SCR DeNOx catalyst considerations when using … DeNOx catalyst considerations when using biomass in power generation 2 / 27 2 Introduction An increased production of electricity from

SCR DeNOx catalyst considerations when using biomass in power generation 1 / 27

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SCR DeNOx catalyst considerations when using biomass in power generation Hans Jensen-Holm1, Francesco Castellino1 and T. Nathan White 1Haldor Topsøe A/S, 55 Nymøllevej, 2800 Lyngby, Denmark 2Haldor Topsøe, Inc., 17629 El Camino Real, Houston, TX 77058, USA 1 Abstract

There is a considerable interest in the utilization of biomass and municipal and industrial waste in the power production with the increasing requirements for reduction of CO2 emissions. Combined with ever-more-strict environmental regulations that control NOx emissions, the use of biomass fuel represents a unique 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 will be discussed. Alkali metals 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 cases of SCRs implemented to meet these new challenges and will include experience from a variety of plants firing biomass in Europe and USA. Operating experience from installations firing 100% biomass as well as installations co-combusting biomass with coal will be discussed. These experiences demonstrate that choosing the right plant configuration and proper catalyst formulation can limit the negative effects. Up to 20 weight per cent biomass can be co-combusted 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 minimize the amount of poisoning species entering the SCR. A low combustion temperature in grate combustion of wood and a low temperature in the SCR reduce the catalyst deactivating impact of the fuel’s content of alkali metals and phosphorous.

F:\CAT\SEK\WINWORD\HES\DNX materiale\Papers\Nyt layout\SCR DeNOx catalyst considerations when using biomass in power generation (2012).doc

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.


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2 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 utilization 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, coffee ground Animal wastes: Poultry litter, meat bone meal Municipal waste, sewage sludge, paper waste

Energy crops (quick growing): Switchgrass, miscanthus, poplar and willow, bamboo Peat The drivers for biomass in power production are political and social concerns about global warming as well as economical incentives. Figure 1 shows a projection of fuels in Danish energy supply; it is seen that in 2020 use of coal will be halved compared to 2010.




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Coal

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Figure 1 Projection of fuels in Danish energy supply (source: Force

Technology, Denmark). Biomass and coal have fundamentally different fuel properties. Biomass contains larger 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 utilized. Also for the operation of SCR systems there are technical challenges associated with co-combustion 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 sulfate- 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 2 shows examples of 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




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by Haldor Topsøe that considers ash deposition rate, pore plugging development and chemical impact on active sites. Deactivation rate will depend on actual fuel specification and boiler type.

Table 1. Ash characteristics of various fuels The figures are examples only and can vary widely

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Ash wt% dry 15 5 0.1 3.8 0.5 4 K2O wt % 1.2 0.6 0.2 1.1 8 17 Na2O wt % 0.1 1.5 2 0.1 0.7 0.1 Nickel wt % - - 5 0.01 - - Vanadium wt % - - 20 0.01 - - P2O5 wt % 0.4 0.8 - 2.7 3 2.3 CaO wt % 2 22 2 9.8 29 8 SCR catalyst deactivation % per 10,000 hours (model simulation cf. Figure 2)

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Figure 2 Examples of deactivation of SCR catalyst for various fuel types.




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Catalyst deactivation in SCRs on coal-fired boilers is in the range 8%-20% per 10,000 hours depending on the coal type whereas it for biomass can be three times higher depending on type of biomass and boiler type. Due to lower combustion temperature deactivation on SCRs on grate-fired boilers is lower than in SCRs on pulverized-fuel fired boilers. 3 Catalyst deactivation mechanisms

Vanadium pentoxide based SCR catalysts may deactivate during operation to both chemical and physical mechanisms of deactivation, such as poisoning, fouling, sintering, pore blocking and channel blocking. The contribution of each of these mechanisms to the overall deactivation rate will, however, depend on the particular application. In the case of 100% biomass combustion, deactivation by alkali metals (e.g. potassium and sodium) and phosphorous are seen as the main responsible for deactivation. Pore blocking and fouling of the catalyst surface by fly ash will also contribute to the measured deactivation rate. 3.1 Alkali metals

The deactivation mechanism of alkali and alkaline-earth metals has been widely studied during the last decade, mainly by doping SCR catalysts with these metals by wet impregnation methods. It is now well understood that the poisoning mechanism occurs via chemical bonding of the alkali metal to the crucial –V-OH sites, the so-called Brønsted-acid sites. These sites are responsible for the adsorption of ammonia on the catalyst surface, which constitutes the first step in the SCR reaction mechanism. In the presence of an alkali metal, very stable –V-OM (M being K or Na) complexes are formed, thereby blocking the acid cycle (see Figure 3).




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V O V

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Figure 3 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 oxidized by either NO or O2 to regenerate the original V=O species.1

The doping methods applied in laboratory investigations of alkali-metal induced deactivation ensure both uniform distribution of poison and close contact with the active sites. However, in real applications the process of alkali accumulation in the catalyst is completely different, and several parameters (e.g. maximum temperature of combustion, SCR temperature, etc.) play an active role in the overall deactivation rate measured. In fact, depending on the combustion technology adopted, a significant fraction of the alkali metals present in the biomass is readily released into the gas phase. During cooling down to the typical SCR temperatures these species undergo both homo- and heterogeneous condensation. The flue gas entering the SCR reactor is therefore characterized by the presence of sub-micron particles rich in alkali metals. The deposition of these particles on the catalyst surface is the first step of the deactivation process. This fact has been clearly demonstrated by different works2,3 carried out during the last decade in collaboration with the Technical University of Denmark (DTU), where commercial full-size Haldor Topsøe catalyst monoliths have been exposed to both a flue gas containing KCl particles and at the Masnedø power plant firing 100% straw in a grate-fired boiler. These studies have shown that the sole presence of sub-micron K-containing particles, to some extent irrespective of the total mass concentration of potassium in the flue gas, is a necessary condition for the deactivation to happen.




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However, evidence for the real mechanism for penetration of potassium into the catalyst wall and subsequent poisoning of the vanadium active sites is still lacking.

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Figure 4 Potassium accumulation in the catalyst wall is supposed to

follow a three-step process involving (1) particle deposition; (2) particle-catalyst reaction and (3) potassium surface diffusion

Recent research performed at Haldor Topsøe by exposing different catalyst compositions to KCl-particle-containing flue gas has shown that K atoms are dragged out from deposited particles by reactions occurring between the deposited particle and the catalyst surface. Once on the surface, K is readily redistributed by surface diffusion (Figure 4). Most interestingly, it has been shown that the vanadium itself is responsible for the occurrence of this reaction. As shown in Figure 5, more potassium is accumulated in catalysts with higher vanadium contents, implying that increasing the catalyst’s vanadium content beyond a certain value is not a viable way for limiting the alkali metal deactivation rate.




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Figure 5 The amount of vanadium in the fresh catalyst has an effect on the potassium

accumulation rate. Catalysts with different vanadium contents were simultaneously exposed to a KCl-particle-containing flue gas for 850 hours at 662°F (350°C). A linear dependency between the V-content and the catalyst’s uptake of potassium has been found (left). As a result, at the end of the exposure the residual activity is independent of the catalyst’s vanadium content beyond a well-defined value (right).

3.2 Phosphorous

In several biomass fuels (e.g. meat bone meal, sewage sludge) phosphorous is organically bound and is released in the gas phase during combustion. Depending on the combustion temperature and atmosphere, gaseous polyphosphoric acids are formed and will condense during cooling of the flue gas at temperatures below 850°C (1472°F). At the SCR reactor inlet, this latter will be then characterized by the presence of liquid aerosols which will readily deposit on the catalyst outer surface and penetrate the catalyst wall by capillary forces, thus reducing the total pore volume and surface area of the catalyst, see Figure 6. A recent study4 has showed that polyphosphoric acid deposited on a vanadium-based SCR catalyst is also able to reduce the redox properties of the catalyst, thus deactivating it by reacting with the redox V=O sites participating in the “activation” of adsorbed ammonia (see Figure 3 page 6). In some cases 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.




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Fresh catalyst surface Exposed to 10 ppm H3PO4

The porous structure of titania is still distinguishable

Exposed to 1000 ppm H3PO4 The surface is uniformly covered by a glassy layer

Figure 6 Fouling of catalyst surface by polyphosphoric acid4

4 Catalyst deactivation in boilers co-firing biomass

As described in the previous sections, catalyst deactivation by potassium and phosphorous from biomass combustion originates from the release of these elements in the gas phase during combustion and the subsequent formation of sub-micron aerosols via homogeneous nucleation. In combustion of most coals, these elements are to a large extent bound to very stable minerals which will not undergo any transformation during combustion. Co-firing biomass with coal or fly ash has been proved to be an effective measure for limiting the deactivation since the gaseous K- and P-species released from the biomass are readily reacting with the coal fly ash, forming much more stable compounds and larger ash particles. Care must be paid, however, in finding the optimum ratio between the amount of biomass and coal fired since the resulting ash composition properties may differ from the one calculated by a simple weighted average. 4.1 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 modernized 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 tons/hour of straw can be fired through




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each burner. With up to 10% co-firing of straw, the fly ash from the units can be utilized in the concrete industry. 4.1.1 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 coal-reference for the deactivation tests. Three different types of catalysts were investigated. Figure 7 shows the relative specific activity as a function of exposure time for a Topsøe catalyst.

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Figure 7 Relative specific catalyst activity as function of exposure time in a pilot reactor at Studstrup Power Station in Denmark. No significant difference in deactivation trends was observed between firing coal or co-firing coal and 7% straw (energy basis).

There is no distinguishable difference in the deactivation trend from exposure to flue gas from coal-firing and from 7% straw co-firing, thus confirming that the alkali metals released from the straw are effectively, 80-85%, captured as more stable potassium alumino-silicate compounds 5. The observed deactivation was found to be caused by arsenic and formation of surface fouling layers.




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4.1.2 Full scale

In 2007 the two units were retrofitted with SCR DeNOx units with Topsøe DNX® catalyst which were commissioned in 2007. Shortly after start-up of the SCR on Unit 4, a rapid increase in pressure drop was observed and inspection 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 8. 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. After 14,000 hours of operation firing on average 10% straw on an energy basis a catalyst was tested for activity, showing 89% residual activity. This confirms that pilot study that the deactivation when co-firing straw is on par with deactivation experienced with 100% coal firing.

Figure 8 Plugging of the SCR catalyst at Studstrup Power Station’s Unit 4 in

Denmark was caused by large straw-ash particles, seen as black particles in the otherwise grey coal ash.

4.2 E.ON Langerlo Power Station, Belgium

E.ON operates two identical 250 MWe coal-fired boilers at their Langerlo Power Station at Genk, Belgium. As part of the so-called Langerlo 2000 Environmental Project the two units were retrofitted with flue-gas desulfurization and SCR DeNOx systems in the late 1990ies.




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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. The biomass contributes significant amount of phosphorous and alkali metals. The sewage sludge contains high sulfur 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. At the annual stops catalyst elements have been taken for chemical analysis and activity tests. Figure 9 shows the catalyst deactivation trend over a period of 55,000 operating hours. It is remarkable that the activity has stabilized at a level around 75% but this is in good accordance with a deactivation model developed at Haldor Topsøe, cf. Figure 2 on page 4. 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.

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Figure 9 SCR catalyst deactivation at E.ON’s Langerlo Power Station, Belgium




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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. 5 100% biomass-fired boilers

5.1 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 energy efficiency up to 94% and an electrical efficiency of 49%. 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. 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 tons per year (from 2010: 500,000 t/y) of pulverized 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 desulfurization units. It was experienced8 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 sulfur 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 vaporizes during combustion, forming a fine aerosol of KCl and K2SO4 particles. A catalyst deactivation monitoring program was initiated. Experience during wood-oil co-firing showed that the catalyst deactivated rather fast as shown in Figure 10.




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Figure 10 Fuel history and catalyst deactivation on DONG Energy’s Avedøre Power Station Unit 2 in Denmark

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 re-insertion of the catalyst and the outage was reduced to three days 9. In the following years washing was applied several times per year. However, after three years and consumption of 600,000 tons of wood pellets, the activity of the catalyst had been reduced by more than 50%. The second layer of catalyst was added in 2006 after five years of operation. To reduce the impact of wood co-firing, injection of coal fly ash into the furnace was initiated in 2006. The fly ash, up to around six tons/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 sulfuric acid aerosols in the stack were also reduced significantly 7. 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 as shown in Figure 10 this has reduced the catalyst deactivation to less than 15% per 10,000 hours compared to on average approximately 25% per 10,000 hours before fly-ash injection.




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5.2 Vattenfall Nordic, combined heat and power plant, 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 Topsøe DNX® catalyst. The pulverized-fuel boiler is 100% biomass fired, burning peat and up to 30% wood dust. The plant is operating only a few thousands hours per year during winter seasons when there is a demand for heat. Catalyst samples were tested after the first two seasons. The measured residual activity is shown in Figure 11. 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. Recent operating data indicate some leveling out of the deactivation rate.

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Figure 11 SCR catalyst activity versus operating hours after 5,000

hours in Vattenfall Nordic’s Uppsala CHP, Sweden

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% as shown in Figure 12. This is in good accordance with the polyphosphoric acid aerosol condensation mechanism described in the section on phosphorous deactivation on page 8.




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Figure 12 Example of distribution of phosphorous and potassium through

the catalyst wall of a catalyst sample from Vattenfall Nordic’s Uppsala CHP, Sweden. The phosphorous is predominantly found at the surface.

5.3 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. Activity testing 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 50-60% per 10,000 hours experienced for the pulverized-fuel 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. This high amount of alkali metals would normally result in severe deactivation. It is thus concluded that grate-firing of biomass does not evaporate alkali metals and release the salts as aerosols to the same degree as does pulverized-fuel or CFB firing and thus that this boiler type




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causes less SCR catalyst deactivation. Rather, the alkali-metal salts are found as larger particles with other ash constituents which deposit on the outer surface of the catalyst. This was convincingly demonstrated by results from washing of the catalyst in hot water. Although overall potassium and sodium contents in the catalyst were significantly reduced, catalyst activity dropped from 93% to 79%. The salts had dissolved in the washing water and penetrated the catalyst. A catalyst element was tested after approximately one year of operation. The activity appeared to have been reduced by approximately 25%. Chemical analysis revealed accumulation of significant amounts of alkali metals and calcium. As shown in the scanning electron microscopy photo in Figure 13, the catalyst surface was severely fouled by deposits rich in potassium and calcium. No phosphorous was found to have accumulated on the catalyst in contrast to the catalyst at the peat fired Uppsala CHP. This is ascribed to the lower content of phosphorous in wood (on fuel basis) and to the lower combustion temperature resulting in little formation of polyphosphoric acid.

Figure 13 Scanning electron microscopy (SEM) photo of the

catalyst surface after approximately one year of operation at Sierra Power, CA, USA

S-, K- and Ca-rich

agglomerates

It was decided to clean the catalyst by washing, this time in a combination of pH-adjusted water and demineralized water. A SEM photo of the surface before and after washing is shown in Figure 14.




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Figure 14 SEM photo of the catalyst surface before (left) and

after wash (right). Surface deposits have effectively been removed.

It is clearly seen that the surface deposits have by and large been removed. Chemical analysis confirmed that around two thirds of the potassium had been removed. Some vanadium had been removed as well; still, however, the activity of the washed catalyst was in excess of 95% of fresh catalyst activity. Figure 15 shows microprobe analysis of the catalyst wall before and after washing. The unwashed catalyst has a high surface concentration of potassium. This surface peak is not present in the washed catalyst, showing a uniform potassium concentration throughout the wall cross section at a level similar to that in fresh catalyst.

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Figure 15 Micro-probe analysis of potassium (red line) and phosphorous through the catalyst wall before (left) and after (right) washing. The unwashed catalyst shows a high surface concentration of potassium.




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This confirms that the alkali metal salts from the wood combustion only to a very limited extent have been released as aerosols but rather are found as larger ash particles which deposit on the outer surface of the catalyst. These deposits reduce activity by blocking the surface but can to a large extent be removed by washing. However, the experiences at Sierra Power demonstrated that care must be taken when washing the catalysts. Since most of the deposited alkali metal salts are water soluble, washing by water will initially favor a redistribution of alkali inside the catalyst wall, thus reducing the activity of the washed elements. Plentiful water has to be used during the washing process to minimize the alkali metal concentration in the resulting washing solution and ensure that the catalyst pores are left filled with almost clean water before drying. 5.4 Vattenfall Nordic, Amager Power Station, Denmark

Vattenfall Nordic’s Amager Power Station in Copenhagen, Denmark has three units that are all producing electric power to the grid and district heating. In the period 2004-2009 Unit 1 was out of service and the turbine, the boiler and the environmental control systems were replaced. Wet flue gas desulfurization and SCR system including a regenerative gas-gas heat exchanger has been supplied by Alstom11. The upgraded unit has a firing capacity of 350 MJ/s and was put back in service in late 2009. The unit has been designed to be very flexible regarding fuel mix and can burn coal, pelletized straw, wood pellets, or coal-biomass in an arbitrary mixture (although coal is not foreseen to be used as fuel). 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 1 page 4. 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 minimize the amount of harmful ash constituents reaching the SCR reactor (Figure 16). The operating temperature of the SCR is 550-600°F (288-316°C).




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Figure 16 General layout of Vattenfall Nordic’s Amager Power Station Unit 1

(source: Vattenfall). After some months of operation firing 100% biomass in the form of straw- and wood pellets, the pressure droop across the SCR increased rapidly. It appeared that in spite of regular soot blowing a layer of very fine-particle dust (1-10 μm particle size) covered the entire face of the catalyst, see Figure 17. The dust appeared to have deposited only at the very inlet to the catalyst whereas the channels themselves were clean. The dust contained large amounts of especially potassium and calcium but also some iron and silicon and appeared to be consistent with ash from burning of wood and straw. Since the amounts and origin of the wood and straw fired are not logged separately it has not been possible to attribute the incident to a particular fuel or fuel mix. The catalyst was cleaned and the soot blower pressure and the soot blowing frequency increased. The unit has been in operation until the summer stop in June 2010.




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Figure 17

A thin layer of very fine-particle ash covers the catalyst at Vattenfall Nordic’s Amager Power Station Unit 1, Denmark.

Investigations are in progress to identify the root cause of this incident and to eliminate the possibility of reoccurrence. Among other things, the electric charge on the surface of the particles will be measured to possibly reveal if static electricity plays a role. 5.5 Low-dust wood firing

Topsøe DNX® catalysts are installed in a number of SCR units on small, around 20 MW, wood-fired boilers in the USA. Characteristic of these SCRs is that they are located after an ESP. Dust level in the flue gas to the SCR thus generally is low and they are operating at very low temperatures, 320-420°F (160-215°C). 5.5.1 Choice of catalyst pitch and keeping the catalyst clean

A 16 MW grate-fired boiler, burning wood chips from virgin wood and forest waste material was retrofitted in 2008 with a vertical up-flow SCR located after the ESP. In stead of soot blowers, the SCR was installed with the capability to reverse the flue gas flow to a vertical downward flow for catalyst cleaning. The SCR was loaded with a Topsøe catalyst with a 4 mm pitch on the basis of a flue gas ash content of 15 mg/Nm3. It turned out that actual dust load was significantly higher, several hundred mg/Nm3 and the catalyst needed removal every 3-4 months for cleaning. During an upgrade in 2009 for increased power production, the catalyst was replaced with a larger pitch and different formulation catalyst. The catalyst pressure drop has not seen any rise since the installation of the new catalyst.




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This reverse-flow reactor design has been applied to two other SCRs on wood-fired boilers in New Hampshire, USA as well. These SCRs were originally (in 2008) installed with vertical down-flow and horizontal flow, respectively, but were converted to enable reverse flow operation a year later to maintain catalyst draft loss at an acceptable level. The catalyst pressure drop issues are related to the combination of very fine-particle highly charged ash and the location of the SCR downstream an ESP. Grate combustion of biomass generates particulates with a high concentration in the sub-micrometer range. The particles are charged as they leave the ESP and they do not dissipate the charge before they are introduced to the catalyst face. This is noticeable as the particles in one installation have been observed to cling to the duct walls and structure inside the reactor. The dust deposition pattern on catalysts in low dust applications furthermore is very different from the deposition pattern in high dust applications. The difference occurs from the difference in particle size distributions of the fly ash particles. The collection efficiency of an ESP is generally very high but experience has shown that there is a “penetration window” in the sub-micrometer size range where the collection efficiency can be significantly lower12. This results in a particle size distribution that is shifted towards the finer fraction of the fly ash. Larger particles with a high inertia (fly ash particles larger than 20 μm), would have a cleaning effect by detaching and re-suspending already deposited particles. The build-up of fine particles at the catalyst face reduces the channel opening and continues until equilibrium is reached. With a too small channel size, the build-up may eventually bridge the channel. Thus, although the flue gas after the ESP contains only little fly ash, a relatively large catalyst pitch should be selected. 5.5.2 Discussion of catalyst chemical analysis and activity testing result

Table 2 shows results from activity tests and chemical analyses made on catalyst elements after about 8,000 hours and about 12,000 hours of operation in an SCR at 320-360°F (160-180°C) on a wood fired boiler. The chemical analysis showed significant uptake of potassium, around 0.7% by weight and a slight accumulation of phosphorous, around 0.25% by weight.




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Table 2. Activity and chemical analysis of catalyst samples from the SCR on a grate-fired boiler burning wood

Fresh 8,000 hrs 12,000 hrs

Relative catalyst activity 100% 80% 71%

Potassium, wt% 0.02 0.76 0.61

Phosphorous, wt% 0.15 0.24 0.25

The 20-30% activity loss in 8-12,000 hours corresponds to a deactivation rate of around 25% per 10,000 hours. A potassium content of 0.6-0.8% will in general cause an activity loss of 30-50%. In this case the deactivation is lower. As described for the Sierra Power case (page 16) the reason is that the potassium-containing ash to a large extent is found as deposits on the catalyst surface. This was confirmed by tests of the catalyst from another wood-fired boiler. A catalyst element was tested after 6,500 hours of operation. It showed virtually no deactivation. Chemical analysis showed significant potassium content, however very unevenly distributed. One sample from the element had 0.43% by weight potassium whereas another sample had 1.14% by weight. The phosphorous content was only slightly higher than for fresh catalyst, 0.2-0.3% by weight. Scanning electron microscopy showed the catalyst surface to be generally rather clean but potassium-rich grains could be identified, see Figure 18.

Figure 18

Catalyst surface of sample from a wood fired plant. It appears rather clean (left). The bright grains (white spots) on the surface contain potassium (right).




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An analysis by Quantitative Scanning Electron Microscopy of the catalyst wall cross section, Figure 19, showed virtually no potassium uptake. It can therefore be concluded that the potassium found by bulk chemical analysis originates from potassium-containing deposits on the surface rather than from potassium located in the catalyst pore structure.

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Figure 19

QSEM analysis of the catalyst wall cross section showed no uptake of potassium.

The findings that the alkali-metal salts are primarily found on the catalyst surface and that the catalyst deactivation is relatively modest at 25-30% per 10,000 hours has been confirmed by similar testing of catalysts from other wood-fired boilers. It is remarkable that the catalyst deactivation in these SCR installed on wood-fired boilers are relatively modest. This can be ascribed to three factors:

- The alkali metal salts in wood is only to a minor part released as sub-micron aerosols due to a lower combustion temperature in grate-fired boilers compared to pulverized-fuel boilers

- The relatively low temperature in the SCR reduces the rate of surface diffusion of alkali metals to active vanadium sites in the catalyst

- At low SCR operating temperature the catalyst exhibits a lower susceptibility to alkali metal poisoning than at higher temperatures




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At typical SCR conditions the surface concentration of redox sites which activates the adsorbed ammonia (V=O sites cf. Figure 3 page 6) is much smaller than the surface concentration of NH3-adsorbing acid sites (V+5-OH sites)13. At low temperature the catalyst’s overall rate of reaction therefore is limited by the number of redox sites. At higher temperature the NH3 adsorption equilibrium constant decreases and the number of available acid sites will limit the reaction rate. As the alkali metals ions react selectively with the acid V+5-OH sites, the deactivation will appear higher at higher temperature. The effect of a low temperature in the SCR on the distribution of alkali metals in the catalyst wall has been convincingly demonstrated by activity tests of catalyst samples with ash deposits from an SCR on a wood-fired boiler. The activity was measured at 485°F (252°C) and 665°F (352°C). The catalyst sample was the heated to 850°F (454°C) for one hour and the activity measurements were then repeated at the lower temperatures. The results are shown in Table 3.

Table 3. Activity of a catalyst from an SCR on a wood-fired boiler operating at 420°F

Relative activity compared to fresh catalyst, %

As received After 1 hour exposure to 850°F

At 485°F (252°C) 57 29

At 665°F (352°C) 49 19

It is seen that the catalyst activity drops by more than 50% by exposing the catalyst to elevated temperature for one hour. The deactivation is ascribed to redistribution by diffusion of the alkali metals, cf. Figure 1 (page 3) as well as a release of additional alkali metals from the surface deposits of wood ash.




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6 Conclusions

Firing of biomass, either 100% or by co-firing with fossil fuel is a simple and widely applied technology for combining renewable and fossil energy utilization to reduce CO2. However, the release of phosphorous and alkali-metal aerosols formed during combustion can cause rapid deactivation of SCR catalysts. The mechanism of phosphorous poisoning has been shown to be primarily a masking of the catalyst surface by condensation of polyphosphoric acid whereas alkali metals penetrate the catalyst via diffusion and react chemically with the ammonia adsorbing vanadium sites. 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. Alkali metal is to a large extent captured by reaction with the coal fly ash.

Injection of coal fly ash in a wood-fired boiler can reduce the catalyst deactivation rate in a high-dust SCR by capturing poisonous constituent of the wood ash, resembling the conditions in coal-biomass co-firing.

For 100% biomass firing the deactivation rate can be as high as 50-60% per 10,000 hours in a pulverized-fuel fired boiler

Due to a low combustion temperature in moving grate wood fired boilers less alkali-metal aerosols are released to the flue gas. Potassium salts deposited in the catalyst in SCRs located downstream an ESP are to a large extent found as surface deposits rather than in catalyst pore system. A low SCR temperature furthermore makes the catalyst less susceptible to alkali metal poisoning. Resulting catalyst deactivation rate is much lower than in PF fired boilers, in the range of 30% per 10,000 hours.

Although wood contains significant amount of phosphorous, no phosphorous uptake on the catalyst is experienced in SCRs on grate-fired boilers. This is in contrast to the experience from a pulverized-fuel boiler firing peat where a main cause for catalyst deactivation was condensation of polyphosphoric acid on the catalyst.

Care must be taken when washing biomass ash contaminated catalysts. The alkali metal salts deposited on the catalyst are water soluble and washing by water can cause redistribution to the catalyst interior, causing additional deactivation. Plentiful water has to be used during the washing process to minimize the alkali metal concentration in the washing solution.

Even though the SCR catalyst is installed in a low-dust position downstream an ESP, provisions for catalyst cleaning must be considered. Furthermore, the sub-micrometer ash fraction downstream the ESP tends to build up on the catalyst face and a relatively large catalyst pitch should be selected.




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7 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, 3-4 (2008), 186

4. F. Castellino, S. B. Rasmussen, A. D. Jensen, J. E. Johnsson, R. Fehrmann, Deactivation of vanadia-based commercial SCR catalysts by polyphosphoric acids, Appl. Catal. B, 83, 1-2 (2008), 110

5. B. Sander (DONG Energy), Experience of co-combustion of biomass and waste in coal-fired power plants, Topsøe Catalysis Forum, 2009

6. Brandenstein, Actual state of knowledge, Catdeact project, E.ON. engineering GmbH (2002)

7. 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)

8. Berg, M. and J.P. Jensen, Biomass and secondary fuels in Denmark, VGB Power Tech (2008)

9. Pedersen, L. S., A. D. Jensen, Selective catalytic reduction of NO in a biomass-fired full scale PF boiler, Dansk Kemiingeniørkonference, Copenhagen, June 2006

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

11. 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, Powergen Europe (2008)

12. Strand, M., J. Pagels, A. Szpila, A. Gudmundsson, E. Swietlicki, M. Bohgard and M. Sanati, “Fly ash penetration through electrostatic Precipitator and flue gas condenser in a 6 MW biomass fired boiler, Energy and Fuels, 16, 1499 (2002)

13. Dumesic, J. A., N.-Y. Topsøe, H. Topsøe, Y. Chen and T. Slabiak, “Kinetics of Selective Catalytic Reduction of Nitric Oxide by Ammonia over Vandadia/Titania, J. Catal., 163, 409-417 (1996)

Presented at: Power Plant Air Pollutant Control "MEGA" Symposium, Washington DC, USA, 2010

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