Ethylene is a diatomic molecule so IR can measure the unique vibrational frequency associated with this molecule VERY quickly and with the minimal amount of sample handling. It is also VERY specific for the gas of interest with other interferents easily kept isolated.
Probably because its one of the most well-researched and accepted methods.
Different elements and compounds absorb different amount of IR light.
By comparing the sample stream to a reference stream you can determine concentration.
I've seen Dual Beam Infra Red analyzers used on quite a few different applications, for both hydrocarbons and emission monitoring.
There's actually two ways:- IR is for flammability. Electrochemical for low ppm levels. IR tech has no consumables, it's cheaper in the long run. Catalytics can also be used which is even cheaper initial cost.
But C2H4 is more flammable than CH4. (It takes 2.6%v/v for C2H4 to become flammable compared to 5% for CH4), therefore you need a fairly sensitive method that responds very fast to detect C2H4.
