The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOxPart I. Pyrolysis and hydrolysis of urea over zeolite catalysts

Eichelbaum, Maik; Farrauto, Robert J.; Castaldi, Marco J.

doi:10.1016/j.apcatb.2010.03.027

Cited by 101 publications

(99 citation statements)

References 23 publications

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“…However, these studies did not consider the formation of deposits resulting from urea decomposition. Deposit formation can lead to backpressure generation, deterioration of the after-treatment system [19] and possible deactivation of the SCR catalyst [20].…”

Section: Introductionmentioning

confidence: 99%

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

2011

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This work aims to develop a multi-component evaporation model for droplets of urea-watersolution (UWS) and a thermal decomposition model of urea for automotive exhausts using the Selective Catalytic Reduction (SCR) systems. In the multi-component evaporation model, the influence of urea on the UWS evaporation is taken into account using a NRTL activity model. The thermal decomposition model is based on a semi-detailed kinetic scheme accounting not only for the production of ammonia (NH 3 ) and isocyanic acid (HNCO), but also for the formation of heavier solid by-products (biuret, cyanuric acid and ammelide). This kinetics model has been validated against gaseous data as well as solid-phase concentration profiles obtained by Lundstroem et al. (2009) andSchaber et al. (2004). Both models have been implemented in IFP-C3D industrial software in order to simulate UWS droplet evaporation and decomposition as well as the formation of solid by-products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas-phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy.

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Section: Introductionmentioning

confidence: 99%

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

2011

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“…The hydrolysis of HNCO with trace amounts of water could produce NH 3 and CO 2 . The lactam compound might be the ammelide (C 3 H 4 O 2 N 4 ), which could be formed by the reaction between HNCO and biuret [36,38]. The formation of H 2 O and CO 2 could be the reason why addition of urea facilitated the formation of CaCO 3 .…”

Section: Possible Mechanism Of Synergetic Inhibitionmentioning

confidence: 99%

“…CO 2 adsorption or release from the particulates benefited the transport of PCP molecules, resulting in sufficient contact between PCP and complex, thereby further improving the inhibition effect. Furthermore, the addition of CaO might decrease the decomposition temperature of urea, promoting urea to decompose more H 2 O and CO 2 [38], thereby more CaCO 3 was formed. This series of circular reaction explained the synergetic inhibition effect of CaO and urea mixture for preventing PCDD/Fs formation from PCP.…”

Section: Possible Mechanism Of Synergetic Inhibitionmentioning

confidence: 99%

Synergetic inhibition of PCDD/F formation from pentachlorophenol by mixtures of urea and calcium oxide

et al. 2016

Journal of Hazardous Materials

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h i g h l i g h t s• Mixed CaO/urea have a synergetic inhibition effect for PCDD/Fs formation from PCP.• 5-20% urea reaction systems obtained the better synergetic inhibition effect.• 2,3,7,8-PCDD congeners were the dominant products in eight reaction systems.• Intermediates could promote the acid-base reaction to realize synergetic inhibition. a r t i c l e i n f o b s t r a c tChlorophenols are structurally similar to PCDD/Fs and have been considered as highly potential precursors for PCDD/Fs formation. The suppressing effects of PCDD/F formation from pentachlorophenol (PCP) were investigated on various mass ratios of CaO and urea. The total concentration of 2,3,7,8-PCDD/Fs, mostly dominated by OCDD, was determined to be 48.58-10186 ng/mg in inhibitor-reaction systems, being much lower than that in blank reaction system (75654 ng/mg). Interestingly, compared with pure CaO and urea reaction system, the concentration and TEQ of formed 2,3,7,8-PCDD/Fs in mixed urea/CaO reaction system were lower, especially with 5-20% urea reaction systems being respectively at decrease by 96.5-99.4% and 99.2-99.7%. The suppression efficiency of TEQ in 5-20% urea reaction systems could be always approximately 100% under 250-350• C. These results suggested that mixed inhibitors, especially 5-20% urea inhibitors, have a synergetic inhibition effect for PCDD/Fs formation from PCP. Mixed inhibitor generated several intermediates, involving CO 2 , H 2 O, NH 3 , Ca(OH) 2 , CaCO 3 , HNCO, biuret and ammelide. The complex between PCP and Ca, N-doped species, lower chlorinated phenols and benzenediol, and organic acids were also determined. Synergetic inhibition mechanism may be attributed to accelerated facilitation of acid-base reaction and N doping. The decomposition of PCP itself also contributes to prevent PCDD/Fs formation.

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“…Lundström et al (2009) showed that urea on a monolith (impregnated with a 32.5% UWS) can totally decompose into NH 3 and HNCO, while urea on a cup will form by-products (i.e., CYA) during the heating process. In thermogravimetric analysis (TGA) experiments, Eichelbaum et al (2010) and Brack et al (2014) found that the geometric structure of the crucible has a significant impact on urea decomposition. All these findings indicate that a quick removal of HNCO can effectively reduce or even prevent the formation of by-products.…”

Section: Introductionmentioning

confidence: 99%

Quasi 1D modeling of two-phase flow and deposit formation for urea-selective catalytic reduction systems

Gan

Yao

et al. 2016

J. Zhejiang Univ. Sci. A

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A quasi 1D model of two-phase flow for a urea-selective catalytic reduction (SCR) system is developed which can calculate not only the generation of reducing agent but also the formation of deposits in the exhaust pipe. The gas phase flow is solved through Euler method, variables are stored on staggered grids, and the semi-implicit method for pressure-linked equation (SIMPLE) algorithm is applied to decouple the pressure and velocity. The liquid phase is treated in a Lagrangian way, which solves the equations of droplet motion, evaporation, thermolysis, and spray wall interaction. A combination of a direct decomposition model and a kinetic model is implemented to describe the different decomposition behaviors of urea in the droplet phase and wall film, respectively. A new 1D wall film model is proposed, and the equations of wall film motion, evaporation, thermolysis, and species transport are solved. The position, weight, and components of deposits can be simulated following implementation of the semi-detailed kinetic model. The simulation results show that a decrease in the exhaust temperature will increase the wall film region and the weight of deposits. Deposit components are highly dependent on temperature. The urea-water-solution (UWS) injection rate can affect the total mass of wall film and expand the film region, but it has little influence on deposit components. An increase in exhaust mass flow can decrease the total weight of deposits on the pipe wall because of the promotion of the mass and heat transfer process both in the droplets and wall film.

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The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOxPart I. Pyrolysis and hydrolysis of urea over zeolite catalysts

Cited by 101 publications

References 23 publications

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

Synergetic inhibition of PCDD/F formation from pentachlorophenol by mixtures of urea and calcium oxide

Quasi 1D modeling of two-phase flow and deposit formation for urea-selective catalytic reduction systems

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