The keys to successfully revamping a fired heater: how to maximise value
Fired heaters are facing scrutiny because they contribute to the global emissions of carbon dioxide (CO2) and nitrogen oxides (NOx). For refineries and petrochemical plants focused on reducing greenhouse gas emissions, it makes sense to target CO2 from fired heaters since they usually have a fuel efficiency between 70-90% and are a major contributor of the site’s overall emission production.
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However, revamping fired heaters only to reduce emissions, whether for NOx or CO2, is unattractive for operators, since it would not generate additional value. A revamp that improves heater capacity, reliability, and availability creates much more value. Revamping a fired heater can be a complex task, especially when dealing with multiple problems simultaneously. A structured methodology is required to successfully determine what problems need to be resolved, in what priority, and what decisions must be made to find the best possible solution.
An example of a successful revamp using this methodology will be used to demonstrate that the goal of emissions reductions can be achieved while improving the bottom line.
Elements of a successful revamp
The Kepner-Tregoe Matrix is ideal for analysing a complex revamp and consists of four steps:
1. Situational Analysis — a high-level definition of the concerns to address
2. Problem Analysis — narrowing in on the specifics and determine root causes
3. Decision Analysis — collect and evaluate alternatives on the merits and risks, decide which option is the best possible choice
4. Potential Problem Analysis — a preventative step to evaluate the impact of the decisions on the system and avoid new problems
This approach is demonstrated for a recent retrofit of a vertical cylindrical heater by XRG Technologies.
The situation analysis is usually performed by the owner/operator. In the case of our example, the high-level issue for the owner was the heater capacity. A plant study showed that the output could be increased by 20-30% if the crude heater could be debottlenecked. However, secondary issues also had to be resolved before a capacity increase could even be considered. The most obvious was that many radiant tubes showed damage such as high-temperature oxidation, tube wall thinning, and creep. Another concern was the short time between decoke cycles, indicative of a potentially high coking rate. Any solutions needed to consider stringent NOx emission limits as well as high CO2 taxes that would be imposed after the year 2020.
Fired heaters are complex pieces of equipment that can be difficult to analyse. The key to a successful retrofit starts with an engineering study to understand all aspects of the heater. Most heater studies begin with a survey in the field to record the present mechanical condition of the heater, observe the flames, measure stack emissions, and collect operating data. This is also an excellent opportunity to interview operators about difficulties they experience controlling the heater. This information is used to generate thermohydraulic models to identify bottlenecks in the present or future operation. Comparing the model results with actual operating data can reveal issues that are not readily observed, like fouling inside the tubes or loss of heat transfer in the convection section.
Key findings of the survey of the VC heater were:
- Temperature/heat flux maldistribution. The peak skin temperature difference between the hottest and coldest pass was approximately 50°C (90°F) at Start of Run (SOR). This imbalance increased to about 150°C (270°F) at End of Run (EOR).
- Short run length. The radiant tube skin temperatures increased by ~120°C (216°F) in only three months.
- Low fuel efficiency. The field data indicated an efficiency between 78 and 79%, despite good mechanical condition of the heater and efficient operation of the burners.
- Loss of heat transfer. Comparison with the thermal models showed that the actual fuel efficiency at SOR (i.e., in clean condition) should have been 2% higher.
Visual observation of the firebox showed flame interactions. Flame interactions almost always result in a performance penalty. Flames rolling into tubes cause hotspots, excessive coke formation, oxidation and tube damage from oxidation, creep, and carburisation. Unruly flames are a safety concern, produce high emissions, and shorten the life of many components.
The root cause of the flame interactions was investigated by a CFD analysis of the heater. CFD (Computational Fluid Dynamics) is a common tool to evaluate the performance before and after a fired heater revamp.
In the original design of the firebox, the natural draft burners were arranged in two circles. The outer ring consisted of ten burners while the inner ring consisted of five burners. This layout caused poor flue gas recirculation patterns and flames coalescing in an unsteady flame cloud. Additionally, the outer burners were too close to the radiant tubes causing flames to locally impinge, creating hotspots. The CFD model replicated the poor flame patterns and explained the difference in coil temperatures, high coking rates, and tube degradation issues.
Armed with information from an engineering study and CFD analysis, the contractor and the operator jointly evaluate the potential revamp opportunities to reach an achievable and optimal retrofit target. For a heater revamp, these include (but are not limited to):
- Improve safety, reliability, operability
o Eliminate flame interaction problems
o Simplify burner control
o Eliminate ambient effects (e.g., wind, rain)
- Reduce Operating Costs
o Increase fuel efficiency
o Reduce fouling in tubes
- Increase profitability
o Increase capacity and throughput
o Switch to more economic fuels or feedstocks
- Reduce emissions
o Nitrogen oxides (NOx)
o Greenhouse gases (CO2)
o Unburned hydrocarbons (CO, CxHy)
Ideally, several scenarios are developed including estimates for CAPEX and OPEX, while considering credits or penalties for CO2 emissions. For this example, the following four solutions were considered in Table 1.
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