Fired assets in the hydrocarbon processing industries use a combustion heat source to heat process fluids in the presence of a catalyst. In the case of a Steam Methane Reformer (SMR), the fluid and catalyst are in tube banks where a reaction takes place converting steam and lighter hydrocarbons into hydrogen and carbon monoxide. Fired assets frequently use a mix of fuels, enabling them to capture the heating value of waste gases and byproducts (see figure).
Asset performance for fired assets needs to consider performance both on the hydrocarbon side and the combustion side of the unit’s operation, but these are not independent. This report will examine how performance improvements to combustion control can provide benefits to multiple aspects of the unit’s operation.
Performance Considerations for Fired Assets
There are several major areas that must be evaluated to develop overall performance metrics. The most important of these are:
- Safety – Safe operation is always a most important consideration as these assets carry an inherent risk of explosion or other accidents, which can be extremely serious.
- Fuel efficiency – Efficient combustion is critical for providing economical and stable operation. The difficulty of this is compounded by the variable makeup of the fuel, which will impact its heating value and change its stochiometric fuel/air ratio.
- Emissions – These units are a major source of emissions to the atmosphere. Better process control can reduce these and reduce the demands on downstream flue gas conditioning systems (if used).
- Production and operability – The unit needs to maintain an optimal operating point with respect to combustion and needs to operate in a stable manner so that it can run very close to its bounded capacity, or it may become a production bottleneck.
- Asset life – There are a variety of assets under consideration. Catalysts have a relatively short life of a few years, while other heat transfer components may have a 20-year life. The overall unit will have an even longer expected life of 30-40 years. All these lifespans can be extended through more stable operation and can be shortened by transients and by operating at poorly chosen or accidental operating points.
Broadly speaking, the overall goal is to operate such units with greater stability – that is with a much smaller variation in key operating parameters and close to the optimal operating points (see figure next page). The greater stability reduces the thermal cycles and resulting stresses on all the equipment, including the catalysts. Optimal operating points improve thermal conversion metrics but also reduce adverse impact on equipment and catalysts from phenomenon such as coking.
Routes to Asset Performance Improvement
The routes to better asset performance and lifespan involve operating practices and improvements to measurements, actuators, and control systems. Leaving aside operating practices, let us examine the benefits possible through improvements in measurements, actuators, and control systems.
Improved Measurements, Actuation, and Control
The measurements of fuel and air flow are fundamental to combustion control, but in these units the fuels vary greatly because they contain variable amount of waste byproducts. Fuel measurement thus needs to include a real-time analysis of the fuel’s lower heating value (LHV). This is critical because it enables the control system to deliver a consistent heat input to the unit.
Besides fuel and air flow, critical measurements are the oxygen and carbon monoxide (O2, CO) in the flue gas stream. Here greatly improved and much faster measurement is available by using online analyzers with tunable diode laser spectroscopy (TDLS) analyzers that measure gas composition across a section, as opposed to sampling analyzers that draw gas samples, often from a single point. TDLS analysis can be performed in regions of the furnace that are too hot for sampling probes. Usually, these hotter locations are superior measuring points because they are in areas where combustion should be (but may not be) complete and where tramp air input is not a factor.
These units have a wide variety of draft schemes and damper actuators. It is important that all these be operable. Units with multiple burners require a good balance of heat release and fuel/air ratio across each burner to deliver more stable and uniform conditions in the heat transfer zones.
Advanced control is a “must” to leverage these better measurements of furnace conditions. The control system should utilize the real-time fuel LHV analysis. It should also employ a dynamic fuel/air ratio and CO/O2 supervisory optimization. The TDLS gas analysis can provide these measurements in real-time with minimal time delay, and the advanced control enables the unit to operate consistently at optimum thermal efficiency and to maintain the combustion process only in the areas where the furnace has been designed for it.
The existing DCS controls need not be replaced. Rather, the advanced control can be applied as supervisory control system managing the existing DCS controls, provided the improved measurements and functional actuations are in place.
It is important to take all the steps toward improvement, including the advanced control. Some firms have improved the measurements but left them “open loop.” They rely on their operators to optimize the unit operation, which is a mistake when a reliable and economical advanced control system could free the operators from this routine (yet important) task. Sustainable and optimized benefits can be realized over time only when operating in closed loop advanced control. Using a turnkey solution, for example Yokogawa’s holistic CombustionOne offering, can assure that all the required steps are implemented.
As mentioned above, performance improvements on the combustion side of the unit will also improve performance on the hydrocarbon side and in other areas, especially unit safety. For example:
- TLDS analyzers can also measure furnace methane gas concentrations directly. Safe furnace conditions during unit startup can be further assured by using this real-time methane measurement as a permissive in the burner management system (BMS).
- Sampling analyzers contain high temperature components that operate above the ignition point of methane. They can be (and have been) a source of ignition during accidents and abnormal conditions. Using TLDS analyzers eliminates this potential hazard.
- Advanced combustion control using both O2 and CO measured directly in the furnace prior to the convection zone, enables consistent, safe, and optimum operation and prevents transients from causing unsafe or suboptimal operating conditions.
- Combustion does not carry over from the radiant to the convective heat transfer zone. This lengthens the life of both tubes and catalysts.
- A fast and accurate calculation of required fuel/air ratio not only improves efficiency but also prevents transient operation from causing unsafe conditions.
Besides the combustion improvements, more stable combustion delivers benefits in other areas of the unit. Stable operation reduces NOX emissions, reducing the load on downstream flue gas conditioning systems that remove NOX. On the hydrocarbon side, reduced coil outlet temperature (COT) variance also reduces coke formation. Reduced thermal cycling reduces thermal stress on tubes, catalysts, and other unit mechanical equipment.
- Maintain an up-to-date census of your fired assets. This should include detailed data about analyzer technology and capabilities, combustion controls, APC implementations, and safety systems.
- For investment decisions, consider the possible benefits of advanced control in multiple areas such as safety, fuel efficiency, production/ operability, and longer asset life. Evaluate benefits to both the combustion and the hydrocarbon sides of the unit.
- On the cost side of investment decisions, evaluate both in-house and turnkey solutions consistently. Be sure to estimate all the associated support costs. Internal support costs often are not measured by existing accounting systems.
- Do not stop halfway. Implement an advanced control system to ensure consistent benefits over time.
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Keywords: Advanced Control, Analyzers, APM, Combustion, Reformers, Tunable Diode Laser Spectroscopy, ARC Advisory Group.