Run length improvement and emissions reduction in delayed coker heaters
For many refineries, the delayed coker unit is a vital component of refinery profitability. The tubes used in delayed coker units foul and must be decoked periodically.
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The shutdown days used to decoke a heater remove it from service. The burner flames used to deliver heat to the tubes contribute to heat flux non-uniformity in the radiant section, accelerating coking. Computational Fluid Dynamics (CFD) modelling is used to explore innovations in combustion that improve temperature uniformity, increase run length, and reduce NOx emissions.
Background: Delayed coker units use fired heaters to heat vacuum residual oil (VRO) to the onset of thermal cracking. The process outlet temperature is typically 900°F but depends on the feedstock and operating conditions. The radiant section flue gas outlet temperature ranges from 1,400°F to 1,600°F. Ideally, the onset of the thermal cracking and coke production (coking) do not occur until the exit of the heater. In practice, this is never the case, so heaters must be decoked to remove coke products built up inside the heater coils. Operators can decoke the heater by pigging, spalling, or steam-air decoking.
There is always non-uniformity in the temperature of the VRO inside the heater tube. Even in ideal situations where uniform heat is applied at the outside of the process tube, the film temperature of the VRO next to the tube wall is always higher than the bulk fluid temperature. The flames and emitting gases inside the heater radiate to the tubes non-uniformly as a function of each VRO tube shadowing the adjacent tube (Hottel, 1983). The shadowing results in higher temperature on the ‘hot side’ of the tube, facing the flames, as opposed to portions not exposed to direct radiation. Finally, variation in the flame pattern itself results in further variation in VRO temperature inside heater tubes. When these sources of non-uniformity occur simultaneously, peak heat flux can be 2.3 plus times higher than the average heat flux, as in Figure 1.
The high temperature and low velocity in the boundary layer next to the tube wall cause coke to form and deposit at the tube wall (Ebert, 1997). The high-temperature flames emanating from the burners result in high NOx generation. A linear increase in flame temperature results in an exponential increase in NOx production. Efficiency-improving methods, such as increasing combustion air temperature, increase both the flame temperature and the resulting NOx from the coker.
The peak tube metal temperature limits the run length of most coker heaters. This peak temperature occurs in a localized spot and is determined by the interaction between the heated VRO and the heat transfer from the burner flames. Reducing this local peak temperature, the run length of a coker heater can be increased.
High-temperature flames generate high NOx emissions and localised heating of the tubes. The temperature of a flame is a function of the composition of the reactants, which is determined by the mixing at the flame front introduced by the burner. A burner can lower the flame temperature by mixing more air or flue gas into the flame front. However, if too much air or flue gas is mixed into the reaction zone, it becomes impossible to maintain a flame.
By intentionally changing the combustion regime such that the fuel is thermally oxidised rather than burned in a flame front, the temperature at which the fuel is oxidised can be greatly reduced. Oxidation of the fuel at a temperature that is close to the exit temperature of the heater produces effectively no NOx and more uniform radiation to the process tubes.
One cannot easily apply thermal oxidation of the fuel – or flameless combustion – to fired heaters for several reasons. Combustion without a flame is only supported when the reactants have a sufficient temperature, a low oxygen concentration, and sufficiently diluted fuel (Capocelli). Because the tubes carrying the VRO in a coker heater are cold compared to the flue gas, a sufficient temperature cannot be maintained throughout the combustion chamber. The oxygen concentration varies by necessity when burners are used because fresh air is introduced at each burner. The fuel is initially undiluted. In the case of ultra-low NOx burners, some fuel is diluted with flue gas before combustion, but there is a limit that cannot be surpassed if a flame is to be maintained.
There are existing burner management systems (BMS), safety instrumentation systems (SIS), and start-up and shutdown procedures for fired heaters. These systems and practices are incompatible with flameless combustion, because they rely on the detection of flame to determine the condition of the firebox. Any practical use of flameless combustion should account for these systems and practices rather than circumvent them.
An alternative mode of combustion maintains a baseload of conventional combustion in the burners and the remaining portion of the combustion is carried out by thermal oxidation of the fuel. Here, the BMS, SIS, and start-up and shutdown procedures remain the same as any conventionally fired heater. Dispersed combustion results in lower emissions, more uniform heat transfer, and potentially longer run lengths in coker heaters.
Dispersed Combustion Nozzles
To achieve practical dispersed combustion, all of the combustion air is introduced through the burners while a portion of the fuel remains in the burners and a portion is diverted to specialised nozzles to be mixed with flue gas. One starts up and shuts down using only the burners and not the nozzles. When the radiant section flue gas meets certain conditions, fuel is diverted from the burners to the flue-gas mixing nozzles. Figure 2 shows a diagram of the nozzles placed in the radiant section of a coker heater.
Figure 3 shows the nozzle used to introduce fuel for dispersed combustion into a fired heater. These nozzles are not burners. They have been designed to entrain and mix flue gas, rather than air, with a portion of the fuel gas used to heat the process.
The shape of the nozzle increases mass flow of the entrained flue gas as well as the uniformity of the mixture in the near field of the nozzle, so that a flame can never be formed, regardless of the fuel composition. Once mixed, the fuel is oxidised within the radiant section with excess air from the burners. The nozzles are oriented to maintain and reinforce desired flue gas recirculation patterns and to ensure that fuel is oxidised completely before leaving the firebox.
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