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Calorific value

Calorific value

Calorific value, understood as the quantity of heat produced in the complete combustion of the units of weight or volume of fuel, is the verdict for an energy vector because it reveals, with a figure, how valid it is as such. Compared to how big it is, above all, given that the problem of many energy carriers is precisely the size, in mass in the case of solids and liquids, in volume for gases. The concept, and the clutter problem, remains, and the calorific value helps to orientate. It is impalpable and hardly imaginable, as a quantity, but knowing it we will love it.

Calorific value: what it means

The calorific value, by definition, is the maximum quantity of energy that can be obtained by completely converting a unit mass of an energy vector under standard conditions. As mentioned introducing it, the calorific value makes a sort of evaluation of energy yield for the same mass, so as not to find an enormous quantity of an energy vector from which it can then obtain a miserable energy. If the conversion of the vector into energy occurs through combustion, the calorific value coincides with thestandard mass enthalpy o fuel combustion volume.

Although ancient, we will find theunit of measurement of Kcal / Kg, we also take as a reference that of MJ / kg indicated by the International System, with the precaution that when we are dealing with a gas, its quantity is indicated with in Nm³. Generally the calorific value is divided into upper (ΔcHso) and lower (ΔcHio)

Higher calorific value

The higher calorific value is the quantity of heat obtained with a complete combustion at constant pressure of the fuel, of one of its unit mass elements, when the combustion products are brought back to the initial temperature of the fuel and the comburent. To obtain an estimate, you can measure it with ad hoc tools or throw yourself into its calculation, based on the elements that make up the fuel in question.

This second way involves a lot of work: you need to know the quantity of mass of the various elements (carbon, hydrogen, sulfur ...) contained in a 1 kg of fuel to then evaluate the heat input provided by each. All that remains is to add them, then, and here is an approximate estimate of calorific value of the fuel, to get the exact one you should take into account the strength of the chemical bonds in the fuel molecules.

The direct method of obtaining the higher calorific value provides for the use of Mahler's calorimeter bomb, apparatus in which a complete stoichiometric reaction is made between oxygen and a certain amount of fuel. A known quantity of water is taken and the heat obtained from the reaction is absorbed and its temperature variation is measured, then it is a question of a formula. The one of Dulong.

Lower calorific value

Conventionally, when it comes to calorific value of a fuel and the efficiency of a thermal machine is defined as "the higher calorific value minus the heat of condensation of water vapor during combustion".

This is because usually in combustion the temperature of the products is higher than that of the starting fuel and part of the theoretically available heat is 'dispersed' by the heating of the fumes and, above all, by the vaporization of the water produced by the combustion. This is what we want to account for with the lower calorific value.

With the refinement of condensing boiler technologies, for example, not all of the latent heat of water vapor it is lost and it is possible to have a nominal efficiency equal to 100% from a fuel, even if the loss in fumes is currently irrecoverable. Never say never, though.

As for the superior, also for the inferior we proceed with the elementary analysis to get a numerical estimate. This time we have to pass by higher calorific value and subtract 2.5 MJ for each kg of water vapor contained in the fumes.

Calorific value: oil

The calorific value of crude oil in kcal / kg is 10,000, to get an idea of ​​what it means we can compare it with that of other fuels similar to it or its competitors. We have the G.P.L. with 11,000 kcal / kg above him, and also petrol and diesel with 10,500 and 10,200 kcal / kg respectively. Below the crude oil threshold, or better than yours calorific value, there are fuel oil, very close, with 9,800, and natural gas immediately following, with 9,200. With hard coal you have to "settle" for 7,400 kcal / kg.

Calorific value: methane

The calorific value of methane is approx 8200-9200 kcal / Sm3 at a temperature of zero degrees centigrade. This means that if one cubic meter of methane is burned at a temperature of 0 ° C and atmospheric pressure, 8,200 to 9,200 kcal are obtained.

As we know this figure is used to get an idea of ​​the heat obtainable from methane as a fuel, to be able to interpret gas consumption in terms of energy produced and not only in cubic meters which may not be a particularly interesting quantification, seen by us "users" and "payers".

The calorific value, in the case of methane as for other gases, it makes us understand if it is “good” or not: the higher it is, the happier we can be. For methane this is a rather important assessment because it is the fuel generally used in Home heating. It is right to understand if it is worth paying for it and, even before that, to choose it. Methane is also used in power plants for the production of electric energy, in cars as a fuel and in some industrial processes: its calorific value is of wide public interest.

Calorific value: wood

It is difficult to speak and above all to define the calorific value of wood, it is a quantity intrinsically linked to the different types of wood and which very much depends on their humidity. It is from this variable that we start to then understand the power of boilers or stoves that cannot be evaluated without taking into account the type of wood and the conditions of the wood we use.

In order to make a comparison with other fuels even of a completely different nature, we can indicate how average calorific value of wood well seasoned the quantity of 3200 kcal / kg. Humidity plays a very strong power, when it is at 15%, for example, there are 3490 Kca / kg, with a jump in humidity up to the value of 40% it collapses to 2300 kcal / kg.

Calorific value: coal

We move from wood to coal, always considering that we are making an estimate with an elementary analysis that does not describe the real complexity of the whole. But it is of great help to us. There are links between elements, which certainly are not isolated in the fuel, but there are also many other factors, including the calorific value of coal. For example the percentage of water, that of volatile substances and ashes. And then there are parameters and measures such as the free swelling index and the agglutinating power.

Settling for an estimate in spans of calorific value keeping in mind "only" of the different types of hard coal distinguishable by their age: the oldest is anthracite, then litantrax, then lignite and the youngest, peat. Of course, you can't interview them to ask them carbongenesis, for which the elemental chemical composition is used: gradually with age, coal is enriched with carbon and loses oxygen, and it is seen that their calorific value increases from peat to anthracite with the decrease of its ratio H / C.

For peat with an H / C of 1.30 the calorific value and of 3000 - 4500 Kcal / Kg, then we have lignite with the ratio H / C 0.97 and 4000 - 6200 Kcal / Kg as an interval, followed by litantrax (H / C = 0.72 and calorific value 7600 - 9000) and anthracite (H / C 0.29 and calorific value 8300 - 9000 Kcal / Kg).


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