I have use steam cleaners that instead of having a huge boiler used a coil of steel pipe with a fire at the bottom. Cold water is pumped into the coil using a diaphragm low volume higher pressure water pump. By the time it gets to the other end of the coil it is high pressure steam. This technique is used today at lower temperature and pressure on some low thermal lag water heaters. One may want to consider using one or more of these items modified for the steam source.
Offered by Mike.