# How is it possible that combustion of coal releases similar energy as TNT explosion while intuitively we would not expect that?

There are some marked differences that make $$\text{TNT}$$ far more suitable than the combustion of coal for explosives purposes.

Firstly, the decomposition reaction of $$\text{TNT}$$:

$$2 \text{C}_7\text{H}_5\text{N}_3\text{O}_6 \to 3 \text{N}_2 + 5 \text{H}_2 + 12 \text{CO} + 2 \text{C}$$

proceeds far faster than the combustion reaction of coal:

$$\text{C}+\text{O}_2 \to \text{CO}_2$$

Secondly, the decomposition of $$\text{TNT}$$ produces far more gaseous reaction products than the combustion of coal: respectively $$10\text{ mol}$$ of gas per $$\text{mol}$$ of $$\text{TNT}$$ for $$1\text{ mol}$$ of gas per $$\text{mol}$$ of coal (and the latter requires $$1\text{ mol}$$ of $$\text{O}_2$$ for the combustion to take place).

It's the production of gaseous reaction/decomposition products that make a good explosive: the super-fast build-up of gas inside the shell makes the pressure increase until the shell bursts, releasing all its energy at once.

Probably because the energy released by the TNT explosion is for a very short time whilst the energy released from combustion of coal is over a much longer time. It’s the energy release rate of the TNT that’s greater

Thank you. But do i understand that the increase of temperature is bigger with coal than with TNT ?

That's hard to say since it would be difficult to specify a single temperature of the rapidly expanding gases and particles of the explosion. There would be significant temperature (and pressure) gradients in an explosion compared to the burning of coal. The temperature of the burning coal would be more uniform.

Hope this helps

First, there is a key difference between the energy values of TNT and coal: you miss the oxygen needed to burn the coal.

Burning 3kg of coal needs some 8kg of oxygen and the oxygen is not included in the calorific value above. The oxygen is also the factor limiting how fast the coal releases its energy. It is limited by the supply of oxygen.

That's why coal by itself cannot do much of destruction.

In contrast, when detonated, TNT decomposes by itself. It can also burn pretty well, but that's another story completely.

OTOH, there are a class of explosives made of some fuel (like coal) and a liquid oxygen. They are not routinely used anymore because of safety and logistics concerns, but they are more or less comparable to TNT in regard to their explosive action.

When you have everything for the chemical reaction to proceed in close proximity, an explosion is possible.

Edit: Another, much more known and widely used explosive that uses coal as a main ingredient and a main source of combustion energy powering the explosion is the black powder.