\chapter{Introduction}
Norske Skog Skogn is the largest producer of newsprint in Norway, and one of the largest producers in Europe. They have three paper machines with a combined production capacity of around 590 000 tons. The factory exports most of its paper to Europe, with the UK as its main market. \cite{Skogn_picture} 

\begin{figure}[h!]
\centering
\makebox[\textwidth][c]{\includegraphics[width=1\textwidth]{Figures/Skogn.png}}
\caption{Norske Skog's paper mill at Skogn. \cite{Skogn_picture}}
\label{fig:skognPicture}
\end{figure}


The paper machines require heat in the form of steam, and apart from heat recovery from the earlier stages of the process, the factory has 3 potential steam producers: An Electric boiler, a bark fired boiler (Boiler5), and a circulating fluidized bed (CFB) boiler (Boiler6). The CFB boiler is the main steam producer.

The CFB technology is currently widely used in coal fired power plants. The technology has certain advantages compared to other technologies in terms of emissions, and it has therefore, because of recent years focus on air pollution and CO2-emissions, gained increase attention as a "cleaner" alternative to other technologies used in coal fired power plants. \cite{BasuPrabir2015CFBB}

Although the main application for the CFB technology traditionally has been coal fired power plants, it is also implemented successfully, and increasingly, for other applications such as biomass and waste fired power plants. \cite{Valmet}

The CFB at Norske Skog (Boiler6) is a Compact CFB delivered from Foster Wheeler in 1997. It has a rated capacity of 50 MW$_{\text{th}}$, although it is currently constrained below this capacity.  Having Boiler6, the main steam provider at Norske Skog Skogn, constrained below rated capacity is not an ideal situation. Especially taking into account that this boiler generally burns the fuel with the lowest cost. It is therefore desirable from the company's side to look into ways to quantify the potential losses, as well as investigating possible improvements to the existing operation.  

\section{Scope of work}

The main objectives of this thesis is therefore first to get an overview of Boiler6; how it works, how it is operated, what the active constraints are, etc. This is to be achieved through visits to the factory, investigation of the boiler and its manuals, conversations with the engineers and operators, and investigations of the control system. The second objective is to identify possible improvements to how it is operated and try to quantify the economic loss resulting from having the boiler operating below its rated capacity.


