How about natural gas?
A diesel engine won't run on 100% nat gas, as its octane is too high, and it won't self ignite. However, a little shot of diesel will ignite, and makes a great "spark plug" for the natural gas/air charge. Again......... a local light plant with old Fairbanks-Morse engines does this. They start and warm up their engines on diesel, grid tie them, and then turn on the gas, and run them on 90-95% natural gas, with 5-10% of the power coming from the drastically decreased diesel as the "spark plug" ignition source. The diesel smoke vanishes from the power plant stacks when they turn on the natural gas.
Is anyone running their Listeroids on 70-90% natural gas with the remaining 10-30% being fuel/waste/veggie oil for the ignition source? With the increased talk of cogeneration and combined heat and power, natural gas is about 60% as expensive per BTU than number 2 fuel oil. And it takes less effort than hunting/gathering WVO. In a residential/light commercial/light industrial (heat the shop) environment where natural gas is available (already being used as a heat source?) it seems like burning it in an already grid tied listeroid would be a winning idea. It also burns very clean.
Since they don't need to load follow, no fueling governor connection would be required for the nat gas in a grid tie situation. Just pipe gas to the intake, and open the gas flow with a gate valve and adjust the governor (liquid fueling) to decreases to where the engine operates in a stable fashion on the minimal amount of liquid fuel. Use an electric solenoid to shut down gas in event of loss of grid. If the belt broke or the system was to island somehow, the engine wouldn't run away, because the governor would shut down the liquid fuel as the RPMS increased to the point where the natural gas would go through unburned. (no liquid fuel = no 'spark' no burn, no power, no increase in RPMS.) It might hunt quite a bit with excess gas, no mechanical load, and the liquid fuel turning on and off to maintain RPMs, but at least it wouldn't destroy itself.