Natural gas exhaust components?

[BioHazard]

I've always wanted to have a large greenhouse to extend the growing season around here, and I've been reading more and more about the benefits of using CO2 exhaust gasses as "fertilizer" for the plants in a greenhouse. I know they sell CO2 generators for greenhouses that simply burn NG/propane for the exhaust only. Seems wasteful...

As I understand it, in a perfect world, the only byproduct from combustion of natural gas or propane is CO2 and water vapor. Incomplete combustion can create CO.

On the other hand, if you've ever spent any time in a warehouse with a propane forklift, you know the exhaust has a certain smell. CO2, CO and water are odorless, so what am I smelling? What other components are found in natural gas engine exhaust? When I read about (enourmous) greenhouse cogeneration setups, they usually talk about "scrubbing" the exhaust, but don't elaborate much on the process.

What would it take to make natural gas engine exhaust clean enough to vent directly into a greenhouse for plant fertilization? Is it possible to tune an engine just right so literally nothing comes out of the exhaust besides CO2 and water? Maybe a catalytic convert is all that is needed?

[TimSR2]

I think it's the ethyl mercaptan odorant.  A propane vehicle with a properly functioning catalytic converter has no objectionable odor to the exhaust, if the mix is adjusted properly.

An NG burner has almost no odor , but can be slightly rotten egg smelling if tuned way too rich and no or bad cat. I think this is also due to the  added odorant.

In any case, noxious exhaust is a sign of rich mixture or incomplete combustion.

[BioHazard]

So, when a large commercial cogeneration system says they are "scrubbing" the exhaust, is that just a fancy way of saying catalytic converter?