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Effects of sewage sludge and meat and bone meal Co-combustion on SCR catalysts
Applied Catalysis B: Environmental 49 (2004) 15–25
Effects of sewage sludge and meat and bone meal Co-combustion on SCR catalysts Jens Beck a,∗ , Jürgen Brandenstein b , Sven Unterberger a , Klaus R.G. Hein a a
Institut für Verfahrenstechnik und Dampfkesselwesen (IVD), Universität Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany b E.On Engineering, Bergmannsglückstraße 41-43, 45896 Gelsenkirchen, Germany Received 1 July 2003; received in revised form 30 October 2003; accepted 15 November 2003
Abstract DeNOx catalysts for the selective catalytic reaction (SCR) in coal fired power plants are deactivated by catalyst poisons such as As, Na, K, P, etc. The deactivation rate depends on the fuel quality and the concentration of catalyst poisons in the fuel. However, operational parameters such as O2 content, residence time, combustion and flue gas temperature also have an influence on decreasing catalyst activity. Equilibrium calculations were carried out to identify possible catalyst deactivation reactions. The calculations concentrated on the elements phosphorus and sodium as primary catalyst poisons in animal residues or sewage sludge. A set of deactivated catalyst samples from different power plants were analysed with respect to surface area, chemical composition and permeability. From the results obtained, deactivation mechanisms were derived which seem to cause increased catalyst deactivation due to P-rich secondary fuels. The samples showed a high P and alkali concentration on the surface. The surface area and the pore volume decreased. Compared to pure coal combustion this decrease was significant for meat and bone meal co-combustion and was also noticed for samples exposed to sewage sludge co-combustion. A correlation between relative activity and Na and P concentration is established. The results of the surface analyses indicate pore condensation as a major deactivation mechanism. © 2003 Elsevier B.V. All rights reserved. Keywords: DeNOx ; Catalyst; SCR; Deactivation; Phosphorus; Alkali; Secondary fuel; Co-combustion
1. Introduction The economic operation of coal fired power plants requires a cost effective fuel supply. Recently, the thermal recycling of residues as secondary fuel is of increasing interest for power plant operators. Consequently, a variety of coal blends and secondary fuels such as sewage sludge, straw, wood or meat and bone meal (MBM) is combusted. The resulting effects on the SCR DeNOx reactor with respect to the fuel type and the operating conditions of the plant have not yet been thoroughly investigated. The activity of the catalyst, which is a measure for the ability to reduce NOx emissions, can rapidly decrease. Alkali metals for instance were found to have a high deactivating potential as has been described by Chen et al. [1] and others
∗
Corresponding author. Tel.: +49-711-685-7760. E-mail address: [email protected] (J. Beck).
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.11.007
[2,3]. Investigations have shown that the dominant parameter for deactivation is the reactivity of the compound, not the elemental concentration of the poison in the fuel. For purely coal fired power plants, the effect of alkali deactivation is negligible since the alkali content is relatively low and the alkali fractions in the fuel are usually bound in very stable compounds such as silicates which do not react even at high temperatures [4,5]. Considering renewable fuels like straw and wood, the alkali metals, especially K, have a considerable effect on catalyst deactivation. In these fuels, the K is bound as sulphate or chloride which have low boiling points. The deactivation effects and possible countermeasures are described elsewhere in further detail [2,3,6]. Phosphorus is also another typical constituent of coal. The total concentration of P is below 1 wt.% so that no significant deactivating effect can be attributed to P because effects from other poisons prevail. The deactivation potential of P has been described by Chen et al. [1]. The behaviour of catalysts extruded with H3 PO4 has been investigated by Blanco [7].
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
His experiments were conducted on catalysts used in HNO3 plants where H3 PO4 was used as an extruder. A decrease of surface area and pore volume was observed. XRD analysis immediately taken after exposure suggested the presence of vanadyl phosphates. The later analysis of spent catalysts did not show any crystalline phosphates. Therefore, it was assumed that these phosphates are unstable. Although the experimental conditions of Blanco’s experiments differ from the situation in power plants, operational experience shows that the co-combustion of P-rich sewage sludge or MBM has a substantial deactivating effect on catalysts. Previous XRD analysis of spent catalysts however showed no crystalline structures which might be assigned to P compounds. However, a correlation between the relative activity and the P concentration was evident. The aim of the current investigation is to identify the parameters affecting the deactivation of high dust SCR catalysts caused during the co-combustion of sewage sludge and MBM. Previous examinations performed on catalysts exposed to full scale co-combustion conditions have shown phosphorus to be a primary deactivating compound; therefore, it will be a focus of this investigation. However, since sewage sludge has roughly the same P content as MBM, the deactivation rate during its co-combustion is considerably lower. MBM in addition contains also high quantities of alkalis which also need consideration. In order to gain information on possible reaction mechanisms, the origin of the P, Na and K compounds in the fuel is studied. On this basis, possible reaction schemes on the formation of deactivating compounds are determined by equilibrium calculations, upon which derived deactivation mechanisms are based. For comparison, deactivated plate type and honeycomb type catalyst samples of different power plants are analysed. The relative activity is determined, XRF analysis of the bulk and the surface composition is carried out, BET and a gas permeability measurement give information on the surface properties. From
the comparison of the data, conclusions on the prevailing deactivation mechanisms are drawn.
2. Experimental A set of samples composed of five deactivated honeycomb catalysts and seven deactivated plate type catalysts were analysed. The samples were subject either to sludge or MBM co-combustion and were taken from DeNOx reactors operating in full-scale high dust plants of dry ash as well as of wet bottom PC boilers. Two reference samples of pure coal combustion have also been included. For an estimate of possible reaction mechanisms thermal equilibrium calculations were carried out. As a basis for the calculations a literature study concerning the nature of P and alkali compounds contained in both secondary fuels was made. With this information, the flue gas concentration of P and Na compounds for usual fuel mixes was approximated. The catalyst samples were analysed by XRF, BET and permeability and the results were evaluated with regard to major deactivation mechanisms (Table 1). 2.1. Catalyst samples Table 2 gives an overview of the samples, exposure, fuel mix and relative activity k/k0 . Type A are honeycomb catalysts; type C are plate types from two different manufacturers. The relative activity of the samples was determined. Analyses of the chemical composition of the surface and of the bulk material at inlet and outlet were carried out. This analysis was supplemented by surface characterisation using BET and permeability measurements. The contribution of the exposure time to the loss of activity was not possible since, due to the restraints of full-scale conditions, the samples were not exposed to co-combustion over the whole operation.
Table 1 Operating conditions and remaining relative activities of the sample set Sample
A.1 A.2 A.3 A.4 A.5 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
Boiler type: wet bottom (w) dry ash (d)
Operating time (h)
Relative activity after exposure, k/k0
Fuel mix (thermal basis) (%) Coal
MBM
Sludge
d d w w d d d w d d d w
14000 46000 18500 55000 n.a. 24500 41500 65000 2000 50000 50000 39000
0.86 0.62 0.53 0.37 n.a. 0.8 0.64 0.58 0.71 0.67 0.67 0.49
X X X X X X X X Oil X X X
0 0 ca. ca. ca. 0 0 ca. ca. 0 0 0
0 ca. 0 0 0 ca. ca. 0 0 ca. ca. 0
Note: Co-combustion was not carried out over the total operating time. a Animal fat.
4 4 4
4 20a
4
4 4
4 4
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25 Table 2 Comparison of coal and secondary fuels [8–10]
LHV (raw) (MJ/kg) Moisture (raw) (%) Volatile matter (dry) (%) Ash (dry) (%)
Hard coal
Sewage sludge
MBM
30.18 2.03 33.16 9.03
10.58 3 49.52 45.1
15.7 8.3 65 26.5
23.6 13.4 17.8 10.3 0.4 0.7 14.2 2.2a
2.95 41.7 0.4 0.5 5.9 2.75 36.2
500 ◦ C)
Ash composition (prepared at Si as SiO2 (%) 40.09 Ca as CaO (%) 5.48 Fe as Fe2 O3 (%) 13.3 23.6 Al as Al2 O3 (%) Na as Na2 O (%) 1.51 K as K2 O (%) 2.91 0.19 P as P2 O5 (%) S as SO3 (%) 7.4 a
Data from E.On.
2.2. Analytical methods 2.2.1. Equilibrium calculations For the analysis primary P and alkali compounds in MBM and sewage sludge were considered. Primary flue gas constituents SO2 , H2 O and CO together with the fly ash components CaO and Al2 O3 and SiO2 were regarded as reaction partners in the relevant temperature range of 400–1400 ◦ C. Na was considered for reactions with P and with the active components of the catalyst V2 O5 , WO3 and MoO3 . The thermal equilibrium calculation software package FactSage Version 5.0 was used for the equilibrium calculations. 2.2.1.1. Sewage sludge. The P content in sewage sludge after the waste water treatment is approximately 7–10%. Ninety percent of the phosphorus in sludge is organically bound (i.e. nucleic acids, phosphorus proteins, phosphorus lipids and adenosintriphosphate ATP, etc.) and only 10% are insoluble salts [11]. The organically bound P-fractions are assumed to be thermally unstable so that the gaseous P compounds will be released at low temperatures around 500 ◦ C. It is from these compounds, as it is described below, that phosphorus oxides, calcium phosphates or phosphoric acid will be formed. During the chemical waste water treatment Ca, Fe and Al salts are added to extract the P chemically from the water, forming either Ca2 HPO4 (OH)2 , Ca10 (PO4 )6 (OH)2 , FePO4 or AlPO4 . According to Corbrigde [12], AlPO4 is highly insoluble, is stable and has a high melting point of 2000 ◦ C. Some P2 O5 is lost at temperatures of 1700 ◦ C, which exceed the maximum boiler temperature of commercial PC plants. AlPO4 is therefore not regarded in these investigations. Similarly to AlPO4 , iron phosphate FePO4 is also inert. In the presence of CaO contained in the ash, calcium phosphate may form at high temperatures typical during combustion, and then can be reduced to phosphorus oxide. The alkali content in sewage sludge is usually negligible.
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Approximate P4 O10 concentrations in the flue gas are ex3 assuming a 4% co-firing thermal pected to be 3500 mg/mN share of sewage sludge with a P-content of 7%. This corresponds to a solid-gas phase conversion of about 50%. The 3. alkali concentrations would amount to 30 mg/mN 2.2.1.2. Meat and bone meal. The largest share of P compounds in MBM have the primary constituents of bone, Apatite Ca5 [OH(PO4 )]3 and Whitekite Ca3 (PO4 )2 . Both have a high melting point of greater than 1800 ◦ C and are insoluble in water. Organically bound phosphates will react similarly as described above. MBM contains a high share of alkalis of approximately 2.5 wt.%. Unlike in coal, the compounds are primarily low boiling point soluble salts such as NaCl, KCl, Na2 SO4 and K2 SO4 . These compounds were identified as strong catalyst poisons as described in [2,3]. Approximate P4 O10 concentrations in the flue gas are ex3 assuming a 5–6% thermal share pected to be 2500 mg/mN of MBM with a P-content of 10%, half of which is then transferred into the gas phase. Making the same assumptions as for P, the maximum possible alkali concentration during MBM co-combustion 3 at combustion temperatures. would amount to 500 mg/mN For comparison, measured values for alkali concentration (Na, K) in the flue gas of pure coal combustion at temperatures above 1100 ◦ C are 7.6 ppm [13]. 2.2.2. Determination of the relative activity To quantify the deactivation of a catalyst, the ratio of the remaining activity k and the initial activity k0 of a new catalyst is expressed as relative activity k/k0 . The initial activity of a reference catalyst monolith of standard size 150 mm × 150 mm × 800 mm and the remaining activity of the spent catalyst, also a monolith were determined in a bench scale reactor according to [14]. For the testing of plate catalysts, plates in original length were cut to appropriate width to fit the bench test rig and put into a carrier box. The volume flow as well as the gas composition of the bench test reactor were adapted to the conditions of the respective full scale plant, 150 N m3 /h (wet), the flue gas composition was 600 ppm NO, 600 ppm NH3 , 1500 ppm SO2 , 5% O2 and 7% H2 O. 2.2.3. X-ray fluorescence analysis The X-ray fluorescence (XRF) analysis was carried out for the catalyst surface and for the bulk material. The inlet and the outlet section of each catalyst were analysed. For the surface analysis, the honeycomb samples were ground to get a smooth plane. The plate type samples were analysed as received. The XRF gives the information of elements to a depth of 50 nm. Thus, the elements enriched on the surface by deposition or surface condensation are quantified. For the bulk analysis the catalyst material was ground and beads were prepared for the analysis. The bulk analysis provides information on elements throughout the whole catalyst
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structure. This permits statements concerning pore condensation or chemical deactivation by penetrating compounds. 2.2.4. BET analysis The BET analysis was carried out using a Micromeritics ASAP 2010. The samples of the honeycomb catalysts were cut into small sections. The plate catalyst samples were prepared by separation of the ceramic carrier material from the metal support. Samples were taken from the inlet and the outlet section of each catalyst. The samples were evacuated and heated at 120 ◦ C for 4 h before analysis. The surface area was determined according to the BET multipoint method, the pore size distribution was analysed according to the BJH and t-plot methods. The procedures for these analyses are well documented in literature. The active sites of the catalyst are located on the surface of the porous carrier material. The higher the surface area, the more active sites that are available for the SCR reaction. In this respect, the surface area may serve as a parameter for the activity of the catalyst. It is expected that surface condensation or pore blocking results in a decrease of specific surface area. As a consequence the activity will decrease. Further information on the catalyst structure is expected from the average pore diameter and the total pore volume. Pore blocking of small pores will shift the value of the average pore diameter towards larger pores, whereas pore condensation is expected to show an adverse effect. The total pore volume will decrease in both cases, whereas the decrease concerning pore blocking is expected to be larger than that attributed to pore condensation. 2.2.5. Permeability measurements As a means for an in-situ characterisation of catalysts, a permeability measurement technique first applied to catalysts at the Universität Hohenheim [15] is adapted to obtain information on the degree of deactivation by evaluating the permeability of gas through the catalyst walls. Good penetration is indicative to good mass transfer through the catalyst which is considered representative for the activity as more the active sites in the pore system are accessible for the SCR reaction. The determination of the gas permeability is carried out ˙ and the pressure by measurement of the volume flow Q drop p over the catalyst sample, similar to Lea and Nurse [16]. According to [16] the permeability coefficient K can be determined allowing for slip since the employed fluid is
a gas: K=
˙ aL Qp A p
(1)
where L is the thickness of the catalyst sample, pa the ambient pressure, and A is the cross-section of the flow channel of the probe. The catalyst samples were inserted between two rubber seals and were compressed by two flanges to avoid torsion or bending. The sample diameter was much larger than the thickness so that radial conduction could be neglected. The sample was then loaded with compressed air and the inlet and outlet pressures and the volume flow were recorded. The outlet pressure was held constant at 0.2 bar relative pressure to ensure exact volume flow measurement according to the manufacturer’s requirements. For the pressure measurement, two pressure transducers were used while the volume flow was measured by a ball ˙ was expected. flow meter. A linear relation of p and Q Therefore, measurements of pin and pout were carried out at volume flow rates of 1, 2, 3, 4 and 5 l/h. Measurements at a constant flow rate were not possible for catalysts with low permeability (honeycomb). Therefore, a reference volume was created which could be sealed with a valve and the two pressures were recorded until equilibrium is reached (Fig. 1). 2.2.5.1. Gas permeability of plate type catalysts. The gas permeability of these catalyst types was the result of the micro cracks, the catalyst pore structure, and of macro cracks. In order to provide a basis for a comparison of honeycomb and plate catalysts, two values for the gas permeability K3 and K5 were determined. The K3 value was obtained with a flow area of 3 mm in diameter. This proved to be small enough to avoid marco cracks and large enough to obtain a quantifiable volume flow. This value served as an equivalent to honeycomb catalysts. To get a more representative value of the effective gas permeability, the larger cracks had to be considered. This was accomplished by using a flow area of 5 mm in diameter (K5 value). Deviations occurred due to the differently structured metal support material; however, attention was directed on measurement between the metal support. 2.2.5.2. Gas permeability of honeycomb catalysts. In comparison to plate type catalysts, honeycomb types have a significantly smaller permeability so that the measurement
Fig. 1. Permeability measurement.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
of gas flow, especially in case of deactivated samples, was difficult to quantify. For the volume flow through the catalyst wall, isothermal conditions were assumed. A reference volume Vref was incorporated in the test set-up, the inlet pressure was kept constant at 2.4 bar. The outlet valve was closed and the time was measured until an equilibrium pressure in the reference volume was reached. The permeability was measured at the first 5 mm of the inlet section, at 30 mm from the inlet and 5 and 30 mm from the outlet respectively. The obtained p measurement over time t was empirically approximated by the following function: p(t) = C1 e−t/T1/2
(2)
where C1 is a constant and T1/2 is the time in which p is half its initial value. Since Vref and pin are kept constant, the only variable is the time t which is proportional to the permeability according to Darcy’s law. The error which results from the free space between catalyst sample and outer seal is neglected. For a qualitative comparison of the permeability coefficients of the both catalyst types, the time constant T1/2 was directly related to the permeability coefficient in order to compare the results of honeycomb and plate type catalysts. The advantage of this method is its ease of application for in-situ measurements in power plants if a relation of permeability and degree of deactivation can be established.
species in the flue gas and on the catalysts. H2 SO4 and CO were identified as major reactants at all studied temperatures while at combustion conditions above 1300 ◦ C, SiO2 , CaO, and carbon were also identified as major reactants. The following formulae show the assumed main reactions during combustion. The results agree with literature [12,17]. The results of the calculations show in all cases the formation of phosphorus oxides or, in the presence of water vapour, phosphoric acid. At temperatures around 500 ◦ C, additional alkali polyphosphates were found to form during the case of MBM co-combustion. In the zone of the boiler exceeding temperatures of 1100 ◦ C, calcium phosphates react with carbon and SiO2 : 2Ca5 OH(PO4 )3 (s) + 6SiO2 (s) + 10C(s) + 2 O2 (g) → 6CaSiO3 (s) + 1.33Ca3 (PO4 )2 (s) + 10 CO(g) + H2 (g) + 0.66P2 O3 (g) + P2 (g)
3.1. Equilibrium calculations Concerning the deactivation mechanisms of P on the catalyst, its transformation from stable compounds in the fuel into reactive gaseous compounds in the flue gas was analysed. Possible formations of reactive P components during the combustion process are described. Subsequently, four reaction paths are shown which correspond to four different deactivation mechanisms: • • • •
Deactivation by solid calcium phosphate. Deactivation by condensed phosphoric acid. Deactivation by phosphorus oxide. Formation of alkaline polyphosphate or vanadyl phosphate glass.
All hint either at pore blocking, where a solid deposition layer prevents the mass transfer of the reactants, or to pore condensation, where P compounds condense inside the catalyst pore system, thus also blocking mass transfer. 3.2. Formation of reactive phosphorus compounds during combustion Equilibrium calculations in the temperature range 400–1400 ◦ C were carried out to estimate the formation of
(3)
2Ca3 (PO4 )2 (s) + 6SiO2 (s) + 10C(s) → 6CaSiO5 (s) + 10CO(g) + P4 (g)
(4)
The reaction goes to completion above 1500 ◦ C. At the same time, the elemental phosphorus reacts with the excess O2 : P4 (g) + 5O2 (g) → P4 O10 (g)
(5)
The phosphorus oxide then reacts with steam to form phosphoric acid: P4 O10 (g) + 6 H2 O (g) → 4 H3 PO4 (g)
3. Results and discussion
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(6)
3.3. Deactivation mechanisms At temperatures present at the SCR catalysts or slightly above (400–500 ◦ C), the gaseous phosphorus acid forms polyphosphoric and metaphosphoric acid. The phosphoric acid reacts with the alkali compounds forming polyphosphates, probably with an intermediate step forming hydrogenous phosphate [17]. From the equilibrium calculations four scenarios for the deactivation by P compounds seem possible. The reaction paths are shown in Fig. 2. (a) Deactivation by calcium phosphate: Disregarding Eqs. (3) and (5), the majority of calcium phosphates are solid throughout the flue gas path. Furthermore, these compounds are chemically very stable at catalyst operating temperatures and will block the pores as fly ash. (b) Deactivation by phosphoric acid: At temperatures of 300–400 ◦ C polyphosphates are formed, consisting of O–P–O chains which may form surface layers by pore blocking or pore condensation. Chemical deactivation by reactions with the catalyst active material forming V or Mo phosphates is also a possible deactivation mechanism. (c) Deactivation by phosphorus (V) oxide: Besides H3 PO4 , P4 O10 is another dominant P compound in the flue gas. The deactivation mechanism is assumed to be pore blocking and the formation of phosphate chains.
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Fig. 2. Proposed reactions of calcium phosphates along the flue gas path.
(d) Formation of alkaline polyphosphate or vanadyl phosphate glasses: In the presence of alkali salts, alkaline polyphosphates may form as shown in Fig. 2. Effects will be similar to the ones describes in (b) and (c). In addition, reactions with the active catalyst components may take place, forming insoluble compounds as already described by Blanco [7]. Trimetaphosphate has been reported to form a ternary system of metaphosphate glass incorporating alkali metals (V2 O5 )n —(Na2 O·P2 O5 )(1−n) at temperatures of approximately 700 ◦ C. Another possibility seems the formation of glassy structures like Na4 VP3 O12 [18]. 3.4. XRF analysis The analysis of the results of the XRF measurements are focussed on P, Na and K. The concentration of the elements Si, Al, Ca, S and As, which also have an influence on the catalyst deactivation are additionally given in Table 3. All elements were converted into their oxide form. In general, the analysis shows an increase in all elements when compared to the fresh catalysts. The concentration is highest on the surface of the inlet section and decreases along the
catalyst length. The bulk concentrations show a similar behaviour but the absolute values are lower. Arsenic is the only element where the bulk concentration sometimes exceeded the surface concentrations. For a detailed description of this phenomenon, refer to literature [19]. Compared to coal mono-combustion (A.1 and C.2.3), the P surface concentrations are high for all samples exposed to co-combustion (exceptions: A.2 and A.5). In the bulk phase, no major difference was observed except for sample C.1.4, which also had high bulk P concentrations of 1.66 wt.% at the inlet section compared to 0.52 wt.% for C.1.3. This is attributed to the animal oil and fat co-combustion the sample was exposed to. In this case, high P loads accumulated in a short operation time of 2000 h. Similar surface P loads accumulated on catalyst samples C.2.1 and C.2.2 during sludge co-combustion. Except for sample A.2, the alkali surface concentrations found during sludge co-combustion were generally higher than in coal combustion. The alkali surface concentrations were also higher than the bulk concentrations. The concentration of the samples C.1.3 and C.1.4 exceeded 1 wt.% which results, according to Kamata et al. [2], in a significant loss of activity. This is consistent with the activity measurements shown in Table 2. The four samples A.3, A.4,
Table 3 XRF analysis of the catalyst surface, inlet section
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2 C.2.1 C.2.2 C.2.3
SiO2 (wt.%)
Al2 O3 (wt.%)
CaO (wt.%)
SO3 (wt.%)
P2 O5 (wt.%)
Na2 O (wt.%)
K2 O (wt.%)
Fe2 O3 (wt.%)
2.7–9.1 10.3 9.8 17.5 22.1 17.1 2...4.7 14.9 23.1 16 3.7 2...3.4 16.4 16.7 7
0.35–2.0 3.4 1.9 6 5.7 4.1 0.43–4.2 5.8 7.7 7.9 1.1 3.9–4.2 10.3 10.9 6.7
0.54. . . 1.4 1.4 4.1 7 9 3 <0.65 2.1 2.8 2.8 0.63 <0.03 1.5 1.1 0.32
1.383.5 2.5 20.6 18.7 20.6 12.8 0.44–3.6 8.2 15.3 12.8 14.5 3.4–3.6 10 8.9 4.1
<0.09 0.56 0.29 2 2 0.43 0.09–0.15 1.5 1.2 1.8 4.4 <0.03 4.9 4.4 0.73
<0.03 0.17 0.24 0.58 0.31 0.42 <0.03 0.41 0.77 1.2 1.5 <0.03 0.27 0.33 0.35
<0.05 0.12 0.15 0.65 0.5 0.25 <0.03 0.22 0.59 1.1 0.06 <0.03 0.26 0.28 0.14
<0.22 0.25 0.43 0.9 0.94 0.36 0.11–0.25 0.71 1.2 4.1 0.98 0.14–0.24 0.7 0.65 0.24
Samples A, C.1 and C.2 give the range of values for new catalysts.
As (ppm) <50 <50 <50 1670 2500 3100 <50 504 0.8 6300 <50 <50 3470 3380 46000
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
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Fig. 3. Comparison of major deactivating elements of typical catalysts; (left) surface composition, (right) bulk concentration.
A.5 and C.1.3 exposed to MBM co-combustion also seem to accumulate alkalis in the bulk phase. Bulk alkalis values ranged from 0.52–0.84 wt.% compared to 0.09–0.49 wt.% for sewage sludge and 0.25 wt.% for coal combustion in the inlet section. Values of the surface accumulation are given in Table 3. The loss in activity of samples A.1 and A.2 of 14 and 38% respectively cannot be clarified by the chemical analyses since no significant accumulations of catalyst poisons were detected. This should be noted especially for catalyst sample A.2 which was exposed to sludge co-combustion and showed no significant accumulation of either P or alkalis. Fig. 3 shows the P, Na and K concentrations of typical honeycomb and plate catalysts exposed either to pure coal, MBM or sewage sludge co-combustion. Since no recordings of the exposure time to co-combustion were made, the absolute values of the different samples cannot be compared.
The sample A.3 and C.1.3 have high P surface concentrations which decrease along the catalyst length The bulk alkali concentrations are also high, this seems typical for MBM co-combustion. The high bulk P concentration of samples C.1.3 in relation to the surface concentration probably depends on a higher permeability which increases the mass transfer to the bulk phase. This would describe the different behaviour of the catalyst poisons in sample C.1.2. For the honeycomb samples, a relation between the relative activity k/k0 , the surface P and the alkali concentration can be established. Furthermore, a correlation between alkalis and P content was observed. Another strong correlation between Ca and the relative activity was also observed and is therefore shown in Fig. 4. For plate type catalysts, the alkali content shows a strong correlation to k/k0 (see Fig. 4), the Ca correlation is moderate and no correlation to the P content can be made. A correlation of alkalis and the P content
Fig. 4. Relation of surface P, alkali and Ca concentration compared to k/k0 of honeycomb and plate catalysts.
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Table 4 Results of the BET analysis Sample
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
BET surface area (m2 g−1 )
Average pore diameter 4V/A (nm)
Total pore volume (cm3 /g)
Inlet
Inlet
Inlet
Outlet 55.3
42.1 54.4 45.2 39.8 45.8
46.1 54.0 52.1 45.4 43.8
16.48 15.62 19.00 16.12 17.07
45.9 54.3 55.2 45.3 51.4 59.9 72.3
17.44 17.55 14.43 17.71 10.29 9.14 8.32
64.5 49.5 47.2 51.2 52.1 47.2 56.1 63.0
Outlet 18.14 18.05 15.06 16.26 12.47 16.87
0.17 0.21 0.21 0.16 0.20
17.33 17.07 14.10 18.70 9.94 9.33 7.74
0.22 0.20 0.18 0.23 0.12 0.13 0.13
13.20
is also not obvious. Due to the strong correlation between Ca and activity for honeycomb catalysts, fly ash blocking seems a major deactivation mechanism. A distinction between pore blocking or pore condensation concerning the predominant effect of phosphorus however is not possible. Alkalis also have a large influence on the activity which can be by chemical deactivation or by the formation of surface layers by glassy phosphorus compounds. The strong relation of activity and alkali content at the plate type catalysts suggests chemical deactivation predominantly by alkali poisoning. A certain contribution by fly ash seems possible whereas the influence of P seems negligible. 3.5. BET analysis Regarding the specific surface area determined by BET, a decrease for all samples A.X and C.1.X was observed compared to the new catalysts A and C.1 (see Table 4). For the catalyst type C.2, no new samples were available for surface characterisation. The results are included since they illustrate the same behaviour of both plate type catalysts C.1.X and C.2.X. Besides the general decrease in surface area of deactivated catalyst samples, a rise in the surface area from the inlet to the outlet section was observed. Due to the turbulent flow in the inlet section, the mass transfer is locally increased. Hence, deactivation mechanisms by pore blocking or pore condensation also increase. Both deactivation mechanisms reduce the surface area which can be observed from the results according to Table 4. For samples A.2 and A.5, no significant difference between the inlet and outlet surface area can be observed. The differences are within the measurement accuracy. The samples C.1.1 and C.1.4. have an opposite behaviour; the surface area decreases from inlet to outlet. In the first case, severe crack formations may be an explanation. For the sample C.1.4, where the difference is larger, the reason is seen in the liquid fuel and the
Outlet 0.23 0.21 0.20 0.21 0.14 0.18 0.21 0.20 0.23 0.19 0.21 0.13 0.14 0.14
resulting flue gas composition, which cannot be compared to the plants of the other samples where solid fuels which generate higher fly ash loads were used. The sample C.1.3 has a higher surface area than sample C.1.2 (see Fig. 5). This may be the reason for better mass transfer to the bulk and could explain the high bulk concentration of P, Na and K as shown in Fig. 3. The data for sample A.5, which was exposed to MBM co-combustion, is given for comparative purposes only since no recordings concerning relative activity or operation time were available. The samples A.3 and A.4 represent MBM deactivation of honeycomb catalysts. Both samples show a large decrease in surface area compared to the new catalyst. They also have the largest surface area difference between the inlet and outlet section. Pore blocking by fly ash cannot be the only cause for this severe decrease since other catalysts were also subjected to the fly ash pore blocking and do not show a comparable behaviour. Additional pore condensation effects attributed to phosphorus compounds is therefore assumed.
Fig. 5. Comparison BET surface area and average pore diameter (APD) of typical catalysts.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Furthermore, a decrease of the average pore diameter and the total pore volume over operating time suggest that catalyst deactivation is due to pore condensation. Sample A.4, which had the longest operation time, is the best example of this phenomenon. From the results, a clear distinction between either severe pore condensation which fills the whole pore volume with condensate, or pore blocking is not possible. Sample A.2 also had a long operation time and shows a similar behaviour concerning the average pore diameter but the total pore volume does not decrease significantly. This clearly hints at pore condensation effects. The plate type catalyst C.1 showed a rise in average pore diameter combined with a total pore volume similar to that of a new catalyst. This behaviour is independent from the operation time, since the samples were taken between 2000 and 65,000 h in operation. The crack formation typical for plate type catalysts is considered a reason for the shift towards larger average pore diameters. However, the average pore diameter of sample C.1.3 only slightly rose whereas the total pore volume decreased. If an average pore diameter of 17 nm, as shown in Table 4, is considered usual for plate type catalysts in operation, the low value of 14.4 nm together with the decrease in pore volume is similar to the results of the honeycomb sample A.4. If this effect could be observed in further MBM deactivated samples, it would suggest a typical deactivation effect: pore condensation due to the decrease in pore diameter and in total pore volume, possibly in combination with pore blocking. 3.6. Permeability The permeability index K is a value for the gas permeability of a porous layer. The higher K, the higher is the permeability of the specific layer. From the results given in Table 5, the principle difference in the measurement method between plate and honeycomb
23
catalysts can be observed. Plate catalysts have a much higher permeability. Due to the inhomogeneous surface which is a result of micro and macro cracks and the metal support, repetitive measurements are difficult so that the shown results give only a limited impression. However, a significant decrease in permeability compared to the new material can be observed. The permeability method seems more applicable for honeycomb catalysts since these have a homogeneous structure. Here, similar to the plate type catalysts, a large decrease in comparison to the new catalyst is observed. Sample A.1 could not be measured due to high impermeability. Severe pore blocking by fly ash within an operation time of only 14,000 h is probably the reason which would also explain the decrease in BET surface area. The other results indicate a trend along the catalyst length similar to the BET measurements. This is expected due to the increased mass transfer at the turbulent inlet section of the catalyst as described previously. The samples A.3 and A.4 especially show a rising permeability along the catalyst length. Fig. 6 illustrates this trend with the exception of sample A.2 which shows first a decline and than a small rise. A comparison with the results of the surface characterisation in Table 4 shows a slight drop of the surface area and the total pore volume along the catalyst length. The difference is within the accuracy of the measurement method. Assuming a correlation of permeability and surface area, the permeability measurement would support the BET results. The surface area analysis of sample A.5 does not show the significant increase from inlet to outlet section that samples A.3 and A.4 did, although the average pore diameter and the total pore volume of all three samples have the same trend. However, the permeability measurement gives the same results for all MBM deactivated samples A.3, A.4 and A.5. The latter differs in the similar surface area of inlet and outlet section, whereas the values for average
Table 5 Results of the permeability measurement K5 (10−3 m2 /s) Inlet
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
K3 (10−3 m2 /s) Outlet
Inlet
Outlet
5 mm
30 mm
1.7E−3 447.4E−6 434.7E−6 182.1E−6
203.5E−6 616.9E−6 854.2E−6 499.0E−6 215.1E−6
30 mm
5 mm
530.9E−6 857.7E−6 601.7E−6 273.1E−6
717.0E−6 1.6E−3 717.0E−6 256.3E−6
43.7E−3
5.7941 1.9758 0.0762 0.5192 1.5725 0.1440 0.2029 0.6025
1.3252 0.1448 0.2898 0.9322 0.2128 0.5844 1.1747
0.9625 0.2866 0.0868 0.5023 0.9263 0.8906 0.3033 0.8082
0.5534 0.1971 0.1267 0.6507 0.2008 0.4860 0.6571
24
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Fig. 6. Permeability along honeycomb catalysts.
pore diameter and total pore volume show the same trend as the other MBM deactivated honeycomb catalysts. In this respect, the permeability measurements support the general results of the BET surface area measurements since they indicate the same trend for MBM deactivation. In cases where the conventional surface measurement does not show clear results, it may serve as an additional tool for further detailed characterisation.
4. Conclusion and outlook For a basic understanding of possible reactions in the flue gas for the temperature range of 1400–400 ◦ C, equilibrium calculations including major flue gas constituents as well as the deactivating components Na and P were carried out. The results indicate the presence of orthophosphoric acid at catalyst temperatures as well as polyphosphorus oxides and alkali polyphosphates which may condensate at catalyst operating temperatures or react with catalyst compounds forming phosphorus glasses. Four possible deactivation mechanisms caused by phosphorus were derived: (a) pore blocking by solid calcium phosphates, (b) deactivation by pore blocking or pore condensation by H3 PO4 or (c) phosphorus oxides and (d) formation of phosphorus glasses in combination with pore condensation and eventually chemical deactivation. Subsequently, a set of three different types of deactivated catalysts, exposed to co-combustion of P rich fuels, either animal residues or sewage sludge, were examined. The relative activity k/k0 was determined and the chemical composition of surface and bulk material was analysed by XRF. Surface characterisation including specific surface area, average pore diameter and total pore volume and an adapted permeability measurement method were carried out. Concerning honeycomb catalysts, the applied permeability measurements can be an easily applicable means for in-situ surface characterisation. For a secure statistical basis
however, much more samples need to be analysed, which were exposed to defined conditions as a reference in order to derive definite relations. Due to the inhomogeneous structure of plate type catalysts, the permeability method does not give repetitive results and will not be applied further. The chemical results of the analyses showed an enrichment of P on the catalyst surface of all samples. The enrichment of alkalis on the surface was higher for samples exposed to MBM co-combustion. These samples also had considerable bulk enrichments of Na and K. These concentrations alone were capable of a severe deactivation. Accordingly, a close relation between alkali content and relative activity could be established for all catalysts. Concerning P, a relation was only observed for the honeycomb catalyst samples. The surface characterisation of all MBM deactivated samples showed an increase of surface area along the catalyst length as well as a shift of the average pore diameter towards smaller values. The total pore volume also decreased slightly which might be a typical effect of catalyst samples exposed to MBM co-combustion. Pore condensation in combination with blocking caused by P was considered as the prevailing deactivation mechanism. During sewage sludge co-combustion, pore condensation had a stronger influence than other mechanisms. The results support the thesis that P tends to form a surface layer or condensates in the pores. The large difference in the deactivation rate between deactivated catalyst samples obtained during MBM or sludge co-firing may be attributed to the different alkali concentration in the fuel. A distinction between the assumed deactivation mechanisms was not possible. In operational conditions in power plants, complex reactions take place forming a range of catalyst poisons. As a result, several mechanisms contribute to the catalyst deactivation beside the considered compounds P, Na and K. Due to the different operating and exposure times to co-combustion, a comparison of the relative activities was difficult. Furthermore, deactivation mechanisms other than those caused by P or alkalis may have occurred. To exclude other influences, future work will include lab-scale investigations to examine the independent contribution of P or Na to catalyst deactivation. The basis will be the major fuel compounds and flue gas concentrations identified. A comparison with the surface characteristics as well as with the remaining gas permeability is expected to show general trends for P or the combination of P and Na. The samples will also serve as reference material for permeability measurements. In order to verify the theory on flue gas concentration and the formation of deactivating P compounds in the flue gas and in the fly ash, in-flame measurements as well as fly ash analyses shall be carried out. In addition, information on the condensation behaviour of fly ash particles along the flue gas duct shall be gathered and the formation of phosphorus compounds will be investigated.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Acknowledgements The work was supported by the EESD programme of the European Commission within the RTD project “Influences from Biofuel (Co-)Combustion on Catalytic Converters in Coal-Fired Power Plants” contract no.: ENK5-CT-200100559. For further information about the project please refer to http://www.eu-projects.de/CATDEACT. The funding for this work is gratefully acknowledged by the authors. Many thanks to Annette and Gabi for the BET analyses. References [1] J.P. Chen, M.A. Buzanowski, R.T. Yang, J.E. Cichanowicz, Deactivation of the Vanadia catalyst in the selective reduction process, J. Air Waste Manage. Assoc. 40 (1990) 1403–1409. [2] H. Kamata, K. Takahashi, I. Odenbrand, The role of K2 O in the selective reduction of NO with NH3 over a V2 O5 (WO3 )/TiO2 commercial selective catalytic reduction catalyst, J. Mol. Catal. A 139 (1999) 189–198. [3] D.A. Bulushev, F. Rainone, L. Kiwi-Minsker, A. Renken, Influence of potassium doping on the formation of Vanadia species in V/Ti oxide catalysts, Longmuir 17 (2001) 5276–5282. [4] T. Reichelt, Freisetzung gasförmiger Alkaliverbindungen bei atmosphärischer und druckaufgeladener Verbrennung, Ph.D. Thesis, Universität Stuttgart, VDI Verlag Düsseldorf, 2001. [5] H. Schürmann, S. Unterberger, K.R.G. Hein, P. Monkhouse, U. Gottwald, The influence of fuel-additives on the behaviour of gaseous alkali compounds during pulverised coal combustion, Faraday Discuss. 119 (2001) 433–444. [6] R. Khodayari, Selective Catalytic Reduction of NOx : Deactivation and Regeneration Studies and Kinetic Modelling of Deactivation, Dissertation, Lund University, 2001.
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[7] J. Blanco, et al., Influence of phosphorus in vanadium-containing catalysts for NOx removal, Appl. Catal. B 55 (1989) 151– 164. [8] T. Heinzel, J. Maier, J. Baum, H. Spliethoff, K.R.G. Hein, Slagging and Fouling in Dry and Molten Ash PFC, Final Report Joule III— OPTEB, European Commission, 1999. [9] R. Cenni, Heavy Metals Behavior in Co-Combustion of Coal and Sewage Sludge, Ph.D. Thesis, Universität Stuttgart, VDI Verlag Düsseldorf, 2001. [10] A. Nottrodt, J. Chibiorz, Technische Anforderungen und allgemeine Empfehlungen für die Entsorgung von Tiermehl und Tierfett in Verbrennungsanlagen, Study for Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, 2001. [11] J. Ottow, W. Bidlingmaier, Umweltbiotechnologie G. Fischer, Stuttgart, 1997. [12] D.E.C. Corbridge, Phosphorus, Elsevier, Amsterdam, 1995. [13] H. Schürmann, S. Unterberger, K.R.G. Hein, P. Monkhouse, On the application of laser spectroscopy for detection of gaseous alkalis during pulverised coal combustion, in: Proceedings of the Presentation at the 6th European Conference on Industrial Furnaces and Boilers, INFUB, 2–5 April 2002. [14] VGB Guideline VGB-R 302 He: Guideline for the Testing of DeNOx Catalysts, VGB Kraftwerkstechnik GmbH, 1998. [15] H. Schreiber, Untersuchungen zur Auswirkung von Oberflächenschichten in DeNOx -Katalysator-Elementen, Permeabilitätsmessungen, Internal Reports 1 and 2, 1995. [16] P.C. Carman, Flow of Gases Through Porous Media, Butterworths, London, 1956. [17] E. Riedel, Anorganische Chemie, de Gruyter, Berlin, New York, 1999. [18] A. Chrissanthopoulos, C. Pouchan, G. Papatheodorou, Structural investigation of vanadium–sodium metaphosphate glasses, Zeitschrift f. Naturf. 56a (2001) 773–776. [19] H. Gutberlet, Einfluß der Feuerungsart auf die Vergiftung von DeNOx -Katalysatoren durch Arsen, VGB Kraftwerkstechnik 68 (3) (1988) 287–293.
Effects of sewage sludge and meat and bone meal Co-combustion on SCR catalysts Jens Beck a,∗ , Jürgen Brandenstein b , Sven Unterberger a , Klaus R.G. Hein a a
Institut für Verfahrenstechnik und Dampfkesselwesen (IVD), Universität Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany b E.On Engineering, Bergmannsglückstraße 41-43, 45896 Gelsenkirchen, Germany Received 1 July 2003; received in revised form 30 October 2003; accepted 15 November 2003
Abstract DeNOx catalysts for the selective catalytic reaction (SCR) in coal fired power plants are deactivated by catalyst poisons such as As, Na, K, P, etc. The deactivation rate depends on the fuel quality and the concentration of catalyst poisons in the fuel. However, operational parameters such as O2 content, residence time, combustion and flue gas temperature also have an influence on decreasing catalyst activity. Equilibrium calculations were carried out to identify possible catalyst deactivation reactions. The calculations concentrated on the elements phosphorus and sodium as primary catalyst poisons in animal residues or sewage sludge. A set of deactivated catalyst samples from different power plants were analysed with respect to surface area, chemical composition and permeability. From the results obtained, deactivation mechanisms were derived which seem to cause increased catalyst deactivation due to P-rich secondary fuels. The samples showed a high P and alkali concentration on the surface. The surface area and the pore volume decreased. Compared to pure coal combustion this decrease was significant for meat and bone meal co-combustion and was also noticed for samples exposed to sewage sludge co-combustion. A correlation between relative activity and Na and P concentration is established. The results of the surface analyses indicate pore condensation as a major deactivation mechanism. © 2003 Elsevier B.V. All rights reserved. Keywords: DeNOx ; Catalyst; SCR; Deactivation; Phosphorus; Alkali; Secondary fuel; Co-combustion
1. Introduction The economic operation of coal fired power plants requires a cost effective fuel supply. Recently, the thermal recycling of residues as secondary fuel is of increasing interest for power plant operators. Consequently, a variety of coal blends and secondary fuels such as sewage sludge, straw, wood or meat and bone meal (MBM) is combusted. The resulting effects on the SCR DeNOx reactor with respect to the fuel type and the operating conditions of the plant have not yet been thoroughly investigated. The activity of the catalyst, which is a measure for the ability to reduce NOx emissions, can rapidly decrease. Alkali metals for instance were found to have a high deactivating potential as has been described by Chen et al. [1] and others
∗
Corresponding author. Tel.: +49-711-685-7760. E-mail address: [email protected] (J. Beck).
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.11.007
[2,3]. Investigations have shown that the dominant parameter for deactivation is the reactivity of the compound, not the elemental concentration of the poison in the fuel. For purely coal fired power plants, the effect of alkali deactivation is negligible since the alkali content is relatively low and the alkali fractions in the fuel are usually bound in very stable compounds such as silicates which do not react even at high temperatures [4,5]. Considering renewable fuels like straw and wood, the alkali metals, especially K, have a considerable effect on catalyst deactivation. In these fuels, the K is bound as sulphate or chloride which have low boiling points. The deactivation effects and possible countermeasures are described elsewhere in further detail [2,3,6]. Phosphorus is also another typical constituent of coal. The total concentration of P is below 1 wt.% so that no significant deactivating effect can be attributed to P because effects from other poisons prevail. The deactivation potential of P has been described by Chen et al. [1]. The behaviour of catalysts extruded with H3 PO4 has been investigated by Blanco [7].
16
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
His experiments were conducted on catalysts used in HNO3 plants where H3 PO4 was used as an extruder. A decrease of surface area and pore volume was observed. XRD analysis immediately taken after exposure suggested the presence of vanadyl phosphates. The later analysis of spent catalysts did not show any crystalline phosphates. Therefore, it was assumed that these phosphates are unstable. Although the experimental conditions of Blanco’s experiments differ from the situation in power plants, operational experience shows that the co-combustion of P-rich sewage sludge or MBM has a substantial deactivating effect on catalysts. Previous XRD analysis of spent catalysts however showed no crystalline structures which might be assigned to P compounds. However, a correlation between the relative activity and the P concentration was evident. The aim of the current investigation is to identify the parameters affecting the deactivation of high dust SCR catalysts caused during the co-combustion of sewage sludge and MBM. Previous examinations performed on catalysts exposed to full scale co-combustion conditions have shown phosphorus to be a primary deactivating compound; therefore, it will be a focus of this investigation. However, since sewage sludge has roughly the same P content as MBM, the deactivation rate during its co-combustion is considerably lower. MBM in addition contains also high quantities of alkalis which also need consideration. In order to gain information on possible reaction mechanisms, the origin of the P, Na and K compounds in the fuel is studied. On this basis, possible reaction schemes on the formation of deactivating compounds are determined by equilibrium calculations, upon which derived deactivation mechanisms are based. For comparison, deactivated plate type and honeycomb type catalyst samples of different power plants are analysed. The relative activity is determined, XRF analysis of the bulk and the surface composition is carried out, BET and a gas permeability measurement give information on the surface properties. From
the comparison of the data, conclusions on the prevailing deactivation mechanisms are drawn.
2. Experimental A set of samples composed of five deactivated honeycomb catalysts and seven deactivated plate type catalysts were analysed. The samples were subject either to sludge or MBM co-combustion and were taken from DeNOx reactors operating in full-scale high dust plants of dry ash as well as of wet bottom PC boilers. Two reference samples of pure coal combustion have also been included. For an estimate of possible reaction mechanisms thermal equilibrium calculations were carried out. As a basis for the calculations a literature study concerning the nature of P and alkali compounds contained in both secondary fuels was made. With this information, the flue gas concentration of P and Na compounds for usual fuel mixes was approximated. The catalyst samples were analysed by XRF, BET and permeability and the results were evaluated with regard to major deactivation mechanisms (Table 1). 2.1. Catalyst samples Table 2 gives an overview of the samples, exposure, fuel mix and relative activity k/k0 . Type A are honeycomb catalysts; type C are plate types from two different manufacturers. The relative activity of the samples was determined. Analyses of the chemical composition of the surface and of the bulk material at inlet and outlet were carried out. This analysis was supplemented by surface characterisation using BET and permeability measurements. The contribution of the exposure time to the loss of activity was not possible since, due to the restraints of full-scale conditions, the samples were not exposed to co-combustion over the whole operation.
Table 1 Operating conditions and remaining relative activities of the sample set Sample
A.1 A.2 A.3 A.4 A.5 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
Boiler type: wet bottom (w) dry ash (d)
Operating time (h)
Relative activity after exposure, k/k0
Fuel mix (thermal basis) (%) Coal
MBM
Sludge
d d w w d d d w d d d w
14000 46000 18500 55000 n.a. 24500 41500 65000 2000 50000 50000 39000
0.86 0.62 0.53 0.37 n.a. 0.8 0.64 0.58 0.71 0.67 0.67 0.49
X X X X X X X X Oil X X X
0 0 ca. ca. ca. 0 0 ca. ca. 0 0 0
0 ca. 0 0 0 ca. ca. 0 0 ca. ca. 0
Note: Co-combustion was not carried out over the total operating time. a Animal fat.
4 4 4
4 20a
4
4 4
4 4
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25 Table 2 Comparison of coal and secondary fuels [8–10]
LHV (raw) (MJ/kg) Moisture (raw) (%) Volatile matter (dry) (%) Ash (dry) (%)
Hard coal
Sewage sludge
MBM
30.18 2.03 33.16 9.03
10.58 3 49.52 45.1
15.7 8.3 65 26.5
23.6 13.4 17.8 10.3 0.4 0.7 14.2 2.2a
2.95 41.7 0.4 0.5 5.9 2.75 36.2
500 ◦ C)
Ash composition (prepared at Si as SiO2 (%) 40.09 Ca as CaO (%) 5.48 Fe as Fe2 O3 (%) 13.3 23.6 Al as Al2 O3 (%) Na as Na2 O (%) 1.51 K as K2 O (%) 2.91 0.19 P as P2 O5 (%) S as SO3 (%) 7.4 a
Data from E.On.
2.2. Analytical methods 2.2.1. Equilibrium calculations For the analysis primary P and alkali compounds in MBM and sewage sludge were considered. Primary flue gas constituents SO2 , H2 O and CO together with the fly ash components CaO and Al2 O3 and SiO2 were regarded as reaction partners in the relevant temperature range of 400–1400 ◦ C. Na was considered for reactions with P and with the active components of the catalyst V2 O5 , WO3 and MoO3 . The thermal equilibrium calculation software package FactSage Version 5.0 was used for the equilibrium calculations. 2.2.1.1. Sewage sludge. The P content in sewage sludge after the waste water treatment is approximately 7–10%. Ninety percent of the phosphorus in sludge is organically bound (i.e. nucleic acids, phosphorus proteins, phosphorus lipids and adenosintriphosphate ATP, etc.) and only 10% are insoluble salts [11]. The organically bound P-fractions are assumed to be thermally unstable so that the gaseous P compounds will be released at low temperatures around 500 ◦ C. It is from these compounds, as it is described below, that phosphorus oxides, calcium phosphates or phosphoric acid will be formed. During the chemical waste water treatment Ca, Fe and Al salts are added to extract the P chemically from the water, forming either Ca2 HPO4 (OH)2 , Ca10 (PO4 )6 (OH)2 , FePO4 or AlPO4 . According to Corbrigde [12], AlPO4 is highly insoluble, is stable and has a high melting point of 2000 ◦ C. Some P2 O5 is lost at temperatures of 1700 ◦ C, which exceed the maximum boiler temperature of commercial PC plants. AlPO4 is therefore not regarded in these investigations. Similarly to AlPO4 , iron phosphate FePO4 is also inert. In the presence of CaO contained in the ash, calcium phosphate may form at high temperatures typical during combustion, and then can be reduced to phosphorus oxide. The alkali content in sewage sludge is usually negligible.
17
Approximate P4 O10 concentrations in the flue gas are ex3 assuming a 4% co-firing thermal pected to be 3500 mg/mN share of sewage sludge with a P-content of 7%. This corresponds to a solid-gas phase conversion of about 50%. The 3. alkali concentrations would amount to 30 mg/mN 2.2.1.2. Meat and bone meal. The largest share of P compounds in MBM have the primary constituents of bone, Apatite Ca5 [OH(PO4 )]3 and Whitekite Ca3 (PO4 )2 . Both have a high melting point of greater than 1800 ◦ C and are insoluble in water. Organically bound phosphates will react similarly as described above. MBM contains a high share of alkalis of approximately 2.5 wt.%. Unlike in coal, the compounds are primarily low boiling point soluble salts such as NaCl, KCl, Na2 SO4 and K2 SO4 . These compounds were identified as strong catalyst poisons as described in [2,3]. Approximate P4 O10 concentrations in the flue gas are ex3 assuming a 5–6% thermal share pected to be 2500 mg/mN of MBM with a P-content of 10%, half of which is then transferred into the gas phase. Making the same assumptions as for P, the maximum possible alkali concentration during MBM co-combustion 3 at combustion temperatures. would amount to 500 mg/mN For comparison, measured values for alkali concentration (Na, K) in the flue gas of pure coal combustion at temperatures above 1100 ◦ C are 7.6 ppm [13]. 2.2.2. Determination of the relative activity To quantify the deactivation of a catalyst, the ratio of the remaining activity k and the initial activity k0 of a new catalyst is expressed as relative activity k/k0 . The initial activity of a reference catalyst monolith of standard size 150 mm × 150 mm × 800 mm and the remaining activity of the spent catalyst, also a monolith were determined in a bench scale reactor according to [14]. For the testing of plate catalysts, plates in original length were cut to appropriate width to fit the bench test rig and put into a carrier box. The volume flow as well as the gas composition of the bench test reactor were adapted to the conditions of the respective full scale plant, 150 N m3 /h (wet), the flue gas composition was 600 ppm NO, 600 ppm NH3 , 1500 ppm SO2 , 5% O2 and 7% H2 O. 2.2.3. X-ray fluorescence analysis The X-ray fluorescence (XRF) analysis was carried out for the catalyst surface and for the bulk material. The inlet and the outlet section of each catalyst were analysed. For the surface analysis, the honeycomb samples were ground to get a smooth plane. The plate type samples were analysed as received. The XRF gives the information of elements to a depth of 50 nm. Thus, the elements enriched on the surface by deposition or surface condensation are quantified. For the bulk analysis the catalyst material was ground and beads were prepared for the analysis. The bulk analysis provides information on elements throughout the whole catalyst
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
structure. This permits statements concerning pore condensation or chemical deactivation by penetrating compounds. 2.2.4. BET analysis The BET analysis was carried out using a Micromeritics ASAP 2010. The samples of the honeycomb catalysts were cut into small sections. The plate catalyst samples were prepared by separation of the ceramic carrier material from the metal support. Samples were taken from the inlet and the outlet section of each catalyst. The samples were evacuated and heated at 120 ◦ C for 4 h before analysis. The surface area was determined according to the BET multipoint method, the pore size distribution was analysed according to the BJH and t-plot methods. The procedures for these analyses are well documented in literature. The active sites of the catalyst are located on the surface of the porous carrier material. The higher the surface area, the more active sites that are available for the SCR reaction. In this respect, the surface area may serve as a parameter for the activity of the catalyst. It is expected that surface condensation or pore blocking results in a decrease of specific surface area. As a consequence the activity will decrease. Further information on the catalyst structure is expected from the average pore diameter and the total pore volume. Pore blocking of small pores will shift the value of the average pore diameter towards larger pores, whereas pore condensation is expected to show an adverse effect. The total pore volume will decrease in both cases, whereas the decrease concerning pore blocking is expected to be larger than that attributed to pore condensation. 2.2.5. Permeability measurements As a means for an in-situ characterisation of catalysts, a permeability measurement technique first applied to catalysts at the Universität Hohenheim [15] is adapted to obtain information on the degree of deactivation by evaluating the permeability of gas through the catalyst walls. Good penetration is indicative to good mass transfer through the catalyst which is considered representative for the activity as more the active sites in the pore system are accessible for the SCR reaction. The determination of the gas permeability is carried out ˙ and the pressure by measurement of the volume flow Q drop p over the catalyst sample, similar to Lea and Nurse [16]. According to [16] the permeability coefficient K can be determined allowing for slip since the employed fluid is
a gas: K=
˙ aL Qp A p
(1)
where L is the thickness of the catalyst sample, pa the ambient pressure, and A is the cross-section of the flow channel of the probe. The catalyst samples were inserted between two rubber seals and were compressed by two flanges to avoid torsion or bending. The sample diameter was much larger than the thickness so that radial conduction could be neglected. The sample was then loaded with compressed air and the inlet and outlet pressures and the volume flow were recorded. The outlet pressure was held constant at 0.2 bar relative pressure to ensure exact volume flow measurement according to the manufacturer’s requirements. For the pressure measurement, two pressure transducers were used while the volume flow was measured by a ball ˙ was expected. flow meter. A linear relation of p and Q Therefore, measurements of pin and pout were carried out at volume flow rates of 1, 2, 3, 4 and 5 l/h. Measurements at a constant flow rate were not possible for catalysts with low permeability (honeycomb). Therefore, a reference volume was created which could be sealed with a valve and the two pressures were recorded until equilibrium is reached (Fig. 1). 2.2.5.1. Gas permeability of plate type catalysts. The gas permeability of these catalyst types was the result of the micro cracks, the catalyst pore structure, and of macro cracks. In order to provide a basis for a comparison of honeycomb and plate catalysts, two values for the gas permeability K3 and K5 were determined. The K3 value was obtained with a flow area of 3 mm in diameter. This proved to be small enough to avoid marco cracks and large enough to obtain a quantifiable volume flow. This value served as an equivalent to honeycomb catalysts. To get a more representative value of the effective gas permeability, the larger cracks had to be considered. This was accomplished by using a flow area of 5 mm in diameter (K5 value). Deviations occurred due to the differently structured metal support material; however, attention was directed on measurement between the metal support. 2.2.5.2. Gas permeability of honeycomb catalysts. In comparison to plate type catalysts, honeycomb types have a significantly smaller permeability so that the measurement
Fig. 1. Permeability measurement.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
of gas flow, especially in case of deactivated samples, was difficult to quantify. For the volume flow through the catalyst wall, isothermal conditions were assumed. A reference volume Vref was incorporated in the test set-up, the inlet pressure was kept constant at 2.4 bar. The outlet valve was closed and the time was measured until an equilibrium pressure in the reference volume was reached. The permeability was measured at the first 5 mm of the inlet section, at 30 mm from the inlet and 5 and 30 mm from the outlet respectively. The obtained p measurement over time t was empirically approximated by the following function: p(t) = C1 e−t/T1/2
(2)
where C1 is a constant and T1/2 is the time in which p is half its initial value. Since Vref and pin are kept constant, the only variable is the time t which is proportional to the permeability according to Darcy’s law. The error which results from the free space between catalyst sample and outer seal is neglected. For a qualitative comparison of the permeability coefficients of the both catalyst types, the time constant T1/2 was directly related to the permeability coefficient in order to compare the results of honeycomb and plate type catalysts. The advantage of this method is its ease of application for in-situ measurements in power plants if a relation of permeability and degree of deactivation can be established.
species in the flue gas and on the catalysts. H2 SO4 and CO were identified as major reactants at all studied temperatures while at combustion conditions above 1300 ◦ C, SiO2 , CaO, and carbon were also identified as major reactants. The following formulae show the assumed main reactions during combustion. The results agree with literature [12,17]. The results of the calculations show in all cases the formation of phosphorus oxides or, in the presence of water vapour, phosphoric acid. At temperatures around 500 ◦ C, additional alkali polyphosphates were found to form during the case of MBM co-combustion. In the zone of the boiler exceeding temperatures of 1100 ◦ C, calcium phosphates react with carbon and SiO2 : 2Ca5 OH(PO4 )3 (s) + 6SiO2 (s) + 10C(s) + 2 O2 (g) → 6CaSiO3 (s) + 1.33Ca3 (PO4 )2 (s) + 10 CO(g) + H2 (g) + 0.66P2 O3 (g) + P2 (g)
3.1. Equilibrium calculations Concerning the deactivation mechanisms of P on the catalyst, its transformation from stable compounds in the fuel into reactive gaseous compounds in the flue gas was analysed. Possible formations of reactive P components during the combustion process are described. Subsequently, four reaction paths are shown which correspond to four different deactivation mechanisms: • • • •
Deactivation by solid calcium phosphate. Deactivation by condensed phosphoric acid. Deactivation by phosphorus oxide. Formation of alkaline polyphosphate or vanadyl phosphate glass.
All hint either at pore blocking, where a solid deposition layer prevents the mass transfer of the reactants, or to pore condensation, where P compounds condense inside the catalyst pore system, thus also blocking mass transfer. 3.2. Formation of reactive phosphorus compounds during combustion Equilibrium calculations in the temperature range 400–1400 ◦ C were carried out to estimate the formation of
(3)
2Ca3 (PO4 )2 (s) + 6SiO2 (s) + 10C(s) → 6CaSiO5 (s) + 10CO(g) + P4 (g)
(4)
The reaction goes to completion above 1500 ◦ C. At the same time, the elemental phosphorus reacts with the excess O2 : P4 (g) + 5O2 (g) → P4 O10 (g)
(5)
The phosphorus oxide then reacts with steam to form phosphoric acid: P4 O10 (g) + 6 H2 O (g) → 4 H3 PO4 (g)
3. Results and discussion
19
(6)
3.3. Deactivation mechanisms At temperatures present at the SCR catalysts or slightly above (400–500 ◦ C), the gaseous phosphorus acid forms polyphosphoric and metaphosphoric acid. The phosphoric acid reacts with the alkali compounds forming polyphosphates, probably with an intermediate step forming hydrogenous phosphate [17]. From the equilibrium calculations four scenarios for the deactivation by P compounds seem possible. The reaction paths are shown in Fig. 2. (a) Deactivation by calcium phosphate: Disregarding Eqs. (3) and (5), the majority of calcium phosphates are solid throughout the flue gas path. Furthermore, these compounds are chemically very stable at catalyst operating temperatures and will block the pores as fly ash. (b) Deactivation by phosphoric acid: At temperatures of 300–400 ◦ C polyphosphates are formed, consisting of O–P–O chains which may form surface layers by pore blocking or pore condensation. Chemical deactivation by reactions with the catalyst active material forming V or Mo phosphates is also a possible deactivation mechanism. (c) Deactivation by phosphorus (V) oxide: Besides H3 PO4 , P4 O10 is another dominant P compound in the flue gas. The deactivation mechanism is assumed to be pore blocking and the formation of phosphate chains.
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Fig. 2. Proposed reactions of calcium phosphates along the flue gas path.
(d) Formation of alkaline polyphosphate or vanadyl phosphate glasses: In the presence of alkali salts, alkaline polyphosphates may form as shown in Fig. 2. Effects will be similar to the ones describes in (b) and (c). In addition, reactions with the active catalyst components may take place, forming insoluble compounds as already described by Blanco [7]. Trimetaphosphate has been reported to form a ternary system of metaphosphate glass incorporating alkali metals (V2 O5 )n —(Na2 O·P2 O5 )(1−n) at temperatures of approximately 700 ◦ C. Another possibility seems the formation of glassy structures like Na4 VP3 O12 [18]. 3.4. XRF analysis The analysis of the results of the XRF measurements are focussed on P, Na and K. The concentration of the elements Si, Al, Ca, S and As, which also have an influence on the catalyst deactivation are additionally given in Table 3. All elements were converted into their oxide form. In general, the analysis shows an increase in all elements when compared to the fresh catalysts. The concentration is highest on the surface of the inlet section and decreases along the
catalyst length. The bulk concentrations show a similar behaviour but the absolute values are lower. Arsenic is the only element where the bulk concentration sometimes exceeded the surface concentrations. For a detailed description of this phenomenon, refer to literature [19]. Compared to coal mono-combustion (A.1 and C.2.3), the P surface concentrations are high for all samples exposed to co-combustion (exceptions: A.2 and A.5). In the bulk phase, no major difference was observed except for sample C.1.4, which also had high bulk P concentrations of 1.66 wt.% at the inlet section compared to 0.52 wt.% for C.1.3. This is attributed to the animal oil and fat co-combustion the sample was exposed to. In this case, high P loads accumulated in a short operation time of 2000 h. Similar surface P loads accumulated on catalyst samples C.2.1 and C.2.2 during sludge co-combustion. Except for sample A.2, the alkali surface concentrations found during sludge co-combustion were generally higher than in coal combustion. The alkali surface concentrations were also higher than the bulk concentrations. The concentration of the samples C.1.3 and C.1.4 exceeded 1 wt.% which results, according to Kamata et al. [2], in a significant loss of activity. This is consistent with the activity measurements shown in Table 2. The four samples A.3, A.4,
Table 3 XRF analysis of the catalyst surface, inlet section
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2 C.2.1 C.2.2 C.2.3
SiO2 (wt.%)
Al2 O3 (wt.%)
CaO (wt.%)
SO3 (wt.%)
P2 O5 (wt.%)
Na2 O (wt.%)
K2 O (wt.%)
Fe2 O3 (wt.%)
2.7–9.1 10.3 9.8 17.5 22.1 17.1 2...4.7 14.9 23.1 16 3.7 2...3.4 16.4 16.7 7
0.35–2.0 3.4 1.9 6 5.7 4.1 0.43–4.2 5.8 7.7 7.9 1.1 3.9–4.2 10.3 10.9 6.7
0.54. . . 1.4 1.4 4.1 7 9 3 <0.65 2.1 2.8 2.8 0.63 <0.03 1.5 1.1 0.32
1.383.5 2.5 20.6 18.7 20.6 12.8 0.44–3.6 8.2 15.3 12.8 14.5 3.4–3.6 10 8.9 4.1
<0.09 0.56 0.29 2 2 0.43 0.09–0.15 1.5 1.2 1.8 4.4 <0.03 4.9 4.4 0.73
<0.03 0.17 0.24 0.58 0.31 0.42 <0.03 0.41 0.77 1.2 1.5 <0.03 0.27 0.33 0.35
<0.05 0.12 0.15 0.65 0.5 0.25 <0.03 0.22 0.59 1.1 0.06 <0.03 0.26 0.28 0.14
<0.22 0.25 0.43 0.9 0.94 0.36 0.11–0.25 0.71 1.2 4.1 0.98 0.14–0.24 0.7 0.65 0.24
Samples A, C.1 and C.2 give the range of values for new catalysts.
As (ppm) <50 <50 <50 1670 2500 3100 <50 504 0.8 6300 <50 <50 3470 3380 46000
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
21
Fig. 3. Comparison of major deactivating elements of typical catalysts; (left) surface composition, (right) bulk concentration.
A.5 and C.1.3 exposed to MBM co-combustion also seem to accumulate alkalis in the bulk phase. Bulk alkalis values ranged from 0.52–0.84 wt.% compared to 0.09–0.49 wt.% for sewage sludge and 0.25 wt.% for coal combustion in the inlet section. Values of the surface accumulation are given in Table 3. The loss in activity of samples A.1 and A.2 of 14 and 38% respectively cannot be clarified by the chemical analyses since no significant accumulations of catalyst poisons were detected. This should be noted especially for catalyst sample A.2 which was exposed to sludge co-combustion and showed no significant accumulation of either P or alkalis. Fig. 3 shows the P, Na and K concentrations of typical honeycomb and plate catalysts exposed either to pure coal, MBM or sewage sludge co-combustion. Since no recordings of the exposure time to co-combustion were made, the absolute values of the different samples cannot be compared.
The sample A.3 and C.1.3 have high P surface concentrations which decrease along the catalyst length The bulk alkali concentrations are also high, this seems typical for MBM co-combustion. The high bulk P concentration of samples C.1.3 in relation to the surface concentration probably depends on a higher permeability which increases the mass transfer to the bulk phase. This would describe the different behaviour of the catalyst poisons in sample C.1.2. For the honeycomb samples, a relation between the relative activity k/k0 , the surface P and the alkali concentration can be established. Furthermore, a correlation between alkalis and P content was observed. Another strong correlation between Ca and the relative activity was also observed and is therefore shown in Fig. 4. For plate type catalysts, the alkali content shows a strong correlation to k/k0 (see Fig. 4), the Ca correlation is moderate and no correlation to the P content can be made. A correlation of alkalis and the P content
Fig. 4. Relation of surface P, alkali and Ca concentration compared to k/k0 of honeycomb and plate catalysts.
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J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Table 4 Results of the BET analysis Sample
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
BET surface area (m2 g−1 )
Average pore diameter 4V/A (nm)
Total pore volume (cm3 /g)
Inlet
Inlet
Inlet
Outlet 55.3
42.1 54.4 45.2 39.8 45.8
46.1 54.0 52.1 45.4 43.8
16.48 15.62 19.00 16.12 17.07
45.9 54.3 55.2 45.3 51.4 59.9 72.3
17.44 17.55 14.43 17.71 10.29 9.14 8.32
64.5 49.5 47.2 51.2 52.1 47.2 56.1 63.0
Outlet 18.14 18.05 15.06 16.26 12.47 16.87
0.17 0.21 0.21 0.16 0.20
17.33 17.07 14.10 18.70 9.94 9.33 7.74
0.22 0.20 0.18 0.23 0.12 0.13 0.13
13.20
is also not obvious. Due to the strong correlation between Ca and activity for honeycomb catalysts, fly ash blocking seems a major deactivation mechanism. A distinction between pore blocking or pore condensation concerning the predominant effect of phosphorus however is not possible. Alkalis also have a large influence on the activity which can be by chemical deactivation or by the formation of surface layers by glassy phosphorus compounds. The strong relation of activity and alkali content at the plate type catalysts suggests chemical deactivation predominantly by alkali poisoning. A certain contribution by fly ash seems possible whereas the influence of P seems negligible. 3.5. BET analysis Regarding the specific surface area determined by BET, a decrease for all samples A.X and C.1.X was observed compared to the new catalysts A and C.1 (see Table 4). For the catalyst type C.2, no new samples were available for surface characterisation. The results are included since they illustrate the same behaviour of both plate type catalysts C.1.X and C.2.X. Besides the general decrease in surface area of deactivated catalyst samples, a rise in the surface area from the inlet to the outlet section was observed. Due to the turbulent flow in the inlet section, the mass transfer is locally increased. Hence, deactivation mechanisms by pore blocking or pore condensation also increase. Both deactivation mechanisms reduce the surface area which can be observed from the results according to Table 4. For samples A.2 and A.5, no significant difference between the inlet and outlet surface area can be observed. The differences are within the measurement accuracy. The samples C.1.1 and C.1.4. have an opposite behaviour; the surface area decreases from inlet to outlet. In the first case, severe crack formations may be an explanation. For the sample C.1.4, where the difference is larger, the reason is seen in the liquid fuel and the
Outlet 0.23 0.21 0.20 0.21 0.14 0.18 0.21 0.20 0.23 0.19 0.21 0.13 0.14 0.14
resulting flue gas composition, which cannot be compared to the plants of the other samples where solid fuels which generate higher fly ash loads were used. The sample C.1.3 has a higher surface area than sample C.1.2 (see Fig. 5). This may be the reason for better mass transfer to the bulk and could explain the high bulk concentration of P, Na and K as shown in Fig. 3. The data for sample A.5, which was exposed to MBM co-combustion, is given for comparative purposes only since no recordings concerning relative activity or operation time were available. The samples A.3 and A.4 represent MBM deactivation of honeycomb catalysts. Both samples show a large decrease in surface area compared to the new catalyst. They also have the largest surface area difference between the inlet and outlet section. Pore blocking by fly ash cannot be the only cause for this severe decrease since other catalysts were also subjected to the fly ash pore blocking and do not show a comparable behaviour. Additional pore condensation effects attributed to phosphorus compounds is therefore assumed.
Fig. 5. Comparison BET surface area and average pore diameter (APD) of typical catalysts.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Furthermore, a decrease of the average pore diameter and the total pore volume over operating time suggest that catalyst deactivation is due to pore condensation. Sample A.4, which had the longest operation time, is the best example of this phenomenon. From the results, a clear distinction between either severe pore condensation which fills the whole pore volume with condensate, or pore blocking is not possible. Sample A.2 also had a long operation time and shows a similar behaviour concerning the average pore diameter but the total pore volume does not decrease significantly. This clearly hints at pore condensation effects. The plate type catalyst C.1 showed a rise in average pore diameter combined with a total pore volume similar to that of a new catalyst. This behaviour is independent from the operation time, since the samples were taken between 2000 and 65,000 h in operation. The crack formation typical for plate type catalysts is considered a reason for the shift towards larger average pore diameters. However, the average pore diameter of sample C.1.3 only slightly rose whereas the total pore volume decreased. If an average pore diameter of 17 nm, as shown in Table 4, is considered usual for plate type catalysts in operation, the low value of 14.4 nm together with the decrease in pore volume is similar to the results of the honeycomb sample A.4. If this effect could be observed in further MBM deactivated samples, it would suggest a typical deactivation effect: pore condensation due to the decrease in pore diameter and in total pore volume, possibly in combination with pore blocking. 3.6. Permeability The permeability index K is a value for the gas permeability of a porous layer. The higher K, the higher is the permeability of the specific layer. From the results given in Table 5, the principle difference in the measurement method between plate and honeycomb
23
catalysts can be observed. Plate catalysts have a much higher permeability. Due to the inhomogeneous surface which is a result of micro and macro cracks and the metal support, repetitive measurements are difficult so that the shown results give only a limited impression. However, a significant decrease in permeability compared to the new material can be observed. The permeability method seems more applicable for honeycomb catalysts since these have a homogeneous structure. Here, similar to the plate type catalysts, a large decrease in comparison to the new catalyst is observed. Sample A.1 could not be measured due to high impermeability. Severe pore blocking by fly ash within an operation time of only 14,000 h is probably the reason which would also explain the decrease in BET surface area. The other results indicate a trend along the catalyst length similar to the BET measurements. This is expected due to the increased mass transfer at the turbulent inlet section of the catalyst as described previously. The samples A.3 and A.4 especially show a rising permeability along the catalyst length. Fig. 6 illustrates this trend with the exception of sample A.2 which shows first a decline and than a small rise. A comparison with the results of the surface characterisation in Table 4 shows a slight drop of the surface area and the total pore volume along the catalyst length. The difference is within the accuracy of the measurement method. Assuming a correlation of permeability and surface area, the permeability measurement would support the BET results. The surface area analysis of sample A.5 does not show the significant increase from inlet to outlet section that samples A.3 and A.4 did, although the average pore diameter and the total pore volume of all three samples have the same trend. However, the permeability measurement gives the same results for all MBM deactivated samples A.3, A.4 and A.5. The latter differs in the similar surface area of inlet and outlet section, whereas the values for average
Table 5 Results of the permeability measurement K5 (10−3 m2 /s) Inlet
A A.1 A.2 A.3 A.4 A.5 C.1 C.1.1 C.1.2 C.1.3 C.1.4 C.2.1 C.2.2 C.2.3
K3 (10−3 m2 /s) Outlet
Inlet
Outlet
5 mm
30 mm
1.7E−3 447.4E−6 434.7E−6 182.1E−6
203.5E−6 616.9E−6 854.2E−6 499.0E−6 215.1E−6
30 mm
5 mm
530.9E−6 857.7E−6 601.7E−6 273.1E−6
717.0E−6 1.6E−3 717.0E−6 256.3E−6
43.7E−3
5.7941 1.9758 0.0762 0.5192 1.5725 0.1440 0.2029 0.6025
1.3252 0.1448 0.2898 0.9322 0.2128 0.5844 1.1747
0.9625 0.2866 0.0868 0.5023 0.9263 0.8906 0.3033 0.8082
0.5534 0.1971 0.1267 0.6507 0.2008 0.4860 0.6571
24
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Fig. 6. Permeability along honeycomb catalysts.
pore diameter and total pore volume show the same trend as the other MBM deactivated honeycomb catalysts. In this respect, the permeability measurements support the general results of the BET surface area measurements since they indicate the same trend for MBM deactivation. In cases where the conventional surface measurement does not show clear results, it may serve as an additional tool for further detailed characterisation.
4. Conclusion and outlook For a basic understanding of possible reactions in the flue gas for the temperature range of 1400–400 ◦ C, equilibrium calculations including major flue gas constituents as well as the deactivating components Na and P were carried out. The results indicate the presence of orthophosphoric acid at catalyst temperatures as well as polyphosphorus oxides and alkali polyphosphates which may condensate at catalyst operating temperatures or react with catalyst compounds forming phosphorus glasses. Four possible deactivation mechanisms caused by phosphorus were derived: (a) pore blocking by solid calcium phosphates, (b) deactivation by pore blocking or pore condensation by H3 PO4 or (c) phosphorus oxides and (d) formation of phosphorus glasses in combination with pore condensation and eventually chemical deactivation. Subsequently, a set of three different types of deactivated catalysts, exposed to co-combustion of P rich fuels, either animal residues or sewage sludge, were examined. The relative activity k/k0 was determined and the chemical composition of surface and bulk material was analysed by XRF. Surface characterisation including specific surface area, average pore diameter and total pore volume and an adapted permeability measurement method were carried out. Concerning honeycomb catalysts, the applied permeability measurements can be an easily applicable means for in-situ surface characterisation. For a secure statistical basis
however, much more samples need to be analysed, which were exposed to defined conditions as a reference in order to derive definite relations. Due to the inhomogeneous structure of plate type catalysts, the permeability method does not give repetitive results and will not be applied further. The chemical results of the analyses showed an enrichment of P on the catalyst surface of all samples. The enrichment of alkalis on the surface was higher for samples exposed to MBM co-combustion. These samples also had considerable bulk enrichments of Na and K. These concentrations alone were capable of a severe deactivation. Accordingly, a close relation between alkali content and relative activity could be established for all catalysts. Concerning P, a relation was only observed for the honeycomb catalyst samples. The surface characterisation of all MBM deactivated samples showed an increase of surface area along the catalyst length as well as a shift of the average pore diameter towards smaller values. The total pore volume also decreased slightly which might be a typical effect of catalyst samples exposed to MBM co-combustion. Pore condensation in combination with blocking caused by P was considered as the prevailing deactivation mechanism. During sewage sludge co-combustion, pore condensation had a stronger influence than other mechanisms. The results support the thesis that P tends to form a surface layer or condensates in the pores. The large difference in the deactivation rate between deactivated catalyst samples obtained during MBM or sludge co-firing may be attributed to the different alkali concentration in the fuel. A distinction between the assumed deactivation mechanisms was not possible. In operational conditions in power plants, complex reactions take place forming a range of catalyst poisons. As a result, several mechanisms contribute to the catalyst deactivation beside the considered compounds P, Na and K. Due to the different operating and exposure times to co-combustion, a comparison of the relative activities was difficult. Furthermore, deactivation mechanisms other than those caused by P or alkalis may have occurred. To exclude other influences, future work will include lab-scale investigations to examine the independent contribution of P or Na to catalyst deactivation. The basis will be the major fuel compounds and flue gas concentrations identified. A comparison with the surface characteristics as well as with the remaining gas permeability is expected to show general trends for P or the combination of P and Na. The samples will also serve as reference material for permeability measurements. In order to verify the theory on flue gas concentration and the formation of deactivating P compounds in the flue gas and in the fly ash, in-flame measurements as well as fly ash analyses shall be carried out. In addition, information on the condensation behaviour of fly ash particles along the flue gas duct shall be gathered and the formation of phosphorus compounds will be investigated.
J. Beck et al. / Applied Catalysis B: Environmental 49 (2004) 15–25
Acknowledgements The work was supported by the EESD programme of the European Commission within the RTD project “Influences from Biofuel (Co-)Combustion on Catalytic Converters in Coal-Fired Power Plants” contract no.: ENK5-CT-200100559. For further information about the project please refer to http://www.eu-projects.de/CATDEACT. The funding for this work is gratefully acknowledged by the authors. Many thanks to Annette and Gabi for the BET analyses. References [1] J.P. Chen, M.A. Buzanowski, R.T. Yang, J.E. Cichanowicz, Deactivation of the Vanadia catalyst in the selective reduction process, J. Air Waste Manage. Assoc. 40 (1990) 1403–1409. [2] H. Kamata, K. Takahashi, I. Odenbrand, The role of K2 O in the selective reduction of NO with NH3 over a V2 O5 (WO3 )/TiO2 commercial selective catalytic reduction catalyst, J. Mol. Catal. A 139 (1999) 189–198. [3] D.A. Bulushev, F. Rainone, L. Kiwi-Minsker, A. Renken, Influence of potassium doping on the formation of Vanadia species in V/Ti oxide catalysts, Longmuir 17 (2001) 5276–5282. [4] T. Reichelt, Freisetzung gasförmiger Alkaliverbindungen bei atmosphärischer und druckaufgeladener Verbrennung, Ph.D. Thesis, Universität Stuttgart, VDI Verlag Düsseldorf, 2001. [5] H. Schürmann, S. Unterberger, K.R.G. Hein, P. Monkhouse, U. Gottwald, The influence of fuel-additives on the behaviour of gaseous alkali compounds during pulverised coal combustion, Faraday Discuss. 119 (2001) 433–444. [6] R. Khodayari, Selective Catalytic Reduction of NOx : Deactivation and Regeneration Studies and Kinetic Modelling of Deactivation, Dissertation, Lund University, 2001.
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