That's my understanding as well. Three stage catalytic converters reduce nitrogen oxide emissions through reduction, so it makes sense that a reducing flame (oxygen starved) would produce less nitrogen oxides than an oxidizing flame.

yes, though if it's sufficiently oxidizing (sufficient excess of air) the flame temperature starts to drop, which will also cut your nitrogen oxide emissions to almost zero

Both my Range Rovers are converted to run on propane, and unlike on petrol the LPG system runs the engine very lean. If you use a special glass sparkplug (those old "Colortune ones!) you can see the flame is a lovely bright blue on propane and quite orange on petrol, with the mixtures set correctly.

Measured at the downpipe with the sniffer in the lambda sensor hole it shows no (or almost no) CO and a bit more NOx than you'd really want, but after the cat there's no measurable NOx either. The tailpipe emits basically just warm carbon dioxide and water vapour.

>Both my Range Rovers are converted to run on propane

What is the downside to this? Obviously availability and ease of filling is part of it, but any other downsides?

shorter range with the same size gas tank maybe

There's a big metal tank where the spare wheel goes, so you lose a bit of space. Availability can be a problem occasionally, particularly where I moved to, but filling is just as easy as filling a petrol tank.

In theory the engine makes a bit less power flat out, but I never drive flat out. It's quieter, runs cooler and cleaner, and has a bit more low end torque because propane burns with a bigger slower bang.