The process of FCC is that the raw oil is heated and cracked under the action of catalyst to generate dry gas, liquid hydrocarbon, gasoline, diesel oil, slurry and coke. Coke produced in the reaction process is attached to the surface of the catalyst to be generated, and is transported to the regenerator for coke burning through inclined pipe, so that the catalyst can be restored to activity and can be recycled. The flue gas discharged from the regenerator is separated from a part of catalyst by three and four turns, and then it is discharged to the atmosphere through the chimney after the waste heat is recovered by the waste heat boiler.

NOx source and influencing factors

The raw materials of FCC unit generally contain nitrogen heterocyclic compounds and aromatic compounds, of which nitrogen content is generally 0.05% - 0.5% (W), which is adsorbed to the intergranular acid position of catalyst molecular sieve during the catalytic cracking reaction. It is found in coke in the form of complex aromatic ring with large molecule, and 30% - 50% of the raw materials The nitrogen is brought into coke and enters the regenerator with the catalyst to be generated. During the regeneration of catalyst, 10% - 30% of nitrogen is converted into NOx, and the rest nitrogen is released in nitrogen form.

The concentration of NOx in flue gas of catalytic regeneration is related to the nitrogen content of feedstock oil, the regeneration mode of catalyst and the main operating conditions of catalytic cracking unit

1) Nitrogen content of raw materials. The nitrogen content of the catalyst increases with the increase of the nitrogen content of the raw oil, and the NOx concentration in the flue gas increases.

2) The control of excess oxygen content in flue gas of catalytic regeneration is the influence

The structure of regenerator and the regeneration mode of catalyst also have a great influence on NOx generation. When the regenerator is operated with oxygen, the CO content as reducing agent decreases, resulting in a significant increase in NOx concentration. Two stage regeneration helps to reduce NOx concentration in flue gas. Generally, the upper part of regenerator adopts incomplete regeneration, and the lower section adopts complete regeneration. After NOx generated in the oxygen rich environment of the lower section enters the upper regenerator, it contacts with the Co rich flue gas and C on the catalyst to make NOx turn into N2. The NOx concentration in flue gas with two-stage regeneration method is 20% - 25% lower than that of traditional regeneration.

3) Main operating conditions. The literature research shows that the higher operating pressure and lower operating temperature are beneficial to reduce NOx generation in regeneration process. With the increase of operating pressure, the precipitation rate of nitrogen volatiles in the porous skeleton structure of catalyst is restrained, and NOx production is reduced correspondingly. Meanwhile, the increase of operating pressure will lead to the residence time of flue gas in catalyst bed and the increase of CO and C concentration, which enhances the reduction effect of CO on NOx and reduces NOx emission. However, with the increase of regeneration temperature, the precipitation rate of nitrogen compounds in coke increases, which makes NOx production increase gradually.