Atomizing Medium for Combustion

You are atomizing the diesel at start-up with air because you don't have any steam yet. Once extraction steam is available you use it to heat your non-distillate fuel oil and spray it into the fire box. The non-distillate fuel oil must be heated to make it flow otherwise it cannot be pumped, let alone atomized.

The temperature in the fire box precludes the formation of water/moisture. Instead; it goes up the stack with the the combustion products.

Air can be used for atomization. However, were you to try it, I think you would find that the passages in your unmodified burner are not large enough to deliver enough air for good atomization of your heavy fuel. In addition, compressed air is not free - you have the installation, maintenance and operating cost and reliability of the air compressor to consider. Besides, if you're not careful, you can introduce water with the air, too. The plant that I'm aware of that tried air soon returned to steam.

You are correct to be concerned about delivering dry steam to the burner. Pipe and hoses should be completely insulated, trapped, and flushed during start-up.

"to atomize" means to transfer enough heat to the cold liquid fuel, from some source, to completely vaporize it. Heat of Vaporization (HoV) must be supplied.

Air is an extremely poor medium for heat transfer as its specific heat capacity is extremely small, 3222 times less than water, if I remember correctly.

A high pressure discharge nozzle might work better? Thats not "vaporized" but a fine spray is better than large droplets.

Also consider pre-heating the cold fuel, that provides most of the HoV already in the fuel.

You are using diesel oil initial start up which can also be done by gas cylinders. The steam thus generated is then being utilized to heat up the day oil HFO tank and the oil heat exchanger to develop the pumping viscosity of the HFO fuel pump of the Oil burner. These fuel pumps are at >30 bar which actually automizes the oil to start up the Boiler on HFO. The initial start up system is then shut off.

You cannot use just compressed air because the HFO is cold and thicker in viscoscity.

Steam has latent heat to give up at constant temperature. Air only has sensible heat and cannot sustain the temperature as it gives up its heat. It's as simple as that.

You just calculate the heat required for automizing the FO and to supply that much heat through air, what will be cost of operation for compressing the air and heating it to the required temperature to impart heat to FO, as compared to the negligible cost of steam for the purpose.

When compared to air and steam as an atomizing medium, there is a possibility of moisture formation with steam when oil flow comes down in any worst case.

Steam is not directly injected for automizing. Steam is just indirectly heating the coils of the day oil tank and tubular heat exchanger till the HFO oil temperature rise > 85c. Then diesel/gas firing is switched off and the burner fuel pump >30b bar takes over and automizes the HFO. The diesel firing/gas firing is just for initial cold start. There is no steam going into the burner.