NO_x Emissions in Fluid Catalytic Cracking Catalyst Regeneration

Abstract

The formation of nitrogen oxides (NO x ) during the regeneration of fluid catalytic cracking (FCC) catalyst can be attributed to oxidation of nitrogen in coke. Thermal NO x does not occur to any significant extent at FCC regenerator temperature. NO x is primarily comprised of nitric oxide (NO) with only a small concentration of NO2 detected. Under typical FCC regenerator conditions, only 10% of the nitrogen in coke evolves from the regenerator as nitric oxide with flue gases. Our findings show that ∼90% of nitrogen in coke is converted to dinitrogen (N2) in the regenerator. The authors suggest that dinitrogen is a secondary product produced by the reduction of NO with carbon and/or carbon monoxide in the regenerator. NO x emissions are lower in controlled burn regenerators where exposure to excess oxygen is minimized. Two-stage regeneration was found to lower NO x emissions by conversion to N2 during recontacting of flue gases and catalyst in the dense bed.

Reaction Mechanism for the Formation of Nitrogen Oxides (NOx) During Coke Oxidation in Fluidized Catalytic Cracking Units

“…In all of the cases studied here, the routes leading to NO 2 removal from coke (CS4 → CS9, CS14 → CS21, CS26 → CS29, CS33 → CS37, and CS51 → CS53) had significantly lower rates than the routes leading to NO formation. This is in line with the experimental observation where the concentration of NO 2 in the exhaust gas from FCC regenerator is lower than that of NO (Babich et al, 2005;Dishman et al, 1998;Zhao et al, 1997). In this figure, the differences of several orders of magnitude in rate constants for NO and NO 2 routes also indicate that there may be other routes for the formation of NO 2 , such as the reactions of NO with O 2 , CO, or CO 2 in the gas phase.…”

“…The concentration of NO x in the flue gas exiting FCC can vary between 50 and 500 ppm (Babich et al, 2005;Dishman et al, 1998;Iliopoulou et al, 2004;Stevenson et al, 2005) depending upon operating conditions. However, even at such concentrations, these reactive compounds can harm the environment and human health.…”

“…During coke oxidation at temperatures in the range of 650-750°C (close to FCC regenerator temperature of about 715°C), it was found that, along with CO and CO 2 , NO was also formed, and some NO molecules reacted further with coke and CO to form N 2 . In Dishman et al (1998), commercial catalysts were coked in a fluidized bed using vacuum heavy gas oil doped with pyridine, indole, carbazole, quinoline, and hydroxypyridines. The study showed that most of the emitted NO x contained NO, while some of them were reduced to N 2 .…”

NOx selective catalytic reduction at high temperatures with mixed oxides derived from layered double hydroxides

“…In the FCC regenerator, the coke is burnt off in the presence of oxygen producing nitrogen oxides. These emissions represent 50% of the total NO x emissions of the refinery [3][4], therefore, the amount of the NO x emitted from a FCC unit should be controlled. The use of additives that catalyze the NO reduction or decomposition in the FCC regenerator unit is probably the most simple and cost-effective method.…”

Active Catalysts for the NO x Reduction in a FCC unit

“…In the FCC regenerator, the coke is burnt off in the presence of oxygen, and a mixture of nitrogen oxides (90% NO, 10% NO 2 ) is formed [4][5][6]. Therefore, the amount of the NO x emitted from a FCC unit can be controlled by lowering the concentration of the nitrogencompounds present in the feed, either by choosing appropriate petroleum sources or by a hydrotreatment.…”