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Author Topic: Why Does Coal Burn For So Long ?  (Read 19041 times)

Offline neilep

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Why Does Coal Burn For So Long ?
« on: 05/10/2009 13:14:04 »


Dearest Coalologists,

As a sheepy I of course luff coal. Coal is made from plant stuff and is well old...even older that 100 years and that's well old !

Look here's some :



Here's Some Coal.

Being delivered next Tuesday


A few questions about coal.

Are there different types of coal found in the world ?..ie: are the ingredients different  and does this have an effect on it's combustibility?

Why does coal burn for so long ?

Is coal found at the same geological circumstances the world over ? ie depth?  geological features ?

How is coal made ?
Are there underground coal construction facilities ?

I know that is a lot of questions and I don't expect them all answered but , a few will do.

As a firm believer in empirical study I plan to sneek into my neighbours house at 3am tomorrow morning as I know he has a Nat King Cole collection so he should be able to assist me, and willingly at that !

In the mean time, if ewe know an answer or three then gratitude with a warm hug will be your reward !



Hugs & shmishes



mwah mwah mwah



Neil
There's Coal in Them Thar Hills
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



 

Offline Mazurka

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Why Does Coal Burn For So Long ?
« Reply #1 on: 05/10/2009 16:30:57 »
To answer your questions back to front;

Broadly speaking coal is made from swamps or bogs that have been buried.  It tends to occur in sedimentary basins that have filled up over millions of years (it is not a quick process!) 

Imagine a mangrove or a peat bog suddenly buried by sand and mud brought down by a catastrophic flood.  Often, after the flood, more mangrove or bog forms and is buried by the next major flood (again and again).  This leads to an identifiable sequence of rocks, which are sometimes referred to as coal measures. 

As this sandwich of vegetable matter and sand builds up, the weight of the material above the lower layers puts pressure on the peaty material which starts to squeeze out water and some of the more volatile substances.  If sufficient pressure is applied to peat for sufficient time, it will metamorphose into brown coal - also known as Lignite.   

If the basin continues to fill, and the lignite is buried further, thus subjected to more pressure and a bit more heat, more water is removed and more volatile substances driven out, leading to sub bituminous and then  bituminous coals.  Given enough pressure and heat, bituminous coal will become anthracite, the highest grade of coal.  (If pushed further it will become graphite, but this is a devil to set on fire so is not so good as a fuel.)  Whilst the coal has increased in grade, the surrounding rock will have become harder and the grains of sand will have been forced together, forming a sandstone.

Coal can best be graded by the proportions of (for want of a better word:) Coal minerals - Vitrinite (shiny highest grade) Inertinite (duller looks more like charcoal) and Exinite (which has some new fancy name).  These minerals are subdivided depending on what the source material was - e.g. wood, leaves, fungus, algae, pine resins etc.

To the best of my knowledge coal is formed this same way across the world, although sometimes instead of sandstones, calcium rich mud's form limestones around the coal.

Coal is found at a variety of depths - from the surface - to considerable depths.  The deepest mine that I know about in the South Wales coal field was more than 700m deep although I suspect that elsewhere people will have gone deeper.

Given that most coal formed at around 300 million years ago, current depth tends to be controlled by subsequent tectonic movement.  Coal that is removed by opencast quarrying did not form that close to the surface.  (As an aside google "Bagger288" to see the largest (strip mining) machine in the world!) 

Broadly speaking the higher grade the coal the better (hotter and longer) it burns - so yes it is kind of affected by composition, but the depth/ temperature it reached during formation is more important.

Why it burns for so long I guess is related to its formation - as a rule of thumb, around 10m thickness of woody/ peaty material is thought to produce around 1m of anthracite - so you have simply squashed more energy into a smaller volume.  I vaguely recall (and cannot be bothered to google) that anthracite yields a minimum of 9kw per kg whereas lignite about produces up to about 5kW per kg.

(apologies for length)
 

Offline Bored chemist

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Why Does Coal Burn For So Long ?
« Reply #2 on: 05/10/2009 19:17:13 »
Coal burns very quickly in the right circumstances. Coal fired power stations burn it in (at a guess) under a second. It can even burn so fast it explodes.
What usually limits the speed of reaction is that it's in pretty big lumps and the air for combustion can only reach the surface. Also once you have burned the surface it is covered by a layer of ash which further impedes the air so that also slows the burning.
 

Offline RD

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Why Does Coal Burn For So Long ?
« Reply #3 on: 05/10/2009 19:24:01 »
Coal burns very quickly in the right circumstances. Coal fired power stations burn it in (at a guess) under a second. It can even burn so fast it explodes.
What usually limits the speed of reaction is that it's in pretty big lumps and the air for combustion can only reach the surface...


Quote
In plants that burn pulverized coal, silos feed coal pulverizers (coal mills) that take the larger 2-inch pieces, grind them to the consistency of face powder.
http://en.wikipedia.org/wiki/Fossil_fuel_power_plant#Fuel_processing
 

Offline chris

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Why Does Coal Burn For So Long ?
« Reply #4 on: 06/10/2009 09:36:20 »
To answer your questions back to front;

Broadly speaking coal is made from swamps or bogs that have been buried.  It tends to occur in sedimentary basins that have filled up over millions of years (it is not a quick process!) 

Imagine a mangrove or a peat bog suddenly buried by sand and mud brought down by a catastrophic flood.  Often, after the flood, more mangrove or bog forms and is buried by the next major flood (again and again).  This leads to an identifiable sequence of rocks, which are sometimes referred to as coal measures. 

As this sandwich of vegetable matter and sand builds up, the weight of the material above the lower layers puts pressure on the peaty material which starts to squeeze out water and some of the more volatile substances.  If sufficient pressure is applied to peat for sufficient time, it will metamorphose into brown coal - also known as Lignite.  

If the basin continues to fill, and the lignite is buried further, thus subjected to more pressure and a bit more heat, more water is removed and more volatile substances driven out, leading to sub bituminous and then  bituminous coals.  Given enough pressure and heat, bituminous coal will become anthracite, the highest grade of coal.  (If pushed further it will become graphite, but this is a devil to set on fire so is not so good as a fuel.)  Whilst the coal has increased in grade, the surrounding rock will have become harder and the grains of sand will have been forced together, forming a sandstone.

Coal can best be graded by the proportions of (for want of a better word:) Coal minerals - Vitrinite (shiny highest grade) Inertinite (duller looks more like charcoal) and Exinite (which has some new fancy name).  These minerals are subdivided depending on what the source material was - e.g. wood, leaves, fungus, algae, pine resins etc.

To the best of my knowledge coal is formed this same way across the world, although sometimes instead of sandstones, calcium rich mud's form limestones around the coal.

Coal is found at a variety of depths - from the surface - to considerable depths.  The deepest mine that I know about in the South Wales coal field was more than 700m deep although I suspect that elsewhere people will have gone deeper.

Given that most coal formed at around 300 million years ago, current depth tends to be controlled by subsequent tectonic movement.  Coal that is removed by opencast quarrying did not form that close to the surface.  (As an aside google "Bagger288" to see the largest (strip mining) machine in the world!) 

Broadly speaking the higher grade the coal the better (hotter and longer) it burns - so yes it is kind of affected by composition, but the depth/ temperature it reached during formation is more important.

Why it burns for so long I guess is related to its formation - as a rule of thumb, around 10m thickness of woody/ peaty material is thought to produce around 1m of anthracite - so you have simply squashed more energy into a smaller volume.  I vaguely recall (and cannot be bothered to google) that anthracite yields a minimum of 9kw per kg whereas lignite about produces up to about 5kW per kg.

(apologies for length)


Superb answer, thank you.

Chris
 

Offline neilep

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Why Does Coal Burn For So Long ?
« Reply #5 on: 06/10/2009 11:02:01 »
To answer your questions back to front;

Broadly speaking coal is made from swamps or bogs that have been buried.  It tends to occur in sedimentary basins that have filled up over millions of years (it is not a quick process!) 

Imagine a mangrove or a peat bog suddenly buried by sand and mud brought down by a catastrophic flood.  Often, after the flood, more mangrove or bog forms and is buried by the next major flood (again and again).  This leads to an identifiable sequence of rocks, which are sometimes referred to as coal measures. 

As this sandwich of vegetable matter and sand builds up, the weight of the material above the lower layers puts pressure on the peaty material which starts to squeeze out water and some of the more volatile substances.  If sufficient pressure is applied to peat for sufficient time, it will metamorphose into brown coal - also known as Lignite.  

If the basin continues to fill, and the lignite is buried further, thus subjected to more pressure and a bit more heat, more water is removed and more volatile substances driven out, leading to sub bituminous and then  bituminous coals.  Given enough pressure and heat, bituminous coal will become anthracite, the highest grade of coal.  (If pushed further it will become graphite, but this is a devil to set on fire so is not so good as a fuel.)  Whilst the coal has increased in grade, the surrounding rock will have become harder and the grains of sand will have been forced together, forming a sandstone.

Coal can best be graded by the proportions of (for want of a better word:) Coal minerals - Vitrinite (shiny highest grade) Inertinite (duller looks more like charcoal) and Exinite (which has some new fancy name).  These minerals are subdivided depending on what the source material was - e.g. wood, leaves, fungus, algae, pine resins etc.

To the best of my knowledge coal is formed this same way across the world, although sometimes instead of sandstones, calcium rich mud's form limestones around the coal.

Coal is found at a variety of depths - from the surface - to considerable depths.  The deepest mine that I know about in the South Wales coal field was more than 700m deep although I suspect that elsewhere people will have gone deeper.

Given that most coal formed at around 300 million years ago, current depth tends to be controlled by subsequent tectonic movement.  Coal that is removed by opencast quarrying did not form that close to the surface.  (As an aside google "Bagger288" to see the largest (strip mining) machine in the world!) 

Broadly speaking the higher grade the coal the better (hotter and longer) it burns - so yes it is kind of affected by composition, but the depth/ temperature it reached during formation is more important.

Why it burns for so long I guess is related to its formation - as a rule of thumb, around 10m thickness of woody/ peaty material is thought to produce around 1m of anthracite - so you have simply squashed more energy into a smaller volume.  I vaguely recall (and cannot be bothered to google) that anthracite yields a minimum of 9kw per kg whereas lignite about produces up to about 5kW per kg.

(apologies for length)


As that bloke Chris said !...Fantastic answer. Thank ewe very much Mazurka
 

Offline neilep

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Why Does Coal Burn For So Long ?
« Reply #6 on: 06/10/2009 11:05:39 »
Coal burns very quickly in the right circumstances. Coal fired power stations burn it in (at a guess) under a second. It can even burn so fast it explodes.
What usually limits the speed of reaction is that it's in pretty big lumps and the air for combustion can only reach the surface. Also once you have burned the surface it is covered by a layer of ash which further impedes the air so that also slows the burning.

That's very interesting and the 'speed burn' puts a perspective that I would never have thought of . Thanks Bored Chemist Those coal fired power stations must get through a tremendous amount ! Anybody know how much coal your typical coal powered fire station gets through in a year ?
 

Offline neilep

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Why Does Coal Burn For So Long ?
« Reply #7 on: 06/10/2009 11:11:52 »
Coal burns very quickly in the right circumstances. Coal fired power stations burn it in (at a guess) under a second. It can even burn so fast it explodes.
What usually limits the speed of reaction is that it's in pretty big lumps and the air for combustion can only reach the surface...


Quote
In plants that burn pulverized coal, silos feed coal pulverizers (coal mills) that take the larger 2-inch pieces, grind them to the consistency of face powder.
http://en.wikipedia.org/wiki/Fossil_fuel_power_plant#Fuel_processing


Wooo...I think RD has answered my question via that link...4000 tons an hour ????...oh my !!
 

Offline JimBob

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Why Does Coal Burn For So Long ?
« Reply #8 on: 06/10/2009 15:46:34 »
Need more people like Polish Dance (Mazurka) here on the geology section. Very nice - I couldn't have done better!
 

Offline Bored chemist

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Why Does Coal Burn For So Long ?
« Reply #9 on: 06/10/2009 18:54:43 »
Coal fired power stations use a lot of coal.
"Both Ferrybridge C and Fiddlers Ferry power
stations comprise four 500-MW steam turbines, using 800 tonnes of coal
and 218 million liters of water per hour for cooling."
Quoted from
http://www.reuters.com/article/pressRelease/idUS91848+26-Jun-2008+MW20080626
 

Offline Mazurka

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Why Does Coal Burn For So Long ?
« Reply #10 on: 07/10/2009 08:33:46 »
 [:I]I fank yew, for the kind words.
 

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Why Does Coal Burn For So Long ?
« Reply #10 on: 07/10/2009 08:33:46 »

 

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