Great information athomas236!
Given the time frames and given the normal problems with HFO which are probably worse for orimulsion, I am assuming that fuel heating was temperature controlled based on laboratory samples.
The drawbacks to heater control by this method, even with HFO, is the variability of the quality. Usually this means a combination of strategies to try and optimise efficiency. This would include excess oxygen flow to try and ensure complete combustion, laboratory sampling to regularly re-assess the target temperature and flame inspection.
Orimulsion is not a newtonian fluid so temperature alone is not a good indicator of the optimum temperature for injection.
At the optimum temperature the fuel is atomised into a well dispersed spray which mixes well with the air stream and burns in the optimum region for heating.
If the temperature is too low the higher viscosity results in larger droplets which (a) take longer to burn and (b) are projected further. This can lead to the fuel burning on the fire tubes and the excessive build up of soot. Fire tube burn through is a higher risk and soot blowing more frequent.
If the temperature is too high the droplet size is smaller and a non-dispersive spray forms which again does not mix well with the air and which burns closer to the nozzles.
This is the problem with HFOs in burners and in engines similar problems could occur. In most engines, however, fuel heater control doesn't use fuel temperature for a control point but uses an inline viscometer between the heater and the engine.
Fuel quality problems with burner operations (often a dirtier fuel and without cleaning)meant that viscometer was not an option at the time but is now. With orimulsion and fuel water emulsions the non-newtonian behaviour makes it more difficult to predict the optimum fuel temperature as when the fuel is sheared e.g. at the nozzles, its viscosity will change even at the same temperature.
It would be interesting to look at the strategy for viscosity control for orimulsion. There are significant efficiency savings to be made in HFO operation which is a simpler fuel. There is an article published about the Huntsman Chemical plant (Wilton, UK) change from temperature control to viscosity control which suggests some impressive cost benefits. Orimulsion may present more of a problem than HFO in a temperature based control strategy and the benefits of viscosity control might be even more marked... or very difficult to achieve., but athomas236's comments on cleaning are a possible consequence of incompete combustion; or the non-combustible residues of the orimulsion?
Some modern viscometers are very effective even with dirty fuels, for example are used with black liquor in paper plants which is also non-newtonian (several manufacturers) and bitumen emulsions (for road surface dressing heater control).
Athomas236, do you have any insights into the fuel heater strategies with orimulsion and do you know what temperatures the fuel was typically heated to?
As a footnote to the large diesel engine applications, FWE may run to 25% water or better. Water misting, fuel water emulsions and wet compession technology for turbines (Diesel & Gas Turbine Worldwide, May 2004)all bring benefits to fuel combustion. The trick, as ever, is to find the optimum benefit to cost ratio.
JMW
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