Modeling of Monolith Reactor Washcoated with CuZSM5 Catalyst for Removing

沸石分子筛在脱硝上的应用

Modeling of Monolith Reactor Washcoated with CuZSM5Catalyst for Removing NO from Diesel Engine by Urea

Joon Hyun Baik,Sung Dae Yim,†and In-Sik Nam*

Department of Chemical Engineering/School of En V ironmental Science and Engineering,

Pohang Uni V ersity of Science and Technology(POSTECH),san31Hyoja-dong,Pohang790-784,Korea

Young Sun Mok

Department of Chemical Engineering,Cheju National Uni V ersity,66Jejudaehakno,Jeju690-756,Korea

Jong-Hwan Lee,Byong K.Cho,and Se H.Oh

General Motors R&D and Planning Center,Warren,Michigan48090-9055

Two sets of reaction kinetics based upon the experimental data independently obtained for NH3-SCR and

urea decomposition reactions over CuZSM5catalyst have been deduced to design the urea-SCR process for

application to heavy-duty diesel engines.They were finally employed simultaneously to simulate the commercial

performance of a monolith reactor for the urea-SCR process.The monolith reactor washcoated with the

CuZSM5catalyst was prepared and its SCR activity was evaluated to confirm the reaction kinetics and the

reactor model developed in the present study for its commercial application.The monolith reactor model

including the mass-transfer resistance directly employs the kinetic parameters obtained from the kinetic study

over a packed-bed flow reactor containing20/30mesh powder type of CuZSM5catalyst.The kinetic and

monolith reactor models developed in the present study well predict the reactor performance of the urea-SCR

process.The model is also capable of describing the effect of the reaction conditions,a critical issue for the

commercial operation of the urea-SCR process to the automotive engine,including the reactor space velocity,

NH3(and/or urea)/NO feed ratio,NH3and urea slips,and the temperature of the thermal decomposition

reactor on the NO removal activity.

Introduction

Selective catalytic reduction(SCR)by NH3is generally recognized as the most effective method for reducing the emission of nitrogen oxide(NO x)from stationary sources.1 Recently,urea-SCR,the selective catalytic reduction of NO x using urea as an alternative reducing agent for its easy transportation and handling,has been reported as one of the most promising way to reduce or control NO x emissions originating from heavy-duty diesel engines.2,3Baik et al.reported that the removal of NO x by urea-SCR over the CuZSM5catalyst was competitive to that by NH3-SCR,indicating urea can be effectively utilized in an SCR reactor system as a reducing agent for NO x from automotive engine.4

The urea-SCR process over the CuZSM5catalyst is mainly based upon the following four reactions:(1)urea thermal decomposition,(2)HNCO hydrolysis,(3)NO reduction,and (4)NH3oxidation reaction.Urea is thermally decomposed into ammonia and isocyanic acid,and then the isocyanic acid formed by reaction1is easily hydrolyzed on the catalyst surface, producing an additional mole of ammonia and carbon dioxide by reaction2.2,5-7Consequently,the complete decomposition of one mole of urea produces two moles of ammonia and one mole of carbon dioxide.NH3is a direct reductant for SCR reaction by reaction3,while urea is an indirect reducing agent to produce NH3for the overall deNO x reaction.On the other hand,the NH3oxidation reaction can occur at a reaction temperature>350°C and mainly produce N2,particularly over CuZSM5catalyst by reaction4.It should be noted that N2is the main product by NH3oxidation reaction over the Cu ion-exchanged zeolite.8-10

However,unreacted NH3decomposed from urea can be a secondary air pollutant because of its toxicity and the formation of ammonium salts inside the reactor as well as in the atmosphere.Searching optimal operating conditions for mini-mum NH3emissions(e.g.,urea/NO feed ratio)generally involves a tradeoff between NO removal activity and NH3slip.11 The development of the kinetic model may resolve this problem more efficiently,and it will also provide critical information for the optimal design and operation of a commercial urea-SCR reactor for its automotive application.

A kinetic model for the simultaneous thermal and catalytic decomposition of urea over CuZSM5catalyst was developed by a unified approach based upon the power-law kinetic model.5 A model containing three main reactions,including the thermal decomposition of urea,the catalytic hydrolysis of HNCO,and the catalytic oxidation of ammonia during the course of the decomposition of urea,adequately described the experimental data.For NH3-SCR,Chae et al.developed a honeycomb reactor

*To whom correspondence should be addressed.Tel.:82-54-279-2264.Fax:82-54-279-8299.E-mail:isnam@postech.ac.kr.

†Present address:Fuel Cell Research Center,Korea Institute of Energy Research(KIER),71-2Jang-Dong,Daejeon305-343,Korea.

NH

2

-CO-NH

2

f NH3+HNCO(1)

HNCO+H

2

O f NH

3

+CO

2

(2)

4NO+4NH

3

+O

2

f4N2+6H2O(3)

4NH

3

+3O

2

f2N2+6H2O(4)

5258Ind.Eng.Chem.Res.2006,45,5258-5267

10.1021/ie060199+CCC:$33.50©2006American Chemical Society

Published on Web06/22/2006

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