Lower and Higher Heating Values of Coal: Why do they exist?

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clustro
Hello friends,

The balanced chemical equation for the combustion of carbon is:

C(solid) + O2(gas) -> CO2(gas)

Now, the lower and higher heating value of hydrocarbon fuels (such as methane) exists because in the LOWER heating value, you are subtracting away the latent heat of vaporization of water.

However, in the combustion of carbon, no water is produced.

Why then, are there reported values for LHV and HHV for coal? The concept doesn't seem like it should apply to coal.

The best answer my co-worker and I could come up with was as follows:

1. The coal may have some residual water in it, which would lower the effective heating value because that water would subsequently be vaporized.
2. The coal maybe have other hydrocarbons embedded in it. These would produce water when combusted.

The HHV for natural gas about 10% higher than the LHV.
The HHV for coal is about 5% higher than LHV. This is why we believe points 1 and 2 to be the case.

Any help you can offer friends is most appreciated. Thanks in advance.

-clustro
 
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clustro said:
Hello friends,

The balanced chemical equation for the combustion of carbon is:

C(solid) + O2(gas) -> CO2(gas)

Now, the lower and higher heating value of hydrocarbon fuels (such as methane) exists because in the LOWER heating value, you are subtracting away the latent heat of vaporization of water.

However, in the combustion of carbon, no water is produced.

Why then, are there reported values for LHV and HHV for coal? The concept doesn't seem like it should apply to coal.

The best answer my co-worker and I could come up with was as follows:

1. The coal may have some residual water in it, which would lower the effective heating value because that water would subsequently be vaporized.
2. The coal maybe have other hydrocarbons embedded in it. These would produce water when combusted.

The HHV for natural gas about 10% higher than the LHV.
The HHV for coal is about 5% higher than LHV. This is why we believe points 1 and 2 to be the case.

Any help you can offer friends is most appreciated. Thanks in advance.

-clustro
The difference is due to the presence of hydrocarbons in coal. Pyrolyzed coal (coke) is closer to pure carbon, and would have LHV=HHV.

http://www.nap.edu/read/4918/chapter/16
 
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James Demers said:
The difference is due to the presence of hydrocarbons in coal. Pyrolyzed coal (coke) is closer to pure carbon, and would have LHV=HHV.

http://www.nap.edu/read/4918/chapter/16

That makes much more sense now. Thank you so much for your reply.

-clustro