it possible to determine the Calorific Value of coal from these equations? Why is more energy released by the combustion of Hydrogen than Carbon or any other elements?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30464
Good Answers: 819
#1

Re: Energy released in combustion

12/06/2012 3:39 AM

A1: yes.

A2: it just is.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Member

Join Date: Dec 2012
Posts: 8
#2
In reply to #1

Re: Energy released in combustion

12/06/2012 5:34 AM

Mr. Slack, I mean to say, how to calculate the calorific value of coal using the energy liberated in combustion reaction of the various elements in coal. What the procedure actually is.?

Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30464
Good Answers: 819
#3
In reply to #2

Re: Energy released in combustion

12/06/2012 5:53 AM

It's not the elements present; it's the combustible materials present. Silicon dioxide, present in coal, does not burn, for example.

An analytical approach:

  • Analyse a statistically-significant sample of the coal for combustible materials
  • Obtain a percentage for each of the combustible species present.
  • Multiply each percentage by the heat of combustion of each species
  • Summate to obtain a total.

An engineering approach:

  • Look up typical values in Kempe's Engineers' Yearbook or Perry, "The Chemical Engineer's Handbook", or any other published text.

A commercial approach:

  • Telephone the coal suppliers and obtain information from their sales offices.
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply Score 1 for Good Answer
Guru

Join Date: Apr 2011
Posts: 7968
Good Answers: 284
#4
In reply to #2

Re: Energy released in combustion

12/06/2012 6:15 AM

For a simplified version, essentially, the enthalpy of formation of the reactants is compared to that of the products.

.

As to the other part of your question: part of the reason hydrogen is releases more energy on a per mass basis than carbon is that the atomic mass is far lower.

So even though two hydrogen atoms are used for every oxygen atom, while a carbon atom can react with two oxygen atoms, four hydrogen atoms have a combined mass far less than one carbon atom.

Another factor is the difference in enthalpy of formation of the products.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Register to Reply
Guru

Join Date: Sep 2007
Location: East Texas
Posts: 1389
Good Answers: 31
#5

Re: Energy Released in Combustion

12/06/2012 9:24 PM

This is the opinion of a retired railroader.

As an engineer the following is something you already know.

The term is "Heat of Combustion." (HOC)

The easiest means of getting a rough estimation for the "Heat of Combustion" for "Coal" is to simply google "Heat of Combustion - Coal."

Hopefully you kept your college freshman Physics Text. A table in mine gives 33 MJ/KG or 8000Kcal/ KG for anthracite coal. For lignite its about 25kJ/g as given by an internet search. I will let you do the conversions.

Multi sourcing the information gets a small variation in values.

The actual work you get from it will be dependent upon the Carnot Efficiency of your combustion process, the thermal losses in converting that thermal energy to a working fluid of some type (steam - etc), the efficiency of your turbines, transmission losses - etc etc. Coal is a very inefficient way of producing electricity. Cheap - yes, ignoring the environmental costs - but still very inefficient.

Co-generation brings that efficiency up some - but its still a Neanderthal way of producing energy.

I believe that it would be possible to estimate the "Heat of Combustion" for a specific type of coal if the chemical makeup is known. I believe it would simply be the sum of the component HOCs.

If were a power plant engineer I would just pick up the phone, call my supplier, and ask him what the "Heat of Combustion" is for the coal they are selling. I think if would matter; especially in scaled applications.

In terms of thermal efficiency; a car powered by a good Internal Combustion Engine has a higher efficiency than an all electric vehicle charged from a coal fired grid.

I have been wanting to take the time to explore the miles per gallon equivalent given for electric vehicles; and not just for the reason given above.

Gavilan

__________________
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark." -- Michelangelo
Register to Reply
Guru
Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 3230
Good Answers: 444
#6

Re: Energy Released in Combustion

12/06/2012 10:51 PM

I believe your answer is found in this old paper:

"FORMULAS FOR CALCULATING THE HEATING VALUE OF COAL AND COAL CHAR: DEVELOPMENT,TESTS AND USES"

that states:

"...A new five-term formula for calculating the heating value of coal from its carbon, hydrogen, sulfur and ash content was obtained by regression analysis of data on 775 samples of U. S. coals of all ranks..."

Why did you think it couldn't be done? After all, it was presented in an engineering handbook. Or is this yet another cleverly worded interview/homework question?

__________________
“Tell me and I forget. Teach me and I remember. Involve me and I learn.” Ben Franklin.
Register to Reply
Guru
Engineering Fields - Mechanical Engineering - New Member India - Member - New Member

Join Date: Aug 2008
Location: City of destiny, INDIA
Posts: 775
Good Answers: 67
#7

Re: Energy Released in Combustion

12/07/2012 4:06 AM

Incidentally long back I also started my carrier working as an Engineer in coal handling plant.

1. Yes, it is possible to determine the Calorific Value of coal from your first equation only. But first you should know the carbon content in coal which varies widely from type to type.

2. All the equations given by you are examples of exothermic reactions. The heat released depend upon heat of formation of new product due to reaction. So, energy released by the combustion of hydrogen is different from others. In fact your second equation give the calorific value of hydrogen, because it is also a fuel.

Register to Reply
Guru
Engineering Fields - Mechanical Engineering - New Member

Join Date: May 2008
Location: CHENNAI, TAMIL NADU, INDIA.
Posts: 1851
Good Answers: 64
#8

Re: Energy Released in Combustion

12/07/2012 4:14 AM

Dear Mr.vramanisowmya143,

What you have stated is the absolute HEAT that can be released from the constituents But the coal or fuel or gas contains C, H2, S etc. indifferent proportions or Percentages.

You have to use STOICHOMETRIC EQUATIONS for the perfect combustion and derive the NETT HEAT to released by the Fuel.

It is NATURE that has provided MORE HEAT RELEASE from the HYDROGEN. Our SUN is a fuel cell BURNING HYDROGEN in several Thousands of Tonnes/Sec. and releases the Heat ENERGY which is essential for survival in this planet.

DHANAYANDHAN.S

Register to Reply Score 1 for Off Topic
3
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30464
Good Answers: 819
#9
In reply to #8

Re: Energy Released in Combustion

12/07/2012 7:32 AM

<....It is NATURE that has provided MORE HEAT RELEASE from the HYDROGEN. Our SUN is a fuel cell BURNING HYDROGEN in several Thousands of Tonnes/Sec. and releases the Heat ENERGY which is essential for survival in this planet....>

It is important to distinguish between chemical reactions, of which combustion is an example, and nuclear reactions, of which fusion is an example. The distinction is blurred in that sentence.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply Good Answer (Score 3)
Guru
Engineering Fields - Mechanical Engineering - New Member

Join Date: May 2008
Location: CHENNAI, TAMIL NADU, INDIA.
Posts: 1851
Good Answers: 64
#11
In reply to #9

Re: Energy Released in Combustion

12/07/2012 12:05 PM

Thank you Mr. PWSlack, for your comments.

Indeed, COMBUSTION is Result of CHEMICAL REACTION. Nuclear Reaction also releases Heat Energy. I should have referred about this. At the time of my reply - only COMBUSTION which is a chemical Reaction was in my mind and Nuclear FISSION and or FUSION did not engage my attention.

When NUCLEAR REACTIONS, be it FISSION or FUSION, releases tremendous quantity of heat HEAT ENERGY and my statement "It is NATURE that has provided MORE HEAT RELEASE from the HYDROGEN" clearly missed the distinction.

How evever the matter is putforth by you, to this CR4 FORUM, and I am extremely thankful to you.

DHAYANANDHAN.S

Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 30464
Good Answers: 819
#13
In reply to #11

Re: Energy Released in Combustion

12/10/2012 3:31 AM

<...Mr.....>

How abstruse.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru

Join Date: Apr 2011
Posts: 7968
Good Answers: 284
#14
In reply to #13

Re: Energy Released in Combustion

12/10/2012 5:36 PM

<.....<...Mr.....>

How abstruse.......>

.

How exsufflicate.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Register to Reply
2
Power-User

Join Date: Dec 2006
Posts: 442
Good Answers: 32
#10

Re: Energy Released in Combustion

12/07/2012 9:20 AM

While it is possible to determine the heat value of coal by these equations, it requires that you have the chemical composition of your specific coal sample, if you want any kind of accuracy.

It is much more difficult to determine the chemical composition than it is to measure the Calorific Value directly.

The process of measuring the value directly is known as calorimetry.

Register to Reply Good Answer (Score 2)
2
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14331
Good Answers: 161
#12

Re: Energy Released in Combustion

12/07/2012 12:15 PM

A. It is only approximately possible to determine the heating value(s) of coal (of which there is a lower heat value and an upper heat value, by using a regression formula for the type of coal, the listed heats of combustion (so you stated), and the fractional portion of elements that are combustible with air.

To obtain the precise value(s), one must have a "four corners" sample of the coal, properly ground and blended together to make a composite sample (and trust me, in this sampling is everything). Then one adds a very accurately and precisely weighed quantity of the coal to a combustion calorimeter (also known as a combustion bomb). Oxygen is then applied to the contents by pressure, and ignition is provided by a hot wire (as I recall). The combustion calorimeter is immersed in a known mass of water at a known temperature before ignition, and the total mass of the metal and heat capacity is also known. By linear combination, the final temperature after combustion allows one to back-calculate the heat value of the coal. Lower heat value and upper heat value are related to the oxygen result (but lower typically). Conditions in a furnace or boiler will not completely combust the coal, and this is why there is always a small percentage of carbon remaining in the ash.

B. Hydrogen is lighter and also has a different bond strength with oxygen than does carbon, hence the difference in heat of combustion.

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply Good Answer (Score 2)
Register to Reply 14 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

dhayanandhan (2); Gavilan (1); James Stewart (1); pritam (1); PWSlack (4); RAMConsult (1); Tad (1); truth is not a compromise (2); vrmanisowmya_143 (1)

Previous in Forum: Liquid Argon Conversion   Next in Forum: Question on Piping Designengineer

Advertisement