You need to find info or books on boiler design to find out how to
calculate flow and heat absorbtion. Years ago I helped build several
tube and fin-tube boilers for steam engines. There are many critical
parts that you need if this is going to be safe and work with any efficiency.
It sounds like you figured out the fuel part, But now need burn rates
and pressure systems info. You may also need high pressure or temperature
controlled valves and automatic dampers to shut things down if stuff goes wrong.
Hot water can hurt you seriously, Steam will kill you if you make the tiniest
mistake with a design. Its explosive if not handled correctly.
If your heater messes up you could get superheated steam. In the U.S. you need a license to handle live steam systems and that's because of the education needed for handling boilers properly and their design.
I know that you are on a roll with this stuff, I wouldn't presume that I or anyone else knows enough about where you may be headed. I quit messing with boilers for a very sane reason... death !!! So as you follow your ideas, Please take time to study where you are going and dig into the physics and science a bit more.. We want you around to pick on for a lot longer if that's OK with YOU !!!