Steam atomisation of HSFO 2

[efreo]

Can anyone help explain the basis of steam atomisation of heavy fuel oil, and why the steam quality is specified as either dry saturated or minimal superheat. I'm particularly interested in how the steam and water mix, and in how the steam/water droplets behave in the flame zone.

 

[Flareman]

The basis of steam atomization is primarily mechanical. For fuel oil to burn decently, it needs to be in small droplets. Then the droplets vaporize quickly rather than overheating and generating soot (which occurs in the middle of the droplet if it's too big).

The oil guns contain chambers where the steam flow impacts the oil and breaks it into droplets before it is sprayed into the burner. The presence of the water/steam also helps to mechanically separate the droplets.

Once the oil is in the flame zone it can evaporate and burn as gas(es).
The flame zone itself is like a "soup" of hydrocarbon radicals, steam, steam radicals and air.
As the temperature goes up, the droplets vaporize but, if the heat transfer into the droplet exceeds the vaporizing capability, the droplet itself can overheat and begin to "crack" (decompose) before it has had chance to become a gas and mix with the air. That forms carbon chains in the droplet and leads to sooting.
The steam in the flame zone is inert so it does not contribute to the flame temperature. In fact it reduces the flame temperature slighly, thus tending to retard the "cracking" of the hydrocarbon and suppress the carbon formation a little. So, if you have a high superheat, you're working against this cooling effect.
Also, some of the steam itself "cracks" so that you get -OH floating around in the flame. If any stray carbon atoms come along they can be swept up by the -OH into -CHO (a gaseous radical) rather than joining up with more C to make solid carbon which again doesn't burn out too well.

You don't need to use steam for atomization. Some guns are mechanically atomised and use the oil pressure and the spray pattern to break up the oil. Usually the pressures needed for this are fairly high (100 psig and up) whereas steam atomization can be successful at lower pressures.

Hope that is useful

David

 

[efreo]

Some literature suggests that the steam and oil actually form an emulsion in the mixing chamber of the atomiser, and that the emulsion helps further breakdown the droplet in the flame zone. ie smaller droplet, better combustion. Is that why the steam is usually specified as dry sat or min superheat?

 

[op9]

There are two basic types of dual fluid atomising nozzles, whether it uses air or steam as the atomising medium.
1) Internal mix or 2) External mix.

Firstly Internal mix

Types of these are I-Jets and T-Jets

Requires quite a large proportion of steam or air to fuel. Typically 0.2lb to 0.3lb medium/ lb of fuel.

There is a common mixing chamber before the steam/ fuel or air/ fuel exits so the medium pressure must always be higher than the fuel pressure to prevent oil entering the atomising med. supply lines. This is typically kept 15-30psi above the oil pressure by some form of differential press control valve. Typical oil pressure would be 90psi, but can be higher or lower.

Here the steam pressure augments the lower oil pressure to assist with atomisation. In fact there are certain types which turn off the steam at higher fuel pressures when there is sufficient mechanical energy available with fuel pressure alone.

Atomisation is excellent with turndowns typically 8 or 10 to 1.

External mix.

The well known Y-Jet is of this type.

Requires less medium consumption 0.05lb to .07lb/ lb fuel.

Medium pressure does not have to be higher than fuel as the interaction is very "local" just before emission out of the nozzle. Supply of the fuel through drillings at an angle to the steam drillings causes the fuel to enter into the high velocity steam which efficiently "tears away" the fuel finely atomising it. There are a number of these drilling combinations terminating in common exit orifices(say 6 to 10 per nozzle).

Atomisation is slightly less efficient compared to the internal mix, but still excellent and capable of 10 to 1 turndowns.

As you can see there is mainly a mechanical effect, but having the steam mixed with the oil assists the effect by its expansion on exiting also "blowing the fuel apart"

The steam must be dry or the free water will cause pulsations in burner operation.

In all types of assisted atomisation the fuel pressure can be much lower than pure pressure atomisation and the viscosity of the fuel much less.


There is another type of atomising which uses this emulsification property. Here a metered quantity of WATER is fed into the heavy oil (up to 6% of the fuel). It must be well mixed and not come through as slugs. The nozzle is a pure pressure jet, but the effect of water turning to steam achieves the enhanced atomisation.

Hope this helps,
Rod Nissen.
Combustion & Engineering Diagnostics
canded@iprimus.com.au

http://www.canded.com.au