Plant managers seeking to achieve across-the-board improvements in their process operations should look no further than their fired assets. Alkylation furnaces, boilers, catalytic reformer units, catalytic hydrocracking heaters, steam methane reformers and other fired equipment represent significant potential for improvements in areas such as equipment performance, asset lifespan, emissions reductions, energy efficiency and safety.
In the process industries, fired assets are the largest energy consumers, and they have a direct impact on site-wide steam, fuel and power balances. These assets are major drivers of plant profitability. As such, they present opportunities for improvement in the areas of fuel-usage efficiency, production capacity, yield management and emissions performance.
While enterprises can realize huge benefits by overhauling site-wide fired-asset management, capital-expenditure budgets might not allow that — at least in a single project. Fortunately, process operators can take a holistic approach by implementing improvements in stages, many of which require little or no capital expenditures (figure 1). For instance, one simple step — upgrading a differential-pressure (DP) transmitter on a boiler drum, or replacing a zirconium oxide probe in the convection section of a fired asset with tunable diode laser spectroscopy (TDLS) technology installed in the radiant section, for instance — could make a major difference in terms of fuel efficiency, emissions reductions and safety.
In fact, addressing measurement technology is one of the first steps in a holistic process. After an analyzing the equipment operation, design and existing instrumentation to determine the limitations vs. optimal operation, fired-asset improvements can be tackled in three stages: measurement, control and optimization. Each improvement is applied on a per-heater basis to ensure code compliance and high levels of operational effectiveness.
FIGURE 1. Implementation of new measurement, controls and optimization technologies will make a positive impact on fired asset energy efficiency, emissions, trip/downtime prevention, asset lifecycles and safety.
Common Issues with Fired Assets
During day-to-day operations, process operators typically deal with an array of issues throughout their fired assets and associated process units. Unmeasured or overlooked process conditions can result in an assortment of problems but also present opportunities for improvements.
Many fired assets do not meet industry recommended best practices for safety and control when utilizing existing instrumentation and manual control of the air supply. Plants not meeting new industry guidelines risk experiencing noncompliance and incidents.
Often, in lieu of accurate and frequent post-combustion measurements of oxygen (O2) and carbon monoxide (CO) concentrations, operators tend to allow surplus excess air in the fired asset. They make this choice due to concerns about the safety issues related to a fuel-rich condition; however, this reduces thermal efficiency. In negative-pressure induced-draft and natural-draft fired assets, exhaust-gas-composition measurements are arbitrarily skewed by tramp air.
Particularly during startup and shutdown, a fired asset presents a major safety risk. Systems lacking the required start permissives — and with operational hazards such as fuel-rich conditions — could experience an explosion. Despite these concerns, many assets do not meet industry standards such as NFPA 87 as they relate to existing instrumentation and controls. This exposes the plant and equipment as incident risks.
Most fired assets lack a real-time measurement of the fuel’s heating value. A major factor affecting fired-asset performance in refineries is the use of a mixture of natural gas and refinery gas as fuel. The composition of this gas fluctuates and is strongly dependent on the composition of the crude oil and the refinery processes. Common components include butane, butylene, methane, ethane and ethylene.
If the fuel consists of blended components, the heating value can change rapidly and cause problems such as wide swings in the coil outlet temperature or O2 content in the flue gas, along with frequent system trips. Not only do trips result in unplanned and costly downtime, but they also stress components in the fired asset or other process equipment. This results in reduced lifetime and the need for repair work sooner than anticipated.
In modern refining operations, the composition of the process feedstock and the flow rate are subject to change. The transition from one crude type to another as well as scheduling changes to the feed rate pose problems.
Even when the feed is in line with desired throughput, variation of the outlet temperature and the behavior of the process flow distribution controller can cause disturbances. Variations in the inlet temperature of the feed can result in further disturbances. Any overheating of the outlet coil temperature has a significant impact on the fuel consumption and leads to reduced efficiency. Altogether, these adverse conditions affect the stability and safety of the operation and the energy efficiency of the entire process.
Heat distribution between the convection and radiant sections of the fired asset is another key issue. For fired assets such as vacuum or thermal cracking heaters, operators must take into consideration the correct heat distribution and, as a result, the temperature distribution along the tubes in the radiant and convection sections.
If a fired asset is running with higher excess air, the temperature profile will change and result in premature coking in the tube. Alternately, thermal cracking can start prematurely and create an overcracking condition. Tubes and tube hangers exposed to higher temperatures can prematurely fail. Therefore, it is important to maintain the designed tube metal temperature profile.
Finally, in a downstream process unit such as a tower, higher temperatures increase the pressure in the process. This, in turn, influences evaporation inside the heater tubes, thus causing an additional disturbance to combustion control.
These and other issues can be addressed by implementing the following three steps.
FIGURE 2. Simply updating fired-asset measurement with different instruments, such as a differential-pressure transmitter for boiler drum level or a tunable diode laser spectroscopy (TDLS) analyzer for post-combustion composition, will yield significant improvements.
1. Addressing Issues with Fired Assets Using Measurement Technology
Often, fired-asset managers need only to replace aging instrumentation or add measurement instrumentation to monitor overlooked variables.
For instance, steam boiler operators can realize improvements by replacing an aging differential-pressure transmitter for drum-level measurement. Traditionally, differential-pressure transmitters are affected by swings in the static pressure that can shift the level reading during startup and shut down. Additionally, pressure spikes can cause shifts in the zero reading. Such issues result in poor control performance and inefficiency or trips and safety risks.
Differential-pressure transmitters offer accuracy and stability, enabling efficient operation with little or no downtime (figure 2). Improvements in static-pressure effects reduce operating expenses by eliminating the constant attention required by operations and maintenance personnel for calibration adjustments. In addition, some differential-pressure technologies provide improved overpressure protection, increasing efficiency and safety.
Fired-asset operators could realize a significant improvement by installing tunable diode laser spectroscopy (TDLS) technology in the radiant section of the heater just downstream of the combustion zone. The traditional approach to O2 content measurement in combustion gases — using zirconium oxide probes — can present a safety hazard because the analyzers operate above the CH4 ignition temperature. As a result, the probes cannot be located in high temperature radiant sections and, thus, may not provide accurate, timely measurements in non-homogenous combustion gases.
When placed after the convection section or in the stack, long measurement delays are introduced. Also, tramp air in the stack can cause an inaccurate composition measurement. For these reasons, it could be said that using zirconium oxide technology can contribute to excessive fuel consumption and emissions along with decreased production.
In contrast, a TDLS analyzer installed in the radiant section provides near real-time O2 and CO measurements. Depending on the fired-asset application, additional live measurements by the TDLS analyzer can include CH4 as a startup permissive and ammonia (NH3) to detect ammonia slip. The TDLS avoids biasing the air/fuel ratio with an arbitrary O2 setpoint based on an error-prone O2 reading as could happen with a zirconium oxide probe. The rapid air/fuel measurement of the TDLS, combined with measurements of the fuel density and mass flow on a real-time basis, enables precise control. This allows fired-asset operators to safely control the loop close to the CO breakthrough point (more on this later).
In the past, TDLS installation required the sender and receiver to be installed on opposite ends of a duct. New TDLS technology has simplified installation. A single-flange design allows installation where cross-duct placement is not feasible due to obstructions or budget constraints. The single-flange design also removes any issues associated with alignment. Even under capital-expenditures restrictions, plant managers can accomplish the installation on a quick turnaround.
The addition of a real-time measurement for the fuel heating value in applications that use waste fuels can also have positive impacts on fired-asset efficiency, asset lifecycle and safety. A real-time heating value estimate enables continuous adjustment of the air/fuel ratio to stabilize combustion and heat transfer into the tubes. This simplifies fired-asset operation while minimizing the thermal stresses on the tubes even under conditions such as wide swings in demand or fuel heating value. Application of the appropriate instrumentation provides a density reading, allowing an immediate calculation of the heating value to adjust real-time control.
Fired-asset managers with the need to monitor noncritical temperature and pressure variables can add those measurements through the deployment of wireless technology. Wireless temperature and pressure transmitters for quick installation in locations such as hazardous areas where wiring could present problems. If the budget permits, additional measurement instrumentation can include stack flow, wind compensation and continuous emissions monitoring.
2. Applying Modern Automation Technology
When aging controllers do not allow program changes, are no longer supported by their manufacturers or do not provide adequate insight into the operation or the cause of trips, fired-asset managers have a number of new controller technologies from which to choose.
For example, in a burner-management system, a SIL 3 safety PLC uses internal diagnostics to ensure the validity of inputs and outputs. Using multiple, internal watchdog timers to detect that the program, processor or I/O are in a fault condition, a safety PLC will transition the process to a known safe state. Controllers and safety PLCs with built-in HART communication also allow fired-asset managers to check the health of field devices before a failure occurs.
In terms of automation priorities, air/fuel ratio control remains at the forefront. The air/fuel ratio control loop usually is biased by an O2 trim loop. The O2 trim setpoint balances safety with efficiency. Because safety takes precedence, a surplus of excess air ensures consumption of all combustibles. Typically, the excess air is kept at a potentially unnecessarily high level that results in excessive fuel consumption, excessive CO2 and NOX emissions and reduced production efficiency.
The optimal O2 level can be determined only if the CO breakthrough point is known. That is when O2 is reduced to a point where CO spikes, with the air/fuel mixture biased by cross limited O2 and CO trim. This breakthrough point is unique for every fired heating asset and is affected by fuel composition, burners and other factors. As noted in recommended practices, when concentrations are measured by a TDLS, safe combustion of gas at 1 percent O2 or lower is possible (figure 3).
Stabilized combustion reduces tube deposits, which accelerate at high temperatures. By minimizing temperature peaks, fired-asset operators reduce the amount of decoking and fuel consumption. This results in increased fuel efficiency and reduced operating expenses for maintenance.
Substantially fewer trips and increased asset life result from stabilized coil outlet temperature and O2 content in flue gas. For fired assets such as steam methane reformers, which utilize catalyst in the tubes, avoiding trips can be critical to extending the life of the catalyst and delaying an expensive catalyst-change turnaround.
FIGURE 3. By determining the CO breakthrough point, a holistic fired-asset control strategy improves fuel efficiency, increases production capacity and extends fired heating asset lifespans. The figure shows an oil burning heater safely operating at 1.6 percent O2.
3. Fired-Asset and Plant-Wide Optimization
Once an operator has completed the measurement and control steps, the third step — optimization — can take many directions. The process industries have many advanced analytics, automation and modeling technologies at their disposal for improvements ranging from feedwater pH optimization in a single fired asset to enterprise-wide digital transformation.
Digital transformation technology enables plant management to optimize the operation of fired-asset portfolios to improve overall site-wide economic performance. Instead of constraints and objectives of individual fired assets, updated operations take site-wide constraints and objectives into account. For predictive asset management, digital twins can be applied to individual assets such as fired assets, other process equipment and across the entire value chain.
In conclusion, fired-asset operators and plant management can realize numerous benefits, including:
- Increased energy efficiency.
- Reduction in greenhouse gas emissions.
- Reduction of unscheduled production downtime.
- Reduction in operation and maintenance costs.
- Reduction in total cost of ownership.
- Improvements in safety.
- Improvements in asset performance management.
These benefits can be achieved by by taking steps as simple as replacing aging measurement instrumentation or modifying control strategies. When capital expenditures are limited, many of the improvements can be made with small capital expenditures or by using the operating expenses budget. Returns on investments can be justified and ensured with the application of a comprehensive, collaborative approach to fired-asset management. This includes an engineering analysis of the fired asset and the resolution of performance issues by the strategic application of hardware and software.