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

In summary, the lower and higher heating values for hydrocarbon fuels exist because of the subtraction of latent heat of vaporization of water. However, coal may have residual water or other hydrocarbons embedded in it, leading to reported values for LHV and HHV. This is why HHV for coal is about 5% higher than LHV. Pyrolyzed coal, closer to pure carbon, would have LHV=HHV.
  • #1
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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  • #2
Coal is not just carbon.
 
  • #3
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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Likes clustro
  • #4
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
 

1. What is the difference between lower and higher heating values of coal?

The lower heating value (LHV) and higher heating value (HHV) of coal are two measurements that represent the amount of heat energy produced when a specific amount of coal is burned. The difference between the two values lies in the treatment of water vapor in the combustion process. LHV considers water vapor in the exhaust gas to be in a gaseous state, while HHV assumes that the water vapor is condensed and the heat released by its condensation is recovered.

2. Why do lower and higher heating values exist?

Lower and higher heating values exist because they provide different ways of measuring the amount of heat energy that can be obtained from coal. LHV is used in Europe and Asia, while HHV is used in the United States. Both values are important in determining the efficiency of coal-fired power plants and the amount of emissions they produce.

3. How are lower and higher heating values of coal determined?

The lower and higher heating values of coal are determined through laboratory tests called proximate and ultimate analyses. These tests measure the physical and chemical properties of coal, such as moisture, ash, volatile matter, and fixed carbon. The results of these tests are used to calculate the LHV and HHV of coal.

4. Which heating value is more accurate?

The accuracy of lower and higher heating values depends on the specific application and location. In the United States, HHV is considered to be more accurate for calculating the efficiency of coal-fired power plants because it takes into account the heat released by water vapor condensation. However, LHV is considered to be more accurate in other countries because it reflects the actual heat energy released during combustion.

5. How do lower and higher heating values affect coal usage?

The lower and higher heating values of coal play a significant role in determining the economic and environmental impacts of using coal. The HHV of coal is generally higher than the LHV, which means that the same amount of coal will produce more heat energy when burned under HHV conditions. As a result, coal-fired power plants that use HHV to determine their efficiency may appear to be more efficient than those that use LHV. However, this does not necessarily mean that they are more environmentally friendly, as HHV does not account for the emissions released during condensation of water vapor.

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