Impact of biomass co-combustion 

on SCR DeNOx operation 

by: 

Hans Jensen-Holm 

Joakim Reimer Thøgersen

Peter Lindenhoff 

Haldor Topsøe A/S 

Presented at Power-Gen Europe, Germany, 2009 

Best paper award within the conference track 

‘Coal, Nuclear & Biomass Generation’
Impact of biomass co-combustion on SCR DeNOx operation Page 2 of 23

Abstract 

With increasing requirements for reductions of CO2 emissions from the power-generating industry, there is a considerable interest in the utilisation of biomass and municipal and industrial 

waste in power plant boilers. Combined with still tighter limits on emission of NOx, the use of

biomass fuel represents a challenge to the designers of Selective Catalytic Reduction (SCR) 

units since the useful life of the SCR catalyst can be significantly reduced. 

The mechanisms responsible for the catalyst deactivation are discussed. Alkali-metal aerosols 

formed during combustion of biomaterial can at worst cause complete deactivation of the catalyst within a few thousand hours of operation. Other components as e.g. phosphorous can 

cause severe fouling of the catalyst surface. 

The paper describes a number of SCRs implemented to meet these new challenges and includes experience from a variety of plants firing biomass in Europe and the USA. Results from

installations in units firing 100% biomass as well as installations in units with co-combustion of

biomass with coal and oil are presented. 

These experiences demonstrate that choosing the right plant configuration and proper catalyst 

formulation, the negative effect can be limited. Up to 20 weight per cent biomass can be cocombusted with the coals without any noticeable impact on catalyst lifetime. The use of SCR on

100% biomass-fired boilers may be done as a tail-end installation to minimise the amount of 

poisoning species entering the SCR. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 3 of 23

Contents 

Introduction.................................................................................................................................... 4 

SCR deactivation mechanisms ..................................................................................................... 6 

Alkali and alkaline-earth metals ................................................................................................ 6 

Phosphorous ........................................................................................................................... 10 

Arsenic .................................................................................................................................... 11 

Examples of experience from biomass-fired boilers ................................................................... 11 

DONG Energy, Avedøre Power Station, Denmark ................................................................. 11 

DONG Energy, Studstrup Power Station, Denmark ............................................................... 13 

Electrabel, Langerlo Power Station, Belgium ......................................................................... 16 

Vattenfall, Combined Heat and Power, Sweden..................................................................... 17 

Sierra Power, California, USA................................................................................................. 19 

Vattenfall, Amager Power Station, Denmark .......................................................................... 20 

Conclusions................................................................................................................................. 22 

References .................................................................................................................................. 23 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 4 of 23

Introduction 

An increased production of electricity from renewable energy sources is being promoted and 

biomass co-firing with coal in existing coal-fired power plants represents an option for combined

renewable and fossil energy utilisation to reduce CO2 emissions. Biomass is considered CO2

neutral with respect to the greenhouse gas balance if the use of fossil fuel in harvesting and 

transporting the biomass is not considered. Direct co-firing, i.e. firing the coal and biomass in 

the same boiler, is the simplest and most widely applied technology for co-firing biomass. 

Possible biomass sources comprise e.g.: 

Wastes and residues

- Wood and bark: forest or tree trimmings, sawdust, demolition wood, crates

- Agricultural residues: straw, olive residue, coconut chips 

- Animal wastes: Poultry litter, meat bone meal 

- MSW, sewage sludge, paper waste 

Peat 

Agricultural energy crops: Switchgrass, miscanthus, poplar and willow, bamboo

Biomass and coal have fundamentally different fuel properties. Biomass (and low-rank coals) 

contains larger relative quantities of alkali and alkaline-earth elements (potassium, sodium, calcium, magnesium), phosphorous and chlorine than coal. As all the constituents of the biomass 

enter the boiler, several technical concerns arise. Higher fuel chlorine contents can lead to 

greater high-temperature corrosion in boilers. Accelerated fouling and slagging can occur when

high potassium containing fuels are utilised. 

Also for the operation of SCR systems there are technical challenges associated with cocombustion of biomass and coal in boilers designed for coal. The constituents of biomass have 

the potential to impair the SCR as the SCR catalysts are susceptible to ‘poisoning’ due to condensation of volatile inorganic species on the catalyst surface. Formation of sulphate- or phosphate-based deposits on the catalyst surface or reaction with the catalyst’s active species can

significantly reduce catalyst activity, resulting in shorter lifetime. Biomass in the form of wastes

may also contribute elements such as arsenic. 

Table 1 shows examples of ‘poisonous’ components in the ash of various fuels and the related

SCR catalyst deactivation rates. Figure 1 shows the deactivation trend for various fuel types as

activity relative to fresh-catalyst activity versus operating time. The activity curves have been 

calculated using a deactivation model developed by Haldor Topsøe that considers ash deposition rate, pore plugging development and chemical impact on active sites. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 5 of 23

Bituminous

coal 

PRB 

coal 

Heavy 

fuel oil 

Peat 

PC boiler

Wood 

CFB boiler

Ash mg/Nm3 15,000 6,000 100 6,000 1,500

Soluble potassium wt % 0.15 0.5 - 0.3 5.2 

Soluble sodium wt % 0.12 1.5 1.5 0.2 0.7 

Nickel wt % - - 20 0.01 -

Vanadium wt % - - 20 0.01 -

Phosphorous wt % 0.1 0.8 - 1 15

Calcium wt % 0.5 22 1.1 15 5

SCR catalyst deactivation 

% per 10,000 hours 

(model simulation cf. Figure 1)

12 20 5 60 46

Table 1 Fly ash characteristics of various fuels 

0%

20%

40%

60%

80%

100%

0 10,000 20,000 30,000 40,000 50,000 60,000

Operation hours

R

elativ

e catalyst activity,

k/k0

Wood

Bituminous coal 

Heavy fuel oil

Light fuel oil 

PRB coal

Figure 1 Expected deactivation of SCR catalyst for various fuel types 

For biomass firing the SCR catalyst deactivation rate is significantly higher. Typical deactivation 

rates for installation in a high-dust position are shown in Table 2. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 6 of 23

Fuel Deactivation rate

100% straw 50% per 1,000 hours 

Wood: 

 - Pulverised-fuel boiler 

 - CFB boiler 

 - Grate-fired boiler 

60% per 10,000 hours 

45% per 10,000 hours 

25% per 10,000 hours 

Peat-fired boiler 30% per 10,000 hours 

Coal-fired boiler 10% per 10,000 hours 

Heavy fuel oil fired boiler 5% per 10,000 hours 

Gas-fired boiler 2% per 10,000 hours 

Table 2 Typical SCR catalyst deactivation rates. Actual deactivation rate 

depends on specific fuel composition 

SCR deactivation mechanisms 

Alkali and alkaline-earth metals 

Alkali-metal aerosols formed during combustion of biomaterial are well known to be able to severely impact the performance of SCR catalysts. The sub-micron particles adhere to the surface 

of the catalyst and are readily transported to the active sites by surface diffusion. Spectroscopic 

studies suggest that the poisoning mechanism occurs via chemical bonding of the alkali metal 

to the crucial –V-OH sites, the so-called Brønsted-acid sites which also adsorb and “activate” 

the ammonia, forming –V-OM (M being K or Na) and thereby blocking the catalytic cycle (see 

Figure 2). Exposure to alkali-metal aerosols can at worst cause complete loss of activity of the 

catalyst within a few thousand hours of operation. 

V O V

O O

H

V O V

O O

M

M

Acid-base Redox

Figure 2 Basically, the SCR reaction involves both acid-base and redox catalytic functions. The reaction

is initiated by the adsorption of ammonia on Brønsted acid sites (V5+-OH), followed by “activation” of ammonia via reaction with redox sites (V=O). This “activated” form of ammonia then 

reacts with gaseous or weakly adsorbed NO, producing N2 and H2O while releasing V4+-OH. 

To complete the catalytic cycle, the V4+-OH species is oxidised by either NO or O2 to regenerate the original V=O species (from [1]) 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 7 of 23

Cf. Table 2, exposure to flue gas from straw combustion results in a very rapid catalyst deactivation. Studies have shown the flue gas to have a high mass concentration (up to more than 2 

g/Nm3

, depending on type of straw) of submicron particles of 0.15-0.30 μm size (number geometric mean diameter). The particles, which are mostly compact agglomerates, consist of almost pure potassium chloride and sulphate with minor amounts of sodium, phosphorous and 

calcium [2]. 

In a study by the CHEC group at the Technical University of Denmark in collaboration with Haldor Topsøe, full-length monolith catalyst samples were exposed to aerosols of pure potassium

chloride and -sulphate in a bench-scale reactor shown in Figure 3 [3]. An aerosol of the salt was 

then generated by injecting the solution of the salt into the flue gas close to the burner where

the temperature was 1000–1150°C. Activities of the catalysts were measured in situ at 350°C 

typically 2–3 times per day. The catalyst activities as a function of exposure time to the flue gas 

are shown in Figure 4. 

Figure 3 Experimental set-up for catalyst exposure tests (from [3]) 

Figure 4 (left) shows that after exposure to KCl aerosols at 350°C for 715 hours, the catalyst 

relative activity was about 69%, which corresponds to a deactivation of about 1.0% per day. 

There was also a fast decrease of the adsorption capacity for NH3 on the catalyst. Then the 

system was cleaned and without addition of KCl the catalyst was tested for another 235 hours.

During this period the catalyst activity remained at about 69% and the amount of chemisorbed

ammonia also levelled off. This confirms that catalyst deactivation stops when addition of KCl is 

ceased. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 8 of 23

The catalyst activity as a function of exposure time to K2SO4 is shown in Figure 4 (right). The

relative activity decreased to 86% during the first 300 hours, corresponding to a deactivation 

rate of 1.1% per day. 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. This difference is ascribed to the difference in aerosol particle size since studies with direct impregnation of catalyst with KCl and K2SO4 showed these to have equal poisoning effect. 

Similar to the KCl-aerosol exposure test, the catalyst capacity for NH3 chemisorption decreased

faster than the relative activity. The active sites within the catalyst structure, although not participating in the SCR reaction, react with potassium resulting in a faster decrease in the catalyst 

capacity for NH3 chemisorption than the observed activity which is impeded by the mass transfer resistances.

KCl aerosols K2SO4 aerosols

KCl deposition stopped

KCl aerosols K2SO4 aerosols

KCl deposition stopped

KCl aerosols K2SO4 aerosols

KCl deposition stopped

Figure 4 Catalyst activity and chemisorbed NH3 on the catalyst as a function of exposure time to 

potassium chloride and potassium sulphate aerosols (from [3]) 

SEM-EDX analyses showed that potassium partly deposited on the catalyst outer wall but 

mainly penetrated into the catalyst wall and the NH3 chemisorption measurements revealed that 

the Brønsted acid sites had reacted with potassium and were rendered inactive for NH3 adsorption and subsequent reaction with NO. The conclusion was that chemical poisoning of active 

sites is the dominating deactivation mechanism but physical blocking of the surface may also

contribute to the loss of activity. 

Figure 4 (right) shows for comparison the deactivation trend of a catalyst exposed to the flue 

gas from a straw-fired power plant, Masnedø, in Denmark. The deactivation rate is approximately 50% per 1,000 hours, very similar to what was observed in the KCl-aerosol exposure 

test. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 9 of 23

Figure 5 shows results from activity test and analysis of catalyst samples from a pilot installation 

on a CFB wood-fired boiler at Nässjö, Sweden. It is seen that at an uptake in excess of 0.2-0.3 

% by weight of (soluble) alkali metals, the impact on catalyst activity is very significant and at 

about 1% an almost complete loss of activity is observed. 

0%

20%

40%

60%

80%

100%

0.0 0.2 0.4 0.6 0.8 1.0

Weight percent potassium equivalents on catalyst

wt% K + 1.7×wt% Na

Relative specific catalyst activity

Figure 5 Deactivation of Topsøe DNX®

 SCR catalyst as function of uptake of soluble alkalimetals. Sodium and potassium are weighted according to their molar weight 

The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but fouling of the surface area may also contribute to the loss of activity in a practical application. 

CaSO4 FOULING LAYER

Figure 6 High content of calcium in fuel can cause deactivation of the catalyst in the SCR unit. 

The scanning electron microscopy picture shows a typical fouling layer on the catalyst surface seen in SCR units firing high-calcium sub-bituminous coals. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 10 of 23

Fouling is a physical blockage of the catalyst pore system by aerosols or other sub-micron ash 

particles. Figure 6 shows how a catalyst exposed to calcium-containing fly ash from burning of 

sub-bituminous Powder River Basin (PRB) coal has developed a dense layer of calcium sulphate which prevents the NOx in the flue gas to reach the active sites in the interior of the catalyst. 

Phosphorous 

Organically bound phosphorous in fuel is released as gas-phase phosphorous during the combustion process. The phosphorous reacts in the gas phase to form polyphosphoric-acid aerosols which during cooling-off condenses in the catalyst pores. Reactions with e.g. calcium and 

alkali metals form sub-micron particles of pyro- or polyphosphates which deposit on the catalyst 

surface, causing pore blocking and surface masking with an ensuing loss of catalyst activity. 

In some cases severe deactivation has been reported. Up to 50% deactivation has been observed after 4,000 operation hours when 5% bone residue or sewage sludge has been co-fired 

with bituminous coal [4]. These investigations reveal that phosphor seems to accumulate in the 

catalyst surface.

Figure 7 shows accumulation of phosphorous in the outer 50-100 microns of the wall of a catalyst from a PRB-fired boiler. In this case phosphor penetrates deeper into the catalyst structure 

than the sub micron calcium-oxide aerosols, which demonstrates that the phosphorous compounds have a high mobility. 

0

1

2

3

4

5

0 200 400 600 800

Distance (μm)

C

onc

e

ntr

ation, wt %

Phosphorous

Sulphur

Figure 7 Sub-surface phosphorous layer in the outer 50-100 microns of the surface 

of a catalyst exposed to flue gas from phosphorous-containing fuel

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 11 of 23

Arsenic 

Arsenic (As) is a component present in most of the world’s coal and co-combustion of e.g. 

waste and sewage sludge can contribute to fuel arsenic content. When the coal is burned, arsenic is released in the furnace as a gaseous oxide, As2O3. As2O3 condenses in the porous 

catalyst structure and may then block the access to the catalytically active sites. Arsenic in 

gaseous form can be a main contributor to deactivation of catalysts in SCR DeNOx units on 

coal-fired boilers. 

The susceptibility of the catalyst to poisoning from exposure to arsenic depends strongly on the 

pore structure of the catalyst. A highly porous catalyst with a diverse pore structure has a high

resistance towards arsenic poisoning as the presence of macro-pores ensure access to active

sites even when the micro-pores have become plugged by arsenic. 

Examples of experience from biomass-fired boilers 

DONG Energy, Avedøre Power Station, Denmark 

DONG Energy’s Avedøre Power Station Unit 2 is a combined heat and power plant, located in

Copenhagen, Denmark. It was commissioned in 2001 and is one of the world’s most efficient

power plants with an energy efficiency up to 94%. 

Straw

Natural gas

Oil

Natural gas Electricity

District hea

Steamturbine

Biomassboiler

Main -

bioler :

Straw

Natural gas

Oil

Natural gas Electricity

District hea

Steamturbine

Biomassboiler

Main -

bioler :

475 MW

2 x 55 MW

40 MW

Gasturbines

45 MW

390 MW

Wood pellets

Biomass

 boiler 

 100 MWth

Main

boiler 

800 MWth

Gas turbines 

Fuel oil Steam Turbine 

Coal fly ash 

Natural gas 

Wood pellets

Natural gas 

Straw

Electricity

District Heating 

Figure 8 DONG Energy’s Avedøre Power Station Unit 2, Denmark [5] 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 12 of 23

Avedøre Unit 2 uses a multi-fuel concept developed by DONG Energy. The unit consists of an

800 MWth main boiler, a 100 MWth straw-fired boiler and 2×135 MWth gas turbines (see Figure 

8). The ultra-supercritical boiler was originally developed to burn natural gas and heavy fuel oil 

but was retrofitted in 2002 to fire up to 300,000 tonnes per year of pulverised wood pellets as an

additional fuel. The furnace is designed for coal firing and has a tangential firing system with 

sixteen low-NOx burners. The unit is equipped with dust filter, SCR DeNOx and desulphurisation units. 

It was experienced [6] that co-firing of wood pellets and fuel oil resulted in deposits in the convective part of the boiler and in the air-preheater. The deposits consisted of alkali from biomass 

and sulphur and vanadium from the fuel oil. Such deposits are corrosive and catalyses the formation of SO3. This resulted in plugging of the air-preheater, in corrosion in the electrostatic 

precipitator and gas preheater as well as deactivation of the SCR. 

An accelerated deactivation was expected as the wood’s relatively high content of potassium 

and chloride partly vaporises during combustion, forming a fine aerosol of KCl and K2SO4 particles. A catalyst deactivation monitoring programme was initiated. Experience during wood-oil 

co-firing showed that the catalyst deactivated rather fast as shown in Figure 9. 

0

50.000

100.000

150.000

200.000

250.000

300.000

350.000

2001 2002 2003 2004 2005 2006 2007

Year

Tonnes

0

50.000

100.000

150.000

200.000

250.000

300.000

350.000

1000 N

m3

Fuel oil

Wood pellets

Fly ash

Natural gas

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 5.000 10.000 15.000 20.000 25.000 30.000 35.000

Operating Hours

Relative catalyst activity, k/k0

Fuel: Oil/gas

Fuel: Oil/gas/wood pellets

Deactivation rate: 25% / 10000 hours

New layer - 2006

Old layer - 2001

Deactivation rate:

15% / 10000 hours

Figure 9 Fuel history and catalyst deactivation on DONG Energy’s Avedøre Power Station Unit 2 

It was decided to apply washing of the catalyst. The washing process was required to be fast as 

it would have to be done frequently due to the rapid deactivation. An in-situ washing process 

using available untreated water was developed to avoid time consuming removal and reinsertion of the catalyst and the outage was reduced to three days [7]. In the following years

washing was applied several times per year. However, after three years and consumption of 

600,000 tonnes of wood pellets, the activity of the catalyst had been reduced by more than

50%. It was concluded that the washes had not been able to effectively counter deactivation of 

the catalyst and the catalyst was replaced in 2006 after five years of operation. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 13 of 23

To reduce the impact of wood co-firing, injection of coal fly ash into the furnace was initiated in 

2006. The fly ash, around six tonnes/hour, is injected through the four “over burner air ports”, 

above the third burner level. This solved the above-mentioned problems with deposits, and the

sulphuric acid aerosols in the stack were also reduced significantly [5]. Furthermore, the coal fly 

ash to some degree captures wood-ash components that poisons the SCR catalyst such as 

alkali-metal aerosols, calcium and phosphorous and this has reduced the catalyst deactivation 

to around 15% per 10,000 hours compared to on average approximately 25% per 10,000 hours 

before fly-ash injection as shown in Figure 9. 

DONG Energy, Studstrup Power Station, Denmark 

DONG Energy’s Studstrup Power Station has two 350 MWe PC boilers – Units 3 and 4 – which 

were put into operation in 1984 and 1985. The units have since then been modernised regularly 

and in 2002 a conversion to commercial operation with co-combustion of ground straw was 

initiated. In each of the two units, four out of 24 burners were modified into ground-straw firing

by using the core air pipes of the coal-burners for the air/straw suspension. Up to five tonnes/hour of straw can be fired through each burner. With up to 10% co-firing of straw, the fly 

ash from the units can be utilised in the concrete industry. 

Pilot tests

As deactivation of high-dust SCR catalyst is one the critical issues of straw co-firing, slip-stream 

reactors were installed on Units 3 and 4. An exposure time of 5,000 hours was achieved in the 

period 2002-2003. On Unit 4 the average straw share was 11% on a weight basis and 7% on 

energy basis. Unit 3 was 100% coal-fired and the test reactor on this unit was used as a coalreference for the deactivation tests. 

0.5

0.6

0.7

0.8

0.9

1.0

0 1,000 2,000 3,000 4,000 5,000 6,000

Exposure time, hours

Relative specific catalyst activity, k/k0

Coal reference

7% straw co-firing

Figure 10 Relative specific catalyst activity as function of exposure time 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 14 of 23

Three different types of catalysts were investigated. Figure 10 shows the relative specific activity 

as a function of exposure time for a Topsøe catalyst. There is no distinguishable difference in 

the deactivation trend from exposure to flue gas from coal-firing and from 7% straw co-firing. 

The observed deactivation was found to be caused by arsenic and formation of surface fouling

layers. 

Full scale

In 2007 the two units were retrofitted with SCR DeNOx units with Topsøe DNX®

 catalyst. Shortly 

after start-up of the DeNOx plant on Unit 4, a rapid increase in pressure drop was observed and 

inspection of the reactor revealed severe plugging of the catalyst. Detailed investigation of the 

plugged catalyst showed that the seminal reason was large ash particles, up to 10 mm in 

length, primarily originating from the knots of the ground straw, cf. Figure 11. 

Figure 11 Severe plugging of the SCR catalyst Studstrup Power Station’s Unit 4 caused by large 

straw-ash particles, seen as black particles in the otherwise grey coal ash 

The catalyst was unloaded for washing on site. The cleaning was done by submersion of the 

catalyst modules into hot water, approximately 50°C, combined with aeration (a “hot bubble 

bath”, see Figure 12). The washing was by and large successful, however still leaving some 15-

20% of the channels plugged. This could be ascribed to the fact that the fly ash had partly hardened. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 15 of 23

Figure 12 Washing of catalyst modules at Studstrup Power Station 

To avoid future incidents, fine-meshed screens were built into the flue gas duct at the exit of the 

boiler to filter out larger ash particles, see Figure 13. The co-firing of straw on Unit 3 had been 

suspended until sufficient experience had been collected on Unit 4. After five months of satisfactory performance of Unit 4 with no increase in pressure drop across the catalyst, it was decided to equip Unit 3 with similar screens. 

Figure 13 Installation of large particle ash screens in the flue gas duct on Studstrup Power 

Station Units 3 and 4 (adopted from [8]) 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 16 of 23

Electrabel, Langerlo Power Station, Belgium 

Electrabel operates two identical 250 MWe coal-fired boilers at their Langerlo Power Station at 

Genk, Belgium. The two boilers were originally (in the mid 1970ies) constructed as oil-fired boilers but were converted to coal firing in 1985-86. As part of the so-called Langerlo 2000 Environmental Project the two units were retrofitted with flue-gas desulphurisation and SCR DeNOx 

systems in the late 1990ies. 

Since 2003 biomass has been co-fired with the coal and the amount now constitutes 10-20% on 

mass basis. On average the fuel blend is composed of 86% coal, 3% olive residue, 4% dried 

sewage sludge and 7% wood dust from hardboard production. Two thirds of the coal fired is

low-alkali South African coal to limit the overall alkali content of the fuel blend. Table 3 shows 

the fuel and ash analyses. It is seen that the biomass contributes significant amount of phosphorous and alkali metals. Sewage sludge contains high sulphur levels and heavy metals can

be in high concentrations. Sewage sludge also has much higher ash content than biomass and 

to avoid problems with slagging and to maintain ash quality, co-firing rate is generally limited. 

Proximate analysis

South 

African

US

CONSOL

Sewage

sludge

Olive

residue Wood Blend 

Moisture % wet 8.45 6.61 8.18 15.95 5.45 7.90 

Ash % dry 14.83 8.18 45.07 10.82 1.12 12.81

Volatiles % dry 24.01 38.35 47.93 72.79 79.15 34.47

Lower heating value kJ/kg 25,045 29,109 10,727 15,910 17,368 24,951 

Hardgrove index HGI 53 55 40 

Ultimate analysis

Carbon, total % dry 71.49 77.17 30.58 46.76 47.74 69.36

Hydrogen % dry 3.93 5.21 7.50 5.95 5.90 4.64 

Oxygen % dry 7.49 5.80 13.98 34.66 37.20 10.10

Nitrogen % dry 1.67 1.43 0.54 1.22 7.88 1.98 

Sulphur % dry 0.59 2.21 1.54 0.06 0.07 1.06 

Chlorine ppm dry 30 930 1100 10,000 160 646

Fluorine ppm dry 211 83 300 107 71 163

Ash analysis

SiO2 % dry 50.56 44.80 41.00 28.93 8.20 47.47

Al2O3 % dry 38.91 22.70 13.00 3.00 20.45 31.59

TiO2 % dry 2.27 1.03 1.10 0.20 44.50 2.1 

Fe2O3 % dry 3.42 19.10 11.00 3.94 8.02 7.4 

CaO % dry 1.20 4.35 13.00 23.28 8.65 3.9 

MgO % dry 0.25 0.97 1.50 12.80 2.68 0.87 

K2O % dry 0.77 1.58 1.70 17.34 2.46 1.47 

Na2O % dry 0.07 0.98 1.50 4.54 1.06 0.54 

P2O5 % dry 1.28 0.39 12.0 3.4 16.6 2.6 

As ppm dry 35 

Table 3 The fuel blend fired at Electrabel’s Langerlo Power Station, Belgium 

At the annual stops catalyst elements have been taken for chemical analysis and activity tests. 

Figure 14 shows the catalyst deactivation trend over a period of 55,000 operating hours. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 17 of 23

It is remarkable that the activity has stabilised at a level around 75% but this is in good accordance with a deactivation model developed at Haldor Topsøe. After an initial deactivation period, the deactivation almost ceases. The arsenic content in the catalyst increased with operating time until a level of 0.25% by weight at which no further uptake occurred, corresponding to

establishment of equilibrium with the flue gas concentration of gaseous arsenic. This level of 

arsenic has only a marginal influence on the activity of the high-porosity Topsøe catalyst. 

0%

20%

40%

60%

80%

100%

0 10,000 20,000 30,000 40,000 50,000 60,000

Operation hours

Relative catalyst activity, k/

k0

Unit 1 elements

Unit 2 elements

Deactivation model

Figure 14 SCR catalyst deactivation at Electrabel’s Langerlo Power Station, Belgium 

Likewise, chemical analyses have shown only a very modest uptake of alkali metals, around 

0.2% by weight sodium plus potassium, and less than 0.1% by weight phosphorous has been

found. After the initial accumulation, these levels have been constant and do not differ from 

what can be observed in catalyst on 100% coal-fired units. A slight fouling of the catalyst surface with fly ash seems to function as a ‘passivating’ layer in which the alkali metals have low 

mobility. 

Vattenfall, Combined Heat and Power, Sweden 

Uppsala CHP is a 400 MWth/125 MWe combined heat and power plant located in Uppsala, 

Sweden. In 2005, the existing SNCR was retrofitted with an up-flow high-dust SCR unit 

equipped with Topsoe DNX®

 catalyst. The pulverised-fuel boiler is 100% biomass fired, burning 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 18 of 23

peat and up to 30% wood dust. The plant is operating 2-3,000 hours per year during winter seasons when there is a demand for heat. 

Catalyst samples were tested in 2007 and 2008 after the first two seasons. The measured residual activity is shown in Figure 15. It is seen that during 5,000 operating hours the catalyst has 

lost 37% of its activity, corresponding to an exponential deactivation rate of 60% per 10,000 

hours. 

0%

20%

40%

60%

80%

100%

0 5,000 10,000 15,000 20,000

Operating Hours

R

elative specific activity, k/

k0

 Element activity tets

Exponential deactivation trend

Figure 15 SCR catalyst activity versus operating hours after 5,000 hours in Vattenfall’s Uppsala CHP 

The peat and wood contain 1-2 wt% potassium and around 1 wt% phosphorous in the ash. 

Chemical analysis of catalyst samples showed only a modest uptake of soluble alkali metals

(around 0.1 wt%) whereas 0.5-0.8 wt% iron and 0.2-0.4 wt% magnesium was found. The content of phosphorous had increased gradually from 0.26 wt% after 2,000 hours to 0.66 wt% after 

5,000 hours. The phosphorous was found primarily in the outer 200 microns of the catalyst wall 

at a level up to 1 wt%, cf. Figure 16. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 19 of 23

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 500 1000 1500 2000

Distance (μm)

Weight-%

Phosphorous

Potassium

Figure 16 Example of distribution of phosphorous and potassium through the catalyst wall of a catalyst 

sample from Vattenfall Uppsala CHP, Sweden. The phosphorous is predominantly found at 

the surface. 

Sierra Power, California, USA 

Sierra Power Corporation in Terra Bella, California, USA operates a small 9 MW grate-fired 

boiler at a saw mill, fuelled by wood and demolition wood. The plant provides a disposal site for 

local green waste lumber manufacturers and pallet manufacturers. The plant started up in 1986. 

In 2008 the plant was retrofitted with SCR DeNOx located downstream a cyclone. 

A catalyst element was tested after 3,000 operating hours. The element was covered with fine

ash and was mechanically cleaned with air before testing. The activity showed 93% activity 

relative to the fresh catalyst, corresponding to a deactivation rate of 22% per 10,000 hours. This

is far lower than the deactivation rate of 60% per 10,000 hours experienced for the pulverisedfuel boiler at Uppsala CHP described above. 

Chemical analysis revealed that the catalyst had accumulated more than 1% by weight soluble 

potassium and also minor amounts of sodium (Table 4). It was therefore decided to test washing of the catalyst in hot water to possibly remove the alkali-metal salts. The washing resulted in 

more than 60% reduction of the alkali metal content. Surprisingly, however, the catalyst activity 

dropped from 93% for the air-cleaned catalyst to 79% for the water-washed catalyst. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 20 of 23

Service hours 0 3,000 

Air cleaned Water washed

Catalyst activity 

% relative to fresh 100 93 79

Potassium, mg/kg 200 12,720 4,820 

Sodium, mg/kg 500 1,760 500 

Phosphorous, wt % 0.33 0.25 

Table 4 Catalyst activity and chemical analysis of catalyst from Sierra Power, USA

It is concluded that grate-firing of biomass does not evaporate alkali metals and release the 

salts as aerosols to the same degree as does pulverised-fuel or CFB firing and thus that this 

boiler type causes less SCR catalyst deactivation, cf. Table 2. Rather, the alkali-metal salts are 

found as larger particles with other ash constituents which may deposit on the outer surface of 

the catalyst. This explains the rather modest decrease in activity for the Sierra Power catalyst in 

spite of accumulation of more than 1 wt% potassium. Washing of the catalyst results in dissolution of the salts and transport to the catalyst interior (see Figure 17) and bonding to the vanadium active sites. The residual activity of 79% is in fair accordance with the potassium level of 

around 0.5%, cf. Figure 5. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000 1200

Distance (μm)

Weight-%

Figure 17 Potassium profile through the catalyst wall after water washing 

Vattenfall, Amager Power Station, Denmark 

Vattenfall’s Amager Power Station in Copenhagen, Denmark has three units that are all producing electric power to the grid and district heating. Units 1 and 2 were originally identical and 

taken into service in 1970/1971. Unit 3 was taken into service in 1991. Since 2004 Unit 1 has 

been out of service and the turbine, the boiler and the environmental control systems are being

replaced. Wet flue gas desulphurisation and SCR system including a regenerative gas-gas heat 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 21 of 23

exchanger has been supplied by Alstom [9]. The upgraded Unit 1 will have a firing capacity of 

350 MJ/s and be in service again in 2009. 

The unit has been designed to be very flexible regarding fuel mix and can burn coal, pelletised

straw, wood pellets, or coal-biomass in an arbitrary mixture. Fuel oil can be used as start-up or 

emergency fuel. The straw has high potassium and low ash contents with the potential of a 

rapid deactivation of the SCR catalyst, cf. Table 2. Comparing the composition of ashes from

coal and from straw it is seen that the potassium content differ one order of magnitude (Table 

5), also on an energy basis (Table 6). 

Reference coal Straw

SiO2 47 30-60

Al2O3 31 0.4-1.7

Fe2O3 6.3 0.3-1.0

CaO 4.2 7-14

MgO 1.7 1.7-2.3

TiO2 1.1 -

Na2O 0.9 0.4-1.2

K2O 2.0 14-29

Table 5 Composition of coal and straw ashes (% by weight) 

Reference coal Straw

LHV MJ/kg 22.8-26.2 15

Ash content % by weight 6-18 4.5 

K2O in ash % by weight 2.0 14-29

K2O per MJ g/MJ 0.046 – 0.137 0.42 – 0.87 

Table 6 Ash content of potassium on basis of fuel energy content 

To enable coal-straw co-firing in any desired ratio, the SCR DeNOx system was chosen as a 

tail-end installation, located downstream the ESP and wet FGD to minimise the amount of 

harmful ash constituents reaching the SCR reactor (Figure 18). 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 22 of 23

Figure 18 General layout of Vattenfall Amager Power Station Unit 1 (from [9]) 

Conclusions 

Direct co-firing of biomass with fossil fuel is a simple and widely applied technology for combining renewable and fossil energy utilisation to reduce CO2. However, the release of alkali-metal 

aerosols formed during combustion and phosphorous ash can cause rapid deactivation of SCR 

catalysts. The experience from a number of installations of Topsøe’s DNX®

 catalyst shows that:

− Co-firing of up to 20% biomass can be done without causing extraordinary deactivation of 

the catalyst compared to 100% coal firing. 

− For 100% biomass firing the deactivation rate can be as high as 60% per 10,000 hours in a 

pulverised-fuel fired boiler. In a grate-fired boiler the deactivation rate is lower, 22% per 

10,000 hours, presumably because less alkali-metal aerosols are released. 

− It is not possible to remove alkali metals bonded to the catalyst’s active sites by simple water washing. On the contrary, alkali-metal salts deposited in the outer surface of the catalyst 

may be transported to the interior, causing additional deactivation. 

− The formation of large-particle ash (LPA) during combustion of biomass must considered, 

necessitating installation of LPA screens.

− The use of SCR on 100% biomass-fired boilers may be done as a tail-end installation to 

minimise the amount of poisoning species entering the SCR. 

− Injection of coal fly ash in a wood-fuel oil co-fired boiler can reduce problems with deposits 

in the convective part of the boiler and in the air-preheater and significantly reduce catalyst

deactivation rate by capturing poisonous constituent of the wood ash. 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.
Impact of biomass co-combustion on SCR DeNOx operation Page 23 of 23

References

1. Topsøe, N.Y, Infrared Spectroscopic Investigations of Environmental DeNOx and 

Hydrotreating Catalysts, Doctoral dissertation, Technical University of Denmark (1998) 

2. Christensen, K.A., The formation of submicron particles from the combustion of straw, Ph.D. 

thesis, Technical University of Denmark (1995) 

3. Zheng, Y., A. D. Jensen, J. E. Johnson, J. R. Thøgersen, Deactivation of V2O5-WO3-TiO2

SCR catalyst at biomass fired power plants, Appl. Catal. B, 83 (2008), 186 

4. Brandenstein, Actual state of knowledge, Catdeact project, E.ON. engineering GmbH 

(2002) 

5. Jensen, J. P., H. D. Sørensen, J. Hansen, S. L. Christensen, Deactivation of high dust 

deNOx catalyst when co-combusting heavy fuel oil and biomass, Electric Power 

Conference, Chicago 2007 

6. Berg, M. and J.P. Jensen, Biomass and secondary fuels in Denmark, VGB Power Tech 

(2008) 

7. Pedersen, L. S., A. D. Jensen, Selective catalytic reduction of NO in a biomass-fired full 

scale PF boiler, Dansk Kemiingeniørkonference, Copenhagen, June 2006 

8. Bille, T., Biomass co-firing, development and experiences with operation, Expperts Conference, Brussels (2008)

9. Crespi, M., M. Porle, A.-C. Larsson, R. La Civita, A. R. Nielsen, The influence of biomass 

burning in the design of an SCR installation, Power-Gen Europe (2008) 

Presented at Power-Gen Europe, Germany, 2009

Best paper award within the conference track ‘Coal, Nuclear & Biomass Generation’

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by

or disclosed to third parties without written approval of Haldor Topsøe A/S.