\chapter{Operation and Control of Boiler6}

This chapter present the current operation mode and control structures for Boiler6, with a focus on combustion control, water side control and emission control. 

\section{Operation objectives}
The operation objective of Boiler6 is to produce steam according to the demand of the paper mill while staying below emission limits for \ce{CO} and \ce{NO_x}. 


\subsection{Throughput manipulator}
\label{sec:TPM}
The throughput manipulator (TPM) of a process plant dictates where to set the production rate. In this case, the TPM of the boiler is the desired steam production, which is decided by the steam demand of the plant. However, to increase the flexibility of the plant, and ensure enough steam for the paper mills in case of boiler trip, steam is stored in an accumulator (i.e. a buffer tank to decouple the steam production in the boiler and the paper mills). Therefore, the TPM is actually the level in the accumulator. 
In current operation mode, the steam production is manually controlled by adjusting the fuel flow rate setpoint. 

However, the steam production is currently limited by the flue gas fan. At high loads, the flue gas fan reaches its maximum power due to air leaking into the system as described in Section \ref{sec:leaks}, and it becomes the bottleneck of the process. In this scenario, the steam production has to be reduced below its design rate.

\subsection{Degrees of freedom}

The degrees of freedom (DOF) for operation and control of a process represent the available valves, pumps and fans that can be manipulated (therefore are also called manipulated variables (MVs)). The MVs for Boiler6 are:

\begin{description}
\item[MV1] Fuel
\item[MV2] Primary air
\item[MV3] Secondary air fan
\item[MV4] Flue gas fan
\item[MV5] Recycle gas fan
\item[MV6] Feedwater flowrate
\item[MV7] Attemperation flowrate 
\item[MV8] Shunt pump
\item[MV9] Steam Air preahetrs
\end{description}
Note MV8 has a small capacity (Section \ref{sec:shuntPumpBal}), and MV9 is not available for the current operation mode due to damaged equipment (Section \ref{sec:SAP})

\subsection{Operational constraints and active constraints}

Boiler6 is operated to a series of constraints related to safety, equipment capacity and emission regulations. These are:
\begin{itemize}
    \item High enough steam pressure and level in the steam accumulator such that the steam production does not become the bottleneck for the downstream paper mills.
    \item Furnace pressure (e.g.$P\leq-0.1\si{mbar}$) to avoid dangerous gases such as \ce{CO} leaking into the boiler chamber.
    \item Bed temperature should be $T\leq$870 \si{\celsius} to avoid \ce{NO_x}-formation and to avoid sintering of sand and alkali that would deposit in measurement instruments, or on equipment walls. 
    \item Exhaust gas temperature should be $T\geq$ 130\si{\celsius} to avoid corrosion due to \ce{SO_2}. 
    \item Having the superheated steam temperature to high, or allowing large variation of the steam temperature causes high thermal stresses.
    \item Ratings of the flue and recycle gas fans.
    \item Emission levels (e.g. \ce{CO}, \ce{NO_x}, \ce{SO_2}) have to be lower than the allowed limits.
    \item The pressure of the superheated steam has to be lower than the drum pressure to allow the steam to flow.
    \item The drum level has to be within its safety limits.
\end{itemize}

At the optimal operating point, a few of the constraints will be active (i.e. are at their minimum or maximum allowed value). To determine which constraints are active, a steady-state model of the plant can be used to solved an optimization problem. However, common engineering knowledge can also be use to analyze what constraints should be active at the optimal point, and this is what it is done in this work. The active constraints are:
\begin{itemize}
    \item The feedwater temperature should be at its minimum, to maximize the temperature difference in the economizer, which maximizes the heat transfer. It is however important to keep in mind that the tubes in the economizer have a temperature close to the water temperature (see Section \ref{sec:shuntPump}). This means that the temperature of the flue gas (which must be above 130\si{\celsius}) creates a lower limit when the it has a high \ce{SO_2}-concentration. 
    \item Flue gas temperature should be at its minimum to maximize heat recovery.
    \item The furnace pressure should be at its maximum to avoid energy losses related to creating vacuum in the boiler.
    \item The Bed temperature should be at the maximum to get a good reaction rate.
\end{itemize}

The plant is however subject to disturbances, and in order to be sure that the active constraints are not exceeded during transient operation, a back-off from optimality is required. 



\subsection{Control variables}
The control variables are variables that should be kept at their desired setpoint, firstly to operate the plant safetly on a short time scale, and secondly to achieve optimal operation on a large time scale. In deciding what variables should be controlled, one has to always control the active constraints, and then control the variables to keep the plant near optimal operation in case of disturbances \citep{Skogestad2004}.  
In addition to the already mentioned active constraints, the superheated steam temperature should also be controlled to avoid large temperature variations, or high thermal stresses. Considering that the pressure in the drum is varying, the steam temperature will fluctuate accordingly, and therefore the steam temperature has to be controlled after the superheater. 
The control variables for Boiler6 are:

\begin{description}
\item[CV1] Steam pressure (and flowrate)
\item[CV2] Drum level
\item[CV3] Superheated steam temperature
\item[CV4] Bed temperature
\item[CV5] Furnance pressure
\item[CV6] Exhaust gas temperature
\item[CV7] Feedwater temperature
\item[CV8] \ce{CO} level in flue gas
\item[CV9] \ce{NO_x} level in flue gas
\item[CV10] \ce{O_2} level in flue gas
\end{description}
 
The control system for Boiler6 was originally designed to control the exhaust gas temperature with the steam air preheaters (see Section \ref{sec:SAP}). However, due to large leaks in these heat exchangers, they are not currently in operation, which leads to not having a degree of freedom to control this CV. 



\subsection{Disturbances} The main disturbances in operating Boiler6 are:
\begin{description}
\item[D1] Fuel composition
\item[D2] Air leaking into the boiler
\item[D3] Feedwater temperature
\end{description}


\section{Combustion Control}
\label{sec:combustionControl}

\begin{figure}[h!]
\centering
\makebox[\textwidth][c]{\includegraphics[width=1.2\textwidth]{Figures/boiler6RegAir.png}}
\caption{Simplified scheme of how the combustion is controlled.}
\label{fig:boiler6RegAir}
\end{figure}

Figure \ref{fig:boiler6RegAir} gives a representation of how the combustion is controlled. Note that the temperature control of the flue gas after the ECO/AP by using the steam air preheaters is included although they are not currently in use because of leaks. 

As can be seen from the figure, temperature in the flue gas bed is controlled by the the flue gas recycle fan, pressure in the furnace is controlled by the flue gas fan, primary air is controlled by the valve before the primary air fan, and the \ce{O_2}-level in the flue duct is controlled by the composition controller which gives a correction factor to the amount of secondary air. The correction factor is allowed to vary between 0.6 and 1.2. 

In addition to the control loops, Figure \ref{fig:boiler6RegAir} also shows two calculation blocks. The information in these blocks are not changed during normal operation, and the only inputs are show in the figure. This means that the only input to the secondary air distribution is the total amount of secondary air calculated, and that the inputs to the air amount calculation is the measured speed of the two  fuel screws, and a fuel/air-ratio setpoint that is used to control the emissions (see Section \ref{sec:emissionControl}).

The parts that are not shown in the figure, are the fuel screws. The speed of thees are manually set by the operators so that a desired steam production is achieved. The desired steam production is based on the level in the downstream accumulators, which is the TPM for the process (see Section \ref{sec:TPM}).

\section{Water Side control}
\label{sec:waterControl}
\begin{figure}[h!]
\centering
\makebox[\textwidth][c]{\includegraphics[width=1.2\textwidth]{Figures/boiler6Control.png}}
\caption{Simplified scheme of how the water/steam side is controlled.}
\label{fig:boiler6RegWater}
\end{figure}

Figure \ref{fig:boiler6RegWater} gives a simplified representation of the control loops of the water/steam side of boiler6. 

The outlet temperature of the superheated steam is controlled by using the attemperator. For simplicity, this loop is shown as a single feedback loop, although it actually is a cascade loop where the TC shown in the figure gives a setpoint to an inner control loop. This inner TC measures the steam temperature between the attemperator and SH2, and adjusts the attemperator accordingly. This setup gives a faster response, but because the inner TC is situated closer to the attemperator, it might have some extra disturbances because of uneven evaporation of the water that is added to the steam.

The water inlet temperature of ECO1 is controlled by the shunt pump. When burning fuel with low sulfur content, the shunt pump should be operated at its minimum value to ensure maximum heat recovery (see Section \ref{sec:shuntPumpBal}). But if the fuel has a higher sulfur content (such as fuel oil), it might be wiser to to have a higher water inlet temperature to reduce the risk of corrosion (see Section \ref{sec:shuntPump}).

The level in the drum is controlled by the feed water. The way the setup works is that the feed water is set equal to the superheated steam flow plus a correction term from the drum level controller. This setup is called a three element drum water level control.\cite{JenoKovacs} By using the steam flow as a feedforward signal input to the control structure, dynamic phenomena such as "shrink" and "swell" have a reduced effect on the feed water regulation. 

"Shrink" and "swell" are phenomena that occurs when steam flow out of the drum are either decreased or increased. Because of the pressure change in the drum that results from the change in steam outflow, the saturation temperature changes, and there is a change in evaporation rate. If the steam flow increases, the pressure and saturation temperature drops, the evaporation increases resulting a "swelling" of the level. with no input signal from the steam flow rate, there would be an inverse response for the water feed flow in this case. "Shrinking" describes the opposite phenomena.


\section{Emission Control}
\label{sec:emissionControl}
This section presents the main control loop for emission control. 

In addition to producing the desired steam amount, it is also an objective to keep emission of pollutants below the limits set by the government. The two main pollutants that require continuous surveillance by the operators, are \ce{CO} and \ce{NO_X}. They need to be kept below limits of 75 mg/Nm\textsuperscript{3} and 340 mg/Nm\textsuperscript{3} respectively. The limits are average values over 24 hours, meaning that short spikes above the limits are usually okay. 

\ce{CO} and \ce{NO_X} are usually varying inversely of each other. This is because \ce{NO_X}-formation is favoured at high \ce{O_2}-levels and high temperatures, while CO is favoured at lower temperatures and low \ce{O_2}-levels. This means that usually, high CO-levels means low \ce{NO_X}-levels and vice versa. 

The main way of making sure emissions stay below limits is therefore to change the air/fuel-ratio setpoint. If the \ce{NO_X}-level is to high, it usually means that there is too much \ce{O_2} (and therefore air), while a high CO-level usually means that there is too little \ce{O_2}.

If changing the air/fuel-ratio setpoint does not give the desired reduction in emissions, the emissions is usually caused by an uneven mixture of fuel and air. This can happen both because of a bad ratio between secondary and primary air, and because of an uneven mixing of the fuel that enters the furnace through two different fuel screws.  

The bad ratio of primary and secondary air can be remedied by changing the setpoint for the \ce{O_2} composition controller, which changes the amount of secondary air. Because this controller only affects the absolute amount secondary air (Figure \ref{fig:boiler6RegAir}), it also indirectly affects the ratio between primary and secondary air. If the cause is uneven fuel distribution, this can to some degree be remedied by changing the ratio between the speed of the two fuel screws.

The challenge by using these two approaches (when changing the air/fuel-ratio fails), is that it is no way of knowing exactly what the reason for the increased emissions is. This is therefore bound to be a process of trial and error where different ratios are changed up and down until the emission levels are reduced again.

%\section{Notes}
%\begin{itemize}
%    \item What is the control objective? ie produce steam for the paper machines and also to operate in a safe way. 
%    \item Identify the TPM, ie steam mass flow
%    \item Identify degrees of freedom, those are the manipulated variables. Then control variables. What are the active constraints: the fan is the bottleneck, due to the leaked air (false air). One DOF has to be used to control the TPM. Other DOf have to be used to control the active constraints (steam temperature, feedwater temperature (related to the shunt pump)) drum level \ for safety reasons.  etc). What are the DOf left?
%    \item O2, NOx control.
%    \item air to fuel ratio? do they use the cross linked control structure?
%    \item drum level control. is it the 3 elements control structure?
%\end{itemize}


%More CVs than MVs, need to use selectors and logic. 


%Show what you control with what. 

%CV's: steam production(TPM), NOx(should be active, need backoff), CO(same ass NOX), O2(CV, not active constraint, energy waste, unburnt material, boiler explosion), sec. air pressure(CV, not active), boiler pressure(flue gas leakage, flue gas fan), bed temperature(sintering, nox, reaction rate), steam drum level(securoty), steam temperature(thermal stress), ECO1 inlet temperature(backoff in tube temperature, should be as low as possible), flue gas temperature(should be as possible, but still above dew point)