Why does coal have a more energy than wood?

[Elite Engineer]

According to wikipedia, the energy density of coal is from 26-49 MJ/L. Meanwhile, for wood, it's only 13 MJ/L. Is this because of the tight molecular structure of coal, compared to the pentose rings lined adajcent in wood (cellulose, lignocellulose?).

 

~EE

[arc]

Coal is simply an end product of a multi MY process of concentrating the wood and other derived source carbon through heat and pressure at great depths within the Earth. The wood could also be put through an artificial refining process of heat and pressure that would reduce it to the same MJ/L as the coal.

[John Cuthber]

Wood is porous and the air spaces drop the average density- wood floats; coal sinks.
But also the carbon has a higher heat of combustion- gram for gram- than cellulose.

Also the combustion process is the reaction with oxygen. The cellulose already contains oxygen so in a sense it is already partly combusted.

Equivalently, you can consider cellulose to be carbon with added water.
In burning it, you have to "waste" heat boiling off that water.

[saraheil]

A non-renewable energy source is one which, once it is used, cannot be replaced. It is gone forever. Many of the fuels we use today fall into this category – oil, gas, petrol and coal. Once these are burned to heat homes or power cars, their energy is lost forever.A renewable energy source is one that can be replaced and so is less bad for the environment. Whereas coal is non-renewable – it consists of the compressed wood deposits of millions of years ago – wood grown in fast growing forests for the purpose of fuel is renewable. The land can be replanted with new trees and the 'wood' crop can be reproduced. Of course, it takes quite a long time.Other forms of renewable energy are even better from an environmental point of view. Energy generated by wind turbines or hydroelectric powerare renewable – and they can be constantly and instantly replenished. Solar energy is another good renewable energy source.

[studiot]

Hello, EE let us look into this more deeply.

 

Here is the table from Wikipedia, it contains more 'surprises' and items of interest.

 

Note that TNT and gunpowder have a rather lower energy density than either coal or wood! (They are near the bottom of the table).

 

They get their destructive power from mechanical energy (PV expansion), not heat production, which underlines the relationship between heat and mechanical energy and how much more mechanical 'effect' you can obtain than heating 'effect' from a given quantity of energy.

 

As to coal and wood,

 

Coal is a little over twice as dense as wood so a given volume (you have mentioned volumetric densities) so there is more 'matter' in a given volume of coal than wood.

 

So you would automatically expect the coal to produce more energy since the chemical reaction producing the heat is dependent on the mass not the volume.

 

Note that Wiki gives both volumetric and gravimetric densities.

Note further that on a gravimetric comparison there is substantially less difference between cola and wood.

 

Nevertheless coal still wins on a gravimetric basis.

 

This is because of the difference of the combustion processes as JC has already noted.

 

 

Edit I just noticed the date on the original parts of this thread..

 

Sarah, you caught me

 

Edited September 7, 2016 by studiot

[BabcockHall]

The oxidation number of carbon in coal is zero. The average oxidation number of carbon in glucose (and therefore in polymers such as cellulose) is also zero. I am not sure that one is more combusted than the other, at least to the rough approximation given by oxidation numbers. The difference because of boiling off the water is an attractive explanation.

Edited September 19, 2016 by BabcockHall