Fouling Deposits from Residual Biomass with High Sodium Content in

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Fouling Deposits from Residual Biomass with High Sodium Content in Power Plants Magín Lapuerta,*,† Anselmo Acosta,‡ and Amparo Pazo† †

Escuela Técnica Superior de Ingenieros Industriales and ‡Facultad de Químicas, Universidad de Castilla La-Mancha, Avda. Camilo José Cela s/n, 13071 Ciudad Real, Spain ABSTRACT: Unusually large fouling deposits on the heat transfer tubes of a power plant using a residual product from olive extraction, which led to boiler shutdowns, have been attributed to an unusually high sodium content in the feed biomass. To confirm this and to evaluate the mechanisms of deposit formation, different techniques were used to analyze feed biomass and mature deposits sampled at different locations of the convection section: inductively coupled plasma, fusion test, energy dispersive X-ray fluorescence spectrometry, and scanning electron microscopy. All the deposits sampled were divided into two layers: a thin inner layer mainly composed of potassium and chlorine and a more porous outer layer with higher content in less volatile elements such as Ca, Si, Al, and Fe, both layers showing high sodium content. Differences in composition and morphology were also found depending on the temperature of the region where the deposits were formed and on the sampling position, frontside or backside, with respect to gas stream. Frontside deposits showed higher porosities and were mainly composed of granular and spherical structures, whereas backside ones showed higher compaction and are mainly composed of crystalline structures formed on the deposit surface. Structures were categorized based on their shape, and some relationships between shape and composition were proposed, which supported hypotheses about their formation mechanisms. Structures defined as amorphous molten were sticky, therefore increasing the capturing efficiency and tenacity of the deposit and leading to large deposits difficult to remove. The high sodium content of these structures was associated with the high sodium content of the feed biomass. Limiting this element is recommended to prevent uncontrolled deposit formation in the boiler.

1. INTRODUCTION Combustion of solid fuels in power plants involves formation of ash particles and gas species which may generate deposits on heat transfer surfaces. Deposits can be classified into two main types:1 (a) slagging deposits, usually located in the sections of the boiler with highest temperature, with direct exposure to radiation from the flame; these deposits do not show particle-based structure but a continuous structure, as a consequence of melting or assimilation of particles in a liquid phase; and (b) fouling deposits, usually located on convective tubes of a boiler. These deposits usually contain a lower proportion of molten phases than slagging deposits. Regardless the type, deposits act as a heat transfer resistance and may therefore reduce heat transfer to the water−steam system, reduce the boiler thermal performance, cause corrosion of heat transfer surfaces, and, in severe cases, completely block the gas channels and subsequently cause boiler shutdowns and unscheduled stops of the power plant.1 The main mechanisms of fouling deposit formation are eddy and Brownian diffusion, thermophoresis, condensation, impaction with particle capture (inertial impaction and eddy turbulent impaction on the frontside tube surface and eddy turbulent impaction on the backside), and heterogeneous chemical reactions between the surrounding gas and the deposits.2,3 The prevalence of these mechanisms depends on the gas temperature, the gas flow velocity and its incident angle, the content of inert compounds, the vapor pressure of the volatile materials, the geometry of the boiler, and the characteristics (size and viscosity) of the ash particles and the deposits previously formed. In turn, these parameters are determined by the plant configuration, the location of the surface within the combustor, the operational conditions, and the biomass characteristics.1,4,5 © XXXX American Chemical Society

Figure 1. Main mechanisms controlling deposition and maturation of ash deposits.

Figure 1 shows the characteristic two layer (inner and outer layer) deposits formed at the heat transfer tubes arranged in cross-flow and the main mechanisms controlling formation and maturation of these deposits. Several authors [e.g., refs 5−12] have pointed out the relation between fouling, slagging, corrosion, sintering, and agglomeration Received: February 13, 2015 Revised: June 16, 2015

A

DOI: 10.1021/acs.energyfuels.5b00356 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (last at FBCs) with the biomass content on alkali (K and Na), chlorine, and sulfur. Also the silica content has been shown to have significant influence.5,11 Among the factors affecting the deposit growth and the difficulty to be removed, the most important ones are chlorine concentration5,8 and flue gas temperature.5 Exhausted olive cake (orujillo) is composed of deoiled pulp and kernels from olive fruit. It is the final residue produced in olive oil extraction. It is a deoiled and dried solid residue obtained after extraction with solvents of the residual olive oil from the olive cake, which is a byproduct of the olive oil mechanical extraction process. In southwest European Mediterranean areas, this fuel is used as a unique fuel in medium-sized boilers to generate steam for on-site thermal use at olive oil industries13 and also in power plants (in Andalusia with powers ranging from 8 to 16 MWe14,15). Furthermore, it is widely used in cofiring in large coal plants in Europe (in the U.K. it is one of the most widely used16,17) because of its easy grinding, which allows its incorporation in coal pretreatment systems with minimal modifications16 and because of its easy feeding, with no flowing problems given its granular shape and particle size (mostly below 8 mm). In addition, its lower heating value (LHV) is high (between 14 and 17.5 MJ/kg aras received), and it is usually marketed under low moisture values (between 9 and 16% ar). Orujillo has relatively high potassium and chlorine contents (typically: K = 1.75% and Cl = 0.1−0.4% dry basis18), which requires a careful definition of the boiler conditions to avoid deposit formation and malfunctions. Occasionally, it may reach high content in chlorine and sodium derived from external contamination or specific treatments used in olives being dressed with salt (NaCl solution) or soda (NaOH solution, i.e., lye) for table olives.19 When this occurs, serious problems often arise in the boilers. The European standard for solid fuels alerts about the eventual increase in the Na content when the biomass is chemically treated with lye. Other external contaminations with NaCl can also take place in biomass from areas exposed to seawater or road salting.18 Other fuels with biomass origin and with high Na content are crude glycerol from biodiesel production, using sodium hydroxide or methoxide as catalyst, and black liquor. Sodium content in crude glycerol could exceed 1%20 and in black liquor can reach up to 20% in the dry solid21,22 leading to fouling deposits entirely composed of Na2CO3 and Na2SO4.23 Although different research works have been found in the literature from pilot-scale suspension-fired plants using straw, wood, olive cake, or exhausted olive cake,24−26 no research studies were found about deposits from large-scale plants with exhausted olive cake.

Table 1. Plant Operational Characteristics and Fuel Characteristics normal operation unit capacity electrical capacity usual gross electrical output usual steam production total air flow rate superheater steam temperatures flue gas temperatures at superheater inlet flue gas temperatures at boiler generator bank oulet soot blowing (of dry steam) frequency fuel

feed biomass particle size feed biomass moisture usual biomass consumption biomass consumption per year fly ash production per year bottom ash production per year

55 MWth 16 MW 14−14.5 MW 61−62 tons/h 65−66 tons/h 280 to 450 °C 650−700 °C 390−420 °C 3 to 6 times per 8 h primary: Orujillo ≥ 95% (in mass); secondary: wood, forest chips, exhausted grape cake, almond shell, etc. ≤ 5% (in mass) ≤1.2 mm 12−17% (wet basis) 12.5 tons/h (wet basis) 90 × 103 tons/year (wet basis) 7 × 103 tons/year (wet basis) 5 × 103 tons/year (wet basis)

shutdown, as a consequence of fouling deposit formation. The fuel usually employed is 95% mass basis of exhausted olive cake and 5% of other biomass (wood, exhausted grape cake, almond shell, and others). During the month before the unscheduled stop, the biomass consumed was 98% exhausted olive cake mix and 2% wood chips, with an extremely high sodium content. This biomass mixture, as well as its main components, were studied here. A scheme of the boiler with the deposit sampling locations is presented in Figure 2, and images of the corresponding deposits are shown in Figure 3 together with the steam temperature at sampling locations. The studied deposits are identified as • S1: Sample of deposit formed at the backside of the tubes located downstream of the secondary superheater. • S2: Sample of deposit formed at the frontside of the tubes located upstream of the primary superheater (area of complete block). • S3: Sample of deposit formed at the frontside of the tubes located upstream of the boiler generator bank.

2. POWER PLANT, BIOMASS USED, AND DEPOSIT SAMPLING LOCATIONS The deposits studied were sampled from a 16 MW suspensioń de la Mancha, S.A.) located in fired boiler power plant (Energias Arenas de San Juan, Spain. This plant is based on a Rankine cycle and includes a multistage horizontal steam turbine, an electric generator of 20 000 kV·A, and an air-cooled water condenser, among other equipment. In the boiler generator bank and in the superheaters, the heat transfer tubes are arranged in cross-flow. Table 1 summarizes the plant operational characteristics, including the main fuel characteristics: kind of fuels, particle size, and usual moisture. The power plant suffered a complete flue gas blockage in the primary superheater and therefore an unscheduled boiler

3. ANALYTICAL TECHNIQUES, SAMPLES, AND SAMPLE PREPARATION 3.1. Analysis of Biomass. Five different biomass samples were analyzed: • The feed biomass (FB), a subsample of the combined sample composed of the biomass increments taken daily (from the delivery system) during the month before the unscheduled stop. • Four different orujillos, OA to OD, received from different oil processing companies around the plant, which are the main components of the mixture, summing around 80 wt % of the whole FB sample. B


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Samples from the deposits were extracted and milled for the determination of their bulk chemical composition by inductively coupled plasma optical emission spectrometry (ICP-OES). Their thermal response was determined in the fusion test and the presence of crystalline inorganic species was obtained by XRD. A few unmilled pieces were used for their structural and chemical analyses in the scanning electron microscope and energy dispersive X-ray fluorescence spectrometry (SEM-EDX). All analyses were made on two different parts of the deposit samples: the inner layer and the outer layer. Additionally, sublayers within these layers were also analyzed with SEM. Table 2 summarizes the methods used to analyze the deposits, and the methods described below are those not subject to standard. Morphological structure and chemical composition were studied with scanning electron microscopy−energy dispersive X-ray spectroscopy (SEM-EDX). The specimen to analyze was mounted on a carbon adhesive tape, and then it was gold-covered to improve its conductivity. The electron microscope used is a high-vacuum microscope. The chemical analysis was performed over an area (area analysis) of the sample or over a specific point (spot analysis) selected as those presenting the flattest surface and most intense signal. However, some quantitative errors could be derived from surfaces not being completely flat. For a better comparison between analytical techniques (ICP and EDX), carbon content is also included among the results despite its lower accuracy since it is close to the lower detection limit. The crystalline phases were detected with a powered X-ray diffractometer. Identification of compounds was made using the Joint Committee on Powder Diffraction Standards (JCPDS) database. Fusion test was also made on powdered deposit samples, similarly as described for biomass ash in Section 3.1.

Figure 2. Schematic view of the boiler with deposit sampling locations (S1, S2, and S3).

4. RESULTS AND DISCUSSION 4.1. Biomass Properties. The results from the physicochemical analysis of the biomass samples (from feed biomass and from its components) are summarized in Table 3. The range of values for feed biomass consumed during the first semester of 2012 is also presented as a reference of biomass leading to usual and no problematic operation (right column in Table 3). Although major element values are often highly variable within the same type of biomass, certain values of OA and OC samples fall outside of the mentioned usual range. It is worthy to remark that the high sodium content in these two components (OA and OC) and, consequently, in the feed biomass (FB) are 2 orders of magnitude higher than the usual values. Likewise, the potassium content is low for these two samples. For the OC sample, the high ash content and the low lower heating value are also remarkable. The silicon values are low for OA and OC and a little high for the other samples. The high sodium contents of OA and OC (and to a lower extent also of OB) can be explained either by a negligent contamination or by any olive pretreatment with soda (NaOH) rather than with sodium chloride (since there is not an increase with respect to the usual Cl content13). In any case, this increased Na content leads to a decrease in the proportion of the others elements, as mentioned above, and to a great increase in the base/acid ratio with respect to that of the original biomass. The values shown in Table 3 for the (K2O + Na2O)/SiO2 index are indicative of probable fouling for all samples, according to Magasiner et al.27 thresholds ((K2O + Na2O)/SiO2 > 1 leading to probable fouling), but clearly with highest probability for OA and OC, both with highest Na content. This effect is confirmed in the fusion tests. Figure 4 shows the resulting heights of the specimens (made from the biomass ash produced at 550 °C in a muffle furnace from each sample) after being subjected to fusion tests. Images of the specimens are also shown at 580 °C (around the expected gas temperature at the S1 and S2 locations, as shown in Figure 3). The dotted line for OC indicates that, at this temperature range,

Figure 3. Approximate temperature distribution under normal operation and pictures of sampled deposits at each location. The parameters determined were moisture, ash and volatile content, chemical analysis, heating value, and fusion test. The standards followed and the equipment used are listed in Table 2. The fusion test was carried out following an internal procedure based on the technical specification CEN-TS 15370-1. A cylindrical pellet (2 mm base diameter, 3 mm height) of biomass ash (obtained by ashing biomass until 550 °C) was heated up in an air atmosphere, and its form and height were continuously recorded. The ash melting temperature is established when the height of the ash pellet reaches a third of the initial height, as proposed in DIN 51730-1994. 3.2. Analysis of Deposits. Morphological structure, including crystalline phases, chemical composition, and fusion behavior were determined for the deposit samples. The description and position of the three deposits studied (S1, S2, and S3) are indicated in Section 2. Here the sample preparation and the analysis performed are described. C


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Energy & Fuels Table 2. Summary of Methods for Characterization methoda

standard

equipment

Biomass moisture as received proximate analysis: volatiles ash chemical analysis: C, H, N/S

EN 147774-2

drying oven

EN 15148 EN 14775

muffle furnace muffle furnace

EN 15104/ EN 15289

Cl

EN 15289

Al, Ca, Fe, K, Mg, Na, P, Si

EN 15290

heating value

EN 14918

fusion test

-

heating of a cylindrical pellet of biomass ash (generated at 550 °C) in an air atmosphere

morphology and chemical composition (Cl, S, Al, Ca, Fe, K, Mg, Na, P, Si) Al, Ca, Fe, K, Mg, Na, P, Si EN 15290 fusion test crystalline phases a

high oxygen combustion−gas analysis: -C, H, S infrared absorption -N thermal conductivity biomass microwave acid digestion−liquid ion chromatrography analysis biomass microwave acid digestion−inductively coupled plasma optical emission spectrometry

-

Deposits scanning electron microscopy−energy dispersive X-ray fluorescence spectrometry (SEM-EDX) deposit microwave acid digestion−inductively coupled plasma optical emission spectrometry heating of a cylindrical pellet in an air atmosphere X-ray diffraction (XRD)

elemental analyzer (LECO - TruSpec)

microwave−ion chromatograph (883 compact IC Pro, Metrohm) optical emission spectrometer (ICP-OES 715-ES, Varian) bomb calorimeter (6100 Calorimeter, Parr) optical heating microscope (Misura 3)

Philips XL30 optical emission spectrometer (ICP-OES 715-ES, Varian) Optical heating microscope (Misura 3) X-ray diffractometer (X’Pert MPC, Panalytical)

The method used is indicated only when it is chosen among the various methods considered as the standard or when there is no standard method.

Table 3. Physicochemical Properties of Biomass Samples content in the mixture (vol %) moisture as received (wt %)a proximate analysis (wt %)b volatiles ash chemical analysis (wt %)b C H N S Cl Si Al Fe Mg Ca Na K Ti P LHVp (MJ/kg)b fouling indices (K2O + Na2O)ash/HHVvc (kg/GJ)b (K2O + Na2O)/SiO2 a

OA

OB

OC

OD

FB

range of values during one semester

12.00 11.52

40.00 18.08

15.00 24.71

15.00 14.62

18.29

7.53−19.68

71.44 8.21

72.46 10.39

68.63 13.46

71.92 11.73

72.06 9.91

70.62−72.31 8.11−9.96

50.21 5.48 1.32 0.06 0.24 0.10 0.02 0.03 0.08 0.45 1.54 1.18 0.00 0.10 17.92

50.85 5.59 1.69 0.14 0.23 0.47 0.10 0.07 0.41 0.97 0.33 2.79 0.01 0.21 18.63

47.40 5.90 1.45 0.06 0.40 0.21 0.04 0.04 0.12 0.65 4.04 0.66 0.00 0.08 16.86

51.35 5.44 1.62 0.14 0.41 0.46 0.15 0.09 0.20 0.48 0.02 2.84 0.01 0.15 18.72

51.17 6.31 1.49 0.06 0.30 0.58 0.16 0.09 0.27 0.71 0.52 2.42 0.01 0.16 18.32

48.73−49.90 5.78−5.94 1.35−2.58 0.11−0.13 0.25−0.29 0.33−0.43 0.06−0.10 0.05−0.07 0.18−0.27 0.52−0.69 0.01 2.47−3.37 0.00−0.01 0.14−0.24 18.30−18.65

1.83 15.76

1.91 3.77

3.44 13.82

1.73 3.48

1.84 2.93

1.52−2.08 2.92−5.05

On a wet basis. bOn a dry basis. cK2O and Na2O are given in % in ash.

(only historical limits are shown with dotted-dashed lines). OB and OD height profiles also are not far from the previous profiles mentioned. On the contrary, the ash specimens from samples corresponding to components OA and OC show shorter heights and greater fluidity at the expected temperature, meaning that particles belonging to these biomass components would be more

the height of the sample was not recorded because of the intense release of volatile species, which obscured the image. This figure shows a height profile for the feed biomass (FB) which, in the temperature range at the superheaters inlet (around 675 °C, and in any case, below 800 °C), is not far from other previous profiles obtained from the feed biomass of this power plant D


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Figure 5. Details of S1 and S3 inner layer. (a) Innermost sublayer and (b) innerleast sublayer. The black line shows the limit between sublayers.

chemical and morphological differences associated with their formation processes. During the preparation of the samples, S1 was found to be the sample with highest compaction and density and the most difficult to be crushed, indicating high tenacity. The outer layer of sample S2, especially the flat external surface (with molten appearance), is also very difficult to crush, while the outer layer of S3 is less tenacious. Deposits S1 and S2 (outer sublayer) should be difficult to remove by soot blowers, owing to their high density and tenacity.29 On the contrary, inner layers of samples S2 and S3 were powdery (as other authors have pointed out for inner layers of straw deposits30,31), fragile, and easy to crush and disintegrate. Figure 6 shows the morphology of different layers and sublayers for each sample. The texture and porosity of the observed materials are indicative of their formation processes. The internal layers, mainly formed by condensation, diffusion, and thermophoresis, have less porosity and smaller singular shapes (particles). In contrast, the outer layers, for which impaction processes are more relevant, have higher porosities (bigger empty spaces) and are composed of larger particles. Similar configurations have been observed in deposits sampled from power plants fed with coal32 and straw,6,31 where higher porosities were also found at the outer layers. Consistently with its highest compactness, the morphology of the deposits located at the backside (S1) is different to that of those sampled from the frontside (S2 and S3). Although the porosity increases as the distance from the water tube increases (as in all cases) sample S1 showed melted undefined shapes, which are indicative of fluidity and showed less porosity than S2 and S3. Sample S1 also showed the most clear crystalline structure among the samples, especially in the innermost sublayer where cubic shapes are visible. This can be associated with KCl crystals, as can be confirmed from the XRD results (Table 4), which show, for the inner layer (composed of the innermost and innerleast sublayers), highest diffraction intensity for KCl. Another confirmation is derived from the chemical analysis of the innermost sublayer, which showed an atomic ratio K/Cl very close to unity. In the outer layers, spherical shapes can be observed in all samples. These spherical particles are composed of ash, which was previously melted in the combustion chamber and further solidified before reaching the superheaters. These spherical shapes are then incorporated into the deposits by inertial impaction as a consequence of their increased size (>10 μm). Jiménez and Ballester25 observed these type of spherical particles in the coarse fraction (with particle diameters above 1 μm) of the size distributions of fly ash from the combustion of pulverized exhausted olive cake. Bashir et al.9 found similar structures at the outer layer of frontside deposits collected from straw suspension firing.

Figure 4. Specimen images at 580 °C (above) and specimen heights during the fusion test (below). For sample OA the initial specimen height was taken at 100 °C and for the rest of samples at 550 °C.

likely to reach the superheaters in a molten or semimolten state, therefore with great stickiness. Consequently, it is expected that these particles would get adhered to the deposits, therefore reducing the viscosity of their external surface and increasing the effectiveness of particle capture and deposit growth.1 However, these components just contribute in a small extent to reduce the height profile from the fusion test of the FB sample. This indicates that encouraging results from the fusion test (such as those from FB) do not necessarily guarantee that the tested biomass will not cause fast deposit formation and consequently malfunctions in the power plant. In fact, biomass particles from pulverized biomass are very disperse, and therefore reactions involving condensed phases are limited,28 differently as in the fusion test, where the whole ash material is compacted, and thus, the condensed phases of each component may contribute to the formation of deposits independently. 4.2. Deposit Properties. The deposits formed on the first superheater (S2) and the boiler generator bank (S3) are very thick, 9.5 cm for S2 and 7.5 cm for S3. These thickness values are greater than the distance between tubes (5.1 cm in the cross gas flue direction) indicating the possibility of blocking if this thickness is projected in the cross gas flue direction. In fact at first superheater there were larger areas completely blocked. Deposits formed at the backside of second superheater (S1) are less thick, 5.5 cm for the S1 sample, and the distance between tubes at this point are 14.6 cm in the cross gas flue direction. In all deposits two different parts are visually detectable and easily separable: the inner layer and the outer layer. Additionally, sublayers within these layers were also detected. At the inner layers, a dense sublayer located in the innermost position from the tube can be distinguished from an innerleast sublayer, in which protuberances are observed in samples S2 and S3 (Figure 5). Also, in the outer layer from sample S2, three sublayers could be distinguished (O1, O2, and O3, from the inner layer to the external surface). These different sublayers were analyzed with SEM-EDX. The determination of the layers and sublayers was made visually. The analyses showed significant E


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Figure 6. Morphology of samples with differentiation between layers and sublayers. In all cases, the images are oriented so that the metal tube is placed at the bottom of the image (not showed).

Table 4. Identified Major (**) and Minor (*) Crystalline Phasesa S1 crystalline phase KCl K0.2Na0.8Cl CaMgSiO4 (monticellite) Ca3Mg(SiO4)2 (merwinite) Na4CaSi3O9 K2CO3 KNaCO3 Mg3Ca(CO3)4 (Huntite)

I

S2 O

I

Sylvite ** ** ** Halite Potassium * Silicates * **

S3 O

I

O

**

**

**

*

*

** *

**

* Carbonates *

*

*

*

**

**

* Sulfates

K2SO4 (arcanite) Oxides KFeO2 Ca3Al2O6 Ca3Fe2TiO8 β-Ca4Fe14O25 a

* *

* * *

*

I = inner layer, O = outer layer.

Figure 7. Outer sublayers for sample S2. In all cases, the images are oriented with the metal tube (not shown) at the bottom of the image.

Figure 7 compares the different shapes observed along different sections of the outer layer from sample S2. The spherical particles found in the internal sublayer of the outer layer (O1) appear to be embedded in a highly compact material with round edges, indicative of fluidity. As the distance from the sublayers (O2 and O3) to the internal layer increases, the spherical shapes become clearer and more regular. Additionally, from sublayer O2 outward a certain amount of dusty acicular or prismatic shapes can be observed on the surface of the spherical particles. Table 4 shows the identified crystalline phases at inner and outer layers without sublayers distinction. Of course, these identified crystalline phases do not correspond to the entire deposits because many of them were molten while they were in

the boiler, thus changing their mineralogical structure and crystallinity into a glassy material, which cannot be identified with XRD. The synthetic sylvite (KCl) was the most intense phase in all layers. The intensity of KCl is higher at inner layers, especially at S1 (consistently with the important presence of more grown crystal cubic shapes). At S2 and S3 inner layers, another major phase can be observed such as the synthetic arcanite (K2SO4). For all samples, at the outer layers, the presence of silicates is increased with respect to the inner layers. As an example, diffractograms for the S2 sample are shown in Figure 8, where the peaks of the identified crystalline phases at the two layers (inner and outer) are marked. F


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usual temperature range in the boiler, indicating high adhesivity and high ability for particle capturing. Table 5 and Figure 10 show the bulk chemical analysis of inner and outer layers of different deposits (performed with ICP-OES, as recommended by ref 33 for the main elements present on these samples), together with the inorganic composition of the feed biomass. Figure 11 shows deposit chemical analysis performed with a different technique (EDX average area analysis) and including sublayer distinction in the inner layer. The results obtained are not comparable, not only because of the differences between the techniques but also because the studied layers are not strictly the same (inner layer is studied as a whole in Figure 10, but two inner sublayers analyzed in Figure 11). Consistently with the composition of the feed biomass, the most significant result derived from the chemical analysis of the deposits is the high sodium content (Table 5). High sodium contents tend to promote ash sintering and deposit growth, as proved by Benson and Sondreal34 in their experiments with coal combustion in pulverized fuel fired boilers. In high temperature zones (above 1038 °C) sodium composing deposits in this type of boiler is usually found in amorphous molten phase rather in crystalline phase.34 However, in this work, deposits were sampled from zones at lower gas temperature (below 675 °C), and therefore, sodium was observed in both molten and crystalline phases. The high Na and K contents in the original biomass lead to molar ratios (K + Na)/(2S + Cl) much higher than unity (Table 5). This implies an excess in K and Na to form KCl and NaCl, with KOH (g) and NaOH (g) remaining in the gas stream, which may react in the surface of solid fly ash particles to form alkali silicates,35 with low melting temperatures, and therefore present in molten phase. This hypothesis is supported by the presence of molten areas on the surface of deposits observed with SEM, as commented above. Figure 10 shows that the potassium content is higher in the inner layers while the rest of elements are found in higher concentrations in the outer layers. Sodium content varies erratically: in S1 and S3 it is lower in the inner layer than in the outer layer, whereas in S2, this trend is opposite. The results are consistent with the formation mechanisms of the different layers, indicating that at the outer layers the impaction of particles is the predominant mechanism, while at the inner layers it is the condensation of vapor species and the deposition of submicrometer particles.36 Such submicrometer particles usually have high content of volatile species (K, Cl, S) and can be formed by homogeneous condensation of volatile species or may be coated on the same condensates. Jiménez and Ballester25 found that the composition from submicrometer particles of pulverized orujillo was mainly K, S, and Cl (apart from elements with atomic number below 11, not detected). The content of inorganic elements obtained from the area analysis with EDX (Figure 11) is, in general, lower than that obtained by ICP-OES analysis. In general, similar trends to those observed with ICP-OES are obtained, but for Na the trend is opposite to that observed with ICP-OES and its amount increases with the distance from the tube, with increasing values toward the outer layer in all samples. Contents of S and Cl are determined with EDX. The absence of sulfur in sample S1, taken at the backside of the tubes, can be remarked upon. For the other samples (S2 and S3) the sulfur content decreases when moving away from the tube. This is consistent with the results of the XRD analysis, where crystalline sulfur species (K2SO4) were only found in the inner layer. These results are in agreement with

Figure 8. X-ray diffractogram of S2 outer and inner layers.

Figure 9. Specimen images at 500 and 580 °C and fusion tests. For the outer layers the initial specimen height was taken at 550 °C and for the inner layers at 100 °C.

Figure 9 shows the specimen heights of the deposit samples after being subjected to fusion tests. Images of the specimens are also shown at 500 and 580 °C (expected temperatures at S3 location and at S1/S2 location, respectively). The high fusion temperatures (∼1200−1300 °C) of the outer layer are consistent with the assumption that the spherical particles are formed from fly ash melted at the combustion chamber (around 1300 °C25) and later vitrified while flowing in the gas stream. Results from the fusion test show that the inner layers melt at temperatures below 700 °C for all samples (including sample S3, whose inner layer does not reach 33% of the initial height up to 1180 °C, but its fluidity is evident even below 700 °C). Especially, sample S2 shows an extremely high fluidity at the G


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Energy & Fuels Table 5. Chemical Analysisa of Biomass Ashb and Deposits (wt %, dry basis) sample

Si

Al

Fe

Mg

Ca

Na

K

P

S

Cl

molar ratio (K + Na)/(2S + Cl)

OA (in ash) OB (in ash) OC (in ash) OD (in ash) FB (in ash) S1 inner layer S1 outer layer S2 inner layer S2 outer layer S3 inner layer S3 outer layer

1.27 4.52 1.57 3.95 5.83 3.91 9.50 3.07 10.61 5.62 9.91

0.25 1.00 0.28 1.31 1.57 0.88 1.95 0.73 2.15 1.28 1.96

0.36 0.69 0.31 0.79 0.95 0.63 1.47 0.51 1.48 0.93 1.36

1.02 3.93 0.86 1.68 2.77 1.48 6.21 1.62 5.06 2.90 5.58

5.50 9.31 4.86 4.10 7.17 5.39 15.03 4.50 13.50 8.14 14.27

18.81 3.14 29.99 0.18 5.23 8.80 5.85 5.35 7.85 11.45 7.85

14.39 26.82 4.93 24.21 24.42 42.30 21.36 45.24 20.44 36.44 24.08

1.18 2.01 0.61 1.29 1.66 1.32 3.32 1.17 3.19 1.89 3.35

0.77 1.39 0.45 1.19 0.60

2.87 2.23 2.99 3.47 2.99

9.20 5.51 12.75 3.64 6.99

a

Determination of Si, Al, Fe, Mg, Ca, Na, K, and P was done by ICP-OES, determination of S by high oxygen and temperature combustion and infrared absorption gas analysis (elemental analyzer), and Cl by ion chromatrography. Analysis of S and Cl was not performed for deposits because analytical techniques are less reliable for the deposit matrix. bThe chemical analyses of the biomass samples were performed directly on the biomass (Table 3), and the weight percentages in the ash (at 550 °C) were calculated.

contrary, for samples S2 and S3, this trend is not clearly observed. However, the expected decrease in chlorine content toward the outer layers is very clear if the outer sublayers of S2 are compared, as shown in Figure 12.

Figure 10. Chemical analysis of deposits by ICP-OES (wt %, dry basis). Figure 12. Chemical analysis with EDX of S2 outer sublayers (wt %, dry basis).

The sampling location has a significant effect on the chemical composition. As the temperature at which the deposits were formed decreases, the carbon and oxygen content in the deposits decrease and that of Cl, S, Ca, Si, Mg, Al and Fe increases. Similar results were found by Jimenez and Ballester,26 who observed higher KCl condensation in the exhaust gas at 360 °C than at 560 °C. Observation of the sample with higher magnification permits distinguishing different structures, which are classified as shown in Table 6. The prevalence of each type of structure in the different deposit samples and layers is variable. Spherical shapes are dominant in the outer layers of frontside samples (S2 and S3), consistently with the high contribution of the inertial impaction at this layers. Amorphous molten shapes, that their aspect is indicative of molten or softness, are scarce in the inner layers, and in contrast, granular shapes are frequent in these layers. Some of these granular structures are composed of round granules which are indicative of a certain molten state. Cubic and polyhedral structures can be mainly found in the internal layer of S1 (especially cubic shapes at the innermost sublayer) and in the cavities of the outer layers. Prismatic structures are mainly found in the outer layer and especially in sublayer O2 of S2.

Figure 11. Chemical analysis of deposits by EDX (wt %, dry basis).

those obtained by Hansen et al.31 on straw mature deposits and with Jimenez and Ballester25,26 on fly ash of pulverized orujillo, who found higher sulfur content (as K2SO4) at submicrometer particles (mainly present at inner layers) than at coarse particles (above 1.9 μm), where no appreciable amounts of S were obtained. The chlorine content decreases as the distance from the water tube increases for sample S1, in agreement with results obtained by Hansen et al.31 on straw mature deposits. On the H


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The chemical analysis shows that similar structures have, in general, similar compositions. Figure 13 (wt %) and Figure 14 (atoms) show the average composition of the structures identified regardless of the deposit where they were found. Cubic and polyhedric shapes are mainly composed of K and Cl with an atom K/Cl ratio close to unity. From both their chemical composition and their shape and size (from 11.7 to 40 μm) it can be inferred that these structures correspond to potassium chloride formed by heterogeneous condensation on the deposit surface, rather than by homogeneous condensation and subsequent thermophoresis or diffusion transport. Potassium chloride aerosols would have led to smaller crystals (from 0.01 to 1 μm) instead.4 In the case of polyhedrons, the crystal boundaries are often undefined, which may be the consequence of either some surface fusion after being formed or some embedment into a molten material or some accidental rupture when the sample was manipulated prior to microscopy observation. Tabular structures have also a repeated composition (three samples analyzed). This composition is quite close to that of monticellite (CaMgSIO4), which is among the crystalline phases identified with XRD (Table 4). Sodium shows highest concentration in prismatic shapes, which are typically observed in the outer layer of sample S2. The atomic ratio indicates the presence of sodium carbonate. The formation of these crystalline structures is probably associated with heterogeneous condensation of sodium carbonate on the deposit surface. Fibrous structures are rich in the same elements composing the prismatic structures, although in this case K and Cl have also high concentrations. However, it is hypothesized that such concentration is adsorbed from other neighbor structures rich in K and Cl. On the contrary, the observed concentration in sodium (also high) is probably inherent to fibrous structures, since no Na-rich structures are located closely. Although the composition of spherical, amorphous molten and granular structures is more variable, some common patterns can be found. Chlorine content is minor in both spherical and amorphous molten structures, whereas phosphorus content is higher than that of other structures. Calcium and sodium contents are high, but calcium content is higher in spheres while sodium is higher in amorphous molten (from 7 to 23% weight). Also silicon and magnesium contents are high in spherical structures, although not as high as in tabular ones. Similar composition was found by Bashir et al.9 for spherical structures. This pattern indicates that both spherical and amorphous structures were formed from fly ash particles, when they were

Table 6. Identified Structures (black asterisk indicates the location of the structure)

The presence of softness or molten structures (mainly amorphous molten shapes in our case) in the deposits is indicative of the ability for capturing particles2 and thus to rapidly increase the width of the deposit. Also the liquid content can bind solid ash particles,1 so the higher the liquid content, the denser the deposit and more tenacious it becomes and more difficult to be removed during the boiler cleaning operations.29 The high rate of deposition and the stiffness of the deposits increase the probability of blocking the flue gas.

Figure 13. Average chemical analysis, by EDX spot analysis, of the identified structures (wt %, dry basis). I


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Figure 14. Average chemical analysis, by EDX spot analysis, of the identified structures (atoms %, dry basis).

condensation and diffusion and thermophoresis transport. In these layers, granular structures composed of large amounts of fine agglomerates were observed in sampling regions S2 and S3, whereas in region S1 (located backside), crystalline structures of KCl were probably formed by condensation on the deposit surface. In the outer layers, calcium, silicon, magnesium, and other elements composing fly ash have increased concentrations, and the dominant structures are (differently to the inner layers) spherical and amorphous molten. Spherical structures are formed by fusion, and ulterior vitrification of solid particles impacting the deposit surface and amorphous structures are derived from ash particles which deposited on the surface at molten state. The sampling region also affects the physical, morphological, and chemical properties of the deposits. Deposits formed in the lowest temperature region (S3) are less stiff, and consequently their removal is easier. Backside deposits (S1) are less porous and more crystalline. Differently, frontside deposits (S2 and S3) are more porous and have similar morphologies. However, the external layer from S2 is much stiffer than that of S3 (temperature at S2 was higher). Carbon and oxygen contents decrease as the temperature decreases, whereas those of chlorine, sulfur, calcium, silicon, magnesium, aluminum, and iron increase. Amorphous molten structures showed high sodium content (7−23% weight), causing its molten state. These structures increase the capturing efficiency and the tenacity of the deposits, leading to large deposits, difficult to be removed. This confirms the important role of this element on the deposit formation mechanisms and proves that it is necessary to prevent the consumption of biomass with high sodium content. Fusion temperatures high enough in the feed biomass do not necessarily guarantee avoiding the formation of deposits. In fact, biomass particles from pulverized biomass could be burnt independently in the boiler, and thus, the solid phase of each component (some of them with negative results in the fusion test) may contribute independently to the formation of deposits. In biomass plants only the moisture content and heating value are usually measured to determine the price of the fuel and eventually the chlorine content to avoid contamination or deposit formation. From the results obtained, including the sodium content in the quality control of plants consuming residual products from olive, processing is recommended.

molten in the combustion chamber. The difference between them is that spheres were vitrified before deposition, whereas amorphous structures were deposited on the surface at molten state. Despite granular structures showing highly variable compositions, K/Cl atomic ratios close to unity were observed in several samples (3 out of 8 averaged samples). This indicates that homogeneously condensed KCl aerosols may have reached the deposits by diffusion or thermophoresis. This is in agreement with the results obtained by Broström et al.,37 who observed (from deposits sampled at frontside) that pure KCl gas nucleated homogeneously at tube temperatures below 550 °C (such as the tube temperatures at the sampling points in the present study). It is expected that, in hotter regions, condensation would mainly occur by direct condensation on the deposit surface. Finally, some of the granular structures showed high sulfur concentration (up to 6.3% weight) and relatively high K and O contents, which could either indicate the transport of K2SO4 condensed on the fly ash (due to the presence of Si, Al, and Fe) upstream of the sampling region (at higher temperatures) or the presence of sulfation reactions on the deposit surfaces.2,5 This could explain why 5 of the 8 averaged samples showed K/Cl ratios higher than unity. Nielsen et al.36 also found condensed K2SO4 on their granular structures.

5. CONCLUSION A complete chemical and morphological characterization of the deposits formed in the water tubes of the superheaters and the generator bank of a boiler fed with exhausted olive cake was performed after the consumption of a feedstock with unusually high sodium content. Also, the feed biomass was analyzed, together with the different exhausted olive cakes which composed the feed biomass. The high presence of Na in some of the analyzed biomass leads to a decrease in the proportion of the other elements and to a great increase on the (K2O + Na2O)/SiO2 index, indicative of probable fouling. Both chemical and morphological differences were observed between different sampling zones (and thus different sampling temperatures and positions relative to the flow) and within each deposit as a function of the distance from the tube surface (i.e., as a function of the layer position). Differences were associated with the predominant deposit formation mechanisms. In the inner layers, potassium and chloride are the most abundant (apart from O and C), and the dominant structures are those associated with J


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AUTHOR INFORMATION

Corresponding Author

*(M.L.) E-mail: [email protected]. Telephone: +(34) 926295431. Fax: +(34) 926295361. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ́ de la Mancha, S.A., is gratefully The company Energias acknowledged for funding this study and providing biomass and deposit samples as well as information about the plant operation.

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REFERENCES

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Fouling Deposits from Residual Biomass with High Sodium Content in

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