I've seen natural gas fired backup generators which have the obvious advantage of fuel being piped in rather than stored on the premises. Propane, that I have not seen other than for small portable generators and some that are used in RVs where you already have propane available. I think diesel pretty much owns the large backup generator market, the engines are the same as used for things like semi trucks, motor yachts and locomotives. I don't think anybody is making huge spark ignition engines anymore although there have been some really big ones in the past.
I agree diesel-powered industrial generators are probably the most common now. However, I think that will change in favor of natural gas and propane where they are readily available. Energy costs, emissions limits and fuel stability will bring it about.
Natural gas has the advantage of not having to store it on site. In fact, storing it on site, as CNG or LNG is an expensive proposition. However, as recent events in Texas have shown, natural disasters can stop the flow of natural gas. That low temperatures did it is primarily a reflection of inappropriate penny-pinching in engineering and building pumping stations. Even with appropriate design and installation, natural disasters can interrupt flow … like in an earthquake area where I live.
Large industrial standby generators powered by propane or natural gas are already available. For example, Generac sells 150 kW propane generators and 500 kW dual fuel (natural gas/ diesel). And no, separate engines aren’t required for natural gas/diesel gensets. They are diesel engines modified to aspirate an air/gas mixture, which provides most of the energy. Small diesel injections provide ignition, but the engines can run on diesel alone if necessary. Generac’s largest natural gas generator is 1 MW and powered by a 12-cylinder, 49 L spark-ignited engine.
Mike in California