Deactivation of V2O5-WO3-TiO2 SCR catalyst at biomass fired power plants:

Elucidation of mechanisms by lab- and pilot-scale experiments

Yuanjing Zheng a, Anker Degn Jensen a,

*, Jan Erik Johnsson a

, Joakim Reimer Thøgersen b

aDepartment of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark

b Haldor Topsøe A/S, P.O. Box 213, Nymøllevej 35, DK-2800 Kgs. Lyngby, Denmark

1. Introduction

Selective catalytic reduction (SCR) was developed for reducing

NOx from stationary sources burning fossil fuels. However, the use

of biomass for power generation has increased in recent years to

reduce the net CO2 emissions, and in some countries such as

Sweden and Denmark, the SCR process is now being used on plants

burning pure biomass or mixtures of biomass and others fuels such

as coal, gas or oil. Unfortunately, it has been found that the SCR

catalysts may deactivate much faster under biomass firing than

when used on fossil fired systems [1–3]. The present authors

investigated the operation of full-length monolith catalysts in a

slipstream high-dust reactor installed at a straw-fired power plant

[1]. A high deactivation rate of about 1% of the relative activity per

day was observed when the power plant ran continuously.

Potassium,which is present in high levels in the fly ash of biomass

fired systems, has been proposed as an important element for

deactivation of SCR catalysts [1–3]. Although the potassium in fullscale systems is present as an aerosol of potassium chloride and

sulphate, most previous investigations of SCR catalyst deactivation

have been carried out in laboratory using wet impregnation with

alkali oxides [4–7]. Our laboratory study using wet impregnation

with potassium chloride and sulphate shows that also these

compounds are strong poisons for the SCR catalyst [8].

Beside KCl and K2SO4 the aerosol in the flue gas from biomass

fired power plants also contains minor amounts of sodium,

phosphorous, and calcium, which are also known poisons to the

SCR catalyst [4,7]. This makes it more difficult to interpret the

Applied Catalysis B: Environmental 83 (2008) 186–194

ARTICLE INFO

Article history:

Received 21 December 2007

Received in revised form 18 February 2008

Accepted 21 February 2008

Available online 26 February 2008

Keywords:

Selective catalytic reduction (SCR)

NOx

V2O5-WO3-TiO2

Potassium

Aerosol

Deactivation

Biomass combustion

ABSTRACT

In this work, deactivation of a commercial type V2O5-WO3-TiO2 catalyst by aerosols of potassium

compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of

KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure

KCl or K2SO4 aerosols in a bench-scale reactor. Exposed samples were characterized by activity

measurements, SEM-EDX, BET/Hg-porosimetry, and NH3 chemisorption. The work was carried out to

support the interpretation of observations of a previous study in which catalysts were exposed on a fullscale biomass fired power plant and to reveal the mechanisms of catalyst deactivation.

Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at

350 8C for 2397 h. No deactivation was found for catalyst plates exposed for 2970 h to fly ash (consisting

mainly of KCl and K2SO4) collected from an SCR pilot plant installed on a straw-fired power plant. A fast

deactivation was observed for catalysts exposed to pure KCl or K2SO4 aerosols at 350 8C in the bench-scale

reactor. The deactivation rates for KCl aerosol and K2SO4 aerosol exposed catalysts were about 1% per day

and 0.4% per day, respectively.

SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt

partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH3 into the

catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5–0.75)

in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst

capacity for NH3 chemisorption decreased as a function of exposure time, which reveals that Brønsted

acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle.

The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but

physical blocking of the surface area may also contribute to the loss of activity in a practical application.

The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired

systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].

2008 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +45 45 25 28 41; fax: +45 45 88 22 58.

E-mail address: aj@kt.dtu.dk (A.D. Jensen).

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journal homepage: www.elsevier.com/locate/apcatb

0926-3373/$ – see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2008.02.019
results of deactivation tests on full-scale plants, and so it is

important to carry out more well-defined experiments where the

catalysts are exposed to aerosols of single well-defined species.

Only few such studies have been conducted in which catalyst

powder or a small catalyst piece was exposed to aerosols of

potassium compounds [9,10]. The exposure time in these

experiments however, was only 30–100 h which is much too

short to observe significant effects on the catalyst activity.

In this work, a layer of commercial KCl powder or ash collected

from the SCR pilot reactor at the biomass fired Masnedø power

plant [1] was loaded on one side of a series of catalyst plates placed

in a lab-scale oven. The idea was to simulate the effect of the

deposits observed previously in the pilot-scale tests [1]. Furthermore, full-length catalyst elements were exposed in a bench-scale

reactor to aerosols of pure KCl or K2SO4 in long-term tests in order

to investigate the relative contribution of these compounds to

catalyst deactivation and compare the results with the observed

deactivation in full-scale [1]. The results of these tests are reported

and the deactivation mechanisms are discussed.

2. Experimental

2.1. Catalysts

Two types of V2O5-WO3-TiO2 catalysts obtained from Haldor

Topsøe A/S were investigated, i.e., plates and full-length monoliths.

The catalyst plates contained 1 wt.% V2O5 and about 7 wt.% WO3 on

a fiber reinforced titania (TiO2) carrier. The active elements were

uniformly distributed over the wall thickness. The plates had

dimensions of 1.3 mm 50 mm 160 mm and were cut into

10 mm 20 mm pieces for exposure and activity test. The

corrugated-type monoliths were similar to the plates but had a

V2O5-content of 3 wt.%. The dimensions of the monoliths were

75 mm 75 mm 500 mm and the hydraulic diameter of the

channels was 6.44 mm. The total specific outer surface area of the

monolith was 455 m2

/m3 and the void factor of the monolith was

0.732. Table 1 shows that the BET surface area of the fresh monolith

catalyst is about 59 m2

/g.

2.2. Exposure test for catalyst plates

Catalyst plates were exposed to a layer of commercial KCl

particles (a mean diameter of about 360 mm, Merck, 104936) in a

lab-scale quartz reactor. Fig. 1 shows the setup and the catalyst

location. About 200 N ml/min compressed air passed through a

water bath at room temperature to be saturated with H2O(g)

(about 3 vol.%), and was then mixed with SO2 to get a concentration of 1000 ppmv SO2. Finally, the gas mixture passed through the

reactor at 350 8C containing nine pieces of the catalyst plate. Only

the three pieces in the middle were covered with a layer of KCl

particles with a thickness of about 2 mm. The three plates in the

upstream position were assumed to have no contact with the

potassium salts or vapor and were used as baseline for comparison.

The three plates in the middle were used to study the effect of the

deposit layer on the catalyst surface. The last three plates in the

downstream position were used to investigate the effect of flue gas

which had been exposed to potassium salts, i.e. the effect of

potassium salt vapor.

In separate experiments catalyst plates were exposed to a 2-

mm-thick layer of fly ash (contained about 55 wt.% KCl and 44 wt.%

K2SO4) collected at the wall of the catalyst box just above the inlet

of the catalyst mounted at the Masnedø SCR pilot plant [1]. In these

tests no SO2 was present in the gas in order not to promote further

sulphation of the KCl in the ash.

After a certain time, the catalysts at the different positions were

tested for activity in the fixed bed quartz reactor described

elsewhere [8]. The gas for these activity tests contained 500 ppmv

NO, 600 ppmv NH3 vol.% O2, and 5 vol.% H2O in N2 and the flow

rate was about 1.2 N l/min. The activities of the catalysts were

measured at 200 8C, 250 8C, 300 8C, 350 8C, and 400 8C, respectively. When NH3 is in excess, as is the case in this work (NH3/

NO = 1.2), the reaction effectively reduces to first order with

respect to NO and zero order with respect to ammonia [11–13]. The

observed catalyst activity that includes the influence of external

and internal mass transfer can then be calculated from [8]:

k 1⁄4  Fgas

mcat

lnð1  XÞ (1)

where Fgas is the gas flow rate (ml/s), mcat is the weight of catalyst

(g), and X is the fractional conversion of NO (%).

2.3. Exposure test for monolith catalyst

2.3.1. Bench-scale reactor

To expose the catalyst to poisons under more realistic conditions

than in the laboratory experiments and study the effect of given

compound, full-length monolith catalysts were placed in a benchscale reactor shown in Fig. 2 and exposed to well-defined aerosols of

KCl or K2SO4. Hot flue gas was produced by a natural gas burner and

the NO level in the flue gas was adjusted by addition of NH3 to the

combustion air. A solution of the salt in distilled water (7.4 g/l KCl or

K2SO4) was prepared and mixed several days before being added to

the system. An aerosol of the salt was then generated by injecting the

solution through a two-fluid nozzle into the flue gas close to the

burner where the temperature was 1000–1150 8C. A bayonet heat

exchanger was inserted into the main duct downstream the

formation of the desired aerosol particles to cool down the flue

gas before the un-lined steel tubes to avoid accelerated corrosion

caused by the potassium compounds.

Upstream of the catalyst, NH3 was added to react with the NO

produced by NH3 oxidation in the burner. The catalyst box was

well insulated and heated by an electrical heating wire. The

exposure temperature was kept at 350 8C and the temperature

difference over the cross section of the catalyst box at the catalyst

inlet was within 4 8C and the temperature difference between the

catalyst inlet and outlet was kept less than 5 8C. A soot blower and

a steel grid were installed 20 cm and 5 cm, respectively, above the

Table 1

BET surface area, BET and Hg-porosimetry pore volume and pore diameter of the

fresh and the first KCl exposed samples

Fresh KCl aerosol exposed

Surface area (m2

/g) 59.43 55.13

Total pore volume (BET) (cm3

/g) 0.17 0.19

Total pore volume (Hg) (cm3

/g) 0.84 0.48

Average pore diameter (BET) (nm) 11.5 13.9

Average pore diameter (Hg) (nm) 97.6 81.7

Fig. 1. Schematic overview of the experimental setup for catalyst plate exposure

test.

Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194 187
catalyst to minimize plugging problems. The soot blower consists

of a steel pipe with a diameter of 16 mm and a hole of 10 mm, a

time controller and connects to compressed air at a pressure of

5 bar. Two additional soot blowers were placed just above the steel

grid to blow away the particles deposited on the mesh. Soot

blowing was carried out by 3–5 s of blowing with compressed air

at an interval of 30 min. The total flow rate at the outlet of the

burner was about 60 N m3

/h and the flow rate through the catalyst

was kept at 40 N m3/h by adjusting the bypass valve, which

corresponds to a channel velocity of 6 m/s at 350 8C, similar to that

of full-scale catalyst operation.

2.3.2. Activity measurement

Activities of the catalysts were measured in situ at 350 8C

typically 2–3 times per day with a flue gas consisting of 500 ppmv

NO, 600 ppmv NH3, 6.5 vol.% O2, 7 vol.% CO2, and about 10–15 vol.%

H2O. As NH3 is in excess, the observed catalyst activity can be

approximated by an observed rate constant k0 [8]:

k0 1⁄4 SV

Ap

lnð1  XÞ (2)

where SV is the space velocity which represents the flow rate of the

flue gas per unit volume of the catalyst element, Ap is the geometric

surface area per unit volume (455 m2

/m3), and X is the fractional

NO conversion. To compare the deactivation rates of different

catalysts, the relative activity was used, which was defined as:

Relative activity 1⁄4 k0

k0

0

(3)

where k0 and k0

0 is the observed rate constant of the NO reduction

reaction for the exposed and fresh catalyst, respectively.

2.3.3. Aerosol measurement

The aerosol particle concentration and size distribution were

measured by a Berner-type low pressure impactor (Hauke model

LPI 25/0.018/2). The impactor is a multistage device in which

aerosol particles are separated into aerodynamic size classes with a

range of 0.028–12.8 mm according to inertia characteristics. It

consists of one pre-impactor stage for removal of large particles

and 10 stages where particles are collected for subsequent

gravimetric analysis. Aluminum foils greased with a thin layer

of Apiezon H high-vacuum grease were used as collection

substrate. The sampling flow rate was 23 N l/min and samples

were taken from about 10 cm above the catalyst entrance through

a straight cylindrical tube perpendicular to the flue gas duct (see

Fig. 2). Prior to sampling, the gas bypassed the impactor for a

period of time to approach steady-state conditions in the sampling

line. Heating tape and mineral wool were wrapped around the

sampling tube and the impactor to keep a temperature of 120 8C in

order to avoid condensation of water vapor.

2.3.4. Ammonia chemisorption

To investigate the mechanisms of deactivation, chemisorption

studies similar to those conducted in our previous laboratory and

pilot-scale studies [1,8], were made typically once per day. Flue gas

produced without NH3 addition to the burner and thus containing

a low level of NO of about 25 ppm and with 600 ppm NH3 added

was first passed over the catalyst for a period of time to saturate the

active sites. The effect of NH3 saturation time was investigated and

it was found that 30 min was sufficient to saturate the catalyst

with NH3 at the current conditions. After the saturation period, the

NH3 addition before the catalyst was shut off, and shortly after

500 ppmv NO was added to the flue gas through NH3 combustion

in the burner. Reaction between NO and chemisorbed NH3 then

took place and the amount of NH3 on the catalyst was calculated

from the amount of NO reduced.

2.4. Catalyst characterization

The distributions of potassium in the exposed catalysts were

investigated at the Centre for Microstructure and Surface Analysis,

Danish Technological Institute by SEM-EDX on a LEO 440

microscope. The samples were prepared by using an MT-990

rotary microtome. The samples were kept dry to prevent any

dissolution, recrystallization, and removal of potassium compounds. Before analysis, the samples were covered with a very thin

carbon film to become conductive. To compare the different

spectra and samples, both carbon and oxygen are excluded from

the analysis results and the amounts of other elements are

recalculated on a carbon and oxygen free basis.

Surface areas by nitrogen adsorption at 77 K were measured on

a Micromeritics ASAP 2000 automatic surface area and pore size

distribution apparatus. About 0.4–0.6 g of sample was cut into

8 mm 8 mm pieces and heated at 350 8C for 1 h before analysis.

The total surface area was calculated by the BET method while the

average pore diameter was calculated from the surface area and

the BET pore volume. To determine the whole range of pore sizes,

the pore diameter and distribution were also measured by Hgporosimetry using a Micromeritics Autopore 9220.

3. Results

3.1. Deactivation of plate catalyst samples

Fig. 3 shows the activities of the catalyst plates at different

positions after exposure of the catalysts in the middle position to

(1) KCl particles at 350 8C for 2397 h including exposure to SO2 in

the period 0–670 h (lower figure), and (2) after exposure to ash

from the Masnedø power plant for 2970 h (upper figure). For the

catalyst plates exposed to KCl particles, the activities of the

catalysts at the ‘before’ and ‘after’ positions were almost the same

as the fresh sample indicating that no or little deactivation has

taken place. The relative activities of the KCl exposed catalysts in

the middle position were about 87% at 350 8C indicating that some

deactivation has taken place. For catalyst plates exposed to

Masnedø ash at 350 8C for 2970 h, no loss in activity was observed.

3.2. Characteristics of aerosol produced in the bench-scale reactor

To compare the aerosol concentration in the flue gas from the

bench-scale reactor with that at the Masnedø power plant, the

measured aerosol concentration was converted to a reference state

of dry gas at 273.15 K, 101.3 kpa, and an O2 concentration of

6 vol.%. Initially, it was planned to add at least half of the aerosol

Fig. 2. Schematic drawing of the bench-scale reactor for catalyst monolith

deactivation test.

188 Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194
load at the Masnedø SCR plant, which was about 580 mg/(N m3

)

[1]. When 835 ml/h of 37.3 g/l KCl solution was added to 60 N m3

/h

flue gas, corresponding to an aerosol load of 520 mg/(N m3

), a

concentration of 197 mg/(N m3

) aerosol was measured at the inlet

of the catalyst. Similarly 111 mg/(N m3

) aerosol was measured at

the catalyst inlet when nominally adding 260 mg/(N m3

). In both

cases only about 40% of the KCl particles reached the catalyst while

the remaining were deposited in the reactor system before the

catalyst.

When adding 835 ml/h of 37.3 g/l KCl the reactor could only run

continuously for about 10 days. Then the heat exchangers became

plugged and the pressure in the burner became too high and the

burner automatically shut down for safety reasons. Therefore, the

addition of KCl was decreased and only 436 ml/h of 7.4 g/l KCl

solution corresponding to a calculated aerosol concentration of

53 mg/(N m3

) was added. The gas temperature at the point of KCl

solution injection was 1050 8C. This allowed continuous operation

for about two months. The same loading was used for the addition

of K2SO4, but the gas temperature was raised to 1120 8C to enhance

vaporization.

To keep track of the aerosol concentration at the catalyst inlet, it

was measured at a typical frequency of once per week by sampling

3.5 N l/min gas just above the catalyst inlet through a heated

aerosol filter and weighing the mass change of the filter. The

aerosol concentration increased over time from about 20 mg/

(N m3

) to 40 mg/(N m3) during the first exposure test of KCl

aerosols. This was probably due to the accumulation of particles in

the system that results in re-entrainment of the previously

deposited KCl.

Fig. 4 shows the measured mass distributions of the aerodynamic diameters for KCl and K2SO4 aerosols in the bench-scale

reactor and for the aerosol sampled at the Masnedø power plant.

For the KCl and Masnedø power plant aerosols (lower figure), the

measured size distributions were unimodal. The geometric mean

mass diameter was about 0.3 mm for KCl which is smaller than the

0.7 mm measured at the Masnedø SCR plant. However, similar mass

mean diameter of 0.3–0.5 mm were obtained in other Danish

straw-fired grate boiler [14].

The mass distribution of the K2SO4 aerosol is also shown in Fig. 4

(upper figure). In contrast to the KCl aerosol, there were two distinct

peaks at 0.07 mm and 1.05 mm, respectively. The mass mean

diameter was 0.55 mm. The different size distributions of KCl and

K2SO4 aerosol could be due to the different aerosol dynamics of KCl

and K2SO4. The concentration of the K2SO4 aerosol was almost

constant at 20 mg/(N m3

) for the first 2100 h, but then increased to

30 mg/(N m3

) at the end of the test. Besides re-entrainment of

previously deposited K2SO4particles, another possible reason for the

change in load could be the change of the nozzle. In the last exposure

period, the nozzle was corroded and a new nozzle of the same type

was inserted. The data in Fig. 4 were obtained using the first nozzle.

3.3. Deactivation of monolith catalyst by exposing to KCl aerosol

Two monolith catalysts were exposed to KCl aerosols at 350 8C

and the catalyst activities as a function of exposure time to the flue

gas are shown in Figs. 5 and 6. As shown in Fig. 5 the catalyst

activity was constant during the first 340 h when no KCl was

added. There was a decrease of the activity (9%) at 412 h which

might be caused by the removal of the fresh catalyst from the

reactor for about 130 h during maintenance of the burner. The

catalyst was further removed for about 1300 h during a later

maintenance of the reactor and aerosol measurement in the empty

reactor. There was a sharp drop (12%) of the activity at an exposure

time of 436 h when the catalyst was re-installed after KCl (load of

260–520 mg/(N m3

)) had been added for 3 days to make the

aerosol measurement. When KCl was added continuously at a level

Fig. 3. Catalytic activities of the 1 wt.% V2O5-WO3-TiO2 catalyst plates as a function

of temperature: (downside) catalyst plates exposed to the KCl particles for 2397 h

including exposure to gaseous SO2 for 670 h, (upside) catalyst plates exposed

2970 h to ash collected at the catalyst box just above the catalyst inlet at the SCR

pilot plant installed at Masnedø straw-fired power plant.

Fig. 4. Aerosol mass size distributions measured by the Berner-type multijet low

pressure impactor at the entrance of the catalyst in the bench reactor, converted to the

reference state, which is a dry gas at 273.15 K, 101.3 kPa, and an O2 concentration of

6 vol.%. Upper figure: K2SO4 aerosol produced by adding 436 ml/h of 7.4 g/l K2SO4

solution 1120 8C. Lower figure: KCl aerosol produced by adding 420 ml/h of 37.3 g/l

KCl solution at 1050 8C and aerosol measured at Masnedø SCR pilot plant.

Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194 189
of 53 mg/(N m3

), the activity kept decreasing. At about 750 h,

1000 ppmv SO2 was added for 3 h to check the system flow rate,

and the relative activity increased from 51% to 56%. This is in good

agreement with the Swedish study [15] where sulphation of the

catalyst by SO2 was conducted to regenerate the catalyst by

increasing its acidity. The activity gain lasted for 3 days and then

disappeared.

After about 1250 h, the pressure drop over the catalyst became

high and automatic soot blowing every 30 min was initiated. The

enhanced soot blowing also gave a rise of the activity of 5% probably

due to cleaning of partly blocked channels. After exposure to the

pure KCl aerosols for about 1830 h (total exposure time of 2440 h),

the relative activity was only 18%. At the end of the test the reactor

was opened and both the plugged channels of the catalyst and the

reactor system were cleaned. The catalyst was tested for another

400 h without KCl addition and the relative activity leveled off at 20%

as mentioned. The concentration of KCl aerosols in the bench-scale

reactor was 10–20 times lower than in the SCR pilot plant at the

Masnedø straw-fired power plant. It is interesting to note that in

spite of the much lower aerosol concentration in the bench-scale

reactor during addition of pure KCl a deactivation rate similar to that

at Masnedø of about 1% per day, was observed.

In the first test reported above, the final activity level was very

low. A second test was carried out to investigate the repeatability

of the experiment, and to confirm that catalyst deactivation stops

when no further KCl is added, also when a significant part of the

catalyst activity is still remaining. The soot blowing and the NH3

chemisorption tests were carried out from the beginning of the

second test. Fig. 6 shows that after exposing to KCl aerosols at

350 8C for 715 h the catalyst relative activity was about 69%, which

corresponds to a deactivation of about 1.0% per day, in good

agreement with the first test. There was also a fast decrease of the

adsorption capacity for NH3 on the catalyst as shown in Fig. 6. Then

the system was cleaned and without addition of KCl the catalyst

was tested for another 235 h. In this period the catalyst activity

remained at about 69% and the amount of chemisorbed ammonia

also leveled off.

3.4. Deactivation of monolith catalyst by exposing to K2SO4 aerosol

The catalyst activity as a function of exposure time to K2SO4 is

shown in Fig. 7. The relative activity decreased to 86% during the

first 300 h, corresponding to a deactivation rate of 1.1% per day. As

shown in a later section, this could be due to the deposited KCl in

the system from previous test. Elemental analysis revealed that

about 0.50 wt.% Cl was found in the catalyst structure. Apparently,

the system has not been cleaned thoroughly enough between the

tests. During the next 200 h, the relative activity fluctuated around

90%. Then the relative activity kept decreasing at a rate of about

0.4% per day; i.e. only 40% of the deactivation rate observed for

catalysts exposed to KCl at a similar aerosol load and at Masnedø

power plant. The pressure drop through the catalyst was almost

constant at about 1.6 mbar except for a slight increase at the end.

Similar to the KCl aerosol exposure test, the catalyst capacity for

NH3 chemisorption decreased faster than the relative activity.

Calculation of the effectiveness factor of the monolith shows that it

is very low and the reaction takes place mainly close to the outer

surface. Also, the fractional resistance to reaction by external mass

transfer is around 70%. The active sites within the catalyst

Fig. 5. Activities, chemisorbed NH3, and pressure drop on the catalyst during the

first KCl aerosols exposure test at 350 8C as a function of exposure time.

Fig. 6. Activities and chemisorbed NH3 on the catalyst during the second KCl

aerosols exposure test at 350 8C as a function of exposure time.

Fig. 7. Activities and chemisorbed NH3 on the catalyst during the K2SO4 aerosols

exposure test at 350 8C as a function of exposure time. Activities of catalyst exposed

at Masnedø SCR plant for 1140 h at 350 8C are shown for comparison.

190 Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194
structure, although not participating in the SCR reaction, do react

with potassium resulting in a faster decrease in the catalyst

capacity for NH3 chemisorption than the observed activity which is

damped by the mass transfer resistances. Although there was a

somewhat higher aerosol load in the last exposure period the

deactivation rate did not increase.

3.5. BET and Hg-porosimetry measurement

As shown in Table 1 the BET surface area of the exposed

catalysts was slightly smaller than those of the fresh samples. The

Hg-porosimetry measurement shows that total pore volume and

average pore diameter decreased after exposing to KCl aerosols,

indicating that some macropores are plugged by KCl aerosols.

3.6. SEM-EDX analysis

Fig. 8a and b shows the SEM images of the upper and

undersurface of the catalyst plate exposed to a layer of KCl

particles at 350 8C for 2397 h (with 1000 ppmv SO2 for 670 h),

respectively. Few particles were found on the catalyst undersurface (Fig. 8b). These particles were probably from the catalyst

material during the cutting of the sample before analysis.

Elemental analysis showed that there was no potassium

and chloride on the undersurface or in the dispersed particles,

confirming that the few particles on the undersurface were

from the catalyst matrix. It can be seen clearly from Fig. 8a that

there were sintered KCl particles on the upper surface. There were

several big particles with diameters of 2–4 mm and many small

particles agglomerated on the surface. The average size of the

commercial KCl particle was about 360 mm and the KCl layer was

pressed onto the catalyst to avoid blowing it away by the air stream

in the reactor. There was almost no chloride in the catalyst

structure, and potassium chloride was sulphated to potassium

sulphate on the catalyst. Fig. 9 shows the potassium distribution

along the catalyst thickness and it can be seen that potassium

penetrated into about 10% of the wall thickness. However, for the

catalyst exposed to ash collected at Masnedø, no potassium was

found on either upper or undersurfaces.

Fig. 8c presents the surface analysis of the catalyst exposed to

pure KCl aerosols for 1830 h (total exposure time of 2440 h) in the

bench-scale reactor. There were some loose particles deposited on

the surface and large pores were partly filled. SEM analysis at high

magnification (not shown here) showed that the size of the

particles were quite homogeneous and in agreement with the

measurement by the low pressure impactor.

Somewhat surprisingly, the elemental analysis by EDX showed

that the aerosols deposited on the first KCl aerosol exposed

catalyst were mainly in the form of potassium sulphate. The

average sulphur content at the surface (analysis down to a depth

of 73 mm) was 3.35 wt.% on carbon and oxygen free basis, while

the average sulphur content through the thickness of the wall was

about 0.9 wt.%. Compared to the sulphur content in the fresh

catalyst (about 1 wt.%) [16–18], the addition of sulphur to the

catalyst by exposing the catalyst to 1000 ppmv SO2 for 3 h was

negligible. However, the promoting effect of surface sulphates

was significant. When 1000 ppmv SO2 was added for 3 h to

calibrate the system flow rate, the activity increased from 51% to

56%. This is probably because sulphate species on the catalyst

support interact with adsorbed water to form Brønsted acid sites

on the catalyst surface under SCR conditions, promoting the

adsorption of ammonia [19–21]. Studied by in situ Raman

spectroscopy suggests that the SO2 presence affects the molecular

structure of catalysts with low vanadia coverage through driving

Fig. 8. SEM images of the exposed catalysts: (a) upper surface of the catalyst plate in the middle position exposed to a layer of KCl particles at 350 8C for 2397 h (with

1000 ppmv SO2 for 670 h), (b) undersurface of the catalyst plate in the middle position exposed to a layer of KCl particles at 350 8C for 2397 h (with 1000 ppmv SO2 for 670 h),

(c) surface of monolith catalyst exposed to KCl aerosol in the bench-scale reactor for 1830 h (total exposure time of 2440 h) and (d) surface of monolith catalyst exposed to

K2SO4 aerosol in the bench-scale reactor for 2707 h.

Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194 191
the dispersed vanadia species in a state of ‘virtually’ high surface

density by crowding them together, thereby providing more

adjacent V sites for activation of NH3 in SCR reaction conditions

[22]. In contrast to the distribution of sulphur, the contents of

chlorine at the surface and in the catalyst structure were similar

with an average content of about 0.33 wt.%.

Fig. 9 shows the profile of K/V molar ratio along the wall

thickness of the first KCl aerosol exposed catalyst. There was a fast

decrease of the K/V ratio from the surface into the catalyst structure.

At a depth of about 200 mm, there was an average K/V ratio of about

0.75 in the catalyst structure. Such a K/V ratio is high enough for a

significant chemical deactivation. Our lab-scale investigation [8]

showed that the remaining activity of a 3 wt.% V2O5 catalyst doped

with a K/V of 0.8 was about 10%. As discussed earlier, the remaining

activity after exposure to the pure KCl aerosols for about 1830 h was

only 18%, in reasonable agreement with the lab-scale data [8].

SEM analysis of the surface of catalyst exposed to K2SO4

aerosols for 2707 h in the bench reactor also showed loose particles

deposited on the surface as shown in Fig. 8d. Elemental analysis

showed that the sulphur content on the catalyst surface and in the

deposit was 0.94 wt.% and 3.16 wt.%, respectively, indicating that

sulphur level within the catalyst did not increase due to exposure

to K2SO4. The cross-sectional analysis showed that the average K/V

molar ratio in the catalyst structure was about 0.49, which was

lower than that in the catalyst exposed to KCl aerosols for 1830 h

(as shown in Fig. 9). There was about 0.3–0.5 wt.% Cl in the catalyst

structure. As mentioned earlier, this could be due to exposure to

KCl even in the K2SO4 test because the reactor was difficult to clean

completely after the addition of the KCl.

3.7. Catalyst deactivation mechanisms

The catalyst deactivation can be caused by several mechanisms,

namely, a decrease of the number of active sites, a decrease of the

turn over frequency of the active sites, and loss of accessibility of

the pore space [23]. The possible influence of each of these

mechanisms in the deactivation of SCR catalysts when exposed to a

potassium-containing aerosol is discussed as following.

3.7.1. Deactivation by fouling and channel plugging

The experiments and the SEM analysis showed that the

catalysts had a high risk of fouling and channel plugging, and

powerful and frequent soot blowing was necessary to keep the

catalyst clean. Thus, fouling appears to be a more severe problem

than in pure coal-fired applications. This could be due to the fine

KCl and K2SO4 aerosols produced in the bench-scale reactor, which

causes more particles to deposit by Brownian and turbulent

diffusion. Furthermore there are no big particles in the flue gas to

sand blast the catalyst surface. It was observed that plugging of

channels takes place mainly close to the inlet. This may be caused

by impaction of particles against the edge of the channel, which

then accelerates the building up of deposits [24]. Additionally, KCl

and K2SO4 have a lower melting point than typical coal fly ash and

this may enhance sticking of the particles to the catalyst surface. It

is clear that if a channel becomes plugged it will not contribute

much to the activity. This may be a severe deactivation mechanism

in full-scale where soot blowing cannot be carried out as frequent

as in the present experiment. In the present experiments, channel

plugging did not contribute much to the observed loss of activity,

but fouling may have contributed to an unknown, but rather small

extent.

3.7.2. Deactivation by poisoning and mechanisms of potassium

penetration and accumulation

The SEM-EDX analysis shows that potassium penetrated into

the catalyst wall and the NH3 chemisorption measurement

revealed that the Brønsted acid sites had reacted with poison

and rendered inactive for NH3 adsorption and subsequent reaction

with NO. This chemical poisoning has been proposed by our

previous study using potassium doped catalyst [8]. This mechanism can be supported by investigation of the potassium doped

catalysts at molecular level [25,26]. In situ Raman spectroscopy

revealed that potassium alters the vanadium surface species in the

SCR reaction, lengthening the V O bond. However, the detailed

mechanisms by which the potassium penetrates the catalyst to the

active sites are unknown. Based on the investigation at the

Masnedø SCR pilot plant, several possible mechanisms for

potassium accumulation and penetration were proposed by the

present authors [1] and will be discussed below in the light of the

results of the present work.

Deactivation due to diffusion of potassium vapor into the

catalyst was ruled out in [1] since the calculated vapor pressures of

KCl and K2SO4 at 350 8C indicate that this mechanism is much too

slow. This is supported by the observation in [1] and the present

work that without aerosols in the flue gas deactivation of the

monolith catalysts due to potassium salt fouling layers does not

take place. The fact that no deactivation of the catalyst plates in the

‘after’ position during the lab-scale exposure test further excludes

deactivation by potassium vapor at 350 8C.

In [1] we also discussed whether the deactivation could be due to

formation of liquid potassium-vanadium-pyrosulphates. Such

species are the active phase in SO2 to SO3 oxidation catalysts, and

could in principle be formed in the present systems thereby

destroying the active sites in the SCR reaction. In full-scale tests SO2

will always be present, but with the bench-scale reactor of this

study, experiments in complete absence of SO2 are possible. The

results in Fig. 5 shows that the catalyst activity does decrease by

exposure to pure KCl aerosols in the absence of SO2, i.e. under

conditions when the pyrosulphate mechanism could not have been

active. The rate of deactivation, about 1% per day, is similar in fullscale and in the present experiments indicating that the pyrosulphate mechanism did not play a role in the full-scale tests either.

The slight deactivation of catalyst plates by direct contact with

pure KCl or ash collected at the Masnedø SCR pilot plant indicates

Fig. 9. Potassium distribution along the thickness of the catalysts exposed at 350 8C

to KCl aerosol for 1830 h, K2SO4 aerosol for 2707 h, and KCl particles for 2397 h.

192 Y. Zheng et al. / Applied Catalysis B: Environmental 83 (2008) 186–194
that the deactivation by deposits on the catalyst is a slow process.

When KCl addition in the bench-scale reactor was disrupted, the

catalyst relative activity and chemisorbed ammonia leveled off

both at a low and high level of poisoning when the relative

activities were 20% and 69%, respectively. This supports the

findings of our previous work where it was observed that when the

power plant ran few hours daily the catalyst activity leveled off [1].

If the pyrosulphates mechanism was active it would be expected

that deposits on the catalyst surface would cause continuous

deactivation. However, the results show that without an aerosol in

the flue gas, no further deactivation takes place. This indicates that

the potassium in the deposits on the catalyst wall is not capable of

penetrating and deactivating the catalyst, or at least it is very slow.

As mentioned above a continuous flow of aerosol through the

catalyst is required for deactivation, while the deposits of Merck

KCl that contain rather large particles only cause slow deactivation

(about an activity decrease of 0.1% per day with only one side

contacting the deposit). This indicates that it is only the finest

fraction of the aerosol that is involved in the deactivation. These

particles are easily deposited due to their high diffusion coefficient,

and the contact area with the wall, per mass of aerosol will be large.

Fig. 10 shows the aerosol diffusion coefficient at 350 8C, the

distribution of the catalyst pore size and the size distribution of

K2SO4 aerosol produced at the bench reactor. The fine aerosol

particles may directly diffuse into the catalyst structure since there

is some overlap between the size distribution of the fine aerosols

and pore sizes of the catalyst. This mechanism is further supported

by a recent Swedish study [27]. They suggested that mainly alkali

in ultra fine particles (<100 nm) in the flue gas caused the alkali

accumulation on the catalyst samples exposed to the flue gas from

biofuel and peat fired boilers. The catalysts investigated in our

work use fiber reinforced TiO2 as carrier and have larger pores

compared to extruded or plate type catalysts. It is expected that

fine particles diffuse into the large pores (>1 mm) of the catalyst

used in the present work. Within the catalyst wall potassium reacts

with the V–OH group and turns the active sites to –V–O–K and

through surface diffusion these are distributed throughout the

wall.

Our results from the bench-scale reactor show that deactivation

of SCR catalyst by KCl aerosol is faster than that by K2SO4 aerosol.

However, our lab-scale investigation of the SCR catalyst deactivation by wet impregnation showed that K2SO4 is only a slightly

weaker poison compared to KCl [8]. For potassium in the form of

aerosol, the different mobility (Tamman and Hu ̈ttig temperature)

of KCl and K2SO4 could be the reason for the difference between the

deactivations caused by KCl and K2SO4. For KCl the THu ̈ttig and

TTamman temperatures are 40 8C and 249 8C and for K2SO4 the

temperatures are 129 8C and 397 8C, respectively. This implies that

the mobility of KCl is larger than K2SO4 and therefore penetrates

the catalyst faster. Another reason could be that the mass mean

diameter of K2SO4 aerosols were higher than that of KCl aerosols.

The bigger particles have more difficulty to penetrate into the

catalyst and also less is deposited.

4. Conclusions

Deactivation of a commercial type V2O5-WO3-TiO2 catalyst

was investigated by exposing the catalyst plates to a layer of KCl

particles or ash at a lab-scale reactor and full-length monolith

catalysts at a bench-scale reactor with pure KCl or K2SO4

aerosols.

Slow deactivation (about 10%) was observed for catalyst plates

exposed to a layer of KCl particles at 350 8C for 2397 h with SO2

addition for 670 h. After 1970 h, no deactivation was found for

catalyst plates exposed to ash collected from a SCR pilot plant

installed at a straw-fired power plant. SEM-EDX analysis showed

that potassium penetrated only about 10% of the catalyst thickness

from the contacting surface of KCl but no potassium on the surface

and in the structure of the catalyst plates exposed to ash was

found.

A fast deactivationwas observed for catalysts exposed to pure KCl

or K2SO4 aerosols at 350 8C in the bench-scale reactor. The

deactivation rates for KCl and K2SO4 aerosols exposed catalysts

were about 1% per day and 0.4% per day, respectively. Deactivated

samples were characterized by BET, Hg-porosimetry, SEM-EDX, NH3

chemisorption. SEM analysis revealed that potassium partly

deposited on the catalyst outer wall which may have slightly

decreased the diffusion rate of NO and NH3 into the catalyst.

However, potassium also penetrated into the catalyst wall and the

average K/V ratios (0.5–0.75) in the catalyst structure were high

enough to explain most or all of the activity loss by chemical

deactivation. Chemical poisoning was further supported by

ammonia chemisorption measurements showing that the amount

of chemisorbed NH3 on the catalyst decreased as a function of

exposure time, which reveals that Brønsted acid sites had reacted

with poison and rendered inactive. Overall, the conclusion is that

chemical poisoning of active sites is the main cause of the

deactivation while physical blocking of the outer wall surface area

also contribute to the loss of activity. The results support our

previous conclusion that the deactivation of SCR catalyst exposed to

potassium-containing aerosols is caused by diffusion of fine

potassium-containing aerosol into the catalyst pores, and subsequent diffusion into the structure, for example by surface diffusion.

Acknowledgements

This work is supported by the EU project ‘Influences from

Biofuel (Co-) Combustion on Catalytic Converters in Coal-fired

Power Plants’ (NNE5-2001-00164). The authors thank Bodil

Taarnskov for the BET and Hg-porosimetry analysis. Many thanks

to the aerosol group at the Department of Chemical Engineering for

help with the aerosol measurement.

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