Calculate the theoretical flame temperature when ethyleneat 298.15 K(25^∘ C) is burned with: (a

Calculate the theoretical flame temperature when ethyleneat...

Key Concepts

Stoichiometric Combustion

This concept involves determining the exact proportion of fuel and oxidizer required for complete combustion, where neither fuel nor oxidizer remains in excess. It is essential to calculate the correct chemical equation for burning a fuel so that all components react fully under ideal conditions, providing a baseline for determining flame temperature and energy release.

Adiabatic Flame Temperature

The adiabatic flame temperature is the maximum temperature that can be achieved by a combustion reaction when no heat is lost to the surroundings. It is calculated based on the energy released during the reaction and the specific heat capacities of the products, making it a critical parameter in combustion analysis and reactor design.

Excess Air

Excess air refers to the additional amount of air provided over the stoichiometric requirement for complete combustion. While it ensures complete oxidation of fuel, it also dilutes the combustion products, which can lead to a lower flame temperature. Understanding the effects of different levels of excess air is vital for optimizing combustion processes and controlling emissions.

Energy Balance

Energy balance in combustion is based on the principle of conservation of energy, where the heat released from chemical reactions is balanced by the increase in enthalpy of the combustion products. This concept is applied to derive the theoretical flame temperature by equating the energy of reactants with that of the products, considering all contributions from thermal energy.

Effect of Preheated Air

Preheated air in combustion systems carries an increased amount of thermal energy into the reaction. Using preheated air can raise the overall flame temperature by reducing the energy required to heat the incoming oxidizer, resulting in more efficient combustion. This concept is crucial when designing systems for higher energy output or to achieve specific temperature profiles.

Transcript

00:01 Problem we have acetylene gas and let's see here we have that's c2 h2 and we're going to burn it with a hundred percent theoretical air and then a hundred percent theoretical oxygen at 25 c and 100 kilopascals so in the the reference case for air we wind up with a 2 .5 kilowatts of oxygen per kilomol fuel so we get 2 .2 kilomoles of co2 a kilomole of h2 and 9 .4 kilomoles of n2 per kilomole fuel.

00:45 The entropy of the reactants is the entropy of formation of them and that we can calculate or look up and that is for acetaline is 227 megajoules per kilomole.

01:00 Now the entropy of the products, we have the entropy of formation for the co2 and for the h2o, and then their change is in entropy because of their increase in temperature because of the combustion.

01:19 So we can figure out that the change in the entropy of the products must be, let's see here, must be 1 ,260 megajoules per kilimol.

01:32 And if we look up what that would be for these products at 2 ,800, calvin, we get about 1 ,200 megajoules per kilomol, which is a little too low.

01:45 And if we, let's see here, that should not be, let's see here, 12, that's, i missed a 3 here.

01:53 So let's see here, that should be 3 megajoules per.

01:59 Kilo more.

02:01 Okay, so we have, if we look, go to the next value of 3 ,000 kelvin, we get a number that is a little too high.

02:10 And we can interpolate and we get a temperature of the adiabetic flame temperature of the products is 2 ,909 kelvin.

02:22 Now, when we burn it with pure oxygen, we have to do a little more, we have to actually extrapolate in this case because the atabatic flame temperature is higher than what we're our tables go to.

02:36 So what we want up doing is here, 30 geometric combustion, we just have two co2s, one h2o.

02:45 So basically, again, we're getting, we're not, don't have any of this nitrogen messing around here.

02:52 The change in enthalpy of the products is the same.

02:57 Same thing here, right? because we have, let's see here...

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