[XeLa.]

Hello,

I'm currently working on a question in which two hydrocarbons (CH₄ and C₂H₂) react with oxygen gas over time. (A valve will introduce air to the hydrocarbon mixture for a given period of time, and then it will be closed off.) I was just wondering how a limit of oxygen gas might react with the two hydrocarbons present? My hydrocarbon mixture is 0.65-mol. % methane - would the oxygen gas react in a way where 65-mol. % might react with the methane and 35-mol. % with the ethyne? (I've provided data in the question based on this assumption, but would this be realistic?)

EDIT: I've tried researching the topic, but I can't find anything definitive about how oxygen reacts with a hydrocarbon mixture. Is there a particular sequence - a mechanism - or is it just random (like entropy)?

Any help would be appreciated!
XeLa

[Enthalpy]

Hi XeLa,

My first interrogation is whether there is enough oxygen to burn both gases when they are ignited.

If not, you'll get soot under usual conditions. How much of each gas reacts would be extremely difficult to tell. You might suppose that acetylene is more reactive, but this won't bring clear consequences.

I insist on "when they are ignited". If the oxygen is introduced slowly and the mix is immediately ignited, you get soot, which does not burn afterwards, even if hot and with oxygen.

If the oxygen suffices to burn both gases towards CO and H₂O, then you get a complex mix of H₂, CO, CO₂, H₂O, O₂ and many more. It even needs more oxygen than this minimum, because the flame in oxygen is much hotter than in air, and the reactions are very incomplete. The temperature keeps a significant amount of H₂ and CO together with O₂. In air, the big amount of N₂ dilutes the heat, the flame is less hot, and the reactions proceed completely to CO₂ and H₂O if the amount of O₂ suffices.

With enough oxygen and a hot flame, the detailed composition of the fuel is unimportant. The temperature suffices to destroy completely the fuel molecules, so the composition of the oxidized products depends just on how much the fuel mix brings of C, H and heat of formation.

[billnotgatez]

@Enthalpy
I guess it is implied, but I do not see where ignition is part of @XeLa question.
It would be of interest as to when and how the ignition is applied.

[XeLa.]

Thanks for the replies, @Enthalpy and @Billnotgatez! The question was written with the intent of a valve introducing air into a piston-and-cylinder apparatus containing only methane and acetylene at increments. Using these increments, a linear model could be produced and the question could be solved from there. This is assuming complete combustion even with lack thereof oxygen gas (an unrealistic assumption, but nonetheless necessary for a solvable highschool-level question).

I was mainly just wondering if oxygen gas would react in some predictable manner with the mixture, but - as you've explained - this doesn't appear to be the case. As such, I've decided to simplify the question to consider only a single, unknown gaseous hydrocarbon in order so that this complexity can be avoided!

Just one other question though, you've brought to my attention the ignition source I intended on using. I was thinking of igniting the mixture electronically - via. a spark - but I'm not sure if that would work/provide the most efficient source in reality. I was wondering what the most efficient means of an ignition source might be to ignite a gaseous compound in a piston?

Thanks for the help.
XeLa

[Enthalpy]

Well then, no nice combustion. With too little oxygen, the hydrocarbon(s) will soot a lot. How much is very hard to predict, as it depends on every details of the mixing process.

Such a question is absolutely impossible to answer, except maybe by experiment.

It is my personal and strong opinion that questions with wrong assumptions should not be given to students, because they remember these wrong assumptions in their professional life. Especially for sooting, about which too many people have misconceptions, like "carbon burns".

There are enough true-life processes that can be computed, no need for artificial conditions. Injection in a Diesel engine for instance. Or why a gas turbine combustor burns kerosene near stoechio proportions and dilutes the flame thereafter. And so on.

A spark is an efficient way to light flammable gas mixtures. It will work practically as soon as the mixture is within the "flammability limit" in the zone around the spark. Heating beyond the autoignition temperature, for instance by compression in a Diesel engine, is an other one. Future engines (one manufacturer develops one) are to pre-mix the fuel and air in the cylinder and tune the compression ratio finely (through the closing time of the inlet valve) so that autoignition happens just before the compression end.

[XeLa.]

Well then, no nice combustion. With too little oxygen, the hydrocarbon(s) will soot a lot. How much is very hard to predict, as it depends on every details of the mixing process.

Such a question is absolutely impossible to answer, except maybe by experiment.

It is my personal and strong opinion that questions with wrong assumptions should not be given to students, because they remember these wrong assumptions in their professional life. Especially for sooting, about which too many people have misconceptions, like "carbon burns".

There are enough true-life processes that can be computed, no need for artificial conditions. Injection in a Diesel engine for instance. Or why a gas turbine combustor burns kerosene near stoechio proportions and dilutes the flame thereafter. And so on.

A spark is an efficient way to light flammable gas mixtures. It will work practically as soon as the mixture is within the "flammability limit" in the zone around the spark. Heating beyond the autoignition temperature, for instance by compression in a Diesel engine, is an other one. Future engines (one manufacturer develops one) are to pre-mix the fuel and air in the cylinder and tune the compression ratio finely (through the closing time of the inlet valve) so that autoignition happens just before the compression end.

Thanks for the reply, Enthalpy! I'll definitely follow your advice and see what I can re-make out of my question.
XeLa