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Process-limited fired heaters that operate below design capacity represent significant lost opportunity costs.
Fired heaters are the largest fuel consumers in a refinery, emitting an estimated 400 to 500 million tons of CO2 every year, so even minor improvements in their efficiency can lead to significant savings. To put it into perspective, if any fired heater is 1 or 2% inefficient, it can consume an additional $1M in fuel over a year.
When a fired heater becomes process-limited, many operators will contemplate expensive revamps. However, several cost-effective solutions can restore performance without significant capital investment.
Understanding process limitations in fired heaters
When a heater becomes process-limited, it typically manifests through:
- Inability to achieve design throughput
- Higher than design bridgewall temperatures (BWT)
- Elevated stack temperatures
- Increased fuel consumption per unit of production
- Reduced convection section efficiency
- CO2 emission limits are being approached or exceeded
Before considering a major revamp, it's essential to identify the root causes of these limitations. Process limitations often stem from correctable inefficiencies rather than fundamental design flaws.
Common causes of process limitations
1. Radiant section inefficiencies
Process tube oxidation
Severe scaling due to oxidation on steel alloy tubes is standard in fired heaters operating at high temperatures. Above approximately 500°C, 9Cr-1Mo materials tend to oxidize rapidly. This oxidation creates an insulating effect that reduces heat transfer efficiency to the process fluid.
Refractory surface degradation
A significant portion of radiant energy interacts with refractory surfaces. As these surfaces degrade over time, their emissivity decreases, resulting in less efficient heat transfer to process tubes.
Carburization
Operating fired heaters at lower excess oxygen levels to save fuel can increase the potential for carburization of radiant section tubes. This leads to grain boundary penetration, carbide formation, embrittlement and eventual crack formation.
2. Convection section inefficiencies
Fouling build-up
Deposit accumulation on tube or fin surfaces in the convection section is a primary cause of reduced heat transfer efficiency. This fouling occurs when airborne particulates from the surrounding environment are drawn into the heater.
Excessive convection section fouling results in:
- High fuel consumption
- Decreased heater efficiency
- Increased flue gas and stack temperatures
- Reduced heater throughput
- Potential equipment damage
Non-revamp solutions to restore design capacity
1. Radiant section restoration
Online tube descaling
Services like Integrated Global Services (IGS) Hot-tek™ can remove oxidation scale from tube surfaces without a shutdown. This immediately improves radiant section heat transfer efficiency and reduces bridge wall temperature. While this provides a short-term improvement, it should be complemented with preventative measures.
Ceramic coatings for process tubes
Applying ceramic coatings to the outer surfaces of process tubes prevents oxidation, corrosion, and carburization while maintaining thermal conductivity close to new tube conditions. These coatings can be applied to existing tubes during a scheduled shutdown or to new tubes at a remote facility.
The average benefit in catalytic reformer heaters includes:
- 6.6% increase in radiant section efficiency
- 6.6% reduction in CO2 emissions
- Approximately 20% reduction in NOx emissions
High emissivity refractory coatings
Specialized ceramic coatings with emissivity values above 0.9, such as Cetek® ceramic coatings, can significantly enhance the radiation characteristics of refractory surfaces. Benefits include:
- Up to 5% improvement in radiant section efficiency
- Corresponding CO2 emission reductions
- NOx emission reduction in specialized units of up to 30%
2. Convection section restoration
Advanced robotic cleaning technology
Modern robotic cleaning technologies can remove more than 90% of convection section tube fouling during planned turnarounds. These remotely controlled systems can access even the most difficult-to-reach convection banks, penetrating deep between tube rows to remove any type or level of fouling.
Typical results include:
- 3-5% increase in thermal efficiency
- Up to 15% reduction in CO2 emissions
- Significant decrease in stack temperature
- Increased steam production (where applicable)
Case study
Tüpras, Izmir refinery
This refinery in Turkey applied Cetek’s High Emissivity Coating in conjunction with Tube Tech’s convection section cleaning service to achieve several operational benefits, which are discussed in this case study overview.
The challenge
The plant was experiencing reduced radiant heat transfer efficiency, higher fuel consumption, and an increase in the flue gas temperature of its steam methane reformer (SMR). The problems arose from the adsorbents in a Pressure Swing Absorber Unit, part of the refinery’s Hydrogen Manufacturing Unit (HMU), being changed during a turnaround.
Once started up, some of these adsorbents were transported by the tail gas stream and facilitated by misplaced screens in one of the beds; the result was an accumulation of fouling on the SMR's convection bank tubes.
The solution
Process tubes before high emissivity coating
Part one: Increasing SMR radiant heat transfer efficiency
In the SMR, the burners are directed to the angled walls in the radiant sections, and the radiation is transferred to the process tubes. The efficiency of the radiant heat transfer is related to the emissivity of the refractory surfaces. The higher the emissivity value, the greater the radiant heat transfer efficiency.
Process tubes after high emissivity coating
Applying a Cetek high-emissivity coating onto the refractory surface increased the heat re-radiated to the process tubes in the radiant section. As a result, more heat is absorbed by the tubes, but less heat travels to the convection section, slightly decreasing steam production.
Post coating evaluation
Tüpras reports that the bridge wall temperature has decreased by 26 °C, a positive indication that the high emissivity ceramic coating is performing well. The amount of heat transferred to the radiant section has increased by 2.9%, leading to the reported decrease in energy consumption of 6.8% (released) and 8.3% (absorbed).
Economic returns
Applying Cetek high-emissivity refractory coating has helped the refinery achieve targeted fuel savings, pay-out, and return on investment. The coating will continue to deliver benefits for approximately two turnarounds or eight years. After this period, it may require reapplication to continue providing fuel savings or increased capacity benefits.
Part two: Restoring convection section heat transfer efficiency
The decision was taken to carry out mechanical cleaning in the convection section of the furnace to increase heat transfer and, therefore, save energy. In addition, and as further motivation for the project, no mechanical cleaning had been carried out in the convection tubes since 1993, the unit's first start-up, due to their inaccessible locations.
Tüpras contacted TubeTech™ to deploy its robotic convection section fouling removal robot. The technology works remotely by penetrating deep between tube rows to remove the most tenacious fouling deposits.
The results
Hydrogen production cost and ROI
The average cost of hydrogen production before and after cleaning was calculated at $943.90 and $919.30 per ton, respectively. After cleaning, the production cost of hydrogen decreased by $24.6/t, and the project's payback period was less than 60 days.
Flue gas temperature
Before convection section cleaning
The average flue gas temperature decreased from 278°C before cleaning to 220°C after cleaning, meaning the furnace’s overall thermal efficiency increased by approximately 3%.
After convection section cleaning
Steam production
Steam production increased by approximately 20%, and the generated steam temperature increased by 10°C-15°C.
Implementation strategy for fired heater restoration
Assessment
Comprehensively evaluate current heater performance through thermal efficiency studies, infrared imaging to identify hot/cold spots, flue gas composition analysis to assess combustion efficiency, and measurement of tube skin temperatures and thickness for oxidation and metallurgical concerns.
Prioritization
First, target interventions with the highest impact and feasibility. Prioritize online maintenance activities like hot refractory repair and tube descaling. Align with planned maintenance schedules to minimize disruptions and perform ROI analysis for each potential solution.
Implementation
Schedule interventions strategically around seasonal demand patterns and processing alternatives. Combine complementary solutions for synergistic benefits. Document baseline performance metrics thoroughly before making changes.
Monitoring & optimization
Establish efficiency-related KPIs and implement continuous monitoring systems for real-time data on critical parameters. Regularly adjust firing patterns, air-fuel ratios and process controls to maintain optimal performance as conditions change.
Restoring a process-limited fired heater to design capacity doesn't necessarily require a significant revamp. By identifying and addressing efficiency losses in the radiant and convection sections, implementing advanced cleaning technologies and enhancing process controls, refineries can achieve substantial performance improvements with relatively modest investments.
These efficiency improvements represent immediate operational benefits through reduced fuel consumption and long-term strategic advantages through improved reliability and extended equipment life.
To speak to a fired heater performance expert, visit integratedglobal.com
