Pyrolysis Heater Firing Pattern for Uniform Heat Flux
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Solution Overview
Problem
Conventional pyrolysis heaters face challenges in maximizing onstream time due to fouling and high tube metal temperatures, which limit heat transfer efficiency and lead to uneven heat flux profiles, resulting in reduced coil utilization and increased NOx production.
Innovation Solution
A firing pattern is implemented in pyrolysis heaters where hearth burners operate with greater than stoichiometric air and wall burners with less than stoichiometric air, distributing air and fuel to achieve a more uniform heat release profile, reducing tube metal temperatures and maximizing heat flux distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional firing patterns are used with stoichiometric air distribution, then complete combustion is achieved, but tube metal temperatures become excessive and heat flux distribution becomes uneven
Solution Approach 1:
The patent applies local quality by distributing air to different burner sections (wall burners vs. hearth burners) at different rates. Wall burners receive less than stoichiometric air while hearth burners receive greater than stoichiometric air, creating localized variations in combustion intensity that result in more uniform heat flux distribution and reduced peak tube metal temperatures.
2Productivity
If more heat is added to maintain efficiency as coke builds up, then process performance is maintained, but tube metal temperatures increase beyond safe limits
Solution Approach 1:
The patent changes the air-to-fuel ratio parameters across different burner sections. By operating wall burners with air/fuel ratios below stoichiometric and hearth burners above stoichiometric, the system achieves better heat flux distribution that maintains process efficiency while preventing excessive tube metal temperature rise during extended operation.
3Productivity
If shorter residence times are used in the pyrolysis coil, then selectivity to ethylene and propylene increases, but heat transfer requirements increase leading to higher tube metal temperatures
Solution Approach 1:
The patent creates local quality variations in the combustion zone by differential air distribution to wall and hearth burners. This produces a more uniform heat flux profile along the coil that can support shorter residence times for higher olefin selectivity without creating localized hot spots that would exceed tube metal temperature limits.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach extends the operating cycle of pyrolysis heaters by reducing tube metal temperatures, improving heat flux uniformity, and lowering NOx production, thereby enhancing coil utilization and overall furnace efficiency.
Implementation Method 1
introducing a first air and fuel mixture to a wall burner section, the first air and fuel mixture having less than the stoichiometric quantity of air for combustion, introducing a second air and fuel mixture to a hearth burner section in the heater in a direction generally parallel to the length of the heating coil, the second air and fuel mixture having more than the stoichiometric quantity of air for combustion; and combusting the fuel and air in the radiant heating zone
Implementation Method 2
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100° C. The heat transfer to the coils is primarily by radiation.
Data Source
AI summary
A furnace, firing pattern and method of operating a heater that employs a combination of hearth burners and wall burners for the cracking of hydrocarbons is described. The firing pattern leads to improvements in the uniformity of the coil metal temperatures and vertical heat flux profiles over the firebox elevation. The hearth burners operate with a stoichiometric excess of air while the wall burners operate with less than the stoichiometric amount of air.