\chapter{Current Operation}
\textit{This should be a part of the process description and/or current operation} \\

A lot of the differences between current operation and how it is supposed to be operated (according to the manufacturer) are due to leaks in the flue duct and low quality fuel(??utdyp hva som menes med low qualtiy fuel).

\section{Leaks}
Leaks at various points in the air/flue gas-system leads to false air leaking into the the system. This have the following effects:

(??legg til tegning av prosessen som illustrerer lekkasjer)

\begin{itemize}
    \item Perhaps the most important effect of the false air is the increased mass flow through the flue duct, something that forces the flue gas fan to operate at maximum capacity. This is considered the main reason why steam production is limited to  (??10-12kg/s ref). The amount of false air is calculated to represent a roughly 50\% increase (ref??, ta med usikkerheter i beregningen) in the mass flow through the fan. There is also a power loss from having the fan operating with a higher load.
    \item Another effect is the energy loss stemming from the fact that false air is heated from 35 degrees ??ref) to 15? ??ref. When the flue gas fan is operating at its maximum capacity the loss is calculated to be ??ref.
    \item False air also causes the oxygen level in the recycle stream to be considerable higher than it should be. The level is... (??outlet oxygen level. Could also do an estimation of extra O2 level leakage in the flue?), compared to (??O2 before economizer) if there had been no leaks. The extra oxygen leads to increased burning at the bottom of the bed, resulting in higher heat production. Because the recycle is used to lower the temperature at the bottom of the bed, the extra oxygen reduces the effect of the recycle, and hence forces the recycle stream to be larger than it could have been. The larger recycle stream requires more power from the recycle fan and flue gas fan, and adds to the problem of having the flue gas fan as an upper constraint for the steam production.
    \item There is also a leakage in the steam preheaters. This is not a contributor to false air, but rather a leakage of steam into the primary and secondary air streams and the boiler room. This leakage renders the steam preheaters useless, and they are therefore not used during normal operation. This means that the flue gas temperature after the economizer and air preheater is not regulated by the steam preheaters as intended in the design.
    \item Corrosion in the economizer/LUFO have prompted the engineers/operators/company?? to close down some of the tubes in the heat exchangers. This has reduced the heat exchanger are and hence the possible heat recovery of the system. If the leaks were to be fixed, the temperature of the exhaust would be much higher (??include calculation of energy loss from extra air and represent it as possible increase in the  
\end{itemize}   

\section{Fuel}

\section{Control Loops}

\subsection{Steam/Water system}

\subsubsection{Recycle Feed Water}
MV: Circulation pump (Eko851-041) \\ 
CV: Feed water temperature at economizer inlet \\
Comment: Desirable to have high enough feed water inlet temperature to make sure that the flue gas exit temperature is around \SI{145}{\celsius}. \\ 
The set point is \SI{160}{\celsius}, but the pump is at 100\% and the feed water temperature is \SI{138}{\celsius}. Flue gas exit temperature is \SI{152}{\celsius}(insert temperature from data set)

\subsubsection{Liquid level steam drum}
MV: Feed water valves \\
CV: Liquid level steam drum \\

\subsubsection{Overheated steam temperature}
MV: Valve regulating water flow from Dolezal. \\
CV: Temperature after the second overheater.\\ 
Comment: The water is injected before  second overheater

\subsubsection{Steam pressure}
MV: Valves opening up to accumulators. \\
CV: Overheated steam pressure. \\
Comment: Pressure was 53.5 bar during visit. The pressure is allowed to vary about 1.5 bars. Max and min constraints are 40-(60 or 65)bar. The operators can choose to either let the steam flow or the steam pressure be controlled automatically by the downstream valves (See steam flow).

\subsubsection{Steam production}
MV: Fuel input \\
CV: Overheated steam flow \\
Comment: This regulation is performed manually by the operators. The time it takes from a change in fuel input to a change in steam production is around 5-10 minutes. They can choose to control either the steam flow or the steam pressure (See steam pressure).


\subsection{Air/fuel system}

\subsubsection{Flue gas fan}
MV: Flue gas fan (variable speed) \\
CV: Pressure in bed \\
Comment: The flue gas fan might be a TPM for high steam production. The capacity is 29.3 Nm3/s. False air (estimated to be around 40\% of original flue gas amount) makes the the amount of gas higher than it should be.

\subsubsection{Recycle fan}
MV: Recycle fan (variable speed) \\
CV: Temperature at the bottom of the bed \\
Comment: The temperature is measured as the average of two of the four temperature sensors in the bottom (the highest and lowest temperature is excluded). \\
The recycle flue gas has a higher oxygen level than it should because of air leaking into the flue duct. Based on oxygen concentration before economizer and inside the exit chimney, the amount of false air is calculated to be about 50\% of original flue gas. The extra oxygen has the effect of increased burning(and hence increased temperature), (and might also contribute to increased NOx-production.??) \\
The recycle fan has a very slow response (15-20 minutes for a change from 60\% to 20\%).

\subsubsection{Air}
MV's: Primary air fan (valve regulated), secondary air fan (variable speed), several valves to control distribution of secondary air.\\

Cv's: Air/fuel-ratio, primary/secondary-ratio.\\

\subsubsection{Fuel}

MV's: Left srew, Right screw \\
CV's: Left/right-ratio, steam production \\
Comment: The operators adjust the fuel according to desired steam production. 

