Alkali deactivation of high-dust SCR catalysts used for NOx reduction

exposed to flue gas from 100 MW-scale biofuel and peat fired

boilers: Influence of flue gas composition

A ̊sa Kling a,

*, Christer Andersson b

, A ̊se Myringer c, David Eskilsson d, Sven G. Ja ̈ra ̊s e

a Vattenfall AB Va ̈rme Uppsala, SE-753 82 Uppsala, Sweden b Vattenfall AB Va ̈rme Norden, SE-814 26 A ̈lvkarleby, Sweden c Vattenfall Utveckling AB, SE-162 87 Stockholm, Sweden d Swedish National Testing and Research Institute, SP, Box 857, SE-501 15 Bora ̊s, Sweden eKTH, Royal Institute of Technology, Department of Chemical Engineering and Technology,

Chemical Technology, SE-100 44 Stockholm, Sweden

Received 8 November 2005; received in revised form 15 March 2006; accepted 24 March 2006

Available online 24 August 2006

Abstract

Deactivation of vanadium–titanium deNOx SCR (selective catalytic reduction) catalysts in high-dust position have been investigated in three

100 MW-scale boilers during biofuel and peat combustion. The deactivation of the catalyst samples has been correlated to the corresponding flue

gas composition in the boilers. Too investigate the effect on catalyst deactivation a sulphate-containing additive was sprayed into one of the

furnaces. Increased alkali content on the SCR catalyst samples decreased the catalytic deNOx activity. The study has shown a linear correlation

between exposure time in the boilers and alkali concentration (mainly potassium) on the samples. The results imply that mainly alkali in ultra fine

particles (<100 nm) in the flue gas increased the alkali accumulation on the catalyst samples. Low correlation was found between particles larger

than 100 nm and the catalyst deactivation. It was not possible to decrease the deactivation of the catalyst samples by the sulphate-containing

additive. Although the additive had an effect in sulphating potassium chloride to potassium sulphate, it did not decrease the amount of potassium in

ultra fine particles or the deactivation of the catalyst samples.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Selective catalytic reduction; Nitrogen oxides; Combustion; Biofuel; Alkali; Potassium; Particles; Deactivation

1. Introduction

Selective catalytic reduction (SCR) of nitrogen oxides

(NOx) has been recognised as an effective post-combustion

method for reducing NOx from stationary sources. One of the

commercial SCR catalysts contains TiO2 for high surface area

support and V2O5 as active catalytic compound. The catalyst

is promoted by tungsten or molybdenum. By addition of

ammonia, the SCR catalyst can reduce more than 90% of the

NOx emissions in the flue gas. At least five biofuel-fired

combined heat and power plants in Sweden are today using

this kind of SCR in high-dust positions for NOx reduction. The

experiences from these boilers are high catalyst deactivation,

2–4 times higher compared with coal-combustion and the

main deactivating compound was found to be potassium [1].

Similar deactivation of titanium–vanadium SCR-catalysts has

been observed during co-combustion of straw and wood in

Denmark [2].

One of the strongest poisons for SCR catalysts is potassium

[1–9]. Potassium preferentially co-ordinates to the hydroxyl

groups of V2O5 and probably also to the promoter and the

catalytic activity decreases with the decrease of both the

number and the strength of the Brønsted acid sites [9]. A

number of studies on wet impregnation with different

potassium compounds on SCR catalysts showed that K2O,

K2SO4 and KCl all give high catalyst deactivation [4–7,9–11].

The combustion method has been found to have a major

impact on the catalyst deactivation in wood-fired boilers [1].

Catalyst samples showed a more severe deactivation during

pulverised fuel combustion exposure than with exposure during

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 69 (2007) 240–251

* Corresponding author. Tel.: +46 18 269788; fax: +46 18 146657.

E-mail address: asa.kling@vattenfall.com (A ̊ . Kling).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.03.022
circulating fluidised bed combustion. The results indicated that

only specific forms of potassium deactivated the catalyst [1].

The total amount of potassium in the flue gas did not correlate

to the catalyst deactivation.

Potassium is usually the dominant alkali metal in biofuels,

unlike in coal where sodium dominates. The potassium is also,

in biomass, bound predominantly as salts or organically bound

in ion-exchangeable form, which results in high volatilisation

during combustion. Depending on the fuel composition the

potassium is bound mainly to either sulphate or chloride in the

flue gas. As the flue gas temperature decreases the potassium

forms submicron particles by homogeneous or heterogeneous

condensation. At SCR catalyst operation temperatures,

between 300 and 400 8C, no or low amounts of potassium

vapour are thermodynamically, stable [12]. The smallest

particles (<0.5 mm) will however be dominated by potassium,

sulphur and chloride [12–17].

The aim of this work was to investigate, in full-scale boilers,

the correlation between SCR catalyst deactivation and the

different alkali aerosol components in the flue gas. The aim was

also to investigate the possibility of using a sulphate-additive to

decrease the deactivation and prolong the catalyst lifetime.

2. Experimental

2.1. Catalyst exposure

Three types of honeycomb V2O5/TiO2 deNOx SCR catalysts

have been used in the study (type A, B and C). The main

difference between the types is the vanadium content. The type

A catalyst has an activity typical for coal-fired applications

(high SO2 content in the flue gas) with low vanadium content

and low deNOx activity per m3

. The type B is a ‘bio-optimised’

catalyst with increased vanadium content for catalysts used in

applications with low SO2 content in the flue gas. Type C is a

catalyst tested for high alkali metal flue gas environments with

very high vanadium content.

The samples have been exposed from 100 to 3000 h using a

simple testing device called a sample bench. The sample-bench

can expose up to 36 honeycomb samples with a size of

20 mm 20 mm inlet area and a length of 90 mm. The samples

each consist of nine channels. The bench holds the samples

directly in the flue gas channel, Fig. 1. The sample bench was

placed between the economisers, at a temperature of 300–

400 8C, in ‘high-dust’ position before dust collection. The

pressure drop over the catalyst samples influences the flue gas

flow through the samples. However, at least 50% of the flue gas

passing over the sample area will go through the samples

(calculated on the pressure balance in the system). Exposure of

this kind of short samples gives a larger deactivation compared

to a full-length catalyst [1,4]. That is mainly a consequence of

turbulence at the inlet of the catalyst, before the laminar flow is

attained.

Three full-scale combined heat and power plants have been

used for the tests. Johannes, situated in Ga ̈vle, has a thermal

capacity of 90 MW and is using bubbling fluidised bed

combustion with a fuel-mix of bark and demolition wood.

The combustion temperature is about 800 8C. Brista, situated

outside Stockholm, has a thermal capacity of 120 MW and is

using forest residues as fuel with circulating fluidised bed

combustion. The combustion temperature is 850–880 8C. Brista

has a combined deNOx cleaning system with a SNCR (Selective

Non-Catalytic Reduction) in combination with a SCR (35 m3

).

The Uppsala combined heat and power plant has a thermal

capacity of 400 MW burning a fuel-mix of peat and wood powder

with pulverised fuel combustion in a Benson boiler. The

combustion temperature is about 1500 8C. An SCR-catalyst

(100 m3

) was installed during 2005 in combination with the

existing SNCR-system. In Table 1 the different boilers, their

thermal capacity, combustion principle, fuels, furnace additive,

A ̊. Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251 241

Fig. 1. Schematic overview of a catalytic sample bench in a plant with bubbling fluidised bed combustion.
sample bench exposure temperatures, flue gas velocities and

SCR sample space velocities (flue gas in relation to volume of

catalyst) are given. The SCR samples have been exposed during a

total period of 5 years but only during the cold period of the year,

from December until April called combustion season in the

paper, these seasons have been named A, B, etc.

No complete fuel analyses were conducted during the

sample exposure periods. Average fuel compositions have been

summarised in Table 2. Fuel samples (forest residue) from the

Brista combined heat and power plant were analysed for

chlorine content. The fuel (0.1–0.3 g) was adsorbed with

hydrogen peroxide and sodium hydroxide and combusted with

oxygen (Scho ̈niger combustion method). The solution was

analysed using ion chromatography.

Sulphate, a patented additive [22], was added in the Brista

combined heat and power plant for two catalyst sample test

periods. The additive was primarily used to reduce superheater

corrosion by sulphation of KCl to K2SO4, which decreases

chlorine-induced corrosion in biofuel boilers [21,22]. The

sulphate was added to the furnace before the superheaters at a

temperature of 800–900 8C, in a water suspension. As the water

suspension reaches the furnace it dissolves into (gaseous) SO3.

The sulphate was added with sulphur to chloride mol ratio of

1:1 (chloride content in the form of potassium chloride

compared with the added sulphate).

The catalytic activity was measured using a small laboratory

test-rig. The gas stream contained (on a volume basis) 500 ppm

NO, 600 ppm NH3 and 4% of oxygen in N2. No water was

added. The total flow rate was 7.5 l/min. Mass flow controllers

that provide a constant flow controlled the flow rates of the

individual gases, Fig. 2. The space velocity of the experiments

was 6000 h1

. The reactor was aluminium-based with an

internal surface of stainless steel. The activity of the catalyst

was measured at 300 8C. The catalytic activity was calculated

according to the rate expression (1)

kNO 1⁄4 AV lnð1  hÞ (1)

where kNO is the catalytic activity, AV = Q/A; Q flow (cm3

/min)

and A area (cm2

) and 1  h = NOout/NOin.

As the flue gas in the test rig was not representative for the

flue gases in the full-scale boilers the absolute activity has not

A ̊ 242 . Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251

Table 1

Boilers used in the study: thermal capacity, fuel, additive, sample exposure temperature, flue gas velocity over the sample bench and SCR sample space velocity

Name Seasona Boiler

type

Maximal thermal

capacity (MW)

Fuel Furnace

additive

SCR-sample

temperature (8C)

Flue gas

velocity (m/s)

Sample space

velocity (h1

)

Johannes A BFBb 90 Bark + 15% demolition

wood (B + 15% DW)

270 3.9 156000

Johannes B BFBb 90 Bark + 30% demolition

wood (B + 30% DW)

270 3.9 156000

Brista A CFBc 120 Forest residues (FR) 370 3.2 128000

Brista B CFBc 120 Forest residues (FR) 370 3.2 128000

Brista C CFBc 120 Forest residues (FR) Sulphur (S) 370 3.2 128000

Brista D CFBc 120 Forest residues (FR) Sulphur (S) 370 3.2 128000

Uppsala A PCd 400 Peat + 30% wood

(P + 30% W)

370 10 400000

Uppsala B PCd 400 Peat + 20% wood

(P + 20% W)

370 10 400000

Uppsala C PCd 400 Peat (P) 370 10 400000

a The cold period of the year from December until April. b Bubbling fluidised bed.

c Circulating fluidised bed. d Pulverised combustion.

Fig. 2. Measurement equipment for catalytic deNOx activity.

Table 2

Standard fuel content of potassium, chlorine, sulphur, sodium and ash for the different fuels used in the study, the different shares of fuels have been calculated based

on energy content [19,20]

Fuel K (mg/kg ds) Cl (mg/kg ds) S (mg/kg ds) Na (mg/kg ds) Ash content (%)

Peat + 15% wood (1) 434 100 2300 218 5.4

Forest residues (11) 1500  480 211  66 384  130 275  190 3.1

Demolition wood (32) 1165  580 641  550 788  790 665  300 5
be used for the evaluation of data. Instead the relative activity

has been used in the paper. The relative activity has been

defined as the activity of a deactivated catalyst compared to the

corresponding fresh sample.

2.2. Catalyst characterisation

The distribution of different chemical species on the catalyst

samples was investigated by scanning electron microscope/

energy-dispersive X-ray (SEM-EDX) on a Hitachi S-3000N,

with an Oxford 7021 X-ray diffractor. A standard resolution of

100 was used but up to 3500 was used for some samples.

Wet chemical analyses were made for potassium, sodium,

chlorine, sulphate, ammonium, lead and zinc. The samples

were analysed with ion chromatography after water leaching of

the samples (285 ml of deionised water in 16 h at a temperature

of 90 8C). For lead and zinc the samples were also placed in

(100 ml, 7 M) nitric acid and analysed using atom absorption

spectrophotometer.

Surface area and nitrogen adsorption data at 77 K were

measured on a Micrometrics ASAP 2000 automatic surface

area and pore-size distribution apparatus. The total surface area

was evaluated by the Brunauer–Emmett–Teller (BET) method,

while the average pore diameter was calculated from the

surface area and the BET pore volume.

2.3. Flue gas characterisation

The flue gas composition of SO2, O2 and CO was

continuously analysed by the instruments for operations

monitoring in all plants. Measurement of alkali chloride

content was performed in the Brista CHP using in situ alkali

chloride monitoring (IACM), developed at Vattenfall Utveckling AB [23]. The instrument measures the sum of the sodium

and potassium chloride concentrations and cannot differentiate

between these two species. The instrument also measures

sulphur dioxide. IACM is an optical UV instrument based on

absorption spectroscopy and has a sampling time of 5–10 s.

Sodium chloride absorption is seen between 194 and 235 nm in

the temperature interval 850–1350 8C, the corresponding

absorption for potassium chloride appears at 198–246 nm

and is observed between 800 and 1300 8C. Each absorption

spectrum was evaluated by DOAS, differential optical

absorption spectroscopy, a technique for evaluating absorption

spectra of complex mixtures. The light source in IACM is a

xenon lamp; the light is sent across the furnace or flue gas

channel by several optical units, Fig. 3.

Particles were sampled in the same measurement position as

the catalyst sample bench was placed. The fine particles

originating from combustion were sampled iso-kinetically

through a heated sampling line. The mass size distributions

were measured using two pre-cyclones in series with a lowpressure impactor (DLPI, dekati low-pressure impactor) and a

filter. The pre-cyclones were used for separating the larger

particles (>2.0 mm) in high concentration in order to obtain a

longer measurement time with the DLPI. Nine different filter

sizes were used with an aerodynamic cut diameter from 0.030

to 2.0 mm. An additional filter was used to collect the smallest

particles leaving the impactor (0.008–0.030 mm). The sample

line, the pre-cyclones, the DLPI and the filter were heated to

120 8C. After collecting the particles, mass size distributions

were obtained by weighing the filters and the particles were

analysed by scanning electron microscope/energy-dispersive

X-ray (SEM-EDX) on a Hitachi S-3000N, with an Oxford 7021

X-ray diffractor. The particle mass collected on the filter was

however so low that the filter could not be weighed. Instead, the

filter was chemically analysed in order to determine mass of the

collected particles using inductive coupled plasma-mass

spectroscopy, ICP-MS.

3. Results

3.1. Catalyst deactivation

The SCR catalyst samples decreased between 5 and 50% in

catalytic activity after 100–3000 h of exposure in the flue gas,

Table 3. No or little plugging was found in the catalyst samples

during the exposure time. The deactivation of the catalyst

samples cannot directly be compared with the deactivation of a

full-length catalyst. The poisonous components accumulate at

the inlet of the catalyst and the catalyst deactivation will be

higher on the short SCR samples. The deactivation rate in

different boilers and with different fuels can however be

compared with this method. To compare the different samples

the loss of activity was calculated as a linear decrease where the

various measurements (exposed more than 500 h) have been

interpolated to 1500 h.

During combustion of peat the loss of activity for the

samples was 5–15%, see Fig. 4, an increased co-combustion

A ̊. Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251 243

Fig. 3. Schematic sketch of a system for IACM monitoring, the light source is

placed in the boiler wall opposite the receiver.
with wood fuel (up to 30%) did not increase the deactivation of

the samples. The loss of activity during combustion of forest

residues was between two and three times higher compared

with peat combustion after 1500 h. An increased deactivation

was measured during co-combustion of bark and demolition

wood (in a bubbling fluidised bed) compared with combustion

of forest residues (in a circulating fluidised bed). The effect of

the sulphur-containing additive on the catalyst deactivation was

small; the same deactivation was measured during the periods

with and without additive.

A ̊ 244 . Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251

Table 3

Fuels, boiler types, additive, sample exposure season, exposure time and relative catalytic activity for SCR sample exposures

Fuel Boiler type Additive Season Exposure time (h) Relative catalytic activity (%)

Type B Type A Type C

Peat + 30% wood PCa A 864 91  1 (4)

Peat + 30% wood PCa A 1340 92  1 (2)

Peat + 30% wood PCa A 2850 93  6 (3)

Peat + 15% wood PCa B 768 88  2 (2)

Peat + 15% wood PCa B 1488 85  2 (4) 80  2 (2)

Peat PCa C 526 96 (1) 91 (1)

Peat PCa C 929 93 (1) 84 (1)

Peat PCa C 1455 95 (1)

Forest residues CFBb A 1230 70 (1)

Forest residues CFBb A 1650 71  4 (4)

Forest residues CFBb A 2880 55  3 (4) 46 (1) 81  8 (4)

Forest residues CFBb B 120 83 (1)

Forest residues CFBb B 864 88 (1) 105 (1)

Forest residues CFBb B 1584 64  6 (2) 49 (1) 89  6 (2)

Forest residues CFBb C 648 73  2 (2)

Forest residues CFBb C 984 79  3 (4)

Forest residues CFBb Sulphate C 1320 73  5 (4) 56  2 (2)

Forest residues CFBb Sulphate C 2820 55 (1)

Forest residues CFBb Sulphate D 502 84  1 (2) 71  1 (2)

Forest residues CFBb Sulphate D 1038 76  1 (2) 60  1 (2)

Forest residues CFBb Sulphate D 1540 71  2 (2) 54  1 (2)

Bark + 15% demolition wood BFBc A 720 72  1 (2) 97 (1)

Bark + 15% demolition wood BFBc A 1150 95  11 (2) 94 (1)

Bark + 15% demolition wood BFBc A 1870 66  3 (3) 87  19 (2)

Bark + 30% demolition wood BFBc B 100 93  3 (3) 93  4 (3)

Bark + 30% demolition wood BFBc B 500 86  7 (3) 85  4 (3)

Bark + 30% demolition wood BFBc B 1000 67  5 (3) 88  4 (2)

Bark + 30% demolition wood BFBc B 1400 59  4 (2)

Bark + 30% demolition wood BFBc B 1500 58  0 (3) 48  1 (3) 79  4 (2)

All the activity analyses were conducted at a temperature of 300 8C. a Pulverised combustion. b Circulating fluidised bed.

c Bubbling fluidised bed.

Fig. 4. Loss of catalytic activity (%) of type B samples per 1500 h of exposure in flue gas, the activity analyses have been conducted at a gas temperature of 300 8C. P,

peat; W, wood; FR, forest residues; B, bark; DW, demolition wood.
The most commonly used catalyst type in biofuel boilers in

Scandinavia is a bio-optimised type with a higher vanadium

content than for applications with higher sulphur content in the

flue gas, such as coal combustion. One bio-optimised catalyst is

the type B—used in this study. The increased amount of

vanadium in the sample increases the deNOx capacity of the

catalyst. When the deactivation of the bio-optimised sample

(type B) was compared with the deactivation of the low

vanadium content sample (type A) the deactivation was 1.2–1.9

times faster for the sample with the lower vanadium content,

Fig. 5. The same result was observed when the type B samples

were compared with a sample with high vanadium content (type

C). The deactivation of the type C sample was 0.4–0.7 times the

deactivation of the type B sample after a 1500-h exposure

period (type C had 30–70% lower deactivation compared with

type B). This indicates that these bio-optimised samples not

only have a higher initial deNOx activity, the deactivation is also

considerable less, which gives a longer lifetime and higher

deNOx capacity per hour in the flue gas.

The elementary analyses using SEM-EDX on the catalyst

samples showed an increased amount of mainly potassium but

also of sodium. Lead was found on the samples exposed to flue

gas from co-combustion of demolition wood. Higher concentrations of the deactivating compounds were found at the

inlet of the catalyst samples compared with the outlet. As the

SEM-EDX also detects the metals in the catalyst material the

signals from the deactivating compounds were very low.

Therefore, the SEM-EDX investigations were mainly used as

guidance for choice of wet chemical analyses and not for any

quantitative measurement. By splitting the catalyst wall the

distribution of the deactivating compounds through the wall

was detected by SEM-EDX line-scan. As seen in earlier studies

[1,4] the potassium was relatively evenly distributed throughout

the wall, Fig. 6. No accumulations of potassium were found on

the outer surface of the samples. Mapping (correlation between

the different ions on the catalyst surface) was done with the

SEM-EDX for potassium, sodium, sulphur, chlorine, oxygen,

titanium and vanadium on samples exposed in all plants in the

study. No correlation was found between the deactivating

compounds and the different possible corresponding anions.

Wet chemical analyses were conducted on samples from

Uppsala (seasons B and C), Brista (seasons C and D) and

A ̊. Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251 245

Fig. 5. Loss of catalytic activity of type A and type C samples compared with type B samples after 1500 h of exposure in flue gas. The analyses were conducted at a

temperature of 300 8C.

Fig. 6. Distribution of potassium through the catalyst wall on a type B sample exposed for 1500 h during combustion of forest residues in a circulating fluidised bed

(season C).
Johannes (seasons A and B). The results are presented in Fig. 7.

Potassium and sodium were accumulated linearly on the

samples. The highest accumulation of potassium was measured

on the samples exposed to co-combustion of bark and

demolition wood, about 0.2 wt.% after 1500 h of exposure in

the flue gas. The accumulation of potassium from forest

residues (with and without sulphate additive) was about 0.10–

0.13 wt.% after 1500 h while combustion of peat gave a

potassium accumulation of 0.05 wt.% after 1500 h. The

accumulation of sodium was low for all samples (less than

0.05 wt.%). About 1.5 wt.% of sulphur was found on the fresh

catalyst samples. During combustion of peat and co-combustion of bark and demolition wood the amount of sulphur on the

samples decreased by about 0.1–0.5 wt.%. During combustion

of forest residues (both with and without sulphate additive) the

amount of sulphur on the samples decreased by about 1 wt.%.

The sulphur level decreased during the first 500 h of exposure,

after that the sulphur level on the samples was more or less

constant. A low amount of chlorine was detected on the

samples. Wet chemical analyses on lead and zinc were only

conducted on the samples exposed to flue gas from combustion

of demolition wood; the accumulation of both zinc and lead was

low on the samples. No wet chemical analysis on zinc and lead

were conduced on the other samples, earlier studies have not

shown an accumulation of these metals during biofuel

combustion nor did the analyses with the SEM-EDX.

Surface area and pore-size distribution were measured on

two different samples exposed during co-combustion bark and

30% demolition wood. These samples were compared with a

fresh sample. No analysis was conducted on the samples that

had been exposed during combustion of peat and forest

residues. Earlier studies have shown that the loss in surface area

was low on similar samples [1,2]. The average pore size for the

fresh sample was 13 nm, 85% of the pores were in the size

range of 10–23 nm and 99% of the pores were smaller than

100 nm. The BET analyses showed a small decrease in surface

area, 6%, on the sample exposed to bark and demolition wood,

Table 4. No change in micropore volume was detected.

A ̊ 246 . Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251

Fig. 7. Accumulation of potassium, sodium, chlorine, sulphur, zinc and lead on catalyst samples as function of the catalyst exposure time in flue gas: W, wood; FR,

forest residues; B, bark; DW, demolition wood.
3.2. Flue gas analysis

The flue gas content of sulphur varied between the different

fuels. During peat combustion the sulphur dioxide content in

the flue gas was about 200 ppmwg (measured on volume basis)

and during combustion of forest residues and co-combustion of

bark and demolition wood about 3–10 ppmwg. The alkali (sum

of potassium and sodium) chloride content was continuously

measured during combustion of forest residues for two seasons.

During the period when no additive was used the average level

of alkali chloride was about 6 ppmwg and the concentration of

SO2 4 ppmwg. This can be compared to the two periods when

the sulphate additive was used when the alkali chloride content

decreased to 2 and 1 ppmwg, respectively, and the SO2

concentration was 3 and 7 ppmwg, respectively. This was

probably a consequence of the sulphation of alkali chlorides

and the resulting formation of alkali sulphates.

Impactor measurements were conducted in Uppsala (peat

and 15% wood powder, season B), Brista (forest residues with

and without sulphate additive, seasons C and D) and Johannes

BFB (bark and 30% demolition wood, season B). The amount

of particles (mg/N m3

) varied between the different boilers and

between the different size fractions (0.008–2 mm), see Fig. 8.

The largest amount of submicron particles (particles with a size

smaller than 1 mm) was found in Johannes BFB during cocombustion of bark and demolition wood. The lowest amount

of particles was found in the Uppsala PC boiler during cocombustion of peat and wood. There was a significant

difference in the total amount of particles smaller than 2 mm

with and without sulphur additive during combustion of forest

residues, larger amounts or particles were found during the

periods when the additive was used. However, the amount of

ultra fine particles (smaller than 100 nm) was the same during

the periods with and without the additive, Table 5.

By using the results from the low pressure impactor; the

different chemical compounds analysed with SEM-EDX and

the weighted amount of particles in each filter step, the amounts

of potassium, sodium, chlorine, sulphur, lead, zinc, phosphorus

and calcium was calculated, Fig. 9 and Table 5. The dominating

chemical compounds in the ultra fine particles were potassium,

sodium, chlorine, sulphur and oxygen. In particles larger than

0.5 mm calcium and phosphorus was also found. The amount of

potassium in the flue gas was, for the ultra fine particles, highest

during combustion of bark and demolition wood, Table 5. The

sulphate additive did not change the amount of potassium in the

ultra fine particles significantly. The amount of alkali in the fine

particles during peat combustion was low. The amount of

sodium, in the fine particles, was low compared with the

A ̊. Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251 247

Fig. 8. Fly ash mass size distribution (mg/N m3) in the flue gas: P, peat; W,

wood; FR, forest residues; B, bark; DW, demolition wood.

Table 5

Fly ash mass size distribution (mg/N m3

) of particles in flue gas (detected with DLPI) with an aerodynamic particle diameter smaller than 100 nm (mg/N m3) and

corresponding measurements with IACM during the impactor-sampling period: alkali chloride and sulphur dioxide

Peat + 15%

wood

Forest residues

(season C)a

Forest residues

(season D)

Forest residues +

sulphate (season C)

Forest residues +

sulphate (season D)

Bark + 30% demolition

wood (season B)

Potassium (mg/N m3

) 4.4 10.5 6.7 6.2  0.2a 6.8 16.7

Sodium (mg/N m3

) 0.8 1.1 1.3 0.9  0.1a 2.5 5.8

Chlorine (mg/N m3

) 0.6 2.5 1.1 0.0  0a 0 4.3

Sulphur (mg/N m3

) 2.7 4.9 2.8 3.3  0.3a 3.8 2.1

Lead (mg/N m3

) 00 0 0a 0 0

Zinc (mg/N m3

) 0 0 0.1 0a 0.2 0.1

Phosphorous (mg/N m3

) 1.1 0 0 0a 0 0.1

Calcium (mg/N m3

) 2.2 0 0 0a 0 0

Total particle amount (mg/N m3

) 17.9 8.6  5.0a 11.6 6.0  0.9a 12.5 32.7

(K + Na)Cl (ppmwg) 5.2 8.0 1.6 2.4

SO2 (ppmwg) 5.4 0.0 7.1 6.3

a Average of two measurements during the same day.

Table 4

BET surface on fresh sample, sample exposed during co-combustion of bark and demolition wood (500 h) and exposed during combustion of refuse energy fuel

(100 h)

Catalyst. type Fuel Relative catalytic

activity (%)

Surface

area (m2

/g)

Micro pore volume

(cm3/g)

Average pore

diameter (A ̊ )

Type B Fresh 100 66.8 0.0016 128

Type B Bark + 30% demolition wood 79 62.7  1.7 0.0024 138
amount of potassium during combustion of all fuels. The

highest amount of sodium was found during combustion of bark

and demolition wood.

There was a significant decrease in chlorine content in the

fine particles during the periods when the sulphate additive was

used. This can be compared with the amount of KCl measured

in the gas phase using IACM, see Table 5. A decreased amount

of KCl in the gas phase corresponded to a decreased amount of

chlorine in the fine particles. The amount of chlorine in the ultra

fine particles was two times higher during combustion of bark

and demolition wood compared with the combustion of forest

residues. The amount of sulphur in the ultra fine particles was

highest during combustion of forest residues. There was low

correlation between the amount of sulphur in the fine particles

and the amount of sulphur dioxide in the flue gas. The sulphur

in the particles probably correlates with the particle sulphate

compounds such as potassium sulphate, sodium sulphate or

other metal sulphates. Low amounts of lead, zinc, phosphorus

and calcium were found in the ultra fine particles.

Five forest residues fuel samples from Brista, season C, were

analysed for chlorine content. The samples were then compared

with the corresponding amount of alkali chloride in the gas

A ̊ 248 . Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251

Fig. 9. Fly ash mass size distribution (mg/N m3) for potassium, sodium, chlorine and sulphur, lead, zinc, phosphorus and calcium: P, peat; W, wood; FR, forest

residues; B, bark; DW, demolition wood.
phase measured in the flue gas when the samples were

collected. There was a strong correlation between the chlorine

content in the fuel and the alkali chloride concentration in the

flue gas, see Fig. 10. The chlorine content in the fuel varied

between 0.005 and 0.045 wt.% during the period.

4. Discussion

4.1. Catalyst deactivation

The wet chemical analyses showed that potassium and

sodium were the main chemical components that accumulate

on and in the catalyst material. There was a strong correlation

between the alkali content on the samples and the corresponding catalytic activity, Fig. 11, which implies that the main

deactivation effect probably was alkali poisoning. This has

been concluded in earlier studies on SCR catalyst deactivation

during biofuel combustion [1–4]. It was difficult to conclude in

what form the alkali penetrates and reacts with the catalyst

material by the analyses conducted on the catalyst material. No

anion was found that correlates to the potassium on the catalyst

surface. The fact that the potassium, also in this study, was

found relatively evenly distributed throughout the catalyst

material and not in higher concentrations on the outer surface of

the catalyst material indicates that it is probably transported

into the material in small particles or in gas phase.

Sulphur has a promoting effect on the SCR catalyst deNOx

activity [27–32]. The decrease in sulphur content in the catalyst

material, during exposure of the catalyst samples in the lowsulphur gas flow, was probably a part of the catalyst

deactivation during the biofuel combustion. Studies on

regeneration of catalyst deactivated during biofuel combustion

have shown that sulphation, exposure of the samples to gaseous

SO2, increased the activity of the catalyst samples from a

relative activity of about 50% up to a relative activity of about

80% [29].

4.2. Flue gas composition

By measuring the flue gas composition before and near the

catalyst surface differences in flue gas composition can be

detected for different fuel types and combustion techniques.

Earlier studies have shown that the flue gas chemical

compositions are highly dependent on fuel, combustion

temperature and combustion technique [12,14–35]. The

combustion aerosols from fuels with a high content of volatile

sodium or potassium compounds are often dominated by a large

submicron fraction [12,14]. These particles are formed by

recondensation of alkali volatilised in the combustion zone, and

their main constituent components are chlorides and sulphates

of the alkali metals. The occurrence of chlorine in the

submicron particles has been found to mainly represent the

occurrence of alkali chlorides during combustion of biomass

[16]. The chlorine content in the submicron particles differed

between the different boilers and for the different fuels. The

chlorine content in the fuel seems to correlate with the chlorine

content in the submicron particles; increased amounts of

chlorine in the fuel corresponded to increased amounts of

chlorine in the submicron particles. These results correspond to

the fuel analyses and the on-line measurements of alkali

chlorides during combustion of forest residues in the circulating

fluidised bed. Increased amount of chloride in the fuel

corresponded to increased amount of alkali chlorides in the

flue gas. When sulphur in the form of SO3 was added to the

boiler at a temperature of 800–900 8C this resulted in a

sulphation of the potassium chlorides to potassium sulphate, the

chlorine content in the submicron particles decreased to very

low values and the on-line measurement with IACM indicated a

decrease in gaseous alkali chlorides by more than 50%. The

addition of the sulphate additive did however not decrease the

total amount of alkali in the fine particles.

4.3. Impact of flue gas composition on SCR catalyst

deactivation

As can be seen in Fig. 12 there was a linear correlation

between the alkali content in the ultra fine particles and the

alkali accumulation on the catalyst samples with an R2 value of

0.76. The linear correlation between the accumulation of alkali

accumulation on the samples and the amount of alkali in the

flue gas decreases with increased particles sizes. The

correlation for particles smaller than 200 nm had a R2 value

of 0.56, the correlation with particles smaller than 300 nm had a

R2 value of 0.14, particles smaller than 450 nm had a R2 value

of 0.05 and for particles smaller than 2000 nm the R2 value was

A ̊. Kling et al. / Applied Catalysis B: Environmental 69 (2007) 240–251 249

Fig. 10. Chlorine content in the forest residue fuel (Brista, season C) compared

with the amount of alkali chloride in the flue gas monitored by IACM.

Fig. 11. Accumulation of alkali on the SCR catalyst samples as function of the

corresponding catalytic deNOx activity (measured at a temperature of 300 8C).

P, peat; FR, forest residues; B, bark; DW, demolition wood; S, sulphur.
0.10. The correlation between the particles smaller than 450 nm

and the accumulation of alkali on the samples are presented in

Fig. 13. The largest catalyst pores are about 100 nm, larger

alkali components can only be transported into the catalyst by

solid-state diffusion. This should lead to a concentration profile

in the material with higher concentrations in the outer part of

the material, which was not found in this study. The uncertainty

in the correlation between the impactor measurements and the

alkali accumulation on the catalyst samples were however

large. Only one or two impactor measurement of flue gas

composition, each with about 20 min sampling time, has been

conducted for each catalyst exposure season—in total up to

3000 h. Only a very small fraction, less then 0.3% calculated

for combustion of forest residues in Brista, of the total alkali

content in the ultra fine particles smaller than 100 nm

accumulated on the catalyst surfaces. The fuel analyses during

season C in the Brista CHP imply that the variation in fuel

quality and the resulting variation in flue gas composition could

be large.

5. Conclusions

The study has shown that alkali, mainly potassium, accumulates linearly on the catalyst samples during the exposure

time in the biofuel boilers. Increased alkali content on the catalyst

samples decreased the catalytic deNOx activity; 40% loss in

catalytic activity correlated to an alkali accumulation of about

2 wt.%. An increased amount of vanadium in the catalyst

material decreased the catalyst deactivation rate significantly.

The results imply that large amounts of alkali in ultra fine

particles (<100 nm) in the flue gas increased the alkali

accumulation on the catalyst. Low correlation was found

between particles larger than 100 nm and the catalyst deactivation. It was not possible to decrease the deactivation of the

catalyst samples by the sulphate-containing additive. Although

the additive had an effect in sulphating potassium chloride to

potassium sulphate, this did not decrease the amount of

potassium in ultra fine particles or the deactivation of the

catalyst samples.

Acknowledgements

Financial support from the European Commission through

the project CATDEACT and Va ̈rmeforsk, Sweden, is gratefully

acknowledged. We also wish to thank the staff at Brista,

Johannes and Uppsala CHP for kind help with the full-scale

deactivation tests.

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