Higher and Lower heat value of combustion of Coal?

[Ahmed Navy]

hello my friends ,
i have two question about calorific value of coal
why Lower heating value decrease with increase oxygen content but increase with increasing hydrogen , sulfur and carbon content ? Do this elements influence the higher heating value or not ?

 

[dRic2]

I have checked this out some time ago, but I do not remember it perfectly so what I'm going to say it half memory and half guessing.

Some of the energy of combustion comes from the oxidizing reactions
$$C + O \rightarrow CO + O \rightarrow CO_2 $$
which should be exothermic (release heat).

If you increase the amount of oxigen in coal, you will have lots of $C-O$ bonds already in the coal and this mean that some of the carbon atoms are already oxidized: this is not good because they won't be able to react with oxygen like in the reaction above, thus less energy will be released. From the same reason if you increase the percentage of Carbon atoms you will get better performances.

About Hydrogen: $H \cdot$ is a very nice radical which speeds up the combustion reactions and prevent them from dying out.

I don't know about Sulfur

 

[256bits]

Do this elements influence the higher heating value or not ?

What is your reflection on this?
What is the difference between the lower heating value and the higher heating value?

 

[Ahmed Navy]

What is your reflection on this?
What is the difference between the lower heating value and the higher heating value?

lower heating value is net calorific value
higher heating value is gross calorific value

 

[Ahmed Navy]

I have checked this out some time ago, but I do not remember it perfectly so what I'm going to say it half memory and half guessing.

Some of the energy of combustion comes from the oxidizing reactions
$$C + O \rightarrow CO + O \rightarrow CO_2 $$
which should be exothermic (release heat).

If you increase the amount of oxigen in coal, you will have lots of $C-O$ bonds already in the coal and this mean that some of the carbon atoms are already oxidized: this is not good because they won't be able to react with oxygen like in the reaction above, thus less energy will be released. From the same reason if you increase the percentage of Carbon atoms you will get better performances.

About Hydrogen: $H \cdot$ is a very nice radical which speeds up the combustion reactions and prevent them from dying out.

I don't know about Sulfur

i read in scientific paper about calorific value as the following :
" LHV of a fuel increases with increasing the hydrogen content due to cause combustion
water. The LHV of a fuel increases with increasing the sulfur content due to cause SOx
gases absorbed by water".
what do meaning of combustion water and SOx gases absorbed by water ?
can you assist me to understand what do he mean ?

 

[Asymptotic]

Consider kerosene instead of coal to simplify the question, and more easily understand the difference between HHV and LHV.

In the USA, 1-K and 2-K kerosene grades are defined by ASTM D3699-90, and are identical except that 2-K has a higher sulfur content (0.30% max, by weight) while 1-K can have no more than 0.04% sulfur. In practice, due to stricter environmental standards evolving over time, 1-K kerosene sulfur content is typically no more than 15 ppm (0.0015% by weight) in many jurisdictions.

Both grades have identical distillation temperature profiles, and thus the same heat of combustion specifications. This is why it is a "simpler" fuel than coal, which has different heating values depending upon the type (lignite, bituminous, anthracite, etc), wide variations in sulfur content and other constituents, and more pronounced aging effects (freshly mined coal has a higher hydrogen content).

2-K kerosene can only be burned in flue-connected appliances which vent combustion gases including the water vapor resulting from combustion outside of a dwelling, and here the LHV (43.1 MJ/kg) is used. The sulfur content in 1-K kerosene is low enough that it can be burned in unvented appliances where combustion byproducts and water vapor are released to ambient air, and the HHV value (46.2 MJ/kg) is used.

The main difference between the two is the HHV value includes energy recovered by the phase change from combustion-produced water vapor to it's liquid form.

https://en.wikipedia.org/wiki/Heat_of_combustion

You'll want to do further research on how this applies to coal.