Apr-2012

Useful tips for fired heater optimisation

Model predictive control that is native to the regulatory control processor provides high-speed, redundant and robust control for process fired heaters

NIKKI BISHOP and BARBARA HAMILTON
Emerson Process Management

Article Summary

Fired heaters are used extensively throughout refineries for heating, vapourisation and thermal cracking of various process fluids. These heaters are essential for refinery operation. Figure 1 shows a typical process fired heater.

The main objective for fired heater optimisation is to 
safely control to a target the combined coil outlet temperature while minimising energy costs, emissions and overall variability. To achieve this objective, the heater must be able to operate safely near constraints and demonstrate stable and robust performance. A holistic approach that examines the field devices, basic control strategy and loop tuning is recommended to establish a solid foundation. Advanced process control methods coupled with a solid combustion control strategy provide an effective means 
of safely operating near constraints. Even further, 
pre-engineered application solutions offer great benefits in the ease of implementation and support.

Back to basics: the instruments
Just as a building is only as good as its foundation, a process control system is only as good as the instruments that provide the measurement data and final control. It is important to address and mitigate process and mechanical issues. Necessary measurement and actuator improvements are made in order to reduce control loop variability. Reducing variability on key loops enables the heater to operate closer to the constraint conditions, which increases throughput, efficiency and safety. For example, it is essential to tightly control the air and fuel flows for safe and efficient combustion. Figure 2 shows typical fired heater measurements.
 
Beyond base regulatory: the full solution
Many fired heaters are instrumented with only the minimum instrumentation necessary for operation. While this strategy allows for operation of the heaters, it lacks the sophistication necessary for optimal performance. A full solution, including a burner management system (BMS) for safety and the necessary instruments, final control elements and damper actuator, is the ideal recipe for optimal heater performance. 

Ultimate control strategy
The ultimate goal for a fired heater is to heat a process fluid to a desired temperature. Maintaining a constant outlet temperature is critical for the process. Variations in outlet temperature introduce variability into the overall process. Since the optimum operation will almost always be near constraints (for instance, maximum tube temperatures, minimum excess air), variation in the process requires the operator to stay further away from the actual limit to provide the necessary buffer or safety margin to handle any unexpected process upsets. As a result, manufacturers are not always able to achieve the most efficient operation of their assets. Figure 3 shows a representation of the impact of variability on cost.

Reducing variability means ensuring a robust and stable control strategy is in place. Implementing a fully automatic, regulatory control system ensures the coil outlet temperature is maintained at the desired setpoint, while simultaneously balancing pass temperatures and airflow. Pass balancing adjusts the flows through each tube so that no pass runs hotter than the others, reducing hot spots within the coils, which translates to longer run lengths and the potential to operate at a higher severity.

Bringing the heat: combustion controls
As previously noted, the ultimate goal for a fired heater is to safely heat a process fluid to a desired temperature. The required energy is provided by combustion of a fuel to heat the coils. The efficiency of the combustion process determines the efficiency of the heater. A single-knob combustion control strategy allows for safe, efficient firing at any desired charge rate. With single-knob control, the desired coil outlet temperature is maintained while operating within mechanical and thermodynamic heater constraints at all times.

A proven strategy for safe combustion is to employ an air and fuel cross-limiting solution. Cross-limiting is the traditional approach of increasing airflow first on an increasing fuel demand and decreasing fuel first on a decreasing demand. This strategy prevents unsafe, fuel-rich environments from being introduced by the automation system. Typically, this is done by empirically determining the air, fuel and stack oxygen levels at various demand points. A linear relationship is typically used between demand and fuel flow. The relationship between demand and airflow follows the empirically derived curve so that the oxygen target at each demand point ensures complete combustion.

Typically, the lowest stack oxygen level that does not produce carbon monoxide (CO) in the stack is best for both efficiency and emissions compliance. CO in the stack is an indication that the fuel is not completely burned.

Advanced combustion controls allow for maximum capture of the available heat in the fuel with less variability. This is important even if the fuel is a refinery waste gas that is free because it reduces greenhouse gas emissions, makes the fuel available for other uses such as boilers or co-gen plants, and allows for more throughput in a capacity-constrained situation. It is impossible to safely drive stack oxygen levels down without robust combustion controls. When air and fuel are not well coordinated, not only is efficiency affected, but a fuel-rich environment risks an explosion if furnace oxygen levels get too low. Figure 4 shows the relationship between excess air and available heat.

The economic savings resulting from operating a furnace at lower excess air can easily be calculated based on the extra energy used to heat the extra air from ambient to the stack exhaust conditions. The stack oxygen concentration is an indication of how much excess air there is. If all the oxygen is perfectly consumed, the oxygen concentration will be 0. This, however, is not safe, so most furnaces operate with oxygen concentrations of 1% or higher. Decreasing the oxygen concentration in the stack from the current level to a lower value results in energy savings that can be substantial, depending on the starting point and the cost of fuel. Figure 5 illustrates the relationship between reduced stack oxygen, efficiency and energy savings.