The COD content in the flue gas slurry from catalytic cracking desulfurization is high
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Our plant is a catalytic cracking unit; the denitration unit is located in the residual boiler, and the unit that follows it is for desulfurization, using EDV wet desulfurization technology from Berg； Earlier on, due to a fault with the flue gas butterfly valve, there was a short circuit in the flue gas; some of it flowed directly into the desulfurization scrubber, while a small amount went into the residual boiler. Due to the low temperature, ammonia injection was stopped in the denitrification unit. However, one issue was observed: when denitrification was in operation, the COD level in the desulfurization slurry was around 100, but after denitrification was stopped, the COD level in the slurry gradually increased to around 200–300. Is this related to the fact that denitrification was stopped? Additionally, due to the shutdown of the denitration system, in order to maintain acceptable levels of NOX in the flue gas, our plant strictly controls the main air flow, keeping the oxygen content in the flue gas at 1%. This ensures that the net NOX level in the flue gas remains between 80 and 100. However, this approach leads to poor burning conditions and there is a risk of residual combustion. Is this related to the poor burning conditions? Here, COD refers only to the COD in the scrubber slurry.
For the problem of poor oxidation, an appropriate amount of deoxidizing agent can be added
Under normal conditions, a small amount of organic matter enters the denitration unit along with the flue gas, and then proceeds to the desulfurization unit. In your description, denitration is in operation and the COD level in the slurry decreases; this may be due to alkaline hydrolysis occurring under ammonia injection, with the resulting substances remaining in the residual boiler. A high COD level can also be caused by low oxygen content or insufficient residence time.
The NOx in the flue gas is related to the bed temperature of the regenerator; a lower bed temperature facilitates the reduction reaction of NOx. The ammonia nitrogen level in the slurry is high – don’t we have a aeration tank?
Nitrogen oxides are related to the charred form
Because there is less oxygen in the flue gas, natural oxidation weakens. After absorption, sulfur dioxide exists in the slurry as sulfurous acid or bisulfite