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Q & A - BOILERS
 
 

B01-Q. How does Moletron prevent slag build up in Boilers' screen tubes, super-heater's banks, etc.?
PREAMBLE. Slag build up occurs when the aggregation of elements (or their compounds) likes Vanadium, Aluminium, Silicon, Nickel, Iron, Calcium, Sodium, etc, contained in the F.O. oxidizes during a combustion process that takes place with an amount of excess air over 10-13 % ( or whenever, more than 1.9-2.2 % of free oxygen is found in flue gases).
These slag-forming elements tend to aggregate in form of asphalt micelles. They bind together with heavy parts of hydrocarbon, constituting a shield, which prevents the oxygen of air to properly oxidize the H and C of the hydrocarbon particles. The results of this improper combustion are:

  • delay of the combustion process, that prolongs the flames' shape and sends flames to impinge against screen tubes,
  • The necessity of supplying an excess of combustion air enough to keep a low smoke opacity, overlaps the threshold limit of the slag forming phenomena, because the necessity to burn the hydrocarbons encompassed within the shield of V-Al-Si-Ni-Fe-Ca-Na (in order to avoid visible smoke) involve an huge excess of combustion air.
  • The aggregation of those slag forming elements has a melting point of about 600-650C, this means that in the last parts of flames, the a.m. elements have already reached the status of liquefied slag, then, as soon as flames impinge against screen tubes, the tubes area having a temperature below 600 C is a suitable area on which slag solidifies. Same slag build-up occurs when flue gases touch the tubes of super-heater banks.

A. Moletron, by disrupting and dispersing the aggregation of slag forming elements, destroys the shield that hindered the oxygen to reach hydrocarbons. Instantly, the whole combustion process results accelerated and flames shapes appear very concentrated and shorter than flames named in the PREAMBLE.

  • The smoke density drops down suddenly, thus claiming a re-adjustment of air-fuel ratio.
  • In turn, the quantity of oxygen involved in the combustion process is greatly reduced.
  • Moreover, with the dispersion through the fuel flow of the "aggregated slag forming elements", converted them from status of "aggregated" into single elements, each element has re-gained its own melting point (V 1710 C-Fe about 1500C- Ca 810C- Na 97.5C- Ni 1452C- Si 1420C-etc.)
  • Shorter flames allow to extinguish themselves before they reach screen tubes.
  • Slag forming elements leave flames already in solid status (except for Na) and leave the combustion chamber in form of powdered ashes. Minor slag traces in combustion areas are the result of too-high excess air kept during burners' ignition or when some burners are shut-down (ship's maneuvering, etc.).

B02-Q. Topic: Visible smoke at funnel versus Oxygen level in flue gases.
Preamble: By keeping the lowest value of oxygen in flue gases, the air/F.O. ratio is set in a way that gases result colorless. This setting is quite close to the limit of acceptance, in fact a slight reduction of the inlet air or a rise of F.O. quantity starts to produce a pale gray visible smoke.
Q. Since F.O. parameters remain constant (quantity, viscosity, pre-heating temp., same bunker lot) versus the air ratio adjustment, how do you explain that in night time the oxygen content in gases rises?
Or conversely: why a night set of air/F.O. ratio produces visible smoke during the day?

A. It is true that the inlet air quantity depends on the position of the fan damper, but the real quantity of inlet air is influenced also by the air temperature changes.

  • Assuming that during the night the air temperature drops, the fan intakes a greater weight of air, despite the fact that the adjustment of the fan damper remains constant, which means a constant volume of air, while the parameter affecting the combustion is the air weight (not the air volume): This is the reason for which several manufacturers keep constant also the air temperature inlet, by means of steam heat exchangers.
  • Another factor that may influence the combustion is the AIR DAMPNESS, usually the moisture rises during the night (and like a small percentage of water -duly emulsified with fuel oil- improves the combustion process), the presence of moisture in the combustion air allows to decrease the excess of air necessary to keep free the combustion process from visible smoke. Therefore a night set of air/F.O. ratio may need to be re-adjusted during the day hours, in case the smoke density tends to rise.

B03-Q. Instead of slag deposits, boilers of this ship suffer of carbon deposits on side tubes and front screen tube of combustion chamber. The F.O. in use is quite heavy (500 cts), but its vanadiun is below 50 ppm. while sodium is less than 10 ppm., pre-heated oil reach 140C (285f) . The ship is steaming at 70% max. output. Can Moletron help in stopping carbon deposits? How is it possible to evaluate the return on the investment? With a year pay-off, Moletron installation could take place during next dry-dock.
Combustion data:…omissis…

A. Moletron can help a lot since the flames' volume sharply decreases after Moletron installation.
Also smoke opacity lowers, then free oxygen in flue gases can be lowered by decreasing excess of air, that in turn decreases flames' volume and heat losses at stack.
However, before drafting a pay-off scheme and installing Moletron also on this ship of yours, we deem you should first try a flames' re-shape adjustment, in order to contain flames within the space foreseen into the combustion chamber.

Flames impingement on screen tubes - sideward or frontward - may depend on poor oil-air mixing due to deterioration of: burners' tips, or air swirl impellers, or burners' throats, or a wrong position of the air swirl versus the burner tips or throat, or a combinations among the mentioned defects.

A check on the reliability of the burners' components is recommended. Testing the whole combustion system including fuel-air ratio actuator, smoke opacimeter and O2 analyzer is also advisable.

Revert on us with the results of the a.m. checks and combustion data; we will be in better position to draft a pay-off scheme and then foresee the Moletron installation.

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