\chapter{Discussion}
\label{sec:discussion}

It is assumed in the calculation of heat loss because of air leaking into the flue gas (Section \ref{sec:heatingExtraAir}) that all of the heat that is currently used to heat extra air can be recovered in the economizer/air preheater if the leaks were to be fixed.  This means that the temperature of the flue gas after the ESP (Figure \ref{fig:boiler6Schematic}) is assumed to remain constant after a potential fix.

In reality, not all of this heat would be recovered in the event of a fix, and the temperature after the ESP would increase. How much this increase would be is hard to predict, but the temperature is already quite high (148\si{\celsius}, Figure \ref{fig:boiler6Schematic}), so if it were to rise a lot it would get too hot for the flue gas fan that operates in the temperature range of 130\si{\celsius}-160\si{\celsius}. 

The fact that the temperature after the ESP already is quite high (and could be even higher if the leaks are fixed), indicates either poor heat recovery or delayed combustion (or both). If there is considerable delayed combustion, it means that the amount of leakage is underestimated (because the calculation is based on the amount of \chemfig{O_2}). An underestimation of the amount of leakage also means that the economic loss is underestimated. \\

The economic losses in Section \ref{sec:heatingExtraAir} and \ref{sec:shuntPumpResults} are calculated using the average electricity price for the data period (21 feb - 8 mar, 2018), and then extrapolated to get a monthly price. 

It is possible to argue that using the electricity price for this calculation might not be the most accurate thing to do taken into account the other two options of covering the lost heat: using the Bark Boiler or increasing the load of Boiler6. However, for the data period used in this calculation the electric heater is used every day, so using the electricity price is actually quite accurate here. Regardless, it should be stressed that both the electricity price and the use of the electric heater changes during the year, so the economic loss should not be extrapolated over the whole year. \\

The Dolezal-scheme for temperature control of the superheated steam had a couple of challenges described in Section \ref{sec:dolezal}. Both of the problems listed could be avoided if the superheated steam temperature was instead controlled by the method of injecting feed water directly into the overheated steam. The feed water has both a higher pressure and a lower temperature, resulting in both better water dispersion by the attemperator and larger controller gain. This is done in all other applications of the CFB boiler. \cite{2009Cg&s} However, the prerequisite for doing this is that the feed water has a high enough purity so that corrosion in the steam network is avoided. The CFB-boilers that use direct injection of feed water to control the steam temperature usually have a closed system for the steam/water-cycle, something that allows them to maintain a high enough water quality. 

This closed steam cycle setup is unfortunately not applicable for the pulp and paper industry for a nmber of reasons:
\begin{itemize}
    \item Complex and large steam networks with multiple sources and sinks makes follow-up on steam contamination difficult.
    \item Steam critical equipment such as turbines and drying cylinders sets high demands for purity and are costly to repair or replace.
\end{itemize}

The cost/benefit for a closed steam cycle simply favors open systems. It seems therefore that the Dolezal-system, which avoids the corrosion problem by only using condensed steam to cool the superheated steam, is the best choice for the boiler at Norske Skog.

There is still possible to do something about the temperature problem without switching out the Dolezal system. If the feed water that makes up the cold side of the heat exchanger is taken from before ECO1 instead of after, the temperature would be much lower (112\si{\celsius} instead of 207\si{\celsius}), resulting in a more efficient condensation and a larger controller gain. The reason that this is not done for Boiler6 might have something to do with where pipes and other components are physically placed at the site.
