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Modeling deactivation of catalysts for selective catalytic reduction of NOx by KCl aerosols Brian Kjærgaard Olsen, Francesco Castellino, and Anker Degn Jensen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01239 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Industrial & Engineering Chemistry Research

Modeling deactivation of catalysts for selective catalytic reduction of NOx by KCl aerosols Brian K. Olsen,† Francesco Castellino,‡ and Anker D. Jensen*,† †

Department of Chemical and Biochemical Engineering, Technical University of Denmark,

Building 229, DK-2800 Kgs. Lyngby, Denmark ‡

Haldor Topsøe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark

Abstract: A detailed model for the deactivation of a V2O5-WO3/TiO2 based SCR monolith catalyst by potassium poisoning has been developed and validated. The model accounts for deposition of KCl aerosol particles present in the flue gas on the external catalyst surface, the reaction of the deposited particles with the catalyst at the surface of the monolith wall, the transport and accumulation of potassium, bound to Brønsted acid sites, throughout the catalyst wall, and the resulting loss in SCR activity. Using an experimentally measured KCl aerosol size distribution as input, the model can replicate the observed deactivation rate of a 3 wt.% V2O5-7 wt.% WO3/TiO2 monolith catalyst, exposed to a KCl aerosol at 350 °C for about 1000 hours, as well as the resulting potassium to vanadium molar ratios in the catalyst wall. Simulations show that the particle deposition rate, and the deactivation rate, decreases if the particle size of the incoming aerosol is increased. The model provides, for the first time, a mechanistic framework for understanding and modelling SCR catalyst deactivation by KCl that may be applicable also for deactivation by other salts and at different operating conditions.

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1. Introduction Selective catalytic reduction (SCR) is a widely adopted measure for limiting the nitrogen oxide (NOx) emissions from fossil fuel fired heat and power plants. In this process, the NOx in the flue gas reacts with added ammonia (NH3) over a suitable catalyst, usually operated at 300-400 °C.1,2 Typical SCR catalysts consist of honeycomb monoliths of vanadia (V2O5) supported on titania (TiO2), promoted with either tungsten oxide (WO3) or molybdenum oxide (MoO3) in order to improve the activity and thermal stability of the catalyst.2,3 In Danish combined heat and power plants, fossil fuels are gradually being substituted with biomass, such as straw, wood chips and wood pellets. This is done in order to meet both national and European targets regarding reduction of the emission of greenhouse gases. However, the relatively high amounts of potassium present in e.g. straw,4 may cause accelerated deactivation of the SCR catalyst if released to the flue gas, as potassium is a strong poison for vanadia based SCR catalysts.5,6 Gaseous potassium species formed at high temperature in the combustion chamber may condense into submicron particles as the temperature of the flue gas decreases towards the SCR reactor. During straw firing, such particles will almost exclusively consist of potassium chloride (KCl) and/or potassium sulfate (K2SO4).7,8 Salt bound potassium from particles that deposit on the catalyst surface, can react with and poison the active Brønsted sites of the catalyst.9-18 Poisoning occurs as potassium enters the interior of the catalyst wall through surface diffusion.5,6,13,18 Modeling of this deactivation process will help predicting the life-time of SCR catalysts in biomass fired power plants, and may aid the development of a remedy to the problem. Khodayari and Odenbrand19 have previously proposed a model for accumulation of poison in SCR catalysts and the resulting deactivation. This model, however, assumes the poison to be in the gas phase.


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For potassium poisoning during biomass firing, such an assumption is hardly realistic. At 350 °C, the vapor pressure of KCl is only about 3.6·10-9 kPa20 and will be even lower for K2SO4 due to its higher melting point (1069 °C for K2SO4 versus 771 °C for KCl)21. This work presents a model for the selective catalytic reduction of NO by NH3 in a single catalyst channel of a monolith during deposition of KCl particles of a known size distribution, and the resulting continuous deactivation of the catalyst. The model calculates the NO and NH3 concentrations along the monolith channel as well as inside the catalyst wall. Furthermore, the deposition of potassium containing particles on the external surface, the reaction between potassium in the particles and the Brønsted sites at the catalyst outer surface and the subsequent transport of potassium into the wall by surface diffusion, causing deactivation of Brønsted sites inside the catalyst, are being accounted for. The model has been compared with an experimentally measured deactivation profile, obtained in one of our previous studies.22 Finally a parameter study with the model is performed to reveal those most important for the rate of deactivation. To the best knowledge of the authors, this is the first model presented in the literature which includes all relevant steps in the deactivation of SCR catalysts due to aerosol particles of KCl. 2. SCR and deactivation model 2.1. Assumptions. The following assumptions have been made for the derivation and solution of the model: •

The catalyst channel is a part of a monolith, and is hence surrounded by channels exposed to identical conditions.

•

The catalyst walls are thin compared to the channel diameter and are treated in planar geometry. There is symmetry in the concentration profiles at the center of the wall.


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•

A developing laminar velocity profile exists along the catalyst channel.

•

Complete mixing of NH3 and flue gas at the catalyst inlet.

•

A 1:1 reaction between NO and NH3 following the reaction scheme: 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O

•

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(1)

The reaction between NO and NH3 follows an Eley-Rideal mechanism where NH3 adsorbs on the Brønsted acid sites of the catalyst and NO reacts from the gas phase.

•

The change over time in the amount of adsorbed NH3 on the catalyst during deactivation is slow, i.e. in pseudo-steady state.

•

Isothermal operation – no significant production of heat during reaction.

•

Particle deposition by film diffusion assuming a zero particle concentration in the gas phase at the wall.

•

The flow pattern in the channel is unaltered by particle build-up on the wall as the layer is thin.

•

All the depositing particles are spherical and consist of KCl.

•

Potassium bound in deposited particles reacts irreversibly with Brønsted sites on the catalyst surface: K-particles + H-O-surface → K-O-surface + HCl (g)

•

(2)

Potassium is transported into the catalyst wall by surface diffusion over -OH sites.

2.2. Axial concentration profiles. We first consider the concentrations of NO and NH3 in the bulk flue gas, flowing with a mean velocity of U (in m/s) along a single monolith channel of length L and hydraulic diameter dh. The flow direction is denoted z in the following. The transient concentration profiles of NO and NH3 in the axial direction are given by: ,

= −

,

−

,

, − ,


(3)

4

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,

= −

,

−

,

(, − , )

(4)

Where Ci,b is the bulk concentration and Ci,s surface concentration of component i (both in mol/m3), while kg,i is the mass transfer coefficient of component i (in m/s). Since the flow is assumed to develop from turbulent at the catalyst inlet, to laminar along the channel, the mass transfer coefficient will be a function of the axial coordinate, z. At t = t0, no deactivation has occurred and the system is assumed to be at steady-state. Hence, the steady-state concentrations, Cssi,b , at all axial positions are required as initial conditions: ( ) , ( )!" = ,

(5)

#

The inlet concentrations of NO and NH3 are assumed to be constant at all times and are defined as: % , ($)!"% = ,

(6)

By introducing a reference concentration Cref, a reference distance zref and a reference time tref, the differential equations can be written in dimensionless form: &, &

=−

'()* &,

&, &

=−

'()* &,

+

()*

+

()*

−

, ()*

−

(&, − &, )

, ()*

(&, − &, )

(7) (8)

With the initial and boundary conditions: % &, ( ̃ )!&"& = &, ( ̃ ) and &, ($̃)!+"% = &,

(9, 10)

& = - , ̃ = and $̃ =

(11, 12, 13)

#

Where: ()*

()*

()*

The following will be used as reference parameters: % ./0 = , ,

./0

2

= 1 and $./0 = '


(14, 15, 16)

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2.3. Concentration profiles in the wall. We now consider the concentration of NO and NH3 inside the catalyst wall which have the characteristic width (half thickness) V/S (in m), where V is the catalyst wall volume and S is the catalyst wall external surface area. Symmetry is assumed at the center of the wall (x = 0). As the catalyst wall is thin, radial coordinates can be approximated to rectangular coordinates. The transient concentration profiles of the two gas phase components in the wall are given by: 3

3

= 4/,

5 6 5

+ (1 − 3)

+ 8

:;<

= 4/,

(17) 5 6 5

+ 8

(18)

Where Ci is the gas phase concentration of component i at a given position inside the wall (mol/m3), CNH3ads is the amount of NH3 adsorbed on the catalyst (mol/m3), ε is the porosity of the catalyst wall, and De,i is the effective diffusivity of component i (m2/s), which is assumed to be constant throughout the catalyst wall. It is furthermore assumed that adsorbed NH3 does not diffuse over the solid phase. The reaction between NO and NH3, with the rate –ri, is assumed to follow an Eley-Rideal expression23-26: >

−8 = −8 = −8 = −8 = =. ?@>

(19)

Where kr is the rate constant (s-1) while KNH3 is the adsorption equilibrium constant of NH3 on Brønsted sites (in m3/mol). These parameters are temperature dependent and can be expressed by a pre-exponential factor, k0 or K0, and an activation energy, Ea, or enthalpy of adsorption, ∆Had: D:

=. = =% · B CEF G = G% · B C

(20) ∆I:; EF

(21)

Where R is the gas constant (in J/mol/K) and T is the temperature in Kelvin. Table 1 presents


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examples of the activation energy, adsorption enthalpy as well as the pre-exponential factors, reported in the literature.25,26 As the catalyst deactivates, fewer sites becomes available for NH3 to adsorb on. In order to account for this, we now introduce the fraction of available Brønsted sites, φ, and modify the rate expression: J=

#

(22) >

−8 = =. J ?@>

(23)

Where C0OH and COH are the initial and current surface concentration of Brønsted sites (in mol/m2 internal surface) at a given position inside the wall. During the deactivation, which occurs over a time frame of hundreds of hours, the local rate of change in the NO and NH3 concentrations will be small. In the following we assume that the concentration of adsorbed NH3 is at pseudo steady-state, i.e.: :;<

($, K) ≈ 0

(24)

The gas phase concentrations of NO and NH3 could in principle also be assumed to be in pseudo steady-state, however, it has been chosen to keep all differential equations in transient form to avoid mixed differential-algebraic equations. These changes yield the following set of partial differential equations which describe the concentration profiles of NO and NH3 inside the catalyst wall during depletion of Brønsted acid sites: 3

3

= 4/,

5 6 5

= 4/,

>

− =. J ?@>

5 6 5

>

− =. J ?@>

(25) (26)

As for the axial concentration profiles, the steady-state concentrations for a fresh catalyst, Cssi, are required as initial conditions:


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(K)|"# = (K)

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(27)

Since we assume symmetry at the center of the wall, the following boundary condition must be fulfilled: -

O

6 6"%

=0

(28)

The flux of component i from the surface (x = V/S), into the catalyst, is equal to the flux from the bulk gas to the surface: -

4/,

O = =P, , − ,

(29)

6

The system can be written in a dimensionless form upon introduction of the reference distance xref = V/S: 3

& &

3

& &

=

Q), ()* 5 & 5 6()*

=

6+ 5

− $./0 =. & J

Q), ()* 5 & 5 6()*

6+ 5

R

> & @> &

S()*

− $./0 =. & J

R

> & @> &

S()*

(30)

(31)

With the initial and boundary conditions: & (K+)!&"& = & (K+)

(32)

#

&-

O 6+

6"%

Q

= 0 and 6 ),-

()*

&6+

T = =P, &, − &,

(33, 34)

Where: K+ = 6

6

(35)

()*

2.4. Particle deposition and external potassium accumulation. When an aerosol with a given size distribution passes through a catalyst channel, particles will diffuse towards and deposit on the channel walls. The flux of particles to the surface depends on their size as well as the flow conditions in the channel. In the following it is assumed that the axial concentration


8

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profile of each particle class in a discrete, polydisperse distribution is constant with respect to time (i.e. at steady-state). It is furthermore assumed that the particles are transported to the catalyst surface solely by film diffusion and that the particle flux is independent of the amount of particles already accumulated on the surface. The bulk concentration of particles in size class i, Wi,b (in #/m3), along a catalyst channel, is then described by the following differential equation: U-,

V,-

= −

W,

'

(36)

Where kp,i is the mass transfer coefficient (in m/s) of particles in size class i. This differential equation has the following solution: % B W, = W,

C

\ X Z ([)[ ; Y # V,-

(37)

Where W0i,b is the concentration of particles in size class i at the channel inlet. For each size class, the flux of particles to the surface at a given axial position, Np,i (in #/m2/s), is given by: ]^, = =^, W,

(38)

The rate at which particles of a given size accumulate on the channel wall is given by the particle flux to the surface minus the amount that is being consumed by reaction with surface -OH sites. We now introduce the surface concentration of potassium bound in particles of size i, CK,p,i (in mol/m2 external surface). The accumulation rate of this parameter is given by: _,V,

=`

`V,-

a,_bc

hV,-

] − =def ,g h ^,

& |®¯ @> & |®¯

S()*

(73) v O & 6+

=

Q< ()* v|®¯©ª®¯ Cov|®¯ @v|®¯°ª®¯ 5 6()*

(§6+)5

(74)

At the catalyst center (x = 0), the following applies: &- |®¯©ª®¯ C&- |®¯°ª®¯ o§6+

=0


(75)

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v|®¯©ª®¯ Cv|®¯°ª®¯ o§6+

=0

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(76)

At the catalyst surface (x = V/S), the boundary conditions become: &- |®¯©ª®¯ C&- |®¯°ª®¯ o§6+

v|®¯©ª®¯ Cv|®¯°ª®¯ o§6+

=

6()* ,Q),-

=−

&, !+@§¨+ − & !6+

6()* _bc `a,_bc oQ

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