Why does temperature not increase by putting more logs into the fire

Question

Why does temperature not increase by putting more logs into the fire ? It is a very common observation which I have seen but after some research online I found that it is not completely true.

One reason which I think is that if I put more and more logs into the fire , it wouldn’t increase the temperature because the kinetic energy of molecules for all the logs would be same. There is no extra kinetic energy inside the logs or in fire that would increase the total energy of the system. Therefore , I feel hot is because I just see more of that heat present because of the logs in extra amount. Therefore , their temp is same and just that the logs come more near to me. I feel hot. Therefore , the heat which comes on our body depends on the kinetic energy of air particles. So , if their K.E is same.

I am very curious to know if there is a different reason behind this ?

Answer 1

Adding more fuel doesn't permit an arbitrarily high combustion temperature because of an unavoidable intrinsic limitation: The reaction has to heat its products. (In the case of wood, this is mostly carbon dioxide and water.)

In the most efficient flame, we might mix the reactants in perfect proportion and eliminate heat losses as best we can, but we can't get around the limitation of heating the products of the combustion reaction. The idealized maximum temperature is called the adiabatic flame temperature and is about 2000°C for wood burning in air. You can estimate this temperature yourself for any reaction if you know the enthalpy of the reaction and the heat capacity of the products.

Other limitations can arise in practice: you can't pack fuel infinitely densely (or, put another way, two identical systems placed side by side aren't twice as hot, as temperature is an intensive variable), the outgoing heat flux increases vastly with increasing temperature, and combustion inefficiencies always exist. But the constraint described above is the ultimate limitation.

Answer 2

I agree with Chemodynamics, and I will try to add a different perspective (or maybe two).

The fire process is a process where you add a fuel (i.e. energy in the system), and that fuel is gradually consumed. The rate at which it is consumed releases the chemical energy. An interesting thing here is that:

• the log is a three dimensional object
• the fire will only occur on the external surfaces of the wood.

The above reason is why, if you put small pieces (or shredded) of wood in a fire (because the surface area is larger) the fire seems to grow stronger much faster and the flame is hotter (compared to a log). What happens then is that the flame releases more energy in the unit of time, which heats up the surrounding material (providing the required activation energy) and sustains the combustion.

Perspective No 1:

The energy released by the flame is governed by a square law of the dimensions of the object, while the thermal capacity is governed by the mass (which follows a cube law wrt the dimensions of the object).

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At this point its noteworthy that in order to understand fully a combustion process you need to be familiar with concepts like internal energy, enthaphy, specific work due to volumetric changes and others. However, I will try to explain it a more descriptive manner.

The flame of a self sustaining combustion process releases at least enough energy to heat up the surrounding material just enough to be able to ignite (this is called activation energy). A rest of the remaining energy raises a) the temperature of the fuel (and oxidizing agent) even further, and also b) it heats up the environment (which is the useful part).

What you feel as heat when you burn a log is either convective or radiative. The convective heat transfer has to do with the movement of combustion gases (and it increases with the speed of the gases and also the temperature of the gases). The radiative is only depended on the temperature and increases by a 4th order of the temperature. That means that doubling the temperature (in Kelvin) results in 16 times increase in radiative heat losses to the environment.

So assuming 300 K is the ambient temperature, then at 1823 °C (or approx 2100 K), the radiative losses would increase by an order of 7⁴(= 2400).

Perspective No 2:

As the temperature of a flame increases the radiative losses to the environment increase with a power law (4th order). So, despite adding more logs to the pile, once the temperatures reaches a high, the radiative losses are equal to the energy released and there is not enough energy to heat up more the remaining mass in the fire.

Answer 3

I think a review of the definition of specific heat will help:

Specific heat is the amount of energy required to raise one gram of a pure substance by one degree Centigrade.

$C_P = \frac {Q}{m\Delta T}$,

$Q = Amount$ of $heat$

$m = Mass$

$T = Temperature$ of the $substance$

The specific heat is a constant for each specific material, and the formula can be rearranged as,

$Q = C_Pm\Delta T = m\Delta T$ for $C_P = 1$

Now we can express the relationship between the temperature and the mass pictorially:

In the illustration above, we note that an increase in temperature will increase the heat proportionally, as well as an increase in mass, and the temperature is independent of the mass but the heat.

Hope this helps.

Answer 4

Adding more wood does not change the temperature at which wood burns, but it does give you more heat.

Think of it this way: a 15,000 BTU furnace may heat a small apartment, but you'll need a 100,000 BTU furnace to heat a house. They both burn natural gas at the same temperature, the latter just burns more of it at the same time. So the air coming out of both will be the same temperature, but you can get a lot more of it from the larger furnace.

Answer 5

Let’s say I have a single molecule of CH₄ and I want to burn this molecule.

CH₄ + 2 O₂ -> CO₂ + 2 H₂O

How much energy is released? Let’s say it releases 1 unit. How much of that unit will be in the form of kinetic energy? Let’s say all of it: 1 unit. How fast is that CO₂ moving? How fast are those 2 H₂O moving?

Let’s say heat is more or less the random motion of molecules.

So, if I burn 2 molecules of CH₄, would the resulting CO₂ and H₂O molecules be going faster? Nope. There would be double the energy released. You can heat a bigger room (double the size).

I’m just giving a simplified explanation as to why the temperature of the products (the flame) stays the same.

If you want to build a furnace and want to achieve higher temperatures, that’s another story that involves using refractory bricks to isolate (keep the heat in instead of losing it to your surroundings).

//EDIT: Changing Let’s say kinetic energy is more or less the random motion of molecules. to Let’s say heat is more or less the random motion of molecules.