Crude oil heaters are critical components in refineries and chemical processing, consuming a significant portion of a plant’s total fuel. Their primary function is to heat crude oil to specific temperatures, reducing its viscosity for efficient transportation and facilitating separation processes like distillation. Given their substantial energy consumption, even small improvements in burner efficiency can lead to considerable cost savings, reduced emissions, and enhanced operational reliability.
Optimizing these fired heaters involves a multifaceted approach, addressing burner design, combustion control, heat transfer mechanisms, and overall system maintenance. The goal is to maximize the heat transferred to the process fluid while minimizing fuel consumption and environmental impact.
Advanced Burner Technologies for Enhanced Efficiency
Modern burner designs play a pivotal role in achieving higher efficiency and lower emissions. The evolution of oil burners has continuously aimed to improve efficiency and capacity to handle modern heating demands.
Low-NOx Burners
Low-NOx burners are engineered to reduce the formation of nitrogen oxides (NOx), harmful pollutants, while maintaining or improving combustion efficiency. They achieve this by staging the combustion process, which reduces available oxygen, temperature, or residence time to limit NOx formation. Implementing low-NOx burners is a key strategy for environmental compliance and can be integrated with advanced control systems for optimal performance.
Forced Draft Burners
Unlike natural draft burners that rely on stack effects for air supply, forced draft burners use a fan to supply combustion air. This provides more precise control over the air-fuel ratio, allowing for optimized combustion and preventing issues like flashback. Forced air supply with micro-positive pressure combustion can significantly improve combustion efficiency and reduce pollution.
Optimizing Combustion Air and Fuel Management
The heart of burner efficiency lies in precise control over the combustion process, specifically the air-fuel ratio.
Precise Air-Fuel Ratio Control
Maintaining an optimal air-fuel ratio is paramount. Too much excess air reduces flame temperature and efficiency, leading to higher energy prices, while too little air can result in incomplete combustion, soot formation, and equipment damage. Continuous measurement of oxygen (O2) and combustibles in the flue gas, particularly in the radiant section, provides crucial data for effective heater operation. A 10% reduction in excess air can improve efficiency by 1-2%.
Advanced control systems, such as linear regulation control strategies, can adapt to required values based on measured temperatures, contributing to improved energy efficiency by minimizing excess air. Some studies indicate that implementing such control strategies can lead to significant fuel savings and a reduction in CO2 emissions.
Fuel Gas Optimization and Variability Management
Fired heaters in refineries often use a mix of fuels, including natural gas, refinery off-gases, and residual fuels. The varying calorific value and composition of these fuels can lead to performance decline. Optimizing the combustion process by incorporating more stable fuels or by using real-time gas chromatography-mass spectrometry (GC-MS) data to accurately calculate mass fractions of individual compounds can allow for more precise burner flow rate determinations. This approach can lead to substantial energy savings by tailoring the fuel supply to the specific thermal needs of each processing unit. Using mass flow control instead of volumetric flow control for fuel gas can also help stabilize energy content and air requirements, leading to lower energy costs and emissions.
Enhancing Heat Transfer in Heater Sections
Efficient heat transfer is fundamental to overall crude oil heater performance. A fired heater comprises a radiant section and a convection section, each playing a distinct role in heat transfer.
Radiant Section Optimization
The radiant section is where most of the heat transfer occurs directly from the flame to the tubes via radiation. It accounts for approximately 70% to 85% of the total heat absorbed. Strategies for optimizing this section include:
- Uniform Heat Flux Distribution: Irregular heat flux can lead to hot spots, process fluid degradation, coke formation, and potentially tube failure. Designing for uniform heat flux density around the process coil reduces coking and prevents scorching of crude oil. Optimization of extended surfaces on tubes can significantly improve the uniformity of heat flux distribution.
- Preventing Flame Impingement: Short flame burners prevent flames from directly impinging on the tubes, which can cause localized overheating and damage.
Convection Section Improvements
The convection section recovers additional heat from the flue gases before they exit the stack, preheating the crude oil before it enters the radiant tubes.
- Economizers and Waste Heat Recovery Units (WHRUs): Installing economizers in the convection section is a highly effective method for recovering waste heat. WHRUs capture thermal energy from hot exhaust and gas discharge, repurposing it for heating oil or other media, thus significantly increasing overall efficiency. This can lead to efficiency increases of 5-10%.
- Finned Tubes: In the convection section, finned tubes can enhance heat transfer by increasing the surface area available for heat exchange.
- Maintaining Cleanliness: Fouling in the convection section can impede heat transfer. Regular cleaning, potentially using soot blowers, is essential to maintain efficiency.
Operational Best Practices and Maintenance
Beyond design, daily operational practices and diligent maintenance are crucial for sustaining high burner efficiency.
Regular Maintenance and Inspections
Routine maintenance, including cleaning the burner and heat exchanger, checking and replacing filters, and inspecting fuel lines and tanks, is vital for efficient operation. Soot and debris buildup can significantly reduce heat transfer effectiveness. Heaters should be periodically inspected to ensure refractory materials are well-fixed, and all manways and peepholes are properly closed to prevent air leakage.
Advanced Control Systems
Implementing advanced control systems, such as predictive control, allows for dynamic optimization of burner operation. These systems can continuously monitor and adjust parameters like fuel flow, air intake, and temperature based on real-time data, ensuring optimal combustion and heat transfer efficiency. Linear regulation control strategies have shown to improve energy efficiency by decreasing exhaust gas temperature and reducing CO2 emissions.
Draft Control
Proper draft control is essential for optimal combustion and reducing emissions. Maintaining the correct negative pressure at the convection section inlet and regulating combustion air flow by adjusting stack dampers and burner registers is critical. Excess draft or insufficient draft can lead to problems like reduced flame temperature, incomplete combustion, or heater damage.
Conclusion
Improving burner efficiency in crude oil heaters is a multifaceted endeavor that offers substantial economic and environmental benefits for the chemical processing and oil and gas industries. By integrating advanced burner technologies, meticulously managing combustion air and fuel, optimizing heat transfer across radiant and convection sections, and adhering to rigorous operational and maintenance practices, companies can significantly reduce fuel consumption, lower emissions, and enhance the overall reliability and lifespan of their critical heating infrastructure.
