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There have been lately agonizing questions about the adequacy of the exhaust gas boiler to produce steam under part load operating conditions like the ones that we encounter in slow steaming. It is well known and indicated in the Energy Balance Sankey Diagrams that about 50% of energy and fuel is wasted in form of thermal energy in exhaust gases and cooling water. The primary candidate for further utilization and improvement of energy, are the exhaust gases. It is proven that the quality of energy in the exhaust gases, the exergy as it is called, is higher compared to the others.
By using waste heat recovery systems, the efficiency is increased to about 55%. This is a very good number, compared to steam turbines of the past. Of course there is no free lunch, so a slight reduction in the efficiency of the engine is noticed, due to increase of the pressure drop in the system and a slight reduction in the scavenge air pressure.
With WHR efficiency increases to about 55%
The centrepiece of an exhaust gas system is the exhaust gas boiler, downstream turbocharger and the exhaust gas header. Potentially at slow steaming conditions, we bypass the boiler to avoid passing the gases at low speeds which is not very healthy for the boiler.
There are two types of exhaust gas boilers; the water tube boiler, which is the most popular one, and the smoke tube boiler. The water tube boiler actually has no steam drum, has high risk of fire and soot deposition and needs soot blowers for cleaning. The smoke tube boiler has tubes surrounded by water, lower risk of fire, but it is imperative that the gases are passed at high speed to achieve the cleaning.
A blessing in disguise for the newer regulation of the low sulphur content in fuel is the reduction of the dew point. That means that we can cool the exhaust gases even further. It is advisable to monitor temperatures upstream and downstream a turbocharger, because the monitoring can indicate problems. Monitoring of temperatures is advisable also in the exhaust gas boiler. Gradual increase in exhaust gas temperature after boiler indicates fouling. Sudden increase indicates minor fire.
In the time of slow steaming, the exhaust gas boiler will operate with exhaust gases of lower flow rate. We expect that the capacity of the boiler to produce steam is affected, as it is definitely decreased. There is dependence in the production of steam from the exhaust gas parameters, namely flow rate and temperature, from feed water temperature, the boiler design and maintenance and of course the ambient conditions.
In part load operation we have lower flow rates along with lower fuel consumption which means lower available thermal energy in the exhaust gases. This also means lower gas velocities, higher propensity for soot deposition and you notice that the temperature of the gases can actually increase at lower loads of the engine, but what decreases is their flow rates. The radical solution of part load is to bypass the boiler. Further to that, we need to add guide vanes in the piping leading the exhaust flow to ensure gas velocity.
The target is to estimate the flow rate and enthalpy of the exhaust gases; usually this information is available by the manufacturer. Complicated thermodynamic calculations can be avoided, by assuming that the exhaust gases are clean air and neglecting the combustion products, with a full acceptable error of 2%. The estimation of boundary temperatures is based on pinch point temperature and boiler pressure. The introduction of very efficient engines these days has resulted in temperature reduction downstream the turbocharger. But efficiency has its price! We cannot get enough heat in the economizer. In the way to find a solution, we notice that the thermal content of the exhaust gases increases as the speed increases. In general, there is a 50% approximate drop in steam generated capacity for a 5 knots vessel speed reduction. It is very critical.
In conclusion, slow steaming up to 50 to 55 % load can be done without harm in long term if certain precautions are taken. That generally is the magic number below which load percentage the auxiliary blowers come in. When we contemplate slow steaming it is necessary to check the steam production capacity of the exhaust gas boiler. Potentially heat recovery from other sources should be considered. It has to be pointed out that an oil fired boiler launched at low speed is fully acceptable during maneuvering, river and channel crossing. But it is considered to be a pathological situation in case of sea travel. We should not be obliged to fire heavy fuel in order to produce steam at sea.
Above article is an edited version of Dr. Kokarakis’ presentation during the 2013 Green4Sea Forum
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