MechChem Africa

Robbie Frank of Boksburg-based Elemental Analytics, a company specialising in analytical instrumentation applications, talks about combustion monitoring and control in fired heating systems.

Elemental Analytics FluegasExact 2700Fired heaters are integral to hydrocarbon processing (HP). Specifically designed for the reaction of fuel and air to produce extremely high gas temperatures, heaters transfer this energy to potentially highly flammable process fluids via heat exchangers. They consume large quantities of fuel, produce large quantities of emissions and are a potential safety hazard to personnel and plant. However, they are currently irreplaceable within many HP processes – so they warrant the highest levels of understanding and care in their operation and control.

Recent improvements in burner technology to reduce NOx emissions require closer monitoring of the process, as newer burners often operate under narrower process control conditions than older, larger nozzle diameter gas burners. Demands have also grown on plant operators to improve safety practices, increase plant efficiency and reduce environmental emissions. As a consequence, more accurate and reliable instrumentation is required to support the control of the process.

New and improved technologies are available to control fired heater combustion with ever greater accuracy and reliability, but the correct selection and effective use of these technologies requires understanding of a complex and delicate process. The cornerstones of well-controlled combustion processes are optimised air-to-fuel ratios and efficient fuel consumption.

Before analyser technologies were developed to measure excess air in the products of combustion, fired heaters were run in conditions of high excess air. Although this meant inefficient and costly fuel consumption, it was the only way to avoid the creation of low-oxygen, fuel rich conditions that could potentially lead to dangerous explosions.

Unfortunately, the reduction of excess air poses a new problem; the process moves to the tipping point at which incomplete combustion takes place. The introduction of zirconium oxide technologies in the late 1960s allowed engineers to obtain reliable and continuous measurements of excess air, enabling them to reduce the air-to-fuel ratio closer to that of the theoretical stoichiometric combustion mix.

The control and safety systems that run fired heaters, however, must therefore perform an extremely complicated balancing act. It is often not enough to just increase excess air levels when incomplete combustion is detected, as the complex interactions of oxygen and unburned fuel can lead to flammable mixtures igniting further down from the burners. Such conditions can lead to a number of negative process control conditions, including excess heat at the process tubes, which causes damage and leaks; carbon deposits, which decreases efficiency and heat transfer; and, in extreme cases, potentially dangerous combustion events can occur.

If a process problem is detected either by analytical instruments or other safety devices, however, it is inadvisable to simply switch off the fuel supply to the burners. Abrupt stops, restarts and light off conditions are the most common time for furnace incidents to occur. It is safer to bring the process carefully and correctly under control than to fully shutdown and restart the process and, as such, good quality and comprehensive analysis of the products of combustion, or lack of, is vital. Read more…

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