The difference between high calorific value and low calorific value of gases
Thread Content
The high calorific value of a gas refers to the total amount of heat released when a unit volume of that gas burns completely, and its exhaust gases are cooled back to their initial temperature, with water vapor being expelled in the form of condensed water; in other words, it is the heat generated during the complete combustion of the fuel, as well as the reaction heat that occurs when the water vapor produced by combustion condenses into water； Low calorific value refers to the total amount of heat released when a unit volume of fuel gas is completely burned, and its exhaust gases are cooled back to their initial temperature, with the water vapor being expelled in vapor form. That is, the heat of reaction when the fuel burns completely and the water vapor in the combustion products remains in gaseous form. According to the above definition, there is the following relationship between high calorific value and low calorific value: the lower calorific value (i.e., the low calorific value) equals the higher calorific value (high calorific value) minus the heat required for water evaporation and for heating the substances during fuel combustion. Obviously, low calorific value is of greater significance in practical applications. The conversion relationship between the two is as follows: Hn = H0 * 5.85, where WHn represents the low calorific value, H0 represents the high calorific value, I represents the content of inert substances, W represents the surface humidity of the fuel, and Wl represents the residual and hygroscopic humidity
I still don’t understand what the reference conditions for high calorific value and low calorific value are It mentions the final discharge state of water; what is the difference between these two values when there is no hydrogen or hydrogen compounds present? I also don’t quite understand what that additional heating amount at the higher value refers to. I earnestly request the original poster to provide a more detailed explanation. Thanks
Yes, I really don’t quite understand. OP, could you explain it in more detail?
I don’t know if you’ve figured it out, but I have； Hehe, sometimes my mind gets stuck on one idea and I can’t think straight. For gases containing alkanes or hydrogen, H2O is produced as a byproduct; the state in which this water is released results in differences in its heat value. Additionally, after fuel combustion, everything cools back to its initial temperature. H2O with a high heat value is released in a liquid state, meaning its initial temperature is low enough to allow the water to turn into a liquid. To enable the combustion of such fuel, it is necessary to supply a certain amount of heat to bring it to its ignition point. If there is no hydrogen or hydrides, such as CO, then its high and low calorific values are equal.
Low calorific value means that the heat released by the condensation of water vapor is not taken into account
Generally, is the high calorific value or the low calorific value used when calculating the fuel gas consumption?
This post was last edited by hangzhuozyh on 2011-7-31 at 10:59. Reply to 4# Peili: I would like to ask whether, if the initial temperature of the burning gas is above 100 degrees, the reaction heat should always be of low calorific value; in that case, what’s the point of having a high calorific value? Also, I would like to ask what the low calorific value and high calorific value are for – is it to calculate how much combustion gas is needed to produce a specified amount of heat?
This post was last edited and replied to by Běn Lí on 2011-7-31 at 14:56. Reply 7# hangzhuozyh: The high or low calorific value depends on the state in which the water is discharged. If its initial temperature allows the water to be discharged in the form of steam, then it has a low calorific value. In practice, the lower calorific value is more significant than the higher one, as the latent heat released during fuel combustion is much greater than the sensible heat. It is also not feasible to cool the combustion products to an extremely low temperature in order to make full use of the fuel’s high calorific value, as this would result in a waste of heat. As for the high calorific value, when calculating the energy consumption of a device, all different types of fuels are typically converted into a standard substance for comparison, such as standard oil or standard coal; it is this high calorific value of the fuel that is used in such calculations. This is also my own personal understanding, for discussion purposes.
If water is present in the fuel, and if the water exiting the flue gases is in the form of steam, then the calorific value of the fuel is its lower heating value, as part of the heat is contained in the water vapor; otherwise, it is the higher heating value.
Usually, it is. . . . . . . . low calorific value