Yes, why is it needed, this specific heat of combustion

After adapting the fire in ancient times to his needs, man was constantly engaged in the search for some new options for the use of fire and new fuels. As a fuel, various types of combustible substances were used: solid, liquid, gaseous. And where is the specific heat of combustion in all this?

By definition, the amount of heat obtained by burning 1 kg of solid matter or 1 cu. Meter of gaseous matter - this is the specific heat of combustion. To measure it, Joule is used per kilogram (or per cubic meter) or calorie per kilogram.

It is already clear from the definition itself that such a concept as specific heat of combustion allows comparing different types of fuel among themselves. In fact, this means how much energy in the form of heat can be obtained from a unit of fuel. For each substance this value will be different. For a more clear understanding, let us turn to the directories.

It turns out that wood has a specific heat of combustion of 8.4-11 MJ / kg, but for coal it will be 15-16 MJ / kg. What does this mean?

And here everything is very simple. One kilogram of coal during combustion produces heat almost twice as much as is produced by burning a kilogram of firewood, and here it is a question of firewood with a certain humidity. Because if the humidity of the wood is high, then some of the heat energy will go to the evaporation and vaporization of the water contained in the wood, and the final result will be even worse.

One can also consider the practical application of this concept to engineering. Let's go back a bit, in the era of steam locomotives and steamers. Already from what we see, the specific heat of combustion of fuel (for coal and firewood) is different, from this it becomes clear that on the same path a steam locomotive or steamer will need more firewood than coal. Of course, if we assume that the energy they spend on the road is the same, regardless of the type of fuel.

And another conclusion arises from the above example. Knowing the specific heat of combustion, you can determine how many kilometers can pass a locomotive or swim a steamer, having a supply of fuel coal or wood. If you make a mistake with the fuel supply, then all the traffic along the route will end.

Here already there is a practical application at first glance of an incomprehensible and unnecessary parameter. It is possible to compare different substances and determine the possibility of using them in a particular situation as a fuel, and also to assess the consequences of this application.

Lately, there has been a lot of talk about switching to other sources of energy instead of gasoline. If we look at this problem from the point of view of the fuel's energy potential, then, without touching any other aspects of this issue, the situation becomes clear and transparent. So, for example and further reasoning, the specific heat of combustion of gasoline is 44-47 MJ / kg, ethyl alcohol - 27 MJ / kg. And although ethyl alcohol is suitable for use as a fuel for ICE, when comparing the data given, the situation is approximately the same as described above, with wood and coal. Ethanol is needed more, the car will pass at one gas station less, and the engine power will drop when using alcohol.

True, this applies to ethanol or methyl alcohol. If, however , butyl alcohol is used as a fuel , and whose specific heat of combustion is the same as that of gasoline, then the practical experiments of automakers confirm the possibility of using it as a motor fuel instead of gasoline, but this does not concern the issue at hand.

So, it becomes clear that the specific heat of combustion is the energy characteristic of the substance, in particular, of the fuel. This value allows to compare the possibilities of different substances and materials, to evaluate their advantages and disadvantages.